Closed Synthetic Cycle for Nickel‐Based Dihydrogen Formation

Dihydrogen evolution was observed in a two‐step protonation reaction starting from a Ni(0) precursor with a tripodal N‐heterocyclic carbene (NHC) ligand.  Upon the first protonation, a Ni(II) monohydride complex is formed, isolated, and fully characterized.  Subsequent protonation yields H2via a transient intermediate (INT) and an isolable Ni(II) acetonitrile complex.  The latter can be reduced to regenerate its Ni(0) precursor.  The mechanism of H2 formation was investigated by using a deute­rated acid and scrutinized by 1H NMR spectroscopy and gas chromatography.  Remarkably, the second protona­tion forms a rare nickel dihydrogen complex, detected and identified in solution, and characterized by 1H NMR spectroscopy. DFT‐based computational analyses were employed to propose a reaction profile and a molecular structure of a Ni‒H2 complex.  Thus, a dihydrogen‐evolving, closed‐synthetic cycle is reported with a rare Ni–H2 species as a key intermediate.