Steric factors affecting the Bronsted acidity of aluminosilsesquioxanes

By reacting AlMe3 with two equivalents of (c-C5H9)(7)Si7O9(OH)(2)OSiR3, the corresponding Bronsted acidic aluminosilsesquioxanes [(c-C5H9)(7)Si7O11(OS iR3)]Al[(c- C5H9)(7)Si7O10(OH)OSiR3] [SiR3 = SiMe3 (1a), SiMePh2 (1b)] are obtained. These complexes readily react with triethylamine to yield the cor responding ammonium salts {[(c-C5H9)(7)Si7O11(OSiR3)](2)Al}(-)(HNEt3)(+) (2 a,b). Hydrogen bonding between the acidic SiO(H)-->Al proton and the pendan t silyl ether function is effectively reduced by increasing the steric bulk of the silyl ether substituents, resulting in a higher acidity for 1b comp ared to that of 1a. With the silsesquioxane ligand (c-C5H9)(8)Si8O11 (OH)(2 ), which lacks pendant silyl ether functions, the acidic proton cannot sati sfactorily be stabilized and this renders the putative Bronsted acid [(c-C5 H9)(8)Si8O13]Al[(c-C5H9)(8)Si8O12(OH)] unstable. In the absence of proton a ccepters, the disproportionation product [(c-C5H9)(8)Si8O13](3)Al-2 (3) is formed instead of the Bronsted acid. However, in the presence of triethylam ine, the initially formed Bronsted acid readily transfers its proton to the amine, affording the ammonium salt {[(c-C5H9)(8)Si8O13](2)Al}(-){HNEt3}(+) (4).