Fluoride Ion Interactions in Alkali-Metal Fluoride-Diol Complexes

Abstract

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.