title: Cooperative anion recognition in copper(II) and zinc(II) complexes with a ditopic tripodal ligand containing a urea group.
abstract: The ability of Cu(II) and Zn(II) complexes of the ditopic receptor H2L [1-(2-((bis(pyridin-2-ylmethyl)amino)methyl)phenyl)-3-(3-nitrophenyl)urea] for anion recognition is reported. In the presence of weakly coordinating anions such as ClO4(-), the urea group binds to the metal ion (Cu(II) or Zn(II)) through one of its nitrogen atoms. The study of the interaction of the metal complexes with a variety of anions in DMSO shows that SO4(2-) and Cl(-) bind to the complexes through a cooperative binding involving simultaneous coordination to the metal ion and different hydrogen-bonding interactions with the urea moiety, depending on the shape and size of the anion. On the contrary, single crystal X-ray diffraction studies show that anions such as NO3(-) and PhCO2(-) form 1:2 complexes (metal/anion) where one of the anions coordinates to the metal center and the second one is involved in hydrogen-bonding interaction with the urea group, which is projected away from the metal ion. Spectrophotometric titrations performed for the Cu(II) complex indicate that this system is able to bind a wide range of anions with an affinity sequence: MeCO2(-) ∼ Cl(-) (log K11 > 7) > NO2(-) > H2PO4(-) ∼ Br(-) > HSO4(-) > NO3(-) (log K11 < 2). In contrast to this, the free ligand gives much weaker interactions with these anions. In the presence of basic anions such as MeCO2(-) or F(-), competitive processes associated with the deprotonation of the coordinated N-H group of the urea moiety take place. Thus, N-coordination of the urea unit to the metal ion increases the acidity of one of its N-H groups. DFT calculations performed in DMSO solution are in agreement with both an anion-hydrogen bonding interaction and an anion-metal ion coordination collaborating in the stabilization of the metal salt complexes with tetrahedral anions.
authors: Departamento de Química Fundamental, Universidade da Coruña , Campus da Zapateira, Rúa da Fraga 10, 15008 A Coruña, Spain.
authors: Israel
authors: I
authors: Carreira-Barral
authors: Teresa
authors: T
authors: Rodríguez-Blas
authors: Carlos
authors: C
authors: Platas-Iglesias
authors: Andrés
authors: A
authors: de Blas
authors: David
authors: D
authors: Esteban-Gómez
mesh_terms: D000838:Anions / Q000737:chemistry*; D056831:Coordination Complexes; D003300:Copper / Q000737:chemistry*; D008024:Ligands; D009682:Magnetic Resonance Spectroscopy; D011789:Quantum Theory; D014508:Urea / Q000737:chemistry*; D015032:Zinc / Q000737:chemistry*
chemical_list: D000838:Anions; D056831:Coordination Complexes; D008024:Ligands; D003300:Copper; D014508:Urea; D015032:Zinc