Designed assembly of heterometallic cluster organic frameworks based on Th6 cluster

Four novel compounds based on hexanuclear thorium cluster were synthesized and characterized. Compound 1 [Th6(HPyC)8(HCOO)4] is formed by replacing formate ligands of preassembled thorium cluster [Th6O4(OH)4(H2O)6(HCOO)12] with eight H2PyC (4-pyrazolecarboxylic acid) under solvothermal conditions. Each of the HPyC− ligands is coordinated with one Cu2+ to form the (4, 8)-connected scu-net structure of compound 2 [(CuCl2)2Th6(HPyC)8(HCOO)4]. In compound 3 [(CuCl2)2Th6(HPyC)10(HCOO)4], ten of the formate ligands of preassembled Th6 cluster are replaced by HPyC− ligands. Compared with compound 2, the two extra HPyC− ligands in the equatorial plane of the Th6 cluster in compound 3 are not further connected to copper ions. Therefore, the topology structure of compound 3 is same with that of compound 2. Compound 4 [(Cu3Cl2)(CuCl2)Th6(PyC)3(HPyC)4(HCOO)5] contains three kinds of metal nodes, Th6 cluster, Cu3 cluster and mononuclear Cu2+, and exhibits a novel (5, 7)-connected net structure, which was first discovered in actinide MOFs. Furthermore, considering the satisfactory stability of compound 4 and its unsaturated metal nodes and Lewis acid sites, the catalysis of cycloaddition of CO2 was further studied. We found that this thorium-copper heterometallic cluster organic framework can be used as a potential actinide functional material for catalyzing the efficient CO2 conversion to value-added products.