Electrocatalytic Interconversions of CO₂ and Formate on a Versatile Iron-Thiolate Platform

Abstract

Exploring bidirectional CO₂/HCO₂^- catalysis holds significant potential in constructing integrated (photo)electrochemical formate fuel cells for energy storage and applications. Herein, we report selective CO₂/HCO₂^- electrochemical interconversion by exploiting the flexible coordination modes and rich redox properties of a versatile iron-thiolate platform, Cp*Fe(II)L (L = 1,2-Ph₂PC₆H₄S^-). Upon oxidation, this iron complex undergoes formate binding to generate a diferric formate complex, [(L^-)₂Fe(III)(μ-HCO₂)Fe(III)]⁺, which exhibits remarkable electrocatalytic performance for the HCO₂^--to-CO₂ transformation with a maximum turnover frequency (TOF_max) ∼10³ s^-1 and a Faraday efficiency (FE) ∼92(±4)%. Conversely, this iron system also allows for reduction at -1.85 V (vs Fc^+/0) and exhibits an impressive FE ∼93 (±3)% for the CO₂-to-HCO₂^- conversion. Mechanism studies revealed that the HCO₂^--to-CO₂ electrocatalysis passes through dicationic [(L₂)^-•Fe(III)(μ-HCO₂)Fe(III)]²⁺ generated by unconventional oxidation of the diferric formate species taking place at ligand L, while the CO₂-to-HCO₂^- reduction involves a critical intermediate of [Fe(II)-H]^- that was independently synthesized and structurally characterized.