The incompletely condensed monosilylated silsesquioxanes (c-C5H9)7Si7O9(OSiRR'2)(OH)2 (SiRR'2 = SiMe3, SiMe2C(H)CH2, SiMePh2) were reacted with SiCl(4) in the presence of an amine which yielded the dichloro compounds (c-C5H9)7Si7O9(OSiRR'2)O2SiCl2 (1-3). These compounds could be hydrolyzed into the corresponding silsesquioxanes containing geminal silanols, (c-C5H9)7Si7O9(OSiRR'2)O2Si(OH)2 (4-6). At elevated temperatures, the geminal silsesquioxanes 4 and 5 undergo condensation reactions and form the closed-cage silsesquioxane monosilanol, (c-C5H9)7Si8O12(OH). The more sterically hindered geminal silsesquioxane 6 undergoes in solution intermolecular dehydroxylation, yielding the thermodynamically stable dimeric disilanol, [(c-C5H9)7Si7O9(OSiMePh2)(O2Si(OH)-)]2-(mu-O) (7). NMR and FT-IR studies show that the two silanols of the geminal silsesquioxanes 4-6 are different from each other with respect to hydrogen bonding, both in solution and in the solid state. Hydrogen bonding of the geminal silanol-containing silsesquioxanes was examined and compared to hydrogen bonding in silsesquioxanes possessing vicinal or isolated silanol groups. The relative Brønsted acidity of the geminal silanols was determined using pK(ip) (ion-pair acidity) measurements in THF with UV-vis. These acidities were compared with those of other silsesquioxanes containing silanol groups. Acidities of 4-6 were found to be among the lowest known for silsesquioxanes.
The hydroxysilsesquioxanes (c-C 5 H 9 ) 7 Si 8 O 12 (OH) (I) and (c-C 5 H 9 ) 7 Si 7 O 9 (OH) 2 OSiMePh 2 (II) have been studied as model supports for silica-grafted aluminum alkyl species. Treatment of AlMe 3 with I gave polymeric {[(c-C 5 H 9 ) 7 Si 8 O 13 ]AlMe 2 } n (1a), which is readily transformed into the corresponding monomeric pyridine adduct, {[(c-C 5 H 9 ) 7 Si 8 O 13 ]AlMe 2 ‚Py (1b). When AlMe 3 was reacted with II, noticeable amounts of the 2:and the Brønsted acidic 1:2 product {[(c-C 5 H 9 ) 7 Si 7 O 11 (OSiMePh 2 )] 2 -Al -}{H + } (III) were obtained besides the main product of the reaction, {[(c-C 5 H 9 ) 7 Si 7 O 11 -(OSiMePh 2 )]AlMe} 2 (3a-c). The main product is a mixture of three dimeric conformational isomers all with the aluminum methyls trans to each other. The difference of the conformers originates from the different orientation of the silsesquioxane ligands. Reaction of the Brønsted acid III with AlMe 3 yielded the kinetic product {[(c-C 5 H 9 ) 7 Si 7 O 11 (OSiMePh 2 )] 2 Al 2 -Me 2 (4). The kinetic and thermodynamic stability of the three conformeric methyl aluminosilsesquioxanes {[(c-C 5 H 9 ) 7 Si 7 O 11 (OSiMePh 2 )]AlMe} 2 (3a-c) and their chemical isomer {[(c-C 5 H 9 ) 7 Si 7 O 11 (OSiMePh 2 )] 2 Al 2 Me 2 ( 4) has been investigated. Isomerization experiments showed that 3a isomerizes to 3b, which subsequently isomerizes to 3c, affording the thermodynamically most stable mixture with a 3a:3b:3c ratio of 1:4:4 after 400 h at 76 °C. Isomerization of 3a to 3b is considerably faster than from 3b to 3c. Direct conversion of 3a into 3c was not observed. Complex 4 slowly isomerizes into 3c, which consecutively isomerizes into the thermodynamic most stable isomeric mixture (1000 h at 76 °C, E a ) 117 kJ‚mol -1 ). Treating Cp 2 ZrMe 2 with the Brønsted acid III gave clean transfer of a silsesquioxane ligand to zirconium, yielding [(c-C 5 H 9 ) 7 Si 7 O 11 (OSiMePh 2 )]ZrCp 2 (5). The methyl aluminosilsesquioxanes 1a and 2-4 are not Lewis acidic enough to effectively abstract a substituent X from Cp 2 ZrX 2 (X ) Me, CH 2 Ph, Cl). Though, 3a-c and 4 definitely interact with Cp 2 ZrX 2 . Dependent on the substituent X, the zirconocene can accelerate the rate of isomerization over 2 orders of magnitude (3a, 1.5 h; 4, 8 h at 76 °C). Surprisingly, complex 4 also reacts with the strongly Lewis acidic B(C 6 F 5 ) 3 . As soon as all 4 has been converted into 3a-c, the accelerating effect stops, which demonstrates that Lewis acids have no effect on the isomerization of 3a-c. Complexes 2, 3a, 3c, 4, and 5 have been structurally characterized.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
Copyright © 2025 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.