Chloroimidazolium Deoxyfluorination Reagent with H2F3- Anion as a Sole Fluoride Source.

In the study, we introduce an air-stable NHC-based deoxyfluorination reagent ImCl[H2F3], offering a promising avenue for deoxyfluorination across various substrates. Reagent efficiently fluorinates benzyl alcohols, carboxylic acids, and P(V) compounds without external fluoride sources. A mechanistic study reveals a two-step process involving benzyl chloride as an intermediate, shedding light on the two-step reaction pathway. The Hammet plot provides insights into reaction mechanisms with different substrates, enhancing our understanding of this versatile deoxyfluorination method.

Modified aryldifluorophenylsilicates with improved activity and selectivity in nucleophilic fluorination of secondary substrates.

Nucleophilic fluorination of secondary aliphatic substrates, especially of halides, still remains a challenge. Among the available reagents, TBAT belongs to one of the best choices due to its stability, affordable price and low toxicity. With the aim to improve its selectivity, we synthesized three analogues modified in the aryl part of the TBAT reagent with one or two electron donating methoxy groups or with one electron withdrawing trifluoromethyl group. All three reagents are air-stable compounds and their structure was confirmed by a single crystal X-ray analysis. In testing the reactivity and selectivity of the reagents with a library of secondary bromides, as well as of other selected primary and secondary substrates, we found that substitution with methoxy groups mostly improves both reactivity and selectivity compared to TBAT, while the substitution with trifluoromethyl group leads to inferior results. Difluorosilicates modified by more than two electron donating methoxy groups proved to be unstable and decomposed spontaneously to the HF2 - anion. DFT calculations of tetramethylammonium analogues of the studied reagents disclosed that the substitution of the phenyl group with the methoxy substituent lowers the transitions state energy of the decomposition to a fluorosilane-fluoride complex, while the substitution with the trifluoromethyl group has an opposite effect.

Quaternary ammonium fluorides and difluorosilicates as nucleophilic fluorination reagents.

TBAT (tetrabutylammonium difluorotriphenylsilicate) is an excellent homogeneous nucleophilic fluorination reagent, but a high excess of the reagent was reported to be essential. We hence optimized the reaction conditions and compared its nucleophilic fluorination reactivity with that of other common commercial nucleophilic fluorination reagents, such as anhydrous TBAF and TASF (tris(dimethylamino)sulfonium difluorotrimethylsilicate). As the substrates, we employed a standard set of primary and secondary octyl substrates under identical conditions. To eliminate the possibility of hydrogen fluoride elimination in the above reagents, we prepared four quaternary ammonium fluorides lacking ß-elimination possibility in the hydrocarbon chain, transformed them to the corresponding difluorotriphenylsilicates, and compared their reactivity with that of the commercial reagents. Furthermore, attempts to isolate analogous tetrabutylammonium difluoromethyldiphenylsilicate or difluorodimethylphenylsilicate failed, as was confirmed by comparison of the published experimental data with computed 19F NMR spectra. Finally, we studied the transition states of decomposition of various tetramethylammonium methylphenyldifluorosilicates by DFT methods and found that their relative energies increase with an increasing number of phenyl groups. The formation of difluorosilicates is a nearly barrierless process.

Low-temperature modification of Ba(BF4)2(H2O)3.

The crystal structure of the low-temperature modification of Ba(BF4)2(H2O)3, barium bis(tetra-fluorido-borate) trihydrate, was determined at 150 K. In contrast to the room-temperature modification, which crystallizes in the space group C2221 [a = 7.1763 (6), b = 18.052 (2), c = 7.1631 (6) Å, V = 927.93 (15) Å3 at 300 K, Z = 4; Charkin et al. (2023 ▸). J. Struct. Chem. 64, 253-261], the low-temperature phase is monoclinic, space group P21 [a = 7.0550 (4), b = 7.1706 (3), c = 9.4182 (6) Å, ß = 109.295 (7)o, V = 449.68 (5) Å3, Z = 2]. The structure of the low-temperature modification of Ba(BF4)2(H2O)3 features O-H⋯F and O-H⋯O hydrogen bonding between water mol-ecules and BF4 - anions. One of the coordinating water mol-ecules in the low-temperature modification is disordered over two sets of sites.

Aldiminium Cations as Countercations to Discrete Main Group Fluoroanions.

The reactions of group 14 tetrafluorides (SiF4, GeF4, and SnF4) and group 15 pentafluorides (PF5, AsF5, and SbF5) with the CAAC-based trifluoride reagent [MeCAACH][F(HF)2] led to the isolation of salts containing discrete 5- or 6-coordinated fluoroanions. The syntheses of [MeCAACH][SiF5], [MeCAACH][GeF5], [MeCAACH][(THF)SnF5], and the structurally related [MeCAACH][(dioxane)SnF5], [MeCAACH][PF6], [MeCAACH][AsF6], and [MeCAACH][SbF6] are effective, selective and in high yield. All compounds were characterized by X-ray single-crystal structure analysis, NMR and Raman spectroscopy. It is worth noting that the synthesized [MeCAACH][GeF5] is a rare example of a structurally characterized compound with discrete [GeF5]- anion, while [MeCAACH][(THF)SnF5] and [MeCAACH][(dioxane)SnF5] represent the first compounds with discrete octahedrally coordinated tin fluoride anions with incorporated solvent molecules. Finally, the aldiminium-based cation [MeCAACH]+ proved to be suitable for the stabilization of rare discrete main group fluoride anions.

Fluoride and Aluminium in Tea (Camellia sinensis L.)-Tea Quality Indicators and Risk Factors for Consumers.

In recent years, the quality and sourcing of tea have gained importance in Europe, but information remains scarce. The aim of this study was to determine the concentrations of fluoride (F-) and total aluminium (Al) species in infusions of commercially available teas in Slovenia, and thus in Europe, and to relate them to tea quality and their impact on consumer safety. F- concentrations were determined using a fluoride-ion-selective electrode and Al concentrations using inductively coupled plasma optical emission spectroscopy. A comparison of the results obtained for four selected tea samples using the calibration curve and a standard addition technique showed good agreement, with no interferences caused by the sample matrix. The concentrations of 35 commercial teas ranged from 0.34 to 4.79 and 0.51 to 8.90 mg/L for F- and Al, respectively. The average concentrations of the two elements followed the same descending order: black filter > green filter > black leaves ≈ green leaves. Single and multivariate statistical methods supported the categorisation of teas by packaging but not by type, with tea in filter bags being more expensive than loose tea. The linear relationship between F- and Al concentrations in infusions (C(Al) = 1.2134 · C(F-)) allows for the determination of one element and estimation of the other, leading to a significant reduction in laboratory effort and cost. This research advances tea assessment by proposing Al concentration alongside F- as a quality indicator and provides the basis for tea-monitoring protocols. Finally, the daily consumption of larger quantities of tea (≈1 L) with elevated F- and Al concentrations could potentially pose a health risk.

From cyclic (alkyl)(amino)carbene (CAAC) precursors to fluorinating reagents. Experimental and theoretical study.

Addition of anhydrous HF to the hydrochloride [MeCAACH][Cl(HCl)0.5] resulted in the formation of salts with high HF content. By stepwise removal of HF in vacuo, we selectively prepared [MeCAACH][F(HF)2] (3) and [MeCAACH][F(HF)3] (4). We also characterised a salt with [F(HF)4]- anions within the structure of [MeCAACH][F(HF)3.5] (5). Compounds with a lower content of HF were not accessible under vacuum conditions. MeCAAC(H)F (1) was selectively prepared by abstraction of HF from 3 with CsF or KF, while [MeCAACH][F(HF)] (2) was prepared by mixing 3 and 1 in a 1 : 1 ratio. Compound 2 proved to be quite unstable as it tends to disproportionate into 1 and 3. This observation triggered our computational study, in which the structural relationships between CAAC-based fluoropyrrolidines and dihydropyrrolium fluorides were investigated using different DFT methods. The study showed that the results were very sensitive to the computational method used. For a correct description, the quality of the triple-ζ basis set was crucial. Surprisingly, the isodesmic reaction of [MeCAACH][F] + [MeCAACH][F(HF)2] → [MeCAACH][F(HF)] + [MeCAACH][F(HF)] did not confirm the low thermodynamic stability of 2. Furthermore, the use of 3 as a nucleophilic fluorinating reagent was tested on a range of organic substrates, as it is the most stable compound in this series. It was found to have the potential to fluorinate benzyl bromides, 1- and 2-alkyl bromides, silanes and sulfonyls with good to excellent yields of the target fluorides.

Deoxyfluorination of Electron-Deficient Phenols.

In this study, we report a facile synthesis of 2-chloro-1,3-bis(2,6-diisopropylphenyl)imidazolium salts in aqueous media under ambient conditions using hypochlorite as a chlorinating agent. In addition, an air-stable and moisture-insensitive deoxyfluorination reagent based on poly[hydrogen fluoride] salt is presented, which is capable of converting electron-deficient phenols or aryl silyl ethers into the corresponding aryl fluorides in the presence of DBU as a base, with good to excellent yields and high tolerance to functional groups.

Synthesis of imidazolium-based pentacoordinated organofluorosilicate and germanate salts.

The syntheses of a new triphenyldifluorogermanate and various pentacoordinated organofluorosilicates are presented. The fluorogermane and fluorosilane compounds were obtained from the corresponding chlorosilanes and chlorogermane by halogen substitution with KF. Subsequent reaction with the imidazolium-based fluoride reagent [IPrH][F] (1,3-bis(2,6-diisopropylphenyl)imidazolium fluoride) led to the formation of [IPrH][Ph3SiF2] (1), [IPrH][Ph2SiF3] (3), [IPrH][Et2SiF3] (4), [IPrH][PhSiF4] (5), [IPrH][EtSiF4] (6) and [IPrH][Ph3GeF2] (7). All the products obtained were characterised by NMR, Raman spectroscopy and X-ray diffraction. The results were supported by DFT calculations of the structurally optimised compounds.

Renewable Reagent for Nucleophilic Fluorination.

Herein, we report a study on the reactivity of three 1,3-diarylimidazolium-based fluoride reagents, with a general formula of [IPrH][F(HF)n] (n = 0, 1, or 2), that tackle the challenges of limited solubility, hygroscopicity, instability, and laborious preparation procedures of nucleophilic fluoride reagents. Fluorination of 4-tert-butylbenzyl bromide reveals that trifluoride [IPrH][F(HF)2] is the most selective reagent. Microwave-assisted activation coupled with the addition of sterically hindered amine DIPEA or alkali metal fluorides increases the rate of fluorination with [IPrH][F(HF)2], making it an excellent reagent for the fluorination of various organic substrates. The scope of substrates includes benzyl bromides, iodides, chlorides, aliphatic halides, tosylates, mesylates, α-haloketones, a silyl chloride, acyl and sulfuryl chlorides, and a nitroarene. The exceptional stability of the air-stable and nonhygroscopic [IPrH][F(HF)2] reagent is illustrated by its convenient synthesis and detailed experimental regeneration protocol using hydrofluoric acid without organic solvents.

Coordination of a Neutral Ligand to a Metal Center of Oxohalido Anions: Fact or Fiction?

Can a neutral ligand bond to a metal center of a square pyramidal oxohalido anion at the available sixth octahedral position? Crystal structures of some compounds indeed suggest that ligands, such as THF, pyridine, H2O, NH3, and CH3CN, can interact with the central metal atom, because they are oriented with their heteroatom toward the metal center with distances being within the bonding range. However, this assumption that is based on chemical intuition is wrong. In-depth analysis of interactions between ligands and oxohalido anions (e.g., VOX4-, NbOCl4-) reveals that the bonding of a neutral ligand is almost entirely due to electrostatic interactions between the H atoms of a ligand and halido atoms of an anion. Furthermore, ab initio calculations indicate that the ligand-VOF4- interactions represent only about one-quarter of the total binding of the ligand within the crystal structure, whereas the remaining binding is due to crystal packing effects. The current study therefore shows that relying solely on the structural aspects of solved crystal structures, such as ligand orientation and bond distances, can lead to the wrong interpretation of the chemical bonding.

Influence of Anodization-Electrolyte Aging on the Photocatalytic Activity of TiO2 Nanotube Arrays.

TiO2 nanotubular films prepared using the anodic oxidation process applied to various forms of metal titanium are promising materials for photocatalytic applications. However, during successive anodizations in batch-anodization cells, the chemical composition of the NH4F- and water-based ethylene glycol electrolyte changes with each subsequent anodization, which greatly affects the final photocatalytic properties of the annealed TiO2 nanotubular films. In the present study, 20 titanium discs (Φ 90 mm) were sequentially anodized in the same anodization electrolyte. The chemical composition of the electrolyte was measured after each anodization and correlated with the anodization current density, temperature, electrical conductivity, and pH of the electrolyte and with the morphology, structure, composition, and photocatalytic activity of the resulting TiO2 nanotube films. It was found that the length of the TiO2 nanotubes decreased with the age of the electrolyte due to its lower conductivity. The subsurface chemical composition was evaluated by time of flight secondary ion mass spectrometry (ToF SIMS) analyses, and the integrated ToF SIMS signals over a depth of 250 nm for the TiO2 nanotube films showed that the concentration of F- in the annealed TiO2 film increased with each subsequent anodization due to the increased pH value of the electrolyte. As a consequence, the concentration of the OH- and O2 - species decreased, which is a major reason for the reduced photocatalytic activity of the TiO2 films. It is proposed that the length of the TiO2 nanotubes does not play a decisive role in determining the photocatalytic activity of the TiO2 nanotube films. Finally, the best measured degradation results of 60% for caffeine were thus achieved for the first anodized titanium discs. After that the efficiency gradually decreased for each subsequent anodized disc.

Fluorine in vegetation due to an uncontrolled release of gaseous fluorides from a glassworks: A case study of measurement uncertainty, dispersion pattern and compliance with regulation.

This study was initiated after the appearance of chlorotic and necrotic lesions on vegetation in the vicinity of a glassworks. The aim was to establish whether the cause was an uncontrolled release of gaseous fluorides. Five different plant species (Norway spruce, peach, common hornbeam, common bean, common grape vine) were collected in the influenced area, and the fluorine (F) content was determined by a fluoride ion selective electrode after prior total sample decomposition by alkaline carbonate fusion. The measurement results were reported together with their measurement uncertainties (MUs), which were evaluated according to the Guide to the Expression of Uncertainty in Measurement. The F contents at comparable distances from the emitter and in a clean area, free from natural or anthropogenic fluoride emissions, were 87-676 and 10 µg g-1, respectively, thereby confirming the release of gaseous fluorides from the glassworks. The F contents in samples of Norway spruce taken at various radial distances from the emitter suggest that the emitted gaseous fluorides were spread about evenly in all directions from the source following an inverse-power function. Estimated distances at which the F content would decrease to 50 µg g-1 (allowed maximum content of F in feeding stuffs) and 21 µg g-1 (maximum fluoride content in vegetables and fruits in relation to the upper limit of fluoride intake for humans) were 378 m and 571 m, respectively, from the emitter. Evaluation of our results for compliance with specification revealed a lack of regulation on fluoride content in the diet of humans and animals as well as a lack of guidelines on how to take into account MU.

Reactivity of VOF3 with N-Heterocyclic Carbene and Imidazolium Fluoride: Analysis of Ligand-VOF3 Bonding with Evidence of a Minute π Back-Donation of Fluoride.

Reaction of vanadium(V) oxide trifluoride (VOF3) and the new "naked" fluoride reagent [(LDipp)H][F] (LDipp = 1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-2 H-imidazol-2-ylidene) leads to the isolation of [(LDipp)H][VOF4] (1) where the long sought discrete [VOF4]- anion was finally obtained. The neutral [(LDipp)VOF3] (2) complex was synthesized by a similar reaction between VOF3 and bulky N-heterocyclic carbene (NHC) ligand LDipp. In this context, we analyzed, by means of DFT calculations, intermolecular interactions between [(LDipp)VOF3] (2) complexes in the crystal structure and realized that these interactions have a significant effect on the V-Ftrans bond length. We further scrutinized ligand bonding within [(LDipp)VOF3] (2) and related complexes, because, in this kind of complexes, a rather short distance between CNHC and cis-halogen atoms has spurred some discussion about the type of interactions between them. We provide evidence of a minute π back-bonding into NHC ligands, which is larger for chloride [(NHC)VOCl3] than fluoride [(NHC)VOF3] complexes, although the fluoride ions are, counterintuitively and to a larger degree, involved in back-bonding than chloride ions. The influence of π back-bonding on V-Ftrans and V-Fcis bond lengths was also rationalized. Finally, the hydrolysis of [(LDipp)VOF3] (2) product was studied and [(LDipp)H][VO2F2] (3) salt was obtained and characterized as the most stable product in this system.

Discrete GeF5- Anion Structurally Characterized with a Readily Synthesized Imidazolium Based Naked Fluoride Reagent.

The recently prepared novel naked fluoride reagent 1,3-bis(2,6-diisopropylphenyl)imidazolium fluoride ([(LDipp)H][F]), treated with an excess of MF4 (M = Si, Ge), results in isolation of [(LDipp)H][MF5] products with the elusive trigonal bipyramidal MF5- anions. Specific steric characteristics of the [(LDipp)H]+ cation readily support isolation of monomeric and discrete trigonal bipyramidal fluorido anions of silicon and germanium. Based on combination of experimental results and DFT calculations, we demonstrate that the role of bulky cation is not solely due to steric hindering but also due to electrostatic effects, which are important in the design of such uncommon species. The discrete GeF5- anion was characterized by X-ray single-crystal diffraction for the first time. We report the missing 19F NMR entries for the discrete GeF5- and GeF62- anions in acetonitrile. All the products were also characterized by Raman spectroscopy and elemental analysis and supported by quantum-mechanical calculations.

Small molecule activation: SbF3 auto-ionization supported by transfer and mesoionic NHC rearrangement.

The reactivity of antimony trifluoride, SbF3, in non-polar aprotic solvents has been unexplored due to its negligible solubility. The reaction of the [SbF3(tmen)] complex (2) with the N-heterocyclic carbene, 1,3-bis(2,6-diisopropylphenyl)imidazole-2-ylidene, (LDipp), probably leads to the formation of a neutral [(LDipp)SbF3] intermediate (3) in non-polar solvents. Mild heat treatment of the reaction mixture unexpectedly activates the SbF3 molecules to undergo the auto-ionization reaction. In this process, one of the (LDipp) ligands undergoes rearrangement into the mesoionic carbene (MIC) and further coordinates to the cation to form [(LDipp)2SbF2]+[SbF4]-, product (4). Studies have confirmed that N,N,N',N'-tetramethylethane-1,2-diamine (tmen) present in the solution is not involved in the conversion mechanism and serves solely as an auxiliary ligand to increase the solubility of SbF3. The compound (4) shows a rare pnictogen(iii) halide auto-ionization process with the first mesoionic NHC rearrangement on any metal/metalloid fluoride compound and the first reported reaction system where a N-heterocyclic carbene ligand facilitates the auto-ionization process on a fluorido substrate.

Domain-wall conduction in ferroelectric BiFeO3 controlled by accumulation of charged defects.

Mobile charged defects, accumulated in the domain-wall region to screen polarization charges, have been proposed as the origin of the electrical conductivity at domain walls in ferroelectric materials. Despite theoretical and experimental efforts, this scenario has not been directly confirmed, leaving a gap in the understanding of the intriguing electrical properties of domain walls. Here, we provide atomic-scale chemical and structural analyses showing the accumulation of charged defects at domain walls in BiFeO3. The defects were identified as Fe4+ cations and bismuth vacancies, revealing p-type hopping conduction at domain walls caused by the presence of electron holes associated with Fe4+. In agreement with the p-type behaviour, we further show that the local domain-wall conductivity can be tailored by controlling the atmosphere during high-temperature annealing. This work has possible implications for engineering local conductivity in ferroelectrics and for devices based on domain walls.

Oxidation of Ruthenium and Iridium Metal by XeF2 and Crystal Structure Determination of [Xe2F3][RuF6]·XeF2 and [Xe2F3][MF6] (M = Ru, Ir).

Salts containing [Xe2F3]+ cations and [MF6]- anions (M = Ru, Ir) were synthesized by the oxidation of metal with excess of XeF2 in anhydrous hydrogen fluoride (aHF) as a solvent. Single crystals of [Xe2F3][RuF6]·XeF2, [Xe2F3][RuF6] and [Xe2F3][IrF6] were grown by slow evaporation of the solvent. [Xe2F3][RuF6]·XeF2 crystallizes in a triclinic P-1 space group (a = 8.3362(1) Å, b = 8.8197(2) Å, c = 9.3026(4) Å; α = 68.27(1)°, ß = 63.45(1)°, γ = 82.02°, V = 568.09(9) Å3 (Z = 2)). Discrete [Xe2F3]+, XeF2 and [RuF6]- units are found in the asymmetric unit. [Xe2F3][RuF6] and [Xe2F3][IrF6] compounds are isostructural and crystallize in a monoclinic Cc space group (a = 14.481(3) Å (Ru); 14.544(3) Å (Ir); b = 8.0837(8) Å (Ru), 8.0808(7) Å (Ir), c = 10.952(2) Å (Ru), 11.014(2) Å (Ir); ß = 136.825(6)° (Ru), 139.954(7)°, V = 877.2(3) Å3 (Ru), 883.6(3) Å3 (Ir); Z = 4). The asymmetric unit in the [Xe2F3][MF6] (M = Ru, Ir) consists of one [Xe2F3]+ and one [MF6]- unit.

Direct solvothermal preparation of nanostructured fluoride aerogels based on AlF3.

AlF3-based aerogels, a new class of inorganic aerogels, are prepared in a novel direct process that combines fluoride sol-gel synthesis with high temperature supercritical drying. The bulk structure of the solid products depends decisively on the applied solvent(s); very voluminous bulk aerogels are obtained only with MeOH that is used either alone or in combination with some other polar solvents. MeOH acts as a methoxylation agent; and formed methoxy (MeO) species are remarkably stable and deactivate the surface acidic sites. Removal of MeO species under moderate conditions results in catalytically active fluorides with a preserved nanostructure. In preparations with MeOH, preferential growth of anisotropic nanoparticles (nanorods) is the key step that leads to the formation of very open aerogel structures. Another process, dehydration of alcohols, results in some hydroxylation and hydration that lead to the formation of distinctive surface and bulk OH/H2O species. The structure of AlF3-based aerogels is consistent with the hexagonal tungsten bronze (HTB) ß-AlF3 although their composition corresponds to a formula AlF3-x(OH, OMe)x·yH2O (x = 0.1 ± 0.05). Some other characteristics of the fluoride nanoparticles, like crystallinity, particle size, and uniformity, can be effectively controlled by the temperature of the solvothermal process. The described methodology allows a controllable preparation of catalytically active fluorides in the form of regularly shaped and uniformly sized nanoparticles.

Fluorination of mixed γ-alumina/γ-gallia xerogels with trifluoromethane: some effects on bulk and surface characteristics.

Interaction of single γ-Al2O3 and γ-Ga2O3, and mixed γ-Al2O3/γ-Ga2O3 xerogels with CHF3 at intermediate temperatures results in partial fluorination. Fluorinated oxides remain amorphous and retain a considerable part of the initial surface area; for the fluorinated Al-based materials surface areas in all cases exceed 100 m2 g-1. Lewis acidity of mixed oxides, either before or after fluorination, is strongly influenced by the presence of surface Ga3+ ions, mainly due to their strong preference to replace highly acidic Al3+ ions in tetrahedral positions. Ion replacement leads to the formation of acidic sites with lower strengths what is confirmed by the model catalytic reaction, isomerisation of CCl2FCClF2. XPS investigations indicate that fluorination of mixed oxides is accompanied by substantial surface reconstructions and preferential formation of Al-F based phases with Ga remaining mainly in O environments. Further segregation processes, such as slow crystallisation of Al(F,OH)3·nH2O phases, are probably promoted by water adsorption.