Electrocatalytic Reduction of CO₂ to Formate by an Iron Schiff Base Complex

The synthesis, structural characterization, and reactivity of an iron(III) chloride compound of 6,6'-di(3,5-di-tert-butyl-2-hydroxybenzene)-2,2'-bipyridine (Fe(^tbudhbpy)Cl), under electrochemically reducing conditions is reported. In the presence of carbon dioxide (CO₂) under anhydrous conditions in N,N-dimethylformamide (DMF), this complex mediates the 2e^- reductive disproportionation of two equivalents of CO₂ to carbon monoxide (CO) and carbonate (CO₃^2-). Upon addition of phenol (PhOH) as a proton source under CO₂ saturation, catalytic current is observed; product analysis from controlled potential electrolysis experiments shows the majority product is formate (68 ± 4% Faradaic efficiency), with H₂ as a minor product (30 ± 10% Faradaic efficiency) and minimal CO (1.1 ± 0.3% Faradaic efficiency). On the basis of data obtained from cyclic voltammetry and infrared spectroelectrochemistry (IR-SEC), the release of CO from intermediate Fe carbonyl species is extremely slow and undergoes competitive degradation, limiting the activity and lifetime of this catalyst. Mechanistic studies also indicate the phenolate moieties coordinated to Fe are sensitive to protonation in the reduced state, suggesting the possibility of cooperative pendent proton interactions being involved in CO₂ reduction.