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Published July 25, 2007 | Supplemental Material
Journal Article Open

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

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.

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.

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August 19, 2023
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