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-thiosemi-carbazonato), 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 H-2. 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 H-2 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(L-1.)](-) with H+ generating H-2. The correctness of proposed mechanism was confirmed by location of corresponding transition state (TS) having single imaginary frequency (i1789 cm(-1)).