Benzylpentaquochromium (III)-chromiun (II) exchange, and the reduction of some benzyl compounds by chromiun (II)


Ashbrook, Allan W.




Rates of electron transfer between benzylpentaaquochromium(ITT) ions and chromium(TT) ions in methanol-water solution have been studied. The reaction pathway was found to be first order in each reactant and to be hydrogen ion independent. Rate constants for substituted and unsubstituted benzylpentaaquochromiiim (T IT) ions were similar, -2 -1 -1 about 1 x 10 M sec at 0°. Activation parameters in the case of the unsubstituted ion were AS* = -29 eu and AH* = 9 kcal mole"1. The electron transfer mechanism is discussed with emphasis on comparisons with reactions of established mechanism. It is concluded that electron transfer occurs by an inner-sphere mechanism via a hydrogen-bridged activated complex. Isotope exchange reactions between two benzylpentaaquochromium(TII) ions have also been studied. The exchange is again first order in each - 7> -1 reactant, with a rate constant of 1.2 x 10 M sec at 0° in methanol-water solvent. Actuation parameters were similar to those given above. The proposed mechanism for this exchange reaction is one involving a four-centered transition state involving both reactants in which both chromium atoms have a formal oxidation state of +2. Studies on reactions between benzylpentaaquochromium(ITI) ion and benzyl halides are reported. These reactions were studied from the point of view of the chromium-containing species. Rate constants for the reactions involved are given, and the calculated and experimental data compared. The actual mechanisms involved have not been completely elucidated and still appear to be rather complex. Organometallic (benzyl halide)chromium(O) tricarbonyl complexes are shown to be reduced by chromous ion with overall second order kinetics and to form metastable, water-soluble (benzylchromium(ITT))chromium(0) tricarbonyl ions, [(PhCH2CrTII)Cr°(CO)3]2+, as the reaction intermediate. While these reduction reactions appear similar to those for the chromous ion reduction of benzyl halides, the reduction rates for the series of tricarbonyls (chloride, bromide, and iodide) are -3 -1 essentially constant at approximately 6.0 x 10 M sec at 25°. This is interpreted as being due to the formation, in each case, of a reaction intermediate formed from a free radical in which the electron of the radical goes into a non-bonding orbital of the chromium(0) atom. The reaction of this species with chromous ion is taken as being the rate determining step in the formation of the intermediate. The decomposition of (benzylchromium(III))chromium(0) tricarbonyl ion in acid solution gives toluenechromium(0) tricarbonyl in almost quantitative yield and there is no evidence for the formation of dimeric products. Chromous ion reduction of a,a'-dibromoxylenes gives rise to the formation of yellow, metastable, water-soluble intermediates; o-(bromomethyl)benzylchromium(TII) ion from a,a"-dibromo-o-xylene and the para from.


Chromium compounds.
Reduction (Chemistry)




Carleton University

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