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

Three new Si-substituted polyoxovanadates (POVs), [Cd 2 (dien) 2 ][Cd(dien)][Cd(Hdien) 2 ][V 15 Si 6 O 46 (OH) 2 (H 2 O)]·7H 2 O (1), [Co(enMe) 2 ] 3 [Co 2 (enMe) 2 (H 2 O) 2 ][V 16 Si 4 O 44 (OH) 2 (H 2 O)]·6H 2 O (2), and [Co(teta)] 4 [V 16 Si 4 O 42 (OH) 4 (H 2 O)]·10H 2 O (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 {V 15 Si 6 O 48 }/{V 16 Si 4 O 46 } clusters of compounds 1-3 were formed by replacing {VO 5 } square pyramids in the classical {V 18 O 42 } cluster with {Si 2 O 7 } units. Compound 1 is a 2D bilayer structure formed by the [V 15 Si 6 O 46 (OH) 2 (H 2 O)] 10- cluster and two types of bridging Cd complexes containing binuclear groups [Cd 2 (dien) 2 ] 4+ . Compound 2 is a 3D framework constructed from the [V 16 Si 4 O 44 (OH) 2 (H 2 O)] 10- cluster and two types of Co complex fragments including binuclear [Co 2 (enMe) 2 (H 2 O) 2 ] 4+ . In compound 3, the [V 16 Si 4 O 42 (OH) 4 (H 2 O)] 8- cluster is connected with bridging [Co(teta)] 2+ 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 {V 15 Si 6 O 48 }/{V 16 Si 4 O 46 } sites and Lewis acid Cd 2+ /Co 2+ 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.