Kinetic studies are described for the activation of carbon monoxide, molecular hydrogen, and molecular oxygen by rhodium halide complexes in solution. Carbon monoxide reacts with bromorhodate(III) and bromocarbon-ylrhodate(III) complexes to produce the anionic species, [Rh(C0)2Br2]⁻. The reaction of [RhBr4(H2O)2]²⁻ in acid bromide solutions involves initial formation of a RhIII(C0) substitution product, which then undergoes
reductive carbonylation: [Eqn 1 and 2]
Further production of RhI is autocatalytic due to a more effective reduction via a CO independent path that involves a [RhIII…Br…RhI(CO)2I] bridged intermediate. Reductive carbonylation of the pentabromocarbonyl-rhodate(III) complex in aqueous HBr solution proceeds in a similar manner. Decomposition of RhIIICO) by water to produce Rh1 species (equations 1 and 2) results in the autocatalytic formation of [Rh(C0)2Br2]⁻ through a step that again involves a bromide-bridged intermediate, [(CO)RhIII…Br ---RhI(C0)2] . Carbon monoxide can be catalytically activated by rhodium(III) for the reduction of substrates such as ferric ion. Kinetic studies sug-gest a mechanism involving reduction of [Rh(C0)Br5]²⁻ by water. Data are consistent with the following scheme: [Eqn 3]
Acid solutions of RhBr3•2H20 activate H2 for the reduction of FeIII . Formation of a metal hydride in the rate-determining step appears to involve substitution by an associative mechanism: [Eqn 4]
Activation parameters for the reaction of [RhBr4(H20)2]⁻ with H2 are similar to those found for the corresponding reaction with CO (equation 1). The reaction of [Rh(C0)2Cl2]⁻ with 02 in 3M HC1 initiallyforms [RhIII(C0)C151]²⁻ by a path thought to involve formation of an oxygen complex: [Eqn 5]
The observed kinetics indicate that further RhIII(CO) is autocatalytically produced according to the following sequence: [Eqn 6, 7, 8]
Subsequent slower reaction to give chlororhodate(III) species 2-occurs via slow hydration of [Rh(C0)Cl5]²⁻. [Eqn 9,10]
Studies on the oxidation of [Rh(CO)2C12]⁻ in LiCl/DMA by molecular 02 suggest the involvement of hydroperoxide free radicals for the production of RhII. Evidence for a carbonato intermediate formed during the oxidation process has been obtained. The RhII solutions were found to contain active, though inefficient, catalysts for the autoxida-tion of DMA. The presence of RhII - 02 has been detected by esr, and the superoxide species may be responsible for the autoxidation reaction. A paramagnetic chloride-bridged rhodium(II) anion [Rh0Clc(DMA)2]²⁻ was isolated from the oxygenated DMA solutions of [Rh(C0)2Cl2]⁻ containing excess chloride. / Science, Faculty of / Chemistry, Department of / Graduate
| Identifer | oai:union.ndltd.org:UBC/oai:circle.library.ubc.ca:2429/19533 |
| Date | January 1974 |
| Creators | Rosenberg, George Nathan |
| Source Sets | University of British Columbia |
| Language | English |
| Detected Language | English |
| Type | Text, Thesis/Dissertation |
| Rights | For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use. |
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