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

Abstract

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 (τ₄ = 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 (i_cat) 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 [Ni^II(L_H)₂]²⁺ and the formation of the hydride Ni(II) complex [Ni^II(L_H)₂H]⁺, 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.