Investigation of Substitution Reactions Between Zinc(II) Complexes with Different Geometries and N-bonding Nucleophiles
Aims: Investigation of interactions between zinc(II) complexes with different geometrical structures and relevant nitrogen donor nucleophiles at physiological pH. Background: The lack of clear distinction between the therapeutic and toxic doses of platinum drugs is a major challenge for the design of novel non-platinum DNA and protein targeting metal-based anticancer agents. The non-platinum antitumor complexes could be alternatives to platinum-based drugs due to their better characteristics and different mechanism of action. Objective This study could provide more information for the design of future zinc-based anticancer drugs, as well as providing a better understanding of the mechanism of interactions between Zn(II) complexes and nitrogen-donor nucleophiles (important from a medical point of view) and clarifies the changes in geometrical structures of zinc(II) that are referred to structure-reactivity correlation Methods Mole-ratio method and UV-Vis spectroscopic kinetic method were applied in this study. Objective : This study could provide more information for the design of future zinc-based anticancer drugs, as well as providing a better understanding of the mechanism of interactions between Zn(II) complexes and nitrogen-donor nucleophiles (important from a medical point of view) and clarifies the changes in geometrical structures of zinc(II) that are referred to structure-reactivity correlation Methods Mole-ratio method and UV-Vis spectroscopic kinetic method were applied in this study. Result: The results indicated additional coordination of chlorides in the first coordination sphere with changes in coordination geometry and formation of the octahedral complex anion [ZnCl4(en)]2- while an excess of chloride didn’t affect the square-pyramidal structure of [ZnCl2(terpy)]. The substitutions of studied complexes and relevant nucleophiles proceed in two consecutive reaction steps that depend on the nucleophile concentration. Octahedral complex anion [ZnCl4(en)]2- forms rapidly, and all substitution processes of this complex species should be considered. We assume that the first reaction step is accompanied by the dissociation of chloride ligands. Nucleophile 1,2,4-triazole have shown the highest affinity toward [ZnCl2(en)], and rates of both steps are almost the same value, which indicates parallel reactions. Conclusion: The different order of reactivity of relevant N-donor ligands toward [ZnCl2(en)] and [ZnCl2(terpy)] complexes for the first reaction step occurred due to the influence of different geometrical structures of complexes. In contrast, low reaction rates for the second reactions of [ZnCl2(en)] complex with imidazole and pyrazine were a consequence of interconversion between octahedral and tetrahedral structure during substitution processes.