Binuclear Highly Distorted Iron(III) Porphyrins Bridged by the Dianions of Hydroquinones: Role of the Bridge in Electronic Communication

Abstract

A family of binuclear highly substituted and distorted ferric porphyrins bridged through a variety of p-hydroquinone dianions is reported. Two iron(II) porphyrins are oxidized with a consequence of 2 e reduction of the p-benzoquinone unit, which produces the binuclear ferric complex bridged with dianions of hydroquinone. The electronic structures of the dimeric complexes have been confirmed by using UV/Vis, IR, 1H NMR, EPR and Mössbauer spectroscopy as well as single-crystal X-ray diffraction studies. The hydroquinone plane is remarkably tilted to form a Fe–O–C angle of approximately 132° and a dihedral angle of approximately 53° with the porphyrin mean plane. Variable-temperature magnetic susceptibility measurements were carried out between 5 and 300 K in the solid state and the iron centres are found to be involved in a weak antiferromagnetic exchange interaction through the bridge. The coupling constant, J, varies between –6.46 and –14.12 cm–1 for the complexes reported here just upon changing the substituent on the bridge. It was observed that an increase in the electron-withdrawing property of the substituent on the p-benzoquinone ring reduces the antiferromagnetic coupling between the two iron centres whereas electron-donating substituents increase it. The exchange interactions between two iron centres, thus, depend on the electronic nature of the bridge.