Electrochemical hydrogen generation by a four-coordinate square-planar Ni(<sc>ii</sc>) complex with an N2P2-type ligand

A Ni(ii) complex with an N2P2-type ligand, [Ni(L-H)(2)](BF4)(2) (L-H = 2-((diphenylphosphino)methyl)-pyridine), was prepared and characterized structurally, spectroscopically, and electrochemically. Its electrochemical hydrogen production capability was investigated and compared with that of a previously reported Ni(ii) complex bearing an amino group in the ligand, [Ni(L-NH2)(2)](BF4)(2) (L-NH2 = 6-((diphenylphosphino)methyl)-pyridin-2-amine). The X-ray crystal structure was revealed to be a four-coordinate square planar structure (tau(4) = 0.25) in the cis form, with the counter anion BF4- weakly coordinated to the Ni(ii) ion. The structure in the solution was assessed on the basis of UV-vis and NMR spectral features, which showed a four coordinate square planar structure in dichloromethane and a five- or six-coordinate structure bound with solvent molecules in acetonitrile. The electrochemical hydrogen production reaction using AcOH as a proton source showed a similar behaviour to that of [Ni(L-NH2)(2)](BF4)(2), with the catalytic current (icat) proportional to the square root of the concentration of AcOH added. This indicates that the reaction mechanism is EECC and that the rate-determining step is the reaction of the two-electron reduced Ni(0) species with the approaching proton to form the Ni(ii)-H- species. The TOF and overpotential values, when evaluated under the same conditions as in a previous study (complex: 1 mM, electrolyte [n-Bu4N](ClO4): 0.1 M in MeCN (3 mL), AcOH = 145 equiv. (pK(a) = 22.3 in MeCN)), were found to be 1060 s(-1) and 710 mV, respectively. These values were higher for the overpotential and smaller for TOF, as compared to those of [Ni(L-NH2)(2)](BF4)(2 )(TOF 8800 s(-1), overpotential 430 mV). The structure of the starting material [NiII(L-H)(2)](2+) and the formation of the hydride Ni(ii) complex [NiII(L-H)(2)H](+), a reaction intermediate in the hydrogen evolution reaction, were evaluated by DFT calculations. The results of the hydrogen evolution behaviour of these two complexes show that the electron-donating amino group plays an important role in the hydrogen evolution reaction, not only capturing protons but also increasing the basicity of the pyridyl N atom.