Electrocatalytic hydrogen evolution at low overpotentials by cobalt macrocyclic glyoxime and tetraimine complexes

Cobalt complexes supported by diglyoxime ligands of the type Co(dmgBF(2))(2)(CH(3)CN)(2) and Co(dpgBF(2))(2)(CH(3)CN)(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(3)CN; n = 1 with X = Br and n = 3 with X = CH(3)CN), have been observed to evolve H(2) electrocatalytically at potentials between -0.55 V and -0.20 V vs SCE in CH(3)CN. 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(3)CN)(2), Co(dpgBF(2))(2)(CH(3)CN)(2), [Co(Tim(Me))Br(2)]Br, and [Co(Tim(Me))(CH(3)CN)(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(3)CN)(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(3)CN)(2), [Co(Tim(Me))Br(2)](+), and [Co(Tim(Me))(CH(3)CN)(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(3)CN, with the smallest overpotential being 40 mV for Co(dmgBF(2))(2)(CH(3)CN)(2) determined by electrochemistry. Consistent with this small overpotential, Co(dmgBF(2))(2)(CH(3)CN)(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.