Substituent‐Generated Electric Fields and Ligand Non‐Innocence Drive the Record‐High Efficiency of Iron Tetraphenylporphyrin Catalysts for CO ₂ ‐to‐CO Conversion

Abstract

Abstract Electrochemical CO 2 ‐to‐CO conversion is a critical step in sustainable catalysis, with iron‐tetraphenylporphyrin (Fe–TPP) derivatives recognised as benchmark catalysts, reaching turnover frequencies (TOFs) up to 10 6 s −1 with trimethylammonium (TMA) substitution. However, substituent effects remain unpredictable: sulfonyl groups render the catalyst inactive, while others enhance activity, yielding over 50 analogues without coherent design principle. Using density functional theory (DFT) and response theory (including electric field effects), we identify factors governing this variability. Electronic structure analysis reveals the redox non‐innocent behaviour of TPP ligand, forming [Fe(II)TPP •• ] 2− species, stabilised by antiferromagnetic coupling ( J = −656 cm −1 ). The ligand's ability to transiently hold and transfer electrons during the catalytic cycle enhances catalytic performance. Energetic‐span model analysis for Fe–TPP ( 1 ), Fe–p‐TMA ( 2 ), and Fe–o‐TMA ( 3 ) yields barriers of 195.5, 184.4, and 162.2 kJ mol −1 , respectively, matching experiments 3 > 2 > 1 . Crucially, local electric fields (LEFs), arising from charged substituents dominate reactivity, surpassing traditional steric or inductive considerations. Applying an oriented electric field of + 0.21 V Å −1 to 1 reproduces the performance of 3 , confirming LEF as a unifying descriptor. Guided by this, we in silico design next‐generation catalysts, setting a new paradigm in CO 2 reduction catalysis.