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Synthesis, characterization of hetero-bimetallic complex and application in the polymerization of ε-caprolactone
Inorganic Chemistry Communications 14 (2011) 763–766
Contents lists available at ScienceDirect
Inorganic Chemistry Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i n o c h e
Synthesis, characterization of hetero-bimetallic complex and application in the polymerization of ε-caprolactone Xiaobo Pan a,b,⁎, Ai Liu b, Lihui Yao b, Lei Wang b, Jinfeng Zhang b, Jincai Wu b,⁎⁎, Xuebo Zhao a, Chu-Chieh Lin c a b c
Qingdao Institute of Bioenergy and Bioprocess Technology, CAS, PR China Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China Department of Chemistry, National Chung Hsing University, Taichung 402, Taiwan, ROC
a r t i c l e
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Article history: Received 1 December 2010 Accepted 2 March 2011 Available online 13 March 2011 Keywords: Hetero-bimetallic complex ε-Caprolactone Catalyst Ring-opening polymerization
a b s t r a c t The reaction of 2,2′-ethylidenebis(4,6-di-tert-butylphenol)(EDBP-H2) with sodium, Al(Me)3 and BnOH with a molar of ratio of 1:1:1:1 in THF gives hetero-bimetallic complex of [(EDBP)Al(CH3)(μ2-OBn)Na(THF)3](1) in 85% yield. In order to investigate the different anion effects on the ROP of ε-caprolactone, complexes [(EDBP)2Al][(Li)(THF)2] (2) and [(EDBP)2Al][(Na)(THF)2] (3) were prepared in an analogous manner to that of 1 in 63% and 71% yields respectively. Experimental results show that hetero-bimetallic complex 1 is an efficient catalyst for the ring-opening polymerization of ε-caprolactone. © 2011 Elsevier B.V. All rights reserved.
Biodegradability, biocompatibility, and permeable properties of aliphatic polyesters, such as poly(ε-caprolactone) (PCL) [1], poly (lactide) (PLA) [2] and poly(trimethylenecarbonate)(PTMC) [3] and their copolymers show their potential applications in the medical field as biodegradable surgical sutures or as a delivery medium for controlled release of drugs [4]. Therefore, there has been increasing interest in the development of efficient catalytic systems for the preparation of PCL, PLA and PTMC. The major polymerization method used to synthesize these polymers has been the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL). An important task for developing new catalytic systems is to make the catalyst more compatible with the purpose of biomedical application. Though several effective initiators that initiate ROP of CL have been reported [5], the cytotoxicity and difficulties in removal of the catalyst from the resulting polymer have limited their utilization. Recently we reported some metal aryloxides supported with 2,2′-ethylidenebis(4,6-di-tert-butylphenol)(EDBPH 2 ), such as EDBP-Li [6], EDBP-Na [7], EDBP-K [8], EDBP-Al [9], are excellent catalysts with good controlled features for ROP of cyclic ester through the activation of alcohol. And EDBP-Al [9a] molecular even can catalyze the synthesis of 400-fold polylactone chains in the ROP of ε-caprolactone. EDBP-H2 is an attractive ligand because it has ⁎ Correspondence to: Qingdao Institute of Bioenergy and Bioprocess Technology, CAS, PR China. ⁎⁎ Corresponding author. E-mail addresses: [email protected] (X. Pan), [email protected] (J. Wu). 1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2011.03.003
been approved as an indirect food additive (as an antioxidant in polymer packaging) by the U.S. Food and Drug Administration [10]. Moreover, sodium and aluminum are nontoxic and are essential for life. Thus, aluminum and sodium complexes supported with EDBPH2 ligand have attracted considerable interest in terms of biomedical purposes. All of these encourage us to design new EDBP nontoxic metal complexes as catalysts for ROP of ε-CL. Herein we report one new hetero-bimetallic complex to catalyze ring-opening polymerization of ε-CL and results show it is an efficient catalyst. The reaction of EDBP-H2 with sodium, Al(Me)3 and BnOH with a molar of ratio of 1:1:1:1 in THF gives complex of [(EDBP)Al(CH3) (μ2-OBn)Na(THF)3] (1) (Scheme 1) in 85% yield. 1H NMR spectra and microanalyses of complex 1 are consistent with our expectations. This is further verified by the crystal structure studies of complex 1. In order to investigate the different cation effects on the ROP of lactone, complexes [(EDBP) 2 Al][(Li)(THF) 2 ] (2) and [(EDBP) 2 Al][(Na)(THF) 2 ] (3) were prepared in an analogous manner to that of 1 in 63% and 71% yield respectively. Further reaction of compound 3 with excess BnOH in toluene produces 4 in 78% yields. Fine colorless crystals were obtained by crystallization from THF/hexane (5 mL:15 mL). The ORTEP drawing of 1 is given in Fig. 1. The [(EDBP)Al(CH3)(μ2-OBn)Na(THF)3] (1) complex is crystallized in triclinic space group P-1. Two oxygen atoms of the EDBP ligand are nonequivalent. One oxygen atom bridges a sodium atom and an aluminum atom, and the other one is coordinated to the aluminum atom. The geometry around Al1 is distorted tetrahedral, with coordination by two oxygen atoms of the phenoxy
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Scheme 1. Preparation of compounds 1–4.
group, one bridging oxygen atom of the benzyl alkoxide group and one carbon atom of methyl with the bond lengths of Al1-O1 1.767 (2), Al1-O2 1.722(3), Al1-O3 1.759(3) and Al1-C30 1.948(5) Å, respectively. The geometry around Na1 is distorted square pyramid, with coordination by one bridging oxygen atom of O1 from ligand, and one bridging oxygen atom of O3 from the benzyl alkoxide group and three oxygen atoms O4, O5, O6 from three THF molecules, and with the bond lengths of Na1-O1 2.515(3), Na1-O3 2.331(3), Na1-O4 2.379(4), Na1-O5 2.307(4) and Na1-O6 2.339(4) Å respectively.
Single crystals of 2 and 4 suitable for X-ray structural determination were obtained from hexane solution. The ORTEP drawing of the molecular structure of 2 and 4 are given in Figs. 2 and 3. In the two structures, the geometry around Al is distorted tetrahedral and the aluminum atom is firmly fixed by four oxygen atoms of two EDBP ligands with average Al–O bond lengths of 1.735(1) and 1.736(8) Å for complex 2 and 4 respectively. Two oxygen atoms of the each EDBP ligand are nonequivalent. One just coordinates to Al1 and the other one bridges aluminum and
Fig. 1. Molecular structure of 1 as 30% ellipsoids (methyl carbons of the tert-butyl groups and all of the hydrogen atoms are omitted for clarity). Selected bond lengths (Å): Al1-O1 1.767(2), Al1-O2 1.722(3), Al1-O3 1.759(3), Na1-O1 2.515(3), Na1-O3 2.331(3), Na1-O4 2.379(4), Na1-O5 2.307(4) and Na1-O6 2.339(4).
Fig. 2. Molecular structure of 2 as 20% ellipsoids (methyl carbons of the tert-butyl groups and all of the hydrogen atoms are omitted for clarity). Selected bond lengths (Å): Al1-O1 1.682(8), Al1-O2 1.767(8), Al1-O3 1.718(9), Al1-O4 1.770(8), Li1-O2 2.07 (3), Li1-O4 1.94(3), Li1-O5 2.13(3), Li1-O6 1.93(2), C1-O1 1.334(12), C13-O2 1.409 (13), C31-O3 1.372(13) and C43-O4 1.352(13).
X. Pan et al. / Inorganic Chemistry Communications 14 (2011) 763–766
Fig. 3. Molecular structure of 4 as 20% ellipsoids (methyl carbons of the tert-butyl groups and all of the hydrogen atoms are omitted for clarity, except for the hydroxy of benzyl alcohol). Selected bond lengths (Å): Al1-O1 1.707(8), Al1-O2 1.725(8), Al1-O3 1.811(9), Al1-O4 1.701(8), Na1-O1 2.309(10), Na1-O3 2.459(10), Na1-O5 2.271(15), Na1-O6 2.37(2), C1-O13 1.402(9), C1-O2 1.367(8), C31-O3 1.376(8) and C43-O4 1.396(9).
lithium/sodium. The structure of 2 is somewhat different from that of 4, in complex 2 two THF molecules coordinates to Li1, and in complex 4 two benzyl alcohol molecules coordinate to Na1 instead of THF. Ring-opening polymerization of ε-caprolactone (ε-CL) employing 1 as an initiator is systematically examined under a dry nitrogen atmosphere. Conversion of CL is determined on the basis of 1H NMR spectroscopic studies. The molecular weight and polydispersity of PCL are measured by gel permeation chromatography (GPC). A typical polymerization experiment exemplified for PCL-94 ([M]0/ [I]0 = 94) is described as follows. To a rapidly stirring solution of 1 in toluene was added ε-CL (0.20 mL, 1.85 mmol). After the reaction was quenched by the addition of excess 0.35 N acetic acid aqueous solutions, the polymer was precipitated into n-hexane. Polymerizations of ε-CL under same reaction conditions (entries 1–4) have been systematically examined as shown in Table 1. It was found that the PDIs of polyesters initiated by 1 range from 1.40 to 1.47, and a linear relationship between the number average molecular weight (Mn) and the monomer-to-initiator ratio ([M]0/[I]0) exists as shown in Fig. 4 (Table 1, entries 1–4). While in these experiments, the experimental value of Mn (Mn(obsd)) obtained from the GPC analysis is generally lower than the theoretical Mn value (calculated on the assumption that each BnOH initiates the polymerization) owe to the occurrence of the back-biting reaction. The cyclic polymerization side reactions were also found for the lactide
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Fig. 4. Polymerization of ε-CL catalyzed by 1 in toluene at 120 °C. The relationship between Mn(■) (PDI (●)) of the polymer and the initial mole ratio [M]0/[complex]0 is shown.
polymerization with the catalysts published. [11] Analysis of PCL47 (Table 1, entry 1) produced from 1 at an initial [M]0/[I]0 ratio of 47 by 1H NMR (Fig. 5) shows a characteristic benzyl peak Hb at 5.11 ppm and hydroxy end group peak Hg at 3.65 ppm in CDCl3, indicating polymers are linear capped with two end groups of the benzyl alkoxy and hydroxy group respectively. In fact the complexes 2, 3, 4 [12] also were applied to the ROP of εCaprolactone, the experimental results show they are not active catalysts which can be attributed to the low basic complexes and cannot activate BnOH to initiate the ROP reaction. In conclusion, a new hetero-bimetallic complex has been synthesized and characterized. The hetero-bimetallic complex 1 is an efficient catalyst for ring-opening polymerization of lactone.
Acknowledgments We thank the financial supports from the National Natural Science Foundation of China (No. 21071069 and 20601011), Science Foundation of Gansu Province of China (0803RJZA103), Scientific Research Foundation for the Returned Overseas Chinese Scholars and the Fundamental Research Funds of the Central Universities of China, State Education Ministry (lzujbky-2009-26).
Appendix A. Supplementary material Supplementary data to this article can be found online at doi:10.1016/j.inoche.2011.03.003.
Table 1 Ring-opening polymerization of ε-caprolactone catalyzed by 1a. Entry
[CL]0/[Complex]0
Time (h)
PDI
Mn (obsd)b
Mn (calcd)c
Conversion (%)
1 2 3 4
47 94 141 188
16 16 16 16
1.42 1.47 1.43 1.40
9170 9887 11,368 16,600
5400 10,300 15,200 19,600
98 95 94 91
a b c
(5300) (6500) (7600) (9600)
Conditions: 0.02 mmol of complex, 10 mL of toluene, 120 °C reflux. Obtained from GPC analysis, and calibrated by polystyrene standard (corrected values listed in brackets). Calculated from the molecular weight of ε-CL times [CL]0/[Complex]0 times conversion yield plus the molecular weight of BnOH.
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Fig. 5. 1H NMR of PCL-47 (from Table 1, entry 1) in CDCl3.
Reference [1] (a) M. Endo, T. Aida, S. Inoue, “Immortal” polymerization of ε-caprolactone initiated by aluminum prophyrin in the presence of alcohol, Macromolecules 20 (1987) 2982–2988; (b) A. Duda, Z. Florjanczyk, A. Hofman, S. Slomkowski, S. Penczek, Living pseudoanionic polymerization of ε-caprolactone. Poly(ε-caprolactone) free of cyclics and with controlled end groups, Macromolecules 23 (1990) 1640–1646; (c) A. Kowalski, A. Duda, S. Penczek, Kinetics and mechanism of cyclic esters polymerization initiated with tin(II) octoace, 1. Polymerization of εcaprolactone, Macromol. Rapid Commun. 19 (1998) 567–572; (d) Z. Gan, T.F. Jim, M. Jim, Z. Yuer, S. Wang, C. Wu, Crystallization under strain and resultant orientation of poly(ε-caprolactone) in miscible blends, Macromolecules 32 (1999) 1218–1225. [2] (a) B.M. Chamberlain, Y. Sun, J.R. Hagadorn, E.W. Hemmesch, V.G. Young Jr., M. Pink, M.A. Hillmyer, W.B. Tolman, Discrete yttrium(III) complexes as lactide polymerization catalysts, Macromolecules 32 (1999) 2400–2402; (b) T.M. Ovitt, G.W. Coates, Stereoselective ring-opening polymerization of mesolactide: synthesis of syndiotactic poly(lactide acid), J. Am. Chem. Soc. 121 (1999) 4072–4073; (c) X. Chen, R.A. Gross, Versatile copolymers from [L]-lactide and [D]-xylofuranose, Macromolecules 32 (1999) 308–314. [3] (a) M. Helou, O. Miserque, J.M. Brusson, J.F. Carpentier, S.M. Guillanme, Poly (trimethylene carbonate) from biometals-based initiators/catalysts: highly efficient immortal ring-opening polymerization processes, Adv. Synth. Catal. 351 (2009) 1312–1324; (b) M. Helou, O. Miserque, J.M. Brusson, J.F. Carpentier, S.M. Guillanme, Highly effective and green catalytic approach toward α,ω-dihydroxy telechelic poly (trimethylenecarbonate), Macromol. Rapid Commun. 30 (2009) 2128–2135. [4] (a) B. Jeong, Y.H. Bae, D.S. Lee, S.W. Kim, Biodegradable block copolymers as injectable drug-delivery systems, Nature 388 (1997) 860–862; (b) R. Gref, Y. Minamitake, M.T. Peracchia, V. Trubetskov, V. Torchilin, R. Langer, Biodegradable long-circulating polymeric nanospheres, Science 263 (1994) 1600–1603. [5] (a) A. Sawhney, C.P. Pathak, J.A. Hubbell, Bioerodible hydrogels based on photopolymerized poly(ethyleneglycol)-co-poly(α-hydroxy acid) diacrylate macromers, Macromolecules 26 (1993) 581–587; (b) X. Zhang, D.A. MacDonald, M.F.A. Goosen, K.B. Mcauley, Mechanism of lactide polymerization in the presence of stannous octoate: the effect of hydroxyl and carboxylic acid substances, J. Polym. Sci A Polym. Chem. 32 (1994) 2965–2970; (c) D.K. Han, J.A. Hubbell, Lactide-based poly(ethylene glycol) polymer networks for scaffolds in tissue engineering, Macromolecules 29 (1996) 5233–5235. [6] (a) B.T. Ko, C.C. Lin, Synthesis, characterization, and catalysis of mixed-ligand lithium aggregates, excellent initiators for the ring-opening polymerization of L-lactide, J. Am. Chem. Soc. 123 (2001) 7973–7977; (b) B.H. Huang, B.T. Ko, T. Athar, C.C. Lin, Synthesis, characterization, and structural determination of polynuclear lithium aggregates and factors affecting their aggregation, Inorg. Chem. 45 (2006) 7348–7356;
[7]
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(c) W.C. Hung, C.C. Lin, Preparation, characterization, and catalytic studies of magnesium complexes supported by NNO-tridentate schiff-base ligands, Inorg. Chem. 48 (2009) 728–734. H.Y. Chen, J.B. Zhang, C.C. Lin, J.H. Reibenspies, S.A. Miller, Efficient and controlled polymerization of lactide under mild conditions with a sodium-based catalyst, Green Chem. 10 (2007) 1038–1040. X.B. Pan, A. Liu, X.Z. Yang, J.C. Wu, N. Tang, Synthesis, characterization of potassium bulky phenolate and application in ring-opening polymerization of Llactide, Inorg. Chem. Commun. 13 (2010) 376–379. (a) Y.C. Liu, B.T. Ko, C.C. Lin, A highly efficient catalyst for the “living” and “immortal” polymerization of ε-caprolactone and L-lactide, Macromolecules 34 (2001) 6196–6201; (b) H.L. Chen, B.T. Ko, B.H. Huang, C.C. Lin, Reactions of 2,2′-methylenebis (4chloro-6-isopropyl-3-methylphenol) with trimethylaluminum: highly efficient catalysts for the ring-opening polymerization of lactones, Organometallics 20 (2001) 5076–5083; (c) M.L. Shueh, Y.S. Wang, B.H. Huang, C.Y. Kuo, C.C. Lin, Reactions of 2,2′methylenebis (4-chloro-6-isopropyl-3-methylphenol) and 2,2′-ethylidenebis(4,6-di-tert-butylphenol) with MgnBu2: Efficient catalysts for ring-opening polymerization of ε-caprolactone and L-lactide, Macromolecules 37 (2004) 5155–5162; (d) C.T. Chen, C.A. Huang, B.H. Huang, Aluminum complexes supported by tridentate aminophenoxide ligand as efficient catalysts for ring-opening polymerizayion of ε-caprolactone, Macromolecules 37 (2004) 7968–7973. U. S. F. D. A. Code of Federal Regulations, Title 21, Volume 3, Section 178. 2010, Revised April 1, (2006); Federal Register, 51(193) 1986. 35511; Federal Register, 48(132) 1983. 1382; Federal Register, 47(101, Bk. 1) 1982. 22512. (a) B.T. Ko, C.C. Lin, Efficient “living” and “immortal” polymerization of lactones and diblock copolymer of ε-CL and δ-VL catalyzed by aluminum alkoxides, Macromolecules 32 (1999) 8296–8300; (b) C.H. Lin, L.F. Yan, F.C. Wang, Y.L. Sun, C.C. Lin, Intramolecular C···H–O hydrogen bond controlling the conformation of heterocycles: synthesis, structure and catalytic reactivity of aluminum aryloxides, J. Organomet. Chem. 587 (1999) 151–159; (c) B.T. Ko, Y.C. Chao, C.C. Lin, Conformation of heterocycles controlled by the existence of unusual C-H···X hydrogen bonds: syntheses and structure determination of aluminum aryloxides, Inorg. Chem. 39 (2000) 1463–1469; (d) B.T. Ko, C.C. Wu, C.C. Lin, Preparation, characterization, and reactions of [(EDBP)Al(μ-OiPr)]2, a novel catalyst for MPV hydrogen transfer reactions, Organometallics 19 (2000) 1864–1869. A typical procedure of the polymerization: Compared to complex 1, it does use additional one equiv. BnOH as an initiator for complexes 2 and 3, and other reaction conditions same to complex 1. While complex 4 does not use additional BnOH as an initiator in ROP of ε-caprolactone with same reaction conditions to complex 1. The final experimental results show complexes 2–4 are not efficient catalyst for ROP of ε-caprolactone.
Contents lists available at ScienceDirect
Inorganic Chemistry Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i n o c h e
Synthesis, characterization of hetero-bimetallic complex and application in the polymerization of ε-caprolactone Xiaobo Pan a,b,⁎, Ai Liu b, Lihui Yao b, Lei Wang b, Jinfeng Zhang b, Jincai Wu b,⁎⁎, Xuebo Zhao a, Chu-Chieh Lin c a b c
Qingdao Institute of Bioenergy and Bioprocess Technology, CAS, PR China Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China Department of Chemistry, National Chung Hsing University, Taichung 402, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 1 December 2010 Accepted 2 March 2011 Available online 13 March 2011 Keywords: Hetero-bimetallic complex ε-Caprolactone Catalyst Ring-opening polymerization
a b s t r a c t The reaction of 2,2′-ethylidenebis(4,6-di-tert-butylphenol)(EDBP-H2) with sodium, Al(Me)3 and BnOH with a molar of ratio of 1:1:1:1 in THF gives hetero-bimetallic complex of [(EDBP)Al(CH3)(μ2-OBn)Na(THF)3](1) in 85% yield. In order to investigate the different anion effects on the ROP of ε-caprolactone, complexes [(EDBP)2Al][(Li)(THF)2] (2) and [(EDBP)2Al][(Na)(THF)2] (3) were prepared in an analogous manner to that of 1 in 63% and 71% yields respectively. Experimental results show that hetero-bimetallic complex 1 is an efficient catalyst for the ring-opening polymerization of ε-caprolactone. © 2011 Elsevier B.V. All rights reserved.
Biodegradability, biocompatibility, and permeable properties of aliphatic polyesters, such as poly(ε-caprolactone) (PCL) [1], poly (lactide) (PLA) [2] and poly(trimethylenecarbonate)(PTMC) [3] and their copolymers show their potential applications in the medical field as biodegradable surgical sutures or as a delivery medium for controlled release of drugs [4]. Therefore, there has been increasing interest in the development of efficient catalytic systems for the preparation of PCL, PLA and PTMC. The major polymerization method used to synthesize these polymers has been the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL). An important task for developing new catalytic systems is to make the catalyst more compatible with the purpose of biomedical application. Though several effective initiators that initiate ROP of CL have been reported [5], the cytotoxicity and difficulties in removal of the catalyst from the resulting polymer have limited their utilization. Recently we reported some metal aryloxides supported with 2,2′-ethylidenebis(4,6-di-tert-butylphenol)(EDBPH 2 ), such as EDBP-Li [6], EDBP-Na [7], EDBP-K [8], EDBP-Al [9], are excellent catalysts with good controlled features for ROP of cyclic ester through the activation of alcohol. And EDBP-Al [9a] molecular even can catalyze the synthesis of 400-fold polylactone chains in the ROP of ε-caprolactone. EDBP-H2 is an attractive ligand because it has ⁎ Correspondence to: Qingdao Institute of Bioenergy and Bioprocess Technology, CAS, PR China. ⁎⁎ Corresponding author. E-mail addresses: [email protected] (X. Pan), [email protected] (J. Wu). 1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2011.03.003
been approved as an indirect food additive (as an antioxidant in polymer packaging) by the U.S. Food and Drug Administration [10]. Moreover, sodium and aluminum are nontoxic and are essential for life. Thus, aluminum and sodium complexes supported with EDBPH2 ligand have attracted considerable interest in terms of biomedical purposes. All of these encourage us to design new EDBP nontoxic metal complexes as catalysts for ROP of ε-CL. Herein we report one new hetero-bimetallic complex to catalyze ring-opening polymerization of ε-CL and results show it is an efficient catalyst. The reaction of EDBP-H2 with sodium, Al(Me)3 and BnOH with a molar of ratio of 1:1:1:1 in THF gives complex of [(EDBP)Al(CH3) (μ2-OBn)Na(THF)3] (1) (Scheme 1) in 85% yield. 1H NMR spectra and microanalyses of complex 1 are consistent with our expectations. This is further verified by the crystal structure studies of complex 1. In order to investigate the different cation effects on the ROP of lactone, complexes [(EDBP) 2 Al][(Li)(THF) 2 ] (2) and [(EDBP) 2 Al][(Na)(THF) 2 ] (3) were prepared in an analogous manner to that of 1 in 63% and 71% yield respectively. Further reaction of compound 3 with excess BnOH in toluene produces 4 in 78% yields. Fine colorless crystals were obtained by crystallization from THF/hexane (5 mL:15 mL). The ORTEP drawing of 1 is given in Fig. 1. The [(EDBP)Al(CH3)(μ2-OBn)Na(THF)3] (1) complex is crystallized in triclinic space group P-1. Two oxygen atoms of the EDBP ligand are nonequivalent. One oxygen atom bridges a sodium atom and an aluminum atom, and the other one is coordinated to the aluminum atom. The geometry around Al1 is distorted tetrahedral, with coordination by two oxygen atoms of the phenoxy
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Scheme 1. Preparation of compounds 1–4.
group, one bridging oxygen atom of the benzyl alkoxide group and one carbon atom of methyl with the bond lengths of Al1-O1 1.767 (2), Al1-O2 1.722(3), Al1-O3 1.759(3) and Al1-C30 1.948(5) Å, respectively. The geometry around Na1 is distorted square pyramid, with coordination by one bridging oxygen atom of O1 from ligand, and one bridging oxygen atom of O3 from the benzyl alkoxide group and three oxygen atoms O4, O5, O6 from three THF molecules, and with the bond lengths of Na1-O1 2.515(3), Na1-O3 2.331(3), Na1-O4 2.379(4), Na1-O5 2.307(4) and Na1-O6 2.339(4) Å respectively.
Single crystals of 2 and 4 suitable for X-ray structural determination were obtained from hexane solution. The ORTEP drawing of the molecular structure of 2 and 4 are given in Figs. 2 and 3. In the two structures, the geometry around Al is distorted tetrahedral and the aluminum atom is firmly fixed by four oxygen atoms of two EDBP ligands with average Al–O bond lengths of 1.735(1) and 1.736(8) Å for complex 2 and 4 respectively. Two oxygen atoms of the each EDBP ligand are nonequivalent. One just coordinates to Al1 and the other one bridges aluminum and
Fig. 1. Molecular structure of 1 as 30% ellipsoids (methyl carbons of the tert-butyl groups and all of the hydrogen atoms are omitted for clarity). Selected bond lengths (Å): Al1-O1 1.767(2), Al1-O2 1.722(3), Al1-O3 1.759(3), Na1-O1 2.515(3), Na1-O3 2.331(3), Na1-O4 2.379(4), Na1-O5 2.307(4) and Na1-O6 2.339(4).
Fig. 2. Molecular structure of 2 as 20% ellipsoids (methyl carbons of the tert-butyl groups and all of the hydrogen atoms are omitted for clarity). Selected bond lengths (Å): Al1-O1 1.682(8), Al1-O2 1.767(8), Al1-O3 1.718(9), Al1-O4 1.770(8), Li1-O2 2.07 (3), Li1-O4 1.94(3), Li1-O5 2.13(3), Li1-O6 1.93(2), C1-O1 1.334(12), C13-O2 1.409 (13), C31-O3 1.372(13) and C43-O4 1.352(13).
X. Pan et al. / Inorganic Chemistry Communications 14 (2011) 763–766
Fig. 3. Molecular structure of 4 as 20% ellipsoids (methyl carbons of the tert-butyl groups and all of the hydrogen atoms are omitted for clarity, except for the hydroxy of benzyl alcohol). Selected bond lengths (Å): Al1-O1 1.707(8), Al1-O2 1.725(8), Al1-O3 1.811(9), Al1-O4 1.701(8), Na1-O1 2.309(10), Na1-O3 2.459(10), Na1-O5 2.271(15), Na1-O6 2.37(2), C1-O13 1.402(9), C1-O2 1.367(8), C31-O3 1.376(8) and C43-O4 1.396(9).
lithium/sodium. The structure of 2 is somewhat different from that of 4, in complex 2 two THF molecules coordinates to Li1, and in complex 4 two benzyl alcohol molecules coordinate to Na1 instead of THF. Ring-opening polymerization of ε-caprolactone (ε-CL) employing 1 as an initiator is systematically examined under a dry nitrogen atmosphere. Conversion of CL is determined on the basis of 1H NMR spectroscopic studies. The molecular weight and polydispersity of PCL are measured by gel permeation chromatography (GPC). A typical polymerization experiment exemplified for PCL-94 ([M]0/ [I]0 = 94) is described as follows. To a rapidly stirring solution of 1 in toluene was added ε-CL (0.20 mL, 1.85 mmol). After the reaction was quenched by the addition of excess 0.35 N acetic acid aqueous solutions, the polymer was precipitated into n-hexane. Polymerizations of ε-CL under same reaction conditions (entries 1–4) have been systematically examined as shown in Table 1. It was found that the PDIs of polyesters initiated by 1 range from 1.40 to 1.47, and a linear relationship between the number average molecular weight (Mn) and the monomer-to-initiator ratio ([M]0/[I]0) exists as shown in Fig. 4 (Table 1, entries 1–4). While in these experiments, the experimental value of Mn (Mn(obsd)) obtained from the GPC analysis is generally lower than the theoretical Mn value (calculated on the assumption that each BnOH initiates the polymerization) owe to the occurrence of the back-biting reaction. The cyclic polymerization side reactions were also found for the lactide
765
Fig. 4. Polymerization of ε-CL catalyzed by 1 in toluene at 120 °C. The relationship between Mn(■) (PDI (●)) of the polymer and the initial mole ratio [M]0/[complex]0 is shown.
polymerization with the catalysts published. [11] Analysis of PCL47 (Table 1, entry 1) produced from 1 at an initial [M]0/[I]0 ratio of 47 by 1H NMR (Fig. 5) shows a characteristic benzyl peak Hb at 5.11 ppm and hydroxy end group peak Hg at 3.65 ppm in CDCl3, indicating polymers are linear capped with two end groups of the benzyl alkoxy and hydroxy group respectively. In fact the complexes 2, 3, 4 [12] also were applied to the ROP of εCaprolactone, the experimental results show they are not active catalysts which can be attributed to the low basic complexes and cannot activate BnOH to initiate the ROP reaction. In conclusion, a new hetero-bimetallic complex has been synthesized and characterized. The hetero-bimetallic complex 1 is an efficient catalyst for ring-opening polymerization of lactone.
Acknowledgments We thank the financial supports from the National Natural Science Foundation of China (No. 21071069 and 20601011), Science Foundation of Gansu Province of China (0803RJZA103), Scientific Research Foundation for the Returned Overseas Chinese Scholars and the Fundamental Research Funds of the Central Universities of China, State Education Ministry (lzujbky-2009-26).
Appendix A. Supplementary material Supplementary data to this article can be found online at doi:10.1016/j.inoche.2011.03.003.
Table 1 Ring-opening polymerization of ε-caprolactone catalyzed by 1a. Entry
[CL]0/[Complex]0
Time (h)
PDI
Mn (obsd)b
Mn (calcd)c
Conversion (%)
1 2 3 4
47 94 141 188
16 16 16 16
1.42 1.47 1.43 1.40
9170 9887 11,368 16,600
5400 10,300 15,200 19,600
98 95 94 91
a b c
(5300) (6500) (7600) (9600)
Conditions: 0.02 mmol of complex, 10 mL of toluene, 120 °C reflux. Obtained from GPC analysis, and calibrated by polystyrene standard (corrected values listed in brackets). Calculated from the molecular weight of ε-CL times [CL]0/[Complex]0 times conversion yield plus the molecular weight of BnOH.
766
X. Pan et al. / Inorganic Chemistry Communications 14 (2011) 763–766
Fig. 5. 1H NMR of PCL-47 (from Table 1, entry 1) in CDCl3.
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