Oxidation of immobilized iron(II)-1,10-phenanthroline complexes by cerium(IV): a probe into the site accessibility of metal complexes covalently attached to silica sol–gels

Mixed-ligand Fe(II)-1,10-phenanthroline complexes containing zero, one, two, or three ligands with triethoxysilyl termini were incorporated into Ce(IV)-containing silica sol–gels by doping the sols prior to gelation. The alkoxysilyl termini ensured that the complexes were covalently bound to the silica gels. Complexes attached to the gels were significantly more stable towards oxidation by Ce(IV) than complexes bearing no tethers. The mixed ligand complexes can be prepared only as a mixture of species due to the unusual trend for binding constants between phenanthroline and iron (H. Irving, D.H. Mellor, J. Chem. Soc. (1962) 5222). Statistical analysis of the data for the mixtures revealed a sigmoid dependence of the stabilization with respect to the number of tethers. Contributions from inner-sphere pathways are believed to account for this non-monotonic dependence.