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Activation of carbon monoxide, hydrogen and oxygen by rhodium halide complexes in solution Rosenberg, George Nathan

Abstract

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.

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