Hydrogen Evolution Catalyzed by Cobaloximes

Abstract

Natural photosynthesis uses sunlight to drive the conversion of energy-poor molecules (H_2O, CO_2) to energyrich ones (O_2, (CH_2O)_n). Scientists are working hard to develop efficient artificial photosynthetic systems toward the "Holy Grail" of solar-driven water splitting. High on the list of challenges is the discovery of molecules that efficiently catalyze the reduction of protons to H_2. In this Account, we report on one promising class of molecules: cobalt complexes with diglyoxime ligands (cobaloximes). Chemical, electrochemical, and photochemical methods all have been utilized to explore proton reduction catalysis by cobaloxime complexes. Reduction of a Co^(II)-diglyoxime generates a Co^I species that reacts with a proton source to produce a Co^(III)-hydride. Then, in a homolytic pathway, two Co^(III) hydrides react in a bimolecular step to eliminate H_2. Alternatively, in a heterolytic pathway, protonation of the Co^(III)-hydride produces H_2 and Co^(III). A thermodynamic analysis of H_2 evolution pathways sheds new light on the barriers and driving forces of the elementary reaction steps involved in proton reduction by Co^I-diglyoximes. In combination with experimental results, this analysis shows that the barriers to H_2 evolution along the heterolytic pathway are, in most cases, substantially greater than those of the homolytic route. In particular, a formidable barrier is associated with Co^(III)-diglyoxime formation along the heterolytic pathway. Our investigations of cobaloxime-catalyzed H_2 evolution, coupled with the thermodynamic preference for a homolytic route, suggest that the rate-limiting step is associated with formation of the hydride. An efficient water splitting device may require the tethering of catalysts to an electrode surface in a fashion that does not inhibit association of Co^(III)-hydrides.

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