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New Synthetic Routes to High Nuclearity Manganese Carboxylate Clusters from the Employment of Pyrazole-Based Alkoxide Ligands

Permanent Link: http://ufdc.ufl.edu/UFE0024060/00001

Material Information

Title: New Synthetic Routes to High Nuclearity Manganese Carboxylate Clusters from the Employment of Pyrazole-Based Alkoxide Ligands
Physical Description: 1 online resource (71 p.)
Language: english
Creator: Pringouri, Konstantina
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2008

Subjects

Subjects / Keywords: carboxylates, chelates, clusters, magnetochemistry, manganese, structures
Chemistry -- Dissertations, Academic -- UF
Genre: Chemistry thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Molecular chemistry provides a 'bottom-up' approach to nanoscale materials, and is thus complementary to the more widespread 'top-down' approach. It brings all the advantages of molecular synthesis to the fabrication, manipulation, modification and study of nanomaterials. In particular, the synthesis of paramagnetic manganese carboxylate clusters has become the focus of much research since the discovery that some molecules can function as single-domain nanoscale magnetic particles, the so-called single-molecule magnets (SMMs). For such reasons and more, we have continued to seek new synthetic methods to new structural types of high-nuclearity Mn carboxylate clusters that might have interesting structural and/or magnetic properties. In the present thesis, the syntheses, crystal structures and magnetochemical characterization of five new manganese clusters are reported, from the employment of 3,5-dimethylpyrazole-1-methanol (dmpmH) as a chelating/bridging ligand in manganese carboxylate chemistry. The carboxylate group used in the reaction affects the identity of the product. The reactions of various preformed triangular Mn carboxylate clusters with 12 equivalents of dmpmH in MeCN afforded a new family of Mn/carboxylato complexes; two octadecanuclear (1 and 2), one octanuclear (3) and two hexanuclear clusters (4 and 5). All complexes are mixed-valence and have descriptions: 4Mn(II), 14Mn(III) for 1 and 2, 6Mn(II), 2Mn(III) for 3, and 4Mn(II), 2Mn(III) for 4 and 5. The core of complexes 1 and 2 consists of a 'disc-shaped' metal topology, whereas 3 has an interesting 'edge-linked cubane' core. Finally, both complexes 4 and 5 possess an 'edge-sharing tetrahedra' core. Variable-temperature, solid-state dc and ac magnetization studies were carried out on complexes 1-5 in the 5.0-300 K range. For complexes 1 and 2, the exact determination of the ground state spin value is currently unfeasible due to their complicated structures, compound 3 has a significant ground state spin value of S = 6, and complexes 4 and 5 have a ground state spin value of S = 0. The combined results demonstrate the ligating flexibility of dmpmH and its usefulness in the synthesis of a variety of Mn molecular species.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Konstantina Pringouri.
Thesis: Thesis (M.S.)--University of Florida, 2008.
Local: Adviser: Christou, George.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2010-12-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2008
System ID: UFE0024060:00001

Permanent Link: http://ufdc.ufl.edu/UFE0024060/00001

Material Information

Title: New Synthetic Routes to High Nuclearity Manganese Carboxylate Clusters from the Employment of Pyrazole-Based Alkoxide Ligands
Physical Description: 1 online resource (71 p.)
Language: english
Creator: Pringouri, Konstantina
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2008

Subjects

Subjects / Keywords: carboxylates, chelates, clusters, magnetochemistry, manganese, structures
Chemistry -- Dissertations, Academic -- UF
Genre: Chemistry thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Molecular chemistry provides a 'bottom-up' approach to nanoscale materials, and is thus complementary to the more widespread 'top-down' approach. It brings all the advantages of molecular synthesis to the fabrication, manipulation, modification and study of nanomaterials. In particular, the synthesis of paramagnetic manganese carboxylate clusters has become the focus of much research since the discovery that some molecules can function as single-domain nanoscale magnetic particles, the so-called single-molecule magnets (SMMs). For such reasons and more, we have continued to seek new synthetic methods to new structural types of high-nuclearity Mn carboxylate clusters that might have interesting structural and/or magnetic properties. In the present thesis, the syntheses, crystal structures and magnetochemical characterization of five new manganese clusters are reported, from the employment of 3,5-dimethylpyrazole-1-methanol (dmpmH) as a chelating/bridging ligand in manganese carboxylate chemistry. The carboxylate group used in the reaction affects the identity of the product. The reactions of various preformed triangular Mn carboxylate clusters with 12 equivalents of dmpmH in MeCN afforded a new family of Mn/carboxylato complexes; two octadecanuclear (1 and 2), one octanuclear (3) and two hexanuclear clusters (4 and 5). All complexes are mixed-valence and have descriptions: 4Mn(II), 14Mn(III) for 1 and 2, 6Mn(II), 2Mn(III) for 3, and 4Mn(II), 2Mn(III) for 4 and 5. The core of complexes 1 and 2 consists of a 'disc-shaped' metal topology, whereas 3 has an interesting 'edge-linked cubane' core. Finally, both complexes 4 and 5 possess an 'edge-sharing tetrahedra' core. Variable-temperature, solid-state dc and ac magnetization studies were carried out on complexes 1-5 in the 5.0-300 K range. For complexes 1 and 2, the exact determination of the ground state spin value is currently unfeasible due to their complicated structures, compound 3 has a significant ground state spin value of S = 6, and complexes 4 and 5 have a ground state spin value of S = 0. The combined results demonstrate the ligating flexibility of dmpmH and its usefulness in the synthesis of a variety of Mn molecular species.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Konstantina Pringouri.
Thesis: Thesis (M.S.)--University of Florida, 2008.
Local: Adviser: Christou, George.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2010-12-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2008
System ID: UFE0024060:00001


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1 NEW SYNTHETIC ROUTES TO HIGH NUCLEARITY MANGANE SE CARBOXYLATE CLUSTERS FROM THE EMPLOYMENT OF PYRAZOLE-BASED ALKOXIDE LIGANDS By KONSTANTINA V. PRINGOURI A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2008

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2 2008 Konstantina V. Pringouri

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3 To my family

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4 ACKNOWLEDGMENTS During m y graduate studies at the Department of Chemistry of the University of Florida, I was honored to have as my advisor Professor George Christou. I woul d like to thank him because in his laboratory, and with my participat ion in his graduate courses, I had a great chance to extend my knowledge in the field of inorgani c chemistry. Professor Christou is an excellent scientist and amazing teacher and he will always be an inspiration for the way I should treat science and teaching. I, finally, thank him for his guidance, pedagogy, and understanding those years. Also, I would like to acknowledge Dr. Kha lil A. Abboud and all the staff at the Center for X-ray Crystallography at UF for all the crystal structures and the patience and time the x-ray analysis requires. I am pleased to mention the sp ecial contribution to my scientific life of Dr. Theocharis C. Stamatatos; since my undergraduate st udies, he has been next to me and as a great scientist he shared with me his knowledge as we ll as his enthusiasm and passion for the science we serve. I remember him as a textbook where I always find answers to my questions. I would like to thank the entire Christou group for their support and thoughtfu l discussions about chemistry. I mostly appreciate the friendship and talking from two special people for me: Jennifer and Shreya. Last but not least, I would like to thank my parents, Vasileio and Georgia; and my brother, Gianni, whose love and patience were essent ial for the completion of my masters degree, as well as the endless conversati ons that kept me motivat ed in this long journey of my studies.

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ............................................................................................................... 4LIST OF TABLES ...........................................................................................................................6LIST OF FIGURES .........................................................................................................................7ABSTRACT ...................................................................................................................... ...............9 CHAP TER 1 INTRODUCTION .................................................................................................................. 112 NEW SYNTHETIC APPROACH TO HIGH NUCLEARITY MANGANESE CAR BOXYLATE CLUSTERS: MN6, MN8 AND MN18 COMPLEXES UTILIZING 3,5-DIMETHYLPYRAZOLE-1-METHANOL ..................................................................... 212.1 Introduction .......................................................................................................................212.2 Experimental Section ........................................................................................................252.2.1 Syntheses ................................................................................................................252.2.1.1 [Mn18O14(O2CMe)18(dmpm)4(dmpmH)2(H2O)2] ( 1) .................................... 252.2.1.2 [Mn18O14(O2CEt)18(dmpm)4(dmpmH)2(H2O)2] ( 2) ...................................... 252.2.1.3 [Mn8O2(O2CPh)10(dmpm)4(H2O)2] ( 3) ......................................................... 262.2.1.4 [Mn6O2(O2CBut)10(py)4] ( 4) ......................................................................... 262.2.1.5 [Mn6O2(O2CPhCl)10(py)4] ( 5) ...................................................................... 262.2.2 X-ray Crystallography ............................................................................................ 272.2.3 Physical Measurements ..........................................................................................302.3 Results and Discussion .................................................................................................... .302.3.1 Syntheses ................................................................................................................302.3.2 Description of Structures ........................................................................................352.3.2.1 [Mn18O14(O2CMe)18(dmpm)4(dmpmH)2(H2O)2] ( 1) .................................... 352.3.2.2 [Mn18O14(O2CEt)18(dmpm)4(dmpmH)2(H2O)2] ( 2) ...................................... 412.3.2.3 [Mn8O2(O2CPh)10(dmpm)4(H2O)2] ( 3) ......................................................... 452.3.2.4 [Mn6O2(O2CBut)10(py)4] ( 4) and [Mn6O2(O2CPhCl)10(py)4] ( 5) ................. 482.4 Magnetochemistry .......................................................................................................... ..552.4.1 Direct Current Magnetic Susceptibility Studies ..................................................... 552.4.2 Alternating Current Magnetic Susceptibility Studies .............................................592.5 Conclusions .......................................................................................................................63LIST OF REFERENCES ...............................................................................................................65BIOGRAPHICAL SKETCH .........................................................................................................71

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6 LIST OF TABLES Table page 2-1 Crystallographic data for 1MeC N, 2MeCN, 3 MeCN, 4 MeCN, and 5 ..................28 2-2 Selected interatomic distances () and angles (deg) for 1MeCN ................................... 36 2-3 Bond valence sum (BVS) calculations for Mn and selected oxygen atom s in 1 ...............38 2-4 Selected interatomic distances () and angles (deg) for 2MeCN ................................... 42 2-5 Bond valence sum (BVS) calculations for Mn and selected oxygen atom s in 2 ...............44 2-6 Selected interatomic distances () and angles (deg) for 3MeCN ................................... 47 2-7 Bond valence sum (BVS) calculations for Mn and selected oxygen atom s in 3 ...............48 2-8 Selected interatomic distances () and angles (deg) for 4MeCN ................................... 50 2-9 Selected interatomic distances () and angles (deg) for 5 ................................................50 2-10 Bond valence sum (BVS) calculations for Mn and selected oxygen atom s in 4 and 5 .....51

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7 LIST OF FIGURES Figure page 1-1 Three-dimensional, ordered array of m onodisperse, identically oriented molecules within a c rystal. Each molecule comprises seven iron atoms (yellow) connected by oxygen atoms (red) and surrounded by organi c groups (grey). The box defines the repeating unit of the crystal ................................................................................................15 1-2 Types of magnetic materials depending on th e na ture of the interaction between spin carriers ...................................................................................................................... ..........16 1-3 Structure of the Mn12 molecule ..........................................................................................18 1-4 The [Mn12O12]16+ core, emphasizing with green thick lines the central [Mn4O4]8+ cubane subcore ...................................................................................................................18 1-5 Structure of the Mn84 molecule ..........................................................................................19 1-6 Magnetic hysteresis loop s for a single crystal of Mn12 at the indicated temperatures showing the steps due to the quantum behavior ................................................................ 19 2-1 The protonated precursors to the chelating ligands reported herein ..................................24 2-2 Molecular structure of complex 1 ......................................................................................36 2-3 Labeled PovRay representation of the Mn18 core .............................................................. 40 2-4 Molecular structure of complex 2, with hydrogen atom s omitted for clarity .................... 41 2-5 Molecular structure of complex 3 ......................................................................................46 2-6 Labeled PovRay representation of the complete [Mn8( 4-O)2( 3-OR)4( -OR)4]6+ core of 3 .....................................................................................................................................47 2-7 Partially labelled PovRay representation of complex 4, with h ydrogen atoms omitted for clarity ............................................................................................................................49 2-8 Partially labelled PovRay representation of complex 5, with h ydrogen atoms omitted for clarity ............................................................................................................................49 2-9 Labeled PovRay representation of the complete [Mn6( 4-O)2( -OR)4]6+ core of 4 ..........52 2-10 Labeled PovRay representation of the [Mn6O2]10+ core of 4 .............................................54 2-11 Three ways of describing the [MnII 4MnIII 2( 4-O)2]10+ core present in complexes 4 and 5..........................................................................................................................................54 2-12 Plot of MT vs T for complex 1 in a 1 kG dc field .............................................................56

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8 2-13 Plot of MT vs T for complex 2 in a 1 kG dc field .............................................................56 2-14 Plot of MT vs T for complex 3 in a 1 kG dc field .............................................................57 2-15 Plot of MT vs T for complex 4 in a 1 kG dc field .............................................................58 2-16 The ac magnetic susceptibility measurements of complex 1 in a 3.5 G field oscillating at the i ndicated frequencies ..............................................................................60 2-17 Plot of the in-phase ( M) (as MT ) ac susceptibility signals of complex 2 in a 3.5 G field oscillating at the indicated frequencies ...................................................................... 61 2-18 Plot of the in-phase ( M) (as MT ) ac susceptibility signals of complex 3 in a 3.5 G field oscillating at the indicated frequencies ...................................................................... 62 2-19 Plot of the out-of-phase ( M) ac susceptibility signals of complex 3 in a 3.5 G field oscillating at the i ndicated frequencies ..............................................................................63

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9 Abstract of Thesis Presen ted to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science NEW SYNTHETIC ROUTES TO HIGH NUCLEARITY MANGANE SE CARBOXYLATE CLUSTERS FROM THE EMPLOYMENT OF PYRAZOLE-BASED ALKOXIDE LIGANDS By Konstantina V. Pringouri December 2008 Chair: George Christou Major: Chemistry Molecular chemistry provides a bottom-up appr oach to nanoscale mate rials, and is thus complementary to the more widespread top-down approach. It brings all the advantages of molecular synthesis to the fabric ation, manipulation, modification a nd study of nanomaterials. In particular, the synthesis of paramagnetic manganese carboxylate clusters has become the focus of much research since the discovery that some molecules can function as single-domain nanoscale magnetic particles, the so-called sing le-molecule magnets (SMMs). For such reasons and more, we have continued to seek new synt hetic methods to new st ructural types of highnuclearity Mn carboxylate clusters that might have interesti ng structural and/or magnetic properties. In the present thesis, the synthe ses, crystal structures and magnetochemical characterization of five new manganese clusters are report ed, from the employment of 3,5dimethylpyrazole-1-methanol (dmpmH) as a chel ating/bridging ligand in manganese carboxylate chemistry. The carboxylate group used in the reaction affects the identity of the product. The reaction of [Mn3O(O2CR)6(py)2(H2O)] (R = Me, Et, Ph, But, PhCl; py = pyridine) with 12 equivalents of dmpmH in MeCN afforded a new family of Mn/carboxylato complexes, [Mn18O14(O2CR)18(dmpm)4(dmpmH)2(H2O)2] (R = Me ( 1), Et ( 2)), [Mn8O2(O2CPh)10(dmpm)4(H2O)2] ( 3) and [Mn6O2(O2CR)10(py)4] (R = But ( 4), PhCl ( 5)). All

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10 complexes are mixed-valence and have descriptions: 4MnII, 14MnIII for 1 and 2, 6MnII, 2MnIII for 3, and 4MnII, 2MnIII for 4 and 5. The core of complexes 1 and 2 consists of a disc-shaped metal topology, whereas 3 has an interesting edge-linked cubane Mn8O2 core. Finally, both complexes 4 and 5 possess an edge-sharing tetrahedra co re. Variable-temperature, solid-state dc and ac magnetization studies were carried out on complexes 15 in the 5.0-300 K range. For complexes 1 and 2, the exact determination of the ground state spin value is currently unfeasible due to their complicat ed structures, compound 3 has a significant ground state spin value of S = 6, and complexes 4 and 5 have a ground state spin value of S = 0. The combined results demonstrate the ligating flexibility of dmpmH and its usefulness in the synthesis of a variety of Mnx molecular species.

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11 CHAPTER 1 INTRODUCTION The health and progress of ci vilizations has often depended on the design and construction of novel devices and m achines. A device is something fabricated for a specific purpose and a machine is some combination of mechanisms for harvesting, utilizing, modifying, applying or transmitting energy, in a simple or complicated way.1 Generally speaking, devices and machines are assemblies of components designed to achieve a special function. The potential applications of that particular device or machine derive from the combined actions of each component of the assembly, from a simple mousetrap to a complicated automobile. In the last fifty years, a great number and a wide variety of new devices a nd machines for collecting, processing, displaying, and storing information have been develope d. The stunning advance in information technology has been heavily dependent on the dramatic mini aturization of the components employed in the construction of such devices and machines.2 From the first computer weighing some 30 tons, to the present state-of-the art supercomputer containing more than 40 million microprocessors, the progress has been simply stupendous.3 One can easily wonder whether and how we can keep on taking this top-down approach of making things smaller and smaller, especially since the degree of miniaturization is rapidly approaching the lower limit represente d by the size of atoms and discrete assemblies of atoms called mol ecules. Further advantages of continuing miniaturization are unquestioned, in computing pow er and numerous other technologies such as those capable of providing major breakthroughs in medicine, producing a wealth of new materials towards renewable energy sources, and solv ing the environmenta l pollution problems.4 Alternatively, one can look at th e challenge of further miniatur ization from the bottom and say that the bottom-up approach is reaching the size scale at which molecular chemists have been working for centuries. Thus, maybe an easier, or at least complementary, approach to further

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12 miniaturization is to re-direct th is vast expertise in molecular science towards the synthesis of larger molecules and related species that can be employed in new ge nerations of nanoscale devices and machines. This is the thrust of a growing number of re searchers around the world, and we have coined the name Molecu lar Nanoscience for this approach. The miniaturization of components for the c onstruction of useful devices is currently pursued by the traditional top-dow n approach. This has been sp ectacularly successful and has underpinned much of the progress of our modern society. Scientists and engineers have learnt to fabricate and organize progressive ly smaller and smaller pieces of matter using techniques such as photolithography, laser, etc.5 However, for instance, photolith ography is typical of many topdown approaches in being of limited applicab ility for dimensions much smaller than 100 nanometers or so. Of course, this size is st ill very small by the standards of our everyday experience, but is very large on the scale of atoms (tenths of nanometers) and molecules (up to a few nanometers). Thus, although Feyn man famously said that there is plenty of room at the bottom, it seems unlikely that the top-down approach will by itself be capable of fulfilling this challenge and opportunity.6 To continue to proceed towards fu rther miniaturization, both science and technology may have to find new ways. The molecular approach mentioned provides one such potential solution to the problem. It should be noted that molecular scientists often st ate that we have always been working at the nanoscale, we started nanoscience, or some such variant of th is, but the truth is that only recently have their efforts begun to be directed in earnest at providing molecular variants of materials potentially important and applicable to device and machine miniaturization, what was defined as Molecular Nanoscience above. This approach begins with sources of individual atoms or small molecules and uses them to synthesize large molecular nanostructures with the desired

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13 and targeted properties for the goal at hand. So, a molecular-level (or molecule-based) device can be denoted as an assembly of some number of molecular components designed to achieve a specific function. Each molecular component pe rforms a single act, while the entire multimolecular (or so-called supram olecular) assembly performs a more complex function, which results from the harmonious cooperation of the various components.7 Molecular-level devices operate through atomic and/or electronic rearrangem ents and, like macroscopic devices, they need energy and signals to operate a nd communicate with the operator, respectively. Thus, extension of the general idea of devices to molecular level is of particular interest not only for basic research, but also for the development of nano science and the progress of nanotechnology.8 The molecular nanoscience approach depends on the ability of chemists to synthesize from small, simple starting materials the target molecu les with the desired properties. In most cases, these are previously unknown molecules that ar e new forms of matter and maybe patented as such. They usually also require the devel opment of new synthetic methodology for their synthesis, and it is this that makes synthetic chemistry a most enjoyable act of creation as much akin to art as to science. The essence of synthetic chemistry, and particularly synthetic (supra)molecular chemistry, finds its full expression in the words of that epitome of the artistscientist Leonardo da Vinci, as quoted by Nobel laureate J ean-Marie Lehn in his book on supramolecular chemistry9: Where Nature finishe s producing its own species, man begins, using natural things and with the help of this nature, to create an infinity of species. It would be pertinent at this point to summarize what adva ntages a molecular approach brings to the field of nanoscale materials. In a general sense, the answer is that molecular synthesis is almost always carried out under mild conditions and at room temperature, or nearly so. These low-energy conditions allow a high level of synthetic c ontrol and an increased ability

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14 to tailor the molecules as required. More specifically, molecular advantages include the following:10 (i) once purified, all the molecules in the sample are identical. Such a single-size (monodisperse) collection of nanosized particles is extremely difficult to achieve with traditional top-down approaches, and is important because th ere is a greater variation of properties with size at the nanoscale than is found at the everyday macroscale. Thus, a distribution of nanoparticle sizes means a significant distribut ion of properties. In contrast, each molecule in a monodisperse collection will have identical properties; (ii) the low-ener gy synthetic methods allow the periphery of the molecule to comprise organic gr oups of various types. These so-called ligands (organic groups attached to meta l atoms) may be altered at will, and this provides a means to finetune important properties of the molecule, such as their solubility, which is important to their purification, crystallizati on, and study; (iii) molecula r solubility is a major advantage, in contrast to classical nanoparticles which form colloidal suspensions. True solubility provides major benefits for purification, processing (i.e. depos ition on surfaces, removal from surfaces, etc) and controlled modification; (iv) th e formation of crystals repres ents the formation of threedimensional ordered arrays of identical, monodisper se particles, each usually even with the same orientation (Figure 1-1). They thus all display the same response to an external influence, such as an applied magnetic field. Crystallization of traditional nanoparticles is extremely difficult to achieve; (v) the peripheral organi c groups (ligands) provide a pr otective coat that prevents significant interactions between adjacent molecules, and ensures the absence of surface variations, roughness, defects, etc, common with traditional nanoparticles. The molecular approach to nanoscience cu rrently encompasses all common types of materials, such as (i) metals and metal alloys, (ii) metal oxides, sulphi des, and other binary materials, and (iii) carbon-based organic substan ces. Thus, their potential applications similarly

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15 span a wide area, from sensors, conductors, electronics and magnets to pharmaceuticals, cosmetics, and many more. Figure 1-1. Three-dimensional, or dered array of monodisperse, id entically oriented molecules within a crystal. Each molecule comprises seven iron atoms (yellow) connected by oxygen atoms (red) and surrounded by organi c groups (grey). The box defines the repeating unit of the crystal. In fact, one could argue that most of the pharmaceu tical industry is alread y molecular nanoscience since most drugs, from the anti-cancer cisplatin to the venerable aspirin are molecular. As the molecular approach to nanoscience matures in th e future, its areas of potential application are likely to encompass many, if not most, of those currently dominated by top-down approaches. As is usually the case, once materials have become available and their advantages realized, then the applications follow. Magnetic materials find widespread utility in numerous areas of a modern society, from sensors, switches, speakers, and computer hard-dri ves, to medical applications such as magnetic resonance imaging. Note that, traditional ma gnetic materials that are used in modern

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16 technological applications consist of three-dime nsional lattices of meta ls or ions possessing unpaired electrons, the spins of which interact in a specific manner. Depending on the nature of these multi-dimensional long range interactions, several types of magnetic materials can be identified (Figure 1-2). If the nature of the in teraction between the individual spins of the unpaired electrons aligns them in a parallel fash ion, the material is called a ferromagnet, while antiparallel alignment results in an antiferroma gnet. If the individual spins interacting in antiparallel fashion are of diffe rent magnitudes, a ferrimagnet re sults. When no interactions between spins are present, the material is calle d paramagnet and this cau ses a random orientation of the spins in the material. Since antiferromagnets and paramagnets give a total net spin of zero, magnets can only be built from either ferromagnets or ferrimagnets, where net spin is present. Figure 1-2. Types of magnetic materials depending on the nature of the interaction between spin carriers. Miniaturization of magnetism-based devices and machines is a major technological imperative, and thus a bottom-up molecular source of nanosized magnets would be invaluable. In fact, exactly this became available in the early 1990s when it was discovered that individual molecules can function as magnets, and these were named single-molecule magnets (SMMs).10,11 In SMMs, each molecule is a single-domain magnetic particle that, below a certain temperature (termed as blocking temperature), exhibits the classical macr oscale property of a magnet, namely magnetization hysteresis.12 In addition, SMMs straddle the classical/quantum interface in also displaying quantum tunneli ng of magnetization and quantum phase interference, which are Ferromagnets Antiferromagnets Ferrim agnets Paramagnets

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17 properties of the microscale.13 SMMs have various potential ap plications, including very ultradensity information storage and sp intronics at the molecular level, with each bit stored as the magnetization orientation of an individual molecule, and as quantum bits for quantum computing14 by taking advantage of the quantum super position of states provided by the quantum tunneling of magnetization. Again, these represent a proof-of-feasibility, and the challenge for the future is, as in all areas of molecular nanosci ence, to build on this foundation and prepare new SMMs that operate at higher temperatures and/or are better tailored to applications within devices. The first SMMs were the [Mn12O12(O2CR)16(H2O)4] (Mn12; R = various groups) family (Figure 1-3)15 comprising a magnetic core with diameter of ~1 nm (Figure 1-4) surrounded by organic (carboxylate) ligands. Many others have since been disc overed, with one thrust again being to make larger ones of several nanometers. In fact, the largest known to date is a remarkable Mn84 molecule with a torus structure of ~4.4 nm diameter (Figure 1-5).16 One interesting aspect of such Mn12 and Mn84 molecules is that their study as monodisperse, crystalline assemblies allows quantum properties to be observed that ar e not easy to detect for traditional top-down nanomagnets. Figure 1-6 shows magne tic hysteresis loops for a Mn12 crystal at different temperatures: hysteresis loops are the diagnostic signature of a magnet and must therefore also be demonstrated by molecular magnets, which they are. However, those for Mn12 in Figure 1-6 are not smooth (as seen for traditional magnets) but show step-like features at various positions. These steps are the evidence of certain quantum properties (quantum tunneling) displayed by these molecular species. This is a clear demons tration of an interesti ng aspect of molecular nanoscience: it can provide not just an alternative route to nanoscale materials for miniaturization of known technologies, but also be the basis of new technologies that take advantage of the

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18 properties that result from the small size and/or molecular nature. In this case, theoretical physicists have postulated that the quantum prope rties of SMMs might prove a basis for the new generation of computing tec hnology called quantum computing.14 Figure 1-3. Structure of the Mn12 molecule. Color scheme: MnIII blue; MnIV green; O red; C gray. Figure 1-4. The [Mn12O12]16+ core, emphasizing with green thick lines the central [Mn4O4]8+ cubane subcore. Color scheme: MnIII blue; MnIV green; O red.

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19 Figure 1-5. Structure of the Mn84 molecule. Color scheme: MnIII blue; O red; C gray. -1 -0.5 0 0.5 1 -1-0.500.51M/Ms0H (T) 0.002 T/s 3.6 K 3.2 K 3.0 K 2.8 K 2.7 K 2.6 K 2.5 K 2.4 K 2.3 K 2.2 K 2.1 K 3.6 K 3.2 K 3.0 K 2.8 K 2.7 K 2.6 K 2.4 K 2.3 K 2.2 K 2.1 K 2.5 K M/Ms1 0.5 0 -0.5 -1 2 mT/s H0(T) 1 0.5 0 -0.5 -1 Figure 1-6. Magnetic hysteresis l oops for a single crystal of Mn12 at the indicated temperatures showing the steps due to the quantum behavior.

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20 From all the above and more, it becomes apparent that there is a con tinuing interest in the synthesis of new, high-nuclearity SMMs. Progress in the field of high-nuclearity molecules and the chances of identifying new SMMs will both benefit from the development of new synthetic methodologies to Mn carboxylate clusters.10 Discovering new preparative routes is thus of great interest not only for the isolation of completely new complexes but also as a means of building up families of related Mn carboxylate species so that structure-property relations can be developed. The choice of ligands in such st udies is obviously crucial, beca use the versatility, flexibility, chelate bite size(s) and other lig and properties are of great importa nce in determining the structure of the product. One synthetic methodology that has proven to be very useful for the synthesis of new polynuclear Mn complexes is the reaction of a ch elating ligand with simple Mn carboxylate sources, e.g. Mn(O2CR)2 (R = Me, Et, Ph), or with a pref ormed Mn carboxylate cluster that does not already contain any chelating ligands.17 Chelates that have prev iously proven useful are, amongst others, 2,2-bipyridine,18 2-picolinate19 and the anion of dibenzoylmethane.20 Thus, for example, the reaction of 2,2 -bipyridine (bpy) with [Mn3O(O2CR)6(py)3]+ was the original way by which the tetranuclear butterfly complexes [Mn4O2(O2CR)7(bpy)2]+ were obtained.21 The present dissertation deals with a new synthetic ro ute towards the isolation of new, high-nuclearity Mn carboxylate clusters, involvi ng the employment of an unexplored pyrazole-based alkoxide ligand.

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21 CHAPTER 2 NEW SYNTHETIC APPROACH TO HIGH NU CLE ARITY MANGANESE CARBOXYLATE CLUSTERS: MN6, MN8 AND MN18 COMPLEXES UTILIZING 3,5DIMETHYLPYRAZOLE-1-METHANOL 2.1 Introduction The synthes is and characterization of molecular 3d-metal clusters continue to be a major research area of many groups around the world b ecause of their fascinating physical properties and the intrinsic architectural b eauty and complexity of their structures. In particular, the fascination of inorganic chemists with Mn coordination chemistry ov er the last two decades or so has been primarily driven by the relevan ce to two fields, bioinorganic chemistry22 and molecular magnetism.23 First, the ability of manganes e to exist in a number of oxida tion states (II-IV) under normal conditions has resulted in this me tal being at the active sites of several redox enzymes, the most important of which is the water oxidizing complex (WOC) on the donor site of photosystem II in green plants and cyanobacteria.22,24 The WOC comprises a tetranucle ar Mn cluster, whose exact structure is still unclear, and is responsible for the light-dri ven, oxidative coupling of two molecules of water into dioxygen.25 In addition, one Ca plays a crucial role in the OxygenEvolving Complex (OEC) activity; without Ca, the OEC does not advance to the S3 state.26 Although there is a considerab le uncertainty about the Mn4Ca structure obtained from crystallography due to the current resolution,27 there is a little doubt th at the OEC is indeed a heterometallic [Mn4CaOx] cluster on the basis of other spectroscopic studies (i.e. XRD22a,27 and EXAFS28). Second, polynuclear Mn compounds containing MnIII have also been found to have large and sometimes abnormally large, ground state spin values ( S), which combined with a large and negative magnetoanisotropy (as reflected in a larg e and negative zero-field splitting parameter,

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22 D ) have led to some of these species being able to function a single-molecule magnets (SMMs).11 These are individual molecules that behave as magnets below a certain blocking temperature.12,29 Thus, they represent a molecular, bottom-up approach to nanomagnetism.10,30 SMMs have a significant energy barrier (vs kT where k is the Boltzmann constant) to magnetization relaxation, and the upper limit to the barrier ( U ) is given by S2| D | or ( S2-1/4)| D | for integer and half-integer spin, respectively. To date, the most studied S MMs are the mixed-valent [Mn12O12(O2CR)16(H2O)4] (MnIII 8MnIV 4; R = various) family with an S = 10 ground state.31 There are now also many other structural types displaying SMM be havior, the vast majority of th em being Mn complexes. This is because Mn clusters often di splay relatively large ground state S values, as well as relatively large (and negative) D values associated with the presence of Jahn-Teller distorted MnIII atoms. However, the concomitant presence of MnII atoms, which give weak ex change interactions with MnIII, often lead to SMMs with low-lying excite d states and relativel y increased relaxation rates.32 Thus, the SMMs with the largest relaxation barriers are currently either MnIII x or mixedvalent MnIII/MnIV species. The former currently comprise Mn2,33 Mn4,34 Mn6,35 Mn26,36 and Mn84 16 complexes, including the Mn6 complex with the highest barrier yet discovered.37 Towards this end, many new routes have been explored and successfully developed for the synthesis of polynuclear Mn complexes,38 with nuclearities currently up to 84.16 These procedures have included comproportionati on reactions of simple starting materials, 31d-h aggregation of clusters of smaller nuclearity,39 fragmentation of highe r nuclearity clusters,40 reductive aggregation or fragmentation of preformed clusters,15, 41 and electrochemical oxidation,32e disproportionation,42 or ligand substitution of preformed species,43 among others. The synthetic methodology that was used in the pr esent work, and has proven to be successful

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23 for the synthesis of new polynuclear Mn complexe s, involved the reaction of a chelating/bridging ligand with a preformed Mn car boxylate cluster that does not already incorporate any such groups. Some of the most useful starting materi als employed for this purpose are the triangular [Mn3O(O2CR)6L3]0/+ (L = terminally-bound, neutral groups) complexes, which have previously afforded many new complexes with a wide range of metal nuclearities. As part of this work, we have also explored a wide variety of potentially chelating and/or bridging ligands that might foster formation of high nuclearity products. One such family is the pyridyl alcohols, which have prove d to be extremely versatile N,Ox (x = 1, hmpH; x = 2, pdmH2; Figure 2-1) chelating and bridgi ng groups that have yielded a num ber of 3d metal clusters with various structural motifs, large S values, and SMM behaviors.44 As an extension to this work with hmpH and pdmH2, we have now asked what kind of pr oducts might result from the general replacement of their pyridyl groups by a pyrazo le-based aromatic ring. One of the resulting molecules that has been used in the present work was 3,5dimethylpyrazole-1-methanol (dmpmH, Figure 2-1), where the analogy between the hmpH/dmpmH pair can be clearly seen. Clearly, a major chemical differe nce between those two ligands is the size of the aromatic ring, and thus the angle that the alkoxide arm (RO-) is flexible to move. The size of the dmpmHs aromatic ring is smaller and so is the angle, a property that makes dmpmH more rigid than hmpH and could potentially alter the identity of th e products. The choice of dmpmH was mainly based on two reasons: (i) the ab ility of its anion (dmpm-) to coordinate to metal ions through both the pyrazole N atom and the O atom of the alkoxide group, acting as bident ate chelating and/or bridging ligand, the latter mode appearing extremely fruitful fo r the isolation of polynuclear metal complexes, and (ii) the unexplored coordi nation chemistry of this organic group with Mn metal ions. Indeed, the only previously re ported example of dmpmH in 3d-metal cluster

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24 chemistry is the cubane-like [Ni4Cl4(dmpm)4(EtOH)4] complex, containing anionic dmpmgroups N,O-chelate to a NiII ion and bridge through their deprotonated arms to two adjacent NiII atoms.45 N OH N OH OH hmpH pdmH2dmpmH N N Me Me OH Figure 2-1. The protonated precursors to the chelating ligands reported herein. Thus, we anticipated that the use of dm pmH in polynuclear transition metal cluster chemistry would give products distinctly differe nt from those with hmpH and related pyridyl alcoholate ligands, and we have therefore explored its use ini tially in Mn carboxylate chemistry. In the present investigations, we have deliber ately targeted higher nuclearity Mn products by exploring the reactions between dmpmH and va rious Mn carboxylate starting materials under basic conditions. This has successfully led to Mn6, Mn8, and Mn18 cluster products containing deprotonated dmpm-. We have further shown that the ch emical identity of the resulting molecules is strongly dependent on the nature of the carboxylate group em ployed. The syntheses, structures, and magnetochemical characterizatio n of these complexes are described herein.

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25 2.2 Experimental Section 2.2.1 Syntheses All m anipulations were performed under aerobic conditions using chemicals and solvents as received, unless ot herwise stated. [Mn3O(O2CR)6(py)2(H2O)] (R = Me, Et, Ph, But, PhCl; py = pyridine) were prepared as described elsewhere.46 2.2.1.1 [Mn18O14(O2CMe)18(dmpm)4(dmpmH)2(H2O)2] (1) A stirred brown solution of [Mn3O(O2CMe)6(py)2(H2O)] (0.04 g, 0.05 mmol) in MeCN (30 mL) was treated with solid dmpmH (0.08 g, 0.60 mmol). The resulting dark brown solution was stirred for 30 min, filtered, and the filtrate left undisturbed to concentrate slowly by evaporation. After three days, dark red prisms of 1MeCN were collected by filtration, washed with cold MeCN (2 x 3 mL) and Et2O (2 x 5 mL), and dried under vacuum; the yield was 45 % (based on the ligand). Anal. Calcd. for 1 (solvent-free): C, 28.22; H, 3.75; N, 5.48 %. Found: C, 28.68; H, 3.97; N, 5.53 %. Selected IR data (cm-1): 3418 (mb), 2927 (m), 1611 (m), 1580 (s), 1545 (s), 1418 (s), 1341 (w), 1248 (w), 1119 (m), 1045 (mb), 839 (w), 777 (w), 715 (w), 671 (w), 615 (s). 2.2.1.2 [Mn18O14(O2CEt)18(dmpm)4(dmpmH)2(H2O)2] (2) A stirred brown solution of [Mn3O(O2CEt)6(py)2(H2O)] (0.26 g, 0.33 mmol) in MeCN (30 mL) was treated with solid dmpmH (0.50 g, 3.96 mmol). The resulting dark brown solution was stirred for 30 min, filtered, and the filtrate was left undisturbed to concentrate slowly by evaporation. After five da ys, dark red crystals of 2MeCN were collected by filtration, washed with cold MeCN (2 x 3 mL) and Et2O (2 x 5 mL), and dried under vacuum; the yield was 55 % (based on the ligand). Anal. Calcd. for 2 (solvent-free): C, 32.59; H, 4.56; N, 5.07 %. Found: C, 32.52; H, 4.48; N, 5.05 %. Selected IR data (cm-1): 3416 (mb), 2975 (m), 2933 (w), 2876 (w), 1577 (s), 1540 (s), 1422 (s), 1294 (m), 1250 (w), 1121 (m), 1074 (m), 980 (w), 883 (w), 812 (w), 712 (m), 617 (s), 544 (w), 414 (w).

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26 2.2.1.3 [Mn8O2(O2CPh)10(dmpm)4(H2O)2] (3) A stirred brown solution of [Mn3O(O2CPh)6(py)2(H2O)] (0.36 g, 0.33 mmol) in MeCN (30 mL) was treated with solid dmpmH (0.50 g, 3.96 mmol). The resulting dark red solution was stirred for 90 min, filtered, and the filtrate was left undisturbed to concentrate slowly by evaporation. After two da ys, orange crystals of 3MeCN were collected by filtration, washed with cold MeCN (2 x 3 mL) and Et2O (2 x 5 mL), and dried under vacuum; the yield was 70 % (based on the ligand). Anal. Calcd. for 3 (solvent-free): C, 50.87; H, 4.09; N, 5.05 %. Found: C, 50.63; H, 4.07; N, 5.12 %. Selected IR data (cm-1): 3371 (wb), 2362 (w), 1601 (m), 1554 (m), 1399 (s), 1262 (w), 1169 (w), 1077 (w), 834 (w), 787 (w), 719 (m), 679 (w), 624 (w), 429 (w). 2.2.1.4 [Mn6O2(O2CBut)10(py)4] (4) A stirred brown solution of [Mn3O(O2CBut)6(py)2(H2O)] (0.29 g, 0.30 mmol) in MeCN (30 mL) was treated with solid dmpmH (0.45 g, 3.60 mmol). The resulting dark red solution was stirred for 90 min, filtered, and the filtrate was left undisturbed to concentrate slowly by evaporation. After two days, red crystals of 4 MeCN were collected by filtration, washed with cold MeCN (2 x 3 mL) and Et2O (2 x 5 mL), and dried under vacuum; the yield was 70 % (based on the total available Mn). Anal. Calcd. for 4 (solvent-free): C, 49.77; H, 6.56; N, 3.32 %. Found: C, 49.66; H, 6.41; N, 3.37 %. Selected IR data (cm-1): 3424 (wb), 2962 (m), 2924 (m), 2866 (m), 1571 (m), 1484 (m), 1455 (m), 1418 (m), 1367 (m), 1332 (w), 1226 (s), 1155 (w), 1121 (m), 1037 (m), 982 (w), 889 (w), 834 (w), 787 (m), 700 (w), 653 (w), 619 (w), 504 (w), 419 (m). 2.2.1.5 [Mn6O2(O2CPhCl)10(py)4] (5) A stirred brown solution of [Mn3O(O2CPhCl)6(py)2(H2O)] (0.34 g, 0.30 mmol) in MeCN (30 mL) was treated with solid dmpmH (0.45 g, 3.60 mmol). The resulting dark red solution was stirred for 90 min, filtered, and the filtrate was left undisturbed to concentrate slowly by

PAGE 27

27 evaporation. After two days, red crystals of 5 were collected by filtration, washed with cold MeCN (2 x 3 mL) and Et2O (2 x 5 mL), and dried under vacuum; the yield was 65 % (based on the total available Mn). Anal. Calcd. for 5: C, 48.40; H, 2.71; N, 2.51 %. Found: C, 48.45; H, 2.77; N, 2.33 %. Selected IR data (cm-1): 3367 (w), 3063 (w), 1607 (s), 1467 (w), 1396 (s), 1223 (w), 1159 (w), 1126 (w), 1044 (m), 952 (w), 843 (w ), 801 (w), 751 (m), 707 (m), 648 (w), 609 (m), 563 (w), 467 (w). 2.2.2 X-ray Crystallography Data were collected on a Siem ens SMAR T PLATFORM equipped with a CCD area detector and a graphite m onochromator utilizing Mo-K radiation ( = 0.71073 ). Suitable crystals of 1MeCN, 2 MeCN, 3MeCN, 4MeCN, and 5 were attached to glass fibers using silicone grease and transferre d to a goniostat where they we re cooled to 173 K for data collection. An initial search of recipr ocal space revealed a triclinic cell for 1MeCN, 2MeCN and 3MeCN, and a monoclinic cell for 4MeCN and 5; the choices of space groups P 1 ( 1MeCN, 2MeCN and 3MeCN), P 21/ n ( 4MeCN) and C 2/ c ( 5) were confirmed by the subsequent solution and refinement of the struct ures. Cell parameters were refined using up to 8192 reflections. A full sphere of data (1850 frames) was collected using the -scan method (0.3 frame width). Absorption corrections by integration were applied based on measured indexed crystal faces. The structures were solved by direct methods in SHELXTL6 ,47 and refined on F2 using full-matrix least-squares. The non-H at oms were treated anisotropically, whereas the H atoms were placed in calculated, ideal positio ns and refined as riding on their respective C atoms. Unit cell parameters and structure solution and refinement data are listed in Table 2-1. For 1MeCN, the asymmetric unit consists of half the Mn18 cluster and two and a half MeCN molecules of crystalliza tion. The solvent molecules were disordered (one of which

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28 Table 2-1. Crystallographic data for 1MeCN, 2 MeCN, 3 MeCN, 4MeCN, and 5 Parameter 1 2 3 f 4 5 Formulaa C82H127Mn18N17O58 C102H168Mn18N18O58C98H96Mn8N10O28 C72.50H117.50Mn6N5.50O23C90H60Cl10Mn6N4O22 Fw, g mol-1 a 3267.93 3563.46 2301.41 1763.86 2233.56 Crystal system Triclinic Triclini c Triclinic Monoclinic Monoclinic Space group P 1 P 1P 1P 21/ n C 2/ c 15.3733(14) 14.7851(19) 13.782(2) 14.2402(16) 20.543(2) b, 15.5402(14) 15.884(2) 14.899(7) 28.024(3) 21.589(2) c 16.1036(15) 17.908(2) 15.175(6) 22.919(3) 20.984(2) deg 103.610(2) 106.377(2) 106.48(1) 90 90 deg 98.917(2) 109.805(2) 112.95(4) 90.153(2) 100.898(2) deg 118.453(2) 102.106(2) 103.86(3) 90 90 V 3 3122.8(5) 3572.6(8) 2525.8(4) 9146.1(18) 9138.6(17) Z 1 1 1 4 4 T K 173(2) 173(2) 173(2) 173(2) 173(2) Radiation, b 0.71073 0.71073 0.71073 0.71073 0.71073 calc, g cm-3 1.738 1.656 1.122 1.281 1.623 mm-1 1.845 1.620 0.870 1.172 R1 c, d 0.0416 0.0366 0.0853 0.0617 0.0480 wR2 e 0.0836 0.0932 0.1686 0.1321 a Including solvate molecules. b Graphite monochromator. c I > 2 ( I ). d R1 = (|| Fo| | Fc||)/ | Fo|. e wR2 = [ [w( Fo 2 Fc 2)2]/ [w( Fo 2)2]]1/2, w = 1/[ 2( Fo 2) + [(0.0680 p)2 + 3.6073 p], where p = [max( Fo 2, 0) + 2Fc 2]/3. f Data from partiall y-solved structure.

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29 around an inversion center) and could not be modeled properly, thus program SQUEEZE,48 a part of the PLATON49 package of crystallographic software was used to calculate the solvent disorder area and remove its c ontribution to the overall intensity data. A total of 725 parameters were refined in the final cycle of refinement on F2 using 6774 reflections with I > 2 ( I ) to yield R1 and wR2 of 4.16 and 8.36 %, respectively. For 2MeCN, the asymmetric unit consists of half the Mn18 cluster and three MeCN molecules of crystallization. Th e latter molecules were disordered and could not be modeled properly, thus program SQUEEZE wa s used to calculate the solvent disorder area and remove its contribution to the overall intensity data. The protons on O27 and O28 were obtained from a Difference Fourier map and were treated riding on their parent atoms. A total of 808 parameters were refined in the final cycle of refinement on F2 using 12509 reflections with I > 2 ( I ) to yield R1 and wR2 of 3.66 and 9.32 %, respectively. For 3MeCN, the asymmetric unit consists of half the Mn8 cluster and one MeCN molecule of crystallization. A total of 643 para meters were refined in the final cycle of refinement on F2 using 6516 reflections with I > 2 ( I ) to yield R1 of 8.53 %. For 4MeCN, the asymmetric unit consists of the complete Mn6 cluster and two MeCN molecules of crystallization. Both the solvent molecules are disordered. One was refined in two parts and one had the N atom common to both part s. The cluster has several disorders. Two tbutyl groups were disordered and refined in tw o parts each with 50 % occupancy. One pyridine ligand is disordered against a MeCN molecu le with 50 % occupancy with another 50 % occupancy MeCN near it in the la ttice but non-coordinated. The last disorder involves a pyridine ligand against a pivalic acid. The hydroxyl proton of the latter was not found and not included in the final refinement. Additionally, the methyl prot ons of the disordered MeCN molecule, sharing

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30 the methyl carbon, were also not included. A total of 865 parameters were refined in the final cycle of refinement on F2 using 14589 reflections with I > 2 ( I ) to yield R1 and wR2 of 6.17 and 16.86 %, respectively. For 5, the asymmetric unit consists of half the Mn6 cluster located on a 2-fold rotation axis. Two of the oxygen atoms of two ligands (O3 and O12) lie on the rotation axis, thus they are disordered in two parts. All disordered atoms we re refined with 0.5 occupation factors. A total of 637 parameters were refined in the final cycle of refinement on F2 using 8545 reflections with I > 2 ( I ) to yield R1 and wR2 of 4.80 and 13.21 %, respectively. 2.2.3 Physical Measurements Infrared spectra were recorded in th e solid state (KBr pellets) on a Nicolet Nexus 670 FTIR spectrometer in the 400-4000 cm-1 range. Elemental analyses (C, H, and N) were performed by the in-house faci lities of the University of Florida Chemistry Department. Variable-temperature direct current (dc) and alte rnating current (ac) magne tic susceptibility data were collected at the University of Fl orida using a Quantum Design MPMS-XL SQUID susceptometer equipped with a 7 Tesla magnet and operating in the 1.8 K range. Samples were embedded in solid eicosane to prevent to rquing. Pascals constants were used to estimate the diamagnetic corrections, which were subtracted from the experimental su sceptibilities to give the molar paramagnetic susceptibilities (M). 2.3 Results and Discussion 2.3.1 Syntheses Many synthetic procedures to polynuclear m a nganese clusters rely on the reaction of triangular [Mn3O(O2CR)6L3] species with a potentially chela ting ligand. This was the approach chosen in the present work usi ng dmpmH as the chelating/bridging ligand. In such reactions, the [Mn3O] core of the trinuclear, oxo-bridged mangan ese complexes serves as a building block for

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31 higher nuclearity species. As is almost always the case in Mn cluster chemistry, the reaction solutions contain a complicated mixture of several species in equilibrium, with factors such as relative solubility, la ttice energies, crystallization kinetics, protonation/deprotonation, oxidation/reduction, structural rearrangements and others determining the identity of the isolated products. In the current work, we have found that one of those fa ctors is the identity of the carboxylate group employed. The reaction of the mixed-valence [Mn3O(O2CMe)6(py)2(H2O)] (MnII, 2MnIII) compound with 12 equivalents of dmpmH in MeCN resulted in the formation of a dark brown solution from which were obtained well-formed, dark red prismatic crystals of the mixed-valence [Mn18O14(O2CMe)18(dmpm)4(dmpmH)2(H2O)2] (1; 4MnII, 14MnIII) complex in good yields (~45%). The formation of 1 is summarized in Equation 2-1. 6 [Mn3O(O2CMe)6(py)2(H2O)] + 6 dmpmH + 4 H2O [Mn18O14(O2CMe)18(dmpm)4(dmpmH)2(H2O)2] + 18 MeCO2 + 12 py + 20 H+ + 2 e(2-1) Decreasing the amount of dmpmH still gave complex 1, but the product was not crystalline. In excess of the ligand, the reaction did not give 1 but instead pale yellow solutions, indicating the presence of exclusively MnII species; we have not pursued char acterization of these products. In alcoholic media, such as MeOH and EtOH, the same synthetic route gave instead the known, torus-like [Mn84O72(O2CMe)78(OMe)24(MeOH)12(H2O)42(OH)6]16 and [Mn70O60(O2CMe)70(OEt)20(EtOH)16(H2O)22]50 clusters in fairly good yields of ~30 and ~50 %, respectively. If the carboxylate employed was propionate in stead of acetate, then the product from the same reaction scheme was the pr opionate analogue of complex 1, [Mn18O14(O2CEt)18(dmpm)4(dmpmH)2(H2O)2] ( 2 ; 4MnII, 14MnIII), obtained as dark red

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32 prismatic crystals in very good yi elds (~55%). The formation of 2 is summarized in Equation 22. 6 [Mn3O(O2CEt)6(py)2(H2O)] + 6 dmpmH + 4 H2O [Mn18O14(O2CEt)18(dmpm)4(dmpmH)2(H2O)2] + 18 EtCO2 + 12 py + 20 H+ + 2 e(2-2) The same product was also obtained from the treatment of [Mn3O(O2CEt)6(py)2(H2O)] with less equivalents of dmpmH, but again it had the form of a non-crystalline powder. The two complexes 1 and 2 are structurally very similar w ith only few differences in their metric parameters (bond distances and angles). The structures have many O2groups most likely created by the deprotonation of H2O molecules, which are presen t in the starting materials, solvents and/or air. The partial depr otonation of dmpmH to its anionic dmpmform, and that of H2O molecules to O2ions, is occurred by the basic MeCO2 and EtCO2 ions; note that the soformed weak acids (MeCO2H and EtCO2H) do not decompose the oxide-bridged Mn18 clusters. For both compounds, the addition of base, such as NEt3, led to the isolation of the same products in higher yields (~60% for 1 and ~70% for 2). The presence of a base, which also has the role of a proton acceptor, facilita tes the deprotonation of H2O and this could be a rationalization of the observed higher yields. In contrast to the octadecanuclear products fr om the use of acetate and propionate reagents, the use of benzoate groups did not lead to the isolation of the benzoate anal ogue of compounds 1 and 2, but instead to an octanuclear product. Treatment of [Mn3O(O2CPh)6(py)2(H2O)] with dmpmH in the molar ratio of 1:12 in MeCN led to the subsequent isolation of the mixed-valence [Mn8O2(O2CPh)10(dmpm)4(H2O)2] ( 3; 6MnII, 2MnIII) complex in excellent yields (~70%). The formation of 3 is summarized in Equation 2-3. 8 [Mn3O(O2CPh)6(py)2(H2O)] + 12 dmpmH + 10 e3 [Mn8O2(O2CPh)10(dmpm)4(H2O)2] + 18 PhCO2 + 4 H2O + 16 py + 8 H+ (2-3)

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33 The addition of NEt3 as a base improved the yiel d of the isolable product 3 (>75%). The use of the preformed, butterfly-like (NBu4 n)[Mn4O2(O2CPh)9(H2O)]51 complex as starting material, gave the same product 3 in lower yield and the quality of the crystals was not satisfactorily. The reactions between triangular [Mn3O(O2CR)6(py)3](ClO4) (MnIII 3) species, where R = Me, Et, and Ph, as the Mn source and dmpmH in various Mn3:ligand ratios gave, unfortunately, complexes 1, 2, and 3, respectively, in comparable overall yi elds (45-70%). Thus, the presence of more MnIII ions within the triangular starting reag ents, compared to the previous, mixedvalence MnII/III 3 species, as well as the presence of ClO4 counteranions did not cause any dramatic influence on the identity of the products (vide infra). The use of the benzoate Mn3 reagent, which gave a product with different nuclearity than the acetate and the propionate anal ogues, led to the conclusion th at the basicity and the bulkiness of the carboxylate group affect the products id entity. So, we decided to employ several other carboxylate groups in order to investigate to what extent the identity of the isolated product might again be affected. Thus, using our standard synthetic procedure, the pivalate (R = But) analogue of the [Mn3O(O2CR)6(py)2(H2O)] triangles was treated with 12 equivalents of dmpmH in MeCN, resulting in the formation of red crystals of the mixed-valence [Mn6O2(O2CBut)10(py)4] ( 4; 4MnII, 2MnIII) compound in the high-yield of 65%; complex 4, however, does not incorporate the dmpmH ligand, and its formation is summarized in Equation 2-4. 2 [Mn3O(O2CBut)6(py)2(H2O)] + 2 e[Mn6O2(O2CBut)10(py)4] + 2 ButCO2 + 2 H2O (2-4) In our attempts to further alte r the nuclearity of the product, we employed an even bulkier carboxylate, the chlorobenzoate gro up. An identical reaction, as before, led to the isolation of the same [Mn6O2]10+ core, with the chlorobenzoate groups in the periphery and the absence of the

PAGE 34

34 dmpmH ligand. Consequently, th e chemical formulation of the new, high-yield (~65-70%) product is depicted as [Mn6O2(O2CPhCl)10(py)4] ( 5; 4MnII, 2MnIII), and its formation is summarized in Equation 2-5. 2 [Mn3O(O2CPhCl)6(py)2(H2O)] + 2 e[Mn6O2(O2CPhCl)10(py)4] + 2 PhClCO2 + 2 H2O (2-5) Various changes in the Mn3:dmpmH molar ratio, in the last tw o reactions that led to complexes 4 and 5, gave the same products in lower yields (1 5-25%), and proved further the thermodymamic stability of their [Mn6O2]10+ core. Moreover, the products id entity does not change in the presence of several equivalents of base. Several approaches leading to the isolation of hexanucle ar Mn complexes with the [MnII 4MnIII 2O2]10+ core are known.52,53,54 These methods can be divided into four groups. The first one is based on MnII(O2CR)2 oxidation with oxygen,53 MnO4 52,53 or bis(trimethylsilyl)peroxide.53 The second involves reduction and subsequent coupling of trinuclear [MnIIMnIII 2O]6+ species resulting in formation the [MnII 4MnIII 2O2]10+ core. The reducing agents were sodium acenaphthylenide,54a phenolic molecules54a or toluene.54b The third method involves construction of the [MnII 4MnIII 2O2]10+ unit by reduction of a species containing the [MnIII 4O2]8+ core52 or by reductive deaggregation (cl eavage) of complexes containing the [MnIII 8MnIV 4O12]16+ core,55 the latter being described by the authors as thermal decarboxylation. In all of the above-mentioned cas es, the carboxylate ions were already present in the Mn starting materials or in the reaction mixtures, and were not involved in any redox process. Recently Quahab, Pavlishchuk and co-workers56 reported an easy one-pot redox reaction for the preparation of complexes containing the [MII 4MnIII 2O2]10+ core (M= Mn, Co, Ni). The reaction of metal nitrates with benzaldehyde unde r aerobic conditions result s in direct in situ

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35 generation of both MnIII ions and benzoate anions in one step by an oxidation process of MnII and benzaldehyde with nitrates. 2.3.2 Description of Structures 2.3.2.1 [Mn18O14(O2CMe)18(dmpm)4(dmpmH)2(H2O)2] (1) The partially labeled molecular stru cture and stereoview of complex 1 are depicted in Figure 2-2. Selected interatomic distances a nd angles are listed in Table 2-2. Complex 1MeCN crystallizes in the triclinic space group P 1. The structure consists of one [Mn18] unit, which is lying on a crystallographic inversion center. Char ge considerations indicate a mixed-valence MnII 4MnIII 14 description with the four MnII ions being Mn5, Mn9, Mn5, and Mn9. This is based, primarily, on considerati on of the structural parameters and bond valence sum (BVS)57 calculations listed in Tables 2-2 and 2-3, respectively. All Mn centers are six-coordi nate with distorted octahedr al geometry except Mn8 and Mn8 that are five-coordinate a nd possess distorted square pyramidal geometry. Analysis of the shape-determining angles using the Reedijk-Addison58 approach yields a trigonality index value of 0.07 for the five-coordinate metal atoms, indicating a slightly distorted coordination polyhedron ( = 0 and 1 for perfect square pyramidal and trigonal bipyra midal geometries, respectively). The fourteen MnIII atoms show the Jahn-Teller (JT) distortion expected for a highspin d4 ion in a near octahedral ge ometry, taking the form of an axial elongation (see Table 2-2). Thus, as is almost always the case, the JT elongation axes avoid the MnIII-O2bonds, the shortest and strongest in the molecule.59 The [Mn18] unit can be described as disc-shaped with ten of the metal ions in a nearly planar arrangement, and Mn2, Mn4, Mn5, and Mn9 (and their symmetry equivalents) lying above and below the plane.

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36 Figure 2-2. Molecular st ructure of complex 1. A) Partially labelled P ovRay representation. B) Stereopair. Hydrogen atoms omitted for clarity. Color scheme: MnII, yellow; MnIII, blue; O, red; N, green; C, gray. Table 2-2. Selected interatomic distances () and angles (deg) for 1MeCNa Parameter Parameter Mn1Mn2 2.936(2) Mn3Mn7 7.861(8) Mn1Mn3 3.071(3) Mn3Mn8 5.408(5) Mn1Mn4 3.219(6) Mn3Mn9 5.783(5) Mn1Mn5 3.147(6) Mn4Mn5 3.121(3) Mn1Mn6 3.059(3) Mn4Mn6 5.459(7)

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37 Table 2-2 Continued. Mn1Mn7 5.231(7) Mn4Mn7 8.302(1) Mn1Mn8 3.397(6) Mn4Mn8 6.565(1) Mn1Mn9 3.526(3) Mn4Mn9 5.210(1) Mn2Mn3 2.813(2) Mn5Mn6 3.233(7) Mn2Mn4 2.848(4) Mn5Mn7 6.524(1) Mn2Mn5 4.360(5) Mn5Mn8 5.784(1) Mn2Mn6 5.214(4) Mn5Mn9 3.707(4) Mn2Mn7 7.520(5) Mn6Mn7 3.365(7) Mn2Mn8 5.845(4) Mn6Mn8 3.533(5) Mn2Mn9 6.309(4) Mn6Mn9 3.427(2) Mn3Mn4 3.188(6) Mn7Mn8 2.819(9) Mn3Mn5 5.368(3) Mn7Mn9 5.059(1) Mn3Mn6 6.124(6) Mn8Mn9 3.992(9) Mn1-O1 1.910(3) Mn5-O27 2.202(3) Mn1-O2 1.953(2) Mn5-O28 2.138(3) Mn1-O3 1.922(3) Mn5-N3 2.247(3) Mn1-O5 1.889(2) Mn6-O1 1.887(3) Mn1-O9 2.270(3) Mn6-O7 1.883(2) Mn1-O11 2.225(3) Mn6-O11 2.444(3) Mn2-O2 2.232(3) Mn6-O19 1.956(3) Mn2-O3 1.877(2) Mn6-O21 2.154(3) Mn2-O4 1.933(3) Mn6-O28 1.918(2) Mn2-O12 1.984(3) Mn7-O6 1.860(2) Mn2-O13 2.170(3) Mn7-O7 1.874(2) Mn2-O15 1.956(3) Mn7-O8 2.300(3) Mn3-O3 1.883(3) Mn7-O20 1.965(3) Mn3-O4 1.914(2) Mn7-O23 2.228(3) Mn3-O9 2.352(3) Mn7-O26 1.968(3) Mn3-O14 2.175(3) Mn8-O5 1.877(2) Mn3-O25 1.963(3) Mn8-O6 1.906(2) Mn3-O6 1.889(2) Mn8-O7 1.940(2) Mn4-O2 1.859(3) Mn8-O24 2.213(3) Mn4-O4 1.914(3) Mn8-O5 1.920(2) Mn4-O9 2.455(2) Mn9-O1 2.135(3) Mn4-O16 2.146(3) Mn9-O10 2.160(3) Mn4-O27 1.901(3) Mn9-O18 2.115(3) Mn4-N1 2.016(3) Mn9-O22 2.183(3) Mn5-O1 2.326(2) Mn9-O29 2.402(3) Mn5-O2 2.172(3) Mn9-N5 2.281(3) Mn5-O17 2.087(3) Mn1-O2-Mn2 88.9(1) Mn3-O6-Mn7 130.6(1) Mn1-O3-Mn2 101.2(1) Mn3-O6-Mn8 132.3(2) Mn1-O3-Mn3 107.6(1) Mn4-O2-Mn5 101.2(1) Mn1-O9-Mn3 83.2(1) Mn4-O27-Mn5 98.8(1)

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38 Table 2-2 Continued. Mn1-O2-Mn4 115.2(2) Mn5-O1-Mn6 99.7(1) Mn1-O9-Mn4 85.8(1) Mn5-O28-Mn6 105.6(1) Mn1-O1-Mn5 95.5(1) Mn5-O1-Mn9 112.3(1) Mn1-O2-Mn5 99.3(1) Mn6-O7-Mn7 127.3(2) Mn1-O1-Mn6 107.4(1) Mn6-O7-Mn8 135.0(1) Mn1-O11-Mn6 81.7(1) Mn6-O1-Mn9 116.8(1) Mn1-O5-Mn8 128.8(2) Mn7-O6-Mn8 96.9(1) Mn1-O5-Mn8' 130.4(1) Mn7-O7-Mn8 95.3(1) Mn1-O1-Mn9 121.2(2) Mn8-O5-Mn8' 99.4(1) Mn2-O3-Mn3 96.9(1) O9-Mn1-O11 164.9(1) Mn2-O4-Mn3 94.0(1) O2-Mn2-O13 168.5(1) Mn2-O2-Mn4 87.7(1) O9-Mn3-O14 174.2(1) Mn2-O4-Mn4 95.5(1) O9-Mn4-O16 166.9(1) Mn2-O2-Mn5 163.8(1) O11-Mn6-O21 173.5(1) Mn3-O4-Mn4 112.8(1) O8-Mn7-O23 172.4(1) Mn3-O9-Mn4 83.0(8) a Primed and unprimed atoms are related by symmetry. Table 2-3. Bond valence sum (BVS)a b calculations for Mn and selected oxygen atoms in 1 Atom MnII MnIII MnIV Mn1 3.19 2.92 3.06 Mn2 3.14 2.87 3.02 Mn3 3.23 2.96 3.10 Mn4 3.29 3.04 3.14 Mn5 1.98 1.83 1.85 Mn6 3.21 2.94 3.09 Mn7 3.20 2.93 3.07 Mn8 3.00 2.75 2.88 Mn9 1.91 1.77 1.83 BVS AssignmentGroup O1 1.77 O2O2O2 1.76 O2O2O3 1.84 O2O2O4 1.70 O2O2O5 1.83 O2O2O6 1.89 O2O2O7 1.82 O2O2O8 0.21 H2O H2O O27 1.90 ROdmpm-O28 2.03 ROdmpmO29 1.16 ROH dmpmH a The italized value is the one closest to the charge for which it was calculated. The oxidation state of a particular atom can be taken as th e nearest whole number to the italized value. b A BVS in the ~1.8-2.0, ~1.0-1.2, and ~0.2-0.4 ranges for an O atom is indicative of non-, singleand double-protonation, re spectively, but can be altered somewhat by hydrogen bonding.

PAGE 39

39 The compound consists of an overall [Mn18( 4-O)4( 3-O)10( 3-OR)2( -OR)6]14+ core (Figure 23A), with the four 4-O atoms (O1, O2, O1, and O2) and ten of the 3-O atoms (O3, O4, O5, O6, O7, O3, O4, O5, O6, and O7) being O2ions (see Table 2-3). The remaining two 3-O atoms (O9 and O9) are from 4-MeCO2 groups, an extremely rare binding mode for this ligand in molecular cluster chemistry. Of the six atoms, four (O27, O28, O 27, and O28) are part of the alkoxide arm (RO-) of the dmpmligands, and the remaining two (O11 and O11) are from 3-bridging, syn,syn,anti-MeCO2 groups. The core can be alternatively described as a central [Mn4O6] unit (containing a linear Mn4 chain) linked by its 32ions to two [Mn7O8] units, one on each side (Figure 2-3B). Each of the latter comprises a face-sharing set of one [Mn4O4] cubane and two [Mn3O4] partial cubanes. The four dmpmanions bind as bidentate -bridging and the two neutral dmpmH as bidentate chelates. Peripheral ligation about the core is provided by eighteen bridging MeCO2 groups, fourteen of them binding in their common 1: 1: mode, two in their rather unusual 1: 2: 3 mode, and the remaining two in the rare 1: 3: 4 mode. BVS calculations were also performed on selected oxygen atoms of 1 to identify their protonation level (Table 2-3); this established O8 and O8 atoms to be part of two terminal wa ter molecules, which complete the coordination sphere of Mn7 and Mn7, respectively, and al so confirmed the protonated form of the two dmpmH ligands (O29 and O29). The determined protonation levels and metal oxidation states are consistent with the overall, neutral [Mn18] complex, as revealed by the crystal structure (vide infra).

PAGE 40

40 Figure 2-3. Labeled PovRay representation of the Mn18 core. A) The complete [Mn18( 4-O)4( 3O)10( 3-OR)2( -OR)6]14+ core of 1. B) Its simplified [Mn18( 4-O)4( 3-O)10]22+ core. Color scheme: MnII, yellow; MnIII, blue; O, red; C, gray.

PAGE 41

41 2.3.2.2 [Mn18O14(O2CEt)18(dmpm)4(dmpmH)2(H2O)2] (2) The molecular structure of complex 2 (Figure 2-4) is almost identical to that of 1, and thus will be only briefly discussed. Selected interatomic distances and angles are listed in Table 2-4. Complex 2MeCN crystallizes in the triclinic space group P 1. The structure is lying on a crystallographic inversion cen ter and consists of a [Mn18] unit. The compound can be again described as disc-shaped with ten of the metals in a nearly planar arrangement and the remaining eight Mn atoms lying above and below the plan e. It also consists of an overall [Mn18( 4-O)4( 3O)10( 3-OR)2( -OR)6]14+ core, with the four 4-O and ten of the 3-O atoms being O2ions and the remaining two 3-O atoms arising from 4-EtCO2 groups. Of the six atoms, four are part of the dmpmligand and the other two are from 3-bridging, syn,syn,anti-EtCO2 groups. Figure 2-4. Molecular st ructure of complex 2, with hydrogen atoms omitted for clarity. Color scheme: MnII, yellow; MnIII, blue; O, red; N, green; C, gray.

PAGE 42

42 Table 2-4. Selected interatomic distances () and angles (deg) for 2MeCNa Parameter Parameter Mn1Mn2 3.218(3) Mn3Mn7 5.848(3) Mn1Mn3 5.197(6) Mn3Mn8 6.316(4) Mn1Mn4 6.129(1) Mn3Mn9 7.518(5) Mn1Mn5 5.444(7) Mn4Mn5 3.200(3) Mn1Mn6 3.056(7) Mn4Mn6 3.080(7) Mn1Mn7 3.522(5) Mn4Mn7 5.448(5) Mn1Mn8 3.420(2) Mn4Mn8 5.795(8) Mn1Mn9 3.371(3) Mn4Mn9 7.898(1) Mn2Mn3 4.399(6) Mn5Mn6 3.218(3) Mn2Mn4 5.381(1) Mn5Mn7 6.586(5) Mn2Mn5 3.127(6) Mn5Mn8 5.247(1) Mn2Mn6 3.145(3) Mn5Mn9 8.314(8) Mn2Mn7 5.769(3) Mn6Mn7 3.410(3) Mn2Mn8 3.689(4) Mn6Mn8 3.514(3) Mn2Mn9 6.513(5) Mn6Mn9 5.250(5) Mn3Mn4 2.818(4) Mn7Mn8 3.954(3) Mn3Mn5 2.849(2) Mn7Mn9 2.820(6) Mn3Mn6 2.939(1) Mn8Mn9 5.042(4) Mn1-O2 1.972(2) Mn5-O17 2.490(2) Mn1-O3 1.887(2) Mn5-O20 1.920(2) Mn1-O4 1.892(2) Mn5-N3 2.015(2) Mn1-O5 1.914(2) Mn6-O4 1.907(2) Mn1-O6 2.414(2) Mn6-O6 2.222(2) Mn1-O25 2.138(2) Mn6-O8 1.952(2) Mn2-O4 2.306(2) Mn6-O16 1.929(2) Mn2-O5 2.145(2) Mn6-O17 2.262(2) Mn2-O8 2.190(2) Mn6-O19 1.886(2) Mn2-O20 2.209(2) Mn7-O3 1.932(2) Mn2-O23 2.098(2) Mn7-O19 1.887(2) Mn2-N1 2.256(2) Mn7-O21 2.196(2) Mn3-O7 1.979(2) Mn7-O29 1.919(2) Mn3-O8 2.245(2) Mn7-O19 1.905(2) Mn3-O10 1.949(2) Mn8-O4 2.127(2) Mn3-O11 1.931(2) Mn8-O18 2.189(2) Mn3-O12 2.169(2) Mn8-O24 2.122(2) Mn3-O16 1.873(2) Mn8-O26 2.217(2) Mn4-O11 1.920(2) Mn8-O27 2.384(2) Mn4-O13 2.195(2) Mn8-N5 2.287(2) Mn4-O14 1.969(2) Mn9-O1 1.961(2) Mn4-O16 1.878(2) Mn9-O3 1.882(2) Mn4-O17 2.382(2) Mn9-O22 2.212(2) Mn4-O29 1.888(2) Mn9-O28 2.281(2) Mn5-O8 1.860(2) Mn9-O29 1.855(2) Mn5-O9 2.174(2) Mn9-O15 1.980(2)

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43 Table 2-4 Continued. Mn5-O11 1.907(2) Mn1-O4-Mn2 99.61(8) Mn3-O8-Mn6 88.6(7) Mn1-O5-Mn2 104.7(9) Mn3-O16-Mn6 101.3(9) Mn1-O4-Mn6 107.1(9) Mn4-O11-Mn5 113.5(8) Mn1-O6-Mn6 82.4(6) Mn4-O17-Mn5 82.1(5) Mn1-O3-Mn7 134.5(9) Mn4-O16-Mn6 108.0(9) Mn1-O4-Mn8 116.6(9) Mn4-O17-Mn6 83.0(6) Mn1-O3-Mn9 126.9(1) Mn4-O29-Mn7 132.7(1) Mn2-O8-Mn3 165.4(9) Mn4-O29'-Mn9' 130.4(9) Mn2-O8-Mn5 100.8(8) Mn5-O8-Mn6 115.2(9) Mn2-O20-Mn5 98.2(8) Mn5-O17-Mn6 85.1(6) Mn2-O8-Mn6 98.7(7) Mn6-O19-Mn7 129.3(1) Mn2-O4-Mn6 96.1(7) Mn6-O19-Mn7' 129.7(9) Mn2-O4-Mn8 112.6(8) Mn6-O4-Mn8 121.1(9) Mn3-O11-Mn4 94.1(8) Mn7-O19-Mn7' 99.5(8) Mn3-O16-Mn4 97.4(8) Mn7-O3-Mn9 95.3(8) Mn3-O8-Mn5 87.4(7) Mn7-O29-Mn9 96.7(8) Mn3-O11-Mn5 95.9(9) a Primed and unprimed atoms are related by symmetry. Close examination of the metric parameters (Table 2-4) and BVS calculations for the Mn atoms (Table 2-5) reveals a mixedand trappe d-valence oxidation state description of 4MnII and 14MnIII centers. All Mn ions are six-coordinate with distorted octahedral geometry except Mn7 and Mn7 which are five-coordina te with distorted square pyram idal geometry. A trigonality index value of 0.06 for the five-coordinate meta l atoms indicates a slightly distorted coordination polyhedron. A ll the six-coordinate MnIII atoms show the expect ed JT distortion of high-spin MnIII in near-octahedral geomet ry, and this takes the form of an axial elongation in every case. The four dmpmanions bind again as bidentate -bridging and the two neutral dmpmH as bidentate chelates. Peripheral liga tion is accomplished by eighteen propionate groups, fourteen of them binding in their common 1: 1: mode, two in their rather unusual 1: 2: 3 mode, and the remaining two in the rare 1: 3: 4 mode. Two terminal water groups complete ligation on the MnIII atoms, Mn9 and Mn9. The determ ined protonation levels and metal

PAGE 44

44 oxidation states (Table 2-5) are cons istent with the overall neutral [Mn18] complex, as revealed by the crystal structure (vide infra). Neither complex 1 nor 2 form any significant intermolecular hydrogen-bonds, only weak intermolecular contacts between C-H bonds and the -system of dmpmH and dmpmgroups. Table 2-5. Bond valence sum (BVS)a b calculations for Mn and selected oxygen atoms in 2 Atom MnII MnIII MnIV Mn1 3.21 2.93 3.08 Mn2 1.95 1.80 1.86 Mn3 3.16 2.89 3.04 Mn4 3.19 2.92 3.06 Mn5 3.22 2.98 3.08 Mn6 3.20 2.93 3.07 Mn7 3.01 2.76 2.89 Mn8 1.86 1.72 1.78 Mn9 3.21 2.93 3.08 BVS AssignmentGroup O3 1.81 O2O2O4 1.78 O2O2O5 1.93 ROdmpmO8 1.73 O2O2O11 1.71 O2O2O16 1.84 O2O2O19 1.84 O2O2O20 1.88 ROdmpmO27 1.23 ROH dmpmH O28 0.22 H2O H2O O29 1.88 O2O2a The italized value is the one closest to the charge for which it was calculated. The oxidation state of a particular atom can be taken as th e nearest whole number to the italized value. b A BVS in the ~1.8-2.0, ~1.0-1.2, and ~0.2-0.4 ranges for an O atom is indicative of non-, singleand double-protonation, re spectively, but can be altered somewhat by hydrogen bonding. Complexes 1 and 2 join only a handful of structural ly characterized Mn clusters of nuclearity eighteen, which currently co mprise the metal oxidation states MnII 2MnIII 16,60 MnII 3MnIII 15,61 MnIII 18,62 and MnIII 16MnIV 2,63 and thus become the first members of the MnII 4MnIII 14 subfamily. The [Mn18O14] core of 1 and 2 is similar to that in the [Mn18O14(O2CMe)18(hep)4(hepH)2(H2O)2]2+ cation,60 where hepH is 2-(hydroxyethyl)pyridine,

PAGE 45

45 but the oxidation st ate description (2MnII, 16MnIII) and the magnetic properties (vide infra) are distinctly different. 2.3.2.3 [Mn8O2(O2CPh)10(dmpm)4(H2O)2] (3) The partially labeled structur e and stereoview of complex 3 are shown in Figure 2-5. Selected interatomic distances and angl es are listed in Table 2-6. Complex 3 crystallizes in the triclinic space group P 1 with the octanuclear molecule in general position. The structure consists of a [Mn8] unit, which is lying on a crystallogr aphic inversion cente r. The overall [Mn8( 4O)2( 3-OR)4( -OR)4]6+ core of 3, is depicted in Figure 2-6. The two 4-O atoms (O1 and O1) are O2ions, the four 3-O (O3, O4, O3, and O4) are from the alkoxide arm (RO-) of dmpmligands, and the four -O atoms (O8, O13, O8, and O13) belong to two different PhCO2 groups. The core of 3 can be described as two distorted c ubanes, edge-linked together at the Mn4-O1 and Mn4-O1 edges, with O8 and O8 providing additional bridges. Charge considerations and an inspection of the metric parameters indicated a 6MnII, 2MnIII description for 3. This was confirmed quantitatively by bond valence sum (BVS) calculations (Table 2-7), which identified Mn4 and Mn4 as the MnIII ions, and the others as MnII. The former was also consistent with the Jahn-Teller (JT) axial elongations, as e xpected for high-spin d4 ions in near-octahedral geometr y, which again avoid the O2ions and lie on O8 and O14 (O8-Mn-O14 = 164.2(1)o) from the PhCO2 ligands. The four dmpmanions bind as bidentate bridging in the 3: 1: 3 mode. The protonation levels of the O atom s were also confirmed by BVS calculations (Table 2-7), and established deprotonation of all the dmpmgroups, as also expected from their bridging modes. Peripheral ligation about the core is provided by ten PhCO2 ions and two terminal H2O (O2 and O2) molecules on Mn1 and Mn1. The ten PhCO2 groups are arranged into three classes; two are bridging in their common 1: 1: mode, four in the 1: 2: 3 mode, and two are monodentate, acting as terminal liga nds on Mn3 and Mn3. The dangling O atoms (O10

PAGE 46

46 and O10) of the latter groups form two str ong, intramolecular hydrogen-bonds with the terminal water molecules (O10O2 = 2.729(1) ). Figure 2-5. Molecular st ructure of complex 3. A) Partially labelled P ovRay representation. B) Stereopair. Hydrogen atoms omitted for clarity. Color scheme: MnII, yellow; MnIII, blue; O, red; N, green; C, gray.

PAGE 47

47 Figure 2-6. Labeled PovRay repres entation of the complete [Mn8( 4-O)2( 3-OR)4( -OR)4]6+ core of 3. Color scheme: MnII, yellow; MnIII, blue; O, red; C, gray. Table 2-6. Selected interatomic distances () and angles (deg) for 3MeCNa Parameter Parameter Mn1Mn2 3.406(3) Mn2Mn3 3.304(2) Mn1Mn3 3.388(6) Mn2Mn4 3.128(3) Mn1Mn4 3.489(5) Mn3Mn4 3.906(3) Mn1-O1 2.223(3) Mn3-O3 2.179(2) Mn1-O2 2.152(3) Mn3-O4 2.461(3) Mn1-O3 2.206(5) Mn3-O6 2.119(6) Mn1-O8 2.230(2) Mn3-O9 2.045(3) Mn1-O12 2.109(2) Mn3-O13 2.214(6) Mn1-O13 2.156(1) Mn3-N3 2.182(2) Mn2-O1 2.213(5) Mn4-O1 1.881(6) Mn2-O3 2.236(3) Mn4-O4 1.964(1) Mn2-O4 2.256(2) Mn4-O8 2.429(3) Mn2-O5 2.130(4) Mn4-O11 1.956(1) Mn2-O7 2.071(3) Mn4-O14 2.233(1) Mn2-N1 2.184(1) Mn4-O1' 1.892(1) Mn1-O1'-Mn2 100.3(3) Mn2-O4-Mn3 88.8(5) Mn1-O3-Mn3 101.2(2) Mn2-O4-Mn4 95.4(3) Mn1-O13-Mn3 101.6(2) Mn2-O1'-Mn4 99.0(3) Mn1-O1'-Mn4 115.8(5) Mn2-O1'-Mn4' 142.2(2) Mn1-O1-Mn4' 103.9(7) Mn3-O4-Mn4 123.5(4) Mn1-O8'-Mn4' 88.0(4) Mn4-O1'-Mn4' 96.1(2) Mn2-O3-Mn3 97.0(4) a Primed and unprimed atoms are related by symmetry.

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48 Table 2-7. Bond valence sum (BVS)a b calculations for Mn and selected oxygen atoms in 3 Atom MnII MnIII MnIV Mn1 1.97 1.80 1.89 Mn2 2.06 1.90 1.96 Mn3 2.04 1.88 1.94 Mn4 3.07 2.81 2.95 BVS AssignmentGroup O1 1.81 O2O2O2 0.33 H2O H2O O3 1.93 ROdmpm-O4 1.88 ROdmpma The italized value is the one closest to the charge for which it was calculated. The oxidation state of a particular atom can be taken as th e nearest whole number to the italized value. b A BVS in the ~1.8-2.0, ~1.0-1.2, and ~0.2-0.4 ranges for an O atom is indicative of non-, singleand double-protonation, re spectively, but can be altered somewhat by hydrogen bonding. Complex 3 joins a family of Mn clusters of nucle arity eight, which currently comprise the metal oxidation levels MnII 8,64 MnII 6MnIII 2,65 MnII 4MnIII 4,17b,66 MnII 2MnIII 6,51,67 MnIII 8,39b,68 and MnIII 2MnIV 6,69 and thus becomes the fourth member of the MnII 6MnIII 2 subfamily. The octanuclear Mn clusters possess a variety of metal topologies su ch as rodlike, serpentine, rectangular, linked butterfly units linked tetrahedral, edge-s haring cubane units with an additional Mn ion etc., but none have reproduced exactly the core of 3. 2.3.2.4 [Mn6O2(O2CBut)10(py)4] (4) and [Mn6O2(O2CPhCl)10(py)4] (5) The molecular structures of complexes 4 and 5 are shown in Figures 2-7 and 2-8, respectively. Selected interatomic distances and angles for 4 and 5 are listed in Tables 2-8 and 29, respectively. The structures of the two hexanuc lear complexes are similar in many aspects, except from the nature of the carboxylate ligation (ButCO2 ( 4) vs. PhClCO2 ( 5)); thus, only the structure of 4 will be described in detail. The structure of complex 4 consists of six Mn ions arranged as two edge-sharing tetrahedra. The overall [Mn6( 4-O)2( -OR)4]6+ core of 4 is depicted in Figure 2-9.

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49 Figure 2-7. Partially labelled PovR ay representation of complex 4, with hydrogen atoms omitted for clarity. Color scheme: MnII, yellow; MnIII, blue; O, red; N, green; C, gray. Figure 2-8. Partially labelled PovR ay representation of complex 5, with hydrogen atoms omitted for clarity. Color scheme: MnII, yellow; MnIII, blue; Cl, purple; O, red; N, green; C, gray.

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50 Table 2-8. Selected interatomic distances () and angles (deg) for 4MeCN Parameter Parameter Mn1Mn2 3.170(1) Mn2Mn6 3.484(2) Mn1Mn3 6.010(1) Mn3Mn4 3.739(2) Mn1Mn4 4.849(1) Mn3Mn5 3.497(2) Mn1Mn5 3.504(2) Mn3Mn6 4.842(1) Mn1Mn6 3.729(1) Mn4Mn5 3.193(1) Mn2Mn3 3.170(1) Mn4Mn6 6.030(1) Mn2Mn4 3.515(2) Mn5Mn6 3.177(2) Mn2Mn5 2.803(8) Mn4-O1 2.135(3) Mn1-O2 2.308(3) Mn4-O4 2.348(3) Mn1-O3 2.135(3) Mn4-O13 2.160(3) Mn1-O7 2.164(3) Mn4-O20 2.125(3) Mn1-O18 2.110(3) Mn4-O23 2.190(2) Mn1-O24 2.184(2) Mn4-N1 2.321(3) Mn1-N2 2.297(6) Mn5-O4 2.231(3) Mn2-O2 2.253(3) Mn5-O12 2.237(3) Mn2-O5 1.959(3) Mn5-O14 1.951(3) Mn2-O8 1.945(3) Mn5-O16 1.969(3) Mn2-O10 2.242(3) Mn5-O23 1.890(2) Mn2-O23 1.885(2) Mn5-O24 1.887(2) Mn2-24 1.888(2) Mn6-O9 2.121(3) Mn3-O6 2.175(3) Mn6-O12 2.332(3) Mn3-O10 2.327(3) Mn6-O15 2.181(3) Mn3-O11 2.121(3) Mn6-O17 2.119(3) Mn3-O19 2.102(3) Mn6-O24 2.169(2) Mn3-O23 2.183(2) Mn6-N4 2.380(6) Mn3-N3 2.305(3) Mn1-O2-Mn2 88.03(9) Mn2-O24-Mn5 95.9(1) Mn1-O24-Mn2 102.0(1) Mn2-O24-Mn6 118.2(1) Mn1-O24-Mn5 118.6(1) Mm3-O23-Mn4 117.6(1) Mn1-O24-Mn6 117.9(1) Mn3-O23-Mn5 118.1(1) Mn2-O10-Mn3 87.8(9) Mn4-O4-Mn5 88.4(9) Mn2-O23-Mn3 102.1(1) Mn4-O23-Mn5 102.8(1) Mn2-O23-Mn4 119.1(1) Mn5-O12-Mn6 88.1(9) Mn2-O23-Mn5 95.9(1) Mn5-O24-Mn6 102.9(1) Table 2-9. Selected interatomic distances () and angles (deg) for 5a Parameter Parameter Mn1Mn2 3.173(6) Mn2Mn3 3.170(6) Mn1Mn3 6.042(1) Mn3Mn3' 3.878(3) Mn1-O2 2.195(2) Mn2-O9 1.978(2) Mn1-O3 2.100(2) Mn2-O11 1.889(2) Mn1-O4 2.189(2) Mn2-O1' 2.207(2) Mn1-O6 2.220(1) Mn3-O10 2.192(2)

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51 Table 2-9 Continued. Mn1-O7 2.241(2) Mn3-O11 2.220(2) Mn1-N1 2.260(2) Mn3-O12 2.126(2) Mn2-O5 1.948(2) Mn3O1' 2.265(2) Mn2-O6 1.890(2) Mn3O8' 2.166(2) Mn2-O7 2.221(2) Mn3-N2 2.253(3) Mn1-O6-Mn1' 120.0(2) Mn2-O11-Mn2' 95.7(2) Mn1-O6-Mn2 100.8(2) Mn2-O11-Mn3 100.7(2) Mn1-O7-Mn2 90.7(7) Mn2-O11-Mn3' 118.0(2) Mn1-O6-Mn2' 119.0(2) Mn2-O1'-Mn3' 90.3(7) Mn2-O6-Mn2' 95.7(2) Mn3-O11-Mn3' 121.7(2) a Primed and unprimed atoms are related by symmetry. Table 2-10. Bond valence sum (BVS)a b calculations for Mn and selected oxygen atoms in 4 and 5 4 Atom MnII MnIII MnIV Mn1 1.95 1.80 1.87 Mn2 3.19 2.92 3.06 Mn3 1.95 1.80 1.86 Mn4 1.89 1.75 1.81 Mn5 3.18 2.91 3.05 Mn6 1.88 1.73 1.80 BVS Assignment O23 1.93 O2O24 1.94 O25 Atom MnII MnIII MnIV Mn1 1.93 1.79 1.85 Mn2 3.20 2.92 3.07 Mn3 1.92 1.77 1.83 BVS Assignment O6 1.88 O2O11 1.88 O2a The italized value is the one closest to the charge for which it was calculated. The oxidation state of a particular atom can be taken as th e nearest whole number to the italized value. b A BVS in the ~1.8-2.0, ~1.0-1.2, and ~0.2-0.4 ranges for an O atom is indicative of non-, singleand double-protonation, re spectively, but can be altered somewhat by hydrogen bonding. At the center of each tetrahedron lies a 4-O2ion (Figure 2-10). Peripheral ligation about the core is accomplished by ten bridging pivala te groups and four term inal pyridine ligands.

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52 Figure 2-9. Labeled PovRay repres entation of the complete [Mn6( 4-O)2( -OR)4]6+ core of 4. Color scheme: MnII, yellow; MnIII, blue; O, red; C, gray. Each Mn center is six-coor dinate and possesses distorted octahedral geometry. Compound 4 is a mixed-valence MnII 4MnIII 2 cluster and the MnIII centers are assigned as the central Mn2 and Mn5. The MnII/MnIII oxidation states were confirmed using a combination of charge balance configurations, BVS calculations (Table 2-10), a nd the presence of JT distortions, expected for octahedral MnIII ions. Thus, Mn(2,5)-O(23,24) distances (average 1.888 ) are appreciably shorter than Mn(1,3,4,6)-O(23,24) (a verage 2.182 ), consistent with the higher oxidation state in the former. Similarly, Mn(2,5)-Ocarboxylate distances (average 2.098 ) are noticeably shorter than Mn(1,3,4,6)-Ocarboxylate distances (average 2.185 ). The MnIII-MnIII pair is consequently bridged by two 4-O2(see Table 2-10), whereas each MnII-MnIII pair is bridged only by one 4O2-. The ten ButCO2 groups are separated in to two classes: six are with each of their

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53 carboxylate oxygen atoms being terminally ligated to a Mn center, while the remaining four are 3 with one carboxylate oxygen atom terminally ligated to a MnII, whereas the other carboxylate oxygen is bridging a MnII-MnIII pair; the latter four oxygen atoms are O(2,4,10,12). The two MnIII atoms also show the JT distortion e xpected for an octahedral high-spin d4 ion, taking the form of an axial elongation with the -oxygen atoms O(2,4,10,12) of the 3-carboxylate groups occupying axial positions. Finally, each remaining terminal position at the MnII sites is occupied by the four pyridine groups. In addition to the edge-sharing te trahedra description of the Mn6O2 core (Figure 2-10), two alternative ways of describing it can be presented that emphasi ze its structural relationship to smaller nuclearity Mn/O units: (i) the Mn6O2 unit can be considered as two [Mn3O]5+ units, joined together by each of the 3-O2atoms becoming 4 by ligating to the MnIII center of the adjacent Mn3O unit. The two [MnII 2MnIIIO]5+ units comprising the [Mn6O2]10+ core of 4 are Mn(1,5,6)O(24) and Mn(2,3,4)O(23) or, alternativ ely, Mn(1,2,6)O(24) and Mn(3,4,5)O(23). (ii) The [Mn6O2]10+ core can be considered to contain the [MnII 2MnIII 2O2]10+ core of [Mn4O2(O2CMe)6(bpy)2].70 The latter possesses a planar Mn4 rhombus with the two 32bridges, one above and one below the plane. This unit is found in 4 Mn(1,2,3,5)O(23,24) or Mn(2,4,5,6)O(23,24), and completion of the Mn6O2 core then requires merely the conversion of the two 3-O2to 4-O2by ligation to an additional MnII center. The short MnIIIMnIII (2.803(8) ) and long MnIIMnIII (3.17-3.52 ) distances compar e favorably with those in [Mn4O2(O2CMe)6(bpy)2] (2.779(1) and 3.3-3.5 respectivel y). Also note that, although the oxidation levels do not correspond, the Mn6O2 unit also contains the nonplanar butterfly-like Mn4O2 unit found in [MnIII 4O2(O2CMe)7(bpy)2]+.70 Such a unit in 4 would be that formed by Mn(1,2,5,6)O(23,24) or Mn(2,3,4,5)O(23,24), with Mn (2,5) representing the hinge or

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54 backbone positions, and completion of Mn6O2 again requires conversion of 3-O2to 4-O2by ligation to additional Mn sites. Thus, the planar and butterfly-like Mn4O2 units represent the products from two possible ways of removing two Mn atoms from the [Mn6O2]10+ core (Figure 2-11). The third possibility would yield a [Mn4( -O)(4-O)]6+ unit; this is currently unknown. Figure 2-10. Labeled PovRay representation of the [Mn6O2]10+ core of 4. Color scheme: MnII, yellow; MnIII, blue; O, red. O Mn Mn Mn Mn O Mn Mn planar Mn4O2 O Mn Mn Mn Mn O Mn Mn O Mn Mn Mn Mn O Mn Mn butterfly Mn4O2 Figure 2-11. Three ways of describing the [MnII 4MnIII 2( 4-O)2]10+ core present in complexes 4 and 5.

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55 2.4 Magnetochemistry 2.4.1 Direct Current Magnetic Susceptibility Studies Variable-te mperature magnetic susceptibility measurements were performed on powdered polycrystalline samples of 15, restrained in eicosane to preven t torquing, in a 1 kG (0.1 T) field and in the 5.0-300 K range. The obtained data are shown as T vs T plot in Figures 2-12 2-15. The magnetic data for the hexanuclear complexes 4 and 5 are identical, and therefore only those for the former will be discussed. The T value for 1 steadily decreases from 40.73 cm3Kmol-1 at 300 K to 6.99 cm3Kmol-1 at 5.0 K (Figure 2-12), while T for 2 shows a similar magnetic re sponse and therefore steadily decreases from 41.31 cm3Kmol-1 at 300 K to a value of 6.74 cm3Kmol-1 at 5.0 K (Figure 2-13). The 300 K value for both octadecanuclear complexes 1 and 2 is much less than the spin-only ( g = 2) value of 59.5 cm3Kmol-1 for four MnII and fourteen MnIII non-interacting ions, indicating the presence of predominant an tiferromagnetic exchange inter actions. The 5.0 K value for both 1 and 2 is consistent with a low, but possibly non-zero, ground state S value. Given the size and low-symmetry of the Mn18 molecules, and the resulting num ber of inequivalent exchange constants, it is not possible to apply the Kambe method71 to determine the individual pairwise Mn2 exchange interaction parameters, and we c oncentrated instead on characterizing the ground state spin, S, and the zero-field splitting parameter, D Thus, magnetization (M ) data were collected in the magnetic field and temperat ure ranges 1-70 kG and 1.8-10.0 K. However, we could not get an acceptable fit using data collected over the whole field range, which is a common problem caused by low-lying excited states, especially if some have an S value greater than that of the ground state, as is the case for 1 and 2 on the basis of Figures 2-12 and 2-13, respectively.

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56 Temperature (K) 050100150200250300 MT (cm3 mol-1 K) 5 10 15 20 25 30 35 40 45 Figure 2-12. Plot of MT vs T for complex 1 in a 1 kG dc field. Temperature (K) 050100150200250300 MT (cm3 mol-1 K) 5 10 15 20 25 30 35 40 45 Figure 2-13. Plot of MT vs T for complex 2 in a 1 kG dc field. A common solution is to only use da ta collected with low fields ( 1.0 T), as we previously reported for many mixed-valence MnII/MnIII clusters.72 However, it was still not possible to obtain a satisfactory fit assuming that only the ground state is populated in this temperature range. This suggests that low-ly ing excited states are populated, even at these relatively low temperatures.

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57 The T value for 3 gradually decreases from 28.58 cm3Kmol-1 at 300 K to 18.11 cm3Kmol-1 at 5.0 K (Figure 2-14). The 300 K value is slightly less than the spin-only ( g = 2) value of 32.25 cm3Kmol-1 for six MnII and two MnIII non-interacting ions, indicating the presence of dominant antiferromagnetic exchange interactions. The 5.0 K value is consistent with an S = 6 ground state, with a g factor slightly less than 2.0, as expected for a MnII/MnIII complex; the spin-only value for S = 6 is 21.0 cm3Kmol-1. Temperature (K) 050100150200250300 MT (cm3 mol-1 K) 16 18 20 22 24 26 28 30 Figure 2-14. Plot of MT vs T for complex 3 in a 1 kG dc field. Again, the high nuclearity and low symmetry of the complex make it extremely difficult to evaluate the various exchange parameters using the Kambe approach, and we concentrated instead on characterizing the ground state spin, S and the zero-field splitting parameter, D Attempted fits of magnetization ( M ) data collected at various fiel ds and at low temperature (<10 K), and assuming that only the ground state is populated, were poor, s uggesting again population of low-lying excited states, as expected for high nuclearity complexes, especially given its large content of MnII atoms, since these usually give very weak exchange interactions and small energy splitting.

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58 The T vs T data for the representative hexanuclear complex 4 are shown in Figure 2-15. Very similar behavior to the previ ous complexes was observed, with the T decreasing from 17.46 cm3Kmol-1 at 300 K to 1.11 cm3Kmol-1 at 5.00 K. The rate of decrease increases below ~50 K. The room temperature T value is less than the 23.5 cm3Kmol-1 spin-only value ( g = 2.0) expected for a 4MnII, 2MnIII complex with non-interacting centers, indicati ng the presence of appreciable antiferromagnetic in teractions between the Mn ions. 050100150200250300 0 2 4 6 8 10 12 14 16 18 20 M (cm3 mol-1 K)Temperature (K) Figure 2-15. Plot of MT vs T for complex 4 in a 1 kG dc field. The T vs T data appear to be heading for 0 at 0 K and thus suggest an S = 0 ground state. Because the complex contains multiple MnII ions that are known to promote weak exchange interactions, clear iden tification of the ground state by magnetiz ation versus field data would be precluded by the low-lying excited states crossing with the ground state in the applied fields of several Tesla. An S = 0 ground state indicated for 4 (and 5, vide infra) is not surp rising given its structural similarity to the family of previous hexanuclear compounds containing the [MnII 4MnIII 2( 4)2]10+ core, which have been found to have S = 0. Since typically MnIIMnII and MnIIMnIII

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59 exchange interactions are weak and antiferromagnetic, the apparent S = 0 ground state of 4 and 5 seems to be a consequence of the well establ ished strong antiferroma gnetic coupling at the central MnIII 2O2 unit of the molecules. This strong, dominating coupling will give an S = 0 ground state for the complete molecules. 2.4.2 Alternating Current Magnetic Susceptibility Studies As we have describ ed before on multiple occasions,73 alternating current (ac) susceptibility studies are a powerful complement to dc studies for determini ng the ground state of a system, because they preclude any complications arising fr om the presence of a dc field. We thus carried out detailed ac studies on complexes 13, which clearly possess nonzero ground state spin values S, as an independent determin ation of their ground states S, and also to study magnetization dynamics. Ac studies were performed in the 1.8-15 K ra nge using a 3.5 G ac field oscillating at frequencies in the 50-1500 Hz ra nge. If the magnetization vector can relax fast enough to keep up with the oscillating field, then there is no imaginary (out-of-phase) susceptibility signal ( M), and the real (in-phase) susceptibility ( M) is equal to the dc susceptibility. However, if the barrier to magnetization relaxation is significant compared to the thermal energy (kT ), then M decreases and there is a non-zero M. In addition, M will be frequency-dependent. Such frequency-dependent M signals are a characteristic signa ture of the superparamagnetic-like properties of a SMM, but by themselves do not prove the SMM behavior.74 For complexes 1 and 2, the ac data are shown in Figures 2-16 and 2-17, respectively, and reveal several per tinent features: (i) MT for both 1 (Figure 2-16A) and 2 (Figure 2-17) decreases linearly with decreasing temperature in the whol e temperature range, indicating depopulation of a high density of excite d states with spin S greater than that of the ground state, in agreement with the conclusion from the dc studies;

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60 Temperature (K) 0246810121416 M'T (cm3 mol-1 K) 0 2 4 6 8 10 12 14 16 18 20 1500 Hz 1000 Hz 500 Hz 250 Hz 50 Hz A Temperature (K) 0246810121416 M'T (cm3 mol-1 K) 0 2 4 6 8 10 12 14 16 18 20 1500 Hz 1000 Hz 500 Hz 250 Hz 50 Hz A Temperature (K) 0246810121416 M'' (cm3 mol-1) -2 -1 0 1 2 1500 Hz 1000 Hz 500 Hz 250 Hz 50 Hz B Temperature (K) 0246810121416 M'' (cm3 mol-1) -2 -1 0 1 2 1500 Hz 1000 Hz 500 Hz 250 Hz 50 Hz B Figure 2-16. The ac magnetic susceptibility measurements of complex 1 in a 3.5 G field oscillating at the indica ted frequencies. A) Plot of the in-phase ( M) (as MT ) signals. B) Plot of the out-of-phase ( M) ac signals. (ii) extrapolation of the MT data from above ~3.0 K to 0 K gives a value of ~2.6 cm3Kmol-1 for both 1 and 2 indicative of an S = 2 ground state with g ~ 1.86. We cannot rule out the possibility that it is an S = 0 or 1 ground state with very low-lying S > 0 or 1 excited states, respectively. In

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61 any event, complexes 1 and 2 clearly have a very small ground state spin. As expected, there were no out-of-phase ac signals down to 1.8 K (Figure 2-16B). Temperature (K) 0246810121416 M'T (cm3 mol-1 K) 0 2 4 6 8 10 12 14 16 18 20 1000 Hz 250 Hz 50 Hz Figure 2-17. Plot of the in-phase ( M) (as MT ) ac susceptibility signals of complex 2 in a 3.5 G field oscillating at the indicated frequencies. Point (i) rationalizes the fact that we were unable to get a satisfactory fit of the dc magnetization data. The fitting procedure assu mes only the ground state is populated, and its failure is clearly due to a combination of (a) many low-lying excited states whose population is difficult to avoid even at the lowest te mperatures employed, and (b) the larger S value compared with the ground state of at le ast many of these low-lying ex cited states will mean that MS levels of the former will approach and even cross those of the latter, exacerbating the fitting difficulties. Of course, both (a) and (b) are to be anticip ated for such high nuclearity complexes as 1 and 2, especially, as we mentioned be fore, given its content of MnII atoms, since these usually give very weak exchange interactions and small energy spl itting. The ac experiment simplifies the situation by dispensing with the dc field completely: lowlying excited states then simply appear as a sloping MT vs T plot, and extrapolation to 0 K, at wh ich only the ground state will be populated, yields the true ground state S value. The only problem that might occasionally appear is the

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62 presence of weak, usually antiferromagnetic, inte rmolecular exchange interactions, but even this can be circumvented by avoiding beginning the extrapolation at too low a temperature. For complex 3, the in-phase MT signal below 15 K again decreases linearly with decreasing temperature to the 4 5 K region, be fore further decreasi ng (more rapidly) at T < 5 K (Figure 2-18). Extrapolation of the plot to 0 K from above 5 K (to avoid the decrease at lower T due to anisotropy, Zeeman effects, weak intermol ecular interactions, etc) gives a value of ~18 cm3Kmol-1. This indicates an S = 6 ground state with g ~ 1.85, in excellent ag reement with the dc magnetic susceptibility studies. Unfort unately, given the small number of MnIII atoms (the basic origin of the expected molecular anisotropy of a Mn system) present in the molecule, complex 3 did not exhibit an out-of-phase ac magnetic su sceptibility signal down to 1.8 K (Figure 2-19), indicating that it does not e xhibit a barrier large enough (vs kT ) to show the superparamagnetlike slow relaxation of its magnetization vector, i.e. it is not a SMM. Studies at much lower temperatures would be required to search for what would at best be a tiny relaxation barrier. Temperature (K) 0246810121416 M'T (cm3 mol-1 K) 16 17 18 19 20 21 22 23 24 1000 Hz 500 Hz 250 Hz 50 Hz Figure 2-18. Plot of the in-phase ( M) (as MT ) ac susceptibility signals of complex 3 in a 3.5 G field oscillating at the indicated frequencies.

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63 Temperature (K) 0246810121416 M'' (cm3 mol-1) -1.0 -0.5 0.0 0.5 1.0 1000 Hz 250 Hz 50 Hz Figure 2-19. Plot of the out-of-phase (M) ac susceptibility signals of complex 3 in a 3.5 G field oscillating at the i ndicated frequencies. 2.5 Conclusions We have reported the employm ent of a new N, O-based bidentate chelat e in transition metal cluster chemistry, one that emphasizes the coordinating ability and versatility of pyrazole-based alkoxide ligands in coordinati on chemistry. The resulting anion of 3,5-dimethylpyrazole-1methanol, dmpmH, has been employed in mangan ese chemistry, and it has provided clean access to three new polynuclear, valenc e-trapped, Mn(II/III) clusters, 13, as well as to two chemicallyrelated hexanuclear molecules. In particular, the reaction between [Mn3O(O2CR)6(py)2(H2O)] (R = Me, Et) and dmpmH has led to the octadecanuclear complexes [Mn18O14(O2CR)18(dmpm)4(dmpmH)2(H2O)2] (R = Me ( 1), Et ( 2)), both possessing a rare discshaped metal topology. In contrast, a simila r reaction but with the benzoate-analogue, [Mn3O(O2CPh)6(py)2(H2O)] led instead to the octanuclear [Mn8O2(O2CPh)10(dmpm)4(H2O)2] ( 3) cluster with an interes ting edge-linked cubane Mn8O2 core. Moreover, when the carboxylate employed was pivalate or chlorobenzoate the dmpmH-free, [Mn6O2(O2CR)10(py)4] (R = But ( 4),

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64 PhCl ( 5)) products were resulted, both with an edge-sharing tetrahedra core As a result of the coordination characteristics of dmpmligand, complex 3 has a significant ground state spin value of S = 6, while for complexes 1 and 2 the exact determination of the ground state spin value is currently unfeasible due to their complicated stru ctures and presence of spin frustration effects within the many triangular units. It will be interes ting to determine, as this work is extended, to what extent dmpmwill continue to provide a route to ne w metal clusters and to what extent these are related to clusters provided by si mple pyrazole (pzH) or pyridyl alcohols (hmpor hep-) alone. We have actually no reason to believe that this research area is exhausted of new results. Indeed, ongoing studies are producing additional and interesting pr oducts, and our belief is that we have scratched only the surface of metal cluster chemistry based on pyrazole-based alkoxide ligands.

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71 BIOGRAPHICAL SKETCH Konstantina Pringouri w as born and raised in Athens, Greece in 1983. She finished her high school studies in 2001 and at the same year she entered the Department of Chemistry at the University of Patras in Greece. During the 4th year of her chemistry studies, she carried out undergraduate research in Professors Spyros P. Perlepes laboratories in the context of her Diploma Thesis. She was involved in the area of 3d-metal cluster chemistry, and particularly concentrated her synthetic efforts in cobalt and chromium carboxylate chemistry. She received her Bachelor of Science degree in April 2006. In the fall of the same year, Konstantina joined the group of Professor George Christou at the Chemistr y Department of the University of Florida. Her current research interests involve the synthesis and magnetochemical characterization of polynuclear manganese carboxylate clus ters in search for high-nuc learity, high-spin molecules and single-molecule magnets.