The [Ni(PR2NR′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 NiI complex possessing a sterically demanding variation of the ligand. Due to the paramagnetic nature of the NiI oxidation state, the protonated NiI 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 NiI complex is protonated at a pendant amine that is endo to Ni, which suggests the presence of an intramolecular NiI⋯HN bonding interaction. Using density functional theory, the hydrogen bond was found to involve three doubly-occupied, localized molecular orbitals: the 3dxz, 3d z 2, and 3dyz orbitals of nickel. These studies provide the first direct experimental evidence for this critical catalytic intermediate, and implications for catalytic H2 production are discussed.
ASJC Scopus subject areas
- Organic Chemistry