Ligand Orientation Control in Low-Spin Six-Coordinate (Porphinato)iron(II) Species

The synthesis of a low-spin six-coordinate iron(II) porphyrinate in which the two axial ligands are forced to have
a relative perpendicular orientation has been successfully accomplished for the first time. The reaction of four-coordinate (tetramesitylporphinato)iron(II) with 2-methylimidazole leads to the preparation of [Fe(TMP)(2-MeHIm)2]
which cocrystallizes with five-coordinate [Fe(TMP)(2-MeHIm)]. The six-coordinate complex accommodates the sterically
crowded pair of imidazoles with a strongly ruffled core and relative perpendicular orientation. This leads to shortened
equatorial bonds of 1.963(6) Å and slightly elongated axial Fe−N bond lengths of 2.034(9) Å that are about 0.04
Å shorter and 0.03 Å longer, respectively, in comparison to those of the bis-imidazole-ligated iron(II) species with
parallel oriented axial ligands. The Mössbauer spectrum shows a pair of quadrupole doublets that can be assigned
to the components of the cocrystallized crystalline solid. High-spin five-coordinate [Fe(TMP)(2-MeHIm)] has ΔEQ
= 2.25 mm/s and δ = 0.90 mm/s at 15 K. The quadrupole splitting, ΔEQ, for [Fe(TMP)(2-MeHIm)2] is 1.71 mm/s,
and the isomer shift is 0.43 mm/s at 15 K. The quadrupole splitting value is significantly larger than that found for
low-spin iron(II) derivatives with relative parallel orientations for the two axial ligands. Mössbauer spectra thus
provide a probe for ligand orientation when structural data are otherwise not available.