Structural, Vibrational, Thermal, Phase Transitions, and Properties Review of Alkylammonium, Alkylenediammonium, and Aminoacid Hexafluorosilicate Salts

Abstract

The present paper aims to review and discuss structural, thermal, phase transitions, solubility, catalytical, anticaries and toxicity properties of alkylammonium [R_4 – nNH_n]₂SiF₆, alkylenediammonium NH₃(CH₂)_nNH₃SiF₆, and amino acid hexafluorosilicate salts, which have been previously published. Structural comparative analyses are made on the crystal structures, which have been determined for these compounds. The effect of hydrogen bonding on the Si···F elongation bonds is reviewed in terms of solid-state relationship. For the solution event, the hydrogen bonds are discussed in relation to the hexafluorosilicate compounds solubility. The vibrational and thermal properties of the hexafluorosilicates are reviewed and discussed, which lead us to highlight as well as the mechanisms and the characters of the phase transitions undergone by these compounds. The NH₃(CH₂)_nNH₃SiF₆ (n = 4, 6) and (N₂H₅)₂SiF₆ salts are found to be highly efficient novel catalysts in the synthetic protocols for organic compounds such as benzimidazoles, benzothiazole and benzoxazole, 3,4-dihydropyrimidin-2-ones/thiones and quinoxalines derivatives, which is considered as a novel tendency in the hexafluorosilicate researchs, particularly in the green chemistry field. The toxicological and anticaries properties of some hexafluorosilicate compounds are reviewed. Besides, the identification and quantification methods of new potential medical substances based on these salts are discussed.

INTRODUCTION

Hexafluorosilicate compounds of the general formula [R_4 – nNH_n]₂SiF₆ (R = alkyl, n = 0–3) studied over the two last decades [1–19] have been found to present interesting crystal dynamic and phase transitions involving as well as hydrogen bonds and reorientational motions of alkylammonium cations [1–8]. The temperature-dependent phase transitions of such compounds [3–5, 16–18] have been studied by DSC, dielectric, and Raman spectra at different temperatures. The alkylenediammonium salts NH₃(CH₂)_n-NH₃SiF₆ constitute a new important family of hexafluorosilicate compounds [20–27]. Among this family, the NH₃(CH₂)₂NH₃SiF₆ is the first compound to be studied [20]. More recently, research reporting X-rays, DSC-TGA, vibrational, DFT modeling, and catalytical properties have been developped on NH₃(CH₂)_nNH₃SiF₆ (n = 3, 4, 6) compounds [22–27]. The research interest in the hexafluorosilicates becomes to shift to the catalysts, optical materials, fluorinating agents, and pharmaceutical substances fields.

The first serious review study, which leads to summarize and systematize the research results published mainly after 2000 on the synthetic protocols, structure and spectral characteristics, solubility and hydrolysis, properties and the practical application aspects of the hexafluorosilicate salts has been developed by Gelmboldt et al. [13]. The review study reflected the various aspects state of chemistry and applied use of hexafluorosilicate salts obtained up 2019: modifiers of zeolite catalysts, fluoridating agents, activating additives and catalysts, preparation of amorphous silica, ionic liquids, and electro-optical materials.

The important work review has focused mainly on the synthesis and the properties of hexafluorosilicate salts [13]. Nevertheless, the structural data, vibrational characteristics, phase transitions, and thermal properties have not been fully reviewed and detailed for these compounds. Hence, the present review aims to highlight the structural, vibrational, thermal, and catalytical, toxicological, and anticaries studies, which have been developped up to now on alkylammonium, alkylenediammonium, and amino acid hexafluorosilicate compounds. The vibrational spectroscopy is used as a powerful tool to describe the behaviour of the cations, particularly in terms of their ability to form hydrogen bonds, and their contribution to the distortion of the ${\text{SiF}}_{6}^{{2-}}$ octahedral geometry from its free symmetry O_h in the crystals.

EXPERIMENTAL

Synthesis methods. We give here just the principle of the synthesis methods via some examples, because of a detailed review study on the different synthesis methods of the hexafluorosilicate compounds was developed in [13]. Indeed, the alkylammonium hexafluorosilicate [R_4 – nNH_n]₂SiF₆ crystals were obtained generally by slow evaporations at room temperature of aqueous solutions containing stoichiometric amounts of organic alkylamine (or hydroxide alkylammonium) and H₂SiF₆ acid [1–5]. Another synthesis method of these compounds consists to prepare firstly a solution of alkylammonium fluoride by adding an alkylamine drop wise and slowly to hydrogen fluoride. Secondly, the silica gel was slowly added to the stirred alkylammonium fluoride solution. After straining the mixture solution, the solvent was slowly evaporated, which yield crystalline salts [14]. Alkylenediammonium NH₃(CH₂)_nNH₃SiF₆ crystals were obtained by slow evaporation at room temperature of aqueous solutions containing stoichiometric amounts of the appropriate alkylenediamine and the H₂SiF₆ acid [20–24, 27].

Structural data analysis. The crystallographic data established elswhere on alkylammonium [1–5, 11, 12, 14–18, 28–30] and alkylenediammonium [20–23, 27, 29, 31–35] hexafluorosilicate compounds are shown in Tables S1 and S2, respectively. The crystal structures characteristics are discussed for these compounds.

Alkylammonium salts review. The crystal structures have not been fully determined for several alkylammonium hexafluorosilicate salts given in the Table S1 [1–5, 12, 15–18]. Only the crystallographic data of the unit cells are determined for the cited compounds. However, the structures of some hexafluorosilicate salts with nitrogen-containing organic cations are determined and well described [11, 28–30]. The Table S1 regroups hexafluorosilicate compounds as well as with aliphatic [1–5, 11, 12, 14–16] and cyclic cations [14, 17, 18, 28, 29].

The tetramethylammonium hexafluorosilicate [(CH₃)₄N]₂SiF₆ has been studied by X-ray diffraction, IR and Raman analyses [1]. Its structure was purposed at ambient temperature in the faces-centred cubic (antifluorite structure type) with the space group Fm3m (Z = 4) and parameter: a = 11.172 Å [1]. The ${\text{SiF}}_{6}^{{2-}}$ anions were expected to occupy the O_h sites symmetry, while the (CH₃)₄N⁺ cations were assumed to have the T_d symmetry. The octahedral anions and the tetrahedral cations have been considered as not distorted from their ideal symmetries.

When one –CH₃ group of [(CH₃)₄N]₂SiF₆ is substitued by a hydrogen atom, then the obtained compound [(CH₃)₃NH]₂SiF₆ crystallized in the cubic system with space group P4₁32 (Z = 2) and the parameter a = 11.688 Å, which is slightly increased rather than that observed in the [(CH₃)₄N]₂SiF₆ compound. Based on structural phase transitions develooped theoretically in the crystal with Fm3m symmetry [36], the [(CH₃)₃NH]₂SiF₆ structure may be considered as derived by a possible structural phase transition taken place at the W-point of the Brillouin zone in the [(CH₃)₄N]₂SiF₆ of Fm3m symmetry.

The substitution of all –CH₃ groups by –C₂H₅ gives rise to the tetraethylammonium hexafluorosilicate [(C₂H₅)₄N]₂SiF₆ compound, crystallizing in the monoclinic system of with the space group P2₁ (Z = 2) and the parametrs: a = 13.430 Å, b = 13.825 Å, c = 12.940 Å, β = 90.894° [3, 5]. The space group P2₁ is considered as a sub-group of Fm3m, implying that the [(C₂H₅)₄N]₂SiF₆ crystal structure may be derived by a structural phase transition taken place at the X-point of the Brillouin zone of the [(CH₃)₄N]₂SiF₆ compound [36]. In [(C₂H₅)₄N]₂SiF₆ structure, as well the ${\text{SiF}}_{6}^{{2-}}$ anions and (C₂H₅)₄N⁺ cations were expected to occupy the C₁ sites symmetry, meaning that the anions and the cations were extensively distorted in this crystal [3, 5].

The diethylammonium hexafluorosilicate [(C₂H₅)₂NH₂]₂SiF₆ crystallized in the P2₁ space group (a = 7.22 Å, b = 10.27 Å, c = 11.70 Å, β = 102.90°) [16]. It is clearly noted that a, and β parametrs changed more remarkably, when passing from tetraethylammonium to diethylammonium structures, all monoclinic (P2₁ space group).

Concerning hexafluorosilicates containing mono-alkylammoniums with –NH₃ group, the [(C₂H₅)-NH₃]₂SiF₆ and [(C₄H₉)NH₃]₂SiF₆ compounds have too the trigonal symmetry: $P\bar {3}m$; a = 10.429, c = 10.065 Å [15] and P321 (Z = 1); a = 8.861, c = 5.038 Å [2, 16], respectively. Since the P321 is a sub-group of the $P\bar {3}m$ space group, then there is relationship symmetry between the two monoalkylammonium structures. So, the butylammonium salt structure may be derived from the ethylammonium structure by losing the m mirror, and the c parameter passes from 10.065 Å in ethylammonium structure to 5.038 Å in the derived butylammonium structure.

For the aromatic-alkylammoniums hexafluorosilicate, the [C₆H₅NH₃]₂SiF₆ compound is crystallized in the orthorhombic system with Pmmm (Z = 2) as space group and parametrs: a = 5.540, b = 9.196, c = 17.165 Å, β = 90.894° [17, 18]; the others aromatic-compounds given in Table S1 are crystallized in the monoclinic and triclinic systems [28, 29]. As an example, the [C₃H₆N]₂SiF₆ was found to be monoclinic of space group P2₁/C (Z = 4) and parametrs: a = 9.444(1), b = 11.511(1), c = 10.167(2) Å, β = 100.62(4)° [28].

The hexafluorosilicate salts of protonated methyl substituted pyridines, R₂SiF₆ (R = 2-picolinium and 2,6-lutidinium) have been studied, especially their structural and vibrational characterisations. The 2,6-lutidinium [(CH₃)₂C₅H₃NH]₂SiF₆ compound is crystallized in the monoclinic system with the space group C2/m (Z = 2; a = 15.8171(4), b = 6.4567(2), c = 8.5075(2) Å, β = 114.694(1)°) [29]. In the cationic part of this compound, there are two –CH₃ groups linked to the aromatic ring, C₅H₃NH. Besides, they are found chains consisting of alternating 2,6-lutidinium cations and ${\text{SiF}}_{6}^{{2-}}$ anions along b-axes (Fig. 1). Within these chains, the cations and anions were linked by bifurcated N–H···F hydrogen bonds of 2.14(1) Å order (for H···F lengths). Noting that only two fluorine atoms of each ${\text{SiF}}_{6}^{{2-}}$ anion is involved in the hydrogen bond with the protonated nitrogen of the cations in this compound.

Fig. 1.

The crystal structure scheme of [(CH₃)₂C₅H₃NH]₂SiF₆ viewed along the [0 0 1] direction. The hydrogen atoms on methyl groups and aromatic rings have been removed for clarity [29].

The 2-picolinium [(CH₃)C₅H₄NH]₂SiF₆ salt containing one –CH₃ group linked to the aromatic-ring was found to be crystallized in the monoclinic system, but with a different space group P2₁/c (Z = 2) and the following parametrs: a = 6.8401(2), b = 13.3041(4), c = 7.9192(2) Å, β = 91.499(2)° [29]. The structure of this compound is built from hydrogen-bonded units consisting of two cations and the anion, as illustrated in the Fig. 2. In contrast to the 2,6-lutidinium compound, four fluorine atoms of each ${\text{SiF}}_{6}^{{2-}}$ anion are involved in four bifurcated N–H···F hydrogen bonds with two 2-picolinium cations, in the way that two fluorine atoms considered in apical positions F(1) form strong (1.89(2) Å) and two equatorial F(2) fluorine atoms form weak (2.735(2) Å) hydrogen bonds to the picolinium cations.

Fig. 2.

The crystal structure scheme of [(CH₃)C₅H₄NH]₂SiF₆ along the [1 0 0] direction highlighting the π-π interactions between the 2-picolinium cations (with A–A₀ length = 4.013 Å). The hydrogen atoms on methyl groups and aromatic rings have been removed for clarity [29].

The structural change between these two compounds is accompanied by an important distortion of the β angle from 91.499(2)° in the compound with one –CH₃ group to 114.694(1)° in the compound with two –CH₃ groups. The distortion is traduced by the lowering symmetry when passing from the C2/m (Z = 2) phase to the P2₁/c (Z = 2) one, accordingly to the modification of the Bravais lattice symmetry of C to P.

Recently, we have synthesised and characterised by X-ray, IR, and Hirshfeld surfaces analyses a novel hexafluorosilicate based on hydrazinium (1+), (N₂H₅)₂SiF₆ [11]; the crystal structure of this compound is formed by two ${\text{(}}{{{\text{N}}}_{{\text{2}}}}{{{\text{H}}}_{{\text{5}}}}{\text{)}}_{{\text{2}}}^{{{\text{2 + }}}}$ cations and an anion ${\text{SiF}}_{6}^{{2-}}$, which are linked together through hydrogen bonds N–H···F. The surface analysis (2D fingerprint plot,) showed that the F···H/H···F intermolecular interactions occupy the important area of the total Hirshfeld surfaces (75.5%), which implied that hydrogen bonds are predominant as intermolecular contacts in the crystal [11].

Comparative study of alkylenediammonium salts. The crystal structures of the reviewed alkylenediammonium hexafluorosilicate salts were completely determined [21–23], which allows us to make such comparative study between the determined compounds’ crystal structures. The comparative study concerns mainly the compounds of general formula NH₃(CH₂)_nNH₃SiF₆ (n = 0, 2, 3, 4, 6). The crystallographic data of these hexafluorosilicate are given in Table S2.

The hydrazinium (2+) hexafluorosilicate NH₃NH₃SiF₆ crystallizes at room temperature in the orthorhombic system with the space group Pbca (Z = 4) and the parametrs: a = 7.603, b = 7.594, c = 8.543 Å [20, 31]. Its structure consists of centrosymmetric ${{{\text{N}}}_{{\text{2}}}}{\text{H}}_{6}^{{2 + }}$ and ${\text{SiF}}_{6}^{{2-}}$, ions arranged in a NaC1-type packing and connected by hydrogen bonds N–H···F ranging from 1.889 to 2.207 Å, and forming three-dimensional hydrogen-bonding networks [31]. The ${\text{SiF}}_{6}^{{2-}}$ anions are observed in inversion centres compatibly to the Wyckoff positions 4a (1/2, 0, 1/2) of the space group Pbca. For the hydrazinium ⁺NH₃N${\text{H}}_{3}^{ + }$ cation, the bond N(1)–Ni centre is situated on the crystallographic inversion centre, corresponding to the 4b Wyckoff positions.

The ethylendiammonium hexafluorosilicate NH₃(CH₂)₂NH₃SiF₆ [21] crystallized in the monoclinic structure P2₁/n (Z = 2, a = 7.351(3), b = 8.732(3), c = 5.819(2) Å, β = 93.84(1)°), which is consisted of ${\text{SiF}}_{6}^{{2-}}$ octahedra located on inversion centres corresponding to the Wyckoff positions: 2d (1/2, 1/2, 0); 2c (0, 0, 1/2) and ethylendiammonium cations either placed on inversion centres: 2a (0, 0, 0); 2b (1/2, 1/2, 1/2).

In the structure of NH₃(CH₂)₄NH₃SiF₆, crystallized in the triclinic system ($P\bar {1}$, Z = 1, a = 5.796(1), b = 5.889(1), c = 7.774(2) Å, α = 87.02(1)°, β = 82.15(1)°, γ = 61.87(1)°), the ${\text{SiF}}_{6}^{{2-}}$ anion is observed on an inversion center C_i(1) compatibly with Wyckoff the position 1d (1/2, 0, 0), the centre of the bond C(2)–C(2i) of the butylammonium cation is situated on another crystallographic inversion centre [22].

The crystal structure of NH₃(CH₂)₆NH₃SiF₆ monoclinic $P\bar {1}$ (Z = 2, a = 5.8965(2), b = 13.6946(5), c = 14.4945(5) Å, α = 91.379(2)°, β = 92.797(2)°, γ = 90.906(2)°), is built up from two ${\text{SiF}}_{6}^{{2-}}$ anions and two organic cations ⁺NH₃(CH₂)₆N${\text{H}}_{3}^{ + }$ [23]. An anion Si(1) observed in the general position, i.e. in the Wyckoff position (x, y, z) corresponding to C₁ symmetry and two moiety of anions located at inversion centres of the $P\bar {1}$ space group: Si(2) on the Wyckoff position 1g (0, 1/2, 1/2) and Si(3) on 1a (0, 0, 0). The unit cell contains one organic cation and two halves of cations located about an inversion centre.

It is mentioning that the monoclinic structure (with n = 2) becomes triclinic of space group $P\bar {1}$ with Z = 1 and Z = 2, when n is equal to 4 and 6, respectively. In the P2₁/n space group, there are four sets of non-equivalent inversion centres 4C_i(2), while in the $P\bar {1}$ there are night nonequivalents inversion centres 8C_i(1).

In all NH₃(CH₂)_nNH₃SiF₆ structures, as well as ⁺NH₃(CH₂)_nN${\text{H}}_{3}^{ + }$ cations and ${\text{SiF}}_{6}^{{2-}}$ anions were linked with N–H⋅⋅⋅F hydrogen bonding, ensuring the three-dimensional cohesion of the crystals. Indeed, four significant N–H···F hydrogen bonding (2.864(2), 2.890(2), 3.017(2), 3.134(2) Å) have been determined by X-ray’s study for the compound (n = 4) [22]. When n increases to 6, the compound presents 17 N–H···F hydrogen bonding ranging from 2.832(3) to 3.095(3) Å [23]. Hence, the compound (n = 6) presents several and strong hydrogen bonds rather than the compound with n = 4. The hydrogen bonding scheme illustrations in the two compounds are given in the Figs. 3 and 4.

Fig. 3.

The molecular structure of the NH₃(CH₂)₄-NH₃-SiF₆ compound with the atom-labeling scheme. Symmetry codes : (i) –x – 1, –y + 1, –z + 1; (ii) –x + 1, ‒y, –z [22].

Fig. 4.

The molecular structure of the NH₃(CH₂)₆NH₃SiF₆ compound with the atom-labeling scheme. Symmetry codes : (i) ‒x, 1 – y, –z ; (ii) –x, 2 – y, –z ; (iii) –x, 1 – y, 1 – z ; (iv) –x, 2 –y, 1 – z [23].

For the compounds containing aliphatic alkylenediammonium, the V/Z variation (V: unit cell volume, Z: number of motifs per unit cell) versus the –CH₂ groups’ number (n = 0, 2, 4, 6) in the H₃N-(CH₂)_nNH₃SiF₆ compounds drawn and given in Fig. 5. The curve variation is not linear; the V/Z value decreases from 124 ų for n = 0 (orthorhombic) to 74 Å³ for n = 2 (monoclinic). When passing to the triclinic system, the V/Z value increases to reach 234 ų when n = 4, and 580 ų at n = 6. The number of CH₂ groups impact of on the unit cell parametrs and volume values is clearly shown. The observed change takes place when passing from the orthorhombic system (n = 0) to the monoclinic one (n = 2), and from the monoclinic to the triclinic system (n = 4).

Fig. 5.

The variation of the unit cell volume (V)/number of motifs per cell (Z) versus the number of CH₂ groups in the hexafluorosilicate NH₃(CH₂)_nNH₃SiF₆ (n = 0, 2, 4, 6).

Recently, a primary single crystal X-ray diffraction characterization made at ambient temperature showed that the NH₃(CH₂)₃NH₃SiF₆ compound crystallizes in the monoclinic system with the space group P2₁/c (Z = 4) and parametrs : a = 18.871(4), b = 5.879(2), c = 17.345(3) Å; β = 117.331(3)° [27]. The full crystal structure is not yet determined.

The phenylenediammonium hexafluorosilicate has been synthetised by studying the interactions in the H₂SiF₆−o-phenylenediamine−H₂O (FSA−PDA−H₂O) system at 25°C, using the isothermal solubility method [32]. The solid phase obtained in this system, formulated as (o-PDAH₂)SiF₆ (23.70–44.60 wt % H₂SiF₆), crystallized in the orthorhombic, with the space group Pmmn (Z = 2), and the parametrs: a = 5.5269(6), b = 7.6648(7), c = 11.2594(9) Å. In this structure, the o-PDA cations and the ${\text{SiF}}_{6}^{{2-}}$ anions were linked through hydrogen bonds of NH···F type (N···F: 2.865(2)–2.967(2) Å) to form a two-dimensional network.

The crystal structure of (C₄N₂H₁₂)SiF_6, monoclinic (C2/c (Z = 4); a = 10.577(5), b = 7.317(5), c = 11.598(5) Å, β = 99.168(5)°), consisted of isolated ${\text{SiF}}_{6}^{{2-}}$ octahedra anions and di-protonated piperazine cations, which are connected to each other via hydrogen bonds to form a three-dimensional network [33].

The hexafluorosilicate salts [2,2'(C₅H₄)₂(NH)₂]-SiF₆ and [4,4'(C₅H₄)₂(NH)₂]SiF₆ [34] crystallized in the centrosymmetric monoclinic I2/a (Z = 4, a = 12.396(4), b = 6.6042(7), c = 13.475(3) Å, β = 92.46(4)°) and orthorhombic Pbcn (Z = 4; a = 11.897(2), b = 7.188(2), c = 12.882(3) Å) space groups, respectively. As observed, the a, b, and c parametrs are not greatly changed between the two compounds structures. The passage from the orthorhombic to the monoclinic system was done by a slight distortion, traduced by the 92.46(4)° value of β in the monoclinic structure of [2,2'(C₅H₄)₂(NH)₂]SiF₆. In these structures, the ${\text{SiF}}_{6}^{{2-}}$ anions occupied inversion centres positions, and the bipyridinium ligands resided on the two-fold axes that cross the middle of single C–C bonds of the dications. The ${\text{SiF}}_{6}^{{2-}}$ anions in these structures were distorted from octahedron geometry, with the Si-F distances varying from 1.658(2) to 1.706(2) Å. The anionic and cationic species were held together via strong N−H···F hydrogen bonds, of 1.77(3) Å in [2,2'(C₅H₄)₂(NH)₂]SiF₆ and 1.82(3) Å in [4-4'(C₅H₄)₂(NH)₂]SiF₆ compounds. These compounds have been characterized by the high tendency to hydrolysis in dilute aqueous solutions with the formation of silica and fluoride anions that allowed considering these salts as potential caries-protected agents [34].

The crystallographic data of [(CH₃)₂NH(CH₂)₂-NH(CH₃)₂]SiF₆·2H₂O [29], which is an example of hexafluorosilicate salts of protonated methyl substituted pyridines and tetramethylethylenediamine is given in the Table S2. This compound crystallized in the triclinic system $P\bar {1}$ (Z = 1), with the parameters a = 6.3753(2), b = 7.6256(2), c = 7.8004(3) Å, α = 117.849(2)°, β = 104.368(2)°, γ = 91.689(2)°.

The crystal structure of the N,N-dimethylbiguanidinium hexafluorosilicate salt [(CH₃)₂NC(NH₂)-NHC(NH₂)NH₂]SiF₆ has been determined and studied [35]. This compound is of monoclinic system with the space group P2₁/c (Z = 4) and the parameters: a = 7.4346(10), b = 12.7628(10), c = 11.0828(10) Å, β = 104.080(10)°. The crystal structure of this salt is consisted of ${\text{SiF}}_{6}^{{2-}}$ anions and N,N-dimethylbiguanidinium cations, which are combined in a framework by medium to strong interionic H-bonds of N–H···F type, varying from 1.85(4) to 2.27(3) Å for the N–H···F bonds.

Others hexafluorosilicate salts. The present study is interested not only in alkylammonium and alkylenediammonium hexafluorosilicate compounds, but it aims also to extend reviewing to other interesting topic of hexafluorosilicates [8, 14, 37–47].

The crystal structures of an important class containing over 40 hexafluorosilicate salts of amino acids have been established and studied [43–47]. Indeed, the crystal structure of N-methylpiperidine betaine hexafluorosilicate has been determined at 100 K in monoclinic system with space group P2₁/c (Z = 4, a = 15.038(3), b = 10.688(2), c = 13.950(3) Å, β = 112.93(3)°) [43]; it consisted of two protonated betaines and one ${\text{SiF}}_{6}^{{2-}}$ anion. In the cations, the piperidinium ring was found to be of chair conformations, but the CH₂COOH group is axial in one cation and equatorial in the other. In the crystal, two medium-strong hydrogen bonds of O–H···F type with d(O···F) equal to 2.565(1) and 2.617(Å) were taken place between the two protonated betaines and ${\text{SiF}}_{6}^{{2-}}$ anion.

The hexafluorosilicate salts of α-alanine have investigated in [44]. Both L-alanine (L-Ala) and DL-alanine (DL-Ala) form hexafluorosilicate salts according to the main 2A⁺${\text{SiF}}_{6}^{{2-}}$ type: 2L-Ala⁺${\text{SiF}}_{6}^{{2-}}$· 3H₂O and 2DL-Ala⁺${\text{SiF}}_{6}^{{2-}}$·2H₂O. The L-alanine formed a new A⁺(A···A⁺)${\text{SiF}}_{6}^{{2-}}$ salt type: L-Ala⁺(L-Ala···L-Ala⁺)${\text{SiF}}_{6}^{{2-}}$·H₂O, where A···A⁺ is dimeric cation held together by a strong hydrogen bond. The structural and vibrational spectroscopic propreties have been described and discussed for these aminoacid hexafluorosilicate salts [44].

Three types of salts of amino acid hexafluorosilicates with singly charged cations have been also investigated in [45]: (i) 2A⁺${\text{SiF}}_{6}^{{2-}}$ (2Gly⁺${\text{SiF}}_{6}^{{2-}}$, 2L-Phe⁺-${\text{SiF}}_{6}^{{2-}}$, 2L-Val⁺${\text{SiF}}_{6}^{{2-}}$, 2L-Glu⁺${\text{SiF}}_{6}^{{2-}}$, 2L-His⁺-${\text{SiF}}_{6}^{{2-}}$), (ii) A²⁺${\text{SiF}}_{6}^{{2-}}$ (L-His²⁺${\text{SiF}}_{6}^{{2-}}$, L-Lys²⁺${\text{SiF}}_{6}^{{2-}}$, L-Orn²⁺${\text{SiF}}_{6}^{{2-}}$), and (iii) 2(A⁺···A)${\text{SiF}}_{6}^{{2-}}$ (2(Bet··· Bet⁺)${\text{SiF}}_{6}^{{2-}}$), where A is an amino acid in zwitterionic state, A⁺ is a singly charged cation, A²⁺ is a doubly charged cation and (A···A⁺) is a dimeric cation with a short hydrogen bond. The indicated amino acids are: glycine (Gly), L-phenylalanine (L-Phe), L‑valine (L‑Val), L-glutamine (L-Glu), L-histidine (L-His), L-lysine (L-Lys), L-ornithine (L-Orn), betaine (Bet).

Single crystal data of eight hexafluorosilicate salts of amino acids: β alanine (β-Ala), betaine (Bet), L‑ornithine (L-Orn), L-serine (L-Ser), L-leucine (L-Leu), L-isoleucine (L-Ile), L-methionine (L‑Met) and L-aspartic acid (L-Asp) have presented and discussed in [46]. The corresponding crystals obtained are 2(β-AlaH)SiF₆, 2(BetH)SiF₆, 2(L-OrnH)SiF₆, 2(L-SerH)SiF₆, 2(L-LeuH)SiF₆·2H₂O, 2(L-IleH)SiF₆·2H₂O, 2(L-MetH)SiF₆·H₂O, and 2(L-AspH)SiF₆·2H₂O. Among these salts, four structures were found anhydrous, and other four ones were hydrated. Analysing the structural properties of these compounds and 22 other amino acid hexafluorosilicates [46], the authors have found that the ${\text{SiF}}_{6}^{{2-}}$ anions, generally ordered, are in octahedral coordination in the majority of the studied and reviewed salts. The ${\text{SiF}}_{6}^{{2-}}$ anions possess more or less octahedral geometry with well-defined fluorine positions. In some cases, the ordered and centrosymmetric anions are located on special positions, which are expressed in their local symmetry (Fig. 6a), however, the most anions, ordered and non-centrosymmetric, were located on general positions with point symmetry C₁ (Fig. 6b). Nevertheless, the rotational disorder behaviour occurred in four cases (2(L-AlaH)SiF₆· 3H₂O, 4(BetH)SiF₆, 2(L-OrnH)SiF₆, 2(L-ValH)-SiF₆) for the equatorial fluorine atoms (Fig. 6c). For the (Gly)₂H₂SiF₆·2H₂O compound, the disorder was found for all atoms as shown in the Fig. 6d.

Fig. 6.

The ${\text{SiF}}_{6}^{{2 - }}$ anions in amino acids: ordered and centrosymmetric, e.g. in (b-Ala)₂H₂SiF₆ (a); ordered, but non symmetric, e.g. in (L-Leu)₂H₂SiF₆·2H₂O (b); rotational disorder, e.g. in (L-Ala)₂H₂SiF₆·3H₂O (c); complete disorder in (Gly)₂H₂SiF₆· 2H₂O [46] (d).

It was concluded that the majority of salts of amino acids with ${\text{SiF}}_{6}^{{2-}}$ anion belong to the 2A⁺${\text{SiF}}_{6}^{{2-}}$ type; such compounds tend to form crystal hydrates, and the ${\text{SiF}}_{6}^{{2-}}$ anion is often disordered within the structures [45, 46]. Analysing all structural data obtained on amino acid hexafluorosilicates, it has been showed that the disorder correlates with the degree of hydrogen bonding [46], so that more hydrogen bonds stabilize the fluorine positions and lead to ordered structures. The authors revealed that the amount of hydrogen bonding towards the ${\text{SiF}}_{6}^{{2-}}$ anion is an indication of disorder; they concluded that in all crystals with 0.6 or less hydrogen bond per fluorine atom are disordered.

Three hexafluorosilicate salts belonging to the sarcosine−H₂SiF₆−H₂O system, which are 2Sar⁺${\text{SiF}}_{6}^{{2-}}$, Sar⁺(Sar···Sar⁺)${\text{SiF}}_{6}^{{2-}}$·2H₂O and 2(Sar···Sar⁺)${\text{SiF}}_{6}^{{2-}}$ have synthesized and studied in [47]. The crystal structures at 296 K as well as the thermal expansion of the three crystals have been determined. The compounds crystallized in the system monoclinic P2₁/n (Z = 2), orthorhombic Pnma (Z = 4), and monoclinic P2₁/c (Z = 2) respectively. The structure of the second compound is beside determined at 150 K in the system orthorhombic, but with the space group P2₁2₁2₁ (Z = 4). In the structure of Sar⁺(Sar…Sar⁺)${\text{SiF}}_{6}^{{2-}}$·2H₂O, the disorder was observed in all three carboxyl groups between the protonated –COOH and deprotonated ‒COO^– states. Thus, the symmetry-related pair of two moieties (Fig. 7b) comprise a dimeric sarcosinium/sarcosine pair, whereas the third sarcosinium moiety (Fig. 7a) extends its O–H hydrogen bond toward a water molecule. Determining the structure of the dihydrate hexafluorosilicate salt at 150 K, it was established [47] that this compound had a lower symmetry (space group P2₁2₁2₁, Z = 4), where occurred the lower temperature ordering of the disordered hydrogen, which means that a phase transition occurred to a lower symmetry structure. Although the atom positions remain in their place, the disorder of the acid hydrogen atom of the dimeric unit disappears. The disordered water molecule O(2w) also was found to be ordered at 150 K, and placed slightly off the mirror plane in the respective higher space group. A comparison of the hydrogen bond situation at 296 and 150 K is shown in the Fig. 8.

Fig. 7.

View of the hydrogen bond arrangement in Sar⁺(Sar…Sar⁺)${\text{SiF}}_{6}^{{2 - }}$·2H₂O at 296 (a) and 150 K (b) [47].

Fig. 8.

Atom labelling in the molecular structure of Sar⁺(Sar…Sar⁺)${\text{SiF}}_{6}^{{2 - }}$·2H₂O. The disorder is observed for the hydrogen atom H(1B) in the dimeric unit and for the O(2w) hydrogen atom [47].

Recently, the synthesis, the crystal structure studies, the physicochemical properties, and anticaries activity of novel potential anticaries hexafluorosilicate substances are developped [48, 49]. The 2-amino-4,6-dihydroxypyrimidinium hexafluorosilicate (L¹H)₂-SiF₆) crystallized in the monoclinic system (I2/a, Z = 4). The structure of this salt was constructed on the basis of two L¹H⁺ pyrimidinium cations and an ${\text{SiF}}_{6}^{{2-}}$ anion, which are linked by the NH···F and CH···F hydrogen bond contacts [48].

The structures of three pyridinium hexafluorosilicate compounds (LH)₂SiF₆ (L = 2-,3-,4-carboxymethylpyridine) with the general formula (C₁₄H₁₆N₂O₄)SiF₆ have been studied [49]. The X-ray results showed that the compound with 2-,3-,4-carboxymethylpyridine crystallized in the (P2₁/n, Z = 2), (P2₁/c, Z = 2), and (I2/a, Z = 4), respectively. The ${\text{SiF}}_{6}^{{2-}}$ anion occupied positions on inversion centres in 2-,3-carboxymethylpyridine compounds, and on a two-fold axis in 4-carboxymethylpyridine salt. These compounds were stabilized by the interplay of intermolecular interactions including strong charge assisted and conventional hydrogen bonds of NH···F and OH···O types along with CH···F contacts and π–π stacking interactions. The different supramolecular motifs in these compounds are due to the different arrangement of the principal binding sites in the isomeric carboxymethylpyridines, as shown in the Fig. 9. These hexafluorosilicate salts have been found to be efficient in caries prevention [49].

Fig. 9.

Ortep plots with labeling scheme of the formula units in the (LH)₂SiF₆ ((a, b, c), where L = 2-,3-,4-carboxymethylpyridine) compounds [49].

Hydrogen bonding in hexafluorosilicate salts. The specificity of these compounds lies in the dominant H-bonds role in the formation of hexafluorosilicate salt structures with alkylammonium, alkylenediammonium and amino acid cations. It was concluded that the high H-acceptor ability of the ${\text{SiF}}_{6}^{{2-}}$ anions leads to a noticeable influence of the H-bond effects on macroscopic properties of hexafluorosilicates, as water solubility and thermal stability [13].

The most known hexafluorosilicate structures with nitrogen-containing organic cations exhibit extensive hydrogen-bonding networks that incorporate all fluorine atoms of ${\text{SiF}}_{6}^{{2-}}$ [13, 29, 37, 38, 48, 49]. The variations in hydrogen bonding interactions in these salts result in the formation of discrete cation–anion unit in the crystal such as in the [(CH₃)C₅H₄NH]₂SiF₆ compound, the formation of chain structure as in [(CH₃)₂C₅H₃NH]₂SiF₆, and the formation of polymeric layer as in [(CH₃)₂NH(CH₂)₂NH(CH₃)₂]SiF₆· 2H₂O [29].

The elongation of Si–F bonds as a result of participation of the fluorine atoms in the hydrogen bond D–H···F have been observed and correlated with hydrogen bond strength. Even if the Si–F bond lengths in the ${\text{SiF}}_{6}^{{2-}}$ anions in these compound structures increase due to hydrogen bonding, the FSF angles of the octahedral anions are found to be closed to 90° and 180° [29]. The unperturbed octahedral geometry of ${\text{SiF}}_{6}^{{2-}}$ anion was also found in some hexafluorosilicate complexes with alkylammonium cations and with other organic cations [14, 37, 39].

In the pyridinium hexafluorosilicates (LH)₂SiF₆ (L = 2-,3-,4-carboxymethylpyridine) [49], the ionic components are held together via a couple of charge-assisted NH···F hydrogen bonds, for which N···F distances vary in the 2.683(2)–2.825(4) Å range. While each anion binds four cations, each cation bridges two ${\text{SiF}}_{6}^{{2-}}$ anions acting besides the hydrogen bonds of NH···F type, also via COOH···F hydrogen bonds, for which the O···F distances vary between 2.583(3) and 2.6525(18) Å. These motifs are realized as three-dimensional H-bonded network in (LH)₂SiF₆ (L = 2‑carboxymethylpyridine), as one-dimensional double tapes in the compound with (L = 3-carboxymethylpyridine) and as two-dimensional layers in the salt (L = 4- carboxymethylpyridine).

Thermal, vibrational, and phase transitions properties. As mentioned in the structural part, the crystal structures of several hexafluorosilicate compounds are not yet fully determined [1–5, 12, 15–18, 27]. The structural characteristics of these compounds, such as the hydrogen bonding, the anions and cations symmetry in the crystals, and the phase transitions, have been investigated using the IR and Raman spectra [1–5, 12, 15–18, 27]. The assignment of the observed bands has been discussed for the studied compounds, showing that the cations and anions, weakly to strongly hydrogen-bonded to each other, are generally distorted inside the crystals, from their free symmetries.

In the infrared spectra of the hexafluorosilicate compounds [1–5, 12, 15–18, 27], weak to medium bands were observed generally at lower frequencies of NH and CH stretching spectral regions, which have been assigned to the combination and overtone modes. In several cases, these non-fundamental bands normally possess appreciable intensities, due to the Fermi resonance. The observation of these bands was considered as indication of the presence of medium to strong hydrogen bonds in the hexafluorosilicate compounds, connecting the cations and the anions. Generally, the CH stretching frequencies are lower than NH ones; the formation of N–H…F hydrogen bonding causes shifting of NH stretching toward lows frequencies, which make it difficult to distinguish between NH and CH stretching modes [12].

The vibrational studies of the alkylammonium hexafluorosilicate compounds spectra were made on the basis of theoretical group analyses, which have developped on the supposed sites symmetry of anions and cations compatibly with the space groups determined by X-ray diffraction studies (Table 1), which lead to interpretate and discuss the vibrational spectra of these hexafluorosilicate compounds. The IR and Raman spectra of the compounds NH₃(CH₂)_nNH₃SiF₆ (n = 0, 4, 6) with fully determined structures are interpreted knowing exactly the local symmetry of anions and cations [20, 24], as indicated in the Table 1. The site symmetry of the anions and cations are recently predicted in the NH₃(CH₂)₃NH₃SiF₆ structure [27].

Table 1. Point group and site group correlations considered for anions and cations in the assignment of vibrational bands of hexafluorosilicate salts

As an example, the tetramethylammonium hexafluorosilicate IR and Raman spectra have been interpreted in terms of symmetry O_h for ${\text{SiF}}_{6}^{{2-}}$ anions and T_d for (CH)₄N⁺ cations, compatibly with the highly symmetric space group Fm3m, which means that as well as the anions and the cations are considered as preserving their free symmetry in the crystal belonging to the antifluorite cubic structure type [1]. In other study based on the electron paramagnetic resonance spectroscopy at room temperature, it is concluded that the radicals induced by δ-irradiation in the on the [(CH₃)₄N]₂SiF₆ compound is (CH₃)₃N⁺ [9]. So, each methyl groups rotate around the C_3v-axis, and they also rotate around the C_3v-axis of (CH₃)₃N⁺ radical, which rotate around the c-axis of the crystal since the EPR spectra did not change in the (a, b) plane.

It is to note that the octahedral ${\text{SiF}}_{6}^{{2-}}$ anions are considered of O_h symmetry and their internal vibrational modes are described as: 1A_1g(Ra) +1E_g(Ra) + 1F_2g(Ra) + 2F_1u(IR) + 1F_2u(In). For an isolated anion, the vibrational modes (A_1g, E_g, F_2g) are theoretically Raman-active, the vibrational modes of F_1u symmetry are IR active and should appear as single bands. The symmetry lowering take place generally when going from the ions considered as isolated to the ions inside the crystal structures, which gives in terms of spectroscopy the splitting of the characteristic degenerate vibrational modes of ${\text{SiF}}_{6}^{{2-}}$ octahedron, particularly in the Raman spectra of hexafluorosilicate compounds [1–5, 12, 15–18]. The inactive T_2u modes could be Raman and (or) infrared actives in particular situations with the lowering symmetry of the hexafluorosilicate.

The infrared spectra of the ${\text{SiF}}_{6}^{{2-}}$ anion (O_h symmetry) are characterized by the presence of two F_1u vibrations ν₃(ν_as Si–F) and ν₄(δ_as FSiF) observed around 730 and 470 cm^–1; while the Raman spectra are known by three characteristic active vibrations ν₁(ν_s Si–F, A_1g), ν₂(ν_as Si–F, E_g) and ν₅(δ_s FSiF) (F_2g) observed generally around 660, 470 and 400 cm^–1 [1]. In the hexafluorosilicate crystals, the ${\text{SiF}}_{6}^{{2-}}$ anions are generally distorted and their symmetry is lowered from O_h to a determined site-symmetry (Table 1). For example, in the case of the [(CH₃)₃NH]₂SiF₆ compound, the symmetry of the anion becomes (D₃) in the site group [12], which results in splitting of the degenerate vibrational modes ν₅(δ_s FSiF) into two components observed at 360 and 345 cm^–1. The band observed at 760 cm^–1 in the Raman spectrum of [(CH₃)₃NH]₂SiF₆ includes the IR vibrational modes ν₄(δ_as FSiF), which becomes active in the Raman spectrum as a result of lowering symmetry of the anion ${\text{SiF}}_{6}^{{2-}}$ inside the crystal. More details about the lowering symmetry site and its effect on the splitting and the activation of vibrational modes as well as in infrared and Raman spectra are available for the hexafluorosilicate compounds [1, 3, 5, 8, 12, 15–18, 20, 24].

For the majority of hexafluorosilicate studied by us [3, 12, 15–17, 24], the experimental IR and Raman frequencies have been compared to semi-empirical PM3 and DFT calculations. The optimal geometries and full thermodynamic properties of these compounds are also characterized, particularly for the alkylammoniums hexafluorosilicates. For the compounds [16, 17], the thermodynamic properties have been calculated using PM3 frequencies, where T (temperature in K), S (entropy in J mol^–1 K^–1), C_p (heat capacity at constant pressure in kJ mol^–1 K^–1), and ΔH = H°–$H_{{298.15}}^{ \circ }$ (enthalpy content, in kJ mol^–1), T₁ = 100 K, T₂ = 298.15 K, and T₃ = 1000 K. In the Table 2, we give the computational methods used, the energy ionisation (IE), and the band gap (BG) calculated for some hexafluorosilicates. The ionization energy of the [C₆H₅NH₃]₂SiF₆ system was calculated at 10.41 eV (PM3) and 7.25 eV (DFT), its band gap was determined at 9.51 eV (PM3) and 6.614 eV (DFT). These values are considered as indicative of a well delocalized aromatic-type of system [17], which is considered as more stable than [C₄H₉NH₃]₂SiF₆, [C₃H₇NH₃]₂SiF₆, and [C₂H₅)₂NH₂]₂SiF₆ [16].

Table 2. Computational methods used, band gap (BG), and ionisation energy (IE) calculated for some hexafluorosilicate salts

The IR and Raman spectra of NH₃(CH₂)_nNH₃SiF₆ (n = 4, 6) in solid state, where three-dimensional N–H···F hydrogen bonds network is formed, are interpreted on the basis of periodic DFT calculations of the model complexes, at fixed unit cell parameters [24] as found from XRD determination, which leads to suggest stronger N-H⋅⋅⋅F hydrogen bonding (HB) for n = 6 in comparison to n = 4 in agreement with the crystallographic finding [22, 23]. The vibrational frequencies of these compounds were calculated using the density functional perturbation theory (DFPT) (linear response (LR) theory), developed for crystal and determining the Hessian matrix of the second derivatives of the energy with respect to the atomic positions [24]. The Gaussian set of programs computes directly infrared intensities and infrared wavenumbers. The simulated Raman spectra using the calculated Raman scattering activity coefficient, which are not the real intensities, could not be compared to the experimental Raman spectra. Hence, the calculated Raman activities were converted into Raman scattering intensities (comparable with the experimental Raman intensities) with the help of Chemcraft program [24].

The HB interactions effect on the geometrical and vibrational characteristics of the ions in these compounds was studied. Indeed, the stretching Si–F modes frequencies could be used as characteristic spectroscopic data to estimate the HB relative strength in the alkylenediammonium hexafluorosilicate compounds.

In hexafluorosilicate-based phase transition materials, the phase transitions are caused generally by cations or protons, and sometimes by the ${\text{SiF}}_{6}^{{2-}}$ anions considered as structurally ordered. Indeed, the [C₄H₉NH₃]₂SiF₆ compound showed a phase transition at 268/261 K, studied by DSC (heating/cooling) and Raman spectroscopy at different temperatures [2]. The observed phase transition is interpretate as a first order transition of order-disorder character, due to the tetrabutylammonium cations dynamic in the system. The changes observed in the Raman spectra at different temperatures for the majority of the internal vibrations of cations indicated that the disordered butylammonium cations exhibit reorientational motions at high temperatures. The butylammonium becomes ordered at low temperatures, and their motions are found to be frozen below phase transition temperature [2].

The [C₃H₇NH₃]₂SiF₆ compound exhibited a first order structural phase transition at around 229/216 K, which was detected by DSC (heating/cooling) and Raman spectroscopy [4, 16]. It is important to note the splitting of the DSC peaks observed as well as in heating and cooling, and the 229/216 K are considered as the centres of the observed doublets. This phenomenon is up to now inexplicable since the crystal structure of this compound is not yet determined. However, important changes are observed for the majority of the propylammonium internal vibrations observed in the Raman spectra at different temperatures near and below the phase transition temperature. The reorientational motions of these cations were considered as the contributor in the mechanism of the observed order−disorder phase transition in this hexafluorosilicate compound [2].

The [(C₂H₅)₄N]₂SiF₆ compound exhibited three structural phase transitions, which have been studied by DSC, Raman spectroscopy, and dielectric measurements [3, 5]. The phase transitions are observed in DSC (heating/cooling) at 30/15, 72/40, and 195/186°C. However, only two-phase transitions were detected by dielectric measurements at 30 and 195°C. The Raman spectra at different temperatures showed that the change in the dynamical state of the tetraethylammonium cations and ${\text{SiF}}_{6}^{{2-}}$ anions contributed to the order-disorder mechanism of the observed phase transitions [5].

The [C₆H₅NH₃]₂SiF₆ compound presents two phase transitions at low temperatures (286/280 and 230/226 K), which have been studied by DSC (heating/cooling) and Raman spectroscopic at different temperatures [18]. The transition observed at 286 K is considered as a second-order phase transition, whereas that observed at 230 K is interpretate as a first-order character. From the Raman spectroscopy as function of temperature, it was concluded that as well as the hexafluorosilicate ${\text{SiF}}_{6}^{{2-}}$ and the phenylammonium cations contribute in the first-order phase transition observed at 230/226 K; but only the cationic motions contribute to the mechanism of the second-order phase transition observed at 286/280 K [18].

The [(CH₂OD)₃CND₃]₂SiF₆ ferroic crystal undergone a solid-solid phase transition of the first order at 185 K, which has been investigated by the optical (linear birefringence) and thermal (DSC) measurements [8]. The infrared spectra of this compound were studied in the temperature domain (308–33 K). The temperature changes of frequencies, width, position and intensity of the IR bands were analyzed to clarify the phase transition mechanism and the contribution of ${\text{SiF}}_{6}^{{2-}}$ and CH₂OD groups to the рhase transition [8]. The Raman spectra of [(CH₂OD)₃CND₃]₂SiF₆ at room temperature were discussed on the basis of the theoretical calculations made based on DFT [8].

In the [(CH₂OH)₃CNH₃]₂SiF₆ salt, trigonal of space group P$\bar {3}$ (a = 7.699(1), c = 7.818(2) Å) at room temperature, the ${\text{SiF}}_{6}^{{2-}}$ anions located in large cavities formed by hydrogen bonded cations are strongly disordered [7]. The DSC measurements revealed a first-order phase transition at low temperature (177 K), which was confirmed by a sharp increase of the linear birefringence below the phase transition temperature. The domain structure observed in the low temperature phase under a polarizing microscope, and the temperature dependent changes of birefringence indicate that this hexafluorosilicate salt has potential ferroelastic properties. The mechanism of the first-order phase observed in this compound is related to the freezing of the reorientational motions of the ${\text{SiF}}_{6}^{{2-}}$ anions below the temperature phase transition 177 K.

The hexafluorosilicate moiety has been used to assemble the phase transitions in others hybrid materials based on amino acids [40, 47]. The bis(betainium)hexafluorosilicate bi(betaine) [40] is an example, which well illustrates the ${\text{SiF}}_{6}^{{2-}}$ anions motions during the order-disorder phase transition since its structure has been determined at two different temperatures. At room temperature, the crystal is orthorhombic with space group Fddd (a = 13.2237, b = 19.3136, c = 22.0443 Å, V = 5630.05 ų), where the ${\text{SiF}}_{6}^{{2-}}$ part exhibits partial disorder originated from a uniaxial wheel-like rotation [40]. When the temperature decreases, a total freeze of this wheel-like motion results in a second-order phase transition at 250 K, and the low temperature phase becomes monoclinic with C2/c space group (a = 13.022, b = 21.9525, c = 11.4831 Å, β = 123.5238°, V = 2736.6 ų). The DSC measurements, variable temperature single-crystal X‑ray diffractions, and theoretical computation of potential energies for the wheel-like motion confirmed that the solid-state phase transition is derived by the order-disorder transformation of the anionic rotor. The basic structural unit of this compound consists of one ${\text{SiF}}_{6}^{{2-}}$ anion and two dimeric (betaine-H-betaine)⁺ cations, which are well illustrated in the two phases in Fig. 10. During the phase transition, the small variation in O−H···O the hydrogen bonding interactions changing from 2.429 to 2.432 Å, indicates that proton dynamic is unrelated to the phase transition. Further detailed structural changes concerning the order-disorder transformation of the wheel-like motion in this compound is illustrated in the Fig. 11. The dynamic changes of the crystalline in-plane rotor give rise to anisotropic and switchable dielectric constants.

Fig. 10.

Molecular structures of the bis(betainium)hexafluorosilicate bi(betaine) at room temperature (RTP) and low (LTP) temperature phases [40].

Fig. 11.

Structural changes during the order−disorder transition of the wheel-like motion in bis(betainium) hexafluorosilicate bi(betaine) [40].

The tetraalkylammonium hexafluorosilicates were used as precursors for silicon films. The electrochemical methods of silicon coatings deposition from low-temperature ion-organic melts have been discovered for the first time by Gudymenko et al. [10]. For this goal, the authors have synthesized the hexafluorosilicate salts [(CH₂)₂(C₂H₅)₂N)]₂SiF₆, [(CH₂)₃(C₃H₇)-N]₂SiF₆ and [(CH₂)(C₂H₅)₃N]₂SiF₆, which have studied their structure by X-rays diffraction, IR, NMR spectroscopy, and also by DTA/DSC methods. The studied tetraalkylammonium salts showed their stability enough to be suit able for electrochemical deposition of silicon coatings under temperatures at least up to 200°C. The temperatures of the beginning of thermal decomposition of the studied complexes were close to each other (~270−280°C) which means that energies of the cation-anion ionic bonds are practically identical and do not depend on the structure of the cation. The initial complex composition change during the thermal decomposition leads to the formation of the binary system (tetraalkylammonium fluoride-tetraalkylammonium fluorosilicate), which re-sults in the formation of lower temperature melts. The use of background melts to produce silicon coatings at temperatures up to 200°C is possible by their respective thermal stability.

For the NH₃(CH₂)₃NH₃SiF₆ crystal recently characterized [27], the DSC and TGA-dTGA techniques recorded in the 25−250°C temperature domain showed the compound decomposition above 30°C, which is described by as the following equation:

$$\begin{gathered} N{{{\text{H}}}_{{\text{3}}}}{{\left( {{\text{C}}{{{\text{H}}}_{{\text{2}}}}} \right)}_{{\text{3}}}}{\text{N}}{{{\text{H}}}_{{\text{3}}}}{\text{Si}}{{{\text{F}}}_{{6{\text{ }}(s)}}} \\ \to \,\,{\text{Si}}{{{\text{F}}}_{{4(s)}}}~ + {\text{ }}2{\text{H}}{{{\text{F}}}_{{(g)}}}~ + {\text{ }}{{{\text{N}}}_{{2{\text{ }}(g)}}}~ + {\text{ }}3{{{\text{C}}}_{{(g)}}}~ + {\text{ 5}}{{{\text{H}}}_{{2(g)}}}. \\ \end{gathered} $$

Water solubility. The relationship between the hexafluorosilicate structures and solubility was studied in several works [30, 41, 49] and reviewed for the majority of hexafluorosilicate compounds in [13]. As an example, the relationship between the solubility and the structure of the pyridinium (C₅H₅NH)₂SiF₆, 2-methylpyridinium (2-CH₃C₅H₄NH)₂SiF₆, and others hexafluorosilicates with substituted pyridinium cations was discussed [41]. The solubility has been investigated in relation to the number of interionic H-bonds in the salt structures [40], which implied that the propagation of a number of short interionic contacts of X−H···F (X = N, O) type is accompanied by a diminution of solubility of relevant hexafluorosilicates.

For a comparative assessment of the effect of interionic H-bonds on the solubility of pyridinium hexafluoridosilicates, an empirical parameter h was purposed [28]. Indeed, the interionic H-bond effects on the solubility of hexafluorosilicate salts have been estimated by using a parameter h, expressed as:

$$h = \frac{n}{{d{{{{\text{(D}}\cdot\cdot\cdot{\text{A)}}}}_{{{\text{av}}}}}}},$$

where n is a number of short interionic contacts D···A ≤ 3.2 Å, d(D···A)_av is an average donor-acceptor distance in the complex structure; the strong and moderate H-bonds following the classification [30]. The solubility data and pH values for 0.001 M solutions of the hexafluorosilicate compounds, (LH)₂SiF₆ (L: 2-aminopyridine, 3-aminopyridine, 2,6-diaminopyridine) and (LH)₂SiF₆·H₂O (L: 4-aminopyridine) in comparison with other hexafluorosilicate salts of functionalized pyridinium cations have been studied by means of calculated h parameters [30].

Generally, the calculated h values for structurally characterized hexafluoridosilicates with pyridinium cations as well as related heterocyclic cations containing pyridine nitrogen atoms are reviewed [13], which conclude that an increase in h values, reflecting an increase in the intensity of interionic H-interactions in salt structures, leads to an exponential decrease in the solubility of the corresponding ammonium hexafluoridosilicates. Furthermore, the solubility of arylammonium hexafluorosilicates in water and h values is reported in the review study [13], which claimed that the tendency to decrease in solubility of hexafluoridosilicates where cations bear additional H-donor groups or for hydrated forms.

Catalytical properties. The hexafluorosilicate of NH₃(CH₂)₂NH₃SiF₆ (n = 4, 6) are found to be highly efficient solid catalysts in the easy synthetic protocols for benzimidazoles, benzothiazole and benzoxazole, 3,4-dihydropyrimidin-2-ones/thiones and quinoxalines derivatives [25, 26]. The advantages of the hexafluorosilicate catalysis methods are: no solvent; mild conditions (room temperature); use of recyclable catalyst, easy work-up procedure, short reaction times (2–7 min), low catalyst content (1 mol %), scalability, solvent free conditions, excellent yields of target products (90–99%), easy reusability, and use of an eco-friendly catalyst. These obvious advantages are the important and significant attributes and of the practical features of the protocols using the two novel hexafluorosilicates compounds in catalysis field [25, 26].

Recently, the new hybrid crystal (N₂H₅)₂SiF₆ is used as a heterogeneous catalyst, in a simple, effective, green and non-toxic protocol, for the Knoevenagel condensation and the biscoumarin derivatives synthesis [19]. This compound examined for five successive cycles without significant loss of catalytic activity, presents several advantages such as a short reaction time and exceptional catalytic activity.

Anticaries and toxicity properties. The caries prophylactic efficacy (CPE) of some fluoride-containing solids has been determined in experiments developped on rats [47, 48]. It is worthy to note that the CPE was calculated as: CPE = [(A – B)/A] × 100%, where A is the number of caries lesions to the teeth in rats receiving the CID (caries-inducing diet) and B is the number of caries lesions in rats receiving the CID + a fluorinated compound [48].

The CPE results obtained for fluorinated compounds showed that (NH₄)₂SiF₆ and the 2-аmino-4,6-dihydroxypyrimidine hexafluorosilicate (L¹H)₂SiF₆ decreased significantly the caries lesions numbers, by 22.7 and 45.5%, respectively [48]; the (L¹H)₂SiF₆ compound had a high CPE, five times greater than that of sodium fluoride NaF (9.1%). In contrast to NaF and (NH₄)₂SiF₆, the compound (L¹H)₂SiF₆ has significant differences in terms of its mechanism of biological activity, and then showed significant increase in alanine aminotransferase (ALT) activity (39%), considered as an evidence of a hepatotoxic action [48]. The disadvantage of this compound as a caries prophylactic agent is the presence of a hepatotoxic action, linked with the specific effects of the pyrimidinium cation.

However, the use of carboxymethylpyridinium hexafluorosilicates results in the significant decrease in the carious lesions number, in the following order: (LH)₂SiF₆ (L = 2-carboxymethylpyridine) by 6.8%, (LH)₂SiF₆ (L = 3-carboxymethylpyridine) by 11.4%, and (LH)₂SiF₆ (L = 4-carboxymethylpyridine) by 45.5%. The latest compound showing the highest CPE, also being 5 times higher than that of NaF, reduce the depth and the number of dental caries and provide efficiency of caries prevention [49]. The findings showed that these fluoride compounds devoid of hepatotoxic effect, which implies that the (LH)₂SiF₆ (L = 4-carboxymethylpyridine) salt is particularly of interest as a promising anti-caries agent.

It has been stated that the sodium hexafluorosilicate Na₂SiF₆, hexafluorosilicic acid H₂SiF₆ and NaF compounds have similar toxicity of fluoridation [50]. Although the effects of the fluoride compounds have been found to be varied among the 3 biological endpoints, no differences were found between the three compounds, relative to the fluoride ion concentration, in any of the assays. Furthermore, the hexafluorosilicic H₂SiF₆ acid and the ammonium hexafluorosilicate (NH₄)₂SiF₆ are toxic chemicals [51, 52]. The (NH₄)₂SiF₆ compound is known by the following toxicological informations [52]:

Health hazard: the inhalation of (NH₄)₂SiF₆ dust can cause pulmonary irritation and can be fatal in some cases; the ingestion may also prove fatal. Contact with dust causes irritation of eyes as well as irritation or ulceration of the skin.

Hazard statement: the (NH₄)₂SiF₆ has been characterised by: H301 (Toxic if swallowed), H311 (Toxic in contact with skin), and H331 (Toxic if inhaled).

In regard to the catalysis field undertaken recently with the alkylenediammonium NH₃(CH₂)_nNH₃SiF₆ (n = 4, 6) [25, 26] and hydrazinium [19] hexafluorosilicates, it seems important to study the toxicity of these compounds, for which any toxicological study has not yet done. Since these compounds are belonging to the hexafluorosilicate salts, then these novel catalysts, could present such toxicological properties as in (NH₄)₂SiF₆, since the silicofluorides compounds have generally similar toxicity of fluoridation.

Thus, the structural, vibrational, thermal properties, and phase transitions of alkylammonium, alkylenediammonium, and amino acid hexafluorosilicate salts are reviewed and discussed. For the alkylenediammonium salts NH₃(CH₂)_nNH₃SiF₆, the structural comparative analysis is developped on the determined crystal structures (n = 0, 2, 4, 6). The effect of hydrogen bonding is discussed as well as on the elongation of the Si–F bonds, and in relation to the solubility of hexafluorosilicate compounds. As a novel tendency in the hexafluorosilicate field research, NH₃(CH₂)_nNH₃SiF₆ (n = 4, 6) salts are found to be as highly efficient novel catalysts in the synthetic protocols for organic compounds such as benzimidazoles, benzothiazole and benzoxazole, 3,4-dihydropyrimidin-2-ones/thiones and quinoxalines derivatives.

As a perspective of this review, it seems very important to study as well as the acute toxicity of the hexafluorosilicate compounds and the development of methods for identification and quantification of new potential medical substances of hexafluorosilicates.