Si-Si Single Bond Research Articles

Diverse Reactions of o-Carborane-Fused Silylenes with C≡E (E = C, P) Triple Bonds.

The reactivities of o-carborane-fused silylenes toward molecules with C≡E (E = C, P) bonds are reported. The reactions of bis(silylene) [(LSi:)C]2B10H10 (1a, L = PhC(NtBu)2) with arylalkynes afforded bis(silylium) carborane adducts 2 and 3, showing a Si(μ-C2)Si structure with an open-cage nido-carborane backbone. In contrast, the reaction of 1a with a phosphaalkyne AdC≡P (Ad = 1-adamantyl) smoothly furnished compound 4, comprising fused CPSi rings with a C=Si double bond and Si-Si single bond, and the related formation mechanism was investigated by DFT calculations. Furthermore, when monosilylene [(LSi:)C]CHB10H10 (1b) was employed to react with AdC≡P, compound 5 was isolated. The structure of 5 features a 1,2,3-triphosphetene core. All products were characterized by NMR spectroscopy and/or X-ray crystallography.

Disilane-bridged architectures: an emerging class of molecular materials.

Disilanes are organosilicon compounds that contain saturated Si-Si bonds. The structural characteristics of Si-Si single bonds resemble those of C-C single bonds, but their electronic structure is more similar to that of C[double bond, length as m-dash]C double bonds, as Si-Si bonds have a higher HOMO energy level. These organosilicon compounds feature unique intramolecular σ electron delocalization, low ionization potentials, polarizable electronic structure, and σ-π interaction. It has been demonstrated that the employment of disilane units (Si-Si) is a versatile and effective approach for finely adjusting the photophysical properties of organic materials in both solution and solid states. In this review, we present and discuss the structure, properties, and relationships of novel σ-π-conjugated hybrid architectures with saturated Si-Si σ bonds. The application of disilane-bridged σ-conjugated compounds as optoelectronic materials, multifunctional solid-state emitters, CPL, and non-linear optical and stimuli-responsive materials is also reviewed.

Five coordinated Mn in Ba4Mn2Si2Te9: synthesis, crystal structure, physical properties, and electronic structure.

We report the synthesis of single-crystals of a new transition metal-containing quaternary chalcogenide, Ba4Mn2Si2Te9, synthesized by the solid-state method at 1273 K. A single-crystal X-ray diffraction study shows that it crystallizes in the orthorhombic crystal system (space group: Pbam) with cell constants of a = 13.4690(6) Å, b = 8.7223(4) Å, and c = 10.0032(4) Å. The asymmetric unit of the structure consists of eight unique crystallographic sites: one Ba, two Mn, one Si, and four Te sites. In this structure, the two Mn sites, Mn(1) and Mn(2), are disordered, each with fractional occupancy of 50%. The short distance of 2.170(3) Å between Mn(1) and Mn(2) implies that both Mn sites are not occupied simultaneously. The Mn atoms show two types of polyhedra: unique Mn(1)Te5 units along with traditional Mn(2)Te4 tetrahedra. The main motifs of the Ba4Mn2Si2Te9 structure are dimeric Si2Te6 units (with Si-Si single bond), Mn(1)Te5, and Mn(2)Te4 polyhedra. The structure can be described as pseudo-two-dimensional if only Mn(1) atoms are present and one-dimensional when only Mn(2) atoms are filled in the structure. The extended 2∞[Mn(1)Si2Te9]10- layers and 1∞[Mn(2)Si2Te8]8- chains are separated by Ba2+ cations. The direct bandgap for the polycrystalline Ba4Mn2Si2Te9 sample is 0.6(1) eV, as determined from an optical absorption study consistent with the sample's black color. The resistivity study of the polycrystalline Ba4Mn2Si2Te9 also confirms the semiconducting behavior. The thermal conductivity (κ) values are extremely low and decrease with increasing temperature up to 0.46 W m-1 K-1 at 773 K. The DFT studies suggest that the computed bandgap depends on the magnetic ordering of Mn magnetic moments, and the value varies from ∼0.3-1.0 eV. Relative inter-atomic bond strengths of pertinent atom pairs have been analyzed using the crystal orbital Hamilton populations (COHP).

Isolation of two bis(silyl)nickel complexes with Si-Si bond formation in a single-crystal-to-single-crystal fashion.

A unique penta-coordinate nickel complex [{1,2-C6H4(SiMe2)(SiH)}2Ni(PEt3)(depe)] (6) that generates two new Si-Si single bonds has been prepared exclusively by the reaction of a chelating silyl hydride 1-(dimethylsilyl)-2-silylbenzene (4) and Ni(depe)(PEt3)2 (depe = Et2PCH2CH2PEt2) in a 2 : 1 ratio. Complex 6 is the first example of mononuclear silyl nickel complexes containing a Si-Si single bond formed in situ. Interestingly, framework 6 exhibits reversible single-crystal-to-single-crystal transformations upon the removal and rebinding of the coordinating PEt3 molecule, which give rise to a PEt3-free complex [{1,2-C6H4(SiMe2)(SiH)}2Ni(depe)] (6'), concomitant with the alteration of the coordination geometry of central metal atoms.

Silane and Germane Molecular Electronics.

This Account provides an overview of our recent efforts to uncover the fundamental charge transport properties of Si-Si and Ge-Ge single bonds and introduce useful functions into group 14 molecular wires. We utilize the tools of chemical synthesis and a scanning tunneling microscopy-based break-junction technique to study the mechanism of charge transport in these molecular systems. We evaluated the fundamental ability of silicon, germanium, and carbon molecular wires to transport charge by comparing conductances within families of well-defined structures, the members of which differ only in the number of Si (or Ge or C) atoms in the wire. For each family, this procedure yielded a length-dependent conductance decay parameter, β. Comparison of the different β values demonstrates that Si-Si and Ge-Ge σ bonds are more conductive than the analogous C-C σ bonds. These molecular trends mirror what is seen in the bulk. The conductance decay of Si and Ge-based wires is similar in magnitude to those from π-based molecular wires such as paraphenylenes However, the chemistry of the linkers that attach the molecular wires to the electrodes has a large influence on the resulting β value. For example, Si- and Ge-based wires of many different lengths connected with a methyl-thiomethyl linker give β values of 0.36-0.39 Å-1, whereas Si- and Ge-based wires connected with aryl-thiomethyl groups give drastically different β values for short and long wires. This observation inspired us to study molecular wires that are composed of both π- and σ-orbitals. The sequence and composition of group 14 atoms in the σ chain modulates the electronic coupling between the π end-groups and dictates the molecular conductance. The conductance behavior originates from the coupling between the subunits, which can be understood by considering periodic trends such as bond length, polarizability, and bond polarity. We found that the same periodic trends determine the electric field-induced breakdown properties of individual Si-Si, Ge-Ge, Si-O, Si-C, and C-C bonds. Building from these studies, we have prepared a system that has two different, alternative conductance pathways. In this wire, we can intentionally break a labile, strained silicon-silicon bond and thereby shunt the current through the secondary conduction pathway. This type of in situ bond-rupture provides a new tool to study single molecule reactions that are induced by electric fields. Moreover, these studies provide guidance for designing dielectric materials as well as molecular devices that require stability under high voltage bias. The fundamental studies on the structure/function relationships of the molecular wires have guided the design of new functional systems based on the Si- and Ge-based wires. For example, we exploited the principle of strain-induced Lewis acidity from reaction chemistry to design a single molecule switch that can be controllably switched between two conductive states by varying the distance between the tip and substrate electrodes. We found that the strain intrinsic to the disilaacenaphthene scaffold also creates two state conductance switching. Finally, we demonstrate the first example of a stereoelectronic conductance switch, and we demonstrate that the switching relies crucially on the electronic delocalization in Si-Si and Ge-Ge wire backbones. These studies illustrate the untapped potential in using Si- and Ge-based wires to design and control charge transport at the nanoscale and to allow quantum mechanics to be used as a tool to design ultraminiaturized switches.

A Triatomic Silicon(0) Cluster Stabilized by a Cyclic Alkyl(amino) Carbene.

Reduction of the neutral carbene tetrachlorosilane adduct (cAAC)SiCl4 (cAAC=cyclic alkyl(amino) carbene :C(CMe2)2 (CH2)N(2,6-iPr2C6H3) with potassium graphite produces stable (cAAC)3Si3, a carbene-stabilized triatomic silicon(0) molecule. The Si-Si bond lengths in (cAAC)3Si3 are 2.399(8), 2.369(8) and 2.398(8) Å, which are in the range of Si-Si single bonds. Each trigonal pyramidal silicon atom of the triangular molecule (cAAC)3Si3 possesses a lone pair of electrons. Its bonding, stability, and electron density distributions were studied by quantum chemical calculations.

Trisilaallene and the Relative Stability of Si3H4 Isomers.

A theoretical quantum-mechanical study of trisilaallene, H2Si [Formula: see text] Si [Formula: see text] SiH2, and of 15 other Si3H4 isomers was carried out using ab initio and DFT methods with a variety of basis sets. Values given below are at B3LYP/6-31G(d,p). Unlike H2C [Formula: see text] C [Formula: see text] CH2 which is linear, H2Si [Formula: see text] Si [Formula: see text] SiH2 is highly bent at the central silicon atom, with a SiSiSi bending angle of 69.4°. The Si [Formula: see text] Si bond length is 2.269 Å, longer than a regular Si [Formula: see text] Si double bond (2.179 Å) but shorter than a Si-Si single bond (2.351 Å). The distance between the terminal silicon atoms is 2.583 Å, significantly longer than a Si-Si single bond. The geometry and electronic properties of H2Si [Formula: see text] Si [Formula: see text] SiH2 are similar to those of the corresponding trisilacyclopropylidene, which is only 2.7 kcal/mol higher in energy. A barrier of only 0.1 kcal/mol separates trisilacyclopropylidene and trisilaallene which can be described as bond-stretch isomers. Sixteen minima were located on the Si3H4 PES, most of them within a narrow energy range of ca. 10 kcal/mol. Six of the Si3H4 isomers are analogous to the classic C3H4 minima structures; however, the other Si3H4 isomers do not have carbon analogues, and they are characterized by hydrogen-bridged structures.

Control of stability through overlap matching: closo-carborynes and closo-silaborynes.

The matching of ring and cap orbitals for overlap is used to arrive at the best carborynes among the many possibilities. Accordingly, 1,2-carboranes, 1,2-silaboranes (C2BnHn+2, and Si2BnHn+2, n = 4, 5, 8, and 10), and their dehydrogeno derivatives were studied with use of the Density Functional Theory (B3LYP/6-311+G*). The dehydrogenation of 2,3-C2B5H7 (6a) to 2,3-C2B5H5 (13a) is estimated to be even less endothermic than those of benzene and 1,2- C2B10H12 (1a) to benzyne and 1,2-C2B10H10 (8a) by more than 21 kcal/mol. This is due to the extra stabilization gained through better overlap of the C2B3H3 ring with the 2 BH caps. The relatively larger size of the Si atom leads to overlap requirements in silaboranes that are different from those in carboranes. The lower Si-Si single bond energy and the preference of Si for lower coordination result in unusual structures in dehydrogenosilaboranes. One of the Si atoms moves away from the surface in Si2B10H10 (15), Si2B8H8 (16, 17, and 18), and 1,2-Si2B5H5 (19). One Si atom forms a bridge to a trigonal surface in 2,3-Si2B5H5 (20) and 1,2-Si2B4H4 (21). Estimates of three-dimensional aromaticity with NICS calculations show that the exohedral double bond does not influence three-dimensional aromaticity.

Disupersilyldisilane R*X 2SiSiX 2R*: Darstellung, Charakterisierung und Strukturen; sterische Effekte der Substituenten

Disupersilyldisilanes R*X 2SiSiX 2R* (R*=supersilyl=Si t-Bu 3; two diastereomers in case of R*XX′SiSiX′XR*) are prepared in organic solvent (i) by dehalogenations of supersilylhalosilanes with Na, NaC 10H 8 or NaR*; (ii) by reactions of disupersilylhalodisilanes with H − (Hal/H exchange); (iii) by reactions of disupersilyldisilanes with Hal 2 (H/Hal exchange); (iv) by reactions of disupersilylhalodisilanes first with NaR* (Hal/Na exchange) then with agents for protonation, alkylation, silylation (Na/H, Na/Me, Na/SiMe 3 exchange); or (v) by reactions of disupersilyldisilenes (here R*PhSiSiPhR*) with HHal or Hal 2 (formation of addition products). As discussed, ( 29Si) of the SiX 2 groups of R*X 2SiSiX 2R* depends strongly on the nature of X. The disupersilyldisilanes are in part moisture sensitive (compounds with SiX 2=SiHHal), in part sensitive against oxygen (compounds with SiX 2=SiHal 2, SiHHal; Hal=Br, I). X-ray structure analyses of the disupersilyldisilanes R*X 2SiSiX 2R* (X 2=H 2, Cl 2, Br 2, BrH, IH, MeBr, PhH, PhCl) as well as R*PhClSiSiHPhR* and R*PhBrSiSiClPhR* show a staggered conformation. Due to steric repulsions of groups R*/X/X, the SiSi bond lengths are longer than 2.34 Å (normal SiSi single bond; exception: R*H 2SiSiH 2R*). From the extent of SiSi bond elongation it is concluded that the bulkiness of X increases in direction H<Cl<Br, I, and Br≈Me<Ph. Trans-configurated diastereomers are sterically more crowded than corresponding gauche-configurated compounds ( trans and gauche refer to the configuration of the heaviest groups after trans-configurated R* in the disilanes). Indeed, bromination or iodination of R*H 2SiSiH 2R* leads exclusively to gauche-R*HalHSiSiHHalR*, but chlorination gives in addition R*Cl 2SiSiH 2R*. Hydridation occurs faster with trans-R*PhClSiSiHPhR* than with the gauche-diastereomer. The R*/R* groups of the disupersilyldisilanes occupy exactly trans positions in R*X 2SiSiX 2R*, trans-R*XX′SiSiX′XR* and trans-R*PhClSiSiHPhR*, whereas they occupy nearly trans positions in gauche-R*XX′SiSiXX′R* (exception gauche-R*MeBrSiSiBrMeR* with R* exactly trans).

Supersilylsilane R*SiX3: Darstellung, Charakterisierung und Strukturen; sterische und van-der-Waals Effekte der Substituenten X [1] / Supersilylsilanes R*SiX3: Syntheses, Characterization and Structures; Steric and van-der-Waals Effects of Substituents X [1

Supersilylsilanes R*SiX3 (R* = supersilyl = SitBu3; X = H, Me, tBu, Ph, SiMe3, F, Cl, Br, I, OMe, OSO2CF3) are prepared (i) by reactions of supersilylhalosilanes with supersilyl sodium NaR* (Hal/R* exchange), (ii) by reactions of supersilylhalosilanes with hydride H- (Hal/H exchange), (iii) by reactions of supersilylsilanes with halogens Hal2 (H/Hal exchange, R*/Hal exchange), (iv) by reactions of supersilylhalosilanes with nucleophiles like F- , MeO- (Hal/F or Hal/OMe exchange) and (v) by reactions of supersilylsilanes with strong acids (H/OSO2CF3 exchange). NMR chemical shifts δ (29Si) of the SiX3 groups of R*SiX3 strongly depends on the nature of X. The supersilylsilanes R*SiX3 are in part moisture sensitive (especially compounds with SiX3≡SiHHal2 and SiH2Hal), in part sensitive against oxygen (compounds with SiX≡SiBr or SiI), and some of them react with supersilyl sodium by supersilanidation (for example R*SiF3, R*SiH2Cl, R*SiMeHCl) or by reduction (for example R*SiCl3, R*SiMeBr2, R*SiPhBrCl). X-ray structure analyses of disilanes tBu3Si-SiX3 with SiX3≡SitBu3, SiPh3, Sil3, SiPhCl2 show a staggered conformation. Due to steric repulsion of the tBu3Si and SiX3 groups as well as van-der-Waals attraction of the substituents tBu and X in tBu3Si-SiX3 the SiSi bonds are longer than 2.34 Å (the normal SiSi single bond length) and the torsion angles are smaller/larger than 60° (the ideal staggered conformation). From the extent of SiSi bond elongation and CSiSiX angle compression it is concluded that the bulkiness of X increases in direction Cl &lt; I &lt; Ph &lt; SiMe3 &lt; CMe3 (tBu3Si-SitBu3 has to date the longest SiSi bond of all disilanes) and the van-der-Waals forces between tBu/X increase in direction tBu/I &lt; tBu/tBu &lt; tBu/Ph.

Theoretical Calculation of Thermochemistry for Molecules in the Si−P−H System

Ab initio molecular orbital calculations have been performed on species belonging to the Si-P-H system. These computations have been coupled to a bond additivity correction procedure to obtain heats of formation for 27 species. The Si-P single bond energy was found to be nominally about 300 kJ/mol, which is somewhat weaker than a Si-Si single bond. Multiple bond character in Si-P was found to be relatively weak. 11 refs., 7 tabs.

Cyclic Polychalcogenide Compounds with Silicon and Germanium Atoms

Abstract A new trithiadisilabicyclo[1.1.1]pentane 1-S3 as well as its selenium analogue 1-Se3 were prepared by dechalcogenation of the corresponding tetrachalcogena[2.1.1]hexanes. The X-ray analysis clearly prooved that the distance between the bridging silicon atoms of 1-S3 and 1-Se3 are 2.407 and 2.515 A, respectively, values which are within the range of common Si-Si single bonds. Under appropriate conditions trihydro- and trihalosilanes react with elemental chalcogens respective chalcogen transfer reagents to give adamantane-type silachalcogenanes or bicyclopolychalcogenasilanes. E. g. treatment of tert-butyl-germanium trichloride with ammonium pentasulfide afforded a double decker type germathiane 7, which thermally rearranged to the corresponding adamantane-type isomer 5.

Structure of the layer compound ZrSiTe

Zirconium silicon telluride, ZrSiTe, crystallizes is a layer-type structure which is related to the PbFCl type. The Zr atom is coordinated by four Si and four Te atoms. The Si and Te atoms are arranged in square planes. These planes are situated at opposite sides of the Zr atom; the coordination polyhedron may be described as a distorted quadratic antiprism. The distances between the Si atoms [2.614 (1) A] are longer than that of a Si-Si single bond (2.35 A), but comparable with the Si-Si separations in transition metal disilicides. The Te-Te separations within the Te 4 plane give no indication of significant interactions