Theoretical Analysis of Cobalt Hangman Porphyrins: Ligand Dearomatization and Mechanistic Implications for Hydrogen Evolution

The design of molecular electrocatalysts for hydrogen evolution has been targeted as a strategy for the conversion of solar energy to chemical fuels. In cobalt hangman porphyrins, a carboxylic acid group on a xanthene backbone is positioned over a metalloporphyrin to serve as a proton relay. A key proton-coupled electron transfer (PCET) step along the hydrogen evolution pathway occurs via a sequential ET-PT mechanism in which electron transfer (ET) is followed by proton transfer (PT). Herein theoretical calculations are employed to investigate the mechanistic pathways of these hangman metalloporphyrins. The calculations confirm the ET-PT mechanism by illustrating that the calculated reduction potentials for this mechanism are consistent with experimental data. Under strong-acid conditions, the calculations indicate that this catalyst evolves H₂ by protonation of a formally Co­(II) hydride intermediate, as suggested by previous experiments. Under weak-acid conditions, however, the calculations reveal a mechanism that proceeds via a phlorin intermediate, in which the <i>meso</i> carbon of the porphyrin is protonated. In the first electrochemical reduction, the neutral Co­(II) species is reduced to a monoanionic singlet Co­(I) species. Subsequent reduction leads to a dianionic doublet, formally a Co(0) complex in which substantial mixing of Co and porphyrin orbitals indicates ligand redox noninnocence. The partial reduction of the ligand disrupts the aromaticity in the porphyrin ring. As a result of this ligand dearomatization, protonation of the dianionic species is significantly more thermodynamically favorable at the <i>meso</i> carbon than at the metal center, and the ET-PT mechanism leads to a dianionic phlorin species. According to the proposed mechanism, the carboxylate group of this dianionic phlorin species is reprotonated, the species is reduced again, and H₂ is evolved from the protonated carboxylate and the protonated carbon. This proposed mechanism is a guidepost for future experimental studies of proton relays involving noninnocent ligand platforms