New polymeric, dimeric and mononuclear dioxidomolybdenum(VI) complexes with an ONO donor ligand: crystal structures, DFT calculations, catalytic performance and protein binding study of the ligand

Reaction of [MoO2(acac)2] with a new ONO donor Schiff base ligand (H2L), derived from 2-aminobenzoylhydrazide and benzoyl acetone in methanol under refluxing conditions, yields polymeric [MoO2L]n (1) and dimeric [MoO2L]2·THF (2) dioxidomolybdenum(VI) complexes. Complexes 1 and 2 undergo reaction with monodentate Lewis bases forming mononuclear complexes [MoO2L(3-pic)] (3), [MoO2L(1-allylimz)] (4) and [MoO2L(DMSO)] (5). In these complexes, the ligand is coordinated to the cis-[MoO2]2+ cores through enolic oxygens and an azomethine nitrogen. In complexes 3 and 4 the remaining sixth coordination site is occupied by the nitrogen atoms and in complex 5 by the oxygen atom of the Lewis base. The crystal structures of these complexes (1–5) have been determined by single crystal X-ray diffraction. The molybdenum atom exhibits a distorted octahedral coordination sphere. In complex 1, a 1D zigzag polymeric chain is formed through OMo–O interactions. In complex 2, a centrosymmetric dimer is formed through weak interaction of the non-coordinating amine nitrogen with the molybdenum centre. In contrast complexes 3, 4 and 5 are monomeric. The investigated complexes have also been characterized by elemental analysis, various spectroscopic techniques, thermogravimetric analysis and cyclic voltammetry. Supportive DFT calculations on the complexes have been carried out. The dioxidomolybdenum(VI) complexes have been tested for catalytic epoxidation and oxidative bromination of styrene. Fluorescence spectral measurements carried out by varying the concentration of the ligand indicate strong fluorescence resonance energy transfer (FRET) from BSA to the ligand and from the binding studies it is revealed that BSA has two binding sites for the ligand.