Nanoscale Double-Core Oligosilane Dendrimers: Synthesis, Structure, and Electronic Properties

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.

Regioselective Derivatization of Silylated [20]Silafulleranes

Silafulleranes with endohedral Cl^- ions are a unique, scarcely explored class of structurally well-defined silicon clusters and host-guest complexes. Herein, we report regioselective derivatization reactions on the siladodecahedrane [nBu₄N][Cl@Si₂₀(SiCl₃)₁₂Cl₈] ([nBu₄N][1]), which has its cluster surface decorated with 12 SiCl₃ and 8 Cl substituents in perfect T_h symmetry. The room-temperature reaction of [nBu₄N][1] with excess iBu₂AlH in ortho-difluorobenzene (oDFB) furnishes perhydrogenated [nBu₄N][Cl@Si₂₀(SiH₃)₁₂H₈] ([nBu₄N][2]) in 50% yield; the non-pyrophoric [2]^- is the largest structurally authenticated (by X-ray diffraction) hydridosilane known to date. A simple switch from pure oDFB to an oDFB/Et₂O solvent mixture suppresses core hydrogenation and results in the formation of [nBu₄N][Cl@Si₂₀(SiH₃)₁₂Cl₈] ([nBu₄N][3]). In addition to the exhaustive Cl/H exchange at all 44 Si-Cl bonds of [1]^- and the regioselective 36-fold silyl group hydrogenation, we achieved the simultaneous introduction of Me substituents at all 8 SiCl vertices along with the conversion of all 12 SiCl₃ to SiH₃ groups by treating [nBu₄N][1] with Me₂AlH/Me₃Al in oDFB ([nBu₄N][Cl@Si₂₀(SiH₃)₁₂Me₈], [nBu₄N][4]; 73%). Quantum-chemical free-energy calculations find an S_N2-Si-type hydrogenation of the exohedral SiCl₃ moieties in [1]^- (trigonal-bipyramidal intermediate) slightly preferred over metathesis-like S_Ni-Si substitutions (four-membered transition state). Cage hydrogenation likely occurs via S_Ni-Si processes. The experimentally demonstrated influence of an Et₂O co-solvent, which drastically increases the respective reaction barriers, is attributed to the increased stability of the resulting iBu₂AlH-OEt₂ adduct and its higher steric bulk compared to free iBu₂AlH.

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.