Enhancement of anion recognition exhibited by a zinc-imidazole-based ion-pair receptor composed of C-H hydrogen- and halogen-bond donor groups.

A 2-haloimidazole-tetraphenylethylene ion-pair receptor 1 is shown to recognise only HSO4- anions in the presence of a cobound Zn2+ cation guest species, which induced a remarkable increase with concomitant blue shift of the emission band of the complex [1 2Zn](4+) whereas no affinity of the free receptor 1 by the anions is observed. In addition, the downfield shifts observed by H-1 NMR of the H-a, H-b and H-c protons of the complex [1 2Zn](4+) upon the addition of HSO4- anions indicate their participation in the recognition event. According to DFT studies, upon chelating a Zn2+ cation with two imidazole nitrogen atoms, receptor 1 adopts a conformation ideally fitted to recognise HSO4- through a combination of C(sp(2))-HO and C(sp(3))-HO hydrogen bondings, C+(sp(2))-BrO halogen bonding and C(sp(2))O tetrel bonding.