Electronic Structure of a Formal Iron(0) Porphyrin Complex Relevant to CO Reduction.

Iron porphyrins can act as potent electrocatalysts for CO functionalization. The catalytically active species has been proposed to be a formal Fe(0) porphyrin complex, [Fe(TPP)] (TPP = tetraphenylporphyrin), generated by two-electron reduction of [Fe(TPP)]. Our combined spectroscopic and computational investigations reveal that the reduction is ligand-centered and that [Fe(TPP)] is best formulated as an intermediate-spin Fe(II) center that is antiferromagnetically coupled to a porphyrin diradical anion, yielding an overall singlet ground state. As such, [Fe(TPP)] contains two readily accessible electrons, setting the stage for CO reduction.