Imidazole-induced self-assembly of polyoxovanadate cluster organic framework for efficient Knoevenagel condensation under mild conditions

Polyoxometalate (POM) cluster organic frameworks (POMCOFs) are an emerging class of catalytic materials, providing an opportunity to develop fascinating catalysts with well-defined, tunable, active sites for a range of chemical transformations. Although remarkable progress on numerous POMCOF materials has been reported, the synthesis of polyoxovanadate (POV) cluster organic frameworks (POVCOFs) has seldom been reported. Thus, the exploration of novel POVCOFs still attracts considerable research interest. Additionally, Knoevenagel condensation as a versatile tool for the construction of C=C bonds still continues to be a challenge at room-temperature and under solvent-free conditions. Herein, three POV-based clusters, [Ni(1-bIm)(4)](2)(V4O12)center dot H2O (1), [Ni(1-eIm)(H2O)(4)](2)(V10O28)(1-eImH)(2)center dot 2H(2)O (2) and [Ni(H2O)(5)](2)(V10O28)(1-pImH)(2)center dot 4H(2)O (3) (1-bIm = 1-butylimidazole; 1-eIm = 1-ethylimidazole and 1-pIm = 1-propylimidazole), have been made.1was controllably achieved by an imidazole-induced self-assembly strategy and exhibited a 2D POVCOF in which circular [V4O12](4-)clusters are bridged by four six-coordinated Ni(ii) atoms (NiN(4)O(2)model).2and3are obtained at lower pH values. Notably, among the three POV-based clusters, POVCOF1displays highly efficient heterogeneous solvent-free catalytic activity (yields of up to 99%) and excellent recyclability in Knoevenagel condensation at room temperature.