Gerhard Roewer

Organosilicon polymers-synthesis, architecture, reactivity and applications

Tailoring of polysilanes with given architectures and reactivities is a great challenge in the field of SiC pre-ceramic polymers. This paper reviews recent polysilane and related copolymer synthesis reactions. It is shown that the Wurtz-type polymerization of dichloro-, trichloro- or tetrachloro-silanes, so far the most extensively studied, enables access to a large variety of architectures ranging from one- to three-dimensional (3D) topologies, and based on secondary >SiR2, tertiary RSi(Si)3 or quaternary Si(Si)4 silicon units in the polymer backbone. These polysilanes usually present an intrinsic low reactivity, detrimental for fiber processing. Examples are given to illustrate how this reactivity can be increased by secondary substitution reactions, which create reactive entities that can favor further crosslinking reactions. Secondly a novel route involving heterogeneously catalyzed disproportionation of chloromethyldisilanes, developed in our laboratory, is reviewed which offers a direct access to polysilyne-type 3D architecture constituted by arrangements of fused rings. The Lewis-base catalyzed disproportionation mechanism is discussed and seems to involve donor-stabilized silylenes as key intermediates in the polymer formation process. The experimental results are supported by ab-initio quantum chemical calculations. Silylenes attack the Si sites of higher functionality causing a high regioselectivity for the exclusive formation of branched oligosilanes. The oligomers undergo thermally induced branching and crosslinking reactions leading to poly(chloromethylsilane)s. Obviously, there are analogies to the oligomer and polymer formation of the transition-metal complex catalyzed dehydropolymerzation of methyldisilanes. Poly(chloromethylsilane)s exhibit a high reactivity due to the presence of Si–Cl bonds. Disproportionation of chloromethyldisilanes in presence of olefins such as styrene provides promising polymer precursors for SiC fibers. Their rheological properties have been investigated for various styrene contents. The polymer fibers spun from melt are cured under ammonia, and then pyrolyzed to silicon carbide fibers, showing temperature resistance up to 1500 °C.

METHYLCHLOROOLIGOSILANES AS PRODUCTS OF THE BASECATALYSED DISPROPORTIONATION OF VARIOUS METHYLCHLORODISILANES

Abstract The methylchlorodisilanes SiCl 2 MeSiCl 2 Me ( 1 ), SiCl 2 MeSiClMe 2 ( 2 ) and SiClMe 2 SiClMe 2 ( 3 ) disproportionate in the presence of a basic catalyst into methylchloromonosilanes and various methylchlorooligosilanes. Oligosilanes involving up to seven silicon atoms were identified by means of 29 Si-, 13 C- 1 H-NMR and GC-MS measurements. Formation of methylchlorooligosilanes is thoughtto take place via silylene intermediates.

Silicon Carbide — A Survey of Synthetic Approaches, Properties and Applications

The challenge to develop tailor-made precursor molecules for silicon carbide has significantly intensified the progress in synthesis of organosilicon polymers. A critical overview is presented regarding useful synthesis routes towards such molecules with appropriate chemical composition as well as controllable architecture (metal condensation of halogenosilanes or silahalogenocarbons, disproportionation of disilanes, dehydrocoupling of hydrosilanes, hydrosilylation of olefins).

Octahedral Adducts of Dichlorosilane with Substituted Pyridines: Synthesis, Reactivity and a Comparison of Their Structures and 29Si NMR Chemical Shifts

H(2)SiCl(2) and substituted pyridines (Rpy) form adducts of the type all-trans-SiH(2*)Cl(2)2 Rpy. Pyridines with substituents in the 4- (CH(3), C(2)H(5), H(2)C=CH, (CH(3))(3)C, (CH(3))(2)N) and 3-positions (Br) give the colourless solids 1 a-f. The reaction with pyrazine results in the first 1:2 adduct (2) of H(2)SiCl(2) with an electron-deficient heteroaromatic compound. Treatment of 1 d and 1 e with CHCl(3) yields the ionic complexes [SiH(2)(Rpy)(4)]Cl(2*)6 CHCl(3) (Rpy=4-methylpyridine (3 d) and 4-ethylpyridine (3 e)). All products are investigated by single-crystal X-ray diffraction and (29)Si CP/MAS NMR spectroscopy. The Si atoms are found to be situated on centres of symmetry (inversion, rotation), and the Si-N distances vary between 193.3 pm for 1 c (4-(dimethylamino)pyridine complex) and 197.3 pm for 2. Interestingly, the pyridine moieties are coplanar and nearly in an eclipsed position with respect to the SiH(2) units, except for the ethyl-substituted derivative 1 e, which shows a more staggered conformation in the solid state. Calculation of the energy profile for the rotation of one pyridine ring indicates two minima that are separated by only 1.2 kJ mol(-1) and a maximum barrier of 12.5 kJ mol(-1). The (29)Si NMR chemical shifts (delta(iso)) range from -145.2 to -152.2 ppm and correlate with the electron density at the Si atoms, in other words with the +I and +M effects of the substituents. Again, compound 1 e is an exception and shows the highest shielding. The bonding situation at the Si atoms and the (29)Si NMR tensor components are analysed by quantum chemical methods at the density functional theory level. The natural bond orbital analysis indicates polar covalent Si-H bonds and very polar Si-Cl bonds, with the highest bond polarisation being observed for the Si-N interaction, which must be considered a donor-acceptor interaction. An analysis of the topological properties of the electron distribution (AIM) suggests a Lewis structure, thereby supporting this bonding situation.

Preparation, characterization and properties of dipolar 1,2-N,N-dimethylaminomethylferrocenylsilanes

Abstract A series of substituted 1,2-N,N-dimethylaminomethylferrocenyl compounds were synthesized and characterized by 1H-NMR, 13C-NMR, 29Si-NMR, ES–MS, IR, UV–vis and 57Fe-Mossbauer spectroscopy. The new (R,S)-2-(N,N-dimethylaminomethyl)ferrocenyl-(aryl)silanes (R,S)-FcNSiMen(C6H4X)m (n=2–0, m=1, X=p-F (5); m=2, X=p-F (6); m=3, X=p-F (7) and m=1, X=p-Br (14) were formed by the reaction of 2-dimethylaminomethylferrocenyllithium FcNLi (1) with chloroarylsilanes ClSi(Me)n(C6H4X)m (n=2–0, m=1, X=p-F (2); m=2, X=p-F (3); m=3, X=p-F (4) and m=1, X=p-Br (13)). The treatment of 5, 6 and 14 with gaseous hydrogen chloride or picric acid resulted in the formation of the hydrochloride complexes 9, 10, 15 and the picrates 11, 12 and 16. The treatment of 14 with LiR or Mg and DMF resulted in the formation of (R,S)-2-(N,N-dimethylaminomethyl)ferrocenyl(4-formylphenyl)dimethylsilane (18). The crystal structures of 7, 12 and 15 were determined by single crystal X-ray analyses. 57Fe-Mossbauer spectroscopy gives evidence of a significant electronic coupling between the ferrocenyl unit and the organic acceptor moiety of the molecules in the ground state.

Preparation of oligosilanes containing perhalogenated silyl groups and their hydrogenation by stannanes

Abstract Starting from methylphenylsubstituted oligosilanes the disilanes SiX 3 -SiX i Me 3− i ( i = 0, 1, 2; X = Cl, Br), trisilanes SiX 2 (SiX i Me 3− i ) ( i = 0, 1) and branched tetrasilanes SiX(SiXMe 2 ) 3 were synthesized and their behavior towards the Lewis-base catalyzed hydrogenation by stannanes was investigated. In the case of methylchlorodisilanes SiCl 3 -SiCl i Me 3− i Si-Si bond cleavage competes with the hydrogenation reaction.

Mechanism of the silicide-catalysed hydrodehalogenation of silicon tetrachloride to trichlorosilane

A mechanism of transition metal silicide-catalysed hydrodehalogenation of silicon tetrachloride to trichlorosilane is proposed. The overall reaction includes electron transfer steps from the metal silicide catalyst to adsorbed silicon tetrachloride molecules. A silylene species (SiCl2), which is formed from SiCl4 and the catalyst surface, is proposed. A hydrogen molecule injects electrons into the solid, whereby hydrogen chloride is generated on the surface. Trichlorosilane results from the oxidative addition of HCl to chemisorbed SiCl2.

Formation and characterization of cyclic and polycyclic silthianes containing Si Si bonds

Abstract The reactions of several organochlorosilanes and -oligosilanes with H2S and NEt3 have been investigated. Different bicyclic silthianes with bis-cyclopentyl, bicyclo-[3,3,0]-octane, bicyclo-[2,2,1]-heptane (norbornane), bicyclo-[3,2,1]-octane, bicyclo-[2,2,2]-octane and bicyclo-[3,2,1]-nonane skeletons were formed and have been characterized by MS and 1H-, 13C- and 29Si-NMR. The reaction of 1,1,2,2-tetrachlorodimethyldisilane with H2S and NEt3 yields 1,3,5,7,9,11-hexamethyl-1,3,5,7,9,11-hexasila-2,4,6,8,10,12-hexathiatetracyclo-[5,5,03,11,05,9]-dodecane (4c) containing three disilane units. Density functional theory calculations proved the general observation that compounds with Si3S2 five-membered rings are preferred. The crystal structures of 4c, 1,3,3,5,7,7-hexamethyl-1,3,5,7-tetrasila-2,4,6,8-tetrathiabicyclo-[3,3,0]-octane (6) and 1,2,2,4,4,5,6,6,8,8-decamethyl-1,2,4,5,6,8-hexasila-3,7-dithiabicyclo-[3,3,0]-octane (9) have been determined.

Hexacoordinate Silicon-Azomethine Complexes: Synthesis, Characterization, and Properties

N,N′-Ethylene-bis(2-hydroxyacetophenoneimine) = salen*H2 and N,N′-ethylene-bis(3,5-di-tert-butyl-salicylideneimine) = salen′H2 react both with SiCl4 under formation of hexacoordinate silicon compounds ((salen′)SiCl2′ salen′ = salen* or salen‡). The analogous fluoro derivatives (salen′)SiF2 have been prepared by reaction of (salen′)SiCl2 with ZnF2. X-ray structure analysis of (salen*)SiF2 clearly demonstrates the octahedral coordination of silicon. Salen* acts as a tetradentate chelating ligand, two halogen atoms remaining at the silicon atom. The reduction of (salen*)SiCl2 by alkaline metal affords polysilanes containing main chain hexacoordinate silicon. Coupling with acetylides results in polycarbosilanes with a Si-C≡C-Si backbone.

Interactions of chloromethyldisilanes with tetrakis(dimethylamino)ethylene (TDAE), formation of [TDAE]+ [Si3Me2Cl7]−

Abstract The chlorodisilanes SiClMe2SiClMe2 (1), SiCl2MeSiCl2Me (2), SiCl3SiCl3 (3) and a 9:1 mixture of 2 and SiCl3SiCl2Me (4) were reacted with the electron-rich alkene tetrakis-(dimethylamino)-ethylene (TDAE) in n-hexane as well as in polar solvents. While 1 gave no reaction at all, 3 underwent a disproportionation reaction into SiCl4 and Si(SiCl3)4. Also 2 and mixtures of 2 and 4 were disproportionated into MeSiCl3 (2a) and methylchlorooligosilanes. Additionally a crystalline mixture of Si3Me3Cl6·TDAE (5a) plus Si3Me2Cl7·TDAE (5b) was obtained by reaction of a 9:1 mixture of 2 and 4 with TDAE in n-hexane as well as in 1,2-dimethoxyethane. The reaction of 2 with TDAE in acetonitrile (MeCN) led to a crystalline precipitation of [TDAE]Cl2·MeCN (6·MeCN) in addition to MeSiCl3 and methylchlorooligosilanes. The structures of 5b and 6·MeCN were determined by X-ray crystallography beside their NMR and IR spectroscopic characterization. Compound 5b crystallizes in the monoclinic space group P2/c (Z=4), 6·MeCN in the orthorhombic space group Pna21 (Z=4). The structure of 5b reveals a [TDAE] + radical cation and a 1,2-Me2Si3Cl7− anion with a pentacoordinated central silicon atom.