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, presenting the opportunity to develop fascinating catalysts with well-defined, tunable, active sites for a range of chemical transformations. Despite remarkable progress on numerous POMCOFs materials has been reported, the synthesis of polyoxovanadate (POV) cluster organic frameworks (POVCOFs) is relatively seldom reported. Thus, exploring novel POVCOFs still attracts considerable research interest. Additionally, Knoevenagel condensation as a versatile tool for construction of C=C bonds still remains continues challenge at room-temperature and solvent-free conditions. Herein, three POV-based clusters, [Ni(1-bIm)4]2(V4O12)·H2O (1), [Ni(1-eIm)(H2O)4]2(V10O28)(1-eImH)2·2H2O (2) and [Ni(H2O)5]2(V10O28)(1-pImH)2·4H2O (3) (1-bIm = 1-butylimidazole; 1-eIm = 1-ethylimidazole and 1-pIm = 1-propylimidazole), have been made. 1 was controllably achieved by imidazole-induced self-assembly strategy and exhibited a 2D POVCOF in which circle-shaped [V4O12]4− clusters are bridged by four six-coordinated Ni(II) atoms (NiN4O2 model). 2 and 3 are obtained at lower pH value, respectively. Notably, among of three POVs-based clusters, POVCOF 1 displays highly efficient heterogeneous solvent-free catalytic activity (yields up to 99%) and excellent recyclability in the Knoevenagel condensation at room temperature.