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Coinage Metal Hydrides: Reactive Intermediates in Catalysis and Significance to Nanoparticle Synthesis

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Coinage Metal Hydrides: Reactive Intermediates in Catalysis and Significance to Nanoparticle Synthesis Coinage Metal Hydrides: Reactive Intermediates in Catalysis and Significance to Nanoparticle Synthesis Athanasios Zavras ORCID: 0000-0003-2797-9303 Submitted in total fulfillment of the requirements of the degree of Doctor of Philosophy May 2019 School of Chemistry The University of Melbourne Abstract 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 i 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 2 revealed two discrete tetranuclear silver clusters [(μ2-dcpm)Ag2(μ2-O2CH)(η -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. ii The mechanisms for nanocluster to nanoparticle transformation are explored in chapter 7. Isolable [Ag3(μ3-H)(BH4)(dppm)3]BF4 is proposed as a starting material in the isolation of the [{Cl@Ag12}@Ag48(dppm)12] nanoparticle. The mechanism of decomposition for [Ag3(μ3- H)(BH4)(dppm)3]BF4, known to occur via liberation of H2 (Chapter 2), is suggested to reduce silver(I) to silver(0) thus triggering the formation of the nanoparticle. iii iv Declaration This is to certify that i. the thesis comprises only my original work towards the PhD except where indicated in the Preface, ii. due acknowledgement has been made in the text to all other material used, iii. the thesis is less than 100 000 words in length, exclusive of tables, maps, bibliographies and appendices. Athanasios Zavras May 2019 v vi Preface I certify that I have contributed > 50% of the content in each of the publications included in this thesis, am the “primary author”, have written the “initial draft” and edited the revisions. The following are peer review publications included and the role of the co-authors: Chapter 2 Zavras, A., Ariafard, A., Khairallah, G. N., White, J. M., Mulder, R. J., Canty, A. J., O'Hair, R. A. J. Synthesis, structure and gas-phase reactivity of the mixed silver hydride borohydride Ph nanocluster [Ag3(μ3-H)(μ3-BH4)L 3]BF4 (LPh = bis(diphenylphosphino)methane). Nanoscale, 2015, 7 (43), 18129-18137. DOI: 10.1039/c5nr05690j Ariafard, A.: Assistance with the theoretical calculations and advisory role Khairallah, G. N.: Advisory role White, J. M.: X-ray crystallography of crystalline material and advisory role Mulder, R. J.: Assistance with the NMR Canty, A. J.: Advisory role O'Hair, R. A. J.: Advisory role Chapter 3 Zavras, A., Khairallah, G. N., Krstic, M., Girod, M., Daly, S., Antoine, R., Maitre, P., Mulder, R. J., Alexander, S.-A., Bonacic-Koutecky, V., Dugourd, P., O’Hair, R. A. J. Ligand-induced + substrate steering and reshaping of [Ag2(H)] scaffold for selective CO2 extrusion from formic acid. Nat. Commun. 2016, 7, 11746. DOI: 10.1038/ncomms11746 Khairallah, G. N.: Assistance with the IRMPD and UVPD and advisory role Krstic, M.: Assistance with the theoretical calculations Girod, M.: Assistance with the UVPD vii Daly, S.: Assistance with the UVPD Antoine, R.: Assistance with the UVPD Maitre, P.: Assistance with the IRMPD Mulder, R. J.: Assistance with the NMR Alexander, S.-A.: Assistance with the NMR Bonacic-Koutecky, V.: Advisory role Dugourd, P.: Advisory role O’Hair, R. A. J.: Advisory role Chapter 4 Zavras, A., Krstic, M., Dugourd, P., Bonacic-Koutecky, V., O’Hair, R. A. J. Selectivity Effects in Bimetallic Catalysis: Role of the Metal Sites in the Decomposition of Formic Acid into H2 and + CO2 by the Coinage Metal Binuclear Complexes [dppmMM'(H)] . Chem. Cat. Chem., 2017, 9, 1298-1302 Krstic, M.: Assistance with the theoretical calculations Dugourd, P.: Advisory role Bonacic-Koutecky, V.: Advisory role O’Hair, R. A. J.: Advisory role Chapter 5 2 Zavras, A., White, J. M., O’Hair, R. A. J. An unusual co-crystal [(μ2-dcpm)Ag2(μ2-O2CH)(η - NO3)]2·[(μ2-dcpm)2Ag4(μ2-NO3)4] and its connection to the selective decarboxylation of formic acid in the gas phase. Dalton Trans., 2016, 45, 19408-19415 White, J. M.: X-ray crystallography of crystalline material and advisory role O’Hair, R. A. J.: Advisory role viii Chapter 6 Zavras, A., Ghari, H., Ariafard, A., Canty, A. J., O’Hair, R. A. J. Gas-Phase Ion-Molecule – – Reactions of Copper Hydride Anions [CuH2] and [Cu2H3] . Inorg. Chem., 2017, 56 (5), 2387- 2399 Ghari, H.: Assistance with the theoretical calculations Ariafard, A.: Advisory role Canty, A. J.: Advisory role O’Hair, R. A. J.: Advisory role Chapter 7 Zavras, A. Mravak, A., Buzancic, M., White, J. M., Bonacic-Koutecky, V., O’Hair, R. A. J. Structure of the Ligated Ag60 Nanoparticle [{Cl@Ag12}@Ag48(dppm)12] (where dppm = bis(diphenylphosphino)methane). CJCP, 2019 Manuscript ID CJCP1812285 Accepted Mravak, A.: Assistance with the theoretical calculations Buzancic, M.: Assistance with the theoretical calculations White, J. M.: X-ray crystallography of crystalline material and advisory role Bonacic-Koutecky, V.: Advisory role O’Hair, R. A. J.: Advisory role ix x xi xii xiii xiv xv xvi xvii xviii xix All co-authors must complete this ·rorm. By signing below co-authors agree to the listed publication being included in the student's thesis and that the student contributed greater than 50% of the content of the publication and is the "primary author" ie. the student was responsible primarily for the planning, execution and preparation of the work for publication. In cases where all members of a large consortium are listed as authors of a publication, only those that actively collaborated with the student on material contained within the thesis should complete this form. This form is to be used in conjunction with the Declaration for a thesis with publication form. Students must submit this form, along with the Declaration for thesis with publicationform, when the thesis is submitted to the Thesis Examination System: https:lltes.app.unimefb.edu.au/ Further information on this policy and the requirements is available at: h e a e l •• ,. I _ Q[8_c/r��f!.8(C :.'-!r.!'!1._ /b_;.f!.cl.'!.·. l!.IPr.
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