Stabilisation of silicenium ylids by adduct formation with aluminium trihalides: the crystal structure of [(Me3C)2SiNCMe3]2AlClF2

Lithium salts of t-butylamino-diorganylfluorosilanes [RR′SiFN(CMe3)Li] react with aluminium trichloride in petroleum ether, eliminating LiF, to give AlCl3 adducts of silicenium ylids, (RR′SiNCMe3)AlCl3. Hydrolysis of these adducts leads to Me3CNH2·AlCl3 and (RR′SiO)3). The compound [(Me3C)2SiNCMe3]2AlClF2 has been obtained as the major product of the reaction of (Me3C)2SiFN(CMe3)Li with Al2Cl6 in tetrahydrofuran. It forms orthorhombic crystals, space group Pnna, with a 16.063(2), b 21.896(2), c 17.987(2) Å, Z = 8. The crystal structure has been determined from 3389 unique diffractometer-measured intensities, and refined to R=0.070. The molecular structure and NMR spectra support the formulation of this compound as a 2:1 adduct of a silicenium ylid with AlClF2. The hydrolysis product Me3CNH2·AlCl3 forms monoclinic crystals, space group P21/n, with a 8.348(5), b 10.908(4), c 11.322(9) Å, β 103.63(10°, Z = 4, R = 0.052 for 1067 reflections. Molecules are monomeric in the solid state.