Home About us Contact
|
Home About us Contact | ||
|
|
||
Redistribution Reactions (redistribution + reaction)
Selected AbstractsRole of Redistribution Reactions in the Polymer Route to Silicon,Carbon,Oxygen CeramicsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 5 2002P. Hubert Mutin Redistribution reactions are used in the synthesis of several preceramic polymers. Moreover, redistributions play a major role in the pyrolysis of these precursors. Examples of thermal redistributions involving the exchange of Si,O/Si,X bonds (X = O, H, C, ,) in polysiloxanes, precursors to silicon,oxygen,carbon ceramics, are given. Redistributions account for the escape of volatile organosilicon compounds, decreasing the yield and modifying the composition of the final ceramic. Moreover, they deeply modify the environment of Si atoms in the residue. At ,900°C, Si,O/Si,C redistributions have reached equilibrium, leading to a random, entropically controlled distribution of sites. At higher temperatures, redistribution equilibria are displaced by the crystallization of SiC. Thus, the structure of the silicon oxycarbide phase (environment of Si atoms) is dependent on the O/Si ratio of the glass and the pyrolysis temperature, but is not directly dependent on the structure of the precursor. [source] An Unusual Reaction of (,-Dimethylaminoethoxy)triethyltin with Phenyltin Trichloride.EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 21 2006Aryl; X, Symmetrical Dimers [R2SnXY], The First X-ray Structural Evidence of the Existence of Complexes R2SnXY·R2SnXY (R = Alkyl, Y = Hal, Y) Both as Unsymmetrical Adducts [R2SnX2·R2SnY2] Abstract The substituent exchange reaction of PhSnCl3 with [Et3Sn(OCH2CH2NMe2)] gives rise unexpectedly to the unsymmetrical adduct [Ph2SnCl2·Ph2Sn(OCH2CH2NMe2)2] (2). It has been unambiguously proved for the first time that compounds of the RSnX3 type are able to undergo the hydrocarbon substituent redistribution reaction. The analogous tin complexes [Et2SnCl2·Et2Sn(OMe)2] (5) and [Bu2Sn(OAc)2·Bu2Sn(OMe)2] (6), which have ligands other than ,-dimethylaminoethoxy and could be considered as "organotin analogs of Grignard reagents" have symmetrical dimeric structures, i.e., can be formulated as [Bu2Sn(OMe)(OAc)]2 and [Et2Sn(OMe)Cl]2, respectively. Both types of structures, viz., unsymmetrical adduct (2) and symmetrical dimer (5, 6), have been characterized by X-ray diffraction analysis. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] The reaction of mixtures of [Rh4(CO)12] and triphenylphosphite with carbon monoxide or syngas as studied by high-resolution, high-pressure NMR spectroscopy,MAGNETIC RESONANCE IN CHEMISTRY, Issue S1 2008Gillian Overend Abstract The fragmentation and redistribution reactions of [Rh4(CO)12,x{P(OPh)3}x] (x = 1,4) with carbon monoxide have been studied using high-resolution, high-pressure NMR spectroscopy. Under the conditions of efficient gas mixing in a high-pressure NMR bubble column, [Rh4(CO)9{P(OPh)3}3] fragments to give mainly [Rh2(CO)6{P(OPh)3}2]; [Rh4(CO)11{P(OPh)3}] is also observed, implying redistribution of the phosphite ligand and/or recombination of the dimers to tetrameric clusters. Fragmentation of [Rh4(CO)10{P(OPh)3}2] is found to be pressure-dependent giving predominantly [Rh2(CO)6{P(OPh)3}2] at low CO pressure (1,40 bar), and increasing amounts of [Rh2(CO)7{P(OPh)3}] at higher (40,80 bar) pressure. Using Syngas (CO : H2 (1:1)) instead of CO in the above fragmentations, homolytic addition of H2 to the dimer [Rh2(CO)6{P(OPh)3}2] to give [RhH(CO)3{P(OPh3}] and [RhH(CO)2{P(OPh)3}2] is observed. The distribution of tetrameric species obtained is similar to that obtained under the same partial pressure of CO. On depressurisation/out-gassing of the sample, the original mixture of tetrameric clusters is obtained. Copyright © 2008 John Wiley & Sons, Ltd. [source] | ||||||||||