Kinetic studies of a number of interesting and significant reactions involving activation of molecular hydrogen, simple olefins and carbon monoxide by solutions of rhodium trichloride trihydrate are described. RhCl₃∙3H₂O in N,N-dimethylacetamide solution is an effective catalyst for the homogeneous hydrogenation of a variety of substituted ethylenes as well as for ethylene itself. The kinetics of the RhCl₃∙3H₂O catalyzed hydrogenation of maleic acid in dimethylacetamide media suggest a mechanism involving an initial hydrogen reduction of rhodium (III) to rhodium (I), which is stabilized in solution by rapid complexing with maleic acid. The rhodium (i) maleic acid complex undergoes a subsequent reaction with hydrogen to produce succinic acid and rhodium (l) again, via an intermediate containing both coordinated maleic acid and hydrogen. In aqueous acid-chloride solution, rhodium (I) olefin complexes also form, but no subsequent homogeneous hydrogenation is apparent. Solutions of RhCl₃∙3H₂O in dimethylacetamide react with ethylene to form a rhodium (I) ethylene complex. Examination of the kinetics of the reaction suggest that the reaction proceeds through an initial dissociation of a chloro rhodium (III) species. A resulting intermediate rhodium (III) ethylene complex is decomposed by water to rhodium (I) which is stabilized by rapid reaction with further ethylene as an olefin complex. The resulting rhodium (I) ethylene complex subsequently acts as a dimerization catalyst for the production of butenes. Ethylene catalyzes at ambient temperatures the production of the [Rh(H₂O)₄Cl₂ ]⁺ cation from aqueous solutions of RhCl₃∙3H₂O; at higher temperatures metallic rhodium is formed. Carbon monoxide reacts with chlororhodate (III) complexes in aqueous HCl solution to form the anionic species [Rh(I)(CO)₂Cl₂]⁻. The observed kinetics indicate that [Rh(I)(CO)₂Cl₂]⁻ is autocatalytically produced; the mechanism postulated involves initial production of some [Rh(I)(CO)₂Cl₂]⁻ through a CO "insertion" reaction with a chlororhodate (III) complex. The [Rh(I)(CO)₂Cl₂]⁻ species Is then involved in a two electron transfer process via a chloride bridged [Rh(I)...Cl…Rh(III) ] intermediate to produce further Rh(I) more efficiently than by the initial direct insertion-reduction process. Carbon monoxide is catalytically activated through coordination to rhodium for subsequent reduction of rhodium (III) resulting from the electron transfer process. An inorganic substrate, ferric chloride, is catalytically reduced by [Rh(I)(CO)₂Cl₂]⁻ in this way. Kinetic studies of direct carbonylation of RhCl₃∙3H₂O in dimethylacetamide solutions have shown the importance of the presence of a water molecule for the production of the [Rh(I)(CO)₂Cl₂]⁻ species from an initially formed Rh(III)(CO) complex. The introduction of carbonyl groups into chlororhodate complexes is found to inhibit catalytic activity for the hydrogenation of olefinic substrates. / Science, Faculty of / Chemistry, Department of / Graduate

In the presence of iron (III) and other oxidants, HCl solutions of RhCl₃.3H₂O under mild conditions catalyze the oxidation of ethylene to acetaldehyde. The kinetics of the reaction measured by gas-uptake techniques indicate the presence of both ethylene dependent and independent steps. Hydroxy species such as RhCl₅(OH)³ ̄and RhCl₄(OH) (H₂O)² ̄, although present in very small concentrations, are significantly reactive towards ethylene. A mechanism based on that postulated for a similar Pd(II) system is presented. This involves the rearrangement of a rhodium (III) hydroxy π - ethylene complex to a σ - complex, followed by the production of acetaldehyde and rhodium (I). However, unlike in the Pd(II) system where the rate determining step is the conversion of the π - to σ-C₂H₄ complex, the rate determining step in the Rh(III) system is thought to involve the production of the π- complex. Iron (III) oxidizes Rh(I) back to Rh(III), giving the net reaction: C₄H₄ + H₂O + 2Fe(III) Rh(III) ⇥ CH₃CHO + 2H+ + 2Fe(II) / Science, Faculty of / Chemistry, Department of / Graduate

The work in this thesis includes crystallographic investigations with low-energy electron diffraction (LEED) for the surface structures designated Rh(111)-(√3x√3)30°-S, Rh(111)-(2x2)-0, Zr(0001)-(1x1)-0 and Zr(0001)-(1x1)-N. In each case intensity-versus-energy (I(E)) curves for a set of diffracted beams were measured with a video LEED analyzer, and then compared with the results of multiple scattering calculations made for various structural models. Levels of correspondence between experimental and calculated 1(E) curves were assessed with the reliability index proposed by Pendry, and surface geometries were determined by the conditions for the best correspondence. The LEED intensity analyses for both the Rh(111)-(√3x√3)30°-S and Rh(111)-(2x2)-0 surface structures indicate that S and 0 atoms adsorb respectively 1.53A and 1.23A above the "expected" hollow sites of three-fold coordination. These values correspond to nearest-neighbor Rh-S and Rh-0 bond distances equal to 2.18 and 1.98A respectively. For the Zr(0001)-(1x1)-0 and Zr(0001)-(1x1)-N surface structures studied, the multiple-scattering analyses suggest that the first involves 0 atoms occupying octahedral holes between successive bulk Zr layers, and that the substrate Zr layers undergo a fee type reconstruction. By contrast the N atoms in Zr(0001)-(1x1)-N appear to just occupy octahedral holes between the first and second layers of hep zirconium, exactly as reported by Shih et al. for the analogous structure formed on titanium. The LEED-determined Zr-0 and Zr-N bond distances are 2.30 and 2.27A respectively, in very close agreement with the values determined by X-ray crystallography for bulk ZrO (2.31A) and bulk ZrN (2.29A). A preliminary study of oxygen chemisorption on the Zr(0001) surface has been made in the low-exposure regime with Auger electron spectroscopy (AES) and with measurements of the width of a half-order LEED beam. Some observations and conclusions are: (i) the diffusion of 0 atoms to the bulk effectively starts at around 236°C; (ii) oxygen adsorbs in a disordered state at room temperature but orders sufficiently to show a (2x2)-type LEED pattern on heating to 220°C; (iii) with increasing 0 exposure, 1/4, 1/2 and 3/4 of the available sites can be systematically filled, prior to the establishment of an ordered (1x1)-0 surface; (iv) the process in (iii) can be reversed by starting with the (1x1)-O surface and heating above 236°C. LEED and AES have also been used to compare the adsorption and coadsorption of 0₂ and H₂S on the Zr(0001) surface for exposures in the one to five Langmuir regime. The new observations made are: (i) sulfur forms a stable (3x3) surface structure after heating to 600°C; (ii) the Zr(0001) surface with high 0 coverage can still adsorb H₂S, whereas the Zr(0001) surface with high S coverage does not adsorb oxygen in detectable amounts; (iii) for surfaces with adsorbed H₂S the 150 eV to 92 eV Auger peak ratio suddenly increases on heating to 530°C. Observation (iii) has been tentatively interpreted in terms of hydrogen desorption. Finally, a set of 1(E) curves were measured for normal incidence on the Zr(0001)-(3x3)-S surface. / Science, Faculty of / Chemistry, Department of / Graduate

The original purpose of this work was to investigate the catalytic properties of a series of Rh₂(CO)₄(P-P)₂ complexes (where P-P = ditertiary phosphines of the type PR₂(CH₂)nPR₂, R = alkyl or aryl) for hydroformylation. The preparation of Rh₂(CO)₄(P-P)₂ involves the synthesis of Rh(P-P)₂Cl, followed by reaction with NaBH₄ to give RhH(P-P)₂, which when treated with CO in benzene yields Rh₂(CO)₄(P-P)₂, as reported in the literature. The dimer, Rh₂(CO)₄(dpp)₂, where dpp = PPh₂(CH₂)₃PPh₂, was prepared and examined for its interaction with H₂, and H₂/CO, in order to test its capabilities for catalytic homogeneous hydroformylation. The interaction of Rh₂(CO)₄(dpp)₂ (49) with H₂, and the reaction of CO with RhH(dpp)₂ (52) to yield 49, are summarized as follows: [Formula Omitted] All the species shown, except 59, have been detected by ¹H and ³¹P{¹H} NMR spectroscopy. Formation of the monomeric hydride, 50, from 49, occurs at high [dpp]. The reaction of Rh₂(CO)₄(dpp)₂ and 6 equivalents dpp with synthesis gas (H₂ : CO = 1 : 1) gives initially 50 and R₂(CO)₄(dpp)₂ reforms after 30 minutes of interaction. This is consistent with the previous finding of low turnover rate for hydroformylation of 1-hexene using as catalyst the co-ordinatively saturated Rh₂(CO)₄(dpp)₂. Treatment of 52 in toluene with ~1 atm CO, followed by treatment with ~1 atm H₂, sets up the following equilibria (where dpp* = monodentate dpp): [Formula Omitted] The homogeneous hydrogenation of 1-hexene at 31° C, - 1 atm H₂, catalyzed by "the RhH(dpp)₂/CO/H₂ system" in toluene is ascribed to the formation of an unidentified "RhH" from 50 and/or 51. The H₂-uptake curve displayed an initial ("inductive") period required for the generation of an active species "RhH", a second period of maximum rate, and a final slowing down period. The mechanism suggested for homogeneous hydrogenation of 1-hexene catalyzed by the "RhH(dpp)₂/CO/H₂ system" is presented as follows: [Formula Omitted] The corresponding rate law for the maximum rate, consistent with the kinetic data, is given by: [Formula Omitted] where ["Rh"]t is total concentration of the active "RhH" catalyst. At high [1-hexene], where k₃[1-hexene] >> k₋₃ + k₄[H₂], the rate law is simplified to: Rate = k₄[H₂],["RhH"]t where ["RhH"]t ~ total rhodium concentration in solution. The values of k₃ and k₄ at 31° C were found to be 0.42 M⁻¹ s⁻¹ and 20 M⁻¹ s⁻¹ respectively. The Rh(dcpe)₂ ⁺X⁻ complexes (X = CI, BF₄, PF₆; dcpe PCy₂(CH₂)₂PCy₂) were prepared and found to have no reactions with NaBH₄ or LiAlH₄. Consequently, the dcpe carbonyl dimer could not be prepared. The Rh(p = p)₂ ⁺Cl⁻ complex, where p = p = PPh₂C₂H₂PPh₂, was isolated and characterized; its reaction with NaBH₄ was incomplete, partially generating RhH(p=p)₂. Treatment of the mixture with CO gave partially Rh(CO)(p=p)₂ ⁺Cl⁻ and another uncharacterized carbonyl complex. A single crystal X-ray structure determination of Rh(dcpe)₂ ⁺Cl⁻ showed that the geometry around Rh is distorted square planar. Also, the extremely air-sensitive species [RhCl(dcpe)• solv]n (solv = THF or 0.1 C₆H₆) and RhCl(dcpe)(CH₂Cl₂)•C₆H₆ were isolated. The interaction of Rh(dcpe)₂ ⁺Cl⁻ with small gas molecules was studied in order to test its potential as a catalyst. There is interaction between Rh(dcpe)₂ ⁺Cl⁻ and HCI, Cl₂, and CO, in CH₂C1₂. The reaction with HCI to give cis-RhHCl(dcpe)₂ ⁺Cl⁻ is extremely rapid. The use of stopped-flow kinetics and UV-VIS spectrophotometric techniques at 25° C gave an equilibrium constant of 4.2 x 10⁷ M⁻¹ for the reaction. The forward reaction was first-order in both [Rh(dcpe)₂ ⁺Cl⁻] and [HCI], indicating a concerted oxidative addition reaction. The RhHCl(dcpe)₂⁺ species reacts further with HCI to give RhHCl₂(dcpe) and the diphosphonium salt, dcpe(HCl)₂. The Rh(dcpe)₂ ⁺Cl⁻ complex reacts with Cl₂ to give RhCl₂(dcpe)₂ ⁺Cl⁻, which was also obtained by prolonged treatment of RhHCl(dcpe)₂ ⁺Cl⁻ with CDCl₃ The reaction of Rh(dcpe)₂ ⁺Cl⁻ with CO to give Rh(CO)(dcpe)₂ ⁺Cl⁻ yielded k on and k off values of 2.2 x 10⁻² M⁻¹ s⁻¹ and 5.02 x 10⁻⁴ s⁻¹ respectively at 25° C. The Rh(dcpe)₂ ⁺Cl⁻, complex was inactive as a catalyst for decarbonylation of benzaldehyde, or hydrogenation of 1-hexene. / Science, Faculty of / Chemistry, Department of / Graduate

The present work was directed toward the synthesis of a new chiral catalyst for asymmetric homogeneous hydrogenation. Efficient ways to synthesize the ferrocenylphosphine ligands (R,S)- and (S,R)-⍺-[2-diphenylphosphinoferrocenyl]ethyldimethylamine ((R,S)- and (S,R)-FcNP) and their cationic rhodium complexes [(diene)Rh(±)FcNP]⁺A⁻ were developed. Structural data for the ligand and models of its metal complex have been used to rationalize the stereochemical approach of the substrate to the metal complex, and hence predict the absolute configuration of the product. The rate of catalytic hydrogenation is dependent on the substrate as is the optical yield of the product alkane. High optical yields are obtained when ⍺-acetamidocinnamlc acid is hydrogenated at 1 atm H₂ and 32°. / Science, Faculty of / Chemistry, Department of / Graduate

No description available.

The aim of this study was to synthesise mono-phosphite complexes of type [Rh(OX)(CO){P(OY)3}], (where Y = the different phosphites that were used and OX = 8-hydroxyquinoline) and to do a kinetic study of iodine oxidative addition to these rhodium(I) square planar complexes in order to determine the rate constants and the reaction mechanism. Part of the characterization was X-ray crystallographic structure determinations which were done on two complexes, namely the [Rh(OX)(CO){P(O(2,4-t-BuPh))3}] and the [Rh(OX)(CO){ P(O(2,6-diMePh))3}]. From the characterization methods it can be said with certainty that the synthesis of the mono-phosphite rhodium(I) complexes was successfully achieved. Table 1 - Selected crystal data as obtained for the two Rh(I) crystal structures solved in this study. [Rh(OX)(CO){P(O(2,4-t- / Prof. A. Roodt

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

published_or_final_version / Chemistry / Master / Master of Science

published_or_final_version / Chemistry / Master / Master of Philosophy