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Orbital Interactions in Group 6 Imido Diamido Complexes

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PAGE 1

ORBITAL INTERACTIONS IN GROUP 6 IMIDO DIAMIDO COMPLEXES By ELON AYINDE ISON A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2004

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Dedicated to my grandparents, John and Lucille McNeil.

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ACKNOWLEDGMENTS There are inevitably more people who have contributed to my experiences at the University of Florida than there is time or space to enumerate. However, I must acknowledge a few people without whom this work would not be possible. First and foremost, I must acknowledge my advisor, Dr. James Boncella, whose hands off but steady guidance was ideal in allowing me to learn and develop as an organometallic chemist. The patience and guidance of two men, Dr. Khalil Abboud who performed all the crystallographic experiments, and Dr. Ion Ghiviriga for his help in NMR, cannot go unnoticed. In fact, this thesis would not be possible if not for their efforts, I must also acknowledge Dr. Adrian Roitberg and Dr. Stephen Fau, at the University of Florida QTP facility for the help with computational chemistry and for giving us access to their facilities. I would be remiss if I did not acknowledge my past and present group members. In particular I would like to, acknowledge Alison Knefely, the last of the Boncella clan at the University of Florida. In the short time we worked together, she surely provided a pleasant working environment. She is a great lab-mate and friend, even though she is frequently on the wrong side of political issues. Her friendship and personality have surely been missed. Finally, I would like to thank my family. My parents Patsy, and Walter, have been supportive throughout this entire experience. My work ethic and values are almost totally iii

PAGE 4

attributable to their parenting. I would also like to acknowledge my little sister Aysha. I would not have made it through this experience if it were not for my wife Ana. In fact, if it were not for her I would literally never have figured out this template. She has been an absolute rock throughout this entire experience and most importantly has remained my best friend. There have been times throughout this experience when I thought that the struggle was not worth it. It was at these times that my daughter Mya has helped me keep it in perspective. iv

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TABLE OF CONTENTS page ACKNOWLEDGMENTS.................................................................................................iii LIST OF TABLES...........................................................................................................viii LIST OF FIGURES...........................................................................................................ix ABSTRACT......................................................................................................................xii CHAPTER 1 INTRODUCTION AND BACKGROUND.................................................................1 Theoretical Calculations and Methodology..................................................................3 ONIOM Calculations....................................................................................................4 Organoimido Chemistry and the Concept of Loading..............................................5 Bonding in Loaded Imido Complexes...............................................................6 Early Examples of loaded complexes................................................................9 Group 6 Imido Diamido Complexes...........................................................................10 Molybdenum and Tungsten Alkyl and Alkylidene Complexes..........................12 Complexes of Mo and W Imido Diamido Complexes Containing donor Ligands.............................................................................................................16 Scope of the Dissertation............................................................................................18 2 STRUCTURE REACTIVITY AND BONDING OF MOLYBDENUM IMIDO DIAMIDO COMPLEXES..........................................................................................20 Introduction.................................................................................................................20 Structure Dynamics and Bonding of Imido Diamido Alkyne Complexes.................21 Summary.....................................................................................................................33 Dynamics and Bonding of Molybdenum Imido Diamido Olefin Complexes............33 Diamide Ligand Folding in d 2 vs d 0 Group 6 Imido Complexes................................39 Conclusions.................................................................................................................44 3 SYNTHESIS AND REACTIVITY OF GROUP 6 IMIDO DIAMIDO METALLACYCLES..................................................................................................45 Introduction.................................................................................................................45 v

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Synthesis and Reactivity of Mo Imido Diamido Metallacyclopentenes and Metallacyclopentadienes........................................................................................46 Synthesis of metallacyclopentene complexes.....................................................47 Kinetics and Mechanism of the Thermal Rerrangement of 25............................51 Computational Studies on the Thermal Rearrangement of 25............................55 Synthesis and reactivity of a metallacyclopentadiene complex.................................65 Summary and Conclusions.........................................................................................68 4 SYNTHESIS AND REACTIVITY OF Mo(VI) COMPLEXES WITH ALKYL AND ARYL ISOCYANIDES....................................................................................70 Introduction.................................................................................................................70 Synthesis of Isocyanide Complexes and Insertion into the Metal-Diamide Bond.....70 Synthesis, Structure and Dynamics of a Chelating Imino Carbamoyl Complex........83 [2+2+1] Cycloaddition reactions of metallacycles with alkyl isocyanides RNC.......89 Summary and Conclusions.........................................................................................91 5 ALKYL ALUMINUM INDUCED DIAMIDE TRANSFER FROM GROUP 6 IMIDO DIAMIDO COMPLEXES.............................................................................92 Introduction.................................................................................................................92 Reactions of Mo Dialkyl Complexes with Trimethyl Aluminum (TMA)..................93 Reactivity of Alkyl Aluminum Halides with Group 6 Imido Diamido Olefin Complexes..............................................................................................................98 Reaction of metallacyclopentanes with alkyl aluminum halides......................100 Reactivity of alkyl aluminum reagents with styrene complexes................104 Reactivity with alkyl aluminum halides.....................................................108 Reactivity of an isobutylene complex with EtAlCl 2 .................................109 Summary...................................................................................................................114 Implications of Aluminum Induced Diamide Transfer to Olefin Dimerization Catalysis...............................................................................................................114 Reactions of W complexes with alkyl aluminum reagents...............................116 Diamide transfer to other W complexes............................................................117 Conclusions...............................................................................................................119 6 EXPERIMENTAL....................................................................................................121 General Methods.......................................................................................................121 Synthesis of 2 alkyne complexes..........................................................................121 Synthesis of bis-Isocyanide Complexes...................................................................126 Synthesis of a Chelating Imino Carbamoyl Complex, (33)......................................127 Reactions with alkyl aluminum reagents..................................................................128 Synthesis of [( 2 -ethylene)(Et)Mo(NPh) 4 -(o-(SiMe 3 N) 2 C 6 H 4 )Al(CH 3 ) 2 ] (42b)...130 Synthesis of [( 2 -styrene)(Me)Mo(NPh) 4 -(o-(SiMe 3 N) 2 C 6 H 4 ))Al(CH 3 ) 2 ] (39)....131 Synthesis of [( 2 -styrene)(Me)Mo(NPh) 4 -(o-(SiMe 3 N) 2 C 6 H 4 )}Al(Cl) 2 ] (40)......131 Synthesis of [( 2 NPh)Mo(Cl)( 4 -(Cl 2 Al((o-(Me 3 SiN) 2 -C 6 H 4 ))] 2 (41)...................133 Kinetic Study............................................................................................................134 vi

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Computational Studies..............................................................................................135 X-ray Experimental..................................................................................................137 X-ray Experimental for Complex 26b...............................................................137 X-ray Experimental for Complex 29.................................................................140 X-ray Experimental for Complex 25.................................................................142 X-ray Experimental for Complex 30b...............................................................144 X-ray Experimental for Complex 33.................................................................146 X-ray Experimental for Complex 34.................................................................148 X-ray Experimental for Complex 36.................................................................150 X-ray Experimental for Complex 41.................................................................152 LIST OF REFERENCES.................................................................................................155 BIOGRAPHICAL SKETCH...........................................................................................163 vii

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LIST OF TABLES Tables page 2-1 1 H and 13 C NMR data for alkyne complexes...........................................................22 2-2 Comparison of selected bond lengths and angles between model compounds 22 and 23 and the reported crystal structure for 21f b ...................................................27 2-3 NLMO analysis of alkyne bond............................................................................31 2-4 NBO analysis of MoN(imido) bonds...................................................................31 2-5 NBO analysis of the MoN(Imido) bond in 20c....................................................39 2-6 Fold angles for some Mo Imido Diamido Complexes.............................................40 2-7 NLMO analysis of MoN(Imido) and MoN(Diamide) bonds in 23 and 24......43 3-1 Dependence of DEAD on the formation of 26c.......................................................54 3-2 Selected bond lengths () and angles ( 0 ) for ONIOM optimized structures for the thermal rearrangement of 25..............................................................................60 4-1 Selected bond Lengths() and Angles( 0 ) for 31 and 32..........................................78 6-1 Kinetic analysis for the reaction of DEAD with 25...............................................135 6-2 Crystal data and structure refinement for 26b........................................................139 6-3 Crystal data and structure refinement for 29..........................................................141 6-4 Crystal data and structure refinement for 29..........................................................143 6-5 Crystal data and structure refinement for 30b........................................................145 6-6 Crystal data and structure refinement for 33..........................................................147 6-7 Crystal data and structure refinement for 34..........................................................149 6-8 Crystal data and structure refinement for 36..........................................................151 viii

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LIST OF FIGURES Figures page 1-1 The ONIOM extrapolation scheme ...........................................................................4 1-2 Limiting VB description of a metal imido linkage.................................................7 1-3 perpendicular set of N(p) orbitals in threefold symmetry....................................10 2-1 Thermal ellipsoid plot of 21e...................................................................................24 2-2 Variable temperature 1 H NMR spectrum of the SiMe 3 region for 21b....................25 2-3 B3LYP optimized models for alkyne complexes 22 and 23 and their transition states ........................................................................................................................27 2-4 Bonding interactions in between Mo, imido, and the alkyne fragment...................30 2-5 NLMO plots (isocontour 0.04) of MoN(imido) bond in 22 and TS 22 ..............32 2-6 Optimized ONIOM structures for 20c.....................................................................37 2-7 HOMO of 23 and 20c showing backbonding of the metal fragment to the olefin in 23 or acetylene in 20c................................................................................37 2-8 HOMO of TS23 and TS20c showing backbonding of the metal fragment to the olefin...................................................................................................................38 2-9 Occupied molecular orbitals of 24...........................................................................43 3-1 Thermal ellipsoid plot of 26b ..................................................................................52 3-2 First order kinetics for the formation of 26c............................................................54 3-3 Eyring plot for the reaction of 25 with DEAD.........................................................55 3-4 Thermal ellipsoid plot of 25.....................................................................................56 3-5 Optimized structures (mPW1K/LANL2DZ) for the thermal rearrangement of 25..58 3-6 Reaction profile G 0 298 (K) for the thermal rearrangement of 25 at the mPW1k/SDD-Aug-cc-pVDZ level of theory...........................................................58 ix

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3-7 ONIOM (mPW1K/LANL2DZ:mPW1K/LANL2MB) optimized structures for the thermal rearrangement of 25..............................................................................59 3-8 Occupied molecular orbitals(B3LYP/LANL2DZ) for 25........................................62 3-9 Occupied molecular orbitals (B3LYP/LANL2DZ) for 25b.....................................63 3-10 Occupied molecular orbitals (B3LYP/LANL2DZ) for 25c.....................................63 3-11 Thermal ellipsoid plot of 29.....................................................................................67 4-1 Thermal ellipsoid plot of 30b (30% ellipsoids).......................................................74 4-2 Thermal ellipsoid plot of 31.....................................................................................76 4-3 Optimized structure (ONIOM B3LYP/LANL2DZ:B3LYP/LANL2MB) for 31 and 32.......................................................................................................................79 4-4 3-Center-4-electron bonding between the diamide lone pair electrons the metal d orbitals and the orbitals of a acceptor ligand................................................79 4-5 Interaction of diamide lone pairs and isocyanide ligand with metal d xy orbital. ....81 4-6 Interaction of diamide lone pairs and CO ligand with metal d xy orbital. ...............81 4-7 Thermal ellipsoid plot of 33.....................................................................................85 4-8 Space filling diagram generated from the crystal structure of 33............................87 4-9 Variable temperature 1 H NMR spectrum (C 7 D 8 ) of 33 ...........................................88 5-1 Thermal ellipsoid plot of 34 ....................................................................................95 5-2 Thermal ellipsoid plot of 36.....................................................................................96 5-3 Optimized B3LYP structures for model complexes Me 2 Al((o-(Me 3 N) 2 -pda) -1 37, and (CH 2 ) 4 Mo(CH 3 )(NH)(o-(Me 3 N) 2 C 6 H 4 )Al(CH 3 ) 2 38 and their molecular orbitals......................................................................................................................98 5-4 1 H NMR spectrum of 42a, ( 2 -ethylene)Mo(Me)(NPh) 4 -(Cl 2 Al((o-(Me 3 SiN) 2 -C 6 H 4 )) in C 6 D 6. ................102 5-5 Proposed structure for 42b showing selected carbon and (parentheses) and proton chemical shifts assigned by NMR spectroscopy.........................................102 5-6 X-ray structure determination of 39a.....................................................................105 5-7 Proposed structure for 39a showing selected carbon and (parentheses) and proton chemical shifts assigned by NMR spectroscopy........................................107 x

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5-8 1 H NOESY spectrum of 39a..................................................................................107 5-9 Proposed structure for 39b showing selected carbon and (parentheses) and proton chemical shifts as assigned by NMR spectroscopy....................................108 5-10 1 H NOESY spectrum of 39b..................................................................................108 5-11 Thermal ellipsoid plot of 41...................................................................................112 5-12 1 H NMR spectrum of W metallacyclopentane methyl complex............................117 5-13 1 H NMR spectrum for the reaction of 12, with TMA............................................120 xi

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Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy ORBITAL INTERACTIONS IN GROUP 6 IMIDO DIAMIDO COMPLEXES By Elon Ayinde Ison August 2004 Chair: James M. Boncella Major Department: Chemistry The orbital interactions of group 6 imido diamido complexes have been investigated. The synthesis of alkyne complexes, 21, ( 2 -alkyne)Mo(NPh)(o-( Me 3 SiN) 2 C 6 H 4 ) by the reaction of alkynes with 20a, ( 2 -isobutylene)Mo(NPh)(o-( Me 3 SiN) 2 C 6 H 4 ), has been reported, along with the X-ray crystal structure of 21e, and the dynamic solution behavior of these complexes via low temperature. DFT calculations have been used to study the bonding in these complexes and an NBO analysis was used to determine the extent of donation by the alkyne ligand. DFT calculations were also performed on 20c and 24. The synthesis of metallacyclopentene complexes, 26, [(NPh)Mo(C(R)C(R)CH 2 CH 2 )(o-(Me 3 SiN) 2 C 6 H 4 )], by the sequential ligand exchange of ethylene from 25 and an alkyne is reported. An X-ray structure of the metallcyclopentene complex 26b, [(NPh)Mo(C(H)C(Ph)CH 2 CH 2 ){o-(Me 3 SiN) 2 C 6 H 4 }], is reported. The synthesis of the metallacyclopentadiene complex xii

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[(NPh)Mo(C(Ph)CHC(Ph)){o-(Me 3 SiN) 2 C 6 H 4 }], 29, a by the [2+2] cycloaddition reactions of two alkynes is reported. The thermal rearrangements of 25, was examined by kinetics and DFT. The synthesis and characterization of bis-isocyanide complexes, (RNC) 2 Mo(NPh)(o-( Me 3 SiN) 2 C 6 H 4 ), 30 of Mo and the subsequent reactivity of these complexes with excess isocyanide yielding tris-isocyanide complexes in the case of t BuNC and an unusual chelating iminocarbamoyl bis-isocyanide complex in the case of 2,6 dimethyl phenyl isocyanide are reported. X-ray structures of the bis-isocyanide complex 30a and the iminocarbamoyl bis-isocyanide complex are also reported. The reactivity of alkyl aluminum reagents with alkyl, olefin and alkynes has been demonstrated. Treatment of an orange solution of Mo(NPh)(o-(Me 3 SiN) 2 C 6 H 4 )(CH 2 ) 4 25, in toluene, with 1 equivalent of trimethyl aluminum (TMA) yields the metallacyclopentane methyl complex, (CH 2 ) 4 Mo(NPh)4 -[(o-(SiMe 3 N) 2 C 6 H 4 )}Al(CH 3 ) 2 34. A similar reaction occurs when an orange solution of the diphenyl acetylene complex, 35, is treated with 1 equivalent of TMA and yields the diphenyl acetylene methyl complex, ( 2 -diphenyl acetylene)(CH 3 )Mo(NPh) 4 -[(o(SiMe 3 N) 2 C 6 H 4 )}Al(CH 3 ) 2 36. The metallacyclopentane complex 25, reacts with alkyl aluminum halides to yield the olefin alkyl complexes ( 2 -ethylene)(R)Mo(NPh) 4 -[(o-(SiMe 3 N) 2 C 6 H 4 )}Al(CH 3 ) 2 42. The reaction of the styrene complex 20c, with ethyl aluminum dichloride or trimethyl aluminum results in the formation of complexes 39, ( 2 -styrene)Mo(Et)(NPh) 4 -(Cl 2 Al((o-(Me 3 SiN) 2 -C 6 H 4 )) or 40,( 2 -styrene)Mo(CH 3 )(NPh) 4 -((CH 3 ) 2 Al((o-(Me 3 SiN) 2 -C 6 H 4 )), respectively. In contrast, the xiii

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reaction of ethyl aluminum dichloride with 20a, results in the the displacement of isobutylene and the formation of the briging imido dimer [( 2 NPh)Mo(Cl)( 4 -(Cl 2 Al((o-(Me 3 SiN) 2 -C 6 H 4 ))] 2 41. The X-ray crystal structure of 41 was determined. xiv

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CHAPTER 1 INTRODUCTION AND BACKGROUND The transition metal series of the periodic table enables chemists to have access to a wide variety of compounds and chemical reactions that are otherwise inaccessible in main-group chemistry. Thus, a tremendous amount of research has been devoted to the development of new and interesting organometallic complexes with an emphasis on the understanding of the structure, bonding and reactivity of the transition metal species. These efforts were devoted to the synthesis and structural characterization of new organometallic complexes that are supported by various types of ancillary ligands including cyclopentadienyl, (Cp), trispyrazoyl borate (Tp), amides, alkoxides and imines. 1 Significant advances in the field of catalysis and synthetic organic chemistry have occurred as a result of these studies; the number and type of organometallic catalysts are too innumerable to be discussed here. This pioneering work has provided insight into the bonding and general understanding of organometallic mechanisms. The reactivity observed at a given metal center is highly dependant on the electronic as well as steric requirements of the ancillary ligands that support a particular metal species. Consequently, synthetic organometallic chemists have focused on the derivitization and discovery of new ligands, with the goal of tuning the reactivity at a metal-center. The development of molecular orbital (MO) theory has allowed the structural and reactivity trends to be discussed in terms of specific interactions of molecular orbitals. 2,3 The accuracy of these methods depends on the way in which interactions between electrons are handled a concept defined as electron correlation. Hartree Fock (HF) 1

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2 theory, for example, provides a method for converting the many-electron Schrdinger equation, into many simpler one-electron equations that can be solved to yield single electron wavefunctions called orbitals. However, significant errors may arise because electron correlation in these methods is treated in an average sense. Technological advances have allowed for the successful employment of gradient corrected density functional theory (DFT) in calculating molecules, particularly involving transition metals, and in the use of small core relativistic effective core potentials (ECPs). DFT offers an advantage over traditional HF methods because it implicitly treats electron correlation. This has set the stage for the calculation of geometries, bond energies, vibrational spectra, activation energies for chemical reactions and other important properties of transition metal complexes. 4-7 These achievements have truly enhanced the understanding of the nature of the bonding seen in transition metal complexes and have allowed researchers to accurately make predictions about the existence of intermediates, the stabilities of species and the accessibility of kinetic pathways in reaction mechanisms. In this dissertation we describe our latest efforts in an ongoing study of group 6 imido complexes supported by a chelating ancillary ligand. To study these complexes we employ the techniques of X-ray crystallography, NMR spectroscopy and computational chemistry(DFT), in order to properly assess the effects of ligand donation on the properties and reactivity of these complexes. This chapter begins with a brief overview of the computational methods used to study these complexes and then proceeds to describe bonding in transition metal imido species and the concept of loading. A brief

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3 account of the group 6 imido diamido complexes previously synthesized in the Boncella lab along with an outline for the dissertation is also presented. Theoretical Calculations and Methodology Unlike Hartree-Fock calculations, density functional theory (DFT) implicitly treats electron correlation by utilizing an energy functional, some with empirically adjusted parameters 4-6 : E xc () = E x () + E c () where E xc is the exchange correlation functional E x is the exchange functional E c is the correlation functional Modern functionals such as the generalized gradient approximation (GGA) or gradient corrected functionals and the so called hybrid functionals employ an admixture of exact Hartree-Fock (HF) exchange functionals and correlational exchange functionals, and have been shown to provide accurate geometries and reasonable energetics for a wide variety of systems at a computational cost similar to HF theory. DFT has a distinct advantage over traditional wave-function based techniques (ab initio) because the explicit addition of electron correlation to these techniques becomes quite computationally expensive. In performing calculations on complex systems involving transition metals it is sometimes necessary to simplify the ligands of a given system not only to expedite the calculations but also to perform the number of calculations necessary to appropriately describe the system under study. However, these simplifications can and sometimes do lead to problematic interpretations and predictions of molecular systems as the steric and/or electronic consequences of ligand trimming must be considered. The electronic

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4 consequences of ligand trimming are separate from and can compound with an incomplete basis set and an imperfect energy functional. ONIOM Calculations An obvious solution for the treatment of complex molecular systems is the partitioning of the system into two or more parts or layers, where the interesting or difficult part of the system (inner layer) is treated at a high level of theory and the rest of the system with a computationally less demanding method. The ONIOM (Our owN n-layered Integrated molecular Orbital + molecular mechanics Method) developed by Morokuma and co-workers is a hybrid method designed to enable different levels of theory to be applied to different parts of a molecule/system and combined to produce a consistent energy expression. 8-12 The basic idea behind the ONIOM approach can be explained as an extrapolation scheme in a two dimensional space with the size of the system on the x-axis and the level of theory on the y-axis (Figure 1-1). Figure 1-1. The ONIOM extrapolation scheme for a molecular system portioned into two (left) and three (right) layers

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5 In order to describe the real system at the highest level of theory, point 4 in a system partitioned in two layers and point 9 in a system partitioned in three layers, the extrapolated energy is defined as E ONIOM2 = E 3 E 1 + E 2 where E 3 is the energy of the entire (real) system calculated at the low level method and E 1 and E 2 are the energies of the model system determined at the low and high level of theory respectively. E ONIOM2 is an approximation to the true energy of the real system E 4 : E 4 = E ONIOM2 + D. If the error, D, of the extrapolation procedure is constant for two different structures their relative energy E 4 will be evaluated correctly by the ONIOM method. The accuracy of the ONIOM method depends on two factors: the choice of the lower level of theory and the model system. Thus the model system and level of theory must be chosen so as to accurately describe the energetics of the real system. Morukuma, Milstein, 13,14 Landis 15,16 and others have successfully employed this method to transition metal systems. We have successfully employed this method throughout this dissertation in describing the consequences of ligand donation in group 6 imido diamido complexes. Organoimido Chemistry and the Concept of Loading Organoimido transition metal chemistry has received attention in recent years because of its implication in several catalytic processes such as such as propylene ammoxidation and nitrile reduction. Imido complexes have also been shown to function as imido transfer intermediates in the aziridination and amination of olefins. There have been several reviews in recent years; the definitive work on imido chemistry occurs in Nugent and Mayers Metal-Ligand Multiple Bonds, 17 in addition there have been more recent reviews by Wigley 18 and Eikey and Abu-Omar 19 et al.

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6 Recent interest in imido chemistry has focused on the electronic structure and reactivity of imido complexes towards CH bond activation. This involved repeated coordination of imido ligands to a single metal center, a strategy defined as loading by Wigley group 20-24 The premise behind the loading strategy is that repeated coordination of more than one strongly -bonding ligand will lead to increased competition for metal dacceptor orbitals. This will result in the localizition of charge at one of these donor ligands and hence weakened bonding and increased reactivity at this ligand. The Wolczanski and Horton groups have studied d 0 bis-imido complexes of Ta and V, and have demonstrated their ability to activate methane. Cundari and co-workers have compared methane activation by three-coordinate group 4, and group 5, and bis-imido amido complexes. 25-27 The results of these early studies demonstrate clearly that the strategy of loading is a potentially useful method for generation of highly reactive organometallic species. Bonding in Loaded Imido Complexes The imido ligand can be considered to bond to a transition metal with one bond and either one or two bonds. The simplest model for bonding in complexes employing these ligands is a simple limiting valence bond (VB) model (Figure 1-2 ). 18 Figure 1-2 depicts the three limiting structures predicted by the simple VB model. Structure A depicts an sp 2 hybridized nitrogen leading to a M=N double bond (1, 1) and a bent MNR linkage with the lone pair residing in a N(sp 2 ) orbital. In this formalism the imido behaves as a 4e -1 donor. In structure B, the M=N (double bond) is maintained if symmetry restrictions or a severe electronic mismatch with the available metal orbitals does not allow lone pair donation.

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7 MNRMNRMNN (sp2)[NR]24e-N (sp)[NR]24e-N (sp)[NR]26e-ABC R Figure 1-2. Limiting VB description of a metal imido linkage Most of the imido complexes described in the literature can be described by Structure C, where the imido ligand is formally a six electron donor (1, 2). There are very few examples of bent imido complexes in the literature. The classic example is found in the bis-imido complex Mo(NPh) 2 (S 2 CNEt 2 ) 2 1, 28 which contains one bent imido (MoNC =139.4(4) 0 ) and one linear imido (MoNC = 169.4(4) 0 ) (Scheme 1-1). MoNSSNEtEtSNEtEtSN1 Scheme 1-1. Mo(NPh) 2 (S 2 CNEt 2 ) 2 a classic example of a bent imido complex

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8 The bent imido can be described by the limiting structure A, whilst the linear imido can be described by the limiting structure C, and this allows the complex to attain an 18-electron configuration. Several researchers have attempted to use similar VB arguments to describe the bonding in imido complexes and implied that the MN bond length and MNC bond angles should reflect the limiting structures in Figure 1-2. However, it has been demonstrated by Cundari et al., 25-27 that the VB description of the metal imido bond is not sufficient to describe the bonding in these complexes. Using ab initio calculations on a series of imido complexes these authors have shown that eight different resonance forms may be used to describe the MN bond (Scheme 1-2). The results of these studies clearly demonstrate that the MN (imido) bond cannot be described by a single resonance form as suggested by the VB description and a molecular orbital approach is often needed to accurately describe the bonding in metalimido complexes. MMNRMMNNRNRRMMNRMMNNRNRR Scheme 1-2. Eight principal resonance contributors for the MN (imido) bond. In this representation a straight line represents a covalent bond (i.e one electron in a TM-centered AO-like MO and one in the N centered counterpart), an arrow pointing towards the metal represents a dative bond and an arrow pointing towards the N represents a back bond. The bottom line (or arrow) describes the bond and the other lines components of the MN linkage.

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9 Early Examples of loaded complexes. Early examples of heavily -loaded complexes involve multiple coordination of imido ligands. The threecoordinate complex Os(N-2,6-C 6 H 3 -i-Pr 2 ) 3 2, by Schrock et al. 29 and the four-coordinate anion [W(N-2,6-C 6 H 3 -i-Pr 2 ) 3 Cl] 3, 23 are the classic -electron complexes (Scheme 1-3). Both complexes are characterized by a ligand based, non-bonding MO comprised of a set of ligand orbitals oriented roughly perpendicular to the molecules C 3 axis which results in these complexes being described as 18-electron complexes. For example, the [W(N-2,6-C 6 H 3 -i-Pr 2 ) 3 Cl] molecule is best described as an 18-electron complex, as one combination of the imido nitrogen p orbitals has a 2 symmetry, and there is no corresponding orbital metal d orbital that can interact with this orbital (Figure 1-3). NOsNNPriiPriPriPrClWNNNPriPriiPriPriPriPr-Wigley et.al.J Am. Chem. Soc.1991, 113, 6326 6328Schrock et.al.J Am. Chem. Soc.1990, 112, 1643 164523 Scheme 1-3. Examples of loaded complexes

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10 .ZZset Figure 1-3. Pi perpendicular set of N(p) orbitals in threefold symmetry for 3 This loading strategy may be applied to complexes employing ligands other than imido or a combination of imido and another donor ligand such as alkynes and amides. The Boncella group has employed this loading strategy in the synthesis of imido diamido complexes. The conflict that arises because of the presence of several donor ligands is responsible for the interesting chemistry of these molecules. Group 6 Imido Diamido Complexes The Boncella group has developed extensive chemistry in the area of group 6 imido diamido complexes starting from the two metal dichloride species Mo(NPh)(o-(Me 3 SiN) 2 C 6 H 4 )Cl 2 (THF), 4, 30 and W(NPh)(o-(Me 3 SiN) 2 C 6 H 4 )Cl 2 5. 31 Their syntheses are shown in Schemes 1-4 and Scheme 1-5. The Mo dichloride complex 4 is synthesized by treating a suspension of the bisimido complex, Mo(NPh) 2 Cl 2 (DME) 6, with a pentane solution of the diamine o-(SiMe 3 N) 2 (-C 6 H 4 ), and allowing the suspension to stir for 24hrs. The aniline complex Mo(NPh)(o-(Me 3 SiN) 2 C 6 H 4 )Cl 2 (aniline), 7, is generated by an apparent transfer of the diamine protons to one of the imido ligands in 6. Lewis bases readily displace aniline in this complex. Thus stirring 7 in THF readily generates 4 quantatatively.

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11 NH2NH2NHNH(i) 2 equivs SiMe3Cl 2 NEt3,Et2O(ii) Mo(NPh)2Cl2(DME), 6, pentane (24 hrs)(iii) THFSiMe3SiMe3MoNClClNNPhMe3SiMe3SiH2NMoNClClNNPhMe3SiMe3SiO(i)(ii)(iii)aniline +74 Scheme 1-4. Synthesis of Molybdenum imido-diamido dichloride, (4). The W dichloride complex is synthesized by treating the dilithiated salt of the diamine with a pentane solution of the W tetrachloride complex (NPh)WCl 4 (OEt 2 ), 8 Scheme 1-5. These complexes serve as starting materials for some extremely rich and diverse chemistry that stems from the electron rich environment provided by the diamide and imido ligands. Molecular orbital calculations have demonstrated that in addition to the imido ligand donating its 6 electrons (1, 2), the diamide ligand could donate its lone pair electrons into a metal d orbital of appropriate symmetry. 32,33 Thus, all the t 2g orbitals on the metal are occupied. The addition of an additional donor ligand to the metal would result in a competition for available d orbitals ,i.e., a loaded environment on the metal center.

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12 NH2NH2NHNH(i) 2 equivs SiMe3Cl 2 NEt3,Et2O(ii) (a) 1. 2 equivs n-BuLi (b) (NPh)WCl4(OEt2), 8, pentaneSiMe3SiMe3WNClClNNPhMe3SiMe3Si(i)(ii)5 Scheme 1-5. Synthesis of Tungsten imido-diamido dichloride, (5). Molybdenum and Tungsten Alkyl and Alkylidene Complexes The dichlorides 4 and 5 were easily converted to alkyl complexes, 9, and 10, respectively, upon treatment with non--hydrogen-containing alkyl magnesium reagents (Scheme 1-6). The generation of the bis-neopentyl complexes M(NPh)(o(Me 3 SiN) 2 C 6 H 4 )(CH 2 (C(CH 3 ) 3 ) 2 (M = Mo, 9c; M = W, 10c) provided an avenue into alkene metathesis chemistry. When compounds 9c and 10c, were heated in the presence of PMe 3, the alkylidene species M(NPh)(CHCMe 3 )(PMe 3 )(o-(Me 3 SiN) 2 C 6 H 4 ) (M = Mo, 11; M = W 12) are generated via abstraction (Scheme 1-7). Compound 12 has been shown to be an active catalyst for the methathesis of terminal olefins but competing decomposition processes drastically limit the catalysts lifetime. 34

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13 M = Mo: R = Me (9a), Ph (9b), CH2CMe3 (9c), CH2Ph (9d), CH2SiMe3 (9e); M = W: R = Me (10a), Ph (10b), CH2CMe3 (10c), CH2Ph (10d), CH2SiMe3 (10e), CH2CMe2Ph (10f).+ 2.0 Mg2+ Cl-MoNClClNNPhMe3SiMe3SiWNClClNNPhMe3SiMe3SiMNRRNNPhMe3SiMe3SiO45 Scheme 1-6. Synthesis of molybdenum and tungsten dialkyl complexes. M = Mo (9c), W (10c)70 oC+ CMe4PMe3WPMe3NNNMe3SiMe3SiPhMNNNPhMe3SiMe3SiM = Mo (11), W (12) Scheme 1-7. Formation of molybdenum and tungsten alkylidene adducts.

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14 70 oCTolueneWNNNMe3SiMe3SiPhWNNNMe3SiSiPhWPMe3NNNMe3SiMe3SiPh12+ PMe3ethyleneWNNNPhMe3SiMe3Si1312b12c Scheme 1-8. Reversible metallation of 12 in the absence of PMe 3 and the formation of 13 in the presence of excess ethylene Removal of PMe 3 from complex 12 results in a reversible metalation of one of the SiMe 3 groups of the o-(Me 3 SiN)(C 6 H 4 ) ligand as shown in Scheme 1-8. When 12 is treated with excess ethylene the metallacyclopentane complex 13, W(NPh)(o-(Me 3 SiN) 2 C 6 H 4 )(CH 2 ) 4 is formed. The mechanistic details of the formation of 13 from 12 in the presence of excess ethylene have been detailed in the literature and will not be discussed here. 34 For the purposes of this discussion it will suffice to say that the reaction 12 with ethylene to form 13 is a deactivation pathyway when 12 is used as an olefin metathesis catalyst for terminal olefins. It is noteworthy that the Schrock group has also observed metallacyclopentanes in olefin metathesis reactions. 35-42 However, the metallacycles formed with these species are particularly unstable and decompose via hydrogen abstraction/transfer mechanisms.

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15 The difference in reactivity between Schrocks complexes (Scheme 1-9) and ours originates from the nature of the ancillary ligands. The bis-alkoxide ancillary ligands in Schrocks complexes, 43 14 and 15, are not very good at stabilizing the metal center via donation. In our complexes the diamide ligands have a high propensity to donate nitrogen p electrons to metal orbitals of appropriate symmetry. The implications of these interactions are the primary focus of this dissertation. WNOORRa) R = CCH3(CF3)2b) R= tBuiPrPriMoNOORRiPrPri1415 Scheme 1-9. Examples of Schrocks metathesis catalysts Reaction With Alkyl Isocyanides The reactivity of tungsten alkyls 10a and 10c has been explored with tert-butyl isocyanide (Bu t NC) (Scheme 1-10). 44 Complexes 10a and 10c inserted Bu t NC into each metal alkyl bond, affording the 2 -imino-acyl complexes 16 and 17. When heated, 16 underwent a carbon-carbon coupling reaction giving 18. This insertion into metal alkyl bonds and the subsequent coupling chemistry have been well documented for early metals with CO or isocyanides. 45 However, the related insertions of isocyanides into metal amide bonds are less well studied, and given the ligand centered nature of the

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16 highest occupied molecular orbitals in these complexes, isocyanide insertion into the metal diamide bond might be expected. R.TButNCToluene85 oC, 2h(only 16)R = Me (16), CH2CMe3(17)WNNNNNPhMe3SiMe3SiWNNNNPhMe3SiMe3SiNWNRRNNPhMe3SiMe3SiR = Me (10a), CH2CMe3 (10c)18 Scheme 1-10. Reactivity of tungsten dialkyls with t BuNC. Complexes of Mo and W Imido Diamido Complexes Containing donor Ligands Alkyl complexes containing hydrogens (10g, R = Phenethyl; 10h, R = Ethyl) can be synthesized and isolated with W. These complexes are stable at room temperature, but the treatment of 10g and 10h with a Lewis Base (PMe 3 ) results in the formation of the olefin complexes 19 (Scheme 1-11). 32 The formation of the olefin complexes 19, has been showed to proceed by a -hydrogen transfer from the ethyl and phenethyl groups in 10g and 10h respectively. This reaction is unusual as it is promoted by the association of a PMe 3 ligand whereas

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17 hydrogen transfer reactions are usually promoted by the dissociation of a ligand. In contrasts, Mo alkyls containing -hydrogens are not stable and undergo -hydrogen transfer reactions at 0 C resulting in the formation of stable isolable olefin complexes 20 (Scheme 1-12). 46 1. 2.0 equivs RMgCl, -780C2. Room Temp3. R = Et, Phenethyl2.0 equivs PMe3Room TempWNClClNNPhMe3SiMe3SiWNMe3PPMe3NNPhMe3SiSiMe3WNRRNNPhMe3SiMe3Si+ RH510g and 10hR19a, R = H; 19b, R = Ph Scheme 1-11. Synthesis of W olefin complexes by PMe 3 promoted -hydrogen transfer 1. 2.0 equivs RMgCl, -780C2. Room Tempa. R1= Me R2=Meb. R1= Me R2= Hc. R1= Ph R2= HR= ethyl, propyl, i-butylMoNClClNNPhMe3SiMe3SiMoNNNPhMe3SiMe3SiR1R2 + RH420 Scheme 1-12. Synthesis of olefin complexes of Mo

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18 The olefin complexes, 20, have served as useful starting materials for the synthesis of several complexes incorporating donor ligands. These include arenes, imines, 47 butadienes, 48 alkynes, and pyridine ligands. 49 These complexes all exhibit unusual structural properties as well as interesting chemical reactivity. These properties originate from the electron rich environment at the metal center provided by the diamide and imido ligands. Our interest in studying these complexes stems from a desire to understand the implications of loading in these complexes, an in so doing, produce species that may be used as catalysts in organic synthesis. Scope of the Dissertation The implications of competitive ligand donation ( loading) in group 6 imido diamido complexes are systematically discussed in this manuscript. We have attempted to do this by utilizing a series of techniques (DFT, X-ray crystallography, NMR spectroscopy, and kinetic and mechanistic studies), in an attempt to attain a deeper understanding of the influence of the diamide and imido ligands on the properties of these complexes. The effects of loading competition between alkyne and imido ligands, as well as the electronic origins of ligand folding in d 0 and d 2 complexes are discussed in Chapter 2. The synthesis of metallacyclopentenes via ligand exchange and the kinetics and mechanism of the thermal rearrangement of metallacyclopentanes are studied by DFT and kinetic studies and are presented in Chapter 3. Reactions at the diamide ligand are discussed in Chapters 4 and 5. The synthesis of isocyanide complexes and the isocyanide insertion in the metal diamide bond to form an iminocarbamoyl complex is discussed in Chapter 4, whilst the alkyl aluminum induced diamide transfer from group 6

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19 imido diamido complexes and the implications to olefin dimerization/codimerization catalysis are discussed in Chapter 5.

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CHAPTER 2 STRUCTURE REACTIVITY AND BONDING OF MOLYBDENUM IMIDO DIAMIDO COMPLEXES Introduction Species containing transition metal to ligand multiple bonds are widespread and play an important role in organometallic chemistry. 17 These species usually contain ligands such as imido, nitrene, oxo and alkylidene s that bind to the metal via overlap of the ligand p and metal d orbitals. When there are several donors on a given metal center, ligand to metal bonding results in a competition for available unfilled metal d orbitals. The term loading has been used by Wigley and others to describe this phenomenon, and it has been suggested that the competition for metal orbitals can lead to increased reactivity of organometallic complexes. 20-27 While the structural and electronic effects of loading is well documented for both tetrahedral and octahedral (bis)imido and mixed oxo-imido complexes, 50-53 there is a sparsity of similar studies on ability of alkyne ligands to compete as donors on a given metal center in a loaded environment. Although, it has been demonstrated that alkynes can stabilize high oxidation states of transition metals via donation of their electrons, it is not clear whether these interactions are strong enough to compete with traditional donors. Recently, our group has investigated the chemistry of group 6 imido complexes of the form M(NPh)(o-(Me 3 SiN) 2 C 6 H 4 )X 2 (M=Mo and W) which incorporate the o-pda (o-phenylenediamine) ligand. 30,31 This work has demonstrated that the diamide ligands are 20

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21 involved in a multicenter donor interaction with the metal center via the diamide lone pair electrons. The success in the synthesis of high oxidation state olefin complexes, 34,46 inspired us to pursue the synthesis and reactivity of alkyne complexes, and in so doing answer the question: Can donation of the alkyne electrons effectively compete with the imido and diamide ligands on the metal center for empty metal d orbitals? In this chapter, the synthesis of alkyne complexes, 21, ( 2 -alkyne)Mo(NPh)(o-( Me 3 SiN) 2 C 6 H 4 ), along with the X-ray crystal structure of 21e, and their solution behavior via low temperature 1 H NMR spectroscopy is discussed. DFT calculations (B3LYP) have been used to study the bonding in these complexes and an NBO analysis was used to determine the extent of donation by the alkyne ligand. Structure Dynamics and Bonding of Imido Diamido Alkyne Complexes Synthesis of Alkyne Complexes Base-free molybdenum alkyne complexes of the type [ 2 -(alkyne)Mo(NPh)-o-(Me 3 SiN) 2 C 6 H 4 )], 21, were prepared by treating pentane solutions of 20 with the appropriate alkyne, followed by removal of solvent under reduced pressure (Scheme 2-1). The alkyne reactants displace the bound olefin with complete conversion to products within 15 min at 20 C. The 1 H NMR spectra of 21 are consistent with monomeric alkyne complexes. As shown in Table 2-1, the 13 C NMR resonances of the carbons of the alkyne fragment are significantly deshielded suggesting the involvement of the alkyne electrons in the Malkyne bonding. 54-60 Alkyne complexes 21a-e are air sensitive, but stable in solution and in the solid-state. In contrast, the alkyne complex 21f is thermally unstable and decomposes in solution at C within ca. 12 h.

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22 Table 2-1. 1 H and 13 C NMR data for alkyne complexes 13 C NMR, ppm 1 H NMR, ppm Tc, K G Kcal/mol Complex R 1 R 2 C 1 C 2 3a CO 2 Me H 175 177 9.2 257 13.2 3b Ph Me 185 180 -268 13.4 3c SiMe 3 Me 200 181 -268 13.4 3d Ph Ph 180 ----3e SiMe 3 SiMe 3 205 ----3f Me Me 187 ----1. 2.0 equivs RMgCl, -780C2. Room Temp1.0 equiv alkynea. R1= Me R2=Meb. R1= Me R2= Hc. R1= Ph R2= Ha. R2=CO2Me R3= Hb. R2= Ph R3=Mec. R2=SiMe3 R3= Med. R2= Ph R3=Phe. R2=SiMe3 R3=SiMe3f. R2= Me R3=MeR= ethyl, propyl, i-butylMoNClClNNPhMe3SiMe3SiMoNNNPhMe3SiMe3SiR2R3MoNNNPhMe3SiMe3SiR1R2O42120+ RH,+ 2.0 equivs Cl-, Mg2+ Scheme 2-1. Synthesis of alkyne complexes A single crystal X-ray analysis was performed on a single crystal of 21e. This crystal structure was originally reported by a previous graduate student, Tom Cameron, 61 however, it is included here for the purposes of discussion. As shown in the thermal ellipsoid plot in Figure 2-1, the alkyne complex adopts a pseudo square pyramidal structure with the imido ligand occupying the apical position. The alkyne ligand is oriented perpendicular to the Mo=N bond of the cis imido ligand and the MoC(19) and

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23 MoC(20) bond lengths, 2.078 and 2.079 respectively, are consistent with MoC single bonds. The C(19)C(20) bond length for 21e, 1.305 is close to the generally accepted value for CC double bonds. Pronounced back bonding is responsible for the lengthening of this alkyne bond, and the structure of 21e is thus best described as having a large contribution from a metallacyclopropene structure. In this resonance form, the alkyne ligand can be considered as a dianionic ligand and the formal oxidation state of the metal would be best described as Mo(VI). In addition, the diamide ligand is folded through concerted torsion of the NR group about the CN bond of the pda ring. The fold angle (angle between the N(2) Mo N(3) plane and the plane of the pda ring) is 133 0 This folding has been attributed to donation of the diamide lone pair electrons into the empty metal d x 2 y 2 orbital. 32,33,62 In solution, the alkyne complexes exhibit C s symmetry just as in the solid state. In complexes 21d-f, the alkyne substituents are the same, and a plane of symmetry bisecting the CC bond of the alkyne, containing the N=M bond of the imido ligand, and bisecting the o-pda ring causes the SiMe 3 protons to become chemically equivalent. These protons appear as a broad peak at 0.4-0.6 ppm. However, in complexes 21a-c where the alkyne substituents are different, there is no longer a plane of symmetry and the SiMe 3 protons are no longer equivalent. These protons appear as a broad singlet in the 1 H NMR spectra at room temperature. However, cooling a C 7 D 8 solution to 0 C, results in the splitting of this peak into two singlets (Figure 2-2). Using the two site exchange approximation, 63 the activation barrier for this process has been measured to be 13.2 Kcal/mol. Interestingly, the barrier is the same regardless of the alkyne substituent (Table 2-1).

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24 This fluxional process can be explained by rotation of the alkyne fragment about the Moalkyne centroid axis. This motion takes the alkyne ligand through a transition state that is also of C s symmetry (Scheme 2-2) where the alkyne is oriented parallel to the imido ligand. Figure 2-1. Thermal ellipsoid plot of 21e, [Mo(NPh)( 2 bis-trimethysilylacetylene){o-(Me 3 SiN) 2 C 6 H 4 }], (40% probability thermal ellipsoids) selected bond lengths (): MoN(1), 1.745(2); MoN(2), 2.023 (2); MoN(3), 2.009 (2); MoC(19), 2.078 (3); MoC(20), 2.079 (3); C(19)C(20), 1.305, (4).

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25 MoNNNMe3SiMe3SiPhR2R3MoNNNPhMe3SiMe3SiR3R2MoNNNPhMe3SiMe3SiR2R3 Scheme 2-2. Alkyne rotataion in 21 MoNNNPh(B)Me3Si(A)Me3SiR3R2AB-8 0C-20 0C Figure 2-2. Variable temperature 1 H NMR spectrum of the SiMe 3 region from to 8 0 C for complex 21b

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26 Molecular orbital calculations To elucidate the bonding pattern of the alkyne fragment in these complexes, density functional theory calculations (DFT) were performed. Initial calculations were carried out on model compound 22 (Figure 2-3), which is a simplified version of 21 where the o-phenylene group (o-C 6 H 4 ) that links the two N atoms of the diamido ligand was simplified to a CH=CHcarbon chain. The organic groups on the nitrogen atoms (SiMe 3 of the amido and phenyl group of the imido) were replaced by hydrogen atoms for simplicity. Propyne was used as the alkyne fragment in this model. The calculated structure for 22 is generally in good agreement with the X-ray crystal structure data obtained for 21e and 21f. 48 The only exception is that the ligand is significantly less folded in this model than in the actual compounds (147 0 vs 133 0 ). This is due to the inability of the hydrogen atoms on the diamide ligand to successfully model the bulky SiMe 3 group. In order to obtain a more accurate model for the alkyne complexes we employed the ONIOM approach (developed by Morokuma 8-12 et.al 9,10,11,12 ) to model the complete ligand system with all substituents used in the experiment (Figure 2-3). We defined a two-layer model, (model compound 23) for the 2-butyne complex 21f. The outer layer consisted of the entire complex, including the entire o-pda ligand, and phenyl and SiMe 3 substituents on the imido and diamide ligand respectively, and the inner layer comprised of 22, with acetylene as the alkyne fragment. The inner layer was modeled using the B3LYP 64,65 /LANL2DZ 66,67 basis set, whilst the B3LYP/LANL2MB basis set was used to model the substituents. As shown in Table 2-2 below there is excellent agreement between the calculated structure 23, and the X-ray crystal structure for 21f.

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27 Table 2-2. Comparison of selected bond lengths and angles between model compounds 22 and 23 and the reported crystal structure for 21f b Bond Lengths and Angles 22 TS 22 23 TS 23 Xray data 21f b MoN(1) 1.755 1.798 1.776 1.818 1.745(2) MoN(2) 2.018 2.053 2.003 2.005 2.009(2) MoN(3) 2.018 2.037 2.020 1.991 2.022(2) MoC(19) 2.087 2.089 2.086 2.087 2.078(3) MoC(20) 2.071 2.079 2.083 2.075 2.079(3) C(19)C(20) 1.331 1.346 1.333 1.353 1.305(4) Fold angle a 147 0 141 0 137 0 138 0 133 0 a. The fold angle is defined as the angle between the planes made by Mo, N(2), N(3), and the plane defined by the benzenoid portion of the o-pda ring. b. See reference 49 and 61 for crystallographic information. 22TS 2223TS 23 Figure 2-3. B3LYP optimized models for alkyne complexes 22 and 23 and their transition states The transition states corresponding to alkyne rotation about the Mo alkyne bond in both 22 and 23 have been located. In both these transition states the alkyne ligand is now oriented parallel to the MoN(imido) bond. This involves rotation of the alkyne ligand in 22 and 23 by 90 0 about the Mo-alkyne bond. The activation energy obtained

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28 from these calculations is 16.9 Kcal/mol for the rotation of 22 and 14.4 Kcal/mol for the rotation of 23. The ONIOM model, in particular, compares favorably to the experimentally obtained activation barrier of 13.2 Kcal/mol. In order to better understand the nature of the bonding in these complexes, we undertook a qualitative MO analysis and applied Weinholds Natural bond orbital (NBO) 68-70 method to the complex. Scheme 2-3 shows the most important orbital interactions between a metal L 3 M fragment and o-(SiMe 3 N) 2 C 6 H 4 2ligand assembled in C s symmetry, with a mirror plane directly bisecting the o-pda ligand and containing the MoN(imido) bond. The frontier orbitals of a L 3 M fragment have been determined by Albright et al. 71 and are represented on by the left side of Scheme 2-3. The metal t 2g orbitals in this non-standard orientation are comprised of one orbital of a symmetry primarily d xy and two orbitals of a symmetry primarily d x 2 -y 2 and d z 2 The metal e g orbitals become a, primarily d yz and a primarily d xz In the case of Mo, the three t 2g derived orbitals lie relatively high in energy and above the chelates populated orbital. The d x 2 y 2 orbital is closest in energy to this orbital and is responsible for the folding of the diamide ligand, seen in these complexes. The two filled orthogonal p orbitals on the imido nitrogen, destabilize the metal t 2g orbitals of a (d z 2 ) and a(d xy ) symmetry. Donor electrons on the alkyne alter the MO diagram in Scheme 2-3. These electrons compete for empty metal orbitals with the imido ligand i.e. the alkyne electrons compete with the imido for the metal d z 2 orbital in the ground state and the d xy orbital in the transition state, resulting in a 3-center-4-electron bonding interaction between the imido, the metal center and the alkyne ligand. As is typical for three center interactions, three orbitals can be expected for the combination of the three fragments, i.e.

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29 (a bonding, nonbonding, and antibonding combination of the orbitals from the 3 fragments). The bonding combination for the interaction of the alkyne, the imido nitrogen lone pair and the metal d z 2 orbital is depicted in Figure 2-4. NPht2gLNPhLLNPhLLNPhLLLNPhLLa'a'a''a''a'xyzLLNPhLLPha'a'a''egLLrepresents the (SiMe3N)2C6H42-a' Scheme 2-3. The effects of the consideration of contributions of the diamide and imido ligands (right side) on a typical ML 3 fragment (left side). In this non-standard orientation the metal d orbitals have a different composition than that usually used for octahedral complexes. This is due to the coordinate system shown in the top right corner of this figure. In a pure ML 3 fragment the z-axis coincides with a threefold rotational axis of an ocatahedron. The atomic composition of of the metal d orbitals, are mixed so that the orbitals are reorientated to lay between the ML bonds. For example the a component of the e g set becomes 1/3( x 2 -y 2 ) + 2/3 yz For full details on the atomic composition of a pure ML 3 fragment see. Albright et al. 71

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30 NMoPhNNMe3SiMe3SiNMoPhNNMe3SiMe3Si21TS 21 Figure 2-4. Bonding interactions in between Mo, imido, and the alkyne fragment NBO population analysis More detailed information about the alkynemetal interactions can be obtained from an NBO population analysis. The NBO 68 program diagonalizes the one and two center blocks of the first order reduced density matrix in such a way that natural bonds are obtained that are said to represent the best Lewis structure of a molecule. This involves a sequence of transformations from a given basis set to various localized sets: natural atomic orbitals (NAOs), natural hybrid orbitals (NHOs), natural bond orbitals (NBOs), and natural localized molecular orbitals (NLMOs). The NLMOs can then be transformed to occupied MOs. Delocalization effects appear as weakly occupied antibonding or Rydberg orbitals. given basis setsNAOsNHOsNLMOs Lewis structures obtained from NBO analyses accounted for approximately 97% of the electron density. As shown in Table 24, the NLMO analysis can be used to quantify the delocalization of the alkyne electrons. In the transition states TS 22 and TS 23, a greater proportion of the electron density of the parent NBO of the alkyne CC bond

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31 is delocalized onto molybdenum. In addition, the occupancy of this bond is also lower in the transition state. Table 2-3. NLMO analysis of alkyne bond Model % CC % Mo Occupancy 22 86.9 9.2 1.738 TS22 80.1 13.8 1.660 23 86.9 8.5 1.738 TS23 78.9 14.0 1.640 Table 2-4, shows the population analysis of the Moimido bond. The bond type, percent contribution from each atom and occupancy of each chemical bond are presented. The Mo=N(imido) bond in model complexes 22 and 23, where the alkyne is aligned in an orthogonal orientation, consists of a -bond and two -bonds as would be expected for a metalnitrogen triple bond. All bonds are substantially polarized towards nitrogen with contributions from nitrogen (68-77%) and (22-32%) from molybdenum. Table 2-4. NBO analysis of MoN(imido) bonds Model Bond Type %Mo %N Occupancy Covalent Bond order b 22 22.9 77.1 1.94 27.3 72.7 1.83 31.7 68.3 1.90 0.960 23 26.2 73.8 1.97 22.1 77.9 1.76 31.6 68.4 1.83 0.949 TS 22 37.9 62.1 1.97 39.3 60.7 1.92 n a 6.4 89.5 1.80 0.928 TS 23 37.1 62.9 1.92 36.4 63.6 1.88 n a 6.7 86.5 1.74 0.868 a n, lone pair b Atom-Atom overlap-weighted NAO bond order However, this bond in transition state structures TS 22, and TS 23, consists of one -bond and one -bond, the third -bond is significantly localized on the nitrogen atom (N contribution 87-90%) and is best described as a lone pair with some delocalization

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32 onto the molybdenum center (6.4-6.8%). This data, in addition to the lower covalent bond orders of the MoN bonds for model complexes 22 and 23, (Table 2-4) suggests a weakening of the MoN(imido) bonds in the transition state. The NLMOs of 22 and TS 22 are represented pictorially in Figure 2-5. The larger contributions of the Mo dorbital in the transition state TS 22 is evident in Figure 2-5h versus the significantly smaller contributions from these orbitals in Figure 2-5d. Also the localization of the third MoN(imido) bond is evident in Figure 2-5g. Thus, it is only in the transition state that the alkyne electrons compete effectively for metal d orbitals. This is evidently a higher energy process, as the stronger metal nitrogen bonds are replaced by weaker metal carbon bonds, leading to the observed activation barrier associated with alkyne rotation. (a)(b)(c)(d) (e)(f)(g)(h) Figure 2-5. NLMO plots (isocontour 0.04) of MoN(imido) bond in 22 (a-c): a = imidoMo dz 2 (), b = imidoMo (dz 2 dyz hybrid, ), c = imidoMo (dxy, ) and TS 22 (e-g): e = imidoMo dz 2 (), f = imidoMo (dxy, ), g = imido lone pair. Plot d = CC bond Mo(dz 2 dyz hybrid) in 22, h = CC bond Mo (dxz, dxy hybrid) in TS 22.

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33 Summary We have demonstrated the synthesis of novel high oxidation state molybdenum imido alkyne complexes that show significant interactions between the alkyne electrons and the metal center. The implications of this interaction on the structure and dynamics of these complexes have been explored via DFT (B3LYP) and NBO calculations. These calculations reveal that the alkyne donation in these complexes occurs at the expense of MoN(imido) bonding. This is an unfavorable interaction since the stronger MoN bonds, are replaced by weaker MoC bonds. The perpendicular orientation of the alkyne ligand in the ground state of these complexes arises as the molecule minimizes the alkyne to Mo donation and maximizes the imido to Mo donation. It is clear that the interplay of donor ligands in this class of compounds plays a crucial role in determining their properties and suggests that a judicious choice of the donors may offer a way to modify the chemistry of these compounds in a desirable fashion. Dynamics and Bonding of Molybdenum Imido Diamido Olefin Complexes The chemistry of group 6 metal imido diamido complexes has resulted in the isolation and characterization of a number of olefin complexes of both W and Mo. 32,46 These olefin complexes are formed when a dialkyl complex containing hydrogen atoms undergoes hydrogen transfer and the loss of one molecule of an alkane. In the case of W, hydrogen transfer is induced by the addition of a Lewis base (Scheme 2-4) whilst, in the case of Mo spontaneous hydrogen transfer occurs at room temperature. Crystal structures of olefin complexes of both W and Mo have been reported. 72,73 Like the alkyne complexes they are characterized by a significant amount of back

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34 bonding, a fact that is manifest for these complexes in both the solid-state structural features, and in solution. For example, in the crystal structure of the styrene complex of Mo, (Mo(NPh)( 2 -styrene)(o-(Me 3 SiN) 2 C 6 H 4 ), 20c, a long CC bond (1.46) is observed for the styrene ligand. Further, in both the 1 H and 13 C NMR spectra of this compound, the resonances for the styrene ligand are shifted significantly upfield, suggestive of a metallacyclopropane like structure for these olefin complexes. 1. 2.0 equivs RMgCl, -780C2. Room Temp3. R = Et, Phenethyl2.0 equivs PMe3Room TempWNClClNNPhMe3SiMe3SiWNMe3PPMe3NNPhMe3SiSiMe3WNRRNNPhMe3SiMe3Si+ RH510g and 10hR19a, R = H; 19b, R = Ph Scheme 2-4. Synthesis of W olefin complexes Unlike the alkyne complexes, olefin rotation is much slower and is not observed on the NMR timescale. Complex 20c in particular exists in solution as two isomers; the phenyl substituents of one isomer are anti to the imido ligand and while this substituent in the other isomer is syn to the imido ligand. No intercoversion of these two isomers were observed even when a C 6 D 6 solution of 20c was heated at 80 0 C. 74

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35 In order to investigate the dynamics of olefin rotation in these complexes, ONIOM calculations were performed. For these calculations a two-layer oniom system was employed as has been described earlier in this chapter (Figure 2-6). The core of these calculations consisted of the CH=CHcarbon chain replacing the o-phenylene group (o-C 6 H 4 ) and with hydrogens in place of the organic groups on the nitrogen atoms (SiMe 3 of the amido and phenyl group of the imido) as described earlier. Ethylene was used as the olefin fragment in this model. This layer was optimized within the ONIOM program using the B3LYP/LANL2DZ method. The outer layer consisted of the full system and was modeled with the computationally less expensive B3LYP/LANL2MB basis set. There was good agreement between the calculated structure for 20c and the experimental X-ray structure of this compound. Using this method we were able to calculate an activation barrier of for olefin rotation of 25.4 Kcal/mol. This is consistent with the experimental observation that olefin rotation is slow on the NMR timescale yet rapid enough on the chemical timescale to give two isomers as the product of the reaction. The energy barrier can also be explained by examining the molecular orbitals, and revisiting the molecular orbital diagram of Scheme 2-3 (Shown again below). Recall that the fragment orbitals L 3 M fragment are comprised of a t 2g set that has one a symmetry (d xy ) orbital and two orbitals of a symmetry d x 2 -y 2 and d z 2 whilst the e g set are composed of a (d yz ) orbital and a (d xz ) orbital. As the alkyne or olefin fragment approaches the L 3 M fragment perpendicular to the MN bond of the imido ligand the a(d xy and d xz ) orbitals mix, one linear combination of these orbitals directs the lobes of the metal orbital towards the orbital of the incoming ligand, the other linear

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36 combination directs the lobes of the d orbitals towards the imido ligand. The olefinic fragment is thus stabilized by back bonding from the metal a orbitals to the orbital of the fragment. This is clearly seen by examining the HOMO of 23 and 20c (Figure 2-7). NPht2gLNPhLLNPhLLNPhLLLNPhLLa'a'a''a''a'xyzLLNPhLLPha'a'a''egLLrepresents the (SiMe3N)2C6H42-a' Scheme 2-3. The effects of the consideration of contributions of the diamide and imido ligands (right side) on a typical ML 3 fragment (left side). In this non-standard orientation the metal d orbitals have a different composition than that usually used for octahedral complexes. This is due to the coordinate system shown in the top right corner of this figure. In a pure ML 3 fragment the z-axis coincides with a threefold rotational axis of an ocatahedron. The atomic composition of of the metal d orbitals, are mixed so that the orbitals are reorientated to lay between the ML bonds. For example the a component of the e g set becomes 1/3( x 2 -y 2 ) + 2/3 yz For full details on the atomic composition of a pure ML 3 fragment see. Albright et al. 71

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37 core20C syn20C antiTS 20C Figure 2-6. Optimized ONIOM structures for 20c NNNPhPhMe3SiMe3SiNNNPhMe3SiMe3Si20 C23 Figure 2-7. HOMO of 23 and 20c showing backbonding of the metal fragment to the olefin in 23 or acetylene in 20c

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38 Rotation of the olefinic fragment by 90 0 results in the symmetry of the orbital of this fragment changing from a to a as it is moved into the mirror plane of the molecule. This orbital can now interact with the metal d orbitals of a symmetry, specifically a linear combination of the d z 2 and d yz orbitals. Recall however, that the imido lone pair electrons were donated into the d z 2 orbital. ImidoMo lone pair donation in this case is made energetically unfavorable by the presence of d electrons in this orbital. This is clearly seen when the HOMO of TS 20c and TS 23 are examined (Figure 2-8). The imido lone pair is clearly non-bonding in TS 20c and TS 23. NNNPhPhMe3SiMe3SiNNNPhMe3SiMe3SiTS 20cTS 23 Figure 2-8. HOMO of TS23 and TS20c showing backbonding of the metal fragment to the olefin.

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39 The weakening of the MoN(Imido) bonds is confirmed by an NBO analysis(Table 2-5). The covalent bond orders for the imido bond are clearly smaller in the TS. Further, the NBO representation of the imido bond reveals that the delocalization of 2 onto Mo is much less in the TS(7.3), than in 6(18.2), i.e., the lone pair on the imido ligand is more greatly polarized toward the nitrogen atom of the imido ligand when the olefin is oriented parallel to the imido ligand (85.7%) than when the olefin is oriented perpendicular to the imido ligand. Table 2-5. NBO analysis of the MoN(Imido) bond in 20c Model Bond Type %Mo %N Occupancy Covalent Bond order 34.1 66.0 1.96 35.0 65.3 1.90 20c 18.2 76.2 1.64 0.99 35.2 64.8 1.88 30.6 69.4 1.79 TS 20c 7.3 85.7 1.73 0.88 These results demonstrate that cis perpendicular orientation of ligands is the most stable in these systems as this orientation maximizes the stabilization of the metal center arising from the imido ligand. Orientation of these ligands parallel to this bond generates interactions that result in the weakening of the MoImido bond. Diamide Ligand Folding in d2 vs d0 Group 6 Imido Complexes Computational and structural studies on d 0 diamide imido complexes have demonstrated the importance of diamide lone pair donation in the bonding in these complexes. In these structures, the diamide lone pair electrons are donated to an empty metal d orbital of appropriate symmetry through concerted torsion of the sp 2 nitrogens in order to achieve effective lone pair pd overlap. This torsion results in the folding of the o-pda ligand as depicted in Table 2-6.

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40 It is evident from Table 2-6 that there is a distinct correlation between the fold angle of the diamide ligand and the oxidation state of the metal. In the d 0 dialkyl complexes like the metallcyclopentane complex 25, the diamide ligand is considerably folded. As shown in Table 2-6, in the alkyne complex 21f, and the alkene complex 20c, the diamide ligand folds to an extent that is comparable to 25. This occurs because there is significant back bonding in these complexes, and they are best described as d 0 metallacyclopropene and metallcyclopropane complexes respectively (vide supra). The folding of the diamide ligand is minimized in the d 2 complexes, [cis-(pyridine) 2 Mo(NPh)-o-(Me 3 SiN) 2 C 6 H 4 }, 44, [cis-(2,6-dimethylphenyl isocyanide) 2 Mo(NPh)-o-(Me 3 SiN) 2 C 6 H 4 }, 30b, and [ 4 -(butadiene)Mo(NPh)-o-(Me 3 SiN) 2 C 6 H 4 }, 24. The synthesis and characterization of 24 and 44 has been reported whilst the synthesis of 30b is discussed in chapter 4. However, it is clear from Table 2-6, electronic properties of the metal prevent diamide folding in complexes in the Mo(IV) oxidation state. 30,31,48,49,75,76 Table 2-6. Fold angles for some Mo Imido Diamido Complexes Molecule Fold Angle 0 [ 2 -(2-butyne)Mo(NPh)-o-(Me 3 SiN) 2 C 6 H 4 }, 21f 133.0 [ 2 -(styrene)Mo(NPh)-o-(Me 3 SiN) 2 C 6 H 4 }, 20c 129.2 Mo(NPh)(o-(Me 3 SiN) 2 C 6 H 4 )(CH 2 ) 4 25 132.5 [ 4 -(butadiene)Mo(NPh)-o-(Me 3 SiN) 2 C 6 H 4 }, 24 167.8 [cis-(pyridine) 2 Mo(NPh)-o-(Me 3 SiN) 2 C 6 H 4 }, 44 175.5 [cis-(2,6-dimethylphenyl isocyanide) 2 Mo(NPh)-o-(Me 3 SiN) 2 C 6 H 4 }, 30b 173.7 In order to investigate the electronic origin of this phenomenon we compared the molecular orbitals of the alkyne complex 23 with the model butadiene complex 24. The model compound 24 was again optimized using the ONIOM method as described earlier.

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41 There was good agreement between the calculated metric parameters of 24 and the reported X-ray crystal structure of 24. 48 The structure and bonding of cis-butadiene complexes can vary between two extremes, the 2 and the 2 -designations (Scheme 2-6). The bonding in 2 complexes is best described by a synergistic model i.e., the ligand acts as a donor, as well as a acceptor, as the metal donates a pair of d electrons in the ligands orbital. In 2 -type complexes, the butadiene ligand is considered a dianionic dialkyl, a result of considerable back bonding. The oxidation state of the transition metal, as well as, the ancillary ligands of the complex, dictates which structure type will be adopted. In the Boncella labs butadiene complexes of both W and Mo have been synthesized. The butadiene complex of Mo, 24, may be best described as 2 d 2 Mo(IV) complex. In contrast, the corresponding complex with W may be described with a 2 designation. This is attributable to the greater tendency of 5d transition metals to back-bond. MoNNNPhMe3SiMe3Si = Ligand Fold Angle0 Scheme 2-5. Fold angle in group 6 diamido complexes

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42 MM22, Scheme 2-6. The 2 and 2 limiting structures for cis-butadiene complexes Figure 2-9 shows the molecular orbitals for the HOMO and HOMO-2 for 24. Unlike the d 0 complexes where the diamide lone pair was donated into the d x 2 -y 2 orbital, this is not possible in this case since this orbital is occupied and is used in back bonding to the butadiene ligand (HOMO-2). The diamide lone pairs are of the correct symmetry to interact with the d z 2 orbital; this is evident in the HOMO. The d z 2 orbital is raised in energy by donation from one of the imido lone pair electrons. Therefore, donation by the diamide lone pair electrons into this orbital results in a competition for the available d z 2 orbital between the diamide and the imido ligand. This would result in a weakening of the MoN(Imido) bond in favor of MoN (diamide) bonding. The imido ligand however is a much stronger donor ligand, thus the MoN(Imido) bonding is favored over MoN(diamide) bonding. This is clearly evident from the NLMO analysis (Table 2-7). The diamide lone pairs n 1 and n 2 are only slightly delocalized unto Mo (1.81 and 0.99% respectively). In contrast, the delocalization of these lone pair electrons onto Mo in 21 is significantly greater (8.2 and 7.4% respectively). The MoN(Imido) bond is slightly weaker in 24 (bond order 0.93) than in 23 (0.95). However the bonds in the imido bond are polarized towards nitrogen to a greater extent than in the d 0 alkyne complex.

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43 Table 2-7.NLMO analysis of MoN(Imido) and MoN(Diamide) bonds in 23 and 24 Model Bond Type %Mo %N Occupancy Covalent Bond order b 24 19.3 80.7 1.88 19.7 80.3 1.82 Imido n 16.5 78.6 1.64 0.93 n 1 1.81 81.4 1.64 Diamide n 2 0.99 81.8 1.64 0.50 23 Imido 26.2 73.8 1.97 22.1 77.9 1.76 31.6 68.4 1.83 0.95 n 1 8.2 82.7 1.67 Diamide n 2 7.4 83.3 1.68 0.50 HOMOHOMO 2NNNPhMe3SiSiMe3NNNPhMe3SiSiMe3 Figure 2-9. Occupied molecular orbitals of 24

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44 Conclusions The results presented in this chapter clearly demonstrate the importance of donor interactions by both the diamide and imido ligands in our complexes. These interactions contribute greatly to the stability of our complexes in that they stabilize the highly electropositive metal centers. Competition for available metal d orbitals can lead to reactivity of these complexes and as is demonstrated in later chapters, this reactivity may either be metal centered or ligand centered. These data suggest that the reactivity of these complexes can be carefully modulated by the judicious choice of ancillary ligands.

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CHAPTER 3 SYNTHESIS AND REACTIVITY OF GROUP 6 IMIDO DIAMIDO METALLACYCLES Introduction Metallacyclopentane complexes were originally observed in our labs as species that result from the deactivation of W(VI) alkylidenes in olefin metathesis reactions. 34 Since these initial studies we have been able to successfully isolate and characterize metallacyclopentane complexes of W and Mo by more direct methods. 34,46,74 Group 6 imido metallacyclopentane complexes are quite rare, and our complexes are unusual in that their inherent stability towards decomposition by either hydrogen elimination/abstraction or carbon-carbon bond cleavage allows them to be readily isolated and characterized. Schrock and co-workers have also observed group 6 imido metallacyclopentane complexes that resulted from the decompostion of alkylidene species. 73,77 These complexes however lacked the inherent stability of those seen in our group and decompose via hydrogen transfer resulting in the formation of butene. The fundamental difference in our complexes and the complexes studied in the Schrock group is the chelating ancillary ligand. As discussed in the previous chapter, the diamide ancillary ligand, {o-(Me 3 SiN) 2 C 6 H 4 } 2, is crucial in stabilizing the high oxidation state of the metal by donation of the diamide lone pairs. The stabilization via donation is not afforded by the alkoxide ligands in Schrocks metathesis catalysts, as they possess 45

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46 electron-withdrawing substituents on the alkoxide ligand in addition to the fact that the more electronegative O atom is less likely to donate its lone pair electrons. 35,36,43 73,78,79 In this chapter, we examine the chemistry of the metallacyclopentane complex, Mo(NPh)(o-(Me 3 SiN) 2 C 6 H 4 )(CH 2 ) 4 25. We begin examining the synthesis of metallacyclopentene complexes by the sequential ligand exchange of ethylene from the metallacyclopentane complex with an alkyne. We also examine the synthesis of metallacyclopentadiene complexes by the [2+2] cycloaddition reactions of two alkyne moieties. The thermal rearrangements of 25, are also examined by kinetics and DFT using small models as well as larger (experimentally exact) systems with quantum (B3LYP) and hybrid methods (ONIOM), respectively. We examine the role that diamide donation plays in influencing the reactivity of these complexes. Synthesis and Reactivity of Mo Imido Diamido Metallacyclopentenes and Metallacyclopentadienes Transition metal metallacycles have been implicated as important species in many catalytic and stoichiometric conversions of organic fragments. One such process is the metal mediated cyclooligomerization reaction of alkynes. In the well-known mechanism for this reaction, metallacyclopentenes, metallacyclopentadienes, as well as alkyne and arene complexes have been cited as key intermediates. 80,55 Recent research in this field has focused on stereoselectively controlling the products of the cyclooligimerization reaction. This has been achieved by developing ligand sets that promote highly selective carbon-carbon bond forming reactions in which low valent transition metals mediate the formation of metallacycles from saturated organic substrates. Takahashi, 72,81-84 and Ladipo 85,86 have utilized this chemistry in the synthesis of substituted arenes, pyridines, and other useful organic molecules. In Takahashis work, the preparation of

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47 multisubstituted benzene and pyridine derivatives was achieved in a one pot synthesis by the use of unsymmetrical metallacyclopentadienes obtained from the intermolecular cross coupling reaction of two different alkynes, and the subsequent treatment with a third alkyne, or nitrile, in the presence of CuCl or Ni(PPh 3 ) 2 Cl 2 The development of methodologies that would afford the stereoselective synthesis of metallacycles is an important area of research and warrants further study. A method for the synthesis of olefin complexes, from Mo(NPh)(o-(Me 3 SiN) 2 C 6 H 4 )Cl 2 (THF) 4, was recently developed in our labs 30,31 and the tendency of these complexes to form metallacycles has been demonstrated. 34,46 In exploring the reactivity of these complexes, we became interested in developing the chemistry of group 6 metallacycles containing imido and bis-amido chelating ligands because of the potential importance of these compounds in organic synthesis. 87 81,82,84,88 Synthesis of metallacyclopentene complexes Heating a toluene solution of the metallacyclopentane complex 25 at 80 0 C with one equiv of an alkyne results in the synthesis of the metallacyclopentene complex 26 (Scheme 3-1). The reaction with phenyl acetylene results in a 1:4 mixture of the two regioisomers 26a and 26b. The phenyl substituent in the metallacycle in 26a is in the position whilst the phenyl substituent in 26b is in the position. The equilibrium between metallacyclopentanes and bis-ethylene complexes has been extensively studied. 89-92 It has been demonstrated in our labs that heating the metallacyclopentane complex 25, in the presence of a Lewis base (PMe 3 ) results in the formation of the ethylene complex 27 as a phosphine adduct (Scheme 3-2). 34,46 These

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48 results demonstrate that 25, is a precusor to the ethylene complex 25a, and that this complex can be trapped in the presence of Lewis bases. 26a. R1= Ph R2= H26b. R2 = H R2= Ph26c. R1= CO2Et R2= CO2EtMoNNNPhMe3SiMe3SiMoNNNPhMe3SiMe3SiR2R1R2800CR125ethylene Scheme 3-1. Synthesis of metallacyclopentene complexes Complex 25a, can also be generated from the dichloride complex 1, and ethyl magnesium chloride as outlined in Scheme 3-3. Again this ethylene complex can be trapped in the presence of donor ligands such as PMe 3 and acetylenes. This strategy was employed to synthesize 26b as a single isomer. The resonances for the metallacycle fragment for this compound were observed as eight line patterns at 3.70, 3.38(2H overlapping) and 1.35ppm in the 1 H NMR spectrum. The vinylic proton from the acetylene ligand was observed downfield at 8.47ppm. In addition two peaks were observed for the SiMe 3 resonances at 0.43 and 0.40ppm.

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49 2525aPMe3toluene, 800CethyleneMoNMe3PPMe3NNPhMe3SiSiMe3MoNNNPhMe3SiMe3SiMoNNNPhMe3SiMe3SiHHHH27 Scheme 3-2. Lewis base generation of olefin bis-PMe 3 olefin complex 27 The strategy employed in Scheme 3-3 takes advantage of the steric environment around the metal central center to promote the stereoselective formation of the metallacycle. Phenyl acetylene adds to 25a, in a manner that minimizes interaction between the phenyl substituent and the bulky SiMe 3 group. This results in the phenyl substituent of the acetylene ligand bonding preferentially to the Mo center. Similarly, complex 26a can be isolated as a single isomer by taking advantage of the steric environment around the metal center. As outlined in Scheme 3-4, exposure of the phenyl acetylene complex, 28, generated in situ, from the isobutylene complex 20a, to ethylene results in the isolation of 26a. The resonances for the metallacycle fragment of 26a are observed at 3.31, 3.06, 3.98 and 1.34 ppm in the 1 H NMR spectrum. The

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50 vinylic proton is observed as a broad singlet at 6.87ppm. Resonances for the SiMe 3 protons are observed as singlets at .06 and 0.32ppm. MoNNNPhMe3SiMe3SiHHHHMoNClClNNPhMe3SiMe3SiMoNNNPhMe3SiMe3SiO 2 eqvs EtMgCl-780CPhPhenyl acetylene+ Ethane 2.0 eqvs Cl-, Mg2++ ethylene425a26b Scheme 3-3. Synthesis of 26b X-ray crystal structure of 26b The identity of 26b was also confirmed by X-ray diffraction analysis. X-ray quality crystals were obtained by slow evaporation of a diethyl ether solution of 26b. Figure 3-1, shows the thermal ellipsoid plot of 26b. Complex 26b, exhibits a pseudo square pyramidal structure with the imido ligand occupying the apical position of the square pyramid. The MoN(1) bond length of 1.731(2) is comparable to MoN lengths in similar complexes. The bond lengths in the metallacycle confirm the predicted arrangement of double and single bonds i.e the arrangement of two long CC bonds (C(20)C(21) 1.504(3) and C(21)C(22) 1.531(3) ), and one short CC bond (C(19)C(20) 1.344(3) ) confirms the

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51 metallacyclopentene description for this complex. As described for d 0 dialkyl complexes of this type, the diamide ligand is substantially folded (fold angle =135.6 0 ) and as discussed in the previous chapter, this has been ascribed to donation of the diamide lone pair electrons into a metal d orbital of appropriate symmetry. MoNNNPhMe3SiMe3SiHMoNClClNNPhMe3SiMe3SiMoNNNPhMe3SiMe3SiO 2 eqvs iBuMgCl, -780CPhenyl acetylenePhethylene+ isobutane, isobutylene,2.0 eqvs, Cl-, Mg2+Ph42826a Scheme 3-4. Synthesis of 26a Kinetics and Mechanism of the Thermal Rerrangement of 25 The observation that heating the metallacyclopentane complex, 25, can induce CC bond cleavage of the metallacycle and the elimination of ethylene is consistent with the observation that metallacyclopentane complexes may exist in equilibrium with a bis-ethylene species as depicted in Scheme 3-5.

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52 Figure 3-1. Thermal ellipsoid plot of 26b (50% probability thermal ellipsoids), selected bond lengths (): MoN (1) 1.7305, MoC (19) 2.1520 (2), MoC (22) 2.175 (2), C (19)C (20) 1.344 (3), C (20)C (21) 1.504(3), C (21)C (22) 1.531(3). MoMo(VI)(IV) Scheme 3-5. Equilibrium between metallacyclopentane species and bis-ethylene species The reaction depicted in Scheme 3-5 represents a formal reduction of the Mo center by two electrons as CC bond cleavage reduces the metal center from Mo(VI) to

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53 Mo(IV). The synthesis of complexes 26 and 27 in Schemes 3-2 to 3-4, suggest that heat is required in order to induce CC bond cleavage of the metallacyclopentane. This implies that the equilibrium depicted in Scheme 3-5 lies largely towards the metallacyclopentane species. In order to confirm the existence of this equilibrium, and to clarify the mechanism of ligand exchange in these metallacyclopentane complexes, we undertook kinetic studies for the reactions of 25 with diethyl acetylene dicarboxylate (DEAD). The kinetics of the thermal disruption of the metallacycle of 25, in C 7 D 8 were examined by following the disappearance of the SiMe 3 peaks of the starting material using 1 H NMR spectroscopy in the presence of excess DEAD. The data points were obtained by plotting the value of the intergral for this peak with respect to time for more than three half-lives. Activation parameters for the conversion of 25 to 26c were determined by reacting C 7 D 8 solutions (0.020M) of 25 with DEAD (1.40M) in the NMR probe at temperatures between 335 and 356K (three samples at each temperature). The disappearance of 25 follows first order kinetics (Figure 3-2) with a rate constant at 342K of k = 3.3 x10 -4 s -1 (t 1/2 = 2.1 x10 3 s). The reaction rate is independent of the concentration of DEAD (Table 3-1). The energy of activation, G corresponds to17.5 Kcal/mol at 342K. Activation parameters (Figure 3-3) obtained from an Eyring plot( S = R[(intercept) .76]; H =-R(slope)), for the formation of 26c from 25 are H = 20.5(1.0) Kcal/mol and S = -14.7(3.0) cal (mol K) -1

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54 00.511.522.5010002000300040005000600070008000Time (s)k(1/sec)335K342K349K356K Figure 3-2. First order kinetics for the formation of 26c Table 3-1. Dependence of DEAD on the formation of 26c [DEAD]/M Rate k(sec -1 ) X 10 4 0.974 3.19(4) 1.44 2.86(8) 1.99 2.98(3) 2.60 2.98(11) The observation that the formation of 26c does not depend on the concentration of substrate is consistent with the proposed decomposition of the metallacyclopentane fragment via CC bond cleavage to a bis-ethylene species pior to the rate determining step. The relatively large positive activation barrier H and a negative entropy of activation, S is consistent with a multi-step mechanism, where the metallacyclopentane rearranges and the alkyne binds to this new species (bis-ethylene) prior to the loss of ethylene. The observed value for S is a combination of the metallacycle rearrangement steps and the binding of DEAD to the activated complex.

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55 Figure 3-3. Eyring plot for the reaction of 25 with DEAD Computational Studies on the Thermal Rearrangement of 25 The synthesis of the metallacyclopentane complex 25, has been reported, however, X-ray structural studies on this compound were not performed at that time. We were interested in the solid-state structure of this complex and an X-ray study was performed on a single crystal of 25 grown from a concentrated pentane solution at 0 C. The thermal ellipsoid plot of 25 is depicted in Figure 3-4. Complex 25, crystallizes with a pseudo square pyramidal structure with the imido ligand occupying the apical position. The MoN(1) bond (1.728) is within the normal range for a MoN triple bond. The Modiamide bonds, MoN(2) and MoN(3), (2.010 and 2.015) are also within the normal range for MoC single bonds. The MoC(13) and MoC(16) bonds (2.187 and 2.191) are typical of MoN single bonds.

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56 Figure 3-4. Thermal ellipsoid plot of 25, (50% probability thermal ellipsoids), selected bond lengths (): Mo(1)N(1) 1.728(2), Mo(1)N(2) 2.010(2), Mo(1)N(3) 2.015(2), Mo(1)C(13) 2.187(2), Mo(1)C(16) 2.191(2) Although the equilibrium between metallacyclopentanes and bis-ethylene species has been observed in the past, we have not, until now, been able to confirm that such a process occurs in our systems since no bis-ethylene species has been detected. We therefore turned to computational chemistry in order to investigate the nature of any intermediates that may form during the thermolysis of 25. For these studies we employed small models as well as larger (experimentally exact) systems. In the model system, the o-phenylene group (o-C 6 H 4 ) that links the two N atoms of the diamido ligand was simplified to a CH=CHcarbon chain. The organic groups on the nitrogen atoms (SiMe 3 of the amido and phenyl group of the imido) were replaced by hydrogen atoms for simplicity. For this model, the recently developed mPW1K

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57 (modified Perdew-Wang 1 parameter for kinetics) exchange correlation functional of Truhlar and coworkers was employed. This functional was recently shown to yield more reliable barrier heights than other exchange-correlation functionals. 93,94 62,95 Using this method, three minima were located on the potential energy surface for rerrangement of the metallacycle fragment in 25 (Figure 3-5). Two of these minima (25b and 25c) possess a pseudo trigonal bipyramidal geometry about Mo. One of the diamide nitrogens is oriented trans to the imido ligand. The molecule 25b can best be described as a trigonal bipyramidal (TBP) metallacyclopentane complex whilst complex 25c can best be described as a trigonal bipyramidal (TBP) bis-olefin complex. We were also able to locate the transition states corresponding to the transformations from 25-25b ( TS 25-25b) and 25b-25c (TS 25b-25c). The relative free energies G 0 298 (relative to 25), for these transformations are depicted in Figure 3-6. In order to obtain a more accurate description of the minima during the thermal rearrangement of 25, ONIOM calculations were performed on larger experimentally exact systems. For these ONIOM calculations a two-layer system was employed, the inner layer consisted of the model complexes described above in Figure 3-5 and were modeled at the mPW1K/LANL2DZ level of theory. The outer layer consisted of the complete system including all substituents on the diamide and imido ligands and was modeled with the mPW1K/LANL2MB basis set. Optimized structures for the minima obtained from these calculations are depicted in Figue 3-7. Unfortunately, all attempts at attaining optimized structures for the transition states for these molecules using the experimentally exact systems failed to converge once the large SiMe 3 groups were introduced. However, because we were able to successfully model these transition states

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58 using the small model system and since we are only interested in a qualitative description of the mechanism for CC bond cleavage in these systems the TSs obtained using the small model system will suffice for the purposes of this discussion. 2525cTS 25-25b25bTS 25b-25c Figure 3-5. Optimized structures (mPW1K/LANL2DZ) for the thermal rearrangement of 25 Figure 3-6. Reaction profile G 0 298 (K) for the thermal rearrangement of 25 at the mPW1k/SDD-Aug-cc-pVDZ level of theory

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59 2525b25c Figure 3-7. ONIOM (mPW1K/LANL2DZ:mPW1K/LANL2MB) optimized structures for the thermal rearrangement of 25 As shown in Table 3-2 there is good agreement between the calculated structures and the experimentally obtained X-ray structure for 25. As expected, the ONIOM calculations successfully reproduced the experimentally observed fold angle in 25. This ligand folding is also seen in the TBP metallacyclopentane complex, 25b and the TBP bis-ethylene complex 25c. In both TBP species the strong trans influence of the imido ligand results in the lengthening of the MoN(2) diamide bond (2.118 and 2.174 for 25b and 25c respectively) relative to the MoN(1) bond (1.989 and 2.029). The CC bond lengths of the ethylene fragment (1.401 and 1.423) in 25c reflect a significant amount of back bonding from the metal to the orbital of the olefinic fragment. However, these bond distances are shorter than the other olefin complexes seen in our group. 46,74 Further, the MC (average distance from both carbons of the 2 ethylene fragment) bond distances of 2.306 and 2.429 are significantly longer that MC(olefin) bonds seen in our group. Thus it appears that the

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60 binding of the olefin to the TBP bis-ethylene complex 25c is weaker than other SP olefin complexes seen in our labs. It seems reasonable then, that olefin loss from this molecule will be feasible under the reaction conditions. Table 3-2. Selected bond lengths () and angles ( 0 ) for ONIOM optimized structures for the thermal rearrangement of 25 Bond 25 25(X-ray) 25c 25b MoC(1) 2.213 2.187(2) 2.306* 2.159 MoC(4) 2.186 2.191(2) 2.429* 2.132 MoN(3) 1.763 1.728(2) 1.798 1.801 MoN(2) 2.041 2.015(2) 2.174 2.118 MoN(1) 2.032 2.010(2) 2.029 1.989 C(1)C(2) 1.548 1.544(8) 1.401 1.568 C(2)C(3) 1.541 1.496(2) NA 1.577 C(3)C(4) 1.538 1.563(1) 1.423 1.565 Fold Angle 137.6 132.5 152.4 133.3 Average MC distance from both carbons of the 2 ethylene fragment Insight into the nature of the metalcarbon bonds within these complexes can be obtained by inspection of the molecular orbitals. For the bis-ethylene complex 25c, the metal d orbitals are depicted in Scheme 3-6. The in phase combination of the diamide lone pair electrons is also depicted in this Scheme. This molecule possesses pseudo C s symmetry with a mirror plane that bisects the diamide chelate and the two ethylene fragments. The linear combination of diamide lone pairs is of a symmetry. .These lone pair electrons are of appropriate symmetry to interact with the d yz and d xz orbitals (see coordinates in Scheme 3-6). However, the energy of these orbitals is perturbed by strong donation from the imido (d xz ) and the orbitals of the ethylene fragment (d yz ). Thus the overlap of the diamide lone pairs with the empty metal d orbitals will not be as significant in this case.

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61 NPhMetal a'NPhNPhNPhNPhMetal a''ZYXrepresents the chelating diamide ligand's in phasecombination of p orbitals Scheme 3-6. Symmetry adapted orbitals of 25c. The diminished importance of diamide donation is clearly evident by inspection of the occupied molecular orbitals. The highest two occupied molecular orbitals of the SP metallacyclopentane complex 25, the TBP metallacyclopentane complex 25b, and the TBP bis-ethylene complex 25c, are depicted in Figures 3-8, 3-9, and 3-10 respectively. As shown in the HOMO of Figure 3-8, the importance of diamide lone pair donation in the SP metallacyclopentane complex 25, is clearly evident. As discussed in the previous chapter this donation is responsible for the folding of the diamide ligand seen in d 0 complexes.

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62 NPhMe3SiMe3Si NPhMe3SiMe3SiHOMOHOMO -1 Figure 3-8. Occupied molecular orbitals(B3LYP/LANL2DZ) for 25 The diminished importance of this interaction is clearly evident in the HOMO and HOMO of the TBP metallacyclopentane complex 25b (Figure 3-9). The perturbations to the a orbitals (d xz and d yz see Scheme 3-6) by the imido and orbitals of the metallacyclopentane fragment raises the energy of these orbitals and results in a weaker interaction between the diamide lone pair electrons and the metal d orbitals. The diamide lone pairs remain primarily ligand centered in the HOMO and HOMO-1 orbitals. The fact that the ligand folds quite significantly, (fold angle 133.3 0 ) suggests that this interaction is not negligible in these complexes, however, the energetics of the interaction are clearly more important in 25 than in 25b.

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63 PhNSiMe3Me3Si PhNNSiMe3NMe3SiHOMOHOMO -1 Figure 3-9. Occupied molecular orbitals (B3LYP/LANL2DZ) for 25b HOMOHOMO -1 Figure 3-10. Occupied molecular orbitals (B3LYP/LANL2DZ) for 25c The diminished importance of diamide lone pair donation is also evident in the HOMO of 25c. In addition the HOMO shows the ethylene fragments stabilized by back bonding from the metal to the a linear combination of the orbitals of two ethylene fragments (see Scheme 3-6). As mentioned above, 25c shows lengthened Mo

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64 C (ethylene) bonds relative to other SP Mo olefin complexes synthesized in our group and is suggestive of a weaker interaction between the metal and ethylene. Decreased donation by the diamide ligand in these complexes reduces the amount of electron density on the metal that is available for back bonding and thus there is a reduction in the stabilization of the ethylene ligands in these complexes, in addition to the fact that two ethylene molecules require electron density. Ethylene ligand exchange from 25c, is therefore possible as the complex can now readily lose a molecule of ethylene as it is less tightly bound in this complex. These results demonstrate the importance of ligand donation in stabilizing the high oxidation state of Mo in these complexes. The metallacyclopentane complexes seen in our labs are stable to decomposition by both -hyrdogen elimination/ transfer reactions and CC cleavage of the metallacyclopentane complexes. Rearrangement of the metallcyclopentane from a SP structure to a TBP structure diminishes the MN(diamide) interactions; as a result CC bond cleavage is induced and the complex can react via ethylene exchange. It is noteworthy that these ligand exchange reactions are significantly slower for the reactions of the W analogue of 25 with and donors. For example, the reaction of the W metallacyclopentane complex W(NPh)(o-(Me 3 SiN) 2 C 6 H 4 )(CH 2 ) 4 13, with PMe 3 proceeds to only 90% completion in 90 days, 34 this is slow in contrast to the facile cleavage of metallacyclopentanes with PMe 3 (1 hr) for the Mo complex. 46,74 These results suggests that the barrier for the CC bond cleavage and the thermal rearrangement of the W metallacyclopentanes is significantly higher.

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65 Synthesis and reactivity of a metallacyclopentadiene complex. Our success in synthesizing metallacyclopentenes inspired us to pursue the subsequent synthesis of metallacyclopentadiene complexes. One can envision the formation of metallacyclopentadienes from metallacyclopentenes by the sequential ligand exchange of an ethylene molecule from a metallacyclopentene as outlined in Scheme 3-7. However, no CC bond cleavage of metallacyclopentenes can be induced when 26 is heated at 80 0 C in the presence an alkyne for weeks. MoMoMoMoR1R2R1R2MoMoPhHPhHPhPh Scheme 3-7. Proposed formation of metallacyclopentadienes from metallacyclopentene complexes. Ancillary ligands have been removed for clarity. Metallacyclopentadienes can be synthesized however, from the [2+2] coupling reactions of terminal acetylenes. Thus, treatment of a pentane solution of the isobutylene complex, ( 2 -isobutylene)Mo(NPh)(o-(SiMe 3 N) 2 C 6 H 4 ), 20a with two equivalents of phenyl acetylene afforded the metallacyclopentadiene complex 29, [(NPh)Mo(C(Ph)CHCHC(Ph)){o-(Me 3 SiN) 2 C 6 H 4 }] in 40% yield (Scheme 3-8). The 1 H

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66 NMR spectrum of 29, showed a singlet (-0.07ppm, 18H) for the SiMe 3 protons. A sharp singlet (6.46ppm, 2H) assigned to the protons of the metallacycle characterizes the orientation of the phenyl substituents. An examination of the reaction mixture revealed that small amounts of the and isomers may also form during the course of this reaction however we were not able to isolate any of these isomers from the reaction mixture and the low yield of these complexes in this reaction prevented their complete spectroscopic characterization. phenyl acetylene2 equivs.20aMoNNNPhMe3SiMe3Si29MoNNNPhMe3SiMe3Si Scheme 3-8. Formation of the metallacyclopentadiene complex 29 Single crystals of 29, were obtained from slow evaporation of a concentrated diethyl ether solution (Figure 3-11). Complex 29 was found to have a pseudo square pyramidal geometry in which the imido group occupies an apical position. The MoN(1) bond length is 1.731 is consistent with a metal nitrogen triple bond. The MoN(2) and MoN(3) bonds (2.008 and 2.004) are consistent with MoN(diamide) bonds. The MoC(22) and MoC(19) bonds are with range for MoC(sp) bonds. The bond lengths within the metallacycle reflect the localization expected for a metallacyclopentadiene fragment, i.e. two short CC bonds

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67 (C(19)C(20) 1.335 and C(21)C(22) 1.339) and one long CC bond C(20)C(21) 1.455. Figure 3-11. Thermal ellipsoid plot of 29, (50% probability thermal ellipsoids), selected bond lengths (): MoN (1), 1.731(2), MoN(2) 2.008(2), MoN(3) 2.002(2), MoC (19), 2.197(3), MoC (22), 2.188(3), C (19)C (20), 1.335(4), C (20)C (21), 1.455(4), C (21)C (22), 1.339(4). Metallacyclopentadiene complexes have been implicated as important intermediates in the cyclotrimerization of acetylenes. 81 78,85,86 When 29 (0.008 mmol) was allowed to react in an NMR tube with excess phenyl acetylene(1.56mmol) at 80 0 C for 24hrs, a 50:50 mixture of two cyclotrimerized products, 1,2,4 triphenyl acetylene, and 1,3,5 triphenyl acetylene were produced (Scheme 3-9). We are currently investigating the utility of 29 as a potential cyclotrimerization catalyst.

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68 29MoNNNPhMe3SiMe3Si1: 1 mixture of 2 isome r sxs phenyl acetylene+ Scheme 3-9. Cyclotrimerization reactions of 29 Summary and Conclusions In the previous chapter the structural consequences of ligand competition on Mo imido diamido complexes were highlighted. We revisited this theme here and have emphasized the importance of diamide donation in the stabiliztion of metallacyclopentane species. Any structural pertubation of the molecule that leads to a reduction in the donor ability of the diamide ligands can lead to increased reactivity. This suggests that more active metallacyclopentane complexes may be synthesized by carefully modulating the substituents on the diamide ligand so as to decrease its donor ability. The frontier molecular orbitals of the complexes seen in this chapter and the previous one are all largely ligand centered. It should come as no surprise then that

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69 reactivity at the ligand in these complexes may also be induced. This is the subject of the next two chapters.

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CHAPTER 4 SYNTHESIS AND REACTIVITY OF MO(VI) COMPLEXES WITH ALKYL AND ARYL ISOCYANIDES Introduction In the previous chapters the importance of ligand donation in stabilizing the high oxidation state of the metals has been emphasized. We have shown that the competition for available d orbitals on the metal results in interesting structural features as well as reactivity. The highest occupied orbitals in these complexes are primarily ligand centered; therefore, it should come as no surprise that reactivity at the ancillary ligands may be induced. In this and the subsequent chapter, we examine the reactivity of the diamide ligands with isocyanides, and in chapter 5, with alkyl aluminum reagents. We also revisit here the concept of loading but in this case with a d 2 six coordinate isocyanide complex. Synthesis of Isocyanide Complexes and Insertion into the Metal-Diamide Bond Compared to the well-known insertion of CO and isocyanides, RNC, into metal alkyl bonds there are fewer examples of the insertion reaction into metal amide bonds. 96-99 The resulting metal-iminocarbamoyl derivatives that are formed are important in the general context of amination of organic substrates. 87 Alkyl isocyanides (RNC) can be regarded as being isoelectronic with CO, and their increased ability to act as Lewis bases makes them good candidates to interact with electrophillic metal centers. Recent work in our group has focused on the use of the chelating disubstituted phenylenediamide group {o-(Me 3 SiN) 2 C 6 H 4 } 2[(TMS) 2 pda]. The facile high yield 70

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71 synthesis of several group(VI) dialkyl complexes of Mo and W has allowed us to investigate the properties of these metal complexes and determine the influence the ancillary ligands have on their structure, stability and reactivity. 30,31 We have demonstrated that donation by these ligands is important in stabilizing the metal center and influencing the reactivity of these complexes. 32,48,49,100 Alkyl isocyanide insertion into W alkyl bonds has been shown to occur readily at room temperature affording 2 imino-acyl complexes that subsequently react via CC coupling reactions of the 2 -imino-acyl group resulting in the formation of diamide ligands. 44 Until now we have not seen any evidence for isocyanide insertion into the metal diamide bonds. Isocyanide insertion has been shown to occur preferentially at metal alkyl bonds as opposed to metal amide bonds presuamably because of the stronger metal amide bond. 77,83 In this chapter, we report the synthesis and characterization of bis-isocyanide complexes of Mo and the subsequent reactivity of these complexes with excess isocyanide yielding tris-isocyanide complexes in the case of t BuNC and an unusual chelating iminocarbamoyl bis-isocyanide complex in the case of 2,6 dimethyl phenyl isocyanide. X-ray structures of a bis-isocyanide complex and the iminocarbamoyl bis-isocyanide complex are reported. We also examine the effects on loading in d 2 six coordinate complexes and also attempt to determine the nature of the MC (isocyanide) bond. Synthesis of bis-Isocyanide Complexes Treatment of a pentane solution of 20a, ( 2 -isobutylene)Mo(NPh)(o-( Me 3 SiN) 2 C 6 H 4 ), with two equivalents of tert-butyl isocyanide resulted in the precipitation of 30a, as green microcrystals (Scheme 4-1). The structure of 30a has been assigned by 1 H, 13 C, and IR spectroscopy and is consistent with

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72 a square pyramidal structure with the imido occupying the apical position as depicted in Scheme 4-1. Resonances for the SiMe 3 protons are observed at (0.75ppm, 18H), whilst, the t BuNC protons are observed at (1.06ppm, 18H). The phenyl imido protons are observed at 7.48(dd, 2H, 7.33, 1.47Hz), 6.93(t, 1H, 7.33Hz) and 6.81(tt, 2H, 7.33, 1.47Hz). The o-pda ligand protons are observed as a pair of doublet of doublets (5.57, 3.3Hz) at 7.33 and 7.05ppm, respectively. Similarly, treatment of 20a with two equivalents of 2,6-dimethyl phenyl isocyanide, afforded 30b as a dark red crystalline material. Resonances for the SiMe 3 protons are observed at 0.73ppm (18H) whilst the RNC (R = 2,6dimethyl phenyl isocyanide) protons are observed at 2.11ppm (12H). The phenyl imido protons are observed at 7.43ppm(d, 2H), 6.99ppm(t, 2H, 7.24Hz) and 6.88ppm(t, 1H, 7.28Hz). Protons for the RNC phenyl group are observed as a multiplet at 6.63ppm. The o-pda ligand protons are observed as a pair of doublets of doublets at (5.65, 3.3Hz) at 7.51 and 7.06ppm. Two isocyanide stretches in the IR spectrum of 30a are observed at 2122 (CN symmetric stretch) and 2082 (CN asymmetric stretch). The appearance of these stretches at a lower frequency than the free isocyanide ligand (CN) = -9 and -50 cm -1 respectively) where CN)stretching frequency of free isocyanide) (stretching frequency of complex)) is indicative of back bonding from the metal to the isocyanide ligand. By comparison, the isocyanide stretches for 30b are observed at 2083 cm -1 and 2022 cm -1 (CN) = -31 and respectively). The larger CN) values observed for 30b is reflective of the greater propensity for 2,6dimethylphenylisocyanide to act as a

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73 acceptor ligand. This is in agreement with the observation that aryl isocyanides are better acceptor ligands than alkyl isocyanides. 20a30MoNNNPhMe3SiMe3SiLMoNLNNPhMe3SiMe3Si2 eqvs RNCisobutylenea. L = tBu isocyanideb. L = 2,6-dimethylphenyl isocyanideR = tBu, 2,6 -dimethylphenyl Scheme 4-1. Synthesis of bist BuNC complex X-ray crystal structure of 30b Single crystals of 30b were obtained by slow diffusion of pentane into a toluene solution of the complex. In addition to the complex, the asymmetric unit has one half of a toluene molecule located on an inversion center. The thermal ellipsoid plot of 30b is depicted in Figure 4-1. The geometry of 30b is best described as a distorted square pyramidal structure with the phenyl imido ligand occupying the axial position. The MoN(Imido) bond (1.746) is in the normal range for a MoN triple bond and the MoN (amido) bonds (2.064 and 2.093) are typical Mo-N bond lengths. The MoC(isocyanide) bonds (2.088 and 2.093) are shorter than typical MoC(sp) (2.2) bonds in related complexes suggestive of back bonding from the metal to the orbital of the isocyanide ligand. The isocyanide ligand is linear with C-N-C angles of (173 0 and 177 0 ).

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74 As has been reported for other terminal isocyanide complexes, there is no significant increase in the CN bond distance (Average 1.17). Like the previously reported d 2 complexes, the diamide ligand in this complex is flat (fold angle 173.7 0 ). 49 As discussed in chapter 2, this fold angle results from the fact that donation of the diamide lone pair electrons into the metal d xy is prevented because this orbital is occupied in these complexes. Figure 4-1. Thermal ellipsoid plot of 30b (30% ellipsoids). Selected bond lengths () and degrees ( 0 ): MoN(1) 1.746(2), MoN(2) 2.064(2), MoN(3) 2.093(2), MoC(19) 2.088(3), MoC(26) 2.085(3), C(19)N(4) 1.160(3), C(26)N(5) 1.166(3), C(19)N(4)C(20) 173.4(3), C(27)N(5)C(26) 176.9(2) Synthesis of tris-Isocyanide Complex The tris-isocyanide complex, ( t BuNC) 3 Mo(NPh)(o-(Me 3 SiN) 2 C 6 H 4 ), 31, was previously synthesized and reported in our labs. 74 It is included in this chapter because of its relevance to the subsequent

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75 discussion. When 20a, was allowed to react with 4 equivs of CN(t-Bu), a fast color change from green to purple occurred, and purple microcrystals of 31 precipitated from solution and were easily isolated by filtration. 20aMoNNNPhMe3SiMe3Si+ tBuNCisobutyleneLMoNLLNNPhMe3SiSiMe331 Scheme 4-2. Synthesis of trist BuNC complex 31 The pseudo-octahedral structure of 31 is shown in Scheme 4-2, was proposed on the basis of the observed NMR data and confirmed by X-ray crystallography. Resonances corresponding to inequivalent SiMe 3 protons were observed at 0.58ppm (br s, 9H) and 0.77ppm (br s, 9H). Broad singlets at 0.93 and 1.17ppm were assigned to inequivalent tert-butyl isocyanide peaks. A plane of symmetry containing the imido nitrogen, o-pda nitrogens, and one isocyanide ligand makes the remaining isocyanide ligands chemically equivalent. The broadening observed in the 1 H NMR spectrum of 31 has been correlated to an equilibrium involving fast, reversible ligand dissociation. As shown in Scheme 4-3, loss of one RNC ligand induces a plane of symmetry in the bis-isocyanide complex, 30a, that is formed. Thus the remaining isocyanide ligands and the SiMe 3 ligands become

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76 equivalent in 30a. Using the two-site exchange approximation, an activation barrier of G = 15.0 Kcal/mol for the ligand dissociation was calculated. + tBuNCLMoNLLNNPhMe3SiSiMe330aLMoNLNNPhMe3SiMe3SitBuNC+LL = tBuNC31 Scheme 4-3. Dissociative equilibrium observed for 31 Figure 4-2. Thermal ellipsoid plot of 31, (30% probability thermal ellipsoids), selected bond lengths () and angles 0 : MoN (1) 1.804(4), MoN (2) 2.166 (5), MoN (3) 2.130 (5), MoC (24) 2.067 (5), MoC (29) 2.158 (5), MoC (19) 2.137 (2)N (2) 1.182 (3), C(2)N(2)C(8) 152.8(5).

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77 X-ray crystal structure of 31. As shown in Figure 4-2, 31 was found to have a distorted octahedral geometry. Three isocyanide ligands were found to occupy meridianal positions. The imido-nitrogen was found to be trans to one of the isocyanide groups. The shorter bond length of the MoC(2) and longer bond length of the C(2)N(2) is attributed to backbonding from the metal d orbitals to the isocyanide ligands. Molecular orbital calculations We were interested in describing the bonding of the isocyanide ligand in our complexes and comparing the acceptor capabilities of RNC with carbonyls. Therefore, DFT calculations were performed on 31 and the previously reported carbonyl complex [Mo(NPh)(PMe 3 ) 2 (CO)(o-(Me 3 SiN) 2 C 6 H 4 )], 32 61 (Figure 4-3). For these calculations a two-layer ONIOM method was employed. For the inner layer the o-phenylene group (o-C 6 H 4 ) that links the two N atoms of the diamido ligand was simplified to a CH=CHcarbon chain. The organic groups on the nitrogen atoms (SiMe 3 of the amido and phenyl group of the imido) were replaced by hydrogen atoms for simplicity; and the subsitituents on the phosphine ligand in 32, and the isocyanide ligand in 31 were replaced by hydrogen atoms. For this layer the B3LYP/LANL2DZ method was employed. The outer layer consisted of the entire complex including all substituents on the imido, phenylene diamide, isocyanide and phosphine ligand and was modeled B3LYP/LANL2MB method. As shown in Figure 4-3, and Table 4-1, there is good agreement between the calculated geometries and the crystal structure of 31. The crystal structure of 32 has not been reported but the structure was confirmed as a minimum by a frequency calculation (number of imaginary frequencies=0).

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78 Table 4-1. Selected bond Lengths() and Angles( 0 ) for 31 and 32 31 32 Bond ONIOM X-ray Bond ONIOM MoN(1) 1.824 1.805(3) MoN(1) 1.833 MoN(2) 2.178 2.166(3) MoN(2) 2.186 MoN(3) 2.141 2.130(3) MoN(3) 2.137 MoC(1) 2.149 2.137(5) MoC(1) 1.998 MoC(2) 2.013 2.067(5) MoP(1) 2.479 MoC(3) 2.131 2.158(5) MoP(2) 2.477 C(4)N(5)C(2) 175.5 153.0(4) Our interest in the bonding in 31 and 32 stemmed from some interesting features in the structures of these molecules. In addition to the six electrons that can be donated by the imido ligand (one and two ), six electrons may also be donated by the diamide ligand through the lone pair electrons on nitrogen (two and one as has been mentioned before for d 0 and d 2 five coordinate complexes in previous chapters). However, a conflict arises because these complexes are Mo(IV) and one of the metal d orbitals is occupied. Thus donation of the diamide lone pair electrons into this orbital would result in an energetically unfavorable filled-filled interaction. The presence of a strong acceptor ligand trans to the diamide lone pair electrons may stabilize this situation as the diamide ligand, the metal orbital (d xy ), and the orbital of the acceptor ligand (CNR in 31 or CO in 32) can engage in 3 center-four-electron bonding as shown in Figure 4-4. Molecules 31 and 32 possess pseudo C s symmetry, thus, there are two sets of metal t 2g orbitals one with a symmetry (d xy and d yz ) and another with a symmetry (Scheme 4-4). A linear combination of the diamide lone pair electrons results in two orbitals of a symmetry ( 3 and 2 ) and these orbitals can interact with orbitals on the metal of appropriate symmetry i.e. the d xy and d yz orbital. The metal orbitals may also interact via back bonding to the acceptor ligand (carbonyl or isocyanide) that is arranged trans to

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79 the one of the diamide nitrogen lone pairs. The occupied molecular orbitals generated by DFT(B3LYP) are a good illustration of these interactions Figures (4-5) and (4-6). 3132 Figure 4-3. Optimized structure (ONIOM B3LYP/LANL2DZ:B3LYP/LANL2MB) for 31 and 32 MMMdiamide ligandmetal dxy (CO) or CNRBondingNon BondingAnti-Bonding Figure 4-4. 3-Center-4-electron bonding between the diamide lone pair electrons the metal d orbitals and the orbitals of a acceptor ligand

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80 PhMe3SiSiMe3LNNLLNPhMe3SiSiMe3MoLNLLPhMe3SiSiMe3MoLNLLNPhMe3SiSiMe3LNNLLNPhMe3SiSiMe3NNLLLNPhMe3SiSiMe3MoCNNLLOa'' (dxy)a'' (dyz)a' (dxz)a'' (3)a''(2)a'' CO (*)metal t2gdiamideoccupied orbitals Scheme 4-4. Group orbitals for interactions in 31 and 32, CO is used as the acceptor ligand in this example

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81 PhNButNCNCNButSiMe3Me3Si PhNButNCCNButSiMe3Me3Si PhNButNCCNButSiMe3Me3SiHOMOHOMO -1HOMO -2 Figure 4-5. Interaction of diamide lone pairs and isocyanide ligand with metal d xy orbital. (Orbitals are displayed at the 0.05 au isocontour level) PhMe3SiSiMe3NPMe3Me3PO PhMe3SiSiMe3NPMe3Me3PHOMOHOMO -1HOMO -2 PhMe3SiSiMe3NPMe3Me3P Figure 4-6. Interaction of diamide lone pairs and CO ligand with metal d xy orbital. (Orbitals are displayed at the 0.05 au isocontour level)

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82 The presence of a lone pair electron on the metal limits the orbital donation by the diamide ligand, thus in the HOMO for both 31 and 32 the diamide lone pairs are primarily ligand centered. The occupied orbitals HOMO and HOMO represent the aforementioned 3-center-4-electron interaction between the diamide lone pairs, the metal, and the orbital of the isocyanide or carbonyl ligand. Bonding between the diamide ligand and the metal d xy orbital in HOMO-2 is negated by antibonding to this ligand in HOMO 1. Thus, the net effect is that the diamide lone pair electrons remain ligand centered in the HOMO and the metal dxy orbital remains largely metal centered. Also evident from the molecular orbital diagrams are the differing acceptor capabilities of CO versus the CNR ligand. The coefficient at the isocyanide carbon in 31 is very small suggesting very little contribution from this carbon in the metalisocyanide back bond. In bonding to a acceptor ligand like CO or CNR the metal d orbitals are perturbed by both the and orbitals of the acceptor ligand (Scheme 4-5) and the net result shows cancellation of the electron density at the carbon and reinforcement at the heteroatom (N or O). 101 Scheme 4-5. Interaction of a metal d-orbital with a acceptor ligand In the case of CO the contribution from is greater than the contribution from the orbital, (because of the difference in electronegativity between carbon and oxygen there

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83 is a larger coefficient at carbon in *) and CO is an overall acceptor. The difference in electronegativity is smaller in between carbon and nitrogen thus there is a smaller coefficient on carbon in the orbital and this results in cancellation of most of the electron density at this atom in the bonding MO. These differences are clearly evident in the HOMO -1 and HOMO 2 orbitals in Figures 4-5 and 4-6. The contribution at the carbon in CO in 32, is significantly greater than in the isocyanide ligand in 31. Synthesis, Structure and Dynamics of a Chelating Imino Carbamoyl Complex Treatment of a C 7 D 8 solution of 30b with two equivalents of 2,6-dimethyl phenyl isocyanide results in the slow conversion (24 hrs) to the imino carbamoyl complex, 33 (Scheme 4-6). Chemical shifts for the SiMe 3 protons occur at the (0.44 ppm, s, 9H) and (0.59 ppm, bs 9H). Signals for the 2,6 dimethyl phenyl isocyanide ligands are observed at (1.94 ppm, s 12H) and two signals were observed for the 2,6-dimethyl phenyl protons at (2.45 ppm, s, 6H) and (1.86 pp s 6H). As shown in Scheme 4-6 the imino carbamoyl ligand is coordinated in 1 and fashion to Mo. The chemical shifts for the quaternary carbons of the iminocarbamoyl ligands are observed at (208.2 ppm) and (181.4 ppm) for the and 1 carbons respectively. One peak was observed in the IR spectrum for the asymmetric CN stretch (2088 cm -1 ) that occurred at a lower frequency than the free isocyanide ( = 25 cm -1 ) again, suggestive of net back bonding in these complexes. Isocyanide insertion into metal amide bonds is less extensively studied than the related insertion reactions of metal alkyl bonds. In recent years, studies of isocyanide insertions into Ta amide bonds have been reported. We propose that the formation of 33 occurs by initial coordination of the of two isocyanide ligands followed by the rapid

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84 insertion of the isocyanides into the metal amide bond. A 1,3 siltropic shift then follows and the resulting iminocarbamoyl species is rapidly trapped by two molecules of 2,6dimethyl isocyanide (Scheme 4-7). 30b33SiMe3PhNNRMoCCNNRRNNNMe3SiR2.0 eqvs2,6dimethylphenyl isocyanideLMoNLNNPhMe3SiMe3SiL = 2,6 -dimethylphenyl isocyanideR = 2,6 -dimethylphenyl24 hours Scheme 4-6. Synthesis of iminocarbamoyl complex, 33 PhNNRMoNNNMe3SiR30bLMoNLNNPhMe3SiMe3SiSiMe3PhNNRMoNNNMe3SiRMe3SiSiMe3PhNNRMoCCNNRRNNNMe3SiR332.0 equivs2,6dimethylphenyl isocyanideisocyanide insertion1,3 silatropic shiftL = 2,6 dimethylphenyl isocyanide Scheme 4-7. Proposed mechanism for the formation of 33

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85 X-ray Crystal Structure of 33 Single crystals of 33 were obtained by a cooling a concentrated pentane solution of this molecule (Figure 4-7). The geometry about Mo may best be described as a distorted trigonal bipyramid with the isocyanide ligands occupying the axial position and the imino carbamoyl fragments and the imido ligand occupying the equatorial plane. A plane of symmetry bisects the two trans isocyanide ligands and passes through the imido and iminocarbamoyl fragments. The imino carbamoyl fragment is flat (rms = 0.02), consequently the substituents on the amido nitrogens (N(5) and N(4)) are sterically encumbered (vide infra). Figure 4-7. Thermal ellipsoid plot of 33, (30% ellipsoids). Selected bond lengths () and angles( 0 ) MoN(1) 1.767(2), MoN(2) 2.108(2), MoC(57) 2.110(3), MoC(48) 2.131(3), MoC(14) 2.147(3), MoC(7) 2.052(3), N(2)C(7) 1.274(3), N(3)C(14) 1.318(4), N(4)C(7) 1.365(3), N(4)C(15) 1.454(3), C(21)N(5)C(14) 124.4(3), C(14)N(5)Si(2) 113.4(2), Si(2)N(5)C(21) 122.1, C(57)N(7)C(58) 166.9, C(48)N(6)C(49) 171.8(3)

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86 The MoN(1) bond length (1.767 ) is within normal range for a molybdenum nitrogen triple bond. One imino carbamoyl fragment binds 1 to the Mo center and shows localized CN bonds i.e., one short C=N bond, C(14)N(3), (1.318 ) and one CN single bond (C(14)N(5), (1.403). The 2 imino carbamoyl fragment exhibits a very short CN bond (C(7)N(2), (1.274)) and a long C(7)N(4) (1.365). Both amido nitrogens N(4) and N(5) are sp 2 hybridized with carbon nitrogen bond lengths shorter than typically seen for CN single bonds (1.48) and the sum of the angles around each nitrogen is equal to 359.9 0 This suggests delocalization within the CNC framework of the imino carbamoyl. The bond lengths for the isocyanide ligands (Average 2.12) are shorter than typical MoC (sp 2 ) bonds suggestive of back bonding to isocyanide in these complexes. The CNC isocyanide bond angles deviate only slightly from linearity (Average 169.4 0 ). The space-filling model of 33, generated from the X-ray study, reveals a very sterically congested region in the plane of the imino carbamoyl fragment (Figure 4-8). This suggests that a free rotation about the carbon nitrogen bonds C(7)N(4) and C(14)N(5) is restricted because of steric hindrance around the nitrogen atoms. For this same reason free rotation of the xylyl fragments about N(4) and N(5) are restricted because of steric interactions. Silicon atom Si(2), is located close to two nitogens; this atom is directly bond to N(5) (bond length = 1.78), but is also close to N(3) (2.53) this distance is less than the sum of the Van der Waals radii of the a nitrogen atom (3.60) and as is evident from the space filling diagram Si(2) interacts with both N(3) and N(5). These close contacts are responsible for the fluxionality of these molecules in solution (vide infra).

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87 Si atom in close contact with 2 nitrogen atoms Figure 4-8. Space filling diagram generated from the crystal structure of 33. The SiN bond distances (1.78) and (2.53) are within the sum of the Van der Waals radii of a N and Si atoms (3.60) Dynamic behavior of 33 in solution Complex 33 is fluxional in solution. At room temperature two SiMe 3 protons can be observed at (0.44 ppm, s, 9H) and (0.59 ppm, bs, 9H). When a C 7 D 8 solution of is heated to 348K the broad singlet sharpens and grows in intensity. The resonance observed at 0.59 ppm broadens into the baseline as this sample is cooled (Figure 4-9). Below the coalesence temperature (218K) two peaks reemerge at 1.13 ppm and 0.09 ppm in a 5:1 ratio. This fluxionality can be explained by a 1,3 silatropic rearrangement of the SiMe 3 group that has close contacts to two nitrogen atoms (Scheme 4-8).

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88 Figure 4-9. Variable temperature 1 H NMR spectrum (C 7 D 8 ) of 33. a) Broadening of the SiMe 3 protons as temperature is cooled. At 348K two peaks are observed for the SiMe 3 protons labeled Si(1) and Si(2) Upon cooling the sample the protons for Si(1), broadens into the baseline. b) 1 H NMR spectrum (C 7 D 8 ) of 33 at the low temperature limit (188K). Below the coalasence temperature, two peaks are observed for the Si(1) protons at 1.13 and 0.09 ppm in 5:1 ratio.

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89 SiMe3(B)PhNNRMoNN(a)N(b)( A )Me3SiRSiMe3(B)PhNNRMoNN(a)N(b)(A)Me3SiR1,3 silatropic rearrangement Scheme 4-8. 1,3 silatropic rearrangement in imino carbamoyl ligand. The axial isocyanide ligands have been removed for clarity Recall, from Figure 4-8 that the group Si(2) has close contacts between both N(a) and N(b). Below the coalescence temperature the equilibrium shown in Scheme 4-8 will be frozen in solution and two species will be detected that differ in the position of SiMe 3 (A). In one position the group SiMe 3 (A) is bonded to N(a) and in the other position it is bonded to N(b). The population of these sites are not equal thus the ratio of the two site populations N(a):N(b) is 5:1. Above the coalescence temperature the chemical shifts for the SiMe 3 are observed as an average of the two sites i.e. above T c the SiMe 3 group feels both N(a) and N(b). The exchange of unequally populated doublets has been described in more detail by Sandstrom in Dynamic NMR Spectroscopy. 102 [2+2+1] Cycloaddition reactions of metallacycles with alkyl isocyanides RNC. Our previous work with W alkyls and t BuNC seemed to suggest that isocyanide insertion in the case of W, proceeded selectively into W alkyl bonds over the W diamide bond. 44 We were interested in investigating whether isocyanide insertion was also competitive in the case of Mo alkyl complexes so we pursued the reactions of the Mo metallacyclopentane complex 25 with isocyanides. Addition of 4.0 equivalents of t BuNC to 25 results in the formation of 30a and the iminocyclopentane molecule, 44 (Scheme 4-9). Whilst the addition of 5.0 equivalents of 2,6 dimethyl phenyl isocyanide to 25, results in the formation of 44, and 30b. Thus, the insertion of isocyanides into metal

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90 amide bonds seems to occur at a rate that is competitive with metal alkyl insertion in these complexes. MoNNPhNMe3SiMe3SiNRR = tBu, 2,6 dimethylphenylisocyanide31and3325 xs RNC44 Scheme 4-9. Isocyanide insertion into the metallacyclopentane complex 25 The reactions outlined in Scheme 4-9, presents a rare example of the metal mediated intermolecular [2+2+1] cycloaddtion reactions of alkenes and alkynes with alkyl isocyanides. Compared with the related carbonylative cyclization, reports of related isocyanide-inserted cyclization is quite rare. 54,59,103 Tamoa et al. published the pioneering work of the Ni(0) induced cyclizations of diynes in 1989. 72 Since then isocyanide insertion into zirconacyclopentadiene, has been demonstrated by Takahashi et al.. 56 The Buchwald group 52,104-108 and the Shibata group 55 have demonstrated the only two examples of catalytic coupling of alkynes and alkenes with alkyl isocyanides. We are currently exploring the catalytic possibilities of our complexes in the [2+2+1] cyclization of alkynes, alkenes, dienes and diynes with alkyl isocyanides.

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91 Summary and Conclusions In this chapter, the role of the chelating diamide fragment has been expanded from that of ancillary ligand to that of a ligand that can take part in the chemistry in these complexes. Again we have emphasized the importance of ligand interactions and there role influencing the outcome of chemical reactivity. The insertion reactions seen between 2,6dimethyl phenyl isocyanide and the diamide ligand results from the conflict that arises between the diamide nitrogen lone pairs and an occupied d orbital that results in the weakening of the MoN (diamide) bond and ultimately a lowering of the barrier to insertion. These results are promising because they suggest that these insertion reactions may lead to the development of a catalyst for the carbonylation of amines. These species have been shown useful as synthetic organic precursors. We are currently investing the potential catalytic abilities of our complexes.

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CHAPTER 5 ALKYL ALUMINUM INDUCED DIAMIDE TRANSFER FROM GROUP 6 IMIDO DIAMIDO COMPLEXES. Introduction In the previous chapters, the importance of the diamide donation in group 6 imido diamido complexes has been emphasized. The highest occupied molecular orbitals (HOMO) of these complexes are primarily ligand centered and can interact with Lewis Acids. In this chapter, we examine the reactivity of these complexes with alkyl aluminum reagents. Species derived from these interactions are potentially useful as olefin dimerization/oligomerization catalysts. Neutral aluminum complexes of the form AlX 3 (x=halide, alkyl etc.) have been extensively utilized in organic synthesis as Lewis acid reagents and as catalysts and co-catalysts in organotransition metal mediated polymerizations or oligimerizations. 109 Examples of this include Ziegler-Natta and metallocene based polymerization systems wherein a transition metal complex is treated with an aluminum reagent e.g. EtAlCl 2 or MAO(methyl aluminoxane) in the presence of olefin at high pressure. In these cases, the aluminum reagent acts as an alkylating agent, however, the exact nature of the catalytic species that arise from the interaction of the transition metal with the aluminum reagent are still under some debate. 110 In our labs we have had success in synthesizing high oxidation state imido complexes of the form M(NPh)(o-( Me 3 SiN) 2 C 6 H 4 )X 2 (M=Mo and W) 30,31 31 incorporating the ((o-(Me 3 Si) 2 -pda) (pda = phenylene diamide) ligand. We have shown 92

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93 that this ligand stabilizes the metal center via donation of the diamide lone pair electrons and we have been able to synthesize complexes incorporating various functional groups e.g. metallacycles, 30,46,100 31 alkyls, olefins, 46 and alkynes with both Mo and W. In an attempt to produce a catalyst that selectively generates 1-hexene from propylene, Exxon 111 revisited a catalytic system first developed by Lawson 112 and Menapace 88,113,114 involving an aniline/WCl 6 /Al mixture with a low Al to W ratio (5:1). Their observation that metallacyclopentanes were formed during this catalysis inspired us to examine the reaction of our metallacyclopentane complexes with aluminum reagents to investigate the catalytic activity of the resultant products. Reactions of Mo Dialkyl Complexes with Trimethyl Aluminum (TMA) Treatment of an orange solution of Mo(NPh)(o-(Me 3 SiN) 2 C 6 H 4 )(CH 2 ) 4 25, in toluene, with 1 equivalent of trimethyl aluminum (TMA) results in a green solution after 15 minutes. Concentration of the solution and subsequent crystallization from pentane (at -78 0 C) results in the isolation of green crystals of 34, a metallacyclopentane methyl complex (Scheme 5-1). A similar reaction occurs when an orange solution of the diphenyl acetylene complex, 35, is treated with 1 equivalent of TMA. The resultant yellow diphenyl acetylene methyl complex, 36, precipitates from pentane after stirring for 24hrs. This transformation is similar to the reaction between alkyl aluminum reagents and lanthanide or early transition metal amides which produce aluminate 115-120 or hydride 121 complexes. The reaction is driven by the formation of the Al-N bond of the aluminum amide complex that is also formed in the reaction. There are apparently no examples of

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94 such an exchange reaction involving an alkyl aluminum reagent and a diamide ligand bound to a transition metal or lanthanide. As shown in (Scheme 5-1) the R 2 Al((o-(Me 3 SiN) 2 -pda) anion that is formed in the reaction is trapped by coordination of its arene ring to the molybdenum center. The 1 H NMR spectra of 34 and 36 reveal the presence of highly shielded aromatic protons (4.7-5.5ppm), suggesting coordination of the aromatic ring to the Mo center. The metallacyclopentane ring protons in 34 appear as multiplets (1.8-3.2ppm). Three different methyl groups and two signals for the protons of the SiMe 3 groups are also observed, while the phenyl imido protons appear as a multiplet at 6-8ppm. The proton NMR spectrum of 36 is also consistent with the presence of a coordinated arene ring and a Mo methyl group. These structures were confirmed with single crystal X-ray diffraction studies. The thermal ellipsoid plots of 34 and 36 are presented in Figures 5-1 and 5-2. A distorted square pyramid with the o-pda ring occupying the apical position best describes the coordination geometry about molybdenum. The aluminum nitrogen bond lengths are typical of AlN single bonds (1.925 ). The pda phenyl ring binds to the metal in an 4 -fashion in both complexes but this bonding is not symmetrical. The metal carbon bond distances MoC10 (34) and MoC24 (36) are significantly shorter (2.334 and 2.348 respectively), than the adjacent metal carbon bonds MoC9 and MoC11 (2.414 and 2.464 respectively) in 34 and MoC23 and MoC25 (2.463 and 2.392) in 36. The 4 th metal carbon bond MoC12(34) and MoC26(36) of this butadienyl fragment is significantly longer (2.605 and 2.549, respectively).

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95 Al(CH3)325353436MoNNNPhMe3SiMe3SiMoNNNPhMe3SiMe3SiPhPhMoMe3SiSiMe3AlNNPhNMoMe3SiSiMe3AlNNPhNPhPh Scheme 5-1. Reaction of metacyclopentane complex with trimethyl aluminum Figure 5-1. Thermal ellipsoid plot of 34 (40% ellipsoids) The hydrogen atoms have been omitted for clarity. Selected bond lengths : MoN(1) 1.725(3), C(7)C(8) 1.469(4), C(8)C(9) 1.421(4), C(9)C(10) 1.408(4), C(10)C(11) 1.384(5), C(11)C(12) 1.400(4), C(12)C(7) 1.418(4), N(2)C(8) 1.335(4), N(3)C(7) 1.346(4), AlN(2) 1.925(2), AlN(3) 1.926(2).

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96 Figure 5-2. Thermal ellipsoid plot of 36 (40% ellipsoids). Hydrogens have been omitted for clarity. Selected bond lengths : Mo (1)N(1) 1.752(2), C(22)C(23) 1.403(4), C(23)C(24) 1.405(4), C(24)C(25) 1.398(4), C(25)---C(26) 1.404(4), C(26)C(27) 1.416(4), C(22)C(27) 1.478(4), C(22)N(3) 1.349(3), C(27)N(2) 1.339(3), Al(1)N(3) 1.923(2), Al(1)N(2) 1.926(2). The carbon nitrogen bond lengths (average 1.341 and 1.344 for 34 and 36 respectively) are slightly shorter than typical for diamide (1.38-1.42) 122 complexes but longer than diimine complexes (1.28). 123 Also, the carbon-carbon bonds of the benzenoid framework are characterized by long CC bonds (1.469 and 1.478). These data suggest that contributions from the two resonance forms A and B shown below describe the interaction between the arene fragment and the Mo center. These resonance structures account for the shortened C-N bonds (relative to a diamide structure) and the relatively long CC bonds that are consistent with a single bond between two sp 2 carbon atoms.

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97 NNRRAlNNRRAlAB DFT calculations 124 were performed on the model compounds [Me 2 Al((o-(Me 3 N) 2 -pda)] 37 and (CH 2 ) 4 Mo(CH 3 )(NH)(o-(Me 3 N) 2 C 6 H 4 )Al(CH 3 ) 2 38, in order to understand the structure and bonding in these new complexes. Optimized structures for these models are depicted in Figure 5-3 along with the molecular orbital plot of the HOMO (Figure 5-3c) of this anion. The benzenoid portion of the HOMO is of symmetry, ideal, for interaction with the empty metal d orbitals. The metal center being asymmetric binds strongly with three atoms in 38 while the 4 th carbon is less strongly bound due to the trans influence of the imido ligand as is observed in the structures of 34 and 36 (Figure 5-3d). In summary, we have demonstrated the first example of a diamide transfer reaction between trimethyl aluminum and an early transition metal. This transformation involves the formation of a very unusual aluminum anion that stabilizes the molybdenum via donor interactions. Transfer of carbon to aluminum is known to be an important chain transfer process in aluminum activated -olefin polymerization reactions. The similar transfer of other ligands, to Al such as diamides has not been investigated 125 These results show the NAl transfer is possible in early transition metal complexes and suggests that such processes may be possible in diamide containing polymerization catalysis. 126 Preliminary results show that 34 catalyzes the dimerization of ethylene

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98 giving 1-butene at room temperature and 1atm of ethylene pressure. We are currently investigating the utility of these complexes as olefin dimerization catalysts and their relevance to group 4 non-metallocene 125,127 polymerization systems. Figure 5-3. Optimized B3LYP structures for model complexes Me 2 Al((o-(Me 3 N) 2 -pda) -1 37, and (CH 2 ) 4 Mo(CH 3 )(NH)(o-(Me 3 N) 2 C 6 H 4 )Al(CH 3 ) 2 38 and their molecular orbitals. Reactivity of Alkyl Aluminum Halides with Group 6 Imido Diamido Olefin Complexes Alkyl olefin complexes are believed to be intermediates in the Cosse mechanism 128-130 where an alkene ultimately inserts into a metal carbon bond. This mechanism predicts that the type of carbon-carbon bond forming process that may follow olefin insertion i.e. olefin polymerization, oligomerization or dimerization, depends on the rate of chain growth (k g ), versus chain termination via elimination (k t ). There are innumerable examples of compounds that appear to catalyze olefin polymerization 101,110,125,127,131 and follow the the Cosse mechanism, however, the

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99 isolation of d 0 alkyl/alkene complexes has been rare because the absence of d electrons prevents stabilization of the olefin and leads to rapid insertion of the olefin into the metalalkyl bond. 128,129,132 During the last decade there has been considerable interest in the development of new non-metallocene catalysts in order to harness the potential of other metals to polymerize ethylene. 125,133 This research has led to the development of a wide array of catalysts virtually spanning the entire transition metal series and encompassing a plethora of non-Cp based ligands, including chelating diamides, imido, diketiminates, alkoxides and others. Chemistry in our group in recent years has focused on group 6 imido complexes incorporating the chelating ((o-(Me 3 Si) 2 -pda) (pda = phenylene diamide ligand). We have demonstrated the utility of these ligands in the synthesis of stable isolable high oxidation state alkyl, 30,31 olefin, 32,46 and alkyne complexes, and have emphasized the importance of diamide donation in stabilizing the metal center. In exploring the reactivity of these complexes with Lewis acids, we discovered serendipitously the unusual diamide transfer reaction of Mo dialkyl complexes with trimethyl aluminum (Scheme 5-1). 134 In these reactions, the chelating diamide ligand is transferred to the aluminum center accompanied by alkylation of Mo, resulting in the isolation of an unusual bimetallic complex. Given the prevalence of alkyl aluminum reagents in olefin oligomerization/polymerization catalysis, we were interested in exploring the reactivity of other molybdenum complexes with alkyl aluminum halides in the hope of elucidating the initial stages of early transition metal mediated olefin polymerization/oligomerization catalysis.

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100 Here we report the reactivity of alkyl aluminum reagents with olefin complexes of the form ( 2 -olefin)Mo(NPh)(o-(Me 3 SiN) 2 C 6 H 4 ). We begin with the reactivity of the metallacyclopentane complex 25, Mo(NPh)(o-(Me 3 SiN) 2 C 6 H 4 )(CH 2 ) 4 with alkyl aluminum halides. This metallacyclopentane reacts by initial CC bond cleavage to form a putative Mobis-ethylene complex that then reacts with the alkyl aluminum halide via diamide transfer as outlined in Scheme 5-1. The reaction of the styrene complex ( 2 -styrene)Mo(NPh)(o-( Me 3 SiN) 2 C 6 H 4 ) 20C, with ethyl aluminum dichloride or trimethyl aluminum results in the formation of complexes 39, ( 2 -styrene)Mo(Et)(NPh) 4 -(Cl 2 Al((o-(Me 3 SiN) 2 -C 6 H 4 )) or 40,( 2 -styrene)Mo(CH 3 )(NPh) 4 -((CH 3 ) 2 Al((o-(Me 3 SiN) 2 -C 6 H 4 )), respectively. In contrast, the reaction of ethyl aluminum dichloride with 20a, ( 2 -isobutylene)Mo(NPh)(o-(Me 3 SiN) 2 C 6 H 4 ), that contains the sterically demanding isobutylene ligand results in the the displacement of isobutylene and the formation of the bridging imido dimer [( 2 NPh)Mo(Cl)( 4 -(Cl 2 Al((o-(Me 3 SiN) 2 -C 6 H 4 ))] 2 41. The structure of 41 was confirmed by a single crystal X-ray diffraction study. Reaction of metallacyclopentanes with alkyl aluminum halides. Treatment of a red/orange toluene solution of 25, with an alkyl aluminum halide results in a color change from red/orange to yellowish brown in 15 minutes. Removal of solvent under reduced pressure followed by washing the brown solid several times with pentane results in the isolation of 42, ( 2 -ethylene)Mo(R)(NPh) 4 -(Cl 2 Al((o-(Me 3 SiN) 2 -C 6 H 4 )) as a brown powder (Scheme 5-2). Several attempts to crystallize 42 from a variety of solvents have resulted in the

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101 isolation of powder. Therefore the structure of 42 was elucidated by the extensive use of NMR spectroscopy (Figure 5-4). The protons of the o-pda ligand are shifted significantly upfield (4.6.6 ppm) upon coordination of the anion Cl 2 Al((o-(Me 3 SiN) 2 (C 6 H 4 ) ) to molybdenum. Four distinct signals are assigned to the ethylene protons at 1.04, 1.21, 2.84 and 3.05 ppm with coupling constants suggestive of a metallacyclopropane type structure (J = 14, 10, 2 Hz). RAlCl2, pentanea. R = Methylb. R = Ethylc. R = i-butyl25MoMe3SiSiMe3AlNNRNClClHHHHMoNNPhNMe3SiMe3Si42+ethylene Scheme 5-2. Reaction of metallacyclopentane complex with alkyl aluminum halides NMR experiments An array of 2D NMR spectroscopic techniques (gHMQC, gHMBC, NOESY) was utilized to establish the structure of 42b ( 2 -ethylene)Mo(Et)(NPh) 4 -(Cl 2 Al((o-(Me 3 SiN) 2 -C 6 H 4 )) (Figure 5-5). The protonated carbons were assigned to their corresponding protons via gHMQC spectra. Long-range coupling of the ethylene carbons (49.0 and 49.7 ppm) to the CH 2 protons of the ethyl fragment (2.22 ppm) were observed in the gHMBC spectrum. Strong NOEs in the 2D 1 H NOESY spectrum, places the proton at (5.23ppm) on the same side as SiMe3 protons (0.32ppm) and the proton at (4.70ppm) on the same side as SiMe3 protons (0.26ppm).

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102 MoMe3SiSiMe3AlNNNClClHHHHHHHHABCDEFGHIJKLABCD + EFGHIJKL Figure 5-4. 1 H NMR spectrum of 42a, ( 2 -ethylene)Mo(Me)(NPh) 4 -(Cl 2 Al((o-(Me 3 SiN) 2 -C 6 H 4 )) in C 6 D 6. MoMe3SiSiMe3AlNNNClClHHHHCH30.32, (-0.41)0.26, (0.0)1.85 (22.5)2.22 (27.4)1.21, (49.7)3.05 (49.7)1.04, 49.02.41, (49.0)HHHH4.70, (95.2)5.30, (105.7)6.59, (95.7)5.23, (96.4) Figure 5-5. Proposed structure for 42b showing selected carbon and (parentheses) and proton chemical shifts assigned by NMR spectroscopy. The reactivity of 25 with alkyl aluminum halides differs from the analogous reaction with halide free alkyl aluminum reagents such as trimethyl aluminum. The proposed reaction sequence is shown in Scheme 5-3. The formation of a heterobimetallic

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103 complex by coordination of aluminum to the diamide lone pair electrons is plausible as Lewis acid/base adducts have been isolated and characterized with lanthanide metals. 117,135 However, no intermediates are seen in this reaction even at 0 C. In the case of alkyl aluminum halides CC bond cleavage seems to be induced resulting in the loss of ethylene. This reaction proceeds via prior rearrangement of the metallacycle as shown in path B of Scheme 5-3. The difference in reactivity seen between alkyl aluminum halides and trialkyl aluminum, results from the accessibility of the diamide lone pair electrons. Computational studies done by Galindo et al., 62 and in our labs, 32 have revealed that the nitrogen lone pairs on the chelating diamide ligand in d 0 complexes are involved in bonding to the metal center via donation into the empty metal d x 2 -y 2 orbital, and is responsible for the folding of the diamide ligand. This occurs because the HOMO of the chelate is close in energy to the empty metal d orbitals. As outlined in Scheme 5-3, the square pyramidal (SP) metallacylopentane complex, undergoes a rearrangement to a trigonal bipyramidal metallacyclopentane complex, which then undergoes CC bond cleavage to form the bis-ethylene complex. In the (TBP) complexes the d x 2 -y 2 orbital lies in the node of diamide lone pair electrons and does not interact significantly with this orbital (the linear combination of diamide lone pair electrons is of a symmetry compared to the a d x 2 -y 2 orbital). Thus there is a greater stabilization of the diamide chelates HOMO in 25, than in the TBP intermediates. The diamide lone pair electrons on the metallacyclopentane complex requires the more acidic trialkyl aluminum reagents 109 in order for diamide transfer to be induced.

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104 MoNNNPhMe3SiSiMe3MoNNNPhMe3SiSiMe3MoNNNPhMe3SiMe3SiMoMe3SiSiMe3AlNNPhRNClClHHHHSPTBPTBPAl(CH3)3RAlCl2ethylenePath APath BMoMe3SiSiMe3AlNNPhN Scheme 5-3. Two pathways for the reaction of metallacyclopentanes with alkyl aluminum reagents Reactivity of alkyl aluminum reagents with styrene complexes Inspired by reactivity of alkyl aluminum reagents, with 25, we investigated the reactivity of olefin complexes with alkyl aluminum reagents. Treatment of a toluene solution of 20C, ( 2 -styrene)Mo(NPh)(o-( Me 3 SiN) 2 C 6 H 4 ) with 1.2 molar equivalents of trimethyl aluminum, results in diamide transfer and the bimetallic complex 39 as a mixture of two isomers (Scheme 5-5). The two isomers differ in the orientation of the methyl group transferred from trimethyl aluminum. In 39a the methyl group is oriented on the same side as the phenyl substituent of the styrene ligand, whilst the methyl group is on the same side of the methylene fragment of the styrene ligand in 39b. The two isomers can be separated by crystallization from pentane. An X-ray crystal structure determination of 39a, was obtained and confirms the stereochemistry of 39a. However we were not able to obtain

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105 crystals of a quality necessary for an accurate structure refinement, therefore, 39a is depicted in Figure 5-6 for the purposes of confirming the stereochemistry. Nipop123*3* of the o-pda ligand donates to the empty metal d orbitalsNPhMetal a'NPhNPhNPhNPhMetal a''NNNNMetal a'Metala''PhPhPhPhPhZYXzyx Scheme 5-4. Orbital interactions in square pyramidal (SP) metallacyclopentane complex and trigonal bipyramidal metallacyclopentane complex Figure 5-6. X-ray structure determination of 39a

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106 Al(CH3)3pentaneMoMe3SiSiMe3AlNNH3CNCH3CH3MoMe3SiSiMe3AlNNH3CNCH3CH3MoNNPhNMe3SiMe3Si39a39b20C Scheme 5-5. Reactivity of styrene complex with trimethyl aluminum 2D 1H NMR studies. The stereochemistry of 39 was also determined by 1 H NMR spectroscopy. Structures for 39a and 39b as determined by gHMQC, gHMBC, and NOESY spectroscopy are depicted in Figures 5-7 and 5-9. The methyl proton in 39a (1.29ppm), displays NOes to two phenyl ortho protons (6.55 and 7.00 ppm), two protons from the o-pda ligand (4.71 and 5.51 ppm) and one proton from the styrene fragment (2.84 ppm) (Figure 5-8). In contrast, the NOESY spectrum of 39b the methyl proton (1.32 ppm) showed NOes to an ortho phenyl proton (6.20), two o-pda ring protons (4.96, 5.48ppm), and the two methylene protons (2.99, and 1.97ppm) (Figure 5-10). These results confirm the assignment of the methyl group cis to the methylene group in 39b, and cis to the phenyl group in 39a. The NOESY spectrum did not display any chemical exchange peaks between 39a and 39b at 25 0 C.

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107 MoMe3SiSiMe3AlNNH3CNCH3CH3HcHaHbHiHeHfHgHh-0.33,-5.9-0.03, -1.350.14, -1.350.21, -0.722.84, (62.0)3.34, (41.2)1.64, (41.2)Hd(146.0)7.0, (125.2)1.29, (20.9)4.71,( 89.0)6.55, (125.3)(155.0)5.51, (98.4)5.54, (98.4)4.32, (89.0)13C NMR shifts are in parenthesesDashed lines represent NOE'sgHMBCMethyl proton (1.29 ppm) shows 3 bond correlations to the methylene carbon at 41.2 ppmMetallcycle proton 2.84 shows 2-bond correlations to the methylene carbon at 41.2 and tothe phenylene carbon at 146.0ppm Figure 5-7. Proposed structure for 39a showing selected carbon and (parentheses) and proton chemical shifts assigned by NMR spectroscopy. Figure 5-8. 1 H NOESY spectrum of 39a

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108 MoMe3SiSiMe3AlNNH3CNCH3CH3HaHcHb2.99, (44.5)1.97, (44.5)3.61, (59.1)HeHfHgHhHi1.32, (20.2)Hj6.207.0, (125.2)4.96, (85.3)5.48, (94.2)5.86, 99.63.77, (92.1)0.08, (-1.24)0.2, (0.79)(148.7)gHMBCMethyl proton (1.32ppm) shows 3 bond correlations to the methylene carbon at 44.5ppmMetallcycle proton 3.61 shows 2-bond correlations to the methylene carbon at 44.5 the phenylene carbon at 148.7ppm13C NMR shifts are in parenthesesDashed lines represent NOE's0.01,(-1.24)-0.31, (-5.77) Figure 5-9. Proposed structure for 39b showing selected carbon and (parentheses) and proton chemical shifts as assigned by NMR spectroscopy. Figure 5-10. 1 H NOESY spectrum of 39b Reactivity with alkyl aluminum halides. Treatment of 20c with 1.2 equivalents of methyl aluminum dichloride, results in the bimetallic complexes 40a and 40b. In this case we were not able to separate the two

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109 isomers but there structures were confirmed by NMR spectroscopy. Again, the methyl group in 40a was oriented cis to the phenyl group of the styrene ligand, whilst this group is oriented cis two the methylene fragment in 40b. MeAlCl2pentaneMoMe3SiSiMe3AlNNH3CNClClMoMe3SiSiMe3AlNNH3CNClClMoNNPhNMe3SiMe3Si40a40b20c Scheme 5-6. Reaction of styrene complex with methyl aluminum chloride Reactivity of an isobutylene complex with EtAlCl 2 When the isobutylene complex 20a, ( 2 -isobutylene)Mo(NPh)(o-( Me 3 SiN) 2 C 6 H 4 ), is treated with one equivalent of ethyl alumnum dichloride in pentane. The dimeric complex 41, [( 2 NPh)Mo(Cl)( 4 -(Cl 2 Al((o-(Me 3 SiN) 2 -C 6 H 4 ))] 2 precipitates as a green powder (Scheme 5-7).

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110 MoSiMe3ClNNPhAlClEtSiMe3NSiMe3NMoSiMe3ClNNPhAlClEtMoNNPhNMe3SiMe3SiEtAlCl2, pentane-isobutyleneMoSiMe3ClNNPhAlClEtSiMe3NSiMe3NMoSiMe3ClNNPhAlEtClMoSiMe3ClNNPhAlClEtSiMe3NSiMe3NMoSiMe3ClNNPhAlClEt20a41 Scheme 5-7. Reaction of isobutylene complex with ethyl aluminum chloride Complex 41 exists as a mixture of three isomers that are not resolved at 25 0 C in the 300 MHz 1 H NMR spectrum. However, the spectrum taken on a 500 MHz instrument revealed the presence of three molecules. The 2D 1 H NOESY spectrum of 41, revealed consisted of three dimeric complexes. Three distinct resonances for three different SiMe 3 groups are detected (0.28, 0.29, 0.30 ppm). These protons show NOes in the 2D NOESY (Scheme 5-8) to three distinct aluminate anion [EtAl(Cl)((o-(Me 3 N) 2 -o-pda)] -1 rings (Scheme 5-8). The sterically demanding isobutylene ligand in 20a is easily displaced by the by the EtAlCl 2. After undergoing diamide transfer, the molecule dimerizes through the imido groups as shown in Scheme 5-9. The relative dispositions of the chloride ligands on the Mo and the Cl and Et groups on Al give rise to 4 possible isomers. Three of these isomers are seen in our system; however, because of extensive overlap between the

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111 proton resonances of these isomers it was not possible to assign unambiguously the absolute configuration for each isomer. NSiMe3NMoSiMe3ClNAlClNSiMe3NMoSiMe3ClNAlClNSiMe3NMoSiMe3ClNAlClHHHHHH0.28 ppm5.59 ppm4.17ppm1.02 ppm0.065 ppm1.02 ppm0.066ppm0.29 ppm7.2ppm5.59 ppm4.16 ppm0.30ppm5.63 ppm4.15 ppm0.96 ppm0.72 ppm Scheme 5-8. NOEs of isomers of 41 MoSiMe3ClNNPhAlClEtSiMe3NSiMe3NMoSiMe3ClNNPhAlClEtMoSiMe3ClNNPhAlClEtSiMe3NSiMe3NMoSiMe3ClNNPhAlClEtMoSiMe3ClNNPhAlClEtSiMe3NSiMe3NMoSiMe3ClNNPhAlEtCl[2+2][2+2]MoSiMe3ClNNPhAlEtClSiMe3NSiMe3NMoSiMe3ClNNPhAlClEtNMoLNMoClLClNMoLNMoLClCl Scheme 5-9. [2+2] cycloaddition reaction of the imido ligand

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112 Crystal Structure of 41 X-ray quality crystals of 41 were obtained by slow diffusion of pentane into a concentrated toluene solution of 41 (Figure 5-11). The asymmetric unit of 41 consists of of the dimer. The molecule possesses a C 2 axis and a mirror plane. The Al(Cl)(C 2 H 5 ) moiety is disordered and is refined in two parts. Their site occupation factors were dependently refined to 0.29(1) for the major part, and consequently 0.21(1) for the minor part. These moieties lay on the mirror plane thus the site occupation factors site above add up to 50%. The coordination geometry about each molybdenum atom is best described as a distorted square pyramid, with the chloride ligands occupying the apical position and the 4 aluminate anion occupying two coordination sites. Figure 5-11. Thermal ellipsoid plot of 41, (40% ellipsoids). Selected Bond Lengths (): MoCl (3), 2.337; MoC (7), 2.269; MoN (1), 1.931; MoC (6), 2.487; C(5)N(2), 1.335; C(7)C(7C), 1.409; C(5)C(5C), 1.501.

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113 The bonding of the aluminate anion, [EtAl(Cl)((o-(Me 3 N) 2 -o-pda)] to molybdenum in 41, is similar to other Mo complexes synthesized in our group where an alkyl aluminum reagent induces transfer of the diamide ligand. The crystal structure of 41 suggests that there is localization of the charge in the benzenoid portion of the o-pda ring (compare the C(5)C(5C) bondlength (1.501) with the C(7)C(7C) bond length (1.409). Also the C(5)N(2) (1.335) bond length is more closely associated with a semibenzoquinone diimine type structure than a diamide structure. 136-140 There has been a tremendous amount of research on the unsubstituted o-phenylenediamido, [C 6 H 4 (NH) 2 -o] 2, ligand and its non-innocence when encountered in transition metal chemistry. In addition to the dianionic diamido form, complexes possessing the monoanionic semibenzoquinone diimine, and neutral benzoquinone diimine ligands have all been observed. In all complexes involving these ligands the diamide lone pair electrons are intimately involved in the stabilization of the metal center. In group 6 complexes for example, the diamide lone pair electrons are donated into the empty metal d orbitals and the ligand subsequently folds, whereas in group 8 metal complexes the metal d orbitals back bond into the ligand CN orbital and subsequently the ligand is flat. 136,141 Unlike the case with transition metals, aluminum does not posess d orbitals that are low enough in energy to interact with the diamide orbitals. Furthermore, the trivalent nature of the aluminum ion results in an overall negative charge on the newly formed aluminate anion. The excess charge density in this anion is localized onto the benzenoid portion of the o-pda ring which subsequently interacts with the molybdenum center through its electrons. This type of bonding interaction has been observed in DFT

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114 calculations (B3LYP/LANL2DZ) on the model complex (CH 2 ) 4 Mo(CH 3 )(NH)(o-(Me 3 N) 2 C 6 H 4 )Al(CH 3 ) 2 134 Summary We have extended the unique diamide transfer reactivity of Mo dialkyls with trimethyl aluminum to other aluminum reagents (alkyl aluminum halides) and olefin complexes. The observed difference in reactivity between alkyl aluminum reagents and alkyl aluminum halides in their reactions with metallacyclopentanes results from the decreased availability of the diamide lone pair electrons in the square pyramidal metallacyclopentane complex. This reactivity leads to the synthesis of stable isolatable alkyl olefin complexes. The alkyl aluminum induced diamide transfer reaction is quite general for molybdenum and we are currently exploring the implications of this reaction in olefin dimerization/oligomerization reactions. Implications of Aluminum Induced Diamide Transfer to Olefin Dimerization Catalysis Previous work from our group showed that exposure of a mixture of W(CH 2 ) 4 (NPh)(o-(Me 3 SiN) 2 ), 13, and EtAlCl 2 to a positive pressure of ethylene resulted in the dimerization and codimerization of the olefin to form a mixture of C 6 products (Scheme 5-10). 142 This olefinic product distribution shown in Scheme 5-10 is not consistent with the well-known Cossee mechanism of olefin polymerization; instead a metallacyclic mechanism must be invoked to explain the observed product selectivity (Scheme 5-11). The exact nature of the catalytic species is still not known for this process. Therefore, the preceeding work on Mo was attempted in order to attain some insight into the mechanism of these dimerization reactions. In the case of Mo, it seems clear that diamide transfer is an important step in the interaction of the metallacyclopentane with

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115 alkyl aluminum reagents. However, unlike Mo, treatment of the W metallacyclopentane with an alkyl aluminum halide does not result in the disruption of the metallacycle and the liberation of ethylene but instead proceeds with the liberation of 1-butene i.e CC bond cleavage was not induced in the case of W. We investigated stoichiometric reactions between various aluminum reagents and W metallacyclopentanes in order determine the nature of any intermediates in this catalysis and in order to investigate their catalytic activity. AlAl = EtAlCl2, Et2AlCl, AlCl3ethylene35%60%5%"W imido catalyst"13WNNNPhMe3SiMe3Si Scheme 5-10. Lewis acid promoted dimerization of ethylene WWWWButene couples to ethylene with the ethyl fragment in the positionButene couples to ethylene with the ethyl fragment in the position Scheme 5-11. Olefin dimerization based on metallacycle mechanism

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116 Reactions of W complexes with alkyl aluminum reagents Synthesis of metallacyclopentane methyl complex 42. In a resealable NMR tube, treatment of a C 6 D 6 solution of the metallacyclopentane complex 13, with 1.2 equivalents of trimethyl aluminum slowly results in the formation of the metallacyclopentane complex 42 after two days (Scheme 5-12). Al(CH3)31342WNNNPhMe3SiMe3SiWMe3SiSiMe3AlNNPhN Scheme 5-12. Reaction of W metallacyclopentane with trimethyl aluminum Attempts to isolate 42 on a preparative scale gave an oil that was difficult to purify. However an examination of the proton spectrum of 42 reveals that it is very similar to the metallacyclopentane methyl complex obtained for Mo (Figure 5-12). The benzenoid protons of the aluminate anion are clearly evident (4.6-5.5 ppm). Three methyl groups (1.2, 0.08, -0.2ppm) and two SiMe 3 protons(0.19, 0.17ppm) are also observed. Reaction with EtAlCl 2 Treatment of a C 6 D 6 solution of 13 with 1.2 equivalents of EtAlCl 2 results in the transfer of the o-pda ligand (Scheme 5-13). The exact nature of the species formed could not be determined because the new species quickly decomposes (overnight) in solution. It is clear that an aluminate type species very similar to those seen with Mo is formed during this reaction because of the appearance of peaks between 5.4-6.6 ppm as in the proton NMR spectrum, which are characteristic of the arene coordination observed in these compounds. Also significant, is the observation that

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117 unlike the reaction of alkyl aluminum halides with the Mo metallacyclopentane which undergoes CC cleavage, the reaction between alkyl aluminum halide and 13 retains the metallacycle framework and no ethylene is released during this reaction. Interestingly, treatment of 13, with two equivalents of EtAlCl 2 results in the liberation of butene and a material is formed that then goes on to catalyze the formation of the C6 products identified in the catalysis. Figure 5-12. 1 H NMR spectrum of W metallacyclopentane methyl complex Diamide transfer to other W complexes. Diamide transfer to aluminum from other W complexes can also be observed. Treatment of the diphenyl acetylene complex 43, and the neopentylidene complex 12 with 2 equivalents of trimethyl aluminum also results in the transfer of the diamide ligand (Scheme 5-13). The exact nature of the species formed in these reactions is still unclear as these compounds have not been isolated. However, in the case of 12, it is clear that two new alkylidene species are formed as two new resonances for the alkylidene protons

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118 (9.3 and 9.1ppm) are observed in the 1 H NMR spectrum along with the peaks for the aluminate anion, [Al(CH 3 ) 2 ((o-(Me 3 N) 2 -o-pda)] (Figure 5-13) These results demonstrate that the transfer of the diamide group to alkyl aluminum reagents is a general reaction pathway in this class of group 6 imido diamide complex. The generality of this reaction may have important implications on the function of other early transition metal amide complexes that function as olefin polymerization catalysts when they are activated by alkyl aluminum reagents. The transfer of a diamide ligand to aluminum has never been considered as a part of the catalyst activation process in these materials. Al(CH3)3WMe3SiSiMe3AlNNPhN12WNNNPhMe3SiMe3SiPMe343WNNNPhMe3SiMe3SiPhPh13WNNNPhMe3SiMe3SiWMe3SiSiMe3AlNNPhNClClEtAlCl2+ buteneexact identity of these species are not known Scheme 5-13. Diamide transfer reactions of W complexes

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119 Conclusions We have demonstrated in this chapter the novel diamide transfer of the o-pda ligand in group 6 metals induced by alkyl aluminum halides. These findings have the potential to be of great interests to a significant portion of the organometallic chemistry community not only because of the possibilities of discovering new catalytic systems but also because they may provide insight into the mechanisms of traditional catalysis i.e. Ziegler Natta and/or metallocene olefin polymerization that use high concentrations of aluminum reagents as cocatalysts in there systems. The discovery that the diamide transfer is also induced in W is significant as it offers the potential of elucidating the reaction mechanisms of the olefin dimerization/codimerization of ethylene seen in our group. Work is currently underway attempting to isolate some of the species derived from the stoichiometric reaction of W complexes with alkyl aluminum reagents.

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120 ppm (f1) 0.0 5.0 10.0 ppm (f1) 5.00 5.50 6.00 aluminate protons ppm (f1) 9.50 10.00 10.50 alkylidene protons Al(CH3)3WMe3SiSiMe3AlNNPhN12WNNNPhMe3SiMe3SiPMe3 Figure 5-13. 1 H NMR spectrum for the reaction of 12, with TMA, showing resonances for the aluminate anion, [Al(CH 3 ) 2 ((o-(Me 3 N) 2 -o-pda)] -1 (right inset), and two alkilidene species (left inset).

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CHAPTER 6 EXPERIMENTAL General Methods All reactions were conducted under a dry argon atmosphere using standard Schlenk techniques and all compounds were handled in a nitrogen-filled dry box. All solvents were distilled under nitrogen from sodium or sodium benzophenone ketyl or passed over activated alumina, stored over molecular sieves, and degassed prior to use. Compounds 4, 20a, 20c, and 25 were synthesized according to published procedures. 30,46,74 All aluminum reagents were purchased from Aldrich and used without further purification. NMR spectra were obtained on Varian Gemini 300, VXR 300, or Mercury 300 instruments with C6D6, C 7 D 8 or CDCl 3 as solvents, as noted, and referenced to residual solvent peaks. A Varian Inova 500 or a Mercury 300bb equipped with an indirect detection probe was used as indicated for the GHMQC, GHMBC, and NOESY experiments. The full X-ray diffraction data for all compounds are stored on a database that can be accessed at http://xray.chem.ufl.edu/chemIndex.htm Contact Dr. Khalil A. Abboud at 352-392-5948 for more information. Synthesis of 2 alkyne complexes R 1 = H R 2 = CO 2 Me, (21a) Methyl propiolate 0.17ml(1.94mmol) was added drop-wise with stirring to a pentane solution of 20a 0.959g (1.94 mmol). An immediate color change from green to orange occurred. The reaction mixture was allowed to stir for hour after which the solution was concentrated by removal of the solvent in vacuo. The 121

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122 pentane solution was reduced to ca. 10ml and then cooled to 0 C for two hours. Orange crystals appeared and after filtration [Mo (NPh)( 2 methyl propiolate)(o-(Me 3 SiN) 2 C 6 H 4 )], 21a was obtained in 82% yield. Recrystallization from concentrated pentane solutions of 21a allowed for further purification. 1 H NMR (C 7 D 8 20 0 C): 9.13 (s 1H alkyne proton) 7.33 (d 2H J = 8.1 Hz phenyl imido ortho protons) 7.20 (bs 2H o-pda ring protons) 6.80 (t 1H phenyl imido para proton J = 7.7 Hz) 3.44 (s 3H CO 2 Me protons) 0.48 (s 18H SiMe 3 protons). One o-pda and one phenyl imido resonance is overlapping with sovent. 1 H NMR (C 7 D 8 -50 0 C): 9.08 (s 1H alkyne proton) 7.42 (d, 2H J = 7.2 Hz phenyl imido ortho protons) 7.27 (d 2H J = 7.8 Hz o-pda ring protons) 6.93 (t 1H J = 7.5 Hz phenyl imido meta proton) 6.80 (t 1H J = 7.5 Hz phenyl imido para proton) 3.42 (s 3H CO 2 Me protons) 0.49 (s 9H SiMe 3 protons) 0.45 (s 9H SiMe 3 protons). One o-pda resonance is overlapping with solvent. 13 C NMR (C 6 D 6 20 0 C): 1.8, 51.8, 122.9, 124.1, 125.7, 126.2, 129.3, 133.4, 138.3, 157.6, 173.8, 176.8. Anal. Calcd for C 22 H 33 MoN 3 O 2 Si 2 : C, 50.66; H, 6.35; N, 8.02; Found C, 51.40; H, 6.20; N, 7.98. R 1 = Ph R 2 = Me (21b) 1-Phenyl-1-propyne (0.31ml, 2.48mmol) was added drop-wise with stirring to a pentane solution of 20a (1.00g, 2.03mmol). An immediate color change from green to orange occurred. The reaction mixture was allowed to stir for hour after which the solution was concentrated by removal of the solvent in vacuo. The pentane solution was reduced to ca. 10ml and then cooled to 0 C for two hours. Orange crystals appeared and after filtration [Mo(NPh)( 2 -phenyl-1propyne)(o-(Me 3 SiN) 2 C 6 H 4 )], 21b was obtained in 84% yield. Recrystallization from concentrated pentane solutions of 21b allowed for further

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123 purification. 1 H NMR (C 6 D 6 20 0 C): 7.36 (d 2H phenyl imido ortho protons) 7.35 (d 2 H alkyne phenyl ortho proton J = 8.1 Hz, 4.9 Hz) 7.24 (bs 2H o-pda ring protons) 7.14 (t 2H phenyl imido meta proton J = 6.6 Hz) 7.04 (t 2H alkyne phenyl meta proton J = 7.5 Hz) 7.00 (t 1H phenyl imido para proton J = 6.3 Hz) 6.91 (s o-pda ring protons) 6.80 (t 1H J = 7.4 Hz alkyne phenyl para proton) 2.41 (s 3H CH 3 CCPh protons) 0.48 (s 18H SiMe 3 protons). 1 H NMR (C 7 D 8 -40 0 C) 7.38 (d 2H J = 9.0 Hz phenyl imido ortho proton) 7.35 (d 2H J = 8.0 Hz alkyne phenyl ortho protons) 7.11 (t 2H J = 8.2 Hz alkyne phenyl meta protons) 6.88 (dd 2H J = 6.4 Hz, 2.4 Hz o-pda ring protons) 6.83 (t 1H J = 8.2 Hz alkyne phenyl para proton) 6.77 (t 1H J = 7.2 Hz phenyl imido para proton) 2.40 (s 3H PhCCMe proton) 0.52 (s 9H SiMe 3 ) 0.43 (s 9H SiMe 3 ) C NMR (C 13 6 D 6 20 0 C): 2.4, 18.9, 122.9, 123.8, 124.5, 127.9, 128.8, 129.2, 129.8, 139.5, 158.2, 179.9, 185.4. Anal. Calcd for C 27 H 35 MoN 3 Si 2 : C, 58.57; H, 6.37; N, 7.59; Found C, 58.44; H, 6.35; N, 7.52. R 1 = SiMe 3, R 2 =Me, (21c) One equivalent of trimethylsilyl propyne (0.141, 0.95mmol) was added drop-wise with stirring to a pentane solution of 20a (0.47g, 0.95mmol) of. The reaction mixture was allowed to stir for hour after which the solution was concentrated by removal of the solvent in vacuo. The pentane solution was reduced to dryness and [Mo(NPh)( 2 trimethylsilyl propyne)(o-(Me 3 SiN) 2 C 6 H 4 )], 21c was obtained as a green oil in 95% yield. Further attempts to crystallize this compound were unsuccessful. 1 H NMR (C 6 D 6 20 0 C): 0.44 (s, 9H, SiMe 3 ), 0.46 (s, 18H, SiMe 3 ), 2.44 (s, 3H, Me), 6.82-7.33 (aromatics), 10.11 (s, 1H, alkyne), 1 H NMR (C 6 D 6 0 C) 7.33 (d 2H J = 7.8 Hz phenyl imido ortho protons) 7.22 (m 2H o-pda ring protons) 7.04 (t 2H J = 8.2 Hz phenyl imido meta protons) 6.91 (dd 2H J = 7.8 Hz, 3.6 Hz o-pda ring

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124 protons) 6.80 (t 1H J = 7.5 Hz phenyl imido para proton) 2.45 (s 3H SiMe 3 CCMe protons) 0.51 (s 9H SiMe 3 protons) 0.48 (s 9H SiMe 3 protons) 0.23 (s 9H SiMe 3 protons). 13 C NMR 0.2, 2.03, 20.7, 123.4, 124.0, 128.7, 157.8, 180.7, 199.6 [(NPh)Mo(C(Ph)C(H)CH 2 CH 2 )(o-(Me 3 SiN) 2 C 6 H 4 )], (26a) In a sealed ampule, a pentane solution of 20a (0.500g, 1.013 mmol) was frozen and evacuated. The vessel was then cooled and the mixture thawed. The neck of the vessel was flushed with dry ethylene for three minutes. Ethylene gas was then added (15psi) to the solution and the mixture was stirred for 12 hrs. The volume was reduced in vacuo and cooled to 0 C. Orange crystals of 26a quickly precipitated from solution and were isolated in 80% yield. 1 H NMR (C 6 D 6 ): -0.07 (s, 9H, SiMe 3 ), 0.33 (s, 9H, SiMe 3 ), 1.66 (dt, 1H, 11.8Hz, 5.9Hz), 2.74 (dddd, 1H, 17.9Hz, 7.2Hz, 5.1Hz, 2.8Hz), 3.05 (dddd, 1H, 17.6Hz, 7.2Hz, 6.6Hz, 2.3Hz), 3.32 (dddd, 1H, 11.7Hz, 5.4Hz, 2.7Hz, 0.9Hz), 6.84 (t, 1H, 2.4Hz), 6.87-7.55 (aromatic), 7.24 (dd, 1H, phenyl ortho proton, 8.5Hz, 1.5Hz), 7.33 (dd, 1H, o-phenylene diamine proton, 8.0Hz, 1.6Hz), 7.37 (dd, 1H, o-phenylene diamine proton, 8.0Hz, 1.6Hz) 7.54 (dd, 1H, ortho-phenyl imido proton, 8.4Hz, 1.3Hz). 13 C NMR: 1.1, 1.5, 40.1, 56.5, 122.7, 125.0, 125.4, 122.8, 126.0, 126.5, 127.4, 129.4, 132.6, 135.1, 151.8, 157.2, 158.0, 188.6. Anal. Calcd for C 30 H 42 MoN 3 OSi 2 : C, 58.80; H, 6.91; N, 6.86; Found C, 58.24; H, 6.79; N, 7.37. [(NPh)Mo(C(H)C(Ph)CH 2 CH 2 )(o-(Me 3 SiN) 2 C 6 H 4 )], (26b) Method 1 A purple Et 2 O solution of 4 (3.00g, 5.17mmol) was cooled to 0 C and treated with of (5.17 ml) EtMgCl 2 (2.0M solution in Et 2 O). An immediate color change from purple to red occurred and the solution was stirred at 0 C for hr. Phenyl acetylene (0.52ml, 6.20mmol) was then added and the solution was allowed to warm to room temperature.

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125 The solution was stirred overnight and the solvent was then removed in vacuo. Complex 26b was then extracted with a toluene/ pentane solution. The solvent was then removed in vacuo and a red solid was obtained. Washing this solid with cold pentane afforded 26b in 40% yield. Method 2 In a sealed ampoule a toluene solution of 25 (0.50g, 1.013 mmol) was heated for 12hrs at 72 0 C with phenyl acetylene (0.27ml, 1.21mmol). The vessel was then cooled and the toluene removed in vacuo to afford an orange solid that consisted of a mixture of 26b (20%) and 26a (80%). The orange solid was then dissolved in pentane and recrystalized at 0 C to afford 26b. 1 H NMR (C 6 D 6 ): 0.40 (s, 9H, SiMe 3 ), 0.43 (s, 9H, SiMe 3 ), 1.34 (dt, 1H, 12.2Hz, 6.1Hz), 3.35 (ddd, 1H, 11.9Hz, 6.1Hz, 1.2Hz), 3.41 (ddd, 1H, 16.8Hz, 7.2Hz, 1.4Hz), 3.70 (dddd, 1H, 17.1Hz, 8.2, Hz, 6.0Hz, 2.3Hz) 6.82 (tt, 1H, phenyl imido para proton, 7.4Hz, 1.2Hz), 6.99-7.14 (ov, mult, 7H,) 7.36-7.46 (ov, mult, 4H), 7.54 (dt, 2H, phenyl imido ortho proton, 7.2Hz, 1.8Hz,) 8.47 (d, 1H, vinylic proton, 2.5Hz). 13 C NMR(C 7 D 8 ): 1.4, 42.8, 56.0, 122.8, 123.7, 125.9, 125.6, 126.0, 126.2, 126.4, 127.5, 129.2, 135.0, 135.2, 141.2, 147.8, 158.1, 168.5, 172.0. Anal. Calcd for C 28 H 37 MoN 3 Si 2 : C, 59.24; H, 6.57; N, 7.40; Found C, 60.68; H, 6.76; N, 6.93. [(NPh)Mo(C(CO 2 Et)C(CO 2 Et)CH 2 CH 2 )(o-(Me 3 SiN) 2 C 6 H 4 )], (26c) In a sealed ampoule a toluene solution of 25 (0.521g, 1.06 mmol) was heated for 12hrs at 72 0 C with diethylacetylene dicarboxylate (0.186ml, 1.16mmol). The vessel was then cooled and the toluene removed in vacuo to afford an orange solid of 26c. The orange solid was then dissolved in pentane and recrystalized at 0 C to afford 26c as an orange solid in 62% yield. 1 H NMR (C 6 D 6 ): s, 9H, SiMe 3 ), 0.33 (s, 9H, SiMe 3 ), 0.87 (t, 3H, (CH 3 ), J = 7.1 Hz),1.17 (m 1H metallacycle proton) 1.27 (t, 3H, (CH 3 ), J = 7.1 Hz), 3.09 (ddd 1H

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126 J = 15.8 6.1 3.8 Hz metallacycle proton) 6.81 (m 1H phenyl imido para proton) 7.00 (overlapping multiplets 4H, o-pda protons and phenyl imido meta protons) 7.20 (d 1H J = 7.9 Hz o-pda proton) 7.58 (d 1H J =7.4Hz phenyl imido ortho proton) 7.67 (d 1H J =8.2 Hz o-pda proton) 13 C NMR(C 7 D 8 ): 184.1, 176.1, 164.2, 157.9, 150.1, 134.3, 130.6, 128.2, 128.9, 127.8, 127.7, 126.4, 125.6, 122.0, 60.3, 60.1, 53.7, 41.2, 14.8, 14.5, 1.43 [(NPh)Mo(C(Ph)CHC(Ph))(o-(Me 3 SiN) 2 C 6 H 4 )], (29) To a pentane solution of 1.00g (2.03mmol) of 20a, phenyl acetylene 0.90ml(4.06mmol) was added drop-wise with stirring. Immediate color change from green to yellowish brown occurred. The reaction mixture was allowed to stir for hour after which the solution was concentrated by removal of the solvent in vacuo. Cooling to 0 C resulted in the appearance of brown crystals of 29 in 40% yield. 1 H NMR (C 6 D 6 ): -0.07 (s, 18H, SiMe 3 ), 6.48 (s, 2H, metallacycle) 6.85-7.78 (aromatic). 13 C NMR (C 6 D 6 ): 1.3, 124.7, 125.9, 126.6, 127.4, 129.5, 130.7, 142.2, 154.2, and 192.7. Anal. Calcd for C 34 H 39 MoN 3 Si 2 : C, 63.63; H, 6.12; N, 6.55; Found C, 62.74; H, 6.24; N, 6.61. Synthesis of bis-Isocyanide Complexes [(cis tBuNC) 2 Mo(NPh)(o-( Me 3 SiN) 2 C 6 H 4 )], (30a) A pentane solution of 20a, (1.00g, 2.03 mmol) was charged with tertButyl isocyanide (0.24ml, 4.06mmol). Green crystals of 30a quickly precipitated. The suspension was filtered and washed several times with pentane. Excess solvent was removed in vacuo to afford 30a as a green microcrystalline material in 52% yield. Complex 30a is unstable in solution and disproportionates to 30a and an unidentified material at 20 0 overnight. Complex 30a can be stored as a solid at 0 C. 1 H NMR (C 6 D 6 ): 0.73(s, 18H, SiMe 3 ), 1.06(s, 18H,

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127 t BuNC), 6.81(tt, 2H, phenyl imido meta proton 7.4, 1.2 Hz), 6.93(t, 1H, phenyl imido para proton, 7.4 Hz), 7.05 (dd, 2H, o-phenylene diamide proton, 5.8, 3.3 Hz), 7.32(dd, 2H, phenyl imido ortho proton, 7.5 1.3 Hz), 7.48( dd, 2H, o-phenylene diamide proton, 5.6, 3.4 Hz). 13 C NMR(C 7 D 8 ): 234.9, 187.1, 151.7, 128.6, 124.3, 123.7, 117.2, 117.1, 58.7, 30.3, 5.5. [(cis 2-6 dimethyl phenyl-NC) 2 Mo(NPh)(o-( Me 3 SiN) 2 C 6 H 4 )], (30b) A pentane solution of 30b, (1.00g, 2.03 mmol) was charged with 2-6-dimethyl phenyl isocyanide (0.37g, 2.82mmol). Dark red crystals of 30b quickly precipitated. The suspension was filtered and washed several times with pentane. Excess solvent was removed in vacuo to afford 30b as a dark red microcrystalline material in 72% yield. 1 H NMR (C 6 D 6 ): 7.52 (dd, 2H, o-phenylene diamide proton 5.7 3.5 Hz), 7.46 (d, 2H, phenyl imido ortho proton 7.4 Hz ), 7.07 (dd, 2H, o-phenylene diamide proton 5.8, 3.4 Hz) 6.99 (t, 2H, phenyl imido meta proton 7.2 Hz), 6.89 (t, 1H phenyl imido para proton,7.3 Hz), 6.64 (ov multiplet, 6H, isocyanide phenyl protons), 2.12 (s, 12H, (CH 3 )), 0.74 (s, 18H, SiMe 3 ), 13 C NMR(C 7 D 8 ): 200.9, 151.4, 131.5, 128.8, 125.2, 124.3, 117.8, 117.5, 18.8, 5.5. Anal. Complex 30b loses a molecule of 2-6-dimethyl phenyl isocyanide during the elemental analysis. Calcd C 27 H 36 MoN 4 Si 2 C, 57.02; H, 6.38; N, 9.85. Found C,57.20; H, 6.00; N, 9.80. Synthesis of a Chelating Imino Carbamoyl Complex, (33) A pentane solution of 25 (1.00g, 2.03mmol) was treated with 2,6-dimethyl phenyl isocyanide (1.16g, 8.92mmol) and the solution was allowed to stir overnight and a yellowish brown powder precipitated. Washing this powder several times with cold pentane followed by recrystalization of the solids from pentane, afforded 33 as red

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128 crystals in 46% yield. 1 H NMR (C 6 D 6 ): 8.04 (dd, 2H, phenyl imido ortho proton, 8.2, 1.6 Hz.), 7.23 (t, 2H, 2,6 dimethyl phenyl isocyanide para proton, 8.4, 1.3 Hz), 7.2-6.5 (overlapping aromatic peaks 16H), 5.43(dd, 1H, phenyl imido para proton, 8.2, 1.6Hz) 2.50(s, 6H, (CH 3 ) (imino carbamoyl), 1.94(s, 12H, (CH 3 ), 2,6-dimethyl phenyl isocyanide), 1.87(s, 6H, (CH 3 ), imino carbamoyl), 0.66(bs, 9H, SiMe 3 ), 0.45(s, 9H, SiMe 3 ). 13 C NMR (C 7 D 8 ): 203.3, 176.6, 157.7, 146.6, 146.2, 145.3, 137.9, 137.4, 136.4, 133.4, 129.1, 128.8, 127.3, 126.6, 125.3, 125.1, 125.0, 124.7, 124.0, 121.0, 120.8, 119.3, 119.1, 115.3, 20.0, 18.8, 18.2, 5.5, 0.78. Anal. Calcd C 54 H 64 MoN 7 Si 2 : C, 67.33; H, 6.70; N, 10.18. Found C, 67.72; H, 6.95, N, 10.07. Reactions with alkyl aluminum reagents Synthesis of (CH 2 ) 4 Mo(NPh)4-(o-(SiMe 3 N) 2 C 6 H 4 )Al(CH 3 ) 2 (34) Trimethyl aluminum (2.44mmol, 0.233ml) was added to an orange solution of 25(1.00g 2.03mmol) in toluene. The solution turned green after 15 minutes and the solvent was removed in vacuo. The green solid was dissolved in pentane and the solution was concentrated. Cooling to 0 C resulted in the isolation of 34, as green crystals. 1 H NMR (C 6 D 6 ): .26 (s, 3H Al(CH 3 ) 2 ), 0.00 (s, 3H, Al(CH 3 ) 2 ), 0.19 (s, 9H, SiMe 3 ), 0.22 (s, 9H, SiMe 3 ), 1.36(s, 3H, Mo(CH 3 )), 1.96(ddd, J=10.7Hz, 9.7Hz, 4.1Hz, 1H, metallacycle), 2.16(m, 1H, metallacycle), 2.36(m, 2H, metallacycle), 2.61(m, 2H, metallacycle), 2.79(m, 1H, metallacycle), 3.25(m, 1H, metallacycle), 4.79(dd J = 7.7 Hz, 1.6 Hz, 1H, o-pda) 4.86(dd J = 7.5 Hz, 1.6 Hz, 1H, o-pda), 5.28(td J=7.2Hz, 1.5Hz, 1H, o-pda), 5.40(td J=7.3Hz, 1.5Hz, 1H, o-pda), 6.81 (t, J = 7.1 Hz, 1H, imido para protons), 6.88 (t J = 7.3 Hz, 1H, imido, meta, protons ), 7.00 (d, J = 8.9 Hz, 1H, imido, ortho, protons). 13 C NMR(C 6 D 6 ) -0.80, -0.27, 0.16, 0.18, 35.9, 38.6,51.0, 59.6,

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129 91.0, 93.1, 102.9, 105.5, 124.3, 128.8, 126.6, 155.4, 160.0. Anal. Calcd for C 36 H 51 AlMoN 3 Si 2 : C, 61.34; H, 7.29; N, 5.96. Found: C, 61.32; H, 7.03; N, 5.85. Synthesis of (2-diphenyl acetylene)(CH 3 )Mo(NPh)4[(o(SiMe 3 N) 2 C 6 H 4 )}Al(CH 3 ) 2, (36) Trimethyl aluminum (2.03mmol, 0.195ml) was added to an orange solution of 35(1.04g, 1.69mmol) in pentane. The solution was stirred overnight; during which time a yellow precipitate formed. The mixture was then filtered and the yellow powder was washed several times with pentane. This yellow powder was dried under vacuum and compound 36 was obtained in 40% yield. 1 H NMR (C 6 D 6 ): -0.27 (s, 3H, Al(CH 3 ) 2 ), 0.02 (s, 3H, Al(CH 3 ) 2 ), 0.03 (s, 9H, SiMe 3 ), 0.35 (s, 9H, SiMe 3 ), 1.71 (s, 3H, Mo(CH 3 )), 4.03 (dd J = 7.0 Hz, 2.5 Hz, 1H, o-pda), 5.51 (dd J = 6.8 Hz, 2.4 Hz, 1H, o-pda), 6.07 (td J = 7.0 Hz, 2.1 Hz, 1H, o-pda), 6.10 (td J = 6.7 Hz, 2.3 Hz, 1H, o-pda), 6.67 (tt J = 7.2 Hz, 1.2 Hz, 1H, imido para proton), 6.79 (t J = 7.9 Hz, 2H, imido meta protons), 6.84 (dd J = 8.5 Hz, 1.7 Hz, 2H, imido ortho protons), 7.05 ( tt J = 7.6 Hz, 1.2 Hz, 1H phenyl para proton), 7.14 (tt J = 7.5 Hz, 1.2 Hz, 1H, phenyl para proton), 7.20 (t J = 7.7 Hz, 2H, phenyl meta proton), 7.27 (t J = 7.7 Hz, 2H, phenyl meta proton), 7.44 (dd J = 8.1 Hz, 1.2 Hz, 2H, phenyl ortho proton), 7.88 (dd J = 8.2 Hz, 1.2 Hz, 2H, phenyl ortho proton). 13 C NMR: 159.6 159.6 156.4 152.1 150.9 137.7 137.2 129.9 129.1 128.6 128.2 127.0 125.5 123.9 ,104.8 97.0 94.0 90.4 23.7 -0.1 -0.2 -0.4 -5.3. Anal. Calcd. C 26 H 46 AlMoN 3 Si 2 : C, 53.86; H, 8.00; N, 7.45. Found C, 53.98; H, 7.72; N, 7.45.

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130 Synthesis of [(2-ethylene)(CH 3 )Mo(NPh)4-(o-(SiMe 3 N) 2 C 6 H 4 ))Al(CH 3 ) 2 } (42a) Methyl aluminum dichloride (2.43ml of a 1.0M solution in hexanes)was added to a toluene solution of 25, (1.00g, 2.03mmol). The solution was allowed to stir for one hour and toluene was removed in vacuo yielding a red solid. The red solid was washed several times with pentane to yield 42b in 62.4% yield. 1 H NMR (C 6 D 6 ): 6.77 (overlapping multiplet, 5H, phenyl imido) 5.62(td, 1H, J = 1.9, 7.2Hz o-pda) 5.42(td, 1H, J =1.9, 7.2Hz o-pda), 5.10(dd, 1H, J =1.9, 7.2 Hz), 4.35 (dd, 1H, J=1.8, 7.3 Hz), 2.78(ddd, 1H, J = 1.8, 10.5, 13.8 Hz 2 -ethylene), 2.38(ddd, 1H, J = 2.0, 7.7, 11.7Hz 2 -ethylene), 1.74(overlapping multiplet, 2H, 2 -ethylene), 1.45(s, 3H, CH 3 ), 0.35(s, 3H, SiMe 3 ), 0.24(s, 3H, SiMe 3 ). 13 C NMR (C 7 D 8 ), 156.2, 154.8, 129.2, 127.3, 125.2, 99.0, 98.7, 93.1, 91.4, 48.1, 43.8, 17.6, 0.31, 0.23. Anal. Calcd. C 23 H 39 AlCl 2 MoN 3 OSi 2 : C, 44.30; H, 6.30; N, 6.74. Found C, 44.57; H, 5.74; N, 7.05 Synthesis of [(2-ethylene)(Et)Mo(NPh)4-(o-(SiMe 3 N) 2 C 6 H 4 )Al(CH 3 ) 2 ] (42b) Ethyl aluminum dichloride (0.352 ml, 2.92mmol) was added to a toluene solution of 25, (1.21g, 2.43mmol). The solution was allowed to stir for one hour and toluene was removed in vacuo yield a brown solid. The brown solid was washed several times with pentane to yield 42b in 63.7% yield. 1 H NMR (C 6 D 6 ): 6.20 (overlapping mulitiplet, 5H phenyl imido) 5.59 (td, 1H, J =7.2 Hz, 1.9 Hz o-pda) 5.39 (td, 1H, J = 7.3 Hz, 2.0 Hz, o-pda) 5.23 (dd, 1H, J = 7.3 Hz, 1.9 Hz, o-pda) 4.70 ( dd, 1H, J = 7.5 Hz, 1.9 Hz, o-pda) 3.05 ( ddd, 1H, J = 14.3 Hz, 11.1 Hz, 4.2 Hz, 2 -ethylene) 2.41 ( ddd, 1H, J = 13.2 Hz, 10.9 Hz, 5.5 Hz, 2 -ethylene) 2.22 ( q, 2H, J = 7.4 Hz, Et) 1.94 (ddd, 1H, J = 14.0 Hz, 11.5 Hz, 5.5 Hz, 2 -ethylene) 1.85 (t, 3H, J = 7.4 Hz, Et(CH 3 )) 1.21 (ddd, 1H, J = 13.4 Hz, 11.5 Hz, 4.3 Hz, 2 -ethylene) 0.32 (s, 9H, SiMe 3 ) 0.26 (s,

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131 9H, SiMe 3 ) 13 C NMR (C 7 D 8 ), 154.9, 153.0,150.6, 128.7, 105.7, 96.4, 95.7, 95.2, 49.7, 49.0, 27.4, 22.5, 0.0, -0.41. Synthesis of [(2-styrene)(Me)Mo(NPh)4-(o-(SiMe 3 N) 2 C 6 H 4 ))Al(CH 3 ) 2 ] (39) Trimethyl aluminum (0.20 ml, 2.09mmol) was added to a toluene solution of 20c, (0.93g, 1.72mmol). The green solution quickly turned red and was allowed to stir for one hour, and then toluene was removed in vacuo to yield a red solid. Analysis of the 1 H NMR spectrum revealed that 39 was formed as a 50:50 mixture of the two isomers 39a and 39b in 64.0 % yield. Recrystalization from an 80:20 mixture of pentane/ether solution afforded 39a in 24.8 %yield. Compound 39b was obtained in low purity. The stereochemistry of 39 was determined by gHMQC, gHMBC, and NOESY spectroscopy. 1 H NMR (C 6 D 6 ): (39a) 7.00-6.84(10H aromatic protons overlapping), 5.60 (3H o-pda protons overlapping with 39b), 4.75 (dd, 1H J = 6.7, 2.6 Hz, o-pda proton), 4.35 (dd 1H J = 6.5 2.5 Hz o-pda proton), 3.38 (dd 1H J = 13.8 5.7 Hz 2 styrene proton), 2.90 (dd 1H J =13.6 11.2 Hz 2 styrene proton), 1.69 (d 1H J = 10.9 5.5 Hz 2 styrene proton), 1.33 (s 3H (CH 3 )), 0.25 (s 9H SiMe 3 ) 0.19 (s 9H SiMe 3 ) .03 (s 3H (CH 3 )), -0.33 (s 3H (CH 3 ) ), 13 C NMR (C 7 D 8 ) chemical shifts were determined by gHMQC and gHMBC (see Figures 5-7 for details). (39b) 7.00 (10H aromatic protons overlapping), 5.89 (tt 3H J= 7.1 1.9 Hz o-pda proton) 5.53 (3H o-pda protons overlapping with 39a), 5.00 (dd, 1H J = 7.1, 1.6 Hz, o-pda proton), 3.80 (dd 1H J = 7.2 2.2 Hz o-pda proton), 3.65 (dd 1H J = 12 9 11.0 Hz 2 styrene proton), 3.06 (dd 1H J =13.1 6.9 Hz 2 styrene proton), 2.21 (d 1H J = 10.9 5.5 Hz 2 styrene proton), 1.41 (s 3H (CH 3 )), 0.30 (s 9H SiMe 3 ) 0.12 (s 9H SiMe 3 ) 0.01 (s 3H (CH 3 ) ), -0.31 (s 3H (CH 3 )) 13 C NMR (C 7 D 8 ) chemical shifts were determined by gHMQC and gHMBC (see Figures 5-9 for details).

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132 Synthesis of [(2-styrene)(Me)Mo(NPh)4-(o-(SiMe 3 N) 2 C 6 H 4 )}Al(Cl) 2 ] (40) Methyl aluminum dichloride (2.66ml of a 1.0M solution in hexanes) was added to a green toluene solution of 20c, (1.20g, 2.22mmol). The solution was allowed to stir for one hour and toluene was removed in vacuo yield a reddish brown solid. The red solid was washed with pentane and excess solvent was removed in vacuo to yield 40 as a mixture of two isomers. The stereochemistry of 40 was determined by gHMQC, gHMBC, and NOESY spectroscopy. MoMe3SiSiMe3AlNNH3CNClClHbHaHcHfHgHhHiHeHd1.93, (44.1)3.60, (44.1)2.88, (60.4)6.94, (125.2)6.141.42, (21.2)4.03, (94.1)5.81, (100.2)5.47, (96.1)5.13, (88.7)0.18, -0.740.31, -0.74gHMBCMethyl proton (1.42ppm) shows 3 bond correlations to the methylene carbon at 44.1ppmMetallcycle proton 2.88 shows 2-bond correlations to the methylene carbon at 44.1 and 3-bond correlations to the phenylene carbon at 147.0ppm(147.0)13C NMR shifts are in parenthesesDashed lines represent NOE's40a

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133 MoMe3SiSiMe3AlNNH3CNClClHcHaHbHiHeHfHgHhHd1.32 (22.1)6.90 (125.5)6.48 (125.3)2.91 (63.8)3.33 (41.6)1.71 (41.6)4.91 (92.1)5.49 ( 99.8)5.49 (99.8)4.55 (92.1)0.23 (-0.68)0.31 (-0.68)(154.1)(154.5)gHMBCMethyl proton (1.32ppm) shows 3 bond correlations to the methylene carbon at 41.6ppmMetallcycle proton 2.91 shows 2-bond correlations to the methylene carbon at 41.6 and to the phenylene carbon at 145.5ppm(145.5)40b13C NMR shifts are in parenthesesDashed lines represent NOE's Synthesis of [(2 NPh)Mo(Cl)(4-(Cl 2 Al((o-(Me 3 SiN) 2 -C 6 H 4 ))] 2 (41) Ethyl aluminum dichloride (0.352 ml, 2.92mmol) was added to a green pentane solution of 20a, (1.65g, 2.43mmol). The solution was allowed to stir overnight, during which time a green powder precipitated. The suspension was filtered and washed several times with pentane. Excess pentane was then removed in vacuo. The green powder was redissolved in CH 2 Cl 2 the solution concentrated and pentane added to precipitate 41, in 30% yield. The 1 H NMR spectrum of 41 revealed that this compound exists as a mixture of 3 isomers (see text for details). 1 H NMR (CDCl 3 ) 7.49, 7.39, 7.17, 5.58, 4.14, 0.97, 0.27, 0.26, 0.25. 13 C NMR (CDCl 3 ) 129.7, 119.2, 95.6, 91.0, 9.6, 6.8, 4.8, 3.4, 0.4,

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134 0.07, -4.20, -4.50. Anal. Calcd. C 41 H 88 Al 2 Cl 6 Mo 2 N 6 Si 4 C 40.50, H 5.64, N 6.91 Found C 41.08, H 5.91, N 7.02 Kinetic Study NMR tube samples were prepared by dissolving 25 (ca. 10mg, 0.020mmol) and mesitylene (13.88l), added as an internal standard, in 1.0ml of C 7 D 8 in the glove box and Diethyl acetylene dicarboxylate was then added to the NMR tube, the tube was then capped. The concentration of DEAD was obtained by plotting the values of the integral for the (DEAD) ethyl group for known concentrations DEAD versus the integrals for the (CH 3 ) protons of mesitylene. The NMR probe was equilibrated to the desired temperature and the sample was then loaded into the probe. Approximately 120 spectra were recorded over the duration of three half-lives. The value of the integral for the SiMe 3 protons of the starting material was recorded for each spectrum. The observed rate constants were obtained by plotting the Ln(integral) vs time. Three samples were performed at each temperature and the rate constants reported were the average of three runs. Activation parameters were obtained by measuring the rate constants at four different temperatures. A plot of Ln(k/T) vs (1/T (K)), results in a line of with a slope of -H /R and an intercept of [(S /R) + 23.76]. Reported errors in the rate constants represent the standard deviation from the least squares fit of the experimental data. Reported errors in the activation parameters are obtained from all experimental data (12 points in the Eyring plot), using 2.

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135 Table 6-1. Kinetic analysis for the reaction of DEAD with 25 T(K) k(run#1)* 10 4 k(run#1) 10 4 k(run#1) 10 4 355.8 9.81 10.6 11.9 349.5 5.78 7.02 7.72 342.5 3.79 3.49 2.73 334.8 1.67 1.67 1.74 Eyring Plot Analysis: S = -14.7 +/-3.0 cal/K.mol; H = 20.5 +/1.0 Kcal/mol Computational Studies. All Density functional theory (DFT) calculations were performed using the Gaussian 98W program package. 143 In most cases molecules were optimized using Becke's hybrid three-parameter functional (B3LYP). The Los Alamos effective core potential, (ECP), plus a standard double-zeta basis set (LANL2DZ) 67 was used to describe the molybdenum center. The Dunning/Huzinaga 66 full double zeta basis set (D95) was used to describe all other atoms. Molecules were visualized with the MOLDEN program. 131 Alkyne complexes 22 and 23. Model compound 22 and TS 22 were optimized at the B3LYP/LANL2DZ level. Using the same level of theory and basis set, vibrational frequency calculations and NBO population analyses were performed on the stationary points to confirm their existence as minima (number of imaginary frequencies =0 or maxima (transition structures) # imaginary frequencies =1, and to analyze (MOs). An IRC calculation was performed to confirm that the transition state is linked to the minima on the potential energy surface. Single-point energies were calculated using the B3LYP level of theory and the LANL2DZ+P basis set, where +P stands for the addition of polarization functions to the D95 basis set as implemented in the Gaussian 98W program. Energy differences (E) are electronic energies i.e. they are taken straight off of the potential energy surface and do not include zero point corrections.

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136 Complex 23 and TS 23 were optimized with the two-layer ONIOM method developed by Morokuma et al. 8-12 Using this method the inner layer consisted of 22 and was modeled with the LANL2DZ basis set. The outer layer consisted of 23 and was comprised of the entire (complete) system and was modeled with the LANL2MB method. The LANL2MB 144,145 basis set consists of the The Los Alamos effective core potential (ECP) plus a standard double-zeta basis set (LANL2DZ) on the Molybdenum atom and the STO-3G basis set on all other atoms. Single-point energies were calculated using the B3LYP level of theory and the LANL2DZ basis set. Using the same level of theory and basis set, vibrational frequency calculations, NBO population analyses were performed on the stationary points to confirm there existence as minima (number of imaginary frequencies =0 or maxima (transition structures) # imaginary frequencies =1, and to analyze (MOs). IRC calculations are not possible within the current implementation of the ONIOM program, however, confirmation that the transition state is linked to the minima on the potential energy surface, was obtained by visualization of the imaginary frequency using the MOLDEN program. Single-point energies were calculated using the B3LYP level of theory and the LANL2DZ basis set. Energy differences (E) are electronic energies i.e. they are taken straight off of the potential energy surface and do not include zero point corrections. Complex 20c and 24 These complexes were optimized in an analogous fashion to 23. Metallacyclopentane complex 25. Model compounds 25, 25b, 25c and TS 25-25b and TS 25b-25c were optimized at the mPW1K/LANL2DZ level. In the model system, the o-phenylene group (o-C 6 H 4 ) that links the two N atoms of the diamido ligand was

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137 simplified to a CH=CHcarbon chain. The organic groups on the nitrogen atoms, (SiMe 3 of the amido and phenyl group of the imido), were replaced by hydrogen atoms for simplicity. The mPW1K method that has been recently developed by Truhlar and co-workers has been recently shown to give more reliable barrier heights than other functionals. 146 Using the same level of theory and basis set, vibrational frequency calculations, were performed on the stationary points to confirm their existence as minima (number of imaginary frequencies =0 or maxima (transition structures) # imaginary frequencies =1, and to analyze (MOs) and to calculate zero point energies. An IRC calculation was performed to confirm that the transition state is linked to the minima on the potential energy surface. Single-point energies were calculated using the B3LYP level of theory and Dunnings Aug-cc-pVDZ basis set. 66,147,148 Relative free energy differences reported (G 0 298 ) include the effects of zero point energy corrections... For ONIOM calculations the inner layer was optimized with the mPW1K/LANL2DZ level. The outer layer was optimized with the mPW1K/LANL2MB level of theory. Molecules 37 and 38 The molecules were optimized with the B3LYP/LANL2DZ method. The SiMe 3 substituents on the diamide ligand were replaced by (CH 3 ) and the substituents on the phenyl imido ligand with H for simplicity. X-ray Experimental X-ray Experimental for Complex 26b. Single crystals of 26b were obtaimed from slow evaporation of a diethyl ether solution. Data were collected at 173 K on a Siemens SMART PLATFORM equipped with A CCD area detector and a graphite monochromator utilizing MoK radiation ( =

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138 0.71073 ). Cell parameters were refined using 6673 reflections. A hemisphere of data (1381 frames) was collected using the -scan method (0.3 frame width). The first 50 frames were remeasured at the end of data collection to monitor instrument and crystal stability (maximum correction on I was < 1 %). Absorption corrections by integration were applied based on measured indexed crystal faces. The structure was solved by the Direct Methods in SHELXTL5 149 and refined using full-matrix least squares. The non-H atoms were treated anisotropically, whereas the hydrogen atoms were calculated in ideal positions and were riding on their respective carbon atoms. A total of 313 parameters were refined in the final cycle of refinement using 5684 reflections with I > 2(I) to yield R 1 and wR 2 of 3.03% and 7.23%, respectively. Refinement was done using F 2

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139 Table 6-2. Crystal data and structure refinement for 26b. Identification code ei01 Empirical formula C28 H37 Mo N3 Si2 Formula weight 567.73 Temperature 173(2) K Wavelength 0.71073 Crystal system Monoclinic Space group P2(1)/n Unit cell dimensions a = 11.2646(5) = 90. b = 9.7968(4) = 92.659(1). c = 25.675(1) = 90. Volume 2830.4(2) 3 Z 4 Density (calculated) 1.332 Mg/m3 Absorption coefficient 0.569 mm-1 F(000) 1184 Crystal size 0.20 x 0.17 x 0.17 mm3 Theta range for data collection 1.59 to 27.50. Index ranges -14 h 14, -11 k 12, -33 l 28 Reflections collected 18462 Independent reflections 6460 [R(int) = 0.0547] Completeness to theta = 27.50 99.4 % Absorption correction Integration Max. and min. transmission 0.9191 and 0.8987 Refinement method Full-matrix least-squares on F2 Data / restraints / parameters 6460 / 0 / 313 Goodness-of-fit on F2 1.058 Final R indices [I>2sigma(I)] R1 = 0.0303, wR2 = 0.0723 [5684] R indices (all data) R1 = 0.0361, wR2 = 0.0766 Largest diff. peak and hole 0.382 and -0.525 e.-3 R1 = (||Fo| |Fc||) / |Fo| wR2 = [w(Fo2 Fc2)2] / wFo22]]1/2 S = [w(Fo2 Fc2)2] / (n-p)]1/2w= 1/[2(Fo2)+(0.0370*p) 2 +0.31*p], p = [max(Fo2,0)+ 2* Fc2]/3

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140 X-ray Experimental for Complex 29 Single crystals were obtained by slow evaporation of a diethl ether solution of 29. Data were collected at 173 K on a Siemens SMART PLATFORM equipped with A CCD area detector and a graphite monochromator utilizing MoK radiation ( = 0.71073 ). Cell parameters were refined using 7620 reflections. A hemisphere of data (1381 frames) was collected using the -scan method (0.3 frame width). The first 50 frames were remeasured at the end of data collection to monitor instrument and crystal stability (maximum correction on I was < 1 %). Absorption corrections by integration were applied based on measured indexed crystal faces. The structure was solved by the Direct Methods in SHELXTL5, and refined using full-matrix least squares. The non-H atoms were treated anisotropically, whereas the hydrogen atoms were calculated in ideal positions and were riding on their respective carbon atoms. The asymmetric unit consists of two chemically equivalent but crystallographically independent molecules. A total of 733 parameters were refined in the final cycle of refinement using 10788 reflections with I > 2(I) to yield R 1 and wR 2 of 3.55% and 6.87%, respectively. Refinement was done using F 2

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141 Table 6-3. Crystal data and structure refinement for 29. Identification code ei02 Empirical formula C34 H39 Mo N3 Si2 Formula weight 641.80 Temperature 193(2) K Wavelength 0.71073 Crystal system Orthorhombic Space group Pbcn Unit cell dimensions a = 25.544(1) = 90. b = 18.7420(8) = 90. c = 27.641(1) = 90. Volume 13233.4(9) 3 Z 16 Density (calculated) 1.289 Mg/m3 Absorption coefficient 0.495 mm-1 F(000) 5344 Crystal size 07 x .05 x .04 mm3 Theta range for data collection 1.54 to 27.50. Index ranges -32 h 33, -24 k 24, -35 l 35 Reflections collected 112367 Independent reflections 15199 [R(int) = 0.0776] Completeness to theta = 27.50 100.0 % Absorption correction Empirical Min. & Max. Transmission 0.966, 0.980 Refinement method Full-matrix least-squares on F2 Data / restraints / parameters 15199 / 0 / 733 Goodness-of-fit on F2 1.076 Final R indices [I>2sigma(I)] R1 = 0.0355, wR2 = 0.0687 [10788] R indices (all data) R1 = 0.0681, wR2 = 0.0836 Largest diff. peak and hole 0.372 and -0.607 e.-3 R1 = (||Fo| |Fc||) / |Fo| wR2 = [w(Fo2 Fc2)2] / wFo22]]1/2 S = [w(Fo2 Fc2)2] / (n-p)]1/2 w= 1/[2(Fo2)+(0.020*p) 2 +11.94*p], p = [max(Fo2,0)+ 2* Fc2]/3

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142 X-ray Experimental for Complex 25 Single crystals of 25 were obtained by cooling a concentrated pentane solution of this complex at 0 C. Data were collected at 173 K on a Siemens SMART PLATFORM equipped with A CCD area detector and a graphite monochromator utilizing MoK radiation ( = 0.71073 ). Cell parameters were refined using up to 8192 reflections. A full sphere of data (1850 frames) was collected using the -scan method (0.3 frame width). The first 50 frames were remeasured at the end of data collection to monitor instrument and crystal stability (maximum correction on I was < 1 %). Absorption corrections by integration were applied based on measured indexed crystal faces. The structure was solved by the Direct Methods in SHELXTL6, and refined using full-matrix least squares. The non-H atoms were treated anisotropically, whereas the hydrogen atoms were calculated in ideal positions and were riding on their respective carbon atoms. The C14-C15 moiety is disordered and was refined in two parts with their site occupation factors dependently refined. A total of 278 parameters were refined in the final cycle of refinement using 4876 reflections with I > 2(I) to yield R 1 and wR 2 of 3.07 % and 7.33%, respectively. Refinement was done using F 2

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143 Table 6-4. Crystal data and structure refinement for 29. Identification code ei11 Empirical formula C22 H35 Mo N3 Si2 Formula weight 493.65 Temperature 173(2) K Wavelength 0.71073 Crystal system Monoclinic Space group P2(1)/n Unit cell dimensions a = 10.2921(6) = 90. b = 18.1227(11) = 103.385(2). c = 13.8423(8) = 90. Volume 2511.7(3) 3 Z 4 Density (calculated) 1.305 Mg/m3 Absorption coefficient 0.630 mm-1 F(000) 1032 Crystal size 0.34 x 0.19 x 0.11 mm3 Theta range for data collection 1.88 to 27.50. Index ranges -13 h 13, -23 k 18, -14 l 17 Reflections collected 16103 Independent reflections 5669 [R(int) = 0.0460] Completeness to theta = 27.50 98.3 % Absorption correction Integration Max. and min. transmission 0.9397 and 0.8176 Refinement method Full-matrix least-squares on F2 Data / restraints / parameters 5669 / 0 / 278 Goodness-of-fit on F2 1.054 Final R indices [I>2sigma(I)] R1 = 0.0307, wR2 = 0.0733 [4876] R indices (all data) R1 = 0.0384, wR2 = 0.0767 Largest diff. peak and hole 0.376 and -0.476 e.-3 R1 = (||Fo| |Fc||) / |Fo| wR2 = [w(Fo2 Fc2)2] / wFo22]]1/2 S = [w(Fo2 Fc2)2] / (n-p)]1/2 w= 1/[2(Fo2)+(m*p) 2 +n*p], p = [max(Fo2,0)+ 2* Fc2]/3, m & n are constants.

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144 X-ray Experimental for Complex 30b Single crystals of 30b were obtained by slow diffusion of pentane into a concentrated toluene solution of 30b.Data were collected at 173 K on a Siemens SMART PLATFORM equipped with A CCD area detector and a graphite monochromator utilizing MoK radiation ( = 0.71073 ). Cell parameters were refined using up to 8192 reflections. A full sphere of data (1850 frames) was collected using the -scan method (0.3 frame width). The first 50 frames were re-measured at the end of data collection to monitor instrument and crystal stability (maximum correction on I was < 1 %). Absorption corrections by integration were applied based on measured indexed crystal faces. The structure was solved by the Direct Methods in SHELXTL6, and refined using full-matrix least squares. The non-H atoms were treated anisotropically, whereas the hydrogen atoms were calculated in ideal positions and were riding on their respective carbon atoms. In addition to the complex, the asymmetric unit has a half toluene molecule located on an inversion center. Its methyl group is thus disordered over two equivalent positions (The center of the phenyl group lies on the inversion center. A total of 444 parameters were refined in the final cycle of refinement using 6271 reflections with I > 2(I) to yield R 1 and wR 2 of 3.49% and 7.58%, respectively. Refinement was done using F 2

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145 Table 6-5. Crystal data and structure refinement for 30b. Identification code ei19 Empirical formula C39.50 H49 Mo N5 Si2 Formula weight 745.96 Temperature 173(2) K Wavelength 0.71073 Crystal system Triclinic Space group P-1 Unit cell dimensions a = 11.5484(9) = 102.802(2). b = 11.8609(9) = 93.928(2). c = 14.8731(12) = 94.868(2). Volume 1971.4(3) Z 2 Density (calculated) 1.257 Mg/m3 Absorption coefficient 0.426 mm-1 F(000) 782 Crystal size 0.27 x 0.26 x 0.11 mm3 Theta range for data collection 1.41 to 27.50. Index ranges -14 h 11, -15 k 13, -19 l 19 Reflections collected 12375 Independent reflections 8614 [R(int) = 0.0441] Completeness to theta = 27.50 95.1 % Absorption correction Integration Max. and min. transmission 0.9542 and 0.9004 Refinement method Full-matrix least-squares on F2 Data / restraints / parameters 8614 / 0 / 444 Goodness-of-fit on F2 0.915 Final R indices [I>2sigma(I)] R1 = 0.0349, wR2 = 0.0758 [6271] R indices (all data) R1 = 0.0520, wR2 = 0.0788 Largest diff. peak and hole 0.615 and -0.644 e.-3 R1 = (||Fo| |Fc||) / |Fo| wR2 = [w(Fo2 Fc2)2] / wFo22]]1/2 S = [w(Fo2 Fc2)2] / (n-p)]1/2 w= 1/[2(F2)+(m*p)2+n*p], p = [max(Fo2,0)+ 2* Fc2]/3, m & n are constants.

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146 X-ray Experimental for Complex 33 Single crystals of 33 were obtained by cooling a petane solution of this complex to 0 C. Data were collected at 173 K on a Siemens SMART PLATFORM equipped with A CCD area detector and a graphite monochromator utilizing MoK radiation ( = 0.71073 ). Cell parameters were refined using up to 8192 reflections. A full sphere of data (1850 frames) was collected using the -scan method (0.3 frame width). The first 50 frames were re-measured at the end of data collection to monitor instrument and crystal stability (maximum correction on I was < 1 %). Absorption corrections by integration were applied based on measured indexed crystal faces. The structure was solved by the Direct Methods in SHELXTL6, and refined using full-matrix least squares. The non-H atoms were treated anisotropically, whereas the hydrogen atoms were calculated in ideal positions and were riding on their respective carbon atoms. The asymmetric unit consists of the Mo complex and two ether molecules (all ether molecules are located on inversion centers). The latter were disordered and could not be modeled properly, thus program SQUEEZE, a part of the PLATON package of crystallographic software, was used to calculate the solvent disorder area and remove its contribution to the overall intensity data. One of the Si atoms is disordered in two positions, Si2 and Si3. Accordingly, their methyl groups and the dimethylphenyl moiety are also disordered and were refined in two parts, with their site occupation factors dependently refined. A total of 621 parameters (with 36 restraints) were refined in the final cycle of refinement using 6995 reflections with I > 2(I) to yield R 1 and wR 2 of 4.84% and 9.81%, respectively. Refinement was done using F 2

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147 Table 6-6. Crystal data and structure refinement for 33. Identification code ei17 Empirical formula C58 H73 Mo N7 O Si2 Formula weight 1036.35 Temperature 173(2) K Wavelength 0.71073 Crystal system Triclinic Space group P-1 Unit cell dimensions a = 11.4653(12) = 90.006(2). b = 13.7990(14) = 100.229(2). c = 20.264(2) = 112.571(2). Volume 2905.1(5) 3 Z 2 Density (calculated) 1.185 Mg/m3 Absorption coefficient 0.310 mm-1 F(000) 1096 Crystal size 0.33 x 0.18 x 0.14 mm3 Theta range for data collection 1.02 to 27.50. Index ranges -14 h 14, -17 k 14, -24 l 26 Reflections collected 18326 Independent reflections 12774 [R(int) = 0.0823] Completeness to theta = 27.50 95.7 % Absorption correction Integration Max. and min. transmission 0.9629 and 0.8726 Refinement method Full-matrix least-squares on F2 Data / restraints / parameters 12774 / 36 / 621 Goodness-of-fit on F2 0.814 Final R indices [I>2sigma(I)] R1 = 0.0484, wR2 = 0.0981 [6995] R indices (all data) R1 = 0.0868, wR2 = 0.1038 Largest diff. peak and hole 0.653 and -0.813 e.-3 R1 = (||Fo| |Fc||) / |Fo| wR2 = [w(Fo2 Fc2)2] / wFo22]]1/2 S = [w(Fo2 Fc2)2] / (n-p)]1/2 w= 1/[2(Fo2)+(m*p) 2 +n*p], p = [max(Fo2,0)+ 2* Fc2]/3, m & n are constants.

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148 X-ray Experimental for Complex 34 Data were collected at 173 K on a Siemens SMART PLATFORM equipped with A CCD area detector and a graphite monochromator utilizing MoK radiation ( = 0.71073 ). Cell parameters were refined using up to 8192 reflections. A full sphere of data (1850 frames) was collected using the -scan method (0.3 frame width). The first 50 frames were remeasured at the end of data collection to monitor instrument and crystal stability (maximum correction on I was < 1 %). Absorption corrections by integration were applied based on measured indexed crystal faces. The structure was solved by the Direct Methods in SHELXTL5, and refined using full-matrix least squares. The non-H atoms were treated anisotropically, whereas the hydrogen atoms were calculated in ideal positions and were riding on their respective carbon atoms. By interchanging their positions, ligands CH3 and C4H8 are disordered where atoms C21, C24 and C25 have full occupancy while atoms C22 and C23 on one hand are disordered and were refined in a second part as C22 C23. Their site occupation factors were dependently refined to 0.66(1) fro the major part, and consequently 0.34(1) for the minor part. C22 and C23 are refined with isotropic thermal parameters only and were constrained to maintain a similar geometry to the C22-C23 unit. A total of 306 parameters were refined in the final cycle of refinement using 4097 reflections with I > 2(I) to yield R 1 and wR 2 of 4.09% and 9.66%, respectively. Refinement was done using F 2

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149 Table 6-7. Crystal data and structure refinement for 34. Identification code ei08 Empirical formula C25 H44 Al Mo N3 Si2 Formula weight 565.73 Temperature 193(2) K Wavelength 0.71073 Crystal system Triclinic Space group P-1 Unit cell dimensions a = 11.1574(7) = 68.598(2). b = 11.4579(7) = 80.593(2). c = 12.7057(8) = 79.021(2). Volume 1476.75(16) 3 Z 2 Density (calculated) 1.272 Mg/m3 Absorption coefficient 0.572 mm-1 F(000) 596 Crystal size 0.11 x 0.11 x 0.05 mm3 Theta range for data collection 1.73 to 27.50. Index ranges -14 h 14, -14 k 14, -16 l 16 Reflections collected 13438 Independent reflections 6642 [R(int) = 0.0500] Completeness to theta = 27.50 98.0 % Absorption correction Integration Max. and min. transmission 0.9720 and 0.9325 Refinement method Full-matrix least-squares on F2 Data / restraints / parameters 6642 / 4 / 306 Goodness-of-fit on F2 0.913 Final R indices [I>2sigma(I)] R1 = 0.0409, wR2 = 0.0966 [4097] R indices (all data) R1 = 0.0751, wR2 = 0.1038 Largest diff. peak and hole 0.918 and -0.609 e.-3 R1 = (||Fo| |Fc||) / |Fo| wR2 = [w(Fo2 Fc2)2] / wFo22]]1/2 S = [w(Fo2 Fc2)2] / (n-p)]1/2 w= 1/[2(Fo2)+(0.047*p) 2 ], p = [max(Fo2,0)+ 2* Fc2]/3

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150 X-ray Experimental for Complex 36 Crystals of 36 were obtained by slow evaporation of a diethyl ether solution of the compound. Data were collected at 173 K on a Siemens SMART PLATFORM equipped with A CCD area detector and a graphite monochromator utilizing MoK radiation ( = 0.71073 ). Cell parameters were refined using up to 8192 reflections. A full sphere of data (1850 frames) was collected using the -scan method (0.3 frame width). The first 50 frames were remeasured at the end of data collection to monitor instrument and crystal stability (maximum correction on I was < 1 %). Absorption corrections by integration were applied based on measured indexed crystal faces. The structure was solved by the Direct Methods in SHELXTL5, and refined using full-matrix least squares. The non-H atoms were treated anisotropically, whereas the hydrogen atoms were calculated in ideal positions and were riding on their respective carbon atoms. A total of 404 parameters were refined in the final cycle of refinement using 31376 reflections with I > 2(I) to yield R 1 and wR 2 of 3.68% and 8.84%, respectively. Refinement was done using F 2

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151 Table 6-8. Crystal data and structure refinement for 36. Identification code ei10 Empirical formula C35 H46 Al Mo N3 Si2 Formula weight 687.85 Temperature 173(2) K Wavelength 0.71073 Crystal system Monoclinic Space group P2(1)/n Unit cell dimensions a = 14.1426(8) = 90. b = 11.3076(7) = 107.361(2). c = 23.6743(16) = 90. Volume 3613.5(4) 3 Z 4 Density (calculated) 1.264 Mg/m3 Absorption coefficient 0.480 mm-1 F(000) 1440 Crystal size 0.23 x 0.15 x 0.05 mm3 Theta range for data collection 1.80 to 27.50. Index ranges -18 h 18, -14 k 14, -30 l 30 Reflections collected 31376 Independent reflections 8249 [R(int) = 0.0645] Completeness to theta = 27.50 99.2 % Absorption correction Integration Max. and min. transmission 0.9761 and 0.9082 Refinement method Full-matrix least-squares on F2 Data / restraints / parameters 8249 / 0 / 404 Goodness-of-fit on F2 1.002 Final R indices [I>2sigma(I)] R1 = 0.0368, wR2 = 0.0884 [5840] R indices (all data) R1 = 0.0644, wR2 = 0.0988 Largest diff. peak and hole 1.007 and -0.692 e.-3 R1 = (||Fo| |Fc||) / |Fo| wR2 = [w(Fo2 Fc2)2] / wFo22]]1/2 S = [w(Fo2 Fc2)2] / (n-p)]1/2 w= 1/[2(Fo2)+(m*p) 2 +n*p], p = [max(Fo2,0)+ 2* Fc2]/3, m & n are constants.

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152 X-ray Experimental for Complex 41 Data were collected at 173 K on a Siemens SMART PLATFORM equipped with A CCD area detector and a graphite monochromator utilizing MoK radiation ( = 0.71073 ). Cell parameters were refined using up to 8192 reflections. A full sphere of data (1850 frames) was collected using the -scan method (0.3 frame width). The first 50 frames were remeasured at the end of data collection to monitor instrument and crystal stability (maximum correction on I was < 1 %). Absorption corrections by integration were applied based on measured indexed crystal faces. The structure was solved by the Direct Methods in SHELXTL5, and refined using full-matrix least squares. The non-H atoms were treated anisotropically, whereas the hydrogen atoms were calculated in ideal positions and were riding on their respective carbon atoms. The systematic absences could not differentiate between space groups C2, Cm and C2/m. In fact the structure was solved and refined in all three and the same disorders were observed in all of them. Space group C2/m provided the best refinement and thus it was used to fully refine the structure. The asymmetric unit consists of of the dimer and of a toluene solvent molecule. Each of which is located on a 2/m symmetry site. The toluene molecule could not be modeled properly, thus program SQUEEZE, a part of the PLATON package of crystallographic software, was used to calculate the solvent disorder area and remove its contribution to the overall intensity data. The Al(Cl)(C2H5) moiety is disordered and is refined in two parts. Their site occupation factors were dependently refined to 0.29(1) for the major part, and consequently 0.21(1) for the minor part. These moieties lay on the mirror plane thus the site occupation factors site above add up to 50%. A total of 160 parameters were refined

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153 in the final cycle of refinement using 2795 reflections with I > 2(I) to yield R 1 and wR 2 of 3.05% and 7.91%, respectively. Refinement was done using F 2

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154 Table 6-9. Crystal data and structure refinement for 41. Identification code ei07 Empirical formula C40 H66 Al2 Cl4 Mo2 N6 Si4 Formula weight 1130.99 Temperature 193(2) K Wavelength 0.71073 Crystal system Monoclinic Space group C2/m Unit cell dimensions a = 20.6565(9) = 90. b = 15.0381(7) = 107.917(2). c = 9.7848(4) = 90. Volume 2892.1(2) 3 Z 2 Density (calculated) 1.299 Mg/m3 Absorption coefficient 0.763 mm-1 F(000) 1164 Crystal size 0.13 x 0.12 x 0.04 mm3 Theta range for data collection 1.71 to 27.50. Index ranges -26 h 26, -19 k 19, -12 l 12 Reflections collected 13047 Independent reflections 3444 [R(int) = 0.0400] Completeness to theta = 27.50 99.4 % Absorption correction Integration Max. and min. transmission 0.9694 and 0.8663 Refinement method Full-matrix least-squares on F2 Data / restraints / parameters 3444 / 0 / 160 Goodness-of-fit on F2 1.013 Final R indices [I>2sigma(I)] R1 = 0.0305, wR2 = 0.0791 [2795] R indices (all data) R1 = 0.0390, wR2 = 0.0807 Largest diff. peak and hole 1.014 and -0.543 e.-3 R1 = (||Fo| |Fc||) / |Fo| wR2 = [w(Fo2 Fc2)2] / wFo22]]1/2 S = [w(Fo2 Fc2)2] / (n-p)]1/2 w= 1/[2(Fo2)+(0.0469*p) 2 ], p = [max(Fo2,0)+ 2* Fc2]/3

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BIOGRAPHICAL SKETCH Elon Ison was born on February 2 nd ,1974, in San Fernando, Trinidad and Tobago. The first of two children of Walter and Patricia Ison, he spent his first 19 years in his native country. In 1995 he came to the United States to pursue a degree in chemistry at Kean University in New Jersey. His hard work and determination paid off when he received his baccalaureate degree with honors in 1999. The following summer he married Ana Bitanga and together they enrolled into the graduate school at the University of Florida. In December of the same year his daughter Mya was born. Elon joined the group of Dr. James Boncella in 2000 and pursued research in organometallic chemistry. This dissertation summarizes his five years of research in the Boncella labs. Elon graduated with a Ph.D. in the summer of 2004. He looks forward to a long and challenging career in scientific research. 163


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ORBITAL INTERACTIONS IN GROUP 6 IMIDO DIAMIDO COMPLEXES


By

ELON AYINDE ISON













A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA


2004

































Dedicated to my grandparents, John and Lucille McNeil.














ACKNOWLEDGMENTS


There are inevitably more people who have contributed to my experiences at the

University of Florida than there is time or space to enumerate. However, I must

acknowledge a few people without whom this work would not be possible. First and

foremost, I must acknowledge my advisor, Dr. James Boncella, whose hands off but

steady guidance was ideal in allowing me to learn and develop as an organometallic

chemist.

The patience and guidance of two men, Dr. Khalil Abboud who performed all the

crystallographic experiments, and Dr. Ion Ghiviriga for his help in NMR, cannot go

unnoticed. In fact, this thesis would not be possible if not for their efforts, I must also

acknowledge Dr. Adrian Roitberg and Dr. Stephen Fau, at the University of Florida QTP

facility for the help with computational chemistry and for giving us access to their

facilities.

I would be remiss if I did not acknowledge my past and "present" group members.

In particular I would like to, acknowledge Alison Knefely, the last of the Boncella clan at

the University of Florida. In the short time we worked together, she surely provided a

pleasant working environment. She is a great lab-mate and friend, even though she is

frequently on the wrong side of political issues. Her friendship and personality have

surely been missed.

Finally, I would like to thank my family. My parents Patsy, and Walter, have been

supportive throughout this entire experience. My work ethic and values are almost totally









attributable to their parenting. I would also like to acknowledge my little sister Aysha. I

would not have made it through this experience if it were not for my wife Ana. In fact, if

it were not for her I would literally never have figured out this template. She has been an

absolute "rock" throughout this entire experience and most importantly has remained my

best friend. There have been times throughout this experience when I thought that the

struggle was not worth it. It was at these times that my daughter Mya has helped me

"keep it in perspective."















TABLE OF CONTENTS

page

A C K N O W L E D G M E N T S ......... .................................................................................... iii

L IST O F TA B LE S ......... .. .......... ............ ............. ............. viii

LIST OF FIGURES ......... ........................................... ............ ix

ABSTRACT ........ ........................... .. ...... .......... .......... xii

CHAPTER

1 INTRODUCTION AND BACKGROUND ..................................... ...............

Theoretical Calculations and M ethodology............................................................... 3
ON IOM Calculations ............... .. .... .... .. ......... ................ ...... .. .. ... .. ........ ..4
Organoimido Chemistry and the Concept of 7r Loading ................... .............
B onding in 7r Loaded Im ido Com plexes ........................................ ....................6
Early Examples of n loaded complexes. ........................... .......... .............. 9
Group 6 Imido Diamido Complexes........................................ 10
Molybdenum and Tungsten Alkyl and Alkylidene Complexes ........................12
Complexes of Mo and W Imido Diamido Complexes Containing 7r donor
L ig a n d s ...................................................................... 1 6
Scope of the D issertation .......................................................................... 18

2 STRUCTURE REACTIVITY AND BONDING OF MOLYBDENUM IMIDO
DIAM IDO COM PLEXES ............................................................ ............... 20

Introduction .................. ......... ... ............ ... ............... ... ... .............. .... 20
Structure Dynamics and Bonding of Imido Diamido Alkyne Complexes .................21
Su m m ary .................. ................................... .... ..... ........... ......... ................ 3 3
Dynamics and Bonding of Molybdenum Imido Diamido Olefin Complexes............33
Diamide Ligand Folding in d2 vs do Group 6 Imido Complexes.............................39
C o n clu sio n s.................................................... .................. 4 4

3 SYNTHESIS AND REACTIVITY OF GROUP 6 IMIDO DIAMIDO
M ETALLACYCLES ...... ................................................................. ............... 45

Introduction .............. ...... .............. .................................. 45









Synthesis and Reactivity of Mo Imido Diamido Metallacyclopentenes and
M etallacyclopentadienes ..................................................................... 46
Synthesis of metallacyclopentene complexes ..................................................47
Kinetics and Mechanism of the Thermal Rerrangement of 25............................51
Computational Studies on the Thermal Rearrangement of 25 ............................55
Synthesis and reactivity of a metallacyclopentadiene complex. .............................65
Sum m ary and C onclu sions .............................................................. .....................68

4 SYNTHESIS AND REACTIVITY OF Mo(VI) COMPLEXES WITH ALKYL
AND ARYL ISOCYANIDES .................................. ............................ 70

Intro du action ................ .............. ................... ............... ........ ..........7 0
Synthesis of Isocyanide Complexes and Insertion into the Metal-Diamide Bond .....70
Synthesis, Structure and Dynamics of a Chelating Imino Carbamoyl Complex........83
[2+2+1] Cycloaddition reactions of metallacycles with alkyl isocyanides RNC ......89
Sum m ary and C onclu sions .............................................................. .....................9 1

5 ALKYL ALUMINUM INDUCED DIAMIDE TRANSFER FROM GROUP 6
IMIDO DIAMIDO COMPLEXES....................... .... ........................... 92

Introduction ......... ............. ..... .. .......... ........... ......... ......... ............... .. 92
Reactions of Mo Dialkyl Complexes with Trimethyl Aluminum (TMA)..................93
Reactivity of Alkyl Aluminum Halides with Group 6 Imido Diamido Olefin
C om p lex es................................. ..... ....... .......... ............. ............ 9 8
Reaction of metallacyclopentanes with alkyl aluminum halides. .....................100
Reactivity of alkyl aluminum reagents with styrene complexes..............104
Reactivity with alkyl aluminum halides ..................................................108
Reactivity of an isobutylene complex with EtAlCl2 ..............................109
Su m m ary ......................... ........................ ... ............. ...................... ..... 114
Implications of Aluminum Induced Diamide Transfer to Olefin Dimerization
Catalysis ..................................................... ......... .. ................. 114
Reactions of W complexes with alkyl aluminum reagents .............................1116
Diamide transfer to other W complexes ........... ................117
C o n c lu sio n s......................................................................................................... 1 1 9

6 EXPERIM EN TAL ............................................................ .................... 121

G general M ethods........... .... ............................................ ................ .......... ....... 12 1
Synthesis of r2 -alkyne complexes ............................. 121
Synthesis of bis-Isocyanide Com plexes .............................................. ..................126
Synthesis of a Chelating Imino Carbamoyl Complex, (33) ..................................... 127
R actions w ith alkyl alum inum reagents ..................................................................128
Synthesis of [(rl2-ethylene)(Et)Mo(NPh)r 4-(o-(SiMe3N)2C6H4)A(CH3)2], (42b)...130
Synthesis of [(r2-styrene)(Me)Mo(NPh)r4-(o-(SiMe3N)2C6H4))Al(CH3)2], (39).... 131
Synthesis of [(rl2-styrene)(Me)Mo(NPh)r 4-(o-(SiMe3N)2C6H4)}Al(C1)2], (40) ......131
Synthesis of [(t2 NPh)Mo(Cl)(r4-(Cl2Al((o-(Me3SiN)2-C6H4))]2, (41)...................133
K in etic S tu d y ...................................................................... 13 4









C om putational Studies........................................... ....................................... 135
X -ray E xperim mental ................ .. .................................................... ....................137
X-ray Experimental for Complex 26b..................... ...............37
X-ray Experimental for Complex 29 .............. ..........................................140
X-ray Experimental for Complex 25 .............. ..........................................142
X-ray Experimental for Complex 30b..........................................................144
X-ray Experimental for Complex 33.........................................................146
X-ray Experimental for Complex 34 .................. ....... .............................. 148
X-ray Experimental for Complex 36 .................. ....... .............................. 150
X-ray Experimental for Complex 41 ............. ............. ...............................152

LIST OF REFEREN CES ........................................................... .. ............... 155

BIOGRAPHICAL SKETCH ............................. ............................. ............... 163














LIST OF TABLES
Tables p

2-1 1H and 13C NM R data for alkyne complexes ................................. ............... 22

2-2 Comparison of selected bond lengths and angles between model compounds 22
and 23 and the reported crystal structure for 21f ................................................27

2-3 NLMO analysis of alkyne 7t bond ...................................................................................31

2-4 NBO analysis of Mo-N(imido) bonds ..... ................................................31

2-5 NBO analysis of the Mo-N(Imido) bond in 20c.................................................... 39

2-6 Fold angles for some Mo Imido Diamido Complexes ..........................................40

2-7 NLMO analysis of Mo-N(Imido) and Mo-N(Diamide) bonds in 23 and 24 ......43

3-1 Dependence of DEAD on the formation of 26c............... ............. ............... 54

3-2 Selected bond lengths (A) and angles (0) for ONIOM optimized structures for
the therm al rearrangem ent of 25 ...........................................................................60

4-1 Selected bond Lengths(A) and Angles(0) for 31 and 32 .......................................78

6-1 Kinetic analysis for the reaction of DEAD with 25 .............................................135

6-2 Crystal data and structure refinement for 26b....................................................... 139

6-3 Crystal data and structure refinement for 29. .....................................................141

6-4 Crystal data and structure refinement for 29. .....................................................143

6-5 Crystal data and structure refinement for 30b......... ................................ 145

6-6 Crystal data and structure refinement for 33. .....................................................147

6-7 Crystal data and structure refinement for 34. .....................................................149

6-8 Crystal data and structure refinement for 36. .....................................................151















LIST OF FIGURES
Figures page

1-1 The ON IOM extrapolation schem e ................................................ .............. 4

1-2 Limiting VB description of a metal imido linkage .............................................7

1-3 n perpendicular set of N(p) orbitals in threefold symmetry .............. .................10

2-1 Therm al ellipsoid plot of 21e. .............................................................................24

2-2 Variable temperature H NMR spectrum of the SiMe3 region for 21b ..................25

2-3 B3LYP optimized models for alkyne complexes 22 and 23 and their transition
state s ............................................................................... 2 7

2-4 Bonding interactions in between Mo, imido, and the alkyne fragment ...................30

2-5 NLMO plots (isocontour 0.04) of Mo-N(imido) bond in 22 and TS 22 ..............32

2-6 Optimized ONIOM structures for 20c ........................................ ............... 37

2-7 HOMO of 23 and 20c showing 7r backbonding of the metal fragment to the
olefin in 23 or acetylene in 20c ........................................... ......................... 37

2-8 HOMO of TS23 and TS20c showing nr backbonding of the metal fragment to
the olefin ......... ......... ..................................38

2-9 O occupied m olecular orbitals of 24 ........................................................................43

3-1 Therm al ellipsoid plot of 26b .............. ......................................... ...................... 52

3-2 First order kinetics for the formation of 26c ......................................................54

3-3 Eyring plot for the reaction of 25 with DEAD ......................................................55

3-4 T herm al ellipsoid plot of 25 .......................................................... ............ ... 56

3-5 Optimized structures (mPW1K/LANL2DZ) for the thermal rearrangement of 25..58

3-6 Reaction profile AGO 298 (K) for the thermal rearrangement of 25 at the
mPW k/SDD-Aug-cc-pVDZ level of theory ............. .....................................58









3-7 ONIOM (mPW1K/LANL2DZ:mPW1K/LANL2MB) optimized structures for
the thermal rearrangement of 25 .............. ....... .......... ..... ............... 59

3-8 Occupied molecular orbitals(B3LYP/LANL2DZ) for 25 ....................................62

3-9 Occupied molecular orbitals (B3LYP/LANL2DZ) for 25b................................. 63

3-10 Occupied molecular orbitals (B3LYP/LANL2DZ) for 25c ................... ................63

3-11 Therm al ellipsoid plot of 29 ....................................................................................67

4-1 Therm al ellipsoid plot of 30b (30% ellipsoids) ............................... ... ..................74

4-2 T herm al ellipsoid plot of 3 1 .......................................................... .....................76

4-3 Optimized structure (ONIOM B3LYP/LANL2DZ:B3LYP/LANL2MB) for 31
a n d 3 2 ............................................................................ 7 9

4-4 3-Center-4-electron bonding between the diamide lone pair electrons the metal
d orbitals and the 7t* orbitals of a 7n acceptor ligand ..........................................79

4-5 Interaction of diamide lone pairs and isocyanide ligand with metal dxy orbital. ....81

4-6 Interaction of diamide lone pairs and CO ligand with metal dxy orbital. .............81

4-7 T herm al ellipsoid plot of 33 .......................................................... ............ ... 85

4-8 Space filling diagram generated from the crystal structure of 33 ............................87

4-9 Variable temperature 1H NMR spectrum (C7Ds) of 33 ..........................................88

5-1 Therm al ellipsoid plot of 34 ................................................................................... 95

5-2 Therm al ellipsoid plot of 36. ....................................................... .................... 96

5-3 Optimized B3LYP structures for model complexes Me2Al((o-(Me3N)2-pda)-1, 37,
and (CH2)4Mo(CH3)(NH)(o-(Me3N)2C6H4)Al(CH3)2, 38 and their molecular
o rb ita ls ............................................................................ 9 8

5-4 1H NMR spectrum of 42a,
(r2 -ethylene)Mo(Me)(NPh) r4 -(Cl2Al((o-(Me3SiN)2-C6H4)) in C6D6 ................ 102

5-5 Proposed structure for 42b showing selected carbon and (parentheses) and
proton chemical shifts assigned by NMR spectroscopy...................................102

5-6 X-ray structure determination of 39a......... ....... ................ ............... 105

5-7 Proposed structure for 39a showing selected carbon and (parentheses) and
proton chemical shifts assigned by NMR spectroscopy.................... ........ 107









5-8 1H N OESY spectrum of 39a ..... .................................................... ...............107

5-9 Proposed structure for 39b showing selected carbon and (parentheses) and
proton chemical shifts as assigned by NMR spectroscopy ..................................108

5-10 1H N O E SY spectrum of 39b ............................................................................. 108

5-11 Therm al ellipsoid plot of 41........................................................ ............... 112

5-12 H NMR spectrum of W metallacyclopentane methyl complex ........................117

5-13 H NMR spectrum for the reaction of 12, with TMA ................. ................... .. 120














Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy


ORBITAL INTERACTIONS IN GROUP 6 IMIDO DIAMIDO COMPLEXES

By

Elon Ayinde Ison

August 2004

Chair: James M. Boncella
Major Department: Chemistry

The orbital interactions of group 6 imido diamido complexes have been

investigated. The synthesis of alkyne complexes, 21,

(rq2-alkyne)Mo(NPh)(o-( Me3SiN)2C6H4) by the reaction of alkynes with 20a,

(rj2-isobutylene)Mo(NPh)(o-( Me3SiN)2C6H4), has been reported, along with the X-ray

crystal structure of 21e, and the dynamic solution behavior of these complexes via low

temperature. DFT calculations have been used to study the bonding in these complexes

and an NBO analysis was used to determine the extent of x donation by the alkyne

ligand. DFT calculations were also performed on 20c and 24.

The synthesis of metallacyclopentene complexes, 26,

[(NPh)Mo(C(R)C(R)CH2CH2)(o-(Me3SiN)2C6H4)], by the sequential 7x ligand exchange

of ethylene from 25 and an alkyne is reported. An X-ray structure of the

metallcyclopentene complex 26b, [(NPh)Mo(C(H)C(Ph)CH2CH2) {o-(Me3SiN)2C6H4}],

is reported. The synthesis of the metallacyclopentadiene complex









[(NPh)Mo(C(Ph)CHC(Ph)){o-(Me3SiN)2C6H4}], 29, a by the [2+2] cycloaddition

reactions of two alkynes is reported. The thermal rearrangements of 25, was examined

by kinetics and DFT.

The synthesis and characterization of bis-isocyanide complexes,

(RNC)2Mo(NPh)(o-( Me3SiN)2C6H4), 30 of Mo and the subsequent reactivity of these

complexes with excess isocyanide yielding tris-isocyanide complexes in the case of

tBuNC and an unusual chelating iminocarbamoyl bis-isocyanide complex in the case of

2,6 dimethyl phenyl isocyanide are reported. X-ray structures of the bis-isocyanide

complex 30a and the iminocarbamoyl bis-isocyanide complex are also reported.

The reactivity of alkyl aluminum reagents with alkyl, olefin and alkynes has been

demonstrated. Treatment of an orange solution of Mo(NPh)(o-(Me3SiN)2C6H4)(CH2)4,

25, in toluene, with 1 equivalent of trimethyl aluminum (TMA) yields the

metallacyclopentane methyl complex,

(CH2)4Mo(NPh)-rq4-[(o-(SiMe3N)2C6H4)} Al(CH3)2, 34. A similar reaction occurs when

an orange solution of the diphenyl acetylene complex, 35, is treated with 1 equivalent of

TMA and yields the diphenyl acetylene methyl complex,

(r2-diphenyl acetylene)(CH3)Mo(NPh)rl4-[(o(SiMe3N)2C6H4)} Al(CH3)2 36. The

metallacyclopentane complex 25, reacts with alkyl aluminum halides to yield the olefin

alkyl complexes (r2-ethylene)(R)Mo(NPh)rl4-[(o-(SiMe3N)2C6H4)}Al(CH3)2, 42.

The reaction of the styrene complex 20c, with ethyl aluminum dichloride or

trimethyl aluminum results in the formation of complexes 39,

(rq2-styrene)Mo(Et)(NPh)rl4-(Cl2Al((o-(Me3SiN)2-C6H4)) or 40,(r2-

styrene)Mo(CH3)(NPh)rl4-((CH3)2Al((o-(Me3SiN)2-C6H4)), respectively. In contrast, the









reaction of ethyl aluminum dichloride with 20a, results in the the displacement of

isobutylene and the formation of the bringing imido dimer

[(Qt2 NPh)Mo(Cl)(r4-(Cl2Al((o-(Me3SiN)2-C6H4))]2, 41. The X-ray crystal structure of 41

was determined.














CHAPTER 1
INTRODUCTION AND BACKGROUND

The transition metal series of the periodic table enables chemists to have access to

a wide variety of compounds and chemical reactions that are otherwise inaccessible in

main-group chemistry. Thus, a tremendous amount of research has been devoted to the

development of new and interesting organometallic complexes with an emphasis on the

understanding of the structure, bonding and reactivity of the transition metal species.

These efforts were devoted to the synthesis and structural characterization of new

organometallic complexes that are supported by various types of ancillary ligands

including cyclopentadienyl, (Cp), trispyrazoyl borate (Tp), amides, alkoxides and

imines.1 Significant advances in the field of catalysis and synthetic organic chemistry

have occurred as a result of these studies; the number and type of organometallic

catalysts are too innumerable to be discussed here. This pioneering work has provided

insight into the bonding and general understanding of organometallic mechanisms. The

reactivity observed at a given metal center is highly dependant on the electronic as well

as steric requirements of the ancillary ligands that support a particular metal species.

Consequently, synthetic organometallic chemists have focused on the derivitization and

discovery of new ligands, with the goal of "tuning" the reactivity at a metal-center.

The development of molecular orbital (MO) theory has allowed the structural and

reactivity trends to be discussed in terms of specific interactions of molecular orbitals.2'3

The accuracy of these methods depends on the way in which interactions between

electrons are handled a concept defined as electron correlation. Hartree Fock (HF)









theory, for example, provides a method for converting the many-electron Schrodinger

equation, into many simpler one-electron equations that can be solved to yield single

electron wavefunctions called orbitals. However, significant errors may arise because

electron correlation in these methods is treated in an average sense.

Technological advances have allowed for the successful employment of gradient

corrected density functional theory (DFT) in calculating molecules, particularly involving

transition metals, and in the use of small core relativistic effective core potentials

(ECP's). DFT offers an advantage over traditional HF methods because it implicitly

treats electron correlation. This has set the stage for the calculation of geometries, bond

energies, vibrational spectra, activation energies for chemical reactions and other

important properties of transition metal complexes.47 These achievements have truly

enhanced the understanding of the nature of the bonding seen in transition metal

complexes and have allowed researchers to accurately make predictions about the

existence of intermediates, the stabilities of species and the accessibility of kinetic

pathways in reaction mechanisms.

In this dissertation we describe our latest efforts in an ongoing study of group 6

imido complexes supported by a chelating ancillary ligand. To study these complexes we

employ the techniques of X-ray crystallography, NMR spectroscopy and computational

chemistry(DFT), in order to properly assess the effects of ligand '7 donation on the

properties and reactivity of these complexes. This chapter begins with a brief overview

of the computational methods used to study these complexes and then proceeds to

describe bonding in transition metal imido species and the concept of 7T loading. A brief









account of the group 6 imido diamido complexes previously synthesized in the Boncella

lab along with an outline for the dissertation is also presented.

Theoretical Calculations and Methodology

Unlike Hartree-Fock calculations, density functional theory (DFT) implicitly treats

electron correlation by utilizing an energy functional, some with empirically adjusted

parameters4: Ex(p) = E(p) + Ec(p) where

Ex" is the exchange correlation functional

Ex is the exchange functional

EC is the correlation functional

Modem functionals such as the generalized gradient approximation (GGA) or

gradient corrected functionals and the so called hybrid functionals employ an admixture

of exact Hartree-Fock (HF) exchange functionals and correlational exchange functionals,

and have been shown to provide accurate geometries and reasonable energetic for a wide

variety of systems at a computational cost similar to HF theory. DFT has a distinct

advantage over traditional wave-function based techniques (ab initio) because the explicit

addition of electron correlation to these techniques becomes quite computationally

expensive.

In performing calculations on complex systems involving transition metals it is

sometimes necessary to simplify the ligands of a given system not only to expedite the

calculations but also to perform the number of calculations necessary to appropriately

describe the system under study. However, these simplifications can and sometimes do

lead to problematic interpretations and predictions of molecular systems as the steric

and/or electronic consequences of ligand trimming must be considered. The electronic









consequences of ligand trimming are separate from and can compound with an

incomplete basis set and an imperfect energy functional.

ONIOM Calculations

An obvious solution for the treatment of complex molecular systems is the

partitioning of the system into two or more parts or layers, where the interesting or

difficult part of the system (inner layer) is treated at a high level of theory and the rest of

the system with a computationally less demanding method.

The ONIOM (Our owN n-layered Integrated molecular Orbital + molecular

mechanics Method) developed by Morokuma and co-workers is a hybrid method

designed to enable different levels of theory to be applied to different parts of a

molecule/system and combined to produce a consistent energy expression.8-12 The basic

idea behind the ONIOM approach can be explained as an extrapolation scheme in a two

dimensional space with the size of the system on the x-axis and the level of theory on the

y-axis (Figure 1-1).


2 4 4 7 9
-High



25 8
-Medium




Low


I I I I ,l -
Model Real Small Intermediate Real
Model Model
Figure 1-1. The ONIOM extrapolation scheme for a molecular system portioned into two
(left) and three (right) layers









In order to describe the real system at the highest level of theory, point 4 in a

system partitioned in two layers and point 9 in a system partitioned in three layers, the

extrapolated energy is defined as EONIOM2 = E3 El + E2 where E3 is the energy of the

entire (real) system calculated at the low level method and El and E2 are the energies of

the model system determined at the low and high level of theory respectively. EONIOM2 is

an approximation to the true energy of the real system E4: E4 = EONIOM2 + D. If the error,

D, of the extrapolation procedure is constant for two different structures their relative

energy AE4 will be evaluated correctly by the ONIOM method. The accuracy of the

ONIOM method depends on two factors: the choice of the lower level of theory and the

model system. Thus the model system and level of theory must be chosen so as to

accurately describe the energetic of the real system. Morukuma, Milstein,13'14

Landis15'16 and others have successfully employed this method to transition metal

systems. We have successfully employed this method throughout this dissertation in

describing the consequences of ligand '7 donation in group 6 imido diamido complexes.

Organoimido Chemistry and the Concept of '7 Loading

Organoimido transition metal chemistry has received attention in recent years

because of its implication in several catalytic processes such as such as propylene

ammoxidation and nitrile reduction. Imido complexes have also been shown to function

as imido transfer intermediates in the aziridination and amination of olefins. There have

been several reviews in recent years; the definitive work on imido chemistry occurs in

Nugent and Mayer's Metal-LigandMultiple Bonds,17 in addition there have been more

recent reviews by Wigley18 and Eikey and Abu-Omar19 et al.









Recent interest in imido chemistry has focused on the electronic structure and

reactivity of imido complexes towards C-H bond activation. This involved repeated

coordination of imido ligands to a single metal center, a strategy defined as '7 loading by

Wigley group20 24. The premise behind the '7 loading strategy is that repeated

coordination of more than one strongly i7-bonding ligand will lead to increased

competition for metal d7T acceptor orbitals. This will result in the localizition of charge at

one of these '7 donor ligands and hence weakened '7 bonding and increased reactivity at

this ligand. The Wolczanski and Horton groups have studied do bis-imido complexes of

Ta and V, and have demonstrated their ability to activate methane. Cundari and co-

workers have compared methane activation by three-coordinate group 4, and group 5, and

bis-imido amido complexes.2527 The results of these early studies demonstrate clearly

that the strategy of 7T loading is a potentially useful method for generation of highly

reactive organometallic species.

Bonding in '7 Loaded Imido Complexes

The imido ligand can be considered to bond to a transition metal with one C bond

and either one or two 7t bonds. The simplest model for bonding in complexes employing

these ligands is a simple limiting valence bond (VB) model (Figure 1-2 ).18 Figure 1-2

depicts the three limiting structures predicted by the simple VB model. Structure A

depicts an sp2 hybridized nitrogen leading to a M=N double bond (lo, 17t) and a bent

M-N-R linkage with the lone pair residing in a N(sp2) orbital. In this formalism the

imido behaves as a 4e-1 donor. In structure B, the M=N (double bond) is maintained if

symmetry restrictions or a severe electronic mismatch with the available metal orbitals

does not allow lone pair donation.









0 _
MM--N R M N- R
J\

A B C
N (sp2) N (sp) N (sp)
[NR]2- 4e- [NR]2- 4e- [NR]2- 6e-

Figure 1-2. Limiting VB description of a metal imido linkage
Most of the imido complexes described in the literature can be described by
Structure C, where the imido ligand is formally a six electron donor (lo, 2x7).
There are very few examples of bent imido complexes in the literature. The classic
example is found in the bis-imido complex Mo(NPh)2(S2CNEt2)2, 1,28 which contains one
bent imido (MoNC =139.4(4)) and one linear imido (MoNC = 169.4(4)0) (Scheme 1-1).
S Et




N ---- Et





Et
Scheme 1-1. Mo(NPh)2(S2CNEt2)2, a classic example of a bent imido complex
Scheme 1-1. Mo(NPh)2(S2CNEt2)2, a classic example of a bent imido complex









The bent imido can be described by the limiting structure A, whilst the linear imido

can be described by the limiting structure C, and this allows the complex to attain an 18-

electron configuration.

Several researchers have attempted to use similar VB arguments to describe the

bonding in imido complexes and implied that the M-N bond length and MNC bond

angles should reflect the limiting structures in Figure 1-2. However, it has been

demonstrated by Cundari et al.,25-27 that the VB description of the metal imido bond is not

sufficient to describe the bonding in these complexes. Using ab initio calculations on a

series of imido complexes these authors have shown that eight different resonance forms

may be used to describe the M-N bond (Scheme 1-2). The results of these studies

clearly demonstrate that the M-N (imido) bond cannot be described by a single

resonance form as suggested by the VB description and a molecular orbital approach is

often needed to accurately describe the bonding in metal-imido complexes.

M 1 N R M N-R

M~N MN


R R



R R



M N--R M N-- R

Scheme 1-2. Eight principal resonance contributors for the M-N (imido) bond. In this
representation a straight line represents a covalent bond (i.e one electron in a
TM-centered AO-like MO and one in the N centered counterpart), an arrow
pointing towards the metal represents a dative bond and an arrow pointing
towards the N represents a back bond. The bottom line (or arrow) describes
the cy bond and the other lines 7t components of the M-N linkage.









Early Examples of '7 loaded complexes.

Early examples of heavily 7t-loaded complexes involve multiple coordination of

imido ligands. The three-coordinate complex Os(N-2,6-C6H3-i-Pr2)3, 2, by Schrock et

al.29 and the four-coordinate anion [W(N-2,6-C6H3-i-Pr2)3C1]-, 3,23 are the classic "20-

electron" complexes (Scheme 1-3). Both complexes are characterized by a ligand based,

non-bonding MO comprised of a set of ligand 7t orbitals oriented roughly perpendicular

to the molecules C3 axis which results in these complexes being described as 18-electron

complexes. For example, the [W(N-2,6-C6H3-i-Pr2)3C1]- molecule is best described as an

18-electron complex, as one combination of the imido nitrogen pjt orbitals has a2

symmetry, and there is no corresponding orbital metal d orbital that can interact with this

orbital (Figure 1-3).



CI

N
WP 'Pr
S'Pr'Pr N N
Osp iPr NN


Pr N
Pr Pr' PrPr
r 2 3



Schrock et.al. Wigley et.al.
J. Am. Chem. Soc.1990, 112, 1643- 1645 J. Am. Chem. Soc.1991, 113, 6326- 6328

Scheme 1-3. Examples of 7 loaded complexes











L set








Figure 1-3. Pi perpendicular set ofN(p) orbitals in threefold symmetry for 3

This 7T loading strategy may be applied to complexes employing ligands other than

imido or a combination of imido and another '7 donor ligand such as alkynes and amides.

The Boncella group has employed this '7 loading strategy in the synthesis of imido

diamido complexes. The 7t conflict that arises because of the presence of several 7t donor

ligands is responsible for the interesting chemistry of these molecules.

Group 6 Imido Diamido Complexes

The Boncella group has developed extensive chemistry in the area of group 6 imido

diamido complexes starting from the two metal dichloride species Mo(NPh)(o-

(Me3SiN)2C6H4)C12(THF), 4,30 and W(NPh)(o-(Me3SiN)2C6H4)C12 5.31 Their syntheses

are shown in Schemes 1-4 and Scheme 1-5. The Mo dichloride complex 4 is synthesized

by treating a suspension of the bisimido complex, Mo(NPh)2C12(DME) 6, with a pentane

solution of the diamine o-(SiMe3N)2(-C6H4), and allowing the suspension to stir for

24hrs. The aniline complex Mo(NPh)(o-(Me3SiN)2C6H4)Cl2(aniline), 7, is generated by

an apparent transfer of the diamine protons to one of the imido ligands in 6. Lewis bases

readily displace aniline in this complex. Thus stirring 7 in THF readily generates 4

quantatatively.











SiMe3
NH2 Me3Si

VNH2 N f ^W ^
( ji) NH (ii) ^ i I
NH2 NH H2N CI
7
SiMe3


(iii)
Ph
(i) 2 equivs SiMe3CI 2 NEt3,Et20 N/
(ii) Mo(NPh)2CI2(DME), 6, pentane (24 hrs) Me3Si
(iii) THF Me3MSiN. Mo.
aniline +N I NK




Scheme 1-4. Synthesis of Molybdenum imido-diamido dichloride, (4).

The W dichloride complex is synthesized by treating the dilithiated salt of the

diamine with a pentane solution of the W tetrachloride complex (NPh)WCl4(OEt2), 8

Scheme 1-5.

These complexes serve as starting materials for some extremely rich and diverse

chemistry that stems from the electron rich environment provided by the diamide and

imido ligands. Molecular orbital calculations have demonstrated that in addition to the

imido ligand donating its 6 electrons (lo, 27), the diamide ligand could donate its lone

pair electrons into a metal d orbital of appropriate symmetry.32'33 Thus, all the t2g orbitals

on the metal are occupied. The addition of an additional 7t donor ligand to the metal

would result in a competition for available d orbitals ,i.e., a 7i loaded environment on the

metal center.











SiMe3 N

NH2 ~Me3S i(i



SiMe3





(i) 2 equivs SiMe3CI 2 NEt3,Et20

(ii) (a) 1. 2 equivs n-BuLi
(b) (NPh)WCl4(OEt2), 8, pentane
Scheme 1-5. Synthesis of Tungsten imido-diamido dichloride, (5).

Molybdenum and Tungsten Alkyl and Alkylidene Complexes

The dichlorides 4 and 5 were easily converted to alkyl complexes, 9, and 10,

respectively, upon treatment with non- P-hydrogen-containing alkyl magnesium reagents

(Scheme 1-6). The generation of the bis-neopentyl complexes

M(NPh)(o(Me3SiN)2C6H4)(CH2(C(CH3)3)2 (M = Mo, 9c; M = W, 10c) provided an

avenue into alkene metathesis chemistry.

When compounds 9c and 10c, were heated in the presence of PMe3, the alkylidene

species M(NPh)(CHCMe3)(PMe3)(o-(Me3SiN)2C6H4) (M = Mo, 11; M = W 12) are

generated via ac abstraction (Scheme 1-7). Compound 12 has been shown to be an active

catalyst for the methathesis of terminal olefins but competing decomposition processes

drastically limit the catalyst's lifetime.34













Me3Si,
MeqSi


Me3Si,


+ 2.0 Mg2+ CI-


Me3Si M = Mo: R = Me (9a), Ph (9b), CH2CMe3 (9c),
Me3Si .. CH2Ph (9d), CH2SiMe3 (9e);

SCI M = W: R = Me (10a), Ph (10b), CH2CMe3 (10c),
CH2Ph (10d), CH2SiMe3 (10e), CH2CMe2Ph (10f).




Scheme 1-6. Synthesis of molybdenum and tungsten dialkyl complexes.


PMe3


70 C


'Me3


Me3Si.


Ph


+ CMe4


M = Mo (9c), W (10c)


M = Mo (11), W(12)
Scheme 1-7. Formation of molybdenum and tungsten alkylidene adducts.












XN"/ /N NN h + PMe3

N 12 Vh 12b


ethylene


MS Me3Si\ / Ph
Me3Si


/ 13 12c


Scheme 1-8. Reversible metallation of 12 in the absence of PMe3 and the formation of 13
in the presence of excess ethylene

Removal of PMe3 from complex 12 results in a reversible metalation of one of the

SiMe3 groups of the o-(Me3SiN)(C6H4) ligand as shown in Scheme 1-8. When 12 is

treated with excess ethylene the metallacyclopentane complex 13,

W(NPh)(o-(Me3SiN)2C6H4)(CH2)4, is formed. The mechanistic details of the formation

of 13 from 12 in the presence of excess ethylene have been detailed in the literature and

will not be discussed here.34 For the purposes of this discussion it will suffice to say that

the reaction 12 with ethylene to form 13 is a deactivation pathyway when 12 is used as an

olefin metathesis catalyst for terminal olefins.

It is noteworthy that the Schrock group has also observed metallacyclopentanes in

olefin metathesis reactions.3542 However, the metallacycles formed with these species

are particularly unstable and decompose via 3 hydrogen abstraction/transfer mechanisms.









The difference in reactivity between Schrock's complexes (Scheme 1-9) and ours

originates from the nature of the ancillary ligands. The bis-alkoxide ancillary ligands in

Schrock's complexes,43 14 and 15, are not very good at stabilizing the metal center via '7

donation. In our complexes the diamide ligands have a high propensity to donate

nitrogen p electrons to metal orbitals of appropriate symmetry. The implications of these

interactions are the primary focus of this dissertation.


Pri- Pr PriN 'Pr
N N
AN


/ 14 0 15
R R



a) R = CCH3(CF3)2
b) R= tBu
Scheme 1-9. Examples of Schrock's metathesis catalysts


Reaction With Alkyl Isocyanides

The reactivity of tungsten alkyls 10a and 10c has been explored with tert-butyl

isocyanide (ButNC) (Scheme 1-10).44 Complexes 10a and 10c inserted ButNC into each

metal alkyl bond, affording the r2-imino-acyl complexes 16 and 17. When heated, 16

underwent a carbon-carbon coupling reaction giving 18. This insertion into metal alkyl

bonds and the subsequent coupling chemistry have been well documented for early

metals with CO or isocyanides.45 However, the related insertions of isocyanides into

metal amide bonds are less well studied, and given the ligand centered nature of the









highest occupied molecular orbitals in these complexes, isocyanide insertion into the

metal diamide bond might be expected.

Ph Ph

N N
Me3Si BuNCMe3Si N
Me3Sii\, Me3Si\ W
N ', ,R R.T N



R = Me (10a), CH2CMe3 (10c)


Toluene
R = Me (16), CH2CMe3
85 oC, 2h
(17)
(only 16)
Ph

N
Me3SiS i


18 N



Scheme 1-10. Reactivity of tungsten dialkyls with tBuNC.

Complexes of Mo and W Imido Diamido Complexes Containing 7: donor Ligands

Alkyl complexes containing 3 hydrogens (10g, R = Phenethyl; 10h, R = Ethyl) can

be synthesized and isolated with W. These complexes are stable at room temperature, but

the treatment of 10g and 10h with a Lewis Base (PMe3) results in the formation of the

olefin complexes 19 (Scheme 1-11).32

The formation of the olefin complexes 19, has been showed to proceed by a 3-

hydrogen transfer from the ethyl and phenethyl groups in 10g and 10h respectively. This

reaction is unusual as it is promoted by the association of a PMe3 ligand whereas 3-










hydrogen transfer reactions are usually promoted by the dissociation of a ligand. In

contrasts, Mo alkyls containing 3-hydrogens are not stable and undergo P-hydrogen

transfer reactions at -200 C resulting in the formation of stable isolable olefin complexes

20 (Scheme 1-12).46

Ph
Ph


1. 2.0 equivs RMgCI, -780C N
2. Room Temp /
3. R = Et, Phenethyl
Me3S iK\, W
XN FR
// R
10g and
10h

Ph Z 2.0 equivs PMe3
/ Room Temp


SPMe3 + RH

SiMe3 19a, R = H; 19b, R = Ph
SiMe3

Scheme 1-11. Synthesis ofW olefin complexes by PMe3 promoted P-hydrogen transfer
Ph Ph


Me3Si
Me3Si\ Mo


4 \


1. 2.0 equivs RMgCI, -780C
2. Room Temp

R= ethyl, propyl, i-butyl


N
Me3Si /
Me3Si \ Mo R1

20 2 R2

a. R1= Me R2=Me
b. R1= Me R2= H
c. R= Ph R2=H



+ RH


Scheme 1-12. Synthesis of olefin complexes of Mo


Me3Si,
Me3Si









The olefin complexes, 20, have served as useful starting materials for the synthesis

of several complexes incorporating 7t donor ligands. These include arenes, imines,47

butadienes,48 alkynes, and pyridine ligands.49 These complexes all exhibit unusual

structural properties as well as interesting chemical reactivity. These properties originate

from the electron rich environment at the metal center provided by the diamide and imido

ligands. Our interest in studying these complexes stems from a desire to understand the

implications of 7 loading in these complexes, an in so doing, produce species that may be

used as catalysts in organic synthesis.

Scope of the Dissertation

The implications of competitive ligand '7 donation (7t loading) in group 6 imido

diamido complexes are systematically discussed in this manuscript. We have attempted

to do this by utilizing a series of techniques (DFT, X-ray crystallography, NMR

spectroscopy, and kinetic and mechanistic studies), in an attempt to attain a deeper

understanding of the influence of the diamide and imido ligands on the properties of these

complexes. The effects of 7T loading competition between alkyne and imido ligands, as

well as the electronic origins of ligand folding in do and d2 complexes are discussed in

Chapter 2. The synthesis of metallacyclopentenes via '7 ligand exchange and the kinetics

and mechanism of the thermal rearrangement of metallacyclopentanes are studied by

DFT and kinetic studies and are presented in Chapter 3. Reactions at the diamide ligand

are discussed in Chapters 4 and 5. The synthesis of isocyanide complexes and the

isocyanide insertion in the metal diamide bond to form an iminocarbamoyl complex is

discussed in Chapter 4, whilst the alkyl aluminum induced diamide transfer from group 6






19


imido diamido complexes and the implications to olefin dimerization/codimerization

catalysis are discussed in Chapter 5.














CHAPTER 2
STRUCTURE REACTIVITY AND BONDING OF MOLYBDENUM IMIDO
DIAMIDO COMPLEXES

Introduction

Species containing transition metal to ligand multiple bonds are widespread and

play an important role in organometallic chemistry.17 These species usually contain

ligands such as imido, nitrene, oxo and alkylidenes that bind to the metal via overlap of

the ligand p 7t and metal d 7t orbitals. When there are several 7t donors on a given metal

center, ligand to metal 7t bonding results in a competition for available unfilled metal d

orbitals. The term "7t loading" has been used by Wigley and others to describe this

phenomenon, and it has been suggested that the competition for metal i7 orbitals can lead

to increased reactivity of organometallic complexes.20-27

While the structural and electronic effects of "T loading" is well documented for

both tetrahedral and octahedral (bis)imido and mixed oxo-imido complexes,5053 there is a

sparsity of similar studies on ability of alkyne ligands to compete as 7t donors on a given

metal center in a 7r loaded environment. Although, it has been demonstrated that alkynes

can stabilize high oxidation states of transition metals via donation of their i7t electrons, it

is not clear whether these interactions are strong enough to compete with traditional

7 donors.

Recently, our group has investigated the chemistry of group 6 imido complexes of

the form M(NPh)(o-(Me3SiN)2C6H4)X2 (M=Mo and W) which incorporate the o-pda (o-

phenylenediamine) ligand.30'31 This work has demonstrated that the diamide ligands are









involved in a multicenter '7 donor interaction with the metal center via the diamide lone

pair electrons. The success in the synthesis of high oxidation state olefin complexes,34'46

inspired us to pursue the synthesis and reactivity of alkyne complexes, and in so doing

answer the question: Can donation of the alkyne 7_L electrons effectively compete with

the imido and diamide ligands on the metal center for empty metal d orbitals?

In this chapter, the synthesis of alkyne complexes, 21, (r2-alkyne)Mo(NPh)(o-(

Me3SiN)2C6H4), along with the X-ray crystal structure of 21e, and their solution behavior

via low temperature 1H NMR spectroscopy is discussed. DFT calculations (B3LYP)

have been used to study the bonding in these complexes and an NBO analysis was used

to determine the extent of 7T donation by the alkyne ligand.

Structure Dynamics and Bonding of Imido Diamido Alkyne Complexes

Synthesis of Alkyne Complexes. Base-free molybdenum alkyne complexes of the

type [r2-(alkyne)Mo(NPh)-o-(Me3SiN)2C6H4)], 21, were prepared by treating pentane

solutions of 20 with the appropriate alkyne, followed by removal of solvent under

reduced pressure (Scheme 2-1). The alkyne reactants displace the bound olefin with

complete conversion to products within 15 min at 20 oC. The 1H NMR spectra of 21 are

consistent with monomeric alkyne complexes. As shown in Table 2-1, the 13C NMR

resonances of the ac carbons of the alkyne fragment are significantly deshielded

suggesting the involvement of the alkyne 7 electrons in the M-alkyne bonding.54-60

Alkyne complexes 21a-e are air sensitive, but stable in solution and in the solid-state. In

contrast, the alkyne complex 21f is thermally unstable and decomposes in solution at -40

C within ca. 12 h.









Table 2-1. H and 13C NMR data for alkyne complexes
13C NMR, ppm H NMR, ppm Tc, K AG Kcal/mol
Complex R1 R2 C1 C2
3a CO2Me H 175 177 9.2 257 13.2
3b Ph Me 185 180 -- 268 13.4
3c SiMe3 Me 200 181 -- 268 13.4
3d Ph Ph 180 -- -- -- --
3e SiMe3 SiMe3 205 -- -- -- --
3f Me Me 187 -- -- -

Ph Ph


Me3Si, Me3Si R1
Me3S\.i Me3Sio NMO>/
N 1. 2.0 equivs RMgCI, -780C 20 R2
S CI
S2. Room Temp a. R1= Me R2=Me
4 R= ethyl, propyl, i-butyl b. Ri= Me R2= H
4 0c. R1= Ph R2= H
+ RH,
Ph + 2.0 equivs Cl-, Mg2+
/ 1.0 equiv alkyne

Me3Si /I
Me3Si,\ -Mo/ R3
Me3SN R3 a. R2=CO2Me R3= H
N b. R2= Ph R3=Me
c. R2=SiMe3 R3= Me
/ 21 R2 d. R2= Ph R3=Ph
e. R2=SiMe3 R3=SiMe3
f. R2= Me R3=Me

Scheme 2-1. Synthesis of alkyne complexes

A single crystal X-ray analysis was performed on a single crystal of 21e. This

crystal structure was originally reported by a previous graduate student, Tom Cameron,61

however, it is included here for the purposes of discussion. As shown in the thermal

ellipsoid plot in Figure 2-1, the alkyne complex adopts a pseudo square pyramidal

structure with the imido ligand occupying the apical position. The alkyne ligand is

oriented perpendicular to the Mo=N bond of the cis imido ligand and the Mo-C(19) and









Mo-C(20) bond lengths, 2.078 and 2.079 A respectively, are consistent with Mo-C

single bonds. The C(19)-C(20) bond length for 21e, 1.305 A, is close to the generally

accepted value for C-C double bonds. Pronounced back bonding is responsible for the

lengthening of this alkyne bond, and the structure of 21e is thus best described as having

a large contribution from a metallacyclopropene structure. In this resonance form, the

alkyne ligand can be considered as a dianionic ligand and the formal oxidation state of

the metal would be best described as Mo(VI).

In addition, the diamide ligand is folded through concerted torsion of the NR group

about the C-N bond of the pda ring. The fold angle (angle between the N(2) Mo N(3)

plane and the plane of the pda ring) is 1330. This folding has been attributed to donation

of the diamide lone pair electrons into the empty metal d2-y2 orbital.32'33'62

In solution, the alkyne complexes exhibit Cs symmetry just as in the solid state. In

complexes 21d-f, the alkyne substituents are the same, and a plane of symmetry bisecting

the C-C bond of the alkyne, containing the N=M bond of the imido ligand, and

bisecting the o-pda ring causes the SiMe3 protons to become chemically equivalent.

These protons appear as a broad peak at 0.4-0.6 ppm. However, in complexes 21a-c

where the alkyne substituents are different, there is no longer a plane of symmetry and

the SiMe3 protons are no longer equivalent. These protons appear as a broad singlet in

the 1H NMR spectra at room temperature. However, cooling a C7D8 solution to -500C,

results in the splitting of this peak into two singlets (Figure 2-2). Using the two site

exchange approximation,63 the activation barrier for this process has been measured to be

13.2 Kcal/mol. Interestingly, the barrier is the same regardless of the alkyne substituent

(Table 2-1).









This fluxional process can be explained by rotation of the alkyne fragment about

the Mo-alkyne centroid axis. This motion takes the alkyne ligand through a transition

state that is also of Cs symmetry (Scheme 2-2) where the alkyne is oriented parallel to the

imido ligand.


Figure 2-1. Thermal ellipsoid plot of 21e, [Mo(NPh)(q2-bis-trimethysilylacetylene) o-
(Me3SiN)2C6H4}], (40% probability thermal ellipsoids) selected bond lengths
(A): Mo-N(1), 1.745(2); Mo-N(2), 2.023 (2); Mo-N(3), 2.009 (2); Mo-
C(19), 2.078 (3); Mo-C(20), 2.079 (3); C(19)-C(20), 1.305, (4).










Ph
Me3Si N
Me3Si W,-NMo k R2


R3


Scheme 2-2. Alkyne rotation in 21
Ph
(B)Me3Si N
(A)Me3SiM\ Mo. R2

-20 R3
-20 OC


-8 C O


0.80 0.75 0.70 0.65 0.60 0.55 0.50 0.45 0.40 0.35 ppm
Figure 2-2. Variable temperature 1H NMR spectrum of the SiMe3 region from -20 to -
80C for complex 21b









Molecular orbital calculations. To elucidate the bonding pattern of the alkyne

fragment in these complexes, density functional theory calculations (DFT) were

performed. Initial calculations were carried out on model compound 22 (Figure 2-3),

which is a simplified version of 21 where the o-phenylene group (o-C6H4) that links the

two N atoms of the diamido ligand was simplified to a -CH=CH- carbon chain. The

organic groups on the nitrogen atoms (SiMe3 of the amido and phenyl group of the

imido) were replaced by hydrogen atoms for simplicity. Propyne was used as the alkyne

fragment in this model. The calculated structure for 22 is generally in good agreement

with the X-ray crystal structure data obtained for 21e and 21f.48 The only exception is

that the ligand is significantly less folded in this model than in the actual compounds

(1470 vs 133). This is due to the inability of the hydrogen atoms on the diamide ligand

to successfully model the bulky SiMe3 group. In order to obtain a more accurate model

for the alkyne complexes we employed the ONIOM approach (developed by Morokuma8-

12 et.al 9,10,11,12) to model the complete ligand system with all substituents used in the

experiment (Figure 2-3).

We defined a two-layer model, (model compound 23) for the 2-butyne complex

21f. The outer layer consisted of the entire complex, including the entire o-pda ligand,

and phenyl and SiMe3 substituents on the imido and diamide ligand respectively, and the

inner layer comprised of 22, with acetylene as the alkyne fragment. The inner layer was

modeled using the B3LYP64'65/LANL2DZ66'67 basis set, whilst the B3LYP/LANL2MB

basis set was used to model the substituents. As shown in Table 2-2 below there is

excellent agreement between the calculated structure 23, and the X-ray crystal structure

for 21f.








Table 2-2. Comparison of selected bond lengths and angles between model compounds
22 and 23 and the reported crystal structure for 21fb
Bond Lengths Xray data
and Angles 22 TS 22 23 TS 23 2f
and Angles 21fb
Mo-N(1) 1.755 1.798 1.776 1.818 1.745(2)
Mo-N(2) 2.018 2.053 2.003 2.005 2.009(2)
Mo-N(3) 2.018 2.037 2.020 1.991 2.022(2)
Mo-C(19) 2.087 2.089 2.086 2.087 2.078(3)
Mo-C(20) 2.071 2.079 2.083 2.075 2.079(3)
C(19)-C(20) 1.331 1.346 1.333 1.353 1.305(4)
Fold angle 1470 1410 1370 1380 1330
a. The fold angle is defined as the angle between the planes made by Mo, N(2),
N(3), and the plane defined by the benzenoid portion of the o-pda ring.
b. See reference 49 and 61 for crystallographic information.

.r f


22 TS 22








23 TS 23





Figure 2-3. B3LYP optimized models for alkyne complexes 22 and 23 and their
transition states

The transition states corresponding to alkyne rotation about the Mo -alkyne bond

in both 22 and 23 have been located. In both these transition states the alkyne ligand is

now oriented parallel to the Mo-N(imido) bond. This involves rotation of the alkyne

ligand in 22 and 23 by 900 about the Mo-alkyne bond. The activation energy obtained









from these calculations is 16.9 Kcal/mol for the rotation of 22 and 14.4 Kcal/mol for the

rotation of 23. The ONIOM model, in particular, compares favorably to the

experimentally obtained activation barrier of 13.2 Kcal/mol.

In order to better understand the nature of the bonding in these complexes, we

undertook a qualitative MO analysis and applied Weinhold's Natural bond orbital

(NBO)68-70 method to the complex. Scheme 2-3 shows the most important orbital

interactions between a metal L3M fragment and o-(SiMe3N)2C6H42- ligand assembled in

Cs symmetry, with a mirror plane directly bisecting the o-pda ligand and containing the

Mo-N(imido) bond. The frontier orbitals of a L3M fragment have been determined by

Albright et al.71 and are represented on by the left side of Scheme 2-3. The metal t2g

orbitals in this non-standard orientation are comprised of one orbital of a" symmetry

primarily dxy, and two orbitals of a' symmetry primarily dx2y2, and dz2. The metal eg

orbitals become a", primarily dyz, and a' primarily dxz. In the case of Mo, the three t2g

derived orbitals lie relatively high in energy and above the chelate's populated 7t* orbital.

The dx2-2 orbital is closest in energy to this orbital and is responsible for the folding of

the diamide ligand, seen in these complexes. The two filled orthogonal p, orbitals on the

imido nitrogen, destabilize the metal t2g orbitals of a' (dz2) and a"(dxy) symmetry.

Donor electrons on the alkyne alter the MO diagram in Scheme 2-3. These

electrons compete for empty metal orbitals with the imido ligand i.e. the alkyne 7'L

electrons compete with the imido for the metal dz2 orbital in the ground state and the dx

orbital in the transition state, resulting in a 3-center-4-electron bonding interaction

between the imido, the metal center and the alkyne ligand. As is typical for three center

interactions, three orbitals can be expected for the combination of the three fragments, i.e.








(a bonding, nonbonding, and antibonding combination of the orbitals from the 3

fragments). The bonding combination for the interaction of the alkyne, the imido

nitrogen lone pair and the metal dz2 orbital is depicted in Figure 2-4.
z


NPh a" a NPh



a'


t2g


SNPhaNpPh NP a
a' a' a"ll


'_--i-Ph

L represents the (SiMe3N)2C6H42- a l "
_L )Ph
Scheme 2-3. The effects of the consideration of 7 contributions of the diamide and imido
ligands (right side) on a typical ML3 fragment (left side). In this non-standard orientation
the metal d orbitals have a different composition than that usually used for octahedral
complexes. This is due to the coordinate system shown in the top right corner of this
figure. In a pure ML3 fragment the z-axis coincides with a threefold rotational axis of an
ocatahedron. The atomic composition of of the metal d orbitals, are mixed so that the
orbitals are reorientated to lay between the M-L bonds. For example the a' component
of the eg set becomes 11/3(x2y2) + '/2/3 yz. For full details on the atomic composition of a
pure ML3 fragment see. Albright et al.71









Ph Ph





Me3Si N-,,,,, M,,Si- N,,.
Me3Si NN MeSi N



21 TS 21
Figure 2-4. Bonding interactions in between Mo, imido, and the alkyne fragment

NBO population analysis. More detailed information about the alkyne-metal '7

interactions can be obtained from an NBO population analysis. The NBO68 program

diagonalizes the one and two center blocks of the first order reduced density matrix in

such a way that natural bonds are obtained that are said to represent the best Lewis

structure of a molecule. This involves a sequence of transformations from a given basis

set to various localized sets: natural atomic orbitals (NAOs), natural hybrid orbitals

(NHOs), natural bond orbitals (NBOs), and natural localized molecular orbitals

(NLMOs). The NLMOs can then be transformed to occupied MOs. Delocalization

effects appear as weakly occupied antibonding or Rydberg orbitals.

given basis sets NAOs NHOs NLMOs

Lewis structures obtained from NBO analyses accounted for approximately 97% of

the electron density. As shown in Table 2- 4, the NLMO analysis can be used to quantify

the delocalization of the alkyne 1tL electrons. In the transition states TS 22 and TS 23, a

greater proportion of the electron density of the parent NBO of the alkyne C-C 7 bond









is delocalized onto molybdenum. In addition, the occupancy of this bond is also lower in

the transition state.

Table 2-3. NLMO analysis of alkyne '7 bond
Model %7r C-C % Mo Occupancy
22 86.9 9.2 1.738
TS22 80.1 13.8 1.660
23 86.9 8.5 1.738
TS23 78.9 14.0 1.640

Table 2-4, shows the population analysis of the Mo-imido bond. The bond type,

percent contribution from each atom and occupancy of each chemical bond are presented.

The Mo=N(imido) bond in model complexes 22 and 23, where the alkyne is aligned in an

orthogonal orientation, consists of a o-bond and two 7t-bonds as would be expected for a

metal-nitrogen triple bond. All bonds are substantially polarized towards nitrogen with

contributions from nitrogen (68-77%) and (22-32%) from molybdenum.

Table 2-4. NBO analysis of Mo-N(imido) bonds
Model Bond Type %Mo %N Occupancy Covalent Bond order
22 C 22.9 77.1 1.94
7i 27.3 72.7 1.83 0.960
72 31.7 68.3 1.90
23 C 26.2 73.8 1.97
7i 22.1 77.9 1.76 0.949
7T2 31.6 68.4 1.83
TS 22 c 37.9 62.1 1.97
7i 39.3 60.7 1.92 0.928
na 6.4 89.5 1.80
TS 23 c 37.1 62.9 1.92
7i 36.4 63.6 1.88 0.868
na 6.7 86.5 1.74
a n, lone pair b Atom-Atom overlap-weighted NAO bond order

However, this bond in transition state structures TS 22, and TS 23, consists of one

o-bond and one 7t-bond, the third 7t-bond is significantly localized on the nitrogen atom

(N contribution 87-90%) and is best described as a lone pair with some delocalization









onto the molybdenum center (6.4-6.8%). This data, in addition to the lower covalent

bond orders of the Mo-N bonds for model complexes 22 and 23, (Table 2-4) suggests a

weakening of the Mo-N(imido) bonds in the transition state.

The NLMOs of 22 and TS 22 are represented pictorially in Figure 2-5. The larger

contributions of the Mo d-7T orbital in the transition state TS 22 is evident in Figure 2-5h

versus the significantly smaller contributions from these orbitals in Figure 2-5d. Also the

localization of the third Mo-N(imido) 7t bond is evident in Figure 2-5g.

Thus, it is only in the transition state that the alkyne i7 electrons compete effectively

for metal d orbitals. This is evidently a higher energy process, as the stronger metal

nitrogen 7 bonds are replaced by weaker metal carbon i7 bonds, leading to the observed

activation barrier associated with alkyne rotation.









(a) (b) (c) (d)










(e) (f) (g) (h)
Figure 2-5. NLMO plots (isocontour 0.04) of Mo-N(imido) bond in 22 (a-c): a =
imido-Mo dz2 (o), b = imido-Mo (dz2, dyz hybrid, i7), c = imido-Mo
(dxy, 7r) and TS 22 (e-g): e = imido-Mo dz2 (0), f= imido-Mo (dxy, 7r), g
= imido lone pair. Plot d = C-C r bond -Mo(dz2, dyz hybrid) in 22, h = C-
C 7i bond -Mo (dxz, dxy hybrid) in TS 22.









Summary

We have demonstrated the synthesis of novel high oxidation state molybdenum

imido alkyne complexes that show significant interactions between the alkyne 7'

electrons and the metal center. The implications of this interaction on the structure and

dynamics of these complexes have been explored via DFT (B3LYP) and NBO

calculations. These calculations reveal that the alkyne 7t donation in these complexes

occurs at the expense of Mo-N(imido) bonding. This is an unfavorable interaction since

the stronger Mo-N bonds, are replaced by weaker Mo-C bonds. The perpendicular

orientation of the alkyne ligand in the ground state of these complexes arises as the

molecule minimizes the alkyne to Mo 7t donation and maximizes the imido to Mo 7t

donation. It is clear that the interplay of 7T donor ligands in this class of compounds plays

a crucial role in determining their properties and suggests that a judicious choice of the

7 donors may offer a way to modify the chemistry of these compounds in a desirable

fashion.

Dynamics and Bonding of Molybdenum Imido Diamido Olefin Complexes

The chemistry of group 6 metal imido diamido complexes has resulted in the

isolation and characterization of a number of olefin complexes of both W and Mo.32,46

These olefin complexes are formed when a dialkyl complex containing 3 hydrogen atoms

undergoes 3 hydrogen transfer and the loss of one molecule of an alkane. In the case of

W, P hydrogen transfer is induced by the addition of a Lewis base (Scheme 2-4) whilst,

in the case of Mo spontaneous 3 hydrogen transfer occurs at room temperature.

Crystal structures of olefin complexes of both W and Mo have been reported.72'73

Like the alkyne complexes they are characterized by a significant amount of 7T back









bonding, a fact that is manifest for these complexes in both the solid-state structural

features, and in solution. For example, in the crystal structure of the styrene complex of

Mo, (Mo(NPh)(l2-styrene)(o-(Me3SiN)2C6H4), 20c, a long C-C bond (1.46A) is

observed for the styrene ligand. Further, in both the 1H and 13C NMR spectra of this

compound, the resonances for the styrene ligand are shifted significantly upfield,

suggestive of a metallacyclopropane like structure for these olefin complexes.

Ph
Ph
N 1.2.0 equivs RMgCI, -78OC N
Me3Si / 2. Room TempMei
Me3S \ i W 3. R = Et, Phenethyl M3
N \I Me3Si N

/ 5 CI R
/ 10g and
S10h

Ph 2.0 equivs PMe3
Room Temp
N
Me3P .

Me3Si" -

/ PMe3 + RH

SiMe3 19a, R = H; 19b, R = Ph
/SiMe3

Scheme 2-4. Synthesis ofW olefin complexes

Unlike the alkyne complexes, olefin rotation is much slower and is not observed on

the NMR timescale. Complex 20c in particular exists in solution as two isomers; the

phenyl substituents of one isomer are anti to the imido ligand and while this substituent

in the other isomer is syn to the imido ligand. No intercoversion of these two isomers

were observed even when a C6D6 solution of 20c was heated at 80 OC.74









In order to investigate the dynamics of olefin rotation in these complexes, ONIOM

calculations were performed. For these calculations a two-layer oniom system was

employed as has been described earlier in this chapter (Figure 2-6). The core of these

calculations consisted of the -CH=CH- carbon chain replacing the o-phenylene group (o-

C6H4) and with hydrogens in place of the organic groups on the nitrogen atoms (SiMe3 of

the amido and phenyl group of the imido) as described earlier. Ethylene was used as the

olefin fragment in this model. This layer was optimized within the ONIOM program

using the B3LYP/LANL2DZ method. The outer layer consisted of the full system and

was modeled with the computationally less expensive B3LYP/LANL2MB basis set.

There was good agreement between the calculated structure for 20c and the experimental

X-ray structure of this compound.

Using this method we were able to calculate an activation barrier of for olefin

rotation of 25.4 Kcal/mol. This is consistent with the experimental observation that

olefin rotation is slow on the NMR timescale yet rapid enough on the chemical timescale

to give two isomers as the product of the reaction. The energy barrier can also be

explained by examining the molecular orbitals, and revisiting the molecular orbital

diagram of Scheme 2-3 (Shown again below).

Recall that the fragment orbitals L3M fragment are comprised of a t2g set that has

one a symmetry (dxy) orbital and two orbitals of a symmetry dx2y2 and dz2, whilst the eg

set are composed of a (dyz) orbital and a (dxz) orbital. As the alkyne or olefin fragment

approaches the L3M fragment perpendicular to the M-N bond of the imido ligand the

a'"(dx and dxz) orbitals mix, one linear combination of these orbitals directs the lobes of

the metal orbital towards the 7t* orbital of the incoming ligand, the other linear









combination directs the lobes of the d orbitals towards the imido ligand. The olefinic

fragment is thus stabilized by 7t back bonding from the metal a" orbitals to the 7t* orbital

of the fragment. This is clearly seen by examining the HOMO of 23 and 20c (Figure 2-

7).
z

NPh a" a' Nr Ph



a'


t2g


NPh NPh NPh a'
a' a' a"


the ha

L represents the (SiMe3N)2C6H42- a"
_L Ph
Scheme 2-3. The effects of the consideration of 7: contributions of the diamide and imido
ligands (right side) on a typical ML3 fragment (left side). In this non-standard
orientation the metal d orbitals have a different composition than that usually
used for octahedral complexes. This is due to the coordinate system shown in
the top right corer of this figure. In a pure ML3 fragment the z-axis coincides
with a threefold rotational axis of an ocatahedron. The atomic composition of
of the metal d orbitals, are mixed so that the orbitals are reorientated to lay
between the M-L bonds. For example the a' component of the eg set
becomes 11/3(x2_y2) + /2/3 yz. For full details on the atomic composition of a
pure ML3 fragment see. Albright et al.1

















core


20C anti


TS 20C


Figure 2-6. Optimized ONIOM structures for 20c


Me3Si,
Me3Si


Me3Si,
Me3Si


Figure 2-7. HOMO of 23 and 20c showing 7 backbonding of the metal fragment to the
olefin in 23 or acetylene in 20c









Rotation of the olefinic fragment by 900 results in the symmetry of the 7r* orbital of

this fragment changing from a" to a' as it is moved into the mirror plane of the molecule.

This orbital can now interact with the metal d orbitals of a' symmetry, specifically a

linear combination of the dz2 and dyz orbitals. Recall however, that the imido lone pair

electrons were donated into the dz2 orbital. Imido-Mo lone pair donation in this case is

made energetically unfavorable by the presence of d electrons in this orbital. This is

clearly seen when the HOMO of TS 20c and TS 23 are examined (Figure 2-8). The

imido lone pair is clearly non-bonding in TS 20c and TS 23.


Ph

Nl


Me3Si,
Me3Si


TS 20c


Me3Si,
Me3Si.


SN(


s2


TS 23


Figure 2-8. HOMO of TS23 and TS20c showing 7t backbonding of the metal fragment to
the olefin.









The weakening of the Mo-N(Imido) bonds is confirmed by an NBO

analysis(Table 2-5). The covalent bond orders for the imido bond are clearly smaller in

the TS. Further, the NBO representation of the imido bond reveals that the delocalization

of 72 onto Mo is much less in the TS(7.3), than in 6(18.2), i.e., the lone pair on the imido

ligand is more greatly polarized toward the nitrogen atom of the imido ligand when the

olefin is oriented parallel to the imido ligand (85.7%) than when the olefin is oriented

perpendicular to the imido ligand.

Table 2-5. NBO analysis of the Mo-N(Imido) bond in 20c
Model Bond Type %Mo %N Occupancy Covalent Bond order
20c C 34.1 66.0 1.96
7i 35.0 65.3 1.90 0.99
72 18.2 76.2 1.64
TS20c cy 35.2 64.8 1.88
7r 30.6 69.4 1.79 0.88
72 7.3 85.7 1.73

These results demonstrate that cis perpendicular orientation of 7T ligands is the most

stable in these systems as this orientation maximizes the stabilization of the metal center

arising from the imido ligand. Orientation of these ligands parallel to this bond

generates interactions that result in the weakening of the Mo-Imido bond.

Diamide Ligand Folding in d2 vs do Group 6 Imido Complexes

Computational and structural studies on do diamide imido complexes have

demonstrated the importance of diamide lone pair donation in the bonding in these

complexes. In these structures, the diamide lone pair electrons are donated to an empty

metal d orbital of appropriate symmetry through concerted torsion of the sp2 nitrogens in

order to achieve effective lone pair p-d overlap. This torsion results in the folding of

the o-pda ligand as depicted in Table 2-6.









It is evident from Table 2-6 that there is a distinct correlation between the fold

angle of the diamide ligand and the oxidation state of the metal. In the do dialkyl

complexes like the metallcyclopentane complex 25, the diamide ligand is considerably

folded. As shown in Table 2-6, in the alkyne complex 21f, and the alkene complex 20c,

the diamide ligand folds to an extent that is comparable to 25. This occurs because there

is significant 7t back bonding in these complexes, and they are best described as do

metallacyclopropene and metallcyclopropane complexes respectively videe supra).

The folding of the diamide ligand is minimized in the d2 complexes,

[cis-(pyridine)2Mo(NPh)-o-(Me3SiN)2C6H4}, 44,

[cis-(2,6-dimethylphenyl isocyanide)2Mo(NPh)-o-(Me3SiN)2C6H4}, 30b, and

[rq4-(butadiene)Mo(NPh)-o-(Me3SiN)2C6H4}, 24. The synthesis and characterization of

24 and 44 has been reported whilst the synthesis of 30b is discussed in chapter 4.

However, it is clear from Table 2-6, electronic properties of the metal prevent diamide

folding in complexes in the Mo(IV) oxidation state.30'31'48'49'75'76

Table 2-6. Fold angles for some Mo Imido Diamido Complexes
Fold
Molecule Angl
Angle
[rq2-(2-butyne)Mo(NPh)-o-(Me3SiN)2C6H4}, 21f 133.0
[rq2-(styrene)Mo(NPh)-o-(Me3SiN)2C6H4}, 20c 129.2
Mo(NPh)(o-(Me3 SiN)2C6H4)(CH2)4, 25 132.5
[rq4-(butadiene)Mo(NPh)-o-(Me3SiN)2C6H4}, 24 167.8
[cis-(pyridine)2Mo(NPh)-o-(Me3SiN)2C6H4}, 44 175.5
[cis-(2,6-dimethylphenyl isocyanide)2Mo(NPh)-o-(Me3SiN)2C6H4}, 30b 173.7

In order to investigate the electronic origin of this phenomenon we compared the

molecular orbitals of the alkyne complex 23 with the model butadiene complex 24. The

model compound 24 was again optimized using the ONIOM method as described earlier.









There was good agreement between the calculated metric parameters of 24 and the

reported X-ray crystal structure of 24.48

The structure and bonding of cis-butadiene complexes can vary between two

extremes, the 7T2 and the o2, i7-designations (Scheme 2-6). The bonding in 7T2 complexes

is best described by a synergistic model i.e., the ligand acts as a o donor, as well as a 7t

acceptor, as the metal donates a pair of d electrons in the ligand's 7t* orbital. In o2,

rT-type complexes, the butadiene ligand is considered a dianionic dialkyl, a result of

considerable back bonding. The oxidation state of the transition metal, as well as, the

ancillary ligands of the complex, dictates which structure type will be adopted. In the

Boncella labs butadiene complexes of both W and Mo have been synthesized. The

butadiene complex of Mo, 24, may be best described as 72 d2 Mo(IV) complex. In

contrast, the corresponding complex with W may be described with a o2, '7 designation.

This is attributable to the greater tendency of 5d transition metals to 7t back-bond.

Sigand Fold Angleh


/ eM3Si ....
/M =Si N--- oM




/ \I\ 9 = Ligand Fold Angleo


Scheme 2-5. Fold angle in group 6 diamido complexes
















2 2
Scheme 2-6. The 7T2 and o2, 7 limiting structures for cis-butadiene complexes

Figure 2-9 shows the molecular orbitals for the HOMO and HOMO-2 for 24.

Unlike the do complexes where the diamide lone pair was donated into the dx2y2 orbital,

this is not possible in this case since this orbital is occupied and is used in 7 back bonding

to the butadiene ligand (HOMO-2). The diamide lone pairs are of the correct symmetry

to interact with the dz2 orbital; this is evident in the HOMO. The dz2 orbital is raised in

energy by donation from one of the imido lone pair electrons. Therefore, donation by the

diamide lone pair electrons into this orbital results in a competition for the available dz2

orbital between the diamide and the imido ligand. This would result in a weakening of

the Mo-N(Imido) bond in favor ofMo-N (diamide) bonding. The imido ligand

however is a much stronger 7t donor ligand, thus the Mo-N(Imido) bonding is favored

over Mo-N(diamide) bonding. This is clearly evident from the NLMO analysis (Table

2-7). The diamide lone pairs ni and n2 are only slightly delocalized unto Mo (1.81 and

0.99% respectively). In contrast, the delocalization of these lone pair electrons onto Mo

in 21 is significantly greater (8.2 and 7.4% respectively). The Mo-N(Imido) bond is

slightly weaker in 24 (bond order 0.93) than in 23 (0.95). However the bonds in the

imido bond are polarized towards nitrogen to a greater extent than in the do alkyne

complex.










Table 2-7.NLMO
Model
24
T. .1 *


analysis of Mo-


Bond Type


imiao a
7i1
n
Diamide nl
n2
23
Imido CY
7il
712
Diamide nl
n2


-N(Imido) and Mo-N(Diamide) bonds in 23 and 24


%Mo %N


19.3
19.7
16.5
1.81
0.99

26.2
22.1
31.6
8.2
7.4


80.7
80.3
78.6
81.4
81.8

73.8
77.9
68.4
82.7
83.3


Occupancy

1.88
1.82
1.64
1.64
1.64

1.97
1.76
1.83
1.67
1.68


Covalent Bond order


0.93


0.50



0.95


0.50


Me3Si, Ph


HOMO 2


Me3Si


Ph N


HOMO
Figure 2-9. Occupied molecular orbitals of 24









Conclusions

The results presented in this chapter clearly demonstrate the importance of 7 donor

interactions by both the diamide and imido ligands in our complexes. These interactions

contribute greatly to the stability of our complexes in that they stabilize the highly

electropositive metal centers. Competition for available metal d orbitals can lead to

reactivity of these complexes and as is demonstrated in later chapters, this reactivity may

either be metal centered or ligand centered. These data suggest that the reactivity of these

complexes can be carefully modulated by the judicious choice of ancillary ligands.














CHAPTER 3
SYNTHESIS AND REACTIVITY OF GROUP 6 IMIDO DIAMIDO
METALLACYCLES

Introduction

Metallacyclopentane complexes were originally observed in our labs as species that

result from the deactivation of W(VI) alkylidenes in olefin metathesis reactions.34 Since

these initial studies we have been able to successfully isolate and characterize

metallacyclopentane complexes of W and Mo by more direct methods.34'46'74 Group 6

imido metallacyclopentane complexes are quite rare, and our complexes are unusual in

that their inherent stability towards decomposition by either 3 hydrogen

elimination/abstraction or 3 carbon-carbon bond cleavage allows them to be readily

isolated and characterized.

Schrock and co-workers have also observed group 6 imido metallacyclopentane

complexes that resulted from the decompostion of alkylidene species. 73,77 These

complexes however lacked the inherent stability of those seen in our group and

decompose via 3 hydrogen transfer resulting in the formation of butene. The

fundamental difference in our complexes and the complexes studied in the Schrock group

is the chelating ancillary ligand. As discussed in the previous chapter, the diamide

ancillary ligand, {o-(Me3SiN)2C6H4}2-, is crucial in stabilizing the high oxidation state of

the metal by 7r donation of the diamide lone pairs. The stabilization via 7r donation is not

afforded by the alkoxide ligands in Schrock's metathesis catalysts, as they possess









electron-withdrawing substituents on the alkoxide ligand in addition to the fact that the

more electronegative O atom is less likely to donate its lone pair electrons.35'36'43 73,78,79

In this chapter, we examine the chemistry of the metallacyclopentane complex,

Mo(NPh)(o-(Me3SiN)2C6H4)(CH2)4, 25. We begin examining the synthesis of

metallacyclopentene complexes by the sequential '7 ligand exchange of ethylene from the

metallacyclopentane complex with an alkyne. We also examine the synthesis of

metallacyclopentadiene complexes by the [2+2] cycloaddition reactions of two alkyne

moieties. The thermal rearrangements of 25, are also examined by kinetics and DFT

using small models as well as larger (experimentally exact) systems with quantum

(B3LYP) and hybrid methods (ONIOM), respectively. We examine the role that diamide

7T donation plays in influencing the reactivity of these complexes.

Synthesis and Reactivity of Mo Imido Diamido Metallacyclopentenes and
Metallacyclopentadienes

Transition metal metallacycles have been implicated as important species in many

catalytic and stoichiometric conversions of organic fragments. One such process is the

metal mediated cyclooligomerization reaction of alkynes. In the well-known mechanism

for this reaction, metallacyclopentenes, metallacyclopentadienes, as well as alkyne and

arene complexes have been cited as key intermediates.80'55 Recent research in this field

has focused on stereoselectively controlling the products of the cyclooligimerization

reaction. This has been achieved by developing ligand sets that promote highly selective

carbon-carbon bond forming reactions in which low valent transition metals mediate the

formation of metallacycles from saturated organic substrates. Takahashi,72,81-84 and

Ladipo85'86 have utilized this chemistry in the synthesis of substituted arenes, pyridines,

and other useful organic molecules. In Takahashi's work, the preparation of









multisubstituted benzene and pyridine derivatives was achieved in a one pot synthesis by

the use of unsymmetrical metallacyclopentadienes obtained from the intermolecular cross

coupling reaction of two different alkynes, and the subsequent treatment with a third

alkyne, or nitrile, in the presence of CuCl or Ni(PPh3)2C12. The development of

methodologies that would afford the stereoselective synthesis of metallacycles is an

important area of research and warrants further study.

A method for the synthesis of olefin complexes, from Mo(NPh)(o-

(Me3SiN)2C6H4)C12(THF) 4, was recently developed in our labs30'31 and the tendency of

these complexes to form metallacycles has been demonstrated.34'46 In exploring the

reactivity of these complexes, we became interested in developing the chemistry of group

6 metallacycles containing imido and bis-amido chelating ligands because of the potential

importance of these compounds in organic synthesis.87 81,82,84,88

Synthesis of metallacyclopentene complexes

Heating a toluene solution of the metallacyclopentane complex 25 at 800C with one

equiv of an alkyne results in the synthesis of the metallacyclopentene complex 26

(Scheme 3-1). The reaction with phenyl acetylene results in a 1:4 mixture of the two

regioisomers 26a and 26b. The phenyl substituent in the metallacycle in 26a is in the ac

position whilst the phenyl substituent in 26b is in the 3 position.

The equilibrium between metallacyclopentanes and bis-ethylene complexes has

been extensively studied.89-92 It has been demonstrated in our labs that heating the

metallacyclopentane complex 25, in the presence of a Lewis base (PMe3) results in the

formation of the ethylene complex 27 as a phosphine adduct (Scheme 3-2).34,46 These









results demonstrate that 25, is a precusor to the ethylene complex 25a, and that this

complex can be trapped in the presence of Lewis bases.

Ph
Ph
N
Me3Si Me3Si
Me3SMR R2 Me3Sii Mo
R1 -ethylene 2 Me3i


25 800C





26a. R1= Ph R2= H
26b. R2 = H R2= Ph
26c. R1= CO2Et R2= CO2Et
Scheme 3-1. Synthesis of metallacyclopentene complexes

Complex 25a, can also be generated from the dichloride complex 1, and ethyl

magnesium chloride as outlined in Scheme 3-3. Again this ethylene complex can be

trapped in the presence of donor ligands such as PMe3 and acetylenes. This strategy was

employed to synthesize 26b as a single isomer. The resonances for the metallacycle

fragment for this compound were observed as eight line patterns at 3.70, 3.38(2H

overlapping) and 1.35ppm in the 1H NMR spectrum. The vinylic proton from the

acetylene ligand was observed downfield at 8.47ppm. In addition two peaks were

observed for the SiMe3 resonances at 0.43 and 0.40ppm.









Ph Ph

N N
Me3Si Me3Si
MeSi Mo toluene, 800C Me3Si M H
N N H
S- ethylene H
25a H
25



PMe3
Ph /

Me3P N

Me3Si N /Mo

N I\PMe3


SiMe3
S27
Scheme 3-2. Lewis base generation of olefin bis-PMe3 olefin complex 27

The strategy employed in Scheme 3-3 takes advantage of the steric environment

around the metal central center to promote the stereoselective formation of the

metallacycle. Phenyl acetylene adds to 25a, in a manner that minimizes interaction

between the phenyl substituent and the bulky SiMe3 group. This results in the phenyl

substituent of the acetylene ligand bonding preferentially 3 to the Mo center.

Similarly, complex 26a can be isolated as a single isomer by taking advantage of

the steric environment around the metal center. As outlined in Scheme 3-4, exposure of

the phenyl acetylene complex, 28, generated in situ, from the isobutylene complex 20a,

to ethylene results in the isolation of 26a. The resonances for the metallacycle fragment

of 26a are observed at 3.31, 3.06, 3.98 and 1.34 ppm in the 1H NMR spectrum. The








vinylic proton is observed as a broad singlet at 6.87ppm. Resonances for the SiMe3

protons are observed as singlets at -0.06 and 0.32ppm.

Ph Ph
N N

Me3Si M 2 eqvs EtMgCI Me3Si,\ Mo H
N 0 N /H
0 \ 25a H

4 + Ethane

Ph 2.0 eqvs Cl-,
/ Phenyl acetylene Mg2+
Me3Si X
Me3Si\,^Mo
NMe3Si + ethylene

26b Ph


Scheme 3-3. Synthesis of 26b

X-ray crystal structure of 26b. The identity of 26b was also confirmed by X-ray

diffraction analysis. X-ray quality crystals were obtained by slow evaporation of a

diethyl ether solution of 26b. Figure 3-1, shows the thermal ellipsoid plot of 26b.

Complex 26b, exhibits a pseudo square pyramidal structure with the imido ligand

occupying the apical position of the square pyramid. The Mo-N(1) bond length of

1.731(2) A, is comparable to Mo-N lengths in similar complexes. The bond lengths in

the metallacycle confirm the predicted arrangement of double and single bonds i.e the

arrangement of two long C-C bonds (C(20)-C(21) 1.504(3) A, and C(21)-C(22)

1.531(3) A), and one short C-C bond (C(19)-C(20) 1.344(3) A) confirms the








metallacyclopentene description for this complex. As described for do dialkyl complexes

of this type, the diamide ligand is substantially folded (fold angle =135.60) and as

discussed in the previous chapter, this has been ascribed to donation of the diamide lone

pair electrons into a metal d orbital of appropriate symmetry.

Ph
/ Ph
N /
Me3Si /\ N
Me3S Me3Si
Me3Si ,;Mo,. 2 eqvs BuMgCI, -780C Me3
N b '- Me3Si ,_Mo Ph
S Cl Phenyl acetylene N
0 28
0 H

Ph
N ethylene
Me3Si / + isobutane,
Me3Si N.Mo isobutylene,
S Ph 2.0 eqvs, Cl, Mg2

26a

Scheme 3-4. Synthesis of 26a

Kinetics and Mechanism of the Thermal Rerrangement of 25

The observation that heating the metallacyclopentane complex, 25, can induce 3

C-C bond cleavage of the metallacycle and the elimination of ethylene is consistent

with the observation that metallacyclopentane complexes may exist in equilibrium with a

bis-ethylene species as depicted in Scheme 3-5.

























C22


C19 C20


Figure 3-1. Thermal ellipsoid plot of 26b (50% probability thermal ellipsoids), selected
bond lengths (A): Mo-N (1) 1.7305, Mo-C (19) 2.1520 (2), Mo-C (22)
2.175 (2), C (19)-C (20) 1.344 (3), C (20)-C (21) 1.504(3), C (21)-C (22)
1.531(3).




Mo Mo

(VI) (IV)

Scheme 3-5. Equilibrium between metallacyclopentane species and bis-ethylene species

The reaction depicted in Scheme 3-5 represents a formal reduction of the Mo center

by two electrons as 3 C-C bond cleavage reduces the metal center from Mo(VI) to









Mo(IV). The synthesis of complexes 26 and 27 in Schemes 3-2 to 3-4, suggest that heat

is required in order to induce 3 C-C bond cleavage of the metallacyclopentane. This

implies that the equilibrium depicted in Scheme 3-5 lies largely towards the

metallacyclopentane species. In order to confirm the existence of this equilibrium, and to

clarify the mechanism of 7T ligand exchange in these metallacyclopentane complexes, we

undertook kinetic studies for the reactions of 25 with diethyl acetylene dicarboxylate

(DEAD).

The kinetics of the thermal disruption of the metallacycle of 25, in C7D8 were

examined by following the disappearance of the SiMe3 peaks of the starting material

using H NMR spectroscopy in the presence of excess DEAD. The data points were

obtained by plotting the value of the intergral for this peak with respect to time for more

than three half-lives.

Activation parameters for the conversion of 25 to 26c were determined by reacting

C7Ds solutions (0.020M) of 25 with DEAD (1.40M) in the NMR probe at temperatures

between 335 and 356K (three samples at each temperature). The disappearance of 25

follows first order kinetics (Figure 3-2) with a rate constant at 342K of k = 3.3 x10-4 s-1

(t/2 = 2.1 x103 s). The reaction rate is independent of the concentration of DEAD (Table

3-1). The energy of activation, AG corresponds tol7.5 Kcal/mol at 342K. Activation

parameters (Figure 3-3) obtained from an Eyring plot( AS = R[(intercept) -23.76]; AHW

=-R(slope)), for the formation of 26c from 25 are AH = 20.5(1.0) Kcal/mol and AS = -

14.7(3.0) cal (molK)1.










2.5


2-
349K
356K
1.5 -

342K



0.5 335K



0 1000 2000 3000 4000 5000 6000 7000 8000
Time (s)

Figure 3-2. First order kinetics for the formation of 26c


Table 3-1. Dependence of DEAD on the formation of 26c
[DEAD]/M Rate k(sec-) X 104
0.974 3.19(4)
1.44 2.86(8)
1.99 2.98(3)
2.60 2.98(11)

The observation that the formation of 26c does not depend on the concentration of

substrate is consistent with the proposed decomposition of the metallacyclopentane

fragment via 3 C-C bond cleavage to a bis-ethylene species pior to the rate determining

step. The relatively large positive activation barrier AHT, and a negative entropy of

activation, AS is consistent with a multi-step mechanism, where the

metallacyclopentane rearranges and the alkyne binds to this new species (bis-ethylene)

prior to the loss of ethylene. The observed value for AS is a combination of the

metallacycle rearrangement steps and the binding of DEAD to the activated complex.













-12.5 AM+ = zu +/- I .u caiimoi
ASt = -14.7 +/- 3.0 cal/molK
-130


"" -135
-135 -

-14.0 -


-14.5 -.


-150 -,,,
2.8121e-3 2.B62e-3 2.9206e-3 2.9886e-3

1/T sec-1
Figure 3-3. Eyring plot for the reaction of 25 with DEAD

Computational Studies on the Thermal Rearrangement of 25

The synthesis of the metallacyclopentane complex 25, has been reported, however,

X-ray structural studies on this compound were not performed at that time. We were

interested in the solid-state structure of this complex and an X-ray study was performed

on a single crystal of 25 grown from a concentrated pentane solution at -300C. The

thermal ellipsoid plot of 25 is depicted in Figure 3-4.

Complex 25, crystallizes with a pseudo square pyramidal structure with the imido

ligand occupying the apical position. The Mo-N(1) bond (1.728A) is within the normal

range for a Mo-N triple bond. The Mo-diamide bonds, Mo-N(2) and Mo-N(3),

(2.010 and 2.015A) are also within the normal range for Mo-C single bonds. The Mo-

C(13) and Mo-C(16) bonds (2.187A and 2.191A) are typical of Mo-N single bonds.


































Figure 3-4. Thermal ellipsoid plot of 25, (50% probability thermal ellipsoids), selected
bond lengths (A): Mo(1)-N(1) 1.728(2), Mo(1)-N(2) 2.010(2), Mo(1)-
N(3) 2.015(2), Mo(1)-C(13) 2.187(2), Mo(1)-C(16) 2.191(2)

Although the equilibrium between metallacyclopentanes and bis-ethylene species

has been observed in the past, we have not, until now, been able to confirm that such a

process occurs in our systems since no bis-ethylene species has been detected. We

therefore turned to computational chemistry in order to investigate the nature of any

intermediates that may form during the thermolysis of 25. For these studies we employed

small models as well as larger (experimentally exact) systems.

In the model system, the o-phenylene group (o-C6H4) that links the two N atoms of

the diamido ligand was simplified to a -CH=CH- carbon chain. The organic groups on

the nitrogen atoms (SiMe3 of the amido and phenyl group of the imido) were replaced by

hydrogen atoms for simplicity. For this model, the recently developed mPW1K









(modified Perdew-Wang 1 parameter for kinetics) exchange correlation functional of

Truhlar and coworkers was employed. This functional was recently shown to yield more

reliable barrier heights than other exchange-correlation functionals.93'94 62,95

Using this method, three minima were located on the potential energy surface for

rerrangement of the metallacycle fragment in 25 (Figure 3-5). Two of these minima (25b

and 25c) possess a pseudo trigonal bipyramidal geometry about Mo. One of the diamide

nitrogens is oriented trans to the imido ligand. The molecule 25b can best be described as

a trigonal bipyramidal (TBP) metallacyclopentane complex whilst complex 25c can best

be described as a trigonal bipyramidal (TBP) bis-olefin complex. We were also able to

locate the transition states corresponding to the transformations from 25-25b ( TS 25-

25b) and 25b-25c (TS 25b-25c). The relative free energies AGO 298 (relative to 25), for

these transformations are depicted in Figure 3-6.

In order to obtain a more accurate description of the minima during the thermal

rearrangement of 25, ONIOM calculations were performed on larger experimentally

exact systems. For these ONIOM calculations a two-layer system was employed, the

inner layer consisted of the model complexes described above in Figure 3-5 and were

modeled at the mPW1K/LANL2DZ level of theory. The outer layer consisted of the

complete system including all substituents on the diamide and imido ligands and was

modeled with the mPW1K/LANL2MB basis set. Optimized structures for the minima

obtained from these calculations are depicted in Figue 3-7. Unfortunately, all attempts at

attaining optimized structures for the transition states for these molecules using the

experimentally exact systems failed to converge once the large SiMe3 groups were

introduced. However, because we were able to successfully model these transition states









using the small model system and since we are only interested in a qualitative description

of the mechanism for 3 C-C bond cleavage in these systems the TS's obtained using the

small model system will suffice for the purposes of this discussion.


N1'
N3,


J c 6 N2 C 1
C6 25.5


8-72 NI
V"4 C C N25b N1



TS 25-25b TS 25b-25c

. Optimized structures (mPW1K/LANL2DZ) for the thermal rearrangement of
25


7 1 rN ilb

Figure 3-6. Reaction profile AGO 298 (K) for the thermal rearrangement of 25 at the
mPW k/SDD-Aug-cc-pVDZ level of theory


N3/














Figure 3-5















N Mc 0
C3J


C6 C2 2 2C5b


NN
N' Iv



C 52 25c
S12


Figure 3-7. ONIOM (mPW1K/LANL2DZ:mPW1K/LANL2MB) optimized structures for
the thermal rearrangement of 25

As shown in Table 3-2 there is good agreement between the calculated structures

and the experimentally obtained X-ray structure for 25. As expected, the ONIOM

calculations successfully reproduced the experimentally observed fold angle in 25. This

ligand folding is also seen in the TBP metallacyclopentane complex, 25b and the TBP

bis-ethylene complex 25c. In both TBP species the strong trans influence of the imido

ligand results in the lengthening of the Mo-N(2) diamide bond (2.118 and 2.174A for

25b and 25c respectively) relative to the Mo-N(1) bond (1.989 and 2.029A).

The C-C bond lengths of the ethylene fragment (1.401A and 1.423A) in 25c

reflect a significant amount of 7n back bonding from the metal to the x* orbital of the

olefinic fragment. However, these bond distances are shorter than the other olefin

complexes seen in our group.46,74 Further, the M-C (average distance from both

carbons of the r2 ethylene fragment) bond distances of 2.306 and 2.429A are

significantly longer that M-C(olefin) bonds seen in our group. Thus it appears that the









binding of the olefin to the TBP bis-ethylene complex 25c is weaker than other SP olefin

complexes seen in our labs. It seems reasonable then, that olefin loss from this molecule

will be feasible under the reaction conditions.

Table 3-2. Selected bond lengths (A) and angles (0) for ONIOM optimized structures for
the thermal rearrangement of 25
Bond 25 25(X-ray) 25c 25b
Mo-C(1) 2.213 2.187(2) 2.306* 2.159
Mo-C(4) 2.186 2.191(2) 2.429* 2.132
Mo-N(3) 1.763 1.728(2) 1.798 1.801
Mo-N(2) 2.041 2.015(2) 2.174 2.118
Mo-N(1) 2.032 2.010(2) 2.029 1.989
C(1)-C(2) 1.548 1.544(8) 1.401 1.568
C(2)-C(3) 1.541 1.496(2) NA 1.577
C(3)-C(4) 1.538 1.563(1) 1.423 1.565
Fold Angle 137.6 132.5 152.4 133.3
* Average M-C distance from both carbons of the r2 -ethylene fragment


Insight into the nature of the metal-carbon bonds within these complexes can be

obtained by inspection of the molecular orbitals. For the bis-ethylene complex 25c, the

metal d orbitals are depicted in Scheme 3-6. The in phase combination of the diamide

lone pair electrons is also depicted in this Scheme. This molecule possesses pseudo Cs

symmetry with a mirror plane that bisects the diamide chelate and the two ethylene

fragments.

The linear combination of diamide lone pairs is of a" symmetry. .These lone pair

electrons are of appropriate symmetry to interact with the dyz and dxz orbitals (see

coordinates in Scheme 3-6). However, the energy of these orbitals is perturbed by strong

donation from the imido (dxz) and the oy orbitals of the ethylene fragment (dyz). Thus the

overlap of the diamide lone pairs with the empty metal d orbitals will not be as significant

in this case.









Ph Ph
N N Ph
Ph
N N
Ph .

N


Metal a"


Metal a'

/ --Y

Z



I represents the chelating diamide ligand's in phase
combination of p orbitals


Scheme 3-6. Symmetry adapted orbitals of 25c.

The diminished importance of diamide 7t donation is clearly evident by inspection

of the occupied molecular orbitals. The highest two occupied molecular orbitals of the SP

metallacyclopentane complex 25, the TBP metallacyclopentane complex 25b, and the

TBP bis-ethylene complex 25c, are depicted in Figures 3-8, 3-9, and 3-10 respectively.

As shown in the HOMO -1 of Figure 3-8, the importance of diamide lone pair

donation in the SP metallacyclopentane complex 25, is clearly evident. As discussed in

the previous chapter this 7 donation is responsible for the folding of the diamide ligand

seen in do complexes.




















SHOMO WJ HOMO-1

Ph Ph

N N
Me3Si Me3Si
Me3Sii Me3Si








Figure 3-8. Occupied molecular orbitals(B3LYP/LANL2DZ) for 25

The diminished importance of this interaction is clearly evident in the HOMO and

HOMO -1 of the TBP metallacyclopentane complex 25b (Figure 3-9). The perturbations

to the a" orbitals (dxz and dyz see Scheme 3-6) by the imido and o orbitals of the

metallacyclopentane fragment raises the energy of these orbitals and results in a weaker

interaction between the diamide lone pair electrons and the metal d orbitals. The diamide

lone pairs remain primarily ligand centered in the HOMO and HOMO-1 orbitals. The

fact that the ligand folds quite significantly, (fold angle 133.30) suggests that this

interaction is not negligible in these complexes, however, the energetic of the interaction

are clearly more important in 25 than in 25b.


















HOMO HOMO-1



Ph
N N Ph

Me3Si Me3Si .




SiMe3 SiMe3

Figure 3-9. Occupied molecular orbitals (B3LYP/LANL2DZ) for 25b


















HOMO HOMO-1
Figure 3-10. Occupied molecular orbitals (B3LYP/LANL2DZ) for 25c

The diminished importance of diamide lone pair donation is also evident in the

HOMO of 25c. In addition the HOMO -2 shows the ethylene fragments stabilized by nt

back bonding from the metal to the a linear combination of the t* orbitals of two

ethylene fragments (see Scheme 3-6). As mentioned above, 25c shows lengthened Mo-









C (ethylene) bonds relative to other SP Mo olefin complexes synthesized in our group

and is suggestive of a weaker interaction between the metal and ethylene. Decreased 7t

donation by the diamide ligand in these complexes reduces the amount of electron

density on the metal that is available for '7 back bonding and thus there is a reduction in

the stabilization of the ethylene ligands in these complexes, in addition to the fact that

two ethylene molecules require electron density. Ethylene ligand exchange from 25c, is

therefore possible as the complex can now readily lose a molecule of ethylene as it is less

tightly bound in this complex.

These results demonstrate the importance of 7T ligand donation in stabilizing the

high oxidation state of Mo in these complexes. The metallacyclopentane complexes seen

in our labs are stable to decomposition by both P-hyrdogen elimination/ transfer reactions

and P C-C cleavage of the metallacyclopentane complexes. Rearrangement of the

metallcyclopentane from a SP structure to a TBP structure diminishes the M-

N(diamide) interactions; as a result 3 C-C bond cleavage is induced and the complex

can react via ethylene exchange.

It is noteworthy that these '7 ligand exchange reactions are significantly slower for

the reactions of the W analogue of 25 with oy and '7 donors. For example, the reaction of

the W metallacyclopentane complex W(NPh)(o-(Me3SiN)2C6H4)(CH2)4, 13, with PMe3

proceeds to only 90% completion in 90 days,34 this is slow in contrast to the facile

cleavage of metallacyclopentanes with PMe3(1 hr) for the Mo complex.46'74 These results

suggests that the barrier for the 3 C-C bond cleavage and the thermal rearrangement of

the W metallacyclopentanes is significantly higher.









Synthesis and reactivity of a metallacyclopentadiene complex.

Our success in synthesizing metallacyclopentenes inspired us to pursue the

subsequent synthesis of metallacyclopentadiene complexes. One can envision the

formation of metallacyclopentadienes from metallacyclopentenes by the sequential 7t

ligand exchange of an ethylene molecule from a metallacyclopentene as outlined in

Scheme 3-7. However, no 3 C-C bond cleavage of metallacyclopentenes can be

induced when 26 is heated at 800C in the presence an alkyne for weeks.



Mo ~ Mo Mo R2 Mo

R1 RP








Ph Ph


Mo H M


P )Ph
Scheme 3-7. Proposed formation of metallacyclopentadienes from metallacyclopentene
complexes. Ancillary ligands have been removed for clarity.
Metallacyclopentadienes can be synthesized however, from the [2+2] coupling

reactions of terminal acetylenes. Thus, treatment of a pentane solution of the isobutylene

complex, (rj2 -isobutylene)Mo(NPh)(o-(SiMe3N)2C6H4), 20a with two equivalents of

phenyl acetylene afforded the metallacyclopentadiene complex 29,

[(NPh)Mo(C(Ph)CHCHC(Ph)){o-(Me3SiN)2C6H4}] in 40% yield (Scheme 3-8). The 1H









NMR spectrum of 29, showed a singlet (-0.07ppm, 18H) for the SiMe3 protons. A sharp

singlet (6.46ppm, 2H) assigned to the 3 protons of the metallacycle characterizes the ca,a'

orientation of the phenyl substituents. An examination of the reaction mixture revealed

that small amounts of the a,3 and 3,3' isomers may also form during the course of this

reaction however we were not able to isolate any of these isomers from the reaction

mixture and the low yield of these complexes in this reaction prevented their complete

spectroscopic characterization.

Ph
Ph /

N /N
Me3Si Me3Si M

Me3Si ....... M. Me3Si. .

N N 29
phenyl acetylene

20a 2 equivs.



Scheme 3-8. Formation of the metallacyclopentadiene complex 29

Single crystals of 29, were obtained from slow evaporation of a concentrated

diethyl ether solution (Figure 3-11). Complex 29 was found to have a pseudo square

pyramidal geometry in which the imido group occupies an apical position.

The Mo-N(1) bond length is 1.731 A is consistent with a metal nitrogen triple bond.

The Mo-N(2) and Mo-N(3) bonds (2.008 and 2.004A) are consistent with

Mo-N(diamide) bonds. The Mo-C(22) and Mo-C(19) bonds are with range for

Mo-C(sp) bonds. The bond lengths within the metallacycle reflect the localization

expected for a metallacyclopentadiene fragment, i.e. two short C-C bonds









(C(19)-C(20) 1.335A and C(21)-C(22) 1.339A) and one long C-C bond

C(20)-C(21) 1.455A.


Figure 3-11. Thermal ellipsoid plot of 29, (50% probability thermal ellipsoids), selected
bond lengths (A): Mo-N (1), 1.731(2), Mo-N(2) 2.008(2), Mo-N(3)
2.002(2), Mo-C (19), 2.197(3), Mo-C (22), 2.188(3), C (19)-C (20),
1.335(4), C (20)-C (21), 1.455(4), C (21)-C (22), 1.339(4).

Metallacyclopentadiene complexes have been implicated as important

intermediates in the cyclotrimerization of acetylenes.81 78'85'86When 29 (0.008 mmol) was

allowed to react in an NMR tube with excess phenyl acetylene(1.56mmol) at 800C for

24hrs, a 50:50 mixture of two cyclotrimerized products, 1,2,4 triphenyl acetylene, and

1,3,5 triphenyl acetylene were produced (Scheme 3-9). We are currently investigating

the utility of 29 as a potential cyclotrimerization catalyst.
















Me3Si


-.."N '/ xs phenyl acetylene
N 29














1: 1 mixture of 2
isomers
Scheme 3-9. Cyclotrimerization reactions of 29

Summary and Conclusions

In the previous chapter the structural consequences of '7 ligand competition on Mo

imido diamido complexes were highlighted. We revisited this theme here and have

emphasized the importance of diamide donation in the stabilization of metallacyclopentane

species. Any structural pertubation of the molecule that leads to a reduction in the donor

ability of the diamide ligands can lead to increased reactivity. This suggests that more

active metallacyclopentane complexes may be synthesized by carefully modulating the

substituents on the diamide ligand so as to decrease its 7t donor ability.

The frontier molecular orbitals of the complexes seen in this chapter and the

previous one are all largely ligand centered. It should come as no surprise then that






69


reactivity at the ligand in these complexes may also be induced. This is the subject of the

next two chapters.














CHAPTER 4
SYNTHESIS AND REACTIVITY OF MO(VI) COMPLEXES WITH ALKYL AND
ARYL ISOCYANIDES

Introduction

In the previous chapters the importance of ligand '7 donation in stabilizing the high

oxidation state of the metals has been emphasized. We have shown that the competition

for available d orbitals on the metal results in interesting structural features as well as

reactivity. The highest occupied orbitals in these complexes are primarily ligand

centered; therefore, it should come as no surprise that reactivity at the ancillary ligands

may be induced. In this and the subsequent chapter, we examine the reactivity of the

diamide ligands with isocyanides, and in chapter 5, with alkyl aluminum reagents. We

also revisit here the concept of 7T loading but in this case with a d2 six coordinate

isocyanide complex.

Synthesis of Isocyanide Complexes and Insertion into the Metal-Diamide Bond

Compared to the well-known insertion of CO and isocyanides, RNC, into metal

alkyl bonds there are fewer examples of the insertion reaction into metal amide bonds.96-

99 The resulting metal-iminocarbamoyl derivatives that are formed are important in the

general context of amination of organic substrates.87 Alkyl isocyanides (RNC) can be

regarded as being isoelectronic with CO, and their increased ability to act as Lewis bases

makes them good candidates to interact with electrophillic metal centers.

Recent work in our group has focused on the use of the chelating disubstituted

phenylenediamide group {o-(Me3SiN)2C6H4}2- [(TMS)2pda]. The facile high yield









synthesis of several group(VI) dialkyl complexes of Mo and W has allowed us to

investigate the properties of these metal complexes and determine the influence the

ancillary ligands have on their structure, stability and reactivity.30'31 We have

demonstrated that 7t donation by these ligands is important in stabilizing the metal center

and influencing the reactivity of these complexes.32,48,49,100 Alkyl isocyanide insertion

into W alkyl bonds has been shown to occur readily at room temperature affording r2

imino-acyl complexes that subsequently react via C-C coupling reactions of the r2-

imino-acyl group resulting in the formation of diamide ligands.44

Until now we have not seen any evidence for isocyanide insertion into the metal

diamide bonds. Isocyanide insertion has been shown to occur preferentially at metal

alkyl bonds as opposed to metal amide bonds presumably because of the stronger metal

amide bond.77'83 In this chapter, we report the synthesis and characterization of bis-

isocyanide complexes of Mo and the subsequent reactivity of these complexes with

excess isocyanide yielding tris-isocyanide complexes in the case of tBuNC and an

unusual chelating iminocarbamoyl bis-isocyanide complex in the case of 2,6 dimethyl

phenyl isocyanide. X-ray structures of a bis-isocyanide complex and the iminocarbamoyl

bis-isocyanide complex are reported. We also examine the effects on '7 loading in d2 six

coordinate complexes and also attempt to determine the nature of the M-C (isocyanide)

bond.

Synthesis of bis-Isocyanide Complexes. Treatment of a pentane solution of 20a,

(rj2-isobutylene)Mo(NPh)(o-( Me3SiN)2C6H4), with two equivalents of tert-butyl

isocyanide resulted in the precipitation of 30a, as green microcrystals (Scheme 4-1). The

structure of 30a has been assigned by 1H, 13C, and IR spectroscopy and is consistent with









a square pyramidal structure with the imido occupying the apical position as depicted in

Scheme 4-1. Resonances for the SiMe3 protons are observed at (0.75ppm, 18H), whilst,

the tBuNC protons are observed at (1.06ppm, 18H). The phenyl imido protons are

observed at 7.48(dd, 2H, 7.33, 1.47Hz), 6.93(t, 1H, 7.33Hz) and 6.81(tt, 2H, 7.33,

1.47Hz). The o-pda ligand protons are observed as a pair of doublet of doublets (5.57,

3.3Hz) at 7.33 and 7.05ppm, respectively.

Similarly, treatment of 20a with two equivalents of 2,6-dimethyl phenyl

isocyanide, afforded 30b as a dark red crystalline material. Resonances for the SiMe3

protons are observed at 0.73ppm (18H) whilst the RNC (R = 2,6- dimethyl phenyl

isocyanide) protons are observed at 2.1 Ippm (12H). The phenyl imido protons are

observed at 7.43ppm(d, 2H), 6.99ppm(t, 2H, 7.24Hz) and 6.88ppm(t, 1H, 7.28Hz).

Protons for the RNC phenyl group are observed as a multiple at 6.63ppm. The o-pda

ligand protons are observed as a pair of doublets of doublets at (5.65, 3.3Hz) at 7.51 and

7.06ppm.

Two isocyanide stretches in the IR spectrum of 30a are observed at 2122 (CN

symmetric stretch) and 2082 (CN asymmetric stretch). The appearance of these stretches

at a lower frequency than the free isocyanide ligand (Av(CN) = -9 and -50 cm-1

respectively) where Av (CN) = (stretching frequency of free isocyanide) (stretching

frequency of complex)) is indicative of back bonding from the metal to the isocyanide

ligand. By comparison, the isocyanide stretches for 30b are observed at 2083 cm-1 and

2022 cm-1 (Av(CN) = -31 and -92 respectively). The larger Av(CN) values observed for

30b is reflective of the greater propensity for 2,6-dimethylphenylisocyanide to act as a 7t









acceptor ligand. This is in agreement with the observation that aryl isocyanides are better

7T acceptor ligands than alkyl isocyanides.

Ph
Ph
N
Me3Si N
Me3S\ 2 eqvs RNC Me3Si
MeSiMe3S\ M L
N MeNSi N 7
isobutylene N N L

R = Bu, 2,6 -dimethylphenyl L
20a / 30






a. L = tBu isocyanide
b. L = 2,6-dimethylphenyl isocyanide
Scheme 4-1. Synthesis of bis-tBuNC complex

X-ray crystal structure of 30b. Single crystals of 30b were obtained by slow

diffusion of pentane into a toluene solution of the complex. In addition to the complex,

the asymmetric unit has one half of a toluene molecule located on an inversion center.

The thermal ellipsoid plot of 30b is depicted in Figure 4-1.

The geometry of 30b is best described as a distorted square pyramidal structure

with the phenyl imido ligand occupying the axial position. The Mo-N(Imido) bond

(1.746A) is in the normal range for a Mo-N triple bond and the Mo-N (amido) bonds

(2.064A and 2.093A) are typical Mo-N bond lengths. The Mo-C(isocyanide) bonds

(2.088A and 2.093A) are shorter than typical Mo-C(sp) (2.2A) bonds in related

complexes suggestive of 7: back bonding from the metal to the 7t* orbital of the

isocyanide ligand. The isocyanide ligand is linear with C-N-C angles of (1730 and 177).









As has been reported for other terminal isocyanide complexes, there is no significant

increase in the C-N bond distance (Average 1.17A). Like the previously reported d2

complexes, the diamide ligand in this complex is flat (fold angle 173.70).49 As discussed

in chapter 2, this fold angle results from the fact that 7t donation of the diamide lone pair

electrons into the metal dxy is prevented because this orbital is occupied in these

complexes.


Figure 4-1. Thermal ellipsoid plot of 30b (30% ellipsoids). Selected bond lengths (A)
and degrees (0): Mo-N(1) 1.746(2), Mo-N(2) 2.064(2), Mo-N(3)
2.093(2), Mo-C(19) 2.088(3), Mo-C(26) 2.085(3), C(19)-N(4) 1.160(3),
C(26)-N(5) 1.166(3), C(19)N(4)C(20) 173.4(3), C(27)N(5)C(26) 176.9(2)

Synthesis of tris-Isocyanide Complex. The tris-isocyanide complex,

(tBuNC)3Mo(NPh)(o-(Me3SiN)2C6H4), 31, was previously synthesized and reported in

our labs.74 It is included in this chapter because of its relevance to the subsequent









discussion. When 20a, was allowed to react with 4 equivs of C=N(t-Bu), a fast color

change from green to purple occurred, and purple microcrystals of 31 precipitated from

solution and were easily isolated by filtration.

Ph

Ph N
L
N
Me3Me3 MeSi Mo
Me3Si M + tBuNC Me3
N L N
2 isobutylene iNe
20a
// 31 SiMe3



Scheme 4-2. Synthesis of tris-tBuNC complex 31

The pseudo-octahedral structure of 31 is shown in Scheme 4-2, was proposed on

the basis of the observed NMR data and confirmed by X-ray crystallography.

Resonances corresponding to inequivalent SiMe3 protons were observed at 0.58ppm (br s,

9H) and 0.77ppm (br s, 9H). Broad singlets at 0.93 and 1.17ppm were assigned to

inequivalent tert-butyl isocyanide peaks. A plane of symmetry containing the imido

nitrogen, o-pda nitrogens, and one isocyanide ligand makes the remaining isocyanide

ligands chemically equivalent.

The broadening observed in the H NMR spectrum of 31 has been correlated to an

equilibrium involving fast, reversible ligand dissociation. As shown in Scheme 4-3, loss

of one RNC ligand induces a plane of symmetry in the bis-isocyanide complex, 30a, that

is formed. Thus the remaining isocyanide ligands and the SiMe3 ligands become






76


equivalent in 30a. Using the two-site exchange approximation, an activation barrier of

AG = 15.0 Kcal/mol for the ligand dissociation was calculated.


Ph

N


L


- tBuNC

+ tBuNC


Me3Si


SiMe3


L

L= tBuNC


Scheme 4-3. Dissociative equilibrium observed for 31


Figure 4-2. Thermal ellipsoid plot of 31, (30% probability thermal ellipsoids), selected
bond lengths (A) and angles0: Mo-N (1) 1.804(4), Mo-N (2) 2.166 (5),
Mo-N (3) 2.130 (5), Mo-C (24) 2.067 (5), Mo-C (29) 2.158 (5), Mo-C
(19) 2.137 (2)-N (2) 1.182 (3), C(2)N(2)C(8) 152.8(5).









X-ray crystal structure of 31. As shown in Figure 4-2, 31 was found to have a

distorted octahedral geometry. Three isocyanide ligands were found to occupy

meridianal positions. The imido-nitrogen was found to be trans to one of the isocyanide

groups. The shorter bond length of the Mo-C(2) and longer bond length of the C(2)-

N(2) is attributed to '7 backbonding from the metal d orbitals to the isocyanide ligands.

Molecular orbital calculations. We were interested in describing the bonding of the

isocyanide ligand in our complexes and comparing the 7t acceptor capabilities of RNC

with carbonyls. Therefore, DFT calculations were performed on 31 and the previously

reported carbonyl complex [Mo(NPh)(PMe3)2(CO)(o-(Me3SiN)2C6H4)], 3261 (Figure 4-

3). For these calculations a two-layer ONIOM method was employed. For the inner

layer the o-phenylene group (o-C6H4) that links the two N atoms of the diamido ligand

was simplified to a -CH=CH- carbon chain. The organic groups on the nitrogen atoms

(SiMe3 of the amido and phenyl group of the imido) were replaced by hydrogen atoms

for simplicity; and the subsitituents on the phosphine ligand in 32, and the isocyanide

ligand in 31 were replaced by hydrogen atoms. For this layer the B3LYP/LANL2DZ

method was employed. The outer layer consisted of the entire complex including all

substituents on the imido, phenylene diamide, isocyanide and phosphine ligand and was

modeled B3LYP/LANL2MB method.

As shown in Figure 4-3, and Table 4-1, there is good agreement between the

calculated geometries and the crystal structure of 31. The crystal structure of 32 has not

been reported but the structure was confirmed as a minimum by a frequency calculation

(number of imaginary frequencies=0).









Table 4-1. Selected bond Lengths(A) and Angles(0) for 31 and 32
31 32
Bond ONIOM X-ray Bond ONIOM
Mo-N(1) 1.824 1.805(3) Mo-N(1) 1.833
Mo-N(2) 2.178 2.166(3) Mo-N(2) 2.186
Mo-N(3) 2.141 2.130(3) Mo-N(3) 2.137
Mo-C(1) 2.149 2.137(5) Mo-C(1) 1.998
Mo-C(2) 2.013 2.067(5) Mo-P(1) 2.479
Mo-C(3) 2.131 2.158(5) Mo-P(2) 2.477
C(4)N(5)C(2) 175.5 153.0(4)

Our interest in the bonding in 31 and 32 stemmed from some interesting features in

the structures of these molecules. In addition to the six electrons that can be donated by

the imido ligand (one o and two 71), six electrons may also be donated by the diamide

ligand through the lone pair electrons on nitrogen (two o and one 7t as has been

mentioned before for do and d2 five coordinate complexes in previous chapters).

However, a conflict arises because these complexes are Mo(IV) and one of the metal d

orbitals is occupied. Thus donation of the diamide lone pair electrons into this orbital

would result in an energetically unfavorable filled-filled interaction. The presence of a

strong '7 acceptor ligand trans to the diamide lone pair electrons may stabilize this

situation as the diamide ligand, the metal orbital (dx), and the 7t* orbital of the '7 acceptor

ligand (CNR in 31 or CO in 32) can engage in 3 center-four-electron bonding as shown in

Figure 4-4.

Molecules 31 and 32 possess pseudo Cs symmetry, thus, there are two sets of metal

t2g orbitals one with a" symmetry (dxy and dyz) and another with a' symmetry (Scheme 4-

4). A linear combination of the diamide lone pair electrons results in two orbitals of a"

symmetry (73 and 712) and these orbitals can interact with orbitals on the metal of

appropriate symmetry i.e. the dxy and dyz orbital. The metal orbitals may also interact via

71 back bonding to the 7t acceptor ligand carbonyll or isocyanide) that is arranged trans to









the one of the diamide nitrogen lone pairs. The occupied molecular orbitals generated by

DFT(B3LYP) are a good illustration of these interactions Figures (4-5) and (4-6).



C6

N6
C3 N1
SI
M. C2 N5 P N1

Mo
N3 C1



SI




31 32
Figure 4-3. Optimized structure (ONIOM B3LYP/LANL2DZ:B3LYP/LANL2MB) for
31 and 32


diamide ligand


metal dxy





Anti-Bonding





Non Bonding


Bonding
Figure 4-4. 3-Center-4-electron bonding between the diamide lone pair electrons the
metal d orbitals and the t* orbitals of a t acceptor ligand


7* (CO) or
CNR





8-












Me3Si



SiMe3


a" (dxy)
Ph
N

Me3SiN L


L
SiMe3
a' (dxz)


Me3Si


SiMe3


lo-L

i L


diamide
Ph occupied
/ orbitals


Me3Si
N-Mo


LSMe3

SiMe3


SiMe3
a" CO (7i*)
a"(E72)
Scheme 4-4. Group orbitals for 7: interactions in 31 and 32, CO is used as the 7t acceptor
ligand in this example


a" (dyz)


metal t2g


Me3Si


a" (713)


N


Me3Si























HOMO 1


Me3Si.


Figure 4-5. Interaction of diamide lone pairs and isocyanide ligand with metal dxy orbital.
(Orbitals are displayed at the 0.05 au isocontour level)


HOMO


HOMO -1


HOMO -2


Me3Si Me3,
Me3Si.


Me3 R
Me3Si .,


Figure 4-6. Interaction of diamide lone pairs and CO ligand with metal dxy orbital.
(Orbitals are displayed at the 0.05 au isocontour level)


Me3PR


Me3Si


SiMe3


HOMO


HOMO 2









The presence of a lone pair electron on the metal limits the 7r orbital donation by

the diamide ligand, thus in the HOMO for both 31 and 32 the diamide lone pairs are

primarily ligand centered. The occupied orbitals HOMO -1 and HOMO -2 represent the

aforementioned 3-center-4-electron interaction between the diamide lone pairs, the metal,

and the tr* orbital of the isocyanide or carbonyl ligand. Bonding between the diamide

ligand and the metal dxy orbital in HOMO-2 is negated by antibonding to this ligand in

HOMO 1. Thus, the net effect is that the diamide lone pair electrons remain ligand

centered in the HOMO and the metal dxy orbital remains largely metal centered.

Also evident from the molecular orbital diagrams are the differing 7r acceptor

capabilities of CO versus the CNR ligand. The coefficient at the isocyanide carbon in 31

is very small suggesting very little contribution from this carbon in the metal-isocyanide

7r back bond. In bonding to a 7r acceptor ligand like CO or CNR the metal d orbitals are

perturbed by both the 7r and tr* orbitals of the acceptor ligand (Scheme 4-5) and the net

result shows cancellation of the electron density at the carbon and reinforcement at the

heteroatom (N or 0).101













Scheme 4-5. Interaction of a metal d-orbital with a 7r acceptor ligand

In the case of CO the contribution from tr* is greater than the contribution from the

7r orbital, (because of the difference in electronegativity between carbon and oxygen there









is a larger coefficient at carbon in 7t*) and CO is an overall 7t acceptor. The difference in

electronegativity is smaller in between carbon and nitrogen thus there is a smaller

coefficient on carbon in the 7t* orbital and this results in cancellation of most of the

electron density at this atom in the bonding MO. These differences are clearly evident in

the HOMO -1 and HOMO 2 orbitals in Figures 4-5 and 4-6. The contribution at the

carbon in CO in 32, is significantly greater than in the isocyanide ligand in 31.

Synthesis, Structure and Dynamics of a Chelating Imino Carbamoyl Complex

Treatment of a C7D8 solution of 30b with two equivalents of 2,6-dimethyl phenyl

isocyanide results in the slow conversion (24 hrs) to the imino carbamoyl complex, 33

(Scheme 4-6). Chemical shifts for the SiMe3 protons occur at the (0.44 ppm, s, 9H) and

(0.59 ppm, bs 9H). Signals for the 2,6 -dimethyl phenyl isocyanide ligands are observed

at (1.94 ppm, s 12H) and two signals were observed for the 2,6-dimethyl phenyl protons

at (2.45 ppm, s, 6H) and (1.86 pp s 6H). As shown in Scheme 4-6 the imino carbamoyl

ligand is coordinated in q1 and r2 fashion to Mo. The chemical shifts for the quaternary

carbons of the iminocarbamoyl ligands are observed at (208.2 ppm) and (181.4 ppm) for

the r2 and q1 carbons respectively.

One peak was observed in the IR spectrum for the asymmetric CN stretch (2088

cm-1) that occurred at a lower frequency than the free isocyanide (Av = 25 cm-1) again,

suggestive of net '7 back bonding in these complexes.

Isocyanide insertion into metal amide bonds is less extensively studied than the

related insertion reactions of metal alkyl bonds. In recent years, studies of isocyanide

insertions into Ta amide bonds have been reported. We propose that the formation of 33

occurs by initial coordination of the of two isocyanide ligands followed by the rapid










insertion of the isocyanides into the metal amide bond. A 1,3 siltropic shift then follows

and the resulting iminocarbamoyl species is rapidly trapped by two molecules of 2,6-

dimethyl isocyanide (Scheme 4-7).

R


Me3Si ....R
2.0 eqvs N
2,6- dimethylphenyl isocyanide N--,
24 hoMo
24 hours N


30b

L = 2,6 -dimethylphenyl isocyanide


N Ph
N--




Me3


R = 2,6 -dimethylphenyl


Scheme 4-6. Synthesis of iminocarbamoyl complex, 33


/h Me3Si

Me3Sii isocyanide insertion N
Me3S\i Mo

30b L N
Me3Si


N ---R


Mo N-Ph


N-R


L = 2,6 dimethylphenyl isocyanide


1,3 silatropic
shift


N
Me3SiN -R
CN 2.0 equivs
N ,' 2,6- dimethylphenyl isocyanide
~T L '2N--h Ph


;iMe3


33 \
R
Scheme 4-7. Proposed mechanism for the formation of 33


Me3,

Me


r









X-ray Crystal Structure of 33. Single crystals of 33 were obtained by a cooling a

concentrated pentane solution of this molecule (Figure 4-7). The geometry about Mo

may best be described as a distorted trigonal bipyramid with the isocyanide ligands

occupying the axial position and the imino carbamoyl fragments and the imido ligand

occupying the equatorial plane. A plane of symmetry bisects the two trans isocyanide

ligands and passes through the imido and iminocarbamoyl fragments. The imino

carbamoyl fragment is flat (rms = 0.02), consequently the substituents on the amido

nitrogens (N(5) and N(4)) are sterically encumbered videe infra).


Figure 4-7. Thermal ellipsoid plot of 33, (30% ellipsoids). Selected bond lengths (A)
and angles(0) Mo-N(1) 1.767(2), Mo-N(2) 2.108(2), Mo-C(57) 2.110(3),
Mo-C(48) 2.131(3), Mo-C(14) 2.147(3), Mo-C(7) 2.052(3), N(2)-C(7)
1.274(3), N(3)-C(14) 1.318(4), N(4)-C(7) 1.365(3), N(4)-C(15) 1.454(3),
C(21)N(5)C(14) 124.4(3), C(14)N(5)Si(2) 113.4(2), Si(2)N(5)C(21) 122.1,
C(57)N(7)C(58) 166.9, C(48)N(6)C(49) 171.8(3)









The Mo-N(1) bond length (1.767 A) is within normal range for a molybdenum

nitrogen triple bond. One imino carbamoyl fragment binds r1 to the Mo center and

shows localized C-N bonds i.e., one short C=N bond, C(14)-N(3), (1.318 A) and one

C-N single bond (C(14)-N(5), (1.403A). The r2 imino carbamoyl fragment exhibits a

very short C-N bond (C(7)-N(2), (1.274A)) and a long C(7)-N(4) (1.365A). Both

amido nitrogens N(4) and N(5) are sp2 hybridized with carbon nitrogen bond lengths

shorter than typically seen for C-N single bonds (1.48A) and the sum of the angles

around each nitrogen is equal to 359.90. This suggests delocalization within the CNC

framework of the imino carbamoyl. The bond lengths for the isocyanide ligands

(Average 2.12A) are shorter than typical Mo-C (sp2) bonds suggestive of 7T back

bonding to isocyanide in these complexes. The CNC isocyanide bond angles deviate

only slightly from linearity (Average 169.40).

The space-filling model of 33, generated from the X-ray study, reveals a very

sterically congested region in the plane of the imino carbamoyl fragment (Figure 4-8).

This suggests that a free rotation about the carbon nitrogen bonds C(7)-N(4) and

C(14)-N(5) is restricted because of steric hindrance around the nitrogen atoms. For this

same reason free rotation of the xylyl fragments about N(4) and N(5) are restricted

because of steric interactions. Silicon atom Si(2), is located close to two nitogens; this

atom is directly bond to N(5) (bond length = 1.78A), but is also close to N(3) (2.53A) this

distance is less than the sum of the Van der Waals radii of the a nitrogen atom (3.60A)

and as is evident from the space filling diagram Si(2) interacts with both N(3) and N(5).

These close contacts are responsible for the fluxionality of these molecules in solution

videe infra).