Axial Ligand Orientations in a Distorted Porphyrin Macrocycle: Synthesis, Structure, and Properties of Low-Spin Bis(imidazole)iron(III) and Iron(II) Porphyrinates†Dedicated to Prof. Animesh Chakravorty on the occasion of his 75th birthday.

Abstract

We have reported here, for the first time, the parallel and perpendicular orientation preferences of two planar and unhindered imidazoles as axial ligands (L) while coordinated toward iron(III) and iron(II) porphyrins, respectively, in a nonplanar porphyrinic environment. The synthesis and characterization of low-spin FeIII(tn-OEP)(L)2·ClO4 and FeII(tn-OEP)(L)2 are reported. FeIII(tn-OEP)(L)2·ClO4 shows rhombic electron paramagnetic resonance (EPR) spectra (at 77 K) in both solid and solution phases that are very characteristic for low-spin (S = 1/2) iron porphyrins with two axial imidazole ligands aligned parallel to each other. Single-point energy calculation is also performed on FeIII(tn-OEP)(1-MeIm)2+ using density functional theory (DFT), which shows that the relative parallel orientations of two 1-MeIm are more stable than the perpendicular orientations. X-ray structures of FeII(tn-OEP)(1-MeIm)2 and FeII(tn-OEP)(1-MeIm)2·THF are reported that demonstrate, for the first time, the near-perpendicular axial ligand orientation (80.9 and 89.8°, respectively) for iron(II) porphyrins in a distorted macrocyclic environment. Even starting from parallel axial orientations of 1-MeIm, geometry optimization using DFT converged well to the perpendicular axial alignment with a 82.54° dihedral angle, which is in close agreement with experiment. This is in sharp contrast to all earlier reports, in which sterically crowded imidazole (such as 2-MeIm) or a nearly planar porphyrin core with a “picket fence” environment that restricts the rotation of the axial ligands is required for perpendicular orientation. Electrochemical data obtained from a cyclic voltammetric study for FeII(tn-OEP)(L)2 reveal one-electron oxidation at very high positive potential, which readily explains why the complexes are so stable in air. Bulk oxidation of FeII(tn-OEP)(1-MeIm)2 at a constant potential of 0.69 V in dichloromethane with 0.1 M tetrabutylammonium perchlorate as the supporting electrolyte generates FeIII(tn-OEP)(1-MeIm)2·ClO4, which has the same EPR spectrum and which upon reduction at 0.29 V regenerates FeII(tn-OEP)(1-MeIm)2 again. Thus, we have demonstrated here, for the first time, that iron(II) and iron(III) porphyrinates with two planar and unhindered axial imidazoles have different orientation preferences in a nonplanar porphyrinic environment.