Novel Aluminum Hydride Derivatives from the Reaction of H₃Al·NMe₃ with the Cyclosilazanes [Me₂SiNH]₃ and [Me₂SiNH]₄

The amine hydrogen atoms of the cyclic trimeric silazane [Me2SiNH]3 are readily replaced by the H2Al· NMe3 group in a simple aminolyis reaction of [Me2SiNH]3 with H3Al·NMe3 to afford the aluminum amides (Me2SiNAlH2·NMe3)n(Me2SiNH)3-n (1, n = 3; 2, n = 1; 4, n = 2). The monosubstituted amide 2 could not be isolated, because it undergoes condensation to the tricyclic compound 1,1‘,2,2‘-(HAlNMe3)2 (3). Contrary to these results the analogous reactions of the more flexible cyclic tetrameric silazane [Me2SiNH]4 with H3Al·NMe3 did not give simple aluminum amides, but complicated mixtures were obtained from which the interesting polycyclic species Al5C22H73N10Si8·C6H6 (5) and Al6C22H76N10Si8·1/4 C6H14 (6) could be isolated in low yields. A key step in the formation of 5 and 6 is a low-temperature dehydrosilylation reaction which leads to cleavage of the silazane ring. Compounds 1, 3, and 4 were characterized spectroscopically (1H, 13C, 27Al NMR and FTIR) and by single crystal X-ray diffraction, whereas 5 and 6 were characterized by X-ray diffraction only. Thermolysis experiments involving 1 and 3 indicate that the onset of Al−N bond formation via dehydrosilylation is accompanied by loss of trimethylamine and formation of larger aggregates, which are stable to further silane elimination to at least 620 °C.