Synthesis, Antimicrobial, DNA Interactions, In Vitro Cytotoxicity, and Cell Cycle Analysis Efficiency of Newly Nano-Sized Cu (II), Ni (II), Co (II), and Zn (II) Thio-Schiff Base Complexes of 2-((E)-((4,5-Dimethyl-2-(((E)-4-Methylbenzylidene)Amino)Phenyl)imino)Methyl)-Benzenethiol

Nano-sized bivalent metal complexes, specifically [M(L)2] with L = 2-((E)-((4,5-dimethyl-2-(((E)-4-methylbenzylidene)amino)phenyl)-imino)methyl)benzenethiol] (C₂₃H₂₂N₂S) and M = Cu (II) (C1), Co (II) (C2), Ni (II) (C3), and Zn (II) (C4), underwent synthesis and subsequent characterization. Elemental analyses, infrared, NMR, mass, electronic, magnetic susceptibility, conductivity assessments, and X-ray diffraction studies were used to assess our bivalent metal complexes. DFT studies were used to study the tautomeric equilibrium of the tridentate thio-Schiff base ligand (HL), via the DFT\B3LYP method in connection with a 6–311G* correlation consistent basis set. Two tautomers, which are thione and thiol forms, were studied to estimate the predominant one. The metal formed four coordinated with the tridentate N₂S donor thio-Schiff base to form octahedral geometry complexes. The SEM, TEM, XRD, AFM, and EDX of the studied complexes unveiled distinct and strong diffraction peaks, signifying their crystalline nature and providing evidence of their particle sizes being within the nano-size. The crystal sizes calculated for all complexes were determined to be ranging from 27.73 to 76.39 nm. The interactions between metal complexes and calf thymus DNA, and their potential for mimicking insulin activity, were investigated in a controlled lab setting by measuring their ability to inhibit alpha-amylase. The antimicrobial potential of thio-Schiff base ligand (HL) plus complexes (C1–C4) were tested. The viscosity and UV–Vis absorption determinations were utilized to assess the calf thymus DNA (CT-DNA) interaction with the nano-sized metal (II) chelates. Our flow cytometry data indicate significant levels of apoptosis and cell cycle arresting in both liver and breast cancer cell lines.