Contents lists available at ScienceDirect Journal of Fluorine Chemistry journal homepage: www.elsevier.com/locate/ fluor Synthesis, crystal structures, properties and caries prevention efficiency of 2-, 3-, 4-carboxymethylpyridinium hexafluorosilicates Vladimir O. Gelmboldt a , Vladimir Yu. Anisimov a , Ivan O. Shyshkin a , Marina S. Fonari b, ⁎ , Victor Ch. Kravtsov b a Odessa National Medical University, Odessa, Ukraine b Institute of Applied Physics Academy of Sciences of R. Moldova, Chisinau, Republic of Moldova ARTICLE INFO Keywords: Hexafluorosilicates Pyridinium cations Crystal structure Hydrogen bonding Solubility Caries prevention efficiency ABSTRACT Three pyridinium hexafluorosilicates with the compositions (LH) 2 [SiF 6 ](I, II, III, where L = 2-, 3-, 4-carbox- ymethylpyridine) were obtained as crystalline solids upon interaction of hexafluorosilicic acid with isomeric carboxymethylpyridines. All compounds were characterized by elemental analysis, IR, NMR 19 F and mass- spectrometry, solubility data, and X-ray crystallography. The binary ionic solids are stabilized by the interplay of intermolecular interactions including strong charge assisted and conventional hydrogen bonds of the NH⋯F, and OH⋯O types along with CH⋯F contacts and π–π stacking interactions. The different supramolecular motifs in I- III are dictated by the different arrangement of the principal binding sites in the isomeric carboxymethylpyr- idines. The relationship between the salts structure and solubility data is discussed. All hexafluorosilicates re- duce the depth and the number of dental caries, and provide efficiency of caries prevention up to 45%. 1. Introduction Nowadays, the tooth decay is one of the most common diseases [1], while for the childhood this pathology ranks first among chronic dis- eases and has a pandemic character. In the arsenal of treatment and prevention of caries, the key positions occupy inorganic fluorides, in- cluding sodium and potassium fluorides, tin difluoride, sodium mono- fluorophosphate, and silver diammine fluoride, [Ag(NH 3 ) 2 ]F (SDF) [2]. In recent years, researchers from Japan and the USA have announced ammonium hexafluorosilicate, (NH 4 ) 2 [SiF 6 ] (AHF) as a promising caries preventive agent [3–9]. Unlike SDF, AHF does not cause staining of hard tooth tissues [10] and provides acid resistance of tooth enamel and dentin [3], prolonged occlusion of tubul dentin [4], and increases the crystallinity of tooth enamel [5,6]. The specificity of the action of AHF as a caries prophylactic and hyposensitive agent is as follows: in saliva (biological fluid) environment, AHF is hydrolyzed to form a so- luble form of silica which catalyzes the precipitation of a mixture of fluorapatite and calcium fluoride [4,7], and the formation of the crys- talline precipitate stable in saliva. In principle, the effectiveness of the anti-caries action of salts with the [SiF 6 ] 2− anion can be increased by using hexafluorosilicates with biologically active “onium” cations. In this case, it becomes possible to enhance the caries-preventive action of the fluoride component as a result of specific (pharmacological, phy- sicochemical) cation effects [11]. In particular, the search for new potential anti-caries agents in the series of hexafluorosilicates whose cations exhibit bactericidal and anti-inflammatory activity is important in a complex of measures in the treatment and prevention of caries [12,13]. This contribution is addressed to the synthesis, study of the struc- ture, properties and caries prevention effect of 2-, 3-, 4-carbox- ymethylpyridinium hexafluorosilicates, whose organic cations are ex- pected to exhibit anti-inflammatory activity based on the PASS- prognosis [14]. 2. Results and discussion 2.1. Crystal structures All compounds were obtained by interaction of relevant N-base with hexafluorosilicic acid in aqueous solution. Crystal structure and re- finement data for I–III are given in Table 1, hydrogen bonding geo- metry is summarized in Table 2. The formula units for I-III are shown in Fig. 1. Compounds I–III have the 1:2 acid : base ratio and crystallize in the monoclinic centrosymmetric space groups P2 1 /n (I), P2 1 /c (II), and I2/a (III) with the [SiF 6 ] 2− anion occupying positions on inversion centers in I, II and on a two-fold axis in III. On the molecular level, the changes in the geometry of components are identical to those pre- viously reported for the related hexafluorosilicates, since the proton https://doi.org/10.1016/j.jfluchem.2017.11.004 Received 13 October 2017; Received in revised form 13 November 2017; Accepted 15 November 2017 ⁎ Corresponding author. E-mail address: fonari.xray@phys.asm.md (M.S. Fonari). Journal of Fluorine Chemistry 205 (2018) 15–21 Available online 16 November 2017 0022-1139/ © 2017 Elsevier B.V. All rights reserved. T