Boron, Aluminum, and Gallium Silsesquioxane Compounds, Homogeneous Models for Group 13 Element-Containing Silicates and Zeolites

The incompletely condensed hydroxysilsesquioxanes R7Si7O9(OH)3 and R7Si7O9(OH)2OSiMePh2 (R = c-C5H9, c-C6H11) were used to form Lewis and Bronsted acidic B and Ga silsesquioxane compds. Treatment of (c-C5H9)7Si7O9(OH)3 with BI3 in the presence of an amine yielded the dimeric {[(c-C5H9)7Si7O12]B}2 (1). 1 Was also obtained when (c-C5H9)7Si7O9(OH)3 reacted with LiBH4, eliminating LiH. However, treatment of LiBH4 with 2 equiv of (c-C5H9)7Si7O9(OH)2OSiMePh2 gave the solvent-free Li borate salt {[(c-C5H9)7Si7O11(OSiMePh2)]2B}-{Li}+ (2). Reaction of GaCl3 with (c-C6H11)7Si7O9(OH)3 in the presence of NEt3 afforded the thermally robust amine adduct [(c-C6H11)7Si7O12]Ga.NEt3 (3). A similar reaction of GaCl3 with 2 equiv of (c-C5H9)7Si7O9(OH)2OSiMePh2 gave the ammonium gallate {[(c-C5H9)7Si7O11(OSiMePh2)]2Ga}-{Et3NH}+ (4). Attempts to obtain the corresponding Bronsted acid of 4, as could be obtained for Al, invariably failed. Nevertheless, ammonium salt 4 reacts as a masked Bronsted acid with the basic GaMe3 to yield the di-Me Ga gallate [(c-C5H9)7Si7O11(OSiMePh2)]2Ga(GaMe2) (5). The equimolar reaction between GaMe3 and R7Si7O9(OH)3 selectively yielded {[R7Si7O11(OH)]GaMe}2 (R = c-C5H9 (6a), c-C6H11 (6b)). The fact that in 6a/b Ga Me and silanol functionalities coexist clearly demonstrates the lower reactivity of Ga-alkyls compared to Al-alkyl groups. The equimolar reaction of GaMe3 with the monosilylated (c-C5H9)7Si7O9(OH)2OSiMePh2 yielded {[(c-C5H9)7Si7O11(OSiMePh2)]GaMe}2 (7) as a 5:1:40 mixt. of three conformational isomers, similar to what was obsd. for the corresponding Me Al silsesquioxanes. The kinetic and thermodn. stability of 7 and its chem. isomer 5 was studied. Isomerization expts. show that both 5 and the main conformer of 7 isomerize to a thermodn. stable mixt. of isomers of 7 with a 2:1:2 ratio. The di-Me Ga silsesquioxane {[(c-C5H9)7Si7O11(OSiMePh2)]2(GaMe2)4} (8) was formed quant. upon treating {[(c-C5H9)7Si7O9(OH)2OSiMePh2]} with 2 equiv of GaMe3. Likewise, reacting {[(c-C5H9)7Si7O9(OH)3]} with 3 equiv of GaMe3 yielded the dimeric {[(c-C5H9)7Si7O12]2(GaMe2)6} (9). The high tendency of Ga to obtain a tetrahedral coordination results in a crowded structure 9 with remarkable low fluxionality. 2, 4, 5, 6B, 8, and 9 were structurally characterized by x-ray crystallog