Synthesis, characterization and theoretical studies of three similar nickel (II) complexes: Biological and photocatalytic applications

Three similar nickel(II) complexes [Ni(HL)L]Cl⋅2CH₃OH⋅4H₂O (1), [Ni(HL)L]NO₃⋅3H₂O (2) and [Ni(HL)L]Br⋅4H₂O (3) of quinoline-2-carbaldehyde-N(4)-cyclohexylthiosemicarbazone (HL) were prepared and characterized physicochemically, while computational analysis was done by density functional theory (DFT). Metal ions are coordinated by the ligands in both neutral and mono deprotonated forms in all the complexes. In vitro and in silico antibacterial studies against Escherichia coli and Staphylococcus aureus have revealed the efficacy of the complexes compared to the free ligand. In vitro analysis showed higher activity of the chloride complex compared to the nitrate and bromide counterpart complexes. Also, the ligand HL and the complexes were examined for their in vitro cytotoxicity on MCF‐7 breast cancer cell line. The ligand with greater potency exhibits an IC₅₀ value of 109.44 μM compared to complex 1 (IC₅₀ 360.55 μM), complex 2 (IC₅₀ 134.89 μM) and complex 3 (IC₅₀ 227.23 μM), which is also in agreement with in silico molecular docking study with DNA. The DNA binding studies of the complexes with CT-DNA confirm the groove mode of binding, while gel electrophoresis indicates endonucleolytic cleavage and fragmentation of DNA due to complex 1. Solid-state energy gaps of complexes 1–3 are evaluated through the Kubelka-Munk model, while intense charge transfer bands are also observed at ∼2.65 eV. Furthermore, the photocatalytic activity of HL and complexes were performed against methylene blue (MB) in presence of sunlight. After 120 min, the degradation efficiency of MB reached 22, 47, 58 and 55 % with HL and respective complexes 1, 2 and 3.