How much do coulombic interactions stabilize a mesophase? Ion pair and non-ionic binary isosteric derivatives of monocarbaborates and carboranes †

Replacement of the B (cid:1) atom in the monocarbaborate anion, 1[10] or 1[12] , and the N + atom in the pyridinium cation [Pyr] of a liquid crystalline ion pair with C atoms leads to an isoelectronic and isosteric non-ionic binary liquid crystalline mixture of carborane ( 2[10] or 2[12] ) and benzene ( [Ph] ) derivatives lacking coulombic interactions. A comparison of mesogenic properties of ion pairs, 1[10]c – [Pyr]c and 1 [12]c – [Pyr]c , with their analogous non-ionic mixtures, 2[10]c – [Ph]c and 2[12]c – [Ph]c , shows a 181 K higher clearing temperature, T c , for the ion pair. This corresponds to a DFT-calculated di ﬀ erence in association energy DD H a ¼ 24.5 kcal mol (cid:1) 1 in a typical dielectric medium ( 3 ¼ 2.5). Pure compounds and binary mixtures were characterized using thermal, optical, and XRD methods.


Introduction
Ionic liquid crystals (ILC) are capable of anisotropic ion transport, [1][2][3][4] and for this reason they are becoming increasingly attractive electrolytes for ion batteries, 5 photovoltaic 6-8 and other applications. 9,10In contrast to non-ionic mesogens, typical ILC are binary systems, comprising of a cation and an anion.Consequently, mesogenic properties of ILC are controlled not only by the anisometry of the two components, but also by electrostatic interactions. 11While the rst factor is well understood, 12 the effect of coulombic interactions on phase stability is poorly characterized and has only recently become the subject of theoretical considerations. 13,14Despite the large number of examples of ILC, 10 there exists no single report of a molecular system that permits experimental evaluation of the coulombic component of the phase stability in the absence of other factors.We now report such a molecular system and for the rst time assess the effect of coulombic interactions, as the only variable, on mesophase stability in ILC.
[17][18] Due to the high delocalization of the charge and the resulting weak nucleophilicity of the boron cluster anions, 19 the ion interactions in such ILC, e.g.1a-[Pyr]a (Fig. 1), 16 are non-specic and mainly electrostatic.Such ion pairs typically form SmA and so crystalline phases, although some exhibit a nematic phase, e.g.azo derivative 1 [12]b-[Pyr]a (Fig. 1). 16eplacement of the B À (12) atom in anion 1 [12]b and the N + atom in cation [Pyr]a with C atoms leads to a non-ionic pair consisting of carborane 2 [12]b and benzene derivative [Ph]a.An equimolar mixture of 2 [12]b and [Ph]a represents an isosteric, non-coulombic analogue of the mesogenic ion pair 1 [12]b-[Pyr]a.
Here, we report the synthesis and detailed thermal, optical and structural (XRD) analysis of ion pairs 1 [10]-[Pyr] and 1 [12]-[Pyr] as well as their electrically neutral analogues, 2 and [Ph].We emphasize the comparison of the mesogenic behaviour of ion pairs 1-[Pyr] with the isosteric, non-ionic equimolar mixtures 2-[Ph].The experimental results are supported with DFT modelling of the association of ions and non-ionic analogues in a dielectric medium.

Thermal analysis
Transition temperatures and enthalpies of the newly prepared compounds were determined by differential scanning calorimetry (DSC) and results are shown in Tables 1 and 2.
Single component materials.All six non-ionic carborane derivatives 2 form a nematic phase (Table 1), which is consistent with nematogenic behavior of their benzene analogues.The three-ring compounds 2a and 2b exhibit a wide temperature range enantiotropic phase with clearing temperatures around 200 C.A comparison of the carborane derivatives with their benzene analogues demonstrates that the phase stability follows the order [10] < [12] < [Ph], which is in agreement with our previous ndings. 16,25The two-ring mesogens, 2c, exhibit monotropic phases with signicantly lower N-I transition temperatures.
The phenyl ether [Ph]c is a new member of a known homologous series 27 and it exhibits a mesophase identied by XRD analysis as a so crystalline B phase (Cr 32 SmB cryst 42 I, Fig. 2a).
DSC, POM, and XRD 28 analyses revealed that pyridinium bromide [Pyr]c-Br exhibits a SmA phase and another more ordered unidentied phase.Unfortunately, the salt decomposes at about 165 C. The shorter analogue, [Pyr]b-Br, is not mesogenic.
Two-component materials.Ion pairs 1a-[Pyr]a and 1b-[Pyr]a were reported before, and they display SmA and nematic phases. 16Previous results demonstrated that ions pairs 1c-[Pyr]a, in which the total number of rings is 3, melt about 110 C and do not form liquid crystalline phases. 16Inclusion of the cyclohexyl ring in the structure of the pyridinium cation increased the   2).Extension of the butyl chain in the [Pyr]b cation to undecyl in [Pyr]c resulted in the appearance of an enantiotropic SmA phase above the E phase in both ion pairs, 1 [10]c-[Pyr]c and 1 [12]c-[Pyr]c, with a clearing temperature of about 210 C (e.g.Fig. 3 and 4).DSC analysis of 1 [12]c-[Pyr]c also revealed another so-crystalline phase X below the E phase.
These ndings are in agreement with our general observations that a total of 4 rings in the ion structures are necessary for induction of mesogenic behavior in ILC of the general structure 1. 16,18 For instance, while 1c-[Pyr]a, a 3-ring ionic compound, does not exhibit mesogenic behaviour, 4-ring ion pairs 1a-[Pyr]a and 1b-[Pyr]a do exhibit enantiotropic SmA and N phases. 16or comparison purposes, equimolar mixtures of non-ionic analogues of 1-[Pyr] were investigated.Initial studies of 2a-[Ph]a and 2b-[Ph]a demonstrated phase separation upon cooling from the isotropic phase.It was considered that the signicant difference in the molecular length and mesogenic behaviour of the two components leads to their incompatibility.Therefore, carboranes 2c and phenyl ether [Ph]c were considered, since the total length of the alkyl chains in each compound is 16 atoms.
POM analysis of equimolar mixtures 2     consistent with a lamellar structure (Fig. 5).Considering the calculated molecular length 28 of about 33.8 Å for both anions 1 XRD patterns obtained for oriented samples of 1 [10]c-[Pyr]c and 1 [12]c-[Pyr]c in the phase formed below the SmA phase could be indexed as an E phase (Table 3 and Fig. 6).However, signals from the full size of the in-plane unit cell were also visible, 28 which are typically forbidden for E phase.Signals (100) and (010) are observed due to the binary nature of the ionic materials.This causes the molecule in the center of the unit cell to be different than those in the corners resulting in a unit cell that is not centered.There is also a slight difference between patterns for 1 [10] XRD analysis of the mesophase formed by phenyl ether [Ph]c revealed a so crystalline B phase (SmB cry ), which is consistent with a relatively large enthalpy of the isotropic transition (12.7 kJ mol À1 ) and the optical texture (Fig. 2).The measured layer spacing is 31.2Å which correlates well with the calculated molecular length of 32.8 Å (including van der Waals radius for H atoms).

Molecular modelling
The energy of coulombic interactions in ion pairs 1c-[Pyr]c was assessed computationally by comparison of the association energy for 1 [12]c-[Pyr]c and 2 [12]c-[Ph]c for an arbitrary chosen relative orientation of the molecules in the pair.The calculations were conducted using the M06-2x functional, which reasonably well reproduces non-covalent interactions. 29nalysis of fully optimized individual structures revealed a nearly linear anion 1 [12]c and a bent cation [Pyr]c (Fig. 7).The hexyl chain and the carbonyl group in the former are staggered relative to the {closo-1-CB 11 } cage.The two carboxyl groups are nearly co-planar with the benzene ring rendering the "organic" half of the molecule planar (Fig. 7a).The angle between the planes of the terminal alkyl chains is 15 , and the molecular length (L, as the H/H distance) is 31.6Å.The carborane analogue 2 [12]c has a nearly identical molecular length (L ¼ 31.3Å).The main difference between 1[12]c and 2 [12]c is that the carbonyl group nearly eclipses the B-H bond in the latter, which result in the wider interplanar angle of 27 .
The undecyl chain in the pyridnium cation [Pyr]c is orthogonal to the ring plane, which results in a bent molecular structure as shown in Fig. 7b.The interplanar angle is 78 and   ).In contrast, the dielectric medium signicantly reduced the exotherm of formation of ion pair 1 [12]c-[Pyr]c (DH a ¼ À27.9 kcal mol À1 ).As a result, the difference in association energy, DDH a , of the non-polar pair and ion pair is 24.5 kcal mol À1 in a weakly dielectric medium.Increasing 3 to 10.0, which is reasonable for an ionic liquid, 30,31 the DDH a value for the two pairs falls to 7.0 kcal mol À1 .Thus, the calculated signicant difference in association enthalpy in the two model binary systems is consistent with the observed 181 K higher thermal stability of the mesophase in ILC than in the non-ionic binary mixture.Also, the observed tendency for the formation of high-temperature lamellar phases (SmA and E) by the ion pair instead of a nematic phase is a consequence of the tight molecular arrangement dictated by signicant coulombic interactions.

Conclusions
For the rst time, we have assessed experimentally the impact of coulombic interactions on mesophase stability by comparison of ionic (1c-[Pyr]c) and isosteric non-ionic (2c-[Ph]c) systems in the absence of other factors.Thus, replacement of appropriate carbon atoms in an equimolar binary mixture with B À and N + fragments leads to an isosteric and isoelectronic ion pair.Results show that the difference in the clearing temperature between the two binary systems is 181 K, which corresponds to a difference in association enthalpy of 7 kcal in appropriate dielectric media.The boron cluster size in the ionic material appears to have little effect on the magnitude of this phenomenon.It is possible, however, that the size of the ions is the determining factor.A comparison was demonstrated for components of approximately equal length due to their best compatibility of the electrically neutral compounds (i.e.formation of a homogeneous nematic phase).
The presented concept of isosteric and isoelectronic replacement of C,C with B À ,N + is applicable to other molecular systems and provides unique experimental models to support theoretical considerations of the liquid crystalline state.

Computational details
Quantum-mechanical calculations were carried out using a Gaussian 09 suite of programs. 32Geometry optimizations for unconstrained conformers of appropriate molecules and ions with the most extended molecular shapes were performed at the M06-2x/3-21G* level of theory 29 using default convergence limits.The alkoxy group was set in all-trans conformation maintaining co-planarity with the aromatic ring in the input structure.The aromatic ring and the carboxyl group were staggered with respect to the carborane cage as found experimentally and computationally in related structures.The orientation of the substituents on the cyclohexane ring was set according to results of conformational analysis for model cyclohexane derivatives.Optimized structures of non-polar molecules served as starting points for optimization of the ionic analogues aer replacing the carbon atoms with B or N.
For calculations of binary system 2 [12]c-[Ph]c, molecules of 2 [12]c and [Ph]c at equilibrium geometry (M06-2x/3-21G*) were set parallel to each other at a 4 Å distance, and the geometry of the pair was minimized.The resulting energy was corrected for basis set superposition error (BSSE) by running single point calculations (M06-2x/3-21G*//M06-2x/3-21G*) at the equilibrium geometry with the keyword COUNTERPOISE ¼ 2, 33 and compared to that of isolated molecular components.The optimized geometry of non-polar pair 2 [12]c-[Ph]c served as the starting point for calculations involving ion pair 1 [12]c-[Pyr]c, aer replacement of the two carbon atoms with B and N. Conformational search was not attempted.For each pair, the association energy was calculated as a difference between the energy of the molecular pair and isolated molecules (DH a ¼ H pair À (H cat + H an )).
Thermodynamic parameters were requested with the FREQ keyword.The PCM model 34 was implemented using the SCRF (Solvent ¼ Generic, Read) keyword and specied "epsinf ¼ 2.25" and "eps ¼ 2.5" or "eps ¼ 10" and the total energies were obtained in single point calculations with the same method.

Experimental part
General NMR spectra were obtained at 128 MHz ( 11 B) and 400 MHz ( 1 H) in CDCl 3 or CD 3 CN.Chemical shis were referenced to the solvent ( 1 H) or to an external sample of B(OH) 3 in MeOH ( 11 B, d ¼ 18.1 ppm).Optical microscopy and phase identication were performed using polarized optical microscopy (POM) and a hot stage.Thermal analysis was obtained using a DSC with samples of about 0.5-1.0mg and a heating rate of 5 K min À1 under a ow of nitrogen gas.

General procedure for the preparation of 1-[Pyr]
To a solution of carboxylic acid 4 [12]-[NEt 4 ] or 4[10]-[NMe 4 ] (0.1 mmol) in dry CH 2 Cl 2 (2 mL) was added oxalyl chloride (5 eq) and a catalytic amount of DMF.The mixture was stirred at rt for 30 min, and the mixture was evaporated to complete dryness.Dry CH 2 Cl 2 was added (3 mL) followed by phenol 3 (0.1 mmol) and Et 3 N (0.2 mmol), and the reaction was stirred overnight.Organic products were extracted (CH 2 Cl 2 ), the organic layer was dried (Na 2 SO 4 ) and the solvent was evaporated.The residue was passed through a silica gel plug (MeCN/CH 2 Cl 2 , 1 : 5) giving crude product as either the [NMe 4 ] or [NEt 4 ] salt.N-Alkyl-4alkoxypyridinium bromide ([Pyr]-Br, 1.2 equivalent) was added to a solution of the crude ester 1[10]-[NMe 4 ] or 1 [12]-[NEt 4 ] (1 eq) in CH 2 Cl 2 upon which a white precipitate formed.Water was added, and the biphasic system was stirred overnight.The CH 2 Cl 2 layer was separated, and the aqueous layer was extracted with additional CH 2 Cl 2 .The CH 2 Cl 2 layers were combined, washed with H 2 O, dried (Na 2 SO 4 ), and the solvent was evaporated.The crude product was puried on a silica gel plug (CH 2 Cl 2 ) and recrystallized from EtOH, EtOH/EtOAc and hexane/EtOAc mixtures providing pure ion pair.

General procedure for preparation of esters 2
Method A. To a solution of carboxylic acid 5 (0.1 mmol) in dry CH 2 Cl 2 (2 mL) was added oxalyl chloride (5 eq) and a catalytic amount of DMF.The mixture was stirred at rt for 30 min, and the mixture was evaporated to complete dryness.Dry CH 2 Cl 2 was added (3 mL) followed by appropriate phenol 3 (ref. 16) (0.1 mmol) and Et 3 N (0.2 mmol), and the mixture was stirred overnight.Organic products were extracted (CH 2 Cl 2 ), organic layer was dried (Na 2 SO 4 ) and the solvent was evaporated.The residue was passed through a silica gel plug (hexane/CH 2 Cl 2 , 4 : 1) giving crude ester 2, which was puried further by recrystallization.
Method B. A solution of carboxylic acid 5 (0.1 mmol) and appropriate phenol 3 (ref.16) (0.11 mmol) was treated with N,N 0dicyclohexylcarbodiimide (DCC, 31 mg, 0.15 mmol) and a catalytic amount of 4-dimethylaminopyridine.The reaction mixture was stirred overnight.The reaction mixture was then washed with 10% HCl, the CH 2 Cl 2 layer separated, dried (Na 2 SO 4 ), and evaporated.The crude material was passed through a silica gel plug (CH 2 Cl 2 ) giving crude ester 2, which was puried further by recrystallization.

Note added after first publication
This article replaces the version published on 15th October 2014, which contained errors in the formulae of the monocarbaborates in the Introduction section.

Fig. 1 Scheme 1 Scheme 2
Fig.1The structure of ions(1 and [Pyr]) and non-ions (2 and [Ph]).In the cage structures, each vertex represents a BH fragment, and the sphere is a carbon atom.

Fig. 2
Fig. 2 The optical texture of (a) B phase in [Ph]c at 29 C and (b) nematic phase at 25 C in 2[12]c-[Ph]c obtained on cooling from the isotropic phase.
[10]c and 1[12]c (including van der Waals radius for the terminal H atoms) the measured layer spacing of about 27 Å indicates approximately 20% of interdigitation and/or imperfect orientational order of molecular long axes and molecular folding in both ion-pairs.The wide-angle region of the diffractograms shows an unsymmetric broad halo, which can be deconvoluted into two signals with the maxima about 4.7 Å and 5.1 Å for 1[10]c-[Pyr]c, and 4.8 and 5.3 for 1[12]c-[Pyr]c.The diffused signals correlate with the mean distance between the alkyl chains (former) and the separation between boron clusters (latter).
c-[Pyr]c and 1[12]c-[Pyr]c.The pattern for the 12-vertex analogue indicates a regular ABA type stacking of the smectic layers, which leads to a doubling of the unit cell dimension along layer normal (thus doubling of the Miller index l, see Fig 6b).The structure of the E phase formed by ion pair 1[12]c-[Pyr]b is similar to that of 1[10]c-[Pyr]c.

Fig. 4
Fig. 4 The optical textures of (a) SmA phase (205 C) and (b) E phase (175 C) obtained for 1[12]c-[Pyr]c on cooling from the isotropic phase in the same region of the sample.