Carbosilanes

Carbosilanes are compounds in which the elements carbon and silicon occupy alternate positions in the molecular framework. Formal replacement of every second carbon atom in the diamond lattice by a silicon atom gives silicon carbide, which exists, however, in several modifications characterized by different stacking orders. The SiC lattice is the basis of most carbosilanes.^[1] These are divided on the basis of structural differences into carborundanes, silascaphanes, and molecules that no longer contain elements of the silicon carbide structure. In the carborundanes the Si–C six-membered rings are arranged in the chair form, in the silascaphanes in the boat form. The molecular framework of the third group is derived from barrelane and asterane. The reactivity of the carbosilanes is determined mainly by the substituents on the skeletal atoms. The presence of SiH or Si–halogen groups leads to a high reactivity, while derivatives with organic groups on the silicon atoms and CH₂ or CH groups in the skeleton are much less reactive. However, even such CH₂ and CH groups permit certain characteristic reactions. Because of their structural features and the range of substituents that can be introduced, the carbosilanes are presently of considerable interest with respect to technological developments as precursors for ceramics.     

Synthesis and structure of silyl acetonitriles

An efficient route for the preparation of Mes₂HSiCH₂CN (1) and (Mes₂HSi)₂CHCN (2) is reported (Mes = 2,4,6-trimethylphenyl). Although the X-ray analyses for 1 and 2 reveal the CH_nCN functionality (n = 1, 2) to be shielded by the Mes₂HSi group, hydrolysis under basic conditions gave exclusively Mes₂HSiOH (3), as a result of hydroxide-induced Si-C bond cleavage. In the solid state 3 is a tetramer forming an eight membered ring by H-bond interactions.     

Synthesis and Properties of Backbone Silylated Imidazol-2-thiones

Imidazole-2-thiones have attracted considerable interest in the past as materials for potential applications in the pharmaceutical and chemical industries. Herein, the synthesis of a series of backbone silylated 1,3-dialkylimidazol-2-thiones is reported. The developed synthesis protocol involves the silylation of N,N-dimethylimidazol-2-thione 1 followed by the addition of organochlorosilanes R_nSiCl_4–n (R=Me, Ph; n=0–4) and enabled the synthesis of the C-silylated derivatives with monocyclic, silyl-bridged or fused tricyclic structures. Reactivity studies performed with N,N-dimethyl-4,5-bistrimethylsilylimidazole-2-thione as a model substance showed surprisingly stable silicon-vinyl bonds and reactivity patterns closely related to those observed for the unsilylated species 1 . Combined UV-spectroscopic and computational studies revealed only minor impact of the silyl substituents on the electronic structure of the imidazol-2-thione ring.     

A theoretical study of structure and bonding of chlorinated silaethanes and 1,3-disilapropanes

Results of extended basis set treatments on the SCF level of theory are reported for all C-chlorinated, Si-chlorinated, symmetrically C- and Si-chlorinated silaethanes, and some chlorinated 1,3-disilapropanes. These molecules are considered as models for carbosilane compounds in general. Computed geometric structure constants are in good agreement with experiment as far as a comparison is possible. The stability and reactivity of molecules considered is discussed by means of computed bond distances, isodesmic reaction energies, and especially by results of population analysis. Si-chlorination yields a stabilization of the Si-C skeleton in carbosilanes whereas C-chlorination reduces this stability to a large extent.     

Reactions of methyl(pentafluorophenyl)- and methyl(pentafluorophenyl)phenylsilanes with electrophiles. A convenient preparative route to halogeno(methyl)pentafluorophenylsilanes C6F5SiMe2X and C6F5SiMeX2 (X=F, Cl and Br)

Halogeno(methyl)pentafluorophenylsilanes C₆F₅SiMe_nX_3−n (n=1, 2) (X=F, Cl, Br) were prepared in good yields from the corresponding phenylsilanes C₆F₅SiMe_nPh_3−n by reactions with the electrophiles aHF, HCl–AlCl₃, Br₂–AlBr₃ or AlX₃ (X=Cl, Br) halogenated hydrocarbons. Additionally, reactions of C₆F₅SiMe₃ and (C₆F₅)₂SiMe₂ with selected electrophiles were studied.     

Synthesis and characterisation of two new binaphthyl trisilanes

The synthesis and characterisation of two binaphthyl trisilanes is described. Reaction between 2,2′-dilithio-1,1′-binaphthyl and 1,3-dichlorohexamethyltrisilane gave 3,3,4,4,5,5-hexamethyl-4,5-dihydro-3H-3,4,5-trisilacyclohepta[2,1-a;4,3-a′]binaphthalene (3). Compound 3 was characterised by ¹H, ¹³C and ²⁹Si NMR spectroscopy and a crystal structure analysis. Reaction between 2,2′-dilithio-1,1′-binaphthyl and 1,3-bis(trifluoromethanesulfonyloxy)-1,1,2,3,3-pentamethyltrisilane, generated in situ by treatment of 1,3-diphenyl-1,1,2,3,3-pentamethyltrisilane with trifluormethanesulfonic acid, gave 3,3,4,5,5-pentamethyl-4,5-dihydro-3H-3,4,5-trisilacyclohepta[2,1-a;4,3-a′]binaphthalene (7). Analysis by ¹H, ¹³C and ²⁹Si NMR spectroscopy revealed that 7 had a very similar structure to 3.     

The Synthesis of tert‐Butyl(dichloromethyl)bis(trimethylsilyl)silane and (Dibromomethyl)ditert‐butyl(trimethylsilyl)silane and their Reactions with Organolithium Derivatives

tert‐Butyl(dichloromethyl)bis(trimethylsilyl)silane ( 4 ), prepared by the reaction of tert‐butylbis(trimethylsilyl)silane with trichloromethane and potassium tert‐butoxide, reacted with 2,4,6‐triisopropylphenyllithium (TipLi) (molar ratio 1 : 2) at room temperature to give (after hydrolytic workup) the silanol tBu(2,4,6‐iPr₃C₆H₂)Si(OH)–CH(SiMe₃)₂ ( 15 ). The formation of 15 is discussed as proceeding through the indefinitely stable silene tBu(2,4,6‐iPr₃C₆H₂)Si=C(SiMe₃)₂ ( 13 ), but attempts to isolate the compound failed. Treatment of (dibromomethyl)ditert‐butyl(trimethylsilyl)silane ( 7 ), made from tBu₂(Me₃Si)SiH, HCBr₃ and KOtBu, with methyllithium (1 : 3) at –78 °C afforded tBu₂MeSi–CHMeSiMe₃ ( 19 ); 7 and phenyllithium (1 : 3) under similar conditions gave tBu₂PhSi–CH₂SiMe₃ ( 20 ). The reaction paths leading to 15 , 19 and 20 are discussed. Reduction of 7 with lithium in THF produced the substituted ethylene tBu₂(Me₃Si)SiCH=CHSitBu₂SiMe₃ ( 21 ). For 21 the results of an X‐ray structural analysis are given.     

Aspects of the synthesis, Characterization and metal–ligand stoichiometry of aminopropyl, nitrobenzamide, and 8-quinolinol silica gels

Surface modification of several silica gels with a number of silanes was studied, as well as further reactions of the modified surfaces. Complete reaction of aminopropylsilane was easily achieved if surface silanols were in excess. Triethoxysilylpropyl-p-nitrobenzamide (TESPN) reacts with difficulty, and complete reaction occured only with long reaction time (at least 12h). The maximum coverage of 100-Å pore silica gel by TESPN was found to be about 1.7 μmol/m², considerably less than the 3–4 /gmmol/m² of available silanol sites, probably due to steric effects of the large silane. Even at this maximum coverage, further reaction of the modified silica with trimethylsilyl capping groups occured, with additional coverage of about 1.0 μmol/m². The maximum amount of silylpropylamido-p-phenylazo-8-quinolinol that could be bound to silica gel was found to be about 1.0 μmol/m², again probably limited by steric effects of the large 8-quinolinol (oxine) moiety. Virtually complete conversion of parent nitrobenzamide silica gel (NBSG) to 8-quinolinol silica gel (QSG) could be achieved if NBSG coverage was less than about 0.7 μmol/m². QSG materials were found to have a 2 oxine to 1 copper(II) stoichiometry, even with surface coverages as low as 0.16 μmol/m².     

Direct Synthesis of Organohalogenosilanes

The direct synthesis of organohalogenosilanes, particularly methyl- and phenylchlorosilanes, is the main process for the production of the monomers required for silicone polymers. This complex heterogeneous catalytic reaction involves the interaction of a gaseous component (the organic halide) with two solid components (silicon and copper), the relative quantities of which change as the reaction proceeds. The mechanism of the direct synthesis has not yet been conclusively established. The free-radical mechanism originally proposed supposes the formation of organic free radicals. On the other hand, the chemisorption mechanism suggested later postulates the dissociative adsorption of the organic halide on the silicon-copper double center (the silicon and the copper being joined by a chemical bond). The results obtained so far indicate that the chemisorption mechanism is of decisive importance, but it is not impossible that both mechanisms operate simultaneously.     

Selective lithiation and crystal structures of G1-carbosilane dendrimers with dimethoxybenzene functionalities

Synthetic routes for the following dendrimers are described: Si[CH₂CH₂Si(Me)₂CH₂-3,5-(MeO)₂-C₆H₃]₄ (2) and Si[CH₂CH₂Si(Me)₂-2,4-(MeO)₂-C₆H₃]₄ (4). The crystal structures of these dendrimers have been determined by single crystal X-ray diffraction. One of the dendrimers (2) shows crystallographic S₄-symmetry and intramolecular C-H?O bonding, whereas the other (4) shows crystallographic C₂-symmetry and is imbedded in a zipper-like network of intermolecular C-H?O bonds. Direct lithiation of these dendrimers with n-BuLi in Et₂O yields complete and selective lithiation in the aryl ring between the methoxy substituents. However, dendrimer 2 is also partially lithiated in the benzylic group. For the solid state structure of such lithiated dendrimers, a diamond-type assembly is predicted.     

Bulky Phenyl Modifications of the Silanide Ligand Si(SiMe3)3 – Synthesis and Reactivity

The synthesis and structural characterization of bulkier variations of the very common organometallic compound MSi(SiMe₃)₃, namely MSi(SiMe₃)(SiPh₃)₂ 3 and MSi(SiPh₃)₃ [M = Li ( 1 ), K ( 5 )] are presented, which can be synthesized via a step by step exchange of SiMe₃ groups by bulkier SiPh₃ groups. This synthetic route is high selective and is performed in good yields via the silanes Si(SiMe₃)₂(SiPh₃)₂ ( 2 ) and Si(SiMe₃)(SiPh₃)₃ ( 4 ). Additionally, the corresponding silanes HSi(SiMe₃)(SiPh₃)₂ ( 6_H ) and HSi(SiPh₃)₃ ( 7_H ) are obtained via the reaction of 3 and 5 with aqueous HCl, respectively. Oxidation of 6_H with CCl₄ gives the chlorsilane Cl‐Si(SiMe₃)(SiPh₃)₂ ( 6_Cl ). The bulkiest chlorsilane Cl–Si(SiPh₃)₃ ( 7 ) is obtained by the reaction of 5 with ECl₂ (E = Sn, Pb).     

Synthesis, structure, and photoluminescence of organosilicon based compounds containing stilbene, butadiene or styrene subunits

Seven silicon based model compounds that contain stilbene, butadiene or styrene subunits with different molecular structures were synthesized and characterized by NMR spectroscopy and single crystal X-ray crystallography. The UV-Vis absorption and photoluminescence spectra were measured in THF solution as well as in the solid state. The interpretation of the spectra reveals that the absorption and emission properties of the compounds originate from the stilbene or butadiene molecular subunits. This investigation provides basic information about the influence of silicon groups, molecular structures and substituents at silicon to absorption and emission properties in organic compounds. Furthermore, due to the functionality of the phenylethynyl substituents at silicon, these compounds may serve as optical active tools and precursors for the introduction of interesting physical properties into new materials for different applications.     

Transient Silenes as Versatile Synthons – The Reaction of Dichloromethyl‐tris(trimethylsilyl)silane with Organolithium Reagents

Treatment of dichloromethyl‐tris(trimethylsilyl)silane (Me₃Si)₃Si–CHCl₂ ( 1 ), prepared by the reaction of tris(trimethylsilyl)silane with chloroform in presence of potassium tertbutoxide, with organolithium reagents (molar ratio 1 : 3) affords the bis(trimethylsilyl)methyl‐disilanes Me₃SiSiR₂–CH(SiMe₃)₂ ( 12 a–d ) ( a : R = Me, b : R = n‐Bu, c : R = Ph, d : R = Mes). The formation of 12 a–d is discussed as proceeding through an exceptional series of isomerization and addition reactions involving intermediate silyl substituted carbenoids and transient silenes. The carbenoid (Me₃Si)₂PhSi–C(SiMe₃)LiCl ( 8 c ) is moderately stable at low temperature and was trapped with water to give (Me₃Si)₂PhSi–CH(SiMe₃)Cl ( 9 c ) and with chlorotrimethylsilane affording (Me₃Si)₂PhSi–CCl(SiMe₃)₂ ( 7 c ). For 12 d an X‐ray crystal structure analysis was performed, which characterizes the compound as a highly congested silane with bond parameters significantly deviating from standard values.     

Formation and Properties of Solvated Electrons

In the formulation of many chemical reactions, electrons are regarded as readily transferable particles, though their participation in these reactions cannot be directly observed. However, the discovery that electrons can be produced in various ways in suitable solutions and that they are stabilized by solvation and can thus be studied directly has recently led to a rapid growth of interest in these, the simplest and most reactive particles of chemistry. The solvated electron has physical properties that permit its detection by various methods even at very low concentrations, so that it is also possible to follow its many reactions, most of which are extremely fast.     

银团簇的形成及性质研究*

本文综述了在固相、液相和气相中形成银团簇的方法, 银团簇的主要性质如吸收光谱、ESR、氧还电位、电子亲和势与电离能, 以及银团簇在光解水和照相显影过程中的催化作用。总结了银团簇的理论计算方法及计算得到的主要性质如稳定几何结构、电子亲和势与电离能。最后展望了今后的发展趋势。     

Synthesis of model long-chain ω-alkenyltrichlorosilanes and triethoxysilanes for the formation of self-assembled monolayers

The synthesis of model long-chain hydrocarbons (C₁₃ and C₁₉) carrying a vinyl group and a trichloro- or a triethoxysilyl group at each end is reported. These compounds are suitable for linkage to a hydroxylated silicon surface and at the other end with vinyl group for further functionalization and multilayer formation.     

Zur analytischen und präparativen Trennung von funktionellen Carbosilanen und Phosphanen mit Hilfe der SFC (Supercritical-Fluid-Chromatography)

Analytical and Preparative Separation of Functional Carbosilanes and Phosphanes by Means of SFC (Supercritical Fluid Chromatography) Analytical and preparative separations of functional silanes, carbosilanes and phosphanes by means of SFC (supercritical fluid chromatography) using CO₂ or CF₃Cl as mobile phases are reported. The detecting system (an IR high pressure cell) of the apparatus which was developed by us is depicted. The separation of mixtures containing silanes with strongly polarizing groups proceeds successfully with Nucleosil-C₁₈ and CO₂ according to the molecular weights. Large differences in the polarity of the compounds give rise to a separation according to molecular weights, smaller ones to the types of compounds. The advantages of a separation by SFC are demonstrated using a mixture of 1,3,5-trisilacyclohexanes with SiH, CCl₂ and SiF groups. The preparative separation is demonstrated using a mixture of 4 different groups of a toral 22 carbosilanes. Eight fractions were obtained each of which containing the compounds with the same number of Si atoms. The final separation was achieved by means of a pressure program (Nucleosil-C₁₈, CO₂). The preparative isolation of a cyclic phosphinoborane from a mixture of three components of the same type as well as the isolation of (Me₃Si)₃P from a mixture of P-rich silylphosphanes is reported.     

Novel pentacoordinate silicon compounds bearing [Si-N-C-N-C-N] chelate ring derived from biguanide ligands

The reactions between secondary carbosilanes 1-3 and 1-propylbiguanide/1-phenylbiguanide, H₆bigR proceed via SiH/NH dehydrocoupling and afford 1,4-bis(silyl)-5-propyl/phenylbiguanides with the general formula (R¹R²₂SiCH₂CH₂SiMeH)₂·H₄bigR, [R¹=Ph, R²=Me, R=ⁿPr (4), R=Ph (5); R¹=Me, R²=Ph, R=ⁿPr (6), R¹=R²=Et, R=Ph (7)]. These compounds represent a new family of pentacoordinate silicon derivatives comprising of [Si-N-C-N-C-N] chelate ring and have been characterized by elemental analysis, FAB mass, IR and multinuclear (¹H, ¹³C and ²⁹Si) NMR spectral studies.     

Controlled Synthesis and Molecular Structures of Methoxy-, Amino-, and Chloro-Functionalized Disiloxane Building Blocks

Functionalized disiloxane units with defined structures are interesting molecular models for investigating the reactivity and chemoselectivity in transformations that are of interest in synthesis, surface chemistry, and materials science. (Mes)PhSi(OMe)₂ ( 1 ) (Mes = mesityl) and (Mes)PhSiCl₂ ( 5 ) were chosen as starting compounds for the controlled synthesis of methoxy-, amino-, and chloro-functionalized unsymmetric disiloxanes. Two synthesis routes towards (Mes)PhSi(OMe)(OSiPh₃) ( 3 ) were followed, one via the aminomethoxysilane (Mes)PhSi(OMe)(NC₄H₈) ( 2 ) and the other via the chlorodisiloxane (Mes)PhSiCl(OSiPh₃) ( 6 ). The amino-substituted disiloxane (Mes)PhSi(NC₄H₈)(OSiPh₃) ( 4 ) was obtained from the chloro derivative 6 with N-pyrrolidinyllithium, but the same reaction starting from compound 3 was not successful. All provided disiloxanes were structurally characterized by X-ray crystallography.     

Highly efficient and chemoselective reduction of sulfoxides using the system silane/oxo-rhenium complexes

This work reports a novel method for the reduction of sulfoxides with silanes catalyzed by high valent oxo-rhenium(V) and (VII) complexes. The catalytic system PhSiH₃/ReIO₂(PPh₃)₂ (1 mol %) proved to be highly efficient for the reduction of a wide range of sulfoxides in excellent yields under mild conditions. This novel methodology is also highly chemoselective, tolerating several functional groups such as –CHO, –CO₂R, –Cl, –NO₂, and double or triple bonds.