Effects of axial pyridine coordination on a saddle-distorted porphyrin macrocycle: 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 FeIII(tn-OEP)Cl/FeIII(tn-OEP)ClO4 that first form high-spin FeIII(tn-OEP)(L)2.X (X: Cl, ClO4) which, on longer exposure, spontaneously auto reduce to a series of air stable FeII(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 FeIII(tn-OEP)(L)2+ in a saddle distorted macrocyclic environment. The synthesis and characterization of high-spin FeIII(tn-OEP)(L)2.X and low-spin FeII(tn-OEP)(L)2 are reported. The X-ray structures of FeII(tn-OEP)(py)2, FeII(tn-OEP)(4-CNpy)2 and FeII(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 FeII(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 FeIII(tn-OEP)(L)2.X. Molecular orbital calculations using DFT for five coordinate FeIII(tn-OEP)Cl shows a2u-like HOMO that is expected for a saddle distorted porphyrin but for six coordinate FeIII(tn-OEP)(L)2.X results in switch of the HOMO from a2u to a1u. However, metal dx2−y2 and porphyrin a1u 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 FeIII(tn-OEP)(H2O)2.ClO4 with a core size of 2.061 Å while, for FeII(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.