Alkali Metal Fluorides Research Articles

Electrochemical Fluorination of Organic Compounds Using Alkali Metal Fluoride Salt with Fluorinated Alcohol-Based Solvent System

Fluorinated alcohols, such as 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) and 2,2,2-trifluoroethanol (TFE), are quite useful in organic chemistry including electrosynthesis, due to their unique features such as high oxidation tolerance, high polarity, low nucleophilicity and strong donor ability in hydrogen bonding. Fluorinated alcohols are also capable of dissolving alkali metal salt such as KF and CsF, which is one of the least expensive fluoride ion sources, in high concentration. In this research, we report the electrochemical fluorination of various organic compounds using alkali metal fluoride salt dissolved in a mixed solvent containing HFIP for the first time. Alkali metal fluorides acted both as a fluoride ion source and a supporting electrolyte to enable the electrochemical fluorination of C-H and C-S bonds efficiently.

Short Fluoride Cycle in Tungsten Technology

It is found that when a tungsten anode is electrochemically dissolved in a acidic fluorides of alkali metals (K,Na)H2F3 and hydrogen fluoride at a temperature of t ~ 37°C, the resulting atomic fluorine reacts completely with tungsten to form WF6. The latter dissolves in the melt, forming complex compounds (K,Na)2WF8 and (K,Na)WF7, which is accompanied by an increase in the melting point of the electrolyte. The addition of up to 23 mol % LiF and saturation of the electrolyte by WF6 lowered its melting temperature below 18°C, which, in an electrochemical process at a temperature of 35–40°C and an anode current density of 0.3–0.5 A/cm2, made it possible to obtain simultaneously gaseous WF6 at the anode and H2 at the cathode. During the gas-phase deposition of tungsten, dense layers are formed from the resulting gaseous mixture with a stoichiometric ratio of components at a temperature of 550–600°C, and the resulting HF is captured by an electrolyte and used to produce a mixture of WF6 + H2, ensuring the circulation of reagents and the absence of stored waste. Based on the results, a short fluoride cycle in tungsten technology is presented. It uses two operations: the electrochemical synthesis of a gaseous mixture of WF6 + H2 in an electrolyzer with a filling anode made of fragments of metal tungsten and the reduction of WF6 by hydrogen with capture the resulting HF, allowing one to reduce the chain of technological devices in the cycle by almost 2 times with a significant reduction in production costs. The hardware and technological scheme of the production chain for the environmentally friendly production of tungsten products with a capacity of ~48.5 t/year, which can be replicated and modified to produce the necessary products, is presented.

Short fluoride cycle in tungsten technology

It is found that when a tungsten anode is electrochemically dissolved in a acidic fluorides of alkali metals (K,Na)H2F3 and hydrogen fluoride at a temperature of t ~ 37°C, the resulting atomic fluorine reacts completely with tungsten to form WF6. The latter dissolves in the melt, forming complex compounds (K,Na)2WF8 and (K,Na)WF7, which is accompanied by an increase in the melting point of the electrolyte. The addition of up to 23 mol % LiF and saturation of the electrolyte by WF6 lowered its melting temperature below 18°C, which, in an electrochemical process at a temperature of 35–40°C and an anode current density of 0.3–0.5 A/cm2, made it possible to obtain simultaneously gaseous WF6 at the anode and H2 at the cathode. During the gas-phase deposition of tungsten, dense layers are formed from the resulting gaseous mixture with a stoichiometric ratio of components at a temperature of 550–600°C, and the resulting HF is captured by an electrolyte and used to produce a mixture of WF6 + H2, ensuring the circulation of reagents and the absence of stored waste. Based on the results, a short fluoride cycle in tungsten technology is presented. It uses two operations: the electrochemical synthesis of a gaseous mixture of WF6 + H2 in an electrolyzer with a filling anode made of fragments of metal tungsten and the reduction of WF6 by hydrogen with capture the resulting HF, allowing one to reduce the chain of technological devices in the cycle by almost 2 times with a significant reduction in production costs. The hardware and technological scheme of the production chain for the environmentally friendly production of tungsten products with a capacity of ~48.5 t/year, which can be replicated and modified to produce the necessary products, is presented.

Pyrene‐Tagged Alcoholic Ionic Liquids as Phase Transfer Catalysts for Nucleophilic Fluorination

Functional group−activity relationships of pyrene‐tagged ionic liquid (PTIL)‐based organocatalysts for nucleophilic fluorination using alkali metal fluorides (MFs) are described, which demonstrate that the pyrene, oligoether and alcohol moieties on the imidazolium ring are vital for efficient catalysis. Further investigation of these findings led to the discovery of new strategy, which showed superior catalyst separation process, i.e., catalyst is effortlessly separated from the reaction mixture using reduced graphene oxide. The catalytic efficiency of the PTIL as a phase transfer catalyst was demonstrated by the high conversion of the reactants up to 98% fluorinated yield using MFs in CH3CN or t‐amyl alcohol. Importantly, the catalyst not only enhanced the reactivity of bimolecular nucleophilic substitutions (SN2) within a short reaction time and reduces the formation of by‐products but also affords high yield with easy isolation and separation under mild conditions.

An ultra-thin inorganic interlayer strategy for achieving efficient inverted planar perovskite solar cells and modules with high fill factor

The fill factor (FF) is a fatal parameter to determine the power conversion efficiency (PCE) of perovskite solar cells. Especially, for the large area solar cells and modules, the low FF is the bottleneck towards the high PCE. The FF is highly correlated with the carrier recombination caused by defects at interfaces. Herein, we propose that ultra-thin inorganic alkali metal fluorides interlayer passivate the interface defects by vapor evaporation method at the perovskite/electron transport layer (ETL) interface. We confirmed that the alkali metal fluorides (AF) layer can function as both defect passivation layer and physical barrier layer, showing the significant enhancement of PCE and ambient stability. Eventually, we achieve the highest FF (80.6%) for 1 cm 2 size and highest FF (73.1%) for 10.8 cm 2 size perovskite solar cell by potassium fluoride (KF) as passivation layer. This functional interlayer strategy paves the way to resolve the industrial requirements with low cost, high fill factor large area planar perovskite solar cells. • As for the alkali metal fluorides as passivation layer, the introduce of alkali metal fluorides can effectively reduce the defects at the perovskite surface: the alkali metal fluorides can improve fluorescence intensity and fluorescence lifetime, so that to decrease the surface defect evoked by the surface vacancy. • As for the alkali metal fluorides as physical barrier layer, the introduce of alkali metal fluorides can effectively reduce the carrier recombination at perovskite/transport layer interfaces and improve ambient stability: The inserted inorganic layer can selectively conduct one type of charges while block the other type, which spatially separates photogenerated electrons and holes to reduce their recombination • The inserted inorganic layer increases the contact angle, which obviously improves the device stability.

The synthesis of newly developed Li(1-x-y)NaxKyYF4:Yb3+/Er3+ and its excellent upconversion properties

Abstract In this work, multiple ions co-doping is performed to synthesize a new series of upconversion materials Li(1-x-y)NaxKyYF4:Yb3+/Er3+ based on ternary alkali metal fluoride matrix material by solvothermal method. It can be found that both the mophology and the luminescence properties of them are greatly affected by the doping ratio of Li, Na and K ions in the matrix. And most surprisingly, when the mol ratio of the three ions was 10:6:9, the luminous intensity was 20 times higher than that of NaYF4:Yb3+/Er3+prepared under the same condition. This study testifies that, multiple ions co-doping is a promising way to enhance the upconversion luminescent efficiency, and opens up a new way to improve the properties of similar upconversion materials. And this new series of Li(1-x-y)NaxKyYF4:Yb3+/Er3+ exhibit great potential in the application of new developed upconversion materials.

Hydrogen Bonding Phase-Transfer Catalysis with Ionic Reactants: Enantioselective Synthesis of γ-Fluoroamines.

Ammonium salts are used as phase-transfer catalysts for fluorination with alkali metal fluorides. We now demonstrate that these organic salts, specifically azetidinium triflates, are suitable substrates for enantioselective ring opening with CsF and a chiral bis-urea catalyst. This process, which highlights the ability of hydrogen bonding phase-transfer catalysts to couple two ionic reactants, affords enantioenriched γ-fluoroamines in high yields. Mechanistic studies underline the role of the catalyst for phase-transfer, and computed transition state structures account for the enantioconvergence observed for mixtures of achiral azetidinium diastereomers. The N-substituents in the electrophile influence the reactivity, but the configuration at nitrogen is unimportant for the enantioselectivity.

Synthesis and Characterization of A[W2O2F9] (A = Li – Cs)

Herein, we report upon the in situ formation of WOF4 and its subsequent reaction with AF (A = Li – Cs) in aHF to produce A[W2O2F9] (A = Li – Cs). Structural analysis of the obtained compounds was performed using single‐crystal X‐ray diffraction. A comparison of the single‐crystal structures obtained here to the previously reported A[Mo2O2F9] (A = Li – Cs) and H3O[W2O2F9] salts is given. Additionally, IR and Raman spectroscopy were used to characterize each salt. These spectra were compared to calculated ones for the solid‐state structure of each salt using the DFT‐PBE0 density functional method. The calculated spectra were used to assign bands in the experimental spectra. Both MoOF4 and WOF4 gave similar reaction products under similar reaction conditions. It was concluded that the oxyfluorides show similar Lewis acid‐base reactions with the alkali metal fluorides in aHF solutions.

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.

Synthesis and Characterization of the Hexafluoridomolybdates(V) A[MoF6] (A = Li – Cs)

The hexafluoridomolybdates(V), A[MoF6] (A = Li – Cs), were synthesized by reacting molybdenum pentafluoride (MoF5) with the alkali metal fluorides in anhydrous hydrogen fluoride (aHF) solution. Single crystals of A[MoF6] (A = Li, Na, Rb, Cs) were grown from solution and analyzed by single‐crystal X‐ray diffraction. The vibrational spectrum of each salt is reported in detail and compared to previously reported spectra. During analysis of the cubic modification of Na[MoF6] it was noticed that a phase transition occurs around 228 K. Some details regarding this phase transition are discussed. Attempts to determine the structure of the low‐temperature phase of Na[MoF6] lead to the observation of the Na[MoF6]·2HF solvate. Additionally, in an attempt to synthesize Na[MoF7] by the oxidation of Na[MoF6] in chlorine trifluoride (ClF3), a tetragonal modification of Na[MoF6] was observed. The crystal structures of the Na[MoF6]·2HF solvate and tetragonal modification of Na[MoF6] are also discussed in detail.

Development of the Foundations of a Technology for Producing Low-Ash Graphite from High-Carbon Rocks of Primorskii Krai

The potential of purification of natural graphite by a hydrodifluoride technology was studied by the example of treatment of deposits of the Southern Far East of Russia. It was determined that impurities contained in graphite can be nearly completely fluorinated with a 20% excess of NH4HF2. The formed complex ammonium fluoride salts and alkali metal fluorides are removed from the fluorinated concentrate by leaching with water. Treatment of the purified product with ammonium sulfate at 300–350°C gives graphite containing up to 99.9% carbon.

Synthesis and Characterization of the Tetrafluoridochlorates(III) A[ClF4] (A = K, Rb, Cs)

Single‐crystalline tetrafluoridochlorates(III) A[ClF4] (A = K, Rb, Cs) were synthesized from solvolysis reactions of alkali metal fluorides in liquid chlorine trifluoride. The structures were examined by means of single‐crystal X‐ray diffraction. K[ClF4] crystallizes in the K[BrF4] structure type, whereas the Rb and Cs compounds crystallize in the Li[AuF4] structure type. The compounds were further characterized by Raman and IR spectroscopy. Solid‐state quantum‐chemical calculations with hybrid density functional methods reproduced the experimental structures and enabled the interpretation of the experimental Raman and IR spectra.

Boosting the efficiency and stability of perovskite solar cells through facile molecular engineering approaches

Not only the poor interaction but also trap states at interfaces and grain boundaries are suspected to be responsible for carrier losses in perovskite solar cell (PSC) architecture, leading to inferior photovoltaic performance and long-term stability. Here, facile and effective molecular engineering approaches have been reported by employing a CsF-doped SnO2 electron-transporting layer (ETL) and inserting zwitterion molecules as building blocks between perovskite and hole-transporting layer (HTL). The modification of SnO2 by alkali metal fluoride significantly improved the opto-electronic properties, indicating rapid extraction of photogenerated electrons and better light-harvesting. On the other hand, zwitterion interlayer demonstrated a considerable passivation in multiple defect states at grain boundaries of perovskite film. This strategy yielded an open-circuit voltage (VOC) of 1.23 V for triple-cation perovskite composition with the loss in potential of only 0.37 V. As a result, a considerable efficiency of 21.7% was achieved with negligible hysteresis. More importantly, such engineering approaches exhibited an admissible long-term stability under continuous light soaking at the maximum power point (MPP) tracking by retaining 90% of initial efficiency after ~800 h. In short, these initiatives have simultaneously improved the photovoltaic performance and long-term stability of PSCs. This work severely highlights the utility of molecular engineering approaches in perovskite devices and provides the basis for facilitating industrial applications in the near future.

Harnessing Ionic Interactions and Hydrogen Bonding for Nucleophilic Fluorination.

We review recent works for nucleophilic fluorination of organic compounds in which the Coulombic interactions between ionic species and/or hydrogen bonding affect the outcome of the reaction. SN2 fluorination of aliphatic compounds promoted by ionic liquids is first discussed, focusing on the mechanistic features for reaction using alkali metal fluorides. The influence of the interplay of ionic liquid cation, anion, nucleophile and counter-cation is treated in detail. The role of ionic liquid as bifunctional (both electrophilic and nucleophilic) activator is envisaged. We also review the SNAr fluorination of diaryliodonium salts from the same perspective. Nucleophilic fluorination of guanidine-containing of diaryliodonium salts, which are capable of forming hydrogen bonds with the nucleophile, is exemplified as an excellent case where ionic interactions and hydrogen bonding significantly affect the efficiency of reaction. The origin of experimental observation for the strong dependence of fluorination yields on the positions of -Boc protection is understood in terms of the location of the nucleophile with respect to the reaction center, being either close to far from it. Recent advances in the synthesis of [18F]F-dopa are also cited in relation to SNAr fluorination of diaryliodonium salts. Discussions are made with a focus on tailor-making promoters and solvent engineering based on ionic interactions and hydrogen bonding.

The Effects of Structural Modifications of Bis‐tert‐alcohol‐Functionalized Crown‐Calix[4]arenes as Nucleophilic Fluorination Promotors and Relations with Computational Predictions

We prepared four structurally modified bis‐tert‐alcohol functionalized crown ether‐strapped calix[4]arene promoters, that is, bis‐tert‐alcohol functionalized crown‐5‐calix[4]arene (BA5C), bis‐tert‐alcohol functionalized crown‐7‐calix[4]arene (BA7C), bis‐1,1‐dimethylbutanol functionalized crown 6‐calix[4]arene (B3A6C), and bis‐1,1‐dimethylhexanol functionalized crown 6‐calix[4]arene (B5AC6C). Nucleophilic fluorinations were conducted using alkali metal fluorides to investigate phase‐transfer catalytic activities with respect to the sizes of crown ether‐strapped calix[4]arene (CEC) subunits and lengths of alkyl chain spacers between tert‐alcohol and CEC subunits. The reactivity of potassium fluoride was significantly enhanced by BA5C promoter because its crown‐5‐calix[4]arene subunit had cavity size appropriate for K+ capture. In contrast, BA7C did not efficiently enhance cesium fluoride SN2 fluorination because its crown‐7‐calix[4]arene cavity was too large to capture Cs+. The alkyl spacer of bis‐tert‐alcohol functionalized CEC provided sufficient distance between F– and Cs+. In particular, the three‐component alkyl carbon chain of B3A6C was optimal in terms of increasing CsF reactivity. Systematic quantum chemical analysis predicted that fluorination reactions facilitated by bis‐tert‐alcohol‐substituted crown‐6‐calix[4]arene (BACCA) and B3A6C might proceed via separated ion‐pair type pre‐reaction complexes, which suggested that these two promotors act as Lewis bases that essentially separate CsF ion‐pairs. The lower (by 0.4 kcal/mol) Gibbs free energy of activation for the reaction promoted by B3A6C than that facilitated by BACCA concurred with the experimentally observed slightly greater efficiency of B3A6C than BACCA.

The Impact of the Alkali Metal Ion on the Crystal Structure and (Mechano)luminescence of Terbium(III) Tetrakis(β‐diketonates)

The novel terbium(III) tetrakis(β‐diketonates) M[TbL4] with different alkali metal counterions (Na, K, Cs) based on functionalized trifluoromethylated β‐diketonate are reported. An efficient approach to alkali metal β‐diketonates ML (M = Na, K, Cs) is proposed, which does not require the alkaline media promoting the destruction of 1,3‐dicarbonyl compounds. The transformation of lithium β‐diketonate LiL into the corresponding alkali metal β‐diketonate ML has been achieved under mild conditions using alkali metal fluorides. Organic ligand generates the multiplicity of coordination modes through a diketonate fragment, methoxy group and trifluoromethyl substituent. The alkali metal ion changes the geometry of coordination environment of the terbium(III) ion and affects the crystal packing of complexes. The additional coordination centers in the ligand coupled with a small atomic radius of sodium ion resulted in a construction of a rare example of discrete lanthanide tetrakis(β‐diketonate) Na[TbL4]. Moreover, this complex exhibited bright luminescence and mechanoluminescence in contrast to polymeric TbIII tetrakis(β‐diketonates) with K and Cs.

Existence of Al2F7− in molten MF–AlF3 (M = K, Cs) systems as determined by Raman spectroscopy and structural simulation

The molten mixtures of alkali metal fluorides and aluminum fluoride are applied as aluminum electrolytes or brazing fluxes. However, the presence of Al2F7− in such molten systems is disputed. In the present study, MF–AlF3 (M = K, Cs) systems with molar ratios < 1 were studied by in-situ Raman spectroscopy and molecular simulation. The results show that, in addition to AlF63−, AlF52−, and AlF4−, the systems also contained Al2F7−. The characteristic bands in the Raman spectra belonging to Al2F7− were located at about 225 cm−1, 315 cm−1, 479 cm−1, and 720 cm−1. There are two possible structures of Al2F7−, which belong to the D3d and D3h point groups. Both of these structures are linear, and their single-point energies were found to differ by only 0.31 kcal/mol.

Liquid Immiscibility and Problems of Ore Genesis: Experimental Data

The paper reports the results of an experimental study of phase relations and distribution of elements in silicate melt–salt melt systems (carbonate, phosphate, fluoride, chloride), silicate melt I – silicate melt II, and fluid–magmatic systems in the presence of alkali metal fluorides. Extraction of a number of ore elements (Y, REE, Sr, Ba, Ti, Nb, Zr, Ta, W, Mo, Pb) by salt components was studied in liquid immiscibility processes within a wide temperature range of 800–1250°С and pressure of 1–5.5 kbar. It is shown that partition coefficients are sufficient for concentration of ore elements in amounts necessary for the genesis of ore deposits. In a fluid-saturated trachyrhyolite melt, the separation into two silicate liquids has been determined. The partition coefficients of a number of elements (Sr, La, Nb, Fe, Cr, Mo, K, Rb, Cs) between phases L1 and L2 have been obtained. The interaction processes of a heterophase fluid in the granite (quartz)–ore mineral–heterophase fluid (Li, Na, K-fluoride) system were studied at 650–850°C and P = 1 kbar. The formation of the phase of a highly alkaline fluid-saturated silicate melt concentrating Ta and Nb is shown as a result of the interaction of the fluid with rock and ore minerals.

Liquid immiscibility and problems of ore genesis (according to experimental data)

The results of an experimental study of phase relations and distribution of elements in silicate melt–salt systems (carbonate, phosphate, fluoride, chloride) melt, silicate melt I–silicate melt II, and also in fluid – magmatic systems in the presence of alkali metal fluorides are presented. Salt extraction of a number of ore elements (Y, REE, Sr, Ba, Ti, Nb, Zr, Ta, W, Mo, Pb) was studied in liquid immiscibility processes in a wide temperature range of 800–1250°С and pressure of 1–5.5 kbar. It is shown that the partition coefficients are sufficient for the concentration of ore elements in the quantity necessary for the genesis of ore deposits. In the fluid-saturated melt of trachyrhyolite, the separation into two silicate liquids has been determined. The partition coefficients of a number of elements (Sr, La, Nb, Fe, Cr, Mo, K, Rb, Cs) between phases L1 and L2 has been obtained. The interaction processes of a heterophase fluid in the granite (quartz)–ore mineral–heterophase fluid (Li, Na, K-fluoride) system were studied at 650–850°C and P = 1 kbar. The formation of the phase of a highly alkaline fluid–saturated silicate melt – Ta and Nb concentrator is shown as a result of the reaction of the fluid with the rock and ore minerals.

Lewis Acidic Behavior of MoOF4 towards the Alkali Metal Fluorides in Anhydrous Hydrogen Fluoride Solutions

Previous studies performed on samples of MoOF4 dissolved in anhydrous hydrogen fluoride (aHF) solutions have indicated the presence of the [Mo2O2F9]– anion. Building upon this earlier work, MoOF4, and MF (M = Li–Cs) were dissolved in aHF solutions to produce M[Mo2O2F9] (M = Li–Cs) salts. Structural analysis of the obtained compounds was performed using single‐crystal X‐ray diffraction. This study provides the first single crystal structures for the [MoVI2O2F9]– anion. Additionally, IR and Raman spectroscopy were used to characterize each salt. These spectra were compared to calculated ones for the solid‐state structure of each salt using the DFT‐PBE0 density functional method. The calculated spectra were used to give band assignments in the experimentally obtained spectra.