Deactivation of a Cobalt Catalyst for Water Reduction through Valence Tautomerism

The activity of the water reduction catalyst [Co III (L 1 )(pyr) 2 ]PF 6 ( 1 ), where (L 1 ) 2− is a bis‐amido pyridine ligand and pyr is pyrrolidine, is investigated. Catalyst 1 has an overpotential of 0.54 V and a high observed TOF of 23 min −1 , albeit for a relatively short time. Considering the significant activity of 1 and aiming to improve catalyst design, a detailed structural and electronic study is performed to understand the mechanisms of deactivation. Experimental and theoretical evidence support that the metal‐reduced [Co I (L 1 )] − is in tautomeric equilibrium with the ligand‐reduced [Co II (L 1 . )] − species. While [Co I (L 1 )] − favors formation of a Co III −H − relevant for catalysis, the [Co II (L 1 . )] − species leads to ligand protonation, structural distortions, and, ultimatley, catalyst deactivation.