Silsesquioxane models for geminal silica surface silanol sites. A spectroscopic investigation of different types of silanols

The incompletely condensed monosilylated silsesquioxanes (c-C5H9)(7)Si7O9(OSiRR'(2))(OH)(2) (SiRR'(2) = SiMe3, SiMe2C(H)CH2, SiMePh2) were reacted with SiCl4 in the presence of an amine which yielded the dichloro compounds (c-C5H9)(7)Si7O9(OSiRR'(2))O2SiCl2 (1-3). These compounds could be hydrolyzed into the corresponding silsesquioxanes containing geminal silanols, (c-C5H9)(7)Si7O9(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)(7)Si8O12(OH). The more sterically hindered geminal silsesquioxane 6 undergoes in solution intermolecular dehydroxylation, yielding the thermodynamically stable dimeric disilanol, [(c-C5H9)(7)Si7O9(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 Bronsted 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.