A range of lanthanide compounds incorporating soft bridging ligands or alkoxide ligands have been synthesised and their magnetic properties investigated. These two classes of compound have shown promise as single molecule magnets but have not been widely studied; this thesis aims to expand on this area of research. Softer bridging ligands are found to slightly increase superexchange interactions between metal centres compared to harder bridging ligands. The introduction to this thesis covers the basic properties of the lanthanides, paying special attention to their chemistry with soft donor ligands and alkoxide ligands. Also included is an introduction to the field of single-molecule magnetism and the role of lanthanide complexes in the study of this behaviour. In Chapter 2, four complexes are reported: the phosphine adducts [Cp'3Ln(H2PMes)] and the phosphide-bridged trimers [(Cp'2)Ln(μ-PHMes)]3 (Ln = Er, Gd). Their structures and magnetic properties are characterised. In Chapter 3, the novel dodecametallic thiolate-bridged lanthanide macrocycles [(Cp'2Ln)3({μ-SCH2}3CMe)]4 (Ln = Dy, Y, Gd) are reported and characterised by X-ray crystallography, NMR spectroscopy and magnetometry. [(Cp'2Dy)3({μ-SCH2}3CMe)]4 is shown to be a single-molecule magnet with Ueff = 69 cm-1. In Chapter 4, the novel thiolate-bridged lanthanide dimers [Cp'2Ln(μ-SCH2{C4H7S2})]2 (Ln = Dy, Y, Gd) are reported, showing sulfur-sulfur bonding leading to ring cyclisation of the bridging ligand [MeC(CH2S)3]3-. These complexes are characterised by X-ray crystallography, NMR spectroscopy and magnetometry. Extra NMR spectroscopic studies were performed to investigate the mechanism of ring closure on the bridging ligand. [Cp'2Dy(μ-SCH2{C4H7S2})]2 is shown to be a single-molecule magnet with Ueff = 87 cm-1. In Chapter 5, four new lanthanide siloxide clusters incorporating alkali metals are reported: the trigonal bipyramidal [Dy2K3(OSiMe3)9]; the octahedral [Dy2K4(OSiMe3)10]; the bi-capped cuboid [Y4K6O6(OSiMe3)12]6-; and the [Dy3K8O3(OSiMe3)12]- 'burger' cluster. All clusters are structurally characterised by X-ray crystallography and [Dy2K4(OSiMe3)10] is magnetically characterised. The synthetic rationalisation for formation of these diverse structures is investigated.

Single molecule magnets (SMMs) are molecules of mesoscopic scale which exhibit quantum properties such as quantum tunneling of magnetization, quantum interference, spin filtering effects, strong spin-phonon coupling and strong hyperfine Stark effects. These effects allow applications of SMMs to high-density information storage, molecular spintronics, and quantum information science. Therefore, SMMs are of interest to physicists, chemists, and engineers. Recently, experimental fabrication of individual SMMs within transistor set-ups have been achieved, offering a new method to examine magnetic properties of individual SMMs. In this thesis, two types of SMMs, specifically Eu2(C8H8)3 and Ni9Te6(PEt3)8, are theoretically investigated by simulating their electron transport properties within three-terminal transistor set-ups. An extended metal atom chain (EMAC) consists of a string of metallic atoms with organic ligands surrounding the string. EMACs are an important research field for nanoelectronics. Homometallic iron-based EMACs are especially attractive due to the high spin and large magnetic anisotropy of iron(II). We explore the exchange coupling of iron atoms in two EMACs: [Fe2(mes)2(dpa)2] and [Fe4(tpda)3Cl2]. Chapter 1 provides an introduction to SMMs, electron transport experiments via SMMs and an introduction to density functional theory (DFT). Chapter 2 presents a theoretical study of electron transport via Eu2(C8H8)3. This type of molecule is interesting since its magnetic anisotropy type changes with oxidation state. The unique magnetic properties lead to spin blockade effects at zero and low bias. In other words, the current through this molecule is completely suppressed until the bias voltage exceeds a certain value. Chapter 3 discusses a theoretical study of electron transport via Ni9Te6(PEt3)8. The magnetic anisotropy of this magnetic cluster has cubic symmetry, which is higher than most SMMs. With appropriate magnetic anisotropy parameters, in the presence of an external magnetic field, uncommon phenomena such as low-bias blockade effects, negative conductance and discontinuous conductance lines, are observed. In Chapter 2 and 3 DFT-calculated magnetic anisotropy parameters are used and electron transport properties are calculated by solving master equations at low temperature. Chapter 4 examines the exchange coupling between iron ions in EMACs [Fe2(mes)2(dpa)2] and [Fe4(tpda)3Cl2]. The exchange coupling constants are calculated by using the least-squares fitting method, based on the DFT-calculated energies from different spin configurations. / Ph. D. / Single molecule magnets (SMMs) are molecules of mesoscopic scale which exhibit quantum properties. Its quantum effects are used to describe the behavior of SMMs at the smallest scales. These quantum properties could also be used to reveal possible applications of SMMs to high-density information storage, molecular spintronics, and quantum information science. Thus SMMs are of interest to physicists, chemists, and engineers. Recently, electron transport via individual SMMs was achieved in experiments. Electron transport is obviously affected by the magnetic properties of the SMM, thus one can examine magnetic properties of an SMM indirectly by measuring electron transport via the SMM. In this thesis, two types of SMMs, Eu2(C8H8)3 and Ni9Te6(PEt3)8, are investigated theoretically by simulating their electron transport properties. An extended metal atom chain (EMAC) consists of a string of metallic atoms with organic ligands surrounding the string. EMACs are an important research field for nanoelectronics. Homometallic iron-based EMACs are especially attractive due to the high spin and large magnetic anisotropy of iron(II). If a molecule has magnetic anisotropy, its magnetic properties change with the direction of its magnetic moment. We explore how iron atoms interact with each other in the EMACs [Fe2(mes)2(dpa)2] and [Fe4(tpda)3Cl2]. Chapter 1 provides an introduction to SMMs, electron transport experiments via SMMs and an approximation method, density functional theory (DFT). DFT is a method to approximate electronic structure and magnetic properties of various many-body systems. Chapter 2 investigates theoretical electron transport via Eu2(C8H8)3. Eu2(C8H8)3 changes its type of magnetic anisotropy when it obtains an extra electron, which is different from most SMMs. If the Eu2(C8H8)3 is short of an extra electron, its magnetization direction is in-plane, that is, its magnetic energy is lowest when its magnetic moment is along any direction in a specific plane. If an extra electron is captured by Eu2(C8H8)3, its magnetization direction becomes out-of-plane, and its lowest energy is obtained when its magnetic moment is along the direction normal to the specific plane. The unique magnetic properties lead to blockade effects at low bias: the current through this molecule is completely suppressed until the bias voltage exceeds a certain value. The bias voltage on a molecule equals the electrical potential difference between two ends of the molecule. Chapter 3 investigates theoretical electron transport via Ni9Te6(PEt3)8. Magnetic anisotropy of Ni9Te6(PEt3)8 is cubic symmetric, and its symmetry is higher than most SMMs. With appropriate magnetic anisotropy parameters, in the presence of an external magnetic field, uncommon phenomena are observed. These phenomena include (1) current is completely suppressed when bias is low; (2) current via SMM decreases while bias on SMM increases; (3) there are discontinuous lines in the figures that describe electrical conductance of current. Chapter 4 examines the iron atoms’ interaction strength in both [Fe2(mes)2(dpa)2] and [Fe4(tpda)3Cl2]. Reasonable spin Hamiltonians are used to describe the energy of EMACs. Considering all possible directions of the spins of iron atoms in two EMACs, we calculate the energy of every possible spin configuration using DFT. The energy of each spin configuration can be expressed as an equation containing one or more coupling constants. We apply the least-squares fitting method to obtain the values of the coupling constants in the spin Hamiltonians.

This dissertation describes the single-molecule magnet (SMM) behaviour of f-block cyclooctatetraenyl sandwich complexes. Chapter one introduces the concepts that dictate SMM behavior particularly in f-elements. The emphasis is to understand the origin of magnetic behaviour and the properties that make lanthanide elements particularly interesting to explore. Current strategies used to predict such behaviour are discussed and a literature review on the subject is provided. Chapter Two describes the magnetic properties of eight isostructural lanthanide sandwich complexes utilizing 1,4-bis(trimethylsilyl)cyclooctatetraenyl dianion as the ligand, [Li(DME)3][LnIII(COT”)2] (Ln = Ce, Nd, Gd, Tb, Dy, Ho, Er, Tb, COT” = 1,4-bis(trimethylsilyl)cyclooctatetraenyl dianion, DME = dimethoxyethane). The complexes display a wide range of magnetic behaviour. The best performing SMM was the erbium complex, which had a blocking temperature of 8 K. Investigating different lanthanide ions with the same ligand enabled us to evaluate our findings in relation to current models used to predict SMM behaviour in lanthanide complexes. Chapter three extends the discussion of lanthanide sandwich complexes to include higher symmetry cyclooctatetraenyl complexes of ErIII and DyIII, [K(18-C-6)][LnIII(COT)2] (18-C-6 = 1,4,7,10,13,16-hexaoxacyclooctadecane, COT = cyclooctatetraene).The change in symmetry evoked by removing the trimethylsyl- (TMS) groups on the ligand greatly influenced the magnetic properties of both complexes. Ab initio calculations revealed that the magnetic relaxation in the ErIII complex occurs via the second excited state which contributes to the very high blocking temperature of 10 K in this complex. Chapter four presents an organometallic building block approach to create triple decker lanthanide COT” complexes of GdIII, DyIII and ErIII with a molecular formula of LnIII2(COT”)3. Synthetically, we couple together the sandwich complexes discussed in Chapter 2 by oxidatively removing one ligand to produce linear complexes where the two metals are bridged by an aromatic COT” ligand. The magnetic properties of all complexes are compared to their respective mononuclear analogs. Most interesting is the unprecedented 4 K increase in blocking temperature of the triple decker ErIII analog compared to the ErIII mononuclear sandwich complex discussed in Chapter 2. This increase is due to a ferromagnetic dipole-dipole interaction between the ErIII ions through the COT” ring. The aromatic bridging ligand provides a GdIII - GdIII interaction of J = -0.448(1) cm-1. Chapter five extends the discussion of magnetic exchange coupling to include linear K2(THF)4[LnIII2(COT)4] (Ln = Gd, Dy, Er, COT = cyclooctatetraenyl dianion, THF = tetrahydrofuran) complexes of GdIII, DyIII and ErIII. Each complex is composed of two LnCOT2 units bridged linearly by a potassium ion. The magnetic interaction between metal ions is much weaker than in the triple decker complexes discussed in Chapter 4, with a GdIII-GdIII interaction of J = − 0.007(4) cm–1. The magnetic properties of the quadruple decker complexes were compared to their mononuclear equivalents (Chapter 3). Surprisingly, the ErIII complex showed an increase in magnetic blocking temperature over its mononuclear analog despite the large ErIII-ErIII separation of 8.819 Å. Ab initio calculations revealed that this increase is due to single ion effects, most likely an increase in symmetry. Chapter six deviates from lanthanide magnetism to study the magnetic properties of uranium sandwich complexes with multiple ligand systems and oxidation states. Prior to this study the SMM behaviour of uranium sandwich complexes was unknown. We report the synthesis, structure and magnetic properties of both uranium-COT” sandwich complexes and uranium-cycloheptatrienyl complexes with oxidation states spanning (III)-(V). None of the complexes showed zero-field SMM behaviour, indicating a sandwichtype ligand is not appropriate for harnessing the SMM character in uranium. We compared the slow magnetic relaxation of isostructural and valence isoelectronic uranium and neodymium complexes. The improved energy barrier in the uranium complex further motivates the use of uranium in SMM design due to its large spin-orbit coupling.

This thesis will describe the magnetic behavior found in a scaling array of magnetic nanomaterials that have been uniquely designed, synthesized and characterised in order to better understand their properties with regards to potential future applications. Within Chapter 1 will be a detailed, yet accessible, introduction to nanomagnetism and the fundamental principles and practical techniques essential to the study of this unique mélange of physics and chemistry. This chapter will be designed to give the reader the necessary tools to understand key literature concepts found in Chapter 2, as well as the work presented in the following chapters. Chapter 2 will provide an overview of relevant literature in the field of magnetic nanomaterials, including: nanoparticles, single-molecule magnets, single-chain magnets and metal-organic frameworks. Chapter 3 will describe work performed on nanoparticles doped with lanthanide ions in order to explore their resulting size, shape, crystallinity and magnetic properties. The relevance of the chosen particles (NaYF4) pertains to their proposed use in a variety of applications due to their known luminescent properties, which we sought to hybridize with interesting magnetic properties, thus creating multimodal imaging capabilities. Doping with a variety of desired ratios of lanthanide ions (GdIII, TbIII, DyIII, ErIII and YbIII) was successful, producing crystalline nanoparticles with tunable size and shape. Magnetic measurements displayed a clear absence of superparamagnetic behavior, indicating that these materials have the potential to be well-suited to applications in biomedicine as multimodal imaging probes and MRI contrast agents. Chapter 4 will build on the previously explored doped nanomaterials through creating a hybrid nanomaterial by tethering lanthanide-based magnetic molecules to the surface of nanoparticles. This is performed through the synthetic design of a SMM with two anisotropic DyIII ions, which was synthesized and designed to bear terminal S-groups in order to promote the binding of the magnetic molecule to capping agent free gold nanoparticles. Upon confirmation of the successful surface attachment of the molecules, magnetic measurements displayed that the magnetic molecules maintained their static properties, however, their dynamic properties were altered. This system was the first example of this type of novel approach to the study of magnetic molecules on surfaces for data storage, spintronics, and quantum computing applications. Chapter 5 will expand on the previous study of ordering arrays of magnetic molecules on the surface of nanoparticles by tethering them into 1D chain networks. We successfully synthesized chain networks with YIII, EuIII, GdIII, TbIII and DyIII lanthanide ions. Magnetic characterisation revealed slow relaxation of the magnetization with no significant interactions between magnetic ions, thus these are discrete magnetic molecules in 1D. Rather surprisingly, the isotropic GdIII analogue displayed field induced slow relaxation of the magnetisation, necessitating the use of ab initio calculations in order to shed light on the potential causes of this unexpected behavior. Overall, through the formation and study of these structures, we observed a new potential method of SMM assembly for the study of ordered arrays of molecular magnets. Chapter 6 will focus on ordering of discrete magnetic systems in 3D. With this in mind, we successfully isolated the first Co8 cuboctahedron MOF. Magnetic measurements displayed that each SBU was well-isolated, with significant antiferromagnetic coupling between CoII ions, leading to an S = 0 ground state. These interactions were then modelled using density functional theory. This type of study promotes the future development of novel high-nuclearity MOF structures with interesting and tuneable magnetic properties, as well as the potential for assembly of discrete molecular magnetic units in 3D using MOFs. Chapter 7 utilizes the principles of Chapter 3, wherein magnetic ions are doped into a diamagnetic material; in this case, MOF-5. We sought to isolate one CoII ion in each SBU, and build upon this by adding additional magnetic ions and probing their interactions. Through magnetic measurements we observed a scaling magnetic moment with CoII content, and with higher dopant percentages we began to observe magnetic interactions occurring within the SBUs. Interestingly, we also observed a change in coordination environment with higher dopant percentages, likely as a result of the previously suggested capability of one ZnII ion within the MOF-5 SBU to become hexacoordinate, allowing CoII doping up to a maximum of 25%. Consequently, this study points to the cause of the structural instability that plagues MOF-5 in the presence of air and moisture. We probed this system further in Chapter 8 using FeIII as a dopant ion, and were able to obtain the first crystallographic evidence of the coordination change of ZnII in MOF-5. Furthermore, the structure obtained with FeIII was the first example of metal ion addition within a MOF that bound two interpenetrated frameworks together. This new MOF was found to have the potential to be a more practical material for gas storage and separation, and/or for catalysis. Thus, this study was informative in regards to the inherent instability of the parent framework, as well as a new method of metal addition to a known MOF structure. Chapter 9 will conclude the work with a discussion of what was performed in, and learned from, each thesis section, as well as provide an outlook and perspective on the novel work that may be derived from these projects going forward.

In this thesis I am presenting a detailed study to optimize the deposition of magnetic molecules and gold nanoparticles in atomically flat surfaces by self-assembling them from solution. Epitaxially grown and atomically flat gold surface on mica is used as substrate for this study. These surfaces have roughness of the order one tenth of a nanometer and are perfect to image molecules and nanoparticles in the 1-10 nanometers range. The purpose of these studies is to find the suitable parameters and conditions necessary to deposit a monolayer of nano-substance on chips containing gold nanowires which will eventually be used to form single electron transistors by electromigration breaking of the nanowire. Maximization of the covered surface area is crucial to optimize the yield of finding a molecule/nanoparticle near the gap formed in the nanowire after electromigration breaking. Coverage of the surface by molecules/nanoparticles mainly depends on the deposition time and concentration of the solution used for the self-assembly. Deposition of the samples under study was done for different solution concentrations and deposition times until a self-assembly monolayer covering most of the surface area is obtained. Imaging of the surfaces after deposition was done by tapping-mode AFM. Analysis of the AFM images was performed and deposition parameters (i.e. coverage or molecule/particle size distribution) were obtained. The subjects of this investigation were a molecular polyoxometalate, a single-molecule magnet and functionalized gold nanoparticles. The obtained results agree with the structure of each of the studied systems. Using the optimized deposition parameters found in this investigation, single-electron transport measurements have been carried out. Preliminary results indicate the right choice of the deposition parameters.

In this dissertation, we study the most important microscopic aspects that grant molecules such as Single Molecule Magnets (SMMs) their preferential spin direction. We do so by proposing and solving a model that includes correlations between electrons occupying atomic orbitals. In addition, we study the relation between the non-equilibrium electronic transport signatures in a SMM model weakly coupled to a three-terminal single electron transistor device, and the interference features of the SMM model in the presence of a magnetic field. Finally, we investigate the equilibrium transport features in a giant-spin model of a SMM in the Kondo regime. We study how the magnetic field modulation of the energy in a highly anisotropic molecule can affect the conductance of the molecule in the Kondo regime.

Ce travail de thèse décrit (i) la synthèse de complexes hétérométalliques d’ions 3d et 4f à partir de précuseurs de Mn, Fe et Co, de sels de lanthanides et de ligands organiques et (ii) l'étude de leurs structures et propriétés. 41 complexes polynucléaires ont été synthétisés dans le cadre de ce travail. Les structures moléculaires de tous les composés ont été déterminées par diffraction des rayons X. Les propriétés magnétiques de 22 complexes ont été étudiées, dont quatre montrent une relaxation lente de leur aimantation considérée comme la signature d’un comportement de molécule-aimant. L'activité catalytique du complexe {Mn4Dy6Li2} calciné a aussi été étudiée et s'est avérée efficace pour l'oxydation du monoxyde de carbone. L'étude systématique de complexes isostructuraux de lanthanides a montré que l'incorporation d’ions 4f peut introduire de l’anisotropie magnétique et que l’ion DyIII est généralement le meilleur candidat pour le ciblage de molécules-aimants hétérométalliques 3d- 4f. / This dissertation describes the syntheses of 3d-4f-metal complexes starting from preformed compounds of Mn, Fe and Co, lanthanide salts and organic ligands and also the investigation of their structures and properties. 41 new polynuclear heterometallic metal complexes were synthesised in the course of this work with different interesting properties. The structures of all obtained compounds have been confirmed using X-ray diffraction. The magnetic properties of 22 complexes were studied, of which four show frequency dependent out-of-phase signals as expected for SMMs. The catalytic activity of calcinated {Mn4Dy6Li2} was investigated and proved effective for the oxidation of CO. It was established, that the use of precursors leads to new families of compounds. Moreover the study of isostructural compounds across the lanthanide series showed 1) that the incorporation of 4f ions introduces magnetic anisotropy and 2) DyIII is usually the best candidate for targeting 3d-4f-SMMs.

Silylamide ligands have been used throughout the Periodic Table since the 1960s. They have delivered landmark complexes by providing the first three co-ordinate f-element complexes, the first trigonal planar f-element complexes and the first near-linear f-element complexes. This area is reviewed in Chapter 2.Herein, this work presents the first uses of several novel bis-silylamide ligands developed at Manchester which take the form {N(SiR3)2} where R = Me, iPr or tBu to afford four novel ligands: N ʹ, {N(SiMe3)(SiiPr3)}; N**, {N(SitBuMe2)2}; N* {N(SitBuMe2)(SiiPr3)}; and N , {N(SiiPr3)2}. Group 1 and 2 complexes of all of these ligands are presented along with the previously reported N*ʹ [N*ʹ = {N(SitBuMe2)(SiMe3)}]; which show variable bonding motifs based on the steric bulk. The N** and N ligands have formed the bulk of the work presented and were used to stabilise the first trigonal planar actinide complex [U(N**)3], as well as the first near-linear Ln(II) (Ln = lanthanide) complexes [Ln(N )2] (Ln = Sm, Eu, Yb, Tm). Additionally the trigonal planar Ln(II) complexes [K(2.2.2-cryptand)][Ln(N**)3] (Ln = Sm, Eu, Yb, Tm) have also been synthesised to compare the physicochemical properties of trigonal planar and near-linear geometries on the same elements with similar ligands.

The work presented herein describes efforts to synthesize and characterize cyanide-bridged molecular compounds with high-spin ground states. This investigation focused primarily on the assembly of hexacyanometallate units with convergent cationic metal complexes that are coordinated to capping ligands. In this manner, a family of related compounds was developed that serve as models for understanding the role of magnetic exchange interactions and anisotropy in nanomagnetic materials. The work presented in Chapter II describes the successful incorporation of the [Fe(CN)6]3- building block into planar geometries with nuclearities ranging from three to ten metal centers. In Chapter III, this methodology was optimized to yield two pentanuclear FeIII/NiII clusters, namely, the trigonal bipyramidal unit, {[Ni(tmphen)2]3[Fe(CN)6]2}, and the extended square, {[Ni(bpy)2(H2O)][Ni(bpy)2]2-[Fe(CN)6]2}. Magnetic measurements on pure phases of these samples revealed that each system exhibits ferromagnetic coupling between the L.S. FeIII and NiII centers, but neither exhibits slow paramagnetic relaxation effects down to T=2K. In Chapter IV, this chemistry was extended to the [Mn(CN)6]3-building block in order to increase magnetic exchange coupling and anisotropy in this cluster type, efforts that resulted in the isolation of the molecule, {[Mn(tmphen)2]3[Mn(CN)6]2}. This cluster exhibits intramolecular antiferromagnetic exchange interactions between the Mn centers which lead to an S=11/2 ground state and a negative ZFS value (D=-0.348 cm-1), parameters that support the experimental observation of Single-Molecule Magnet (SMM) behavior at low temperatures. A detailed investigation of the physical and structural properties of {[Co(tmphen)2]3[Fe(CN)6]2} in Chapters V and VI led to the realization that the cluster exhibits sensitivity to temperature and humidity. The molecule exists in three different electronic isomeric forms in the solid state and undergoes a charge-transfer induced spin-transition (CTIST) under the influence of temperature. The results presented in Chapter VI describe the behavior of this same cluster in solution, the highlight of which is the discovery that water reacts with the cluster to form a fourth electronic isomer. Finally, it is described in Chapter VII that this Co/Fe trigonal bipyramidal unit can be used as a building block for systematically incorporating three metal types into a family of 1-D chain and cluster compounds.

Ce travail de thèse décrit (i) la synthèse de complexes hétérométalliques d'ions 3d et 4f à partir de précuseurs de Mn, Fe et Co, de sels de lanthanides et de ligands organiques et (ii) l'étude de leurs structures et propriétés. 41 complexes polynucléaires ont été synthétisés dans le cadre de ce travail. Les structures moléculaires de tous les composés ont été déterminées par diffraction des rayons X. Les propriétés magnétiques de 22 complexes ont été étudiées, dont quatre montrent une relaxation lente de leur aimantation considérée comme la signature d'un comportement de molécule-aimant. L'activité catalytique du complexe {Mn4Dy6Li2} calciné a aussi été étudiée et s'est avérée efficace pour l'oxydation du monoxyde de carbone. L'étude systématique de complexes isostructuraux de lanthanides a montré que l'incorporation d'ions 4f peut introduire de l'anisotropie magnétique et que l'ion DyIII est généralement le meilleur candidat pour le ciblage de molécules-aimants hétérométalliques 3d- 4f.