New Tetraaza Schiff Base Ligands and Their Complexes: Synthesis, Characterization and Thermodynamic Studies

Some new symmetrical tetraaza Schiff base ligands containing 2-quinolinecarboxaldehyde moiety formed from condensation reaction of 2-quinolinecarboxaldehyde and o-phenylenediamine derivatives, such as N,Nˊ-bis(2-quinolylmethylidene)-4-methoxy-1,2- phenylenediimine (L^1), N,Nˊ-bis(2-quinolylmethylidene)-4-methy-1,2-phenylenediimine (L^2), N,Nˊ-bis(2-quinolylmethylidene)-4-chloro- 1,2-phenylenediimine (L3), N,Nˊ-bis(2-quinolylmethylidene)-4-carboxylic-1,2-phenylenediimine (L^4), N,Nˊ-bis(2-quinolylmethylidene)-4- nitro-1,2-phenylenediimine (L^5), and their Ni(II), Cu(II), Zn(II) complexes, have been synthesized. The ligands and their metal complexes have been characterized by molar conductance, elemental analysis, IR, 1H NMR and UV-Vis spectroscopy. The formation constant and the free energy of the complexes were determined using electronic spectroscopic titration at constant ionic strength 0.1 M (NaClO4), at 25 °C in MeOH solvent. The comparison of the formation constants and the thermodynamic parameters show that the trend of complex formation toward a given ligand is as follows: L^2 L^1 L^3 L^5 L^4. The trend for complex formation of the metal ions with a given ligand is as follows: Zn(II) Cu(II) Ni(II).