Ruthenium substituted Keggin type polyoxomolybdates: Synthesis, characterization and use as bifunctional catalysts for the epoxidation of alkenes by molecular oxygen

The ruthenium substituted polyoxomolybdate of the Keggin structure, Q₄PRu^III(H₂O)Mo₁₁O₃₉ (Q=n-Bu₄N), has been synthesized and characterized. The IR spectra show that this compound is isostructural with the known manganese and cobalt analogs. The cyclic voltammogram showed similar redox potentials and the UV-vis spectra showed similar energies for the d-d transitions compared to the corresponding tungstate, Q₄PRu^III(H₂O)W₁₁O₃₉. The catalytic activity of the molybdate versus tungstate in reactions with molecular oxygen was, however, significantly different. IR and ³¹P NMR evidence indicated that treatment of Q₄PRu^III(H₂O)Mo₁₁O₃₉ with oxygen showed no structural changes whereas, for Q₄PRu^III(H₂O)W₁₁O39, a clear change was observed. This finding probably explains the lack of catalytic activity for the latter in the co-oxidation of cumene and 1-octene to cumyl alcohol and 1-octene oxide. For the molybdenum compound, this reaction took place by a kinetic balance of ruthenium metal-catalyzed autooxidation of cumene to cumene hydroperoxide and the molybdenum catalyzed oxygen transfer from cumene hydroperoxide to 1-octene to yield the products. High catalyst loading led to reaction inhibition whereas low loading and excess cumene led to increased autooxidation.