Proton-hydride tautomerism in hydrogen evolution catalysis

Abstract

Efficient generation of hydrogen from renewable resources requires development of catalysts that avoid deep wells and high barriers. Information about such features can be obtained by chem. characterization of catalytic intermediates, but few have been obsd. to date. Here, we have mapped the energy landscape of hydrogen evolution in a 2e- + 2H+ reaction sequence by chem. characterization of the intermediate formed upon initial protonation. This intermediate, obtained by treatment of Cp*Rh (bpy) (Cp* = η^5-pentamethylcyclcopentadienyl; bpy = κ^2- 2, 2'- bipyridyl) with acid, is not a hydride species but rather bears [η^4-Cp*H] as a ligand. The preserved Rh^I center can readily be protonated, leading to evolution of H_2 and reformation of η^4-Cp* bound to rhodium(III) . To the best of our knowledge, this is the first direct observation of a proton- relay role for the ubiquitous Cp* ligand, which is revealed here to minimize energy barriers by enabling proton- hydride tautomerism. These low barriers enable reversible catalysis; interconversion of H+ and H_2 can be achieved by minor variation of base concn. in the system. In this presentation, the energy landscape of the system will be discussed, including details from spectroscopic studies of the newly detected intermediate.

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