Highly Active Chromium Complexes Supported by Constrained Schiff-Base Ligands for Cycloaddition of Carbon Dioxide to Epoxides

Article abstract of DOI:10.1021/acs.inorgchem.0c03732

Novel constrained Schiff-base ligands (inden) were developed based on the well-known salen ligands. Chromium complexes supported by the constrained inden ligands were successfully synthesized and used as catalysts for the synthesis of cyclic carbonates from epoxides and carbon dioxide (CO2). The catalyst having tert-butyl (tBu) groups as substituents in combination with tetrabutylammonium bromide (TBAB) as a cocatalyst exhibited very high catalytic activity with a turnover frequency of up to 14800 h-1 for the conversion of CO2 and propylene oxide into propylene carbonate exclusively at 100 °C and 300 psi of CO2 under solvent-free conditions. The catalyst was found to be highly active for various epoxide substrates to produce terminal cyclic carbonates in 100% selectivity.

Full text of DOI:10.1021/acs.inorgchem.0c03732

pubs.acs.org/IC
Communication
Highly Active Chromium Complexes Supported by Constrained
Schiff-Base Ligands for Cycloaddition of Carbon Dioxide to Epoxides
Jiraya Kiriratnikom, Nattiya Laiwattanapaisarn, Kunnigar Vongnam, Nopparat Thavornsin,
Pornpen Sae-ung, Sophon Kaeothip, Anucha Euapermkiati, Supawadee Namuangruk,
and Khamphee Phomphrai*
Cite This: Inorg. Chem. 2021, 60, 6147−6151
Read Online
sı
*
Metrics & More
Article Recommendations
Supporting Information
ACCESS
ABSTRACT: Novel constrained Schiff-base ligands (inden) were developed based on the well-known salen ligands. Chromium
complexes supported by the constrained inden ligands were successfully synthesized and used as catalysts for the synthesis of cyclic
carbonates from epoxides and carbon dioxide (CO2). The catalyst having tert-butyl (tBu) groups as substituents in combination with
tetrabutylammonium bromide (TBAB) as a cocatalyst exhibited very high catalytic activity with a turnover frequency of up to 14800
h−1 for the conversion of CO2 and propylene oxide into propylene carbonate exclusively at 100 °C and 300 psi of CO2 under
solvent-free conditions. The catalyst was found to be highly active for various epoxide substrates to produce terminal cyclic
carbonates in 100% selectivity.
Scheme 1. Chromium(III) Complexes Containing (a)
Regular Salen and (b) Inden Ligands (this work)
ecause of public concerns of global warming as a result of
increasing atmospheric carbon dioxide (CO2) levels, CO2
B
has been explored extensively as a promising feedstock for
various important chemical productions such as urea,
methanol, cyclic carbonates, and polycarbonates.1−3 Among
them, the cycloaddition reaction of CO2 to epoxides, affording
cyclic carbonates, is a commercially important reaction already
exploited in numerous applications such as polar aprotic
solvents for catalytic reactions,4 electrolytes for lithium-ion
batteries used in electric vehicles and mobile devices,5,6 and
precursors for valuable polymers and chemicals.6−8 The
cycloaddition synthesis route of cyclic carbonates is a much
greener alternative process compared to that using phosgene.
However, CO2 is highly stable and has low reactivity.
Therefore, highly active catalysts are required for this simple
but significant 100% atom economy transformation.
Several catalytic systems have been explored for CO2/
epoxides cycloaddition reactions employing metal complexes
such as Al, Cr, Co, Zn, and Fe ligated by a variety of well-
known supporting ligands such as porphyrin, salicylimine
(salen), and Schiff-base ligands.9−14 Among these ligands,
dinitrogen dioxide-chelating salen ligands have been exten-
sively studied as supporting ligands in catalysis because of
simple ligand syntheses and tunable modifications for
electronic and steric properties (Scheme 1).11,15−19 Among
those active metal complexes, chromium complexes were
found to be remarkable catalysts for the preparation of cyclic
carbonates from CO2 and a wide range of epoxides. They are
highly stable and can be easily prepared from commercially
available chromium(III) acetate or chloride as a starting
material. The first chromium catalysts for cycloaddition were
reported by Kruper and Dellar using a porphyrin ligand
system.20 Shortly after that, a large variety of chromium
complexes based on salen ligands (Scheme 1a) were found to
efficiently catalyze the cycloaddition reaction of CO2 to
epoxides.11,21 In 2001, Paddock and Nguyen investigated
binary (salen)CrIII complexes/4-(dimethylamino)pyridine
(DMAP) catalyst systems based on a cooperative mechanism
requiring both (salen)CrIII complexes as a Lewis acid for
epoxide activation and DMAP cocatalyst as a Lewis base for
CO2 activation.22 This binary system was very active for the
propylene oxide (PO)/CO2 cycloaddition reaction, obtaining a
high turnover frequency (TOF) of 916 h−1 using [PO]:[Cr] =
1333:1 and 100 psi of CO2 at 100 °C. In 2008, Sun et al.
reported a bifunctional one-component catalyst system based
on pyrrolidine (salen)CrIIIX complexes containing an electro-
philic site (the metal center) and a nucleophilic unit (the
strong organic base) in the same molecule.23 This innovative
intramolecular two-centered cooperative catalyst was highly
active even at a high [PO]/[Cr] ratio of 5000:1 and 290 psi of
Received: December 21, 2020
Published: March 5, 2021
https://dx.doi.org/10.1021/acs.inorgchem.0c03732
© 2021 American Chemical Society
Inorg. Chem. 2021, 60, 6147−6151
6147

Inorganic Chemistry
pubs.acs.org/IC
Communication
Figure 1. Synthetic procedure for (inden)CrIII complexes.
CO2 at 80 °C, providing the highest TOF for chromium
catalysts of 2120 h−1. Motivated by the high potentials of cyclic
carbonates in industry, we have set out to design a new highly
active catalyst for the CO2/epoxides cycloaddition reactions
based on the famous and successful salen ligand framework.
Conventional modifications of salen ligands have concen-
trated on variations of the aromatic substituents and diamine
backbones such as R1−4 in Scheme 1a, giving ligands that can
be systematically tuned for electron deficiency, steric
hindrance, and chirality.11,15−18 While the development of
salen-type ligands seems matured, we found that constraint
modification around the aromatic ring and imine bond, as
shown in Scheme 1b, has never been investigated. This novel
ligand design has several promising advantages over the
existing salen framework. For example, the coordinating N and
O atoms on both sides are now restricted to being coplanar
with respect to the phenyl rings, giving a more rigid
coordination environment compared to salen ligands. The
N−O distances could be lengthened due to constraint of the 5-
membered ring, resulting in a wider bite angle to the metal
center and a larger active site available for substrate
coordination. Therefore, the novel indanone-based ligands
(inden) 1a−1c were synthesized via the condensation reaction
of the corresponding substituted 7-hydroxy-1-indanone with
ethylenediamine in moderate-to-high yield (see the Supporting
Information, SI). Ligands 1b and 1c were characterized
crystallographically (Figures S4 and S5). The average N−O
distance (e.g., N1−O1) in ligand 1c is 2.663(4) Å, which is
significantly longer (>0.1 Å) than the reported average N−O
distance in the related nonconstrained salen ligand (2.555
Å).24
Figure 2. Structural comparison of compounds (a) A and (b) 2c with
calculated bond distances (Å).
t
distance between the two Bu groups in 2c is about 0.036 Å
longer than that in compound A (2.712 vs 2.748 Å), giving a
wider pocket at Cr atom.
The (inden)CrIIICl complexes 2a−2c were synthesized
following a common preparation in high yield by reacting
ligands 1a−1c with NaH, followed by CrCl3·3THF in
tetrahydrofuran (THF; Figure 1). Unfortunately, several
attempts to crystallize the complexes for X-ray structural
studies were unsuccessful. Therefore, computational studies of
the related nonconstrained (salen)CrCl complex [compound
A: Scheme 1a; R1−2 = tert-butyl (tBu) and R3−4 = H] in
comparison with complex 2c were undertaken (Figure 2 and
Table S2). The calculated charges on the Cr (+1.11 vs +1.10)
and Cl (−0.42 vs −0.43) atoms are only slightly different in
both compounds, suggesting that the electronic difference of
the two compounds may not be significant. However, the
ligand coordinations to Cr atoms are significantly different.
The average N−O distance (e.g., N1−O1) in 2c is about 0.06
Å longer than that in compound A because of the more
constrained 5-membered rings, giving a wider pocket at Cr
atom. The average Cr−N bond distance in 2c is slightly longer
(about 0.013 Å), while the average Cr−O and Cr−Cl bond
distances are very similar to compound A. The shortest H−H
The catalytic activities of complexes 2a−2c for the PO/CO2
cycloaddition reactions to cyclic propylene carbonate (PC)
were then tested under the neat PO condition, as summarized
in Table 1, entries 1−3, using a PO/cocatalyst/catalyst mole
ratio of 5000:1:1 at 80 °C and 300 psi of CO2 for 1 h.
Tetrabutylammonium bromide (TBAB) was used as a
cocatalyst. Very low catalyst and cocatalyst loadings were
used (0.02 mol %). All complexes were active for the
cycloaddition reactions, giving conversions exclusively to PC
at 18%, 19%, and 38% for complexes 2a−2c, respectively.
Complex 2b having an ethylene backbone has a slightly higher
TOF of 950 h−1 compared to complex 2a having a phenylene
backbone. When tBu groups were incorporated into the ligand
as in 2c, an exceptionally high TOF of 1900 h−1 was achieved.
This unoptimized condition already gave a TOF comparable to
the record-high TOF for chromium catalysts having intra-
molecularly linked electrophilic and nucleophilic centers [TOF
= 2120 h−1; P(CO2) = 290 psi; 80 °C].23 The reaction without
the catalyst (entry 4) did not produce appreciable product,
revealing that the presence of 2c is necessary. For comparison,
6148
https://dx.doi.org/10.1021/acs.inorgchem.0c03732
Inorg. Chem. 2021, 60, 6147−6151

Inorganic Chemistry
pubs.acs.org/IC
Communication
between complexes A and 2c, as shown in Figure 3. The
reaction starts with PO adsorption on the Cr atom (IC) having
similar free energies for both compounds A and 2c. The Br−
ion then attacks PO as the ring-opening step, requiring free-
energy barriers of 10.09 and 11.67 kcal/mol for A and 2c,
respectively. The free-energy barrier for the subsequent CO2
cycloaddition step is, however, much larger than the first step.
The free-energy barrier for this step catalyzed by A (25.21
kcal/mol) is higher than that for 2c (22.96 kcal/mol), in line
with the observed lower activity for A compared to 2c.
The cycloaddition reaction was further optimized using
catalyst 2c in subsequent reactions. In addition to TBAB, other
cocatalysts were investigated, as shown in Table S4 for
tetrabutylammonium iodide, DMAP, bis(triphenylphosphine)-
iminium chloride, and 1,8-diazabicyclo[5.4.0]undec-7-ene
under the same reaction conditions, giving TOFs ranging
from 150 to 950 h−1. However, these cocatalysts were still
outperformed by TBAB. The equivalents of TBAB were then
optimized by increasing them from 1 to 10. As anticipated, the
TOF increased dramatically to 4300 h−1 (entry 6). Higher
TOFs can be further achieved by increasing the amounts of PO
(5000−20000 equiv) and TBAB (10−200 equiv) (entries 7−
10). At this point, the amount of chromium catalyst is
extremely low (0.005 mol %) but still gave the highest TOF at
11400 h−1 for a PO/TBAB/2c ratio of 20000:200:1 (entry
10). Only a small background conversion (1%) from TBAB
was observed at 80 °C (entry 9). When the temperature was
increased from 80 to 100 °C (entry 12), the TOF increased
dramatically to finally reach 14800 h−1. When the reaction time
was increased to 3 h, 100% conversion of PO was obtained
with 94% isolated yield by flash chromatography.
Table 1. Cycloaddition of CO2 to PO by Chromium
Catalysts
a
c
d
PO/cocatalyst/
catalyst
conv
(%)
TOF
(h−1
b
entry catalyst cocatalyst
)
1
2
3
4
5
6
7
8
2a
2b
2c
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
5000:1:1
5000:1:1
5000:1:1
5000:5:0
5000:1:1
5000:10:1
10000:10:1
20000:100:1
20000:200:0
20000:200:1
20000:200:0
20000:200:1
18
19
38
trace
25
86
40
46
1
900
950
1900
A
1250
4300
4000
9200
1
2c
2c
2c
2c
2c
2c
2c
9
e
f
10
11
12
57
4
74
11400
4
14800
f
a
b
Conditions: neat PO, 300 psi of CO2, 80 °C, and 1 h. Cocatalysts.
c
d
1
Determined by H NMR of the crude reaction mixture. Moles of
e
PC produced per mole of chromium catalyst per hour. Reported
conversion already subtracted background conversion from entry 9.
f
Reaction temperature: 100 °C. Reported conversion in entry 12
already subtracted background conversion from entry 11.
the cycloaddition using A as a catalyst was carried out under
the same conditions, giving a TOF of only 1250 h−1 (Table 1,
entry 5). It is evident that 2c having constrained 5-membered
rings is significantly more active, giving a TOF of over 50%
higher than that using the related salen complex A.
To gain insight information on the cycloaddition reaction,
computational calculations were performed and compared
The high efficiency of the 2c/TBAB binary system was
further demonstrated for other terminal epoxides such as
epichlorohydrin, hexene oxide, 3,4-epoxy-1-butene, styrene
oxide, and 3-phenoxypropylene oxide, as shown in Table S5.
Figure 3. Free energy profiles for PO/CO2 cycloaddition reactions using compounds A and 2c at 353.15 K.
6149
https://dx.doi.org/10.1021/acs.inorgchem.0c03732
Inorg. Chem. 2021, 60, 6147−6151

Inorganic Chemistry
pubs.acs.org/IC
Communication
All cycloaddition reactions gave very high TOF (2750−16600
h−1) with 100% selectivity to cyclic carbonates. The highest
TOF of 16600 h−1 belongs to the cycloaddition with
epichlorohydrin. The cycloaddition with internal epoxides
such as pentene oxide and cyclohexene oxide exhibited only
AUTHOR INFORMATION
■
Corresponding Author
Khamphee Phomphrai − Department of Materials Science
and Engineering, School of Molecular Science and
Engineering, Vidyasirimedhi Institute of Science and
Technology (VISTEC), Wang Chan, Rayong 21210,
Thailand; orcid.org/0000-0002-3132-680X;
Email: khamphee.p@vistec.ac.th
low-to-moderate activity (TOF of 390 and 168 h−1
,
respectively) possibly because of the steric hindrance during
the nucleophilic ring-opening of the epoxide.23,25 However, the
CO2 cycloaddition to cyclohexene oxide gave both cyclic
carbonate and atactic polycarbonate (without ether linkage) in
the ratio of 64:36. The gel permeation chromatography trace
of poly(cyclohexene carbonate) exhibited bimodal distribu-
tions with Mn of 11900 Da (Đ = 1.02) and 5800 Da (Đ = 1.04)
(see Figure S23). The presence of bimodal distributions was
commonly found for the copolymerization of cyclohexene
oxide with CO2 typically attributed to the trace of 1,2-
cyclohexanediol impurities.26 Comparisons of the activities of
our catalyst with other reported chromium catalyst systems for
these epoxides are rather difficult because very different
reaction conditions were applied in the literature [for example,
T = 25−130 °C and P(CO2) = 1−780 atm].20,22,25,27−29
However, the previously reported TOFs (h−1) in general are
on the order of hundreds rather than thousands, as shown in
this work.
Authors
Jiraya Kiriratnikom − Department of Materials Science and
Engineering, School of Molecular Science and Engineering,
Vidyasirimedhi Institute of Science and Technology
(VISTEC), Wang Chan, Rayong 21210, Thailand
Nattiya Laiwattanapaisarn − Department of Materials Science
and Engineering, School of Molecular Science and
Engineering, Vidyasirimedhi Institute of Science and
Technology (VISTEC), Wang Chan, Rayong 21210,
Thailand
Kunnigar Vongnam − Department of Materials Science and
Engineering, School of Molecular Science and Engineering,
Vidyasirimedhi Institute of Science and Technology
(VISTEC), Wang Chan, Rayong 21210, Thailand
Nopparat Thavornsin − Department of Materials Science and
Engineering, School of Molecular Science and Engineering,
Vidyasirimedhi Institute of Science and Technology
(VISTEC), Wang Chan, Rayong 21210, Thailand
Pornpen Sae-ung − Department of Materials Science and
Engineering, School of Molecular Science and Engineering,
Vidyasirimedhi Institute of Science and Technology
(VISTEC), Wang Chan, Rayong 21210, Thailand
Sophon Kaeothip − Corporate Innovation, Science and
Innovation, PTT Global Chemical Public Company Ltd.,
Chatuchak, Bangkok 10900, Thailand
In summary, the novel constrained (inden)CrIII complexes
were successfully developed and found to be highly active in
the cycloaddition reactions of CO2 to epoxides in the presence
of TBAB as a cocatalyst. The CrIII/TBAB catalysts are highly
active even at low catalyst loading of 0.005 mol %, giving PC
exclusively with a very high TOF of 14800 h−1. The binary
catalyst system can be extended to other terminal and internal
epoxides with high catalytic activities. The ease of ligand
synthesis and modification of the catalyst’s structure is the
major advantage of this type of ligand that will allow a
systematic tailoring of the catalytic activities and selectivity.
The reported novel constrained inden ligand framework was
proven to be the major successful development over the
existing well-known salen ligands. The constrained inden metal
complexes could also be applied in place of several other salen-
based metal complexes already exploited in a wide range of
catalytic processes.
Anucha Euapermkiati − Corporate Innovation, Science and
Innovation, PTT Global Chemical Public Company Ltd.,
Chatuchak, Bangkok 10900, Thailand
Supawadee Namuangruk − National Nanotechnology Center,
National Science and Technology Development Agency, Klong
Luang 12120, Pathum Thani, Thailand
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c03732
ASSOCIATED CONTENT
Notes
■
sı
* Supporting Information
The authors declare no competing financial interest.
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c03732.
ACKNOWLEDGMENTS
■
We acknowledge financial support from the PTT Global
Chemical Public Company Ltd. Instrumental support from
Frontier Research Center, VISTEC, is gratefully acknowledged.
This work was partially supported by the NANOTEC,
NSTDA, Ministry of Science and Technology, through its
program of Research Network NANOTEC.
Complete details for the crystallographic study of ligands
1b−1c, details for the experimental synthesis, cyclo-
addition reactions, characterization of 1a−1c, 2a−2c,
and the corresponding cyclic carbonates and polycar-
bonate from CO2/epoxide cycloaddition, and computa-
tional studies (PDF)
Accession Codes
REFERENCES
■
CCDC 2050712 and 2050713 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
(1) Chauvy, R.; Meunier, N.; Thomas, D.; De Weireld, G. Selecting
Emerging CO2 Utilization Products for Short- to Mid-Term
Deployment. Appl. Energy 2019, 236, 662−680.
(2) Otto, A.; Grube, T.; Schiebahn, S.; Stolten, D. Closing the Loop:
Captured CO2 as a Feedstock in the Chemical Industry. Energy
Environ. Sci. 2015, 8, 3283−3297.
6150
https://dx.doi.org/10.1021/acs.inorgchem.0c03732
Inorg. Chem. 2021, 60, 6147−6151

Inorganic Chemistry
pubs.acs.org/IC
Communication
(3) Sun, Y.; Lin, Z.; Peng, S. H.; Sage, V.; Sun, Z. A Critical
Perspective on CO2 Conversions into Chemicals and Fuels. J. Nanosci.
Nanotechnol. 2019, 19, 3097−3109.
(25) Cuesta-Aluja, L.; Djoufak, M.; Aghmiz, A.; Rivas, R.; Christ, L.;
Masdeu-Bultó, A. M. Novel Chromium (III) Complexes with N4-
Donor Ligands as Catalysts for the Coupling of CO2 and Epoxides in
Supercritical CO2. J. Mol. Catal. A: Chem. 2014, 381, 161−170.
(26) Della Monica, F.; Maity, B.; Pehl, T.; Buonerba, A.; De Nisi, A.;
Monari, M.; Grassi, A.; Rieger, B.; Cavallo, L.; Capacchione, C.
[OSSO]-Type Iron(III) Complexes for the Low-Pressure Reaction of
Carbon Dioxide with Epoxides: Catalytic Activity, Reaction Kinetics,
and Computational Study. ACS Catal. 2018, 8, 6882−6893.
(27) Castro-Osma, J. A.; Lamb, K. J.; North, M. Cr(salophen)
Complex Catalyzed Cyclic Carbonate Synthesis at Ambient Temper-
ature And Pressure. ACS Catal. 2016, 6, 5012−5025.
(28) Adolph, M.; Zevaco, T. A.; Altesleben, C.; Walter, O.; Dinjus,
E. New Cobalt, Iron and Chromium Catalysts Based on Easy-to-
Handle N4-Chelating Ligands for the Coupling Reaction of Epoxides
with CO2. Dalton Trans. 2014, 43, 3285−3296.
(29) Dean, R. K.; Devaine-Pressing, K.; Dawe, L. N.; Kozak, C. M.
Reaction of CO2 with Propylene Oxide and Styrene Oxide Catalyzed
by a Chromium(III) Amine-Bis(phenolate) Complex. Dalton Trans.
2013, 42, 9233−9244.
(4) Schäffner, B.; Schäffner, F.; Verevkin, S. P.; Börner, A. Organic
Carbonates as Solvents in Synthesis and Catalysis. Chem. Rev. 2010,
110, 4554−4581.
(5) Nie, M.; Chalasani, D.; Abraham, D. P.; Chen, Y.; Bose, A.;
Lucht, B. L. Lithium Ion Battery Graphite Solid Electrolyte Interphase
Revealed by Microscopy and Spectroscopy. J. Phys. Chem. C 2013,
117, 1257−1267.
(6) Kamphuis, A. J.; Picchioni, F.; Pescarmona, P. P. CO2-Fixation
into Cyclic and Polymeric Carbonates: Principles and Applications.
Green Chem. 2019, 21, 406−448.
(7) Maeda, C.; Miyazaki, Y.; Ema, T. Recent Progress in Catalytic
Conversions of Carbon Dioxide. Catal. Sci. Technol. 2014, 4, 1482.
(8) Taherimehr, M.; Pescarmona, P. P. Green Polycarbonates
Prepared by the Copolymerization of CO2 with Epoxides. J. Appl.
Polym. Sci. 2014, 131, 41141.
(9) North, M.; Pasquale, R.; Young, C. Synthesis of Cyclic
Carbonates from Epoxides and CO2. Green Chem. 2010, 12, 1514.
(10) Buttner, H.; Longwitz, L.; Steinbauer, J.; Wulf, C.; Werner, T.
Recent Developments in the Synthesis of Cyclic Carbonates from
Epoxides and CO2. Top. Curr. Chem. 2017, 375, 50.
(11) Decortes, A.; Castilla, A. M.; Kleij, A. W. Salen-Complex-
Mediated Formation of Cyclic Carbonates by Cycloaddition of CO2
to Epoxides. Angew. Chem., Int. Ed. 2010, 49, 9822−9837.
(12) Shaikh, R. R.; Pornpraprom, S.; D’Elia, V. Catalytic Strategies
for the Cycloaddition of Pure, Diluted, and Waste CO2 to Epoxides
under Ambient Conditions. ACS Catal. 2018, 8, 419−450.
(13) Martín, C.; Fiorani, G.; Kleij, A. W. Recent Advances in the
Catalytic Preparation of Cyclic Organic Carbonates. ACS Catal. 2015,
5, 1353−1370.
(14) Qin, Y.; Guo, H.; Sheng, X.; Wang, X.; Wang, F. An Aluminum
Porphyrin Complex with High Activity and Selectivity for Cyclic
Carbonate Synthesis. Green Chem. 2015, 17, 2853−2858.
(15) Cozzi, P. G. Metal-Salen Schiff Base Complexes in Catalysis:
Practical Aspects. Chem. Soc. Rev. 2004, 33, 410−421.
(16) Shaw, S.; White, J. D. Asymmetric Catalysis Using Chiral Salen-
Metal Complexes: Recent Advances. Chem. Rev. 2019, 119, 9381−
9426.
(17) Matsumoto, K.; Saito, B.; Katsuki, T. Asymmetric Catalysis of
Metal Complexes with Non-Planar ONNO Ligands: Salen, Salalen
and Salan. Chem. Commun. 2007, 3619−3627.
(18) Baleizao, C.; Garcia, H. Chiral Salen Complexes: An Overview
̃
to Recoverable and Reusable Homogeneous and Heterogeneous
Catalysts. Chem. Rev. 2006, 106, 3987−4043.
(19) Coates, G. W.; Moore, D. R. Discrete Metal-Based Catalysts for
the Copolymerization of CO2 and Epoxides: Discovery, Reactivity,
Optimization, and Mechanism. Angew. Chem., Int. Ed. 2004, 43,
6618−6639.
(20) Kruper, W. J.; Dellar, D. D. Catalytic Formation of Cyclic
Carbonates from Epoxides and CO2 with Chromium Metal-
loporphyrinates. J. Org. Chem. 1995, 60, 725−727.
(21) Darensbourg, D. J. Making Plastics from Carbon Dioxide: Salen
Metal Complexes as Catalysts for the Production of Polycarbonates
from Epoxides and CO2. Chem. Rev. 2007, 107, 2388−2410.
(22) Paddock, R. L.; Nguyen, S. T. Chemical CO2 Fixation: Cr(III)
Salen Complexes as Highly Efficient Catalysts for the Coupling of
CO2 and Epoxides. J. Am. Chem. Soc. 2001, 123, 11498−11499.
(23) Zhang, X.; Jia, Y.-B.; Lu, X.-B.; Li, B.; Wang, H.; Sun, L.-C.
Intramolecularly Two-Centered Cooperation Catalysis for the
Synthesis of Cyclic Carbonates from CO2 and Epoxides. Tetrahedron
Lett. 2008, 49, 6589−6592.
(24) Darensbourg, D. J.; Mackiewicz, R. M.; Rodgers, J. L.; Fang, C.
C.; Billodeaux, D. R.; Reibenspies, J. H. Cyclohexene Oxide/CO2
Copolymerization Catalyzed by Chromium(III) Salen Complexes and
N-Methylimidazole: Effects of Varying Salen Ligand Substituents and
Relative Cocatalyst Loading. Inorg. Chem. 2004, 43, 6024−6034.
6151
https://dx.doi.org/10.1021/acs.inorgchem.0c03732
Inorg. Chem. 2021, 60, 6147−6151