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Computational Study of Anomalous Reduction Potentials for Hydrogen Evolution Catalyzed by Cobalt Dithiolene Complexes

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posted on 2012-09-19, 00:00 authored by Brian H. Solis, Sharon Hammes-Schiffer
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 H2 at the most negative potential) in this series, even though it has the most strongly electron-withdrawing substituents and the least negative CoIII/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 CoIII/II reduction. One fewer sulfur atom is protonated in the CoII(mnt)2 complex than in the other three complexes in the series. As a result, the subsequent CoII/I reduction step occurs at the most negative potential for Co­(mnt)2. According to the proposed mechanism, the resulting CoI complex undergoes intramolecular proton transfer to form a catalytically active CoIII-hydride that can further react to produce H2. Understanding the impact of ligand protonation on electrocatalytic activity is important for designing more effective electrocatalysts for solar devices.

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