Proton-hydride tautomerism in hydrogen evolution catalysis

Efficient generation of hydrogen from renewable resources requires development of catalysts that avoid deep wells and high barriers. Information about the energy landscape for H-2 production can be obtained by chemical characterization of catalytic intermediates, but few have been observed to date. We have isolated and characterized a key intermediate in 2e(-) + 2H(+) -> H-2 catalysis. This intermediate, obtained by treatment of Cp*Rh(bpy) (Cp*, eta(5)-pentamethylcyclopentadienyl; bpy, kappa(2)-2,2'-bipyridyl) with acid, is not a hydride species but rather, bears [eta(4)-Cp*H] as a ligand. Delivery of a second proton to this species leads to evolution of H-2 and reformation of eta(5)-Cp* bound to rhodium(III). With suitable choices of acids and bases, the Cp* Rh(bpy) complex catalyzes facile and reversible interconversion of H+ and H-2.