Guanidine Derived Transition Metal Complexes: Catalysts for C–H Activation and Potent Antibacterial Agents for Shelf-Life Enhancement of Fruits

Guanidine-based transition metal complexes, owing to their strong cationic character and hydrogen-bonding ability, have emerged as promising systems for both catalysis and antimicrobial applications. A series of guanidine derived biguanide nickel(II) complexes (NiL1–NiL4) synthesised and characterised by X-ray diffraction, UV–Vis spectroscopy, cyclic voltammetry, and HR-MS. These water-soluble complexes exhibit excellent catalytic activity (TON 680 for cyclohexane) for selective C(sp³)-H bond chlorination and bromination under mild conditions using NaOCl as a green oxidant, affording high turnover numbers with remarkable selectivity under ambient conditions using water as solvent at room temperature within short reaction time of 30 minutes. Mechanistic studies indicated the involvement of a Ni(III) intermediate.

Among the complexes, NiL1 (Ni-ethylenebisbiguanide) showed potent antibacterial activity against E. coli (MIC: 250 μg/mL) via membrane disruption, with low cytotoxicity. This inhibits the proliferation of E.Coli. due to enhanced cell wall penetration. Furthermore, its incorporation into a chitosan-based coating enhances the shelf life of tomatoes and lemons by reducing microbial growth and moisture loss.