Ligand-Assisted Metal-Centered Electrocatalytic Hydrogen Evolution upon Reduction of a Bis(thiosemicarbazonato)Ni(II) Complex

In this study, we report the electrocatalytic behavior of the neutral, monomeric Ni­(II) complex of diacetyl-bis­(N-4-methyl-3-thiosemicarbazonato), NiL1, for ligand-assisted metal-centered hydrogen evolution in acetonitrile (ACN) and dimethylformamide (DMF). Using foot-of-the-wave analysis (FOWA), NiL1 displays a maximum turnover frequency (TOF) of 4200 and 1200 s–1 for acetic acid (CH3COOH) in ACN and DMF, whereas for trifluoroacetic acid (CF3COOH) the TOFs are 1300 and 120 s–1 in ACN and DMF, respectively. In ACN, the overpotentials are 0.53 and 0.67 V for CH3COOH and CF3COOH, respectively. In DMF, the overpotential is 0.85 V for CH3COOH. First-order dependence with respect to the catalyst is established. NiL1 displays a minimum Faradaic efficiency of 87% from controlled potential electrolysis. Gas analysis from controlled potential electrolysis in both ACN and DMF using CH3COOH and CF3COOH confirms NiL1 as an electrocatalyst to produce H2. In ACN, TONs of 48 and 24 were obtained for CH3COOH and CF3COOH, respectively in 4 h. In DMF, TONs of 13 and 3 were obtained for CH3COOH and CF3COOH, respectively. The H2 evolution reaction was evaluated using deuterated acid, demonstrating an inverse kinetic isotope, which is consistent with formation of a metal hydride intermediate. A proposed ligand-assisted metal-centered mechanism for HER is supported by computational investigations. All catalytic intermediates in the proposed mechanism were structurally and energetically characterized using density functional theory (DFT), with the B3LYP/6-311g­(d,p) and BP86/TZV/P in solution modeled via polarizable continuum model. The final step of catalysis involves the reaction of [HNi­(L1·)]− with H+ generating H2. The correctness of proposed mechanism was confirmed by location of corresponding transition state (TS) having single imaginary frequency (i1786 cm–1)