Deactivation of a Cobalt Catalyst for Water Reduction through Valence Tautomerism.

The activity of the water reduction catalyst [CoIII (L1 )(pyr)2 ]PF6 (1), where (L1 )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 [CoI (L1 )]- is in tautomeric equilibrium with the ligand-reduced [CoII (L1. )]- species. While [CoI (L1 )]- favors formation of a CoIII -H- relevant for catalysis, the [CoII (L1. )]- species leads to ligand protonation, structural distortions, and, ultimatley, catalyst deactivation.