Elucidating the origins of enhanced CO₂ reduction in manganese electrocatalysts bearing pendant hydrogen-bond donors

Complexes of the general form [Mn(X)(CO)₃bpy] (X = a variety of monodentate ligands, bpy = 2,2′-bipyridine) have been reported to act as electrocatalysts for the reduction of CO₂ to CO. In this work, a series of phenol and anisole substituted bipyridine ligands were synthesized and ligated to a manganese metal center in order to probe for an intramolecular hydrogen-bonding interaction in the transition state of CO₂ reduction. Ligands without the ability to intramolecularly hydrogen bond displayed decreased catalytic current density compared to those with the ability to hydrogen bond with CO₂. Electrocatalysis was studied by performing voltammetric and bulk electrolysis experiments under argon or CO₂ environments. Measurements of catalytic rates using hydrogen vs. deuterium for the intramolecular H/D-bonding step show that there is an isotope effect associated with the catalysis. The data presented herein suggest a mechanism involving two subsequent equilibrium isotope effects in combination with a primary kinetic isotope effect.