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

Abstract

The reactions (CuTL + 'R -LCun-R and the mechanism of decomposition of the transient complexes LCu11-R were studied using the pulse-radiolysis technique (L1 = H2O, L* 12 = 2,5,8,ll-tetramethyl-2,5,8,ll-tetraazadodecane; 'R = 'CH3, 'CH2COOH, 'CH(CH3)COOH, •-CH2CH2COOH.) The kinetics of formation of the transient complexes obey pseudo-firstorder 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 7*-1 x 10s 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 a bond for L2. 3. For R = CH2COOH and CH(CHs)-COOH the transient complexes decompose via heterolysis of the copper-carbon a bonds forming CunL. The effect of the nature of R and L on the kinetics of these reactions is discussed.

Understanding the mechanisms of transition metal catalysed redox reactions

“…It is known that alkyl 228 and aliphatic radicals rapidly coordinate copper(I) in aqueous solutions both in the absence 230 It is also interesting to note that the position of the OMRP equilibrium is always shifted towards the organometallic complex (kd,OMRP >> ka,OMRP) consistent with the expectations for RT OMRP.…”

Ligand Effects on the Reactivity of CuIL Complexes Towards Cl3CCO2−

“…Figure 5. Computer output of the absorbance vs. time; experimental conditions: Ar-saturated solutions containing 8.0·10 Ϫ4  (CuL 2 ) ϩ at pH ϭ 6.5 were mixed with Ar-saturated solutions containing 0.02  Cl 3 CCO 2 Ϫ and 0.5  NaClO 4 at the same pH measured at λ ϭ 350 nm Based on these results and on the assumption that the absorption band at λ ϭ 350 nm is due to the formation of the transient complex L 2 Cu II ϪCCl 2 CO 2 Ϫ (complexes with Cu II ϪC σ-bonds are known to absorb in this wavelength region [15] ) it is reasonable to propose that the following reactions and kinetic equations describe the system: [RX] and therefore it is concluded that the large error limit is due to the observation that k IV [Cu II L] is still not large enough and that [Cu II L] Ͼ 0 even when no Cu II L was added to the solutions.…”

“…The transient L 2 Cu II ϪCCl 2 CO 2 Ϫ is the most stable LCu II -alkyl complex in aqueous solution, observed to date. [15,19] …”

Quantum chemistry of organocuprates as intermediates of catalytic and photochemical reactions

“…[3] Some experimental evidences of formation of adducts between copper complexes and organic radicals were obtained, [4][5][6][7][8][9][10][11][12][13][14] but the information on their structure was very scarce. Over the last years organometallic chemistry of Cu(I) and Cu(III) was vastly in progress [15,16] in contrast to Cu(II); only few stable organocuprates (II) are known to date.…”