Utilizing Mechanistic Insight for the Improved Design of Homogeneous Manganese Electrocatalysts for CO2 Reduction

Abstract

Soluble manganese complexes were studied using voltammetry and preparative techniques for their ability to electrocatalytically reduce carbon dioxide to carbon monoxide. The key observation that these catalysts require water for their ability to perform catalytic CO2 reduction led to the hypothesis that the second coordination sphere of the manganese complex could be favorably influenced by placing a hydrogen-bonding proton near the binding site for carbon dioxide. This proposition was tested by synthesizing the species MnBr(6-(2-hydroxyphenol)-2,2’-bipyridine)(CO)3, which includes a ligand framework with a phenolic proton in close proximity to the CO2 binding site that allows for facile proton-assisted C-O bond cleavage. This new complex not only exhibited a 46-fold increase in turnover frequency, but also displayed an overpotential of 440 mV, which is excellent relative to other state-of-the art CO2 reduction electrocatalysts. Phenolic methylation led to a complex with catalytic properties similar to the parent complex, MnBr-(2,2’-bipyridine)(CO)3, which suggests a key role for the phenolic proton in catalysis. The mode of action of the pendant phenolic proton was investigated by synthesizing regioisomeric complexes where the phenol was systematically moved away from the metal center by placing it at different positions around the 2,2’-bipyridine ring. These complexes did not exhibit enhanced ability to perform electrocatalytic CO2 reduction leading to the conclusion that the precise placement of the phenol is crucial for enhanced reactivity. Additional information was garnered from voltammetric experiments, including the observation the a pendant phenol allows for catalysis without the need for an additional proton source, confirming that the phenolic proton plays a role in the C—O bonding break step during catalysis.