CO2capture capacity of five- and six-membered cyclic amidines bearing silatranyl group under dry conditions

Article abstract of DOI:10.1016/j.tet.2017.01.012

CO₂capture and release behaviors of three amidines bearing silatranyl group in DMSO solution were evaluated under dry conditions containing a very small amount of water. A six-membered cyclic amidine with silatranyl group captured CO₂at 25?°C under atmospheric pressure quantitatively, and the trapped CO₂was released at 60?°C under Ar atmosphere. A five-membered cyclic amidine with silatranyl group also captured CO₂, but less efficiently, under the same conditions as above. In contrast, an acyclic amidine with silatranyl group did not capture CO₂at all, as expected from the poor CO₂-capturing ability of the acyclic amidine moiety.

Full text of DOI:10.1016/j.tet.2017.01.012

Accepted Manuscript
CO capture capacity of five- and six-membered cyclic amidines bearing silatranyl
2
group under dry conditions
Naoto Aoyagi, Takeshi Endo
PII:
S0040-4020(17)30025-X
10.1016/j.tet.2017.01.012
TET 28382
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 14 October 2016
Revised Date: 5 January 2017
Accepted Date: 6 January 2017
Please cite this article as: Aoyagi N, Endo T, CO capture capacity of five- and six-membered
2
cyclic amidines bearing silatranyl group under dry conditions, Tetrahedron (2017), doi: 10.1016/
j.tet.2017.01.012.
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ACCEPTED MANUSCRIPT
CO2 capture capacity of five- and six-
Leave this area blank for abstract info.
membered cyclic amidines bearing silatranyl
group under dry conditions
Naoto Aoyagi, and Takeshi Endo*
Molecular Engineering Institute, Kindai University, 11-6 Kayanomori, Iizuka, Fukuoka 820-8555, Japan

ACCEPTED MANUSCRIPT
Tetrahedron
journal homepage: www.elsevier.com
CO2 capture capacity of five- and six-membered cyclic amidines bearing silatranyl
group under dry conditions
Naoto Aoyagi and Takeshi Endo
Molecular Engineering Institute, Kindai University, 11-6 Kayanomori, Iizuka, Fukuoka 820-8555, Japan
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
CO2 capture and release behaviors of three amidines bearing silatranyl group in DMSO solution
were evaluated under dry conditions containing a very small amount of water. A six-membered
cyclic amidine with silatranyl group captured CO2 at 25 °C under atmospheric pressure
quantitatively, and the trapped CO2 was released at 60 °C under Ar atmosphere. A five-
membered cyclic amidine with silatranyl group also captured CO2, but less efficiently, under the
same conditions as above. In contrast, an acyclic amidine with silatranyl group did not capture
CO2 at all, as expected from the poor CO2-capturing ability of the acyclic amidine moiety.
Available online
Keywords:
Amidine
Silatrane
2009 Elsevier Ltd. All rights reserved
.
Carbon dioxide
Capture
Release
———
Corresponding author. Tel.: +81-948-22-9210; fax: +81-948-21-9032; e-mail: tendo@moleng.fuk.kindai.ac.jp

2
Tetrahedron
ACCEPTED MANUSCRIPT
1. Introduction
The increasing concentration of CO2 in the atmosphere by
anthropogenic consumption of fossil fuels has brought about
serious environmental problems,1 whereas CO2 is also considered
to be a cheap, green C1 source, owing to its harmless nature and
usefulness in organic transformations.2 Development of efficient
methods for CO2 capture from waste gases and/or the atmosphere
has become an important research area in recent years. One of the
most studied processes relies on chemically reversible CO2
capture by primary or secondary amines at ambient temperature
to give ammonium carbamates, and CO2 release upon heating.3,4
For example, styrene- or siloxane based polymers have been
studied as reversible CO2 capturing materials.4 However, these
processes require 2 equivalents of amines for CO2 capture and
high temperature for CO2 release.
Chart. Chemical structures of amidines bearing silatranyl group.
by the reaction with (3-chloropropyl)trimethoxysilane (3) to
afford N1-(3-(trimethoxysilyl)propyl)propane-1,3-diamine (4).
Treatment of 4 with N,N-dimethylformamide dimethyl acetal
gave the six-membered cyclic amidine with trimethoxysilyl
group (2a). The trimethoxysilane (2a) was then allowed to react
with triethanolamine in the presence of a catalytic amount of
KOH in benzene to afford the cyclic amidine with silatranyl
group (1a) quantitatively. The five-membered cyclic amidine
with trimethoxy group (2b) was obtained by the reaction of
Amidines and guanidines are considered more attractive as
reversible CO2 capture-release compounds, since their CO2
capture requires 1 equivalent of the amidines and the CO2 release
proceeds readily at low temperature.5-7 Previously, we reported
that polystyrene derivatives bearing cyclic amidine pendant
groups absorbed CO2 at a pressure of 1 atm in the solid state as
well as in solution, and desorbed the CO2 gas under N2
atmosphere.5a,5b
The
CO2-capturing
reaction
occurs
commercially
available
3-(2-
simultaneously with the capture of water or alcohol molecules to
yield bicarbonates and have been recently applied as switchable
ionic liquids and surfactants.6 On the other hand, the amidines
cannot efficiently trap CO2 molecules under dry conditions and a
certain amount of water (e.g., 700 ppm for 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU)) is necessary for the CO2
capture through formation of the bicarbonate salts.6a It is
important to search for amidine derivatives that can capture CO2
under dry conditions containing a very small amount of water
from a standpoint of developing catalytic systems for CO2-
insertion reactions of water-labile substrates. We have recently
aminoethylamino)propyltrimethoxysilane
(5)
with
N,N-
dimethylformamide dimethyl acetal in toluene. The
trimethoxysilyl group of 2b was then converted to silatranyl
group by the reaction with triethanolamine to afford 1b in 98%
yield. The acyclic amidine with silatranyl group (1c) was
synthesized in good yield by the reaction of 3-
aminopropyltrimethoxysilane (6) with N,N-dimethylformamide
dimethyl acetal in toluene, followed by silatranylation with
triethanolamine in the presence of KOH catalyst in benzene.
found that
a
six-membered cyclic amidine, 1,4,5,6-
tetrahydropyrimidine, captured CO2 quantitatively under dry
conditions, where DBU captured CO2 much less efficiently.5g
Silatranes are chemically stable pentavalent organic silicon
compounds, and their chemical properties and biological
activities have been studied since the 1970s.8 The silatranyl
group has been recently employed to functionalize silica
surfaces.8,9 In general, alkyltrialkoxysilanes are used for such use.
However, they are very susceptible to hydrolysis, so that it is
difficult to get desired thickness and morphology of coatings. On
the other hand, the silatranyl groups are more stable toward
hydrolysis when compared to related triethoxysilanes and react
with silanols on silicon surfaces to form a monolayer in aqueous
solution, thereby being excellent anchors for functionalizing
silicon surfaces. Moreover, much attention has been paid to
silatranes in the field of materials science.10,11 For example,
owing to their large dipole moments along the transannular
coordinate bond between silicon and nitrogen, silatranes were
found to dissolve Li salts to form amorphous complexes, which
showed good ion conductivity and thermal stability.11 Thus, it
should be of quite interest to design and synthesize amidine
compounds bearing silatranyl groups from viewpoints of both
surface chemistry as well as materials science. In this study, we
have designed and synthesized amidines bearing silatranyl group
and investigated their CO2 capture and release behavior (Chart).
Scheme 1. Synthesis of the amidines with silatranyl group (1a–
c). Reagents and conditions: (i) 1,3-propanediamine (5 eq.), 90
°C, 1 h; (ii) Me2NCH(OMe)2 (1.1 eq.), toluene, 80 °C, 24 h; (iii)
N(CH2CH2OH)3 (1 eq.), KOH (1 mol%), benzene, 90 °C, 24 h;
(iv) Me2NCH(OMe)2 (1.05 eq.), 50 °C, 1 h.
2. Results and discussion
The amidines bearing silatranyl group (1a–1c) were
synthesized as shown in Scheme 1. Trimethyoxysilyl group was
introduced to one of the nitrogen atoms of 1,3-propanediamine
Characterization of the amidines with silatranyl group was
1
achieved by H, 13C, and 29Si NMR, and FT-IR spectroscopies

along with elemental analysis. Characteristic data
ACCEPTEaDre MANUSCRIPT
summarized in Table 1. The characteristic absorption peaks of
the silatrane structure, the stretch of the transannular N–Si, were
clearly observed at 576–579 cm–1.10c–10e The existence of the
silatrane unit was also confirmed by 29Si NMR spectra, which
showed single peaks at high magnetic field of ca. –67 ppm, being
consistent with their five-coordinate structures of the
1
alkylsilatrane moiety.10a,10d The H NMR spectra showed singlets
at low magnetic field of ca. 7 ppm assignable to the methine
protons of the amidine moieties. The signals of the methine
carbons were observed at 150–158 ppm in the 13C NMR spectra.
These 1H and 13C signals of the amidine moieties were almost the
same as those of the amidines with trimethoxysilyl group,
indicating that there is virtually no interaction between the
amidine units and the silatrane units.
Table 1. Spectral data for 1a–1c.
1a
1b
1c
Fig. 1. CO2 capture-release behavior of 1a (solid line) and 1b
(dashed line). a) Flow rate of CO2 and Ar: 200 mL/min; b) The
CO2 fixation efficiency was calculated based on the bicarbonate
1a or 1b.
FTꢀIR (ATR): νSiꢀN/cm–1
νC=N/cm–1
576
576
579
1615
–67.1
6.96
150.6
1595
–67.1
6.80
158.0
1644
–64.8
7.24
154.7
29Si NMR:
1H NMR:
13C NMR:
δ
(
Si)/ppma
(C
=N)/ppma
δ(C
H=N)/ppma
δ
H
solution. The CO2 capture and release cycle could be repeated
once more. CO2 capture and release behavior of 1b in DMSO
was examined similarly by monitoring the weight change of the
solution. In contrast to the case of 1a, the maximum amount of
the captured CO2 per amidine unit remained only 9%. The
captured CO2 was completely expelled out under Ar flow at 60
°C. The maximum amount of the captured CO2 at the second run
(16%) was higher than that at the first CO2 uptake. This is
probably because water was accumulated in the solution from the
CO2 and gas during the first cycle bubbling. A solution of the
acyclic formamidine with silatranyl group (1c) in DMSO did not
uptake CO2 at all as expected from our previous result that a
similar acyclic amidine, N,N-dimethyl-N'-octylformamidine, did
not capture CO2 in solution at ambient temperature.5g
a Measured in CDCl3 at 25 °C.
We next investigated the capture and release of CO2 by the
silatranyl amidines in solution (Fig. 1). The amidines bearing
trimethoxysilyl group could not be employed for this use,
because the trimethoxysilyl group is highly susceptible to
hydrolysis in the presence of bicarbonate and amidinium ions,
and water, which leads irreversibly to polymerization to form
large aggregates. The capture of CO2 by 1a was carried out by
bubbling CO2 gas (0.2 L/min, H2O <5 ppm) into a solution of 1a
in DMSO at 25 °C and the weight gain was monitored. CO2 was
captured very rapidly and the amount of the captured CO2 per
amidine moiety reached nearly 100% within 30 min. The
resulting bicarbonate salt of 1a was insoluble and precipitated
from the solution. The bicarbonate salt could not be isolated
because it was highly deliquescent. Then, Ar gas (H2O <1 ppm)
was introduced into the mixture at a flow rate of 0.2 L/min at 60
°C. All the trapped CO2 was released within 30 min, and the
mixture reverted to its original state, i.e., a colorless clear
We further investigated the CO2 binding to and release from
1a in DMSO-d6 by 1H NMR spectroscopy (Fig. 2). Upon
bubbling the CO2 into a solution of 1a in DMSO-d6 at 25 °C for 1
h, the signal of the methine proton (N=CH-N) shifted to a lower
magnetic field of 8.47 ppm and all the proton signals of two
propylene groups also shifted to lower magnetic fields, which
indicated the formation of amidinium bicarbonate. The CO2 was
1
Fig. 2. H NMR spactra (400 MHz, 25 °C) of the CO2 capture and release of 1a in DMSO-d6 (0.1 M). Flow rates of CO2 and Ar
were 200 mL/min.

4
Tetrahedron
ACCEPTED MANUSCRIPT
not bound to the nitrogen atom of the silatranyl ring, because
evaporated in vacuo, and the residue was added toluene. The
the signals of the aminomethyl protons in silatranyl ring
precipitate was filtered off, and the filtrate was evaporated to
obtain the crude title compound 4 (> 99%) as a pale yellow oil.
The crude product was purified by distillation under reduced
pressure to give the pure title compound 4 (40.7 g, 86%) as a
(Si(OCH2CH2)3N) did not change after bubbling CO2 gas. Then,
Ar gas was introduced into the mixture of [1aH]+HCO3 in
–
DMSO-d6 at 60 °C for 1h. All the proton signals of the product
were consistent with the H NMR spectrum of 1a before CO2
1
1
colorless liquid; Bp: 112-114 °C/1 mmHg; H NMR (400 MHz,
adsorption. This indicated that the capture and release of CO2
occurred without decomposition of the silatrane moiety as a
result of its high stability toward hydrolysis.
CDCl3, 25 °C) δ (ppm): 0.66 (t, J = 9.2 Hz, 2H, CH2Si), 1.12
(brs, 3H, NH, NH2), 1.56-1.67 (m, 4H, CH2CH2Si, NH2CH2CH2),
2.61 (t, J = 7.3 Hz, 2H, CH2CH2CH2Si), 2.67 (t, J = 7.3 Hz, 2H,
NH2CH2CH2CH2), 2.76 (t, J = 6.9 Hz, 2H, NH2CH2), 3.57 (s, 9H,
(OCH3)3); 13C NMR (100 MHz, CDCl3, 25 °C) δ (ppm): 6.54
(CH2Si), 22.9 (CH2CH2Si), 33.9 (NH2CH2CH2), 40.4
(NH2CH2CH2), 47.6 (NH2CH2CH2CH2), 50.4 ((OCH3)3), 52.6
(CH2CH2CH2Si); 29Si NMR (79.5 MHz, CDCl3, 25 °C) δ (ppm): -
41.7 (Si(OCH3)3); IR (cm-1): 3362 (NH2), 3295 (NH2, NH), 2936
(CH), 2839 (OCH3), 1190 (CN), 1078 (CO), 828 (SiO), 771
(CSi); EI-MS (m/z): 236 (M+).
3. Conclusion
CO2 capture and release behaviors of the silatranyl amidines
in DMSO solution were investigated under dry conditions
containing a very small amount of water by monitoring the mass
change of the reaction systems. The six-membered cyclic
amidine with silatranyl group captured CO2 quantitatively at 25
°C under atmospheric pressure to form the corresponding
amidinium bicarbonate with adventitious water, and the trapped
CO2 was released at 60 °C under Ar atmosphere. The excellent
ability of the six-membered cyclic amidine moiety was
undoubtedly reflected in this CO2 capture and release behavior.
The five-membered cyclic amidine with silatranyl moiety also
captured CO2, but less efficiently, under the same conditions as
above. In contrast, the acyclic amidine with silatranyl group did
not capture CO2 at all, as expected from the poor ability of the
acyclic amidine moiety in CO2 capture. Thus, the silatranyl
pendant group does not affect the CO2 capture-release behavior
of the amidine moieties, thereby being a candidate for an
excellent anchoring group to silicon surfaces. In addition, ionic
conductive materials could be designed and synthesized based on
the amidines with silatranyl group, and such studies are ongoing
in our laboratory.
4.2.2. Preparation of 1-[3-(Trimethoxysilyl)propyl]-1,4,5,6-
tetrahydropyrimidine (2a)
N,N-Dimethylformamide dimethyl acetal (13.1 g, 110 mmol)
was added to the mixture of 4 (23.6 g, 100 mmol) in anhydrous
toluene (100 mL) at room temperature, and the mixture was
stirred for 24 h at 80 °C. The mixture was evaporated in vacuo,
and the residue was distilled under reduced pressure to give the
title compound 2a (23.9 g, 97%) as a colorless liquid; Bp: 109
1
°C/1 mmHg; H NMR (400 MHz, CDCl3, 25 °C) δ (ppm): 0.58
(t, J = 8.2 Hz, 2H, CH2Si), 1.56-1.66 (m, 2H, CH2CH2Si), 1.83
(quin, J = 6.0 Hz, 2H, CH=NCH2CH2CH2N), 3.03 (t, J = 6.9 Hz,
2H, CH2CH2CH2Si), 3.12 (t,
CH=NCH2CH2CH2N), 3.29 (t,
J
J
=
=
6.0 Hz, 2H,
6.0 Hz, 2H,
CH=NCH2CH2CH2N), 3.58 (s, 9H, (OCH3)3), 6.97 (s, 1H,
N=CHN); 13C NMR (100 MHz, CDCl3, 25 °C) δ (ppm): 5.74
(CH2Si), 21.1 (CH=NCH2CH2CH2N), 21.4 (CH2CH2Si), 43.0
(CH=NCH2CH2CH2N), 43.2 (CH=NCH2CH2CH2N), 50.5
((OCH3)3), 55.2 (CH2CH2CH2Si), 150.1 (N=CHN); 29Si NMR
(79.5 MHz, CDCl3, 25 °C) δ (ppm): -42.5 (Si(OCH3)3); IR (cm-1):
2929 (CH), 2841 (OCH3), 1629 (C=N), 1103 (CN), 1034 (CO),
814 (SiO), 784 (CSi); EI-MS (m/z): 246 (M+).
4. Experimental section
4.1. Materials and instruments
CO2 (>99.999%, H2O <5 ppm) and Ar (>99.9999%, H2O <1
ppm) were obtained from Yamagata Sanso (Yamagata, Japan).
All other reagents were purchased from Aldrich, Wako Pure
Chemical Industries (Osaka, Japan), Tokyo Kasei Kogyo (Tokyo,
Japan), and Kanto Chemical (Tokyo, Japan). All starting
materials (3, 5, 6) and solvents were distilled prior to use. 1,3-
Diaminopropane was dried over CaH2, and then distilled prior to
use. All other reagents were used as received without further
purification. Proton (1H, 400 MHz), Carbon-13 (13C, 100 MHz),
and Silicon-29 (29Si, 79.5 MHz) nuclear magnetic resonance
(NMR) spectroscopy measurements were performed on a JEOL
ECS-400/SS instrument using tetramethylsilane as an internal
standard at 25 °C. The infrared (IR) spectra were recorded by
attenuated total reflection (ATR) method on a Thermo Fisher
Scientific Nicolet iS10 spectrometer. Mass spectroscopy was
4.2.3.
Synthesis
of
1-[3-(Silatranyl)propyl]-1,4,5,6-
tetrahydropyrimidine (1a)
A solution of 2a (7.39 g, 30 mmol) in anhydrous benzene (15
mL) was added dropwise for 5 min to the stirring mixture of
triethanolamine (4.48 g, 30 mmol) and potassium hydroxide (20
mg, 0.3 mmol) in anhydrous benzene (30 mL) at 60 °C. After the
mixture was stirred at 90 °C for 24 h, the mixture was evaporated
in vacuo. The residue was purified by recrystallization from
toluene to give the title compound 1a (8.97 g, > 99%) as a white
1
solid; Mp: 131-134 °C; H NMR (400 MHz, CDCl3, 25 °C) δ
(ppm): 0.33 (t, J = 8.2 Hz, 2H, CH2Si), 1.55-1.63 (m, 2H,
CH2CH2Si), 1.80 (quin, J = 6.0 Hz, 2H, CH=NCH2CH2CH2N),
2.81 (t, J = 6.0 Hz, 6H, Si(OCH2CH2)3N), 2.97 (t, J = 7.3 Hz, 2H,
CH2CH2CH2Si), 3.13 (t, J = 6.0 Hz, 2H, CH=NCH2CH2CH2N),
3.25 (t, J = 6.0 Hz, 2H, CH=NCH2CH2CH2N), 3.76 (t, J = 6.0
Hz, 6H, Si(OCH2CH2)3N), 6.97 (s, 1H, N=CHN); 13C NMR (100
performed on
a Shimadzu GCMS-QP5050A in electron
ionization (EI) mode. Electrospray ionization high-resolution
mass spectra (ESI-HRMS) were obtained on a JEOL AccuTOF
JMS-T100LP.
MHz, CDCl3, 25 °C)
δ
(ppm): 12.9 (CH2Si), 21.3
(CH=NCH2CH2CH2N),
(CH=NCH2CH2CH2N), 43.4 (CH=NCH2CH2CH2N), 51.0
24.2 (CH2CH2Si), 43.1
4.2. Synthesis of amidines bearing silatranyl group
4.2.1. Preparation of N1-(3-(trimethoxysilyl)propyl)propane-1,3-
diamine (4)
(Si(OCH2CH2)3N),
56.4
(CH2CH2CH2Si),
57.7
(Si(OCH2CH2)3N), 150.5 (N=CHN); 29Si NMR (79.5 MHz,
CDCl3, 25 °C) δ (ppm): -67.1 (Si(OCH2CH2)3N); IR (cm-1): 2921
(CH), 2868 (OCH2), 1615 (C=N), 1120 (CN), 1085 (CO), 746
(CSi), 576 (SiN). ESI-HRMS (m/z): calcd for C13H26N3O3Si
(M+H)+ 300.1743, found 300.1765.
(3-Chloropropyl)trimethoxysilane (39.8 g, 200 mmol) was
added to a solution of anhydrous 1,3-propanediamine (74.1 g,
1.00 mol) at room temperature under nitrogen atmosphere, and
the mixture was stirred for 1 h at 90 °C. The mixture was

4.2.4.
imidazoline (2b)
Preparation
of
1-[3-(Trimethoxysilyl)propyl]-2-
triethanolamine (2.98 g, 20 mmol) and potassium hydroxide (13
ACCEPTED MANUSCRIPT
mg, 0.2 mmol) in anhydrous benzene (20 mL) at 60 °C. After the
mixture was stirred at 90 °C for 24 h, the mixture was evaporated
in vacuo. The residue was purified by recrystallization from n-
hexane/ethyl acetate (5 : 1) to give the title compound 1c (5.64 g,
98%) as a white solid; Mp: 61-64 °C; 1H NMR (400 MHz,
CDCl3, 25 °C) δ (ppm): 0.37 (t, J = 8.2 Hz, 2H, CH2Si), 1.55-
N,N-Dimethylformamide dimethyl acetal (6.55 g, 55 mmol)
was added to the mixture of 5 (11.1 g, 50 mmol) in anhydrous
toluene (100 mL) at room temperature, and the mixture was
stirred for 24 h at 80 °C. The mixture was evaporated in vacuo,
and the residue was distilled under reduced pressure to give 2b
1.64 (m, 2H, CH2CH2Si), 2.79 (t,
J = 6.0 Hz, 6H,
1
(11.4 g, 98%) as a colorless liquid; Bp: 100-101 °C/1 mmHg; H
Si(OCH2CH2)3N), 2.79 (s, 6H, N(CH3)2), 3.19 (t, J = 7.2 Hz, 2H,
CH2CH2CH2Si), 3.75 (t, J = 6.0 Hz, 6H, Si(OCH2CH2)3N), 7.24
(s, 1H, N=CHN); 13C NMR (100 MHz, CDCl3, 25 °C) δ (ppm):
13.4 (CH2Si), 28.1 (CH2CH2Si), 37.0 (N(CH3)2), 51.0
NMR (400 MHz, CDCl3, 25 °C) δ (ppm): 0.63 (t, J = 8.4 Hz, 2H,
CH2Si), 1.60-1.68 (m, 2H, CH2CH2Si), 3.09 (t, J = 7.2 Hz, 2H,
CH2CH2CH2Si), 3.19 (t, J = 9.6 Hz, 2H, CH=NCH2CH2N), 3.57
(s, 9H, (OCH3)3), 3.81 (dt,
J = 10.0, 2.0 Hz, 2H,
(Si(OCH2CH2)3N),
57.7
(Si(OCH2CH2)3N),
59.6
CH=NCH2CH2N), 6.80 (s, 1H, N=CHN); 13C NMR (100 MHz,
CDCl3, 25 °C) δ (ppm): 6.11 (CH2Si), 21.9 (CH2CH2Si), 48.2
(CH=NCH2CH2N), 49.9 (CH=NCH2CH2N), 50.5 ((OCH3)3), 55.0
(CH2CH2CH2Si), 157.8 (N=CHN); 29Si NMR (79.5 MHz, CDCl3,
25 °C) δ (ppm): -42.4 (Si(OCH3)3); IR (cm-1): 2939 (CH), 2839
(OCH3), 1602 (C=N), 1185 (CN), 1073 (CO), 803 (SiO), 771
(CSi); EI-MS (m/z): 232 (M+).
(CH2CH2CH2Si), 154.7 (N=CHN); 29Si NMR (79.5 MHz, CDCl3,
25 °C) δ (ppm): -64.8 (Si(OCH2CH2)3N); IR (cm-1): 2923 (CH),
2869 (OCH2), 1644 (C=N), 1121 (CN), 1096 (CO), 709 (CSi),
579 (SiN). ESI-HRMS (m/z): calcd for C12H26N3O3Si (M+H)+
288.1743, found 288.1768.
4.3. Typical reaction for the CO2 capture-release behavior of
1a-c.
4.2.5. Synthesis of 1-[3-(Silatranyl)propyl]-2-imidazoline (1b)
CO2 (H2O <5 ppm) and Ar (H2O <1 ppm) gases were passed
through calcium chloride and silica gel (20 × 400 mm) before use
for the reaction. The mixture of amidine (5 mmol) in anhydrous
DMSO (10 mL) was preheated at 60 °C under bubbling dry Ar
for 30 min before the initial CO2 fixation step. Dry CO2 gas was
bubbled into a solution (0.2 L/min) of the amidine in anhydrous
DMSO at 25 °C for 1 h. Then, dry Ar gas was introduced into the
mixture at a flow rate of 0.2 L/min at 60 °C for 1 h. The above
CO2 fixation-release steps were repeated one more time. The CO2
capture rates were estimated from the weight change of the
reaction mixture by using a blank anhydrous DMSO (10 mL)
solution.
A solution of 2b (4.65 g, 20 mmol) in anhydrous benzene (10
mL) was added dropwise for 5 min to the stirring mixture of
triethanolamine (2.98 g, 20 mmol) and potassium hydroxide (13
mg, 0.2 mmol) in anhydrous benzene (20 mL) at 60 °C. After the
mixture was stirred at 90 °C for 24 h, the mixture was evaporated
in vacuo. The residue was purified by recrystallization from
toluene to give the title compound 1b (5.61 g, 98%) as a white
1
solid; Mp: 113-115 °C; H NMR (400 MHz, CDCl3, 25 °C) δ
(ppm): 0.37 (t, J = 8.6 Hz, 2H, CH2Si), 1.55-1.69 (m, 2H,
CH2CH2Si), 2.82 (t, J = 6.0 Hz, 6H, Si(OCH2CH2)3N), 3.05 (t, J
= 7.6 Hz, 2H, CH2CH2CH2Si), 3.21 (t, J = 10.0 Hz, 2H,
CH=NCH2CH2N), 3.77 (t, J = 6.0 Hz, 6H, Si(OCH2CH2)3N),
3.70-3.81 (m, 2H, CH=NCH2CH2N), 6.80 (s, 1H, N=CHN); 13C
NMR (100 MHz, CDCl3, 25 °C) δ (ppm): 13.2 (CH2Si), 24.3
(CH2CH2Si), 48.2 (CH=NCH2CH2N), 50.7 (CH=NCH2CH2N),
References and notes
1. (a) Haszeldine, R. S. Science 2009, 325, 1647-1652; (b) Hunt, A.
J.; Sin, E. H. K.; Marriott, R.; Clark, J. H. ChemSusChem 2010, 3,
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Chem. Res. 2010, 43, 751-760.
50.9
(Si(OCH2CH2)3N),
54.8
(CH2CH2CH2Si),
57.6
(Si(OCH2CH2)3N), 158.0 (N=CHN); 29Si NMR (79.5 MHz,
CDCl3, 25 °C) δ (ppm): -67.1 (Si(OCH2CH2)3N); IR (cm-1): 2921
(CH), 2872 (OCH2), 1595 (C=N), 1118 (CN), 1086 (CO), 753
(CSi), 576 (SiN). ESI-HRMS (m/z): calcd for C12H24N3O3Si
(M+H)+ 286.1586, found 286.1623.
4.2.6.
Preparation
of
N,N-dimethyl-N'-(3-
(trimethoxysilyl)propyl)formamidine (2c)
4. (a) Tsuda, T.; Fujiwara, T. J. Chem. Soc., Chem. Commun. 1992,
1659-1661; (b) Tsuda, T.; Fujiwara, T.; Taketani, Y.; Saegusa, T.
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N,N-Dimethylformamide dimethyl acetal (5.01 g, 42 mmol)
was added to a solution of 6 (7.17 g, 40 mmol) at room
temperature, and the mixture was stirred for 1 h at 50 °C. The
mixture was evaporated in vacuo, and the residue was distilled
under reduced pressure to give the title compound 2c (8.91 g,
95%) as a colorless liquid; Bp: 87-88 °C/3 mmHg. 1H NMR (400
MHz, CDCl3, 25 °C) δ (ppm): 0.63 (t, J = 8.4 Hz, 2H, CH2Si),
1.58-1.66 (m, 2H, CH2CH2Si), 2.83 (s, 6H, N(CH3)2), 3.22 (t, J =
6.8 Hz, 2H, CH2CH2CH2Si), 3.57 (s, 9H, (OCH3)3), 7.27 (s, 1H,
N=CHN); 13C NMR (100 MHz, CDCl3, 25 °C) δ (ppm): 6.26
(CH2Si), 25.4 (CH2CH2Si), 36.9 (N(CH3)2), 50.2 ((OCH3)3), 58.7
(CH2CH2CH2Si), 154.8 (N=CHN); 29Si NMR (79.5 MHz, CDCl3,
25 °C) δ (ppm): -41.1 (Si(OCH3)3); IR (cm-1): 2937 (CH), 2839
(OCH3), 1648 (C=N), 1188 (CN), 1077 (CO), 814 (SiO), 775
(CSi); EI-MS (m/z): 234 (M+).
6. (a) Heldebrant, D. J.; Jessop, P. G.; Thomas, C. A.; Eckert, C. A.;
Liotta, C. L. J. Org. Chem. 2005, 70, 5335-5338; (b) Jessop, P. G.;
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4.2.7.
Synthesis
of
N,N-dimethyl-N'-[3-
(Silatranyl)propyl]formamidine (1c)
A solution of 2c (4.69 g, 20 mmol) in anhydrous benzene (10
mL) was added dropwise for 5 min to the stirring mixture of

6
Tetrahedron
7. (a) Zhang, Q.; Wang, W.-J.; Lu, Y.; Li, B.-G.; Zhu, S.
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ACCEPTED MANUSCRIPT
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