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 H2 and
reduction of O2, N2, 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.