Do all ionic liquids need organic cations? Characterisation of [AlCl 2·nAmide]+ AlCl4- and comparison with imidazolium based systems

Article abstract of DOI:10.1039/c0cc04989a

The addition of a simple amide to AlCl₃ causes the formation of a liquid of the form [AlCl₂·nAmide]⁺ AlCl ₄^-. The material thus produced is liquid over a wide temperature range, is relatively insensitive to water and has the properties of an ionic liquid. This ionic liquid is shown to be a suitable medium for the acetylation of ferrocene and the electrodeposition of aluminium and demonstrated that quaternary ammonium cations are not always needed to form ionic liquids.

Full text of DOI:10.1039/c0cc04989a

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COMMUNICATION
Do all ionic liquids need organic cations? Characterisation of
[
+
AlCl ÁnAmide] AlCl and comparison with imidazolium based systems
À
2
4
ab a a a
Hadi M. A. Abood, Andrew P. Abbott,* Andrew D. Ballantyne and Karl S. Ryder
Received 16th November 2010, Accepted 25th January 2011
DOI: 10.1039/c0cc04989a
+
form ZnCl and ZnCl although they have relatively low
À
The addition of a simple amide to AlCl causes the formation of
3
3
+
À
13
a liquid of the form [AlCl ÁnAmide] AlCl4 . The material thus
conductivities.
2
produced is liquid over a wide temperature range, is relatively
insensitive to water and has the properties of an ionic liquid. This
ionic liquid is shown to be a suitable medium for the acetylation
of ferrocene and the electrodeposition of aluminium and
demonstrated that quaternary ammonium cations are not always
needed to form ionic liquids.
3
In the current study it is shown that AlCl can also undergo
disproportionation by forming a complex with acetamide or
urea and the complex is liquid at ambient temperature with a
3 to 4 fold higher conductivity than the corresponding zinc
3
chloride eutectics. The AlCl based liquids only form with a
limited range of amides and only over a relatively small
compositional range.
The ubiquitous application of ionic liquids is evidenced
3
An AlCl eutectic was made by slowly mixing aluminium
3
1
through their use in catalysis, electrochemistry, biotechnology
2
chloride (2.1 g, 15.75 mmol) with acetamide (0.93 g, 15.75 mmol)
in a Schlenk tube under a blanket of dry nitrogen gas making a
1 : 1 mole ratio. The solids reacted immediately without
external heating and reached a temperature of ca. 75 1C.
The liquid was left to cool gradually to room temperature
producing transparent, yellow colored, free flowing liquid as
shown in Fig. 1. This liquid was found to have a density of
4
and material processing. One of the only factors linking all of
these ionic liquids is the use of organic cations, most of which
are nitrogen based with the vast majority containing imid-
azolium moieties. Metal containing ionic liquids have
primarily incorporated the active metal centre as a chloro-
metallate anion. Aluminium-based ionic liquids are well
known and exemplars using imidazolium and pyridinium
À3
À1
1.4 g cm , a conductivity of 0.804 mS cm and a viscosity of
60 cP at 25 1C. When this liquid was examined by differential
scanning calorimetry it underwent a glass transition around
À63 1C and was stable up to 100 1C.
5
,6
chlorides are amongst the first studied ionic liquids. In all
4À
cases aluminium forms anionic species including AlCl and
À 7
2 7
Al Cl . One issue associated with this is the Lewis acidity
and general reactivity of the metal centre which is largely
affected by the relative content of the Lewis base in the liquid.
Cationic aluminium complexes are less common than their
anionic counterparts and are certainly unknown in ionic
liquids. They are generally formed from the reaction of alkyl
An analogous liquid was made using equimolar amounts of
urea in place of acetamide (Fig. 1). Numerous amides and
polyols were tested and the only other amide that produced a
liquid at room temperature was N,N-dimethyl urea. Attempts
to use tetramethylurea, dimethylformamide and benzamide as
the amide did not result in a homogeneous liquid presumably
due to their inability to act as complexing agents. No polyols
8
–10
aluminium halides with a Lewis acid, LA or base, LB
e.g.
(1)
[acid–Cl] (2)
+
À
2 2
[R AlCl ]
R
2
Al(m-Cl)]
2
+ nBase - [R
2
Al(base)
n
]
+
À
[
R
2
AlCl] + LB + LA - [R
2
2
Al(base) ]
2
The area of cationic aluminium complexes has been
1
1
reviewed by Atwood. All of the complexes of this form
synthesized to date are solids at ambient temperature.
The majority of ionic liquids are based on quaternary
1
2
ammonium or phosphonium cations,
recently shown that room temperature eutectics could also
be formed between ZnCl and simple amides and diols. These
however it was
2
2
form ionic liquids through the disproportionation of ZnCl to
a
Chemistry Department, University of Leicester, Leicester, LE1 7RH,
UK. E-mail: apa1@le.ac.uk; Fax: +44 (0)116 252 3789;
Tel: +44 (0)116 252 2087
Chemistry Department, College of Science, Al-Nahrain University,
P.O.B. 64055, Jadria, Baghdad, Iraq
b
3 3
Fig. 1 1 : 1 AlCl –acetamide (left) and 1 : 1 AlCl –urea (right).
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 3523–3525 3523

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Fig. 2 Plots of viscosity and conductivity as a function of temperature for the 1 : 1 AlCl
vs. conductivity for both systems (c).
3
–urea (a), 1 : 1 AlCl
3
–acetamide (b) and a plot of fluidity
formed homogeneous liquids but instead reacted with AlCl3
frequencies were shifted from 1673 and 1631 to 1662 and
À1
yielding HCl. To characterize the AlCl Áamide complexes
1557 cm . Similar shifts were also observed for the urea-
3
formed a variety of analytical techniques were applied to the
liquids. FAB-MS showed several aluminium containing
based liquid and these suggest that complexation to the
aluminium occurs through the oxygen on the amide.
The liquid only forms in the compositional range 1 : 1 o
+
+
species [AlCl
2
(Acet)
2
]
2
(m/z = 215) and [AlCl (Acet)]
(
m/z = 156) (or more probably the dimer with bridging
amide : AlCl
either AlCl
3
o 1 : 2. Outside these limits a suspension of
ligands). Slight traces of aluminium containing cations
3
or amide in the eutectic forms. This is clearly
were also found with m/z = 121 and 179 (which could be
related to the stoichiometry of the amide : AlCl
+
3
complex; once
2+
[
AlCl(Acet)
5
]
). There was one primary aluminium containing
À
the AlCl
2
is saturated with amide no further material will
anionic species AlCl (m/z = 169) with only small traces of
4
dissolve in the liquid. It should however be noted that this is
À
Al Cl7 . Analogous complexes were found using urea as the
2
+
amide although the signal for [AlCl (U)] was comparatively
very different from the corresponding ZnCl : acetamide
2
complex where the eutectic composition was at 1 : 4 and
liquids did not form below a stoichiometry of 1 : 2. It can
therefore be concluded that the reaction occurring to produce
the aluminium-based ionic liquids is:
2
small compared with the acetamide case. Jacobs and No
¨
cker
À
could be
+
2 3 4 4
(NH ) ] [AlCl ]
showed that the complex [AlCl
1
4
formed but this was a solid with a melting point of 150 1C.
Similar studies with pyridine and tetrahydrofuran yielded
analogous complexes with 4 neutral ligands surrounding the
aluminium centre and 2 additional axial chloride ligands i.e.
+
ÁnAmide] AlCl
+
2
AlCl
3
+ nAmide # [AlCl
2
4
(3)
1
5,16
an octahedral geometry.
The speciation observed from
The liquids are optically transparent and highly fluid. Fig. 2
shows plots of viscosity and conductivity for the 1 : 1
AlCl : acetamide and 1 : 1 AlCl : urea as a function of
FAB-MS suggests that the lower melting points of these
complexes are due to their coordination geometry.
3
3
2
7
Al NMR spectroscopy of the liquid shows the presence of
main peaks at d = 101.1, 88.6 and 73.6 ppm. This technique
has been used previously to quantify aluminium species in
temperature. Comparison of these systems with the imid-
azolium chloroaluminates for example shows that they have
similar but lower conductivities and higher viscosities (1 : 1
3
1
7
solution. Most previous studies have been made in solution
of a molecular solvent and so direct comparison is not possible
3-butyl-1-methyl imidazolium chloride : AlCl
3
; viscosity =
À1
27 cP, conductivity 10 mS cm ). For all of these systems
plots of conductivity vs. fluidity are linear since the mobile
species are similar in size (Fig. 2c). The properties of these
since the solvent significantly affects the peak position. The
À
peak at 88.6 is thought to correspond to AlCl
4
since it
1
2
integrates to the size of the other two peaks. Also, the addition
of MgCl causes it to increase in size of this peak while the
liquids are akin to those of most common ionic liquids. The
liquid is qualitatively observed to hydrolyse less readily in moist
air than the chloroaluminate liquid. This is in keeping with the
2
other two decrease in size. It is assumed that the peaks at
+
À
d = 101.1 and 73.6 ppm correspond to the complexes
report by Atwood et al. that crystals of [AlCl (THF) ] [AlCl ]
2
4
4
+ +
AlCl (Acet) ] and [AlCl (Acet)] respectively. The inte-
16
[
could be easily handled in the open laboratory.
2
2
2
grated areas under these two peaks were found to be 1.6 : 1
which is in close agreement with the signal intensities from
FAB-MS. While integration of these two peaks does not give
quantitative information about the exact amount of species
present this correlation in relative peak intensities can be used
for species identification.
AlCl is a Lewis acid and has been used in ionic liquids for a
3
1
2
range of catalytic reactions. To demonstrate the efficacy of
the AlCl : amide systems the acetylation of ferrocene was
carried out in an analogous method to that published using
3
1
8
1emimI : 2AlCl
3
.
For this study the reactions were carried
at 0 1C for 2 h with subsequent
out in a Schlenk flask under N
2
The method of complexation can be determined through
analysis of the IR spectra of the liquid and pure amide.
Significant shifts in the stretching frequencies of the N–H were
work up by quenching with 2 M HCl solution and extraction
with dichloromethane (4 Â 25 ml). The individual products
were then isolated by column chromatography (Scheme 1).
Table 1 shows the yield of the respective components from
the acetylation of ferrocene. The acetylferrocene and
shifted from 3300 and 3151 in pure acetamide to 3435 and
À1
3
361 cm
in the eutectic liquid. The CQO stretching
3
524 Chem. Commun., 2011, 47, 3523–3525
This journal is c The Royal Society of Chemistry 2011

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Scheme 1 Acetylation of ferrocene.
Table 1 Percentage yields of products from acetylation of ferrocene
with two equivalents of acetic anhydride in liquids of 1 : 1.5
urea : AlCl
3 3
or 1 : 1.5 acetamide : AlCl
Mole % AlCl
3
Acetylferrocene Diacetylferrocene
Urea
Acetamide
40
40
50
55
58
67
75
26
8
0
89
38
16
9
36
56
0
0
38
40
19
1
1
1
1
1
8
8
8
8
8
emimI : AlCl
emimI : AlCl
emimI : AlCl
emimI : AlCl
emimI : AlCl
3
3
3
3
3
Fig. 3 Cyclic voltammogram of 1 : 1 aluminium trichloride with
acetamide (1) and urea (2) at room temperature. (Inset: aluminium
coating obtained from bulk deposition (see text for details).)
3
In conclusion this work has shown that an AlCl : amide
eutectic can be created with aluminium containing cations and
anions. The physical properties are akin to those of other ionic
liquids based on quaternary ammonium cations. These liquids
have the advantage that they are easy to prepare and less water
sensitive than other aluminium containing liquids.
1
1
13
diacetylferrocene were characterized by H NMR, { H} C
NMR, elemental analysis, electrospray MS, and IR spectro-
scopy. Interestingly, the two amide-based liquids show
differing reactivity. The urea liquid gave moderate yield of
each of the compounds whereas the acetamide liquid gave a
yield of 56% of the diacetylated product with low quantities of
the monoacetylated intermediate. This latter result is an
The authors would like to thank the CARA and SRF for a
fellowship for HMAA and EU FP6 through the project
IONMET for funding this work.
improvement on that reported for the emim AlCl I liquid
3
despite the fact that there is significantly less AlCl . The fact
3
Notes and references
that no product was obtained using 1 : 1 emimI : AlCl
À
3
suggests that AlCl
3
I
is not very Lewis acidic. It also suggests
1 V. I. Parvulescu and C. Hardacre, Chem. Rev., 2007, 107, 2615.
2 M. Armand, F. Endres, D. R. MacFarlane, H. Ohno and
B. Scrosati, Nat. Mater., 2009, 8, 621.
that the cationic aluminium containing species is more Lewis
À
acidic than Al
2
Cl
7
.
3
4
F. Van Rantwijk and R. A. Sheldon, Chem. Rev., 2007, 107, 2757.
A. P. Abbott, G. Frisch, J. Hartley and K. S. Ryder, Green Chem.,
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Another technological application of aluminium-based
1
ionic liquids has been for batteries or metal deposition.
9,20
Fig.
3
shows the cyclic voltammogram for the 1 : 1
5 J. S. Wilkes, J. A. Levisky, R. A. Wilson and C. L. Hussey, Inorg.
Chem., 1982, 21, 1263.
AlCl
3
: acetamide and 1 : 1 AlCl : urea systems at 25 1C in a
3
6
7
J. S. Wilkes, ACS Symp. Ser., 2002, 818, 214.
L. Heerman and W. D’Olislager, Inorg. Chem., 1985, 24, 4704.
1
mm radius Pt disc electrode. The shape is generally similar to
that of the butyl-methyl imidazolium chloride : AlCl system
3
8 D. J. Darensbourg, E. L. Maynard, M. W. Holtcamp,
K. K. Klausmeyer and J. H. Reibenspies, Inorg. Chem., 1996, 35,
2682.
although there is no under-potential deposition presumably
À 19,20
due to the lack of Al Cl
2
.
7
There is also no evident
9
Z. Florjanczyk, W. Bury, E. Zygadzo-Monikowska, I. Justyniak,
R. Balawender and J. Lewinski, Inorg. Chem., 2009, 48, 10892.
2
0
deposition of nano-particulate aluminium and the over-
potential required to deposit metal is larger than in the
imidazolium-based liquid. Electrodeposition of aluminium
cannot be carried out from Lewis basic chloroaluminate melts
10 H. Jacobs and B. No
¨
cker, Z. Anorg. Allg. Chem., 1993, 614, 73.
11 D. A. Atwood, Coord. Chem. Rev., 1998, 176, 407.
12 P. Wasserscheid and T. Welton, Ionic Liquids in Synthesis, 2nd edn,
Wiley-VCH, Weinheim, 2008.
(
x
AlCl3 o 0.5) and this is thought to be due to the speciation of
13 A. P. Abbott, J. C. Barron, K. S. Ryder and D. Wilson,
Chem.–Eur. J., 2007, 13, 6495.
À
aluminium which is predominantly AlCl
4
. Bulk electrolysis
1
4 H. Jacobs and B. No
5 P. Pullman, K. Hensen and J. W. Bats, Z. Naturforsch., B, 1982,
7, 1312.
¨
cker, Z. Anorg. Allg. Chem., 1992, 614, 25.
of the 1 : 1 AlCl
3
: acetamide liquid was carried out at 25 1C by
1
À2
applying a constant current of 2 mA cm
with a stable
3
voltage of 0.4 volt was applied between a brass rod cathode
radius = 5 mm) and an aluminium anode for one hour. An
16 N. C. Means, C. M. Means, S. G. Bott and J. L. Atwood, Inorg.
Chem., 1987, 26, 1466.
(
1
7 J. Derouault, P. Granger and M. T. Forel, Inorg. Chem., 1977, 16,
214.
adherent, grey metallic coating was obtained on the cathode as
shown in Fig. 3 (inset). Electrochemical quartz crystal micro-
balance showed an electrodeposition efficiency of 97%. The
ability to electrodeposit aluminium from these liquids suggests
that the cationic species are those predominantly reduced at
the electrode surface.
3
18 S. Stark, B. L. MacLean and R. D. Singer, J. Chem. Soc., Dalton
Trans., 1999, 63.
1
9 Electrodeposition of Metals from Ionic Liquids, ed. F. Endres,
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0 A. P. Abbott, H. M. A. Abood, R. M. Ali, F. Qiu and K. S. Ryder,
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2
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 3523–3525 3525