Fluorine Ligands Research Articles

Synthesis and Stabilization of Highly Reactive Haloacetylium Ions.

The reactions of selected haloacetyl fluorides with the strong Lewis acids AsF5 and SbF5 were investigated in the aprotic solvent SO2ClF. Depending on the stoichiometric ratio of AsF5 or SbF5 and the haloacetyl fluorides, either O-coordinated adducts or the respective haloacetylium ions were isolated as solids. The compounds were characterized by low-temperature vibrational spectroscopy and single-crystal X-ray diffraction. [CClH2CO][Sb2F11] and [CH2FCO][Sb2F11] crystallize in the monoclinic space group P21 with two formula units per unit cell. The reactivity of the haloacetyl fluorides under Lewis acidic conditions is discussed based on quantum chemical calculations of fluoride ion affinities on the BP/def2SVPP level of theory. The haloacetylium ions are stabilized in the solid-state phase by intermolecular C···F contacts and electron donation of the fluorine ligands of the anions as well as intramolecular hyperconjugation.

Strongly bound anions featuring bismuth fluoride building blocks

The stability of polynuclear superhalogen anions composed of BiF5 building blocks was investigated using ab initio electronic structure methods and flexible basis sets. A comprehensive exploration of the ground state potential energy surfaces of (Bi2F11)−, (Bi3F16)− and (Bi4F21)− anions, which can be viewed as comprising BiF5 fragments and an additional fluorine atom, led to the identification of their isomeric structures. It was found that the most stable isomers, predicted to dominate at room temperature, correspond to chain-like extended structures containing BiF6 subunits, with fluorine ligands arranged octahedrally around Bi atoms, sharing F atoms to form Bi–F–Bi bridging linkages. The vertical electron detachment energies of the (BinF5n+1)− anions (n = 1–4) were found to be very high (ranging from 10.91 to 13.36 eV) and increased with the number of bismuth atoms (n) and thus the BiF5 building blocks involved in the structure. Thermodynamic stability of the (BinF5n+1)− anions (i.e., their susceptibility to fragmentation) was also verified and discussed.

Formation of Peroxynitrite, [O-N-O-O]-, via a Cascade of Reactions between Ozonide and Ammonia.

We report on an unexpected reaction between ammonia and potassium ozonide dissolved in liq. NH3 resulting in the formation of peroxynitrite, [ONOO]-, which exclusively happens in the presence of a specific partially fluorinated aniline-based ammonium cation. High-resolution structural data of the peroxynitrite anion in cis-conformation have been obtained. We further studied this molecule anion by single crystal Raman spectroscopy. The cis and trans isomers of peroxynitrite were analysed computationally with respect to their relative energies, the cis-trans transition barrier and their decomposition pathways to the nitrate anion. By experimentally examining cations decorated with fluorine ligands to different degrees, we demonstrate that fluorine specific interactions play a crucial role in the unexpected formation of peroxynitrite and as a conspicuously structure directing factor for the underlying crystalline solid phases, exhibiting distinct micro-separations of fluorine and hydrogen enriched regions.

Coinage metal-ethylene complexes of sterically demanding 1,10-phenanthroline ligands.

Phenanthroline-based ligands with bulky aryl groups flanking the metal binding pocket enabled the synthesis and detailed investigation of ethylene complexes of copper(I), silver(I), and gold(I), including structural data of [{2,9-bis(2,4,6-triisopropylphenyl)-1,10-phenanthroline}M(C2H4)][SbF6] (M = Cu, Ag, Au), Additionally, a related copper(I)-ethylene complex with a highly fluorinated ligand is also reported. Gold(I) affects the ethylene moiety significantly as evident from the notable upfield coordination shifts of ethylene carbon signals in the NMR and lengthening of the ethylene CC bond length. Silver(I) forms the weakest bond with ethylene in this series of isoleptic, group 11 metal-ethylene complexes. Preliminary catalytic investigations underscore the potential of copper complexes, particularly those with weakly coordinating supporting ligands, as effective catalysts for C(sp3)-H functionalization through trifluoromethyl carbene insertion.

The Crucial Role of Sb2F10 in the Chemical Synthesis of F2.

The purely chemical synthesis of fluorine is a spectacular reaction which for more than a century had been believed to be impossible. In 1986, it was finally experimentally achieved, but since then this important reaction has not been further studied and its detailed mechanism had been a mystery. The known thermal stability of MnF4 casts serious doubts on the originally proposed hypothesis that MnF4 is thermodynamically unstable and decomposes spontaneously to a lower manganese fluoride and F2. This apparent discrepancy has now been resolved experimentally and by electronic structure calculations. It is shown that the reductive elimination of F2 requires a large excess of SbF5 and occurs in the last reaction step when in the intermediate [SbF6][MnF2][Sb2F11] the addition of one more SbF5 molecule to the [SbF6]- anion generates a second tridentate [Sb2F11]- anion. The two tridentate [Sb2F11]- anions then provide six fluorine bridges to the Mn(II) cation thereby facilitating the reductive elimination of the two fluorine ligands as F2.

Die entscheidende Rolle von Sb2F10 bei der chemischen Synthese von F2

AbstractDie Synthese von elementarem Fluor auf rein chemischem Weg ist spektakulär und galt länger als ein Jahrhundert als unmöglich. Im Jahr 1986 war man schließlich erfolgreich. Seitdem wurde diese wichtige Reaktion nicht weiter untersucht und ihr detaillierter Mechanismus blieb ein Rätsel. Die ursprüngliche Hypothese eines thermodynamisch instabilen MnF4, welches sich spontan zu einem Manganfluorid niedrigerer Oxidationsstufe und elementarem Fluor zersetzt, wird durch die bekannte thermische Stabilität von MnF4 in Frage gestellt. Diese scheinbare Diskrepanz konnte nun experimentell und durch quantenchemische Rechnungen aufgeklärt werden. Die reduktive Eliminierung von F2 aus MnF4 erfordert einen großen Überschuss an SbF5. Sie findet im letzten Reaktionsschritt statt, wenn sich an das Zwischenprodukt [SbF6][MnF2][Sb2F11] ein weiteres SbF5‐Molekül unter Bildung eines weiteren tridentaten [Sb2F11]−‐Anions anlagert. Die beiden [Sb2F11]−‐Anionen stellen dem Mn‐Atom so sechs verbrückende F‐Atome zur Verfügung und ermöglichen damit die reduktive Eliminierung der beiden terminalen Fluoridoliganden als F2.

Theoretical Measurements of Quantitative Effects Caused by Spectator Ligands on Palladium-catalyzed C–H Activation

Ligands can definitely influence C–H activation at the metal center. A ligand not directly participating in the reaction is called a spectator ligand. We attempt to quantitatively characterize the effects of diverse spectator ligands on C-H activation at palladium. We designed a model palladium catalyst and selected an array of spectator ligands, such as methoxyl, amide, methyl, phenyl, cyanide, fluorine, chlorine, and several neutral ligands, and performed density functional theory calculations on the mechanism and energetics of C–H activation reactions of benzene with different catalysts. Univalent ligands have substantially larger effects than neutral ligands, and strongly σ-donating ligands (e.g., methyl and phenyl) severely hinder the C-H activation in progress. A ligand trans to the reaction site influences C–H activation more than that cis to the reaction site, indicating electronic effects to be at work. For example, the existence of a methyl ligand raises the barrier height of C-H activation by 6.4 or 14.4 kcal/mol when it is placed at the position cis or trans to the C–H activation site. The effects of poorly σ-donating ligands are not significant and similar to those of the κ1-acetate ligand. Some σ-donating and π-accepting ligands, such as cyanide and isonitrile, hinder the C–H activation trans to them but appear to facilitate the C–H activation cis to them. On the basis of molecular orbital analyses, a chemical model is proposed to understand the observed ligand effects. Lastly, the conclusions are applied to explain the plausible mechanism of the dehydrogenative Heck coupling.

Unexpected d–p orbital covalent interaction between the non-d-block main-group metal tellurium and fluorine at high pressure

As a 5p-block main-group metal, tellurium generally loses its outermost electrons and forms ionic compounds upon encountering more electronegative elements under ambient conditions. Here, we report an unexpected d–p orbital covalent interaction between the 5p-block metal tellurium and the most electronegative element F at high pressure. In tellurium fluorides, the strong fluorine ligand field affects the effective nuclear charge of the metal atom and then affects the valence orbital energies of the central tellurium atom, incorporating unoccupied 5d orbitals of tellurium into the valence orbitals and interacting strongly with the valence orbitals of F atoms under high pressure. An electronic structure analysis of tellurium fluorides reveals the existence of strong d–p covalent interactions in the Te−F system. This study is extremely valuable for advancing our understanding of chemical bonding and high-pressure main-group chemistry.

Novel first-principles insights into graphene fluorination

Fluorination of graphene sheets with xenon difluoride leads to the formation of the widest bandgap Gr derivative, namely, fluorographene. Accurate experimental observations distinguish two stages of mechanism in the fluorination procedure: the half-fluorination stage, wherein one side of the Gr sheet is rapidly fluorinated, and the full-fluorination stage, involving much slower fluorination of the opposite side of the sheet [R. J. Kashtiban et al., Nat. Commun. 5, 5902 (2014)]. Here, we perform comprehensive density functional calculations to illustrate accurate microscopic insights into the much slower rate of the full-fluorination stage compared with the half-fluorination one. The calculated minimum energy paths for the half- and full-fluorination processes demonstrate much enhanced fluorine adsorption after the half-fluorination stage, which sounds inconsistent with the experimental picture. This ambiguity is explained in terms of significant chemical activation of the graphene sheet after half-fluorination, which remarkably facilitates the formation of chemical contaminants in the system and, thus, substantially slows down the full-fluorination procedure. After considering the binding energy and durability of the relevant chemical species, including hydrogen, oxygen, and nitrogen molecules and xenon atom, it is argued that oxygen-fluorine ligands are the most likely chemical contaminants opposing the complete fluorination of a graphene sheet. Then, we propose an oxygen desorption mechanism to carefully explain the much enhanced rate of the full-fluorination procedure at elevated temperatures. The potential photocatalytic application of the pristine and defected samples in water splitting and carbon dioxide reduction reactions is also discussed.

Intermediates in Friedel‐Crafts Acylation of Fumaryl Halides

AbstractFumaryl chloride and fluoride were reacted with different Lewis acids to synthesize the intermediates of the Friedel‐Crafts acylation. The salt of the monoacyl cation [C4H2FO2]+[Sb3F16]− was obtained from the reaction of fumaryl fluoride with SbF5 in SO2ClF solutions. The reaction was repeated using fumaryl chloride as starting material, which reacted under halogen exchange to obtain the salt of the monoacyl cation [C4H2FO2]+[SbCl2F4]−. In addition, the reaction of fumaryl chloride with SbCl5 in SO2ClF was studied. The covalent donor‐acceptor complex C4H2Cl2O2 ⋅ 2 SbCl5 was formed, containing oxygen‐bonded Lewis acids. The compounds were characterized by low‐temperature vibrational spectroscopy. Single‐crystal X‐ray structure analyses were conducted for [C4H2FO2]+[Sb3F16]− as well as for C4H2Cl2O2 ⋅ 2 SbCl5. In the solid state of [C4H2FO2]+[Sb3F16]− C⋅⋅⋅O and C⋅⋅⋅F contacts are observed and the origin of these interactions is discussed by means of ESP maps and NBO analysis. The monoacyl cation is stabilized by electrostatic attraction and electron back‐donation from oxygen and fluorine ligands to the positive ring‐structured π‐hole at the oxocarbenium center. Besides, the formation of the diacyl cation is not observed, which is based on small distances between the positive charges involving charge‐charge repulsion. The great advantage of using fumaryl halides in Friedel‐Crafts acylation is featured by the possibility to synthesize ketones keeping an acyl fluoride moiety.

Superhalogen Anions Supported by the Systems Comprising Alternately Aligned Boron and Nitrogen Central Atoms.

Using DFT/(B3LYP/wB97XD/B2PLYPD) and OVGF electronic structure methods with flexible atomic orbital basis sets, we examined the series of polynuclear superhalogen anions matching the (BF3(BN) n F4n+1)– formula (for n = 1-10,13,18-20) containing alternately aligned boron and nitrogen central atoms decorated with fluorine ligands. It was found that the equilibrium structures of these anions correspond to fully extended chains (with each B and N central atom surrounded by four substituents arranged in a tetrahedral manner) and thus mimic the globally stable fully extended (all-trans) conformations of higher n-alkanes. The vertical electron detachment energies of the (BF3(BN) n F4n+1)– anions were found to exceed 8 eV in all cases and gradually increase with the increasing number of n. The approximate limiting value of vertical electron binding energy that could be achieved for such polynuclear superhalogen anions was estimated as equal to ca. 10.7 eV.

Binding Sites of a Positron Emission Tomography Imaging Agent in Alzheimer's β-Amyloid Fibrils Studied Using 19F Solid-State NMR.

Amyloid imaging by positron emission tomography (PET) is an important method for diagnosing neurodegenerative disorders such as Alzheimer's disease. Many 11C- and 18F-labeled PET tracers show varying binding capacities, specificities, and affinities for their target proteins. The structural basis of these variations is poorly understood. Here we employ 19F and 13C solid-state NMR to investigate the binding sites of a PET ligand, flutemetamol, to the 40-residue Alzheimer's β-amyloid peptide (Aβ40). Analytical high-performance liquid chromatography and 19F NMR spectra show that flutemetamol binds the current Aβ40 fibril polymorph with a stoichiometry of one ligand per four to five peptides. Half of the ligands are tightly bound while the other half are loosely bound. 13C and 15N chemical shifts indicate that this Aβ40 polymorph has an immobilized N-terminus, a non-β-sheet His14, and a non-β-sheet C-terminus. We measured the proximity of the ligand fluorine to peptide residues using 19F-13C and 19F-1H rotational-echo double-resonance (REDOR) experiments. The spectra show that three segments in the peptide, 12VHH14, 18VFF20, and 39VV40, lie the closest to the ligand. REDOR-constrained docking simulations indicate that these three segments form multiple binding sites, and the ligand orientations and positions at these sites are similar across different Aβ polymorphs. Comparison of the flutemetamol-interacting residues in Aβ40 with the small-molecule binding sites in other amyloid proteins suggest that conjugated aromatic compounds preferentially bind β-sheet surface grooves lined by aromatic, polar, and charged residues. These motifs may explain the specificity of different PET tracers to different amyloid proteins.

Theoretical Insight into the Effect of Fluorine-Functionalized Metal-Organic Framework Supported Palladium Single-Site Catalyst in the Ethylene Dimerization Reaction.

Ethylene dimerization reaction is one of the most common mechanisms for the production of 1-butene. Recently, metal-organic frameworks (MOFs) have received extensive attention in this area since they combine all the advantages of homogeneous and heterogeneous catalysts in a single compound. Here a computational mechanistic study of MOF-supported palladium single-site catalyst for ethylene dimerization reaction is reported. Catalytic systems with both biphenyl-type backbone as organic ligand and its fluorine-functionalization have been investigated to reveal the origin of ligand effects on the catalytic activity and selectivity. The calculations revealed that the nonfluorinated palladium MOF catalyst undergoes dimerization over isomerization reaction. Then the influence of the fluorine-functionalized organic ligand was compared in the dimerization catalytic cycle, which was strongly favored in terms of activity and selectivity. Catalyst-substrate interactions were analyzed by energy decomposition analysis revealing the critical role of ligand backbone functionalization on the activity. This theoretical analysis identified three chemically meaningful dominant effects on these catalysts; steric, electrostatic and charge transfer effects. The steric effects promote nonfluorinated MOF catalyst, whereas the electrostatic effects are the dominant factor that promotes its fluorinated counterpart. This theoretical study provides feedback with future experimental studies about the role of fluorine ligand functionalization in palladium MOF catalysts for ethylene dimerization reaction.

Syntheses, Structures, and Bonding of NgF2 ⋅CrOF4 , NgF2 ⋅2CrOF4 (Ng=Kr, Xe), and (CrOF4 )∞.

The noble-gas difluoride adducts, NgF2 ⋅CrOF4 and NgF2 ⋅2CrOF4 (Ng=Kr and Xe), have been synthesized and structurally characterized at low temperatures by Raman spectroscopy and single-crystal X-ray diffraction. The low fluoride ion affinity of CrOF4 renders it incapable of inducing fluoride ion transfer from NgF2 (Ng=Kr and Xe) to form ion-paired salts of the [NgF]+ cations having either the [CrOF5 ]- or [Cr2 O2 F9 ]- anions. The crystal structures show the NgF2 ⋅CrOF4 adducts are comprised of Ft -Ng-Fb - - -Cr(O)F4 structural units in which NgF2 is weakly coordinated to CrOF4 by means of a fluorine bridge, Fb , in which Ng-Fb is elongated relative to the terminal Ng-Ft bond. In contrast with XeF2 ⋅2MOF4 (M=Mo or W) and KrF2 ⋅2MoOF4 , in which the Lewis acidic, F4 (O)M- - -Fb - - -M(O)F3 moiety coordinates to Ng through a single M- - -Fb -Ng bridge, both fluorine ligands of NgF2 coordinate to CrOF4 molecules to form F4 (O)Cr- - -Fb -Ng-Fb - - -Cr(O)F4 adducts in which both Ng-Fb bonds are only marginally elongated relative to the Ng-F bonds of free NgF2 . Quantum-chemical calculations show that the Cr-Fb bonds of NgF2 ⋅CrOF4 and NgF2 ⋅2CrOF4 are predominantly electrostatic with a small degree of covalent character that accounts for their nonlinear Cr- - -Fb -Ng bridge angles and staggered O-Cr- - -Fb -Ng-Ft dihedral angles. The crystal structures and Raman spectra of two CrOF4 polymorphs have also been obtained. Both are comprised of fluorine-bridged chains that are cis- and trans-fluorine-bridged with respect to oxygen.

Structure and Energetics of Ligand-Fluorine Interactions with Galectin-3 Backbone and Side-Chain Amides: Insight into Solvation Effects and Multipolar Interactions.

Multipolar fluorine–amide interactions with backbone and side‐chain amides have been described as important for protein–ligand interactions and have been used to improve the potency of synthetic inhibitors. In this study, fluorine interactions within a well‐defined binding pocket on galectin‐3 were investigated systematically using phenyltriazolyl‐thiogalactosides fluorinated singly or multiply at various positions on the phenyl ring. X‐ray structures of the C‐terminal domain of galectin‐3 in complex with eight of these ligands revealed potential orthogonal fluorine–amide interactions with backbone amides and one with a side‐chain amide. The two interactions involving main‐chain amides seem to have a strong influence on affinity as determined by fluorescence anisotropy. In contrast, the interaction with the side‐chain amide did not influence affinity. Quantum mechanics calculations were used to analyze the relative contributions of these interactions to the binding energies. No clear correlation could be found between the relative energies of the fluorine–main‐chain amide interactions and the overall binding energy. Instead, dispersion and desolvation effects play a larger role. The results confirm that the contribution of fluorine–amide interactions to protein–ligand interactions cannot simply be predicted, on geometrical considerations alone, but require careful consideration of the energetic components.

Novel oxidation state — zinc(III) in chelate with 3,7,11,15-tetraazaporphine and one fluorine ligand: Quantum-chemical modeling

Based on the results of a quantum chemical calculation using the DFT method, the possibility of the existence of a zinc heteroligand complex with 3,7,11,15-tetraazaporphine and fluoride ion with an oxidation state of Zn(III), unusual for the given element, is shown. Data on the structural parameters and multiplicity of the ground state of this complex are also presented.

About possibility of stabilization of unusual copper(IV) oxidation state in complexes with porphyrazine and two fluorine ligands: Quantum-chemical design

Based on the results of a quantum chemical calculation using the DFT method, the possibility of the existence of a copper heteroligand complex with porphyrazine and two fluoride ions with an oxidation state of IV that is unusual for this element has been shown. Also, data on the structural parameters and multiplicity of the ground state of this complex have been presented.

Terminal and bridging fluorine ligands in TiF4 as studied by 19F NMR in solids

To examine bonding nature of fluorine ligands in a metal coordinated system, 19F high-resolution solid-state NMR has been applied to TiF4, which bears both bridging and terminal fluorines. Observed 12 isotropic signals are assigned to 12 crystallographically different fluorines (6 terminal and 6 bridging fluorines) in TiF4 by referring to the calculated isotropic shifts using density functional theory (DFT). The isotropic chemical shift (δiso) for terminal F (FT) appears at high frequency (420–480 ppm from δ(CCl3F) = 0 ppm) with large shielding anisotropy Δσ ∼ 850 ppm. Whereas the δiso and Δσ values for bridging F (FB) are moderate; δiso ∼ 0–25 ppm and Δσ ∼ 250 ppm. The origin of the observed high-frequency shift for FT is ascribed to the second-order paramagnetic shift with increased covalency, shorter Ti–F bonds, and smaller energy difference between the occupied and vacant orbitals. Examination of the orientation of the shielding tensor relative to the molecular structure shows that the most deshielded component of the shielding tensor is oriented along the Ti–F bond. The characteristic orientation is consistent with a Ti–F σ bond formed by dYZ of Ti and pz of F. Further, we show that the selectively observed spinning sideband patterns and the theoretical patterns with the calculated Δσ and η (shielding asymmetry) values are not consistent with each other for FB, indicating deficiency of the present DFT calculation in evaluating Δσ.

Fluorine shifts from sulfur in dimethylaminodifluorosulfane complexes of cyclopentadienyl metal carbonyls of chromium, molybdenum, and tungsten

Previous theoretical studies predict metal SF3 complexes to be disfavored relative to shift of a fluorine atom from the SF3 ligand to the metal atom to give complexes with separate SF2 and fluorine ligands. We now explore metal complexes of the Me2NSF2 ligand hoping that substituting one of the electron-withdrawing fluorine atoms with an electron releasing dimethylamino group will strengthen the sulfur-fluorine bonds thereby inhibiting fluorine shift from sulfur to metal. The cyclopentadienylmetal carbonyl systems of the group 6 metals chromium, molybdenum, and tungsten were chosen for this study since CpM(CO)2SF2NMe2 (M = Cr, Mo, and W) derivatives with an intact Me2NSF2 ligand would be analogs of the very stable nitrosyls CpM(CO)2(NO). However, for all of the CpM(CO)nSF2NMe2 systems fluorine shift from sulfur to the metal atom occurred in all of the low-energy structures giving structures having separate Me2NSF and fluorine ligands. Isomeric CpM(CO)2SF2NMe2 structures with intact Me2NSF2 ligands bonded to the metal through both sulfur and nitrogen analogous to the known metal nitrosyls CpM(CO)2(NO) lie 13–18 kcal/mol in energy above the lowest energy isomers with split Me2NSF + F ligands.

Ammonium hexafluorosilicates: a new type of anti-caries agents

It is known that dental caries is one of the most common diseases; for children, this pathology ranks first among chronic diseases and has the character of a pandemic. In the modern arsenal of means for the treatment and prevention of caries, fluoride preparations – sodium, potassium fluoride, tin difluoride, sodium monofluoridephosphate, diammine silver fluoride – occupy the leading positions. In recent decades, ammonium hexafluorosilicate and hexafluorosilicates of organic ammonium cations, which have certain advantages over traditional fluoride preparations, have been actively studied as promising anti-caries and hypo-sensitive agents.
 The aim of this review is to systematize and analyze literature data, reflecting existing experience in the field of synthesis methods, studying the structure, physicochemical properties and biological activity of ammonium hexafluorosilicates as potential anti-caries agents.
 The general method for the synthesis of ammonium hexafluorosilicates is the interaction of the corresponding base, or its salt, in methanol, ethanol or another solvent with stoichiometric or excess amount of hydrofluoric acid. The structure of ammonium hexafluorosilicates, which are typical supramolecular compounds, is described. Using the example of pyridinium hexafluorosilicates, the relationship between water solubility and structural characteristics of salts was demonstrated. Identification methods have been proposed for the combination of cetylpyridinium hexafluorosilicate. The results of determination of acute toxicity for a number of hexafluorosilicates in animal experiments are given. Studies of the antibacterial activity of hexafluorosilicates are described. It is shown that hexafluorosilicates significantly reduce the number of carious lesions and their depth, and significantly exceed sodium fluoride in all indicators. The feasibility of their use for the prevention of periodontitis is demonstrated. A technological scheme for obtaining a gel containing cetylpyridinium hexafluoro silicate has been proposed.
 General methods for the synthesis of ammonium hexafluorosilicates include the interaction of an organic base or the corresponding hydrochloride in a solvent medium with an excess of hydrofluoric acid silica. In the crystal structures of hexafluorosilicates, hydrogen bond systems with the participation of fluorine ligands and H-donor fragments of cations perform the main structure-forming and stabilizing function. The solubility of hexafluorosilicates in water varies widely, which is determined by the nature of the ammonium cation and, in the case of pyridinium salts, is antibatically correlated with the number of strong and medium H-bonds in the salt structure. The degree of hydrolysis of hexafluoro silicates in 1×10–4 М aqueous solutions is high and in some cases reaches practically quantitative values. The results of the determination of the acute toxicity of ammonium hexafluorosilicates make it possible to classify these compounds as moderately toxic substances. All studied ammonium hexafluorosilicates exhibit high caries preventive activity, significantly reducing the number and depth of carious lesions, while simultaneously significantly improving the biochemical parameters of dental pulp and periodontal bone tissue. Ammonium hexafluorosilicates can be used as new effective anti-caries and hypo-sensitive agents.