Industrial Preparation of Phosphonium Ionic Liquids - American

Chapter 4

Industrial Preparation of Phosphonium Ionic Liquids 1

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Downloaded by PENNSYLVANIA STATE UNIV on July 18, 2012 | http://pubs.acs.org Publication Date: August 26, 2003 | doi: 10.1021/bk-2003-0856.ch004

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Christine J. Bradaric , Andrew Downard *, Christine Kennedy , Allan J . Robertson , and Yuehui Zhou 5

Cytec Canada Inc., P.O. Box 240, Niagara Falls, Ontario L2E 6T4, Canada [email protected] Correspondingauthor: phone: 905-356-9000; fax: 905-374-5819); [email protected] [email protected] [email protected] [email protected] 1

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Abstract While a great deal of attention has been given to imidazolium ionic liquids in recent years, very few investigations involving phosphonium ionic liquids have been reported in the open literature. Here we present an account of our research into ionic liquids from the perspective of a future, large-scale producer of ionic liquids for industrial applications.

© 2003 American Chemical Society In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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1 - Introduction The field and phenomenon of room temperature ionic liquids [7] are now well past infancy, but much work remains to be done to fulfill the true potential of these neoteric solvents. While ionic liquids containing quaternary nitrogen-based cations have undergone extensive investigation in a myriad of applications over the last several years (see reviews by the groups of Gordon [2], Rogers [3], Seddon [4], Sheldon [5], Welton [6], Wasserscheid [7], and others [8]), studies involving quaternary phosphonium systems are much rarer [9]. As the global leader in the production of phosphine and phosphine derivatives, Cytec Industries has a great deal of experience in the manufacture of quaternary phosphonium salts that translates naturally to the manufacture of ionic liquids. For instance, we routinely produce tetraalkylphosphonium halides such as the ionic liquid trihexyl(tetradecyl)phosphonium chloride, [(C Hi3) P(Ci4H29)]Cl (trade names: CYPHOS® 3653 and CYPHOS IL 101) [10], in tonne quantities. Over the past several years our research program has developed a diverse range of phosphonium ionic liquids, pairing tetraalkylphosphonium cations with anions such as halides, tetrafluoroborate, hexafluorophosphate, dicyanamide, bis(trifluoromethanesulfonyl)amide, carboxylates, phosphinates, tosylates, alkylsulfates, and dialkylphosphates, among others (Figure 1). The history of ionic liquids chemistry has been described in detail elsewhere [2,11] but is worth reviewing briefly here. The first report of a room temperature molten salt was made by Walden in 1914, who noted the physical properties of ethylammonium nitrate (mp: 12-14°C) formed by the reaction of ethylamine with concentrated nitric acid [12]. This discovery evidently did not arouse great or immediate interest in the scientific community of the day. Nonetheless, the next half century saw sporadic reports of the use of ionic liquids as media for electrochemical studies and, less commonly, as solvents for organic reactions [2], Much of this work involved eutectic mixtures of chloroaluminate-based salts such as AlCl -NaCl and pyridinium hydrochloride [13]. To our knowledge, no quaternary phosphonium cations were employed for such work during this period. Ionic liquids didn't reach a more general audience until seminal research efforts by the groups of Osteryoung [14] and Wilkes [77, 75] in the 1970s, and Hussey [16] and Seddon [77] in the 1980s. This period also saw the first use of ionic liquids as reaction media for organic synthesis [18], and, in 1990, for biphasic catalysis [19]. In the early 1990s, a report by Wilkes and coworkers describing the first air and moisture stable imidazolium salts, based on tetrafluoroborate, [BF ]", and hexafluorophosphate, [PF ]" [20], fueled further interest in the field. This interest has seen explosive growth during the past decade [21], expanding to include diverse applications such as catalysis [2, 5, 8a], separations [22], electrochemistry [23], electrodeposition [24], photochemistry [25], liquid crystals [26], C 0 capture [27], desulfiirization of fuel [28], enzymatic syntheses [5], lubrication [29], rocket propulsion [30], and 6

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In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.


tetrafluoroborate

ρ

tosylate

hexafluorophosphate

Q F

bromide

Br©

dialkylphosphate

Ο II

dicyanamide

Θ

decanoate

A®

Θ

alkylsulfate

Ο \\

bis(trifluoromethanesulfonyl)amide

©

phosphinate

Ι-

Figure 1: Examples of anions that can be paired with tetraalkylphosphonium cations to produce ionic liquids. (Adapted with permission from reference 9a. ©2003 The Royal Society of Chemistry.)

chloride

CI©


44 thermal storage devices [31] to name a just a few. Reflecting this, the number of papers published on ionic liquids has grown from approximately 40 per year in the early 1990s to multiple hundreds per year today [21]. Relative to their quaternary nitrogen based cousins, specific accounts of ionic liquids containing quaternary phosphorus cations are quite rare, a lacuna which is somewhat surprising given how often phosphonium ionic liquids are mentioned in review articles [2-8]. One recent report described some trialkylphosphonium salts, [HPR ] X", that are liquid at room temperature [32]. Three accounts of catalysis in phosphonium ionic liquids have been reported: 1) tetraalkylphosphonium tosylates as solvents for hydroformylation [9]; 2) tetraalkylphosphonium halides as solvents for palladium catalyzed Heck reactions [33]; and 3) trihexyl(tetradecyl)phosphonium chloride as a solvent for palladium mediated Suzuki cross-coupling reactions [34]. We note also that many low melting tetraalkylphosphonium salts are already well known as phase transfer catalysts [35]. +


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2 - Features and Advantages of Phosphonium Ionic Liquids +

Asymmetrical tetraalkylphosphonium halides, [R'PR ]X" are typically prepared by nucleophilic (S 2) addition of tertiary phosphines, [PR ], to haloalkanes, [R'X (X = Cl, Br, I)] (Eq. 1) [36], although other methods have been reported [37]. 3

N

PR

3

+

R'X

3

+

> [R'PR ] X" 3

(1)

While the pK s for tertiary phosphines are typically lower than the corresponding amines, their larger radii and more polarizable lone pair make them more nucleophilic. Hence the kinetics of salt formation are, in general, much faster than for amines [38, 39]. The requisite tertiary phosphine starting materials can be prepared viafreeradical addition of phosphine gas (PH ) [40] to alpha olefins [41], often in the presence of a suitable promoter such as DuPont's Vazo® series [42]. The large number of commercially available haloalkanes and trialkylphosphines suggests a potentially large number of possible tetraalkylphosphonium salts. In our experience, however, there are some practical restrictions that limit the number of systems than can be synthesized easily. For example, only primary haloalkanes have reasonably fast kinetics. Furthermore, alkylphosphines containing branched alkyl chains also tend to be rather slow to react for what we believe are steric reasons. These restrictions notwithstanding, there are still a very large number of phosphonium salts that can be made by quaternization and/or anion exchange reactions. While quaternization reactions such as those discussed above are well known (particularly for the syntheses of transfer catalysts and Wittig reagents), a

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45 the realization that judicious selection of anion and cation can produce true room temperature ionic liquids is comparatively recent. For example, we have found that quaternizing PR (R = pentyl, hexyl, octyl) with 1-chloro- or 1bromotetradecane produces phosphonium halides that are liquid at room temperature[45]. In addition, other phosphonium salts such as tetrabutylphosphonium chloride (mp: 67 °C), tetraoctylphosphonium bromide (mp: 45 °C) and tributyl(tetradecyl)phosphonium chloride (mp: 60 °C) are also low melting and fall within the generally accepted, broader definition of ionic liquids, i.e. salts which melt below ~100 °C. Though innumerable phosphonium cations can be imagined as constituents of phosphonium ionic liquids, we have utilized the trihexyl(tetradecyl)phosphonium cation, [(C Hi ) P(Ci H29)] , in much of our work. This is for reasons of cost and convenience, and because we have found it works well in many cases. As is well known for imidazolium halides [4, 6, 11] salts containing chlorometallate anions (e.g. A1C1 7A1 C1 ", FeCl ", etc.) addition of metal chlorides (e.g. A1C1 , FeCl , etc.) to phosphonium chlorides [43]. For example, we have prepared trihexyl(tetradecyl)phosphonium tetrachloropalladate, [(C H ) P(Ci H29)]2[PdCl ] by simple addition of PdCl to two equivalents of [(C H, ) P(Ci H 9)]Cl. This deep red ionic liquid has a melting point of -50 °C, with onset of decomposition occurring above 400 °C. Althoughfreeflowing well below room temperature, the viscosity of trihexyl(tetradecyl)phosphonium tetrachloropalladate is approximately an order of magnitude greater than that of trihexyl(tetradecyl)phosphonium chloride (e.g. 104 Ρ versus 12 Ρ at 30 °C). It's composition has been confirmed by NMR spectroscopy and elemental analysis [44]. Phosphonium halides can also be converted by metathesis methods (see Eq. 2-3) to other anions such as phosphinate, carboxylate, tetrafluoroborate, hexafluorophosphate, etc. (Figure 1) [45].


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[R'PR ] X" + M A [R'PR ] X- + HA + MOH

> [R'PR ] A- + M X > [R'PR ] A" + M X + H 0

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+

3

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2

(2) (3)

[R, R' = alkyl; X = halogen; M = alkali metal; A = anion] While such methods are both powerful and versatile, the ionic liquids thus produced inevitably contain residual halide ions [2-8, 46], making them unsuitable for many applications. Halogen free systems can be produced by direct reaction of tertiary phosphines with alkylating agents such as alkyltosylates, dialkysulfates, and trialkylphosphates, among others. As suggested by Eq. 1, typical phosphonium cations have the general formula [R'PR ] , in which three of the alkyl groups are identical while the fourth is different. However, this does not have to be the case. Primary and secondary alkylphosphines (RPH , R PH) are also available and can be +

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46 converted to asymmetric tertiary phosphines (RR' P or R R'P ) through free radical addition to olefins [41]. The resulting phosphonium cations have generic formulas of RR* R"P and R R ' R " P . This path offers another way to tune the properties of phosphonium ionic liquids. One difference between phosphonium and ammonium salts is their stability with respect to degradation under various conditions [9, 47]. For example, although both can decompose by internal displacement at higher temperatures (Eq. 4), phosphonium salts are generally more thermally stable than ammonium salts in this respect [47]. 2

+

+

2


2

2

—A-»

[R^fX

R E + R-X (E = N,P)

(4)

3

Unlike their ammonium counterparts, which can undergo facile Hoffmann- or β-elimination in the presence of base [48], phosphonium salts decompose to yield a tertiary phosphine oxide and alkane under alkaline conditions (Eq. 5) [49]. Alternatively, depending on the nature of R and R \ stable phosphoranes can be formed (Eq. 6); these are well known as Wittig reagents. While the decomposition of phosphonium salts by these pathways may occur even at room temperature in some cases, contrasting examples are known where tetraalkylphosphonium halides can be combined with concentrated sodium hydroxide well above room temperature without any degradation [47] (eg.[(C H33)P(C H )]Br[5i?]). 16

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[R P-CH -R'] [R P~CH -R'] 3

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2

2

+

+ +

OH" OH"

> R P=0 + CH -R' (5) > R3P=CHR' + H 0 (6) 3

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While the decomposition point of neat phosphonium ionic liquids on heating varies somewhat depending on the anion, thermogravimetric analyses (TGA) indicate dynamic thermal stability in excess of 300 °C for many species [57]. Figure 2 shows TGA data for trihexyl(tetradecyl)phosphonium tetrafluoroborate, which exhibits a profile typical of most phosphonium salts. This enhanced thermal stability relative to quaternary nitrogen based salts is an important factor when, for example, reaction products must be removed from an ionic liquid by high temperature distillation. Viscosity is a particularly important characteristic for solvents being considered in industrial applications. Phosphonium based ionic liquids tend to have viscosities somewhat higher than their ammonium counterparts, especially at or near room temperature. However, on heating from ambient to typical industrial reaction temperatures (e.g. 70-100 °C) their viscosities generally decrease to < 1 P. This is shown for trihexyl(tetradecyl)phosphonium chloride in Figure 3. Ionic liquid viscosities are also very sensitive to solutes [52], and the addition of reactants and/or catalysts can be expected to further reduce viscosity.



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Figure 2: Stability with respect to temperature, as demonstrated by thermogravimetric analysis (TGA), for trihexyl(tetradecyl)phosphonium tetrafluoroborate, [(C Hi3) P(C| H 9)][BF ]. (Reproduced with permission from reference 9a. ©2003 The Royal Society of Chemistry.) 6

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-i

2 1 0

ι

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1

1

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-40

-20

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60

80

100

1

120

e

T(C) -•-neat - e - + 1 % hexane

water -*~+1% toluene

Figure 3: Viscosity with respect to temperature for trihexyl(tetradecyl)phosphonium chloride, [(C Hi3) P(Ci H 9)]Cl. (Reproduced with permission from reference 9a. ©2003 The Royal Society of Chemistry.) 6

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For example, mixing trihexyl(tetradecyl)phosphonium chloride with 1% (w/w) of hexane, water, or toluene decreases viscosity at all temperatures (Figure 3). While the densities of many imidazolium ionic liquids have been reported previously [52], few phosphonium ionic liquids are available for comparison. In contrast to most imidazolium ionic liquids (which generally have densities > 1 g/mL) [52], tetraalkylphosphonium salts tend to have densities in

American Chemical Society Library IBth St., Solvents; N.W. Rogers, R., et al.; In Ionic1155 Liquids as Green Washington, DC 20036 ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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1-2η

0.95

"Ι

0

!

20

1

1

40 60 Temperature (C)

r

!

80

100

Figure 4: Density as a function of temperature for several tetraalkylphosphonium tosylates. (Reproduced with permission from reference 9a. ©2003 The Royal Society of Chemistry.)

Figure 5: Density as a function of temperature for several salts of the trihexyl(tetradecyl)phosphonium cation. (Reproduced with permission from reference 9a. ©2003 The Royal Society of Chemistry.)


49 the range 0.8 to 1,2 g/mL range with densities < 1 g/mL being the norm. Phosphonium cations containing more carbon atoms have slightly lower densities, as illustrated in Figure 4, which shows density as a function of temperature for several tetraalkylphosphonium tosylates. The anion employed also has an impact on density; for example, Figure 5 shows the following order with respect to density for trihexyl(tetradecyl)phosphonium cations paired with several anions: Cl"

Industrial Preparation of Phosphonium Ionic Liquids - American

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