Cationic clustering influences the phase behaviour of ionic liquids

“Unlike charges attract, but like charges repel”. This conventional wisdom has been recently challenged for ionic liquids. It could be shown that like-charged ions attract each other despite the powerful opposing electrostatic forces. In principle, cooperative hydrogen bonding between ions of like-charge can overcome the repulsive Coulomb interaction while pushing the limits of chemical bonding. The key challenge of this solvation phenomenon is to establish design principles for the efficient formation of clusters of like-charged ions in ionic liquids. This is realised here for a set of well-suited ionic liquids including the same hydrophobic anion but different cations all equipped with hydroxyethyl groups for possible H-bonding. The formation of H-bonded cationic clusters can be controlled by the delocalization of the positive charge on the cations. Strongly localized charge results in cation-anion interaction, delocalized charge leads to the formation of cationic clusters. For the first time we can show, that the cationic clusters influence the properties of ILs. ILs comprising these clusters can be supercooled and form glasses. Crystalline structures are obtained only, if the ILs are dominantly characterized by the attraction between opposite-charged ions resulting in conventional ion pairs. That may open a new path for controlling glass formation and crystallization. The glass temperatures and the phase transitions of the ILs are observed by differential scanning calorimetry (DSC) and infrared (IR) spectroscopy.

cationic clusters strongly influences the properties of ionic liquids. If substantial amount of cationic clusters is available, the ILs can be supercooled and form glasses at temperatures between 193 and 213 K. If cationic clusters are only present in trace amounts, the ILs can be crystallized by forming hydrogen-bonded 'ion pairs' between cations and anions solely 30 . The glass transition temperatures and the phase transition temperatures were measured by differential scanning calorimetry (DSC). If crystallization occurs, the infrared spectra show the typical sharp features of cation-anion interaction only and the broad redshifted bands of the cationic clusters completely disappear. For these ILs the structures could be obtained from X-ray crystallography 30 . The formation of clusters of like-charged ions is influenced by the shape and interaction strength of the ions and may provide a new path for controlling supercooling and crystallization.

Results and Discussion
First, we studied the formation and stability of cationic clusters for a set of specifically selected ionic liquids (see SI1). The ILs are based on ammonium, piperidinium, pyrrolidinium, pyridinium, and imidazolium cations all including a hydroxyethyl group for H-bond formation to the anion or to other cations resulting in cationic clusters. The cations are shown in Fig. 1 Fig. 1). The ILs II, II and IV were synthesized in our lab (see SI1), the ILs I and V were purchased form IoLiTec.
In respect to structure and charge distribution within the cation, the ILs can be divided into two groups. ILs I, II and III represent saturated compounds, characterized by positive charge localization at the nitrogen atom. In contrast, delocalized positive charge within the nitrogen heterocycle is typical for ILs IV and V. The transmission infrared spectra for all ILs I-V are shown as a function of temperature in Fig. 2. We started at the highest temperature of about 353 K for all compounds and went down below the phase transition temperature for each IL ranging between 253 K for ILs I, II and III, 213 K for IL IV and 193 K for IL V.
For all ILs I-V the observed OH stretch vibrational bands above 3500 cm −1 can be exclusively referred to the hydrogen bond enhanced OH … O=S cation-anion (c-a) interaction. In all compounds I-V these vibrational bands have almost similar frequencies and intensities at 353 K, which indicates that the c-a hydrogen bonds are independent of the nature of the cation (see SI2). For all ILs vibrational intensity is also observed below 3500 cm −1 but to different extent depending on the type of cation in the ILs I-V. These redshifted vibrational features can be assigned to the OH … OH hydrogen bonds among the cations (c-c) indicating the presence of cationic clusters (see Fig. 2). Recently, both structural motifs have been observed for isolated cationic complexes of IL V by using a combined approach of photodissociation mass spectrometry and double resonance techniques 31 .
For ILs I and II the IR spectra are mainly characterized by the vibrational bands at 3540 cm −1 indicating pure c-a interaction along the OH … O=S hydrogen bond. Only little redshifted contributions can be observed for these ILs. Below their melting points, the IR spectra in the OH frequency range become highly symmetric. (see SI3) Even the trace amounts of redshifted intensities disappear and the ionic interaction is described by c-a hydrogen bonds only. The interpretation of the vibrational bands is supported by DFT calculated frequencies of c-a and c-c type clusters including one up to four ion pairs at the B3LYP-D3/6-31 + G* level of theory (see SI4) [32][33][34][35] . Figure 3 shows the spectrum of [HEMIm]NTf 2 ] (V) at the lowest (213 K) and highest (353 K) temperatures. Additionally the calculated averaged frequencies of a dimer, trimer and tetramer of purely c-a bound clusters along with those of a dimer, linear trimer and cyclic tetramer of purely c-c bound clusters are shown. The calculated c-a frequencies lie clearly above 3500 cm −1 , whereas the c-c cluster frequencies are further redshifted with increasing cluster size strongly supporting the existence of two types of cluster species. The agreement between the calculated and measured frequencies also underlines that the c-c hydrogen bonds are stronger than those in the c-a clusters. That is quite surprising because the c-a hydrogen bonds are additionally enhanced by Coulomb attraction, whereas the c-c hydrogen bonds have to compete with the repulsive Coulomb forces between like-charged ions. Overall,  the comparison with the experimental spectra suggest that the existence of linear trimer and cyclic tetramers are required to represent the measured IR spectra.
We assume that the cation-anion (I ca ) and the cation-cation (I cc ) hydrogen bond intensities arise from cations of these two general classes of H-bonding configurations, for which relative populations are a function of absolute temperature only. Consequently, a plot of ln(I cc /I ca ) versus 1/T should yield a straight line with a slope that is proportional to the average difference in energy between the two classes of H-bond configurations. All ILs show a linear dependence between ln(I cc /I ca ) and 1/T. The slope is positive for all ILs indicating that the formation of the cationic clusters resembles an exothermic reaction (see Fig. 4). Obviously, we observe two types of cations involved in different types of clusters, c-c and c-a. The slopes for IL I and II are significantly smaller than those for the other ILs (1.6 vs 1.0). The low reaction enthalpy for the ILs comprising saturated cations with localized charge indicates that only small clusters are formed in accord with the small redshifted contributions in the IR spectra. The other ILs in particular IV and V form larger clusters with enhanced stability which cannot be destroyed in favor of crystallization.
The interpretation of the IR spectra at low temperatures is also supported by the DSC measurements (see also SI5). During cooling from 373 to 193 K at 2, 5 and 10 K·min −1 , only a heat capacity change corresponding to the glass transition (T g ) could be observed in the DSC profiles of IL IV and V. It means that the supercooled liquids are fairly stable. For ILs I, II and III complex phase transition with melting (T fus ) and solid/solid phase transition (T ss ) are observed. Figure 5 shows the DSC traces related to the temperature dependent IR spectra of ILs II and V. After phase transition, the IR spectra for IL II only exhibit c-a vibrational bands, whereas the minor c-c spectral features typical for the liquid phase completely disappear. Obviously, the disappearance of the cationic clusters and the presence of pure c-a bonded ion pairs are related to fusion at 276.2 K and two solid/solid phase transitions at 266.2 and 251.6 K, respectively. In contrast, the strong formation of cationic clusters in IL V is not changing the spectral features with decreasing temperature. The liquid can be supercooled and show glass transition between 195 and 198 K. From the combined IR and DSC experiments, we have clear evidence that the formation of cationic clusters prevents the ILs from crystallization and liquid/solid phase transition. The resulting material is a glass.
Both, IR and DSC experiments suggest that ILs I and II should give crystal structures. The crystal structure for IL I is known from the literature and shows one cation-anion pair in the asymmetric unit and four ion pairs in the unit cell 30 . As expected, the choline cation and the [NTf 2 ] − anion are held together by hydrogen bonds with an interatomic distance between the hydroxyl proton and the sulfonyl oxygen atom of about r(H…O=S) = 205 pm. Unfortunately, we failed to obtain the crystal structure for IL II. We found a low-temperature structure at 12 K (see SI6) that showed neither H-bonding between anions and cations, nor H-bonding among the cations. This structure showed a delocalized positive charge, which is spherically surrounded by the negative partial charges of the anions. The DCS traces show a second crystalline phase of IL II at 267 K between the low-temperature phase and the amorphous (glassy) phase (see Fig. 5). Probably due to high thermal motion and resulting decrease in scattering power, we were not able to solve the crystal structure of this phase.
Overall, the IR and DSC measurements clearly indicate different phase transition behavior for these two types of ILs. If substantial cationic clusters are formed, the liquid IL will supercool and becomes a glass at low temperatures as in ILs IV and V. No liquid/solid phase transition is observed. Usually, crystallization can be avoided only, if liquids are cooled sufficiently fast. Molecules will then arrange so slowly that they cannot adequately sample configurations in the available time allowed by the cooling rate. The liquid's structure, therefore, appears 'frozen' . The resulting material is a glass 36 . Here, we show that substantial cationic cluster formation prevents crystallization and glass formation for ILs. Hydrogen bonding immobilizes the cations and no solid configurations can be formed.

Conclusions
In this work, we could show for a set of well-suited ionic liquids that the presence of like-charge attraction is governed by the charge delocalization of the cation. If the positive charge is delocalized and available to the counterion, cooperative hydrogen bonding between the like charges is possible resulting in cationic clusters up to cyclic tetramers. As shown in the IR spectra, these clusters are characterized by redshifted vibrational bands which increase in intensity with decreasing temperature. The ILs exhibiting substantial amounts of cationic clusters form glasses as shown by DSC measurements. The ILs showing only traces of these structures can be crystallized or solidified, resulting in pure cation-anion hydrogen bonds. The glass and liquid/solid phase transitions are also observed in the IR experiments. Obviously, the amount of cationic clusters influence the glass formation and crystallization. For the first time we could show, that like-charge attraction modifies the macroscopic properties of ILs. The formation of clusters of like-charged ions is influenced by the shape and interaction strength of the ions and may provide a new path for controlling crystallization and glass formation.