Metal-coordinated Aldazine Schiff Base: synthesis, spectroscopic characterisation, and evaluation of antimicrobial and antioxidant potentials of CU (II), ZN (II) and AG(I) complexes

Abstract

Antimicrobial resistance poses a growing threat to global health as multidrug-resistant pathogens render many conventional drugs ineffective. This has created an urgent need for alternative therapeutic approaches. In response, an aldazine Schiff base and Cu(II), Zn(II) and Ag(I) complexes were synthesised, spectroscopically characterised and evaluated for antimicrobial and antioxidant potentials. The ligand, 3EHH, was prepared from hydrazine and 3-ethoxy-2-hydroxybenzaldehyde. The Cu(II) (Cu3E), Zn(II) (Zn3E), and Ag(I) (Ag₂3E) complexes were synthesised and characterised by magnetic moments, NMR, IR and UV/Vis spectra. The in vitro antimicrobial and antioxidant properties were investigated using agar well diffusion, DPPH radical scavenging and ferric reducing antioxidant power (FRAP) assays, respectively. Formation of 3EHH was confirmed by the bis-azomethine protons signal at 8.69 δ ppm and the absence of amino and carbonyl protons. Coordination via azine nitrogen and phenolate oxygen was supported by shifts in v(C=N) from 1622 to 1631–1616 cm^–1 and v(C–O) from 1214 to 1210–1193 cm^–1, loss of phenolic protons in Zn3E NMR spectrum, and changes in π→π* transitions, together with new LMCT (423–405 nm) and d→d bands (662 nm) in the UV-Visible spectra. The inhibition zones and MIC were in the ranges of 8–24 mm and 10–0.125 mg/mL, respectively. The DPPH and FRAP antioxidant assays showed IC₅₀ 138.62–486.82 µg/mL and 9.240–22.890 AAE mg/g at 1000 µg/mL, respectively. Chelation of 3EHH with Cu (II), Zn(II), and Ag(I) improved its bioactivity, with the Cu (II) complex showing the greatest potential as a dual antimicrobial and antioxidant agent.