published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

An atomically thin film of semiconducting transition metal dichalcogenides (TMDCs) is emerging as a class of key materials in chemistry and physics due to their remarkable chemical and electronic properties. The TMDCs are layered materials with weak out-of-plane van der Waals (vdW) interaction and strong in-plane covalent bonding enabling scalable exfoliation into two-dimensional (2D) layers of atomic thickness. The growth techniques to prepare these 2D TMDC materials in high yield and large scale with high crystallinity have attracted intensive attention recently because of the new properties and potentials in nano-elctronic, optoelectronic, spintronic and valleytronic applications. In this thesis, I develop methods for the chemical synthesis of 2D TMDCs films. The relevant growth mechanism and material characteristics of these films are also investigated. Molybdenum disulfide (MoS2) is synthesized by using molybdenum trioxide (MoO3) and sulfur (S) powder as the precursor. The films are formed on substrate pre-treated with reduced graphene oxide as the catalyst. However, this method cannot be extended to other TMDC materials such as molybdenum diselenide (MoSe2) and tungsten diselenide (WSe2) because reduced graphene oxide (rGO) reacts with selenium to form alloy materials rather than TMDC films. At the same time, the conversion of MoO3 to MoSe2 or that of tungsten trioxide (WO3) to WSe2 without the assistance of hydrogen in the chemical reaction is not thermodynamically feasible because the oxygen in the metal oxide cannot be replaced by selenium due to lower reactivity of the latter. On the other hand, I demonstrate that MoSe2 film can be synthesized directly by using MoSe2 and Se powder. Furthermore, the method of sulfurization or selenization of pre-deposited metal film can be promising due to precise thickness/size controls. Finally, some perspectives on the engineering challenges and fabrication methods of this family of materials will be given. / published_or_final_version / Physics / Master / Master of Philosophy

Graduation date: 2003

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

Chong Kim Hung. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2005. / Includes bibliographical references. / Abstracts in English and Chinese. / Table of contents --- p.vi / Acknowledgements --- p.i / Abstract --- p.ii / 摘要 --- p.iv / List of Compounds --- p.ix / Synthesized / Abbreviations --- p.x / Chapter Chapter 1 --- Reactivity of Pyridyl-1-azaallyl Enamido Germanium(II) Chloride / Chapter 1.1 --- Introduction --- p.1 / Chapter 1.1.1 --- General Aspects of Reactivity of Heteroleptic Germylenes --- p.1 / Chapter 1.1.2 --- Synthesis of Pyridyl-1 -azaallyl Germanium(II) Chloride Complex --- p.10 / Chapter 1.1.3 --- Objectives of This Work --- p.12 / Chapter 1.2 --- Results and Discussion --- p.14 / Chapter 1.2.1.1 --- Synthesis of Chalcogenonyl Halide Complexes --- p.15 / Chapter 1.2.1.2 --- Spectroscopic Properties of 33 and 34 --- p.15 / Chapter 1.2.1.3 --- "Molecular Structures of [Ge(E){N(SiMe3)C(Ph)- C(SiMe3)(C5H4N-2)}Cl] (E = S (33), Se (34))" --- p.16 / Chapter 1.2.2.1 --- Synthesis of Group 11 Transition Metal-Pyridyl-1- Enamido Germanium(II) Chloride Complexes --- p.20 / Chapter 1.2.2.2 --- Spectroscopic Properties of 35 and 36 --- p.21 / Chapter 1.2.2.3 --- Molecular Structures of [Ge(CuI){N(SiMe3)- C(Ph)C(SiMe3)(C5H4N-2)}Cl(THF)2]4 (35) and [Ge(AuI){N(SiMe3)C(Ph)C(SiMe3)(C5H4N-2)}Cl] (36) --- p.22 / Chapter 1.2.3.1 --- "Reaction of Pyridyl-l-azaallyl Germanium(II) Chloride with 3,5-di-tert butyl-o-benzoqumone: Synthesis of [Ge{O(2,4-di-Bu'-C6H2)O}{N(SiMe3)C(Ph)C(SiMe3)- (C5H4N-2)}C1] (37)" --- p.27 / Chapter 1.2.3.2 --- Spectroscopic Properties of 37 --- p.27 / Chapter 1.2.3.3 --- "Molecular Structure of [Ge{0(2,4-di-Bu'- C6H2)O} {N(SiMe3)C(Ph)C(SiMe3)(C5H4N-2)}Cl] (37)" --- p.28 / Chapter 1.2.4.1 --- Synthesis of Boron-Germanium(II) Hydride Adduct --- p.31 / Chapter 1.2.4.2 --- Spectroscopic Properties of 38 --- p.31 / Chapter 1.2.4.3 --- Molecular Structure of [Ge(BH3){N(SiMe3)C(Ph)- C(SiMe3)(C5H4N-2)}H] (38) --- p.32 / Chapter 1.2.5.1 --- Substitution Reaction of Pyridyl-l-azaallyl Germanium(II) Chloride with Lithium Phenylacetylide --- p.34 / Chapter 1.2.5.2 --- Spectroscopic Properties of 39 --- p.34 / Chapter 1.2.5.3 --- Molecular Structure of [Ge{N(SiMe3)C(Ph)C(SiMe3)- (C5H4N-2)}(CCPh)] (39) --- p.35 / Chapter 1.2.6.1 --- Reaction of Pyridyl-l-azaallyl Germanium(II) Chloride with excess lithium; the formation of [GeC(Ph)C(SiMe3)(C5H4N-2)]2 (40) --- p.38 / Chapter 1.2.6.2 --- Spectroscopic Properties of 40 --- p.38 / Chapter 1.2.6.3 --- Molecular Structure of [GeC(Ph)C(SiMe3)(C5H4N-2)]2 (40) --- p.39 / Chapter 1.3 --- Experimental for Chapter 1 --- p.43 / Chapter 1.4 --- References for Chapter 1 --- p.50 / Chapter Chapter 2 --- Synthesis of Late Transition Metal Pyridyl-l-azaallyl Complexes / Chapter 2.1 --- Introduction --- p.55 / Chapter 2.1.1 --- General Aspects of 1 -azaallyl Metal Complexes --- p.55 / Chapter 2.1.2 --- Synthesis of Pyridyl-l-azaallyl Metal Complexes --- p.61 / Chapter 2.2 --- Results and Discussion --- p.68 / Chapter 2.2.1 --- Synthesis of Late Transition Metal Pyridyl-l-azaallyl Complexes --- p.68 / Chapter 2.2.2 --- Spectroscopic Properties of 55-59 --- p.70 / Chapter 2.2.3 --- Molecular Structures of Compounds 55-59 --- p.71 / Chapter 2.3 --- Experimental for Chapter 2 --- p.80 / Chapter 2.4 --- References for Chapter 2 --- p.83 / Appendix I / Chapter A. --- General Procedures --- p.86 / Chapter B. --- Physical and Analytical Measurements --- p.86 / Appendix II / Table A.1. Selected Crystallographic Data for Compounds 33-36 --- p.89 / Table A.2. Selected Crystallographic Data for Compounds 37-40 --- p.90 / Table A.3. Selected Crystallographic Data for Compounds 56-58 --- p.91 / Table A.4. Selected Crystallographic Data for Compound 59 --- p.92

This thesis deals with the applications of organopalladium and organochromium chemistry to the functionalisation of the benzopyran ring system, at a variety of oxidation levels. Section I demonstrates the functionalisation of 3-, 6-, and 8-bromochromones via palladium (0) insertion into the C-Br bond. The resultant arylpalladium species are shown to undergo addition to the least substituted end of a variety of olefins including methyl acrylate, acrylonitrile and styrene. Subsequent palladium-hydride elimination leads to overall palladium catalysed vinylation of the chromone and the synthesis of a number of novel compounds. Vinylation occurs regiospecifically at the site of chromone bromination and is shown to allow clean substituent introduction into each of the three sites. The palladium catalysed reaction of 3,6-dibromo-chromone with methyl acrylate leads to vinylation at both the C3 and C6 positions. Carbonylation of the 6-bromochromone in ethanol or butanol leads to the 6-ethyl or 6-butyl esters respectively. The palladium catalysed vinylation of the 6-bromochromone with ethyl vinyl ether leads to a mixture of products from addition of the chromone to either end of the olefin. With p-bromophenol or p-bromo-N,N-dimethylaniline the reaction gives exclusively the acetylated product arising from addition to the more substituted end of the olefin. This change in orientation is rationalised by considering the polarisation of the olefin and the arylpalladium species. Section II demonstrates the functionalisation of chroman and 4-chromanol via coordination to the Cr(CO)₃ moiety. (η⁶-Chroman)Cr(CO)₃ is synthesised and is shown to undergo regiospecific ring deprotonation at C8 under kinetic conditions or regiospecific benzylic deprotonation at C4 under thermodynamic conditions. The resultant anions are quenched with alkyl halides, aldehydes, Eschenmoser's salt and methyl disulphide resulting in selective functionalisation of either site. No mixed products are observed. The uncomplexed arene is shown to be totally unreactive under identical conditions. (η⁶-4-Chromanol)Cr(CO)₃ is synthesised and is shown to undergo regiospecific C8 ring deprotonation by comparison with authentic samples of the C5 and C8 methylated alcohols. Protection of the hydroxyl group as its methyl, t-butyldimethylsilyl or methoxymethyl ethers is found not to alter the regiochemistry of deprotonation. The 4-chromanol t-butyldimethylsilyl and tri-i-propylsilyl ethers are synthesised and coordinated to the metal unit. Cleavage of the silyl ethers is shown to proceed with loss of stereochemistry, indicating C-0 bond cleavage.

published_or_final_version / abstract / Chemistry / Doctoral / Doctor of Philosophy

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

Ph.D. / This thesis deals with the synthesis of nitrogen-donor compounds and their reaction with metal ions. The first type of nitrogen-donor compounds are the unconjugated diimines (N,N´-bis(diphenylmethylene)ethylenediamine (L1) and (N,N´-bis(diphenylmethylene)propylenediamine (L2). Compounds L1 and L2 were reacted with [NiBr2(DME)] or [NiCl2·6H2O] to form complexes (2.1a), (2.2a), (2.3a) and (2.4a). These nickel complexes were characterized by IR spectroscopy, elemental analysis and mass spectrometry. When the complexes were left in chloroform for prolonged periods, hydrolysis of the diimine ligand took place, leading to the formation of nickel complexes 2.1b, 2.2b, 2.3b and 2.4b. The identity of the hydrolysed nickel complexes 2.1b and 2.2b was confirmed by single crystal X-ray crystallography. Complex 2.1b crystallised in the P21/n space group, whilst 2.2b crystallised in the P-1 space group. Compounds L1 and L2 were also reacted with [PdClMe(MeCN)2] to form the palladium complexes (3.1) and (3.2). The palladium complexes were characterized by NMR spectroscopy, elemental analysis and single crystal X-ray crystallography. Attempts to recrystallize 3.1 from a dichloromethane solution led to the formation of 3.1a. Both complexes 3.1a and 3.2 crystallised in the P21/n space group. Complexes 3.1 and 3.2 were tested as catalysts for the Heck coupling reaction of iodobenzene with methyl acrylate or butyl acrylate at 80 C. The products from the coupling reactions were characterized by GC and NMR spectroscopy. These complexes were found to be highly active with 100% conversions observed in some instances. The second type of ligands that were prepared are the benzoylpyrazolyl compounds, (3,5-dimethylpyrazol-1-yl)phenylmethanone (C1), (3,5-ditertiarybutylpyrazol-1-yl)phenylmethanone (C2), (3,5-dimethylpyrazol-1-yl)-o-toluoylmethanone (C3), (3,5-ditertiarybutylpyrazol-1-yl)-o-toluoylmethanone (C4), (2-chlorophenyl)-(3,5-dimethylpyrazol-1-yl)methanone (C5), (2-chlorophenyl)-(3,5-ditertiarybutylpyrazol-1-yl)methanone (C6), (2-flourophenyl)-(3,5-dimethylpyrazol-1-yl)methanone (C7), (2-flourophenyl)-(3,5-ditertiarybutylpyrazol-1-yl)methanone (C8). These compounds were fully characterized using NMR spectroscopy, IR spectroscopy and elemental analysis. Compounds C1, C3, C5 and C7 were reacted with [NiBr2(DME)] to form nickel complexes (4.31-4.34). These nickel complexes were found to be insoluble in all common organic solvents and hence were characterized only by IR spectroscopy and elemental analysis. Compounds C1-C8 were also reacted with [PdCl2(MeCN)2] to form palladium complexes (4.35-4.42). Complexes 4.35-4.42 were characterized using NMR spectroscopy, IR spectroscopy, elemental analysis and in selected cases single crystal X-ray crystallography. Complex 4.39 crystallised in the C2/n space group and complex 4.42 crystallised in the P21/n space group. Attempts to recrystallize 4.37a led to the formation of 4.37b, which contains both 3,5-dimethylpyrazol-1-yl)-o-toluoylmethanone and 3,5-dimethylpyrazole as ligands. Complex 4.37b was confirmed by NMR spectroscopy and single crystal X-ray crystallography. Complex 4.37b crystallised in the Pbca space group. The formation of 4.37b is attributed to hydrolysis of 3,5-dimethylpyrazol-1-yl)-o-toluoylmethanone ligand in 4.37a due to the presence of adventitious water in the solvent. The palladium complexes (4.35-4.42) were tested as catalysts for the Heck coupling reaction of iodobenzene with butyl acrylate and also for the Suzuki coupling reaction of iodobenzene with phenylboronic acid or 4-chlorophenylboronic acid. In these reactions, complexes 4.35-4.42 were found to be highly active at 120 C. The pyrazolyl nickel and palladium complexes were further tested as catalysts in ethylene oligomerization reactions using EtAlCl2 as the co-catalyst. The nickel complexes were found to be the most active reaching TONs of 10.8105 g mol-1 h-1. The palladium analogues only gave TONs of up to 3.9105 g mol-1 h-1. The oligomers were characterized by GC and NMR spectroscopy and were found to be in the C10-C16 range, with C16 the most abundant olefin.

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