Combined Spectroscopic and Electrochemical Detection of a Ni I ⋅⋅⋅HN Bonding Interaction with Relevance to Electrocatalytic H 2 Production

The [Ni(P R 2 N R′ 2 ) 2 ] 2+ family of complexes are exceptionally active catalysts for proton reduction to H 2 . 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 H 2 production are discussed.