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Activation of hydrogen, olefins, oxygen and carbon monoxide by rhodium complexes in non-aqueous solvents Ng, Flora Tak Tak

Abstract

Kinetic studies of a number of interesting and significant reactions involving activation of hydrogen, olefins, oxygen and carbon monoxide by solutions of rhodium complexes containing sulphur and/or chloride ligands are described. The cis 1,2,3-trichlorotris(diethylsulphide)rhodium (III) complex, RhCl(EtS₃and the corresponding dibenzyl sulphide complex, RhCl₃(Bz₂S)₃in N,N -dimethylacetamide (DMA) solution were found to be effective catalysts for the homogeneous hydrogenation of maleic, fumaric and trans-cinnamic acids. The kinetic data are consistent with a dissociation of a sulphur ligand prior to the hydrogen reduction of rhodium (III) to rhodium (I). The rhodium (I) is stablized in solution by rapid complexing with the olefin to produce a Rh¹(olefin)(Ln) complex (L = auxiliary ligands) which then reacts with H₂ in a rate determining step to produce the saturated paraffin and rhodium (I). In some instances, more complex kinetics resulted when one of the auxiliary ligands in the Rh¹ (olefin)(Ln) complex dissociates prior to reaction with H(2); a unique apparent zero order in catalyst concentration has been observed. Isomerization was observed in the RhCl(EtS)₃catalyzed hydrogenation of fumaric acid and a mechanism involving rhodium (III) alkyl intermediate seems likely. The cyclooctene complex, [Rh(C8H14)₂C1], in DMA was found to be a convenient source for preparing rhodium (I) complexes "in situ" by adding the desired ligands, for example, chloride or diethyl sulphide. Kinetic data obtained using such solutions are in good agreement with the hydrogenation data obtained by starting from the corresponding rhodium (III) complexes. This result confirms that rhodium (I) intermediates are involved in the catalytic hydrogenation starting from rhodium (III) complexes. During studies to investigate the effect of solvent on catalytic hydrogenation of olefins by rhodium (III) complexes, dimethyl sulphoxide (DMSO) was found to be catalytically reduced by hydrogen to dimethyl sulphide and water in the presence of RhCl(EtS)₃ and RhCl‧3H2O. The kinetics were consistent with a rate determining heterolytic splitting of H₂ by Rh(III)(DMSO) to produce Rh[III](DMSO)H¯ which then decomposes to the products in a fast step. RhCl₃‧3H₂O also catalyzed the oxidation of DMSO to dimethyl sulphone using a mixture of oxygen and hydrogen. The solution of [Rh(C8H14)₂C1]₂ in DMA containing LiCl was found to be a versatile catalyst, for besides the activation of hydrogen and olefins, oxygen and carbon monoxide could also be activated. The formation of a rhodium (I) molecular oxygen complex, Rh(I)(O2) and a subsequent catalyzed oxidation of the DMA solvent and cyclooctene were studied in detail. The formation of the Rh(I)(O2) complex appears to be irreversible. An E.S.R. signal, possibly due to species such as Rh(II)(O2¯) was also observed. The kinetics of the oxidation suggest the equilibrium formation of the Rh(I)(O2) complex followed by a rate determining step to give the products. A free radical mechanism seems likely. Solutions of [Rh(C8H14)₂C1]₂ in LiCl/DMA readily reacted with carbon monoxide to form a Rh(I)(CO)₂ species. A solution of the oxygen complex was converted more slowly to the Rh(I)(CO)₂ species in a reaction whose observed rate was determined by the dissociation of the coordinated oxygen. Preliminary studies indicated that a mixture of CO and O₂ is converted catalytically to CO₂ by a solution of [Rh(C8H14)₂C1]₂ in LiCl/DMA.

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