Selective binding of anions by rigidified nanojars: sulfate vs. carbonate.

Selective binding and transport of highly hydrophilic anions is ubiquitous in nature, as anion binding proteins can differentiate between similar anions with over a million-fold efficiency. While comparable selectivity has occasionally been achieved for certain anions using small, artificial receptors, the selective binding of certain anions, such as sulfate in the presence of carbonate, remains a very challenging task. Nanojars of the formula [anion⊂{Cu(OH)(pz)}n]2− (pz = pyrazolate; n = 27–33) are totally selective for either CO32− or SO42− over anions such as NO3−, ClO4−, BF4−, Cl−, Br− and I−, but cannot differentiate between the two. We hypothesized that rigidification of the nanojar outer shell by tethering pairs of pyrazole moieties together will restrict the possible orientations of the OH hydrogen-bond donor groups in the anion-binding cavity of nanojars, similarly to anion-binding proteins, and will lead to selectivity. Indeed, by using either homoleptic or heteroleptic nanojars of the general formula [anion⊂Cun(OH)n(L2–L6)y(pz)n−2y]2− (n = 26–31) based on a series of homologous ligands HpzCH2(CH2)xCH2pzH (x = 0–4; H2L2–H2L6), selectivity for carbonate (with L2 and with L4–L6/pz mixtures) or for sulfate (with L3) has been achieved. The synthesis of new ligands H2L3, H2L4 and H2L5, X-ray crystal structures of H2L4 and the tetrahydropyranyl-protected derivatives (THP)2L4 and (THP)2L5, synthesis and characterization by electrospray-ionization mass spectrometry (ESI-MS) of carbonate- and sulfate-nanojars derived from ligands H2L2–H2L6, as well as detailed selectivity studies for CO32−vs. SO42− using these novel nanojars are presented.