Lanthanide clusters as highly efficient catalysts regarding carbon dioxide activation

Article abstract of DOI:10.1039/c9nj05831a

A series of tetranuclear lanthanide clusters supported by organic ligands ([Ln₄L₆(NO₃)₄]·4(MeCN), Ln = La, Nd, Sm) has been synthesized and characterized. The Lewis acidic Ln³⁺ sites were investigated as highly efficient catalysts regarding CO₂ activation. The clusters showed significant thermal stability after 4 reaction cycles of CO₂ insertion into epoxides to form cyclic carbonates, with TOF up to 6700 h^-1. The catalytic system also displays a wide substrate scope and high catalytic activity. Unfortunately, the catalytic efficiency of the catalysts for some sterically hindered reaction substrates is not very satisfactory.

Full text of DOI:10.1039/c9nj05831a

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Lanthanide clusters as highly efficient catalysts
regarding carbon dioxide activation†
Cite this:DOI: 10.1039/c9nj05831a
Wei Hou, Gang Wang, Xiaojing Wu, Shuoyi Sun, Chunyang Zhao,
Wei-Sheng Liu * and Fuxing Pan*
A series of tetranuclear lanthanide clusters supported by organic ligands ([Ln
4
L
6
(NO
3
)
4
]Á4(MeCN), Ln = La,
3+
Received 22nd November 2019,
Accepted 19th February 2020
Nd, Sm) has been synthesized and characterized. The Lewis acidic Ln sites were investigated as highly
efficient catalysts regarding CO
2
activation. The clusters showed significant thermal stability after 4
À1
reaction cycles of CO
2
insertion into epoxides to form cyclic carbonates, with TOF up to 6700 h . The
DOI: 10.1039/c9nj05831a
catalytic system also displays a wide substrate scope and high catalytic activity. Unfortunately, the
catalytic efficiency of the catalysts for some sterically hindered reaction substrates is not very satisfactory.
rsc.li/njc
Introduction
Crystalline lanthanide clusters are one of the most promising
1
classes of inorganic–organic hybrid materials. Besides their
high structural diversity, they have shown interesting physical
and chemical properties in the fields of luminescence, magnetism
2
and catalysis. Most of these applications benefit from the unique
f-orbitals of rare earth ions compared with transition metal ions.
On the other hand, with the use of fossil fuels, a large amount of
Scheme 1 Self-assembly of lanthanide clusters 1–3 (N blue, O red).
greenhouse gas carbon dioxide (CO
considered the main cause of global warming. However CO
also be regarded as an environmentally friendly C1 reagent, self-assembled with the aid of CO
2
) was released, which has been
3
2
can
2
À
3
ions derived from atmo-
. However, further activation of CO with lanthanide
7
compared to toxic chemicals like carbon monoxide, isocyanates or spheric CO
2
2
4
3+
phosgene. The chemical fixation of CO2 into the value-added clusters is still relatively rare. As isolated molecules, multiple Ln
chemical reagent cyclic carbonate is among the most significant sites with strong Lewis acidity are able to be sufficiently exposed
topics in green and sustainable chemistry. To date, numerous to the substrates. Thus, the generation and study of functional
nanomaterials and metal–organic frameworks have been synthe- lanthanide clusters with novel structures has become very
sized and applied in the reaction between epoxides and CO
obtain cycle carbonates with good results. However, in many
2
to attractive in the field of contemporary materials research.
Triggered by the structural diversity and abundance of Schiff
5
cases, the inner catalytic sites of these materials cannot be exposed base ligands, we designed and synthesized a series of novel
to the substrates. Thus, they usually require more catalyst and tetranuclear lanthanide clusters supported by hydrazide Schiff
relatively harsh reaction conditions to accomplish the conversion. base ligands ([Ln
Lanthanide clusters have preliminarily shown high efficiency These clusters were isolated by introducing Ln(NO
regarding CO capture. Tang et al. reported the fixation of CO
acetonitrile and methanol mixed solutions (1 : 2) of H
with tetranuclear lanthanide clusters. Our group has developed existence of a certain amount of triethylamine. Satisfyingly, these
a pair of chiral heptameric lanthanide clusters, which are clusters serve as high-efficiency catalysts in the cycloaddition
of epoxides and CO2 to obtain cyclic carbonates under mild
4
L
6
(NO
3
)
4
]Á4(MeCN), Ln = La, Nd, Sm; Scheme 1).
Á6H O into the
L, with the
3
)
3
2
2
2
3
6
conditions.
Lanzhou University, Key Laboratory of Nonferrous Metals Chemistry and Resources
Utilization of Gansu Province and State Key Laboratory of Applied Organic
Chemistry, College of Chemistry and Chemical Engineering, Lanzhou, China.
Results and discussion
E-mail: liuws@lzu.edu.cn, panfx@lzu.edu.cn
†
Electronic supplementary information (ESI) available: Experimental section,
1
Complexes 1–3 (complex 1: [La
Nd (NO
]Á4(MeCN); complex 3: [Sm
are isostructural, featuring a tetranuclear structure. Cluster 1
4
L
6
(NO
3
)
4
]Á4(MeCN); complex 2:
characterization and H NMR spectra of the substrates and products. CCDC
922297, 1922298 and 1922299. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c9nj05831a
[
4
L
6
3
)
4
4
L
6
(NO
3 4
)
]Á4(MeCN))
1
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was selected as a representative model for discussion, as shown
in Fig. 1b. The asymmetric unit of complex 1 consists of a La
2
core with different coordination modes. The La1 center is nine-
coordinated with [LaO N ] environment in the shape of Tri-hat
7
2
triangular prism, while the La2 center has [LaO N] environ-
8
ment in the shape of Tri-hat triangular prism. La1 and La2 are
connected via three m -O, which are derived from three ligands.
2
Two La
La
2
cores are further connected by two ligands to form the
cluster. In addition, La1 and La2 each have one NO
3
À
4
involved in coordination. The single crystal structure analysis
revealed that complexes 1–3 have potential catalytic sites for the
coupling of epoxides with CO to form cyclic carbonates.
2
Nitrate ions are involved in the coordination of the three
complexes, and the steric hindrance opposite the coordination
À
direction of the NO
3
is weak, which provides a guarantee for
the oxygen in the epoxy compounds to participate in the
3
+ 8
coordination to Ln . In order to determine which of the three
compounds has the best catalytic performance, we started from
Fig. 2 The conversion of cyclocarbonate under different reaction conditions.
Reaction conditions: (a) 10 mmol styrene oxide, 0.01 mol% catalyst, 0.75 mol%
the reaction between styrene oxide and CO . 3.5 mg of 1, 2 and
2
co-catalyst, 120 1C, 1.5 h, 1 MPa CO
catalyst, 0–1 mol% TBAB, 120 1C, 1.5 h, 1 MPa CO
–0.0125 mol% catalyst, 0.75 mol% TBAB as the co-catalyst, 120 1C, 1.5 h,
; (d) 10 mmol styrene oxide, 0.01 mol% catalyst, 0.75 mol% TBAB as
the co-catalyst, 60–140 1C, 1.5 h, 1 MPa CO
2
; (b) 10 mmol styrene oxide, 0.01 mol%
3
, respectively, and 0.075 mol% of TBAB as the co-catalyst were
used for the reaction to obtain cyclic carbonate in solvent-free
conditions at 120 1C and 1 MPa. After reacting for 1.5 h, the 1 MPa CO
2
; (c) 10 mmol styrene oxide,
0
2
2
.
three catalysts show different behavior, which is reflected by
the conversion and selectivity (75%, 85% and 96%, respectively)
À1
À1
À1
and TOF (5076 h , 5753 h and 6497 h , respectively). Since 3
Subsequently, the catalyst was utilized for cycloaddition of
with a range of different epoxides and the results are
showed the best catalytic activity, we chose 3 as a representative CO
2
catalyst to further investigate the reactions. Then, the effects of summarized in Table 1. The epoxides were converted to their
different co-catalysts were tested on this reaction, where TBAB corresponding cyclic carbonates with high selectivity and conver-
showed the best catalytic performance (Fig. 2a). At the same sion due to the good catalytic activity of the catalyst. There are two
time, we also explored the corresponding change in reaction main important factors which affect this reaction, namely the
conversion when the amount of TBAB is changed. From Fig. 2b, steric effect of the reaction substrate and the electronic effect of
9
the conversion of the product increased with the increase of substituent groups in the reaction substrate. The steric effect of
TBAB dosage (0 mol%, 0.25 mol%, 0.5 mol%, 0.75 mol% and the epoxide plays a predominant role in this reaction. The activities
1
0
1
mol%, respectively). The catalyst also showed excellent catalytic are higher when the steric effect is decreased. Conversions are
activity when the amount of catalyst 3 was gradually increased up to 99% for the epoxides with weak steric hindrance, such as
from 0.0025 mol% to 0.015 mol% with 0.75 mol% TBAB, with the propylene oxide, butylene oxide and epoxy chloropropane
À1
11
highest initial TOF up to 6700 h (Fig. 2c). When the content of (Table 1, entries 1, 2 and 4). Due to steric hindrance, the
the catalyst is above 0.01 mol%, the increase in the catalyst cyclic carbonate conversion and selectivity of the internal epoxides
content is not very significant for the reaction conversion, so it is (cyclohexene oxide and cyclopentene oxide) and 1,2-epoxyhexane
12
necessary to select 0.01 mol% as the catalyst amount. Reactions were very low (Table 1, entries 3, 12 and 13). The electron-
at different temperatures (Fig. 2d) showed outstanding catalytic withdrawing effects of the substituents of epoxy chloropropane,
activity with 0.01 mol% catalyst 3, and the TOF was up to epibromohydrin and glycidol (Table 1, entries 4, 5 and 10) favour
À1
À
6
497 h at 120 1C.
nucleophilic attack of Br , which affords the conversion of 99%
13
epoxy carbonate under the investigated reaction conditions.
Similarly, the better activity of t-butyl glycidyl ether and 1-allyloxy-
,3-epoxypropane (Table 1, entries 8 and 9) may be related to the
2
oxygen atom in the molecule. The oxygen atom in an epoxy
compound can strongly attract the electron clouds of neighboring
groups due to its high electronegativity, which leads to the easier
attack of its neighboring epoxide by a nucleophilic agent. Thus, the
existence of O can facilitate the ring opening of epoxide and
14
improve the reaction rate.
Furthermore, the recycling of the rare earth catalyst was
investigated by using epoxypropane as a representative substrate.
After completion of the reaction, the catalyst was easily recovered
by centrifuging at 6000 rpm for 5 min, washing with fresh
Fig. 1 (a) Structural formula of the ligand with potential multicoordination
sites capturing metal ions; (b) molecular structure of complex 1 (hydrogen
atoms and solvent molecules are omitted for clarity).
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Table 1 Various carbonates from different epoxides catalysed by 3
a
À1
Entry
1
Substrate
Product
Conversion (%)
TOF (h
)
99
6700
2
3
4
5
99
75
99
98
6700
5076
6700
6633
2
Fig. 3 Recycling of complex 3 for the cycloaddition of CO .
6
7
97
96
99
6565
6497
6700
8
9
99
99
6700
6700
1
1
0
1
Fig. 4 The PXRD patterns of the as-synthesized (black) complex 3 and
after recycling four times (red).
99
29
34
6700
1963
2301
3+
17
opening step was driven by the activation effect of the Ln site
and the carbon–bromine bond was formed through nucleophilic
1
2
3
À
18
attack by Br on the less hindered side of the epoxide. The
carbon atom in CO was then attacked by the oxygen anion, while
2
1
the electrons in the carbon–oxygen double bond moved toward the
a
Reaction conditions: 10 mmol epoxide, 0.01 mol% catalyst, 0.75 mol%
1
2
TBAB, 1.5 h, 1 MPa CO , 120 1C. Determined by H NMR spectra.
acetonitrile and then drying at 80 1C in an oven. The recovered
catalyst was used for subsequent reactions using fresh substrates
under the conditions of 120 1C, 1.5 h. To solve the problem of loss of
catalyst dosage by washing, we scaled the amount of substrate added
to perform the next experiment. The recovered catalyst was tested for
four subsequent runs and showed almost consistent activity with a
slight decrease in the conversion (Fig. 3). The conversion decrease
may be attributed to the deactivation of a small amount of catalyst in
15
the experiment. As illustrated in Fig. 4, the PXRD pattern of the
recycled catalyst showed that the original structure of the catalyst
remained stable after four catalytic reaction cycles.
Based on the structure of the lanthanide clusters and previous
16
literature reports, a possible mechanism (shown in Fig. 5) could
be used to explain the cycloaddition reaction. First, an epoxide was
3+
captured and activated by the Lewis acidic Ln site. Then the ring Fig. 5 A possible mechanism for the cycloaddition reaction.
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1
9
carbon atom that was bound to bromine. Finally, an acyclic
carbonate molecule was formed, which was converted to a cyclic
carbonate by intramolecular cyclization, releasing the original
catalyst for the next catalytic cycle. The increasing TOF from 1 to
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X. L. Tang, W. H. Wang, W. Dou, J. Jiang, W. S. Liu,
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8
In summary, a series of highly active tetranuclear lanthanide
clusters was synthesized and characterized. The unique structures
(
b) D. Y. Hong, Y. K. Wang, C. Serre, G. Ferey and J. S. Chang,
of these clusters provide catalytically active sites for CO
2
2
1
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2
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least 4 times without significant loss or structural damage.
These results provide new insights for designing lanthanide
9
017, 19, 3769–3779.
1
clusters as efficient CO conversion catalysts.
2
1
Conflicts of interest
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0416–10419; (b) J. Langanke, L. Greiner and W. Leitner, Green
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Chem., 2013, 15, 1173–1182; (c) C. J. Whiteoak, E. Martin,
E. Escudero-Adan and A. W. Kleij, Adv. Synth. Catal., 2013,
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This work was supported by the National Natural Science
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