Bimetallic lanthanide amido complexes as highly active initiators for the ring-opening polymerization of lactides

Article abstract of DOI:10.1039/c2dt31597a

A series of neutral bimetallic lanthanide amido complexes supported by rigid phenylene bridged bis(β-diketiminate) ligands were synthesized, and their catalytic behavior for the polymerization of l-lactide and rac-lactide was explored. The amine elimination reaction of Ln[N(TMS)₂] ₃(μ-Cl)Li(THF)₃ with PARA-H₂, [PARA-H ₂ = 2[2,6-ⁱPr₂C₆H ₃NHC(Me)C(H)C(Me)N]-(para-phenylene)] in a 2:1 molar ratio in THF at 25°C afforded the corresponding bimetallic lanthanide amido complexes PARA-{Ln[N(SiMe₃)₂]₂}₂ [Ln = Nd(1), Sm(2), Y(3)] in high isolated yields. Similar reaction of Nd[N(TMS) ₂]₃(μ-Cl)Li(THF)₃ with META-H₂, [META-H₂ = 2[2,6-ⁱPr₂C₆H ₃NHC(Me)C(H)C(Me)N]-(meta-phenylene)] at 90°C in toluene for about 48 h gave META-{Nd[N(SiMe₃)₂]₂} ₂ (4). Complexes 1-4 were well characterized by elemental analysis, IR spectroscopy, and their definitive structures were confirmed by an X-ray crystal structure analysis. The coordination environment and coordination geometry around the metal atoms are similar in these complexes. Each of the metal atoms is four-coordinated with two nitrogen atoms from the N,N-chelating β-diketiminate unit, and two nitrogen atoms from two (Me ₃Si)₂N- groups to form a distorted tetrahedron. These complexes can serve as highly active initiators for l-lactide polymerization in toluene. In addition, they also showed high activity towards rac-lactide polymerization in THF at room temperature, giving heterotactic-enriched polymers (P_r ≈ 0.70), and complex 4 displays obviously higher activity in comparison with complex 1.

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Bimetallic lanthanide amido complexes as highly
active initiators for the ring-opening polymerization
Cite this: Dalton Trans., 2013, 42, 2870
of lactides†
Song Sun,a Kun Nie,a Yufang Tan,a Bei Zhao,*a Yong Zhang,a Qi Shena and
Yingming Yao*a,b
A series of neutral bimetallic lanthanide amido complexes supported by rigid phenylene bridged bis-
(β-diketiminate) ligands were synthesized, and their catalytic behavior for the polymerization of L-lactide
and rac-lactide was explored. The amine elimination reaction of Ln[N(TMS)2]3(μ-Cl)Li(THF)3 with
PARA-H2, [PARA-H2 = 2[2,6-iPr2C6H3NHC(Me)C(H)C(Me)N]-(para-phenylene)] in a 2 : 1 molar ratio in THF
at 25 °C afforded the corresponding bimetallic lanthanide amido complexes PARA-{Ln[N(SiMe3)2]2}2
[Ln = Nd(1), Sm(2), Y(3)] in high isolated yields. Similar reaction of Nd[N(TMS)2]3(μ-Cl)Li(THF)3 with
META-H2, [META-H2 = 2[2,6-iPr2C6H3NHC(Me)C(H)C(Me)N]-(meta-phenylene)] at 90 °C in toluene for
about 48 h gave META-{Nd[N(SiMe3)2]2}2 (4). Complexes 1–4 were well characterized by elemental analy-
sis, IR spectroscopy, and their definitive structures were confirmed by an X-ray crystal structure analysis.
The coordination environment and coordination geometry around the metal atoms are similar in these
complexes. Each of the metal atoms is four-coordinated with two nitrogen atoms from the N,N-chelating
β-diketiminate unit, and two nitrogen atoms from two (Me3Si)2N– groups to form a distorted tetra-
hedron. These complexes can serve as highly active initiators for L-lactide polymerization in toluene. In
addition, they also showed high activity towards rac-lactide polymerization in THF at room temperature,
giving heterotactic-enriched polymers (Pr ≈ 0.70), and complex 4 displays obviously higher activity in
comparison with complex 1.
Received 18th July 2012,
Accepted 23rd November 2012
DOI: 10.1039/c2dt31597a
www.rsc.org/dalton
distributions, tacticity for poly(rac-lactide), etc.) depend mainly
Introduction
on the catalysts.2–6 Among the most promising candidates
Poly(lactic acid)s (PLAs) have received considerable attention used for lactide polymerization, rare earth metal complexes
during the past decades, because of their biodegradability and are highly efficient initiators for the ROP of lactide with fast
biocompatibility, which gives them wide potential applications initiation speed and, in some case, good stereoselectivity.6
in environmental, biomedical, and pharmaceutical fields.1
It is well known that lanthanide metals have a unique elec-
Ring-opening polymerization (ROP) of lactides initiated by
metal-based complexes has been proved to be the most con-
venient and promising route for synthesis of PLAs with high
molecular weights and narrow molecular weight distributions.
To date, numerous main group, transition metal, and rare
earth metal complexes have been prepared for this purpose,
and it was found that the microstructures of the poly-
mers (including molecular weights and molecular weight
troniv structure, and that the catalytic activities and/or selectiv-
ities of organolanthanide complexes are governed primarily by
the coordination environment around the metal center. This
has provided the opportunity to modify the catalytic property
of lanthanide-based catalysts by selecting suitable ancillary
ligands. Chelating mono-β-diketiminate ligands, whose steric
and electronic properties can be easily tuned by an appropriate
choice of starting materials used in their synthesis, have been
used in organolanthanide chemistry for a long time, and have
led to some successful catalysts/initiators for the polymeriz-
ation of methyl methacrylate,7 lactide8 and ε-caprolactone.9
However, the phenylene bridged bis(β-diketiminate) ligands10
have never been used in organolanthanide chemistry, to our
knowledge.
aKey Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry,
Chemical Engineering and Materials Science, Soochow University, Suzhou 215123,
People’s Republic of China. E-mail: yaoym@suda.edu.cn; Fax: +86 512 65880305;
Tel: +86 512 65882806
bThe Institute of Low Carbon Economy, Suzhou, 215123, People’s Republic of China
†Electronic supplementary information (ESI) available. CCDC 874949 (for 1),
874950 (for 2), 874951 (for 3), 874948 (for 4). For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c2dt31597a
Recently, Harder et al. found that the phenylene bridged
bis(β-diketiminate) groups are useful dinucleating ligands to
2870 | Dalton Trans., 2013, 42, 2870–2878
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stabilize a series of bimetallic calcium and zinc complexes, in temperature (to 90 °C) and a prolonged reaction time (to about
which the bimetallic zinc complexes are efficient catalysts for 48 h), to yield the analogous bimetallic neodymium amido
the copolymerization of cyclohexene oxide and CO2.10 These complex META-{Nd[N(SiMe3)2]2}2 (4) in moderate yield (51%).
results encouraged us to explore the application of phenylene Presumably, this difference should be attributed to the fact
bridged bis(β-diketiminate) ligands in organolanthanide that the preligand META-H2 has a more crowed steric configur-
chemistry. Now four bimetallic lanthanide amido complexes ation in comparison with PARA-H2, which makes the reaction
stabilized by rigid phenylene-bridged bis(β-diketiminate) of Nd[N(TMS)2]3(μ-Cl)Li(THF)3 with META-H2 difficult.
ligands have been synthesized and well characterized. It was
All of these four amido complexes were fully characterized
found that these bimetallic lanthanide amido complexes are by elemental analysis, FT-IR and X-ray structure analysis. The
extremely efficient initiators for the ROP of lactides, in which elemental analysis of the crystals is consistent with the
all of the four amido groups are active for the polymerization.
formula of the bimetallic amide complexes. The IR spectra of
complexes 1–4 exhibited strong absorptions near 1554 and
1510 cm−1, which were consistent with partial CvN double
bond character.11 Complex 3 was further characterized by
NMR spectroscopy. In the 1H NMR spectrum, the singlets of
the two methyne groups of the β-diketiminate units in
complex 3 shift from 4.85 ppm in PARA-H2 to 5.16 ppm, and
the other signals of the β-diketiminate ligand in complex 3 are
changed slightly in comparison with those of the preligand
PARA-H2.10 Notably, in the 1H NMR spectrum of complex 3,
the appearance of a single peak for the protons of amido
groups in high-field region (δ 0.32 ppm) indicated that the
chemical environments of the four amido groups in complex 3
are same. Complexes 1–4 are very sensitive to air and moisture.
The crystals decompose in a few minutes when they are
exposed to air, but both the crystals and the solution are
rather stable when stored under argon. Complexes 1–3 are
freely soluble in THF, and slightly soluble in hexane and
toluene, and complex 4 is freely soluble in THF and toluene,
and is moderately soluble in hexane.
Results and discussion
Syntheses and characterization of bimetallic lanthanide amido
complexes
The amine elimination reaction between lanthanide amido
complexes with preligands is a straightforward method for the
synthesis of lanthanide amido derivatives. Thus, this method
was used to synthesize the desired lanthanide amido com-
plexes. Reactions of Ln[N(TMS)2]3(μ-Cl)Li(THF)3 with para-
phenylene bridged bis(β-diketimine), PARA-H2 [PARA-H2 = 2
[2,6-iPr2C6H3NHC(Me)C(H)C(Me)N]-(para-phenylene)] were
conducted in a 2 : 1 molar ratio in THF at 25 °C. After workup,
the desired bimetallic lanthanide amido complexes PARA-{Ln-
[N(SiMe3)2]2}2 [Ln = Nd(1), Sm(2), Y(3)] were isolated from a
concentrated toluene solution as microcrystals in high yields
(85–89%), as shown in Scheme 1.
It was found that the structure of the preligand has a
profound effect on this exchange reaction. On replacement
of the PARA-H2 by the preligand META-H2 [META-H2
To provide complete structural information for these new
bimetallic phenylene bridged bis(β-diketiminate) lanthanide
amido complexes, single-crystal X-ray structural analyses were
carried out for complexes 1–4 (Table 1). The crystals suitable
for an X-ray structure determination of complexes 1–3 were
obtained from a toluene solution at room temperature,
whereas the crystals for complex 4 were obtained from a mixed
=
2[2,6-iPr2C6H3NHC(Me)C(H)C(Me)N]-(meta-phenylene)], the
reaction of META-H2 with Nd[N(TMS)2]3(μ-Cl)Li(THF)3 did not
occur under the same reaction conditions, and only the
starting materials were isolated. A further study revealed that
this reaction can occur in toluene at an elevated reaction
Scheme 1 Synthesis of bimetallic lanthanide amido complexes.
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Table 1 Crystallographic data for complexes 1–4
Compound
1
2
3
4
Formula
Fw
C
64H124N8Nd2Si8
C64H124N8Sm2Si8
1531.13
C64H124N8Y2Si8
1408.22
C64H124N8Nd2Si8
1518.91
1518.91
T/K
223(2)
Triclinic
0.60 × 0.15 × 0.10
ˉ
P1
223(2)
223(2)
223(2)
Crystal system
Crystal size/mm
Space group
Orthorhombic
0.70 × 0.25 × 0.15
Pnma
Orthorhombic
0.55 × 0.35 × 0.10
Pnma
Monoclinic
0.80 × 0.30 × 0.25
P21/n
a/Å
b/Å
c/Å
α (°)
9.7093(12)
12.6706(17)
17.136(2)
82.717(9)
81.736(9)
82.820(9)
2057.1(5)
4
19.569(3)
13.4929(13)
35.129(3)
19.5917(14)
13.4025(11)
35.085(3)
21.3890(13)
11.4497(6)
34.1506(17)
β (°)
95.069(2)
γ (°)
V/Å3
9275.7(18)
4
1.228
9212.5(13)
4
1.148
8330.7(8)
4
1.211
Z
Dcalcd/g cm−3
1.226
1.402
μ/mm−1
1.398
1.399
1.385
F (000)
θmax/°
796
25.5
3600
25.5
3416
25.5
3184
27.5
Collected
Unique reflns
Obsd reflns [I > 2.0σ(I)]
No. of variables
16 749
7604
6051
27 156
14 571
11 355
823
27 220
13 185
8351
44 629
15 349
13 139
776
377
835
GOF
R
wR
Rint
1.085
1.056
1.047
1.107
0.0622
0.1362
0.0477
0.777, −1.242
0.0802
0.1741
0.0566
3.038, −0.994
0.0888
0.1816
0.0896
1.159, −0.490
0.0416
0.1015
0.0378
1.176, −0.895
Largest diff. peak, hole/e Å−3
solution of hexane and toluene at room temperature. X-Ray N1–C2–C4–N2, respectively. In complex 1, the two neodymium
diffraction analyses displayed that the complexes 1–3 are iso- atoms are situated at opposite positions of the phenylene ring
structural. Thus, only the molecular structure of complex 1 is to reduce the steric congestion of the complex, and the two
shown in Fig. 1. Complex 1 has a completely symmetric bime- planes defined by N(1)C(2)C(4)N(2) and N(2A)C(4A)C(2A)N(1A)
tallic structure. Each of the neodymium atoms is four-coordi- are completely parallel with a dihedral angle of 0°. The dis-
nated with two nitrogen atoms from the N,N-chelating tance between the two neodymium atoms is 9.59 Å, and the
β-diketiminate unit, and two nitrogen atoms from two distances from each of the neodymium atoms to the plane of
(Me3Si)2N– groups. The coordination geometry can be the bridged phenylene ring are the same (about 1.57 Å).
described as a distorted tetrahedron, which is similar to those
In contrast, complex 4 has an approximately C2-symmetric
of the mononuclear species.12 The backbone of the β-diketimi- bimetallic structure, as shown in Fig. 2. The coordination
nate unit (NC3N) and the neodymium atom form a stable six- environment and coordination geometry around the neody-
membered ring in a boat conformation, and the C3 and Nd mium atoms are similar to that in complex 1. The backbone of
atoms lying 0.28 and 1.64 Å out of the plane defined by the β-diketiminate unit (NC3N) and the neodymium atom also
form a stable six-membered ring in a boat conformation, and
Fig. 1 An ORTEP diagram of complex 1 showing an atom numbering scheme.
Thermal ellipsoids are drawn at the 30% probability level, and hydrogen atoms
are omitted for clarity. Complexes 2, 3 are isomorphous with complex 1.
Fig. 2 ORTEP diagram of complex 4 showing an atom numbering scheme.
Thermal ellipsoids are drawn at the 30% probability level, and hydrogen atoms
are omitted for clarity.
2872 | Dalton Trans., 2013, 42, 2870–2878
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for L-lactide polymerization. These complexes represent the
first examples of metal-based initiators displaying such high
activity in L-lactide polymerization.13,6f The ionic radii have a
profound influence on the activity, and the increasing order of
Y < Sm < Nd is consistent with the order of ionic radii
(Table 3, entries 1–19). The polymerization medium played an
important role in influencing the catalytic activity and the mol-
ecular weight of the resultant polymers. The complexes show
higher activity in toluene than in THF. When complex 1 was
used as the initiator, the yield reached 95% in toluene at 50 °C
in 30 min, and gave the polymer with a high molecular weight,
when the [M]0/[I]0 was 16 000, whereas the yield is only 43%
under the same polymerization conditions in THF, even if the
polymerization time prolonged to 12 h, and the polymer has
an apparently low molecular weight (Table 3, entries 4 and 5).
Although there are two metal centers in these bimetallic
complexes, the GPC curves of the resulting polymers were all
unimodal, which indicates the same polymerization behavior
of the two metal centers (see ESI†). To gain some insights into
the polymerization mechanism with these bimetallic com-
plexes, a 1H NMR monitoring reaction of complex 3 with
L-lactide was carried out. Treatment of complex 3 with 15 equiv.
of L-lactide in C6D6 at 25 °C revealed that, after about 30 min,
most of the resonance of –N(SiMe3)2 groups at 0.32 ppm in
complex 3 (Fig. 3) disappeared, and a set of signals in
0.10 ppm was observed (Fig. 4). Presumably, these signals
should be assigned to the end group of –CO–N(SiMe3)2. This
result indicated that the (Me3Si)2N– groups of complex 3 par-
ticipated in the initiation reaction during the polymerization
of L-lactide. However, the resonance of the end group of
Table 2 Selected bond lengths (Å) and bond angles (°) for complexes 1–4
Bond lengths
1
2
3
4
Ln1–N1
Ln1–N2
Ln1–N5
Ln1–N6
Ln2–N3
Ln2–N4
Ln2–N7
Ln2–N8
2.480(5)
2.408(5)
2.306(5)
2.312(5)
2.425(12)
2.343(11)
2.359(16)
2.311(13)
2.463(12)
2.362(11)
2.190(17)
2.267(12)
2.363(12)
2.260(10)
2.258(17)
2.260(10)
2.353(11)
2.296(10)
2.206(17)
2.228(10)
2.469(3)
2.397(3)
2.341(3)
2.325(3)
2.426(3)
2.463(3)
2.336(3)
2.310(3)
Bond angles
N6–Ln1–N2
N6–Ln1–N5
N2–Ln1–N5
N6–Ln1–N1
N2–Ln1–N1
N5–Ln1–N1
N7–Ln2–N8
N7–Ln2–N4
N8–Ln2–N4
N7–Ln2–N3
N8–Ln2–N3
N4–Ln2–N3
121.2(3)
110.4(4)
97.9(4)
106.1(4)
80.8(4)
125.0(4)
112.1(4)
97.3(5)
107.9(4)
77.7(4)
133.6(4)
111.5(4)
95.1(5)
125.4(4)
136.7(5)
107.2(4)
77.3(4)
121.2(3)
110.4(4)
97.9(4)
106.1(4)
80.8(4)
137.5(4)
111.2(4)
97.0(5)
122.6(3)
137.9(4)
105.4(4)
479.6(4)
121.59(11)
111.90(12)
100.58(11)
115.04(12)
74.93(9)
126.78(11)
109.15(11)
135.97(11)
100.34(11)
107.07(11)
127.72(11)
77.23(10)
137.5(4)
C3 and Nd atoms lying 0.26 and 1.83 Å out of the plane
defined by N1–C2–C4–N2, respectively, which is slightly
different from the corresponding values in complex 1. In
complex 4, the two neodymium atoms are also situated at
opposite positions of the phenylene ring, and the distances
from the two neodymium atoms to the plane of the bridged
benzene ring are 0.44 and 1.70 Å, respectively, which is
different to those observed in complex 1. To reduce the steric
congestion, the two planes defined by N(1)C(2)C(4)N(2) and
N(4)C(21)C(19)N(3) are approximately perpendicular with a
dihedral angle of 78.1°. The distance between the two neo-
dymium metals is 9.25 Å, which is slightly shorter than the corre-
sponding distance of 9.59 Å in complex 1. The bond distances
of N(1)–C(2), C(2)–C(3), C(3)–C(4) and N(2)–C(4) lie intermedi-
ate between the corresponding single- and double-bond dis-
tances (see Table 2), which suggest significant delocalization
within the π-system of β-diketiminate backbone. In addition,
the distances from each of the metal ions to the carbon atoms
of the β-diketiminate backbone are in the range 2.89–3.01 and
3.03–3.09 Å, which are shorter than the sum of the van der
Waals radii of Nd and C atoms (rNd = 2.39 Å, rC = 1.70 Å).
These bond parameters indicated a tendency of an η5 bonding
pattern for the β-diketiminate backbone.
1
–CO–N(SiMe3)2 was not observed in the H NMR of the oligo-
mer. In order to understand the structure and the composition
of the PLA obtained, a oligomer of L-lactide initiated by
complex 2 after quenching with the addition of isopropanol
containing 5% of HCl was analyzed by MALDI-TOF mass spec-
trometry (Fig. 5). It was found that the calculated molecular
weights fit very well with the measured ones, if the end group
of the oligomer obtained is an isopropoxo group. Additionally,
in the 1H NMR spectrum of the oligomer, the resonances at
1.21 and 5.05 ppm can be attributed to the isopropoxo group
(Fig. S5†). These results indicated that the end cap of the
polymer is not the –OCH(CH3)CO–N(SiMe3)2, but the –OCH
(CH3)CO–OiPr group. We postulated that there is a further
reaction between the end amide group of –CO–N(SiMe3)2 with
isopropanol, because the amide group –CO–N(SiMe3)2 gener-
ated is unstable.14 On the other hand, the peaks are separated
Lactide polymerization
The polymerization behavior of complexes 1–4 for L-lactide by 72 amu in the MALDI-TOF mass spectrum, indicating a
polymerization was examined, and the preliminary results are high degree of transesterification during the polymerization,
listed in Table 3. It can be seen that complexes 1–4 can initiate which led to lower measured molecule weights than the calcu-
the ROP of L-lactide in toluene with extremely high activity. For lated ones, and wider polydispersivities.
example, using complex 1 as the initiator, the yield is 80%
Generally, the polymerization initiated by rare-earth metal
within 30 min at 50 °C in toluene, when the molar ratio of amide was not well controlled, giving a broadly distributed
L-lactide to initiator is 20 000 (Table 3, entry 6). The yield can polymer, because the silylamide group is less nucleophilic
reach 72% in 30 min at 70 °C, even when the molar ratio of than the alkoxide, which caused the relatively slow initiation.15
monomer to initiator increased to 40 000 (Table 3, entry 9). Thus, the L-lactide polymerization initiated by complex 1 in
The activity of complex 4 is comparable with that of complex 1 the presence of 4 equiv. of 2-propanol was conducted (Table 3,
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Table 3 Polymerization of L-lactide initiated by complexes 1–4a
Entry
Initiator
[M]0/[I]0/[iPrOH]0
T (°C)
t
Yieldc (%)
Mcd (×104)
Mnf (×104)
PDIf
1
2
3
4
5
6
7
8
1
1
1
1
1b
1
1
1
1
2
2
2
2
2
3
3
3
3
3
4
4
4
4
4
4
1
1
1
1
8000/1/0
12 000/1/0
16 000/1/0
16 000/1/0
16 000/1/0
20 000/1/0
32 000/1/0
32 000/1/0
40 000/1/0
12 000/1/0
16 000/1/0
16 000/1/0
20 000/1/0
40 000/1/0
12 000/1/0
16 000/1/0
16 000/1/0
20 000/1/0
40 000/1/0
12 000/1/0
16 000/1/0
16 000/1/0
20 000/1/0
32 000/1/0
40 000/1/0
1000/1/4
50
50
50
50
50
50
50
70
70
50
50
50
50
70
50
50
50
50
70
50
50
50
50
70
70
50
50
50
50
5 min
5 min
5 min
30 min
12 h
30 min
2 h
30 min
30 min
5 min
5 min
30 min
1 h
30 min
5 min
5 min
30 min
1 h
30 min
5 min
5 min
30 min
30 min
30 min
30 min
5 min
5 min
5 min
5 min
95
92
82
95
43
80
52
80
72
94
77
94
74
67
74
48
55
46
57
96
83
95
90
83
80
91
93
99
96
27.3
39.7
47.3
54.8
24.8
57.6
59.9
92.2
103.8
40.6
44.4
54.2
53.3
96.5
40.0
27.6
31.7
33.1
82.1
41.5
47.9
54.7
64.8
95.7
115.3
3.28e
6.70e
14.2e
17.2e
23.8
22.3
24.3
25.7
8.4
1.45
1.47
1.38
1.42
1.45
1.47
1.43
1.52
1.56
1.40
1.45
1.44
1.44
1.53
1.47
1.40
1.49
1.54
1.49
1.42
1.46
1.40
1.45
1.49
1.51
1.40
1.45
1.31
1.32
23.7
17.3
32.8
38.3
29.3
27.4
30.5
27.9
34.2
32.5
32.3
28.8
27.5
28.9
24.9
23.7
28.0
25.9
39.4
32.3
7.27
10.6
14.7
17.2
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
2000/1/4
4000/1/4
5000/1/4
a General polymerization conditions: toluene as the solvent, [L-lactide] = 1 mol L−1 b In THF. c Yield: weight of polymer obtained/weight of
.
monomer used. d Mc = (144.13) × [M]0/[I]0 × (polymer yield)/4. e Mc = (144.13) × [M]0/[I]0 × (polymer yield)/4 + 60. f Measured by GPC relative to
poly-MMA standard.
Fig. 3 The 1H NMR spectrum (300 MHz, C6D6) of PARA-{Y[N(SiMe3)2]2}2.
Fig. 4 The 1H NMR spectrum (400 MHz, C6D6) of reaction of complex 3 + 15
L-lactide after 30 min at room temperature.
entries 26–29). It can be seen that the molecular weights of the
resultant polymers were quite controlled, and there was a good multiple active centers in this system. The initiation mechan-
linear relationship between the molecular weight and the ism initiated by the lanthanide alkoxo complex was also eluci-
molar ratio of monomer to initiator (Fig. 6), when the dated by end-group analysis. The oligomer was prepared by
initiation groups were alkoxides. The molecular weight distri- the polymerization of L-lactide initiated by complex 3 in the
butions also narrowed in comparison with those initiated by presence of
4 equiv. of 2-propanol, and subsequently
lanthanide amides, but they were still broader than those of quenched by the addition of ethanol containing 5% of HCl.
the well-controlled systems, which can be attributed to the The 1H NMR spectrum of the oligomer clearly showed that
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Fig. 7 The 1H NMR spectrum (400 MHz, CDCl3) of the oligomer of L-lactide
initiated by complex 3 in the presence of 4 equiv. of 2-propanol after quenching
by the addition of ethanol containing 5% of HCl. Polymerization conditions:
[L-LA]0/[I]0 = 5, in toluene, 10 min, 25 °C.
Fig. 5 The MALDI-TOF mass spectrum of poly(L-lactide) initiated by complex 2
after quenching by the addition of isopropanol containing 5% of HCl. Polymer-
ization conditions: [L-LA]0/[I]0 = 10, in toluene, 30 min, 25 °C (doped with NaI).
difference should be attributed to their different structures.
Similar difference in activity for the copolymerization of CO2
and cyclohexene oxide was also observed in the bimetallic zinc
systems stabilized by the same ligands, which was attributed
to the effect of the longer metal–metal distance in the para-
linked system on the transition states. Microstructural analysis
of the resulting polymers revealed that these complexes
exerted a moderate level of heterotacticity (Pr = 0.65–0.73). The
stereoselectivity of these bimetallic lanthanide complexes is
comparable with those of the polymers initiated by monome-
tallic β-diketiminate lanthanide complexes.6b
Conclusions
Fig. 6 Polymerization of L-lactide initiated by complex 3 in the presence of 4
equiv. of 2-propanol in toluene at 50 °C. Relationship between the number-
averaged molecular weight (Mn) and the molar ratio of monomer to initiator.
In this work, four bimetallic lanthanide amido complexes
stabilized by phenylene bridged bis(β-diketiminate) ligands
were prepared, and it was found that these complexes showed
extremely high activities for the ROP of lactides. The molecular
structures of these complexes have a profound effect on the
activity in lactide polymerization. The yield can reach 72% in
30 min at 70 °C, when the molar ratio of monomer to initiator
is 40 000 for L-lactide polymerization, and 10 000 equiv. of rac-
lactide also can be polymerized to high conversions within
30 min. These complexes represent the first examples that
metal-based initiators display such high activity in L-lactide
polymerization. These complexes showed moderate stereose-
lectivity for rac-lactide polymerization to give heterotactic-rich
polylactides. The MALDI-TOF mass spectrum indicates that
the end group of the polymer chain is a nitrile group, and
there is a high degree of transesterification side reaction
during the lactide polymerization initiated by these bimetallic
lanthanide amido complexes, giving rise to series of chains
separated by the mass of a lactic acid (72) rather than that of a
lactide unit (144). A 1H NMR monitoring reaction revealed that
the ring-opening polymerization of lactides proceeded via a
there is an isopropoxo group according to the resonances at
1.25 and 5.05 ppm, and no resonance was observed for the
β-diketiminate ligand, as shown in Fig. 7. Obviously, the
isopropoxo group existing in the oligomer must come from
2-propanol, which was introduced by the exchange reaction of
2-propanol with the (Me3Si)2N– groups. These results also indi-
cated that the (Me3Si)2N– groups initiated the polymerization
of L-lactide when using the bimetallic lanthanide amido com-
plexes as catalysts.
Complexes 1 and 4 also showed good activity for the ROP of
rac-lactide, and the preliminary results are listed in Table 4.
Complex 4 displays apparently higher activity in comparison
with complex 1. For example, complex 4 can initiate almost
complete polymerization of rac-lactide when the molar ratio of
monomer to initiator ([M]0/[I]0) is 10 000 within 30 min in
THF; whereas complex 1 gave 71% yield under the same
polymerization conditions (Table 4, entries 6 and 11). This
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Dalton Transactions
Table 4 Ring-opening polymerization of rac-lactide by complexes 1 and 4a
Entry
Initiator
[M]0/[I]0
t
Yield (%)b
Mcc (×104)
Mnd (×104)
PDId
Pre
1
2
3
4
5
6
7
8
1
1
1
1
1
1
4
4
4
4
4
4
600/1
1000/1
1400/1
2000/1
8000/1
10 000/1
600/1
1000/1
1400/1
2000/1
10 000/1
14 000/1
5 min
5 min
30 min
30 min
30 min
30 min
5 min
98
97
98
99
90
71
96
97
97
97
92
71
2.12
3.50
4.94
7.13
25.9
25.6
2.07
3.50
4.89
7.00
33.1
35.8
5.01
7.06
7.68
9.98
23.7
27.9
5.91
6.62
8.49
9.12
25.6
26.8
1.62
1.68
1.60
1.61
1.64
1.59
1.68
1.68
1.63
1.61
1.54
1.56
0.65
0.65
0.68
0.65
0.65
0.68
0.72
0.63
0.70
0.71
0.70
0.72
5 min
9
30 min
30 min
30 min
30 min
10
11
12
a General polymerization conditions: THF as the solvent, the temperature is 25 °C, [rac-LA] = 1 mol L−1 b Yield: weight of polymer obtained/
.
weight of monomer used. c Mc = (144.13) × [M]0/[I]0 × (polymer yield)/4. d Measured by GPC relative to poly-MMA standard. e Measured by
homonuclear decoupled 1H NMR spectroscopy at 20 °C in CDCl3.
coordination–insertion mechanism, and all of the four amido THF was used as an eluent at a flow rate of 1.0 mL min−1
groups in the bimetallic systems participated in the initiation at 40 °C.
reaction. These results provided new insights into the design
and synthesis of highly efficient lanthanide-based catalysts for
homogeneous catalysis. Further studies are ongoing to expand
Synthesis
PARA-{Nd[N(SiMe3)2]2}2 (1). To a THF solution of Nd-
[N(TMS)2]3(μ-Cl)Li(THF)3 (3.54 g, 4.00 mmol) was added a THF
solution of PARA-H2 (10 mL, 1.18 g, 2.00 mmol). The mixture
the synthesis and reactivity of lanthanide complexes based on
these ligand systems in our laboratory.
was stirred overnight at 25 °C, and then THF was evaporated
completely under reduced pressure. Toluene (35 mL) was
added to the residue, and the mixture was stirred at 80 °C for
about 12 h. The precipitate formed was removed by centrifu-
Experimental
General procedures
All of the manipulations were performed under an argon gation, and then the solvent was evaporated completely under
atmosphere, using the standard Schlenk techniques. HN reduced pressure again. Hexane (5 × 3 mL) was added to pre-
(TMS)2, L-lactide, rac-lactide, and n-BuLi are commercially cipitate the product. The powder obtained was dissolved in
available. HN(TMS)2 was dried over CaH2 for 4 days and dis- toluene and light purple crystals were obtained from a con-
tilled before use. Tetrahydrofuran (THF), toluene, and hexane centrated toluene solution (about 25 mL) at 25 °C in a
were dried and freed of oxygen by refluxing over sodium/ben- few days (2.60 g, 86%). Mp: 273–276 °C. Anal. calcd for
zophenone ketyl and distilled prior to use. L-Lactide and rac- C64H124N8Nd2Si8: C, 50.61; H, 8.23; N, 7.38. Found: C, 50.43;
lactide were sublimated under vacuum twice at 50 °C and then H, 8.56; N, 7.48. IR (KBr, cm−1): 3064(m), 2960(s), 2929(s),
recrystallized twice from dry toluene. The ligands PARA-H2
=
2870(s), 1625(s), 1554(s), 1510(s), 1460(s), 1435(s), 1381(s),
2[2,6-iPr2C6H3NHC(Me)C(H)C(Me)N]-( para-phenylene), 1363(m), 1278(s), 1255(s), 1025(w), 842(s), 754(m). The crystals
META-H2 = 2[2,6-iPr2C6H3NHC(Me)C(H)C(Me)N]-(meta-phen- suitable for an X-ray diffraction analysis were obtained by the
ylene),10 LiN(TMS)2, and Ln[N(TMS)2]3(μ-Cl)Li(THF)3 (Ln = Nd, slow cooling of a hot toluene solution.
Sm, Y)16 were prepared according to the published procedures.
PARA-{Sm[N(SiMe3)2]2}2 (2). The synthesis of complex 2 was
Carbon, hydrogen, and nitrogen analyses were performed by carried out in the same way as that described for complex 1,
direct combustion with a Carlo-Erba EA-1110 instrument. The but Sm[N(TMS)2]3(μ-Cl)Li(THF)3 (3.56 g, 2.00 mmol) was used
IR spectra were recorded with a Nicolet-550 FTIR spectrometer instead of Nd[N(TMS)2]3(μ-Cl)Li(THF)3. Light yellow crystals
1
as KBr pellets. The H NMR spectra were recorded in a C6D6 were obtained from concentrated toluene solution (about
solution for complex 3 with a Unity Varian-300 spectrometer, 23 mL) at 25 °C in a few days (2.72 g, 89%). Mp: 269–272 °C.
and in a CDCl3 solution for the oligomer. Because of their Anal. calcd for C64H124N8Si8Sm2: C, 50.20; H, 8.16; N, 7.32.
paramagnetism, no resolvable NMR spectrum for complexes 1, Found: C, 50.15; H, 8.17; N, 7.27. IR (KBr, cm−1): 3061(m),
2 and 4 was obtained. MALDI-TOF mass spectrum was 2960(s), 2870(s), 2891(m), 1625(s), 1554(s), 1510(s), 1460(s),
recorded using a Bruker Reflex II mass spectrometer (Bremen, 1436(s), 1381(s), 1363(m), 1278(m), 1257(m), 1178(w), 933(s),
Germany). The uncorrected melting points of crystalline 883(s), 754(m). The crystals suitable for an X-ray diffraction
samples in sealed capillaries (under argon) are reported as analysis were obtained by the slow cooling of a hot toluene
ranges. Molecular weight and molecular weight distribution solution.
(PDI) were determined against a poly-MMA standard by gel
PARA-{Y[N(SiMe3)2]2}2 (3). The synthesis of complex 3 was
permeation chromatography (GPC) on a PL 50 apparatus, and carried out in the same way as that described for complex 1,
2876 | Dalton Trans., 2013, 42, 2870–2878
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but Y[N(TMS)2]3(μ-Cl)Li(THF)3 (3.31 g, 2.00 mmol) was used Procedure for the oligomerization of L-lactide by complex 3 in
instead of Nd[N(TMS)2]3(μ-Cl)Li(THF)3. Light yellow crystals the presence of 4 equiv. of 2-propanol
were obtained from concentrated toluene solution (about
23 mL) at 25 °C in a few days (2.39 g, 85%). Mp: 274–276 °C.
In a glovebox, to a rapidly stirring solution of complex 3
(100 mg, 0.071 mmol) in toluene (1.5 mL) was added a solu-
Anal. calcd for C64H124N8Si8Y2: C, 54.59; H, 8.88; N, 7.96.
tion of iPrOH in toluene (0.28 mmol, 0.5 mL). After the
Found: C, 54.47; H, 9.01; N, 8.08. IR (KBr, cm−1): 3057(s), 3029(s),
mixture was stirred for about 5 min, L-lactide (51 mg,
2959(s), 2867(m), 1625(s), 1552(s), 1515(s), 1459(s), 1437(s),
0.35 mmol) dissolved in 1 mL toluene was added. The mixture
1384(s), 1364(s), 1277(m), 1173(m), 1103(w), 935(s), 882(s), 842(s),
was stirred vigorously for about 10 min, and was quenched by
758(m). 1H NMR (C6D6, 300 MHz): 7.20 (s, 4H, CHN-aryl),
the addition of ethanol containing 5% of HCl. The resulting
7.09 (s, 6H, CHN-aryl), 5.16 (s, 2H, CH3CNCH), 3.10 (m, 4H, CH
white powder was washed with a large amount of ethanol, and
(CH3)2), 1.98 (s, 6H, CH3CN), 1.68 (s, 6H, CH3CN), 1.35
dried under vacuum at 60 °C overnight.
(s, 12H, CH(CH3)2), 1.12 (s, 12H, CH(CH3)2), 0.32 (s, 72H,
TMS). 13C NMR (75 MHz, d8-THF): δ 2.74 (Si(CH3)3), 18.02
X-Ray crystallographic structure determinations
(CH3CN), 18.57 (CH3CN), 20.23 (CH(CH3)2), 21.81 (CH(CH3)2),
23.47 (CH(CH3)2), 26.45 (CH(CH3)2), 93.88 (CH3CNCH), 96.37
Suitable single crystals of complexes 1–4 were sealed in a thin-
walled glass capillary for determination the single-crystal struc-
(CH3CNCH), 120.88 (CHN-aryl), 121.31 (CHN-aryl), 122.42
tures. Intensity data were collected with a Rigaku Mercury CCD
(CHNaryl), 123.84 (CHNaryl), 126.21 (CHNaryl), 126.97 (CHNaryl),
area detector in ω scan mode using Mo Kα radiation (λ =
137.64 (CHN-aryl), 138.26 (CHN-aryl), 139.90 (CHNaryl), 141.43
0.71070 Å). The diffracted intensities were corrected for
(CHNaryl), 143.31 (CHNaryl), 144.38 (CHNaryl), 154.60 (Cq),
Lorentz/polarization effects and empirical absorption correc-
161.58 (Cq), 162.78 (Cq), 166.06 (Cq). The crystals suitable for
tions. Details of the intensity data collection and crystal data
an X-ray diffraction analysis were obtained by the slow cooling
are given in Table 1.
of a hot toluene solution.
The structures were solved by direct methods and refined
META-{Nd[N(SiMe3)2]2}2 (4). To a toluene solution of Nd
by full-matrix least-squares procedures based on |F|2. All of the
[N(TMS)2]3(μ-Cl)Li(THF)3 (3.26 g, 3.68 mmol) was added a
non-hydrogen atoms were refined anisotropically. The hydro-
toluene solution of META-H2 (12 mL, 1.09 g, 1.34 mmol). The
gen atoms in these complexes were all generated geometrically,
mixture was stirred at 90 °C for about 48 h, and then the pre-
assigned appropriate isotropic thermal parameters, and
cipitate was removed from the reaction mixture by centrifu-
allowed to ride on their parent carbon atoms. All of the hydro-
gation. Subsequently, the solvent was evaporated completely
gen atoms were held stationary and included in the structure
under reduced pressure. Hexane (8 × 3 mL) was added to pre-
factor calculation in the final stage of full-matrix least-squares
cipitate the product. The powder obtained has a moderate
refinement. The structures were solved and refined using
solubility in hexane and light purple crystals were obtained
SHELEXL-97 programs. The selected bond parameters are sum-
from a mixed solution of hexane and toluene (about 10 : 1) at
marized in Table 2.
25 °C in a few days (1.53 g, 51%). Mp: 221–223 °C. Anal. calcd
for C64H124N8Si8Nd2: C, 50.61; H, 8.23; N, 7.38. Found: C,
50.58; H, 8.32; N, 7.41. IR (KBr, cm−1): 3058(m), 2937(s), 2922
Acknowledgements
(s), 2865(s), 1623(s), 1547(s), 1521(s), 1463(s), 1438(s), 1380(s),
1361(m), 1280(s), 1253(s), 1022(w), 837(s), 758(m). The crystals
Financial support from the National Natural Science Foun-
suitable for an X-ray diffraction analysis were obtained by the
slow cooling of a mixed solution of hexane and toluene.
dation of China (Grants 20972108, 21132002, 21174095, and
21172165), PAPD, the Qing Lan Project, and the Natural
Science Foundation of the Jiangsu Higher Education Insti-
tutions (Project 10KJD150005) is gratefully acknowledged.
Typical procedure for the polymerization reaction
Notes and references
The procedures for the polymerization of lactide initiated by
complexes 1–4 were similar, and a typical polymerization pro-
cedure is given below. A 50 mL Schlenk flask, equipped with a
magnetic stirring bar, was charged with the desired amount of
L-lactide and toluene. The contents of the flask were then
stirred at 50 °C until L-lactide was dissolved, and then a solu-
tion of the initiator in toluene was added to this solution by
syringe. The mixture was stirred vigorously at 50 °C for the
desired time, during which time an increase in the viscosity
was observed. The reaction mixture was quenched by the
addition of ethanol and then poured into ethanol to precipi-
tate the polymer, which was dried under vacuum and weighed.
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