Absorption and Resonance Raman Investigations of Ligand Rotation and Nonplanar Heme Distortion in Bis-Base Low-Spin Iron(II)-Tetrakis(opivalamidophenyl)porphyrin Complexes

The absorption and resonance Raman (RR) spectra of the bis-N-methylimidazole, bis-1,5-dicyclohexylimidazole, and bis-pyridine complexes of the meso-(alpha)(alpha)(beta)(beat) and meso-(alpha)(beat)(alpha)(beta) atropisomers of Fe(II)-tetrakis(o-pivalamidophenyl)porphyrins (Fe(II)TpivPP) were obtained in methylene chloride. The different spatial arrangements of the o-pivalamide pickets in these two Fe(II)TpivPP compounds are expected to control the absolute and relative positions of the axial ligand rings with respect to the Fe-N(pyrrole) bonds. In particular, the spectroscopic data obtained for the bis-N-methylimidazole and bis-dicyclohexylimidazole complexes of the Fe(II)[(alpha)(beat)(alpha)(beta)-TpivPP] derivative showed the most important differences. Redshifts of the B and Q absorption bands (+ 4-5 nm) as well as an upshift of the low frequency (nu)8 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 nearly parallel in the bis-N-methylimidazole complex to nearly perpendicular in the bisdicyclohexylimidazole complex. On the basis of stereochemical considerations as well as previous spectroscopic investigations, the data were interpreted in terms of change in porphyrin structure from planar to saddled. Complementing to a parallel study on bis-base Fe(II) "basket handle" porphyrin complexes, this spectroscopic investigation provides an additional means to distinguish planar, ruffled, and saddled conformations for ferrous hemes included in proteins.