Synthesis of new heterocyclic N'-(2‑hydroxy-3-methoxybenzylidene)-4-oxopiperidine-1-carbohydrazide and its mononuclear metal (II) complexes: Spectroscopic characterization, fluorescence, DFT, thermo-kinetic, and antimicrobial studies

New hydrazone Schiff base N'‑(2‑hydroxy-3-methoxybenzylidene)-4-oxopiperidine-1-carbohydrazide (H2L), synthesized by the condensation reaction of 2‑hydroxy-3-methoxybenzaldehyde (HMB) and 4-oxopiperidine-1-carbohydrazide (CPH), form mononuclear complexes, [Mn(HL)(Cl)(H2O)2], [Co(HL)(Cl)(H2O)2], [Ni(HL)(Cl)(H2O)2] and [Cu(HL)(Cl) (H2O)2]. The formation of ligand was confirmed by elemental analysis, melting point, IR, 1H and 13C NMR, and ESI-Mass spectroscopy, Cu(II) complex was further examined by ESR spectroscopy. Other properties, like, molar conductance, IR, electronic spectra, magnetic susceptibility, DFT, powder X-ray diffractometer, fluorescence and thermal analysis have also been studied. Powder X-ray diffraction patterns inferred that compounds are nanocrystalline nature. Surface morphology further support this observation. All compounds show good absorption properties and in excited state at (λ334nm), the fluorochromes of the ligand and its metal (II) complexes emitted a good fluorescence emission with the resonation of 0.2 nm in (λ527–545 nm) region. The DFT B3LYP/6–311G+(d,p) calculations were carried out for the determination of the optimized structure of ligand (H2L) and its metal complexes. The frontier MOs energies data indicated good charge transfer (CT) from HOMO to LUMO. Thermal study of complexes monitored under nitrogen atmosphere led us to determine the activation parameters of various decomposition steps. On the basis of activation energy evaluated by Coat-Redfern relation, thermal stability order was found to be Ni(II)