Spectroscopic and Quantum Chemical Study of the Ni(P^Ph₂N^{C6H4CH2P(O)(OEt)2}₂)₂Electrocatalyst for Hydrogen Production with Emphasis on the Ni^IOxidation State

The bis(diphosphine)nickel catalyst first investigated by DuBois and co-workers [DuBois, M. R.; DuBois, D. L. Chem. Soc. Rev. 2009, 38, 62] is arguably one of the most promising molecular catalysts for hydrogen production. It features a low overpotential and, in its most recent variation, a high turnover number of 105 s–1 [Helm, M. L.; Stewart, M. P.; Bullock, R. M.; DuBois, M. R.; DuBois, D. L. Science 2011, 333, 863]. The complex features two reversible one-electron reductions. It is believed that all accessible oxidation states (2+, 1+, 0) of nickel are involved in the proposed catalytic cycle. In this article we focus on the paramagnetic NiI state, for which few experimental studies have been performed. By a combination of modern EPR and quantum chemical methods, it is established that the stable NiI species does not feature a hydride ligand. Furthermore, hydrogen evolution already starts upon addition of acid to the NiI state even without the presence of additional reducing equivalents. The implications for the catalytic cycle are discussed.