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Activation of hydrogen by rhodium nad iridium chloro complexes containing sulphide or arsine type ligands. Ng, Flora Tak Tak


A kinetic study was undertaken to investigate the potentiality of rhodium chioro complexes containing diethyl sulphide ligands as homogeneous catalysts. Preliminary studies have also been carried out on the analogous iridium complexes and some rhodium complexes containing arsenic ligands. Under mild conditions of temperature and pressure (70-85°C and upto one atmosphere of hydrogen), it was found that cis RhCl₃(Et₂S)₃ in dimethylacetamide solution is an active catalyst for the homogeneous hydrogenation of maleic acid, trans cinnamic acid and ethylene. In benzene solution, no homogeneous hydrogenation occurred. The kinetics of the maleic acid hydrogenation have been studied in detail and it was found that the hydrogenation process consisted of two rate determining steps: the initial hydrogen reduction of Rh(III) to Rh(I) which complexed rapidly with the maleic acid present and the subsequent hydrogenation of the Rh(I) - maleic acid complex to yield succinic acid and Rh(I) again. The detailed mechanisms of these steps, with particular emphasis on solvent effects are discussed. The corresponding IrCl₃(Et₂S)₃ complex and Rh(III) complexes with "difars" and "diars" (two chelating arsenic ligands) were inactive as homogeneous hydrogenation catalysts, however reactions with H₂ were observed with the iridium complex itself and also the rhodium difars complex; these reactions are also discussed.

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