CO₂-to-CO Electrochemical Conversion With an Fe(I) Porphyrin Complex in Water

Iron porphyrin complexes constitute a well-established and versatile class of molecular electrocatalysts for the reduction of CO₂ to CO. In both organic and aqueous media, the reaction mechanism is typically proposed to involve the interaction of CO₂ with a formally defined [(porphyrin)Fe⁰] intermediate. In this work, we performed a mechanistic investigation of CO₂ reduction using the water-soluble complex [(pTMA)Fe^IIICl]Cl₄ under aqueous conditions. In situ scanning spectroelectrochemistry was employed, enabling the synchronized acquisition of UV-vis or IR spectra during cyclic voltammetry experiments. Our results provide strong evidence for CO₂ binding to the electrogenerated [(pTMA)Fe^I]³⁺ species, followed by reductive C─O bond cleavage to yield a stable [(pTMA)(Cl)Fe^II-CO]^{3 +} complex. This process corresponds to an overall two-electron reduction per iron center. This mechanism, which has not been previously considered for molecular iron porphyrins in CO₂ reduction, is proposed to be facilitated by the charged porphyrin periphery and the hydrogen-bonding network of the aqueous medium. These features may open new avenues toward achieving CO₂ reduction at lower overpotentials in water.