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

Five neutral heteroleptic maleonitriledithiolate nickel complexes [RN(PPh2)(2)Ni(mnt)] bearing bis(diphenylphosphanyl)amine ligands (where mnt(2-) = maleonitriledithiolate; R = CH3O(CH2)(3) (1), CH3S(CH2)(3) (2), (S)-CH3CHPh (3), (CH3)(2)CH(CH2)(2) (4), and p-CH3C6H4 (5)) have been synthesized and fully characterized by elemental analysis, FTIR, NMR (H-1, C-13, and P-31) 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 center dot 1.5CH(2)Cl(2) 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 (eta) 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.