Hydrogen Evolution Catalyzed by Cobaloximes

Citation

Dempsey, Jillian Lee (2011) Hydrogen Evolution Catalyzed by Cobaloximes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/XK4M-DZ27. https://resolver.caltech.edu/CaltechTHESIS:01252011-134049886

Abstract

Cobaloximes are among a promising class of small molecules which catalytically evolve hydrogen at modest overpotentials. Motivated by the imminent need to develop efficient solar energy conversion processes, a number of research groups have recently revisited the catalytic activity of cobaloximes, which was initially reported by Espenson almost three decades ago. Both Espenson’s seminal work and the studies reported during this recent resurgence are chronicled in the introductory Chapter 1. The next three chapters introduce photochemical methods for detecting catalytic intermediates and determining kinetics associated with the elementary steps of hydrogen evolution. Four catalytic pathways are considered; each beginning with the reduction of a Co^II-diglyoxime to generate Co^I, which reacts with a proton donor to produce a Co^III-hydride. In a homolytic pathway, two Co^III-hydrides react in a bimolecular step to eliminate H₂. Alternatively, in a heterolytic pathway, protonation of Co^III-hydride produces H₂ and Co^III. The Co^III-hydride may also be reduced further to a Co^II-hydride, which can react via analogous heterolytic or homolytic pathways. In Chapter 2, kinetics of electron transfer reactions of a Co-diglyoxime complex are presented. These experimental results, along with a detailed thermodynamic analysis of proposed hydrogen evolution pathways, shed new light on the barriers and driving forces of the elementary reaction steps involved in proton reduction. A strong thermodynamic preference for a Co^III-hydride homolytic pathway over a Co^III-hydride heterolytic route is identified as the key finding from this work. In Chapter 3, phototriggered hydride generation utilized in conjunction with time-resolved spectroscopy is introduced as a novel method for mechanistic investigations. Here, excited-state proton transfer from an organic photoacid to a Co^I-diglyoxime triggers the formation of the reactive Co^III-hydride. This and the subsequent reactivity of Co^III-hydride are monitored spectroscopically. The reaction kinetics are consistent with a heterolytic route for hydrogen evolution that proceeds via a Co^II-hydride intermediate. Chapter 4 extends these mechanistic investigations to aqueous media by employing photoionization and pulse radiolysis methods to trigger Co^II-diglyoxime reduction. Chapters 5 and 6 focus on the design and construction of second generation cobaloximes. In Chapter 5, the thermodynamic preference for bimolecular reactivity of two Co^III-hydrides is probed with a binuclear cobaloxime. A covalent alkyl tether is used to decrease the volume required for diffusional collisions. Electrocatalytic activity is consistent with a rate-limiting step associated with the formation of the hydride, as seen in mononuclear catalysts, and thus no enhancement of catalytic activity is observed. However, as an efficient water splitting device may require the tethering of catalysts to an electrode surface, this ligand should allow binuclear association of immobilized catalysts. A strategy for covalently grafting cobaloxime derivatives to silicon electrodes is introduced in Chapter 6. A terminal olefin is incorporated into a glyoxime backbone, a functionality amenable to surface-based coupling reactions. The bifunctional cobaloxime is an active catalyst, and initial efforts to prepare the chemically modified electrode are discussed. Three appendices are provided, including work on the photochemical generation of powerful Os^II reductant, electron transfer reactions of N,N’,3,3’-tetramethyl-4,4’-bipyridinium, and annotated MATLAB scripts utilized for kinetics analysis.

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