Recent experiments have shown that junctions consisting of individual single-molecule magnets (SMMs) bridged between two electrodes can be fabricated in three-terminal devices, and that the characteristic magnetic anisotropy of the SMMs can be affected by electrons tunneling through the molecule. Vibrational modes of the SMM can couple to electronic charge and spin degrees of freedom, and this coupling also influences the magnetic and transport properties of the SMM. The effect of electron-vibron coupling on transport has been extensively studied in small molecules, but not yet for junctions of SMMs. The goals of this thesis will be two-fold: to present a novel approach for studying the effects of this electron-vibron coupling on transport through SMMs that utilizes both density functional theory calculations and model Hamiltonian construction and analysis, and to present a software framework based on this hybrid approach for the simulation of transport across user-defined SMMs. The results of these simulations will indicate a characteristic suppression of the current at low energies that is strongly dependent on the overall electron-vibron coupling strength and number of molecular vibrational modes considered. / Master of Science

This thesis will present the synthesis, characterization and magnetic measurements of lanthanide complexes with varying nuclearities (Ln, Ln2, Ln3 and Ln4). EuIII, GdIII, TbIII, DyIII, HoIII and YbIII have been selected as the metal centers. Eight polydentate Schiff-base ligands have been synthesized with N- and mostly O-based coordination environments which chelate 7-, 8- or 9-coordinate lanthanide ions. The molecular structures were characterized by single crystal X-ray crystallography and the magnetic properties were measured using a SQUID magnetometer. Each chapter consists of crystal structures and magnetic measurements for complexes with the same nuclearity. There are eight DyIII SMMs in this thesis which are discrete molecules that act as magnets below a certain temperature called their blocking temperature. This phenomenon results from an appreciable spin ground state (S) as well as negative uni-axial anisotropy (D), both present in lanthanide ions owing to their f electron shell, generating an effective energy barrier for the reversal of the magnetization (Ueff). The ab initio calculations are also included for the SMMs with high anisotropic energy barriers to understand the mechanisms of slow magnetic relaxation in these systems.

This thesis will present the synthesis, characterization and magnetic measurements of lanthanide complexes with varying nuclearities (Ln, Ln2, Ln3 and Ln4). EuIII, GdIII, TbIII, DyIII, HoIII and YbIII have been selected as the metal centers. Eight polydentate Schiff-base ligands have been synthesized with N- and mostly O-based coordination environments which chelate 7-, 8- or 9-coordinate lanthanide ions. The molecular structures were characterized by single crystal X-ray crystallography and the magnetic properties were measured using a SQUID magnetometer. Each chapter consists of crystal structures and magnetic measurements for complexes with the same nuclearity. There are eight DyIII SMMs in this thesis which are discrete molecules that act as magnets below a certain temperature called their blocking temperature. This phenomenon results from an appreciable spin ground state (S) as well as negative uni-axial anisotropy (D), both present in lanthanide ions owing to their f electron shell, generating an effective energy barrier for the reversal of the magnetization (Ueff). The ab initio calculations are also included for the SMMs with high anisotropic energy barriers to understand the mechanisms of slow magnetic relaxation in these systems.

The field of molecular magnetism has expanded rapidly since the discovery of single molecule magnets (SMMs) in the 1990’s and has witnessed extraordinary advances in the last several decades. One of the current trends in molecular magnetic research is to incorporate metal ions that have pronounced single-ion anisotropy in an effort to improve magnetic exchange interactions. The 4d and 5d transition metal ions have large spin-orbit coupling parameters which contribute to the orbital angular momentum effects that lead to anisotropic behavior. The work herein describes efforts to synthesize and characterize new cyanide-bridged molecular materials incorporating 4d and 5d transition metal ions, specifically the [Os(CN)_(6)]^(3-), [Mo(CN)_(6)]^(3-) and [W(CN)_(8)]^(3-) ions. The 5d hexacyanometallate [Os(CN)_(6)]^(3-) was incorporated into a trinuclear cyanide bridged molecule and the [Fe(CN)_(6)]^(3-) analog was prepared as a reference compound for assessing the effect of the 5d versus 3d metal ion on the magnetic properties. Both molecules exhibit SMM bistability with a pronounced increase (~90 %) in the blocking temperature (TB) of the OsIII analogue. In addition to typical SMM behavior, both compounds exhibit exchange-biased SMM behavior, a shift in the quantum tunneling of the magnetization (QTM) from zero field. This exchange-bias can be turned “on” or “off” depending on the presence of interstitial methanol molecules. New trigonal bipyramidal (TBP) molecules incorporating the rarely studies hexacyanomolybdate(III) ion are presented in chapter III of this dissertation. The molecules of general formula [M(tmphen)_(2)]_(3)[Mo(CN)_(6)]_(2) (M = V^(II), Mn^(II) and Fe^(II); tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline), represent additions to a large homologous family of TBP molecules reported by the Dunbar group over the years. The [Mo(CN)_(6)]^(3-) ion was prepared in situ by loss of one cyanide ligand from [Mo(CN)_(7)]^(4-). Of particular interest among the compounds reported is the V_(3)Mo_(2) analog which exhibits extraordinarily strong antiferromagnetic coupling (estimated J = -134 cm^(-1). The observed exchange coupling parameter is more than twice the current record for the antiferromagnetic coupling parameter for a cyanide-bridged magnetic molecule. Another set of results were obtained using the octacyanometallate anion [WV(CN)_(8)]^(3-) as a building block for the synthesis and magnetic studies of a family of new cyanide-bridged magnetic materials. The compounds exhibit several different structural motifs including three 0-D molecular compounds (two pentanuclear molecules and a linear trinuclear molecule) and a 1-D chain, findings that illustrate the structural versatility of the octacyanotungstate(V) ions. The TBP molecule, [Mn(tmphen)_(2)]_(3)[W(CN)_(8)]_(2), exhibits evidence for an out-of-phase signal when subjected to ac measurements in zero applied field. The 1-D chain also reveals evidence for the beginning of an out-of-phase signal under zero applied field which hints at single chain magnet behavior.

This thesis will present the synthesis, characterization and magnetic measurements of lanthanide complexes with varying nuclearities (Ln, Ln2, Ln3 and Ln4). EuIII, GdIII, TbIII, DyIII, HoIII and YbIII have been selected as the metal centers. Eight polydentate Schiff-base ligands have been synthesized with N- and mostly O-based coordination environments which chelate 7-, 8- or 9-coordinate lanthanide ions. The molecular structures were characterized by single crystal X-ray crystallography and the magnetic properties were measured using a SQUID magnetometer. Each chapter consists of crystal structures and magnetic measurements for complexes with the same nuclearity. There are eight DyIII SMMs in this thesis which are discrete molecules that act as magnets below a certain temperature called their blocking temperature. This phenomenon results from an appreciable spin ground state (S) as well as negative uni-axial anisotropy (D), both present in lanthanide ions owing to their f electron shell, generating an effective energy barrier for the reversal of the magnetization (Ueff). The ab initio calculations are also included for the SMMs with high anisotropic energy barriers to understand the mechanisms of slow magnetic relaxation in these systems.

Single-molecule magnets (SMMs) have been lauded for their application in next generation devices for their enhanced information storage capabilities, increased processing speeds, and increased storage densities compared to bulk magnets. However, the success of SMMs in such applications and their technological readiness is hindered by their operation temperatures and memory lifetimes. SMMs are molecular species that possess a bistable ground state and magnetic anisotropy, which together result in an energy barrier to the reorientation of the magnetic moment. The magnetic memory response relies on its ability to retain magnetization in the absence of an external field. To this end, lanthanide ions with their large inherent magnetic anisotropy combined with well-defined crystal field microstates are attractive candidates for eliciting higher operation temperatures and lifetimes. This dissertation focuses on the use of lanthanide ions in the development of high barrier SMMs with a close emphasis on the magnetic anisotropy and crystal field manipulation through geometry, design, and modification. In the pursuit of lanthanide (Ln)-based SMMs, two cyclooctatetraenyl (COT2-) complexes of the non-Kramers ion, TmIII, [TmIII(η8-COT)I(THF)2] and [K(18-C-6)(THF)2][TmIII(η8-COT)2], were isolated. As an ion that possess an integer angular momentum projection (J = 6), it was vital that a highly symmetric local environment was utilized to observe field-induced slow magnetic relaxation. The static and dynamic properties of TmIII(η8-COT)I(THF)2] and [K(18-C-6)(THF)2][TmIII(η8-COT)2] were characterized revealing Ueff of 7.93 K and 53.3 K, respectively. More importantly, the effect of increased symmetry was observed on the rate of quantum tunneling of the magnetization (QTM), where the rate was two orders of magnitude faster in the heteroleptic complex. This emphasized the importance of local symmetry for non-Kramers ions and contributed to the rare class of TmIII SMMs. Due to the prevalent role of QTM in Ln-based SMMs, a common strategy is to induce magnetic communication between Ln ions to overcome its detrimental effects. To this end, bridging units should be sufficiently small enough to bring the Ln ions close in proximity, yet the surrounding environment of the metal center should still promote uniaxial magnetic anisotropy. We compared the effect of ancillary ligands on the magnetic properties of two dinuclear DyIII compounds with the same {μ-Cl}2 core bridge. The complexes [DyIII{N(SiMe3)2}2(μ-Cl)(THF)]2 and [DyIII(η8-COT)(μ-Cl)(THF)]2 were characterized with static and dynamic magnetic measurements. The well-matched ligand field of the silyl amide ligands with the DyIII ion, precluded the observation of zero field tunneling. While both complexes are characterized by antiferromagnetic coupling, it is evident that peripheral ligands also play a vital role in determining the performance of multinuclear SMMs. Magnetic coupling between 4f centers is classically weak; however, the use of ligands with diffuse electron clouds may penetrate the shielded 4f orbitals to effectively promote communication. One such ligand that had not previously been investigated for its ability to couple the magnetic moment of Ln ions was the trianionic cycloheptatrienyl. Utilizing Ln silyl amides, in situ deprotonation afforded the dinuclear complexes [KLnIII2(η7-C7H7){N(SiMe3)2}4] (Ln = GdIII, DyIII, ErIII). The static and dynamic magnetic characterization revealed rare and highly sought-after ferromagnetic coupling in a Ln-based system. The ancillary silyl amide ligands were a necessity for the isolation of these dinuclear species yet did not provide a synergistic ligand field for the Ln ions when combined with the cycloheptatrienyl bridge, ultimately preventing the observation of slow relaxation in some of the variants studied. Pseudo-linear complexes, those molecules with strong axial donors have shown immense promise in the design of highly efficient SMMs. Our work has shown that amides are effective in directing the anisotropy of the Ln ions, thus the removal of the central organometallic bridge from the previous compounds would effectively create a highly anisotropic complex. This was achieved in our study of a formally five-coordinate complex of a ferrocene diamide ligated DyIII ion, [(NNTBS)DyIIII(THF)2]. The static and dynamic magnetic properties were characterized, yielding Ueff = 771 K with open magnetization hysteresis loops at zero-field, due in part to the axial disposition of the nitrogen atoms of the diamide ligand. Computational analysis of the parent compound and its fragments was completed. Our results indicated that the presence of equatorially coordinated solvent molecules such as THF, influence the axiality in the crystal field microstates more significantly than the coordinated halide. The removal of coordinated solvent such as THF, is imperative to improve the performance of DyIII SMMs. By way of a bulky bisanilide ligand that precludes the approach of solvent to the metal center, combined with a large bite angle, [K(DME)n][LArDyIII(X)2], a formally four coordinate complex, was investigated. In contrast to the complex of the ferrocene diamide ligand, retention of the magnetic moment was not observed at zero-field, despite the fact that the slow relaxation dynamics occurred over a greater temperature range for which Ueff = 1278-1334 K. In addition, variants of the bound halide (X = Cl, I) were examined for their effect on the static and dynamic magnetic properties, revealing zero field relaxation times that were on average 5.6x longer for the heavier congener. The collective results of the findings presented herein are being utilized to synthesize new low-coordinate Ln-based SMMs. Combining divalent and redox chemistries with bulky amido ligands will ideally elicit even larger energy barriers to spin reversal and higher blocking temperatures, supporting the push towards Ln-based SMMs with increased technological readiness.

The effect of pressure on a number of magnetically interesting compounds such as single-molecule magnets and dimeric copper and manganese molecules has been investigated to probe the validity of ambient magneto-structural correlations. The first chapter is an introduction to the equipment and methodologies that have been adopted to carry out the experimental high-pressure work. The second chapter reports the first combined high-pressure single crystal X-ray diffraction and high pressure magnetism study of four single-molecule magnets (SMMs). At 1.5 GPa the structures [Mn6O2(Et-sao)6(O2CPh(Me)2)2(EtOH)6] (1) – an SMM with a record effective anisotropy barrier of ~86 K – and [Mn6O2(Etsao) 6(O2C-naphth)2(EtOH)4(H2O)2] (2) both undergo significant structural distortions of their metallic skeletons which has a direct effect upon the observed magnetic response. Up to 1.5 GPa pressure the effect is to flatten the Mn-N-O-Mn torsion angles weakening the magnetic exchange between the metal centres. In both compounds one pairwise interaction switches from ferro- to antiferromagnetic, with the Jahn-Teller (JT) axes compressing (on average) and re-aligning differently with respect to the plane of the three metal centres. High pressure dc χMT plots display a gradual decrease in the low temperature peak value and slope, simulations showing a decrease in |J| with increasing pressure with a second antiferromagnetic J value required to simulate the data. The “ground states” change from S = 12 to S = 11 for 1 and to S = 10 for 2. Magnetisation data for both 1 and 2 suggest a small decrease in |D|, while out-of-phase (χM //) ac data show a large decrease in the effective energy barrier for magnetisation reversal. The third SMM is the complex [Mn3(Hcht)2(bpy)4](ClO4)3·Et2O·2MeCN (3·Et2O·2MeCN) that at 0.16 GPa loses all associated solvent in the crystal lattice, becoming 3. At higher pressures structural distortions occur changing the distances between the metal centres and the bridging oxygen atoms making |J| between the manganese ions weaker. No significant variations are observed in the JT axis of the only MnIII present in the structure. Highpressure dc χMT plots display a gradual decrease in the low temperature peak value and slope. Simulations show a decrease in J with increasing pressure although the ground state is preserved. Magnetisation data do not show any change in |D|. The fourth SMM, [(tacn)6Fe8O2(OH)12](ClO4)3.9Br4.1⋅6H2O, (4) is the largest inorganic compound ever studied at high-pressure. Up to 2.0 GPa the conformation of the complex remains largely unaffected, with the counter ions and water molecules moving around to accommodate a compression of the unit cell volume. High pressure magnetic susceptibility data collected up to 0.93 GPa confirm minimal changes in the intra-molecular exchange interactions. The third chapter focuses on three hydroxo-bridged CuII dimers: [Cu2(OH)2(H2O)2(tmen)2](ClO4)2 (5), [Cu2(OH)2(tben)2](ClO4)2 (6) and [Cu2(OH)2(bpy)2](BF4)2 (7) have been structurally determined up to 2.5, 0.9 and 4.7 GPa, respectively. 6 and 7 have never been reported before. Pressure imposes important distortions in the structures of all three complexes, particularly on the bond distances and angles between the metal centres and the bridging hydroxo groups. 5 undergoes a phase transition between 1.2 and 2.5 GPa caused by the loss of a coordinated water molecule. This leads to a loss of symmetry and dramatic changes in the molecular structure of the complex. The structural changes are manifested in different magnetic behaviours of the complexes as seen in dc susceptibility measurements up to ~0.9 GPa: J becomes less antiferromagnetic in 5 and 6 and more ferromagnetic in 7. The fourth chapter shows the compression of two oxo-bridged MnII/MnIII mixed valence dimers: [Mn2O2(bpy)4](ClO4)3⋅3CH3CN, (8) has been squeezed up to 2.0 GPa whilst [Mn2O2(bpy)4](PF6)3⋅2CH3CN⋅1H2O, (9) could be measured crystallographically up to 4.55 GPa. 9 has never been reported before, while 8 has been reported in a different crystallographic space group. The application of pressure imposes significant alterations in the structures of both complexes. In particular, in 8 the Mn-Mn separation is reduced by the contraction of some of the Mn-O bond distances, 9 shows essentially analogous behaviour: the Mn-Mn distance and nearly all the Mn-N bonds shrink significantly. The magnetic behaviour of the complexes has been measured up to 0.87 GPa for 8 and 0.84 GPa for 9, but neither display any significant differences with respect to their ambient data.

The work presented herein focuses on the synthesis and characterization of new cyanide bridged molecular magnetic materials that form discrete molecules as well as three dimensional networks. This research is inspired by the recognition that the Prussian blue (PB) family exhibits a wide range of interesting magnetic properties such as photomagnetism, spin crossover, and high TC magnets owing to the presence of the cyanide bridge that promotes magnetic communication between adjacent metal spins. An underexplored facet of this research is the systematic development of the topic with anisotropic metal ions research that was undertaken as part of this dissertation. The resulting discoveries are materials that exhibit a wide range of bistable magnetic properties, including photomagnetism, long range magnetic ordering, SMM, and exchange-biased SMM behavior. New Prussian Blue analogs are presented in Chapter II of this thesis that are based on the nearly unexplored hexacyanoosmate(III) ion. A family of CoII PB derivatives of OsIII were found to exhibit photomagnetic and charge transfer induced spin transition (CTIST) behavior and a study of alkali metal cation dependence revealed marked differences in both the photomagnetic and CTIST properties, with the highest ordering temperature being observed for the K+ analog which exhibits a TC of 28.5 K. The phenomenon of linkage isomerism reported for PB analogs and other molecular materials that incorporate the [Cr(CN)6]3- ion wherein the CN ligand reverses its binding mode between the two metal centers was studied in detail as described in Chapter III. Small molecule models that incorporate [Cr(CN)6]3- and CoII ions were investigated by single crystal X-ray crystallography, magnetism, and solution IR studies and the data led to useful mechanistic information about the nature of the cyanide reversal process. The use of the anisotropic hexacyanoosmate(III) anion to form a trinuclear species with MnIII was undertaken in the study described in Chapter IV. The first SMM based on the hexacyanoosmate(III) ion was discovered and found to exhibit a very rare exchange biased SMM phenomena in one of its crystal forms. In Chapter V new building blocks with the pentadentate MPPA ligand are described which are ideally suited for the preparation of a range of model compounds of the dinuclear and trinuclear variety.

Single-Molecule Magnets (SMMs) are discrete molecules that exhibit slow relaxation of magnetization. Unlike conventional magnets that rely on the long range magnetic ordering in the form of domains, these molecules act as magnets independently, that is without the influence of neighbouring molecules. SMMs have intrigued physicists and chemists alike for over twenty years with their potential future applications in data storage quantum computing, and with this communal interest there has been significant collaboration between the two fields of research. SMMs have brought forth an opportunity for coordination chemists to muster their creativity and synthetic expertise in the rational design and development of these magnetic materials. From these new and fascinating compounds, both experimental and theoretical physicists have sought to develop and refine our understanding of the aspects of these molecular magnets in order to improve their performance at higher temperatures. In this work, new topologies for lanthanide complexes are explored using a novel Schiff base ligand. The magnetic properties of dinuclear, tetranuclear and octanuclear lanthanide complexes are discussed and correlated to their structural properties. The rational design of tetrazine-based Schiff base ligands for magnetic studies is also discussed in hopes of developing high performance SMMs.

This thesis describes the synthesis, characterization and magnetic investigation of homometallic lanthanide complexes based on two different o-vanillin-derived Schiff base ligands: H2ovph and H2ovgrd. The studies were performed using single crystal X-ray diffractometry, Powder XRD and SQUID magnetometry. Chapter 2 focuses on dinuclear systems 1-8 coordinated to the ligand H2ovph and presents their structural and magnetic properties, mainly with respect to their intramolecular interactions. Chapter 3 describes two hexanuclear systems, 9 (DyIII) and 10 (GdIII), with trigonal prism-assembled core structures. A structural comparison to other similar complexes in the literature is performed. A series of dinuclear complexes, 11-15, based on the ligand H2ovgrd are described in Chapter 4, focusing on the synthetic strategy, crystal structures and magnetism. The presence of the lanthanide contraction is evident in this system of complexes and is consistent with the intrinsic lanthanide contraction property.