Substituent dependence of imidazoline derivatives on the capture and release system of carbon dioxide

Article abstract of DOI:10.1039/c7nj03133e

The reversible capture-release behaviors of CO₂ by imidazoline derivatives bearing several 2-substituents were investigated under dry conditions containing a very small amount of water. The imidazoline having a cyclic guanidine moiety only captured CO₂ quantitatively, and then the cyclic guanidine derivative gave the guanidinium bicarbonate by CO₂ fixation together with a slight amount of water. In contrast, the imidazolines bearing n-butylthiol and n-pentyl groups never captured CO₂ even with the addition of an equimolar amount of water. Furthermore, the binding structure between CO₂ and the cyclic guanidine derivative was studied in detail by elemental analysis, FTIR-ATR, solid-state NMR, TGA, and DFT calculations in order to predict the CO₂ capture-release mechanisms depending on the imidazoline structure.

Full text of DOI:10.1039/c7nj03133e

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Substituent Dependence of Imidazoline Derivatives on Capture
and Release System of Carbon Dioxide
Received 00th January 20xx,
Accepted 00th January 20xx
Yoshiaki Yoshida, Naoto Aoyagi, Takeshi Endo*
The reversible capture-release behaviors of CO2 by imidazoline derivatives bearing several 2-substituents were
investigated under dry conditions containing a very small amount of water. The imidazoline having a cyclic guanidine
moiety only captured CO2 quantitatively, and then the cyclic guanidine derivative gave the guanidinium bicarbonate by CO2
fixation together with a slight amount of water. In contrast, the imidazolines bearing n-butylthio and n-pentyl groups
never captured CO2 even by the addition of equimolar amount of water. Furthermore, the binding structure between CO2
and cyclic guanidine derivative was studied in detail by elemental analysis, FTIR-ATR, solid-state NMR, TGA, and DFT
calculation in order to predict CO2 capture-release mechanisms depending on the imidazoline structure.
DOI: 10.1039/x0xx00000x
www.rsc.org/
inefficient uptake for CO2 of five-membered cyclic amidines.
Actually, we recently reported that a six-membered cyclic
amidine with silatranyl group captured CO2 quantitatively,
1. Introduction
During the past twenty years, the increasing concentration of
carbon dioxide (CO2) in the atmosphere has given rise to the
serious environmental issues.1-2 Recently, many researchers
have reported the efficient methods for CO2 capture by using
various basic reagents, e.g. primary or secondary amines,
amidines, guanidines, and so on.3-12 Especially, amidines and
guanidines, such as 1,8-diazabicyclo [5.4.0]undec-7-ene (DBU)
and 1,5,7-triazabicyclo [4.4.0]dec-5-ene (TBD), are well known
as significantly efficient basic reagents for reversible CO2
capture-release processes at low temperature.6-12 Previously,
we have reported reversible CO2 fixation by polyamidine and
polystyrene derivatives bearing cyclic amidines under wet
conditions.13-16 On the other hand, we also found that six-
membered cyclic amidines captured CO2 quantitatively under
dry conditions.17-19 The difference of wet and dry conditions
affects significantly adsorption property for CO2, because some
amidines efficiently trap CO2 together with a certain amount of
water.20-23 Such the CO2-binding mechanisms have been
clarified by solid-state NMR study, single crystal X-ray analysis,
and computational evaluation, which commonly shown as
formation of a bicarbonate salt through a zwitterionic adduct
although a five-membered cyclic amidine with silatranyl group
captured CO2 much less efficiently and an acyclic amidine with
silatranyl group never captured CO2.19 The low CO2 fixation
efficiency of five-membered cyclic amidine is considered
because of the high ring strain of CO2-binding five-membered
cyclic amidine compared with that of six-membered cyclic
amidine.40 This result means that the geometric and steric
structure of cyclic amidines contributes to the CO2 fixation
efficiency of them. However, the effect of ring and substituent
structures for CO2 fixation efficiency has never been reported
within the cyclic amidine derivatives. Hence, we investigated
the reversible capture-release behaviors of CO2 by imidazoline
derivatives bearing several 2-substituebts (Scheme 1). In
addition, the binding structures between CO2 and imidazoline
derivatives were studied in detail by elemental analysis, FTIR-
ATR, solid-state NMR, TGA, and DFT calculation.
(
Scheme 1
).24-32 Although these previous works have
demonstrated CO2 capture-release behaviors for bicyclic and
six-membered cyclic amidines and guanidines, CO2 adsorption
for simple five-membered cyclic compounds such as 2-
substituted imidazolines have hardly been studied compared
with acyclic and alkanol amidines and guanidines, ionic liquid,
and polymer.33-39 One of the reasons is perhaps because of
Scheme 1. Reversible reaction of CO2 and imidazoline
derivatives.
2. Results and discussion
2.1. CO2 fixation behavior of imidazoline derivatives depending on
substituents
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Adsorption reaction of CO2 by imidazoline derivatives, 1a
,
1b
,
to the protonated imine moiety was also observed at 1667 cm-
1
and 1c, was carried out with dry CO2 bubbling in anhydrous
(
Fig. 2a). These peaks disappeared completely after CO2
acetonitrile (MeCN) at 25 °C for 2 h under ambient moisture fixation for 1c, and then the peaks due to the guanidinium and
Scheme 2). Although these reaction mixtures remained as the bicarbonate moieties were observed at 1675 cm-1 and 1603
(
homogeneous and colorless solution before CO2 bubbling, the cm-1, respectively (Fig. 2b). On the other hand, each nitrogen
white solid was obtained during CO2 bubbling only in the case atom in 3c was observed definitely as different chemical shift
of 1c. On the other hand, the white solids of 1a and 1b were at 72.6 ppm, 76.6 ppm, and 78.7ppm by 15N-CPMAS NMR
never obtained even by the addition of equimolar amount of spectrum (Fig. 2c).41
water after CO2 bubbling. In fact, 13C NMR spectra of 1a and 1b
before and after bubbling CO2 in 0.1 mol/L CD3CN solution
showed that the chemical shift of main carbon peaks was
never observed before and after bubbling CO2, and the free
CO2 was determined at 125.8 ppm after CO2 bubbling (Figs.
S1a and S2a). Therefore, 1c having N-butyl amino group as the
2-substituent of imidzoline exhibited fairly high adsorption
property for CO2 compared with that of the imidazoline
derivatives having the other substituents. This result suggests
that the guanidine structure, which exhibits strong base due to
the stable positive charge delocalized on nitrogen, is
significantly efficient for CO2 fixation. Furthermore, the
compositional formula of the obtained white solid was
confirmed by elemental analysis, and the structure was
predicted as the bicarbonate 3c (Fig. 1). The bicarbonate was
produced by a slight amount of water contained in air, CO2 gas,
and solvent.
Figure 2. FTIR-ATR spectra of (a) 1c and (b) 3c expanded at the
range of 1500-1800 cm-1. (c) 15N-CPMAS NMR spectrum of 3c
expanded at the range of 60-90 ppm.
Furthermore, the geometric structures of guanidinium and
bicarbonate moieties were also revealed by 13C NMR
measured in D2O and solid-state (Figs. 3 and S3). Although the
carbon peak due to the guanidine moiety of 1c was observed
at 164.1 ppm in D2O (Fig. 3a), the peak shifted to 161.5 ppm
indicating preparation of guanidinium moiety after CO2
Scheme 2. Adsorption reaction of CO2 (0.2 L/min) by
imidazoline derivatives in dry MeCN.
fixation for 1c
bicarbonate moiety was observed at 160.7 ppm after CO2
fixation for 1c Fig. 3b). The peak-shift behavior observed by
NMR spectra in D2O agreed with that of IR spectra before and
after CO2 fixation for 1c Figs. 2 and ). Moreover, these peaks
(Fig. 3b). On the other hand, the peak due to
(
(
3
due to the guanidinium and bicarbonate moieties were also
observed definitely at 162.7 ppm and 161.1 ppm by 13C-
CPMAS NMR, respectively (Fig. 3c). These results indicate that
the fairly stable guanidinium cation fixes CO2 and H2O
molecules as the bicarbonate anion. On the other hand, 1b
having n-pentyl group as the 2-substituent captured CO2 only
in high concentration solution (1.2 mol/L in CH3CN) under wet
condition (Fig. S2), although 1a having n-butylthio group as the
2-substituent never fixed CO2 even under high concentration
and wet condition (Fig. S1).
Figure 1. Bicarbonate structure of 3c confirmed by elemental
analysis.
2.2. Structure analysis of bicarbonate 3c by FTIR-ATR and solid-
state NMR (15N and 13C)
IR spectra of 1c and 3c were measured by FTIR-ATR methods
(Fig. 2a and 2b). The spectrum of 1c showed clearly the peaks
assigned to the imine and secondary amine moieties at 1629
cm-1 and 1538 cm-1, respectively, although the weak peak due
2 | J. Name., 2012, 00, 1-3
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Table 1. Bond lengths, bond angles, and dihedral angles of 2a, 2b, and 2c estimated by DFT calculation (B3LYP/6-31G*).
Imidazoline
Bond length (Å)
CO2−N
Bond angle (deg)
O−C−O
Dihedral angle (deg)
NCH2−CH2N
C−N−Ca
150.31
132.60
126.36
CN−CXb
-25.81
1.57
2a
2b
2c
3.022
2.803
1.546
176.84
175.18
135.91
20.96
19.80
0.00
0.00
a Bond angle between CO2 and imine bond. b Dihedral angle between CO2 and X-substituted moiety (X = S; 2a, CH2; 2b, NH; 2c).
This result means that 1a cannot capture CO2 and the CO2 Furthermore, the ring-distortions of 2a and 2b were confirmed
fixation efficiency of 1b is fairly low compared with that of 1c
However, the predicted pKa value of imidazoline derivatives dihedral angles of imidazoline rings (20.96 º and 19.80 º)
did not necessarily explain the relation between CO2 fixation Table 1). On the other hand, the almost bonds of 2c was
efficiency and basicity of them, because the predicted pKa ordered on the same plane and the hydrogen on the
value of 1b was higher than that of 1c Fig. S4). Therefore, butylamino group fairly came close to the oxygen on trapped
.
by the CO2-imine bond angle (150.31 º and 132.60 º) and the
(
(
these results suggest that the CO2 fixation efficiency of CO2 (1.687 Å), which the trapped CO2 was stabilized by the
imidazoline derivatives depends on their substituents and ring hydrogen bond with the proton in butylamino group. HOMO
geometric structures.
level of 1c was also high significantly compared with 1a and 1b,
meaning that 1c can capture CO2 efficiently due to the high
nucleophilicity based on the resonance structure and
symmetric ring of cyclic guanidine moiety (Fig. S4).40 These
results mean that 2c has the strong interaction between 1c
and CO2. Additionally, even if 2a and 2b could capture CO2, the
trapped CO2 would release immediately due to the strained
geometric structure between imidazoline rings and trapped
CO2 in 2a and 2b. On the other hand, the calculated geometry
for bicarbonate 3c showed absolutely planar structure and
close position between the imidazolinium and bicarbonate
moieties as well as 2c (Fig. 4). This result suggests that the
anionic oxygen on the bicarbonate is stabilized by the
hydrogen on the imidazolinium moiety. Therefore, CO2
adsorption behavior of 1c and the stable structure of 3c
predicted by NMR and IR spectra were supported strongly by
the computational evaluation.
Figure 3.
13C NMR spectra observed in D2O containing 1,4-
dioxane (0.4%) of (a) 1c and (b) 3c expanded at the range of
150-170 ppm. (c) 13C-CPMAS NMR spectrum of 3c expanded at
the range of 150-170 ppm.
2.3. Computational evaluation for CO2 adsorption behavior of
imidazoline derivatives
The CO2 adsorption behavior of imidazoline derivatives was
predicted by the optimized geometries of 2a, 2b, 2c, and 3c
with DFT calculation (Fig. 4 and Table 1). Specifically, the
calculated geometry for the zwitterionic adduct 2c showed the
short CO2-imine nitrogen bond length (1.546 Å) and the bent O
Figure 4. The optimized geometries of 2a
, 2b, 2c, and 3c
estimated by DFT calculation (B3LYP/6-31G*).
−C−O bond angle (135.91 º) of trapped CO2. In contrast, the
2.4. Thermal decomposition behavior of bicarbonate 3c
calculated geometry for 2a and 2b showed the longer CO2-
The weight loss behavior of bicarbonate 3c was shown by TG
and DTG traces (Fig. 5). The weight loss of bicarbonate 3c
exhibited tree-steps behavior before the thermal
imine nitrogen bond lengths (3.022 Å and 2.803 Å) and the
almost straight O−C−O bonds (176.84 º and 175.18 º).
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decomposition of 1c. This tree-steps behavior of weight loss NMR, and DFT calculation. Furthermore, CO2 capture and
suggested the dimer form of 3c prepared with CO2 fixation, release cycle could be repeated a few times with the same
because the total weight loss of the tree-steps disagreed with efficiency.
the weight loss value due to the just elimination of CO2 and
H2O from the 3c monomer. Pérez et al and Villiers et al have
reported that the bicarbonate salt of the guanidine derivatives
formed the dimer by two hydrogen bonds between two
hydrogen in two guanidinium cations and two oxygen atoms in
3c dimer).25,28 Therefore, the first (i)
−
two HCO3 anions (Fig. 5
,
and second (ii) weight loss occurred at 74 ºC and 107 ºC due to
the elimination of one pair of CO2 and H2O from the
bicarbonoate moiety in 3c dimer, although the analytical value
of the first weight loss was half value of the second one (Anal.
(i) 7.9% and (ii) 15.8%, Calc. 15.3%). Accordingly, the
intermediate complex of 3c and 1c, which formed by the two
hydrogen bonds between O
H
−
N
and
O
−H
N in the bicarbonate
salt 3c and the free guanidine 1c
(
Fig. 5, 3c+1c), released
another one pair of CO2 and H2O at 158 ºC (Anal. (iii) 30.3%,
Calc. 30.6%). This thermal decomposition behavior indicated
that the fixed CO2 was fairly stabilized due to the multi
hydrogen bond between the guandinium and bicarbonate
moieties as mentioned above.
Figure 6. CO2 capture-release behaviors of 1c under CO2 or Ar
flows (0.2 L/min each) at 25 °C or 60 °C, the CO2 fixation
efficiency was calculated based on the bicarbonate 3c
.
3. Conclusion
CO2 fixation of imidazoline derivatives having different
substituents, 1a 1b, and 1c, were investigated with dry CO2
,
bubbling in anhydrous MeCN under the ambient moisture. The
only 1c captured CO2 quantitatively and the bicarbonate 3c
was obtained by CO2 fixation together with a slight amount of
water, which structure was determined by elemental analysis,
FTIR-ATR, solid-state NMR, TGA, and DFT calculation. In
contrast, 1a and 1b never captured CO2 even by the addition
of equimolar amount of water, because these imidazoline
moieties could not capture CO2 stably owing to the strained
geometric structure between imidazoline rings and trapped
CO2 in 2a and 2b predicted by DFT calculation. Furthermore,
CO2 capture-release behaviors of 1c were also investigated
with dry CO2 and Ar bubbling in anhydrous DMSO. Although 1c
could capture CO2 quantitatively only within 10 min, the
equivalent release of trapped CO2 needed at least 60 min. The
cycle behavior of CO2 capture-release for 1c exhibited the
same efficiency during a few cycle times. In conclusion, the
imidazoline derivative such as 1c, which was one of the five-
membered cyclic amidines, was excellently efficient for CO2
capture, because 3c was significantly stable due to the strong
interaction between the imidazolinium and bicarbonate
moieties based on the specific basicity of guanidine moiety.
Figure 5. TGA trace of bicarbonate 3c observed under N2 flow
(0.1 L/min) at heating rate of 5 ºC/min.
2.5. CO2 capture-release behaviors of 1c
The behavior of CO2 capture-release cycle for 1c was observed
in anhydrous DMSO by bubbling the dry CO2 at 25 ºC and
bubbling Ar at 60 ºC in an alternate manner for 1 h (Fig. 6). The
CO2 capture property was significantly efficient because of
nearly 100% fixation within 10 min. On the other hand, almost
trapped CO2 was released within 60 min under Ar flow.
Although both CO2 capture and release proceeded
quantitatively by bubbling CO2 and Ar, respectively, 1c could
capture CO2 very rapidly compared with the release rate of
trapped CO2. This result means that the guanidine moiety
exhibits a highly efficient CO2 capture together with an
extremely low amount of water, and also the obtained
complex from guanidinium and bicarbonate moieties is
significantly stable salt as predicted by FTIR-ATR, solid-state
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Scheme 3. Synthesis of imidazoline derivatives.
residue was washed with Et2O (50 mL), and the precipitate was
dried in vacuo for 12 h at 40 °C to give S-butylisothiouronium
iodide as a white solid. The obtained white solid was added to
a 40% NaOHaq (20 mL). To the mixture was added Et2O (40 mL),
and was vigorously stirred at room temperature for 15
minutes. The layers were separated and the aqueous layer was
extracted with Et2O (10 mL x 3). The combined organic phase
was dried with Na2SO4, then evaporated under reduced
pressure. The residue was dried in vacuo for 12 h at 40 °C to
give 1a as a white solid.
4. Experimental Section
4.1. Materials and instruments
All starting materials and dehydrated solvents were purchased
from Wako Pure Chemical Industries (Osaka, Japan) and Tokyo
Chemical Industry (Tokyo, Japan). CO2 (>99.999%, H2O < 5
ppm) and Ar (>99.9999%, H2O < 1 ppm) were obtained from
Yamagata Sanso (Yamagata, Japan) and Fukuhou Teisan
(Fukuoka, Japan), respectively. The NMR spectra were
obtained using a JEOL ECS-400 spectrometer operating at 400
2-(Butylthio)-4,5-dihydro-1H-imidazole hydroiodide:
Yield 14.3 g, 100%. 1H NMR (400 MHz, DMSO-d6, 25 °C) δ
(ppm): 0.90 (t, 3H, J = 7.2 Hz, CH3), 1.38 (sext, 2H, J = 7.5 Hz,
CH3CH2), 1.62 (quin, 2H, J = 7.5 Hz, CH3CH2CH2), 3.18 (t, 2H, J =
7.3 Hz, SCH2), 3.87 (s, 4H, CH2CH2), 10.0 (br s, 2H, 2(NH)). 13C
NMR (100 MHz, DMSO-d6, 25 °C) δ (ppm): 13.3 (CH3), 20.9
(CH3CH2), 30.3 (CH3CH2CH2), 30.7 (SCH2), 45.1 (CH2CH2), 169.1
(N=C-S).
1
MHz for H and 100 MHz for 13C. The elemental analyses were
performed by the Perkin Elmer 2400II CHNS/O Analyzer. FTIR
spectra were recorded on a Thermo Scientific Nicolet iS10
spectrometer equipped with
a Smart iTR diamond ATR
sampling accessory in the range of 4000−650 cm-1. Solid state
NMR, 13C and 15N cross-polarization (CP)/magic angle spinning
(MAS) (99.5 and 40.1 MHz, respectively) measurements were
performed on a JEOL JNM-ECX 400 spectrometer, at a spinning
speed of 10 kHz. Thermogravimetric analysis (TGA) was
performed on a Seiko Instrument TG-DTA 6200 using an
aluminum pan in the temperature range of 30−350 °C at a
heating rate of 5 °C/min under a nitrogen atmosphere (flow
2-(Butylthio)-4,5-dihydro-1H-imidazole:
1
Yield 7.68 g, 97%. H NMR (400 MHz, CDCl3, 25 °C) δ (ppm):
0.92 (t, 3H, J = 7.3 Hz, CH3), 1.4 (sext, 2H, J = 7.3 Hz, CH3CH2),
1.67 (quin, 2H, J = 7.3 Hz, CH3CH2CH2), 3.06 (t, 2H, J = 7.3 Hz,
SCH2), 3.47 (br s, 2H, NHCH2), 3.84 (br s, 2H, NCH2), 4.37 (br s,
1H, NH). 13C NMR (100 MHz, CDCl3, 25 °C) δ (ppm): 13.6 (CH3),
21.9 (CH3CH2), 31.0 (CH3CH2CH2), 31.5 (SCH2), 45.7 (NHCH2),
55.5 (NCH2), 164.4 (N=C-S).
rate 100 mL/min). Geometry optimized energy for 2a, 2b, and
2c was estimated by using DFT calculation with B3LYP/6-31G*
method (Wavefunction, Inc., Spartan’06 Windows version
1.1.0).42
4.2.2. 2-Pentyl-4,5-dihydro-1H-imidazole (1b)
A solution of hexanal (2.0 g, 20 mmol) in CH2Cl2 (100 mL ) and
ethylenediamine (1.3 g, 21 mmol) were mixed and stirred at
0 °C for 30min, and N-bromosuccinimide (21 mmol) was added
to the mixture and the resulting solution was allowed to warm
to rt. After stirring for 12 h, 40% NaOHaq (10 mL) was added to
the reaction mixture, and the mixture was vigorously stirred at
room temperature for 15 minutes. The layers were separated
and the aqueous layer was extracted with CH2Cl2 (10 mL x 3).
The combined organic phase was dried with Na2SO4, then
evaporated under reduced pressure. The residue was dried in
vacuo for 12 h at 40 °C to give 1b as a white solid.
4.2. Synthesis and characterization of imidazoline derivatives
Imidazoline derivatives, 1a, 1b, and 1c, were synthesized
according to the literatures (Scheme 3).43-44
4.2.1. 2-(Butylthio)-4,5-dihydro-1H-imidazole (1a)
A solution of 1-iodobutane (11.0 g, 60 mmol) was added
dropwise for
5
min to the stirring suspension of
ethylenethiourea (5.1 g, 50 mmol) in dry ethanol (50 mL) at
room temperature. After the mixture was stirred at 50 °C for
12 h, the mixture was evaporated under reduced pressure. The
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2-Pentyl-4,5-dihydro-1H-imidazole:
(10 mL) at 25 °C. After 2 h under bubbling CO2, anhydrous Et2O
1
Yield 2.76 g, 98%. H NMR (400 MHz, CDCl3, 25 °C) δ (ppm): (20 mL) was added to a suspension of the salt in MeCN. The
0.90 (t, 3H, J = 6.8 Hz, CH3), 1.33 (m, 4H, J = 7.4 Hz, CH3CH2CH2), resulting white precipitate was filtered off, washed with
1.62 (quin, 2H, J = 7.4 Hz, CH3(CH2)2CH2), 2.23 (t, 2H, J = 7.8 Hz, anhydrous Et2O (10 mL x 3) and dried in a stream of CO2 to
CH3(CH2)3CH2), 3.58 (s, 4H, NCH2CH2NH), 4.25 (br s, 1H, NH). give imidazolium bicarbonate as a white powder.
13C NMR (100 MHz, CDCl3, 25 °C) δ (ppm): 14.0 (CH3), 22.4 2-(1-Butylamino)-4,5-dihydro-1H-imidazolium bicarbonate (3c).
1
(CH3CH2), 26.4 (CH3CH2CH2), 29.4 (CH3(CH2)2CH2), 31.6 Yield 925 mg, 91%. H NMR (400 MHz, D2O, 25 °C) δ (ppm):
(CH3(CH2)3CH2), 49.7 (NCH2CH2NH), 168.2 (N=C-N).
0.88 (t, 3H, J = 7.3 Hz, CH3), 1.32 (sext, 2H, J = 7.6 Hz, CH3CH2),
1.52 (quin, 2H, J = 7.1 Hz, CH3CH2CH2), 3.16 (t, 2H, J = 7.1 Hz,
NHCH2), 3.65 (s, 4H, NCH2CH2NH). 13C NMR (100 MHz, D2O,
4.2.3. 2-(1-Butylamino)-4,5-dihydro-1H-imidazole (1c)
A solution of methyl iodide (8.5 g, 60 mmol) was added 0.4% dioxane (67.4 ppm), 25 °C) δ (ppm): 13.7 (CH3), 20.0
dropwise for min to the stirring suspension of (CH3CH2), 31.1 (CH3CH2CH2), 43.0 (NHCH2), 43.4 (NCH2CH2NH),
5
−
ethylenethiourea (5.1 g, 50 mmol) in dry ethanol (100 mL) at 160.7 (N=C−N), 161.5 (HCO3 ). Anal. Calcd for C8H17N3O3: C,
room temperature. After the mixture was stirred at room 47.28; H, 8.43; N, 20.68. Found: C, 47.12; H, 8.47; N, 20.61.
temperature for 6 h, the mixture was evaporated under
reduced pressure. The residue was washed with Et2O (50 mL), 4.3.2. CO2 capture-release behavior of 1a, 1b, and 1c
and the precipitate was dried in vacuo for 12 h at 40 °C to give CO2 (>99.999%, H2O < 5 ppm) and Ar (>99.9999%, H2O < 1
S-methylisothiouronium iodide as a white solid. A butylamine ppm) gases were passed through calcium chloride and silica gel
(4.0 g, 55 mmol) was added to a stirring suspension of the (20 x 400 mm) before use for the reaction. The solution of
obtained white solid in anhydrous THF (50 mL) at room imidazoline (4 mmol) in anhydrous DMSO (10 mL) was
temperature. The mixture was stirred at 40 °C for 24 h, then preheated at 60 °C under bubbling dry Ar for 30 min before
evaporated under reduced pressure. The residue was washed the initial CO2 fixation step. Dry CO2 gas was bubbled into a
with Et2O, and the precipitate was dried in vacuo for 12 h at solution (0.2 L/min) of the imidazoline in anhydrous DMSO at
40 °C to give N-butylguanidine hydroiodide as pale yellow oil. 25 °C for 1 h. Then, dry Ar gas was introduced into the mixture
The hydroiodide was added to a 40% NaOHaq (20 mL). To the at a flow rate of 0.2 L/min at 60 °C for 1 h. The above CO2
mixture was added CH2Cl2 (40 mL), and was vigorously stirred fixation-release steps were repeated one more time. The CO2
at room temperature for 15 min. The layers were separated, capture rates were estimated from the weight change of the
and the aqueous layer was extracted with CH2Cl2 (10 mL x 3). reaction mixture by using a blank anhydrous DMSO (10 mL)
The combined organic phase was dried with Na2SO4, then solution.
evaporated under reduced pressure. The residue was dried in
vacuo for 12 h at 40 °C to give 1c as a pale yellow solid.
2-(Methylthio)-4,5-dihydro-1H-imidazole hydroiodide:
1
Yield 12.2 g, 100%. H NMR (400 MHz, DMSO-d6, 25 °C) δ
Conflicts of interest
(ppm): 2.63 (s, 3H, SCH3), 3.86 (s, 4H, CH2CH2), 9.96 (brs, 2H,
2(NH)). 13C NMR (100 MHz, DMSO-d6, 25 °C) δ (ppm): 13.6
(SCH3), 45.2 (CH2CH2), 170.4 (N=C-N).
There are no conflicts to declare.
2-(1-Butylamino)-4,5-dihydro-1H-imidazole hydroiodide:
1
Yield 13.5 g, 100%. H NMR (400 MHz, CDCl3, 25 °C) δ (ppm):
0.94 (t, 3H, J = 7.3 Hz, CH3), 1.45 (sext, 2H, J = 7.5 Hz, CH3CH2), Acknowledgements
1.64 (quin, 2H, J = 7.5 Hz, CH3CH2CH2), 3.40 (t, 2H, J = 7.0 Hz,
This work was financially supported by JSR Co., Ltd.
NHCH2), 3.76 (s, 4H, NCH2CH2NH), 7.48 (br s, 3H, 3(NH)). 13C
NMR (100 MHz, CDCl3, 25 °C) δ (ppm): 13.7 (CH3), 19.9
(CH3CH2), 30.9 (CH3CH2CH2), 42.6 (NCH2CH2NH), 43.9 (NHCH2),
159.7 (N=C-N).
Notes and references
1
2
3
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6 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C7NJ03133E
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DOI: 10.1039/C7NJ03133E
GRAPHICAL ABSTRACT
Substituent Dependence of Imidazoline Derivatives on Capture and
Release System of Carbon Dioxide
Yoshiaki Yoshida, Naoto Aoyagi, Takeshi Endo*
The imidazoline having a cyclic guanidine moiety only captured CO2 quantitatively, and
then the cyclic guanidine derivative gave the guanidinium bicarbonate by CO2 fixation
together with a slight amount of water.
1