DIMETHYLSILOXY-ALUMINIUMDIHALIDE DIMERS - MOLECULAR-STRUCTURES OF [ME2HSIOALX2]2 (X = CL, I) AND [ME3SIOAL(BH4)2]2

sym-Tetramethyldisiloxane Me2HSiOSiHMe2 is cleaved by aluminium trihalides AlX3 (X = Cl, Br, 1) to give dimethylhalosilanes Me2HSiX and alumosiloxane dimers [Me2HSiOAlX2]2 (x = (Cl, 1; Br, 2; I, 3). Treatment of 1 with four equivalents of methyllithium yields the dimeric tetramethylsiloxalane [Me2HSiOAlMe]2, 4, which is also available from Me2HSiO-SiHMe2 and Me2AlCl. Compounds 1-4 have been identified and characterized by their physical data, analytical data, NMR and mass spectra. The crystal structures of the dichloride and the diiodide dimers have been determined by single crystal X-ray diffraction studies. The molecules have a planar, centrosymmetrical SiOAl2OSi skeleton with three-coordinate oxygen atoms and pseudo-tetrahedral four-coordinate aluminium atoms. Gallium trichloride cleaves methyl groups from sym-tetramethyldisiloxane to give methylgallium dichloride and polymeric by-products. Dimeric trimethylsiloxyaluminium-bis(tetrahydridoborate), 5, prepared in previous studies, has also been investigated by an X-ray diffraction study. The SiOal2OSi skeleton of the dimer [Me3SiOAl(BH4)2]2 resembles that of 1 and 3, but the aluminium atoms are hexacoordinated by two bidentate BH4 groups and the two siloxy oxygen atoms.