Selective binding of anions by rigidified nanojars: sulfate carbonate

• Published in: Organic & biomolecular chemistry Vol. 19; no. 35; pp. 7641 - 7654
• Main Authors: Al Isawi, Wisam A, Salome, Austin Z, Ahmed, Basil M, Zeller, Matthias, Mezei, Gellert
• Format: Journal Article
• Published: 15.09.2021
• ISSN: 1477-0520; 1477-0539
• DOI: 10.1039/d1ob01318a

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 CO 3 2− or SO 4 2− over anions such as NO 3 − , ClO 4 − , BF 4 − , 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⊂Cu n (OH) n ( L2-L6 ) y (pz) n −2 y ] 2− ( n = 26-31) based on a series of homologous ligands HpzCH 2 (CH 2 ) x CH 2 pzH ( x = 0-4; H 2 L2 -H 2 L6 ), selectivity for carbonate (with L2 and with L4-L6 /pz mixtures) or for sulfate (with L3 ) has been achieved. The synthesis of new ligands H 2 L3 , H 2 L4 and H 2 L5 , X-ray crystal structures of H 2 L4 and the tetrahydropyranyl-protected derivatives (THP) 2 L4 and (THP) 2 L5 , synthesis and characterization by electrospray-ionization mass spectrometry (ESI-MS) of carbonate- and sulfate-nanojars derived from ligands H 2 L2 -H 2 L6 , as well as detailed selectivity studies for CO 3 2− vs. SO 4 2− using these novel nanojars are presented. By using nanojars as anion binding agents, selectivity for either carbonate or sulfate can be achieved upon rigidification of the nanojar outer-shell with tethers of varying lengths between pairs of pyrazole ligands.