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1 NEW MANGANESE-CERIUM CLUSTERS WI TH NOVEL TOPOLOGIES: SYNTHESIS, XRAY CRYSTALLOGRAPHY, AND MAGNETIC CHARACTERIZATION OF TWO UNPRECEDENTED HEXAAND DECA METALLIC MIXED METAL CAGES By CHRISTOS LAMPROPOULOS 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 2007
2 2007 Christos Lampropoulos
3 To my parents Athanasios and Aggeliki
4 ACKNOWLEDGMENTS I would like to thank m y advisor, Professor George Christou for his guidance, help and understanding all these years. Also, I would like to acknowledge Associate Professor Stephen Hill and Dr. Benjamin Smith for serving as my gr aduate committee, for all the help, as well as for all the interesting and stimulating discussions. Also, I would like to thank Dr. Khalil A. Abboud and all the staff of the Center for X-ra y Crystallography at UF, for all the crystal structures, and the patience and time that the x-ray analysis requi res. Additionally, my work was highly stimulated by two post-docto ral fellows that I had the pleas ure to work with, and learn a lot from, Dr. Muralee Murugesu and Dr. Theochar is C. Stamatatos. Especially, I would like to acknowledge Theocharis for his unconditional friendship and support throughout my career, as well as the vast amount of information that he bombarded me with every day in the lab, during coffee breaks, and at home (since he was my roo mmate). Finally, I thank my parents for all the support, love and patience througho ut the years I have lived away from them, for always being there in every situation, every bad moment, as well as for every celebration in my life.
5 TABLE OF CONTENTS page ACKNOWLEDGMENTS...............................................................................................................4LIST OF TABLES................................................................................................................. ..........6LIST OF FIGURES.........................................................................................................................7ABSTRACT.....................................................................................................................................9CHAPTER 1 INTRODUCTION..................................................................................................................102 MANGANESE-CERIUM CLUSTER CHEMIS TRY: A POSSIBLE ROUTE TO NEW DENOX CATAL YSTS AND MIXED-METAL SMMS....................................................... 242.1 Introduction............................................................................................................... ....242.1.1 MnIV-CeIV Complexes: Synthetic an d Structural Aspects................................ 262.1.2 Mn-Ce SMMs: A Magnetism Overview.......................................................282.2 Experimental Section.................................................................................................... 292.2.1 Synthesis........................................................................................................... 2220.127.116.11 [Mn4Ce6O12(O2CMe)10(NO3)4(py)4] py2H2O (1pyH2O).............. 218.104.22.168 [Mn4Ce2O2(O2CMe)6(NO3)4(hmp)4]H2O (22H2O)........................ 302.2.2 X-ray Crystallography....................................................................................... 302.3 Results and Discussion.................................................................................................. 332.3.1 Synthesis........................................................................................................... 332.3.2 Description of Structures.................................................................................. 322.214.171.124 [Mn4Ce6O12(O2CMe)10(NO3)4(py)4] py2H2O...................................3126.96.36.199 [Mn4Ce2O2(O2CMe)6(NO3)4(hmp)4]..................................................382.4 Magnetic Susceptibility Studies.................................................................................... 412.4.1 Direct Current Magnetic Susceptibility Studies................................................ 4188.8.131.52 [Mn4Ce6O12(O2CMe)10(NO3)4(py)4] py2H2O...................................4184.108.40.206 [Mn4Ce2O2(O2CMe)6(NO3)4(hmp)4]H2O........................................442.4.2 Alternating Current Magnetic Susceptibility Studies....................................... 4220.127.116.11 [Mn4Ce6O12(O2CMe)10(NO3)4(py)4] py2H2O...................................418.104.22.168 [Mn4Ce2O2(O2CMe)6(NO3)4(hmp)4]H2O:....................................... 492.5 Conclusions................................................................................................................ ...51LIST OF REFERENCES...............................................................................................................53BIOGRAPHICAL SKETCH.........................................................................................................58
6 LIST OF TABLES Table page 2-1 Crystal data and structure re finem ent parameters for complex 1 ......................................322-2 Crystal data and structure re finement parameters for complex 2 ......................................322-3 Selected bond distances ( ) and angles () for complex 1 ................................................362-4 Bond valence sum calculations and assignments for the Ce ions in 1 ...............................362-5 Bond valence sum calculations a nd assignments for the Mn ions..................................... 382-6 Bond valence sum calculations and assignments for the triply-bridging oxygen ions...... 382-7 Selected bond distances ( ) and angles () for complex 2 ................................................392-8 Bond valence sum calculations and assi gnments for the Mn and Ce ions in 2 .................39
7 LIST OF FIGURES Figure page 1-1 Representation of the different ways the m agnetic field lines of flux change in the absence of a sample, in the presence of a di amagnetic sample, and in the presence of a paramagnetic sample....................................................................................................... 111-2 Plot of the MT product ( M is the molar susceptib ility) vs. temperature.......................... 121-3 Magnetic dipole arrangements in different types of materials........................................... 131-4 Typical hysteresis loop of a magnet................................................................................... 141-5 ORTEP representation in Pov-Ray fo rmat showing the structure of the [Mn12O12(O2CMe)16(H2O)4] complex................................................................................181-6 ORTEP representation in Pov-Ray form at of the metal core of a typical [Mn12O12(O2CR)16(H2O)4] complex..................................................................................191-7 Representative plots of the potential energy barier for an SMM....................................... 201-8 In-phase (as M T ) and out-of-phase (as M ) AC susceptibility signals for a dried, microcrystalline sample of [Mn12O12(O2CR)16(H2O)4].....................................................211-9 Magnetization hysteresis loops for a typical [Mn12O12(O2CR)16(H2O)4] complex in the 1.3-3.6 K temperature range at a 4 mT/s field sweep rate........................................... 221-10 Schematic representation of the change in energy of ms sublevels as the magnetic field is swept from zero to a non-zero value...................................................................... 222-1 ORTEP representations in Pov-Ray format of several Mn-Ce complexes........................ 272-2 ORTEP representations in Pov-Ray format at the 50% probability level of the X-ray crystal structures of two members of the Mn8Ce family of complexes.............................292-3 ORTEP representations in th e PovRay format of complex 1 ............................................372-4 PovRay representations of complex 2 ................................................................................402-5 Schematic representation of the metal core of 2 and the key structural features............... 412-6 Ordering of the energy states for 1, using the calculated exchange parameter J and the Van Vleck equation......................................................................................................442-7 Plot of MT vs temperature for a dried, microcrystalline sample of complex 1 .................452-8 Plot of MT vs temperature for a dried, microcrystalline sample of complex 2 .................46
8 2-9 Plot of the the in-phase component ( M T) of the AC magnetic susceptibility of complex 1 versus temperature........................................................................................... 482-10 Plot of the out-of-phase ( M) AC susceptibility signal versus temperature for a microcrystalline sample of complex 1 ...............................................................................492-11 Plot of the the in-phase component ( M T) of the AC magnetic susceptibility of complex 2 versus temperature........................................................................................... 502-12 Plot of the out-of-phase ( M) AC susceptibility signal versus temperature for a microcrystalline sample of complex 2 ...............................................................................51
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 MANGANESE-CERIUM CLUSTERS WITH NOVEL TOPOLOGIES: SYNTHESIS, XRAY CRYSTALLOGRAPHY, AND MAGNETIC CHARACTERIZATION OF TWO UNPRECEDENTED HEXAAND DECA METALLIC MIXED METAL CAGES By Christos Lampropoulos December 2007 Chair: George Christou Major: Chemistry Polynuclear discrete clusters incorporating transition metals have been attracting a great deal of attention, due to their ae sthetically pleasing chemical structures, as well as the fascinating magnetic properties they often possess. In particular, many of these cl usters exhibit singlemolecule magnetism (SMM) behavior, a property wh ich is intrinsic to the individual molecules, and makes them capable of functioning as nanos cale magnetic particles. Inorganic chemical research has been also directed towards th e synthesis of high oxi dation-state molecular compounds, such as mixed-3d, 3d/4f, and, 3d/5f clus ters. Such complexes provide a wide range of applications in diverse ar eas involving inorganic, organic, environmental, and industrial chemistry; i.e. MnIV and CeIV oxides are effective catalysts for the conversion of nitric oxide to N2 and O2. In search for new mixed-metal SMMs, and effective catalysts for the NOx conversion process, mixed Mn-Ce compounds were judged to be worth exploring. Keep ing all this in mind, a series of mixed metal Mn-Ce molecular clusters were synthesized and characterized, including two new hexaand decanuclear Mn/Ce molecular cages with many novel structural features.
10 CHAPTER 1 INTRODUCTION Magnetic materials have been influencing hum anity sin ce the ancient times. The discovery and subsequent utilization of magne tic materials have been centra l to significant technological advancements that have aff ected civilization and humankind.1 The ancient Greeks were the first to recognize that the mixe d-valent iron oxide (Fe2O3), magnetite, attracts elemental iron. Since then, magnetic materials have been in the spotlight of research worldwide because magnetism is one of the few phenomena that thrust the technological developmen t. Historically, approximately two thousand years ago the Chinese developed the first magnetic device: the compass. Knowing the importance and contribution of the compass to world trade and travel vi a land, sea or air, one can easily realize the role of magnetic materials to the progre ss of our civilization. Following this early development, magnets have almost become essential to every modern society and have found uses in several magnetomechanical applic ations, acoustic instruments, information and telecommunication devices, electrical motors a nd generators, magnetic shielding, as well as numerous others.2 Modern day magnetic materials in clude magnetic alloys and oxides, particularly ferrites, such as MgFe2O4, which can function in transformer cores, magnetic recording and/or information storage devices.2,3 The motion of electrical charge and specifically the spin a nd orbital angular momenta of electrons within the atoms of a ma terial are responsible for the ma gnetic field associated with the magnetic substance. One would expect that sinc e all matter is composed of electrons, there should be plenty of magnetic materials. However, this is not true and ther e are actually very few materials that behave as magnets. Closed electr on shells is the most common configuration of electrons in most substances, i.e. electrons with magnetic fields aligned in opposite directions are paired with each other and there is no overall net-magnetization. Such materials with negligible
11 magnetic moment are called diamagnets.4,5 Hence, the crucial element that distinguishes a magnetic substance, or a paramagnet, from a diamagnet is the existence of a magnetic moment that arises from at least one unpaired electron. a b c Figure 1-1. Representation of the different ways the magnetic fiel d lines of flux change in the absence of a sample, in the presence of a di amagnetic sample, and in the presence of a paramagnetic sample. (a) magnetic field lines of flux (i.e. contour lines of constant field values) in vacuum; (b) the lines of flux for a diamagnetic substance in a magnetic field; (c) the lines of flux for a paramagnetic substance in a magnetic field There are various types of magnetic material s depending on their response to an external applied magnetic field. As a conseq uence, the magnetic susceptibility, of the material also changes. In Figure 1-1, a, the magnetic field lin es of flux are shown for a magnet in vacuum. Interaction of the electron pair s of a diamagnet and an applied field, generates a repulsive field which weakly repels the diamagnet from the a pplied field and causes th e flux lines to divert around the diamagnetic substance (Figure 1-1, b); the sign of is negative. In contrast, a paramagnet is drawn by an applie d magnetic field; the sign of is positive and hence, the lines of flux are attracted by the paramagneti c substance (Figure 1-1, c). The strength of the attraction is governed both by the number of unp aired electrons in the material as well as the nature of the interactions of its electron spins.1,2,6,7 The various types of para magnetism are distinguished by both the temperature dependence as well as the absolute magnitude of .7 Simple paramagnetic behavior is observed in substa nces in which the magnetic mome nts of unpaired electrons are S N N S N S S N S N N S N S N S N S
12 independent of each other. In the absence of a magnetic field, individu al magnetic moments are randomly oriented. The electron spins of paramagneti c materials align parall el, albeit weakly, to any applied magnetic field, and this effort is opposed to the randomizi ng effect of thermal energy. Removal of the field results in the subseq uent randomization of the spins, which is an entropy-driven process. Therefore, availability of unpaired electrons does not necessarily result in a magnetic material, since a paramagne t can have a net zero magnetization. antiferromagnet M T ferromagnet T antiferromagnet M T ferromagnet T Figure 1-2. Plot of the MT product ( M is the molar susceptibility) vs. temperature. (a) Plot of MT vs. T for a ferromagnet. (b) Plot of MT vs. T for an antiferromagnet In paramagnetic substances that contain multiple metal centers, the magnetic moments of the unpaired spins are not independent, but rather in teract with each other, either in a cooperative manner when there is a parallel alignment of the magnetic moments, or in a non-cooperative way, when there is an overall anti-parallel a lignment of the magnetic moments. The former describes ferromagnetic behavior while the latter is associated with an antiferromagnetic or ferrimagnetic response; antiferromagnetism refers to a complete canceling of magnetic moments while ferrimagnetism corresponds to the situation in which magnetic moments align in an antiparallel fashion but result ing in a non-zero magnetization.1,2,4,7,8 Examples of classical ferromagnets include iron, cobalt, nickel and seve ral rare earth metals and their alloys while ferromagnet antiferromagnet a b
13 magnetite, Fe3O4, is a ferrimagnet.4 Plots of the MT product, where M is the molar susceptibility, vs. temperature (T) are comm only used as a probe for ferromagnetic or antiferromagnetic responses in molecular and/or classical systems. Ferromagnets exhibit increase of the MT signals as temperature decreases (Figure 1-2, a), whereas antiferromagnets show decrease of the MT signals with decreasing temperat ure, as shown in Figure 1-2, b. At all temperatures, ferro-, antiferroand fe rrimagnets are composed of domains, or tiny regions in which all the spins ar e aligned parallel. Each domain aligns randomly with respect to its neighbors. Application of a strong external magnetic fiel d induces the ali gnment of all the domains with the field. As alignment occurs, th e interactions of spin s become strong enough to overcome dipole interactions and entropy considerations that maintain the random alignment of the domains. Effectively, a domain of a specific alignment grows at the expense of a neighboring domain and thus the material becomes magneti zed. Finally, when the magnetization reaches a saturation value all the spins align pa rallel in one part icular direction. ParamagneticFerromagneticAntiferromagnetic Ferrimagnetic Figure 1-3. Magnetic dipole arrangeme nts in different types of ma terials. (a) Paramagnetic, (b) ferromagnetic, (c) antiferromagnetic, and (d) ferromagnetic materials Preservation of this parallel spin alignment af ter the applied field is removed (i.e. remnant magnetization) is not surprisingl y dependent on the systems temperature. The system should be lying below a blocking temperature TB, in order to retain its fe rromagnetic response, in the absence of an external magnetic field. Above TB, the thermal energy ( kT ) is large enough to cause the random alignment of th e electron spins and change the behavior of the material to
14 simple paramagnetic; this fact also explains the MT vs. T behavior of ferroand antiferromagnets at higher temperatures (F igure 1-2). For the decay of the remnant magnetization, a coercive field in the opposite direction is applied, inducing the realignment of the spins in the opposite direction and resulting in a hysteresis l oop (Figure 1-4). For information storage, a small coercive field (high permeability) with a relatively rectangular-shaped hysteresis loop is crucial so that the two ma gnetic orientations of the spin can represent zero and one in the binary digital system used by current technology.4,7,8 One of the requirements for information storage is the retention of the system at a temper ature in which the material exhibits hysteresis while the removal of the stor ed information involves heating to a temperature above TB.8 M H Ms Ms Figure 1-4. Typical hysteresis loop of a magnet, where M is magnetization, H is the applied magnetic field and Ms is the saturation value of the magnetization The United States National Nanotechnology Ini tiative website defines nanotechnology as "the understanding and control of matter at dimensions of r oughly 1 to 100 nanometers, where unique phenomena enable novel applications". The first hard disk drive, RAMAC, introduced by IBM in 1957 had a storage capacity of only 2000 bits in-2 while storage density reached approximately 10 Gbits in-2 in 2000 an increase by a factor of five million.9 Due to the
15 increasing need for the storage of greater quanti ties of digital information on smaller surfaces areas, the development of magnetic particles of nanoscale dimens ions is one of the current intensive interests. Progress in that direction i nvolves the use of smaller materials of nanoscale dimensions that behave as permanent magnets with functional temperatures in the practical range for technological use. In order for scientis ts to bring magnetic materials to the range of nano, two main approaches have been followed: the top-down and the bottom-up approach. The top-down route involves the fragme ntation of bulk ferroor ferrima gnets to a size smaller than a single domain (20-200nm), therefore all the spins within the particle always remain parallel. These particles, also known as superparamagne ts, are composed of randomly oriented spins unless induced by an applied magnetic field. Supe rparamagnets retain their magnetization when their magnetic relaxation is slowed below TB. Unfortunately this approach has been problematic due to the wide distribution of shap es and sizes of these nanoparticles.10 Besides, there is a distribution of barrier heights for the interconversi on of the spins and these materials are insoluble in organic solvents and thus unstable for some applications. Recently, new fragmentation techniques base d on scanning tunneling microsc opy and biomineralization have been devised as a method of improvement of the non-uniform particle size obstacle.11,12 Yet another strategy curr ently being explored in detail is the development of new magnets using molecules as building blocks. Materials of this kind are also called molecule-based magnets, and have the potential to demonstrate characteristics that are unattainable by the conventional metal/intermetallics and metal-oxide magnets used so far. These materials present properties such as low-temperatur e processability, high magnetic su sceptibilities, high solubility, compatibility with polymers for composites, biocompatibility, transparency, semiconducting and insulating properties, high remnant magnetizat ions, as well as several other desirable
16 characteristics.1,13 Paramagnetic organic or inorganic mol ecules with a large number of unpaired electrons are typically used as the building blocks for the preparation of these molecule-based magnets, which rely on long-range intermolecula r interactions to account for their magnetic behavior. Reported in 1967, the firs t molecule-based magnet, [Fe(dtc)2Cl], where dtc = diethyldithiocarbamato, was found to have an S = 3/2 ground state with ferromagnetic ordering at 2.46 K.8,14 This area of multidisciplinary scientific research was subsequently silent, as far as scientific publication is concerned, until 1987 when Miller and co-workers reported a molecular ferromagnet composed of alternating st acks of metallocenium donor cations (D+) and organic radical acceptor anions (A-), each with a single unpaired electron. For the complex [Fe(C5Me5)2]+, where D+ is the decamethylferrocenium cation and Ais the tetracyanoethylene anion, [TCNE]-, the ordering temperature was found to be 4.8 K.15,16 From these studies, it was determined that the position of adjacent chains re lative to one another ha d a significant effect on the bulk magnetic properties of the material. Thus Kahn and coworkers studied the effect of the chain arrangement on magneti c properties, by designing, sy nthesizing and studying the magnetism of a series of mixed-metal CuII -L MnII chains, where L is a bridging ligand.17 For example, antiferromagnetic coupling was observed between the chains of [MnIICuII(pba)(H2O)3], where pba = 1,3-propylenebis(oxamate). By changing the ligand only very slightly, contrasting data we re observed as the compound, [MnIICuII(pbaOH)(H2O)3], where pbaOH = 2-hydroxy-1,3-propylenebis(oxamate), s howed overall ferromagnetic coupling with Tc = 4.6 K.17 Such studies emphasized the importance of the selected bridging groups in the formation of 2D or 3D lattices and as well as the communication between the magnetic centers in the molecular building blocks.
17 After this original work, an attractive di rection became apparent: the synthesis of molecules containing several tr ansition metal ions, such as Mn, Fe, V, Ni, and Co, can potentially exhibit behavior similar to that of su perparamagnets. After year s of struggle with the reactivity of metal sources, redox chemistry that was taking place during the synthesis of such compounds, and finding the appropriate combination of solvents, molar ratios, and crystallization techniques, in 1993 the first example of a molecule [Mn12O12(O2CMe)16(H2O)4]( 1), abbreviated as Mn12ac, able to behave as a magnet by itself was discovered.18,19 This discovery led to a totally new approach: the synthe sis of nanoscale magnetic materials in which the magnetism was intrinsic to the molecule and not due to inte ractions between molecules. This hypothesis was consequently proven true by performing magne tic studies in polyethylene matrix of Mn12ac and showed the absence of any kind of long -range three-dimensi onal interactions. Ever since several polynuclear metal comp lexes that behave as nanoscale magnetic particles have been prepared, re sulting in the rapid development of a venerable new area of highspin metal clusters, termed single-molecule magnets (SMMs).20 For numerous reasons, SMMs represent a stimulating area of research, promising several advantages over conventional nanoscale magnetic particles. Such advantages in clude (i) the preparati on of purified compounds by solution methods, resulting in a product with a single, sharply defined size; (ii) possible variation in peripheral carboxyl ate ligation such that small or bulky, hydrophilic or hydrophobic ligands can be incorporated into the synthesis; (iii) solubility in several organic solvents, providing access to a variety of indus trial applications; and (iv) the possibility of reaching subnanoscale dimensions, resulting in the potential development of even better memory storage devices.20
18 The magnetic behavior of a SMM arises from the combination of a large ground state spin, S, and a large and negative magnetic anisotr opy as gauged by the axial zero-field splitting parameter (ZFS), D .21,22 Both in the fields of inorganic and organic chemistry, there is an intense search underway for such potentially useful high-spin molecules. The first SMM was Mn12ac and it was shown that this exceptional combination of high-spin ground stat e and large, negative magnetic anisotropy displayed by the complex, resu lted in nanoscale-like magnetic behavior and the subsequent classification of th e molecule as a SMM (Figure 1-5).18,19,21,23 Probably the most intensely studied SMM, [Mn12O12(O2CMe)16(H2O)4] has an S = 10 ground state spin and is just one in a class of well-characterized Mn12 complexes of the general formula, [Mn12O12(O2CR)16(H2O)4], where R = Me, Et, Ph, as well as numerous other groups.20 (a) (b) Figure 1-5. ORTEP representation in Pov-Ra y format showing the structure of the [Mn12O12(O2CMe)16(H2O)4] complex. (a) The [MnIII 8MnIV 4( 3O2-)12]16+ core and (b) the [Mn12O12(O2CMe)16(H2O)4] complex with acetates as peripheral ligands. MnIV green; MnIII blue; O red; C gray In these dodecanuclear mixed-va lence Mn(III/IV) complexes, a non-planar ring composed of eight alternating MnIII and eight triply bridging oxi de ions surrounds a central [Mn4 IVO4]8+ cubane unit. Sixteen bridging car boxylate ligands and four terminal water molecules complete the peripheral ligation. The large ground state spin (S=10) arises from exchange interactions
19 between the S = 3/2 spins of the MnIV ions and the S = 2 spins of the MnIII ions. Each of the eight MnIII ions on the outer periphery of the comple x undergoes a Jahn-Teller (JT) distortion as expected for a high-spin d4 ion in near-octahedral geometry. The distortion takes place in the form of an elongation of two trans bonds. The approximately pa rallel alignment of the elongation axes of the eight MnIII ions accounts for a high degree of molecular anisotropy. Molecular anisotropy is defined by the tensorial sum of the single ion anisotropies of the metal ions and therefore the overall an isotropy of a cluster is primarily a consequence of the single-ion anisotropies of the constituent ions within the cluster and of the relative orientations of the magnetic axes of these ions with respect to each other (Figure 1-6). Figure 1-6. ORTEP representati on in Pov-Ray format of the metal core of a typical [Mn12O12(O2CR)16(H2O)4] complex, showing the rela tive disposition of the JT elongation axes indicated as solid black bonds. MnIV green; MnIII blue; O red Hence, the magnetic anisotropy of a Mn12 cluster is primarily of the axial type, with the x and y directions approximately equivalent to each other while different from the z direction. Consequently the magnetic moment of an individual Mn12 molecule preferentially lies in the z direction, or the easy-axis, of the molecule. As a re sult of this Ising type of zero-field splitting is that the S = 10 ground state spin is divided into 21 (2S + 1) sublevels, each characterized by a spin projection quantum number, ms, where S ms S. The energy of each sublevel is given as E (ms) = ms 2D, giving rise to a double well potential (Fig ure 1-7). Because the value of the axial
20 ZFS parameter D for a SMM is negative (i.e. D= -0.50 cm-1 for 1), the ms = sublevels lie lowest in energy while the ms = 0 sublevel lies highest. For the reversal of the spin direction from spin-up (ms = -10) to spin-down (ms = +10) orientations of the magnetic moment, there is a potential energy barrier, defined as U = S2|D| for integer spins, such as Mn12ac (S=10) and U = S2|D| for half-integer spins as it will be shown later for other molecular systems. ms= +10 +9 +8 +7 +6 +5 +4 +3 +2 +1 ms= 0 -1 -2 -3 -4 -5 -6 -7 -8 -9 ms= -10 ms Energy 100|D| ms= 0 ms= +10 +9 +8 +7 +6 +5 +4 +3 +2 ms= -10 -9 -8 -7 -6 -5 -4 -3 -2 +1 -1Energy Orientation of msvector ( ) (a)(b) Figure 1-7. Representative plots of the potential energy barier for an SMM. (a) Plot of the energy versus the orientation of the ms vector ( ) along the z axis and (b) plot of the energy versus the ms sublevel for a Mn12 complex with an S = 10 ground state, experiencing zero-field splitting,2S Dz For 1, the potential energy barrier for the reversal of the spin orientation is 72 K. For this reason, SMMs exhibit slow magnetization relaxati on at low temperatures. Experimental evidence for this behavior is supported by the a ppearance of frequency-dependent signals ( M signals) in out-of-phase AC magnetic susceptibility meas urements, as shown in Figure 1-8b, and of hysteresis loops in magnetization ve rsus DC field scans (Figure 1-9).21 In contrast to the hysteresis loops of traditional ferrior ferromagnetic materials, such as those of magnetite or chromium dioxide, re spectively, the plots of magnetization versus magnetic field of SMMs show steps that correspond to an increase in the relaxation rate of
21 magnetization that occurs when the energy of ms sublevels coincide on the opposite sides of the potential energy barr ier (Figure 1-10).20,24-27 Such predicted, but neve r before observed behavior was first reported in 1996 for molecules of [Mn12O12(O2CMe)16(H2O)4]2MeCO2H4H2O.24 Hence, the relaxation of the magnetization of a SMM occurs not just by thermal activation over the energy barrier, but also by quantum tunne ling of the magnetiza tion through the energy barrier. Temperature (K) 246810 0 10 20 30 40 50 Temperature (K) 246810 0.0 0.5 1.0 1.5 2.0 2.5 3.0 1000 Hz 250 Hz 50 Hz 1000 Hz 250 Hz 50 Hz MT (cm3K mol-1) M(cm3mol-1)(a) (b) Figure 1-8. In-phase (as MT) and out-of-phase (as M ) AC susceptibility signals for a dried, microcrystalline sample of [Mn12O12(O2CR)16(H2O)4] at the indicated oscillation frequencies This quantum phenomenon is not unique to 1, but is also exhibited by many other SMMs, including the octanuclear FeIII oxo-hydroxo cluster, [Fe8O2(OH)12(tacn)6]8+ (tacn = 1,4,7triazacyclononane), where ground state tunneling was first observed, i.e., tunneling between the lowest energy ms levels.28 In the case of 1, quantum tunneling of magnetization (QTM) is thermally assisted and occurs between ms sublevels higher in energy than the lowest lying ms = 10 states. Although tunneling provides a route for ra pid reversal of magnetiz ation and, hence, a less attractive memory storage device, an inves tigation into the gap between the quantum and classical understanding of magnetism can be made using these complexes as models.
22 -1 -0.5 0 0.5 1 -1-0.500.51 M/Ms 0H (T) 0.004 T/s 1.3 K 1.8 K 2 K 2.2 K 2.4 K 2.6 K 2.8 K 3 K 3.6 K M/Ms -1 -0.5 0 0.5 1 H0(T) 4 mT/s 1.3 K 1.8 K 2 K 2.2 K 2.4 K 2.6 K 2.8 K 3 K 3.6 K -1 0 0.5 1 -0.5 Figure 1-9. Magnetization hystere sis loops for a typical [Mn12O12(O2CR)16(H2O)4] complex in the 1.3-3.6 K temperature range at a 4 mT/s field sweep rate. M is normalized to its saturation value, Ms H = 0 H = 0 H = 0 Figure 1-10. Schematic representation of the change in energy of ms sublevels as the magnetic field is swept from zero to a non-zero valu e. Resonant magnetization tunneling occurs when the ms sublevels are aligned between the two halves of the diagram
23 Due to the strong need for new SMMs with even larger S values and more negative D values, numerous synthetic strategies aiming at the improvement of these materials have been employed. However, even if these strategies seem feasible on paper, they have been proven a considerable challenge to synthetic chemists worldwide. One of the primary goals of this research is the development of new synthetic methods for the subsequent preparation of new SMMs. One of the strategies employed towards this goal is the introduction of bulky organic ligands to the synthesis of polynuclear compound s and stabilization of new geometries and nuclearities which might afford new SMMs. Also, a relatively new approach to this effort is the incorporation of heavier metals, such as lantha nides and actinides, in polynuclear clusters. The advantages of such mixed-metal complexes include the incorporation of highly anisotropic metal ions that could aid our effort s for better SMMs, the high coordination afinity of such heavier metals, which could lead to inte resting metal assemblies with new coordination environments and exciting magnetic properties.
24 CHAPTER 2 MANGANESE-CERIUM CLUSTER CHEMISTRY: A P OSSIBLE ROUTE TO NEW DENOX CATALYSTS AND MIXED-METAL SMMS 2.1 Introduction A well established area of inorganic chemical research has been the synthesis of high oxidation-state molecular compounds or polymeric oxides of Mn or Ce. These molecules provide a wide range of applications in diverse areas involving inorganic, organic, environmental, and industrial chemistry owing to th eir ability to oxidize both inor ganic and organic substrates.29-51 Some of the many occurrences of high oxidation-stat e clusters is the tetranuclear Mn cluster that is present near Photosystem II of green plants which is responsible for the oxidation of H2O to O2.29-32,51 Consequently, a great deal of attention has been given to the synthesis of molecular species that could possibly mimic this very efficient natural catalyst.29-31 In the homogeneous and heterogeneous catalysis of water to molecular dioxygen by Ru complexes, CeIV is used. MnO4 and CeIV are known in inorganic synthesis for their oxidative ability, and have been employed in the past in the formation of MnIII and/or MnIV clusters, or mixed-valent CoIII,IV complexes. In organic chemistry MnVII, MnIV and CeIV (as (NH4)2[Ce(NO3)6]) have been also used extensively as oxidizing agents for a vast variety of organic substrates.39-42 Furthermore, MnIII, MnIV and CeIV oxides have been widely used, either alone or as mixed Mn/Ce oxides, in a number of heterogeneous catalytic oxidation processes.43-50 For example, CeIV/MnIV composite oxides are being widely used in suband supercritical catalytic wet oxidations for the treatment of wastewater containing t oxic organic pollutants such as amm onia, acetic acid, pyridine, phenol, polyethylene glycol, and others.45-50 Another application of MnIV and CeIV composite oxides is ca talysis of the de-NOx process in car exhausts. Active research in this directi on has shown that Mn and Ce oxides seem to have considerable catalytic activity in the conversion of nitric oxide from the exhaust gases of cars
25 into nitrogen and oxygen. Selective Catalytic Re duction (SCR) of nitrogen oxides using a zeolite catalyst impregnated with Fe, Ce and Mn was found to reduce N2O by as much as 94% at 147C from a gas mixture 4:1 NO and N2O 52. Car exhausts are not the only site of NOx production. Virtually any combustion process in the high temperature zones produces NOx mixtures of gases. Other contributors to the NOx emissions, apart of the internal combustion engines are the coal or gas-fired or oil-fired furnaces, boi lers and incinerators. Finally, NOx gases are also produced during a variety of chemical pro cesses including the manufacture of nitric acid, the nitration of organic chemicals, the production of adipic acid and the reprocessi ng of spent nuclear fuel rods. Even though some of the above mentioned NOx emitters produce a mixture of those toxic gases in low concentrations, the aggr egate amount discharged in indus trial and/or highly populated areas tends to cause problems. Theref ore, it is evident that a de-NOx catalyst is very much needed and various agencies seem to be in terested to invest in related research. The US Environmental Protection Agency in the near future will require greater levels of NOx abatement from mobile and stationary emi ssion sources; for light trucks and cars this requirement will be approximately 0.07grams/mile, which is about 90% lower than todays maximum level.52 Technological improvement is happening constantly, but improvement by 90% would require a great amount of effort. One possible direction in the synthesis of a wellbehaved catalyst in the de-NOx process is the use of preformed MnIV/CeIV clusters on zeolite. Such a system has the advantage of the clusters staying more on the surface of the zeolite, due to their size, instead of infusing to the center of the porous material, and therefore provides greater surface area for better catalytic activity. Heterometalic compounds containing first-row transition metals and lanthanides are known in the inorganic literatu re, but are relatively rare Our group has previously reported a series of
26 mixed-metal Mn-Ce compounds53,54 as well as Mn/Ln (where Ln = Eu, Gd, Tb, Dy, Ho) compounds55-60, some of which behave as superparamagnets below their blocking temperature.55,56 Other groups have also e xplored the reactivity of CeIV with various first-row transition metal reagents 61 and great effort was invested in this chemistry both for fundamental understanding of the reactivity, and possible cat alytic applications. Following, we will be focused on a brief synthetic and magnetostructural exploration of the Mn/Ce compounds recently reported in the literature. 2.1.1 MnIV-CeIV Complexes: Synthetic and Structural Aspects As part of our investigation in Mn-Ce chem istry, our group has previously reported three different MnIV-CeIV compounds. They were all obtained as pa rt of a larger investigation of the oxidation of MnII reagents by CeIV under a variety of conditions. The reaction of Mn(NO3)2xH2O and 2,2 -bipyridine (bpy) with (NH4)2[Ce(NO3)6] in a 1:1:2 molar ratio in 25% aqueous acetic acid gave [CeIVMnIV 2O3(O2CMe)(NO3)4(H2O)2(bpy)2](NO3)2H2O (1H2O) (Figure 2-1, a) in 40% yield. Similarly, the reaction of Mn(O2CMe)2H2O and 6-methyl-2hydroxypyridine (mhpH)62,63 with (NH4)2[Ce(NO3)6] in a 4:9:6 molar ratio in 30% aqueous acetic acid gave [CeIV 3MnIV 2O6(O2CMe)6(NO3)2(mhpH)4]2H2O (2H2O) (Figure 2.1, b) in 24% yield. From a similar reaction and using 2-pyrol lidone instead of mhpH, a similar cluster to complex 2 was formed, [CeIV 3MnIV 2O6(O2CMe)6(NO3)2(pyro)2(H2O)3] (Figure 2-1, c). Finally, the reaction of a solution of Mn(O2CMe)2H2O in 50% aqueous acetic acid with Ce(ClO4)4 (0.5M solution in HClO4) in a 2:3 molar ratio gave [CeIVMnIV 6O9(O2CMe)9(MeOH)(H2O)2](ClO4)H2O0.5MeCO2H (3H2O0.5MeCO2H) in ~50% yield. Charge considerations, inspecti on of metric parameters and bond valence sum calculations establish that all the Ce and Mn ions of 13 are in oxidation state 4+.64
27 a b c d Figure 2-1. ORTEP representa tions in Pov-Ray format of several Mn-Ce complexes. (a) 1, (b) 2, (c) 3, (d) 4. Color scheme: MnIV purple, CeIV yellow, N blue, O red, C grey. H atoms have been omitted for clarity Extending this chemistry, a different synthetic route to heterometallic Mn-Ce compounds was followed and involved the reaction of prefor med oxo-bridged Mn clusters with various CeIV sources, in hope for some new complexes with novel structural features, possible magnetic
28 and/or catalytic properties. We were le d to this direction by the fact that CeIV is a good oxidizing agent, which could break the preformed Mn co mpounds and reassemble them into a different complex with hopefully unique structural features. CeIII and/or CeIV ions have the advantage of many coordination sites, which can vary from 6-1264, as well as the tendency to bind to O2-/ORions (oxophilicity). This propert y of Ce was attractive since it could potentially promote aggregation to large clusters. 2.1.2 Mn-Ce SMMs: A Magnetism Overview As part of our continuing search for new synthetic routes towards novel structural types which can function as SMMs, we have joined ongoing efforts in mixed-metal cluster chemistry. Recently, there has been a spurt of research activity in the scientific community towards heterometallic systems which can behave as SMMs.65-71 Our group had contributed to this relatively nascent field with the successful characterization of the Mn8Ce,54 Mn11Dy4,55 Mn2Dy2,56 and Fe2Dy2 58 SMMs. Indeed, we had earlier reported the template synthesis of the Mn8Ce SMM which possessed an S = 16 ground state spin. A repr esentation of the common core of a whole family of Mn8Ce clusters, as well as a representati on of the full structure of one of the members of the family ([Mn8CeO8(O2CPh)12(MeCN)4]) are shown in Figure 2-2. Although the Ce ion (CeIV) in the complex was diamagnetic, it acted as a template around which eight ferromagnetically coupled MnIII ions wrapped. The ground-state sp in was one of the largest ever reported for a Mn cluster, and th is was the first Mn-Ce SMM. Therefore, in an attempt to extend this rich ch emistry, we decided to start from preformed, relatively complex Mn sources, and explore the oxidative strength of CeIV. Our efforts were collectively motivated by the possible catalytic applications of Mn-Ce compounds, and their magnetic behavior in search of new examples of Mn-Ln single-molecule magnets (SMMs).
29 Figure 2-2. ORTEP representations in Pov-Ray format at the 50% probability level of the X-ray crystal structures of two members of the Mn8Ce family of complexes. (a) [Mn8CeO8(O2CMe)12(py)4]. (b) [Mn8CeO8(O2CPh)12(MeCN)4]. Color scheme: Mn green, Ce cyan, N dark blue, O red, C grey H atoms have been omitted for clarity. The complexes have a four-fold symmetry axis with an inversion center 2.2 Experimental Section 2.2.1 Synthesis All manipulations were performed under aer obic conditions using ch emicals as received, unless otherwise stated. [Mn12O12(O2CH3)16(H2O)4] was prepared as described elsewhere.34 22.214.171.124 [Mn4Ce6O12(O2CMe)10(NO3)4(py)4] pyH2O (1pyH2O): Method 1: To a stirred solution of [Mn12O12(O2CH3)16(H2O)4] (2.5g, 0.25mmol) in MeCN (20ml) was slowly added solid (NH4)2[Ce(NO3)6] (0.3g, 0.5mmol). The solution was stirred for 30 minutes during which time the color changed sli ghtly from dark brown to reddish brown. The solution was filtered, layered with pyridine a nd left undisturbed for 3 days during which time black needles emerged. The crystals were kept in mother liquor for the X-ray analysis and dried under vacuum for other solid state studies (i.e. magnetism, IR spectroscopy, etc.). a b
30 Method 2: To a stirred solution of [Mn12O12(O2CH3)16(H2O)4] (2.5g, 0.25mmol) in a mixture of MeCN (15ml) and pyridine (10ml) was slowly added solid (NH4)2[Ce(NO3)6] (0.3g, 0.5mmol) and the solution changed color slight ly from dark brown to reddish brown. The reaction mixture was then filtered and left undist urbed for a period of three days. Concentration of the solution by slow evaporation produced single black needle-like crystals, suitable for single crystal X-ray diffraction st udies. The crystals of 1 were maintained in the mother liquor for Xray crystallography and other single-crystal studies or collected by filtration, washed with Et2O, and dried in vacuo for miscellaneous solid -state studies. Anal. Calcd % (Found) for 1py2H2O: C, 21.92 (21.80); H, 2.41 (2.36); N, 2.84 (2.97). Selected IR data (KBr, cm-1): 3422(bm), 2362(sm), 2336(sm), 1635(bw), 1523(bm), 1384(v s), 1022(bw), 668(sm), 612(bm), 532(bm). 126.96.36.199 [Mn4Ce2O2(O2CMe)6(NO3)4(hmp)4]H2O (22H2O) To a stirred solution of (NH4)2[Ce(NO3)6] (0.55g, 1mmol) in MeCN (20ml) was added hmpH (0.10mL, 1mmol). After stirring for five minutes, Mn(O2CMe)2 .4H2O (0.25 g, 1 mmol) was added and the solution became redish brown. The reaction mixture was kept under magnetic stirring for five more minutes, and then filtered. The filtrate was left undisturbed for a period of 5 days, during which time dark brown plate-like cr ystals formed suitable for single-crystal X-ray analysis. The crystals of 2 were maintained in mother liquor for X-ray crystallography and other single-crystal studies, or collected by filtration, washed with Et2O, and dried in vacuo for several other solid-state studies. Anal. Calcd % (Found) for 2H2O: C, 27.96 (28.21)%; H, 3.00 (2.95)%; N, 3.62 (3.69)%. Selected IR data (KBr, cm-1): 3387(bm), 1613(sm), 1446(s), 1386(s), 1051(sm), 825(sw), 771(sm), 667(sm), 563(bm). 2.2.2 X-ray Crystallography Data were collected at 173 K on a Siemens SMART PLATFORM equipped with a CCD area detector and a graphite monochromator utilizing MoK radiation ( = 0.71073 ). Cell
31 parameters were refined using up to 8192 reflections. A full sphe re of data (1850 frames) was collected using the -scan method (0.3 frame width). The first 50 frames were re-measured at the end of data collection to monitor instrument and crystal stability (maximum correction on I was < 1 %). Absorption corrections by integrat ion were applied based on measured indexed crystal faces. Crystal data and structural refinement parameters for complexes 1 and 2 are listed in Tables 2.1 and 2.2 respectively The structure of complex 1 was so lved by the Direct Methods in SHELXTL6, and refined using full-matrix least squares. The non-H at oms were treated anisotropically, whereas the hydrogen atoms were calculated in ideal positio ns and were riding on their respective carbon atoms. The asymmetric unit consists of a half Mn4Ce6 cluster, one pyridine molecule, two acetonitrile molecules and two half molecules of acetonitrile molecu les (located on or close to symmetry elements). A total of 567 parameters we re refined in the final cycle of refinement using 25901 reflections with I > 2(I) to yield R1 and wR2 of 7.17% and 16.76%, respectively. Refinement was done using F2. The structure of complex 2 was solved by the Direct Methods in SHELXTL6, and refined using full-matrix least squares. The non-H at oms were treated anisotropically, whereas the hydrogen atoms were calculated in ideal positio ns and were riding on their respective carbon atoms. The asymmetric unit cons ists of a half cluster, one ha lf water and three acetonitrile molecules. The latter were disordered and could not be modeled properly, thus program SQUEEZE, a part of the PLATON package of crystallographic so ftware, was used to calculate the solvent disorder area and remove its contribution to the overall intensity data.
32 Table 2-1. Crystal data and structur e refinement parameters for complex 1 Empirical formula C62 H78 Ce6 Mn4 N16 O44 Formula weight 2811.88 Crystal system Monoclinic Space group P21/n a, 14.0419(16) b, 14.3165(16) c, 24.551(3) a, deg 90 b, deg 104.253(2) g, deg 90 Volume, 3 4783.5(9) Z 2 Temperature, K 173(2) Radiation, 0.71073 calcd, mg/m3 1.952 mm-1 3.394 Wr2 0.1795 Table 2-2. Crystal data and structur e refinement parameters for complex 2 Empirical formula C48 H62 Ce2 Mn4 N14 O31 Formula weight 1831.12 Crystal system Triclinic Space group P-1 a, 11.5998(9) b, 12.0750(9) c, 13.1809(10) a, deg 108.886(1) b, deg 104.727(1) g, deg 90.137(1) Volume, 3 1682.2(2) Z 1 Temperature, K 173(2) Radiation, 0.71073 calcd, mg/m3 1.808 mm-1 2.149 Wr2 0.1009 Variable-temperature dc and ac magnetic su sceptibility data were collected at the University of Florida using a Quantum Desi gn MPMS-XL SQUID susceptometer equipped with a 7 T magnet and operating in the 1.8-300 K range. Samples were embedded in eicosane to prevent torquing. Magnetization ve rsus field and temperature da ta was fit using the program
33 MAGNET.72 Pascals constants were used to estimate the diamagnetic correction, which was subtracted from the experimental susceptibility to give the molar paramagnetic susceptibility (M). 2.3 Results and Discussion 2.3.1 Synthesis (NH4)2[Ce(NO3)6] has been extensively used in organic synthesis as an oxidizing agent for a plethora of organic substrates.39-42 Clearly, this remarkable oxidizer could potentially attack even more complicated systems, and in this case CeIV attacked the [Mn12O12(O2CH3)16(H2O)4] cluster, and reassembled a completely different high-nuclearity, high-oxidation state product. During this process, the potential ly high coordination number of CeIV, as well as its preference in binding to the oxygen-rich nitrat e anions, had a major effect on the product, and thus CeIV ions were incorporated into the final cluster. Th e presence of pyridine was also crucial for the stabilization and therefore isolation of the cl uster, since the coordi nation sphere of the manganese ions was completed by one pyr idine per manganese center. Copmplex 1 seems eventually as the most thermodynamically favored product, the formation of which is assisted by the presence of the solvent mixture. Crysta l lattice dynamics is far complicated, and the formation of a product like this is nearly impossible to predict. In the case of compound 2 the synthesis follows a very fru itful synthetic pathway, widely used in manganese cluster chemistry. This synthetic route involves the reaction of a chelating ligand, in this case hmpH, with meta l-carboxylate precursors, such as Mn(O2CMe)2 .4H2O. However, in our case, and since employment of Ce ions was also desired, a source of the second metal was added to the reaction mixture. The or der in which the reactants are added during the course of the reaction, follows a common and successfully-used in the past scheme. First the lanthanide non-carboxylate metal source was dissol ved in the desired solvent (i.e. MeCN), then
34 the organic pyridyl alcoholate ligan d was added in order to increas e the pH of the solution, and finally the manganese carboxylate was incorporated, with a direct noticeabl e color change of the solution to dark red, due to the oxidation of the MnII ions to MnIII and/or MnIV, as a consequence of the basic conditions domina ting the reaction mixture. In both the above mentioned examples, nitrates are needed to complete the coordination sphere of the CeIV or CeIII ion. The same reactions were performed using Ce(ClO4)4, but due to the absence of nitrates, the clusters were not stab ilized and isolated. The latter experimental fact does not imply that the product did not form in solution; however, the analogous compound did not properly crystallize and was not further char acterized. Also, performing the same reaction but changing the carboxylate liga tion (propionates, benzoates, tert butyl acetates, pivalates etc.), also did not produce any crystall ine product, possibly due to ster ic restrictions within the structure or other thermodynamic and kinetic factors which we are unable to analyze without any other experimental fact. 2.3.2 Description of Structures 188.8.131.52 [Mn4Ce6O12(O2CMe)10(NO3)4(py)4] pyH2O A representation of the full structure of 1 as well as a partially labeled presentation of the core of the cluster, are shown in Figure 2-3. Sele cted interatomic distances and angles are listed in Table 2-3. Compound 1 can be described as one central octahedron of six CeIV ions with the equatorial Ce atoms belonging al so to two outer distorted MnIII/IV 2CeIV 2] cubanes, with the latter core held together by two quadruply-bridging oxides, and two triply -bridging oxides. The oxidation states of the manganese and cerium ions, as well as the protonation level of the triplybridging oxide bridges were established by bond-va lence sum calculations (p rovided in Tables 24, 2-5, 2-6 respectively), charge considerations and the presence of JahnTeller (JT) distortions on the MnIII ions. Mn4 and its symmetr y equivalent partner (Mn4 ) have JT axial elongations
35 with, as expected, the elongated MnIII-O and MnIII-N bonds being at least 0.1-0.2 longer than the other MnIII-O/N bonds. The equatorial plane of the molecule is define d by the square planar arrangement of Ce2, Ce3 and their symmetry-related partners at the equa torial plane of the central octahedron, and is extended to include the four coplanar trip ly-bridging O2in the two cubane-like units at the opposite ends of the cage. The distance between th e axial Ce1 and its symmetry equivalent from this plane is 2.614 Within the Ce6 octahedron, the distance between the axial Ce ions to the equatorial ones is on average 3.670 In the equa torial plane of the centr al octahedron, there are two short and two longer Ce-Ce distan ces, namely 3.628 for the Ce2 Ce3 pair and its symmetry equivalent, and 3.800 for the Ce2 Ce3 pair and its symmetry equivalent, respectively. The angle between the two best mean planes defined by Ce2, Ce3 and their symmetry-related partners, and Ce1, O7, O9 and th eir symmetry-equivalent partners, is equal to 89.06. Another interesting feature of this molecule is the fact that the pyridine rings are not parallel to the plane of the e quator and the distance between N4 (which is bound to Mn5) and the equatorial plane is 1.638, whereas the distan ce between N3 (which is bound to Mn4) and the same plane is 1.506 stacking interactions between the clusters in the unit cell is present, since the distance between two para llel pyridines belongi ng to two different molecules is in the range of the regular stacking distance of 3.5-3.8 (N4C13 = 3.604 and N3C18 = 3.543 ). The cuboidal structure construc ted by two Mn and two Ce ions, to the best of our knowledge, has never been observed before in the lit erature. Also, the octahedral arrangement of the six CeIV ions is novel, giving this compound its characteristic structural signature. Compounds such as 1 are of particular interest due to their high nuclearity, symmetry and architectural beauty, as well as their fascin ating properties that we always aim for.
36 Table 2-3. Selected bond distances () and angles () for complex 1 Ce1 O9 2.139(8) Ce3 O10 2.402(8) Ce1 O7 2.153(8) Ce3 O12 2.407(7) Ce1 O8 2.332(8) Ce3 O20 2.414(9) Ce1 O5 2.427(11) Mn4 O12 1.879(8) Ce1 O4 2.443(11) Mn4 O11 1.885(8) Ce1 O2 2.468(11) Mn4 O13 1.943(9) Ce1 O1 2.475(11) Mn4 O17 1.954(9) Ce1 O10 2.506(8) Mn4 O8 2.256(7) Ce1 N2 2.816(17) Mn4 N3 2.313(10) Ce1 N1 2.880(16) Mn5 O11 1.835(8) Ce2 O7 2.245(7) Mn5 O12 1.856(9) Ce2 O9 2.280(8) Mn5 O10 1.910(8) Ce2 O8 2.340(8) Mn5 O15 1.948(9) Ce2 O10 2.372(8) Mn5 O19 1.965(9) Ce2 O21 2.376(8) Mn5 N4 2.062(11) Ce2 O14 2.383(9) Ce1 Ce3 3.6669(10) Ce2 O16 2.387(9) Ce1 Ce2 3.6732(11) Ce2 O11 2.390(7) Ce2 Mn5 3.2690(19) Ce3 O7 2.251(8) Ce2 Mn4 3.3684(18) Ce3 O9 2.251(7) Ce2 Ce3 3.6284(9) Ce3 O8 2.289(8) Ce3 Mn5 3.2766(19) Ce3 O22 2.382(8) Ce3 Mn4 3.3711(19) Ce3 O18 2.385(9) Mn4 Mn5 2.809(3) Ce1-O7-Ce2 116.7(3) Mn5-O10-Ce2 99.0(3) Ce1-O7-Ce3 112.7(3) Mn4-O12-Ce3 103.0(3) Ce2 -O7-Ce3 107.6(3) Mn5-O10-Ce3 98.2(3) Mn4-O8-Ce3 95.8(3) Ce2-O10-Ce3 105.5(3) Mn4-O8-Ce1 145.5(4) Mn5-O10-Ce1 149.4(4) Ce3-O8-Ce1 105.0(3) Ce2-O10-Ce1 100.2(3) Mn4-O8-Ce2 94.2(3) Ce3-O10-Ce1 99.4(3) Ce3-O8-Ce2 110.4(3) Mn5-O11-Mn4 98.0(4) Ce1-O8-Ce2 103.7(3) Mn5-O11-Ce2 100.6(3) Ce1 -O9-Ce3 117.0(3) Mn4-O11-Ce2 103.4(3) Ce1 -O9-Ce2 112.4(3) Mn5-O12-Mn4 97.5(4) Ce3-O9-Ce2 106.4(3) Mn5-O12-Ce3 99.7(3) Mn4-O12-Ce3 103.0(3) Symmetry code: = 1 -x+1,-y+1,-z Table 2-4. Bond valence sum calculations and assignments for the Ce ions in 164 CeIII CeIV Assignment Ce1 4.315087 3.80034 CeIV Ce2 4.287562 3.776098 CeIV Ce3 4.301192 3.788102 CeIV
37 Figure 2-3. ORTEP representations in the PovRay format of complex 1. (a) The complete molecule, and (b) a partially labeled representation of the metal core. Color scheme: MnIII green, MnIV purple, CeIV orange, N blue, O red, C grey. H atoms have been omitted for clarity
38 Table 2-5. Bond valence sum calculations and assignments for the Mn ions MnII MnIII MnIV Assignment Mn4 3.788102 2.899136 3.043662 MnIII Mn5 3.926952 3.591875 3.770935 MnIV Table 2-6. Bond valence sum calculations and assignments for the triply-bridging oxygen ions BVS Assignment O7 2.006347 O2O9 1.982799 O2O11 1.834913 O2O12 1.784175 O184.108.40.206 [Mn4Ce2O2(O2CMe)6(NO3)4(hmp)4] A partially labeled representation of the full structure of 2 as well as two different views of the core of the cluster showing the positions of the metal centers and the key structural features of the Mn4Ce2 cage, are shown in Figure 2-4. Selected interatomic distances and angles can be found in Table 2-7. The oxidation states of the manganese and cerium ions were established by bond-valence sum calculations (provided in Table 28), charge considerations and the presence of Jahn-Teller (JT) distortions on the MnIII ions. Mn4 and its symmetry equivalent partner (Mn4 ) have JT axial elongations with as expected, the elongated MnIII-O and MnIII-N bonds being at least 0.1-0.2 longer than the other MnIII-O/N bonds. Compound 2 can be described as a compressed di storted octahedron comprising of four manganese ions in the equatorial plane (Mn1, Mn 2 and their symmetry related partners), and two cerium centers located at the axial positions, positioned 2.026 above and below the plane, respectively. The angle between the Mn4 plane and the plane which includes both the cerium centers and two diagonally opposing manganese ions (Mn2 and its symmetry related partner), is 86.72. The parallelogram that the four manganese ions form is 5.115 in length and 3.198 in width, as seen in Figure 2-5. The core of 2 is held together by four -O2CMeand two 2:2: 3
39 O2CMeligands. The latter coordination mode is relatively rare in the inorganic literature; a representation emphasizing the bindi ng mode of these acetate ligands is shown in Figure 2-4, c. Table 2-7. Selected bond distances () and angles () for complex 2 Ce O14 2.470(2) Mn2-O11 1.927(2) Ce O15 2.153(8) Mn2-N4 2.005(7) Ce O 12.557(2) Mn2-O9 2.157(3) Ce O 22.561(3) Mn2-O13 2.329(2) Ce O1 2.563(2) Mn1 Mn2 3.1984(7) Ce O6 2.566(2) Mn2 Ce 3.5337(5) Ce O5 2.619(2) Ce O3 2.628(3) Ce O13 2.638(2) Mn2-O1-Mn1 115.62(10) Ce O12 2.716(2) Mn2-O1-Ce 113.12(9) Mn1 O15 1.879(2) Mn1-O1-Ce 104.94(8) Mn1 O1 1.893(2) Mn2-O1-Ce 104.10(8) Mn1 O8 1.927(2) Mn1-O1-Ce 113.35(9) Mn1 N3 2.039(3) Ce-O1-Ce 105.45(7) Mn1 O10 2.160(2) Mn1 -O12-Ce 96.73(8) Mn1 O12 2.274(2) Mn2-O13-Ce 97.02(8) Mn2 O1 1.887(2) Mn2-O14-Ce 107.61(9) Mn2 O14 1.888(2) Mn1-O15-Ce 107.63(9) Symmetry code: = -x+1,-y+1,-z Table 2-8. Bond valence sum calculations and assignments for the Mn and Ce ions in 264 Each of the CeIII centers is ten-coordinate (bicapped square antiprism) and their coordination parameters within.sphere is comple ted by two bidentate chel ating nitrate ligands on each CeIII ion. The alcoxide arm of each hmpbridges one Mn and on Ce atom, while the nitrogen of the pyridyl backbone is bonded to each of the MnIII ions, forming a stable 5membered Mn-N-C-C-O-Mn chelating ring. II III IV Assignment Mn1 3.16 2.89 3.04 MnIII Mn2 3.17 2.90 3.04 MnIII Ce 2.99 2.63 CeIII
40 Mn2 Mn1` Mn2` Mn1 Ce Mn2 Mn1` Mn2` Mn1 Ce Figure 2-4. PovRay repr esentations of complex 2. (a) The complete complex 2, (b) a part of 2 where the hmpligands are omitted for clarity, and (c) the metal core emphasizing in dashed line the unusual 2:2:3 O2CMeligand. Color scheme: MnIII green, CeIII orange, N blue, O red, C grey. H atoms have been omitted for clarity a b c O4 C9
41 5.115 3.198 2.026 86.72 5.115 3.198 2.026 86.72 Figure 2-5. Schematic representation of the metal core of 2 and the key structural features There is no significant interaction between clus ters, since the shortest contact between O4 of one molecule and C9 of its neighbor is 3.197; there are no inter-and/or-intramolecular hydrogen bonding or stacking interactions within the molecules in the crystal structure. 2.4 Magnetic Susceptibility Studies 2.4.1 Direct Current Magnetic S usceptibility Studies 220.127.116.11 [Mn4Ce6O12(O2CMe)10(NO3)4(py)4] pyH2O Solid state, variable-temperature magnetic su sceptibility measurements were performed on vacuum-dried microcrystalline samples of complex 1, suspended in eicosane to prevent torquing. The dc magnetic susceptibility (M) data were collected in the 5.0-300 K range in a 0.1 T (1000
42 G) magnetic field. Figure 2-7 shows the molar magnetic susceptibility (M) of complex 1 as a MT versus T plot. The MT has a value of ~7.2 cm3 K mol-1 at 300 K, gradually decreasing with decreasing temperature to ~3.1 cm3 K mol-1 at 5.0 K. The MT value at 300 K is lower than ~9.8 cm3 K mol-1 expected for four non-inte racting Mn centers with th e oxidation states found in complex 1 (2MnIII and 2MnIV atoms), since CeIV is diamagnetic (f 0) and has no magnetic contribution. The exchange parameter J between the Mn centers, gauging the interaction within the MnIII-O2-MnIV units of 1 is highly affected by the concurrent bonding to the bridging O2of the strong Lewis acidic CeIV ion which weakens the Mn-O bonds that mediate the MnIIIMnIV superexchange interaction. Complex 1 was expected to exhibit the magnetic response of two independent dimeric units; the large diamagnetic oc tahedral composed of six CeIV ions in the middle, would result in a negligible long range exchange pathway between the dimers at the two opposite ends of the cluster. The isotropic (Heisenberg) spin Ha miltonian describing an exchange-coupled MnIIIMnIV dimeric unit, which is effectively what we expect for complex 1, is given by equation 2-1, where J is the exchange coupling paramete r between the two Mn ions, and S1 = 2 and S2 = 3/2. = 2J(12) (2-1) The eigenvalues of the spin Hamiltonian may be determined using equation 2-2, where ST is each energy state given by the relationship ST=|S1+S2|, |S1+S2 1|,, |S1 S2|; in this case the energy states are ST(1) = 7/2, ST(2) = 5/2, ST(3) = 3/2, ST(4) = 1/2. E(ST) = J [ST(ST + 1)] (2-2) A theoretical M versus T expression was derived for complex 1 from the use of the Van Vleck equation,73,74 which was modified to include a fraction (p) of paramagnetic impurity (assumed to be mononuclear MnII), and temperature-independent paramagnetism (TIP). The
43 latter was kept constant at 300 10-6 cm3 K mol-1. Data below 15 K were ignored, due to zerofield splitting (ZFS), weak interm olecular interactions, and Zeeman effects from the use of the DC field. The modified Van Vleck equation (e quation 2-3) was used to fit the observed temperature dependence of the molar magnetic su sceptibility as a function of the exchange coupling parameter J, and an isotropic g value. The parameterin equation 2-3 is the degeneracy of the ST state, but for dinuclear compounds is always 173,74. The obtained fit (solid line in Figure 2-7) gave J = -45.65 cm-1, g = 1.95 and p = 0.012 and a ground-state spin value, ST=1/2 (Figure 2-6). The goodness of the fit is evident from the low statistical error, R2=0.999. The MnIII-MnIV moiety in 1 is bridged by two 3-O2ligands, where the third metal atom is a diamagnetic CeIV atom (strong Lewis acid). Therefore, th e exchange coupling is expected to be lower than the case of MnIII-(2-O2-)2-MnIV dimers where the exchange pathway is mediated by two 2-O2ions, and higher than the case of MnIII-(3-O2-)-MnIV units when there is only one monoatomic bridge between the metal ions. The value of J in the case of complex 1 is, as predicted, significantly lower than the lite rature values reported for independent MnIII-(2-O2-)2MnIV dimers, which range from -102 cm-1 to -220 cm-1,75,76 and higher than MnIII-(3-O2-)2-MnIV dimeric units within a larger cluster, with only one bridging 3-O2and J=-18.7 cm-1, -21 cm-1, and -36 cm-1.77 kT ) T E(S)exp T (S1) T (2S )( exp)()12)(1( T S kT 3 2 2 Ng )1( M kT T SE T S T S T S p TIP p kT12 2 2 35Ng (2-3)
44 Figure 2-6. Ordering of the energy states for 1, using the calculated exchange parameter J and the Van Vleck equation. All states were corrected by 34.24cm-1 in order for the ground state to be at 0 18.104.22.168 [Mn4Ce2O2(O2CMe)6(NO3)4(hmp)4]H2O The magnetic data for complex 2 revealed overall antiferromagnetic interactions within the cluster, since the MT decreases sharply with decreasing temperature. The presence of CeIII atoms in the structure of 2, further complicates the understanding of the magnetic data for this compound, since CeIII has significant spin-orbit coupling. For the lanthanides, paramagnetism is due to unpaired electrons in the f orbitals, wh ich are more buried (low er in energy) than d orbitals and are not as affected by the ligands. Thus, the orbital contribution is not quenched
45 significantly. The spin-o rbit coupling for the unpaired electr ons in the f orbitals of the lanthanides is a lot stronger than unpaired electrons in the d orbitals of the transition metals. 050100150200250300 0 2 4 6 8 10 12 M T [cm 3 mol -1 K]Temperature [K] Figure 2-7. Plot of MT vs temperature for a dried, mi crocrystalline sample of complex 1. M is the DC molar magnetic susceptibility measured in a 1.0 kG field. The solid line is the fit of the data to the theoretical equation; see the text for the fit parameters Consequently, it is expected that the experime ntal values obtained fo r trivalent lanthanides would not be in good agreement with the spin-only values obtained from theory. In this case, the effective magnetic moment, denoted as eff, can be calculated from equation 2-4. eff = g[J(J+1)] (2-4) where J is the angular moment um quantum number which incl udes spin-orbit coupling. The relationship of eff and MT is given in in equation 2-5.
46 eff = (8MT)1/2 (2-5) 050100150200250300 0 2 4 6 8 10 12 M T [cm 3 mol -1 K]Temperature [K] Figure 2-8. Plot of MT vs temperature for a dried, mi crocrystalline sample of complex 2 in eicosane. M is the DC molar magnetic susceptibility measured in a 1.0 kG field Therefore, it is feasible to calculate the MT for a given lanthanide. In this manner, CeIII has a MT of 0.8 for the free ion.8 Thus, at 300K we can expect a MT value corresponding to four MnIII and two CeIII, namely MT300K = 4MTMn(III) + 2MTCe(III) = [4*SMn(III)(SMn(III)+1)] /2 + 0.8*2, or 13.6 cm3mol-1K. The room-temperature MT value of approximately 10.9 cm3mol-1K is below that expected for 4MnIII and 2CeIII ions (13.6 cm3mol-1K), which indicates the presence of dominant antiferromagnetic in teractions, even at 300K. The va lue then drops, slowly with decreasing temperature to a value of ~7.6 cm3mol-1K at 130 K. Below 130 K, MT decreases
47 more sharply to a value of ~0.9 cm3mol-1K at 5.0 K. This behaviour is consistent with overall antiferromagnetic exchange interactions between the metal centers with a diamagnetic spin ground state (ST = 0). 2.4.2 Alternating Current Magnetic Susceptibility Studies 22.214.171.124 [Mn4Ce6O12(O2CMe)10(NO3)4(py)4] pyH2O In order to determine the ground state spin of complex 1, alternating current (AC) magnetic susceptibility measurements were pe rformed in the 1.8 K temperature range in a 3.5 G AC field oscillating at 50 Hz. The in-phase compone nt of the ac susceptibility contains two regions, which is usually observe d for systems behaving as SMMs; there is a linearly decreasing part of the sus ceptibility at higher temperatures, namely for T>7.5 K in this case, followed by a change of the slope of th e decrease at lower temperatures (Fig. 2-9). However, the small ST value of the cluster minimizes the possibi lities of this system to behave as a single-molecule magnet. It is interesting to note that extrapolation of the data to 0 K from the higher temperature region (>7.5 K) corresponds to an S=5/2 groundstate, whereas extrapolation from the lower temperature region (1.8 7.5 K) supports a ground state of S=1/2 or 1. The ground state of the cluster is more likely to be 1/2, since it was found that the two Mn centers are coupled in a non-cooperative fashion, and the in teraction between the dimers seems to be negligible (see DC magnetic suscep tibility studies). At lower temperatures it is possible to detect the long range interaction between the two dimeric moieties in the structure, and/or any kind of weak intermolecular inter actions between the clusters via the significant stacking. In the outof-phase plot, shown in Figure 2-10, there is no evident signal present, and as expected for such a low spin system, this complex is not an SMM.
48 0246810121416 0 2 4 6 8 10 12 1000Hz 500Hz 250Hz M T [cm 3 mol -1 K]Temperature [K] Figure 2-9. Plot of the the in-phase component (M T) of the AC magnetic susceptibility of complex 1 versus temperature, at the indicated oscilating frequencies
49 0246810121416 -2 0 2 4 1000Hz 500Hz 250Hz M '' [cm 3 mol -1 ]Temperature [K] Figure 2-10. Plot of the out-of-phase (M ) AC susceptibility signal versus temperature for a microcrystalline sample of complex 1 126.96.36.199 [Mn4Ce2O2(O2CMe)6(NO3)4(hmp)4]H2O: AC susceptibility measurements for complex 2H2O were performed in the 1.8 K temperature range in a 3.5 G AC field oscillating at 50 Hz. The in-phase component of the ac susceptibility for complex 2 exhibits a linear decrease of the signal, with decreasing temperature, confirming also the dominant antiferromagnetic interactions within the molecule (Fig. 2-11). In contrast to complex 1, the M T product for 2 is decreasing at a constant rate, and reaches a value of ~0.7 cm3mol-1K at 1.8 K. Extrapolation of M T to 0 K gives a value of ~0.6 cm3mol-1K suggestive of an S=1/2 or an S=0 ground state spin value, with the latter value being
50 more possible due to the symmetry of the mol ecule, as well as the accordance to the DC magnetic susceptibility data. In the case of the fo rmer S=1/2 ground state, we believe that is due to the spin frustration effects expected within the many Mn3 triangular units of the Mn4Ce2 structure, as well as some residual magnetism from the CeIII ions. We define spin frustration here in its more general form as competing exchange interactions of comparable magnitude that prevent (frustrate) the preferred pa irwise antiparallel spin alignments that would give small (0 or 1/2) ground states. From the absence of any si gnal in the out-of-phase component of the AC (Fig.2-12), it is evident that this complex does not behave as an SMM, as expected for such a low spin system. 0246810121416 -2 -1 0 1 2 3 4 1000 Hz 500 Hz 250 Hz 50 Hz M 'T [cm 3 mol -1 K]Temperature [K] Figure 2-11. Plot of the the in-phase component (M T) of the AC magnetic susceptibility of complex 2 versus temperature, at the indicated oscilating frequencies
51 0246810121416 -2 0 2 4 1000 Hz 500 Hz 250 Hz 50 Hz '' M [cm 3 mol -1 ] Temperature [K] Figure 2-12. Plot of the out-of-phase (M ) AC susceptibility signal versus temperature for a microcrystalline sample of complex 2 2.5 Conclusions The versatility of manganese c oordination chemistry, in conj unction with the coordination abilities of cerium ions produced two new clusters, namely MnIII 2MnIV 2CeIV 6 and MnIII 4CeIII 2, with unprecedented structural characteristics an d interesting magnetic responses. Specifically, the octahedral arrangement of any lantha nide within a cluster, as found in Mn4Ce6, as well as the cubane-like motif c onsisting of two MnIII/IV and two CeIV ions, have never been observed in the literature. Even though these molecules are not single-molecule magnets, they possess aesthetically pleasing ca ge-like metal architectures. This area of mixed metal-lanthanide
52 molecular clusters can be extended to di fferent metal center combinations and/or metal/lanthanide ratios and is likely to produce more aesthetically pleasing and eventually interesting polymetalic assemblies, and therefore in-depth investigation of this chemistry is essential.
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58 BIOGRAPHICAL SKETCH Christos Lampropoulos was bor n in Patras, Greece on May 13th, 1981. He performed his high school studies at th e General Lyceum in Kamares and graduated with highest honors and GPA 19.8 out of 20. He then travelled to the United States and began his undergraduate studies at Harper Community College in Palatine, IL. Af ter two semesters at Harper, he transferred to the University of Illinois at Chicago (UIC) wher e he was a biology major for the first year. Since his sophomore year at UIC, he was performing un dergraduate research in the group of Prof. John A. Morrison on the synthesis of trifluorom ethyl-containing organom etalic compounds of platinum, and molybdenum. His in terest in synthetic inorganic chemistry made him change his major and he received a bachelors degree with h onors in chemistry. In the fall of 2004, Christos joined the group of Drago Professor George Ch ristou at the Chemistry Department of the University of Florida, where he worked on the synthesis and the physical and magnetic characterization of polynuclear transition meta l, and mixed transition metal-lanthanide complexes in search for new single-molecule ma gnets and effective catal ysts for the conversion of toxic NOx gases to simple N2 and O2.