Trinuclear Dioxidomolybdenum(VI) Complexes of Tritopic Phloroglucinol-Based Ligands and Their Catalytic Applications for the Selective Epoxidation of Olefins

Four trinuclear dioxidomolybdenum(VI) complexes, [{(MoO2)-O-VI(H2O)}(3)ptk(bhz)(3)] (1), [{(MoO2)-O-VI(H2O)}(3)ptk(fah)(3)] (2), [{(MoO2)-O-VI(H2O)}(3)ptk(inh)(3)] (3), and [{(MoO2)-O-VI(H2O)}(3)ptk(nah)(3)] (4), based on the tritopic central 2,4,6-triacetylphloroglucinol (H(3)ptk) ligands H(6)ptk(bhz)(3) (I), H(6)ptk(fah)(3) (II), H(6)ptk(inh)(3) (III) and H(6)ptk(nah)(3) (IV) (Hbhz = benzoylhydrazide, Hfah = 2-furanoylhydrazide, Hinh = isonicotinoylhydrazide and Hnah = nicotinoylhydrazide), respectively, are presented. All of the synthesized ligands, as well as their complexes, have been characterized by elemental, thermal, and electrochemical analyses, spectroscopic techniques (FTIR, UV/Vis, H-1 and C-13 NMR), and single-crystal X-ray studies of [{(MoO2)-O-VI(H2O)}{(MoO2)-O-VI(MeOH)}(2)ptk(bhz)(3)]2H(2)O1.25MeOH (1a) and [{(MoO2)-O-VI(EtOH)}(3)ptk(fah)(3)]3EtOH (2a). Each pocket of the ligands coordinates in a dibasic tridentate fashion through two oxygen atoms and one nitrogen atom to each metal center. Due to the presence of tridentate binding pockets in the ligands, each metal center conserves its octahedral structure by coordinating with water molecules in the synthesized complexes or by other solvent(s) in the crystal structures. These complexes were evaluated for the epoxidation of terminal and internal alkenes in the presence of H2O2 using NaHCO3 as a promoter. Under the optimized reaction conditions, all alkenes were converted to the corresponding epoxides selectively in good yield and high turnover number.