Ammonium hexafluorosilicates as potential anti-caries agents: the problem of cation selection

In the last decade, ammonium hexafluorosilicate (AHFS) and ammonium hexafluorosilicates with biologically active cations (AHBAC), which have certain advantages over traditional fluoride medicinal substances, have been actively studied as anti-caries agents. In particular, an important feature of the action of AHFS is its ability to cause prolonged occlusion of the dentinal tubules with a precipitate of calcium fluoride; when using AHBAС there is a possibility of strengthening the anti-caries activity of the substance due to the pharmacological potential of the cation. The purpose of the review is to analyze the effects of the cation on the physicochemical properties and biological activity of ammonium hexafluorosilicates as potential anti-caries agents. Research methods – bibliosemantic, content analysis. It was drew the attention to the peculiarity of the AHBAC structure: salt structures are formed on the basis of systems of strong interionic H-bonds, mainly of the NH···F type, which have a significant effect on the properties of hexafluorosilicates in the crystalline state and their behavior in solutions. It was demonstrated the non-trivial nature of the change of solubility in water of AHBAC with heterocyclic and aromatic cations, which consists in the decrease of solubility with increasing number of hydrophilic fragments in the structure of cations. Adequate 2D QSPR models for interpretation and virtual screening of AHBAC water solubility have been constructed. Accounting for the effect of H-bonds on the solubility of AHBAC was detailed. It was assumed that the process of hydrolysis of AHBAC in aqueous solutions can be stimulated by elongation of the Si–F anion bonds due to the effects of H-bonds. It is shown that the thermal stability of AHBAC with pyridinium cations symbatically correlates with the number of strong and medium H-bonds in salt structures. The action of the pharmacological effects of the cation on the biological activity of AHBAC is manifested in the form of an increase in the caries-prophylactic efficacy of AHBAC in comparison with a similar effect of AHFS. Attempts to establish a relationship between the anti-caries activity of AHBAC and a certain pharmacological action of the cation have led to mixed results. This obviously reflects the complex mechanism of the influence of the biological activity of the cation on the caries-prophylactic efficacy of hexafluorosilicates, which is not limited to any one, albeit dominant, type of activity.

Successive Crystallization of Organically Templated Uranyl Sulfates: Synthesis and Crystal Structures of [pyH](H3O)[(UO2)3(SO4)4(H2O)2], [pyH]2[(UO2)6(SO4)7(H2O)], and [pyH]2[(UO2)2(SO4)3]

Three new uranyl sulfates, [pyH](H3O)[(UO2)3(SO4)4(H2O)2] (1), [pyH]2[(UO2)6(SO4)7(H2O)] (2), and [pyH]2[(UO2)2(SO4)3] (3), were produced upon hydrothermal treatment and successive isothermal evaporation. 1 is monoclinic, P21/c, a = 14.3640(13), b = 10.0910(9), c = 18.8690(17) Å, β = 107.795(2), V = 2604.2(4) Å3, R1 = 0.038; 2 is orthorhombic, C2221, a = 10.1992(8), b = 18.5215(14), c = 22.7187(17) Å, V = 4291.7(6) Å3, R1 = 0.030; 3 is orthorhombic, Pccn, a = 9.7998(8), b = 10.0768(8), c = 20.947(2) Å, V = 2068.5(3) Å3, R1 = 0.055. In the structures of 1 and 2, the uranium polyhedra and SO4 tetrahedra share vertices to form ∞3[(UO2)3(SO4)4(H2O)2]2− and ∞3[(UO2)6(SO4)7(H2O)]2− frameworks featuring channels (12.2 × 6.7 Å in 1 and 12.9 × 6.5 Å in 2), which are occupied by pyridinium cations. The structure of 3 is comprised of ∞2[(UO2)2(SO4)3]2− layers linked by hydrogen bonds donated by pyridinium cations. The compounds 1–3 are formed during recrystallization processes, in which the evaporation of mother liquor leads to a stepwise loss of hydration water.

Crystal structure of pyridinium tetraisothiocyanatodipyridinechromium(III) pyridine monosolvate

In the crystal structure of the title compound, (C5H6N)[Cr(NCS)4(C5H5N)2]·C5H5N, the CrIII ions are octahedrally coordinated by four N-bonding thiocyanate anions and two pyridine ligands into discrete negatively charged complexes, with the CrIII ion, as well as the two pyridine ligands, located on crystallographic mirror planes. The mean planes of the two pyridine ligands are rotated with respect to each other by 90°. Charge balance is achieved by one protonated pyridine molecule that is hydrogen bonded to one additional pyridine solvent molecule, with both located on crystallographic mirror planes and again rotated by exactly 90°. The pyridinium H atom was refined as disordered between both pyridine N atoms in a 70:30 ratio, leading to a linear N—H...N hydrogen bond. In the crystal, discrete complexes are linked by weak C—H...S hydrogen bonds into chains that are connected by additional C—H...S hydrogen bonding via the pyridinium cations and solvent molecules into layers and finally into a three-dimensional network.

Crystal structure, Hirshfeld surface analysis and physicochemical characterization of bis[4-(dimethylamino)pyridinium] di-μ-chlorido-bis[dichloridomercurate(II)]

The title molecular salt, (C7H11N2)2[Hg2Cl6], crystallizes with two 4-(dimethylamino)pyridinium cations (A and B) and two half hexachloridodimercurate(II) anions in the asymmetric unit. The organic cations exhibit essentially the same features with an almost planar pyridyl ring (r.m.s. deviations of 0.0028 and 0.0109 Å), which forms an inclined dihedral angle with the dimethyamino group [3.06 (1) and 1.61 (1)°, respectively]. The dimethylamino groups in the two cations are planar, and the C—N bond lengths are shorter than that in 4-(dimethylamino)pyridine. In the crystal, mixed cation–anion layers lying parallel to the (010) plane are formed through N—H...Cl hydrogen bonds and adjacent layers are linked by C—H...Cl hydrogen bonds, forming a three-dimensional network. The analyses of the calculated Hirshfeld surfaces confirm the relevance of the above intermolecular interactions, but also serve to further differentiate the weaker intermolecular interactions formed by the organic cations and inorganic anions, such as π–π and Cl...Cl interactions. The powder XRD data confirms the phase purity of the crystalline sample. Furthermore, the vibrational absorption bands were identified by IR spectroscopy and the optical properties were studied by using optical UV–visible absorption spectroscopy.