Kinetics of Electron Transfer Reactions of H_2-Evolving Cobalt Diglyoxime Catalysts
Abstract
Co−diglyoxime complexes catalyze H_2 evolution from protic solutions at modest overpotentials. Upon reduction to Co^I, a Co^(III)-hydride is formed by reaction with a proton donor. Two pathways for H_2 production are analyzed: one is a heterolytic route involving protonation of the hydride to release H_2 and generate Co^(III); the other is a homoytic pathway requiring association of two Co^(III)-hydrides. Rate constants and reorganization parameters were estimated from analyses of laser flash−quench kinetics experiments (Co^(III)−Co^(II) self-exchange k = 9.5 × 10^(−8) − 2.6 × 10^(−5) M^(−1) s^(−1); λ = 3.9 (±0.3) eV: Co^(II)−Co^(I) self-exchange k = 1.2 (±0.5) × 10^5 M^(−1) s^(−1); λ = 1.4 (±0.05) eV). Examination of both the barriers and driving forces associated with the two pathways indicates that the homolytic reaction (Co^(III)H + Co^(III)H → 2 Co^(II) + H_2) is favored over the route that goes through a Co^(III) intermediate (Co^(III)H + H+ → Co^(III) + H_2).
Additional Information
© 2009 American Chemical Society. Received September 21, 2009. Publication Date (Web): December 31, 2009. We thank Bruce Brunschwig, Xile Hu, Jay Labinger, and Jonas Peters for insightful discussions. We also thank Etsuko Fujita for generous assistance in obtaining in situ absorption spectra. This work was supported by the NSF Center for Chemical Innovation (Powering the Planet, CHE-0802907 and CHE- 0947829), the Arnold and Mabel Beckman Foundation, CCSER (Gordon and Betty Moore Foundation), and the BP MC2 program. JLD is an NSF Graduate Research Fellow.Attached Files
Supplemental Material - ja9080259_si_001.pdf
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Additional details
- Eprint ID
- 17867
- Resolver ID
- CaltechAUTHORS:20100406-102958526
- CHE-0802907
- NSF
- CHE- 0947829
- NSF
- Arnold and Mabel Beckman Foundation
- Gordon and Betty Moore Foundation
- BP MC2 program
- Caltech Center for Sustainable Energy Research
- NSF Graduate Research Fellowship
- Created
-
2010-04-06Created from EPrint's datestamp field
- Updated
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2021-11-08Created from EPrint's last_modified field
- Caltech groups
- CCI Solar Fuels