Three Polyoxometalate-Based Ag–Organic Compounds as Heterogeneous Catalysts for the Synthesis of Benzimidazoles

Three new POM-based compounds, with formulae [Na_0.63Ag₃(Htba)_2.37(tba)_0.63(H₂O)₂(PMo₁₂O₄₀)]·4H₂O (Ag₃PMo), [Ag₄(Htba)₄(H₂O)₂(PMo₁₂O₄₀)](NO₃)·H₂O (Ag₄PMo), and [Ag₃(Htba)₂(tba)(PW₁₂O₄₀)_0.5](NO₃)_0.5·13H₂O (Ag₃PW), were prepared with a 3-(4H-1,2,4-triazol-4-yl)benzoic acid (Htba) ligand, Keggin-type anions ([PMo₁₂O₄₀]^3-/[PW₁₂O₄₀]^3-), and a silver ion (Ag⁺). The structural features of these compounds are particularly different from the multinuclear subunits, which are [Ag₃(tba)₃] clusters in Ag₃PMo, [Ag₄(tba)₃] chains in Ag₄PMo, and [Ag₃(tba)₃]₂ clusters in Ag₃PW, connected by multidonor atom tba ligands and Ag⁺ ions. Meanwhile, in these compounds, polyanions act as polydentate ligands to link adjacent Ag-tba metal-organic units and expand their spatial dimensions. These compounds, as heterogeneous catalysts, exhibit high stability and excellent catalytic activity to construct benzimidazoles. Ag₃PMo could efficiently catalyze the condensation of benzene-1,2-diamines and benzaldehydes and produce benzimidazoles in good yields. In addition, Ag₃PMo could be reused up to 7 times and was suitable for gram-scale reactions.