Thesis (Ph. D.)--University of Hong Kong, 1988.

The use of N-heterocyclic carbenes (NHCs) as ligands in catalysis is one of the most significant developments in modern catalysis and organometallic chemistry. One way to extend the scope of NHC ligand tuning is by means of annulation of carbocyclic and heterocyclic rings to the NHC backbone. The bis(imino)acenaphthene-supported N-heterocyclic carbene [IPr(BIAN)] has been synthesized and can be regarded as originating from the fusion of a naphthalene ring to an NHC. Several metal complexes of IPr(BIAN), including those incorporating copper(I), silver(I), gold(I), or iridium(I) have been synthesized and characterized, including single-crystal X-ray diffraction studies. The doncity of IPr(BIAN) was investigated using the Tolmen Electronic Parameter (TEP) method. A TEP value of 2042 cm-1 was calculated for the IPr(BIAN) ligand using the Ir(CO)2Cl complex which indicates that IPr(BIAN) is a relatively strong electron donating NHC ligand. The well-behaved redox chemistry of the BIAN ligand class rendered IPr(BIAN) an excellent candidate for exploration of the relationship between ligand charge and carbene donicity. The electrochemical reduction of IPr(BIAN) was studied by cyclic voltammetry (CV) in a THF solution and a reversible reduction wave was detected at - 1.79 V vs SCE. Spectroelectrochemical IR studies were also undertaken to further characterize the nature of the reduced state. IPr(BIAN) was found to be a stronger electron donating ligand in the reduced state in comparison with the neutral state of the ligand. IPr(BIAN) was also chemically reduced using potassium graphite and the resulting radical anion was studied by electron paramagnetic resonance (EPR) techniques. An isotropic EPR signal was observed at a g value of 2.0112. Due to the known antimicrobial activities of silver and gold NHCs, the activities of the silver and gold complexes of IPr(BIAN) and the imidazolium salts of several BIAN ligands were investigated using the minimum inhibitory concentration test. The silver(I) and gold(I) complexes of IPr(BIAN) were found to be moderately active. The most active compounds were found to be the imidazolium salts, with MIC values ranging between < 0.6 μg/mL and 78 μg/mL for the diisopropylphenyl(BIAN) and the mesityl(BIAN) imidazolium chlorides against S. aureas, B. subtilis, E. coli, and P. aeruginosa. The preparation of nanofibers impregnated with IPr(BIAN)AuCl by the process of electrospinning was also explored. The antimicrobial activities of the resulting nanofiber mats were determined on the basis of the inhibition zone test, and a localized antimicrobial activity was observed for the Gram-positive bacteria M. leuteus. / text

In an effort to develop broadly applicable photoswitchable catalysts, we have reported a method for modulating N-heterocyclic carbene (NHC) donicity using light by incorporating a photochromic diarylethene (DAE) into the backbone of a NHC scaffold. UV irradiation of 4,5-dithienylimidazolone or an analogous NHC-Ir(CO)₂Cl complex effected a photocyclization between the two thiophene rings, which led to a change in the electron donating ability of the NHC scaffold. Subsequent exposure to visible light reversed the photocyclization reaction. The concept of photo-modulating NHC donicity in this manner enabled photoswitchable NHC organocatalysis. The catalytic activity of a DAE-annulated imidazolium pre-catalyst in transesterification and amidation reactions was successfully switched between the active and nearly inactive states ([kappa]vis/[kappa]UV = 100) upon alternate UV ([lambda]irr = 313 nm) or visible ([lambda]irr > 500 nm) irradiation. The photoswitchable NHC organocatalysis was later extended to facilitating ring-opening polymerizations of cyclic esters, the rates of which were controlled via external light stimuli. Additionally, a photochromic dithienylethene-annulated N-heterocyclic carbene (NHC)-Rh(I) complex was synthesized and enabled photoswitching of the catalytic activity in series of hydroboration reactions. All of the examples demonstrate extremely rare instances of photomodulating a catalyst's activity by tuning its electronic properties. Furthermore, by taking advantage of the versatility of NHCs in both organo- and organometallic catalysis, we have developed novel photoswitchable catalysts for a variety of applicable transformations. Nanoparticles that can be transported in subsurface reservoirs at high salinities and temperatures are expected to have a major impact on enhanced oil recovery and electromagnetic imaging. We have developed an approach that will facilitate nanopaticle transport through porous media at high salinity by adsorbing or grafting rationally designed co-polymers on platform nanoparticles. Notably, co-polymers of acrylic acid with either 2-acrylamido-2-methylpropanesulfonate or styrenesulfonate have been electrostatically adsorbed or covalently grafted onto iron oxide nanoclusters. The presence of sulfonate groups on the iron oxide surface enabled long-term colloidal stability of the particles in extremely concentrated brine (8% wt. NaCl + 2% wt. CaCl₂) at elevated temperatures (90 °C) and minimized their adsorption on model mineral surfaces. / text

A library of luminescent cyclometalated gold(III) complexes containing various bis-cyclometalating R-C^N^C ligands derived from 2,6-diphenylpyridine and ancillary alkynyl ligands, [Au(R-C^N^C)(C≡CR’)], has been synthesized. Some of the complexes have been structurally determined by X-ray crystallography. These complexes were found to exhibit intense emission in dichloromethane solution at 298 K, originating from metal-perturbed triplet intraligand (IL) excited states of the R-C^N^C ligand, with substantial charge transfer character from the aryl moiety to the pyridyl ring. In the presence of electron-rich alkynyl ligands, the emission origin could be switched to an alkynyl-to-cyclometalating ligand ligand-to-ligand charge transfer (LLCT) excited state. This class of complexes was also demonstrated to show rich electroluminescence properties as emitters in organic light-emitting devices. In addition, the supramolecular assembly of this class of complexes has also been revealed in gelation studies. N-Heterocyclic carbenes have been incorporated into the gold(III) metal center to prepare a series of luminescent mononuclear and dinuclear gold(III) complexes, [{Au(C^N^C)}n(NHC)](PF6)n and [{Au(tBuC^N^CtBu)}n(NHC)]- (PF6)n (n = 1, 2). The X-ray crystal structures of most of the complexes have been determined. The emissions of these complexes were assigned to originate from the metal-perturbed intraligand excited state of the bis-cyclometalating ligand. One of the C2-bridged dinuclear complexes was found to exhibit two distinct reduction couples, tentatively correlated to the presence of significant intramolecular π-π interaction in the complex. Two novel series of luminescent mononuclear alkynylgold(III) complexes, [Au(C^N)(C≡CR)2] (HC^N = 2-phenylpyridine (Hppy) and derivatives) and [Au(C^N^N)(C≡CR)]PF6 (HC^N^N = 6-phenyl-2,2’-bipyridine and derivatives), have been synthesized. Some of the X-ray crystal structures have been determined. The former class of complexes with bidentate C^N ligands has been observed to show tunable emission spanning across the visible spectrum from 462 to 697 nm. With the exception of [Au(ppy)(C≡C-C6H4-NH2-p)2] which showed a low-energy band originated from a 3LLCT [π(C≡C-C6H4-NH2)→π*(C^N)] excited state, all complexes exhibited vibronic-structured emission bands originated from the intraligand transition of the cyclometalating C^N ligand in dichloromethane solution at 298 K. On the other hand, most of the complexes with the tridentate C^N^N-type ligand have been observed to exhibit vibronic-structured emission bands attributed to the intraligand transition of the C^N^N ligand in low-temperature butyronitrile glass. / published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

Despite previous extensive study of the widely-employed ruthenium-catalysed olefin metathesis reaction, the finer mechanistic details have not been elucidated. An area that is noticeably lacking is spectroscopic exploration of the relevant complexes. In this work, organometallic ruthenium complexes of importance to olefin metathesis have been investigated using Ru K-edge X-ray absorption spectroscopy. The lowest energy feature in the Ru K-edge spectrum has been unambiguously assigned as due to Ru 4d←1s transitions. These electric-dipole-forbidden transitions are extremely sensitive to geometry. For centrosymmetric complexes, the pre-edge feature has very low intensity because it is limited by the weak electric quadrupole mechanism. By contrast, non-centrosymmetric complexes exhibit a substantial increase in pre-edge intensity because Ru 5p-4d mixing introduces electric-dipole-allowed character to the Ru 4d←1s transitions. The energy of the edge feature in the Ru K-edge spectrum corresponds to ionisation of 1s electrons and is a good indicator of the charge on the metal centre. Unexpectedly, we found that the first-generation (L = PCy₃) Grubbs precatalyst (1) has a higher 1s ionisation energy than the second-generation (L = H₂IMes) complex (2). This effect provides a compelling rationale for the unexplained differences in phosphine dissociation kinetics for complexes 1 and 2: the phosphine dissociation rate of 2 is slower than 1 because the metal centre is more electron-deficient in 2. Density functional theory calculations confirm the charge differences and offer some insight into the nature of bonding in these complexes, particularly with regard to the N-heterocyclic carbene and trialkylphosphine ligands. On the basis of these results, we propose that, for this system, the NHC ligand is a weaker σ-charge donor than the phosphine ligand, and that the NHC accepts significant π-electron density from the metal; both interactions function to reduce the electron density on the ruthenium centre. An ultimate goal is to investigate reactive species in the olefin metathesis mechanism; accordingly, we have made considerable progress toward collecting XAS data for a metallacyclobutane species, and we are pursuing methods to trap the four-coordinate intermediate in the metathesis cycle.

The palladium-mediated cross-coupling of aryl electrophiles and amines (Buchwald-Hartwig amination) has become a widely used method of constructing arylamine frameworks. A crucial aspect of the advancement of this chemistry has been the design of ancillary ligands that are able to promote enhanced reactivity in challenging amination reactions. Despite significant ligand development within the field, challenges in this chemistry remain. Chemoselective aminations, wherein one amine substrate undergoes preferential arylation in the presence of multiple reactive amines has remained an underexplored area of Buchwald-Hartwig amination chemistry. This thesis describes the use of [Pd(cinnamyl)Cl]2 and N-[2-di(1-adamantylphosphino)phenyl]morpholine (Mor- DalPhos) in an extensive study of chemoselective Buchwald-Hartwig aminations, with 62 examples of structurally diverse di-, tri-, and tetraamines obtained in synthetically useful yields at reasonable catalyst loadings (1-5 mol % Pd). The coordination chemistry of [(Mor-DalPhos)Pd] species was also explored, as were complementary chemoselective aminations with the isomeric p-Mor-DalPhos ligand, leading to divergent product formation in some instances. The same [Pd(cinnamyl)Cl]2/Mor-DalPhos catalyst system used in the chemoselectivity study was also employed in a series of Buchwald-Hartwig aminations conducted under aqueous and solvent-free conditions, another underexplored area of this chemistry. A total of 52 amine products were isolated using these methodologies, moderate catalyst loadings (3 mol % Pd), and without the use of any additional additives, co-solvents, or rigorous exclusion of air. The synthesis of low-coordinate palladium complexes featuring both NHC and dialkylchlorophosphine ligands is also discussed herein. These complexes are prepared via a previously unreported and straightforward methodology involving an unusual net PCl bond reductive elimination, and represent a potential new class of pre-catalysts forpalladium-mediated reactions.

Isobenzofuran has since its discovery proven an important intermediate in the synthesis of novel organic compounds, due to its high reactivity in Diels-Alder reactions. This thesis details the synthesis, purification and characterization of cyclophane compounds derived from a novel analogue of isobenzofuran, 5,5'-bi(isobenzofuran). Because of the relative rarity of molecules containing multiple isobenzofuran units, 5.5'-bi(isobenzofuran) is an excellent candidate for study. Prelminary analysis of an additional novel analogue of isobenzofuran, 5.5'-bis(isobenzofuranyl) acetylene, is also presented. / x, 119 leaves : ill. ; 29 cm.

The side-on dioxygen adducts of N-heterocyclic carbene (NHC) containing rhodium complexes, [ClRh(IPr)2(O2)] and [ClRh(IMes)2(O2)], previously synthesized in our laboratories possess a square planar geometry and O-O bond lengths of 1.323(3) and 1.341(4) Å, respectively. Both of these attributes are uncharacteristic of Rh(O2) complexes, which are typically octahedral and possess O-O bond lengths of approximately 1.45 Å. Full characterization by NMR, IR, Raman, DFT and XAS confirmed the short O-O bond lengths of these structures and revealed that they were rhodium(I) coordination complexes of singlet oxygen with no net oxidation/reduction process having taken place. The unique bonding mode appears to result from the interaction of a filled Rh d orbital with one of the two degenerate O2 * orbitals, which causes splitting of the O2 * orbitals, favoring spin pairing in the O2 HOMO, and the inability of Rh to donate electron density to the empty * orbital. Initial investigations of these complexes as catalysts for the reduction and oxidation of C-O bonds, as well as singlet oxygen generation were also undertaken. Rh(IPr)2 coordination complexes of N2, H2 and CO were also synthesized and characterized by X-ray crystallography, NMR and elemental analysis. Interestingly, the addition of hydrogen gas to rhodium did result in oxidation of the metal. A Rh(NHC) complex featuring an anionic acetate ligand, [(AcO)Rh(IPr)(CO)2], was synthesized and characterized by NMR, IR and X-ray crystallography. This complex proved to be an effective catalyst for the regioselective hydroformylation of aliphatic and aromatic alkenes, which occurred without isomerization of the alkene. Initial rates of hydroformylation with our catalyst were compared to the chloride analogue, [ClRh(IPr)(CO)2], and demonstrated the beneficial nature of replacing the halide with a carboxylate ligand, which is less inhibiting of the reaction. The synthesis of a bifunctional hydrogenation catalyst featuring a protic-NHC was attempted by addition of benzimidazoles to [Cl2Ru(diphosphine)]. Although these attempts were unsuccessful, a large number of complexes of the formula [Cl2Ru(diphosphine)(-N3-benzimidazole)2] were synthesized and proved to be effective catalysts for the chemoselective hydrogenation of ketones versus alkenes. Use of chiral diphosphines and 1-triphenylmethylbenzimidazole yielded catalysts capable of producing secondary alcohols with moderate enantioselectivity. / Thesis (Ph.D, Chemistry) -- Queen's University, 2010-09-17 12:44:52.686

In the last decade N-heterocyclic carbene (NHC) ligands have become important in organometallic chemistry and homogeneous catalysis, rivalling the well established phosphines. Most of the current attention to date has focused on the NHC complexes of the platinum group metals (rhodium, palladium and nickel) plus ruthenium based system, but the chemistry of NHC systems of iron which is relatively inexpensive and environmentally friendlier is considerably less developed. Thus, this project involves the design, synthesis, characterization and application in catalytic transfer hydrogenation of NHC ligands and their iron(II) complexes. The motivation for the choice of NHC as a ligand stems from the ability to systematically tune the ligand both electronically and sterically in addition to the stability and robustness of the ligand to stabilize metal centres in various environments. In this research imidazolium based NHCs are generated. Thus, three different series of imidazolium salts were synthesized and their iron(II) complexes was obtained. All the compounds were characterized by spectroscopic and crystallographic methods. These are: (a) 1,3-dialkylimidazolium salts (b) 1,3-diarylimidazolium salts and (c) ferrocenylimidazolium salts bearing methyl and phenyl spacers between the ferrocenyl and the imidazolium moieties. A total of 20 novel compounds were synthesized and are reported in this thesis. Furthermore, the application of the new compounds as transfer hydrogenation catalysts was investigated using 17 saturated and unsaturated ketones as substrates, in the presence of KOH as the base and 2-propanol as the hydrogen source. The dialkylated NHC iron(II) complexes showed excellent yields, and TON values of up to 200 were achieved under the optimized reaction conditions. Without complexation with iron, the 1,3-diarylimidazolium and ferrocenylimidazolium series of salts were also found to be active catalysts for the transfer hydrogenation reaction of ketones in alcoholic media. In the case of ferrocenylimidazolium salts a TON value up to 1880 was achieved. Notably, two of the unsaturated ketones were successfully converted at a high yield with a high selectivity to the corresponding saturated ketones only. In addition, the stability of NHC ligands to moisture was investigated, since an understanding of the stability of various deprotonated NHC-based imidazolium cations to attack by moisture resulting in hydrolysis products is very important to understanding the coordination chemistry of the ligands on to metal centres. Four novel ionic diamino aldehyde compounds were obtained by moisture attack on saturated NHC ligands. The route to the formation of the hydrolysed compounds is formulated to occur via an imidazolinium ring opening process. On the other hand the unsaturated counterparts were more stable towards hydrolysis yielding adducts with the iron(II) precursors. Finally, the electrochemical properties of the ferrocenylimidazolium salts were investigated using cyclic voltametry. By comparing the relative shifts in the formal electrode potentials of the ferrocene/ferrocenium coupled with the ferrocenylimidazolium salts, it was easy to evaluate the influence of the substituents on the carbene containing imidazolium moiety on the electrochemical properties of the iron centres. The formal electrode potential of the ferrocenylimidazolium salts shifted to higher positive potentials as compared to ferrocene, indicating a high electron withdrawing effect of the imidazolium salts. This makes the metal centres more vulnerable to attack by nucleophiles. The electrochemical studies have enabled a structure-activity correlation to be drawn for the various ferrocenylimidazolium salts. / Thesis (Ph.D.)-University of KwaZulu-Natal, Westville, 2011.

Unable to copy abstract. / Thesis (M.Sc.)-University of KwaZulu-Natal, Westville, 2011.