Zwitterionic pyridinium derivatives of [closo-1- CB9H10] and [closo-1-CB11H12] as high D3 additives to a nematic host†

Fig. 1 The structures of the [closo-1-CB9H10] and [closo-1Substituted closo-carbaborate–pyridinium zwitterions were prepared in 35–50% yield by reacting 1-amino-closo-1-carbaborates with 4-alkoxypyrylium triflates. Two of the newmaterials, 1[6]d and 2[10]b, exhibit a high temperature SmA phase, whose stability is driven by dipolar interactions. Solution studies in a nematic host, ClEster, demonstrated high positive dielectric anisotropy of these new compounds (D3z +50) resulting from a longitudinal molecular dipole moment of about 20 D. CB11H12] anions (A and B) and their zwitterionic 1,10-(IA, IIA) and 1,12disubstituted (IB, IIB) derivatives. Q represents an onium fragment such as ammonium, sulfonium or pyridinium. Each vertex represents a BH fragment and the sphere is a carbon atom. Introduction


Introduction
Polar liquid crystals and additives enable electro-optical switching 1 and are essential components of materials for liquid crystal display (LCD) applications. 2 Recently, we have demonstrated that zwitterionic derivatives of the [closo-1-CB 9 H 10 ] À cluster (A, Fig. 1) have high dielectric anisotropy D3 and are useful additives to nematic materials for LCD. In this context, we developed a synthetic methodology and prepared 1-sulfonium 3,4 and 1-quinuclidinium, 3 and also 10-sulfonium 4-6 and 10-pyridinium 7 zwitterions, compounds of the general structure IIA and IA. Some of them exhibit nematic behavior and D3 reaching a record high value of 113.5(!) in nematic solutions. 7 The preparation of 1-pyridinium derivatives of the [closo-1-CB 9 H 10 ] À (A) and [closo-1-CB 11 H 12 ] À (B) clusters however was very inefficient due to the mechanistic issue in the former, 8 and instability of the key intermediate in the latter case. 9 Such pyridinium derivatives IIA and IIB (Fig. 1, Q ¼ Pyr) are predicted to have signicant longitudinal dipole moments, and, consequently, high positive D3. In addition, they are expected to have lower melting points and be more soluble than the quinuclidinium analogues.
Here we demonstrate a simple method for preparation of 1-pyridinium zwitterions of anions A and B and their use as high D3 additives to liquid crystal materials for LCD applications.

Results and discussion
Synthesis Compounds 1[n], 10-vertex derivatives of type IIA, and 2[n], 12vertex derivatives of type IIB, were obtained by adapting a general method for converting pyrylium salts to N-substituted pyridinium derivatives 10 and following a single example of using 4-alkoxypyrylium in this context. 11 Thus, a reaction of 1-amino derivatives 3[n] and 4[n] with 4-alkoxypyrylium salts 5 in anhydrous THF gave 1[n] and 2[n], respectively, in 35-50% yields (Scheme 1). 4-Alkoxypyryliums are very rare, 12 however triate salts 5 were conveniently obtained by alkylation of 4H-pyran-4one with appropriate alkyl triate 6. In addition to three primary alkoxy derivatives 5a, 5b and 5d, the new method was demonstrated also for a secondary alkoxy derivative. Thus, (S)-2octanol was converted to triate 6c and subsequently to pyrylium salt 5c with apparent partial racemization (ee ¼ 35%), as evident from the analysis of the pyridinium product 1 [6]c. This Fig. 1 The structures of the [closo-1-CB 9 H 10 ] À and [closo-1-CB 11 H 12 ] À anions (A and B) and their zwitterionic 1,10-(IA, IIA) and 1,12disubstituted (IB, IIB) derivatives. Q + represents an onium fragment such as ammonium, sulfonium or pyridinium. Each vertex represents a BH fragment and the sphere is a carbon atom.
indicates that the electrophilic O-alkylation of 4H-pyran-4-one with 6c proceeds through an ion pair and partial scrambling of the stereocenter. Triate 6c was signicantly more reactive than primary alkyl triates and synthesis of 1 [6]c and 2 [6]c was performed at lower temperatures.

Electronic absorption
Pyridinium derivatives 1[n] and 2[n] are colorless solids. Spectroscopic analysis demonstrated that the 12-vertex derivative 2 [6]c is more transparent in the UV region than its analogue 1 [6] c, however both compounds exhibit relatively strong p / p* absorption bands at l max ¼ 265 nm (calcd at 260 nm, f ¼ 0.16) 14 for 2 [6]c and at l max ¼ 282 nm (calcd at 309 nm, f ¼ 0.25) for 1 [6] c (Fig. 2). The origin of this absorption is an efficient intramolecular charge transfer from the HOMO, localized on the cluster, to the LUMO on the pyridinium fragment as shown for 1 [6]b and 2 [6]b in Fig. 3. 8 Interestingly, the HOMO of the latter has lower energy and signicantly greater contribution from the B-alkyl chain than observed in the 10-vertex analogue 1 [6]b.

Thermal analysis
All six compounds melt above 100 C ( Table 1). The lowest melting points were observed for the branched (2-octyloxy)pyridinium derivatives [6]c, which is in agreement with results obtained for bis-zwitterionic derivatives of the [closo-B 10 H 10 ] 2À cluster. 15 It is considered that the branching methyl group close to the pyridinium ring disrupts efficient packing in the solid state driven by coulombic interactions. 15 The highest melting point among the six compounds (216 C) is exhibited by threering derivative 1 [6]d. Data in Table 1 also suggest that derivatives of the [closo-1-CB 9 H 10 ] À cluster (A) have lower melting points than the 12-vertex analogues (e.g. 1 [6]c vs. 2 [6]c), which is in agreement with general trends in mesogenic derivatives of 10-and 12-vertex carboranes. 16,17 Polarizing optical microscopy (POM) and differential scanning calorimetry (DSC) revealed that 2 [10]b and 1 [6]d exhibit an   enantiotropic SmA phase with the clearing temperature of 202 C and above 270 C, respectively (Table 1 and Fig. 4).

XRD data
The formation of the SmA phase was conrmed by powder XRD measurements for 2 [10]b. A diffractogram of the mesophase obtained at 195 C showed a series of sharp commensurate reections consistent with the lamellar structure with a layer spacing of 25.61Å (Fig. 5). Considering the calculated molecular length of 31.75Å, 14 the observed layer spacing indicates 19% of interdigitation. The wide-angle region of the diffractogram shows an unsymmetric broad halo, which can be deconvoluted into two signals with the maxima 4.5Å and 5.4Å. The diffused signals correlate with the mean distance between the molten alkyl chains (former) and the mean separation between the carborane cages (latter).
Temperature dependence studies demonstrated that the SmA phase has a negative thermal expansion coefficient, k ¼ À0.0030 (1)Å K À1 , while the thermal expansion coefficient of the Cr 2 phase is positive (k ¼ +0.00598 (3)Å K À1 ). 14 Smectic behavior of boron cluster-derived mesogens is very rare 16 even for polar compounds, 6,7 and the observed hightemperature SmA phase for 2 [10]b and 1 [6]d results presumably from strong lateral dipole-dipole interactions of the zwitterions. This is supported by a comparison of 2 [10]b with its nonpolar isosteric analogue 11 [10]b, 7 a low temperature nematogen (T NI ¼ 25 C) derived from p-caraborane.

Binary mixtures
Three of the new compounds were investigated as additives to ClEster, which forms a nematic phase at ambient temperature characterized by a small negative D3. Results demonstrated that the two-ring zwitterions are more soluble in the host than the three-ring derivative 1 [6]d, and 2 [10]b forms stable 6 mol% solutions in the host. 14 Extrapolation of the virtual [T NI ] for 2 [10]b from the solution data gives the N-I transition at 82 AE 4 C, which is signicantly lower than the SmA-I transition at 202 C. This difference further supports the notion that SmA stability originates from dipolar interactions between the zwitterions. The branched derivative 1 [6]c signicantly disrupts the nematic order of the host and its extrapolated [T NI ] is below À100 C.

Dielectric measurements
Dielectric permittivity values change non-linearly with the concentration of the additives in ClEster as shown for 2 [10]b in Fig. 6. This indicates some aggregation of the polar molecules in the solution, which is similar, albeit to lesser extent, to that previously observed for sulfonium (1-Sulf) and quinuclidinium (1-Quin) derivatives of type IIA. 3 Therefore, dielectric parameters for the pure additives were extrapolated from dilute about 3 mol% solutions and results are shown in Table 2. Analysis of data in Table 2 demonstrates that all three pyridinium derivatives exhibit substantial dielectric anisotropy. Zwitterions 1 [6]d and 2 [6]b have D3 values 54 and 49, respectively, which, for comparable concentrations, are higher by about 10 than those for the previously investigated 1-Sulf and 1-Quin derivatives. 3 The value D3 ¼ 35 extrapolated for 1 [6]c with the branched alkyl chain is the lowest in the series, which is presumably related to the low order parameter as evident from dramatic destabilization of the nematic phase of the host.
Computational results for 1 [6]b and 2 [6]b in Table 3 indicate that the value and orientation of the calculated dipole moment for both series of pyridinium zwitterions are essentially the same: 20 D oriented about 6 relative to the long molecular axis. Consequently, assuming a typical order parameter S ¼ 0.65 and the Kirkwood factor g ¼ m eff 2 /m 2 ¼ 0.50, the calculated dielectric anisotropy values in ClEster are D3 ¼ 115 (3 || ¼ 140) for 1 [6]b and D3 ¼ 107 (3 || ¼ 130) for 2 [6]b, according to the Maier-Meier relationship between molecular and bulk parameters of the nematic phase. 18 Thus, the observed differences in the extrapolated dielectric parameters for the pyridinium zwitterions in Table 2 reect different compatibility with the host: the degree of aggregation (Kirkwood factor g) and the impact on the order parameter (S app ).

Conclusions
We have developed a method for efficient preparation of two types of 1-pyridinium zwitterions derived from the [closo-1-CB 9 H 10 ] À (A) and [closo-1-CB 11 H 12 ] À anions (B) that exhibit mesogenic properties and are suitable for low concentration, high D3 additives to nematic hosts. The method appears to be general and opens access to a variety of derivatives of the general structure II where Q ¼ 4-alkoxypyridine (Fig. 1). The method permits manipulation with the structure of the R group and the alkoxy substituent for tuning properties of the compounds, especially for improving solubility in the nematic hosts. Dielectric measurements indicate that the pyridinium zwitterions 1[n] and 2[n] are signicantly more effective dipolar additives to nematic hosts than those previously investigated (1-Sulf and 1-Quin).

Computational details
Quantum-mechanical calculations were carried out using Gaussian 09 suite of programs. 19 Geometry optimizations for unconstrained conformers of 1 [6]b and 2 [6]b with the most extended molecular shapes were undertaken at the B3LYP/6-31G(d,p) level of theory using default convergence limits. The alkoxy group was set in all-trans conformation co-planar with the pyridine ring in the input structure. The orientation of the alkyl substituents on the alicyclic ring and carborane cage in the input structure was set according to conformational analysis of the corresponding 1-ethyl derivatives. No conformational search for the global minimum was attempted.
Calculations in solvent media using the PCM model 20 were requested with the SCRF (solvent ¼ generic, read) keyword and eps ¼ 3.07 and epsinf ¼ 2.286 input parameters.
Electronic excitation energies for 1 [6]b and 2 [6]b in MeCN dielectric medium were obtained at the B3LYP/6-31G(d,p) level using the time-dependent DFT method 21 supplied in the Gaussian package. Solvent calculations using the PCM model 20 were requested with the SCRF (solvent ¼ CH 3 CN) keyword. Selected molecular orbitals involved in these transitions are shown in Fig. S5 and S6. †

Experimental part
General Reactions were carried out under Ar and subsequent manipulations were conducted in air. NMR spectra were obtained at 128 MHz ( 11 B) and 400 MHz ( 1 H) in CDCl 3 or CD 3 CN. 11 B  General methods for preparation of pyrylium salts 5 Method A. A neat mixture of 4H-pyran-4-one (1 mmol) and alkyl triate 6 (1 mmol) was stirred at 60 C for 1 h under Ar resulting in brown oil. The mixture was cooled to room temperature and used without further purication.
Method B. A modied Method A by using CH 2 Cl 2 (1 mL) as a solvent. Aer 1 h, the solvent was removed in vacuo and the product was used without further purication.
Method C. A modied Method B by conducting the reaction at 0 C to prevent decomposition of the secondary alkyl triate. 1 H NMR data are provided in the ESI. † General methods for preparation of alkyl triates 6 Method A. Following a general method for alkyl triates, 22 to a vigorously stirred solution of triic anhydride (1.2 mmol) in CH 2 Cl 2 (15 mL) at 0 C, a solution of pyridine (1 mmol) and primary alcohol (1 mmol) in CH 2 Cl 2 (10 mL) was added dropwise over a 15 min period and the mixture was stirred for an additional 1 h at 0 C. The solution was washed with ice-cold H 2 O, dried (Na 2 SO 4 ) and evaporated to dryness to give the appropriate alkyl triate 6 as a colorless liquid that quickly began to darken. The resulting mixture was ltered through a cotton plug and used without further purication.
Method B. To a vigorously stirred mixture of a secondary alcohol (1 mmol) and pyridine (1 mmol) at À78 C in CH 2 Cl 2 (25 mL) was added dropwise triic anhydride (1 mmol). The mixture was stirred for 10 min at À78 C and then kept at 0 C until the alcohol was consumed (by TLC). The mixture was washed with ice-cold water, dried (Na 2 SO 4 ) and the solvent was removed in vacuo at 0 C. The resulting triate 6 was kept at 0 C and quickly used in the next step. 1 H NMR data are provided in the ESI. † Preparation of 1-decyl-12-(4-heptyloxyphenyl)-p-carborane (11 [10]