Kinetics of the reaction between substituted thioureas and the palladium (II) aquocomplex of 1, 1, 7, 7-tetraethyldiethylenetriamine in water and in aqueous micellar solutions

Abstract

The kinetics of water replacement in the complex [Pd(Et₄dien)(H₂O)](NO₃)₂, where Et₄ dien = Et₂N[CH₂]₂NH[CH₂]₂ NEt₂, by thiourea and its methyl, ethyl, n-butyl, N, N′-dimethyl, N, N′-diethyl, N, N′-di-n-butyl and p-tolyl derivatives have been studied at 25 °C and ionic strength 0.03 mol dm^–3 in water and in the presence of cationic (CTAN) and anionic (SDS) micelle-forming surfactants. All the reactions studied are first order with respect to both reactant species. The reaction rates in water depend on both the steric hindrance and the inductive effects of the alkyl groups present in the parent thiourea molecule. The substitution reactions are inhibited by the CTAN micelles and take place in the bulk water, the nucleophiles being partitioned between the aqueous and micellar pseudo-phases. Anionic SDS micelles accelerate the overall reactions, which occur in the micellar phase with the same mechanism as in water. The estimated binding constants of the nucleophiles are larger with CTAN micelles than with SDS micelles and depend on the hydrophobic nature and number of the alkyl groups present in the unsubstituted thiourea. In the case of the CTAN surfactant the standard transfer free energies of the thioureas from water in the micelle suggest that binding of the solubilizates occurs in the palizade layer of the micelle.