Efficient coupling of CO2 and epoxides with bis(phenoxyiminato) cobalt(III)/Lewis base catalysts

Article abstract of DOI:10.1016/j.molcata.2009.07.009

A series of unbridged bis(phenoxyiminato) cobalt(III)complexes were synthesized and studied as catalysts for the coupling of CO₂ and epoxide. Dimethylamino pyridine (DMAP) and tetrabutyl ammonium bromide (Bu₄NBr), were used as co-cat

Full text of DOI:10.1016/j.molcata.2009.07.009

Journal of Molecular Catalysis A: Chemical 312 (2009) 87–91
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Efficient coupling of CO2 and epoxides with bis(phenoxyiminato)
cobalt(III)/Lewis base catalysts
Ahlam Sibaouih, Paul Ryan, Kirill V. Axenov, Markku R. Sundberg, Markku Leskelä, Timo Repo∗
Laboratory of Inorganic Chemistry, Department of Chemistry, University of Helsinki, PO Box 55, FIN-00014 Helsinki, Finland
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of unbridged bis(phenoxyiminato) cobalt(III)complexes were synthesized and studied as cata-
lysts for the coupling of CO2 and epoxide. Dimethylamino pyridine (DMAP) and tetrabutyl ammonium
bromide (Bu4NBr), were used as co-catalysts in the reaction. Both Co(III)/DMAP and Co(III)/Bu4NBr sys-
tems showed high catalytic activity and selectivity in the coupling of n-hexyl oxide and CO2, resulting
in n-hexyl carbonate. Several mono- and di-substituted terminal epoxides were also tested. The reaction
mechanism of the Co(III)/DMAP system was investigated in detail using ESI-MS revealing that in addition
to the main product, cyclic carbonate, traces of oligomers are formed.
Received 23 April 2009
Received in revised form 3 July 2009
Accepted 6 July 2009
Available online 15 July 2009
Keywords:
Carbon dioxide
Epoxide
© 2009 Elsevier B.V. All rights reserved.
Catalyst
Cyclic carbonate
Mechanism
1. Introduction
2. Experimental
The conversion of carbon dioxide to desirable, economically
competitive products has received much attention because of its
potential use as an abundant C1 resource and its indirect role as an
environmental pollutant [1–3]. The synthesis of cyclic carbonate
by the coupling of CO2 and epoxides is one of the most interesting
reactions in this area (Scheme 1), and in recent decades numer-
ous catalyst systems have been developed for this transformation
[4–9]. Apart from a number of efficient Zn complexes [10–12], the
most common catalyst precursors reported for the synthesis of both
cyclic carbonates and polycarbonates are based on salen and por-
phyrin complexes of Co(III) [4,5,12–17], Cr(III) [6,18–22,19,23] and
Al(III) [7,18,24–28]. Square pyramidal geometry is characteristic of
both salen and porphyrin complexes. This involves planar coordi-
nation of the ligand framework around the metal center with the
square pyramid coordination sphere being completed by a halide at
the apical position. As an augmentation for these previous studies,
we report herein a convenient method of cyclic carbonate synthe-
sis by utilizing catalysts based on unbridged bis(phenoxyiminato)
cobalt(III) complexes.
2.1. Reagents
The epoxides were purchased from Fluka and Aldrich and used
without further purification. Solvents were used after distillation.
The carbon dioxide of purity 99.99% was purchased from AGA and
used as received. Anhydrous cobalt(II) chloride (CoCl2), tetrabutyl
ammonium bromide (Bu4NBr), dimethylamino pyridine (DMAP),
salicylaldehyde,
2ꢀ-hydroxyacetophenone,
3,5-di-tert-butyl-
2-hydroxybenzaldehyde, aniline, p-toluidine, 4-chloroaniline,
benzylamine, 4-fluorobenzylamine, 2-phenethylamine, sodium
hydride, iodine were purchased from Aldrich.
2.2. Typical reaction procedure
All of the coupling reactions were conducted in a 50 ml
stainless-steel autoclave equipped with a glass liner and a magnetic
stirring bar. In a typical reaction set-up the catalyst (0.01 mmol),
dichloromethane (0.5 ml), epoxide (16.6 mmol), and co-catalyst
(0.04 mmol) were loaded into the glass liner. The reactor was then
pressurizedwiththeappropriateamountofCO2 (10 bar)andheated
to desired temperature (145 ◦C). After the required reaction time
(2 h), the reactor was cooled in an ice bath and degassed. For the GC
analysis thereaction solution was dilutedanda standard (acetophe-
non) was added. A sample from the reaction solution was taken and
analyzed with a gas chromatograph (Agilent 6890N). The product
formation and distribution was verified with a mass spectrometer
(Agilent 5973).
∗
Corresponding author. Tel.: +358 9191 50194; fax: +358 9191 50198.
E-mail address: timo.repo@helsinki.fi (T. Repo).
1381-1169/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2009.07.009

88
A. Sibaouih et al. / Journal of Molecular Catalysis A: Chemical 312 (2009) 87–91
lated 535.1790, found 535.1785, error 1.09 ppm). MS(EI+): m/z (%)
535 (96, M+), 297 (100, M+–C16H16NO), 239(100, ligand).
Bis(N-phenylethyl-salicylaldiminato)Cobalt(II): Orange, Anal.
Calc. For C30H28CoN2O2: C, 71.00; H, 5.56; N, 5.52. Found C, 71.09;
H, 5.88; N, 5.47%. HRMS (ESI-TOF): m/z [C30H28CoN2O2] 507
(calculated 507.1477, found 507.1470, error 1.34 ppm). MS(EI+): m/z
(%) 507 (76, M+), 283 (100, M+–C15H14NO), 225(46, ligand).
Bis(N-phenylethyl-3-tert-butyl-salicylaldiminato)Cobalt(II): Light
green, Anal. Calc. for C46H60CoN2O2·H2O: C, 73.67; H, 8.33; N,
3.74. Found C, 73.63; H, 8.36; N, 3.76%. HRMS (ESI-TOF): m/z
[C46H60CoN2O2] 731 (calculated 731.3981, found 731.3974, error
0.94 ppm). MS(EI+): m/z (%) 731 (100, M+), 395 (50, M+–C23H30NO),
336(50, ligand).
Scheme 1. Coupling reaction of CO2 and epoxides.
2.3. General procedure for cobalt(II) complexes
The Schiff base ligands were prepared via the simple conden-
sation reaction of a primary amine with either salicylaldehyde or
2ꢀ-hydroxyacetophenone in ethanol. The relevant Schiff base lig-
and was dissolved in dry THF and added via a syringe to a solution
of NaH in THF under argon. This solution was stirred for 10 min at
room temperature to deprotonate the ligand. CoCl2 was then intro-
duced to the reaction mixture. The reaction was heated to 60 ◦C
and stirred for 12 h under argon. The reaction solution was sub-
sequently filtered and the solvent was evaporated under vacuum.
The residue was then washed either with methanol or a mixture
of THF and hexane to remove any unreacted ligand salt and CoCl2.
Finally this mixture was filtered and the solvent was evaporated
under vacuum.
2.4. General procedure for cobalt(III) complexes
A previously prepared cobalt(II) complex was treated with a
half equivalent of I2 and dissolved in 5 ml of CH2Cl2. The solu-
tion mixture was stirred at room temperature for 12 h after which
the solvent was evaporated and the excess iodine was removed by
sublimation under vacuum for 24 h.
Bis(N-phenyl-salicylaldiminato) Iodo Cobalt(III): 1 Dark red, Anal.
Calc. For C26H20CoIN2O2·2H2O: C, 50.83; H, 3.94; N, 4.56%. Found
C, 50.87; H, 3.22; N, 4.51%. HRMS (ESI-TOF): m/z (%): [M+–I] 451
(calculated 451.0851, found 451.0847, error 1 ppm).
Bis(N-4-methylphenyl-salicylaldiminato) Iodo Cobalt(III): 2 Dark
red, Anal. Calc. For C28H24CoIN2O2·H2O: C, 53.86; H, 4.20; N, 4.49%.
Found C, 53.67; H, 3.78; N, 4.57%. HRMS (ESI-TOF): m/z (%): [M+–I]
479 (calculated 479.1164, found 479.1165, error 0.25 ppm).
Bis(N-4-chlorophenyl-salicylaldiminato) Iodo Cobalt(III): 3 Brown,
Anal. Calc. for C26H18 CoICl2N2O2: C, 48.25; H, 2.80; N, 4.33%. Found
C, 48.24; H, 2.72; N, 4.45%. HRMS (ESI-TOF): m/z (%): [M+–I] 519
(calculated 519.0072, found 519.0070, error 0.42 ppm).
Bis{2-[(benzylimino)ethyl]phenoxy} Iodo Cobalt(III): 4 Green,
Anal. Calc. For C30H28CoIN2O2·H2O: C, 55.23; H, 4.63; N, 4.29%.
Found C, 55.19; H, 4.53; N, 4.33%. HRMS (ESI-TOF): m/z (%): [M+–I]
507 (calculated 507.1477, found 507.1453, error 1.32 ppm).
Bis(N-benzyl-salicylaldiminato) Iodo Cobalt(III): 5 Green, Anal.
Calc. For C28H24CoIN2O2: C, 55.46; H, 3.99; N, 4.62%. Found C, 55.04;
H, 3.85; N, 4.72%. HRMS (ESI-TOF): m/z (%): [M+–I] 479 (calculated
479.1164, found 479.1163, error 0.29 ppm).
Bis(N-phenyl-salicylaldiminato)Cobalt(II): Orange, Anal. Calc. for
C
26H20CoN2O2·H2O: C, 66.53; H, 4.72; N, 5.97%. Found C,
66.48; H, 4.19; N, 5.88%. HRMS (ESI-TOF): m/z [C26H20CoN2O2]
451 (calculated 451.0840, found 451.0851, error 0.43 ppm)
[C26H20CoN2O2 + Na] 474 (calculated 474.0749, found 474.0746,
error 0.60 ppm) MS(EI+): m/z (%) 451 (100, M+), 255 (51,
M+–C13H10NO), 196(51, ligand).
Bis(N-4-methylphenyl-salicylaldiminato)Cobalt(II): Red, Anal.
Calc. for C28H26CoN2O2·H2O: C, 67.60; H, 5.27; N, 5.63%. Found
C, 67.99; H, 5.09; N, 5.57%. HRMS (ESI-TOF): m/z [C28H26CoN2O2]
479 (calculated 479.1164, found 479.1170, error −1.14 ppm)
[C28H26CoN2O2 + Na] 502 (calculated 502.1062, found 502.1056,
error 1.10 ppm). MS(EI+): m/z (%) 479 (60, M+), 269 (18,
M+–C14H13NO), 210(65, ligand).
Bis(N-4-chlorophenyl-salicylaldiminato)Cobalt(II): Orange, Anal.
Calc. for C26H18 CoN2O2Cl2·H2O: C, 58.01; H, 3.74; N, 5.20%.
Found C, 58.18; H, 3.61; N, 5.04%. HRMS (ESI-TOF): m/z
[C26H18 CoN2O2Cl2] 519 (calculated 519.0072, found 519.0074, error
−0.50 ppm), [C26H18 CoN2O2Cl2 + Na] 541 (calculated 541.9970,
found 541.9968, error 0.35 ppm). MS(EI+): m/z (%) 519 (77, M+),
289(35, M+–C13H9NOCl), 230 (60, ligand).
Bis{2-[(benzylimino)ethyl]phenoxy}Cobalt(II): Green, Anal. Calc.
for C30H28CoN2O2: C, 71.00; H, 5.56; N, 5.52%. Found C, 71.12; H,
5.69; N, 5.31%. (ESI-TOF): m/z [C30H28 N2O2Co] 507 (calculated
507.1477, found 507.1476, error 0.17 ppm) [C30H28 N2O2Co + Na] 530
(calculated 530.1375, found 530.1376, error −0.10 ppm). MS(EI+):
m/z (%) 507(55, M+), 284(88, M+–C15H14NO), 225(100, ligand).
Bis(N-benzyl-salicylaldiminato)Cobalt(II): Orange, Anal. Calc. For
Bis(N-4-fluorobenzyl-salicylaldiminato) Iodo Cobalt(III): 6 Dark
green, Anal. Calc. for C28H22CoIF2N2O2: C, 52.36; H, 3.45; N, 4.36%.
Found C, 52.28; H, 3.42; N, 4.04%. HRMS (ESI-TOF): m/z (%): [M+–I]
515 (calculated 515.0976, found 515.0972, error 0.77 ppm).
Bis{2-[(phenylethylimino)ethyl]phenoxy} Iodo Cobalt(III): 7 Dark
green, Anal. Calc. for C32H32CoIN2O2: C, 58.02; H, 4.87; N, 4.23.
Found C, 58.18; H, 4.97; N, 4.05. HRMS (ESI-TOF): m/z (%): [M+–I]
535 (calculated 535.1790, found 1794, error 0.75 ppm).
Bis(N-phenylethyl-salicylaldiminato) Iodo Cobalt(III):
8 Dark
green, Anal. Calc. For C30H28CoIN2O2·H2O: C, 55.23; H, 4.63;
N, 4.29%. Found C, 55.62; H, 4.28; N, 4.32%. HRMS (ESI-TOF):
m/z (%): [M+–I] 507 (calculated 507.1477, found 507.1478, error
0.23 ppm).
Bis(N-phenylethyl-3-tert-butyl-salicylaldiminato) Iodo Cobalt(III):
9 Dark brown, Anal. Calc. For C46H60CoIN2O2·H2O: C, 63.01; H, 7.13;
N, 3.19%. Found C, 63.22; H, 7.23; N, 3.08%. HRMS (ESI-TOF): m/z (%):
[M+–I] 731 (calculated 731.3981, found 731.3993, error 1.58 ppm).
C
28H24CoN2O2·H2O: C, 67.60; H, 5.27; N, 5.63%. Found C, 67.61; H,
5.11; N, 5.50% HRMS (ESI-TOF): m/z [C28H24CoN2O2] 479 (calculated
479.1164, found 479.1161, error 0.69 ppm), [C28H24CoN2O2 + Na]
502 (calculated 502.1062, found 502.1062, error 2.17 ppm). MS(EI+):
m/z (%) 479 (100, M+), 269 (100, M+–C14H12NO), 211(77, ligand).
Bis(N-4-fluorobenzyl-salicylaldiminato) Cobalt(II): Orange, Anal.
Calc. for C28H22CoF2N2O2: C, 65.25; H, 4.30; N, 5.43%. Found C,
65.50; H, 4.30; N, 5.31%. HRMS (ESI-TOF): m/z [C28H22CoF2N2O2]
515 (calculated 515.0976, found 515.0966, error 1.96 ppm),
[C28H22CoF2N2O2 + Na] 538 (calculated 538.0866, found 538.0874,
error 1.42 ppm). MS(EI+): m/z (%) 515 (100, M+), 287 (100,
M+–C14H11 NO), 228(55, ligand).
3. Result and discussion
3.1. The unbridged bis(phenoxyiminato) cobalt complexes
Bis{2-[(phenylethylimino)ethyl]phenoxy}Cobalt(II): Orange, Anal.
Calc. for C32H32CoN2O2: C, 71.77; H, 6.02; N, 5.23%. Found C, 70.71;
H, 6.03; N, 5.19%. HRMS (ESI-TOF): m/z [C32H32CoN2O2] 535 (calcu-
For this study
bis(ketiminato) complexes were synthesized. Although resembling
a series of Co(III) bis(aldiminato) and

A. Sibaouih et al. / Journal of Molecular Catalysis A: Chemical 312 (2009) 87–91
89
Fig. 1. Schematic structure of the unbridged bis(phenoxyiminato) cobalt(III) com-
plexes used as catalyst precursors in this study.
the salen type complexes, unbridged bis(phenoxyiminato) cobalt
complexes have not been so far reported for the activation of CO2
(Fig. 1). According to performed quantum chemistry calculations,
the unbridged bis(phenoxyiminato) Co(III) complexes prefer a
distorted square pyramidal geometry wherein iodide lies in the
main plane and one of the oxygens from the phenoxy groups
is at the apical position (Fig. 2). The main difference between
the unbridged bis(phenoxyiminato) and salen type transition
metal complexes is that in later ones the ligand usually adopt
the thermodynamically more favored planar configuration [29].
However, in the chemical transformations catalyzed by salen type
complexes the actual configuration catalytic species could often
be far different from planar [29–31]. In this account, the unbridged
bis(phenoxyiminato) Co(III) and salen type complexes [4] would
have similar catalytic behavior in CO2 activation reactions.
Fig. 3. Catalytic activity of unbridged bis(phenoxyiminato) cobalt(III) complexes
1–9.
3.3. Different substrates
In addition to n-hexyl oxide, the 4/DMAP and 4/Bu4NBr catalytic
systems were studied with a range of terminal and cyclic epox-
ides. Using 4/Bu4NBr catalyst the corresponding cyclic carbonates
were obtained in pure state with good yields. While having a simi-
lar activity to 4/Bu4NBr, 4/DMAP tends to produce oligomeric side
products particularly with cyclohexyl oxide and cyclopentene oxide
(Table 1).
3.4. Mechanism
The coupling reaction of propylene oxide (PO) and CO2 was fur-
ther investigated using ESI-MS which turned out to be a highly
beneficial method to get an insight of the actual processes occurred
in course of the catalytic reaction. The original solution con-
taining 8/DMAP and excess of PO was maintained under CO2
atmosphere (10 bar) and heated for 1 h. In the resulted mixture,
besides cyclic carbonate and the initial cobalt complex, various
cobalt(III) cations including [8 + DMAP]+, [8 + DMAP + PO]+, [8 + 2×
3.2. Coupling reaction of CO2 and n-hexyl oxide
Complexes 1–9, when used together with tetrabutyl ammonium
bromide (Bu4NBr) or dimethylamino pyridine (DMAP) showed high
catalytic activity and selectivity and turnover numbers of up to 1500
were recorded for the coupling of CO2 and n-hexyl oxide in 2 h
(Fig. 3). Catalysts 4 and 7 bearing the bis(ketiminato) ligands with-
out tert-butyl substituents were found to be slightly more active
than the aldiminato complexes. In general, differences in the ligand
substitution pattern in the unbridged bis(phenoxyiminato) com-
plexes do not have a great effect on catalytic activity. This suggests
that the overall coordination sphere in these unbridged catalyst
precursors is favorable for the efficient coupling reaction and the
nature and size of ligand substituents in the series have only a minor
influence on the course of the reaction. It is worth mentioning that
Bu4NBr and DMAP alone can also catalyze the coupling of CO2 and
epoxides, but their activities remained moderate in the absence of
cobalt complexes.
Table 1
Reactivity of 4/DMAP (a) and 4/Bu4NBr (b) towards terminal and cyclic epoxides.
Substrate
Product
TOF(a/b), h−1
750/900
760/910
600/640
100a/420
20a/210
Reaction conditions: 4 (0.01 mmol), co-catalyst (0.06 mmol), 4/DMAP (a) 4/Bu4NBr
(b) and, epoxide (10 mmol, 1000 equiv.), CO2 (10 bar), CH2Cl2 (0.5 ml), 145 ◦C, 1 h.
Yields were determined by comparing the product with an external standard in
GC-Fid.
Fig. 2. Calculated structure of bis(salicylaldiminato) complex 4 (left) shows a dis-
torted square pyramidal coordination geometry for Co(III) (right).
a
Oligomeric side products.

90
A. Sibaouih et al. / Journal of Molecular Catalysis A: Chemical 312 (2009) 87–91
Scheme 2. Proposed reaction mechanism for Co(III)/DMAP catalyst 8 in PO/CO2 coupling reaction.
DMAP + PO + CO2]+, [8 + DMAP + PO + CO2]+ were identified using
high-resolution mode (Scheme 2) (see supporting information).
The results show that under applied conditions the initial Co com-
plex 8 may coordinate either one dimethylamino pyridine (DMAP)
molecule (complex b) or propylene oxide (PO) together with DMAP
(complex c), which led presumably to the dissociation of Co–I bond
and shift of the iodine atom into outer coordination sphere.
Due to the flexibility of the ligand surrounding of
bis(phenoxyiminato) cobalt(III) complexes, the metal center
can host both epoxide and DMAP cis to each other. Previous reports
on coupling and alternating copolymerization of CO2 and epoxides
catalyzed by salen type systems have shown that the opening of
the epoxide is an important step in the mechanism and it proceeds
through a bimetallic process [19–22].
As described earlier, nucleophilic attack of an external DMAP
or coordinated nucleophile on the less hindered carbon atom of
coordinated PO causes the ring opening and apparently leads to
the formation of (d) (Scheme 2) [23]. Unfortunately the species (d)
was undetectable by ESI-MS. Therefore, the ring opening of epox-
ide might also occur via bimetallic process. Insertion of CO2 into

A. Sibaouih et al. / Journal of Molecular Catalysis A: Chemical 312 (2009) 87–91
91
the Co–alkoxide bond affords the carbonate intermediate species
References
(e) and subsequent intramolecular cyclisation generates species (f).
Following elimination provides propylene carbonate and regener-
ates the species (b).
[1] X. Yin, J.R. Moss, Coord. Chem. Rev. 181 (1999) 27–59.
[2] D.J. Darensbourg, R.M. Mackiewicz, A.L. Phelps, D.R. Billodeaux, Acc. Chem. Res.
37 (2004) 836–844.
Intriguingly, Co-complexes other than a–f were also identi-
fied. The presence of [e + PO] (i), [e + PO + CO2] (g), as well as
[c + PO + DMAP] (h) are clear fingerprints for the leaching of cat-
alyst selectivity (see supporting information). The cyclic carbonate
also can be formed from depolymerization of the growing polymer
chain by backbiting reaction. In this case the carbonate end groups
attack the nearest alkoxy carbon in the chain. During the process of
cyclic carbonate formation, the polymer chain may be either free
or coordinated to the metal center (from g to e) [23].
[3] G.W. Coates, D.R. Moore, Angew. Chem., Int. Ed. 43 (2004) 6618–6639.
[4] R.L. Paddock, S.T. Nguyen, Chem. Commun. (2004) 1622–1623.
[5] L. Jin, H. Jing, T. Chang, X. Bu, L. Wang, Z. Liu, J. Mol. Catal. A: Chem. 261 (2007)
262–266.
[6] R.L. Paddock, S.T. Nguyen, J. Am. Chem. Soc. 123 (2001) 11498–11499.
[7] X.-B. Lu, Y.-J. Zhang, B. Liang, X. Li, H. Wang, J. Mol. Catal. A: Chem. 210 (2004)
31–34.
[8] J. Sun, S.-I. Fujita, M. Arai, J. Organomet. Chem. 690 (2005) 3490–3497.
[9] J.-Q. Wang, D.-L. Kong, J.-Y. Chen, F. Cai, L.-N. He, J. Mol. Catal. A: Chem. 249
(2006) 143–148.
[10] J. Sun, L. Wang, S. Zhang, Z. Li, X. Zhang, W. Dai, R. Mori, J. Mol. Catal. A: Chem.
256 (2006) 295–300.
[11] F. Li, L. Xiao, C. Xia, B. Hu, Tetrahedron Lett. 45 (2004) 8307–8310.
[12] Y.-M. Shen, W.-L. Duan, M. Shi, J. Org. Chem. 68 (2003) 1559–1562.
[13] S.-W. Chen, R.B. Kawthekar, G.-J. Kim, Tetrahedron Lett. 48 (2007) 297–300.
[14] R.L. Paddock, Y. Hiyama, J.M. Mckay, S.T. Nguyen, Tetrahedron Lett. 45 (2004)
2023–2026.
[15] X.-B. Lu, L. Shi, Y.-M. Wang, R. Zhang, Y.-J. Zhang, X.-J. Peng, Z.-C. Zhang, B. Li, J.
Am. Chem. Soc. 128 (2006) 1664–1674.
[16] C.T. Cohen, T. Chu, G.W. Coates, J. Am. Chem. Soc. 127 (2005) 10869–10878.
[17] C.T. Cohen, C.M. Thomas, K.L. Peretti, E.B. Lobkovsky, G.W. Coates, Dalton Trans.
(2006) 237–249.
[18] P. Chen, M.H. Chisholm, J.C. Gallucci, X. Zhang, Z. Zhou, Inorg. Chem. 44 (2005)
2588–2595.
[19] D.J. Darensbourg, J.C. Yarbrough, C. Ortiz, C.C. Fang, J. Am. Chem. Soc. 125 (2003)
7586–7591.
[20] D.J. Darensbourg, R.M. Mackiewicz, J. Am. Chem. Soc. 127 (2005) 14026–14038.
[21] D.J. Darensbourg, J.C. Yarbrough, J. Am. Chem. Soc. 124 (2002) 6335–6342.
[22] K.B. Hansen, J.L. Leighton, E.N. Jacobsen, J. Am. Chem. Soc. 118 (1996)
10924–10925.
4. Conclusion
The results herein underline that activated by either DMAP
or Bu4NBr co-catalysts, the unbridged bis(phenoxyiminato) Co(III)
complexes are effective and selective catalysts for the chemical
fixation of CO2 and epoxide leading to the synthesis of the cyclic
carbonate. Thebis(phenoxyiminato)Co(III)complexesco-catalyzed
with Bu4NBr produced only cyclic carbonate while with DMAP
traces of oligomers were formed together with the major product,
cyclic carbonate. The proposed mechanism accounts well for the
experimental data obtained and corroborates earlier mechanistic
proposals as well as sheds light on the new catalytic system.
[23] G.A. Luinstra, G.R. Haas, F. Molnar, V. Bernhart, R. Eberhardt, B. Rieger, Chem.
Eur. J. 11 (2005) 6298–6314.
Acknowledgements
[24] J. Melendez, M. North, R. Pasquale, Eur. J. Inorg. Chem. (2007) 3323–3326.
[25] X.-B. Lu, R. He, C.-X. Bai, J. Mol. Catal. A: Chem. 186 (2002) 1–11.
[26] M. North, R. Pasquale, Angew. Chem. Int. Ed. 48 (2009) 2946–2948.
[27] X.-B. Lu, Y.-J. Zhang, K. Jin, L.-M. Luo, H. Wang, J. Catal. 227 (2004) 537–541.
[28] M. Alvaro, C. Baleizao, E. Carbonell, M. El Ghoul, H. García, B. Gigante, Tetrahe-
dron 61 (2005) 12131–12139.
[29] K. Matsumoto, B. Saito, T. Katsuki, Chem. Commun. (2007) 3619–3627.
[30] M. Calligaris, G. Manzini, G. Nardin, L. Randaccio, J. Chem. Soc., Dalton Trans.
(1972) 543–547.
We are grateful for the financial support of Fortum Foundation
and to the Academy of Finland (project #210733).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molcata.2009.07.009.
[31] R.B. Lauffer, R.H. Heistand, L. Que, Inorg. Chem. 22 (1983) 50–55.