Synthesis and characterization of lanthanide amides bearing aminophenoxy ligands and their catalytic activity for the polymerization of lactides

Article abstract of DOI:10.1021/om300036a

A series of neutral lanthanide complexes supported by aminophenoxy ligands were synthesized, and their catalytic behavior in the polymerization of l-lactide and rac-lactide was explored. The amine elimination reactions of equimolar amounts of Ln[N(TMS)₂]₃(ν-Cl)Li(THF) ₃ and aminophenol [HONH]¹ {[HONH]¹ = 2,6-Me₂-C₆H₄NHCH₂(3,5- ^tBu₂-C₆H₂-2-OH)} in toluene gave the dimeric lanthanide complexes {[ON]¹Ln[ONH]¹}₂ (Ln = La (1), Nd (2)), whereas the similar reactions of La[N(TMS) ₂]₃(THF)₂ or Ln[N(TMS)₂] ₃(ν-Cl)Li(THF)₃ (Ln = Nd, Sm) with the aminophenols [HONH]² {[HONH]² = (μ-OCH₃-C ₆H₄)NHCH₂(3,5-^tBu₂-C ₆H₂-2-OH} and [HONH]³ {[HONH]³ = (NC₅H₄)NHCH₂(3,5-^tBu ₂-C₆H₂-2-OH)} generated the neutral aminophenoxy lanthanide amides {[ON]²Ln[N(TMS)₂]} ₂ [Ln = La (3), Nd (4), Sm (5)] and {[ON]³Ln[N(TMS) ₂](THF)}₂ [Ln = La (6), Nd (7), Sm (8)], respectively, in high isolated yields. These complexes have been fully characterized. X-ray structural determination revealed that complexes 1 and 2 have unsolvated centrosymmetric dimeric structures, in which one hydrogen atom belonging to the amino group of the ligand is reserved. Complexes 3-5 are isostructural and have an unsolvated dimeric structure. The coordination geometry around each of the lanthanide metal atoms can be described as a distorted trigonal bipyramid. Complexes 6 and 7 have a solvated dimeric structure, and the lanthanide metal centers have distorted capped trigonal-prismatic geometries. It was found that complexes 3-8 are highly efficient initiators for the ring-opening polymerization of l-lactide and rac-lactide, affording polymers with high molecular weights.

Full text of DOI:10.1021/om300036a

Article
pubs.acs.org/Organometallics
Synthesis and Characterization of Lanthanide Amides Bearing
Aminophenoxy Ligands and Their Catalytic Activity for the
Polymerization of Lactides
Jie Zhang,† Jinshui Qiu,† Yingming Yao,*,†,‡ Yong Zhang,† Yaorong Wang,*,† and Qi Shen†
†Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering & 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 complexes supported by amino-
phenoxy ligands were synthesized, and their catalytic behavior in the
polymerization of L-lactide and rac-lactide was explored. The amine elimination
reactions of equimolar amounts of Ln[N(TMS)2]3(μ-Cl)Li(THF)3 and amino-
phenol [HONH]1 {[HONH]1 = 2,6-Me2-C6H4NHCH2(3,5-tBu2-C6H2-2-OH)}
in toluene gave the dimeric lanthanide complexes {[ON]1Ln[ONH]1}2 (Ln = La
(1), Nd (2)), whereas the similar reactions of La[N(TMS)2]3(THF)2 or
Ln[N(TMS)2]3(μ-Cl)Li(THF)3 (Ln = Nd, Sm) with the aminophenols
[HONH]2 {[HONH]2 = (ο-OCH3-C6H4)NHCH2(3,5-tBu2-C6H2-2-OH} and
[HONH]3 {[HONH]3 = (NC5H4)NHCH2(3,5-tBu2-C6H2-2-OH)} generated the
neutral aminophenoxy lanthanide amides {[ON]2Ln[N(TMS)2]}2 [Ln = La (3),
Nd (4), Sm (5)] and {[ON]3Ln[N(TMS)2](THF)}2 [Ln = La (6), Nd (7), Sm
(8)], respectively, in high isolated yields. These complexes have been fully
characterized. X-ray structural determination revealed that complexes 1 and 2
have unsolvated centrosymmetric dimeric structures, in which one hydrogen atom belonging to the amino group of the ligand is
reserved. Complexes 3−5 are isostructural and have an unsolvated dimeric structure. The coordination geometry around each of
the lanthanide metal atoms can be described as a distorted trigonal bipyramid. Complexes 6 and 7 have a solvated dimeric
structure, and the lanthanide metal centers have distorted capped trigonal-prismatic geometries. It was found that complexes 3−8
are highly efficient initiators for the ring-opening polymerization of L-lactide and rac-lactide, affording polymers with high
molecular weights.
by the reduction of the corresponding Schiff bases, which can
be prepared from inexpensive and readily available starting
materials. These ligand systems have been utilized in main
group and transition-metal coordination chemistry, and some
of these metal complexes show good activity in homogeneous
catalysis.4 The utilization of such ligands in organolanthanide
chemistry, however, has not been studied to any significant
degree.5 Furthermore, most of the reported aminophenoxy
lanthanide metal complexes are isolated as unexpected products
from the reactions of organolanthanide complexes with Schiff
bases and not from aminophenols.5 Recently, we reported the
first examples of lanthanide metal amides bearing dianionic
aminophenoxy ligands synthesized from aminophenol starting
materials and found that these aminophenoxy lanthanide metal
complexes could initiate the ring-opening polymerization of
lactide.6 These results indicated that the dianionic N,O-chelate
ligands have great potential for the design and synthesis of
organolanthanide metal catalysts for homogeneous catalysis.
INTRODUCTION
■
Throughout the past decade, significant effort has been invested
in the development of alternative ligands to replace the
traditional set of bis(cyclopentadienyl) ancillary ligands often
used in organolanthanide chemistry,1 because the reactivity of
organolanthanide metal complexes depends mainly on the
coordination environment around the metal centers. Lantha-
nide metals are considered to be hard acids and have an
oxophilic nature, which leads to a preference for hard oxygen-
or nitrogen-based ligands. On the other hand, it has been
demonstrated that bridged dianionic ligands are efficient
ancillary ligands for the stabilization of single-site catalysts
based on trivalent lanthanide metals. Thus, the utilization of
bridged bis(phenolato) and bis(amido) ligands in the design of
organolanthanide metal catalysts has attracted significant
attention, and some of these lanthanide metal complexes
have shown high levels of activity in polymerization reactions
and organic transformations.2,3
The aminophenoxy group is a type of dianionic N,O-chelate
ligand, which combines the properties of oxygen- and nitrogen-
based ligands. Aminophenol pro-ligands can be easily accessed
Received: January 17, 2012
Published: March 23, 2012
© 2012 American Chemical Society
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Organometallics
Article
pressure, the residue was crystallized in hexane, and a solid was
isolated (23.9 g, 70%). Anal. Calcd for C22H31NO2: C, 77.38; H, 9.15;
N, 4.10. Found: C, 77.53; H, 9.32; N, 3.86. H NMR (400 MHz,
However, the aminophenoxy lanthanide amide complexes
obtained were anionic species [ON]2Ln[N(TMS)2][Li-
(THF)]2 {[ON]2− = [p-CH3C6H4NCH2(3,5-tBu2C6H2-2-
O)]2−}, in which two aminophenoxy groups and one amido
group were found to be coordinated to the central metal. The
crowded coordination environments around the lanthanide
metals could potentially decrease their catalytic activity because
steric hindrance can have a significant impact on the catalytic
activity of lanthanide complexes. Thus, we synthesized neutral
aminophenoxy lanthanide amides to improve their catalytic
performance. In this paper, three aminophenoxy ligands
(Figure 1) were introduced to organolanthanide chemistry,
1
CDCl3): 8.67 (s, 1H, Ph-OH), 7.32 (s, 1H, Ph), 7.04 (s, 1H, Ph),
6.98−6.78 (m, 4H, Ph), 4.53 (s, 1H, NH), 4.37 (s, 2H, CH2), 3.84 (s,
t
t
3H, Me), 1.45 (s, 9H, Bu), 1.33 (s, 9H, Bu). 13C NMR (100 MHz,
CDCl3): 157.00(Ph), 152.87(Ph), 145.59(Ph), 141.41(Ph),
138.17(Ph), 137.91(Ph), 126.62(Ph), 125.48(Ph), 123.84(Ph),
112.69(Ph), 110.23(CH2), 42.55(OMe), 35.37(C(CH3)3), 34.32-
(C(CH3)3), 31.83(C(CH3)3), 29.93(C(CH3)3).
Synthesis of (NC5H4)NHCH2(3,5-tBu2-C6H2-2-OH) {[HONH]3}.
A procedure similar to that described for [HONH]2 was used, but with
2-pyridinamine (9.4 g, 100 mmol) instead of o-anisidine. A solid was
isolated (20.3 g, 65%). Anal. Calcd for C20H28N2O: C, 76.89; H, 9.03;
1
N, 8.96. Found: C, 76.61; H, 8.89; N, 8.75. H NMR (400 MHz,
CDCl3): 11.76 (s, 1H, Ph-OH), 8.06 (d, J = 5.1 Hz, 1H, Ph), 7.40−
7.32 (m, 1H, Ph), 7.29 (d, J = 2.4 Hz, 1H, Ph), 7.09 (d, J = 2.4 Hz, 1H,
Ph), 6.59−6.52 (m, 1H, Ph), 6.41 (d, J = 8.5 Hz, 1H), 5.09 (s, 1H,
NH), 4.46 (d, J = 6.6 Hz, 2H, CH2), 1.45 (s, 9H), 1.32 (s, 9H). 13C
NMR (100 MHz, CDCl3): 157.31(Ph), 153.19(Ph), 145.91(Ph),
141.73(Ph), 138.49(Ph), 138.22(Ph), 137.51(Ph), 126.93(Ph),
125.80(Ph), 124.16(Ph), 113.01(Ph), 110.55(CH2), 35.68(C(CH3)3),
34.64(C(CH3)3), 32.14(C(CH3)3), 30.25(C(CH3)3).
Figure 1. Aminophenoxy ligands.
Synthesis of {[ON]1La[ONH]1]}2 (1). To a stirred toluene solution
of La[N(TMS)2]3(μ-Cl)Li(THF)3 (20 mL, 1.76 g, 2.00 mmol) was
added a toluene solution of [HONH]1 (20 mL, 0.68 g, 2.00 mmol).
The mixture was stirred for 2 h at 90 °C, and then the precipitate
formed (LiCl) was separated by centrifugation. Toluene was
evaporated to about 15 mL under reduced pressure. Colorless block
microcrystals were obtained from a concentrated toluene solution in a
few days (1.39 g, 85%). Mp: 315−317 °C. Anal. Calcd for
C92H126N4La2O4: C, 67.80; H, 7.79; N, 3.44; La, 17.04. Found: C,
67.58; H, 7.83; N, 3.69; La, 17.31. 1H NMR for 1·2toluene (400 MHz,
d8-THF+C6D6): 7.38−7.27 (m, 4H, Ph−CH3), 7.22−7.11 (m, 8H,
Ph), 6.87−7.03 (m, 6H, Ph−CH3), 6.64−6.84 (m, 6.3 Hz, 8H, Ph),
6.34−6.52 (m, 4H, Ph), 4.21 (s, 4H, CH2), 3.99 (d, J = 25.4 Hz, 4H,
CH2), 3.76 (s, 2H, NH), 2.31 (s, 6H, CH3−Ph), 2.14−1.96 (m, 24H,
Me), 1.46 (s, 36H, tBu), 1.17−1.08 (s, 36H, tBu). 13C{1H} NMR (100
MHz, d8-THF+C6D6): 163.37(Ph), 147.24(Ph), 145.74(Ph),
137.53(Ph), 135.94(Ph), 135.78(Ph), 131.04(Ph), 129.51(Ph),
129.32(Ph), 128.55(Ph), 126.90(Ph), 125.68(Ph), 125.00(Ph),
124.64(Ph), 123.91(Ph), 123.72(Ph), 57.11(CH2Ar), 55.47(CH2Ar),
35.68(C(CH3)3), 34.13(C(CH3)3), 32.01(C(CH3)3), 30.58(C-
(CH3)3), 23.08(CH3), 21.38(CH3), 18.92(CH3), 14.32(CH3). IR
(KBr, cm−1): 3036(s), 2881(s), 2375(s), 1816(s), 1639(m), 1685(m),
1457(s), 1389(s), 1249(s), 998(s), 844(m), 748(m), 601(w), 532(s).
Synthesis of {[ON]1Nd[ONH]1]}2 (2). The synthesis of complex 2
was carried out in the same way as that described for complex 1, but
Nd[N(TMS)2]3(μ-Cl)Li(THF)3 (1.77 g, 2.00 mmol) was used instead
of La[N(TMS)2]3(μ-Cl)Li(THF)3. Blue crystals were obtained at
room temperature in several days (1.25 g, 76%). Mp: 279−281 °C.
Anal. Calcd for C92H126N4Nd2O4: C, 67.36; H, 7.74; N, 3.41; Nd,
17.58. Found: C, 67.77; H, 7.84; N, 3.07; Nd, 17.81. IR (KBr, cm−1):
3063(s), 2858(s), 2446(s), 2350(s), 1635(s), 1492(m), 1442(m),
1238(s), 1130(w), 1083(s), 1000(m), 836(s), 709(w), 597(s), 532(s).
Synthesis of {[ON]2La[N(TMS)2]}2 (3). To a stirred toluene
solution of La[N(TMS)2]3(THF)2 (20 mL, 1.53 g, 2.00 mmol) was
added a toluene solution of [HONH]2 (20 mL, 0.68 g, 2.00 mmol).
The mixture was stirred for 2 h at 90 °C, and then the toluene solution
was concentrated to about 30 mL under reduced pressure. Colorless
crystals were obtained at room temperature in several days (2.63 g,
90%). Mp: 274−276 °C. Anal. Calcd for C56H94La2N4O4Si4: C, 52.65;
H, 7.42; N, 4.38; La, 21.75. Found: C, 52.51; H, 7.58; N, 3.98; La,
and a series of aminophenoxy lanthanide complexes were
synthesized and subsequently characterized. As anticipated, the
neutral organolanthanide amido complexes showed high
activity in the ring-opening polymerization of L-lactide and
rac-lactide.
EXPERIMENTAL SECTION
■
General Procedures. The complexes described below are
sensitive to air and moisture. Therefore, all manipulations were
performed under pure argon with rigorous exclusion of air and
moisture using Schlenk techniques. Solvents were dried and freed of
oxygen by refluxing over sodium/benzophenone ketyl and distilled
prior to use. HN(TMS)2, L-lactide, rac-lactide, and nBuLi are
commercially available. HN(TMS)2 was dried over CaH2 for 3 days
and distilled before use. Aminophenol [HONH]1 {[HONH]1 = (2,6-
Me2C6H4)NHCH2(3,5-tBu2-C6H2-2-OH)},7 La[N(TMS)2]3(THF)2,8
and Ln[N(TMS)2]3(μ-Cl)Li(THF)3 (Ln = La, Nd, Sm)9 were
prepared according to the literature. Rare-earth metal analyses were
performed by ethylenediaminetetraacetic acid titration with a xylenol
orange indicator and a hexamine buffer.10 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 FTIR spectrometer as KBr pellets. The 1H and 13C NMR
spectra were recorded in a d8-THF/C6D6 solution for complexes 1, 3,
5, 6, and 8 with a Unity Varian spectrometer. Because of their
paramagnetism, no resolvable NMR spectrum for the neodymium
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 at 40 °C.
Synthesis of (ο-OCH3C6H4)NHCH2(3,5-tBu2-C6H2-2-OH)
{[HONH]2}. o-Anisidine (11.3 mL, 100 mmol) was added under
vigorous stirring to a solution of 3,5-di-tert-butyl-2-hydroxybenzalde-
hyde (23.4 g, 100 mmol) in refluxing absolute ethanol (250 mL),
which was allowed to react for 48 h. The crystalline orange solid was
filtered and washed with cold absolute ethanol. The precipitate (28.9
g, 85 mmol) was added under vigorous stirring to a suspension of
NaBH4 (4.0 g, 100 mmol) in ethanol (200 mL). The reaction mixture
was stirred for 12 h, resulting in a white suspension, and then the
solvent was completely removed under reduced pressure. The solid
residue was extracted into CHCl3. The organic layer was dried with
Na2SO4 and filtered. After removal of the solvent under reduced
1
21.46. H NMR for 3·2toluene (400 MHz, d8-THF): 7.05−7.20 (m,
4H, Ar-CH3), 7.02−7.04 (m, 8H, Ar), 6.63−6.76 (m, 6H, Ar-CH3),
6.44 (d, J = 7.8 Hz, 2H, Ar), 6.04 (t, J = 7.5 Hz, 2H, Ar), 4.19 (s, 4H,
CH2), 4.11 (s, 6H, MeO), 2.30 (s, 6H, Ar-CH3), 1.45 (s, 18H, tBu-Ar),
t
1.25 (s, 18H, Bu-Ar), 0.03 (s, 36H, N(TMS)2). 13C{1H} NMR (100
MHz, d8-THF): 163.12(Ph), 150.60(Ph), 148.26(Ph), 138.50(Ph),
136.26(Ph), 134.27(Ph), 130.57(Ph), 129.77(Ph), 129.01(Ph),
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Scheme 1. Synthesis of Complexes 1 and 2
124.86(Ph), 124.26(Ph), 121.83(Ph), 109.84(Ph), 109.19(Ph),
108.86(Ph), 107.61(Ph), 58.02(CH2Ar), 50.69(OCH3), 35.82(C-
(CH3)3), 34.59(C(CH3)3), 32.60(C(CH3)3), 31.24(C(CH3)3), 21.65-
(CH3), 4.79(Si(CH3)3). IR (KBr, cm−1): 3635(s), 2945(s), 2336(m),
1604(s), 1535(s), 1458(m), 1358(m), 1290(w), 1200(s), 1150(w),
1086(s), 1083(m), 860(w), 836(s), 745(s), 553(m).
Synthesis of {[ON]3Nd[N(TMS)2](THF)}2 (7). The synthesis of
complex 7 was carried out in the same way as that described for
complex 4, but [HONH]3 (0.62 g, 2.00 mmol) was used instead of
[HONH]2. Blue block microcrystals were obtained from a
concentrated toluene solution in a few days (1.39 g, 89%). Mp:
284−286 °C. Anal. Calcd for C60H104N6Nd2O4Si4: C, 52.44; H, 7.63;
N, 6.11; Nd, 20.99. Found: C, 52.18; H, 7.76; N, 5.89; Nd, 20.63. IR
(KBr, cm−1): 3427(s), 2956(s), 2905(m), 2866(m), 2596(m),
1970(m), 1812(m), 1616(s), 1570(m), 1512(s), 1483(w), 1439(s),
1389(m), 1361(m), 1299(m), 1236(m), 1201(w), 1068(m), 932(m),
884(m), 840(s), 769(s), 735(m), 525(w).
Synthesis of {[ON]2Nd[N(TMS)2]}2 (4). The synthesis of complex
4 was carried out in the same way as that described for complex 2, but
[HONH]2 (20 mL, 0.68 g, 2.00 mmol) was used instead of [HONH]1.
Blue crystals were obtained at room temperature in several days (2.59
g, 88%). Mp: 195−197 °C. Anal. Calcd for C56H94N4Nd2O4Si4: C,
52.21; H, 7.35; N, 4.34; Nd, 22.39. Found: C, 52.10; H, 7.53; N, 3.90;
Nd, 22.59. IR (KBr, cm−1): 3650(s), 2958(s), 2466(s), 1640(s),
1508(s), 1458(s), 1419(m), 1358(m), 1300(w), 1245(s), 1172(w),
1122(s), 1022(m), 883(w), 836(s), 748(s), 532(m).
Synthesis of {[ON]3Sm[N(TMS)2](THF)}2 (8). The synthesis of
complex 8 was carried out in the same way as that described for
complex 5, but [HONH]3 (0.62 g, 2.00 mmol) was used instead of
[HONH]2. Yellow block microcrystals were obtained from a
concentrated toluene solution in a few days (1.39 g, 89%). Mp:
238−240 °C. Anal. Calcd for C60H104N6Sm2O4Si4: C, 51.97; H, 7.56;
Synthesis of {[ON]2Sm[N(TMS)2]}2 (5). The synthesis of complex
5 was carried out in the same way as that described for complex 4, but
Sm[N(TMS)2]3(μ-Cl)Li(THF)3 (1.78 g, 2.00 mmol) was used instead
of Nd[N(TMS)2]3(μ-Cl)Li(THF)3. Red block microcrystals were
obtained from a concentrated toluene solution in a few days (2.55 g,
86%). Mp: 246−248 °C. Anal. Calcd for C56H94N4O4Sm2Si4: C, 51.72;
H, 7.29; N, 4.31; Sm, 23.13. Found: C, 51.43; H, 7.47; N, 4.67; Sm,
1
N, 6.06; Sm, 21.69. Found: C, 51.60; H, 7.72; N, 6.39; Sm, 22.08. H
NMR (400 MHz, C6D6): 8.19 (b, 2H, Ar), 7.56 (b, 2H, Ar), 5.93 (b,
4H, Py), 5.38(b, 4H, Py), 4.70 (b, 2H, CH2), 4.31 (b, 2H, CH2), 2.53
t
t
(b, 8H, THF), 1.89 (s, 18H, Bu-Ar), 1.77 (s, 18H, Bu-Ar), 1.22 (b,
8H, THF), 0.08 (b, 36H, N(TMS)2). IR (KBr, cm−1): 3406s, 2957s,
2904m, 2867m, 2585m, 1933m, 1613s, 1512m, 1476m, 1440s, 1361s,
1300m, 1254m, 1201s, 1156s, 1131m, 1067s, 932s, 881s, 839s, 770s,
736m, 646m, 524m.
1
23.25. H NMR for 5·2toluene (400 MHz, d8-THF + C6D6): 8.57 (s,
2H, Ar), 7.79 (d, J = 7.6 Hz, 2H, Ar), 7.15 (d, J = 24.4 Hz, 2H, Ar),
6.30 (m, 4H, Ar), 6.18 (m, 10H, Ar-CH3), 5.93 (m, 2H, Ar), 5.07(s,
2H, CH2), 4.32 (s, 2H, CH2), 3.47 (s, 6H, MeO), 1.33 (s, 6H, Ar-
Typical Procedure for the Polymerization Reaction. The
procedures for the polymerization of L-lactide or rac-lactide initiated by
complexes 3−8 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
under argon. The contents of the flask were maintained at the desired
temperature until the lactide was dissolved, and then a solution of the
initiator in toluene was added to this solution by syringe. The mixture
was 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 methanol and then poured into methanol
to precipitate the polymer, which was dried under vacuum and
weighed.
X-ray Crystallography. 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. All of the non-hydrogen
atoms were refined anisotropically. The hydrogen atoms in these
complexes were all 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
t
t
CH3), 1.07 (s, 18H, Bu-Ar), 0.28 (s, 18H, Bu-Ar), −2.24 (s, 36H,
N(TMS)2). IR (KBr, cm−1): 3700(s), 2954(s), 2450(s), 1600(s),
1477(s), 1358(m), 1230(m), 1246(s), 1169(w), 1122(s), 984(m),
933(s), 836(s), 744(s), 602(s), 521(m).
Synthesis of {[ON]3La[N(TMS)2](THF)}2 (6). The synthesis of
complex 6 was carried out in the same way as that described for
complex 3, but [HNOH]3 (0.62 g, 2.00 mmol) was used instead of
[HONH]2. Colorless block microcrystals were obtained from a
concentrated toluene solution in a few days (1.42 g, 92%). Mp: 303−
305 °C. Anal. Calcd for C60H104La2N6O4Si4: C, 52.85; H, 7.69; N,
6.16; La, 20.37. Found: C, 52.42; H, 7.81; N, 6.43; La, 19.95. 1H NMR
(400 MHz, d8-THF): 7.48−7.29 (m, 4H, Ph), 7.28−7.00 (m, 8H,
pyridine ring), 4.19−4.36 (m, 4H, CH2), 3.75 (s, α-CH2 THF), 1.90
(s, β-CH2 THF), 1.41 (s, 36H, tBu), 0.21(s, 36H, N(TMS)2). 13C{1H}
NMR (100 MHz, d8-THF): 163.10(Ph), 161.76(Ph), 152.00(pyridine
ring), 141.80(pyridine ring), 136.10(pyridine ring), 135.46(Ph),
129.48(Ph), 125.85(pyridine ring), 123.12(pyridine ring),
121.51(Ph), 111.83(Ph), 111.22(CH2), 67.21(α-CH2, THF), 32.33-
(C(CH3)3), 30.55(C(CH3)3), 26.17(C(CH3)3), 25.10(β-CH2, THF),
23.34(C(CH3)3), 14.33(C(CH3)3), 5.11(Si(CH3)3), 4.41(Si(CH3)3),
2.58(Si(CH3)3). IR (KBr, cm−1): 3410(s), 2956(s), 2906(m),
2868(m), 2588(m), 1990(m), 1910(m), 1614(s), 1572(m), 1512(s),
1478(m), 1440(m), 1361(s), 1252(m), 1201(w), 1162(m), 1066(s),
932(s), 881(s), 837(s), 770(s), 736(m), 682(w), 523(w).
3141
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Scheme 2. Synthesis of Complexes 3−5
Scheme 3. Synthesis of Complexes 6−8
included in the structure factor calculation in the final stage of full-
matrix least-squares refinement.
RESULTS AND DISCUSSION
Synthesis of the Aminophenoxy Lanthanide Com-
plexes. We previously reported that the amine elimination
■
Figure 3. Molecular structure of complex 4 showing the atom-
numbering scheme. Thermal ellipsoids are drawn at the 20%
probability level. Disordered carbon atoms of tert-butyl groups and
hydrogen atoms are omitted for clarity. Complexes 3 and 5 are
isomorphous with complex 4.
Figure 2. Molecular structure of complex 2 showing the atom-
numbering scheme. Thermal ellipsoids are drawn at the 20%
probability level. Disordered carbon atoms of tert-butyl groups and
hydrogen atoms are omitted for clarity. Complex 1 is isomorphous
with complex 2.
mixture of [HONH]1 and Ln[N(SiMe3)2]3(μ-Cl)Li(THF)3 in
toluene at room temperature did not afford the desired neutral
aminophenoxy lanthanide amido complexes or the similar
anionic lanthanide amido complexes reported previously,6 but
instead gave the aminophenoxy lanthanide complexes
{[ON]1Ln[ONH]1}2 (Ln = La (1), Nd (2)), as shown in
Scheme 1. These results indicated that the steric bulk of the
aminophenoxy ligand has a profound effect on the outcome of
the amine elimination reaction of aminophenol with lanthanide
amide. Reaction temperatures of 0 and 90 °C were investigated
and found to have no effect on the outcome of these reactions,
both providing only the homoleptic aminophenoxy lanthanide
reaction of aminophenol [HONH] {[HONH] = [p-
CH3C6H4NHCH2(3,5-tBu2C6H2-2-OH)]} with lanthanide
amide gave the anionic aminophenoxy lanthanide amides.6
We postulated that the ligand used was not sterically imposing
enough to stabilize the neutral aminophenoxy lanthanide
amide. In order to understand the effect of the steric bulk of
the aminophenoxy group on the synthesis of aminophenoxy
lanthanide complexes, a new aminophenol, [HONH]1, was
synthesized. It was found that the reactions of an equimolar
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a
Table 3. Polymerization rac-Lactide by Complexes 3 −8
c
[M]0/
[I]0
T
t
yield
Mn
(×104) PDI
b
c
d
entry initiator
(°C) (min) (%)
Pr
1
3
3
4
4
4
5
5
6
6
7
7
7
8
8
8
8
3000
4000
4000
4000
5000
2000
3000
2000
3000
500
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
20
20
20
1 h
1 h
2 h
2 h
12
12
12
12
12
6
88
82
78
95
85
82
81
94
73
91
87
79
58
80
38
74
33.00
36.41
38.71
43.70
42.38
23.33
15.93
13.42
10.02
11.95
14.62
6.56
1.53
0.80
0.70
0.74
0.71
0.72
0.71
0.70
0.66
0.67
0.70
0.69
0.63
0.63
0.65
0.68
0.71
2
1.43
1.50
1.49
1.54
1.30
1.53
1.54
1.60
1.47
1.59
1.51
1.47
1.60
1.48
1.68
3
4
5
6
7
8
9
10
11
12
13
14
15
16
1000
2000
1000
1000
2000
2000
8.35
12
6
11.52
10.94
21.31
12
a
General polymerization conditions: THF as the solvent, [rac-LA] = 1
Figure 4. Molecular structure of complex 7 showing the atom-
numbering scheme. Thermal ellipsoids are drawn at the 20%
probability level. Disordered carbon atoms of tert-butyl groups and
hydrogen atoms are omitted for clarity. Complex 6 is isomorphous
with complex 7.
b
mol/L. Yield: weight of polymer obtained/weight of monomer used.
c
d
Measured by GPC calibrated with standard polystyrene samples. Pr
is the probability of racemic linkages between monomer units
determined from the methane region of the homonuclear decoupled
1H NMR spectrum.
a
Table 2. Polymerization L-Lactide by Complexes 3−8
1 and 2 were confirmed by elemental analysis, and definitive
molecule structures were provided by an X-ray diffraction
study.
To further explore the effect of ligand structure on the
outcome of the amine elimination reaction, an aminophenol
containing an additional coordination site, [HONH]2, was
synthesized and used to synthesize organolanthanide com-
c
[M]0/
[I]0
T
t
yield
b
Mn
c
entry initiator
(°C) (min)
(%)
(×104)
PDI
1
2
3
4
5
6
7
8
9
3
3
4
4
4
4
4
4
4
4
5
5
5
6
6
7
7
8
8
3000
4000
500
70
70
25
50
70
25
50
25
70
70
70
70
70
70
70
70
70
70
70
10
60
30
30
5
93
23
83
92
98
30
75
69
94
82
97
92
80
100
94
96
90
95
93
11.19
2.70
1.61
2.19
1.69
2.36
1.68
1.25
1.44
2.29
1.52
1.54
1.50
1.51
1.53
1.59
1.57
1.47
1.51
1.44
1.49
12.74
7.45
500
500
6.96
1
plexes. A H NMR monitoring reaction of La[N(TMS)2]3(μ-
d
d
e
500
30
15
5
2.98
Cl)Li(THF)3 with the aminophenol [HONH]2 was carried out.
Treatment of La[N(TMS)2]3(μ-Cl)Li(THF)3 with one equiv-
alent of [HONH]2 in C6D6 at room temperature revealed that a
rapid reaction was taking place, and precipitation of solid
material was noted in the tube. The precipitate disappeared
500
8.33
500
11.92
19.21
18.64
19.46
20.48
18.90
18.72
13.89
17.98
18.62
10.18
12.25
2000
2500
1500
2000
2500
3000
4000
3000
4000
2000
2500
10
10
10
30
30
10
60
60
60
30
30
10
11
12
13
14
15
16
17
18
19
1
when 0.5 mL of THF-d8 was added. The H NMR analysis
revealed that a lanthanum amido complex, [ON]2La[N-
(TMS)2](THF)x, had formed, with the elimination of HN-
(TMS)2. On a preparative scale, attempts were made to
simplify the synthetic procedure by separating the lanthanum
amido complex from LiCl using hot toluene. Thus, the reaction
of an equimolar mixture of La[N(TMS)2]3(μ-Cl)Li(THF)3
with [HONH]2 in toluene at 90 °C was investigated. The
reaction still gave a large amount of precipitate, which indicated
the poor solubility of [ON]2La[N(TMS)2](THF)x in hot
toluene. When the neutral lanthanum amide La[N-
(TMS)2]3(THF)2 was used to replace La[N(TMS)2]3(μ-
Cl)Li(THF)3, the desired neutral aminophenoxy lanthanum
amido complex {[ON]2La[N(TMS)2]}2 (3) was isolated in
high yield from a concentrated toluene solution as block
crystals, as shown in Scheme 2. Therefore, the neutral
lanthanum amide La[N(TMS)2]3(THF)2 was used to synthe-
size aminophenoxy lanthanum complexes. In contrast, the
reaction of an equimolar mixture of [HONH]2 with Ln[N-
(TMS)2]3(μ-Cl)Li(THF)3 (Ln = Nd, Sm) in toluene at 90 °C
conveniently afforded the final products {[ON]2Ln[N-
(TMS)2]}2 [Ln = Nd (4), Sm (5)] in yields of up to 88%,
as shown in Scheme 2. These results indicated that the
presence of an additional coordination site in the amino-
a
General polymerization conditions: toluene as the solvent, [L-LA] = 1
b
mol/L. Yield: weight of polymer obtained/weight of monomer used.
c
d
Measured by GPC calibrated with standard polystyrene samples. In
e
THF. in CH2Cl2.
1
complexes. In the H NMR spectrum of complex 1, a singlet
resonance at δ 3.76 ppm corresponding to the N−H proton
was observed, which was similar to the N−H resonance of the
aminophenol ligand reported in the literature.7b This result
indicated that a hydrogen atom was bonded to a nitrogen atom.
In accordance with the presence of an N−H proton, doublet
resonances at δ 3.99 ppm for one CH2(Ar) group with J = 25.4
Hz were observed because of the coupling interaction with N−
H, whereas a singlet resonance at δ 4.21 ppm for another
CH2(Ar) group was observed. The compositions of complexes
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1
Figure 5. H NMR spectrum of the oligomer of L-lactide initiated by complex 8 in 25 °C in CDCl3.
1
Figure 6. H NMR spectrum of the oligomer of L-lactide initiated by complex 8 in 70 °C in CDCl3.
phenoxy ligand plays a crucial role in the stabilization of neutral
aminophenoxy lanthanide amido complexes. The compositions
of complexes 3−5 were confirmed by elemental analysis and IR
spectroscopy, as well as by NMR spectroscopy in the case of
complexes 3, and 5. The definitive molecular structures of these
complexes were confirmed by single-crystal structure analysis.
Complex 3 is soluble in THF, but insoluble in toluene, whereas
complexes 4 and 5 are soluble in THF and toluene, but
insoluble in hexane.
in the ligand is practical for stabilizing the neutral amino-
phenoxy lanthanide complexes. In order to elucidate the
electronic and steric effects of the ligand on the activity of the
aminophenoxy lanthanide amido complexes, a ligand contain-
ing a pyridine group, [HNOH]3, was synthesized and used to
stabilize the expected lanthanide amido complexes. Similarly,
the reaction of an equimolar mixture of La[N-
(SiMe3)2]3(THF)2 with [HONH]3 in toluene gave the neutral
aminophenoxy lanthanide amide {[ON]2La[N(TMS)2]-
(THF)}2 (6) in high isolated yield, as shown in Scheme 3.
As expected, the reactions of an equimolar mixture of
The successful synthesis of complexes 3−5 demonstrated
that the strategy of introducing an additional coordination atom
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Scheme 4. Possible Mechanism for the Ring-Opening Polymerization of L-Lactide
Ln[N(SiMe3)2]3(μ-Cl)Li(THF)3 with [HNOH]3 in toluene
afforded {[ON]3Ln[N(TMS)2](THF)}2 (Ln = Nd (7), Sm
(8)) in high isolated yields under the same reaction conditions
(Scheme 3). Complexes 6−8 were subsequently characterized
The molecular structures of complexes 3−5 are isomor-
phous. Only the ORTEP diagram of complex 4 is shown in the
figure (Figure 3). Complexes 3−5 have unsolvated dimeric
structures with two toluene molecules involved in the crystal
lattice. Similarly to observations made in complexes 1 and 2,
the two central metal atoms in complexes 3−5 were bridged by
two aryloxo-O atoms. Each of the metal atoms is five-
coordinated by two oxygen atoms and one nitrogen atom
from one aminophenoxy group and one oxygen atom from
another aminophenoxy group, as well as one nitrogen atom
from the hexamethyldisilylamide group. The coordination
geometry around the metal center was distorted trigonal
bipyramidal, in which the O(1) and O(2) atoms occupy the
apical positions. The length of the Ln−N(CH2Ar) bond ranged
from 2.371(10) to 2.211(4) Å, which was in agreement with
the contraction of ionic radii in the lanthanide metal ions. The
average length of the Ln−O(Ar) bonds was in the range
2.437(6)−2.347(3) Å, which was comparable with the Ln−
O(Ar) bond lengths reported in the literature.11 The average
length of the Ln−N(TMS) bonds in complexes 3−5 was
2.392(9), 2.337(3), and 2.307(4) Å, respectively, which was
comparable with the corresponding bond lengths in {(MBMP)-
1
by elemental analysis and IR spectroscopy, as well as H NMR
spectroscopy in the case of complexes 6, and 8. Complexes 6−
8 demonstrated relatively good solubility in toluene in
comparison with complexes 3−5.
Complexes 1 and 2 are isostructural. The molecular diagram
of complex 1 is depicted in the figure (Figure 2). Complexes 1
and 2 have unsolvated centrosymmetric dimeric structures and
possess bridging aminophenoxy-O atoms. Each of the metal
atoms in the complexes is coordinated to one oxygen atom and
one nitrogen atom from a dianionic aminophenoxy ligand and
two oxygen atoms and one nitrogen atom from two
monoanionic aminophenoxy ligands. The coordination envi-
ronment can be described as a distorted trigonal bipyramid with
the O(1) and N(2) atoms occupying the axial positions of the
bipyramid. The terminal Ln−O(Ar) bond distances in
complexes 1 and 2 are 2.248(5) and 2.191(4) Å, respectively.
As expected in these complexes, the Ln−N1(amino) bond
lengths are longer than the Ln−N2(amido) bond lengths.
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Ln[N(TMS)2](THF)2}2e when the difference in ionic radii was
considered.12
polymer was relatively broad (Table 2, entry 8). As expected,
the polymerization temperature affected the polymerization. At
higher temperatures, the polymerization proceeded at a greater
rate (Table 2, entries 4−6). Therefore, all of the polymerization
procedures for L-lactide were carried out in toluene solvent at
70 °C.
The molecular structures of complexes 6 and 7 were
determined by X-ray crystal diffraction. Complexes 6 and 7 are
isomorphous. Only the ORTEP diagram of complex 7 is shown
in Figure 4. Complexes 6 and 7 have solvated dimeric
structures. They consist of two metal ions, two aminophenoxy
groups, two N(TMS)2 groups, and two THF molecules. In
contrast with complexes 3−5, complexes 6 and 7 possess
bridging aminophenoxy-N atoms rather than dimerizing though
the aminophenoxy-O atoms. Each of the lanthanide metal
atoms is seven-coordinated by four nitrogen atoms and one
oxygen atom from two aminophenoxy groups and one nitrogen
atom from the N(TMS)2 group, as well as one oxygen atom
from a THF molecule. The overall coordination geometry
around the lanthanide metal atoms can be considered to be a
distorted capped trigonal prism with the cap position occupied
by the N(2) atom.
The average length of the Ln(1)−O(1) bonds in complexes
6 and 7 was 2.234(3) and 2.187(2) Å, respectively, which was
shorter than the corresponding bond lengths in complexes 3
and 4. This difference was attributed to the formation of
bridged bonds in the latter. As expected, the average length of
the Ln−N(CH2Ar) bonds was longer than the corresponding
lengths in complexes 3 and 4. The lengths of the Ln−N(TMS)
bonds in complexes 6 and 7 are slightly longer than those in
complexes 3 and 4, which was attributed to the increase in the
coordination number.12
Ring-Opening Polymerization of Lactides by Com-
plexes 3−8. Biodegradable polymers, such as poly(ε-
caprolactone) (PCL) and poly(lactide) (PLA), have recently
attracted significant attention as replacements for conventional
synthetic materials because of their biodegradable, biocompat-
ible, and permeable properties.13 Ring-opening polymerization
of cyclic esters initiated by organometallic complexes is a
convenient method for the synthesis of these high molecular
weight polymers.14 To further explore the relationship between
the structures of the catalysts and their catalytic properties and
behaviors, complexes 3−8 were examined in the ring-opening
polymerization of L-lactide and rac-lactide.
As expected, the aminophenoxy lanthanide amides were
efficient initiators in the ring-opening polymerization of L-
lactide. The representative polymerization results are shown in
Table 2. It is clear that L-lactide polymerization can be initiated
by all of the lanthanide complexes, affording polymers with
high molecular weights. Furthermore, the catalytic activities of
these neutral lanthanide amides are greater than those of the
anionic aminophenoxy lanthanide amides.6 For example, using
the neodymium complex 4 as the initiator, a 94% yield was
achieved in 10 min at 70 °C when the molar ratio of monomer
to initiator was 2000 (Table 2, entry 12). When the anionic
aminophenoxy neodymium amide was used as the initiator,
however, a yield of 86% was achieved in 4 h at 70 °C, when the
molar ratio of monomer to initiator was 1800.6 The activities of
these neutral lanthanide amides are also greater than those of
the imidazolidine-bridged bis(phenolate) lanthanide amides.11
As shown in Table 2, the polymerization medium also has a
profound effect on the catalytic activity and the molecular
weight of the resultant polymers. It was found that the
lanthanide metal amido complexes showed greater activity in
toluene than in THF (Table 2, entries 3 and 6). Although a
yield of 69% could be achieved in 5 min using CH2Cl2 as the
solvent, the molecular weight distribution of the resultant
The structure of the aminophenoxy lanthanide amides can
also affect their catalytic behavior for L-lactide polymerization.
The lanthanide amides 6−8 showed greater activity in
comparison with complexes 3−5 and gave polymers with
relatively narrow molecular weight distributions (PDI = 1.44−
1.59) (Table 2, entries 14−19). For example, the lanthanum
amide 6 can polymerize almost completely 4000 equivalents of
L-lactide at 70 °C in one hour (Table 2, entry 15), whereas the
lanthanum amide 3 can polymerize 4000 equivalents of L-
lactide to give a yield of 23% under the same polymerization
conditions (Table 2, entry 2). This difference in activity was
attributed to the different molecular structures of the
lanthanide amides. Although all of complexes 3−8 have dimeric
structures, complexes 3−5 are unsolvated O-bridged dimers,
whereas complexes 6−8 are solvated N-bridged dimers. This
difference implied that the coordination environments around
the lanthanide metals in complexes 3−5 were more crowded
than those in complexes 6−8. In noncoordination solvent, the
dimeric structures are maintained. Thus, complexes 6−8
showed greater catalytic activity.
Stereoselective polymerization of rac-lactide has received
significant interest in recent years from academic and industrial
research groups. The polymerization is extremely sensitive to
the structures of the catalysts, because the environment of the
last unit of the propagating active species must be sterically
proper to incorporate the configurationally opposite enan-
tiomer.15 To further elucidate the effect of the structures of the
ancillary ligands of lanthanide-based catalysts on the polymer-
ization stereoselectivity, the catalytic behavior of complexes 3−
8 in the ring-opening polymerization of rac-lactide was also
tested. The results are summarized in Table 3.
It can be seen that these lanthanide amides smoothly
initiated the ring-opening polymerization of rac-lactide in THF
at 25 °C to afford polymers with high molecular weights and
relatively narrow molecular weight distributions (PDI = 1.30−
1.60). The activity of these neutral aminophenoxy lanthanide
complexes is greater than the anionic ones.6 For example, using
the neodymium complex 4 as the initiator at room temperature,
the polymerization was complete in one hour when the molar
ratio of monomer to initiator was 4000 (Table 3, entry 4).
When the anionic neodymium complex was used as the
initiator, however, a yield of 88% was achieved in six hours
when the molar ratio of monomer to initiator was 500.6 The
ionic size of the lanthanide metal ions has an obvious effect on
the catalytic activity in rac-lactide polymerization. The observed
increasing order in activity is in agreement with the order of
ionic radii, which is consistent with that observed in L-lactide
polymerization. The structures of the aminophenoxy groups
can also affect the catalytic activity of the corresponding
lanthanide complexes in rac-lactide polymerization. Complexes
6−8 performed poorly in comparison to complexes 3−5 in rac-
lactide polymerization. These results are contrary to those
observed for L-lactide polymerization, as mentioned above. It is
worthy to note that the activities of complexes 3−5 for rac-
lactide polymerization are greater than those for L-lactide
polymerization, in spite of the fact that the former polymer-
izations were conducted in THF at room temperature. For
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instance, complex 3 polymerized 4000 equivalents of rac-lactide
in THF at room temperature to give an 82% yield in 20 min,
whereas complex 3 polymerized L-lactide in toluene at 70 °C to
give a 23% yield in one hour. We attributed these differences to
the different structures of these complexes in toluene and in
THF. In the coordinated THF solution, the dimeric structures
of these complexes were broken, whereas in toluene solution
the dimeric structure was maintained, as mentioned above.
The ionic radii have no obvious effect on the stereo-
selectivity, and these complexes showed stereoselectivity in rac-
lactide polymerization, affording heterotactic-rich PLA. The Pr
values of all of the polymers obtained ranged from 0.63 to 0.71.
The ability of these complexes to control the steroechemical
outcome of the polymerization is similar to that of the anionic
aminophenoxy lanthanide complexes.6 Cui and her co-workers
reported that the coordination geometry around the metal
center plays a critical role in governing the stereoselectivity of
rac-lactide polymerization.15a In our experience, however,
although their coordination environments around the lantha-
nide metals are quite different, a discernible difference in
stereoselectivity was not observed when the neutral and the
anionic aminophenoxy lanthanide complexes were used as
initiators. The ability of these complexes to control the
stereochemical outcome of the polymerization is worse than
those of the amine-bridged bis(phenolate) lanthanide com-
plexes, which can polymerize rac-lactide to give high
heterotactic polymers with a Pr value greater than 0.95.15a,c,f
These results revealed that the ancillary ligand plays a crucial
role in controlling the stereoselectivity in rac-lactide polymer-
ization.
role in the stabilization of neutral aminophenoxy lanthanide
amido complexes. These neutral aminophenoxy lanthanide
amides are efficient initiators in L-lactide and rac-lactide
polymerization, and their activities are greater than the
corresponding anionic ones. The structures of the amino-
phenoxy ligands clearly affect the activity of the corresponding
lanthanide amides. Furthermore, the polymerization of rac-
lactide initiated by aminophenoxy lanthanide complexes gave
heterotactic-rich polymers. These results indicated that the
organolanthanide catalysts stabilized by dianionic N,O-chelate
ligands have great potential in homogeneous catalysis. Further
studies are under way in our laboratory.
ASSOCIATED CONTENT
* Supporting Information
This material is available free of charge via the Internet at
http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: yaoym@suda.edu.cn. Fax: (86)512-65880305. Tel:
(86)512-65882806.
■
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.
The initiation mechanism was also elucidated by end-group
analysis of the oligomer of L-lactide, which was prepared by the
polymerization of L-lactide initiated by complex 8 in a 1:10
molar ratio and subsequently quenched by 2-propanol. The 1H
NMR spectrum of the oligomer clearly showed that only the
isopropoxo group was observed, according to the resonances at
1.25 and 5.10 ppm. No resonance signal was observed for the
phenoxyamido ligand at room temperature, as shown in the
figure (Figure 5). However, when the oligomerization was
conducted at 70 °C, the aromatic proton resonances in the
range 6.50−7.75 ppm and the methyl and methylene proton
resonances of the phenoxyamido group at 1.15, 1.47, and 4.75
ppm were observed as shown in Figure 6, indicating that the
phenoxyamido group was involved in the polymerization.
Furthermore, the presence of an isopropoxo group was
indicated by resonances at 1.25 and 5.00 ppm. Obviously, the
isopropoxo group existing in the oligomer must have come
from 2-propanol, which was introduced by the exchange
reaction of 2-propanol with the -N(TMS)2 group.6 These
results revealed that the Ln-N(TMS)2 group initiated the
polymerization at room temperature (Scheme 4, path a),
whereas both the Ln-N(TMS)2 and Ln-N(amido) groups
initiated the polymerization at higher temperatures (Scheme 4,
paths a and b).
REFERENCES
■
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CONCLUSION
■
In summary, a series of lanthanide metal complexes supported
by dianionic aminophenoxy ligands were prepared using a
general amine elimination reaction of bis(trimethylsilyl)amido
lanthanide metal complexes with aminophenols, and their
structural features have been determined by X-ray diffraction
study. It was found that the presence of an additional
coordination site in the aminophenoxy ligand plays a crucial
3147
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