Elimination of Interionic Hydrogen Bonding in the Imidazolium-Based Ionic Liquids

Hydrogen bonding is a phenomenon of paramount importance in room-temperature ionic liquids. The presence or absence of the hydrogen bond drastically alternates self-diffusion, shear viscosity, phase transition points, and other key properties of a pure substance. For certain applications, the presence of cation-anion hydrogen bonding is undesirable. In the present paper, we investigate perspectives of removing the hydrogen...fluorine interionic attraction in the imidazolium borates, the strongest non-covalent interaction in this type of system. Chemical modification of the tetrafluoroborate anion not only eliminates hydrogen bonding but also changes the most thermodynamically preferable orientation of the cation in the vicinity of the anion. Although the most acidic hydrogen atom of the imidazole ring remains the paramount electrophilic center of the cation, it does not engender a strong electrostatically driven coordination pattern with the properly modified anions. The reported new physical insights help compose more robust ionic liquids and tune solvation properties of the imidazolium-based RTILs.


Graphical Abstract
Introduction Room-temperature ionic liquids (RTILs) are a general term to designate an extensive group of organic and organic-inorganic salts that exhibit melting points above 373 K. [1][2][3] RTILs possess somewhat specific physical-chemical properties compared to molecular liquids. 4 They conduct electricity, exhibit small vapor pressures, and demonstrate a robust catalytic activity. Furthermore, some RTILs are environmentally friendly and serve as rather universal solvents for inorganic and biological entities thanks to their amphiphilic structures and a wide liquid range. 5  The well-recognized shortcomings of all RTILs in the context of industry-level applications are high shear viscosities and low self-diffusivities. These drawbacks come not only from more or less long hydrocarbon chains confining the central polarized atom but also from the relatively strong cation-anion coordination in the condensed phase. Nevertheless, the electrostatic attraction in RTILs is significantly weaker than in the case of inorganic molten salts. 6 The cation-anion hydrogen bonding constitutes another powerful factor that increases melting temperatures and shear viscosities of RTILs. 7 Hydrogen bonding can be avoided by employing weakly coordinating ions and chemically modifying the electron density distribution on the cation or the anion by introducing inert, non-polar, and bulky substituents.
The imidazolium-based ionic liquids [8][9][10][11] hold promise for application in the field of energy storage. For instance, they can be used as robust electrolytes in electric double-layer capacitors because the imidazolium-based RTILs possess a much wider potential window, better thermal stabilities, higher power densities, and lower saturated vapor pressures as compared to most of the conventional organic and aqueous electrolyte systems. 12 In the meantime, the imidazolium-based RTILs are inferior to their traditional competitors according to ionic conductivity. Ionic conductivity directly depends on the self-diffusivities of the ions, therefore both enumerated physical properties must be tuned together.
A common practical solution to the high viscosity problem is the preparation of mixtures containing ionic and molecular liquids in certain proportions. 4,[13][14][15][16] Thanks to polar moieties in the structure of many polar solvents, RTILs normally exhibit a strongly negative solvation enthalpy while forming true solutions. The resulting systems exhibit intermediate physical-chemical properties being actually stable and fine-tuned hybrids of their parental systems. The dependence of ionic conductivity versus the molar fraction of RTIL contains a maximum that features interplay between the diffusivity of the ions and their quantity per a unit volume. The electrolytes composed of ionic and molecular liquids hold promise for versatile applications in electrochemical devices. 17 The progress in organic chemistry led to the synthesis of the unusual cations and anions that are referred to as weakly coordinating ions. 18 also simulated as two ion pairs to understand the role of a larger system in the interionic coordination.
The global minimum, local minima and transition points were found on the potential energy surface of the simulated systems according to the following procedure. First, the system was equilibrated at 300 K using PM7-MD simulations. 24 The equations of motion were integrated with a time-step of 5×10 -4 ps according to the algorithm of Verlet. At equal time intervals, the external kinetic energy equivalent to the immediate temperature of 1000 K was provided to the system by randomly increasing the linear velocities of the atoms to maintain Maxwell-Boltzmann distribution. The system was left to relax during 100 time-steps before the Cartesian coordinates of the configuration were saved. The excess energy was subsequently removed from the system by the Andersen thermostat, 25

Results and Discussion
We start from the investigation of the potential energy surfaces ( Figure 1) and the effect of the anion on the number of stationary points. In the [EMIM][BF 4 ] system, only two stationary points were identified, the less thermodynamically favorable one being +4.5 kJ mol -1 inferior to the global minimum state. The global minimum ionic configuration is depicted in Figure 2. The major structural pattern in this system is a formation of the strong hydrogen...fluorine H-bond. Its energy capacity is confirmed by the bond length, 0.191 nm.
This result is in perfect agreement with molecular dynamics and quantum chemistry reports that were published during the last two decades. [40][41][42] In turn, the [EMIM][Me 4 B] system is more conformationally flexible ( Figure 1). Note that a greater number of atoms and chemical bonds in a system generally enhance diversity of its potential energy surface. Furthermore, the absence of the cation-anion coordination pattern fosters the emergence of new stationary states. The stationary points with the potential energies of more than +50 kJ mol -1 are transition states as suggested by the presence of significantly large imaginary vibrational frequencies. They correspond to the non-covalent rearrangements of the electrostatically interacting moieties of the cation and the anion and range between 100 and 400 cm -1 in the complex space. In the present simulations, we did not aim to find transition states that would be responsible for chemical reactions, e.g. bond breakage (stretching). A more aggressive perturbation must be used to locate the mentioned points on the potential energy surface should they be of interest for a computational study.  Figure 3, whereas the selected ionic configurations are exemplified in Figure 4. One notices that larger systems feature larger energetic differences among the local minimum states, while their total number also increases somewhat. Several transition points above +200 kJ mol -1 reveal repacking of ions upon searching for alternative stable configurations.    4 ]. Therefore, the cation-anion attraction forces get must smaller. This is clearly evidenced by the number of stationary points located in the considered systems. The absence of a single preferable coordination pattern gives rise to novel states and more complex molecular dynamics in the condensed phase.      Our above hypothesis gets its confirmation from Figure 7 that summarizes the partial charges on the boron atoms. The boron atom is not sensitive to the chemical nature of the substituents but it is sensitive to the distance of the center of the anion to the imidazole ring.
The partial charges that were derived from the electrostatic potential fitting procedure decrease as the size of the substituent group increases. In turn, the CM5 charges are quite insensitive to the substituent. Whereas the partial charge on boron is +0.  In the present work, it is important to identify the role of the system's size and demonstrate that the systems (Table 1) used to study the potential energy surfaces of the ionic systems can be successfully applied. Figure 9 provides the results of the potential energy surface search, whereas Figure     In the case of the discussed larger system, we used a softer kinetic energy perturbation.
Unlike for other systems, the perturbation was equivalent to 500 K. The procedure was modified because we were not interested in sampling transition states. Instead, we rather aimed to get local minimum states to compare them with the analogous states in the smaller system. As a result, Figure 9 does not contain any transition points. It should be noted that the magnitude of the perturbation in the applied sampling method acts as a limiting factor for the investigation of the potential energy surface. For instance, a rather modest periodic perturbation used in this work does not allow for any chemical reactions to be initiated. The applied kinetic energy perturbation, therefore, must be chosen responsibly and with a phenomenon of interest in mind.

Conclusions
The physical-chemical properties of RTILs represent a many-dimensional function wherein the constituent ions are the cornerstone parameters. The cation-anion interaction heavily depends on the specific coordination sites available in the structures of both involved ions. In this work, we showed that the adjustment of interionic attraction is possible via a straightforward chemical modification of the anion's structure. The modified tetraalkylborate anions foster redistribution of the electron density over all ions, change the major structural pattern and, most importantly, eliminate a strong cation-anion electrostatic attraction that leads to a hydrogen bond formation in 1-ethyl-3-methyl-imidazolium tetrafluoroborate.
The systematic investigation of stationary points is equivalent to the most comprehensive sampling of the phase space in molecular dynamics simulations. Furthermore, an ability to observe ionic configurations that are located far from the global minimum state assures that we considered all relevant points in the simulated systems. The applied method forms a potentially fruitful basis to succeed in the target molecular design and the development of novel task-specific ionic liquids.
In the future, the experimental measurement must be performed to identify an extent to which the melting temperature in the imidazolium-based RTILs decreases thanks to the removal of the strongest interionic attraction pattern that involves an intrinsically acidic hydrogen atom of the positively charged imidazole ring.

Acknowledgments
All reported numerical simulations have been conducted at the P.E.S. computational facility.

Conflict of interest
The author does not have financial or personal relationships that might inappropriately influence (bias) their actions: dual commitments, competing interests, or competing loyalties.

Authors for correspondence
All correspondence regarding the content of this paper shall be directed through electronic mail to the author: vvchaban@gmail.com (Professor Dr. Vitaly V. Chaban).