Iron Nitrosyl “Natural” Porphyrinates: Does the Porphyrin Matter?

The
synthesis and spectroscopic characterization of three five-coordinate
nitrosyliron­(II) complexes, [Fe­(Porph)­(NO)], are reported. These three
nitrosyl derivatives, where Porph represents protoporphyrin IX dimethyl
ester, mesoporphyrin IX dimethyl ester, or deuteroporphyrin IX dimethyl
ester, display notable differences in their properties relative to
the symmetrical synthetic porphyrins such as OEP and TPP. The N–O
stretching frequencies are in the range of 1651–1660 cm–1, frequencies that are lower than those of synthetic
porphyrin derivatives. Mössbauer spectra obtained in both zero
and applied magnetic field show that the quadrupole splitting values
are slightly larger than those of known synthetic porphyrins. The
electronic structures of these naturally occurring porphyrin derivatives
are thus seen to be consistently different from those of the synthetic
derivatives, the presumed consequence of the asymmetric peripheral
substituent pattern. The molecular structure of [Fe­(PPIX-DME)­(NO)]
has been determined by X-ray crystallography. Although disorder of
the axial nitrosyl ligand limits the structural quality, this derivative
appears to show the same subtle structural features as previously
characterized five-coordinate nitrosyls.