Effects of Tuning Intramolecular Proton Acidity on CO2 Reduction by Mn Bipyridyl Species

In order to understand the effect of intramolecular proton acidity on CO2 reduction by Mn bipyridyl species, three fac-Mn(CO)(3) bipyridine complexes containing intramolecular phenol groups of varying acidities were synthesized and electrochemical, spectroscopic, and computational studies were performed. While the phenol group acidity has minimal influence on the metal center, the complex containing a fluoro-substituted (more acidic) phenol, MnBr(F-HOPh-bpy)(CO)(3), exhibits a decreased catalytic to peak current ratio following the second reduction in comparison to the complexes with unsubstituted or methyl-substituted phenol groups (MnBr(HOPh-bpy)(CO)(3) and MnBr(Me-HOPh-bpy)(CO)(3), respectively). A second process is also present in the catalytic wave for MnBr(F-HOPh-bpy)(CO)(3). Furthermore, MnBr(F-HOPh-bpy)(CO)(3) exhibits decreased CO production and increased H-2 production in comparison to MnBr(HOPh-bpy)(CO)(3). Spectroelectrochemistry under an inert atmosphere in the presence of water shows that following the first reduction, for both MnBr(F-HOPh-bpy)(CO)(3) and MnBr(HOPh-bpy)(CO)(3), the major product is a phenoxide-coordinated fac-(CO)(3) species formed from reductive deprotonation and the minor product is a six-coordinate Mn(I) hydride. For both species, the major species following the second reduction is a five-coordinate anion believed to be the active catalyst for CO2 reduction, but the Mn(I) hydride persists as a minor species. The IR assignments are supported by theoretical calculations. These findings show that changes to the acidity of an intramolecular substituent can have significant effects on the catalytic performance and product selectivity of Mn(CO)(3) bipyridine catalysts despite having minimal effect on the metal center, with a more acidic intramolecular substituent increasing H-2 production at the expense of CO2 reduction.