Sulfate encapsulation in three-fold interpenetrated metal-organic frameworks with bis(pyridylurea) ligands

Self-assembly of two flexible bis-pyridylurea ligands, pentane-1,5- diylbis(3-pyridin-3-ylurea) (L⁵) and hexane-1,6-diylbis(3-pyridin-3- ylurea) (L⁶) with ZnSO₄ or CdSO₄ (metal-to-ligand molar ratio 1:2) results in three metal-organic frameworks with (4,4) net structures, {[ML₂(H₂O)₂]SO ₄·2H₂O}_n (1: Zn, L⁵; 2: Cd, L⁵; and 3: Zn, L⁶). Single-crystal X-ray diffraction analyses reveal that all the compounds are characteristic of a 3-fold parallel interpenetrated framework with cavities suitable for encapsulating the sulfate anion despite the different spacer lengths ((CH₂)₅ and (CH₂)₆) of the two ligands and variation of the metal ions. The ligand adopts an anti-anti conformation and functions as a bridging bidentate linker, while the sulfate ion is selectively bound by four urea functionalities via multiple hydrogen bonds.