Thesis (Ph.D.) - University of St Andrews, January 2009.

The reactions of tert-butyl-, sec-butyl-, and n-butyllithium with dimethylfluorovinylsilane include addition to the double bond to give both silene and silenoid intermediates, fluorine substitution, and a novel vinyl substitution. For the tert-butyllithium reaction, product stereochemistry and trapping experiments using both cyclopentadiene and methoxytrimethylsilane show that silenes are not formed in THF. In hexane about 67% of the 1,3-disilacyclobutanes obtained arise from silene dimerization while 33% are formed by silenoid coupling. In hexane the order of reactivity for addition, t-Bu > sec-Bu > n-Bu, is opposite that for fluorine substitution. The vinyl substitution is most significant with secondary alkyllithium reagents including the tert-butyllithium adduct to dimethylfluorovinylsilane and with sec-butyllithium itself. Evidence for the formation of vinyllithium or ethylene in the process could not be obtained.

No description available.

Single-molecule electronics is an emerging subfield of nanoelectronics where the ultimate goal is to use individual molecules as the active components in electronic circuitry. Over the past century, chemists have developed a rich understanding of how a molecule’s structure determines its electronic properties; transposing the paradigms of chemistry into the design and understanding of single-molecule electronic devices can thus provide a tremendous impetus for growth in the field. This dissertation describes how we can harness the principles of organosilicon and organogermanium chemistry to control charge transport and function in single-molecule devices. We use a scanning tunneling microscope-based break-junction (STM-BJ) technique to probe structure-conductivity relationships in silicon- and germanium-based wires. Our studies ultimately demonstrate that charge transport in these systems is dictated by the conformation, conjugation, and bond polarity of the σ-backbone. Furthermore, we exploit principles from reaction chemistry such as strain-induced Lewis acidity and σ-bond stereoelectronics to create new types of digital conductance switches. These studies highlight the vast opportunities that exist at the intersection between chemical principles and single-molecule electronics. Chapter 1 introduces the fields of single-molecule electronics, silicon microelectronics, and physical organosilane chemistry and our motivation for bridging these three worlds. Chapters 2-6 elaborate on the specific approach taken in this dissertation work, which is to deconstruct the molecular wire into three structural modules – the linker, backbone, and substituent – then synthetically manipulate each component to elucidate fundamental conductance properties and create new types of molecular conductance switches. Chapter 2 describes the first single-molecule switch that operates through a stereoelectronic effect. We demonstrate this behavior in permethyloligosilanes with methylthiomethyl electrode linkers; the strong σ-conjugation in the oligosilane backbone couples the stereoelectronic properties of the sulfur-methylene σ-bonds that terminate the molecule. Chapter 3 describes the electric field breakdown properties of C-C, Si-Si, Ge-Ge, Si-O, and Si-C bonds. The robust covalent linkage that the methylthiol endgroup forms with the electrodes enables us to study molecular junctions under high voltage biases. Chapter 4 unveils a new approach for synthesizing atomically discrete wires of germanium and presents the first conductance measurements of molecular germanium. Our findings show that germanium and silicon wires are nearly identical in conductivity at the molecular scale, and that both are much more conductive than aliphatic carbon. Chapter 5 describes a series of molecular wires with π–σ–π backbone structures, where the π–moiety is an electrode–binding thioanisole ring and the σ–moiety is a triatomic α–β–α chain composed of C, Si, or Ge atoms. We find that placing heavy atoms at the α–position decreases conductance, whereas placing them at the β–position increases conductance. Chapter 6 demonstrates that silanes with strained substituent groups can couple directly to gold electrodes. We can switch off the high conducting Au-silacycle interaction by altering the environment of the electrode surface. These chapters outline new molecular design concepts for tuning conductance and incorporating switching functions in single–molecule electrical devices.

Graduation date: 1993

Primary silanes of the type RSiH$ sb3$, where R = phenyl (Ph), benzyl (Bz) and n -hexyl, undergo catalytic dehydrogenative polymerisation in the presence of catalytic amounts of Cp$ sb2$TiMe$ sb2$ (Cp = $ eta sp5$-C$ sb5$H$ sb5)$ to give linear oligomers consisting of about 10 silicon atoms, regardless of the reaction conditions. When the amount of Cp$ sb2$TiMe$ sb2$ was increased, novel organometallic complexes of the type $ {$(Cp$ sb2$Ti)$ sb2( mu$-H)$( mu$-HSiRH)$ }$ and $ {$Cp$ sb2$Ti$( mu$-HSiRH)$ } sb2$ were isolated and characterised. X-ray crystal structural characterisation has confirmed the first Ti-H-Si-Ti bridged systems. Other titanocenes (Cp$ sbsp{2}{ prime}$TiMe$ sb2$(Cp$ sp prime$ = $ eta sp5$-C$ sb5$H$ sb4$Me), (Cp$ sb2$TiH) $ sb{ rm n}$, (Cp$ sb2$Ti) $ sb2$ and Cp$ sb2$Ti(CO)$ sb2)$ also polymerised PhSiH$ sb3,$ and in addition gave analogous complexes under the appropriate conditions. Furthermore PhSiH$ sb3$ undergoes polymerisation in the presence of CpCp$ sp *$TiMe$ sb2$ (Cp$ sp *$ = $ eta sp5$-C$ sb5$Me$ sb5),$ but not Cp$ sbsp{2}{ *}$TiMe$ sb2.$ Polymerisation of RSiH$ sb3$ (R = phenyl and benzyl) also occurs in the presence of Cp$ sb2$ZrMe$ sb2$ and complexes of the type $ {$Cp$ sb2$(SiHMeR)Zr$( mu$-H)$ sb2$Zr(SiH$ sb2$R)Cp$ sb2 }$ have been identified, and isolated (R = phenyl). (Cp$ sb2$ZrH$ sb2$) $ sb{ rm n}$ also polymerises PhSiH$ sb3,$ with the associated production of $ {$Cp$ sb2$Zr$( mu$-H)(SiH$ sb2$Ph)$ } sb2.$ The analogous complex was also observed during the reaction of Cp$ sb2$ZrMe$ sb2$ with BzSiH$ sb3.$ The reactions have been followed by $ sp1$H and $ sp{29}$Si NMR, and possible reaction paths are discussed.

The work described in the two parts of this thesis is concerned with the development of two methods for the synthesis of substituted olefins. Both methods involve elimination reactions of (beta)-hydroxysilanes to form the double bond, therefore in the general introduction these reactions are discussed together with the methods available for the preparation of (beta)-hydroxysilanes ... [see pdf file for full abstract].

Thesis (Ph.D.)--McMaster University (Canada), 1992. / Source: Dissertation Abstracts International, Volume: 54-08, Section: B, page: 4153. Adviser: Adam P. Hitchcock.

Thesis (Ph.D.)--McMaster University, 2005. / Supervisor: William J. Leigh. Includes bibliographical references. Also available online.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1970. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliography.