Silyl Species Research Articles

Homogeneous catalyst systems with organosilicon components for olefin metathesis

Two silicon-containing catalyst systems for the olefin metathesis reaction are described. The first system, WCl 6/Ph 2SiH 2, is active for all kinds of olefinic substrates: acyclic, cyclic and functionalized olefins, and alkadienes. The second system, W(CO) 6/Ph 2SiH 2, is active for the metathesis of terminal olefins, but only under UV irradiation. Upon reaction between the catalyst components, a tungsten–silylene intermediate is formed as the precursor of the active species for metathesis. The product of a [2+2] cycloaddition of the tungsten–silylene complex was isolated. It showed catalytic activity for the metathesis of 1-hexene under UV irradiation, or at elevated temperature, when tungsten–silylene species could be generated from [W(CO) 4] 2[μ 2-Si(C 6H 5) 2] 2. This is the first example of olefin metathesis with metal–silylene activation.

The photochemical interaction of polyphenylferrisiloxane with oligoorganosilanes

The photochemical interaction of polyphenylferrisiloxane with some oligoorganosilanes in benzene solutions has been studied by UV, IR, ESR, and Mossbauer spectroscopy. Silylene species formed during photolysis of oligoorganosilanes react with the high-spin iron(III) to reduce the latter to low-spin iron(II). The formation of the low-spin state of iron(II) was supported by measurements of the magnetic moments. The insertion of the silylene species into the polymer chain to form the Fe−Si bond occurs due to the photoreaction.

Synthesis and Reactivity of d0Bis(imido) Silyl and Germyl Complexes of Molybdenum and Tungsten

The syntheses and reactivities of d0 bis(imido) molybdenum/tungsten silyl chloride complexes (2,6-iPr2C6H3N)2M[Si(SiMe3)3]Cl (1, M = Mo; 2, M = W) and the corresponding germyl complexes (2,6-iPr2C6H3N)2M[Ge(SiMe3)3]Cl (3, M = Mo; 4, M = W) are described. The complex (2,6-iPr2C6H3N)2Mo[Si(SiMe3)3]Cl (1), prepared by the reaction of (2,6-iPr2C6H3N)2MoCl2(dme) with (THF)3LiSi(SiMe3)3, has been structurally characterized. In general, these complexes are rather stable and do not react with CO, H2, or CH3CN. Complex 1 reacts with 2,6-Me2C6H3NC to provide the insertion product (2,6-iPr2C6H3N)2Mo[η2-C(N-2,6-Me2C6H3)Si(SiMe3)3](Cl) (5) and with AgOTf to give the silyl triflate complex (2,6-iPr2C6H3N)2Mo[Si(SiMe3)3]OSO2CF3 (6) in high yield. Complexes 1−6 react with neopentylmagnesium chloride to produce the silyl neopentyl complexes (2,6-iPr2C6H3N)2M[E(SiMe3)3](CH2CMe3) (7, M = Mo, E = Si; 8, M = W, E = Si; 9, M = Mo, E = Ge; 10, M = W, E = Ge). Complex 7, which was characterized by X-ray crystallography, contains an agostic interaction involving the α hydrogen of the neopentyl ligand (d(Mo−H) 2.55(4)Å). The neopentyl complexes 7−10 readily react with hydrogen (1 atm) to generate free neopentane and HSiMe3, probably via hydrogenation of the Mo−C bond to generate a highly unstable silyl hydride intermediate. The mechanism of HSiMe3 formation is unknown but may involve decomposition of the silyl hydride species via a four-membered transition state to generate a highly reactive silylene species. The corresponding tungsten analog 8 undergoes a similar reaction, but at a much slower rate. Attempts to trap the possible silylene intermediates (2,6-iPr2C6H3N)2MSi(SiMe3)2 (M = Mo, W) were unsuccessful.

Hydrocarbon-Bridged Methyldimethoxysilanes as new Co-condensation Agents for the Sol−Gel Process of the Rhodium(I) Complex ClRh(CO)(P∼O)2Containing the Ligand PhP(CH2CH2OCH3)(CH2)3Si(OCH3)31|Pv3

New hydrocarbon bridged co-condensation agents of the type MeSi(OMe)2(CH2)z(MeO)2SiMe [1(D0−Cz−D0), z = length of the flexible bridging group (z = 6, 8, 14)] were synthesized and polycondensed to give the polysiloxanes 1(Di−Cz−Di). These monomers were employed in variable amounts y in the sol−gel processing of the monomeric ether−phosphine ligand PhP(CH2CH2OMe)(CH2)3Si(OMe)3 [2(T0)] and its P-coordinated trimethoxysilyl-(T)-functionalized rhodium(I) complex ClRh(CO)(P∼O)2 [3(T0)2] to yield the polysiloxane-bound ether−phosphine ligands 2(Tn)(Di−Cz−Di) and the rhodium complexes 3(Tn)2(Di−Cz−Di)y [O∼P: η1-P-coordinated ether−phosphine ligand; y = number of co-condensed D−Cz−D molecules; D = D-type silicon atom (two oxygen neighbors), T = T-type silicon atom (three oxygen neighbors); i, n = number of Si−O−Si bonds; i = 0−2; n = 0−3]. The relative amounts of T and D silyl species and the degree of condensation were determined by 29Si CP/MAS NMR spectroscopic investigations. The polysiloxanes 1(Di−Cz−Di) whic...

Comparison of H-H versus Si-H sigma-Bond Coordination and Activation on 16e Metal Fragments. Organosilane, N(2), and Ethylene Addition to the Agostic Complex W(CO)(3)(PR(3))(2) and Dynamic NMR Behavior of the Latter.

Variable-temperature (31)P{(1)H} NMR spectroscopy of the agostic complexes M(CO)(3)(PCy(3))(2) (M = Mo, W) indicates dynamic behavior as evidenced by collapse below -20 degrees C of a singlet to an AB signal plus a shifted singlet. The inequivalency of the phosphines is possibly due to the presence of conformational isomers resulting from hindered rotation of the M-P bond or, less likely, a geometric isomer with pseudo-cis PCy(3) ligands. Further studies on the coordination chemistry of W(CO)(3)(PR(3))(2) (R = iPr, Cy) were performed. The bridging dinitrogen complex [W(CO)(3)(PiPr(3))(2)](2)(&mgr;-N(2)) (1) was cleanly formed in the reaction of W(CO)(3)(PiPr(3))(2) with N(2). Complex 1 was structurally characterized and compared with other bridging dinitrogen compounds of tungsten. The ethylene complex W(CO)(3)(PCy(3))(2)(eta(2)-C(2)H(4)) (2) was synthesized and characterized by X-ray crystallography in order to compare the binding mode of ethylene with that of H(2). Phenylsilane reacted with W(CO)(3)(PR(3))(2) (R = iPr, Cy) to form the thermally unstable oxidative addition (OA) products WH(SiH(2)Ph)(CO)(3)(PR(3))(2) (3, R = Cy; 4, R = iPr). Diphenylsilane reacted with W(CO)(3)(PiPr(3))(2) at 60 degrees C to form the bridging silyl species [W(CO)(3)(PiPr(3))(&mgr;-SiHPh(2))](2) (5), which was confirmed by spectroscopic techniques and X-ray crystallography to have two 3-center 2-electron W.H.Si interactions. Detailed comparisons of the binding and activation of silanes versus H(2) on various 16e metal centers suggest a high degree of similarity, but relative ease of OA depends on the electrophilicity of the metal-ligand fragment and other factors such as bond energetics. Increasing the electrophilicity of the metal center (e.g., adding positive charge) may aid in stabilizing alkane coordination.

Conversion of a Diruthenium μ-Methylene Complex, Cp2Ru2(μ-CH2)(μ-CO)(CO)2, into Methane through Reduction with Hydrosilane: Reactivity of Silyl−μ-Methylene Intermediates Cp2Ru2(μ-CH2)(SiR3)(X)(CO)2(X = H, SiR3) Relevant to Catalytic CO Hydrogenation and Activation of Si−H, C−H, and C−Si Bonds

Thermolysis (170 °C, 3 days) of a diruthenium μ-methylene complex, Cp2Ru2(μ-CH2)(μ-CO)(CO)2 (1), in the presence of HSiMe3 produces methane along with methylsilane (SiMe4) and mononuclear organometallic products, CpRu(H)(SiR3)2(CO) (2) and CpRu(CO)2(SiMe3) (3). The reaction mechanism involving initial CO dissociation has been investigated by using a labile μ-methylene species, Cp2Ru2(μ-CH2)(μ-CO)(CO)(MeCN) (4), the MeCN adduct of the coordinatively unsaturated species resulting from decarbonylation of 1. Treatment of 4 with HSiR3 produces the hydrido−silyl−μ-methylene intermediate Cp2Ru2(μ-CH2)(H)(SiR3)(CO)2 (5) and the disilyl−μ-methylene complex Cp2Ru2(μ-CH2)(SiR3)2(CO)2 (6) successively. Further reaction of 5 and 6 with HSiR3 affords methane under milder conditions (120 °C, 12 h) compared to the methane formation from 1. Meanwhile complicated exchange processes are observed for the silylated μ-methylene species 5 and 6. The dynamic behavior of the hydrido−silyl species 5 giving a 1H-NMR spectrum consistent with an apparent Cs structure at ambient temperature has been analyzed in terms of a mechanism involving intramolecular H- and R3Si-group migration between the two ruthenium centers. It is also revealed that intramolecular exchange reaction of the hydride and μ-CH2 atoms in 5 proceeds via the coordinatively unsaturated methyl intermediate Cp2Ru2(CH3)(SiR3)(CO)2 (9). In addition to these intramolecular processes, the hydride, μ-CH2, and SiR3 groups in 5 and 6 exchange with external HSiR3 via replacement of the η2-bonded H2 or HSiR3 ligand in μ-methylene or μ-silylmethylene intermediates Cp2Ru2(μ-CHX)(μ-CO)(CO)(η2-H-Y) [X, Y = H, SiR3 (7), SiR3, H (18), SiR3, SiR3 (16)] as confirmed by trapping experiments of 7 with L (CO, PPh3) giving Cp2Ru2(μ-CHX)(μ-CO)(CO)(L) [X, L = H, CO (1), H, PPh3 (11), SiR3, CO (12), SiR3, PPh3 (13)]. Hydrostannanes (HSnR3) also react with 4, in a manner similar to the reaction with HSiR3, to give the hydrido−stannyl−μ-methylene intermediate Cp2Ru2(μ-CH2)(H)(SnR3)(CO)2 (20) and the distannyl−μ-methylene complex Cp2Ru2(μ-CH2)(SnR3)2(CO)2 (21) successively (the stannyl analogues of 5 and 6, respectively). The intramolecular exchange processes (H ↔ SnR3, H ↔ μ-CH2) are also observed for 20. But the HSnPh3 adduct 20c is further converted to a mixture containing the μ-η1:η2-phenyl complex Cp2Ru2(μ-Ph)(SnCH3Ph2)(CO)2 (22) and the bis(μ-stannylene) complex Cp2Ru2(μ-SnPh2)2(CO)2 (23) instead of 21c. The isolation of 22 supports viability of the methyl species (9). These results suggest that methane formation from 1 follows (i) CO dissociation, (ii) H−SiR3 oxidative addition giving the hydrido−silyl−μ-methylene intermediate 5, (iii) equilibrium with the methyl intermediate 9, (iv) a second oxidative addition of H−SiR3, and (v) elimination of methane repeating reductive elimination from mono- and dinuclear hydrido−methyl intermediates 27 and 28. The present reaction sequence can be viewed as a model system for methanation via the Fischer−Tropsch mechanism where hydrosilane behaves as a H2 equivalent (pseudo-hydrogen). The molecular structures of 6d,e, 12a, 13a, 21a, and 22 have been determined by X-ray crystallography.

The differing modes of reaction of 1-(8-dimethylamino-1-naphthyl)-1-hydrodisilane and 1-(1-naphthyl)-1-hydrodisilane in nickel-catalyzed reactions with acetylene: formation of a pseudo-pentacoordinate silole via SiSi bond cleavage vs. hydrosilation without SiSi bond cleavage

Pseudo-pentacoordinate 1-(8-dimethylamino-1-naphthyl)-1-hydrodisilanes 1 and 2 and the tetracoordinate counterpart 3 have been prepared. In the presence of an Ni(0) complex as catalyst, 1 and 2 undergo degradation to generate a silylene species and a hydrosilane. The complex with Ni of the silylene species from 1 has been trapped with an excess of diphenylacetylene to give a pseudo-pentacoordinate silole 4. The X-ray structure of 4 indicates that this silole occupies two pseudo-equatorial positions. In contrast, the tetracoordinate hydrodisilane 3 undergoes hydrosilation without SiSi bond cleavage.

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.

Semi-Insulating Polycrystalline Silicon by Low Pressure Chemical Vapour Deposition from Disilane and Nitrous Oxide

Usually, semi-insulating polycrystalline silicon films SiOx are obtained from the chemical decomposition of silane and nitrous oxide. In this paper we describe the study of semi-insulating polycrystalline silicon (SIPOS) films from disilane and nitrous oxide by low pressure chemical vapour deposition. The kinetics of growth and the variation of oxygen content have been investigated relatively to deposition parameters such as temperature, total pressure and gas flow ratio. The oxygen content is assessed by X-ray photoelectron spectroscopy (XPS) and by differential thickness method. We show that the growth rate is higher for “disilane SIPOS films” than “silane SIPOS films” and we explain this behaviour by the strong difference in growth rate of silylene species. We also show that the inhibition effect of nitrous oxide on the growth rate is very low in the case of SIPOS films obtained with disilane. The electrical resistivity depends on the oxygen content and is ranged between 109 and 1013 Ω· cm.

Reaction Pathway of Formation of Methoxysilanes in the Reaction of Silicon with Methanol Catalyzed by Copper(I) Chloride

The reaction of silicon with methanol using copper(I) chloride as a catalyst gives HSi(OCH 3) 3 and Si(OCH 3) 4, the selectivity for HSi (OCH 3) 3 being dependent on the conditions of the treatment of the silicon-catalyst mixture as well as the reaction conditions. The selectivity for HSi(OCH 3) 3 was 64% when a silicon-copper(I) chloride mixture was preheated at 723 K and the reaction was carried out at 513 K. The addition of a small amount of thiophene to the feed increased the selectivity to almost 100%. A separate experiment showed that metallic copper catalyzes the reaction of HSi(OCH 3) 3 and methanol to give Si(OCH 3) 4, this reaction being completely inhibited by thiophene. It is concluded that HSi(OCH 3) 3 is a sole primary product and that Si(OCH 3) 4 is formed by the secondary reaction of HSi(OCH 3) 3 with methanol on metallic copper, which is formed on the silicon surface during the silicon-methanol reaction. Alkyl chlorides such as methyl and propyl chloride also were found to be effective additives for improving the selectivity. Copper(I) chloride serves as the catalyst for forming intermetallic Cu-Si species. The presence of copper(I) chloride in the reaction system also serves to enhance the selectivity for HSi(OCH 3) 3. It is suggested that the chloride ions are transferred to the surface of metallic copper formed in the course of the reaction to poison the catalytic activity for the HSi(OCH 3) 3-methanol reaction. The reaction mechanism involving surface silylene species is proposed for the exclusive formation of HSi(OCH 3) 3.

ChemInform Abstract: Alkyl‐, Silyl‐, and Germyl‐Substituted Thiolate, Selenolate, and Tellurolate Derivatives and Interconversion of Silyl Species by Chalcogen Metathesis.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Closely Approaching the Silylium Ion (R 3 Si + )

The crystal structure of the tri-isopropyl silyl species, i-Pr(3)Si(Br(6)-CB(11)H(6)), where the brominated carborane Br(6)-CB(11) H(6)(-) is perhaps the least nucleophilic anion presently known, has revealed the highest degree of silylium cation character (R(3)Si(+)) yet observed. The average C-Si-C angle is 117 degrees , only 3 degrees short of the planarity expected of a pure silylium ion(120 degrees ). This value compares to 114 degrees recently reported for a toluene-solvated silyl cation, [Et(3)Si(toluene)](+) by Lambert and co-workers. The greater silylium ion character of i-Pr(3)Si(Br(6)-CB(11)H(6)) versus [i-Pr(3)Si(toluene)](+) is also reflected in the larger downfield shift of the silicon-29 nuclear magnetic resonance, 109.8 versus </=94.0 parts per million. The unusual bonding of toluene to R(3)Si(+) in Lambert's compound is reinterpreted as a significant covalent pi interaction.

ChemInform Abstract: Synthesis, Structure, and Reactivity of Trisilanes Containing the 8‐( Dimethylamino)‐1‐naphthyl Group at the Central Silicon Atom. A Novel Nickel‐ or Palladium‐Catalyzed Degradation of Trisilane to Disilane and Silylene Species.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Alkyl-, silyl-, and germyl-substituted thiolate, selenolate, and tellurolate derivatives and interconversion of silyl species by chalcogen metathesis

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTAlkyl-, silyl-, and germyl-substituted thiolate, selenolate, and tellurolate derivatives and interconversion of silyl species by chalcogen metathesisPhilip J. Bonasia, Victor Christou, and John ArnoldCite this: J. Am. Chem. Soc. 1993, 115, 15, 6777–6781Publication Date (Print):July 1, 1993Publication History Published online1 May 2002Published inissue 1 July 1993https://doi.org/10.1021/ja00068a039RIGHTS & PERMISSIONSArticle Views354Altmetric-Citations52LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (666 KB) Get e-AlertsSupporting Info (2)»Supporting Information Supporting Information Get e-Alerts

Synthesis, structure, and reactivity of trisilanes containing the 8-(dimethylamino)-1-naphthyl group at the central silicon atom. A novel nickel- or palladium-catalyzed degradation of trisilane to disilane and silylene species

Synthesis, structure, and reactivity of trisilanes containing the 8-(dimethylamino)-1-naphthyl group at the central silicon atom. A novel nickel- or palladium-catalyzed degradation of trisilane to disilane and silylene species

Si atomic layer epitaxy based on Si 2H 6 and remote He plasma bombardment

Atomic layer epitaxy (ALE) of Si has been demonstrated by using remote He plasma low energy ion bombardment to desorb H from an H-passivated Si(100) surface at low temperaturea and subsequently chemisorbing Si 2H 6 on the surface in a self-limiting fashion. Si substrates were prepared using an RCA clean followed by a dilute HF dip to provide a clean, dihydride-terminated (1 × 1) surface, and were loaded into a remote plasma chemical vapor deposition system in which the substrate is downstream from an r.f. noble gas (He or Ar) glow discharge in order to minimize plasma damage. An in situ remote H plasma clean at 250°C for 45 min was used to remove surface O and C and to provide an alternating monohydride and dihydride termination, as evidenced by a (3 × 1) reflection high energy electron diffraction (RHEED) pattern. It was found necessary to desorb the H from the Si surface to create adsorption sites for Si- bearing species such as Si 2H 6. Remote He plasma bombardment for 1–3 min was investigated over a range of temperature (250°C−410°C), pressures (50–400 mTorr) and r.f. powers (6–30 W) in order to desorb the H and to convert the (3 × 1) RHEED pattern to a (2 × 1) pattern which is characteristic of either a monohydride termination or a bare Si surface. It was found that as He pressures and r.f. powers are raised the plasma potential and mean free paths are reduced, leading to lower He bombardment energies but higher fluxes. Optimal He bombardment parameters were determined to be 30 W at 100 mTorr process pressure at 400°C for 1–3 min. He was found to be more effective than Ar bombardment because of the closer match of the He and H masses compared with that between Ar and H. Monte Carlo TRIM simulations of He and Ar bombardment of H-terminated Si surfaces were performed 3o validate this hypothesis and to predict that approximately 3 surface H atoms were displaced by the incident He atoms, with no bulk Si atom displacement for He energies in the range 15–60 eV. The He bombardment cycles were followed by Si 2H 6 dosing over a range of partial pressures (from 10 −7 Torr to 1.67 mTorr), temperatures (250°C–400°C) and times (from 20s to 3 min) without plasma excitation, because it is believed that Si 2H 6 can chemisorb in a self-limiting fashion on a bare Si surface as two silyl (SiH 3) species, presumably leading to a H-terminated surface once again. The Si 2H 6 dosing pressures and times corresponded to saturation dosing (about 10 6 langmuirs). Alternate Si 2H 6 dosing and He low energy ion bombardment cycles (about 100–200) were performed to confirm the ALE mode of growth. It was found that the growth per cycle saturates with long Si 2H 6 dosing at a level which increases slightly with He bombardment time. At 400°C, for 2 min He bombardment at 100 mTorr and 30 W, the growth per cycle saturates at about 0.1 monolayers cycle −1, while for 3 min He bombardment the Si growth saturates at about 0.15 monolayers cycle −1. It was also confirmed that the growth is achieved only by using alternate He bombardment and Si 2H 6 dosing. He bombardment alone for a comparable time (3 min × 100 cycles) causes a negligible change in the Si film thickness (less than 5 Å). Similarly, thermal growth using Si 2H 6 under these conditions for (3 min × 100 cycles) causes negligible deposition (less than 5 Å).

γ-Ray- and UV-induced generation of methyl(phenyl)silylene and silene species from heptamethyl-2-phenyltrisilane

A benzene solution of heptamethyl-2-phenyltrisilane (1) has been irradiated with γ-rays in the presence of methanol. Hexamethyldisilane, methoxymethyl(phenyl)silane (2) and a 1-methoxytetramethyldisilanyl- and trimethylsilyl-substituted cyclohexadiene (3) were produced. The formation of 2 and 3 can be interpreted in terms of reactions of methanol with methyl(phenyl)silylene and a silicon–carbon double-bonded intermediate (silene), respectively. These intermediates are generated as follows. Benzene is first excited by γ-rays and by collisions the energy transfers to 1. Excited 1 decomposes to give methyl(phenyl)silylene or undergoes rearrangement to a silene intermediate as when 1 is excited by UV light. A kinetic study revealed that the energy transfer in the γ-irradiated system is the same as that in a UV-irradiated system.

Rate constants for silylene reactions

It is only since 1985 that the absolute rate constanss have been measured for some reactions of divalent silylene species. In this article the absolute rate constant data reported to date for the reactions of SiH2, SiMe2, SiMePh, SiHCl, SiCl2, SiF2 and SiBr2 are reviewed and, where possible, mechanistic pathways discussed. The reactivity of silylenes is, in general, much higher than had previously been estimated on the basis of relative rate studies.

An impulsive collision model for the vibrational and rotational excitation of sputtered molecules: R De Jonge et al,Nucl Instrum Meth Phys Res,B30, 1988, 159–172

Atomic layer epitaxy (ALE) of Si has been demonstrated by using remote He plasma low energy ion bombardment to desorb H from an H-passivated Si(100) surface at low temperaturea and subsequently chemisorbing Si2H6 on the surface in a self-limiting fashion. Si substrates were prepared using an RCA clean followed by a dilute HF dip to provide a clean, dihydride-terminated (1 × 1) surface, and were loaded into a remote plasma chemical vapor deposition system in which the substrate is downstream from an r.f. noble gas (He or Ar) glow discharge in order to minimize plasma damage. An in situ remote H plasma clean at 250°C for 45 min was used to remove surface O and C and to provide an alternating monohydride and dihydride termination, as evidenced by a (3 × 1) reflection high energy electron diffraction (RHEED) pattern. It was found necessary to desorb the H from the Si surface to create adsorption sites for Si- bearing species such as Si2H6. Remote He plasma bombardment for 1–3 min was investigated over a range of temperature (250°C−410°C), pressures (50–400 mTorr) and r.f. powers (6–30 W) in order to desorb the H and to convert the (3 × 1) RHEED pattern to a (2 × 1) pattern which is characteristic of either a monohydride termination or a bare Si surface. It was found that as He pressures and r.f. powers are raised the plasma potential and mean free paths are reduced, leading to lower He bombardment energies but higher fluxes. Optimal He bombardment parameters were determined to be 30 W at 100 mTorr process pressure at 400°C for 1–3 min. He was found to be more effective than Ar bombardment because of the closer match of the He and H masses compared with that between Ar and H. Monte Carlo TRIM simulations of He and Ar bombardment of H-terminated Si surfaces were performed 3o validate this hypothesis and to predict that approximately 3 surface H atoms were displaced by the incident He atoms, with no bulk Si atom displacement for He energies in the range 15–60 eV. The He bombardment cycles were followed by Si2H6 dosing over a range of partial pressures (from 10−7 Torr to 1.67 mTorr), temperatures (250°C–400°C) and times (from 20s to 3 min) without plasma excitation, because it is believed that Si2H6 can chemisorb in a self-limiting fashion on a bare Si surface as two silyl (SiH3) species, presumably leading to a H-terminated surface once again. The Si2H6 dosing pressures and times corresponded to saturation dosing (about 106 langmuirs). Alternate Si2H6 dosing and He low energy ion bombardment cycles (about 100–200) were performed to confirm the ALE mode of growth. It was found that the growth per cycle saturates with long Si2H6 dosing at a level which increases slightly with He bombardment time. At 400°C, for 2 min He bombardment at 100 mTorr and 30 W, the growth per cycle saturates at about 0.1 monolayers cycle−1, while for 3 min He bombardment the Si growth saturates at about 0.15 monolayers cycle−1. It was also confirmed that the growth is achieved only by using alternate He bombardment and Si2H6 dosing. He bombardment alone for a comparable time (3 min × 100 cycles) causes a negligible change in the Si film thickness (less than 5 Å). Similarly, thermal growth using Si2H6 under these conditions for (3 min × 100 cycles) causes negligible deposition (less than 5 Å).

Photolysis of organopolysilanes. Insertion of silylene species into a conjugated diene

Photolysis of 2-phenylheptamethyltrisilane in the presence of 2,3-dimethylbutadiene afforded several compounds; their formation can be explained by two competitive pathways, the intermediacy of a 2-alkenyl-1-silacyclopropane derivative and a novel addition of the silicon—silicon bond to the conjugated diene.