Synthesis, structure and properties of a high-spin Fe(III) porphyrin with non-equivalent axial ligands: Implications for the hemoproteins

The synthesis and X-ray structure of a six-coordinate (porphinato)iron(III) derivative having the mixed-axial ligation of perchlorate and dimethylformamide with no displacement of metal in a nonplanar porphyrinic environment is reported for the first time. The Fe-Np (Np: porphinato nitrogen) distances which range from 2.044(2) to 2.070(2) Å, leaves little doubt that the Fe(III) ion is in high-spin state which is also supported by its characteristic axial EPR spectrum at low temperature. The axial Fe-O(DMF) distance is 2.050(2) Å which is ~0.6 Å shorter than the Fe-OClO3 distance of the complex. In the complex, iron sits in the plane of four porphyrinic nitrogens even when metal coordinates to two non-equivalent axial ligands. The present study suggests that the displacement of iron in proteins is the consequence of non-equivalent axial coordination as well as the protein induced deformations at heme.