Electronic Structures and Orbital Interactions of Iron Porphyrin Intermediates Throughout Electrochemical CO ₂ Reduction R

ABSTRACT

Iron porphyrins are among the most efficient molecular electrocatalysts for the CO ₂ reduction reaction (CO ₂ RR). Herein, the electronic structures and bonding interactions of low‐valent iron porphyrin intermediates involved in CO ₂ RR are systematically investigated by density functional theory (DFT) calculation using Fe ^III (TPP)Cl (TPP = tetraphenyl porphyrin dianion) as model. Analysis of spin population and Hirshfeld charge shows the formal Fe ⁰ intermediate has bonding interaction between iron center and porphyrin diradial, and the electron density is sequentially transferred from porphyrin tetraanion to iron and then to CO ₂ during [Fe ^II (TPP)CO ₂ ] ²⁻ adduct formation and followed protonation steps. The calculations also reveal distinct functions of Fe 3d and porphyrin π* orbitals throughout the catalytic cycle. The Fe d _z2 orbital acts as primary electron donor to π* orbital of CO ₂ , backbonding interaction between Fe d _π (both d _xz and d _yz ) orbitals and porphyrin π* orbitals serve as electron reservoir which stabilize the reducing equivalents. Although the reducing equivalents are stored in porphyrin π* orbitals, it is not the electron transfer path way to CO ₂ . These findings provide an orbital‐based strategy for further rational design regarding orbital energy and functions of molecular electrocatalysts featuring non‐innocent ligands.