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) (L-5) and hexane-1,6-diylbis(3-pyridin-3-ylurea) (L-6) 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)]SO4 center dot 2H(2)O}(n) (1: Zn, L-5; 2: Cd, L-5; and 3: Zn, L-6). 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.