Organometallic catalysis has attracted significant interest from both industry and academia due to its wide applications in organic synthetic transformations. Example of such transformations include the reaction of a zinc carbenoid with olefins to form cyclopropanes. The first project is a computational study using both density functional and correlated wavefunction methods of the reaction between ethylene and model zinc carbenoid, nitrenoid and oxenoid complexes (L-Zn-E-X, E = CH2, NH or O, L = X = I or Cl). It was shown that cyclopropanation of ethylene with IZnCH2I and aziridination of ethylene with IZnNHI proceed via a single-step mechanism with an asynchronous transition state. The reaction barrier for the aziridination with IZnNHI is lower than that of cyclopropanation. Changing the leaving group of IZnNHI from I to Cl, changes the mechanism of the aziridination reaction to a two-step pathway. The calculation results from the epoxidation with IZnOI and ClZnOCl oxenoids suggest a two-step mechanism for both oxenoids. Another important example of organometallic catalysis is the formation of alkyl arenes from arenes and olefins using transition metal catalysis (olefin hydroarylation). We studied with DFT methods the mechanism of a novel Rh catalyst (FlDAB)Rh(TFA)(η2–C2H4) [FlDAB = N,N’ -bis(pentafluorophenyl)-2,3-dimethyl-1,4-diaza-1,3-butadiene; TFA = trifluoroacetate] that converts benzene, ethylene and air-recyclable Cu(II) oxidants to styrene. Possible mechanisms are discussed.

In the last two decades, significant strides have been made towards utilizing the scanning tunneling microscope (STM) as a reaction chemistry tool, in addition to its primary use as an imaging instrument. Built off the STM, the STM-break junction (STM-BJ) technique was developed specifically for the reliable and reproducible measurement of properties of a single molecule suspended between two electrodes. These advances are crucial to the fields of molecular electronics and single-molecule reactivity, the latter also relating back to traditional bulk chemistry. By intelligently designing experiments and systems to probe with the STM and STM-BJ, we can begin to understand chemical processes on a deeper level than ever before. Chapter 1 provides an overview of the recent work using the STM and STM-BJ to effect chemical transformations which involve the making and breaking of bonds. We contextualize this progress in terms of single-molecule manipulation and synthetic chemistry, to understand the implications and outlook of this field of study. Seminal surface-based reactions are discussed, in addition to reactions that occur in both solution and within the single molecule junction. Differences between STM and STM-BJ capabilities and limitations are detailed, and the challenges of translating these fundamental experiments into functional reactions are addressed. Chapter 2 describes using the STM-BJ to study the binding of aryl iodides between gold electrodes. Important details regarding these binding modes, which were previously incompletely understood, are revealed via concrete experimental evidence. Our data suggests that this system, which is synthetically accessible, holds promise for forming the sought-after and highly conducting covalent gold-carbon bonds in situ and can be modulated with applied bias. Chapter 3 builds upon the knowledge gained in Chapter 2, and focuses on the reactivity of aryl iodides in the junction. We demonstrate a new in situ reaction of an Ullmann coupling, or dimerization, of various biphenyl iodides. By strategically designing the molecules studied, we are also able to gain mechanistic insight into this process, which in the bulk still remains debated, as well as demonstrate a cross-coupling reaction. This project is ongoing as of the submission of this dissertation, so other findings and continuing experiments are included. Chapter 4 transitions towards a different type of binder to gold, the cyclopropenylidene-based carbene. These amino-functionalized carbenes prove to be stronger linkers than N-heterocyclic carbenes, which are known binders to gold. Using a variety of surface analysis, imaging, and computational techniques, we explore the binding geometries and energies of cyclopropenylidenes, expanding the scope of carbene surface modifiers. Chapter 5 summarizes this body of PhD research, suggests directions for future work, and concludes the dissertation. These works explore the binding and reactivity of molecules on gold surfaces and within the single molecule junction, improving upon the understanding of this newly burgeoning field. This thesis seeks to encourage future work on these and related systems, to continue refining our comprehension of both junction and bulk reaction chemistry processes.

With the advent of photoredox catalysis, new synthetic paradigms have been established with increasing numbers of novel transformations being achieved. Nevertheless, modern photoredox chemistry has several drawbacks in efficiency, scalability, and light penetration. In this dissertation, we document developments in photoredox catalysis that harvest red light (600- 800 nm) and demonstrate its applications in polymer science and chemical biology. First, a method towards the red-light activation and control of olefin metathesis will be discussed. This system employs a mixed, bis-N-heterocyclic carbene coordinated ruthenium indenylidene complex in conjunction with an osmium(II) complex under red light irradiation. Mechanistic investigation suggests a reduction of a cationic Ru species, to lead to the active metathesis species. Expectedly, polymerization through barriers is achieved with red light only. Material penetration, including mammalian tissue, and limited photocytotoxicity brings red light photoredox catalysis in the forefront of biological applications such as photoproximity labeling. This proceeds through the photocatalytic generation of reactive intermediates, the lifetimes of which dictate the spatial resolution of labeling. We thus describe a method to activate aryl(trifluoromethyl) diazos using an osmium(II) catalyst and red light, providing highly reactive, short-lived carbenes. The short lifetime of the carbene is highlighted by its ability to map small molecules to target proteins, a feat even nitrenes cannot achieve. Finally, efforts towards the synthesis of a wash-free fluorogenic photocatalyst with applications for intracellular red light photoproximity labeling will be described.

Electric fields influence reactions by stabilization of charge-separated transition states. While this has been a longstanding hypothesis supported computationally, recent experimental confirmations highlight the potential for leveraging electric field effects to drive small molecule reactions far from equilibrium. Herein we report electric-field catalysis of an alkane solvent-derived acylation reaction in the scanning tunneling microscope-break junction (STM-BJ), providing additional support for this hypothesis. Additionally, the design and reactivity of an internally charged zwitterionic ligand are disclosed. Synthetic access of metal ligands bearing opposing charged functional groups permitted the examination of stochiometric metalation and catalytic behavior of electric field-bearing ligands.While traditionally computation has been used to rationalize why a particular catalyst is successful descriptively, it has been rarely used to screen candidates and prescriptively provide optimal catalyst structure. We report a neural network-enabled catalyst screening platform that dramatically reduce the resource intensity for examining a large chemical space. We leverage this platform to examine azolium N-heterocyclic carbene (NHC) precursors to address the lack of compatibility for electron-rich aryl aldehydes in the NHC-catalyzed Stetter reaction. This led to the discovery of a new class of azolium NHC precursor: oxadiazoliums that proved competent in achieving the target reaction addressing current limitations in Stetter catalysis.

This thesis describes the synthesis and isolation of silver, mercury, ruthenium and palladium complexes of bidentate N-heterocyclic carbenes (NHCs), derived from imidazolium-linked cyclophanes and related bis-imidazolium salts. The cyclophane structures contain two imidazolyl links between ortho- and meta- substituted aromatic rings and the related structures are ortho-, meta- and para-xylyl linked bis-imidazolium salts. The complexes have been characterised by NMR spectroscopy and X-ray crystallography. The synthesis of five new silver complexes has been achieved via a simple complexation reaction of the cyclophane and bis-imidazolium salts with the basic metal source Ag2O. The new silver carbene systems are thermally stable. Three of the complexes are dinuclear, cationic complexes, while two are mononuclear complexes, one cationic and one neutral. A number of mono- and di-nuclear mercury(II)-NHC complexes have been synthesised from the ortho- and meta-linked cyclophanes and the related meta-linked bis-imidazolium salts. The mercury complexes were prepared by direct mercuration method using mercury(II) acetate. The syntheses were perfomed in air and the complexes are stable to air and moisture. Mercury complexes I and II represent the first example of mononuclear metal complexes derived from meta-substituted imidazolium-linked cyclophanes. NHC-ligand transfer reactions from NHC-silver complexes and NHC-mercury complexes are described. An ortho-cyclophane ligand was successfully transfered from a silver complex to its palladium counterpart. Furthermore, palladium complex III, bearing a para-xylyl linked bis-NHC ligand, was made by transmetallation from both a silver and mercury complex. This is the first reported NHC-palladium complex of a para-xylyl linked bis-NHC ligand. A new redox transmetallation method for NHC ligand transfer, using a mercury complex, is presented. A palladium complex was made via redox transmetallation using a mercury complex of an ortho-NHC-cyclophane. A ruthenium(II)-NHC complex, IV containing an ortho-cyclophane ligand has been prepared via silver transmetallation and in situ complexation methods. In the transmetallation route, a silver complex of an ortho-cyclophane was treated with RuCl2(PPh3)3 to form IV. This complex represents the first example of a ruthenium complex bearing an NHC-cyclophane ligand, v and is also the first example of a metal complex derived from an imidazolium-linked cyclophane where the arene unit of the cyclophane is also involved in bonding to the metal centre.

Thesis (Ph.D.(Chemistry))--University of Pretoria, 2004. / Includes summary. Also available online.

Thesis (Ph. D.)--Ohio State University, 2005. / Title from first page of PDF file. Includes bibliographical references (p. 164-174).

The work described herein focuses on the synthesis and characterisation of copper(I) complexes bearing N-heterocyclic carbene (NHC) ligands, their use in catalysis as well as organometallic synthesis and related reaction mechanisms. Two classes of complexes were considered: neutral NHC-Cu(I) species and their cationic analogues. Concerning the former, initial efforts were focused on the development of a general and straightforward synthetic methodology towards complexes of the type [Cu(X)(NHC)] (X = Cl, Br, I). More than 10 NHC-Cu(I) species were synthesised in high yields under mild conditions, in air and using technical grade solvents. These complexes exhibited interesting activity in the catalytic dehydrogenation of formic acid/amine adducts proving in three times more efficiency than the copper salts previously employed in such a reaction. Hydroxide- and tert-butoxide analogues showed to be efficient catalysts in the N-methylation of amines with CO₂ as carbon source, and in the dehydrogenative coupling of silanes and carboxylic acids. Experimental and computational work were carried out in order to elucidate the mechanism of these transformations. Regarding the use of these species in organometallic synthesis, homo- and heteroleptic bis-NHC-Cu(I) complexes were employed as carbene transfer reagents to other transition metals. Aside from well-known cationic gold(I) species, two novel palladium(II) analogues were isolated and fully characterised.

Nowadays, the requirement to design highly valuable compounds is undoubtedly one of the major challenges in the field of organic and organometallic chemistry. The use of the versatile and efficient N-heterocyclic carbenes (NHCs) combined with transition metals represents a key feature in modern organometallic chemistry and homogeneous catalysis. In the course of this thesis, the straightforward design and synthesis of a library of Pd(0) bearing NHC ligands was achieved. Their catalytic performances (Chapter 1) and their phosphorescence properties in solution (Chapter 2) were disclosed. Currently, cross-couplings are some of the most important types of reaction in palladium catalysis. The formation of highly hindered biaryls substrates is one of the main requirements in cross-coupling chemistry. The design and synthesis of a palladium dimer bearing a bulky NHC ligand can fulfil this proposal (Chapter 4). The development of new classes of ligands is a topic of interest. For this reason, normal, abnormal, remote and mesoionic N-heterocyclic carbenes copper complexes were investigated and their reactivity compared in the [3+2] cycloaddition of azides and alkynes (Chapter 7). Air and moisture stable Cu(I)-NHC species have also been compared to their silver analogues for the alkynylation of ketones (Chapter 9). The different reactivity of the two latter organometallic species (Cu and Ag) with ethyldiazoacetate reagent via the formation of carbenes or C-H activated product is presented in Chapter 8. Recently, the development of a bimetallic catalytic system is strongly considered and has high impact. For this reason, two dual catalytic transformations (Pd-NHC and Cu-NHC) were studied for the C-H arylation (Chapter 5) and the synthesis of substituted alkenes products via a relay or cooperative mechanisms (Chapter 6). The isolation of intermediates and mechanistic studies were examined in each of these studies.

Thesis (PhD (Chemistry and Polymer Science))--Stellenbosch University, 2008. / Alkylated acetonitrile that forms during the synthesis of the sulfonium salt, [(Me3)2(MeS)S][BF4], is involved in the formation of new , -unsaturated Fischer-type carbene complexes from (CO)5M=C(OMe)CH2Li (M = Cr, W). Metal migration observed when the substitution product obtained from the reaction of the anionic carbene complexes (CO)5M=C(NMe2)CºC¯ (M = Cr, W) with Ph3PAu+ was left in solution, was also kinetically and theoretically investigated. 1H NMR and quantum mechanical (at the B3LYP level of theory) data indicated a complicated mechanism. The a,b-unsaturated Fischer-type carbene complex, (CO)5Cr=C(OMe)CH=C(Me)NH(Me), obtained from the reaction of (CO)5M=C(OMe)CH2¯ with alkylated acetonitrile, was transformed into the new remote one-N, six-membered, carbene ligand (rN1HC6) complex, (CO)5Cr=C(CH=C(Me)N(Me)CH=C(nBu). The carbene ligand unprecedentedly preferred the softer Rh(CO)2Cl moiety to the Cr(CO)5 metal fragment and transferred readily. A new series of remote and abnormal square planar compounds [r/a(NHC)(PPh3)2MCl]CF3SO3 (M = Pd or Ni) was prepared by oxidative substitution. The various positions for metal-carbon bond formation on a pyridine ring to furnish various ligand types i.e. C2 for nN1HC6, C3 for aN1HC6 or C4 for rN1HC6 received attention. The ligands were arranged in increasing order of carbene character, aNHC < nNHC < rNHC and trans influence, nN2HC5 ~ aN1HC6 ~ nN1HC6 < rN1HC6. In competitive situations, oxidative substitution occurred selectively at C4 of the pyridine ring rather than at C2 and on the aromatic ring containing the heteroatom (C4), rather than on an annealed aromatic ring (C7). Crystal and molecular structure determinations confirmed the preferred coordination sites. Quantum mechanical calculations (at the RI-BP86/SV level of theory) indicated that the chosen carbene ligand has a much larger influence than the metal on the BDE of the M-Ccarbene bond; the farther away the N-atom is from the carbene carbon, the stronger the bond. In complexes that also contain additional external nitrogen atoms, e.g. trans-chloro(N-methyl-1,2,4- trihydro-2-dimethylaminepyrid-4-ylidene)bis(triphenylphosphine)palladium(II) triflate and transchloro( N-methyl-1,2,4-trihydro-2-dimethylaminepyrid-4-ylidene)bis(triphenylphosphine)nickel(II) triflate, stabilisation originates from both the nitrogens. 2-Chloro-1-methyl-1H-pyrid-4-ylidenephenylammonium triflate afforded complexes with both remote as well as normal nitrogen atoms. New azole complexes of palladium and nickel with remote heteroatoms were also prepared from N-methyl-4',4'-dimethyl-2'-thiophen-3-chloro-2-yl-4,5-dihydro-oxazole. Employing the compound 1,5-dichloroanthraquinone, the product of a double oxidative substitution on two Pd centra could be isolated but not alkylated. The fact that the chemical shift of the metal bonded carbon in the 13C NMR spectrum can not be used as absolute measure of carbene character, was emphasised in a compound where the heteroatom was situated seven bonds away from the carbon donor. In efforts to synthesise a sulphur-bridged complex that contains carbene ligands, crystals of transdi- iodobis(1,3-dimethyl-imidazoline-2-ylidene)palladium were obtained. Bridged thiolato complexes with N1HC6 ligands were unexpectedly found in the attempt to substitute the halogen on chosen square planar carbene complexes of palladium, widening the application possibilities of N1HC6 ligands in organometallic chemistry beyond that of catalysis. A trinuclear cluster, [(PdPPh3)3(μ-SMe)3]BF4 was isolated as a by-product of these reactions. A series normal and abnormal thiazolylidene complexes of nickel and palladium were prepared by oxidative substitution of the respective 2-, 4- and 5-bromothiazolium salts with M(PPh3)4 (M = Pd or Ni), and unequivocally characterised. In a preliminary catalytic investigation, all the thiazolinium and simple pyridinium derived palladium complexes showed activity in the Suzuki-Miyaura coupling reaction. Little variation in activity in the order a (N next to carbon donor) > n > a (S next to carbon donor) was found for the former series, whereas decreased activity was exhibited in the sequence r > a > n of the latter group. The pyridinium derived complexes showed superior activity to the thiazolinium ones. The rNHC complex, trans-chloro(N-methyl-1,2,4-trihydro-2- dimethylaminepyrid-4-ylidene)bis(triphenylphosphine)palladium(II) triflate, showed similar Suzuki-Miyaura activity to the standard N2HC5 carbene complex precatalyst, trans-chloro[(1,3- dimethyl-imidazol-2-ylidene)triphenylphosphine]palladium(II) triflate.