Synthesis of guanidinium–sulfonimide ion pairs: towards novel ionic liquid crystals

The recently introduced concept of ionic liquid crystals (ILCs) with complementary ion pairs, consisting of both, mesogenic cation and anion, was extended from guanidinium sulfonates to guanidinium sulfonimides. In this preliminary study, the synthesis and mesomorphic properties of selected derivatives were described, which provide the first example of an ILC with the sulfonimide anion directly attached to the mesogenic unit.

We were thus interested, whether this concept could be also used to generate the corresponding sulfonimide ion pairs 2 and 3 with mesomorphic properties. The results of this preliminary study are discussed below.

Results and Discussion
The synthesis of guanidinium-sulfonimide ion pairs commenced with the commercially available sulfonyl chloride 4 [38], which was treated with trifluoromethanesulfonamide (5a) in the presence of NEt 3 following a procedure by Hesemann and Brunel [40]. Then the fluorinated sulfonimide was converted to the potassium salt 6a after recrystallization from MeOH in 60% yield (Scheme 2).
The corresponding nonfluorinated sulfonimide K + -salt 6b was obtained by deprotonation of methanesulfonamide (5b) with NaH followed by treatment with sulfonyl chloride 4 according to a method by Dick and Townsend [41]. Analogous deprotonation with KOH yielded 6b in 71%. The K + -salts were prepared due to their more convenient isolation as compared to the corresponding protonated compounds. However, the K + -sulfonimides 6a,b were not used for a direct methyl transfer towards the synthesis of the desired guanidinium-sulfonimide ion pairs in a similar way that the arylsulfonic acid methylesters were previously used as methyl transfer reagents [38], because we wanted to avoid the activation with dimethyl sulfate reported by DesMarteau [21]. Therefore we planned an indirect formation of the ion pairs by anion exchange via salt metathesis. In order to be successful, two requirements have to be met. First, the solubility of the sulfonimide salts 6a,b in the solvent must be sufficient. Second, one of the products must be insoluble in order to shift the equilibrium towards complete conversion. In contrast to the sodium 4-alkoxyphenylsulfonates both potassium salts 6a,b are soluble in boiling MeCN, so that both conditions for a successful salt metathesis are fulfilled.
However, this intermediate did not allow a salt metathesis, because the resulting KI is highly soluble in MeCN (and other organic solvents). Therefore, the intermediate was treated with AgNO 3 in MeOH. The resulting N-methylated guanidinium nitrate was then reacted with 6a or 6b in MeCN to the desired ion pairs 2a and 2b in 79% and 75% yield, respectively, while the precipitating KNO 3 shifted the salt metathesis to completion (Scheme 3).
The good solubility of the K + -sulfonimide salts 6a,b in MeCN was further used for a salt metathesis towards the N-protonated  Figure 1.
POM observations of 3b upon cooling from the isotropic liquid revealed fan-shaped textures and homeotropic alignment ( Figure 2) typical for SmA phases.
The mesophase of compound 3b was investigated by X-ray scattering (WAXS and SAXS) at different temperatures. The  XRD experiments revealed diffraction patterns with a single diffraction peak and a diffuse halo at 4.7 Å resulting from the alkyl chains ( Figure 3). These patterns are typical for smectic mesophases and further confirm the assignment of a SmA phase based on POM observations. The exact layer spacing at each temperature was determined by fitting the first-order peak with a Gaussian distribution ( Figure 3 and Supporting Information File 1, Table S1) and decreases with rising temperatures. To allow a comparison with the layer spacings of compounds 1 and 7•Cl, the layer spacing of 3b was determined at a reduced temperature (T red = 0.95•T iso ) by linear extrapolation of the obtained data (  and anion (23-24 Å, Table 2), we propose the formation of mixed double layers with the charged heads of cation and anion pointing to each other. This packing behavior is in good agreement with those reported for guanidinium sulfonate 1 [38].

Conclusion
We have developed a route towards guanidinium-sulfonimide ion pairs in which both anion and cation contain mesogenic units. The replacement of a spherical halide counterion by a calamitic sulfonimide anion indeed led to a decrease of the melting points, the effect being larger for trifluorosulfonimides 2a and 3a as compared to methylsulfonimides 2b and 3b. It should be noted that Strassner has recently introduced a different concept to tune melting points in ionic liquids by electronic effects of the aryl substituent [44,45]. However, the mesogenic sulfonimide resulted in the formation of a SmA mesophase only in the case of 3b, while ion pairs 2a,b and 3a did not show any liquid-crystalline properties. Thus, the presence of mesogenic counterions could not overcome the known tendency of sulfonimides to inhibit mesomorphism.

Experimental
General Information. All reactions were carried out under a nitrogen atmosphere with Schlenk-type glassware and the solvents were dried and distilled under nitrogen prior to use. Characterization of the compounds was carried out by using the following instruments. Compounds 4 and 5a,b are commercially available. Full characterization of compounds 1 and 8•I is given in [38], and for compound 7•Cl in [42]. For compounds 2b, 3a and 7•I the following water content was determined by Karl Fischer titration: 0.38%, 0.36% and 0.13%, respectively (see Supporting Information File 1, Table S2).

4-(Dodecyloxy)-N-((trifluoromethyl)sulfonyl)benzenesulfonamide, potassium salt (6a):
A mixture of trifluoromethanesulfonamide (5a) (207 mg, 1.38 mmol) and 4-(dodecyloxy)benzenesulfonylchloride (4) (500 mg, 1.39 mmol) was dissolved in abs dichloromethane (20 mL). Abs triethylamine (1 mL, 701 mg, 6.93 mmol) was added and the resulting mixture was heated under reflux for 12 h. After cooling to room temperature the solvent was removed in vacuo, the residue was taken up in ethyl acetate (100 mL), and the hot suspension was filtered. The filtrate was evaporated to dryness and the residue was purified by flash chromatography with ethyl acetate as eluent. The resulting solid was taken up in methanol (20 mL

4-(Dodecyloxy)-N-((methylsulfonyl)benzenesulfonamide, potassium salt (6b):
Methanesulfonamide (5b) (277 mg, 2.91 mmol) was given to a suspension of sodium hydride (333 mg, 8.31 mmol) in abs DMF (10 mL) and the mixture was stirred for 1 h. After cooling to 0 °C a solution of 4-(dodecyloxy)benzenesulfonylchloride (4, 1.00 g, 2.77 mmol) in abs THF (5 mL) was added dropwise and the reaction mixture was warmed to room temperature. After being stirred for 3 days, the mixture was brought to pH 1 by the addition of concd hydrochloric acid. The solvents were removed in vacuo and the residue was taken up in dichloromethane (50 mL). The resulting solution was dried with magnesium sulfate and filtered, and the filtrate was evaporated to dryness. The residue was taken up in methanol (30 mL) and treated with potassium hydroxide (156 mg, 2.77 mmol) for 5 min. After cooling the mixture to 0 °C the product 6b precipitated as a colorless solid. Yield: 901 mg (71%); colorless solid; mp > 300 °C; 1  General procedure for the preparation of pentamethylguanidinium ion pairs (2a,b) Potassium carbonate (144 mg, 971 μmol) and methyl iodide (207 mg, 1.46 mmol) were added to a solution of guanidinium chloride (7•Cl, 200 mg, 485 μmol) in acetonitrile (20 mL). The resulting mixture was heated to 50 °C for 12 h and then cooled to room temperature, and the solvent was removed in vacuo.
The residue was taken up in dichloromethane (20 mL) and filtered, and the filtrate was concentrated to dryness. A solution of the residue in methanol (20 mL) was treated with silver nitrate (165 mg, 971 μmol) and stirred for 12 h at room temperature under the exclusion of light. The solvent was removed under reduced pressure, the residue was taken up in dichloromethane (20 mL), and the slurry was filtered by using a Rotilabo-syringe filter. After concentration of the filtrate to dryness, the residue was taken up in acetonitrile (10 mL), and sulfonimide salt 6a or 6b (509 μmol) was added. The mixture was heated under reflux for 5 min, the solvent was removed in vacuo, and the residue was taken up in dichloromethane (20 mL). After filtration with a Rotilabo-syringe filter the solvent was removed in vacuo, and the crude product was recrystallized from ethyl acetate.    N-(4-(Dodecyloxy)phenyl)-N,N',N',N",N"-   General procedure for the preparation of tetramethylguanidinium ion pairs (3a,b)