Tin Oxide Nanoparticles and SnO2/SiO2 Hybrid Materials by Twin Polymerization Using Tin(IV) Alkoxides

Twin polymerization was used to prepare composite materials composed of SnO2 nanoparticles entrapped in a polymer matrix. Novel, well-defined tin-containing molecular precursors, so-called twin monomers, were synthesized by transesterification starting from Sn(OR)4 (R=tBu, tAm) to give Sn(OCH2C4H3O)4 (1), [Sn(OCH2C4H3S)4⋅HOCH2C4H3S]2 (2), [Sn(OCH2-2-OCH3C6H4)4⋅HOCH2-2-OCH3C6H4]2 (3), [Sn(OCH2-2,4-(OCH3)2C6H3)4⋅HOCH2-2,4-(OCH3)2C6H3]2 (4), 2,2′-spirobi[4H-1,3,2-benzodioxastannine] (5), 2,2′-spirobi[6-methylbenzo(4H-1,3,2)-dioxastannine] (6), and 2,2′-spirobi[6-methoxybenzo(4H-1,3,2)dioxastannine] (7). 13C and 119Sn NMR spectroscopy in the solid state and in solution as well as IR spectroscopy and elemental analysis were used to characterize the tin alkoxides. The molecular structures of compounds 2 and 3 were determined by single-crystal X-ray diffraction analysis. The moisture sensitivity of the tin(IV) alkoxides was demonstrated by the formation of the tin oxocluster [Sn3(μ3-O)(μ2-OH)(μ2-OCH2C4H3S)3(OCH2C4H3S)6(HOCH2C4H3S)]2 (2 a), a hydrolysis product of compound 2. Polymerization reactions in the melt (for 1 and 5) and in solution (for 2–4) resulted in cross-linked nanocomposites of the type polymer/SnO2. Subsequent oxidation of the composites gave SnO2 with BET surface areas up to 178 m2 g−1. Simultaneous twin polymerization of compounds 5–7 with the silicon derivative 2,2′-spirobi[4H-1,3,2-benzodioxasiline] resulted in the formation of polymer/SnO2/SiO2 hybrid materials. Oxidation gave porous materials with SnO2 nanoparticles embedded in a silica network with BET surface areas up to 378 m2 g−1. The silica acts as a crystal growth inhibitor, which prevents sintering of the SnO2 nanoparticles 20–32 nm in size.