Electrocatalytic Hydrogen Evolution by Cobalt Complexes with a Redox Non-Innocent Polypyridine Ligand

Novel cobalt and zinc complexes with the tetradentate ppq (8-(1″,10″-phenanthrol-2″-yl)-2-(pyrid-2′-yl)­quinoline) ligand have been synthesized and fully characterized. Electrochemical measurements have shown that the formal monovalent complex [Co­(ppq)­(PPh3)]+ (2) undergoes two stepwise ligand-based electroreductions in DMF, affording a [Co­(ppq)­DMF]−1 species. Theoretical calculations have described the electronic structure of [Co­(ppq)­DMF]−1 as a low-spin Co­(II) center coupling with a triple-reduced ppq radical ligand. In the presence of triethylammonium as the proton donor, the cobalt complex efficiently drives electrocatalytic hydrogen evolution with a maximum turnover frequency of thousands per second. A mechanistic investigation proposes an EECC H2-evolving pathway, where the second ligand-based redox process (E), generating the [Co­(ppq)­DMF]−1 intermediate, initiates proton reduction, and the second proton transfer process (C) is the rate-determining step. This work provides a unique example for understanding the role of redox-active ligands in electrocatalytic H2 evolution by transition metal sites.