Reversible zwitterionic liquids, the reaction of alkanol guanidines, alkanol amidines, and diamines with CO2

Article abstract of DOI:10.1039/b924790d

Alkanolamidines, alkanolguanidines and diamines each react with CO ₂ to form reversible zwitterionic liquids. CO₂ is chemically bound to the alcohol on alkanolamidines and alkanolguanidines as zwitterionic alkylcarbonates, while CO₂ is chemically bound on diamines as zwitterionic carbamates. All three classes of zwitterionic liquids could be reverted to their non-ionic forms by thermally stripping the CO ₂ at temperatures near 50 °C, while one derivative was found to irreversibly form a cyclic carbonate.

Full text of DOI:10.1039/b924790d

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Reversible zwitterionic liquids, the reaction of alkanol guanidines, alkanol
amidines, and diamines with CO2
David J. Heldebrant,*a Phillip K. Koech,a M. Trisha C. Ang,b Chen Liang,b James E. Rainbolt,a
Clement R. Yonkera and Philip G. Jessopb
Received 25th November 2009, Accepted 22nd January 2010
First published as an Advance Article on the web 3rd March 2010
DOI: 10.1039/b924790d
Alkanolamidines, alkanolguanidines and diamines each react with CO2 to form reversible
zwitterionic liquids. CO2 is chemically bound to the alcohol on alkanolamidines and
alkanolguanidines as zwitterionic alkylcarbonates, while CO2 is chemically bound on diamines as
zwitterionic carbamates. All three classes of zwitterionic liquids could be reverted to their
non-ionic forms by thermally stripping the CO2 at temperatures near 50 ◦C, while one derivative
was found to irreversibly form a cyclic carbonate.
would also serve as a switchable solvent and CO2BOL. These
Introduction
molecules could be an amidine–alcohol, a guanidine–alcohol,
Carbon dioxide chemistry is increasingly relevant to real-world
or a diamine (eqn (3) and (4)). The key differences in such
issues, thanks to global warming. Key chemistry issues currently
switchable solvents compared to the earlier examples is that
being studied are the capture, storage, and utilization of CO2.
the polar, CO2-containing form would be zwitterionic. If the
While the relevance of capture and storage are obvious, the rel-
reaction with CO2 is reversible and the zwitterions are liquids,
evance of CO2 utilization is less clear. Although CO2 utilization
then these liquids could offer advantages over the previously-
is unlikely to consume significant quantities of CO2, it can be a
reported switchable solvents including the ease of use associated
significant strategy for the development of sustainable processes.
with having a single liquid component solvent rather than a
As part of our research efforts towards CO2 utilization, some of
mixture, and possibly an enhanced polarity difference between
us invented switchable solvents, meaning solvents that can switch
the ionic and non-ionic forms. There are very few reports of non
reversibly from one version to another.1 Our original version was
switchable zwitterionic liquids in the literature,6-10 and none that
a mixture of an amidine and an alcohol (eqn (1), where B is a
we could find on their polarity, so it was difficult to predict the
liquid amidine), but since then we and others have developed
polarity of these proposed switchable zwitterionic liquids. The
others such as guanidine/alcohol mixtures (eqn (1), where B
zwitterionic switchable solvents could also have advantages for
is a liquid guanidine),2 amidine/primary amine mixtures,3 and
CO2 capture in that the gravimetric CO2 capacity may be higher
secondary amines (eqn (2)).4 Switchable solvents of these types
than in the two-component systems.
all convert from a low-polarity non-ionic liquid to a high-
It is important not to confuse these proposed zwitterionic
polarity ionic liquid form upon exposure to an atmosphere of
liquids with the mythical amidine–CO2 zwitterionic adducts.
CO2 and revert back to the low polarity form when the CO2 is
Some groups have proposed zwitterionic adducts of amidines
removed by heat or flushing with inert gas. We also demonstrated
and guanidines with CO2 in which the basic nitrogen atom
that these switchable solvents can be used as reversible CO2-
binds directly to the CO2 carbon. However such adducts have
binding organic liquids (CO2BOLs) for CO2 capture, and are
never been isolated or even observed spectroscopically; reports
more energy-efficient in that role than aqueous solutions of
ethanolamine.5
of spectroscopic detection of such adducts in the past have been
erroneous due to confusion with the bicarbonate salts, a result
of failure to rigorously remove water from the system before
study.11-15 The zwitterions we report in this paper are entirely
different in structure, having the CO2 coordinated to an alkoxide
oxygen (eqn (4)).
(1)
(2)
Because these switchable solvents consist of a CO2-capturing
group (an alcohol in eqn (1) and one equivalent of amine in eqn
(2)) and a base (B in eqn (1) and the second equivalent of amine
in eqn (2)), we conjectured that a single molecule containing
both a CO2-capturing group and a basic functional group
(3)
(4)
To our knowledge, the only known switchable zwitterionic
liquid was developed in our laboratory.16 Our first generation
switchable zwitterionic liquid was an alkanolamine, which
reversibly captured SO2 as an alkylsulfite salt. We hypothesized
aPacific Northwest National Laboratory, Richland, WA, USA 99352.
E-mail: david.heldebrant@pnl.gov
bDepartment of Chemistry, Queen’s University, Kingston, ON, Canada
K7L 3N6
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Table 1 NMR chemical shifts of 1 in CDCl3 with and without CO2
1H
1 (OH)
2
3
4
5 (NH)
6
8, 10
1a
1b
2.8 (s)
—
3.9 (t)
4.2 (t)
1.6 (m)
1.8 (m)
3.4 (t)
3.7 (t)
—
—
—
2.6 (s), 2.7 (s)
2.8 (s)
6.0–7.0 (s)
13C
1 (CO2)
2
3
4
5
6
8, 10
1a
1b
—
152.2
62.0
66.3
23.3
23.3
42.4
42.5
—
—
158.9
161.3
35.7, 37.0, 39.8
36.9, 39.7
Fig. 1 Synthetic route to 1a n = 0, 2a n = 4, 3a n = 9.
that a similar switchable zwitterionic liquid could be made with
CO2 if a sufficiently strong organic base such as a guanidine or
amidine was paired with an alcohol in a single molecule (eqn
(1), where “B” represents a basic group such as an amidine or
guanidine). Alternatively, we also thought that a diamine, where
one or both of the N atoms are secondary amines, would also
react with CO2 to give a zwitterionic liquid (eqn (3), where B¢
represents a basic group such as an amine). Such systems could
serve as either reversible CO2-capturing agents or switchable
solvents. We now report the preparation and study of these
switchable zwitterionic liquids.
1,1,3,3-tetramethylguanidine (TMG) as our first substrate base
because of its ease of functionalization.
Our first target molecules were alkanolguanidines (com-
pounds 1a, 2a, and 3a in Scheme 1) which were prepared from
the reaction of 1,1,3,3-tetramethylguanidine with trimethylsilyl-
protected bromoalkanols (Fig. 1). The resulting silyl-protected
alkanolguanidines were then cleaved with K2CO3 in methanol.
Isolated compound 1a was a clear and colorless non-viscous
liquid. Spectroscopiccharacterization of 1a was performedusing
both 1H and 13C NMR in CDCl3 (Table 1). The 1H NMR of 1a
in CDCl3 showed two inequivalent CH3 peaks (carbons 8 and 10
using the numbering scheme in Fig. 2) at 2.6 and 2.7 ppm which
=
we attribute to the lack of free rotation around the C N double
Results and discussion
bond. Two triplets were observed at 3.9 and 3.4 ppm, which we
assign to the CH2 carbons 2 and 4 respectively. Carbon 3 was
visible at 1.6 pm as amultiplet andthe alcohol O–H was observed
at 2.8 ppm. The infrared (IR) spectrum of 1a showed a broad
We set out to synthesize a series of compounds (Scheme 1)
that could react with CO2 to make CO2-activated switchable
zwitterionic liquids.
O–H band at 3246 cm-1 as well as strong (C N) absorption
=
bands at 1654 and 1618 cm-1.
Scheme 1 Candidate precursors for preparing switchable zwitterionic
liquids.
Fig. 2 Numerical assignment of 1a.
The reaction of anhydrous 1a with CO2 lead to the formation
of a highly viscous hazy liquid (1b), which we assigned the
zwitterion structure (eqn (5)). The characteristic increase in
viscosity has been observed when mixtures of amidines or guani-
dines and alcohols react with CO2 to form their corresponding
alkylcarbonate salts.1,2,4,5,17,18 The zwitterion 1b was soluble in
common organic solvents such as CDCl3 and MeCN and
was characterized by NMR. The H NMR spectrum (CDCl3)
showed no evidence of the free alcohol OH. The spectrum also
showed downfield shifting of the R–CH2–O protons from 3.9 to
(a) Alkanol-guanidines
Our recent interest in CO2BOLs1,2,5,17,18 led us to explore the
functionalization of an amidine or guanidine with an alcohol to
make a single component CO2BOL. Combining the alcohol and
the base components on one molecule (similar to alkanolamines)
would simplify the binary alcohol/base solvent mixture into
a unimolecular system, and potentially decrease component
volatility through hydrogen bonding while increasing CO2
weight capacities by using shorter alcohol chains. We chose
1
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Table 2 NMR chemical shifts of 2a in CDCl3 with and without CO2
1H
1 (OH)
2
3
4
5 (NH)
6
8, 10
2a
2b
(s)
—
3.5 (t)
3.8 (t)
1.5, 1.7
1.3, 1.5, 1.6
3.1 (t)
3.7 (t)
—
8.8 (s)
—
—
2.5, (s) 2.7 (s)
2.8 (s)
13C
1 (CO2)
2
3
4
5
6
8, 10
2a
2b
—
158.6
62.9
65.1
23.3
23.3
42.4
42.5
—
—
160.8
163.2
35.7, 37.0, 39.8
36.9, 39.7
Table 3 NMR chemical shifts of 3a in CDCl3 with and without CO2
1H
1 (OH)
2
3
4
5 (NH)
6
8, 10
3a
3b
—
—
3.6 (t)
3.8 (t)
1.5 (m) 1.3 (s)
1.2 (s) 1.5 (m) 1.6 (m)
3.1 (t)
3.1 (t)
—
—
—
—
2.5, (s) 2.7 (s)
2.9 (s) broad
13C
1 (CO2)
2
3
4
5
6
8, 10
3a
3b
—
158.9
62.8
64.9
26.0, 27.6, 29.7,32.9
25.9–32.9
49.7
45.5
—
—
160.4
161.6
39.0, 39.8
40.0
4.2 ppm indicating that CO2 was chemically bound through
the alcohol. The spectrum showed a broad singlet centered
=
at 6.5 ppm corresponding to the guanidinium C N–H. The
viscous liquid could not be a bicarbonate salt of 1 because water
was not present. If 1b was a bicarbonate, there would be no
downfield shifting of the alcohol CH2O protons. The neat 13C
spectrum of 1b showed the guanidinium carbon at 161.3 ppm as
well as the alkylcarbonate peak at 152.2 ppm. The guanidinium
and carbonate peaks had the same integrated area, indicating
Fig. 3 The proposed structure of the cyclic carbonate product of 1b
heated above 25 ◦C.
1
stoichiometric absorption of CO2. The H and 13C chemical
carbon at 125 ppm, the CH2–OCO2 carbonate at 66 ppm, and
shifts are in agreement with our previous studies of amidinium
and guanidinium alkylcarbonates.1,2,5,17,18,19
1
the H spectrum showed aliphatic CH2–OCO2– hydrogens at
4.1 ppm and CH2–N hydrogens at 3.3 ppm. The CH3 hydrogens
were observed as an equivalent singlet at 2.7 ppm, and the
internal R–CH2–R hydrogens were observed as a pentet at
1.7 ppm. The cyclic carbonate is a non-viscous liquid that boils
at 100 ◦C and does not release CO2. The cyclic carbonate is
likely forming because of the length of the propanol chain of
the alcohol allowing the carboxylate to be in proximity to form
a thermodynamically favorable 8-membered ring. Compound
1a is not a useful reversible zwitterionic liquid because of its
inability to release CO2.
(5)
The neat infrared (IR) spectrum of 1b on NaCl plates showed
strong bands at 1653 cm-1 and 1615 cm-1, which are comparable
We hypothesized that a longer alkyl spacer between the
alcohol and guanidine would inhibit cyclization, so we syn-
thesized two longer chained TMG derivatives (compounds 2a
and 3a in Scheme 1 and Fig. 4). Isolated compounds 2a and
+
=
to other protonated amidine and guanidine imines [C N–H]
such as [DBUH]Cl (1645 cm-1)1 and [DBUH][OCO2(CH2)5CH3]
(1648 cm-1).2 New strong bands appeared at 1590 cm-1,
1403 cm-1, and 1275 cm-1 which we assign to the (C O) of the
=
1
3a were both hazy white non-viscous liquids. The H NMR
carbonate. We also observed medium intensity N–H stretching
bands at 3160 cm-1. The free O–H of 1a was undetectable when
CO2 was bound.
spectra of compounds 2a and 3a (Tables 2 and 3) were nearly
identical to the compound 1a, with the only difference being the
Gravimetrically, 0.502 grams (3 mmol) of 1a absorbed 0.130
grams (3 mmol) of CO2, corresponding to 20.6% CO2 by weight.
The zwitterion 1b did not off-gas, rather it slowly cyclized
into a cyclic carbonate. All attempts to thermally release CO2
were unsuccessful because of condensation of the carboxylate
to the activated imine carbon (6) to produce the cyclic ester
(Fig. 3). The cyclic ester 13C spectrum (CDCl3) showed a
carboxylate peak at 153 ppm, the N3C–O tetra-coordinated
Fig. 4 Numerical assignment of positions in compounds 2a and 3a.
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6 or 9 aliphatic carbons of the alcohol chains. The infrared
re-contacted with CO2 and then stripped for a total of three
cycles. Each absorption cycle, 2a bound 0.046 grams of CO2,
and each stripping released 0.046 grams of CO2.
(IR) spectrum of 2a and 3a showed broad O–H bands at
3356 cm-1 3381 cm-1 as well as strong (C N) absorption bands
=
at 1604 cm-1. and 1605 cm-1 respectively.
In our previous studies, dual component mixtures of TMG
and linear alcohols in MeCN were unable to capture more than
0.47 molar equivalents of CO2. Previous thermodynamic esti-
mates of the energetics of CO2 binding by mixtures of alcohols
and TMG showed CO2 binding was entropically unfavorable
(on average DH = -160 kJ mol-1 and DS= -530 J mol-1).5
The single component showed slightly similar thermodynamic
properties to our dual component mixtures of alcohols and
guanidines. We estimate values from the Van’t Hoff plot of the
integrations of the hydrogens of the bound (CH2–O–CO2) and
unbound (CH2–OH) in CDCl3 as a function of temperature to
be DH = -87.0 kJ mol-1 and DS= -293 J mol-1 for 2a and DH =
-87 kJ mol-1 and DS= -291 J mol-1 for 3a. In the zwitterionic ex-
periments, K is defined as [2b]/(PCO2 [2a]), which is different from
the K defined in our dual component systems. The estimated
thermodynamics from the single component systems are similar
to estimates of the dual component systems in that the binding of
CO2 is entropically controlled. This suggests that alkanolguani-
dines bind CO2 in a similar manner as dual component systems.
The entropically unfavored binding of CO2 makes TMG a
poor choice for a base component for switchable zwitterionic
liquids at STP, however they may be applicable in high pressure
applications. We are investigating the functionalization of an
amidine base like DBU with a hexanol chain, which we predict is
likely to be more thermodynamically favored and may bind up to
1 molar equivalent of CO2 at STP. The thermodynamic estimates
to date suggest that the limitation of CO2 binding is due to
the thermodynamics of the alkylcarbonate guanidinium binding
mode of CO2BOLs and not the carbamate-like binding mode of
alkanolamines.
Compound 3a converts to a slightly yellow waxy solid in
the presence of CO2 (3b, eqn (6)). The solid was soluble in
common organic solvents such as CDCl3 and MeCN and was
1
characterized by NMR. The H and 13C spectra of this solid
were similar to the propanol derivative 1b except that only 74%
CO2 was absorbed.
(6)
The IR spectrum of the crude 3b on NaCl plates showed what
appeared to be a mixture of carboxylated and non-carboxylated
-1
-1
=
TMG–(CH2)11–OH. New (C N) bands at 1695 cm , 1670 cm ,
-1
-1
-1
-1
=
1605 cm , and (C O) bands at 1591 cm , 1402 cm , 997 cm ,
and 833 cm-1 were observed for the carboxylated species.
N–H stretching bands were not detected because of strong
overlapping O–H bands at 3375 cm-1. The solid does not
melt, rather it decarboxylates completely at 65 ◦C. Attempts
to crystallize the solid only yielded a waxy or gel like solid,
which was not suitable for crystallographic analysis.
Our third target alkanolguanidine was
a
hexanol-
functionalized TMG. The isolated TMG–(CH2)6–OH 2a was
a clear and colorless non-viscous liquid. The 1H and 13C NMR
spectra (CDCl3) are presented in Table 3. The 1H and 13C NMR
spectra showed the same trends as the propanol and undecanol
derivatives. The infrared (IR) spectrum of 2a showed a broad
O–H band at 3356 cm-1 as well as strong (C N) absorption
=
bands at 1604 cm-1.
CO2 is bound through the alcohol moiety in alkanolguani-
dines in contrast to its binding to the amine of an alkanolamine
because of two factors: the reduced nucleophilicity of guanidines
compared to amines, and the higher basicity of amidines
and guanidines compared to amines. To our knowledge, there
are no published accounts of isolated carbaguanidinium or
carbamidinium zwitterions. We believe they are not isolable
because amidines and guanidines are weak nucleophiles and
weak hydrogen bond donors or acceptors20 and are unable to
react with CO2 at standard temperature and pressure unless
in the presence of a weak acid. The alcohol moiety of the
alkanolguanidine is more nucleophilic than the delocalized
guanidine component. Alcohols are known to react with CO2
to form thermodynamically unstable alkylcarbonic acids.21,22
Alkylcarbonic acids have been stabilized by the addition of
strong bases, such as DBU or TMG as is the case with
our previous systems.1,2,5,17,19 We previously hypothesized that
simple amines are not basic enough to accept the proton
from alkylcarbonic acids and do not form alkylcarbonates5 like
amidine and guanidine bases.
The reaction of anhydrous 2a with CO2 leads to the formation
of a highly viscous hazy liquid (2b, eqn (7)). The zwitterion was
found to be soluble in CDCl3 and MeCN as well as acetone.
Nile red indicator dye was found to be insoluble in 2a as well
as 2b, and no polarity data could be measured. No polarity
measurements were performed on 1a,b or 3a,b. Table 3 lists the
1
1H and 13C NMR spectra of the zwitterion in CDCl3. The H
and 13C chemical shifts showed the same trends as the propanol
and undecanol derivatives in the presence of CO2.
(7)
The IR spectrum of 2b on NaCl plates was a mixture of
carboxylated and non-carboxylated material. We observed a new
-1
-1
=
=
strong C N band at 1618 cm , strong C O bands at 1600 cm ,
1403 cm-1, and 1290 cm-1 and broad O–H stretching bands at
3379 cm-1. The N–H band was not detected because it was
dwarfed under the very broad and strong O–H peak of 2a.
Gravimetrically, 0.452 grams (2 mmol) of 2a absorbed 0.046
grams (1 mmol) of CO2, corresponding to 10.2% CO2 by
weight. The 0.5 molar equivalent of chemically bound CO2
were thermally stripped from 2b at 70 ◦C. The 2a was then
(b) Alkanol-Amidines
Amidines are slightly less basic than guanidines but are
still sufficiently basic to serve as the basic group in a
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(8)
zwitterionic liquid precursor. N¢-(2-Hydroxyethoxy)ethyl)-N,N-
dimethylacetamidine (compound 4a in Scheme 1 and Fig. 5)
was prepared from aminoethoxyethanol by treatment with
N,N-dimethylacetamide dimethylacetal and purified by vacuum
distillation. The colorless liquid 4a greatly increases in viscosity
when CO2 is bubbled through it. The product 4b has been
identified spectroscopically to be as shown in eqn (8).
Nile Red dissolved in neat 4a was 544 nm before exposure to
CO2, indicating a polarity comparable to 1-octanol. After CO2
was bubbled through the sample for 20 min at room temperature,
the absorbance shifted to 554 nm, indicating a polarity that is
greater than that of methanol. This amount of polarity change
is comparable to that observed with the DBU/n-propanol
switchable solvent,2 showing that the amidine–alcohol single-
component switchable solvent is just as effective as the DBU/n-
propanol two-component switchable solvent and that there is no
inherent advantage, in terms of increasing the size of the polarity
change, to using a switchable zwitterionic liquid rather than a
switchable ionic liquid.
Fig. 5 Numerical assignment of positions in compound 4.
(c) Diamines containing two secondary amines
Upon the reaction of 4a with CO2, the mass of the liquid phase
increases by 20%. The theoretical mass increase, according to the
stoichiometry of the equation and neglecting dissolved CO2, is
25%. This gravimetric evidence, as well as the NMR evidence,
show that the reaction is incomplete. 13C NMR spectra of a
solution of 4a in DMSO (1 : 1 mixture by volume (Table 4))
show that, as CO2 is bubbled through the solution, two small new
peaks appear at 160.7 and 155.9 ppm, in addition to the peak
for 4a at 160.4 ppm. After further CO2 treatment, the peaks
have shifted to 161.6 and 156.2 and have become larger than
the peak for 4a. We attribute the new peaks to the protonated
amidine group and the alkylcarbonate group of 4b, respectively.
In comparison, the alkylcarbonate peak of [DBUH][O2COR]
ionic liquid appears at 157–159 ppm depending on the alkyl
group.2 Heating the DMSO/4b mixture and bubbling argon
through it restored the 13C-NMR spectrum of 4a.
A dilute solution of 4a in DMSO failed to react significantly
with CO2, suggesting that DMSO as a solvent inhibits the
reaction. The same effect was observed for a dilute solution
of 4a in C6D6.
To determine the polarity change upon conversion from the
amidine alcohol to the zwitterionic liquid, the spectrum of a
solution of solvatochromatic dye Nile Red in liquid 4a was
obtained before and after treatment with CO2, The wavelength
of maximum absorbance (lmax) of a solution of Nile Red is
indicative of the polarity of the solvent.23 The lmax value of
Liquid secondary amines represent another kind of switch-
able solvent, wherein CO2 is bound directly to the nitrogen
atom in a carbamate salt rather than to an oxygen atom in
an alkylcarbonate as is the case for alkanolguanidines and
alkanolamidines.4 Amines such as N-ethyl-N-butylamine, N-
ethyl-N-propylamine, dipropylamine, and benzylmethylamine,
are low-polarity non-ionic liquids that are switched, by exposure
to CO2, into carbamate ionic liquids of increased polarity (eqn
(2)). By heating the viscous liquid to 60 ◦C and purging it with
N2, the liquid can switch back to the low polarity form. In most
cases, especially for primary amines, solid carbamate salts are
formed following the addition of CO2.
We speculated that a diamine in which both amines were
secondary would function as a switchable solvent and as a
potential CO2-capturing liquid (eqn (9)) However, only a liquid
diamine that gave a liquid zwitterion would be acceptable for
these applications. We screened the liquid diamines of the
structure RHN(CH2)nNHR where n = 2 (R = Me), n = 3
(R = Me, Et, i-Pr), and n = 6 (Me, Et, Bu). All gave solids
upon reaction with CO2. Note that in every case the diamines
were used as received and not dried in any way. It is quite
possible, for reasons explained below, that thoroughly dried
amines might give liquids upon reaction with CO2, but this
possibility was not pursued because, for practical reasons, it
is preferable to have a system that is water tolerant. The reaction
Table 4 NMR chemical shifts of 4 in DMSO-d6 with and without CO2
1H
1 (OH)
2–5
6 (NH)
7
8
10
4a
4.7 (br)
7.4 a(br)
5.6 (br)
3.23 (t), 3.4-3.5 (m)
3.3-3.5 a(m)
3.45 (m), 3.48 (m), 3.56 (m)
—
—
9.19 (s)
—
—
—
1.82 (s)
2.03 a(br)
2.31 (s)
2.78 (s)
4b
2.95 a(br)
4a+HCl
3.17 (s), 3.22 (s)
13C
1 (CO2)
2–5
6
7
8
10
4a
—
155.9
—
51.1, 62.4, 73.0, 73.3
47.0, 62.8, 70.2, 70.7
43.8, 60.0, 69.1, 72.3
—
—
—
159.8b
160.7
164.7
12.7
13.3
15.0
38.0
38.6
41.0, 39.8
4b
4a+HCl
a These peaks are broadened due to high viscosity. b In very concentrated solution, this peak is at 160.4 ppm.
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Table 5 NMR chemical shifts of 5 in CDCl3 with and without CO2
1H
1
2 (NH)
3
4
5
6
7
5a
5b
2.3 (s)
2.6 (br)
1.0 (br)
10.6 (br)
2.5 (t)
1.6 (quint)
1.9 (br)
—
3.4 (br)
—
—
—
2.9 (br)a
2.8 (br)a
13C
1
2 (NH)
3
4
5
6
7
5a
5b
36.7
32.1
—
—
50.7
30.2
23.8
—
—
34.2
—
163.9
44.7a
44.7a
a Overlapped.
of MeHN(CH2)3NHMe (5a) with CO2 is described here, and is
representative of the behaviour of these amines.
(9)
The viscosity of neat liquid 5a increased as CO2 was bubbled
through it. Upon completion of the reaction, the liquid turned
into a white solid, to which we assign the zwitterionic struc-
ture [MeH2N+CH2CH2CH2N(Me)CO2 ] (eqn (9)). The ATR-
-
(d) Diamines containing one secondary amine and one tertiary
amine
IR spectrum of the solid showed a band at 1545 cm-1, which is
characteristic of a carbamate anion.24 No peak was observed
at 835 cm-1, which would have indicated the presence of a
Because all diamines having two secondary amines gave solids,
a diamine containing one secondary and one tertiary amine was
tested. Bubbling CO2 through neat liquid N,N¢-trimethyl-1,3-
propanediamine (compound 6a) increased the viscosity of the
liquid greatly, but no solidification was observed. We assign
1
bicarbonate anion. The H NMR spectrum of 5b shows only
minor changes in the chemical shifts of most carbon-bound
protons except at positions 5 and 6 (see numbering scheme
in Fig. 7), where the peaks have shifted downfield by 0.9 and
~0.6 ppm, respectively (Table 5). The N-bound proton has
shifted from 1.0 ppm to 10.6 ppm, consistent with the change
in charge on the N atom from neutral to positive. In the 13C
NMR spectrum of 5b, a new peak appears at 163.9 ppm,
which corresponds to the carbamate central carbon. Crystals
of the product were grown by slow evaporation of a solution
of the compound in hexanes under a CO2 atmosphere. The
crystal structure confirms the formation of a zwitterionic salt
co-crystallized with one water molecule (Fig. 6). The water
molecule is an integral part of the hydrogen-bonding network
in the structure, which suggested to us that if the water had
been absent, the salt might not have solidified. However, treating
thoroughly-dried 5a with CO2 also created a solid.
the zwitterionic structure [Me2HN+CH2CH2CH2N(Me)CO2 ]
-
(6b) to the liquid product (eqn (10)). The ATR-IR spectrum
of the solid showed the disappearance of the N–H band at
3298 cm-1 and the appearance of a band at 1545 cm-1, which
is characteristic of a carbamate anion.24 No peak was observed
at 835 cm-1, which would have indicated the presence of bicar-
bonate anion. The 1H NMR spectrum of 6b shows only minor
changes in the chemical shifts of most carbon-bound protons
except at positions 5 and 7 (see numbering scheme in Fig. 8),
where the peaks have shifted downfield by 0.6 and 0.4 ppm,
respectively. The N-bound proton peak for the secondary amine
disappears and a new peak at 10.6 ppm appears, consistent
with a dialkylammonium cation (Table 6). In the 13C NMR
spectrum, a new peak appears at 163.9 ppm, which corresponds
to the carbamate central carbon. High resolution ESI mass-
spec (positive mode) of [Me2HN+CH2CH2CH2N(Me)CO2 ] was
-
measured and matched the expected M + 1 peak of 161.13 m/z.
(10)
Fig. 6 Crystal structure of zwitterionic [MeH2N+CH2CH2CH2N-
(Me)CO2-]·H2O (5b). Displacement ellipsoids are drawn at the 50%
probability level.
Fig. 8 Numerical assignment of positions in 6a and 6b.
The mass of liquid 6a increased by 28 wt% due to uptake
of CO2. The theoretical increase in mass, assuming complete
conversion but neglecting the mass of dissolved CO2, should be
38%. The lower observed uptake suggests that the conversion of
eqn (9) is incomplete, which may be the reason why the liquid did
Fig. 7 Numerical assignment of positions in 5a and 5b.
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Table 6 NMR chemical shifts of 6 in CDCl3 with and without CO2
1H
1
2 (NH)
3
4
5
6
7
6a
6b
2.2 (s)
2.3 (s)
—
10.5 (br)
2.3 (t)
2.4 (t)
1.6 (quint)
1.8 (m)
2.6 (t)
3.2 (br)
1.4 (br)
—
2.4 (s)
2.8 (br)
13C
1
2 (NH)
3
4
5
6 (CO2)
7
6a
6b
45.4
44.8
—
—
57.9
56.7
27.9
25.1
50.5
47.2
—
161.1
36.5
34.0
not solidify. Removal of CO2 and reversion of the liquid back
to the neutral diamine form was achieved by bubbling argon
through the viscous zwitterionic liquid at 60 ◦C. The reversion
back to the diamine was confirmed by 1H NMR spectroscopy.
To determine the polarity change upon conversion from
the diamine to the zwitterionic liquid, the spectrum of the
solvatochromatic dye Nile Red was obtained while the dye was
in liquid 6 before and after treatment with CO2, The lmax value
of Nile Red dissolved in neat 6a was 523 before exposure to
CO2, indicating a polarity comparable to benzene. After CO2
was bubbled through the sample with CO2 for 20 min at room
temperature, the absorbance shifted to 534 nm, indicating a
polarity that is comparable to that of THF. This amount of
polarity change is comparable to that observed with simple
secondary amines such as ethyl butyl amine,4 showing that there
is no inherent advantage, in terms of increasing size of the
polarity change, to using switchable zwitterionic liquids rather
than switchable ionic liquids.
NMR) spectra for compounds 1Si–3Si, 1a–3b were recorded
with a Varian 300 (75.5 MHz) spectrometer. Chemical shifts are
reported in delta (d) units, parts per million (ppm) relative to
the center of the triplet at 77.00 ppm for deuteriochloroform. 1H
and 13C NMR spectra for compounds 4a–6b were obtained with
a Bruker AVANCE-400 MHz NMR spectrometer where each
sample is referenced to tetramethylsilane. The NMR spectra of
the CO2 treated amines were obtained by bubbling CO2 into a
septa capped NMR tube that had 0.1 mL amine in dry CDCl3.
Data collection for the crystal of 5b was done on a Bruker
SMART APEX II X-ray diffractometer.
N,N,N¢,N¢-Tetramethyl-N¢¢-(3-trimethylsilanyloxypropyl)-
guanidine (1Si)
1,1,3,3-Tetramethylguanidine (TMG) (33.6 mL, 260.4 mmol)
was charged into
a 250 mL round bottom flask and
then xylenes (25 mL) was added, this solution was
heated to reflux. While maintaining the temperature (3-
bromopropoxy)trimethylsilane25 25 g (118.39 mmol) was added.
The resulting white suspension was refluxed overnight. After
which the reaction was allowed to cool to room temperature
and the resulting precipitate was filtered off, and the filtrate was
distilled under vacuum, collecting xylenes followed by a second
fraction compound 1a as a colorless viscous liquid (9 g, 38%). 1H
NMR (300 MHz, CDCl3) d 3.67 (t, J = 5.4 Hz, 2H), 3.46 (t, J =
Experimental
All reactions were run under an atmosphere of argon, un-
less otherwise indicated. Flasks were oven-dried and cooled
under vacuum. All secondary diamines, solvatochromatic
dye, CO2 (supercritical fluid chromatography grade), ar-
gon (99.98%), chlorotrimethylsilane, triethylamine, 3-bromo-
1-propanol, 6-bromo-1-hexanol, 11-bromo-1-undecanol and
1,1,3.3-tetrametylguanidine were purchased from Sigma-
Aldrich and used as received. High-resolution mass spectra
(HRMS) ESI for compounds 2Si–3Si, 1a–3a were obtained
on a Thermo Scientific Exactive (tm) Mass Spectrometer from
Thermo Scientific Company as m/z (relative intensity). High-
resolution mass spectrum (HRMS) ESI for 6b was obtained
on a Qstar XL QqTOF from Applied Biosystems/MDS Sciex.
Accurate masses are reported for the molecular ion (M + 1)
or a suitable fragment ion. IR measurements for compounds
1Si–3Si, 1a–3b were performed on NaCl disks using a Nicolet
Magna-750 spectrometer running on OMNIC software. The IR
spectra for compounds 5a, 5b, 6a, and 6b were obtained on a
Varian Scimitar 1000 FTIR equipped with the Pike MIRacleTM
ZnSe ATR accessory. Proton nuclear magnetic resonance (1H
NMR) spectra for compounds 1Si–3Si, 1a–3b were recorded
with a Varian (300 MHz) spectrometer. Chemical Shifts are
reported in delta (d) units, parts per million (ppm) downfield
from tetramethylsilane. Coupling constants are reported in
1
5.4 Hz, 2H), 2.00 (quin, J = 5.3 Hz, 2H), 0.09 (s, 9H). 13C{ H}
NMR (75.5 MHz, neat) d 160.4, 61.1, 45.9, 39.7, 39.0, 35.6, -0.3.
FTIR (NaCl, film): 2940, 1661, 1653, 1495, 1366, 1249, 1091,
839 cm-1
N¢¢-(3-Hydroxypropyl)-N,N,N¢,N¢-tetramethylguanidine (1a)
1.3 g of compound 1Si was charge into a clean dry 100 mL
RB flask, and then anhydrous MeOH 20 mL was added. The
solution was degassed by bubbling argon for 10 min and then
K2CO3 1.46 g was added, this mixture was stirred at RT for 1 h.
The reaction mixture was then filtered and the resulting filtrate
was distilled under vacuum to afford compound 1a as a colorless
liquid (0.73 g 80% yield). 1H NMR (300 MHz, CDCl3) d 3.85 (t,
J = 5.0 Hz, 2H), 3.33 (t, J = 5.0 Hz, 2H), 2.72 (br s, 1H),2.19 (s,
1
6H), 2.58 (s, 6H) 1.64 (m, 2H). 13C{ H} NMR (75.5 MHz, neat)
d 158.9, 62.0, 42.4, 39.8, 39.0, 35.7, 23.3 FTIR (NaCl, film):
3246, 2927, 1653, 1611, 1496, 1455, 13609, 1330, 1178, 1132,
1062 cm-1 HRMS ESI (positive mode M + H+) calculated for
C8H19N3O 174.1600, found 174.1596.
1
Hertz (Hz). Carbon-13 nuclear magnetic resonance (13C{ H}
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N,N,N¢,N¢-Tetramethyl-N¢¢-(6-trimethylsilanyloxyhexyl)-
guanidine (2Si)
30 min, cooled, and concentrated in a rotary evaporator. A clear
colorless liquid was recovered after vacuum distillation (yield
82%). ESI-MS (positive mode, M + H+) observed: 175.1447,
calculated for C8H19N2O2: 175.1441. IR (neat, cm-1): 3365 m,
2864 s, 1612 s, 1390 m, 1128 m, 1063 m, 1013 w, 891 w, 574 w.
Following the same procedure as for 1Si using, (6-
bromohexoxy)trimethylsilane27 (3.50 g, 13.90 mmol), TMG
(3.80 mL, 30.55 mmol) and xylenes (5 mL) affords 2a (1.8 g,
1
45%) as a pale yellow liquid. H NMR (300 MHz, CDCl3) d
Treatment of N¢-(2-hydroxyethoxy)ethyl)-N,N-
dimethylacetamidine (4a) with CO2
3.53 (t, J = 7.0 Hz, 2H), 3.06 (t, J = 7.0 Hz, 2H), 2.70 (s, 6H),
2.62 (s, 6H), 1.47 (m, 2H) 1.24 (m, 6H), 0.08 (s, 9H). 13C{ H}
1
NMR (75.5 MHz) d 160.1, 62.9, 49.6, 39.8, 39.0, 32.9, 27.4,
25.9, -0.3. FTIR (NaCl, film): 2931, 2859, 1663, 1494, 1453,
1363, 1243, 1095, 840 cm-1 HRMS ESI (positive mode M + H+)
calculated for C14H33N3OSi 288.2465, found 288.2453.
Carbon dioxide was bubbled, via a syringe needle, through a
sample of 4a in a test tube. Although the liquid became quite
viscous after only 0.5 h, the bubbling was continued for another
3 h to ensure maximum possible conversion. IR (neat, cm-1):
3366 m, 2871 m, 1646 s, 1396 m, 1287 m, 1126 m, 1066 m, 1013
m, 836 w, 621 w. NMR data given in Table 4. ESI-MS (negative
mode, M+H+) observed: 217.1200, calculated for C9H17N2O4:
217.1188.
N¢¢-(6-Hydroxyhexyl)-N,N,N¢,N¢-tetramethylguanidine 2a
Following the same procedure as for compound 1a using
compound 2Si (1.70 g, 5.92 mmol), K2CO3 (1.63 g, 11.83 mmol),
and MeOH (15 mL) provides 2a (1.0 g, 78%) as a pale yellow
oil. 1H NMR (300 MHz, CDCl3) d 3.58 (t, J = 6.7H Hz, 2H),
3.06 (t, J = 6.7 Hz, 2H), 2.70 (s, 6H), 2.61 (s, 6H), 1.51 (m,
IR spectroscopic data for the bases before and after CO2
treatment
1
4H) 1.32 (m, 4H). 13C{ H} NMR (75.5 MHz) d 160.8, 62.9,
N,N¢-Dimethyl-1,3-propanediamine (5a). (neat, cm-1) Be-
fore CO2 treatment: 3280 m, 2929 m, 2879 m, 2841 m, 2786 s,
2694 w, 1473 s, 1446 s, 1124 s, 739 s. After CO2 treatment: 3289
w, 2928 m, 2840 m, 2787 m, 2405 m, 2361 m, 1545 s, 1471 s,
1372 s, 1034 s, 1255 s, 1038 s, 807 s, 757 s.
42.4 39.8, 37.0, 35.7, 23.3. FTIR (NaCl, film): 3355, 2995, 2928,
1618, 1501, 1454, 13.74, 1238, 1137, 1102, 751 cm-1 HRMS ESI
(positive mode M + H+) calculated for C11H25N3O 216.2070,
found 216.2061.
N,N,N¢-Trimethyl-1,3-propanediamine (6a). (neat, cm-1)
Before CO2 treatment: 3298 w, 2940 s, 2857 m, 2814 s, 2783 s,
2764 s, 1739 w, 1460 s, 1376 w, 1263 w, 1150 m, 1122 m, 1041 s,
839 s, 743 s. After CO2 treatment: 2941 m, 2814 m, 2762 m, 2462
w, 1683 w, 1624 w, 1545 w, 1460 s, 1376 s, 1305 m, 1262 s, 1040 s,
808 m.
N,N,N¢,N¢-Tetramethyl-N¢¢-(11-trimethylsilanyloxyundecyl)-
guanidine (3Si)
Following the same procedure as for 1Si using, (11-
bromoundecanoxy)trimethylsilane26 (8.1 g, 25.10 mmol), TMG
(7.0 mL, 55.30 mmol) and xylenes (10 mL) provide compound
3Si (3.70 g, 41%) as a pale yellow liquid. 1H NMR (300 MHz,
CDCl3) d. 3.56 (t, J = 7.2 Hz, 2H), 3.04 (t, J = 7.2 Hz, 2H),
2.69 (s, 6H), 2.61 (s, 6H), 1.49 (quintet, J = 7.2 Hz, 4H), 1.23
Gravimetric uptake of CO2 by N,N,N¢-trimethyl-1,3-
propanediamine (6a)
1
(unresolved s, 14H), 0.24 (s, 9H). 13C{ H} NMR (75.5 MHz,
Compound 6a (0.3 mL, 2.05 mmol) was added to a flame-dried
1 dram vial sealed with a rubber septa and massed. CO2 was
bubbled into the amine at room temperature and 1 bar to form
the zwitterion. Mass was taken at 10 min intervals for 100 min
although no further mass increase was observed after the first
10 min.
neat) d 62.8, 49.7, 39.8, 39.0, 32.9, 32.8, 29.7, 27.6, 25.9, 2.1 FTIR
(NaCl, film): 2925, 2853, 1604, 1502, 1456, 1375, 1252, 1136,
1059, 845 cm-1. HRMS ESI (positive mode M + H+) calculated
for C19H43N3OSi 358.3248, found 358.3256.
N¢¢-(11-Hydroxyundecyl)-N,N,N¢,N¢-tetramethylguanidine (3a)
Following the same procedure as for compound 1a using
compound 3Si (2.80 g, 7.83 mmol), K2CO3 (2.12 g, 16.79 mmol),
and MeOH (20 mL) provides 3a (2.16 g, 95%) as a pale yellow
Polarity measurement of N,N,N¢-trimethyl-1,3-propanediamine
(6a)
Approximately 3 mL of the amine was placed in a 1 cm quartz
cuvette where it was dotted with Nile red on a needle tip to give
an absorbance between that of 0.5 and 1.0. The cuvette was then
bubbled with atmospheric CO2 for a period of 20 min or until
the viscosity of the amine changed. A UV/visible spectrum was
measured again. The experiment was done in triplicate, where
the average of all three is the reported measurement.
1
oil. H NMR (300 MHz, CDCl3) d. 3.57 (t, J = 7.5 Hz, 2H),
3.04 (t, J = 7.5 Hz, 2H), 2.85 (s, 1H), 2.70 (s, 6H), 2.62 (s, 6H),
1
1.49 (quintet, J = 6.8 Hz, 4H), 1.23 (unresolved s, 14H).13C{ H}
NMR (75.5 MHz, neat) d 62.8, 49.6, 39.7, 38.9, 32.9, 37.7, 29.6
27.6, 25.9. FTIR (NaCl, film): 3381, 2921, 2852, 1605, 1501,
1455, 1375, 1145, 1063 cm-1. HRMS ESI (positive mode M + H+)
calculated for C16H35N3O 286.2853, found 286.2855.
Preparation of N¢-(2-hydroxyethoxy)ethyl)-N,N-
dimethylacetamidine (4a)
Conclusions
Compounds 2a, 4a, and 6a are switchable solvents, in that
they react reversibly with CO2 to form zwitterionic liquids.
Any of these three compounds can serve as switchable sol-
vents and as organic CO2-capturing agents (CO2BOLs). The
Aminoethoxyethanol (6.1 g) was charged into a round bottom
flask. N,N-Dimethylacetamide dimethylacetal (8.0 g) was added
to reaction mixture, which was then heated to 75 ◦C, stirred for
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liquid zwitterionic salts were all highly viscous and polar. The
change in polarity, upon conversion of the neutral molecule
to the zwitterion, was comparable to that observed with non-
zwitterionic switchable solvents. In terms of gravimetric CO2-
capturing capacity, these liquids are comparable to the two-
component CO2-binding liquids reported previously.
For alkanolguanidines, the chain length of the alkyl group
between the alcohol and guanidine has an affect on the physical
properties and regeneration of the alkanolguanidines. Short
alkyl chains promoted cyclization of the carbonate to form
the cyclic carbonate. Longer alkyl chains produced a reversible
zwitterionic liquid, while very long alkyl chains produced
reversible zwitterionic wax-like materials.
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were found to form reversible zwitterionic carbamate salts,
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Acknowledgements
DJH, PKK and CRY gratefully acknowledge internal funding
from Pacific Northwest National Laboratory (PNNL) Directed
Research and Development. PNNL is proudly operated by
Battelle for the U. S. Department of Energy. PGJ, TA, and CL
thanktheNatural Sciences and Engineering Research Councilof
Canada (NSERC), CFI/OIT, and the Canada Research Chairs
program for funding the research, the equipment, and PGJ’s
salary, respectively. The authors thank Dr Rui Zhang for the
high resolution mass spec. (HRMS) analysis and Dr Ruiyao
Wang for the X-ray crystallography.
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