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(OCH₂ C₄ H₃ O)₄ (1), [Sn(OCH₂ C₄ H₃ S)₄ ⋅HOCH₂ C₄ H₃ S]₂ (2), [Sn(OCH₂ -2-OCH₃ C₆ H₄ )₄ ⋅HOCH₂ -2-OCH₃ C₆ H₄ ]₂ (3), [Sn(OCH₂ -2,4-(OCH₃ )₂ C₆ H₃ )₄ ⋅HOCH₂ -2,4-(OCH₃ )₂ C₆ H₃ ]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). ¹³ C and ¹¹⁹ Sn 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 -OCH₂ C₄ H₃ S)₃ (OCH₂ C₄ H₃ S)₆ (HOCH₂ C₄ H₃ S)]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.