Steric Effects on CO₂ Reduction with Substituted Mn(bpy)(CO)₃X-Type Catalysts on Multiwalled Carbon Nanotubes Reveal Critical Mechanistic Details

A series of Mn-(bpy-R)-(CO)3Br (bpy-R = 4,4'-R-2,2'-bipyridine) complexes with systematic substituent variations (R = H, -Me, -Et, t Bu, and -Ph) are immobilized on multiwalled carbon nanotubes (MWCNTs) and investigated as electrocatalysts for CO2 reduction to study substituent effects on heterogenized molecular electrocatalysis. The electrochemical response and catalytic activity of each heterogenized complex are characterized, unveiling clear trends across the series investigated. Mn-(bpy-Ph)-(CO)(3)Br/MWCNT exhibited the best catalytic performance, producing CO with a Faradaic efficiency of 72% and a current density (J CO) of 7.0 mA/cm(2) at low overpotential (eta = 0.65 V). Adding steric bulk to the bpy ligands is shown to restrict Mn0-Mn0 dimerization and cause a shift to two-electron reduction occurring at less negative potentials. The apparent quantity of electroactive catalyst scales inversely with steric bulk, where Mn-(bpy-Ph)-(CO)(3)Br exhibits no distinguishable Faradaic features in CV under normal conditions. These results indicate that catalytic performance is optimized by the confinement of electroactive species to the MWCNT interface.