CHARACTERIZATION OF ALUMINUM ISOPROPOXIDE AND ALUMINOSILOXANES

Aluminium isopropoxide, in the form of a crystalline solid of melting point 127-degrees-C, has been shown by X-ray diffraction to be a tetrameric molecular species of formula Al[(mu-O(i)Pr)2Al(O(i)Pr)2]3, consistent with earlier proposals and spectroscopic data. A central aluminium achieves coordination number six via two bridging alkoxide groups from each of three Al(O(i)Pr)4- groups. The resulting planar Al(mu-2-O)2Al rings have widened (approximately 132-degrees) angles at oxygen to increase the non-bonded Al-Al distance. While the terminal O(i)Pr groups show the shortest (1.70 angstrom) Al-O distances in the molecule, they are nevertheless strongly bent (approximately 140-degrees) at oxygen. The H-1 NMR spectrum of Al4(O(i)Pr)12 is analysed based on the structure reported here. Characterization of aluminosiloxane derivatives Al(O(i)Pr)3-x(OSiMe3)x (x = 1-3) by H-1 and Al-27 NMR, mass spectrometry and IR spectroscopy is also reported. For x = 1 and 2, these show co-existence of several oligomers. The effect of incorporation of OSiMe3 groups is to favour smaller oligomers.