Imidazole-directed fabrication of three polyoxovanadate-based copper frameworks as efficient catalysts for constructing C–N bonds

Abstract

A synthetic pathway for the directed preparation of three novel polyoxovanadate-based copper frameworks (POVCFs), i.e., [Cu_0.5(1-ipIM)₂]₂[V₄O₁₁] (POVCF 1), [Cu(1-ipIM)₂](VO₃)₂ (POVCF 2) and [Cu(1-pIM)₄](HpIM)₄[V₁₀O₂₈] (POVCF 3) (1-ipIM = 1-isopropylimidazole; 1-pIM = 1-propylimidazole) using bifunctional imidazole molecules as organic ligands and base has been developed. Systematic studies revealed that the variable base enviroment in the reaction is the key step in the preparation of 1D to 3D supermolecular networks of POVCFs 1–3. Single crystal X-ray diffraction analyses demonstrated that the Cu²⁺ atoms of POVCFs 1–2 were coordinated with four imidazole derivative molecules and two different polyoxovanadate {[V₄O₁₁]²⁻ and [V₁₀O₂₈]⁶⁻} clusters, respectively, exhibiting a [CuN₄O₂] binding set and a distorted octahedral geometry. Specifically, POVCF 1 exhibited adjacent [V₄O₁₁]²⁻ that dangle up and down arranged in a parallel 2D network and further coordinated with [Cu(1-ipIM)₄]²⁺ to form a 3D supramolecular structure. However, POVCF 3 presented one tetrahedral coordinated vanadium atom and one four-coordinated copper atom and they further gave rise to a 1D network by the Cu–O and V–O bonds. More importantly, these POVCFs were further studied in the construction of C–N bond reactions of primary amines under mild conditions, and it was found that POVCF 1 displayed efficient heterogeneous catalytic activities in the Chan–Lam reaction (yields up to 89%).