The synthesis of sulfuric acid using the so-called double contact process is one of the most important technical procedures. Within this process, disulfuric acid, H2S2O7, is an important intermediate. Disulfuric acid can be seen as the first member of a series of acids of the general formula H2SnO3n+1 (n = number of sulfur atoms). Formally, they are products of the continuous condensation of H2SO4. Interestingly, neither disulfuric acid nor the higher condensation products have been characterized until the early 1990s. Surprisingly late, in 1991, Hönle determined the crystal structure of H2S2O7 . However, crystal structures of higher polysulfuric acids have remained completely unknown up to now. In addition, the respective salts, i.e., polysulfates, were investigated scarcely, and essentially only disulfates have been reported. These were known for a limited number of mainly alkaline and alkaline earth metals when we started a research project aiming at the characterization of polysulfuric acids and their salts some years ago. This research afforded a plethora of new compounds, especially using oleum and SO3 as reactants. Among these compounds are the tris-(disulfato)-metallates A2[M(S2O7)3] (A = Li, Na, K, NH4, Rb, Cs; M = Si, Ge, Sn) [2–4] and B[M(S2O7)3] (B = Ba, Pb; M = Si, Ge)  and the tetrakis-(disulfato)-metallates Sr2[M(S2O7)4] (M = Si, Ge) , bearing the central atoms in octahedral coordination of disulfate groups. From the reaction of elemental palladium with SO3, the unique disulfate Pd(S2O7) could be gained . This compound shows the palladium atoms in an unusual octahedral coordination of oxygen atoms leading to the unique magnetic properties of the compound. In some cases, also higher polysulfates have been obtained, for example, Pb(S3O10) , the bis-(trisulfato)-aurate anion [Au(S3O10)2]– , the tetrasulfate (NO2)2(S4O13) , and the bis-(tetrasulfato)-palladate K2[Pd(S4O13)2] .
It is obvious that the preparation of polysulfates is getting more difficult with increasing length of the polysulfate chain. This is also confirmed by quantum mechanical calculations that predict decreasing stabilization energy for growing chain lengths of the polysulfate anion [8–10]. In order to elucidate further possibilities for the preparation of polysulfates, especially at lower temperatures, and aiming at compounds with larger polysulfate anions, we recently started to use ionic liquids (ILs) as reaction media. ILs turned out to be extraordinarily versatile reaction media throughout the last 20 years [11–16]. It is especially interesting that some of these ILs are very resistant against oxidation so that reactions even with oleum or SO3 are possible. It turned out that [bmim][HSO4] (bmim=1-butyl-3-methylimidazolium) is a very suitable candidate. This IL is oxidation-resistant against cerium(IV) , but not against ozone . It also turned out to be a probate inert solvent for our efforts. Thus, we could recently report on the disulfates [HPy]3Sm[S2O7]3 and [HPy]Bi[S2O7]2 originating from these investigations . Similar reactions with Eu2O3 as a metal source have now afforded an interesting side product, [HPy]2[S2O7], and the reaction of oleum with the pyridine/SO3 complex Py·SO3 and GeCl4 in [bmim][HSO4] produced [bmim][HPy][S2O7]. Both strategies, the synthesis of polysulfates from ILs and even from neat organic solvents like pyridine, are, to the best of our knowledge, unique until now. Although the formation of the compounds presented was serendipitous, we think that the findings might be of broader interest because both compounds display the [S2O7]2– ion in an essentially uncoordinated, i.e., “naked” situation.
2 Results and discussion
[C8H15N2][C5H6N][S2O7] (in the following abbreviated as [bmim][HPy][S2O7]), obtained as side product of the reaction of oleum, the pyridine/SO3 complex Py·SO3 and GeCl4, crystallizes in the monoclinic space group P21/n with four formula units per cell. In the asymmetric unit, there is one crystallographically independent ion of each type [bmim]+, [Hpy]+ and [S2O7]2–, respectively. [bmim][HPy][S2O7] shows a layer type structure (Fig. 1), with the layers consisting of either [bmim]+ cations or a combination of [HPy]+ cations and disulfate anions.
Astonishingly, disulfate species might also be synthesized in typical organic solvents. Thus, [C5H6N]2[S2O7] (in the following abbreviated as [HPy]2[S2O7]) was obtained from the reaction of Eu2O3 and the pyridine/SO3 adduct Py·SO3 in dry pyridine. In comparison to the crystal structure of [bmim][HPy][S2O7], there is only one type of nitrogen based cation responsible for the charge compensation of the disulfate anion. This binary pyridinium disulfate crystallizes in the orthorhombic space group P212121 with four formula units per cell. The pyridinium cations as well as the disulfate anions are arranged in layers that are stacked alternatingly along the  direction (Fig. 2). Thus, the disulfate anions are embedded between the cationic layers. The [NH] moieties of the organic cations are orientated towards the oxygen atoms of the disulfate groups.
It is remarkable that in both compounds the disulfate anions are essentially uncoordinated by the large and uni-positive, i.e., weakly coordinating cations (Figs. 3 and 4). This is obvious when the S–O bond lengths of the disulfate units are examined (Table 1). The terminal S–O bond lengths lie in the very narrow range from 143.7(1)–144.8(1) pm for [bmim][HPy][S2O7] and from 143.7(1) to 144.9(1) pm for [HPy]2[S2O7], showing no visible elongation towards one of the organic cations in the structure. The S–O–S bridge is almost symmetrical and shows S–O bond lengths of 164.3(1) and 165.7(1) pm for [bmim][HPy][S2O7] and 163.9(1) to 165.3(1) pm for [HPy]2[S2O7]. Such a uniform situation concerning the bond lengths S–O is not observed in typical metal salts of disulfuric acid. Even in (IO2)2(S2O7) that contains the quite weakly coordinating IO2+ cation the contact to the disulfate anion is clearly visible from the S–O distances (Table 1) . The [SO3] moieties of the free disulfate units show staggered orientation with respect to each other with torsion angles O–S–S–O of about 32° for [bmim][HPy][S2O7] and 33° in [HPy]2[S2O7]. In (IO2)2(S2O7) the respective value is 23°.
Selected bond lengths and angles of the disulfate groups for [bmim][HPy][S2O7] and [HPy]2[S2O7] compared to those in (IO2)2(S2O7) .
|S–O bond||[bmim][HPy][S2O7] (pm)||[HPy]2[S2O7] (pm)||(IO2)2(S2O7) (pm)|
|S–O–S bond angle||[bmim][HPy][S2O7] (deg)||[HPy]2[S2O7] (deg)||(IO2)2(S2O7) ( deg)|
Other cation–anion contacts result from hydrogen bonds with the [NH] moieties of the pyridinium cations as donors and the oxygen atoms of the disulfate anions as acceptors. In [bmim][HPy][S2O7], the shortest donor–acceptor distance is 326.7(2) pm between N1 and O12 and the respective angle ∠D–H····A is 161(2)°, hinting at a weak hydrogen bond according to the terminology of Jeffrey . At the [bmim]+ cation, only a [CH3] moiety could function as donor, and such hydrogen bonds are known to be very weak, although not completely negligible . However, the shortest donor–acceptor distances associated with large angles ∠D–H····A were found to be larger than 325 pm. This is clearly above the sum of the van der Waals radii so that there are essentially no hydrogen bonds involving the [bmim]+ cation.
In [HPy]2[S2O7], the nitrogen atoms of the cations show quite short distances to oxygen atoms of the disulfate groups (Fig. 4). The shortest distance of 274.4(1) pm is found between N1 and O11 and hints at medium strong hydrogen bonds. The corresponding angle ∠D–H····A is 149(3)°, and the bond H1–O11 shows a length of 196(3) pm. The second pyridinium cation shows similar values (H2–O12: 203(3) pm) and a donor–acceptor distance of 282.4(2) pm with an angle ∠D–H····A of 144(3)°. This bond is therefore also considered to be medium strong .
Our results show that the IL 1-butyl-3-methylimidazolium hydrogensulfate, [bmim][HSO4], is a suitable medium for reactions with strong oxidizers like SO3. The reaction of GeCl4 and the donor-acceptor complex Py·SO3 performed in this medium did not lead to the intended product, but to the disulfate [bmim][HPy][S2O7]. Similarly, reactions with the Py·SO3 complex and Eu2O3 gave an unexpected side product, namely the disulfate [HPy]2[S2O7]. Both disulfates have in common that the inorganic anions are essentially unaffected by the large and weakly coordinating counterions. Thus, the structures exhibit rather “naked” disulfate anions. In contrast to the disulfates known so far, which are mostly metal salts with significant cation–anion contacts, the compounds under discussion allow for the inspection of undisturbed S2O72– ions. Even hydrogen bonds that might occur due to the presence of [NH] moieties in the pyridinium cations seem to play only a minor role.
4 Experimental section
Water and further impurities were removed from the IL 1-butyl-3-methylimidazolium hydrogensulfate, [bmim][HSO4] under reduced pressure as best as possible. After drying, the IL was handled under strictly anaerobic conditions.
4.2 Synthesis of [bmim][HPy][S2O7]
GeCl4 (214 mg, 1 mmol), [bmim][HSO4] (0.5 mL), Py·SO3 (318.32 mg, 2 mmol), and oleum (65 % SO3) (0.5 mL) were filled in a screwable Pyrex® tube (Duran® glass, 12 × 100 mm; VWR, Germany) with Teflon® inlay, which was placed in a sand bath and held at a temperature of 50 °C for 24 h. After cooling down to room temperature, a number of moisture sensitive single crystals could successfully be separated from the IL and handled under inert conditions.
4.3 Synthesis of [HPy]2[S2O7]
Eu2O3 (50 mg), pyridine (1 mL), [bmim][HSO4] (0.5 mL), and Py·SO3 (100 mg) were loaded into a thick-walled glass ampoule (length = 250 mm, ∅ = 20 mm, thickness of the tube wall = 2 mm). The ampoule was torch-sealed, placed into a tube furnace, and heated up to 60 °C within 24 h. The temperature was maintained for 48 h and finally reduced to 25°C within 120 h. After cooling down to room temperature, a number of colorless single crystals could be separated from the pyridine under inert conditions.
Caution! Oleum and Py·SO3 are strong oxidizers, which need careful handling.
4.4 X-ray crystallography
Several single crystals were transferred into inert oil (AB128333; ABCR, Karlsruhe, Germany). Suitable crystals were selected, mounted onto glass needles (∅ = 0.1 mm), and immediately placed into a stream of cold N2 (–153 °C) inside the diffractometer (κ-APEX II; Bruker, Karlsruhe, Germany). After unit cell determination, the reflection intensities were collected. Structure solution and refinements were carried out with the Shelx program package . Details are summarized in Table 2.
Crystallographic data of [HPy]2[S2O7] and [bmim][HPy][S2O7].
|Formula weight, g mol-1||336.34||395.45|
|Unit cell dimensions|
|Density (calculated), g cm-3||1.63||1.51|
|Absorption coefficient, cm-1||0.4||0.4|
|Crystal size, mm3||0.20 × 0.10 × 0.06||0.15 × 0.15 × 0.14|
|2θ range for data collection, °||4.42–69.94||3.98–70|
|Index range||–11 ≤ h ≤ 12||–13 ≤ h ≤ 13|
|–20 ≤ k ≤ 19||–24 ≤ k ≤ 24|
|–21 ≤ l ≤ 19||–22 ≤ l ≤ 22|
|Reflections collected||13 795||87 582|
|Completeness to theta, %||99.9||99.8|
|R1/wR2 [Io > 2σ(I)]||0.0324/0.0798||0.0398/0.1307|
|R1/wR2 (all data)||0.0390/0.0833||0.0516/0.1368|
|Goodness-of-fit on F2||1.044||1.050|
|Ratio of racemic twin individuals, %||43:57||–|
|Diff. density min./max., e Å–3||0.42/–0.27||1.28/–0.87|
|CCDC number||968 656||968 657|
[bmim][HPy][S2O7] The crystal structure of [bmim][HPy][S2O7] could be solved in the monoclinic space group P21/n (no. 14). Most of the heavy atom positions were determined with the help of Shelx-2013  using Direct Methods. Further atoms could be successfully located by difference Fourier techniques during refinement with Shelxl-2013 . A numerical absorption correction was applied to the data using the program packages X-Red 32  and X-Shape . The nitrogen-bound hydrogen atom was refined freely, whereas all other hydrogen atoms were refined using AFIX codes. Finally, the structure model was refined to R1 = 0.0516 and wR2 = 0.1368 for all data. Selected bond lengths and angles are presented in Table 1.
[HPy]2[S2O7] The crystal structure of [HPy]2[S2O7] could be solved in the orthorhombic space group P212121 (no. 19). The heavy atom positions were determined by Shelxs-2013  using Direct Methods. Further atoms could be successfully located by difference Fourier techniques during refinement with Shelxl-2013 . A multiscan absorption correction was applied to the data using the program package Sadabs-2012/1 . The nitrogen-bound hydrogen atoms were refined freely, whereas all other hydrogens were refined using AFIX codes. The structure was refined as a racemic twin, and the ratio of the two individuals was found to be 43:57. The final residual values are R1 = 0.0390 and wR2 = 0.0833 for all data. Selected bond lengths and angles are presented in Table 1.
CCDC 968656 and 968657 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Financial support of the Deutsche Forschungsgemeinschaft is gratefully acknowledged. J.B. received a stipend of the Stiftung der Metallindustrie im Nordwesten. The authors want to thank Dr. Marc Schmidtmann for the collection of the X-ray data.
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