Kinetics and mechanism of the substitution reactions of some bifunctional palladium(II) complexes with different nitrogen-donor heterocycles

Abstract

© 2015 Springer International Publishing Switzerland. Substitution reactions of three Pd(II) complexes, [Pd(cbdca)Cl2]2- (cbdca = cyclobutane-1,1-dicarboxylato), [Pd(ox)Cl2]2- (ox = oxalato) and [Pd(mal)Cl2]2- (mal = malonato), with different five- and six-membered N-heterocycles, such as pyrazole, 3-amino-4-iodo-pyrazole (pzI), 5-amino-4-bromo-3-methyl-pyrazole (pzBr), 1,2,4-triazole, pyrazine, imidazole, pyridazine and pyrimidine, were investigated in aqueous 0.10 M NaClO4 with the presence of 20 mM NaCl using variable-temperature stopped-flow spectrophotometry. Substitution of these complexes occurs in two consecutive reversible steps, and their reactivity decreases in order [Pd(cbdca)Cl2]2- > [Pd(mal)Cl2]2- > [Pd(ox)Cl2]2-. The most reactive nucleophile among the five-membered heterocycles is imidazole, while pyridazine is the most reactive among the six-membered heterocycles. Activation parameters were determined for all reactions, and negative values for entropy of activation, ΔS≠, support an associative mode of substitution. The reaction between [Pd(mal)Cl2]2- complex and pzBr was investigated by 1H NMR, and the obtained results confirm that substitution with nitrogen-donor nucleophile does not lead to decomposition of the complex.