Synthesis and characterization of rare-earth metal guanidinates stabilized by amine-bridged bis(phenolate) ligands and their application in the controlled polymerization of rac-lactide and rac-β-butyrolactone

Article abstract of DOI:10.1039/c5ra10151d

Eight rare-earth metal guanidinates supported by a versatile family of chelating amine-bridged bis(phenolate) ligands were synthesized. Metathesis reactions of rare-earth metal chlorides [LnClL¹(THF)] stabilized by amine-bridged bis(phenolate)

Full text of DOI:10.1039/c5ra10151d

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Synthesis and characterization of rare-earth metal
guanidinates stabilized by amine-bridged
bis(phenolate) ligands and their application in the
controlled polymerization of rac-lactide and rac-b-
butyrolactone†
Cite this: RSC Adv., 2015, 5, 53161
*
*
*
Tinghua Zeng, Qinqin Qian, Bei Zhao, Dan Yuan, Yingming Yao and Qi Shen
Eight rare-earth metal guanidinates supported by
a versatile family of chelating amine-bridged
bis(phenolate) ligands were synthesized. Metathesis reactions of rare-earth metal chlorides [LnClL1(THF)]
stabilized by amine-bridged bis(phenolate) ligand L1 with in situ generated lithium guanidinates in a 1 : 1
molar ratio in THF afforded ytterbium guanidinates YbL1 [R2NC(NR1)2] [R1 ¼ –iPr, R2N ¼ –NiPr2 (1),
–N(CH2)5 (2)]. Insertion reactions of the yttrium amides bearing bridged bis(phenolate) ligands with 1
equiv of N,N0-diisopropylcarbodiimide (DIC) yielded six yttrium guanidinates YL1 [(SiHMe2)2NC(NiPr)2] (3),
YL2[(SiHMe2)2NC(NiPr)2](THF) (4), YL3[(SiHMe2)2NC(NiPr)2] (5), YL4[(SiHMe2)2NC(NiPr)2] (6), YL5
[(SiHMe2)2NC(NiPr)2] (7), YL6[(SiHMe2)2NC(NiPr)2] (8), respectively. The behaviors of complexes 1–8 in the
polymerization of rac-lactide (LA) and rac-b-butyrolactone (BBL) were also explored. It was found that
complexes 1–8 efficiently initiated the ring-opening polymerization (ROP) of rac-LA and rac-BBL in a
controlled manner, providing highly heterotactic polylactide (Pr up to 0.99) and highly syndiotactic
poly(3-hydroxybutyrate) (Pr up to 0.82). The framework of the bridge played a significant role in
governing the stereoselectivity, while guanidinate groups work as initiating groups.
Received 5th May 2015
Accepted 9th June 2015
DOI: 10.1039/c5ra10151d
www.rsc.org/advances
Amine bridged bis(phenolate)s, as one type of dianionic
chelating ligands, have received considerable attention in rare-
Introduction
earth metal chemistry due to their easy availability, highly
tunable features and outstanding capacity to stabilize metal
centers. Rare-earth metal complexes stabilized by amine
bridged bis(phenolate) ligands have been reported to be effi-
cient initiators for the ROP of cyclic esters, yielding polymers in
high yields.9,11–18,21–27,29–32 Recently, our group has reported a
series of rare-earth metal complexes supported by bridged
bis(phenolate) ligands.21–26,29–36 Lanthanide aryloxides and
alkoxides stabilized by amine-bridged bis(phenolate) ligands
are found to be efficient initiators for the controlled polymeri-
zation of rac-lactide (LA) and rac-b-butyrolactone (BBL).22–25
Moreover, group IV metal complexes bearing an amine-bridged
bis(phenolato) ligand are highly efficient in catalyzing regiose-
lective intermolecular hydroamination reactions.37
With the growing global concern over sustainability, the prep-
aration of biodegradable polymers such as poly(lactic acid)
(PLA) and poly(3-hydroxybutyrate) (PHB) has become increas-
ingly important and opened commercial prospects for a range
of applications in the pharmaceutical, biomedical, and envi-
ronmental elds.1–6 The most straightforward route to prepare
PLA and PHB is the ring-opening polymerization (ROP) of the
corresponding cyclic monomers, promoted by initiators that
achieve high activity with controlled molecular weight, poly-
dispersity, and microstructure of resulting polymers.1,4,7 Among
the variety of metal complexes that have been disclosed for
these polymerizations, rare-earth metal complexes have been
reported to be efficient initiators for the preparation of unique
polyesters with controlled features.8–28
To gain better understanding of the relationship between the
bis(phenolate) ligands and the catalytic property of corre-
sponding rare-earth metal complexes, we have designed and
synthesized new guanidinato complexes stabilized by several
amine bridged bis(phenolate) ligands. Although guanidinato
ligands have already been reported to stabilize rare-earth metal
complexes, they were not employed as initiating groups in ROP
of cyclic esters.8c,38–40 Complexes bearing both bis(phenolate)
and guanidinato ligands are thus tested in initiating the ROP of
Key Laboratory of Organic Synthesis of Jiangsu Province and Suzhou Key Laboratory of
Macromolecular Design and Precision Synthesis, College of Chemistry, Chemical
Engineering and Materials Science, Soochow University, Dushu Lake Campus,
Suzhou, 215123, People's Republic of China. E-mail: zhaobei@suda.edu.cn;
yuandan@suda.edu.cn; yaoym@suda.edu.cn; Fax: +86-512-65880305; Tel: +86-512-
65882806
† CCDC 1047907–1017909 and 1048032 (for complex 1, 2,
4 and 5). For
crystallographic data in CIF or other electronic format see DOI:
10.1039/c5ra10151d
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rac-lactide and rac-b-butyrolactone, and their activities and
capacities in stereocontrol are compared.
Results and discussion
Synthesis and characterization of complexes 1–8
Ligand precursors L1H2–L6H2 bearing various bridges were
synthesized by Mannich condensations of substituted phenols,
formaldehydes, and appropriate amines.41 They were isolated in
moderate to good yields as white powders (Chart 1).
Two general methods for synthesizing rare-earth metal
guanidinates include metathesis reactions between a suitable
rare-earth metal precursor and an alkali guanidinate,38,39 and
the insertion of carbodiimides into a rare-earth metal–nitrogen
bond.40 Ytterbium guanidinato complexes 1–2 were synthesized
from metathesis reactions between the easily available Yb(THF)
ClL1 and freshly prepared lithium guanidinates (Scheme 1).
Yttrium complexes 3–8 were prepared via insertion reactions. In
a two-step sequence, ligand precursors LH2 rst reacted with Y
[N(SiHMe2)2]3(THF)2 to generate yttrium amides, which were
treated with one equivalent of N,N0-diisopropylcarbodiimide
(DIC) affording six yttrium guanidinates 3–8 (Scheme 2 and
Chart 2).
Scheme 1 Synthesis of complexes 1–2.
Complexes 1 and 2 are well soluble in THF, moderately
soluble in toluene, and slightly soluble in hexane. Complexes 3–
8 are moderately soluble in hexane. All complexes are quite air-
and moisture-sensitive, but both the crystals and solution are
quite stable when stored under argon. These complexes were
characterized by elemental analysis, IR spectroscopy, X-ray
diffraction studies in the cases of complexes 1, 2, 4 and 5, and
by NMR spectroscopy for diamagnetic complexes 3–8.
Scheme 2 Synthesis of complexes 3–8.
diffraction analyses show that complexes 1, 2, 4 and 5 have
monomeric structures. The ORTEP diagrams of all four struc-
tures are depicted in Fig. 1–4, respectively. Their crystallo-
graphic data are given in Table 1, and the selected bond lengths
and angles are provided in Table 2.
In complexes 1 and 2, the coordinating environments
around the ytterbium ions are similar. Same as other rare-earth
metal complexes stabilized by the same ligand L1,10,17,22,24,38 the
amino side-arm was also found to bind to the metal center in
the solid state. The ytterbium centre is six-coordinated by two
oxygen atoms and two nitrogen atoms from one amine-bridged
bis(phenolate) ligand (L1), and two nitrogen atoms from one
guanidinate group. The coordination geometry can be
Crystal structure analyses
Crystals suitable for X-ray diffraction analysis of complexes 1
and 2 were obtained from toluene and THF solution at room
temperature, respectively, whereas crystals of complexes 4 and 5
were obtained from hexane solution, respectively. X-ray
Chart 1 Ligand precursors L1H2–L6H2.
53162 | RSC Adv., 2015, 5, 53161–53171
Chart 2 Complexes 3–8.
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Fig. 1 ORTEP diagram of complex 1 showing the atom numbering
scheme. Thermal ellipsoids are drawn at the 20% probability level.
Hydrogen atoms and solvent molecules are omitted for clarity.
Fig. 4 ORTEP diagram of complex 5 showing the atom numbering
scheme. Thermal ellipsoids are drawn at the 10% probability level.
Hydrogen atoms and solvent molecules are omitted for clarity.
described as distorted octahedron, in which O1, O2, N1, and N4
atoms occupy equatorial positions, and N2 and N3 atoms
occupy axial positions. The N2–Yb–N3 angles are found to be
163 and 165ꢀ, respectively.
The yttrium ion in complex 4 is six-coordinated by two oxygen
atoms and one nitrogen atom from the imidazolidine-bridged
bis(phenolate) ligand (L2), two nitrogen atoms from the guani-
dinate group and one oxygen atom from one THF molecule. It is
noteworthy that only one nitrogen atoms from the imidazolidine
ring binds to the metal center in the solid state, which is
different from the yttrium amide bearing the same ligand L2.32
In complex 5, the yttrium center is hexacoordinated by two
oxygen atoms and two nitrogen atoms from the bis(phenolate)
ligand L3, and two nitrogen atoms from the guanidinate group.
Fig. 2 ORTEP diagram of complex 2 showing the atom numbering
scheme. Thermal ellipsoids are drawn at the 20% probability level.
Hydrogen atoms are omitted for clarity.
˚
The average Y–O(Ar) bond lengths in 5 is 2.105 A, which is
1
˚
slightly shorter than those in 4 (2.149 A) and L -ligated yttrium
guanidinates (2.128–2.162 A).38 The bond angles of O1–Y–O2 in
˚
4 (107.0(18)ꢀ) and 5 (105.4(3)ꢀ) are much smaller than the
average angle of 153.2ꢀ in L1-ligated yttrium guanidinates. An
obvious difference is also found for the values of the dihedral
angle between the phenyl rings of the bis(phenolate) ligands,
which are 84.5ꢀ for 4, and 81.8ꢀ for 5. Signicantly larger values
of 142.6–161.3ꢀ were found for L1-ligated yttrium guanidinates,
which suggests that the N-heterocycle bridge in 4 and the eth-
ylenediamine bridge in 5 result in a more distorted geometry of
the complexes. Such a difference might contribute to the
different activity and selectivity of these complexes in initiating
ROP.
Polymerization of rac-LA initiated by complexes 1–8
In general, all complexes 1–8 have been found to be active
initiators for the controlled ROP of rac-LA at room temperature
in THF. More than 70% yields were obtained within 1 h with
monomer-to-initiator ratios of up to 1000 at 25 C. The results
are summarized in Table 3.
Fig. 3 ORTEP diagram of complex 4 showing the atom numbering
scheme. Thermal ellipsoids are drawn at the 10% probability level.
Hydrogen atoms and solvent molecules are omitted for clarity.
ꢀ
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Table 1 Crystallographic data for complexes (2 ꢁ 1)$toluene, 2, 4$toluene and (2 ꢁ 5)$THF
Compound
(2 ꢁ 1)$toluene
101H172N10O4Yb2
1936.57
223(2)
2
4$toluene
(2 ꢁ 5)$THF
Formula
Formula weight
T (K)
C
C46H78N5O2Yb
906.17
223(2)
C55H94N5O3Si2Y
1018.44
223(2)
C94H172N10O5Si4Y2
1812.60
223(2)
Crystal system
Crystal size (mm)
Space group
Monoclinic
0.80 ꢁ 0.40 ꢁ 0.30
P21/c
10.8249(7)
15.1480(10)
31.913(2)
90
98.688(2)
90
5172.9(6)
2
Triclinic
Monoclinic
0.70 ꢁ 0.60 ꢁ 0.40
P21/c
12.0592(16)
16.234(2)
30.021(4)
90
90.787(4)
90
5876.6(13)
4
Monoclinic
1.40 ꢁ 1.00 ꢁ 0.20
P21/c
20.91(5)
9.786(18)
28.19(9)
90
102.70(7)
90
5627(25)
2
0.40 ꢁ 0.40 ꢁ 0.20
ꢀ
P1
˚
a (A)
11.2280(7)
11.8377(7)
20.1349(18)
86.230(7)
82.991(7)
64.118(4)
2389.5(3)
2
1.259
1.995
950
27.49
˚
b (A)
˚
c (A)
a (o)
b (o)
g (o)
3
˚
V (A )
Z
Dcalc. (g cmꢂ3
)
1.243
1.848
2040
27.49
1.151
1.076
2200
27.48
1.070
1.115
1960
27.49
m (mmꢂ1
F (000)
)
qmax (o)
Collected
29 123
11 753
10 165
532
1.132
0.0575
0.1242
22 816
10 772
9609
500
1.087
32 030
13 342
8780
581
1.099
0.1220
0.3109
26 518
12 769
4141
546
1.045
Unique rens
Obsd rens [I > 2.0s(I)]
No. of variables
Goodness-of-t
R
0.0529
0.0992
0.1471
0.3242
wR
Table 2 Selected bond lengths (A˚) and bond angles (o) of complexes 1, 2, 4 and 5
Bond lengths
1
2
Bond lengths
4
Bond lengths
5
Yb1–O1
Yb1–O2
Yb1–N1
Yb1–N2
Yb1–N3
Yb1–N4
2.116(3)
2.132(3)
2.495(4)
2.554(4)
2.293(4)
2.321(4)
2.092(3)
2.105(3)
2.513(3)
2.553(4)
2.282(4)
2.338(4)
Y1–O1
Y1–O2
Y1–O3
Y1–N1
Y1–N3
Y1–N4
2.157(5)
2.141(4)
2.397(5)
2.514(5)
2.396(6)
2.406(5)
Y1–O1
Y1–O2
Y1–N1
Y1–N2
Y1–N3
Y1–N4
2.117(8)
2.093(9)
2.538(10)
2.574(9)
2.312(9)
2.418(10)
Bond angles
1
2
Bond angles
4
Bond angles
5
N2–Yb1–N3
O1–Yb1–N1
O1–Yb1–N4
O2–Yb1–N1
O2–Yb1–N4
O1–Yb1–O2
N1–Yb1–N4
O1–Yb1–N2
N3–Yb1–N4
165.4(15)
78.6(14)
101.9(15)
78.9(13)
100.7(15)
157.0(14)
179.2(15)
90.4(15)
58.0(15)
163.3(12)
78.8(11)
100.3(13)
78.1(11)
103.1(13)
155.8(11)
177.3(12)
88.2(13)
58.2(13)
O1–Y1–O2
O1–Y1–O3
O2–Y1–O3
O1–Y1–N1
O1–Y1–N4
O2–Y1–N1
O2–Y1–N4
O3–Y1–N1
N1–Y1–N3
N3–Y1–N4
107.0(18)
88.7(18)
95.0(19)
77.6(18)
154.3(19)
92.5(18)
98.7(18)
165.9(18)
88.2(19)
55.6(19)
O1–Y1–O2
O1–Y1–N1
O1–Y1–N2
O1–Y1–N3
O1–Y1–N4
O2–Y1–N1
N1–Y1–N2
N1–Y1–N3
N1–Y1–N4
N3–Y1–N4
105.4(3)
79.2(3)
95.3(3)
96.7(3)
150.7(3)
147.4(3)
70.8(3)
100.3(4)
93.6(3)
56.2(3)
Using complex 1 as the initiator, the ROP of rac-LA proceeds Moreover, highly heterotactic PLA (Pr $ 0.97) was obtained.
faster in THF and toluene than in CH2Cl2, and with higher Complexes 1, 2 and 3 bearing the same bis(phenolate) ligand L1
stereoselectivities in THF than in toluene and CH2Cl2 (Table 3, showed similar activities (Table 3, entries 6, 9 and 12), while
entries 1–3). Overall, THF is the optimal solvent, which is complex 1 worked slightly better than the other two in the case
consistent with reported results.24 Almost quantitative yields of high [M]0/[I]0 ratios (Table 3, entries 7, 8, 10, and 12).
were obtained with the ratio of monomer to complex 1 Moreover, Pr values of the polymers are essentially the same.
increasing from 300 to 1200 (Table 3, entries 4–7). A good yield These ndings are similar to results from lanthanide alkyls,
of 84% was still obtained when the ratio was increased to 1500. aryloxides and oxides stabilized by the same phenolate
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Table 3 Polymerization of rac-LA initiated by complexes 1–8a
Yieldb (%)
Mc ( ꢁ 104)
Mnd ( ꢁ 104)
Đd
Pre
c
Entry
Initiator
[M]0/[I]0
1
1
1
1
1
1
1
1
1
2
2
3
3
4
4
5
5
6
6
7
7
8
8
500
500
500
300
700
1000
1200
1500
1000
1500
500
1000
500
1000
500
1000
500
1000
500
1000
500
1000
99
67
99
99
99
98
98
84
97
75
97
90
83
80
76
70
95
92
95
91
94
90
7.1
4.8
7.1
8.9
6.9
8.6
1.16
1.25
1.23
1.15
1.12
1.13
1.11
1.15
1.08
1.08
1.23
1.21
1.68
1.65
1.36
1.35
1.44
1.41
1.66
1.57
1.58
1.63
0.99
0.78
0.75
0.98
0.98
0.99
0.98
0.97
0.99
0.98
0.98
0.97
0.62
0.62
0.69
0.69
0.51
0.52
0.82
0.82
0.82
0.82
2f
3g
4
5
6
7
4.3
5.2
10.0
14.1
16.9
18.2
14.0
16.2
7.0
13.0
6.0
11.5
5.5
10.1
6.9
13.3
6.8
13.1
6.7
13.0
11.5
15.1
17.8
18.8
13.2
15.1
8.8
16.5
8.5
14.6
3.4
12.0
4.2
14.8
3.1
15.3
3.3
15.9
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
a
b
General polymerization conditions: THF as the solvent, [rac-LA] ¼ 1 mol Lꢂ1, at 25 ꢀC, reaction time 1 h. Yield: weight of polymer obtained/
c
d
weight of monomer used. Mc ¼ (144.13) ꢁ [M]0/[I]0 ꢁ (polymer yield) (%). Measured by GPC calibrated with standard polystyrene samples.
e
Pr is the probability of racemic linkages between monomer units and is determined from the methine region of the homonuclear decoupled
1H NMR spectrum. f In CH2Cl2. In toluene.
g
ligand.17–19,22,24 Thus, neither the metal center nor the guanidi-
nate group plays signicant roles in inuencing the polymeri- relatively narrow molecular weight distributions in the range of
zation rates or tacticity.
Mw/Mn ¼ 1.08–1.68. It is noteworthy that the average-number
All PLA obtained in the presence of complexes 1–8 showed
A comparative study of complexes 4–8 on the activity in molecular weights (Mn) values of PLA obtained with
initiating the stereoselective ROP of rac-LA under identical complexes 1–3 are generally in good agreement with theoretical
conditions revealed substantial differences (Table 3). Using Mn values, which indicate that the polymerization proceeded in
complex 4 as the initiator, the yield is 83% and Pr is 0.62 when a living fashion without other signicant side reactions. To
the ratio of monomer to initiator is 500 (Table 3, entry 13), illustrate this living character, the relationship between the
which is similar to that of yttrium amide bearing the same number-average molecular weight (Mn) and the molar ratio of
imidazolidine-bridged bis(phenolate) ligand.21,32 Complex 5 monomer to initiator 1 ([M]0/[I]0) was plotted in Fig. 5. As the
initiated ROP of rac-LA and afforded PLA with Pr value of 0.69 monomer/initiator ratio increases, the molecular weight of the
(Table 3, entries 15 and 16). The alkyl analogue of 5 (ref. 17)
was reported to initiate ROP of rac-LA and afforded PLA of
almost the same Pr value. Complex 6 effectively initiated rac-
LA polymerization and produced PLA with Pr value of 0.51
(Table 3, entries 17 and 18). The stereoselectivity of 6 is similar
to that of homopiperazine-bridged bis(phenolate) Ti(IV)
complex.42 The methoxy-amine-bis(phenolate) yttrium
complex 7 showed the same level of activity and selectivity (Pr
¼ 0.82) as furan-amine-bis(phenolate) yttrium complexes 8
(Table 3, entries 19–22). Based on the ndings discussed
above, it is conclusive that the bis(phenolate) ligands exert
crucial roles in inuencing the tacticity. Overall, complexes
stabilized by ligands with coordinating arms (1, 2, 3, 7, and 8)
show higher stereoselectivity (Pr values of 0.82–0.99), while
those carrying ligands of longer bridges (4, 5, and 6) show
lower stereoselectivity (Pr values of 0.51–0.69). This nding
emphasizes again the importance of bis(phenolate) ligands on
tacticity.8f,8u,9,23–26
Fig. 5 Polymerization of rac-LA initiated by complex 1 in THF at 25 ꢀC.
Relationship between the number-averaged molecular weight (Mn)
and the molar ratio of monomer to initiator.
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resulting polymer increased linearly, while molecular weight stereoselectivity to produce highly syndiotactic PHB in toluene.
distributions were kept almost unchanged (1.12–1.16), strongly Representative results are summarized in Table 4.
suggesting that the polymerization process is controllable.
A full conversion of rac-BBL was achieved within 2 min in the
To investigate the mechanism of ROP of rac-LA initiated by ratio of [M]0/[I]0 ¼ 200 (Table 4, entry 1) in toluene at 25 ꢀC.
these guanidinate complexes, oligomerization of rac-LA initi- Moreover, highly syndiotactic PHB (Pr ¼ 0.82) was obtained.
ated by complex 1 in a [M]0/[I]0 ratio of 15 was carried out. Increasing the monomer to initiator ratio to 1000 still led to
However, no guanidinate or bis(phenolate) group was identi- almost full conversion within 10 min (Table 4, entries 2–5). A
ed from the 1H NMR spectrum of the oligomer. Considering lower of yield of 88% was obtained when the monomer to
the instability of the oligomer with the guanidinate end cap,38 initiator ratio was raised to 1500 (Table 4, entry 6). The solvent
a new oligomer was prepared by quenching the oligomeriza- plays a signicant role in inuencing the activity and stereo-
tion described above with benzyl alcohol. End group analysis selectivity, as the ROP proceeded much slower in THF and
by 1H NMR spectroscopy clearly shows the existence of a afforded PHB of lower Pr value (Table 4, entries 2 and 7).
benzyloxy group and a HOCH(CH3)CO– group according to the Decreasing the monomer concentration also slowed down the
resonances at about 5.18, 7.33 ppm assignable to the former polymerization rate (Table 4, entries 2 and 8). These ndings are
and at 1.23, 2.57 and 4.34 ppm assignable to the latter, as consistent with those observed in the amino-bridged bis(phe-
shown in Fig. 6. The benzyloxy group is believed to come from nolate) lanthanide systems.14,15,24
the exchange reaction of benzyl alcohol with the guanidinate
Complex 2 showed similar activities and stereoselective
group. Meanwhile, no resonances corresponding to the control in initiating the ROP of rac-BBL (Table 4, entries 9 and
bis(phenolate) ligand L1 was observed in the 1H NMR spec- 10). Both ytterbium complexes 1 and 2 are more active than the
trum, which ruled out the possibility that the amine-bridged yttrium complex 3 (Table 4, entries 11 and 12), which may be
bis(phenolate) group is the initiating group in the polymeri- attributed to the rare-earth metal centres of different ionic radii.
zation process. These results revealed that the guanidinate However, the stereoselectivity is not at all inuenced, which is
group in these complexes acted as the initiating group in the quite different from that observed in rac-LA polymerization
ROP of rac-LA, while the bis(phenolate) group works as the (vide supra).
spectator ligand controlling the stereoselectivity of the poly-
merization process.24,25
The activities and stereoselective control of complexes 4–8 in
initiating the ROP of rac-BBL were examined and compared
under identical conditions. Complex
4
bearing an
imidazolidine-bridged bis(phenolate) ligand is less active than
complex 3, and the Pr value of the resulting PHB is much lower
(Table 4, entries 13 and 14). The activity of complex 5 (Table 4,
entry 15) is the lowest among complexes 1–8, and is obviously
lower than that of the analogous yttrium amide.9 Complex 6
ROP of rac-BBL initiated by complexes 1–8
ROP of BBL is generally much more challenging than that of LA
despite of their larger ring strains,43 and most of the catalysts
show low stereoselectivity and a high prevalence of chain
termination events, such as transesterication and chain
transfer.41,44 Complexes 1–8 proved efficient in initiating the
ROP of rac-BBL, and are in general capable of controlling
bearing
a homopiperazine-bridged bis(phenolate) ligand
showed higher activity than that of 4, while resulting PHB have
Fig. 6 1H NMR spectrum of rac-LA oligomer initiated by complex 1 after quenching with benzyl alcohol.
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Table 4 Polymerization of rac-BBL initiated by complexes 1–8a
Yieldb (%)
Mc (ꢁ 104)
Mnd (ꢁ 104)
Đd
Pre
c
Entry
Initiator
[M]0/[I]0
t
1
2
3
4
1
1
1
1
1
1
1
1
2
2
3
3
4
4
5
6
6
7
7
8
8
200
400
600
800
1000
1500
400
400
400
1000
200
400
200
400
200
200
400
200
400
200
400
2 min
10 min
10 min
10 min
10 min
30 min
30 min
10 min
10 min
10 min
1 h
1 h
10 h
10 h
10 h
10 h
10 h
10 h
10 h
100
98
98
98
96
88
60
83
100
92
97
80
70
55
16
96
85
96
75
63
56
1.7
3.4
5.1
6.7
8.3
11.4
2.1
2.9
3.4
7.9
1.7
2.4
1.2
1.9
0.3
1.7
2.9
1.7
2.6
1.1
1.9
2.5
3.5
6.5
8.2
9.7
13.5
3.0
3.3
4.2
9.7
2.5
3.1
2.3
3.0
—
1.35
1.33
1.31
1.36
1.42
1.37
1.35
1.30
1.29
1.44
1.35
1.31
1.78
1.86
—
0.82
0.82
0.82
0.82
0.82
0.82
0.67
0.82
0.82
0.82
0.82
0.82
0.58
0.58
—
5
6
7f
8g
9
10
11
12
13
14
15
16
17
18
19
20
21
1.8
3.5
1.7
3.3
2.3
3.2
1.83
1.78
1.86
1.80
1.78
1.82
0.57
0.57
0.81
0.81
0.79
0.79
10 h
10 h
a
General polymerization conditions: toluene as the solvent, [rac-BBL] ¼ 2 mol Lꢂ1, at 25 ꢀC. b Yield: weight of polymer obtained/weight of monomer
c
d
e
used. Mc ¼ 86.09 ꢁ [M]0/[I]0 ꢁ (polymer yield) (%). Measured by GPC calibrated with standard polystyrene samples. Pr is the probability of
racemic linkages between monomer units and is determined from the carbonyl region of the 13C{1H} NMR spectroscopy at 25 ꢀC. In THF.
f
g
[rac-BBL] ¼ 1 mol Lꢂ1
.
almost the same Pr values (Table 4, entries 16 and 17). The
methoxy-amine-bis(phenolate) complex 7 was found to be more
active than the furan-amine-bis(phenolate) complex 8, and
these two complexes show similar stereoselective control (Pr ¼
0.79–0.81). The same inuence exerted by bis(phenolate)
ligands on stereoselectivity has been observed as that for poly-
merization of rac-lactide (vide supra).
It was found that the PHB obtained with complexes 1–3 had
relatively narrow molecular weight distributions (1.13 < Mw/Mn
< 1.44) and average-number molecular weights (Mn) are gener-
ally in good agreement with the theoretical Mn values. For the
ROP of rac-BBL initiated by complex 1, the number-average
molecular masses (Mn) values of the resultant PHB increases
as the monomer-to-initiator ratio increases, while the values of
Mw/Mn were kept almost unchanged, indicating that the poly-
merization process is also controllable (Table 4, entries 1–5).
However, the PHB produced with 4–8 in toluene had relatively
broad molecular weight distributions (Mw/Mn ¼ 1.31–1.86),
which may be mainly attributed to the transesterication reac-
tions during the polymerization process or to the relatively slow
initiation step.
The ROP mechanism of rac-BBL was elucidated by using
the method similar to that of rac-LA. The oligomer of rac-BBL
was prepared by the reaction of complex 1 with rac-BBL in a
1 : 20 molar ratio which is quenched by benzyl alcohol. In
addition to the resonances characteristic for the HOCH(CH3)
CH2CO– group at 4.19, 2.30 and 1.38 ppm, respectively, the 1H
NMR spectrum in CDCl3 clearly showed the existence of the
benzyloxy group according to the resonances at about 7.34
and 5.12 ppm (Fig. 7), which arises from the exchange
Fig. 7 1H NMR spectrum of rac-BBL oligomer initiated by complex 1
after quenching with benzyl alcohol.
reaction between benzyl alcohol with the guanidinate group.
Meanwhile, additional resonances at 6.95 and 5.81 ppm were
observed, which may correspond to trans-crotonate groups
that may result from the elimination side reactions in the
polymerization progress. A similar elimination reaction in
rac-BBL polymerization has been observed using amino-
bridged bis(phenolate)lanthanide alkoxo complexes as the
initiators.24
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(3.05 g, 3.8 mmol) in 20 mL of THF. The mixture was stirred
overnight at room temperature, and the solvent was removed in
a vacuum. The residue was extracted with toluene, and LiCl was
removed by centrifugation. Yellow crystals were obtained from
toluene solution at room temperature in a few days (1.79 g,
51%). Anal. calc. for C47H82N5O2Yb (922.23): C, 61.21; H, 8.96;
N, 7.59; Yb, 18.76. Found: C, 60.98; H, 9.11; N, 7.77; Yb, 18.66. IR
(KBr, cmꢂ1): 2961 (s), 2898 (s), 2867 (s), 2373 (w), 1629 (s), 1561
(m), 1478 (s), 1414 (m), 1360 (m), 1309 (s), 1255 (m), 1237 (m),
1202 (m), 1166 (m), 1134 (m), 1027 (m), 876 (m), 838 (m), 744
(m), 694 (m), 528 (m), 446 (m).
Conclusion
In summary, eight rare-earth metal guanidinates supported by a
series of amine-bridged bis(phenolate) ligands were synthe-
sized according to either metathesis reactions, or insertion
reactions of carbodiimides. All eight complexes have been fully
characterized, and the solid state structures of four of them
have been determined. A comparative study on these complexes
in initiating the ROP of rac-LA and rac-BBL revealed that
bis(phenolate) ligands play critical roles in governing the ster-
eoselectivity. ROP initiated by complexes 1–3 bearing the
amine-bridged bis(phenolate) ligand L1 afforded highly heter-
otactic PLA (Pr values of more than 0.97) and syndiotactic PHB
(Pr values of 0.82). End group analysis of the oligomer revealed
that in the ROP of rac-LA and rac-BBL the guanidinate groups
serve as initiating groups.
Synthesis of L1Yb[(CH2)5NC(NiPr)2] (2). Following the
procedure for the synthesis of complex 1, Li[(CH2)5NC(NiPr)2]
(3.8 mmol), which was formed in situ by the reaction of
LiN(CH2)5 with DIC, reacted with L1YbCl(THF) (2.81 g, 3.5
mmol) in THF (40 mL) afforded the nal product as yellow
crystals aer crystallization in toluene solution (1.84 g, 58%).
Anal. calc. for C46H78N5O2Yb (906.18): C, 60.97; H, 8.68; N, 7.73;
Yb, 19.10. Found: C, 60.72; H, 8.86; N, 7.88; Yb, 18.99. IR (KBr,
cmꢂ1): 2952 (s), 2864 (s), 2361 (w), 1630 (s), 1599 (m), 1513(m),
Experimental
Reagents and general procedures
All manipulations requiring dry atmosphere were performed 1478 (s), 1414 (m), 1360 (m), 1308 (m), 1257 (m), 1237 (m), 1202
under a puried argon atmosphere by use of standard Schlenk (m), 1166 (m), 1131 (m), 1085 (m), 1028 (m), 875 (m), 838 (m),
techniques or in a glovebox. Solvents were degassed and distilled 744 (m), 529 (m), 446 (m).
from sodium benzophenone ketyl under argon prior to use.
Synthesis of L1Y[(SiHMe2)2NC(NiPr)2] (3). Reatment of the
Ligands L1H2–L6H2,38 lanthanide precursors L1YbCl(THF)30,45 and homoleptic Y[N(SiHMe2)2]3(THF)2 with 1 equiv of L1H2 afforded
L1Y[N(SiHMe2)2](THF) to L6Y[N(SiHMe2)2](THF)9 were prepared the corresponding bridged bis(phenolate) yttrium amide L1Y
by modications of the methods reported in the literature. Benzyl [N(SiHMe2)2](THF). A solution of DIC (0.15 g, 1.2 mmol) in
˚
alcohol were dried over 4 A molecular sieves for 1 week and then hexane (5 mL) was added by syringe at room temperature to a
distilled before use. rac-Lactide (rac-LA) was recrystallized twice stirred solution of L1Y[N(SiHMe2)2](THF) (0.98 g, 1.2 mmol) in
from dry toluene and then sublimed under vacuum at 50 ꢀC. rac- hexane (20 mL). The reaction mixture was stirred for 12 h at
b-Butyrolactone (rac-BBL) was freshly distilled from CaH2 under room temperature. The solvent was removed in a vacuum, and a
nitrogen and degassed thoroughly by freeze–pump–thaw cycles hexane (4 mL)/toluene (1 mL) solution was added. The resulting
prior to use.
Lanthanide
solution was kept at room temperature for crystallization, and
ethyl- colorless crystals were obtained in a few days (0.63 g, 60%).
analyses
were
performed
by
enediaminetetraacetic acid titration with a xylenol orange Anal. calc. for C45H82N5O2Si2Y (870.24): C, 62.11; H, 9.50; N,
indicator and a hexamine buffer. Carbon, hydrogen, and 8.05; Y, 10.22. Found: C, 61.75; H, 9.75; N, 7.86; Y, 10.05. IR
nitrogen analyses were performed by direct combustion with a (KBr, cmꢂ1): 2956 (s), 2906 (s), 2122 (w), 1622 (s), 1551 (m), 1471
Carlo-Erba EA-1110 instrument. The IR spectra were recorded (s), 1375 (m), 1307 (m), 1248 (m), 1166 (m), 1133 (m), 1050 (m),
with a Nicolet-550 Fourier transform IR spectrometer as KBr 1028 (m), 940 (m), 912 (m), 838 (m), 805 (m), 779 (m), 744 (m),
pellets. The H and 13C NMR spectra were recorded in a C6D6 528 (m), 446 (m). 1H NMR (400 MHz, C6D6, 25 ꢀC): d 7.60 (s, 2H,
1
solution for complexes 3–6 with a Unity Varian spectrometer. ArH), 7.11 (s, 2H, ArH), 4.89 (m, 2H, N(SiHMe2)2), 4.26 (br, 2H,
Molecular weights and molecular weight distributions were ArCH2N), 3.72 (m, 2H, CH(CH3)2), 2.99 (br, 2H, ArCH2N), 2.39
determined against polystyrene standards by gel permeation (br, 2H, NCH2CH2N), 2.08 (br, 2H, NCH2CH2N), 1.84 (br, 6H,
chromatography (GPC) on a PL 50 apparatus, and THF was used N(CH3)2), 1.73 (s, 18H, C(CH3)3), 1.47 (s, 18H, C(CH3)3), 1.31–
as an eluent at a ow rate of 1.0 mL minꢂ1 at 40 ꢀC. The 1.26 (m 12H, CH(CH3)2), 0.35 (m, 12H, N(SiHMe2)2). 13C{1H}
microstructures of PLAs and PHBs were measured by homo- NMR (100 MHz, C6D6, 25 ꢀC): d 167.6 (CN3), 162.4, 136.4, 135.7,
decoupling 1H NMR spectroscopy at 20 ꢀC in CDCl3 and by 13C 125.8, 124.8, 124.4(Ar-C), 65.3 (ArCH2N), 59.8 (N(CH2)2N), 48.4
{1H} NMR spectroscopy at 40 ꢀC in CDCl3, respectively, on a (N(CH3)2), 46.5 (CH(CH3)2), 35.6 (C(CH3)3), 34.3 (C(CH3)3), 27.6
Unity Varian AC-400 spectrometer.
(CH(CH3)2), 23.7 (CH(CH3)2), 2.6 (NSiHMe2).
Synthesis of L1Yb[iPr2NC(NiPr)2] (1). A Schlenk ask was
Synthesis of L2Y[(SiHMe2)2NC(NiPr)2](THF) (4). Following
charged with HNiPr2 (0.38 g, 3.8 mmol), THF (20 mL), and a the procedure for the synthesis of complex 3, L2Y
stirring bar. The solution was cooled to 0 C, and n-BuLi (1.9 [N(SiHMe2)2](THF) (0.96 g, 1.2 mmol), which was formed by the
ꢀ
mL, 3.8 mmol, 2 M in hexane) was added by syringe. The reaction of L2H2 with Y[N(SiHMe2)2]3(THF)2, reacted with DIC in
solution was stirred for 1 h at 0 ꢀC, and then N,N0-diisopro- hexane (20 mL), affording the nal product as colorless crystals
pylcarbodiimide (DIC) (0.48 g, 3.8 mmol) was added. The aer crystallization in toluene/hexane solution (0.67 g, 65%).
resulting solution was slowly warmed to room temperature, and Anal. calc. for C48H86N5O3Si2Y (926.30): C, 62.24; H, 9.36; N,
stirred for 1 h, and then was added slowly to L1YbCl(THF) 7.56; Y, 9.60. Found: C, 61.99; H, 9.55; N, 7.73; Y, 9.85. IR (KBr,
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cmꢂ1): 2956 (s), 2915 (m), 2869 (m), 2210 (m), 2109 (m), 1614 (s), 122.3, 118.5 (Ar-C), 63.2 (ArCH2N), 52.1 (NCH2CH2N), 49.4
1473 (s), 1363 (m), 1305 (s), 1241 (s), 1165 (m), 1129 (m), 1016 (NCH2CH2CH2N), 48.2 (CH(CH3)2), 40.0 (NCH2CH2CH2N), 35.1
(w), 915 (m), 835 (m), 792 (w), 646 (w), 527 (w), 407 (w). 1H NMR (C(CH3)3), 34.8 (C(CH3)3), 32.2 (C(CH3)3), 31.5 (C(CH3)3), 31.3
ꢀ
(400 MHz, C6D6, 25 C): d 7.44 (s, 2H, ArH), 6.80 (s, 2H, ArH), (CH(CH3)2), 29.8(CH(CH3)2), 2.6 (NSiHMe2).
2
4.89 (m, 2H, N(SiHMe2)2), 4.23 (d, JHH ¼ 14.1 Hz, 2H, ArCH2),
Synthesis of L5Y[(SiHMe2)2NC(NiPr)2] (7). Following the
3.72 (m, 2H, CH(CH3)2), 3.70 (br. s, 4H, a-CH2 THF), 3.61 (d, procedure for the synthesis of complex 3, L5Y[N(SiHMe2)2](THF)
2JH,H ¼ 7.5 Hz, 1H, NCH2N), 3.54 (br. s, m, 2H, NCH2CH2N), 2.85 (0.96 g, 1.2 mmol), which was formed by the reaction of L5H2
2
2
(d, JHH ¼ 14.1 Hz, 2H, ArCH2), 2.1 (d, JHH ¼ 7.4 Hz, 1H, with Y[N(SiHMe2)2]3(THF)2, reacted with DIC in hexane (20 mL),
NCH2N), 1.90 (br. m, 2H, NCH2CH2N), 1.56 (s, 18H, C(CH3)3), affording the nal product as colorless crystals aer crystalli-
1.41 (s, 18H, C(CH3)3), 1.35 (br. s, 4H, b-CH2 THF), 1.31–1.26 (m zation in hexane solution (0.65 g, 63%). Anal. calc. for C44H79
-
12H, CH(CH3)2), 0.35 (m, 12H, N(SiHMe2)2). 13C{1H} NMR (100 N4O3Si2Y (857.2): C, 61.65; H, 9.29; N, 6.54; Y, 10.37. Found: C,
MHz, C6D6, 25 ꢀC): d 168.3 (CN3), 160.6, 137.2, 136.7, 124.3, 61.35; H, 9.38; N, 6.83; Y, 10.13. IR (KBr, cmꢂ1): 2956 (s), 2906
123.9, 121.8 (Ar-C), 71.3 (NCH2N), 69.0 (a-CH2 THF), 56.9 (m), 2863 (m), 2122 (w), 1622 (s), 1551 (m), 1471 (s) 1310 (s),
(N(CH2)2N), 51.8 (ArCH2N), 48.4 (CH(CH3)2), 35.7 (C(CH3)3), 1248 (m), 1167 (m), 1077(s), 1028 (w), 913 (m), 834 (m), 803 (m),
35.6 (C(CH3)3), 34.1 (C(CH3)3), 32.1 (C(CH3)3), 25.23 (b-CH2 742 (m), 528 (m), 452 (w). 1H NMR (400 MHz, C6D6, 25 ꢀC): d 7.56
THF), 27.6 (CH(CH3)2), 23.6 (CH(CH3)2), 2.6 (NSiHMe2).
(s, 2H, ArH), 6.97 (m, 2H, ArH), 4.80 (m, 2H, N(SiHMe2)2), 3.84
Synthesis of L3Y[(SiHMe2)2NC(NiPr)2] (5). Following the (m, 2H, CH(CH3)2), 3.59, 3.26 (m, 4H, NCH2Ar), 2.98 (s, 3H,
procedure for the synthesis of complex 3, L3Y[N(SiHMe2)2](THF) OCH3), 2.76, 2.37 (m, 4H, NCH2CH2O), 1.74 (s, 18H, C(CH3)3),
(0.98 g, 1.2 mmol), which was formed by the reaction of L3H2 1.43 (s, 18H, C(CH3)3), 1.30 (m, 12H, CH(CH3)2), 0.35 (d, 12H,
with Y[N(SiHMe2)2]3(THF)2, reacted with DIC in hexane (20 mL), N(SiHMe2)2). 13C{1H} NMR (100 MHz, C6D6, 25 ꢀC): d 168.5
affording the nal product as colorless crystals aer crystalli- (CN3), 161.4, 136.5, 125.4, 124.1, 123.9, 121.5 (Ar-C), 73.1
zation in hexane solution (0.63 g, 60%). Anal. calc. for C45H82
-
(NCH2CH2O), 64.5 (ArCH2N), 60.4 (OCH3), 49.6 (NCH2CH2O),
N5O2Si2Y (870.24): C, 62.11; H, 9.50; N, 8.05; Y, 10.22. Found: C, 48.4 (CH(CH3)2), 35.5 (C(CH3)3), 34.6 (C(CH3)3), 32.8 (C(CH3)3),
61.86; H, 9.68; N, 8.33; Y, 10.12. IR (KBr, cmꢂ1): 2958 (s), 2906 32.1 (C(CH3)3), 28.6 (CH(CH3)2), 27.6 (CH(CH3)2), 2.6 (NSiHMe2).
(m), 2869 (m), 2124 (w), 1620 (s), 1554 (m), 1472 (s), 1373 (m),
Synthesis of L6Y[(SiHMe2)2NC(NiPr)2] (8). Following the
1310 (s), 1247 (m), 1171 (w), 1128 (w), 1073 (w), 1014 (m), 913 procedure for the synthesis of complex 3, L6Y[N(SiHMe2)2](THF)
1
(m), 836 (m), 802 (m), 740 (w), 529 (m), 444 (m). H NMR (400 (0.99 g, 1.2 mmol), which was formed by the reaction of L6H2
MHz, C6D6, 25 ꢀC): d 7.63 (s, 2H, ArH), 6.986.93 (m, 2H, ArH), 5.56, with Y[N(SiHMe2)2]3(THF)2, reacted with DIC in hexane (20 mL),
5.22, 4.89 (m, 2H, N(SiHMe2)2), 4.70, 4.06 (m, 2H, NCH2Ar), 3.45, affording the nal product as colorless crystals aer crystalli-
3.32 (m, 2H, CH(CH3)2), 3.25, 2.53 (m, 2H, NCH2Ar), 2.99, 2.81 (t, zation in hexane solution (0.70 g, 66%). Anal. calc. for C46H81
-
4H, NCH2CH2N), 2.15 (s, 3H, NCH3), 1.93 (s, 3H, NCH3), 1.90, N4O3Si2Y (883.24): C, 62.55; H, 9.24; N, 6.34; Y, 10.07. Found: C,
1.80 (s, 18H, C(CH3)3), 1.46, 1.41 (s, 18H, C(CH3)3), 1.11, 0.84, 0.70 61.33; H, 9.18; N, 6.37; Y, 10.32. IR (KBr, cmꢂ1): 2956 (s), 2902
(m, 12H, CH(CH3)2), 0.47–0.14 (m, 12H, N(SiHMe2)2). 13C{1H} (m), 2122 (m), 1624 (s), 1548 (m), 1473 (s), 1373 (m), 1310 (s),
NMR (100 MHz, C6D6, 25 ꢀC): d 171.0 (CN3), 161.5, 160.8, 158.5, 1246 (m), 1168 (w), 1129 (w), 1022 (m), 912 (m), 837 (m), 743 (m),
136.8, 126.3, 125.4, 125.3, 124.9, 124.1, 123.8, 123.5, 117.0 (Ar-C), 526 (m), 447 (m). 1H NMR (400 MHz, C6D6, 25 ꢀC): d 7.58, 7.54 (s,
70.8 (a-CH2 THF), 65.0 (ArCH2N), 56.1 (ArCH2N), 51.3 (N(CH2)2N), 2H, ArH), 7.04 (m, 2H, ArH), 4.86 (m, 2H, N(SiHMe2)2), 4.02 (m,
46.6 (N(CH2)2N), 46.5 (CH(CH3)2), 42.3 (NCH3), 35.6 (C(CH3)3), 1H, CHO), 3.88 (m. 4H, NCH2Ar), 3.72 (m, 2H, CH(CH3)2), 3.50
34.2 (C(CH3)3), 32.2 (C(CH3)3), 32.0 (C(CH3)3), 30.9 (CH(CH3)2), (m, 2H, OCH2CH2), 2.43, 2,08 (m, 2H, NCH2CHO), 1.75 (s, 18H,
30.3 (CH(CH3)2), 26.5 (b-CH2 THF), 2.5 (NSiHMe2).
C(CH3)3), 1.44 (s, 9H, C(CH3)3), 1.42 (s, 9H, C(CH3)3), 1.31–1.26
Synthesis of L4Y[(SiHMe2)2NC(NiPr)2] (6). Following the (m 12H, CH(CH3)2), 1.10 (m, 4H, OCH2CH2CH2CH), 0.40 (d,
procedure for the synthesis of complex 3, L4Y[N(SiHMe2)2](THF) 12H, N(SiHMe2)2). 13C{1H} NMR (100 MHz, C6D6, 25 ꢀC): d 170.2
(0.99 g, 1.2 mmol), which was formed by the reaction of L4H2 (CN3), 162.1, 135.6, 131.6, 130.6, 124.5, 118.1 (Ar-C), 79.6 (CHO),
with Y[N(SiHMe2)2]3(THF)2, reacted with DIC in hexane (20 mL), 67.4 (CH2O), 64.6 (NCH2CHO), 56.6 (ArCH2N), 54.5 (NCH2-
affording the nal product as colorless crystals aer crystalli- CH2N), 46.8 (CH(CH3)2), 35.3 (C(CH3)3), 31.3 (C(CH3)3), 30.2
zation in hexane solution (0.72 g, 68%). Anal. calc. for C46H82
-
(CH(CH3)2), 28.6 (CH(CH3)2), 28.1 (THF-CH2), 25.2 (THF-CH2),
N5O2Si2Y (882.25): C, 62.62; H, 9.37; N, 7.94; Y, 10.08. Found: C, 2.5 (NSiHMe2).
62.35; H, 9.58; N, 7.83; Y, 10.32. IR (KBr, cmꢂ1): 2957 (s), 2906
(m), 2872 (m), 2119 (w), 1623 (s), 1556 (m), 1472 (s), 1362 (m),
General polymerization procedure
659 (w), 519 (w). 1H NMR (400 MHz, C6D6, 25 ꢀC): d 7.54 (s, 2H, The procedures for polymerization of rac-LA and rac-BBL were
1300 (m), 1244 (s), 1167 (m), 1092 (m), 993 (m), 940 (m), 797 (s),
ArH), 6.88 (s, 2H, ArH), 4.89 (m, 2H, N(SiHMe2)2), 4.27 (d, 2JHH
12.6 Hz, 2H, ArCHHN), 3.92 (m, 2H, CH(CH3)2), 3.39, 2.86 (m,
¼
similar and given below:
A 20 mL vial, equipped with a magnetic stirring bar, was
2
4H, NCH2CH2N), 2.68 (d, JHH ¼ 12.6 Hz, 2H, ArCHHN), 1.85, charged in a glovebox with the desired amount of monomer and
(m, 4H, NCH2CH2CH2N) 1.66 (s, 18H, C(CH3)3), 1.43 (s, 18H, solvent. Aer the monomer was dissolved, a solution of the
C(CH3)3), 1.36–1.21 (m, 12H, CH(CH3)2), 1.11 (m, 2H, NCH2- initiator was added to this solution via a syringe. The mixture
CH2CH2N), 0.40–0.28 (m, 12H, N(SiHMe2)2). 13C{1H} NMR (100 was immediately stirred at the desired temperature for the
MHz, C6D6, 25 ꢀC): d 169.3 (CN3), 157.6, 137.5, 127.9, 127.8, desired time. The reaction was quenched by the addition of
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ethanol and then poured into ethanol to precipitate the poly-
mer, which was dried under vacuum to constant weight.
697; (c) N. Ajellal, D. M. Lyubov, M. A. Sinenkov,
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5440–5448;
(d)
M.
Bouyahyi,
T.
Roisnel
and
Oligomer preparation
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C. Bakewell, R. H. Platel, S. K. Cary, S. M. Hubbard,
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Oligomerization of rac-LA and rac-BBL was carried out with
complex 1 as the initiator in THF and toluene, respectively, at
25 ꢀC under the condition of a molar ratio of [monomer]/
[initiator] of 15 or 20. The polymerization mixture was stirred
for 0.5 h and then quenched by adding benzyl alcohol. The
precipitated oligomers were collected, dried under vacuum, and
1
used for H NMR measurement.
X-ray crystallographic structure determinations
Suitable single crystals of complexes 1, 2, 4 and 5 were sealed in
a thin-walled glass capillary for determination of the single-
crystal structures. Intensity data were collected with a Rigaku
Mercury CCD area detector in u scan mode using Mo Ka radi-
˚
ation (l ¼ 0.71070 A). The diffracted intensities were corrected
for Lorentz/polarization effects and empirical absorption
corrections.
The structures were solved by direct methods and rened by
full-matrix least-squares procedures based on |F|2. The
hydrogen atoms in these complexes were generated geometri-
cally, assigned appropriate isotropic thermal parameters, and
allowed to ride on their parent carbon atoms. All of the
hydrogen atoms were held stationary and included in the
structure factor calculation in the nal stage of full matrix least-
squares renement. The structures were solved and rened
using SHELXTL-97 programs.
Acknowledgements
Financial support from the National Natural Science Founda-
tion of China (Grants 21174095, 21132002, 21172165, 21372172
and 21402135), PAPD, the Major Research Project of the Natural
Science Foundation of the Jiangsu Higher Education Institu-
tions (Project 14KJA150007),and the Qing Lan Project is grate-
fully acknowledged.
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