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

Abstract

© 2015 Taylor & Francis. Substitution reactions of five monofunctional Pd(II) complexes with unsaturated N-heterocycles were investigated using variable-temperature stopped-flow spectrophotometry and 1H NMR. The results are useful for better explanation of structure-reactivity relationship of Pd(II) complexes as well as for prediction of potential targets of Pd(II) complexes toward common N-heterocycles, constituents of biomolecules and different N-bonding pharmaceutical agents. Substitution reactions of five monofunctional Pd(II) complexes, [Pd(terpy)Cl]+ (terpy = 2,2′;6′,2″-terpyridine), [Pd(bpma)Cl]+ (bpma = bis(2-pyridylmethyl)amine), [Pd(dien)Cl]+ (dien = diethylenetriamine or 1,5-diamino-3-azapentane), [Pd(Me4dien)Cl]+ (Me4dien = 1,1,7,7-tetramethyldiethylenetriamine), and [Pd(Et4dien)Cl]+ (Et4dien = 1,1,7,7-tetraethyldiethylenetriamine), with unsaturated N-heterocycles such as 3-amino-4-iodo-pyrazole (pzI), 5-amino-4-bromo-3-methyl-pyrazole (pzBr), 1,2,4-triazole, pyrazole, pyrazine, and imidazole were investigated in aqueous 0.10 M NaClO4 in the presence of 10 mM NaCl using variable-temperature stopped-flow spectrophotometry. The second-order rate constants k2 indicate that the reactivity of the Pd(II) complexes decrease in the order [Pd(terpy)Cl]+ > [Pd(bpma)Cl]+ > [Pd(dien)Cl]+ > [Pd(Me4dien)Cl]+ > [Pd(Et4dien)Cl]+. The most reactive nucleophile of the heterocycles is pyrazine, while the slowest reactivity is with pyrazole. Activation parameters were determined for all reactions and negative entropies of activation, ΔS≠, supporting an associative mode of substitution. The reactions between [Pd(bpma)Cl]+ and 1,2,4-triazole, pzI, and pzBr were also investigated by 1H NMR to define the manner of coordination. These results could be useful for better explanation of structure-reactivity relationships of Pd(II) complexes as well as for the prediction of potential targets of Pd(II) complexes toward common N-heterocycles, constituents of biomolecules and different N-bonding pharmaceutical agents.