Iron(II)- or vanadium(IV)-containing siliceous gels

Iron(II)-containing SiO 2 gels can be prepared by the hydrolysis and condensation of tetramethoxysilane in methanolic solutions of iron(II) chloride. These gels show an absorption at 1000 nm indicating the presence of iron(II). In opposition to that, iron(II) cannot be introduced into organically modified gels of the 3-glycidyloxypropyltrimethoxysilane type. Vanadium(IV) can be incorporated into both gels. Unlike the SiO 2 gels, the organically modified gels are compact and elastic. The spectra of the vanadium(IV)-containing gels show a characteristic absorption at about 800 nm. In some cases these gels change theirs colours with time, presumably due to the exchange of ligands in the coordination sphere of vanadium.