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.