Rare Three-Dimensional Uranyl-Biphenyl-3,3'-disulfonyl-4,4'-dicarboxylate Frameworks: Crystal Structures, Proton...

Due to the intrinsic coordination preference of the linear uranyl unit, uranyl-organic frameworks (UOFs) are generally prone to exhibiting low-dimensional structures. Reactions of uranyl nitrate with biphenyl-3,3'-disulfonyl-4,4'-dicarboxylic acid dipotassium salt (K₂H₂BPDSDC) under different conditions led to three UOFs, namely, {(Me₂NH₂)[K₂(UO₂)₃(μ₃-O)(μ₃-OH)₂(μ₂-OH)(BPDSDC)(H₂O)₃]·4DMF} (1), {[K₂(UO₂)(μ₃-O)(BPDSDC)_0.5(H₂O)₂]} (2), and {(Me₂NH₂)_2.5[K_1.5(UO₂)(BPDSDC)_1.5(H₂O)₃]} (3). Compounds 1 and 2 contain one-dimensional (1D) ribbon structures formed from UO₂²⁺ units bridged by μ₃-O atoms and carboxylate groups. The 1D ribbons in 1 are linked by K⁺ atoms to form a two-dimensional (2D) layer, which is further pillared by the biphenyl units to give a three-dimensional (3D) framework. For 2, the oxygen atoms of UO₂²⁺ units in each 1D ribbon bridge the K⁺ atoms to form four -[K-O-K]- infinite chains located above and below the ribbon. The 1D ribbons in 2 are bridged by sulfonate groups to generate a 3D substructure featuring 1D channels occupied by biphenyl moieties. In 3, each mononuclear [(UO₂)(COO)₃] unit is bridged by three K⁺ atoms to form a 3D substructure featuring 1D small left-handed and large righted helical channels occluded by biphenyl moieties. Compound 2 exhibits an excellent proton conductivity with the highest conductivity of 1.07 × 10^-3 S cm^-1. The inner walls of 1D channels of 2 are full of the hydrophilic sulfonate groups, which boost enrichment of the guest water molecules, thus resulting in a high proton conductivity. Finally, temperature dependence of fluorescent studies showed that compounds 1 and 2 display the characteristic uranyl emissions. This work presents the elegant examples of the rarely explored 3D UOFs and expands the potentials of UOFs.