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

Cobalt complexes supported by diglyoxime ligands of the type Co(dmgBF2)2(CH3CN)2 and Co(dpgBF2)2(CH3CN)2 (where dmgBF2 is difluoroboryl-dimethylglyoxime and dpgBF2 is difluoroboryl-diphenylglyoxime), as well as cobalt complexes with 14-tetraene-N4 (Tim) ligands of the type Co(TimR)X2n+ (R=methyl or phenyl, X=Br or CH3CN; n=1 with X=Br and n=3 with X=CH3CN), have been observed to evolve H2 electrocatalytically at potentials between -0.55 V and -0.20 V vs SCE in CH3CN. The complexes with more positive Co(II/I) redox potentials exhibited lower activity for H2 production. For the complexes Co(dmgBF2)2(CH3CN)2, Co(dpgBF2)2(CH3CN)2, Co(TimMe)Br2Br, and Co(TimMe)(CH3CN)2(BPh4)3, bulk electrolysis confirmed the catalytic nature of the process, with turnover numbers in excess of 5 and essentially quantitative faradaic yields for H2 production. In contrast, the complexes Co(TimPh/Me)Br2Br and Co(TimPh/Me)(CH3CN)2(BPh4)3 were less stable, and bulk electrolysis only produced faradaic yields for H2 production of 20-25%. Cyclic voltammetry of Co(dmgBF2)2(CH3CN)2, Co(TimMe)Br2+, and Co(TimMe)(CH3CN)23+ 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 H2 evolution were close to the reported thermodynamic potentials for the production of H2 from protons in CH3CN, with the smallest overpotential being 40 mV for Co(dmgBF2)2(CH3CN)2 determined by electrochemistry. Consistent with this small overpotential, Co(dmgBF2)2(CH3CN)2 was also able to oxidize H2 in the presence of a suitable conjugate base. Digital simulations of the electrochemical data were used to study the mechanism of H2 evolution catalysis, and these studies are discussed.