Molecular precursors for the preparation of homogenous zirconia–silica materials by hydrolytic sol–gel process in organic media. Crystal structures of [Zr{OSi(OtBu)3}4(H2O)2]·2H2O and [Ti(OtBu){OSi(OtBu)3}3]

Abstract

[Zr(OPrⁱ)₄·PrⁱOH] reacts with [HOSi(O^tBu)₃] in anhydrous benzene in 1 : 1 and 1 : 2 molar ratios to afford alkoxy zirconosiloxane precursors of the types [Zr(OPrⁱ)₃{OSi(O^tBu)₃}] (A) and [Zr(OPrⁱ)₂{OSi(O^tBu)₃}₂] (B), respectively. Further reactions of A or B with glycols in 1 : 1 molar ratio afforded six chemically modified precursors of the types [Zr(OPrⁱ)(OGO){OSi(O^tBu)₃}] (1A–3A) and [Zr(OGO){OSi(O^tBu)₃}₂] (1B–3B), respectively [where G = (–CH₂–)₂ (1A, 1B); (–CH₂–)₃ (2A, 2B) and (–CH₂CH₂CH(CH₃–)} (3A, 3B)]. The precursors A and B are viscous liquids, which solidify on ageing whereas the other products are all solids, soluble in common organic solvents. These were characterized by elemental analyses, molecular weight measurements, FAB mass, FTIR, ¹H, ¹³C and ²⁹Si-NMR studies. Cryoscopic molecular weight measurements of all the products, as well as the FAB mass studies of 3A and 3B, indicate their monomeric nature. However, FAB mass spectrum of the solidified B suggests that it exists in dimeric form. Single crystal structure analysis of [Zr{OSi(O^tBu)₃}₄(H₂O)₂]·2H₂O (3b) (R_fac = 11.9%) as well as that of corresponding better quality crystals of [Ti(O^tBu){OSi(O^tBu)₃}₃] (4) (R_fac = 5.97%) indicate the presence of a M–O–Si bond. TG analyses of 3A, B, and 3B indicate the formation of zirconia–silica materials of the type ZrO₂·SiO₂ from 3A and ZrO₂·2SiO₂ from B or 3B at low decomposition temperatures (≤200 °C). The desired homogenous nano-sized zirconia–silica materials [ZrO₂·nSiO₂] have been obtained easily from the precursors A and B as well as from the glycol modified precursors 3A and 3B by hydrolytic sol–gel process in organic media without using any acid or base catalyst, and these were characterized by powder XRD patterns, SEM images, EDX analyses and IR spectroscopy.