Combined Spectroscopic and Electrochemical Detection of a Ni^I⋯H-N Bonding Interaction with Relevance to Electrocatalytic H₂ Production

The [Ni(P^R₂N^R′₂)₂]²⁺ family of complexes are exceptionally active catalysts for proton reduction to H₂. In this manuscript, we explore the first protonation step of the proposed catalytic cycle by using a catalytically inactive Ni^I complex possessing a sterically demanding variation of the ligand. Due to the paramagnetic nature of the Ni^I oxidation state, the protonated Ni^I intermediate has been characterized through a combination of cyclic voltammetry, electron nuclear double resonance (ENDOR) spectroscopy, and hyperfine sublevel correlation (HYSCORE) spectroscopy. Both the electrochemical and spectroscopic studies indicate that the Ni^I complex is protonated at a pendant amine that is endo to Ni, which suggests the presence of an intramolecular Ni^I⋯HN bonding interaction. Using density functional theory, the hydrogen bond was found to involve three doubly-occupied, localized molecular orbitals: the 3d_xz, 3d z 2, and 3d_yz orbitals of nickel. These studies provide the first direct experimental evidence for this critical catalytic intermediate, and implications for catalytic H₂ production are discussed.