CCC-Pincer Bis(carbene) Lanthanide Dibromides. Catalysis on Highly

Jun 2, 2010 - Catalysis on Highly cis-1,4-Selective Polymerization of Isoprene and Active ... Graduate School of the Chinese Academy of Sciences, Beij...
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Organometallics 2010, 29, 2987–2993 DOI: 10.1021/om1002039

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CCC-Pincer Bis(carbene) Lanthanide Dibromides. Catalysis on Highly cis-1,4-Selective Polymerization of Isoprene and Active Species Kui Lv†,‡ and Dongmei Cui*,† †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China, and ‡ Graduate School of the Chinese Academy of Sciences, Beijing 100039, People’s Republic of China Received March 16, 2010

A series of CCC-pincer 2,6-xylenyl bis(carbene)-ligated rare-earth metal dibromides (PBNHC)LnBr2(THF) ((PBNHC) = 2,6-(2,4,6-Me3C6H2NCHCHNCCH2)2C6H3; Ln = Sc (1), Y (2), La (3), Nd (4), Sm (5), Gd (6), Dy (7), Ho (8), Tm (9), Lu (10)) have been synthesized. Upon activation with AlR3 (R = Me, Et, iBu) and [Ph3C]þ[B(C6F5)4]-, complexes 2, 4, 6, 7, and 8 exhibited high activity and cis-1,4 selectivity (99.6%, 25 °C) toward the polymerization of isoprene, although complexes 1, 3, 5, 9, and 10 were inert. The selectivity was not affected by the nature of the central metal and AlR3 and was maintained at elevated temperatures up to 80 °C (97.4%). The yttrium hydrido aluminate cation [(PBNHC)Y(μ-H)2AliBu2]þ was identified as the active species according to NMR spectroscopic analysis.

Introduction To meet the shortage of natural rubber and the increasing demand for high-performance rubbers, the synthesis of polyisoprene, an alternative of natural rubber, having microstructures with a higher than 98% cis-1,4 regularity and a higher molecular weight, has become increasingly important.1-5 Such microstructures can be achieved by rational design of the catalysts applied in the polymerization. Thus for half a century, many efforts have been devoted to innovating highly cis-1,4-selective catalytic systems, leading to the discovery of the Ziegler-Natta catalysts based on transition metals and lanthanide elements and the analogous

molecular lanthanide-aluminate6-8 as well as the singlesite cationic systems comprising lanthanidocenes or non-cyclopentadienyl lanthanide hydrocarbyls and noncyclopentadienyl lanthanide halogens.2,9-12 Despite these achievements, further exploration of new catalyst systems providing over 98% cis-1,4 selectivity and homogeneous, well-defined, straightforward, and high-yielding syntheses and thermodynamic stability, to satisfy requirements from both academic and industrial fields, has remained a fascinating and challenging subject. In the meantime, research has also focused on elucidating the active species or the transition state for this polymerization process, which may facilitate improving the catalytic performances of the existing catalyst systems and designing new ones, and,

*Corresponding author. Fax: þ86-431-85262773. E-mail: dmcui@ ciac.jl.cn. (1) Kirk-Othmer Encyclopedia of Chemical Technology, 2nd ed.; Wiley: New York, 1965; Vol. 7. (2) (a) Dolgoplosk, B. Polym. Sci. U.S.S.R. 1971, 13, 367. (b) Marechal, J.; Dawans, F.; Teyssie, P. J. Polym. Sci., Part A: Polym. Chem. 1970, 1993. (c) Dawans, F.; Duran, J.; Teyssie, P. J. Polym. Sci., Part B: Polym. Phys. 1972, 10, 493. (3) (a) Terry, L. Chem. Week 1989, 144, 40. (b) Terry, L. Chem. Week 1999, 161, 18. (4) Evans, W. J.; Giarikos, D. G.; Ziller, J. W. Organometallics 2001, 20, 5751. (5) Zhao, J.; Ghebremeskel, G. N. Rubber Chem. Technol. 2001, 74, 409. (6) (a) Shen, Z.; Ouyang, J.; Wang, F.; Hu, Z.; Yu, F.; Qian, B. J. Polym. Sci., Polym. Chem. Ed. 1980, 18, 3345. (b) Mazzei, A. Makromol. Chem., Suppl. 1981, 4, 61. (c) Wilson, D.; Jenkins, D. Polym. Bull. 1992, 27, 407. (7) (a) Dong, W.; Masuda, T. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 1838. (b) Dong, W.; Endo, K.; Masuda, T. Macromol. Chem. Phys. 2003, 204, 104. (c) Dong, W.; Masuda, T. Polymer 2003, 44, 1561. (d) Friebe, L.; Nuyken, O.; Windisch, H.; Obrecht, W. Macromol. Chem. Phys. 2002, 203, 1055. (e) Kobayashi, E.; Hayashi, N.; Aoshima, S.; Furukawa, J. J. Polym. Sci., Polym. Chem. 1998, 36, 1707. (f) Bonnet, F.; Violante, C.; Roussel, P.; Mortreux, A.; Visseaux, M. Chem. Commun. 2009, 23, 3380. (g) Visseaux, M.; Mainil, M.; Terrier, M.; Mortreux, A.; Roussel, P.; Mathivet, T.; Destarac, M. Dalton Trans. 2008, 4558.

(8) For a selected review, see: (a) Shen, Z. Inorg. Chim. Acta 1987, 140, 7. (b) Shen, Z.; Ouyang, J. Handbook of the Physics and Chemistry of Rare Earth; Gschneidner, K., Fleming, L., Jr., Eds.; Elsevier: Amsterdam, 1987; Chapter 61 (Rare earth coordination catalysts in stereospecific polymerization). (c) Kuran, W. Principle of Coordination Polymerization; John Wiley and Sons Ltd.: New York, 2001. (d) Friebe, L.; Nuyken, O.; Obrecht, W. Adv. Polym. Sci. 2006, 204, 1. (e) Fischbach, A.; Anwander, R. Adv. Polym. Sci. 2006, 204, 155, and references therein. (9) For the active species [{[(C5Me5)La{(μ-Me)2AlMe(C6F5)}][Me2Al(C6F5)2]}2] for trans-1,4 polymerization of isoprene, see: Zimmermann, M.; T€ ornroos, K.; Anwander, R. Angew. Chem., Int. Ed. 2008, 47, 775. (10) (a) Fischbach, A.; Meermann, C.; Eickerling, G.; Scherer, W.; Anwander, R. Macromolecules 2006, 39, 6811. (b) Meermann, C.; T€ornroos, K.; Nerdal, W.; Anwander, R. Angew. Chem., Int. Ed. 2007, 46, 6508. (c) Fischbach, A.; Perdih, F.; Herdtweck, E.; Anwander, R. Organometallics 2006, 25, 1626. (d) Fischbach, A.; Klimpel, M.; Widenmeyer, M.; Herdtweck, E.; Scherer, W.; Anwander, R. Angew. Chem., Int. Ed. 2004, 43, 2234. (e) Maiwald, S.; Weissenborn, H.; Windisch, H.; Sommer, C.; M€uller, G.; Taube, R. Macromol. Chem. Phys. 1997, 198, 3305. (f) Maiwald, S.; Sommer, C.; M€uller, G.; Taube, R. Macromol. Chem. Phys. 2001, 202, 1446. (11) Arndt, S.; Beckerle, K.; Zeimentz, P. M.; Spaniol, T. P.; Okuda, J. Angew. Chem., Int. Ed. 2005, 44, 7473. (12) Zhang, L.; Suzuki, T.; Luo, Y.; Nishiura, M.; Hou, Z. Angew. Chem., Int. Ed. 2007, 46, 1909.

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moreover, is crucial to explain the mechanism.13 For the Ziegler-Natta-type catalyst systems, it is commonly accepted that the formation of the active species, although lacking definition, involves the generation of a metalcarbon bond or a metal-hydride bond followed by cationization via Al-to-Ln chloride transfer,14 while for the single-site systems the ligand- or electron-donor-coordinated metal-alkyl cations (or dications) have been considered as quasi-living intermediates, although they are usually inert alone.9-12 Therefore, to date, the molecular structures of the active species still remain ambiguous, especially for the more complicated systems incorporating both aluminum alkyls and organoborate cocatalysts. On the other hand, the “pincer” architecture, such as pincer phosphines and amines,15-17 provides a preorganized backbone capable of blocking meridional (or pseudo-meridional) coordination sites of the metal center, leaving the remainder available for catalysis. Meanwhile, functionalized N-heterocyclic carbene (NHC) ligands are hemilabile and covalently bond to the metal centers, which allow easily adjusting the coordination sphere and rigidity and chirality of the attached metal centers.18-21 Thus pincer carbenes,22-26 the combination of the pincer chelating effect with the strong electron-donating ylidene carbon, should provide complexes with enhanced stability and a precisely controlled metal coordination sphere, which makes the complexes potential homogeneous catalysts. To date CNC-, CPC-, and rare CCC-pincer bis(NHC)s attached transition metal complexes have been isolated and (13) The active species of the multinuclear complex with the composition Nd(OiPr)3-AlEt3-AlEt2Cl was reported 20 years ago, which represented a kind of active species. See: (a) Boisson, C.; Barbotin, F.; Spitz, R. Macromol. Chem. Phys. 1999, 200, 1163. (b) Shan, C.; Lin, Y.; Ouyang, J.; Fan, Y.; Yang, G. Makromol. Chem. 1987, 188, 629. (14) (a) Evans, W.; Ulibarri, T.; Ziller, W. J. Am. Chem. Soc. 1990, 112, 2314. (b) Fischbach, A.; Perdih, F.; Sirsch, P.; Scherer, W.; Anwander, R. Organometallics 2002, 21, 4569. (c) Dietrich, H.; Zapilko, C.; Herdtweck, E.; Anwander, R. Organometallics 2005, 24, 5767. (d) Evans, W.; Champagne, T.; Ziller, J. Chem. Commun. (Cambridge, U.K.) 2005, 5925. (e) Evans, W.; Champagne, T.; Ziller, J. Organometallics 2005, 24, 4882. (15) (a) van Koten, G. Pure Appl. Chem. 1989, 61, 1681–1694. (b) Albrecht, M.; van Koten, G. Angew. Chem., Int. Ed. 2001, 40, 3750. (c) Rybtchinski, B.; Milstein, D. Angew. Chem., Int. Ed. 1999, 38, 870. (d) Jensen, C. Chem. Commun. 2000, 2443. (16) (a) Peris, E.; Mata, J.; Loch, J.; Crabtree, R. Chem. Commun. 2001, 201. (b) Kamalesh Babu, R.; Babu, R.; McDonald, R.; Decker, S.; Klobukowski, M.; Cavell, R. Organometallics 1999, 18, 4226. (c) Kamalesh Babu, R.; Cavell, R. G.; McDonald, R. Chem. Commun. 2000, 481. (17) Hu, X.; Castro-Rodriguez, I.; Meyer, K. Organometallics 2003, 22, 3016. (18) D€ ohring, A.; G€ ohre, J.; Jolly, P.; Kryger, B.; Rust, J.; Verhovnik, G. Organometallics 2000, 19, 388. (19) (a) Aihara, H.; Matsuo, T.; Kawaguchi, H. Chem. Commun. 2003, 2204. (b) Jones, N.; Liddle, S. T.; Wilson, C.; Anold, P. Organometallics 2007, 26, 755. (20) (a) Spencer, L.; Winston, S.; Fryzuk, M. Organometallics 2004, 23, 3372. (b) Spencer, L.; Fryzuk, M. J. Organomet. Chem. 2005, 690, 5788. (21) (a) Wang, Z.; Sun, H.; Yao, H.; Shen, Q.; Zhang, Y. Organometallics 2006, 25, 4436–4438. (b) Li, W.; Sun, H.; Chen, M.; Wang, Z.; Hu, D.; Shen, Q.; Zhang, Y. Organometallics 2005, 24, 5925. (22) Edworthy, I.; Blake, A.; Wilson, C.; Arnold, P. Organometallics 2007, 26, 3684. (23) Pugh, D.; Danopoulos, A. Coord. Chem. Rev. 2007, 251, 610. (24) (a) Mata, J.; Poyatos, M.; Peris, E. Coord. Chem. Rev. 2007, 251, 841. (b) Hahn, F. E.; Jahnke, M. C.; Pape, T. Organometallics 2007, 26, 150. (25) Mas-Marza, E.; Poyatos, M.; Sanau, M.; Peris, E. Organometallics 2004, 23, 323. (26) Peris, E.; Mata, J.; Loch, J.; Crabtree, R. Chem. Commun. 2001, 201. (27) Tulloch, A.; Danopoulos, A.; Tizzard, G. J.; Coles, S.; Hursthouse, M.; Hay-Motherwell, R.; Motherwell, W. Chem. Commun. 2001, 1270. (28) Chen, J.; Lin, I. Dalton Trans. 2000, 839. (29) Gr€ undemann, S.; Albrecht, M.; Loch, J.; Faller, J.; Crabtree, R. Organometallics 2001, 20, 5485.

Lv and Cui Table 1. Selected Bond Lengths (A˚) and Bond Angles (deg) for Complexes 2, 4, 6, and 7

Ln-C(7) Ln-C(22) Ln-C(36) Ln-O(1) Ln-Br(1) Ln-Br (2) C(7)-Ln (1)-C(22) Br(1)-Ln(1)-Br(2)

Ln = Y

Ln = Nd

Ln = Gd

Ln = Dy

2.541(3) 2.541(3) 2.479(4) 2.403(3) 2.7689(3) 2.7689(3) 156.89(15) 172.82(2)

2.628(9) 2.640(9) 2.613(9) 2.574(6) 2.8197(14) 2.8005(14) 150.1(3) 165.42(5)

2.569(10) 2.579(10) 2.552(10) 2.515(8) 2.7400(16) 2.7653(15) 152.9(4) 165.89(6)

2.520(9) 2.536(9) 2.502(10) 2.476(7) 2.7087(14) 2.7188(14) 156.1(3) 167.23(5)

showed activity toward Suzuki coupling reaction;22-24,27-30 however few examples of catalytic performance for polymerization have been reported.31 We found recently that the NCNpincer bis(imido) ligand stabilized lanthanide complexes exhibited an excellent cis-1,4 selectivity for the polymerization of 1,3-conjugated dienes, which contributed significantly to the C2 symmetric geometry of the ligands.32 Intrigued by this result, we employed the CCC-pincer bis(NHC)s to prepare the corresponding Sc, Sm, and Lu dibromide complexes.33 Unfortunately these complexes are inert for the polymerization of isoprene. Herein we report the analogous complexes based on other lanthanide elements and their distinguished cis-1,4 selectivity (99.6%) with high activity toward isoprene polymerization in the presence of AlR3 and organoborate. The role that the CCC-pincer bis(NHC) ligand plays in tuning the specific selectivity of the attached metal center will be shown. Moreover, the isolation and identification of the yttrium hydrido aluminate bimetallic cation, the probable active species arising from a ternary catalytic system, will be discussed.

Results and Discussion Preparation and Characterization of CCC-Pincer Bis(NHC)-Ligated Rare-Earth Metal Dibromides. The one-pot reaction of (2,6-(2,4,6-Me 3C 6 H2 -NCHCHNCCH2 )2-1Br-C6H3) 3 2HBr ((PBNHC-Br) 3 2HBr) with LnCl3 and 3.0 equiv of nBuLi in THF at room temperature under vigorous stirring afforded the title CCC-pincer bis(NHC)-ligated rareearth metal dibromides (PBNHC)LnBr2(THF) (Ln = Sc (1),33 Y (2), La (3), Nd (4), Sm (5),33 Gd (6), Dy (7), Ho (8), Tm (9), Lu (10)33) in moderate to high yields. 1H and 13C NMR spectral analysis of the yttrium complex 2 showed the absence of the ylidene proton from the ligand and formation of M-σ-C bond between the yttrium ion and the ylidene carbons (δ 188.46-192.44), which was comparable to its lutetium analogue 10 reported previously (complexes 4, 6, and 7 are paramagnetic).33 The molecular structures of complexes 2, 4, 6, and 7 were further confirmed by X-ray diffraction analyses as THF-solvated isomers. The monoanionic CCC-pincer bis(NHC) ligand coordinates to the central metal ion in a κC:κC:κC0 tridentate mode with the two NHC moieties anti-arranged, adopting a pseudomeridional conformation around the central metal. The two N-aryl rings and the coordinating THF ring are almost parallel. The two bromo groups are disposed almost inline, forming a large Br-Ln-Br angle that varies from 165.42(5)° to 172.82(2)° for complexes 2, 4, 6, and 7, respectively. The Br-Ln-Br angle is bisected by the xylenyl plane (Table 1). The trend of (30) Andavan, G. T. S.; Bauer, E. B.; Letko, C. S.; Hollis, T. K.; Tham, F. S. J. Organomet. Chem. 2005, 690, 5938. (31) Poyatos, M.; Mata, J. A.; Peris, E. Chem. Rev. 2009, 109, 367. (32) Gao, W.; Cui, D. J. Am. Chem. Soc. 2008, 130, 4984. (33) Lv, K.; Cui, D. Organometallics 2008, 27, 5438.

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Table 2. Polymerization of Isoprene under Various Conditionsa microstructurec entry

catalyst

AlR3

[IP]/ [Ln]

Tp (°C)

time (min)

yield (%)

Mnb (10-4)

Mw/Mn

cis-1,4

3,4-

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15d 16e 17f 18 19 20 21g

2 (Y) 2 (Y) 2 (Y) 4 (Nd) 4 (Nd) 4 (Nd) 6 (Gd) 6 (Gd) 6 (Gd) 7 (Dy) 7 (Dy) 7 (Dy) 8 (Ho) 7 (Dy) 7 (Dy) 7 (Dy) 7 (Dy) 7 (Dy) 7 (Dy) 7 (Dy) Y-H-Al

AliBu3 AlEt3 AlMe3 AliBu3 AlEt3 AlMe3 AliBu3 AlEt3 AlMe3 AliBu3 AlEt3 AlMe3 AliBu3 AliBu3 AliBu3 AliBu3 AliBu3 AliBu3 AliBu3 AliBu3

500 500 500 500 500 500 500 500 500 500 500 500 500 2000 3000 4000 5000 1000 1000 1000 500

25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 40 60 80 25

300 300 300 90 15 30 180 20 30 200 30 60 120 180 300 600 900 60 60 60 100

50 100 80 100 100 100 100 100 100 100 100 100 20 100 100 100 100 71 86 64 40

14.4 9.4 17.4 28.0 19.0 62.2 23.7 26.5 32.4 15.1 11.2 13.7 33.3 31.1 54.1 83.0 95.6 9.4 6.1 3.9 10.2

3.81 2.22 3.08 3.09 1.77 1.70 3.87 1.73 1.62 2.61 2.27 2.45 3.29 2.95 2.37 2.12 2.07 2.44 1.65 1.40 2.83

99.6 99.4 99.3 97.3 96.3 96.7 98.6 97.8 97.8 99.3 98.6 99.1 99.5 99.2 99.4 99.3 99.1 98.4 98.0 97.6 99.5

0.4 0.6 0.7 2.7 3.7 3.3 1.4 2.2 2.2 0.7 1.4 0.9 0.5 0.8 0.6 0.7 0.9 1.6 2.0 2.3 0.5

a C6H5Cl (5 mL), complex (20 μmol), RT. [Ln]0/[AlR3]0/[B]0 = 1:20:1 (B = [Ph3C]þ[B(C6F5)4]-). The catalytic systems were obtained in the following reaction sequence: the Ln compound was first reacted with AlR3 and then B was added. b Determined by gel permeation chromatography (GPC) with respect to a polystyrene standard. c Determined by the 13C NMR spectrum of polyisoprene. d C6H5Cl (5 mL). e C6H5Cl (8 mL). f C6H5Cl (10 mL). g The polymerization was performed with the oil isolated from a mixture of PBNHCYBr2(THF) with 20 equiv of AliBu3 and 1.0 equiv of [Ph3C]þ[B(C6F5)4]-.

the Ln-C(36) bond lengths, 2.341(8) A˚ in 1 < 2.443(8) A˚ in 10 < 2.479(4) A˚ in 2 < 2.502(10) A˚ in 7 < 2.552(10) A˚ in 6 < 2.575(8) A˚ in 5 < 2.613(9) A˚ in 4, is in good agreement with that of ionic radii of the corresponding lanthanide elements. Accordingly the Ccarbene-Ln-Ccarbene bond angle is 150.1(3)° in 4 < 152.4(2)° in 5 < 152.9(4)° in 6 < 156.1(3)° in 7 < 156.89(15)° in 2 < 158.9(3)° in 10 < 163.9(3) in 1, a trend inverse that of ionic radii, suggesting that the smaller the ionic radius, the closer the two mesityl rings of the ligand are drawn. This close arrangement of the mesityl rings leads to a very crowded coordination sphere around the metal centers, which exerts significant influence on the activity and cis-1,4 selectivity of the complexes (vide infra). Isoprene Polymerization. All these CCC-pincer bis(NHC)ligated rare-earth metal dibromides 1-10 were tested for the polymerization of isoprene under the activation of AlR3 (R = Me, Et, iBu) and [Ph3C]þ[B(C6F5)]-. Although featuring a similar pincer architecture to the reported aryldiimine-ligated rare-earth metal dichlorides,32 complexes 1-10 behaved differently, as shown in Table 2. The catalyst activity is strongly dependent on the central metal type. The Nd complex activated by AlEt3 exhibited the highest activity, finishing the polymerization within 15 min at room temperature (Table 2, entry 5). Under the same conditions, Gd and Dy analogues showed slightly lower activities, giving complete conversion within 30 min. The Y and Ho counterparts were the least active, in which cases much longer times were needed for completion (Table 2, entries 8 and 11), while those complexes with the largest (La) and the smallest (Sc, Tm, Lu) metal centers were inert. The samarium complex was also nonactive, and was presumed to be reduced to the inert divalent counterpart by AlR3, evidenced by the formation of a purple oil. The bulkiness of aluminum alkyls also influenced obviously the activity in the trend AlEt3 > AlMe3.AliBu3 regardless of the central metal type. Moreover these systems demonstrated the controllable nature. When 7 (Dy) was chosen as the precursor, the polymerization could be performed smoothly

Figure 1. Perspective view of complex 7 with thermal ellipsoids drawn at the 30% probability level. Hydrogen atoms are omitted for clarity.

under various monomer-to-initiator ratios ranging from 500 to 5000, and correspondingly the molecular weight of the resultant polyisoprene increased linearly while the molecular weight distribution kept almost constant (Table 2, entries 14-17). In some systems a broad molecular weight distribution was found, which could be attributed to the slow initiation and fast polymer-chain propagation, a common feature of nonliving coordination polymerization systems. Remarkably, these systems displayed excellent cis-1,4 selectivity. The most active neodymium complex (4) was slightly lower in cis-1,4 selectivity compared to the gadolinium counterpart 6. The yttrium (2) and the dysprosium (7) analogues exhibited extremely high cis-1,4 selectivities, up to 99.6% higher than that of the NCN-pincer-ligated rare-earth metal dichloride system (98.8%)32 (Table 2, entries 1-3, 10-17, Figure 2). This could be attributed to the more crowded environment around the active metal center caused

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Figure 2. 13C NMR spectrum of the polymer with 99.6% cis-1,4 regularity obtained with 2/20AliBu3/[Ph3C]þ[B(C6F5)4]- at room temperature (entry 10).

Figure 3. Combined 1H NMR spectra (C6D6, 25 °C) of AliBu3 (A), (PBNHC)YBr2(thf)2 (B), (PBNHC)YBr2(THF)/2AliBu3 (C), and (PBNHC)YBr2(THF)2/2AliBu3/[Ph3C]þ[B(C6F5)4]- (D).

by the two much closer N-phenyl rings of the ligands in 2 and 7 than that in 4 and 6, which was proved by the larger C(7)Ln-C(22) bond angles (156.89(15)° (2) and 167.23(5)° (7) vs 165.42(5)° (4) and 165.89(6)° (6)) (vide supra). The selectivity seemed to be affected slightly by the bulkiness of the aluminum alkyls, which was in contrast to the previously reported systems, where changing the type of AlR3 caused a change in the selectivity, for instance, from cis-1,4 to trans-1,4.34 Noteworthy was that the distinguished cis-1,4 selectivity was almost maintained at elevated polymerization temperatures up to 80 °C. To date, few catalytic systems are known to be so stable at such a high temperature, indicating the stable nature of these cationic species at least in the presence of monomer, which should be attributed to the strong (34) Kaita, S.; Hou, Z.; Wakatsuki, Y. Macromolecules 1999, 32, 9078.

coordination of electron-donating carbene carbon atoms to the central lanthanide ions. Active Species. The formation of active species was monitored by means of NMR spectroscopy technique. Complex 2 was chosen as the precursor, which was treated with 2.0 equiv of AliBu3 in C6D6 at room temperature. The anticipated alkylation of 2 by AliBu3 did not take place but gave an AliBu3-adduct product (I), as shown by the spectra A, B, and C in Figure 3. The coordinating THF molecule in 2 was taken by AliBu3 according to the changes of the corresponding chemical shifts of THF (C in Figure 3). Increasing the amount of AliBu3 to 10.0 equiv still could not initiate the alkylation, whereas upon addition of [Ph3C]þ[B(C6F5)]- to the system 2/2AliBu3, the reaction took place immediately, which was monitored with NMR spectroscopy. The resultant mixture separated into two portions: a red oil deposited at the bottom of the flask and an upper clear solution. The oil

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Figure 4. 1H NMR spectrum (C6D4Cl2, 25 °C) of the intermediate II.

Figure 5. 1H NMR spectrum (tol-d8, 25 °C) of the active species VI (*: possibly the signals for isobutyl-bridged yttrium aluminate; Δ: impurities from borate).

might be the bromo-bridged bimetallic cation [(PBNHC)Y( μ-Br)2AliBu2]þ[B(C6F5)4]- (II), which has low solubility in C6D6 (Figure 4); the simultaneously formed Ph3CH (δ 5.61) and isobutene (δ 4.94 and δ 1.78) dissolved in the upper solution (D in Figure 3).32,35 The intermediate II was proved to be inactive. On the basis of our previous study on the NCNpincer-type rare-earth metal halogen complexes that more than 5.0 equiv of AlR3 was requisite to generate the active bis(alkyl)bridged bimetallic species,32 in this work, we found that an excess amount of AlR3 was also requisite to generate the active species. Thus a mixture of 2/10AliBu3/[Ph3C]þ[B(C6F5)]- was employed and the reaction performed in benzene. As expected, a red oil appeared at the bottom of the reaction mixture and turned pure red after decanting the colorless supernatant, (35) For propylene polymerization, Zr- and Hf-based hydrido aluminates have been defined to be the active species. See: (a) Bryliakov, K. P.; Talsi, E. P.; Semikolenov, N. V.; Zakharov, V. A.; Brand, J.; AlonsoMoreno, C.; Bochmann, M. J. Organomet. Chem. 2007, 692, 859. (b) Carr, A. G.; Dawson, D. M.; Thornton-Pett, M.; Bochmann, M. Organometallics 1999, 18, 2933. (c) Bryliakov, K. P.; Talsi, E. P.; Voskoboynikov, A. Z.; Lancaster, S. J.; Bochmann, M. Organometallics 2008, 27, 6333.

Scheme 1. Synthesis of CCC-Pincer Bis(NHC) Lanthanide Dibromides via in Situ Reaction

which strikingly showed the same catalytic performances as compared to the in situ generated ternary system (Table 2, entries 1 and 21). Although the oil did not have good solubility in benzene, it can partly dissolve in toluene-d8, which allowed us to identify the active species by 1H NMR by giving a clear spectrum (the impurities were assignable to the reaction agents) (Figure 5). The multiple peaks around δ 1.68 (CH) that display

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Lv and Cui

Scheme 2. Probable Mechanism for Generation of the Active Species

coupling with the ABX signal at δ 0.04 (CH2) and the two doublets (CH3) at δ 1.06 and 1.13 are assigned to the AlCH2CHMe2 methine proton. The resonances arising from the PBNHC ligand are obviously different from those in the neutral complex 2. For instance, the signals of methylene groups (NCH2) appear at δ 4.96 and 5.28, which are invisible in 2; two sets of resonances are observed for the mesityl groups compared with one set in 2. Meanwhile the integrated intensity ratio of CH2CHMe2 to PBNHC is 2:1, suggesting that there is one ligand and two CH2CHMe2 groups in an active species molecule. Two broad resonances appearing in the high-field region δ -0.41 and -0.99 that give mutual correlation but do not correlate with any other hydrogen and carbon atoms in the 1H-1H COSY and 1H-13C HMQC spectra (Figure S8, Figure S9) are assigned to the two inequivalent Y-H hydrides, which is consistent with the distorted structure of VI and is consistent with that reported previously.24b According to this 1 H NMR spectrum analysis, the oil was defined as the novel hydride-bridged yttrium-aluminate ion pair [(PBNHC)Y( μ-H)2AliBu2]þ[B(C6F5)]- (VI), which might be the active species of this system. On the basis of this information, the reaction pathway could be depicted as shown in Scheme 2: addition of AliBu3 to 2 affords the adduct I, which is cationized by [Ph3C]þ[B(C6F5)]- to give the ion pair II with release of Ph3CH and isobutene (19F NMR spectral analysis reveals that no coordination of the anionic [B(C6F5)]- to the metal center is found (Figure S7));36 further addition of AliBu3 leads to alkylation, to provide the mixed bromo/isobutyl yttrium aluminate (III) by extruding AliBu2Br; alkylation of III by another equivalent of AliBu3 affords yttrium diisobutylbridged aluminate IV; β-H transfer of the isobutyl moiety in IV generates the mixed H/isobutyl species V, which transfers into the yttrium bis(hydride)-bridged aluminate (VI) via the second β-H transfer reaction. As far as we are aware, there is only one related cationic hydride-bridged ansa-zirconocene aluminate, which is generated in a similar reaction pathway and is characterized by NMR spectral analysis, showing (36) (a) Porri, L.; Ricci, G.; Shubin, N. Macromol. Symp. 1998, 128, 53. (b) Oehme, A.; Gebauer, U.; Gehrke, K.; Beyer, P.; Hartmann, B.; Lechner, M. Macromol. Chem. Phys. 1994, 195, 3773. (c) Wilson, D. Macromol. Chem., Macromol. Symp. 1993, 66, 273. (37) (a) Erofeev, A. B.; Bulychev, B. M.; Bel’skii, V. K.; Soloveichik, G. L. J. Organomet. Chem. 1987, 335, 189. (b) Bel'skii, V. K.; Bulychev, B. M.; Erofeev, A. B.; Soloveichik, G. L. J. Organomet. Chem. 1984, 268, 107. (c) Lobkovsky, E. B.; Solveychik, G. L.; Bulychev, B. M.; Erofeev, A. B.; Gusev, A. I.; Kirillova, N. I. J. Organomet. Chem. 1983, 254, 167. (d) Lobkovskii, E. B.; Soloveichik, G. L.; Erofeev, A. B.; Bulychev, B. M.; Bel'skii, V. K. J. Organomet. Chem. 1982, 235, 151. (e) Arndt, S.; Beckerle, K.; Hultzsch, K. C.; Sinnema, P.; Voth, P.; Spaniol, T. P.; Okuda, J. J. Mol. Catal A: Chem. 2002, 190, 215. (f) Li, X.; Baldamus, J.; Nishiura, M.; Tardif, O.; Hou, Z. Angew. Chem. 2006, 118, 8364.

activity for propylene polymerization,35 even though some neutral hydrido-bridged yttrium aluminate adducts have been reported (no activity was known).37 To date, the active species of the conventional ZieglerNatta catalytic systems based on lanthanide carboxylates has remained ambiguous, although a two-step activation sequence involving the formation of a reactive Ln-alkyl bond or Ln-hydride bond and the Al-to-Ln chlorotransfer (cationization) has been suggested.14 For the modified ZieglerNatta catalysts of the neutral molecular lanthanide aluminate, [Me2LnCl]n and [MeLnCl2]n are considered as the actual initiating species.36,38 Regarding the single-site metalalkyl-based cationic catalytic systems, the quasi-living intermediates have been isolated and characterized as [{[(C5Me5)La{(-Me)2AlMe(C6F5)}][Me2Al-(C6F5)2]}2],9 [Nd-(C3H5)(C4H6)n]2þ,10 [YMe(sol)6]2þ,11 and [(PNP)Y(CH2SiMe3)(thf)x]þ.12 We also reported recently a new type of cationic Ziegler-Natta catalytic system based on NCN-pincer lanthanide dichlorides; the probable active metal centers are bis(alkyl) bridged lanthanide aluminates albeit without characterization.32 Obviously the active species [(PBNHC)Y( μ-H)2AliBu2]þ arising from the ternary system based on the present CCC-pincer lanthanide dibromides is unprecedented for diene polymerization and distinctly different from the above-mentioned catalyst systems.

Conclusion We have demonstrated, for the first time, that the xylenylbridged bis(N-heterocyclic carbene)-ligated lanthanide dibromides are distinguished precursors for the polymerization of isoprene upon activation with AlR3 and organoborates to provide high activity and excellent cis-1,4 selectivity within a broad temperature range (20-80 °C). The ligand featuring distorted pincer architecture plays a crucial role in governing the specific selectivity of the precursors, while the electrondonating carbene carbon atoms generate strong coordination to the central metal, endowing them excellent thermostability. By monitoring the stepwise stoichiometric reactions among the catalyst components with NMR spectroscopy techniques, the active species arising from the combination (38) (a) Fischbach, A.; Meermann, C.; Eickerling, G.; Scherer, W.; Anwander, R. Macromolecules 2006, 39, 6811. (b) Meermann, C.; T€ornroos, K.; Nerdal, W.; Anwander, R. Angew. Chem., Int. Ed. 2007, 46, 6508. (c) Fischbach, A.; Perdih, F.; Herdtweck, E.; Anwander, R. Organometallics 2006, 25, 1626. (d) Fischbach, A.; Klimpel, M.; Widenmeyer, M.; Herdtweck, E.; Scherer, W.; Anwander, R. Angew. Chem., Int. Ed. 2004, 43, 2234. (39) Zimmermann, M.; T€ ornroos, K. W.; Anwander, R. Angew. Chem., Int. Ed. 2008, 47, 775.

Article

Organometallics, Vol. 29, No. 13, 2010

of 2/AliBu3/[Ph3C]þ[B(C6F5)]- has been identified as the unprecedented yttrium hydrido aluminate cation [LLn(μ-H)2AliBu2]þ. This represents the first living intermediate isolated from a ternary catalyst system. These results not only shed new light on the further investigation of the mechanism for diene polymerization but also open up a new pathway for designing efficient catalysts.

Experimental Section General Methods. All reactions were carried out under a dry and oxygen-free nitrogen atmosphere by using Schlenk techniques or under a nitrogen atmosphere in a glovebox. All solvents were purified from an MBraun SPS system. 1H and 13C NMR spectra were recorded on a Bruker AV400 (FT, 400 MHz for 1 H; 100 MHz for 13C) and Bruker AV600 (FT, 600 MHz for 1H; 150 MHz for 13C) spectrometer. NMR assignments were confirmed by 1H-1H COSY (correlation spectroscopy), 1H-13C HMQC (1H-detected heteronuclear multiple quantum coherence), and 1H-13C HMBC (1H-detected heteronuclear multiple-bond correlation) experiments when necessary. Isoprene (99%, Acros) was dried over CaH2 while stirring for 48 h and distilled before use. The molecular weight and molecular weight distribution of the polymers were measured by TOSOH HLC-8220 GPC (column: Super HZM-H3) at 40 °C using THF as eluent (flow rate 0.35 mL/min) against polystyrene standards. Elemental analyses were performed at National Analytical Research Centre of Changchun Institute of Applied Chemistry. 2,6-Dimethylaniline was obtained from Aldrich and purified by distillation before use. X-ray Crystallographic Studies. Crystals for X-ray analysis were obtained as described in the preparations. The crystals were manipulated in a glovebox. Data collections were performed at -86.5 °C on a Bruker SMART APEX diffractometer with a CCD area detector, using graphite-monochromated Mo KR radiation (λ = 0.71073 A˚). The determination of crystal class and unit cell parameters was carried out by the SMART program package. The raw frame data were processed using SAINT and SADABS to yield the reflection data file. The structures were solved by using the SHELXTL program. (PBNHC)YBr2(THF) (2). The complex was synthesized following the same procedure we reported previously. Under a nitrogen atmosphere, nBuLi (1.60 M in hexane, 2.60 mL, 4.20 mmol) was added dropwise to a THF suspension of YCl3 (0.27 g, 1.39 mmol) and PB(NHC-Br) 3 2HBr (1.0 g, 1.39 mmol) at room temperature. Removal of the solvent under reduced pressure followed by extracting the residue with toluene and then removal of the toluene afforded complex 2 as yellowish powders (0.53 g, 48.2%). Single crystals for X-ray analysis grew from the mixture of THF and hexane at -30 °C overnight as yellow blocks. 1H NMR (400 MHz, C6D6, 25 °C): δ 1.46 (br, 4H, THF), 2.17 (s, 18H, C6H2(CH3)3), 3.61 (br, 4H, THF), 6.01 (multi, 2H, NCHCHN), 6.37 (multi, 2H, NCHCHN), 6.79 (s, 4H, C6H2(CH3)3), 7.20-7.25 (multi, 3H, C6H3). 13C NMR (100 MHz, C6D6, 25 °C): δ 19.16 (s, 4C, o-N-C6H2(CH3)3), 21.87 (s, 2C, p-N-C6H2(CH3)3), 26.19 (s, 2C, THF), 60.79, (2C, CH2), 71.18 (s, 2C, THF), 119.52, 124.95 (4C, NCHdCHN), 122.31, 126.38, 129.43, 129.78, 137.76, 138.45, 138.86, 147.29 (18C, C6H2(CH3)3, C6H3), 188.46 (d, J = 15.4, 1C, Y-C), 192.44 (dd, 2C, NCN). Anal. Calcd for C36H41Br2N4OY (%): C, 54.43; H, 5.20; N, 7.05. Found: C, 53.70; H, 5.37; N, 6.93; Synthesis of Complexes 3, 4, and 6-9. Following a similar procedure to that described previously, the complexes (PBNHC)LnBr2(thf) (Ln = La, Nd, Gd, Dy, Ho, Tm) were synthesized from LnCl3: La, yellow microcrystals, yield 54.2%; Nd, yellow

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microcrystals, yield 62.1%; Gd, purple microcrystals, yield 35.2%; Dy, yellow microcrystals, yield 65.8%; Ho, yellow microcrystals, yield 54.3%; Tm, yellow microcrystals, yield 64.2%. The NMR spectra of these complexes were not available due to paramagnetism. Anal. Calcd for C36H41Br2N4OLa (%): C, 51.20; H, 4.89; N, 6.63. Found: C, 51.70; H, 5.37; N, 6.83. Anal. Calcd for C36H41Br2N4ONd (%): C, 50.88; H, 4.86; N, 6.59; Found: C, 50.40; H, 4.37; N, 6.03. Anal. Calcd for C36H41Br2N4OGd (%): C, 50.11; H, 4.79; N, 6.49. Found: C, 50.30; H, 5.27; N, 6.33. Anal. Calcd for C36H41Br2N4ODy (%): C, 49.81; H, 4.76; N, 6.45. Found: C, 49.47; H, 4.97; N, 6.95. Anal. Calcd for C36H41Br2N4OHo (%): C, 49.67; H, 4.75; N, 6.44. Found: C, 50.13; H, 5.07; N, 6.03. Anal. Calcd for C36H41Br2N4OTm (%): C, 49.44; H, 4.73; N, 6.41. Found: C, 49.71; H, 4.93; N, 6.03. Polymerization of Isoprene. In a glovebox, (PBNHC)DyBr2(THF) (20 μmol) was dissolved in 5 mL of chlorobenzene in a 25 mL flask; then 400 μmol of AlR3 and 20 μmol of borate ([Ph3C]þ[B(C6F5)4]-) were added to generate the active species followed by the addition of 1.0 mL of isoprene. After a designated time, ethanol was injected into the system to quench the polymerization. The mixture was poured into a large quantity of ethanol to precipitate the white solids. After filtration and drying under vacuum at 40 °C for 24 h, polyisoprene was given at a constant weight (1.36 g, 100%). Preparation of the CCC-Pincer Bis(carbene)-Ligated Yttrium Aluminate Hydride [(PBNHC)Y(μ-H)2AliBu2]þ[B(C6F5)4]- for NMR Measurement. In a glovebox, (PBNHC)YBr2(THF) (0.15 g, 0.19 mmol) was mixed with AliBu3 (0.37 g, 1.9 mmol) in benzene (2 mL) in a 25 mL flask; then [Ph3C]þ[B(C6F5)4]- was added slowly, after which a red oil precipitated at the bottom of the flask. Removal of the colorless upper layer and washing the oil with benzene several times afforded [(PBNHC)Y(μ-H)2AliBu2]þ[B(C6F5)4]- as a white powder (0.063 g, 24%). 1H NMR (600 MHz, toluene-d8, 25 °C): δ -0.99 (br, 1H, Y-H-Al), -0.41 (br, 1H, Y-H-Al), 0.04 (multi, 4H, AlCH2CH(CH3)2), 1.06 (d, 6H, AlCH2CH(CH3)2), 1.13 (d, 6H, AlCH2CH(CH3)2), 1.68 (multi, 2H, AlCH2CH(CH3)2), 1.81 (s, 6H, o-N-C6H2(CH3)3), 2.01 (s, 6H, o-N-C6H2(CH3)3), 2.32 (s, 3H, p-N-C6H2(CH3)3), 2.42 (s, 3H, p-N-C6H2(CH3)3), 4.96 (s, 2H, CH2), 5.28 (s, 2H, CH2), 6.08, 6.14, 6.60, 6.62 (s, 4H, NCHdCHN), 6.83 (s, 2H, C6H2(CH3)3), 6.99 (s, 2H, C6H2(CH3)3), 7.00 (d, JH-H = 7.8, 2H, m-C6H3), 7.34 (t, JH-H = 7.8, 1H, p-C6H3) . 13C NMR (151 MHz, toluened8, 25 °C): δ 17.74 (s, 2C, o-N-C6H2(CH3)3), 18.53 (s, 2C, o-N-C6H2(CH3)3), 21.75 (s, 2C, p-N-C6H2(CH3)3), 23.02 (s, 2C, AlCH2CH(CH3)2), 28.50 (s, 2C, AlCH2CH(CH3)2), 29.68 (d, 4C, AlCH2CH(CH3)2), 56.79 (s, 1C, CH2), 58.79 (s, 1C, CH2), 122.76, 123.53, 123.61, 124.60 (4C, NCHdCHN), 126.45, 129.30, 129.58, 130.23, 130.64, 131.01 (17C, C6H2(CH3)3, C6H3). The signals for ylidene carbons that were directly bonded to the yttrium were not observed because of the low solubility of the cationic species in toluene.

Acknowledgment. We thank the National Natural Science Foundation of China (No. 20934006) and the Ministry of Science and Technology of China (Nos. 2005CB623802 and 2009AA03Z501) for financial support. Supporting Information Available: 1H and 13C NMR spectra of selected polyisoprene samples; 19F, 1H-1H COSY, and 1 H-13C HMQC NMR spectra for complex VI; ORTEP drawings for molecular structures of 2, 4, and 6; X-ray crystallographic data and refinement for 2, 4, 6, and 7 in CIF format. These materials are available free of charge via the Internet at http://pubs.acs.org.