H. Sik Kim, H. Gy. Jang et al.
FULL PAPER
Chemical Co. and used without further purification. Pyridine, 2-methyl-
pyridine, 3-methylpyridine, 4-methylpyridine, 2,2'-dipyridyl, 4-dimethyl-
aminopyridine, ZnCl2, and ZnI2 were purchased from Aldrich Chemical
Co. ZnBr2 was purchased from Fluka Chemical Co. CO2 and 13CO2 were
purchased from Sin Yang Gas Co. and Aldrich Chemical Co., respectively.
[L2ZnX2] (X: Cl, Br, I;L pyridine or substituted pyridine) was prepared
according to the published procedure.[36] IR spectra were recorded on a
Mattson Infinity Spectrometer with MCT detector. 1H and 13C NMR
spectra were recorded on a Varian Unity 600 superconducting high-
resolution spectrometer. Elemental analysis was carried out by the
Advanced Analytical Center at KIST using a Perkin Elmer 2400 CHNS
analyzer.
Conclusion
Coupling reactions of CO2 and epoxides to produce cyclic
carbonates have been performed in the presence of a catalyst
[L2ZnX2] (L pyridine or substituted pyridine;X Cl, Br, I).
The effects of pyridine and halide ligands on the catalytic
activity and the formation of active species have been
investigated. The catalysts with electron-donating substitu-
ents on pyridine ligands exhibit higher activity than those with
unsubstituted pyridine ligands. On the other hand, the
catalysts with electron-withdrawing substituents on pyridine
ligands show no activity;this demonstrates the importance of
the nucleophilicity of the pyridine ligands. A zinc complex
containing a strongly chelating 2,2'-dipyridyl ligand (1g) was
found to be totally inactive, indicating that ligand dissociation
is also an important factor in the catalysis process.
Unlike the most of metal halide-catalyzed coupling reac-
tions, halide ions in 1 are found to play a role in controlling the
dissociation of the pyridine ligand, but not in the ring opening
of epoxides as nucleophiles. The catalytic activity of [L2ZnX2]
is found to decrease with increasing electronegativity of the
halide ligands. A series of di- and trinuclear zinc complexes
with bridging pyridinium alkoxy ion ligands have been
synthesized from the reactions of [L2ZnX2] and epoxides with
or without the presence of CO2. The structural character-
ization of these complexes by X-ray crystallography clearly
demonstrates the role of the pyridine ligands in the formation
of active species in the coupling reactions of CO2 with
epoxides performed in the presence of [L2ZnX2]. The
aggregation state of these complexes is likely to be largely
determined by the kind of epoxide, not by the pyridine ligand
in [L2ZnX2]: dinuclear zinc complexes are formed with
propylene oxide and trinuclear zinc complexes with ethylene
oxide. The dinuclear zinc complexes adopt a square-planar
geometry of the Zn2O2 core with two bridging pyridinium
propoxy ion ligands, while trinuclear zinc complexes adopt a
boat geometry for the Zn3O3 core with three bridging
pyridinium alkoxy ion ligands. These di- and trinuclear zinc
complexes show higher activities for the coupling reactions of
CO2 and epoxides than the corresponding precursors
[L2ZnBr2], indicating the presence of an induction period.
NMR and FTIR experiments showed that zinc complexes
bridged by pyridinium alkoxy ions rapidly interact with CO2
to give zinc carbonate species, which in turn react with
additional epoxides to generate further bridged zinc com-
plexes and cyclic carbonates. Pyridine exchange through a
Synthesis of [Zn2Br4{m-OCH(CH3)CH2-NC5H5}2] (2a-PO): A solution of
1a (2.00 g, 5.22 mmol) in methylene chloride (30 mL) was treated with
propylene oxide (0.50 mL, 7.15 mmol) in a 60 mL high-pressure glass
reactor and stored at room temperature for 10 h. The precipitate was
filtered onto a glass filter and dried under vacuum to give a white solid.
Yield: 85%;elemental analysis calcd (%) for C 16H22Br4N2O2Zn2: C 26.52,
H 3.06, Br 44.10, N 3.87, Zn 18.04;found: C 25.90, H 2.95, N 3.79, Zn 19.51;
1H NMR (600 MHz, [D6]DMSO, 258C): d 1.20 (d, 2J(H,H) 6 Hz, 3H;
CH3), 4.04 (m, 1H;CH), 4.36 (dd, 2J(H,H) 13 Hz, 1H;CH 2), 4.73 (dd,
3
2J(H,H) 13 Hz, 1H;CH ), 8.15 (t, J(H,H) 6 Hz, 2H;C H5N), 8.61 (t,
2
5
3J(H,H) 8 Hz, 1H;C 5H5N), 8.97 ppm (d, 2J(H,H) 6 Hz, 2H;C 5H5N).
Synthesis of [Zn2Br4{m-OCH(CH3)CH2-NC5H4(2-CH3)}2] (2b-PO): Com-
plex 2b-PO was prepared in a manner similar to that employed in the
preparation of 2a-PO by simply replacing 1a with 1b. Yield: 70%;
elemental analysis calcd (%) for C18H26Br4N2O2Zn2: C 28.72, H 3.48, Br
42.46, N 3.72, Zn 17.37;found: C 28.70, H 3.65, N 3.73, Zn 17.41; 1H NMR
(600 MHz, [D6]DMSO, 258C): d 1.20 (d, 2J(H,H) 6 Hz, 3H;CH 3), 2.80
(s, 3H;CH 3), 4.03 (m, 1H;CH), 4.37 (dd, 2J(H,H) 12 Hz, 1H;CH 2), 4.71
(dd, 2J(H,H) 13 Hz, 1H;CH 2), 8.13 (t, 3J(H,H) 7 Hz, 1H;C 5H5N), 8.19
(d, 2J(H,H) 8 Hz, 1H;C 5H5N), 8.64 (t, 3J(H,H) 6 Hz, 1H;C 5H5N),
2
8.98 ppm (d, J(H,H) 6 Hz, 1H;C H5N).
5
Synthesis of [Zn3Br6(m-OCH2CH2-NC5H5)3] (2a-EO): Complex 2a-EO
was prepared in a manner analogous to that described above for complex
2a-PO by replacing propylene oxide with ethylene oxide. Yield: 87%;
elemental analysis calcd (%) for C21H27Br6N3O3Zn3: C 24.14, H 2.60, Br
45.88, N 4.02, Zn 18.77;found: C 23.52, H 2.57, N 3.92, Zn 17.54; 1H NMR
3
(600 MHz, [D6]DMSO, 258C): d 3.90 (t, J(H,H) 5 Hz, 2H;CH 2), 4.71
(t, 3J(H,H) 5 Hz, 2H;CH 2), 8.20 (t, 3J(H,H) 7 Hz, 2H;C 5H5N), 8.66 (t,
3J(H,H) 8 Hz, 1H;C 5H5N), 9.05 ppm (d, 2J(H,H) 6 Hz, 2H;C 5H5N).
Synthesis of [Zn3Br6{m-OCH2CH2-NC5H4(2-CH3)}3] (2b-EO): This com-
plex was prepared in a manner analogous to that described above for
complex 2a-EO by simply replacing 1a with 1b. Yield: 78%;elemental
analysis calcd (%) for C24H33Br6N3O3Zn3: C 26.52, H 3.06, Br 44.10, N 3.87,
Zn 18.04;found: C 26.02, H 3.00, N 3.73, Zn 17.52; 1H NMR (600 MHz,
[D6]DMSO, 258C): d 2.96 (s, 3H;CH 3), 3.96 (t, 3J(H,H) 6 Hz, 2H;
CH2), 4.75 (t, 3J(H,H) 6 Hz, 2H;CH 2), 8.07 (t, 3J(H,H) 7 Hz, 1H;
C5H5N), 8.14 (d, 2J(H,H) 8 Hz, 1H;C 5H5N), 8.58 (t, 3J(H,H) 6 Hz, 1H;
2
C5H5N), 8.99 ppm (d, J(H,H) 6 Hz, 1H;C H5N).
5
Synthesis of [Zn3Br6{m-OCH2CH2-NC5H4(4-CH3)}3] (2c-EO): Complex
2c-EO was prepared by reacting 1c with ethylene oxide in methylene
chloride (30 mL) at room temperature for 10 h in a 60 mL high-pressure
glass reactor. After the reaction, the solution was dried under vacuum to
give
a white solid. Yield;68%;elemental analysis calcd (%) for
À
C N bond cleavage in the pyridinium alkoxy ion ligands and
C24H33Br6N3O3Zn3: C 26.52, H 3.06, Br 44.10, N 3.87, Zn 18.00;found: C
25.82, H 3.10, N 3.79, Zn 17.83; 1H NMR (600 MHz, [D6]DMSO, 258C):
d 2.70 (s, 3H;CH 3), 3.93 (t, 3J(H,H) 5 Hz, 2H;CH 2), 4.68 (t, 3J(H,H)
6 Hz, 2H;CH 2), 8.08 (d, 2J(H,H) 6 Hz, 2H;C 5H5N), 8.94 ppm (d,
the structural transformation between di- and trinuclear
complexes have not been observed during the coupling
reactions. This suggests that zinc complexes bridged by
pyridinium alkoxy ions are the active species in the catalysis.
2J(H,H) 7 Hz, 2H;C H5N).
5
Reaction of CO2 with [Zn2Br4{m-OCH(CH3)CH2-NC5H5}2] (3a-PO): A
60 mL high-pressure glass reactor containing solution of 2a-PO (0.50 g,
0.66 mmol) in DMSO (10 mL) was charged with 0.5 MPa CO2 and stored at
room temperature for 3 h. The precipitate obtained was filtered, washed
with CH2Cl2, and dried under vacuum to give a white solid (0.21 g).
Experimental Section
Method and materials: All manipulations were carried out under argon
unless otherwise stated, with glassware thatwas flame-dried prior to use.
The solvents were freshly distilled before use according to literature
procedures. Ethylene oxide was purchased from Hyundai Petrochemical
Co. and used as received. Propylene oxide was purchased from Aldrich
Coupling reactions of epoxides and CO2: All the coupling reactions were
conducted in a 100 mL stainless-steel bomb equipped with a stirring bar
and an electrical heater. The reactor was charged with the appropriate
catalyst and epoxide, and pressurized with CO2 (ꢀ 1.4 MPa). The bomb
was then heated to 1008C with the addition of CO2 from a reservoir tank to
684
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Chem. Eur. J. 2003, 9, No. 3