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Keywords:

  • Ionic liquids;
  • Sulfur trioxide;
  • Sulfuric acid;
  • Potassium;
  • Crystal structure

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Experimental Section
  6. Acknowledgements
  7. Supporting Information

The ionic liquid 1-butyl-3-methylimidazolium hydrogensulfate, [bmim]HSO4, turned out to be resistant even to strong oxidizers like SO3. Thus, it should be a suitable solvent for the preparation of polysulfates at low temperatures. As a proof of principle we here present the synthesis and crystal structure of K2(S2O7)(H2SO4), which has been obtained from the reaction of K2SO4 and SO3 in [bmim]HSO4. In the crystal structure of K2(S2O7)(H2SO4) (orthorhombic, Pbca, Z = 8, a = 810.64(2) pm, b = 1047.90(2) pm, c = 2328.86(6) pm, V = 1978.30(8) Å3) two crystallographically unique potassium cations are coordinated by a different number of monodentate and bidentate-chelating disulfate anions as well as by sulfuric acid molecules. The crystal structure consists of alternating layers of [K2(S2O7)] slabs and H2SO4 molecules. Hydrogen bonds between hydrogen atoms of sulfuric acid molecules and oxygen atoms of the neighboring disulfate anions are observed.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Experimental Section
  6. Acknowledgements
  7. Supporting Information

Some years ago we started to investigate the reactivity of sulfuric acid, oleum or even neat SO3 under harsh conditions. We were able to present several unprecedented disulfates like B[M(S2O7)3] (B = Sr, Ba, Pb, M = Si, Ge),1 A2[Si(S2O7)3] (A = Na, K, Rb, NH4, Ag), A2[Ge(S2O7)3] (A = Li, Na, K, Rb, Cs, NH4, Ag), A2[Sn(S2O7)3] (A = Na, K, NH4, Ag), and the unique germanate Hg2[Ge(S2O7)3]Cl2 with cationic [HgCl2/2]+ chains.24 All of these compounds were obtained via reactions in oleum and very seldom a higher polysulfate has been obtained under these conditions. The trisulfate Pb(S3O10) is one of the few examples.5 In order to prepare even higher polysulfates we expanded our research to reactions with neat SO3, which lead us to partly unprecedented new sulfates and polysulfates. In this way we obtained the first palladium disulfate, Pd(S2O7), in which the octahedral oxygen coordination of palladium leads to a ferromagnetic behavior at low temperature.6 Moreover, we presented the first tetrasulfate in the structure of (NO)2[S4O13]7 and the first complex of this anion in the structure of K2[Pd(S4O13)2].8 Nevertheless, it must be stated that we do not have a really satisfying preparative route to polysulfates at the moment. However, our theoretical investigations reveal that the stability of polysulfate anions decreases remarkably with the chain length of the anion, and that long chain polysulfates are favored at lower temperature.7 As one option we decided to use ionic liquids (ILs) as low temperature solvents. Due to their extraordinary properties ionic liquids are frequently used also in preparative inorganic chemistry. To quote some special properties, the ionic liquids reveal a very low vapor pressure, are fluid over a broad temperature range (–50 to +400 °C), are mostly non-flammable, non-explosive and reveal a broad electrochemical window (–4 to +4 V). Even handling of elemental fluorine (E0 = +3.1 V) and elemental cesium (E0 = –2.9 V) is possible for selected ionic liquids.920 Thus ionic liquids might be, in contrast to typical organic solvents, also stable against the oxidation power of SO3. In a first attempt we choose the ionic liquid 1-butyl-3-methylimidazolium hydrogensulfate, [bmim]HSO4, as a reaction medium. In a typical reaction we have now obtained the potassium disulfate adduct K2(S2O7)(H2SO4) at low temperature. This reaction shows that polysulfates can be indeed obtained from suitable ILs. In the presented case the anion of the IL is obviously a reactant while the imidazolium cation is not found in the reaction product.

Results and Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Experimental Section
  6. Acknowledgements
  7. Supporting Information

K2(S2O7)(H2SO4) crystallizes in the orthorhombic space group Pbca with eight formula units per unit cell. The asymmetric unit consists of two potassium cations, K1 and K2, one disulfate anion and one H2SO4 molecule. K1 is coordinated by two chelating and three monodentate disulfate ions as well as by one molecule of sulfuric acid resulting in a coordination number of 8. Interestingly, one of the chelating disulfate ions is coordinated via the bridging oxygen atom (O121, cf. Figure 1). The distances K1–O are found in the typical region between 275.3(1) and 300.68(9) pm if distances up to 350 pm are taken into account. The second potassium atom, K2, shows a coordination number of 10 with distances K2–O between 280.05(9) and 331.3(1) pm. The oxygen atoms belong to two chelating and one monodentate disulfate ion, and to two chelating and one monodentate H2SO4 molecule. The function of a sulfuric acid molecule as a chelating ligand is not observed very often, but has already been seen during the systematic studies of alkaline and alkaline earth metal hydrogensulfates and their H2SO4 adducts.2130 The H2SO4 molecules in K2(S2O7)(H2SO4) can easily be identified by the respective S–O bond lengths. They are enlarged by about 10 pm, indicating that the oxygen atoms O33 and O34 are part of the OH groups of the molecule (Table 1). The presence of H2SO4 molecules in the compound is further corroborated by the vibration energies located in the IR spectrum (see Exp. Sect.). The observed distances fit quite well the findings for the crystal structures of H2SO4. The distances and angles within the S2O72– ion show typical values, i.e. the terminal S–O bonds range from 143.21(9) to 146.2(1) pm while the O–S–O bridge is essentially symmetric with bond lengths of 163.83(9) (S1–O121) and 163.54(9) pm (O2–S121), respectively.

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Figure 1. Coordination of the two crystallographically different K+ ions (K1 and K2) in the crystal structure of K2(S2O7)(H2SO4). The ellipsoids are drawn at a 75 % probability level. Labeling of the atoms is in accordance with Table 2.

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Table 1. Experimental bond lengths for K2(S2O7)(H2SO4).
AtomAtomLength /pm AtomAtomLength /pm
 
K1O1216300.68(9) K2O32280.05(9)
K1O11273.39(10) K2O322280.68(10)
K1O116276.87(10) K2O313291.96(10)
K1O121284.39(10) K2O333297.68(11)
K1O211268.88(10) K2O133286.47(10)
K1O217278.39(10) K2O113307.05(10)
K1O224282.65(10) K2O12280.42(10)
K1O31275.31(10) K2O224301.84(10)
 K2O234296.54(10)
S1O121163.83(9) K2O31331.34(10)
S1O13144.60(10) 
S1O11144.06(9) S3O33154.67(10)
S1O12143.21(9) S3O34151.89(11)
S2O121163.54(9) S3O32143.05(9)
S2O21143.50(9) S3O31142.88(10)
S2O22143.37(10)    
S2O23146.22(10)    
    
 D–H···AH–DH···AD–A[ang] D–H–A 
 O34–H1–O2375.3180.7253.7(1)163.23(1) 
 O33–H2–O1379.2180.7259.4(1)170.35(1) 

In the crystal structure of K2(S2O7)(H2SO4) layers of the composition [K2(S2O7)] are stacked alternating with H2SO4 molecules along the [001] direction (Figure 2). As mentioned above the H2SO4 molecules are partly coordinated to potassium ions, especially to K2. Furthermore, the sulfuric acid molecules are linked via hydrogen bonds to disulfate ions. With donor acceptor distances of 253.7(1) and 259.4(1) pm, respectively, and bond angles of 163.23(1) and 170.35(1)° these hydrogen bonds can be classified as strong.31 The [K2(S2O7)] slabs show some similarities to the well-known disulfate K2(S2O7).32 However, the latter exhibits a three-dimensionally linked structure.

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Figure 2. The crystal structure of K2(S2O7)(H2SO4) is composed of [K2(S2O7)] slabs and H2SO4 layers which alternate along the [001] direction (left). The H2SO4 molecules are coordinated to the K+ ions (especially to K2) but show also strong hydrogen bonds to non-coordinating oxygen atoms of disulfate groups (emphasized as dashed lines in the right part of the Figure).

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In this contribution we could show that the ionic liquid 1-butyl-3-methylimidazolium hydrogensulfate, [bmim]HSO4, is a suitable medium to perform reactions with the strong oxidizer SO3. Compared to our previous reactions which have been performed in oleum or even with neat SO3 this medium allows a much easier adjustment of the SO3 concentration. Furthermore the presented reaction occurred at a much lower temperature. This opens especially new options for the preparation of polysulfates with larger chain lengths. We think that ILs, especially containing HSO4 as an anion, might be a highly suitable way for the preparation of this class of compounds. With respect to this option we are currently investigating in more detail the potential of this specific ionic liquid. Moreover, other ionic liquids that bear also a high oxidation resistivity will be tested.

Experimental Section

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Experimental Section
  6. Acknowledgements
  7. Supporting Information

Synthesis of K2(S2O7)(H2SO4): For the generation of SO3 a 500 mL three-necked flask was filled with P4O10 (30 g, 97 %, Merck, Darmstadt, Germany) and a dropping funnel filled with oleum (30 mL, 65 % SO3, puriss, Merck, Darmstadt, Germany) was connected. By heating to 150 °C SO3 (approx. 2 g) was driven off and condensed into a Schlenk flask by cooling the latter with liquid nitrogen. K2(S2O7)(H2SO4) was obtained by the reaction of K2SO4 (50 mg) with SO3 and the ionic liquid 1-butyl-3-methylimidazolium hydrogensulfate, [bmim]HSO4 (1 mL, used as obtained). The reaction was performed in a thick-walled screwable glass ampoule (Pyrex© Duran glass tube, 12 × 100 mm, with Teflon© inlay) whereas the SO3 was added via a glass applicator. The ampoule was screwed, placed in a sand bath and heated up to 60 °C. The temperature was held at 60 °C for four days. Finally the furnace was switched off and the ampoule was cooled to room temperature. A number of very moisture sensitive colorless single crystals were obtained.

Caution! Oleum and SO3 are strong oxidizers, which need careful handling. During and even after the reaction the tube might be under remarkable pressure.

X-ray Crystallography: Several single crystals were transferred into inert oil (AB128333, ABCR, Karlsruhe, Germany). The crystals are enormous moisture sensitive and thus difficult to prepare. A suitable crystal was mounted onto a glass needle (Ø = 0.1 mm) and immediately placed into a stream of cold N2 (–120 °C) inside the diffractometer (κ-APEX II, Bruker, Karlsruhe, Germany). After unit cell determination, the reflection intensities were collected. Structure solution and refinements were done with the SHELX program package.33

K2(S2O7)(H2SO4): The crystal structure of K2(S2O7)(H2SO4) could be solved in the orthorhombic space group Pbca (no. 61) (Table 2). The heavy atom positions were determined by SHELXS-9733 using direct methods. Further atoms could be successfully located by difference Fourier techniques during refinement with SHELXL-97.33 A numerical absorption correction was applied to the data using the program packages X-RED 1.2234 and X-SHAPE.35 Finally, the structure and model refined to R1 = 0.0354 and wR2 = 0.0519 for all data. Selected bond lengths are presented in Table 1.

Table 2. Crystallographic data of K2(S2O7)(H2SO4).
FormulaK2(S2O7)(H2SO4)
Formula weight352.40
Temperature /K153(2)
Crystal systemorthorhombic
Space groupPbca (no. 61)
a /pm810.64(2)
b /pm1047.90(2)
c /pm2328.86(6)
Volume /Å31978.30(8)
Z8
ρcalc /mg mm–32.366
Absorption coefficient /cm–116.40
F(000)1408
Crystal size /mm0.23 × 0.11 × 0.03
2Θ range for data collection3.50–69.69°
Index ranges–13 ≤ h ≤ 13, –16 ≤ k ≤ 16, –37 ≤ l ≤ 37
Reflections collected49812
Independent reflections4350 [R(int) = 0.0593]
Completeness to θ = 34.845°1.000
Absorption correctionnumerical
Refinement methodFull-matrix least-squares on F2
Data/restraints/parameters4350 / 0 / 153
Goodness-of-fit on F20.945
Final R indexes [I> = 2σ (I)]R1 = 0.0223, wR2 = 0.0495
Final R indexes [all data]R1 = 0.0354, wR2 = 0.0519
Largest diff. peak/hole /e·Å–30.565 / –0.483
ICSD number426362

Further details of the crystal structure investigation may be obtained from the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (Fax: +49-7247-808-666; E-Mail: crysdata@fiz-karlsruhe.de, or at http://www.fiz-karlsruhe.de/request_for_deposited_data.html) on quoting the CSD number CSD-426362 http://www.ccdc.cam.ac.uk/data_request/cif .

Vibrational Spectroscopy: Several crystals were mechanically selected in a glove box, directly transferred to into a closed sample holder, and immediately measured within a range of 6500–550 cm–1 with a Bruker Tensor 27 Spectrometer using the ATR-method (attenuated total reflection). The IR-data were processed with the OPUS 2.0.5 program.36 Important IR energies: 3321 s (νOH), 1385 s (νas SO3), 1296 w (νas SO3), 1186 s (νs SO3), 1086 m (νs SO3), 1003 m (νs SO3), 941 s (νs SO3), 883 s (νs SO3), 773 m (νs SOS), 725 m (νas SOS), 682 s (δas SO3), 658 s (δas SO3), 599 m (δas SO3) cm–1.

Supporting Information (see footnote on the first page of this article): Atomic coordinates, thermal displacement parameters, and a full list of distances and angles for K2(S2O7)(H2SO4).

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Experimental Section
  6. Acknowledgements
  7. Supporting Information

The authors like to thank Dr. Marc Schmidtmann for the collection of the X-ray data. J. B. gratefully acknowledges a stipend from the Stiftung der Metallindustrie im Nordwesten.

Supporting Information

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Experimental Section
  6. Acknowledgements
  7. Supporting Information

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