Nonplanar Distortions of Bis-Base Low-Spin Iron(II)−Porphyrinates: Absorption and Resonance Raman Investigations of Cross-Trans-Linked Iron(II)−Basket-Handle Porphyrin Complexes

Publisher Website
Google Scholar
Abstract
Electronic absorption and Soret-excited resonance Raman (RR) spectra are reported for bis-N-alkylimidazole and bis-pyridine complexes of various cross-trans-linked iron(II)−“basket-handle” porphyrins (Fe(II)−BHP) in methylene chloride. These compounds enable us to characterize the spectroscopic properties of ruffled six-coordinated low-spin Fe(II)−porphyrin complexes. The visible absorption spectra show that the Q and B bands are progressively red-shifted when the handles are shortened and/or when the steric hindrance of the axial ligands is increased. This effect is accompanied by both a decrease in RR frequency of the ν2 mode and an increase in frequency of the ν8 and νs(Fe-ligand2) modes. More precisely, an inverse linear correlation is found between the frequencies of the ν2 and ν8 modes. For each ligation state, the positions of the absorption bands are also linearly correlated with the frequency of the ν2 or ν8 mode. All of these spectroscopic data reveal that the degree of ruffling of the Fe(II)−BHP complexes is increased by the N-methylimidazole → pyridine axial substitutions, presumably because the mutual steric strains between the axial ligand rings, the porphyrin macrocycle and the porphyrin handles are increased. The present study provides a first basis for discerning ruffled conformations from planar and other nonplanar structures in ferrous heme proteins.

This publication has 69 references indexed in Scilit: