Dihydrogen Evolution by Protonation Reactions of Nickel(I).
Nickel-mediated formation of H(2) by protonation of Ni(I) has been established and the kinetics of the process investigated. The diamagnetic complex [Ni(II)(psnet)](BF(4))(2) was prepared and reduced to [Ni(I)(psnet)](BF(4)) with NaBH(4) 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(z)()2)(1) ground state. Under appropriate conditions, [Ni(psnet)](+) reacts with acids in nonaqueous media to give near-quantitative yields of H(2) according to the stoichiometry Ni(I) + H(+) --> Ni(II) + (1)/(2)H(2). Dihydrogen production was demonstrated to be directly related to Ni(I) oxidation. The reaction system [Ni(psnet)](+)/HCl/DMF, which gives H(2) yields of greater, similar90%, was subjected to a kinetics analysis. The overall reaction [Ni(psnet)](+) + HCl --> [Ni(psnet)Cl](+) + (1)/(2)H(2) proceeds by two parallel pathways dependent on chloride concentration. Addition of Bu(4)NCl accelerates the reaction, whereas (Bu(4)N)(PF(6)) 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(-) --> H(2) + 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.
James
Thomas L.
TL
Department of Chemistry, Harvard University, Cambridge, Massachusetts 02138.
Cai
Lisheng
L
Muetterties
Mark C.
MC
Holm
R. H.
RH