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

Abstract

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)X2] n + (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)Br2]Br, 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)Br2]Br 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)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 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.

Theoretical Modeling of Low‐Energy Electronic Absorption Bands in Reduced Cobaloximes

“…[3,4,5] of their promising hydrogen-evolving catalytic capability. Today, cobaloximes, and the related diimine-dioxime cobalt complexes [6,7,8,9], are recognized as some of the most efficient molecular catalysts for electro-and photo-catalytic hydrogen evolution [10,11,12].…”

“…These compounds are known to be powerful nucleophiles in their reduced Co(I) state. It is accepted that the catalytic cycle for hydrogen evolution proceeds via protonation of the Co(I) species, yielding a Co(III)-H hydridocobaloxime intermediate that, after further reduction to the Co(II)-H state, can evolve dihydrogen through either protonation of the hydride moiety or bimolecular reductive elimination [1,2,3,4,5,10,11,6,13,14,15,16,17,18].…”

“…These compounds are known to be powerful nucleophiles in their reduced Co(I) state. It is accepted that the catalytic cycle for hydrogen evolution proceeds via protonation of the Co(I) species, yielding a Co(III)-H hydridocobaloxime intermediate that, after further reduction to the Co(II)-H state, can evolve dihydrogen through either protonation of the hydride moiety or bimolecular reductive elimination [1,2,3,4,5,10,11,6,13,14,15,16,17,18].Recent reports from the groups of Muckerman [19], and Jiang [21] have addressed aspects of the catalytic activity of these compounds using quantum chemical approaches and confirm the role of the Co(I) species. Experimentally, the spectroscopic signatures of Co(I) intermediates have been observed during the course of electro-and photo-catalytic experiments [4,5,22], and the Co(I) species [14,23,24,9], and its protonated Co(III)-H form [25,26,27] Figure 1).…”

Singly versus Doubly Reduced Nickel Porphyrins for Proton Reduction: Experimental and Theoretical Evidence for a Homolytic Hydrogen‐Evolution Reaction

“…Unlike mononuclear mechanisms, only a few examples of bimetallic pathways B and E have been suggested to likely be involved in HER 28, 31. For example, in a Co corrole system reported by Dey, Gross, and co‐workers, pathways A and B are suspected to be possible on the basis of density functional theory (DFT) calculations 37.…”

“…These results highlighted the significant effect of pentafluorophenyl groups in regulating the redox chemistry of Ni porphyrins. As electrocatalytic proton reduction is closely related to the reduction of catalysts, meso substituents with strong electron‐withdrawing properties are considered to be able to cut the energy cost for generating H 2 , and thus to benefit H 2 evolution, despite the fact that they might decrease the basicity of the metal center and make the metal center less reactive to protons 28, 34. This result may explain why the Co pophyrins without strong electron‐withdrawing groups reported by Fukuzumi and co‐workers are catalysts for O 2 reduction but not as efficient for proton reduction, as O 2 reduction is initiated by Co II , whereas Co I or Co 0 are required for proton reduction.…”