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Gas Phase Structure and Reactivity of Zirconocene Cations


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GAS PHASE STRUCTURE AND REACTI VITY OF ZIRCONOCENE CATIONS By ALEXANDER A. AKSENOV A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2005

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This work is dedicated to my loving wife Karina

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iii ACKNOWLEDGMENTS Author expresses his deepest gratitude to Dr. John R. Eyler for his knowledge, help and concern and Dr. David E. Richardson for his sin qua non expertise and fruitful discussion.

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iv TABLE OF CONTENTS page ACKNOWLEDGMENTS.................................................................................................iii LIST OF TABLES............................................................................................................vii LIST OF FIGURES.........................................................................................................viii ABSTRACT....................................................................................................................... ..x CHAPTER 1 CHEMISTRY OF ZIRCONOCENES AND ZIRCONOCENE CATIONS IN THE CONDENSED PHASE................................................................................................1 1.1 Introduction and Historical Background................................................................1 1.2 Structure of Zirconocenes.......................................................................................2 1.3 Reactivity of Zirconocene Derivatives...................................................................4 1.3.1 Formation of Zirconium-Carbon Bonds.......................................................6 1.3.1 Cleavage of Zirconium-Carbon Bonds.........................................................7 1.3.2 Other Organozirconium Derivatives............................................................7 1.4 Cationic Zirconocene Species.................................................................................9 1.4.1 Structure and Reactivity of Zirconocene Cations.........................................9 1.4.2 Nucleophilic Addition Reactions...............................................................10 1.4.3 Cationic Zirconocenes as Lewis Acid Catalysts........................................11 1.5 Single-Center Metal Polymerization....................................................................12 1.5.1 Ziegler-Natta Polymerization.....................................................................12 1.5.2 Metallocene-Catalyzed Olefin Polymerization..........................................13 2 FTICR MS: METHOD OVERVIEW.........................................................................15 2.1. Introduction and Historical Background.............................................................15 2.2 Theory and Instrumentation..................................................................................16 2.2.1 Ion Cyclotron Motion.................................................................................18 2.2.2 Acquisition of Mass Spectra.......................................................................20 2.3 Ionization Methods...............................................................................................23 2.3.1 Electron Ionization.....................................................................................24 2.3.2 Chemical Ionization....................................................................................25 2.3.3 Electrospray Ionization...............................................................................25 2.3.4 Matrix Assisted Lase r Desorption/Ionization.............................................26

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v 2.3.5 Atmospheric Pressure Chemical Ionization...............................................27 2.3.6 Cationization...............................................................................................28 3 GAS-PHASE CHEMISTRY OF META L IONS AND IONIC COMPLEXES........29 3.1 Gas-Phase vs. Solution Chemistry........................................................................29 3.2 Gas-Phase Reactivity of Bare and OxoMetal Ions.............................................30 3.3 Gas-phase Reactivity of Ligated Metal Ions........................................................32 3.4 Reactions of the Bis( 5-cyclopentadienyl)methylzirc onium Cation in the Gas Phase.......................................................................................................................34 3.4.1 Reactions with Alkenes..............................................................................34 3.4.2 Allyl Complex Formation: Mech anistic Differences of Reaction Patterns in the Gas and Liquid Phases.............................................................38 4 GAS-PHASE REACTIONS OF BIS( 5-CYCLOPENTADIENYL) METHYLZIRCONIUM CATIONS WI TH KETONES AND ALDEHYDES.........41 4.1 Rationale...............................................................................................................41 4.2 Experimental Procedures......................................................................................43 4.3 Reaction with Ketones..........................................................................................51 4.3.1 Reactions of Cp2ZrCH3 +/Cp2ZrCD3 + with Acetone/d6-Acetone...............51 4.3.2. The Reactions of Cp2ZrCH3 +/Cp2ZrCD3 + with 2-Butanone, Methyl Isobutyl Ketone and Cyclohexanone...............................................................61 4.3.3 Reactions with Other Ketones....................................................................70 4.4 Reaction with Aldehydes......................................................................................72 4.5 Comments on Suggested Reaction Mechanisms..................................................85 5 GAS-PHASE REACTIONS OF BIS( 5-CYCLOPENTADIENYL) METHYLZIRCONIUM CATIONS WITH IMINES................................................92 5.1 Synthesis and Reactivity of Imines.......................................................................92 5.2 Gas-phase Reactions of the Bis( 5-Cyclopentadienyl)methylzirconium Cation with Imines..............................................................................................................93 5.2.1 Reactions of Cp2ZrCH3 +/Cp2ZrCD3 + with 2,2,4,4-Tetramethyl-3Pentanone Imine...............................................................................................94 5.2.2 Reactions of Cp2ZrCH3 +/Cp2ZrCD3 + with Aryl Substituted Imines..........98 6 CONCLUSIONS AND FUTURE WORK...............................................................102 6.1 Conclusions.........................................................................................................102 6.2 Future Work........................................................................................................103 6.2.1 IRMPD Spectroscopy...............................................................................103 6.2.2 Other gas-phase studies of organometallic compounds...........................108

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vi LIST OF REFERENCES.................................................................................................111 BIOGRAPHICAL SKETCH...........................................................................................132

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vii LIST OF TABLES Table page 3.1. Reactions of Cp2ZrCH3 + with alkenes.......................................................................34 4.1. Isotopes of zirconium.................................................................................................45 4.2. Major reaction products obs erved in the reactions of 1 and 2 with 2-butanone, and methyl isobutyl ketone at various reaction times (1 to 5 seconds)....................63 4.3 Major reaction products obs erved in the reactions of 1 and 2 with diethyl carbonate at various reaction times (1 to 5 seconds)................................................71 4.4. Major reaction products obs erved in the reactions of 1 and 2 with acetaldehyde, benzaldehyde, propanal and n -hexanal at various reaction times (1 to 5 seconds)..73 4.5. The absolute electronic energy (kcal/mol) of the most stable structure is given, and the relative energy difference is gi ven for the remaining structures (in kcal/mol). Lettering corresponds to the structures pictured in Figure 4.16, a) through f)..................................................................................................................91 5.1. Imines used in this work............................................................................................94

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viii LIST OF FIGURES Figure page 1.1. Structural types of Cp2Zr(IV) compounds (adapted from ref. [23]); examples of each structural type are list ed in the right column.....................................................2 1.2. Dewar-Chatt-Duncanson model of alke ne bonding by a metal complex (adapted from ref. [28]).............................................................................................................4 1.3. Two-electron reactions of zirconocene complexes......................................................6 1.4. Reactions of Cp2ZrC(O)RCl (adapted from Ref. [23])................................................8 2.1. Schematic representation of a cylindrical FTICR analyzer cell (adapted from Ref. [104])........................................................................................................................1 8 2.2. Schematic representation of a cross s ection of an FT-ICR analyzer cell, where ions are being excited by the RF potentia l applied to the excitation electrodes (adapted from Ref. [104]).........................................................................................21 2.3. Stages of data in the FTICR experiment....................................................................22 3.1. A reaction profile for hydrocarbon oxida tion by a transition metal oxo-ion.............31 3.2. Reactions of Cp2ZrCH3 + with alkenes (adapted from Ref. [147]).............................35 3.3. Comparison of qualitative potentia l energy surfaces for the reaction of Cp2ZrCH3 + with ethylene in solution and the gas phase (adapted from Ref. [147])........................................................................................................................3 6 3.4. Mass spectrum for the reaction of Cp2ZrCH3 + with cyclopentane............................38 3.5. Proposed structure of the complex with m/z 287.......................................................38 4.1. Schematic diagram of the mass spectrometer used in this work...............................44 4.2. Schematic diagram of the electron ionization source................................................44 4.3. EI mass spectrum of Cp2Zr(CH3)2: Cp2ZrCH3 + (m/z 235-242), Cp2Zr+(m/z 220227) and Cp2ZrOH+ (m/z 237-244)..........................................................................46 4.4. Example of a SWIFT experiment..............................................................................48

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ix 4.5. Binuclear complexes Cp4Zr2(CH3)n + (n = 0 3)........................................................48 4.6. EI mass spectrum of Cp2Zr(CD3)2 ( Noise peaks)...................................................50 4.7. Mass spectra of ions formed in the reaction of Cp2ZrMe+ with acetone...................52 4.8. Mass spectra of ions formed in the reaction of Cp2ZrMe+ with acetone...................54 4.9. The a) 3-enolate and b) 3-allyl complexes..............................................................56 4.10. Mass spectra of ions fo rmed in the reaction of Cp2ZrMe+ with methyl isobutyl ketone.......................................................................................................................62 4.11. Ions produced in the reaction of Cp2ZrMe+ with benzoquinone: products of coordination of quinone molecule by the binuclear complexes Cp4Zr2(CH3)n + (n = 0 3)......................................................................................................................7 1 4.12. Mass spectra of ions fo rmed in the reaction of Cp2ZrMe+ with acetaldehyde, 1 second reaction delay...............................................................................................75 4.13. Mass spectra of ions with m/z 279 and 280 formed in the reaction of Cp2ZrCD3 + with acetaldehyde, at a) 1 second an d b) 5 seconds reaction delays........................76 4.14. Ions produced in the reaction of Cp2ZrMe+ with benzaldehyde at 1 second reaction delay...........................................................................................................76 4.15. Qualitative potential surface for the reaction of 1 with acetaldehyde/generic aldehyde in the gas phase and in soluti on. The lettering corresponds to that in Scheme 4.21. Gas-phase estimates of ener getic effects are based on data from Ref. [173].................................................................................................................80 4.16. The most stable theoretically calculated structures for the Cp2ZrC3H5O+ cation. Canonical structures shown were calculated with the MPW1PW91/6311+G(d,p) functional/basis se t. The calculated absolute electronic energies of structures from a) through f) are listed in Table 4.5.................................................90 5.1. EI mass spectrum of benzophenone imine (molecular weight 181 amu)..................99 6.1. IRMPD spectrum of Cr+(Et2O)2 (adapted from Ref. [204])...................................104 6.2. Theoretical IR absorption spectra for th e key structures discussed in the present work........................................................................................................................106 6.3. Depletion spectrum of ion with m/z 282 (postulated as [Cp2Zr(CH3CHO)(H2O)]+).....................................................................................108

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x Abstract of Dissertation Pres ented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy GAS PHASE STRUCTURE AND REACTI VITY OF ZIRCONOCENE CATIONS By Alexander A. Aksenov December 2005 Chair: John R. Eyler Major Department: Chemistry The reactions of bis( 5-cyclopentadienyl)methylzirconi um cation with aldehydes, ketones and imines in the gas phase have b een studied by Fourier transform ion cyclotron resonance mass spectrometry. Reactions of bis( 5-cyclopentadienyl)methylzirconium cation with a majority of the ketones studied resulted in consecutiv e addition of one and two substrate molecules and/or elimination of neutral. The key ionic products of the elimination reactions were identified as 3-enolate complexes. Similar product ion structures are also postulated for the reacti ons with aldehydes, where these complexes are either the only or the major reaction product. Deuterium-labeled substrates and methyl zirconocene were used to investigate mechanistic details. Results indicate a multip le-step mechanism, with migratory insertion of an aldehyde molecule into th e methyl zirconocene cation, followed by -H elimination and, via a 6-membered cyclic transition state, fo rmation of the resulting enolate complex. When a -H elimination pathway is not available for ketones, the reaction is proposed to

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xi proceed instead via direct nucleophilic attack of th e metal-bound alkyl preceded by a fast migratory insertion equilibrium. In reactions with the majority of imines, the reactivity toward bis( 5cyclopentadienyl)methylzirconium cation is gove rned by the availability of a labile hydrogen. This leads to reaction products diffe rent from those which might be expected in analogy with reaction products ob served for ketones and aldehydes, i.e. the azomethyne/benzylidene species instead of the en amines. Also, due to lack of lability of the aryl groups, the migratory insertion equilibrium was not po ssible for reactions of arylsubstituted imines. Consequently, only meth ane elimination was observed. However, a general reaction mechanism resembles that proposed for ketones. The proposed mechanisms account very well for all of the observed reaction features.

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1 CHAPTER 1 CHEMISTRY OF ZIRCONOCENES AND ZIRCONOCENE CATIONS IN THE CONDENSED PHASE 1.1 Introduction and Historical Background Among other transition metals, Zr and its complexes, especia lly cyclopentadienyl derivatives, have special signifi cance in view of their potent ial to serve as catalysts or reagents for multitudes of synthetically in teresting reactions. Even though zirconium occurs to the extent of 0.022% in the lithosphere (more abundant than carbon but less abundant than Ti, 0.63%) and is one of the l east expensive metals, its application in organic syntheses of any kind was virtually unknown until the 1970s (apart from use as a rather unsuccessful substitute for Ti salts in the Ziegler-Natta polymerization) [1]. However, over the last twenty years orga nozirconium compounds (almost exclusively zirconocenes) have become some of the most widely used organometallics, along with other metals such as Ni, Cr, Rh, and Fe. The explosive growth of this area is documented in multiple reviews (a few of the most recent are given [2-15]) and the chemistry of organozirconium and other group 4 transition metals has been extensively discussed in some monographs [16, 17]. Most importantly, the d0 group 4 metallocenes are of great interest as precursors of alke ne polymerization catalysts [18, 19], and can be tuned to make polymers of very specific tacticities. Single-center metal-catalyzed polymerization has attracted ample attention and evolved from an area of purely academic interest into commercially important technology [20]. The rema inder of this chapter reviews some of the most important aspects of the ch emistry of organozirconium compounds.

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2 1.2 Structure of Zirconocenes The majority of known Zr compounds are derivatives of the so-called “Cp2Zr unit” (Cp = cyclopentadienyl or related ligand) with Zr in +4 or, less often, +2 oxidation states. Zr (+3) compounds are also known, but are genera lly considered as unwanted by-products [21]. Figure 1.1. Structural types of Cp2Zr(IV) compounds (adapted from ref. [23]); examples of each structural type are listed in the right column.

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3 Structural types of Cp2Zr(IV) compounds are shown in Fig. 1.1. In addition to the structures shown in Fig. 1.1, it is well known for the coordinatively unsaturated 14 and 16 electron zirconocene species to form dime rs, oligomeric species, or polymeric aggregates. It is common for the “Cp2Zr unit” to remain intact in the majority of chemical transformations. However, under certain conditions, for example, treatment with an excess of very strong nucleophiles su ch as alkyl lithium compounds, the Cp ring (or both) can be displaced [22]. Zirconocene (IV) derivatives repres ented by the general formula Cp2ZrR1R2 (both R1 and R2 being X ligands), such as zirconocene dichloride, Cp2ZrCl2, the Schwartz reagent Cp2ZrHCl [24, 25], Cp2ZrMe2, etc., are d0, 16-electron compounds, with an empty valence-shell orbital, but with no filled orbitals av ailable for interaction. This makes these complexes inherently electrophil ic (Lewis acidic), but not nucleophilic. The empty 2d orbital of zirconium is able to interact with and bonding electrons, as well as nonbonding electron pairs, and this interaction is usually a fi rst step in the reaction of Zr derivatives. Formation of the -complexes of transition metals is described by the Dewar-Chatt-Duncanson model [26, 27] (Figure 1.2). In this model, a -type donation from the C=C orbital with concomitant -backbonding into an empty orbital of the alkene results in synergis tic bonding: the greater the -donation to the metal, the greater the -backbonding. Also, the greater the elec tron density back-donated into the orbital of the alkene, the greater the reduction in the C=C bond order, which can ultimately lead to formation of 2 alkene insertion product. This, in tu rn, implies that only metals with at least one filled nonbonding orbital can form such complexes. For the d0 zirconocenes, only donation of electrons is possible from al kenes, so these complexes act solely as

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4 Lewis acids, thus forming only unstable (usually transient) species. This interaction is more efficient for the cationic zirconocenes. Figure 1.2. Dewar-Chatt-Duncanson model of alkene bonding by a metal complex (adapted from ref. [28]) 1.3 Reactivity of Zirconocene Derivatives The reactivity of various zirconocene deriva tives can be generalized into several distinct types of reactions, as shown on Fi gure 1.3. The most important and relevant reactions are discussed below.

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5 a) -Complexation and -dissociation b) -Complexation and -dissociation c) Hydrozirconation and dehydrozirconation d) Carbozirconation and decarbozirconation e) Oxidative addition and reductive elimination f) -Bond metathesis

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6 g) Migratory insertion a nd migratory deinsertion Figure 1.3. Two-electron reactions of zirconocene complexes. 1.3.1 Formation of Zirconium-Carbon Bonds Apart from transmetallation, the most synt hetically important r eaction resulting in zirconium-carbon bond formation is hydrozir conation [25, 29] (Fig. 1.2c), which provides an easy and convenient way of conve rting alkenes and alkyne s into alkyland vinylzirconium derivatives, respec tively [30]. The commonly used “Cp2Zr” precursor is the Schwartz reagent, Cp2ZrHCl, in conjunction with an aluminium hydride, such as LiAlH4 [24]. The formation of the insertion product occurs with good regioand stereoselectivity (anti-Mar kovnikov, syn addition, with the formation of the E-alkenyl complex in reactions with alkynes) [31]. The availability of -hydrogens in the insertion product can possibly lead to in tramolecular rearrangement. Another synthetically important mean s of formation of organozirconocene derivatives is oxidative addition (Fig. 1.2e). It should be noted, however, that since the Cp2Zr(IV) complexes have d0 configuration, the oxidative addition process is not possible, since it requires that both empty and filled orbitals be available for interaction.

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7 It has been shown that the actual reaction is oxidative c oupling-elimination [32]. The oxidative addition reaction is va luable for converting allyl et hers into allylzirconocenes [33], as well as for conversion of alkenyl chlorides into alkenylzirconocene chlorides. 1.3.1 Cleavage of Zirconium-Carbon Bonds The zirconocene alkyl/alkenyl insertion product can be converted by protonolysis/deuterolysis and halogenolysi s into the correspo nding (halogenated) alkane/alkene. Cleavage of the Zr-C bond occurs in relatively mild conditions: protonolysis requires treatment with diluted acid or even water, while halogenolysis occurs under treatment with bromine or iodine or, when an alkene group is present, succinimides (NBS or NCS) at room temeperatu re. The configuration of alkene is usually preserved [34]; however, some examples of concomitant skeletal rearrangement are known [25, 29]. Another path fo r Zr-C bond cleavage is treatm ent with peroxides, which leads to formation of alcohols [35]. The inherent low intrinsic nucleophilicit y of alkyl/alkenyl zirconocenes renders them unable to react with a majority of co mmon electrophilic substr ates, with the only exception known to be the reaction with aldehydes [36]. On the other hand, dienezirconocenes can react with a variety of substrates, such as alkenes, alkynes, aldehydes/ketones and ev en nitriles [37]. 1.3.2 Other Organozirconium Derivatives An important class of compounds contai ning a Zr-C bond is the acylzirconocenes, which can be easily and straightforwardly obtained via treatmen t of alkyl/alkenyl derivatives with carbon monoxide at atmospheri c pressure, resulting in a generally clean reaction of CO inserting into the Zr-C bond [34, 38] The stability of the acylzirconocenes depends strongly on the ligand of the given derivative, with the

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8 acylzirconocene chlorides being most stab le (not losing CO even at pressures significantly lower than atmospheric). Acylzi rconocenes can be further involved in a variety of other reactions shown in Figur e 1.4. All of those reactions proceed via migratory insertion [34, 39] (Fig. 1.2g). Figure 1.4. Reactions of Cp2ZrC(O)RCl (adapted from Ref. [23]). Another very important class of organozirconium compounds are the complexes, which can be considered as zirconocyclopropanes. The -complexation (or, rather, oxidative -complexation) can be considered as a two-electron red-ox process with the alkene being reduced by zirconium. The required Zr(II) species can be generated by a reducing agent (alkali metal amalgam) prior to reaction, or in situ [39-41]. The resultant zirconacycle, called the “Negishi reagent” is a valuable reagent, enabling synthesis of various zirconocycles and serv ing as a very convenient source of the “Cp2Zr” unit. The sequence of reactions involving formation of zirconium -complexes is called the “Negishi protocol,” and was shown to be a concerted non-dissociative process ( the Cp2Zr species is not generate d at any stage [42]). Along with the more recently developed Erker-Buchwald protocol [4347], these two protocols, often used complementarily to each other, provide synt hetic routes to tri-me mber zirconocycles and a variety of compounds obtained from them.

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9 1.4 Cationic Zirconocene Species Zirconocene cations are not found as isolat ed species in the solution phase. The high reactivity of such species usually l eads to their immediate conversion/reaction. Furthermore, if such cations are indeed produced they always exist as an ionic pair with varying degree of coordination by a counterion. In general, it is impossible to define a discrete cation as part of a salt: in most cas es, no indication of a cationic nature is found either (by the means of X-ray) in the solid st ate, or in solution, and the only indication of the zirconocene cation’s presence is the ch aracteristic chemistry taking place. Normally, the ionic species are formed from a suitable precursor with appropriate in situ activation. The most widely used met hods in organic synthesis include halide abstraction from Cp2ZrCl2 with Ag salts, such as AgClO4, AgAsF6, AgSbF6, etc. (the counterion must be non-nucleophi lic) and protonation of met hyl zirconocenes with acids containing the BARF (tetraki s(3,5-bis(trifluoremethyl)phenyl )borate) [48] anion. The variety of methods used for the generation of bis( 5-cyclopentadienyl)methylzirconium and related cations employed in polymerization catalysis is considered below in Chapter 1.4. A vast number of studies have been carried out [49]. 1.4.1 Structure and Reactivity of Zirconocene Cations The zirconocene cations genera ted in solution always exist in their solvated form and/or ion pairs with a c ounterion. Depending on the nucleop hilicity of the anion, the degree of coordination can vary and th e free cation is never observed. Weakly coordinating counterions such as BARFprovide the only way to utilize the reactivity of the zirconocene cation, while common nucleophilic anions such as Clcan completely shut down the reactivity in most cases. Mo reover, these cationi c species exhibit a tendency to abstract chlorine ion from chlorinated solven ts, and show even greater

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10 tendency to abstract and bind fluoride (usually forming Cp2ZrX2[50]) (so counterions like BF4 cannot be used[51]). This feature is used for synthetic purposes for C-F bond activation in glycosyl fluorides [52, 53]. The observed rate for reactions invo lving zirconocene cationic species Cp2ZrX(L)+ depends on the rate of dissociation of the coordinated ligand to give the 14electron species. The latter complexes ar e the most reactive ones, since their electrophilicity is un impeded by electron donation from the base. Of course, such species tend to coordinate solvent molecule(s), thus establishing a solvent associationdissociation equilibrium which dete rmines the cation’s reactivity. 1.4.2 Nucleophilic Addition Reactions The Cp2ZrRCl complex is not electrophilic enough to react with the most substrates efficiently, i.e. cleanly and with a sufficient rate; thus, the synthetically interesting addition reactions may involve zirconocene cations as intermediates. Alkyl and alkenyl zirconocene cations can be produ ced from the hydrozirconation products of alkenes and alkynes by the Schw artz reagent via chloride abstraction by a catalytic amount of (most commonly) AgClO4 [54]. In aldehyde addition reactions of Cp2ZrRCl complexes, a mechanism involving generation of alkyl/alkenyl zirconocene wa s proposed [54]. Th e resultant reaction products are alcohols formed through condens ation of the aldehyde and R ligand of zirconocene. Another app lication of silver-mediated additi on is synthesis of homologues of unsaturated aldehydes by hydrozirconation of alkene/alkyne ethers with the Schwartz reagent and addition of aldehyde followed by acidic hydrolysis. This method allows 2 and 4 carbon homologization with very good to excellent yields [55]. Also, utilizing

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11 addition of aldehydes by a bimetallic 3-me thylsilyl-1-propenylzirconocene chloride allows synthesis of such valuable or ganic reagents as 1,3 dienes [56, 57]. Other reactions of practical importance are the ring opening of oxiranes producing alcohols [58] and carbometallati on of alkenes and alkynes. The interesting feature of the latter is that only single add ition is achieved through the me tal-alkyl addition, rather than polymerization. Among possible carbometalla tions, carboalumination is the most common; this method is highly stereoa nd regioselective and can produce various products depending on the starti ng alkene/alkyne and the subs equent Al-C bond cleavage method [59-61]. 1.4.3 Cationic Zirconocenes as Lewis Acid Catalysts As in nucleophilic addition reactions, ca tionic zirconocene species are produced by abstraction of an anion from a neutral complex, usually zirconocene dichloride or triflate, by AgClO4. The main application of such sp ecies is catalysis of Diels-Alder reactions of -unsaturated aldehydes or ketones and common dienes [62]. Zirconocene catalysts are advantageo us over the usual AlCl3 Lewis acid catalyst because of their lower susceptibility to water and oxygen, thus eliminating the need for extensive drying of reaction solvents, and generally leading to significantly lower amounts of catalyst required (<1 mol%). Albe it efficient (up to 106 reaction acceleration), the catalysts provide rather poor reaction product regioand stereosele ctivities [63]. The major drawback, however, is the tendency to catalyze polymerization reactions instead, resulting in the competing side reaction of a diene polymerization, thus limiting possible application of such catalysts to only very reactive diene/di enophile pairs. Other reactions catalyzed by cationic zirconocene species in clude various rearrangements/isomerization

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12 reactions, such as, for instan ce, rearrangement of epoxides into corresponding aldehydes [58], which proceeds with generally good yields. 1.5 Single-Center Metal Polymerization The main practical application of zirc onocenes is undoubtedly in polymerization catalysis. The active species is generally ac cepted to be a coordinatively unsaturated cation such as LL’MCH3 + (M=Ti, Zr; L= 5-cyclopentadienyl or related ligand). The extensive interest in this cl ass of compounds was triggered not only by very high catalytic activities (comparable to those of enzymes!), but also by the ability to exert stereocontrol of the resultant polymer by modification of the “Cp” ligands, thus creating polymers with a wide array of properties unattaina ble by other means [64]. Single-center metal polymerization has stemmed from conventiona l Ziegler-Natta polymerization, which is reviewed briefly below. 1.5.1 Ziegler-Natta Polymerization In classical heterogeneous Ziegler-Natta polymerization, the process takes place at the structural defects (dis locations and edges) of TiCl3 crystals [65]. The proposed mechanism for such reactions pos tulates polymer chain growth by cis -insertion of the olefin into a Ti-C bond on the surface [66, 67]. The non-uniform ity of the active sites in heterogeneous catalysts is partially responsib le for the polymer’s high polydispersity. The more advanced version of the Ziegle r-Natta catalyst, developed on the basis of group 4 metallocenes, was comprised of mixtures of Cp2TiCl2 and AlR2Cl and exhibited moderate activity [68]. The propos ed mechanism of its action involves olefin coordination and insertion into th e Ti-C bond of electron-deficient Cp2Ti +R species. The latter are formed via ligand exchange between aluminum and titanium and formation of Cp2TiRCl, followed by coordination of the Lewi s acidic aluminum to the titanium’s

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13 chloride and polarization of the Ti +-Cl bond. The Ti-C bond “prepared” in this way readily inserts ethylene [69]. This mechanism is similar to that postulated for heterogeneous Ziegler-Natta catalysis: in both cases the Lewis acidity of the metal center plays a key role, and cis insertion of the olefin into th e M-R bond is observed. Originally the nature of the active center was not fu lly established; however there were early indications that Cp2TiR+ was an actual catalyst [70]. Unambiguous proof of this premise was obtained when the ionic species [Cp2ZrMe(L)]+[BPh4]was found to be a very active ethylene polymerization catal yst [71]. It was shown th at this compound generates cationic metallocene species that are capable of olefin polymerization without the aluminum alkyl activators. Further studies confirmed the crucial role of metallocene cations as catalytically active species in homogeneous polymerization catalysis [72-83]. 1.5.2 Metallocene-Catalyzed Olefin Polymerization The major drawback of homogeneous pol ymerization catalysts, which impeded their industrial application, was unacceptably low efficiency and an inability to polymerize alkenes other than ethylene. A breakthrough was achieved with the application of methylaluminoxa ne (MAO) [84, 85], which, at the present time, is used almost exclusively in cataly tic olefin polymerization. Unlik e in Ziegler-Natta catalysis, treatment of the metallocene/Al alkyl catalyst with miniscule, but exact, amounts of water can immensely augment the catalyst’s activ ity as well as providing the ability to polymerize alkenes more complex than ethyl ene. Various otherwise inactive systems were shown to achieve exceptional ac tivity upon such treatment [84, 85]. The currently accepted mechanism of MAO formation assumes partial hydrolysis of the solid AlMe3 resulting in formation of extended networks of (AlMeO)n. Introduction of catalytic amounts of Cp2MR2 (Cp = cyclopentadienyl or related ligand; M

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14 = Ti, Zr; R = Cl, Me) into an excess of MAO generates Cp2MMe+ species via initial methylation of metal-halide bonds and furthe r abstraction of a methyl anion from the Cp2MMe2. The MAO serves as a very low-c oordinating counterion, thus forming a [Cp2MMe]+[MAO-Me]“salt”. Very low coordination of the anion enables high activity of the catalyst. The importance of the zirconocene cation being “free” in orde r to exhibit its activity, also discussed in Ch apter 1.3, has led to development of various ways for preparation of weakly coor dinating anions. Such system s are called ‘single site’ metallocene catalysts, implying that polymeriza tion occurs solely at the metal center. Large anions such as [B(C6H4R)]and carboranes have been used as counterions, but they exhibit rather strong interactions with the cation [86]. Less than tw o decades ago, a very weakly coordinating perfluorinated tetrap henylborate anion was introduced as a counterion [79] which afforded exceptionally high activity of the single-site zirconocene catalyst for alkene polymerization, exceeding that of the catalysts with MAO as a counterion (co-catalyst). Various co-catalysts for metal-catalyzed olefin polymerization have been reviewed [87].

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15 CHAPTER 2 FTICR MS: METHOD OVERVIEW 2.1. Introduction and Historical Background Valuable insights into reaction mechan isms, including those of polymerization reactions, can be obtained using Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS) [88-92] for studies of the intrinsic gas-phase reactivity of transition metal complexes. The spatial separation of reacting species under highvacuum conditions has certain advantages, mo st importantly the absence of complicating factors prevailing in solution su ch as association by ion pairi ng, interactions with solvent, and intraand intermolecular processes leading to destruction/modification of the active species. This is especially relevant in catal ytic studies, since active species are often present in trace amounts and very short-lived. On the other hand, reaction mechanisms in the gas phase may be different from those observed in the condensed phase. However, information about such processes can provi de a valuable description of reactions occurring at extreme conditions and assi st in explanation of possible reaction intermediates/side products which might occur in traditional chemistry in the liquid phase. The key issues that can be addressed by the gas-phase studies are: the effects of solvent and intrinsic reactivity, as well as the effects of ancillary lig ands and impact of aggregation and ion-pairing on reactivity of ionic species; the thermochemical determinations of various gas-phase r eaction energies; and, importantly, direct observation of key reactive intermediates, as well as identification of solution species in

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16 general (speciation). It is po ssible to refine our understa nding of the condensed-phase chemistry through the study of analogous gas-phase chemistry. FTICR has become very important tec hnique due to the method’s ability to provide mass resolution and accuracy unmatche d by any other type of mass spectrometer. Another important factor is the ability to combine FTICR mass spectrometry with various ionization techniques. Rapid growth of the number of FTICR mass spectrometers installed in laboratories around the world has occurred after the disc overy of electrospray ionization (ESI), which triggered extensive application of FTICR mass spectrometry in studies of biomolecules [93, 94]. Another adva ntage of this method is the capability of trapping and storing ions inside the spectrometer, which enables investigation of the ion chemistry. The principles of Fourier transform ma ss spectrometry are based on the cyclotron resonance theory, developed in the 1930s [95]. The firs t attempt at employing the principle of ion cyclotron re sonance in a mass spectromete r was undertaken in 1950 [96], while the first FTICR mass spectrometer was built in 1974 by Comisarow and Marshall [97] At the present time, FTICR mass spect rometry is used widely for studies and characterization of a broad range of system s. A large number of reviews have been written on this topic [92, 98-104]. The principl es of the FTICR MS technique are briefly reviewed below. 2.2 Theory and Instrumentation All FTICR mass spectrometers are comprise d of certain components. Since the ion cyclotron motion is caused by a magnetic field, a magnet is required. The magnets can be permanent, electromagnetic or supercond ucting. The first of these three have very low field strengths, unacceptable in many cases for the purposes of FTICR. The best of

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17 electromagnets are capable of producing fields in the 1-2 Tesla range, which is suitable for ions of relatively low mass-to-charge ra tio. Since the performa nce of the FTICR MS correlates with the magnetic field st rength, superconducting magnets capable of producing fields ranging from 2 to 9.4 Tesl a are commonly used. The highest magnetic field used in any of the current ly operational mass spectrometers is 14.5 T at the National High Magnetic Field Laboratory in Tallahassee, Florida [105]. Another main component of any FTICR ma ss spectrometer is the analyzer cell. All of the manipulations with ions (storage, ion/molecule re actions etc.) and ion signal detection are carried out inside it. Various cell designs have been used, with cubic and cylindrical cells being the most common ones. The cubic cell is composed of six plates: two plates are perpendicular to the magnetic fi eld and used to confine the ions inside the cell, hence the “trapping” plates name. Norma lly the trapping plates of cubic cells have openings for the electrons or ions to be introduced into the ce ll. The other four plates are the ion excitation and detection plates. The f unction of those is discussed below. Other types of cells, though different in design, are comprised of plates or cylinders which perform the same functions as in the cubic cell. In the cylindrical cell, excitation and detection plates form a cylinder, with the tra pping plates remaining parallel to each other (Fig 2.1), while in the open cylindrical ce ll trapping plates are replaced by two open cylinders at either side of the cell. The advant age of such designs is better suitability for positioning in the bore of a superconducting magnet. Various other cell designs have been developed [92].

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18 Figure 2.1. Schematic representation of a cylindr ical FTICR analyzer cell (adapted from Ref. [104]). 2.2.1 Ion Cyclotron Motion The magnetic field exerts force on a charge d particle moving in it, if a component of the particle’s velocity is perpendicular to the magnetic field axis. This force is called the “Lorentz force”. For a charged particle its velocity, magnetic field and the exerted Lorentz force are mutually perpendicular. The value of the Lorentz force acting on a charged particle in the magnetic field is: F = qvBsin where q is the charge on the particle, v is the velocity of the particle, B is the magnetic field strength (in Tesla) and is the angle between the velo city of the charged particle and the direction of the ma gnetic field. In case when = /4 (particle moves perpendicularly to the magnetic field) this expression simplifies to F = qvB The total charge on an ion is equal to: q = ze

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19 where e is the charge of electron and z is the total number of electronic charges on the ion. The resultant trajectory of an ion will be circular, so it will be precessing in the plane perpendicular to the magnetic field in a fa shion called “cyclotron motion”. The frequency of such motion depends on the mass-to-charg e ratio of the ion a nd strength of the magnetic field: or Here m is the mass of the ion and is the angular frequency, f is the cyclotron frequency. Therefore, ions of lower m / z have higher cyclotron freque ncies than ions of higher m / z As mentioned above, the FTICR analyzer ce ll (Fig. 2.1) consists of two excitation and two detection plates along w ith two trapping plates in orde r to confine the ions inside the cell. Potentials applied to the trapping pl ates are usually low (~ 1 volt), but they leads to the side effect of a non-zer o potential at the ce nter of the FT-ICR analyzer cell due to electrostatic fields from the trapping plates. This can offset the center of the cyclotron motion and result in the “magnetron motion” [92, 103] in addition to the orbiting motion. The magnetron frequency can be calculated as follows: Here fm is the magnetron frequency (in Hz), Vtrap is the potential applied to the trapping plates, a is the distance between the trapping plates and is a geometry factor of the

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20 FTICR analyzer cell [92, 106]. Magnetron fr equencies are usually of an order of magnitude of a few hundred Hz (unlike cyclotro n frequencies, which are typically within the kHz to MHz range). The magnetron fre quency depends upon cell dimensions, design, applied trapping potentials, and magnetic field strength. The resultant trajectory of an ion in the cell is a combination of three co mponents: the cyclotron motion, the magnetron motion, and the oscillation between the trappi ng plates along the magnetic field axis. The magnetron motion [92, 106] and the overall complex ion motion [102, 107-109] are discussed in detail in the literature. 2.2.2 Acquisition of Mass Spectra When the packets of ions in cyclotron mo tion repeatedly move past two detection plates, charge moves within the detection circuit to counteract the proximity of the passing ions. The voltage change between the detection plates can be measured as a function of time. The resultant data are calle d “transient”, “time-do main” data, or “free induction decay” (Fig 2.3a). Prior to detecti on, however, it is necessary to perform ion “excitation” application of a radio frequency voltage to the two excitation plates at the frequency resonant with the cyclotron fre quency of individual ions. The excitation is necessary due to two factors: when the ions are first formed in, or enter into the FTICR cell, the radii of their cyclotr on orbits are too small to be de tectable and the ions therefore must be excited to detectable radii (Fig 2.2). Also, prior to the exci tation, initial cyclotron motion of ions is incoherent, and the effect of ions passing opposite detection plates cancels out so no transient can be observed. Appl ication of the RF volta ge drives the ions to larger orbits along with colle ction of ions into “ion packet s”, which results in coherent motion, i.e the cyclotron motion of an ion cloud, rath er than individual ions. Such an ion

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21 packet will induce a potential in the detection plates which alternates with the cyclotron frequency of ions with various m/z Figure 2.2. Schematic representation of a cross section of an FT-ICR analyzer cell, where ions are being excited by the RF potentia l applied to the excitation electrodes (adapted from Ref. [104]). The magnitude of the signal induced on dete ction plates is proportional to the total charge and the proximity of the ions to the detection plates (orbital radius), and is independent of the magnetic field strength. Su ch a waveform is comprised from different cyclotron frequencies of all of th e ions in the cell. The next step is, therefore, extraction of data about the different ion packets. Th is is achieved throu gh application of the “Fourier transform” operation (FT)[110] wher e frequency information is obtained from time-domain data (Fig 2.3b).

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22 a) Time-domain waveform. b) Frequency domain data. c) Resultant spectrum cal ibrated in terms of m / z. Figure 2.3. Stages of data in the FTICR experiment.

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23 A resultant spectrum is a function of signa l magnitude from cyclotron frequencies of the ions present. As shown above, the cyclotron frequency depends on the m / z ratio, so it is feasible to plot signal magnitude as a function of m / z (Fig 2.3c). An excitation waveform consisting of a linear increase or sweep through the resonant frequency range of inte rest at constant amplitude is known as a “chirp”. Ions of different m / z values are excited to orbits of the same radius, even though their cyclotron frequencies differ. Such application of a complex frequency signa l to the excitation plates, which causes the tra pped ions to absorb energy at their specific resonance frequency, can also be used for selective ejectio n of ions from the cell. If a broad range of frequencies is applied to cover the mass range of interest with amplitude and duration adjusted to drive all ions to cyclotron orbits still contained within the cell dimensions, the optimal image current can be obtained. However, such excitation also allows ejection of all of the ions from the cell if their orbits reach the electrodes. It is possible, therefore, to construct a complex signal that misse s one or more frequency windows of a predetermined width. Such an approach is called “stored waveforms inverse Fourier transform” or SWIFT [111]. Applying such a sign al to the trapped ions causes ejection of all ions with exception of a select ed cluster of ions with given m/z 2.3 Ionization Methods Formation of ions can be achieved by multiple means: ions can be formed either by removal/addition of an electron leading to ions M+/M-, by protonation/deprotonation leading to protonated/deprotonated molecules MH+/(M-H)-, or by addition of another small ion leading to cationized molecules such as MNa+. Ions can be formed in an MS ion source from a neutral precursor, or pre-exis t in a solution phase. Also, ionization can be occurring within the homogeneous high magnetic field region (internal ionization

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24 sources) or externally to the magnetic field. In th e latter case, transfer of ions into the MS cell is required. Despite sometimes limited understanding of the exact means of ion formation [112] the use of various ioni zation methods enables mass spectroscopic analysis of a wide array of species from sma ll molecules to large pr oteins. A brief review of some of the ionization methods is given below. 2.3.1 Electron Ionization The process of electron ionization (EI) involves interaction of gas phase molecules with electrons accelerated to about 10-100 eV. As a primary product, EI forms an excited odd elec tron ion-radical M+.. These ions are additionally destabilized by having an unpaired electron and, usually, ar e capable of high-energy, sometimes quite complex, reactions. Although the stability of ions may be increased by lowering the energy of the ionizing electrons, most of the time ion fragmentation is observed. In general, the presence of molecu lar ions in spectra is an exception rather than a rule. Some structural characteristics, su ch as those favoring charge lo calization, may have a profound effect on fragmentation and may increase the relative stability of parent ions or channel fragmentation processes. For example, for a series of normal hydrocarbons, the intramolecular vibrational and rotational super-cooling of molecules in a supersonic molecular beam (SMB) lead to a significant increase in the stability of molecular ions formed by EI [113]. With conventional EI normal hydrocarbons show only barely detectable M+., while such ions dominated in the spectra obtained with SMB. In a variation on the EI method, metastab le atom bombardment (MAB), gas phase chemi-ionization (not chemical ionization!) can be induced by metastable atoms of noble gases. Employing one of the five of them (w ith excitation energy decreasing from 20 to 8

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25 eV from helium to xenon, it is possible to cont rol the energy transferred to the molecules being ionized. 2.3.2 Chemical Ionization Chemical ionization (CI) is a gas phase process involving ion-molecule reactions with species derived from a reagent gas (or gas mixture) that is first ionized by EI at high electron energy. Typically, the even electron protonated molecule MH+ is formed. An efficient CI process, in the case of ion sources producing a cons tant beam of ions, requires relatively high pressure of the reagent gas (~1 Torr), but in instruments that trap ions for an extended time, ion sources ma y work at much lower pressure. Due to collisional cooling of the nascent ions by mol ecules of the reagent gas the CI process is relatively soft. The ionization efficiency and type of ions formed depend on the specificity of ion-molecule reactions. For ex ample, a high electron affinity can provide very efficient ionization leading to Mions. Other types of CI ion formation include association with species derived from the r eagent gas or from “true” chemical reactions [114]. 2.3.3 Electrospray Ionization Electrospray ionization (ESI) [115-118] is the ionization technique most frequently used in conjunction with FTICR mass spectrometry. ESI has the advantage of being a soft ionization technique, resulting in th e vaporization and ionization of a sample, at the same time minimizing fragmenta tion. The latter is particularly important for labile molecules such as biological mol ecules, and for the study of complex mixtures where fragments must not interfere with the mass spectrum so the original constituents can be determined. ESI frequently leads to formation of multiply charged ions, particularly in the case of large biomolecu les. Since mass spectrometry is based upon the

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26 determination of m / z not mass alone, it is frequently th e case that a mass spectrum will contain the same molecules but in a variety of charge stat es, and therefore at several different m / z values. As ions become more highly charged, the m / z becomes lower and the spacing between the “isotopomers” become s narrower. As a result, it becomes more difficult to resolve the signals and the reso lution of the mass analyzer becomes more important. The charge state of an ion can be determined by examining the spacing between the isotopomer signals. The increasing importance of the electrospray ionization technique is promoting the growth of the FTICR mass spectrometry, due to the ultra-high resolution of FTICR mass spectrometers For these reasons, the ESI FTICR mass spectrometry has become increasingly importa nt for study of biological molecules in recent years. 2.3.4 Matrix Assisted Laser Desorption/Ionization An alternative mean of ion formation is desorption of ionic species from the solid phase with desorption energy supplied by a lase r. Direct laser deso rption of analyte can only be effective if the max of the analyzed compound matc hes the laser output. Matrix assisted laser desorption/ionization (MAL DI), on the other hand, does not pose this restriction and provides good i onization efficiency at high mass ranges [119]. In MALDI, the light energy is first absorbed by an excess (usually >1000) of a cr ystalline matrix with the absorption maximum close to the laser out put. A short burst of ions formed by a single laser pulse is followed by ion analysis. In a simplifie d model, matrix evaporation forms excited and ionized species that in turn ionize other components. As in the case of other soft ionization methods, neutral species which are pres ent in the plume (before its dissipation in vacuum) provide a cooling effect, thus contributi ng to the stability of ions

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27 formed. Many compounds have been tested as matrices for MALDI, but only some work well. For different classes of compounds, ma trices show striking differences in the ionization efficiency and the difference in th eir suitability for different method of ion analysis. For instance, matrices working we ll with MALDI TOF may not be suitable for MALDI FTICR. MALDI may show lack of re producibility resulting from the matrix selection, matrix-to-compound ratio, form of ma trix crystallization, and possible presence of impurities including those formed by a degrading matrix. MALDI and other soft ionization methods are highly selective and subject to strong suppression effects. Although this selectivity may facilitate detection of co mpounds with well recognized ionization characteristics, at the same time it may create an uncertainty in characterizing novel compounds as well as in evaluating pur ity. Suppression effects may completely prevent some or all components from producing ions and may lead to a situation in which ions observed represent only the most easily ionized components a nd not those that are quantitatively important. As a result, the rem oval of salts, organic buffers, and detergents is important in avoiding sample unrelat ed ions and suppression effects [120]. 2.3.5 Atmospheric Pressure Chemical Ionization Atmospheric pressure chemical ioniza tion (APCI) is most commonly used in combination with liquid chromatography. Th is ionization technique involves high temperature nebulization of solutions in the presence of a corona discharge or -emitter while solvents or solvent additives (such as for example, ammonium acetate) play the role of a reagent gas. Ions formed at the atmospheric pressure are transferred to the high vacuum part of a mass spectrometer via se veral stages of pumping and mass analyzed [121].

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28 2.3.6 Cationization The formation of cationized ions has been recognized as a very useful in the analysis of compounds that resist protona tion [122]. Even traces of alkali metals, especially the ubiquitous s odium ion, may lead to full or partial cationization, but dissolving salts or hydroxides in a matrix al lows much better control of this process. Elements with well recognizable isotopic patt erns, for example silver or thallium, both with two naturally occurring isotopes (107Ag 51.3% and 109Ag 48.7%; 203Tl 29.5% and 205Tl 70.5%), produce easily disc ernable attachment ions [ 123]. Analogously, mixtures of alkali metal salts can produce well recognizable patterns. It is also possible to facilitate cationization by using special derivatives inco rporating groups with a high affinity for alkali ions (such as, for example, crown ethers) [124].

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29 CHAPTER 3 GAS-PHASE CHEMISTRY OF METAL IONS AND IONIC COMPLEXES 3.1 Gas-Phase vs. Solution Chemistry It is quite apparent that ions studied under the conditions inside an FTICR mass spectrometer will exhibit higher reactivity than those in the solution due to a combination of two factors. The isolated charge on the ions due to the absen ce of counterions and solvent, coupled with high-energy collisions result in the accumulation of significant energy that cannot be discarded into a solven t bath. Furthermore, the possible formation of excited states during high energy ionization events, such as collisions with “hot” electrons (exceeding the appearance energy (A E) by numerous electron volts) during EI, can lead to a distribution of excited states. Often the ground states species comprise less than a half of the total io n population [125-130]. Application of special techniques, like surface ionization (SI) or high-pressure ionizat ion sources, is required to overcome this complication. The value of such gas-phase studies, therefore, is not only in the possibility of determining thermodynamic and kinetic data (which often cannot be directly applied to the processes in solution). It is also in the possibility of obtaining a precise and thorough description of the impact of f actors which would be very hard to account for in the actual reaction in the solution phase due to interaction of species of interest with solvent, clustering, nonhomogeneity and so on. Exampl es of such factors are the effects of ligands, electronic configuration and periodic trends, as well as the detection of possible elusive reaction intermediates, etc. A brief review of some aspects of the gas phase chemistry of metal ions and simple organometallics is given below.

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30 3.2 Gas-Phase Reactivity of Bare and OxoMetal Ions Significant interest in the chemistry of metal ions in the gas phase was triggered by a discovery of their abili ty to perform specific activ ation of carbon-hydrogen and carbon-carbon bonds [131]. Both activation of unstrained carbon-carbon bonds by bare transition metals [132, 133] as well as by oxide cations, MO+ [134] have been discussed in number of reviews. A recent review provides an update of the chemistry of alkane activation in the gas phase w ith a focus on thermochemical data for various metal-carbon bond energies [135]. An alkane activation reacti on is promoted when there is an empty orbital on the metal which accepts electrons from the car bon-carbon bond being cleaved. If such an orbital is filled, the repulsive interact ion makes the reaction inefficient [130, 136]. Simultaneously, the electrons on the metal’s orbital with -symmetry are back-donated into the orbital of the breaking bond, thus leading to its dissoc iation if the reactants are capable of getting sufficiently close. For e fficient activation both of the interactions described above are required. Interest in the chemistry of metal oxo-i ons stems from their potential role as reaction intermediates in vari ous oxidation processes, in particular the oxidation of hydrocarbons. Since alkane oxidation by a transition metal oxo-cation is generally exothermic (Fig. 3.1), a catalytic cycle can pot entially be devised if the metal cation can be reoxidized in situ by a suitable reagent (such as oxygen, hydrogen peroxide, nitrogen oxides, etc.).

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31 Figure 3.1. A reaction profile for hydrocar bon oxidation by a transition metal oxo-ion. An example of methane activation by FeO+, a most widely studied species, is shown in Scheme 3.1 [134]. Scheme 3.1 An example of a true gas-phase catalytic cycle for the oxid ation of ethylene by nitrogen dioxide with CeO2 + as a catalyst is shown in Scheme 3.2 [137]. However, the turnover rate was limited to only 8 cycles per CeO2 + ion due to reaction of the latter with the hydrocarbon contaminants present in the NO2 gas, resulting in generation of CeO2H+ species, which were unreactive toward ethylene.

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32 Scheme 3.2 3.3 Gas-phase Reactivity of Ligated Metal Ions Even though studies of ligated metal ions in the gas phase are not as comprehensive as those of bare ions, there is a significant number of publications on this subject, especially for the complexes of gr oup 8-10 metals, with the main focus on Fe and Co. The simplest complexes, metal hydrides, can react with alkanes and alkenes in the gas phase to form either metal alkyl or allyl complexes by the loss of one or two molecules of dihydrogen (or methane) [138]. Co(allyl)+ is generally more reactive than Fe(allyl)+. Both complexes react rapidly with alkanes (excluding methane) by a C-H bond activation mechanism resulting in th e formation of one or two dihydrogen molecules [139]. Among MCH3 + and LMCH3 + ions, FeCH3 + and CoCH3 + ions are also the most widely studied. The latter is capable of reac ting with alkanes larger than ethane by an initial insertion into a C-H bond. Su bsequent methane loss is followed by dehydrogenation or alkane elimination resulti ng in cobalt allyl complexes [140]. Both FeCH3 + and CoCH3 + react with cyclic alkanes.

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33 The reaction of MCH3 + ions with alkenes is particularly important with regard to Ziegler-Natta polymerization. The Cp2ZrCH3 + complex exhibits distinct reactivity with unsaturated hydrocarbons in the gas phase. Reactions of Cp2ZrCH3 + are reviewed in the following sub-chapter. FeCH3 + is unreactive with ethylene, while CoCH3 + forms an insertion product which decomposes via lo ss of dihydrogen to form a stable allyl complex. Such M(allyl)+ complexes are also formed by FeCH3 + and CoCH3 + ions from higher alkenes as a result of methane loss and possible elimination of dihydrogen or alkene [139]. Alkene complexes Me(alkene)+ of Fe+[141], Co+ and Ni+ [142] primarily exhibit ligand displacement and condensation reactions However, bis(allyl) complexes resulting from dehydrogenation by double allylic C-H ac tivation could also be formed. These complexes were not observed for the bare me tal ions. Also, some ligand coupling, similar to metal-assisted Diels-Alder reactions, was reported for Co(alkene)+ and to a small extent for Fe(alkene)+ complexes. Studies of cyclopentadienyl complexes, in particular CpCo+, in reactions with aliphatic alkanes larger than methane reve aled formation of mainly dehydrogenation products along with a small amount of products resulting from C-C bond cleavages [143]. In the reaction of CpNi+ with a variety of aldehydes, formation of decarbonylation products was reported [144]. It was shown that Cp2Zr+ is capable of abstracting a chlorine atom from chlorosubstituted methanes, indicating that the disso ciation energy of the Zr-Cl bond is at least 81 kcal/mol [145]. In addition, the cyclopentad ienyl complexes of iron and nickel were

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34 shown to undergo a metal-switching reaction with a number of metal ions (Ti, Rh, Nb) [146]. 3.4 Reactions of the Bis( 5-cyclopentadienyl)methylzirco nium Cation in the Gas Phase The following sub-chapter reviews applicati on of FTICR MS as a primary tool for studying species involved in the process of me tallocene-catalyzed ol efin polymerization. The physico-chemical properties of zirconocene ions, their intrinsic reactivity, and their reaction patterns have been studied. The effect of the solvent on reactivity can be estimated by comparison of gas-phase results with data for condensed phase single-site polymerization catalysis. 3.4.1 Reactions with Alkenes The first study of reactivity of Cp2ZrCH3 + with unsaturated hydrocarbon in the gas phase [147] indicated that no polymerizati on took place at all, but rather that the reaction proceeded exclusively via eliminati on of dihydrogen or alkene and formation of an 3-allyl complex (Table 3.1) [147]. Table 3.1. Reactions of Cp2ZrCH3 + with alkenes (adapted from Ref. [147]).

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35 The suggested reaction mechanisms involve insertion/dehydrogenation accompanied by statistical H/D scrambli ng with deuterated ethylene or a -bond metathesis (Fig. 3.2). a) Insertion/dehydrogena tion reaction mechanism b) -bond metathesis reaction mechanism Figure 3.2. Reactions of Cp2ZrCH3 + with alkenes (adapted from Ref. [147]). The observed reaction pathways were rationalized by the insufficient thermalization of “hot” ions produced the during electron ionization event (Fig. 3.3) [147-151]. The observed loss of a neutral molecule (H2, CH4) in the gas-phase reactions can be suppressed. Three ways can be suggest ed so the reaction pathway observed in the liquid phase would be approached : first, increasing the pressure by, for example, pulsing an inert gas, to quench excess internal en ergy by collisions; second, by coordination of a weakly bound molecule (for example, a solv ent) by the coordinatively unsaturated

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36 metallocene cation. The loss of a solvent mol ecule will result in excess energy being taken away as translational energy of a departing moiety; or third, by increasing the size and complexity of coordinated ligands, creating an “energy sink” where the excess energy can be efficiently redi stributed among a multitude of available vibrational states, thus decreasing the probability of allyl complex formation. Figure 3.3. Comparison of qualitative potential energy surfaces for the reaction of Cp2ZrCH3 + with ethylene in solution and the gas phase (adapted from Ref. [147]).

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37 The first approach was implemented by Plattner e t al [152]. Stable solutions of the tetrakis(pentafluorophenyl)borate sa lt of the methylzirconocene cation, Cp2ZrCH3 +, in a mixture of acetonitrile and CH2Cl2 were electrosprayed with a "high" pressure, 10 mTorr, of thermalization gas (Ar). This led to insertion of up to four molecules of alkene before deactivation. The observed rate cons tant of propylene polymerization under these conditions was estimated to be about 6 orders of magnitude higher than that in solution [152, 153], which is an excellent illustration of intrinsic high reactivity of an isolated cation in the absence of a coordinating count erion or coordination by solvent molecules. The loss of hydrogen was more difficult to suppr ess –reaction with et hylene still led to allyl complex formation. Besides reactions of metallocenes with unsaturated hydrocarbons, previous gasphase studies have involv ed investigation of the reactivity patterns of bis( 5cyclopentadienyl)methylzirconium cations with nitriles [154] and e ffects of ancillary ligands on LL’ZrCH3 + cation reactivity with dihydrogen and ethyl ene [150] and other alkenes [151]. Also, the intrinsic reactiv ity of hydroxyzirconocenes was studied as a model for metal-hydroxide complex ion reactio ns with a range of compounds [155]. Our experimental data (F igure 3.4) indicate the abili ty of C-H bond activation by the Cp2ZrCH3 + complex in cyclic alkanes with cons ecutive formation of a complex with cyclic allyl structure. In the reaction w ith cyclopentane, a co mplex with m/z 287 was observed. The experimental procedure and a ppearance of mass spectra of zirconocene complexes are discussed in detail in Chap ter 4. The ion with m/z 287 was presumably formed via loss of dihydrogen and methane (S cheme 3.3). The proposed structure of the complex is shown on Figure 3.5.

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38 Figure 3.4. Mass spectrum for the reaction of Cp2ZrCH3 + with cyclopentane. Cp2ZrCH3 + + C5H10 Cp2ZrC5H7 + + CH4 + H2 Scheme 3.3 Formation of such a complex assumes initia l activation of a C-H bond by a metal center. Figure 3.5. Proposed structure of the complex with m/z 287. 3.4.2 Allyl Complex Formation: Mechanistic Di fferences of Reaction Patterns in the Gas and Liquid Phases One of the main drawbacks of metallocene catalysts is a gradual decrease in the rate of monomer uptake with time, which occu rs on the timescale of several hours, or, in some cases, even minutes [156, 157]. The exact nature of this phenomenon is still debatable; to a certain degree, the deactivati on results from factors such as presence of impurities and high-temperature decomposition. Recently, it has been postulated that a m/z 290 280 270 260 250 240 230 220 Abundance 100 90 80 70 60 50 40 30 20 10

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39 possible cause may be accumulation of a less active or inactive zirconium-allyl complex [72, 151, 152, 158-162]. The formation of this complex was observed or inferred on several occasions in both (a s discussed above) gas-phase ex periments [148, 154] and in the solution phase [72, 151, 152, 158161, 163-165]. Formation of H2 gas during the polymerization reactions cataly zed by zirconocenes is a possi ble indication of the allyl complex formation [166]. Various theoretical studies of the bis( 5-cyclopentadienyl)methylzirconium cation reactivity toward dihydrogen and alkenes [167-171] or even alkanes [172, 173] have been reported. However, the subsequent transformati on of the allyl complex after its formation in the reaction mixture is not completely cl ear. Initially, it was presumed that this complex is catalytically in active [72]. As shown by Zieg ler and co-workers [174, 175], the allyl complex can be reactivated by alke ne molecule insertion; their theoretical studies of zirconocene and tita nocene cations indicate that the limiting step in the reaction of the allyl complex formation reaction is a -hydrogen elimination and that it has an activation barrier of ~ 11 kcal/mol for the zi rconium and ~ 13 kcal/mol for the titanium complex. The liquid-phase studies by Britzinger and co-workers of the Cp2ZrMe(methallyl) complex [162] showed that its perfluorotriphenyl borane adduct, the contact ion pair [Cp2Zr(methallyl)]+[MeB(C6F5)3]-, reacts with propene with rates substantially lower than those of cationic Zr -alkyl species. The prope ne molecule inserts between Zr and one of the allylic termini. Their DFT calculations have indicated that insertion of propene directly into an 3-coordinated Zr-allyl unit occurs with lower activation energy than insertion into an 1-bound Zr-allyl species and that the lowest-

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40 energy pathway for the reactiv ation of a cationic zirconocene allyl species is its reconversion to the corresponding Zr-alkyl species by H2. Nevertheless, experimental proof of the allyl complex reactivation has yet to be demonstrated. It is expected that formation of the transiti on state for alkene insertion by the allyl complex and its consecutive transf ormation into a vinyl-terminated product can be controlled by various means. It also appears to be necessary to test the intrinsic reactivity of the Cp2Zr(allyl)+ complex by studying its reac tivity under the gas phase lowpressure conditions of an FTICR MS spectrome ter, thus eliminating various perplexing factors such as effects of impurities, elect ric permeability of the solvent, ligands exchange, etc.

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41 CHAPTER 4 GAS-PHASE REACTIONS OF BIS( 5-CYCLOPENTADIENYL) METHYLZIRCONIUM CATIONS WI TH KETONES AND ALDEHYDES 4.1 Rationale Despite rapid growth in the gas-phase chemistry of organometallic complexes and, especially, metal ions [134, 135, 176, 177], and the major importance of metallocene catalysis, gas-phase studies of metallocenes are still rather scarce. The purpose of the work reported in this chapter was to gain a deeper understanding of this chemistry, with a focus on reaction mechanisms of the gas-phase chemistry of metallocenes, and to attempt to draw general conclusions about these pr ocesses. For example, based on the work reported in Chapter 3, it may be postu lated that the formation of an 3-allyl complex is an instance of a general process which takes place under high-vacuum, non-equilibrium conditions. Scheme 4.1 shows for the reaction mechan ism suggested in earlier studies [151] for the reactions of methyl zirconocen es with terminal olefins, Y = CH2. For reactions with olefins such as isobutene or styrene, this scheme can be applied for the Y=C(R1)R2 substrate (R1 = H or R). In the case of 1,1 disubstitu ted alkenes, elimination of methane was observed [147]. At the same time, in the liquid phase, the 2 alkene insertion products of zirconocenes are gene rally stable and have no tendency to eliminate a neutral molecule.

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42 Scheme 4.1 In the case of aldehydes and ketones, the group Y is the carbonyl oxygen. Compared to alkenes, the carbonyls are expect ed to be coordinate d more strongly to Zr(IV) by donation of an electron lone pair from oxygen (as opposed to -bonding electrons in the alkenes) to an empty d-orb ital of the metal. Ther efore, initial bonding of the substrate with the metal is expected to occur via the substrate’s carbonyl oxygen. In the gas phase, under high-v acuum, high-energy collision conditions, the reactivity

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43 observed might possibly also be described by the general Scheme 4.1. This would be quite different from the known chemistry in th e solution phase, as discussed in the last chapter. For example, for silver-mediated (i.e chloride abstraction from Schwarz reagent by a catalytic amount of AgClO4 [54]) aldehyde addition reactions of Cp2ZrRCl complexes, a mechanism involving generation of alkyl/alkenyl zirconocene was proposed [54]. The resultant reaction products are alcohols formed through condensation of the aldehyde and R ligand of zirconocene. The chemistry of zirconocene carbonyl, acyl and related species presents interest due to, for example, the potential of complexes like [LnM{CHClCH2C(=O)R]+ to carry out insertion polymerization/copolymerizati on of vinyl chloride, thus enabling better control of PVC polymer structure and propert ies than is possible by conventional radical methods [178]. The work presented in this chapter repres ents a study of the reactivity pathways of bis( 5-cyclopentadienyl)methylzirconium ( 1 ), chosen as a model compound, with a series of simple ketones and aldehydes in the gas phase. 4.2 Experimental Procedures Ion-molecule reactions were studied using Fourier transform ion cyclotron resonance mass spectrometry. The primary ma ss spectrometer used in the experiments was based on a 2.0 Tesla magnet with an inte rnal electron ionizati on (EI) ion generation source (Fig. 4.1). A common experiment sequence was used: 1) Quench: application of +/9.5 V to tr apping plates in order to eliminate any ions present prior to the experiment.

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44 Figure 4.1. Schematic diagram of the mass spectrometer used in this work. 2) Electron beam: irradiation of the neutra l substrates with the flux of electrons produced by the filament of the EI (Electr on Ionization) source (Fig. 4.2). The number of electrons was controlled by the filament current, typically in the range of 1-2 A. The electron energy distribution, controlled by the voltages on the repeller plate and the grid, was varied from 15 to 50 eV in order to produce an optimal number of ions. Figure 4.2. Schematic diagram of the electron ionization source. 3) Ion ejection (optional): if necessary, the ejection of ions with selected m/z from the MS cell by applying SWIFT waveform [179] could be carried out.

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45 4) Reaction delay: time delay varying from 0 to10 s allowing unejected ions to react with the neutral(s) present in the cell. 5) Excitation: excitation of the reaction product ions. 6) Detection: detection of the image curre nt with subsequent Fourier transform of the transient to obtain the mass spectrum. The above sequence of events (scan) was repeated for each experiment 20 to 40 or more times in order to achieve a higher signal to noise ratio by a signal averaging. Table 4.1. Isotopes of zirconium. Isotope Natural Abundance, % 90Zr 51.45 91Zr 11.22 92Zr 17.15 93Zr Radioactive 94Zr 17.38 95Zr Radioactive 96Zr 2.80 Zirconium has five stable isotopes (T able 4.1), which in superposition with 13C/12C isotopes result in a unique pattern of seve n peaks in mass spectra for all of the organozirconium compounds studied in this work, enabling unambiguous identification of the presence of Zr. Electron ionization of Cp2Zr(CH3)2 resulted in the formation of two ions: Cp2ZrCH3 + and Cp2Zr+. The typically obtained mass spectra of Cp2Zr(CH3)2 are shown in Fig. 4.3

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46 m/z 240 235 230 225 220 Abundance 100 90 80 70 60 50 40 30 20 10 a) No reaction delay m/z 240 235 230 225 220 Abundance 100 90 80 70 60 50 40 30 20 10 b) Reaction delay 0.5 s. Figure 4.3. EI mass spectrum of Cp2Zr(CH3)2: Cp2ZrCH3 + (m/z 235-242), Cp2Zr+(m/z 220-227) and Cp2ZrOH+ (m/z 237-244) Since the Cp2Zr+ ion and its reaction products compli cate the spectrum, in order to distinguish products from these two possible parents, additional studies using selective SWIFT ejection were implemented when neces sary. An example of a SWIFT experiment is shown in Figure 3.4. When one possible parent ion Cp2ZrMe+ (m/z 235-242) is ejected,

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47 the reaction products’ pe aks (m/z 277-284) disappear. On the other hand, when another possible parent Cp2Zr+ (m/z 220-227) is ejected, no change is observed. This indicates that the product is formed from the Cp2ZrMe+ ion. An incomplete removal of either complex by SWIFT, es pecially of the Cp2ZrMe+ ion, stems from a partial re-formation during a reaction delay event vi a collisions of neutral Cp2Zr(CH3)2, which is present in the cell at all times, with Cp2Zr+ ions. a) No ions ejected b) Cp2Zr+ (m/z 220-227) is ejected m/z 290 280 270 260 250 240 230 220 210 Abundance 100 90 80 70 60 50 40 30 20 10 m/z 290 280 270 260 250 240 230 220 210 200 Abundance 100 90 80 70 60 50 40 30 20 10

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48 m/z 300 290 280 270 260 250 240 230 220 210 Abundance 100 90 80 70 60 50 40 30 20 10 c) Cp2ZrMe+ (m/z 235-242) is ejected Figure 4.4. Example of a SWIFT experiment. The typical background pressure on the 2.0 T instrument was ~ 210-9 Torr; pressure of zirconocene was ~ 10-7 Torr, decreasing throughout e xperiment at the rate of approximately 10-8 Torr/hour due to the sample evapor ation. A set of experiments at different substrate pressures of 0.510-7, 110-7, 210-7 and 410-7 Torr and common reaction times of ~ 0, 0.5, 1, 2, 5 s and l onger were carried out for each reaction. The formation of binuclear metal ions Cp4Zr2(CH3)n + (n = 0 3) in reactions of both parent ions with the neutral complicated the spectrum, especia lly at longer reaction times (Fig. 4.5), which impeded the ability to st udy reactions with rate coefficients below ~ 109 1/Ms. m/z 500 490 480 470 460 450 440 Abundance 5 4 3 2 1 Figure 4.5. Binuclear complexes Cp4Zr2(CH3)n + (n = 0 3)

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49 The presence of water also decreased the amount of the ion of interest as it reacted very rapidly to form a hydroxyl complex (m/z 237-244) (see Fig. 4.3): Cp2ZrCH3 + + H2O Cp2ZrOH+ + CH4 Therefore, precautions had to be taken to minimize the extent of hydrolysis. At prolonged reaction times the Cp2ZrOH+ ion and its reaction products dominated the spectrum. In some cases hydrolysis interfered with the identificati on of reaction products even at typical reaction times of 1-5 seconds. Cp2Zr(CH3)2 and all of the aldehydes, ketones, and imines used in these studies were purchased from commercial sources. Th e liquids were purifie d by repeated freezepump-thaw cycles. Cp2Zr(CD3)2 was synthesized from commercially purchased precursors in accordance with a preparation procedure described in the literature [180] via the following reaction: Cp2ZrCl2 + 2CD3MgI Cp2Zr(CD3)2 + 2MgICl A Schlenk line was used at all times due to the extreme air and moisture sensitivity of both the reactants and the product. The reacti on was carried out in diethyl ether solvent at room temperature under a flow of dry ni trogen in thoroughly desiccated glassware. The reaction product’s precipitate was dissolved in ether/pentane mixture for the final product extraction followed by a recrysta llization from benzene at -20 C. The mass spectra of the Cp2Zr(CD3)2 obtained from this procedure are shown in Fig. 4.6. As expected, the Cp2ZrCH3 + (m/z 235) complex is replaced with the Cp2ZrCD3 + (m/z 238) complex, while the hydrolysis product, Cp2ZrOH+ (m/z 237), is unaltered.

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50 m/z 245 240 235 230 225 220 Abundance 100 90 80 70 60 50 40 30 20 10 a) No reaction delay m/z 245 240 235 230 225 220 Abundance 100 90 80 70 60 50 40 30 20 10 b) Reaction delay 0.5 s. Figure 4.6. EI mass spectrum of Cp2Zr(CD3)2 ( Noise peaks). Ab initio (Hartree-Fock) as well as Density Functional Theory (DFT) calculations were carried out by Cesar S. Contreras for the zirconocene complexes to establish the most stable structures.

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51 4.3 Reaction with Ketones For the sake of simplicity, a ll of the seven Zr-containi ng isotopic ions are further reported as a single mass correspond ing to the most abundant (51.45%) 90Zr isotope. In cases where there is an overlap of severa l series of peaks co rresponding to different products, a detailed analysis of the contributions from indivi dual ions has been carried out. Since the isotopic abundances of the c onstituent elements are known, and at least one peak containing the lightest 90Zr isotope and thus of lowest m/z does not overlap with other peaks, the heights of individua l series of peaks can be estimated. In previous studies under similar conditi ons it has been demonstrated that the cyclopentadienyls are solely spectator ligands – as no Cp “switching” reactions and HD elimination for the complexes with fully deut erated ligands but non-deuterated Cp rings have been observed [147]. Therefore, the Cp2Zr unit was presumed to remain intact throughout all of the experiments. Where ques tions exist about the structures of the product ions, their formulae ar e reported in the form Cp2ZrCxHyOz + with the corresponding m/z ratio. Since all of the reported ions we re singly charged the term “empirical formula” is used throughout the text Detailed structural considerations are presented later in this chapter. 4.3.1 Reactions of Cp2ZrCH3 +/Cp2ZrCD3 + with Acetone/d6-Acetone The reaction of 1 (m/z 235) with acetone (nominal molecular weight 58 amu) proceeds with formation of several products (Fig. 4.7). Two principal ones with m/z 293 and 351 correspond to the consecutive addition of one and two substrate molecules, as shown in Scheme 4.2. Addition of the s econd acetone molecule becomes pronounced only after 4 seconds of reaction time (at a total pressure of approximately 210-7 torr). A variety of structures can be pos tulated for the observed adducts, e.g. 1 vs. 2 complexes,

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52 etc., (only one of the likely structures is show n), and, therefore, theoretical geometry optimizations and/or spectroscopic data are needed for more definitive “proof” of structure. a) 1 second reaction time b) 5 seconds reaction time Figure 4.7. Mass spectra of ions formed in the reaction of Cp2ZrMe+ with acetone. An ion with m/z 277 (Fig.4.7; the peak with m/z 278 corresponds to the product depicted in Scheme 4.2), correspond ing to the empirical formula Cp2ZrC3H5O+, apparently resulting from elim ination of methane from the i on with m/z 293, is formed to a lesser extent than the latter, and its peak height decrea ses at longer reaction times. It is coordinatively unsaturated and adds an additional acetone molecule, forming an ion with m/z 335 (Scheme 4.3, Figure 4.8). m/z 350 300 250 200 Abundance 100 90 80 70 60 50 40 30 20 10 m/z 277 m/z 350 300 250 200 Abundance 100 90 80 70 60 50 40 30 20 10 m/z 293 m/z 351

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53 Scheme 4.2 Scheme 4.3

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54 Figure 4.8. Mass spectra of ions formed in the reaction of Cp2ZrMe+ with acetone The hydrolysis product of 1 (m/z 235), bis(cyclopenta dienyl)zirconium hydroxyl cation Cp2ZrOH+ (m/z 237, Fig. 4.3), formed by reacti on with trace amounts of water present in the mass spectrometer, reacts w ith acetone to give an addition product with m/z 295, which becomes predominant at very long reaction times (5 seconds or more). This indicates a slower react ion rate and, for these species reacting with acetone, is consistent with the demonstrated intrinsi c lower reactivity of the hydroxide derivative compared to the methylzirconium cation itsel f [155]. The formation of binuclear metal ions Cp4Zr2(CH3)n + (n = 0 3) and their derivatives complicated th e spectrum, especially at longer reaction times and had to be taken into account as well. Reaction of 1 with acetone proceeds differen tly for different stages of thermalization of the reaction mixture. The consecutive addition of two acetone molecules by the Cp2ZrCH3 + ion results in a coordina tively saturated 18 electron complex. The loss of methane and the subse quent generation of the complex with m/z 277 results from the reaction of 1 ions with sufficient in ternal energy (Scheme 4.4): m/z 280 15 10 5 m/z 345 340 335 2 1

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55 Cp2ZrCH3 + + O=C(CH3)2 Cp2ZrC4H9O+ Cp2ZrC3H5O+ + CH4 m/z 235 m/z 58 m/z 293 m/z 277 Scheme 4.4 Consideration of the gasphase chemistry of bis( 5-cyclopentadienyl) methylzirconium cations described in the lite rature suggests that the species with m/z 277 have structures similar to those pos tulated for the gas-phase reaction of 1 with unsaturated hydrocarbons, i.e. 3-allyl complexes [147, 154] The possibility of 3-allyl complex formation is well documented [72, 151, 152, 158-161, 163, 164]. Density functional theory studies also confirm the plau sibility of allyl intermediate formation as a possible side reaction in ho mogeneous single-site olef in polymerization [174, 175]. However, there is no direct evidence supporting 3-allyl structures postulated for the reaction of 1 with acetone. Also, it is difficult to suggest tenable reaction mechanisms which would lead to the formation of such complex(es). Our DFT calculations, carried out by Cesar S. Contreras, sugg est that the most stable structure is not an 3-allyl complex, but rather that of the complex shown in Fig. 4.9, which is an 3-enolate complex. The higher stability of this complex can be understood: the 3-enolate complex is isoelectronic with the 3-allyl complex and both the allyl and enolate complexes in eith er resonance form participate in both and dative bonding of the ligand to a metal center, thus co ntributing to high stab ility of the complex. The ability of the oxygen atom in the 3-enolate complex (even though it is weaker donor compared to the met hylene group) in the oxygen -bonded resonance form to provide additional bonding by partially transferri ng its available lone pair into empty Zr 2d orbital results in increased total bond st rength. In another re sonance form, dative bonding from the oxygen atom is substantially more efficient than -complexation of the

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56 C=C bond by a metal center. According to the Dewar-Chatt-Duncanson model discussed above [26, 27], an efficient -bonding occurs when a -type donation from the C=C orbital is synergistic with concomitant -backbonding into an empty orbital of the alkene which in turn implies that only me tals with at least one filled nonbonding orbital can efficiently form such complexes. For the d0 zirconocene, only donation of electrons is possible from the C=C bond, making the -complexation rather inefficient. a) b) Figure 4.9. The a) 3-enolate and b) 3-allyl complexes. In the reactions of Cp2ZrMe+ with d6-acetone, normal addition products of d6acetone (nominal molecular weight 64 amu) to 1 are expected to have m/z 299 and m/z 363, 6 and 12 mass units, respectively, higher th an for the products seen with the nondeuterated substrate. Both of these ions were observed, and their intensities obeyed the same trend as in case of nondeuterated acetone: only one molecule was coordinated initially and the product with two acetone molecules became the major peak after few

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57 seconds reaction time. In addition, form ation of a product ion with m/z 302 was observed, which became predominant after 5 s. The 3 mass unit gain can be assigned to the following product (Scheme 4.5): Scheme 4.5 This ion results, apparently, from CH3/CD3 scrambling between the metal center and carbonyl substrate. Three ions with m/z 279, m/z 280 (major product), and m/z 282, corresponding to the empirical formula Cp2ZrC3H5-nDnO+, n = 2, 3 and 5 (approximate peak height ratios ~ 2:5:1), replace Cp2ZrC3H5O+ (m/z 277), the ion reporte d above in reactions of 1 with acetone (postulated to be an 3-enolate complex). The observed CH3/CD3 scrambling and lack of H/D scrambling suggest reversible transfer of the methyl group between the Zr and a coordinated acetone molecule as a unit, rather than transfer of individual H atoms. This in turn implies that in the complex with m/z 280 the CD3 group remains intact (Scheme 4.6): Scheme 4.6

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58 The transfer of the deuterated met hyl group introduces only a negligible secondary isotope effect, so the migratory insertion-deinsertion equilibrium remains unaffected for all of the studied comb inations of zirconocene/acetone: in the approximation of a E arising only from a change in ze ro-point energy of the stretching vibration of the Zr-C bond the predicted isotope effect is: kCD3/kCH3 = e-, where 3 3 ~1 2CD CHkT hc [181] Assuming ~(Zr-C) ~ 500 cm-1, the calculated value for is ~ 1 10-3, thus kCD3/kCH3 1. Cp2ZrCD3 +, ( 2 ), reacted with acetone similarly to 1 with a corresponding shift of all the peaks by 3 mass units. However, two ions (m/z 277 and m/z 280, corresponding to the empirical formulae Cp2ZrC3H5O+ and Cp2ZrC3H2D3O+)) with comparable peak heights, and a small amount of an ion with m/z 279 were also formed. Similarly, the reaction of 2 with d6-acetone resulted in formation of ions with m/z 302 and 366 (addition of one and two molecule s of d6-acetone respectively) and an ion with m/z 282, corresponding to the empirical formula Cp2ZrC3D5O+ (Scheme 4.7). Cp2ZrCD3 + + O=C(CD3)2 Cp2ZrC4D9O+ Cp2ZrC3D5O+ + CD4 m/z 238 m/z 64 m/z 302 m/z 282 Scheme 4.7 This complex is analogous to the product with m/z 277 ( 3-enolate complex), but fully deuterated (except for the Cp rings). Formation of the complex with m/z 282 proceeded with a deuterium kinetic isotope effect as the apparent rate of formation of the corresponding complex (m/z 277) in the reaction of 1 and acetone was approximately twice as high. The primary isotope effect indicates C-D bond cleavage in the rate-

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59 determining step in the reaction sequence leading to the forma tion of the enolate complex, since, in this case there are no C-H bonds availabl e in the reacting ligands. All other observed reactivity trends were similar to those described above for the reactions of 1 : increasing reaction time led to increasing ad dition of a second acetone molecule to the zirconocene complex, disappear ance of the m/z 282 comple x along with concomitant hydration and hydrolysis reactions due to inev itable traces of water, prevalent at very long reaction times (5 seconds and above). The proposed reaction mech anism for the reaction of 1 with d6-acetone as an example is shown in Schemes 4.8 a) and b). The second step, the migratory insertion of a coordinated ketone molecule, is reversible and leads to CH3/CD3 scrambling. This equilibrium is fast and proceeds via the intermediate insertion product where all three substituents at the oxygen-bound carbon are equi valent, thus leading to a statistical distribution of methyl and me thyl-d3 groups between the metal and coordinated ketone (Scheme 4.8a). When a ketone molecule is coordinate d by the metal center, the Brnsted-Lowry acid-base reaction can take place. The me tal-bound alkyl group is a good Brnsted base, the -proton of a ketone is slight ly acidic (and significantly more acidic in the ketone molecule coordinated by a meta l) and the nascent enolate is expected to be a reasonably good leaving group. The combination of these f actors enables nucleophi lic attack of the alkyl on the -proton of a ketone. This reaction is a ssumed to be the slowest step and to proceed via the least strained 6-membered cyc lic transition state. If two different groups are available at the carbonyl group, two different products can be formed, which, in combination with products formed in the reaction of either the CH3or CD3nucleophile,

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60 will comprise the observed set of products (Scheme 4.8b). The products resulting from reaction involving nucleophilic attack on a deut eron are expected to be less abundant due to the primary isotope effect resulting from C-D bond cleavage. Scheme 4.8a The expected product abundance ratios ar e completely consistent with those observed in the experiment.

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61 Scheme 4.8b 4.3.2. The Reactions of Cp2ZrCH3 +/Cp2ZrCD3 + with 2-Butanone, Methyl Isobutyl Ketone and Cyclohexanone. The product distributions in reactions of 1 and 2 with ketones other than acetone are also entirely consistent with th e proposed reaction mechanism. Both 1 and 2 sequentially add two substrat e molecules (Scheme 4.9 illustr ates the case for methyl isobutyl ketones as a substrate, exampl es of spectra are shown in Fig. 4.10).

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62 a) 1 second reaction time b) 5 seconds reaction time Figure 4.10. Mass spectra of ions formed in the reaction of Cp2ZrMe+ with methyl isobutyl ketone. m/z 400 350 300 250 Abundance 100 90 80 70 60 50 40 30 20 10 m/z 335 m/z 277 m/z 319 m/z 435 m/z 400 350 300 250 200 Abundance 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 m/z 277 m/z 335 m/z 319 m/z 235 m/z 237 m/z 220

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63 Scheme 4.9 The major products observed are summarized in Table 4.2 Table 4.2. Major reaction products observed in the reactions of 1 and 2 with 2-butanone, and methyl isobutyl ketone at various reaction times (1 to 5 seconds). Observed Reaction Products Reaction m/z Suggested Empirical Formula Neutral eliminated 307 [Cp2ZrCH3 O=C(CH3)C2H5]+ 379 [Cp2ZrCH3 2O=C(CH3)C2H5]+ 277 Cp2ZrC3H5O+ C2H6 1 (m/z 235) + 2butanone (nominal molecular weight 72 amu) 291 Cp2ZrC4H7O+ CH4

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64 Table 4.2 Continued Observed Reaction Products Reaction m/z Suggested Empirical Formula Neutral eliminated 310 [Cp2ZrCD3 O=C(CH3)C2H5]+ 382 [Cp2ZrCD3 2O=C(CH3)C2H5]+ 279 Cp2ZrC3H3D2O+ C2H5D 280 Cp2ZrC3H2D3O+ C2H6 291 Cp2ZrC4H7O+ CHD3 293 Cp2ZrC4H5D2O+ CH3D 2 (m/z 238) + 2butanone (nominal molecular weight 72 amu) 294 Cp2ZrC4H4D3O+ CH4 335 [Cp2ZrCH3 O=C(CH3)i-C4H9]+ 435 [Cp2ZrCH3 2O=C(CH3)i-C4H9]+ 277 Cp2ZrC3H5O+ C4H10 319 Cp2ZrC6H11O+ CH4 1 (m/z 235) + methyl isobutyl ketone (nominal molecular weight 100 amu) 377 [Cp2ZrC3H5O+ O=C(CH3)i-C4H9]+ 338 [Cp2ZrCD3 O=C(CH3)i-C4H9]+ 438 [Cp2ZrCD3 2O=C(CH3)i-C4H9]+ 279 Cp2ZrC3H3D2O+ C4H9D 2 (m/z 238) + methyl isobutyl ketone (nominal molecular weight 100 amu) 280 Cp2ZrC3H2D3O+ C4H10

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65 Table 4.2 Continued Observed Reaction Products Reaction m/z Suggested Empirical Formula Neutral eliminated 319 Cp2ZrC6H11O+ CHD3 321 Cp2ZrC6H9D2O+ CH3D 2 (m/z 238) + methyl isobutyl ketone (nominal molecular weight 100 amu) 322 Cp2ZrC6H8D3O+ CH4 333 [Cp2ZrCH3 O=C(CH2)5]+ 431 [Cp2ZrCH3 2O=C(CH2)5]+ 1 (m/z 235) + cyclohexanone (nominal molecular weight 98 amu) 317 Cp2ZrC6H9O+ CH4 336 [Cp2ZrCD3 O=C(CH2)5]+ 434 [Cp2ZrCD3 2O=C(CH2)5]+ 2 (m/z 235) + cyclohexanone (nominal molecular weight 98 amu) 317 Cp2ZrC6H9O+ CHD3 In the reaction of 1 with 2-butanone, a complex with m/z 277, (most likely resulting from the loss of ethane from the single adduct ion with m/z 307), was produced to a lesser extent than in the reactions with acetone. An ion with m/z 291 (most likely produced by methane loss from the m/z 307 ion) was nearly as abundant as the m/z 277 ion. The intensities of both p eaks appeared to change with time in a similar fashion. Substantial fragmentation of the coordinated ligands resulted in multiple product peaks, increasing with reaction time. Scheme 4.10 s hows as an example possible origination of the ion with m/z 293 in the reaction of 1 with 2-butanone:

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66 Scheme 4.10 The reaction of 1 with methyl isobutyl ketone also produced a m/z 277 complex (presumably resulting from C4H10 loss by the m/z 335 ion), which was the major reaction product for reaction times less than one sec ond. The complex with m/z 277 was nearly three times more abundant than the complex with m/z 319 (presumably resulting from the loss of methane by the m/z 335 ion). For both ke tones, variations of peak heights with reaction time followed the same trends as for complexes with m/z 277, 291 and 319. As in the reaction with 2-butanone, apparent ligand frag mentation was also observed. The loss of a zirconocene methyl gr oup (and probable elimination of a neutral alkene), possibly in a multiple-step pro cess, was commonly observed (Scheme 4.11). Scheme 4.11 The coordinatively unsaturated complex with m/z 293 reacted further by addition of a methyl isobutyl ketone molecule, resu lting in a product with m/z 399. The reactions of 2 with 2-butanone and methyl isobutyl ke tone produced fragmentation patterns that were remarkably similar to those produced in reaction of 1 A series of peaks with same

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67 m/z were observed, due to the common loss of the deuterated/nondeuterated methyl group of the metallocene along with the fragmentation. Scheme 4.12 The complex with m/z 277 is presumed to be the same reaction product for both of these ketones and acetone, i.e. the 3-enolate complex. The complexes with m/z 291 and m/z 319 were formed in comparable amounts with m/z 277, and, according to the reaction mechanism proposed above, differ from the 3-enolate complex with m/z 277 Cp2ZrCD3 + + O=C(CH3)C2H5 m/z 238 m/z 72 m/z 310 [Cp2ZrCH3 O=C(CD3)C2H5]+Cp2ZrOC(C2H5)=CD2 + + CH3D [Cp2ZrCD3 O=C(CH3)C2H5 ]+[Cp2ZrC2H5 O=C(CH3)CD3]+[Cp2ZrCH3 O=C(CD3)C2H5]+ Cp2ZrC5H8D3O+ Cp2ZrOC(CD3)=CHCH3 + + CH4 m/z 293 m/z 294 Cp2ZrOC(C2H5)=CH2 + + CHD3 [Cp2ZrCD3 O=C(CH3)C2H5 ]+ Cp2ZrOC(CH3)=CHCH3 + + CHD3 m/z 291 m/z 291 Cp2ZrOC(CD3)=CH2 + + C2H6 [Cp2ZrC2H5 O=C(CH3)CD3] + Cp2ZrOC(CH3)=CD2 + + C2H6 m/z 280 m/z 279

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68 (carrying the methyl group) only by subs titution of the corre sponding alkyl (Et, i Bu,) at the central carbon, or both carbons (Me, Me and Me, i Pr) of the enolate ligand. Thus, the ketones other than acetone, R1-C(=O)-R2 (R1 = Me, R2 = Et, i Bu) react with Cp2ZrCH3 +/Cp2ZrCD3 + in a similar manner, but two differe nt substituents at the carbonyl carbon enable additional possible reaction pathway(s), with the loss of one of the alkyls as the corresponding alkane, as shown in Scheme 4.12 for the reaction of 2 with 2butanone. The expected product abundances are quite consistent with the experimental values, except for higher abundance of the complex with m/z 277 in the reactions of 1 and 2 with methyl isobutyl ketone, which was almost three times more abundant than would be expected from a pur ely statistical distribution of alkyl groups and an assumed isotope effect kH/kD ~ 2. This possibly results from a combination of two factors: higher nucleophilicity of the isopropyl as compared to the methyl group and release of steric tension when the relatively bulky i Bu group is removed from the metal coordination sphere. Scheme 4.13

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69 Scheme 4.14 Cyclohexanone (nominal molecular weight 98 amu) reacted with 1 forming simple addition products of m/z 333 and 431 onl y, with no other complexes and just two products: the loss of methane leads to form ation of complexes with m/z 317 and 415 respectively. The same enol ate product (m/z 317) is fo rmed in the reaction of 2 with cyclohexanone. This is additional evidence fo r the proposed reaction mechanism, as two

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70 groups at the ketone’s carbonyl are locked in to the ring and unable to migrate so that CD3 group scrambling is impossible, and the deuterated group is eliminated as methane resulting in the same comp lex as for the reaction of 1 (Scheme 4.13). The fragmentation reactions which do not conform to the stoichiometry of the expected products of reactions fitting the pa ttern of Scheme 4.8 for the reactions of both 1 and 2 with all of the ketones studied result in minor amounts of products and were pronounced only for the more branched methyl isobutyl ketone and very long reaction times. The observed fragmentation product p eaks most likely result from ions with different isomeric structures with the same m/z. One of the possible pathways is shown in Scheme 4.14 for the reaction of 1 with 2-butanone. The formation of a carbocation via C-H bond scission enables further rearrangements[182] and at some point is followed by elimination of neutral(s) lead ing to the variety of fragmentation products. 4.3.3 Reactions with Other Ketones In the reactions of Cp2ZrCH3 + with 1,4-benzoquinone and 1,4-naphthoquinone, no interesting reactivity was observed (except for very small amounts of the complex with m/z 277 formed in reaction with 1,4-benzoquinone). However, both substrates exhibited an unusual tendency to coordinate readily with the binuclear metal complexes (Fig. 4.11) In the reaction of Cp2ZrCH3 + with diethyl carbonate, the peaks corresponding to the complex with m/z 471 (additi on of two substrate molecule s) overlap with the region of the binuclear metal ions Cp4Zr2(CH3)n + (n = 0-3), which severely impairs quantitative analysis. The observed peaks are summarized in the Table 4.3

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71 Figure 4.11. Ions produced in the reaction of Cp2ZrMe+ with benzoquinone: products of coordination of quinone molecule by the binuclear complexes Cp4Zr2(CH3)n + (n = 0 3) Table 4.3 Major reaction products observed in the reactions of 1 and 2 with diethyl carbonate at various reaction times (1 to 5 seconds). Reaction m/z Empirical Formula 353 [Cp2ZrCH3 O=C(OC2H5)2]+ 471 [Cp2ZrCH3 2O=C(OC2H5)2]+ 1 + diethyl carbonate (nominal molecular weight 118 amu) 277 Cp2ZrC3H5O+ 356 [Cp2ZrCD3 O=C(OC2H5)2]+ 474 [Cp2ZrCD3 2O=C(OC2H5)2]+ 2 + diethyl carbonate (nominal molecular weight 118 amu) 279/280 [Cp2ZrC3H3D2O+/ Cp2ZrC3H2D3O]+ The complex with m/z 277 was formed in smaller amounts than in the reaction with acetone. This complex was still abunda nt enough for the addition product (m/z 395) to be observed. Several ligand fragmentation products, always accompanied by the loss of a methyl (or methyl-d 3 in the reactions of 2 ) group were observed; a few of the possible resulting structures of some of the observed ions are shown in Scheme 4.15: m/z 550 500 Abundance 15 10 5

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72 Scheme 4.15 In the case of the reaction of 2 with diethyl carbonate, in stead of the unsaturated complex with m/z 277, two series of peaks, m/z 279 and 280, with relative intensities 1:3 were observed. This intensity ratio was inde pendent of time. The total intensity obeyed the same trends observed before: a maximum in tensity is reached at 2 seconds of reaction time with gradual disappearan ce at longer reaction times. 4.4 Reaction with Aldehydes Reaction patterns for 1 and 2 with all of the studied aldehydes were remarkably different from those observed in the reactions with various ketones: often no simple substrate molecule addition products were formed at all. Instead, at short reaction times a single (or major) product, resulting from di hydrogen loss from a postulated short-lived carbonyl molecule addition product, was obtai ned in all of the cases (Table 4.4). In reaction of 1 with acetaldehyde (Fig. 4.12), increasing the reaction time led to the addition of another substrate molecule by the single reaction product, the postulated 3-enolate complex (m/z 277), forming an ion with an m/z 321. The combined abundance of the ions with m/z 277 and 321 decreased si gnificantly at longer reaction times due to water contamination of the ace taldehyde sample, and formati on of the hydration product of the ion with m/z 277 (m/z 295). The bis( 5-cyclopentadienyl) zirconium hydroxide cation, Cp2ZrOH+ was also formed in significant amounts, resulting in an eventual

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73 intensity shift into peaks belonging to th e hydroxyl cation coordi nating one substrate molecule (m/z 281), Scheme 4.16. Table 4.4. Major reaction products observed in the reactions of 1 and 2 with acetaldehyde, benzaldehyde, propanal and n -hexanal at various reaction times (1 to 5 seconds). Observed Reaction Products Reaction m/z Suggested Empirical Formula 277 [Cp2ZrC3H5O+] 1 (m/z 235) + acetaldehyde (nominal molecular weight 44 amu) 321 [Cp2ZrC3H5O+ CH3CHO]+ 279/280 (ion abundance ratio ~1/3) [Cp2ZrC3H3D2O]+/[Cp2ZrC3H2D3O]+ 2 (m/z 238) + acetaldehyde (nominal molecular weight 72 amu) 323/324 (ion abundance ratio ~1/3) [Cp2ZrC3H3D2O CH3CHO]+/ [Cp2ZrC3H2D3O CH3CHO] + 1 (m/z 235)+ benzaldehyde (nominal molecular weight 106 amu) 339 [Cp2ZrC2H2(C6H5)O]+ 2 (m/z 238) + benzaldehyde (nominal molecular weight 106 amu) 341 [Cp2ZrC2D2(C6H5)O]+

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74 Table 4.4 Continued Observed Reaction Products Reaction m/z Suggested Empirical Formula 291 [Cp2ZrC4H7O]+ 293 [Cp2ZrCH3 C2H5CHO]+ 1 (m/z 235) + propanal (nominal molecular weight 58 amu) 349 [Cp2ZrC4H7O C2H5CHO]+ 293/294 (ion abundance ratio ~1/1) [Cp2ZrC4H5D2O]+/ [Cp2ZrC4H4D3O]+ 295 [Cp2ZrCD3 C2H5CHO]+ 2 (m/z 238) + propanal (nominal molecular weight 58 amu) 351/352 (ion abundance ratio ~1/1) [Cp2ZrC4H5D2O C2H5CHO]+/ [Cp2ZrC4H4D3O C2H5CHO]+ 333 [Cp2ZrC7H13O]+ 335 [Cp2ZrCH3 C5H11CHO]+ 1 (m/z 235) + n -hexanal (nominal molecular weight 100 amu) 433 [Cp2ZrC7H13O C5H11CHO]+ 335/336 (ion abundance ratio ~3/2) [Cp2ZrC7H11D2O]+/ [Cp2ZrC7H10D3O]+ 338 [Cp2ZrCD3 C5H11CHO]+ 2 (m/z 238) + n -hexanal (nominal molecular weight 100 amu) 435/436 (ion abundance ratio ~3/2) [Cp2ZrC7H11D2O C5H11CHO]+/ [Cp2ZrC7H10D3O C5H11CHO]+

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75 Figure 4.12. Mass spectra of ions formed in the reaction of Cp2ZrMe+ with acetaldehyde, 1 second reaction delay. Scheme 4.16 The intensity distribution of the two products (m/z 279 and 280) formed in reaction of Cp2ZrCD3 + with acetaldehyde, was approximate ly 1:3, nearly independent of reaction time (Fig. 4.13). Increasing the react ion time resulted in addition of another acetaldehyde molecule by these complexes (m/z 323/324), with the intensity distribution remaining unaffected. Interestingly, the same in tensity ratios were observed for the water molecule addition products, (m/z 297/298). Th is implies that, since these ions have similar reactivity, both reaction products of 2 with acetaldehyde have similar structures, m/z 400 350 300 250 200 Abundance 100 90 80 70 60 50 40 30 20 10 m/z 277

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76 i.e. both are 3-enolate complexes, and they diffe r only by the number of deuterium atoms (Cp2ZrC3H3D2O+ and Cp2ZrC3H2D3O+ respectively). a)b) Figure 4.13. Mass spectra of ions with m/ z 279 and 280 formed in the reaction of Cp2ZrCD3 + with acetaldehyde, at a) 1 second and b) 5 seconds reaction delays. In the reaction pattern of 1 with benzaldehyde (Fig.4.14) the complex with m/z 339 (Cp2ZrC2H2(C6H5)O+) is coordinatively unsaturate d, analogous to the complex with m/z 277. Figure 4.14. Ions produced in the reaction of Cp2ZrMe+ with benzaldehyde at 1 second reaction delay. m/z 350 300 250 Abundance 100 90 80 70 60 50 40 30 20 10 m/z 290 285 280 275 Abundance 30 25 20 15 10 5 m/z 285 280 275 Abundance 60 50 40 30 20 10

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77 The reaction of 2 with benzaldehyde resulted in formation of the same set of products as in reaction of 1 with a two mass unit shif t corresponding to the two deuterium atoms in the products, incl uding the hydration product (m/z 359, Cp2ZrC2H2D2(C6H5)O2 +). The product of benzaldehyde molecule addition to Cp2ZrOH+ ion (m/z 343) dominated in the spectrum at l ong reaction times (>5 s.). In the reaction of 1 with propanal, a few minor products were obs erved at longer reaction times (unlike in reaction with acetaldehyde), but the peak with m/z 291 was always the major peak in the mass spectrum. In the reaction of 1 with n -hexanal, at reaction times 5 seconds and longer, the direct addition of aldehyde molecule(s) to 1 (m/z 335 and 435) was noticeable. Increasing the reac tion time led to extensive liga nd fragmentation resulting in a multitude of peaks corresponding to the loss of dihydrogen or a vari ety of alkenes by various ions. Analogous to the reaction pattern s with other aldehydes, the ion with m/z 333 also coordinated a water molecule, fo rming a hydration product with m/z 351, which was predominant at reaction times of 5 seconds and longer. The reaction of 2 with n hexanal showed the same trend of extensive ligand fragmentation at increasing reaction times as in reaction of 1 with n -hexanal. In summary, all three aldehydes reacted with 1 in the same fashion (Scheme 4.17): Cp2ZrCH3 + + RCHO Cp2ZrC2H2RO+ + H2 Scheme 4.17 where R = Me, Et, Ph, C5H11. These complexes are presumed to be the postulated 3enolate complexes with the corresponding Rsubstituent, and all of their isomers which are possible according to the reaction mechan ism. Two products were formed in the

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78 reaction of 2 with all aldehydes (except for benz aldehyde), apparently differing only by the number of deuterium atoms in the structure. The slowest step in the r eaction mechanism, as in the case of reactions with ketones, is presumed to be a nucleophilic attack on the -hydrogen (deuterium) of the coordinated carbonyl. The key feat ure of aldehydes is the pres ence of a hydrogen atom at the carbonyl carbon instead of an alkyl gr oup. This leads to th e possibility of -H elimination provided the reaction proceeds thr ough the same initial equilibrium as that with ketones, i.e., coordination of a substr ate molecule and migratory insertion. This hydrogen further acts as nucleophile. Since th e attack on the hydroge n in the phenyl ring in the reactions of 1 and 2 with benzaldehyde will lead to the disruption of aromaticity, only deuteriums of the CD3 bond are available for the reac tions, hence only one product is observed. The proposed reaction mechanism, which demonstrates this for the example of reaction of 2 with acetaldehyde, is shown in Scheme 4.18. The -H elimination is known to be a very facile process [183]. T hus, when in the rate limiting step of nucleophilic attack on an -H of the coordinated ketone an alkyl is replaced by hydrogen, a better nucleophile, while th e leaving group remains the same, the reduction of the activation barrier is significant enough to ma ke this reaction pathwa y favorable over any other available for a given system. The 6member transition state preceding dihydrogen elimination is similar to that postulated for the reaction with ketones, and is assumed to be the least strained. The obser ved kinetic isotope effect kD/kH 0.3 is consistent with that typically observed for C-D vs. C-H bond cleavage for relatively linear transition states [184, 185]. The noticeable effect of reduction in the amount of the complex carrying 3 deuterium atoms in the series ac etaldehyde – propanal – hexanal (resulting

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79 from the attack on hydrogen of -CH3, -C2H5 and -C5H9 groups correspondingly) suggests certain geometrical constrains in the reacti ons with bulkier radical s – the lower steric availability of the secondary hydrogen atoms and/or removal from the reactive center of more accessible primary ones. A postulated reaction potential surface asso ciated with this mechanism, for the reaction of 1 with acetaldehyde, is shown in Fig. 4.15a. a) Gas-phase

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80 b) Solution Figure 4.15. Qualitative potential surface for the reaction of 1 with acetaldehyde/generic aldehyde in the gas phase and in soluti on. The lettering corresponds to that in Scheme 4.21. Gas-phase estimates of ener getic effects are based on data from Ref. [173]. Formation of ions with m/z 357 and 359 in the reactions of 1 and 2 with benzaldehyde, and similar ions for other aldehyd es, is most likely due to inevitable traces of water in the instrument, leading to hydrolysis of the 3-enolate complex according to the following proposed scheme (Scheme 4.19). This hydrolysis is the first directly observed gas-phase reaction of the 3-enolate complex. The reactivity of the isoelectronic 3-allyl complex has been studied on several occasions in the solution [162] and in the gas phase [175, 186]. However, no studies have been carried out so far on the gas-phase reactivity of the 3-enolate complex, leaving this t opic open for future investigation.

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81 Scheme 4.18a The liquid-phase studies [187-193] indicate potential significance of such species in polymerization catalysis: for example, in the investigation of th e polymerization of the methylmethacrylate (MMA) with the neutral enolate Cp2ZrMe[OC(OMe)=CMe2] in the absence and in the presence of zirconocene cations Cp2ZrMe+ to bypass the rate-limited initiation of the system, it was shown that by itself the enolate is not active in polymerization, but as soon as the reacti on system contains the cation, MMA is quantatively converted into syndiotactic-r ich PMMA with high molecular weight (Mn > 100,000 g/mol) and very narrow mol ecular weight distribution (Mw/Mn < 1.05) [187].

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82 Scheme 4.18b The gas-phase studies presented in th is work augment our understanding of related processes taking place in the condensed phase. Individual reactions stages can be more easily discerned. The reaction mechanis m postulated in the or iginal study of the

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83 silver-mediated addition of al kenyland alkylzirconocene chlo ride to aldehyde [54] is shown in Scheme 4.20. Scheme 4.19 The precursor alkyl/alkenyl zirconocene chloride complex is activated via chloride abstraction by AgClO4 and resultant cation reacts w ith the aldehyde molecule. The process leading from complex (IIIA) to co mplex (IVA) and regeneration of the active species (IIA) (transfer of the R’ group) can be either interor intramolecular in nature. Since only the structures of final products ( RR’COH alcohols, formed after treatment of the reaction mixture with NaHCO3 aqueous solution) were established, the detailed reaction mechanism is not completely clear.

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84 Scheme 4.20 The reaction mechanism which is consiste nt with the gas-phase studies data presented earlier in this chapter is show n in Scheme 4.21. According to the reaction mechanism postulated for th e gas-phase reactions of 1 with aldehydes (Scheme 4.18), the coordinated aldehyde molecule under goes migratory insertion followed by -H elimination. This suggests formation of a si de product (VI). Rege neration of the active species (IIB) occurs via chloride abstrac tion by cationic complex (IVB) from neutral precursor (I). Even though this process is e ndothermic (cation IVB is more stable than IIB), the equilibrium can be shifted toward sp ecies (V) and (IIB), as the concentration of (V) is much lower than that of (I) at all times. Hydrogen elimination and formation of 3enolate complex do not take place in the liqui d phase due to efficient energy dissipation in the presence of solvent bath (Figure 4.15b).

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85 Scheme 4.21 As can be seen, the gas-phase studies can provide some comprehensive insight into the reaction mechanism by direct monito ring of possible reac tion intermediates and eliminating perplexing factors associated with the solution chemistry. 4.5 Comments on Suggested Reaction Mechanisms Several inter-related factors need to be considered in understanding the reactivity of the bis( 5-cyclopentadienyl)methylzirconium cation. Having a d0 configuration, the Zr metal center cannot be involved in further ox idation; therefore th e number of possible reaction pathways is reduced. Traces of bac kground water invariably lead to hydrolysis products which must be accounted for, especia lly in experiments with deuterium-labeled compounds. The hydrolysis reaction leads to s ubstitution of the sp ecies of interest, 1 or 2,

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86 by the Cp2ZrOH+ cation thus decreasing total ion abundanc es of these cations and their reaction products. However, the significan tly lower reactivity of the hydroxyl complex [155] compared to 1 and 2 reduces possible competition for the substrate and does not hinder reaction product identifica tion for times less than 5 seconds at the pressures used in these studies. Extended reaction times lead to eventual thermalization of r eactant ions through collisions. Ignoring long range interactions at short reaction times, the number of molecular collisions under the high-vacuum conditions found in the FTICR cell is insignificant: since kT p c zrel, where kT crel8 and b a b am m m m when assuming collisional crossection for zirc onocene cation and carbonyl molecule2d ) ( 2 1b ad d d being approximately 1 nm2, for typical masses of Cp2ZrL+ ~ 235 amu and higher and of carbonyl ~ 70 amu, ~ 910-26 kg, and relc 337 m/s; thus, for T = 295 K and p = 110-7 torr (1.3310-5 Pa), z 1.1. However, this number is actually significantly higher (approximately an order of magnitude or more) for charged species due to a long-range interaction of the positive charge of the cation wi th electron cloud of the neutral. The ion-induced dipole (“Langevi n”) potential and the ion-neutral collision rate constant can be calculated. The simp lest case of the Lange vin theory considers spiraling elastic collisions for a point-charg e induced dipole force, with the attraction potential, 4 22 ) ( r e r V Here, is the average polarizability of the neutral molecule and r is ion-molecule internuclear separation. Th e ion mobility, associated with this potential can be converted into the momentum transfer collision rate constant:

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87 m M Kr 1010 97 9 ) ( Here Mr is the reduced mass of the system and m is the ion mass in amu [194]. Better agreement with the experimental values, especially in the case of anisotropic nonpolar molecules can be achie ved if the average polarizability of the neutral is replaced by the maximum component of the diagonalized polarizability tensor ’ (this component is the same for spheri cal molecules and 5-20% larger for most anisotropic molecules [195]. This substitution gives a modified expression for the rate constant: m M Kr' 10 97 9 ) (10 The collision rate constant estimated for an ion of 100 amu colliding with a nitrogen molecule, k ~ 6.611*10-10 cm3 s-1 [92]. By multiplying this number by the concentration of neutral, the number of collisions per second per ion can be obtained. The limited number of collisions will l ead to non-equilibrium conditions, which will result in kinetically controlled reacti on products. At long reaction times, when equilibrium conditions are more closely appr oached, the observed products will result from thermodynamically controlled reactions. Here, the term “kin etically controlled reaction products” does not refer to irreversible reaction(s), bu t rather to those resulting from processes far from equilibrium. Caution must be used when comparing th e gas-phase reactivity of metallocenes to the known chemistry of their condensed-phase analogs. Very few zirconocene complexes are known in the liquid phase without triphe nylphosphine or similar stabilizing ligands and such complexes react with some substrat es in an unusual manne r. The reactivity of direct liquid-phase analogs of the system of interest, i.e. the bis( 5cyclopentadienyl)methylzirconium cation, as di scussed above, is greatly affected by the

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88 nature of the counterion. These catalytic systems were quite thoroughly studied with respect to their function in al kene polymerization reactions, but the reactivity of such systems with various substrates has not been studied systematically. Thus, perhaps the best approach for examining the gas-phase reactivity of Cp2ZrMe+ cations with carbonyls would be to compare the reactivity of this complex with alkenes in both the gaseous and liquid phases. The reaction of olefin insertion into an M-X bond (X = H, R) proceeds via equilibrium of the alkene coordination comp lex and the alkene insertion product, which is exothermic by 25 kcal/mol. Both complexes are expected to be more stable for the coordination and insertion of the car bonyl group. Even though there is no -backbonding possible for the d0 Zr4+ metal center, the lone electr on pair of the carbonyl oxygen is more accessible than electrons of the alkene, making it a stronger Lewis base. Insertion of the carbonyl into the M-X bond resu lts in formation of the metal-oxygen bond, which is typically significan tly stronger than an M-C bond. This compensates for the disruption of the C=C bond, which is usually stronger than that of C=O by ~ 30 kcal/mol. Even though oxygen is a weaker -donor than the methyl ene group, it provides additional bonding by partially transferring its available lone pair into the empty Zr d2orbital, thus increasing tota l bond strength. Theoretical ca lculations of ground state energies clearly indicate higher stability of the enolate complex compared to other possible isomers (Fig. 4.16, Table 4.5)

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89 a) b) c) d)

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90 e) f) Figure 4.16. The most stable theoretica lly calculated structures for the Cp2ZrC3H5O+ cation. Canonical structur es shown were calculated with the MPW1PW91/6311+G(d,p) functional/basis se t. The calculated absolute electronic energies of structures from a) through f) are listed in Table 4.5. The unsaturated nature of the bis( 5-cyclopentadienyl)methylzirconium cation results in a propensity to coor dinate at least two carbonyl mo lecules in order to increase its electron count from 14 to a closed-shell 18 electrons. The extent of the insertion reaction equilibrium depends on various factors su ch as the nature of the substituents at the metal center, and for the reaction of olefin insertion into an M-X bond, Keq varies by several orders of magnitude. On the othe r hand, the reaction of carbonyl insertion, compared to the reaction with alkenes, can be expected to be shifted in the direction of the substrate coordinated by the da tive bond rather than the M-O-CR1R2R3 insertion product, due to the higher stab ility of the carbonyl adduct. This assumption is supported by the experiments reported here – no addi tion of a third molecule of carbonyl was

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91 observed in any reaction of the carbonyls inve stigated, i.e. the C= O group acts as an Lligand, rather than an X-ligand in the inse rtion product, thus li miting the number of coordinated entities to two. Table 4.5. The absolute electronic energy (kcal/mo l) of the most stable structure is given, and the relative energy difference is gi ven for the remaining structures (in kcal/mol). Lettering corresponds to the st ructures pictured in Figure 4.16, a) through f). Method Basis Sets Ea b c d e f MPW1PW91 sd(97), -393187 9.6 18.4 24.4 26.1 27.7 cep-121g, -393112 18.4 crenbl, -393103 lanl2dz -392738 33.5 B3LYP sd(97), -393269 9.9 20.6 30.1 31.0 lanl2dz, -392866 8.6 25.6 32.0 34.9 34.0 lanl2dz -392806 11.8 23.2 21.7 --34.3 RHF lanl2dz -390076 11.4 22.0 13.4 32.4 35.2

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92 CHAPTER 5 GAS-PHASE REACTIONS OF BIS( 5-CYCLOPENTADIENYL) METHYLZIRCONIUM CATIONS WITH IMINES 5.1 Synthesis and Reactivity of Imines The general reaction mechanism suggested in chapter 4 for th e reactions of the methyl zirconocenes with R1R2C=X compounds was shown to encompass both the reactions with alkenes and aldehydes, and, w ith the appropriate modi fications, ketones. A further advance toward generality would suggest extension of this mechanism to reactions with the next logical clas s of compounds cont aining C=N bonds, i.e. imines. However, the chemistry of imines is significan tly different from that of both alkenes and carbonyls. These compounds are usually observe d as transitional sp ecies and very few imines can be isolated and stored. Therefore, before consideration of the reactivity of imines with organometallic species, traditional liquid phase chemistry of this class of compounds needs to be considered and discussed. Nucleophilic addition of ammonia/amines to aldehydes or ketones is the primary means of imine synthesis; however, only a few of these reactions are synthetically useful. For example, the addition of ammonia to al dehydes or ketones leads to two initial products, hemiaminals (“aldehyde ammonias”) and imines: R1R2C=O + NH3 R1R2C(NH2)OH + R1R2C=NH Both of these compounds are unstable. A ma jority of imines with a hydrogen on the nitriogen atom polymerize s pontaneously[196] (the simple st homologue, methanimine CH2=NH is stable in solution for several hours at -95C, but rapidly decomposes at -80C

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93 [196, 197]). The final reaction products are the result of various conde nsation reactions of combinations of hemiaminals and imines, and/or of imine with ammonia or carbonyl compounds The most important example of such products is hexamethylenetetramine, prepared from ammonia and formaldehyde. Ar omatic aldehydes give hydrobenzamides ArCH(N=CHAr). The addition of primary, secondary and tertiary amines to aldehydes and ketones results in various of products [198], but only the reaction of primary amines gives imines: R1R2C=O + R3NH2 R1R2C=NR3 The imines with a group at the nitrogen atom are, in general, more stable and sometimes can be isolated. However, some imines, espe cially those with a simple R group, tend to rapidly decompose or polymerize, unless there is at least one aryl group on the nitrogen and/or the carbon. Such com pounds are called Schiff bases. The formation of the Schiff bases occurs via initial fo rmation of hemiaminals followed by loss of water. 5.2 Gas-phase Reactions of the Bis( 5-Cyclopentadienyl)methylzirconium Cation with Imines Given the instability of the imines, only a few are available commercially, thus limiting our choice of substrates The reactions with four compounds were investigated: 2,2,4,4-tetramethyl-3-pentanone imine, benz ophenone imine, N-benzylidene aniline and N-benzylidenebenzyl amine (Tab le 5.1). All four compounds c ontain either aryl, or bulky alkyl groups, with or without the group on the nitrogen.

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94 Table 5.1. Imines used in this work. Imine Structure Molecular weight (amu) 2,2,4,4-tetramethyl-3-pentanone imine 141.26 Benzophenone imine 181.24 N-benzylidene aniline 181.24 N-benzylidenebenzyl amine 195.26 5.2.1 Reactions of Cp2ZrCH3 +/Cp2ZrCD3 + with 2,2,4,4-Tetramethyl-3-Pentanone Imine 2,2,4,4-tetramethyl-3-pentanone imine is the only stable alip hatic imine. The experimental setup and conditions were similar to those described in the previous chapter for the reaction with ketones and aldehydes: the pressure of bis( 5cyclopentadienyl)dimethylzirconium was in the 5*10-8 – 1*10-7 Torr range, and the substrate was leaked in to ~ 1*10-7 Torr pressure. The reaction time was varied in the 0 – 5 second range and SWIFT ejection was selectiv ely applied to different possible parent ions in order to determine the orig in of various reaction products.

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95 Scheme 5.1a In the reaction of 1, three major products were formed: m/z 376, m/z 360 and m/z 318. The first one is the product of addition of one imine molecule to zirconocene with no neutral eliminated. The latter two result from elimination of methane (m/z 360) and (most probably) isobutane (m/z 318). At the same time, reaction of 2 leads to the products with m/z 379, m/z 360 and m/z 321, i.e. the deuterated methyl group is either

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96 lost as methane (formation of m/z 360) or re tained and isobutene is eliminated (formation of m/z 321). Scheme 5.1b The course of these reactions appears to be quite similar to that of the reactions with ketones (Scheme 4.8). Elimination of e ither methane or isobutene strongly suggests, as shown in Scheme 4.8a, fast migratory in sertion equilibrium preceding the elimination (Scheme 5.1a). The next step in the react ion mechanism (Scheme 4.8) is nucleophilic attack of the Zr-bound group on the -hydrogen of a coordinated substrate. This, however, is not the most efficien t pathway in the case of imines. For coordinated 2,2,4,4-tetramethyl-3-p entanone imine, the least strongly bound hydrogen is expected to be that of the N-H bond. Even though the N-H bond is normally

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97 6 kcal/mol weaker than the C-H bond, due to coordination of the imine by the metal center, donation of a nitrogen atom’s lone pair results in an induced + charge which depletes N-H bond’s electron density and reduces the bond order. Therefore, nucleophilic attack in this case is expe cted to target the nitrogen -bound hydrogen of imine (Scheme 5.1b). Such attack leads to fina l product(s) different from those suggested by Schemes 4.1 and 4.8, as the enamine (analogously w ith the enolate) complex can no longer be formed. The resultant azomethine structures have been postulated in earlier gas-phase studies of reactions of bis( 5-cyclopentadienyl)methylzirconiu m cations with nitriles (as opposed to nitriles coordinate d by the methyl zirconocene ca tion) [154], as well as in liquid phase studies of cati onic Zr and Ti complexes [81, 199, 200]. The enamine complex can be formed via facile H shift fr om the methyl carbon to the nitrogen (Scheme 5.2). The calculated absolute electronic energi es (as described in pr evious section; used functional/basis set: MPW1PW91/ 6-311+G(d,p)) of both structur es are almost equal (in kcal/mol): for the azomethine complex, Scheme 5.2 (left), Etot=-380330.7; for the enamine complex, Scheme 5.2 (right), Etot=-380333.0. This suggests that these structures are in equilibrium with each other and both complexes contribute to the observed peak with m/z 318. In order for the enamine stru cture to be formed from the azomethine complex with m/z 360, the methyl group transfer needs to take place. This is far less likely than the hydrogen transfer, so the forma tion of the enamine structure in this case cannot be expected.

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98 Scheme 5.2 It also possible that the alternative is omeric complexes shown in Scheme 5.3 (coordinated nitriles) also part ially account for the observed p eaks. It has been discussed that such complexes are not in equilibriu m with azomethine species, but interconversion between them is possible under CID conditions [154]. The nitrile coordination product, however, is the most plausible complex in reactions of the concomitant parent ion, Cp2Zr+ (m/z 220) as shown in Scheme 5.4. Scheme 5.3 5.2.2 Reactions of Cp2ZrCH3 +/Cp2ZrCD3 + with Aryl Substituted Imines The interesting chemistry exhi bited in the reactions of 1 and 2 with 2,2,4,4tetramethyl-3-pentanone imine was not obs erved for the other imines studied.

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99 Scheme 5.4 It was found that under EI conditions used in these experiments, the benzophenone imine has an apparent tende ncy to decompose, with the biphenyl (molecular weight 154 amu) being the most abundant product of decomposition (Figure 5.1). m/z 185 180 175 170 165 160 155 150 Abundance 55 50 45 40 35 30 25 20 15 10 5 Figure 5.1. EI mass spectrum of benzophe none imine (molecular weight 181 amu). The resultant spectra of reactions of 1 and 2 with the benzophenone imine appear to be quite complex, partially due to be nzophenone imine fragments which retained

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100 positive charge forming complexes with the neutral Cp2Zr(CH3)2 (or Cp2Zr(CD3)2). However, only the ion with m/z 400 wa s produced from the parent ions Cp2ZrCH3 + (m/z 235)/Cp2ZrCD3 + (m/z 238) and, most probably, result ed from elimination of methane by the complex with m/z 416 (not observed or observed to a very small extent) (Scheme 5.5). Scheme 5.5 An identical pattern is ob served in reactions of 1 and 2 with N-benzylidene aniline: the only produc t originating from Cp2ZrCH3 + (m/z 235)/Cp2ZrCD3 + (m/z 238) had m/z 400, most probably al so resulting from the methane elimination by the complex with m/z 416 (observed in very low abundance), i.e. the elimination reaction proceeds almost to completion. Unlike the case of the benzophenone imine, there is no hydrogen atom bound to nitrogen in the N-benzylidene aniline. Thus the carbon-bound hydrogen of the imine group is the only lab ile electrophilic hydr ogen available. Nucleophilic attack in the reaction with N-benzylidene aniline, therefore, will be directed toward this hydrogen, and, due to the lack of lab ility of the phenyl group, lead to a different reaction product (Scheme 5.6).

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101 Scheme 5.6 N-benzylidenebenzyl amine exhibits an almost complete absence of reactivity with the methyl zirconocene cation as well as absence of coordination by the metal center. It is apparent that the very low lability of the aryl groups precludes the migratory insertion equilibrium, which locks imine groups at the nitrogen. Consequently, only methane elimination can be observed. Furt hermore, this also makes improbable any rearrangement leading to isom erization of the product struct ures in the Schemes 5.5 and 5.6 into one another. In summary, it is cl ear that the reactivity of imines with bis( 5cyclopentadienyl)methylzirconium cations is governed by the availabi lity of a labile hydrogen in the respective imine. This leads to reaction produ cts different from expected (Scheme 4.1), i.e. the azomet hyne/benzylidene species instead of the enamines. However, the reaction mechanism in general resembles that proposed for ketones (Scheme 4.8).

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102 CHAPTER 6 CONCLUSIONS AND FUTURE WORK 6.1 Conclusions The reactions and proposed reaction mech anisms reported here illustrate the reactivity of bis( 5-cyclopentadienyl)methylzirconium ca tions toward ketones, aldehydes and imines under gas-phase, low-pressure cond itions in the FTICR analyzer cell. These reactions exhibit distinct pa tterns for all of th e compounds studied. In reactions with ketones, addition of up to two molecules of the various carbonyls (resulting in formation of 18ecomplexes for Cp2ZrMe+) was observed, along with fo rmation of the postulated 3enolate complex whenever pos sible. In the reactions wi th aldehydes, due to the possibility of -H elimination, the Zr-bound hydr ogen acts as nucleophile, and the reaction sequence leading to th e formation of the proposed 3enolate complex is very efficient, so in all the cases this comple x was the only or major product observed. Even though the reaction with ketones pro ceeds by a similar mechanism, since H elimination is not possible the metal-bound al kyl group acts as nucleophile instead, which results in a major decrease in the amount of 3enolate complex(es) and multiple competing reaction pathways leading to severa l enolate products, as well as products of ligand fragmentation, especially pronounced in reactions with long and branched substrates. In reactions with the majority of imines, the reactivity toward bis( 5cyclopentadienyl)methylzirconium cation is gove rned by the availability of a labile hydrogen. This leads to reaction products diffe rent from those which might be expected in analogy with reaction products ob served for ketones and aldehydes, i.e.

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103 azomethyne/benzylidene species instead of the en amines. Also, due to lack of lability of the aryl groups, the migratory insertion equilibrium was not po ssible for reactions of arylsubstituted imines. Consequently, only meth ane elimination was observed. However, a general reaction mechanism resembles that proposed for ketones. The proposed mechanisms account very well for al l the observed reaction features. 6.2 Future Work Our data, presented earlier in Chap ter 4, indicate formation of an 3-enolate complex isoelectronic to an allyl in reactions of Cp2ZrMe+ with aldehydes and ketones. Neither the structure no r the reactivity of the 3-enolate complex has ever been studied in the gas phase. Even though the DFT calculations indicate that the 3-enolate structure is the most stable among likely structures, it is still necessary to demonstrate unambiguously that such structur es are, indeed, formed. Furthe rmore, the majority of the observed gas-phase species re ported in the literature ar e postulated based on observed reactivity trends, data from MS/MS studies or other indirect means. There is very little definitive, spectroscopic evidence of structure for gas-phase ionic species. For example, even though the allyl complex was one of the most commonly postulated structures formed in reactions of various metal complexe s with a wide array of organic substrates, there is no spectroscopic eviden ce confirming that conjecture. 6.2.1 IRMPD Spectroscopy Infrared multiple photon dissociation (IRMPD) spectroscopy provides a very convenient way of obtaining infrared absorpti on spectra of gas phase ions. One of the currently operational facilities is built ar ound a continuously tunabl e Free Electron Laser (FEL) which is located at the FOM Institut e "Rijnhuizen" in Nieuwegein, the Netherlands

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104 [201]. The principles of operation and experi mental setup are disc ussed in details in reference [202]. Figure 6.1. IRMPD spectrum of Cr+(Et2O)2 (adapted from Ref. [204]). The possibility of confirming postulated ionic structures by obtaining infrared spectra of the ions in question at the new FELIX-FTICR facility is quite exciting. The current ionization capabilities for the FTICR mass spectrometer located at FELIX [203] include electron ionization (E I), multiphoton ionization (MPI), matrix assisted laser desorption ionization (MALDI) and electrospr ay ionization (ESI) with an octopole ion guide and a pumping station fo r external ion sources. An ex ample of a spectrum obtained at the facility for the complex of chromium and diethyl ether is s hown in Figure 6.1. On the spectrum, the inset shows mass spectra taken with the FEL off and on the resonance near 1010 cm-1. Observed dissociation channels at m/z=126 and m/z=52 correspond to detachment of one and two ether ligands, respectively. Zooming in on the m/z = 126

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105 channel reveals the pattern due to the naturally occurring Cr isotopes, showing the level of detail typically obtainable with FT-ICR mass spectrometry. Significant success has been achieved in the application of IRMPD spectroscopy [205] for the studies of ions in the gas phase by using the FELIX free electron laser interfaced to a FTICR spectrometer. For example, in the recent investigation of gas-phase Cr+ complexes for both the monomer complex [Cr(aniline)]+ and the dimer complex [Cr(aniline)2]+, the spectra showed features indica ting binding of the metal ion to the aromatic cloud, as opposed to the N atom, thus resolving an ambiguity in computational predictions of the preferred binding site [204]. As was di scussed in Chapter 3, the Cp2ZrC3H5 + ion which has been seen during th e studies of the reactions of bis( 5cyclopentadienyl)methylzirconium(l+) with dihydrogen, ethylene, and propylene [147], was postulated to contain an allyl moiety. Even though this has been strongly suggested by earlier work [154, 166], there is no experimental (spectroscopic) evidence. Comparison of experimentally observed vibr ational frequencies to those known for the allyl species and/or predicte d by theoretical calcu lations for the Zr complex ion should give definitive proof to the postulated stru cture. For example, when an allyl group is bonded to a metal, three bands of medium to strong intensity are seen in the 1375 1510 cm-1 region, and three more, corresponding to me tal-ligand stretching vi brations, are seen in the 280 570 cm-1 region [206]. These are substant ially shifted (and increased in number) from the carbon-carbon double and tr iple bond stretching frequencies of 1550 1610 cm-1 and 2000 2070 cm-1, respectively, and metal-liga nd frequencies closer to 500 cm-1 which should be seen for si gma metal-allyl bonds [206].

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106 Figure 6.2. Theoretical IR absorption spectra for the key structures discussed in the present work. A similar approach could be used for postu lated product ions seen in other studies as well. Since the ions of interest, such as zirconocene 3-allyl or 3-enolate can be (and Zirconocene-methyl ZrC11H13+0 50 100 150 200 250 300 0500100015002000 frequency (cm-1)Intensity (km/mol) Zirconocene-allyl ZrC13H15+0 50 100 150 200 250 300 350 0500100015002000 frequency (cm-1)Intensity (km/m o Zirconocene-enolate ZrC13H15O0 50 100 150 200 250 300 350 0500100015002000 frequency (cm-1)Intensity (km/m o

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107 were) formed by EI, they should be amenab le to study at the F ELIX-FTICR facility. A probable complication is the strong coordination of the lig ands by a metal center. The binding energy of the 3-allyl ligand in the Cp2Zr( 3-allyl)+ moiety is estimated to be 22 kcal/mol above that of the corr esponding alkene in the cation Cp2Zr(H)( 2-alkene)+ [162], thus requiring at least ~ 55-60 kcal/mol or greater to be provi ded in order for the dissociation of the 3-allyl ligand to occur. This is close to the upper limit for bond energy for bonds successfully dissociated at th e FELIX facility. In cases like that, instead of monitoring the lowest energy dissocia tion pathway, the lowest energy reaction pathway with a suitable reagent could be followed. For example, a possible choice of such a reagent for the zirconocene 3-allyl complex could be H2, which is known to react in the gas phase with the zirconocene 3-allyl forming a single reaction product (zirconocene alkyl) with an act ivation barrier of ~ 11 kcal/m ol, well within the reach of the laser power at the FELIX facility. The calculated spectra for the most important species involved in the work presented in this thesis are shown on Figure 6.2. Our preliminary data indicate that such spectra can possibly be obtained experimentally. As can be seen in Figure 6. 3, the most prominent feature for zirconocene spectra, in-phase rocking motion of Cp ring hydrogen atoms at ~ 12.5 m is reproduced for the Zr(III) complex. The quality and reprodu cibility of the spectra can be dramatically enhanced if the spectra acqui sition were carried out in th e dissociation (monitoring of fragments’ intensities) rather than in the depletion (monitoring of parent ion’s disappearance) mode.

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108 Figure 6.3. Depletion spectrum of ion with m/z 282 (postulated as [Cp2Zr(CH3CHO)(H2O)]+). 6.2.2 Other gas-phase studies of organometallic compounds As was mentioned above, several liquidphase studies indicate that zirconocene enolate complexes have potenti al importance as polymerizat ion catalysts [187-193]. This calls for more thorough investigation of such complexes both in liquid and gas phases. By applying the FTICR MS approach, with all the advantages di scussed above, it is possible to enrich our understand ing of the reactivity of the 3-enolate complex, which has never been studied in the gas phase. Vary ing structural comple xity of the aldehydes reacting with the bis( 5-cyclopentadienyl)methylzirconium cation enabled us, as described above, to control the substituent at th e central carbon of the enolate ligand. It is

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109 feasible to carry out series of experiments on the reactivity of the 3-enolate complex toward alkenes and other molecules, such as water, alcohols, dihydrogen, etc. This will provide valuable insight into the reactivity of such complexes and will extend our knowledge of gas-phase metallocene chemistry. The exciting possibi lity of imitating of the liquid-phase reactions of methylmethacrylate polymeri zation in the low-pressure environment of FTICR mass spectrometer should help clarify the reaction mechanism. However, the potential of FTICR MS studies could also be applied to solve other unanswered questions. It is possible in genera l to investigate the e ffect of weakly bound ligands (such as solvent molecules) on the metallocene catalysts LL*MCH3 + (L, L* = Cp or related ligands; M = Ti, Zr) reactivity. An electrospray ionizat ion (ESI) ion source could be used. The suitable precursor (such as, for example, [Cp2ZrMe]+[C6H5N(CH3)2B(C6F5)4]-) [79] in the solvent of interest can be introduced directly, or the oxidation of the correspondi ng neutral in ESI sour ce in order to produce Cp2ZrMeSn + (n = 1, 2) cation can be carried out. The choice of solvents is somewhat limited by their suitability for electrospray, as well as, of course, the possibility of reaction with the metallocene. Study of the reactivity of meta llocene species with alkenes, apart from the aspects discussed above, will also help to discern the effect of 16electron or closed 18-elect ron shells vs. the 14-electr on shell of coordinatively unsaturated Cp2MMe+. These experiments would somewhat mimic the effect of the solvent shell in solution-phase catalysis. Theref ore, it can be anticipat ed that the reaction may lead to inserting of a second alkene mo lecule rather than formation of the allyl complex. The presence of the solvent molecule s in the ICR cell may help to resolvate the metallocene complex cation. The probability of this process is negligible at typical ICR

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110 cell operating pressures, but can increase to a noticeable value at elevated pressures available in ion traps. If the metallocene cation has already undergone alkene insertion, resolvating will lead to a consecutive insertion, ultimately resulting in oligomerization. Experimental proof of this hypothesis is a very interesting challenge. In order to discern the factors most affecting the exte nt of allyl complex formation vs. dimerization/oligomerization it is essential to investigate a range of metallocenes with a variety of solvent ligands. It is also possible to use differe nt precursors LL*M(CH3)2 for EI or [LL*MCH3]+X(L, L* = Cp or related ligand; X= [C6H5N(CH3)2B(C6F5)4][207224] or other suitable counter ion; M = Ti, Zr) for ESI. Further advances toward the unders tanding liquid-phase processes could potentially be obtained from on-line ESI-FTI CR MS studies of actua l reaction mixtures and real-time monitoring of cationic species. Change in concentra tions of (potential) active species as well as mon itoring of growing polymer chai ns would provide important clues and mechanistic details about cataly zed homogeneous polymerization processes. Coupling of ESI-FTICR MS studies of actual reaction mixtures (not necessarily of polymerization reactions) with IRMPD spectro scopy both at FELIX and, potentially, with simpler CO2 laser based systems would create an immense opportunity for studying structure and functions of vari ous elusive species which bear a charge. Due to the unique sensitivity of the FTICR MS technique it may be possible to detect and identify transient species which cannot be characterized by ot her means. This will tremendously enhance our understanding of various catalytic processe s, as well as the intricacies of reaction mechanisms.

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132 BIOGRAPHICAL SKETCH I was born in 1977 in Molodechno, Be lorussia former USSR, now the independent republic of Belarus. My fa mily moved to Russia in 1988. In 1994, I graduated from my high school and was accepte d into the Chemistry Department of the Moscow State University in Moscow, Russia. I graduated in August 1999 with B.S. in Chemistry with the Honor of Excellence Diplom a. Part of my dipl oma project research was carried out in the School of Chemistr y of the University of Nottingham, in Nottingham, United Kingdom. In the fall of 2001 I started my PhD program at the Chemistry Department of the University of Florida.


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Permanent Link: http://ufdc.ufl.edu/UFE0012946/00001

Material Information

Title: Gas Phase Structure and Reactivity of Zirconocene Cations
Physical Description: Mixed Material
Copyright Date: 2008

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
System ID: UFE0012946:00001

Permanent Link: http://ufdc.ufl.edu/UFE0012946/00001

Material Information

Title: Gas Phase Structure and Reactivity of Zirconocene Cations
Physical Description: Mixed Material
Copyright Date: 2008

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
System ID: UFE0012946:00001


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Full Text












GAS PHASE STRUCTURE AND REACTIVITY OF ZIRCONOCENE CATIONS


By

ALEXANDER A. AKSENOV













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


2005

































This work is dedicated to my loving wife Karina















ACKNOWLEDGMENTS

Author expresses his deepest gratitude to Dr. John R. Eyler for his knowledge, help

and concern and Dr. David E. Richardson for his sin qua non expertise and fruitful

discussion.
















TABLE OF CONTENTS



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

LIST OF TABLES .................. .................. ................. ............ .............. .. vii

L IST O F FIG U R E S .............. ............................ ............. ........... ........... viii

A B ST R A C T .......... ..... ...................................................................................... x

CHAPTER

1 CHEMISTRY OF ZIRCONOCENES AND ZIRCONOCENE CATIONS IN THE
C O N D EN SED PH A SE ..................................................... ............................... 1

1.1 Introduction and H historical B background ........................................ .....................1
1.2 Structure of Zirconocenes......... ................................................... ............... 2
1.3 Reactivity of Zirconocene Derivatives ..... .................. ...............4
1.3.1 Formation of Zirconium-Carbon Bonds ........ .......................................6
1.3.1 Cleavage of Zirconium-Carbon Bonds.................................... ............... 7
1.3.2 Other O rganozirconium D erivatives ........................................ .................7
1.4 Cationic Zirconocene Species.......................................................................... 9
1.4.1 Structure and Reactivity of Zirconocene Cations.....................................9
1.4.2 Nucleophilic Addition Reactions ........................................ ........ ... 10
1.4.3 Cationic Zirconocenes as Lewis Acid Catalysts .............. ...................11
1.5 Single-Center M etal Polym erization ....................................... ............... 12
1.5.1 Ziegler-Natta Polymerization .......... ............ .. .................12
1.5.2 Metallocene-Catalyzed Olefin Polymerization .......................................13

2 FTICR M S: M ETHOD OVERVIEW ..................................... ........................ 15

2.1. Introduction and Historical Background .................................. ............... 15
2.2 T theory and Instrum entation ....................................................... ..................... 16
2.2.1 Ion Cyclotron Motion ................................................... 18
2.2.2 Acquisition of M ass Spectra ............................ ..... .. ............... 20
2.3 Ionization M methods ................................................................. ......... 23
2.3.1 Electron Ionization .............................................................................. 24
2.3.2 C hem ical Ionization......................................................... ............... 25
2.3.3 Electrospray Ionization................ .. .......... ............... 25
2.3.4 Matrix Assisted Laser Desorption/Ionization...........................................26









2.3.5 Atmospheric Pressure Chemical Ionization ............................................ 27
2.3.6 Cationization..................... ........................ .........28

3 GAS-PHASE CHEMISTRY OF METAL IONS AND IONIC COMPLEXES ........29

3.1 G as-Phase vs. Solution Chem istry ......... ........................ .....................29
3.2 Gas-Phase Reactivity of Bare and Oxo- Metal Ions.................. ............... 30
3.3 Gas-phase Reactivity of Ligated M etal Ions .................................. ...................32
3.4 Reactions of the Bis(5 -cyclopentadienyl)methylzirconium Cation in the Gas
P h a se ................ ...................... ....................... ................ 3 4
3.4.1 R actions w ith A lkenes ....................... ........ .... ................. .... 34
3.4.2 Allyl Complex Formation: Mechanistic Differences of Reaction
Patterns in the Gas and Liquid Phases .......................................... ........38

4 GAS-PHASE REACTIONS OF BIS(Ql-CYCLOPENTADIENYL)
METHYLZIRCONIUM CATIONS WITH KETONES AND ALDEHYDES .........41

4.1 R rationale ............................................................... ..... ........... 41
4.2 Experim ental Procedures .......................................... ....... ............................ 43
4.3 R action w ith K etones ................................................................................. .... 51
4.3.1 Reactions of Cp2ZrCH3+/Cp2ZrCD3+ with Acetone/d6-Acetone ...............51
4.3.2. The Reactions of Cp2ZrCH3+/Cp2ZrCD3+ with 2-Butanone, Methyl
Isobutyl Ketone and Cyclohexanone. ................................... ............... 61
4.3.3 Reactions with Other Ketones ..... ...................... ..............70
4.4 Reaction with Aldehydes ........................ .... ............................ 72
4.5 Comments on Suggested Reaction Mechanisms ...............................................85

5 GAS-PHASE REACTIONS OF BIS(Ql-CYCLOPENTADIENYL)
METHYLZIRCONIUM CATIONS WITH IMINES ............. ............... 92

5.1 Synthesis and R activity of Im ines.................................................. .................. 92
5.2 Gas-phase Reactions of the Bis(fs-Cyclopentadienyl)methylzirconium Cation
w ith Im in e s ................................. ......... .... .... ......................... .......................9 3
5.2.1 Reactions of Cp2ZrCH3+/Cp2ZrCD3+ with 2,2,4,4-Tetramethyl-3-
P entanone Im ine .................... ......... ... ..................... ......... ...... ....... ... 94
5.2.2 Reactions of Cp2ZrCH3+/Cp2ZrCD3+ with Aryl Substituted Imines ..........98

6 CONCLUSIONS AND FUTURE WORK........................................................102

6 .1 C o n clu sio n s................................................ .................. 10 2
6.2 Future W ork................................................................... ......... 103
6.2.1 IRM PD Spectroscopy ........ ....... .......... ...... ............... 103
6.2.2 Other gas-phase studies of organometallic compounds .........................108









LIST OF REFEREN CE S ............................................................. ........................ 111

BIOGRAPHICAL SKETCH ............................................................. ..................132















LIST OF TABLES


Table page

3.1. Reactions of Cp2ZrCH3+ with alkenes. ................................ ..................34

4.1. Isotopes of zirconium ........... ..................................... ........ .. .. ........ .... 45

4.2. Major reaction products observed in the reactions of 1 and 2 with 2-butanone,
and methyl isobutyl ketone at various reaction times (1 to 5 seconds) ..................63

4.3 Major reaction products observed in the reactions of 1 and 2 with diethyl
carbonate at various reaction times (1 to 5 seconds)....................................71

4.4. Major reaction products observed in the reactions of 1 and 2 with acetaldehyde,
benzaldehyde, propanal and n-hexanal at various reaction times (1 to 5 seconds)..73

4.5. The absolute electronic energy (kcal/mol) of the most stable structure is given,
and the relative energy difference is given for the remaining structures (in
kcal/mol). Lettering corresponds to the structures pictured in Figure 4.16, a)
through f)................................... ................................. ........... 91

5.1. Im ines u sed in this w ork ........................................ .............................................94















LIST OF FIGURES


Figure page

1.1. Structural types of Cp2Zr(IV) compounds (adapted from ref. [23]); examples of
each structural type are listed in the right column. .................................................2

1.2. Dewar-Chatt-Duncanson model of alkene bonding by a metal complex (adapted
from ref. [2 8]) .................................................... ............... ..... 4

1.3. Two-electron reactions of zirconocene complexes................................................6

1.4. Reactions of Cp2ZrC(O)RCl (adapted from Ref [23]) ............................................8

2.1. Schematic representation of a cylindrical FTICR analyzer cell (adapted from Ref.
[104]) .............................................................................................18

2.2. Schematic representation of a cross section of an FT-ICR analyzer cell, where
ions are being excited by the RF potential applied to the excitation electrodes
(adapted from R ef. [104])........................................................................... .... 2 1

2.3. Stages of data in the FTICR experiment......................................... ...............22

3.1. A reaction profile for hydrocarbon oxidation by a transition metal oxo-ion.............31

3.2. Reactions of Cp2ZrCH3+ with alkenes (adapted from Ref. [147]).............................35

3.3. Comparison of qualitative potential energy surfaces for the reaction of
Cp2ZrCH3+ with ethylene in solution and the gas phase (adapted from Ref.
[14 7 ]). ................................................................................3 6

3.4. Mass spectrum for the reaction of Cp2ZrCH3+ with cyclopentane. ...........................38

3.5. Proposed structure of the complex with m/z 287................... ................... ................38

4.1. Schematic diagram of the mass spectrometer used in this work. ............................44

4.2. Schematic diagram of the electron ionization source. .............................................44

4.3. El mass spectrum of Cp2Zr(CH3)2: Cp2ZrCH3+ (m/z 235-242), Cp2Zr+(m/z 220-
227) and Cp2ZrOH+ (m/z 237-244)............................................ ... .................. 46

4.4. Example of a SW IFT experiment. ........................................................................... 48









4.5. Binuclear complexes Cp4Zr2(CH3)n+ (n = 0 3) ................................... ...............48

4.6. El mass spectrum of Cp2Zr(CD3)2 (* Noise peaks)............................................. 50

4.7. Mass spectra of ions formed in the reaction of Cp2ZrMe+ with acetone..................52

4.8. Mass spectra of ions formed in the reaction of Cp2ZrMe+ with acetone...................54

4.9. The a) l3-enolate and b) l3-allyl complexes .......................................................56

4.10. Mass spectra of ions formed in the reaction of Cp2ZrMe with methyl isobutyl
ketone. .............................................................................. 62

4.11. Ions produced in the reaction of Cp2ZrMe+ with benzoquinone: products of
coordination of quinone molecule by the binuclear complexes Cp4Zr2(CH3)n+ (n
= 0 3 ) ...................................... ................................ ................ 7 1

4.12. Mass spectra of ions formed in the reaction of Cp2ZrMe with acetaldehyde, 1
second reaction delay ........................ .................... .. ...... ... ............... 75

4.13. Mass spectra of ions with m/z 279 and 280 formed in the reaction of Cp2ZrCD3
with acetaldehyde, at a) 1 second and b) 5 seconds reaction delays......................76

4.14. Ions produced in the reaction of Cp2ZrMe+ with benzaldehyde at 1 second
reaction delay. ........................................................................76

4.15. Qualitative potential surface for the reaction of 1 with acetaldehyde/generic
aldehyde in the gas phase and in solution. The lettering corresponds to that in
Scheme 4.21. Gas-phase estimates of energetic effects are based on data from
R ef [173]. ................................................................................80

4.16. The most stable theoretically calculated structures for the Cp2ZrC3H5O+ cation.
Canonical structures shown were calculated with the MPW1PW91/6-
311+G(d,p) functional/basis set. The calculated absolute electronic energies of
structures from a) through f) are listed in Table 4.5..................................... ...... 90

5.1. El mass spectrum of benzophenone imine (molecular weight 181 amu). ...............99

6.1. IRMPD spectrum of Cr+(Et20)2 (adapted from Ref [204]). ................................. 104

6.2. Theoretical IR absorption spectra for the key structures discussed in the present
w o rk ...................................... .................................. ................ 1 0 6

6.3. Depletion spectrum of ion with m/z 282 (postulated as
[Cp2Zr(CH 3CH O )(H 20 )] ........................... .............................. ........................ 108















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

GAS PHASE STRUCTURE AND REACTIVITY OF ZIRCONOCENE CATIONS

By

Alexander A. Aksenov

December 2005

Chair: John R. Eyler
Major Department: Chemistry

The reactions of bis(l5-cyclopentadienyl)methylzirconium cation with aldehydes,

ketones and imines in the gas phase have been studied by Fourier transform ion cyclotron

resonance mass spectrometry. Reactions of bis(l5-cyclopentadienyl)methylzirconium

cation with a majority of the ketones studied resulted in consecutive addition of one and

two substrate molecules and/or elimination of neutral. The key ionic products of the

elimination reactions were identified as r3-enolate complexes. Similar product ion

structures are also postulated for the reactions with aldehydes, where these complexes are

either the only or the major reaction product.

Deuterium-labeled substrates and methylzirconocene were used to investigate

mechanistic details. Results indicate a multiple-step mechanism, with migratory insertion

of an aldehyde molecule into the methyl zirconocene cation, followed by P-H elimination

and, via a 6-membered cyclic transition state, formation of the resulting enolate complex.

When a P-H elimination pathway is not available for ketones, the reaction is proposed to









proceed instead via direct nucleophilic attack of the metal-bound alkyl preceded by a fast

migratory insertion equilibrium.

In reactions with the majority of imines, the reactivity toward bis(r5-

cyclopentadienyl)methylzirconium cation is governed by the availability of a labile

hydrogen. This leads to reaction products different from those which might be expected

in analogy with reaction products observed for ketones and aldehydes, i.e., the

azomethyne/benzylidene species instead of the enamines. Also, due to lack of liability of

the aryl groups, the migratory insertion equilibrium was not possible for reactions of aryl-

substituted imines. Consequently, only methane elimination was observed. However, a

general reaction mechanism resembles that proposed for ketones. The proposed

mechanisms account very well for all of the observed reaction features.














CHAPTER 1
CHEMISTRY OF ZIRCONOCENES AND ZIRCONOCENE CATIONS IN THE
CONDENSED PHASE

1.1 Introduction and Historical Background

Among other transition metals, Zr and its complexes, especially cyclopentadienyl

derivatives, have special significance in view of their potential to serve as catalysts or

reagents for multitudes of synthetically interesting reactions. Even though zirconium

occurs to the extent of 0.022% in the lithosphere (more abundant than carbon but less

abundant than Ti, 0.63%) and is one of the least expensive metals, its application in

organic syntheses of any kind was virtually unknown until the 1970s (apart from use as a

rather unsuccessful substitute for Ti salts in the Ziegler-Natta polymerization) [1].

However, over the last twenty years organozirconium compounds (almost exclusively

zirconocenes) have become some of the most widely used organometallics, along with

other metals such as Ni, Cr, Rh, and Fe. The explosive growth of this area is documented

in multiple reviews (a few of the most recent are given [2-15]) and the chemistry of

organozirconium and other group 4 transition metals has been extensively discussed in

some monographs [16, 17]. Most importantly, the do group 4 metallocenes are of great

interest as precursors of alkene polymerization catalysts [18, 19], and can be tuned to

make polymers of very specific tacticities. Single-center metal-catalyzed polymerization

has attracted ample attention and evolved from an area of purely academic interest into

commercially important technology [20]. The remainder of this chapter reviews some of

the most important aspects of the chemistry of organozirconium compounds.











1.2 Structure of Zirconocenes

The majority of known Zr compounds are derivatives of the so-called


"Cp2Zr unit" (Cp = cyclopentadienyl or related ligand) with Zr in +4 or, less often, +2

oxidation states. Zr (+3) compounds are also known, but are generally considered as


unwanted by-products [21].




1etc.

IV
Setc.


IV





: ." .. ,.etc.
i |
















etc.
(.
." .: .-: '. i i = r **






i '










Figure 1.1. Structural types of Cp2Zr(IV) compounds (adapted from ref [23]); examples
of each structural type are listed in the right column.









Structural types of Cp2Zr(IV) compounds are shown in Fig. 1.1. In addition to the

structures shown in Fig. 1.1, it is well known for the coordinatively unsaturated 14 and 16

electron zirconocene species to form dimers, oligomeric species, or polymeric

aggregates. It is common for the "Cp2Zr unit" to remain intact in the majority of

chemical transformations. However, under certain conditions, for example, treatment

with an excess of very strong nucleophiles such as alkyl lithium compounds, the Cp ring

(or both) can be displaced [22].

Zirconocene (IV) derivatives represented by the general formula Cp2ZrRiR2 (both

R1 and R2 being X ligands), such as zirconocene dichloride, Cp2ZrCl2, the Schwartz

reagent Cp2ZrHCl [24, 25], Cp2ZrMe2, etc., are do, 16-electron compounds, with an

empty valence-shell orbital, but with no filled orbitals available for interaction. This

makes these complexes inherently electrophilic (Lewis acidic), but not nucleophilic. The

empty 2d orbital of zirconium is able to interact with a- and 7t- bonding electrons, as well

as nonbonding electron pairs, and this interaction is usually a first step in the reaction of

Zr derivatives. Formation of the 7t-complexes of transition metals is described by the

Dewar-Chatt-Duncanson model [26, 27] (Figure 1.2). In this model, a o-type donation

from the C=C 7t orbital with concomitant 7t-backbonding into an empty 7* orbital of the

alkene results in synergistic bonding: the greater the o-donation to the metal, the greater

the 7t-backbonding. Also, the greater the electron density back-donated into the 7* orbital

of the alkene, the greater the reduction in the C=C bond order, which can ultimately lead

to formation of r2 alkene insertion product. This, in turn, implies that only metals with at

least one filled nonbonding orbital can form such complexes. For the do zirconocenes,

only donation of electrons is possible from alkenes, so these complexes act solely as









Lewis acids, thus forming only unstable (usually transient) species. This interaction is

more efficient for the cationic zirconocenes.






r)



d-orbital ethylene
pi-ortiital






C \
















(adapted from ref [28])

1.3 Reactivity of Zirconocene Derivatives

The reactivity of various zirconocene derivatives can be generalized into several

distinct types of reactions, as shown on Figure 1.3. The most important and relevant

reactions are discussed below.










~'I* Z~


+ goY
,_i .. -IL


I.


a) o-Complexation and o-dissociation


x


Y
I ,,' i' '- -.-. -"
X


*I
X


b) nt-Complexation and nt-dissociation


c_/


*'


Ii


c) Hydrozirconation and dehydrozirconation


C'


\ /
/ \


car'boz~::aircon tio


d) Carbozirconation and decarbozirconation


K,' '7<


oxi atix
i ~


e) Oxidative addition and reductive elimination


X -Y



f) -Bond metathesis

I) a-Bond metathesis


.' Zr H

i- .i~\ -


7r,

Tn Ii


....


T


rr


cv
I













1.3.1 Formation of Zirconium-Carbon Bonds





























LiAlH4 [24]. The formation of the insertion product occurs with good regio- and
Y Y

g) Migstereoselectivitatory (ainsertion and migratory deinsertion, with the formation of the E-alkenyl

compFigure 1.3. Two-electron reactions with alkynes) [3]. Thzirconocene availability of mplexes.ns in the insertion










Figure 1.3. Two-electron ry lead actions of zirconocene complexes.
1.3.1 Formation of Zirconium-Carbon Bonds

Apart from transmetallation, the most synthetically important reaction resulting in

zirconium-carbon bond formation is hydrozirconation [25, 29] (Fig. 1.2c), which

provides an easy and convenient way of converting alkenes and alkynes into alkyl- and

vinyl- zirconium derivatives, respectively [30]. The commonly used "Cp2Zr" precursor is

the Schwartz reagent, Cp2ZrHC1, in conjunction with an aluminium hydride, such as

LiA1H4 [24]. The formation of the insertion product occurs with good regio- and

stereoselectivity (anti-Markovnikov, syn addition, with the formation of the E-alkenyl

complex in reactions with alkynes) [31]. The availability of P3-hydrogens in the insertion

product can possibly lead to intramolecular rearrangement.

Another synthetically important means of formation of organozirconocene

derivatives is oxidative addition (Fig. 1.2e). It should be noted, however, that since the

Cp2Zr(IV) complexes have do configuration, the oxidative addition process is not

possible, since it requires that both empty and filled orbitals be available for interaction.









It has been shown that the actual reaction is oxidative coupling-elimination [32]. The

oxidative addition reaction is valuable for converting allyl ethers into allylzirconocenes

[33], as well as for conversion of alkenyl chlorides into alkenylzirconocene chlorides.

1.3.1 Cleavage of Zirconium-Carbon Bonds

The zirconocene alkyl/alkenyl insertion product can be converted by

protonolysis/deuterolysis and halogenolysis into the corresponding (halogenated)

alkane/alkene. Cleavage of the Zr-C bond occurs in relatively mild conditions:

protonolysis requires treatment with diluted acid or even water, while halogenolysis

occurs under treatment with bromine or iodine, or, when an alkene group is present,

succinimides (NBS or NCS) at room temperature. The configuration of alkene is usually

preserved [34]; however, some examples of concomitant skeletal rearrangement are

known [25, 29]. Another path for Zr-C bond cleavage is treatment with peroxides, which

leads to formation of alcohols [35].

The inherent low intrinsic nucleophilicity of alkyl/alkenyl zirconocenes renders

them unable to react with a majority of common electrophilic substrates, with the only

exception known to be the reaction with aldehydes [36]. On the other hand,

dienezirconocenes can react with a variety of substrates, such as alkenes, alkynes,

aldehydes/ketones and even nitriles [37].

1.3.2 Other Organozirconium Derivatives

An important class of compounds containing a Zr-C bond is the acylzirconocenes,

which can be easily and straightforwardly obtained via treatment of alkyl/alkenyl

derivatives with carbon monoxide at atmospheric pressure, resulting in a generally clean

reaction of CO inserting into the Zr-C bond [34, 38]. The stability of the

acylzirconocenes depends strongly on the ligand of the given derivative, with the









acylzirconocene chlorides being most stable (not losing CO even at pressures

significantly lower than atmospheric). Acylzirconocenes can be further involved in a

variety of other reactions shown in Figure 1.4. All of those reactions proceed via

migratory insertion [34, 39] (Fig. 1.2g).

dil. HCI







--, Rr,^
MeOH

Figure 1.4. Reactions of Cp2ZrC(O)RCl (adapted from Ref [23]).

Another very important class of organozirconium compounds are the 7t-

complexes, which can be considered as zirconocyclopropanes. The 7t-complexation (or,

rather, oxidative 7t-complexation) can be considered as a two-electron red-ox process

with the alkene being reduced by zirconium. The required Zr(II) species can be generated

by a reducing agent (alkali metal amalgam) prior to reaction, or in situ [39-41]. The

resultant zirconacycle, called the "Negishi reagent" is a valuable reagent, enabling

synthesis of various zirconocycles and serving as a very convenient source of the

"Cp2Zr" unit. The sequence of reactions involving formation of zirconium 7t-complexes is

called the "Negishi protocol," and was shown to be a concerted non-dissociative process (

the Cp2Zr species is not generated at any stage [42]). Along with the more recently

developed Erker-Buchwald protocol [43-47], these two protocols, often used

complementarily to each other, provide synthetic routes to tri-member zirconocycles and

a variety of compounds obtained from them.









1.4 Cationic Zirconocene Species

Zirconocene cations are not found as isolated species in the solution phase. The

high reactivity of such species usually leads to their immediate conversion/reaction.

Furthermore, if such cations are indeed produced, they always exist as an ionic pair with

varying degree of coordination by a counterion. In general, it is impossible to define a

discrete cation as part of a salt: in most cases, no indication of a cationic nature is found

either (by the means of X-ray) in the solid state, or in solution, and the only indication of

the zirconocene cation's presence is the characteristic chemistry taking place.

Normally, the ionic species are formed from a suitable precursor with appropriate

in situ activation. The most widely used methods in organic synthesis include halide

abstraction from Cp2ZrCl2 with Ag salts, such as AgC104, AgAsF6, AgSbF6, etc. (the

counterion must be non-nucleophilic) and protonation of methyl zirconocenes with acids

containing the BARF (tetrakis(3,5-bis(trifluoremethyl)phenyl)borate) [48] anion. The

variety of methods used for the generation of bis(rl-cyclopentadienyl)methylzirconium

and related cations employed in polymerization catalysis is considered below in Chapter

1.4. A vast number of studies have been carried out [49].

1.4.1 Structure and Reactivity of Zirconocene Cations

The zirconocene cations generated in solution always exist in their solvated form

and/or ion pairs with a counterion. Depending on the nucleophilicity of the anion, the

degree of coordination can vary and the free cation is never observed. Weakly

coordinating counterions such as BARF- provide the only way to utilize the reactivity of

the zirconocene cation, while common nucleophilic anions such as C1- can completely

shut down the reactivity in most cases. Moreover, these cationic species exhibit a

tendency to abstract chlorine ion from chlorinated solvents, and show even greater









tendency to abstract and bind fluoride (usually forming Cp2ZrX2[50]) (so counterions like

BF4- cannot be used[51]). This feature is used for synthetic purposes for C-F bond

activation in glycosyl fluorides [52, 53].

The observed rate for reactions involving zirconocene cationic species

Cp2ZrX(L)+ depends on the rate of dissociation of the coordinated ligand to give the 14-

electron species. The latter complexes are the most reactive ones, since their

electrophilicity is unimpeded by electron donation from the base. Of course, such species

tend to coordinate solvent moleculess, thus establishing a solvent association-

dissociation equilibrium which determines the cation's reactivity.

1.4.2 Nucleophilic Addition Reactions

The Cp2ZrRCl complex is not electrophilic enough to react with the most

substrates efficiently, i.e., cleanly and with a sufficient rate; thus, the synthetically

interesting addition reactions may involve zirconocene cations as intermediates. Alkyl

and alkenyl zirconocene cations can be produced from the hydrozirconation products of

alkenes and alkynes by the Schwartz reagent via chloride abstraction by a catalytic

amount of (most commonly) AgC104 [54].

In aldehyde addition reactions of Cp2ZrRCl complexes, a mechanism involving

generation of alkyl/alkenyl zirconocene was proposed [54]. The resultant reaction

products are alcohols formed through condensation of the aldehyde and R ligand of

zirconocene. Another application of silver-mediated addition is synthesis of homologues

of unsaturated aldehydes by hydrozirconation of alkene/alkyne ethers with the Schwartz

reagent and addition of aldehyde followed by acidic hydrolysis. This method allows 2

and 4 carbon homologization with very good to excellent yields [55]. Also, utilizing









addition of aldehydes by a bimetallic 3-methylsilyl-l-propenylzirconocene chloride

allows synthesis of such valuable organic reagents as 1,3 dienes [56, 57].

Other reactions of practical importance are the ring opening of oxiranes producing

alcohols [58] and carbometallation of alkenes and alkynes. The interesting feature of the

latter is that only single addition is achieved through the metal-alkyl addition, rather than

polymerization. Among possible carbometallations, carboalumination is the most

common; this method is highly stereo- and regioselective and can produce various

products depending on the starting alkene/alkyne and the subsequent Al-C bond cleavage

method [59-61].

1.4.3 Cationic Zirconocenes as Lewis Acid Catalysts

As in nucleophilic addition reactions, cationic zirconocene species are produced

by abstraction of an anion from a neutral complex, usually zirconocene dichloride or

triflate, by AgC104. The main application of such species is catalysis of Diels-Alder

reactions of a,P-unsaturated aldehydes or ketones and common dienes [62]. Zirconocene

catalysts are advantageous over the usual AlC13 Lewis acid catalyst because of their

lower susceptibility to water and oxygen, thus eliminating the need for extensive drying

of reaction solvents, and generally leading to significantly lower amounts of catalyst

required (<1 mol%). Albeit efficient (up to 106 reaction acceleration), the catalysts

provide rather poor reaction product regio- and stereoselectivities [63]. The major

drawback, however, is the tendency to catalyze polymerization reactions instead,

resulting in the competing side reaction of a diene polymerization, thus limiting possible

application of such catalysts to only very reactive diene/dienophile pairs. Other reactions

catalyzed by cationic zirconocene species include various rearrangements/isomerization









reactions, such as, for instance, rearrangement of epoxides into corresponding aldehydes

[58], which proceeds with generally good yields.

1.5 Single-Center Metal Polymerization

The main practical application of zirconocenes is undoubtedly in polymerization

catalysis. The active species is generally accepted to be a coordinatively unsaturated

cation such as LL'MCH3 (M=Ti, Zr; L= r5-cyclopentadienyl or related ligand). The

extensive interest in this class of compounds was triggered not only by very high catalytic

activities (comparable to those of enzymes!), but also by the ability to exert stereocontrol

of the resultant polymer by modification of the "Cp" ligands, thus creating polymers

with a wide array of properties unattainable by other means [64]. Single-center metal

polymerization has stemmed from conventional Ziegler-Natta polymerization, which is

reviewed briefly below.

1.5.1 Ziegler-Natta Polymerization

In classical heterogeneous Ziegler-Natta polymerization, the process takes place

at the structural defects (dislocations and edges) of TiC13 crystals [65]. The proposed

mechanism for such reactions postulates polymer chain growth by cis-insertion of the

olefin into a Ti-C bond on the surface [66, 67]. The non-uniformity of the active sites in

heterogeneous catalysts is partially responsible for the polymer's high polydispersity.

The more advanced version of the Ziegler-Natta catalyst, developed on the basis

of group 4 metallocenes, was comprised of mixtures of Cp2TiCl2 and A1R2C1 and

exhibited moderate activity [68]. The proposed mechanism of its action involves olefin

coordination and insertion into the Ti-C bond of electron-deficient Cp2Ti6+R species. The

latter are formed via ligand exchange between aluminum and titanium and formation of

Cp2TiRC1, followed by coordination of the Lewis acidic aluminum to the titanium's









chloride and polarization of the Ti6+-C1I- bond. The Ti-C bond "prepared" in this way

readily inserts ethylene [69]. This mechanism is similar to that postulated for

heterogeneous Ziegler-Natta catalysis: in both cases the Lewis acidity of the metal center

plays a key role, and cis insertion of the olefin into the M-R bond is observed. Originally

the nature of the active center was not fully established; however there were early

indications that Cp2TiR+ was an actual catalyst [70]. Unambiguous proof of this premise

was obtained when the ionic species [Cp2ZrMe(L)]+[BPh4]- was found to be a very active

ethylene polymerization catalyst [71]. It was shown that this compound generates

cationic metallocene species that are capable of olefin polymerization without the

aluminum alkyl activators. Further studies confirmed the crucial role of metallocene

cations as catalytically active species in homogeneous polymerization catalysis [72-83].

1.5.2 Metallocene-Catalyzed Olefin Polymerization

The major drawback of homogeneous polymerization catalysts, which impeded

their industrial application, was unacceptably low efficiency and an inability to

polymerize alkenes other than ethylene. A breakthrough was achieved with the

application of methylaluminoxane (MAO) [84, 85], which, at the present time, is used

almost exclusively in catalytic olefin polymerization. Unlike in Ziegler-Natta catalysis,

treatment of the metallocene/Al alkyl catalyst with miniscule, but exact, amounts of water

can immensely augment the catalyst's activity as well as providing the ability to

polymerize alkenes more complex than ethylene. Various otherwise inactive systems

were shown to achieve exceptional activity upon such treatment [84, 85].

The currently accepted mechanism of MAO formation assumes partial hydrolysis

of the solid AlMe3 resulting in formation of extended networks of (AlMeO)n.

Introduction of catalytic amounts of Cp2MR2 (Cp = cyclopentadienyl or related ligand; M









= Ti, Zr; R = Cl, Me) into an excess of MAO generates Cp2MMe species via initial

methylation of metal-halide bonds and further abstraction of a methyl anion from the

Cp2MMe2. The MAO serves as a very low-coordinating counterion, thus forming a

[Cp2MMe]+[MAO-Me]- "salt". Very low coordination of the anion enables high activity

of the catalyst.

The importance of the zirconocene cation being "free" in order to exhibit its

activity, also discussed in Chapter 1.3, has led to development of various ways for

preparation of weakly coordinating anions. Such systems are called 'single site'

metallocene catalysts, implying that polymerization occurs solely at the metal center.

Large anions such as [B(C6H4R)]- and carboranes have been used as counterions, but they

exhibit rather strong interactions with the cation [86]. Less than two decades ago, a very

weakly coordinating perfluorinated tetraphenylborate anion was introduced as a

counterion [79] which afforded exceptionally high activity of the single-site zirconocene

catalyst for alkene polymerization, exceeding that of the catalysts with MAO as a

counterion (co-catalyst). Various co-catalysts for metal-catalyzed olefin polymerization

have been reviewed [87].














CHAPTER 2
FTICR MS: METHOD OVERVIEW

2.1. Introduction and Historical Background

Valuable insights into reaction mechanisms, including those of polymerization

reactions, can be obtained using Fourier transform ion cyclotron resonance mass

spectrometry (FTICR MS) [88-92] for studies of the intrinsic gas-phase reactivity of

transition metal complexes. The spatial separation of reacting species under high-

vacuum conditions has certain advantages, most importantly the absence of complicating

factors prevailing in solution such as association by ion pairing, interactions with solvent,

and intra- and intermolecular processes leading to destruction/modification of the active

species. This is especially relevant in catalytic studies, since active species are often

present in trace amounts and very short-lived. On the other hand, reaction mechanisms in

the gas phase may be different from those observed in the condensed phase. However,

information about such processes can provide a valuable description of reactions

occurring at extreme conditions and assist in explanation of possible reaction

intermediates/side products which might occur in traditional chemistry in the liquid

phase. The key issues that can be addressed by the gas-phase studies are: the effects of

solvent and intrinsic reactivity, as well as the effects of ancillary ligands and impact of

aggregation and ion-pairing on reactivity of ionic species; the thermochemical

determinations of various gas-phase reaction energies; and, importantly, direct

observation of key reactive intermediates, as well as identification of solution species in









general speciationn). It is possible to refine our understanding of the condensed-phase

chemistry through the study of analogous gas-phase chemistry.

FTICR has become very important technique due to the method's ability to

provide mass resolution and accuracy unmatched by any other type of mass spectrometer.

Another important factor is the ability to combine FTICR mass spectrometry with various

ionization techniques. Rapid growth of the number of FTICR mass spectrometers

installed in laboratories around the world has occurred after the discovery of electrospray

ionization (ESI), which triggered extensive application of FTICR mass spectrometry in

studies ofbiomolecules [93, 94]. Another advantage of this method is the capability of

trapping and storing ions inside the spectrometer, which enables investigation of the ion

chemistry.

The principles of Fourier transform mass spectrometry are based on the cyclotron

resonance theory, developed in the 1930s [95]. The first attempt at employing the

principle of ion cyclotron resonance in a mass spectrometer was undertaken in 1950 [96],

while the first FTICR mass spectrometer was built in 1974 by Comisarow and Marshall

[97] At the present time, FTICR mass spectrometry is used widely for studies and

characterization of a broad range of systems. A large number of reviews have been

written on this topic [92, 98-104]. The principles of the FTICR MS technique are briefly

reviewed below.

2.2 Theory and Instrumentation

All FTICR mass spectrometers are comprised of certain components. Since the

ion cyclotron motion is caused by a magnetic field, a magnet is required. The magnets

can be permanent, electromagnetic or superconducting. The first of these three have very

low field strengths, unacceptable in many cases for the purposes of FTICR. The best of









electromagnets are capable of producing fields in the 1-2 Tesla range, which is suitable

for ions of relatively low mass-to-charge ratio. Since the performance of the FTICR MS

correlates with the magnetic field strength, superconducting magnets capable of

producing fields ranging from 2 to 9.4 Tesla are commonly used. The highest magnetic

field used in any of the currently operational mass spectrometers is 14.5 T at the National

High Magnetic Field Laboratory in Tallahassee, Florida [105].

Another main component of any FTICR mass spectrometer is the analyzer cell.

All of the manipulations with ions (storage, ion/molecule reactions etc.) and ion signal

detection are carried out inside it. Various cell designs have been used, with cubic and

cylindrical cells being the most common ones. The cubic cell is composed of six plates:

two plates are perpendicular to the magnetic field and used to confine the ions inside the

cell, hence the "trapping" plates name. Normally the trapping plates of cubic cells have

openings for the electrons or ions to be introduced into the cell. The other four plates are

the ion excitation and detection plates. The function of those is discussed below. Other

types of cells, though different in design, are comprised of plates or cylinders which

perform the same functions as in the cubic cell. In the cylindrical cell, excitation and

detection plates form a cylinder, with the trapping plates remaining parallel to each other

(Fig 2.1), while in the open cylindrical cell trapping plates are replaced by two open

cylinders at either side of the cell. The advantage of such designs is better suitability for

positioning in the bore of a superconducting magnet. Various other cell designs have

been developed [92].























Figure 2.1. Schematic representation of a cylindrical FTICR analyzer cell (adapted from
Ref. [104]).

2.2.1 Ion Cyclotron Motion

The magnetic field exerts force on a charged particle moving in it, if a component

of the particle's velocity is perpendicular to the magnetic field axis. This force is called

the "Lorentz force". For a charged particle its velocity, magnetic field and the exerted

Lorentz force are mutually perpendicular. The value of the Lorentz force acting on a

charged particle in the magnetic field is:


F = qvBsinO

where q is the charge on the particle, v is the velocity of the particle, B is the magnetic

field strength (in Tesla) and 0 is the angle between the velocity of the charged particle

and the direction of the magnetic field. In case when 0=7c/4 (particle moves

perpendicularly to the magnetic field), this expression simplifies to


F =qvB

The total charge on an ion is equal to:

q =ze









where e is the charge of electron and z is the total number of electronic charges on the

ion. The resultant trajectory of an ion will be circular, so it will be processing in the plane

perpendicular to the magnetic field in a fashion called "cyclotron motion". The frequency

of such motion depends on the mass-to-charge ratio of the ion and strength of the

magnetic field:






or





Here m is the mass of the ion and co is the angular frequency,fis the cyclotron frequency.

Therefore, ions of lower m/z have higher cyclotron frequencies than ions of higher m/z.

As mentioned above, the FTICR analyzer cell (Fig. 2.1) consists of two excitation

and two detection plates along with two trapping plates in order to confine the ions inside

the cell. Potentials applied to the trapping plates are usually low (-1 volt), but they leads

to the side effect of a non-zero potential at the center of the FT-ICR analyzer cell due to

electrostatic fields from the trapping plates. This can offset the center of the cyclotron

motion and result in the "magnetron motion" [92, 103] in addition to the orbiting motion.

The magnetron frequency can be calculated as follows:






Here fm is the magnetron frequency (in Hz), Vp is the potential applied to the trapping

plates, a is the distance between the trapping plates and a is a geometry factor of the









FTICR analyzer cell [92, 106]. Magnetron frequencies are usually of an order of

magnitude of a few hundred Hz (unlike cyclotron frequencies, which are typically within

the kHz to MHz range). The magnetron frequency depends upon cell dimensions, design,

applied trapping potentials, and magnetic field strength. The resultant trajectory of an ion

in the cell is a combination of three components: the cyclotron motion, the magnetron

motion, and the oscillation between the trapping plates along the magnetic field axis. The

magnetron motion [92, 106] and the overall complex ion motion [102, 107-109] are

discussed in detail in the literature.


2.2.2 Acquisition of Mass Spectra

When the packets of ions in cyclotron motion repeatedly move past two detection

plates, charge moves within the detection circuit to counteract the proximity of the

passing ions. The voltage change between the detection plates can be measured as a

function of time. The resultant data are called "transient", "time-domain" data, or "free

induction decay" (Fig 2.3a). Prior to detection, however, it is necessary to perform ion

"excitation" application of a radio frequency voltage to the two excitation plates at the

frequency resonant with the cyclotron frequency of individual ions. The excitation is

necessary due to two factors: when the ions are first formed in, or enter into the FTICR

cell, the radii of their cyclotron orbits are too small to be detectable and the ions therefore

must be excited to detectable radii (Fig 2.2). Also, prior to the excitation, initial cyclotron

motion of ions is incoherent, and the effect of ions passing opposite detection plates

cancels out so no transient can be observed. Application of the RF voltage drives the ions

to larger orbits along with collection of ions into "ion packets", which results in coherent

motion, i.e. the cyclotron motion of an ion cloud, rather than individual ions. Such an ion










packet will induce a potential in the detection plates which alternates with the cyclotron

frequency of ions with various m/z.















"'"...... .....,"
.. .. ... ................ .......









Figure 2.2. Schematic representation of a cross section of an FT-ICR analyzer cell, where
ions are being excited by the RF potential applied to the excitation electrodes
(adapted from Ref. [104]).

The magnitude of the signal induced on detection plates is proportional to the total

charge and the proximity of the ions to the detection plates (orbital radius), and is

independent of the magnetic field strength. Such a waveform is comprised from different

cyclotron frequencies of all of the ions in the cell. The next step is, therefore, extraction

of data about the different ion packets. This is achieved through application of the

"Fourier transform" operation (FT)[110] where frequency information is obtained from

time-domain data (Fig 2.3b).













i l'' -


,.II.IL :







S4 IL -



,* .. ..-. .. .....






a) Time-domain waveform.






















b) Frequency domain data.






















c) Resultant spectrum calibrated in terms of m/z.


Figure 2.3. Stages of data in the FTICR experiment.









A resultant spectrum is a function of signal magnitude from cyclotron frequencies

of the ions present. As shown above, the cyclotron frequency depends on the m/z ratio, so

it is feasible to plot signal magnitude as a function of m/z (Fig 2.3c).

An excitation waveform consisting of a linear increase or sweep through the

resonant frequency range of interest at constant amplitude is known as a "chirp". Ions of

different m/z values are excited to orbits of the same radius, even though their cyclotron

frequencies differ. Such application of a complex frequency signal to the excitation

plates, which causes the trapped ions to absorb energy at their specific resonance

frequency, can also be used for selective ejection of ions from the cell. If a broad range of

frequencies is applied to cover the mass range of interest with amplitude and duration

adjusted to drive all ions to cyclotron orbits still contained within the cell dimensions, the

optimal image current can be obtained. However, such excitation also allows ejection of

all of the ions from the cell if their orbits reach the electrodes. It is possible, therefore, to

construct a complex signal that misses one or more frequency windows of a

predetermined width. Such an approach is called "stored waveforms inverse Fourier

transform" or SWIFT [111]. Applying such a signal to the trapped ions causes ejection of

all ions with exception of a selected cluster of ions with given m/z.

2.3 Ionization Methods

Formation of ions can be achieved by multiple means: ions can be formed either

by removal/addition of an electron leading to ions M /M-, by protonation/deprotonation

leading to protonated/deprotonated molecules MH+/(M-H)-, or by addition of another

small ion leading to cationized molecules such as MNa Ions can be formed in an MS

ion source from a neutral precursor, or pre-exist in a solution phase. Also, ionization can

be occurring within the homogeneous high magnetic field region (internal ionization









sources) or externally to the magnetic field. In the latter case, transfer of ions into the MS

cell is required. Despite sometimes limited understanding of the exact means of ion

formation [112] the use of various ionization methods enables mass spectroscopic

analysis of a wide array of species from small molecules to large proteins. A brief review

of some of the ionization methods is given below.

2.3.1 Electron Ionization

The process of electron ionization (EI) involves interaction of gas phase

molecules with electrons accelerated to about 10-100 eV. As a primary product, El forms

an excited odd electron ion-radical M+'. These ions are additionally destabilized by

having an unpaired electron and, usually, are capable of high-energy, sometimes quite

complex, reactions. Although the stability of ions may be increased by lowering the

energy of the ionizing electrons, most of the time ion fragmentation is observed. In

general, the presence of molecular ions in spectra is an exception rather than a rule. Some

structural characteristics, such as those favoring charge localization, may have a profound

effect on fragmentation and may increase the relative stability of parent ions or channel

fragmentation processes. For example, for a series of normal hydrocarbons, the

intramolecular vibrational and rotational super-cooling of molecules in a supersonic

molecular beam (SMB) lead to a significant increase in the stability of molecular ions

formed by El [113]. With conventional El normal hydrocarbons show only barely

detectable M+', while such ions dominated in the spectra obtained with SMB.

In a variation on the El method, metastable atom bombardment (MAB), gas phase

chemi-ionization (not chemical ionization!) can be induced by metastable atoms of noble

gases. Employing one of the five of them (with excitation energy decreasing from 20 to 8









eV from helium to xenon, it is possible to control the energy transferred to the molecules

being ionized.

2.3.2 Chemical Ionization

Chemical ionization (CI) is a gas phase process involving ion-molecule reactions

with species derived from a reagent gas (or gas mixture) that is first ionized by El at high

electron energy. Typically, the even electron protonated molecule MH is formed. An

efficient CI process, in the case of ion sources producing a constant beam of ions,

requires relatively high pressure of the reagent gas (-1 Torr), but in instruments that trap

ions for an extended time, ion sources may work at much lower pressure. Due to

collisional cooling of the nascent ions by molecules of the reagent gas the CI process is

relatively soft. The ionization efficiency and type of ions formed depend on the

specificity of ion-molecule reactions. For example, a high electron affinity can provide

very efficient ionization leading to M- ions. Other types of CI ion formation include

association with species derived from the reagent gas or from "true" chemical reactions

[114].

2.3.3 Electrospray Ionization

Electrospray ionization (ESI) [115-118] is the ionization technique most

frequently used in conjunction with FTICR mass spectrometry. ESI has the advantage of

being a "soft" ionization technique, resulting in the vaporization and ionization of a

sample, at the same time minimizing fragmentation. The latter is particularly important

for labile molecules such as biological molecules, and for the study of complex mixtures

where fragments must not interfere with the mass spectrum so the original constituents

can be determined. ESI frequently leads to formation of multiply charged ions,

particularly in the case of large biomolecules. Since mass spectrometry is based upon the









determination of m/z, not mass alone, it is frequently the case that a mass spectrum will

contain the same molecules but in a variety of charge states, and therefore at several

different m/z values. As ions become more highly charged, the m/z becomes lower and

the spacing between the "isotopomers" becomes narrower. As a result, it becomes more

difficult to resolve the signals and the resolution of the mass analyzer becomes more

important. The charge state of an ion can be determined by examining the spacing

between the isotopomer signals. The increasing importance of the electrospray ionization

technique is promoting the growth of the FTICR mass spectrometry, due to the ultra-high

resolution of FTICR mass spectrometers. For these reasons, the ESI FTICR mass

spectrometry has become increasingly important for study of biological molecules in

recent years.

2.3.4 Matrix Assisted Laser Desorption/Ionization

An alternative mean of ion formation is desorption of ionic species from the solid

phase with desorption energy supplied by a laser. Direct laser desorption of analyte can

only be effective if the ,max of the analyzed compound matches the laser output. Matrix

assisted laser desorption/ionization (MALDI), on the other hand, does not pose this

restriction and provides good ionization efficiency at high mass ranges [119]. In MALDI,

the light energy is first absorbed by an excess (usually >1000) of a crystalline matrix with

the absorption maximum close to the laser output. A short burst of ions formed by a

single laser pulse is followed by ion analysis. In a simplified model, matrix evaporation

forms excited and ionized species that in turn ionize other components. As in the case of

other soft ionization methods, neutral species which are present in the plume (before its

dissipation in vacuum) provide a cooling effect, thus contributing to the stability of ions









formed. Many compounds have been tested as matrices for MALDI, but only some work

well. For different classes of compounds, matrices show striking differences in the

ionization efficiency and the difference in their suitability for different method of ion

analysis. For instance, matrices working well with MALDI TOF may not be suitable for

MALDI FTICR. MALDI may show lack of reproducibility resulting from the matrix

selection, matrix-to-compound ratio, form of matrix crystallization, and possible presence

of impurities including those formed by a degrading matrix. MALDI and other soft

ionization methods are highly selective and subject to strong suppression effects.

Although this selectivity may facilitate detection of compounds with well recognized

ionization characteristics, at the same time it may create an uncertainty in characterizing

novel compounds as well as in evaluating purity. Suppression effects may completely

prevent some or all components from producing ions and may lead to a situation in which

ions observed represent only the most easily ionized components and not those that are

quantitatively important. As a result, the removal of salts, organic buffers, and detergents

is important in avoiding sample unrelated ions and suppression effects [120].

2.3.5 Atmospheric Pressure Chemical Ionization

Atmospheric pressure chemical ionization (APCI) is most commonly used in

combination with liquid chromatography. This ionization technique involves high

temperature nebulization of solutions in the presence of a corona discharge or p-emitter

while solvents or solvent additives (such as, for example, ammonium acetate) play the

role of a reagent gas. Ions formed at the atmospheric pressure are transferred to the high

vacuum part of a mass spectrometer via several stages of pumping and mass analyzed

[121].









2.3.6 Cationization

The formation of cationized ions has been recognized as a very useful in the

analysis of compounds that resist protonation [122]. Even traces of alkali metals,

especially the ubiquitous sodium ion, may lead to full or partial cationization, but

dissolving salts or hydroxides in a matrix allows much better control of this process.

Elements with well recognizable isotopic patterns, for example silver or thallium, both

with two naturally occurring isotopes (107Ag 51.3% and 109Ag 48.7%; 203T1 29.5% and

205T1 70.5%), produce easily discernable attachment ions [123]. Analogously, mixtures of

alkali metal salts can produce well recognizable patterns. It is also possible to facilitate

cationization by using special derivatives incorporating groups with a high affinity for

alkali ions (such as, for example, crown ethers) [124].














CHAPTER 3
GAS-PHASE CHEMISTRY OF METAL IONS AND IONIC COMPLEXES

3.1 Gas-Phase vs. Solution Chemistry

It is quite apparent that ions studied under the conditions inside an FTICR mass

spectrometer will exhibit higher reactivity than those in the solution due to a combination

of two factors. The isolated charge on the ions due to the absence of counterions and

solvent, coupled with high-energy collisions, result in the accumulation of significant

energy that cannot be discarded into a solvent bath. Furthermore, the possible formation

of excited states during high energy ionization events, such as collisions with "hot"

electrons (exceeding the appearance energy (AE) by numerous electron volts) during EI,

can lead to a distribution of excited states. Often the ground states species comprise less

than a half of the total ion population [125-130]. Application of special techniques, like

surface ionization (SI) or high-pressure ionization sources, is required to overcome this

complication. The value of such gas-phase studies, therefore, is not only in the possibility

of determining thermodynamic and kinetic data (which often cannot be directly applied to

the processes in solution). It is also in the possibility of obtaining a precise and thorough

description of the impact of factors which would be very hard to account for in the actual

reaction in the solution phase due to interaction of species of interest with solvent,

clustering, nonhomogeneity and so on. Examples of such factors are the effects of

ligands, electronic configuration and periodic trends, as well as the detection of possible

elusive reaction intermediates, etc. A brief review of some aspects of the gas phase

chemistry of metal ions and simple organometallics is given below.









3.2 Gas-Phase Reactivity of Bare and Oxo- Metal Ions

Significant interest in the chemistry of metal ions in the gas phase was triggered

by a discovery of their ability to perform specific activation of carbon-hydrogen and

carbon-carbon bonds [131]. Both activation of unstrained carbon-carbon bonds by bare

transition metals [132, 133] as well as by oxide cations, MO+ [134] have been discussed

in number of reviews. A recent review provides an update of the chemistry of alkane

activation in the gas phase with a focus on thermochemical data for various metal-carbon

bond energies [135].

An alkane activation reaction is promoted when there is an empty orbital on the

metal which accepts electrons from the carbon-carbon bond being cleaved. If such an

orbital is filled, the repulsive interaction makes the reaction inefficient [130, 136].

Simultaneously, the electrons on the metal's orbital with 7t-symmetry are back-donated

into the c* orbital of the breaking bond, thus leading to its dissociation if the reactants are

capable of getting sufficiently close. For efficient activation both of the interactions

described above are required.

Interest in the chemistry of metal oxo-ions stems from their potential role as

reaction intermediates in various oxidation processes, in particular the oxidation of

hydrocarbons. Since alkane oxidation by a transition metal oxo-cation is generally

exothermic (Fig. 3.1), a catalytic cycle can potentially be devised if the metal cation can

be reoxidized in situ by a suitable reagent (such as oxygen, hydrogen peroxide, nitrogen

oxides, etc.).











MO- RH

E




[PrH-MIO] L
M- ROH


R-M--OH
Rxn coordinate

Figure 3.1. A reaction profile for hydrocarbon oxidation by a transition metal oxo-ion.

An example of methane activation by FeO a most widely studied species, is

shown in Scheme 3.1 [134].


FeOC+ CH4 [Fe-CH3]
OH



FeOH* + CH3. Fe+ + CH3OH
57% FeCH2I +H2O 41%
2%


Scheme 3.1

An example of a true gas-phase catalytic cycle for the oxidation of ethylene by

nitrogen dioxide with CeO2 as a catalyst is shown in Scheme 3.2 [137]. However, the

turnover rate was limited to only 8 cycles per CeO2 ion due to reaction of the latter with

the hydrocarbon contaminants present in the NO2 gas, resulting in generation of CeO2H

species, which were unreactive toward ethylene.










Ceo,' +NOz1 C. eO)++NO


+ 100% +
._, + CH4 10- CeO + Cz-+ O


RH
CeO-F


Scheme 3.2

3.3 Gas-phase Reactivity of Ligated Metal Ions

Even though studies of ligated metal ions in the gas phase are not as

comprehensive as those of bare ions, there is a significant number of publications on this

subject, especially for the complexes of group 8-10 metals, with the main focus on Fe and

Co.

The simplest complexes, metal hydrides, can react with alkanes and alkenes in the

gas phase to form either metal alkyl or allyl complexes by the loss of one or two

molecules of dihydrogen (or methane) [138]. Co(allyl) is generally more reactive than

Fe(allyl)+. Both complexes react rapidly with alkanes (excluding methane) by a C-H

bond activation mechanism resulting in the formation of one or two dihydrogen

molecules [139].

Among MCH3 and LMCH3 ions, FeCH3 and CoCH3 ions are also the most

widely studied. The latter is capable of reacting with alkanes larger than ethane by an

initial insertion into a C-H bond. Subsequent methane loss is followed by

dehydrogenation or alkane elimination resulting in cobalt allyl complexes [140]. Both

FeCH3 and CoCH3 react with cyclic alkanes.









The reaction of MCH3+ ions with alkenes is particularly important with regard to

Ziegler-Natta polymerization. The Cp2ZrCH3+ complex exhibits distinct reactivity with

unsaturated hydrocarbons in the gas phase. Reactions of Cp2ZrCH3+ are reviewed in the

following sub-chapter. FeCH3 is unreactive with ethylene, while CoCH3 forms an

insertion product which decomposes via loss of dihydrogen to form a stable allyl

complex. Such M(allyl) complexes are also formed by FeCH3 and CoCH3 ions from

higher alkenes as a result of methane loss and possible elimination of dihydrogen or

alkene [139].

Alkene complexes Me(alkene)+ of Fe+[141], Co+ and Ni+ [142] primarily exhibit

ligand displacement and condensation reactions. However, bis(allyl) complexes resulting

from dehydrogenation by double allylic C-H activation could also be formed. These

complexes were not observed for the bare metal ions. Also, some ligand coupling, similar

to metal-assisted Diels-Alder reactions, was reported for Co(alkene)+ and to a small

extent for Fe(alkene)+ complexes.

Studies of cyclopentadienyl complexes, in particular CpCo+, in reactions with

aliphatic alkanes larger than methane revealed formation of mainly dehydrogenation

products along with a small amount of products resulting from C-C bond cleavages [143].

In the reaction of CpNi+ with a variety of aldehydes, formation of decarbonylation

products was reported [144].

It was shown that Cp2Zr+ is capable of abstracting a chlorine atom from chloro-

substituted methanes, indicating that the dissociation energy of the Zr-Cl bond is at least

81 kcal/mol [145]. In addition, the cyclopentadienyl complexes of iron and nickel were










shown to undergo a metal-switching reaction with a number of metal ions (Ti, Rh, Nb)

[146].

3.4 Reactions of the Bis(lq5-cyclopentadienyl)methylzirconium Cation in the Gas
Phase

The following sub-chapter reviews application of FTICR MS as a primary tool for

studying species involved in the process of metallocene-catalyzed olefin polymerization.

The physico-chemical properties of zirconocene ions, their intrinsic reactivity, and their

reaction patterns have been studied. The effect of the solvent on reactivity can be

estimated by comparison of gas-phase results with data for condensed phase single-site

polymerization catalysis.

3.4.1 Reactions with Alkenes

The first study of reactivity of Cp2ZrCH3+ with unsaturated hydrocarbon in the

gas phase [147] indicated that no polymerization took place at all, but rather that the

reaction proceeded exclusively via elimination of dihydrogen or alkene and formation of

an l3-allyl complex (Table 3.1) [147].

Table 3.1. Reactions of Cp2ZrCH3+ with alkenes (adapted from Ref. [147]).
r., r..,-r'.
UstlrlP' t H. cini -F'ra: rT.

I ."- J' I, -" -r(i-H 1 00
L-bu n H2 ._ Ir C
-'Ler1 11 / ..i I r n 5

1-hene IT L 52 7
2H- Lp:ZrL.,H, H J s7
*:1. :p (." H ._I I -I. Ij II|U
I.r.! I r.lr t 14 p:Z r1. ,I ll4;'
niLH'.h -. t t- ,[;- i(' tH ln1 i iti


CH4 r31 2

t( Hi
f i ,











The suggested reaction mechanisms involve insertion/dehydrogenation

accompanied by statistical H/D scrambling with deuterated ethylene or a C-bond

metathesis (Fig. 3.2).


-+ ,. H
-> H







I ,'
'I J H


+ n.


a) Insertion/dehydrogenation reaction mechanism




+ I


b) o-bond metathesis reaction mechanism

Figure 3.2. Reactions of Cp2ZrCH3+ with alkenes (adapted from Ref [147]).

The observed reaction pathways were rationalized by the insufficient

thermalization of "hot" ions produced the during electron ionization event (Fig. 3.3)

[147-151]. The observed loss of a neutral molecule (H2, CH4) in the gas-phase reactions


can be suppressed. Three ways can be suggested so the reaction pathway observed in the

liquid phase would be approached: first, increasing the pressure by, for example, pulsing

an inert gas, to quench excess internal energy by collisions; second, by coordination of a


weakly bound molecule (for example, a solvent) by the coordinatively unsaturated


A- L
HIF~


+. H
-' ;.-


I: *_+







36


metallocene cation. The loss of a solvent molecule will result in excess energy being

taken away as translational energy of a departing moiety; or third, by increasing the size

and complexity of coordinated ligands, creating an "energy sink" where the excess

energy can be efficiently redistributed among a multitude of available vibrational states,

thus decreasing the probability of allyl complex formation.





1_L -V ''


.11


1'T


i
~

t
~,
t,
t( '


P1







I.Z


*

<-F ~ ,*----~

*1


Figure 3.3. Comparison of qualitative potential energy surfaces for the reaction of
Cp2ZrCH3+ with ethylene in solution and the gas phase (adapted from Ref.
[147]).


7P
r


1.
r.
z
~









The first approach was implemented by Plattner et al. [152]. Stable solutions of

the tetrakis(pentafluorophenyl)borate salt of the methylzirconocene cation, Cp2ZrCH3+, in

a mixture of acetonitrile and CH2C12 were electrosprayed with a "high" pressure, 10

mTorr, of thermalization gas (Ar). This led to insertion of up to four molecules of alkene

before deactivation. The observed rate constant of propylene polymerization under these

conditions was estimated to be about 6 orders of magnitude higher than that in solution

[152, 153], which is an excellent illustration of intrinsic high reactivity of an isolated

cation in the absence of a coordinating counterion or coordination by solvent molecules.

The loss of hydrogen was more difficult to suppress -reaction with ethylene still led to

allyl complex formation.

Besides reactions of metallocenes with unsaturated hydrocarbons, previous gas-

phase studies have involved investigation of the reactivity patterns of bis(q5-

cyclopentadienyl)methylzirconium cations with nitriles [154] and effects of ancillary

ligands on LL'ZrCH3 cation reactivity with dihydrogen and ethylene [150] and other

alkenes [151]. Also, the intrinsic reactivity of hydroxyzirconocenes was studied as a

model for metal-hydroxide complex ion reactions with a range of compounds [155].

Our experimental data (Figure 3.4) indicate the ability of C-H bond activation by

the Cp2ZrCH3+ complex in cyclic alkanes with consecutive formation of a complex with

cyclic allyl structure. In the reaction with cyclopentane, a complex with m/z 287 was

observed. The experimental procedure and appearance of mass spectra of zirconocene

complexes are discussed in detail in Chapter 4. The ion with m/z 287 was presumably

formed via loss of dihydrogen and methane (Scheme 3.3). The proposed structure of the

complex is shown on Figure 3.5.










Cp2ZrMe+


100
90
80
70 CP2Zr+ Cp2ZiOH+
u 60
*" 50
.0 40 287
30
20-
10-

220 230 240 250 260 270 280 290
m/z

Figure 3.4. Mass spectrum for the reaction of Cp2ZrCH3+ with cyclopentane.

Cp2ZrCH3+ + C5Ho > Cp2ZrCH7+ + CH4 + H2
Scheme 3.3

Formation of such a complex assumes initial activation of a C-H bond by a metal center.


Cp




Cp
Figure 3.5. Proposed structure of the complex with m/z 287.

3.4.2 Allyl Complex Formation: Mechanistic Differences of Reaction Patterns in the
Gas and Liquid Phases

One of the main drawbacks of metallocene catalysts is a gradual decrease in the

rate of monomer uptake with time, which occurs on the timescale of several hours, or, in

some cases, even minutes [156, 157]. The exact nature of this phenomenon is still

debatable; to a certain degree, the deactivation results from factors such as presence of

impurities and high-temperature decomposition. Recently, it has been postulated that a









possible cause may be accumulation of a less active or inactive zirconium-allyl complex

[72, 151, 152, 158-162]. The formation of this complex was observed or inferred on

several occasions in both (as discussed above) gas-phase experiments [148, 154] and in

the solution phase [72, 151, 152, 158-161, 163-165]. Formation of H2 gas during the

polymerization reactions catalyzed by zirconocenes is a possible indication of the allyl

complex formation [166].

Various theoretical studies of the bis(l5-cyclopentadienyl)methylzirconium cation

reactivity toward dihydrogen and alkenes [167-171] or even alkanes [172, 173] have been

reported. However, the subsequent transformation of the allyl complex after its formation

in the reaction mixture is not completely clear. Initially, it was presumed that this

complex is catalytically inactive [72]. As shown by Ziegler and co-workers [174, 175],

the allyl complex can be reactivated by alkene molecule insertion; their theoretical

studies of zirconocene and titanocene cations indicate that the limiting step in the reaction

of the allyl complex formation reaction is a P-hydrogen elimination and that it has an

activation barrier of- 11 kcal/mol for the zirconium and 13 kcal/mol for the titanium

complex. The liquid-phase studies by Britzinger and co-workers of the

Cp2ZrMe(methallyl) complex [162] showed that its perfluorotriphenylborane adduct, the

contact ion pair [Cp2Zr(methallyl)]+[MeB(C6F5)3]-, reacts with propene with rates

substantially lower than those of cationic Zr-alkyl species. The propene molecule inserts

between Zr and one of the allylic termini. Their DFT calculations have indicated that

insertion of propene directly into an r3-coordinated Zr-allyl unit occurs with lower

activation energy than insertion into an rl-bound Zr-allyl species and that the lowest-









energy pathway for the reactivation of a cationic zirconocene allyl species is its

reconversion to the corresponding Zr-alkyl species by H2.

Nevertheless, experimental proof of the allyl complex reactivation has yet to be

demonstrated. It is expected that formation of the transition state for alkene insertion by

the allyl complex and its consecutive transformation into a vinyl-terminated product can

be controlled by various means. It also appears to be necessary to test the intrinsic

reactivity of the Cp2Zr(allyl) complex by studying its reactivity under the gas phase low-

pressure conditions of an FTICR MS spectrometer, thus eliminating various perplexing

factors such as effects of impurities, electric permeability of the solvent, ligands

exchange, etc.














CHAPTER 4
GAS-PHASE REACTIONS OF BIS({S-CYCLOPENTADIENYL)
METHYLZIRCONIUM CATIONS WITH KETONES AND ALDEHYDES

4.1 Rationale

Despite rapid growth in the gas-phase chemistry of organometallic complexes

and, especially, metal ions [134, 135, 176, 177], and the major importance of metallocene

catalysis, gas-phase studies of metallocenes are still rather scarce. The purpose of the

work reported in this chapter was to gain a deeper understanding of this chemistry, with a

focus on reaction mechanisms of the gas-phase chemistry of metallocenes, and to attempt

to draw general conclusions about these processes. For example, based on the work

reported in Chapter 3, it may be postulated that the formation of an r3-allyl complex is an

instance of a general process which takes place under high-vacuum, non-equilibrium

conditions.

Scheme 4.1 shows for the reaction mechanism suggested in earlier studies [151]

for the reactions of methyl zirconocenes with terminal olefins, Y = CH2. For reactions

with olefins such as isobutene or styrene, this scheme can be applied for the Y=C(Ri)R2

substrate (R1 = H or R). In the case of 1,1 disubstituted alkenes, elimination of methane

was observed [147]. At the same time, in the liquid phase, the 12 alkene insertion

products of zirconocenes are generally stable and have no tendency to eliminate a neutral

molecule.






42





S ch eH 3 Z 4













-1














Scheme 4.1

In the case of aldehydes and ketones, the group Y is the carbonyl oxygen.

Compared to alkenes, the carbonyls are expected to be coordinated more strongly to

Zr(IV) by donation of an electron lone pair from oxygen (as opposed to 21-bonding

electrons in the alkenes) to an empty d-orbital of the metal. Therefore, initial bonding of

the substrate with the metal is expected to occur via the substrate's carbonyl oxygen. In

the gas phase, under high-vacuum, high-energy collision conditions, the reactivity









observed might possibly also be described by the general Scheme 4.1. This would be

quite different from the known chemistry in the solution phase, as discussed in the last

chapter. For example, for silver-mediated (i.e. chloride abstraction from Schwarz reagent

by a catalytic amount of AgC104 [54]) aldehyde addition reactions of Cp2ZrRCl

complexes, a mechanism involving generation of alkyl/alkenyl zirconocene was proposed

[54]. The resultant reaction products are alcohols formed through condensation of the

aldehyde and R ligand of zirconocene.

The chemistry of zirconocene carbonyl, acyl and related species presents interest

due to, for example, the potential of complexes like [LM {CHCICH2C(=O)R] to carry

out insertion polymerization/copolymerization of vinyl chloride, thus enabling better

control of PVC polymer structure and properties than is possible by conventional radical

methods [178].

The work presented in this chapter represents a study of the reactivity pathways of

bis(T5-cyclopentadienyl)methylzirconium (1), chosen as a model compound, with a series

of simple ketones and aldehydes in the gas phase.

4.2 Experimental Procedures

Ion-molecule reactions were studied using Fourier transform ion cyclotron

resonance mass spectrometry. The primary mass spectrometer used in the experiments

was based on a 2.0 Tesla magnet with an internal electron ionization (El) ion generation

source (Fig. 4.1).

A common experiment sequence was used:

1) Quench: application of +/- 9.5 V to trapping plates in order to eliminate any

ions present prior to the experiment.











ICR Cell


2T Magnet
/


El Source


Diffusion Pump


Figure 4.1. Schematic diagram of the mass spectrometer used in this work.

2) Electron beam: irradiation of the neutral substrates with the flux of electrons

produced by the filament of the El (Electron Ionization) source (Fig. 4.2). The number of

electrons was controlled by the filament current, typically in the range of 1-2 A. The

electron energy distribution, controlled by the voltages on the repeller plate and the grid,

was varied from 15 to 50 eV in order to produce an optimal number of ions.

Rhenium Filam ent
Repeller Plate






Grid


Figure 4.2. Schematic diagram of the electron ionization source.

3) Ion ejection (optional): if necessary, the ejection of ions with selected m/z from

the MS cell by applying SWIFT waveform [179] could be carried out.


Valves





Solids Probe
Port









4) Reaction delay: time delay varying from 0 tolO s allowing unejected ions to

react with the neutral(s) present in the cell.

5) Excitation: excitation of the reaction product ions.

6) Detection: detection of the image current with subsequent Fourier transform of

the transient to obtain the mass spectrum.

The above sequence of events (scan) was repeated for each experiment 20 to 40 or

more times in order to achieve a higher signal to noise ratio by a signal averaging.

Table 4.1. Isotopes of zirconium.
Isotope Natural Abundance, %

90Zr 51.45

9Zr 11.22

92Zr 17.15

93Zr Radioactive

94Zr 17.38

95Zr Radioactive

96Zr 2.80
Zirconium has five stable isotopes (Table 4.1), which in superposition with 13C/12C

isotopes result in a unique pattern of seven peaks in mass spectra for all of the

organozirconium compounds studied in this work, enabling unambiguous identification

of the presence of Zr. Electron ionization of Cp2Zr(CH3)2 resulted in the formation of two

ions: Cp2ZrCH3+ and Cp2Zr+. The typically obtained mass spectra of Cp2Zr(CH3)2 are

shown in Fig. 4.3




















SI


220 225 230 235 240
m/z


No reaction delay

100
90
80
70
a,
u 60
2 50
I 40
30
20-
10-
-- -i ^J


A A .


230
m/z


.AL


n)h1


b) Reaction delay 0.5 s.

Figure 4.3. El mass spectrum of Cp2Zr(CH3)2: Cp2ZrCH3+ (m/z 235-242), Cp2Zr+(m/z
220-227) and Cp2ZrOH+ (m/z 237-244)

Since the Cp2Zr+ ion and its reaction products complicate the spectrum, in order to

distinguish products from these two possible parents, additional studies using selective

SWIFT ejection were implemented when necessary. An example of a SWIFT experiment

is shown in Figure 3.4. When one possible parent ion Cp2ZrMe (m/z 235-242) is ejected,


I I







47


the reaction products' peaks (m/z 277-284) disappear. On the other hand, when another

possible parent Cp2Zr (m/z 220-227) is ejected, no change is observed. This indicates


that the product is formed from the Cp2ZrMe+ ion. An incomplete removal of either

complex by SWIFT, especially of the Cp2ZrMe ion, stems from a partial re-formation

during a reaction delay event via collisions of neutral Cp2Zr(CH3)2, which is present in

the cell at all times, with Cp2Zr ions.


III I


230 240 250
m/z


260 270 280 290


a) No ions ejected


1 I


1111


210 220 230 240

b) Cp2Zr (m/z 220-227) is ejected


260 270


280 290


100
90
80
70
60
50
40
30
20
10
200


I.__ ....._ __


------------------- --------------


111,


, 1 l .1
















20
101
210 220 230 240 250 260 270 280
m /
c) Cp2ZrMe+ (m/z 235-242) is ejected
Figure 4.4. Example of a SWIFT experiment.

The typical background pressure on the 2.0 T instrument was 2-10-9 Torr;

pressure of zirconocene was 10-7 Torr, decreasing throughout experiment at the rate of

approximately 10-8 Torr/hour due to the sample evaporation. A set of experiments at

different substrate pressures of 0.5-10-7, 1-10-7, 2-107 and 410-7 Torr and common

reaction times of 0, 0.5, 1, 2, 5 s and longer were carried out for each reaction.

The formation of binuclear metal ions Cp4Zr2(CH3)n+ (n = 0 3) in reactions of

both parent ions with the neutral complicated the spectrum, especially at longer reaction

times (Fig. 4.5), which impeded the ability to study reactions with rate coefficients below

~ 109 1/M-s.


290 300


~kLAAA.~AAA~


fnAJ\I\A.,


440 450 460 470
m/z


480 490 500


Figure 4.5. Binuclear complexes Cp4Zr2(CH3)n' (n = 0 3)


kL i t u k 6









The presence of water also decreased the amount of the ion of interest as it reacted very

rapidly to form a hydroxyl complex (m/z 237-244) (see Fig. 4.3):

Cp2ZrCH3+ + H20 Cp2ZrOH+ + CH4

Therefore, precautions had to be taken to minimize the extent of hydrolysis. At

prolonged reaction times the Cp2ZrOH+ ion and its reaction products dominated the

spectrum. In some cases hydrolysis interfered with the identification of reaction products

even at typical reaction times of 1-5 seconds.

Cp2Zr(CH3)2 and all of the aldehydes, ketones, and imines used in these studies

were purchased from commercial sources. The liquids were purified by repeated freeze-

pump-thaw cycles. Cp2Zr(CD3)2 was synthesized from commercially purchased

precursors in accordance with a preparation procedure described in the literature [180]

via the following reaction:

Cp2ZrCl2 + 2CD3MgI Cp2Zr(CD3)2 + 2MgICl

A Schlenk line was used at all times due to the extreme air and moisture sensitivity of

both the reactants and the product. The reaction was carried out in diethyl ether solvent

at room temperature under a flow of dry nitrogen in thoroughly desiccated glassware. The

reaction product's precipitate was dissolved in ether/pentane mixture for the final product

extraction followed by a recrystallization from benzene at -20 OC. The mass spectra of the

Cp2Zr(CD3)2 obtained from this procedure are shown in Fig. 4.6. As expected, the

Cp2ZrCH3+ (m/z 235) complex is replaced with the Cp2ZrCD3+ (m/z 238) complex, while

the hydrolysis product, Cp2ZrOH+ (m/z 237), is unaltered.




















m/z


a) No reaction delay


A n A A


A IL


m/z


b) Reaction delay 0.5 s.
Figure 4.6. El mass spectrum of Cp2Zr(CD3)2 (* Noise peaks).
Ab initio (Hartree-Fock) as well as Density Functional Theory (DFT) calculations
were carried out by Cesar S. Contreras for the zirconocene complexes to establish the


most stable structures.


nn~, ,~ i .nnl.h..


''''''"'""""''''''' "









4.3 Reaction with Ketones

For the sake of simplicity, all of the seven Zr-containing isotopic ions are further

reported as a single mass corresponding to the most abundant (51.45%) 90Zr isotope. In

cases where there is an overlap of several series of peaks corresponding to different

products, a detailed analysis of the contributions from individual ions has been carried

out. Since the isotopic abundances of the constituent elements are known, and at least one

peak containing the lightest 90Zr isotope and thus of lowest m/z does not overlap with

other peaks, the heights of individual series of peaks can be estimated.

In previous studies under similar conditions it has been demonstrated that the

cyclopentadienyls are solely spectator ligands as no Cp "switching" reactions and HD

elimination for the complexes with fully deuterated ligands but non-deuterated Cp rings

have been observed [147]. Therefore, the Cp2Zr unit was presumed to remain intact

throughout all of the experiments. Where questions exist about the structures of the

product ions, their formulae are reported in the form Cp2ZrCxHyOz with the

corresponding m/z ratio. Since all of the reported ions were singly charged the term

"empirical formula" is used throughout the text. Detailed structural considerations are

presented later in this chapter.

4.3.1 Reactions of Cp2ZrCH3+/Cp2ZrCD3+ with Acetone/d6-Acetone

The reaction of 1 (m/z 235) with acetone (nominal molecular weight 58 amu)

proceeds with formation of several products (Fig. 4.7). Two principal ones with m/z 293

and 351 correspond to the consecutive addition of one and two substrate molecules, as

shown in Scheme 4.2. Addition of the second acetone molecule becomes pronounced

only after 4 seconds of reaction time (at a total pressure of approximately 2-10-7 torr). A

variety of structures can be postulated for the observed adducts, e.g. rl vs. r2 complexes,











etc., (only one of the likely structures is shown), and, therefore, theoretical geometry


optimizations and/or spectroscopic data are needed for more definitive "proof" of


structure.

100
90
80
70
60 m/z 277

40
30
20
10

200 250 300 350
m/z


a) 1 second reaction time


m/z 293
100
90
80
S70
60
S5o m/z 351
40
<30 /

10.I..._

200 250 300 350
rfz
b) 5 seconds reaction time
Figure 4.7. Mass spectra of ions formed in the reaction of Cp2ZrMe+ with acetone.

An ion with m/z 277 (Fig.4.7; the peak with m/z 278 corresponds to the product


depicted in Scheme 4.2), corresponding to the empirical formula Cp2ZrC3HsO+,


apparently resulting from elimination of methane from the ion with m/z 293, is formed to


a lesser extent than the latter, and its peak height decreases at longer reaction times. It is


coordinatively unsaturated and adds an additional acetone molecule, forming an ion with


m/z 335 (Scheme 4.3, Figure 4.8).

















4 .


7


+ 0
I):


Ir


Scheme 4.2


:7'


Scheme 4.3


/Z


7
A


/.


-- K.^'),


7~~


.












15 2

10




280 335 340 345
m/z m/z


Figure 4.8. Mass spectra of ions formed in the reaction of Cp2ZrMe with acetone

The hydrolysis product of 1 (m/z 235), bis(cyclopentadienyl)zirconium hydroxyl

cation Cp2ZrOH (m/z 237, Fig. 4.3), formed by reaction with trace amounts of water

present in the mass spectrometer, reacts with acetone to give an addition product with

m/z 295, which becomes predominant at very long reaction times (5 seconds or more).

This indicates a slower reaction rate and, for these species reacting with acetone, is

consistent with the demonstrated intrinsic lower reactivity of the hydroxide derivative

compared to the methylzirconium cation itself [155]. The formation of binuclear metal

ions Cp4Zr2(CH3)n+ (n = 0 3) and their derivatives complicated the spectrum, especially

at longer reaction times and had to be taken into account as well.

Reaction of 1 with acetone proceeds differently for different stages of

thermalization of the reaction mixture. The consecutive addition of two acetone

molecules by the Cp2ZrCH3 ion results in a coordinatively saturated 18 electron

complex. The loss of methane and the subsequent generation of the complex with m/z

277 results from the reaction of 1 ions with sufficient internal energy (Scheme 4.4):









Cp2ZrCH3+ + O=C(CH3)2 Cp2ZrC4H90+ Cp2ZrC3HsO+ + CH4

m/z 235 m/z 58 m/z 293 m/z 277
Scheme 4.4

Consideration of the gas-phase chemistry of bis(T5-cyclopentadienyl)

methylzirconium cations described in the literature suggests that the species with m/z 277

have structures similar to those postulated for the gas-phase reaction of 1 with

unsaturated hydrocarbons, i.e. l3-allyl complexes [147, 154]. The possibility of 3-allyl

complex formation is well documented [72, 151, 152, 158-161, 163, 164]. Density

functional theory studies also confirm the plausibility of allyl intermediate formation as a

possible side reaction in homogeneous single-site olefin polymerization [174, 175].

However, there is no direct evidence supporting r3-allyl structures postulated for the

reaction of 1 with acetone. Also, it is difficult to suggest tenable reaction mechanisms

which would lead to the formation of such complex(es).

Our DFT calculations, carried out by Cesar S. Contreras, suggest that the most

stable structure is not an r3-allyl complex, but rather that of the complex shown in Fig.

4.9, which is an r3-enolate complex. The higher stability of this complex can be

understood: the l3-enolate complex is isoelectronic with the r3-allyl complex and both

the allyl and enolate complexes in either resonance form participate in both a and dative

bonding of the ligand to a metal center, thus contributing to high stability of the complex.

The ability of the oxygen atom in the r3-enolate complex (even though it is weaker C-

donor compared to the methylene group) in the oxygen o-bonded resonance form to

provide additional bonding by partially transferring its available lone pair into empty Zr

2d orbital results in increased total bond strength. In another resonance form, dative

bonding from the oxygen atom is substantially more efficient than t7-complexation of the









C=C bond by a metal center. According to the Dewar-Chatt-Duncanson model discussed

above [26, 27], an efficient 7t-bonding occurs when a a-type donation from the C=C 7t

orbital is synergistic with concomitant 7t-backbonding into an empty 7* orbital of the

alkene which in turn implies that only metals with at least one filled nonbonding orbital

can efficiently form such complexes. For the do zirconocene, only donation of electrons is

possible from the C=C bond, making the 7t-complexation rather inefficient.










-2A42 2.44












a) b)

Figure 4.9. The a) l3-enolate and b) l3-allyl complexes.

In the reactions of Cp2ZrMe+ with d6-acetone, normal addition products of d6-

acetone (nominal molecular weight 64 amu) to 1 are expected to have m/z 299 and m/z

363, 6 and 12 mass units, respectively, higher than for the products seen with the

nondeuterated substrate. Both of these ions were observed, and their intensities obeyed

the same trend as in case of nondeuterated acetone: only one molecule was coordinated

initially and the product with two acetone molecules became the major peak after few






57


seconds reaction time. In addition, formation of a product ion with m/z 302 was

observed, which became predominant after 5 s. The 3 mass unit gain can be assigned to

the following product (Scheme 4.5):











Scheme 4.5

This ion results, apparently, from CH3/CD3 scrambling between the metal center and

carbonyl substrate.

Three ions with m/z 279, m/z 280 (major product), and m/z 282, corresponding to

the empirical formula Cp2ZrC3H5-nDnO+, n = 2, 3 and 5 (approximate peak height ratios ~

2:5:1), replace Cp2ZrC3HsO+ (m/z 277), the ion reported above in reactions of 1 with

acetone (postulated to be an l3-enolate complex).

The observed CH3/CD3 scrambling and lack of H/D scrambling suggest reversible

transfer of the methyl group between the Zr and a coordinated acetone molecule as a unit,

rather than transfer of individual H atoms. This in turn implies that in the complex with

m/z 280 the CD3 group remains intact (Scheme 4.6):







Scheme 4.6
Scheme 4.6









The transfer of the deuterated methyl group introduces only a negligible

secondary isotope effect, so the migratory insertion-deinsertion equilibrium remains

unaffected for all of the studied combinations of zirconocene/acetone: in the

approximation of a AE arising only from a change in zero-point energy of the stretching

vibration of the Zr-C bond the predicted isotope effect is:


hcv C3 [181]
kcD3/kcH3 = e- where CD [181]


Assuming V (Zr-C) 500 cm-1, the calculated value for A is 1.10-3, thus kCD3/kcH3 1.

Cp2ZrCD3+, (2), reacted with acetone similarly to 1, with a corresponding shift of

all the peaks by 3 mass units. However, two ions (m/z 277 and m/z 280, corresponding to

the empirical formulae Cp2ZrC3HsO+ and Cp2ZrC3H2D30+)) with comparable peak

heights, and a small amount of an ion with m/z 279 were also formed.

Similarly, the reaction of 2 with d6-acetone resulted in formation of ions with m/z

302 and 366 (addition of one and two molecules of d6-acetone respectively) and an ion

with m/z 282, corresponding to the empirical formula Cp2ZrC3DO+ (Scheme 4.7).

Cp2ZrCD3+ + O=C(CD3)2 Cp2ZrC4D9O+ Cp2ZrC3DsO+ + CD4

m/z 238 m/z 64 m/z 302 m/z 282

Scheme 4.7

This complex is analogous to the product with m/z 277 (q3-enolate complex), but

fully deuterated (except for the Cp rings). Formation of the complex with m/z 282

proceeded with a deuterium kinetic isotope effect as the apparent rate of formation of the

corresponding complex (m/z 277) in the reaction of 1 and acetone was approximately

twice as high. The primary isotope effect indicates C-D bond cleavage in the rate-









determining step in the reaction sequence leading to the formation of the enolate

complex, since, in this case, there are no C-H bonds available in the reacting ligands. All

other observed reactivity trends were similar to those described above for the reactions of

1: increasing reaction time led to increasing addition of a second acetone molecule to the

zirconocene complex, disappearance of the m/z 282 complex along with concomitant

hydration and hydrolysis reactions due to inevitable traces of water, prevalent at very

long reaction times (5 seconds and above).

The proposed reaction mechanism for the reaction of 1 with d6-acetone as an

example is shown in Schemes 4.8 a) and b). The second step, the migratory insertion of a

coordinated ketone molecule, is reversible and leads to CH3/CD3 scrambling. This

equilibrium is fast and proceeds via the intermediate insertion product where all three

substituents at the oxygen-bound carbon are equivalent, thus leading to a statistical

distribution of methyl and methyl-d3 groups between the metal and coordinated ketone

(Scheme 4.8a).

When a ketone molecule is coordinated by the metal center, the Bronsted-Lowry

acid-base reaction can take place. The metal-bound alkyl group is a good Bronsted base,

the a-proton of a ketone is slightly acidic (and significantly more acidic in the ketone

molecule coordinated by a metal) and the nascent enolate is expected to be a reasonably

good leaving group. The combination of these factors enables nucleophilic attack of the

alkyl on the a-proton of a ketone. This reaction is assumed to be the slowest step and to

proceed via the least strained 6-membered cyclic transition state. If two different groups

are available at the carbonyl group, two different products can be formed, which, in

combination with products formed in the reaction of either the CH3- or CD3- nucleophile,






60


will comprise the observed set of products (Scheme 4.8b). The products resulting from

reaction involving nucleophilic attack on a deuteron are expected to be less abundant due

to the primary isotope effect resulting from C-D bond cleavage.




S C HC H 3
CH3



I 4













CD3















Scheme 4.8a

The expected product abundance ratios are completely consistent with those

observed in the experiment.










L+


\
L., -


+

,CH3


,. ,-2
\


*7/I
'7


H
\-H
l-I


o1R /.


'1



i i\
:


+

2
.L
'LZ

" *


.7 i
2 82


:1 +


I I











280





CH3


CD4



1'o
-7. nI
K A'


Scheme 4.8b

4.3.2. The Reactions of Cp2ZrCH3+/Cp2ZrCD3+ with 2-Butanone, Methyl Isobutyl
Ketone and Cyclohexanone.

The product distributions in reactions of 1 and 2 with ketones other than acetone

are also entirely consistent with the proposed reaction mechanism. Both 1 and 2

sequentially add two substrate molecules (Scheme 4.9 illustrates the case for methyl

isobutyl ketones as a substrate, examples of spectra are shown in Fig. 4.10).


"I,.


-7 H3


Zr


+i


I









m/z 235


85
80
75
70
65
60
S55
c 50
'o 45
2 40
30
25
20
15
10
5
200
200


m/z 237


m/z 277

I




11.1 ~. I


m/z 319





.. I il


m/z 335

/
I. 1


250 300 350 400
m/z


a) 1 second reaction time


11 j i L


m/z 319



I


m/z 277


300
m/z


m/z 335

/


Ill. ...


jili jI


m/z 435




I \.. ,


b) 5 seconds reaction time

Figure 4.10. Mass spectra of ions formed in the reaction of Cp2ZrMe with methyl
isobutyl ketone.


m/z 220


\I


. L


I


I i


IIIIIIII


= I










7
A
/


+


+i
-,11


7r


+


/'


CH3
_/.


.7


i-Bu


I I I1


Scheme 4.9

The major products observed are summarized in Table 4.2

Table 4.2. Major reaction products observed in the reactions of 1 and 2 with 2-butanone,
and methyl isobutyl ketone at various reaction times (1 to 5 seconds).
Reaction Observed Reaction Products

m/z Suggested Empirical Formula Neutral

eliminated

1 (m/z 235) + 2- 307 [Cp2ZrCH3 O=C(CH3)C2Hs]

butanone (nominal 379 [Cp2ZrCH3 20=C(CH3)C2Hs]

molecular weight 72 277 Cp2ZrC3HO+ C2H6

amu) 291 Cp2ZrC4H70 CH4


'7


K


+ .








Table 4.2 Continued
Reaction Observed Reaction Products

m/z Suggested Empirical Formula Neutral

eliminated

2 (m/z 238) + 2- 310 [Cp2ZrCD3 O=C(CH3)C2H5]

butanone (nominal 382 [Cp2ZrCD3 20=C(CH3)C2H5]+

molecular weight 72 279 Cp2ZrC3H3D20+ C2HD

amu) 280 Cp2ZrC3H2D30+ C2H6

291 Cp2ZrC4H70+ CHD3

293 Cp2ZrC4H5D20 CH3D

294 Cp2ZrC4H4D30+ CH4

1 (m/z 235) + methyl 335 [Cp2ZrCH3 O=C(CH3)i-C4H9]+

isobutyl ketone 435 [Cp2ZrCH3 20=C(CH3)i-C4H9]

(nominal molecular 277 Cp2ZrC3HO+ C4H1o

weight 100 amu) 319 Cp2ZrC6H10+ CH4

377 [Cp2ZrC3HsO O=C(CH3)i-C4H9]

2 (m/z 238) + methyl 338 [Cp2ZrCD3 O=C(CH3)i-C4H9]+

isobutyl ketone 438 [Cp2ZrCD3 20=C(CH3)i-C4H9]

(nominal molecular 279 Cp2ZrC3H3D20+ C4H9D

weight 100 amu) 280 Cp2ZrC3H2D30+ C4Ho









Table 4.2 Continued
Reaction Observed Reaction Products

m/z Suggested Empirical Formula Neutral

eliminated

2 (m/z 238) + methyl 319 Cp2ZrC6H10+O CHD3

isobutyl ketone 321 Cp2ZrC6H9D20+ CH3D

(nominal molecular 322 Cp2ZrC6H8D30+ CH4

weight 100 amu)

1 (m/z 235) + 333 [Cp2ZrCH3 O=C(CH2)5]

cyclohexanone 431 [Cp2ZrCH3 20=C(CH2)5]

(nominal molecular 317 Cp2ZrC6H90+ CH4

weight 98 amu)

2 (m/z 235) + 336 [Cp2ZrCD3 0O=C(CH2)5]+

cyclohexanone 434 [Cp2ZrCD3 20=C(CH2)5]

(nominal molecular 317 Cp2ZrC6H90+ CHD3

weight 98 amu)

In the reaction of 1 with 2-butanone, a complex with m/z 277, (most likely

resulting from the loss of ethane from the single adduct ion with m/z 307), was produced

to a lesser extent than in the reactions with acetone. An ion with m/z 291 (most likely

produced by methane loss from the m/z 307 ion) was nearly as abundant as the m/z 277

ion. The intensities of both peaks appeared to change with time in a similar fashion.

Substantial fragmentation of the coordinated ligands resulted in multiple product peaks,

increasing with reaction time. Scheme 4.10 shows as an example possible origination of

the ion with m/z 293 in the reaction of 1 with 2-butanone:










.*'/ ,CHs
A' ,


AY
.,. j


H/"
\


+


3 5 1 ,' "' '

Scheme 4.10

The reaction of 1 with methyl isobutyl ketone also produced a m/z 277 complex

(presumably resulting from C4H10 loss by the m/z 335 ion), which was the major reaction

product for reaction times less than one second. The complex with m/z 277 was nearly

three times more abundant than the complex with m/z 319 (presumably resulting from the

loss of methane by the m/z 335 ion). For both ketones, variations of peak heights with

reaction time followed the same trends as for complexes with m/z 277, 291 and 319.

As in the reaction with 2-butanone, apparent ligand fragmentation was also

observed. The loss of a zirconocene methyl group (and probable elimination of a neutral

alkene), possibly in a multiple-step process, was commonly observed (Scheme 4.11).


.7 .H :7





Scheme 4.11

The coordinatively unsaturated complex with m/z 293 reacted further by addition

of a methyl isobutyl ketone molecule, resulting in a product with m/z 399. The reactions

of 2 with 2-butanone and methyl isobutyl ketone produced fragmentation patterns that

were remarkably similar to those produced in reaction of 1. A series of peaks with same









m/z were observed, due to the common loss of the deuterated/nondeuterated methyl

group of the metallocene along with the fragmentation.


Cp2ZrCD3+ + O=C(CH3)C2H5 ~ Cp2ZrC5H8D30+ -
m/z 238 m/z 72 m/z 310


[Cp2ZrCH3 O=C(CD3)C2H5]








[Cp2ZrCD3 O=C(CH3)C2H5]


[Cp2ZrC2H5 O=C(CH3)CD3]


- [Cp2ZrCH3 O=C(CD3)C2H5]+


-- [Cp2ZrCD3 O=C(CH3)C2H5 ]


-- [Cp2ZrC2H5 O=C(CH3)CD3]


Cp2ZrOC(C2H5)=CD2 + CH3D
m/z 293

Cp2ZrOC(CD3)=CHCH3+ + CH4

m/z 294


Cp2ZrOC(C2H5)
m/z 291

Cp2ZrOC(CH3)=

m/z 291


Cp2ZrOC(CD3)
m/z 280

Cp2ZrOC(CH3)

m/z 279


=CH2 + CHD3



CHCH3+ + CHD3


=CH2+ + C2H6



=CD2+ + C2H6


Scheme 4.12

The complex with m/z 277 is presumed to be the same reaction product for both of

these ketones and acetone, i.e. the l3-enolate complex. The complexes with m/z 291 and

m/z 319 were formed in comparable amounts with m/z 277, and, according to the

reaction mechanism proposed above, differ from the l3-enolate complex with m/z 277









(carrying the methyl group) only by substitution of the corresponding alkyl (Et, iBu,) at

the central carbon, or both carbons (Me, Me and Me, iPr) of the enolate ligand. Thus, the

ketones other than acetone, Ri-C(=O)-R2 (R1 = Me, R2 = Et, iBu) react with

Cp2ZrCH3+/Cp2ZrCD3+ in a similar manner, but two different substituents at the carbonyl

carbon enable additional possible reaction pathway(s), with the loss of one of the alkyls

as the corresponding alkane, as shown in Scheme 4.12 for the reaction of 2 with 2-

butanone.

The expected product abundances are quite consistent with the experimental

values, except for higher abundance of the complex with m/z 277 in the reactions of 1

and 2 with methyl isobutyl ketone, which was almost three times more abundant than

would be expected from a purely statistical distribution of alkyl groups and an assumed

isotope effect kH/kD ~ 2. This possibly results from a combination of two factors: higher

nucleophilicity of the isopropyl as compared to the methyl group and release of steric

tension when the relatively bulky iBu group is removed from the metal coordination

sphere.










Scheme 4.13












~7H /


,, +

4/c, I_

/' c-it






7 .H-



,' ItI
"\



,,.. +

.****yH \ >cjH,

/ ,-, .


S+


I -_


,7. ,' H -.- ,

/ /
H, ., *




Scheme 4.14

Cyclohexanone (nominal molecular weight 98 amu) reacted with 1 forming

simple addition products of m/z 333 and 431 only, with no other complexes and just two

products: the loss of methane leads to formation of complexes with m/z 317 and 415

respectively. The same enolate product (m/z 317) is formed in the reaction of 2 with

cyclohexanone. This is additional evidence for the proposed reaction mechanism, as two


~---------i ~


== .









groups at the ketone's carbonyl are locked into the ring and unable to migrate so that CD3

group scrambling is impossible, and the deuterated group is eliminated as methane

resulting in the same complex as for the reaction of 1 (Scheme 4.13).

The fragmentation reactions which do not conform to the stoichiometry of the

expected products of reactions fitting the pattern of Scheme 4.8 for the reactions of both 1

and 2 with all of the ketones studied result in minor amounts of products and were

pronounced only for the more branched methyl isobutyl ketone and very long reaction

times. The observed fragmentation product peaks most likely result from ions with

different isomeric structures with the same m/z. One of the possible pathways is

shown in Scheme 4.14 for the reaction of 1 with 2-butanone. The formation of a

carbocation via C-H bond scission enables further rearrangements[182] and at some point

is followed by elimination of neutral(s) leading to the variety of fragmentation products.



4.3.3 Reactions with Other Ketones

In the reactions of Cp2ZrCH3+ with 1,4-benzoquinone and 1,4-naphthoquinone, no

interesting reactivity was observed (except for very small amounts of the complex with

m/z 277 formed in reaction with 1,4-benzoquinone). However, both substrates exhibited

an unusual tendency to coordinate readily with the binuclear metal complexes (Fig. 4.11)

In the reaction of Cp2ZrCH3+ with diethyl carbonate, the peaks corresponding to

the complex with m/z 471 (addition of two substrate molecules) overlap with the region

of the binuclear metal ions Cp4Zr2(CH3)n+ (n = 0-3), which severely impairs quantitative

analysis. The observed peaks are summarized in the Table 4.3











15-

C
. 10-


5-


m/z


Figure 4.11. Ions produced in the reaction of Cp2ZrMe with benzoquinone: products of
coordination of quinone molecule by the binuclear complexes Cp4Zr2(CH3)n+
(n= 0-3)
Table 4.3 Major reaction products observed in the reactions of 1 and 2 with diethyl
carbonate at various reaction times (1 to 5 seconds).
Reaction m/z Empirical Formula

1 + diethyl carbonate 353 [Cp2ZrCH3 O=C(OC2H5)2]

(nominal molecular weight 471 [Cp2ZrCH3 20=C(OC2H5)2]+

118 amu) 277 Cp2ZrC3HsO

2 + diethyl carbonate 356 [Cp2ZrCD3 O=C(OC2H5)2]+

(nominal molecular weight 474 [Cp2ZrCD3 20=C(OC2H5)2]

118 amu) 279/280 [Cp2ZrC3H3D20+/ Cp2ZrC3H2D30]+

The complex with m/z 277 was formed in smaller amounts than in the reaction

with acetone. This complex was still abundant enough for the addition product (m/z 395)

to be observed. Several ligand fragmentation products, always accompanied by the loss

of a methyl (or methyl-d3 in the reactions of 2) group were observed; a few of the

possible resulting structures of some of the observed ions are shown in Scheme 4.15:


11.
















', H.
265


Scheme 4.15

In the case of the reaction of 2 with diethyl carbonate, instead of the unsaturated

complex with m/z 277, two series of peaks, m/z 279 and 280, with relative intensities 1:3

were observed. This intensity ratio was independent of time. The total intensity obeyed

the same trends observed before: a maximum intensity is reached at 2 seconds of reaction

time with gradual disappearance at longer reaction times.

4.4 Reaction with Aldehydes

Reaction patterns for 1 and 2 with all of the studied aldehydes were remarkably

different from those observed in the reactions with various ketones: often no simple

substrate molecule addition products were formed at all. Instead, at short reaction times a

single (or major) product, resulting from dihydrogen loss from a postulated short-lived

carbonyl molecule addition product, was obtained in all of the cases (Table 4.4).

In reaction of 1 with acetaldehyde (Fig. 4.12), increasing the reaction time led to

the addition of another substrate molecule by the single reaction product, the postulated

r3-enolate complex (m/z 277), forming an ion with an m/z 321. The combined abundance

of the ions with m/z 277 and 321 decreased significantly at longer reaction times due to

water contamination of the acetaldehyde sample, and formation of the hydration product

of the ion with m/z 277 (m/z 295). The bis(rl-cyclopentadienyl)zirconium hydroxide

cation, Cp2ZrOH+ was also formed in significant amounts, resulting in an eventual









intensity shift into peaks belonging to the hydroxyl cation coordinating one substrate

molecule (m/z 281), Scheme 4.16.

Table 4.4. Major reaction products observed in the reactions of 1 and 2 with
acetaldehyde, benzaldehyde, propanal and n-hexanal at various reaction times
(1 to 5 seconds).
Reaction Observed Reaction Products

m/z Suggested Empirical Formula

1 (m/z 235) + acetaldehyde 277 [Cp2ZrC3HsO+]

(nominal molecular weight 321 [Cp2ZrC3H5O+ CH3CHO]+

44 amu)

2 (m/z 238) + acetaldehyde 279/280 (ion [Cp2ZrC3H3D20]+/[Cp2ZrC3H2D30]+

(nominal molecular weight abundance ratio

72 amu) -1/3)

323/324 (ion [Cp2ZrC3H3D20 CH3CHO]+/

abundance ratio [Cp2ZrC3H2D30 CH3CHO]

-1/3)

1 (m/z 235)+ benzaldehyde 339 [Cp2ZrC2H2(C6H5)O]

(nominal molecular weight

106 amu)

2 (m/z 238) + benzaldehyde 341 [Cp2ZrC2D2(C6Hs)O]+

(nominal molecular weight

106 amu)









Table 4.4 Continued
Reaction Observed Reaction Products

m/z Suggested Empirical Formula

1 (m/z 235) + propanal 291 [Cp2ZrC4H70]+

(nominal molecular weight 293 [Cp2ZrCH3 C2H5CHO]

58 amu) 349 [Cp2ZrC4H70 C2H5CHO]

2 (m/z 238) + propanal 293/294 (ion [Cp2ZrC4H5D20]+/ [Cp2ZrC4H4D30]+

(nominal molecular weight abundance ratio

58 amu) -1/1)

295 [Cp2ZrCD3 C2H5CHO]

351/352 (ion [Cp2ZrC4H5D20 C2H5CHO]+/

abundance ratio [Cp2ZrC4H4D30 C2H5CHO]

-1/1)

1 (m/z 235) + n-hexanal 333 [Cp2ZrC7H130]+

(nominal molecular weight 335 [Cp2ZrCH3 C5H11CHO]

100 amu) 433 [Cp2ZrC7Hi30 C5H11CHO]+

2 (m/z 238) + n-hexanal 335/336 (ion [Cp2ZrC7Hi1D20] / [Cp2ZrC7HloD30]

(nominal molecular weight abundance ratio

100 amu) -3/2)

338 [Cp2ZrCD3 C5H11CHO]

435/436 (ion [Cp2ZrC7HllD20 C5H11CHO]+/

abundance ratio [Cp2ZrC7HioD30 C5H11CHO]

-3/2)










100
90 m/z 277
80
70
S60
r-
50
0 40
30
20
10

200 250 300 350 400
m/z

Figure 4.12. Mass spectra of ions formed in the reaction of Cp2ZrMe+ with acetaldehyde,
1 second reaction delay.

+ +


SZr-OH + o 7r ,


zH H

237 44 281

Scheme 4.16

The intensity distribution of the two products (m/z 279 and 280) formed in

reaction of Cp2ZrCD3+ with acetaldehyde, was approximately 1:3, nearly independent of

reaction time (Fig. 4.13). Increasing the reaction time resulted in addition of another

acetaldehyde molecule by these complexes (m/z 323/324), with the intensity distribution

remaining unaffected. Interestingly, the same intensity ratios were observed for the water

molecule addition products, (m/z 297/298). This implies that, since these ions have

similar reactivity, both reaction products of 2 with acetaldehyde have similar structures,






76


i.e. both are r3-enolate complexes, and they differ only by the number of deuterium

atoms (Cp2ZrC3H3D20 and Cp2ZrC3H2D30+ respectively).


i I I


275 280 285
mi/7


i...I 1. .


A..1 .. 1I


275 280 285 290
mlz,


a) b)

Figure 4.13. Mass spectra of ions with m/z 279 and 280 formed in the reaction of
Cp2ZrCD3 with acetaldehyde, at a) 1 second and b) 5 seconds reaction delays.

In the reaction pattern of 1 with benzaldehyde (Fig.4.14), the complex with m/z

339 (Cp2ZrC2H2(C6Hs)O+) is coordinatively unsaturated, analogous to the complex with

m/z 277.


60

.~ 40
30"
20i
10

250 300 350
m/z

Figure 4.14. Ions produced in the reaction of Cp2ZrMe with benzaldehyde at 1 second
reaction delay.


60-

50-
40-
-
- 30-

< 20-
10.


,I I









The reaction of 2 with benzaldehyde resulted in formation of the same set of

products as in reaction of 1, with a two mass unit shift corresponding to the two

deuterium atoms in the products, including the hydration product (m/z 359,

Cp2ZrC2H2D2(C6H5)02+). The product of benzaldehyde molecule addition to Cp2ZrOH

ion (m/z 343) dominated in the spectrum at long reaction times (>5 s.). In the reaction of

1 with propanal, a few minor products were observed at longer reaction times (unlike in

reaction with acetaldehyde), but the peak with m/z 291 was always the major peak in the

mass spectrum. In the reaction of 1 with n-hexanal, at reaction times 5 seconds and

longer, the direct addition of aldehyde molecules) to 1 (m/z 335 and 435) was

noticeable. Increasing the reaction time led to extensive ligand fragmentation resulting in

a multitude of peaks corresponding to the loss of dihydrogen or a variety of alkenes by

various ions. Analogous to the reaction patterns with other aldehydes, the ion with m/z

333 also coordinated a water molecule, forming a hydration product with m/z 351, which

was predominant at reaction times of 5 seconds and longer. The reaction of 2 with n-

hexanal showed the same trend of extensive ligand fragmentation at increasing reaction

times as in reaction of 1 with n-hexanal.

In summary, all three aldehydes reacted with 1 in the same fashion (Scheme 4.17):

Cp2ZrCH3+ + RCHO Cp2ZrC2H2RO+ + H2

Scheme 4.17

where R = Me, Et, Ph, C5H11. These complexes are presumed to be the postulated r3

enolate complexes with the corresponding R- substituent, and all of their isomers which

are possible according to the reaction mechanism. Two products were formed in the









reaction of 2 with all aldehydes (except for benzaldehyde), apparently differing only by

the number of deuterium atoms in the structure.

The slowest step in the reaction mechanism, as in the case of reactions with

ketones, is presumed to be a nucleophilic attack on the a-hydrogen (deuterium) of the

coordinated carbonyl. The key feature of aldehydes is the presence of a hydrogen atom at

the carbonyl carbon instead of an alkyl group. This leads to the possibility of P-H

elimination provided the reaction proceeds through the same initial equilibrium as that

with ketones, i.e., coordination of a substrate molecule and migratory insertion. This

hydrogen further acts as nucleophile. Since the attack on the hydrogen in the phenyl ring

in the reactions of 1 and 2 with benzaldehyde will lead to the disruption of aromaticity,

only deuteriums of the CD3 bond are available for the reactions, hence only one product

is observed. The proposed reaction mechanism, which demonstrates this for the example

of reaction of 2 with acetaldehyde, is shown in Scheme 4.18. The P-H elimination is

known to be a very facile process [183]. Thus, when in the rate limiting step of

nucleophilic attack on an a-H of the coordinated ketone an alkyl is replaced by hydrogen,

a better nucleophile, while the leaving group remains the same, the reduction of the

activation barrier is significant enough to make this reaction pathway favorable over any

other available for a given system. The 6-member transition state preceding dihydrogen

elimination is similar to that postulated for the reaction with ketones, and is assumed to

be the least strained. The observed kinetic isotope effect kD/kH 0 0.3 is consistent with

that typically observed for C-D vs. C-H bond cleavage for relatively linear transition

states [184, 185]. The noticeable effect of reduction in the amount of the complex

carrying 3 deuterium atoms in the series acetaldehyde propanal hexanal (resulting






79


from the attack on hydrogen of -CH3, -C2H5 and -C5H9 groups correspondingly) suggests

certain geometrical constrains in the reactions with bulkier radicals the lower steric

availability of the secondary hydrogen atoms and/or removal from the reactive center of

more accessible primary ones.

A postulated reaction potential surface associated with this mechanism, for the

reaction of 1 with acetaldehyde, is shown in Fig. 4.15a.


H3
I-1;


\ H
/'


12


1-
H


I.-


~ obiservd


H

.. H,
rl11
,'."


-1


a) Gas-phase


K'--







80




R- 'V




H' S



Scheme 4.21. Gas-phase estimates of energetic effects are ba sed on ed



















Ref. [173].
H "/S r H-+















benzaldehyde, and similar ions for other aldehydes, is most likely due to inevitable traces
-' R II.

of w tX= I' less c

b) Solution

Figure 4.15. Qualitative potential surface for the reaction of 1 with acetaldehyde/generic
aldehyde in the gas phase and in solution. The lettering corresponds to that in
Scheme 4.21. Gas-phase estimates of energetic effects are based on data from
Ref. [173].

Formation of ions with m/z 357 and 359 in the reactions of 1 and 2 with

benzaldehyde, and similar ions for other aldehydes, is most likely due to inevitable traces

of water in the instrument, leading to hydrolysis of the r3-enolate complex according to

the following proposed scheme (Scheme 4.19). This hydrolysis is the first directly

observed gas-phase reaction of the r3-enolate complex. The reactivity of the isoelectronic

r3-allyl complex has been studied on several occasions in the solution [162] and in the

gas phase [175, 186]. However, no studies have been carried out so far on the gas-phase

reactivity of the r3-enolate complex, leaving this topic open for future investigation.












+




H


H
r- / __


insrtiu~n


4-


/ -7
[> "jr'


HI


Scheme 4.18a

The liquid-phase studies [187-193] indicate potential significance of such species

in polymerization catalysis: for example, in the investigation of the polymerization of the

methylmethacrylate (MMA) with the neutral enolate Cp2ZrMe[OC(OMe)=CMe2] in the

absence and in the presence of zirconocene cations Cp2ZrMe+ to bypass the rate-limited

initiation of the system, it was shown that by itself the enolate is not active in

polymerization, but as soon as the reaction system contains the cation, MMA is

quantatively converted into syndiotactic-rich PMMA with high molecular weight (Mn >

100,000 g/mol) and very narrow molecular weight distribution (Mw/Mn < 1.05) [187].


a...
_/


I /


I I h r 11 1 1 f I









-1+


'p


/\t


A''
/ n


i-
LA-V

K:-./


I I


Scheme 4.18b

The gas-phase studies presented in this work augment our understanding of

related processes taking place in the condensed phase. Individual reactions stages can be

more easily discerned. The reaction mechanism postulated in the original study of the


'I


:./ ~V


U\
Ii LU2


;


-H
i.
rp
\'
"S


I


\






83


silver-mediated addition of alkenyl- and alkylzirconocene chloride to aldehyde [54] is

shown in Scheme 4.20.


+


C'


+ V I


+


9
IC)
A-
Ny '


/~
Ar t


II


'A-


Scheme 4.19

The precursor alkyl/alkenyl zirconocene chloride complex is activated via chloride

abstraction by AgC104 and resultant cation reacts with the aldehyde molecule. The

process leading from complex (IIIA) to complex (IVA) and regeneration of the active

species (IIA) (transfer of the R' group) can be either inter- or intramolecular in nature.

Since only the structures of final products (RR'COH alcohols, formed after treatment of

the reaction mixture with NaHCO3 aqueous solution) were established, the detailed

reaction mechanism is not completely clear.


-






84


X ., / | -
.10 .- R
*,^ {_ .- ^ .- X =,2 rCr-x



IVA








nH





Scheme 4.20

The reaction mechanism which is consistent with the gas-phase studies data

presented earlier in this chapter is shown in Scheme 4.21. According to the reaction

mechanism postulated for the gas-phase reactions of 1 with aldehydes (Scheme 4.18), the

coordinated aldehyde molecule undergoes migratory insertion followed by P-H

elimination. This suggests formation of a side product (VI). Regeneration of the active

species (IIB) occurs via chloride abstraction by cationic complex (IVB) from neutral

precursor (I). Even though this process is endothermic (cation IVB is more stable than

IIB), the equilibrium can be shifted toward species (V) and (IIB), as the concentration of

(V) is much lower than that of (I) at all times. Hydrogen elimination and formation of 3-

enolate complex do not take place in the liquid phase due to efficient energy dissipation

in the presence of solvent bath (Figure 4.15b).








R

C 1047H
--7+




/

/ |II B
-, /> ,* R




'i \


/ /








Scheme 4.21

As can be seen, the gas-phase studies can provide some comprehensive insight

into the reaction mechanism by direct monitoring of possible reaction intermediates and

eliminating perplexing factors associated with the solution chemistry.

4.5 Comments on Suggested Reaction Mechanisms

Several inter-related factors need to be considered in understanding the reactivity

of the bis(ql-cyclopentadienyl)methylzirconium cation. Having a do configuration, the Zr

metal center cannot be involved in further oxidation; therefore the number of possible

reaction pathways is reduced. Traces of background water invariably lead to hydrolysis

products which must be accounted for, especially in experiments with deuterium-labeled

compounds. The hydrolysis reaction leads to substitution of the species of interest, 1 or 2,









by the Cp2ZrOH cation, thus decreasing total ion abundances of these cations and their

reaction products. However, the significantly lower reactivity of the hydroxyl complex

[155] compared to 1 and 2 reduces possible competition for the substrate and does not

hinder reaction product identification for times less than 5 seconds at the pressures used

in these studies.

Extended reaction times lead to eventual thermalization of reactant ions through

collisions. Ignoring long range interactions, at short reaction times, the number of

molecular collisions under the high-vacuum conditions found in the FTICR cell is


Crel p F- IkT Mam
insignificant: since z =-- where Cre; = and u = b when assuming
kT V rn ma + mb

collisional crossection for zirconocene cation and carbonyl molecule = d2,

d = (da + db) being approximately 1 nm2, for typical masses of Cp2ZrL -235 amu


and higher and of carbonyl 70 amu, t ~ 9.10-26 kg, and Crel = 337 m/s; thus, for T =

295 K and p = 1.10-7 torr (1.33-10-5 Pa), z = 1.1. However, this number is actually

significantly higher (approximately an order of magnitude or more) for charged species

due to a long-range interaction of the positive charge of the cation with electron cloud of

the neutral. The ion-induced dipole ("Langevin") potential and the ion-neutral collision

rate constant can be calculated. The simplest case of the Langevin theory considers

spiraling elastic collisions for a point-charge induced dipole force, with the attraction

2
ae
potential, V(r) = Here, a is the average polarizability of the neutral molecule and
2r4

r is ion-molecule internuclear separation. The ion mobility, associated with this potential

can be converted into the momentum transfer collision rate constant:










K(a) = 9.97 x 1010 Here M is the reduced mass of the system and m is the ion
m

mass in amu [194]. Better agreement with the experimental values, especially in the case

of anisotropic nonpolar molecules can be achieved if the average polarizability of the

neutral is replaced by the maximum component of the diagonalized polarizability tensor

a' (this component is the same for spherical molecules and 5-20% larger for most

anisotropic molecules [195]. This substitution gives a modified expression for the rate


constant: K(a) = 9.97 x 10 10 The collision rate constant estimated for an ion of
m

100 amu colliding with a nitrogen molecule, k 6.611*10-10 cm3 s-1 [92]. By multiplying

this number by the concentration of neutral, the number of collisions per second per ion

can be obtained.

The limited number of collisions will lead to non-equilibrium conditions, which

will result in kinetically controlled reaction products. At long reaction times, when

equilibrium conditions are more closely approached, the observed products will result

from thermodynamically controlled reactions. Here, the term kineticallyy controlled

reaction products" does not refer to irreversible reactionss, but rather to those resulting

from processes far from equilibrium.

Caution must be used when comparing the gas-phase reactivity of metallocenes to

the known chemistry of their condensed-phase analogs. Very few zirconocene complexes

are known in the liquid phase without triphenylphosphine or similar stabilizing ligands

and such complexes react with some substrates in an unusual manner. The reactivity of

direct liquid-phase analogs of the system of interest, i.e. the bis(rl5-

cyclopentadienyl)methylzirconium cation, as discussed above, is greatly affected by the









nature of the counterion. These catalytic systems were quite thoroughly studied with

respect to their function in alkene polymerization reactions, but the reactivity of such

systems with various substrates has not been studied systematically. Thus, perhaps the

best approach for examining the gas-phase reactivity of Cp2ZrMe+ cations with carbonyls

would be to compare the reactivity of this complex with alkenes in both the gaseous and

liquid phases.

The reaction of olefin insertion into an M-X bond (X = H, R) proceeds via

equilibrium of the alkene coordination complex and the alkene insertion product, which

is exothermic by 25 kcal/mol. Both complexes are expected to be more stable for the

coordination and insertion of the carbonyl group. Even though there is no t-backbonding

possible for the do Zr4 metal center, the lone electron pair of the carbonyl oxygen is

more accessible than 7t electrons of the alkene, making it a stronger Lewis base. Insertion

of the carbonyl into the M-X bond results in formation of the metal-oxygen a bond,

which is typically significantly stronger than an M-C c bond. This compensates for the

disruption of the C=C 7t bond, which is usually stronger than that of C=O by 30

kcal/mol. Even though oxygen is a weaker o-donor than the methylene group, it provides

additional bonding by partially transferring its available lone pair into the empty Zr d2-

orbital, thus increasing total bond strength. Theoretical calculations of ground state

energies clearly indicate higher stability of the enolate complex compared to other

possible isomers (Fig. 4.16, Table 4.5)







89












219 /221














a) b)












242 244













c) d)