Linear Silanes Research Articles

An Electrochemical Strategy to Synthesize Disilanes and Oligosilanes from Chlorosilanes.

Silanes are important compounds in industrial and synthetic chemistry. Here, we develop a general approach for the synthesis of disilanes as well as linear and cyclic oligosilanes via the reductive activation of readily available chlorosilanes. The efficient and selective generation of silyl anion intermediates, which are arduous to achieve by other means, allows for the synthesis of various novel oligosilanes by heterocoupling. In particular, this work presents a modular synthesis for a variety of functionalized cyclosilanes, which may give rise to materials with distinct properties from linear silanes but remain challenging synthetic targets. In comparison to the traditional Wurtz coupling, our method features milder conditions and improved chemoselectivity, broadening the functional groups that are compatible in oligosilane preparation. Computational studies support a mechanism whereby differential activation of sterically and electronically distinct chlorosilanes are achieved in an electrochemically driven radical-polar crossover mechanism.

An Electrochemical Strategy to Synthesize Disilanes and Oligosilanes from Chlorosilanes**

AbstractSilanes are important compounds in industrial and synthetic chemistry. Here, we develop a general approach for the synthesis of disilanes as well as linear and cyclic oligosilanes via the reductive activation of readily available chlorosilanes. The efficient and selective generation of silyl anion intermediates, which are arduous to achieve by other means, allows for the synthesis of various novel oligosilanes by heterocoupling. In particular, this work presents a modular synthesis for a variety of functionalized cyclosilanes, which may give rise to materials with distinct properties from linear silanes but remain challenging synthetic targets. In comparison to the traditional Wurtz coupling, our method features milder conditions and improved chemoselectivity, broadening the functional groups that are compatible in oligosilane preparation. Computational studies support a mechanism whereby differential activation of sterically and electronically distinct chlorosilanes are achieved in an electrochemically driven radical‐polar crossover mechanism.

Catalytic activation of remote alkenes through silyl-rhodium(III) complexes.

The tandem isomerization-hydrosilylation reaction is a highly valuable process able to transform mixtures of internal olefins into linear silanes. Unsaturated and cationic hydrido-silyl-Rh(III) complexes have proven to be effective catalysts for this reaction. Herein, three silicon-based bidentate ligands, 8-(dimethylsilyl)quinoline (L1), 8-(dimethylsilyl)-2-methylquinoline (L2) and 4-(dimethylsilyl)-9-phenylacridine (L3), have been used to synthesize three neutral [RhCl(H)(L)PPh3] (1-L1, 1-L2 and 1-L3) and three cationic [Rh(H)(L)(PPh3)2][BArF4] (2-L1, 2-L2 and 2-L3) Rh(III) complexes. Among the neutral compounds, 1-L2 could be characterized in the solid state by X-ray diffraction showing a distorted trigonal bipyramidal structure. Neutral complexes (1-L1, 1-L2 and 1-L3) failed to catalyze the hydrosilylation of olefins. On the other hand, the cationic compound 2-L2 was also characterized by X-ray diffraction showing a square pyramidal structure. The unsaturated and cationic Rh(III) complexes 2-L1, 2-L2 and 2-L3 showed significant catalytic activity in the hydrosilylation of remote alkenes, with the most sterically hindered (2-L2) being the most active one.

Steric Effects in the Catalytic Tandem Isomerization‐Hydrosilylation Reaction

AbstractThe selective synthesis of linear silanes from remote alkenes is reported. Four new silane‐thioether bidentate proligands [SiMe2H(o‐C6H4SR)] (R=iBu, pentyl, benzyl, neopentyl) have been synthesized and used to form unsaturated and cationic 16‐electron hydrido‐silyl‐RhIII complexes. These compounds are efficient catalysts for the tandem catalytic alkene isomerization‐hydrosilylation reaction at room temperature under solvent‐free conditions. The different size of the substituent on the sulfur atom results on a difference in the activity of this tandem reaction. Experimental observations demonstrate that the isomerization process is the rate‐determining step of this catalytic transformation. This process would be of value to the chemical industry because mixtures of internal aliphatic olefins are substantially cheaper and more readily available than the pure terminal isomers.

Large Variations in the Single-Molecule Conductance of Cyclic and Bicyclic Silanes.

Linear silanes are efficient molecular wires due to strong σ-conjugation in the transoid conformation; however, the structure-function relationship for the conformational dependence of the single-molecule conductance of silanes remains untested. Here we report the syntheses, electrical measurements, and theoretical characterization of four series of functionalized cyclic and bicyclic silanes including a cyclotetrasilane, a cyclopentasilane, a bicyclo[2.2.1]heptasilane, and a bicyclo[2.2.2]octasilane, which are all extended by linear silicon linkers of varying length. We find an unusual variation of the single-molecule conductance among the four series at each linker length. We determine the relative conductance of the (bi)cyclic silicon structures by using the common length dependence of the four series rather than comparing the conductance at a single length. In contrast with the cyclic π-conjugated molecules, the conductance of σ-conjugated (bi)cyclic silanes is dominated by a single path through the molecule and is controlled by the dihedral angles along this path. This strong sensitivity to molecular conformation dictates the single-molecule conductance of σ-conjugated silanes and allows for systematic control of the conductance through molecular design.

From Remote Alkenes to Linear Silanes or Allylsilanes depending on the Metal Center

AbstractThe selective synthesis of linear silanes or allylsilanes from remote alkenes was explored. The cationic, unsaturated, 16‐electron hydrido–silyl–RhIII complex was found to be an efficient catalyst for a tandem catalytic alkene isomerization–hydrosilylation reaction at room temperature under solvent‐free conditions. The analogous hydrido–silyl–IrIII complex was selective for dehydrogenative silylation, which led to the formation of terminal allylsilanes.

Rhodium(III) Catalyzed Solvent‐Free Tandem Isomerization–Hydrosilylation From Internal Alkenes to Linear Silanes

AbstractThe selective synthesis of linear silanes from internal alkenes or alkene mixtures is reported. Unsaturated 16 electrons hydrido–silyl–RhIII complexes are efficient catalysts for a tandem catalytic alkene isomerization–hydrosilylation reaction at room temperature under solvent‐free conditions. Such a process would be of value to the chemical industry, as mixtures of internal aliphatic olefins are substantially cheaper and more readily available than the pure terminal isomers.

Control of Selectivity through Synergy between Catalysts, Silanes and Reaction Conditions in Cobalt-Catalyzed Hydrosilylation of Dienes and Terminal Alkenes.

Readily accessible ( i-PrPDI)CoCl2 [ i-Pr PDI = 2,6-bis(2,6-diisopropylphenyliminoethyl)pyridine] reacts with 2 equivalents of NaEt3BH at -78 °C in toluene to generate a catalyst that effects highly selective anti-Markovnikov hydrosilylation of the terminal double bond in 1,3- and 1,4-dienes. Primary and secondary silanes such as PhSiH3, Ph2SiH2 and PhSi(Me)H2 react with a broad spectrum of terminal dienes without affecting the configuration of the other double bond. When dienes conjugated to an aromatic ring are involved, both Markovnikov and anti-Markovnikov products are formed. The reaction is tolerant of various functional groups such as an aryl bromide, aryl iodide, protected alcohol, and even a silyl enol ether. Reactions of 1-alkene under similar conditions cleanly lead to a mixture of Markovnikov and anti-Markovnikov hydrosilation products, where ratio of the products increasingly favors the latter, as the size of the 2,6-substituents in the iminoylaryl group becomes larger. The complex ( i-PrPDI)CoCl2 gives exclusively the linear silane for a wide variety of terminal alkenes. Mechanistic studies suggest a pathway that involves a key role for an in situ generated metal hydride, (L)Co(I)-H. Exclusive reduction of the terminal double bond (vis-a-vis hydrosilylation) when (EtO)2Si(Me)H is used in the place of PhSiH3 is explained on the basis of an alternate silane-mediated decomposition path for the linear Co(I)-alkyl intermediate.

Aspects of Interfacial Structure of Silane Coupling Agents in Particulate-Filled Polymer Composites and the Reinforcement Effect: A Critical Review

In order to form an effective interphase in a polymer composite, the preparation and characterization of silane structure on particle surface and in the interfacial layer in the composite are reviewed. The polycondensation of silane is affected by the concentration and pH of silane solution and the pH of inorganic particle. Pulsed 1H nuclear magnetic resonance (pulsed NMR) analysis is useful to characterize the structure of silane layer on the particle surface. It is found that the linear chain structure formed from dialkoxy type silane is flexible, whereas the network structure formed from trialkoxy type silane is rigid. To form an effective interphase, the linear chain structure formed from dialkoxy type is more useful than the network structure formed from trialkoxy type in the silica-filled rubber composites. The pre-treatment method and the integral blend method are compared in the silica-filled rubber composites. As a result, the dialkoxy type is found to be suitable for the pre-treatment method, whereas the trialkoxy type is suitable for the integral blend method. In the pre-treatment method, the linear silane chains form entanglements with the rubber chains in the interfacial region and improve the reinforcement effect. In the integral blend of trialkoxy type, the formation of the silane network and the entanglement proceed simultaneously during the preparation process and a well-entangled interfacial region is formed. Pulsed NMR analysis for the silica-filled unvulcanized rubber composites is useful to analyze the interfacial layer in the composites. The surface treatment of CaCO3 with a mixture of amino and mercapto functional silanes is found to be useful to improve the reinforcement effect in rubber/CaCO3 composites.

Tensile properties of styrene-butadiene rubber/silica composites with mercapto functional silane coupling agents: influences of loading method and alkoxy group number

The influences of alkoxy group number and loading method of silane coupling agents on the mechanical properties of a styrene-butadiene rubber/silica composite were investigated. Mercapto functional silane coupling agents with dialkoxy and trialkoxy structures were used. The pre-treatment method and the integral blend method were compared. Both the fracture stress and modulus at 200% strain were higher in the pre-treatment than in the integral blend for dialkoxy type composites. However, they were higher in the integral blend than in the pre-treatment for trialkoxy-type composites. The interaction between the silane chains on the silica surface and the rubber molecular chains at the interfacial region was estimated by 1H pulse nuclear magnetic resonance spectroscopy using an unvulcanized silica/rubber mixture. It was found that the binding of rubber molecular chains by the silane chains was higher in the pre-treatment system for dialkoxy-type composites, whereas it was higher in the integral blend for trialkoxy-type composites. The reason is proposed as follows: in the pre-treatment for dialkoxy type, a linear silane chain formed in the case of multi-layer coverage. The silane chain entangled with the rubber chain in the interfacial region and improved the reinforcement effect. For the trialkoxy type, a network structure formed using the pre-treatment method, lowering the amount of entanglement. However, in the integral blend for trialkoxy type, the formation of the silane network and the entanglement progressed simultaneously during the preparation process. A well-entangled interfacial region was formed.

Effect of chemical structure of silane coupling agent on interface adhesion properties of syndiotactic polypropylene/cellulose composite

AbstractTo improve interaction between syndiotactic polypropylene (SPP) and fibrous cellulose (FC), effects of chemical structure of silane coupling agent on the reactivity for the surface hydroxyl group on the FC were studied by X‐ray photoelectron spectroscopy (XPS) measurement. Among the three kinds of the silane coupling agent, the 3‐aminopropyltrimethoxysilane (APTMS) showed the highest reactivity with the surface hydroxyl group on the FC, and the linear silane compound with methoxyl group was found to be suitable for the reaction. Although the morphology of the SPP/FC composite is hardly affected by the difference in the kinds of the silane coupling agent, the tensile properties were considerably different. In particular, in the case of using higher silane coupling agent solution (over 3 wt %), the chemical structure of silane coupling agent certainly affected the tensile properties of the SPP/silanized FC composite. It was found that the tensile properties were distinctly affected by the reactivity between the surface hydroxyl group on FC and the silane coupling agent. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Coupled-cluster studies of the electronic excitation spectra of silanes

The electronic excitation spectra of unsubstituted linear silanes (n-Si(m)H(2m+2), m = 1-6), cyclopentasilane (c-Si5H10), and neopentasilane (neo-Si5H12) have been studied at the coupled-cluster approximate singles and doubles (CC2) level using Dunning's quadruple-zeta basis sets augmented with diffuse functions (aug-cc-pVQZ). Comparisons with measured ultraviolet spectra for Si2H6 and n-Si3H8 show that CC2 calculations using these basis sets yield excitation energies in good agreement with experiment. The calculated excitation thresholds for Si2H6 and n-Si3H8 of 7.61 and 6.68 eV are only 0.05 eV larger than the gas-phase values of 7.56 and 6.63 eV, respectively. For n-Si4H10, n-Si5H12, and neo-Si5H12, the calculated excitation thresholds of 6.51, 6.14, and 6.87 eV for the lowest dipole-allowed transitions are about 0.4 eV larger than the corresponding liquid-phase data of 6.05, 5.77, and 6.53 eV; the discrepancy can mainly be attributed to solvent effects. The obtained excitation thresholds for n-Si6H14 is 5.85 eV, whereas no experimental data are available for its optical gap. Calculations using the Karlsruhe triple-zeta valence basis sets augmented with single and double sets of polarization functions show that very large basis sets augmented with diffuse functions are needed for obtaining accurate excitation energies. The optical gaps for silanes obtained using the triple-zeta polarization basis sets were found to be 0.4 and 0.2 eV larger than those obtained using Dunning's quadruple-zeta basis sets. Excitation thresholds calculated at density functional theory levels using generalized gradient approximation are 0.7-1.0 eV smaller than the experimental values and by employing hybrid functionals they are 0.3-0.4 eV below the experimental thresholds. By adding the present basis-set correction and environmental effects to the previously calculated CC2 value for the excitation threshold of the Si29H36 silicon nanocluster, the extrapolated absorption threshold is 4.0 eV as compared to the recently reported experimental value of 3.7 eV.

Bright luminescence from silane substituted and bridged silicon nanoclusters

Time-dependent density functional theory (TDDFT) calculations show that silicon nanoclusters (Si-NC) capped by linear silane groups have large oscillator strengths of the same magnitude as reported in recent experimental studies. We propose a mechanism where linear silanes attached to the Si-NC surface affect the optical properties enhancing the oscillator strengths and thereby accounting for the bright luminescence observed in the blue region of the visible spectrum. The anisotropic emission seen experimentally can also be explained by the presence of the silane groups on the cluster surface. The excitation energies are found to be only slightly affected by the silanes, whereas the oscillator strengths increase with the length of the silane chain and are significantly larger than obtained for unsubstituted Si-NCs. In TDDFT studies of Si-NC dimers interconnected by a linear silane bridge, we obtained large oscillator strengths indicating that such structures could be useful light sources for optical devices.

Electrochemical oxidation of cyclic polysilanes

During the past decade, the electrochemical properties of cyclic polysilane derivatives, namely [Mes 2Si] 3 ( I), [ t-Bu(Me)Si] 4 ( IIa), [Et 2Si] 4 ( IIb), [( n-Pr) 2Si] 5 ( IIIa), [Et 2Si] 5 ( IIIb), [Me 2Si] 6 ( IVa), [Et 2Si] 6 ( IVb), [Et 2Si] 7 ( V), [Me 2Si] 8 ( VI), and [Me 2Si] 9 ( VII), have been explored in our laboratory. Various parameters have been investigated, such as anodic peak potentials, the effect of anode material, nature of supporting electrolyte, atmosphere under which electrolyses were conducted, extent of electricity consumption, solvent, ring size, applied potential, and more. It has been found that the type of products emerged from electrolysis was highly sensitive to the nature of electrolyte. Thus, in the presence of BF 4 − salts, cyclic polysilanes underwent ring opening followed by SiSi bond cleavage to afford linear silanes with F [one or two (major)], H or OH moieties at the terminal positions. In the presence of ClO 4 − salts, mostly cyclic siloxanes with different number of oxygen atoms were formed, in addition to linear ones, in some cases. The origin of oxygen atoms in the oxygen insertion process was postulated to stem from both ClO 4 − anions and molecular oxygen. Both mechanisms are discussed. In the presence of non-oxidizing electrolytes, such as HSO 4 − and acetate salts, mostly mono- and dihydroxy linear polysiloxanes and polysilanes, respectively, were generated. When CF 3COO − (TFA) salt was utilized under “oxygen-free” and “water-free” conditions, in the dry box, the reaction became quite selective since the major products were cyclic siloxanes containing only one oxygen atom each. The source of oxygen is assumed to originate from the TFA electrolyte, and a plausible mechanism is suggested. The net effect of ring size was studied with cyclic silanes bearing the same substituents ( IIb, IIIb, IVb, and V). Each derivative afforded a distinctive major five- or six-membered siloxane, containing either one or two oxygen atoms. In addition, whereas IIb and IIIb gave products with retention of the original number of silicon atoms, IVb and V gave also cyclic siloxanes with a loss of 1–3 Si atoms, due to fragmentation.

A molecular approach to self-assembly of trimethylsilylacetylene derivatives on gold.

We recently discovered that a linear multifunctional trimethylsilylacetylene (TMSA) compound forms long-range and highly stable self-assembled monolayers (SAMs) on reconstructed Au(111). To better understand the interactions governing self-assembly in this new system, we synthesized a series of homologue organosilanes and performed scanning tunneling microscopy (STM) measurements at the Au(111)/n-tetradecane interface. The four TMSA-terminated linear silanes that we synthesized self-assemble in similar ways on gold, with the molecules standing upright on the surface. In contrast, compounds with a slightly modified terminal group but the same polyunsaturated linear chain above the TMSA head do not self-assemble. In particular, substituting a methyl group of TMSA with a more bulky one prevents self-assembly. Removing the C triple bond C triple bond of TMSA or substituting the Si atom by a C atom also hinders self-assembly. Finally, substituting one methyl group of TMSA by a hydrogen atom induces self-assembly but in a different geometry, with the molecules lying flat on the gold surface in a quasi-epitaxy mode. Our molecular approach demonstrates the key role played by the TMSA head in self-assembly, its origin being twofold: 1) the TMSA layers are commensurate to the Au(111) adlattice along the <112> direction, and 2) the C triple bond C triple bond of TMSA activates the Si atom and induces the creation of a surface Si-Au chemical bond. The highly stable TMSA-based SAMs appear then as promising materials for applications in surface modification.

Ab initio investigation of the temporary anion states of silane and the linear silanes: (Si nH 2 n+2 ), n=2–5

The stabilization method is used in conjunction with Koopmans' theorem to calculate the energies of the unfilled molecular orbitals appropriate for describing the low-lying anion states of the linear silanes, Si n H 2 n+2 , with n=1–5. In agreement with earlier studies, the lowest energy unfilled molecular orbital for each member of the n⩾3 silanes is found to be largely SiH 2 σ * in character.

New Polyphosphazenes with Unsaturated Side Groups: Use as Reaction Intermediates, Cross-Linkable Polymers, and Components of Interpenetrating Polymer Networks

A number of new poly(organophosphazenes) have been synthesized which bear 2-butenoxy or 4-allyloxyphenylphenoxy side groups. Co-substituent groups included trifluoroethoxy, phenoxy, or benzyloxyphenoxy groups. Species with 4-allyloxyphenylphenoxy units underwent Si--H coupling to linear silanes or siloxanes to extend the side groups and form hybrid phosphazene/organosilicon polymers. A number of these polymers are rubbery elastomers which are readily cross-linked by heat or light. Seven of the mixed-substituent, cross-linked polymers were incorporated into interpenetrating polymer networks (IPN's) with polystyrene, poly(methyl methacrylate), polyacrylonitrile, poly(acrylic acid) and poly(dimethylsiloxane). The phase compatibility characteristics of the IPN's were assessed by DSC, TEM, FT-IR spectroscopy, and 1H and 31P NMR spectroscopy data.