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

Abstract

Dinuclear di(µ-alkoxo) bridged [Fe₂L¹₂] 1 (L¹ = trianion of 1,3-bis(salicylideneamino)propan-2-ol) and [Fe₂L²₂] 2 (L² = trianion of 1,3-bis(salicylamino)propan-2-ol) and di(µ-phenoxo) bridged [Fe₂L³₂] 3 (L³ = trianion of 4-methyl-2,6-bis(salicylideneaminomethyl)phenol) and [Fe₂L⁴₂] 4 (L⁴ = 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 Fe^IIIFe^III to mixed-valence Fe^IIIFe^II and reduced Fe^IIFe^II species while 2 exhibits a single quasireversible reduction peak corresponding to the mixed-valence form.