Design of Carborane Molecular Architectures via Electronic Structure Computations

Quantum-mechanical electronic structure computations were employed to explore initial steps towards a comprehensive design of polycarborane architectures through assembly of molecular units. Aspects considered were (i) the striking modification of geometrical parameters through substitution, (ii) endohedral carboranes and proposed ejection mechanisms for energy/ion/atom/energy storage/transport, (iii) the excited state character in single and dimeric molecular units, and (iv) higher architectural constructs. A goal of this work is to find optimal architectures where atom/ion/energy/spin transport within carborane superclusters is feasible in order to modernize and improve future photoenergy processes

Boron in Catalysis and Materials Chemistry: A Themed Issue in Honor of Professor Todd B. Marder on the Occasion of His 65th Birthday

Boron, a metalloid with rich chemistry, continues to offer a diverse platform in designing novel catalysts and materials for applications in a variety of areas. This book, while celebrating Professor Todd Marder’s contributions to boron chemistry, on the occasion of his 65th birthday in November 2020, highlights and brings into focus some of the important discoveries in this field, through state-of-the-art reviews and research article

Synthesis of Carborane Pincer Complexes and Exploration Of The Reactivity of Metal-Boron Bonds

Exploration of transition metal complexes with metal-boron bonds for the cooperative bond activation of small molecules in catalysis is one of the recent developments in novel synthetic strategies. The use of boron clusters as a modular platform for the formation of metal-boron bond has been successfully utilized by our and other groups. Carboranes are icosahedral boron-carbon molecular clusters that have gained attraction as multifunctional ligands due to their unique geometry, electronic properties, and versatility of functionalization methods. Their 3-D structure exerts a steric bulk around the exohedral substituent, either metal or non-metal center. Furthermore, due to the cluster geometry, five of the cluster atoms are always in proximity with the exohedral metal center vertex resulting in the possibility of 3-center-2-electron B–H…M bridging interactions that support the 2-center-2-electron metal-boryl -bond. The use of rigid tridentate pincer ligands with carborane backbone and heteroatom donor groups allowed the synthesis of transition metal B-carboranyl complexes through the activation of relatively inert cluster B–H bonds. The unique geometry of the cluster with the pincer donor arms has resulted the highly strained, electron rich metal-boron bonds that have the potential to act as nucleophilic reaction centers in the bond activation of small molecules. Transition metal benzyne complexes have been intensively studied due to their ability of small molecule activation. The benzyne chemistry inspired the synthesis of the transition metal carboryne complexes, which can be considered as 3-D inorganic boron-based analogues of benzynes, in many cases similar reactivity. The highly strained and electron-rich metal-boron bonds in the three-membered (BB)\u3eRu metallacycle can act as nucleophilic reaction centers in bond activation of small molecules. In this dissertation, a novel synthetic stratergy was employed in synthesizing (BB)\u3eRu carboryne complexes via neutral ruthenium carborane dichloride complex as an intermediate, which allowed to install several classes of auxiliary ligands in trans- position to boryls: nitriles and phosphines. The replacement of strongly bound π-accepting carbonyls by these predominantly σ-donating and more labile ligands opened several new reactivity manifolds. New examples of the reactivity of the ruthenium carboryne bis(benzonitrile) complex in C-C bond activation of terminal alkynes, C-H bond activation of the phenyl group, N=N bond activation of azides and C-H and C-C bond activation of aldehydes wasstudied, in addition to the unique reactivity observed in ruthenium carboryne bis(diphenylphosphine) complex for the aldehydes. In addition, the presence of an exohedral 2-center-2-electron B-Ru bond and a 3- center-2-electron B-H…Ru bond in the same metal complex with the metal hydride resulted in catalytic activity in the transfer hydrogenation reaction. A series of boryl hydride complexes (POBOP)Ru(H)(L) [L=PPh3, PHPh2, PEt3] were synthesized, which were used as a platform for the assembly of heterometallic complexes. Functionalization of the carbon vertex of the carborane cluster is carried out using alkali metal-containing strongly basic reagents to generate carbon-centered nucleophiles. The careful utilization of metal chelating ligands to capture and remove the metal center from the metalated carbon vertex of the cluster generated strong carbon-centered nucleophiles with uncompensated negative charge on the carbon atom in these separated ion pairs. The alkali metal cation was separated from the deprotonated cluster using metalchelating crown ethers, which allowed us for the first time to isolate, structurally and spectroscopically characterize the highly nucleophilic “naked” 3-D carborane carbanion. Analogously, the doubly deprotonated 1,1′-bis(o-carborane) yielded similar dianionic “naked” carbanionic species serving as the first example of carbdianion

Synthesis and structural studies of metallacarboranes

The properties of some new metallacarboranes are described in this thesis, along with the results of a study to determine structural patterns in compounds published by others. Chapter 1 introduces heteroborane cluster compounds and a description of the bonding in these compounds. The synthesis and structure of supraicosahedral heteroboranes are discussed in detail with reference to literature examples throughout. A brief description of the trans influence is also given. Chapter 2 describes the synthesis and structures of a series of thirteen vertex indenyl cobaltacarboranes and a single fourteen vertex bimetallic indenyl cobaltacarborane. The crystallographically-determined orientation of the exo-polyhedral indenyl ligand in each compound is used to probe the relative strengths of the metal-carborane bonds. Rotation of a related exo-polyhedral ligand about 360o is explored computationally and the results used to help rationalise the orientations of the indenyl ligands. Chapter 3 describes the synthesis and structures of a single twelve vertex and a series of thirteen vertex naphthalene ruthenacarboranes. As for the isoelectronic indenyl cobaltacarboranes, the orientations of the naphthalene ligands are explored crystallographically and computationally. The details of some unexpected side products are also discussed. Chapter 4 is an analysis of the results of a crystallographic database search which showed clear structural patterns in previously published metallacarboranes. Cage atoms which are relatively weakly bonded to the metal tend to lie trans to exo-polyhedral ligands which are relatively strongly bonded to the metal. Chapter 5 describes the structures of thirteen and fourteen vertex bimetallic pentamethylcyclopentadienyl ruthenacarboranes with unconventional electron counts, which were synthesised by metallation followed by direct electrophilic insertion. The structures of an exo-polyhedral metal-bound species and three solvent-bound zwitterionic metallacarboranes prepared from attempted direct electrophilic insertion reactions are also presented. Chapter 6 gives the experimental procedures used to synthesise and purify each new compound and provides details of how they were characterised. Appendix 1 lists the crystallographic data relevant to each compound. Appendix 2 gives details of the literature structures found during the structural database search described in chapter four. Appendix 3 (electronic appendix - see CD-ROM) contains rtf and CIF files (crystallographic data) for all compounds (where available). Appendix 4 (electronic appendix - see CD-ROM) contains CIF and mol2 files for all literature compounds discussed in chapter four.ESPR

FILM STABILIZAITON AND PHOTOPHYSICS OF UNCONVENTIONAL CONJUGATED POLYMERS

Conjugated polymers offer a unique opportunity to develop high performing, flexible, lightweight, and large area electronic devices. With advances in conjugated polymer understanding and synthesis, the use of polymers as active layer materials in electronic applications, rather than just substrate materials, has become more promising. However, defects in morphological stability, as well as imperfect electronic understanding, are still present, limiting the use of these materials in commercializable electronics. Fundamental understanding of structure-property relationships can allow for facile synthetic solutions to major drawbacks of conjugated polymer integration in standard device architectures. Chapter 1 presents background research on the history of conjugated polymer development and the electronic device architectures these materials are typically incorporated in. Chapter 2 presents the use of thiol-ene chemistry to stabilize poly(3-alkylthiophene) films through a grafting-to procedure. This offers a simple way to produce highly oriented, insoluble, semi-conducting films through facile synthetic tuning of the polymers end-groups and side chains. In Chapter 3, the photophysics of carborane containing poly(dihexylfluorene) polymers is discussed. These unique class of materials experience drastic solvatochromism making them highly coveted for simple sensing applications. Through extensive spectroscopic investigations, a complete understanding of the excited-state dynamics is presented. Chapter 4 extends on Chapter 3 by demonstrating a straightforward method to synthesize carborane containing poly(dihexylfluorene)s with emissive properties that change with fluorene conjugation length, allowing for emission color tuning of the polymer solid and solution states. Lastly, Chapter 5 presents the synthesis of novel poly(bisthienyl carborane) and poly(bisthienyl carborane-alt-thiophene) is presented. This is the first example of a soluble conjugated polymer implementing a strong donor aromatic group and a strong acceptor carborane junction in the repeat unit. The fascinating excited-state characteristics are determined through femtosecond and nanosecond spectroscopy, showing the polymer can promote triplet formation on the carborane unit, making it useful for triplet sensitization applications. The work presented in this thesis shows that straightforward synthetic techniques can be used to highly affect the properties of common conjugated polymers, making them more robust or beneficial for electronic applications. Side-chain and backbone engineering is a necessary technique for furthering the development of useful and applicable “plastic” electronics

Bis(carboranes) in catalysis : from chelation to metalation

Chapter 1 is an overview of the chemistry of carboranes and their reactivity with particular focus on metalation and use in catalysis. The chemistry of bis(carboranes) is also discussed in detail. A full scope of thesis is also included. Chapter 2 describes the development of synthetic routes using bis(o-carborane) as a chelating ligand to a ruthenium centre. Novel compounds are characterised and suitable species are tested as catalysts in Diels-Alder cycloaddition reactions. Chapter 3 expands on the synthesis of new catalysts exploring substitution reactions on the cages of bis(o-carborane). Octamethyl-bis(o-carborane) is synthesised and used as a ligand to a metal centre. One product is tested as a catalyst in Diels-Alder cycloaddition reactions. Chapter 4 discusses the displacement of arene ligands from arene-ruthenium derivatives of bis(o-carborane) using phosphine and phosphite ligands. This resulted in an experimental and computational study of a novel isomerisation process. The new characteristics and reactivity of these compounds are discussed and suitable compounds are tested as catalysts in Diels-Alder cycloaddition reactions. Chapter 5 introduces bis(m-carborane) as a different type of ligand to a metal centre. The metalation of bis(m-carborane) is discussed using different metal fragments. Chapter 6 outlines the experimental procedures for all new compounds discussed, including synthesis and analysis. Appendix A explores the use of bis(o-carborane) as a ligand on Au(III) metal centres with a comprehensive study of novel compounds. Appendix B summarises the crystallographic data for all compounds studied by XRD in this thesis

Transition metal complexes of bis(carboranes) and their potential as catalyst precursors

Chapter 1 provides a short overview of heteroborane chemistry with particular attention on the areas of metallacarborane catalysts and bis(o-carborane). It also includes a more detailed scope of the thesis. Chapter 2 reports the first examples of heterometalated bis(carboranes) using cobalt and ruthenium, specifically their preparation and characterisation. Tangential to this is the isolation of a novel 13-vertex/12-vertex species, the former of which possesses the first crystallographically-determined 4,5,1,12-M2C2B9 architecture (where M = CoCp*). Chapter 3 expands the range of heterometalated bis(carboranes) to include those with the catalytically-active rhodium fragment {Rh(PPh3)2H}. Additionally, the first example of a crystallographically-characterised rhodacarborane/carborane species is reported. Chapter 4 discusses the catalytic activity of the rhodacarborane compounds in alkene isomerisation and ketone hydrosilylation. The results of preliminary DFT calculations into the possible mechanisms of these catalytic reactions are also included. Chapter 5 contains the experimental procedures and spectroscopic details for all novel compounds discussed. Appendix A explores the use of phosphites as ligands in rhodacarboranes and Appendix B details the preliminary results in the deboronation/metalation of bis(m-carborane). Appendix C summarises the crystallographic tables of all compounds studied by XRD in this thesis

The search for hypercloso metallacarboranes through ligand manipulation

Chapter one is an introduction to carborane and metallacarborane chemistry, with a particular focus on “non-Wadian”, including hypercloso, heteroboranes. The chapter then discusses the various methods that could be used to synthesise hypercloso boranes and heteroboranes before identifying generating hypercloso metallacarboranes through ligand set manipulation as the objective of this project. Chapter two contains a discussion of attempts to synthesise hypercloso molybdacarboranes through ligand abstraction. A reaction between the species [NEt4][1,7-Me2-2,2,2-(CO)3-2-I-closo-2,1,7-MoC2B9H9] and Ag[BF4] was found unexpectedly to regenerate 1,7-Me2-2,2,2,2-(CO)4-closo-2,1,7-MoC2B9H9 through a process referred to as carbonyl stealing. Further related experiments also produced results consistent with carbonyl stealing. In chapter three efforts are instead directed at synthesising hypercloso ruthenacarboranes. In the process two new routes to closo ruthenacarboranes were developed. The first leads to anions of the general formula [1,2-R2-3-Cl-3,3-(PPh3)2-closo-3,1,2-RuC2B9H9]- whilst the second (which follows from this) was used to synthesise a diverse family of mixed ligand ruthenacarboranes. Attempts to synthesise hypercloso ruthenacarboranes instead produced highly complex product mixtures from which several unusual “wedged” species were isolated. Chapter four describes efforts directed towards synthesising hypercloso ruthenacarboranes using the same methodology as in chapter three but by also exploiting steric factors. This once again resulted in complex product mixtures from which some highly unusual species were isolated, including an additional “wedged” species as well as two “symbiotic” species, one of which is without precedent in the literature. Chapter five contains a summary of the research contained within this thesis whilst chapter six contains experimental details for all of the novel compounds prepared as well as alternative or improved syntheses for some literature compounds