Vanadium-Silsesquioxane Nanocages as Heterogeneous Catalysts for Synthesis of Quinazolinones.

The functionalization of polyoxovanadate clusters is promising but of great challenge due to the versatile coordination geometry and oxidation state of vanadium. Here, two unprecedented silsesquioxane ligand-protected "fully reduced" polyoxovanadate clusters were fabricated via a facial solvothermal methodology. The initial mixture of the two polyoxovanadate clusters with different colors and morphologies (green plate V₁₄ and blue block V₆) was successfully separated as pure phases by meticulously controlling the assembly conditions. Therein, the V₁₄ cluster is the highest-nuclearity V-silsesquioxane cluster to date. Moreover, the transformation from a dimeric silsesquioxane ligand-protected V₁₄ cluster to a cyclic hexameric silsesquioxane ligand-protected V₆ cluster was also achieved, and the possible mechanism termed "ligand-condensation-involved dissociation reassembly" was proposed to explain this intricate conversion process. In addition, the robust V₆ cluster was served as a heterogeneous catalyst for the synthesis of important heterocyclic compounds, quinazolinones, starting from 2-aminobenzamide and aldehydes. The V₆ cluster exhibits high activity and selectivity to access pure quinazolinones under mild conditions, where the high selectivity was attributed to the confinement effect of the macrocyclic silsesquioxane ligand constraining the molecular freedom of the reaction species. The stability and recyclability as well as the tolerance of a wide scope of aldehyde substrates endow the V₆ cluster with a superior performance and appreciable potential in catalytic applications.