New [ONOO]-type amine bis(phenolate) ytterbium(II) and -(III) complexes: Synthesis, structure, and catalysis for highly heteroselective polymerization of rac-lactide

Article abstract of DOI:10.1021/ic401747n

Two divalent ytterbium (Yb^II) complexes, 1 and 2, supported by new [ONOO]-type amine bis(phenolate) ligands L^a,b were synthesized in good yield by an amine elimination reaction of Yb^II(N(SiMe ₃)₂)

Full text of DOI:10.1021/ic401747n

Article
pubs.acs.org/IC
New [ONOO]-Type Amine Bis(phenolate) Ytterbium(II) and -(III)
Complexes: Synthesis, Structure, and Catalysis for Highly
Heteroselective Polymerization of rac-Lactide
Sheng Yang, Kun Nie, Yong Zhang, Mingqiang Xue, Yingming Yao, and Qi Shen*
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
S
* Supporting Information
ABSTRACT: Two divalent ytterbium (YbII) complexes, 1 and 2,
supported by new [ONOO]-type amine bis(phenolate) ligands La,b
were synthesized in good yield by an amine elimination reaction of
YbII(N(SiMe3)2)2(TMEDA) (TMEDA = tetramethylethylenedi-
amine) with one equivalent of the ligand precursor. X-ray structural
determination showed complexes 1 and 2 both have a monomeric
structure. Each adopts distorted octahedral coordination geometry
around the six-coordinate YbII ion. Two new trivalent ytterbium
(YbIII) aryloxide complexes, 4 and 5, bearing [ONOO]-type amine
bis(phenolate) ligands Lc and Ld were prepared by double-
protonation reaction of Yb(C5H5)3THF with one equivalent of
the ligand precursor, then one equivalent of phenol. Complex 4 has
a symmetric dimeric structure with a Yb2O2 core bridging through
the oxygen atoms of the OC6H4-4-CH3 groups. Complex 5 is a THF-solvated monomer. Each six-coordinated YbIII ion in both
complexes adopts a distorted octahedron. All the complexes synthesized together with the known YbII complex 3 were evaluated
in the ring-opening polymerization (ROP) of rac-lactide (rac-LA). Complexes 1 and 4 were found to be extremely active for
controlled ROP of rac-LA, as judged by narrow molar mass distributions (Mw/Mn: 1.07−1.16 for complex 1 and 1.07−1.10 for
complex 4) and experimental molar mass Mn,exp values in good agreement with theoretic Mn,calcd values calculated on a single PLA
chain produced per metal center of initiator. Complex 5 is less controlled. Complexes 1, 4, and 5 exhibited the same high
stereoselectivity to give heterotactic polylactide with a Pr (probability of racemic enchainment of monomer units) ranging from
0.97 to 0.99. Dramatic decreases in activity and stereoselectivity were found for complexes 2 (Pr = 0.82) and 3 (Pr = 0.67),
indicating the framework of L in the unit of [YbL] is crucial for determining activity and stereoselectivity of either YbII or YbIII
complexes.
borohydride, and alkylide) bearing sterically demanding
ancillary ligands, such as dianionic heteroatom-bridged-bi-
sphenolate ligands, β-diiminato ligands, and salalen-type
ligands, are notable as highly active for the polymerization of
rac-LA to highly stereoselective PLAs by a chain-end control
mechanism.6−10
INTRODUCTION
■
Currently, biodegradable polymers as a promising alternative of
synthetic petrochemical-based plastics have attracted consid-
erable attention. Of the variety of biodegradable polymers
known, poly(lactic acid)s (PLAs) derived from biorenewable
resources have become one of the most important candidates
because of their biodegradable, biocompatible nature and
potential wide-ranging commercial applications (e.g., single-use
packaging materials, medical sutures, and drug delivery
systems).1 Stereochemistry in PLAs plays an important role
in determining the mechanical properties, biodegradability, and
ultimately the end use of the material.2 Consequently, many
efforts over the past decade have been made toward the
preparation of discrete, well-defined single-site catalysts for
controlling the stereochemistry of lactide polymerization. A
variety of systems based on metal complexes were successfully
explored to prepare heterotactic and stereoblock isotactic PLA
from the enantiomerically pure monomers, the racemic
mixture, or meso-lactide.3−10 Among the variety of initiators,
trivalent lanthanide (LnIII) complexes (amide, alkoxide,
Systematic studies on LnIII complexes bearing various
bis(phenolate) ligands have demonstrated the bridge and the
ortho-phenolate substituents play a crucial role in achieving
high heteroselectivity for the chain-end-controlled polymer-
ization of rac-LA.7b,6a The LnIII metal also has a great influence
on the activity and selectivity.6a,7a,10f
Contrary to LnIII complexes, the stereoselective polymer-
ization ability of LnII complexes remains largely unexplored,
although these complexes proved to be highly active initiators
for ring-opening polymerization (ROP) of ε-caprolactone and
L-LA as well.11 Recently, we described the synthesis of a series
Received: July 7, 2013
© XXXX American Chemical Society
A
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Inorganic Chemistry
Article
of YbII complexes with tetradentate bis(phenolate) ligands
including amine-amine-bis(phenolate) Le, methoxy-amine bis-
(phenolate) Lf, and piperazidine-bridged bis(phenolate) Lg
(Figure 1) and reported for the first time the rapid, controlled,
procedures. Rare earth metal analysis was performed by ethyl-
enediaminetetraacetic acid titration with a xylenol orange indicator and
a hexamine buffer.18
Instruments and Measurements. Organometallic samples for
NMR spectroscopic measurements were prepared in a glovebox and
1
then sealed by paraffin film. H and 13C NMR spectra were recorded
on a Unity Inova-400 (FT, 400 MHz for 1H; 100 MHz for 13C)
spectrometer. Homonuclear decoupled 1H NMR spectra were
recorded on a Unity Inova-400 spectrometer at 20 °C in CDCl3.
Carbon, hydrogen, and nitrogen analyses were performed by direct
combustion with a Carlo-ErbaEA-1110 instrument. IR spectra were
recorded on a Nicolet-550 Fourier transform IR spectrometer as KBr
pellets. Melting points of the complexes were measured in sealed
capillaries and are uncorrected. MALDI-TOF mass spectra were
recorded using a Bruker Reflex II mass spectrometer (Bremen,
Germany). Molar mass (Mn) and molar mass distribution (Mw/Mn)
were determined against a polystyrene standard by SEC (size exclusion
chromatography) on a PL 50 apparatus, and THF was used as an
eluent at a flow rate of 1.0 mL min−1 at 40 °C. The melting point of
PLA was measured at a heating rate of 2 °C on a DSC 2010TA
instrument.
Synthesis of H2La. A solution of 2-tert-butyl-4-methylphenol (16.4
g, 100 mmol), 2-(aminomethyl)tetrahydrofuran (5.21 g, 50 mmol),
and 36% aqueous formaldehyde (9.7 mL, 100 mmol) in methanol (50
mL) was stirred and refluxed for 40 h. The mixture was cooled, and
the supernatant solution decanted. The solid residue was triturated
with ice cold methanol, filtered, and washed thoroughly with cold
methanol. The ligand precursor H2La was isolated as a white powder
(38.5 g, 85% yield). Anal. Calcd for C29H43NO3 (453.66): C, 76.78; H,
9.55; N, 3.09. Found: C, 76.93; H, 9.29; N, 3.25. 1H NMR (400 MHz,
C6D6, 25 °C): δ 8.78 (s, 2 H, ArOH), 6.90 (s, 2 H, ArH), 6.70 (s, 2 H,
ArH), 4.24 (s, 1 H, CHO), 4.03−3.89 (m, 2 H, CH2O), 3.73 (s, 4 H,
ArCH2NCH2Ar), 2.57−2.50 (m, 2 H, NCH2Furf), 2.23 (s, 6 H,
ArCH3), 1.88 (s, 4 H, CH2CH2), 1.39 (s, 18 H, C(CH3)3).
Figure 1. YbII complexes with Le−g and Lh.15
12
and stereoselective ROP of rac-LA with complexes
YbIILe(TMEDA) and YbIILf(TMEDA), forming high-hetero-
tactic PLA with a Pr (probability of racemic enchainment of
monomer units) as high as 0.99. The importance of the bridge
in these ligands on the control of stereoselectivity was also
demonstrated.12 To expand upon our initial work, we were
interested in investigating the effect of ligand architecture on
rac-LA polymerization behavior of YbII complexes.
THF-amine- and furan-amine-bisphenolate ligands were first
synthesized in 2003 by Moshe Kol and his co-workers. Group
IV metal complexes bearing these ligands were found to be
highly active 1-hexene polymerization catalysts.13 These
[ONOO]-type bis(phenolate) ligands have been used as
ancillary ligands in d-block transition metal and alkaline-earth
metal chemistry.14 However, their application in designing well-
defined single-site lanthanide catalysts has still remained
unexplored. In this paper, we report that the new YbII
complexes with [ONOO]-type bis(phenolate) ligands enable
the initiation of the rapid, controlled, and stereoselective ROP
of rac-LA, giving high-heterotactic PLAs (Pr ranging from 0.97
to 0.98). The synthesis and molecular structure of these
complexes and the proposed polymerization mechanism
including the real active species for the systems with YbII
complexes will be presented. The synthesis and molecular
structure of two new YbIII aryloxide complexes with [ONOO]-
bis(phenolate) ligands and their reactivity for highly hetero-
selective ROP of rac-LA are also described.
Synthesis of H2Lb. A solution of 2-tert-butyl-4-methylphenol (8.2
g, 50 mmol), 2-(aminomethyl)furan (4.86 mL, 50 mmol), and 36%
aqueous formaldehyde (9.7 mL, 100 mmol) in methanol (50 mL) was
stirred and refluxed for 24 h. After cooling, another equivalent of the
phenol (8.2 g, 50 mmol) was added, and the reaction was left stirring
at room temperature for several days. The solvent was evaporated, and
the heavy oil was left at room temperature for another two weeks, then
was triturated with cold hexane, filtered, and washed thoroughly with
cold hexane. The first part of H2Lb was obtained as a white powder.
The mother liquid was evaporated to give a crude oil again after a
week at room temperature, and the oil was then triturated again to give
more product. This procedure was repeated several times until no
more solid precipitated. The total amount of product isolated in this
manner was 19.3 g (43% yield). Anal. Calcd for C29H39NO3 (449.62):
1
C, 77.47; H, 8.74; N, 3.12. Found: C, 77.61; H, 8.39; N, 3.19. H
NMR (400 MHz, C6D6, 25 °C): δ 7.53 (s, 2 H, ArOH), 7.47 (s, 1 H,
CHO), 7.01 (s, 2 H, ArH), 6.76 (s, 2 H, ArH), 6.38 (s, 1 H, CH), 6.27
(s, 1 H, CH), 3.67 (s, 2 H, NCH2Furf), 3.64 (s, 4 H, ArCH2NCH2Ar),
2.24 (s, 6 H, ArCH3), 1.40 (s, 18 H, C(CH3)3).
YbIILa(TMEDA) (1). A Schlenk flask was charged with H2La (1.36 g,
3.00 mmol), toluene (15 mL), and a stirring bar. To this solution was
added a toluene (20 mL) solution of Yb(N(SiMe3)2)2(TMEDA) (1.83
g, 3.00 mmol), and the color of the solution became dark green. The
solution was stirred at room temperature for 12 h. After concentration,
the solution was placed at −5 °C for crystallization. After several days
black-red microcrystals of 1 (1.29 g, 58% based on Yb) were obtained.
Mp: 154−156 °C (dec). Anal. Calcd for YbC35H57N3O3 (740.90): C,
56.74; H, 7.75; N, 5.67; Yb, 23.36. Found: C, 56.93; H, 7.79; N, 5.25;
EXPERIMENTAL SECTION
■
General Methods. All the manipulations were performed in a pure
argon atmosphere using standard Schlenk techniques or under a
nitrogen atmosphere in a glovebox. The solvents were degassed and
distilled from sodium benzophenone ketyl prior to use. The phenols
and amines were purchased from Aldrich or Alfa Aesar. All liquids
were dried over 4 Å molecular sieves for a week and distilled before
use, and solid materials were used without purification. rac-LA
(Aldrich) was recrystallized three times with dry ethyl acetate. Ligands
H2Lc,d,13,16 YbII (N(SiMe3)2)2(TMEDA),12 Yb(C5H5)3(THF),17 and
YbIILh(THF)3 (3)15 were synthesized according to literature
1
Yb, 23.67. H NMR (400 MHz, C6D6, 25 °C): δ 7.01−7.36 (m, 4 H,
ArH), 6.90 (s, 1 H, CH), 4.13−4.25 (m, 2 H, CH2O), 2.98−3.23 (m, 4
H, ArCH2N), 2.62−2.69 (t, 2 H, JH−H = 14 Hz, NCH2), 2.45−2.46 (d,
6 H, J = 4 Hz, ArCH3), 2.11 (s, 12 H, N(CH3)2), 1.96 (s, 4 H,
CH2CH2), 1.74 (s, 18 H, C(CH3)3), 0.89 (s, 4 H, NCH2CH2N). 13C
NMR (100 MHz, C6D6, 25 °C): δ 166.11 (arom-CO), 135.65 (arom-
CCH2N), 131.58 (arom-CBut), 130.55 (arom-CMe), 124.47 (arom-
CH), 118.08 (arom-CH), 79.56 (CH−O), 67.43 (CH2O), 64.60
B
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Article
Table 1. Crystallographic Data for Complexes 1, 2, 4, and 5
1
2
4
5
empirical formula
fw
C35H57N3O3Yb·C7H8
833.01
C35H53N3O3Yb·0.5C7H8
782.91
C84H122N2O8Yb2·C7H8
C49H76NO5Yb
932.15
1726.05
223(2)
orthorhombic
Pbcn
temperature (K)
cryst syst
space group
a (Å)
223(2)
223(2)
223(2)
orthorhombic
P212121
15.387(2)
16.533(2)
18.915(3)
90
monoclinic
P21/c
monoclinic
P21/c
20.276(5)
13.306(3)
16.615(4)
90
13.552(4)
17.092(5)
17.402(6)
90
16.663(16)
20.890(19)
25.771(2)
90
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
108.177(5)
90
104.584(7)
90
90
90
90
90
4258.7(17)
4
3901(2)
4
8974.7(15)
4
4811.8(11)
4
Z
Dcalcd. (mg cm−3
)
1.299
1.333
1.277
1.287
absorp coeff (mm−1
)
2.234
2.434
2.123
1.986
F(000)
1728
1612
3576
1948
θ range (deg)
3.00−25.50
3.11−25.50
3.07−25.50
3.00−27.45
reflns collected/unique 20 523/7876 [R(int) = 0.0374] 18 686/7240 [R(int) = 0.0398] 36 067/8334 [R(int) = 0.0590] 22 834/10791 [R(int) = 0.0353]
data/restraints/params 7876/34/429
7240/15/408
1.142
8334/13/440
1.157
10 791/0/505
1.077
goodness-of-fit on F2
final R [I > 2σ(I)]
wR2 (all data)
1.086
0.0440
0.1303
0.0548
0.0787
0.0427
0.2295
0.1560
0.0843
Table 2. Selected Bond Lengths (Å) and Angles (deg) for Complexes 1 and 2
bond length
1
2
bond length
1
2
Yb(1)−N(1)
Yb(1)−N(2)
Yb(1)−N(3)
bond angle
2.554(5)
2.577(5)
2.564(5)
2.570(6)
2.583(8)
2.632(10)
2
Yb(1)−O(1)
Yb(1)−O(2)
Yb(1)−O(3)
bond angle
2.249(4)
2.246(4)
2.429(4)
2.188(7)
2.239(7)
2.656(7)
2
1
1
N(1)−Yb(1)−N(3)
N(2)−Yb(1)−N(3)
O(1)−Yb(1)−N(3)
O(2)−Yb(1)−N(3)
O(3)−Yb(1)−N(3)
N(1)−Yb(1)−N(2)
O(1)−Yb(1)−O(2)
O(3)−Yb(1)−N(2)
168.78(15)
72.82(16)
103.55(16)
100.01(16)
99.01(15)
118.34(15)
155.55(15)
171.79(14)
114.4(3)
71.3(4)
O(1)−Yb(1)−O(3)
O(2)−Yb(1)−O(3)
O(1)−Yb(1)−N(1)
O(2)−Yb(1)−N(1)
O(3)−Yb(1)−N(1)
O(1)−Yb(1)−N(2)
O(2)−Yb(1)−N(2)
91.32(15)
91.39(16)
78.73(13)
79.43(15)
69.85(14)
89.92(15)
90.84(16)
138.9(2)
87.9(2)
77.6(2)
79.9(2)
68.7(2)
100.3(3)
95.6(3)
92.0(3)
157.0(3)
81.3(3)
173.9(3)
109.2(3)
115.4(3)
(NCH2CHO), 56.56 (ArCH2N), 54.47 (NCH2CH2N), 46.48 (NCH3),
35.25 (CMe3), 30.24 (C(CH3)3), 28.09 (THF-CH2), 25.22 (THF-
CH2), 21.47 (ArCH3). IR (KBr, cm−1): 2949 (s), 2862 (s), 1631 (m),
1460 (s), 1230 (s), 1255 (s), 1035 (m), 825 (m), 632 (w), 510 (m).
YbIILb(TMEDA) (2). The synthesis of complex 2 was carried out in
the same way as that described for complex 1, but Yb(N-
(SiMe3)2)2(TMEDA) (1.83 g, 3.00 mmol) and H2Lb (1.34 g, 3.00
mmol) were used. Red crystals were obtained upon crystallization
from a toluene solution at room temperature (1.39 g, 63% based on
Yb). Mp: 233−235 °C (dec). Anal.Calcd for YbC35H53N3O3 (736.87):
C, 57.05; H, 7.25; N, 5.70; Yb, 23.49. Found: C, 57.40; H, 7.31; N,
5.21; Yb, 23.03. 1H NMR (400 MHz, C6D6, 25 °C): δ 6.90−7.29 (m, 4
H, ArH), 6.58 (s, 1 H, OCH), 5.72 (s, 1 H, CCH), 5.24 (s, 1 H, CH),
[YbIIILc(OC6H4-4-CH3)]2 (4). To a THF solution of Yb-
(C5H5)3(THF) (1.35 g, 3.07 mmol) was added a THF solution of
H2Lc (1.65 g, 3.07 mmol). The reaction mixture was stirred for 1 h at
room temperature, and then HOC6H4-4-CH3 (0.31 mL, 3.07 mmol)
was added by syringe. The mixture was stirred overnight at 50 °C. The
solvent was evaporated to dryness under vacuum, and toluene (10
mL) was added. The resulting solution was kept at room temperature
for crystallization, and yellow crystals of complex 4 were obtained in a
few days (1.75 g, 70% based on Yb). Mp: 192−194 °C (dec). Anal.
Calcd for Yb2C84H120N2O8 (1631.97): C, 61.82; H, 7.41; N, 1.72; Yb,
21.21. Found: C, 61.87; H, 7.39; N, 1.68; Yb, 21.18. IR (KBr, cm−1):
2956 (s), 2870 (s), 1610 (m), 1506 (m), 1478 (m), 1360 (m), 1239
(s), 1156 (s), 877 (w), 835 (m), 743 (m), 639 (m), 506 (s).
YbIIILd(OC6H3-2,6-iPr2)(THF) (5). The synthesis of complex 5 was
carried out in the same way as that described for complex 4, but
HOC6H3-2,6-iPr2 (0.66 mL, 3.55 mmol) was used instead of
HOC6H4-4-CH3. Colorless crystals of complex 5 were obtained
from a toluene solution with a small amount of THF at room
temperature in a few days (2.23 g, 67% based on Yb). Mp: 212−214
°C (dec). Anal. Calcd for YbC49H76NO5 (932.15): C, 63.20; H, 8.12;
N, 1.50; Yb, 18.58. Found: C, 63.23; H, 8.10; N, 1.55; Yb, 18.54. IR
(KBr, cm−1): 2960 (s), 2870 (s), 1602 (m), 1480 (s), 1436 (s), 1361
(m), 1234 (s), 1157 (s), 877 (w), 838 (m), 745 (m), 639 (m), 556
(m), 505 (s).
4.13−4.16 (d, 2 H, J
3.08−3.11 (d, 2 H, J
= 12 Hz, ArCH2N), 3.36 (s, 2 H, CCH2),
= 12 Hz, ArCH2N), 2.48 (s, 6 H, ArCH3),
H−H
H−H
2.11 (s, 12 H, NCH3), 2.01 (s, 4 H, NCH2CH2N), 1.65 (s, 18 H,
C(CH3)3). 13C NMR (100 MHz, C6D6, 25 °C): δ 165.99 (arom-CO),
155.45 (C-O), 139.39 (CH−O), 135.74 (arom-CCH2N), 130.83
(arom-CBut), 129.34 (arom-CMe), 124.11 (arom-CH), 118.47 (arom-
CH), 112.35 (CH), 106.68 (CH), 64.13 (furan-CH2N), 57.13
(ArCH2N), 46.71 (NCH2CH2N), 46.44 (NCH3), 35.29 (CMe3),
30.44 ((CH3)3), 21.43 (ArCH3). IR (KBr, cm−1): 2950 (s), 2846 (s),
1627 (w), 1460 (m), 1225 (s), 1160 (s), 1018 (w), 861 (w), 743 (w),
632 (m), 508 (m).
C
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Inorganic Chemistry
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Table 3. Selected Bond Lengths (Å) and Angles (deg) for Complexes 4 and 5
bond length
4
5
bond length
4
5
Yb(1)−O(1)
Yb(1)−O(2)
Yb(1)−O(3)
bond angle
2.086(6)
2.090(6)
2.336(6)
4
2.112(3)
2.098(3)
2.364(3)
5
Yb(1)−O(4)
Yb(1)−O(5)
Yb(1)−N(1)
bond angle
2.279(5)
2.253(4)
2.434(7)
4
2.071(3)
2.325(4)
2.530(4)
5
O(1)−Yb(1)−O(2)
O(1)−Yb(1)−O(3)
O(1)−Yb(1)−O(4)
O(1)−Yb(1)−O(5)
O(2)−Yb(1)−O(3)
O(2)−Yb(1)−O(4)
O(2)−Yb(1)−O(5)
O(3)−Yb(1)−O(4)
98.9(2)
155.51(13)
93.22(14)
100.40(14)
85.71(13)
90.00(14)
103.63(14)
85.02(13)
93.57(13)
O(3)−Yb(1)−O(5)
O(4)−Yb(1)−O(5)
O(1)−Yb(1)−N(1)
O(2)−Yb(1)−N(1)
O(3)−Yb(1)−N(1)
O(4)−Yb(1)−N(1)
O(5)−Yb(1)−N(1)
101.62(16)
71.0(2)
165.07(12)
101.28(14)
80.44(12)
77.96(12)
69.59(13)
163.14(14)
95.58(13)
147.8(2)
97.85(18)
108.46(17)
85.3(2)
78.5(2)
81.8(2)
70.5(2)
163.23(18)
102.4(2)
102.7(2)
170.95(19)
81.11(17)
a
Table 4. Polymerization of rac-LA by Complexes 1−3
b
c
d
c
e
entry
complex
[M]/[I]
T/°C
t/h
conv (%)
Mn,exp × 104
Mn,calcd × 104
Mw/Mn
Pr
1
2
3
4
5
6
7
8
1
1
1
1
1
1
1
1
1
1
2
3
400
800
20
20
20
20
20
20
20
60
20
20
20
20
20
20
20
20
20
1
1
94
79
96
77
90
53
97
100
86
83
39
26
78
98
100
96
91
6.96
9.85
5.41
9.10
1.09
1.09
1.12
1.07
1.16
1.09
1.16
1.20
1.20
1.15
1.28
1.38
1.07
1.16
1.09
1.10
1.14
0.97
0.99
0.98
0.97
0.98
0.98
0.97
0.91
0.61
0.90
0.82
0.67
0.98
0.99
0.97
0.97
0.52
800
2
10.52
11.73
12.18
14.94
27.39
1.97
11.06
11.09
12.96
15.26
27.94
1.44
1000
1000
2000
2000
100
2
4
6
12
0.4
24
4
f
9
100
2.49
1.24
g
10
100
2.83
1.20
11
12
13
14
15
16
17
100
24
24
2
2.60
0.56
100
1.51
0.37
12
12
YbIILe(THF)2
YbIILe(THF)2
800
8.67
8.99
1000
800
4
13.51
12.45
14.00
2.74
14.11
11.52
13.82
1.31
YbIILf(TMEDA)12
YbIILf(TMEDA)12
3
1000
100
5
12
YbIILg(THF)3
1
a
b
General polymerization conditions: solvent = THF, [LA]0 = 1.0 M. Conversion = weight of polymer obtained/weight of monomer used.
c
d
Determined by SEC against polystyrene standard and corrected by a factor of 0.58 for PLA. Calculated by ([LA]/[I]) × 144.13 × X (X = conv).
Pr is the probability of racemic linkages between monomer units determined from the methine region of the homonuclear decoupled H NMR
e
1
f
g
spectrum. Solvent = toluene. Solvent = DME.
Polymerization of rac-LA. The polymerization proceeded in a
glovebox with a 25 mL round flask. A typical polymerization procedure
is as follows: To a stirred solution of rac-LA (0.55 g, 3.84 mmol) in
3.36 mL of THF was added a THF solution (0.48 mL) of complex 1
(0.0048 mmol, [LA] = 1.00 mol/L). The polymerization took place
immediately, and the system became viscous in a few minutes at room
temperature. The system kept stirring for the desired time and then
was terminated by 1.0 mL of HCl/CH3OH (0.05/10 v/v). The
viscous solution was quenched by an addition of ethanol and then
poured into ethanol to precipitate the polymer. The polymeric product
was filtered, washed with ethanol, dried at 40 °C for 24 h in vacuo, and
weighed (0.53 g, 96%).
mode using Mo Kα radiation (λ = 0.71075 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. All the non-hydrogen
atoms were refined anisotropically. The hydrogen atoms in these
complexes were all generated geometrically (C−H bond lengths fixed
at 0.95 Å), assigned appropriate isotropic thermal parameters, and
allowed to ride on their parent carbon atoms. All the H 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 by using the SHELEXL-97 program.19 CCDC-
884173 (for 1), -884174 (for 2), -936910 (for 4), and -936911 (for 5)
contain the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/
cif.
The tacticity of the PLA was calculated according to the methine
1
region homonuclear decoupling H NMR spectrum. (Table 4, entry
3).
Oligomer Preparation. Oligomerization of rac-LA was carried out
with complex 1 in THF at 25 °C at a molar ratio of [monomer]/
[initiator] of 15. 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 for 1H NMR and
MALDI-TOF mass spectra 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 single-crystal structure determination. Intensity data
were collected with a Rigaku Mercury CCD area detector in ω scan
RESULTS AND DISCUSSION
■
Syntheses of Complexes 1 and 2. The preligands of H2La
and H2Lb were synthesized according to a reported procedure13
as white, microcrystalline materials in moderate yields and were
fully characterized.
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We recently reported that the use of Yb(N-
(SiMe3)2)2(TMEDA) instead of Yb(N(SiMe3)2)2(THF)2 as a
precursor in the reaction with H2Lf is important for the
preparation of the facile isolated complex YbIILf(TMEDA) in
high yield.12 The same method was also used in the present
case. The reaction between Yb(N(SiMe3)2)2(THF)2 and H2La
as well as H2Lb led to the formation of a tar, which prevented
the purification of the product. However, the title complexes
were readily produced by replacing Yb(N(SiMe3)2)2(THF)2
with Yb(N(SiMe3)2)2(TMEDA) in these reactions. Thus, the
reactions of Yb(N(SiMe3)2)2(TMEDA) with H2La,b at a 1:1
molar ratio in toluene took place immediately at ambient
temperature with the release of the amine HN(SiMe3)2. The
reaction mixtures were stirred for 12 h. From the resulting
solution, complexes 1 and 2 were easily isolated as red crystals
in good yields (58% for complex 1, 63% for complex 2)
(Scheme 1) upon the evaporation of volatiles and the washing
of residues with hexane followed by crystallization from toluene
solution.
Figure 2. ORTEP diagram of the molecular structures of complex 1.
Thermal ellipsoids are drawn at the 20% probability level. All
hydrogens and free solvent molecules are omitted for clarity.
Scheme 1. Syntheses of Complexes 1 and 2
Figure 3. ORTEP diagram of the molecular structures of complex 2.
Thermal ellipsoids are drawn at the 20% probability level. All
hydrogens and free solvent molecules are omitted for clarity.
Complexes 1 and 2 have good solubility in THF and toluene
and are soluble in hot hexane, but are not soluble in cold
hexane. They are sensitive to air and moisture. The crystals
decompose within a few minutes upon exposure to air, but the
crystals and the solution are both stable when stored under
argon or nitrogen. The crystals of complex 1 and complex 2
one tetradentate ligand and one TMEDA molecule. In complex
1, O(1), O(2), O(3), and N(2) occupy equatorial positions and
N(1) and N(3) occupy axial positions. The N(1)−Yb−N(3)
angle is slightly distorted from the ideal 180° and is
169.78(15)°. In complex 2, O(1), O(2), O(3), and N(3)
occupy equatorial positions, and N(1) and N(2) occupy axial
sites with a N(1)−Yb−N(2) angle of 173.9(3)°.
1
gave satisfactory elemental analyses. Their H NMR spectra
showed clear peaks that could be assigned to the ligand and the
coordinated TMEDA. The identities of complexes 1 and 2 were
unequivocally established by an X-ray structure determination.
Crystal Structures of Complexes 1 and 2. Crystals
suitable for X-ray structural determinations of complexes 1 and
2 were obtained from a concentrated toluene solution at room
temperature. Complexes 1 and 2 were crystallized as a toluene
solvated 1·toluene and a half toluene solvated 2·0.5 toluene,
respectively. The free toluene in each unit cell evaporates under
vacuum, and, therefore, satisfactory elemental analysis for 1 and
2 without free toluene could be obtained (see the Experimental
Section).
The average Yb−OAr bond lengths are 2.248(4) Å (for 1)
and 2.214(7) Å (for 2), and this is comparable with the
corresponding bond lengths in the reported bridged bis-
(phenolate) YbII complexes YbIILf(TMEDA) [2.215(3) Å]12
and YbIILe(THF)2 [2.277(2) Å].11g These values also compare
with those found in other YbII complexes containing unbridged
aryloxides,20 when the effect of the coordination number on the
effective ionic radii is considered.21 The Yb−O(ether) bond
length of 2.429(4) Å in 1 is ca. 0.05 Å shorter than that found
in YbIILf(TMEDA).12 In contrast, the Yb−O(ether) bond
length (2.656(7) Å) in 2 is particularly long compared with that
found for 1 and YbIILf(TMEDA)12 and in other LnIII
complexes.6 The much longer Yb−O(ether) bond length in 2
may be attributed to a weaker side arm donor resulting in
weaker binding to Yb. Such a long Yb−O(ether) bond length
was also found in the zirconium complex [C4H3OCH2N(CH2-
2-OC6H2-3,5-But2)2]Zr(CH2Ph)2 (Zr−O(ether) = 2.684(2)
Å).13
The molecular structures of complexes 1 and 2 are shown in
Figures 2 and 3, respectively. Selected bond lengths and angles
are listed in Table 2. Overall, the structures of 1 and 2 are
closely related to those of the YbIILf(TMEDA),
15
YbIILe(THF)2,12 and YbIILh(THF)3 complexes. Both com-
plexes have a monomeric structure. Each adopts distorted
octahedral coordination geometry around the ytterbium ion by
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Scheme 2. Syntheses of Complexes 4 and 5
The known complex YbIILh(THF)3 (3) (Figure 1) was also
prepared by amine elimination following a published
procedure15 with the aim of understanding the influence of
the ligand skeleton on the catalytic behavior.
Syntheses of Complexes 4 and 5. We synthesized YbIII
aryloxide complexes supported by the THF-amine-bisphenolate
ligand Lc and the methoxy-amine-bisphenolate ligand Ld, to
compare the catalytic activity and stereoselectivity of the YbII
complex with the YbIII complex in the ROP of rac-LA. The
double-protonation reaction of Ln(C5H5)3(THF) with an
equivalent of proligand and then an equivalent of phenol was
used for the syntheses of the target complexes because this
procedure was recently reported to be an accessible route for
the preparation of salt-free lanthanide aryloxides with a
tetradentate-bridged bisphenolate ligand.10a The treatment of
Yb(C5H5)3(THF) with H2Lc in a 1:1 molar ratio in THF for 1
h and then with one equivalent of HOC6H4-4-CH3 at 50 °C for
another 12 h conveniently afforded the expected complex 4 as
light yellow crystals in 67% yield upon crystallization from a
toluene solution (Scheme 2). An attempt to synthesize isolable
YbIII aryloxide with ligand La was not successful.
Figure 4. ORTEP diagram of the molecular structures of complex 4.
Thermal ellipsoids are drawn at the 20% probability level. All
hydrogens are omitted for clarity.
The successful preparation of complex 4 led us to synthesize
an analogue containing a methoxy-amine-bis(phenolate) ligand
Ld,6i YbIIILd(OC6H4-4-CH3), using the same procedure.
However, the reaction between Yb(C5H5)3(THF), H2Ld, and
HOC6H4-4-CH3 did not afford the isolated complex in the
desired yield. Fortunately, the same reaction with bulkier
phenol HOC6H3-2,6-iPr2 proceeded smoothly, affording
complex 5 as colorless crystals in 70% yield upon crystallization
from a toluene solution with a small amount of THF (Scheme
2).
Complexes 4 and 5 were fully characterized, and their
molecular structures were determined by single-crystal
structure analysis. Both complexes are moderately sensitive to
air and moisture. The crystals decompose in a few minutes
when they are exposed to air, but they are thermally stable.
Complexes 4 and 5 are freely soluble in THF and toluene and
slightly soluble in hexane.
oxygen atoms of the OC6H4-4-CH3 groups. Each YbIII atom is
six-coordinate, and the coordination geometry around the YbIII
atom can be described as a distorted octahedron. The average
bond length of Yb−O(Ar) [2.088(6) Å] is similar to that of the
Ln−O(phenolate) bond.6−10 The bond length of Yb−O(ether)
is 2.336(6) Å, which is much shorter than those found in 1 and
5 and the related LnIII complexes, reflecting the weak trans-
influence of bridged aryloxides. The average bridged Yb−
O(aryloxide) bond length of 2.266(4) Å is ca. 0.20 Å longer
than the unbridged Yb−O(aryloxide) bond in 5. This is
reasonable.
Complex 5 Is a Monomer. The six-coordinated YbIII ion is
ligated by three oxygen atoms and one nitrogen atom from the
Ld ligand, one oxygen atom from the aryloxo group, and one
oxygen atom from a THF molecule in a distorted octahedron.
The molecular structure of complex 5 is similar to that of the
analogue YbIIILe(OC6H3-2,6-iPr2)(THF).10a The average bond
length of Yb−O(Ar) [2.105(3) Å] is comparable with that in
complex 4. The Yb−O(ether) bond length of 2.364(3) Å is
comparable with those found in the related YLd[N(SiHMe2)2]-
(THF) and YLd(CH2SiMe3)(THF) complexes,6i if the
influence of effective ionic radii is considered.21 The Yb−
O(aryloxide) bond length is normal.
Crystal Structures of Complexes 4 and 5. The
molecular structures of complexes 4 and 5 are shown in
Figures 4 and 5, and their selective bond lengths and angles are
listed in Table 3. The side arm functional group in each
complex was found to bind to the metal center. Complex 4 is a
symmetric dimer containing a Yb2O2 core bridging through the
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2000. The Mn,exp values of the resulting PLAs increased
proportionally with the ratio and were very close to the Mn,calcd
values that were calculated considering one polymer chain
induced per metal of complex 1. The molar mass distributions
of all the PLAs remained narrow (Mw/Mn = 1.07−1.16). A
slightly higher Mn,exp (ca. 20%) than Mn,calcd was observed when
the molar ratio of monomer/1 was lower than 800 (Table 4
entry 1). This may impact the use of polystyrene standards for
SEC (size exclusion chromatography) calibration. All the
obtained heterotactic PLAs had high Pr values (0.97−0.99).
Moreover, the high heteroselectivity of complex 1 was not
influenced by the ratio or the conversion. However, longer
polymerization times and an increase in the polymerization
temperature from 20 °C to 60 °C both caused some loss of
heteroselectivity (Table 4, entries 7 and 8) because of
transesterification side reactions. The solvent greatly influenced
both the activity and stereoselectivity. THF was the optimum
solvent and gave high activity and high heterotactic selectivity.
The polymerization performed in DME (dimethoxyethane) at
room temperature gave 83% conversion after 4 h, and the
resulting PLA had a lower Pr value (Pr = 0.90, Table 4, entry
10). However, replacing THF with toluene led to a dramatic
decrease in activity and heterotacticity. The polymerization
became slow, and the resulting PLA was atactic (Pr = 0.61,
Table 4, entry 9). The crucial role of THF in high heterotactic
selectivity has been reported for Y, Zn, Ca, and well-defined
LnIII metal systems.4j,k,5b,6a,9a,10f The far higher activity in THF
than in toluene is in striking contrast to the methoxy-amine-
bisphenolate LnIII alkoxide systems, which are able to initiate an
equally rapid ROP of rac-LA in THF and in toluene.6a,i
However, it is consistent with the amine-amine-bisphenolate
yttrium alkyl9a and the 1,ω-dithiaalkanediyl-bridged bis-
(phenolate) LnIII amide7a systems.
Figure 5. ORTEP diagram of the molecular structures of complex 5.
Thermal ellipsoids are drawn at the 20% probability level. All
hydrogens are omitted for clarity.
ROP of rac-LA Initiated by Complexes 1−3. Complexes
1−3 were evaluated by the ROP of rac-LA (Scheme 3).
Scheme 3
The polymerization of rac-LA with complex 1 has a first-
order kinetic dependence on the concentration of rac-LA
without an induction time (Figure 6), suggesting a mechanism
wherein one molecule of LA per active site of catalyst is
involved. The Mn,exp values of the resulting polymers increase
linearly with monomer conversion, and Mw/Mn remains within
1.09−1.10 (Figure 7). These data indicate that the polymer-
Representative results are summarized in Table 4. All
complexes proved to be active initiators for the ROP of rac-
LA under mild conditions. However, differences in activity and
stereoselectivity were found. For complex 1 almost complete
conversion of the monomer was achieved within 2 h at room
temperature in a molar ratio of monomer to complex 1 of 800.
The Mn,exp value of the resulting PLA was very close to the
Mn,calcd value that was calculated for a single PLA chain
produced per metal center of complex 1. The molar mass
distribution of the PLA was narrow (Mw/Mn =1.12, Table 4,
entry 3). Complex 1 showed a high level of stereoselectivity to
yield heterotactic PLA with a Pr of 0.98 (Table 4, entries 1−7).
The resulting PLA has a Tm of 125 °C and no observable Tg,
which is close to that reported for a heterotactic PLA.2b
However, a significant decrease in activity and stereoselectivity
was observed for complex 2 (Pr = 0.82, Table 4, entry 11).
Complex 3 was even worse and showed no selectivity (Pr =
0.67, Table 4, entry 12). The catalytic behavior of complex 3 is
similar to that of the reported YbIILg(THF)3 (Figure 1), as
shown in Table 4, entry 17. Thus, the catalytic activity and the
heterotactic selectivity of complexes 1−3 improved as the
dative ability of the bridge increased. This significant influence
of the structure of the bridge in the bisphenolate ligand on the
activity and tacticity of the growing polymer chain was pointed
out by Okuda’s group for the ROP of rac-LA using LnIII
complexes.7b
Figure 6. Plots of ln[rac-LA]0/[rac-LA] vs time for ROP of rac-LA
initiated by complex 1. Conditions: [M]/[I] = 800, solvent = THF,
[M] = 1.0 M, T = 20 °C.
The polymerization of rac-LA with complex 1 proceeded
smoothly in monomer/1 molar ratios ranging from 800 to
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trend of the electron-donating ability of their ligands, is in
accordance with the assumption.
1H NMR spectroscopy of the oligomers obtained from the
oligomerization of rac-LA with complex 1 and upon quenching
with methanol and wet hexane, respectively, revealed a methoxy
end group as a singlet at δ 3.74 for the former and a hydroxy
group at δ 4.13 as a singlet for the latter. Obviously, the
methoxy and hydroxy groups in the oligomers were derived
from the corresponding quenching alcohols and water via the
acylation of an electrophilic acyl end group in the original
oligomer chains.11c,12 The appearance of signals for HOC(O)-
1
CH(CH3) or MeOC(O)CH(CH3) in the H NMR spectra of
the oligomers demonstrated that the PLA was linear rather than
cyclic,7c and the polymerization proceeded by a coordination/
insertion mechanism.
A MALDI-TOF mass spectrum of the oligomer was obtained
upon quenching with wet hexane (Figure 8). As shown in
Figure 8, the oligomer contains chains separated by lactides
(144) rather than lactic acid (72) and is capped with a −OH
group at one end of the chain. This indicates that almost no
transesterification side reactions occurred during the lactide
Figure 7. Plots of conversion vs Mn and Mw/Mn for ROP of rac-LA
initiated by complex 1. Conditions: [M]/[I] = 800, solvent = THF,
[M] = 1.0 M, T = 20 °C.
1
polymerization initiated by complex 1. The data from the H
NMR spectrum and the MALDI-TOF mass spectrum are both
in accordance with the assumption that the active species in the
present polymerization system might be the “YbIIILaX” species
(X is a ring-opened LA) that forms via the oxidation of complex
1 by rac-LA and the initiating group is a ring-opened LA.
ROP of rac-LA Initiated by Complexes 4 and 5. The
proposed active species “LnLX” in the LnII complex system11c,f
has a similar skeleton to the corresponding LnIII alkoxide.
Therefore, the stereocontrol of a LnII complex should be similar
to that of its trivalent partner because the LnL motif around the
active initiating group plays a crucial role in determining the
stereocontrol of the catalyst. Indeed, the partners of
ization proceeds in a living fashion, that is, without any
significant side reactions. Overall, complex 1 is as good as the
reported complexes YbIILf(TMEDA) and YbIILe(TMEDA)12
(Figure 1) for the controlled heteroselective ROP of rac-LA
(Table 4, entries 1−7 and 13−16).
It is presumed that the initial oxidation of a LnII complex
(and concomitant monomer reduction) precedes the ROP of
the cyclic ester.11c,f Indeed, the reaction between complex 1 and
rac-LA in a 1:1 molar ratio yields an instant color change from
deep red to pale yellow, which is indicative of YbII oxidation.
Efforts to isolate and characterize the product were
unsuccessful. Although the exact nature of the intermediate
was not determined, a YbIII species “YbIIILaX”, where X is a
ring-opened rac-LA, i.e., a propagating alkoxide group, is
presumably formed by the oxidation of 1 with rac-LA. Thus, the
catalytic activity of a Yb(II) complex should be related to the
electron-donating ability of the ligand, which could improve the
reducing ability of the Yb(II) complex. The activity sequence of
3 < 2 < 1 observed here, which is consistent with the increasing
12
YbIILe(THF)2 and YbIIILe(OCH2CF3)(THF)10f containing
an amine-amine-bis(phenolate) ligand were found to have the
same high heteroselectivity during the polymerization of rac-LA
(Table 4, entry 10, and Table 5, entry 15).
Thus, complexes 4 and 5 were assessed in the ROP of rac-
LA. The results are summarized in Table 5. Both complexes are
highly stereoselective initiators, affording heterotactic PLAs
with a Pr up to 0.98 in monomer/initiator molar ratios from
Figure 8. MALDI-TOF mass spectrum of rac-LA oligomer initiated by complex 1 ([M]/[I] = 20:1, in THF, 20 °C, quenching by wet hexane, doped
with NaI).
H
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a
Table 5. Polymerization of rac-LA by Complexes 4 and 5
b
c
d
c
e
entry
complex
[M]/[I]
T/°C
t/min
conv (%)
Mn,exp × 104
Mn,calcd × 104
Mw/Mn
Pr
1
4
4
4
4
4
4
4
5
5
5
5
400
500
20
20
20
20
20
20
20
20
20
20
20
20
20
25
25
30
30
30
40
40
40
70
40
100
40
70
5
99
99
97
98
97
91
98
58
87
52
60
100
100
98
95
6.03
7.08
5.70
7.13
1.09
1.09
1.09
1.07
1.08
1.09
1.08
1.21
1.34
1.19
1.17
1.33
1.34
1.05
1.04
0.98
0.98
0.97
0.97
0.98
0.97
0.98
0.98
0.97
0.98
0.97
0.80
0.65
0.97
0.99
2
3
600
7.91
8.38
4
800
10.96
12.84
13.39
16.24
16.29
12.69
17.15
23.20
1.55
11.29
13.97
15.72
28.22
4.18
5
1000
1200
2000
500
6
7
8
9
500
6.26
10
11
12
13
14
15
1000
2000
100
7.49
17.28
1.44
6i
YLfN(SiHMe2)2
LaLfN(SiHMe2)2
6i
100
5
2.04
1.44
YLe(OCH2CF3)(THF)10f
1000
1000
60
60
14.11
13.68
12.30
16.90
YbIIILe(OCH2CF3)(THF)10f
a
b
General polymerization conditions: solvent = THF, [LA]0 = 1.0 M. Conversion = weight of polymer obtained/weight of monomer used.
c
d
Determined by SEC against polystyrene standard and corrected by a factor of 0.58 for PLA. Calculated by ([LA]/[I]) × 144.13 × X (X = conv).
e
1
Pr is the probability of racemic linkages between monomer units determined from the methane region of the homonuclear decoupled H NMR
spectrum.
400 to 2000 in THF at room temperature (Table 5, entries 1−
11). The heteroselectivity of complexes 4 and 5 are the same as
those of the corresponding divalent complexes 1 and
YbIILf(TMEDA)12 (Table 4, entries 1−7, 15, and 16). This
shows that the skeleton of the active species in 4 and 1, and in 5
and YbIILf(TMEDA), is similar and the substituent at the 4-
position of the phenyl ring of the tetradentate bis(phenolate)
ligand has almost no influence on the stereoselectivity as
pointed out before.6a
proceeds rapidly to give a complete conversion of the monomer
within 30−70 min (time was not optimized). All the PLAs
produced have narrow molar mass distributions (Mw/Mn =
1.07−1.09) and Mn,exp values close to the Mn,calcd values for a
monomer/ytterbium ratio up to 1000 (Table 5, entries 1−5).
The Mn,exp values deviate from the Mn,calcd values when the
molar ratio of monomer to initiator increases to 1200−2000
(Table 5, entries 6 and 7). A similar phenomenon was also
demonstrated by Carpentier6b and Coates,3k and this mismatch
possibly arises from chain-transfer processes.
It is worth noting that the heterotacticity of the resulting
PLA with complex 5 is much higher than those obtained by the
reported analogous yttrium and lanthanum amides6a,i (Table 5,
entries 12 and 13). This may be caused by differences in the
ionic radius. An increasing sequence of La < Y < Yb was
observed and is consistent with a decrease in the ionic radius of
the metals. This shows the importance of the metal center in
determining the stereoselectivity of a complex, as mentioned
before.12 Interestingly, replacing the N(CH2)2OMe bridge with
N(CH2)2NMe2 did not result in a difference in stereocontrol
between the analogous Y and YbIII complexes (Table 5, entries
14 and 15), demonstrating that the ligand containing a
N(CH2)2NMe2 bridge is less sensitive to the metal center.
Complex 4 could act as a single-site initiator for the
controlled polymerization of rac-LA as found for complex 1.
Polymerization systems with complex 4 gave PLAs with Mn,exp
values very close to the calculated Mn,calcd values for a single
PLA chain produced per metal center and with narrow molar
mass distributions (Mw/Mn = 1.07−1.09, Table 5, entries 1−5).
Complex 4 is more active than complex 1. For example, the
polymerization of rac-LA with complex 4 proceeded rapidly at
20 °C in THF at [rac-LA] = 1.0 M, resulting in the complete
conversion of the monomer within 40 min in a monomer/
initiator molar ratio of 1000 (Table 5, entry 5). The same
polymerization with complex 1 gave 90% conversion after 4 h
(Table 4, entry 5). The lower catalytic activity of 1 vs 4 may be
attributed to a deactivation process from the initial redox
reaction, which precedes the ring-opening polymerization of
rac-LA.
A first-order kinetic dependence on the concentration of rac-
LA without an induction time was observed for the polymer-
ization of rac-LA using complex 4 (Figure 9). The Mn,exp values
increased linearly with the conversion of the monomer and the
Mw/Mn values remained in the range 1.09−1.10 (Figure 10),
indicating that the polymerization proceeded in a controlled
manner.
Complex 5 was found to be worse in terms of activity and
controllability compared with complex 4 and its partner
Figure 9. Plots of ln[rac-LA]0/[rac-LA] vs time for ROP of rac-LA
initiated by complex 4. Conditions: [M]/[I] = 500, solvent = THF,
[M] = 1.0 M, T = 20 °C.
At various monomer/initiator ratios that range from 400 to
2000, the polymerization of rac-LA initiated by complex 4
I
dx.doi.org/10.1021/ic401747n | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: qshen@suda.edu.cn. Fax: (86)512-65880305.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the National Natural Science Foundation of
China (Grants 20972107, 21132002, and 21372171), a project
funded by the Priority Academic Program Development of
Jiangsu Higher Education Institutions, for financial support.
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Figure 10. Plots of conversion vs Mn and Mw/Mn for ROP of rac-LA
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CONCLUSION
■
New YbII complexes 1 and 2 supported by THF-amine-
bis(phenolate) and furan-amine-bis(phenolate), respectively,
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ASSOCIATED CONTENT
* Supporting Information
This material is available free of charge via the Internet at
http://pubs.acs.org.
■
S
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