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

Abstract

Five new coordination compounds, namely [{Cd(μ-L1)(μ-SO₄)(H₂O)₂}·MeOH]_∞ (1), [{Cd(μ-L2)₂(SO₄)(H₂O)}·3.5H₂O]_∞ (2), [{Cd(μ-L3)(SO₄)(H₂O)₃}·MeOH]_∞ (3), [{Cd(L4)₄ (H₂O)₂}·SO₄·8H₂O] (4) and [{Cd(μ-L5)(μ₃-SO₄)(H₂O)}·2H₂O]_∞ (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 SO₄^2–, NO^3–, ClO^4–, AcO^–, Cl^– and Br^– resulted in the isolation of 2, which indicates the selective separation of SO₄^2–, 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 SO₄^2–, play an important role in the selective separation of SO₄^2–. A similar competitive crystallization also resulted in selective SO₄^2–, separation in the form of compound 5, as determined by FTIR spectroscopy and elemental analysis.