High-throughput approach for the identification of anilinium-based ionic liquids that are suitable for electropolymerisation

Article abstract of DOI:10.1039/c5cp02294k

We report the synthesis of new protic ionic liquids (PILs) based on aniline derivatives and the use of high-throughput (HT) techniques to screen possible candidates. In this work, a simple HT method was applied to rapidly screen different aniline derivatives against different acids in order to identify possible combinations that produce PILs. This was followed by repeating the HT process with a Chemspeed robotic synthesis platform for more accurate results. One of the successful combinations were then chosen to be synthesised on a larger scale for further analysis. The new PILs are of interest to the fields of ionic liquids, energy storage and especially, conducting polymers as they serve as solvents, electrolytes and monomers at the same time for possible electropolymerisation (i.e. a self-contained polymer precursor).

Full text of DOI:10.1039/c5cp02294k

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High-throughput approach for the identification
of anilinium-based ionic liquids that are suitable
for electropolymerisation†
Cite this:DOI: 10.1039/c5cp02294k
Muhammad E. Abdelhamid,ab Timothy Murdoch,c Tamar L. Greaves,ac
Anthony P. O’Mullane*d and Graeme A. Snook*b
We report the synthesis of new protic ionic liquids (PILs) based on aniline derivatives and the use of high-
throughput (HT) techniques to screen possible candidates. In this work, a simple HT method was applied to
rapidly screen different aniline derivatives against different acids in order to identify possible combinations
that produce PILs. This was followed by repeating the HT process with a Chemspeed robotic synthesis
platform for more accurate results. One of the successful combinations were then chosen to be synthesised
on a larger scale for further analysis. The new PILs are of interest to the fields of ionic liquids, energy storage
and especially, conducting polymers as they serve as solvents, electrolytes and monomers at the same time
for possible electropolymerisation (i.e. a self-contained polymer precursor).
Received 20th April 2015,
Accepted 15th June 2015
DOI: 10.1039/c5cp02294k
www.rsc.org/pccp
in many ways such as in mixtures of ILs with molecular solvents
or acids.9–11
Introduction
The polymerisation of aniline into PANi can be achieved via
The search for the optimal IL for a specific application, such
chemical or electrochemical oxidative polymerisation.1–3 The as the electropolymerisation of aniline, can be difficult and
resulting conductive polymer films can be applied as super- time consuming, as ILs are composed of cations and anions, of
capacitors, actuators or as photovoltaic devices.1,4 However which there are a vast number of potential combinations.
a significant consideration to achieve both high quality films Furthermore, the incorporation of molecular or organic solvents
and high polymerisation yields is the provision of an excess increases the sample size dramatically due to the vast number
of protons within the polymerisation medium.2,5 The electro- of possible solvents and concentration ratios. However, high-
polymerisation of aniline in some ionic liquids (ILs) results in throughput (HT) techniques, which have become widely acceptable
polyaniline (PANi) films with unique properties. This is because for the rapid study and identification of new drugs and materials,
PANi grows in ILs in a different way than in molecular and can be applied.12–14 The technique involves cross screening and
organic solvents.6 A broad range of ionic liquids (ILs) have been analysis of a large variety of starting materials ultimately leading
used in many applications in electrochemistry and are attrac- to the identification of promising combinations.15 Such, HT
tive because they can serve as solvents as well as electrolytes.7,8 techniques would therefore in principle be useful in IL synthesis
ILs have been utilised as polymerisation media for polyaniline as it would allow for the rapid screening and identification of
potential ILs that are suitable for specific applications.
A particularly interesting subset of ILs, called protic ionic
a School of Applied Sciences, RMIT University, GPO Box 2476, Melbourne,
VIC 3001, Australia
liquids (PILs) have the potential to be highly effective for
the electropolymerisation of polymers that require the presence
of protons. They are produced via proton transfer from a
Brønsted acid to a Brønsted base.16,17 They are cheaper and
easier to synthesise than aprotic ILs as they require only simple
equimolar acid–base reactions.18 This, along with their gener-
ally lower viscosity compared with aprotic ionic liquids,16
makes them highly suitable for the high-throughput synthesis
technique as demonstrated by Greaves et al.16 In addition,
aniline and its derivatives are suitable candidates for a cationic
precursor for PILs as they can act like Brønsted bases as shown
by recent attempts.19
b Mineral Resources, Commonwealth Science and Industrial Research Organisation
(CSIRO), Private Bag 10, Clayton, VIC 3169, Australia.
E-mail: Graeme.Snook@csiro.au
c Manufacturing, Commonwealth Science and Industrial Research Organisation
(CSIRO), Private Bag 10, Clayton, VIC 3169, Australia
d School of Chemistry, Physics and Mechanical Engineering, Queensland University
of Technology, GPO Box 2434, Brisbane, Qld 4001, Australia.
E-mail: anthony.omullane@qut.edu.au
† Electronic supplementary information (ESI) available: NMR spectra, viscosity
measurements and density measurements of n-methylanilinium TFA, n-methyl-
anilinium sulphate, n-ethylanilinium sulphate, and n-ethylanilinium methane-
sulfonate. See DOI: 10.1039/c5cp02294k
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In this paper, a stoichiometric ratio of various combinations
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Eppendorf tube high-throughput screen. In this method, a
of aniline and aniline derivatives (i.e. as bases) with various few drops of aniline and its derivatives were loaded into seven
acids were cross screened in a high throughput manner to short separate Eppendorf tubes. Subsequently, an approximately
list the most promising anilinium-based protic ionic liquids
(PILs), which were then further characterised. A few PILs were
identified and the optimum combination of N-ethylanilinium
and TFA was chosen for full scale synthesis and characterisation.
Table 1 Illustrates the different acids and aniline derivatives that were
screened via two high-throughput methods and their products
The new PIL was synthesised via a previous method used by
Snook et al.4 The benefit of synthesising such PILs is to utilise
them for the direct production of conducting polyaniline
derivatives without the addition of further solvents or proton
donors, such as acids.2 They are a self-contained poly-
merisation system as they act as a polymerisation medium
‘‘solvent and electrolyte’’, and pre-protonated monomer at the
same time, which can in principle be polymerised into the
polyaniline derivative. This method is a greener approach
to polymerise anilines as it reduces waste by eliminating the
need to use other solvents, and removes the acid protonation
step required during conventional polymerisation of aniline.
Furthermore, this highly novel approach results in new morpho-
logies of conducting polymers offering advanced properties of
these materials.
Experimental section
Materials
N-Ethylaniline, n-methylaniline, 2-ethyl-6-methylaniline, 2,5-
dimethylaniline, 2,6-dimethylaniline, 3,5-dimethylaniline, 2-ethyl-
aniline, 3-ethylaniline (Sigma Aldrich), trifluoroacetic acid (Sigma
Aldrich, TFA), hydrochloric acid (Merck Millipore, 65% HNO3),
formic acid (Sigma-Aldrich), nitric acid (Merck Millipore, 37%
HCl), sulphuric acid (Ajax Finechem Pty Ltd, 98% H2SO4) and
methanesulfonic acid (Sigma-Aldrich, 99.5% CH3SO3H) were
used as received. In addition, pentane, absolute ethanol (Merck
Millipore, EMSURE ACS), diethyl ether (Sigma-Aldrich) and
18.2 MO cm Milli-Q water were also utilised.
Synthesis methods
In order to screen the various acids against the aniline deriva-
tives, two high-throughput methods were used. The first method
is the simplest as a predetermined volume of the derivatives
were loaded manually into Eppendorf tubes followed by the
addition of the acids to start the reaction. This method is a quick
and simple way to get preliminary screen data suitable for a large
number of reactants.
The second method is an automated process where the
loading of samples and the reactions are controlled by a robotic
platform. This method is more accurate than the Eppendorf
method as the reaction conditions, such as temperature and
reaction time, are easily controlled and stoichiometric propor-
tions are used of the acids and bases.
Following the two screening techniques, one promising
acid–base combination was chosen to be synthesised on a
(patterned cells are for the test tube high-throughput and solid cells are
Solid, liquid (not IL), dark (oxidised) liquid, and possible PIL
larger scale.
for the Chemspeed robotic synthesis).
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equimolar amount of acid was added drop-wise to each deri- equimolar amounts of n-ethylaniline and trifluoroacetic acid
vative while stirring (Table 1). (TFA). 0.1 mole of n-ethylaniline was added into a round-bottom
Chemspeed robotic synthesis platform. The Chemspeed flask with 5 mL of absolute ethanol. Connected to the round-
robotic synthesis platform contained an ISYNTH reactor block bottom flask was a titration column loaded with 0.1 mole of TFA,
equipped with an array of 24 vials of 20 mL volume. The IL 3 mL of absolute ethanol and 2 mL of milli-Q water. The acid
synthesis protocol was designed to mimic the conventional mixture was added drop-wise to the n-ethylaniline solution
synthesis of protic ionic liquids where ethanol was added to under stirring and controlled temperature under À10 1C in
aniline and its derivative solutions in order to ensure a liquid order to prevent the oxidation of the amine into an amide. After
state during addition of the acid. The concentrations of the the reaction was finished, a yellow-orange viscous liquid was
aniline (derivatives) in ethanol stock solutions were 1.0 M as yielded in the flask. The product was dried under dynamic
well as for the acids stock solutions.
vacuum at 35 1C for 2 hours and then kept under static vacuum
The robotic platform dispenses liquids by means of an overnight at room temperature.
articulated 4-needle tool connected to four individual syringe
The dried product was a yellow to brown slushy solid with a
pumps. This allows simultaneous aspiration/dispensing of ani- melting point of approximately 42 1C. The solid was purified
line (derivatives) or acids to/from four adjacent reactor vials. through dissolving it in pentane and then adding ethyl ether to
Each syringe pump can operate individually to deliver different crash out the product as a white solid. The white solid was
volumes, and at different rates to the other syringes. In this collected and dried via the rotary evaporator at 35 1C for an
particular application, the platform was fitted with four 10 mL hour. The water content of the dried solid was measured to be
syringe pumps, and four stainless steel needles (ID = 0.8 mm). 15 ppm. To obtain a liquid IL solution at room temperature,
The synthesis was conducted in two sequential batches of 24 150 mL of milli-Q water was added to 1 g of the dried white solid
PIL mixtures. The basic experimental steps programmed into and the mixture was heated to about 50 1C with a heat gun. This
the Chemspeed workflow for each batch are described below.
resulted in a light yellow-orange viscous liquid at room tem-
(1) Reagent bases (aniline derivatives + ethanol) and acids perature with a measured water content of 162 ppm.
were loaded into 60 mL reservoir vials and fitted with purge
septa.
Characterisation. All the acid–base mixtures produced by
the Chemspeed robotic platform and the individually synthe-
1
(2) ISYNTH reactors were configured to be open under inert sised n-ethylanilinium TFA were characterised by H NMR and
atmosphere (to prevent condensation) and cooled to a target 13C NMR using Bruker BioSpin Av400X NMR Spectrometers in
temperature of À10 1C.
order to confirm the proton transfer from the acid to the base
(3) Each of the six aniline derivative + ethanol mixtures were and the structure of the products. Furthermore, conductivity
sequentially dispensed to four ISYNTH reactor vials such that measurement of the ILs was undertaken by SevenMulti dual pH
all 24 reactor vials contain the aniline derivative. They were and conductivity meter (Mettler Toledo), the density and visco-
aspirated at 5 mL minÀ1 and dispensed at 20 mL minÀ1
(4) Agitation/shaking of reactors was turned on (400 RPM).
.
sity were measured by Anton Paar density meter with External
Measuring Cell and Anton Paar Rolling-ball viscometer Lovis
(5) Each of the four acids were then added drop-wise to four 2000 in combination with A&D SV-1A tuning fork vibro visco-
ISYNTH reactors such that all 24 reactor vials contained acid + meters respectively.
base. Acids were aspirated at 10 mL minÀ1 and dispensed at
0.1 mL minÀ1 to achieve drop-wise addition over an extended
period of time.
Results
(6) The reactors were agitated for an additional hour follow-
ing the final acid addition. Due to the drop-wise addition of
acid, all reactions had sufficient time to react between 2 and
a solid forms, (2) non-ionic liquid forms (no proton transfer),
6 hours.
Fig. 1, shows n-ethylaniline and its products after the addition
of different acids. Four possible outcomes were considered: (1)
(3) spontaneous polymerisation occurs (dark solid) or (4) the
desired outcome of a protic ionic liquid forms. The combina-
(7) ISYNTH reactors were configured to be closed. Vacuum
pressure in the reactors was stepped down gradually to a target
tions that resulted in potential ILs were then synthesised using
pressure of 1 mbar on the attached vacuum pump. At the same
the Chemspeed robotic platform in a high-throughput manner,
time the temperature was set to 38 1C (43 1C on attached cryostat).
(8) Vacuum, heat, and agitation were continued overnight,
although a review of the log data indicates that minimal
pressure was obtained within 2 hours of starting step 7.
(9) Pressure and temperature were returned to ambient
conditions before the 24 reactor vials were removed, capped
and taken away for analysis.
(10) Steps 1–9 were repeated for batch 2, which used the
same three remaining bases, and the four acids.
Fig. 1 Visual appearance of pure n-ethylaniline and its products as a result
Synthesis of n-ethylanilinium trifluoroacetate. A larger
quantity of n-ethylanilinium TFA was prepared by reacting
of the addition of various acids using the simple Eppendorf tubes high-
throughput screen.
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hence formic acid and hydrochloric acid were omitted. The
visual appearance of n-ethylaniline-acids combinations can be
clearly observed in Fig. 1 where the addition of formic acid
yielded a dark runny liquid that is not a potential IL. On the
other hand, hydrochloric acid produced a solid product when
reacted with n-ethylaniline. However, sulphuric, trifluoroacetic,
and methanesulfonic acids produced relatively viscous liquid
products that are potentially ILs. The nitric acid yielded a
viscous liquid as well, however, the product was consequently
oxidised by the strong acid into a dark black liquid.
The series of 54 aniline derivative–acid combinations that were
prepared using the simple Eppendorf tubes high-throughput
method, and the 36 samples prepared using the Chemspeed
robotic platform were classified according to their visual appear-
ance (i.e. solid or liquid). The results of the reactions carried out
by both high-throughput methods are shown in Table 1 according
to the visual classification scheme. Some results were different
from the simple high-throughput method due to the high
accuracy and precise control of the reaction conditions (i.e.
greater control over the stoichiometry, and due to maintaining
a low temperature during the reaction) by the Chemspeed
robotic platform.
Fig. 3 13C NMR spectra of n-ethylanilinium TFA.
(–COO–) group which are consistent with the formation of a
PIL. Furthermore, the structure of n-ethylaniline did not alter
upon reaction with TFA as concluded from its characteristic
peaks. The combined 1H NMR and 13C NMR data indicate that
the reaction of n-ethylaniline with TFA took place via proton
transfer only, thus forming an ionic compound.
The Chemspeed HT method identified five possible PILs,
however, we chose n-ethylanilinium TFA for the full scale
synthesis as it was the least viscous and easiest to handle.
The formation of n-ethylanilinium TFA PIL was confirmed by
1H NMR and 13C NMR were also used to determine which of
the acid–base compounds produced via the Chemspeed robotic
platform were ionic liquids (Fig. S1 and S2, ESI†). The NMR
spectra show that n-methylaniline yielded ionic liquids with
two acids only (i.e. sulphuric acid and trifluoroacetic acid).
While n-ethylaniline produced three ionic liquids with three
acid combinations (i.e. sulphuric acid, trifluoroacetic acid and
methanesulfonic acid).
The conductivity of the n-ethylanilinium TFA PIL was mea-
sured to be 5.2 mS cmÀ1 at room temperature, compared to
1.0 mS cmÀ1 for N-methylpyrrolidinium TFA and 14.4 mS cmÀ1
for 2-pyrrolidonium TFA,17,21,22 which is consistent with the
1
the H NMR spectra as it shows the proton transfer from the
carboxylic group of TFA to the n-ethylaniline’s nitrogen atom.
Fig. 2 shows the absence of the only peak for TFA usually
observed at 11.5 ppm20 and the appearance of an extra peak at
7.3 ppm (i.e. n-ethylaniline region). This peak shift is attributed
to the proton transfer between the acid and the n-ethylaniline
forming a protic ionic liquid.
Furthermore, 13C NMR measurements were carried out to
confirm the structure of n-ethylaniline and the presence of the
TFA in the PIL (Fig. 3). The multiplet peak at B120 ppm is
associated with carbon atoms surrounded by three fluorine
atoms (–CF3), and the peak at B160 ppm corresponds to a
Fig. 4 Graph showing the density of n-ethylanilinium TFA as a function of
temperature.
Fig. 2 1H NMR spectra of n-ethylanilinium TFA.
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Fig. 6 Illustration showing (a) anilinium salt arranged in an offset p stacking
configuration, (b) n-ethylanilinium salt where the ethyl group is hindering
the p stacking rearrangement by increasing the proximity between the
amine group and the benzene ring, and (c) 3,5-dimethylanilinium salt as
an example for the alkyl groups that are attached to the benzene ring and
extend on the same plane and do not hinder the p stacking rearrangement.
of the p electron cloud of the benzene ring (Fig. 6a), hence
forming solid salts. However, the hydrocarbon chains act like
spacers between the amine group and the benzene ring which
reduces the formation of this offset p stacking of the anilinium
(Fig. 6b), thus preventing the formation of solid salts. It should
Fig. 5 Graph shows the kinematic and dynamic viscosities of n-ethyl-
anilinium TFA as a function of temperature.
formation of an ionic compound. Density measurements were be noted that 7 out of the 9 studied aniline derivatives formed
carried out at various temperatures (i.e. 30 to 80 1C, Fig. 4). It solids and/or non-ILs liquids (for example 2-ethyl-aniline and
shows that the n-ethylanilinium TFA PIL is approx. 20% denser 3-ethyl-aniline) as their hydrocarbon chains are attached to the
than water at near room temperature and its density decreases benzene ring instead of the amine group (Fig. 6c). The hydro-
as a function of temperature.
carbon chains attached to the benzene ring extend initially on
The dynamic and kinematic viscosities were measured at the the same plane and hence do not hinder the p stacking rearrange-
same range of temperatures (Fig. 5) to give values of 24.3 mPa sÀ1 ment. However, the position of the alkyl group on the benzene
and 20.5 mm2 sÀ1 respectively which were much greater than ring is shown to affect the state of the product. By comparing
water viscosity values near room temperature. The melting point the outcome of 2-ethylaniline and 2-ethyl-6-methylaniline, where
for both the solid and the liquid form of n-ethylanilinium TFA the ethyl group is on the ortho position, with 3-ethylaniline,
were measured to be approximately 42 1C and 3 1C respectively.
where the ethyl group is on the meta position, one can conclude
that the derivatives with ortho positioned ethyl groups yielded
almost the same results vs. the type of acids (e.g. H2SO4 & TFA -
solid products/CH3SO3H - liquid products) while meta positioned
derivatives resulted in more or less the opposite outcomes with the
Discussion
The screening process by both the simple Eppendorf tubes and same acids (e.g. H2SO4, TFA - liquid products/CH3SO3H - solid
Chemspeed robotic synthesis platform, resulted in five acid– products). This might be attributed to different steric effects
base combinations being identified as potentially useful PILs exhibited by the ethyl group on the acids.
namely; n-methylanilinium sulphate, n-methyl-anilinium TFA,
The addition of 150 mL (0.015 wt%, 162 ppm measured) of
n-ethylanilinium sulphate, n-ethyl-anilinium TFA, and n-ethyl- water to the product of the full scale synthesis (n-ethylanilinium
anilinium methanesulfonate. Both methods served a comple- TFA) in addition to heating to 50 1C after the purification
mentary role for each other, where the simple Eppendorf tubes process were required to convert it back into liquid form. The
high-throughput enabled very rapid screening to narrow down PIL produced is stable after cooling down to room temperature,
the number of possible acid–base combinations to be tested hence suggesting that water plays a role in lowering the melting
by the Chemspeed robotic synthesis platform. The Chemspeed point of the PIL which was measured to be 42 1C for the solid
high-throughput system then served as an accurate synthesis and 3 1C after water addition.
method.
In this investigation we screened the stoichiometric acid–base
The Chemspeed high-throughput method revealed that only reaction of a broad range of commercially available aniline deriva-
two aniline derivatives yielded PILs where a hydrocarbon chain tives with 6 acids. Combinations of these which led to PILs that
is attached to the amine (i.e. n-methylaniline and n-ethylaniline). were liquid at room temperature have the potential to be poly-
In addition, the longer the hydrocarbon chain is, the less viscous merised into conductive polymers. Of the 54 combinations tested
the PIL is as shown in (Fig. S3, ESI†). This can be explained when in a HT manner there were five identified as possible candidates for
the acid reacts with the aniline base to form an anilinium salt, the future polymerisation trials, and all of these contained an aniline
proton is transferred to the amine group and the nitrogen atom derivative with a –NHR group. None of the aniline derivatives with
becomes positively charged. With many anilinium molecules in –NH2 groups formed PILs which were liquid at room temperature.
close proximity, they tend to form an offset p stacking arrange-
Consequently this HT screening approach indicates that
ment where the positively charged nitrogen atoms align on top future studies should focus on n-alkylanilines with different
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alkyl chain lengths (–NHR), extending to a broader range of Notes and references
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Conclusions
This is the first high throughput combinatorial approach to
anilinium-based PIL design to the best of our knowledge.
Compared to the traditional IL synthesis approaches, this is a
significantly different and efficient method. Traditionally, IL
research has focussed on the rigorous characterisation of a
handful of ILs, and there are few studies which use a large
number of ILs.16 For the aprotic ILs, their synthesis is difficult
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Acknowledgements
The authors acknowledge Dr Shaun C. Howard and Dr Benja-
min W. Muir from the Rapid Automated Materials & Processing 20 S. Budavari, M. J. O’Neil, A. Smith and P. E. Heckelman,
centre at CSIRO for providing the technical support and access
to the Chemspeed robotic synthesis platform. A.O.M. acknowl-
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(FT110100760). M.E.A. acknowledges CSIRO and RMIT for
provision of his PhD studentship. G.A.S. acknowledges the
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2007, 60, 21–28.
funding from the CSIRO Office of the Chief Executive Julius 22 M. Yoshizawa, W. Xu and C. A. Angell, J. Am. Chem. Soc.,
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