H-1-NMR STUDIES OF IRON(III) PORPHYRINS WITH AXIAL PHENOXIDE OR THIOPHENOXIDE LIGANDS

The proton NMR spectra of five-coordinate, high-spin iron(III) complexes of protoporphyrin IX dimethyl ester and tetra-phenylporphyrin with axial p-nitrophenoxy and p-nitrothiophenoxy ligands have been obtained and analyzed. Differences between S and O coordination included an upfield bias for the heme methyls in the phenoxy complexes by ca. 5 ppm and a more sizable upfield shift (by ca. 10 ppm) for the porphyrin meso protons in thiophenoxy complexes. Resonances due to the axial phenoxy ligands have been assigned by methyl and methoxy substitution. Analysis of relaxation properties yields two anomalous results. The axial ligand differential relaxation does not parallel the relative distances from the metal, suggesting that line widths cannot be used to make assignments when a large degree of p spin delocalization is present. The axial ligand and porphyrin resonances give conflicting indexes of the metal-centered relaxivity. The spectrum of the low-spin, six-coordinate complex with axial p-nitrophenoxide and imidazole ligands has been recorded. It differs from that of the bis(imidazole) adduct. Low-field resonances observed in the 1H NMR spectrum of the mutant hemoglobins Iwate and Boston can now be assigned to the methylene and/or meta protons of the axial tyrosinate ligands in high-spin complexes. © 1990, American Chemical Society. All rights reserved.