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Hydrogen Evolution Catalyzed by Cobaloximes

Dempsey, Jillian L. and Brunschwig, Bruce S. and Winkler, Jay R. and Gray, Harry B. (2009) Hydrogen Evolution Catalyzed by Cobaloximes. Accounts of Chemical Research, 42 (12). pp. 1995-2004. ISSN 0001-4842. doi:10.1021/ar900253e.

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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.

Item Type:Article
Related URLs:
URLURL TypeDescription
Brunschwig, Bruce S.0000-0002-6135-6727
Winkler, Jay R.0000-0002-4453-9716
Gray, Harry B.0000-0002-7937-7876
Additional Information:© 2009 American Chemical Society. Received on September 30, 2009. Publication Date (Web): November 23, 2009. We thank Xile Hu, Louise Berben, Brandi Cossairt, and Jonas Peters for discussions and their important contributions to cobaloxime chemistry. This work was supported by an NSF Center for Chemical Innovation (CCI Powering the Planet, Grants CHE-0802907 and CHE-0947829), the Arnold and Mabel Beckman Foundation, CCSER (Gordon and Betty Moore Foundation), and the BP MC2 program. J.L.D. is an NSF Graduate Research Fellow.
Group:CCI Solar Fuels
Funding AgencyGrant Number
Arnold and Mabel Beckman FoundationUNSPECIFIED
Gordon and Betty Moore FoundationUNSPECIFIED
NSF Graduate Research FellowshipUNSPECIFIED
Issue or Number:12
Record Number:CaltechAUTHORS:20100120-084526967
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Official Citation:Hydrogen Evolution Catalyzed by Cobaloximes Jillian L. Dempsey, Bruce S. Brunschwig, Jay R. Winkler, Harry B. Gray Accounts of Chemical Research 2009 42 (12), 1995-2004
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:17228
Deposited By: Ruth Sustaita
Deposited On:25 Jan 2010 17:12
Last Modified:08 Nov 2021 23:33

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