polar and ionic liquid crystals based on the [closo-1-cb9h10]

A plot of the TNI of the [10] series versus the TNI of the [12] analogues for the non-polar (circles) and polar (rubbers) derivatives. A plot of the N–I (dots) and SmA–N (diamonds) transition temperatures for binary mixtures of 1a[Pyr] (black solid) and 1b[Pyr] (gray dotted) in 3a as a function of the mole fraction of the ion pair .

Scope of the work

Access to polar and ionic liquid crystals based on 1 and 2 required efficient cluster functionalization methods. The synthetic approach to 12-pyridinium derivatives of the group [closo-1-CB11H12] - (1), (type 1F), was also very inefficient.

Fundamentals of boron clusters and liquid crystals 2.1 Boron clusters 2.1 Boron clusters

Liquid crystals

Depending on the amount of order in the material, thermotropic liquid crystals form many liquid crystalline phases, called mesophases.36,39 As the temperature increases, the order of the molecules in the phase decreases and vice versa. The reorientation of molecules by an external electric field requires a certain threshold voltage Vth (Equation 4) and is called the Fréedericksz transition (Figure 6).42.

Figure 2. General scheme of a calamitic liquid crystal molecule. L stands for a linking group

Boron clusters as core elements of liquid crystals

Due to the non-polar character of p-carboranes, this type of liquid crystals is not of particular interest and is of little importance in the search for materials for technological applications. Initially, there was limited use of the 10-peaked monocarba-closo-borate anion due to the lack of a cluster functionalization method.

Figure 8. Polar (QP and DP), ionic (IO) and nonpolar (NP) derivatives of boron clusters

The [closo-1-CB 11 H 12 ] – anion
Functionalization of the [closo-1-CB 11 H 12 ] − cluster
Boron substitution
Summary
References

Like the acidity of the C–H bond, the reactivity of the B–H bond is strongly dependent on the cluster substitution. Currently, there are two ways to introduce the cyano group on the carbon tip. The alternative route involves transformation of the carboxyl group of the iodic acid [closo-1-CB11H10-1-COOH-12-I]-(16) to acid chloride 27 using oxalyl chloride (Figure 23).

The previous method involved diazotization of the 1-amine 32 with [NO]+[BF4]-, formation of an unstable dinitrogen intermediate, [closo-1-. This is in contrast to iodination of the [closo-1-CB9H10]− anion, in which electrophilic attack occurs almost exclusively at the B(6) corner. Recently, an improved microwave-assisted synthesis of the B(12) amine 67 was published, in which the product was obtained in shorter time (2 h) and higher yield (70%).53

The route to 10-sulfonium derivatives of [closo-1-CB9H10] anion involves the formation of the protected mercaptan obtained by reacting the stable dinitrogen. In this review, methods for the functionality of the [closo-1-CB11H12] anion in antipodal positions were highlighted.

Figure 10. Examples of zwitterionic (2[10]b, 1c) 14,15 and ionic (2f) 16 liquid crystals derived from 1

Methods and key intermediates to polar liquid crystals 3.1 Introduction 3.1 Introduction

Description and contributions

I was involved in all aspects of this research, including synthesis, compound characterization and optimization of procedures for the iodic acid 1 and its intermediates. I was responsible for research into the amination process of iodic acid [closo-1-CB9H8-1-COOH-10-I]– (1) using Pd(0) catalysts. My work also included the synthesis and investigation of the liquid crystalline properties of ester 3[3]a, prepared from sulfonic acid 2[3] and 4-butoxyphenol.

In the following chapter, the entire text of the manuscript is included for narrative consistency, however, the experimental part contains only experiments performed by me.

The manuscript

Introduction

Partly because of their flux behavior and ready epimerization at the sulfur center, esters 3[5] such as the 4-butoxyphenyl ester 3[5]a are sufficiently soluble in liquid crystalline hosts and are of interest for display applications. However, further exploration of these unusual molecular materials and their practical applications requires significant amounts of iodic acid 1 and is limited by access to [closo-2-CB9H9-2-COOH]-(4). The preparation of closo acid 4 (Scheme 1), described by Kennedy, involves large amounts of solvents and a large excess of reactants [10].

The second step, an oxidation, uses an even larger volume of solvents at 150 ml/g of arachnoic acid 5. The first two reactions used in the production of closoic acid 4 are thus impractical to scale up in their current form.

Figure 1. Acid 1 as a precursor to 1,10-difuctionalized derivatives of the {closo-1-CB 9 } cluster, which include polar (I) and ionic (II) liquid crystals

Reducing the amount of glyoxylic acid by half gave lower yields of the product with comparable purity in the range of 85%-95% (run 2). Further reduction of the amount of glyoxylic acid led to unstable product that was degraded upon isolation (run 4). Given the difficulties in isolating the arachnoic acid 5 with smaller amounts of the reagents, we decided to pursue the concentrated version of the original procedure.

Additional reduction of volume of the reaction was accomplished by increasing the concentration of HCl from 5% in the original procedure to 18%. The desired reduction of the volume of the reaction is achieved by using ~ 6 times more concentrated solution of base and 4 times more concentrated HCl. The oxidation of the arachnoic acid 5 to closo acid 4 was originally carried out using hypoiodite under basic solutions [10,11].

Table 1. Optimization of preparation of [arachno-6-CB 9 H 13 -6-COOH] - NEt 4 + (5)

Transformation of closo acid 4 to iodo acid 1

The preparation of amino acid 8

In general, the amination reaction was conveniently performed on a 15 gram scale from iodic acid 1 using phosphine A as the ligand giving the amino acid 8 in 43%–56% yield. Amino acid 8 was converted to the dinitric acid 10 in a typical yield of 55%–65% by diazotization with NO+BF4-.

Preparation of acid 2[3]

Subsequent thermolysis of 10 in Me2NCHS gave protected mercaptan 11, which was cycloalkylated with dibromide [17] 12 under hydrolytic conditions to form the sulfonic acid 2[3]. In both methods, the crude acid can be additionally purified by conversion to methyl ester 3[3]b, chromatography and basic hydrolysis back to acid 2[3]. Using this method, we also prepared tetramethylenesulfonic acid 13, which was isolated as its methyl ester 14 (Scheme 5).

Preparation of methyl ester 14

Preparation of methyl esters 3[n]b and acid 2[3]

Preparation of ester 3[3]a

Conclusions

We have developed a practical, large-scale preparation of acid [closo-2-CB9H9-2-COOH]-(4), mainly by reducing the volume of the aqueous phase and better controlling the reaction temperature. Our search for a more efficient Pd catalyst was unsuccessful and the amination procedure for iodic acid 1, which yields approximately 50% of amino acid 8, could not be improved. The cycloalkylation reaction and formation of the sulfonic acid 2[n] was improved by using Cs2CO3 as base, nevertheless the yield remains below 50%.

Further increase in overall yield can be achieved by improving the amination and cycloalkylation steps and complete separation of the isomeric iodic acids.

Experimental

The Et2O layer was separated and the aqueous layer was further extracted with Et2O (4 x 50 mL). The Et2O was separated in a 2-L separatory funnel and the aqueous layer was further extracted with additional Et2O (3x. The Et2O layer was separated and the aqueous layer was further extracted with Et2O (4x100 mL).

The remaining Et2O was completely evaporated and the aqueous solution was filtered to remove insoluble material. The aqueous layer was filtered through Celite and the process repeated two more times (the insoluble material apparently contains a B(10)-phosphonium derivative). CH2Cl2 (400 mL) was added and the biphasic system was vigorously stirred overnight until the aqueous layer became clear.

Acknowledgements

Description and contributions

I also performed physicochemical studies, which included UV-vis spectroscopy, thermal analysis, and temperature-dependent NMR spectroscopy of the putative intermediate 14.

The manuscript

Introduction
Results and discussion Synthesis Synthesis
Computational details

The two protocols were used in tandem for the preparation of the 1-pentyl derivative 1c from the parent anion [closo-1-CB11H12]- (B). Further DFT calculations indicate that the stability of the dinitrogen species 14 depends on the nature of the antipodal substituent. We also calculated the reaction mechanism for the formation of the B(12)-substituted pyridinium derivatives of [closo-1-CB11H12]- anion.

The calculations confirmed that dinitrogen derivatives of the [closo-1-CB11H12] anion are generally unstable and therefore difficult to isolate. Nevertheless, 11B NMR evidence for the existence of the dinitrogen derivative 14c as a transient species was presented. Population analysis of the MP2 wave function (MP2//MP2) was performed using the DENSITY(MP2) keyword.

Figure 1. The structures of the [closo-1-CB 9 H 10 ] - and [closo-1-CB 11 H 12 ] - anions (A and B), and their zwitterionic 1,10– (IA, IIA) and 1,12–disubstituted (IB, IIB) derivatives

Ray data collection

Acknowledgements
References

THF was removed in vacuo to give a dark orange liquid, which was extracted with Et2O (3 x 50 mL). The resulting brown residue was redissolved in Et2O, H2O was added (10 mL), and the Et2O was removed in vacuo. The organic layers were combined, dried (Na2SO4), and solvents were evaporated to give 400 mg (81% yield) 4c[NMe4] as a brown solid contaminated with the 7-isomer 8c[NMe4].

The organic layers were combined, H2O (30 mL) was added and the Et2O was removed in vacuo. Water (25 ml) was added to the combined organic layers and Et2 O was removed in vacuo. The organic layers were combined, dried (Na2SO4) and the solvents evaporated, yielding 912 mg of mixture of isomers 6c[NEt4] and 7c[NEt4].

High  materials – synthesis and characterization 4.1 Introduction 4.1 Introduction

Description and contributions

We performed extensive structure-property relationship studies, which include the effect of alkyl chain length on the mesogenic and electro-optical properties. All these derivatives were found to enhance dielectric anisotropy, , of the nematic host due to significant longitudinal dipole moment and consequently large extrapolated  values. In this work I was responsible for all aspects of the synthesis of esters 4[n], including intermediates such as acids, phenols, alcohols.

My role was also to investigate the thermal properties of all esters 4[n] and to prepare and analyze thermal and electro-optical properties of binary mixtures of selected esters 4[n] and derivatives 3[n] in two nematic hosts. Based on the collected data, I also determined the electro-optical parameters of the esters and their virtual temperatures.

The manuscript

Introduction

Here, we report the synthesis and thermal and dielectric characterization of two series of compounds in pure form as well as binary mixtures.

4.2.2.2 Results Synthesis

Conclusions

We have reported a diverse library of two series of polar compounds derived from an inorganic boron cluster, which act as effective additives to nematic materials for modulating dielectric anisotropy, , and thus electro-optical properties. Compounds 3[n] do not exhibit liquid crystalline behavior or enhanced solubility, even with elongated molecular cores up to 3 rings. On the other hand, esters 4[n] have significantly improved compatibility with nematic hosts and exhibit liquid crystalline behavior.

However, compounds with a total of 2 or 3 rings in 4[n] generally do not form liquid crystalline phases. These materials exhibit good host compatibility (reasonable solubility, high g and Sapp values) and a large  of about 70. Additional structure-property relationship studies are needed to further enhance the compatibility of these polar compounds with nematic hosts.

Computational details

Experimental Part

The suspension began to bubble and became homogeneous and was vigorously stirred for 30 min at rt. The product was isolated by column chromatography (SiO2), the eluent was filtered through a cotton plug and the solvent was evaporated to give the desired ester in about 60% yield. The B(10) signals of the cis isomers are shifted up by about 1 ppm relative to the trans isomers in the 11B NMR spectra.

In compounds 4[n]h, the pyrimidine ring of the cis-isomer is also displaced downwards by approx. 0.01 ppm relative to the trans isomer 4[n]h-trans. Following a general literature procedure3, a biphasic mixture of 3-heptylpentane-1,5-diol, 47% aqueous HBr (15 equiv.) and an equal volume of conc. The black reaction mixture was cooled to room temperature, diluted by the addition of half its volume of H2O and extracted with CH2Cl2.