Design and synthesis of heteroleptic Ni(II) dipyrrin complexes for electrochemical proton reduction reactions: Cyclic voltammetric and theoretical studies

The effect of nuclearity on electrochemical hydrogen generation using new heteroleptic Ni(II) complexes containing redox-active dipyrrin and dithiocarbamate ligands has been described. Complexes 1–2 have been meticulously characterized by spectroscopic studies (ESI-MS, IR, 1H, 13C NMR, UV–vis) and their structures unambiguously confirmed by X-ray single crystal analyses. Electrocatalytic properties of the complexes toward hydrogen evolution reaction have been investigated by cyclic voltammetric studies in an organic medium in the presence of acetic acid as a weak proton source. Notably, complexes 1 and 2 produce H2 via doubly reduced Ni(II) species i.e. Ni(0) in the presence of acetic acid. Further, these complexes exhibited significant electrocatalytic activity (TOF: 264 (1) and 650 s−1 (2). Controlled potential electrolysis established a minimum Faradaic efficiency of 92 (1) and 96 % (2). Complex 2 exhibited higher turnover frequency relative to 1, while 1 showed lower overpotential (0.35 V) in comparison to 2 (0.45 V). The stability of the complexes and the amount of produced H2 has been investigated by bulk electrolysis study. A tentative mechanism (ECEC; E, electrons and C, chemical steps) and involved intermediate species for the proton reduction reaction for 1 has been established by theoretical studies.