School of Chemistry - Theses

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    Coinage metal hydrides: reactive intermediates in catalysis and significance to nanoparticle synthesis
    Zavras, Athanasios ( 2019)
    The coinage metal hydrides of copper, silver and gold have applications in catalysis and nanoparticle synthesis. Coinage metal hydrides are key intermediates in the chemical transformations of a range of substrates including fine chemical syntheses and chemical storage of hydrogen. Ranging from mononuclear coinage metal hydrides to clusters and nanoparticles, a fundamental understanding of their atomic and molecular interactions is invaluable in developing innovative solutions to practical problems. The reactive sites can be identified using a range of spectroscopic methods allowing the “tuning” and/or “reshaping” of the reactive site by ligands to control the reactivity. Mass spectrometry provides a means to identify coinage metal hydrides in solution and further allows isolation of discrete coinage metal hydrides that can be: (i) characterised, for example by spectroscopic methods, (ii) reacted with neutral substrates, or (iii) fragmented to generate reactive intermediates in the gas phase. The use of borohydride in nanoparticle synthesis is well-known. Chapter 2 describes a mass spectrometry directed synthesis to afford the first isolable silver hydride borohydride cluster, [Ag3(μ3-H)(μ3-BH4)L3]BF4 (L =bis(diphenylphosphino)methane), structurally characterised by X-ray crystallography. Gas-phase experiments and DFT calculations reveal ligand (L) loss from [Ag3(H)(BH4)L3]+ results in the loss of BH3 and a geometry change of the cluster to yield [Ag3(H)(BH4)Ln]+ (n = 1 or 2). This work reveals links between silver hydride/borohydride and silver hydride nanoclusters adding to our understanding of silver nanoparticle synthesis using borohydride salts. Chapter 3 examines that the reactivity of CO2 with the binuclear silver hydride cation core, [Ag2H]+, can be controlled by design. Reshaping the geometry and reaction environment of [Ag2H]+ using a range of phosphine ligands (bis(diphenylphosphino)methane, 1,2- bis(diphenylphosphino)benzene and bis(diphenylphosphino)ethane) allows “tuning” of the active site’s reactivity toward formic acid to produce H2. Gas-phase ion-molecule reactions, collision-induced dissociation, infrared and ultraviolet action spectroscopy and computational chemistry link structure to reactivity and mechanism. The gas-phase studies were then translated to solution-phase studies using NMR to show that H2 could be produced from solutions comprising well-defined ratios of ligand, AgBF4, NaO2CH and HO2CH at near ambient temperature. Chapter 4 further developed the concept of altering the reactive site by changing the binuclear metal centres of the [LAg2H]+ core to compare all six possible combinations of copper silver and gold i.e. [LAg2H]+, [LCu2H]+, [LAu2H]+, [LCuAgH]+, [LCuAuH]+ and [LAgAuH]+ in the gas phase. DFT calculations, gas-phase ion-molecule reactions and gas-phase energy-resolved collision-induced dissociation showed both metal centres play a role in the reaction with formic acid. One metal site functions as an “anchor” for an oxygen of formic acid or formate while the other facilitates the dehydrogenation step resulting in the formation of H2. It was found that the copper homobinuclear species performed best overall. Attempts to isolate the reactive intermediate [LAg2(O2CH)]+ by using a range of bisphosphine ligands resulted in the isolation of an unusual co-crystal in the case of L = dcpm as described in Chapter 5. Single crystal X-ray diffraction of crystals suitable for crystallographic analysis revealed two discrete tetranuclear silver clusters [(μ2-dcpm)Ag2(μ2-O2CH)(η2-NO3)]2·[(μ2- dcpm)2Ag4(μ2-NO3)4]. The solution-phase studies, tracked by NMR, show that H2 could be produced from solutions comprising well-defined ratios of ligand, AgBF4, NaO2CH and HO2CH at 65⁰C. Gas-phase studies indicate that while the tetranuclear cluster [L2Ag4(O2CH)3]+ undergoes sequential decarboxylation reactions, none of the resultant hydrides react with formic acid. These results highlight important role of the binuclear hydride [LAg2(H)]+ in the catalytic decarboxylation of formic acid. Hydrido cuprate [CuH2]- has been explored for its applications in hydrogen storage. Chapter 6 indicates two chemically induced routes for the liberation of hydrogen when [CuH2]- is reacted with various chemical substrates. One path occurs via homocoupling of both hydride ligands giving the substrate-coordinated copper, the other by protonation with acids.
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    Synthesis, structure and reactivity of ligand stabilized coinage metal nanoclusters
    Zavras, Athanasios ( 2013)
    The coinage metal nanoclusters (CMNCs), defined as copper, silver or gold, constitute an intermediate state of matter that exist between molecules and bulk material. The properties of CMNCs differs to that of molecules and bulk material due to quantum confinement effects. These nanostructured materials have attracted significant attention owing to their fundamentally interesting architectures, and unique properties with applications in areas such as catalysis, optical materials, medical imaging, models for hydrogen storage. Tailoring the properties of such promising materials has proven challenging and requires a fundamental understanding of their assembly, structure and reactivity. The aim of this thesis is: (i) the primary application of mass spectrometric techniques to monitor the formation of CMNCs which result from the addition of sodium borohydride to a solution consisting of a coinage metal salt and the bidentate ligand, bis(diphenylphsphino)methane (dppm) under various synthetic conditions; (ii) to apply this information in developing synthetic approaches to optimize clusters of interest and apply a mass spectrometry (MS) directed synthesis leading to the isolation of crystalline material suitable for structural characterization by X-ray crystallography (iii) apply MS based analysis methods to provide information on the reactivity of CMNCs in solution and the reactivity and structure of mass selected CMNCs in the gas phase. Electrospray ionization mass spectrometry (ESI-MS) and UV-Vis spectroscopy were used to monitor the formation of gold nanocluster cations in the condensed phase via the sodium borohydride (NaBH4) reduction of methanolic solutions containing AuClPPh3 and dppm. ESI-MS highlights the formation of complexes prior to the addition of NaBH4 as [Au2(dppm)2]2+, [Au(PPh3)2]+, [Au2(dppm)3]2+, [Au(dppm)2]+,[Au2Cl(dppm)2]+. The cationic complex product distribution can be monitored over a range of metal to ligand ratios to minimize the colloid precursor [Au(PPh3)2]+. The addition of NaBH4 where the optimized metal to ligand ratio was determined as AuClPPh3:dppm is 1:2 results in the formation of the following types of gold nanoclusters [Au9(dppm)4]3+, [Au9(dppm)5]3+, [Au5(dppm)3(dppm-H+)]2+, [Au10(dppm)4]2+, [Au11(dppm)5]3+, [Au11(dppm)6]3+, [Au13(dppm)6]3+ and [Au14(dppm)6(Ph2PCHPPh2)]3+. The gas phase unimolecular chemistry of these cations was examined by (i) collision induced dissociation (CID) and electron capture dissociation resulting in the gas phase synthesis of the novel clusters [Aux(dppm)y]z+ (x = 2,3 , 6–13; y = 1–6 and z = 1–3) and [Aux(dppm)y(dppm-H+)]z+ (x = 5,14; y= 2,5; z = 2,3) via ligand loss and core fission fragmentation channels. (ii) electron capture dissociation (ECD) of mass selected multiply charged gold cluster cations where an additional fragmentation channel arises due to C-P bond activation. ESI-MS was also applied to study the reactivity that results from silver salts in the presence of dppm, that are treated with sodium borohydride. It was observed by ESI-MS that no all metallic silver clusters had formed. Instead there existed abundant and relatively monodisperse trinuclear silver(I) hydride clusters. The synthesis could be refined by careful MS based analysis to result in the isolation of crystalline material of (i) [Ag3(μ3-H)(μ3-Cl)(dppm)3]BF4, and (ii) [Ag3(μ3-H)(dppm)3](BF4)2. These clusters could be mass selected to generate novel gas phase clusters in the gas phase. The multiply charged cation [Ag3(μ3-H)(dppm)3]2+ was also investigated by ECD and EID. The silver hydride cluster cation [Ag10H8(dppm)6]2+ was observed during the synthesis of trinuclear silver clusters. This cluster has yet to be isolated.