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.

八乙基氧卟啉（octaethyloxophlorin, H3OEPO）配合物中，铁与卟啉大环之间的电子分配问题长期以来都是一个棘手的难题。具体而言，作为血红素降解过程中的重要中间体，(Py)₂Fe(OEPO)曾被提出三种分别对应三价铁、二价铁与一价铁的共振式。为厘清这一问题，研究人员通过单晶X射线衍射、变温磁矩测量以及电子顺磁共振（Electron Paramagnetic Resonance, EPR）与核磁共振（Nuclear Magnetic Resonance, NMR）光谱表征，对(Py)₂FeIII(OEPO)、(Im)₂FeIII(OEPO)·2THF以及(Im)₂FeIII(OEPO)·1.6CHCl3的电子与几何结构开展了系统研究。实验结果清晰表明，两类配合物均以三价铁/氧卟啉三阴离子形式存在，而非此前在(2,6-二甲苯基异氰)₂FeII(OEPO•)中被证实的二价铁/氧卟啉自由基形式。在10 K至300 K的固态条件下，(Py)₂FeIII(OEPO)以高自旋（S = 5/2）状态存在，轴向配体处于平行平面，卟啉环呈平面构型，且轴向Fe−N键长较长。而在溶液中，该配合物主要以低自旋（S = 1/2）形式存在。与之相反，(Im)₂FeIII(OEPO)·2THF与(Im)₂FeIII(OEPO)·1.6CHCl3的结构均表现为卟啉环发生显著褶皱畸变，轴向配体近乎处于垂直平面，且轴向Fe−N键长相对偏短。晶体学、磁学、EPR以及NMR的所有实验结果均证实，(Im)₂FeIII(OEPO)在固态与溶液中均以低自旋三价铁形式存在。