Tetradentate Schiff base ligands of 3,4-diaminobenzophenone: Synthesis, characterization and thermodynamics of complex formation with Ni(II), Cu(II) and Zn(II) metal ions

Some new symmetrical diimino tetradentate Schiff base ligands were synthesized by the reaction of 3,4-diaminobenzophenone with salicylaldehyde derivatives, such as [3,4-bis(((2-hydroxy-4-methoxyphenyl)methylene)amino)phenyl]phenylmethanone (L1), [3,4-bis(((2-hydroxy-5-methoxyphenyl)methylene)amino)phenyl]phenylmethanone (L2), [3,4-bis(((5-bromo-2-hydroxyphenyl)methylene)amino)phenyl]phenylmethanone (L3) and [3,4-bis(((2-hydroxy-5-nitrophenyl)methylene)amino)phenyl]phenylmethanone (L4). Additionally, a tetradentate Schiff base ligand [3,4-bis(((2-hydroxy-3-methoxyphenyl)methylene)amino)phenyl]phenylmethanone (L5) was synthesized. All the Schiff bases and their Ni(II), Cu(II) and Zn(II) complexes were characterized using elemental analysis and infrared, electronic, mass and 1H-NMR spectroscopy. The formation constants of the complexes were measured using UV–Vis spectroscopic titration at constant ionic strength 0.10 M (NaClO4), at 25 °C in dimethylformamide (DMF) as solvent