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) have been used to form Lewis and Br?nsted acidic boron and gallium silsesquioxane compounds. Treatment of (c-C5H9)7Si7O9(OH)3 with BI3 in the presence of an amine yielded the dimeric [(c-C5H9)7Si7O12]B2 (1). Compound 1 was also obtained when (c-C5H9)7Si7O9(OH)3 was reacted with LiBH4, eliminating LiH. On the other hand, treatment of LiBH4 with 2 equiv of (c-C5H9)7Si7O9(OH)2OSiMePh2 gave the solvent-free lithium 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 Br?nsted acid of 4, as could be obtained for aluminum, invariably failed. Nevertheless, ammonium salt 4 reacts as a masked Br?nsted acid with the basic GaMe3 to yield the dimethyl gallium gallate [(c-C5H9)7Si7O11(OSiMePh2)]2Ga(GaMe2) (5). The equimolar reaction between GaMe3 and R7Si7O9(OH)3 selectively yielded [R7Si7O11(OH)]GaMe2 (R = c-C5H9 (6a), c-C6H11 (6b)). The fact that in 6a/b gallium methyl and silanol functionalities coexist clearly demonstrates the lower reactivity of gallium-alkyls compared to aluminum-alkyl groups. The equimolar reaction of GaMe3 with the monosilylated (c-C5H9)7Si7O9(OH)2OSiMePh2 yielded [(c-C5H9)7Si7O11(OSiMePh2)]GaMe2 (7) as a 5:1:40 mixture of three conformational isomers, similar to what was observed for the corresponding methyl aluminum silsesquioxanes. The kinetic and thermodynamic stability of 7 and its chemical isomer 5 has been investigated. Isomerization experiments show that both 5 and the main conformer of 7 isomerize to a thermodynamic stable mixture of isomers of 7 with a 2:1:2 ratio. The dimethyl gallium silsesquioxane [(c-C5H9)7Si7O11(OSiMePh2)]2(GaMe2)4 (8) was formed quantitatively 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 dimerc [(c-C5H9)7Si7O12]2(GaMe2)6 (9). The high tendency of gallium to obtain a tetrahedral coordination results in a crowded structure 9 with remarkable low fluxionality. Compounds 2, 4, 5, 6b, 8, and 9 have been structurally characterized. The incompletely condensed hydroxysilsesquioxanes R7Si7O9(OH)3 and R7Si7O9(OH)2OSiMePh2 (R = c-C5H9, c-C6H11) have been used to form Lewis and Br?nsted acidic boron and gallium silsesquioxane compounds. Treatment of (c-C5H9)7Si7O9(OH)3 with BI3 in the presence of an amine yielded the dimeric [(c-C5H9)7Si7O12]B2 (1). Compound 1 was also obtained when (c-C5H9)7Si7O9(OH)3 was reacted with LiBH4, eliminating LiH. On the other hand, treatment of LiBH4 with 2 equiv of (c-C5H9)7Si7O9(OH)2OSiMePh2 gave the solvent-free lithium 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 Br?nsted acid of 4, as could be obtained for aluminum, invariably failed. Nevertheless, ammonium salt 4 reacts as a masked Br?nsted acid with the basic GaMe3 to yield the dimethyl gallium gallate [(c-C5H9)7Si7O11(OSiMePh2)]2Ga(GaMe2) (5). The equimolar reaction between GaMe3 and R7Si7O9(OH)3 selectively yielded [R7Si7O11(OH)]GaMe2 (R = c-C5H9 (6a), c-C6H11 (6b)). The fact that in 6a/b gallium methyl and silanol functionalities coexist clearly demonstrates the lower reactivity of gallium-alkyls compared to aluminum-alkyl groups. The equimolar reaction of GaMe3 with the monosilylated (c-C5H9)7Si7O9(OH)2OSiMePh2 yielded [(c-C5H9)7Si7O11(OSiMePh2)]GaMe2 (7) as a 5:1:40 mixture of three conformational isomers, similar to what was observed for the corresponding methyl aluminum silsesquioxanes. The kinetic and thermodynamic stability of 7 and its chemical isomer 5 has been investigated. Isomerization experiments show that both 5 and the main conformer of 7 isomerize to a thermodynamic stable mixture of isomers of 7 with a 2:1:2 ratio. The dimethyl gallium silsesquioxane [(c-C5H9)7Si7O11(OSiMePh2)]2(GaMe2)4 (8) was formed quantitatively 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 dimerc [(c-C5H9)7Si7O12]2(GaMe2)6 (9). The high tendency of gallium to obtain a tetrahedral coordination results in a crowded structure 9 with remarkable low fluxionality. Compounds 2, 4, 5, 6b, 8, and 9 have been structurally characterized.