Dinuclear Aluminum Poly(phenolate) Complexes as Efficient Catalysts for Cyclic Carbonate Synthesis

Article abstract of DOI:10.1021/acs.organomet.6b00153

A series of dinuclear aluminum complexes 1-4 stabilized by amine-bridged poly(phenolato) ligands have been synthesized, which are highly active in catalyzing the cycloaddition of epoxides and CO₂. In the presence of 0.3 mol % complex 3 and 0.9 mol % NBu₄Br at 1 bar CO₂ pressure, terminal epoxides bearing different functional groups were converted to cyclic carbonates in 60-97% yields. Complex 3 is one of the rare examples of Al-based catalysts capable of promoting the cycloaddition at 1 bar pressure of CO₂. Moreover, reactions of more challenging disubstituted epoxides also proceeded at an elevated pressure of 10 bar and afforded cyclic carbonates in 52-90% yields.

Full text of DOI:10.1021/acs.organomet.6b00153

Article
pubs.acs.org/Organometallics
Dinuclear Aluminum Poly(phenolate) Complexes as Efficient
Catalysts for Cyclic Carbonate Synthesis
Pengfei Gao, Zhiwen Zhao, Lijuan Chen, Dan Yuan,* and Yingming Yao*
Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science, Dushu
Lake Campus, Soochow University, Suzhou 215123, People’s Republic of China
S
* Supporting Information
ABSTRACT: A series of dinuclear aluminum complexes 1−4
stabilized by amine-bridged poly(phenolato) ligands have been
synthesized, which are highly active in catalyzing the
cycloaddition of epoxides and CO2. In the presence of 0.3
mol % complex 3 and 0.9 mol % NBu4Br at 1 bar CO2
pressure, terminal epoxides bearing different functional groups
were converted to cyclic carbonates in 60−97% yields.
Complex 3 is one of the rare examples of Al-based catalysts
capable of promoting the cycloaddition at 1 bar pressure of
CO2. Moreover, reactions of more challenging disubstituted
epoxides also proceeded at an elevated pressure of 10 bar and afforded cyclic carbonates in 52−90% yields.
1 bar CO2 at room temperature using 2.5 mol % catalyst.
Cooperation of both metal centers is reported to contribute to
the high activity.9a−c Inspired by these reports, we worked on
the design and synthesis of dinuclear aluminum complexes
aiming at developing catalysts active under mild conditions (i.e.,
low temperature or pressure). On the basis of our previous
study on lanthanide complexes bearing ethylenediamine-
bridged tetraphenolato ligands, which efficiently catalyzed the
cycloaddition of CO2 and epoxides,13 multidentate poly-
(phenolato) ligands are employed to stabilize aluminum
centers. Four dinuclear aluminum complexes were prepared
and applied in cyclic carbonate synthesis.
INTRODUCTION
■
CO2 is an abundant, renewable, and nontoxic C1 feedstock, and
transformation of CO2 to value-added chemicals is attracting
increasing interest.1 For instance, coupling reactions of CO2
and epoxides generate cyclic carbonates in a 100% atom-
economic manner (Scheme 1), which find application as
Scheme 1. Synthesis of Cyclic Carbonates from Epoxides
and CO2
RESULTS AND DISCUSSION
■
pharmaceutical intermediates and aprotic solvents.2 A variety of
catalysts have been developed to catalyze this transformation,3
including transition metal4 (e.g., Zn, Cr, Co, and Fe) and main
group metal (e.g., Al5−12) complexes.
Synthesis of Complexes 1−4. Ligand precursors oxy-
ethylamine-bridged bis(phenol) (LnH3)(n = 1, R = Bu; n = 2,
t
R = CH3; and n = 3, R = Cl) and amine-bridged tri(phenol)
(L4H3) were easily prepared according to literature methods.14
Alkane elimination reactions of LnH3 with AlMe3 in THF at 25
°C proceeded straightforwardly and gave the corresponding
aluminum complexes 1−4 in good yields of 70−82% (Scheme
2).
Since aluminum is earth-abundant and nontoxic, application
of Al-based catalysts in CO2 transformation is highly desirable.
Aluminum complexes stabilized by various ligands, including
porphyrin,5 phthalocyanine,6 triphenolate,7 salen,8,9 salalen,10
pyrazole,11 and aminoethanol-derived12 ligands, have been
reported to catalyze this cycloaddition reaction. However, high
temperatures and pressures are in general required. Highly
active Al-based catalytic systems include an aluminum
triphenolate complex reported by Kleij et al., which, in
combination with ammonium iodide, efficiently catalyzed
reactions of mono- and disubstituted epoxides with 10 bar
CO2 at 70−90 °C in the presence of low catalyst loading of
0.05−0.5 mol %.7b Dinuclear aluminum(salen) complexes,9
mainly reported by North and co-workers, are exceptionally
effective in catalyzing the cycloaddition of terminal epoxides at
1H NMR spectra of 1−4 show the absence of signals for
phenolic and alcoholic protons (for 1−3), corroborating
successful complexation. Moreover, no resonances of coordi-
nating methyl groups were observed, suggesting a 1:1 ratio of
ligand to Al in complexes. Methylene protons are found to
resonate in the range of 4.20−2.90 ppm. Solid state structures
of complexes 3 and 4 reveal dinuclear centrosymmetric
structures (Figures 1 and 2). Each central metal ion is five-
Received: February 24, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.6b00153
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Scheme 2. Synthesis of Complexes 1−4
Figure 2. Solid state structure of one independent molecule of
complex 2×4·9THF. Thermal ellipsoids are drawn at the 50%
probability level, and hydrogen atoms and solvent molecules are
omitted for clarity. Selected bond lengths [Å] and bond angles [deg]:
Al1−O1 1.745(2), Al1−O2 1.734(2), Al1−O3 1.871(2), Al1−O6
1.900(2), Al1−N1 2.053(3), Al2−O4 1.728(2), Al2−O5 1.743(2),
Al2−O6 1.868(2), Al2−O3 1.894(2), Al2−N2 2.065(2), O2−Al1−O1
116.87(11), O2−Al1−O3 114.63(11), O1−Al1−O3 128.46(11), O2−
Al1−O6 104.56(10), O1−Al1−O6 91.25(10), O3−Al1−O6 73.96(9),
O2−Al1−N1 96.35(10), O1−Al1−N1 94.11(10), O3−Al1−N1
82.23(9), O6−Al1−N1 153.20(10), O4−Al2−O5 117.52(12), O4−
Al2−O6 115.72(11), O5−Al2−O6 126.69(10), O4−Al2−O3
103.18(10), O5−Al2−O3 91.42(10), O6−Al2−O3 74.16(9), O4−
Al2−N2 96.07(10), O5−Al2−N2 94.45(10), O6−Al2−N2 82.45(9),
and O3−Al2−N2 154.45(10).
Table 1. Catalytic Production of Cyclic Carbonate from CO2
a
and 1,2-Epoxyhexane (5a) Mediated by Complexes 1−4
b,c
entry
catalyst
Co-catalyst
T (°C)
t (h)
conversion
Figure 1. Solid state structure of complex 3·2THF. Thermal ellipsoids
are drawn at the 50% probability level, and hydrogen atoms and
solvent molecules are omitted for clarity. Selected bond lengths [Å]
and bond angles [deg]: Al1−O1 1.749(2), Al1−O2 1.752(2), Al1−O3
1.818(2), Al1−O3A 1.853(2), Al1−N1 2.068(2); O1−Al1−O2
118.66(11), O1−Al1−O3 120.65(11), O1−Al1−O3A 94.31(10),
O1−Al1−N1 95.04(10), O2−Al1−O3 120.68(11), O2−Al1−O3A
97.65(10), O2−Al1−N1 93.70(10), O3−Al1−O3A 77.02(10), O3−
Al1−N1 82.61(9), and O3A−Al1−N1 159.59(10).
1
1
2
3
4
3
3
3
3
3
3
3
3
3
3
3
3
TBAB
TBAB
TBAB
TBAB
100
100
100
100
100
85
18
18
18
18
18
18
18
18
18
12
24
18
18
18
18
18
90
85
96
99
ND
88
85
52
89
82
99
79
66
98
92
88
2
3
4
5
6
TBAB
TBAB
TBAI
7
70
8
100
100
100
100
100
100
100
85
9
TOAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
10
11
coordinated with four donor atoms from one ligand as well as
one bridging oxygen atom from the other ligand, adopting
trigonal bipyramidal geometry. Bond distances between Al and
phenolate oxygen atoms in 3 (Al1−O1 1.749(2) Å and Al1−
O2 1.752(2) Å) are found to be comparable, which are
significantly shorter than those of Al and bridging alcoholic
oxygen atoms (1.818(2) Å). Similar trends have been observed
in complex 4.
Catalytic Activities. The performances of complexes 1−4
were first examined and compared in the cycloaddition of CO2
to 1,2-epoxyhexane under solvent-free conditions at atmos-
pheric pressure (Table 1). At 100 °C, all complexes efficiently
catalyzed the cycloaddition in the presence of 0.6 mol %
NBu4Br as cocatalyst and generated the desired cyclic carbonate
in 85−99% yield (entry 1−4), which is one of the rare examples
of Al-based catalysts that is capable of promoting the
d
12
e
13
f
14
f
15
f
16
70
a
Reaction conditions: 0.3 mol % catalyst and 0.6 mol % cocatalyst.
b
c
Determined by 1H NMR spectroscopy. Selectivity for the cyclic
d
carbonate was all >99%. Reaction conditions: 0.2 mol % catalyst and
0.4 mol % cocatalyst. Reaction conditions: 0.3 mol % catalyst and 0.3
mol % cocatalyst. Reaction conditions: 0.3 mol % catalyst and 0.9 mol
% cocatalyst.
e
f
cycloaddition at 1 bar CO2 pressure.8h,j,9 The activity decreases
in the order of 4 ≈ 3 > 1 > 2, which may be attributed to the
more Lewis acidic Al centers in 3 and 4 due to the presence of
B
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electron-withdrawing chloro-substituents (in 3) and the phenyl
bridge (in 4). Because of its good activity and easy preparation,
complex 3 was employed for further optimization. No
conversion was detected in the absence of cocatalyst (entry
5). Lowering the temperature to 85 and 70 °C resulted in lower
yields of 88 and 85%, respectively (entry 6 and 7). Different
cocatalysts have been tested, and NBu4Br proved to be the best
performer (entries 3, 8, and 9), suggesting the importance of
balancing good nucleophilic and leaving abilities.4d,11b Reaction
time has also been optimized, and 18 h turned to be the
optimal choice (entries 3, 10, and 11). Different loadings of
precatalyst and cocatalyst were screened, and a near
quantitative yield of 98% was obtained in the presence of 0.3
mol % complex 3 and 0.9 mol % NBu4Br (entries 12−14).
Lowering the temperature to 85 °C resulted in a good yield of
>90% (entries 15−16). Overall, the optimal reaction condition
is determined as follows: 0.3 mol % of complex 3, 0.9 mol % of
NBu4Br, 85 °C, 18 h, and 1 bar CO2.
Table 3. Synthesis of Disubstituted Epoxides and CO2
Catalyzed by Complex 3 and NBu4Br
a
a
Reaction conditions: 0.3 mol % catalyst, 0.9 mol % NBu4Br, 40 h, 10
b
c
bar CO2, 85 °C. Isolated yield. Selectivity for the cyclic carbonate
was all >99%. 78% trans and 22% cis isomers. 79% trans and 21% cis
isomers. Only cis-carbonate product was formed. 32% poly-
(carbonate) and 8% poly(ether) were also detected. Reaction
To explore the scope of the cycloaddition, a variety of mono-
and disubstituted epoxides were examined, and the results are
summarized in Tables 2 and 3, respectively. Cyclic carbonates
from monosubstituted epoxides bearing various substituents,
including halogen, aryl, alkenyl, ether, morpholine, hydroxyl,
and ester groups, were obtained in moderate to excellent yields
d
e
f
g
h
conditions: 0.3 mol % catalyst, 40 h, 10 bar CO2, 85 °C. 92%
i
polyether was detected. Reaction conditions: 0.3 mol % catalyst, 3 mol
% NBu4Br, 40 h, 10 bar CO2, 85 °C. 10% poly(ether) was detected,
and no poly(carbonate) was detected.
Table 2. Cycloaddition of Terminal Epoxides and CO2
Catalyzed by Complex 3 and NBu4Br
a
(60 to 97%) (Table 2, entries 1−10). Some substrates
containing substituents which might coordinate to the
aluminum center and deactivate the catalyst (e.g., 5e−j) gave
corresponding cyclic carbonates in 60−97% yields,7a revealing a
good tolerance of complex 3 toward different functional groups.
Cycloaddition of 1,2,7,8-diepoxyoctane (5l) with 2 equiv of
CO2 occurred, which generated the corresponding carbonate 6l
in 70% yield, and a higher yield of 90% after prolonged reaction
time (entry 11). Moreover, reactions of bulky/internal epoxides
(5m−5q) which are generally challenging substrates and
usually give rise to unsatisfactory yields4c−e,7a proceeded
smoothly at elevated pressure (10 bar) after a prolonged
reaction time of 40 h (Table 3, entry 1−5). For substrate 5m,
which is composed of 78% trans and 22% cis isomers, the
corresponding carbonate was obtained in 79% trans and 21% cis
configurations, indicating that the configuration remained
almost unchanged during the reaction (entry 1). Similarly,
cis-configured epoxides 5n and 5o afforded exclusively cis-
carbonates 6n and 6o, which is consistent with previous reports
(entries 2 and 3).4d,13 Reaction of cyclohexene oxide (5p) with
CO2 deserves some comments (entry 4). In the presence of 0.3
mol % precatalyst 3 and 0.9 mol % NBu4Br at 85 °C and 10 bar
of CO2, 52% of cyclic carbonate 6p formed concomitant with
32% poly(carbonate) from copolymerization of 5p and CO2,
and 8% poly(ether) resulting from homopolymerization of 5p.
Adjusting the amount of NBu4Br leads to different results. In
the absence of NBu4Br, no cyclic carbonate or poly(carbonate)
was detected, while 92% poly(ether) formed as the only
product. In comparison, lowering the catalyst to cocatalyst ratio
to 1:10 leads to the formation of 89% cyclic carbonate and 10%
poly(ether), and no poly(carbonate). The influence of
ammonium salts on product distribution of cyclohexene oxide
and CO2 cycloaddition has also been reported by Kleij et al. in a
Fe-based catalytic system4j and by Lamberti et al. in bimetallic
salalen aluminum complex catalyzed reactions.10
a
Reaction conditions: 0.3 mol % catalyst, 0.9 mol % NBu4Br, 18 h, 1
b
c
To further explore the potential of aluminum-based catalyst
3, the initial TOF which was generally accepted as a
bar CO2 (balloon), 85 °C. Isolated yield. Selectivity for the cyclic
carbonate was all >99%. 24 h. 40 h.
d
e
C
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measurement for catalyst capacity7b was studied under 1 bar
CONCLUSIONS
■
4f,7a
CO2
using 1,2-epoxyhexane 5a as a model substrate. A
In summary, a series of dinuclear aluminum complexes
stabilized by amine-bridged bis(phenolato) ligands have been
prepared and characterized. They showed good activities in
catalyzing the cycloaddition of terminal epoxides bearing
different functional groups and CO2 at 1 bar pressure, and
are among the rare examples of Al-based catalysts operative at
atmospheric pressure. In addition, generally accepted challeng-
ing internal epoxides were also transformed into cyclic
carbonates in moderate to good yields at a higher pressure of
10 bar. Further application of aluminum complexes in CO2
fixation is ongoing in our laboratory.
moderate initial TOF of 127 h−1 was obtained within 0.5 h
(Table S1). However, TOF values showed a decreasing trend as
the conversion increased, mainly due to the obviously
decreased concentration of the substrate. A first-order
disappearance of 1,2-epoxyhexane concentration was obtained
from the plot of ln([5a]0/[5a]) (Figure 3).
EXPERIMENTAL SECTION
■
General Procedures. All manipulations were performed under an
argon atmosphere using standard Schlenk techniques, except for ligand
precursor synthesis. Solvents (toluene, hexane, and THF) were
distilled by refluxing over sodium benzophenoneketyl prior to use.
Ligand precursors L1H3, L2H3, and L4H3 were prepared according to
optimization of literature methods.14 1,2-Epoxyhexane was dried over
CaH2 for 2 days and distilled before use. Other epoxide substrates and
carbon dioxide were commercially available and used without further
purification. Carbon, hydrogen, and nitrogen analyses were performed
by direct combustion with a CarloErba EA-1110 instrument.
Aluminum contents were determined by EDTA-based complexometric
titration, and procedures are detailed in Supporting Information. The
structures of aluminum complexes were determined by X-ray single
crystal diffractometry. The structures were solved and refined using
SHELEXL-97 programs.
Figure 3. Semilogarithmic plot of the cycloaddition of 1,2-
epoxyhexane and CO2 (1 bar) catalyzed by complex 3 and NBu4Br.
[5a]0, initial concentration of 1,2-epoxyhexane; [5a], the concen-
tration of 1,2-epoxyhexane. y = 0.1137x + 0.4425; R2 = 0.9817.
t
L1H3 (R = Bu). A mixture of 2,4-di-tert-butylphenol (20.6 g, 100
mmol), ethanolamine (3 mL, 50 mmol), and paraformaldehyde (3.0 g,
100 mmol) in methanol (6 mL) was heated under reflux for 12 h.
After cooling to room temperature, all volatiles were removed under
reduced pressure. Pure product was isolated by washing the residue
with hexane (25 mL) in 60% yield (14.93 g, 30 mmol). The identity of
the product was confirmed by comparison with literature data.14a 1H
NMR (CDCl3, 400 MHz): δ 7.20 (s, 1H, ArH), 7.19 (s, 1H, ArH),
6.89 (s, 1H, ArH), 6.88 (s, 1H, ArH), 3.87 (t, J = 5.3 Hz, 2H,
OCH2CH2N), 3.74 (s, 4H, NCH2Ar), 2.72 (t, J = 5.3 Hz, 2H,
OCH2CH2N), 1.38 (s, 18H, C(CH3)3), 1.26 (s, 18H, C(CH3)3).
L2H3 (R = CH3). A mixture of 2,4-dimethylphenol (12.2 g, 100
mmol), ethanolamine (3 mL, 50 mmol), and paraformaldehyde (3.0 g,
100 mmol) was heated neat for 12 h in 85 °C. After reaction, toluene
(30 mL) was added to the reaction mixture, and the product was
isolated after filtration in 40% yield (6.59 g, 20 mmol). The identity of
the product was confirmed by comparison with literature data.14b 1H
NMR (CDCl3, 400 MHz): δ 6.84 (s, 2H, ArH), 6.69 (s, 2H, ArH),
3.84 (t, J = 5.3 Hz, 2H, OCH2CH2N), 3.72 (s, 4H, NCH2Ar), 2.68 (t, J
= 5.3 Hz, 2H, NCH2CH2O), 2.19 (s, 6H, CH3), 2.17 (s, 6H, CH3).
L3H3 (R = Cl). A mixture of 2,4-dichlorophenol (16.3 g, 100 mmol),
ethanolamine (3 mL, 50 mmol), and paraformaldehyde (3.0 g, 100
mmol) was heated neat for 18 h in 120 °C. The product was purified
by chromatography (eluent: ethyl acetate/hexane = 1:10) and isolated
as a reddish brown oil. A mixture of toluene and hexane was added to
the oil, and the precipitate was isolated by filtration. The crude product
from the precipitate was further purified by recrystallization from
CHCl3. Pure product was obtained as colorless crystals (6.58 g, 16
mmol, 32%) 1H NMR (CDCl3, 400 MHz): δ 7.21 (s, 2H, ArH), 6.93
(s, 2H, ArH), 3.87 (t, J = 5.3 Hz, 2H, OCH2CH2N), 3.76 (s, 4H,
NCH2Ar), 2.71 (t, J = 5.3 Hz, 2H, NCH2CH2O). 13C NMR (CDCl3,
100 MHz): δ 150.9, 129.1, 128.7, 124.8, 124.5, 121.9 (Ar−C), 59.9
(OCH2CH2N), 56.2 (NCH2Ar), 54.8 (NCH2CH2O).
A possible mechanism has been proposed (Scheme 3).9b
Epoxides are activated through coordination to aluminum
Scheme 3. Proposed Mechanism of Cyclic Carbonate
Synthesis
L4H3. 2,4-Di-tert-butylphenol (17.95 g, 87 mmol) was dissolved in
methanol (60 mL), and NaOH (3.83 g, 96 mmol) was added to the
solution. After stirring for half an hour, 37% aqueous formaldehyde
(14 mL, 174 mmol) was added. The resulting mixture was left stirring
for 18 h at 25 °C. Then, 300 mL of water and 30 mL of hydrochloric
acid were added to the solution until the pH value reached 3−4. The
centers, which facilitates the attack of halides of the cocatalyst
to the less hindered carbon atom of epoxides to generate
species B. Carbon dioxide then inserts into the Al−O bond
forming carbonate C. Finally, cyclic carbonates form through
intramolecular cyclization (Scheme 3).
D
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precipitate was isolated and washed with cool hexane to give 3,5-di-
tert-butyl-2-hydroxybenzyl alcohol (16.07 g, 68 mmol, 78%). It was
subsequently treated with dry hydrochloric acid gas in hexane for 7 h
at 50 °C, and all volatiles were removed in vacuo to generate 3,5-di-
tert-2-hydroxybenzyl chloride (13.76g, 54 mmol, 80%). A mixture of 2-
hydroxyaniline (2.95 g, 27 mmol) and 3,5-di-tert-2-hydroxybenzyl
chloride (13.76 g, 54 mmol) in CH2Cl2 (70 mL) was heated at 50 °C.
Triethylamine (9 mL, 65 mmol) was added to the mixture dropwise,
and the reaction was stirred for 12 h. Water was added to the reaction
to remove triethylamine hydrochloride. The organic layer was dried
over sodium sulfate, and the filtrate was concentrated. The product
was obtained (9.57 g, 65%) via recrystallization from acetonitrile. The
overall yield is 41%. The identity of the product was confirmed by
comparison with literature data.14c 1H NMR (CDCl3, 400 MHz): δ
7.21−7.23 (d, J = 8.0 Hz, 1H, ArH), 7.16 (d, 2H, ArH), 6.94 (m, 3H,
ArH), 6.82−6.84 (t, 2H, ArH), 4.10 (s, 4H, NCH2Ar), 1.37 (s, 18H,
C(CH3)3), 1.24 (s, 18H, C(CH3)3).
Complex 1. A mixture of AlMe3 (0.288 g, 4 mmol, in 4 mL THF
solution) and L1H3 (1.99 g, 4 mmol, in 15 mL THF solution) was
stirred for 5 h at room temperature. After the reaction, all volatiles
were removed in vacuo. Twenty milliliters of toluene was added to
dissolve the residue. White powders were obtained at room
temperature from the toluene solution (1.71 g, 1.64 mmol, 82%).
1H NMR (C6D6, 400 MHz): δ 7.51 (s, 2H, ArH), 6.78 (s, 2H, ArH),
4.28 (t, J = 5.3 Hz, 2H, OCH2CH2N), 3.31 (br-s, 4H, NCH2Ar), 2.45
(br-s, 2H, NCH2CH2O), 1.52 (s, 18H, C(CH3)3), 1.41 (s, 18H,
C(CH3)3). 13C NMR (C6D6, 100 MHz): δ 156.3, 139.8, 138.4, 124.9,
124.6, 121.8 (Ar−C), 58.0 (OCH2CH2N), 56.7 (NCH2Ar), 53.4
(OCH2CH2N), 35.6, 34.7 (Ar−C), 32.4 ((CH3)3C), 30.1 ((CH3)3C).
Anal. Calcd for C64H96Al2N2O6: C, 73.67; H, 9.27; N, 2.68; Al, 5.17.
Found: C, 73.89; H, 9.24; N, 2.69; Al, 5.15.
Complex 2. A mixture of AlMe3 (0.288 g, 4 mmol, in 4 mL THF
solution) and L2H3 (1.32 g, 4 mmol, in 15 mL THF solution) was
stirred for 5 h at room temperature. After the reaction, all volatiles
were removed in vacuo. Two milliliters of toluene and 12 mL of
hexane were added to dissolve the residue. White powders were
obtained at room temperature after 2 days from the extract (1.41 g,
1.06 mmol, 75%). 1H NMR (CDCl3, 400 MHz): δ 6.86 (s, 2H, ArH),
6.62 (s, 2H, ArH), 4.20 (s, 2H, OCH2CH2N), 3.81 (d, J = 12.4 Hz,
2H, NCHHAr), 3.66 (d, J = 12.4 Hz, 2H, NCHHAr), 2.82 (s, 2H,
NCH2CH2O), 2.19 (s, 6H, Ar−CH3), 1.98 (s, 6H, Ar−CH3). 13C
NMR (CDCl3, 100 MHz): δ 154.6, 131.7, 128.1, 127.6, 126.2, 120.5
(Ar−C), 57.3 (OCH2CH2N), 56.1 (NCH2Ar), 53.4 (NCH2CH2O),
20.7 (Ar−CH3), 16.2 (Ar−CH3). Anal. Calcd for C40H48Al2N2O6: C,
67.97; H, 6.85; N, 3.96; Al, 7.64. Found: C, 68.17; H, 6.83; N, 3.97; Al,
7.61.
Complex 3. A mixture of AlMe3 (0.288 g, 4 mmol, in 4 mL THF
solution) and L3H3 (1.64 g, 4 mmol, in 15 mL THF solution) was
stirred for 5 h at room temperature. After the reaction, all volatiles
were removed in vacuo. Forty-five milliliters of THF was added to
dissolve the residue. Colorless crystals were obtained at room
temperature after 1 day from the extract (1.34 g, 1.54 mmol, 77%).
1H NMR (CDCl3, 400 MHz): δ 7.29 (d, 2H, ArH), 6.90 (d, 2H, ArH),
4.32 (t, J = 6.0 Hz, 2H, OCH2CH2N), 3.86 (d, J = 13.4 Hz, 2H,
NCHHAr), 3.68 (d, J = 13.4 Hz, 2H, NCHHAr), 2.90 (t, J = 6.0 Hz,
2H, NCH2CH2O). 13C NMR (CDCl3, 100 MHz): δ 152.9, 130.4,
127.9, 125.6, 123.2, 122.6 (Ar−C), 56.8 (OCH2CH2N), 56.3
(NCH2Ar), 53.6 (NCH2CH2O). Anal. Calcd for C32H24Al2Cl8N2O6:
C, 44.17; H, 2.78; N, 3.22; Al, 6.20. Found: C, 44.30; H, 2.77; N, 3.23;
Al, 6.18.
155.3, 150.2, 140.1, 138.2, 135.8, 128.6, 124.4, 122.7, 121.0, 120.5,
119.1 (Ar−C), 35.2, 34.3 (NCH2Ar), 32.0 (C(CH3)3), 29.8
(C(CH3)3). Anal. Calcd for C72H96Al2N2O6: C, 75.89; H, 8.49; N,
2.46; Al, 4.74. Found: C, 76.11; H, 8.46; N, 2.47; Al, 4.72.
Catalytic Procedure for the Cycloaddition of CO2 to
Epoxides under 1 Bar CO2. In a typical experiment, complex 3
(21.7 mg, 0.05 mmol), NBu4Br (48.1 mg, 0.15 mmol), and 1,2-
epoxyhexane (2 mL, 16.63 mmol) were added to a 20 mL flask
equipped with a magnetic stirring bar under argon. The round
bottomed flask was then degassed and filled with CO2. The mixture
was heated at 85 °C for 18−24 h. After reaction, a drop of the resulting
mixture was analyzed by 1H NMR spectroscopy to determine the yield
and selectivity. The final pure product was isolated by column
chromatography and confirmed by comparison with literature data.
Catalytic Procedure for the Cycloaddition of CO2 to
Epoxides under 10 Bar CO2. Experiments were conducted in a
100 mL stainless steel Parr reactor with a stirring bar inside and needle
valve for injection. In a typical experiment, complex 3 (21.7 mg, 0.05
mmol) and NBu4Br (48.1 mg, 0.15 mmol) were dissolved by epoxides.
The mixture was injected into the reactor which was dried before use.
The reactor was pressurized to 10 bar with carbon dioxide and heated
at 85 °C for 40 h. After the reaction, a drop of the resulting mixture
1
was analyzed by H NMR spectroscopy to determine the yield and
selectivity. The final pure product was isolated by column
chromatography and confirmed by comparison with literature data.
Product 6o which was not synthesized before was characterized by
multinuclear NMR spectroscopy.
X-ray Crystallographic Structure Determination. Suitable
single crystals of complexes 3·2THF and 2×4·9THF were sealed in
a thin-walled glass capillary for determination of the single-crystal
structures. Intensity data were collected with a Rigaku Mercury CCD
area detector in ω scan mode using Mo−Kα radiation (λ= 0.71070 Å).
The diffracted intensities were corrected for Lorentz/polarization
effects and empirical absorption corrections.
The structures were solved by direct methods and refined by full-
matrix least-squares procedures based on |F|2. The hydrogen atoms in
these complexes were generated geometrically, assigned appropriate
isotropic thermal parameters, and were allowed to ride on their parent
carbon atoms. All of the hydrogen atoms were held stationary and
included in the structure factor calculation in the final stage of full-
matrix least-squares refinement. The structures were solved and
refined using SHELEXL-97 programs.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00153.
NMR spectra of complexes 1−4 and cyclic carbonates,
and a table of X-ray crystallographic data (PDF)
X-ray crystallographic data for 3 and 4 (CIF)
AUTHOR INFORMATION
■
Corresponding Authors
*(D.Y.) E-mail: yuandan@suda.edu.cn.
*(Y.Y.) E-mail: yaoym@suda.edu.cn.
Notes
Complex 4. A mixture of AlMe3 (0.288 g, 4 mmol, in 4 mL THF
solution) and L4H3 (2.18 g, 4 mmol, in 15 mL THF solution) was
stirred for 5 h at room temperature. After the reaction, all volatiles
were removed in vacuo. Ten milliliters of THF was added to dissolve
the residue. Colorless crystals were obtained at room temperature after
1 day from the extract (1.59 g, 1.4 mmol, 70%). 1H NMR (CDCl3, 400
MHz): δ 7.66−7.64 (dd, J = 8.1 Hz, 1H, ArH), 7.46−7.44 (dd, J = 8.1
Hz, 1H, ArH), 7.28−7.24 (m, J = 7.9 Hz, 1H, ArH), 7.15 (s, 2H, ArH),
7.11−7.07 (d, J = 7.9 Hz, 1H, ArH), 6.84 (s, 2H, ArH), 4.20 (br-s, 4H,
NCH2Ar), 1.23 (s, 36H, C(CH3)3). 13C NMR (CDCl3, 100 MHz): δ
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the National Natural Science
Foundation of China (Grants 21132002, 21372172, and
21402135), the Major Research Project of the Natural Science
of the Jiangsu Higher Education Institutions (14KJA150007),
and PAPD are gratefully acknowledged.
E
DOI: 10.1021/acs.organomet.6b00153
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(e) Metcalfe, I. S.; North, M.; Pasquale, R.; Thursfield, A. Energy
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DOI: 10.1021/acs.organomet.6b00153
Organometallics XXXX, XXX, XXX−XXX