Switching Orientation of Two Axial Imidazole Ligands between Parallel and Perpendicular in Low-Spin Fe(III) and Fe(II) Nonplanar Porphyrinates

We have reported here the synthesis, structure, and properties
of low-spin bis-imidazole-coordinated Fe­(III) and Fe­(II) complexes
of 5,10,15,20-tetrakis­(pentafluorophenyl)-2,3,7,8,12,13,17,18-octachloroporphyrin,
[FeIII(TFPPCl8)­(L)2]­ClO4 and FeII(TFPPCl8)­(L)2 (L = 1-methylimidazole,
4-methylimidazole, imidazole). The X-ray structure of FeII(TFPPCl8)­(1-MeIm)2 is reported here, which
demonstrated the near-perpendicular axial ligand orientation (dihedral
angle between two 1-methylimidazoles is 80.7°) for Fe­(II) porphyrins
in a highly saddle-distorted macrocyclic environment. Oxidation of
FeII(TFPPCl8)­(L)2 using thianthrenium
perchlorate produces [FeIII(TFPPCl8)­(L)2]­ClO4, which was also isolated in the solid state
and characterized spectroscopically. The complex gives rhombic EPR
spectra in both solid and solution phases at 77 K and thus represents
a rare example of nearly parallel axial ligand orientations for the
unhindered imidazoles in a saddle-distorted porphyrin macrocycle.
Geometry optimization using DFT also converged to the parallel axial
alignment when 1-methylimidazole was used as the axial ligand (the
dihedral angle between two axial ligands is 8.6°). The potential
energy surface (PES) scan results also show that the relatively parallel
axial orientations are energetically preferred for Fe­(III), while
perpendicular orientations are preferred for the Fe­(II) complexes
reported here. Bulk oxidation of FeII(TFPPCl8)­(L)2 in dichloromethane at a constant potential under
nitrogen converts it to [FeIII(TFPPCl8)­(L)2]­ClO4, which gives identical EPR spectra at 77
K and which upon reduction regenerates FeII(TFPPCl8)­(L)2 again. Thus, we have demonstrated here very
rare examples of Fe porphyrins in which the relative axial imidazole
orientations switch between parallel and perpendicular just upon changing
the oxidation states of iron from +3 to +2, respectively, in a nonplanar
porphyrinic environment. These observations could be immensely important
for understanding the possible effects of axial histidine orientations
on similar macrocyclic deformations observed in various heme proteins.