Characterization of singly reduced iron(II) porphyrins

Abstract

Optical absorption, electron paramagnetic resonance (EPR), and resonance Raman (RR) spectra are reported for the one-electron reduction products of a series of Fe/sup II/ porphyrins. The porphyrins include 5,10,15,20-tetraphenylporphyrin (TPP), 2,7,12,17-tetrabromo-5,10,15,20-tetraphenylporphyrin (TPPBr/sub 4/), 2,7,12-tricyano-5,10,15,20-tetraphenylporphyrin (TPP(CN)/sub 3/), and 2,7,12,17-tetracyano-5,10,15,20-tetraphenylporphyrin (TPP(CN)/sub 4/). The iron complexes of these porphyrins represent a series in which the reduction potential is successively shifted to more positive values. The RR and EPR data demonstrate that for all of the complexes reduction results in a low-spin configuration for the metal ion. For (FeTPP)/sup -/ and (FeTPPBr/sub 4/)/sup -/, the unpaired electron resides in the metal d/sub z/sup 2// orbital; however, reduction results in the transfer of a significant amount of paired electron density to the macrocyle through ..pi.. back-bonding. Destabilization the metal d/sub z/sup 2// orbital via ligation of a single pyridine or CO molecule is insufficient to push the unpaired electron from the metal ion to the macrocycle. For (FeTPP(CN)/sub 3/)/sup -/ and (FeTPP(CN)/sub 4/)/sup -/, the unpaired electron resides primarily on the porphyrin ring although a small amount of unpaired density is shared with the metal ion through ..pi..-orbital interactions. The extensive interaction between the metal and porphyrin ..pi.. orbitals which is present in all ofmore » the complexes provides a mechanism for enhancing the oscillator strength of formally forbidden charge-transfer transitions. It is suggested that these charge-transfer absorptions are primarily responsible for the complicated optical spectra of the reduced complexes.« less