In recent years, several research groups demonstrated the possible usage of “onium”
hexafluorosilicates as new perspective caries preventive and hyposensitive agents in the
practice of dentistry [1-3]. In present communication synthesis, structural characteristics,
properties and potential pharmaceutical application of “onium” hexafluorosilicates with
heterocyclic cations will be discussed.
The crystalline salts with the composition (L1-4H)2SiF6 (L1 = 2-pyridineacetic acid, I; L2 = 3-
pyridineacetic acid, II; L3 = 4-pyridineacetic acid, III; L4 = 2-amino-4,6-
dihydroxypyrimidine, IV) were obtained by the interaction of hexafluorosilicic acid (45 %)
with methanol solution of corresponding “onium” chlorides (L1-3H)Cl (I – III) or free base L4
(IV). The complexes I – IV were characterized by elemental analysis, 1H, 19F NMR, IR,
mass-spectrometry, thermogravimetric analysis, solubility data and X-ray crystallography. In
crystal structures I – IV anion SiF6
2– occupying position on inversion centers in the
centrosymmetric P21/n (I), P21/c (II), and I2/a (III, IV) space groups. The inter-ionic
interactions of the NH···F and OH···F types released in the structures of salts I – III (NH···F
and NH···O type in the case of IV), and for the anion SiF6
2– the symmetry deviation from the
ideal octahedral one is dictated by its closest environment and an involvement of all or part of
the fluoroligands in H-bonds of different strength. Hexafluorosilicate anion in the structures
has the geometry of the distorted octahedron with the Si–F distances running from 1.6479(18)
till 1.6984(12) Å. The solubility data of I – III in water (25 ºC) are 0.82, 1,02 and 0,58 mol.
%, respectively. The pyridinium hexafluorosilicates revealed a tendency toward solubility
decrease with increase the number of strong and medium H-bonds (D···A ≤ 3,2 Å) [3, 4]. The
19F NMR spectra of compounds I – IV in D2O have one sharp and additional broad resonance
peaks. We assigned these peaks as SiF6
2– anion (δF = –132.90 ÷ –133.00 ppm) and the
hydrolysis intermediate [SiF5(H2O)]– (δF = –132.33 ppm). In dilute water solutions (110–4
М), salts I – IV underwent practically quantitative hydrolysis with formation of fluoride-ions
and soluble form of silicon dioxide, which catalyzed the process of formation of calcium
phosphate sediment from saliva [1].
All hexafluorosilicates reduce the depth and the number of dental caries of animals (Wistar
rats), and provide efficiency of caries prevention up to 45 %. Salts III and IV show the
highest caries prevention efficiency, exceeding the same index for NaF in 5 times. The data of
biochemical tests showed almost complete absence of toxic effects in a given dosing, with the
exception of IV (hepatotoxic compound).
[1] A T. Suge, A. Kawasaki, K. Ishikawa et al., Dent. Mater., 2008, 24, 192-198.
[2] Yu. Hosoya, E. Watanabe, K. Tadokoro et al., J. Oral Sci., 2012, 54, 267-272.
[3] V.O. Gelmboldt, E.V. Ganin, M.M. Botoshansky et al., J. Fluor. Chem., 2014, 160, 57-63.
[4] V.O. Gelmboldt, E.V. Ganin, M.S. Fonari et al., J. Fluor. Chem., 2009, 130, 428-433.