Liquid-Crystalline Phase of Phosphonium Salts with Three Long

Chapter 21

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Liquid-Crystalline Phase of Phosphonium Salts with Three Long n-Alkyl Chains as Ordered Ionic Fluids 1,4

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David J. Abdallah , Hui C. Wauters, Dylan C. Kwait , C. L. Khetrapal , G. A. Nagana Gowda, Allan Robertson, and Richard G. Weiss* 2

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1Department of Chemistry, Georgetown University, Washington, DC 20057-1227 Sanjay Gandhi Post Graduate Institute of Medical Sciences, Lucknow-226 014, India Cytec Industries Inc., Niagara Falls, Ontario, L2E 6T4, Canada Current address: Clariant Corporation, 70 Meister Avenue, Somerville, N J 08876 2

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We describe the syntheses and properties of liquid-crystalline phosphonium salts, especially those with 3 long n-alkyl chains, a shorter substituent, and a variety of anions.

Many ionic liquid crystals are known. Most may be divided into two classes: (1) the organic part is attached to a negatively charged head group, especially as in metal alkanoates; (2) the organic parts are attached to a positively charged head group, especially quaternary salts comprised of a Group V A (Group 15) cationic head group (N.B., Ν or P). Of the latter variety, those with one or two long w-alkyl chains have been studied extensively, and many form either smectic phases when neat or other assemblies, such as micelles, 1

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(a) In this work, the arbitrary definition of a 'long chain' is = 10 carbon atoms. © 2005 American Chemical Society

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In Ionic Liquids IIIB: Fundamentals, Progress, Challenges, and Opportunities; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

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when mixed with water. We have found interesting 'biradial' and 'tetraradiaT packing arrangements for crystalline phases of several Group V A salts containing four equivalent w-alkyl chains with 10 to 18 carbon atoms. All of the salts crystallize as stacked monolayers with an 'ionic plane' consisting of an array of anions and positively-charged Ν or Ρ atoms in the middle of each layer, but none exhibits a liquid-crystalline phase. Recently, we have found that many phosphonium salts (nPmA) with three long equivalent w-alkyl chains (containing m = 10, 14, or 18 carbon atoms each), one shorter chain with η = 0-5 carbon atoms or a benzyl group (Bz), and monovalent anions (A) of various types and sizes do form a liquid-crystalline phase. The greater thermal stability and generally wider mesophase temperature ranges of the phosphonium salts are two important reasons why we have focused our attention on them rather than the more accessible and more easily synthesized ammonium salts. Some ammonium salts have larger liquidcrystalline ranges than their corresponding phosphonium salts. An example is the BzN18Br (T . A2 78.3 °C; smectic range = 12.5 °C) and BzP18Br (T . SmA2 70.8 °C; smectic range = 8 °C) pair. When heated to 140°C, BzN18Br exhibits a broad exothermic transition centered around 130 °C, corresponding to what we believe is a Hofinann-type elimination reaction. Furthermore, differential scanning calorimetry thermograms indicate that large molecular reorganizations accompany the crystal-to-liquid crystal transitions of these (ΔΗ = 145.5 KJ/mol for BzN18Br and 117.7 KJ/mol for BzP18Br during the first heatings) and the other salts discussed here. The additional stability and lack of highly conjugated groups make the nPmA salts excellent candidates to be ordered ionic fluids for performing thermal and photochemical reactions of solute molecules.


5,6

K

Sm

K

9

Synthetic Aspects The phosphonium salts are easily prepared by S 2 reactions from the corresponding phosphines and alkyl halides (chlorides and bromides) in the absence of oxygen. The phosphines react rapidly with molecular oxygen, yielding phosphoranes and other oxidized species. To minimize exposure to molecular oxygen, reagent transfers were performed in a dry box under a nitrogen atmosphere and the solvents employed were saturated with molecular nitrogen prior to their use. Once the phosphorus atoms are quaternized, the salts can be handled in air without problem. However, especially those lPmA with shorter m chains are hygroscopic and must be handled in a dry atmosphere to ensure that hydrates are not formed. N


305 Quantitative anion exchange was usually achieved by converting the chloride or bromide salt to its ethyl xanthate and then adding a stong acid containing the desired anion type (Scheme I). Details of the procedures for purification and characterization of the salts are included within the references 7

d t e d

5 , 6 . 8,9,10,11

[H(CH )J P + RX Downloaded by NORTH CAROLINA STATE UNIV on September 7, 2012 | http://pubs.acs.org Publication Date: March 15, 2005 | doi: 10.1021/bk-2005-0902.ch021

2

[H(CH ) ] PR X-

3

2 m

3

CHCU K "S-C-OCKCH, +

[H(CH ) ] PR " A 2 m

3

+ CS + HOCH CH 2

2

3

HA

[H(CH )J PR -S.C-OCH CH 2

+ KX

3

2

3

S

Scheme J. General synthetic route to nPmA salts.

Structure and packing within crystalline phases From single-crystal X-ray crystallographic measurements, all of the phosphonium and ammonium salts with four equivalent long ^-chains that we have investigated adopt packing arrangements with two chains projected along one side and the other two chains projected along the opposite side of a rough plane defined by a mosaic of cationic head group atoms and anions. When the w-alkyl chains consist of < 12 carbon atoms, the packing arrangement is 'tetraradial (i.e., with the four chains of each cation projected roughly in a tetrahedral arrangement); for salts with > 12 carbon atoms per chain, the arrangement is 'biradial' (i.e., pairs of chains lie next to each other). We have found several variants of the 'biradial' arrangement (Figures 1 and 2), but in all of them, one chain of each pair adopts some gauche bends in order to redirect it along the axis of its partner. 5


306 Biradial Conformations


Tetraradial Conformation

IPlOBr TT/i-ττ \ \ %/4- a

M

12ΝΟ·Η 0 2

\/a

H(CHA)4Y+A=*nYA

€

t ^

12NBr

18PI

η = # carbon atoms in the four long alkyl chains Y = NorP Λ = anion

Figure 1. A tetraradial and three biradial forms ofnYA salts.

IQPBr

18PI

Figure 2. Packing arrangements of crystalline lOPBr and 18PI showing the distance between ionic layers and the distribution of ionic centers within.


307 Fewer examples of the crystal packing by salts with three long w-alkyl chains and one shorter chain are known. In our hands, they are more difficult to crystallize into single crystals suitable for diffraction experiments than salts with four equivalent long chains. A part of this difficulty may be related to the greater complexity of their packing arrangements. The three nPmA and nNmA of this type we have examined thus far adopt a bent "h" shape in which two chains are projected, in parallel, along one side of the rough plane defined by the ionic centers and the other chain is interdigitated with a chain from another molecule. For example, molecules of BzN18Br (Figure 3) pack in alternating interdigitated and non-interdigitated regions of alkyl chains separated by roughly defined ionic planes. Spacings between ionic planes alternate between 19.8 Â (an interdigitated segment) and 30.4 Â (two noninterdigitated segments) and their sum constitutes the length of the c-axis in a unit cell.


9,12

y

C118 C117 » C116 C115 * C114 % C113 ' C112 cm * C110 C109 ' C108 C107 C106 C105 * C104 C103 cite'

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C218 C217 C216 C215 C214 C213 C212 C211 C210 C209 C206 C207 - C206 C205 . C204 C203 >· C202

cWertc? BR1

C4^V*C6

Figure 3. Crystal packing showing two layer distances and structure of BzN18Br.



308 Powder X-ray diffraction patterns of most of the salts at room temperature consisted of peaks that could be indexed as 00/ at low angles and other relatively sharp peaks at higher angles, frequently superimposed on a broad peak. The latter feature indicates that the alkyl chains of the salts are somewhat disordered in the solid phases. It may also explain why so few of the ΙΡηΑ are easily crystallized. In fact, the solids are not mechanically strong; they are easily distorted by applying pressure and differentiation of some of them from liquid crystals is very difficult by polarizing optical microscopy alone. The examples of 1P10C1 (that passes directly from the solid to the isotropic phase) and its hydrate, 1Ρ10Ο·Η Ο (that is smectic at room temperature), are shown in Figure 4. 2

Figure 4. Left: Bâtonnets forming in a fan-like texture in 1P10CI at 40 °C upon cooling (solid phase). Right: Spherulite texture of lP10ChH O at room temperature (smectic phase). 2

Structure and packing within liquid-crystalline phases Based primarily on information from X-ray diffractometry (N.B., one sharp low-angle peak corresponding to a distance greater than one but less than two times the extended length of a single molecule, and a broad high-angle peak) and polarizing optical microscopy (N.B., optical patterns like those reported for other smectic A phases and the appearance of oily streak patterns when the samples are sheared), all of the liquid-crystalline phases of the nPmA salts with


309 'small' η groups are the smectic A (SmA ) type. description of this phase is shown in Figure 5. 2

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A general structural

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Figure 5. Cartoon representation of the smectic A packing phases of nPmA salts. R represents the short η group. 2

Because the sum of the cross-sectional areas of the three long chains is much larger than that of an Ν or Ρ centered head group, assemblies somewhat like those of the symmetrical tetra-n-alkyl salts, in which the ionic parts are arranged in planes that bisect pairs of chains on each molecule, were anticipated. Additionally, we considered incorrectly that nematic phases, in which the molecules retain only orientational order and are not arranged in layered assemblies, were present based on the ease with which these liquid crystals can be aligned in magnetic fields; smectic phases are usually difficult to orient in this way. However, despite their ease of alignment, all of the liquid-crystalline nPmA and the corresponding nNmA investigated are smectic A phases (i.e., mesophase Wlayered assemblies). A typical thermogram of 1P14C10 is displayed in Figure 6. Thermograms from the first heating of the solvent-crystallized morph of most salts contain an additional low ΔΗ solid-solid transition that is not present in 14

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(b) Abréviations for phase transitions are: K - K => solid-solid; K-SmA => solid-Smectic A ; K-I => solid-isotropic; SmA -I => Smectic A -isotropic. 2

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the second heating scan. The reproducibility of the second and subsequent heating thermograms and all cooling thermograms provide compelling evidence for the aforementioned thermal stability of the salts. They also demonstrate that the morphs obtained by solvent recrystallization and melt cooling are not always the same.

Heating ->

U

κ

f λ Γ Solid 2

ν

ν

Solid 1

SmAi

Isotropic

« - Cooling t

β α 4» -40

-20

0

20

40

60

100

80

120

Temperature (°C)

Figure 6. DSC thermogram of IP14C10 . The sample was cooledfrom the isotropic melt and then heated at a rate of 5 °C/min. 4

Although enthalpies of the K-»SmA2 transitions of the enantiotropic (e) liquid-crystalline salts (0P18I, 1P18A (A « Br, I, N 0 , BF , C I O 4 , PF ), 2P18A (A - Br, I), and BzP18Br) vary over a wide range, most are near 80 KJ/mol, and those with lower values have an additional K->K transition near in temperature. The enthalpies of all of the SmA ->I transitions are much lower,

Liquid-Crystalline Phase of Phosphonium Salts with Three Long

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