The Effects of Axial Ligands on Electron Distribution and Spin States in Iron Complexes of Octaethyloxophlorin, Intermediates in Heme Degradation

The results presented here show that the nature of the axial ligand can alter the distribution of
electrons between the metal and the porphyrin in complexes where there is an oxygen atom replacing one
of the meso protons. The complexes (1-MeIm)2FeIII(OEPO) and (2,6-xylylNC)2FeII(OEPO•) (where OEPO
is the trianionic octaethyloxophlorin ligand and OEPO• is the dianionic octaethyloxophlorin radical) were
prepared by addition of an excess of the appropriate axial ligand to a slurry of {FeIII(OEPO)}2 in chloroform
under anaerobic conditions. The magnetic moment of (2,6-xylylNC)2FeII(OEPO•) is temperature invariant
and consistent with a simple S = 1/2 ground state. This complex with an EPR resonance at g = 2.004 may
be considered as a model for the free-radical like EPR signal seen when the meso-hydroxylated heme/heme oxygenase complex is treated with carbon monoxide. In contrast, the magnetic moment of
(1-MeIm)2FeIII(OEPO) drops with temperature and indicates a spin-state change from an S = 5/2 or an
admixed S = 3/2,5/2 state at high temperatures (near room temperature) to an S = 1/2 state at temperatures
below 100 K. X-ray diffraction studies show that each complex crystallizes in centrosymmetric form with
the expected six-coordinate geometry. The structure of (1-MeIm)2FeIII(OEPO) has been determined at 90,
129, and 296 K and shows a gradual and selective lengthening of the Fe−N(axial bond). This behavior is
consistent with population of a higher spin state at elevated temperatures.