Synthetic Control of Thorium Polyoxo-Clusters in Metal–Organic Frameworks toward New Thorium-Based Materials

Abstract

Control over the assembly of thorium-based polyoxo-clusters remains a great challenge because of their quick olation and oxolation reactions. Here, we report the synthesis and isolation of two pure phase thorium polyoxo-cluster-based metal–organic frameworks (Th-MOFs), Th-NU-1008 and Th-NU-1011. Crystal structures of these Th-MOFs reveal two distinct 8-connected polyoxo-clusters: a hexanuclear cluster [Th6(μ3-O)4(OH)4(H2O)6(HCOO)4] was observed in Th-NU-1008 and a tetranuclear cluster [Th4(μ3-O)2(OH)4(DMF)2(H2O)4] in Th-NU-1011 which has never been reported before among thorium species. In both Th-NU-1008 and Th-NU-1011, the tetratopic linker, 1,4-dibromo-2,3,5,6-tetrakis(4-carboxyphenyl) benzene, joins the clusters together, forming two three-periodic networks, each with csq topology. The Brunauer–Emmett–Teller (BET) surface areas calculated from the Ar isotherms are approximately 800 and 700 m2 g–1 for Th-NU-1008 and Th-NU-1011, respectively. Because the linkers are oriented orthogonal to each other in the two respective MOFs, a 32 Å hexagonal channel is observed in Th-NU-1008 while a 15 Å channel is observed in Th-NU-1011. This work opens a new avenue to construct unexplored Th-MOF materials with targeted polyoxo-cluster structures.