Cooperative Anion Recognition in Copper(II) and Zinc(II) Complexes with a Ditopic Tripodal Ligand Containing a Urea Group

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.