Selective recognition of fluoride salts by vasarenes: a key role of a self-assembled in situ dimeric entity via an exceptionally short [O–H–O]− H-bond†
A self-assembled supramolecular dimeric entity via an exceptionally short (2.404 Å) and strong (22.9 kcal mol−1) [O–H–O]− hydrogen bond is the key to the special reactivity of vasarenes with fluoride salts. Vasarene is a self-assembled, vase-shaped compound, obtained by the reaction between ninhydrin and phloroglucinol. Analogous compounds are prepared by replacing the phloroglucinol with other polyhydroxy aromatics. Vasarenes show special affinity towards compounds of the type M+F−, where M being a large monovalent cation, producing ion-pair-vasarene adducts. The first step in the proposed mechanism is the dissociation of the M+F− salt releasing F− to the solution, which may provide an explanation as to why only MF salts, which include large monovalent cations, undergo this reaction. From a practical point of view, the ease of their preparation and their special affinity towards fluoride salts make vasarenes potential means for salt separation. The readily formed dimeric structure with the very short [O–H–O]− negative charge-assisted H-bond (−CAHB) can also be further used as a model in theoretical studies of such systems and understanding their role in biological processes.