Metal(II) Ion Complexes with 5-(Pyrazin-2-yl)-2,4-dihydro-3H-1,2,4-triazole-3-thione; Synthesis, Structural Characterization, Acid-base, and Complexing Properties in Solution

The study reports the synthesis of complexes Co(HL)Cl2 (1), Ni(HL)Cl2 (2), Cu(HL)Cl2 (3), and Zn(HL)3Cl2 (4) with the title ligand, 5-(pyrazin-2-yl)-1,2,4-triazole-5-thione (HL), and their characterization by elemental analyses, ESI-MS (m/z), FT-IR and UV/Vis spectroscopy, as well as EPR in the case of the CuII complex. The comparative analysis of IR spectra of the metal ion complexes with HL and HL alone indicated that the metal ions in 1, 2, and 3 are chelated by two nitrogen atoms, N(4) of pyrazine and N(5) of triazole in the thiol tautomeric form, whereas the ZnII ion in 4 is coordinated by the non-protonated N(2) nitrogen atom of triazole in the thione form. pH potentiometry and UV/Vis spectroscopy were used to examine CoII, NiII, and ZnII complexes in 10/90 (v/v) DMSO/water solution, whereas the CuII complex was examined in 40/60 (v/v) DMSO/water solution. Monodeprotonation of the thione triazole in solution enables the formation of the L:M = 1:1 species with CoII, NiII and ZnII, the 2:1 species with CoII and ZnII, and the 3:1 species with ZnII. A distorted tetrahedral arrangement of the CuII complex was suggested on the basis of EPR and Vis/NIR spectra.