Experimental and theoretical investigation of mixed ligand metal(II) complexes derived from LCH for multifunctional applications

Novel mixed-ligand metal(II) complexes of Cu, Mn, Fe, and Zn were synthesized from Schiff base ligands (LQ-CFNA and HN-MPD) derived from 2-hydroxyl-1-naphthaldehyde, 2-hydroxyl-1,4-naphthoquinone and amine precursors forming LCH system. The novelty of this work lies in the integration of a Schiff base framework with a redox-active naphthoquinone moiety within a mixed-ligand environment, which is expected to enhance the electronic and biological properties of the resulting complexes. The synthesized complexes were characterized using FTIR, UV-Vis spectroscopy, molar conductance, magnetic susceptibility, and EDXRF analysis, while the ligands were further confirmed by 1 H and 13 C NMR spectroscopy. FTIR spectra of the ligands exhibited azomethine (C = N) stretching bands at 1677 cm − 1 (LQ-CFNA) and 1615 cm − 1 (HN-MPD), which shifted to 1599,1644, 1532, and 1539 cm − 1 for the Cu(II), Mn(II), Fe(II), and Zn(II) complexes, respectively, confirming coordination through the imine nitrogen. Molar conductance values of 8.77, 23.58, and 29.17 S.cm 2 .mol − 1 for Cu(II), Mn(II), and Zn(II) complexes indicated non-electrolytic behavior, whereas the Fe(II) complex exhibited a high molar conductance value of 279 S.cm 2 .mol − 1 , suggesting electrolytic nature in solution. UV-Vis spectra and magnetic susceptibility measurements suggested paramagnetic behavior and supported predominantly octahedral geometries across the complexes. Biological evaluation showed enhanced antibacterial and antifungal activities of the complexes compared to the free ligands, with the Mn(II) complex exhibiting the highest antibacterial inhibition zone of 22.5 mm against Klebsiella pneumoniae , while the Fe(II) complex demonstrated superior antifungal activity ( 19.5 mm) against Aspergillus flavus , exceeding that of the standard drug. DNA studies revealed cleavage activity for HN-MPD, whereas LQ-CFNA showed no cleavage effect. Corrosion inhibition studies on mild steel in acidic medium revealed temperature-dependent behavior, with HN-MPD exhibiting better inhibition efficiency at 303k and 62.5 ppm concentration, while LQ-CFNA showed the highest inhibition efficiency of 74.69% at 373k and 62.5 ppm concentration. Density functional theory (DFT) calculations supported the experimental observations, providing insights into electronic structure and reactivity. In conclusion, the findings highlight the multifunctional potential of these mixed-ligand metal(II) complexes for biomedical and corrosion-inhibition applications.