Multifunctional and Sustainable Fe-Iminopyridine Complexes for the Synthesis of Cyclic Carbonates

Article abstract of DOI:10.1002/cssc.201802563

The use of multifunctional and sustainable Fe catalysts for the formation of cyclic carbonates from epoxides and carbon dioxide at 80 °C and 3 bar pressure is presented. The optimal catalyst possesses a halide counteranion and a hydrogen bond donor to activate the epoxide for ring opening, affording a single-component, cocatalyst-free catalytic system.

Full text of DOI:10.1002/cssc.201802563

DOI: 10.1002/cssc.201802563
Communications
Multifunctional and Sustainable Fe-Iminopyridine
Complexes for the Synthesis of Cyclic Carbonates
Eun Young Seong+, Jae Hyung Kim+, Nam Hee Kim, Kwang-Hyun Ahn, and Eun Joo Kang*[a]
The use of multifunctional and sustainable Fe catalysts for the
formation of cyclic carbonates from epoxides and carbon diox-
ide at 808C and 3 bar pressure is presented. The optimal cata-
lyst possesses a halide counteranion and a hydrogen bond
donor to activate the epoxide for ring opening, affording a
single-component, cocatalyst-free catalytic system.
investigated, and current commercial catalysts for cyclic-car-
bonate synthesis are based on hydrogen bond donors (RꢀOH,
RꢀNH2, and its protonated species) with quaternary ammoni-
um or phosphonium salts.[6] However, research on metal-based
systems continues due to their higher intrinsic Lewis acidity, al-
lowing for lower reaction temperatures and lower catalyst
loadings. In recent years, the investigation of multifunctional,
single-component catalysts displaying both Lewis acidic and
nucleophilic components has been considered crucial for a
sustainable approach to CO2 conversion under mild conditions.
To this end, current research on metal-based, single-compo-
nent catalysts has focused on the introduction of a nucleophil-
ic quaternary onium salt tethered to the ligand (Figure 1,
Investigation of chemical reactions that can recycle CO2 as a
chemical feedstock for the production of industrially attractive
chemicals, plastic precursors, and fuels is regarded as a viable
method for reducing the net amount of anthropogenic CO2
present in the atmosphere.[1] The synthesis of cyclic carbonates
by the cycloaddition of CO2 to epoxides represents one of a
few processes employing CO2, and this reaction is thermody-
namically favored by the release of the ring-strain energy con-
tained in epoxides.[2] Cyclic carbonates have become increas-
ingly attractive in academic and industrial arenas, being appli-
cable as polar aprotic solvents that can replace regulated sol-
vents as electrolytes in lithium-ion batteries, and in providing
intermediates for the synthesis of biodegradable polymers.[3]
In the catalytic system for a typical cyclic-carbonate synthe-
sis, Lewis-acidic compounds, such as metal–organic complexes
or hydrogen bond donors, activate the epoxide ring opening
with a remarkable acceleration of the reaction rate. At the
same time, nucleophilic counteranions within quaternary am-
monium salts, phosphonium salts, or ionic liquids facilitate the
epoxide ring opening through a nucleophilic attack. Following
the cooperative ring-opening step, CO2 inserts into the metalꢀ
alkoxide bond to afford the alkoxy carbonate intermediate, the
cyclization of which yields the cyclic carbonate as product. As
a result, many catalysts used in the cycloaddition of CO2 to ep-
oxides utilize binary systems consisting of a Lewis-acidic com-
ponent (“catalyst”) and a nucleophilic component (“cocata-
lyst”). Undoubtedly, metalosalen and porphyrin complexes of
Al, Cr, Mn, Co, or Zn have attracted much attention presuma-
bly owing to the significant features of ligands, including facile
synthesis, ability to chelate most metal ions, and a planar ge-
ometry with empty axial positions for efficient epoxide activa-
tion.[4,5] Alternatively, metal-free systems have been extensively
Figure 1. Roles of functionalized ligands in single-component catalytic
system.
Type I). Bifunctional Al(salen), Co(salen), and Zn(salpyr) com-
plexes bearing an ammonium or pyridinium halide have been
reported to be more active than the equivalent binary cata-
lysts.[7] Metalloporphyrins of Mg and Zn containing an ammo-
nium, phosphonium, or imidazolium bromide have also been
developed that exhibit higher catalytic activity without any co-
catalyst.[8,9] Alternatively, a nucleophilic halide could be dissoci-
ated from the metal center through the thermodynamic rever-
sion in trinuclear Zn complex, the coordination of imidazole to
the metal center in cobalt-porphyrin system, or the ionic inter-
action of imidazolium cation in a SmCl4 complex.[10] Additional-
ly, the tertiary aliphatic amine of the N-methylhomopiperazine
moiety was designed to function as a neutral nucleophile and
directly attack the epoxide in a functionalized Al(salen) system
(Type II).[11]
[a] E. Y. Seong,+ J. H. Kim,+ N. H. Kim, Prof. K.-H. Ahn, Prof. E. J. Kang
Department of Applied Chemistry
Kyung Hee University
Yongin 17104 (Korea)
E-mail: ejkang24@khu.ac.kr
Synergistic effects were also observed when multiple sites
for epoxide activation were introduced (Type III). Diverse hy-
drogen bond donors, such as a phenolic hydroxy, secondary
amine, or protonated tertiary ammonium groups, participated
[+] These authors contributed equally to this work.
Supporting Information and the ORCID identification number(s) for the
author(s) of this article can be found under:
https://doi.org/10.1002/cssc.201802563.
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to supply additional Brønsted-acidic components and improve
the catalytic performance.[12] Research related to hydrogen
bond donors has been extensively performed for organocata-
lytic systems,[13] but only a few investigations on their incorpo-
ration in metal–ligand complex systems have been reported.[12]
Also, despite an increasing interest in iron catalysis,[14] there
have been very few reports of functionalized iron–ligand sys-
tems for the sustainable and carbon-balanced approach to CO2
conversion, which is considered a worthy challenge. Herein we
designed and synthesized multifunctional iron(II) iminopyridine
or bisiminopyridine complexes containing several hydroxy
groups that can interact with the epoxide via hydrogen bond-
ing with this well-designed catalyst structure. Furthermore, the
halide anion in iron complexes can facilitate the ring opening
of the epoxide in a cocatalyst-free system.
iron catalysts were synthesized by introducing two types of li-
gands sequentially (Scheme 1b and d).[17] After selectively in-
troducing one tridentate ligand (terpyridine (tpy) or bisimino-
pyridine), a dihydroxybipyridine (6,6’-dihydroxyl-2.2’-bipyridyl
(dhbpy)) ligand was complexed, resulting in a complex with an
axial halide, thus providing a catalyst with a Lewis-acidic iron
center and a labile halide nucleophile.
Initially, styrene oxide (1a) was selected as a model substrate
to identify the optimal homoleptic iron–iminopyridine catalyst
and optimize other reaction conditions including the catalyst
amount, CO2 pressure, and temperature under cocatalyst- and
solvent-free conditions (Table 1). As expected, FeBr2 did not
show any catalytic activity for the synthesis of cyclic carbo-
nates at 1008C and 10 bar CO2 pressure (entry 1). Fortunately,
when the model reaction was carried out in the presence of
Fe(IPEtOH)3Cl2 and Fe(IPEtOH)3Br2 (0.5 mol%), the desired styrene
carbonate (2a) was obtained in 55% and 96% yield, respec-
tively (entries 2 and 3). The significant difference in the reac-
tion results of Fe(IPEtOH)3SO4 without or with tetrabutylammoni-
um bromide (TBAB) implied that the counteranion of the iron–
iminopyridine complex plays an important role and that the
nucleophilic halide present in the Fe(IPEtOH)3Br2 catalyst allows
this catalyst to function as a single-component catalyst. To clar-
ify the role of the hydroxy groups in the iminopyridine ligand,
we compared the reactivities of iron–ligand complexes con-
Functionalized iminopyridine (IP) or bisiminopyridine (BIP) li-
gands were prepared by the condensation reaction of 2-pyridi-
necarboxaldehyde (picolinealdehyde) or 2,6-pyridine dicarbox-
aldehyde with an amine (Scheme 1).[15] The proposed hydroxy
groups were introduced in the form of several hydroxyalkyl
amines. Interestingly, both the iminopyridine-ligand synthesis
by condensation and the homoleptic iron–ligand complexation
by self-assembly were carried out in a single step, enabling the
control of steric and electronic properties through functional-
group variation of the amine used (Scheme 1a).[16] Heteroleptic
Scheme 1. Typical procedures for the preparation of multifunctional iron(II) complexes: (a) [Fe(IPR)3]X2, (b) [Fe(tpy)(bpy)X]X and its derivatives, (c) BIPR ligands,
and (d) [Fe(BIPR)(dhbpy)X]X.
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Table 1. Synthesis of cyclic carbonate 2a with homoleptic Fe–iminopyri-
Table 2. Synthesis of cyclic carbonate 2a with heteroleptic Fe-bisimino-
dine complexes.
pyridine complexes.
Yield[a]
[%]
Entry
Catalyst[a]
([mol%])
PCO
[bar]
T
[8C]
t
Yield[b]
[h] [%]
Entry
Catalyst
([mol%])
PCO
[bar]
T
[8C]
2
2
1
2
3
4
5
6
7
8
FeBr2 (0.5)
10
10
10
10
10
10
5
5
5
5
5
100
100
100
100
100
100
100
100
100
100
80
– (68)
55 (22)
96
1
2
3
4
5
6
7
8
9
[Fe(tpy)(bpy)Cl]Cl (1)
10
5
5
5
5
3
3
3
3
3
120
100
100
100
80
80
80 15
80
80
80
12
12
12
12
15
15
80
53 (36)
92
Fe(IPEtOH)3Cl2 (0.5)
Fe(IPEtOH)3Br2 (0.5)
Fe(IPEtOH)3SO4 (0.5)
Fe(IPEtOH)3SO4 (0.5)[b]
Fe(IPPh)3Br2 (0.5)
Fe(IPcHex)3Br2 (0.5)
Fe(IPEtOH)3Br2 (0.5)
[Fe(tpy)(dcbpy)Cl]Cl (0.5)
[Fe(tpy)(dhbpy)Cl]Cl (0.5)
[Fe(tpy)(dhbpy)Br]Br (0.25)
[Fe(tpy)(dhbpy)Br]Br (0.25)
[Fe(BIPPrOH)(dhbpy)Br]Br (0.25)
– (69)
96
70
80
91
92
67 (23)
26 (66)
95
66 (28)
76 (20)
95
66 (24)
76 (24)
94 (6)
58 (42)
72 (15)
50 (23)
[Fe(BIPPrðOHÞ )(dhbpy)Br]Br (0.25)
2
Fe(tpy)(dhbpy)I]I (0.25)
15
15
15
Fe(IPPrðOHÞ )3Br2 (0.5)
2
[Fe(BIPPrOH)(dhbpy)I]I (0.25)
9
Fe(IPPrðOHÞ )3Br2 (0.3)
2
10
[Fe(BIPPrðOHÞ )(dhbpy)I]I (0.25)
2
10
11
12
13
14
Fe(IPPrðOHÞ )3Br2 (0.5)
2
Fe(IPPrðOHÞ )3I2 (0.5)
5
5
5
80
80
80
2
[a] bpy=2,2’-bipyridyl, dcbpy=4,4’-dicarboxy-2,2’-bipyridyl, dhbpy=6,6’-
dihydroxyl-2,2’-bipyridyl. [b] Yields were determined by 1H NMR analysis
using 1,1,2,2-tetrachloro-ethane as an internal standard (yield of unreact-
ed styrene oxide in parentheses).
Fe(IPPh)3I2 (0.5)
Fe(IPcHex)3I2 (0.5)
[a] Yields were determined by 1H NMR analysis using 1,1,2,2-tetrachloro-
ethane as an internal standard (yield of unreacted styrene oxide in paren-
theses). [b] Reaction with 1.0 mol% of TBAB.
the amount of the catalyst (0.25 mol%), possibly due to the
dissociation rate of the axial halide and the nucleophilicity of
free halide (entry 4). However, with the above ligand combina-
tion, we observed a marked decrease in catalytic activity when
the temperature was lowered slightly from 100 to 808C. To
solve this problem, the tridentate ligand was switched from
terpyridine to bisiminopyridine, which can be readily modified
through the introduction of desired functional groups. As the
number of hydrogen bond donors (hydroxy groups) increased,
the activity of the BIP-derived catalysts increased, and the [Fe(-
taining aryl, alkyl, monohydroxy, and dihydroxy groups (en-
tries 6–9), and the reactions with hydrogen-bond donors re-
sulted in an efficient conversion even when the pressure of
CO2 was reduced to 5 bar. A sharp decline in the yield of 2a
was observed when the temperature was decreased to 808C
(entry 11). However, the reaction efficiency was increased to
95% when the bromide nucleophile in the catalyst complex
was replaced with an iodide (entry 12). The observed activity
of the counteranions was in the order of their nucleophilicities
as Cl<Br<I; this behavior is likely further strengthened by the
fact that the bromide anion is a stronger hydrogen-bond ac-
ceptor than the iodide.[18] Additionally, the effect of the hydro-
gen bond donor was larger at lower temperature (entries 13
and 14).
BIPPrðOHÞ )(dhbpy)Br]Br catalyst was found to be most effective.
2
Due to the success of using iodide in the homoleptic iron cata-
lyst optimization, iodide was tested in the heteroleptic cata-
lysts as well. However, the yields were considerably lower than
expected (entries 8–10). The final optimized conditions showed
that the amount of catalyst could be reduced from 0.5 to
0.25 mol% and that the CO2 pressure could also be reduced
from 5 to 3 bar, in comparison to the results with the homo-
leptic iron–iminopyridine catalysts.
Due to the lower reactivity of the homoleptic iron–iminopyr-
idine catalysts below 808C, we next investigated the catalytic
performance of heteroleptic iron complexes containing both a
hydrogen bond donor and a halide nucleophile (Table 2). The
effect of introducing a hydroxy group onto the bidentate
ligand was examined in [Fe(tpy)(ligand)Cl]Cl catalysts, the
dhbpy ligand complex containing a 6,6’-dihydroxyl group
showed a significant improvement even under milder condi-
tions, much better than the 4,4’-dicarboxy-2,2’-bipyridyl
(dcbpy) ligand complex containing a 4,4’-dicarboxyl group (en-
tries 1–3). When the chloride ligand was changed to a bro-
mide, the yield was comparable to yields obtained using half
Having established the optimal homoleptic and heteroleptic
Fe systems, a range of mono- and disubstituted epoxides were
treated using these conditions (Table 3). Epoxides bearing halo-
genated phenyl or methyl substituents were converted to their
corresponding cyclic carbonates (2a–d) in good-to-excellent
yields. When the epoxides with long alkyl chains were utilized
(1e–g), poor solubility of the iron complex was observed when
run neat, and a polar solvent (DMF) was necessary to smoothly
perform the reaction. Glycidol (1h), its derivatives (1i–j), and
epichlorohydrin (1l) showed excellent reactivities, affording
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al challenging substrates due to their steric bulk, hindering the
approach of the nucleophilic halide. Their reactions were thus
conducted using a higher amount of the catalyst (1.0–
2.5 mol%), at an elevated pressure of 10 bar, and a tempera-
ture of 1008C. Cycloaddition of cyclopentene oxide with CO2
afforded the desired carbonate 2m in 79–81% yield, and only
the cis-isomer was observed, suggesting a double-inversion
mechanism. While utilizing cyclohexene oxide is reported to
be difficult due to it forming a polyether through self-polymer-
ization or a polycarbonate through copolymerization with CO2,
the related polymer was not detected under Fe catalysis and
2n formed in low yield (28–34%). The selectivities for cyclic
carbonates in Table 3 were all greater than 99%, which reflect-
ed the absence of any other reaction products, including poly-
mers and diols.
Table 3. Cycloaddition of CO2 with various epoxides.
To determine the intermediate species and gain insight into
the mechanism of iron-catalyzed cycloaddition reactions, a sto-
ichiometric reaction of the iron-complex and propylene oxide
(PO) was conducted and monitored by 1H NMR spectrosco-
py.[10c,19] Because the optimal iron complexes in method A and
B exhibited a complicated pattern at 1.0–5.0 ppm, the simpler
[Fe(BIPPh)Cl2] complex was used in this study. As shown in
Figure 2, after mixing the Fe complex and PO in [D6]DMSO at
608C for 30 min, the chloroalkoxy group (labelled “*”), result-
ing from epoxide ring opening, was observed along with un-
reacted PO (labelled “&”), providing evidence for chloride-
mediated ring opening. This result indicates that the halide
counteranion of the Fe catalyst attacks the activated epoxide,
providing the metal alkoxide intermediate and affording a co-
catalyst-free system.
[a] Method A conditions: catalyst (0.5 mol%), CO2 (5 bar), 808C, 20 h.
[b] Method B conditions: catalyst (0.25 mol%), CO2 (3 bar), 808C, 15 h.
[c] DMF (10m). [d] DMF (10m), 1008C. [e] CO2 (5 bar), 508C. [f] Catalyst
(1.0 mol%), CO2 (10 bar). [g] Catalyst (0.5 mol%), CO2 (10 bar), 20 h.
[h] Catalyst (2.5 mol%), CO2 (10 bar), 20 h.
To investigate the effect of integrating multifunctional
groups into a single-component catalyst, the catalytic per-
formance of a simple physical mixture of the individual com-
ponents, such as FeX2 and ligand, was evaluated against the
performance of the preformed catalyst (Table 4). Surprisingly,
when performing the reaction using a mixture of each compo-
corresponding cyclic carbonates (72–99% yields) while sub-
strate 1k with a methacrylate unit showed a tendency to poly-
merize, requiring a lower temperature of 508C with the iron–
bisiminopyridine complex. Disubstituted epoxides are in gener-
1
Figure 2. 1H NMR monitoring of chloroalkoxy species (*) and propylene oxide (&) in [D6]DMSO. H NMR spectrum of (a) Fe(BIPPh)3Cl2 and (b) mixture of Fe(-
BIPPh)3Cl2 and propylene oxide (1:2).
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Fe complex was dissociated and the Lewis-acidic Fe center ac-
tivated the epoxide. The most characteristic feature of the op-
timized geometries of prereaction complex is the presence of
OꢀH···O hydrogen bonding between the alkyl alcohol chain in
iminopyridine ligand and epoxide coordinated to the iron
atom; this suggests that epoxide ring opening is more suscep-
tible to nucleophilic attack of halide. It is also found that the
axially oriented epoxide substrate was biased toward the hy-
drogen bond donor by about 10 degrees. It is also structurally
difficult for the oxygen atom of an alkyl alcohol chain to act as
a ligand for the Fe center.
Table 4. Mechanistic investigation of multifunctional Fe complexes.
Entry
Component
([mol%])
Yield[a]
[%]
method A[b]
[Fe(IPPrðOHÞ )3]I2 (0.5)
95
2
1[b]
2[b]
3[b]
4[b]
FeI2 (0.5), picolinaldehyde (1.5), serinol (1.5)
FeI2 (0.5)
99
– (90)
2 (98)
2 (98)
IPPrðOHÞ (1.5)
2
picolinaldehyde (1.5), serinol (1.5)
On the basis of above-discussed experimental findings, a
plausible mechanism is proposed (Scheme 2). The activity pat-
method B[c]
5[c]
6[c]
[Fe(BIPPrðOHÞ )(dhbpy)Br]Br (0.25)
94 (6)
11 (86)
1 (90)
2
Fe(BIPPrðOHÞ )Br2 (0.25), dhbpy (0.25)
FeBr2 (0.25), BIPPrðOHÞ (0.25), dhbpy (0.25)
2
2
1
[a] Yields were determined by H NMR analysis (yield of unreacted styrene
oxide in parentheses). [b] Reaction conditions: CO2 (5 bar), 20 h. [c] Reac-
tion conditions: CO2 (3 bar), 15 h.
nent, including FeI2, picolinaldehyde, and serinol (2-amino-1,3-
propanediol, entry 1), the reaction was found to be as effective
as method A which utilized the preformed catalyst. Carrying
out the reaction utilizing only one of the necessary compo-
nents (entries 2–4) did not lead to any reaction and only un-
reacted starting material was observed. Although the
[Fe(IPPrðOHÞ )3]I2 complex could be generated in situ by self-as-
2
sembly from the individual components, the heteroleptic cata-
lyst was not amenable to this in situ process. When the multi-
functional performance of the heteroleptic iron–bisiminopyri-
dine system was also tested (entries 5 and 6), simply mixing of
the individual components exhibited almost no catalytic activi-
ty in contrast to the reaction using the preformed [Fe(-
Scheme 2. Plausible mechanism.
BIPPrðOHÞ )(dhbpy)Br]Br catalyst, demonstrating the superiority of
tern of the multifunctional Fe catalyst can be rationalized on
the array of hydrogen bond donors, the Lewis acidic Fe center,
and the nucleophilic halide counteranion in the single-compo-
nent catalytic system. First, dissociation of the imino ligand in
the homoleptic Fe complex or the halide anion in the hetero-
leptic Fe complex occurs,[20] after which the epoxide is activat-
ed by the Lewis-acidic center and the hydroxy groups in the
ligand, accelerating the ring opening of the epoxide. Simulta-
2
integrating the functional groups in a single preformed com-
plex.
DFT calculations were carried out to analyze the molecular
structure of the theoretical reaction complex, demonstrating
the feasibility of the functionalized iron-ligand system. Fig-
ure 3a presents the structure of the epoxide-coordinated Fe
complex, in which one of the imino ligands in the homoleptic
Figure 3. (a) Proposed structure of the epoxide-coordinated Fe complex, (b) DFT calculation result of the prereaction complex model.
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less hindered site of the epoxide, which causes ring opening
and generates the haloalkoxy anion species. Insertion of CO2
into the FeꢀO bond results in the formation of the haloalkoxy
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It has recently been recognized that global supplies of the
most useful metal ores are being exhausted while Fe is one of
the five most abundant metals in the Earth’s crust. The devel-
opment of sustainable Fe catalyst substitutes for the large-
scale production of cyclic carbonates is a meaningful investiga-
tion. Owing to the existence of both the Lewis acidic Fe center
and hydrogen bond donors and in combination with a nucleo-
philic halide in one catalyst, iron(II)–iminopyridine and bisimi-
nopyridine complexes show high catalytic activity for the for-
mation of cyclic carbonates from epoxides and CO2 under mild
conditions. This approach represents a method of moving to-
wards greener CO2 recycling processes to afford valuable
chemicals with potential industrial applications.
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and Technology, National Research Foundation (2017M1A2A20
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Conflict of interest
The authors declare no conflict of interest.
Keywords: carbon dioxide
iminopyridine complex · homogeneous catalyst · hydrogen
bond donor
·
cyclic carbonates
·
Fe-
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Manuscript received: November 7, 2018
Revised manuscript received: November 22, 2018
Version of record online: && &&, 0000
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ChemSusChem 2019, 12, 1 – 8
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&
These are not the final page numbers! ÞÞ

COMMUNICATIONS
E. Y. Seong, J. H. Kim, N. H. Kim, K.-H. Ahn,
E. J. Kang*
All in one-component: A Lewis-acidic
metal center, a functionalized ligand
containing hydrogen bond donors, and
a nucleophilic halide all work efficiently
in one catalyst, FeII–iminopyridine and
bisiminopyridine complexes, through a
well-organized structure. The formation
of cyclic carbonates occurs from epox-
ides and carbon dioxide under mild
conditions in a single-component, coca-
talyst-free system.
&& – &&
Multifunctional and Sustainable Fe-
Iminopyridine Complexes for the
Synthesis of Cyclic Carbonates
&
ChemSusChem 2019, 12, 1 – 8
www.chemsuschem.org
8
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!