Browsing by Author "O'Kennedy, Sean James"

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    Analysis of hydroformylation catalysis mechanisms of rhodium catalyst precursors by computational and instrumental means.
    (Stellenbosch : Stellenbosch University, 2021-05) O'Kennedy, Sean James; Gerber, Wilhelmus J. ; Mapolie, Selwyn Frank ; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
    ENGLISH ABSTRACT: The most industrially prevalent hydroformylation catalysts are based mainly on cobalt or rhodium with various combinations of carbonyl, halide or tertiary phosphine ligands and derivatives thereof. Recently, catalyst designs have branched out to include nitrogenous ligands such as pyridines, imines, triazoles and pyrazoles as well as multidentate combinations of either nitrogen, oxygen and/or tertiary phosphine donor ligands. This dissertation focuses on the mechanistic pathways of hydroformylation catalyst in absence of a detectable hydride intermediate. Due to the transient nature and low abundance of the catalytic intermediates, the study is focused primarily on in situ and in silico modelling of proposed mechanistic pathways. In order to investigate the kinetic trans effect in transition metals an example reaction of the transformation of [RhCl6]3􀀀 was investigated. It has been well documented that the transformation of [RhCl6]3􀀀 to [Rh(H2O)6]3+ is slow (equilibration takes more than 10 years) in acidic solutions at room temperature. However, on the addition of base to the solution, the [RhCl6]3􀀀 complex initially forms the [RhCl5(OH)]3􀀀 complex which then rapidly converts to [Rh(OH)6]3􀀀. We show using computational methods that the transformation of the [RhCl5(OH)]3􀀀 to [Rh(OH)6]3􀀀 occurs via labilisation of the trans chloride to form a 5 coordinate [RhCl4(OH)]2􀀀 species that can readily react with another hydroxide. The transformation to a hydroxido complex causes a cascade of rapid transformations to form the final [Rh(OH)6]3􀀀 complex. The domain based localized pair natural orbital coupled cluster singles doubles and virtual triples (DLPNO-CCSD(T)) was used to calculated accurate transition state energies that agreed well with the experimental data and corroborated our proposed dissociative ligand exchange mechanism. Hydroformylation is a well understood catalytic reaction for the more prevalent cobalt and rhodium phosphine based catalysts that follow a pathway where the catalyst spends most of the catalytic cycle in the Co(I) or Rh(I) oxidation state, respectively. However, we show an alternative pathway where oxidative addition of hydrogen directly to the precursor forms a Rh(III) dihydride which remains in the octahedral geometry throughout most the catalytic cycle. The oxidative addition of hydrogen is shown to form low abundances of transient rhodium dihydride species by computational chemistry, explaining the lack of spectroscopically detectable species in situ. In addition, it is shown via computational fragment analysis and a local energy decomposition scheme that the double electron transfer to form the dihydride occurs on relaxation from the transition state towards the dihydride octahedral complex and not at the transition state itself. The overall catalytic mechanism for a the [[N-(pyridine-_N-methyl)-N0-(3-imine-_N-p-tolyl)]rhodium(I)(_4-1; 5-cyclooctadiene)]+ complex was investigated using in situ 1H NMR in conjunction with UHPLC-MS. The spectroscopic results indicated possible transamination of the imine ligand by reaction with wither the acetone solvent or the formed aldehyde product. Computational chemistry was used to model the proposed hydroformylation mechanism for the [[N-(pyridine-_N-methyl)-N0-(3-imine-_N-p-hydroxyphenyl)]dicarbonylrhodium(I)]+ and [N--(pyridine-_N-methyl)-N0-(3-imine-_N-p-hydroxyphenyl)]chlorocarbonylrhodium(I) complexes, using the [Rh(CO)4]+ and [RhCl(CO)3] complexes as model precursors. The computational results indicated that the overall mechanism is feasible with a dissociative step at each point except that of the alkene addition. Each 5-coordinate intermediate of the proposed dissociative mechanism was stabilised by agostic interaction between the vacant site and the hydrogen of the proximate coordinated alkyl or acyl ligand. Throughout the mechanism, competition is seen between the chloride, hydride, alkyl and acyl ligands. This trans influence was found to destabilise the complex isomers where these ligands are in the trans position. No steric or chiral specificity is expected in these complexes from computationally calculated geometries as there is no hindrance to the coordination of either the linear or branched alkyl ligand explaining the lack of n-iso specificity in experimental results.
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    A kinetic and thermodynamic study of procyanidin oligomer conformation by 1H NMR and DFT
    (Stellenbosch : Stellenbosch University, 2015-12) O'Kennedy, Sean James; Gerber, W. J.; De Villiers, A. J.; Brand, D. J.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
    ENGLISH ABSTRACT: Please refer to full text for abstract