Environmentally friendly rhodium(I) model catalysts
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Date
2019-02
Authors
Morerwa, Zanele Gift
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State
Abstract
The aim of this study was to investigate model rhodium(I) complexes as environmentally friendly water-soluble homogeneous catalysts for processes such as the carbonylation of methanol, a process that is important in industry due to its role in i.e. the production of liquid fuels and bulk chemicals. The flexibility of manipulating tertiary phosphines in terms of bulkiness and Lewis basicity are factors which render them attractive candidates in the modification of square-planar complexes towards applied chemical processes. If the latter are utilized in homogeneous catalytic applications, different fundamental and bench-marking reactions are of importance such as substitution reactions and oxidative addition. A range of 4-(phenylamino)pent-3-en-2-onate (PhonyH) derivatives with various electron withdrawing and donating substituents on the para position on the N-phenyl ring were synthesized and characterised with infrared and NMR spectroscopy. The uncoordinated compounds were then used to synthesis a range of dicarbonyl-[4-(phenylamino)pent-3-en-2onato]-rhodium(I) complexes. Following the synthesis of the dicarbonyl-rhodium(I) complexes, tertiary phosphines, PR3 (PR3 = triphenylphosphine (PPh3) and 1,3,5-triaza-7phosphaadamantane (PTA)) were employed to substitute a CO from the parent complexes, forming carbonyl-[4-(phenylamino)pent-3-en-2-onato]-PR3-rhodium(I) [Rh(N,O-bid)(CO)(PR3)] complexes. Single crystal X-ray crystallographic determinations of the [Rh(4-CH3Phony)(CO)(PPh3)] and [Rh(4-F-Phony)(CO)(PPh3)] complexes were successfully completed and compared with literature. [Rh(4-CH3-Phony)(CO)(PPh3)] crystalized in the triclinic (P1) crystal system, whilst [Rh(4-F-Phony)(CO)(PPh3)] crystalized in the monoclinic space group (P21/c). A preliminary kinetic study of the CO substitution reaction and equilibrium studies were undertaken to evaluate how changes at the rhodium(I) centre could affect the reactivity of the rhodium(I) complex. Similarly, a preliminary kinetic study of the iodomethane oxidative addition to [Rh(4-CH3-Phony)(CO)(PPh3)] and [Rh(4-CH3-Phony)(CO)(PTA)] was undertaken to evaluate the reactivities of these Rh(I) model complexes. Through the substitution kinetic studies it was discovered that a large equilibrium constant (K1) was present favouring the formation of the [Rh(4-CH3-Phony)(CO)(PTA)] as product, but further investigation was identified to clarify some uncertainties due to the presence of other preliminarily identified but unknown species also present. The preliminary kinetic study of the iodomethane oxidative addition showed that the Rh(III) alkyl species which forms from the first product from the [Rh(4-CH3-Phony)(CO)(PTA)] species exhibits a larger equilibrium constant value (K1) than that obtained for the corresponding [Rh(4-CH3-Phony)(CO)(PPh3)], although not that significant. Nevertheless, similar experiments when evaluating the [Rh(4-CH3-Phony)(CO)(PTA)] as reactant confirmed its reactivity similar to the PPh3 analog and hence the potential application for the design of future water-soluble catalyst models.
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Keywords
Dissertation (M.Sc. (Chemistry))--University of the Free State, 2019, Catalysis, Green chemistry, Rhodium(I), Homogeneous catalysts, XRD, Substitution kinetics, Oxidative addition