NMR and EPR studies of low-spin Fe(III) complexes of meso-tetra-(2,6-disubstituted phenyl)porphyrinates complexed to imidazoles and pyridines of widely differing basicities

Bis-axially ligated iron(III) complexes of five tetra-(2,6-disubstituted phenyl)porphyrines, with 1-MeIm, 2-MeHIm, and nine substituted pyridines with pKa(PyH(super +))=9.7 to 1.1 were investigated. The high basicity pyridine complexes exhibited Curie behavior for their pyrrole-H resonance, while the lower basicity pyridine complexes showed significant deviations from simple Curie behavior that are consistent with the existence of a mixed configuration ground state and a thermally accessible excited state.