Computational Analysis of Proton-Coupled Electron Transfer in Hydrotris(triazolyl)borate Mid-Late 3d and 4d Transition Metal Complexes

Design of electrocatalysts for the evolution of H-2 and reduction of O-2, N-2, and CO2, as well as water splitting is essential for the development of alternative energy sources. Typically, the catalytic cycle is controlled by key proton-coupled electron transfer (PCET) processes including sequential or concerted electron transfer (ET) and proton transfer (PT) pathways. Studying the reaction free energies and free energy barriers of PCET processes can thus give insight into the design of more effective electrocatalysts. Herein, the focus is on complexes with the scorpionate ligand hydrotris(1,2,4-triazole-1-yl)borate (Ttz), [M(Ttz)(CO)(3)]. From the reaction free energies of the studied "PCET squares" for converting M(0)(-) to M(I)H+, for Group 6 and 10 complexes, a sequential pathway (PT-ET over ET-PT) is predicted. However, for Group 7-9 metals, a concerted pathway (EPT) is preferred. Analyses of trends in the calculated free energy barriers and reaction free energies of 40 transition-metal complexes suggest that the metal and its electronic structure greatly affect the nature of the PCET processes.