School of Chemistry - Theses
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ItemMetal-anilate coordination polymers( 2018)Crystalline coordination polymers are a class of materials which hold promise as adsorbents, sensors, electronic materials and magnets, as catalysts and separation matrices, or as batteries and electro-catalysts. Many of these applications depend on materials which offer an unusual combination of properties, resulting from the combination of a ligand and metal in a periodic lattice. These properties include stability and permanent porosity upon desolvation, crystallinity, monodisperse pores, open metal sites, flexibility, uptake of and interaction with guest molecules. Coordination polymers derived from redox-active ‘anilate’ ligands such as 2,5-dihydroxy-p-benzoquinone (H2dhbq) and chloranilic acid (H2can) have been studied as a possible mechanism for the introduction of electroactive properties into coordination polymers. No systematic study of the coordination polymers derived from substituted anilate ligands has previously been undertaken. In Chapter 2, the syntheses and crystal structures of novel anilate molecules and derived mononuclear and dinuclear compounds are reported. Ligand syntheses are achieved in one or two steps from commercially available starting materials, achieving sufficient purity upon recrystallisation, and many are observed to form isostructural dinuclear metal complexes. The bond distances within these compounds and similar compounds in the literature are observed to be dependent on oxidation state, and are independent of metal coordination geometry, allowing for the assignment of oxidation state from crystallographic data. In Chapter 3, a number of novel one-dimensional coordination polymers are described with metal centres linked by anilate ligands. These one-dimensional chains are shown to have a few different shapes, dependent on the relative coordination geometries around sequential metal centres in a chain. Two strategies for the formation of porous materials by cross-linking of one-dimensional chains are described, but synthetic attempts ultimately resulted in the formation of non-porous materials. In Chapter 4, structures are shown in which each metal centre links to three bridging anilate ligands, forming two- and three-dimensional coordination polymers, the majority of which showed a two-dimensional hexagonal grid network structure. Of these compounds, anionic coordination polymers of the form [M2(anilate)3]2- were found to show open-type structure, and were investigated for host-guest properties via gas adsorption. Reversible, gas-specific structural transformations were observed with the loading of CO2 and N2O into some of these compounds; by changing the cation, the structure was rigidified and did not collapse on desolvation. Compounds containing iron exhibited metal-to-ligand charge transfer, resulting in enhanced conductivity. This MLCT was found to be a temperature-dependent process in some cases. In Chapter 5, structures are shown in which each metal centre links to four bridging anilate ligands, forming two- dimensional square-grid coordination polymers and three-dimensional diamond-type coordination polymers. The square grid compound (Et4N)[Y(can)2] is shown to maintain structure on loss of solvent, and is investigated for interactions with solid, liquid and gaseous intercalants. Particularly strong interactions with hydrogen, methane and CO2 are investigated further via in situ crystallographic methods. Finally, this thesis is concluded, and these individual results are examined as a whole, including commentary on the limitations of this work and potential future work in this area. Appendices 1–5 contain additional data that supplement the thesis.
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ItemTailor-made covalent organic-inorganic polyoxometalate hybrids: versatile platforms for the elaboration of novel molecular architectures( 2018)Covalent organic-inorganic polyoxometalate (POM or POMs) hybrids constitute versatile platforms for the elaboration of functional molecular architectures. This Ph.D. research project aimed to synthesize novel organic-inorganic POM hybrids using pre- and post-functionalization methods. The synthesis of organic-inorganic hybrids starting from POMs, known as direct functionalization, is a well-established synthetic procedure. However, as the complexity of the targeted functional system increases, a multi-step strategy relying on the post-functionalization of preformed hybrid POMs is necessary. Herein, both approaches are explored. Following hybridization of POM surfaces using organic units, ranging from small groups to large polymeric chains, this work provides a significant step forward in the rational design and synthesis of POMs, which permits the elaboration of POM based nanomaterials. At first, boronic acids and esters ligands were selected for POMs’ post-functionalization. Three organoboron functionalized Anderson-Evans and one organoboron functionalized Lindqvist POM were synthesized using Schiff base chemistry; with the general formulas of the Anderson-Evans POM hybrids being [MnIIIMo6VIO18((OCH2)3)CN=CHC6H4(B(OR)2)2]3− (where R = H, Me), [MnIIIMo6VIO18((OCH2)3)CN=CHC6H4(BO2(CH2)3)2]3-, with the formula of the Lindqvist POM hybrid being [VV6O13{(OCH2)3CN=CHC6H4B(OH)2}2]2-. These compounds have been characterized in the solid state by single-crystal X-ray diffraction (XRD), FT-IR spectroscopy and elemental analysis and in solution using Nuclear Magnetic Resonance (NMR) spectroscopy. This work has further been extended to organosilane functionalized mono and di lacunary Keggin POMs. Two organoboron functionalized Keggin POMs were synthesized using N, N'-dicyclohexylcarbodiimide (DCC) coupling; with the general formulas being [β2-SiW11O39{O(Si(CH2)3NHC=O-C12H17BO2)2}]4- and [γ-SiW10O36{O(Si(CH2)3NHC=OC12H17BO2)2}]4-. These compounds have been characterized in the solid state by FT-IR spectroscopy and elemental analysis and in solution using NMR spectroscopy. Later, the employment of microwave-assisted synthesis permitted the generation of novel mixed metal tris(alkoxo)molybdovanadates. The reaction of [β-Mo8O24]4- and [H3V10O28]3- with pentaerythritol or tris(hydroxymethyl)aminomethane yielded compounds with the general formula [V3Mo3O16(O3-R)]2- where R = C5H8OH or C4H6NH2. Post-synthetic esterification of the alcohol derivative yielded the acylated derivative [V3Mo3O16(O3-R)]2- where R = C7H11O2. Single-crystal X-ray Diffraction (XRD), NMR spectroscopy, High-Resolution Mass Spectrometry (HR-MS) and FT-IR spectroscopy have been used in combination to rationalize the structural isomerization observed within these systems. The rational design and synthesis of two novel covalent organic-inorganic hybrid polymers via Atom Transfer Radical Polymerization (ATRP), composed of either a Lindqvist POM macro initiator of formula [V3Mo3O19{(OCH2)3CNHC=OC(CH3)2Br}]2- or an Anderson-Evans POM macro initiator of formula [MnIIIMo6O18{(OCH2)3CNHC=OC(CH3)2Br}2]3- and pH-responsive poly(2 (diethylamino)ethyl methacrylate) (PDEAEMA) polymer, was investigated. POM macro initiators were characterized using single-crystal X-ray diffraction (XRD), 1H NMR spectroscopy, FT-IR spectroscopy, UV-Vis and elemental analysis; while POM-polymer hybrids were characterized using 1H NMR spectroscopy, FT-IR spectroscopy, thermogravimetric analysis (TGA) and UV-Vis spectroscopy to assess the integrity of the POM units. These POM-polymer hybrids self-assemble into nanoparticles via copolymerization with poly(2-(diethylamino)ethyl methacrylate)-b-poly(ethylene glycol) (PDEAEMA-b-PEG), when the pH is increased above the pKa of PDEAEMA. Dynamic Light Scattering (DLS) studies were conducted to investigate the size distribution of the nanoparticles, while disassembly studies proved that they respond to biologically relevant pH variations. These observations were supported by Cryo-TEM imaging which provided valuable direct visualization of the nanoparticles. Importantly, growing polymer chains from POM macro initiators offers an excellent control over the loading of the POM clusters inside the nanoparticles.
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ItemSynthesis and investigation of polyoxometalate-supported lanthanoid single-molecule magnets( 2017)The crystal field splitting of the literature lanthanoid single-molecule magnet (Ln-SMM) family Na9[Ln(W5O18)2] has been investigated using inelastic neutron scattering (INS). The experimental measurements have been complemented by complete active space self-consistent field (CASSCF) ab initio methods. Chapter 2 of this thesis is devoted to the preliminary study of the TbIII analogue alone. Given the novelty of INS applied to Ln-polyoxometalates (Ln-POMs), and the potential challenges posed by Ln-POM systems (i.e the neutron absorption of the lanthanoid and of tungsten atoms), the choice of the Tb analogue, which is not a SMM, was dictated by the fact that Tb has the lowest neutron absorption cross-section. It was not possible to rationalize the observed INS transitions using current models for the electronic structure of this system, so the INS spectra were interpreted based on crystal field CASSCF ab initio methods, the latter being an unprecedented theoretical approach for Ln-POMs. Ab initio calculations on the isolated polyanion [Tb(W5O18)2]9- (gas phase) allowed attribution of the complex INS spectra of Na9[Tb(W5O18)2] to the presence of two distinct polymorphs, with slightly different structural parameters. A simple magnetostructural model to correlate the INS signal of each polymorph with the structural parameters of the first coordination sphere of TbIII was proposed. In Chapter 3, based on the successful INS study of the TbIII analogue, the spectroscopic investigation was extended to the more challenging – in terms of neutron absorption cross-section - Nd, Ho and Er SMM analogues of the Na9[Ln(W5O18)2] family. Magnetic excitations were observed for all compounds investigated. The INS transitions and the magnetic properties were interpreted based on crystal field methods and the CASSCF ab initio method developed in Chapter 2 was extended to these compounds, including also the Dy analogue, showing that low-symmetry effects play an important role in defining the electronic properties of these systems. The electronic structures calculated ab initio outperform in many cases the current state-of-the-art theoretical models for the Na9[Ln(W5O18)2] family in terms of the ability to reproduce the energy of the low-lying crystal field levels as determined by INS. A model representing the distribution of electrostatic point charges associated to the atoms in the in the crystal lattice was introduced in the CASSCF calculations. This approach gave better agreement with the experimental data with respect to the “gas phase” methodology employed in Chapter 2. Chapter 4 presents the synthesis and characterisation of a new family of organic hybrid Ln-POMs. The compounds have been characterised by single crystal X-ray diffraction, X-ray photoelectron spectroscopy, elemental analysis, and FT-IR spectroscopy. Detailed direct (dc) and alternating current (ac) magnetic characterisation was performed, revealing slow relaxation of the magnetisation in the presence of an external dc magnetic field for one compound.