Zn(II) metal–organic frameworks (MOFs) derived from a bis-pyridyl-bis-urea ligand: Effects of crystallization solvents on the structures and anion binding properties

Abstract

The Reaction of N,N′-bis-(3-pyridyl)ethylene-bis-urea (BPEBU) with ZnSO₄ in 2 : 1 ligand : metal ratio in various crystallization solvents afforded four metal–organic frameworks (MOFs), namely [{Zn(H₂O)₃ (SO₄)(µ-BPEBU)}·EG·2H₂O]_n2, [{Zn(µ-BPEBU)₂(H₂O)₂}.SO₄·2H₂O.2THF}_n3, [{Zn(µ-BPEBU)₂(H₂O)₂}·SO₄·2H₂O.2acetone}n4 and [{Zn(µ-BPEBU)₂(H₂O)₂}·SO₄·2H₂O·2(1,4-dioxane)}_n5, which were mainly characterized by single cyrstal X-ray diffraction. MOFs 3–5 are found to be isomorphous. The study revealed that the crystallization solvents affected the structures; the single crystal structure of the free BPEBU was also discussed in the context of the conformation and ligating topology of the ligand found in the reported MOFs. When the MOFs syntheses were performed in a competitive environment wherein other oxo-counter anions such as NO₃⁻, ClO₄⁻ and CF₃SO₃⁻ were present, the isolated MOFs turned out to be the corresponding sulfate MOFs 2–4 indicating interesting sulfate selectivity.