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

Abstract

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 CC 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)·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 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 (NiN₄O₂ model). 2 and 3 are obtained at lower pH values. Notably, among the three POV-based clusters, POVCOF 1 displays highly efficient heterogeneous solvent-free catalytic activity (yields of up to 99%) and excellent recyclability in Knoevenagel condensation at room temperature.