Fluoride Ion Interactions in Alkali-Metal Fluoride–Diol Complexes

The activity of F– is an important factor in the design of both inorganic and organic reactions involving fluorine compounds. The present study investigates interactions of F– with diols in alkali-metal fluoride–diol complexes. Increases in the reactivities of alkali-metal fluorides and their solubilities in alcohols is observed with increasing cation size. The difference in alkali-metal ion size produces different structural motifs for F–-diol complex salts. The CsF complex salt with ethylene glycol (EG), CsF-EG, has a layered structure, whereas the Rb and K complex salts, (RbF)5-(EG)4 and (KF)5-(EG)4, form columnar structures. Comparison of the CsF complex salts with three different diols— EG, 1,3-propylene glycol (PG13), and 1,4-butylene glycol (PG14)—revealed that the diol chain length affects the bridging mode in their layered structures. EG bridges two OH oxygen atoms within the same CsF layer in CsF-EG, whereas PG13 and BG14 bridge two OH oxygen atoms in different CsF layers in (CsF)2-PG13 and CsF-BG14, respectively. The F– ion coordination environment involves interactions between alkali-metal ions and H atom(s) in the diol OH groups, where the F–···H interactions are more dominant than the F–···M+ interaction, based on Hirshfeld surface analyses. The O–H bond weakening observed by infrared spectroscopy also reflects the strengths of the F–···H interactions in these complex salts.