New Tris(tert-butoxy)siloxy Complexes of Aluminum and Their Transformation to Homogeneous Aluminosilicate Materials via Low-Temperature Thermolytic Pathways

The new tris(tert-butoxy)siloxy complexes of aluminum, Al[OSi(O^tBu)₃]₃(THF) (1), Al[OSi(O^tBu)₃]₃(HOⁱPr)·0.5[Al(OⁱPr)₃]₄ (2), and [(ⁱPrO)₂AlOSi(O^tBu)₃]₂ (3), were synthesized. Compounds 2 and 3 were structurally characterized by single-crystal X-ray crystallography. Thermolysis of a toluene-d₈ solution containing 1 and 0.5 equiv of [Al(OⁱPr)₃]₄ resulted in the formation of 3 and free THF (by ¹H NMR spectroscopy). The low-temperature decompositions of 1, 2, and 3 allowed for their use as single-source precursors to high surface area aluminosilicate xerogels via solution (toluene) thermolyses. The DSC traces of the xerogels derived from the 1:1 Al/Si precursors (2 and 3) exhibited sharp exothermic transitions at ≈1000 °C, attributed to the crystallization of mullite, with that for the xerogel from 3 having a larger associated heat flow. The DSC trace of the xerogel derived from 1 exhibited a weak exothermic transition at a slightly higher temperature. Although these xerogels have similar ²⁷Al MAS NMR spectra, their microstructures appear to be quite different, as indicated by the substantially more efficient transformation to mullite (by PXRD) of the xerogel from 2. All of the xerogels are homogeneous; however, the microstructure of the xerogel from 2 appears to more closely resemble that of mullite, given the enhanced formation of mullite (by PXRD) and the reduced free energy difference associated with mullite formation (by DSC). Acidity measurements characterized the Lewis/Brønsted site ratios as 1.2, 1.6, and 1.0 for the xerogels from 1, 2, and 3, respectively.