Neutral heteroleptic nickel complexes incorporating maleonitriledithiolate and bis(diphenylphosphanyl)amine as robust molecular electrocatalysts for hydrogen evolution

Article abstract of DOI:10.1016/j.ica.2020.119587

Five neutral heteroleptic maleonitriledithiolate nickel complexes [RN(PPh₂)₂Ni(mnt)] bearing bis(diphenylphosphanyl)amine ligands (where mnt^2? = maleonitriledithiolate; R = CH₃O(CH₂)₃ (1), CH₃S(CH₂)₃ (2), (S)-CH₃CHPh (3), (CH₃)₂CH(CH₂)₂ (4), and p-CH₃C₆H₄ (5)) have been synthesized and fully characterized by elemental analysis, FTIR, NMR (¹H, ¹³C, and ³¹P) spectroscopy, UV–Vis spectrum, single crystal X-ray diffraction, thermogravimetric analysis, and density functional theory (DFT) calculation. The nickel atom in 1, 2, 4, and 5?1.5CH₂Cl₂ adopts a slightly distorted square-planar coordination environment with two phosphorus atoms of RN(PPh₂)₂ and two sulfur atoms of mnt^2?, respectively, while the nickel atom in 3 exhibits a pseudo square pyramid with HP₂S₂ chromophore. Furthermore, the electrochemical properties for 1–5 were also investigated by cyclic voltammetry. With the addition of 120 mM trifluoroacetic acid (TFA) to 1–5 in MeCN, the turnover frequency (TOF) values are estimated to be 368.59–585.17 s^?1 with the corresponding overpotential (η) values of 0.59–0.73 V. The electrochemical studies indicate that all complexes can be used as low-cost robust molecular eletrocatalysts for the reduction protons to hydrogen in the presence of TFA and a possible ECEC mechanism for electrocatalytic hydrogen evolution has been also proposed.