Synthesis and characterization of amine-bridged bis(phenolate)lanthanide alkoxides and their application in the controlled polymerization of rac-lactide and rac-β-butyrolactone

Article abstract of DOI:10.1021/ic301746c

A series of neutral lanthanide alkoxides supported by an amine-bridged bis(phenolate) ligand were synthesized, and their catalytic behaviors for the polymerization of rac-lactide (LA) and rac-β-butyrolactone (BBL) were explored. The reactions of (C₅H₅)₃Ln(THF) with amine-bridged bis(phenol) LH₂ [L = Me₂NCH ₂CH₂N{CH₂-(2-OC₆H₂Bu ^t₂-3,5)}₂] in a 1:1 molar ratio in THF for 1 h and then with 1 equiv each of 2,2,2-trifluoroethanol, benzyl alcohol, and 2-propanol gave the neutral lanthanide alkoxides LLn(OCH₂CF ₃)(THF) [Ln = Y (1), Yb (2), Er (3), Sm (4)], LY(OCH ₂Ph)(THF) (5), and LY(OPrⁱ)(THF) (6), respectively. These lanthanide alkoxides are sensitive to moisture, and the yttrium complex [(LY)₂(μ-OPrⁱ)(μ-OH)] (7) was also isolated as a byproduct during the synthesis of complex 6. Complexes 1-6 were well characterized by elemental analyses and IR and NMR spectroscopy in the cases of complexes 1 and 4-6. The definitive molecular structures of all of these complexes were determined by single-crystal X-ray analysis. It was found that complexes 1-6 can initiate efficiently the ring-opening polymerization of rac-LA and rac-BBL in a controlled manner. For rac-LA, polymerization gave polymers with very narrow molecular weight distributions (PDI ≤ 1.12) and very high heterotacticity (P_r up to 0.99). The observed activity-increasing order is in agreement with the order of the ionic radii, whereas the order for stereoselectivity is in the reverse order. For rac-BBL polymerization, the resultant polymers have narrow molecular distributions (PDI ≤ 1.26) and high syndiotacticity (P_r up to 0.83). It is worth noting that the activity-decreasing order Yb > Er > Y ? Sm is observed for rac-BBL polymerization, which is opposite to the order of ionic radii and to the order of activity for rac-LA polymerization. The ionic radii of lanthanide metals have no obvious effect on the stereoselectivity for rac-BBL polymerization, which is quite different from that for rac-LA polymerization. End-group analysis of the oligomer of rac-BBL suggested that elimination side reactions occurred slowly in these systems, which led to chain cleavage and the formation of crotonate (and carboxy) end groups.

Full text of DOI:10.1021/ic301746c

Article
pubs.acs.org/IC
Synthesis and Characterization of Amine-Bridged
Bis(phenolate)lanthanide Alkoxides and Their Application in the
Controlled Polymerization of rac-Lactide and rac-β-Butyrolactone
Kun Nie,† Lei Fang,† Yingming Yao,*,†,‡ Yong Zhang,† Qi Shen,† and Yaorong Wang*,†
†Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science, Dushu
Lake Campus, Soochow University, Suzhou 215123, People’s Republic of China
‡The Institute of Low Carbon Economy, Suzhou 215123, People’s Republic of China
S
* Supporting Information
ABSTRACT: A series of neutral lanthanide alkoxides supported by an amine-
bridged bis(phenolate) ligand were synthesized, and their catalytic behaviors for the
polymerization of rac-lactide (LA) and rac-β-butyrolactone (BBL) were explored.
The reactions of (C5H5)3Ln(THF) with amine-bridged bis(phenol) LH2 [L =
Me2NCH2CH2N{CH2-(2-OC6H2But2-3,5)}2] in a 1:1 molar ratio in THF for 1 h
and then with 1 equiv each of 2,2,2-trifluoroethanol, benzyl alcohol, and 2-propanol
gave the neutral lanthanide alkoxides LLn(OCH2CF3)(THF) [Ln = Y (1), Yb (2),
Er (3), Sm (4)], LY(OCH2Ph)(THF) (5), and LY(OPri)(THF) (6), respectively.
These lanthanide alkoxides are sensitive to moisture, and the yttrium complex
[(LY)2(μ-OPri)(μ-OH)] (7) was also isolated as a byproduct during the synthesis of
complex 6. Complexes 1−6 were well characterized by elemental analyses and IR
and NMR spectroscopy in the cases of complexes 1 and 4−6. The definitive
molecular structures of all of these complexes were determined by single-crystal X-
ray analysis. It was found that complexes 1−6 can initiate efficiently the ring-opening polymerization of rac-LA and rac-BBL in a
controlled manner. For rac-LA, polymerization gave polymers with very narrow molecular weight distributions (PDI ≤ 1.12) and
very high heterotacticity (Pr up to 0.99). The observed activity-increasing order is in agreement with the order of the ionic radii,
whereas the order for stereoselectivity is in the reverse order. For rac-BBL polymerization, the resultant polymers have narrow
molecular distributions (PDI ≤ 1.26) and high syndiotacticity (Pr up to 0.83). It is worth noting that the activity-decreasing order
Yb > Er > Y ≫ Sm is observed for rac-BBL polymerization, which is opposite to the order of ionic radii and to the order of
activity for rac-LA polymerization. The ionic radii of lanthanide metals have no obvious effect on the stereoselectivity for rac-BBL
polymerization, which is quite different from that for rac-LA polymerization. End-group analysis of the oligomer of rac-BBL
suggested that elimination side reactions occurred slowly in these systems, which led to chain cleavage and the formation of
crotonate (and carboxy) end groups.
polymerizations, organolanthanides, especially yttrium com-
plexes, hold a special position, which is particularly efficient
initiators for the preparation of unique polyesters with
controlled features.6,8c,d,9,10a,b,g
INTRODUCTION
■
Biodegradable polymers, such as poly(lactide) (PLA) and
poly(hydroxybutyrate) (PHB), have recently gained great
attention as a replacement for conventional synthetic materials
because of their biodegradable, biocompatible, and permeable
properties.1 The most efficient synthesis, and the commercial
route to PLA and PHB, involves the ring-opening polymer-
ization (ROP) of the related monomers, respectively.1d,g
Because the stereochemistry of the monomeric units in the
polymer chains plays a decisive role in the mechanical, physical,
and degradation properties of PLA or PHB materials, the
design and synthesis of catalysts to prepare different stereo-
specific PLA or PHB architectures is a major topic.2
Throughout the past decade, significant effort has been invested
in the development of discrete, well-characterized metal
complexes to initiate the stereocontrolled ROP of lactide
(LA) and β-butyrolactone (BBL).3−10 Among the numerous
metal-based catalysts that have been disclosed for these
Amine-bridged bis(phenolate)lanthanide derivatives showed
high activity for the polymerization of cyclic esters, giving
polymers both in high yields and with high molecular weights,
and, in some cases, excellent stereoselectivity for rac-LA
polymerization. However, the alkoxides showed good controll-
ability for these polymerizations, in comparison with the
corresponding lanthanide amides and alkyls. Almost all of the
amine-bridged bis(phenolate)lanthanide alkoxides are gener-
ated in situ by alcoholysis reactions of lanthanide amido or alkyl
complexes with alcohols while these lanthanide alkyl and amido
complexes are extremely sensitive, and alcoholysis reactions are,
Received: August 8, 2012
Published: October 2, 2012
© 2012 American Chemical Society
11133
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Inorganic Chemistry
Article
filtered under vacuum prior to use) was used as an eluent at a flow rate
of 1.0 mL min−1 at 40 °C. The samples were prepared by dissolving a
maximum weight of 5 mg of polymer in 5 mL of THF. The
microstructures of PLAs and PHBs were measured by homodecou-
pling 1H NMR spectroscopy at 20 °C in CDCl3 and by 13C{1H} NMR
spectroscopy at 40 °C in CDCl3, respectively, on a Unity Varian AC-
400 spectrometer.
in some cases, uncontrolled. Thus, unexpected products were
usually generated, which led to termination of the active
propagation species in the polymerization of cyclic esters. The
structurally characterized amine-bridged bis(phenolate)-
lanthanide alkoxide is scarce. To our knowledge, there is only
one example of crystallographically characterized yttrium
alkoxide stabilized by a tetradentate salan ligand formed by
alcoholysis reaction in the literature.10a
Synthesis of LY(OCH2CF3)(THF) (1). To a THF solution of
(C5H5)3Y(THF) (2.41 g, 6.76 mmol) was added a THF solution of
LH2 (3.55 g, 6.76 mmol). The reaction mixture was stirred for 1 h at
room temperature, and then 2,2,2-trifluoroethanol (0.49 mL, 6.76
mmol) was added via a syringe. The mixture was stirred for 24 h at 50
°C, and then THF was evaporated completely under vacuum. Toluene
(15 mL) was added to extract the residue, then THF (1 mL) was
added, and colorless crystals were obtained at 5 °C in several days
(3.70 g, 70%). Mp: 190−192 °C. Anal. Calcd for C40H64F3N2O4Y: C,
61.37; H, 8.24; N, 3.58; Y, 11.36. Found: C, 61.09; H, 8.43; N, 3.73; Y,
Recently, we became interested in studying the synthesis and
reactivity of lanthanide alkoxo complexes that are supported by
the bulky bridged bis(phenolate) ligands.11 It was found that
carbon-bridged bis(phenolate)lanthanide alkoxo complexes can
be conveniently synthesized by proton-exchange reactions
using (C5H5)3Ln(THF) as the starting materials, which are
efficient initiators for the controlled polymerization of ε-
caprolactone.11b,c A further study revealed that lanthanide
aryloxides stabilized by the amine-bridged bis(phenolate)
ligand can be prepared in high yields by sequential proton-
exchange reactions using (C5H5)3Ln(THF) as the precur-
sors,10b but attempts to synthesize the desired amine-bridged
bis(phenolate)lanthanum alkoxides by the same method were
unsuccessful.11d We now find that both the acidity of the
alcohols and the ionic radii of the lanthanide metals play crucial
roles in proton-exchange reactions between amine-bridged
bis(phenolate)lanthanide cyclopentadienyl complexes and
alcohols, and the desired lanthanide alkoxides can be prepared
under suitable reaction conditions using (C5H5)3Ln(THF) as
the precursors. It was found that these lanthanide alkoxides are
efficient initiators for the ROP of rac-LA and rac-BBL to give
polymers with narrow molecular distributions and high syndio/
heterotacticity. A systematic study revealed that the ionic radii
of lanthanide metals have no obvious effect on the stereo-
selectivity for rac-BBL polymerization, but the activity-
decreasing order is opposite to the order of the ionic radii,
which is quite different from those for polymerization of other
cyclic esters. Herein we report these results.
1
11.26. H NMR (300 MHz, C6D6, 25 °C): δ 7.57 (s, 2H, ArH), 7.10
(d, 2H, J(H,H) = 11.7 Hz, ArH), 4.35 (s, 2H, OCH2CF3), 3.94 (br,
2H, ArCH2N), 3.85 (br, 4H, α-CH2THF), 2.89 (br, 2H, ArCH2N),
2.26 (br, 2H, N(CH2)2N), 1.71 (s, 18H, C(CH3)3), 1.66 (br, 6H,
N(CH3)2), 1.53 (br, 2H, N(CH2)2N), 1.43 (s, 18H, C(CH3)3), 1.26
(br, 4H, β-CH2THF). 13C{1H} NMR (101 MHz, C6D6, 25 °C): δ
161.8, 136.5, 136.4, 125.8, 124.6, 124.5 (Ar−C), 70.7 (α-CH2, THF),
66.6 (CF3), 65.2 (CH2CF3), 58.8 (ArCH2N), 48.9 (N(CH2)2N), 45.6
(N(CH3)2), 35.5 (C(CH3)2), 34.3 (C(CH3)3), 32.3 (C(CH3)3), 30.4
(C(CH3)3), 25.3 (β-CH2, THF). IR (KBr pellet, cm−1): 2957(s),
2905(m), 2858(m), 1608(w), 1468(s), 1287(m), 1192(m), 1148(m),
1028(m), 995(w), 958(m), 922(w), 877(m), 825(m), 776(w),
739(m).
Synthesis of LYb(OCH2CF3)(THF) (2). The synthesis of complex
2 followed a procedure similar to that described for the preparation of
complex 1, but (C5H5)3Yb(THF) (2.14 g, 4.86 mmol) was used
instead of (C5H5)3Y(THF). Yellow crystals were obtained in a
toluene/THF solution (3.29 g, 71%). Mp: 193−195 °C. Anal. Calcd
for C40H64F3N2O4Yb: C, 55.41; H, 7.44; N, 3.23; Yb, 19.96. Found: C,
55.44; H, 7.56; N, 3.37; Yb, 19.83. IR (KBr pellet, cm−1): 2957(s),
2904(m), 2859(m), 1604(w), 1468(s), 1287(m), 1243(w), 1192(m),
1148(m), 1027(m), 991(w), 960(m), 922(w), 880(m), 838(m),
778(w), 744(m).
Synthesis of LEr(OCH2CF3)(THF) (3). The synthesis of complex 3
followed a procedure similar to that described for the preparation of
complex 1, but (C5H5)3Er(THF) (2.29 g, 4.37 mmol) was used
instead of (C5H5)3Y(THF). Pink crystals were obtained in the
toluene/THF solution (2.52 g, 67%). Mp: 198−200 °C. Anal. Calcd
for C40H64F3N2O4Er: C, 55.79; H, 7.49; N, 3.25; Er, 19.42. Found: C,
55.51; H, 7.57; N, 3.49; Er, 19.56. IR (KBr pellet, cm−1): 2957(s),
2904(m), 2867(m), 1608(w), 1467(s), 1286(m), 1235(w), 1192(m),
1153(m), 1026(m), 991(w), 960(m), 922(w), 879(m), 833(m),
775(w), 740(m).
Synthesis of LSm(OCH2CF3)(THF) (4). The synthesis of complex
4 followed a procedure similar to that described for the preparation of
complex 1, but (C5H5)3Sm(THF) (1.40 g, 3.35 mmol) was used
instead of (C5H5)3Y(THF). Colorless crystals were obtained in a
toluene/THF solution (1.84 g, 65%). Mp: 206−208 °C. Anal. Calcd
for C40H64F3N2O4Sm: C, 56.90; H, 7.64; N, 3.32; Sm, 17.81. Found:
C, 57.21; H, 7.47; N, 3.63; Sm, 17.77. 1H NMR (300 MHz, C6D6, 25
°C): δ 8.14 (d, 4H, ArH), 7.93 (d, 2H, ArCH2N), 5.45 (br, 2H,
ArCH2N), 4.96 (s, 2H, OCH2CF3), 3.28 (s, 18H, C(CH3)3), 2.32 (br,
2H, N(CH2)2N), 1.94 (br, 4H, β-CH2THF), 1.68 (s, 18H, C(CH3)3),
1.22 (br, 4H, α-CH2THF), 0.55 (s, 6H, N(CH3)2), 0.18 (br, 2H,
N(CH2)2N). IR (KBr pellet, cm−1): 2957(s), 2867(m), 1775(w),
1466(s), 1282(m), 1246(w), 1162(m), 991(w), 960(m), 922(w),
878(m), 823(m), 775(w), 742(w).
EXPERIMENTAL SECTION
■
General Procedures. All of the manipulations were performed
under a purified argon atmosphere using standard Schlenk techniques.
The solvents were degassed and distilled from sodium benzophenone
ketyl under argon prior to use. (C5H5)3Ln(THF)12 and the ligand LH2
[L = Me2NCH2CH2N{CH2-(2-OC6H2But2-3,5)}2]13 were prepared
according to the procedures reported in the literature. 2,2,2-
Trifluoroethanol, benzyl alcohol, and 2-propanol were dried over 4
Å molecular sieves for 1 week and then distilled before use. rac-Lactide
(rac-LA) was recrystallized twice from dry toluene and then sublimed
under vacuum at 50 °C. rac-β-Butyrolactone (rac-BBL) was freshly
distilled from CaH2 under nitrogen and degassed thoroughly by
freeze−pump−thaw cycles prior to use. Lanthanide analyses were
performed by ethylenediaminetetraacetic acid titration with a xylenol
orange indicator and a hexamine buffer.14 Carbon, hydrogen, and
nitrogen analyses were performed by direct combustion with a Carlo-
Erba EA-1110 instrument. The IR spectra were recorded with a
Nicolet-550 Fourier transform IR spectrometer as KBr pellets. The 1H
and 13C NMR spectra were recorded in a C6D6 solution for complexes
1 and 4−6 with a Unity Varian spectrometer. Because of their
paramagnetism, no resolvable NMR spectra for the other complexes
were obtained. The uncorrected melting points of crystalline samples
in sealed capillaries (under argon) are reported as ranges. Molecular
weight and molecular weight distribution (PDI) were determined
against a poly(methyl methacrylate) (PMMA) standard by gel
permeation chromatography (GPC) on a PL 50 apparatus equipped
with PLgel 10 μm MIXED-B columns (300 × 7.5 mm) and a refractive
index detector, and tetrahydrofuran (THF; HPLC grade, distilled and
Synthesis of LY(OCH2Ph)(THF) (5). The synthesis of complex 5
followed a procedure similar to that described for the preparation of
complex 1, but benzyl alcohol (0.36 mL, 3.50 mmol) was used instead
of 2,2,2-trifluoroethanol. Colorless crystals were obtained in a toluene
solution (2.13 g, 77%). Mp: 177−179 °C. Anal. Calcd for
C45H69N2O4Y: C, 68.33; H, 8.79; N, 3.54; Y, 11.24. Found: C,
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Inorganic Chemistry
Article
Table 1. Crystallographic Data for Complexes 1−7
1
2
3
4
5
6
7 ·hexane
formula
fw
C40H64F3N2O4Y
782.84
C40H64F3N2O4Yb
866.97
C40H64ErF3N2O4
861.19
C40H64F3N2O4Sm
844.28
C45H69N2O4Y
790.93
C41H69N2O4Y
742.89
C77H130N4O6Y2
1385.67
T/K
223(2)
223(2)
223(2)
223(2)
293(2)
223(2)
223(2)
cryst syst
cryst size/mm
triclinic
triclinic
triclinic
triclinic
orthorhombic
triclinic
orthorhombic
0.50 × 0.20 ×
0.15
0.60 × 0.40 × 0.30 0.50 × 0.10 × 0.10 0.45 × 0.20 × 0.20 0.70 × 0.50 ×
0.60 × 0.40 ×
0.30
0.40 × 0.30 ×
0.30
0.30
space group
a/Å
P1
P1
P1
P1
P212121
P1
̅
P212121
̅
̅
̅
̅
10.8358(7)
11.7873(6)
17.2975(11)
77.769(4)
88.802(5)
76.706(4)
2100.4(2)
2
10.8072(7)
11.7853(6)
17.2989(12)
77.660(4)
88.739(5)
76.945(4)
2096.0(2)
2
10.8288(7)
11.7776(6)
17.2934(11)
77.713(4)
88.780(4)
76.773(4)
2097.0(2)
2
10.891(3)
11.800(3)
17.306(5)
77.981(15)
89.348(18)
76.359(14)
2112.3(9)
2
13.0187(14)
17.8209(18)
19.477(2)
10.823(3)
11.778(2)
17.233(4)
77.929(14)
89.923(16)
77.180(13)
2092.1(9)
2
17.9555(10)
21.0348(12)
21.4682(13)
b/Å
c/Å
α/deg
β/deg
γ/deg
V/Å3
Z
4518.7(8)
4
8108.3(8)
4
D
calcd/g cm−3
1.238
1.374
1.364
1.327
1.163
1.330
1696
25.35
44370
8264
7247
1.179
1.135
1.472
2984
μ/mm−1
1.440
2.283
2.053
1.441
1.432
F(000)
832
894
890
878
800
θmax/deg
27.5
27.5
25.50
25.50
25.50
25.50
37951
14965
11698
colld reflns
unique reflns
17248
17866
18104
17873
17466
7742
7717
7735
7813
7745
obsd reflns [I >
2.0σ(I)]
6600
7250
6812
6184
6220
no. of variables
462
462
438
456
402
420
730
GOF
R
1.101
1.070
1.093
1.074
1.112
1.091
1.125
0.0724
0.1726
0.0560
0.772, −1.132
0.0388
0.0985
0.0345
1.286, −1.561
0.0484
0.1071
0.0497
0.974, −1.348
0.0747
0.1904
0.0619
1.990, −1.753
0.0712
0.1545
0.0614
0.810, −0.458
0.0603
0.1445
0.0435
1.209, −1.352
0.0833
0.1471
0.0790
0.570, −0.467
wR
Rint
largest diff peak,
hole/e Å−3
68.71; H, 8.33; N, 3.25; Y, 11.49. 1H NMR (400 MHz, C6D6, 25 °C):
δ 7.74 (d, 2H, J(H,H) = 7.5 Hz, ArH), 7.63 (d, 2H, J(H,H) = 2.5 Hz,
ArH), 7.39 (m, 2H, ArH), 7.18 (m, 2H, ArH), 7.15 (s, 1H, ArH), 5.37
(s, 2H, OCH2Ph), 4.01 (d, 2H, J(H,H) = 12.3 Hz, ArCH2N), 3.80 (t,
4H, α-CH2THF), 2.96 (d, 2H, J(H,H) = 12.4 Hz, ArCH2N), 2.35 (s,
2H, N(CH2)2N), 1.83 (s, 18H, C(CH3)3), 1.75 (s, 6H, N(CH3)2),
1.63 (s, 2H, N(CH2)2N), 1.47 (s, 18H, C(CH3)3), 1.18 (m, 4H, β-
CH2THF). 13C{1H} NMR (101 MHz, C6D6, 25 °C): δ 162.2, 148.4,
136.3, 136.2, 128.2, 126.5, 125.9, 125.8, 124.7, 124.4 (Ar−C), 69.9 (α-
CH2THF), 65.3 (OCH2Ph), 58.8 (ArCH2N), 48.9 (N(CH2)2N), 45.7
(N(CH3)2), 35.7 (C(CH3)2), 34.3 (C(CH3)3), 32.3 (C(CH3)3), 30.5
(C(CH3)3), 25.4 (β-CH2THF). IR (KBr pellet, cm−1): 2955(s),
2903(m), 2869(m), 1603(w), 1464(s), 1301(m), 1246(w), 1135(m),
1020(m), 991(w), 960(m), 923(w), 876(m), 837(m), 734(w).
1166(m), 1014(m), 991(w), 960(m), 923(w), 881(m), 828(m),
771(w), 746(m).
Synthesis of [(LY)2(μ-OPri)(μ-OH)] (7). During the synthesis of
complex 6, a small amount of colorless crystals was isolated before the
hexane solution was concentrated.
Typical Procedure for the Polymerization Reaction. The
procedures for polymerization of rac-LA and rac-BBL initiated by
complexes 1−6 were similar, and a typical polymerization procedure is
given as follows. A 50 mL Schlenk flask, equipped with a magnetic
stirring bar, was charged with the desired amount of monomer and
solvent. After the monomer was dissolved, a solution of the initiator
was added to this solution via a syringe. The mixture was immediately
stirred vigorously 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 precipitate the
polymer, which was dried under vacuum and weighed.
Synthesis of LY(OPri)(THF) (6). The synthesis of complex 6
followed a procedure similar to that described for the preparation of
complex 1, but 2-propanol (0.33 mL, 4.28 mmol) was used instead of
2,2,2-trifluoroethanol. Colorless crystals were obtained in a concen-
trated hexane solution (2.52 g, 42%). Mp: 170−172 °C. Anal. Calcd
for C41H69N2O4Y: C, 66.29; H, 9.36; N, 3.77; Y, 11.97. Found: C,
66.12; H, 9.30; N, 3.65; Y, 12.23. 1H NMR (300 MHz, C6D6, 25 °C):
δ 7.64 (d, 2H, J(H,H) = 9.9 Hz, ArH), 7.14 (s, 2H, ArH), 4.49 (s, 1H,
OCH(CH3)2), 4.01 (d, 2H, J(H,H) = 12.3 Hz, ArCH2N), 3.88 (s, 4H,
α-CH2THF), 2.95 (d, 2H, J(H,H) = 12.2 Hz, ArCH2N), 2.35 (s, 2H,
N(CH2)2N), 1.81 (s, 18H, C(CH3)3), 1.74 (s, 3H, N(CH3)2), 1.62
(br, 2H, N(CH2)2N), 1.55 (s, 3H, N(CH3)2), 1.50 (s, 18H, C(CH3)3),
1.35 (s, 6H, OCH(CH3)2), 1.21 (br, 4H, β-CH2THF). 13C{1H} NMR
(75 MHz, C6D6, 25 °C): δ 162.4, 136.2, 136.0, 125.8, 124.8, 124.3
(Ar−C), 70.2 (α-CH2THF), 67.3 (OCH(CH3)2), 65.4 (ArCH2N),
58.9 (N(CH2)2N), 48.9 (N(CH2)2N), 45.9 (N(CH3)2), 35.6 (C-
(CH3)2), 34.3 (C(CH3)3), 32.3 (C(CH3)3), 30.6 (C(CH3)3), 29.9
(OCH(CH3)2), 25.4 (β-CH2THF). IR (KBr pellet, cm−1): 2956(s),
2906(m), 2868(m), 1617(w), 1466(s), 1300(m), 1245(w), 1190(m),
Oligomer Preparation. Oligomerization of rac-LA and rac-BBL
was carried out with complex 5 as the initiator in THF and toluene,
respectively, at 25 °C under the condition of a molar ratio of
[monomer]/[initiator] of 20. The reaction was stirred for 0.5 h and
then quenched by adding n-hexane and 1 drop of water. The
precipitated oligomers were collected, dried under vacuum, and used
1
for H NMR measurement.
X-ray Crystallographic Structure Determination. Suitable
single crystals of complexes 1−7 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 ω
scan mode using Mo Kα radiation (λ = 0.71070 Å). The diffracted
intensities were corrected for Lorentz/polarization effects and
empirical absorption corrections. Details of the intensity data
collection and crystal data are given in Table 1.
The structures were solved by direct methods and refined by full-
matrix least-squares procedures based on |F|2. The hydrogen atoms in
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these complexes were generated geometrically, 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 final stage of full-
matrix least-squares refinement. The structures were solved and
refined using SHELEXL-97 programs.
Scheme 1. Synthesis of Complexes 1−4
RESULTS AND DISCUSSION
■
Synthesis of Amine-Bridged Bis(phenolate)lanthanide
Alkoxo Complexes. The lanthanide aryloxo complexes
stabilized by amine-bridged bis(phenolate) ligand L [L =
Me2NCH2CH2N{CH2-(2-OC6H2But2-3,5)}2] can be prepared
by the reactions of amine-bridged bis(phenolate)lanthanide
cyclopentadienyl complexes with phenols under mild con-
ditions,10b but the corresponding lanthanum cyclopentadienyl
complexes cannot react with methanol or 2-propanol even in
refluxed THF or toluene to generate the desired amine-bridged
bis(phenolate)lanthanum alkoxides.11d Taking into account the
difference in acidity among cyclopentadienes, phenols, and
alcohols, 2,2,2-trifluoroethanol was selected to revisit this
proton-exchange reaction. The reactions of amine-bridged
bis(phenolate)lanthanum and -yttrium cyclopentadienyl com-
Scheme 2. Synthesis of Complexes 5 and 6
1
to the fact that the acidity of benzyl alcohol and 2-propanol is
weaker than that of 2,2,2-trifluoroethanol. These results show
that the acidity of alcohols also has a significant effect on the
proton-exchange reaction.
During the synthesis of complex 6, a small amount of 7 was
concomitantly isolated as a colorless crystalline byproduct,
which should be attributed to the presence of trace water in the
reaction mixture (Scheme 3). The compositions of complexes
plexes with 2,2,2-trifluoroethanol were conducted at first. H
NMR monitoring revealed that the reaction of LYCp(THF)
with 1 equiv of 2,2,2-trifluoroethanol in C6D6 at room
temperature took place rapidly and, after a few minutes,
afforded the yttrium alkoxide complex 1 as a neat product, with
elimination of cyclopentadiene, because the resonance at 6.65
ppm for the cyclopentadienyl group disappeared and the
characteristic signals for cyclopentadiene were observed.
However, in the case of LLaCp(THF), the reaction became
more complicated. When the amine-bridged bis(phenolate)
lanthanum cyclopentadienyl complex reacted with 2,2,2-
trifluoroethanol, the splitting peaks for the eliminated cyclo-
Scheme 3. Pathway for the Formation of Complex 7
1
pentadiene were found at 6.49, 6.30, and 2.69 ppm in the H
NMR spectrum, indicating that the exchange reaction took
place. However, the proton-exchange reaction is not complete
because the single sharp resonance at 6.65 ppm for the
cyclopentadienyl group still existed; even after the reaction was
conducted at elevated temperature, the alcohol was in excess.
Furthermore, when the alcohol was in excess, two new peaks
were observed at 9.85 and 8.61 ppm, indicating that some side
reactions occurred. It is reasonable to postulate that there is an
equilibrium between the amine-bridged bis(phenolate)lantha-
num cyclopentadienyl complex and the corresponding
lanthanum alkoxide. These results revealed that the ionic radii
of the lanthanide metals have a significant effect on the
outcome of this proton-exchange reaction. On a preparative
scale, the yttrium alkoxide complex 1 can also be synthesized by
the direct reaction of (C5H5)3Y(THF) with LH2 in a 1:1 molar
ratio and then with 1 equiv of 2,2,2-trifluoroethanol in situ in
THF. A further study revealed that the analogous amine-
bridged bis(phenolato)lanthanide alkoxides LLn(OCH2CF3)-
(THF) [Ln = Yb (2), Er (3), Sm (4)] can be prepared by
sequential exchange reactions using (C5H5)3Ln(THF) (Ln =
Yb, Er, Sm) as the starting materials in THF at 50 °C in
moderate isolated yields (Scheme 1). For the smaller
lanthanide metal, yttrium, the proton-exchange reactions with
benzyl alcohol and 2-propanol were also examined, and it was
found that the yttrium alkoxides 5 and 6 can also be
synthesized by the same method as that shown in Scheme 2.
However, for samarium, the attempts were unsuccessful, and no
pure product was isolated. This difference should be attributed
1−6 were confirmed by elemental analysis and NMR
spectroscopy in the cases of complexes 1 and 4−6. The
definitive molecular structures of all of these complexes were
determined by single-crystal structural analysis. Complexes 1−6
are sensitive to air and moisture. The crystals decompose in a
few minutes when they are exposed to air, but both the crystals
and solution are rather stable when stored under argon. All of
these lanthanide complexes are freely soluble in THF and
toluene and slightly soluble in hexane.
Crystal Structures. To provide complete structural
information for these new amine-bridged bis(phenolate)-
lanthanide species, single-crystal X-ray structural analyses
were carried out for complexes 1−7. Crystals suitable for an
X-ray structure determination of complexes 1−5 were obtained
from a toluene and THF solution at room temperature,
whereas crystals of complexes 6 and 7 were obtained from a
hexane solution. X-ray diffraction analyses show that complexes
1−6 have monomeric structures and 7 has a dinuclear
structure.
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The structures of complexes 1−6 are similar, and the
ORTEP diagrams of complexes 1, 5, and 6 are shown in
Figures 1−3, respectively. The selected bond lengths and angles
Figure 3. ORTEP diagram of complex 6 showing an atom numbering
scheme. Thermal ellipsoids are drawn at the 20% probability level, and
hydrogen atoms are omitted for clarity.
Figure 1. ORTEP diagram of complex 1 showing an atom numbering
scheme. Thermal ellipsoids are drawn at the 20% probability level, and
hydrogen atoms are omitted for clarity. Complexes 2−4 are
isomorphous with complex 1.
the O4−Ln−N2 angle is slightly distorted away from the
idealized position of 180° to about 168° for these complexes.
The overall coordination geometries of complexes 1−6 are
similar to those of LYbCl(THF), LYb(NPh2)(THF), LYbMe-
(THF),15b LY(CH2SiMe3)(THF),9a and LY(OC6H4-4-CH3)-
(THF).10b In complexes 1−6, the average Ln−O(Ar) bond
lengths are 2.158(3), 2.122(3), 2.144(3), 2.214(7), 2.161(4),
and 2.168(3) Å, respectively, following the trend of Sm > Y >
Er > Yb, which is consistent with their ionic radii, and are
comparable with the corresponding values in the complexes
mentioned above, when the differences in the ionic radii are
considered. Similar consequences are also observed from the
Ln−N1, Ln−N2, Ln−O(CH2CF3), and Ln−O(THF) bond
lengths in these complexes. In complexes 1, 5, and 6, the Y−
OR bond lengths are 2.08(2), 2.063(4), and 2.068(3) Å,
respectively, which are apparently shorter than that in (SalanY-
OPri)2 [2.276(2) Å; Salan = MeN(CH2)2MeN{CH2-(2-
OC6H2But2-3,5)}2] because of the formation of a bridge
bond in the latter.10a
The structure of complex 7 (Figure 4) shows a dinuclear
feature containing a Y2O2 core bridging through the oxygen
atoms of one OiPr group and one hydroxyl group. Each of the
yttrium atoms is six-coordinated by two oxygen and two
nitrogen atoms from the dianionic amine-bridged bis-
(phenolate) ligand, one oxygen atom from the OiPr group,
and one oxygen atom from the hydroxyl group to form a
distorted octahedron. The average bond length of Y1−O(Ar)
[2.138(4) Å] is comparable with that of Y2−O(Ar) [2.119(4)
Å], both of which are slightly shorter than the corresponding
values in the amine-bridged bis(phenolate)yttrium complexes
mentioned above. The OiPr group is unsymmetrically
coordinated to the central metal atoms with a deviation of
0.027 Å, but the hydroxyl group is almost symmetrically
coordinated to the yttrium atoms.
Figure 2. ORTEP diagram of complex 5 showing an atom numbering
scheme. Thermal ellipsoids are drawn at the 20% probability level, and
hydrogen atoms are omitted for clarity.
for these complexes are provided in Tables 2 and 3. Like other
amine-bridged bis(phenolate)lanthanide complexes,9a,10b,15 the
side-arm amido group was found to bind to the metal center in
the solid state. Each metal ion is six-coordinated to two oxygen
and two nitrogen atoms from the dianionic amine-bridged
bis(phenolate) ligand, one oxygen atom from the alkoxo group,
and one oxygen atom from one THF molecule. The
coordination geometry around the metal center can be best
described as a slightly distorted octahedron, in which O1, O2,
O3, and N1 can be considered to occupy equatorial positions
within the octahedron. O4 and N2 occupy axial positions, and
ROP of rac-LA by Complexes 1−6. To understand the
effect of the lanthanide ionic radius, and the structure of the
initiating group of amine-bridged bis(phenolate)lanthanide
complexes on the polymerization activity, controllability, and
stereoselectivity, the catalytic behavior of complexes 1−6 for
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Table 2. Selected Bond Lengths (Å) and Bond Angles (deg) for Complexes 1−6
1
2
3
4
5
6
Bond Lengths
Ln1−O1
Ln1−O2
Ln1−O3
Ln1−O4
Ln1−N1
Ln1-N2
2.153(3)
2.162(3)
2.08(2)
2.116(3)
2.127(3)
2.068(18)
2.352(3)
2.549(3)
2.516(4)
2.136(3)
2.152(3)
2.197(7)
2.230(7)
2.09(3)
2.151(4)
2.170(4)
2.063(4)
2.372(4)
2.554(4)
2.522(5)
2.161(3)
2.175(3)
2.068(3)
2.379(3)
2.617(3)
2.561(4)
2.08(2)
2.376(3)
2.588(3)
2.553(4)
2.363(4)
2.432(7)
2.652(8)
2.655(9)
2.567(4)
2.529(5)
Bond Angles
170.53(14)
111.2(6)
O4−Ln1−N2
O1−Ln1−O3
O2−Ln1−O3
O2−Ln1−N1
O1−Ln1−N1
O3−Ln1−N1
O1−Ln1−O2
O1−Ln1−N2
170.61(12)
110.6(6)
170.80(11)
110.8(5)
168.9(3)
110.5(17)
108.6(19)
78.1(3)
168.48(15)
99.94(16)
106.39(17)
79.90(13)
77.17(13)
159.46(15)
153.15(15)
96.17(16)
172.94(11)
107.61(12)
106.86(13)
77.13(10)
78.47(10)
154.07(12)
142.42(11)
94.14(12)
104.4(8)
101.3(6)
102.1(5)
79.06(10)
78.27(10)
148.5(7)
79.80(11)
79.06(11)
152.8(10)
144.09(12)
102.88(12)
79.47(13)
78.77(12)
152.7(12)
142.49(15)
103.46(15)
77.6(2)
145.1(13)
138.7(3)
104.6(3)
142.35(12)
103.01(12)
It was found that the number-average molecular weight (Mn)
values are generally in good agreement with the calculated ones,
when the molar ratio of monomer to initiator is lower than
1000 in THF. However, as the molar ratio of monomer to
initiator increases, the experimental Mn values apparently
deviate from the calculated ones. A similar phenomenon has
been also observed by Carpentier and Coates, and this
mismatch possibly arises from chain-transfer processes or
poor correlation between the PMMA calibration of the gel
permeation chromatograph and the actual molecular weights of
the PLA chains.3k,6b Solvent plays a key role not only in the
polymerization activity but also the stereoselectivity. Using
complex 1 as the initiator (Figure 5), the yield is 98% and Pr is
0.98 (Pr: probability of racemic enchainment) in THF when the
molar ratio of monomer to initiator is 400 (Table 4, entry 2),
whereas the yield is 44% and Pr is 0.74 in toluene under the
same polymerization conditions (Table 4, entry 3). The effects
of the solvent in our cases are consistent with those observed in
the alkoxyamino-bridged bis(phenolate)lanthanide systems.6a,k
The ionic radii of the lanthanide metals have an obvious
effect on the catalytic activity and stereoselectivity but have no
obvious effect on the controllability for rac-LA polymerization.
Using complex 4 as the initiator, the yield is 96% when the
molar ratio of monomer to initiator is 2000 in 1 h (Table 4,
entry 17), whereas the yield is 84% using the ytterbium
complex 2 as the initiator, even when the molar ratio of
monomer to initiator decreased to 1500 under the same
polymerization conditions (Table 4, entry 10). The activity-
decreasing order of Sm > Y ≈ Er > Yb is in agreement with the
order of their ionic radii, which is also consistent with the
activity trend observed in the other lanthanide initiator systems
for LA polymerization.10b,i,11a This may be attributed to the
larger ionic radii resulting in a greater opening of the metal
coordination sphere in the vicinity of the σ ligand, which makes
insertion of rac-LA into Ln−O bonds easier. All of these
lanthanide alkoxo complexes polymerize rac-LA to give highly
heterotactic PLAs. The increasing order in heterotacticity
follows the decreasing order of the ionic radii of the central
metal: Sm (Pr = 0.93−0.94) < Y (Pr = 0.97−0.98) ≤ Er (Pr =
0.98−0.99) ≤ Yb (Pr = 0.99). This result revealed that a
crowded coordination environment around the metal center is
essential for higher stereoselectivity for rac-LA polymerization.
The influence of the ionic radii of the metal center reported
Table 3. Selected Bond Lengths (Å) and Bond Angles (deg)
for Complex 7
bond lengths
Y1−O1
bond angles
O5−Y1−N1
2.117(4)
2.158(4)
2.244(4)
2.267(4)
2.494(5)
2.622(5)
2.107(4)
2.131(4)
2.271(4)
2.278(4)
2.526(5)
2.598(6)
166.00(16)
97.11(17)
94.41(17)
85.05(17)
79.98(17)
172.73(15)
100.77(18)
96.03(18)
79.07(17)
94.08(17)
73.19(14)
72.47(14)
107.46(16)
106.46(16)
Y1−O2
Y1−O5
Y1−O6
Y1−N1
Y1−N2
Y2−O3
Y2−O4
Y2−O5
Y2−O6
Y2−N3
Y2−N4
O1−Y1−O2
O1−Y1−O6
O2−Y1−N2
O6−Y1−N2
O5−Y2−N3
O3−Y2−O4
O3−Y2−O6
O6−Y2−N4
O4−Y2−N4
O5−Y1−O6
O5−Y2−O6
Y1−O5−Y2
Y1−O6−Y2
Figure 4. ORTEP diagram of complex 7 showing an atom numbering
scheme. Thermal ellipsoids are drawn at the 20% probability level, and
hydrogen atoms are omitted for clarity.
the ROP of rac-LA was examined. The representative
polymerization data are summarized in Table 4.
All of these complexes were proven to be active initiators for
the ROP of rac-LA and gave PLAs with high molecular weights.
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a
Table 4. Polymerization of rac-LA Initiated by Complexes 1−6
b
c
d
d
e
entry
catalyst
[M]0/[I]0
t
yield (%)
Mc (×104)
Mn (×104)
PDI
Pr
1
2
1
1
1
1
1
1
1
2
2
2
3
3
3
4
4
4
4
4
5
5
5
6
6
6
200
400
1 h
96
98
44
98
98
98
90
95
95
84
93
94
91
95
95
92
96
73
98
96
88
98
95
89
2.76
5.64
3.67
6.40
1.07
1.07
0.97
0.98
0.74
0.98
0.98
0.97
0.97
0.99
0.99
0.99
0.98
0.99
0.98
0.94
0.93
0.94
0.94
0.93
0.98
0.97
0.98
0.98
0.98
0.97
1 h
f
3
400
1 h
4
600
1 h
8.47
11.29
14.11
19.44
5.47
9.01
10.15
12.30
14.25
7.82
1.05
1.06
1.05
1.09
1.03
1.04
1.05
1.07
1.09
1.09
1.09
1.08
1.10
1.09
1.12
1.06
1.06
1.09
1.06
1.07
1.07
5
800
1 h
6
1000
1500
400
1 h
7
1 h
8
1 h
9
1000
1500
400
1 h
13.68
18.14
5.36
16.90
19.81
5.83
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
1 h
1 h
1000
1500
400
1 h
13.54
19.66
5.47
13.15
14.58
6.09
1 h
10 min
20 min
30 min
1 h
1000
1500
2000
3000
400
13.68
19.87
27.65
31.54
5.64
10.43
12.25
14.06
12.57
5.53
2 h
1 h
1000
1500
400
1 h
13.82
19.01
5.64
10.53
12.11
5.94
1 h
1 h
1000
1500
1 h
13.68
11.25
13.43
1 h
19.22
a
b
General polymerization conditions: THF as the solvent; [rac-LA] = 1 mol L−1, at 25 °C. Yield: weight of polymer obtained/weight of monomer
c
d
e
used. Mc = 144.13 × ([M]0/[I]0) × (polymer yield) (%). Measured by GPC calibrated with standard PMMA samples. Measured by
homodecoupling H NMR spectroscopy at 25 °C. In toluene.
f
1
remained almost unchanged (ranging from 1.05 to 1.09),
indicating that the polymerization process is controlled.
The property of the initiating groups has no obvious effect
on the catalytic activity but has a profound effect on the
controllability for rac-LA polymerization. In comparison with
the amine-bridged bis(phenolate)lanthanide alkyl,9a amido,6a
and aryloxo complexes,10b these lanthanide alkoxides exhibit
similar activity for rac-LA polymerization. However, the
polymerization controllability of these lanthanide alkoxo
complexes is superior to those of the lanthanide complexes
mentioned above and is as good as that of the amine-bridged
bis(phenolate)lanthanide alkoxo complexes generated in situ.6a
These results can be attributed to the fact that the alkoxo
groups are more nucleophilic in comparison with the alkyl and
amido groups, which caused a fast initiation and gave the
polymers narrow PDIs. A comparative study (polymerization
by complexes 1, 5, and 6) revealed that the structures of the
alkoxo groups in these complexes have no obvious effect on the
activity and controllability for rac-LA polymerization. However,
for the stereoselectivity, the situation is complicated. The
stereoselectivity of these yttrium alkoxo complexes for rac-LA
polymerization is similar to those of the corresponding yttrium
alkyls, amides, and aryloxides. However, the samarium alkoxo
complex 4 showed better stereoselectivity than the amine-
bridged bis(phenolate)samarium borohydride complex (the Pr
values are 0.94 and 0.83, respectively),8b which indicates that
the initiating group seems to have an influence on the
stereoselectivity of rac-LA polymerization.
Figure 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.
here is also in agreement with the observations reported by the
Carpentier group in the alkoxyamino-bridged bis(phenolate)-
lanthanide systems.6a,k The polymerizations initiated by
complexes 1−6 gave polymers with very narrow PDIs (1.03−
1.12), which revealed that the congestion around the metal
center has no obvious effect on the controllability. To further
elucidate the controlled character of the polymerization, the
relationship between the number-average molecular weight
(Mn) and the molar ratio of monomer to initiator ([M]0/[I]0)
was measured (Table 4, entries 1, 2, and 4−6). When the molar
ratio of monomer to initiator increased, the molecular weight of
the resultant polymer increased linearly, whereas the PDIs
The initiation mechanism was elucidated by end-group
analysis of the oligomer of rac-LA, which was prepared by the
reaction of complex 5 with rac-LA in a 1:20 molar ratio. End-
1
group analysis by H NMR spectroscopy clearly showed the
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1
Figure 6. H NMR spectrum of rac-LA oligomer initiated by complex 5 in CDCl3 ([rac-LA]0:5 = 20:1, 25 °C).
a
Table 5. Polymerization of rac-BBL Initiated by Complexes 1−6
b
c
d
d
e
entry
catalyst
[M]0/[I]0
t
yield (%)
Mc (×104)
Mn (×104)
PDI
Pr
1
2
3
4
1
1
2
2
2
2
2
2
2
2
2
3
3
4
5
5
6
6
400
1000
200
10 min
10 min
2 min
91
77
3.13
6.62
1.72
3.44
4.46
8.82
2.12
4.20
1.11
1.17
1.25
1.21
0.83
0.83
0.83
0.83
0.67
0.82
0.83
0.83
0.83
0.83
0.83
0.83
0.83
100
100
55
400
2 min
f
5
400
30 min
2 min
g
6
400
87
2.99
5.16
3.36
5.58
1.20
1.21
1.19
1.13
1.13
1.24
1.18
1.25
7
600
10 min
10 min
10 min
30 min
30 min
10 min
10 min
5 h
100
100
100
99
8
800
6.88
7.42
9
1000
1500
2000
400
8.60
10.35
15.65
16.19
3.55
10
11
12
13
14
15
16
17
18
12.77
15.82
3.44
92
100
81
1000
400
6.97
6.13
57
400
10 min
10 min
10 min
10 min
95
3.27
6.88
3.27
7.14
3.51
7.40
3.44
8.06
1.26
1.18
1.21
1.15
0.83
0.82
0.83
0.82
1000
400
80
95
1000
83
a
b
General polymerization conditions: toluene as the solvent; [rac-BBL] = 2 mol L−1, at 25 °C. Yield: weight of polymer obtained/weight of
c
d
e
monomer used. Mc = 86 × ([M]0/[I]0) × (polymer yield) (%). Measured by GPC calibrated with standard PMMA samples. Determined from
f
g
the carbonyl region of the 13C NMR spectrum at 25 °C. In THF. [rac-BBL] = 1 mol L−1.
existence of a benzyloxy group and a HOCH(CH3)CO− group
according to the resonances at about 7.35 ppm for the former
and at 1.26, 2.76, and 4.36 ppm for the latter, as shown in
Figure 6. Meanwhile, no proton resonance of the ligand was
polymerization behavior of complexes 1−6 for polymerization
of rac-BBL has also been examined, and the results are
summarized in Table 5. It can be seen that the polymerizations
of rac-BBL initiated by complexes 1−6 also proceeded in a
controlled fashion, resulting in PHBs with narrow PDIs (Mw/
Mn ≤ 1.26) and a good agreement of the number-average
molecular weights determined by GPC with the calculated Mn
values (Table 5).
1
observed in the H NMR spectrum of the oligomer, which
revealed that the amine-bridged bis(phenolate) group was not
involved in the polymerization process. Thus, polymerization
proceeds via a common “coordination−insertion” mechanism.
ROP of rac-BBL by Complexes 1−6. Highly active and
controlled ROP of BBL to produce high-molecular-weight PHB
is rare5k,6b−i because most of the catalysts showed low activity
toward β-lactone polymerizations, despite their favorable ring
strain,16 and a high prevalence of chain termination events, such
as transesterification and chain transfer.2a,6j In this work, the
The solvent also has a significant influence on both the
polymerization rate and stereoselectivity. Different from that
observed for rac-LA polymerization, toluene is a better
polymerization medium than THF. For example, using complex
2 as the initiator, polymerization in toluene reached 100%
conversion in less than 2 min, and Pr of the resultant polymer is
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Inorganic Chemistry
Article
1
Figure 7. H NMR spectrum of rac-BBL oligomer initiated by complex 5 in CDCl3 ([rac-BBL]0:5 = 20:1, 25 °C).
0.83 (Table 5, entry 4), when the molar ratio of rac-BBL to
complex 2 is 400, whereas polymerization in THF led to 55%
conversion within 30 min, and Pr is 0.67 under the same
polymerization conditions (Table 5, entry 5). It was found that
decreasing the monomer concentration resulted in a slight
decrease in the polymerization rate (Table 5, entries 4 and 6).
The effects of the solvent and the monomer concentration in
our cases are consistent with those observed in the
alkoxyamino-bridged bis(phenolate)lanthanide systems.6b,d
Therefore, all other polymerization reactions were preferably
conducted in toluene at a concentration of rac-BBL of 2 M.
The ionic radii of the lanthanide metals have significant effect
on the catalytic activity for rac-BBL polymerization. For
example, using the ytterbium complex 2 as the initiator, the
yield was 99% in 30 min, when the molar ratio of monomer to
initiator was 1500 (Table 5, entry 10), whereas the yield was
57% in 5 h using the samarium complex 4 as the initiator, even
when the molar ratio of monomer to initiator decreased to 400
under the same polymerization conditions (Table 5, entry 14).
The activity-decreasing order Yb > Er > Y ≫ Sm is opposite to
the order of the ionic radii and the activity order for rac-LA
polymerization. Carpentier et al. found that the lanthanum
amido complexes stabilized by tridentate pyridyine- and
thiophene-linked bis(naphtholate) ligands showed the highest
polymerization rate for rac-BBL polymerization, in comparison
with the corresponding yttrium and scandium amido complex-
es.6i Similarly, bis(guanidinate)neodymium isopropoxide is
much more active than its yttrium and lutetium congeners
for the ROP of rac-BBL.6g These results indicated that the
ancillary ligand can adversely affect the activity order of the
ionic radii on the polymerization, although the real reason still
remains unclear. It is worth noting that the ionic radii of the
lanthanide metals have no obvious effect on the stereo-
selectivity for rac-BBL polymerization in this ligand system,
which is quite different from that observed in rac-LA
polymerization. All of these complexes polymerize rac-BBL to
give syndiotactic-enriched polymers, and the Pr values are about
0.82.
The nature of the alkoxo groups does not affect the activity
and stereoselectivity for rac-BBL polymerization. Complexes 1,
5, and 6 show similar activity and stereoselectivity for the
polymerization. The activity and stereoselectivity of these
lanthanide alkoxo complexes are also comparable with the
alkoxyamino-bridged bis(phenolate)lanthanide alkoxo com-
plexes generated in situ (Pr is about 0.81).6b,d
The initiation mechanism was elucidated by end-group
analysis of the oligomer of rac-BBL, which was prepared by the
reaction of complex 5 with rac-BBL in a 1:20 molar ratio. End-
1
group analysis by H NMR spectroscopy clearly showed the
existence of a benzyloxy group and a HOCH(CH3)CH2CO−
group according to the resonances at about 7.34 and 5.12 ppm
for the former and at 4.19, 2.30, and 1.38 ppm for the latter, as
shown in Figure 7, which confirmed that the rac-BBL ring is
opened via cleavage of the acyl−oxygen bond and then inserted
into the Ln−O bond. Meanwhile, trans-crotonate (and
1
carboxy) groups were also observed in the H NMR spectrum,
which indicated that the elimination side reaction exists in the
polymerization progress. A similar elimination reaction in rac-
BBL polymerization has been observed using bulky β-
diiminatezinc alkoxides,3k Al(OiPr)3, and tin-based complexes
as the initiators,17 but not observed in the polymerization
initiated by the alkoxyamino-bridged bis(phenolate)lanthanide
alkoxo complexes formed in situ.6b
CONCLUSION
■
In summary, a series of neutral amine-bridged bis(phenolate)-
lanthanide alkoxo complexes were successfully synthesized via
proton-exchange reactions using the readily available
(C5H5)3Ln(THF) as the precursors, and their structural
features have been provided by a X-ray diffraction study.
Both the acidity of the alcohols and the ionic radii of the
lanthanide metals significantly affect the proton-exchange
reaction. It was found that these lanthanide alkoxo complexes
are efficient initiators for the ROP of rac-LA and rac-BBL. Both
the ancillary ligand and the ionic radii of the lanthanide metals
have a profound effect on the activity and stereoselectivity. The
nature of the initiating groups has a significant effect on the
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controllability but has no obvious effect on the activity and
stereoselectivity. For rac-LA polymerization, these complexes
displayed good activity, excellent controllability, and high
stereoselectivity to give highly heterotactic PLAs. The observed
activity-increasing order is in agreement with the order of the
ionic radii, but the heterotacticity follows the opposite
sequence. The stereoselectivity is similar to those of the
corresponding lanthanide alkyls, amides, and aryloxides but is
better than that of the samarium borohydride. For rac-BBL
polymerization, these complexes represented one of the best
initiators in activity and stereoselectivity to give syndiotactic-
enriched polymers. It was found that the activity-decreasing
order of Yb > Er > Y ≫ Sm for rac-BBL polymerization is
opposite to the order of the ionic radii for the first time, and the
ionic radii of the lanthanide metals have no obvious effect on
the stereoselectivity, which is quite different from that for rac-
LA polymerization.
ASSOCIATED CONTENT
■
S
* Supporting Information
X-ray crystallographic data in CIF format. This material is
available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: yaoym@suda.edu.cn. Tel.: (86)512-65882806. Fax:
(86)512-65880305.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the National Natural Science
Foundation of China (Grants 20972108, 21072146,
21174095, and 21132002), PAPD, and the Qing Lan Project
is gratefully acknowledged.
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