Synthesis and structure of Lanthanide Aryloxo complexes and their Polymerization behavior for e-Caprolactone

Article abstract of DOI:10.1002/zaac.201300218

Protic exchange reactions of bis(phenolate) lanthanide cyclopentadienyl complexes (C₅H₅)Ln(MBMP)(THF)₂ [MBMP2- = 2,2- methylene-bis(6-tert-butyl-4-methyl-phenolate)] with a series of mono-phenols (including 2,6-dimethylphenol, 4-tert-butylphenol, and 4-methoxyphenol) produce the lanthanide aryloxo complexes (MBMP)Ln(OC₆H3-Me2-2,6)(DME) ₂ [Ln = La (1), Nd (2)], (MBMP)La(OC₆H₄-tBu-4) (DME)₂ (3), [(MBMP)Nd(OC₆H₄-tBu-4)- (THF) ₂]₂ (4), and [(MBMP)Ln(OC₆H₄-OMe-4) (THF)₂]₂ [Ln = La (5), Nd (6)]. All of these complexes were well characterized by elemental analyses, IR spectra, and NMR in the cases of complexes 1, 3, and 5. The definitive molecule structures of complexes 1, 3, and 4 were determined by single-crystal X-ray diffraction. The catalytic behavior of complexes 1-6 for the ring-opening polymerization of e-caprolactone was explored.

Full text of DOI:10.1002/zaac.201300218

ARTICLE
DOI: 10.1002/zaac.201300218
Synthesis and Structure of Lanthanide Aryloxo Complexes and Their
Polymerization Behavior for ε-Caprolactone
Lei Fang,[a] Yingming Yao,*[a] Yong Zhang,[a] Qi Shen,[a] and Yaorong Wang*[a]
Keywords: Bis(phenolate) ligand; Lanthanides; Structure; Catalysis; Polymerization; ε-Caprolactone
Abstract. Protic exchange reactions of bis(phenolate) lanthanide cy-
clopentadienyl complexes (C5H5)Ln(MBMP)(THF)2 [MBMP2– = 2,2Ј-
(THF)2]2 (4), and [(MBMP)Ln(OC6H4-OMe-4)(THF)2]2 [Ln = La (5),
Nd (6)]. All of these complexes were well characterized by elemental
analyses, IR spectra, and NMR in the cases of complexes 1, 3, and
5. The definitive molecule structures of complexes 1, 3, and 4 were
methylene-bis(6-tert-butyl-4-methyl-phenolate)] with
a series of
mono-phenols (including 2,6-dimethylphenol, 4-tert-butylphenol, and
4-methoxyphenol) produce the lanthanide aryloxo complexes determined by single-crystal X-ray diffraction. The catalytic behavior
(MBMP)Ln(OC6H3-Me2-2,6)(DME)2 [Ln La (1), Nd (2)], of complexes 1–6 for the ring-opening polymerization of ε-caprolac-
=
(MBMP)La(OC6H4-tBu-4)(DME)2 (3), [(MBMP)Nd(OC6H4-tBu-4)-
tone was explored.
catalyze the controlled polymerization of l-lactide,[18] and the
highly heteroselective polymerization of rac-lactide.[11,12,14]
Recently, we also found that the lanthanide alkoxo and amido
complexes stabilized by carbon bridged bis(phenolate) groups
are efficient initiators for the ring-opening polymerization of ε-
caprolactone, and the polymerization controllability is greatly
influenced by the property of the initiating group.[20–22] To fur-
ther understand the influence of the initiating group on the
catalytic property of carbon bridged bis(phenolate) lanthanide
complexes, some lanthanide aryloxo complexes stabilized by
2,2Ј-methylene-bis(6-tert-butyl-4-methyl-phenolate)
Introduction
During the past decades, organolanthanide chemistry has
witnessed rapid progress, because many of lanthanide com-
plexes have interesting catalytic properties for organic trans-
formation and polymerization.[1–5] It is well known that the
catalytic properties of organolanthanide complexes depend on
not only the metal itself, but also the coordination environment
around the central metal atom because of the unique electron
structure of the lanthanide metal. Thus, exploration of new an-
cillary ligands and synthesis of organolanthanide complexes
with different structures to systematically study the relation-
ship between structure and catalytic property have received
considerable attention.[6–9]
(MBMP2–) group were synthesized by protic exchange reac-
tion, and their catalytic behavior for the polymerization of ε-
caprolactone was explored. Herein we report these results.
In recent years, the application of bridged bis(phenolate)s as
ancillary ligands has received increasing attention in organol-
anthanide chemistry.[10] The bridged bis(phenolate) groups can
be used as dianionic ligands, which has the advantage of avo-
iding ligand redistribution reactions because of chelating ef-
fects. Meanwhile, these dianionic ligands have an advantage to
Experimental Section
Materials and General Procedures: All manipulations were per-
formed in an argon atmosphere, using the standard Schlenk techniques.
Solvents were distilled from sodium benzophenone ketyl before use.
design single-site catalysts, which is important for controlled ε-Caprolactone was purchased from Acros, dried with CaH2 for 48
h, and distilled under reduced pressure before use. Phenols and 2,2Ј-
methylene-bis(6-tert-butyl-4-methyl-phenol) (MBMPH2) were com-
mercially available. (C5H5)Ln(MBMP)(THF)n (Ln = La, Nd) was pre-
pared according to the literature procedures.[22,23] Lanthanide analyses
were performed by EDTA titration with an xylenol orange indicator
and a hexamine buffer.[24] Carbon and hydrogen analyses were per-
formed by direct combustion with a Carlo-Erba EA-1110 instrument.
The IR spectra were recorded with a Nicolet-550 FT-IR spectrometer
as KBr pellets. NMR spectra were obtained with an INOVA-400MHz
apparatus. Molecular weights and molecular weight distributions were
determined against polystyrene standards by gel permeation
chromatography (GPC) with a PL 50 apparatus, and THF was used as
an eluent at a flow rate of 1.0 mL·min–1 at 40 °C.
polymerization. Indeed, some of the organolanthanide com-
plexes stabilized by bridged bis(phenolate) groups have been
found to show good catalytic activity and controllability in the
polymerization of some polar monomers.[11–19] For example,
the amine bridged bis(phenolate) lanthanide complexes can
* Dr. Y. Yao
Fax: +86-512-65880305
E-Mail: yaoym@suda.edu.cn
* Dr. Y. Wang
E-Mail: yrwang@suda.edu.cn
[a] Key Laboratory of Organic Synthesis of Jiangsu Province
College of Chemistry, Chemical Engineering, and Materials
Science
Dushu Lake Campus
Synthesis of (MBMP)La(OC6H3-Me2-2,6)(DME)2 (1): To a THF
solution (30 mL) of (C5H5)La(MBMP)(THF)3 (2.81 g, 3.70 mmol)
Soochow University
Suzhou 215123, P. R. China
2324
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2013, 639, (12-13), 2324–2330

Lanthanide Aryloxo Complexes and Their Polymerization Behavior for ε-Caprolactone
was added HOC6H3-Me2-2,6 (0.45 g, 3.70 mmol). The mixture was CDCl3): δ = 1.24 (br., 16 H, THF), 1.38 [s, 36 H, C(CH3)3], 2.25 (s,
2
stirred overnight at 50 °C, and further the solvent was removed under 12 H, ArCH3), 3.57 (d, JHH = 14.4 Hz, 4 H, CH2), 3.68 (br., 16 H,
vacuum. Toluene was added to extract the solid, and the precipitate
was removed by centrifugation. The toluene extracts were concentrated
to about 12 mL and about 1 mL of DME was added. Colorless micro-
THF), 3.89 (s, 6 H, OCH3), 7.10–6.96 (m, 12 H, ArH), 7.19 (m, 4 H,
ArH) ppm. 13C NMR (400 MHz, CDCl3): δ = 21.2 (ArCH3), 25.7
(THF), 32.4 [C(CH3)3], 35.5 [C(CH3)3], 36.7 (CH2), 57.2 (OCH3),
crystals were obtained at room temperature in a few days (2.07 g, 72% 71.3 (THF), 116.1, 117.5, 128.1, 129.6, 130.2, 135.8, 141.2, 150.6,
based on La). C39H59LaO7: calcd. C 60.15; H 7.64; La 17.84%; found:
154.8, 160.2 (Ph) ppm. IR (KBr): ν˜ = 2957 (s), 2916 (s), 2871 (m),
1606 (m), 1453 (s), 1378 (m), 1236 (s), 1090 (m), 860 (m), 762 (m),
1
C 59.86; H 7.57; La 17.55%. H NMR (400 MHz, 25 °C CDCl3): δ
= 1.34 [s, 18 H, C(CH3)3], 2.13 (s, 6 H, ArCH3), 2.27 (s, 6 H, ArCH3), 619 (m) cm–1.
2
3.24 (s, 12 H, CH3O), 3.48 (m, 8 H, OCH2), 3.79 (d, JHH = 14.2 Hz,
Synthesis of [(MBMP)Nd(OC6H4-OMe-4)(THF)2]2 (6): The synthe-
2 H, CH2), 6.70 (m, 2 H, ArH), 6.85–7.04 (m, 5 H, ArH) ppm. 13C
NMR (400 MHz, CDCl3): δ = 15.8 (ArCH3), 21.4 (ArCH3), 30.3
[C(CH3)3], 35.1 [C(CH3)3], 35.8 (CH2), 59.8 (CH3O), 71.0 (OCH2),
124.2, 125.5, 127.9, 128.6, 129.7, 130.1, 136.2, 139.3, 152.7, 161.4
(Ph) ppm. IR (KBr): ν˜ = 2956 (s), 2915 (s), 2871 (m), 1608 (m), 1442
(s), 1232 (s), 1156 (s), 861 (m), 768 (m), 619 (m), 582 (s) cm–1. Crys-
tals suitable for an X-ray diffraction analysis were obtained by slow
cooling of a hot DME solution.
sis of complex 6 was carried out in the same way as that described for
complex 2, but HOC6H4-OMe-4 (0.43 g, 3.60 mmol) was used instead
of HOC6H3-Me2-2,6. Pale blue microcrystals were obtained from con-
centrated THF solution at room temperature in a few days (1.76 g,
65% based on Nd). C76H106Nd2O12: calcd. C 60.85; H 7.12; Nd
19.23%; found: C 60.51; H 7.48; Nd 18.91%. IR (KBr): ν˜ = 2957 (s),
2914 (s), 2866 (m), 1615 (m), 1450 (s), 1369 (m), 1237 (s), 1082 (s),
861 (m), 765 (m), 614 (m) cm–1.
Synthesis of (MBMP)Nd(OC6H3-Me2-2,6)(DME)2 (2): The synthe-
sis of complex 2 was carried out in the same way as that described for
complex 1, but (C5H5)Nd(MBMP)(THF)3 (1.93 g, 2.53 mmol) was
used instead of (C5H5)La(MBMP)(THF)3. Pale blue microcrystals
were isolated from the concentrated toluene/DME (7:1) solution
(8 mL) at room temperature (1.35 g, 68% based on Nd). C39H59NdO7:
calcd. C 59.82; H 7.59; Nd 18.42%; found: C 59.53; H 7.62; Nd
18.67%. IR (KBr): ν˜ = 2955 (s), 2916 (s), 2871 (m), 1593 (m), 1442
(s), 1234 (s), 1186 (s), 861 (m), 760 (m), 582 (m) cm–1.
Procedure for the Polymerization Reaction: The procedures for the
polymerization of ε-caprolactone initiated by complexes 1 to 6 were
similar, and a typical polymerization procedure is given below. A
50 mL Schlenk flask, equipped with a magnetic stirring bar, was
charged with a solution of initiator in toluene. To this solution was
added desired amount of ε-caprolactone by syringe. The contents of
the flask were stirred vigorously at 50 °C for desired time, during
which time an increase in viscosity was observed. The reaction mixture
was quenched by the addition of 1 m HCl/ethanol solution, and then
poured into methanol to precipitate the polymer, which was dried un-
der vacuum and weighed.
Synthesis of (MBMP)La(OC6H4-tBu-4)(DME)2 (3): The synthesis
of complex 3 was carried out in the same way as that described for
complex 1, but HOC6H4-tBu-4 (0.32 g, 2.15 mmol) was used instead
of HOC6H3-Me2-2,6. Colorless microcrystals were obtained from con-
centrated toluene/DME solution at room temperature in a few days
(1.41 g, 81% based on La). C41H63LaO7: calcd. C 61.03; H 7.87; La
17.22%; found: C 60.59; H 7.95; La 16.89%. 1H NMR (400 MHz,
CDCl3): δ = 1.30 [s, 9 H, C(CH3)3], 1.38 [s, 18 H, C(CH3)3], 2.25 (s,
6 H, ArCH3), 3.27 (s, 12 H, CH3O), 3.47 (m, 8 H, OCH2), 3.88 (d,
2JHH = 14.4 Hz, 2 H, CH2), 6.75–6.96 (m, 4 H, ArH), 7.19 (m, 4 H,
ArH) ppm. 13C NMR (400 MHz, CDCl3): δ = 21.6 (ArCH3), 30.3
[C(CH3)3], 31.2 [C(CH3)3], 35.4 [C(CH3)3], 35.7 [C(CH3)3], 36.3
(CH2), 59.5 (CH3O), 70.8 (OCH2), 117.1, 126.8, 128.3, 129.5, 130.4,
136.6, 140.3, 147.1, 152.7, 161.4 (Ph) ppm. IR (KBr): ν˜ = 2954 (s),
2915 (s), 2869 (m), 1609 (m), 1447 (s), 1363 (m), 1231 (s), 1159 (m),
860 (m), 766 (m), 617 (m) cm–1. Crystals suitable for an X-ray diffrac-
tion analysis were obtained from the hot toluene/DME solution.
X-ray Crystallography: Suitable single crystals of complexes 1, 3,
and 4 were sealed in a thin-walled glass capillary for determining the
single-crystal structure. 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-ma-
trix least-squares procedures based on |F|2. All the non-hydrogen
atoms were refined anisotropically. The hydrogen atoms in these com-
plexes were all generated geometrically (C–H bond lengths fixed at
0.95 Å), assigned appropriate isotropic thermal parameters, and al-
lowed to ride on their parent carbon atoms. All 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 the SHELEXL-97 program.
Synthesis of [(MBMP)Nd(OC6H4-tBu-4)(THF)2]2 (4): The synthesis
of complex 4 was carried out in the same way as that described for
complex 2, but HOC6H4-tBu-4 (0.72 g, 4.80 mmol) was used instead
of HOC6H3-Me2-2,6. Pale blue crystals were isolated from the concen-
trated THF solution (8 mL) at room temperature (2.64 g, 71% based
on Nd). C82H118Nd2O10: calcd. C 63.45; H 7.66; Nd 18.58%; found:
C 63.86; H 7.74; Nd 18.31%. IR (KBr): ν˜ = 2956 (s), 2912 (s), 2869
(m), 1608 (m), 1443 (s), 1234 (s), 1158 (s), 861 (m), 768 (m), 619 (m)
cm–1.
Crystallographic data (excluding structure factors) for the structures in
this paper have been deposited with the Cambridge Crystallographic
Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK. Copies
of the data can be obtained free of charge on quoting the depository
numbers CCDC-935151 (1), CCDC-935152 (3), and CCDC-935153
(4) (Fax: +44-1223-336-033; E-Mail: deposit@ccdc.cam.ac.uk, http://
www.ccdc.cam.ac.uk).
Synthesis of [(MBMP)La(OC6H4-OMe-4)(THF)2]2 (5): The synthe-
sis of complex 5 was carried out in the same way as that described for
complex 1, but HOC6H4-OMe-4 (0.36 g, 3.00 mmol) was used instead
of HOC6H3-Me2-2,6. Colorless microcrystals were obtained from con-
centrated THF solution at room temperature in a few days (1.57 g,
70% based on La). C76H106La2O12: calcd. C 61.29; H 7.17; La
Results and Discussion
Synthesis and Characterization of Lanthanide Complexes
1–6
We previously reported that the reaction of carbon bridged
18.65%; found: C 61.67; H 7.24; La 19.11%. 1H NMR (400 MHz, bis(phenol) (MBMPH2) with (C5H5)3Ln(THF) in THF in a 1:1
Z. Anorg. Allg. Chem. 2013, 2324–2330
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L. Fang, Y. Yao, Y. Zhang, Q. Shen, Y. Wang
ARTICLE
Table 1. Crystallographic data for complexes 1, 3, and 4.
1
3·0.5(C7H8)·0.5(C4H8O)
4·(C4H8O)
Formula
Fw
T /K
C39H59LaO7
778.77
193(2)
C46.5H71LaO7.5
878.86
223(2)
C86H126Nd2O11
1624.35
193(2)
Crystal system
Space group
monoclinic
P21/n
monoclinic
P21/n
triclinic
P1
¯
Crystal size /mm
a /Å
b /Å
c /Å
α /°
β /°
0.31ϫ0.29ϫ0.20
13.726(1)
15.367(1)
18.943(2)
90.00
98.927(3)
90.00
3947.3(6)
4
0.31ϫ0.22ϫ0.20
15.635(2)
23.143(2)
26.999(3)
90.00
101.656(3)
90.00
9568(2)
8
0.55ϫ0.36ϫ0.18
11.874(2)
13.586(1)
13.586(1)
71.145(5)
88.081(6)
83.720(6)
2061.6(4)
1
γ /°
V /Å3
Z
Dcalcd. /g·cm–3
1.310
1.126
1.220
0.938
1.308
1.301
μ /mm–1
F(000)
1624
3656
850
θmax /°
27.48
25.35
25.35
Collected reflections
43648
92314
20381
Unique reflections
8996
17485
7521
Observed reflections [I Ͼ 2.0σ(I)]
8158
12343
7148
No. of variables
439
908
483
GOF
R
wR
1.117
0.0301
0.0625
1.184
0.1240
0.2451
1.086
0.0316
0.0833
molar ratio gave the bis(phenolate) lanthanide cyclopen- sensitive to air and moisture. These complexes are soluble in
tadienyl complexes (MBMP)Ln(C5H5)(THF)n, which is a use- THF, DME, and toluene, and slightly soluble in hexane.
ful precursor for synthesis of neutral lanthanide alkoxo com-
The definitive molecular structures of complexes 1, 3, and
plexes stabilized by MBMP group by further proton exchange 4 were determined by X-ray diffraction. Crystals of complex
reaction.[21,22] This method has the advantage for avoiding for- 1 suitable for an X-ray structure determination were obtained
mation of “ate” lanthanide derivatives. Therefore, we intend to from a concentrated DME solution. The molecular structure of
expand the scope of this method, and synthesize bis(phenolate) complex 1 is depicted in Figure 1, and the selected bond
lanthanide aryloxo complexes.
lengths and bond angles are provided in Table 2. Complex 1
A NMR-scale reaction was conducted firstly. 1H NMR mon- has a monomeric structure and the lanthanum atom is coordi-
itoring reaction revealed that the reaction of (MBMP)La(C5H5) nated by two oxygen atoms from one MBMP2– group, one
(THF)3 with 2,6-dimethylphenol occurred in 1 h in C6D6 solu- oxygen atom from aryloxo group, and four oxygen atoms from
tion at room temperature, because the single sharp resonance two DME molecules. The coordination number is seven, and
of cyclopentadienyl group at about 6.31 ppm almost disap- the coordination arrangement around lanthanum ion can be de-
peared and the splitting peaks at about 6.47, 6.32, and scribed as a distorted capped trigonal prism, with the capping
2.67 ppm appeared, which can be attributed to the eliminated atom being the oxygen atom O(5). The La(1)–O[bis(phenol-
cyclopentadiene. On a preparative scale, the expected lantha- ate)] and the La(1)–O(Ar) bond lengths are 2.279(2), 2.302(2),
nide aryloxo complexes (MBMP)Ln(OC6H3-Me2-2,6)(DME)2 and 2.259(2) Å, respectively, which is comparable with the
[Ln = La (1), Nd (2)] can be obtained in high isolated yields corresponding bond lengths in complexes (MBMP)La(C5H5)-
from toluene/DME solution. A further study revealed that the (THF)3,[23] (MBMP)3La2(THF)3,[23] and (THF)La(O-2,6-
reactions of (MBMP)Ln(C5H5)(THF)n with 4-tert-butylphenol iPr2C6H3)2(μ-O-2,6-iPr2C6H3)2M(THF)2 (M = Li, Na).[25]
proceeds smoothly, and generated the desired lanthanide aryl-
Complex 3 crystallizes with two crystallographically inde-
oxo complexes (MBMP)La(OC6H4-tBu-4)(DME)2 (3) and pendent but chemically similar molecules (3a and 3b) in the
[(MBMP)Nd(OC6H4-tBu-4)(THF)2]2 (4) from toluene/DME unit cell, and selected bond lengths of both molecules are pro-
and THF solution, respectively. The similar reactions with 4- vided in Table 2. An ORTEP of complex 3a is depicted in
methoxylphenol also produced the expected bridged bis(phen- Figure 2. Complex 3 has a monomeric structure and the lantha-
olate) lanthanide aryloxo complexes [(MBMP)Ln(OC6H4- num ion is coordinated by two oxygen atoms from one
OMe-4)(THF)2]2 [Ln = La (5), Nd (6)] as shown in Scheme 1. MBMP2– group, one oxygen atom from aryloxo group, and
Complexes 1–6 were well characterized by elemental analysis, four oxygen atoms from two DME molecules. The overall co-
IR spectroscopy, and NMR spectrum for the diamagnetism lan- ordination is similar to that in complex 1. The La(1)–O[bis-
thanum complexes 1, 3, and 5. Because of their paramagnetic (phenolate)] and the La(1)–O(Ar) bond lengths are 2.266(7),
properties, no resolvable NMR spectrum for the neodymium 2.220(8), and 2.230(8) Å, respectively. These values are
complexes was obtained. All of these complexes are moderate slightly shorter than the corresponding bond lengths in com-
2326
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Lanthanide Aryloxo Complexes and Their Polymerization Behavior for ε-Caprolactone
Scheme 1. Synthesis of the lanthanide aryloxo complexes.
nated solvent molecule has significant influence on the solid-
state structure. Each of the neodymium ions is six-coordinate
by two oxygen atoms from bis(phenolate) ligand, two oxygen
atoms from two aryloxo groups, and two oxygen atoms from
two THF molecules to form a distorted octahedron. The
Nd(1)–O[bis(phenolate)] bond lengths are 2.169(2) and
2.217(2) Å, respectively, giving the average of 2.193(2) Å,
which is comparable with the corresponding bond lengths in
complexes 1 and 3 when the difference in ionic radii between
neodymium and lanthanum is considered, and the bond length
in [(MBMP)Nd(μ-OiPr)(THF)2]2.[22] The tert-butylphenoxo
group is asymmetrically coordinated to the neodymium ions.
The Nd(1)–O(3) bond length of 2.434(2) Å is about 0.076 Å
longer than the Nd(1)–O(3A) bond length of 2.358(2) Å. The
average Nd(1)–O(Ar) bond length of 2.386(2) Å is comparable
with the Nd–O(alkox) bond length in [(MBMP)Nd(μ-OiPr)-
(THF)2]2,[22] but apparently longer than the La(1)–O(Ar) bond
length in complexes 1 and 3, although the ionic radius of lan-
thanum ion is larger than that of neodymium ion. This differ-
ence can be attributed to the formation of bridging bond in the
Figure 1. ORTEP diagram of complex 1 showing atom-numbering
scheme. Thermal ellipsoids are drawn at the 30% probability level.
Hydrogen atoms are omitted for clarity.
plex 1, reflecting the less steric congestion around lanthanum latter.
ion resulted from the coordination of less bulky 4-tert-butyl-
phenolate group.
Ring-opening Polymerization of ε-Caprolactone
An ORTEP of complex 4 is shown in Figure 3, and the se-
lected bond lengths and bond angles are listed in Table 2.
Poly(ε-caprolactone) and related copolymers have received
Complex 4 shows a centrosymmetric dimeric feature contain- considerable interest in the past ten years, because of their bio-
ing a Nd2O2 core bridging through the oxygen atoms of the degradable property and the wide application in the medical
aryloxo groups. The overall molecule structure is quite dif- field.[26–29] It has been found that many organolanthanide com-
ferent from that of complex 3, which revealed that the coordi- plexes are efficient initiators for the ring-opening polymeriza-
Z. Anorg. Allg. Chem. 2013, 2324–2330
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L. Fang, Y. Yao, Y. Zhang, Q. Shen, Y. Wang
ARTICLE
Table 2. Selected bond lengths /Å and bond angles /° for complexes 1, 3, and 4.
Bond lengths
1
3a
3b
4
Ln(1)–O(1)
Ln(1)–O(2)
Ln(1)–O(3)
Ln(1)–O(3A)
Ln(1)–O(4)
Ln(1)–O(5)
Ln(1)–O(6)
Ln(1)–O(7)
O(1)–C(1)
2.279(2)
2.302(2)
2.259(2)
2.266(7)
2.220(8)
2.230(8)
2.260(7)
2.241(8)
2.241(8)
2.169(2)
2.217(2)
2.434(2)
2.358(2)
2.510(2)
2.585(2)
2.604(2)
2.701(4)
2.691(2)
2.678(2)
1.342(3)
1.334(4)
1.340(3)
2.74(2)
2.66(3)
2.76(2)
2.76(2)
1.36(1)
1.35(1)
1.36(1)
2.67(2)
2.66(3)
2.63(2)
2.66(2)
1.33(1)
1.35(1)
1.34(1)
1.338(4)
1.349(4)
1.361(4)
O(2)–C(7)
O(3)–C(24)
Bond angles
1
3
4
O(1)–Ln(1)–O(2)
O(1)–Ln(1)–O(3)
O(2)–Ln(1)–O(3)
Ln(1)–O(1)–C(1)
Ln(1)–O(2)–C(7)
Ln(1)–O(3)–C(24)
Ln(1)–O(3A)–C(24)
86.40(5)
96.08(6)
176.69(6)
154.2(1)
152.4(1)
174.1(1)
86.8(3)
103.6(3)
96.0(3)
149.8(7)
160.2(7)
172.1(8)
87.8(3)
101.4(3)
97.1(3)
149.6(7)
160.3(9)
170.5(8)
94.22(8)
89.67(8)
103.49(8)
158.2(2)
144.7(2)
105.2(2)
142.7(2)
Figure 2. ORTEP diagram of complex 3a showing atom-numbering
scheme. Thermal ellipsoids are drawn at the 20% probability level.
Hydrogen atoms are omitted for clarity.
Figure 3. ORTEP diagram of complex 4 showing atom-numbering
scheme. Thermal ellipsoids are drawn at the 20% probability level.
Hydrogen atoms and carbon atoms on THF molecules are omitted for
clarity.
tion (ROP) of ε-caprolactone.[20–22,30–32] To further explore the
influence of the initiating group of organolanthanide com-
plexes on the catalytic property, the catalytic behaviors of com-
plexes 1–6 for the ROP of ε-caprolactone were tested
(Scheme 2), and the results are listed in Table 3.
These lanthanide aryloxo complexes can serve as active ini-
tiators to polymerize ε-caprolactone in toluene to give the
polymers with high molecular weights and moderate molecule
weight distributions. The molecule weights of the resultant
Scheme 2. Ring-opening polymerization of ε-caprolactone.
polymers range from 2.61ϫ10–4 to 8.92ϫ10–4 g·mol–1 as the different from the cases initiated by lanthanide alkoxo com-
increase of molar ratio of monomer to initiator, but there is no plexes stabilized by the same bis(phenolate) ligand, and the
linear relationship between the molar ratios of monomer to latter can initiate ε-caprolactone polymerization in a controlled
initiator and the molecular weights. On the other hand, the manner.[21,22] It has been reported that the silylamide initiating
molecule weight distributions range from 1.40 to 1.64. These group is inferior to alkoxo group for controlled polymerization
results indicated that the polymerization initiated by these lan- of lactide, because the silylamide group is less nucleophilic
thanide aryloxo complexes is not well-controlled. This is quite than alkoxo group, which caused the relatively slow initiating
2328
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© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2013, 2324–2330

Lanthanide Aryloxo Complexes and Their Polymerization Behavior for ε-Caprolactone
a)
Table 3. Polymerization of ε-caprolactone initiated by complexes 1 to 6
.
Run
Initiator
[M]0/[I]0
t /h
Tp /°C
Yield /%b)
Mn (obsd)c) /ϫ10–4 g·mol–1
PDI
1
2
3
4
5
6
7
8
1
1
1
1
2
2
2
2
2
3
3
3
4
4
5
5
5
6
6
400
600
800
1000
200
400
600
800
1000
600
800
1000
400
600
400
600
800
400
600
1
1
2
2
1
1
2
2
4
1
2
2
4
4
4
4
4
4
4
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
100
100
96
79
100
87
93
76
81
100
98
82
96
55
100
100
89
93
60
4.35
6.53
8.32
8.67
2.51
4.08
5.84
6.34
8.92
6.13
8.68
8.91
3.74
2.88
4.21
6.29
7.52
3.94
3.55
1.64
1.60
1.53
1.47
1.58
1.49
1.55
1.42
1.45
1.46
1.49
1.43
1.63
1.45
1.56
1.52
1.47
1.48
1.40
9
10
11
12
13
14
15
16
17
18
19
a) General polymerization conditions: toluene as solvent, Vsol/V[M] = 4:1. b) Yield: weight of polymer obtained/weight of monomer used. c)
Measured by GPC in THF calibrated with standard polystyrene samples.
speed.[33] The results in our cases revealed that the aryloxo cies, which is the really active center, when a dimeric complex
group is also inferior to alkoxo group in controllability for was used as the initiator for lactide polymerization.[33]
ε-caprolactone polymerization. It is worthy to note that the
To gain more information about the initiation mechanism,
cyclopentadienyl precursors showed very low activity under the oligomer of ε-caprolactone was prepared by the reaction
the same polymerization conditions, indicating that the reactiv- of ε-caprolactone with complex 6 in a 10:1 molar ratio, and
ity of Ln–Cp bonds is lower than that of Ln–O(Ar) bond.
then quenched by 2-propanol. The oligomer was characterized
The ionic radii of the lanthanide ions have an obvious influ- by 1H NMR spectroscopy as shown in Figure 4. In the 1H
ence on the catalytic activity for ε-caprolactone polymeriza- NMR spectrum, the signals at about 2.96 and 3.63 ppm can be
tion. The catalytic activity decreased apparently with a de- assigned to the end of HOCH2(CH2)4CO–. On the other hand,
crease of the ionic radii. Using the lanthanum complex 1 as the signals at about 1.21 and 5.02 ppm revealed that another
the initiator, the yield was 96% when the molar ratio of mono- end group of the oligomer is an isopropoxo group, but not
mer to initiator is 800 in 2 h (Table 3, entry 3), whereas the the 4-methoxyphenoxo group. We postulated that an exchange
yield was 76% using the neodymium complex 2 as the initiator reaction occurred between aryloxo group and isopropoxo
under the same polymerization conditions (Table 3, entry 8). group during quenching the oligomerization by 2-propanol.
Similar active trend has also been observed in the carbon Similar phenomenon was also observed by Shen et al. in the
bridged bis(phenolate) lanthanide systems for ε-caprolactone polymerization of cyclic carbonate initiated by lanthanide ary-
polymerization,[20–22] and the diketiminato lanthanide bis(al- loxides.[35]
lyl) complexes for lactide polymerization.[34] This difference
can be attributed to that the lanthanide metal with larger ionic
radius has more opening coordination sphere, which is advan-
tageous for the coordination of monomer to the central metal
atom. The bulkiness of the aryloxo group seems to have no
obvious effect on the catalytic activity for ε-caprolactone poly-
merization. Complex 1 showed similar activity for this poly-
merization in comparison with complex 3. In contrast, the so-
lid-state structure of the initiator has an influence on the cata-
lytic activity. The initiator having monomeric structure showed
higher activity than that having dimeric structure. For example,
complexes 1 and 3 can polymerize completely in 2 h when the
molar ratio of monomer to initiator is 800 (Table 3, entries 3
and 11); whereas the yield is 89% even the polymerization
time extends to 4 h under the same polymerization conditions
(Table 3, entry 17). The difference in activity among these lan-
thanide complexes can be attributed to the lower initiating
1
speed, because Okuda et al. has suggested that the dimeric
Figure 4. H NMR spectrum of the oligomer of ε-caprolactone initi-
structure should be cleaved and produced the monomeric spe- ated by complex 6, and quenched by 2-propanol in CDCl3.
Z. Anorg. Allg. Chem. 2013, 2324–2330
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
2329

L. Fang, Y. Yao, Y. Zhang, Q. Shen, Y. Wang
ARTICLE
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Conclusions
Some new carbon bridged bis(phenolate) lanthanide aryloxo
complexes were synthesized, and their structural features were
provided by single-crystal X-ray diffraction. It was found that
the coordination solvent molecule has a profound influence on
the solid-state structure of the bis(phenolate) lanthanide aryl-
oxo complexes. The complexes crystallized from DME-con-
taining solvent have monomeric structures, whereas the com-
plexes crystallized from THF have dimeric structure. These
lanthanide aryloxo complexes are efficient initiators for ε-cap-
rolactone polymerization, but the polymerization is not well
controlled. These results revealed that the aryloxo group is in-
ferior to alkoxo group in controllability for ε-caprolactone po-
lymerization. Furthermore, the complexes having dimeric
structure showed relatively lower activity than the complexes
having monomeric structure.
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Acknowledgements
Financial support from the National Natural Science Foundation of
China (Grants 21072146, 21174095, and 21132002), PAPD, and the
Qing Lan Project is gratefully acknowledged.
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Received: April 25, 2013
Published Online: June 19, 2013
2330
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© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2013, 2324–2330