Stereochemical aspects of axial ligation in ferrous iron-porphyrins probed by resonance Raman spectroscopy

Abstract

Abstract We report the concentration-dependent resonance Raman (RR) studies of the FENIm stretching modes in the photo-reduced iron-octaethyl porphyrin (FeIIOEP) and iron-protoporphyrin-IX dimethylester (FeIIPPDME) complexes with 2-methyl imidazole (2-MeIm) and 1,2-dimethyl imidazole (1,2-Me2Im) as axial ligands. The FeNIm stretching modes in both iron complexes have revealed two components due to the co-existence of the upright and tilted configurations of the FENIm bond with respect to the normal to the heme plane. The frequencies of the two components in both the complexes shift to higher side with an increase in concentration of 2-MeIm as axial ligand. With the more sterically hindered 1,2-Me2Im as axial ligand, the (1,2-Me2Im)FeII-OEP complex exists mainly with the tilted configuration of the FENIm bond while the upright configuration is the dominant species in the (1,2-Me2Im)FeIIPPDME complex. From the comparative study, we infer that the vinyl groups in the protoporphyrin complex play a dominant role in non-bonded interactions with the sterically hindered axial ligands in stabilizing the specific configurations.