Kinetics and Reaction Mechanisms of Copper(I) Complexes with Aliphatic Free Radicals in Aqueous Solutions. A Pulse-Radiolysis Study

The reactions (Cu^IL +. R → LCu^II-R and the mechanism of decomposition of the transient complexes LCu^II—R were studied using the pulse-radiolysis technique (L¹ = H₂O, L²= 2,5,8,11-tetramethy1-2,5,8,11-tetraazadodecane;. R —. CH₃, •CH₂COOH, •CH(CH₃)COOH, •-CH₂CH₂COOH.) The kinetics of formation of the transient complexes obey pseudo-first-order rate laws. The rate constants measured for these reactions are in the range (2–4) x 10⁹ M^-1 s^-1 for L¹ and 6 x 10⁶ –1 x 10⁸ M^-1 s^-1 for L². The mechanisms of decomposition of the transient complexes depend on the nature of R and L as follows: 1. For R = CH₃ ethane is the final product. The kinetics of its formation obey second-order rate laws. Free methyl radicals are not intermediates in these processes. 2. For R = CH₂CH₂COOH the transient complexes decompose via an analogous mechanism to that of R = CH₃ for L¹ and via homolysis of the copper-carbon σ bond for L². 3. For R = CH₂COOH and CH(CH₃)-COOH the transient complexes decompose via heterolysis of the copper—carbon σ bonds forming Cu^IIL. The effect of the nature of R and L on the kinetics of these reactions is discussed.