Rare Three-Dimensional Uranyl–Biphenyl-3,3′-disulfonyl-4,4′-dicarboxylate Frameworks: Crystal Structures, Proton Conductivity, and Luminescence

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}_<i>n</i> (<b>1</b>), {[K₂(UO₂)­(μ₃-O)­(BPDSDC)_0.5­(H₂O)₂]}_<i>n</i> (<b>2</b>), and {(Me₂NH₂)_2.5­[K_1.5(UO₂)­(BPDSDC)_1.5­(H₂O)₃]}_<i>n</i> (<b>3</b>). Compounds <b>1</b> and <b>2</b> contain one-dimensional (1D) ribbon structures formed from UO₂²⁺ units bridged by μ₃-O atoms and carboxylate groups. The 1D ribbons in <b>1</b> 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 <b>2</b>, the oxygen atoms of UO₂²⁺ units in each 1D ribbon bridge the K⁺ atoms to form four −[K–O–K]_<i>n</i>– infinite chains located above and below the ribbon. The 1D ribbons in <b>2</b> are bridged by sulfonate groups to generate a 3D substructure featuring 1D channels occupied by biphenyl moieties. In <b>3</b>, 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 <b>2</b> exhibits an excellent proton conductivity with the highest conductivity of 1.07 × 10^–3 S cm^–1. The inner walls of 1D channels of <b>2</b> 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 <b>1</b> and <b>2</b> display the characteristic uranyl emissions. This work presents the elegant examples of the rarely explored 3D UOFs and expands the potentials of UOFs.