Synthesis, Characterization, and Reactivity of Thermally Stable Anhydrous Quaternary Ammonium Fluorides

Fluoride Ion in Alcohols: Isopropanol vs Hexafluoroisopropanol

The utility of alcohol as a hydrogen bonding donor is considered a providential avenue for moderating the high basicity and reactivity of the fluoride ion, typically used with large cations. However, the practicality of alcohol-fluoride systems in reactions is hampered by the limited understanding of the pertinent interactions between the OH group and F-. Therefore, this study comparatively investigates the thermal, structural, and physical properties of the CsF-2-propanol and CsF-1,1,1,3,3,3-hexafluoro-2-propanol systems to explicate the effects of the fluoroalkyl group on the interaction of alcohols and F-. The two systems exhibit vastly different phase diagrams despite the similar saturated concentrations. A combination of spectroscopic analyses, alcohol activity coefficient measurements, and theoretical calculations reveal the fluorinated alcohol system harbors the stronger OH···F- interactions between the two systems. The diffusion coefficient and ionic conductivity measurements attribute the present results to disparate states of ion association in the two systems.

The pKBHX Hydrogen-Bond Basicity Scale: From Molecules to Anions

The pK_BHX (logarithm of complexation constant K of 4-fluorophenol with bases) hydrogen-bond basicity scale of neutral hydrogen-bond acceptors (HBAs) is extended to anionic HBAs. The scale is constructed for 26 anions through (i) the infrared measurement of K on NBu₄⁺X^- ion pairs in CCl₄, (ii) the estimation of K from linear free energy relationships between measured K values and literature K values for various phenols in polar solvents, and (iii) the computation of K at the density functional theory level in CCl₄. The scale extends on a 9.4 pK unit range from fluoride to tetraphenylborate. Considering a number of anions as organic functions substituted with unipolar substituents, their pK_BHX values can be related to the Hammett-Taft substituent constants σ. Unipolar substituents (O^- and S^-) obey the same pK_BHX versus σ relationships as dipolar ionic (N^-N⁺R₃) and dipolar (OH, CF₃, NR₂, or OR) ones for the nitrile, carbonyl, nitroso, nitro, sulfonyl, and phosphoryl functions. Like dipolar substituents, unipolar substituents at carbon and nitrogen operate by field-inductive and resonance effects, whereas substituents at sulfur and phosphorus operate only by the field-inductive effect.

Tetramethylammonium Fluoride: Fundamental Properties and Applications in C-F Bond-Forming Reactions and as a Base

Nucleophilic ionic sources of fluoride are essential reagents in the synthetic toolbox to access high added-value fluorinated building blocks unattainable by other means. In this review, we provide a concise description and rationale of the outstanding features of one of these reagents, tetramethylammonium fluoride (TMAF), as well as disclosing the different methods for its preparation, and how its physicochemical properties and solvation effects in different solvents are intimately associated with its reactivity. Furthermore, herein we also comprehensively describe its historic and recent utilization, up to December 2021, in C-F bond-forming reactions with special emphasis on nucleophilic aromatic substitution fluorinations with a potential sustainable application in industrial settings, as well as its use as a base capable of rendering unprecedented transformations.

Insights into the Stabilization of Fluoride Ions in Ionic Liquids: Pointers to Better Fluorinating Agents

The fluorination efficiency of a fluorinating agent depends on the free availability of the fluoride ions, which in turn depends on its interaction with its solvation shell. A stable fluoride-based poor solvate ionic liquid (SIL) comprising 1-ethyl-3-methylimidazolium (EMIM) cation and ethylene glycol (EG) was recently reported and demonstrated as a fluorinating agent. Herein, we performed ab initio calculations and ab initio molecular dynamics simulations to gain a microscopic understanding of the intermolecular interactions in this SIL in gas, liquid, and crystalline phases. Ethylene glycol (EG), being capable of forming hydrogen bond(s) with the fluoride ion, prevents the latter from reacting with the EMIM cation. Fluoride forms hydrogen bonds with both the cation and the EG molecule, but it was found to have more affinity toward EG, forming a stronger hydrogen bond with its hydroxyl proton than with the acidic proton of the cation. An optimal concentration of EG in the SIL balances its contribution to stabilizing the fluoride ion and yet making fluoride available for fluorination.

Improved Access to Organo-Soluble Di- and Tetrafluoridochlorate(I)/(III) Salts

A facile one-pot gram-scale synthesis of tetraalkylammonium tetrafluoridochlorate(III) [cat][ClF4 ] ([cat]=[NEt3 Me]+ , [NEt4 ]+ ) is described. An acetonitrile solution of the corresponding alkylammonium chloride salt is fluorinated with diluted fluorine at low temperatures. The reaction proceeds via the [ClF2 ]- anion which is structurally characterized for the first time. The potential application of [ClF4 ]- salts as fluorinating agents is evaluated by the reaction with diphenyl disulfide, Ph2 S2 , to pentafluorosulfanyl benzene, PhSF5 . The CN moieties in acetonitrile and [B(CN)4 ]- are transferred in CF3 groups. Exposure of carbon monoxide, CO, leads to the formation of carbonyl fluoride, COF2 , and elemental gold is dissolved under the formation of tetrafluoridoaurate [AuF4 ]- .

New Insights into the Stability of Anhydrous 2H-Imidazolium Fluoride and its High Dissolution Capability toward a Strongly Hydrogen-Bonded Compound

Fluorides have been widely applied in pharmaceutical, medicinal, and materials science as well as in fine chemical manufacturing. The performance of fluorides, however, can be markedly affected by the water content. One previous study (Maiti, A.; et al. Phys. Chem. Chem. Phys. 2008, 10, 5050) suggested that anhydrous 1,3-dimethylimidazolium fluoride ([DMIm]F) was unstable since the fluoride undergoes a self-decomposition reaction. Herein we first show quantum-chemical calculation evidence that although gas-phase [DMIm]F is unstable, the bulk phase of anhydrous [DMIm]F is quite stable. We then demonstrate the successful synthesis of the anhydrous [DMIm]F compound via the reaction between 1,3-dimethylimidazolium iodide and silver fluoride. Importantly, we find that anhydrous [DMIm]F possesses a high dissolution capability toward 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), although it is known that TATB is hardly dissolved in many common organic solvents. Our Born-Oppenheimer molecular dynamics (BOMD) simulations further show that the high dissolving ability of anhydrous [DMIm]F toward TATB can be attributed to the chemical reaction between the F^- anion and the TATB molecules, which disrupts the strong hydrogen-bonding interaction among the TATB molecules. Alternatively, water molecules in hydrous [DMIm]F tend to form a hydration layer around the F^- anion, thereby preventing F^- from reacting with the TATB molecule. This result explains why TATB is barely dissolved in hydrous [DMIm]F.

Effective neutralization of chemical warfare agents (HD, VX) by Me-DABCOF: a small molecule with dual action

Me-DABCOF, a mild universal, non-corrosive, water-soluble decontamination agent that effectively neutralizes chemical warfare agents (HD, VX).

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.

Nucleophilic fluorination facilitated by a CsF–CaF2 packed bed reactor in continuous flow

A simple to prepare, dry and handle packed bed reactor carrying CsF on CaF₂, towards nucleophilic fluorinations in continuous flow, is reported.

Potassium fluoride activation for the nucleophilic fluorination reaction using 18-crown-6, [2.2.2]-cryptand, pentaethylene glycol and comparison with the new hydro-crown scaffold: a theoretical analysis

Ion-pair binding in the hydro-crown ether leads to more effective catalytic cycle for nucleophilic fluorination.