Electrocatalytic hydrogen production from neutral water using an aqueous Ni(II) pincer complex

Water is the exemplary source for sustainable H-2 production and provides an ideal solvent for this electrocatalytic reaction. In neutral pH, aqueous saline solution, the Ni-II diminopyridiine precatalyst [Ni(kappa(3)-2,6-{PhNCMe}(2)(NC5H3))Br-2], electrocatalytically produced H-2 with an onset voltage of -1.0 V (387 mV overpotential) versus Ag/AgCl. In anhydrous acetonitrile solution, trifluoroacetic acid supplied protons for H-2 production with an overpotential of 555 mV. Computational analysis (DFT) provided insight into the identities of the catalysts and supported the non-innocent role of the ligand in the reduction chemistry. Mechanistic propositions for these observations, also supported computationally, suggested proton transfer to the d(z2)-based orbital of the reduced Ni(I) complex to generate a square planar Ni(II). A second proton transfer led to formation of a Ni(II) dihydrogen complex.