Preparation of macrocyclic Schiff-base ligand and antibacterial activities of transition metal complexes thereof

A macrocyclic Schiff-base (H2L) ligand is prepared via condensation of 2,6-pyridine dicarboxaldehyde with triethylene tetramine. The ligand is characterized using elemental analysis, by mass spectrometry, infrared (IR) spectroscopy, and proton nuclear magnetic resonance spectroscopy. The corresponding 1:1 metal complexes with Cr(III), Fe(III), Co(II), Ni(II), Cu(II), Cd(II), UO2(II), and Th(IV) are additionally characterized by determining their magnetic moment, molar conductance, thermal analysis (thermogravimetry and differential thermoanalysis), and solid reflectance measurements. The complexes have the general formulae [M(H2L)(H2O)](X) n · yH2O (X = Cl or AcO, n = 2,3, y = 2–5) except for the Th(IV) complex having the formula [Th(H2L)(Cl)]Cl3. The molar conductance data reveal that all the metal chelates are electrolytes. IR spectra show that H2L is coordinated to the metal ions in a neutral pentadentate manner with 5N donor sites of the two azomethine–N, pyridine–N, and two amino–NH groups. The magnetic and solid reflectance spectra reveal that the complexes are octahedral. Thermal analysis shows that the complexes decompose in four to five steps. The activation thermodynamic parameters are calculated using the Coats–Redfern method. The parent Schiff base and its eight metal complexes were assayed against four bacterial species, two Gram negative, and two Gram positive. The Schiff base and five of its metal complexes showed antibacterial activity at different rates. The complexes Cr(III) and Cu(II) inhibited Gram-positive bacteria, while Co(II) complex inhibited all tested bacteria greater than the parent Schiff base. Three metal complexes (Ni, Cd, and Th) completely missed antibacterial activity.