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.

Alkali Metal Fluorides in Fluorinated Alcohols: Fundamental Properties and Applications to Electrochemical Fluorination

“…A similar continuous layered structure was reported for an aliphatic diol system. 22 Such continuous structures were obtained presumably due to the strong ionic bonding of cesium and fluoride ions, as supported by the distinct solid-state structure of TBAF/(TFE) 4 .…”

“…Despite a number of precedents of solid-state structures of hydrogen-bonded fluoride ions using TBAF, the solid-state structures of hydrogen-bonded MF complexes remain relatively rare. Matsumoto and co-workers recently reported the solid-state structure of MF/aliphatic diol complexes (M = K, Cs, Rb) . Gouverneur and co-workers reported the solid-state structure of binaphthyl-based bisurea with CsF .…”

“…In the solid-state structures of CsF/(HFIP) 3 and CsF/(TFE) 2 , the oxygen atoms in HFIP and TFE interacted with cesium ions. The interaction of the hydrogen atoms of the OH groups with the fluoride ions also seems feasible based on the precedents. − ,, Thus, the high solubility of MF in fluorinated alcohols is attributed to the multiple interactions of MF with OH groups.…”

“…Matsumoto and co-workers recently reported the solid-state structure of MF/aliphatic diol complexes (M = K, Cs, Rb). 22 Gouverneur and co-workers reported the solidstate structure of binaphthyl-based bisurea with CsF. 13 In this context, we were prompted to investigate the solid-state structures of MF/fluorinated alcohol complexes (M = Cs, K).…”

“…The interaction of the hydrogen atoms of the OH groups with the fluoride ions also seems feasible based on the precedents. [7][8][9]13,22 Thus, the high solubility of MF in fluorinated alcohols is attributed to the multiple interactions of MF with OH groups.…”