title: Computational study of anomalous reduction potentials for hydrogen evolution catalyzed by cobalt dithiolene complexes.
abstract: The design of efficient hydrogen-evolving catalysts based on earth-abundant materials is important for developing alternative renewable energy sources. A series of four hydrogen-evolving cobalt dithiolene complexes in acetonitrile-water solvent is studied with computational methods. Co(mnt)(2) (mnt = maleonitrile-2,3-dithiolate) has been shown experimentally to be the least active electrocatalyst (i.e., to produce H(2) at the most negative potential) in this series, even though it has the most strongly electron-withdrawing substituents and the least negative Co(III/II) reduction potential. The calculations provide an explanation for this anomalous behavior in terms of protonation of the sulfur atoms on the dithiolene ligands after the initial Co(III/II) reduction. One fewer sulfur atom is protonated in the Co(II)(mnt)(2) complex than in the other three complexes in the series. As a result, the subsequent Co(II/I) reduction step occurs at the most negative potential for Co(mnt)(2). According to the proposed mechanism, the resulting Co(I) complex undergoes intramolecular proton transfer to form a catalytically active Co(III)-hydride that can further react to produce H(2). Understanding the impact of ligand protonation on electrocatalytic activity is important for designing more effective electrocatalysts for solar devices.
authors.affiliation: Department of Chemistry, 600 South Matthews Avenue, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA.
authors.forename: Brian H
authors.initials: BH
authors.lastname: Solis
authors.forename: Sharon
authors.initials: S
authors.lastname: Hammes-Schiffer