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

Three new POM-based compounds, with formulae [Na 0.63 Ag 3 (Htba) 2.37 (tba) 0.63 (H 2 O) 2 (PMo 12 O 40 )]·4H 2 O ( Ag 3 PMo ), [Ag 4 (Htba) 4 (H 2 O) 2 (PMo 12 O 40 )](NO 3 )·H 2 O ( Ag 4 PMo ), and [Ag 3 (Htba) 2 (tba)(PW 12 O 40 ) 0.5 ](NO 3 ) 0.5 ·13H 2 O ( Ag 3 PW ), were prepared with a 3-(4 H -1,2,4-triazol-4-yl)benzoic acid (Htba) ligand, Keggin-type anions ([PMo 12 O 40 ] 3- /[PW 12 O 40 ] 3- ), and a silver ion (Ag + ). The structural features of these compounds are particularly different from the multinuclear subunits, which are [Ag 3 (tba) 3 ] clusters in Ag 3 PMo , [Ag 4 (tba) 3 ] chains in Ag 4 PMo , and [Ag 3 (tba) 3 ] 2 clusters in Ag 3 PW , 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. Ag 3 PMo could efficiently catalyze the condensation of benzene-1,2-diamines and benzaldehydes and produce benzimidazoles in good yields. In addition, Ag 3 PMo could be reused up to 7 times and was suitable for gram-scale reactions.