Three Si-substituted polyoxovanadates as efficient catalysts for Knoevenagel condensation and selective oxidation of styrene to benzaldehyde.

Three new Si-substituted polyoxovanadates (POVs), [Cd(dien)][Cd(dien)][Cd(Hdien)][VSiO(OH)(HO)]·7HO (1), [Co(enMe)][Co(enMe)(HO)][VSiO(OH)(HO)]·6HO (2), and [Co(teta)][VSiO(OH)(HO)]·10HO (3) (dien = diethylenetriamine; enMe = 1,2-diaminopropane; teta = triethylenetetramine) were synthesized by the hydrothermal method and characterized. Structural analysis sheds light on the fact that the {VSiO}/{VSiO} clusters of compounds 1-3 were formed by replacing {VO} square pyramids in the classical {VO} cluster with {SiO} units. Compound 1 is a 2D bilayer structure formed by the [VSiO(OH)(HO)] cluster and two types of bridging Cd complexes containing binuclear groups [Cd(dien)]. Compound 2 is a 3D framework constructed from the [VSiO(OH)(HO)] cluster and two types of Co complex fragments including binuclear [Co(enMe)(HO)]. In compound 3, the [VSiO(OH)(HO)] cluster is connected with bridging [Co(teta)] to expand into a 2D network. Compounds 1 and 3 represent the first 2D assemblies based on a vanadosilicate cluster. 1-3 served as heterogeneous catalysts and exhibited highly efficient catalytic activities for the Knoevenagel condensation under mild ambient conditions with low catalyst loading, featuring the open Lewis base {VSiO}/{VSiO} sites and Lewis acid Cd/Co sites. The conversion of benzaldehyde was up to 99.3% in 80 min at room temperature using 1 as a heterogeneous catalyst with only 0.37% catalyst loading. Moreover, compounds 1-3 as catalysts for selective oxidation of styrene to benzaldehyde exhibited excellent catalytic performance, high selectivity and could be readily recycled. Most strikingly, compound 1 showed excellent catalytic performance with 97.6% conversion of styrene and 100% selectivity of benzaldehyde in 15 min. In addition, the catalytic activity of catalyst 1 was well maintained after five cycling reactions.