nComplexes of Crotonaldehyde and N-Benzaldehyde salycylhydrazone with some transition metals and their antimicrobial studies

Complexes of Ni(II), Cu(II), Co(II), Mn(II), and Fe(II) with crotonaldehyde- and N-benzaldehyde-salicylhydrazone (CSH/NBSH) were synthesized and characterized by FTIR, UV–Vis spectroscopy, conductivity, melting point, magnetic moment, and XRD. FTIR indicates tridentate coordination via carbonyl-O, azomethine-N, and amine-N, while electronic spectra and magnetic moments support predominantly octahedral geometries. XRD (Scherrer) reveals nanocrystallites spanning ~ 20–41 nm with dominant reflections at 2θ ≈ 20–29° (d ≈ 4.4–3.1 Å) and low-to-moderate crystallinity (~ 3–33%), with Mn derivatives showing the highest crystalline fractions and Ni derivatives the largest coherent domains. Consistent with these structural features – defect-rich surfaces and accessible metal centers – the metal chelates exhibit enhanced antibacterial activity relative to the free ligands: activity increases are most notable against Staphylococcus aureus and Pseudomonas aeruginosa , whereas responses against Proteus vulgaris , Klebsiella spp., Salmonella Typhi, Escherichia coli , Streptococcus spp., and Bacillus cereus are comparatively weaker. These results highlight a structure–activity relationship in which nanoscale domain size and crystallinity modulate antimicrobial performance, offering levers for tuning future ligand/metal choices.