Syntheses, structures, spectra and redox properties of alkoxo- and phenoxo-bridged diiron(III) complexes

Dinuclear di(mu-alkoxo) bridged [Fe2L21] 1 (L-1 = trianion of 1,3-bis(salicylideneamino)propan-2-ol) and [Fe2L22] 2 (L-2 = trianion of 1,3-bis(salicylamino)propan-2-ol) and di(mu-phenoxo) bridged [Fe2L23] 3 (L-3 = trianion of 4-methyl-2,6-bis(salicylideneaminomethyl)phenol) and [Fe2L24] 4 (L-4 = trianion of 4-methyl-2,6-bis(salicylaminomethyl)phenol) complexes have been synthesized and characterized. The crystal structure of 1 contains two molecules in the asymmetric unit and each molecule has two FeN2O4 distorted octahedral co-ordination units. The structure of 2 consists of a centrosymmetric dimer where the two crystallographically equivalent metal ions are asymmetrically bridged by two alkoxo oxygen atoms. The iron(III) centers have N2O4 co-ordination cores with amine nitrogens and phenolate oxygens in cis position. The crystal structure of 3 contains trans-FeN2O4 distorted octahedral co-ordination units bridged by two phenoxo oxygen atoms. The electronic spectra of all the complexes are characterized by high intensity charge-transfer transitions. Cyclic voltammetric studies of 1, 3 and 4 in dichloromethane solvent reveal stepwise reduction of (FeFeIII)-Fe-III to mixed-valence (FeFeII)-Fe-III and reduced (FeFeII)-Fe-II species while 2 exhibits a single quasireversible reduction peak corresponding to the mixed-valence form.