- Email: [email protected]
Aluminum complexes with benzoxazolphenolate ligands: Synthesis, characterization and catalytic properties for ring-opening polymerization of cyclic esters
Accepted Manuscript Aluminum complexes with benzoxazolphenolate ligands: synthesis, characterization and catalytic properties for ring-opening polymerization of cyclic esters Yunping Zhang, Aihong Gao, Yongfang Zhang, Zhenghe Xu, Wei Yao PII: DOI: Reference:
S0277-5387(16)30032-8 http://dx.doi.org/10.1016/j.poly.2016.03.042 POLY 11907
To appear in:
Polyhedron
Received Date: Accepted Date:
15 February 2016 16 March 2016
Please cite this article as: Y. Zhang, A. Gao, Y. Zhang, Z. Xu, W. Yao, Aluminum complexes with benzoxazolphenolate ligands: synthesis, characterization and catalytic properties for ring-opening polymerization of cyclic esters, Polyhedron (2016), doi: http://dx.doi.org/10.1016/j.poly.2016.03.042
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Aluminum complexes with benzoxazolphenolate ligands: synthesis, characterization
and
catalytic
properties
for
ring-opening
polymerization of cyclic esters Yunping Zhang, Aihong Gao, Yongfang Zhang, Zhenghe Xu*, Wei Yao* School of Resources and Environment, University of Jinan, Jinan 250022, P. R. China
Tel: (86)-531-82769233 Fax: (86)-531-82769233 E-mail: [email protected]
Aluminum complexes with benzoxazolphenolate ligands: synthesis, characterization
and
catalytic
properties
for
ring-opening
polymerization of cyclic esters
Abstract A family of bidentate ligands, 1-3, were synthesized by the reactions of 5-diethylamino-2-nitrosophenol with bromomethylphenol derivatives. Treatment of the ligands 1-3 with one equivalent of AlMe3 gave the corresponding aluminum complexes 4-6, respectively. All the complexes were characterized by 1 H and
13
C NMR
spectroscopy, along with elemental analysis. Single crystal X-ray diffraction analysis of complex 5 revealed that the Al complex has a distorted tetrahedral geometry around the metal center. All the aluminum complexes are efficient initiators for the ring-opening polymerization of ε-caprolactone and lactide in the presence of benzyl alcohol in a living fashion. The aluminum complexes produced PLA with a slight heterotactic bias.
Keywords: Aluminum complex; Benzoxazolphenol; Ring-opening polymerization; Cyclic ester
1. Introduction Poly(ε-caprolactone) (PCL) and polylactide (PLA), as well as their copolymers, are interesting alternative candidates to traditional petrochemical-base polymers, due to their biodegradability and biocompatibility [1,2]. The most promising methodology for the synthesis of these polymers is the ring-opening polymerization (ROP) of cyclic esters initiated by metal-based catalysts, such as Sn [3-8], Al [9-39], Zn [40-49], Mg [50-59], Fe [60-66], Ti [67-76], In [77-81] and rare-earth metals [82-87] with ancillary ligands. Among the reported catalysts, aluminum catalysts have been studied the most
for the ROP of cyclic esters because of their great controllability over the molecular weight and distribution of the polymers by modification of the ancillary ligands [88-92].
Recent
research
indicates
that
aluminum
complexes
with
phenoxy-imine/amine ligands acting as bulky monoanionic ligands have attracted great attention for their successful application in the ROP reaction [9-12]. Nomura’s group reported a facile and efficient system for the ROP of ε-caprolactone (ε-CL) at room temperature using a salicylaldimine-Al complex [9]. In addition, Pappalardo’s group reported the synthesis of dimethyl (salicylaldiminato) aluminum complexes and their reactivity toward the ROP of ε-CL and lactide (LA) [10]. Therefore, some aluminum complexes bearing a similar structure with phenoxy-imine/amine ligands were synthesized and their catalytic properties for the ROP reaction have been investigated. Ko’s
group
showed
that
aluminum
complexes
bearing
sterically
bulky
benzotriazole-phenoxide ligands were efficient catalysts for the ROP of L-LA [13]. Wang’s group also showed that aluminum complexes supported by functionalized phenolate ligands were active in the polymerization of rac-LA [14]. According to the catalytic properties for aluminum complexes bearing bulky monoanionic ligands in the ROP reaction, Al complexes with benzoxazolphenolate ligands were expected to be efficient catalysts in the ROP reaction. In this paper, a family of benzoxazolphenolate ligands and their aluminum complexes have been synthesized. Additionally, their catalytic properties for the ROP of ε-CL and LA have also been examined.
2. Results and Discussion 2.1. Synthesis and characterization The synthesis of the ligands 1-3 and their aluminum complexes 4-6 are presented in Scheme 1. The ligands 1-3 were synthesized according to a literature procedure by the reaction of 5-diethylamino-2-nitrosophenol with the corresponding 2-bromomethylphenol in THF [93]. The ligands 1-3 were characterized by 1H and
13
C NMR
spectra. The NMR spectral signals were consistent with the respective structures. The proton signals of the OH groups of 1-3 appeared in the low field region (δ 11.48-12.29 ppm), showing the existence of intramolecular hydrogen bonds. Treatment of 1-3 with
one equivalent of AlMe3 in toluene afforded the aluminum complexes 4-6 through an alkane elimination reaction. The aluminum complexes 4-6 were also characterized by 1
H and 13C NMR spectra and elemental analyses. The absence of the O–H signal of the
ligands and the appearance of new protons signals for AlMe2 (δ -0.63 to -0.79 ppm) demonstrated the formation of the desired aluminum complexes. Crystals of complex 5 suitable for X-ray crystal structure determination were crystallized from n-hexane at room temperature. Its molecular structure is shown in Fig. 1 with selected bond lengths and angles. X-Ray analysis reveals that the Al centre adopts a slightly distorted tetrahedral geometry, as evidenced by the bond angles for O1–Al1–N1
92.33(1)o,
N1–Al1–C19
105.95(2)o,
C18–Al1–N1
111.18(2)o,
C18–Al1–O1 114.23 (2)o, O1–Al1–C19 114.93(2)o and C18–Al1–C19 115.40(2)o. The six-membered chelating ring is nearly planar, with the aluminum atom lying out of the plane by 0.0894 Å. The torsion angles between the six-membered chelating ring and C19–Al1–C18 is 93.1(2)o. The Al–O bond length [1.782(2) Å] is slight longer than those in salicylaldiminato aluminum complexes [1.7688(2)-1.7792(1) Å]. In contrast, the Al–N bond length of complex 5 [1.960(3) Å] is slight shorter as compared with the salicylaldiminato aluminum complexes [1.9664(1)-1.9896(1) Å] [9-12].There is, however, no significant differences observed in the Al–C bond distances. 2.2. ROP of ε-CL using complexes 4-6 The catalytic performances of complexes 4-6 toward the polymerization of ε-CL were investigated and the results are shown in Table 1. All the aluminum complexes 4-6 showed high activities for the ring-opening polymerization of ε-CL in the presence of benzyl alcohol (BnOH) (Table 1, entries 1-6). The highest catalytic activity was obtained with a BnOH/Al molar ratio of 2:1 (Table 1, entries 2, 5 and 7-8). Under the same conditions, the activities of the complexes were compared with each other. The catalytic activity decreased when the bromine groups at the 3 and 5 position on the phenol ring were replaced by methyl groups (Table 1, entries 4 and 5). Additionally, entry 6 shown that catalytic activity was reduced upon the addition of tertiary butyl groups to the phenol backbone. This could be due to the steric and electronic effects of the substituents. In comparison with related known catalytic systems, the activities of
complexes 4-6 (turnover frequency (TOF): 441.4, 735.0 and 582.0 h–1) were lower than those of the phenolate Al catalyst systems (TOF: 7698.6 h–1) [9]. To investigate the catalytic properties for the ROP of ε-CL, the catalytic precursor 5 was studied under different reaction conditions. A linear relationship between the number average molecular weight (Mn) and ([ε-CL]0/[BnOH]0) indicated the "living" character of the polymerization process, as shown in Fig. 2 (Table 1, entries 5, 9-12). The effect of the BnOH/Al molar ratio on the ROP of ε-CL was also examined (Table 1, entries 5, 7-8). The results imply that the Al complexes initiate the polymerization reaction in a controlled fashion. Remarkably, complex 5 was tolerant to the protic BnOH. The addition of 10 equivalents of BnOH did not degrade the complex (Fig. 3). The 1H NMR spectrum of PCL-50 prepared from ε-CL and 5 ([ε-CL]0/[BnOH]0 = 50) for chain-end studies is shown in Fig. 4. The polymer chain should be capped with one benzyl ester and one hydroxy chain end. 2.3. ROP of rac-LA using complexes 4-6 Complexes 4-6 were also examined with regard to their catalytic behavior in the ROP of LA. The ROP was carried out in toluene in the presence of 2.0 equivalents of BnOH. The results are summarized in Table 2. The experimental results show complexes 4-6 are efficient catalysts for the polymerization of L-LA, giving a linear PLA which is capped with one benzyl ester and one hydroxyl end (Fig. 5). For complexes 4-6, the activity is comparable to Al complexes bearing salicylaldimine ligands (TOF: 4.1, 9.4 and 5.6 h–1) [11]. Complex 5 shows a relatively higher activity (Table 2, entry 2, TOF: 9.8 h–1) than those of complexes 4 and 6 (Table 2, entries 1 and 3, TOF: 6.1 and 6.5 h–1). The same trend was found in the ROP of ε-CL. Complex 5 was selected to evaluate the effect of LA/BnOH on the activity and polymer molecular weights. A plot of Mn versus ([L-LA]0/[Al]0) initiated by 5 exhibited a linear relationship. This demonstrates the "living" character of the polymerization process, as shown in Fig. 6 (Table 2, entries 2, 4-7). Poly(rac-LA) with homonuclear decoupled 1H NMR spectra of the methine region were also investigated. The Pr value indicates that these polymer chains are partially heterotactic (Fig. 7) [19]. It was found that complex 5 has the highest Pr value (0.64)
compared to complexes 4 and 6 (Table 2, entries 8-10) at 30 oC. These results indicate that increasing the electron withdrawing ability of the substituents enhances the catalytic activity. Additionally, the tacticity of the poly(rac-LA) is significantly influenced by the reaction temperature. Upon elevating the temperature from 30 to 70 °C, the Pr value decreased from 0.64 to 0.57 (Table 2, entries 9 and 13).
3. Experimental Section 3.1. General procedures All reactions and manipulations which related to the preparation of the metal complexes were carried out under nitrogen with standard Schlenk-line or glovebox techniques. All solvents were refluxed over the appropriate drying agent and distilled prior to use. 1H and
13
C NMR spectra were measured using a Bruker AVANCE-400
NMR spectrometer. The elemental analyses were performed on a Perkin-Elmer 2400 analyzer. Gel permeation chromatography (GPC) measurement was performed on a TOSOH HLC 8220 GPC at 40 oC using THF as the eluent against polystyrene standards. 3.2. Ring-opening polymerization of cyclic esters In a typical polymerization experiment, the aluminum complex, the required amount of lactone and BnOH in toluene were loaded in a flame-dried vessel containing a magnetic bar. The vessel was placed in an oil bath. After a certain reaction time, the polymer was isolated by precipitation with cold methanol. The precipitate was collected and dried under vacuum at 40 oC for 24 h. For some polymerization reactions, samples were taken for determining the monomer conversion by
1
H NMR
spectroscopy during the reaction. 3.3.1. Synthesis of ligand 1 A solution of 2-bromomethyl-4,6-dimethylphenol (2.31 g, 10.70 mmol) and 5-diethylamino-2-nitrosophenol (2.08 g, 10.70 mmol) in THF (15 mL) in the presence of K2CO3 (1.48 g, 10.70 mmol) was heated to reflux for 12 h and filtered. The THF was evaporated under reduced pressure and the residual material was purified by flash chromatography eluting with 1:4 EA/hexane to afford ligand 1 as a yellow oil. Yield:
1.32 g (65%). Anal. Calcd. for C19H22N2O2 (310.39): C, 73.52; H, 7.14; N, 9.03. Found: C, 73.58; H, 7.19; N, 9.48%. 1H NMR (400 MHz, CDCl3, 293 K) δ, ppm: 11.48 (s, 1H, OH), 7.61 (s, 1H, Ar–H), 7.49 (d, J = 6.4 Hz, 1H, Ar–H), 7.07 (s, 1H, Ar–H), 6.83 (s, 1H, Ar–H), 6.74 (d, J = 6.4 Hz, 1H, Ar–H), 3.42 (q, J = 6.9 Hz, 4H, CH2CH3), 2.30 (s, 3H, CCH3), 2.32 (s, 3H, CCH3), 1.22 (t, J = 6.9 Hz, 6H, CH2CH3).
13
C NMR (100
MHz, CDCl3, 293 K) δ, ppm: 160.7, 154.1, 151.2, 146.9, 134.4, 130.1, 127.8, 125.8, 123.8, 119.0, 110.3, 110.1, 93.0, 45.1, 20.5, 15.9, 12.4. 3.3.2. Synthesis of ligand 2 Ligand 2 was synthesized according to the same procedure as for that of 1, but 2,4-dibromo-6-bromomethylphenol (3.69 g, 10.70 mmol) was used instead of 2-bromomethyl-4,6-dimethylphenol. After a similar work-up, ligand 2 was obtained as a yellow solid. Yield: 1.53 g (67%). Anal. Calcd. for C17H16Br2N2O2 (440.13): C, 46.39; H, 3.66; N, 6.36. Found: C, 46.46; H, 3.72; N, 6.48%. 1H NMR (400 MHz, CDCl3, 293 K) δ, ppm: 12.29 (s, 1H, OH), 8.00 (s, 1H, Ar–H), 7.71 (s, 1H, Ar–H), 7.50 (d, J = 8.1 Hz, 1H, Ar–H), 6.79 (m, 2H, Ar–H), 3.43 (q, J = 6.9 Hz, 4H, CH2CH3), 1.23 (t, J = 6.9 Hz, 6H, CH2CH3).
13
C NMR (100 MHz, CDCl3, 293 K) δ, ppm: 157.9, 153.7, 151.6,
147.5, 137.2, 129.2, 127.8, 119.6, 113.4, 111.8, 111.0, 110.9, 92.5, 45.1, 12.5. 3.3.3. Synthesis of ligand 3 Ligand 3 was synthesized according to the same procedure as for that of 1, but 2-bromomethyl-4,6-di-tert-butylphenol (3.21 g, 10.70 mmol) was used instead of 2-bromomethyl-4,6-dimethylphenol. After a similar work-up, ligand 3 was obtained as a yellow solid. Yield: 1.31 g (63%). Anal. Calcd. for C25H34N2O2 (394.55): C, 76.10; H, 8.69; N, 7.10. Found: C, 76.19; H, 8.75; N, 7.19%. 1H NMR (400 MHz, CDCl3, 293 K) δ, ppm: 11.95 (s, 1H, OH), 7.50 (d, J = 8.0 Hz, 1H, Ar–H), 7.19 (s, 1H, Ar–H), 7.16 (s, 1H, Ar–H), 6.87 (s, 1H, Ar–H), 6.73 (d, J = 8.0 Hz, 1H, Ar–H), 3.42 (q, J = 6.4 Hz, 4H, CH2CH3), 1.49 (s, 9H, C(CH3)3), 1.37 (s, 9H, C(CH3)3), 1.28 (t, J = 6.4 Hz, 6H, CH2CH3).
13
C NMR (100 MHz, CDCl3, 293K) δ, ppm: 154.9, 149.8, 142.9, 140.8,
136.8, 126.1, 125.2, 122.5, 122.2, 120.7, 119.5, 118.9, 90.5, 45.1, 35.2, 34.6, 31.5, 30.0, 29.7, 29.5, 12.4. 3.3.4. Synthesis of complex 4
AlMe3 (1.27 mL, 1.00 M in toluene, 1.27 mmol) was added dropwise to a solution of 1 (0.36 g, 1.27 mmol) in toluene (15 mL) at about 0 oC with stirring. The reaction mixture was allowed to warm to room temperature and was stirred at 70 oC for 12 h. After removal of the solvent in vacuo, the product was recrystallized from n-hexane to give complex 4 as yellow solid. Yield: 0.50 g (85%). Anal. Calcd. for C21H27AlN2O2 (366.43): C, 68.83; H, 7.43; N, 7.64. Found: C, 68.75; H, 7.38; N, 7.50%. 1H NMR (400 MHz, CDCl3, 293K) δ, ppm: 7.55 (s, 1H, Ar–H), 7.51 (d, J = 9.0 Hz, 1H, Ar–H), 7.25 (s, 1H, Ar–H), 6.95 (s, 1H, Ar–H), 6.91 (d, J = 9.0 Hz, 1H, Ar–H), 3.46 (q, J = 7.1 Hz, 4H, CH2CH3), 2.37 (s, 3H, CCH3), 2.35 (s, 3H, CCH3), 1.24 (t, J = 7.1 Hz, 6H, CH2CH3), -0.79 (s, 6H, AlCH3).
13
C NMR (100 MHz, CDCl3, 293 K) δ, ppm: 162.0,
151.4, 151.1, 147.4, 137.6, 133.3, 133.1, 125.5, 125.5, 117.1, 114.8, 113.8, 95.3, 45.9, 20.6, 17.4, 11.8, -7.3. 3.3.5. Synthesis of complex 5 Complex 5 was synthesized according to the same procedure as for that of 4, but 2 (0.79 g, 1.80 mmol) was used instead of 1. After a similar work-up, complex 5 was obtained as a yellow solid. Yield: 0.83 g (81%). Anal. Calcd. for C19H21AlBr2N2O2 (496.17): C, 45.99; H, 4.27; N, 5.65. Found: C, 45.95; H, 4.21; N, 5.69%. 1H NMR (400 MHz, CDCl3, 293 K) δ, ppm: 7.99 (s, 1H, Ar–H), 7.80 (s, 1H, Ar–H), 7.44 (d, J = 9.3 Hz, 1H, Ar–H), 6.83–6.78 (m, 2H, Ar–H), 3.44 (q, J = 7.1 Hz, 4H, CH2CH3), 1.23 (t, J = 7.1 Hz, 6H, CH2CH3), -0.63 (s, 6H, AlCH3). 13C NMR (101 MHz, CDCl3, 293 K) δ, ppm: 156.0, 157.9, 151.4, 148.2, 139.7, 128.5, 124.1, 117.2, 117.0, 112.4, 111.7, 108.3, 92.4, 45.2, 12.4, -9.7. 3.3.6. Synthesis of complex 6 Complex 6 was synthesized according to the same procedure as for that of 4, but 3 (0.56 g, 1.42 mmol) was used instead of 1. After a similar work-up, complex 6 was obtained as a yellow solid. Yield: 0.54 g (76%). Anal. Calcd. for C27H39AlN2O2 (450.59): C, 71.97; H, 8.72; N, 6.22. Found: C, 71.90; H, 8.76; N, 6.28%. 1H NMR (400 MHz, CDCl3, 293 K) δ, ppm: 7.79 (s, 1H, Ar–H), 7.53 (s, 1H, Ar–H), 7.45 (d, J = 9.0 Hz, 1H, Ar–H), 7.05 (s, 1H, Ar–H), 6.92 (d, J = 9.0 Hz, 1H, Ar–H), 3.46 (q, J = 7.1 Hz, 4H, CH2CH3), 1.45 (s, 9H, CCH3), 1.36 (s, 9H, CCH3), 1.23 (t, J = 7.1 Hz, 6H,
CH2CH3), -0.66 (s, 6H, AlCH3). 13C NMR (100 MHz, CDCl3, 293 K): δ, ppm: 165.1, 160.6, 150.1, 145.7, 140.6, 139.3, 130.9, 124.7, 121.3, 119.3, 116.2, 109.5, 98.3, 46.9, 35.5, 34.3, 32.7, 31.4, 29.7, 29.4, 11.3, -9.9. 3.4. X-ray crystallography Diffraction data of complex 5 were collected at 120 K with an Xcalibur, Eos, Gemini diffractometer equipped with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). Using Olex2 [95], the structure was solved with the ShelXS [96] structure solution program using direct methods and refined with the ShelXL [96] refinement package using least squares minimisation.
4. Conclusions The benzoxazolphenolate ligands and their aluminum complexes have been synthesized and fully characterized by 1H and 13C NMR spectra and elemental analyses. The structure of complex 5 was confirmed by X-ray crystallography. The structure reveals that the Al complex has a distorted tetrahedral geometry around the metal center. All of the aluminum complexes were tested in the ROP of ε-CL and LA. The complexes showed high activity in the presence of BnOH. Microstructural analysis of the poly(rac-LA) catalyzed by these complexes revealed that ligands 1-3 have the ability to control the tacticity of the PLA. The substituents on the ligand affect the tacticities of the poly(rac-LA).
Acknowledgments We are grateful for financial support from the National Natural Science Foundation of China (No. 21104026), Public Special Ministry of Water Resources for research on the comprehensive utilization of water resources in coastal and most strict water resources management model (201201116), Shandong Province Water Conservancy research and technology promotion projects (SDSLKY201306), Shandong Agriculture Major Application of Technology Innovation for research on key technology of
comprehensive management of small ecological watershed (2011139), Shandong Province of Major Application of Technology Innovation for research and demonstration of water quality evaluation and risk management of Jinxiuchuan Reservoir (SDSLKY201229)
Appendix A. Supplementary data CCDC 973210 contains the supplementary crystallographic data for complex 5. These
data
can
be
obtained
http://www.ccdc.cam.ac.uk/conts/retrieving.html
free or
of from
charge the
via
Cambridge
Crystallographic Data Center, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected]. References [1] G. Swift, Acc. Chem. Res. 26 (1993) 105. [2] R.E. Drumright, P.R. Gruber, D.E. Henton, Adv. Mater. 12 (2000) 1841. [3] L. Wang, C.E. Kefalidis, S. Sinbandhit, V. Dorcet, J.-F. Carpentier, L. Maron, Y. Sarazin, Chem. Eur. J. 19 (2013) 13463. [4] K.M. Stridsberg, M. Ryner, A.-C. Albertsson, Adv. Polym. Sci. 157 (2002) 41. [5] M. Lahcini, P.M. Castro, M. Kalmi, M. Leskelä, T. Repo, Organometallics 23 (2004) 4547. [6] A. Tullo, Chem. Eng. News. 3 (2000) 13. [7] W. Chen, H. Yang, R. Wang, R. Cheng, F. Meng, W. Wei, Z. Zhong, Macromolecules 43 (2010) 201. [8] V. Poirier, T. Roisnel, S. Sinbandhit, M. Bochmann, J.-F. Carpentier, Y. Sarazin, Chem. Eur. J. 18 (2012) 2998. [9] N. Nomura, T. Aoyama, R. Ishii, T. Kondo, Macromolecules 38 (2005) 5363. [10] D. Pappalardo, L. Annunziata, C. Pellecchia, Macromolecules 42 (2009) 6056. [11] W. Zhang, Y. Wang, W.-H. Sun, L. Wang, C. Redshaw, Dalton Trans. 41 (2010) 11587. [12] J. Liu, N. Iwasa, K. Nomura, Dalton Trans. (2008) 3978.
[13] W.-J. Tai, C.-Y. Li, P.-H. Lin, J.-Y. Li, M.-J. Chen, B.-T. Ko, Appl. Organometal. Chem. 26 (2012) 518 [14] C. Zhang, Z.-X. Wang, J. Organomet. Chem. 693 (2008) 3151. [15] K. Matsubara, C. Terata, H. Sekine, K. Yamatani, T. Harada, K. Eda, M. Dan, Y. Koga, M. Yasuniwa, J. Polym. Sci. Part A: Polym. Chem. 50 (2012) 957. [16] C.-Y. Li, C.-Y. Tsai, C.-H Lin, B.-T. Ko, Dalton Trans. 40 (2011) 1880. [17] J.-T. Issenhuth, J. Pluvinage, R. Welter, S. Bellemin-Laponnaz, S. Dagorne, Eur. J. Inorg. Chem. 2009 (2009) 4701. [18] C.-T. Chen, H.-J. Weng, M.-T. Chen, C.-A. Huang, K.-F. Peng, Eur. J. Inorg. Chem. 2009 (2009) 2129. [19] T.M. Ovitt, G.W. Coates, J. Am. Chem. Soc.124 (2002) 1316. [20] B. Gao, R. Duan, X. Pang, X. Li, Z. Tang, X. Zhuang, X. Chen, Organometallics 32 (2013) 5435. [21] S. Tabthong, T. Nanok, P. Kongsaeree, S. Prabpai, P. Hormnirun, Dalton Trans. 43 (2014) 1348. [22] C.-T. Chen, C.-H. Liao, K.-F. Peng, M.-T. Chen, T.-L. Huang, J. Organomet. Chem. 753 (2014) 9. [23] M.-C. Chang, W.-Y. Lu, H.-Y. Chang, Y.-C. Lai, M.Y. Chiang, H.-Y. Chen, H.-Y. Chen, Inorg. Chem. 54 (2015) 11292. [24] X. Pang, H. Du, X. Chen, X. Wang, X. Jing, Chem. Eur. J. 14 (2008) 3126. [25] A. Pilone, K. Press, I. Goldberg, M. Kol, M. Mazzeo, M. Lamberti, J. Am. Chem. Soc. 136 (2014) 2940. [26] C. Agatemor, A.E. Arnold, E.D. Cross, A. Decken, M.P. Shaver, J. Organomet. Chem. 335 (2013) 745. [27] Y. Gamze, K. Asgar, Appl. Catal. A. 433 (2012) 223. [28] S.L. Hancock, M.F. Mahon, M.D. Jones, Dalton Trans. 42 (2013) 9279. [29] D. Li, Y. Peng, C. Geng, K. Liu, D, Kong, Dalton Trans. 42 (2013) 11295. [30] F. Qian, K. Liu, H. Ma, Dalton Trans. 39 (2010) 8071. [31] A. Meduri, T. Fuoco, M. Lamberti, C. Pellecchia, D. Pappalardo, Macromolecules 47 (2014) 534.
[32] K. Bakthavachalam, N. D. Reddy, Organometallics 32 (2013) 3174. [33] M.-W. Hsiao, C.-C. Lin, Dalton Trans 42 (2013) 2041. [34] M.O. Miranda, Y. DePorre, H. Vazquez-Lima, M.A. Johnson, D.J. Marell, C.J. Cramer, W.B. Tolman, Inorg. Chem. 52 (2013) 13692. [35] L. Postigo, M.C. Maestre, M.E.G. Mosquera, T. Cuenca, G. Jiménez, Organometallics 32 (2013) 2618. [36] P. Hormnirun, E. L. Marshall, V.C. Gibson, A.J.P. White, D.J. Williams, J. Am. Chem. Soc. 126 (2004) 2688. [37] M.P.F. Pepels, M. Bouyahyi, A. Heise, R. Duchateau, Macromolecules 46 (2013) 4324. [38] C.T. Altaf, H. Wang, M. Keram, Y. Yang, H. Ma, Polyhedron 81 (2014) 11. [39] M.D. Jones, L. Brady, P. McKeown, A. Buchard, P.M. Schäfer, L.H. Thomas, M.F. Mahon, T.J. Woodman, J.P. Lowe, Chem. Sci. 6 (2015) 5034. [40] M. Rauch, Y. Rong, W. Sattler, G. Parkin, Polyhedron 103 (2016) 135. [41] A. Gao, W. Yao, Y. Xiao, M Zhang, G. Zhu, N. Zhang, S. Wang, D. Wang, Y.-Y Zhang, Y. Gao, Z. Xu, P. Lu, Z. Zhang, Polyhedron 85 (2015) 537. [42] X.-F. Yu, C. Zhang, Z.-X. Wang, Organometallics 32 (2013) 3262. [43] M.D. Jones, C.G. Keir, A.L. Johnson, M.F. Mahon, Polyhedron 29 (2010) 312. [44] L.R. Collins, L.A. Moffat, M.F. Mahon, M.D. Jones, M.K. Whittlesey, Polyhedron 103 (2016) 121. [45] M. Cheng, A.B. Attygalle, E.B. Lobkovsky, G.W. Coates, J. Am. Chem. Soc. 121 (1999) 11583. [46] W. Yao, Y. Mu, A. Gao, W. Gao, L. Ye, Dalton Trans. (2008) 3199. [47] K. Pang, Y. Rong, G. Parkin, Polyhedron 29 (2010) 1881. [48] R. Petrus, P. Sobota, Organometallics 31 (2012) 4755. [49] A. Gao, W. Yao, Y. Mu, W. Gao, M. Sun, Q. Su, Polyhedron 28 (2009) 2605. [50] H. Xie, Z. Mou, B. Liu, P. Li, W. Rong, S. Li, D. Cui, Organometallics 33 (2014) 722. [51] Y. Gao, Z. Dai, J. Zhang, X. Ma, N. Tang, J. Wu, Inorg. Chem. 53 (2014) 716. [52] A. Garces, L.F. Sanchez-Barba, J. Fernandez-Baeza, A. Otero, M. Honrado, A.
Lara-Sanchez, A.M. Rodriguez, Inorg. Chem. 52 (2013) 12691. [53] Y. Wang, W. Zhao, D. Liu, S. Li, X. Liu, D. Cui, X. Chen, Organometallics 31 (2012) 4182. [54] V. Balasanthiran, M.H. Chisholm, K. Choojun, C.B. Durr, P.M. Wambua, Polyhedron 103 (2016) 235. [55] T. Han, J. Utko, L. B. Jerzykiewicz, P. Sobota, Dalton Trans. 41 (2011) 12660. [56] W.-C. Hung, C.-C. Lin, Inorg. Chem. 48 (2009) 728. [57] J. Wu, B.-H. Huang, M.-L. Hsueh, S.-L. Lai, C.-C. Lin, Polymer 46 (2005) 9784. [58] L. Wang, H. Ma, Macromolecules 43 (2010) 6535 [59] H.-Y. Tang, H.-Y. Chen, J.-H. Huang, C.-C. Lin, Macromolecules 40 (2007) 8855. [60] A.B. Biernesser, B.Li, J.A. Byers, J. Am. Chem. Soc. 135 (2013) 16553. [61] B.J. O’Keefe, L.E. Breyfogle, M.A. Hillmyer, W.B. Tolman, J. Am. Chem. Soc. 124 (2002) 4384. [62] J.B. Chen, J.L. Gorczynski, G.Q. Zhang, C.L. Fraser, Macromolecules 43 (2010) 4909. [63] J. Chen, J.L. Gorczynski, C.L. Fraser, Macromol. Chem. Phys. 211 (2010) 1272. [64] Y.Y. Kang, H.-R. Park, M.H. Lee, J. An, Y. Kim, J. Lee, Polyhedron 95 (2015) 24. [65] X. Wang, K. Liao, D. Quan, Q. Wu, Macromolecules 38(2005) 4611. [66] D.S. Mcguinness, E.L. Marshall, V.C. Gibson, J.W. Steed, J. Polym. Sci. Part A: Polym. Chem. 41 (2003) 3798. [67] L.-C. Liang, S.-T. Lin, C.-C. Chien, Inorg. Chem. 52 (2013) 1780. [68] F. Zhang, H. Song, G. Zi, J. Organomet. Chem. 695 (2010) 1993. [69] S.K. Russell, C.L. Gamble, K.J. Gibbins, K.C.S. Juhl, W.S. MitchellIII, A.J. Tumas, G.E. Hofmeister, Macromolecules 38 (2005) 10336. [70] S.H. Kim, J. Lee, D.J. Kim, J.H. Moon, S. Yoon, H.J. Oh, Y. Do, Y.S. Ko, J.-H. Yim, Y. Kim, J. Organomet. Chem. 694 (2009) 3409. [71] Y. Kim, J.G. Verkade, Organometallics 21 (2002) 2395. [72] T.W. Graham, C. Ong, P. Wei, D.W. Stephan, J. Organomet. Chem. 692 (2007) 4481. [73] Y. Kim, J.G. Verkade, Macromol. Rapid Commun. 23 (2002) 917.
[74] S. Park, Y. Do, J. Organomet. Chem. 692 (2007) 1633. [75] H.-Y. Chen, M.-Y. Liu, A.-K. Sutar, C.-C. Lin, Inorg. Chem. 49 (2010) 665. [76] M.J. Go, J.M. Lee, K.M. Lee, C.H. Oh, K.H. Park, S.H. Kim, M. Kim, H.-R. Park, M.H. Park, Y. Kim, J. Lee, Polyhedron 67 (2014) 286. [77] M. Normand, V. Dorcet, E. Kirillov, J.-F. Carpentier, Organometallics 32 (2013) 1694. [78] M.P. Blake, A.D. Schwarz, P. Mountford, Organometallics 30 (2011) 1202. [79] L.E.N. Allan, G.G. Briand, A. Decken, J.D. Marks, M.P. Shaver, R.G. Wareham, J. Organomet. Chem. 736 (2013) 55. [80] A. Pietrangelo, M.A. Hillmyer, W.B. Tolman, Chem. Commun. (2009) 2736. [81] I. Yu, A. Acosta-Ramírez, P. Mehrkhodavandi, J. Am. Chem. Soc. 134 (2012) 12758. [82] G.G. Skvortsov, G.K. Fukin, S.Y. Ketkov, A.V. Cherkasov, K.A. Lyssenko, A.A. Trifonov, Eur. J. Inorg. Chem. 24 (2013) 4173. [83] B. Liu, T. Roisnel, L. Maron, J.-F. Carpentier, Y. Sarazin, Chem. Eur. J.19 (2013) 3986. [84] P. Benndorf, J. Kratsch, L. Hartenstein, C.M. Preuss, P.W. Roesky, Chem. Eur. J. 18 (2012) 14454. [85] W. Ren, L. Chen, N. Zhao, Q. Wang, G. Hou, G. Zi, J. Organomet. Chem. 758 (2014) 65. [86] W.-K Gu, P.-F. Xu, Y.-R Wang, Y.-M Yao, D. Yuan, Q. Shen, Organometallics 34 (2015) 2907. [87] B.-Y. Li, Y.-M. Yao, Y.-R. Wang, Y. Zhang, Q. Shen, Polyhedron 27 (2008) 709. [88] O. Dechy-Cabaret, B. Martin-Vaca, D. Bourissou, Chem. Rev. 104 (2004) 6147. [89] B.J. O'Keefe, M.A. Hillmyer, W.B. Tolman, J. Chem. Soc., Dalton Trans. 15 (2011) 2215. [90] R.H. Platel, L.M. Hodgson, C.K. Williams, Polym. Rev. 48 (2008) 11. [91] M.J. Stanford, A.P. Dove, Chem. Soc. Rev. 39 (2010) 486. [92] C.M. Thomas, Chem. Soc. Rev. 39 (2010) 165. [93] W. Yao, D. Huang, Org. Lett. 12 (2010) 736.
[94] A. Kowalski, A. Duda, S. Penczek, Macromolecules 31 (1988) 2114. [95] O.V. Dolomanov, L.J. Bourhis, R.J. Gildea, J.A.K. Howard, H. Puschmann, J. Appl. Cryst. 42 (2009) 339. [96] G.M. Sheldrick, SHELXS, Acta. Cryst. A64 (2008) 112.
Scheme 1. Synthesis of complexes 1-6.
Fig. 1. The molecular structure of complex 5. Thermal ellipsoids are shown at the 30% probability level. Hydrogen atoms and solvent are omitted for clarity. Selected bond lengths [Å] and angles [o]: Al1–N1 1.960(3), Al1–O1 1.782(2), Al1–C18 1.950(4), Al1–C19 1.958(4), N1–C7 1.310(4), O1–C1 1.319(4), O1–Al1–N1 92.33(1), N1–Al1–C19 105.95(2), C18–Al1–N1 111.18(2), C18–Al1–O1 114.23(2), O1–Al1–C19 114.93(2), C18–Al1–C19 115.40(2).
Fig. 2. Plot of Mn versus [ε-CL]0/[BnOH]0 for the polymerization of ε-CL by complex 5.
Fig. 3. 1H NMR spectra of ligand 2 (top) and complex 5 (middle), the reaction mixture of complex 5 with 10 molar equivalents of BnOH (bottom).
Fig. 4. 1H NMR spectrum of a typical PCL sample.
Fig. 5. 1H NMR spectrum of a typical PLA sample.
Fig. 6. Plot of Mn versus [L-LA]0/[BnOH]0 for the polymerization of L-LA by complex 5.
Fig. 7. Homonuclear decoupled 1H NMR spectra of the methine region of poly(rac-LA) ((right) by complex 4 Pr = 0.55, Table 2, entry 12; (middle) by complex 5, Pr = 0.64, Table 3, entry 13; (left) by complex 6, Pr = 0.58, Table 3, entry 14, 400 MHz, CDCl3)
Table 1. Ring-opening polymerization of ε-CL catalyzed by complexes 4-6a. Entry
Cat.
[BnOH]0/ [Al]0/[ε-CL]
1 2 3 4 5 6 7 8 9 10 11 12
4 5 6 4 5 6 5 5 5 5 5 5
0:1:100 0:1:100 0:1:100 2:1:100 2:1:100 2:1:100 1:1:100 4:1:100 2:1:150 2:1:200 2:1:300 2:1:350
t (min)
Convb (%)
TOFc (h-1)
M n.calcdd (×103 )
Mne (×103 )
60 60 60 14 8 10 13 15 13 15 20 26
trace trace trace 96 98 97 97 95 96 97 98 97
− − − 441.4 735.0 582.0 447.7 380.0 664.6 776.0 882.0 783.5
− − − 5.6 5.7 5.6 11.2 2.8 8.3 11.2 16.9 19.5
− − − 6.9 7.0 7.1 12.1 4.0 9.1 10.6 15.3 16.8
0
PDIe
− − − 1.14 1.10 1.12 1.19 1.23 1.11 1.19 1.13 1.16
a
Polymerization conditions: catalyst, 30 µ mol; [ε-CL]0 = 0.5 M in toluene; a N2 atmosphere; at 70
o
C. bDetemined by 1H NMR spectra. cMole of monomer consumed per mole of catalyst per hour.
d
Calculated from ([ε-CL]0/[BnOH]0) × conversion × 114.14+108). eDetermined by GPC against
polystyrene standards in THF, multiplied by 0.58 [94].
Table 2. Ring-opening polymerization of lactides catalyzed by complexes 4-6a.
a
Entry
Cat.
1 2 3 4 5 6 7 8 9 10 11 12 13 14
4 5 6 5 5 5 5 4 5 6 5 4 5 6
[BnOH]0/ [Al]0 /[M]0 2:1:100 2:1:100 2:1:100 2:1:150 2:1:200 2:1:300 2:1:350 2:1:100 2:1:100 2:1:100 2:1:100 2:1:100 2:1:100 2:1:100
M L-LA L-LA L-LA L-LA L-LA L-LA L-LA rac-LA rac-LA rac-LA rac-LA rac-LA rac-LA rac-LA
T (oC) 70 70 70 70 70 70 70 70 70 70 50 30 30 30
t (h) 16 10 15 13 16 19 23 16 10 15 36 60 60 60
Convb (%) 97 98 98 96 97 98 95 96 97 97 93 89 91 90
TOFc (h-1 ) 6.1 9.8 6.5 11.1 12.1 15.5 14.5 6.0 9.7 6.5 2.6 1.5 1.5 1.5
Mn.calcdd (×103 ) 7.1 7.2 7.2 10.5 14.1 21.3 24.1 7.0 7.1 7.1 6.8 6.5 6.7 6.6
Mne (×103 ) 7.5 7.7 7.7 11.8 15.6 22.1 27.2 7.7 7.9 7.8 7.6 7.2 7.6 7.6
PDIe 1.04 1.05 1.09 1.13 1.15 1.14 1.13 1.08 1.10 1.11 1.04 1.07 1.05 1.10
Prf − − − − − − − 0.52 0.57 0.53 0.60 0.55 0.64 0.58
Polymerization conditions: catalyst, 30 µmol; [LA]0 = 0.5 M in toluene; a N2 atmosphere. Detemined by 1H NMR spectra. cMole of monomer consumed per mole of catalyst per hour. d Calculated from (([LA]0/[BnOH]0 ) × conversion × 144.13+108). eDetermined by GPC against polystyrene standards in THF, multiplied by 0.58 [94]. fPr is the probability of racemic linkages between monomer units determined by homonuclear decoupled 1H NMR spectroscopy [19]. b
Aluminum complexes with benzoxazolphenolate ligands: synthesis, characterization
and
catalytic
properties
for
ring-opening
polymerization of cyclic esters Yunping Zhang, Aihong Gao, Yongfang Zhang, Zhenghe Xu*, Wei Yao* School of Resources and Environment, University of Jinan, Jinan 250022, P. R. China
O O
O
cat HO
O
O
BnOH
On O O O
O
O
cat HO
O
O
BnOH
O
On
O
O NEt2 cat: O N Al R
O
R
O
4: R = Me 5: R = Br 6: R = tBu
Aluminum complexes with benzoxazolphenolate ligands: synthesis, characterization
and
catalytic
properties
for
ring-opening
polymerization of cyclic esters Yunping Zhang, Aihong Gao, Yongfang Zhang, Zhenghe Xu*, Wei Yao* School of Resources and Environment, University of Jinan, Jinan 250022, P. R. China
Benzoxazolphenolate ligands and their aluminum complexes have been synthesized and fully characterized. All of the aluminum complexes were tested in the ring-opening polymerization of cyclic esters. The aluminum complexes produced PLA with a slight heterotactic bias.
S0277-5387(16)30032-8 http://dx.doi.org/10.1016/j.poly.2016.03.042 POLY 11907
To appear in:
Polyhedron
Received Date: Accepted Date:
15 February 2016 16 March 2016
Please cite this article as: Y. Zhang, A. Gao, Y. Zhang, Z. Xu, W. Yao, Aluminum complexes with benzoxazolphenolate ligands: synthesis, characterization and catalytic properties for ring-opening polymerization of cyclic esters, Polyhedron (2016), doi: http://dx.doi.org/10.1016/j.poly.2016.03.042
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Aluminum complexes with benzoxazolphenolate ligands: synthesis, characterization
and
catalytic
properties
for
ring-opening
polymerization of cyclic esters Yunping Zhang, Aihong Gao, Yongfang Zhang, Zhenghe Xu*, Wei Yao* School of Resources and Environment, University of Jinan, Jinan 250022, P. R. China
Tel: (86)-531-82769233 Fax: (86)-531-82769233 E-mail: [email protected]
Aluminum complexes with benzoxazolphenolate ligands: synthesis, characterization
and
catalytic
properties
for
ring-opening
polymerization of cyclic esters
Abstract A family of bidentate ligands, 1-3, were synthesized by the reactions of 5-diethylamino-2-nitrosophenol with bromomethylphenol derivatives. Treatment of the ligands 1-3 with one equivalent of AlMe3 gave the corresponding aluminum complexes 4-6, respectively. All the complexes were characterized by 1 H and
13
C NMR
spectroscopy, along with elemental analysis. Single crystal X-ray diffraction analysis of complex 5 revealed that the Al complex has a distorted tetrahedral geometry around the metal center. All the aluminum complexes are efficient initiators for the ring-opening polymerization of ε-caprolactone and lactide in the presence of benzyl alcohol in a living fashion. The aluminum complexes produced PLA with a slight heterotactic bias.
Keywords: Aluminum complex; Benzoxazolphenol; Ring-opening polymerization; Cyclic ester
1. Introduction Poly(ε-caprolactone) (PCL) and polylactide (PLA), as well as their copolymers, are interesting alternative candidates to traditional petrochemical-base polymers, due to their biodegradability and biocompatibility [1,2]. The most promising methodology for the synthesis of these polymers is the ring-opening polymerization (ROP) of cyclic esters initiated by metal-based catalysts, such as Sn [3-8], Al [9-39], Zn [40-49], Mg [50-59], Fe [60-66], Ti [67-76], In [77-81] and rare-earth metals [82-87] with ancillary ligands. Among the reported catalysts, aluminum catalysts have been studied the most
for the ROP of cyclic esters because of their great controllability over the molecular weight and distribution of the polymers by modification of the ancillary ligands [88-92].
Recent
research
indicates
that
aluminum
complexes
with
phenoxy-imine/amine ligands acting as bulky monoanionic ligands have attracted great attention for their successful application in the ROP reaction [9-12]. Nomura’s group reported a facile and efficient system for the ROP of ε-caprolactone (ε-CL) at room temperature using a salicylaldimine-Al complex [9]. In addition, Pappalardo’s group reported the synthesis of dimethyl (salicylaldiminato) aluminum complexes and their reactivity toward the ROP of ε-CL and lactide (LA) [10]. Therefore, some aluminum complexes bearing a similar structure with phenoxy-imine/amine ligands were synthesized and their catalytic properties for the ROP reaction have been investigated. Ko’s
group
showed
that
aluminum
complexes
bearing
sterically
bulky
benzotriazole-phenoxide ligands were efficient catalysts for the ROP of L-LA [13]. Wang’s group also showed that aluminum complexes supported by functionalized phenolate ligands were active in the polymerization of rac-LA [14]. According to the catalytic properties for aluminum complexes bearing bulky monoanionic ligands in the ROP reaction, Al complexes with benzoxazolphenolate ligands were expected to be efficient catalysts in the ROP reaction. In this paper, a family of benzoxazolphenolate ligands and their aluminum complexes have been synthesized. Additionally, their catalytic properties for the ROP of ε-CL and LA have also been examined.
2. Results and Discussion 2.1. Synthesis and characterization The synthesis of the ligands 1-3 and their aluminum complexes 4-6 are presented in Scheme 1. The ligands 1-3 were synthesized according to a literature procedure by the reaction of 5-diethylamino-2-nitrosophenol with the corresponding 2-bromomethylphenol in THF [93]. The ligands 1-3 were characterized by 1H and
13
C NMR
spectra. The NMR spectral signals were consistent with the respective structures. The proton signals of the OH groups of 1-3 appeared in the low field region (δ 11.48-12.29 ppm), showing the existence of intramolecular hydrogen bonds. Treatment of 1-3 with
one equivalent of AlMe3 in toluene afforded the aluminum complexes 4-6 through an alkane elimination reaction. The aluminum complexes 4-6 were also characterized by 1
H and 13C NMR spectra and elemental analyses. The absence of the O–H signal of the
ligands and the appearance of new protons signals for AlMe2 (δ -0.63 to -0.79 ppm) demonstrated the formation of the desired aluminum complexes. Crystals of complex 5 suitable for X-ray crystal structure determination were crystallized from n-hexane at room temperature. Its molecular structure is shown in Fig. 1 with selected bond lengths and angles. X-Ray analysis reveals that the Al centre adopts a slightly distorted tetrahedral geometry, as evidenced by the bond angles for O1–Al1–N1
92.33(1)o,
N1–Al1–C19
105.95(2)o,
C18–Al1–N1
111.18(2)o,
C18–Al1–O1 114.23 (2)o, O1–Al1–C19 114.93(2)o and C18–Al1–C19 115.40(2)o. The six-membered chelating ring is nearly planar, with the aluminum atom lying out of the plane by 0.0894 Å. The torsion angles between the six-membered chelating ring and C19–Al1–C18 is 93.1(2)o. The Al–O bond length [1.782(2) Å] is slight longer than those in salicylaldiminato aluminum complexes [1.7688(2)-1.7792(1) Å]. In contrast, the Al–N bond length of complex 5 [1.960(3) Å] is slight shorter as compared with the salicylaldiminato aluminum complexes [1.9664(1)-1.9896(1) Å] [9-12].There is, however, no significant differences observed in the Al–C bond distances. 2.2. ROP of ε-CL using complexes 4-6 The catalytic performances of complexes 4-6 toward the polymerization of ε-CL were investigated and the results are shown in Table 1. All the aluminum complexes 4-6 showed high activities for the ring-opening polymerization of ε-CL in the presence of benzyl alcohol (BnOH) (Table 1, entries 1-6). The highest catalytic activity was obtained with a BnOH/Al molar ratio of 2:1 (Table 1, entries 2, 5 and 7-8). Under the same conditions, the activities of the complexes were compared with each other. The catalytic activity decreased when the bromine groups at the 3 and 5 position on the phenol ring were replaced by methyl groups (Table 1, entries 4 and 5). Additionally, entry 6 shown that catalytic activity was reduced upon the addition of tertiary butyl groups to the phenol backbone. This could be due to the steric and electronic effects of the substituents. In comparison with related known catalytic systems, the activities of
complexes 4-6 (turnover frequency (TOF): 441.4, 735.0 and 582.0 h–1) were lower than those of the phenolate Al catalyst systems (TOF: 7698.6 h–1) [9]. To investigate the catalytic properties for the ROP of ε-CL, the catalytic precursor 5 was studied under different reaction conditions. A linear relationship between the number average molecular weight (Mn) and ([ε-CL]0/[BnOH]0) indicated the "living" character of the polymerization process, as shown in Fig. 2 (Table 1, entries 5, 9-12). The effect of the BnOH/Al molar ratio on the ROP of ε-CL was also examined (Table 1, entries 5, 7-8). The results imply that the Al complexes initiate the polymerization reaction in a controlled fashion. Remarkably, complex 5 was tolerant to the protic BnOH. The addition of 10 equivalents of BnOH did not degrade the complex (Fig. 3). The 1H NMR spectrum of PCL-50 prepared from ε-CL and 5 ([ε-CL]0/[BnOH]0 = 50) for chain-end studies is shown in Fig. 4. The polymer chain should be capped with one benzyl ester and one hydroxy chain end. 2.3. ROP of rac-LA using complexes 4-6 Complexes 4-6 were also examined with regard to their catalytic behavior in the ROP of LA. The ROP was carried out in toluene in the presence of 2.0 equivalents of BnOH. The results are summarized in Table 2. The experimental results show complexes 4-6 are efficient catalysts for the polymerization of L-LA, giving a linear PLA which is capped with one benzyl ester and one hydroxyl end (Fig. 5). For complexes 4-6, the activity is comparable to Al complexes bearing salicylaldimine ligands (TOF: 4.1, 9.4 and 5.6 h–1) [11]. Complex 5 shows a relatively higher activity (Table 2, entry 2, TOF: 9.8 h–1) than those of complexes 4 and 6 (Table 2, entries 1 and 3, TOF: 6.1 and 6.5 h–1). The same trend was found in the ROP of ε-CL. Complex 5 was selected to evaluate the effect of LA/BnOH on the activity and polymer molecular weights. A plot of Mn versus ([L-LA]0/[Al]0) initiated by 5 exhibited a linear relationship. This demonstrates the "living" character of the polymerization process, as shown in Fig. 6 (Table 2, entries 2, 4-7). Poly(rac-LA) with homonuclear decoupled 1H NMR spectra of the methine region were also investigated. The Pr value indicates that these polymer chains are partially heterotactic (Fig. 7) [19]. It was found that complex 5 has the highest Pr value (0.64)
compared to complexes 4 and 6 (Table 2, entries 8-10) at 30 oC. These results indicate that increasing the electron withdrawing ability of the substituents enhances the catalytic activity. Additionally, the tacticity of the poly(rac-LA) is significantly influenced by the reaction temperature. Upon elevating the temperature from 30 to 70 °C, the Pr value decreased from 0.64 to 0.57 (Table 2, entries 9 and 13).
3. Experimental Section 3.1. General procedures All reactions and manipulations which related to the preparation of the metal complexes were carried out under nitrogen with standard Schlenk-line or glovebox techniques. All solvents were refluxed over the appropriate drying agent and distilled prior to use. 1H and
13
C NMR spectra were measured using a Bruker AVANCE-400
NMR spectrometer. The elemental analyses were performed on a Perkin-Elmer 2400 analyzer. Gel permeation chromatography (GPC) measurement was performed on a TOSOH HLC 8220 GPC at 40 oC using THF as the eluent against polystyrene standards. 3.2. Ring-opening polymerization of cyclic esters In a typical polymerization experiment, the aluminum complex, the required amount of lactone and BnOH in toluene were loaded in a flame-dried vessel containing a magnetic bar. The vessel was placed in an oil bath. After a certain reaction time, the polymer was isolated by precipitation with cold methanol. The precipitate was collected and dried under vacuum at 40 oC for 24 h. For some polymerization reactions, samples were taken for determining the monomer conversion by
1
H NMR
spectroscopy during the reaction. 3.3.1. Synthesis of ligand 1 A solution of 2-bromomethyl-4,6-dimethylphenol (2.31 g, 10.70 mmol) and 5-diethylamino-2-nitrosophenol (2.08 g, 10.70 mmol) in THF (15 mL) in the presence of K2CO3 (1.48 g, 10.70 mmol) was heated to reflux for 12 h and filtered. The THF was evaporated under reduced pressure and the residual material was purified by flash chromatography eluting with 1:4 EA/hexane to afford ligand 1 as a yellow oil. Yield:
1.32 g (65%). Anal. Calcd. for C19H22N2O2 (310.39): C, 73.52; H, 7.14; N, 9.03. Found: C, 73.58; H, 7.19; N, 9.48%. 1H NMR (400 MHz, CDCl3, 293 K) δ, ppm: 11.48 (s, 1H, OH), 7.61 (s, 1H, Ar–H), 7.49 (d, J = 6.4 Hz, 1H, Ar–H), 7.07 (s, 1H, Ar–H), 6.83 (s, 1H, Ar–H), 6.74 (d, J = 6.4 Hz, 1H, Ar–H), 3.42 (q, J = 6.9 Hz, 4H, CH2CH3), 2.30 (s, 3H, CCH3), 2.32 (s, 3H, CCH3), 1.22 (t, J = 6.9 Hz, 6H, CH2CH3).
13
C NMR (100
MHz, CDCl3, 293 K) δ, ppm: 160.7, 154.1, 151.2, 146.9, 134.4, 130.1, 127.8, 125.8, 123.8, 119.0, 110.3, 110.1, 93.0, 45.1, 20.5, 15.9, 12.4. 3.3.2. Synthesis of ligand 2 Ligand 2 was synthesized according to the same procedure as for that of 1, but 2,4-dibromo-6-bromomethylphenol (3.69 g, 10.70 mmol) was used instead of 2-bromomethyl-4,6-dimethylphenol. After a similar work-up, ligand 2 was obtained as a yellow solid. Yield: 1.53 g (67%). Anal. Calcd. for C17H16Br2N2O2 (440.13): C, 46.39; H, 3.66; N, 6.36. Found: C, 46.46; H, 3.72; N, 6.48%. 1H NMR (400 MHz, CDCl3, 293 K) δ, ppm: 12.29 (s, 1H, OH), 8.00 (s, 1H, Ar–H), 7.71 (s, 1H, Ar–H), 7.50 (d, J = 8.1 Hz, 1H, Ar–H), 6.79 (m, 2H, Ar–H), 3.43 (q, J = 6.9 Hz, 4H, CH2CH3), 1.23 (t, J = 6.9 Hz, 6H, CH2CH3).
13
C NMR (100 MHz, CDCl3, 293 K) δ, ppm: 157.9, 153.7, 151.6,
147.5, 137.2, 129.2, 127.8, 119.6, 113.4, 111.8, 111.0, 110.9, 92.5, 45.1, 12.5. 3.3.3. Synthesis of ligand 3 Ligand 3 was synthesized according to the same procedure as for that of 1, but 2-bromomethyl-4,6-di-tert-butylphenol (3.21 g, 10.70 mmol) was used instead of 2-bromomethyl-4,6-dimethylphenol. After a similar work-up, ligand 3 was obtained as a yellow solid. Yield: 1.31 g (63%). Anal. Calcd. for C25H34N2O2 (394.55): C, 76.10; H, 8.69; N, 7.10. Found: C, 76.19; H, 8.75; N, 7.19%. 1H NMR (400 MHz, CDCl3, 293 K) δ, ppm: 11.95 (s, 1H, OH), 7.50 (d, J = 8.0 Hz, 1H, Ar–H), 7.19 (s, 1H, Ar–H), 7.16 (s, 1H, Ar–H), 6.87 (s, 1H, Ar–H), 6.73 (d, J = 8.0 Hz, 1H, Ar–H), 3.42 (q, J = 6.4 Hz, 4H, CH2CH3), 1.49 (s, 9H, C(CH3)3), 1.37 (s, 9H, C(CH3)3), 1.28 (t, J = 6.4 Hz, 6H, CH2CH3).
13
C NMR (100 MHz, CDCl3, 293K) δ, ppm: 154.9, 149.8, 142.9, 140.8,
136.8, 126.1, 125.2, 122.5, 122.2, 120.7, 119.5, 118.9, 90.5, 45.1, 35.2, 34.6, 31.5, 30.0, 29.7, 29.5, 12.4. 3.3.4. Synthesis of complex 4
AlMe3 (1.27 mL, 1.00 M in toluene, 1.27 mmol) was added dropwise to a solution of 1 (0.36 g, 1.27 mmol) in toluene (15 mL) at about 0 oC with stirring. The reaction mixture was allowed to warm to room temperature and was stirred at 70 oC for 12 h. After removal of the solvent in vacuo, the product was recrystallized from n-hexane to give complex 4 as yellow solid. Yield: 0.50 g (85%). Anal. Calcd. for C21H27AlN2O2 (366.43): C, 68.83; H, 7.43; N, 7.64. Found: C, 68.75; H, 7.38; N, 7.50%. 1H NMR (400 MHz, CDCl3, 293K) δ, ppm: 7.55 (s, 1H, Ar–H), 7.51 (d, J = 9.0 Hz, 1H, Ar–H), 7.25 (s, 1H, Ar–H), 6.95 (s, 1H, Ar–H), 6.91 (d, J = 9.0 Hz, 1H, Ar–H), 3.46 (q, J = 7.1 Hz, 4H, CH2CH3), 2.37 (s, 3H, CCH3), 2.35 (s, 3H, CCH3), 1.24 (t, J = 7.1 Hz, 6H, CH2CH3), -0.79 (s, 6H, AlCH3).
13
C NMR (100 MHz, CDCl3, 293 K) δ, ppm: 162.0,
151.4, 151.1, 147.4, 137.6, 133.3, 133.1, 125.5, 125.5, 117.1, 114.8, 113.8, 95.3, 45.9, 20.6, 17.4, 11.8, -7.3. 3.3.5. Synthesis of complex 5 Complex 5 was synthesized according to the same procedure as for that of 4, but 2 (0.79 g, 1.80 mmol) was used instead of 1. After a similar work-up, complex 5 was obtained as a yellow solid. Yield: 0.83 g (81%). Anal. Calcd. for C19H21AlBr2N2O2 (496.17): C, 45.99; H, 4.27; N, 5.65. Found: C, 45.95; H, 4.21; N, 5.69%. 1H NMR (400 MHz, CDCl3, 293 K) δ, ppm: 7.99 (s, 1H, Ar–H), 7.80 (s, 1H, Ar–H), 7.44 (d, J = 9.3 Hz, 1H, Ar–H), 6.83–6.78 (m, 2H, Ar–H), 3.44 (q, J = 7.1 Hz, 4H, CH2CH3), 1.23 (t, J = 7.1 Hz, 6H, CH2CH3), -0.63 (s, 6H, AlCH3). 13C NMR (101 MHz, CDCl3, 293 K) δ, ppm: 156.0, 157.9, 151.4, 148.2, 139.7, 128.5, 124.1, 117.2, 117.0, 112.4, 111.7, 108.3, 92.4, 45.2, 12.4, -9.7. 3.3.6. Synthesis of complex 6 Complex 6 was synthesized according to the same procedure as for that of 4, but 3 (0.56 g, 1.42 mmol) was used instead of 1. After a similar work-up, complex 6 was obtained as a yellow solid. Yield: 0.54 g (76%). Anal. Calcd. for C27H39AlN2O2 (450.59): C, 71.97; H, 8.72; N, 6.22. Found: C, 71.90; H, 8.76; N, 6.28%. 1H NMR (400 MHz, CDCl3, 293 K) δ, ppm: 7.79 (s, 1H, Ar–H), 7.53 (s, 1H, Ar–H), 7.45 (d, J = 9.0 Hz, 1H, Ar–H), 7.05 (s, 1H, Ar–H), 6.92 (d, J = 9.0 Hz, 1H, Ar–H), 3.46 (q, J = 7.1 Hz, 4H, CH2CH3), 1.45 (s, 9H, CCH3), 1.36 (s, 9H, CCH3), 1.23 (t, J = 7.1 Hz, 6H,
CH2CH3), -0.66 (s, 6H, AlCH3). 13C NMR (100 MHz, CDCl3, 293 K): δ, ppm: 165.1, 160.6, 150.1, 145.7, 140.6, 139.3, 130.9, 124.7, 121.3, 119.3, 116.2, 109.5, 98.3, 46.9, 35.5, 34.3, 32.7, 31.4, 29.7, 29.4, 11.3, -9.9. 3.4. X-ray crystallography Diffraction data of complex 5 were collected at 120 K with an Xcalibur, Eos, Gemini diffractometer equipped with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). Using Olex2 [95], the structure was solved with the ShelXS [96] structure solution program using direct methods and refined with the ShelXL [96] refinement package using least squares minimisation.
4. Conclusions The benzoxazolphenolate ligands and their aluminum complexes have been synthesized and fully characterized by 1H and 13C NMR spectra and elemental analyses. The structure of complex 5 was confirmed by X-ray crystallography. The structure reveals that the Al complex has a distorted tetrahedral geometry around the metal center. All of the aluminum complexes were tested in the ROP of ε-CL and LA. The complexes showed high activity in the presence of BnOH. Microstructural analysis of the poly(rac-LA) catalyzed by these complexes revealed that ligands 1-3 have the ability to control the tacticity of the PLA. The substituents on the ligand affect the tacticities of the poly(rac-LA).
Acknowledgments We are grateful for financial support from the National Natural Science Foundation of China (No. 21104026), Public Special Ministry of Water Resources for research on the comprehensive utilization of water resources in coastal and most strict water resources management model (201201116), Shandong Province Water Conservancy research and technology promotion projects (SDSLKY201306), Shandong Agriculture Major Application of Technology Innovation for research on key technology of
comprehensive management of small ecological watershed (2011139), Shandong Province of Major Application of Technology Innovation for research and demonstration of water quality evaluation and risk management of Jinxiuchuan Reservoir (SDSLKY201229)
Appendix A. Supplementary data CCDC 973210 contains the supplementary crystallographic data for complex 5. These
data
can
be
obtained
http://www.ccdc.cam.ac.uk/conts/retrieving.html
free or
of from
charge the
via
Cambridge
Crystallographic Data Center, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected]. References [1] G. Swift, Acc. Chem. Res. 26 (1993) 105. [2] R.E. Drumright, P.R. Gruber, D.E. Henton, Adv. Mater. 12 (2000) 1841. [3] L. Wang, C.E. Kefalidis, S. Sinbandhit, V. Dorcet, J.-F. Carpentier, L. Maron, Y. Sarazin, Chem. Eur. J. 19 (2013) 13463. [4] K.M. Stridsberg, M. Ryner, A.-C. Albertsson, Adv. Polym. Sci. 157 (2002) 41. [5] M. Lahcini, P.M. Castro, M. Kalmi, M. Leskelä, T. Repo, Organometallics 23 (2004) 4547. [6] A. Tullo, Chem. Eng. News. 3 (2000) 13. [7] W. Chen, H. Yang, R. Wang, R. Cheng, F. Meng, W. Wei, Z. Zhong, Macromolecules 43 (2010) 201. [8] V. Poirier, T. Roisnel, S. Sinbandhit, M. Bochmann, J.-F. Carpentier, Y. Sarazin, Chem. Eur. J. 18 (2012) 2998. [9] N. Nomura, T. Aoyama, R. Ishii, T. Kondo, Macromolecules 38 (2005) 5363. [10] D. Pappalardo, L. Annunziata, C. Pellecchia, Macromolecules 42 (2009) 6056. [11] W. Zhang, Y. Wang, W.-H. Sun, L. Wang, C. Redshaw, Dalton Trans. 41 (2010) 11587. [12] J. Liu, N. Iwasa, K. Nomura, Dalton Trans. (2008) 3978.
[13] W.-J. Tai, C.-Y. Li, P.-H. Lin, J.-Y. Li, M.-J. Chen, B.-T. Ko, Appl. Organometal. Chem. 26 (2012) 518 [14] C. Zhang, Z.-X. Wang, J. Organomet. Chem. 693 (2008) 3151. [15] K. Matsubara, C. Terata, H. Sekine, K. Yamatani, T. Harada, K. Eda, M. Dan, Y. Koga, M. Yasuniwa, J. Polym. Sci. Part A: Polym. Chem. 50 (2012) 957. [16] C.-Y. Li, C.-Y. Tsai, C.-H Lin, B.-T. Ko, Dalton Trans. 40 (2011) 1880. [17] J.-T. Issenhuth, J. Pluvinage, R. Welter, S. Bellemin-Laponnaz, S. Dagorne, Eur. J. Inorg. Chem. 2009 (2009) 4701. [18] C.-T. Chen, H.-J. Weng, M.-T. Chen, C.-A. Huang, K.-F. Peng, Eur. J. Inorg. Chem. 2009 (2009) 2129. [19] T.M. Ovitt, G.W. Coates, J. Am. Chem. Soc.124 (2002) 1316. [20] B. Gao, R. Duan, X. Pang, X. Li, Z. Tang, X. Zhuang, X. Chen, Organometallics 32 (2013) 5435. [21] S. Tabthong, T. Nanok, P. Kongsaeree, S. Prabpai, P. Hormnirun, Dalton Trans. 43 (2014) 1348. [22] C.-T. Chen, C.-H. Liao, K.-F. Peng, M.-T. Chen, T.-L. Huang, J. Organomet. Chem. 753 (2014) 9. [23] M.-C. Chang, W.-Y. Lu, H.-Y. Chang, Y.-C. Lai, M.Y. Chiang, H.-Y. Chen, H.-Y. Chen, Inorg. Chem. 54 (2015) 11292. [24] X. Pang, H. Du, X. Chen, X. Wang, X. Jing, Chem. Eur. J. 14 (2008) 3126. [25] A. Pilone, K. Press, I. Goldberg, M. Kol, M. Mazzeo, M. Lamberti, J. Am. Chem. Soc. 136 (2014) 2940. [26] C. Agatemor, A.E. Arnold, E.D. Cross, A. Decken, M.P. Shaver, J. Organomet. Chem. 335 (2013) 745. [27] Y. Gamze, K. Asgar, Appl. Catal. A. 433 (2012) 223. [28] S.L. Hancock, M.F. Mahon, M.D. Jones, Dalton Trans. 42 (2013) 9279. [29] D. Li, Y. Peng, C. Geng, K. Liu, D, Kong, Dalton Trans. 42 (2013) 11295. [30] F. Qian, K. Liu, H. Ma, Dalton Trans. 39 (2010) 8071. [31] A. Meduri, T. Fuoco, M. Lamberti, C. Pellecchia, D. Pappalardo, Macromolecules 47 (2014) 534.
[32] K. Bakthavachalam, N. D. Reddy, Organometallics 32 (2013) 3174. [33] M.-W. Hsiao, C.-C. Lin, Dalton Trans 42 (2013) 2041. [34] M.O. Miranda, Y. DePorre, H. Vazquez-Lima, M.A. Johnson, D.J. Marell, C.J. Cramer, W.B. Tolman, Inorg. Chem. 52 (2013) 13692. [35] L. Postigo, M.C. Maestre, M.E.G. Mosquera, T. Cuenca, G. Jiménez, Organometallics 32 (2013) 2618. [36] P. Hormnirun, E. L. Marshall, V.C. Gibson, A.J.P. White, D.J. Williams, J. Am. Chem. Soc. 126 (2004) 2688. [37] M.P.F. Pepels, M. Bouyahyi, A. Heise, R. Duchateau, Macromolecules 46 (2013) 4324. [38] C.T. Altaf, H. Wang, M. Keram, Y. Yang, H. Ma, Polyhedron 81 (2014) 11. [39] M.D. Jones, L. Brady, P. McKeown, A. Buchard, P.M. Schäfer, L.H. Thomas, M.F. Mahon, T.J. Woodman, J.P. Lowe, Chem. Sci. 6 (2015) 5034. [40] M. Rauch, Y. Rong, W. Sattler, G. Parkin, Polyhedron 103 (2016) 135. [41] A. Gao, W. Yao, Y. Xiao, M Zhang, G. Zhu, N. Zhang, S. Wang, D. Wang, Y.-Y Zhang, Y. Gao, Z. Xu, P. Lu, Z. Zhang, Polyhedron 85 (2015) 537. [42] X.-F. Yu, C. Zhang, Z.-X. Wang, Organometallics 32 (2013) 3262. [43] M.D. Jones, C.G. Keir, A.L. Johnson, M.F. Mahon, Polyhedron 29 (2010) 312. [44] L.R. Collins, L.A. Moffat, M.F. Mahon, M.D. Jones, M.K. Whittlesey, Polyhedron 103 (2016) 121. [45] M. Cheng, A.B. Attygalle, E.B. Lobkovsky, G.W. Coates, J. Am. Chem. Soc. 121 (1999) 11583. [46] W. Yao, Y. Mu, A. Gao, W. Gao, L. Ye, Dalton Trans. (2008) 3199. [47] K. Pang, Y. Rong, G. Parkin, Polyhedron 29 (2010) 1881. [48] R. Petrus, P. Sobota, Organometallics 31 (2012) 4755. [49] A. Gao, W. Yao, Y. Mu, W. Gao, M. Sun, Q. Su, Polyhedron 28 (2009) 2605. [50] H. Xie, Z. Mou, B. Liu, P. Li, W. Rong, S. Li, D. Cui, Organometallics 33 (2014) 722. [51] Y. Gao, Z. Dai, J. Zhang, X. Ma, N. Tang, J. Wu, Inorg. Chem. 53 (2014) 716. [52] A. Garces, L.F. Sanchez-Barba, J. Fernandez-Baeza, A. Otero, M. Honrado, A.
Lara-Sanchez, A.M. Rodriguez, Inorg. Chem. 52 (2013) 12691. [53] Y. Wang, W. Zhao, D. Liu, S. Li, X. Liu, D. Cui, X. Chen, Organometallics 31 (2012) 4182. [54] V. Balasanthiran, M.H. Chisholm, K. Choojun, C.B. Durr, P.M. Wambua, Polyhedron 103 (2016) 235. [55] T. Han, J. Utko, L. B. Jerzykiewicz, P. Sobota, Dalton Trans. 41 (2011) 12660. [56] W.-C. Hung, C.-C. Lin, Inorg. Chem. 48 (2009) 728. [57] J. Wu, B.-H. Huang, M.-L. Hsueh, S.-L. Lai, C.-C. Lin, Polymer 46 (2005) 9784. [58] L. Wang, H. Ma, Macromolecules 43 (2010) 6535 [59] H.-Y. Tang, H.-Y. Chen, J.-H. Huang, C.-C. Lin, Macromolecules 40 (2007) 8855. [60] A.B. Biernesser, B.Li, J.A. Byers, J. Am. Chem. Soc. 135 (2013) 16553. [61] B.J. O’Keefe, L.E. Breyfogle, M.A. Hillmyer, W.B. Tolman, J. Am. Chem. Soc. 124 (2002) 4384. [62] J.B. Chen, J.L. Gorczynski, G.Q. Zhang, C.L. Fraser, Macromolecules 43 (2010) 4909. [63] J. Chen, J.L. Gorczynski, C.L. Fraser, Macromol. Chem. Phys. 211 (2010) 1272. [64] Y.Y. Kang, H.-R. Park, M.H. Lee, J. An, Y. Kim, J. Lee, Polyhedron 95 (2015) 24. [65] X. Wang, K. Liao, D. Quan, Q. Wu, Macromolecules 38(2005) 4611. [66] D.S. Mcguinness, E.L. Marshall, V.C. Gibson, J.W. Steed, J. Polym. Sci. Part A: Polym. Chem. 41 (2003) 3798. [67] L.-C. Liang, S.-T. Lin, C.-C. Chien, Inorg. Chem. 52 (2013) 1780. [68] F. Zhang, H. Song, G. Zi, J. Organomet. Chem. 695 (2010) 1993. [69] S.K. Russell, C.L. Gamble, K.J. Gibbins, K.C.S. Juhl, W.S. MitchellIII, A.J. Tumas, G.E. Hofmeister, Macromolecules 38 (2005) 10336. [70] S.H. Kim, J. Lee, D.J. Kim, J.H. Moon, S. Yoon, H.J. Oh, Y. Do, Y.S. Ko, J.-H. Yim, Y. Kim, J. Organomet. Chem. 694 (2009) 3409. [71] Y. Kim, J.G. Verkade, Organometallics 21 (2002) 2395. [72] T.W. Graham, C. Ong, P. Wei, D.W. Stephan, J. Organomet. Chem. 692 (2007) 4481. [73] Y. Kim, J.G. Verkade, Macromol. Rapid Commun. 23 (2002) 917.
[74] S. Park, Y. Do, J. Organomet. Chem. 692 (2007) 1633. [75] H.-Y. Chen, M.-Y. Liu, A.-K. Sutar, C.-C. Lin, Inorg. Chem. 49 (2010) 665. [76] M.J. Go, J.M. Lee, K.M. Lee, C.H. Oh, K.H. Park, S.H. Kim, M. Kim, H.-R. Park, M.H. Park, Y. Kim, J. Lee, Polyhedron 67 (2014) 286. [77] M. Normand, V. Dorcet, E. Kirillov, J.-F. Carpentier, Organometallics 32 (2013) 1694. [78] M.P. Blake, A.D. Schwarz, P. Mountford, Organometallics 30 (2011) 1202. [79] L.E.N. Allan, G.G. Briand, A. Decken, J.D. Marks, M.P. Shaver, R.G. Wareham, J. Organomet. Chem. 736 (2013) 55. [80] A. Pietrangelo, M.A. Hillmyer, W.B. Tolman, Chem. Commun. (2009) 2736. [81] I. Yu, A. Acosta-Ramírez, P. Mehrkhodavandi, J. Am. Chem. Soc. 134 (2012) 12758. [82] G.G. Skvortsov, G.K. Fukin, S.Y. Ketkov, A.V. Cherkasov, K.A. Lyssenko, A.A. Trifonov, Eur. J. Inorg. Chem. 24 (2013) 4173. [83] B. Liu, T. Roisnel, L. Maron, J.-F. Carpentier, Y. Sarazin, Chem. Eur. J.19 (2013) 3986. [84] P. Benndorf, J. Kratsch, L. Hartenstein, C.M. Preuss, P.W. Roesky, Chem. Eur. J. 18 (2012) 14454. [85] W. Ren, L. Chen, N. Zhao, Q. Wang, G. Hou, G. Zi, J. Organomet. Chem. 758 (2014) 65. [86] W.-K Gu, P.-F. Xu, Y.-R Wang, Y.-M Yao, D. Yuan, Q. Shen, Organometallics 34 (2015) 2907. [87] B.-Y. Li, Y.-M. Yao, Y.-R. Wang, Y. Zhang, Q. Shen, Polyhedron 27 (2008) 709. [88] O. Dechy-Cabaret, B. Martin-Vaca, D. Bourissou, Chem. Rev. 104 (2004) 6147. [89] B.J. O'Keefe, M.A. Hillmyer, W.B. Tolman, J. Chem. Soc., Dalton Trans. 15 (2011) 2215. [90] R.H. Platel, L.M. Hodgson, C.K. Williams, Polym. Rev. 48 (2008) 11. [91] M.J. Stanford, A.P. Dove, Chem. Soc. Rev. 39 (2010) 486. [92] C.M. Thomas, Chem. Soc. Rev. 39 (2010) 165. [93] W. Yao, D. Huang, Org. Lett. 12 (2010) 736.
[94] A. Kowalski, A. Duda, S. Penczek, Macromolecules 31 (1988) 2114. [95] O.V. Dolomanov, L.J. Bourhis, R.J. Gildea, J.A.K. Howard, H. Puschmann, J. Appl. Cryst. 42 (2009) 339. [96] G.M. Sheldrick, SHELXS, Acta. Cryst. A64 (2008) 112.
Scheme 1. Synthesis of complexes 1-6.
Fig. 1. The molecular structure of complex 5. Thermal ellipsoids are shown at the 30% probability level. Hydrogen atoms and solvent are omitted for clarity. Selected bond lengths [Å] and angles [o]: Al1–N1 1.960(3), Al1–O1 1.782(2), Al1–C18 1.950(4), Al1–C19 1.958(4), N1–C7 1.310(4), O1–C1 1.319(4), O1–Al1–N1 92.33(1), N1–Al1–C19 105.95(2), C18–Al1–N1 111.18(2), C18–Al1–O1 114.23(2), O1–Al1–C19 114.93(2), C18–Al1–C19 115.40(2).
Fig. 2. Plot of Mn versus [ε-CL]0/[BnOH]0 for the polymerization of ε-CL by complex 5.
Fig. 3. 1H NMR spectra of ligand 2 (top) and complex 5 (middle), the reaction mixture of complex 5 with 10 molar equivalents of BnOH (bottom).
Fig. 4. 1H NMR spectrum of a typical PCL sample.
Fig. 5. 1H NMR spectrum of a typical PLA sample.
Fig. 6. Plot of Mn versus [L-LA]0/[BnOH]0 for the polymerization of L-LA by complex 5.
Fig. 7. Homonuclear decoupled 1H NMR spectra of the methine region of poly(rac-LA) ((right) by complex 4 Pr = 0.55, Table 2, entry 12; (middle) by complex 5, Pr = 0.64, Table 3, entry 13; (left) by complex 6, Pr = 0.58, Table 3, entry 14, 400 MHz, CDCl3)
Table 1. Ring-opening polymerization of ε-CL catalyzed by complexes 4-6a. Entry
Cat.
[BnOH]0/ [Al]0/[ε-CL]
1 2 3 4 5 6 7 8 9 10 11 12
4 5 6 4 5 6 5 5 5 5 5 5
0:1:100 0:1:100 0:1:100 2:1:100 2:1:100 2:1:100 1:1:100 4:1:100 2:1:150 2:1:200 2:1:300 2:1:350
t (min)
Convb (%)
TOFc (h-1)
M n.calcdd (×103 )
Mne (×103 )
60 60 60 14 8 10 13 15 13 15 20 26
trace trace trace 96 98 97 97 95 96 97 98 97
− − − 441.4 735.0 582.0 447.7 380.0 664.6 776.0 882.0 783.5
− − − 5.6 5.7 5.6 11.2 2.8 8.3 11.2 16.9 19.5
− − − 6.9 7.0 7.1 12.1 4.0 9.1 10.6 15.3 16.8
0
PDIe
− − − 1.14 1.10 1.12 1.19 1.23 1.11 1.19 1.13 1.16
a
Polymerization conditions: catalyst, 30 µ mol; [ε-CL]0 = 0.5 M in toluene; a N2 atmosphere; at 70
o
C. bDetemined by 1H NMR spectra. cMole of monomer consumed per mole of catalyst per hour.
d
Calculated from ([ε-CL]0/[BnOH]0) × conversion × 114.14+108). eDetermined by GPC against
polystyrene standards in THF, multiplied by 0.58 [94].
Table 2. Ring-opening polymerization of lactides catalyzed by complexes 4-6a.
a
Entry
Cat.
1 2 3 4 5 6 7 8 9 10 11 12 13 14
4 5 6 5 5 5 5 4 5 6 5 4 5 6
[BnOH]0/ [Al]0 /[M]0 2:1:100 2:1:100 2:1:100 2:1:150 2:1:200 2:1:300 2:1:350 2:1:100 2:1:100 2:1:100 2:1:100 2:1:100 2:1:100 2:1:100
M L-LA L-LA L-LA L-LA L-LA L-LA L-LA rac-LA rac-LA rac-LA rac-LA rac-LA rac-LA rac-LA
T (oC) 70 70 70 70 70 70 70 70 70 70 50 30 30 30
t (h) 16 10 15 13 16 19 23 16 10 15 36 60 60 60
Convb (%) 97 98 98 96 97 98 95 96 97 97 93 89 91 90
TOFc (h-1 ) 6.1 9.8 6.5 11.1 12.1 15.5 14.5 6.0 9.7 6.5 2.6 1.5 1.5 1.5
Mn.calcdd (×103 ) 7.1 7.2 7.2 10.5 14.1 21.3 24.1 7.0 7.1 7.1 6.8 6.5 6.7 6.6
Mne (×103 ) 7.5 7.7 7.7 11.8 15.6 22.1 27.2 7.7 7.9 7.8 7.6 7.2 7.6 7.6
PDIe 1.04 1.05 1.09 1.13 1.15 1.14 1.13 1.08 1.10 1.11 1.04 1.07 1.05 1.10
Prf − − − − − − − 0.52 0.57 0.53 0.60 0.55 0.64 0.58
Polymerization conditions: catalyst, 30 µmol; [LA]0 = 0.5 M in toluene; a N2 atmosphere. Detemined by 1H NMR spectra. cMole of monomer consumed per mole of catalyst per hour. d Calculated from (([LA]0/[BnOH]0 ) × conversion × 144.13+108). eDetermined by GPC against polystyrene standards in THF, multiplied by 0.58 [94]. fPr is the probability of racemic linkages between monomer units determined by homonuclear decoupled 1H NMR spectroscopy [19]. b
Aluminum complexes with benzoxazolphenolate ligands: synthesis, characterization
and
catalytic
properties
for
ring-opening
polymerization of cyclic esters Yunping Zhang, Aihong Gao, Yongfang Zhang, Zhenghe Xu*, Wei Yao* School of Resources and Environment, University of Jinan, Jinan 250022, P. R. China
O O
O
cat HO
O
O
BnOH
On O O O
O
O
cat HO
O
O
BnOH
O
On
O
O NEt2 cat: O N Al R
O
R
O
4: R = Me 5: R = Br 6: R = tBu
Aluminum complexes with benzoxazolphenolate ligands: synthesis, characterization
and
catalytic
properties
for
ring-opening
polymerization of cyclic esters Yunping Zhang, Aihong Gao, Yongfang Zhang, Zhenghe Xu*, Wei Yao* School of Resources and Environment, University of Jinan, Jinan 250022, P. R. China
Benzoxazolphenolate ligands and their aluminum complexes have been synthesized and fully characterized. All of the aluminum complexes were tested in the ring-opening polymerization of cyclic esters. The aluminum complexes produced PLA with a slight heterotactic bias.