Characterization of metal-bound water in bioactive Fe(III)-cyclophane complexes

Binuclear Fe(III) complexes, Fe₂PO and Fe₂PC, have functions of antioxidants as well as superoxide dismutase and peroxidase mimickers. The role of water molecules in the non-cytotoxic properties of these complexes have been studied by thermogravimetric and IR/Raman-spectroscopic methods. The thermogravimetric analysis of Fe₂PO shows the presence of nine water molecules (Fe₂PO·9H₂O), three of which are directly coordinated to the metallic ion; the remaining six molecules occupy the secondary coordination sphere. For Fe₂PC, eight water molecules were detected (Fe₂PC·8H₂O), and only one of them directly coordinates to the metallic ion. IR/Raman spectrum analyses corroborate the presence of water molecules in both metallic complexes and the mode of coordination to the ligand, on the basis of bands characteristic of hydration water at ∼3300 cm⁻¹ and bands of adsorbed water between 430 and 490 cm⁻¹. A pentacoordinate geometry is proposed for Fe₂PO and a hexacoordinated geometry for Fe₂PC. Those results are consistent with theoretical calculations performed through a semiempirical PM7 method. The presence of coordinated water molecules is closely related to the reactivity of Fe₂PO and Fe₂PC in solution.