A new dinuclear rhodium(III) 'sandwich' polyoxometalate, [(WZnRh₂/(III))(ZnW₉O₃₄)₂]^10-. Synthesis, characterization and catalytic activity

The rhodium containing polyoxometalate, [(WZnRh₂/(III))(ZnW₉O₃₄)₂]¹⁰ was prepared by reaction of [(WZn₃)(ZnW₉O₃₄)₂]^12- with RhCl₃. The X-ray diffraction showed this compound to be isostructural to other compounds of this series. This was confirmed by the IR spectra. The elementary analysis and cyclic voltammetry measurements indicated that the polyoxometalate was of high purity. Epoxidation of alkenes with 30% aqueous H₂O₂ in 1,2-dichloroethane/water biphasis systems using [(WZnRh₂/(III))(ZnW₉O₃₄)₂]¹⁰ as catalyst showed high overall activity as compared to other noble metal substituted polyoxometalates of this series. The reactions were highly selective to epoxidation of cyclohexene and similar to previously reported [(WZnRh₂/(II))(ZnW₉O₃₄)₂]^12-, except that hydrogen peroxide yields were measurably higher in the rhodium case. The [(WZnRh₂(III))(ZnW₉O₃₄)₂]¹⁰ catalyst appears to be stable under turnovers conditions. Using IR spectroscopy as a probe to compare active polyoxometalates (Rh and Mn) versus an inactive one (Zn), showed that in the former case peroxo-tungstate intermediates could be identified whereas for the inactive compound no such intermediate was observed. A possible mechanism taking into account reactivity, steric constraints and IR spectra assuming structural integrity under catalytic turnover conditions was put forth.