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Synthesis, structure and catalytic activity of complexes [Ln(EDBP)2(DME)Na(DME)3](Ln=Er, Yb, Sm) for ring-opening polymerization of ε-caprolactone YU JianFang, WU GuangMing, HUANG Jing, SUN WeiLin† & SHEN ZhiQuan† Key Laboratory of Macromolecular Synthesis and Functionalization, Ministry of Education, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China
Discrete ion-pair complexes [Ln(EDBP)2(DME)Na(DME)3] [Ln=Er (1), Yb (2), Sm (3)] have been synthesized by the reaction between sodium salt of 2,2′-ethylidene-bis(4,6-di-tert-butylphenol)(EDBPH2) and Ln(BH4)3·3THF (Ln=Er, Yb, Sm) followed by centrifugation and recrystalization. The complexes were characterized by elemental analysis and FT-IR, and the bonding model of these compounds was confirmed by X-ray single crystal diffraction for complex 1. It was found that four O atoms in two biphenol ligands as well as two O atoms in one ethylene glycol dimethyl ether (DME) molecule connect to the center rare earth metal atom, while sodium exists as counterpart cation to balance the charge. Complexes 1―3 can all be used as single component initiators for the ring-opening polymerization of ε-caprolactone. ring-opening, polymerization, ε-caprolactone, X-ray single crystal, biphenol
1 Introduction The Ln―O bond in rare earth complexes is accounted for some of their outstanding performance in the catalytic , chemistry[1 2]. For the intrinsic oxytropic properties of rare earth metals, the Ln―O bond can be easily formed through metathesis between the rare earth metal precursors (LnCl3) and sodium salt of some oxygen-containing ligands, such as phenol and alcohol. Among these oxygen-containing compounds, the bidental phenols were ― extensively investigated in the coordination chemistry[3 6]. By modifying the substituting groups, a variety of tailormade single-site catalysts have been developed, from which ring-opening polymerization and olefin polymerization benefit most. In lanthanide chemistry, however, the biphenol ligands were not studied a lot, while the rare earth complexes based on steric phenol were reported as highly efficient initiators for the ring-opening polymerization of cyclic esters[4,5,7]. Compared with Ln(OAr)3
catalytic system, the biphenol groups have a relatively rigid framework, which may result in controllable polymerization. Some studies have been done for the ― preparation of biphenol based rare earth complexes[4 6], and EDBPH2 as biphenol ligand was employed to prepare biphenol based rare earth catalysts. However, as it is known, the studies about biphenol based discrete ion-pair complexes in lanthanide chemistry are rarely reported, and the application of such kind of complexes is rarely mentioned[6]. In our previous work, a new EDBPH2 based yttrium complex was reported, which was used as an initiator for the ring-opening polymerization of ε-caprolactone[8]. Here we prepared three complexes [Ln(EDBP)2(DME)Na(DME)3] [Ln=Er (1), Yb (2), Sm (3)] with different central Received June 20, 2009; accepted July 20, 2009 doi: 10.1007/s11426-009-0208-7 † Corresponding author (email:
[email protected];
[email protected]) Supported by the National Natural Science Foundation of China (Grant No. 20376085) and the Major State Basic Research Program of China (Grant No. 2005CB623800)
Sci China Ser B-Chem | Oct. 2009 | vol. 52 | no. 10 | 1711-1714
metal radii, which are discrete ion-pair complexes and can be used as single component initiators for the ringopening polymerization of ε-caprolactone.
2 Experimental 2.1 Materials All the manipulations were conducted using Schlenk and vacuum line techniques. ε-Caprolactone (Alpha product, 99%) was stirred with CaH2 and fresh distilled prior to use. Anhydrous lanthanide chlorides were prepared by heating the mixture of hydrated rare earth chloride and ammonium chloride under reduced pressure[9]. Lanthanide borohydride was prepared according to the reported method[10]. NaH (60% in mineral oil) was washed with hexane several times and dried under vacuum. Toluene and DME were freshly distilled over potassium using benzophenone as indicator. Other chemicals were used as received. 2.2 Measurements IR spectra were recorded with Nujol film discs on a Bruker Fourier transform infrared (FT-IR) spectrometer. The content of rare earth was analyzed by EDTA-titration method. The elemental analysis was performed on a Flash EA1112 (Thermofinnigan, Italy). Size Exclusion Chromatography (SEC) was performed on a Waters 1525/2414 gel permeation chromatography (GPC) system with polystyrene standards used for calibration. Suitable single crystal was sealed in a thin-walled glass capillary, and data collection was performed at 20℃ on a Bruker SMART diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.710 73 Å).
Elemental analysis for C76H128NaO12Er (1): C, 64.10; H, 9.06; Er, 11.75. Found: C, 63.89; H, 9.20; Er, 11.30. FT-IR (Nujol, cm−1): 3489, 2724, 1297, 1196, 1156, 1077, 878, 838, 724, 524. Elemental analysis for C76H128NaO12Yb (2): C, 63.84; H, 9.02; Yb, 12.10. Found: C, 63.60; H, 9.02; Yb, 11.96. FT-IR (Nujol, cm−1): 3489, 3000-2800, 1120, 1080, 838, 779, 523, 449. Elemental analysis for C76H128NaO12Sm (3): C, 64.87; H, 9.17; Sm, 10.69. Found: C, 64.36; H, 9.31; Sm, 10.68. FT-IR (Nujol, cm−1): 3489, 2725, 1298, 1080, 838, 724, 523, 429. Crystal data for complex 1 are summarized in Tables 1 and 2. 2.4 Polymerization In a typical polymerization experiment, the toluene solution of the initiator was injected to a 20 mL vial loaded with 0.5 mL ε-caprolactone (in 0.5 mL toluene) by a syringe. After specified intervals, the polymerization was terminated by ethanol containing 5% HCl. The polymer was washed 4 times by ethanol and then dried under vacuum to constant weight.
3 Results and discussion 3.1 Synthesis of complexes 1―3 The metathesis proceeded smoothly by using lanthanide borohydride as the precursor instead of lanthanide chloride, which can be attributed to the better solubility of lanthanide borohydride in DME. Satisfactory yields can be obtained after centrifugation and recrystalizaiton for complexes 1, 2 and 3 respectively (Scheme 1).
2.3 Synthesis and characterization of complexes 1―3 To a flamed flask purged with argon was added NaH (318.2 mg, 13.2 mmol) and 20 mL of fresh distilled DME, and this suspension was stirred at 0℃ for 10 min. A 20 mL DME solution of EDBPH2 (1576.5 mg, 3.6 mmol) was then added dropwise. The resulting mixture was warmed to room temperature and stirred overnight. It was centrifuged and the clear solution was added to a 20 mL DME solution of Er(BH4)3·3THF (763.5 mg, 1.8 mmol) and stirred at 60℃ for 20 h. This solution was centrifuged and then concentrated to approximately 1/4 volume and cooled to −30℃ to obtain cubic crystals after three days in a yield of 70% based on Er. Complexes 2 and 3 were prepared in similar procedures. 1712
Scheme 1 Synthesis of complexes 1―3.
3.2 Crystal structure Suitable crystals of complex 1 for X-ray diffraction analysis were grown by slowly cooling the DME solution. An ORTEP plot of crystal 1 is presented in Figure 1. Figure 1 reveals an interesting anionic bimetal
YU JianFang et al. Sci China Ser B-Chem | Oct. 2009 | vol. 52 | no. 10 | 1711-1714
Table 1 Crystallographic information for complex 1
Figure 1 ORTEP drawing of complex 1 with displacement ellipsoids at 25% probability level. Hydrogen atoms are omitted for clarity.
structure of complex 1. One DME is bonded to the Er atom by O5 and O6, and the Er―O distances are 2.476 and 2.448 Å, respectively. The Er coordination sphere is completed with four phenolate O atoms (O1―O4) from EDBPH2 ligands, while the Er―O distances span from 2.131 to 2.183 Å. One can also note that Na+ was found to balance the charge, which is surrounded by six O atoms from three DME molecules. The crystallographic information and selected bond distances and angles are listed in Table 1 and 2, respectively. 3.3 Characteristics of the polymerization The catalytic activity of complexes 1―3 for the ringopening polymerization toward the ε-caprolactone was tested and the results are summarized in Table 3. It was found that all the three complexes are effective single component initiators for the polymerization in toluene. At 65℃ in 4 h, PCL with a molecular weight of 2.19 × 104 and a PDI of 1.16 (entry 4) was produced from complex 1. Temperature effect was investigated, as demonstrated
Empirical formula
C76 H128 Er Na O12
Formula weight
1424.03
Temperature (K)
293(2)
Wavelength (Å)
0.71073
Crystal system
Monoclinic
Space group
P2(1)/c
a (Å)
15.5563(11)
b (Å)
21.8882(16)
c (Å)
25.3625(18)
Volume (Å3)
8286.6(10)
Z
4
Dcalc (mg/m3)
1.141
Absorption coefficient (mm−1)
1.070
F(000)
3036 3
Crystal size (mm )
0.466 × 0.398 × 0.367
θ range for data collection (°)
1.84―25.50
Completeness to θ = 25.50 (%)
99.2
Reflections collected/unique
43028/15313
Independent reflections
10993 [Rint = 0.0552]
Absorption correction
Empirical
Maximum and minimum transmission
1.000; 0.71514
Data/restraints/parameters
15313/5/820
Final R indices (I >2σ(I))
R1 = 0.0431, wR2 = 0.0930
R indices (all data)
R1 = 0.0641, wR2 = 0.0984 2
Goodness-of-fit on F
1.017
Largest difference peak and hole (e.Å−3)
1.011; −0.567
Table 2 Selected bond distances (Å) and angles (º) of complex 1 Er-O(1)
2.167(2)
Na-O(7)
2.370(5)
O(3)-Er-O(2) 110.97(10)
Er-O(2)
2.135(2)
Na-O(8)
2.488(5)
O(3)-Er-O(1)
90.08(10)
Er-O(3)
2.131(3)
Na-O(9)
2.424(7)
O(2)-Er-O(1)
93.43(10)
Er-O(4)
2.183(2) Na-O(10) 2.427(6)
O(3)-Er-O(4)
92.68(10)
Er-O(5)
2.476(3) Na-O(11) 2.349(4)
O(2)-Er-O(4)
92.63(10)
Er-O(6)
2.448(3) Na-O(12) 2.481(5)
O(1)-Er-O(4)
171.96(9)
Table 3 Ring-opening polymerization of ε-caprolactone initiated by complexes 1―3 Mw b) × 10−4
Entry a)
Initiator
T (℃)
Time (h)
Conversion (%)
PDI b)
1
1
25
6
n.d. c)
―
―
2
1
45
6
11
1.23
1.11
3
1
65
2
20
1.67
1.14
4
1
65
4
38
2.19
1.16
5
1
65
7
57
2.20
1.27
6
2
65
4
28
1.82
1.39
7
3
65
4
61
4.85
1.28
a) Conditions: toluene 0.5 mL, ε-caprolactone 0.5 mL, molar ratio: [CL]/[Ln]=400. b) Measured by GPC against polystyrene standards. c) n.d. = no detected.
YU JianFang et al. Sci China Ser B-Chem | Oct. 2009 | vol. 52 | no. 10 | 1711-1714
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in entries 1, 2 and 4. Using Complex 1 as an initiator, the polymerization completed with a conversion of 38% in 4 h at 65℃, while only a conversion of 11% was achieved in 6 h at 45℃, which can be ascribed to that relatively higher temperature favors higher polymerization activity. It is interesting to note that the PCLs obtained from complexes 1―3 in different polymerization conditions have moderate molecular weight distributions (PDIs<1.40). Furthermore, it is worth mentioning that the polymerization activities were governed by the radii of the central metals (entries 4, 6 and 7), while an activity sequence of complex Yb < Er < Sm was found. This can be explained by the fact that larger radius of metal favors the coordination process of the monomer to the rare earth center (Yb3+< Er3+< Sm3+), resulting in relatively higher polymerization efficiency. According to our previous report, the ring-opening polymerization may be explained by a complicated M-O,
1
2
Ling J, Shen Z Q, Huang Q H. Novel single rare earth aryloxide ini-
In conclusion, bimetal lanthanide complexes 1―3 were prepared, and the anionic bimetal structure was well established by X-ray single crystal diffraction for complex 1. Complexes 1―3 were used as single component initiators for the ring-opening polymerization of εcaprolactone to give moderate molecular weight distributions. Further investigation concerning the polymerization mechanism and kinetics is underway in our laboratory.
6
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tiators for ring-opening polymerization of 2,2-dimethyltrimethylene
for the living polymerization and block copolymerization of 3
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YU JianFang et al. Sci China Ser B-Chem | Oct. 2009 | vol. 52 | no. 10 | 1711-1714