Herein, a new pyridoxal Schiff base (HL) and its Mn(II), Cu(II) and Zn(II) com- plexes have been synthesized, where the used amine in Schiff base formation was 4-aminopyridine. All of the investigated compounds have been synthesized in meth- anol as well as two choline chloride deep eutectic solvents. The synthesized com- pounds have been characterized by using IR, NMR, Mass and UV–Vis spectroscopic methods. Also, the optimized geometries, the tautomerism, assignment of the NMR chemical shifts and IR bands as well as exploring the frontier orbitals of the syn- thesized species have been calculated using the Density Functional Theory (DFT) approaches. Methanol solution of the HL Schiff base involves its Enol tautomer. The deprotonated L− species acts as an anionic-bidentate ligand in structure of the synthesized [ML2] complexes. The calculated IR wavenumbers and NMR chemical shifts are good in consistent with the corresponding experimental values, confirming validity of the optimized geometries for the investigated compounds. The covalent level and strength of the coordinative bonds as well as the intramolecular H-bonds have been examined using the Atoms In Molecules (AIM) analysis. Complexation to metal ions increased antibacterial activity of the HL Schiff base, where the Mn(II) complex showed a considerable antibacterial activity.

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