Comparative solution equilibria studies of complex formation between Ir(III) ion and antituberculosis drug analogues: Spectroscopic, potentiometric and conductometric approach

Abstract

Abstract The detailed studies concerning the binding mode of bioactive ligands: pyrazine-2-carboxamide (PZA), pyrazine-2-thiocarboxamide (PTCA) and pyrazine-2-amidoxime (PAOX) have been performed by using three independent techniques: UV–Vis spectroscopy, potentiometry and conductometry. The composition and stability constants of new series Ir(III) complexes with antituberculosis drug and its structural analogues have been determined in aqueous and nonaqueous solutions. The hypsochromic shifts of bands and appearance of new peaks during UV–Vis spectrophotometric titration confirmed the interaction of Ir(III) ion with bioligands. In the studied pH range activation of pyrazine derivatives began complexation process. The sharply changed of conductimetric curves slope clearly demonstrated that the ML2 type complexes formation with relatively high stability are favored. Under the experimental conditions, it was found that metal ion interact with ligands leading to the formation of positively charged complexes. The gradual and cumulative stability constants of Ir(III) coordination compounds have been determined. The stability order of Ir(III)– PY systems studied is as follows: PAOX\u202f>\u202fPZA\u202f>\u202fPTCA. The analysis of formation equilibria revealed that mononuclear Ir(III) coordination compounds with metal to ligand molar ratio equals 1:2 are preferable with prominently high solution stability. Comparison of stability constants values showed that an increase in the efficiency of the reaction between metal ion and bioligands was observed with elongation of substituent chain. In the binary complexes, the bidentate nature of bioactive ligands and participation of amine and heterocyclic nitrogens in coordination of central ion were confirmed.