Modeling acidic sites in zeolites and aluminosilicates by aluminosilsesquioxanes

Protolysis of alkylaluminum compounds with silsesquioxanes is an efficient procedure to synthesize both Lewis and Bronsted acidic aluminosilsesquioxanes. Treatment of AlEt3 with (c-C5H9)(7)Si7O9(OH)(3) and (c-C5H9)(7)Si7O9(OSiMe3)(OH)(2) gives the corresponding Lewis acidic aluminosilsesquioxanes, {[ (c-C5H9)(7)Si7O12]Al}(n) (1) and [ (c-C5H9)(7)Si7O11(OSiMe3)]AlEt . NEt3 (2). By allowing AlEt3 to react with two equivalents of (C-C5H9)(7)Si7O9(OSiMe3)(OH)(2), the Bronsted acidic aluminosilsesquioxane [(c-C5H9)(7)Si7O11(OSiMe3)]Al[(c-C5H9)(7)Si7O10(OSiMe3)(OH)] (3) is selectively formed. H-1 NMR and IR spectroscopy and density functional theory (DFT) calculations show that complex 3 contains a strong intramolecular hydrogen bond. Although the high strength of this hydrogen bond reduces the Bronsted acidity of 3 substantially, 3 can easily be deprotonated by amines to yield the corresponding ammonium salts {[(cC(3)H(9))(7)Si7O11(OSiMe3)](2)Al}-X+ (X = Et3NH 4a, PhN(H)Me-2 4b, C5H5NH 4c). The X-ray crystal structure of 4a demonstrates that the ammonium cation is bonded to the aluminosilsesquioxane anion by a hydrogen bond. The corresponding lithium salt {[(c-C5H9)(7)Si7O11(OSiMe3)](2)Al}(-){Li . 2THF}(+) (5) could best be prepared by protolysis of (c-C5W9)(7)Si7O9(OSiMe3)(OH)(2) with half an equivalent of LiAlH4.