Dimerization of Fe(III) Ion in an Aqueous Medium: Mechanistic Modelling and Effects of Ligands

Aqueous iron solutions generally undergo spontaneous hydrolysis followed by aggregation resulting in the precipitation of nanocrystalline oxyhydroxide minerals. The mechanism of nucleation of such multinuclear oxyhydroxide clusters are unclear due to limited experimental evidence. Here, we investigate the mechanistic pathway of dimerization of Fe(III) ions using density functional theory (DFT) in aqueous medium considering effects of other ligands. Two hydrolyzed monomeric Fe(III) ions in aqueous medium may react to form two closely related binuclear products, the μ-oxo and the dihydroxo Fe₂ dimer. Our studies indicate that the water molecules in the second coordination sphere and those co-ordinated to the Fe(III) ion, both participate in the dimerization process. The proposed mechanism effectively explains the formation of dihydroxo and μ-oxo Fe₂ dimers with interconversion possibilities, for the first time. Results show, with only water molecules present in the second co-ordination sphere, dihydroxo Fe₂ dimer is the thermodynamically and kinetically favored product with a low activation free energy. We calculated the step-wise reaction free energies of dimerization in the presence of nitrate ions in the first and second coordination sphere of Fe(III) ion separately, which shows that with nitrate ions in the second co-ordination sphere, the μ-oxo Fe₂ dimer is the kinetically favored product.