Synthesis, spectral characterization, thermal studies and biological activity of (Z)‐5‐((1,5‐dimethyl‐3‐oxo‐2‐phenyl‐2,3‐dihydro‐1H‐pyrazol‐4‐yl)diazenyl)‐6‐hydroxy‐2‐mercaptopyrimidin‐4(3H)‐one and its metal complexes

(Z)‐5‐((1,5‐Dimethyl‐3‐oxo‐2‐phenyl‐2,3‐dihydro‐1H‐pyrazol‐4‐yl)diazenyl)‐6‐hydroxy‐2‐mercaptopyrimidin‐4(3H)‐one (H3L) was exploited as a ligand for the synthesis of a series of metal complexes, where the metal was Mn(II), Co(II), Ni(II), Cu(II), Zn(II), Pd(II), Cr(III) and Ru(III). The structural features of the ligand and the synthesized complexes were characterized using various analytical, spectral and thermal measurements. Infrared spectra showed that the ligand coordinates to all metal ions in a neutral keto–enol–thiol manner. It behaves in a tetradentate fashion in all binuclear complexes except the Cr(III) complex, where it is hexadentate, and in a tridentate fashion in mononuclear complexes. The azo group participates in chelation only in the case of binuclear complexes. All complexes exhibit octahedral geometry except Pd(II) and Cu(II) acetate which are square planar and square pyramidal, respectively. Kinetic parameters were computed from thermal decomposition data. Axial electron spin resonance spectra were obtained for Cu(II) complexes indicating ( dx2−y2)1 as a ground state with small g|| values affording strong interaction between the ligand and metal ions. The biological measurements reveal that the ligand has antioxidant activity relative to the metal complexes, while the metal complexes have cytotoxicity in comparison to the free ligand. (Z)‐5‐((1,5‐Dimethyl‐3‐oxo‐2‐phenyl‐2,3‐dihydro‐1H‐pyrazol‐4‐yl)diazenyl)‐6‐hydroxy‐2‐mercaptopyrimidin‐4(3H)‐one (H3L)