Copper-Containing Polyoxometalate-Based Metal-Organic Frameworks as Heterogeneous Catalysts for the Synthesis of N-Heterocycles.

Three new polyoxometalate-based metal-organic frameworks (POMOFs) [Cu4(µ3-OH)2(tba)3(H2O)5(SiW12O40)0.5](H2SiW12O40)0.5·2.5H2O (CuSiW), [Cu3(µ3-OH)(tba)3(Htba)(H2O)2(HPMo12O40)]·7H2O (CuPMo), and [Cu4(µ3-OH)2(tba)3(H2O)3(PW12O40)0.5]2(PW12O40)·0.5H2O (CuPW) were constructed using multinuclear copper clusters, 3-(4H-1,2,4-triazol-4-yl)benzoic acid (Htba), and Keggin polyoxometalates (POMs). Different POMs regulate the formation of different multinuclear copper clusters ("boat" tetranuclear clusters in CuSiW, trinuclear clusters in CuPMo, and "chair" tetranuclear clusters in CuPW) and different topological structures of CuSiW, CuPMo, and CuPW (3-connected two-dimensional (2D) network for CuSiW, 4-connected 2D network for CuPMo, and (4,6)-connected three-dimensional network for CuPW). CuSiW, CuPMo, and CuPW as heterogeneous catalysts combine the high stability of MOFs in polar solvents and excellent catalytic activity of POMs and could be used for the synthesis of nitrogen-heterocycle compounds. The condensation cyclization reactions of 2-aminophenols/benzenesulfonyl hydrazines with 1,3-diketones produce benzoazoles and pyrazoles in good to excellent yields under the catalysis of CuPMo. Moreover, the catalyst could be reused at least for 7 runs, and this protocol was suitable for gram-scale reactions.