π-Cation radicals of iron(III) derivatives of deformed porphyrins

Abstract

π-Cation radicals of iron(III) derivatives of some deformed porphyrins have been characterised using UV/VIS, IR and ¹H NMR spectroscopy and by magnetic susceptibility measurements. The deformation in these systems has been induced by a covalent attachment of short bridging chains across the porphyrin periphery. Molecular mechanics simulation clearly reveals the enforced deformation in the porphyrin cores of the free bases inferred from the multiplet structure for the pyrrole protons by ¹H NMR spectroscopy. Significant red shifts in Soret (171–340 cm^–1) and Q-band (76–354 cm^–1) absorption maxima for the iron(III) derivatives relative to [Fe(tpp)Cl](H₂tpp = 5,10,15,20-tetraphenylporphyrin) implies the retention of deformation in solution. Collective evidence from IR, UV/VIS and ¹H NMR spectroscopy and magnetic susceptibility measurements suggests for the oxidised derivatives a high-spin iron(III) state (S=5//₂) antiferromagnetically coupled to a porphyrin radical cation (S=1//₂). Magnetic moment values (4.8–5.1 µ_β) measured for solids and CD₂Cl₂ solutions indicate an intramolecular d–π coupling facilitated through an antiferromagnetic exchange. These results substantiate the emerging correlation between the structure of the macrocycle and the metal–ligand magnetic interactions in high-spin iron(III) porphyrin radical-cation complexes.