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.

A nickel complex of a conjugated bis-dithiocarbazate Schiff base for the photocatalytic production of hydrogen

“…Bond lengths of 1.293 Å and 1.314 Å are consistent with C-N double bonds. 4,15 To confirm that this current enhancement corresponds to catalytic hydrogen generation, bulk electrolysis was performed at -1.8 V vs. Fc + /Fc. This is in contrast to what was observed for copper coordination.…”

Computational Redox Potential Predictions: Applications to Inorganic and Organic Aqueous Complexes, and Complexes Adsorbed to Mineral Surfaces

“…(12) where the ∆G (ref) is the experimental redox free energy of the reference complex used as isodesmic model. This method had been successfully applied not only to organic compounds [38][39][40][41] but also for transition metal complexes and reproduced experimentally determined redox potentials accurately within ~0.1 eV [42][43][44]. Moreover, this isodesmic method was employed to predict the redox potentials of actinyl (VI/V) in solution and calculated redox potentials were in good agreement with the experimentally determined redox potentials [45].…”

Ligand‐Based PCET Reduction in a Heteroleptic Ni(bpy)(dithiolene) Electrocatalyst Giving Rise to Higher Metal Basicity Required for Hydrogen Evolution

“…), choice of other more stable sulfur-donating ligands may help researchers to advance the studies on the metal-sulfur systems. In this context, researchers attempting to explore artificial hydrogenase mimics have paid great attention to dithiolene ligands (L; examples are shown in Scheme 1a) in order to examine the catalytic activity of various homoleptic ML 2 -type complexes with M=Fe, [18][19][20] Co, [21][22][23][24] Ni, [25][26][27][28][29][30][31][32][33] Mo, [34] Rh, [35] and W. [36,37] We also reported on the electrocatalytic activity of several NiL 2 -type complexes, showing their unique reaction paths permitting the formation of hydride intermediates via two consecutive ligand-based proton-coupled electron transfer (PCET) processes (Scheme 2(3)). [30,32,33] On the other hand, our previous efforts involve mechanistic studies on the HER catalyzed by various artificial systems with different metal and ligand geometries.…”