Doctoral Degrees (Chemistry)

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Browsing Doctoral Degrees (Chemistry) by Subject "Activation Parameters"

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    A crystallographic, computational and mechanistic study of rhodium enaminoketonado complexes
    (University of the Free State, 2013-01) Venter, Gertruida Jacoba Susanna; Roodt, Andreas; Steyl, Gideon
    English: This study includes the investigation of enaminoketones as ligand systems in rhodium complexes with possible future application in catalysis. In order to evaluate the influence of substituents on the phenyl ring on activity of the complex, a range of 4-(phenylamino)pent-3-en-2-onate (PhonyH) derivatives with chloride substituents on different positions on the phenyl ring were synthesized and characterized through X-ray crystallography as well as infrared and NMR spectroscopy. The compounds crystallize in a range of space groups and varying crystal systems, are stable in air over a period of several years and soluble in most solvents. The optimized structures of these compounds were calculated using DFT methods. The relative energies of the optimized structures adopt a cumulative nature ‒ the relative energy of 2,4-Cl2-PhonyH with regard to unsubstituted PhonyH is roughly equal to the sum of the relative energies of 2-Cl- PhonyH and 4-Cl-PhonyH, while the relative energy of 2,6-Cl2-PhonyH is twice the relative energy of 2-Cl-PhonyH. The distortion of the phenyl ring from the ideal planar position presented in the calculated structures corresponds to the distortion observed in the solid state. The synthesis of the uncoordinated compounds was followed by the synthesis and characterization of a range of substituted dicarbonyl-[4-(phenylamino)pent-3-en-2-onato]- rhodium(I) complexes. The complexes crystallized in varying crystal systems and space groups. The trans influence of nitrogen was confirmed through the difference in the Rh-CO bonds: the Rh-C bond trans to the nitrogen atom is longer than the Rh-C bond trans to oxygen. The impact of the chloride substituents was observed from differences in geometrical parameters and is supported by information from the calculated structures and literature. The optimized structures of these complexes were calculated using DFT methods, and their optimized energies follow the same cumulative trend as observed in the uncoordinated compounds. A range of carbonyl-[4-(phenylamino)pent-3-en-2-onato]-triphenylphosphine-rhodium(I) {[Rh(N,O-Bid)(CO)(PPh3)]} complexes were synthesized and characterized, containing both electron-withdrawing chloride atoms and electron-donating methyl groups. These complexes displayed poor solubility, but once dissolved, were stable over a period of several months. Isomorphism was observed between [Rh(2,6-Cl2-Phony)(CO)(PPh3)] and [Rh(2,6-Me2- Phony)(CO)(PPh3)]. [Rh(2,6-Cl2-Phony)(CO)(PPh3)] and [Rh(2,6-Me2-Phony)(CO)(PPh3)] were chosen to investigate the exchange of triphenylphosphine coordinated in [Rh(N,O-Bid)(CO)(PPh3)] complexes with the uncoordinated phosphine, allowing for the comparison of the electronic effect of the substituents on the phenyl rings. The method chosen for the investigation was magnetization spin transfer, an NMR technique which utilizes the magnetic properties of nuclei and determines the kinetic properties of the exchange reaction by following the rate at which magnetic equilibrium is restored. The rate of the phosphine exchange reaction in [Rh(2,6-Cl2-Phony)(CO)(PPh3)] was determined as approximately three times faster than the rate of reaction for phosphine exchange in [Rh(2,6- Me2-Phony)(CO)(PPh3)]. The decreased electron density surrounding the rhodium atom in [Rh(2,6-Cl2-Phony)(CO)(PPh3)] allows for the reversal of the reaction as indicated by the k-1 values of approximately 11 s-1 calculated from the [Rh(2,6-Cl2-Phony)(CO)(PPh3)] exchange reaction. This value is absent in the reaction of the [Rh(2,6-Me2-Phony)(CO)(PPh3)] complex. The activation parameters of the exchange reaction in [Rh(2,6-Cl2-Phony)(CO)(PPh3)] (ΔH‡ = 25(3) kJ.mol-1 and ΔS‡ = -117(9) J.K-1.mol-1) correlate well with the parameters of the exchange reaction in [Rh(2,6-Me2-Phony)(CO)(PPh3)] (ΔH‡ = 24(4) kJ.mol-1 and ΔS‡ = -124(12) J.K-1.mol- 1). In both cases the value for entropy, ΔS‡, is negative, indicating an associative mechanism. The relative contribution of TΔS‡ to ΔG‡ is approximately 60% for both complexes, whereas the enthalpy (ΔH‡) terms are correspondingly small. This indicates that the activation process is primarily controlled by entropy and involves the formation of a stable, well-ordered transition state while bond weakening is less important. The relatively constant values for ΔG‡ imply that the exchange reaction is not very sensitive to changes in temperature.