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 â 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)] (ÎHâ¡ =
25(3) kJ.mol-1 and ÎSâ¡ = -117(9) J.K-1.mol-1) correlate well with the parameters of the exchange
reaction in [Rh(2,6-Me2-Phony)(CO)(PPh3)] (ÎHâ¡ = 24(4) kJ.mol-1 and ÎSâ¡ = -124(12) J.K-1.mol-
1). In both cases the value for entropy, ÎSâ¡, is negative, indicating an associative mechanism.
The relative contribution of TÎSâ¡ to ÎGâ¡ is approximately 60% for both complexes, whereas the
enthalpy (ÎHâ¡) 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 ÎGâ¡ imply that
the exchange reaction is not very sensitive to changes in temperature.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ufs/oai:etd.uovs.ac.za:etd-07182013-100717 |
| Date | 18 July 2013 |
| Creators | Venter, Gertruida Jacoba Susanna |
| Contributors | Prof G Steyl, Prof A Roodt |
| Publisher | University of the Free State |
| Source Sets | South African National ETD Portal |
| Language | en-uk |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.uovs.ac.za//theses/available/etd-07182013-100717/restricted/ |
| Rights | unrestricted, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to University Free State or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
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