Catalytic oxidation of internal and terminal alkenes by oxidoperoxidomolybdenum(VI) and dioxidomolybdenum(VI) complexes

•Oxidoperoxidomolybdenum(VI) and dioxidomolybdenum(VI) complexes are reported.•Single crystal XRD technique is used for structural elucidation of complexes.•These complexes are tested as catalyst for the oxidation of styrene and cyclohexene.•Catalytic activity of the peroxido complexes are compared with their dioxido analogues.

Reaction of oxidoperoxidomolybdenum(VI) species, generated in situ by the reaction of MoO3 in the presence of H2O2, with three Schiff bases H2hap-inh (I), H2hap-nah (II) and H2hap-bhz (III), (derived from the condensation of 2-hydroxyacetophenone with isonicotinoylhydrazide, nicotinoylhydrazide and benzoylhydrazide) results in the formation of oxidoperoxidomolybdenum(VI) complexes, [MoO(O2)(hap-inh)(MeOH)] (1), [MoO(O2)(hap-nah)(MeOH)] (2) and [MoO(O2)(hap-bhz)(MeOH)] (3), respectively. Their dioxidomolybdenum(VI) analogues [MoO2(hap-inh)]n (4) [MoO2(hap-nah)(MeOH)] (5) and [MoO2(hap-bhz)(MeOH)] (6) have also been prepared by reacting equimolar ratio of [MoO2(acac)2] and corresponding ligands. These complexes have been characterized in the solid state as well as in solution namely by spectroscopic techniques (IR, electronic, 1H and 13C NMR), elemental and thermogravimetric analysis. Structures of complexes 3, 4, and 6a ([MoO2(hap-bhz)(DMSO)]) have been confirmed by single crystal X-ray study which reveals that ligands behave as a dibasic tridentate coordinating through ONO functionalities. Sodium bicarbonate assisted oxidation of styrene and cyclohexene using 30% H2O2 as oxidant has been carried out using these catalysts. Under the optimized reaction conditions styrene gave mainly styrene oxide and minor amount of phenylacetaldehyde. Oxidation of cyclohexene gave cyclohexene epoxide selectively. The corresponding dioxidomolybdenum(VI) complexes have relatively lower catalytic activities.

Graphical abstractOxidoperoxidomolybdenum(VI) complexes, [MoO(O2)(hap-inh)(MeOH)] (1), [MoO(O2)(hap-nah)(MeOH)] (2) and [MoO(O2)(hap-bhz)(MeOH)] (3) and dioxidomolybdenum(VI) analogues [MoO2(hap-inh)]n (4) [MoO2(hap-nah)(MeOH)] (5) and [MoO2(hap-bhz)(MeOH)] (6) have been prepared and their catalytic activities for the oxidation of styrene and cyclohexene evaluated.Figure optionsDownload full-size imageDownload as PowerPoint slide