Unusual Orientation of Axial Ligands in Metalloporphyrins. Molecular Structure of Low-Spin Bis(2-methylimidazole)(meso-tetraphenylporphinato)iron(III) Perchlorate

Article abstract of DOI:10.1021/ja00241a011

The preparation and characterization of the low-spin complex ClO4 is described.The detailed temperature-dependent magnetic susceptibility of two different solvates and the crystal structure determination of the tetrahydrofuran solvate are reported.Solid samples of both solvates of ClO4 have an unusual low-spin EPR spectrum, the so-called strong g_max signal.This strong g_max signal is correlated with a mutually perpendicular alignment of the two axial 2-methylimidazole ligands.This arrangement solves certain stereochemical problems associated with the formation of the low-spin iron(III) complex with the sterically hindered axial ligands.In addition, the short axial bond required for the low-spin state is achieved by a tipping of the axial ligands and a substantial S4 ruffling of the porphinato core.A detailed analysis and comparison of this structure and that of a related high-spin complex with the same axial ligands (ClO4) is given.This analysis shows the subtlety of the steric interactions of the methyl protons with porphinato core atoms.The axial Fe-N bond distance average is 2.012 Angstroem, and the equatorial bond distance average is 1.970 Angsroem.Crystal data for ClO4*THF*H2O: monoclinic, a=26,943 (3) Angstroem, b=16.927 (2) Angstroem, c=23.358 (3) Angsroem, and β=104.76 (1)deg, Z=8, space group C2/c, 6648 observed data.