Cs[Tf2N]: a second polymorph with a layered structure

The structure of the title ionic liquid is layered, with caesium and oxygen atoms forming the center of the layers and fluorine atoms forming the surface of the layers.


Chemical context
Recently, ionic liquids (IL) with melting points below 373 K, known as room temperature ionic liquids (RTIL), have emerged as a novel system that can be used to replace processes utilizing hazardous organic solvents and provide water-free environments (Welton, 1999). The exclusion of water from RTIL can be challenging as their ionic nature predisposes a hygroscopic nature, and even so-called hydrophobic ILs can be difficult to dry (Francesco et al., 2011). Reducing the solubility of water is possible by exchanging constituent ions of the IL for those that are less hydrophilic, but this often results in higher melting points or increased viscosity (Francesco et al., 2011). The ability to change the physicochemical characteristics of ionic liquids has lead them to be praised as 'tunable solvents', but is often more of a challenging act of balancing physical properties.
The substitution of bis(trifluoromethyl)sulfonyl)imide (bistriflimide, Tf 2 N) as the anion in ILs offers a more hydrophobic IL with lower melting point (Matsumoto et al., 2002;Sun et al., 1997). In general, anions of the triflate family are weakly coordinating when in the presence of other ligands, and interactions with metal ions may not be observed when in the presence of water. These weak interactions are due to the delocalization of charge across the molecule. The structure of bistriflimide also allows for multidentate coordination motifs when binding through the oxygen atoms, and often results in coordination of multiple metal cations. When Tf 2 N interactions expand beyond a single central metal atom, the ability to diffuse charge across a structure is highlighted (DesMarteau, 1995). Cs[Tf 2 N] has a desirably low melting point of 398 K which is outside the conventional definition of a RTIL; however, this melting point is in the range of many well-known ILs while ISSN 2056-9890 still being above the boiling point of water to enable convenient drying (Hagiwara et al., 2008;Scheuermeyer et al., 2016). Previous reports of alkali metals and Tf 2 N include Cs[Tf 2 N], which presents as either an anhydrate or a variety of hydrates (Xue et al., 2002). Some common structural similarities can be observed across the A[Tf 2 N]ÁnH 2 O (A = Li, Na, K, Rb, and Cs) series. The most notable feature is the formation of polar and non-polar regions that result from the coordination of multiple metal cations by the Tf 2 N À ion. Each of the sulfonyl groups binds to a metal cation creating a polar chain that may extend to a layer, and orientates the trifluoromethyl groups to create non-polar surfaces. Within the series, only the structures of Cs and K salts as anhydrates have been reported.
Our synthesis and analysis of Cs[Tf 2 N] has revealed a second layered polymorph set in P2 1 /c in addition to the previously reported structure in C2/c (Xue et al., 2002).

Structural commentary
The structure develops from the various ways in which six Tf 2 N molecules coordinate the central 10-coordinate caesium cation (Fig. 1). The simplest coordination mode is monodentate, where one oxygen atom on one of the sulfonyl groups of the Tf 2 N molecule coordinates to the caesium cation. The bidentate coordination mode has two motifs. In end-on coordination, both oxygen atoms of a single sulfonyl group coordinate with Cs + , while in side-on coordination one oxygen on each of the sulfonyl groups within a Tf 2 N molecule coordinate with Cs + . Two of the six distinct Tf 2 N molecules exhibit the side-on coordination mode, and in one of them the nitrogen atom of the Tf 2 N molecule may come close enough to interact with the caesium cation. Examining the bond lengths in the coordination environment of the caesium cation, it is comprised of nine oxygen atoms ranging from 3.060 (2)-3.539 (3) Å and one interaction with a nitrogen atom of 3.280 (3) Å . These three different modes of Tf 2 N binding join the caesium cations together in a complex sheet with layers of trifluoromethyl groups above and below (Fig. 1).
As the Tf 2 N molecule coordinates in the cis conformation, this orients the trifluoromethyl groups in the opposite direc-tion from the sulfonyl groups creating a layer of fluorine atoms. With this layer, trifluoromethyl groups have an intramolecular closest contact of 2.770 (4) Å and an intermolecular closest contact of 2.970 (4) Å . There is a fluorine-fluorine closest contact length of 3.01 Å spanning the void between the non-polar surfaces of adjacent sheets in the layered structure. These layers are easily observed parallel to (100), Fig. 2. Examining the bistriflimide molecule, the S-N-S bond angle is 127.60 (17) resulting in an intramolecular carbon-carbon separation of 4.18 Å .
This structure of alternating layers of hydrophilic alkali metal cations bound by the sulfonyl groups and hydrophobic layers of trifluoromethyl groups closely matches the reported structures of K[Tf 2 N] and Cs[Tf 2 N] (Xue et al., 2002). The noted deviations are in the coordination environment of the Cs + cation. For the previously reported structure of Cs[Tf 2 N], the caesium coordination environment is also 10; however, the oxygen interactions are generally longer by about 0.05 Å , with Cs-O bonds ranging from 3.04 (1) to 3.65 (1) Å . The lone Cs-N bond is 3.39 (1) Å , which is considerably longer than the 3.280 (3) Å bond length observed in the current structure. This extension of bond lengths is reflected in the bistriflimide molecule where the S-N-S bond angle is contracted to 126.38 and the intramolecular carbon-carbon separation is shortened to 4.08 Å . As the molecule shifts, so does the orientation of the trifluoromethyl groups, resulting in an intramolecular closest contact of 2.72 Å and an intermolecular closest contact of 2.96 Å . The shift also extends to the void between the non-polar surface of adjacent sheets, where fluorine-fluorine closest contacts are observed at 2.69 Å spanning the void. While the void space between layers The coordination of the Cs + cation by nine oxygen atoms and one nitrogen atom of six different bistriflimide anions coordinating above and three below, in a view slightly off the (100) plane. Other caesium cations are crystallographically equivalent to Cs1, and are shown to depict how the sheet extends. The displacement ellipsoids are drawn at the 50% probability function with the color scheme of caesium (purple), oxygen (red), sulfur (yellow), nitrogen (blue), carbon (black), and fluorine (green). appears reduced, the overall structure has a calculated density of 2.58 g cm À3 (Xue et al., 2002), less dense than the calculated 2.65 g cm À3 of the more compact current structure.
The Cs[Tf 2 N] purity was confirmed by melting point measurements that closely match literature values, showing an onset temperature of 397 K and complete melting at 399 K (Hagiwara et al., 2008;Scheuermeyer et al., 2016). Additional Raman analysis (Fig. 3), shows a number of features that closely match the reported spectra for Tf 2 N À in water and solid-state measurements made on HTf 2 N, confirming the presence of the Tf 2 N molecule (Rey et al., 1998). To elucidate bands that signify interactions with metal cations, a comparison to the reported Raman spectra of La[Tf 2 N] 3 (H 2 O) 3 (Bhatt et al., 2005) was made. The major bands and assignments of all compounds in the comparisons are reported in Table 1. The additional bands observed at 535 and 507 cm À1 for Cs[Tf 2 N] suggest multiple SO 2 bending modes associated with multiple coordination modes of Tf 2 N. In particular, the band at 507 cm À1 for Cs[Tf 2 N] matches with a 511 cm À1 band in La[Tf 2 N] 3 (H 2 O) 3 suggesting a tentative assignment to a down-shift of the SO 2 bending mode by the bidentate side-on coordination of the Tf 2 N molecule, observed in both structures. Ball and stick model view along [001] showing the layer of the structure arising from the hydrophobic surfaces formed by orientation of the trifluoromethyl groups, comparing (a) the unit cell of the stucture discussed in the paper set in P2 1 /c and (b) that of the previously reported structure set in C2/c (Xue et al., 2002). Atoms are designated as caesium (purple), oxygen (red), sulfur (yellow), nitrogen (blue), carbon (black), and fluorine (green).

Figure 3
Raman spectra of Cs[Tf 2 N] with no other bands observed from 1400 cm À1 to 3200 cm À1 . The additional bands at 535 and 507 cm À1 suggest the multiple SO 2 bending modes associated with multiple coordination modes. Major band assignments are given with (stretching), (bending), ! (wagging), (twisting) and (rocking) followed by the functional group. Planar designations are i.p.for in plane and o.p. for out of plane. minute. After four h, the temperature was reduced to 378 K. While the liquid cooled, the stirring deposited droplets of the ionic liquid on the sides of the beaker resulting in rapid crystallization. These crystals were harvested and suitable crystals were selected for diffraction. Yield was estimated at 95% based on mass.

Experimental
Raman measurements were collected using a Thermo Scientific DXRxi Raman Imaging Microscope. A 532 nm laser was focused on the sample surface through a 10x objective providing a spot size of 1 um and the collection consisted of 200 scans at 10 mW for 0.25 seconds each.
Melting point data were collected on a Bü chi M-560, where two glass sample tubes were filled with 4-5mm of sample and the temperature was ramped at a rate of 0.5 K per minute.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2.

Funding information
Funding for this research was provided by: National Nuclear Security Administration (NA-23).
Comparison of observed Raman shifts in cm À1 from Tf 2 N-containing compounds. Band wavenumbers given in bold are unassigned and in italicized are from reanalysis of reported spectra for La[Tf 2 N] 3 (H 2 O) 3 . Major band assignments are given with (stretching), (bending), ! (wagging), (twisting) and (rocking)

Special details
Experimental. Single crystal data for [Cs][Tf 2 N] were collected on a Bruker D8 Quest diffractometer, with CMOS detector in shutterless mode. The crystal was cooled to 100 K employing an Oxford Cryostream liquid nitrogen cryostat. The diffractometer was equipped with graphite monochromatized MoKa (λ= 0.71073 Å) radiation. A hemisphere of data was collected using omega scans and 0.5° frame widths. Data collection and initial indexing and cell refinement were handled using APEX II1 (Bruker, 2014)software. Frame integration, including Lorentz-polarization corrections, and final cell parameter calculations were carried out using SAINT+ software (Bruker, 2014). The data were corrected for absorption using redundant reflections and the SADABS (Bruker, 2014)program. Decay of reflection intensity was not observed as monitored via analysis of redundant frames. The structure was solved using Direct methods and difference Fourier techniques. Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.