A tricarbonyl rhenium(I) complex with a pendant pyrrolidinium moiety as a robust and recyclable catalyst for chemical fixation of carbon dioxide in ionic liquid

Article abstract of DOI:10.1039/b618423e

A novel Re(I) complex covalently anchored with a pyrrolidinium moiety was successfully synthesized and used as an efficient and recyclable catalyst in the cycloaddition of CO₂ with epoxides under mild reaction conditions to give excellent isolated yield and selectivity of cyclic carbonates in pyrrolidinium ionic liquid. The Royal Society of Chemistry.

Full text of DOI:10.1039/b618423e

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A tricarbonyl rhenium(I) complex with a pendant pyrrolidinium moiety
as a robust and recyclable catalyst for chemical fixation of carbon
dioxide in ionic liquid{
Wing-Leung Wong, Kwong-Chak Cheung, Pak-Ho Chan, Zhong-Yuan Zhou, Kam-Han Lee and
Kwok-Yin Wong*
Received (in Cambridge, UK) 18th December 2006, Accepted 5th February 2007
First published as an Advance Article on the web 28th February 2007
DOI: 10.1039/b618423e
A novel Re(I) complex covalently anchored with a pyrrolidi-
nium moiety was successfully synthesized and used as an
rhenium(I) bipyridine complexes have been found to be able to
catalyze the electro- and photo-chemical reduction of CO
2
7
efficiently. Surprisingly, their applications in the chemical fixation
efficient and recyclable catalyst in the cycloaddition of CO
2
8
with epoxides remain rare. As such, it is highly desirable
of CO
2
with epoxides under mild reaction conditions to give excellent
isolated yield and selectivity of cyclic carbonates in pyrrolidi-
nium ionic liquid.
to explore the potential of the rhenium complexes in catalyzing the
activation and transformation of CO into useful organic
2
compounds such as cyclic carbonates, compounds that are widely
used in a variety of areas including the manufacture of monomers,
polar aprotic solvents, pharmaceutical, biomedical and fine
The development of efficient and recyclable catalytic systems to
convert atmospheric carbon dioxide (CO
and fuels is an important research topic in recent years because
the accumulation of CO in the atmosphere causes serious
2
) into useful chemicals
1
9
chemical intermediates.
In this context, we report the use of a novel tricarbonyl
rhenium(I) complex 1, which is covalently associated with a
pyrrolidinium ionic liquid moiety, as a robust and efficient catalyst
2
environmental problems including the greenhouse effect and
global warming. The use of electrochemical reduction and
chemical fixation to transform this greenhouse gas into high-
energy compounds and useful organic chemicals represents a
2
for the chemical fixation of CO with various epoxides under low
pressure and temperature conditions. To the best of our knowl-
1a–c
promising strategy.
These chemical processes are, however,
edge, complex 1 is the first recyclable rhenium catalyst designed for
usually performed in organic solvents and therefore the precious
catalysts cannot be recycled easily. Room-temperature ionic liquids
the green synthesis of cyclic carbonates with CO
nium bromide (2) as the reaction medium.
Complex 1 was first synthesized by refluxing Re(CO)
with 1-methyl-1-(3-N,N9-bis(2-pyridyl)propylamino)pyrrolidinium
2
with pyrrolidi-
offer an alternative choice of medium for the transformation of
2
solubility, high conductivity, wide
5
Cl
CO
2
because of their high CO
2
3
10
electrochemical window and non-volatility. Moreover, the use of
ionic liquids as reaction media allows the catalysts to be recycled
efficiently. These advantageous properties, therefore, render ionic
3
iodide (L) in ethanol to give Re(CO) (L)Cl. The complex was
then treated with an excess amount of silver triflate in ethanol,
giving a quantitative yield of 1 as the triflate salt, a non-
coordinating anion. The complex was characterized by NMR,
ESI-MS, and elemental analysis.{ The crystal structure of 1 reveals
that the pyrrolidinium ionic liquid moiety is covalently conjugated
with the rhenium(I) complex (Fig. 1). The rhenium(I) ion is
associated with the two pyridines of the ligand, an aquo molecule,
and three CO groups in a facial orientation, forming an octahedral
liquids an ideal reaction medium compared to traditional organic
4
. In order to catalyze this
solvents for the transformation of CO
2
reaction in ionic liquids, the catalyst must be preferentially soluble
in these media. An effective way to achieve this purpose is to
5
conjugate the catalyst with an ionic liquid moiety (Scheme 1).
2
The activation of CO using transition metal complexes as
1
,6
12
catalysts has been extensively studied. In particular, tricarbonyl
complex. The bond lengths in 1 agree with those of the
7
I
3
structurally similar fac-Re (CO) bipyridine complexes. The two
˚
Re–N bonds are very similar in length (2.18 A), and the bond
angles of the metal centre with the coordinated atoms of the
ligands fall in the range of 80–96u, indicating that the complex
adopts a slightly distorted octahedral geometry. This observation
implies that the anchored pyrrolidinium ionic liquid moiety does
not affect significantly the structure of the complex.
The ability of 1 to catalyze the chemical fixation of CO
2
with
Scheme 1 Structure of complex 1 and pyrrolidinium bromide 2.
1,2-epoxy-3-phenoxypropane in the ionic liquid of pyrrolidinium
11
bromide was studied. With 0.8 mol% of 1 (relative to the
Department of Applied Biology and Chemical Technology, The Hong
Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China.
E-mail: bckywong@polyu.edu.hk; Fax: +852 2364 9932;
Tel: +852 3400 8686
{ Electronic supplementary information (ESI) available: Synthesis and
X-ray crystal data for complex 1 and ligand L. See DOI: 10.1039/b618423e
epoxide), this complex was found to be able to convert the epoxide
into 3-phenoxy-1,2-propylene carbonate in the presence of CO
1.0 MPa) with 66% conversion and over 99% selectivity for cyclic
2
(
carbonate within 0.5 h at 80 uC (Table 1). When increasing the
reaction time to 1 h under the same conditions, the conversion was
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13
Fig. 2
C NMR spectra of (A) the reactant and (B) the crude product
collected in the 10th experiment.
Fig. 1 X-Ray crystal structure of 1 (30% probability chosen for the
ellipsoids, O9A atom is at equivalent position (x 2 1, K 2 y, z 2 1/2) of
O9 atom).
was also performed in which the 2,29-dipyridylamine tricarbonyl
rhenium(I) complex was used as the catalyst. Under similar
conditions, this complex was found to be able to catalyze the
cycloaddition but the recyclability was much poorer (conversion
reduced to 88% at the 4th cycle) compared to complex 1 due to the
leaching of catalysts in the product extraction processes. These
observations, therefore, suggest that the anchored pyrrolidinium
moiety in complex 1 can effectively minimize the loss of the
catalyst compared to the pyrrolidinium-free complex.
2
remarkably increased to 95%. When the CO pressure was reduced
to 0.5 MPa, the conversion dropped to 82% (entry 3).
Nevertheless, the reaction was found to be completed within 1 h
2
at 80 uC when the pressure of CO was increased to 1.5 MPa
(Table 1, entry 5). These results indicate that complex 1 is a very
efficient catalyst for the synthesis of cyclic carbonates in ionic
liquids under mild reaction conditions.
As expected, complex 1 can also catalyze the cycloaddition of
We then investigated the possibility of recycling complex 1 in the
synthetic process. The complex was used to catalyze the
2
other terminal epoxides with CO under mild reaction conditions
in the ionic liquid. Epoxides that are highly polar (e.g.
epichlorohydrin and glycidyl methacrylate) give excellent conver-
sion efficiencies (100%) and isolated yields (99%) within 1 h
(Table 2, entries 1 and 2). Styrene oxide was found to be less
reactive, presumably due to the steric hindrance of the phenyl
group; only 86% conversion was obtained in the first hour (Table 2:
entry 3). When this reaction was allowed to proceed for another
cycloaddition of 1,2-epoxy-3-phenoxypropane with CO
2
for
10 cycles, and the extent of conversion of the epoxide was
examined. Surprisingly, both the extents of conversion of the
epoxide in the 1st and 10th experiments were found to be very
similar (95%, Table 1: entry 6), indicating that there was no
significant loss of catalysts in the product extraction processes. The
crude product was characterized by both GC-MS and NMR to
examine the selectivity of the reaction and the purity of the product
in each experiment. As shown in the NMR spectrum (Fig. 2(B)), a
new carbon signal given by the carbonate product appears at
2
h, the conversion was increased to 95% with high selectivity for
cyclic carbonate (.99%). For the aliphatic terminal epoxide 1,2-
epoxyhexane, this molecule, which is relatively non-polar, shows
only moderate reactivity (Table 2: entry 5). For the low boiling
epoxide such as propylene oxide, only 13% of propylene carbonate
was obtained after 1 h under the same reaction conditions. This is
because at 1.5 MPa and 80 uC propylene oxide exists as a gas and
most of which probably stays in the headspace region of the
reaction vessel. The cycloaddition of cyclic epoxides such as
cyclohexene oxide was much less efficient compared to the others;
the conversion was found to be less than 30% even though the
reaction was allowed to proceed for 2 h. The mechanism of the
154.7 ppm. This signal, however, does not appear in the NMR
spectrum of the reactant. For comparison, a control experiment
Table 1 Cycloaddition of 1,2-epoxy-3-phenoxypropane with CO
catalyzed by 1 at different pressures using 2 as the reaction medium
2
a
cycloaddition of epoxides with CO2 catalyzed by complex 1
8
Br, in which
probably follows that proposed for Re(CO)
5
b
Entry Catalyst (mol%) Pressure /MPa Time/h Conv. /%
c
oxidative addition of epoxide to the Re-catalyst to give a cyclic
oxorhenium intermediate was followed by insertion of CO into
2
1
2
3
4
5
6
a
0.8
0.8
0.8
—
0.8
0.8
1.0
1.0
0.5
1.0
1.5
1.5
0.5
1
1
1
1
66
95
82
36
98
the Re–O bond, and finally reductive elimination to produce cyclic
carbonate. As shown in Table 1 (entry 4), the ionic liquid itself can
4
c
catalyze the reaction through a Lewis-acid mechanism with the
2
assistance of Br as the nucleophile, though the conversion is
d
1
95 (10th cycle)
much lower. Whether the formation of cyclic carbonates in this
study occurs via the catalysis of Re-catalyst only or a combination
of both catalytic pathways cannot be differentiated at this stage.
In conclusion, we have successfully prepared a new tricarbonyl
rhenium(I) complex covalently associated with a pyrrolidinium
ionic liquid moiety. This complex can serve as an efficient and
Reaction conditions: epoxide (2.5 mmol, 340 mL), ionic liquid 2
b
2
(2.5 mmol, 520 mg), temperature: 80 uC. CO was charged directly
into an autoclave at room temperature and atmosphere.
Determined by GC-MS. The selectivity for cyclic carbonates were
c
d
over 99% in all cases. Conversion of the 10th experiment in the
recycling studies.
2
176 | Chem. Commun., 2007, 2175–2177
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(
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terminal epoxides at 80 uC
2
(1.5 MPa) with different
a
b
Time/h Conversion (%) Yield (%)
c
Entry Epoxide
1
1
1
100
100
99
99
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86
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86
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5
6
a
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—
79
13
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Some recent examples of activation and transformation of CO in ionic
4
2
Reaction conditions: epoxide (2.5 mmol, 340 mL), catalyst
0.8 mol%), ionic liquid 2 (2.5 mmol, 520 mg). CO (1.5 MPa) was
charged directly into an autoclave at room temperature and
1
liquids: (a) R. Sheldon, Chem. Commun., 2001, 2399; (b) J. J. Peng and
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(
2
b
c
atmosphere. Determined by GC-MS. Isolated yield based on
.5 mmol of epoxides.
2
recyclable catalyst for the green synthesis of cyclic carbonates with
CO in pyrrolidinium ionic liquids under low temperature and
2
pressure conditions. Moreover, the metal complex can be recycled
efficiently in the synthetic process.
We acknowledge the support from The Hong Kong Polytechnic
University and the Research Grants Council (Grant no. PolyU
5 Numerous examples of immobilization of catalysts with ionic liquids
were reported in literatures but only few of them were designed through
covalent linking: (a) N. Audic, H. Clavier, M. Mauduit and
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5
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Notes and references
1
Selected characterization for complex 1: H NMR (400 MHz CD
.05–2.21 (m, 6H), 2.93 (s, 3H), 3.37–3.52 (m, 6H), 4.15 (t, 2H, J = 8 Hz),
{
2
3
CN): d
6 (a) A.-A. G. Shaikh and S. Sivaram, Chem. Rev., 1996, 96, 951; (b)
M. Yoshida and M. Ihara, Chem. Eur. J., 2004, 10, 2886; (c)
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690, 3490.
7 (a) J. Hawecker, J. M. Lehn and R. Ziessel, J. Chem. Soc., Chem.
Commun., 1983, 536; (b) B. P. Sullivan, C. M. Bolinger, D. Conrad,
W. J. Vining and T. J. Meyer, J. Chem. Soc., Chem. Commun., 1985,
1414; (c) S. Cosnier, A. Deronzier and J. C. Moutet, New J. Chem.,
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J. A. Timney, J. Chem. Soc., Dalton Trans., 1992, 9, 1455; (e)
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7.41 (t, 2H, J = 6 Hz), 7.67 (d, 2H, J = 8 Hz), 8.17 (t, 2H, J = 8 Hz), 8.75 (d,
+
2
2
H, J = 6 Hz); ESI-MS m/z: 716.9 (M 2 CF
Re(H O)(CO )(C18 )(CF SO : C, 31.26; H, 3.09; N, 6.34. Found: C,
1.21; H, 3.12; N, 6.61%.
Crystal data for 1 (CCDC 631247): Re(H
(CF SO ), M = 883.81; monoclinic, P2 /c; a = 10.800(2), b = 16.235(3),
c = 18.666(4) A, b = 102.601(3)u; V = 3193.9(10) A ; T = 294(2) K; Z = 4;
3 3
SO ). Anal. Calc. for
2
3
H N
25 4
3
3 2
)
3
2
3 25 4
O)(CO) (C18H N )?
2
3
3
1
3
˚
21
˚
m = 4.027 mm ; reflections collected = 29441; independent reflections =
313 (Rint = 0.0502); final R values [I . 2s(I)]: R1 = 0.0415, wR2 = 0.1008;
R values (all data): R1 = 0.0637, wR2 = 0.1143.
7
+
2
Crystal data for ligand L (CCDC 632736): (C18
25 4
H N ) I , M = 424.32;
˚
1
/n; a = 10.756(2), b = 9.1086(19), c = 19.236(4) A, b =
3 21
monoclinic, P2
4.476(4)u; V = 1878.9(7) A ; T = 294(2) K; Z = 4; m = 1.710 mm
reflections collected = 17082; independent reflections = 4337 (Rint
.0379); final R values [I . 2s(I)]: R1 = 0.0283, wR2 = 0.0685; R values (all
data): R1 = 0.0425, wR2 = 0.0750.
For crystallographic data in CIF or other electronic format see DOI:
0.1039/b618423e
˚
9
;
=
0
8 Only one report on using Re(CO) Br as the catalyst: J. L. Jiang,
5
F. X. Gao, R. M. Hua and X. Q. Qiu, J. Org. Chem., 2005, 70, 381.
9 (a) W. J. Peppel, Ind. Eng. Chem., 1958, 50, 767; (b) A. Behr, Angew.
Chem., Int. Ed. Engl., 1988, 27, 661; (c) A.-A. G. Shaikh and S. Sivaram,
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2003, 42, 663.
1
1
(a) Electrochemical and Electrocatalytic Reactions of Carbon Dioxide, ed.
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(
b) Chemical Fixation of Carbon Dioxide: Methods for Recycling CO
into Useful Products, ed. M. M. Hallman, CRC Press, Boca Raton, FL,
993; (c) Green Chemistry: Frontiers in Benign Chemical Syntheses and
2
10 For the X-ray structural characterization of L, see ESI{.
11 1-Methyl-1-N-propylpyrrolidinium bromide (2) was prepared according
to the literature: D. R. MacFarlane, P. Meakin, J. Sun, N. Amini and
M. Forsyth, J. Phys. Chem. B, 1999, 103, 4164.
1
Processes, ed. P. T. Anastas and T. C. Williamson, Oxford University
Press, New York, 1998; (d) D. H. Gibson, Chem. Rev., 1996, 96, 2063;
12 For the X-ray structural characterization of complex 1, see ESI{.
This journal is ß The Royal Society of Chemistry 2007
Chem. Commun., 2007, 2175–2177 | 2177