Experimental and Theoretical Investigation of Intercalation and Molecular Structure of Organo-Iron Complexes in Montmorillonite

The intercalation of the μ-oxo Fe(III)-phenanthroline 1:1 complex [(OH2)3(Phen)FeOFe(Phen)(OH2)3]+4 inside montmorillonite yielded a nanostructured material with strong and selective entrapping ability toward thiol molecules and hydrogen sulfide. In this work, experiments and computational molecular modeling by means of quantum mechanical calculations has been applied to study the molecular structure and interactions between this complex and the interlayer of montmorillonite. This approach allowed the identification of the geometrical disposition of the complexes inside the interlayer, the characterization of the hydration and coordination water molecules, and the explanation of the physico-chemical properties of these functionalized materials. The antiferromagnetic spin configuration of the Fe(III) ions results in the most stable state. Two conformers of the complex have been considered, having the phenanthroline rings in twisted or in parallel planes, respectively, and the transition of one conformer into the other has been explored by molecular dynamics simulations. The conformer with phenanthroline rings in parallel planes is found to be the favored species for intercalation in montmorillonite. Both experimental nuclear magnetic resonance analysis and adsorption isotherms are consistent with the modeling results. Different complex amount, equal and double of the cation exchange capacity (CEC) of montmorillonite, and hydration states inside the interlayer have been investigated reproducing faithfully the experimental d(001) spacing of the montmorillonite in the different conditions. The complex molecules intercalated over the CEC of montmorillonite adopt a disposition of the phenanthroline rings perpendicular to that of the complex already introduced by cation exchange.