Alkyltrioctylphosphonium chloride ionic liquids: Synthesis and physicochemical properties

Article abstract of DOI:10.1039/c1dt10332f

A series of alkyltrioctylphosphonium chloride ionic liquids, prepared from trioctylphosphine, and the respective 1-chloroalkane (C_nH _2n+1Cl), where n = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12 or 14, is presented. The cynosure of this work is the manner in which the variable chain length impacts the physical properties, such as melting points/glass transitions, thermal stability, density and viscosity. Experimental density and viscosity data were interpreted using QPSR correlations and group contribution methods. We present the first example of an empirical alternation effect for ionic liquids.

Full text of DOI:10.1039/c1dt10332f

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
Dalton
Transactions
Cite this: Dalton Trans., 2011, 40, 12750
www.rsc.org/dalton
PAPER
Alkyltrioctylphosphonium chloride ionic liquids: synthesis and
physicochemical properties
Gabriela Adamova´,a Ramesh L. Gardas,a,b Lu´ıs Paulo N. Rebelo,c Allan J. Robertsond and Kenneth R. Seddona,c
Received 25th February 2011, Accepted 4th July 2011
DOI: 10.1039/c1dt10332f
A series of alkyltrioctylphosphonium chloride ionic liquids, prepared from trioctylphosphine, and the
respective 1-chloroalkane (CnH2n+1Cl), where n = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12 or 14, is presented. The
cynosure of this work is the manner in which the variable chain length impacts the physical properties,
such as melting points/glass transitions, thermal stability, density and viscosity. Experimental density
and viscosity data were interpreted using QPSR correlations and group contribution methods. We
present the first example of an empirical alternation effect for ionic liquids.
The synthesis of phosphonium salts initially involves a nucle-
Introduction
ophilic substitution reaction between chloroalkane and a tertiary
Phosphonium salts represent an important sub-class of ionic
phosphine to produce a tetraalkylphosphonium halide which can
be further converted, via anion exchange, into other ionic liquids.7
liquids and became, over the past decade, an integral part of these
industrially relevant solvents. An increasing trend in the number
The lack of available trialkylphosphines may have been one
of publications enlarged their potential impact on industrial
reason for the historically slow development of phosphonium ionic
applications.1 However, phosphonium-based ionic liquids were
liquids. However, since the mid 80s a variety of trialkylphosphines,
less systematically studied than their nitrogen-based counterparts
as well as a limited number of tetraalkylphosphonium halides
have been commercially available from Cytec Industries, Inc.8
As availability and cost of ionic liquids are paramount to any
industrial applications, commercially available phosphonium ionic
liquids can meet those demands and add many advantages in
comparison with other families of ionic liquids, including for
example higher thermal stability which allows them to be used
in high temperature processes.9 A high thermal stability was
actually one of the reasons why Eastman Chemical Company
(USA) employed [P8 8 8 18]I as a Lewis basic ionic liquid in an
isomerisation process of 3,4-epoxybut-1-ene to 2,5-dihydrofuran
instead of its ammonium analogue, in 1996.10 The process was
successfully running for eight years with a capacity of 1400 metric
tons per year.
(e.g. dialkylimidazolium, dialkylpyrollidinium, tetraalkylammo-
nium, etc.).2–4 In particular, it is somewhat surprising that there
are no published data on ionic liquids of the type, [Px x x n]X, where
n is systematically varied, while keeping x constant. Restricting
the phosphonium alkyl chains to being linear, with x and n having
values between 1 and 20, the number of potential combinations is
400 ionic liquids for any given anion. Here, we present, for the first
time, the systematic series of phosphonium chlorides (see Fig. 1),
based on trioctylphosphine. Related studies of ionic liquids based
on trihexylphosphine5 and tributylphosphine6 are in preparation.
Phosphonium salts are not necessarily liquid at room tem-
perature, but by careful selection of the alkyl substituents, as
well as an appropriate anion, there are many of them which
fall within the broad definition of ionic liquids.11 Traditionally,
phosphonium-based halides represented by tetraethylphospho-
nium bromide or tetraphenylphosphonium chloride, have melt-
Fig. 1 The structure of the alkyltrioctylphosphonium cation, where n =
1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12 or 14.
◦
ing points above 270 C12 and therefore they are not usually
considered, even in the broadest definition, as ionic liquids.
Others, such as tetrabutylphosphonium chloride (m.p. 86 ◦C)
aThe QUILL Centre, The Queen’s University of Belfast, Stranmillis Road,
Belfast, UK, BT9 5AG. E-mail: quill@qub.ac.uk
◦
or tetraoctylphosphonium bromide (m.p. 42 C)13 are moderate
bCurrent address: Department of Chemistry, Indian Institute of Technology
Madras, Chennai, India, 600 036
to low-melting solids, and are termed as ionic liquids. The well
known trihexyl(tetradecyl)phosphonium chloride (CYPHOS IL
101), available on the ton scale for more than a decade, is liquid
at room temperature, which is the case of most of the compounds
presented here.
cInstituto de Tecnologia Qu´ımica e Biolo´gica, Universidade Nova de Lisboa,
Oeiras, Portugal, 2780-901; Web: www.itqb.unl.pt
dCytec Canada Inc, 9061 Garner Rd., Niagara Falls, Ontario, Canada,
L2E 6S5
12750 | Dalton Trans., 2011, 40, 12750–12764
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
The tetraalkylphosphonium ionic liquids, which contain cations
with a distorted tetrahedral structure, have not surprisingly very
different structural and physical properties to those, say, based
on a planar aromatic cation. This has been well illustrated in a
recent paper,14 which predicted a different morphology, and hence
different physicochemical properties for each class of ionic liquid.
We present here the first systematic study of a related series of
tetralkylphosphonium ionic liquids, producing synthetically pure
materials with extensive physicochemical characterisation. An
interesting, and with hindsight perhaps predictable, phenomenon
which emerged from this work was the observation of the effect
of the “even/odd” number of carbons in the fourth alkyl chain
on the physical properties. Systematic studies involving changes
in the alkyl substitution on the cation and/or the anion, have
been previously reported in the literature.15 However, we believe
principally for reasons of cost, published studies have largely
focussed on cations containing alkyl chains with an even number
of carbon atoms; of these the majority have focussed on 1,3-
dialkylimidazolium salts. Of these, remarkably few studies have
included alkyl chains containing both even and odd number of
carbon atoms.16–18
can be controlled either by temperature, pressure or microwave
output (max. 1000 W). The software enables on-line modifications
of any method parameter and the reaction process is monitored
by an appropriate graphical interface. Agitation is achieved by
magnetic stirring (max. 400 rpm) controlled by software.
NMR spectroscopy. All NMR spectra were recorded on a
Bruker Advance spectrometer DPX 300 at 21 ◦C, using deuteriated
propanone as solvent (10–20 mM solutions for 1H and 31P NMR
spectroscopy; 30–35 mM solutions for 13C NMR spectroscopy),
referred to TMS for 1H and 13C NMR spectra and 85% H3PO4 for
31P NMR spectra.
Mass spectrometry. Most samples were analysed by fast atom
bombardment (FAB) spectrometry, performed on with VG-
Autospec (Fisons Instruments) in a positive ion mode. Two of the
samples ([P8 8 8 9]Cl and [P8 8 8 14]Cl) were analysed by electrospray
ionisation mass spectrometry (ESI-MS) on a Waters LCT Premier
mass spectrometer (Micromass Technologies).
Microanalysis. Elemental carbon and hydrogen contents
(wt%) were analysed using a Perkin–Elmer Series II CHNS/O
2400 CHN Elemental Analyser, and provided analysis within 0.3
wt% error. Halide content was determined by dioxygen flask,
combustion followed by halide titration with aqueous mercury(II)
nitrate, using a dicarbazide indicator.
Experimental
Instrumental
Autoclave. The miniclave drive manufactured by Bu¨chi Glas
Uster (Switzerland) was purchased from Ken Kimbel & Co. Ltd.
(United Kingdom). The autoclave consists of a glass reactor with
nominal volume 200 cm3; holder for glass reactor with stainless
steel anti-splinter cage; insert ring; cover plate with 6 welded-in
14 " Swagelok connections, welded on bmd 075 magnetic drive; O-
ring seal; union nut and holding rod. The magnetic coupling (bmd
075) is combined with the cyclone 075 (0–2500 rpm) equipped
with an electric motor, into a hermetically sealed unit. The
controller is equipped with a remote control input for external
speed adjustment. The stationary housing of the magnetic drive
and the cover plate are welded together and built as one common
unit. The cover plate contains pressure relief valve, platinum
resistance thermometer (Pt 100), gas inlet tube, pressure gauge
and an inlet/outlet tube. The maximum temperature is 250 ◦C
and maximum pressure is 10 bar.
Water content. Water content was determined using a Cou-Lo
Compact (version 08.05) Karl Fischer titrator, and titration solu-
tions from Riedel-de-Hae¨n. Ionic liquids were tested immediately
after drying under high vacuum for 48 h, and the water content
was below 450 ppm for all samples.
Differential scanning calorimetry (DSC). The melting points
and glass transitions of the compounds were measured by DSC,
using a TA Instruments Modulated DSC Q 2000 V24.4 Build 116
with refrigerated cooling system RCS 90 capable of controlling
the temperature down to 220 K. Dry dinitrogen gas was purged
through the DSC cell with a flow rate of ca. 20 cm3 min-1.
All samples, due to their hygroscopic character, were prepared
in a glove box, using Tzero hermetic platinum pans to avoid
any contact with air and measured under the same conditions
(standard heating and cooling ramp of 5 K min-1 over five cycles).
Microwave. MicroSYNTHmicrowave used was manufactured
by Milestone (Italy) as the single-mode set up with a 20 cm3
volume vial placed in the TMF safety shield vessel, closed by
TMF screw cap and located in the microwave cavity (43 l) at the
position with the highest energy intensity allowing rapid heating.
Reaction temperature control is provided by an IR sensor (max.
250 ◦C), monitoring the outside surface temperature of the vessel,
mounted in the sidewall of the cavity, about 5 cm above the
bottom and fibre-optic sensor (max. 300 ◦C) immersed within
the vessel. A pneumatic pressure sensor (max. 55 bar) placed
inside the vessel monitors the vapour concentration and overall
pressure. An additional sensor monitors the pressure inside the
cavity. Post-reaction cooling of the reaction mixture is achieved
by a constant airflow (1.8 m3 min-1) through the cavity and a
stream of compressed air. The MicroSYNTH reaction system is
monitored via an external control terminal with a touch screen
display utilising the EasyControl 640 software package. The run
Thermogravimetric analysis (TGA). Decomposition temper-
ature measurements were performed in a TA Instruments Q50
thermogravimetric analyser. The samples were measured in plat-
inum pans, at a heating rate of 10 K min-1, in a dynamic
mode, under a dinitrogen atmosphere. The onset of weight loss
in each thermogram was used as a measure of decomposition
temperature (see Results and Discussion). The sample size was
typically between 7 and 10 mg and sample preparation was similar
to that described for the DSC measurements.
Density measurements. Density was measured with an Anton
Paar vibrating tube densimeter, model DMA 4500, operating at
atmospheric pressure and within the temperature range 273 to
363 K. The internal calibration of the instrument was confirmed
by measuring densities of atmospheric air and doubly distilled
water, according to the recommendations of the manufacturer. The
DMA 4500 cell was embedded in a metallic block, the temperature
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 12750–12764 | 12751
©

View Article Online
of which was controlled by several Peltier units. This arrangement
enabled a temperature stability better than 2 mK.
All ionic liquids used in density determinations were degassed
under vacuum and moderate temperature conditions for periods
longer than 48 h and were injected into the densimeter immediately
after cooling, using non-lubricated disposable syringes.
Viscosity measurements. The viscosity was determined using
an AMVn microviscometer, Anton Paar, that allows measure-
ments from 273 to 363 K at atmospheric pressure, and over a wide
viscosity range (0.3 to 20 000 mPa s). A glass capillary with 4 mm
internal diameter and a 3 mm gold/tungsten ball (placed inside
the capillary) of density 15 g cm-3 was chosen for this study and
an extension of runtime up to 1000 s was required due to the high
viscosity of each sample. Temperature was maintained constant to
within 0.01 K by a capillary block with in-built Peltier elements.
Since these ionic liquids were extremely hygroscopic, the capillary
was always charged in the glove box. The capillary/ball system was
calibrated with standard viscosity oil (1 Pa s) from PRA. Viscosity
measurements were performed in steps of 10 K from 293 K up to
363 K for all ionic liquid samples for which a sufficient amount
(ca. 3.5 cm3) was available. Each sample was measured ten times
in order to check reproducibility.
Fig. 2 Numbering convention used for the cation described in the 1H,
13C and 31P NMR tables. The fourth (variable) alkyl substitution on the
phosphorus is labelled as a secondary chain (a¢, b¢, g¢, etc.) where the
terminal position takes priority, apart from positions a¢ and b¢.
Synthesis of [P8 8 8 n]Cl, n = 1 or 2. The autoclave glass
reactor was charged with trioctylphosphine (see Table 1) under a
dinitrogen atmosphere, placed into the stainless steel cage holder,
and quickly attached to the cover plate. A gas cylinder containing
chloroalkane (chloromethane or chloroethane) was connected to
the autoclave through the gas inlet tube. The gas and inlet valves
were opened after the set up was proved to be tight and the gas flow
was maintained at 2 cm3 min-1. The reaction mixture was slowly
heated over several hours in an oil bath until the temperature
Glove box. All ionic liquids studied here were handled in a
dinitrogen-filled glove box (MBraun LabMaster dp; 0.1 ppm < O2
and H2O) immediately after drying, and stored there until used.
◦
inside of the reactor reached 145 C. The pressure in the reactor
was built up by an excess of chloroalkane and did not exceed
4 bar with increasing temperature. The gas inlet was closed after
approximately one hour and the reaction was maintained at 145 ◦C
for two hours in the case of chloromethane, and for an additional
two hours in the case of chloroethane. After the reaction was
cooled down to room temperature, the pressure was released, and
a sample for 31P NMR spectroscopy was taken to check for the
disappearance of the phosphine signal. The content of the reactor
was then transferred to a round-bottomed flask (100 cm3) and
dried at 65 ◦C in vacuo for 48 h.
Materials
The phosphonium ionic liquids presented in this study were
synthesised following the procedures described in the next
section. Chloromethane (≥99.5%), chloroethane (≥99.7%),
1-chloropropane (98%), 1-chlorobutane (99.5%), 1-chloropentane
(99%), 1-chlorohexane (99%), 1-chloroheptane (99%),
1-chlorooctane (99%), 1-chlorononane (98%), 1-chlorodecane
(98%), 1-chlorododecane (98%) and 1-chlorotetradecane
(98%) were all purchased from Aldrich and used as received.
Trioctylphosphine (97%, r = 0.859 g cm-3) was supplied by Cytec
Industries, Canada.
Synthesis of [P8 8 8 n]Cl, n = 3 or 4. Trioctylphosphine (1 mol
eq.) was mixed with chloroalkane (1.1 mol eq.) in a 20 cm3
glass microwave vial with inserted stirring bar, under an inert
atmosphere (dinitrogen). The reaction mixture was transferred
R
Synthetic procedures
under Suba-Sealꢀ septa (purchased from Sigma Aldrich) avoiding
R
any contact with air while transferring from the Atmosbagꢀ to the
Three different techniques were used to synthesise alkyltri-
octylphosphonium chlorides. An autoclave was used for reac-
tions with volatile gaseous chloroalkanes such as chloromethane
and chloroethane. Syntheses of [P8 8 8 3]Cl and [P8 8 8 4]Cl, which
used chloroalkanes of mid-range volatility, were performed in
a microwave reactor and the remaining compounds requiring
non-volatile chloroalkanes were synthesised via thermal reflux.
Glassware used in the synthetic procedures was charged with
shield vessel. The septum was removed and discarded immediately
before the TMS screw cap was attached to the shield vessel.
They were then placed inside the microwave cavity. The reaction
conditions were: Ramp (5 min) to 180 ◦C; hold at 180 ◦C (30 min),
medium stirring, 500 W (50% max) power; pressure reached 3 bar.
After the reaction was completed and cooled down, the product
was transferred to a round-bottomed flask (25 cm3) and dried at
65 ◦C in vacuo for 48 h.
R
starting materials in an M size Atmosbagꢀ (purchased from
Sigma Aldrich) which was filled with dinitrogen, in order to
provide an inert atmosphere and avoid potential oxidation of the
trioctylphosphine to trioctylphosphine oxide. Details of individual
syntheses are given in Table 1, elemental analytical data are
reported in Table 2. In all cases products were characterised by
31P, 13C and 1H NMR spectroscopy (Tables 4 and 5; Fig. 2) and by
mass spectrometry (Tables 2 and 3) prior to purification.
Synthesis of [P8 8 8 n]Cl, n = 5–9, 10, 12 or 14. Trioctylphosphine
(1 mol eq.) and chloroalkane (1.1 mol eq.) were placed in a
round-bottomed flask (50 cm3). The mixture was stir-heated under
a reflux condenser connected to the Schlenk line flushed with
dinitrogen. The reaction mixture was heated in a silicone oil bath
◦
up to 145 C and maintained from 12 to 16 h depending on the
alkyl chain length of chloroalkane used. In order to remove the
12752 | Dalton Trans., 2011, 40, 12750–12764
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
Table 1 Masses of reactants (m), reaction conditions for synthesis, yield, appearance and water content of [P8 8 8 n]Cl ionic liquids
P8 8 8
CnH2n+1Cl
T/◦C
t/h
Yield/%
Colour
Stateb
H2O/ppm
a
n
V/cm3
m/g
V/cm3
m/g
r/g cm-3
1
2
3
4
5
6
7
8
72.73
72.73
12.12
12.12
12.12
12.12
12.12
12.12
14.55
12.12
12.12
12.12
60
60
10
10
10
10
10
10
12
10
10
10
32.80
32.70
2.61
3.10
3.59
4.10
4.54
5.04
6.66
6.04
7.00
8.05
30
30
0.915
0.918
0.892
0.886
0.882
0.879
0.881
0.875
0.870
0.868
0.867
0.859
140c
140c
180d
180d
145
145
145
145
145
145
145
145
2.5c
4.5c
0.5d
0.5d
12
12
14
14
16
92
94
96
97
96
94
95
96
95
95
92
89
White
s
s
l
l
l
l
l
l
l
l
l
l
—
—
Pale yellow
Colourless
Colourless
Colourless
Colourless
Colourless
Colourless
Colourless
Colourless
Colourless
Pale yellow
2.33
2.78
3.16
3.58
4.00
4.41
5.80
5.25
6.08
6.91
442
425
411
395
340
296
256
233
225
218
9
10
12
14
16
16
16
a Density data provided by supplier. b s = solid; l = liquid. c Synthesis performed in autoclave. d Synthesis performed in microwave.
+
Table 2 Elemental analysis of [P8 8 8 n]Cl and identification of cations by +ve FAB-MS, where peaks were assigned as [P8 8 8 n
]
C/%
calc.
H/%
calc.
Cl/%
calc.
P/%
calc.
m/z
n
exp.
exp.
exp.
exp.
calc.
exp.
1
2
3
4
5
6
7
8
71.30
71.76
72.20
72.60
72.99
73.35
73.69
74.01
74.32
74.61
75.14
75.63
71.36
71.70
72.06
72.76
72.17
73.35
73.26
73.63
73.65
74.82
74.89
75.13
12.92
12.97
13.02
13.06
13.09
13.13
13.17
13.20
13.23
13.26
13.31
13.36
12.72
12.12
13.28
13.09
13.38
12.84
13.21
13.60
13.62
13.65
13.02
13.23
8.42
8.15
7.89
7.65
7.43
7.22
7.02
6.83
6.65
6.48
6.16
5.87
8.48
8.44
7.43
8.08
7.89
7.90
7.61
7.13
6.61
6.52
5.95
6.43
7.36
7.35
6.90
6.69
6.49
6.30
6.13
5.96
5.81
5.66
5.38
5.13
7.44
7.74
7.23
6.07
6.56
5.91
5.92
5.64
5.42
5.01
6.14
5.21
385.4
399.4
413.4
427.4
441.4
455.4
469.5
483.5
—
385.5
399.4
413.4
427.2
441.2
455.2
469.3
483.3
—
9
10
12
14
511.5
539.5
—
511.3
539.3
—
Table 3 [P8 8 8 9]Cl and [P8 8 8 14]Cl were identified by electrospray ionisation spectrometry (ESI-MS)
-
+
{[P8 8 8 n]Cl2}
[P8 8 8 n
ES+
]
n
ES-
ES-
relative intensity %
ES+
exp
relative intensity %
100, 38, 5
theo
exp
theo
9
567.5, 568.5
569.5, 570.5
571.5, 572.5
—
567.5, 568.5
569.5, 570.5
571.6, 572.5
—
100, 25
55, 15
3, 2
497.5, 498.5, 499.5
567.6, 568.6, 569.6
497.5, 498.6, 499.5
567.6, 568.6, 569.6
14
—
100, 40, 5
excess of starting 1-chloroalkane (CnH2n+1Cl), where n ≥ 5, the
purification step involved dissolution of the product in hexane,
followed by extraction with water. All products were dried in vacuo
for 48 h.
Alkylphosphines are often compared to their analogous
amines,19 which are an integral part of the synthetic construction
units of ionic liquids, although there are few similarities between
them when it comes to chemical reactivity. Some of the underlying
differences are the excellent nucleophilicity of alkylphosphines20
towards electro-positive carbon due to their relatively low ion-
isation energies (favouring electron transfer); increased strength
of carbon-phosphorus bonds; and the larger size and higher
polarisability of the phosphorus atom leading to a lower repulsion
energy during the reaction, despite of the fact that the pKa
of trioctylphosphine (9.58) is lower than that of trioctylamine
(10.82).21 Nucleophilic attack of phosphorus at a haloalkane,
RX, depends on the reactivity of the haloalkane, which decreases
with increasing C–X bond strength in a sequence: RF < RCl
< RBr < RI. This explains why in this work all the syntheses
Results and discussion
Synthetic procedures
The tetralkylphosphonium chlorides studied here, were prepared
by nuclephilic (SN2) addition of a trialkylphosphine to linear
chloroalkanes varying from chloromethane to chlorotetradecane:
S
2
P
888 +CnH2n+1Cl ⎯⎯N⎯⎯→[P888n ]+ Cl−
no solvent
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 12750–12764 | 12753
©

View Article Online
◦
1
Table 4 For [P8 8 8 n]Cl, 1H NMR shifts (d/ppm, d6-propanone, 300 MHz, 21 C), with coupling constant (2JPH/Hz) and 31P{ H} NMR shifts (d/ppm,
d6-propanone, 120 MHz, 21 ◦C); peaks were assigned according to Fig. 2
Ha,a¢
Hb,b¢
Hg,g¢-y,y¢
Hw,w¢
31P
n
d/ppm
intensity d/ppm
intensity d/ppm
intensity
24H,6H
d/ppm
intensity d/ppm
1
2.44–2.54 (m),
2.05 (d, J = 14.1)
2.51–2.63 (m)
2.49–2.57 (m)
2.52–2.64 (m)
6H,3H(a¢) 1.50–1.63 (m) 6H
1.06–1.30 (m), 1.33–1.42 (m)
0.74–0.79 (m)
9H
33.46
2
3
4
8H
8H
8H
1.50–1.64 (m) 6H
1.52–1.63 (m) 8H
1.50–1.63 (m) 8H
1.12–1.30 (m), 1.32–1.44 (m)
0.95–0.98 (m), 1.32–1.44 (m)
0.82–0.86 (m), 1.14–1.29 (m), 3H,24H,8H 0.74–0.78 (m)
1.32–1.44 (m)
27H,6H
3H,24H
0.74–0.78 (m)
0.74–0.77 (m)
9H
9H
9H
36.24
34.12
34.47
5
6
7
8
9
10
12
14
2.45–2.55 (m)
2.53–2.63 (m)
2.50–2.60 (m)
2.52–2.62 (m)
2.49–2.59 (m)
2.64–2.74 (m)
2.65–2.75 (m)
2.46–2.56 (m)
8H
8H
8H
8H
8H
8H
8H
8H
1.43–1.56 (m) 8H
1.50–1.65 (m) 8H
1.51–1.63 (m) 8H
1.50–1.60 (m) 8H
1.50–1.61 (m) 8H
1.65–1.78 (m) 8H
1.65–1.77 (m) 8H
1.50–1.63 (m) 8H
1.05–1.35 (m)
1.07–1.42 (m)
1.10–1.41 (m)
1.10–1.42 (m)
1.11–1.41 (m)
1.25–1.55 (m)
1.25–1.45 (m),1.48–1.54 (m)
1.13–1.28 (m),1.32–1.41 (m)
34H
36H
38H
40H
42H
44H
40H,8H
46H,8H
0.66–0.74 (m)
0.74–0.84 (m)
0.74–0.78 (m)
0.74–0.78 (m)
0.74–0.80 (m)
0.88–0.92 (m)
0.88–0.92 (m)
0.73–0.78 (m)
12H
12H
12H
12H
12H
12H
12H
12H
34.47
34.44
34.45
34.46
34.49
34.43
34.43
34.42
◦
1
Table 5 For [P8 8 8 n]Cl, 13C { H} NMR shifts (d/ppm, d6-propanone, 75 MHz, 21 C), with coupling constants (1JPC/Hz), peaks were assigned according
to Fig. 2
Ca,a¢
Cb,b¢
Cg,g¢
Ce,e¢,q,q¢
Ck¢-c¢
Cj,j¢
Cy,y¢
Cw,w¢
n
1
2
3
4
d/ppm
d/ppm
d/ppm
30.30
30.15
30.07
30.10
d/ppm
d/ppm
d/ppm
33.04
33.04
33.00
33.01
d/ppm
22.68
d/ppm
14.90
5.10(d, JPC = 51.75 Hz),
21.27(d, JPC = 48.75 Hz)
13.71(d, JPC = 48.75 Hz),
19.63(d, JPC = 47.10 Hz)
19.92(d, JPC = 47.18 Hz),
21.92(d, JPC = 47.25 Hz)
19.75(d, JPC = 47.40 Hz),
19.96(d, JPC = 47.18 Hz)
19.97(d, JPC = 47.18 Hz)
19.99(d, JPC = 47.18 Hz)
19.99(d, JPC = 47.10 Hz)
19.99(d, JPC = 47.10 Hz)
19.96(d, JPC = 47.10 Hz)
19.96(d, JPC = 47.25 Hz)
19.98(d, JPC = 47.18 Hz)
20.01(d, JPC = 47.03 Hz)
23.79
32.10, 31.90
32.22, 32.02
32.16, 31.96
32.19, 31.99
23.88, 6.90
23.76, 16.68
24.82, 23.78
22.83
14.88
22.73
14.82, 16.37
14.85, 14.30
22.78, 25.17
5
6
23.78, 23.13
23.78, 23.59
23.80
23.80
23.78
23.77
23.78
23.83
30.11,34.01
30.13, 31.79
30.16
30.23
30.34
30.28
30.33
30.16
32.20, 31.99
32.21, 32.01
33.02
33.03
33.04
33.05
33.02
33.02
33.02
33.08
22.83, 22.48
22.87
22.84
22.85
22.80
22.80
22.81
22.88
14.85, 14.65
14.87
14.89
14.89
14.85
14.84
14.84
14.92
7
32.85, 32.00, 32.21
32.22, 32.12
8a
9
32.18, 31.98, 30.49q¢
32.18, 31.97, 30.72q¢,j¢
32.19, 31.99
10
12
14
30.84
30.35, 30.94
32.24, 32.04
a Due to the symmetry of the compound, all the peak assignments are equal.
presented were carried out under harsh reaction conditions (high
temperature, long reaction time). Taking into account the twelve
basic rules of green chemistry (specifically, rule number five, which
states: “The use of auxiliary substances (e.g. solvents, separation
agents, etc.) should be made unnecessary wherever possible and
innocuous when used”)22 and the impact of the solvent on reaction
time, the solvent was excluded from all synthetic procedures.
Reaction time depends also on the number of carbon atoms in
the chloroalkane; the longer the alkyl chain length, the longer the
reaction takes. Syntheses performed in the microwave reactor were
much more time and energy efficient compared to conventional
heating methods. This methodology can be applied to any organic
reaction, regardless of the volatility of starting compounds, and
reaches even higher efficiency using software for predicting and
optimising yields as developed by Strauss.23 The average yield for
all synthesised compounds was 94%, with purification being the
yield reducing step. The purification step was performed to remove
the excess of chloroalkane from the product and consisted in the
dissolution of the ionic liquid in hexane followed by washing with
water and further drying in vacuo for 48 h. Almost all products
were colourless, with just two exceptions ([P8 8 8 2]Cl and [P8 8 8 14]Cl),
which had a pale yellow colour even after purification. The whole
series is hydrophobic, giving two phases when mixed with water.
Nuclear magnetic resonance spectroscopy
1
31P NMR. The 31P{ H} NMR spectra of [P8 8 8 n]Cl (see Table
4) all exhibited a singlet at +34.40 0.33 ppm with the exceptions
of [P8 8 8 1]Cl and [P8 8 8 2]Cl, where the signals were at +33.46 ppm
and +36.24 ppm, respectively. Often the singlet is accompanied
by minor peaks, each of similar intensity, on either side as the
result of the presence of more than one active nucleus, in this case
13C nuclei (“spin–spin splitting”), see Fig. 3. The distance between
adjacent peaks around singlet is a measure of the strength of the
interaction and is denoted by a coupling constant, 1JPC, which was
62.42 0.46 Hz for all compounds studied here.
As the free trioctylphosphine signal is at -29.51 ppm, mon-
itoring of the reactions is very straightforward. For accurate
12754 | Dalton Trans., 2011, 40, 12750–12764
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
phosphonium salt and its isomers account for > 98% of the
phosphorus.
1H and 13C NMR. The cationic charge has a strong influence
on the electron density around the hydrogen atoms close to the
cationic centre, therefore their shifts can be observed at high
magnetic field. In comparison with the corresponding ammonium-
based ionic liquids, phosphonium ionic liquids are more stable.
The difference in shifts of protons closest to the cationic center is
ca. 1 ppm downfield compared to ammonium based ionic liquids.27
A major difference in chemical shifts of protons can be observed
with increasing number of carbon atoms in the fourth alkyl chain
on the cation. The protons of the methyl substituent (Fig. 5-A)
in [P8 8 8 1]Cl can be observed as doublet due to the strong
phosphorus–hydrogen interaction.
1
Fig. 3 31P{ H} NMR spectrum of [P8 8 8 8]Cl in d6-propanone.
interpretation of 31P NMR of phosphonium ionic liquids, it is
important to know the purity of starting phosphine (Fig. 4).
Fig. 4 31P{ H} NMR spectrum (d6-propanone, 120 MHz, 21 ◦C) with
1
Fig. 5 1H NMR spectra(d6-propanone, 120 MHz, 21 ◦C) of (A) [P8 8 8 8]Cl
and (B) [P8 8 8 1]Cl, peaks are assigned according to Fig. 2 (individual peaks
are assigned in Table 4), solvent is assigned as (*).
peak assignments of trioctylphosphine, and its corresponding phosphine
oxide.
Trioctylphosphine is industrially prepared (Cytec, Canada) by
free radical addition24 of PH3 to 1-octene. This process results in
the formation of two isomeric tertiary phosphine by-products.
The major isomeric impurity originates from a Markovnikov
addition25 to the secondary carbon of 1-octene. This isomer,
di(1-octyl)(2-octyl)phosphine, typically accounts for 2.5 to 3.0%
of the tertiary octylphosphines and has a clearly visible down-
field chemical shift as indicated in the 31P NMR spectra of
the commercially supplied trioctylphosphine, see Fig. 4. The
1-octene used above also has traces (<1%) of a branched
1-octene isomer. Addition to this olefin results in the formation
of another, but relatively minor tertiary phosphine which has a
chemical shift slightly up-field from that of the main component
(-34.63 ppm). The spectrum also reveals the presence of a small
amount of trioctylphosphine oxide (as the trialkylphosphines
are easily oxidised when exposed to air26). Trace amounts of
dioctyl octylphosphonate [O P(C8H17)(OC8H17)2], another air
oxidation product, can be detected as well. The percentage of
the trioctylphosphine isomers do not exceed more than 3% in the
starting material. The above tertiary alkylphosphine isomers are
also quaternised along with the tri(1-octyl)phosphine analogue.
The corresponding phosphonium salt derived from di(1-octyl)(2-
octyl)phosphine has a 31P NMR chemical shift slightly further
Fig. 6 13C{ H} NMR spectra (d6-propanone, 120 MHz, 21 ◦C) of
1
(A) [P8 8 8 8]Cl and (B) [P8 8 8 1]Cl, peaks are assigned according to Fig. 3
(individual peaks are assigned in Table 5), solvent is assigned as (*).
As the number of methylene groups increases, the shift of
the methyl group at the end of the fourth alkyl substitution
moves towards to lower magnetic field until reaches the multiplet
down-field from that of the main component (+38.60
0.22
ppm and +34.40 0.33 ppm, respectively). The main component
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 12750–12764 | 12755
©

View Article Online
belonging to methyl groups of three remaining octyl substitutions
on the cation, Fig. 5 (see Table 4). The sensitivity of the chemical
shift of the terminal methyl group as a function of chain length is
noted, see Table 5 and Fig. 6.
Differential scanning calorimetry measurements and
thermogravimetric analysis
The melting and glass transition temperatures were measured
by differential scanning calorimetry. The values of the melting
temperature (Tm) and glass transition (Tg) are shown in Table 6
for all compounds investigated. The trend of Tm or Tg as a function
of the number of carbon atoms in the fourth alkyl substitution on
the phosphonium cation is displayed in Fig. 7.
Fig. 8 DSC scans of (A) [P8 8 8 7]Cl and (B) [P8 8 8 3]Cl, scan speed 5 K min-1.
The glass transition temperatures are in the range of -82.8 ◦C
([P8 8 8 9]Cl) to -58.2 ◦C ([P8 8 8 4]Cl). Samples that were solids at
room temperature displayed distinct melting peaks, specifically
compounds with n = 1–3. This is probably due to the small size
of the cation which leads to closer cation anion contact which in
turn produces a higher lattice energy.28 It was already reported by
Tindale et al.29 that, as the alkyl chain length is increased on the
cation (Me < Bu < Oct) for the series of iodide salts, the melting
point decreases. Our findings agree with their results. The ionic
liquids with the longest chain (n = 12 or 14) are liquid at room
temperature, but exhibit observable melting points as well as glass
transition points.
Fig. 7 Melting (ꢀ) and glass transition ( ) temperatures, Tm and Tg,
substituent on the phosphonium cation.
᭺
respectively, as a function of the number of carbons, n, in the fourth alkyl
Table 6 Decomposition temperatures, Td for [P8 8 8 n]Cl were determined
by dynamic TGA. Tm and Tg obtained by DSC are reported along with
the enthalpies of fusion, DHfus, and enthalpies of glass transition, DHglass-l
n
Tm/◦C
DHfus/J g-1
Tg/◦C
DHglass-l/J g-1
Td/◦C
Unlike their ammonium counterparts, which tend to undergo
facile Hoffmann- or b-elimination in the presence of base,30
phosphonium salts decompose to yield a tertiary phosphine
oxide and an alkane under alkaline conditions.31 In contrast,
tetralkylphosphonium halides can be combined with concentrated
sodium hydroxide well above room temperature without any
degradation.32 While the decomposition point of neat phospho-
nium ionic liquid varies with the counter ion,7 the alkyltri-
octylphosphonium chlorides under investigation here, appear to
show dynamic onset of decomposition at around 320 20 ◦C
(data are reported in Table 6, and typical TGA scans are shown in
Fig. 9). Pyridinium-based salts,33 in contrast, decompose at about
100 ◦C higher temperature.
1
2
3
4
5
6
7
8
89.3
66.2
28.6
—
—
—
—
—
—
—
8.49
20.87
31.24
—
—
—
—
—
—
—
—
—
—
—
—
—
323
317
322
326
310
305
315
317
295
312
320
330
-58.2
-74.9
-74.6
-75.4
-72.5
-82.8
-77.0
-74.8
-64.5
0.65
0.78
0.97
0.36
1.12
2.82
0.24
0.09
0.05
9
10
12
14
-30.0
-20.0
9.96
13.33
The melting point is taken as the onset of an endothermic
peak on heating and the glass transition temperature is taken
as the midpoint of a small heat flow change on heating from the
amorphous glass state to a liquid state. Two examples of DSC
scans are presented in Fig. 8.
Most of the studied compounds simply form a glass at low
temperatures with no apparent freezing or melting transitions.
Density and molar volume
Density measurements were carried out over a broad range of
temperatures (293.15 < T/K < 363.15) at atmospheric pressure,
and data are reported along with the corresponding molar volume,
12756 | Dalton Trans., 2011, 40, 12750–12764
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
Fig. 9 Typical TGA curves for [P8 8 8 n]Cl, examples of [P8 8 8 1]Cl (black
line), [P8 8 8 4]Cl (green line), [P8 8 8 8]Cl (red line) and [P8 8 8 12]Cl (blue line).
Fig. 11 Experimental densities of [P8 8 8 n]Cl ionic liquids at 30 ◦C, where
n = 3, 5, 7 and 9 ( ), n = 4, 6, 8, 10, 12 and 14 ( ).
diagram illustrated in Fig. 7, which for n > 10 shows both Tg and
Tm transitions, indicating the onset of a different structural re´gime.
In the temperature range studied, the density decreases quasi-
linearly with increasing temperature as illustrated in Fig. 12.
Vm, for each studied ionic liquid in Table 7. The repeatability of
the density measurements was better than 5 ¥ 10-5 g cm-3.
The densities of the investigated compounds are in the range
of 0.84 to 0.90 g cm-3 over a temperature range of 293.15 to
363.15 K. It is commonly well known, that tetraalkylphosphonium
halides have densities below 1 g cm-3; the longer the alkyl chains on
the cation, the less dense the phosphonium ionic liquids become.
Obviously, the choice of the anion has a big impact on the density,34
chlorides showing a lower density in comparison to other anions
such as triflate, [OTf]-; bistriflamide, [NTf2]-; tetrafluoroborate,
[BF4]-; or hexafluorophosphate, [PF6]-; when paired with the same
cation.7 On average, the decrease in density is 4.4% for an increase
in temperature from 293.15 to 363.15 K.
The plot of experimental densities for [P8 8 8 n]Cl, as a function of
n, is shown in Fig. 10. The density decreases with increasing chain
length. The even-numbered compounds form a quasi-straight
line, while the odd-numbered compounds deviate from this line
showing another, independent quasi-linear behaviour. This leads
to an apparent alternation effect observed up to [P8 8 8 9]Cl (see red
line in Fig. 11). Fig. 11 also indicates that, when the density data
are partitioned into three sets (n = even up to 8, odd up to 9, and
n > 9), then each of these sets lie upon a straight line as a function
of n, all with very high regression coefficients (R2 > 0.995). The
discontinuity observable for n ≥ 10 corresponds well to the phase
Fig. 12 Experimental density as a function of temperature for the
[P8 8 8 n]Cl ionic liquids series where n = 3 ( ), 4 ( ), 5 ( ), 6 ( ), 7 ( ), 8
(
), 9 ( ), 10 ( ), 12 ( ) and 14 ( ). The lines correspond to the linear fit of
᭹
experimental results obtained in this work (0.99998 £ R2 £ 1).
It is also of interest to express the volumetric behaviour of
phosphonium ionic liquids in terms of their molar volume,
Vm = M/r, where an expected increase of Vm with increasing
temperature is observed, see Fig. 13. Vm as a function of n exhibits
a quasi-linear trend for each represented temperature, Fig. 14. A
detailed analysis of the residuals of the data from Fig. 14 for
3 £ n £ 9, see Fig. 15, clearly shows the see-saw, alternation,
odd/even effect, which had also been observed in the densities,
see Fig. 11. Examination of Fig. 10 and 11 suggests a non-linear
response of density (the empirical parameter) to chain length; the
deviations from linearity are small, but significant. In examining
the molar volumes (the thermodynamically significant parameter),
see Fig. 14, the regular deviation from linearity is much smaller and
therefore can only been revealed unambiguously on the residuals
plotted in Fig. 15.
Fig. 10 Experimental densities for [P8 8 8 n]Cl ionic liquids at 293 K (ꢀ),
303 K (ꢁ), 313 K (᭡), 323 K (ꢀ), 333 K ( ), 343 K ( ), 353 K (᭜) and
᭹
᭺
363 K (᭛) as a function n. Experimental data for compounds, where n = 1
or 2 are not included as they are solids with melting points above 65 ◦C.
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 12750–12764 | 12757
©

View Article Online
Table 7 Experimental density (r/g cm-3) and molar volume (Vm/cm3 mol-1) data for [P8 8 8 n]Cl ionic liquids measured at temperatures T = 293.15 K to
T = 363.15 K. Calculated density values were obtained from the extended Ye and Shreeve equation35 proposed by Gardas and Coutinho.36 DVm values
represent the difference (calc - exp) between Vm values and molar volume data calculated using the fitting eqn (6)
b
b
rexp
/
rcalc
/
(Vm)exp
/
(Vm)calc
/
DVm
/
rexp
/
rcalc
/
(Vm)exp
/
(Vm)calc
/
DVm /
T/K
g cm-3 g cm-3 Deva/% cm3 mol-1 cm3 mol-1 cm3 mol-1 T/K
g cm-3 g cm-3 Deva/% cm3 mol-1 cm3 mol-1 cm3 mol-1
[P8 8 8 3]Cl
[P8 8 8 4]Cl
293.15
—
—
—
—
—
—
293.15 0.8998 0.8977 -0.24
303.15 0.8940 0.8917 -0.25
313.15 0.8882 0.8858 -0.26
323.15 0.8824 0.8800 -0.27
333.15 0.8766 0.8743 -0.26
343.15 0.8707 0.8686 -0.24
353.15 0.8649 0.8631 -0.22
363.15 0.8591 0.8576 -0.18
[P8 8 8 6]Cl
514.78
518.15
521.54
524.95
528.44
531.97
535.55
539.16
513.47
517.22
520.97
524.72
528.47
532.22
535.96
539.71
-1.31
-0.93
-0.57
-0.23
0.03
0.25
0.41
303.15 0.8969 0.8938 -0.35
313.15 0.8912 0.8879 -0.37
323.15 0.8855 0.8821 -0.39
333.15 0.8798 0.8763 -0.39
343.15 0.8740 0.8707 -0.38
353.15 0.8683 0.8651 -0.37
363.15 0.8625 0.8596 -0.34
[P8 8 8 5]Cl
500.83
504.01
507.25
510.57
513.94
517.35
520.79
499.98
503.65
507.32
510.99
514.65
518.32
521.99
-0.85
-0.36
0.07
0.42
0.71
0.97
1.20
0.55
293.15 0.8988 0.8957 -0.34
303.15 0.8929 0.8898 -0.35
313.15 0.8870 0.8839 -0.35
323.15 0.8812 0.8781 -0.35
333.15 0.8753 0.8724 -0.34
343.15 0.8695 0.8668 -0.31
353.15 0.8636 0.8612 -0.28
363.15 0.8578 0.8557 -0.24
[P8 8 8 7]Cl
293.15 0.8938 0.8922 -0.18
303.15 0.8879 0.8862 -0.19
313.15 0.8820 0.8804 -0.19
323.15 0.8762 0.8746 -0.18
333.15 0.8704 0.8689 -0.17
343.15 0.8645 0.8633 -0.14
353.15 0.8587 0.8578 -0.11
363.15 0.8529 0.8523 -0.06
[P8 8 8 9]Cl
530.98
534.48
538.02
541.57
545.20
548.88
552.61
556.37
530.63
534.46
538.29
542.12
545.95
549.78
553.61
557.44
-0.35
-0.02
0.27
0.55
0.75
0.90
1.00
1.07
293.15 0.8951 0.8939 -0.13
303.15 0.8892 0.8880 -0.14
313.15 0.8833 0.8821 -0.14
323.15 0.8775 0.8763 -0.14
333.15 0.8717 0.8706 -0.12
343.15 0.8658 0.8650 -0.10
353.15 0.8600 0.8594 -0.06
363.15 0.8542 0.8540 -0.03
[P8 8 8 8]Cl
293.15 0.8897 0.8905 0.09
303.15 0.8837 0.8846 0.10
313.15 0.8778 0.8788 0.12
323.15 0.8719 0.8730 0.13
333.15 0.8659 0.8674 0.17
343.15 0.8600 0.8617 0.20
353.15 0.8541 0.8562 0.25
363.15 0.8482 0.8507 0.30
[P8 8 8 10]Cl
548.87
552.49
556.15
559.82
563.58
567.39
571.25
575.14
547.78
551.69
555.60
559.52
563.43
567.34
571.25
575.16
-1.09
-0.80
-0.55
-0.30
-0.15
-0.05
-0.00
0.02
565.35
569.09
572.87
576.66
580.54
584.47
588.45
592.48
564.94
568.93
572.92
576.92
580.91
584.90
588.89
592.89
-0.41
-0.16
0.05
0.26
0.37
0.43
0.44
0.41
583.70
587.65
591.64
595.64
599.74
603.87
608.06
612.27
582.09
586.17
590.24
594.31
598.39
602.46
606.54
610.61
-1.61
-1.48
-1.40
-1.33
-1.35
-1.41
-1.52
-1.66
293.15 0.8897 0.8890 -0.08
303.15 0.8837 0.8831 -0.07
313.15 0.8778 0.8773 -0.06
323.15 0.8718 0.8715 -0.03
333.15 0.8659 0.8658 -0.01
343.15 0.8601 0.8602 -0.02
353.15 0.8542 0.8547 -0.06
363.15 0.8483 0.8493 -0.11
[P8 8 8 12]Cl
293.15 0.8866 0.8849 -0.20
303.15 0.8807 0.8790 -0.20
313.15 0.8749 0.8732 -0.19
323.15 0.8690 0.8675 -0.17
333.15 0.8631 0.8618 -0.15
343.15 0.8573 0.8563 -0.12
353.15 0.8515 0.8508 -0.09
363.15 0.8457 0.8453 -0.04
599.45
603.50
607.60
611.76
615.92
620.11
624.36
628.67
599.25
603.40
607.56
611.71
615.87
620.03
624.18
628.34
-0.20
-0.10
-0.04
-0.05
-0.05
-0.08
-0.18
-0.33
293.15 0.8902 0.8876 -0.30
303.15 0.8843 0.8817 -0.30
313.15 0.8784 0.8759 -0.29
323.15 0.8725 0.8701 -0.27
333.15 0.8666 0.8645 -0.25
343.15 0.8608 0.8589 -0.23
353.15 0.8550 0.8534 -0.20
363.15 0.8492 0.8479 -0.16
[P8 8 8 14]Cl
293.15 0.8814 0.8825 0.12
303.15 0.8754 0.8766 0.13
313.15 0.8694 0.8708 0.16
323.15 0.8634 0.8651 0.20
333.15 0.8575 0.8595 0.23
343.15 0.8516 0.8539 0.27
353.15 0.8457 0.8484 0.33
363.15 0.8398 0.8430 0.39
614.90
619.01
623.16
627.38
631.61
635.87
640.19
644.57
616.40
620.64
624.88
629.11
633.35
637.59
641.83
646.06
1.50
1.63
1.72
1.73
1.74
1.72
1.64
1.49
649.01
653.36
657.72
662.20
666.67
671.19
675.77
680.42
650.71
655.21
659.51
663.91
668.31
672.71
677.11
681.51
1.70
1.85
1.79
1.71
1.64
1.52
1.34
1.09
684.67
689.35
694.13
698.93
703.75
708.65
713.61
718. 63
685.02
689.58
694.15
698.71
703.27
707.84
712.40
716.96
0.35
0.23
0.02
-0.22
-0.48
-0.81
-1.21
-1.67
a Dev (%) = [(rcalc - rexp)/rexp] ¥ 100. b DVm = (Vm)calc - (Vm)exp
.
Experimental density data were fitted into the extended Ye and
Shreeve equation35 proposed by Gardas and Couthino,36 which
is applicable over a wide range of temperatures 273–393 K, and
pressures, 0.10–100 MPa, according to eqn (1):
The densities of alkyltrioctylphosphonium ionic liquids were
estimated according to eqn (1) with the molecular volumes of
ions and groups taken from previously reported values36,39,40 where
available. Since a range of values for the molecular volume of the
chloride ion have ben published previously, in this work the value
was estimated by minimising the objective function (OF), given in
eqn (2):
rIL (T ,P) = M NAV mol (a +bT +cP)
(1)
where M is the formula mass in g mol-1, NA is the Avogadro’s
number, Vmol is the molecular volume in units of 1 ¥ 10-27 cm3,
T is the temperature in K, and P is the pressure in MPa. The
values of the coefficients a, b and c were estimated by Gardas and
Coutinho36 as 0.80050 0.00023, (6.6520 0.0069) ¥ 10-4 K-1, and
(-5.919 0.024) ¥ 10-6 MPa-1, respectively, by fitting eqn (1) to the
previously published experimental pressure–volume–temperature
(PVT) data of ILs.37,38
N
p
2
(2)
OF=
(rcalc − rexp
)
N p
∑
i=1
where rcalc is calculated density using eqn (1), rexp is the experimen-
tal density, and Np represents the number of data points, in this
case Np = 79. The calculated density rcalc of the studied ionic liquids
12758 | Dalton Trans., 2011, 40, 12750–12764
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
Fig. 15 Illustration of the see-saw, even/odd alternation effect in the form
of deviations (residuals) between experimental and fitted data as a function
of n, at four different temperatures.
Fig. 13 Molar volumes as a function of temperature for the [P8 8 8 n]Cl
ionic liquids series where n = 3 ( ), 4 ( ), 5 ( ), 6 ( ), 7 ( ), 8 ( ), 9 ( ), 10
᭹
( ), 12 ( ) and 14 ( ). The lines correspond to the linear fit of experimental
results obtained in this work (0.99998 £ R2 £ 1).
Fig. 16 Linear relationship between experimental and calculated density
of herein studied ionic liquids at temperatures T = 293.15 K to T =
363.15 K, using the extended Ye and Shreeve equation proposed by Gardas
and Coutinho.
N
p
r
calc −rexp
rexp
(3)
AAD (%)=
N p
∑
i=1
Rebelo and coworkers41 used a simplified model to predict the
molar volumes of three phosphonium ionic liquids, which share
the same cation ([P6 6 6 14]+) and different anions.
Fig. 14 Experimental molar volume of [P8 8 8 n]Cl as a function of n for
four temperatures. The isotherms are 293 K (ꢀ), 313 K (᭡), 333 K ( ) and
᭹
An “ideal” volumetric behaviour33,42–45 was previously observed
for 1-alkyl-3-methylimidazolium-based ionic liquids, where a high
degree of linearity was noted in the plots of molar volume (Vm) vs.
the number of carbon atoms in the alkyl chain (n). The variation
of the molar volume per addition of two carbon atoms in the
∂V ∂(2n)
353 K (᭜). The lines represent a linear fit of the results for each isotherm.
displays a good agreement with the corresponding experimental
density rexp, where (see Fig. 16):
cation of these ionic liquids, (
) = (34.4 0.5) cm3
m
mol-1 was obtained, irrespective of the anion and the length
of the chain. Equally important, this rule-of-thumb is observed
irrespective of whether the chains are incorporated in either the
cation or the anion. A striking case is that of the 1-alkyl-3-
methylimidazolium alkylsulfonates,15 where the observed linearity
as n increases enabled the establishment of effective volumes
(“molar size”) occupied by all the other anions and cations. The
molar volume (Vm) of an ionic liquid was defined simply as the
rcalc = (0.9988 0.0002)rexp
(R2 = 0.9849 at a 95% level of confidence)
For the density prediction from molar volume, using eqn (1),
an average absolute deviation (AAD) of 0.2% with the maximum
deviation (MD) 0.39% was observed for 79 density data points
studied of 10 ionic liquids, where AAD is defined as, eqn (3):
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 12750–12764 | 12759
©

View Article Online
Table 8 Group contribution parameters associated with eqn (5) used to
predict the effective molar volume of the phosphonium ionic liquids as a
function of temperature from 293.15 to 363.15 K
sum of the effective molar volumes (V*) of the anion (a) and cation
(c), eqn (4):
*
*
(4)
Vm = Vc + Va
Gi
ai/cm3 mol-1
bi/cm3 mol-1 K-1
This model,41 the first totally predictive in nature once an
anchor anion (or cation) is chosen, was developed based on
density data obtained at 298.15 K. Following the logic of Rebelo
and coworkers,41 any cations with the same empirical formulae
should have the same molar volumes, irrespective of how the alkyl
chain lengths are distributed around central phosphorus. Thus,
the molar volume of [P8 8 8 n]Cl should equal that of [P6 6 6 (n + 6)]Cl at
any temperature, and hence the extant calculated molar volume
of [P6 6 6 14]Cl (582.5 cm3 mol-1 at 298.15 K)41 should serve as
a fair estimate for the molar volume of [P8 8 8 8]Cl. In fact, the
experimental value obtained in this work deviates by only 0.5%
from this value. If this model is applied to the data reported here,
although reasonable fits may be obtained, the precision of these
data is greater than the difference between estimated and observed
values and hence, the differences are statistically significant; this
phenomenon will be discussed in detail elsewhere, alongside data
for related series of ionic liquids.46
+
[P1 1 1 1
]
73.776
16.992
24.391
0.0840
0.0082
0.0954
–CH2–
Cl-
Table 9 AAD and MD observed for the molar volumes of studied
phosphonium ionic liquids predicted using eqn (6)
n
AAD/%
MD/%
3
4
5
6
7
8
9
10
12
14
0.13
0.10
0.13
0.07
0.05
0.25
0.02
0.26
0.24
0.09
0.23
0.25
0.20
0.20
0.07
0.28
0.06
0.28
0.27
0.23
For the first time, we would like to introduce a group contri-
bution method for the prediction of molar volume as a function
of temperature. The phosphonium ionic liquids presented in this
study were divided into three groups, see Fig. 17, and each group
is represented as a function of temperature according to the
following eqn (5):
3
(8)
B =
nibi
∑
i=1
where ni is the number of groups of type i, and the parameters ai
and bi here estimated are presented in Table 8.
Calculated molar volumes are in an excellent agreement with
the corresponding experimental values (Vm)exp, where:
Vm(Gi) = A + B(T - 273.15)
(5)
(Vm)cal = (1.0003 0.0002) ¥ (Vm)exp
(R2 = 0.9997 at 95% level of confidence)
The relative deviations between the calculated and experimental
data as a function of experimental molar volume for all data points
used in the current study are shown in Fig. 18 and 19, similar
behaviour to that seen in Fig. 15. For 79 data points of twelve
phosphonium ionic liquids, the overall AAD is 0.13% and MD is
0.28% (Table 9).
Fig. 17 Structure of [P8 8 8 8]Cl divided into groups, distinguished by
colours (red, blue and green), representing molar volumes of each moiety,
Gi (i = 1, 2, or 3).
Vm relates to the structure, Fig. 17, and is considered to be
composed of contributions from [P1 1 1 1]+ (green), –(CH2)n– (blue)
and Cl- (red), hence the overall molar volume is represented by
eqn (6):
3
(6)
Vm =
Vm (Gi )
∑
i=1
The parameters A and B can be obtained by a group contribu-
tion method according to eqn (7) and eqn (8):
Fig. 18 The relative deviations between calculated, using eqn (5) and the
experimental molar volumes as a function of experimental molar volumes
for the phosphonium ionic liquids, where n = 3, 5, 7 and 9 ( ) and n = 4, 6,
8, 10, 12 and 14 ( ).
3
(7)
A =
niai
∑
i=1
12760 | Dalton Trans., 2011, 40, 12750–12764
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
study, a similar behaviour was found: within our studied [P8 8 8 n]Cl
family, the calculated values of a lie between 6.52 ¥ 10-4 K-1 and
6.91 ¥ 10-4 K-1, for [P8 8 8 3]Cl and [P8 8 8 14]Cl, respectively.
This is significantly lower than the ap values of common,
molecular organic liquids – and it is not at all surprising since
a typical ionic liquid at these working temperatures is at a
considerably lower hypothetical reduced temperature47,17 than any
common low-molecular weight organic liquid. Values of ap of the
same order of magnitude (5–6 ¥ 10-4 K-1) were reported by Gu
and Brennecke48 for several ionic liquids containing [BF4]- and
[PF6]- anions and imidazolium and pyridinium-based cations. The
only two literature references dealing with the thermal expansion
coefficient of phosphonium ILs, are those of (i) Esperanc¸a et al.41
who studied [P6 6 6 14]+ in combination with three different anions
([NTf2]-/[O2CMe]-/Cl-) obtaining 6.70 ¥ 10-4 K-1, 6.19 ¥ 10-4
K-1, and 6.04 ¥ 10-4 K-1, respectively, while (ii) Tariq et al.34 with
[NTf2]-/[OTf]-/[O2CMe]- obtained 6.91 ¥ 10-4 K-1, 6.69 ¥ 10-4
K-1, and 6.83 ¥ 10-4 K-1, respectively.
Fig. 19 Illustration of the see-saw, even/odd alternation effect in the
form of deviations (residuals) between experimental and predicted data
according to eqn (6) as a function of n, at four different temperatures.
As shown in the density section, the variation between odd and
even number of carbons in the fourth alkyl substitution (n) on the
cation causes non-linear behaviour. It is satisfying to observe the
internal consistency between the data plotted in Fig. 15 and that
presented in Fig. 19, given the different assumptions made in the
data treatment.
Viscosity
Factors that affect viscosity of ionic liquids are still not fully
understood, but the chemical structure of the anion is known
to have a strong influence. Specifically halides have high viscosity
as they likely take part in hydrogen bonding.49 The viscosity can
be decreased by addition of water or organic solvent.42
Thermal expansion coefficient
Viscosities were measured at temperatures between 293.15 to
363.15 K for ten phosphonium ionic liquids out of the twelve
presented here (see Table 11); the limitation is their liquid range.
The results indicate that the viscosity of the studied phosphonium
ionic liquids decreases markedly with temperature increase, for
example, on increasing temperature by 10 K from (293.15 to
303.15) K one observes a decrease in viscosity for [P8 8 8 8]Cl of 51%.
The decrease in viscosity observed was up to 98% by increasing
the temperature from 293.15 to 363.15 K. Viscosity as a function
of the fourth alkyl chain is illustrated in Fig. 20. To the best of our
knowledge, this is the first time that it is observed that, on average,
the viscosity decreases (although only modestly) with increasing
alkyl chain length.
The evolution of the volumetric properties with temperature can
be expressed by calculating the coefficient of thermal expansion,
ap, defined as, eqn (9):
ap =−[∂ln(r) ∂T ]p = −a
(9)
Plots of ln(r) as a function of temperature, eqn (10), (fitting
equation parameters can be found in Table 10) for the studied
ionic liquids form quasi-linear lines and they are also quite parallel
to each other, therefore, the thermal expansion coefficient, ap, is
almost constant.
ln[ r cm−3 ]=a(T mK)+b
(10)
(
)
In very high-accuracy measurements over an enlarged temper-
ature range, Tariq et al.17 demonstrated that on the homologous
series of [Cnmim][NTf2], ap may slightly decrease with increasing
temperature in the low-T region, to start increasing (also mildly)
in the high-T domain. Additionally, there is a tendency for
an increase of ap with the alkyl chain length. In our present
Table 10 Parameters of the linear fit, according to eqn (10)
n
ap = -a/K-1¥10-3
b
3
4
5
6
7
8
9
10
12
14
0.65213
0.66716
0.66926
0.67984
0.67505
0.66079
0.66787
0.68262
0.67314
0.69145
0.089049
0.089011
0.084025
0.082484
0.077658
0.088278
0.085060
0.083352
0.081092
0.076565
Fig. 20 Experimental viscosities of [P8 8 8 n]Cl ionic liquids at 293.15 K
(ꢀ), 303.15 K (ꢁ), 313.15 K (᭡), 323.15 K (ꢀ), 333.15 K ( ), 343.15 K
᭹
(
), 353.15 K (᭜) and 363.15 K (᭛) as a function of the number of carbon
᭺
atoms in the alkyl chain of the fourth alkyl substitution on the cation.
Having established this behaviour which shows both an alterna-
tion effect and unexpected chain length dependence, the reasons
for this behaviour will be discussed elsewhere.46
= (6.72 0.11) ¥ 10-4 K-1
ap
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 12750–12764 | 12761
©

View Article Online
Table 11 Experimental viscosities (hexp) of phosphonium chlorides as a function of temperature at atmospheric pressure. Deva represents standard
deviation of ten measurements performed with each sample at each given temperature. Calculated viscosities (hcal) were obtained from the VFT equation
and AAD represents agreement between calculated and experimental data
T/K
hexp/mPa s
Dev/%
hcal/mPa s
AAD/%
T/K
hexp/mPa s
Dev/%
hcal/mPa s
AAD/%
[P8 8 8 3]Cl
293.15
303.15
313.15
323.15
333.15
343.15
353.15
363.15
[P8 8 8 5]Cl
293.15
303.15
313.15
323.15
333.15
343.15
353.15
363.15
[P8 8 8 7]Cl
293.15
303.15
313.15
323.15
333.15
343.15
353.15
363.15
[P8 8 8 9]Cl
293.15
303.15
313.15
323.15
333.15
343.15
353.15
363.15
[P8 8 8 12]Cl
293.15
303.15
313.15
323.15
333.15
343.15
353.15
363.15
[P8 8 8 4]Cl
293.15
303.15
313.15
323.15
333.15
343.15
353.15
363.15
[P8 8 8 6]Cl
293.15
303.15
313.15
323.15
333.15
343.15
353.15
363.15
[P8 8 8 8]Cl
293.15
303.15
313.15
323.15
333.15
343.15
353.15
363.15
[P8 8 8 10]Cl
293.15
303.15
313.15
323.15
333.15
343.15
353.15
363.15
[P8 8 8 14]Cl
293.15
303.15
313.15
323.15
333.15
343.15
353.15
363.15
—
—
—
—
3732.66
1708.15
839.82
449.98
258.62
155.35
95.26
1.54
2.58
2.73
4.38
4.75
3.23
2.63
2.19
3732.66
1695.46
841.51
449.98
256.32
154.12
97.10
0.00
0.74
0.20
0.00
0.89
0.79
1.93
4.67
1682.00
812.45
436.34
245.28
148.67
91.19
62.59
1.92
2.55
2.51
2.08
1.93
1.27
1.03
1667.50
827.66
442.57
252.10
151.59
95.50
62.65
0.86
1.87
1.43
2.78
1.96
2.48
0.10
66.82
63.70
2964.65
1245.15
641.65
385.12
206.69
116.95
80.32
0.93
1.70
2.55
4.39
2.08
2.18
3.70
0.86
2705.17
1272.95
650.80
356.83
207.66
127.18
81.42
8.75
2.23
1.43
7.35
0.47
8.75
1.37
2.56
3190.71
1509.85
768.46
422.01
247.54
143.41
98.77
1.98
2.94
2.74
3.11
1.73
2.73
2.04
1.56
3190.69
1506.48
772.36
424.50
247.54
151.88
97.37
0.00
0.22
0.51
0.59
0.00
5.91
1.41
0.46
55.59
54.16
65.17
64.87
2455.34
1156.30
616.10
334.51
199.66
123.27
79.28
2.77
2.98
4.98
5.21
1.86
3.25
3.31
1.03
2455.35
1176.12
610.35
338.92
199.39
123.27
79.55
0.00
1.71
0.93
1.32
0.13
0.00
0.35
1.63
2488.28
1202.16
629.05
348.42
204.88
122.52
83.21
1.80
2.33
3.37
2.76
3.40
2.94
4.22
2.88
2488.28
1200.96
627.22
350.18
206.97
128.47
83.20
0.00
0.10
0.29
0.50
1.02
4.85
0.02
2.02
54.18
53.30
57.07
55.92
2286.52
1102.62
565.36
309.30
187.86
115.82
76.35
1.62
2.13
1.72
1.94
0.61
2.89
3.80
6.48
2286.52
1104.14
576.65
321.95
190.28
118.11
76.49
0.00
0.14
2.00
4.09
1.29
1.98
0.19
0.07
2775.56
1322.93
694.44
361.63
219.74
138.84
90.43
4.65
2.31
5.33
1.47
1.66
4.16
0.82
0.87
2759.52
1322.93
687.02
381.73
224.68
138.97
89.72
0.58
0.00
1.07
5.56
2.25
0.09
0.78
2.69
51.37
51.41
61.80
60.13
2539.44
1233.02
644.20
377.48
215.65
129.29
89.57
1.64
3.74
2.21
3.82
2.22
2.89
1.93
1.26
2516.87
1233.02
652.12
368.00
219.51
137.33
89.56
0.89
0.00
1.23
2.51
1.79
6.22
0.01
0.48
1411.34
719.37
398.69
227.43
14.,53
93.98
1.59
2.09
3.67
1.88
3.11
2.82
2.58
2.50
1468.29
719.37
388.08
226.35
140.77
92.34
4.03
0.00
2.66
0.47
0.17
1.75
1.52
1.90
64.32
44.30
63.35
45.14
60.85
60.55
The Vogel–Fulcher–Tammann equation has been successfully
used to model the temperature dependence of ionic liquid viscosity
for many years.2,50 A viscosity–temperature correlation based on
the (VFT) equation, eqn (11) is proposed:
T0 simultaneously by minimising the following objective function
(OF), eqn (12):
N
2
p
⎡
⎢
⎢
⎣
⎤
⎥
⎥
⎦
B
OF=
A +
− ln(rexp
)
N p
(12)
∑
T −T0
i=1
⎛
⎜
⎜
⎞
⎟
⎟
⎟
⎟
⎠
B
h=A +exp
(11)
⎜
⎜
⎝
T −T
0
The parameters A, B and T0 of eqn (11) are given in Table
12 together with AAD and MD for the viscosity data of all
studied compounds. The values of the parameter T0 are between
126 to 165 K. Gardas and Coutinho,51 obtained 165.06 K as T0
for approximately 500 viscosity data points of 25 imidazolium,
pyridinium and pyrrolidinium based ionic liquids.
where h is the viscosity in mPa s units, T is temperature (K), A,
B and T0 are adjustable parameters. Experimental viscosity as a
function of temperature for the three chosen ionic liquids fitted
into the VFT equation is presented in Fig. 21.
The ratio of parameters B and T0, B/T0, is also known as
the Angell strength parameter. Experimental viscosity data of
the ionic liquids were used to optimise the parameters A, B and
As shown in Fig. 22, good agreement is observed between cal-
culated and experimental viscosity data using the VFT equation,
with all three adjustable parameters (A, B, T0). The calculated
12762 | Dalton Trans., 2011, 40, 12750–12764
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
Fig. 22 Linear relationship between hexp and hcal using the VFT equation,
eqn (11).
Fig. 21 Experimental viscosities as a function of temperature: [P8 8 8 4]Cl
(᭜), [P8 8 8 8]Cl ( ) and [P8 8 8 14]Cl (᭡), fitted into the VFT equation, eqn
(11). Fits are represented by solid lines.
᭹
taken together, significantly expand the range of phosphonium
ionic liquids available for potential industrial application, and are
significantly more stable than their ammonium analogues.
Table 12 Correlation parameters of VFT equation with the deviations of
the fit for the viscosity of [P8 8 8 n]Cl as a function of temperature, determined
from experimental values in the range T = 293.15 to T = 363.15 K
Acknowledgements
n
A
B/K
T0/K
B/T0
AAD/%
MD/%
This work was funded by Cytec Canada, Inc. G.A. would like
to thank Dr Douglas Harris (Cytec) for fruitful comments and
advice at the beginning of this work, Dr Marijana Blesic and
Dr John Holbrey for countless discussions; Prof. Chris Strauss,
Dr Markus Fanselow and Dr Giulia Fiorani for microvawe
assistance and helpful guidance. L.P.N.R. thanks Fundac¸a˜o para
a Cieˆncia e Tecnologia for support under grants PTDC/QUI-
QUI/101794/2008 and PTDC/QUI/71331/2006.
3
4
5
6
7
8
9
10
12
14
-11.95
-11.94
-11.96
-11.76
-11.89
-11.81
-11.90
-11.77
-11.67
-9.47
2095.59
2095.59
2095.38
2095.36
2095.29
2095.25
2095.25
2095.27
2095.17
1263.14
135.12
135.12
131.35
130.98
129.35
128.48
128.48
129.26
126.72
165.00
15.51
15.51
15.95
16.00
16.20
16.31
16.31
16.21
16.53
7.66
2.3
1.2
4.1
1.1
0.8
1.1
1.2
1.6
1.6
1.6
3.7
4.7
8.8
5.9
1.7
4.9
4.1
5.6
6.2
4.0
Notes and references
viscosity (hcal) of studied ILs displays a good agreement with the
corresponding experimental viscosity (hexp), Fig. 22, where:
1 K. J. Fraser and D. R. MacFarlane, Aust. J. Chem., 2009, 62, 309–321.
2 K. R. Seddon, A. Stark and M. J. Torres, in ‘Clean Solvents - Alternative
Media for Chemical Reactions and Processing’ ed. M. A. Abraham
and L. Moens, ACS Symposium Series, Vol. 819, Am. Chem. Soc.,
Washington, 2002, pp. 34–49.
3 A. K. Ziyada, C. D. Wilfred, M. A. Bustam, Z. Man and T. Murugesan,
J. Chem. Eng. Data, 2010, 55, 3886–3890.
4 S. Seki, T. Kobayashi, Y. Kobayashi, K. Takei, H. Miyashiro, K.
Hayamizu, S. Tsuzuki, T. Mitsugi and Y. Umebayashi, J. Mol. Liq.,
2010, 152, 9–13.
5 G. Adamova´, R. L. Gardas, L. P. N. Rebelo, A. J. Robertson and K. R.
Seddon, J. Chem. Soc., Dalton Trans. manuscript in preparation.
6 G. Adamova´, R. L. Gardas, M. Nieuwenhuyzen, A. V. Puga, L. P. N.
Rebelo, A. J. Robertson and K. R. Seddon, J. Chem. Soc., Dalton Trans.
submitted.
ln(hcal) = (0.9929 0.0035)ln(hexp
)
(R2 = 0.9992 at a 95% of confidence)
Relative deviations between the calculated and the experimental
viscosity data are presented in Table 11 along with experimental
viscosity values at wide range of temperatures (293.15–363.15 K).
Conclusions
7 C. J. Bradaric, A. Downard, C. Kennedy, A. J. Robertson and Y. H.
Zhou, Green Chem., 2003, 5, 143–152.
A basic synthetic methodology for a series of related phosphonium
ionic liquids is presented here, along with a detailed study of their
1H, 13C and 31P NMR, mass spectra, thermal stability, density and
viscosity over a wide range of temperatures, melting points/glass
transition temperatures and phase diagrams. For the first time
there is evidence for an alternation effect within a series of ionic
liquids, which reveals itself in both the density and viscosity
data. This is the first series of tetraalkylphosphonium chlorides
fully studied and characterised; another two related papers on
[P6 6 6 n]Cl and [P4 4 4 n]Cl will be submitted shortly. These papers,
8 Anonymous, ‘http://www.cytec.com/specialty-chemicals/phosphine.
htm’.
9 R. E. Del Sesto, C. Corley, A. Robertson and J. S. Wilkes, J. Organomet.
Chem., 2005, 690, 2536–2542.
10 N. V. Plechkova and K. R. Seddon, Chem. Soc. Rev., 2008, 37, 123–150.
11 M. Freemantle, ‘An Introduction to Ionic Liquids’, RSC Publishing,
Cambridge, 2010.
12 G. Wittig, Justus Liebigs Ann. Chem., 1953, 580, 44–57.
13 Anonymous, ‘Aldrich Handbook of Fine Chemicals’, Sigma–Aldrich
Chemical Company Ltd.
14 K. Shimizu, M. F. C. Gomes, A. A. H. Padua, L. P. N. Rebelo and J.
N. C. Lopes, THEOCHEM, 2010, 946, 70–76.
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 12750–12764 | 12763
©

View Article Online
15 M. Blesic, M. Swadz´ba-Kwas´ny, T. Belhocine, H. Q. N. Gunaratne, J.
N. C. Lopes, M. F. C. Gomes, A. A. H. Padua, K. R. Seddon and L. P.
N. Rebelo, Phys. Chem. Chem. Phys., 2009, 11, 8939–8948.
16 M. Tariq, P. J. Carvalho, J. A. P. Coutinho, I. M. Marrucho, J. N. C.
Lopes and L. P. N. Rebelo, Fluid Phase Equilib., 2011, 301, 22–32.
17 M. Tariq, A. P. Serro, J. L. Mata, B. Saramago, J. M. S. S. Esperanc¸a,
J. N. C. Lopes and L. P. N. Rebelo, Fluid Phase Equilib., 2010, 294,
131–138.
36 R. L. Gardas and J. A. P. Coutinho, Fluid Phase Equilib., 2008, 263,
26–32.
37 R. L. Gardas, M. G. Freire, P. J. Carvalho, I. M. Marrucho, I. M. A.
Fonseca, A. G. M. Ferreira and J. A. P. Coutinho, J. Chem. Eng. Data,
2007, 52, 80–88.
38 R. L. Gardas, M. G. Freire, P. J. Carvalho, I. M. Marrucho, I. M. A.
Fonseca, A. G. M. Ferreira and J. A. P. Coutinho, J. Chem. Eng. Data,
2007, 52, 1881–1888.
39 H. D. B. Jenkins, H. K. Roobottom, J. Passmore and L. Glasser, Inorg.
Chem., 1999, 38, 3609–3620.
18 J. D. Holbrey and K. R. Seddon, J. Chem. Soc., Dalton Trans., 1999,
2133–2139.
19 W. A. Henderson and C. A. Streuli, J. Am. Chem. Soc., 1960, 82, 5791–
5794.
20 W. A. Henderson and S. A. Buckler, J. Am. Chem. Soc., 1960, 82,
5794–5800.
21 ChemAxon, ‘www.chemicalize.org’, 2010.
22 P. T. Anastas and J. C. Warner, ‘Green Chemistry: Theory and Practice’,
Oxford University Press, New York, 1998.
23 C. R. Strauss, Aust. J. Chem., 2009, 62, 3–15.
24 M. M. Rauhut, H. A. Currier, A. M. Semsel and V. P. Wystrach, J. Am.
Chem. Soc., 1961, 26, 5138–5145.
40 J. Esperanca, H. J. R. Guedes, M. Blesic and L. P. N. Rebelo, J. Chem.
Eng. Data, 2006, 51, 237–242.
41 J. M. S. S. Esperanc¸a, H. J. R. Guedes, M. Blesic and L. P. N. Rebelo,
J. Chem. Eng. Data, 2006, 51, 237–242.
42 K. R. Seddon, A. Stark and M. J. Torres, Pure Appl. Chem., 2000, 72,
2275–2287.
43 Z. Y. Gu and J. F. Brennecke, J. Chem. Eng. Data, 2002, 47, 339–345.
44 P. A. Z. Suarez, S. Einloft, J. E. L. Dullius, R. F. de Souza
and J. Dupont, J. Chim. Phys. Phys.-Chim. Biol., 1998, 95, 1626–
1639.
25 W. Markownikoff, Justus Liebigs Ann. Chem., 1870, 153, 228–259.
26 S. A. Buckler, J. Am. Chem. Soc., 1962, 84, 3093–3097.
27 K. Tsunashima and M. Sugiya, Electrochem. Commun., 2007, 9, 2353–
2358.
45 L. P. N. Rebelo, V. Najdanovic-Visak, R.G. de Azevedo, J. M. S. S.
Esperanc¸a, M. N. da Ponte, H. J. R. Guedes, Z. P. Visak, H. C. de
Sousa, J. Szydlowski, J. N. C. Lopes and T. C. Cordeiro, in ‘Ionic
Liquids IIIA: Fundamentals, Progress, Challenges, and Opportunities,
Properties and Structure’, ed. R. D. Rogers and K. R. Seddon, ACS
Symposium Series, Vol. 901, Am. Chem. Soc., Washington, 2005, pp.
270–291.
28 A. F. Kapustinskii, Q. Rev. Chem. Soc., 1956, 10, 283–294.
29 J. J. Tindale, C. Na, M. C. Jennings and P. J. Ragogna, Can. J. Chem.,
2007, 85, 660–667.
30 J. March, ‘Advanced Organic Chemistry’, 4th Edit., John Wiley and
46 G. Adamova´, J. N. C. Lopes, L. P. N. Rebelo, A. J. Robertson and K.
R. Seddon, Chem. Sci. manuscript in preparation.
Sons, Inc, New York, 1992.
31 M. Zanger, C. A. Van der Werf and W. E. McEwen, J. Am. Chem. Soc.,
1959, 81, 3806–3807.
32 M. O. Wolff, K. M. Alexander and G. Belder, Chimica Oggi-Chemistry
Today, 2000, 18, 29–32.
33 J. M. Crosthwaite, M. J. Muldoon, J. K. Dixon, J. L. Anderson and J.
F. Brennecke, J. Chem. Thermodyn., 2005, 37, 559–568.
34 M. Tariq, P. A. S. Forte, M. F. C. Gomes, J. N. C. Lopes and L. P. N.
Rebelo, J. Chem. Thermodyn., 2009, 41, 790–798.
35 C. F. Ye and J. M. Shreeve, J. Phys. Chem. A, 2007, 111, 1456–1461.
47 L. P. N. Rebelo, J. N. C. Lopes, J. M. S. S. Esperanc¸a and E. Filipe, J.
Phys. Chem. B, 2005, 109, 6040–6043.
48 Z. Y. Gu and J. F. Brennecke, J. Chem. Eng. Data, 2002, 47, 339–345.
49 S. A. Forsyth, J. M. Pringle and D. R. MacFarlane, Aust. J. Chem.,
2004, 57, 113–119.
50 A. A. Fannin, D. A. Floreani, L. A. King, J. S. Landers, B. J. Piersma,
D. J. Stech, R. L. Vaughn, J. S. Wilkes and J. L. Williams, J. Phys.
Chem., 1984, 88, 2614–2621.
51 R. L. Gardas and J. A. P. Coutinho, AIChE J., 2009, 55, 1274–1290.
12764 | Dalton Trans., 2011, 40, 12750–12764
This journal is
The Royal Society of Chemistry 2011
©