Mechanochemical Green Synthesis, Characterization and Antibacterial Evaluation of the N-Hydroxy-4-Phenylpyridine-2-Carboxamide Ligand and Its Mononuclear Mn (II), Co (II), Ni (II) and Cu (II) Complexes

A solvent-minimised mechanochemical approach was developed for the synthesis of N-hydroxy-4-phenylpyridine-2-carboxamide (HL) and its Mn(II), Co(II), Ni(II) and Cu(II) complexes. The ligand was designed as a multifunctional tridentate N, O′, O donor through the pyridyl nitrogen and the two oxygen atoms of the hydroxamate group. The ligand and its metal complexes were characterized by elemental analysis, FT-IR, UV–Visible spectroscopy, magnetic susceptibility and electron paramagnetic resonance spectroscopy. Spectroscopic changes included shifts in the hydroxamate carbonyl and N–O stretching bands and changes in pyridyl nitrogen vibrations upon coordination. The proposed [ML₂] formulation provides an N₂O₄ donor environment around the metal centre. This is consistent with approximately octahedral geometries for Mn(II), Co(II) and Ni(II), and a Jahn–Teller-distorted octahedral geometry for Cu(II) complexes. The mechanochemical route reduced solvent consumption and reaction time compared with conventional solution synthesis. The antibacterial activities of HPPCA and its Mn(II), Co(II), Ni(II) and Cu(II) complexes were evaluated against representative Gram-positive and Gram-negative bacterial strains. The observed activity order was as follows: [CuL2] ˃ [NiL2] ˃ [CoL2] ˃ [MnL2] ˃ HPPCA (free ligand). The study provides a systematic platform for investigating the relationships among metal-ion identity, coordination environment, spectroscopic properties, and antibacterial activity in hydroxamate-based transition-metal (II) complexes.