Heterobimetallic rare earth metal-zinc catalysts for reactions of epoxides and CO2under ambient conditions

Article abstract of DOI:10.1039/d0dt03772a

Four homodinuclear rare earth metal (RE) complexes1-4bearing a multidentate diglycolamine-bridged bis(phenolate) ligand were synthesized. In addition, seven heterobimetallic RE-Zn complexes5-11were prepared through a one-pot strategy. In these heterobimetallic complexes, two RE centers are bridged by either Zn(OAc)₂or Zn(OBn)₂moieties. All complexes were characterized by single crystal X-ray diffraction, elemental analysis, IR spectroscopy, and multinuclear NMR spectroscopy (in the case of diamagnetic complexes1,4,7and11). Moreover, the multi-nuclear structures of complexes4and11in solution were also studied by¹H DOSY spectroscopy. These complexes were applied in catalyzing the coupling reaction of carbon dioxide (CO₂) with epoxides. Zn(OAc)₂- and Zn(OBn)₂-bridged heterobimetallic complexes showed comparable catalytic activities under ambient conditions and were more active than monometallic RE complexes. Significant synergistic effect in heterobimetallic complexes is observed. Mono-substituted epoxides were converted into cyclic carbonates under 1 atm CO₂at 25 °C in 88-96% yields, whereas di-substituted epoxides reacted under 1 atm CO₂at higher temperatures in 40-80% yields.

Full text of DOI:10.1039/d0dt03772a

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Heterobimetallic rare earth metal–zinc catalysts
for reactions of epoxides and CO2 under ambient
conditions†
Cite this: Dalton Trans., 2021, 50,
1453
a
Kuan Yin,‡a Linyan Hua,‡a Liye Qu,a Quanyou Yao,a Yaorong Wang,
a,b
Dan Yuan, *a Hongpeng Youb and Yingming Yao
*
Four homodinuclear rare earth metal (RE) complexes 1–4 bearing a multidentate diglycolamine-bridged
bis(phenolate) ligand were synthesized. In addition, seven heterobimetallic RE–Zn complexes 5–11 were
prepared through a one-pot strategy. In these heterobimetallic complexes, two RE centers are bridged by
either Zn(OAc)2 or Zn(OBn)2 moieties. All complexes were characterized by single crystal X-ray diffraction,
elemental analysis, IR spectroscopy, and multinuclear NMR spectroscopy (in the case of diamagnetic
complexes 1, 4, 7 and 11). Moreover, the multi-nuclear structures of complexes 4 and 11 in solution were
also studied by 1H DOSY spectroscopy. These complexes were applied in catalyzing the coupling reaction
of carbon dioxide (CO2) with epoxides. Zn(OAc)2- and Zn(OBn)2-bridged heterobimetallic complexes
showed comparable catalytic activities under ambient conditions and were more active than monometal-
lic RE complexes. Significant synergistic effect in heterobimetallic complexes is observed. Mono-substi-
tuted epoxides were converted into cyclic carbonates under 1 atm CO2 at 25 °C in 88–96% yields,
whereas di-substituted epoxides reacted under 1 atm CO2 at higher temperatures in 40–80% yields.
Received 2nd November 2020,
Accepted 15th December 2020
DOI: 10.1039/d0dt03772a
rsc.li/dalton
CO2, and examples conducted under ambient conditions are
very few.5 Among these scarce catalytic systems, most of them
Introduction
Carbon dioxide (CO2) is an abundant, renewable, and in- focused on the reactions of monosubstituted epoxides, which
expensive natural resource that can be transformed into value- include bimetallic Al(salen), Al(acen), Al(amidinate) and Al(etha-
added compounds through chemical fixation.1 Coupling of CO2 nolateamine) complexes,5a–e,l,t Cr(salophen) complex,5f dinuc-
with epoxides forms cyclic carbonates, which is a 100% atom lear macrocyclic iron complex,5g sulfur-bridged bis(phenolato)
economical reaction (Scheme 1). Cyclic carbonates find wide bismuth complex,5h μ-oxotetranuclear zinc and cobalt clusters,5i
applications as non-protic solvents, lithium-ion battery electro- N,N-dialkylcarbamate iron complex,5k metal–porphyrin frame-
lytes, fine chemical intermediates, etc.2 Significant efforts have work,5p metal–salen molecular cages,5q and organocatalysts,5o,r,s
been made to address the general reactivity and selectivity while reactions of more challenging disubstituted epoxides gen-
issues associated with cyclic carbonate synthesis. A variety of erally required elevated temperature/pressure. Notably, the
catalysts including metal complexes3 and organocatalysts4 have calcium–crown ether catalyst reported by Werner and co-work-
been developed to catalyse this transformation. However, they ers,5m and Zn and Cu complexes developed by Muralidharan
in general require high temperature and/or high pressure of and co-workers,5n catalysed the reactions of disubstituted term-
inal epoxides under ambient conditions.
Recently, Liu and co-workers reported heterometallic heli-
aKey Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry,
cates featuring multiple zinc–rare earth metal (Zn–RE) units for
Chemical Engineering and Materials Science, Dushu Lake Campus, Soochow
the ambient chemical fixation of CO2.5j The same group also
University, Suzhou 215123, People’s Republic of China.
E-mail: yuandan@suda.edu.cn, yaoym@suda.edu.cn
bState Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of
Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s
Republic of China
†Electronic supplementary information (ESI) available: Solid state structures,
NMR spectra, and crystallographic data. CCDC 1852420 for 3, 1852421 for 1,
1852422 for 4, 1852637 for 2, 1852727 for 7, 1852728 for 8, 1853071 for 11,
1853073 for 10, 1976682 for 5, 1976683 for 6 and 1976684 for 9. For ESI and crys-
tallographic data in CIF or other electronic format see DOI: 10.1039/d0dt03772a
‡These authors contributed equally to this work.
Scheme 1 Reaction of CO2 and epoxides.
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Scheme 2 Synthesis of rare earth metal complexes 1–4.
synthesized in 82–90% yield by the reaction of RECp3(THF)
with the ligand precursor LH3 (Scheme 2).
Following the strategy developed for Zn(OAc)2-bridged
heterobimetallic complexes,8 one-pot reactions of LH3 with
RECp3(THF) followed by the addition of 0.5 equivalent of Zn
(OAc)2 gave rise to heterobimetallic complexes 5 (RE = Sm) and
6 (RE = Nd) in low yields of 20–25% (Scheme 3). Sm and Nd
complexes were synthesized for comparison with RE–Zn analo-
gous B and C stabilized by an ethanolamine-bridged ligand.
The construction of heterobimetallic Zn(OBn)2-bridged
complexes was achieved through one-pot reactions of LH3 with
RECp3(THF), ZnEt2 and PhCH2OH, which worked more
straightforwardly, and generated complexes 7–11 in good
yields of 77–87% (Scheme 3).
Fig. 1 Heterobimetallic RE–Zn complexes A, B and C (ref. 7 and 8).
reported other heterometallic helicates or coordination polymers
as efficient catalysts for the cycloaddition of epoxides and CO2
under elevated temperature/pressure.3h,6 Our group reported
that RE–Zn complexes were stabilized by ethylenediamine-
bridged tetra(phenolato) ligands (A in Fig. 1), which are active
catalysts under ambient conditions.7 We also reported heterome-
tallic RE–Zn complexes B and C (Fig. 1) stabilized by the ethanol-
amine-bridged bis(phenolato) ligand, which, however, shows
inferior activity as compared to that of homometallic RE metal
complexes.8 Although some above-discussed heterobimetallic
complexes catalysed the cycloaddition of mono-substituted epox-
ides and CO2 under ambient conditions, none of them showed
any activity for reactions of di-substituted epoxides.
To develop more active catalysts, and to further investigate
the structure–activity correlations, the ancillary ligands were
then modified by introducing an extra oxygen donor to the pre-
viously reported ethanolamine-bridged bis(phenolato) ligand.8
We herein report RE–Zn complexes stabilized by the diglycola-
mine-bridged bis(phenolate) ligand, which gave rise to signifi-
cantly improved catalytic activity, and performed better
through the cooperation of the two metal centers. A series of
epoxides, including disubstituted terminal epoxides, were
transformed into cyclic carbonates under ambient conditions.
All complexes were characterized by single crystal X-ray
diffraction, elemental analysis, and IR spectroscopy.
Diamagnetic complexes 1, 4, 7 and 11 were also characterized
by multinuclear NMR spectroscopy.
Solid-state structures of complexes 1–11 are depicted in
Fig. 3, 4, and S1–9.† In complex 4, each La center is bound
with five oxygen atoms and one nitrogen atom from the two
ligands and one THF molecule, adopting a pentagonal bipyra-
midal geometry. The solid-state structure of complex 11 reveals
a trinuclear architecture, involving one Zn and two La centers.
The two [LaO2Zn] coordination planes have a dihedral angle of
70°. The lengths of the La–O bonds of complexes 4 and 11 are
almost identical (i.e., La–OPh 2.30 Å, La–OCH2 2.42 Å, La–O
(CH2)2 2.64 Å), while the Zn–O bonds in complex 11 are ca.
1.94 Å. The distances of the Zn–O bonds in complex 6 fall in
the range of 1.931–1.988 Å, which are in a similar range as
those of the previously reported Zn(OAc)2-bridged Nd–Zn
complex C.8 The Zn–O bonds in complex 10 are shorter than
those of the Zn(OBn)2-bridged Nd–Zn complex D (Fig. 2; 1.932,
1.956 vs. 1.947, 1.979 Å).10
The RE–Zn distances of different heterobimetallic com-
plexes are listed and compared (Table 1).8 The Zn(OBn)2-
bridged complexes have RE–Zn distances of ca. 3.4 Å, whereas
Results and discussion
Complex synthesis
A multidentate bis(phenolate) ligand bearing a diglycolamine the Zn(OAc)2-bridged ones have RE–Zn distances of ca. 3.8 Å.
1
bridge was employed to stabilize multiple metal centers. The
The H NMR spectrum of 4 displays one set of signals for
ligand precursor LH3 was readily prepared via the Mannich the ligand, suggesting a symmetrical structure in solution. In
condensation of phenols and amino alcohol.9 Homo-dinuclear the 1H NMR spectrum of complex 11, additional signals
complexes RE2L2(THF) [RE = Y (1), Yb (2), Nd (3), La (4)] were corresponding to benzyloxy groups are observed. The methyl-
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Scheme 3 Synthesis of heterobimetallic rare earth metal–zinc complexes 5–11.
Fig. 2 Heterobimetallic RE–Zn complex D (ref. 11).
Fig. 3 The molecular structure of complex 4·2tol with thermal ellip-
soids drawn at the 30% probability level. All hydrogen atoms and solvent
molecules are omitted for clarity.
ene group of benzyloxy groups resonates at 5.33 ppm, which
shifts downfield compared to that of free PhCH2OH, corrobor-
ating its coordination.
The diffusion coefficients (D) of complexes 4 and 11 are suggests a larger aggregate of complex 11 than of 4 in solution.
determined to be 4.02 × 10−10 and 3.61 × 10−10 m2 s−1, respect- Moreover, for solutions of LaCp3(THF), H3L, complexes 4 and
ively. The fact that the latter is 10% lower than the former 11 which are of the same concentration, the correlation
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mined to be roughly consistent, further proving that solid-state
and solution structures are consistent.
Catalytic study
With these complexes in hand, the catalytic potential of rare
earth metal complexes 1–4 was first evaluated under atmos-
pheric pressure using 1,2-epoxyhexane as a template substrate
and tetrabutylammonium halide as the co-catalyst.
Under identical conditions (0.2 mol% catalyst, 100 °C),
lanthanum complex 4 gave the highest yield of 90% (Table 2,
entry 4).8,14 The large atomic radius which results in a larger
space for substrate binding may together be the reasons for
the higher activity. Different co-catalysts were studied of which
tetrabutylammonium bromide (TBAB) gave the best yield of
92% at 80 °C under 1 atm CO2 pressure (Table 2, entries 5–10).
The decreasing activity sequence of Br− > I− > Cl− was
observed, possibly because Br− has balanced nucleophilic and
leaving abilities. PPNCl performed better than TBAC (Table 2,
entries 8 and 9), mainly due to the weaker ion pairing of Cl−
with the PPN cation.15 The yield gradually decreased as the
temperature decreased, and only 15% yield was obtained at
25 °C (Table 2, entries 10–13).
Fig. 4 The molecular structure of complex 11·4THF with thermal ellip-
soids drawn at the 20% probability level. All hydrogen atoms and solvent
molecules are omitted for clarity.
Table 1 RE–Zn distances of heterobimetallic complexes
Zn(OBn)2-bridged
complexes
Zn(OAc)2-bridged
complexes
Complex
To conduct reactions under ideal atmospheric conditions
(i.e., 25 °C, 1 atm CO2 pressure), the loadings of catalyst and
co-catalyst were increased to 0.5 mol% and 1 mol%, respect-
ively. Indeed, in the presence of RE–Zn complexes, >60%
yields were achieved under atmospheric conditions (Table 3,
entries 2–8). Zn(OAc)2- and Zn(OBn)2-bridged complexes
showed comparable activities under these conditions (Table 3,
entries 2, 3 vs. 6, 7). For complexes 7–11, a similar trend of
larger ionic radii giving higher activity was observed (vide
supra) (Table 3, entries 4–8).
Distance of RE–Zn Sm–Zn 3.38 (9)
Sm–Zn 3.82 (B)
Nd–Zn 3.84 (C)
Sm–Zn 3.78 (5)
Nd–Zn 3.82 (6)
(Å)
Nd–Zn 3.41 (10)
La–Zn 3.44 (11)
Nd–Zn 3.39 (D)
between lg D and their molecular weights (lg(MW)) shows good
linearity (Fig. 5, Table S1†). The VT NMR experiments of 11 in
d8-toluene were conducted in the range of 25–100 °C, which
revealed no obvious change (Fig. S20†). These findings prove
that the trinuclear structure of 11 is maintained in solution.11
In addition, the hydrodynamic radius rh of complex 11 was
determined to be 12.9 0.5 Å, which was essentially the same
A comparison between heterobimetallic RE–Zn complexes
and monometallic RE complexes showed that the former are
significantly more active (Table 3, entries
1 and 8).
as the average crystalline radius of 12.0
0.5 Å (Table S2,
Fig. S23†).12,13 The hydrodynamic radius (11.6 0.5 Å) and
crystalline radius (10.6 0.5 Å) of complex 4 were also deter-
Table 2 Catalytic production of cyclic carbonate from CO2 with 1,2-
epoxyhexane mediated by dinuclear catalysts 1–4a
Entry
Catalyst
Co-catalyst
T [°C]
Conversionb,c [%]
1
2
3
4
5
6
7
8
1
2
3
4
4
4
4
4
4
4
4
4
4
TBAI
TBAI
TBAI
TBAI
100
100
100
100
80
80
80
80
80
80
60
65
60
80
90
75
83
85
43
81
92
71
40
15
TBAI
BTABd
TOABe
TBAC
PPNCl
TBAB
TBAB
TBAB
TBAB
9
10
11
12
13
40
25
a Reaction conditions: 0.2 mol% catalyst, 0.2 mol% co-catalyst, 1 atm
CO2 pressure, 18 h, solvent free. b Determined by 1H NMR spec-
troscopy. c Selectivity for the cyclic carbonate product were all >99%.
d (2-Bromoethyl)trimethylammonium bromide. e Tetra-n-octylammo-
nium bromide.
Fig. 5 Log–log-relationship between the diffusion coefficients in the
1H DOSY NMR spectra and Mw of compounds. Conditions for DOSY
experiment: samples in THF-d8 with a concentration of 0.056 M, 25 °C,
400 MHz.
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Table 3 Catalytic production of cyclic carbonate from CO2 with 1,2-
epoxyhexane mediated by complexes 4–11a
Table 4 Cycloaddition of CO2 with epoxides catalyzed by complex 11
and NBu4Bra
Entry
Catalyst
Co-catalyst
Conversionb,c [%] Entry
Substrate
Product
Yieldb,c
93%
1
2
3
4
5
6
7
8
4
5
6
7
8
9
10
11
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
—
46
67
73
79
75
60
80
88
6
1
2
95%
96%
9
11
3
10
ZnEt2 + 2PhCH2OH
11
TBAB
TBAB
10
94
11d
a Reaction conditions: 0.5 mol% catalyst, 1 mol% co-catalyst, 25 °C,
24 h, 1 atm CO2 pressure. b Determined by 1H NMR spectroscopy.
c Selectivity for the cyclic carbonate product were all >99%. d Reaction
conditions: 0.5 mol% catalyst, 2 mol% co-catalyst, 25 °C, 24 h, 1 atm
CO2 pressure.
4
5
95%
95%
Heterometallic complexes showed almost one-fold higher activity
than homo-analogues, as evidenced by their kinetic constants
(Fig. 6). It is also noteworthy that the mixture of ZnEt2 and
PhCH2OH gave a low yield of 10% (Table 3, entry 10). These
findings strongly suggest that cooperation between the RE and
Zn centers exists, which accounts for the enhanced activity of
the heterobimetallic complexes. The absence of the co-catalyst
TBAB led to almost complete deactivation (Table 3, entry 9).
The adjustment of the catalyst to co-catalyst ratios gave a
good yield of 94% (Table 3, entry 11) in the presence of 1 atm
CO2 at 25 °C, which in general required shorter reaction times
compared to the RE–Zn helicate catalyst system.5j
6
96%d
7
8
88%
96%
a Reaction conditions: 0.5 mol% complex 11, 2 mol% TBAB, 25 °C, 1
atm CO2, 24 h. b Isolated yield. c Selectivity for the cyclic carbonate
product were all >99% unless otherwise stated. d 40 h.
To explore the scope of the heterobimetallic catalyst, a
variety of epoxides were examined under the aforementioned
optimized conditions, and the results are summarized in
Table 4. All the monosubstituted epoxides with different types
of functional groups, e.g., aryl, ether and ester groups, were
converted into their corresponding cyclic carbonates 12b–12i
in good yields (88–96%) at 25 °C and 1 atm CO2.
Disubstituted epoxides are challenging substrates due to
their increased steric hindrance, which in general require elev-
ated temperature/pressure to ensure good conversions.5
Achieving the transformation of disubstituted epoxides under
ambient conditions remains a challenging task. It is note-
worthy that the reaction of a disubstituted terminal epoxide at
25 °C gave the cyclic carbonate 12j in 87% yield after prolong-
ing the reaction time to 40 h (Table 5, entry 1). Compared to
literature reports, the heterobimetallic complex gave a similar
yield with less catalyst loading.5m,n
In addition, the reactions of disubstituted internal epox-
ides, i.e., cyclopentene oxide and cyclohexene oxide, were con-
verted into cyclic carbonates in the presence of 1 atm CO2 at
an elevated temperature of 80–100 °C in 40–80% yield
(Table 5, entries 2–4). This is one of the few catalysts capable
of converting internal epoxides under 1 atm pressure.5m,n At
an elevated pressure of 30 atm, a higher conversion of cyclo-
hexene oxide and less poly(carbonate) was detected, resulting
in better selectivity (Table 5, entry 3).
The catalytic activities of homodinuclear complex 4 and
heterobimetallic complex 11 were compared in catalyzing the
reactions of disubstituted epoxides and CO2 under identical
conditions. The latter showed higher activity compared to
complex 4 in catalyzing the reactions of 1,1- and 1,2-di-
substituted epoxides (Table 5). The reactions of cyclohexene
oxide and 1,2-epoxy-4-vinylcyclohexane gave rise to mixtures of
cyclic carbonates, poly(ether) and poly(carbonate), and the
Fig. 6 Reaction monitoring with 1H NMR spectroscopy (condition:
Table 3, entries 1 and 8). Rate constants are determined to be 0.0551 h−1
(blue cycle) and 0.0326 h−1 (red square), respectively.
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Table 5 Cycloaddition of CO2 with epoxides catalyzed by complex 4 or 11 and NBu4Bra
Entry
1
Substrate
T (°C)
t (h)
Product
Yieldb (complex 11)
Yieldb (complex 4)
25
40
87%
70%
2
3
80
00
24
24
80%c
74%d
45%g
51%e86%f
4
100
40
40%h
51%i
a Reaction conditions: 0.5 mol% catalyst, 2 mol% TBAB, 1 atm CO2. b Isolated yield. c 13% poly(ether) was detected. d 4% poly(ether) was detected.
e 48% cis and 3% trans isomers, 38% poly(carbonate) and 9% poly(ether) were detected. f 25 °C, 30 atm CO2; 73% cis and 13% trans isomers, 8%
poly(carbonate) and 5% poly(ether) were detected. g 35% cis and 10% trans isomers, 2% poly(carbonate) and 6% poly(ether) were detected. h 50%
poly(ether) was detected. i 10% poly(ether) was detected.
reactions catalyzed by heterobimetallic complex 11 resulted in the extra donor well stabilized the metal centers during the
higher conversions than those of homodinuclear complex 4.
catalytic process. The Zn(OAc)2-bridged complexes 5 and 6 are
Based on the above-discussed results, a synergism between comparably active as Zn(OBn)2-bridged complexes 9 and 10
metal centers is proposed to explain the enhanced activity of (Table 6, entries 1, 2 and 5, 6). Another two hetero-bimetallic
the heterobimetallic system (Fig. S24†). The catalytic cycle complexes D and A showed inferior activity than complexes 6,
starts with the coordination of epoxide with one of the rare 7, and 10 (Table 6, entries 7 and 8).
earth metal centers, followed by the attack of the co-catalyst
These findings prove that the diglycolamine-bridged bis
(NBu4Br) to cause the ring-opening process.16 CO2 may be acti- (phenolate) ligand is a privileged scaffold for highly active het-
vated by the zinc center, which is attacked by the alkoxide erobimetallic complex construction. Ligands of higher electron
group resulting from the ring-opening. In addition, the Zn densities (Table 6, entries 1, 2 vs. 3, 4) resulted in higher yields
center contributes to stabilizing the resulting carbonate, as of cyclic carbonates. Complex D is least active in cyclic carbon-
has been proved computationally. Finally, an intramolecular ate synthesis (Table 6, entry 7), while it is highly active in poly
nucleophilic substitution leads to cyclization along with the (carbonate) synthesis,10 which may be correlated with the least
release of the bromide anion.
electron-donating ligand. It is thus tentatively concluded that
The catalytic activities of a series of heterobimetallic com- more electron-donating ligands may be beneficial for cyclic
plexes were compared under identical conditions (Table 6). carbonate formation, while less electron-donating ligands may
Upon comparing complexes 5 and 6 bearing the diglycola- be beneficial for poly(carbonate) formation. The unambiguous
mine-bridged bis(phenolate) ligand with complexes B and C correlation between the electronic properties of the ligands
bearing the ethanolamine-bridged ligand, the former gave ca. and the catalytic performance of the complexes requires
20% higher yields (Table 6, entries 1–4), demonstrating that further investigation.
Heterobimetallic complexes 5 and 6 have been applied in
the copolymerization of cyclohexene oxide and CO2, which
gave moderate yields of 62% and 37%, respectively (Table 7,
Table 6 Comparison of RE–Zn complexes in catalysing the cyclo-
addition of CO2 with 1,2-epoxyhexanea
Entry
Catalyst
Conversionb,c (%)
Table 7 Comparison of RE–Zn complexes towards the copolymeriza-
tion of CO2 and cyclohexene oxidea
1
2
3
4
5
6
7
8
9
5 (Sm–Zn)
6 (Nd–Zn)
B (Sm–Zn)
C (Nd–Zn)
9 (Sm–Zn)
10 (Nd–Zn)
D (Nd–Zn)
A (Y–Zn)
67
73
50
53
60
80
50
58
79
Entry Initiator Yieldb (%) Polycarbonatec (%) Mn (×104) Đd
1
5
6
B
C
62
37
58
42
99
99
59
59
12.9
14.5
4.1
2.73
8.95
8.42
8.72
2
3e
4e
2.5
7 (Y–Zn)
a Reaction conditions: VTol/VCHO = 1/1, [initiator]/[CHO] = 1/125, 70 °C,
a Reaction conditions: 0.5 mol% catalyst, 1 mol% co-catalyst TBAB, 24 h, 30 bar CO2 pressure. b Isolated yield. c Determined by 1H NMR
25 °C, 24 h, 1 atm CO2 pressure. b Determined by 1H NMR spec- spectroscopy. d Determined by GPC versus polystyrene standards. e Data
troscopy. c Selectivity for the cyclic carbonate product were all >99%.
from ref. 8.
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entries 1 and 2). Upon comparison with complexes B and C, acetate (60 : 1). The pure product was isolated as a white solid.
1
complexes 5 and 6 showed better activity for cyclic carbonate Yield: 47.74 g, 88%. H NMR (400 MHz, CDCl3): δ 7.22 (d, J =
synthesis than for copolymerization, which is consistent with 2.2 Hz, 2H, ArH), 6.89 (d, J = 2.3 Hz, 2H, ArH), 3.86–3.79 (m,
the electron properties of ligands (vide supra).
2H, NCH2CH2), 3.74 (s, 4H, NCH2Ar), 3.66–3.56 (m, 4H,
CH2OCH2), 2.79–2.68 (m, 2H, CH2OH), 1.39 (s, 18H, CH3), 1.27
(s, 18H, CH3). 13C NMR (100 MHz, CDCl3): δ 152.5, 141.4,
136.3, 125.0, 123.8, 121.7 (Ar–C), 73.7, 68.9, 62.0 (NCH2), 58.0,
52.1, 35.0, 34.3 (OCH2), 31.8 (C(CH3)), 29.8 (C(CH3)).
Conclusion
In conclusion, a series of homometallic RE metal complexes
and heterobimetallic RE–Zn complexes stabilized by multiden-
tate ancillary ligands have been prepared and characterized. In
the presence of RE–Zn complexes, the conversion of CO2 into
cyclic carbonates was achieved at room temperature and 1 atm
CO2 pressure in 40–96% yields. Moreover, challenging di-
substituted epoxides were also transformed into cyclic carbon-
ates under 1 atm CO2 pressure at elevated temperatures. The
obvious synergistic effect was proved by the enhanced catalytic
activity of heterometallic RE–Zn complexes. Further studies on
the construction of heterometallic complexes and their cata-
lytic applications are in progress in our laboratory.
General synthetic procedure of dinuclear rare earth metal
complexes RE2L2(THF)n (1–4). The ligand precursor LH3
(1.63 g, 3.00 mmol) was dissolved in dry THF (15 mL). The
THF solution (10 mL) of RECp3(THF) [RE = Y, Yb, Nd, La]
(3.00 mmol) was added and the mixture was stirred for 24 h at
room temperature. The solvent was removed under reduced
pressure, and a large amount of powder was precipitated by
adding hexane (10 mL). The powder was collected and redis-
solved at 120 °C in a mixture of THF (10 mL) and toluene
(8 mL). The dilute solution was concentrated to saturation.
After a few weeks, single crystals of the dinuclear complexes
(1–4) were obtained at about 0 °C.
Yttrium complex Y2L2 (1). Yield: 1.69 g, 90%. Anal. calcd for
C68H104N2O8Y2·3C7H8: C, 69.78; H, 8.42; N, 1.83. Found: C,
69.73; H, 8.43; N, 1.82. 1H NMR (400 MHz, THF-d8): δ
7.05–7.01 (m, 4H, ArH), 6.79–6.78 (m, 4H, ArH), 4.18 (s, 4H,
ArCH2N), 3.69 (s, 8H, ArCH2NCH2), 3.45 (s, 8H, CH2OCH2),
2.75 (s, 4H, CH2OH), 1.38 (s, 36H, C(CH3)3, 1.18 (s, 36H,
C(CH3)3). 13C NMR (100 MHz, THF-d8): δ 162.2, 135.6, 135.3,
124.9, 123.3 (Ar–C), 75.5 (ArCH2N), 35.5, 34.1 (OCH2), 32.1
(C(CH3)), 30.6 (C(CH3)). IR (selected absorbances, cm−1): 2896
(–CH2–), 1475 (–CH3), 1202 (–C–O–C–), 1167 (–C–N–), 875 (Ar).
Ytterbium complex Yb2L2 (2). Yield: 1.75 g, 82%. Anal. calcd
for: C68H104N2O8Yb2·2C7H8: C, 61.25; H, 7.52; N, 1.74. Found:
C, 61.20; H, 7.52; N, 1.73. IR (selected absorbances, cm−1):
2949 (–CH2–), 2897 (C–H), 1475 (–CH3), 1368 (–C–O–C–), 1101
(–C–N–), 875 (Ar).
Experimental
Materials and instrumentation
All manipulations involving air and moisture sensitive com-
pounds were carried out in a glovebox or using standard
Schlenk techniques under argon. The solvents were dried by
refluxing with sodium (tetrahydrofuran, toluene, and hexane)
or CaH2 (1,2-dimethoxyethane). The epoxides were dried over
CaH2. All solid starting materials were dried at 50 °C for more
than one day under vacuum.
Elemental analyses were recorded using an Elementar Vario
EL III Analyzer. The infrared spectra were recorded using a
ThermoFisher
Nicolet
6700
spectrometer
in
the
400–4000 cm−1 region. The NMR (1H, 13C, and 1H Dosy)
spectra were recorded using a Bruker Ascend 400 spectrometer
or an Agilent DD2-600 spectrometer. The crystal data were col-
lected using a Bruker D8 Venture diffractometer (Mo Kα radi-
ation, λ = 0.71073 Å) at 293 K. Data reduction was accom-
plished by the Bruker APEX program. The structures were
solved by direct methods and refined by a full matrix least-
squares technique based on F2 using the SHELXL program. All
of the non-hydrogen atoms were refined anisotropically.
Hydrogen atoms were generated geometrically.
Neodymium complex Nd2L2(THF)2 (3). Yield: 1.96 g, 87%.
Anal. calcd for: C76H120N2O10Nd2: C, 60.44; H, 8.01; N, 1.85.
Found: C, 60.55; H, 8.06; N, 1.85. IR (selected absorbances,
cm−1): 2950 (–CH2–), 1473 (–CH3), 1279 (–C–O–C–), 1166 (–C–
N–), 877 (Ar).
Lanthanum complex La2L2(THF)2 (4). Yield: 2.02 g, 90%.
Anal. calcd for C76H120N2O10La2: C, 60.87; H, 8.07; N, 1.87.
1
Found: C, 60.75; H, 8.15; N, 1.86. H NMR (400 MHz, THF-d8):
δ 7.15 (d, J = 2.6 Hz, 4H, ArH), 6.90 (d, J = 2.8, Hz, 4H, ArH),
4.18 (t, J = 4.7 Hz, 4H, ArCH2N), 4.05 (d, 4H, J = 12.0 Hz,
ArCH2N), 3.61–3.59 (m, 8H, CH2), 3.53 (t, J = 4.4 Hz, 4H,
NCH2CH2O), 3.27 (t, 4H, J = 5.6 Hz, NCH2CH2O), 3.14 (d, J =
Experimental procedure
Synthesis of ligand precursor LH3.9a 2,4-Di-tert-butylphenol 12.2 Hz, 4H, OCH2CH2O), 2.73 (t, J = 5.1 Hz, 4H, OCH2CH2O),
(44.36 g, 215 mmol, 2.15 eq.), 37% aqueous formaldehyde 1.77–1.67 (m, 8H, CH2), 1.47 (s, 36H, C(CH3)3), 1.27 (s, 36H,
(17.53 mL, 216 mmol, 2.16 eq.), 2-(2-aminoethoxy)ethanol C(CH3)3). 13C NMR (100 MHz, THF-d8): δ 164.1, 136.4, 135.9,
(10.52 g, 100 mmol, 1.00 eq.) and toluene (200 mL) were added 127.2, 126.1, 124.4 (Ar–C), 78.1, 72.4, 68.8, 66.0 (ArCH2N), 65.0
into a 500 mL round-bottomed flask that was equipped with a (NCH2), 53.1, 36.4, 35.0 (OCH2), 33.0 (C(CH3)), 31.7 (C(CH3)).
Dean–Stark trap and reflux condenser. The mixture was heated IR (selected absorbances, cm−1): 2951 (–CH2–), 1473 (–CH3),
under reflux overnight which was tracked by TLC (10 : 1 pet- 1280 (–C–O–C–), 1166 (–C–N–), 876 (Ar).
roleum ether : ethyl acetate). The crude product was purified
General synthetic procedure of Zn(OAc)2-bridged heterome-
by column chromatography with petroleum ether and ethyl tallic rare earth metal–zinc complexes (5 and 6). The ligand
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Dalton Transactions
precursor LH3 (0.548 g, 1.00 mmol) was dissolved in dry THF
Samarium–zinc complex Sm2ZnL2(OBn)2(THF)2 (9). 2.08 g,
(15 mL). The THF solution (10 mL) of RECp3(THF) [RE = Nd, 77%. Anal. calcd for: C90H134N2O10Sm2Zn: C, 59.98; H, 7.49; N,
Sm] (1.00 mmol) was added and the mixture was stirred for 1.55. Found: C, 59.97; H, 7.50; N, 1.88. IR (selected absor-
12 h at 50 °C. Zn(OAc)2 (0.092 g, 0.50 mmol) was added into bances, cm−1): 2900 (–CH2–), 1602 (C–H(Ph)), 1475 (–CH3),
the mixture, which gave a clear solution after a 24 h reaction at 1199 (–C–O–C–), 1166 (–C–N–), 875 (Ar).
50 °C. The solvent was removed under reduced pressure, and a
Neodymium–zinc complex Nd2ZnL2(OBn)2(THF)2 (10). Yield:
large amount of powder was precipitated by adding hexane 2.05 g, 76%. Anal. calcd for: C90H134N2O10Nd2Zn: C, 60.39; H,
(15 mL). The powder was collected and redissolved in a 7.55; N, 1.57. Found: C, 60.59; H, 7.56; N, 1.56. IR (selected
mixture of THF (3 mL) and hexane (3 mL). After a few days, the absorbances, cm−1): 2900 (–CH2–), 1602 (C–H(Ph)), 1474
crystals of the heterometallic complexes (5–6) were obtained at (–CH3), 1201 (–C–O–C–), 1166 (–C–N–), 877 (Ar).
about 0 °C.
Lanthanum–zinc complex La2ZnL2(OBn)2(THF)2 (11). Yield:
Samarium–zinc complex Sm2ZnL2(OAc)2 (THF)2 (5). Yield: 2.17 g, 81%. Anal. calcd for: C90H134N2O10La2Zn: C, 60.76; H,
0.217 g, 20%. Anal. calcd for: C80H126N2O14Sm2Zn: C, 56.26; H, 7.59; N, 1.57. Found: C, 60.83; H, 7.57; N, 1.53. 1H NMR
7.55; N, 1.64. Found: C, 57.09; H, 7.42; N, 1.54. IR (selected (600 MHz, THF-d8): δ 7.60 (s, 4H, ArH) 7.35–7.30 (m, 4H, ArH),
absorbances, cm−1): 2948 (–CH2–), 2876 (–CH2–), 1565(CvO), 7.25–7.18 (m, 6H, ArH), 6.99 (s, 4H, ArH), 5.33 (s, 4H, PhCH2O),
1473 (–CH3), 1201 (–C–O–C–), 1165 (–C–N–), 876 (Ar).
4.20–4.05 (m, 8H, ArCH2N), 3.71 (m, 4H, CH2–THF), 3.56 (s, 4H,
Neodymium–zinc complex Nd2ZnL2(OAc)2 (THF)2 (6). Yield: NCH2CH2O), 3.10 (s, 8H, CH2OCH2), 2.74 (s, 4H, CH2O),
0.212 g, 25%. Anal. calcd for: C80H126N2O14Nd2Zn: C, 56.66; H, 1.87–1.80 (m, 4H, CH2–THF), 1.58 (s, 8H, C(CH3)3), 1.35 (s, 56H,
7.61; N, 1.65. Found: C, 57.10; H, 7.41; N, 1.70. IR (selected C(CH3)3). 13C NMR (151 MHz, THF-d8): δ 164.4, 163.8, 147.5,
absorbances, cm−1): 2950 (–CH2–), 2866 (–CH2–), 1563(CvO), 136.6, 129.6, 127.5, 126.8, 126.4, 124.8 (ArC), 77.5, 72.7 (ArCH2N),
1473 (–CH3), 1201 (–C–O–C–), 1166 (–C–N–), 875 (Ar).
69.1 (CH2OCH2), 67.2, 66.3 (NCH2CH2), 52.4 (ArCH2O), 36.6,
General synthetic procedure of Zn(OBn)2-bridged heterome- 36.4, 35.3 (CH2O), 33.4 (C(CH3)), 31.6, 31.1 (C(CH3)), 27.2
tallic rare earth metal–zinc complexes RE2ZnL2(OBn)2(THF)2 (CH2CH2). IR (selected absorbances, cm−1): 2875 (–CH2–), 1602
(7–11). ZnEt2 (1 M in hexane, 1.50 mmol, 1.50 mL) and (C–H(Ph)), 1473 (–CH3), 1205 (–C–O–C–), 1167 (–C–N–), 878 (Ar).
PhCH2OH (1 M in THF, 3.00 mmol, 3.00 mL) were mixed and
General procedure for the reaction of CO2 with epoxides.
stirred for to give suspension. The mixture of Epoxide (10 mmol), catalyst (0.05 mmol), and Bu4NBr (tetra-
4
h
a
RECp3(THF) [RE = Y, Yb, Sm, Nd, La] (3.00 mmol) and LH3 butylammonium bromide) (0.2 mmol) were placed in a 5 mL
(3.00 mmol) in 20 mL of THF was added into the suspension, flask. A balloon filled with CO2 was connected to the flask,
which gave a clear solution after 24 h reaction at 50 °C. The and the mixture was stirred at 25 °C for 24 h. The conversion
solvent was removed under reduced pressure. The resulting of an epoxide to a cyclic carbonate was determined by 1H NMR
solids were dissolved in THF (5 mL) and hexane (8 mL). After spectroscopy. The product was purified by flash chromato-
the filtration and removal of the solvent, heterometallic com- graphy (hexane and ethylene acetate). Cyclic carbonates 12a–l
plexes 7–11 were obtained.
are known compounds, and spectroscopic data are consistent
Yttrium–zinc complex Y2ZnL2(OBn)2(THF)2 (7). Yield: 2.18 g, with those reported in the literature.3–5
87%. Anal. calcd for: C90H134N2O10Y2Zn·4.5C4H10O2: C, 62.22;
4-Butyl-1,3-dioxolan-2-one:.5d 1H NMR (400 MHz, CDCl3) δ
H, 8.65; N, 1.34. Found: C, 62.25; H, 8.62; N, 1.34. 1H NMR 4.72–4.64 (m, 1H, CHO), 4.50 (t, J = 8.8 Hz, 1H, OCH2), 4.05 (t,
(600 MHz, THF-d8): δ 7.62 (s, 3H, ArH), 7.32–7.28 (m, 4H, ArH), J = 8.2 Hz, 1H, OCH2), 1.86–1.76 (m, 1H, CH2), 1.70–1.63 (m,
7.22 (s, 5H, ArH), 7.10 (s, 1H, ArH), 6.91 (s, 3H, ArH), 6.84 (s, 1H, CH2), 1.37–1.29 (m, 4H, CH2), 0.91 (t, J = 6.8 Hz, 3H, CH3).
2H, ArH), 5.32 (dd, J = 14.5, 55.0 Hz, 4H, PhCH2O), 4.36–4.34
4-Phenyl-1,3-dioxolan-2-one (12b).5d 1H NMR (400 MHz,
(m, 1H, NCH2CH2O), 4.22–4.08 (m, 8H, ArCH2N), 3.65–3.64 (m, CDCl3) δ 7.45–7.44 (m, 3H, ArH), 7.38–7.36 (m, 2H, ArH), 5.68
4H, CH2–THF), 3.55–3.50 (m, 3H, NCH2CH2O), 3.09 (s, 4H, (t, J = 8.5 Hz, 1H, OCH2), 4.80 (t, J = 8.7 Hz, 1H, OCH2), 4.35 (t,
NCH2CH2O), 3.02–2.85 (m, 4H, OCH2CH2O), 2.74–2.65 (m, 4H, J = 8.6 Hz, 1H, OCH).
OCH2CH2O), 1.80–1.76 (m, 4H, CH2–THF), 1.60–1.56 (m, 18H,
4-(4-Bromophenyl)-1,3-dioxolan-2-one (12c).5m 1H NMR
C(CH3), 1.30–1.19 (m, 54H, C(CH3)3). 13C NMR (151 MHz, THF- (400 MHz, CDCl3) δ 7.57 (d, J = 8.4 Hz, 2H, ArH), 7.23 (d, J =
d8): δ 161.2, 145.8, 135.8, 135.2, 134.8, 134.6, 127.7, 125.2, 8.4 Hz, 2H, ArH), 5.62 (t, J = 8.5 Hz, 1H, OCH), 4.78 (t, J = 7.5
124.4, 124.1, 122.9, 122.7 (Ar–C), 74.5, 70.1 (ArCH2N), 67.6, Hz, 1H, OCH2), 4.28 (t, J = 9.3 Hz, 1H, OCH2).
67.2 (CH2OCH2), 65.3, 64.4 (NCH2CH2), 52.1 (OCH2Ar), 34.8,
4-(4-Chlorophenyl)-1,3-dioxolan-2-one (12d).5d 1H NMR
34.3, 33.4 (CH2O), 31.4 (C(CH3)), 29.9, 29.0 (C(CH3)), 25.4 (400 MHz, CDCl3) δ 7.41 (d, J = 8.4 Hz, 2H, ArH), 7.29 (d, J =
(CH2CH2). IR (selected absorbances, cm−1): 2989 (–CH2–), 1602 8.5 Hz, 2H, ArH), 5.64 (t, J = 8.7 Hz, 1H, OCH), 4.78 (t, J = 8.6
(C–H(Ph)), 1475 (CH2), 1301 (–C–O–C–), 1168 (–C–N–), 874 Hz, 1H, OCH2), 4.28 (t, J = 8.6 Hz, 1H, OCH2).
(Ar).
4-(Methoxymethyl)-1,3-dioxolan-2-one (12e).5d 1H NMR
Ytterbium–zinc complex Yb2ZnL2(OBn)2(THF)2 (8). Yield: (400 MHz, CDCl3) δ 4.79–4.76 (m, 1H, OCH), 4.45 (t, J = 8.3 Hz,
2.22 g, 80%. Anal. calcd for: C90H134N2O10Yb2Zn: C, 58.51; H, 1H, OCH2), 4.32–4.29 (m, 1H, OCH2), 3.61–3.48 (m, 2H,
7.31; N, 1.52. Found: C, 58.63; H, 7.37; N, 1.52. IR (selected OCH2O), 3.35 (s, 3H, OCH3).
absorbances, cm−1): 2900 (–CH2–), 1602 (C–H(Ph)), 1475
(–CH3), 1202 (–C–O–C–), 1166 (–C–N–), 876 (Ar).
4-(Phenoxymethyl)-1,3-dioxolan-2-one (12f).5n 1H NMR
(400 MHz, CDCl3) δ 7.32 (t, J = 8.3 Hz, 2H, ArH), 7.00 (t, J = 7.0
1460 | Dalton Trans., 2021, 50, 1453–1464
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Hz, 1H, ArH), 6.90 (d, J = 7.7 Hz, 2H, ArH), 5.03–5.02 (m, 1H,
OCH), 4.64–4.54 (m, 2H, OCH2), 4.26–4.13 (m, 2H, OCH2).
(2-Oxo-1,3-dioxolan-4-yl)-methyl 4-(tert-butyl)benzoate (12g).17
1H NMR (400 MHz, CDCl3) δ 7.94 (d, J = 7.8 Hz, 2H, ArH), 7.46
(d, J = 8.7, 2H, ArH), 5.03–5.00 (m, 1H, OCH), 4.62–4.60 (m,
1H, OCH2), 4.58–4.54 (m, 2H, OCH2), 4.51–4.39 (m, 1H, OCH2),
1.32 (s, 9H, C(CH3)3).
(c) M. Cokoja, C. Bruckmeier, B. Rieger, W. A. Herrmann
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A metal–free synthesis of N–aryl carbamates under ambient
conditions, Angew. Chem., Int. Ed., 2015, 54, 11686.
4-(Propoxymethyl)-1,3-dioxolan-2-one (12h).18 1H NMR
(400 MHz, CDCl3) δ 4.82–4.77 (m, 1H, OCH), 4.48 (t, J = 8.4,
1H, OCH2), 4.39–4.36 (m, 1H, OCH2), 3.68–3.60 (m, 2H, CH2O),
3.57–3.48 (m, 2H, OCH2), 1.56–1.51 (m, 2H, CH2CH2),
1.37–1.32 (m, 2H, CH2CH2), 0.90 (t, J = 7.1 Hz, 3H, CH3).
4-((Allyloxy)methyl)-1,3-dioxolan-2-one (12i).5d 1H NMR
(400 MHz, CDCl3) δ 5.92–5.82 (m, 1H, CHvCH2), 5.31–5.21
(m, 2H, CHvCH2), 4.82–4.80 (m, 1H, OCH), 4.50 (t, J = 8.4 Hz,
1H, OCH2), 4.48–4.38 (m, 1H, OCH2), 4.11–4.10 (m, 2H, OCH2),
3.71–3.60(m, 2H, OCH2).
4,4-Dimethyl-1,3-dioxolan-2-one (12j).5d 1H NMR (400 MHz,
CDCl3) δ 4.15 (s, 2H, OCH2), 1.53 (s, 6H, CH3).
cis-Tetrahydro-4H-cyclopenta[d][1,3]dioxol-2-one (12k).5d 1H
NMR (400 MHz, CDCl3) δ 5.11 (s, 2H, OCH2), 2.29–2.19 (m, 2H,
CH2), 1.87–1.73 (m, 2H, CH2), 1.71–1.68 (m, 2H, CH2).
cis-Hexahydrobenzo[d][1,3]dioxol-2-one (12l).5d 1H NMR
(400 MHz, CDCl3) δ 4.72–4.68 (m, 2H, CHO), 1.95–1.84 (m, 4H,
CH2), 1.68–1.56 (m, 2H, CH2), 1.48–1.40 (m, 2H, CH2).
cis-5-Vinylhexahydrobenzo[d][1,3]dioxol-2-one (12m).5d 1H
NMR (400 MHz, CDCl3) δ 5.75–5.66 (m, 1H, CHvCH2),
5.06–4.99 (m, 2H, CH), 4.80–4.61 (m, 2H, CHvCH2), 2.32–2.16
(m, 2H, CH2), 1.65–1.51(m, 2H, CH2), 1.41–1.15(m, 2H, CH2),
1.11–1.24 (m, 1H, CH).
Conflicts of interest
3 (a) B. H. Xu, J. Q. Wang, J. Sun, Y. Huang, J. P. Zhang,
X. P. Zhang and S. J. Zhang, Fixation of CO2 into cyclic car-
bonates catalyzed by ionic liquids: a multi-scale approach,
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CO2 with amines: an entry to formamide, formamidine
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There are no conflicts to declare.
Acknowledgements
We acknowledge the financial support from the National
Natural Science Foundation of China (21871198 and
21674070), the Major Research Project of the Natural Science
of the Jiangsu Higher Education Institutions (19KJA360005),
the Project of Scientific and Technologic Infrastructure of
Suzhou (SZS201708), the Open Research Fund of State Key
Laboratory of Rare Earth Resource Utilization (CIAC), and
PAPD.
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