Lanthanide Alkyl Complexes Supported by a Piperazidine-Bridged Bis

Jul 27, 2010 - It was found that complexes 1−4 are highly efficient initiators for the ... Complex 1 can also initiate rac-lactide polymerization wi...
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Organometallics 2010, 29, 3507–3514 DOI: 10.1021/om100298z

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Lanthanide Alkyl Complexes Supported by a Piperazidine-Bridged Bis(phenolato) Ligand: Synthesis, Structural Characterization, and Catalysis for the Polymerization of L-Lactide and rac-Lactide Yunjie Luo,*,†,‡ Wenyi Li,† Dan Lin,†,‡ Yingming Yao,*,† Yong Zhang,† and Qi Shen† †

Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science, Dushu Lake Campus, Soochow University, Suzhou 215123, People’s Republic of China, and ‡ Organometallic Chemistry Laboratory, Ningbo Institute of Technology, Zhejiang University, Ningbo 315100, People’s Republic of China Received April 13, 2010

A series of lanthanide alkyl complexes supported by a piperazidine-bridged bis(phenolato) ligand were synthesized, and their catalytic activity for the polymerization of L-lactide was explored. The alkane elimination reaction of Ln(CH2SiMe3)3(THF)2 with H2[ONNO] {H2[ONNO] = 1,4-bis(2-hydroxy-3,5di-tert-butylbenzyl)piperazidine} in a 1:1 molar ratio in THF gave the neutral lanthanide alkyl complexes [ONNO]Ln(CH2SiMe3)(THF) [Ln = Y (1), Lu (2), Yb (3), Gd (4)] in high isolated yields. Treatment of a gadolinium tris(alkyl) complex formed in situ from the reaction of anhydrous GdCl3 with 3 equiv of Li(CH2SiMe3) in THF gave a novel “ate” gadolinium alkyl complex, {[ONNO]Gd(CH2SiMe3)(μ-Li)(μ-Cl)}2 (5). All of these complexes are fully characterized including X-ray structural determination. Complexes 1-4 are isomorphous, monomeric, and THF-solvated. The coordination geometry around the lanthanide metals can be best described as a distorted trigonal bipyramid. Complex 5 is dimeric and unsolvated, and each [ONNO]Gd(CH2SiMe3) moiety is connected by two μ-Cl and two μ-Li to form a distorted seven-coordinated capped trigonal-prismatic geometry. It was found that complexes 1-4 are highly efficient initiators for the controlled ring-opening polymerization of L-lactide, giving polymers with high molecular weights and narrow molecular weight distributions, whereas complex 5 exhibited apparently low activity for this polymerization. Complex 1 can also initiate rac-lactide polymerization with high activity, but the stereoselectivity is poor.

Introduction Bridged bis(phenolate) ancillary ligands have received considerable attention because such ligand sets are easily available and tunable, which allows the possibility for a systematic study on the effect of steric and electronic properties of the ligands on the reactivity of the resulting complexes.1,2 In recent years, significant efforts have also been directed toward the design and synthesis of new organolanthanide complexes stabilized by *To whom correspondence should be addressed. E-mail: yaoym@ suda.edu.cn; [email protected]. (1) Wu, J. C.; Yu, T. L.; Chen, C. T.; Lin, C. C. Coord. Chem. Rev. 2006, 250, 602. (2) (a) Braune, W.; Okuda, J. Angew. Chem., Int. Ed. 2003, 42, 64. (b) Ko, B. T.; Chao, Y. C.; Lin, C. C. Inorg. Chem. 2000, 39, 1463. (c) Tshuva, E. Y.; Goldberg, I.; Kol, M. J. Am. Chem. Soc. 2000, 122, 10706. (d) Segal, S.; Goldberg, I.; Kol, M. Organometallics 2005, 24, 200. (e) Gendler, S.; Zelikoff, A. L.; Kopilov, J.; Goldberg, I.; Kol, M. J. Am. Chem. Soc. 2008, 130, 2144. (f) Cohen, A.; Kopilov, J.; Goldberg, I.; Kol, M. Organometallics 2009, 28, 1391. (g) Meppelder, G.-J. M.; Fan, H. T.; Spaniol, T. P.; Okuda, J. Organometallics 2009, 28, 5159. (h) Meppelder, G.-J. M.; Fan, H. T.; Spaniol, T. P.; Okuda, J. Inorg. Chem. 2009, 48, 7378. (i) Capacchione, C.; Proto, A.; Ebeling, H.; Mulhaupt, R.; Moller, K.; Spaniol, T. P.; Okuda, J. J. Am. Chem. Soc. 2003, 125, 4964. (j) Capacchione, C.; Avagliano, A.; Proto, A. Macromolecules 2008, 41, 4573. (k) Tang, L. H.; Wasserman, E. P.; Neithamer, D. R.; Krystosek, R. D.; Cheng, Y.; Price, P. C.; He, Y. Y.; Emge, T. J. Macromolecules 2008, 41, 7306. (l) Ishii, A.; Toda, T.; Nakata, N.; Matsuo, T. J. Am. Chem. Soc. 2009, 131, 13566. r 2010 American Chemical Society

bridged bis(phenolate) ligand systems, as well as the exploration of their applications in homogeneous catalysis. To date, a variety of bis(phenolate) ligands containing C-, N-, and S-bridges have been employed in organolanthanide chemistry.3-5 It has been found that the structure of the bridges has a profound influence both on the synthesis of organolanthanide complexes and on the reactivity of the resulting lanthanide derivatives. The introduction of electron-donating heteroatom(s) into the bridges can not only, via the additional coordination of the heteroatom(s) to the lanthanide atom, prevent ligand redistribution reaction3c,4c but also improve the reactivity of the bis(phenolate) lanthanide complexes.3b,4i Furthermore, the structure of the bridges can also significantly affect the stereoselectivity of the polymerization of rac-lactide.4a,g,k,5d However, among the reported bridged bis(phenolate) ligands, almost all of the bridges are flexible chains, while heterocycle-bridged (3) For recent examples of carbon-bridged bis(phenolate) lanthanide complexes, see: (a) Deng, M. Y.; Yao, Y. M.; Shen, Q.; Zhang, Y.; Sun, J. Dalton Trans. 2004, 944. (b) Yao, Y. M.; Xu, X. P.; Liu, B.; Zhang, Y.; Shen, Q. Inorg. Chem. 2005, 44, 5133. (c) Xu, X. P.; Ma, M. T.; Yao, Y. M.; Zhang, Y.; Shen, Q. Eur. J. Inorg. Chem. 2005, 676. (d) Xu, X. P.; Yao, Y. M.; Hu, M. Y.; Zhang, Y.; Shen, Q. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 4409. (e) Xu, X. P.; Zhang, Z. J.; Yao, Y. M.; Zhang, Y.; Shen, Q. Inorg. Chem. 2007, 46, 9379. (f) Xu, X. P.; Yao, Y. M.; Zhang, Y.; Shen, Q. Chin. Sci. Bull. 2007, 52, 1623. (g) Qi, R. P.; Liu, B.; Xu, X. P.; Yang, Z. J.; Yao, Y. M.; Zhang, Y.; Shen, Q. Dalton Trans. 2008, 5016. Published on Web 07/27/2010

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bis(phenolate) ligands have seldom been introduced to organolanthanide chemistry.6 In addition, the conformation of heterocycle bridging groups in the bis(phenolate) ligands should be constrained when they are coordinated to lanthanide metals, resulting in different coordination spheres around the central metal and thus affecting reactivity of the corresponding complexes. We recently became interested in the synthesis and catalytic behavior of organolanthanide complexes stabilized by N-heterocycle-bridged bis(phenolate) ligands.6 It was found that the reactions of Ln[N(TMS)2]3(μ-Cl)Li(THF)3 with imidazolidinebridged bis(phenol) gave the desired monomeric neutral bis(phenolate) lanthanide amido complexes.6b On the contrary, the similar reactions with amine-bridged bis(phenol)s produced the unexpected zwitterionic amine-bridged bis(phenolate) lanthanide complexes.7 Furthermore, the amine elimination reactions of piperazidine-bridged bis(phenol) with ytterbium tris(amido) complexes could afford the bimetallic bis(phenolate) ytterbium diamido complexes, which are highly efficient initiators for the ring-opening polymerization of L-lactide.6c The piperazidine ring is flexible, but it possesses two main stable conformations, chair and boat forms.8 When the piperazidine bridge adopts chair conformation, bimetallic piperazidine-bridged bis(phenolate) lanthanide complexes could be isolated. In contrast, when the piperazidine ring adopts a boat conformation, mononuclear piperazidine-bridged bis(phenolate) lanthanide complexes are expected. However, the factors in governing the formation of the types of piperazidine-bridged bis(phenolate) lanthanide (4) For recent examples of amine bis(phenolate) lanthanide complexes, see: (a) Cai, C. X.; Amgoune, A.; Lehmann, C. W.; Carpentier, J. F. Chem. Commun. 2004, 330. (b) Kerton, F. M.; Whitwood, A. C.; Willans, C. E. Dalton Trans. 2004, 2237. (c) Yao, Y. M.; Ma, M. T.; Xu, X. P.; Zhang, Y.; Shen, Q.; Wong, W. T. Organometallics 2005, 24, 4014. (d) Boyd, C. L.; Toupance, T.; Tyrrell, B. R.; Ward, B. D.; Wilson, C. R.; Cowley, A. R.; Mountford, P. Organometallics 2005, 24, 309. (e) Bonnet, F.; Cowley, A. R.; Mountford, P. Inorg. Chem. 2005, 44, 9046. (f) Amgoune, A.; Thomas, C. M.; Ilinca, S.; Roisnel, T.; Carpentier, J.-F. Angew. Chem., Int. Ed. 2006, 45, 2782. (g) Amgoune, A.; Thomas, C. M.; Roisnel, T.; Carpentier, J. F. Chem.-Eur. J. 2006, 12, 169. (h) Delbridge, E. E.; Dugah, D. T.; Nelson, C. R.; Skelton, B. W.; White, A. H. Dalton Trans. 2007, 143. (i) Zhou, H.; Guo, H. D.; Yao, Y. M.; Zhang, Y.; Shen, Q. Inorg. Chem. 2007, 46, 958. (j) Guo, H. D.; Zhou, H.; Yao, Y. M.; Zhang, Y.; Shen, Q. Dalton Trans. 2007, 3555. (k) Liu, X. L.; Shang, X. M.; Tang, T.; Hu, N. H.; Pei, F. K.; Cui, D. M.; Chen, X. S.; Jing, X. B. Organometallics 2007, 26, 2747. (l) Amgoune, A.; Thomas, C. M.; Carpentier, J.-F. Macromol. Rapid Commun. 2007, 28, 693. (m) Willans, C. E.; Sinenkov, M. A.; Fukin, G. K.; Sheridan, K.; Lynam, J. M.; Trifonov, A. A.; Kerton, F. M. Dalton Trans. 2008, 3592. (n) Pang, M. L.; Yao, Y. M.; Zhang, Y.; Shen, Q. Chin. Sci. Bull. 2008, 53, 1978. (o) Kramer, J. W.; Treitler, D. S.; Dunn, E. W.; Castro, P. M.; Roisnel, T.; Thomas, C. M.; Coates., G. W. J. Am. Chem. Soc. 2009, 131, 16042. (p) Ajellal, N.; Bouyahyi, M.; Amgoune, A.; Thomas, C. M.; Bondon, A.; Pillin, I.; Grohens, Y.; Carpentier, J. F. Macromolecules 2009, 42, 987. (q) Ajellal, N.; Thomas, C. M.; Carpentier, J.-F. J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 3177. (r) Barroso, S.; Cui, J.; Carretas, J. M.; Cruz, A.; Santos, I. C.; Duarte, M. T.; Telo, J. P.; Marques, N.; Martins, A. M. Organometallics 2009, 28, 3449. (s) Nie, K.; Gu, X. Y.; Yao, Y. M.; Zhang, Y.; Shen, Q. Dalton Trans. 2010, 39, 6832. (5) For recent examples of sulfur-bridged bis(phenolate) and biphenolate lanthanide complexes, see: (a) Arnold, P. L.; Natrajan, L. S.; Hall, J. J.; Bird, S. J.; Wilson, C. J. Organomet. Chem. 2002, 647, 205. (b) Ma, H. Y.; Spaniol, T. P.; Okuda, J. J. Chem. Soc., Dalton Trans. 2003, 4470. (c) Ma, H. Y.; Okuda, J. Macromolecules 2005, 38, 2665. (d) Ma, H. Y.; Spaniol, T. P.; Okuda, J. Angew. Chem., Int. Ed. 2006, 45, 7818. (e) Konkol, M.; Spaniol, T. P.; Kondracka, M.; Okuda, J. Dalton Trans. 2007, 4095. (f) Ma, H. Y.; Spaniol, T. P.; Okuda, J. Inorg. Chem. 2008, 47, 3328. (6) (a) Xu, X. P.; Yao, Y. M.; Zhang, Y.; Shen, Q. Inorg. Chem. 2007, 46, 3743. (b) Zhang, Z. J.; Xu, X. P.; Li, W. Y.; Zhang, Y.; Shen, Q. Inorg. Chem. 2009, 48, 5715. (c) Zhang, Z. J.; Xu, X. P.; Sun, S.; Yao, Y. M.; Zhang, Y.; Shen, Q. Chem. Commun. 2009, 7414. (7) Dyer, H. E.; Huijser, S.; Schwarz, A. D.; Wang, C.; Duchateau, R.; Mountford, P. Dalton Trans. 2008, 32. (8) Lee, C. C.; Hsu, S. C.; Lai, L. L.; Lu, K. L. Inorg. Chem. 2009, 48, 6329.

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complexes are still not well understood to date. In order to get some idea of these factors, we extended this piperazidinebridged bis(phenol) to prepare lanthanide alkyl complexes. It was found that the reactions of this preligand with lanthanide tris(alkyl) complexes Ln(CH2SiMe3)3(THF)2 gave only, instead of the bimetallic lanthanide metal complexes, the mononuclear bis(phenolate) lanthanide mono(alkyl) complexes. A further study revealed that these lanthanide alkyl complexes can polymerize L-lactide with high efficiency and, in some cases, with good controllability. Here we report these results.

Experimental Section General Considerations. All manipulations were performed under pure argon with rigorous exclusion of air and moisture using standard Schlenk techniques and a glovebox. Solvents (toluene, hexane, and THF) were distilled from sodium/benzophenone ketyl, degassed by the freeze-pump-thaw method, and dried over fresh Na chips in the glovebox. Anhydrous LnCl3 were purchased from Strem. LiCH2SiMe3 (1 M solution in pentane) was obtained from Aldrich, and pentane was removed under vacuum before use. L-Lactide and rac-lactide were purchased from Arcos and were recrystallized from hot anhydrous toluene twice. Deuterated solvents (C6D6 and CDCl3) were obtained from CIL. Ln(CH2SiMe3)3(THF)2 (Ln = Y, Lu, Yb, Gd) were prepared according to the literature.9 Ligand N(CH2CH2)2N{CH2-(2-OH-C6H2-tBu2-3,5)}2 ([ONNO]H2) was synthesized by Mannich condensation reaction as used for the preparation of other amine-bridged bis(phenol)s.10 Samples of organolanthanide complexes for NMR spectroscopic measurements were prepared in the glovebox using J. Young valve NMR tubes. NMR (1H, 13C) spectra were recorded on a Bruker AVANCE III spectrometer at 25 °C. Carbon, hydrogen, and nitrogen analyses were preformed by direct combustion with a Carlo-Erba EA 1110 instrument. The IR spectra were recorded with a Nicolet-550 FT-IR spectrometer as KBr pellets. The X-ray structural determination was carried out on a Rigaku Mercury diffractometer. The molecular weight and molecular weight distributions were determined against polystyrene standards by gel permeation chromatography (GPC) at 40 °C with a PL 50 apparatus; THF was used as an eluent at a flow rate of 1.0 mL/min. Synthesis of Complex [ONNO]Y(CH2SiMe3)(THF) (1). To a colorless THF solution (20 mL) of Y(CH2SiMe3)2(THF)2 (0.99 g, 2.00 mmol) was added a THF solution (20 mL) of [ONNO]H2 (1.04 g, 2.00 mmol) slowly at room temperature. The mixture was stirred at room temperature for 3 h, and then the solvent was removed under reduced pressure. The residual oil was dissolved in a mixture of toluene/hexane (1:2 v/v). Colorless crystals were obtained by cooling the above solution to -30 °C over several days (1.08 g, 63%). 1H NMR (C6D6, 400 MHz, ppm): δ -0.73 (s, 2H, CH2TMS), 0.42 (s, 9H, Si(CH3)3), 1.03 (d, J = 16.8 Hz, 2H, ArCH2N), 1.43 (s, 18H, C(CH3)3, overlapped with THF signals), 1.43 (4H, THF-β-CH2), 1.67 (s, 18H, C(CH3)3), 2.11, 2.43, 2.66, 3.21 (br, s, 8H, N(CH2CH2)2N), 4.13 (br, s, 4H, THF-R-CH2), 4.19 (d, J = 16.8 Hz, 2H, ArCH2N), 6.88, 7.56 (s, 4H, Ar-H). 13C NMR (C6D6, 100 MHz, ppm): δ 5.0 (Si(CH3)3), 25.4 (THF-β-CH2), 30.7, 32.2 (C(CH3)3), 32.2, 35.5 (C(CH3)3), 45.3 (CH2SiMe3), 51.6 (ArCH2N), 60.2 (THF-R-CH2), 70.0 (N(CH2)2N), 122.4, 124.1, 125.0, 125.6, 128.5, 129.3, 136.7, 136.9, 137.9, 160.5 (Ar-C). Anal. Calcd for C42H71N2O3SiY: C, 65.60; H, 9.31; N, 3.64; Y, 11.56. Found: C, 65.34; H, 9.14; N, 3.86; Y, 11.26. IR (KBr pellet, cm-1): 2952 (s), 2866 (m), 1603 (s), 1471 (s), 1442 (s), 1413 (s), 1383 (w), 1237 (s), 1204 (s), 1135 (m), 1088 (s), 836 (s). (9) (a) Lappert, M.; Pearce, R. J. Chem. Soc., Chem. Commun. 1973, 126. (b) Luo, Y. J.; Baldamus, J.; Hou, Z. M. J. Am. Chem. Soc. 2004, 126, 13910. (10) Tshuva, E. Y.; Goldberg, I.; Kol, M. Organometallics 2001, 20, 3017.

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Table 1. Crystallographic Data for Complexes 1-5 1 3 toluene

2 3 2toluene

3 3 2toluene

4 3 toluene

5 3 5toluene C111H166Cl2Gd2Li2N4O4Si2 2075.94 293(2) triclinic P1 0.80  0.70  0.40 15.144(3) 15.458(3) 15.565(3) 119.65(2) 108.29(3) 93.98(4) 2893.0(9) 1 1.192 1.250 1088 25.35 22 977 9976 7651 488 1.110 0.0998 0.2504 3.136, -1.183

formula

C49H79N2O3SiY

C56H87LuN2O3Si

C56H87N2O3SiYb

C49H79GdN2O3Si

fw T (K) cryst syst space group cryst size (mm3) a (A˚) b (A˚) c (A˚) R (deg) β (deg) γ (deg) V (A˚3) Z Dcalcd (g cm-3) μ (mm-1) F(000) θmax (deg) collected reflns unique reflns obsd reflns [I>2.0σ(I)] no. of variables GOF R wR largest diff peak, hole (e 3 A˚-3)

861.14 223(2) monoclinic P21/n 0.45  0.40  0.22 16.688(7) 11.356(5) 26.146(11)

1039.34 223(2) monoclinic P21/n 0.80  0.50  0.30 14.747(3) 15.629(3) 24.663(5)

1037.41 293(2) monoclinic P21/n 0.60  0.60  0.30 14.676(2) 15.924(2) 24.664(3)

929.48 223(2) monoclinic P21/n 0.60  0.30  0.22 16.622(1) 11.3484(7) 26.184(2)

90.627(7)

96.423(5)

96.864(4)

90.646(2)

4954(4) 4 1.154 1.240 1856 25.50 19 017 9081 6739 504 1.170 0.0959 0.1852 0.728, -0.783

5648(2) 4 1.222 1.809 2184 25.50 27 988 10 365 7997 509 1.124 0.0901 0.2197 2.833, -1.735

5723(1) 4 1.204 1.695 2180 25.35 53 743 10 466 8313 505 1.174 0.0913 0.1874 1.381, -1.093

4938.7(5) 4 1.250 1.406 1956 27.48 29 899 11 126 9190 503 1.156 0.0651 0.1226 0.873, -1.183

Synthesis of Complex [ONNO]Lu(CH2SiMe3)(THF) (2). Complex 2 was prepared by a procedure similar to that for complex 1. Using [ONNO]H2 (1.04 g, 2.00 mmol) and Lu(CH2SiMe3)3(THF)2 (1.16 g, 2.00 mmol), complex 2 was isolated as colorless crystals (1.34 g, 78%). 1H NMR(C6D6, 400 MHz, ppm): δ -0.54 (s, 2H, CH2TMS), 0.49 (s, 9H, Si(CH3)3), 0.99 (d, J = 16.8 Hz, 2H, ArCH2N), 1.31 (br, s, 4H, THF-β-CH2), 1.44 (s, 18H, C(CH3)3), 1.73 (s, 18H, C(CH3)3), 2.32, 2.59, 2.63, 2.93 (br, s, 8H, N(CH2CH2)2N), 3.57 (br, s, 4H, THF-R-CH2), 4.01 (d, J = 16.8 Hz, 2H, ArCH2N), 6.84, 7.60 (s, 4H, Ar-H). 13C NMR (C6D6, 100 MHz, ppm): δ 4.3 (Si(CH3)3), 25.3 (THF-β-CH2), 30.6, 32.0 (C(CH3)3), 34.0, 35.5 (C(CH3)3), 44.7 (CH2SiMe3), 50.2 (ArCH2N), 59.6 (THF-R-CH2), 68.7 (N(CH2)2N), 121.7, 124.4, 124.7, 125.5, 128.4, 129.2, 137.2, 137.5, 137.9, 160.7 (Ar-C). Anal. Calcd for C42H71N2O3SiLu: C, 58.99; H, 8.37; N, 3.28; Lu, 20.46. Found: C, 58.72; H, 8.33; N, 3.41; Lu, 20.35. IR (KBr pellet, cm-1): 2955 (s), 2866 (m), 1609 (s), 1470 (s), 1442 (s), 1412 (s), 1389 (w), 1238 (s), 1204 (s), 1134 (m), 1092 (s), 833 (s). Synthesis of Complex [ONNO]Yb(CH2SiMe3)(THF) (3). Complex 3 was prepared by a procedure similar to that for complex 1. Using [ONNO]H2 (1.04 g, 2.00 mmol) and Yb(CH2SiMe3)2(THF)2 (0.98 g, 2.00 mmol), complex 3 was isolated as yellow crystals (1.35 g, 72%). Anal. Calcd for C42H71N2O3SiYb: C, 59.13; H, 8.39; N, 3.28; Yb, 20.28. Found: C, 58.87; H, 8.24; N, 3.56; Yb, 20.16. IR (KBr pellet, cm-1): 2955 (s), 2862 (m), 1604 (s), 1474 (s), 1441 (s), 1412 (s), 1384 (w), 1240 (s), 1203 (s), 1136 (m), 1086 (s), 839 (s). Synthesis of Complex [ONNO]Gd(CH2SiMe3)(THF) (4). Complex 4 was prepared by a procedure similar to that for complex 1. Using [ONNO]H2 (1.04 g, 2.00 mmol) and Gd(CH2SiMe3)2(THF)2 (0.95 g, 2.00 mmol), complex 4 was isolated as colorless crystals (0.98 g, 59%). Anal. Calcd for C42H71N2O3SiGd: C, 60.24; H, 8.55; N, 3.35; Gd, 18.78. Found: C, 59.97; H, 8.23; N, 3.58; Gd, 18.66. IR (KBr pellet, cm-1): 2951 (s), 2865 (m), 1604 (s), 1470 (s), 1443 (s), 1412 (s), 1385 (w), 1238 (s), 1205 (s), 1134 (m), 1084 (s), 837 (s). Synthesis of Complex {[ONNO]Gd(CH2SiMe3)(μ-Li)(μ-Cl)}2 (5). To a THF slurry of GdCl3 (0.53 g, 2.00 mmol) was added a THF solution (20 mL) of LiCH2SiMe3 (0.56 g, 6.00 mmol) slowly at room temperature. The resulting clear, colorless solution was stirred at room temperature for 10 min, and then a THF

solution (20 mL) of [ONNO]H2 (1.04 g, 2.00 mmol) was added slowly at room temperature. The mixture was stirred at room temperature for 4 h, and then the solvent was removed under reduced pressure. The residual oil was dissolved in a mixture of toluene/hexane (2:1 v/v). Colorless crystals were obtained at -30 °C in a few days (1.13 g, 68%). Anal. Calcd for C76H126N4O4Si2Gd2Li2Cl2: C, 56.54; H, 7.81; N, 3.47; Cl, 4.40; Gd, 19.47. Found: C, 56.43; H, 7.56; N, 3.58; Cl, 4.52; Gd, 19.25. IR (KBr pellet, cm-1): 2954 (s), 1768 (w), 1604 (m), 1467 (s), 1412 (s), 1389 (m), 1237 (s), 1135 (m), 1085 (s), 832 (s). A Typical Polymerization Procedure. The procedures for the polymerization of L-lactide initiated by complexes 1-5 were similar, and a typical polymerization procedure is given below. A 50 mL Schlenk flask, equipped with a magnetic stirring bar, was charged with the desired amount of L-lactide and toluene. The contents of the flask were then stirred at 70 °C until L-lactide was completely dissolved. Then a toluene solution of the initiator was added to this solution by a syringe. The mixture was stirred vigorously at 70 °C for the desired time, during which time an increase of viscosity was observed. The reaction mixture was quenched by the addition of methanol and then poured into a large amount of methanol to precipitate the polymer, which was dried under vacuum and weighed. X-ray Crystallography. Suitable single crystals of complexes 1-5 were sealed in a thin-walled glass capillary to determine the single-crystal structure. Intensity data were collected with a Rigaku Mercury CCD area detector in ω scan mode using Mo KR radiation (λ = 0.71070 A˚). The diffracted intensities were corrected for Lorentz polarization effects and empirical absorption corrections. Details of the intensity data collection and crystal data are given in Table 1. The structures were solved by direct methods and refined by full-matrix least-squares procedures based on |F|2. All of the non-hydrogen atoms, except the disordered atoms, were refined anisotropically, and the disordered non-hydrogen atoms were refined isotropically. The hydrogen atoms in these complexes were all generated geometrically (C-H bond lengths fixed at 0.95 A˚), were assigned appropriate isotropic thermal parameters, and were allowed to ride on their parent carbon atoms. All of the H atoms were held stationary and were included in the structure factor calculation in the final stage of full-matrix least-squares refinement.

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Scheme 1

The structures were solved and refined using the SHELXL-97 program.

Results and Discussion Synthesis and Characterization of Bis(phenolate) Lanthanide Alkyl Complexes. Since this piperazidine-bridged bis(phenol) can act as an anionic bifunctional ligand, it is expected that the reaction of such ligand with 2 equiv of organolanthanide metal tris(alkyl) complexes Ln(CH2SiMe3)3(THF)2 might give bimetallic lanthanide metal alkyl species {(THF)x(SiMe3CH2)2Ln}2[ONNO], as we have found in synthesizing bimetallic lanthanide metal amido complexes.6c Therefore, a 1H NMR monitoring reaction was initially carried out. Treatment of Y(CH2SiMe3)3(THF)2 with 0.5 equiv of the piperazidine-bridged bis(phenol) [ONNO]H2 in C6D6 at room temperature demonstrated that a rapid reaction took place and afforded a yttrium alkyl complex, [ONNO]YCH2SiMe3(THF)x, as a neat product in a few minutes, with the elimination of SiMe4. No bimetallic species was formed. It seems that the steric shielding of such bis(phenolate) ligand is not bulky enough to stabilize bimetallic organolanthanide metal bis(alkyl) complexes. On a preparative scale, alkane elimination reactions of Ln(CH2SiMe3)3(THF)2 with piperazidine-bridged bis(phenol) [ONNO]H2 in a 1:1 molar ratio in THF at room temperature, after workup, gave the mononuclear bis(phenolate) lanthanide alkyl complexes [ONNO]Ln(CH2SiMe3)(THF) (Ln = Y (1), Lu (2), Yb (3), Gd (4)) in good to high isolated yields, as shown in Scheme 1. The compositions of complexes 1-4 were confirmed by elemental analysis. The definitive molecular structures of these complexes were determined by single-crystal structure analysis (see below). Considering the great difference in the outcomes for the reactions of piperazidine-bridged bis(phenol) with Yb[N(SiMe3)2]36c and with Ln(CH2SiMe3)(THF)2, it is reasonable to postulate, in these cases, that the bulkiness of the amido or alkyl group plays a crucial role in the formation of the bimetallic piperazidine-bridged bis(phenolate) lanthanide derivatives. To simplify the synthetic route, we intended to prepare the bis(phenolate) lanthanide alkyl complex using Ln(CH2SiMe3)(THF)2 formed in situ instead of that isolated. Only with the exception of Gd species, it was found that the reaction of anhydrous GdCl3 with 3 equiv of LiCH2SiMe3 in THF, then with 1 equiv of bis(phenol), gave the unexpected “LiCl”-containing product {[ONNO]Gd(CH2SiMe3)(μ-Li)(μ-Cl)}2 (5), as shown in Scheme 2. Crystal structure determination revealed that complex 5 has a novel dimeric structure. Two chlorine and two lithium atoms, serving as bridges, connected two [ONNO]Gd(CH2SiMe3) moieties, but there is no bonding between lithium and chlorine atoms. Complexes 1-5 are stable in the solid state at room temperature for a couple of months. Crystals of complexes 1-5 suitable for an X-ray diffraction analysis were obtained from a mixture solution of toluene and hexane at -30 °C. Complexes 1-4 have a solvated monomeric structure crystallized in the monoclinic system and are isomorphous. Therefore, the ORTEP diagram of only complex 1 is shown in Figure 1. Their selected bond lengths and bond angles

Figure 1. ORTEP diagram of complex 1 showing the atomnumbering scheme. Thermal ellipsoids are drawn at the 20% probability level, and hydrogen atoms are omitted for clarity. Complexes 2 to 4 are isomorphous with complex 1. Table 2. Selected Bond Lengths (A˚) and Bond Angles (deg) for Complexes 1-4

Ln1-O1 Ln1-O2 Ln1-O3 Ln1-C35 Ln1-N1 Ln1-N2 O1-Ln1-O2 O2-Ln1-C35 C35-Ln1-O1 C35-Ln1-O3 C35-Ln1-N1 C35-Ln1-N2 O3-Ln1-N1 O3-Ln1-N2 N1-Ln1-N2 Ln1-O1-C1 Ln1-O2-C7

1

2

3

4

2.151(4) 2.179(4) 2.436(4) 2.423(6) 2.549(4) 2.533(5)

2.115(6) 2.124(7) 2.377(8) 2.387(11) 2.440(8) 2.470(7)

2.111(6) 2.110(6) 2.391(8) 2.359(11) 2.469(8) 2.479(8)

2.191(4) 2.205(3) 2.471(3) 2.463(5) 2.568(4) 2.557(4)

112.2(2) 126.6(3) 116.3(3) 81.4(4) 92.7(3) 90.1(3) 159.7(3) 139.2(3) 59.7(3) 139.1(6) 145.2(6)

120.1(1) 121.1(2) 113.6(2) 80.7(2) 92.3(2) 93.8(2) 147.6(1) 153.1(1) 58.3(1) 143.5(3) 141.6(3)

118.1(2) 121.4(2) 115.1(2) 80.3(2) 92.1(2) 92.8(2) 147.5(1) 152.6(1) 58.4(1) 144.9(4) 141.8(4)

110.9(2) 127.0(3) 116.9(3) 80.5(4) 93.5(3) 90.6(3) 159.4(3) 139.2(3) 59.9(3) 139.3(6) 145.7(6)

are provided in Table 2. The boat conformation, different from that in the bimetallic piperazidine-bridged bis(phenolate) ytterbium diamido complexes,6c of the piperazidine rings in complexes 1-4 results from the formation of a mononuclear lanthanide alkyl complex. The lanthanide metal atom is six-coordinated by two oxygen atoms, two nitrogen atoms from the bis(phenolate) ligands, one carbon atom from the alkyl group, and one oxygen atom from the THF molecule. The coordination geometry at the central metal can be best described as a distorted trigonal prism, which is different from those observed in amino-amino-bis(phenolate)4k and methoxy-aminobis(phenolate)4a,g,11 lanthanide alkyl complexes. The difference in coordination geometry should be attributed to the constrained geometry of the nitrogen atoms in the piperazidine rings in complexes 1-4. The CH2SiMe3 group is located in a cis position to the THF molecule in complexes 1-4, as indicated by the corresponding bond angle C35-Ln1-O3, ranging from 80.3(2)° to 81.4(4)°. This orientation is quite different from those observed in amino-amino-bridged4k and methoxyamino-bridged11 bis(phenolate) yttrium alkyl complexes, in (11) Cai, C.-X.; Toupet, L.; Lehmann, C. W.; Carpentier, J.-F. J. Organomet. Chem. 2003, 683, 131.

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Scheme 2

Table 3. Selected Bond Lengths (A˚) and Bond Angles (deg) for Complex 5

Figure 2. ORTEP diagram of complex 5 showing the atom-numbering scheme. Thermal ellipsoids are drawn at the 20% probability level, and hydrogen atoms are omitted for clarity. One possible orientation is shown for disordered atoms for clarity.

which the CH2SiMe3 group and the THF molecule are located in trans positions. The CH2SiMe3 group is approximately perpendicular to the plane defined by the two nitrogen atoms of the piperazidine ring and the lanthanide metal atom according to the bond angles of C35-Ln1-N1 and C35-Ln1-N2 (Table 2). Two oxygen atoms of the bis(phenolate) ligand are located in trans positions and two nitrogen atoms are located in cis positions in these complexes because of the boat conformation of the piperazidine ring. In complex 1, the average Y-O(Ar) bond length of 2.165(4) A˚ is comparable to the corresponding bond lengths in amino-amino-bridged bis(phenolate) yttrium silylalkyl complexes [2.141(2) to 2.153(3) A˚],4k but is slightly larger than that in the methoxy-amino-bridged bis(phenolate) yttrium complex [ONOO]Y(CH2SiMe3)(THF) [ONOO = MeOCH2CH2N{CH2(2-O-C6H2-But2-3,5)}2] (2.126(4) A˚).11 The Y-C35 and the average Y-N bond lengths are 2.423(6) and 2.541(4) A˚, respectively, which are in accordance with the corresponding Y-C and Y-N bond lengths in the bridged bis(phenolate) yttrium alkyl complexes mentioned above.4k,11 However, the O1-Y1O2 bond angle of 118.1(2)° is apparently smaller than the corresponding bond angles in the above-mentioned bis(phenolate) yttrium alkyl complexes (range from 148.8(1)° to 155.7(8)°), which can be attributed to the weaker flexibility of the piperazidine-bridged bis(phenolate) group in comparison with amino-amino-bridged and methoxy-amino-bridged bis(phenolate)

Gd1-O1 Gd1-N1 Gd1-Cl1 Gd1-Cl1A Li1-O2A

2.334(6) 2.589(9) 2.860(3) 2.902(3) 2.28(3)

O1-Gd1-O2 C35-Gd1-O1 C35-Gd1-N1 O1-Gd1-N2 Cl1-Gd1-Cl1A Gd1-O1-Li1 O1-Li1-O2A Gd1-O2-C7

149.7(2) 95.5(4) 97.2(4) 131.2(2) 70.73(8) 132.3(11) 128(2) 120.5(6)

Gd1-O2 Gd1-N2 Gd1-C35 Li1-O1

N1-Gd1-N2 C35-Gd1-O2 C35-Gd1-N2 O2-Gd1-N2 Gd1-Cl1-Gd1A Gd1-O2-Li1A Gd1-O1-C1

2.353(6) 2.565(8) 2.131(11) 1.79(2)

57.7(3) 96.1(4) 98.1(4) 74.4(2) 109.27(8) 122.4(7) 121.6(5)

groups. The average Ln-O(Ar), Ln-N, and Ln-C bond lengths in complexes 2-4 are comparable to the corresponding bond lengths in complex 1, if the difference in ionic radii is considered. The ORTEP diagram of complex 5 is shown in Figure 2, and selected bond lengths and bond angles are provided in Table 3. Complex 5 is centrosymmetric dimeric and unsolvated, and it crystallizes in the triclinic system with five cocrystallized toluene molecules in the unit cell. Each of the gadolinium atoms is sevencoordinated, with two oxygen atoms, two nitrogen atoms from the bis(phenolate) ligands, one carbon atom from the alkyl group, and two chlorine atoms to form a distorted capped trigonal-prismatic geometry, with the capping atom being the carbon atom. The overall coordination geometry at the central metal atom is quite different from that in complex 4. Each of the lithium atoms is coordinated with two oxygen atoms from two bis(phenolate) ligands. To our best knowledge, this is the first example of a lithium atom being two-coordinated. The average Gd-O(Ar) bond length of 2.343(7) A˚ is apparently larger than the Gd-O(Ar) bond length in complex 4 (2.198(3) A˚) because of the formation of bridged bonding in the former. The average Gd-N bond length of 2.577(8) A˚ in complex 5 is comparable to the value in complex 4, whereas the Gd-C35 bond length is 2.131(11) A˚, which is apparently smaller than the corresponding value in complex 4 (2.463(5) A˚) and the cationic amidinate gadolinium alkyl complex (2.364(6) A˚).12 Two chlorine atoms are asymmetrically coordinated to the gadolinium atom. The Gd-Cl1 and Gd-Cl1A bond lengths are 2.860(3) and 2.901(3) A˚, respectively. The average Gd-Cl bond length of 2.880(4) A˚ is apparently larger than the values in {Na(μTHF)[(C5Me5)Gd(THF)]2(μ-Cl)3(μ3-Cl)2}2 (2.769(6) A˚)13 and (12) Bambirra, S.; Bouwkamp, M. W.; Meetsma, A.; Hessen, B. J. Am. Chem. Soc. 2004, 126, 9182. (13) Shen, Q.; Qi, M. H.; Lin, Y. H. J. Organomet. Chem. 1990, 399, 247.

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[(ButCp)2Gd(μ-Cl)]2 (2.771(3) A˚),14 which can be attributed to the steric bulkiness of the [ONNO]Gd(CH2SiMe3) moieties. The O1-Gd1-O2 bond angle of 149.7(2)° is apparently larger than the corresponding bond angle in complex 4, but is comparable to the amino-amino-bridged4k and methoxy-amino-bridged bis(phenolate)11 yttrium alkyl complexes mentioned above. The Li-O(Ar) bond lengths are 1.79(2) and 2.28(3) A˚, giving an average of 2.03(3) A˚, which is comparable with the average Li-O(Ar) bond lengths reported for the lithium-bridged bis(phenolate) complexes.15 Polymerization of L-Lactide by Complexes 1-5. Polyesters, such as poly(lactide), poly(glucol), poly(ε-caprolactone), and related copolymers, have been wide applied in the medical field as biodegradable surgical sutures or as a delivery medium for controlled release of drugs due to their biodegradable, biocompatible, and permeable properties.16 Ring-opening polymerization of a cyclic ester catalyzed/initiated by welldefined metal complexes has been proved to be an efficient method for the synthesis of polyesters with high molecular weights and narrow molecular weight distributions.17 Bridged bis(phenolate) lanthanide complexes have been found to be efficient initiators for the ring-opening polymerization of lactide.3-6 To further understand the effect of bis(phenolate) ligands on the polymerization activity and controllability, the catalytic behavior of complexes 1-4 for the ring-opening polymerization of L-lactide was examined. The representative L-lactide polymerization data are summarized in Table 4. It can be seen that all of these lanthanide alkyl complexes can initiate the ring-opening polymerization of L-lactide with very high activity, and the activity is apparently higher than those of the reported bridged bis(phenolate) lanthanide derivatives for this polymerization. For example, complexes 1 and 3 can initiate the complete polymerization of L-lactide within 3 min, even when the molar ratio of monomer to initiator reaches 1100, resulting in the TOF being as high as 22 000 h-1 (Table 4, entries 6 and 17). However, the methoxy-amino-bridged bis(phenolate) yttrium amido complex combined with 1 equiv of 2-propanol could polymerize 1000 equiv of rac-lactide for 1 h,4g the aminoamino-bridged bis(phenolate) yttrium alkyl complexes could polymerize only 300 equiv of rac-lactide,4k and the imidazolidine-bridged bis(phenolato) ytterbium amides could polymerize only 400 equiv of L-lactide in 1 h under the same polymerization conditions.6b Remarkably, the catalytic activity of the piperazidine-bridged bis(phenolate) ytterbium alkyl complex (complex 3) is comparable with that of the bimetallic bis(phenolate) (14) Song, S. P.; Shen, Q.; Jin, S. C.; Guan, J. W.; Lin, Y. H. Polyhedron 1992, 11, 2857. (15) (a) Ko, B. T.; Lin, C. C. J. Am. Chem. Soc. 2001, 123, 7973. (b) Chisholm, M. H.; Lin, C. C.; Gallucci, J. C.; Ko, B. T. Dalton. Trans. 2003, 406. (c) Hsueh, M. L.; Huang, B. H.; Wu, J. C.; Lin, C. C. Macromolecules 2005, 38, 9482. (d) Chen, H. Y.; Zhang, J.; Lin, C. C.; Reibenspies, J. H.; Miller, S. A. Green. Chem. 2007, 9, 1038. (e) Huang, C. A.; Chen, C. T. Dalton Trans. 2007, 5561. (16) (a) Chiellini, E.; Solaro, R. Adv. Mater. 1996, 8, 305. (b) Fujisato, T.; Ikada, Y. Macromol. Symp. 1996, 103, 73. (c) Drumright, R. E.; Gruber, P. R.; Henton, D. E. Adv. Mater. 2000, 12, 1841. (d) Agarwal, S.; Mast, C.; Dehnicke, K.; Greiner, A. Macromol. Rapid Commun. 2000, 21, 195. (e) Mecking, S. Angew. Chem., Int. Ed. 2004, 43, 1078. (17) (a) O’keefe, B. J.; Hillmyer, M. A.; Tolman, W. B. J. Chem. Soc., Dalton Trans. 2001, 2215. (b) Coates, G. W. J. Chem. Soc., Dalton Trans. 2002, 467. (c) Piers, W. E.; Emslie, D. J. H. Coord. Chem. Rev. 2002, 233-234, 131. (d) Dechy-Cabaret, O.; Martin-Vaca, B.; Bourissou, D. Chem. Rev. 2004, 104, 6147. (e) Chisholm, M.; Zhou, Z. J. Mater. Chem. 2004, 14, 3081. (f) Edelmann, F. T. Chem. Soc. Rev. 2009, 38, 2253. Hou, Z. M.; Wakatsuki, Y. Coord. Chem. Rev. 2002, 231, 1. (18) Wang, J. F.; Yao, Y. M.; Zhang, Y.; Shen, Q. Inorg. Chem. 2009, 48, 744.

Luo et al. Table 4. Polymerization of L-Lactide by Complexes 1-5a entry

initiator

[M]0/[I]0

t

yield (%)b

Mcc (104)

Mnd (104)

PDId

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28e

1 1 1 1 1 1 1 2 2 2 2 3 3 3 3 3 3 4 4 4 4 4 4 4 5 5 5 5

300 400 500 600 900 1100 1200 400 500 700 800 300 400 500 800 900 1100 500 600 800 900 1100 1200 1400 100 200 300 300

1 min 1 min 2 min 3 min 3 min 3 min 3 min 3 min 3 min 3 min 3 min 1 min 1 min 1 min 3 min 3 min 3 min 3 min 3 min 3 min 3 min 3 min 3 min 3 min 1 min 1 min 1h 1h

99 97 98 97 96 98 50 100 97 98 79 96 98 97 98 99 99 100 98 97 95 99 100 83 98 93 57 53

4.28 5.59 7.06 8.39 12.45 15.54 8.65 5.77 6.99 9.89 9.11 4.15 5.65 6.99 11.30 12.84 15.70 7.21 8.47 11.18 12.32 15.70 17.30 16.75 1.34 2.68 2.46 2.29

5.00 6.31 7.04 7.53 10.80 12.38

1.25 1.21 1.21 1.14 1.15 1.12

6.76 7.53 9.28 8.21 5.65 6.55 7.17 9.69 11.16 13.26 8.53 9.24 10.36 10.98 12.97 14.35 13.24 2.61 5.22 2.62 1.17

1.41 1.32 1.31 1.27 1.13 1.13 1.14 1.12 1.15 1.13 1.40 1.37 1.35 1.33 1.40 1.41 1.21 1.24 1.24 1.23 1.24

a General polymerization conditions: in toluene, [L-lactide] = 1 mol/ L, 70 °C. b Yield: weight of polymer obtained/weight of monomer used. c Mc = 144.13  [M]0/[I]0  (polymer yield) (%). d Measured by GPC in THF calibrated with standard polystyrene samples and corrected using the Mark-Houwink factor of 0.58. e In THF.

ytterbium amide,6c which is one of the most active ytterbium initiators for L-lactide polymerization reported to date.18 It is reasonable to propose a common catalytic species derived from both the mononuclear and dinuclear complexes around the same metal. The high activity might be attributed to the constrained conformation of the bis(phenolate) groups in these alkyl complexes, which resulted in a more open coordination sphere around the metal center. The ionic radii of the lanthanide metals have a profound effect on the catalytic activity, and the activity increased with the increase of the ionic radius. The gadolinium alkyl complex (4) showed the highest activity among these complexes, and the yield reached 83% in 3 min even when the molar ratio of monomer to initiator increases to 1400 (Table 4, entry 24). The observed activity order is in agreement with the activity trend observed in the imidazolidine-bridged bis(phenolate) lanthanide amido complexes,6b as well as in the bis(allyl) diketiminatolanthanide complexes for L-lactide polymerization.19 In contrast, complex 5 showed rather low activity for this polymerization. The yield is only 57% even when the molar ratio of monomer to initiator decreases to 300 and the polymerization time was prolonged to 1 h (Table 4, entry 27). The dramatic decrease in activity should be attributed to the steric congestion around the gadolinium atom for the formation of dimeric structure. The catalytic activity of complex 5 in THF is similar to that in toluene (Table 4, entry 28), indicating a stable dimeric structure even in THF at 70 °C. (19) Sachez-Barba, L. F.; Hughes, D. L.; Humphrey, S. M.; Bochmann, M. Organometallics 2005, 24, 3792.

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Table 5. Polymerization of rac-Lactide by Complex 1a

Figure 3. Relationship between the number-averaged molecular weight (Mn) and the molar ratio of monomer to initiator. Polymerization was initiated by complex 1 in toluene at 70 °C.

Organolanthanide alkoxides are efficient initiators for the controlled ring-opening polymerization of cyclic esters, such as ε-caprolactone,3b lactide,4g and β-butyllactone.4o Because of the low nucleophilicity of amido and alkyl groups in comparison with alkoxides,5c it was regarded, compared to organolanthanide alkoxides, that organolanthanide amido and alkyl complexes usually are much inferior in controllability for the polymerization of these cyclic esters. However, it was found that these piperazidine-bridged bis(phenolate) lanthanide alkyl complexes are efficient initiators for the polymerization of L-lactide in toluene, giving polymers with high molecular weights and relatively narrow molecular weight distributions (Mw/Mn =1.12-1.41). Meanwhile, the corrected number-averaged molecular weights Mn, with the Mark-Houwink factor of 0.58,20 are close to the calculated Mn, indicating a controllable polymerization. To further elucidate the controllable character of the polymerization, the relationship between the number-averaged molecular weight (Mn) and the molar ratio of monomer to initiator ([M]/[I]) was measured in toluene at 70 °C with complex 1 as an initiator. As depicted in Figure 3, a good linear relationship between Mn and the molar ratio of monomer to initiator was observed, while the molecular weight distributions remained almost unchanged (Table 4, entries 1-6). These results proved that the polymerization proceeded in a controllable fashion. Polymerization of rac-Lactide. Recently, stereoselective polymerization of rac-lactide has received considerable attention. This polymerization is extremely sensitive to the structures of the catalysts, because the environment of the last unit of the propagating active species must be sterically proper to incorporate the configurationally opposite enantiomer.4k It was found that bridged bis(phenolate) lanthanide derivatives, including alkyl,4k amide,4a,5d alkoxide,4g aryloxide,4s and borohydride,4e can initiate the heteroselective polymerization of rac-lactide, giving polymers with medium to very high tacticity depending on the ancillary ligands. To further elucidate the effect of the structures of the bridged bis(phenolate) groups on the polymerization stereoselectivity, the catalytic behavior of complex 1 for the ringopening polymerization of rac-lactide was also tested. For (20) Kowalski, A.; Duda, A.; Penczek, S. Macromolecules 1998, 31, 2114.

entry

[M]0/[I]0

t

yield (%)b

Mcc (104)

Mnd (104)

PDId

Pr/Pme

1 2 3 4 5 6 7 8 9f 10f 11f 12g 13g 14g

100 300 500 700 900 1100 1200 1400 300 500 700 300 500 700

1h 1h 1h 2h 4h 4h 4h 12 h 1h 1h 2h 1h 1h 2h

98 96 95 100 98 98 92 14 97 83 29 94 95 82

1.41 4.15 6.84 10.08 12.70 15.52 15.90

1.37 3.98 4.48 6.30 7.01 7.24 7.44

1.58 1.58 1.48 1.61 1.61 1.60 1.48

0.48/0.52 0.52/0.48 0.58/0.42 0.58/0.42 0.61/0.39 0.59/0.41 0.61/0.39

4.19 5.98 2.93 4.06 6.85 8.27

8.42 8.46 1.60 9.86 9.74 9.48

1.58 1.81 1.83 1.48 1.64 1.65

0.62/0.38 0.62/0.38 0.65/0.35 0.30/0.70 0.26/0.74 0.24/0.76

a General polymerization conditions: THF as the solvent, [rac-lactide] = 1 mol/L. b Yield: weight of polymer obtained/weight of monomer used. c Mc = 144.13  [M]0/[I]0  (polymer yield) (%). d Measured by GPC calibrated with standard polystyrene samples and corrected using the Mark-Houwink factor of 0.58. e Measured by homodecoupling 1 HNMR spectroscopy at 20 °C in CDCl3. f Using imidazolidinebridged bis(phenolato) yttrium amide as the initiator.6b g Using “ate” piperazidine-bridged bis(phenolato) ytterbium amide as the initiator.6c

comparison, the known imidazolidine-bridged bis(phenolato) yttrium amide6b and the “ate” piperazidine-bridged bis(phenolato) ytterbium amide6c were screened for the polymerization of rac-lactide. These results are summarized in Table 5. It can be seen that complex 1 is an efficient initiator for the ring-opening polymerization of rac-lactide, and the polymerizations proceed smoothly in THF at 20 °C. However, the molecular weights of the resulting polymers deviate significantly from the calculated ones, and the molecular weight distributions are relatively broad in comparison with those of L-lactide polymerization initiated by complex 1 (Table 4, entries 1-6). These results indicated that the controllability of complex 1 for rac-lactide polymerization is inferior to that for L-lactide polymerization. Complex 1 showed apparently higher activity for rac-lactide polymerization in comparison with the imidazolidine-bridged bis(phenolato) yttrium amide and the “ate” piperazidine-bridged bis(phenolato) ytterbium amide. For example, complex 1 can polymerize completely 700 equiv of raclactide in 2 h (entry 4), but the yields are 29% and 82% respectively using the imidazolidine-bridged bis(phenolato) yttrium amide and the “ate” piperazidine-bridged bis(phenolato) ytterbium amide as the initiators under the same polymerization conditions (entries 11 and 14). The activity of complex 1 is also higher than those of the amino-amino-bridged bis(phenolate) yttrium alkyl complexes,4k but is slightly lower than that of the methoxy-amino-bridged bis(phenolate) yttrium amido complex combined with 1 equiv of 2-propanol.4g However, complex 1 showed almost no stereoselectivity for rac-lactide polymerization, and the Pr values of the resultant polymers range from 0.48 to 0.61. The stereocontrollability of complex 1 is similar to those of the dithiaalkanediyl-bridged bis(phenolate) rare-earth metal complexes,5f but is worse than those of the amine-bridged bis(phenolate) yttrium alkyl and aryloxo complexes, which give high heterotactic polymers with Pr over 0.95.4g,s Polymerizations of rac-lactide initiated by the imidazolidine-bridged bis(phenolato) yttrium amide also gave atactic polymers with Pr values ranging from 0.62 to 0.65 (entries 9-11). In contrast, the “ate” piperazidine-bridged bis(phenolato) ytterbium amide showed isotactic selectivity for this polymerization. The resulting polymes have Pm values ranging from 0.70 to 0.76

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(entries 12-14). To our best knowledge, examples of isotactic selective polymerization of rac-lactide using organolanthanide complexes as the initiators are rare.21

Conclusion In summary, a series of piperazidine-bridged bis(phenolate) lanthanide metal alkyl complexes were successfully synthesized via alkane elimination reaction, and their structure features have been determined by X-ray diffraction study. On the basis of our previous results, it seems that the steric bulkiness of the amido or the alkyl groups played a critical role in governing the formation of the types of lanthanide complexes. In this case, the reaction of lanthanide tris(trimethylsilylmethyl) complexes Ln(CH2SiMe3)3(THF)2 with piperazidine-bridged bis(phenol) gave only mononuclear lanthanide alkyl complexes instead of the expected bimetallic species. These piperazidine-bridged bis(phenolate) lanthanide alkyl complexes can initiate the ring-opening (21) (a) Zi, G. F.; Wang, Q. W.; Xiang, L.; Song, H. B. Dalton Trans. 2008, 5930. (b) Heck, R.; Schulz, E.; Collin, J.; Carpentier, J.-F. J. Mol. Catal. A: Chem. 2007, 268, 163.

Luo et al.

polymerization of L-lactide with both high activity and good controllability, whereas the yttrium alkyl complex showed poor stereoselectivity for rac-lactide polymerization. Further work exploring the formation and the reactivity of bimetallic organolanthanide derivatives stabilized by piperazidine-bridged bis(phenolate) ligands is in progress.

Acknowledgment. Financial support from the National Natural Science Foundation of China (Grants 20771078, 20972108, and 20632040), the Major Basic Research Project of the Natural Science Foundation of the Jiangsu Higher Education Institutions (Project 07KJA15014), and the Qing Lan Project is gratefully acknowledged. Y.L. is also grateful for the financial support by Zhejiang Provincial Natural Science Foundation (Y4090617), State Key Laboratory of Polymer Physics and Chemistry (200902), and Key Laboratory of Organic Synthesis of Jiangsu Province (KJS0907). Supporting Information Available: Crystallographic data for complexes 1-5 in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.