Dihydrogen Evolution by Protonation Reactions of Nickel(I)

Abstract

Nickel-mediated formation of H₂ by protonation of Ni(I) has been established and the kinetics of the process investigated. The diamagnetic complex [Ni^II(psnet)](BF₄)₂ was prepared and reduced to [Ni^I(psnet)](BF₄) with NaBH₄ in THF (psnet = bis(5-(diphenylphosphino)-3-thiapentanyl)amine). Both complexes were structurally characterized by X-ray diffraction. [Ni(psnet)]¹⁺ was demonstrated to be an authentic Ni(I) complex with a ...(d_z²)¹ ground state. Under appropriate conditions, [Ni(psnet)]⁺ reacts with acids in nonaqueous media to give near-quantitative yields of H₂ according to the stoichiometry Ni^I + H⁺ → Ni^II + ¹/₂H₂. Dihydrogen production was demonstrated to be directly related to Ni(I) oxidation. The reaction system [Ni(psnet)]⁺/HCl/DMF, which gives H₂ yields of ≳90%, was subjected to a kinetics analysis. The overall reaction [Ni(psnet)]⁺ + HCl → [Ni(psnet)Cl]⁺ + ¹/₂H₂ proceeds by two parallel pathways dependent on chloride concentration. Addition of Bu₄NCl accelerates the reaction, whereas (Bu₄N)(PF₆) decreases the rate. A two-term rate law is presented which includes contributions from both pathways, whose common initial step is protonation of Ni(I). Path A (low chloride concentration) involves the formation and collapse of nickel hydride chloride ion pairs; the rate-determining step is the minimal reaction 2Ni^III−H^- → H₂ + 2Ni^II. Path B (high chloride concentration) includes as the rate-limiting step collapse of a nickel hydride dichloride ion pair followed by the bimolecular reaction of two Ni^III−H^- intermediates or reduction to Ni^II−H^- by Ni^I followed by protonation of the hydride. The relation of these results to the reactions of hydrogenase enzymes is considered.