Investigation into the Catalytic Dehydrocoupling of Bridged Disilanes

Abstract

As humanity continues into the 21st century, society continues to progress towards more electronic-centric lifestyles. The use of expensive and rare transition state metals within semiconductors continues to an issue as the rare metals become scarcer. The search to identify highly efficient semiconductor substituents discovered the potential of crystalline silicon. The structural rigidity of the crystalline silicon has limited electronic application and redirected attention towards the development of elastomeric compounds, capable of bending, rolling folding and stretching while maintaining high-performance electronic conductivity. Advancements in silicon chemistry have identified polysilanes, organosilicon polymers possessing of a Si-Si backbone. The σ-conjugation along the polysilane Si-Si backbone provides a semiconducting material with unique electronic properties, meanwhile possessing the flexibility and desirable physical properties of organic compounds. However, the susceptibility of the Si-Si bond to UV irradiation and high moisture conditions facilitates polysilane chain breakdown. Attempts to reinforce the Si-Si backbone through covalent molecular bridges have been developed, where the incorporation of a molecular bridge between two silicon atoms reinforces the disilane bond and increases the overall robustness of the bridged polysilane. Careful selection of the bridging molecule may allow for enhancement of the electronic properties of the bridged disilane. Naphthalenebridge disilanes have been shown to exhibit unique electronic properties due to the σ-π mixing between the bridge and silicon centres. However, limitations regarding disilane dehydrocoupling have prevented successful synthesis of bridged disilane oligosilanes.

This work focuses on the continued development of bridged polysilanes aimed towards the synthesis of bis(phenyl)silyl precursors as disilane building blocks with terminal hydride ends for successive dehydropolymerisation. The first section is directed towards the identification of experimental procedures capable of synthesising benzyl silane bridges from chloro(aryl)silanes and benzyl bromide substrates. These reactions are conducted for the expansion of the potential bridging molecular substrates to non-aromatic molecules. The second section details an intramolecular dehydrocoupling investigations on the 1,8-bis(phenylsilyl)naphthalene precursor with a series of four transition-metal dehydrocoupling catalysts. The four catalysts selected were either rhodium ([Rh(cod)Cl]2,Rh(PPh3)3Cl) or zirconocene-based complexes (Cp2ZrBu2, Cp2ZrMe2). The catalytic investigation aimed to identify potential catalysts for the preparation of bridged disilanes and bridged polysilane chains, with a focus on the [Rh(cod)Cl]2 catalyst. The [Rh(cod)Cl]2 catalyst was confirmed as a highly efficient dehydrocoupling catalyst for intramolecular dehydrocoupling disilane synthesis, significant more efficiency than Rh(PPh3)3Cl under mild conditions. Increased catalytic loading and increased temperature failed to prove tertiary dehydrocoupling with the [Rh(cod)Cl]2 catalyst. In situ zirconocene catalysts were identified as potential catalysts for disilane building blocks in contrast to previous evidence. Investigation into zirconocene-based complexes revealed while the [Cp2Zr] is a capable hydrosilane dehydrocoupling catalyst, direction lithiation of the hydrosilane provides improved yields and conversion rates at the cost of purity. NMR spectroscopy revealed varying stereoselectivity between the zirconocene and rhodium catalysts, with mechanistic insight provided to explain the observed stereoselectivity between the two different dehydrocoupling catalyst classes. The final section details a second intramolecular dehydrocoupling investigation where the previous dehydrocoupling catalysts are applied to two ferrocene-bridged disilyl precursors 1,1-bis(phenylsilyl)ferrocene) and 1,1-bis(diphenyl silyl)ferrocene). Experimental analysis revealed none of the investigated catalysts were capable of reaction success, but that compound degradation was significant for zirconocene catalysts, proposed to be consequential of the use of lithiation additives.