Electron Distribution in Iron Octaethyloxophlorin Complexes. Importance of the Fe(III) Oxophlorin Trianion Form in the Bis-pyridine and Bis-imidazole Complexes

The apportionment of electrons between iron and the porphyrinic macrocycle in complexes of octaethyloxophlorin
(H3OEPO) has been a vexing problem. In particular, for (Py)2Fe(OEPO), which is an important intermediate in
heme degradation, three resonance structures involving Fe(III), Fe(II), or Fe(I), respectively, have been considered.
To clarify this matter, the electronic and geometric structures of (Py)2FeIII(OEPO), (Im)2FeIII(OEPO)·2THF, and
(Im)2FeIII(OEPO)·1.6CHCl3 have been examined by single-crystal X-ray diffraction, measurement of magnetic moments
as a function of temperature, and EPR and NMR spectral studies. The results clearly show that both complexes
exist in the Fe(III)/oxophlorin trianion form rather than the Fe(II)/oxophlorin radical form previously established for
(2,6-xylylNC)2FeII(OEPO•). In the solid state from 10 to 300 K, (Py)2FeIII(OEPO) exists in the high-spin (S = 5/2)
state with the axial ligands in parallel planes, a planar porphyrin, and long axial Fe−N distances. However, in
solution it exists predominantly in a low-spin (S = 1/2) form. In contrast, the structures of (Im)2FeIII(OEPO)·2THF
and (Im)2FeIII(OEPO)·1.6CHCl3 consist of porphyrins with a severe ruffled distortion, axial ligands in nearly
perpendicular planes, and relatively short axial Fe−N distances. The crystallographic, magnetic, EPR, and NMR
results all indicate that (Im)2FeIII(OEPO) exists in the low-spin Fe(III) form in both the solid state and in solution.