Coinage metal hydrides: reactive intermediates in catalysis and significance to nanoparticle synthesis
AffiliationSchool of Chemistry
Document TypePhD thesis
Access StatusOpen Access
© 2019 Dr. Athanasios Zavras
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.
Keywordschemistry; mass spectrometry; coinage metals; hydrides; clusters; nanoparicles; catalysis; hydrogen storage
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