Selective Separation of the Sulfate Anion by In Situ Crystallization of CdII Coordination Compounds Derived from Bis(pyridyl) Ligands Equipped with a Urea/Amide Hydrogen-Bonding Backbone

Five new coordination compounds, namely [{Cd(mu-L1)(mu-SO4)(H2O)(2)}center dot MeOH](infinity) (1), [{Cd(mu-L2)(2)(SO4)(H2O)}center dot 3.5H(2)O](infinity) (2), [{Cd(mu-L3)(SO4)(H2O)(3)center dot MeOH](infinity) (3), [{Cd(L4)(4)(H2O)(2)}center dot SO4 center dot 8H(2)O] (4) and [{Cd(mu-L5)(mu(3)-SO4)(H2O)}center dot H2O](infinity) (5), [L1 = N-(3-picolyl)-N'-(3-pyridyl)urea, L2 = N-(4 -picolyl)-N'-(3-pyridyl)urea, L3 = N,N'-di-3-pyridylurea, L4 = N,N'-di-4-pyridylurea and L5 = N,N'-di-3-pyridylnicotinamide] have been synthesized and characterized by single-crystal X-ray diffraction. It is revealed that the positional isomers L1,L2 and L3,L4 play a crucial role in determining the resultant supramolecular structures of 1-4. Various supramolecular architectures, that is, a 3D coordination polymer (1), a 1D loopedchain coordination polymer (2), a 1D zigzag coordination polymer (3), a discrete coordination complex (4) and a 2D corrugated sheet (5), were observed. In situ crystallization of compound 2 under competitive crystallization conditions in the presence of competing anions such as SO42-, NO3- , ClO4-, AcO-, Cl- and Br- resulted in the isolation of 2, which indicates the selective separation of SO42- from an aqueous mixture of other competing anions, as revealed by FTIR spectroscopy, X-ray powder diffraction and elemental analysis. The X-ray structure of 2 indicates that both intermolecular hydrogen bonding and metal-ligand coordination of SO42- play an important role in the selective separation of SO42-. A similar competitive crystallization also resulted in selective SO42- separation in the form of compound 5, as determined by FTIR spectroscopy and elemental analysis.