Two Crystalline Forms of Low-Spin [Fe(TMP)(5-MeHIm)₂]ClO₄. Relative Parallel and Perpendicular Axial Ligand Orientations

The preparation and characterization of two crystalline forms of [Fe(TMP)(5-MeHIm)₂]ClO₄ with distinctly different molecular structures are reported. Crystal structure analysis shows that paral-[Fe(TMP)(5-MeHIm)₂]ClO₄ has the axial imidazole ligands arranged in a relative parallel orientation (over a slightly S₄-ruffled porphyrin core) and perp-[Fe(TMP)(5-MeHIm)₂]ClO₄ has the axial imidazole ligands arranged in a relative perpendicular orientation (over a considerably S₄-ruffled porphyrin core). The two species have different Mössbauer and solid-state EPR spectra. The small quadrupole splitting (ΔE_q = 1.78(1) mm/s, 120 K) and a single observable EPR g_max value (3.43 at 4.2 K) for perp-[Fe(TMP)(5-MeHIm)₂]ClO₄ are indicative of the relative perpendicular arrangement of the axial ligands. The larger quadrupole splitting (ΔE_q = 2.557(3) mm/s, 120 K) and rhombic g-tensor (g₁ = 2.69, g₂ = 2.34−2.43, and g₃ = 1.75) in the solid state and in frozen DMF−acetonitrile 3:1 (g₁ = 2.64, g₂ = 2.30, and g₃ =1.80) at 4.2 K for paral-[Fe(TMP)(5-MeHIm)₂]ClO₄ are indicative of a relative parallel axial ligand orientation. The actual axial ligand dihedral angles are Δφ = 76° and Δφ = 26 or 30° for perp- and paral-[Fe(TMP)(5-MeHIm)₂]ClO₄, respectively, and thus the dihedral angle at which the EPR spectral type changes from large g_max to rhombic must be 30 < Δφ < 76°. Because the porphyrin and axial ligands are similar for both crystalline forms of [Fe(TMP)(5-MeHIm)₂]ClO₄, a more direct correlation between molecular and electronic structure has been established. Molecular mechanics calculations indicate that nonbonded interactions between the axial ligands and meso-mesityl groups of [Fe(TMP)(5-MeHIm)₂]⁺ destabilize a relative parallel orientation for the axial ligands, yet the parallel orientation is observed in all frozen solution samples as confirmed by EPR investigations. This is believed to be due to the competing stabilization of the electronic state of the rhombically distorted parallel complex with an energy stabilization of 2.8−3.7 kcal/mol, as compared to the energy destabilization of 2.6 kcal/mol obtained from MM calculations.