Synthesis and Characterization of Mn (II), Ni (II), and Cu (II) Complexes Derived From Sulfonamide and <i>o</i>‐Vanillin‐Based Schiff Base and Their Antimicrobial, Antioxidant, and Cytotoxic Properties

ABSTRACT A Schiff base, 4‐[(2‐hydroxy‐3‐methoxy‐1‐methylene)amino]benzenesulfonamide (HL), and mononuclear complexes of this with Cu (II), Ni (II), and Mn (II) have been synthesized and characterized using elemental (CHNS) and metal content analysis, molar conductivity, FT‐IR, 1 H‐NMR, 13 C‐NMR, UV–visible spectral studies, magnetic susceptibility measurement, LC–MS spectral analysis, thermal analysis (TGA, DTG, and DTA), and PXRD method. The proposed formulae for the complexes are [MnL 2 (H 2 O) 2 ], [NiL 2 (H 2 O) 2 ], and [CuL 2 ], where Schiff base acts as a bidentate ON chelating ligand. Octahedral geometry has been proposed for the Mn (II) and Ni (II) complexes and square planar geometry for the Cu (II) complex. The PXRD pattern reveals that Mn (II), Ni (II), and Cu (II) complexes were crystalline, with an average crystallite size ranging from 64.64 to 84.82 nm. In vitro biological activity assessments of the Schiff base and its metal complexes showed their potential bioactivity. The IC 50 values of the complexes indicate that Mn (II) and Cu (II) complexes have more free radical scavenging capacity compared to the parent Schiff base. Antimicrobial screening against multiple multidrug resistant Gram‐positive and Gram‐negative bacteria and fungi demonstrated significant antimicrobial activity for the Schiff base and its metal complexes. The LC 50 values from the brine shrimp lethality bioassay showed that the Schiff base along with the Mn (II) and Cu (II) complexes had a strong cytotoxicity. The Schiff base complexes exhibited a greater binding affinity for hydrogen bonds, π‐sulfur, π‐alkyl, and van der Waals interactions, according to the results of the molecular docking technique.