Calix[8]arene-Based Manganese Complexes for Electrocatalytic CO2 Reduction

Abstract

Transition metal catalysts with modified second-coordination sphere employed in the electrocatalytic CO₂ can result in increased activity or directed product selectivity. Calixarenes can form metal complexes and potentially catalyze reactions within its cavity, taking advantage of the surrounding phenols groups to tune the reactivity by second-coordination sphere effects. Here, we present a Mn(I) bromotricarbonyl complex with phenanthroline-functionalized calix[8]arene ligands capable of electrocatalytically reducing CO₂ into different products with 2,2,2-trifluoroethanol as proton donor. The selectivity of the reaction seems to be affected by the calixarene cavity: two calixarene-free analogous complexes reduce CO₂ to CO almost exclusively, while the calixarene complexes produce primarily CO, H₂. Interestingly, in some cases the less frequently observed CH₄ was also detected, albeit with low Faradaic efficiency. Thus, the manganese center placed within the calixarene cavity promotes the formation of reduced CO₂ products by more than two electrons and two protons, affording CH₄ in some cases.