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

Five new coordination compounds, namely [{Cd(μ‐ L1 )(μ‐SO 4 )(H 2 O) 2 } · MeOH] ∞ ( 1 ), [{Cd(μ‐ L2 ) 2 (SO 4 )(H 2 O)} · 3.5H 2 O] ∞ ( 2 ), [{Cd(μ‐ L3 )(SO 4 )(H 2 O) 3 } · MeOH] ∞ ( 3 ), [{Cd( L4 ) 4 (H 2 O) 2 } · SO 4 · 8H 2 O] ( 4 ) and [{Cd(μ‐ L5 )(μ 3 ‐SO 4 )(H 2 O)} · 2H 2 O] ∞ ( 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 looped‐chain 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 inthe presence of competing anions such as SO 4 2– , NO 3 – , ClO 4 – ,AcO – , Cl – and Br – resulted in the isolation of 2 , which indicates the selective separation of SO 4 2– 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 SO 4 2– play an important role in the selective separation of SO 4 2– . A similar competitive crystallization also resulted in selective SO 4 2– separation in the form of compound 5 , as determined by FTIR spectroscopy and elemental analysis.