First stable nitrate-encapsulated sandwich type polyoxometalate: Synthesis, structural characterization, and catalytic performance

A stable sandwich type Keggin polyoxometalate, Na4K5[K3Cu3(NO3)(A-α-PW9O34)2], has been synthesized and crystallized by a simple reaction method. Elemental analysis, FTIR, UV–vis, X-ray single crystal, thermal gravimetry, and cyclic voltammetry were used for the characterization of synthesized POM. The TBA salt of [K3Cu3(PW9O34)2]9 − revealed high catalytic activity and 74–99% selectivity in the liquid phase epoxidation of alkenes with aqueous H2O2. The X-ray structure of this complex reveals that three Cu(II) ions are sandwiched between two A-α-PW9O34 moieties and that a nitrate monoanion is encapsulated in the same plane as the three Cu(II) atoms. The nitrate is coplanar with the three copper atoms, which are each in a distorted square-pyramidal environment. After recrystallization, the NO3− group in the central belt didnʹt exit, in which this phenomenon is attributed to the type of counterion in the complex (as a result of mixing Na+ and K+ as counterions). This compound showed excellent stability in different pH and also didnʹt dissociate with hydrogen peroxide.