Hydrogen Bonding in Protic Ionic Liquids: Reminiscent of Water

Similarities and differences : Far‐infrared spectra of protic ionic liquids could be assigned to intermolecular bending and stretching modes of hydrogen bonds. The characteristics of the low‐frequency spectra resemble those of water. Both liquids form three‐dimensional network structures, but only water is capable of building tetrahedral configurations. EAN: ethylammonium nitrate, PAN: propylammonium nitrate, DMAN: dimethylammonium nitrate.

     

Spectroscopic Evidence for Clusters of Like‐Charged Ions in Ionic Liquids Stabilized by Cooperative Hydrogen Bonding

Direct spectroscopic evidence for hydrogen‐bonded clusters of like‐charged ions is reported for ionic liquids. The measured infrared O?H vibrational bands of the hydroxyethyl groups in the cations can be assigned to the dispersion‐corrected DFT calculated frequencies of linear and cyclic clusters. Compensating the like‐charge Coulomb repulsion, these cationic clusters can range up to cyclic tetramers resembling molecular clusters of water and alcohols. These ionic clusters are mainly present at low temperature and show strong cooperative effects in hydrogen bonding. DFT‐D3 calculations of the pure multiply charged clusters suggest that the attractive hydrogen bonds can compete with repulsive Coulomb forces.     

Controlling the Subtle Energy Balance in Protic Ionic Liquids: Dispersion Forces Compete with Hydrogen Bonds

The properties of ionic liquids are determined by the energy‐balance between Coulomb‐interaction, hydrogen‐bonding, and dispersion forces. Out of a set of protic ionic liquids (PILs), including trialkylammonium cations and methylsulfonate and triflate anions we could detect the transfer from hydrogen‐bonding to dispersion‐dominated interaction between cation and anion in the PIL [(C₆H₁₃)₃NH][CF₃SO₃]. The characteristic vibrational features for both ion‐pair species can be detected and assigned in the far‐infrared spectra. Our approach gives direct access to the relative strength of hydrogen‐bonding and dispersion forces in a Coulomb‐dominated system. Dispersion‐corrected density functional theory (DFT) calculations support the experimental findings. The dispersion forces could be quantified to contribute about 2.3 kJ mol^?1 per additional methylene group in the alkyl chains of the ammonium cation.     

Dispersion and Hydrogen Bonding Rule: Why the Vaporization Enthalpies of Aprotic Ionic Liquids Are Significantly Larger than those of Protic Ionic liquids

It is well known that gas‐phase experiments and computational methods point to the dominance of dispersion forces in the molecular association of hydrocarbons. Estimates or even quantification of these weak forces are complicated due to solvent effects in solution. The dissection of interaction energies and quantification of dispersion interactions is particularly challenging for polar systems such as ionic liquids (ILs) which are characterized by a subtle balance between Coulomb interactions, hydrogen bonding, and dispersion forces. Here, we have used vaporization enthalpies, far‐infrared spectroscopy, and dispersion‐corrected calculations to dissect the interaction energies between cations and anions in aprotic (AILs), and protic (PILs) ionic liquids. It was found that the higher total interaction energy in PILs results from the strong and directional hydrogen bonds between cation and anion, whereas the larger vaporization enthalpies of AILs clearly arise from increasing dispersion forces between ion pairs.     

Dissecting the Vaporization Enthalpies of Ionic Liquids by Exclusively Experimental Methods: Coulomb Interaction,Hydrogen Bonding,and Dispersion Forces

The quantification of hydrogen bonding and dispersion energies from vaporization enthalpies is a great challenge. Dissecting interaction energies is particularly difficult for ionic liquids (ILs), for which the composition of the different types of interactions is known neither for the liquid nor for the gas phase. In this study, we demonstrate the existence of ion pairs in the gas phase and dissect the interaction energies exclusively from measured vaporization enthalpies of different alkylated protic ILs (PILs) and aprotic ILs (AILs) and the molecular analogues of their cations. We demonstrate that the evaporated ion pairs are characterized by H‐bond‐enhanced Coulomb interaction. The overall interaction energy for the ILs in the bulk phase is composed of Coulomb interaction (76 kJ mol^?1), hydrogen bonding (38 kJ mol^?1), and minor dispersion interaction (10 kJ mol^?1). Thus, hydrogen bonding prominently contributes to the overall interaction energy of PILs, which is reflected in the properties of this class of liquids.     

The Role of Ionic Liquids in Hydrogen Storage

Ionic liquid (IL) based H₂ storage for H₂ generation from NH₃BH₃ derivatives is shown. These systems promote H₂ generation at low temperature, with good reaction rates and high total H₂ yields. The effects of ILs and the H₂ yield in correlation with the basicity, the cations of the ILs, and the role of carbenes are discussed. Furthermore, mechanistic findings on the dehydrogenation are described. IL material blends are competitive with conventional H₂ storage materials with experimental efficiencies of at least 6.5 wt % H₂.     

The Double‐Faced Nature of Hydrogen Bonding in Hydroxy‐Functionalized Ionic Liquids Shown by Neutron Diffraction and Molecular Dynamics Simulations

We characterize the double‐faced nature of hydrogen bonding in hydroxy‐functionalized ionic liquids by means of neutron diffraction with isotopic substitution (NDIS), molecular dynamics (MD) simulations, and quantum chemical calculations. NDIS data are fit using the empirical potential structure refinement technique (EPSR) to elucidate the nearest neighbor H???O and O???O pair distribution functions for hydrogen bonds between ions of opposite charge and the same charge. Despite the presence of repulsive Coulomb forces, the cation–cation interaction is stronger than the cation–anion interaction. We compare the hydrogen‐bond geometries of both “doubly charged hydrogen bonds” with those reported for molecular liquids, such as water and alcohols. In combination, the NDIS measurements and MD simulations reveal the subtle balance between the two types of hydrogen bonds: The small transition enthalpy suggests that the elusive like‐charge attraction is almost competitive with conventional ion‐pair formation.     

Pseudohalogen Chemistry in Ionic Liquids with Non-innocent Cations and Anions

Within the second funding period of the SPP 1708 “Material Synthesis near Room Temperature”,which started in 2017, we were able to synthesize novel anionic species utilizing Ionic Liquids (ILs) both, as reaction media and reactant. ILs, bearing the decomposable and non-innocent methyl carbonate anion [CO₃Me]⁻, served as starting material and enabled facile access to pseudohalide salts by reaction with Me₃Si−X (X=CN, N₃, OCN, SCN). Starting with the synthesized Room temperature Ionic Liquid (RT-IL) [nBu₃MeN][B(OMe)₃(CN)], we were able to crystallize the double salt [nBu₃MeN]₂[B(OMe)₃(CN)](CN). Furthermore, we studied the reaction of [WCC]SCN and [WCC]CN (WCC=weakly coordinating cation) with their corresponding protic acids HX (X=SCN, CN), which resulted in formation of [H(NCS)₂]⁻ and the temperature labile solvate anions [CN(HCN)_n]⁻ (n=2, 3). In addition, the highly labile anionic HCN solvates were obtained from [PPN]X ([PPN]=μ-nitridobis(triphenylphosphonium), X=N₃, OCN, SCN and OCP) and HCN. Crystals of [PPN][X(HCN)₃] (X=N₃, OCN) and [PPN][SCN(HCN)₂] were obtained when the crystallization was carried out at low temperatures. Interestingly, reaction of [PPN]OCP with HCN was noticed, which led to the formation of [P(CN)₂]⁻, crystallizing as HCN disolvate [PPN][P(CN⋅HCN)₂]. Furthermore, we were able to isolate the novel cyanido(halido) silicate dianions of the type [SiCl_0.78(CN)_5.22]²⁻ and [SiF(CN)₅]²⁻ and the hexa-substituted [Si(CN)₆]²⁻ by temperature controlled halide/cyanide exchange reactions. By facile neutralization reactions with the non-innocent cation of [Et₃HN]₂[Si(CN)₆] with MOH (M=Li, K), Li₂[Si(CN)₆] ⋅ 2 H₂O and K₂[Si(CN)₆] were obtained, which form three dimensional coordination polymers. From salt metathesis processes of M₂[Si(CN)₆] with different imidazolium bromides, we were able to isolate new imidazolium salts and the ionic liquid [BMIm]₂[Si(CN)₆]. When reacting [Mes(nBu)Im]₂[Si(CN)₆] with an excess of the strong Lewis acid B(C₆F₅)₃, the voluminous adduct anion {Si[CN⋅B(C₆F₅)₃]₆}²⁻ was obtained.     

Salts and Ionic Liquids of Resonance Stabilized Amides

The synthesis, structure, and bonding of alkali salts of resonance stabilized amides, such as diformylamide (dfa), formylcyanoamide (fca), nitrocyanoamide (nca), and for comparision, the well‐known dicyanoamide (dca), are discussed on the basis of experimental and theoretical data. The first structural reports of K(18‐crown‐6)⁺dfa^?, K(18‐crown‐6)⁺fca^?, Na⁺nca^?, and Li(TMEDA)⁺dca^? are presented. Examination of the X‐ray data reveals almost planar anions with strong cation–anion interactions resulting in network‐like structures in the solid state. For comparison, the X‐ray structures of covalently bound phenyldicyanoamide and diformamide are also discussed. The thermal behavior of the alkali salts of these amides is studied by thermoanalytical experiments. Moreover, several novel ionic liquids based on resonance stabilized amides have been prepared and were fully characterized, namely the dfa, fca, and nca salts of EMIM (1‐ethyl‐3‐methyl‐imidazolium), BMIM (1‐butyl‐3‐methyl‐imidazolium), and HMIM (1‐hexyl‐3‐methyl‐imidazolium). Most of them are liquid at room temperature, except BMIM⁺fca^? that melts at 32 °C. These ionic liquids are neither heat nor shock sensitive, are thermally stable up to over 200 °C, and can be prepared easily in large quantities.     

Role of Hydrogen Bond Defects for Cluster Formation and Distribution in Ionic Liquids by Means of Neutron Diffraction and Molecular Dynamics Simulations

Defects fundamentally govern the properties of all real materials. Correlating molecular defects to macroscopic quantities remains a challenge, particularly in the liquid phase. Herein, we report the influence of hydrogen bonds (HB) acting as defects in mixtures of non-hydroxyl-functionalized ionic liquids (ILs) with an increasing concentration of hydroxyl-functionalized ILs. We observed two types of HB defects: The conventional HBs between cation and anion (c–a), and the elusive HBs between cations (c–c) despite the repulsive Coulomb forces. We use neutron diffraction with isotopic substitution in combination with molecular dynamics simulations for measuring the geometry, strength, and distribution of mobile OH defects in the IL mixtures. In principle, this procedure allows relating the number and stability of defects to macroscopic properties such as diffusion, viscosity, and conductivity, which are of utmost importance for the performance of electrolytes in batteries and other electrical devices.     

Symmetry Breaking in Chiral Ionic Liquids Evidenced by Vibrational Optical Activity

Ionic liquids (ILs) are receiving increasing interest for their use in synthetic laboratories and industry. Being composed of charged entities, they show a complex and widely unexplored dynamic behavior. Chiral ionic liquids (CILs) have a high potential as solvents for use in asymmetric synthesis. Chiroptical methods, owing to their sensitivity towards molecular conformation, offer unique possibilities to study the structure of these chiral ionic liquids. Raman optical activity proved particularly useful to study ionic liquids composed of amino acids and the achiral 1‐ethyl‐3‐methylimidazolium counterion. We could substantiate, supported by selected theoretical methods, that the achiral counterion adopts an overall chiral conformation in the presence of chiral amino acid ions. These findings suggest that in the design of chiral ionic liquids for asymmetric synthesis, the structure of the achiral counter ion also has to be carefully considered.