In situ crystallization of the tetrahydrate of pentafluoro-benzenesulfonic acid, featuring the Eigen ion (H₉O₄)⁺

1 Introduction

Benzenesulfonic acid is a well-known derivative of both, benzene and sulfuric acid. Its preparation by aromatic sulfonation represents one of the classical electrophilic substitution reactions and is of considerable importance in industrial chemistry, where sulfonic acids and their salts serve as intermediates for detergents, dyes, and ion exchange resins. ^{1 , 2 , 3 , 4} Further sulfonation to form polysulfonic acids is possible but more difficult because the [SO₃H] group is an electron-withdrawing substituent, exerting both inductive (–I) and mesomeric (–M) effects that reduce the electron density of the aromatic ring. As we have shown, various benzene polysulfuric acids can nevertheless be prepared, but preferably not by electrophilic substitution at the phenyl ring. ⁵ Similarly, the sulfonation of pentafluorobenzene, C₆F₅H, is hindered by the electron withdrawing presence of the fluorine atoms. According to the early literature, sulfonation of C₆F₅H is possible within 48 hours using oleum containing 20 % of SO₃. ⁶ Later, Sartori et al. described a faster synthesis applying neat SO₃ at a temperature of 100 °C. ⁷ This group also conducted experiments to determine the acidity of the acid. ⁸ To date, neither pentfluorobenzenesulfonic acid, C₆H₅(SO₃H), nor any of its hydrates have been structurally described. For the monohydrate, C₆H₅(SO₃H)·H₂O, a melting point of 101 °C has been reported, making it likely that the compound is an oxonium salt according to (H₃O)[C₆F₅(SO₃)]. ⁷ The acid and some of its salts have been used occasionally in catalytic reactions, ⁹ and the lithium salt was considered as an electrolyte. ¹⁰ Moreover, the [C₆H₅(SO₃)]⁻ anion has been used as a weakly coordination anion, similarly to the well-known triflate anion, CF₃SO₃⁻. ¹¹ Structurally, only a handful of pentafluorobenzenesulfonates are known. ^{9 , 12 , 13} In course of our investigations on sulfonic acids we became also interested in fluorinated sulfonates. ^{14 , 15} The absence of any C–H bonds makes this anion a particularly attractive ligand for the synthesis of lanthanide complexes. This is helpful to suppress quenching effects in the luminescence of such complexes. Pentafluorobenzenesulfonates, in particular the sodium salt, are commercially available but unreasonable expensive compounds. Thus, we decided to prepare the acid ourselves from C₆F₅H and oleum (20 % SO₃). After work-up, a light brown oily liquid was obtained, probably a higher hydrate of the acid compared to the above-mentioned solid mono-hydrate. Based on our experience with in situ crystallization using an optical heating and crystallization device (OHCD), ^{16 , 17} we were able to grow single crystals from the brown oil. It turned out that the compound is a tetrahydrate of the acid according to the formulation (H₉O₄)[C₆F₅SO₃], containing the so-called Eigen ion (H₉O₄)⁺ which is rarely described so far. It consists of a central (H₃O)⁺ ion surrounded by three H₂O molecules according to [(H₃O)(H₂O)₃]⁺ leading to the star-shaped structure of the ion. Interestingly, there is another structural motif for the (H₉O₄)⁺ known, which has a chain-like arrangement an can be seen as the dihydrate of the Zundel cation, (H₅O₂)⁺, according to [(H₅O₂)(H₂O)₂]⁺. ¹⁸

2 Experimental

2.2 In situ crystallization of (H₉O₄)[C₆F₅SO₃]

For the crystallization of (H₉O₄)[C₆F₅SO₃], the compound was placed in glass capillary which was sealed and fixed onto the goniometer of a Bruker D8 Venture Kappa single crystal diffractometer. The sample was cooled to a temperature of 230 K upon which the acid solidified. Subsequently the CO₂ LASER of the OHCD (optical heating crystallization device) is used to heat the shock frozen acid in the capillary in order to provoke zone melting and crystallite formation. Repeated heating and cooling finally leads to the formation of a single crystal. This can be nicely seen from the diffraction pattern which shows the typical Debye rings of a polycrystalline powder in the beginning and then changes to the diffraction pattern of a single crystal (Figure 1). From the collected data the crystal structure could be elucidated.

Figure 1:

Diffraction patterns of (H₉O₄)[C₆F₅SO₃] during CO₂ laser heating as seen by the detector of the single crystal diffractometer. The left image shows that the solidified acid is highly polycrystalline, almost a powder. Repeated treatment with the laser leads to the growth of a single crystal with the defined diffraction pattern shown in the right-hand picture.

3 Results and discussion

The crystal structure (H₉O₄)[C₆F₅SO₃] is triclinic with space group P 1 ‾ and contains two formula units in the cell (Table 1). Crystallographically, there is one independent [C₆F₅SO₃]⁻ anion and one (H₉O₄)⁺ cation. The pentafluorobenzenesulfonate anion displays almost C _s symmetry with the mirror plane located in the plane of the phenyl ring. The bond distances S–O within the [SO₃] moiety of the anion range from 143.6(1) to 145.1(1) pm (Table 2). The longest distance occurs to the oxygen atom (O12) with strong participation in hydrogen bonds to the (H₉O₄)⁺ cation. The S–C bond length is 179.1(2) pm and in line with previous findings. ^{12 , 13} The most interesting feature of the crystal structure of (H₉O₄)[C₆F₅SO₃] is the (H₉O₄)⁺ cation which is composed of a central (H₃O)⁺ ion coordinated by three H₂O molecules. Meanwhile this ion is referred to as Eigen ion, in appreciation of Manfred Eigen´s groundbreaking work on proton transport in aqueous media (occasionally also the H₃O⁺ is named Eigen ion, while its monohydrate, (H₅O₂)⁺, is called Zundel ion). ²³ The (H₉O₄)⁺ has been investigates by several methods, including mass spectrometry, spectroscopy, and theoretical calculations. ^{24 , 25 , 26 , 27 , 28} Contrastingly, structural investigations in the solid state are scarce, especially compared to numerous reports on the (H₃O)⁺ and the (H₅O₂)⁺ ion, respectively. ^{29 , 30 , 31} In the structure of (H₉O₄)⁺ a central (H₃O)⁺ ion is coordinated by three H₂O molecules via hydrogen bonds. With respect to the limitations of hydrogen atom localization by means of X-ray diffraction, the distance of the respective donor (D) and acceptor (A) oxygen atoms is a key parameter to estimate the strength of a hydrogen bond: Shorter D–A hint at stronger hydrogen bonds. ^{32 , 33} To some extent a validation of this assumption has been made additional methods, for example neutron diffraction (which gives accurate H-atom positions) and IR as well as NMR measurements. Thus, for strong hydrogen bonds the distance between donor and acceptor atom should be significantly shorter than the sum of their van der Waals radii, and the angle D–H⋯A should be almost 180°. For hydrogen bonds with oxygen as donor and acceptor atoms, very strong hydrogen bonds show D–A distances of 250 pm or even shorter.

The (H₉O₄)⁺ cation in the crystal structure of (H₉O₄)[C₆F₅SO₃] shows slight deviation from the ideal C _3v symmetry (Figure 2). The distances between the central oxygen atom of the H₃O⁺ ion (O2) and the oxygen atoms of the surrounding H₂O molecules (O1, O3, O4) range from 249.7 to 253.0 pm, proving strong hydrogen bonding. The angles ∠ O–H⋯O are found between 171 and 177° and support the assumption of strong hydrogen bonds, even if it has to be mentioned that the hydrogen atom positions are not refined but calculated (Table 2). With respect to bond angles between the H₃O⁺ oxygen atom and the H₂O oxygen atoms, which show values of 111.1°, 117.1°, and 110.3°, the cations are flattened with respect to a perfect tetrahedral arrangement. This is in line with the previous findings for this cation and it is also reasonable with respect to the high electronegativity of oxygen which makes the lone pair of the H₃O⁺ ion less space filling. In the crystal structure of (H₉O₄)[C₆F₅SO₃] the H₂O molecules of the (H₉O₄)⁺ cation are involved in further hydrogen bonds with oxygen atoms of the [SO₃] groups. These bonds are between 30 and 40 pm longer than observed within the (H₉O₄)⁺ cation. Thus, they have to be classified as weak. Furthermore, there is a weak hydrogen bond between adjacent (H₉O₄)⁺ cations, involving the oxygen atoms O1 and O4 (Figure 2). The respective hydrogen atom in this bond (H4B) is part of another hydrogen bond to the oxygen atom O11. In sum, this results in a so-called bifurcated hydrogen bond, with typical angles ∠O–H–O of 116 and 147°, respectively (Table 2).

Figure 2:

The (H₉O₄)⁺ cation in the crystal structure of (H₉O₄)[C₆F₅SO₃]. The oxygen atoms of the cation are drawn in violet color. Short hydrogen bonds are drawn as black lines, long hydrogen bonds as hatched lines between the donor and acceptor oxygen atoms. The distances are given as bold type values in units of pm. The oxygen atom O4 shows a slight disorder over two positions with almost a 70/30 % ratio and only the major position is shown in the figure. The ellipsoids are set to 70 % probability (except H atoms).

The hydrogen bonding network between (H₉O₄)⁺ cations and [SO₃] groups lead to hydrophilic double layers which extend in the a–b plane of the unit cell (Figure 3). These layers are separated by the hydrophobic fluorinated phenyl rings of the anions. Within the hydrophobic layers the shortest distances between the aromatic rings are 328 pm, hinting at weak π−π interactions. ³⁴

Figure 3:

Crystal structure (H₉O₄)[C₆F₅SO₃]. The hydrogen bonds between the (H₉O₄)⁺ ions (violet oxygen atoms) and the [SO₃] moieties (blue oxygen atoms) of the anions are emphasized by black lines. The layers of the oxygen atoms alternate with layers of the fluorinated phenyl rings of the anions.

4 Conclusions

With the help of in situ grown single crystals of (H₉O₄)[C₆F₅SO₃] the first structure determination of a hydrate of the acid C₆F₅SO₃H has been performed. The compound exhibits the rarely seen star-shaped Eigen ion, (H₉O₄)⁺, which can be seen as the trihydrate of the (H₃O)⁺ ion. The structure of the (H₉O₄)⁺ cation is stamped by strong hydrogen bond. In the layered structure of (H₉O₄)[C₆F₅SO₃] the cations are further connected to the sulfonate groups of the anions by weak hydrogen bonds.