Unusual orientation of axial ligands in metalloporphyrins. Molecular structure of low-spin bis(2-methylimidazole)(meso-tetraphenylporphinato)iron(III) perchlorate

Abstract

The preparation and characterization of the low-spin complex (Fe(TPP)(2-MeHIm)/sub 2/)ClO/sub 4/ 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 (Fe(TPP)(2-MeHIm)/sub 2/)ClO/sub 4/ have an unusual low-spin EPR spectrum, the so-called strong g/sub max/ signal. This strong g/sub 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 S/sub 4/ 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 ((Fe(OEP)(2-MeHIm)/sub 2/)ClO/sub 4/) 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 A, and the equatorial bond distance average is 1.970 A. Crystal data for (Fe(TPP)(2-MeHIm)/sub 2/)ClO/sub 4/ x THF x H/sub 2/O: monoclinic, a = 26.943 (3) A,more » b = 16.927 (2) A, c = 23.358 (3) A, and ..beta.. = 104.76 (1)/sup 0/, Z = 8, space group C2/c, 6648 observed data.« less