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

Sodium difluorophosphate: facile synthesis, structure, and electrochemical behavior as an additive for sodium-ion batteries

Despite the success of difluorophosphate (PO₂F₂⁻, DFP) electrolyte additives in lithium and potassium-ion batteries, their utilization in sodium-ion batteries remains unexplored due to difficulties in the synthesis of sodium difluorophosphates (NaDFP). Thus, in this study, NaDFP salt prepared via ion exchange of KDFP and NaPF₆ is characterized using single-crystal X-ray diffraction, Raman and infrared (IR) spectroscopy, energy dispersive X-ray analysis (EDX), and thermogravimetry-differential thermal analysis (TG-DTA). Electrochemical tests demonstrate enhanced cycle performance of a hard carbon electrode (capacity retention; 76.3% after 500 cycles with NaDFP vs. 59.2% after 200 cycles in the neat electrolyte), achieving a high coulombic efficiency (average of 99.9% over 500 cycles) when NaDFP is used as an electrolyte additive. Further, electrochemical impedance spectroscopy (EIS) using a HC/HC symmetric cell demonstrates significant reduction of the interfacial resistance upon addition of NaDFP. X-ray photoelectron spectroscopy (XPS) indicates presence of stable, Na⁺-conducting solid-electrolyte interphase (SEI) components formed in the presence of NaDFP. This work not only presents a feasible NaDFP synthesis method, but also demonstrates the use of NaDFP as a strategy for optimizing sodium-ion battery performance

Partially naked fluoride in solvate ionic liquids

Truly naked fluoride exists only in the gas phase. Fluoride can be stabilized by a complexing agent and an organic cation, resulting in anhydrous or dehydrated fluoride which is “partially naked.” This partially naked fluoride enables fluorination reactions at much lower temperatures than hydrated fluorides. Here we show a simple method for preparing fluoride-based solvate ionic liquids (SILs) by mixing 1-alkyl-3-methylimidazolium (1-ethyl-3-methylimidazolium or 1-butyl-3-methylimidazolium) bromide, silver fluoride (AgF), and EG (1:1:1 in molar ratio) in dry methanol. Removal of the methanol produced anhydrous SILs, [C₂C₁im]F·EG and [C₄C₁im]F·EG. This is the first SIL reported that comprises fluoride. ¹H NMR and infrared spectroscopy reveal fluoride hydrogen bonds with EG OH groups and cation aromatic H atoms but not cation tail group protons. Fluorination reactions on benzyl bromide show that [C₂C₁im]F·EG has high reactivity with reasonable yield under mild conditions, confirming the fluoride ion is partially naked

Structural evaluation and protium-deuterium exchange in 1-ethyl-3-methylimidazolium halide-ethylene glycol mixtures

A series of equimolar mixtures of 1-ethyl-3-methylimidazolium halide ([C₂C₁im]X: X⁻ = F⁻, Cl⁻, Br and I⁻) and ethylene glycol (EG) are studied by ¹H NMR and IR spectroscopies. The chemical shifts for the protons in EG and imidazolium ring shift towards the downfield in the order of F⁻ >> Cl⁻ > Br⁻ > I⁻, owing to the strength of their respective X…H interactions. Amongst all the studied systems, the fluoride complex ([C₂C₁im]F·EG) shows extremely strong interactions between F⁻ and OH hydrogen of EG, resulting in no “free” EG in the mixture as reflected in the infrared spectra. Quantum chemical calculations suggest several possible geometries for all the halide systems, where the geometry with the EG molecule forming a chelate of halide ion gives the most stable structure. The calculated interaction energies of these geometries also confirm that the fluoride complex has a significantly higher interaction energy than those of the other halide systems. Furthermore, the halide anion affects the selectivity of protium-deuterium exchange site in these systems, and some imidazolium ring hydrogen atoms are activated only in the presence of F⁻