The diversity of transition metal complexes allows for a wide range of chemical processes to be mediated by the metal, from catalysis to surface chemistry. Investigations into the structure and electronic configuration of transition metal complexes allow for tuning of desired species by modifications to the ligands and/or metals to achieve more efficient thermodynamics and kinetics for the process of interest. Transition metals, often used in catalysts for a number of important processes, require detailed descriptions of intermediates, transition states and products to fully characterize a reaction mechanism(s) in order to design more active and efficient catalysts. Computational investigations into inorganic catalysts are explored with the aim of understanding the activity of each species and how modifications of supporting ligands, co-ligands and metals vary the interaction along the reaction pathway. Reported results give important insight into the development of the most active complexes in addition to determining the least active complexes to aid experimental development. This report first investigates the mechanisms of two unique transfer reactions: 1) formation of low coordinate nickel-nitrene ((P~P)Ni=NR; P~P = 1,2-bis(dihydrophosphino)-ethane or 1,2-bis(difluoromethylphosphino)-ethane) complexes as catalysts for nitrogen atom transfer and 2) oxidation of a triphosphorus niobium complex, [(η2-P3SnPh3)Nb(OMe)3], for the transfer of the phosphorus synthon, Ph3SnP3. These reactions have utility in the synthesis of nitrogen and phosphorus containing molecules, respectively, and the results presented provide mechanistic insight into the synthesis of the organometallic intermediates. Additionally, a computational approach towards rational catalyst design was performed on the ruthenium based hydroarylation catalyst TpRu(CO)(Ph) [Tp = hydrido-tris(pyrazolyl)borate]. Targeted modifications at the Tp, metal and co-ligand (CO) sites were studied in order to tune the electronics and sterics of the catalyst. Modifications, through computational methods, provided a more cost- and time-efficient way to study the impact of modifications, which provided direct input into attractive synthetic targets. The research described heir in highlights the use of computational chemistry methodologies, specifically DFT, in collaboration with experimental results, for the accurate description of reaction geometries and factors influencing the thermodynamics and kinetics of the systems. Valuable insight is gained by treating inorganic complexes with theoretical methods and additionally provides a fast, cheap way to predict and understand the chemistry of such complex systems.

The synthesis of transition metal dichalcogenides (TMDs) are crucial to realization of their real-world applications in electronic, optoelectronic and chemical devices. However, the fabrication yield in terms of material quality, crystal size, defect density are poorly controlled. In this work, by employing the up-to-date stack-and-transfer and nano fabrication techniques, synthetic TMDs that obtained from different growth methods with various crystal qualities were studied. In most of the cases, better crystals with lower defect densities and larger crystal domain sizes are preferred. Self-flux method was developed to obtain better quality crystals comparing to the traditional chemical vapor transport, as characterized by lower defect densities. BN encapsulating graphene device platform was utilized and TMDs monolayers with different defect densities was inserted in between the BN/graphene interface, where intrinsic defects from the TMDs disturbed the electronic environment of graphene. With the better TMD crystal insertion, we obtain much better electrical performed device in terms of hysteresis, FWHM of Dirac peak and electron mobility. This device also showed advantage in quantum transport measurements . On the other hand, the presence of defects are not always undesired, especially when it comes to serve as electrocatalysts, in which most of the reactions take place at vacancy sites. However, similar to most of the MoS2 electronic devices, forming barrier-free metal semiconductor contact is the major challenge. We develop a platform that contact resistance could be monitored simultaneously with electrochemical activity. In this platform, the total device resistance is significantly reduced before electrochemical reaction happens while the intrinsic catalytic activity of the MoS₂ can be extracted. With this platform, we found the intrinsic catalytic activity of MoS₂ strongly correlated to H-coverage on its surface. By adding molecular mediator into electrolytes, H-coverage and the resulting HER activity was enhanced via “Catch and Release” mechanism. Molecular simulation was performed to support our experimental results.

Ligand design in general enables the formation of coordination compounds with multiple functionalities within a single framework. To date, two of the most widely studied ligands are 2,2′:6′,2′′-terpyridine (terpy) and acetylacetone (acac), whose tridentate and bidentate coordination pockets, respectively, enables the formation of metallic complexes with various geometries. The Brusso group had been incorporating imidoyl amidine (ImAm) ligands to build different materials such as organic radicals and fluorescent materials. In particular, the ligands N-2-pyridylimidoyl-2-pyridylamidine (Py2ImAm) and N-2-pyrimidylimidoyl-2-pyrimidylamidine (Pm2ImAm) were recently synthesized and have great appeal to build metallic complexes, as they poses two coordination sites similar to those in terpy and acac. The work presented herein represents the first studies involving the coordination of Py2ImAm and Pm2ImAm as discrete ligands. Our results demonstrate the versatility of these ligand frameworks, in which discrete mononuclear complexes, homometallic and heterometallic polynuclear complexes may be realized. Chapter one serves as a brief introduction to transition metal chemistry and has a comprehensive review of the coordination chemistry of the ImAm ligand framework. In chapter two, the selective coordination of first row transition metals into the bidentate or tridentate sites of Py2ImAm is explored. The formation of these mononuclear complexes is acid-base driven, where a weak acid induces coordination to the tridentate site and a weak base leads to coordination in the bidentate site. Coordination to both sides of Pm2ImAm with manganese or iron is explored in chapter three. The results show the formation of unusual tetranuclear complexes with the metal ions in both low spin and high spin configurations. Chapter four covers the coordination to cobalt, and the formation of polynuclear complexes with different geometries using Pm2ImAm. The magnetochemistry of these cobalt polynuclear complexes is also presented and reveal a single molecule magnet behaviour for one of the complexes. Finally, in chapter five, a one-pot synthesis of copper-manganese heterometallic complexes is presented. Overall, these imidoyl amidine ligands are able to build complexes with different geometries, different electronic configurations (i.e. low or high spin), and different metal ions. These results show a great versatility of ImAm ligands and suggest the future use of these ligands by other research groups.

Over the last two decades, two-dimensional (2D) materials such as graphene and transition metal dichalcogenides (TMDCs) have invoked tremendous interest of the scientific community due to their unique electronic and optical properties. While TMDCs hold great promise as a potential replacement for silicon for scaling transistors beyond sub-3 nm technology node, graphene holds great potential as transparent electrodes and optical phase-modulators for next-generation photonic devices. In addition to the aforementioned applications, these 2D devices also provide a great platform for studying novel physical phenomena associated with 2D materials such as Moiré interactions, valley-dependent spintronics, and correlated electron physics. In order to realize high-performance 2D material based devices, advancement of three key aspects are imperative - (1) analytical modeling to gauge insights into the electrostatics and current transport in 2D devices, (2) development of efficient techniques for fabricating 2D devices, and (3) understanding the fundamental limitations of the existing characterization techniques and developing better methods. We started by modeling the unique electrostatics of the 2D lateral p-n junctions, wherein we developed analytical expressions for the electric field, electrostatic potential, and depletion width across 2D lateral p-n junctions. We extend these expressions for use in lateral 3D metal-2D semiconductor junctions and lateral 2D heterojunctions. The results show a significantly larger depletion width (~ 2 to 20x) for 2D junctions compared to conventional 3D junctions. Further, we show that the depletion widths at metal-2D semiconductor junctions can be significantly modulated by the surrounding dielectric environment and semiconductor doping density. Finally, we derived a minimal dielectric thickness for a symmetrically-doped 2D lateral p-n junction, above which the out-of-plane simulation region boundaries minimally affect the simulation results. After electrostatics, we attempted to understand the current transport in 2D material-based devices. Typically used back-gated field-effect transistors (BGFETs) are often modeled as Schottky barrier (SB)-MOSFETs assuming that the current flow is limited by the source-contact in the OFF state, while the channel limits the current in the ON state. Here, using an analytical model and drift-diffusion simulations, we show that the channel limits the overall current in the OFF state and vice versa, contrary to past studies. For top-contacted BGFETs, we modeled different current paths at a top-contacted metal-2D semiconductor junction and illustrated the unique “corner effect”—where the potential change and current transport are dominated by the metal-2D semiconductor edge and the associated lateral region. We determined that the edge transport supersedes the vertical current injection in monolayer TMDCs and hence, to reduce contact resistance in 2D devices degenerate doping of channel region next to contact regions is of paramount importance. After developing models to theoretically analyze these devices, we focused on understanding the shortcomings in the existing characterization techniques affecting the extraction of important device parameters such as contact resistance, SBH, and channel mobility. We prove that the transfer length estimated using the standard TLM measurement techniquecan severely overestimate the true transfer length. We also discuss the large discrepancy in SBH values extracted using the Arrhenius method compared to their theoretical values. Using our analytical modeling, we attribute this to the presence of long channel regions in experimental devices. Furthermore, we highlight that the presence of large contact resistance results in underestimation of channel mobilities which renders Kelvin measurements such as four-probe and Hall-bar measurements imperative for 2D devices. Finally, we introduced a unique etch and doping method using self-limiting oxidation which allows us to design and fabricate various high-performance 2D devices. We first used the method to demonstrate a selective, damage-free atomic layer etch (ALE) that enables layer-by-layer removal of monolayer WSe₂ without altering the physical, optical, and electronic properties of the underlying layers. Using a comprehensive set of characterization techniques, we show that the quality of our ALE processed layers is comparable to that of pristine layers of similar thickness. Further, using graphene as a testbed, we demonstrate the use of a sacrificial monolayer WSe₂ layer to protect the channel, which is etched in the final process step in a technique we call Sacrificial WSe₂ with ALE Processing (SWAP). Furthermore, the top oxidized layer acts like an atomically thin degenerate p-type dopant for a large variety of semiconductors such as graphene, carbon nanotubes, and WSe2. We show that the TOS-doped graphene yields a low sheet resistance due to high mobility at a very high hole density that remains active even at 1.5 K. We apply this principle to improve the transmittance of graphene (>99%) at telecommunication bandwidth (1.5 to 1.6 𝜇m), that makes it a suitable replacement for Indium tin oxide (ITO) as a transparent electrode.

The study involves synthetic approaches towards the preparation of novel NHC complexes of low valent rhenium and manganese transition metals. Diverse methods of synthesis were studied. The direct approach, in which the ylidene obtained from deprotonation of 1,3-bis(2,4,6- trimethylphenyl)imidazolium chloride was added to the metal substrate, proved to be unsuccessful as isolation of the free carbene should rather be performed in an argon filled glove box under extreme inert conditions. By way of further investigation the ylidene was prepared by in situ methods and then quenched with the metal substrate. Different bases for deprotonation purposes and reaction conditions were explored. All routes employed were investigated and compared using group VII transition metal substrates Re(CO)5Br, Mn(CO)5Br, Re2(CO)10 and Mn2(CO)10. Isolation and purification of these products proved to be very challenging due to the insolubility in some organic solvents with consequent problematic spectroscopic analyses of the complexes. The tendency of the products to undergo various side reactions is observed in all reactions. Specifically, hydrolysis of the imidazolium ligand, followed by vinyl formation, yielded the mesitylformamide compound (3). The results obtained for some of the monometal substrates indicated that the target complexes were formed but could not be isolated. However, the synthesis route employing deprotonation by nBuLi as base and [Mn2(CO)10] as dimetal substrate lead to the isolation of the target dinuclear complex [Mn2(CO)9IMes] (9). Other novel complexes obtained during the course of this study include the biscarbene tetrarhenium complex [Re2(CO)9.C(OEt)C4H2OC(OEt)Re2(CO)9] (12) and various side reaction products. In many cases, metal-metal bond cleavage and carbonyl insertion was observed, as is evident in the complex IMesH[ReO4] (6) and ketene product (13). Structural and theoretical studies were performed to investigate the bond character between the carbene ligand and the metal. / Dissertation (MSc)--University of Pretoria, 2010. / Chemistry / unrestricted

No description available.

The investigation of N,N'bis(salicylidene)-1,7-diamino-4-azaheptane, SALLDIPN, N,N'bis(salicylidene)-1,5-diamino-3-azaheptane, SALDIEN, and N,N'bis(salicylidene)-1,5-diamino-3-thiopentanane, SALUAES, as pentadentate ligands is logical in that they offer an analogy to the environment around the cobalt atom in Vitamin B₁₂, I [see Figure I], represented below. These complexes might be expected to give some insight as to the behavior and role of the metal ion as it interacts with the five-donor atoms and hence an insight into the role of the metal ion in Vitamin B₁₂. [See Figure I] Compounds formed by the reaction of Ni(XSAL)₂·2H₂0 with 1,5-diamino-3-azaheptane, DlEN, and Ni(OAc)₂·4H₂O with X-SALDAES were isolated and characterized. In addition to these, alkyl σ-bonded derivatives formed by the reaction of reduced Co(SALDIPN) with R-X were also isolated and characterized. Mass spectra, magnetic moments; X-ray powder patterns, infrared, visible, and near infrared spectra were obtained on the majority of the compounds. Nuclear magnetic resonance spectra were obtained on the majority of the σ-bonded alkyl complexes. It was concluded that the Ni(X-SALDIEN) complexes are square planar whereas the Ni(X-SALDAES) complexes are distorted square planar or possibly a five-coordinate species. Spin state changes were observed for both complexes when they were dissolvea in pyridine which is in contrast to Ni(X-SALEN). The alkyl derivatives were all primary in nature and very stable to light, air, and H₂0 which in contrast to the alkyl derivatives of Co(X-SALEN) which happen to be photolabile. The geometry around the cobalt atom in R-Co-(SALDIPN) was concluded to be pseudo-octahedral. Reasons for certain anomalies (i.e., magnetic moments, etc.) were discussed in detail. / Master of Science

Chelation of first-row transition metals has many useful properties in the biomedical and industrial fields due to the stabilizing and/or property-altering effects that certain chelates can induce in these metals. One such useful design principle for these chelates is the addition of bulky steric groups which can have an added effect on these properties. Chapter I will explore the origins of these effects and show examples of how these effects are leveraged to produce useful complexes in a variety of applications. In Chapter II, we will discuss our choice of ligand design and the development of related synthetic procedures for all organic portions of the complexes. In Chapter III, we then study the effects of the series of bulky ligands with Mn2+ and Zn2+ on the formation of thermodynamically and kinetically inert complexes and investigate the subsequent effects on redox properties. Chapter IV furthers this investigation with Fe2+ and Co2+ using the bulkiest and least bulky versions of our ligand, where these metal complexes are investigated for the effects on redox properties and spin states. Lastly, a brief appendix details work performed on pyridine-imidazole systems bound to Mn2+ for their potential use in water oxidation catalysis.

The reactivity of the [(Phen)Cr(CO)_3] intermediate with phosphines and phosphites (L) has been investigated through ligand-competition studies. This intermediate possesses virtually no ability to discriminate among L. The agreement between reactivity data for the thermal and photochemically-generated intermediates indicates that the same intermediate is produced via each process. Pulsed laser flash photolysis of (n^2-NP)M(CO)_4 (I) (M = Cr, Mo; NP = 1-diethylamino-2-diphenylphosphinoethane) in the absence and presence of P (OPr-i)_3 (L) in 1,2-dichloroethane and chlorobenzene induces unimolecular ring-opening to afford [(n^1-NP)M(CO)_4] (II), in which the bidentate ligand is coordinated through P. Reaction of this intermediate takes place through competitive ring-reclosure and attack at (II) by L to afford (I) and cis-(n^1-NP) (L)M(CO)_4.

A dissertation submitted to the Faculty of Science University of the Witwatersrand, in partial fulfilment of the requirements for the degree of master of science (MSc) School of physics, University of Witwatersrand, 2017. / Intensive study on structural, electronic and optical properties of bulk transition metal dichalcogenides and dipnictogenides (MX2; where M = V, Nb and X = S, Se, Te, P) was undertaken. A relative stability test was done to determine the most stable ground state configuration via calculation of total ground state energy and volume which was fitted to the third order Birch-Murnaghan equation of state to extract lattice parameters. Cohesive energies of the above mentioned MX2 compounds and their elemental solids were then computed from which formation energies were acquired based on their respective equations of reaction between reactants and product. Its significance was to aid in determining if a material is energetically stable. Elastic constants were predicted from which mechanical properties i.e bulk, Young’s and shear moduli and consequently Poisson’s ratio were resolved by feeding the stiffness matrix onto online elastic tensor analysis tool which facilitated verification of their mechanical stability based on the well-known Born stability conditions which varies from one crystal system to another, at a later stage phonon dispersion curves were plotted after performing phonon calculation based on phonopy code to verify if the materials of concern are dynamically stable. After a material had fulfilled all the above stability tests, its structural study was initiated using various functionals. Functional that described best the structural properties of each individual compound considered was then applied in exploring its electronic and optical properties whose motivation was to find out the most stable phase as well as gauge if these materials could be used in various fields that suits their mechanical and optical properties. Furthermore, from carefully calculated optical spectra, plasma frequencies were analyzed which indicated the possibility of applying a material in plasmonic related fields. In addition to above, other factors to be considered when selecting a given electrode material that are crucial for optoelectronics are good chemical and thermal stabilities, high transparency and excellent conductivity. / XL2018