Methyl aluminosilsesquioxanes, models for Lewis acidic silica-grafted methyl aluminum species

The hydroxysilsesquioxanes (c-C5H9)(7)Si8O12(OH) (I) and (c-C5H9)(7)Si7O9(O H)(2)OSiMePh2 (II) have been studied as model supports for silica-grafted a luminum alkyl species. Treatment of AlMe3 with I gave polymeric {[(c-C5H9)( 7)Si8O13]AlMe2}(n) (1a), which is readily transformed into the correspondin g monomeric pyridine adduct, [(c-C5H9)(7)Si8O13]AlMe2. PY (1b). When AlMe3 was reacted with II, noticeable amounts of the 2:1 product {[(c-C5H9)(7)Si7 O11(OSiMePh2)](AlMe2)(2)}(2) (2) and the Bronsted acidic 1:2 product {[(c-C 5H9)(7)Si7O11(OSiMePh2)](2)Al-}{H+} (III) were obtained besides the main pr oduct of the reaction, {[(c-C5H9)(7)Si7O11(OSiMePh2)]AlMe}(2) (3a-c). The m ain product is a mixture of three dimeric conformational isomers all with t he aluminum methyls trans to each other. The difference of the conformers o riginates from the different orientation of the silsesquioxane ligands. Rea ction of the Bronsted acid III with AlMe3 yielded the kinetic product [(c-C 5H9)(7)Si7O11(OSiMePh2)](2)Al2Me2 (4). The kinetic and thermodynamic stabil ity of the three conformeric methyl aluminosilsesquioxanes {[(c-C5H9)(7)Si7 O11(OSiMePh2)]AlMe}(2) (3a-c) and their chemical isomer {[(c-C5H9)(7)Si7O11 (OSiMePh2)(]2)Al2Me2 (4) has been investigated. Isomerization experiments s howed that 3a isomerizes to 3b, which subsequently isomerizes to 3c, afford ing the thermodynamically most stable mixture with a 3a:3b:3c ratio of 1:4: 4 after 400 h at 76 degreesC. Isomerization of 3a to 3b is considerably fas ter than from 3b to 3c. Direct conversion of 3a into 3c was not observed. C omplex 4 slowly isomerizes into 3c, which consecutively isomerizes into the thermodynamic most stable isomeric mixture (1000 h at 76 degreesC, E-a = 1 17 kJ . mol(-1)). Treating Cp2ZrMe2 with the Bronsted acid III gave clean t ransfer of a silsesquioxane ligand to zirconium, yielding [(c-C5H9)(7)Si7O1 1(OSiMePh2)]ZrCP2 (5). The methyl aluminosilsesquioxanes 1a and 2-4 are not Lewis acidic enough to effectively abstract a substituent X from Cp2ZrX2 ( X = Me, CH2Ph, Cl). Though, 3a-c and 4 definitely interact with Cp2ZrX2. De pendent on the substituent X, the zirconocene can accelerate the rate of is omerization over 2 orders of magnitude (3a, 1.5 h; 4, 8 h at 76 degreesC).. Surprisingly, complex 4 also reacts with the strongly Lewis acidic B(C6F5) (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 isomeriza tion of 3a-c. Complexes 2, 3a, 3c, 4, and 5 have been structurally characte rized.