3-D Motions of Iron in Six-coordinate {FeNO}7 Hemes by NRVS

摘要

The vibrational spectrum of a six-coordinate nitrosyl iron porphyrinate, mono-[Fe(TpFPP)(1-MeIm)(NO)], has been studied by oriented single-crystal nuclear resonance vibrational spectroscopy (NRVS). The crystal was oriented to give spectra perpendicular to the porphyrin plane and two in-plane spectra perpendicular or parallel to the projection of the FeNO plane. These enable assignment of the FeNO bending and stretching modes. The measurements reveal that the two in-plane spectra have substantial differences that result from the strongly bonded axial NO ligand. The direction of the in-plane iron motion is found to be largely parallel and perpendicular to the projection of the bent FeNO on the porphyrin plane. The out-of-plane Fe–N–O stretching and bending modes are strongly mixed with each other as well as with porphyrin ligand modes. The stretch is mixed with ν50 as was also observed for dioxygen complexes. The frequency of the assigned stretching mode of eight Fe–X–O (X = N, C, and O) complexes is correlated with the Fe–XO bond distances. The nature of highest frequency band at ~560 cm⁻¹ has also been examined in two additional new derivatives. Previously assigned as the Fe–NO stretch (by resonance Raman), it is better described as the bend as the motion of the central nitrogen atom of the FeNO group is very large. There is significant mixing of this mode. The results emphasize the importance of mode mixing; the extent of mixing must be related to the peripheral phenyl substituents.