Electronic structure analysis of electrochemical CO2 reduction by iron-porphyrins reveals basic requirements for design of catalysts bearing non-innocent ligands

Abstract

Electrocatalytic CO₂ reduction is a possible solution to the increasing CO₂ concentration in the earth’s atmosphere, because it enables storage of energy while using the harmful CO₂ feedstock as a starting material. Notably, iron(II) tetraphenylporphyrin, [Fe^II(TPP)]⁰ (TPP²⁻ = tetraphenylporphyrin tetra-anion diradical), and its derivatives have been established as one of the most promising families of homogeneous catalysts for CO₂ reduction into CO. Our earlier work has demonstrated that [Fe(TPP)]²⁻, a catalytically active species, is best described as an Fe(II) center antiferromagnetically coupled with a TPP⁴⁻ diradical. In fact, [Fe(TPP)]²⁻ represents a prototypical example of a diverse array of highly efficient molecular catalysts that feature non-innocent ligands. To obtain valuable insights for future catalyst design, their outstanding catalytic performance warrants an investigation aimed at elucidating the role played by the ligand non-innocence in the reaction. To this end, the reactivity of [Fe(TPP)]²⁻ was first investigated in detail by using density functional theory calculations, and the theoretical results were then validated by reproducing available experimental kinetic and thermodynamic data. Further in-depth analyses pinpointed the electronic-structure feature of the non-innocent TPP ligand that is responsible for the high efficiency of the reaction. Finally, we analyzed the electronic-structure evolution found for the reactions catalyzed by ten related representative non-innocent systems. Our results revealed that for the reactions under consideration, the reducing equivalents are stored on the non-innocent ligand, while CO₂ functionalization takes place at the metal center. Therefore, all of the transformations invariably entail two synchronized electron-transfer events: (1) a metal-to-CO₂ transfer and (2) a ligand-to-metal electron transfer. The former is affected by σ-donation from the metal d_z² orbital to the CO₂ orbital, and the latter is facilitated by orbital coupling between the ligand and the metal center. Our results suggested that ligand non-innocence plays a fundamental role in stabilizing highly active intermediates while realizing high product selectivity for CO₂ reduction and that the metal–ligand cooperativity is essential to the high reaction kinetics. On the basis of these findings, we proposed fundamental requirements for design of catalysts with non-innocent ligands.