Electrochemical Hydrogen Generation by Four-Coordinate Square-Planar Ni(II) Complex with N2P2-Type Ligand

A Ni(II) complex with N2P2-type ligand, [Ni(LH)2](BF4)2 (LH = 2-((diphenylphosphino)methyl)-pyridine), was prepared and characterized structurally, spectroscopically, and electrochemically. Its electrochemical hydrogen production reaction was investigated, which was compared with that of the previously reported Ni(II) complex bearing amino group in the ligand, [Ni(LNH2)2](BF4)2 (LNH2 = 6-((diphenylphosphino)methyl)-pyridin-2-amine). The X-ray crystal structure reveals to be a four-coordinate square planar structure (τ4 = 0.25) in the cis form, with the counter anion BF4– weakly presenting on the Ni(II) ion. The structure in the solution was assessed on the basis of UV-vis and NMR spectral features, which showed four coordinate square planar structure in dichloromethane and five- or six-coordinate structure bound with solvent molecules in acetonitrile. The electrochemical hydrogen production reaction using AcOH as a proton source showed similar behaviour to that of [Ni(LNH2)2](BF4)2, with the catalytic current (icat) proportional to the square root of the concentration of AcOH added. It indicates that the reaction mechanism is EECC and that the rate-determining step is the reaction stage of the two-electron reduced Ni(0) species with the approaching proton to form the Ni(II)–Hｰ species. The TOF and overpotential values, when evaluated under the same conditions as in the previous study (complex: 1 mM, electrolyte [n-Bu4N](ClO4): 0.1 M in MeCN (3 mL), AcOH = 145 equiv. (pKa = 22.3 in MeCN)), were evaluated to be 1060 s–1 and 710 mV, respectively. These values were higher in the overpotential and smaller in TOF, as compared to those of [Ni(LNH2)2](BF4)2 (TOF 8800 s–1, overpotential 430 mV). The structure of the starting material [NiII(LH)2]2+ and the formation of the hydride Ni(II) complex [NiIIH(LH)2]+, a reaction intermediate in the hydrogen evolution reaction, were evaluated by DFT calculations. Based on the results of the hydrogen evolution behaviour by these two complexes, it was clearly demonstrated that the amino group functions importantly as a proton transfer site in the hydrogen generation reaction.