Aust. J. Chem. 2009, 62, 309–321
Phosphonium-Based Ionic Liquids: An Overview
Kevin J. FraserAand Douglas R. MacFarlaneA,B
ASchool of Chemistry, Monash University, Wellington,VIC 3800, Australia.
BCorresponding author. Email: Douglas.Macfarlane@sci.monash.edu.au
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.
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.The variation in properties between
salts, even those with a common cation but different anions,
is dramatic. For example, butylmethylimidazolium hexafluoro-
phosphate [C4mim][PF6] is immiscible with water, whereas
butylmethylimidazolium tetrafluoroborate [C4mim][BF4] is
water soluble.This sort of variation in physical proper-
ties gave rise to Seddon’s description of ILs as ‘designer
solvents’.The number of potential anion–cation combina-
tions possible reputedly equates to one trillion (1012) different
ILs.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
Kevin J. Fraser received his M.Sc. in 2004 from the University ofAberdeen, 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
Professor Doug MacFarlane leads the Monash Ionic Liquids Group at Monash University. He is also the program leader of
the Energy Program in theAustralian Centre for Electromaterials Science. He was a Ph.D. graduate of Purdue University in
1982 and after postdoctoral work atVictoria UniversityWellington took up a faculty position at Monash University. Professor
MacFarlane was recently awarded anAustralian Research Council Federation Fellowship to extend his work on Ionic Liquids.
He was elected to theAustralianAcademy 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.
Nitrogen-based cations, in particular N-methylimidazolium
and pyrrolidinium salts, have been the subject of many of the
ILs are also available and have a range of useful properties,
but have been much less studied. Early reports regarding phos-
phonium 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 tetrabutylphospho-
nium 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
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.
along with the multitude of available anions, represents an enor-
mous number of possible salts. Those commercially available
as of November 2008, for example, can be found in Table 1.
© CSIRO 200910.1071/CH085580004-9425/09/040309
310 K. J. Fraser and D. R. MacFarlane
Phosphonium ILs available as of November 2008
Supplier (Trade name)
state at RTB
Cytec (CYPHOS IL 101)
Cytec (CYPHOS IL 102)
Cytec (CYPHOS IL 103)
Cytec (CYPHOS IL 104)
Extraction of heavy metals
Cytec (CYPHOS IL 105)
Cytec (CYPHOS IL 106)
Hydro formation of
Cytec (CYPHOS IL 108)
Cytec (CYPHOS IL 109)
Cytec (CYPHOS IL 110)
Heck couplingSuzuki cross
Cytec (CYPHOS IL 111)
Cytec (CYPHOS IL 120)
Solvent in the Diels–Alder
Cytec (CYPHOS IL 162)
Phase Transfer catalyst[51,75,76]
Cytec (CYPHOS IL 163)
Medium for fluorination of
Cytec (CYPHOS IL 164)
Phosphonium-Based Ionic Liquids311
Cytec (CYPHOS IL 166)
Heck coupling reactions[46,65]
Cytec (CYPHOS IL 167)
Cytec (CYPHOS IL 169)
Cytec (CYPHOS IL 201)
Separation of organic materials
Cytec (CYPHOS IL 202)
Separation of organic materials
Cytec (CYPHOS IL 208)
Cytec (CYPHOS IL 239)
Cytec (CYPHOS IL 249)
Cytec (CYPHOS IL 250)
Mixed triisobutyl(methyl)phosphonium and
Cytec (CYPHOS IL 256)
Cytec (CYPHOS IL 257)
Cytec (CYPHOS IL 260)
Cytec (CYPHOS IL 265)
Cytec (CYPHOS IL 268)
Cytec (CYPHOS IL 300)
Cytec (CYPHOS IL 319)
Cytec (CYPHOS IL 320)
Cytec (CYPHOS IL 324)
312K. J. Fraser and D. R. MacFarlane
Supplier (Trade name)
state at RTB
Cytec (CYPHOS IL 325)
Cytec (CYPHOS IL 327)
Cytec (CYPHOS IL 331)
Cytec (CYPHOS IL 349)
Cytec (CYPHOS IL 351)
Solvent for the Regioselective
o-Alkylation of C/OAmbidentate
Electrolyte in double layer
Solvent in the separation of aromatic
hydrocarbons from aliphatic hydrocarbons
Electrolyte in double layer
ATdec, decomposition temperature measured by the step tangent method.BRT, room temperature.CData from Cytec Industries Inc. unless otherwise referenced.
Phosphonium-Based Ionic Liquids 313
These include salts with the traditional halide anions such as tri-
IL 101 and 102), which are liquid at room temperature and
have glass transition temperatures as low as −65◦C.Salts
that contain other anions such as tosylate, dicyanamide, methyl-
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
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
based ILs, the higher thermal stability of phosphonium-based
ILs is useful in processes that operate at greater than 100◦C.
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.Another advantage of phosphonium-
based ILs as compared to their imidazolium cation analogues is
which can in turn lead to carbene formation.Alkylphos-
phonium 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,exfoliation agents for montmorillonite clays,[53–59]
Examples of anions that can be paired with tetraalkylphosphonium cations to produce ILs.
catalysts in epoxy curing,and in high-temperature poly-
Here we overview this family of organic salts and their prop-
review. Other very useful reviews of the field include those by
Zhou et al.and Clyburne et al.
Synthesis of Phosphonium-Based Ionic Liquids
The first phosphonium salts to become available were the [Cl]−
as biocides[83,84]and phase transfer catalysts.[85–87]The ever-
growing interest in phosphonium ILs led Bradaric et al.to
closely examine a range of potential ILs for industrial produc-
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
The synthesis of [P6,6,6,14][Cl] is described by Bradaric
synthesized by adding trihexylphosphine to one equivalent of
1-chlorotetradecane at 140◦C under nitrogen and stirring for
12h. After the reaction is complete, the mixture is vacuum
stripped to remove any volatile components such as tetradecene
isomers, and excess 1-chlorotetradecane.The resultant IL
is a clear, pale yellow liquid, with a typical yield of 94%.
nium hydrochloride and hydrochloric acid; the residual acid can
be an important issue in several applications, but can easily
314 K. J. Fraser and D. R. MacFarlane
? NaOH R ?
trimethylpentyl)phosphinate (redrawn from ref. ).
be removed. Other minor impurities may include tetradecene
isomers and R2PH.
The ion exchange reactions used to produce phosphonium-
based ILs generally fall into two categories (as shown in Eqns 1
[R?PR3]+[X]−+ MA → [R?PR3]+[A]−+ MX
where R, R?=alkyl; X=halogen; M=alkali metal; andA=an
anion such as phosphinate, carboxylate, tetrafluoroborate, and
hexafluorophosphate.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 avail-
able are a useful example of Eqn 2. [P6,6,6,14][bis(2,4,4-
trimethylpentyl)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 con-
tain residual chloride ions, which may adversely affect metal
catalysts[93–105]and/or contaminate reaction products. In addi-
tion, anion exchange processes are typically inefficient and
usually involve the use of environmentally hazardous molec-
ular 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
mogravimetric analysis (10Kmin−1, N2flow, Al pans); (i) [C4mpyr][dca],
(ii) [C4mpyr][tcm], (iii) [P6,6,6,14][dca], and (iv) [C4mpyr][NTf2].
desirable.Bradaric et al.successfully synthesized phos-
phonium ILs in several direct, solvent free, halide free routes by
the quaternization of tertiary phosphines with dialkylsulfates,
dialkylphosphates, and alkylphosphonates as described in Eqns
P(n-Bu)3+ SO2(OR)2→ [RP(n-Bu)3][SO3(OR)]
(R = Me,Et,n-Bu)
(R = Me,Et,n-Bu;R?= n-Bu)
(R = Me,Bu;R?= i-Bu)
P(n-Bu)3+ O=PR(OR)2→ [RP(n-Bu)3][PRO2(OR)]
(R = Me,n-Bu)
3+ O=P(OR)3→ [RPR?
In addition to being halide free, these synthetic routes also
provide a straightforward way to tune the properties of the phos-
phonium 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.have reported the properties
of novel phosphonium ILs that have large anions, for example
Physical Properties of Phosphonium-Based Ionic Liquids
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 dis-
tilled from the IL.Phosphonium ILs have also been shown to
possess greater electrochemical stability than their ammonium
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 ter-
tiary phosphine oxide and alkane under alkaline conditions
[R3P–CH2–R?]++ OH−→ R3P=O + CH3–R?
Phosphonium-Based Ionic Liquids315
sponding ammonium room temperature ionic liquids (ILs) synthesised
byTsunashima et al.
Physical and thermal properties of phosphonium and corre-
(TGA, 10◦C min−1) [◦C]
Alternatively, depending on the nature of R and R?, stable
phosphoranes can be formed (Eqn 7);these are well known
as Wittig reagents.
[R3P–CH2–R?]++ OH−→ R3P=CHR?+ H2O (7)
While the decomposition of phosphonium salts by these
pathways may occur in some cases,contrasting examples
are known where tetraalkylphosphonium halides can be com-
bined with concentrated sodium hydroxide well above room
temperature without any degradation (e.g., [P4,4,4,16][Br]).
ILs in general are often reported to be resistant to thermal
decomposition and thus suitable for high-temperature applica-
tions; however, in most cases, thermal stability is measured by
recording weight loss with rising temperature by thermogravi-
metric analysis (TGA)(Fig. 3). This produces a substantial
overestimation of the operational upper temperature limit at
ing extended periods.The work of Scott et al.has described
be estimated from isothermal TGA measurements.
Factors that affect the viscosity of an IL are poorly under-
stood, 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.
Tsunashima et al.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., 35mPas for
triethyl(methoxymethyl)phosphonium [NTf2]), even lower than
good conductivities and high thermal stability and, therefore,
such as battery electrolytes.
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 sys-
tem 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.
Ionicity of Phosphonium-Based Ionic Liquids
Original observations by Waldenconcluded that, for strong
electrolyte solutions, the molar conductivityis inversely
φ (φ=η−1) of the medium through which the ions move.
These observations were combined into what is now known as
the ‘Walden rule’:
?η = constant(8)
a means of testing data against this relationship.As a point
of reference, a 0.1M aqueous KCl solution is usedbecause
potassium and chloride ions have similar hydrodynamic radii in
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 avail-
able and newly synthesized phosphonium salts prepared by
Fraser et al.over the temperature range of 30–100◦C. For
comparison, [C4mpyr][NTf2], [N1,8,8,8][Cl], and [C2mpyr][dca]
are also presented. The slope of the line with increasing tem-
perature 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 indi-
cating 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 exam-
ple, 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,
point of view, one could consider [P6,6,6,14][Sacc] to be surpris-
ingly 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.It seems that some of the larger phospho-
nium 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
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
degree of ion association generally indicates a lower viscosity
vide a range of potentially useful properties. For example, if ion
pairing is significant, it may be predicted that such compounds
lie between true molecular solvents and true ILs and hence
possess intermediate, but nonetheless tunable properties.
316 K. J. Fraser and D. R. MacFarlane
Ideal KCI line
Increase in temperature
Log [1/Viscosity (Poise?1)]
Log [Molar conductivity (S cm2 mol?1)]
[C2mpyr][dca] are shown for comparison.
Walden plot for phosphonium salts over a temperature range 30–100◦C.[C4mpyr][NTf2], [N1,8,8,8][Cl], and
Ionic liquids synthesized by Tsunashima et al. for use as electrolytes in lithium-based batteries (redrawn from ref. ).
Electrochemical Applications of Phosphonium
temperature ILs have been widely investigated as electrolytes
in lithium batteries and solar cells because of their low vis-
cosities and reasonable conductivites.[125,126]Phosphonium-
based ILs have not been as widely investigated as electrolytes,
because of their typically large cations and lower conductivi-
ties. Tsunashima et al. have synthesized less bulky, phospho-
nium cations,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 100mAhg−1.Com-
pared with this, the [P2,2,2(201)][NTf2] IL had a discharge
capacity of 119mAhg−1.
Solar Cell Electrolytes
Dye-sensitized solar cells (DSCs) are currently receiving signif-
icant attention because of their potential applications as flexible
adsorbed nanoporous titanium dioxide (TiO2) photoelectrode,
an electrolyte that contains an iodide/triiodide redox couple,
and a platinum-coated conducting glass counter-electrode.
Recently Kunugi et al. synthesized [P2,2,2,5][NTf2] for use as an
DSC using [P2,2,2,5][NTf2] was 1.2% under 1 sun illumination
and 3.8% at lower light levels. Interestingly, the device per-
formance was higher for the quaternary phosphonium IL elec-
trolytes than those for the corresponding quaternary ammonium
ILs.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.
Super-capacitors are high power energy sources that are very
attractive for hybrid vehicle applications because of their high
ments of porous carbons using ILs are scarce; in addition, good
Phosphonium-Based Ionic Liquids 317
(CYPHOS IL 101)
(CYPHOS IL 162)
1% Pd2(dba)3 CHCl3
(CYPHOS IL 101)
Suzuki coupling carried out in a phosphonium ionic liquid (redrawn from ref. ).
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.For special
applications, e.g., hybrid vehicles or fuel cells, such a tempera-
ever, applications that require operation at ambient temperature
are in high demand.The first reported phosphonium-based elec-
trolyte in a super-capacitor was in 2005 by Frackowiak et al.
trochemical stability and cyclability. Using [P6,6,6,14][NTf2]
as the electrolyte, the authors were able to construct a
super-capacitor operating at 3.4V and producing an energy
density of ∼40Whkg−1.
The use of phosphate and phosphinate anions as cor-
rosion 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]Follow-
ing 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;specifically 1-ethyl-3-methylimidazolium, N-methyl
N-propylpyrrolidinium, and trihexyl(tetradecyl)phosphonium
IL offered substantially more protection than the imidazolium
treatment using [P6,6,6,14][NTf2], a 50-fold reduction in corro-
better than commonly reported chemical treatments.[141–145]
The mechanism of this corrosion resistance is being further
Phosphonium Ionic Liquids: Useful Synthetic Solvents
stimulated the development of ‘green’ chemistry.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.Phos-
phonium 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.The first reported uses of a phosphonium
et al.The use of tributyl(hexadecyl)phosphonium bromide
palladium-mediated Heck coupling of aryl halides with acrylate
esters was reported (see Fig. 6a).Although high yields were
reported without the use of an additive ligand, reaction condi-
tions 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
provide more successful Heck coupling reactions of deactivated
The reaction requires only 50◦C, is complete within 2h, and the
318K. J. Fraser and D. R. MacFarlane
1 atm CO, 110?C, 18 h
amine using [P6,6,6,14][Br] as the reaction solvent (redrawn from ref. ).
Palladium-catalyzed carbonylation–hydroamination reaction between 1-bromo-2-(phenylethynyl)benzene and benzyl-
of the Heck reaction. While chloride and decanoate anions
such as tetrafluoroborate and hexafluorophosphate result in
significantly lower yields.
The Suzuki cross-coupling reactionhas become a stan-
dard 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
cess reported by McNulty et al.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.Two advantages of this system
are the milder conditions under which the Suzuki coupling takes
of economical, readily available aryl chlorides.
Imidazolium ILs used as solvents in the Suzuki cross-
coupling reaction require ultrasonic irradiation in order to
proceed at 30◦C.The use of imidazolium-type ILs in this
solvents decompose when subject to ultrasonic irradiation.
The purely thermal Suzuki coupling reaction does not proceed
with aryl chlorides even at 110◦C.In contrast, the use of
the phosphonium salt allows very high conversion with aryl
bromides and iodides and electron deficient chlorides at 50 to
Several nitrogen-based ILs[156–158]as well as phosphonium
saltshave 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.Ludley et al.reported in 2001 that phos-
phonium 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 mod-
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,β-
unsaturated esters, aldehydes, and ketones at room temperature
occurs smoothly, with high yield and high stereoselectivity.
Other advantages of using this solvent included the reaction tak-
ing 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 120min, the catalysts from the group of
metal chlorides, triflates, and bis-triflimides being very soluble
of the phosphonium IL, the product can be isolated by distilla-
no major advantage of using phosphonium-based ILs over their
possibly for chiral phosphonium salts in promoting asymmetric
Recently, several reports have explored the use of
tions such as the Grignard reaction.[49,160–162]Ramnial et al.
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
the addition of anhydrous bromine to give exclusive formation
of bromobenzene.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 impor-
tantly, deprotonation of [P6,6,6,14][Cl] to form a phosphorane
was not observed.
do not alter the behaviour of the Grignard reagents. Carbenes
open up the use of phosphonium-based ILs as a reliable reaction
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.
Well known problems associated with C–H activation in imi-
dazolium ions by highly reactive bases have also been observed
for phosphonium ILs.[64,163]Deprotonation reactions can result
Tseng et al.and they report that [P6,6,6,14][Cl] was found to
be 50% deuterium exchanged at the α-position within 30min at
50◦C and 12min at 65◦C.This experiment clearly indicates
that the α-protons on the phosphonium ion, although crowded,
are accessible to small bases such as OH−.However, this
Phosphonium-Based Ionic Liquids 319
exchange reaction does not appear to produce a measurable
acidity of these cations. Further investigation of this reaction
by Ramnial et al. suggests that [Ph3PCH2CH3]+[Br]−can be
deprotonated to form a phosphorane by bases such as potassium
tert-butoxide, as shown by31P–1H NMR studies.In the case
of larger, more bulky bases, there appears to be no reaction with
the phosphonium component.Nonetheless, the deprotona-
tion reactions occur more readily in the imidazolium ILs than
in phosphonium ILs, probably because of both electronic and
derivatives in phosphonium salt ILs have recently been reported
by Alper et al.This class of compounds occur naturally
and are reported to have local anaesthetic activity superior to
that of procaine.For this reason the synthesis of substituted
3-methyleneisoindolin-1-ones has generated considerable inter-
est 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 reac-
tion yields (up to 82%). One other major advantage was that the
based ILs were not suitable as reaction media, because of the
basic conditions and prolonged heating (110◦C). Other tradi-
tional solvents, such as tetrahydrofuran did not produce any
Phosphonium ILs clearly offer, in some cases, several advan-
tages over other types of ILs, including, in specific cases and
stability in strongly basic or strongly reducing conditions. Our
opment 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–
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 Huntshould
provide increasingly valuable insights into these fundamentals.
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.
The authors thank DrAl Robertson of Cytec Industries Inc. for his valuable
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