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Kinetic studies on catalytic properties of rhodium complexes in solution Rempel, Garry Llewellyn

Abstract

Kinetic studies of a number of interesting and significant reactions involving activation of molecular hydrogen, simple olefins and carbon monoxide by solutions of rhodium trichloride trihydrate are described. RhCl₃∙3H₂O in N,N-dimethylacetamide solution is an effective catalyst for the homogeneous hydrogenation of a variety of substituted ethylenes as well as for ethylene itself. The kinetics of the RhCl₃∙3H₂O catalyzed hydrogenation of maleic acid in dimethylacetamide media suggest a mechanism involving an initial hydrogen reduction of rhodium (III) to rhodium (I), which is stabilized in solution by rapid complexing with maleic acid. The rhodium (i) maleic acid complex undergoes a subsequent reaction with hydrogen to produce succinic acid and rhodium (l) again, via an intermediate containing both coordinated maleic acid and hydrogen. In aqueous acid-chloride solution, rhodium (I) olefin complexes also form, but no subsequent homogeneous hydrogenation is apparent. Solutions of RhCl₃∙3H₂O in dimethylacetamide react with ethylene to form a rhodium (I) ethylene complex. Examination of the kinetics of the reaction suggest that the reaction proceeds through an initial dissociation of a chloro rhodium (III) species. A resulting intermediate rhodium (III) ethylene complex is decomposed by water to rhodium (I) which is stabilized by rapid reaction with further ethylene as an olefin complex. The resulting rhodium (I) ethylene complex subsequently acts as a dimerization catalyst for the production of butenes. Ethylene catalyzes at ambient temperatures the production of the [Rh(H₂O)₄Cl₂ ]⁺ cation from aqueous solutions of RhCl₃∙3H₂O; at higher temperatures metallic rhodium is formed. Carbon monoxide reacts with chlororhodate (III) complexes in aqueous HCl solution to form the anionic species [Rh(I)(CO)₂Cl₂]⁻. The observed kinetics indicate that [Rh(I)(CO)₂Cl₂]⁻ is autocatalytically produced; the mechanism postulated involves initial production of some [Rh(I)(CO)₂Cl₂]⁻ through a CO "insertion" reaction with a chlororhodate (III) complex. The [Rh(I)(CO)₂Cl₂]⁻ species Is then involved in a two electron transfer process via a chloride bridged [Rh(I)...Cl…Rh(III) ] intermediate to produce further Rh(I) more efficiently than by the initial direct insertion-reduction process. Carbon monoxide is catalytically activated through coordination to rhodium for subsequent reduction of rhodium (III) resulting from the electron transfer process. An inorganic substrate, ferric chloride, is catalytically reduced by [Rh(I)(CO)₂Cl₂]⁻ in this way. Kinetic studies of direct carbonylation of RhCl₃∙3H₂O in dimethylacetamide solutions have shown the importance of the presence of a water molecule for the production of the [Rh(I)(CO)₂Cl₂]⁻ species from an initially formed Rh(III)(CO) complex. The introduction of carbonyl groups into chlororhodate complexes is found to inhibit catalytic activity for the hydrogenation of olefinic substrates.

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