Computational study of anomalous reduction potentials for hydrogen evolution catalyzed by cobalt dithiolene complexes

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.