published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

Nanostructured noble metals exhibit novel physical, mechanical and chemical behavior, and hold promise for applications such as gas sensing and electron emission. A strong emphasis was placed on the processing and characterization of these materials, in the form of nanoporous or nanocrystalline thin films. Palladium-based and osmium-ruthenium alloys were investigated in this dissertation research and will be presented as follows: (1) Preparation and Characterization of Nanoporous Metal Thin Films (2) Characterization of Osmium-Ruthenium Coatings Nanoporous palladium (np-Pd) thin films were prepared by dealloying co-sputtered palladium-nickel precursor alloys. Nanoporous structures were created with 3-D interconnected ligaments and open pores. Size of ligaments and pores was ~5 nm, achieved with a novel processing method developed in this study. Hydrogen cycling tests performed with np-Pd films demonstrated a significant improvement in sensitivity to hydrogen and response time for sensing. Effects of alloying element (Ni), film thickness, local stress and pore/ligament size on hydrogen cycling behavior were investigated in detail. Additionally, nanoporous gold and gold-palladium thin films were studied to clarify the evolution of microstructure during dealloying, including the formation of nanoporous structure and effects of substrate curvature on dealloying behavior. The results from this project have yielded a new understanding of dealloying as well as an ideal coating material for hydrogen sensing. Nanocrystalline osmium-ruthenium (Os-Ru) thin films were deposited on porous tungsten substrates with varied sputtering parameters. These parameters were mapped to microstructure, film texture and film composition in samples that were comparable to commercial devices. Using this map, Os-Ru films can be produced with higher stability during annealing and/or high-temperature operation. These results should lead to Os-Ru top coatings that increase the lifetime and emission performance of dispenser cathodes.

The reactions of nitro complexes of nickel and palladium with CO have been examined to determine the mechanism(s) by which CO₂ is produced. The solution and solid state structures of square planar Ni(NO₂)₂(L)₂ reactants and pseudotetrahedral Ni(NO₂)(NO)(L)₂ products have been determined and related to their reactivity. Infrared, ³¹P{¹H}, and crystallographic data indicate rapid isomerization between nitro and nitrito bonding modes of the NO⁻₂ ligands. The crystal structures of Ni(NO₂)₂(PPh₂(Ch₂)₂PPh₂) (I), Ni(NO₂) (NO) (PMe₃)₂ (II), and [Ni(ONO) (NO) (PPh₂(CH₂)₂PPh₂]₂ (III), show the NO⁻₂ groups to be N-bonded in I and II and O-bonded in III. The nitrosyl ligands in II and III are non-linear (Ni-N-O = 165.5(8) ° and 153.4(8) °, respectively). Furthermore, III crystallizes as a dimer bridged by two phosphine ligands even though molecular weights show this complex to be monomeric in solution. Each Ni(NO₂) (NO) (L)₂ complex reacts with CO to produce stoichiometric amounts of Ni(NO₂) (NO) (L)₂ and CO₂. Rate date indicate the reaction proceeds associatively through formation of a carbonyl intermediate which has been directly observed in the reaction of Ni(NO₂)₂(P(C₆H₁₁)₃)₂ with CO. The reaction of C¹⁸O with Ni(NO₂)₂(PMe₃)₂ results in no incorporation of ¹⁸O into the nickel product while ¹⁸O is incorporated into CO₂ to form ¹⁸OC¹⁶O. The mechanism consistent with all of the data involves a rapid equilibrium between both forms of NO⁻₂ coordination followed by the reaction of CO with either isomer in the rate determining step to form a monocarbonyl complex. Irreversible oxygen atom transfer to CO and loss of CO₂ terminate the reaction. The corresponding square planar palladium complexes, Pd(NO₂)₂L₂, react with CO to form N₂O, CO₂ and novel tetranuclear palladium clusters (Pd₄(CO)₅L₄). A crystal structure of Pd₄(CO)₅ - (PMePh₂)₄ shows the cluster to be a distorted tetrahedron of metal atoms with one open edge and the five remaining edges each bridged by a carbonyl group.

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The thesis is divided into three parts. The first part looks at the reactivity difference between [Pt(terpy)(OH2)f+ and [Pt(bpma)(OH2)]2+ where terpy is 2,2' :6',2"-terpyridine and bpma is bis(2-pyridylmethyl)amine, towards thiols namely, L-cysteine, DLpenicillamine and glutathione. This is followed by a comparative study of [Pt(bpma)(OH2)]2+ and [pd(bpma)(OH2)f+. Finally the reactivity differences between [Ru(terpy)(bipy)(OH2)f+ and [Ru(terpy)(tmenXOH2)]2+ are reported. Included are the synthesis and characterization ofthe complexes. The substitution behaviour of [pt(terpy)(OH2)]2+and [Pt(bpma)(OH2)f+ was studied as a function of entering thiol concentration and temperature. The reactions between the Ptcomplexes and DL-penicillamine, L-cysteine and glutathione were carried out in a 0.10 mol dm03 aqueous perchloric acid medium using stopped-flow or conventional UV-Vis spectrophotometry as required. The observed pseudo-first-order rate constants for the substitution reactions are given by kobs = k2[thiol] + k 2. The k 2 term represents the reverse solvolysis reaction. This term was found to be zero for Ptn(terpy) which was the most reactive complex. The second-order rate constants, ka; for the three thiols varied between 0.107±0.001 M·l S·l and 0.517±0.025 M"l sol for PtlI(bpma) and 10.7±0.7 M"l S·l to 711.9±18.3 M"l S·l for PtlI(terpy), with glutathione being the strongest nucleophile. Analysis of the activation parameters, Mf' and .1.S", clearly shows that the substitution process is associative in nature. The second study has looked at the substitution of the coordinated water molecule from [Pt(bpma)(OH2)f+ and [pd(bpma)(OH2)f+ by a series of nucleophiles [Nu] viz. TU, DMTU, TMTU and as well as Be", Cl', SCN", and r for the Ptn(bpma) complex. The investigation was conducted under pseudo-first-order conditions as a function of concentration of [Nu] as well as temperature for PtlI(bpma) complex using stopped flow spectrophotometry. Reactions involving PdII(bpma) were done at 10°C. The observed pseudo-first-order rate constants obeyed the equation kobs= k2[Nu]. The second-order rate constants, kz, at 10 "C for the sulfur donor nucleophiles have been found to vary between 70.35 M I sol and 223.06 M I sol for PtII(bpma) and (1.24 ± 0.01) x 105 M I sol to (2.17 ± 0.02) x 105 M-Is-l for PdII(terpy), with DMTV being the strongest nucleophile. The second-order rate constant, ka; at 25 "C fur PtII(terpy) was found to increase in the following order cr < Be" < TMfU < SCN < TV < DMTV < f. This order is in agreement with the polarizability of the nucleophiles, the nucleophilic discrimination factor being 0.38. The temperature studies for PtII(bpma) suggest that the substitution process is associative in nature.n The third part looked at the reactivities of [Ru(terpy)(bipy)(OHz)]z+ and [Ru(terpy)(tmen)(OHz)]z+ where bipy is 2,2'-bipyridine and tmen is N,N,N ',N 'tetramethylethylenediamine with three nucleophiles TV, DMTV and CH3CN. The pKa values for the complexes were found to be 9.99 and 10.27 for [Ru(terpy)(bipy)(OHz)]z+ and [Ru(terpy)(tmen)(OHz)f+, respectively. The substitution of water involving the two complexes was studied under pseudo-first order conditions using UV-Visible Spectrophotometry. The pseudo-first-order rate constant fitted the simple rate law kobs = kz [Nu] + k-z. The k.z term was found to be zero for [Ru(terpy)(bipy)(OHz)f+ but nonzero for [Ru(terpy)(tmen)(OHz)]z+. The values of the second order rate constants (kz) for the three nucleophiles were found to be between (1.08 ± 0.02) x 10-4 M l sol and (15.0 ± 0.27) x 10-4 M-l sol for [Ru(terpy)(bipy)(OHz)]z+ and (0.82 ± 0.04) x 10-4 M-l sol and (21.90 ± 0.69) x 10-4 M-I sol for [Ru(terpy)(tmen)(OHz)]z+. The results suggests that nback donation accounts for the difference in reactivity. / Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 2003.

Chapter 1 reviews the coordination behaviour of the 6-diphenylphosphino-2,2'-bipyridine, 6-anilino-2,2'-bipyridine, 2,2'-bipyridyl-6-one, 6-N-methylanilino-2,2'-bipyridine, 6-piperidyl-2,2'-bipyridine and 2-(phenylamino)-1,10-phenanthroline ligands. These ligands are all tridentate and contain well established chelating fragments viz., 2,2'-bipyridine and 1,10-phenanthroline. Thus the review of their coordination provides insight into the expected coordination of the 2-diphenylphosphino-1,10-phenanthroline (Ph2Pphen) ligand. The synthesis and characterisation of this ligand is described in Chapter 2. Chapter 3 describes the synthesis and characterisation of a range of Ph2Pphen ligand-bridged dicopper(l) complexes. It has been shown that Ph2Pphen reacts with a suitable copper(l) precursor, [Cu(MeCN)4]+, to form the versatile dinuclear [Cu2(u-Ph2Pphen)2(MeCN)2]2+complex cation containing two bridging Ph2Pphen ligands; the structure of the SbF6 salt of this complex has been determined X-ray crystallographically. This complex possesses labile acetonitrile ligands which have been substituted by a variety of neutral and anionic ligands. Complexes prepared this way include [Cu2(u-Ph2Pphen)2(u-CI)]+, [Cu2(u-Ph2Pphen)2(u-I)]+, [Cu2(u-Ph2Pphen)2(py)2]2 +, [Cu2(u-Ph2Pphen)2{u-S2CN(Et)2}]+, [Cu2(u-Ph2Pphen)2(n-bipy)]2+ and [Cu2(u-Ph2Pphen)2(n-phen)]2+. X-ray structure determinations have been completed for [Cu2(u-Ph2Pphen)2{u-S2CN(Et)2}]+ and [Cu2(u-Ph2Pphen)2(n-bipy)]2+. The X-ray crystal structures of these ligand-bridged complexes confirm that the phosphorus atom coordinates to one copper atom while the phenanthroline fragment chelates to the other copper atom with the result that each metal atom has a tetrahedral geometry. Chapter 4 reviews the synthesis and characterisation of palladium and platinum complexes of the 2-diphenylphosphino-1, 10-phenanthroline (Ph2Pphen) ligand. The comproportionation reaction with Pd(II) and Pd(0) afforded the dinuclear complex [Pd2(u-Ph2Pphen)2](BF4)2. The reaction of Ph2Pphen with platinum resulted in ill-defined products that could not be isolated and characterised. / Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 1998

This thesis details the attempt to design and synthesis a range of ligands and organocatalysts based on a common backbone design. Initial results were promising with a number of ligands being generated from our common C2-symmetric backbone. Unfortunately none of the molecules synthesises gave promising results in test reactions. Variations on the initial design also failed to give any encouraging results. More positively, work on phosphorus-nitrogen (P,N) ligands was successful, with a number of different ligands being synthesised and metal complexes prepared. Pleasingly we were able to obtain X-ray crystallography of one of these complexes indicating that the ligand was complexed to the metal via the phosphorus moiety. Work using the Buchwald-Hartwig reaction for coupling aryl bromides to both 1,2,3,4-tetrahydroisoquinoline and 1,2,3,4-tetrahydroquinoline was successful, with methodology being developed which we believe can be applied to the synthesis of Ancistrocladinium A. In particular the coupling between 1,2,3,4-tetrahydroisoquinoline and 1-bromonaphthalene afforded us the full carbon skeleton of the ring system of the natural product in one step, from which we were able to generate the iminium salt. We also investigated an alternative route for the synthesis of Ancistrocladinium A achieving atropoisomerism. Experimental data is provided in chapter three, and all X-ray crystallography structures reported in chapter two are provided in the appendix.

Very little information exists on the marine biogeochemistry of Rh, Pd and Pt, or the platinum group elements (PGE), an emerging group of contaminants whose principal emissions are associated with the abrasion of the catalytic converter in motor vehicles and chemotherapy drugs discharged in hospital wastes. In this study, Rh(III), Pd(II) and Pt(IV) were added individually and in combination to cultures of the marine microalga, Chlorella stigmatophora, maintained in coastal seawater at 15oC and under fluorescence lighting both in the presence and absence of trace nutrients (e.g. Fe, Co, Zn and EDTA). The accumulation of PGE was established under varying conditions (pH, algal biomass, PGE concentration, time) by ICP-MS analysis of seawater and nitric acid digests and EDTA washes of the alga, the latter giving a measure of PGE adsorption by C. stigmatophora. Under all conditions the extent of accumulation was in the order: Rh &gt; Pd &gt;&gt; Pt. In short-term (24-h) exposures, accumulation isotherms were quasi-linear up to PGE concentrations of 30 ug L-1, although Pd displayed convexity, hence saturation of available binding sites, at greater concentrations. The pH, adjusted between about 5.5 and 9.5 by addition of acid or base, did not have a great impact on PGE accumulation, with Rh displaying a moderate increase in accumulation and Pd a moderate reduction with increasing pH. More important, all PGE displayed a significant reduction in accumulation on a weight-normalized basis with increasing concentration of algae, an effect not reported for metal-marine algal interactions previously in the literature. Longer-term experiments showed that the rates of both overall accumulation and internalization were greatest for Pd and least for Pt. Consistent with this observation, the toxicity to C. stigmatophora evaluated by both pigment content and growth rate, was significantly greater for Pd than for Pt. Differences in the biogeochemical behaviours among the PGE are attributed to differences in their aqueous speciation in seawater, different affinities for the algal surface, different tendencies to cross the cell membrane, and especially with regard to Pd and Pt, differences in the rates of these interactions. Thus, although the equilibrium chemistries of Pd and Pt are very similar, their differential biogeochemistries are the result of kinetic constraints on reactions involving the latter. Because the environmental concentrations of PGE are predicted to increase with increasing emissions from vehicles and hospitals, the results of this study make an important contribution to an improved understanding of the likely effects and fates of these emerging contaminants in the marine environment. The results are also more generally important to an improved understanding of the interactions of trace metals with microalgae in seawater.

This Thesis explores the topic of large amplitude motion within molecular machines and the different mechanisms and molecular architectures that are exploited in order to achieve control over the relative positions of submolecular components with respect to one another. Chapter One provides a thorough survey of a vast range of molecular switches and machines that have been developed during the last two decades. The focus is on interlocked and non-interlocked systems that display highly controlled large amplitude motion and the principles that govern their operation. Initially, simple molecular switches and shuttles are described with the chapter finally arriving at complex molecular machines such as motors, ratchets and walking molecules. The importance of understanding the different mechanisms that dictate the operation of switchable molecular machines and their fundamental differences are highlighted throughout the chapter. Chapters Two to Four are devoted to reporting the author’s contributions to novel switchable molecular systems. Chapter Two describes the serendipitous discovery of an ion-pair template which has been exploited in rotaxane formation and the operation of an orthogonal interaction anion-switchable molecular shuttle. The macrocycle moves back and forth along the thread between a cationic pyridinium station and a metal coordinating triazole motif when chloride anions are bound and removed respectively from a palladium centre which is located inside the cavity of the macrocycle. Excellent positional integrity (>98%) of the ring at both stations is achieved due to the orthogonal binding modes in the two states of the shuttle. Chapter Three presents a non-interlocked molecular switch that operates through the manipulation of dynamic covalent chemistry. The switch is comprised of a “two legged”, small organic molecule (a “walking unit”), anchored to a three foothold track via one disulfide and one hydrazone bond. The acid promoted hydrazone exchange allows a specific ratio of the two positional isomers to be achieved at equilibrium. However, the system is also arranged in such a manner that the ratio can be biased towards one positional isomer when the hydrazone exchange is carried out alongside the photoisomerisation (Z  E) of a stilbene motif which is incorporated in the track. The isomerisation alters the relative free energies of the products by increasing the ring strain of one positional isomer with respect to the other, hence introducing bias into the system. The final chapter reports the logical progression of the work presented in Chapter Three and describes the pursuit of a four-station dynamic covalent energy ratchet, of which the net position of the walker unit can be driven away from a steady state, minimum energy distribution by orthogonal disulfide and hydrazone exchange and concomitant stilbene isomerisation. The endeavour towards the successful synthesis of this rather complex molecule is described alongside the principles for its proposed operation. Chapter Two is presented in the form of an article that has already been published in a peer-reviewed journal. No attempt has been made to rewrite this work other than a slight alteration in the order of figures in the text to allow for easier reading and re-formatting to ensure consistency of presentation throughout this thesis. The original paper is reproduced, in its published format in the Appendix. Chapters Two, Three and Four begin with a synopsis to provide a general overview of the work that is presented in addition to a grateful acknowledgement of the contribution of my fellow researchers.

In an effort to obtain more efficient and greener chemical transformations, a substantial amount of research interest has been directed towards the use of arene C-H bonds as functional groups. Hydroarylation of alkynes through direct functionalisation of C-H bonds has been studied in recent years leading to the development of high-yielding metal-mediated processes. The main aim of the current work is the addition of a third component in the hydroarylation of alkynes trough C-H activation, in order to achieve a second C-C bond formation. Attempts at palladium-catalysed three-component reaction of unactivated indoles with alkynes and aryliodides are described. The three-component reaction was studied in the intermolecular mode with both aryliodides and the more reactive diaryliodonium salts. These latter regents are reactive arylating and oxidising agents and have been used in the direct arylation of indoles under mild conditions through a PdII-PdIV catalytic cycle. In both cases the three-component product was not obtained. The intramolecular version of the reaction using alkyne-tethered indoles and diaryliodonium salts is also described. In this case the tandem process was successful, especially when using ethynylbenzyl indole derivatives, the Z-tetrasubstituted olefins could be selectively obtained under mild conditions. Finally, a low-yielding synthesis of chromenes from propargylaryl ethers and diaryliodonium salts is also discussed.