Date on Master's Thesis/Doctoral Dissertation

12-2017

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Chemistry

Degree Program

Chemistry, PhD

Committee Chair

Grapperhaus, Craig

Committee Member

Buchanan, Robert

Committee Member

Zamborini, Francis

Committee Member

Sumanasekera, Gamini

Author's Keywords

proton reduction; alcohol oxidation

Abstract

Hydrogen is a potential alternate to the carbon fuels to meet the future demands for energy in an eco-friendly manner. Hydrogen produced by the electrolysis of water is stored in fuel cells which on demand can be converted into electrical energy upon the reaction with oxygen. In the quest for economically viable electrocatalysts for hydrogen evolution, the work in this dissertation describes evaluation of nickel based catalysts coordinated to redox active ligands, dialkyl/diarylphosphine benzenethiol and bis(thiosemicarbazone) for their ability to generate hydrogen with external acids as a proton source. A nickel complex with the benzenethiol ligand framework follows a traditional metal-hydride route and affords a maximum turnover frequency of 51 s-1 with an overpotential of 1.1 V. A Ni(II) complex with the bis(thiosemicarbazone) ligand framework displayed a turnover frequency of 4161 s-1 operating at an overpotential of 0.52 V. Hydrogen evolution proceeds through metal-hydride intermediate distinct from the typical hydride pathway. DFT studies have been performed in gas phase and in solvent phase using polarizable continuum model (PCM) calculations to probe the mechanism of the hydrogen evolution reactions.To further evaluate the cooperativity between the redox active ligands and metals in catalysis, complexes that facilitate alcohol oxidation to mimic the activity of galactose oxidase were explored. Copper(II) complexes with an N4-ligand framework have been synthesized and evaluated as catalysts for oxidation of benzylic alcohols to benzylic aldehydes under aerobic conditions. Proton transfer from the protonated NMI to the imidazole arm in the ligand framework helps to open the site for the binding of TEMPO, thus driving the reaction forward. The reaction studied over the period of 4 hours resulted in yields of up to 99%. Optimum conditions are 5 mmol alcohol with 5 mol% of catalyst, NMI and TEMPO with respect to substrate in acetonitrile under aerobic conditions. To gain mechanistic insights, the components of the reaction mixture such as benzyl alcohol, catalyst concentration, NMI, oxygen flow rate, TEMPO and solvent have been varied.

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