Closed Synthetic Cycle for Nickel-Based Dihydrogen Formation

Dihydrogen evolution was observed in a two-step protonation reaction starting from a Ni⁰ precursor with a tripodal N-heterocyclic carbene (NHC) ligand. Upon the first protonation, a Ni^II monohydride complex was formed, which was isolated and fully characterized. Subsequent protonation yields H₂ via a transient intermediate (INT) and an isolable Ni^II acetonitrile complex. The latter can be reduced to regenerate its Ni⁰ precursor. The mechanism of H₂ formation was investigated by using a deuterated acid and scrutinized by ¹ H NMR spectroscopy and gas chromatography. Remarkably, the second protonation forms a rare nickel dihydrogen complex, which was detected and identified in solution and characterized by ¹ H NMR spectroscopy. DFT-based computational analyses were employed to propose a reaction profile and a molecular structure of the Ni-H₂ complex. Thus, a dihydrogen-evolving, closed-synthetic cycle is reported with a rare Ni-H₂ species as a key intermediate.