In situ formation of thermally stable, room-temperature ionic liquids from CS2 and amidine/amine mixtures

Article abstract of DOI:10.1021/cm101316h

Amidinium dithiocarbamates salts with diverse structures are prepared in situ by adding one equivalent of CS₂ to an equimolar mixture of two nonionic molecules, an amidine and an amine. Many of the salts made in this way are room temperature ionic liquids (RTILs) and the others (ILs) melt well below the decomposition temperature of the salts, ca. 80 °C. Unlike the analogous amidinium carbamate RTILs, which are made by adding CO₂ to amidine/amine mixtures and decompose near 50 °C, the amidinium dithiocarbamates do not revert to their amidine/amine mixtures when they are heated. The thermal, rheological, conductance, and spectroscopic properties of representative examples from a total of 50 of these ILs and RTILs are reported, comparisons between them and their nonionic phases (as well as with their amidinium carbamates analogues) are made, and the thermolysis pathways of the ammonium dithiocarbamates are investigated.

Full text of DOI:10.1021/cm101316h

5492 Chem. Mater. 2010, 22, 5492–5499
DOI:10.1021/cm101316h
In situ Formation of Thermally Stable, Room-Temperature Ionic Liquids
from CS2 and Amidine/Amine Mixtures.
Tao Yu, Taisuke Yamada,† and Richard G. Weiss*
Department of Chemistry, Georgetown University, Washington, D.C. 20057-1227. †Present address:
Asahi Kasei E-materials, 2-1 Samejima, Fuji, Shizuoka, Japan
Received May 10, 2010. Revised Manuscript Received July 6, 2010
Amidinium dithiocarbamates salts with diverse structures are prepared in situ by adding one
equivalent of CS2 to an equimolar mixture of two nonionic molecules, an amidine and an amine.
Many of the salts made in this way are room temperature ionic liquids (RTILs) and the others (ILs)
melt well below the decomposition temperature of the salts, ca. 80 °C. Unlike the analogous
amidinium carbamate RTILs, which are made by adding CO2 to amidine/amine mixtures and
decompose near 50 °C, the amidinium dithiocarbamates do not revert to their amidine/amine
mixtures when they are heated. The thermal, rheological, conductance, and spectroscopic properties
of representative examples from a total of 50 of these ILs and RTILs are reported, comparisons
between them and their nonionic phases (as well as with their amidinium carbamates analogues) are
made, and the thermolysis pathways of the ammonium dithiocarbamates are investigated.
1. Introduction
for diverse applications.4 A subclass of RTILs can be
“switched” reversibly between their ionic and nonionic
states.5,6 This property has been exploited to separate
products (or catalysts) in a manner that allows the catalyst7
to be reused.8 Previously, we developed a class of thermally
reversible ionic liquids by adding CO2, a neutral triatomic
molecule and a greenhouse gas, to an equimolar mixture of
two nonionic molecules, an aliphatic amidine and an
amine.6 The chemistry associated with the formation of
the amidinium carbamate RTILs9 is shown in eq 1 in
Scheme 1. The RTILs can be transformed quantitatively
back to their nonionic components by passing an unreac-
tive “displacing” gas, such as molecular nitrogen, through
the ionic liquid. The nonionic and ionic states can be
switched repeatedly in this way without detectable degra-
dation of the system. Although the benign nature of this
switching process opens several potential applications for
ionic liquids, such as reversible catalyst formation10 and
Because they exhibit characteristics of “green sol-
vents”1 and offer rather unique physicochemical proper-
ties,2 ionic liquids (ILs) and especially room temperature
ionic liquids (RTILs)3 have become increasingly popular
*Corresponding author. E-mail: weissr@georgetown.edu.
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L. A.; Hancu, D.; Beckman, E. J.; Brennecke, J. F. Nature 1999, 399,
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2010 American Chemical Society
pubs.acs.org/cm
Published on Web 09/17/2010

Article
Chem. Mater., Vol. 22, No. 19, 2010 5493
Scheme 1
CO2 gas sequestration,11 the thermal instability of these
ionic liquids above ca. 50 °C (eq 1), even under one
atmosphere pressure of CO2, imposes some limitations
on their applicability. Thus, RTILs of similar structure
but with greater thermal stability would significantly
expand their potential uses.
Previously, we observed that the gel networks12 made when
a triatomic molecule is added to a “latent” alkylamine gelator
and the ionic cross-links formed similarly within aminopoly-
siloxanes13 are thermally more stable with CS2 (an environ-
mentally harmful liquid14) than with CO2. In addition, others
have investigated the properties of functional materials made
by adding CS2 to liquid-like mixtures.15-18
Here, we report the properties of representative exam-
ples from a total of 50 ILs and RTILs obtained when CS2
was added to different combinations of an amidine (D)
and an amine (A) (Scheme 1). This approach is amenable
to the construction of extensive libraries of ILs whose
properties can be tuned for specific tasks. As expected, the
amidinium dithiocarbamates (D-A-S) are more stable
thermally (to ∼80 °C under air atmospheres) than the
corresponding ammonium carbamates (to ∼50 °C under
CO2 atmospheres) and have other attractive features, but
they are not thermally reversible. However, loss of CS2
can be accomplished at room temperature by adding a
small amount of a solution of dilute aqueous carboxylic
acid. The pathways for reaction of the neat D-A-S above
80 °C have been inferred from their thermolysis products.
The thermal, conductive, and spectroscopic properties of
these RTILs are compared with those of their nonionic
phases and of the corresponding amidinium carbamates.
The latter comparison addresses the extent to which
changing the triatomic molecular adduct influences the
properties of the RTILs and demonstrates further that
environmentally deleterious triatomic molecules can have
beneficial purposes in materials science. Their ease of
preparation of these ILs is perhaps their most important
and beneficial attribute: they can be made in one pot, in
situ, by simply adding the three components;amidine,
amine, and CS2;together.
2. Experimental Section
2.1. Materials. Carbon disulfide (anhydrous, 99.9%) was
purchased from Aldrich. t-Butyl amine (99.5%), n-hexylamine
(99%), n-octylamine (99%), n-decylamine (99%), cyclohexylamine
(99%), dibutylamine (99.5%), and 1,1,3,3-tetramethylguanidine
(TMG) (99%) (all from Aldrich) were used as received. 1,8-
Diazabicyclo[5.4.0]undec-7-ene (DBU) (98% from Aldrich) was
refluxed over CaH2 (Acros) for 5 h and distilled at 67-68 °C/0.2
Torr onto activated 5 A molecular sieves (>99.5% by GC).6a All
˚
organic solvents were HPLC grade (from Aldrich or Fisher).
Prolinol (ProOH), leucinol (LeuOH), isoleucine octyl ester (IleC8),
DAPNE (1-(p-dimethylaminophenyl)-2-nitroethylene), and the
other aliphatic amidines were available from previous studies.6
Chloroform-d (99.8% D) and deuterated water (99.9% D) were
purchased from Cambridge Isotope Laboratory, Inc.
2.2. Amidinium Dithiocarbamate Sample Preparation. CS2
liquid (1 mol equivalent) was added slowly to a stirred amidine
and amine sample in a glass vial. The vial was placed in an
ice-water bath during the CS2 addition to dissipate the heat
from the exothermic reaction. (Warning: CS2 is a toxic liquid
that should be handled carefully.)
2.3. Characterization. NMR spectra were recorded on an
Inova 400 MHz Spectrometer operating at 400 MHz (1H), or
100.5 MHz (13C), respectively. 1H and 13C spectra were indirectly
referenced to TMS using residual solvent signals as internal
standards and CDCl3 as the solvent. IR spectra were obtained
on a Perkin-Elmer Spectrum One FT-IR spectrometer, using an
attenuated total reflection accessory or NaCl plates. Gas chroma-
tographic (GC) analyses were performed on a Hewlett-Packard
5890A gas chromatograph equipped with flame ionization detec-
tors and a DB-5 (15 m ꢀ 0.25 mm) column (J & W Scientific).
Thermal gravimetric analysis (TGA) measurements were per-
formed under a nitrogen atmosphere at a 5 °C/min heating rate
on a TGA Q50 thermogravimetric analyzer (TA Instruments)
interfaced to a computer. UV-vis spectra were recorded on a
Varian CARY 300 Bio UV-visible spectrophotometer in Hellma
(11) (a) Bara, J. E.; Camper, D. E.; Gin, D. L.; Noble, R. D. Acc. Chem.
Res. 2010, 43, 152–159. (b) Camper, D.; Bara, J. E.; Gin, D. L.;
Noble, R. D. Ind. Eng. Chem. Res. 2008, 47, 8496–8498. (c) Bara,
J. E.; Carlisle, T. K.; Gabriel, C. J.; Camper, D.; Finotello, A.; Gin,
D. L.; Noble, R. D. Ind. Eng. Chem. Res. 2009, 48, 2739–2751.
(12) (a) George, M.; Weiss, R. G. J. Am. Chem. Soc. 2001, 123, 10393–
10394. (b) George, M.; Weiss, R. G. Langmuir 2003, 19, 1017–1025.
(c) Carretti, E.; Dei, L.; Weiss, R. G. Soft Matter 2005, 1, 17–22.
(13) Yu, T.; Wakuda, K.; Blair, D. L.; Weiss, R. G. J. Phys. Chem. C
2009, 113, 11546–11553.
(14) Beauchamp, R. O., Jr.; Bus, J. S.; Popp, J. A.; Boreiko, C. J.;
Goldberg, L. CRC Crit. Rev. Toxicol. 1983, 11, 169–278.
(15) Heldebrant, D. J.; Yonker, C. R.; Jessop, P. G.; Phan, L. Eur. J.
Chem. 2009, 15, 7619–7627.
(16) Ochiai, B.; Endo, T. Prog. Polym. Sci. 2005, 30, 183–215.
(17) Valentine, W. M.; Amamath, V.; Graham, D. G.; Anthony, D. C.
Chem. Res. Toxicol. 1992, 5, 254–262.
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1447.

5494 Chem. Mater., Vol. 22, No. 19, 2010
Yu et al.
Table 1. Phasesa of Neat 1/1 (mol/mol) Amidine (D)/Amine (A) mixtures before (b) and after (a) adding CS2 at Room Temperature and at -20 °C in
Parentheses; Melting Points (°C) of Ammonium Dithiocarbamates that are Solids at Room Temperature Are Presented Numerically
amine (A)
prolinol
(ProOH)
leucinol
(LeuOH)
isoleucine
octyl ester (IleC8)
hexylamine
a
octylamine
a
cyclohexylamine
dibutylamine
b a
amidine (D)
b
a
b
a
b
a
b
b
b
a
tC4
C4
C6
C8
C16
DBU
TMG
none
L
L
L
L
S
L
L
L
L(L)
L(L)
L(L)
L
L
L
L
S
L
L
L
L(L)
L(L)
L(L)
L
L
L
L
S
L
L
S
L(L)
L(L)
L(L)
L
L
L
L
L
L
L
L
L(L)
L(L)
L(S)
L
L
L
L
L
L
L
L
L(L)
L(L)
L(S)
L
L
L
L
L
L
L
L
L(L)
L(L)
L(S)
L
L
L
L
L
L
L
L
L(L)
L(L)
L(S)
L(L)
L(L)
L(L)
L(S)
S 39.7-41.3
S 55.8-57.1
S 60.5-61.8
S 41.2-42.5
S
S 34.5-36.1
S 54.0-55.9
S 50.8-51.8
S 37.3-38.9
S
S 38.6-40.1
S 52.8-55.5
S 61.7-63.3
S 42.4-44.0
S
S 48.0-50.2
L(S)
L(S)
S 49.1-51.7
L(S)
L(S)
S 55.8-58.0
S 59.0-60.3
S 42.9-43.8
S
S 47.5-49.9
S 52.2-53.5
S 38.6-39.6
S
S
S
a L = liquid, S = solid.
quartz cells with 0.1 cm path lengths. Viscosity measurements were
performed on an Anton Paar Physica MCR 301 rheometer (Anton
Paar GmbH, Graz, Austria) using a parallel plate (radius 25 mm,
gap 0.5 mm) geometry. Conductivities were measured with a
Yellow Spring Instrument Co. model 31 conductivity meter using
Au electrodes in a cell constructed by Prof. Robert de Levie. GC-
MS was performed on a Varian Saturn 2100 instrument in the
electron ionization (EI) mode using a DB-5 (15 m ꢀ 0.25 mm)
column.
3. Results and Discussion
3.1. Preparation and General Properties of the D-A-S.
The appearances of the phase of the D/A combinations at
room temperature, both before and after adding CS2, are
collected in Table 1. Primary amines, secondary amines,
amino esters and amino alcohols were tested as A compo-
nents. Aliphatic amidines, 1,8-diazabicyclo[5.4.0]undec-7-ene
(DBU), and 1,1,3,3-tetramethylguanidine (TMG) were
employed as the D components. A large number of the
combinations remain liquids at room temperature after
adding CS2. However, none of the combinations with the
longest-chained linear amidine, C16, produced a liquid
dithiocarbamate at room temperature; in general, the shorter
chained D appear to favor the formation of liquid D-A-S,
probably as a result of less ordering by London dispersion
forces. All of the amidinium dithiocarbamates, which are
solids at room temperature, have melting points below
65 °C, well below the temperature at which the amidinium
dithiocarbamates decompose. Furthermore, many of the
RTILs remain noncrystalline and flowing (albeit very viscous
fluids) to at least -20 °C; they have a broad and synthetically
useful liquid range.
As expected, the D/A mixtures were qualitatively less
viscous than the corresponding D-A-S ionic liquids because
of the introduction of strong electrostatic interactions in the
latter. The additional hydrogen bonding and dipolar inter-
actions available to the RTILs with an amino alcohols or an
amino ester component made them more viscous than the
ionic liquids with aliphatic amines. In addition, there was no
evidence by polarized optical microscopy for liquid crys-
tallinity in any of the amidinium dithiocarbamates made
from hexadecylamidine (C16) and several different aliphatic
amines, amino esters and amino alcohols. Unlike the amid-
inium carbamates (i.e., the RTILs made by adding CO2 to
Figure 1. FT-IR spectra of (a) C6/cyclohexylamine and (b) C6-cyclohex-
ylamine-CS2.
D/A mixtures), which remain clear and uncolored, the amid-
inium dithiocarbamates are a yellow or orange color (see
Figure S1 in the Supporting Information), which can be
attributed to n-π* electronic transitions of the dithiocarba-
mate group.19
3.2. Characterization of the D-A-S. The amidinium
dithiocarbamate structures were identified on the basis of
analyses of thermal gravimetric analyses (TGA) and 1H and
13C NMR and FT-IR spectral data of amidine/amine
samples before and after the addition of CS2. Figure 1 shows
the FT-IR spectra of a representative sample, C6/cyclohex-
ylamine, before and after adding CS2; spectra for eight
additional D-A-S are presented in Figure S2 of the Support-
ing Information. After addition of CS2, the typical NdC
stretching band of the amidine at 1629 cm-1 is replaced by a
band at 1645 cm-1, which can be assigned to protonated
amidine. In addition, the bands at 1088 and 977 cm-1 are
assigned to CdS and C-S stretchings, respectively.20
The 1H NMR spectrum of C6/cyclohexylamine changes
markedly upon addition of CS2 (Figure 2a). Exposing the
C6-cyclohexylamine-CS2 amidinium dithiocarbamate to N2
(19) Takahashi, K. J. Am. Chem. Soc. 1976, 98, 7488–7498.
(20) (a) Nakamoto, K.; Fujita, J.; Condrate, R. A.; Morimoto, Y.
J. Chem. Phys. 1963, 39, 423–427. (b) Gosavi, R. K.; Agarwala,
U.; Rao, C. N. R. J. Am. Chem. Soc. 1967, 89, 235–239.

Article
Chem. Mater., Vol. 22, No. 19, 2010 5495
Figure 2. NMR spectra: (a) 1H spectra of C6/cyclohexylamine (black), C6-cyclohexylamine-CS2 (red), and C6-cyclohexylamine-CS2-N2 (blue) in CDCl3
with peak assignments in the inset; (b) 13C spectrum of C6-cyclohexylamine-CS2 in CDCl3.
and gently heating at 50 °C (C6-cyclohexylamine-CS2-N2)
for 30 min, conditions that reconvert the analogous amidi-
nium carbamate to its uncharged amidine/amine mixture,6
results in no discernible changes. Upon addition of CS2, the
13C resonance of the central carbon of the amidine in C6
shifted downfield from 156 to 162 ppm and the resonance of
CS2 moved from 19315 to 210 ppm (Figure 2b).
are stable to at least 80 °C under a nitrogen atmosphere
and weight losses indicate that decomposition above
150 °C does not involve either CS2 or H2S (Caution:
H2S is a poisonous gas that should be handled only under
well controlled and ventilated conditions) as the exclusive
species expelled. The theoretical weight loss from C8-
IleC8-CS2 if one molecule of CS2 or H2S (the molecule
lost when ammonium dithiocarbamates are heated to
their decomposition temperature12b,24) is 14.7 or 6.6%,
respectively. Nearly 16% more weight was lost when C8/
IleC8 was heated under the same conditions (Figure 3a).
Furthermore, only 0.18% weight was lost from C8-IleC8-
CS2 when it was kept at 25 °C under a dry nitrogen gas
flow of 60 cc/min during a 60 min period (Figure 3b), and
only 1.7% of the initial weight was lost after heating the
sample to ca. 100 °C at a rate of 5 °C/min. As mentioned,
the thermal behaviors of this and other D-A-S are in
contrast with those of the corresponding ammonium
carbamates and amidinium carbamates (which are ther-
mally reversible, losing a molecule of CO2 when heated to
ca. 50 °C).6,12b Also, when heated, an amidinium dithio-
carbonate has been found to lose a molecule of CS2,15 and
we established previously that ammonium dithiocarba-
mates lose one molecule of H2S and become thioureas
when heated to ca. 100 °C.12b
3.3. Thermolysis Pathways. To the best of our knowl-
edge, the mechanism of thermolysis of amidinium dithiocar-
bamates has not been investigated.25 To gain insights into the
process, we have isolated and identified the thermolysis
products from one amidinium dithiocarbamate, tC4-t-buty-
lamine-CS2. From them, it is possible to infer a plausible
mechanistic pathway for the thermally induced reactions
(Scheme 2).26 Reaction commences with formation of a
covalent bond between the amidinium carbon atom and a
sulfur from the dithiocarbamate followed by (or synchro-
nous with) addition of a proton from the amine moiety of the
Adding CS2 to a primary amine is known to yield
-
ammonium dithiocarbamate salts (RNHCS2 H3NþR).12b
Given our simple addition method for preparing the D-A-S,
some ammonium dithiocarbamate salts could conceivably be
present as well.21 To test this possibility, a sample of hexy-
lammonium dithiocarbamate was made. To it was added 1
equiv. of C6 amidine, and 0.5 equiv. of CS2 (to obtain the
correct stoichiometry for a D-A-S) were added to the hex-
ylammonium dithiocarbamate in a CDCl3 solution. 1H and
13C NMR spectra recorded before and after the addition of
the amidine and CS2 (see Figure S11 in the Supporting
Information) show that the ammonium dithiocarbamate
was transformed completely (within the limits of detection)
to another species. On the basis of the identical nature of the
NMR spectra of that species and those of C6-hexylamine-
CS2, it is the D-A-S.22 This experiment demonstrates that the
thermodynamically less favored ammonium dithiocarba-
mate (N. B., the amidines are much more basic than the
amines employed in this study23), if formed kinetically to any
extent, equilibrates to D-A-S rapidly when CS2 is added.
We selected a higher boiling amine, IleC8, for TGA
studies. On the basis of the data in Figure 3a, the D-A-S
(21) In fact, the purity of the D-A-S appears to depend upon the purity
of the amidine, amine, and CS2 reagents as well as the accuracy with
which each can be weighed. If slightly more or less than the exact
molar amount of any of the 3 components is added to the mixture,
the IL will contain a small amount of another species. In our
experience, making different batches of on D-A-S, the very small
amounts of the extraneous species (which must be present to some
extent), have no discernible influence on the properties of the IL.
(22) In Figure S11a, the 3 protons from RNH3 at 7.9 ppm and the single
proton from RNHCO2 at 7.75 ppm of the ammonium dithiocar-
bamate were located by adding a small drop of D2O to the sample
tube; only those signals disappeared. Thus, the loss of the 7.9 ppm
peak and the shift of the 7.75 ppm peak after the addition of
amidine and CS2 have been used as the principal identification tools
for the transformation of the ammonium dithiocarbamate to the
amidinium dithiocarbamate, given that the eventual spectra
matched those of the directly prepared D-A-S.
(24) Schroeder, D. C. Chem. Rev. 1995, 55, 181–228.
(25) The closest study we have found is between hindered isonitriles and
thioacids: Stockdill, J. L.; Wu, X.; Danishefsky, S. J. Tetrahedron
Lett. 2009, 50, 5152–5155.
(26) tC4 (which lacks a methyl group at the amidine carbon) and t-
butylamine were selected as substrates. These molecules simplify
the spectral identification of the thermolysis products. As indicated
by the proposed mechanism in Scheme 2, these substrates should
react like the others in Scheme 1.
(23) Ishikawa, T., Ed. Superbases for Organic Synthesis; Wiley: Chippenham,
U.K., 2009; pp 20-31.

5496 Chem. Mater., Vol. 22, No. 19, 2010
Yu et al.
Figure 3. (a) TGA curves of C8/IleC8 (gray) and C8-IleC8-CS2 (black). (b) Isothermal TGA curve of C8-IleC8-CS2 at 25 °C for 60 min.
Scheme 2. Thermolysis of tC4-t-Butylamine-CS2
Acid treatment of dithiocarbamates is known to result in
formation of ammonium ions and CS2.13 Depending on the
pKB of the parent amine, acid-derived decomposition of
alkyl dithiocarbamates can occur by two different mecha-
nisms.29 For parent amines with pKB > 10.5, the decom-
positions occur by a concerted intramolecular S-to-N hydro-
gen transfer with C-N bond cleavage. When pKB < 10.5,
alkyl dithiocarbamates decompose via zwitterionic inter-
mediates whose N-protonation is slower than the C-N
bond breaking (eq 2). The same mechanism should obtain
when a D-A-S is treated with an acid (such as dilute aqueous
acetic acid). Evidence for that chemistry, shown in eq 3,
comes from 13C NMR spectroscopy (see Figure S3 in the
Supporting Information).
dithiocarbamate (blue) to one of the nitrogen atoms of the
amidine. Note that two parallel pathways ensue based on
which of the amidine nitrogen atoms is the proton acceptor.
Some of these steps are reminiscent of the reactions be-
tween carbodiimides and carboxylates in Merrifield peptide
syntheses.27 In Scheme 2, cleavage of the original C-S
bond of the CS2 molecule results in an isothiocyanate (V)
and a zwitterionic species (III or IV), which cleaves into
a thioformamide (VI or VII) and an amine. Cleavage of
the original C-S bond of the CS2 molecule results in V
and a zwitterionic species (III or IV), which cleaves into
VI or VII and an amine. The resulting amines can then
react with V, generating N-tert-butyl-N0,N0-dimethyl-
thiourea (VIII) or N,N0-di-tert-butylthiourea (IX).28 The
proposed pathways are consistent with the presence of
dimethylamine and t-butylamine, which were captured as
their hydrochloride salts (see the Supporting Information).
Furthermore, the relative yields ofVIII and IXsuggest that
the thermolysis prefers slightly the pathway through inter-
mediate I.
3.4. Conductivity Measurements. The conductivity (σ)
of some of the D-A-S and their mixtures with CHCl3 was
investigated at room temperature. The effect on conductivity
of the high ion concentrations upon adding CS2 to D/A
combinations were offset to a large extent in the neat RTILs
by large viscosity increases (Table 2). However, conductiv-
ities increased significantly when CS2 was added to D/A
mixtures in 50 wt % CHCl3 solutions. The increases were
slightly smaller than observed when CO2 was added to either
neat or chloroform solutions of a D/A.6c We attribute these
observations to the higher viscosities of the dithiocarbamates
(vide infra), and less ion pairing30 in the carbamates.
(29) (a) Humeres, E.; Debacher, N. A.; Sierra, M.; Franco, J. D.;
Schutz, A. J. Org. Chem. 1998, 63, 1598–1603. (b) Humeres, E.;
Debacher, N.; Sierra, M. M. S. J. Org. Chem. 1999, 64, 1807–1813.
(c) Humeres, E.; Debacher, N.; Franco, J. D.; Lee, B. S.; Marten-
dal, A. J. Org. Chem. 2002, 67, 3662–3667.
(30) (a) Jarosik, A.; Krajewski, S.; Lewandowski, A.; Radzimski, P. J.
Molecular Liquids 2006, 123, 43–50. (b) Vila, J.; Gines, P.; Rilo, E.;
Cabeza, O.; Varela, L. M. Fluid Phase Equilib. 2006, 247, 32–39.
(27) (a) Merrifield, R. B. J. Am. Chem. Soc. 1963, 85, 2149–2154.
(b) Atherton, E.; Sheppard, R. C. Solid Phase Peptide Synthesis:
A Practical Approach; Oxford University Press: Oxford, U.K., 1989.
(28) Custelcean, R.; Gorbunova, M. G.; Bonnesen, P. V. Chem.;Eur.
J. 2005, 11, 1459–1466.

Article
Chem. Mater., Vol. 22, No. 19, 2010 5497
Figure 4. (a) Conductivities of C8/hexylamine mixtures at room temperature as a function of mol % substrate before (2) and after adding 1 equiv. of CO2
(O) and CS2 (b). (b) Conductivities of neat C8-hexylamine-CS2 (b) and DBU-LeuOH-CS2 (O) at various temperatures.
Table 2. Conductivities before (b) and after (a) adding CS2 to Neat and 50
wt % CHCl3 Solutions of Neat D/A Combinations at Room Temperature
conductivity (μs/cm)
triatomic
molecule
a
(neat)
a (50 wt %
CHCl3)
mol % of
D-A-S
D/A
b
C8/hexylamine
C8/hexylamine
DBU/LeuOH
DBU/LeuOH
CS2
CO2
CS2
CO2
22
22
31
31
39
58
46
54
265
276
198
214
24.1
25.8
25.7
27.7
As shown in Figure 4, for example, the conductivity of
neat C8/hexylamine changed very little as it was diluted with
CHCl3. After adding CS2, the conductivity of the C8-
hexylamine-CS2 only doubled, but it increased further as
small increments of CHCl3 were added as a diluent, reached
a maximum at ca. 10 mol %31 (ca. 20-30 wt % of C8-
hexylamine-CS2), and decreased at still lower concentrations
(higher dilutions) in CHCl3 (Figure 4a). As mentioned, the
rise in conductivity upon initial addition of CHCl3 is attrib-
uted to a convolution of decreasing viscosity (i.e., higher ion
mobility) and less ion pairing (i.e., leading to an increase in
the concentration of free ions).32 Eventually, the decreased
viscosity and ion pairing that accompany the addition of
more CHCl3 are offset by the lower concentration of mobile
ions, and the conductivity decreases. Increased conductivity
with decreasing viscosity is also seen in neat D-A-S as
temperature is raised (Figure 4b).33
A Walden plot34 has been used to evaluate further the ion
mobilities of the ionic liquids. It is based on Walden’s obser-
vation for aqueous solutions of strong electrolytes that the
equivalent conductivity of electrolytes (i.e., the conductivity
per mole of charge), Λ = σVE (where VE is the volume con-
taining one Faraday of positive charge) is large, depends
inversely on viscosity, and changes with temperature in
parallel with the inverse of viscosity. According to Walden’s
Rule, Λ η = k, η is the viscosity, and k is a temperature-
dependent constant. From the Walden plot in Figure 5, C4-
hexylamine-CS2 can be classified as a “good ionic liquid”
Figure 5. Log-log plot of equivalent conductivity (Λ) versus fluidity
(η-1) for C4-hexylamine-CS2. The straight line is determined by data for
1 M aqueous KCl.36
because it lies very close to the ideal line described by aqueous
KCl.33,35 However, because of their relatively high visco-
sities (vide infra), the ammonium dithiocarbamates exhibit
lower conductivities than many other classes of ionic
liquids.33
3.5. Miscibility and Polarity. Initially, the miscibilities
of the samples in their D/A and D-A-S states were deter-
mined empirically as a measure of their polarity.37 Before
adding CS2, D/A mixtures with all solvents examined re-
sulted in solutions (Table 3). Similar to many other types
of classical RTILs, including amidinium carbamates,6c our
D-A-S phases are miscible with very polar solvents, such as
DMSO and ethanol, but not with lower polarity solvents,
such as hexane, toluene, and diethyl ether. Furthermore, the
length of the Cn amidine alkyl chain appears to influence
somewhat the miscibility behavior: C8-hexylamine-CS2 is
partially miscible with diethyl ether and C6-hexylamine-CS2
or C4-hexylamine-CS2 is immiscible with it.
Also, the solvatochromic dye, DAPNE (1-(p-dimethyl-
aminophenyl)-2-nitroethylene),38 was utilized to estimate
the polarity of the D/A and D-A-S. For example, λmax
=
423 nm for C8-hexylamine and 438 and 440 nm for C8-
hexylamine-CO2 and C8-hexylamine-CS2, respectively (see
Table S1 in the Supporting Information); the λmax of toluene
(31) The concentrations are expressed in mol % in order to normalize
the concentrations of ionic liquids and amidine/amine.
(32) Fraser, K. J.; Izgorodina, E. I.; Forsyth, M.; Scott, J. L.; MacFarlane,
D. R. Chem Commun. 2007, 37, 3817–3819.
(36) Angell, C. A.; Byrne, N.; Belieres, J.-P. Acc. Chem. Res. 2007, 40,
1228–1236.
(37) (a) Crosthwaite, J. M.; Aki, S. N. V. K.; Maginn, E. J.; Brennecke,
J. F. J. Phys. Chem. B 2004, 108, 5113–5119. (b) Domanska, U.;
Casas, L. M. J. Phys. Chem. B 2007, 111, 4109–4115.
(38) (a) Reichardt, C. Chem. Soc. Rev. 1992, 21, 147–153. (b) Reichardt,
C. Chem. Rev. 1994, 94, 2319–2358. (c) Richter-Egger, D. L.;
Tesfal, A.; Flamm, S. J.; Tucker, S. A. J. Chem. Educ. 2001, 78,
1375–1378.
(33) Xu, W.; Cooper, E. I.; Angell, C. A. J. Phys. Chem. B 2003, 107,
6170–6178.
(34) (a) Walden, P. Z. Phys. Chem. 1906, 55, 207. (b) MacFarlane,
D. R.; Forsyth, M.; Izgorodina, E. I.; Abbott, A. P.; Annata, G.;
Frasera, K. Phys. Chem. Chem. Phys. 2009, 11, 4962–4967.
(35) Greaves, T. L.; Weerawardena, A.; Fong, C.; Krodkiewska, I.;
Drummond, C. J. J. Phys. Chem. B 2006, 110, 22479–22487.

5498 Chem. Mater., Vol. 22, No. 19, 2010
Yu et al.
Table 3. Miscibility Behaviora of Various Mixtures of 50 vol % D-A-S Ionic Liquids and Organic Solvents at Room Temperature
solvent
hexane
toluene
diethyl ether
CH2Cl2
ethyl acetate
ethanol
DMSO
C4-hexylamine-CS2
C6-hexylamine-CS2
C8-hexylamine-CS2
C4-IleC8-CS2
C6-IleC8-CS2
C8-IleC8-CS2
I
I
I
I
I
I
I
I
I
I
I
I
I
I
P
I
P
P
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
a I = immiscible, M = miscible, P = partially miscible.
Figure 6. (a) Viscosities (Pa s) at 25 °C of neat Cn/cyclohexylamine before (white) and after addition of CO2 (gray) or CS2 (black). (b) Viscosities (Pa s) of
C8-cyclohexylamine-CS2 (white) and C8-IleC8-CS2 (black) at different temperatures.
and N,N-dimethylformamide are 425 and 446 nm, respec-
tively.38 According to this probe, the D/Aare much less polar
than their RTILs, whether they be generated by addition of
CS2 or CO2, but are less polar than many imidazolium-based
ILs.39
25 °C (43 Pa s), which is also lower than the viscosity of
C8-IleC8-CO2 (7.1 Pa s) at 25 °C.
3.7. Comparisons of Phase Types from Dithiocarba-
mate and Carbamate Analogues. Comparisons of the
structural differences between amidinium dithiocarba-
mates and amidinium carbamates offer a means to gain
insights about how they support formation of RTILs.
The data in Table 4 demonstrate that the appearances of
some of the D/A ionic liquids depend on whether CO2 or
CS2 is added. For example, DBU-hexylamine-CO2 is a
liquid, whereas DBU-hexylamine-CS2 is a solid. Ob-
viously, these differences are a manifestation of the
properties of the carbamate and dithiocarbamate anions.
To some extent, those properties can be related to the
physical properties of the triatomic molecules from which
the anions derive, CO2 and CS2. Both are linear molecules
and have zero dipoles.42 However, they possess large qua-
drupole moments: for CO2, 3.00 ꢀ 10-26 esu cm2; for CS2,
1.84 ꢀ 10-26 esu cm2.43 In addition, both the bond lengths
3.6. Viscosity Measurements. Consistent with their
CO2 analogues, the D/A and D-A-S exhibit viscosities
that are virtually independent of shear rate; they behave
like Newtonian liquids (see Figure S4 in the Supporting
Information).40 As shown in Figure 6a, the average
viscosity over the range of shear stress values for C6/
cyclohexylamine is 0.010 Pa s at room temperature. After
exposure to CO2, the viscosities of these samples increase
by orders of magnitude to 3.8 Pa s (C6-cyclohexylamine-
CO2), but the viscosity of the corresponding C6-cyclo-
hexylamine-CS2 is much higher still, ca. 19 Pa s. The
higher viscosities of the dithiocarbamates can be ex-
plained on the basis of their higher polarizabilities and
hard-soft/acid-base41 (HSAB) theory: dithocarbamate
is a softer base than a carbamate and amidinium is a soft
acid. Also, the viscosities increase with increasing chain
length of the Cn: for example, C4-cyclohexylamine-CS2,
C6-cyclohexylamine-CS2, and C8-cyclohexylamine-CS2
˚
and kinetic diameter of CS2 (bond length 1.55 A and
˚
diameter 4.48 A) are larger than those of CO2 (1.16 and
42
˚
3.30 A).
As mentioned above, whereas the amidinium dithio-
carbamates cannot be heated to expel CS2 and regenerate
the nonionic D/A states, gentle heating or passing nitro-
gen gas through an amidinium carbamate does result in
the loss of CO2. This grossly different chemical behavior
can be understood from the enthalpies of formation of
dithiocarbamates and carbamates. The heats of forma-
tion ΔHf are -393.5 kJ mol-142 for CO2 and 115.7 kJ
mol-1 for CS244 in the gas phase. Thus, it is expected from
have viscosities 8.6, 19, and 69 Pa s, respectively. As
3
expected, the viscosities of the amidinium dithiocarba-
mate ionic liquids decrease significantly as temperature is
increased within the range below their decomposition
temperatures (Figure 6b): the viscosity of C8-IleC8-CS2
at 45 °C is 1.6 Pa s, almost 25 times lower than that at
(39) (a) Anderson, J. L.; Ding, J.; Welton, T.; Armstrong, D. W. J. Am.
Chem. Soc. 2002, 124, 14247–14254. (b) Carmichael, A. J.; Seddon,
K. R. J. Phys. Org. Chem. 2000, 13, 591–595.
(40) Khan, S. A.; Royer, J. R.; Raghavan, S. R. Aviation Fuels with
Improved Fire Safety: A Proceeding; National Academy Press:
Washington, D.C., 1997; pp 31-46.
(41) (a) Pearson, R. G. J. Am. Chem. Soc. 1963, 85, 3533–3543.
(b) Chattaraj, P. K.; Lee, H.; Parr, R. G. J. Am. Chem. Soc.
1991, 113, 1855–1856.
(42) Lide, D. R., Ed. CRC Handbook of Chemistry and Physics; 90th ed.;
CRC Press: Boca Raton, FL, 2009.
(43) Gupta, A. D.; Singh, Y.; Singh, S. J. Chem. Phys. 1973, 59, 1999–
2006.
(44) ΔHfo (gas) = ΔHfo (liquid) þ Δvap Hfo (tb) = 89.0 þ 26.7 = 115.7
kJ mol-1. See ref 42, pp 5-20 and 6-117.

Article
Chem. Mater., Vol. 22, No. 19, 2010 5499
Table 4. Phasesa of Neat 1/1 (mol/mol) D/A Mixtures at Room Temperature after Treating with CO2 or CS2
amine (A)
isoleucine octyl ester (IleC8)
leucinol (LeuOH)
hexylamine
cyclohexylamine
6c
CO2
6b
CO2
6a
6a
CO2
amidine (D)
CS2
CS2
CO2
CS2
CS2
C6
C8
DBU
L
L
L
L
L
L
L
L
L
L
L
S
L
L
L
L
L
S
S
S
L
L
L
S
a L = liquid, S = solid.
the Hammond Postulate that the enthalpy of formation
of a dithiocarbamate will be more favored energetically,
although reactions of amines with both CO2 and CS2 are
exothermic. These results indicate that the addition of
CO2 and CS2 can tune the physicochemical properties of
the ILs and RTILs, although they may do so in subtly
different ways.
nonionic D/A mixtures. However, as noted by miscibility
studies, the polarities of the dithiocarbamates do change
subtly when the chain length of the Cn amidine or the
amine structure is changed. As expected, the conductiv-
ities of the structurally analogous neat dithiocarbamates
and carbamates are very similar, they remain similar at
the same dilution factors in chloroform, and they are
significantly higher than the conductivities of their D/A
component mixtures.
4. Conclusions
Equimolar mixtures of an amidine and an amine can be
transformed easily into a new class of room-temperature
ionic liquids, amidinium dithiocarbamates, upon addi-
tion of one molar equivalent of CS2. The amidinium
dithiocarbamates are more stable thermally than their
amidinium carbamate analogues, made by adding an-
other triatomic molecule, CO2, to amidine/amine mix-
tures. In general, the dithiocarbamates are stable to
higher temperatures than their amidine-amine mixtures,
even in air. Unlike their amidinium carbamate analogues
(which lose a molecule of CO2 near their decomposition
temperatures, ca. 50 °C under an atmosphere of CO2), the
amidinium dithiocarbamates do not lose a molecule of
CS2 when heated above their decomposition tempera-
tures, ca. 80 °C. Instead, they undergo a series of struc-
tural changes that lead to several products, including
thioureas and thioformamides, which provide insights
into the thermolysis mechanism. However, CS2 can be
expelled from the amidinium dithiocarbamates in the
presence of a weak acid.
The thermal, polar, viscous, and conductance proper-
ties of the liquid amidinium dithiocarbamates have been
characterized and compared with both their amidine-
amine precursor mixtures and with the analogous
amidinium carbamates. The results show that these phy-
sical and chemical properties can be tuned by changing
the D/A combinations and the nature of the triatomic
molecule added. Viscosities of the Cn-amine-CS2 systems
increase as the amidine chain length n increases and are
more viscous than the corresponding Cn-amine-CO2
RTILs. Although the C4 amidine provided RTILs when
combined with CS2 and all of the amines examined in this
work, only the amidinium dithiocarbamates composed of
an amino alcohol were liquids at room temperature when
the amidine was the bulky and short-chained guanidine
TMG.
Attractive attributes of the amidinium dithiocarbamate
ILs and RTILs (when compared to many other types of
ionic liquids) include their ease of preparation (simple in
situ mixing of commercially available or easily synthe-
sized precursors), their diversity of structures, and their
broad temperature ranges (commencing at subambient
temperatures). This work provides a facile, one-step
approach to utilize CS2, an environmentally unattractive
liquid, to prepare useful and very attractive chemical
materials by very simple manipulations and reactions.
These ILs and RTILs are expected to find extensive use as
media for a broad range of guest molecule reactions,
including some that involve the creation of chiral centers
or the recuperation of expensive metal catalysts.45 Future
research will focus on such applications.
Acknowledgment. We are grateful to the National Science
Foundation for its financial support of this research. We also
acknowledge Prof Daniel L. Blair of the Department of
Physics, Georgetown University, for the use of his rheometer
and aid in collecting the viscosity data and Prof. Judith
(Faye) Rubinson of the Department of Chemistry, George-
town University, for helpful discussions about Walden plots.
Supporting Information Available: Materials, instrumenta-
tion, and sample preparation methods, IR spectra, NMR
spectra, density investigations, viscosities and conductivities at
different temperatures, a Walden plot and photographs of a D/A
and D-A-S, UV-vis λmax values of DAPNE in various solvents,
and viscosities of some D/A and D-A-S as a function of shear
stress, characterizations of thermolysis products, experimental
details of transformation from ammonium dithiocarbamates to
amidinium dithiocarbamates (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
(45) (a) Muldoon, M. J. Dalton Trans. 2010, 39, 337–348. (b) Cole-
Hamilton, D. J.; Tooze, R. P., Eds. Catalyst Separation, Recovery
and Recycling, Chemistry and Process Design; Springer: Dordrecht,
The Netherlands, 2006. (c) Cornils, B.; Herrmann, W. A.; Horvath, I. T.;
Leitner, W.; Mecking, S.; Olivier-Bourbigou, H.; Vogt, D., Eds. Multi-
phase Homogeneous Catalysis; Wiley-VCH: Weinheim, Germany,
2005. (d) Benaglia, M., Eds. Recoverable and Recyclable Catalysts;
Wiley: Weinheim, Germany, 2009.
According to the spectroscopic probe DAPNE, the
dithiocarbamates and carbamates have similar polari-
ties and are much more polar than their corresponding