Silsesquioxanes formation

The exploration of silsesquioxane-based titanium catalysts has resulted in valuable insight into the fundamentals of silsesquioxane formation. Furthermore, an improved synthesis method for the important precursor R7Si7012H3, with R=cyclopentyl, was identified. Finally, new catalytic systems based on R2Si207H4, with R=tert-butyl, were discovered. 

Lee D W and Kawakami Y (2007) Incompletely condensed silsesquioxanes formation and reactivity. Polym J 39 230-238... 

Evaluation of the peak pattern gives evidence of silsesquioxanes and their homo-derivatives as well as high molecular reaction products with ladder-like structures. The presence of cage-like polysiloxanes is remarkable since their formation generally requires completely different preparative conditions [10],... 

Siloxane compounds, in vitreous silica manufacture, 22 414 Siloxane materials, 20 240 Siloxane oligomers, in silicone polymerization, 22 555-556 Siloxanols, silylation and, 22 703 Silsesquioxane hybrids, 13 549 Silsesquioxanes, 15 188, 22 589-590 SilvaGas process, 3 696, 697 Silver (Ag), 22 636-667. See also Silver compounds. See Ag entries Argentothiosulfate complexes Batch desilverizing Lead-silver alloys Palladium-silver alloy membranes analytical methods for, 22 650-651 applications of, 22 636-637, 657-662 as bactericide, 22 656, 657, 660 barium alloys with, 3 344 in bimetallic monetary system, 22 647-648 in cast dental gold alloys, 8 307t coke formation on, 5 266 colloidal precipitation color, 7 343t colloidal suspensions, 7 275 color, 7 334, 335... 

A different strategy for the preparation of silsesquioxanes containing silanol groups has been put forward by Matasumoto et al., which involves the stepwise formation of siloxane linkages and the use of protection groups. In this... 

Scheme 3. Formation of completely and incompletely condensed silsesquioxanes upon hydrolysis of RSiXs pc = Cl, OR) followed by condensation steps.

Several titanocene derivatives containing silsesquioxane ligands have also been prepared and characterized. It soon turned out that reactions of 3 or its mono-silylated derivative 12 with titanocene dihalides are not straightforward and usually lead to the formation of product mixtures. A common feature appears to be the formation of p-oxo species despite the use of carefully dried solvents. Although at this stage the occurrence of partial hydrolysis cannot completely be ruled out, we... 

The fulvene route was also successfully employed in the preparation of a compound, which can be regarded as one of the most advanced molecular models for a catalytically active titanium center on a silica surface. When Cp Ti(C5Me4CH2) was reacted with the monosilylated silsesquioxane precursor 12 in refluxing toluene a color change from deep purple to amber was observed. Crystallization afforded a bright-yellow material, which was subsequently shown to be the novel mo o(pentamethyleyclopentadienyl) titanium(IV) silsesquioxane complex 126 (69% yield). Its formation is illustrated schematically in Scheme 42. 

The surprising outcome of this reaction is the exclusive formation of a Cp Ti complex in which two silsesquioxanes are bonded in different ways to a single Ti... 

While previously reported Cr silsesquioxane complexes contained chromium in the hexavalent oxidation state, the recently synthesized lithium silsesquioxane precursors such as 15-17 enabled the synthesis of an unprecedented heterobimetallic chromium species.Treatment of 16 with anhydrous CrCh in THF resulted in formation of green crystalline 154, which was structurally characterized by X-ray diffraction (Scheme 52). 

Mechanistically, in situ monitoring of the reaction between the starting silsesquioxane la and the organometallic precursor Ta(=CH- Bu)(-CH2- Bu)3 has revealed the formation of the tetraneopentyl tantalum(V) intermediate silsesqui-... 

To model the photochemical reaction of Re2(CO)io with silica in the presence of THE, silanolates other than silsesquioxanes [Et3SiOH, PhsSiOH, Ph2Si(OH)2 and (OH)Ph2SiOSiPh2(OH)j with [Re2(CO)8(THE)2] have also been synthesized [148], pointing to the formation of the surface species [((r-=SiO)((r-H)Re2(CO)8j in the above-mentioned reaction with silica. Rhenium carbonyl of the two diol derivatives listed above where both the SiOH have reacted are proposed as models for surface reaction with a vicinal hydroxyl residue, although no direct surface species is reported [148]. 

Sprung and Guenther were the first to postulate that the hydrolysis of organyltrifunctional monomers XSiYj involves a consecutive formation of linear, cyclic, polycyclic and, finally, polyhedral siloxanes. They assumed the chain growth to be of a random character. This hypothesis was further developed by Brown and Vogt who studied the hydrolysis of cyclohexyl- and phenyltrichloro-silane They suggested the formation of polyhedral silsesquioxanes and their homo... 

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