Low viscosity ionic liquids based on organic salts of the dicyanamide anion

Article abstract of DOI:10.1039/b103064g

New families of salts v/z. quaternary ammonium, N-alkyl-N-methylpyrrolidinium or 1-alkyl-3-methylimidazolium dicyanamides, Cat⁺N(CN)₂^-, are low melting compounds, most being liquid at rt, water-miscible and have low (for i

Full text of DOI:10.1039/b103064g

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Low viscosity ionic liquids based on organic salts of the dicyanamide
anion
Douglas R. MacFarlane,* Jake Golding, Stewart Forsyth, Maria Forsyth and Glen B. Deacon
Centre for Green Chemistry, School of Chemistry, PO Box 23, Victoria 3800, Australia.
E-mail: dmacfarlane@sci.monash.edu.au
Received (in Cambridge, UK) 4th April 2001, Accepted 20th June 2001
First published as an Advance Article on the web 6th July 2001
New families of salts viz. quaternary ammonium, N-alkyl-N-
methylpyrrolidinium or 1-alkyl-3-methylimidazolium di-
cyanamides, Cat+N(CN)22, are low melting compounds,
most being liquid at rt, water-miscible and have low (for
ionic liquids) viscosity at rt, e.g. h = 21 cP for 1-ethyl-
3-methylimidazolium dicyanamide.
known iodide concentrations; the noise limited detection limit
was thereby found to be 0.5% (w/w) I2. No discernable I mass
peaks were found in the dca salts, therefore indicating a residual
I content of < 0.5% (w/w). The same considerations and lack of
a detectable signal indicate residual Ag < 0.5% (w/w). It is
known6 that impurities such as halides and water can have an
effect on properties such as viscosity, though the effect of water
in particular is expected to be more dramatic when the viscosity
Currently, many chemically inert ionic liquids, which are of
interest as media in Green Chemistry,1 have viscosities much
higher than those of solvents normally used in synthesis. Thus
the search for new, more versatile ionic liquids is driven in part
by the need for materials of lower viscosity. High viscosities not
only lead to handling difficulties e.g. in filtration, decantation,
and dissolution, but may also lead to reduced reaction rates and
competitive unimolecular side reactions. One of the most fluid,
inert of families of ionic liquids is that based on the
bis(trifluoromethanesulfonyl)amide, N(Tf)22 ion,2 for example,
1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)-
amide (1) (h = 34 cP 20 °C;2a cf. H2O, 1 cP; toluene 0.59 cP).
6
differs greatly from that of pure water (e.g. emim BF4 ). After
rigorous final drying, compounds were handled in a nitrogen
drybox during preparation of thermal analysis samples. Vis-
cosity measurements were carried out in a drybox.
Thermal properties of the salts were investigated by differ-
ential scanning calorimetry. All of the dca salts, except the N,N-
dimethylpyrrolidinium compound, are liquids at rt with well
defined mps below 0 °C (Table 1), providing a large liquid
range. Notably, the mps are 20–30 °C or even more below those
of the corresponding salts,2a perhaps an unexpected result since
charge delocalisation could be expected to be greater for dca,
producing weaker ion–ion interactions. Many of the dca salts
are glass forming, with very low glass transition temperatures,
Tg (Table 1), and only very sluggish crystallization kinetics at
any temperature. The dimethylpyrrolidinium compound shows
evidence of multiple solid phases, with a solid–solid thermal
transition around 26 °C (Table 1) which consumes a large
fraction of the total entropy of melting. This behaviour is
indicative of plastic crystal phase formation and in the case of
these ionic compounds, the plastic phases are often highly
conductive.7
The viscosities of representative compounds at 25 °C (Table
1) are lower than those of the corresponding salts, e.g. 1-ethyl-
3-methylimidazolium dca (h = 21 cP) and (h = 34 cP); N-
butyl-N-methylpyrrolidinium dca (h = 50 cP) and (h = 85
cP).2c This rather useful trend is possibly related to the smaller
size of the anion and parallels the lower mps of the dca
compounds. In practical terms, it is possible to filter at rt a
precipitate from a suspension in the lower viscosity dca salt with
a sintered glass frit under vacuum.
There is also a need for donor solvent characteristics. We now
report several new series of salts based on the dicyanamide
(dca) anion, viz. quaternary ammonium, pyrrolidinium, and
imidazolium dicyanamides (2–4), most of which are liquids at
rt. These have a different solubility profile from the correspond-
ing bis(trifluoromethanesulfonyl)amides, potential donor char-
acteristics as the anion is a powerful ligand,3 and, in representa-
tive cases, lower viscosity.
Quaternary iodide salts of triethylamine, tributylamine, N-
methylpyrrolidine, and 1-methylimidazole were prepared by
reported methods,2,4 and were converted into the corresponding
dicyanamides by reaction with a slight excess of silver
dicyanamide in water or ethanol (Scheme 1).
Table 1 Thermal properties of dicyanamide saltsa
r/gcm23
5%
(25 °C)
Viscosity/
cP 5%
(25 °C)
Filtration to remove AgI and evaporation of water under
vacuum gave the ionic liquid. Preparative details for two
examples are provided;† more extensive details and discussion
of properties will be published elsewhere.5 Identification was
Tg/
Ts–s
°C
/
Mp/
°C
Compoundb °C
2
2
2
P11(dca)
26
115
—
—
0.92
0.95
0.92
1
by H NMR and positive and negative electrospray MS.† The
P
12(dca)
P13(dca)
14(dca)
P16(dca)
6222(dca)
210
235
255c
211
possible presence of residual I2 and Ag+ were examined via
inspection of the appropriate mass regions of the respective
mass spectra. The sensitivity of the MS measurement to trace
amounts of I2 was determined by spiking the ionic liquids with
45
50
45
P
2106
2100
282
N
N6444(dca)
emim(dca)
243c
221
2104
1.06
21
a Tg = glass transition temperature, Ts–s = solid–solid transition tem-
perature. b P
= N,N-dialkylpyrrolidinium, N = tetraalkylammonium,
subscripts refer to the number of carbons in the alkyl chains. c In these cases
the melting transition is weak and the reported value only approximate.
Scheme 1
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Chem. Commun., 2001, 1430–1431
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b103064g

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Notes and references
† Representative syntheses: 1-ethyl-3-methylimidazolium dicyanamide:
[emim(dca)]. Ag(dca)9 (2.0 g, 11 mmol) was added to a solution of emimI
(2.60 g, 11 mmol) in water (30 ml), and the resulting suspension was stirred
overnight. Filtration and evaporation under vacuum gave the crude title
compound, which was then dissolved in DCM and the solution dried over
anhydrous MgSO4. Evaporation under vacuum gave emim(dca) which was
finally dried under vacuum over SiO2 (yield, 1.51 g, 96%). Anal. Calcd for
C8H11N5: C, 54.2; H, 6.3; N, 39.5%. Found C, 52.7; H, 6.3; N, 38.6%. IR
(liquid film): 3489 (w), 3150 (s), 3106 (s), 2988 (s), 2365 (w), 2232 (v.s),
2195 (v.s), 2132 (v.s), 1637 (w), 1573 (s), 1466 (m), 1427 (w), 1388 (w),
1131 (s), 1170 (s), 1088 (w), 1030 (w), 959 (w), 905 (w), 844 (w), 802 (w),
753 (m), 701 (w), 648 (m), 622 (s) cm21. 1H NMR (300 MHz, d6-DMSO):
d 1.42 (t, CH3), 3.84 (s, n-CH3), 4.19 (q, N-CH2), 7.67 (s, CH), 7.76 (s, CH),
9.10 (s, N-CH-N) ppm. 13C NMR (JMOD) (75 MHz, d6-DMSO): d 14.5 (s,
CH3), 35.2 (s, CH3), 35.2 (s, CH3), 43.6 (s, CH2), 121.4 (s, CH), 123.0 (s,
CH) ppm, N(CN)22 and N–CH–N not cited. Electrospray MS(+ve): m/z 111
(100% 2 emim+) MS (2ve) m/z 66 (100% dca2), 243 (5% [emim-
(dca)2]2.
Fig. 1 Cyclic voltammetry of neat 1-ethyl-3-methylimidazolium dicyan-
amide (emim(dca)) carried out at 100 °C under a nitrogen atmosphere (dry
box), on a glassy carbon microelectrode, with a platinum counter electrode
and Ag/Ag+ pseudo reference electrode.
N-butyl-N-methylpyrrolidinium dicyanamide: [P14(dca)] Ag(dca) (2.10g,
12 mmol) and P14I (2.70g, 10 mmol) gave [P14(dca)] (yield 1.73g, 83%). IR
(liquid film): 3488 (m), 2964 (m), 2876 (w), 2227 (s), 2191 (m), 2131 (v.s),
1466 (m), 1340 (m), 1306 (m), 1004 (v.w), 929 (w), 902 (v.w), 830 (v.w)
cm21. 1H NMR (300 MHz, d6-DMSO): d 0.92 (t, CH3), 1.31 (m, CH2), 1.67
(m, CH2), 2.06 (br-t, 2 3 CH2), 2.96 (s, CH3), 3.24 (q, CH2), 3.43 (br-m, 2
3 CH2) ppm. 13C NMR (75 MHz d6-DMSO): d 13.8 (s, CH3), 19.6 (s, 2 3
CH2), 21.5 (s, CH2), 25.3 (s, CH2), 47.9 (t, CH3), 63.3 (s, CH2), 63.8 (t, 2
3 CH2) ppm, N(CN)2- not observed. Electrospray MS (+ve): m/z 142
(100%, P14+); MS (2ve) m/z 66 (100%, dca2).
1 (a) K. R. Seddon, J. Chem. Tech. Biotechnol., 1997, 68, 351; (b) T.
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The dca compounds which are liquid at rt are hygroscopic
and are completely miscible with water, by contrast with the
water-immiscible analogues.2 The compounds also appear to
absorb carbon dioxide readily. Extended periods at tem-
peratures in excess of 100 °C have produced no evidence of
breakdown. Qualitative tests showed that a number of hydrated
cobalt(II), and copper(II) salts are appreciably soluble in 1-ethyl-
3-methylimidazolium dca at rt with enhanced solubility on
heating to 75 °C, whereas little dissolution of nickel salts and
CuCl occurs. By contrast, the dca-soluble CuCl2·2H2O and
CoCl2·6H2O are insoluble in the corresponding liquid at rt and
only dissolved slightly at 75 °C. The solubility of the salts in the
dca solvent may be due to its donor ligand properties, consistent
with the rich coordination chemistry that is known of the anion,3
though the inert behaviour of nickel salts is surprising. Of
further interest is that glucose is soluble in the dca ionic liquid
but not in the salt. Thus, the corresponding dca and salts show
complementary solvent properties.
The electrochemical behaviour of these salts is illustrated by
the example of the cyclic voltammogram in Fig. 1. The liquid is
stable to quite low potentials, around 22 V vs. Ag/Ag+, in
common with other salts of this cation; the reductive limit
presumably reflecting a reduction reaction of the cation. The
stability in the oxidative range is reduced as compared to the
analogue2 but still leaves a large ( > 3.5 V) window for
electrochemical use. The irreversible oxidation observed at ca.
1.5 V vs. Ag/Ag+ may be indicative of the formation of a neutral
dimer, [N(CN)2]2, a compound of theoretical interest but which
has not been characterized.8
Thus new low melting, water-miscible, ionic liquids of
relatively low viscosity which have considerable potential as a
reaction medium and with a coordinating anion have been
produced.
We are grateful for support from the Australian Research
Council through the Centre for Green Chemistry.
8 (a) M. Sana and G. Leroy, Ann. Soc. Sci. Bruxelles, 1987, 101, 23; G.
Leroy, M. Sana and C. Wilante, Belg. Theochem., 1987, 38, 249; A. M.
Golub, H. Köhler and V. V. Skopenko, Chemistry of Pseudohalides,
Elsevier, Berlin, 1986.
9 D. Britton and Y. M. Chow, Acta Crystallogr., Sect. B, 1977, 33, 697.
Chem. Commun., 2001, 1430–1431
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