Transformation of molecular compounds with Ba(Sr)/Al/Si and Ca(Sr, Ba)/Ti(Zr, Hf)/Si heteroelements as new efficient route to metal silicate materials

Abstract

Silicon-based moieties were anchored in heterometallic alkoxide platforms to obtain molecular clusters based on the M–O–M′–O–Si motif for the generation of mixed-metal silicate materials. Single-source molecular precursors with structures [M{(μ-ddbfo)₂Al(OSiPh₃)₂}₂], where dbbfoH = 2,3-dihydro-2,2-dimethylbenzofuran-7-ol and M = Ba (1), Sr (2), [Sr₄Ti₂(μ₆-O)(μ₃,η²-OCH₂CH₂OMe)₈(η-OCH₂CH₂OMe)₂(OSiPh₃)₄] (3), [Sr₄M′₂(μ₄-OH)(μ₃,η²-OCH₂CH₂OMe)₄(μ,η²-OCH₂CH₂OMe)₆(μ-X)(OSiPh₃)₄] (4, with M′ = Zr and X = Cl; 5 with M′ = Hf and X = OH), [Sr₃Hf₂(μ₅-O)(μ₃,η²-OCH₂CH₂OMe)₄(OCH₂CH₂OMe)₄(OSiPh₃)₄] (6), and [Ca₂M′₂(μ₃-OH)₂(μ,η²-OCH₂CH₂OMe)₄(η²-HOCH₂CH₂OMe)₂(η-OCH₂CH₂OMe)₂(OSiPh₃)₄] for M′ = Zr (7), Hf (8), were prepared by substitution of methyl groups, using Ph₃SiOH or chloride atoms with KOSiPh₃, in [M{(μ-ddbfo)₂AlMe₂}₂] (M = Sr, Ba) or [M₄M′₂(μ₆-O)(μ₃,η²-OCH₂CH₂OMe)₈(OCH₂CH₂OMe)₂(HOCH₂CH₂OMe)_xCl₄] (M = Ca, Sr, Ba, M′ = Ti, Zr, Hf and x = 0, 4). The precursors were characterized by elemental analysis, NMR spectroscopy, and single-crystal X-ray structural analysis. Thermal decomposition of compounds 1–8 at 1000 °C led to formation of ceramic materials that consisted of mixed-metal oxide nanocrystallites embedded in an amorphous SiO₂ matrix. Compounds 1 and 2 decomposed to BaAl₂Si₂O₈ and SrAl₂Si₂O₈ aluminosilicates, 4 gave a mixture of SrSiO₃, ZrO₂, SrZrO₃ and Sr₇ZrSi₆O₂₁, 5 and 6 gave mixtures of various compounds, including SrHfO₃, Hf_0.96Si_0.04O₂, and SiO₂, 7 lead to CaZrO₃, ZrO₂ and Ca₃ZrSi₂O₉ and 8 gave a HfO₂, Ca₂HfSi₄O₁₂ and SiO₂ phase. The thermolysis of compound 3 gave SiO₂ particles of average size 40–90 nm, which contained spherical SrTiO₃, SrSiO₃ and Sr₂TiSi₂O₈ nanocrystallites of diameter 5–16 nm.