Ditopic receptors containing urea groups for solvent extraction of Cu(ii) salts

The ditopic receptor L3 [1-(2-((7-(4-(tert-butyl)benzyl)-1,4,7,10-tetraazacyclododecan-1-yl)methyl)phenyl)-3-(3-nitrophenyl)urea] containing a macrocyclic cyclen unit for Cu(ii)-coordination and a urea moiety for anion binding was designed for recognition of metal salts. The X-ray structure of [CuL3(SO₄)] shows that the sulfate anion is involved in cooperative binding via coordination to the metal ion and hydrogen-bonding to the urea unit. This behaviour is similar to that observed for the related receptor L1 [1-(2-((bis(pyridin-2-ylmethyl)amino)methyl)phenyl)-3-(3-nitrophenyl)urea], which forms a dimeric [CuL1(μ-SO₄)]₂ structure in the solid state. In contrast, the single crystal X-ray structure of [ZnL3(NO₃)₂] contains a 1 : 2 complex (metal : anion) where one anion coordinates to the metal and the other is hydrogen-bonded to the urea group. Spectrophotometric titrations performed for the [CuL3(OSMe₂)]²⁺ complex indicate that this system is able to bind a wide range of anions with an affinity sequence: MeCO₂^- > Cl^- > H₂PO₄^- > Br^- > NO₂^- > HSO₄^- > NO₃^-. Lipophilic analogues of L1 and L3 extract CuSO₄ and CuCl₂ from water into chloroform with high selectivity over the corresponding Co(ii), Ni(ii) and Zn(ii) salts.