Oxidative homolysis reactions between organochromium macrocycles and dihalide radical anions

Abstract

Many electron acceptors are able to oxidize organometals. In the case of organochromium(III) complexes such as (H{sub 2}O){sub 5}CrR{sup 2+} and RCrL(H{sub 2}O){sup 2+} (L = (15)aneN{sub 4} = 1,4,8,12-tetraazacyclopentadecane), studies of the oxidation step have been examined for the acceptors Ru(bpy){sub 3}{sup 3+}, {sup 2}E-Cr(bpy){sub 3}{sup 3+}, Ni((14)aneN{sub 4}){sup 3+}, and IrCl{sub 6}{sup 2{minus}}. As a result of the reaction between R{sm bullet} and I{sub 2}, I{sm bullet} is formed and thus I{sub 2}{sup {sm bullet}{minus}}. The form of the rate law for the reaction between I{sub 2} and RCrL(H{sub 2}O){sup 2+} allowed us to infer that one of the chain-propagating steps was the oxidation of the organometal by I{sub 2}{sup {sm bullet}{minus}}. The study of the I{sub 2} reaction did not give a value of this rate constant, because k for the chain reaction is a composite that contains also the rate constants for the initiation and termination steps. The authors decided to investigate the reaction involving I{sub 2}{sup {sm bullet}{minus}} for several reasons. First, they wanted to confirm that such a reaction occurs rapidly enough for it to be a chain propagation step in the iodine reaction. Second, they wanted to use its rate constant, in conjunctionmore » with the observed constant for the chain reaction, to calculate the rate constant for other elementary steps in the chain. Third, by variation of the group R and by use of other X{sub 2}{sup {sm bullet}{minus}} radicals, they sought to learn more about the nature of the rate-determining step in the reaction. The point behind the use of the macrocycle is that, in donating considerable electron density to the metal, is assists oxidative pathways (for X{sub 2} and X{sub 2}{sup {sm bullet}{minus}}) and decreases solvolytic and homolytic decomposition pathways.« less