Synthetic and Computational Investigations of Highly Reduced Main Group Organometallics of Groups XIII and XIV

Abstract

This thesis concerns the fundamental understanding of low-valent compounds of Groups 13 and 14 through a combination of synthetic and computational investigations. Low-valent main-group species continue to challenge conventional descriptions of chemical bonding and reactivity, displaying unusual electronic structures and reaction pathways. Despite advances in the field, many fundamental questions concerning the stabilisation, electronic structure, and particularly mechanisms of formation and reactivity of these compounds remain unresolved. The work presented herein seeks to address several such questions through the synthesis of novel compounds, detailed electronic structure analyses, and extensive mechanistic investigations using density functional theory. Chapter 1 provides an introduction to low-valent main group chemistry and the computational methods employed throughout this thesis. Particular emphasis is placed upon the concepts of valency, electronic and steric stabilisation, density functional theory, and the interpretation of potential energy surfaces relevant to reaction mechanism elucidation. Chapter 2 presents two acridanide ligands, together with their alkali-metal salts and germanium(II) and tin(II) precursor complexes, were synthesised and characterised. Reduction chemistry involving simple alkali-metal reductants and a magnesium(I) dimer produced a variety of unusual products, including the first dimagnesiogermane. DFT was subsequently employed to investigate the electronic structure and mechanism of formation of this species, revealing a reductive-addition process across an Mg-Mg bond and providing insight into the formation of low-valent Group 14 compounds. Chapter 3 concerns the formation of a hexahydridosilicate dianion and its conversion into the parent silylene dianion. Benchmarking against experimental structural and spectroscopic data allowed the development of density functional methodologies for low-valent chemistry. Electronic structure calculations and kinetic modelling were then used to examine this transformation wherein the hexahydridosilicate loses two equivalents of dihydrogen to form the SiH2 dianion trapped within the molecular assembly. DFT revealed a reaction pathway consistent with experiment and an unexpected autocatalytic process as the origin of the observed reactivity. Chapter 4 investigates oxygen-extrusion reactions observed during attempts to prepare low-valent boron compounds. Exploration of the potential energy surfaces using DFT showed that closed-shell pathways were inaccessible, whereas open-shell pathways involving boryl radicals were consistent with both computational predictions and experimental observations. The resulting mechanism provides an explanation for an unusual reductive skeletal-editing process involving oxygen deletion and ring contraction. Chapter 5 briefly presents two computational studies involving aluminyl compounds. Mechanisms for the 1,4-bis activation of benzene by monomeric and dimeric aluminyl anions are examined and compared with experiment. In addition, the steric and electronic properties of a series of iron-aluminium compounds are investigated using computational methods for quantifying ligand environment and accessibility, as is presented in the two associated publications. Collectively, the investigations described throughout this thesis demonstrate how synthetic chemistry and modern computational methods can be used in concert to explain and predict low-valent main-group chemistry. The work expands the known chemistry of low-valent compounds of Groups 13 and 14 through the synthesis and characterisation of new molecular species, investigation of unusual electronic structures, and the elucidation of reaction mechanisms that are otherwise experimentally inaccessible. These studies contribute to the growing understanding of how low-valent main-group compounds activate chemical bonds and access otherwise challenging reactivity. Show more