Investigation of disulfide-based Schiff base and its mono and binuclear Cu(II) complexes: Green synthesis, crystal structure analysis, Hirshfeld surface analysis, in-vitro biological assessment, DNA binding studies, and computational studies

Abstract

A Schiff base, L (6,6′-((1E)-((disulfanediylbis(2,1-phenylene))bis(azanylylidene))bis(methanylylidene))bis(2-methoxyphenol)), was synthesised via a heatless condensation of 2,2 diaminodiphenyl disulfide and o-vanillin. Its structure was confirmed by single crystal X-ray diffraction. Hirshfeld surface analysis highlighted significant intermolecular interactions among ligand molecules. Three Cu(II) complexes, [CuL(OAc)].OAc, [Cu₂L(SO₄)].H₂O, and [Cu₂L(NO₃)], were synthesised using ultrasonic irradiation. Characterization of L and the metal complexes included elemental analysis, molar conductance, magnetic moments, FTIR, UV–Vis, EPR, mass spectrometry, and Job's Analysis. The molar conductance values of the complexes [CuL(OAc)].OAc, [Cu₂L(SO₄)], [Cu₂L(NO₃)].NO₃ were respectively 25, 14 and 78 Ω⁻¹ cm⁻¹ mol⁻¹, suggesting that [CuL(OAc)].OAc and [Cu₂L(NO₃)].NO₃ are electrolytic in nature. The complexes exhibited paramagnetic behavior with a d⁹ electronic configuration, as indicated by their magnetic moment values ranging from 1.74 to 1.77 BM. Computational methods supported structural and stability assessments. Stability order was [CuL(OAc)].OAc < [Cu₂L(NO₃)].NO₃ < [Cu₂L(SO₄)].H₂O < L. DNA binding studies using UV–vis spectroscopy, fluorescence assays, and viscosity measurements confirmed intercalative binding, with intrinsic binding constants (K_b) and quenching constants (K_sv) values of 10⁵ M⁻¹. [Cu₂L(NO₃)].NO₃ showed the highest antibacterial activity against gram-positive bacteria with a zone of inhibition up to 13 mm and the lowest MIC values. The antioxidant and anti-inflammatory properties of the complex were assessed through DPPH and egg albumin denaturation assays. The complex [Cu₂L(NO₃)].NO₃ exhibited promising activity, with IC₅₀ values of 11.79 µg/mL for the antioxidant assay and 96.13 µg/mL for the anti-inflammatory study. Molecular docking studies on CT-DNA revealed strong binding affinities, with [Cu₂L(NO₃)].NO₃ exhibiting the lowest binding energy of -8.36 kcal/mol. The synthesised compounds demonstrate significant potential for applications in medicinal chemistry and biological studies.