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

Abstract

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).