Effects of axial pyridine coordination on a saddle-distorted porphyrinmacrocycle: stabilization of hexa-coordinated high-spin Fe(iii) and air-stable low-spin iron(ii) porphyrinates

Abstract

We have reported here the effect of axial ligand L (L: pyridine/substituted pyridine) on Fe^III(tn-OEP)Cl/Fe^III(tn-OEP)ClO₄ that first form high-spin Fe^III(tn-OEP)(L)₂.X (X: Cl, ClO₄) which, on longer exposure, spontaneously auto reduce to a series of air stable Fe^II(tn-OEP)(L)₂ complexes. The introduction of four nitro groups into the meso-positions of octaethyl porphyrin (tn-OEP), severely distorts the porphyrin macrocycle which enables the facile isolation of a rare family of high-spin Fe^III(tn-OEP)(L)₂⁺ in a saddle distorted macrocyclic environment. The synthesis and characterization of high-spin Fe^III(tn-OEP)(L)₂.X and low-spin Fe^II(tn-OEP)(L)₂ are reported. The X-ray structures of Fe^II(tn-OEP)(py)₂, Fe^II(tn-OEP)(4-CNpy)₂ and Fe^II(tn-OEP)(3-Clpy)₂ have been determined in which the axial ligands are orientated nearly perpendicular to each other. Electrochemical data obtained from cyclic voltammetric study for Fe^II(tn-OEP)(L)₂ reveals the one electron oxidations at very high positive potentials which readily explains why the complexes are so stable in air. However, spectroscopic characterizations such as magnetic and EPR measurements in both solid and solution, and ¹H NMR in solution demonstrates the high-spin nature of Fe^III(tn-OEP)(L)₂.X. Molecular orbital calculations using DFT for five coordinate Fe^III(tn-OEP)Cl shows a_2u-like HOMO that is expected for a saddle distorted porphyrin but for six coordinate Fe^III(tn-OEP)(L)₂.X results in switch of the HOMO from a_2u to a_1u. However, metal d_x²−y² and porphyrin a_1u bonding interaction is symmetrically unfavorable and thus responsible for high-spin nature of the complexes reported here. The porphyrin cores (tn-OEP) are found to be least distorted in Fe^III(tn-OEP)(H₂O)₂.ClO₄ with a core size of 2.061 Å while, for Fe^II(tn-OEP)(py)₂, the macrocycle is distorted most with lowest core size of 1.961 Å; thus shows a significant and unprecedented core expansion of 0.1 Å in the series.