Computational Study on the Proton Reduction Potential of Co, Rh, and Ir Molecular Electrocatalysts for the Hydrogen Evolution Reaction

In this study, comprehensive density functional theory calculations were conducted to investigate the molecular mechanism of electrocatalytic proton reduction using group 9 transition metal bpaqH (2-(bis(pyridin-2-ylmethyl)amino)-<i>N</i>-(quinolin-8-yl)acetamide) complexes. The goal was to explore how variations in the structural and electronic properties among the three metal centers might impact the catalytic activity. All three metal complexes were observed to share a similar mechanism, primarily characterized by three key steps: heterolytic cleavage of H₂ (HEP), reduction protonation (RPP), and ligand-centered protonation (LCP). Among these steps, the heterolytic cleavage of H₂ (HEP) displayed the highest activation barrier for cobalt, rhodium, and iridium catalysts compared to those of the RPP and LCP pathways. In the RPP pathway, hydrogen evolution occurred from the M^II-H intermediate using acetic acid as a proton donor at the open site. Conversely, in the LCP pathway, H-H bond formation took place between the hydride and the protonated bpaqH ligand, while the open site acted as the spectator. The enhanced activity of the cobalt complex stemmed from its robust σ-bond donation and higher hydride donor ability within the metal hydride species. Additionally, the cobalt complex demonstrated a necessary negative potential in the first (M^III/II) and second (M^II/I) reduction steps in both pathways. Notably, M^III/II-H exhibited a more crucial negative potential for the cobalt complex compared to those of the other two metal complexes. Through an examination of kinetics and thermodynamics in the RPP and LCP processes, it was established that cobalt and rhodium catalysts outperformed the iridium ligand scaffold in producing molecular hydrogen after substituting cobalt metal with rhodium and iridium centers. These findings distinctly highlight the lower-energy activation barrier associated with LCP compared to alternative pathways. Moreover, they offer insights into the potential energy landscape governing hydrogen evolution reactions involving group 9 transition metal-based molecular electrocatalysts.