A crystallographic, computational and mechanistic study of rhodium enaminoketonato complexes

[摘要] 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-C-PhonyH with regard to unsubstituted PhonyH is roughly equal to the sum of the relative energies of 2-CIPhonyH and 4-CI-PhonyH, while the relative energy of 2,6-Cb-PhonyH is twice the relative energy of 2-CI-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 andcharacterization 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: theRh-C bond trans to the nitrogen atom is longer than the Rh-C bond trans to oxygen. The impactof the chloride substituents was observed from differences in geometrical parameters and issupported by information from the calculated structures and literature. The optimized structuresof these complexes were calculated using OFT methods, and their optimized energies follow thesame cumulative trend as observed in the uncoordinated compounds.A range of carbonyl-[4-(phenylamino) pent-3-en-2-onato ]-triphenylphosphine-rhodium(l){[Rh(N,O-Bid)(CO)(PPh3)]} complexes were synthesized and characterized, containing bothelectron-withdrawing chloride atoms and electron-donating methyl groups. These complexesdisplayed poor solubility, but once dissolved, were stable over a period of several months.Isomorphism was observed between [Rh(2,6-Ch-Phony)(CO)(PPh3)] and [Rh(2,6-Me2-Phony)(CO)(PPh3)] .[Rh(2,6-Ch-Phony)(CO)(PPh3)] and [Rh(2,6-Me2-Phony)(CO)(PPh3)] were chosen to investigatethe exchange of triphenylphosphine coordinated in [Rh(N,O-Bid)(CO)(PPh3)] complexes withthe uncoordinated phosphine, allowing for the comparison of the electronic effect of thesubstituents on the phenyl rings. The method chosen for the investigation was magnetizationspin transfer, an NMR technique which utilizes the magnetic properties of nuclei and determinesthe kinetic properties of the exchange reaction by following the rate at which magneticequilibrium is restored.The rate of the phosphine exchange reaction in [Rh(2,6-Ch-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-C12-Phony)(CO)(PPh3)] allows for the reversal of the reaction as indicated by the k-, values of approximately 11 s-1 calculated from the [Rh(2,6-Cl2Phony)(CO)(PPh3)] exchange reaction. This value is absent in the reaction of the [Rh(2,6-Me2Phony)(CO)(PPh3)] complex. The activation parameters of the exchange reaction in [Rh(2,6-C12-Phony)(CO)(PPh3)] (∆H�?= 25(3) kJ.mol-l and ∆S�?= -117(9) J.K-l.mol-l) correlate well with the parameters of the exchange reaction in [Rh(2,6-Me2-Phony)(CO)(PPh3)] (∆H�?= 24(4) kJ.mol-l and ∆S�?= -124(12) J.K-I.mol-1I). In both cases the value for entropy, ∆S�? is negative, indicating an associative mechanism.The relative contribution of T∆S�?to ∆G�?is approximately 60% for both complexes, whereas the enthalpy (∆H�? 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 ∆G�?imply thatthe exchange reaction is not very sensitive to changes in temperature.

[发布机构] University of the Free State