Synthesis of mono(guanidinate) rare earth metal bis(amide) complexes and their performance in the ring-opening polymerization of l-lactide and rac-lactide

Article abstract of DOI:10.1039/c2nj20925j

One-pot salt metathesis reaction of anhydrous LnCl₃ with one equivalent of guanidinate lithium [(SiMe₃)₂NC(NCy) ₂]Li (Cy = cyclohexyl), following the introduction of two equivalents of NaN(SiMe₃)₂ or LiN(SiHMe₂)₂ in THF at room temperature, afforded a family of mono(guanidinate) rare earth metal bis(amide) complexes [(SiMe₃)₂NC(NCy)₂] Ln[N(SiRMe₂)₂]₂(THF)_n (R = Me, n = 0, Ln = Nd (1), Lu (2), Y (3); R = H, n = 1, Ln = Y (4)) in 62-79% isolated yields. These complexes were characterized by elemental analysis, NMR spectroscopy (except for 1 for its strong paramagnetic property of Nd ³⁺), FT-IR spectroscopy, and X-ray single crystal diffraction. Single crystal structural determination revealed that the central metals in 1-3 are four-coordinated with a distorted tetrahedral geometry, while that in 4 is five-coordinated with a distorted trigonal bipyramidal geometry. These complexes could serve as highly active initiators for l-lactide polymerization in toluene at 50 °C. In addition, they also showed high activity towards rac-lactide polymerization in THF at room temperature, affording a moderate level of heterotactic polymers (P_r > 0.70). Employing 4 as an initiator, a controllable polymerization fashion was observed both in l-lactide polymerization and rac-lactide polymerization under certain conditions. The polymerization proceeded via a coordination-insertion mechanism verified experimentally by end group analysis of the oligomer. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012.

Full text of DOI:10.1039/c2nj20925j

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PAPER
Synthesis of mono(guanidinate) rare earth metal bis(amide) complexes
and their performance in the ring-opening polymerization of L-lactide and
rac-lactidew
Yibin Wang,ab Yunjie Luo,*a Jue Chen,a Hanming Xuea and Hongze Liangb
Received (in Victoria, Australia) 2nd November 2011, Accepted 5th January 2012
DOI: 10.1039/c2nj20925j
One-pot salt metathesis reaction of anhydrous LnCl3 with one equivalent of guanidinate lithium
[(SiMe3)2NC(NCy)2]Li (Cy = cyclohexyl), following the introduction of two equivalents of
NaN(SiMe3)2 or LiN(SiHMe2)2 in THF at room temperature, afforded a family of mono(guanidinate)
rare earth metal bis(amide) complexes [(SiMe3)2NC(NCy)2]Ln[N(SiRMe2)2]2(THF)n (R = Me, n = 0,
Ln = Nd (1), Lu (2), Y (3); R = H, n = 1, Ln = Y (4)) in 62–79% isolated yields. These complexes
were characterized by elemental analysis, NMR spectroscopy (except for 1 for its strong paramagnetic
property of Nd3+), FT-IR spectroscopy, and X-ray single crystal diffraction. Single crystal structural
determination revealed that the central metals in 1–3 are four-coordinated with a distorted tetrahedral
geometry, while that in 4 is five-coordinated with a distorted trigonal bipyramidal geometry. These
complexes could serve as highly active initiators for L-lactide polymerization in toluene at 50 1C.
In addition, they also showed high activity towards rac-lactide polymerization in THF at room
temperature, affording a moderate level of heterotactic polymers (Pr 4 0.70). Employing 4 as an
initiator, a controllable polymerization fashion was observed both in L-lactide polymerization and
rac-lactide polymerization under certain conditions. The polymerization proceeded via a
coordination–insertion mechanism verified experimentally by end group analysis of the oligomer.
Introduction
attributed to synthesis difficulty and ligand redistribution.
To date, only one example of mono(guanidinate) rare earth
metal bis(amide) complex [(SiMe3)2NC(NCy)2]La[N(SiMe3)2]2
prepared through insertion of La–N s bonds of La[N(SiMe3)2]3
into a Cy–NQCQN–Cy molecule has ever been reported.3j
However, as far as the catalytic behavior of the guanidinate
rare earth metal bis(amide) complexes is concerned, their
performance in polymerization still remains unexplored to
the best of our knowledge.
Non-Cp ancillary ligands have received intense interest in the
field of coordination and organometallic chemistry in recent
years, and guanidinate anions [R2NC(NR0)2]ꢀ are assumed to be
one of the ideal isoelectronic alternatives to Cp anions because
their steric and electronic properties can be easily tailed by tuning
the organic substituents on nitrogen atoms.1 Compared to the
application of guanidinate frameworks in main group and
d-block transition metals, the employment of such monoanionic
bidentate ligands in rare earth metals was only introduced very
recently.1a Most of the guanidinate-ligated rare earth metal
complexes reported so far are homoleptic2 and bis-guanidinate-
ligated species,3 while neutral mono-guanidinate-ligated rare
earth metal complexes are still rather scarce.3b,j,4 This may be
On the other hand, polylactide (PLA) is a biodegradable
and biocompatible synthetic macromolecule owing to its
mechanical and physical properties suitable for a variety of
applications such as sutures, product packaging, and artificial
tissues.5 The most effective method to prepare PLA is the ring-
opening polymerization (ROP) of lactides using metal-based
initiators.6 Some rare earth metal complexes were found to be
active for lactide polymerization.7 However, the application of rare
earth metal complexes with the formula LLnR2 (L = monoanionic
ancillary ligand, R = alkyl, amide, etc.), which are considered to
possess much open coordination spheres around rare earth metals
and are expected to display unique characters for polymerization,8
in lactide polymerization is relatively limited,3j,9 and the examples
of lactide polymerization initiated solely by rare earth metal
bis(amide) complexes are rather scarce.9a,b,f,g It is also noteworthy
that in the reported lactide polymerization cases, the initial feed
a Organometallic Chemistry Laboratory, Ningbo Institute of Technology,
Zhejiang University, Ningbo 315100, P. R. China.
E-mail: lyj@nit.zju.edu.cn; Fax: +86-574-88130130;
Tel: +86-574-88130085
b School of Materials Science & Chemical Engineering,
Ningbo University, Ningbo, Zhejiang 315211, P. R. China
w Electronic supplementary information (ESI) available: 1H NMR
spectrum of 4, plot of the degree of L-LA polymerization vs. [L-LA]/[4],
1
and H NMR spectrum of an L-lactide oligomer using 4/iPrOH as an
initiator. CCDC reference numbers 803565–803567 and 816943. For
ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c2nj20925j
c
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New J. Chem., 2012, 36, 933–940 933

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molar ratio of [LA]0/[Ln]0 promoted by rare earth metal
complexes was always less than a few thousands.5a
of the N–C–N linkage was confirmed by the stretching of C–N
1
bonds at approximately 1630 cmꢀ1. Room-temperature H and
In our continuous study on the lactide polymerization
initiated by rare earth metal complexes, we found that ligand
scaffolds at the rare earth metals affected dramatically their
reactivity.7b,d,9c,10 Herein we would like to report the synthesis
and characterization of a family of neutral and mononuclear rare
earth metal bis(amide) complexes bearing N,N0-dicyclohexyl-N00-
bis(trimethylsilyl) guanidinate ligand [(SiMe3)2NC(NCy)2]ꢀ. These
structural discrete complexes could act as single-component
initiators for L-lactide and rac-lactide polymerization with
high activity, and under some conditions, in a controllable
polymerization fashion.
13C NMR spectra of 2–3 in C6D6 (except for 1 for its strong
paramagnetic property of Nd3+) showed two singlets for the two
types of SiMe3 groups, suggesting a high symmetry of the
molecules as observed in [(SiMe3)2NC(NCy)2]La[N(SiMe3)2]2.3j
In the case of 4, a doublet resonance at 0.41 ppm (J = 3.2 Hz)
in an 1H NMR spectrum should correspond to the methyl
protons of N(SiHMe2)2 groups, while the signal of an SiH
proton appeared at 5.12 ppm.
Single crystals of 1–4 suitable for X-ray diffraction were
grown from a mixture solution of hexane and toluene at ꢀ30 1C.
Since 1, 2 and 3 are isomorphous, only the molecular structures
of 1 and 4 are shown in Fig. 1 and 2, respectively. The
crystallographic data are summarized in Table 1, and the selected
bond distances and angles are listed in Table 2. The central
metal Ln3+ in 1–3 is four-coordinated by one bidentate
guanidinate ligand through two nitrogen atoms and two
nitrogen atoms from two amide groups, forming a distorted
Results and discussion
Synthesis and characterization of mono(guanidinate) rare earth
metal bis(amide) complexes
Different from the procedure adopted by Arnold and co-workers
to prepare the first mono(guanidinate) rare earth metal
bis(amide) complex [(SiMe3)2NC(NCy)2]La[N(SiMe3)2]2,3j we
managed to employ a salt metathesis strategy to generate the
aimed mono(guanidinate) rare earth metal bis(amide) complexes.
One-pot salt metathesis reaction of anhydrous LnCl3 with one
equivalent of guanidinate lithium [(SiMe3)2NC(NCy)2]Li (formed
in situ by treatment of Cy–NQCQN–Cy with one equivalent of
(SiMe3)2NLi in hexane at room temperature), following addition
of two equivalents of NaN(SiMe3)2 or LiN(SiHMe2)2 in THF at
room temperature, after workup, afforded the corresponding
mono(guanidinate) rare earth metal bis(amide) complexes
[(SiMe3)2NC(NCy)2]Ln[N(SiRMe2)2]2(THF)n (R = Me, n = 0,
Ln = Nd (1), Lu (2), Y (3); R = H, n = 1, Ln = Y (4)) in
62–79% isolated yields, as shown in Scheme 1.
Although 1–4 were prepared from salt metathetical reaction,
they were isolated as neutral and mononuclear species. These
complexes were thermally stable at ambient temperature in a
glovebox. They were soluble in THF, toluene, diethyl ether,
and even in aliphatic solvents such as hexane and pentane.
The formulation of these complexes was supported by elemental
analysis and FT-IR spectroscopy. The delocalized double bond
Fig. 1 Molecular structure of 1 with thermal ellipsoids at 10%
probability. Hydrogen atoms have been omitted for clarity.
Scheme 1 Preparation of the mono(guanidinate) rare earth metal bis(amide) complexes.
c
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934 New J. Chem., 2012, 36, 933–940

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Table 2 Selected bond distances and bond angles for 1–4
1
2
3
4
Bond distances/A
N1–C1
N2–C1
Ln–N1
Ln–N2
1.342(6)
1.339(5)
1.334(5)
2.274(3)
2.271(3)
2.189(3)
2.199(3)
1.328(7)
1.332(7)
2.314(4)
2.313(5)
2.225(5)
2.237(5)
1.344(5)
1.339(5)
2.356(4)
2.328(3)
2.241(4)
2.278(4)
1.337(6)
2.413(4)
2.423(4)
2.314(4)
2.322(4)
Ln–N4
Ln–N5
Bond angles/1
N1–C1–N2
N1–Ln–N2
N4–Ln–N5
113.8(4)
55.29(14)
119.13(16)
114.2(4)
59.18(12)
117.91(14)
114.1(5)
57.70(17)
118.44(18)
114.3(4)
57.54(12)
119.47(14)
for 2, 68.781; for 3, 66.751). This orientation minimizes the
interaction between the SiHMe2 groups and the cyclohexyl group
of the guanidinate ligand. The Y–N(SiHMe2)2 distances and the
Y–N(guanidinate) distances range from 2.241(4) to 2.278(4) A
and from 2.356(4) to 2.328(3) A, respectively. These values are
identical to those of [(SiMe3)2NC(NCy)2]La[N(SiMe3)2]2,3j if
the difference of the effective ionic radii is considered.11 The
bite angle of N1–Y–N2 in 4 (57.54(12)1) is much larger
than that in [(SiMe3)2NC(NCy)2]La[N(SiMe3)2]2 (53.86(9)1),3j
whilst the bond angle of N4–Y–N5 (119.47(14)1) is close to that
in [(SiMe3)2NC(NCy)2]La[N(SiMe3)2]2 (120.2(1)1).3j
Fig. 2 Molecular structure of 4 with thermal ellipsoids at 20%
probability. Hydrogen atoms (except for Si–H) have been omitted
for clarity.
tetrahedral geometry. In comparison, owing to sterically less
demanding N(SiHMe2)2 group, the central metal Ln3+ 4 is five-
coordinated by one guanidinate ligand through two nitrogen
atoms, two nitrogen atoms from two amide groups, and one
oxygen atom from a THF molecule to adopt a distorted
trigonal bipyramidal geometry. Since the structural features of
these complexes are similar, only the structure of 4 is discussed
in detail herein. It is found that the plane formed by Si1N3Si2
of an N(SiMe3)2 group on the central carbon atom of the NCN
moiety and the plane of N2C1N1Y1 are not co-planar (the
dihedral angle is 85.621 (for 1, 78.781; for 2, 75.041; for 3,
76.741)), this militates against the p-overlap between these two
moieties. The dihedral angle of the plane of N1C1N2Y1
and the plane formed by N4Y1N5 is 79.591 (for 1, 65.411;
Lactide polymerization
In order to assess the polymerization behavior of these neutral
mono(guanidinate) rare earth metal bis(amide) complexes, 1–4
were employed firstly as initiators for the ring-opening poly-
merization of L-lactide. As illustrated in Table 3, all these
neutral complexes alone could serve as single-component
initiators for L-lactide polymerization in toluene at 50 1C,
with an activity trend of 4 4 3 4 2 4 1. Remarkably, these
complexes displayed high activity. For instance, using 4 as an
initiator, the conversion could reach up to 93% in two minutes
Table 1 Details of the crystallographic data and refinements for 1–4
1
2
3
4
Formula
FW
T/K
Crystal system
Space group
a/A
b/A
c/A
C
31H76N5NdSi6
C31H76LuN5Si6
862.48
293(2)
Monoclinic
P21/c
9.6391(14)
24.143(4)
20.157(3)
90
91.219(3)
90
4689.8(11)
4
1.222
2.283
1808
C31H76N5Si6Y
776.42
293(2)
Monoclinic
P21/c
9.5883(16)
24.230(4)
20.194(3)
90
91.177(5)
90
4690.7(13)
4
1.099
1.421
1680
C31H76N5OSi6Y
792.42
223(2)
Triclinic
%
P1
12.0258(13)
19.1155(19)
20.642(2)
75.965(5)
87.006(6)
85.555(6)
4587.0(8)
4
831.75
293(2)
Monoclinic
P21/c
9.5611(10)
24.392(3)
20.327(2)
90
91.353(3)
90
4739.3(8)
4
1.166
1.271
1764
0.80 ꢂ 0.60 ꢂ 0.40
32 618
8530
407
1.120
0.0543
0.1071
a/1
b/1
g/1
V/A3
Z
Dcalc/gꢁcmꢀ3
1.147
1.456
m/mmꢀ1
F (000)
1712
0.80 ꢂ 0.50 ꢂ 0.20
Crystal size/mm
Reflections collected
Independent reflections
No. of variables
GOF on F2
R [I 4 2s(I)]
WR
0.80 ꢂ 0.50 ꢂ 0.30
0.60 ꢂ 0.60 ꢂ 0.50
43 812
8565
44 680
8574
39 101
16 946
407
1.173
0.0419
0.0843
407
1.272
0.1070
0.1692
854
1.141
0.0753
0.1266
c
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Table 3 L-Lactide polymerization by 1–4a
observed in the molar ratio range of [LA]0/[Y]0 from 500 to
2000 with 4 as the initiator, as the molecular weights of the
resulting polymers increased almost linearly with the increase
in the molar ratio of monomer-to-initiator while the molecular
weight distributions kept nearly constant (Table 3, runs 8–11).
Furthermore, all these rare earth metal bis(amide) complexes
were also found to be active for the ring-opening polymeriza-
tion of rac-lactide at room temperature in various organic
solvents, as shown in Table 4. Again, 4 demonstrated the
highest activity (Table 4, runs 1–4). Microstructural analysis
of the resulting polymers by homonuclear decoupled 1H NMR
spectroscopy revealed that all these complexes exerted a
moderate level of heterotacticity (Pr 4 0.70).13 Polymerization
solvent affected not only the polymerization activity, but also the
level of stereo-selectivity. For example, using 4 as the initiator in
THF, 1000 equiv. of rac-lactide could be consumed completely in
one hour to produce polymer with Pr = 0.73 (Table 4, run 5).
In comparison, when the polymerization was performed in
toluene, only 90% yield of the polymer with Pr = 0.60 was
obtained even when the polymerization was prolonged to 9 hours
(Table 4, run 9). Notably, a controllable polymerization of
rac-lactide was also observed with 4 when the molar ratio of
[LA]0/[Y]0 was in the range from 500 to 4000 (Table 4, runs 4–7).
Despite the two amide groups bonded to each rare earth
metal in these complexes, GPC curves of the resulting polymers
were all unimodal, indicative of an equivalent polymerization
behavior of the two amide groups (see Fig. S2 in ESIw). To gain
some insights into the polymerization mechanism with these rare
earth metal bis(amide) complexes, oligomerization of L-lactide
by 4 at [LA]0/[Y]0 = 7 in toluene at room temperature was
carried out. No signal for the guanidinate ligand was observed
in the 1H NMR spectrum of the resulting oligomer, suggesting
that the ancillary ligand was not engaged in the initiation stage
(Fig. 3). However, analysis of the oligomer revealed that,
[M]/ t/ Conv.b TOF ꢂ Mn,calcd Mn,obsd Mw/
c
d
10ꢀ4/hꢀ1 ꢂ 10ꢀ4 ꢂ 10ꢀ4 Mn
d
Run Initiator [Ln] min (%)
1
2
3
4
5
6
7
8
1
2
2
2
3
3
3
4
4
4
4
4
4
4
4
4000 2
4000 2
6000 2
10 000 2
4000 2
6000 2
10 000 2
500 2
1000 2
1500 2
2000 2
3000 2
4000 2
6000 2
10 000 2
79
100
93
72
100
96
9.5
12.0
16.7
21.6
12.0
17.3
24.6
1.5
3.0
4.5
6.0
9.0
22.75
28.80
40.20
51.80
28.80
41.47
59.04
3.60
14.84
17.62
14.07
12.91
13.87
19.83
13.13
4.13
1.67
1.62
1.71
1.76
1.52
1.66
1.72
1.65
1.55
1.58
1.61
1.70
1.58
1.63
1.62
82
100
100
100
100
100
100
100
93
9
7.20
7.94
10
11
12
13
14
15
10.80
14.40
21.60
28.80
43.20
66.96
11.43
13.86
11.41
16.28
19.40
17.51
12.0
18.0
27.9
a
Polymerization conditions: [M] = 1.0 M, in toluene, 50 1C.
Conv. = conversion. Mn of PLA calculated from Mn,calcd =
b
c
d
144 ꢂ ([LA]/2[Ln]) ꢂ Conv. (LA). Determined by GPC at 40 1C
in THF relative to polystyrene standards; corrected by the Mark–
Houwink equation [Mn,obsd = 0.58Mn (GPC)].
even when the molar ratio of [LA]0/[Y]0 was increased up to
10 000. This represents the first example that displays such
high activity in lactide polymerization promoted by rare earth
metal-based initiators, and its activity can compete with the
most active metal-based initiator reported so far.12 It is also
noteworthy that, a controllable polymerization fashion was
Table 4 rac-Lactide polymerization by 1–4a
Conv.b (%)
TOF/hꢀ1
Mn,calcd ꢂ 10ꢀ4
Mn,obsd ꢂ 10ꢀ4
Mw/Mn
Pre
c
d
d
Run
Initiator
[M]/[Ln]
Solvent
t/h
1
2
3
4
5
6
1
2
3
4
4
4
4
4
4
4
500
500
500
500
1000
2000
4000
1000
1000
1000
THF
THF
THF
THF
THF
THF
THF
THF
1
1
1
1
1
1
1
0.25
9
90
58
51
100
100
100
88
100
90
450
290
255
500
1000
2000
3520
4000
100
3.24
2.09
1.84
3.60
7.20
14.40
25.34
7.20
6.48
6.05
5.33
15.26
12.15
4.13
1.54
1.69
1.80
1.65
1.65
1.70
1.72
1.91
1.39
1.86
0.72
0.77
0.78
0.76
0.73
n.d.g
n.d.g
0.63
0.60
0.58
6.86
13.91
24.72
5.66
4.89
13.97
7
8f
9
Toluene
CH2Cl2
10
1
84
840
a
b
c
Polymerization conditions: [M] = 1.0 mol Lꢀ1, 25 1C. Conv. = conversion. Mn of PLA calculated from Mn,calcd = 144 ꢂ ([LA]/2[Ln]) ꢂ
d
Conv. (LA). Determined by GPC at 40 1C in THF relative to polystyrene standards; corrected by the Mark–Houwink equation [Mn,obsd =
0.58Mn (GPC)]. Pr is the probability of racemic linkages between monomer units determined from the methine region of the homonuclear
e
1
f
g
decoupled H NMR spectrum at 20 1C in CDCl3. Performed in THF at 60 1C. n.d.: not determined.
c
936 New J. Chem., 2012, 36, 933–940
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Fig. 3 1H NMR spectrum (400 MHz, CDCl3) of an L-lactide oligomer using 4 as an initiator after quenching with ethanol (*, ethanol and
monomer signals). Polymerization conditions: [LA]0/[Y]0 = 7, in toluene, 20 min, 20 1C.
besides the resonances at 4.35, 2.69, and 1.49 ppm for the end
group of HOCH(CH3)CO–, a set of signals in the region of
0.3–0.1 ppm and a multiplet signal at 4.55 ppm could be
assigned to the N(SiHMe2)2 group. However, these signals
disappeared when the oligomerization was carried out in the
presence of 2-propanol (see Fig. S3 in ESIw). In the MALDI-TOF
mass spectrum (Fig. 4), oligomers with the N(SiHMe2)2 end
cap could also be found. These results showed that the
silylamido groups acted as the initiating group in the ring-
opening polymerization of L-lactide with these rare earth metal
bis(amide) complexes, as illustrated in Scheme 2. Remarkably,
the MALDI-TOF mass spectrum also showed that the intra-
molecular transesterification took place extensively to afford
cyclic oligomers. The small amount of oligomers with the
carboxylic acid end might stem from the hydrolysis of
H–[OCH(CH3)CO]n–N(SiHMe2)2.
Fig. 4 MALDI-TOF mass spectrum of an L-lactide oligomer using 4 as an initiator (doped with NaI).
c
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Scheme 2 Proposed mechanism scenario of L-lactide polymerization initiated by mono(guanidinate) rare earth metal bis(amide) complexes.
benzophenone ketyl, degassed by the freeze–pump–thaw
Conclusions
method, and dried over fresh Na chips in a glovebox. Anhydrous
In summary,
a family of neutral and mononuclear
LnCl3 was purchased from STREM. n-BuLi (1.0 M in hexane
solution), HN(SiMe3)2, HN(SiHMe2)2, and carbodiimide
(Cy–NQCQN–Cy) were purchased from Acros, and used as
received. L-Lactide and rac-lactide (99.5%) were obtained
from Shenzhen Brightchina Industrial Co., Ltd. and were
recrystallized from hot toluene. Deuterated solvents (CDCl3,
C6D6) were obtained from CIL.
mono(guanidinate) rare earth metal bis(amide) complexes
[(SiMe3)2NC(NCy)2]Ln[N(SiRMe2)2]2(THF)n (R = Me, n = 0,
Ln = Nd (1), Lu (2), Y (3); R = H, n = 1, Ln = Y (4)) was
prepared by one-pot salt metathesis reaction of anhydrous LnCl3,
guanidinate lithium [(SiMe3)2NC(NCy)2]Li, and NaN(SiMe3)2 or
LiN(SiHMe2)2 in 1 : 1 : 2 molar ratio in THF at room temperature.
No ligand redistribution and ‘‘ate’’ complexes were observed in the
preparation process. All these mono(guanidinate) rare earth
metal bis(amide) complexes alone showed high activity towards
L-lactide polymerization. The polymerization could complete in
few minutes in toluene at 50 1C even when the molar ratio of
[LA]0/[Ln]0 reached up to 10 000, and afforded polymers with
unimodal molecular weight distributions. Furthermore, these
complexes were also active for rac-lactide polymerization to
produce polymers with a moderate level of heterotacticity.
Employing 4 as an initiator, a controllable polymerization
fashion was observed both in L-lactide polymerization and
rac-lactide polymerization under certain conditions. End group
analysis of the oligomer revealed that the ring-opening poly-
merization proceeded via a coordination–insertion mechanism.
Samples of organo rare earth metal complexes for NMR
spectroscopic measurements were prepared in the glovebox
using J. Young valve NMR tubes. NMR (1H, 13C) spectra
were recorded on a Bruker AVANCE III spectrometer at
25 1C. Carbon, hydrogen, and nitrogen analyses were performed
by direct combustion on a Carlo-Erba EA-1110 instrument,
quoted data are the average of at least two independent
determinations. FT-IR spectra were recorded on a Bruker
TENSOR 27 spectrometer. Molecular weight and molecular
weight distribution of the polymers were measured by PL GPC
50 with two mixed-B columns at 40 1C using THF as an eluent
against polystyrene standards, flow rate: 1 mL minꢀ1, sample
concentration: 1 mg mLꢀ1
.
Synthesis of [(SiMe3)2NC(NCy)2]Nd[N(SiMe3)2]2 (1)
Experimental part
All reactions were carried out at room temperature. n-BuLi
(1.0 mL, 1.0 mmol, 1.0 M in hexane) was added drop by drop
to a hexane solution (30 mL) of HN(SiMe3)2 (0.161 g, 1.0 mmol).
After 20 min, the resulting solution (containing LiN(SiMe3)2
formed in situ) was added slowly to a hexane solution of
Cy–NQCQN–Cy (0.206 g, 1.0 mmol). The reaction mixture
was stirred for 0.5 h, and then was added slowly to a THF slurry
Materials and procedures
All manipulations were performed under pure argon with
rigorous exclusion of air and moisture using the standard
Schlenk techniques and argon-filled glovebox. Solvents
(toluene, hexane, and THF) were distilled from sodium/
c
938 New J. Chem., 2012, 36, 933–940
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of NdCl3 (0.251 g, 1.0 mmol). After the resulting clear blue
solution was stirred for 15 min, NaN(SiMe3)2 (1.2 mL,
2.0 mmol, 1.67 M in THF solution) was added via a pipette,
and the reaction mixture was kept for 24 h. Removal of the
volatiles under the reduced pressure gave a blue residue, which
was washed by hexane, and was extracted by toluene (2 ꢂ 10 mL).
Drying up the solvents afforded the aimed product as a blue
powder (0.66 g, 79%). Recrystallization from a mixture solution of
hexane and toluene gave 1 as blue crystals. FT-IR (KBr, cmꢀ1):
2930 (s), 2854 (s), 1634 (s), 1451 (m), 1252 (m), 842 (s). Anal.
calcd for C31H76N5NdSi6: C, 44.76; H, 9.21; N, 8.42. Found:
C, 44.65; H, 9.07; N, 8.63%.
1.83 (d, J = 13.2 Hz, 4H, Cy-CH2), 1.93 (d, J = 11.6 Hz, 4H,
Cy-CH2), 3.39 (m, 2H, Cy-H), 4.01 (m, 4H, THF-a-CH2), 5.12
(m, 4H, N(SiHMe2)2). 13C NMR (100 MHz, C6D6): d 2.7
(CNSiMe3), 3.9 (YNSiHMe2), 25.5 (THF-b-CH2), 26.4, 26.5,
37.5 (Cy-CH2), 54.9 (Cy-CH), 71.2 (THF-a-CH2), 167.9
(NCN). FT-IR (KBr, cmꢀ1): 2932 (s), 2852 (s), 1634 (m),
1451 (s), 1345 (m), 1252 (s), 1194 (m), 1139 (m), 1049 (s), 944 (s).
Anal. calcd for C31H76N5OSi6Y: C, 47.00; H, 9.67; N, 8.84.
Found: C, 46.81; H, 9.49; N, 9.02%.
Typical procedure for the polymerization of L-lactide
The procedures for polymerization initiated by these complexes
were similar, and a typical polymerization procedure is given
below. In a glovebox, the desired amount of toluene and
L-lactide was mixed in a 50 mL Schlenk flask equipped with
a magnetic stirring bar. The flask was taken out from the
glovebox and attached to a well-purged argon Schlenk line.
The flask was placed in an oil bath with a temperature of
50 1C. After L-lactide was dissolved, toluene solution of the
rare earth metal bis(amide) complex was quickly injected into the
flask via a syringe. The mixture was stirred vigorously at 50 1C
for the predetermined time, during which an increase in viscosity
was observed. The polymerization was terminated by addition of
ethanol, and then a large amount of ethanol was poured into the
flask to precipitate the polymer. The resulting polymer was dried
under vacuum at 60 1C overnight and weighted.
Synthesis of [(SiMe3)2NC(NCy)2]Lu[N(SiMe3)2]2 (2)
2 was prepared by a procedure similar to that of 1. Using
n-BuLi (1.0 mL, 1.0 mmol, 1.0 M in hexane), HN(SiMe3)2
(0.161 g, 1.0 mmol), Cy–NQCQN–Cy (0.206 g, 1.0 mmol), LuCl3
(0.281 g, 1.0 mmol) and NaN(SiMe3)2 (1.2 mL, 2.0 mmol, 1.67 M
in THF solution), 2 was isolated as a white powder (0.53 g, 62%).
Recrystallization from a mixture solution of hexane and
toluene gave 2 as colorless crystals. 1H NMR (400 MHz,
C6D6): d 0.23 (s, 18H, CNSiMe3), 0.46 (s, 36H, YNSiMe3),
1.16 (m, 6H, Cy-CH2), 1.59 (m, 6H, Cy-CH2), 1.76 (d, 4H,
Cy-CH2), 1.89 (d, 4H, Cy-CH2), 3.48 (m, 2H, Cy-CH). 13C NMR
(100 MHz, C6D6): d 2.8 (CNSiMe3), 5.5 (YNSiMe3), 26.2, 26.3,
37.9 (Cy-CH2), 55.5 (Cy-CH), 169.4 (NCN). FT-IR (KBr, cmꢀ1):
2929 (s), 2853 (s), 1635 (s), 1451 (m), 1252 (s), 955 (s), 841 (s).
Anal. calcd for C31H76LuN5Si6: C, 43.17; H, 8.88; N, 8.12.
Found: C, 43.05; H, 8.62; N, 8.53%.
Typical procedure for the polymerization of rac-lactide
The procedures for polymerization initiated by these complexes
were similar, and a typical polymerization procedure is given
below. In a glovebox, the desired amount of solvent and
D,L-lactide was mixed in a 50 mL Schlenk flask equipped with
a magnetic stirring bar. After D,L-lactide was completely
dissolved, toluene solution of the rare earth metal bis(amide)
complex was quickly added into the flask via a pipette. The
mixture was stirred vigorously at ambient temperature for the
predetermined time, during which an increase in viscosity was
observed. The polymerization was terminated by addition of
ethanol, and then a large amount of ethanol was poured into
the flask to precipitate the polymer. The resulting polymer was
dried under vacuum at 60 1C overnight and weighted.
Synthesis of [(SiMe3)2NC(NCy)2]Y[N(SiMe3)2]2 (3)
3 was prepared by a procedure similar to that of 1. Using
n-BuLi (1.0 mL, 1.0 mmol, 1.0 M in hexane), HN(SiMe3)2
(0.161 g, 1.0 mmol), Cy–NQCQN–Cy (0.206 g, 1.0 mmol),
YCl3 (0.195 g, 1.0 mmol) and NaN(SiMe3)2 (1.2 mL, 2.0 mmol,
1.67 M in THF solution), 3 was isolated as a white powder
(0.54 g, 70%). Recrystallization from a mixture solution of hexane
1
and toluene gave 3 as colorless crystals. H NMR (400 MHz,
C6D6): d 0.24 (s, 18H, CNSiMe3), 0.43 (s, 36H, YNSiMe3), 1.20
(m, 6H, Cy-CH2), 1.53 (m, 6H, Cy-CH2), 1.77 (d, 4H, Cy-CH2),
1.89 (d, 4H, Cy-CH2), 3.34 (m, 2H, Cy-CH). 13C NMR
(100 MHz, C6D6): d 2.8 (CNSiMe3), 5.2 (YNSiMe3), 26.2,
26.3, 38.2 (Cy-CH2), 55.3 (Cy-CH), 169.6 (NCN). FT-IR
(KBr, cmꢀ1): 2930 (s), 2854 (s), 1634 (s), 1451 (m), 1252 (s),
955 (m), 842 (s). Anal. calcd for C31H76N5Si6Y: C, 47.95;
H, 9.87; N, 9.02. Found: C, 47.74; H, 9.58; N, 9.47%.
Procedure for the oligomerization of L-lactide by 4
In a glovebox, 50 mg of L-lactide and 40 mg of 4 were dissolved
in 1 mL of toluene at 20 1C. The mixture was stirred vigorously
for 20 min, and was quenched by the addition of ethanol
containing 5% of HCl. The resulting white powder was washed
with a large amount of ethanol, and dried under vacuum at
60 1C overnight (20 mg).
Synthesis of [(SiMe3)2NC(NCy)2]Y[N(SiHMe2)2]2(THF) (4)
4 was prepared by a procedure similar to that of 1. Using
n-BuLi (1.0 mL, 1.0 mmol, 1.0 M in hexane), HN(SiMe3)2
(0.161 g, 1.0 mmol), Cy–NQCQN–Cy (0.206 g, 1.0 mmol),
YCl3 (0.195 g, 1.0 mmol) and LiN(SiHMe2)2 (0.276 g, 2.0 mmol,
prepared from n-BuLi and HN(SiHMe2)2 in hexane in 1 : 1
molar ratio), 4 was isolated as a white powder. Recrystallization
from a solution of hexane gave 4 as colorless crystals (0.54 g,
68%). 1H NMR (400 MHz, C6D6): d 0.30 (s, 18H, N(SiMe3)2),
0.41 (d, J = 3.2 Hz, 24H, N(SiHMe2)2), 1.23 (m, 6H,
Cy-CH2), 1.38 (m, 4H, THF-b-CH2), 1.62 (m, 6H, Cy-CH2),
Crystal structural determination
Suitable single crystals of the complexes were sealed in a thin-
walled glass capillary for structural determination. Intensity
data were collected with a Rigaku Mercury CCD area detector in
o scan mode using Mo Ka radiation (l = 0.71070 A). The
diffracted intensities were corrected for Lorentz polarization
effects and empirical absorption corrections. The structures were
solved by direct methods and refined by full-matrix least-squares
c
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New J. Chem., 2012, 36, 933–940 939

View Article Online
procedures based on |F|2. All the non-hydrogen atoms were
refined anisotropically. The structures were solved and
refined using SHELEXL-97 program. CCDC 803565–803567,
and 816943 contains the supplementary crystallographic data for
1–4, respectively.
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This work was partly financially supported by the National
Natural Science Foundation of China (21172195), Science and
Technology Department of Zhejiang Province (2010R10020),
and State Key Laboratory of Rare Earth Resource Utilization
(RERU2011020). The authors are grateful to Mr. Yong
Zhang of Suzhou University for help with X-ray analysis.
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