Multinuclear transition metal-containing polyoxometalates constructed from Nb/W mixed-addendum precursors: synthesis, structures and catalytic performance

Four new transition metal-containing Nb/W mixed-addendum POM trimers with the formula H 19 [M 4 (H 2 O) x (P 2 W 15 Nb 3 O 62 ) 3 ]· m (HCOOH)· n H 2 O (M = Cu, x = 15, m = 0, and n = 21, Cu-POM ; M = Co, x = 7, m = 0, and n = 15, Co-POM ; M = Mn, x = 7, m = 6, and n = 18, Mn-POM ; and M = Zn, x = 7, m = 0, and n = 23, Zn-POM ) have been synthesized by a solvothermal method in a water-ethanol mixed solvent. All the four compounds were characterized by single-crystal X-ray diffraction, powder X-ray diffraction (XRD), IR spectroscopy, thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS). These compounds can serve as efficient heterogeneous catalysts for the cyanosilylation of different carbonyl compounds under ambient temperature and solvent-free conditions, and Cu-POM shows much better catalytic performance than the other three compounds. The cycle experiment showed that Cu-POM can be reused for at least five cycles without significant loss of catalytic activity. The IR spectroscopy and XRD analysis revealed that Cu-POM can retain its integrity after catalysis. Four TM-containing polyoxometalates were isolated by the self-assembly of {P 2 W 15 Nb 3 O 62 } and TM ions, and they can serve as efficient catalysts for cyanosilylation.