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

Three new polyoxometalate-based metal-organic frameworks (POMOFs) [Cu 4 (μ 3 - OH) 2 (tba) 3 (H 2 O) 5 (SiW 12 O 40 ) 0.5 ](H 2 SiW 12 O 40 ) 0.5 ·2.5H 2 O ( CuSiW ), [Cu 3 (μ 3 - OH)(tba) 3 (Htba)(H 2 O) 2 (HPMo 12 O 40 )]·7H 2 O ( CuPMo ), and [Cu 4 (μ 3 - OH) 2 (tba) 3 (H 2 O) 3 (PW 12 O 40 ) 0.5 ] 2 (PW 12 O 40 )·0.5H 2 O ( CuPW ) were constructed using multinuclear copper clusters, 3-(4 H -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.