Synthesis and characterization of amine bridged bis(phenolate) lanthanide aryloxides and their application in the polymerization of lactide

Article abstract of DOI:10.1039/c001888k

A series of neutral lanthanide aryloxides supported by an amine bridged bis(phenolate) ligand were synthesized, and their catalytic behavior for the polymerization of l-lactide was explored. The reactions of (C₅H ₅)₃Ln(THF) with amine bridged bis(phenol)LH₂ [L = Me₂NCH₂CH₂N{CH₂-(2-O-C ₆H₂-Bu^t₂-3,5)}₂] in a 1:1 molar ratio, and then with 1 equivalent of 2,6-diisopropylphenol, p-cresol or 4-methoxyphenol, respectively, in situ in THF gave the neutral lanthanide aryloxides LLn(OC₆H₃-2,6-Prⁱ₂)(THF) [Ln = Nd (1), Sm (2), Yb (3)], LLn(OC₆H₄-4-CH ₃)(THF)_n [Ln = Nd (4), Sm (5), n = 2; Ln = Y (6), n = 1] and LLn(OC₆H₄-4-OCH₃)(THF)_n [Ln = Nd (7), Sm (8), n = 2; Ln = Yb (9), n = 1] in high isolated yields. These complexes were well characterized by elemental analyses, IR spectra and NMR spectroscopy in the case of complex 6. The definitive molecular structures of complexes 1-8 were determined by single-crystal X-ray analyses, which revealed that both the substituents of the aryloxo groups and the ionic radii of the lanthanide metals affect the solid-state structures of the bis(phenolate) lanthanide aryloxides. It was found that complexes 1-9 are efficient initiators for the ring-opening polymerization of l-lactide, and the structures of the aryloxo groups have no obvious effect on the catalytic activity and controllability. A further study revealed that complex 6 can initiate the highly heteroselective ring-opening polymerization of rac-lactide. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c001888k

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www.rsc.org/dalton | Dalton Transactions
Synthesis and characterization of amine bridged bis(phenolate) lanthanide
aryloxides and their application in the polymerization of lactide†
Kun Nie, Xiangyong Gu, Yingming Yao,* Yong Zhang and Qi Shen
Received 28th January 2010, Accepted 5th May 2010
First published as an Advance Article on the web 3rd June 2010
DOI: 10.1039/c001888k
A series of neutral lanthanide aryloxides supported by an amine bridged bis(phenolate) ligand were
synthesized, and their catalytic behavior for the polymerization of L-lactide was explored. The reactions
of (C5H5)3Ln(THF) with amine bridged bis(phenol)LH2 [L = Me2NCH2CH2N{CH2-(2-O-C6H2-
But2-3,5)}2] in a 1 : 1 molar ratio, and then with 1 equivalent of 2,6-diisopropylphenol, p-cresol or
4-methoxyphenol, respectively, in situ in THF gave the neutral lanthanide aryloxides
LLn(OC6H3-2,6-Pri2)(THF) [Ln = Nd (1), Sm (2), Yb (3)], LLn(OC6H4-4-CH3)(THF)n [Ln = Nd (4),
Sm (5), n = 2; Ln = Y (6), n = 1] and LLn(OC6H4-4-OCH3)(THF)n [Ln = Nd (7), Sm (8), n = 2; Ln =
Yb (9), n = 1] in high isolated yields. These complexes were well characterized by elemental analyses, IR
spectra and NMR spectroscopy in the case of complex 6. The definitive molecular structures of
complexes 1–8 were determined by single-crystal X-ray analyses, which revealed that both the
substituents of the aryloxo groups and the ionic radii of the lanthanide metals affect the solid-state
structures of the bis(phenolate) lanthanide aryloxides. It was found that complexes 1–9 are efficient
initiators for the ring-opening polymerization of L-lactide, and the structures of the aryloxo groups have
no obvious effect on the catalytic activity and controllability. A further study revealed that complex 6
can initiate the highly heteroselective ring-opening polymerization of rac-lactide.
are generated in situ by the alcoholysis reactions of lanthanide
Introduction
amido and alkyl complexes with alcohols.6,9,12 To our knowledge,
Amine bridged bis(phenol)s, as a type of dianionic chelate ligands,
there is only one example of structurally characterized amine
bridged bis(phenolate) lanthanide alkoxide in the literature.14
Furthermore, the effect of the structures of the alkoxo groups
on the catalytic behavior has seldom been studied up to date.
Recently, we became interested in studying the synthesis and
reactivity of lanthanide complexes that are supported by the
bulky bridged bis(phenolate) ligands.15–18,22–27 It was found that
the carbon-bridged bis(phenolate) lanthanide alkoxo complexes
can be conveniently synthesized by the protolytic exchange
reactions using (C5H5)3Ln(THF) as the starting materials,24,25 but
attempts were unsuccessful to synthesize the desired amine bridged
bis(phenolate) lanthanide alkoxides through the same method.28
A further study revealed that the lanthanide aryloxides stabilized
by the amine bridged bis(phenolate) ligand can be prepared
in high yields by the sequential protolytic exchange reactions
using (C5H5)3Ln(THF) as the precursors, and a series of neutral
amine bridged bis(phenolate) lanthanide aryloxo complexes were
synthesized and well characterized. Furthermore, the catalytic
behavior of these lanthanide aryloxides for the ring-opening
polymerization of lactide was elucidated. Herein we report these
results.
have received considerable attention in organolanthanide chem-
istry in recent years, and some of these complexes have been found
to be efficient initiators for the ring-opening polymerization of
cyclic esters.1–14 These ligand systems have attractive features, such
as easily available and tunable, which allows it to be possible for the
systematic study on the effect of the steric and electronic properties
of the ligand on the reactivity of the corresponding lanthanide
complexes.15–19 It has been found that the presence of heteroatom(s)
on the bridged bis(phenolate) ligands has significant influence
on the reactivity of the corresponding lanthanide complexes, and
the coordination of the electron-donating heteroatoms to the lan-
thanide ions improves their catalytic activity for the polymeriza-
tion ofcyclicesters inmost cases.20 Onthe other hand, the initiating
groups in bridged bis(phenolate) lanthanide complexes also have
profound effect on their catalytic behavior for the polymerization
of cyclic esters.21 For example, amine bridged bis(phenolate)
lanthanide alkoxides showed better controllability for cyclic ester
polymerizations in comparison with the corresponding lanthanide
amides and alkyls, giving the polymers with high molecular weight
and narrow molecular weight distributions. However, almost
all of the amine bridged bis(phenolate) lanthanide alkoxides
Experimental
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.
E-mail: yaoym@suda.edu.cn; Fax: (86)512-65880305; Tel: (86)512-
65882806
General
All of the manipulations were performed in a purified argon
atmosphere using standard Schlenk techniques. The solvents
were degassed and distilled from sodium benzophenone ketyl
† CCDC reference numbers 763744–763751. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/c001888k
6832 | Dalton Trans., 2010, 39, 6832–6840
This journal is
The Royal Society of Chemistry 2010
©

under argon prior to use. (C5H5)3Ln(THF)29 and ligand LH2
[L = Me2NCH2CH2N{CH2-(2-O-C6H2-But2-3,5)}2]30 were pre-
pared according to the procedures that are recommended in
the literatures. All phenols were pre-dried, and recrystallized, or
distilled before use. L-Lactide and rac-lactide were recrystallized
twice from dry toluene and was then sublimed under vacuum
at 50 ◦C. Lanthanide analyses were performed by EDTA titration
with an xylenol orange indicator and a hexamine buffer.31 Carbon,
hydrogen, and nitrogen analyses were performed by direct combus-
tion with a Carlo-Erba EA-1110 instrument. The IR spectra were
recorded with a Nicolet-550 FTIR spectrometer as KBr pellets.
The 1H and 13C NMR spectra were recorded in a C6D6 solution for
complex 6 with a Unity Varian spectrometer. Because of their para-
magnetism, no resolvable NMR spectrum for other complexes was
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 polystyrene standard by gel permeation chromatography
(GPC) 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. Microstructures of PLAs◦were
measured by homodecoupling 1H NMR spectroscopy at 20 C in
CDCl3 on a Bruker AC 500.
LNd(OC6H4-4-CH3)(THF)2 (4). The synthesis of complex 4
was carried out in the same way as that described for complex
1, but p-cresol (0.51 mL, 4.91 mmol) was used instead of 2,6-
diisopropylphenol. Blue crystals were obtained at -5 ◦C in a
few days (3.65 g, 81%), mp 171–173 ◦C; (Found: C, 63.73; H, 8.58;
N, 2.94; Nd, 15.65. C49H77N2O5Nd (918.37) requires C, 64.09; H,
8.45; N, 3.05; Nd, 15.71); nmax/cm-1 2953 s, 2905 s, 2867 s, 1603 s,
1500 s, 1476 s, 1436 w, 1414 w, 1359 s, 1328 w, 1280 s, 1238 m,
1203 w, 1165 m, 1132 w, 1042 m, 879 m, 859 m, 763 m, 742 m (KBr
pellet).
LSm(OC6H4-4-CH3)(THF)2 (5). The synthesis of complex 5
was carried out in the same way as that described for complex
4, but (C5H5)3Sm(THF) (2.02 g, 4.84 mmol) was used instead of
(C5H5)3Nd(THF). Colorless crystals were obtained in the toluene
solution at -5 ◦C (3.71 g, 83%), mp 167–168 ◦C; (Found: C, 63.19;
H, 8.52; N, 2.91; Sm, 16.49. C49H77N2O5Sm (924.48) requires C,
63.66; H, 8.39; N, 3.03; Sm, 16.26); nmax/cm-1 2953 s, 2905 s, 2868 s,
1603 s, 1505 s, 1476 s, 1437 w, 1414 w, 1359 s, 1329 w, 1278 s, 1239 m,
1202 w, 1165 m, 1132 w, 1042 m, 879 m, 833 m, 763 m, 742 m (KBr
pellet).
LY(OC6H4-4-CH3)(THF) (6). The synthesis of complex 6 was
carried out in the same way as that described for complex 4,
but (C5H5)3Y(THF) (1.70 g, 4.77 mmol) was used instead of
(C5H5)3Nd(THF). Colorless crystals were obtained in the hexane–
THF solution (3.13 g, 83%), mp 148–150 ◦C; (Found: C, 68.22; H,
8.97; N, 3.13; Y, 11.35. C45H69N2O4Y (790.93) requires C, 68.34;
H, 8.79; N, 3.54; Y, 11.24); nmax/cm-1 2954 s, 2905 s, 2868 s, 1608 s,
1507 s, 1478 s, 1437 w, 1415 w, 1359 s, 1329 w, 1282 s, 1238 m, 1202
w, 1166 m, 1132 w, 1044 m, 874 m, 837 m, 772 m, 739 m (KBr
pellet). 1H NMR (300 MHz, C6D6, 25 ◦C): d 7.65 (d, 4J(H, H) =
2.2 Hz, 2H, ArH), 7.19 (s, 2H, ArH), 7.06 (m, 4H, ArH), 4.03 (d,
2J(H, H) = 12.4 Hz, 2H, ArCH2N), 3.92 (br, 4H, a-CH2 THF),
2.99 (d, 2J(H, H) = 12.5 Hz, 2H, ArCH2N), 2.31 (s, 3H, ArCH3),
2.15 (br, 2H, NCH2CH2N), 1.86 (s, 18H, C(CH3)3), 1.76 (s, 6H,
N(CH3)2), 1.50 (s, 18H, C(CH3)3), 1.42 (br, 2H, NCH2CH2N), 1.25
(br, 4H, b-CH2 THF). 13C{1H} NMR (300 MHz, C6D6, 25 ◦C): d
163.5, 162.1, 138.2, 137.0, 136.6, 130.5, 129.6, 128,9, 128.7, 128.3,
126.1, 126.0, 125.0, 124.9, 120.1 (Ar–C), 71.0 (a-CH2 THF), 65.6
(ArCH2N), 59.1 (N(CH2)2N), 49.3 (N(CH2)2N), 45.9 (ArCH3),
36.0 (N(CH3)2), 34.6 (N(CH3)2), 32.6 (C(CH3)3), 31.0 (C(CH3)3),
25.6 (b-CH2 THF), 21.8 (C(CH3)3), 21.2 (C(CH3)3).
Syntheses
LNd(OC6H3-2,6-Pri2)(THF) (1). To
a THF solution of
(C5H5)3Nd(THF) (1.46 g, 3.55 mmol) was added a THF solution
of LH2 (1.86 g, 3.55 mmol). The reaction mixture was stirred for 1 h
at room temperature, and then 2,6-diisopropylphenol (0.66 mL,
3.55 mmol) was added by a syringe. The mixture was stirred
overnight at 50 ◦C, and then the solvent was evaporated under
vacuum. Toluene (10 mL) was added to the residue, blue crystals
were obtained at room temperature in a few days (2.09 g, 72%),
mp 207–209 ◦C; (Found: C, 65.75; H, 8.75; N, 2.94; Nd, 15.89.
C50H79N2O4Nd (916.39) requires C, 65.53; H, 8.69; N, 3.06; Nd,
15.74); nmax/cm-1 2956 s, 2905 s, 2868 s, 1601 m, 1587 m, 1478 s,
1414 m, 1388 w, 1360 w, 1301 m, 1272 m, 1204 m, 1165 m, 1131 m,
1117 m, 1042 m, 911 m, 857 m, 837 m, 760 m, 743 m (KBr pellet).
LSm(OC6H3-2,6-Pri2)(THF) (2). The synthesis of complex 2
was carried out in the same way as that described for complex
1, but (C5H5)3Sm(THF) (1.32 g, 3.16 mmol) was used instead of
(C5H5)3Nd(THF). Colorless crystals were obtained in the toluene–
◦
THF solution (2.16 g, 74%), mp 231–233 C; (Found: C, 64.82;
LNd(OC6H4-4-OCH3)(THF)2 (7). The synthesis of complex 7
was carried out in the same way as that described for complex
1, but 4-methoxyphenol (0.58 g, 4.64 mmol) was used instead
of 2,6-diisopropylphenol. Blue crystals were obtained at room
temperature in a few days (3.29 g, 76%), mp 203–205 ◦C; (Found:
C, 62.61; H, 8.44; N, 2.90; Nd, 15.58. C49H77N2O6Nd (934.37)
requires C, 62.99; H, 8.31; N, 3.00; Nd, 15.44); nmax/cm-1 2952 s,
2905 s, 2867 s, 1603 m, 1499 s, 1477 s, 1436 m, 1413 m, 1384 w,
1359 w, 1328 m, 1290 m, 1261 m, 1227 m, 1167 m, 1042 m, 912 m,
879 m, 831 m, 753 m, 743 m (KBr pellet).
H, 8.55; N, 2.83; Sm, 16.06. C50H79N2O4Sm (922.50) requires C,
65.10; H, 8.63; N, 3.04; Sm, 16.30); nmax/cm-1 2955 s, 2905 s, 2864 s,
1601 m, 1588 m, 1476 s, 1414 m, 1388 w, 1359 w, 1302 m, 1271 m,
1204 m, 1165 m, 1131 m, 1116 m, 1042 m, 911 m, 859 m, 838 m,
760 m, 743 m (KBr pellet).
LYb(OC6H3-2,6-Pri2)(THF) (3). The synthesis of complex 3
was carried out in the same way as that described for complex
1, but (C5H5)3Yb(THF) (1.35 g, 3.07 mmol) was used instead of
(C5H5)3Nd(THF). Yellow crystals were obtained in the toluene–
◦
THF solution (2.32 g, 80%), mp 208–210 C; (Found: C, 62.95;
LSm[OC6H4(OCH3)](THF)2 (8). The synthesis of complex 8
was carried out in the same way as that described for complex
7, but (C5H5)3Sm(THF) (1.84 g, 4.41 mmol) was used instead of
(C5H5)3Nd(THF). Colorless crystals were obtained in the toluene–
H, 8.19; N, 2.80; Yb, 18.05. C50H79N2O4Yb (945.19) requires C,
63.54; H, 8.42; N, 2.96; Yb, 18.31); nmax/cm-1 2956 s, 2905 s, 2865 s,
1602 m, 1588 s, 1479 s, 1415 m, 1388 w, 1359 w, 1305 m, 1274 m,
1204 m, 1165 m, 1132 m, 1113 m, 1042 m, 911 m, 862 m, 840 m,
765 m, 745 m (KBr pellet).
◦
THF solution (3.28 g, 79%), mp 206–208 C; (Found: C, 62.17;
H, 8.37; N, 2.87; Sm, 15.31. C49H77N2O6Sm (940.48) requires C,
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 6832–6840 | 6833
©

Table 1 Crystallographic data for complexes 1–4
Compound
1
2
3
4
Formula
Fw
C50H79N2O4Nd
916.39
C50H79N2O4Sm
922.50
C50H79N2O4Yb
945.19
C49H77N2O5Nd
918.37
T/K
223(2)
223(2)
223(2)
223(2)
Crystal system
Crystal size/mm
Space group
Orthorhombic
0.60 ¥ 0.45 ¥ 0.20
P212121
15.2696(6)
16.8752(7)
19.2287(8)
4954.8(4)
4
1.228
1.089
1940
25.50
Orthorhombic
0.30 ¥ 0.30 ¥ 0.20
P212121
15.3534(14)
16.8680(15)
19.187(2)
4968.9(8)
4
1.233
1.223
1948
25.50
Orthorhombic
0.22 ¥ 0.20 ¥ 0.20
P212121
15.604(3)
16.456(3)
19.273(3)
4948.9(15)
4
1.269
1.931
1980
25.50
Orthorhombic
0.40 ¥ 0.35 ¥ 0.28
Pnma
21.889(2)
20.6138(18)
10.7149(9)
4834.8(7)
4
1.262
1.118
1940
25.50
˚
a/A
˚
b/A
˚
c/A
3
˚
V/A
Z
Dc/g cm-3
m/mm-1
F(000)
◦
qmax
/
Collected
14970
16070
14662
20198
Unique reflns
Obsd reflns [I > 2.0s(I)]
No. of variables
8503
7930
495
8999
6447
454
8865
8018
478
4607
4255
276
GOF
1.033
0.0381
0.0902
0.0246
-0.005(14)
0.601, -0.780
1.091
0.1032
0.2604
0.0930
0.04(3)
2.391, -1.202
1.056
0.0512
0.1097
0.0410
0.006(12)
1.731, -1.247
1.119
0.0489
0.1240
0.0349
R
wR
Rint
Absolute structure parameter
-3
˚
Largest diff. peak, hole/e A
1.333, -0.697
62.58; H, 8.25; N, 2.98; Sm, 15.99); nmax/cm-1 2951 s, 2905 s, 2865 s,
1603 m, 1500 s, 1479 s, 1437 m, 1413 m, 1384 w, 1359 w, 1328 m,
1290 m, 1276 m, 1238 m, 1167 m, 1042 m, 912 m, 879 m, 832 m,
778 m, 742 m (KBr pellet).
structures. Intensity data were collected with a Rigaku Mercury
CCD area detector in w scan mode using Mo-Ka radiation
˚
(l = 0.71070 A). The diffracted intensities were corrected for
Lorentz/polarization effects and empirical absorption correc-
tions. Details of the intensity data collection and crystal data are
given in Tables 1 and 2.
LYb[OC6H4(OCH3)](THF)2 (9). The synthesis of complex 9
was carried out in the same way as that described for complex
7, but (C5H5)3Yb(THF) (2.14 g, 4.86 mmol) was used instead of
(C5H5)3Nd(THF). Yellow crystals were obtained in the toluene–
The structures were solved by direct methods and refined by
full-matrix least-squares procedures based on |F|2. During refine-
ment, constraints are used in some cases because of the presence
of disordered atoms. In complex 6, the methyl groups (C24–C26)
are disordered, and were refined isotropically. In complexes 7 and
8, the benzene rings (C20–C25) and the methoxyl groups (O3 and
C26) are disordered, and were refined isotropically. The other non-
hydrogen atoms, including disordered carbon atoms (C32–C34,
C42, C43 in complexes 1–3; C9–C11 and C17–C19 in complexes
4, 5, 7, and 8) were refined anisotropically. The hydrogen atoms
on the disordered carbon atoms and C8 in complexes 7 and 8 were
not generated. The other hydrogen atoms in 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.
◦
THF solution (3.29 g, 76%), mp 215–217 C; (Found: C, 60.92;
H, 7.72; N, 2.88; Yb, 19.02. C45H69N2O5Yb (891.08) requires C,
60.66; H, 7.80; N, 3.14; Yb, 19.42); nmax/cm-1 2951 s, 2898 s, 2866 s,
1604 m, 1503 s, 1476 s, 1441 m, 1415 m, 1384 w, 1359 w, 1326 m,
1291 m, 1263 m, 1226 m, 1169 m, 1043 m, 913 m, 877 m, 829 m,
777 m, 743 m (KBr pellet).
Typical procedure for the polymerization reactions
The procedures for the polymerization of L-lactide initiated
by complexes 1–9 were similar, and a typical polymerization
procedure is given below. A 50 mL Schlenk flask, equipped with
a magnetic stirring bar, was charged with the desired amount of
L-lactide and toluene. The contents of the flask were then stirred
at 70 ◦C until L-lactide was dissolved, and then a solution of
the initiator in toluene was added to this solution by syringe.
The mixture was stirred vigorously at 70 ◦C for the desired time,
during which time an increase in the viscosity was observed. The
reaction mixture was quenched by the addition of methanol and
then poured into methanol to precipitate the polymer, which was
dried under vacuum and weighed.
Results and discussion
Synthesis of amine bis(phenolate) lanthanide aryloxides
We previously found that the reactions of (C5H5)3Ln(THF) with
amine bridged bis(phenol) in THF gave the amine bis(phenolate)
cyclopentadienyl lanthanide complexes, but these lanthanide
complexes can not further react with methanol or isopropanol
even in refluxed THF or toluene to generate the desired amine
X-Ray crystallographic structure determinations†
Suitable single crystals of complexes 1–8 were sealed in a
thin-walled glass capillary for determination the single-crystal
6834 | Dalton Trans., 2010, 39, 6832–6840
This journal is
The Royal Society of Chemistry 2010
©

Table 2 Crystallographic data for complexes 5–8
Compound
5
6·toluene
7
8
Formula
Fw
C49H77 N2O5Sm
924.48
C52H77N2O4Y
883.07
C49H77N2O6Nd
934.37
C49H77N2O6Sm
940.48
T/K
223(2)
223(2)
223(2)
223(2)
Crystal system
Crystal size/mm
Space group
Orthorhombic
0.20 ¥ 0.20 ¥ 0.20
Pnma
21.8277(13)
20.6671(13)
10.6996(6)
Monoclinic
0.30 ¥ 0.20 ¥ 0.18
P21/n
14.5340(12)
21.4857(15)
16.2519(13)
98.143(2)
5023.9(7)
4
1.168
1.203
1896
25.50
Orthorhombic
0.26 ¥ 0.18 ¥ 0.14
Pnma
21.9718(14)
20.6271(13)
10.7530(8)
Orthorhombic
0.55 ¥ 0.30 ¥ 0.26
Pnma
21.9170(12)
20.6367(10)
10.7274(5)
˚
a/A
˚
b/A
˚
c/A
b/◦
3
˚
V/A
4826.8(5)
4
1.272
1.261
1948
25.50
17609
4578
4873.4(6)
4
1.273
1.112
1972
25.50
15115
4645
4851.9(4)
4
1.287
1.257
1980
25.50
16286
4610
Z
Dc/g cm-3
m/mm-1
F(000)
◦
qmax
/
Collected
25721
9314
Unique reflns
Obsd reflns [I > 2.0s(I)]
No. of variables
4093
276
6470
490
4142
266
4259
265
GOF
R
wR
Rint
1.106
0.0449
0.1157
0.0411
1.004, -0.672
1.101
0.0923
0.2127
0.0720
1.166
0.0570
0.1460
0.0362
0.964, -0.722
1.132
0.0526
0.1260
0.0331
1.065, -0.995
-3
˚
Largest diff. peak, hole/e A
0.866, -0.732
bis(phenolate) lanthanide alkoxides.28 Now, a further study re-
vealed that the amine bis(phenolate) cyclopentadienyl lanthanide
complexes generated in situ can react smoothly with phenols
under mild conditions to give the amine bridged bis(phenolate)
lanthanide aryloxo complexes in high isolated yields. Thus, a series
of lanthanide aryloxides were prepared using (C5H5)3Ln(THF)
as the starting materials. The reactions of (C5H5)3Ln(THF)
with amine bridged bis(phenol) LH2 in a 1 : 1 molar ratio, and
then with 1 equivalent of 2,6-diisopropylphenol, p-cresol or 4-
methoxyphenol, respectively, in situ in THF at 50 ◦C, after workup,
gave the desired neutral lanthanide aryloxides LLn(OC6H3-2,6-
Pri2)(THF) [Ln = Nd (1), Sm (2), Yb (3)], LLn(OC6H4-4-
CH3)(THF)n [Ln = Nd (4), Sm (5), n = 2; Ln = Y (6), n = 1]
and LLn(OC6H4-4-OCH3)(THF)n [Ln = Nd (7), Sm (8), n = 2;
Ln = Yb (9), n = 1] in high isolated yields (72–83%) as shown in
Scheme 1. The compositions of complexes 1–9 were confirmed
by elemental analysis and NMR spectroscopy in the case of
complex 6. The definitive molecular structures of complexes 1–
8 were determined by single-crystal structure analysis. It was
found that both the ionic radii of the lanthanide metals and
the bulkiness of the aryloxo groups have effect on the solid
state structures of these complexes. When the aryloxo group is
diisopropylphenolate, complexes (1–3) have similar structures, and
only one THF molecule is coordinated to the metal center. For the
less bulky 4-methylphenolate and 4-methoxyphenolate, there are
two THF molecules to coordinate to the larger neodymium and
samarium ions (complexes 4, 5, 7 and 8); whereas there is one THF
molecule to coordinate to the small yttrium and ytterbium ions
(complexes 6 and 9). Complexes 1–9 are moderately sensitive to
air and moisture. The crystals decompose in a few minutes when
they are exposed to air, but both the crystals and the solution
are rather stable when stored under argon. All of the lanthanide
aryloxo 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 to 8. X-Ray
diffraction analyses displayed that all of these complexes have
monomeric structures, and the difference in overall structures
among these complexes is the number of the coordinated THF
molecules.
The crystals suitable for an X-ray structure determination of
complexes 1 to 3 were obtained from concentrated toluene solution
at room temperature. Complexes 1 to 3 are isomorphous, and
only the ORTEP diagram of complex 3 is shown in Fig. 1.
The selected bond lengths and angles for these molecules are
provided in Table 3. Like other amine bridged bis(phenolate)
lanthanide complexes,3,8 the sidearm amido group was found to
bind to the metal center in the solid state. Each metal ion is six-
coordinated to two oxygen atoms and two nitrogen atoms from
the dianionic amine bridged bis(phenolate) ligand, one oxygen
atom from diisopropylphenoxo 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 O(1), O(2), O(4) and N(2) can be considered to occupy
equatorial positions within the octahedron. O(3) and N(1) occupy
axial positions, and the O(3)–Ln–N(1) angle is slightly distorted
away from the idealized position of 180◦ to about 169◦ for these
complexes. The overall coordination geometries of complexes
1–3 are similar to those of LYbCl(THF), LYbNPh2(THF) and
LYbMe(THF).3 The average Ln–O(Ar) bond lengths range from
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 6832–6840 | 6835
©

Scheme 1 Synthesis of amine bridged bis(phenolate) lanthanide aryloxides.
◦
˚
Table 3 Selected bond lengths (A) and bond angles ( ) for complexes 1–3
and 6
Bond lengths
1
2
3
6
Ln1–O1
Ln1–O2
Ln1–O3
Ln1–O4
Ln1–N1
Ln1–N2
2.224(3)
2.232(4)
2.212(4)
2.498(4)
2.676(4)
2.687(4)
2.175(9)
2.096(5)
2.120(5)
2.084(6)
2.358(5)
2.519(7)
2.500(6)
2.144(4)
2.159(4)
2.098(4)
2.365(4)
2.526(5)
2.514(5)
2.200(11)
2.170(11)
2.449(12)
2.618(11)
2.820(16)
Bond angles
O3–Ln1–N1
O1–Ln1–N2
N2–Ln1–O2
O2–Ln1–O4
O4–Ln1–O1
N1–Ln1–O4
O1–Ln1–O2
168.80(13)
86.19(14)
94.48(14)
84.10(15)
82.46(13)
88.58(14)
148.79(13)
168.7(4)
88.0(5)
93.5(4)
82.9(5)
83.7(4)
90.2(4)
149.7(4)
168.76(19)
94.0(2)
90.7(3)
85.09(19)
82.43(19)
89.1(2)
160.03(16)
92.66(16)
91.99(16)
85.37(15)
84.66(15)
96.38(16)
153.70(15)
155.4(2)
˚
2.228(3) (for Nd) to 2.108(5) A (for Yb, see Table 3), which reflected
the usual lanthanide contraction from Nd3+ to Yb3+.32 Similar
consequences of the decrease of the ionic radii of the Ln3+ ions
from Nd3+ to Yb3+ are also observed from the Ln–O(Ph) and Ln–
O(THF) bond lengths in these complexes.
Fig. 1 ORTEP diagram of complex 3 showing an atom numbering
scheme. Thermal ellipsoids are drawn at the 30% probability level.
Disordered carbon atoms of methyl groups and hydrogen atoms are omi-
tted for clarity. Complexes 1 and 2 are isomorphous with complex 3.
˚
In complex 1, the average Nd–O(Ar) bond length of 2.228(3) A
is comparable to the corresponding bond lengths in bridged
˚
2.212(4) A, is comparable with the Nd–O(Ar) bond lengths
bis(phenolate) neodymium complexes, such as [(MBMP)Nd(m-
observed in the aforementioned complexes, and (ArO)3Nd(THF)
2-
33
OPri2)(THF)2]2 (2.215(6) A, MBMP = 2,2¢-methylenebis(6-
(ArO = 2,6-di-tert-butyl-4-methylphenolate) (2.176 A). In
˚
˚
tert-butyl-4-methylphenoxo),24
(MBMP)Nd[N(TMS)2](THF)2
complexes 2 and 3, the average Sm–O(Ar) and Yb–O(Ar) bond
26
˚
˚
(2.200(2) A, TMS = SiMe3), and [ONNO]Nd[N(TMS)2](THF)
length are 2.189(10) and 2.108(5) A, respectively, which are
˚
{2.229(5) A, H2[ONNO]
=
1,4-bis(2-hydroxy-3,5-di-tert-
comparable to the corresponding bond lengths in complex 1,
when the difference in ionic radii is considered, and other amine
butyl-benzyl)-imidazolidine}.12 The Nd–O(Ph) bond length of
6836 | Dalton Trans., 2010, 39, 6832–6840
This journal is
The Royal Society of Chemistry 2010
©

◦
˚
be attributed to the less bulkiness of 4-methylphenoxo group
than 2,6-diisopropylphenoxo group. The Ln–O(Ar) and Ln–
O(Ph) bond lengths in complexes 4 and 5 are comparable with
the corresponding values in complexes 1 and 2. The molecular
structure of complex 6 is shown in Fig. 3. In complex 6, only one
THF molecule is coordinated to yttrium ion. Thus, the yttrium
ion is six-coordinated to form a slightly distorted octahedral
geometry, which is different from those of complexes 4 and 5. The
overall coordination geometry of complex 6 is similar to those of
Table 4 Selected bond lengths (A) and bond angles ( ) for complexes 4,
5, 7 and 8
4
5
7
8
Ln1–O1
Ln1–O2
Ln1–O3
Ln1–O4
Ln1–O5
Ln1–N1
Ln1–N2
O1–Ln1–O2
O2–Ln1–O1A
O1–Ln1–O1A
O2–Ln1–N1
O1–Ln1–N1
O2–Ln1–N2
O1–Ln1–N2
N1–Ln1–N2
2.239(3)
2.232(5)
2.568(5)
2.622(4)
2.212(3)
2.202(4)
2.595(4)
2.663(4)
2.246(3)
2.223(6)
2.219(3)
2.189(5)
2.580(6)
2.616(5)
2.695(6)
2.722(7)
105.32(9)
105.32(9)
148.66(18)
141.2(2)
76.06(9)
75.9(2)
2.560(6)
2.590(5)
2.656(6)
2.691(7)
105.02(9)
105.02(9)
149.49(18)
141.6(2)
76.63(9)
75.9(2)
2.688(4)
2.721(5)
2.663(4)
2.684(5)
˚
complexes 1–3. The Y–O(Ar) bond length of 2.152(4) A accords
105.70(7)
105.70(7)
148.02(15)
141.73(17)
75.86(7)
76.15(18)
90.85(8)
105.25(7)
105.25(7)
149.08(15)
141.94(17)
76.47(7)
75.83(17)
91.27(8)
with the values in amine bridged bis(phenolate) yttrium com-
8
˚
plexes LY(CH2SiMe3)(THF) [2.147(2) A], LY(CH2C6H4NMe2-
36
˚
o)[2.141(3) A],
[ONOO]Y(CH2SiMe3)(THF) [ONOO
=
t
˚
MeOCH2CH2N{CH2-(2-O-C6H2- Bu2-3,5)}2] [2.129(4) A], and
37
˚
[ONOO]YN(SiHMe2)2(THF) [2.151(2) A]. The Y–O(Ph) bond
90.59(10)
65.29(19)
91.10(10)
65.74(19)
length is comparable with those values in complexes 1–5, when the
65.58(15)
66.11(15)
difference in ionic radii is considered.
bridged bis(phenolate) samarium10 and ytterbium complexes.3
The Sm–O(Ph) and Yb–O(Ph) bond lengths of 2.170(11) and
˚
2.084(6) A, respectively, are in agreement with the corresponding
34
˚
values in (ArO)3Sm(THF) (2.155 A) and (ArO)2YbCl(THF)
35
˚
(2.080(1) A).
The definitive molecular structures of complexes 4–6 were
determined. All of these complexes have monomeric structures,
and their selected bond parameters are listed in Tables 3 and 4,
respectively. Complexes 4 and 5 are isomorphous, and only the
ORTEP diagram of complex 4 is shown in Fig. 2. In complexes 4
and 5, each metal ion is coordinated to two oxygen atoms and two
nitrogen atoms from the amine bridged bis(phenolate) ligand, one
oxygen atom from 4-methylphenoxo group, and two oxygen atoms
from two THF molecules. The coordination geometry around the
metal center can be best described as a slightly distorted capped
trigonal prism, with the capping atom being the oxygen atom
of 4-methylphenoxo group. The overall coordination geometry is
similar to that of amine bridged bis(phenolate) yttrium chloride,19
but different from those of complexes 1–3. This difference can
Fig. 3 ORTEP diagram of complex 6 showing an atom numbering
scheme. Thermal ellipsoids are drawn at the 30% probability level.
Disordered carbon atoms of methyl groups and hydrogen atoms are
omitted for clarity.
Complexes 7 and 8 are isomorphous, and only the ORTEP
diagram of complex 7 is shown in Fig. 4. In complexes 7 and 8,
each metal ion is surrounded by one amine bridged bis(phenolate)
ligand, one 4-methoxyphenoxo group and two THF molecules.
The overall coordination geometries of complexes 7 and 8 are iden-
tical with those of complexes 4 and 5. As expected, the Ln–O(Ar)
and Ln–O(Ph) bond lengths in complexes 7 and 8 are comparable
with the corresponding values in complexes 4 and 5.
Ring-opening polymerization of lactide
Biodegradable polymers, such as poly(e-caprolactone) (PCL)
and poly(lactide) (PLA), have recently gained great atten-
tion as a replacement for conventional synthetic materials be-
cause of their biodegradable, biocompatible, and permeable
properties.38,39 Ring-opening polymerization of cyclic esters ini-
tiated by organometallic complexes is a convenient method for
the synthesis of these polymers with high molecular weights.40–46
To understand the effect of initiating groups on the polymeriza-
tion activity and controllability of amine bridged bis(phenolate)
Fig. 2 ORTEP diagram of complex 4 showing an atom numbering
scheme. Thermal ellipsoids are drawn at the 30% probability level.
Disordered carbon atoms and hydrogen atoms are omitted for clarity.
Complex 5 is isomorphous with complex 4.
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 6832–6840 | 6837
©

give the polymers with high molecular weights and relatively
narrow molecular weight distributions (Mw/Mn = 1.10–1.59). For
example, using complex 1 as the initiator, the polymerization gives
88% yield in 2 h even for [M]0/[1]0 = 1000 (entry 2), and produced
PLA with a molecular weight of 13.90 ¥ 104 and a PDI of 1.47.
The polymerization temperature affects the activity. As expected,
the higher the temperature was, the faster the polymerization
proceeded. For instance, using complex 6 as the initiator, almost
complete yield could reach at 70 ◦C in 1 h when the molar ratio of
monomer to initiator ([M]/[I]) is 400 (entry 17), whereas the yield
was 45% at 50 ◦C in 1 h (entry 18) under the same polymerization
conditions.
The ionic radii of the lanthanide metals have a significant effect
on the catalytic activity for L-lactide polymerization. Upon a
decrease of the ionic radii, the catalytic activity decreased. Using
neodymium complex 1 as the initiator, the yield was 92% when the
molar ratio of monomer to initiator is 800 in 1 h (entry 1), whereas
it was 24% using an ytterbium complex 3 as the initiator in 2 h,
even when the molar ratio of monomer to initiator decreases to 600
under the same polymerization conditions (entry 7). The observed
activity decreasing order, Nd > Sm > Yb ª Y, is in agreement
with the order of ionic radii, which is consistent with the active
trend observed in the methylene-linked bis(phenolate) lanthanide
systems for the polymerization of e-caprolactone,24 imidazolidine-
bridged bis(phenolate) rare-earth metal amides12 and the bis(allyl)
diketiminatolanthanide complexes for lactide polymerization.47
On the other hand, using the yttrium aryloxo complex as the
initiator (entries 16–19), the polymerizations give the polymers
with apparently small PDI values, which indicated that the central
metal ions also affect the polymerization controllability.
Fig. 4 ORTEP diagram of complex 7 showing an atom numbering
scheme. Thermal ellipsoids are drawn at the 30% probability level.
Disordered carbon and oxygen atoms and hydrogen atoms are omitted
for clarity. Complex 8 is isomorphous with complex 7.
lanthanide complexes, the catalytic behavior of complexes 1–9 for
the ring-opening polymerization of L-lactide was examined.
As expected, the amine bridged bis(phenolate) lanthanide ary-
loxides are efficient initiators for the ring-opening polymerization
of L-lactide. The representative L-lactide polymerization data are
summarized in Table 5. It can be seen that all of these lanthanide
aryloxides can initiate L-lactide polymerization in toluene to
In comparison with the amine bridged bis(phenolate) lan-
thanide alkyl6,8 and amido complexes,48 these aryloxo complexes
Table 5 Polymerization of L-lactide initiated by complexes 1–9a
T/◦C
t
Yield (%)b
Mc (¥ 104)
Mn d(¥ 104)
PDId
c
Entry
Initiator
[M]0/[I]0
1
2
3
4
5
6
7
8
1
1
2
2
2
3
3
4
4
4
5
5
5
5
5
6
6
6
6
7
7
8
8
9
9
800
1000
400
600
800
400
600
600
800
1000
400
600
800
1000
800
200
400
400
600
800
1000
600
800
400
600
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
50
70
70
70
70
70
70
70
1 h
2 h
1 h
2 h
2 h
1 h
2 h
1 h
1 h
2 h
1 h
2 h
2 h
3 h
1 h
1 h
1 h
1 h
3 h
1 h
2 h
2 h
2 h
1 h
2 h
92
88
98
96
95
98
24
99
96
86
98
98
98
27
48
99
99
45
22
94
83
98
98
98
20
10.60
12.67
5.64
8.29
10.94
5.64
2.51
8.55
11.06
12.38
5.64
8.47
11.29
3.89
5.53
2.85
5.70
2.59
1.73
10.83
11.95
8.47
11.29
5.64
—
11.34
13.90
10.37
10.64
12.06
9.81
4.31
7.22
8.80
8.02
6.32
8.53
8.77
3.21
4.69
4.09
4.56
2.53
1.51
9.77
8.74
9.86
10.34
5.09
—
1.38
1.47
1.59
1.54
1.59
1.58
1.21
1.54
1.55
1.42
1.59
1.51
1.44
1.37
1.42
1.30
1.29
1.10
1.14
1.45
1.54
1.45
1.43
1.22
—
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
a General polymerization conditions: toluene as the solvent, [L-LA] = 1 mol L-1. b Yield: weight of polymer obtained/weight of monomer used. c Mc =
(144.13) ¥ [M]0/[I]0 ¥ (polymer yield) (%). d Measured by GPC calibrated with standard polystyrene samples.
6838 | Dalton Trans., 2010, 39, 6832–6840
This journal is
The Royal Society of Chemistry 2010
©

Table 6 Polymerization of L-lactide initiated by complexes 2 and 5a
Entry
Initiator
[M]0/[I]0
T/◦C
t
Yield (%)b
Mc c(¥ 104)
Mn d(¥ 104)
PDId
1
2
3
4
5
6
7
8
2
2
2
2
2
2
5
5
5
5
5
5
800
800
800
800
800
800
800
800
800
800
800
800
70
70
70
70
70
70
70
70
70
70
70
70
10 min
20 min
30 min
1 h
1.5 h
2 h
10 min
20 min
30 min
1 h
1.5 h
2 h
17
20
28
75
88
95
40
55
68
93
98
98
1.96
2.30
3.23
8.64
10.14
10.94
4.61
6.34
7.83
3.06
3.13
4.98
9.12
10.86
12.06
4.39
5.02
6.01
7.71
9.42
8.77
1.22
1.20
1.24
1.38
1.44
1.59
1.12
1.22
1.25
1.36
1.43
1.44
9
10
11
12
10.71
11.29
11.29
a General polymerization conditions: toluene as the solvent, [L-LA] = 1 mol L-1. b Yield: weight of polymer obtained/weight of monomer used. c Mc =
(144.13) ¥ [M]0/[I]0 ¥ (polymer yield) (%). d Measured by GPC calibrated with standard polystyrene samples.
Table 7 Polymerization of rac-lactide initiated by complex 6a
Entry
[M]0/[I]0
T/◦C
t
Yield (%)b
Mc c(¥ 104)
Mn d(¥ 104)
PDId
Pre
1
2
3
4
5
6
7
8
100
100
200
300
400
500
600
700
700
1000
20
20
20
20
20
20
20
20
20
20
10 min
1 h
1 h
1 h
1 h
1 h
2 h
3 h
4 h
93
1.34
1.44
2.88
4.32
5.76
7.20
7.60
6.65
10.08
11.81
2.99
3.50
4.34
5.25
5.29
6.75
7.03
4.24
7.46
7.65
1.12
1.17
1.14
1.15
1.16
1.11
1.09
1.14
1.14
1.11
0.98
0.95
0.97
0.95
0.96
0.98
0.98
0.97
0.98
0.97
100
100
100
100
100
88
66
100
82
9
10
5 h
a General polymerization conditions: THF as the solvent, [L-LA] = 1 mol L-1. b Yield: weight of polymer obtained/weight of monomer used. c Mc =
(144.13) ¥ [M]0/[I]0 ¥ (polymer yield) (%). d Measured by GPC calibrated with standard polystyrene samples. e Measured by homodecoupling 1H NMR
spectroscopy at 20 ◦C in CDCl3.
exhibit similar activity for lactide polymerization. The amine
bridged bis(phenolate) yttrium alkyl and amido complexes can
initiate the complete polymerization of lactide when the molar
ratio of the monomer to the initiator is 500 within 1 h or
80 min, respectively; The yield is 99% using the amine bridged
bis(phenolate) yttrium aryloxide 3 as the initiator at 70 ◦C in
1 h (entry 6). However, compared with the bridged bis(phenolate)
lanthanide alkoxo complexes generated in situ,6,12 complexes 1–9
show relatively poor controllability for L-lactide polymerization,
and give the resultant polymers with relatively broad molecular
weight distributions. These results indicated that the electronic
properties of the Ln–O bonds have profound effect on the control-
lability of lactide polymerization. A comparative study revealed
that the structures of the aryloxo groups in these complexes
have no obvious effect on the activity and controllability for L-
lactide polymerization. The activity and controllability initiated
by the complexes with the same central metal ion is very similar
(entries 4, 12 and 22). However, the measured molecular weights of
the polymers initiated by complexes 1–3 are all superior from the
theoretical values, whereas the measured molecular weights of the
polymers initiated by complexes 4–9 are near to the calculated
ones. These results indicated that the initiation efficiency is low
for the former. To further understand this phenomenon, the
polymerization kinetics was measured using complexes 2 and 5
as the initiators, respectively, and the results are summarized in
Table 6. It can be seen that the polymerization initiated by complex
5 is apparently faster than that initiated by complex 2 under the
same polymerization conditions. The yield is 40% after 10 min
using complex 5 as the initiator (entry 7), whereas the yield is only
17% using complex 2 as the initiator (entry 1). In addition, the
molecule weight distributions of the resultant polymers broadened
as prolonging the polymerization time using both complexes as
the initiators, which indicated the presence of transesterification
under the current polymerization conditions. Based on these
results, we postulated that the coordination of the bulky 2,6-
diisopropylphenoxo group to lanthanide ion increases the steric
hindrance around the metal center, which blocks the coordination
of the monomer at initiation step. Thus, the polymerization
kinetics initiated by complexes 1–3 are slow initiation and fast
propagation in comparison with those initiated by complexes 4–9,
which decreases the initiation efficiency.
Because complex 6 showed relatively good controllability for
the ring-opening polymerization of L-lactide, its catalytic behavior
for rac-lactide polymerization was also tested, and the preliminary
results are listed in Table 7. It can be seen that complex 6 can initiate
the controlled highly heteroselective ring-opening polymerization
of rac-lactide. The polymerizations proceed smoothly in THF
at 20 ◦C, and give the polymers with high molecular weights
and narrow molecular weight distributions (Mw/Mn = 1.09–
1.17). The catalytic activity of complex 6 is similar to that of
the amine bridged bis(phenolate) yttrium alkyl complexes,8 but
is lower than that of the methoxyl-amino bridged bis(phenolate)
yttrium alkoxide.6 For example, complex 6 and the yttrium alkyl
complex can polymerize completely 500 equivalents of rac-lactide
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The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 6832–6840 | 6839
©

in 1 h, whereas the yttium alkoxide complex can polymerize 1000
equivalents of rac-lactide within 40 min under the same polymer-
ization conditions. However, the controllability of complex 6 for
this polymerization is as good as that of the yttrium alkoxide,6 and
is better than the yttrium alkyl complexes8 mentioned above, which
is consistent with the general rule suggested by Okuda et al.21 It
is worthy to note that complex 6 showed high stereoselectivity for
rac-lactide polymerization to give heterotactic polymers with Pr
over 0.95. The stereocontrol ability of complex 6 is similar to that
of the amine bridged bis(phenolate) yttrium alkyl complexes (Pr
over 0.94), but is better than that of the methoxyl-amino bridged
bis(phenolate)yttrium alkoxo complex (Pr = 0.80). These results
revealed that the initiating group has no obvious effect on the
stereoselectivity, whereas the ancillary ligand plays a crucial role.
7 E. E. Delbridge, D. T. Dugah, C. R. Nelson, B. W. Skelton and A. H.
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11 C. E. Willans, C. E. Sinenkov, G. K. Fukin, K. Sheridan, J. M. Lynam,
A. A. Trifonov and F. M. Kerton, Dalton Trans., 2008, 3592.
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Luo, Inorg. Chem., 2009, 48, 5715.
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Conclusion
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3743.
In summary, a series of neutral amine bridged bis(phenolate)
lanthanide aryloxo complexes were successfully synthesized via
protolytic ligand exchange reactions using the readily available
(C5H5)3Ln(THF) as the precursors, and their structure features
have been provided by X-ray diffraction study. These complexes
represent the first structurally characterized examples of amine
bridged bis(phenolate) lanthanide aryloxo complexes. These re-
sults indicated that the acidity of the phenols has a profound
influence on the reactivity of amine bridged bis(phenolate) cy-
clopentadienyl lanthanide complexes, because the amine bridged
bis(phenolate) lanthanide alkoxides cannot be prepared by this
method. These lanthanide aryloxo complexes can initiate ef-
ficiently the ring-opening polymerization of L-lactide, and the
observed activity increasing order is in agreement with the
order of the ionic radii. In comparison with the amine bridged
bis(phenolate) lanthanide alkoxides, these lanthanide aryloxides
show inferior controllability for L-lactide polymerization. The
structures of the aryloxo groups have no obvious effect on the
catalytic activity and controllability of L-lactide polymerization,
but the steric bulkiness of the aryloxo group can affect the
initiation step.
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Acknowledgements
Financial support from the National Natural Science Foundation
of China (Grants 20771078, 20972108 and 20632040), the Major
Basic Research Project of the Natural Science Foundation of the
Jiangsu Higher Education Institutions (Project 07KJA15014), and
the Qing Lan Project is gratefully acknowledged.
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©