Effect of sterically inhibited axial azaferrocene ligands on the physical properties of iron(III) porphyrins. Crystal structures of bis(azaferrocene) complexes of iron(III) and cobalt(III) porphyrinates

Abstract

The crystal structures of [Fe(oep)L₂][O₃SCF₃] and [Co(oep)L₂][ClO₄] [H₂oep = 2,3,7,8,12,13,17,18-octaethylporphyrin, L = [Fe(C₅H₅)(C₄H₄ N)] have been determined. The azaferrocene ligands L in both compounds are seen to be parallel. The axial M–N (azaferrocene) bonds are longer than those found in analogous imidazole and pyridine complexes. Rhombic EPR spectra for bis(azaferrocene) complexes of iron(III) porphyrins are postulated to be a result of steric interactions of the azaferrocene H 2,5 atoms splitting the degeneracy of the d_xz and d_yz orbitals. The Mössbauer spectra of these iron complexes were recorded and interpreted as evidence for σ bonding of the azaferrocene nitrogen atom to the porphyrin iron core with little or no π back bonding from the porphyrinic iron to the N atom. This is in agreement with the long bond lengths found in the structures. The rhombic EPR spectra observed are in keeping with a parallel arrangement of the ligand planes, as are the magnitudes of the Mössbauer quadrupole splittings. It is shown that the large g_z values of the complexes are not unusual and must relate to long axial bonds and to steric effects splitting the d_xz and d_yz orbitals.

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