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Electrocatalytic Hydrogen Evolution at Low Overpotentials by Cobalt Macrocyclic Glyoxime and Tetraimine Complexes

Hu, Xile and Brunschwig, Bruce S. and Peters, Jonas C. (2007) Electrocatalytic Hydrogen Evolution at Low Overpotentials by Cobalt Macrocyclic Glyoxime and Tetraimine Complexes. Journal of the American Chemical Society, 129 (29). pp. 8988-8998. ISSN 0002-7863. doi:10.1021/ja067876b. https://resolver.caltech.edu/CaltechAUTHORS:20140730-123328207

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Abstract

Cobalt complexes supported by diglyoxime ligands of the type Co(dmgBF_2)_2(CH_3CN)_2 and Co(dpgBF_2)_2(CH_3CN)_2 (where dmgBF_2 is difluoroboryl-dimethylglyoxime and dpgBF_2 is difluoroboryl-diphenylglyoxime), as well as cobalt complexes with [14]-tetraene-N_4 (Tim) ligands of the type [Co(Tim^R)X_2]^(n+) (R = methyl or phenyl, X = Br or CH_3CN; n = 1 with X = Br and n = 3 with X = CH_3CN), have been observed to evolve H_2 electrocatalytically at potentials between −0.55 V and −0.20 V vs SCE in CH_3CN. The complexes with more positive Co(II/I) redox potentials exhibited lower activity for H_2 production. For the complexes Co(dmgBF_2)_2(CH_3CN)_2, Co(dpgBF_2)_2(CH_3CN)_2, [Co(Tim^(Me))Br2]Br, and [Co(Tim^(Me))(CH_3CN)_2](BPh_4)_3, bulk electrolysis confirmed the catalytic nature of the process, with turnover numbers in excess of 5 and essentially quantitative faradaic yields for H_2 production. In contrast, the complexes [Co(Tim^(Ph/Me))Br_2]Br and [Co(Tim^(Ph/Me))(CH_3CN)_2](BPh_4)_3 were less stable, and bulk electrolysis only produced faradaic yields for H_2 production of 20−25%. Cyclic voltammetry of Co(dmgBF_2)_2(CH_3CN)_2, [Co(Tim^(Me))Br_2]^+, and [Co(Tim^(Me))(CH_3CN)_2]^(3+) in the presence of acid revealed redox waves consistent with the Co(III)−H/Co(II)−H couple, suggesting the presence of Co(III) hydride intermediates in the catalytic system. The potentials at which these Co complexes catalyzed H_2 evolution were close to the reported thermodynamic potentials for the production of H_2 from protons in CH_3CN, with the smallest overpotential being 40 mV for Co(dmgBF_2)_2(CH_3CN)_2 determined by electrochemistry. Consistent with this small overpotential, Co(dmgBF_2)_2(CH_3CN)_2 was also able to oxidize H_2 in the presence of a suitable conjugate base. Digital simulations of the electrochemical data were used to study the mechanism of H_2 evolution catalysis, and these studies are discussed.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1021/ja067876bDOIArticle
http://pubs.acs.org/doi/abs/10.1021/ja067876bPublisherArticle
http://pubs.acs.org/doi/suppl/10.1021/ja067876bPublisherSupporting Information
ORCID:
AuthorORCID
Hu, Xile0000-0001-8335-1196
Brunschwig, Bruce S.0000-0002-6135-6727
Peters, Jonas C.0000-0002-6610-4414
Additional Information:© 2007 American Chemical Society. Received November 3, 2006; Publication Date (Web): June 28, 2007. We acknowledge support from an NSF Chemical Bonding Center (grant CHE-0533150) and from the Beckman Institute Molecular Materials Research Center. We thank Prof. Nathan S. Lewis, Prof. Harry B. Gray, and Dr. Jay Winkler for insightful discussions. We also thank Prof. Alex Sessions and Dr. Chao Li for their generous help with the gas chromatography measurements and Neal Mankad and Larry Henling for help with crystallographic studies.
Funders:
Funding AgencyGrant Number
NSFCHE-0533150
Caltech Beckman InstituteUNSPECIFIED
Issue or Number:29
DOI:10.1021/ja067876b
Record Number:CaltechAUTHORS:20140730-123328207
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20140730-123328207
Official Citation:Hu, X., Brunschwig, B. S., & Peters, J. C. (2007). Electrocatalytic Hydrogen Evolution at Low Overpotentials by Cobalt Macrocyclic Glyoxime and Tetraimine Complexes. Journal of the American Chemical Society, 129(29), 8988-8998. doi: 10.1021/ja067876b
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:47675
Collection:CaltechAUTHORS
Deposited By: Joanne McCole
Deposited On:30 Jul 2014 21:38
Last Modified:10 Nov 2021 17:49

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