Silanols complexes with bases

Infrared Spectroscopic Data for Silanols and Their Complexes with Bases... 

The silanol complex 57 exhibits a Si H M agostic interaction characterized by a /(Si-H) of 41 Hz and a Si-H distance of 1.70(7) It would be incautious to interpret such a low value of the Si-H coupling in terms of a significant Si-H bond activation, because the Si-H bond forms rather acute angles with the Si-C and Si-Si bonds (about 82 and 101°, respectively) and thus must have a considerable p character on silicon, which should contribute to the decrease of /(Si-H). The silanol ligand is -coordinate to ruthenium and the Ru-Si bond of 2.441(3) A is not exceptional, but the Si(SiMe3)3 deviates from the silanol plane by 19.0°, probably as a result of the Si-H interaction. Deprotonation of 57 by strong bases affords a neutral ruthenocene-like product. 

It has recently been shown that within the same reaction series the mechanism of catalysis by uncharged Lewis bases may change with variation of the structure (275). Tandem kinetic and IR hydrogen bond studies revealed that silylation reaction of a silanol catalyzed with triethylamine is first order with respect to the silanol-amine hydrogen bond complex [Eq. (75).] The same reaction was shown to be effectively catalyzed by... 

Condensation catalysts include both acids and bases, as well as organic compounds of metals. Both tin(II) and tin(IV) complexes with carboxjdic acids are extremely useful. It has been suggested that the tin catalyst is converted to its active form by partial hydrolysis followed by reaction with the hydrolyzable silane to yield a tin—silanolate species (eqs. 22 and 23) (193,194). 

Similarly to Bianchini s approach, De Rege [26] also immobilized cationic [((R,R)-Me-duphos (26))Rh-(COD)]OTf complex noncovalently by the hydrogenbonding interaction of triflate counterion with surface silanols ofMCM-41 support. In contrast to the results obtained by Bianchini et al. [25c], the catalytic activity and selectivities of the immobilized 26-Rh complex on MCM-41 were equal to or greater than the homogeneous counterparts (Scheme 2.7). Moreover, the catalysts were recyclable (up to four times, with no loss of activity) and did not leach. Here again, the counteranion was very important for the successful immobilization of the catalyst onto MCM-41. Whereas, the DuPhos-Rh complex with triflate anion was effectively immobilized (6.7 wt% based on Rh), tlie analogous complex with the lipophilic BArp anion [BArp = R(3,l-((. i )2-C J I i was not loaded onto the support. 

The anionic polymerization of cyclosiloxanes is a complex process. For the alkali metal silanolate catalysts the weight of experimental evidence supports a mechanism based on growth from the metal silanolate ion pair. The ion pair is in dynamic equilibrium with ion-pair dimers which, for the smaller alkali metal ions like lithium and sodium, are themselves in dynamic equilibrium with ion-pair dimer aggregates. The fractional order in catalyst which is observed is a direct result of the equilibria between ion pairs, ion-pair dimers and ion-pair dimer aggregates. Polar solvents break down the aggregates and increase the concentration of ion-pair dimers and hence the concentration of ion pairs. Species like crown ethers and the [2.1.1] cryptate which form strong complexes with the metal cation increase the dissociation of ion-pair dimers into ion pairs. In the case of the lithium [2.1.1] cryptate dissociation into ion pairs is complete and the order in catalyst is unity. 

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