Dehydrohalogenations of cis- and trans-1-bromo-2-flourocyclohexanes

Reactions of both diastereomeric vicinal bromofluorocyclohexanes with sodamide, sodium methoxide, potassium tert-butoxide, and triethylamine were studied. cis-1-Bromo-2-fluorocyclohexane eliminated almost exclusively hydrogen bromide giving mainly 1-fluorocyclohexene and a small amount of 3- fluorocyclohexene. trans-1-Bromo-2-fluorocyclohexane eliminated preferentially hydrogen fluoride on treatment with sodamide. Potassium tert- butoxide eliminated hydrogen bromide and yielded 3-fluorocyclohexene and a small amount of 1,3-cyclohexadiene, whereas sodium methoxide converted trans-1-bromo-2-fluorocyclohexane to 3-fluorocyclohexene and 3-methoxycyclohexene. Possible mechanisms and stereochemistry of the preferential elimination of hydrogen fluoride are discussed and new interpretations are offered.     

Enhancement of Demasking of Fluoride Ion From its Aluminum Complexes by Dilution in the Determination With a Fluoride Ion-Selective Electrode

Abstract

In the determination of fluoride ion with an ion-selective electrode the permissible concentration of aluminum increases markedly with decreasing the fluoride concentration in 0.5M citrate buffer solutions at pi I 6. As the accuracy is constant over a concentration range down to 10^?5 M F^? where a Nernstian relation holds fluoride determinations can be accomplished by the successive dilution method. The dilution steps are repeated until the last two results are in agreement, while the concentration of citrate is kept constant. As the ionic strength is kept constant with citrate, the addition of sodium chloride to the buffer is not necessary. Several samples, especially glasses, have been analyzed successfully.     

Ion Selective Electrode Determination of Free Versus Total Fluoride Ion in Simple and Fluoroligand Coordinated Hexafluoropnictate (PnF<Subscript>6</Subscript><Superscript>−</Superscript>, Pn = P,As, Sb,Bi) Salts

Interpretation of the results of determinations of free fluoride (F_f⁻) and total fluoride (F_t⁻) obtained with fluoride ISE while conducting elemental chemical analysis of bulk material of newly synthesized inorganic fluoride compounds is of crucial importance for the purpose of determination of purity and stoichiometry of these compounds. Knowledge of the properties and behavior of these compounds in aqueous media is therefore essential. Observations are presented on the determinations of the amounts of F_t⁻ and F_f⁻ in fluorinated compounds, in the particular hexafluoropnictate salts (PnF₆⁻, Pn = P, As, Sb, Bi) as found in aqueous media and in some compounds with XeF₂, AsF₃ ligands. A critical look at the determined amounts of F_f⁻, F_t⁻and calculated amounts of bound fluoride (F_b⁻) is provided.     

Sol-gel-fluorination synthesis of amorphous magnesium fluoride

The sol-gel fluorination process is discussed for the reaction of magnesium alkoxides with HF in non-aqueous solvents to give X-ray amorphous nano-sized magnesium fluoride with high surface areas in the range of 150-350 m²/g (HS-MgF₂). The H2 type hysteresis of nitrogen adsorption-desorption BET-isotherms is indicative for mesoporous solids. A highly distorted structure causes quite high Lewis acidity, shown by NH₃ temperature-programmed desorption (NH₃-TPD) and catalytic test reactions. XPS data of amorphous and conventionally crystalline MgF₂ are compared, both show octahedral coordination at the metal site. Thermal analysis, F-MAS NMR- and IR-spectroscopy give information on composition and structure of the precursor intermediate as well as of the final metal fluoride. The preparation of complex fluorides, M⁺MgF₃⁻, by the sol-gel route is reported. From the magnesium fluoride gel of the above process thin films for optical application are obtained by, e.g., spin coating.     

Structure and magnetic properties of the cubic oxide fluoride BaFeO2F

Fluorination of the parent oxide, BaFeO_3−δ, with polyvinylidine fluoride gives rise to a cubic compound with a=4.0603(4) Å at 298 K. ⁵⁷Fe Mössbauer spectra confirmed that all the iron is present as Fe³⁺. Neutron diffraction data showed complete occupancy of the anion sites, indicating a composition BaFeO₂F, with a large displacement of the iron off-site. The magnetic ordering temperature was determined as T_N=645±5 K. Neutron diffraction data at 4.2 K established G-type antiferromagnetism with a magnetic moment per Fe³⁺ ion of 3.95 μ_B. However, magnetisation measurements indicated the presence of a weak ferromagnetic moment that is assigned to the canting of the antiferromagnetic structure. ⁵⁷Fe Mössbauer spectra in the temperature range 10-300 K were fitted with a model of fluoride ion distribution that retains charge neutrality of the perovskite unit cell.     

Selective Fluoride Electrode Mechanism Studies

Abstract

We wanted to determine the mechanism of charge transport through the LaF₃ crystal in Orion's 94-09 and 94-09A fluoride electrodes. Voltage was applied to the electrode while it was in 15% Sodium Acetate Solution (pH 6.3) with or without (F¹⁸). We concluded that the mechanism of conduction through LaF₃ was one of ionic diffusion. F¹⁸ accumulated on the electrode surface very rapidly. Diffusion into the body of the LaF₃ (deeper than 300 Å) was also very rapid.     

Preparation and optical properties of CeF3-containing oxide fluoride glasses

Cerium-containing oxide fluoride glasses CeF₃-BaF₂-AlF₃-SiO₂ were prepared under CO and Ar atmospheres. The glass prepared in a CO atmosphere exhibited blue emission under UV irradiation because the hydrolysis of CeF₃ to a Ce⁴⁺-containing compound during heating process was controlled. The emission spectrum was separable into three peak components by peak analysis. X-ray photoelectron spectroscopy (XPS) spectra were measured to investigate the valency of the elements in the glass. Oxide and fluoride ions in the glass respectively have different electronic states from those in CeO₂ and CeF₃. The electronic state of cerium ion in the oxide fluoride glass differed from that in CeO₂ and CeF₃.     

Measurement of pH by NMR Spectroscopy in Concentrated Aqueous Fluoride Buffers

An NMR spectroscopic technique has been developed to give rapid, accurate pH measurements on tenth-milliliter samples of concentrated acidic aqueous solutions buffered by fluoride ion in the pH 1.5-4.5 range. The fluoride ¹⁹F chemical shift has been calibrated as a function of pH at 0.1 and 1.0 M concentration by reference to an internal 3-fluoropyridine standard. Subsequent measurements of fluoride buffer pH required no additives and only two NMR spectra in the presence of an external reference standard.     

On the Triple Role of Fluoride Ions in Palladium‐Catalyzed Stille Reactions

The mechanism of Stille reactions (cross‐coupling of ArX with Ar′SnnBu₃) performed in the presence of fluoride ions is established. A triple role for fluoride ions is identified from kinetic data on the rate of the reactions of trans‐[ArPdBr(PPh₃)₂] (Ar=Ph, p‐(CN)C₆H₄) with Ar′SnBu₃ (Ar′=2‐thiophenyl) in the presence of fluoride ions. Fluoride ions promote the rate‐determining transmetallation by formation of trans‐[ArPdF(PPh₃)₂], which reacts with Ar′SnBu₃ (Ar′=Ph, 2‐thiophenyl) at room temperature, in contrast to trans‐[ArPdBr(PPh₃)₂], which is unreactive. However, the concentration ratio [F^?]/[Ar′SnBu₃] must not be too high, because of the formation of unreactive anionic stannate [Ar′Sn(F)Bu₃]^?. This rationalises the two kinetically antagonistic roles exerted by the fluoride ions that are observed experimentally, and is found to be in agreement with the kinetic law. In addition, fluoride ions promote reductive elimination from trans‐[ArPdAr′(PPh₃)₂] generated in the transmetallation step.     

Mechanism for the gas‐phase hydrogen fluoride‐mediated decomposition of peroxyacetyl nitrate (PAN) studied by DFT method

Density functional theory has been used to study the mechanism of the decomposition of peroxyacetyl nitrate (CH₃C(O)OONO₂) in hydrogen fluoride clusters containing one to three hydrogen fluoride molecules at the B3LYP/6‐311++G(d,p) and B3LYP/6‐311+G(3df,3pd) levels. The calculations clarify some of the uncertainties in the mechanism of PAN decomposition in the gas phase. The energy barrier decreases from 30.5 kcal mol^?1 (single hydrogen fluoride) to essentially 18.5 kcal mol^?1 when catalyzed by three hydrogen fluoride molecules. As the size of the hydrogen fluoride cluster is increased, PAN shows increasing ionization along the O? N bond, consistent with the proposed predissociation in which the electrophilicity of the nitrogen atom is enhanced. This reaction is found to proceed through an attack of a fluorine to the PAN nitrogen in concert with a proton transfer to a PAN oxygen. On the basis of our calculations, an alternative reaction mechanism for the decomposition of PAN is proposed. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Triarylboron‐Linked Conjugated Microporous Polymers: Sensing and Removal of Fluoride Ions

A luminescent conjugated microporous polymer (BCMP‐3) has been synthesized in high yield by a carbon–carbon coupling reaction using triarylboron as a building unit. BCMP‐3 was fully characterized by using powder X‐ray diffraction analysis, Fourier transform infrared spectroscopy, ¹³C solid‐state NMR spectroscopy, field emission scanning electron microscopy, thermogravimetric analysis, and nitrogen and carbon dioxide adsorption. The new three‐dimensional conjugated framework possess a high Brunauer–Emmett–Teller (BET) specific surface area up to 950 m² g^?1 with a pore volume of 0.768 cm³ g^?1, good stability, and abundant boron sites in the skeleton. Under excited‐light irradiation, BCMP‐3 exhibits strong fluorescent emission at 488 nm with a high absolute quantum yield of 18 % in the solid state. Polymer BCMP‐3 acts as a colorimetric and fluorescent chemosensor with high sensitivity and selectivity for F^? over other common anions. In addition, the polymer also works as an adsorbent for F^? removal and shows good adsorption capacities of up to 24 mg g^?1 at equilibrium F^? concentrations of 16 mg L^?1 and a temperature of 298 K. The adsorption kinetics and isotherm were analyzed by fitting experimental data with pseudo‐second‐order kinetics and Langmuir equations. Furthermore, we highlight that BCMP‐3 is an adsorbent for fluoride removal that can be efficiently reused many times without loss of adsorption efficiency.     

Complexation of fluoride anion and its ion pairs with alkali metal cations by tetra-substituted lower rim calix[4]arene tryptophan derivative

Abstract

Selective binding of fluoride anion and its alkali metal cation ion pairs by a fluorescent calix[4]arene (L) derivative bearing tryptophan moieties was studied in acetonitrile at 25 °C. It was found that LF^? and LF₂^2? complexes were formed and their stability constants were determined by means of UV and ¹H NMR titrations. Both amide and indole NH groups were involved in the stabilisation of the fluoride complexes, i.e. L afforded two anion-binding sites. ¹H NMR titrations provided evidence of simultaneous complexation of both Na⁺ cation and fluoride anion resulting in the formation of a ternary NaLF complex. In this ion pair complex, the Na⁺ cation was most probably bound primarily by the calixarene ether and amide oxygen atoms, whereas the indole NH groups interacted with the fluoride ion. A highly favourable ion pairing between Na⁺ and F^– in acetonitrile was studied using potentiometric measurements. The results pointed out the importance of fluoride pairing with alkali metal cations in aprotic organic solvents and the necessity of taking these reactions into account in the course of speciation studies of such solutions.     

Epoxy-based oligomeric probe bearing naphthalene units for selective turn-OFF fluorescent sensing of fluoride anion

A simple epoxy-based oligomer 1 bearing naphthalene unit at the chain-ends is reported to be highly selective ON–OFF type fluorescent probe for fluoride anion. The titled oligomer displayed fluorescence quenching upon addition of F^?, resulting in selective detection of fluoride anion over other anions, such as AcO^?, Cl^?, Br^?, I^?, HSO₄^?, NO₃^? and H₂PO₄^? in CH₃CN. Fluorescence experiments suggest the significant influence of the oligomer chain on the sensitivity and selectivity of 1 towards fluoride anion.     

Ba3(BO3)(CO3)F: The First Borate Carbonate Fluoride Synthesized by the High-Temperature Solution Method

In contrast to the well-investigated halogen-containing borates and carbonates, very few halogen-containing borate carbonate compounds have been reported. Specifically, no example of borate carbonate fluoride has been synthesized successfully until now. Herein, the planar π-conjugated units [BO₃]³⁻ and [CO₃]²⁻ and the F⁻ ions are introduced simultaneously into one crystal structure resulting in the first borate carbonate fluoride, Ba₃(BO₃)(CO₃)F, by the high-temperature solution method in the atmosphere. Its structure features a hexagonal channel formed by the [BO₃]³⁻ and [CO₃]²⁻ units with the [F₃Ba₈]¹³⁺ trimers filled in the channel. Various characterizations including single crystal- and powder-XRD, EDX, IR, UV-vis-NIR, and TG-DSC, together with the first principles calculation have been carried out to verify the structure and fully understand the structure–property relationships.     

Highly Sensitive and Selective Fluoride Ion Chemosensing,Fluoroalkylated Polysilane

Summary: This paper reports a new and simple approach for the amplified detection of fluoride ions by a polymer‐based chemosensor made of stiff fluoroalkylated polysilane, poly(3,3,3‐trifluoropropylmethylsilane). This chemosensor exhibited extreme sensitivity and selectivity towards fluoride ions by a decrease in the photoluminescence (PL) intensity with parts per billion concentrations of fluoride ions in solution in tetrahydrofuran (THF). The quenching constant for fluoride ions (K = 1.35 × 10⁷ M ⁻¹) was found to be exceptionally high.

Schematic representation of the polymer amplification mechanism after addition of fluoride ion in tetrahydrofuran.     

Pyrolytic synthesis and Eu→Eu reduction process of blue-emitting perovskite-type BaLiF3:Eu thin films

Perovskite-type barium lithium fluoride (BaLiF₃) was synthesized by pyrolysis of metal trifluoroacetates. The reaction temperature necessary for producing a single-phase material was found to be 600°C, which was lower than that for a conventional solid-state reaction or a melting method. Eu-doped BaLiF₃ was also prepared and characterized to examine the suitability of trifluoroacetates for precursors in synthesizing homogeneous complex metal fluoride materials. It was demonstrated that trivalent Eu³⁺, which was used as acetate for a starting material, was reduced to divalent Eu²⁺ in the pyrolysis process of BaLiF₃, as indicated by a broad blue emission due to an allowed 4f⁶5d→4f⁷ transition at 408 nm with a ultraviolet excitation at 254 nm. The concentration quenching of the blue emission occurred at 5 at% of Eu in BaLiF₃, indicating that Eu was homogeneously dispersed in the BaLiF₃ host lattice. Mechanisms of the formation and reduction process of BaLiF₃ were discussed based on pertinent chemical reactions.     

Influence of Surface Coating on Fluoride Removal by Magnetite

Al(OH)₃- and ZrO(OH)₂-coated magnetites were prepared and used for fluoride removal from aqueous samples. The influence of pH, sorbent mass, and ions such as chloride, sulfate, and phosphate on the removal of fluoride was characterized. The sorption process was highly pH dependent, and the optimal sorption was obtained from pH 4 to 5 for ZrO(OH)₂- and pH 4 to 7 for Al(OH)₃-coated magnetites. The sorption isotherm was well described by the Langmuir equation for the sorbents. The maximum adsorption capacity of ZrO(OH)₂-coated magnetite (57.47?mg-F?g^?1-sorbent) was higher than for Al(OH)₃-coated magnetite (23.87?mg-F?g^?1-sorbent). The ion-exchange reaction occurred in 5?min and more than 99% of fluoride was removed from solution. When the ZrO(OH)₂-coated magnetite was used, the presence of foreign ions negatively affected the fluoride removal. The prepared sorbents showed an excellent performance for the removal of fluoride in water samples.     

Thermal stability and degradation kinetic study of white and colored cotton fibers by thermogravimetric analysis

Complexation of neptunium(V) with fluoride in aqueous solutions at elevated temperatures was studied by spectrophotometry and microcalorimetry. Two successive complexes, NpO₂F(aq) and NpO₂F₂⁻, were identified by spectrophotometry in the temperature range of 10–70°C. Thermodynamic parameters, including the equilibrium constants and enthalpy of complexation between Np(V) and fluoride at 10–70°C were determined. Results show that the complexation of Np(V) with fluoride is endothermic and that the complexation is enhanced by the increase in temperature — a two-fold increase in the stability constants of NpO₂F(aq) and more than five-fold increase in the stability constants of NpO₂F₂⁻ as the temperature is increased from 10 to 70°C.     

Field determination of fluoride in drinking water using a polymeric aluminium complex of 5-(2-carboxyphenylazo)-8-hydroxyquinoline impregnated paper

A simple field method which allows the determination of fluoride in drinking water with a small handheld instrument called Arsenator was developed. Arsenator is a commercially available instrument which was used successfully for reliable determination of arsenic. In the proposed method the functionality of the Arsenator which is based on a photometric measurement of a spot on the reagent paper is expanded to analyse fluoride. A polymeric aluminium complex of 5-(2-carboxyphenylazo)-8-hydroxyquinoline (LH₂) has been prepared as a new specific reagent for fluoride. Job's method of continuous variation was adopted for the determination of the composition of the coloured complex, which was further characterized by UV-VIS spectroscopic studies. The molar absorptivity of the complex formation is 8.48?×?10³?L?mol^?1?cm^?1 at 410?nm. The coloured complex reacts with fluoride on an impregnated paper where its colour changes are dependent on the concentration of fluoride in water samples. The change in the colour was measured using the Arsenator. The method allows a reliable determination of fluoride in the range 0.3 to 2.0?mg?L^?1. Further spectophotometric determinations of fluoride in drinking water were also studied. The determination is based on the reaction of aluminium complex with fluoride in the examined samples. Beer's law is obeyed in the range 0.3 to 2.0?mg?L^?1 of fluoride at 495?nm. Sensitivity, detection limit and quantitation limit of the method were found to be 0.251?±?0.007?µg^?1?mL, 0.1?mg?L^?1 and 0.3?mg?L^?1, respectively. The optimum reaction conditions and other analytical conditions were evaluated. The effect of interfering ions on the determination is described. There is no interference by nitrate or chloride. Sulphate interfered only at high concentrations which are not expected in drinking water.     

Fluoride‐ion‐conducting Polymers: Ionic Conductivity and Fluoride Ion Diffusion Coefficient in Quaternized Polysulfones

We describe the three‐step synthesis of a new polymeric fluoride ion conductor based on the fully aromatic polymer polysulfone (PSU). In the first step, PSU is chloromethylated (CM‐PSU) using reagents (i.e., stannic chloride, paraformaldehyde, and trimethylchlorosilane) that are less toxic than those used in the standard procedure. In the second step, CM‐PSU reacts with a tertiary amine (trimethylamine or 1,4‐diazabicyclo[2.2.2]octane) to form quaternary ammonium groups fixed on the PSU backbone and mobile chloride counter‐anions. The chloride ions can, in a third step, be exchanged with fluoride ions by immersion of the ionomer in NaF solution. The fluoride ion conductivity reaches 3–5 mS cm^?1 at 25 °C and 5–10 mS cm^?1 at 40 °C. We determined the F^? diffusion coefficient in these ionomers by pulsed gradient spin‐echo (PGSE) high‐resolution magic angle spinning (HRMAS) nuclear magnetic resonance (NMR) spectroscopy and by impedance spectroscopy using the Nernst–Einstein relation. The diffusion coefficients determined by the two methods are in good agreement, ranging from 2 to 4×10^?10 m² s^?1. The porosity and tortuosity of the ionomer membranes can be estimated.