The metal complexes of new Schiff bases containing phosphonate groups and catalytic properties for alkane oxidation

In this study, two new salicylidene phosphonate ligands (HL1 and HL2) and their metal complexes (Cu2+, VO2+ and La3+) were synthesized and characterized by spectroscopic and analytical methods. The molecular structure of the ligand HL1 was determined by single-crystal X-ray diffraction study. In the structure of the ligand, there is an intramolecular phenol-imine hydrogen bond. The synthesized compounds exhibit only one emission maximum upon excitation at 270-295 nm range. Complexation of the Schiff base ligands with metal ions did not cause a considerable quenching effect. Finally, the complexes prepared were used as catalysts in cyclohexane oxidation under microwave irradiation. The complexes showed high conversion rates (> 90%) for cyclohexane oxidation; however, poor selectivity was observed for all complexes. The La3+ complexes showed better selectivity for cyclohexane -> cyclohexanol transformation with about 45% selectivity.