The NMR shifts are not a measure for the nakedness of the fluoride anion

Fluoride-Based Deep Eutectic Solvents with Amide Dual-Hydrogen-Bond Donors

Developing F--containing electrolytes is crucial for electrochemical and chemical fluorination. However, balancing the F- concentration and electrochemical stability of the electrolytes remains a challenge. In this study, fluoride-based deep eutectic solvents (F-DESs) were obtained by using amide hydrogen-bond donors (HBDs) containing dual N-H bonds. The obtained F-DES, [TMA]F·3.5[1,3-DMU], was prepared by facilely mixing solid compounds of tetramethylammonium fluoride ([TMA]F) and 1,3-dimethylurea (1,3-DMU), resulting in a high F- concentration (2.6 mol dm-3) and a wide electrochemical window (3.1 V) at room temperature. The electrochemical window was much wider than that of [TMA]F·3.5[EG] (EG, ethylene glycol) as another F-DES with an alcohol HBD (1.9 V). Moreover, [TMA]F·3.5[1,3-DMU] exhibited an ionic conductivity that was 2 orders of magnitude higher than that of [TMA]F·3.5[1,3-DMTU] (1,3-DMTU, 1,3-dimethylthiourea) around room temperature because of the bifurcated hydrogen bonds between the dual N-H bonds of 1,3-DMU and one F-. Thus, [TMA]F·3.5[1,3-DMU] was demonstrated to be applicable to electrochemical fluorination.

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.

Solvent-in-Salt Electrolytes for Fluoride Ion Batteries

The fluoride ion battery (FIB) is a promising post-lithium ion battery chemistry owing to its high theoretical energy density and the large elemental abundance of its active materials. Nevertheless, its utilization for room-temperature cycling has been impeded by the inability to find sufficiently stable and conductive electrolytes at room temperature. In this work, we report the use of solvent-in-salt electrolytes for FIBs, exploring multiple solvents to show that aqueous cesium fluoride exhibited sufficiently high solubility to achieve an enhanced (electro)chemical stability window (3.1 V) that could enable high operating voltage electrodes, in addition to a suppression of active material dissolution that allows for an improved cycling stability. The solvation structure and transport properties of the electrolyte are also investigated using spectroscopic and computational methods.

Where Are Sodium Ions in AOT Reverse Micelles? Fluoride Anion Probes Nanoconfined Ions by 19F Nuclear Magnetic Resonance Spectroscopy

Confining water to nanosized spaces creates a unique environment that can change water's structural and dynamic properties. When ions are present in these nanoscopic spaces, the limited number of water molecules and short screening length can dramatically affect how ions are distributed compared to the homogeneous distribution assumed in bulk aqueous solution. Here, we demonstrate that the chemical shift observed in ¹⁹F NMR spectroscopy of fluoride anion, F^-, probes the location of sodium ions, Na⁺, confined in reverse micelles prepared from AOT (sodium dioctyl sulfosuccinate) surfactants. Our measurements show that the nanoconfined environment of reverse micelles can lead to extremely high apparent ion concentrations and ionic strength, beyond the limit in bulk aqueous solutions. Most notably, the ¹⁹F NMR chemical shift trends we observe for F^- in the reverse micelles indicate that the AOT sodium counterions remain at or near the interior interface between surfactant and water, thus providing the first experimental support for this hypothesis.

Understanding the Chemical Shifts of Aqueous Electrolyte Species Adsorbed in Carbon Nanopores

Interfaces between aqueous electrolytes and nanoporous carbons are involved in a number of technological applications such as energy storage and capacitive deionization. Nuclear magnetic spectroscopy is a very useful tool to characterize ion adsorption in such systems thanks to its nuclei specificity and the ability to distinguish between ions in the bulk and in pores. We use complementary methods (density functional theory, molecular dynamics simulations, and a mesoscopic model) to investigate the relative importance of various effects on the chemical shifts of adsorbed species: ring currents, ion organization in pores of various sizes, specific ion-carbon interactions, and hydration. We show that ring currents and ion organization are predominant for the determination of chemical shifts in the case of Li⁺ ions and hydrogen atoms of water. For the large Rb⁺ and Cs⁺ ions, the additional effect of the hydration shell should be considered to predict chemical shifts in agreement with experiments.

Fluorination and hydrolytic stability of water-soluble platinum complexes with a borane-bridged diphosphoramidite ligand

The high fluorophilicity of borane-containing ligands offers promise for accessing new metallodrug candidates capable of bifunctional [¹⁸F]-positron emission tomography (PET) imaging, but this requires water soluble and hydrolytically stable ligands that can be fluorinated under mild conditions. Toward this goal, here we report the synthesis and characterization of water-soluble Pt(II) complexes containing a triaminoborane-bridged diphosphoramidite ligand called ^MeOTBDPhos that can be fluorinated using simple fluoride salts. NMR and XRD studies show that (^MeOTBDPhos)PtCl₂ (1) dissolves in water with cooperative H-OH addition across the bridgehead N-B bond to form 1-H2O. The B-OH bond in 1-H2O undergoes rapid displacement with fluoride (<10 min) when treated with CsF in MeCN to form 1-HF. 1-HF can also be prepared in <10 min by addition of KF to 1 in the presence Kryptofix® 222 and (HNEt₃)Cl in MeCN. In addition to using fluoride salts, we show how mononuclear 1 can be fluorinated with HBF₄·Et₂O to form dinuclear [(^MeOTBDPhos-HF)Pt(μ-Cl)]₂(BF₄)₂ (4-HF). Comparative studies show that the B-F bond in 1-HF undergoes hydrolysis as soon as it is dissolved in water or saline, but the B-F bond persists for hours when the pH of the solution is lowered to pH ≤ 2. In contrast to 1-HF, the B-F bond in dinuclear 4-HF persists for days when dissolved in water, which may be attributed to slow, sacrificial release of fluoride from the BF₄^- anion. The results show how cooperative N-B reactivity on the ligand can be leveraged to rapidly fluorinate water-soluble ^MeOTBDPhos complexes under mild conditions and afford suggestions for how to enhance hydrolytic B-F stability, as required for use in biomedical applications.

Unusually Large Effects of Charge-assisted C-H⋅⋅⋅F Hydrogen Bonds to Anionic Fluorine in Organic Solvents: Computational Study of 19 F NMR Shifts versus Thermochemistry

A comparison of computed ¹⁹ F NMR chemical shifts and experiment provides evidence for large specific solvent effects for fluoride-type anions interacting with the σ*(C-H) orbitals in organic solvents like MeCN or CH₂ Cl₂ . We show this for systems ranging from the fluoride ion and the bifluoride ion [FHF]^- to polyhalogen anions [ClF_x ]^- . Discrepancies between computed and experimental shifts when using continuum solvent models like COSMO or force-field-based descriptions like the 3D-RISM-SCF model show specific orbital interactions that require a quantum-mechanical treatment of the solvent molecules. This is confirmed by orbital analyses of the shielding constants, while less negatively charged fluorine atoms (e. g., in [EF₄ ]^- ) do not require such quantum-mechanical treatments to achieve reasonable accuracy. The larger ¹⁹ F solvent shift of fluoride in MeCN compared to water is due to the larger coordination number in the former. These observations are due to unusually strong charge-assisted C-H⋅⋅⋅F^- hydrogen bonds, which manifest beyond some threshold negative natural charge on fluorine of ca. < -0.6 e. The interactions are accompanied by sizable free energies of solvation, in the order F^- ≫[FHF]^- >[ClF₂ ]^- >[ClF₄ ]^- . COSMO-RS solvation free energies tend to moderately underestimate those from the micro-solvated cluster treatment. Red-shifted and intense vibrational C-H stretching bands, potentially accessible in bulk solution, are further spectroscopic finger prints.

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.

Unbalanced-Ion-Pair-Catalyzed Nucleophilic Fluorination Using Potassium Fluoride

An unbalanced ion pair promoter (e.g., tetrabutylammonium sulfate), consisting of a bulky and charge-delocalized cation and a small and charge-localized anion, greatly accelerates nucleophilic fluorinations using easy handling KF. We also successfully converted an inexpensive and commercially available ion-exchange resin to the polymer-supported ion pair promoter (A26-SO₄^2-), which could be reused after filtration. Moreover, A26-SO₄^2- can be used in continuous flow conditions. In our conditions, water is well-tolerated.

Designing boron and metal complexes for fluoride recognition: a computational perspective

Fluoride anions (F^-) may have beneficial or harmful effects on the environment depending on their concentration. Here, we shed light on F^- recognition by compounds containing boron, tellurium and antimony, which were experimentally demonstrated to be capable of interacting with the F^- ion in a partially aqueous medium. Boron and metal complexes recognize F^- anions primarily using electrostatic energy along with important contributions from orbital interaction energy. The natural orbitals for chemical valence (NOCV) methodology indicates that the main orbital interactions behind fluoride recognition are σ bonds between the receptors and the F^- anions. The charged receptors, which provide (i) two B atoms, (ii) one B atom and one Sb atom, or (iii) one B atom and one Te atom to directly interact with the F^- ions, appear to be some of the best structures for the recognition of F^- anions. This is supported by the combination of favorable electrostatic and σ bond interactions. Overall, the presence of electron donor groups, such as -CH₃ and -OH, in the receptor structure destabilizes the fluoride recognition because it decreases the attractive electrostatic energy and increases the Pauli repulsion energy in the receptor⋯F^- bonds. Notably, electron acceptor groups, for example, -CN and -NO₂, in the receptor structure favor the interaction with the F^- ions, due to the improvement of the electrostatic and σ bond interactions. This study opens the way to find the main features of a receptor for F^- recognition.

NMR studies of adsorption and diffusion in porous carbonaceous materials

Porous carbonaceous materials have many important industrial applications including energy storage, water purification, and adsorption of volatile organic compounds. Most of their applications rely upon the adsorption of molecules or ions within the interior pore volume of the carbon particles. Understanding the behaviour and properties of adsorbate species on the molecular level is therefore key for optimising porous carbon materials, but this is very challenging owing to the complexity of the disordered carbon structure and the presence of multiple phases in the system. In recent years, NMR spectroscopy has emerged as one of the few experimental techniques that can resolve adsorbed species from those outside the pore network. Adsorbed, or "in-pore" species are shielded with respect to their free (or "ex-pore") counterparts. This shielding effect arises primarily due to ring currents in the carbon structure in the presence of a magnetic field, such that the observed chemical shift differences upon adsorption are independent of the observed nucleus to a first approximation. Theoretical modelling has played an important role in rationalising and explaining these experimental observations. Together, experiments and simulations have enabled a large amount of information to be gained on the adsorption and diffusion of adsorbed species, as well as on the structural and magnetic properties of the porous carbon adsorbent. Here, we review the methodological developments and applications of NMR spectroscopy and related modelling in this field, and provide perspectives on possible future applications and research directions.

From medium to endoplasmic reticulum: Tracing anticancer phenolato titanium(IV) complex by 19F NMR detection

Titanium(IV) complexes of diaminobis(phenolato)-bis(alkoxo) ligands are promising anticancer drugs, showing marked in-vivo efficacy with no toxic side-effects in mice, hence, it is of interest to elucidate their mechanism of action. Herein, we employed a fluoro-substituted derivative, FenolaTi, for mechanistic analysis of the active species and its cellular target by quantitative ¹⁹F NMR detection to reveal its biodistribution and reactivity in extracellular and intracellular matrices. Upon administration to the serum-containing medium, FenolaTi interacted with bovine serum albumin. 20 h post administration, the cellular accumulation of FenolaTi derivatives was estimated as 37% of the administered compound, in a concentration three orders-of-magnitude higher than the administered dose, implying that active membrane transportation facilitates cellular penetration. An additional 19% of the administered dose that was detected in the extracellular environment had originated from post-apoptotic cells. In the cell, interaction with cellular proteins was detected. Although some intact Ti(IV) complex localized in the nucleus, no signals for isolated DNA fractions were detected and no reactivity with nuclear proteins was observed. Interestingly, higher accumulation of FenolaTi-derived compounds in the endoplasmic reticulum (ER) and interaction with proteins therein were detected, supporting the role of the ER as a possible target for cytotoxic bis(phenolato)-bis(alkoxo) Ti(IV) complexes.

Using solid-state nuclear magnetic resonance to rationalize best efficiency of 2,6-dihydroxybenzoic acid over other 2,X-dihydroxybenzoic acid isomers in solvent-free matrix-assisted laser desorption/ionization of poly(ethylene glycol)

RATIONALE

Among isomers of dihydroxybenzoic acid (DHB), 2,5-DHB is often the most efficient matrix in matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) for a great variety of compounds. Yet, when performing solvent-free MALDI, 2,6-DHB yields better results for poly(ethylene glycol [PEG]). This intriguing feature is explored here using solid-state nuclear magnetic resonance (NMR).

METHODS

Ternary mixtures were prepared by grinding 2,X-DHB (X = 3-6), poly(ethylene glycol) (M_n = 2000 g mol^-1 ) and lithium fluoride (LiF) in a matrix/analyte/salt molar ratio of 50/1/10 for 16 min under a controlled atmosphere. After mixing, a few grains were applied to the MALDI target for MS analysis, whereas the major part of the ground sample was transferred into rotors to perform ¹³ C, ⁷ Li, and ¹⁹ F NMR experiments.

RESULTS

Lithiated PEG chains are mainly formed with 2,6-DHB in solvent-free MALDI, but their abundance increases with 2,3-DHB and 2,4-DHB when water uptake is favored by a humid atmosphere. Solid-state NMR shows that grinding 2,6-DHB-based samples in atmospheric conditions leads to a solid phase in which the matrix, PEG, and salt molecules exhibit a high mobility compared with systems involving other 2,X-DHB isomers. This mobile environment would favor (as a solvent) LiF dissociation and best promote PEG cationization.

CONCLUSIONS

Complementary data in ¹³ C, ⁷ Li, and ¹⁹ F NMR spectra are consistent with the formation of a solid phase of high mobility composed of 2,6-DHB, PEG, and the two salt components that ultimately favor the production of lithiated PEG chains.

Impact of Multiple Hydrogen Bonds with Fluoride on Catalysis: Insight from NMR Spectroscopy

Hydrogen-bonding interactions have been explored in catalysis, enabling complex chemical reactions. Recently, enantioselective nucleophilic fluorination with metal alkali fluoride has been accomplished with BINAM-derived bisurea catalysts, presenting up to four NH hydrogen-bond donors (HBDs) for fluoride. These catalysts bring insoluble CsF and KF into solution, control fluoride nucleophilicity, and provide a chiral microenvironment for enantioselective fluoride delivery to the electrophile. These attributes encouraged a ¹H/¹⁹F NMR study to gain information on hydrogen-bonding networks with fluoride in solution, as well as how these arrangements impact the efficiency of catalytic nucleophilic fluorination. Herein, NMR experiments enabled the determination of the number and magnitude of HB contacts to fluoride for thirteen bisurea catalysts. These data supplemented by diagnostic coupling constants ^1hJ_{NH···F^–} give insight into how multiple H bonds to fluoride influence reaction performance. In dichloromethane (DCM-d₂), nonalkylated BINAM-derived bisurea catalyst engages two of its four NH groups in hydrogen bonding with fluoride, an arrangement that allows effective phase-transfer capability but low control over enantioselectivity for fluoride delivery. The more efficient N-alkylated BINAM-derived bisurea catalysts undergo urea isomerization upon fluoride binding and form dynamically rigid trifurcated hydrogen-bonded fluoride complexes that are structurally similar to their conformation in the solid state. Insight into how the countercation influences fluoride complexation is provided based on NMR data characterizing the species formed in DCM-d₂ when reacting a bisurea catalyst with tetra-n-butylammonium fluoride (TBAF) or CsF. Structure–activity analysis reveals that the three hydrogen-bond contacts with fluoride are not equal in terms of their contribution to catalyst efficacy, suggesting that tuning individual electronic environment is a viable approach to control phase-transfer ability and enantioselectivity.

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.

The value of universally available raw NMR data for transparency, reproducibility, and integrity in natural product research

Covering: up to 2018With contributions from the global natural product (NP) research community, and continuing the Raw Data Initiative, this review collects a comprehensive demonstration of the immense scientific value of disseminating raw nuclear magnetic resonance (NMR) data, independently of, and in parallel with, classical publishing outlets. A comprehensive compilation of historic to present-day cases as well as contemporary and future applications show that addressing the urgent need for a repository of publicly accessible raw NMR data has the potential to transform natural products (NPs) and associated fields of chemical and biomedical research. The call for advancing open sharing mechanisms for raw data is intended to enhance the transparency of experimental protocols, augment the reproducibility of reported outcomes, including biological studies, become a regular component of responsible research, and thereby enrich the integrity of NP research and related fields.

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.

NMR shielding constants in group 15 trifluorides

By combining large basis and complete basis set (CBS) extrapolations of the coupled-cluster equilibrium geometry results with rovibrational and relativistic corrections, we demonstrate that it is possible to achieve near-quantitative accuracy for the NMR shielding constants in three group 15 trifluorides – NF₃, PF₃ and AsF₃.

Development of an Inner Filter Effects-Based Upconversion Nanoparticles-Curcumin Nanosystem for the Sensitive Sensing of Fluoride Ion

This paper describes a novel ratiometric fluorescence-based sensor for the detection of fluoride ion. Yb³⁺, Er³⁺, and Tm³⁺ codoped NaYF₄ upconversion nanoparticles (UCNPs), which can emit fluorescence at 546, 657, 758, and 812 nm under the 980 nm single wavelength excitation, were synthesized, amino-modified and applied as the fluorescent signal indicator. The natural chemical curcumin served as specific recognition element and mixed with UCNPs to make a nanosystem. In this nanosystem, the absorption peak of curcumin shows a bathochromic shift when F^- was added, causing an upconversion fluorescence quenching at 546 and 657 nm through inner filter effects (IFE), whereas the upconversion emission at 758 and 812 nm remained unchanged. Thus, the fluorescence ratio I₅₄₆/I₇₅₈ was inversely proportional to F^- concentration. Meanwhile, the large absorption bathochromic shift also lead to a color change, based on the colorimetric analysis of F^- by the naked eye. Under the optimized conditions, the developed UCNPs-curcumin mixed system achieved the colorimetric and ratiometric fluorescence sensing toward F^- in the linear range of 25-200 μM and 5-200 μM, with the detection limits as low as 25 μM (ca. 0.48 ppm) and 5 μM (ca. 0.10 ppm), respectively. The developed nanosystem also has high selectivity and antijamming ability. Furthermore, this method showed promising practical applications in spiked real samples (ex., tap water and milk) with recoveries of 79.58% to 134.02% and RSD values in the range of 0.94% to 22.11%, which confirmed its great potential in harmful substance detection.

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

The synthesis and properties of a new class of anhydrous quaternary ammonium fluorides, based on the rigid skeleton azabicyclo[2.2.2]octane, is described. Compounds 2a-d were easily prepared by passing the corresponding ammonium iodides over fluoride-based resin followed by drying their hydrated form at 100 or 140 °C under reduced pressure. The stability (experimental and theoretical study), solubility, reactivity, and characterization by solution and solid-state MAS NMR are discussed.

Simple One-Pot Syntheses and Characterizations of Free Fluoride- and Bifluoride-Containing Polymers Soluble in Non-Aqueous Solvents

One of the problems that arise with bifluoride- or fluoride-containing compounds is their poor solubility in non-aqueous solvents. We report herein a facile one-pot synthesis and the chemical analysis of fluoride/bifluoride-containing polymers, which are soluble in MeCN. Different polymers, such as Polyvinylacetate or Polyethylene imine and saccharides, such as maltodextrin, were complexed with ammonium (bi)fluoride using hydrogen bonds to form the desired (bi)fluoride-containing compounds. The newly formed hydrogen bonding (bi)fluoride-doped polymer matrices were analyzed using infrared and nuclear magnetic resonance spectroscopies, and X-ray diffraction. The promising materials also underwent impedance spectroscopy, conductivity measurements and preliminary tests as electrolytes for room temperature fluoride ion batteries along with an analysis of their performance.

Dehydration of Ions in Voltage-Gated Carbon Nanopores Observed by in Situ NMR

Ion transport through nanochannels is of fundamental importance in voltage-gated protein ion channels and energy storage devices. Direct microscopic observations are critical for understanding the intricacy of ionic processes in nanoconfinement. Here we report an in situ nuclear magnetic resonance study of ion hydration in voltage-gated carbon nanopores. Nucleus-independent chemical shift was employed to monitor the ionic processes of NaF aqueous electrolyte in nanopores of carbon supercapacitors. The state of ion hydration was revealed by the chemical shift, which is sensitive to the hydration number. A large energy barrier was observed for ions to enter nanopores smaller than the hydrated ion size. Increasing the gating voltage above 0.4 V overcomes this barrier and brings F(-) into the nanopores without dehydration. Partial dehydration of F(-) occurs only at gating voltage above 0.7 V. No dehydration was observed for Na(+) cations, in agreement with their strong ion hydration.

Squeezing fluoride out of water with a neutral bidentate antimony(V) Lewis acid

Because of hydration, fluoride ions in water typically elude complexation by neutral Lewis acids. Here, we show how this limitation can be overcome with a bidentate Lewis acid containing two antimony(V) centers. This derivative (2) is obtained by the simple reaction of 4,5-bis(diphenylstibino)-9,9-dimethylxanthene (1) with two equivalents of 3,4,5,6-tetrachlorobenzoquinone (o-chloranil). It features two square-pyramidal stiborane units oriented in a face-to-face fashion. Titration experiments show that this new bidentate Lewis acid binds fluoride in aqueous solutions containing 95% water with a binding constant (K) of 700±30 M(-1). The structure of the fluoride adduct confirms fluoride anion chelation between the two antimony centers.

Highlights on contemporary recognition and sensing of fluoride anion in solution and in the solid state

The fluoride anion has recently gained well deserved attention among the scientific community for its importance in many fields of human activities, but also for concerns on its effect on health and the environment. Although surprisingly overlooked in systematic studies in the past, fluoride has nowadays become a topical target in the field of anion recognition. A multitude of scientific reports are published every year where the establishment of efficient and specific interaction with fluoride is sought in polar and aqueous media. Here, the emphasis is directed to a detailed description of the most interesting contemporary studies in the field, with a particular focus given to those published in the last few years.

Intermolecular and intramolecular hydrogen bonds involving fluorine atoms: implications for recognition, selectivity, and chemical properties

A correlation between 19F NMR isotropic chemical shift and close intermolecular F⋅⋅⋅H-X contacts (with X=N or O) has been identified upon analysis of the X-ray crystal structures of fluorinated molecules listed in the Cambridge Structural Database (CSD). An optimal F⋅⋅⋅X distance involving primary and shielded secondary fluorine atoms in hydrogen-bond formation along with a correlation between F⋅⋅⋅H distance and F⋅⋅⋅H-X angle were also derived from the analysis. The hydrogen bonds involving fluorine are relevant, not only for the recognition mechanism and stabilization of a preferred conformation, but also for improvement in the permeability of the molecules, as shown with examples taken from a proprietary database. Results of an analysis of the small number of fluorine-containing natural products listed in the Protein Data Bank (PDB) appear to strengthen the derived correlation between 19F NMR isotropic chemical shift and interactions involving fluorine (also known as the "rule of shielding") and provides a hypothesis for the recognition mechanism and catalytic activity of specific enzymes. Novel chemical scaffolds, based on the rule of shielding, have been designed for recognizing distinct structural motifs present in proteins. It is envisaged that this approach could find useful applications in drug design for the efficient optimization of chemical fragments or promising compounds by increasing potency and selectivity against the desired biomolecular target.

Mechanistic studies on the enzymatic processing of fluorinated methionine analogues by Trichomonas vaginalis methionine γ-lyase

TFM (L-trifluoromethionine), a potential prodrug, was reported to be toxic towards human pathogens that express MGL (L-methionine γ-lyase; EC 4.4.1.11), a pyridoxal phosphate-containing enzyme that converts L-methionine into α-oxobutyrate, ammonia and methyl mercaptan. It has been hypothesized that the extremely reactive thiocarbonyl difluoride is produced when the enzyme acts upon TFM, resulting in cellular toxicity. The potential application of the fluorinated thiomethyl group in other areas of biochemistry and medicinal chemistry requires additional studies. Therefore a detailed investigation of the theoretical and experimental chemistry and biochemistry of these fluorinated groups (CF3S− and CF2HS−) has been undertaken to trap and identify chemical intermediates produced by enzyme processing of molecules containing these fluorinated moieties. TvMGL (MGL from Trichomonas vaginalis) and a chemical model system of the reaction were utilized in order to investigate the cofactor-dependent activation of TFM and previously uninvestigated DFM (L-difluoromethionine). The differences in toxicity between TFM and DFM were evaluated against Escherichia coli expressing TvMGL1, as well as the intact human pathogen T. vaginalis. The relationship between the chemical structure of the reactive intermediates produced from the enzymatic processing of these analogues and their cellular toxicity are discussed.

Solid state nuclear magnetic resonance as a tool to explore solvent-free MALDI samples

Solid-state Nuclear magnetic resonance (NMR) was used here to explore structural characteristics of samples to be subjected to matrix-assisted laser desorption/ionization (MALDI) and prepared without the use of any solvent. The analytical systems scrutinized in NMR were mixtures of a 2,5-dihydroxybenzoic acid (2,5-DHB) matrix and caesium fluoride (CsF), used as the cationization agent in synthetic polymer MALDI mass analysis, at different molar ratios (1:1, 5:1, and 10:1). Complementary information could be obtained from 13C, 133Cs, and 19F NMR spectra. Grinding the matrix together with the salt in the solid state was shown to induce a strong modification in the molecular organization within the MALDI sample. The evidenced mechano-induced reactions allow strong interactions between the matrix and the cation, up to the formation of a salt, and only occur in the presence of some water molecules. Addition of a poly(ethylene oxide) polymer as the analyte did not further modify the observed molecular organizations. Although relative matrix and salt concentrations in the scrutinized samples were unusual for MALDI analysis, mass spectra of good quality could be obtained and revealed that cation attachment on polymers during the MALDI process is not a matrix-independent event since a lower ionization efficiency was obtained from highly organized solid samples, mostly consisting of 2,5-DHB caesium salt species.

Anhydrous Phosphazenium Fluorides as Sources for Extremely Reactive Fluoride Ions in Solution

Several peralkylated polyaminophosphazenium cations were evaluated for the generation of novel anhydrous F- salts. Two of them have been characterized by X-ray analysis and are particularly soluble, even in apolar aprotic solvents like benzene or THF, one of them even at -30 degrees C. Such solutions probably represent the most basic metal-free and stable media known to date. Comparison of these fluorides with known F- sources demonstrates that they are of unprecedented reactivity and selectivity in E2 elimination reactions.

Tetramethylphosphonium fluoride: "naked" fluoride and phosphorane

Me(4)PF was investigated in the solid state, in the gas phase, and in solutions. Vibrational spectra of the solid and a single-crystal structure show an ionic tetramethylphosphonium fluoride. The compound crystallizes in the space group Pbca with a = 1016.0(1), b = 1018.0(1), c = 1205.8(4) pm, and Z = 8. The fluoride ion is nearly trigonal planar surrounded by three Me(4)P+ cations forming six H...F contacts between 218 and 240 pm. The compound is stable below 120 degrees C and sublimes in a vacuum. It possesses a phosphorane structure in the gas phase that was studied by electron diffraction and vibrational spectra, and additionally by theoretical calculations. The Me(4)PF molecule has a trigonal bipyramidal structure with one methyl group and the fluorine atom in axial positions and bond lengths of d(PC(eq)) = 182.6(4) pm, d(PC(ax)) = 188.4(8) pm, and d(PF) = 175.3(6) pm. The compound is remarkably soluble in acetonitrile, water, and alcohols, and slightly soluble in benzene, dimethyl ether, and diethyl ether. The solutions were studied by (1)H, (13)C, (19)F, and (31)P NMR spectroscopy. The hygroscopic Me(4)PF forms a tetrahydrate which crystallizes in the space group I4(1)/a with a = 1106.1(1) pm, c = 816.3(1) pm, and Z = 4. The fluoride ion in Me(4)PF.4 H(2)O is surrounded by four water molecules. These units form a three-dimensional network in which the Me(4)P+ cations are embedded without any contacts.

On the preparation of fluorine-18 labelled XeF(2) and chemical exchange between fluoride ion and XeF(2)

A recent report claims to have prepared [18F]XeF2 by exchange between a large stoichiometric excess of XeF2 and no-carrier-added 18F-, as salts of the [2,2,2-crypt-M+] (M = K or Cs) cations, in CH2Cl2 or CHCl3 solvents at room temperature. Attempts to repeat this work have proven unsuccessful and have led to a critical reinvestigation of chemical exchange between fluoride ion, in the form of anhydrous [N(CH3)4][F] and [2,2,2-crypt-K][F], and XeF2 in dry CH2Cl2 and CH3CN solvents. It was shown, by use of 19F and 1H NMR spectroscopies, that [2,2,2-crypt-K][F] rapidly reacts with CH3CN solvent to form HF2-, and with CH2Cl2 solvent to form HF2-, CH2ClF, and CH2F2 at room temperature. Moreover, XeF2 rapidly oxidizes 2,2,2-crypt in CH2Cl2 solvent at room temperature to form HF and HF2-. Thus, the exchange between XeF2 and no-carrier-added 18F- reported in the prior work arises from exchange between XeF2 and HF/HF2-, and does not involve fluoride ion. However, naked fluoride ion has been shown to undergo exchange with XeF2 under rigorously anhydrous and HF-free conditions. A two-dimensional 19F-19F EXSY NMR study demonstrated that [N(CH3)4][F] exchanges with XeF2 in CH3CN solvent, but exchange of HF2- with either XeF2 or F- is not detectable under these conditions. The exchange between XeF2 and F- is postulated to proceed by the formation of XeF3- as the exchange intermediate.