Theoretical Study on the Mechanism of the Electrocatalytic CO₂ Reduction to Formate by an Iron Schiff Base Complex

The iron(III) chloride compound 6,6'-di(3,5-ditert-butyl-2-hydroxybenzene)-2,2'-bipyridine (Fe(^tbudhbpy)Cl) can effectively catalyze the electrochemical CO₂ reduction in N,N-dimethylformamide. Density functional calculations were conducted to investigate the mechanism and unravel the governing factors of product selectivity. The results suggest that the initial catalyst, Fe(^tbudhbpy)Cl (formally Fe^III-Cl), undergoes two reduction steps, accompanied by the dissociation of Cl^-, leading to the formation of the active ferrous radical intermediate 2 (formally Fe^I). Without phenol, 2 attacks CO₂ to generate the Fe^III-carboxylate intermediate Fe^III-CO₂, followed by a one-electron reduction to generate Fe^II-CO₂, which reacts with another CO₂ to produce CO. This aligns with the experimental result that CO is the main product when the phenol is absent. In contrast, when phenol is presented, the triple reduced species 3 is protonated at its ligand N site to yield 3pt(N) (formally Fe⁰-NH), which subsequently performs a nucleophilic attack on CO₂ to afford formate. This process occurs via an orthogonal electron/proton transfer mechanism, where two electrons and one proton are transferred from the ligand to the CO₂ moiety. The redox noninnocent nature of the ligand is thus crucial for formate formation, as it facilitates electron and proton shuttling, enabling 3pt(N) to attack CO₂ through this unusual mechanism effectively.