Highly water stable lanthanide metal-organic frameworks constructed from 2,2′-disulfonyl-4, 4′-biphenyldicarboxylic acid: Syntheses, structures, and properties

A series of three-dimensional (3D) lanthanide metal-organic frameworks (LnMOFs), (Me₂NH₂)[LnL- (H₂O)] (Ln = Eu (1), Gd (2), Tb (3), Dy (4); H₄L = 2, 2′- disulfonyl-4, 4′-biphenyldicarboxylic acid) have been successfully synthesized from H₄L and Ln(NO₃)₃·6H₂O under solvothermal conditions. Single crystal X-ray diffraction (SCXRD) shows that all LnMOFs 1-4 are isomorphous and isostructural with a hepta-coordinated Ln(III) being connected through the carboxylate groups of the L^4- ligands, resulting in the formation of an one-dimensional (1D) inorganic rod-like [Ln(-COO)₂)]_n⁺ chain along the c axis. The infinite 1D chains are further linked by the sulfonate and biphenyl groups, leading to formation of a uninodal 5-connected 3D network with bnn topology. The present LnMOFs are the first example of anionic 3D bnn-net constructed on Ln-O-C rods with channels being filled with (Me₂NH₂)⁺ cations. The L^4- ligand shows a pentadentate coordination mode with two bound sulfonate groups. All the LnMOFs are insoluble in water and highly stable against moisture. Cation-exchange with Li⁺, Na⁺, and K⁺ ions can be easily performed at room temperature (RT). In addition, LnMOFs 1 and 3 display characteristic photoluminescence of Eu(III) and Tb(III) ions upon excitation at 394 and 353 nm, respectively. The investigation of magnetism demonstrates relatively weak antiferromagnetic interactions between Gd(III) ions (J = -0.0042(5) cm^-1) in 2, and between Dy(III) ions (θ = -0.20(2) K) in 4. The proton conductivity of 1 is 4.14 × 10^-8 S cm^-1 at 95% relative humidity (RH) and 25 °C.