Ambient chemical fixation of CO2 using a highly efficient heterometallic helicate catalyst system

Article abstract of DOI:10.1039/c7cc09092g

Two novel heptanuclear 3d-4f helicates have been synthesized and characterized. The helicates act as catalysts and show high catalytic activity for the coupling of CO₂ and epoxides to obtain cyclic carbonates with a wide substrate scope at ambient temperature and pressure.

Full text of DOI:10.1039/c7cc09092g

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc
first methanofullerene derivatives of Sc
3
N@C2n (2n
=
68 and 80). Synthesis of the
3
N@D5h-C80
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Ambient chemical fixation of CO by using highly efficient
2
heterometallic helicates catalyst system†
Received 00th January 20xx,
Accepted 00th January 20xx
Li Wang, Cong Xu, Qingxin Han, Xiaoliang Tang, Panpan Zhou, Ruilian Zhang, Guoshu Gao, Benhua
Xu, Wenwu Qin and Weisheng Liu*
DOI: 10.1039/x0xx00000x
www.rsc.org/
7
Two novel heptanuclear 3d-4f helicates have been synthesized synthesis reactions . These results show that: on the one hand,
and characterized. The helicates act as catalysts show high close proximity of different metal centers leads to potential
catalytic activity for coupling of CO
2
and epoxides to obtain cyclic cooperativity and synergy, which mainly account for the high
carbonates with a wide substrate scope at ambient temperature catalyst reactivity and selectivity; on the other hand, precise
and pressure.
control of the supramolecular structures would modulate the
catalytic activity of the catalysts and extend the wide of the
catalysts. So far, a lot of hetero-multinuclear systems have
found wide applications in a range of organic transformations.
However, there are no literatures reported for the ambient
2
The conversion of CO into useful chemicals has boast great
interest from chemists and industry due to its economic and
1
environmental aspects . In this respect, the coupling of CO
2
and epoxides to obtain cyclic carbonates are environmentally-
friendly reactions for reducing carbon dioxide emissions and
chemical fixation CO
2
to obtain cyclic carbonates by using
hetero-multinuclear system.
2
saving energy . Although a variety of catalyst systems have
8
Helices are the central structural motif in biosystem .
Helical-metal complex (helicates) particularly lanthanide
helicates, acting as a biomimetic model have received much
attention because of their fascinating structures and have
various potential applications in magnetic, optical, biology and
medicine. Many efforts have been made to construct various
lanthanide helicates by Lehn, Bünzli, Piguet, Gunnlaugsson and
been applied to catalyze these reactions, there are still face
some limitations such as low catalyst reactivity with turnover
-1
frequencies (TOFs) rarely >300 h , air sensitivity, the use of
o
additional solvents, even at elevated temperatures (>130 C)
3
and high pressure conditions .
Mimicking Nature seems the smartest strategy to design
novel catalysts. Cooperative activation through multiple metal
4
centers is a common feature in enzyme catalysis . In many
9
a few other groups . What’s more, the rational design of
hetero-multinuclear helicates and further exploration of their
0
catalytic applications are challenging and promising .
1
enzymes, two or more metal centers in close proximity in the
active site are able to activate multiple reacting species leading
to exceptional efficiency and unparalleled selectivity. Inspired
by these natural systems, there have been great interests in
controlling the cooperative effects between the metal centers
in multinuclear complexes to improve the catalytic
Driven by the ultimate goal of nature mimetic, we report
here the design and synthesis of heterometallic helicates
3
featuring multiple Zn-Ln units Zn Ln L (Ln =Tb
+
3+
3+
(1), Dy (2))
4
3 4
and their catalytic applications in the cycloaddition of CO and
2
epoxides to obtain cyclic carbonates under ambient conditions
5
performance . Among them, bimetallic system that
(
Scheme 1).
incorporate alkalis or transition metals and lanthanides can be
used as promising building blocks for the preparation of
6
hetero-multinuclear cooperative catalysts . Such as
heterobimetallic Schiff base Ln-M complexes and Ln-M3
complexes have been developed by Shibasaki and other
groups act as catalysts for a wide variety of asymmetric
Scheme 1. Self-assembly of the heterometallic helicates as catalysts for
CO conversion reactions.
2
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Herein, a “helical” ligand
H
3
L
with the C=N and C=O donor
In this 3d-4f helicate system, the two metal centers of Ln
groups which show different binding preferences for and Zn are situated in a proper range to enable the
2
+
3+
3+
3+
3+
selectively coordinated with Zn and Ln (Ln =Tb
)) ions has been first synthesized to construct two novel 3d- multimetalllic sites act as Lewis acid activators, suggesting
heptanuclear helicates Zn Ln Single-crystal X-ray their promising potential for catalytic applications. Thus, we
structural analysis revealed that the complexes and are initially utilize 0.025 mol % and 2, respectively, in
(
1
), Dy
intramolecular synergistic action and the exposed
(
2
4
f
4
3 4
L .
1
2
1
isostructural and crystallized in the monoclinic P2/c space combination with 0.75 mol % tetrabutyl-ammonium bromide
group. In parallel, the two complexes also have been well (TBAB) as binary catalysts for the cycloaddition of styrene
characterized by elemental analysis, infrared spectrum (IR) oxide and CO
2
to obtain cyclic carbonate with solvent-free
Figure S1) and powder X-ray diffraction (PXRD) (Figure S2). under 120 C, 1 MPa. Both of them achieved excellent yields
Thermo gravimetric analysis (TGA) (Figure S3) results show (96 %, 95 %, respectively) and exhibited high catalytic activity
o
(
-1
that the helicates display high thermal stability below 260 °C.
Complex 1 was selected as a representative example for 3800 h after 1 hour reactions (Table 1 entries 3-4).
with initial turnover frequencies (TOFs) as high as 3840 h and
1 was
-1
crystal structure description. The unit cell consists of an equal used as a representative catalyst to examine the reactions.
proportion of the M and P enantiomers, and therefore the Different co-catalysts have been tested with TBAB being the
crystal system is achiral (Figure S1). Each strand is wrapped best performer (Table 1 entries 3, 10 and 11). Neither
1 nor
around Zn-Tb pairs in a head-to-tail manner (Figure 1a). The TBAB alone could catalyse the reaction with satisfactory yield
center Tb2 ion has eight-coordinated trigondodecahedron (0 %, 25 %, respectively), while the yield was increased to 96 %
geometry with two methoxy oxygen atoms, two amido oxygen with the co-existence of
1 and TBAB. With lower loading of
atoms and four phenoxo oxygen atoms from four ligands. The TBAB (0.5 mol %, 0.4 mol %, 0.25 mol %) still obtained good
-
-
1
-1
two Tb1 ions on the ends are coordinated in the same way: yields with high initial TOF as high as 3400 h , 3200 h , 2160 h
1
both of them are ten-coordinated with one methoxy oxygen
(Table S6 entries 3-5), respectively. The yields remained
atom, one amido oxygen atom, and two phenoxo oxygen almost unchanged when the pressure was decreased from 1.0
atoms from two ligands. Furthermore, they are coordinatively to 0.7 MPa (Table 1 entries 3 and 5). It is noteworthy that the
saturated by three bidentate nitrates, which terminate the substrate has been used without prior removal of either
helicate and prevent it from extending infinitely. All of the four oxygen or moisture and in the catalytic process air or even
zinc ions have five-coordinated rectangular pyramid geometry water (volume ratio= 20 %) (Table 1 entries 11-12) also had
with the N1O3 pockets of the two ligand strands, which are little effect on the reactions. Reactions at different
similar to common N2O2 pockets. A nitrate ion with temperatures (Table 1 entries 6-8) revealed that the catalyst
monodentate coordination or one H O molecule occupies the also show excellent catalytic activity with relative high initial
2
-1
axial ligation site of the Zn metals. Each zinc ion adjacent to a TOFs >600 h under mild conditions, which promote us to
Tb atom is linked by two phenoxo oxygen atoms from two further study their catalytic performance even under ambient
ligands, and the Zn-Tb distances measured were 3.418 Å, 3.518 condition. With loading 0.125 mol % of
1 after 48 h reaction, a
Å for Zn1-Tb1 and Zn2-Tb2, respectively. To the best of our yield of 96 % for propylene carbonate was achieved with TOF
a
-1
knowledge, the heptanuclear 3d-4f helical structure assembled 16 h at ambient condition (Table 2 entry 1 ). The TOF is
by two tetranuclear double-strand helicates sharing the same higher than that of the Co-CMP and Al-CMP catalysts, which
-1
-1
rare earth ion Tb2 is the largest nuclearity helicate among the were 3.5 h and 3.4 h calculated from experimental data of
discrete 3d-4f complexes.
the reported literature under the same conditions,
1
respectively .
1
Table 1. Synthesis of cyclic carbonate via insertion of CO
a
2
to styrene oxide
catalysed by complexes 1 and 2
Co-cat
Entry
Cat.
T
(
P
Time
(h)
1
1
1
1
1
1
1
Yield
TOF
-1
(h )
o
b
C)
(bar)
10
10
10
10
7
10
10
10
10
10
10
10
10
(%)
25
0
1
2
3
4
5
6
7
8
9
-
Bu
-
4
NBr
120
120
120
120
120
100
80
-
-
1
1
2
1
1
1
1
1
1
1
1
1
Bu
4
Bu
4
Bu
4
Bu
4
Bu
4
Bu
4
Bu
4
Bu
4
Bu
4
Bu
4
Bu
4
NBr
NBr
NBr
NBr
NBr
NBr
NI
96
95
93
84
50
15
84
61
93
81
67
3840
3800
3720
3360
2000
600
3360
2440
3720
3240
26800
60
1
1
120
120
120
120
120
10
11
12
13
NCl
NBr
NBr
NBr
1
1
1
1
c
d
e
a. Reaction conditions: 10 mmol styrene oxide, 0.025 mol %catalyst, 0.75
Figure 1. a. Coordination model of the ligand; b. Space-filling
representation of the tetranuclear doublestrand helicate; c. Molecular
structure of the extended heptanuclear helicate. For clarity, each ligand is
represented in a different color. Hydrogen atoms, coordinated solvent
molecules and nitrates, external counteranions are omitted.
1
mol % co-catalyst; b. Determined by H NMR analysis. Selectivity of cyclic
carbonates were all >99%; c. Not excluding the air inside the reaction
2
system; d. 0.2ml of H O was added to the reaction system; e. Reaction
condition: 10 mmol styrene oxide, 0.0025 mol %catalyst, 0.5 mol %TBAB.
2
| J. Name., 2012, 00, 1-3
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To value the high catalytic performance of this 3d-4f epoxide molecules due to the size selective catalysis of their
helicates system, we compared it with the representative special nano-cage structures.
salen-Zn and lanthanide complexes catalysts under various
conditions (Table S4). The results show that it exhibits higher
activity than the most used salen-Zn catalyst including some
1
2
multinuclear Zn catalysts such as Zn
1
3
, [L1Zn
2 4
] complexes and
Table 2. Various carbonates from different epoxides catalysed by various
3
lanthanide catalysts , suggesting that the high efficiency of catalysts under different conditions
a
b
Entry
1
Substrate
Product
Yield(%)
Yield(%)
98
this heterometallic catalyst system. Gratifyingly, with a load of
.0025 mol % and 0.5 mol % TBAB for the cycloaddition of
c
96(79 )
0
1
o
styrene oxide under 120 C, 1MPa after 1 h reaction resulted in
a moderate yield of 67 % that was obviously enhanced
compared with that of 19 % catalyzed only by TBAB and show
2
3
85
64
95
96
93
-1
a remarkable TOF as high as 26 800 h (Table 1 entry 13). This
value can be comparable to that of these most activity metal
1
catalysts such as Zn-porphyrin , Mg-porphyrin , Al-aminotri-
4
15
4
5
95
96
1
6
17
18
2
phenolate , Zn-CMP and Zn(OPO) catalysts (Table S5).
To explore the wider applicability of the novel 3d-4f helicate
catalyst system, a series of monosubstituted terminal epoxides
and internal epoxide substrate were tested under different
conditions. It can be seen that all the functionalized terminal
d
81(76 )
6
7
93
21
97
epoxides could obtained with excellent yields (>90%) and high
o
selectivity (99 %) under 120 C and 1 MPa CO
after 1 h
e
37(91 )
2
b
reactions (Table 2 entries 1-6 ). The internal epoxides, such as
cyclohexene oxide exhibited lower activity compared with
terminal counterparts, but performed with relatively high
Reaction conditions: a.10 mmol epoxide, 0.0125 mol catalyst, 0.72 mmol
catalyst loading and a long time to afford the products in fairly TBAB, r.t, 48 h, 1 atm; b. 10 mmol epoxide, 0.0025 mol catalyst, 0.075
b
good yield (91%) (Table 2 entry 7 ), being ascribed to the steric
o
mmol TBAB, 120 C, 1 h, 10 atm; c. 10 mmol epoxide, 0.25 mmol TBAB, r.t.,
4 h, 10 atm; d. 10 mmol epoxide, 0.0125 mol catalyst, 0.36 mmol TBAB,
r.t, 48 h, 1 atm; e. 10 mmol epoxide, 0.005 mol catalyst, 0.075 mmol TBAB,
2
effect. Moreover, a series of experiments were carried on with
o
1
1
(0.125mol %) and different loading of TBAB for the
1
20 C, 8 h, 10 atm. Yields were determined by H NMR. Selectivity of cyclic
cycloaddition of styrene oxide under ambient condition after carbonates were all >99%.
4
8 h reactions (Table S7). Substrate scope experiments also
Based on the present results and the studies of reported
20
multimetallic catalysts , multi-metallic synergisms may arise
simultaneously (Table 2 entries 1-7a and Table S8). It is show
that the catalyst also display high catalytic for the conversion
of these epoxides into cyclic carbonates under ambient
condition with a wide substrates scope. To examine its
recyclability, taking the cycloaddition with styrene oxide as an
example under ambient conditions. Notably, the catalyst can
be easily separated from the reaction mixture by
centrifugation and it could be reused more than five times
without a significant decrease in catalytic activity (Table S9)
from the heterometallic helicates system and two plausible
pathways are proposed (Figure S17). As shown in Zn2-Ln2,
there may only Zn2 in activating the epoxide. Coordination of
epoxide with Zn2 is the first step and with subsequent epoxide
-
ring opening by Br . Then CO insert the Zn2-O
bonds and
epoxide
2
cyclized by internal attack of the CO on the C-Br bond with re-
2
generation of the co-catalyst occur. For Zn1-Ln1, both Zn1 and
Ln1 ions may play a role in activating the components of the
-
reaction. Thus, ring-opening of the epoxide by Br results in an
III
and retained bright Tb luminescence (Figure S7, Table S3).
III
The characteristic green emission of Tb complex under UV
Ln1-Oepoxide. Coordination of CO to the Zn1 ion, followed by
2
excitation can be easily recognized by the naked eyes, which
could be a convenient proof for characterizing the recycled
catalyst. The recycled catalyst was also characterized by PXRD
and IR, which are in agreement with that as synthesized
intramolecular transfer of the alkoxide onto the coordinated
CO gives a zinc carbonate intermediate which can cyclize to
2
form the cyclic carbonate product and regenerate catalyst. In
addition, since the Lewis-base sites (N atom) groups may also
21
(Figure S9-10), indicating that the 3d-4f helicate is a relatively
contribute to the cycloaddition , H3L was utilized as a catalyst
robust catalyst. In addition, the decent heterogeneous
catalytic performance also be favored by comparison with the
representative heterogeneous catalyst systems such as CMPs
and MOFs under similar reaction conditions (Table S10).
Control experiments with HKUST-1 used as a catalyst were
carried on in the reactions (Figure S12). As shown in Table S10,
the helicate performed higher activity with good reusability
and display wider substrate scopes than most of the reported
with TBAB for the model reaction. The yield of 47% was
obviously enhanced compared with that of 25% catalyzed only
by TBAB (Table S6, entries 8-9). Thus, we speculate that the
catalyst could increase of the CO affinity via the introduction
2
of the acylamide groups into the structure and combined the
multiple Lewis acid sites which may account for the high
catalytic activity of the catalysts.
In summary, two biomimetic catalysts that featuring unique
multiply helical structures have been first synthesized. They
1
9
MOFs , as some of them were limited diffusion of larger
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display high catalytic activities and selectivity for CO
2
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1
without a significant decrease in catalytic activity and retained
III
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