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

Substitution reactions of five monofunctional Pd(II) complexes with unsaturated N-heterocycles were investigated using variable-temperature stopped-flow spectrophotometry and H-1 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(Me(4)dien)Cl](+) (Me(4)dien = 1,1,7,7-tetramethyldiethylene triamine), and [Pd(Et(4)dien)Cl](+) (Et(4)dien = 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 k(2) 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, Delta S-not equal, supporting an associative mode of substitution. The reactions between [Pd(bpma) Cl]+ and 1,2,4-triazole, pzI, and pzBr were also investigated by H-1 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.