all-anti-Pentasilane: Conformation Control of Oligosilanes Based on the Bis(tetramethylene)-Tethered Trisilane Unit

Iterative Synthesis of Oligosilanes Using Methoxyphenyl- or Hydrogen-Substituted Silylboronates as Building Blocks: A General Synthetic Method for Complex Oligosilanes

Organosilanes have attracted the attention of researchers for more than 150 years due to their unique properties, and they have become indispensable industrial assets. However, many synthesized oligosilanes with multiple Si-Si bonds are relatively simple, i.e., they often only contain a single repeating unit. More laborious customized synthetic routes can lead to more complex oligosilanes, but compared to carbon-based molecules, their structural diversity remains limited. The development of effective and practical synthetic routes to complex oligosilanes that contain mixed substituents constitutes a long-standing challenge. Here, we describe an iterative synthesis of oligosilanes using methoxyphenyl- or hydrogen-substituted silylboronates, which were obtained via transition-metal-catalyzed Si-H borylation reactions. The first key reaction is a cross-Si-Si bond-forming reaction between chloro(oligo)silanes and silylboronates activated by MeLi. The second key reaction is the selective chlorination of the methoxyphenyl group or the hydrogen atom at the terminal of the oligosilanes. Iteration of these two key reactions enables the synthesis of various oligosilanes that are otherwise difficult to access. As a demonstration of the synthetic utility of this iterative synthetic approach, oligosilanes with different sequences were prepared by simply changing the order of the reaction of four different silicon units. Furthermore, a bespoke tree-shaped oligosilane is easily obtained via the present iterative synthesis. The solid-state structures of several of these oligosilanes were unequivocally determined using single-crystal X-ray diffraction analysis.

β-Amino- and Alkoxy-Substituted Disilanides

Our recent study on formal halide adducts of disilenes led to the investigation of the synthesis and properties of β-fluoro- and chlorodisilanides. The reaction of the functionalized neopentasilanes (Me3Si)3SiSiPh2NEt2 and (Me3Si)3SiSiMe2OMe with KOtBu in the presence of 18-crown-6 provided access to structurally related β-alkoxy- and amino-substituted disilanides. The obtained Et2NPh2Si(Me3Si)2SiK·18-crown-6 was converted to a magnesium silanide and further on to Et2NPh2Si(Me3Si)2Si-substituted ziroconocene and hafnocene chlorides. In addition, an example of a silanide containing both Et2NPh2Si and FPh2Si groups was prepared with moderate selectivity. Also, the analogous germanide Et2NPh2Si(Me3Si)2GeK·18-crown-6 could be obtained.

Incorporating Methyl and Phenyl Substituted Stannylene Units into Oligosilanes. The Influence on Optical Absorption Properties

Molecules containing catenated heavy group 14 atoms are known to exhibit the interesting property of σ-bond electron delocalization. While this is well studied for oligo- and polysilanes the current paper addresses the UV-absorption properties of small tin containing oligosilanes in order to evaluate the effects of Sn–Si and Sn–Sn bonds as well as the results of substituent exchange from methyl to phenyl groups. The new stannasilanes were compared to previously investigated oligosilanes of equal chain lengths and substituent pattern. Replacing the central SiMe₂ group in a pentasilane by a SnMe₂ unit caused a bathochromic shift of the low-energy band (λ_max = 260 nm) of 14 nm in the UV spectrum. If, instead of a SnMe_2, a SnPh₂ unit is incorporated, the bathochromic shift of 33 nm is substantially larger. Keeping the SnMe₂ unit and replacing the two central silicon with tin atoms causes shift of the respective band (λ = 286 nm) some 26 nm to the red. A similar approach for hexasilanes where the model oligosilane [(Me₃Si)₃Si]₂(SiMe₂)₂ (λ_max = 253 nm) was modified in a way that the central tetramethyldisilanylene unit was exchanged for a tetraphenyldistannanylene caused a 50 nm bathochromic shift to a low-energy band with λ_max = 303 nm.

σ-Bond Electron Delocalization in Oligosilanes as Function of Substitution Pattern, Chain Length, and Spatial Orientation

Polysilanes are known to exhibit the interesting property of σ-bond electron delocalization. By employing optical spectroscopy (UV-vis), it is possible to judge the degree of delocalization and also differentiate parts of the molecules which are conjugated or not. The current study compares oligosilanes of similar chain length but different substitution pattern. The size of the substituents determines the spatial orientation of the main chain and also controls the conformational flexibility. The chemical nature of the substituents affects the orbital energies of the molecules and thus the positions of the absorption bands.

σ-Bond electron delocalization of branched oligogermanes and germanium containing oligosilanes

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Oligogermanes and germaoligosilanes isostructural to oligosilanes were synthesized.

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UV absorption data and X-ray diffraction revealed only marginal differences.

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Conformational flexibility is directly reflected in the UV absorption spectrum.

In order to evaluate the influence of germanium atoms in oligo- and polysilanes, a number of oligosilane compounds were prepared where two or more silicon atoms were replaced by germanium. While it can be expected that the structural features of thus altered molecules do not change much, the more interesting question is, whether this modification would have a profound influence on the electronic structure, in particular on the property of σ-bond electron delocalization.

The UV-spectroscopic comparison of the oligosilanes with germanium enriched oligosilanes and also with oligogermanes showed a remarkable uniform picture. The expected bathochromic shift for oligogermanes and Ge-enriched oligosilanes was observed but its extent was very small. For the low energy absorption band the bathochromic shift from a hexasilane chain (256 nm) to a hexagermane chain with identical substituent patterns (259 nm) amounts to a mere 3 nm.

Electronic transitions in conformationally controlled tetrasilanes with a wide range of SiSiSiSi dihedral angles

Unlike π-electron chromophores, the peralkylated n-tetrasilane σ-electron chromophore resembles a chameleon in that its electronic spectrum changes dramatically as its silicon backbone is twisted almost effortlessly from the syn to the anti conformation (changing the SiSiSiSi dihedral angle ω from 0 to 180°). A combination of UV absorption, magnetic circular dichroism (MCD), and linear dichroism (LD) spectroscopy on conformationally controlled tetrasilanes 1-9, which cover fairly evenly the full range of angles ω, permitted a construction of an experimental correlation diagram for three to four lowest valence electronic states. The free chain tetrasilane n-Si4 Me10 (10), normally present as a mixture of three enantiomeric conformer pairs of widely different angles ω, has also been included in our study. The spectral trends are interpreted in terms of avoided crossings of 1B with 2B and 2A with 3A states, in agreement with SAC-CI calculations on the excited states of 1-7 and conformers of 10.

1,1,4,4-Tetra-tert-butyl-1,4-dichloro-2,2,3,3-tetra-phenyl-tetra-silane

The title compound, C(40)H(56)Cl(2)Si(4), was synthesized by the coupling of 1,1-di-tert-butyl-1,2-dichloro-2,2-diphenyl-disilane with lithium. The asymmetric unit contains one half-mol-ecule, which is completed by an inversion centre. In the mol-ecule, the tetra-silane skeleton adopts a perfect anti conformation and the Si-Si bonds [2.4355 (5) and 2.4328 (7) Å] are longer than the standard Si-Si bond length (2.34 Å). The Si-Si-Si angle [116.09 (2)°] is larger than the tetra-hedral bond angle (109.5°). These long bond lengths and the wide angle are favorable for reducing the steric hindrance among the tert-butyl and the phenyl groups. The dihedral angle between the phenyl rings in the asymmetric unit is 37.36 (8)°.

Rearrangement/Fragmentation Reactions of Oligosilanes with Aluminum Chloride

Reinvestigation of the Lewis acid catalyzed rearrangement of some open-chain permethyloligosilanes with the Al(Fe)Cl(3) catalyst system exhibited several cases of additional reactivity: namely, a fragmentation/cyclization reaction. Introduction of (trimethylsilyl)methyl substituents into the oligosilane substrates strongly facilitated this reaction, yielding cyclic or bicyclic carbacyclosilanes. Investigations concerning the composition of the catalyst system indicated that the incorporation of about 0.1% FeCl(3) into the AlCl(3) lattice provided an effective catalyst.

Conformational Control of Polysilanes: Use of CH(2) Spacers in the Silicon Backbone

By the reaction of a number of oligosilyl potassium compounds with (trimethylsilyl)chloromethane, derivatives containing the (trimethylsilyl)methyl substituent were prepared. Using X-ray single-crystal structure analysis and UV spectroscopy the conformational properties of some of the compounds were studied. It was found that the (trimethylsilyl)methylated examples exhibit UV absorption properties which correspond to lower energy transitions in comparison to those of analogous trimethylsilylated molecules. The influence of this effect decreases, however, with increasing chain lengths.

Electronic excitations of 1,4-disilyl-substituted 1,4-disilabicycloalkanes: a MS-CASPT2 study of the influence of cage size

We present a multistate complete active space second-order perturbation theory computational study aimed to predict the low-lying electronic excitations of four compounds that can be viewed as two disilane units connected through alkane bridges in a bicyclic cage. The analysis has focused on 1,4-disilyl-1,4-disilabicyclo[2.2.1]heptane (1a), 1,4-bis(trimethylsilyl)-1,4-disilabicyclo[2.2.1]heptane (1b), 1,4-disilyl-1,4-disilabicyclo[2.1.1]hexane (2a), and 1,4-bis(trimethylsilyl)-1,4-disilabicyclo[2.1.1]hexane (2b). The aim has been to find out the nature of the lowest excitations with significant oscillator strengths and to investigate how the cage size affects the excitation energies and the strengths of the transitions. Two different substituents on the terminal silicon atoms (H and CH3) were used in order to investigate the end group effects. The calculations show that the lowest allowed excitations are of the same character as that found in disilanes but now red-shifted. As the cage size is reduced from a 1,4-disilabicyclo[2.2.1]heptane to a 1,4-disilabicyclo[2.1.1]hexane, the Si...Si through-space distance decreases from approximately 2.70 to 2.50 A and the lowest allowed transitions are red-shifted by up to 0.9 eV, indicating increased interaction between the two Si-Si bonds. The first ionization potential, which corresponds to ionization from the Si-Si sigma orbitals, is lower in 1b and 2b than in Si2Me6 by approximately 0.9 and 1.2 eV, respectively. Moreover, 1b and 2b, which have methyl substituents at the terminal Si atoms, have slightly lower excitation energies than the analogous species 1a and 2a.

all-anti-Octasilane: Conformation Control of Silicon Chains Using the Bicyclic Trisilane as a Building Block

The perfect all-anti-octasilane composed of two bicyclic trisilane units with trimethylsilyl groups at the termini has been synthesized. Its X-ray crystal structure and spectroscopic data demonstrate the effective sigma-delocalization over the silicon framework, which is a definitive difference from the unconstrained n-Si8Me18.

Novel double-cored oligosilane dendrimers—conformational dependence of the UV absorption spectra

The novel double-cored oligosilane dendrimers {[Me(Me3Si)2Si-Me2Si]2SiMe-SiMe2}2 (4) and {[Me(Me3Si)2Si-Me2Si]2SiMe}2 (5) have been shown to display unusual UV absorption behaviour as a function of conformation and steric interaction in the longest oligosilane chains.

Structurally and conformationally defined small methyl polysilanes

Possessing the property of sigma-bond electron delocalisation, polysilanes are a class of compounds with unique properties. In recent years major progress has been achieved in the theoretical understanding and the synthesis of polysilanes. Much insight into the connection between conformation and electronic properties has been gained from studies of defined small polysilane molecules. The transition from Wurtz type coupling reactions to the stepwise construction of polysilane molecules employing silyl anions as key intermediates has permitted access to defined compounds with a higher degree of structural complexity. Besides this, several methods have been developed to control the conformational properties and thus gain control over the electronic properties of polysilanes.