Characterization of aluminium isopropoxide and aluminosiloxanes

Abstract

Aluminium isopropoxide, in the form of a crystalline solid of melting point 127°C, has been shown by X-ray diffraction to be a tetrameric molecular species of formula Al[(μ-O iPr) 2Al(O iPr) 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 iPr) 4 − groups. The resulting planar Al(μ 2-O) 2Al rings have widened (∼ 132°) angles at oxygen to increase the non-bonded AlAl distance. While the terminal O iPr groups show the shortest (1.70 Å) AlO distances in the molecule, they are nevertheless strongly bent (∼ 140°) at oxygen. The 1H NMR spectrum of Al 4(O iPr) 12 is analysed based on the structure reported here. Characterization of aluminosiloxane derivatives Al(O iPr) 3- x (OSiMe 3) x ( x = 1−3) by 1H and 27Al 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 OSiMe 3 groups is to favour smaller oligomers.