Iso- and mixed-valent phenoxy bridged binuclear macrocyclic complexes of cobalt, iron and manganese

Abstract

The binuclear complexes of a tetraaminodiphenol macrocyclic ligand (H₂L) of cobalt(II) and manganese(II) with the composition [M₂LX₂]βH₂O (X = Cl or Br) and of iron(II) with the composition [Fe₂L(imidazole)₂(CH₃OH)₂](ClO₄)₂·2CH₃OH have been synthesized and spectroscopically characterized. Cyclic voltammetric measurements of the dicobalt(II) complexes have indicated that the generation of Co^III-Co^II and Co^III-Co^III species take place quasi-reversibly or irreversibly at about 0.38 and 0.90 V vs SCE, respectively. The dimanganese(II) complexes undergo quasi-reversible one-electron oxidation at about 0.32 V, but further oxidation does not occur up to 1.2 V. The iron(II) complex irreversibly gets oxidized at ca 1 V. The dicobalt(II) and dimanganese(II) complexes can be oxidized with bromine and the mixed-valence complexes [M^IIIM^IILCl₂Br(CH₃OH)]·solvent have been isolated. Variable-temperature magnetic measurements of [Co^IIICo^IILCl₂Br (CH₃OH)]·0.5CH₂Cl₂, [Mn₂^IILBr₂(CH₃OH)₂]·H₂O and [Fe₂^IIL(imidazole)₂(CH₃OH)₂] (ClO₄)₂·2CH₃OH have indicated that the mixed-valence cobalt complex behaves as a S = 3/2 system, while both the diiron(II) and dimanganese(II) complexes exhibit antiferromagnetic exchange coupling with J =-6.3 and -6.1 cm^-1, respectively.