Phosphonium-Based Ionic Liquids: An Overview

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RESEARCH FRONT Review

CSIRO PUBLISHING

Aust. J. Chem. 2009, 62, 309–321

www.publish.csiro.au/journals/ajc

Phosphonium-Based Ionic Liquids: An Overview Kevin J. FraserA and Douglas R. MacFarlaneA,B A School

of Chemistry, Monash University, Wellington, VIC 3800, Australia. author. Email: [email protected]

B Corresponding

Phosphonium cation-based ionic liquids (ILs) are a readily available family of ILs that in some applications offer superior properties as compared to nitrogen cation-based ILs. Applications recently investigated include their use as extraction solvents, chemical synthesis solvents, electrolytes in batteries and super-capacitors, and in corrosion protection. At the same time the range of cation–anion combinations available commercially has also been increasing in recent years. Here, we provide an overview of the properties of these interesting materials and the applications in which they are appearing. Manuscript received: 19 December 2008. Final version: 9 March 2009.

Introduction According to current convention, a salt that melts below the normal boiling point of water is known as an ‘ionic liquid’ (IL) or by one of many synonyms including low/ambient/room temperature molten salt, ionic fluid, liquid organic salt, fused salt, and neoteric solvent.[1] The variation in properties between salts, even those with a common cation but different anions, is dramatic. For example, butylmethylimidazolium hexafluorophosphate [C4 mim][PF6 ] is immiscible with water, whereas butylmethylimidazolium tetrafluoroborate [C4 mim][BF4 ] is water soluble.[2] This sort of variation in physical properties gave rise to Seddon’s description of ILs as ‘designer solvents’.[3] The number of potential anion–cation combinations possible reputedly equates to one trillion (1012 ) different ILs.[1] ILs have received much attention of late because of their potential application in green chemistry and as novel electrochemical materials. They have indeed become ‘designer solvents’, with many ILs now being designed for a specific application, for example as potential electrolytes for various electrochemical devices,[4–16] including rechargeable lithium cells,[17,18] solar cells,[19–21] actuators,[22–24] and double layer capacitors.[25–27]

Nitrogen-based cations, in particular N-methylimidazolium and pyrrolidinium salts, have been the subject of many of the publications in the field. A number of phosphonium cation-based ILs are also available and have a range of useful properties, but have been much less studied. Early reports regarding phosphonium ILs were published in the 1970s by Parshall using stannate and germanate salts[28–33] and by Knifton et al.[34–40] in the 1980s centering on the use of molten tetrabutylphosphonium bromide as an ionic solvent. To some extent the slower uptake of work on phosphonium ILs can be attributed to the difficulty in synthesizing the starting materials, for example tributylphosphine. Although phosphine derivatives have been available on a commercial scale since 1971, it was not until 1990 that tributylphosphine became available on a large scale.[41] Since then tetrabutylphosphonium chloride and bromide have become widely available on a multi-ton scale, along with many other trialkylphosphines and their corresponding quaternary phosphonium salts, in particular from Cytec Industries Inc.[41] Variation of the four substituents on the phosphonium cation, along with the multitude of available anions, represents an enormous number of possible salts. Those commercially available as of November 2008, for example, can be found in Table 1.

Kevin J. Fraser received his M.Sc. in 2004 from the University of Aberdeen, Scotland. He recently completed his Ph.D. under the supervision of Professor D. R. MacFarlane entitled ‘Physical Properties of Phosphonium Based Ionic Liquids’ at Monash University, Melbourne. His current interests include air electrodes for use in fuel cells and ionic liquid-based chemical sensors.

Professor Doug MacFarlane leads the Monash Ionic Liquids Group at Monash University. He is also the program leader of the Energy Program in the Australian Centre for Electromaterials Science. He was a Ph.D. graduate of Purdue University in 1982 and after postdoctoral work at Victoria University Wellington took up a faculty position at Monash University. Professor MacFarlane was recently awarded an Australian Research Council Federation Fellowship to extend his work on Ionic Liquids. He was elected to the Australian Academy of Sciences in 2007. His research interests include the chemistry and properties of ionic liquids and solids and their application in a wide range of technologies from electrochemical (batteries, fuel cells, solar cells and corrosion prevention), to biotechnology (drug ionic liquids and protein stabilisation) and biofuel processing.

© CSIRO 2009

10.1071/CH08558

0004-9425/09/040309

806 256[68]

409 312[68]

0.882 0.955 0.888 0.892 0.898[68] 1.070 1.067 1.064[68] 1.030 0.938 — — — 0.930

Cytec (CYPHOS IL 101)

Cytec (CYPHOS IL 102)

Cytec (CYPHOS IL 103)

Cytec (CYPHOS IL 104)

Cytec (CYPHOS IL 105)

Cytec (CYPHOS IL 106)

Cytec (CYPHOS IL 108)

Cytec (CYPHOS IL 109)

Cytec (CYPHOS IL 110)

Cytec (CYPHOS IL 111)

Cytec (CYPHOS IL 120)

Cytec (CYPHOS IL 162)

Cytec (CYPHOS IL 163)

Cytec (CYPHOS IL 164)







310 (N2 )

310 (N2 )

300 (N2 )



380 (N2 )

340 (N2 )

400[69] (N2 )



320 (N2 )

395[69] (N2 )

340 (N2 )

380 (N2 )

320 (N2 )

350 (N2 )

Solid (82)

Solid (102)

Solid (57)

Solid (73)

Liquid

Solid (50)

Liquid

36

Solid (45)

Liquid

Liquid

Liquid

Liquid

Liquid

Physical state at RTB (Tm [◦ C])

Hydrophilic

Hydrophilic

Hydrophilic

Hydrophilic

Hydrophobic (1.8%)

Hydrophobic (2.2%)

Hydrophobic (0.7%)



Hydrophilic

Hydrophobic (3.1%)

Hydrophobic (20.6%)

Hydrophobic (21.1%)

Hydrophobic (4.5%)

Hydrophobic (8%)

Hydrophobic/ hydrophilic (Max water capacity)

Water/alcohols

Water/alcohols

Water/alcohols

Non-polar/polar aprotic solvents Non-polar/polar aprotic solvents Non-polar/polar aprotic solvents Water/THF/hexane



Non-polar/polar aprotic solvents Non-polar/polar aprotic solvents Non-polar/polar aprotic solvents Non-polar/polar aprotic solvents Non-polar/polar aprotic solvents Water/alcohols

Miscibility

Medium for fluorination of arylchlorides[77] Petrochemical applications[37]

Solvent in the Diels–Alder reaction[74] Phase Transfer catalyst[51,75,76]

Grignard reagent[64] Corrosion Inhibitor[73] Heck coupling[65] Suzuki cross coupling[46] Hydroformylation reactions[41]

Hydro formation of Olefins[71] Reaction media[72]

Bio-transformations[70]

Suzuki/Heck coupling reactions[46,65] Heck coupling[65] Henry nitroaldo reaction[66] Extraction of heavy metals[67]

Grignard reagent[64]

Potential use

310



787





319

2094

1824

Tdec [◦ C]A (Atm: N2 /Air)

Physical properties Viscosity (25◦ C) [mPa s]

Tetradecyl(trihexyl)phosphonium chlorideC Tetradecyl(trihexyl)phosphonium bromideC Tetradecyl(trihexyl)phosphonium decanoateC Tetradecyl(trihexyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinateC Tetradecyl(trihexyl)phosphonium dicyanamide Triisobutyl(methyl)phosphonium tosylateC Tributyl(methyl)phosphonium methylsulfateC Tetradecyl(trihexyl)phosphonium bistriflamideC Tetradecyl(trihexyl)phosphonium hexafluorophosphateC Tetradecyl(trihexyl)phosphonium tetrafluoroborateC Tributyl(methyl)phosphonium tosylateC Tributyl(hexadecyl)phosphonium bromideC Tetrabutylphosphonium bromideC Tetrabutylphosphonium chlorideC

Density (25◦ C) [g cm−3 ]

Supplier (Trade name)

Chemical name

Table 1. Phosphonium ILs available as of November 2008

RESEARCH FRONT

K. J. Fraser and D. R. MacFarlane

Tetraoctylphosphonium bromideC Tetradecyl(tributyl)phosphonium chlorideC Ethyltri(butyl)phosphonium diethylphosphateC Tetradecyl(tributyl)phosphonium dodecylsulfonateC Tetradecyl(trihexyl)phosphonium dodecylsulfonateC Tetrabutylphosphonium glycolateC Tri(mixed isobutyl/butyl(methyl) phosphonium)tosylateC Tri(mixed isobutyl/butyl(methyl) phosphonium)tosylateC Tri(mixed hexyl/octyl(ethyl)phosphonium) diethyl phosphateC Mixed triisobutyl(methyl)phosphonium and triisobutyl(ethyl)phosphonium tosylatesC Triethyl(methoxyethyl)phosphonium bromide (Trihexyl)tetradecylphosphonium diisobutylmonothiophosphateC Tri(i-butyl)methylphosphonium chlorideC Triethyl[2-(2-methoxyethoxy)ethyl] phosphonium bromideC Tri(i-butyl)methyl phosphonium dimethyl phosphateC Trihexylmethylphosphonium tosylateC Trihexylethylphosphonium tosylateC Tetrabutylphosphonium benzoateC 1.066 1.07

Cytec (CYPHOS IL 239)

Cytec (CYPHOS IL 249)

— — — — — —

Cytec (CYPHOS IL 268)

Cytec (CYPHOS IL 300)

Cytec (CYPHOS IL 319)

Cytec (CYPHOS IL 320)

Cytec (CYPHOS IL 324)

0.92

Cytec (CYPHOS IL 265)

Cytec (CYPHOS IL 260)





Cytec (CYPHOS IL 208)

Cytec (CYPHOS IL 257)



Cytec (CYPHOS IL 202)





Cytec (CYPHOS IL 201)

Cytec (CYPHOS IL 256)

1.007

Cytec (CYPHOS IL 169)





Cytec (CYPHOS IL 167)

Cytec (CYPHOS IL 250)

0.938

Cytec (CYPHOS IL 166)









1045



624





2339

4030

4027

259





541

















260 (Air)



















350 (N2 )

290 (N2 )

Solid (38)

Solid (60.2)

Solid (73)

Solid (65)

Liquid

Solid (115)

Liquid

Solid (31.7)

Solid (N/A)

Liquid

Liquid

Solid (33)

Liquid

Liquid

Liquid

Liquid

Solid (45)

Solid (42)













Hydrophobic





Hydrophilic

Hydrophilic

Hydrophilic



Hydrophobic

Hydrophobic



Hyrdrophilic

Hydrophobic











Non-polar/polar aprotic solvents —



Water/polar organic solvents Water/polar organic solvents Water/polar organic solvents —

Non-polar/polar aprotic solvents Non-polar/polar aprotic solvents —

Water/polar organic solvents —

Non-polar solvents

























(Continued)

Separation of organic materials from water[78] Separation of organic materials from water[78] —

Grignard reagent[64] Suzuki/ Heck coupling reactions[46,65] Suzuki/Heck coupling reactions[46,65] —

RESEARCH FRONT

Phosphonium-Based Ionic Liquids 311

AT

dec ,

— — —

Sigma–Aldrich

Sigma–Aldrich

Sigma–Aldrich



























256 (Air)

283 (Air)

279 (Air)





Tdec (Atm: N2 /Air)

— —

Hydrophobic[82]

Liquid[81]

Liquid[82]









Hyrdrophilic[80]

Solid (98)[80] —



Hydrophobic

Hydrophobic

Hydrophobic





Solid (59–62)

Liquid

Liquid

Liquid

Liquid

Liquid

Physical state at RTB (Tm [◦ C])

Hydrophobic/ hydrophilic (Max water capacity)











Methanol[80]

Non-polar/polar aprotic solvents Non-polar/polar aprotic solvents Non-polar/polar aprotic solvents —





Miscibility

decomposition temperature measured by the step tangent method. B RT, room temperature. C Data from Cytec Industries Inc. unless otherwise referenced.





Sigma–Aldrich

Merck



Sigma–Aldrich



706

916

616

18870

4799

Viscosity (25◦ C) [mPa s]

[◦ C]A

Physical properties

Solvent in the separation of aromatic hydrocarbons from aliphatic hydrocarbons[81] Electrolyte in double layer capacitors[82]





Solvent for the Regioselective o-Alkylation of C/O Ambidentate Nucleophiles[79] Electrolyte in double layer capacitors[80] —











Potential use

312

Trihexyl(tetradecyl)phosphonium bis[oxalato(2-)]borate

Tetrabutylphosphonium tetrafluoroborate Tributylmethylphosphonium dibutyl phosphate Tributylmethylphosphonium methyl carbonate Triethylmethylphosphonium dibutyl phosphate Trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl)phosphinate

0.89

Cytec (CYPHOS IL 351) —

0.91

Cytec (CYPHOS IL 349)

Sigma–Aldrich

0.93



Cytec (CYPHOS IL 327)

Cytec (CYPHOS IL 331)



Cytec (CYPHOS IL 325)

Tetrabutylphosphosphonium tridecylsulfosuccinateC N-Methyl-N-ethyl morpholinium dimethyl phosphateC (Trihexyl)tetradecylphosphonium diisobutyldithiophosphateC (Trihexyl)tetradecylphosphonium bis(2-ethylhexyl)phosphateC (Trihexyl)tetradecylphosphonium 2-ethylhexanoateC Tetrabutylphosphonium methanesulfonate

Density (25◦ C) [g cm−3 ]

Supplier (Trade name)

Chemical name

Table 1. (Continued)

RESEARCH FRONT

K. J. Fraser and D. R. MacFarlane

RESEARCH FRONT

Phosphonium-Based Ionic Liquids

313

catalysts in epoxy curing,[60] and in high-temperature polycarbonate reactions.[61,62] Here we overview this family of organic salts and their properties, as well as some of their applications. Our goal is to provide a guide to the field and its literature, rather than a comprehensive review. Other very useful reviews of the field include those by Zhou et al.[41] and Clyburne et al.[63]

These include salts with the traditional halide anions such as trihexyl(tetradecyl)phosphonium chloride and bromide (CYPHOS IL 101 and 102), which are liquid at room temperature and have glass transition temperatures as low as −65◦ C.[42] Salts that contain other anions such as tosylate, dicyanamide, methylsulfate, diethylphosphate, phosphinate, bistriflamide ([NTf2 ]− ), tetrafluoroborate, and carboxylates are also available (Table 1). Of course, not all such phosphonium salts are liquid at room temperature, but by careful selection of R and R# as well as the appropriate anion, there are many phosphonium salts that can be prepared that have sub-ambient melting points and many more that fall within the broader general definition of ILs (Tm < 100◦ C). Reasons why one might consider a phosphonium IL in an industrial process include availability and cost. Phosphonium salts can meet both of these demands as they are already manufactured on a multi-ton scale.[41] In comparison to nitrogenbased ILs, the higher thermal stability of phosphonium-based ILs is useful in processes that operate at greater than 100◦ C.[43] A good example where phosphonium salts out-perform their ammonium counterparts is the biphasic conversion of aromatic chlorides to fluorides using potassium fluoride at temperatures that exceed 130◦ C.[44] Another advantage of phosphoniumbased ILs as compared to their imidazolium cation analogues is that the C2 proton of the latter tends to make them slightly acidic, which can in turn lead to carbene formation.[45] Alkylphosphonium salts are generally less dense than water, which can be beneficial in product work-up steps that involve decanting aqueous layers that contain inorganic salt by-products. For these reasons phosphonium ILs are now appearing in applications as solvents,[46–49] phase transfer catalysts,[49–51] electrochemical media,[52] exfoliation agents for montmorillonite clays,[53–59] F

Synthesis of Phosphonium-Based Ionic Liquids The first phosphonium salts to become available were the [Cl]− and [Br]− salts.[41] Historically these compounds have been used as biocides[83,84] and phase transfer catalysts.[85–87] The evergrowing interest in phosphonium ILs led Bradaric et al.[41] to closely examine a range of potential ILs for industrial production. In doing so they synthesized a range of phosphonium-based salts that were liquid at, or near, room temperature. More recently, the readily available trihexyl(tetradecyl)phosphonium chloride ([P6,6,6,14 ][Cl]) has been chosen as a starting material for the synthesis of numerous phosphonium-based ILs by anion exchange reactions.[67] The synthesis of [P6,6,6,14 ][Cl] is described by Bradaric et al.[41] and a similar methodology can be applied to the synthesis of other tetraalkylphosphonium halides. [P6,6,6,14 ][Cl] can be synthesized by adding trihexylphosphine to one equivalent of 1-chlorotetradecane at 140◦ C under nitrogen and stirring for 12 h. After the reaction is complete, the mixture is vacuum stripped to remove any volatile components such as tetradecene isomers, and excess 1-chlorotetradecane.[41] The resultant IL is a clear, pale yellow liquid, with a typical yield of 94%. Potential impurities for this synthesis included trihexylphosphonium hydrochloride and hydrochloric acid; the residual acid can be an important issue in several applications, but can easily O

! N

! B

C

F

F

C

F3C

N

Dicyanamide

O

Tosylate

S

O

O

! O

O

C R

O

Carboxylate

O !

R R Phosphinate

RO

OR

Dialkylphosphate Fig. 1.

!O

F

O S

O

!

F OR

Alkylsulfate

! O

P

!

O P

CF3

Bis(trifluoromethanesulfonyl)amide

O

O S

S

N

F Tetrafluoroborate

O

! N

F

F P

F

F

Hexafluorophosphate

Examples of anions that can be paired with tetraalkylphosphonium cations to produce ILs.

RESEARCH FRONT

314

K. J. Fraser and D. R. MacFarlane

! Cl

P

P

"

C6H13 C6H13

C6H13

80

OH

R R

Trihexyl(tetradecyl) phosphonium chloride

Weight %

"

100

O

C14H29

Bis(2,4,4-trimethylpentyl) phosphinic acid

(i)

C6H13

! H2O

R#

40

P

C6H13

R

! O

200

300 Temp [$C]

400

500

Fig. 3. Degradation of ionic liquids measured by temperature-ramped thermogravimetric analysis (10 K min−1 , N2 flow, Al pans); (i) [C4 mpyr][dca], (ii) [C4 mpyr][tcm], (iii) [P6,6,6,14 ][dca], and (iv) [C4 mpyr][NTf2 ].[113]

desirable.[106] Bradaric et al.[41] successfully synthesized phosphonium ILs in several direct, solvent free, halide free routes by the quaternization of tertiary phosphines with dialkylsulfates, dialkylphosphates, and alkylphosphonates as described in Eqns 3–5.[41]

O

P C6H13

(iv)

20

C14H29 "

(iii)

60

0 100 " NaOH

(ii)

" NaCl

R

Trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinate Fig. 2. Synthesis of trihexyl(tetradecyl)phosphonium trimethylpentyl)phosphinate (redrawn from ref. [41]).

bis(2,4,4-

be removed. Other minor impurities may include tetradecene isomers and R2 PH.[41] The ion exchange reactions used to produce phosphoniumbased ILs generally fall into two categories (as shown in Eqns 1 and 2). [R # PR3 ]+ [X]− + MA → [R # PR3 ]+ [A]− + MX

(1)

[R # PR3 ]+ [X]− + HA + MOH → [R # PR3 ]+ [A]− + MX + H2 O (2) where R, R# = alkyl; X = halogen; M = alkali metal; and A = an anion such as phosphinate, carboxylate, tetrafluoroborate, and hexafluorophosphate.[41] ILs that contain the anions shown in Fig. 1 can be synthesized by one or other of these routes. The series of phosphonium phosphinates that are available are a useful example of Eqn 2. [P6,6,6,14 ][bis(2,4,4trimethylpentyl)phosphinate] (trade name CYPHOS IL 104), is produced commercially by this route, as shown in Fig. 2. The phosphinic acid reactant in Fig. 2 is a well known and popular solvent for the extraction of cobalt from nickel in both sulfate and chloride media,[88,89] and is currently used to produce more than half of the western world’s cobalt.[90–92] Materials prepared from the chloride salt inevitably contain residual chloride ions, which may adversely affect metal catalysts[93–105] and/or contaminate reaction products. In addition, anion exchange processes are typically inefficient and usually involve the use of environmentally hazardous molecular solvents. Factors such as these, increase the final cost of ILs and in some cases can limit their application range. For these reasons, chloride-free routes to phosphonium salts are

P(n-Bu)3 + SO2 (OR)2 → [RP(n-Bu)3 ][SO3 (OR)] (R = Me, Et, n-Bu)

(3)

PR3# + O=P(OR)3 → [RPR3# ][PO2 (OR)2 ] (R = Me, Et, n-Bu; R # = n-Bu) (R = Me, Bu; R # = i-Bu)

(4)

P(n-Bu)3 + O=PR(OR)2 → [RP(n-Bu)3 ][PRO2 (OR)] (R = Me, n-Bu) (5) In addition to being halide free, these synthetic routes also provide a straightforward way to tune the properties of the phosphonium ILs by varying the size of the anion (through the size of the alkyl groups attached to the anion moiety). This ability is useful for tailoring properties to various application requirements. Del Sesto et al.[69] have reported the properties of novel phosphonium ILs that have large anions, for example [P6,6,6,14 ][CuPc(SO)]− 4 and [P6,6,6,14 ][C(CN)3 ]. Physical Properties of Phosphonium-Based Ionic Liquids Thermal Stability Previous research on phosphonium ILs has shown that, although both phosphonium and ammonium salts can decompose by internal displacement at high temperatures, phosphonium ILs are generally more stable in this respect. This is an important factor when, for example, reaction products need to be distilled from the IL.[41] Phosphonium ILs have also been shown to possess greater electrochemical stability than their ammonium analogues.[107] Phosphonium ILs, unlike their ammonium counterparts which can undergo facile Hoffmann- or β-elimination in the presence of base,[108–110] tend to decompose to yield a tertiary phosphine oxide and alkane under alkaline conditions[111] (Eqn 6). [R3 P–CH2 –R # ]+ + OH− → R3 P=O + CH3 –R #

(6)

RESEARCH FRONT

Phosphonium-Based Ionic Liquids

315

Table 2. Physical and thermal properties of phosphonium and corresponding ammonium room temperature ionic liquids (ILs) synthesised by Tsunashima et al.[115] IL [P2,2,2(12) ][NTf2 ] [N2,2,2(12) ][NTf2 ] [P2,2,2(101) ][NTf2 ] [N2,2,2(101) ][NTf2 ] [P2,2,2(201) ][NTf2 ] [N2,2,2(201) ][NTf2 ]

Viscosity (25◦ C) [mPa s]

Decomposition temperature (TGA, 10◦ C min−1 ) [◦ C]

180 316 35 69 44 85

408 390 388 287 404 384

Original observations by Walden[117] concluded that, for strong electrolyte solutions, the molar conductivity[118] is inversely proportional to viscosity and directly proportional to the fluidity φ (φ = η−1 ) of the medium through which the ions move.[117] These observations were combined into what is now known as the ‘Walden rule’: #η = constant

Alternatively, depending on the nature of R and R# , stable phosphoranes can be formed (Eqn 7);[41] these are well known as Wittig reagents. [R3 P–CH2 –R # ]+ + OH− → R3 P=CHR # + H2 O

Ionicity of Phosphonium-Based Ionic Liquids

(7)

While the decomposition of phosphonium salts by these pathways may occur in some cases,[87] contrasting examples are known where tetraalkylphosphonium halides can be combined with concentrated sodium hydroxide well above room temperature without any degradation (e.g., [P4,4,4,16 ][Br][112] ). ILs in general are often reported to be resistant to thermal decomposition and thus suitable for high-temperature applications; however, in most cases, thermal stability is measured by recording weight loss with rising temperature by thermogravimetric analysis (TGA)[113] (Fig. 3). This produces a substantial overestimation of the operational upper temperature limit at which an IL may be expected to retain its structural integrity during extended periods. The work of Scott et al.[113] has described methods by which longer term stability at high temperatures can be estimated from isothermal TGA measurements. Viscosity Factors that affect the viscosity of an IL are poorly understood, but the chemical structure of the anion is known to have a particularly strong influence. The lowest viscosity ILs are formed from small anions that have a diffuse negative charge and are unlikely to take part in any hydrogen bonding.[114] Tsunashima et al.[115] synthesized a range of low viscosity ILs of the triethylalkylphosphonium cations together with the bis(trifluoromethylsulfonyl)amide anion. By incorporating a methoxy group in one of the alkyl chains, they produced ILs with low viscosities at room temperature (e.g., 35 mPa s for triethyl(methoxymethyl)phosphonium [NTf2 ]), even lower than their ammonium counterparts[115] (see Table 2). The ILs showed good conductivities and high thermal stability and, therefore, should be advantageous in various electrochemical applications, such as battery electrolytes.[115] The high viscosity of some phosphonium ILs can be decreased by an increase in temperature and/or addition of a diluent. At reaction temperatures of 80 to 100◦ C and with the addition of 10% of a small molecule reactant, the entire system becomes quite water-like. This phenomenon is not unusual; small amounts of solutes or diluents can have a profound effect on the viscosity of most ILs.[116]

(8)

Angell has described the use of a plot of log # versus log η as a means of testing data against this relationship.[119] As a point of reference, a 0.1 M aqueous KCl solution is used[119] because potassium and chloride ions have similar hydrodynamic radii in aqueous solution. An ‘ideal line’ that represents the behaviour of an ideal electrolyte can then be constructed which runs through this point with unity slope, as suggested by the Walden rule. Any deviation from the ‘ideal’ line is thought to indicate a lack of complete ion dissociation of the salt, or, in other words, a low degree of ionicity. Fig. 4 shows a Walden plot for a family of readily available and newly synthesized phosphonium salts prepared by Fraser et al.[68] over the temperature range of 30–100◦ C. For comparison, [C4 mpyr][NTf2 ], [N1,8,8,8 ][Cl], and [C2 mpyr][dca] are also presented. The slope of the line with increasing temperature for each compound is one, which indicates that the conductivity/fluidity relationship remains constant. It has been demonstrated that the degree of dissociation for many neat ILs is almost independent of temperature.[120–124] A dashed line has also been drawn one log unit below the ideal line, thus indicating the situation where the liquid is exhibiting only 10% of the ‘ideal’ molar conductivity that it should possess for a given viscosity. One could consider liquids that lie below this line as being predominantly associated in some way, for example, as ion pairs and/or aggregates. Thus three of the eight ILs shown ([P6,6,6,14 ][Cl], [P6,6,6,14 ][Cyc], and [P6,6,6,14 ][dbsa]) lie in this ‘associated IL’ region. [P6,6,6,14 ][Sacc] and [N1,8,8,8 ][Cl] lie on the borderline. As an example of the impact of this, [P4,4,4,4 ][Sacc], although being almost three times more viscous than [P6,6,6,14 ][Sacc], nonetheless has a similar conductivity,[68] presumably because the degree of ion correlation and association is lower in the former case. Looking at this from a rather different point of view, one could consider [P6,6,6,14 ][Sacc] to be surprisingly fluid given the size of the cation, but on consideration of its rather low conductivity, one realises that it is not behaving as a true IL and is better thought of as reflecting the properties of an associated IL.[68] It seems that some of the larger phosphonium cations are capable of forming such associated species because the anion is able to approach the centre of positive charge more closely than it can in the equivalent nitrogen-based cation. The fact that some of these liquids lie in the associated IL zone of the Walden plot does not necessarily mean that they are not of interest as solvents or media. Certainly their conductivity is lower than a true IL of comparable viscosity, however, the high degree of ion association generally indicates a lower viscosity than might have otherwise been the case. Hence such liquids provide a range of potentially useful properties. For example, if ion pairing is significant, it may be predicted that such compounds would exhibit higher vapour pressures. Generally speaking, they lie between true molecular solvents and true ILs and hence possess intermediate, but nonetheless tunable properties.

RESEARCH FRONT

316

K. J. Fraser and D. R. MacFarlane

3 [P6,6,6,14][Sacc] [P6,6,6,14][Cyc]

Log [Molar conductivity (S cm2 mol!1)]

2

[P4,4,4,4][Sacc] C lK

ea

[P6,6,6,14][CI] 1

ne

I li

[N1,8,8,8][CI] Id

[P6,6,6,14][NTf2] [P6,6,6,14][dca] [P6,6,6,14][dbsa]

0

d te cia uid o q s li As nic n io gio re

[C4mpyr][NTf2] [C2mpyr][dca] [P6,6,6,14][Ace]

!1

[N8,8,8,8][CI] re

tu

!2 e

as

e cr

!3 !3

in

p

m

te

a er

In

!2

!1

0 1 Log [1/Viscosity (Poise!1)]

2

3

Fig. 4. Walden plot for phosphonium salts over a temperature range 30–100◦ C.[68] [C4 mpyr][NTf2 ], [N1,8,8,8 ][Cl], and [C2 mpyr][dca] are shown for comparison.

"

O

" N

P

F3C O

O

[P2,2,2(201)]" Fig. 5.

[DEME]"

! N

O

S

S

O

O

CF3

[NTf2]!

Ionic liquids synthesized by Tsunashima et al. for use as electrolytes in lithium-based batteries (redrawn from ref. [127]).

Electrochemical Applications of Phosphonium Ionic Liquids Battery Electrolytes Pyrrolidinium and imidazolium [NTf2 ]− based roomtemperature ILs have been widely investigated as electrolytes in lithium batteries and solar cells because of their low viscosities and reasonable conductivites.[125,126] Phosphoniumbased ILs have not been as widely investigated as electrolytes, because of their typically large cations and lower conductivities. Tsunashima et al. have synthesized less bulky, phosphonium cations,[127] for example, [P2,2,2(201) ][NTf2 ] (Fig. 5), and studied these in lithium battery applications. The discharge capacity of a battery based on an a nitrogen cation-based IL, [DEME][NTf2 ], after 50 cycles was 100 mA h g−1 .[127] Compared with this, the [P2,2,2(201) ][NTf2 ] IL had a discharge capacity of 119 mA h g−1 .[127] Solar Cell Electrolytes Dye-sensitized solar cells (DSCs) are currently receiving significant attention because of their potential applications as flexible

and low-cost solar cells.[128,129] A typical DSC consists of a dyeadsorbed nanoporous titanium dioxide (TiO2 ) photoelectrode, an electrolyte that contains an iodide/triiodide redox couple, and a platinum-coated conducting glass counter-electrode.[129] Recently Kunugi et al. synthesized [P2,2,2,5 ][NTf2 ] for use as an electrolyte in a DSC.[130] The energy conversion efficiency of the DSC using [P2,2,2,5 ][NTf2 ] was 1.2% under 1 sun illumination and 3.8% at lower light levels. Interestingly, the device performance was higher for the quaternary phosphonium IL electrolytes than those for the corresponding quaternary ammonium ILs.[130] In a further attempt to incorporate phosphonium-based ILs into DSCs, Ramirez et al. constructed solar cells based on aliphatic, asymmetric, and mid-size chain length phosphonium iodide salts. An overall efficiency of 5.7% was reported under moderate light conditions.[131] Super-Capacitor Electrolytes Super-capacitors are high power energy sources that are very attractive for hybrid vehicle applications because of their high power density and long durability.[132–134] Capacitance measurements of porous carbons using ILs are scarce; in addition, good

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Phosphonium-Based Ionic Liquids

317

(a) Br Pd/Et3N catalyst

Co2t-Bu

[P4,4,4,16][Br] (CYPHOS IL 162)

"

R Co2t-Bu 51–99% yields

R (b)

R&

X " R%

Pd(OAc)2 catalyst [P6,6,6,14][Cl] (CYPHOS IL 101)

R& R

R

R%

81–98% yields Fig. 6.

(a) [P4,4,4,16 ][Br] and (b) [P6,6,6,14 ][Cl] ionic liquids reported as solvents in the Heck coupling reaction (redrawn from refs [65] and [46], respectively).

X " R%

R

Fig. 7.

B(OH)2

1% Pd2(dba)3 CHCl3 [P6,6,6,14][Cl] (CYPHOS IL 101) K3PO4 50$C

R

R% 76–100%

Suzuki coupling carried out in a phosphonium ionic liquid (redrawn from ref. [48]).

wetting of highly porous carbons by ILs is complicated because of their high viscosity. Some authors overcome this issue by operating the capacitor at higher temperatures.[135] For special applications, e.g., hybrid vehicles or fuel cells, such a temperature enhancement for capacitor performance is acceptable. However, applications that require operation at ambient temperature are in high demand. The first reported phosphonium-based electrolyte in a super-capacitor was in 2005 by Frackowiak et al.[136] The ILs [P6,6,6,14 ][NTf2 ] and [P6,6,6,14 ][dca] provided good electrochemical stability and cyclability. Using [P6,6,6,14 ][NTf2 ] as the electrolyte, the authors were able to construct a super-capacitor operating at 3.4 V and producing an energy density of ∼40 W h kg−1 .[136] Corrosion Inhibitors The use of phosphate and phosphinate anions as corrosion inhibitors and in conversion coatings is generally well accepted.[137,138] Recent efforts by Forsyth et al. have shown the utility of a series of novel and commercially available phosphonium-based ILs using [P(O)2 (OR)2 ]− and [P(O)2 (R)2 ]− anions in combination with the [P6,6,6,14 ]+ cation as corrosion inhibitors for magnesium alloys.[73,139,140] Following initial studies of coatings on Li, preliminary investigations and screening compared the degree of protection afforded by a range of [NTf2 ]− -based ILs on commercial Mg alloy AZ31;[139] specifically 1-ethyl-3-methylimidazolium, N-methyl N-propylpyrrolidinium, and trihexyl(tetradecyl)phosphonium cations were investigated. The authors report that weight loss and electrochemical measurements indicate that the [P6,6,6,14 ][NTf2 ] IL offered substantially more protection than the imidazolium or pyrrolidinium ILs. In one example, by controlling the transfer and electrochemical reactions on magnesium alloys by chemical

treatment using [P6,6,6,14 ][NTf2 ], a 50-fold reduction in corrosion rate in a NaCl solution was found,[139] which is significantly better than commonly reported chemical treatments.[141–145] The mechanism of this corrosion resistance is being further investigated.[73] Phosphonium Ionic Liquids: Useful Synthetic Solvents Environmental pressure to reduce waste and re-use materials has stimulated the development of ‘green’ chemistry.[146] Recent reviews have covered these emerging fields[101,147] and it is apparent that one of the most difficult areas to make more environmentally friendly is solution phase chemistry.[49] Phosphonium ILs may have a role to play in this effort, as described in the following examples. The Heck cross-coupling reaction is a common reaction for the formation of carbon–carbon bonds between alkenes and organic halides.[148] The first reported uses of a phosphonium salt as an IL solvent in a synthetic reaction was that of Kaufmann et al.[65] The use of tributyl(hexadecyl)phosphonium bromide ([P4,4,4,16 ][Br], CYPHOS IL 162) as a recyclable medium for the palladium-mediated Heck coupling of aryl halides with acrylate esters was reported (see Fig. 6a).[65] Although high yields were reported without the use of an additive ligand, reaction conditions were not ideal (100◦ C), and the reaction favoured only the more activated aryl halides. In a bid to reduce selectivity of the aryl halides, more recent attempts using phosphonium ILs have been promising, as shown by McNulty et al.[149–151] The use of [P6,6,6,14 ][Cl] was shown to provide more successful Heck coupling reactions of deactivated and sterically demanding aryl halides in high yields (Fig. 6b).[46] The reaction requires only 50◦ C, is complete within 2 h, and the solvent can be effectively re-used.[46] Further screening of phosphonium ILs showed that the anion plays a major role in the yield

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318

K. J. Fraser and D. R. MacFarlane

R1 I " X

R2NH2

PdCL2(PPh3)2/DBU/[P6,6,6,14][Br]

N

R2

1 atm CO, 110$C, 18 h O

Fig. 8. Palladium-catalyzed carbonylation–hydroamination reaction between 1-bromo-2-(phenylethynyl)benzene and benzylamine using [P6,6,6,14 ][Br] as the reaction solvent (redrawn from ref. [165]).

of the Heck reaction. While chloride and decanoate anions[46] coupled with the [P6,6,6,14 ]+ cation produced high yields, anions such as tetrafluoroborate and hexafluorophosphate result in significantly lower yields.[46] The Suzuki cross-coupling reaction[152] has become a standard method for carbon–carbon bond formation between an sp2 carbon or non-β-hydride-containing electrophile and a boronic acid derivative. Recently, the use of [P6,6,6,14 ][Cl] has been reported as a useful medium for the Suzuki cross-coupling of aryl alides with boronic acid derivatives, as seen in Fig. 7.[48] The process reported by McNulty et al.[48] requires a soluble palladium catalyst precursor such as Pd2 (dba)3 ·CHCl3 , that is dissolved in the phosphonium IL, to produce a dark orange solution. This solution was stable in the absence of oxygen for an extended period of time and could be recycled after solvent extraction of the biaryl reaction products.[48] Two advantages of this system are the milder conditions under which the Suzuki coupling takes place in comparison to imidazolium-based ILs and the reactivity of economical, readily available aryl chlorides. Imidazolium ILs used as solvents in the Suzuki crosscoupling reaction require ultrasonic irradiation in order to proceed at 30◦ C.[153] The use of imidazolium-type ILs in this manner is further compromised by the recent discovery that these solvents decompose when subject to ultrasonic irradiation.[154] The purely thermal Suzuki coupling reaction does not proceed with aryl chlorides even at 110◦ C.[155] In contrast, the use of the phosphonium salt allows very high conversion with aryl bromides and iodides and electron deficient chlorides at 50 to 70◦ C. Several nitrogen-based ILs[156–158] as well as phosphonium salts[74] have been studied with regard to solvents in the Diels– Alder reaction. Using ILs as solvent media in the Diels–Alder reaction generally produced yields that are good to high in both imidazolium and phosphonium-based salts. Several reactions have been investigated with both acyclic 1,3-dienes as well as cyclopentadiene and with a variety of acrolein and acrylic acid dienophiles.[74] Ludley et al.[74] reported in 2001 that phosphonium tosylates are very good solvents for the Diels–Alder reaction of isoprene with oxygen-containing dienophiles. The reaction proceeded with high regioselectivity, even without the use of a Lewis acid catalyst. The reaction temperatures are moderate (between 60 and 120◦ C) and the solvents can be reused.[74] In a recent publication involving [P6,6,6,14 ][NTf2 ], results have shown that in the presence of catalysts, the Diels–Alder reaction between cyclopentadiene and dienophiles in the form of α,βunsaturated esters, aldehydes, and ketones at room temperature occurs smoothly, with high yield and high stereoselectivity.[159] Other advantages of using this solvent included the reaction taking place at room temperature, the time of the reaction needed to obtain quantitative yields of the product being relatively short and varying from 30 to 120 min, the catalysts from the group of

metal chlorides, triflates, and bis-triflimides being very soluble in the phosphonium IL and, because of the high thermal stability of the phosphonium IL, the product can be isolated by distillation from the IL and the catalyst.[159] Although there seems to be no major advantage of using phosphonium-based ILs over their nitrogen-based counter-parts, there is potential for phosphonium salts, in general, as both catalyst and solvent for the reaction, and possibly for chiral phosphonium salts in promoting asymmetric cycloadditions.[159] Recently, several reports have explored the use of phosphonium-based ILs as media for strong base-mediated reactions such as the Grignard reaction.[49,160–162] Ramnial et al.[64] reported that phosphonium ILs can dissolve other important carbon-centred ligands, can be used as solvents for generating N-heterocyclic carbenes and their metal complexes, and can be used as a solvent medium for the generation of Wittig reagents. The reactivity of [P6,6,6,14 ][Cl] in Grignard reagent solutions with a tetrahydrofuran-to-[P6,6,6,14 ][Cl] ratio of 1:3 was tested by the addition of anhydrous bromine to give exclusive formation of bromobenzene.[64] After 1 month, when the aged solutions were treated with Br2 , 5% of biphenyl was detected along with bromobenzene. Benzene by products were not observed (which indicates deprotonation did not occur); however, more importantly, deprotonation of [P6,6,6,14 ][Cl] to form a phosphorane was not observed.[64] The use of phosphonium-based solvents in Grignard solutions do not alter the behaviour of the Grignard reagents. Carbenes dissolved in [P6,6,6,14 ][Cl] can also be used to prepare complexes of both main group and transition metals.[64] These observations open up the use of phosphonium-based ILs as a reliable reaction media for a wide variety of basic reagents. Also, it must be noted that the use of certain phosphonium ILs also facilitates product separation because of the triphasic nature of some water, IL, and hexane combinations. This creates the possibility of limiting the use of ethereal solvents in this class of reactions, thus allowing for a general ‘greening’ of Grignard chemistry.[49] Well known problems associated with C–H activation in imidazolium ions by highly reactive bases have also been observed for phosphonium ILs.[64,163] Deprotonation reactions can result in the decomposition of the IL ions.To test if phosphonium-based ILs undergo deprotonation, Ramnial et al.[64] carried out a simple test that consisted of a solution of the phosphonium IL dissolved in a 50/50 D2 O/D-ethanol mixture that contained a small amount of NaOD. Over a period of days, the 1 H signal of the α-CH2 protons decreased relative to the other protons on the alkyl chains on the phosphonium IL.[64] Similar experiments were carried out by Tseng et al.[163] and they report that [P6,6,6,14 ][Cl] was found to be 50% deuterium exchanged at the α-position within 30 min at 50◦ C and 12 min at 65◦ C.[163] This experiment clearly indicates that the α-protons on the phosphonium ion, although crowded, are accessible to small bases such as OH− .[64] However, this

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Phosphonium-Based Ionic Liquids

exchange reaction does not appear to produce a measurable acidity of these cations. Further investigation of this reaction by Ramnial et al. suggests that [Ph3 PCH2 CH3 ]+ [Br]− can be deprotonated to form a phosphorane by bases such as potassium tert-butoxide, as shown by 31 P–1 H NMR studies.[64] In the case of larger, more bulky bases, there appears to be no reaction with the phosphonium component.[64] Nonetheless, the deprotonation reactions occur more readily in the imidazolium ILs than in phosphonium ILs, probably because of both electronic and steric reasons.[64] Two efficient approaches for the synthesis of isoindolin-1-one derivatives in phosphonium salt ILs have recently been reported by Alper et al.[164] This class of compounds occur naturally and are reported to have local anaesthetic activity superior to that of procaine.[165] For this reason the synthesis of substituted 3-methyleneisoindolin-1-ones has generated considerable interest over the past several decades.[166–168] The reaction is based on the sequential use of Sonogashira coupling, carbonylation, and hydroamination chemistry as shown in Fig. 8. Using [P6,6,6,14 ][Br] as the reaction solvent provided good stereo selectivity to certain isomers and produced high reaction yields (up to 82%). One other major advantage was that the synthesis can be conducted as a ‘one-pot’reaction.[164] Nitrogenbased ILs were not suitable as reaction media, because of the basic conditions and prolonged heating (110◦ C). Other traditional solvents, such as tetrahydrofuran did not produce any reaction.[164] Concluding Remarks Phosphonium ILs clearly offer, in some cases, several advantages over other types of ILs, including, in specific cases and applications, higher thermal stability, lower viscosity, and higher stability in strongly basic or strongly reducing conditions. Our overview here has attempted to present the current state of development of the field, as reported in the open literature. There is a need for considerable further study of their physical and chemical properties in order to understand better the structure– property relationships at play in these liquids. Much also remains to be done to broaden our understanding of the differences between the nitrogen- and phosphonium-based cations; the computational work of Izgorodina[68,169] and Hunt[170] should provide increasingly valuable insights into these fundamentals. Additional Comment For an excellent collation of scholarly phosphonium-based IL references please visit: http://www.chem.monash.edu.au/ ionicliquids/downloads.html pdf file courtesy of Cytec. Acknowledgements The authors thank Dr Al Robertson of Cytec Industries Inc. for his valuable insights and helpful comments. D.R.M. is grateful to the Australian Research Council for a Federation Fellowship.

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