| Abstract
| - The hydroxysilsesquioxanes (c-C5H9)7Si8O12(OH) (I) and (c-C5H9)7Si7O9(OH)2OSiMePh2 (II)have been studied as model supports for silica-grafted aluminum alkyl species. Treatmentof AlMe3 with I gave polymeric {[(c-C5H9)7Si8O13]AlMe2}n (1a), which is readily transformedinto the corresponding monomeric pyridine adduct, {[(c-C5H9)7Si8O13]AlMe2·Py (1b). WhenAlMe3 was reacted with II, noticeable amounts of the 2:1 product {[(c-C5H9)7Si7O11(OSiMePh2)](AlMe2)2}2 (2) and the Brønsted acidic 1:2 product {[(c-C5H9)7Si7O11(OSiMePh2)]2Al-}{H+} (III) were obtained besides the main product of the reaction, {[(c-C5H9)7Si7O11(OSiMePh2)]AlMe}2 (3a−c). The main product is a mixture of three dimeric conformationalisomers all with the aluminum methyls trans to each other. The difference of the conformersoriginates from the different orientation of the silsesquioxane ligands. Reaction of theBrønsted acid III with AlMe3 yielded the kinetic product {[(c-C5H9)7Si7O11(OSiMePh2)]2Al2Me2 (4). The kinetic and thermodynamic stability of the three conformeric methyl aluminosilsesquioxanes {[(c-C5H9)7Si7O11(OSiMePh2)]AlMe}2 (3a−c) and their chemical isomer {[(c-C5H9)7Si7O11(OSiMePh2)]2Al2Me2 (4) has been investigated. Isomerization experimentsshowed that 3a isomerizes to 3b, which subsequently isomerizes to 3c, affording thethermodynamically 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 3ainto 3c was not observed. Complex 4 slowly isomerizes into 3c, which consecutively isomerizesinto the thermodynamic most stable isomeric mixture (1000 h at 76 °C, Ea = 117 kJ·mol-1).Treating Cp2ZrMe2 with the Brønsted acid III gave clean transfer of a silsesquioxane ligandto zirconium, yielding [(c-C5H9)7Si7O11(OSiMePh2)]ZrCp2 (5). The methyl aluminosilsesquioxanes 1a and 2−4 are not Lewis acidic enough to effectively abstract a substituent X fromCp2ZrX2 (X = Me, CH2Ph, Cl). Though, 3a−c and 4 definitely interact with Cp2ZrX2.Dependent on the substituent X, the zirconocene can accelerate the rate of isomerizationover 2 orders of magnitude (3a, 1.5 h; 4, 8 h at 76 °C). Surprisingly, complex 4 also reactswith the strongly Lewis acidic B(C6F5)3. As soon as all 4 has been converted into 3a−c, theaccelerating effect stops, which demonstrates that Lewis acids have no effect on theisomerization of 3a−c. Complexes 2, 3a, 3c, 4, and 5 have been structurally characterized.
- Silsesquioxane ligands have been applied to model the interaction of AlMe3 with silica surface silanols. Methylaluminosilsesquioxane complexes including {[(c-C5H9)7Si7O11-(OSiMePh2)]AlMe}2 and {[(c-C5H9)7Si7O11(OSiMePh2)]2Al2Me2 were synthesized, and their isomerization behavior and reactivity toward zirconocenes and the Lewis acidic B(C6F5)3 were investigated.
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