| Abstract
| - The relationship between axial ligand orientation and porphyrin structure and spectra for bis-base low-spin Fe(II)−porphyrins has been investigated by using the meso-ααββ and meso-αβαβ atropisomers of picket fence porphyrin to control ligand orientation. The absorption and resonance Raman data obtained for these complexes provide new spectral diagnostic tools in the characterization of the nonplanar distortions of tetrapyrroles in proteins.
- The absorption and resonance Raman (RR) spectra of the bis-N-methylimidazole, bis-1,5-dicyclohexylimidazole,and bis-pyridine complexes of the meso-ααββ and meso-αβαβ atropisomers of Fe(II)−tetrakis(o-pivalamidophenyl)porphyrins (Fe(II)TpivPP) were obtained in methylene chloride. The different spatial arrangements of the o-pivalamidepickets in these two Fe(II)TpivPP compounds are expected to control the absolute and relative positions of theaxial ligand rings with respect to the Fe−N(pyrrole) bonds. In particular, the spectroscopic data obtained for thebis-N-methylimidazole and bis-dicyclohexylimidazole complexes of the Fe(II)[αβαβ-TpivPP] derivative showed themost important differences. Redshifts of the B and Q absorption bands (+ 4−5 nm) as well as an upshift of the lowfrequency ν8 RR mode (+ 5 cm-1) were observed. No shift of the skeletal high frequency modes was detected.These spectral effects were associated with a change in relative position of the axial imidazole rings from nearlyparallel in the bis-N-methylimidazole complex to nearly perpendicular in the bis-dicyclohexylimidazole complex. Onthe basis of stereochemical considerations as well as previous spectroscopic investigations, the data were interpretedin terms of change in porphyrin structure from planar to saddled. Complementing to a parallel study on bis-baseFe(II) “basket handle” porphyrin complexes, this spectroscopic investigation provides an additional means to distinguishplanar, ruffled, and saddled conformations for ferrous hemes included in proteins.
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