Synthesis, Structures, and Polymerization Catalytic Properties of

Apr 23, 2009 - ... (μ-oxo)titanium complex (Me2C)(Me2Si)(C5H3)2(μ-O)(CpTiCl)2 (1) was ... Minxiong Li , Haibin Song , Shansheng Xu , and Baiquan Wan...
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Organometallics 2009, 28, 3109–3112

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Synthesis, Structures, and Polymerization Catalytic Properties of Doubly Bridged Bis(cyclopentadienyl) Dinuclear (µ-Oxo)titanium Complexes Shuang Luo,† Bo Shen,† Bin Li,† Haibin Song,† Shansheng Xu,† and Baiquan Wang*,†,‡ State Key Laboratory of Elemento-Organic Chemistry, College of Chemistry, Nankai UniVersity, Tianjin 300071, People’s Republic of China, and State Key Laboratory of Organometallic Chemistry, Chinese Academy of Sciences, Shanghai 200032, People’s Republic of China ReceiVed February 4, 2009 Summary: The doubly bridged dinuclear (µ-oxo)titanium complex (Me2C)(Me2Si)(C5H3)2(µ-O)(CpTiCl)2 (1) was synthesized by the reaction of the lithium compound of the doubly bridged bis(cyclopentadienyl) ligand with (CpTiCl2)2O. Treatment of 1 with concentrated HCl or HBr gaVe the corresponding Cpdecoordinated products (Me2C)(Me2Si)(C5H3)2[CpTiX(µO)TiX2] (X ) Cl (2), Br (3)). All these titanium complexes were characterized by 1H NMR, 13C NMR, MS spectra, and elemental analysis. The crystal structures of (CpTiCl2)2O, 1, and 3 were determined by X-ray diffraction. Their catalytic properties for ethylene polymerization were also studied in the presence of MAO. Dinuclear metallocene complexes have attracted considerable attention in academic and industrial fields over the past few decades.1 In these complexes the two reactive metal centers are held in close proximity to offer the opportunity to produce cooperative electronic and chemical interaction, and the complexes have been envisaged to be useful for homogeneous catalysis.1b In order to control the distance between the coordination sites, a certain degree of rigidity of the ligand is required. Ideally, the two Cp units should be arranged in a sterically properly fixed manner and at the same time at a moderately short distance from each other; additionally, the bridging group(s) should allow for potential electron density delocalization. If two Cp rings are bridged by a single unit, the relative orientation between the two coordination sites may vary in an uncontrolled manner because of the rotation about the E-Cp bond (E ) bridge atom). On the other hand, doubly bridged bis-Cp ligands restrict the relative orientation of the two metals in the corresponding dinuclear complexes and offer a rigid system in which the metal could be locked to the same (cis) or opposite (trans) faces of the ligand.2 The structure and rigidity of the bridge between the two Cp rings greatly influence * To whom correspondence should be addressed. Fax: +86-22-23504781. E-mail: [email protected]. † Nankai University. ‡ Chinese Academy of Sciences. (1) For examples, see: (a) Reddy, K. P.; Petersen, J. L. Organometallics 1989, 8, 2107. (b) Buzinkai, J. F.; Schrock, R. R. Inorg. Chem. 1989, 28, 2837, and references cited therein. (c) Nifant’ev, I. E.; Borzov, M. V.; Churakov, A. V.; Mkoyan, S. G.; Atovmyan, L. O. Organometallics 1992, 11, 3942. (d) Ju¨ngling, S.; Mu¨lhaupt, R. J. Organomet. Chem. 1993, 460, 191. (e) Manriquez, J. M.; Ward, M. D.; Reiff, W. M.; Calabrese, J. C.; Jones, N. L.; Carroll, P. J.; Bunel, E. E.; Miller, J. S. J. Am. Chem. Soc. 1995, 117, 6182. (f) Qian, Y.; Huang, J.; Bala, M. D.; Lian, B.; Zhang, H.; Zhang, H. Chem. ReV. 2003, 103, 2633. (g) Gurubasavaraj, P. M.; Roesky, H. W.; Oswald, R. B.; Dolle, V.; Pal, A. Organometallics 2007, 26, 3346. (2) Siemeling, U.; Jutzi, P.; Neumann, B.; Stammler, H. G.; Hursthouse, M. B. Organometallics 1992, 11, 1328.

the catalytic behavior of these catalysts,3 and these doubly bridged bis-Cp dinuclear metallocene catalysts have been extensively studied.4-14 However, there is not a good example for the cooperative effect between the two metal centers of the doubly bridged bis(cyclopentadienyl) dinuclear metallocene catalysts, because nearly all the studied complexes are trans isomers, which always are the single products or major products. To obtain the cis dinuclear metallocene complex, we chose the dinuclear (µ-oxo)titanium complex (CpTiCl2)2O as the starting material and synthesized the doubly bridged bis(cyclopentadienyl) dinuclear (µ-oxo)titanium complex (Me2C)(Me2Si)(C5H3)2(µ-O)(CpTiCl)2 (1). However, its further reactions with concentrated HCl or HBr afforded the corresponding Cp-decoordinated products (Me2C)(Me2Si)(C5H3)2[CpTiX(µ-O)TiX2] (X ) Cl (2), Br (3)), instead of the expected cis dinuclear metallocene complexes.

Results and Discussion The doubly bridged dinuclear (µ-oxo)titanium complex (Me2C)(Me2Si)(C5H3)2(µ-O)(CpTiCl)2 (1) was prepared by the reaction of the lithium compound of the doubly bridged bis(cyclopentadienyl) ligand with (CpTiCl2)2O in 65% yield (Scheme 1). Complex 1 was found to be stable to air and moisture in the solid state and could be exposed to air for several hours without obvious decomposition, but it decomposed readily in solution when exposed to air. Complex 1 is quite soluble in benzene, toluene, THF, and CH2Cl2, while being slightly soluble in n-pentane and n-hexane. The 1H NMR spectrum of 1 showed two sharp singlets for both Me2Si and Me2C groups, one singlet (3) Herrmann, W. A.; Morawietz, M. J. A.; Ku¨ber, H. F. H. J. Organomet. Chem. 1996, 509, 115. (4) Cano, A.; Cuenca, T.; Go´mez-Sal, P.; Royo, B.; Royo, P. Organometallics 1994, 13, 1688. (5) Corey, J. Y.; Huhmann, J. L.; Rath, N. P. Inorg. Chem. 1995, 34, 3203. (6) Lang, H.; Blau, S.; Much, A.; Weiss, K.; Neugebauer, U. J. Organomet. Chem. 1995, 490, C32. (7) Herzog, T. A.; Zubris, D. L.; Bercaw, J. E. J. Am. Chem. Soc. 1996, 118, 11988. (8) Cano, A.; Cuenca, T.; Go´mez-Sal, P.; Manzanero, A.; Royo, P. J. Organomet. Chem. 1996, 526, 227. (9) Huhmann, J. L.; Corey, J. Y.; Rath, N. P. Organometallics 1996, 15, 4063. (10) Veghini, D.; Henling, L. M.; Burkhardt, T. J.; Bercaw, J. E. J. Am. Chem. Soc. 1999, 121, 564. (11) Cano, A. M.; Cano, J.; Cuenca, T.; Go´mez-sal, P.; Manzanero, A.; Royo, P. Inorg. Chim. Acta 1998, 280, 1. (12) Xu, S.; Dai, X.; Wu, T.; Wang, B.; Zhou, X.; Weng, L. J. Organomet. Chem. 2002, 645, 212. (13) Xu, S.; Feng, Z.; Huang, J. J. Mol. Catal. A: Chem. 2006, 250, 35. (14) Xu, S.; Jia, J.; Huang, J. J. Polym. Sci. A: Polym. Chem. 2007, 45, 4901.

10.1021/om900089w CCC: $40.75  2009 American Chemical Society Publication on Web 04/23/2009

3110 Organometallics, Vol. 28, No. 10, 2009

Notes

Scheme 1

Scheme 2

for C5H5 protons, four groups of multiplets, and two groups of triplets for the C5H3 protons. Due to the two different bridges, the two C5H3 units are inequivalent, resulting in an ABCDEF splitting pattern. In the MS spectrum of 1 no molecular ion peak was detected. Its base peak was at m/z 473, corresponding to [M+ - C5H5], indicating the weak coordination of the η5-C5H5 to the metal, which agreed with the following reaction results. When complex 1 was treated with an excess of concentrated HCl or HBr in THF, the Cp ligand was decoordinated and the corresponding products 2 and 3 were obtained (Scheme 2). Reaction of 1 with AlEtCl2 was also carried out, and only the Cp-decoordinated complex 2 was obtained. To our surprise, the Ti-O bonds were extremely strong and could not be cleaved easily. Complexes 2 and 3 were found to be very stable to air and moisture in the solid state, and they could be kept below 0 °C for several weeks in solution under an argon atmosphere. Complexes 2 and 3 are moderately soluble in THF and CH2Cl2 but insoluble in n-pentane and n-hexane. The 1H NMR spectra of 2 and 3 are similar, and both showed two singlets for SiMe2, two singlets for CMe2, a singlet for C5H5, and five groups of singlets for C5H3 protons. The chemical shift of C5H3 protons in complex 2 shifted noticeably upfield from those of complex 3, mainly due to the influence of halogen atoms. The molecular ion was detected in the MS spectrum for 2 but not for 3. The base peaks in the MS spectra are at m/z 443 and 577 for 2 and 3, respectively, both corresponding to [M+ - C5H5]. The crystal structures of (CpTiCl2)2O, 1, and 3 were determined by X-ray diffraction analysis (Figures 1-3, respectively). The molecular structure of (CpTiCl2)2O has Ci symmetry with a Ti-O-Ti angle of 180°.15 The Ti-Cen (Cen represents the centroid of the Cp ring) distance is 2.012 Å. In complex 1 the two Cp ligands are in a trans configuration with a Ti-O-Ti angle of 171.97(15)°. The Ti- - -Ti separation (3.667 Å) is longer than that in (CpTiCl2)2O (3.558 Å), due to the rigidity of the doubly bridged bis(cyclopentadienyl) ligand with a dihedral angle of 27.7° and the interaction between the two Cp ligands. The Ti-Cen distances (2.096, 2.110, 2.100, 2.097 Å) are much longer than those in (CpTiCl2)2O (2.012 Å) and Cp2TiCl2 (2.059 Å).16 The Ti-O (1.840(2), 1.837(2) Å) and Ti-Cl (2.418(1), (15) (a) Corradini, P.; Allegra, G. J. Am. Chem. Soc. 1959, 81, 5510. (b) Thewalt, U.; Schomburg, D. J. Organomet. Chem. 1977, 127, 169. (16) Clearfield, A.; Warner, D. K.; Saldarriaga-Molina, C. H.; Ropal, R.; Bernal, I. Can. J. Chem. 1975, 53, 1622.

Figure 1. ORTEP diagram of (CpTiCl2)2O. Thermal ellipsoids are shown at the 30% probability level. Selected bond lengths (Å) and angles (deg): Ti(1)-O(1) ) 1.7791(8), Ti(1)-Cl(1) ) 2.2397(16), Ti(1)-Cl(2) ) 2.2421(17), Ti(1)-Cen ) 2.012; Ti(1A)-O(1)-Ti(1) ) 180.0 (Cen represents the centroid of the Cp ring).

Figure 2. ORTEP diagram of 1. Thermal ellipsoids are shown at the 30% probability level. Selected bond lengths (Å) and angles (deg): Ti(1)-O(1) ) 1.840(2), Ti(2)-O(1) ) 1.837(2), Ti(1)-Cl(1) ) 2.4183(12), Ti(2)-Cl(2) ) 2.4135(13), Ti(1)-Cen(C1-C5) ) 2.096, Ti(1)-Cen(C6-C10) ) 2.110, Ti(2)-Cen(C13-C17) ) 2.100, Ti(2)-Cen(C21-C25) ) 2.097; Ti(1)-O(1)-Ti(2) ) 171.97(15).

2.413(1) Å) bond lengths are also much longer than those in (CpTiCl2)2O (Ti-O ) 1.779(8) Å; Ti-Cl ) 2.239(1), 2.242(1) Å). This could be attributed to the greater intramolecular interaction in the molecule of 1. The framework structure of 3 is similar to that of 1, but with a smaller Ti-O-Ti angle of 165.7(2)°. The Ti- - -Ti separation (3.633 Å) is slightly shorter than that in 1, due to the reduced intramolecular interaction. The dihedral angle of the doubly bridged bis(cyclopentadienyl) ligand is 26.3°. The Ti(2)-Cen distances (2.058, 2.061 Å) are close to that in Cp2TiCl2 (2.059 Å), while the Ti(1)-Cen distance (2.036 Å) is slightly shorter, but still longer than that in (CpTiCl2)2O (2.012 Å). All these distances are much decreased in comparison with those in 1 (2.096-2.110 Å), as the result of decoordination of a Cp ligand. The Ti(2)-O(1) (1.942(4) Å) and Ti(2)-Br(3) (2.512(1) Å) bonds are much longer than the Ti(1)-O(1) (1.719(4) Å) and Ti(1)-Br (2.443(1),

Notes

Organometallics, Vol. 28, No. 10, 2009 3111 Table 1. Ethylene Polymerization Catalyzed by Complexes 1-3 and (CpTiCl2)2O/MAOa cat.

yield (g)

10-5Ab

1 2 3 (CpTiCl2)2O

1.86 1.78 1.83 1.70

1.86 1.78 1.83 1.70

10-4Mnc 29.2 0.352 8.87

10-4Mwc

Mw/Mnc

43.3

1.5

0.621 34.2

1.8 3.9

a Polymerization conditions: in 50 mL of toluene, Tp ) 60 °C, cat. 5.0 µmol, [Al]/[Ti] ) 2000, Pethylene ) 0.1 MPa, t ) 1 h. b A ) activity in units of (g of polymer) (mol of Ti)-1 h-1. c Mn, Mw, and Mw/Mn were determined by GPC.

Figure 3. ORTEP diagram of 3. Thermal ellipsoids are shown at the 30% probability level. Selected bond lengths (Å) and angles (deg): Ti(1)-O(1) ) 1.719(4), Ti(2)-O(1) ) 1.942(4), Ti(1)-Br(1) ) 2.4433(12), Ti(1)-Br(2) ) 2.4386(11), Ti(2)-Br(3) ) 2.5127(12), Ti(1)-Cen(C1-C5) ) 2.036, Ti(2)-Cen(C9-C13) ) 2.058, Ti(2)-Cen(C16-C20) ) 2.061, Ti(1)-O(1)-Ti(2) ) 165.7(2).

2.438(1) Å) bonds, respectively, due to the unsymmetrical structure of 3. It was reported that the Ti-O bonds in (Cp2TiX)2O, (fulvalene)(Cl2TiOTiCl2), and [(Me2Si(C5H4)2](Cl2TiOTiCl2) could be cleaved by reactions with an excess of HX or AlEtCl2.17 However, in our cases only the Cp-bond cleavage was observed, instead of the Ti-O bond cleavage. In fact, we have tried these reactions under more severe reaction conditions such as heating to reflux, but the results were the same. The further reaction of the Cp-decoordinated complex 2 with excessive HCl was also carried out, but the complex remained unchanged. The other Cp ligand seems to be coordinated to the metal very firmly. By comparison of the structural parameters of 2 and 3, we assume that the first Cp ligand could be easily cleaved mainly due to the greater intramolecular interaction, especially between a Cp ligand and other ligands. After loss of a Cp ligand, the structural parameters of the remaining Cp2Ti unit in 3 are very similar to those of Cp2TiCl2. The stable structure unit and the reduced intramolecular interaction make the other Cp ligand less reactive, so that it is not cleaved even under more severe reaction conditions. The unusual stability of the Ti-O bonds in 1-3 may be related to the rigidity of the doubly bridged bis(cyclopentadienyl) ligand. When they were activated with MAO, all of these titanium complexes were effective catalysts for ethylene polymerization, and the results are given in Table 1. The polymerization experiment catalyzed by (CpTiCl2)2O/MAO was also carried out under identical conditions for comparison. As can be seen from Table 1, all these complexes show similar catalytic activities, but the molecular weight of polyethylene obtained with 1 (Mn ) 29.2 × 104) is much higher than those with 3 (Mn ) 0.35 × 104) and (CpTiCl2)2O (Mn ) 8.87 × 104). The molecular weight distribution of polyethylene obtained with 1 (MWD ) 1.48) is also much narrower than those with 3 (MWD ) 1.76) and (CpTiCl2)2O (MWD ) 3.86). This indicated that (17) (a) Nath, D.; Sharma, R. K.; Bhat, A. N. Inorg. Chim. Acta 1976, 20, 109. (b) Alvaro, L. M.; Cuenca, T.; Flores, J. C.; Royo, P.; Pellinghell, M. A.; Tiripicchio, A. Organometallics 1992, 11, 3301. (c) Ciruelos, S.; Cuenca, T.; Flores, J. C.; Go´mez, R.; Go´mez-Sal, P.; Royo, P. Organometallics 1993, 12, 944.

the introduction of the doubly bridged bis(cyclopentadienyl) ligand had little effect on the catalytic activity but great effect on the properties of the produced polyethylene. The broader MWD with (CpTiCl2)2O implied that the two metal centers of the catalyst are not equal or that the catalyst may decompose. Thus, the introduction of the rigid doubly bridged bis(cyclopentadienyl) ligand may stabilize the catalytically active species and result in an increase of molecular weight and decrease of molecular weight distribution. The much lower molecular weight of polyethylene with 3 can be attributed to the unsymmetrical structure of the catalyst, which may result in a high rate of β-hydride elimination. Only a few (µ-oxo)titanium catalysts have been reported.18 Chien et al. reported that the (µ-oxo)titanium complex (CpN · HClCpTiCl)2(µ-O) (CpN · HCl ) C5H4CH2CH2NMe2 · HCl) and the mononuclear titanium complex CpNCpTiCl2 showed almost the same activities for ethylene polymerization with MAO as cocatalyst, but their activities differed by a factor of 3 when the cocatalyst was [Ph3C]+[B(C6H5)4]-/TIBA.18a They considered that the µ-oxo atom could be removed by the trimethylaluminum within MAO, and these two systems produced the same catalytic species. A similar mechanism was also suggested for the dinuclear (µ-oxo)titanium catalyst [Me2Si(C 5 Me 4 )(3-tert-butyl-5-methyl-2-phenoxy)TiCl] 2 (µ-O)/ MAO system.18bSince MAO is employed in this study at an Al/Ti ratio of 2000, according to this mechanism it is reasonable that the Ti-O bond might be cleaved on activation with MAO, and the active species of 1/MAO may be the dinuclear cationic complex [(Me2C)(Me2Si)(C5H3)2][CpTi+(Me)]2[MeMAO]-2, while 2/MAO and 3/MAO might produce the same catalytic species [(Me2C)(Me2Si)(C5H3)2][CpTi+(Me)][Ti+(Me)2][Me-MAO]-2. In summary, the doubly bridged bis(cyclopentadienyl) dinuclear (µ-oxo)titanium complex 1 was synthesized. Its reactions with concentrated HCl or HBr did not give the cis dinuclear metallocene complex but afforded the corresponding η5-Cp ligand decoordinated products. The Ti-O bonds were much stronger than the Cp-Ti bond. Ethylene polymerization results showed that the introduction of the doubly bridged bis(cyclopentadienyl) ligand had little effect on the catalytic activity but great effect on the properties of the produced polyethylene.

Experimental Section General Considerations. All experimental manipulations were carried out under an atmosphere of dry argon using standard Schlenk techniques. THF and n-hexane were refluxed under nitrogen over Na/benzophenone and distilled before use. CH2Cl2 was refluxed under nitrogen over calcium hydride and distilled before use. Methylaluminoxane (MAO, 10% solution in toluene) was purchased (18) (a) Flores, J. C.; Chien, J. C. W.; Rausch, M. D. Macromolecules 1996, 29, 8030. (b) Hanaoka, H.; Hino, T.; Nabika, M.; Kohno, T.; Yanagi, K.; Oda, Y.; Imai, A.; Mashima, K. J. Organomet. Chem. 2007, 692, 4717.

3112 Organometallics, Vol. 28, No. 10, 2009 from Arbemarle Co. Polymerization grade ethylene (Daqing Petrochemical Co., People’s Republic of China) was used without further purification. 1H and 13C NMR spectra were recorded on a Bruker AV300 or Varian AS-400 spectrometer and EI mass spectra on a VG ZAB-HS instrument. Elemental analyses were performed on a Perkin-Elmer 240C analyzer. Gel permeation chromatography (GPC) measurements were carried out on PL-GPC-220 in trichlorobenzene (TCB). (Me2C)(Me2Si)(C5H4)219 and (CpTiCl2)2O20 were synthesized according to literature procedures. Synthesis of 1. To a solution of (Me2C)(Me2Si)(C5H4)2 (1.60 g, 7.0 mmol) in hexane (50 mL) was gradually added n-BuLi (14.0 mmol, 2.0 M in hexane) at 0 °C. After it was slowly warmed to room temperature and stirred for 6 h, the resulting suspension was filtered and evaporated to dryness. Then THF (60 mL) and (CpTiCl2)2O (2.69 g, 7.0 mmol) were added and the reaction mixture was stirred at room temperature overnight. After removal of solvent under reduced pressure the residue was extracted with CH2Cl2. Upon concentration, addition of hexane, and cooling, 2.40 g (65%) of 1 was obtained as dark red crystals. Mp: 246-247 °C. Anal. Calcd for C25H28Cl2OSiTi2: C, 55.69; H, 5.23. Found: C, 55.37; H, 5.50. 1H NMR (300 MHz, 293 K, CDCl3): δ 6.93 (m, 1H, C5H3), 6.83 (m, 1H, C5H3), 6.78 (m, 1H, C5H3), 6.74 (m, 1H, C5H3), 6.34 (s, 10H, C5H5), 5.75 (t, J1 ) 2.53 Hz, J2 ) 3.24 Hz, 1H, C5H3), 5.63 (t, J1 ) 2.63 Hz, J2 ) 2.89 Hz, 1H, C5H3), 1.76 (s, 3H, C-Me), 1.34 (s, 3H, C-Me), 0.72 (s, 3H, Si-Me), 0.33 (s, 3H, Si-Me) ppm. 13C NMR (100 MHz, 293 K, CDCl3): δ 148.1, 145.4, 137.1, 132.0, 118.4, 117.2, 117.1, 107.9, 107.4, 106.9, 38.2, 36.8, 25.6, 2.5, -2.3 ppm. MS (EI): m/z 503 (10, M+ - Cl), 473 (100, M+ - C5H5), 309 (8, [(Me2C)(Me2Si)(C5H3)2TiCl]+), 148 (8, [CpTiCl]+), 65 (13, [C5H5]+). Synthesis of 2. To a solution of 1 (0.485 g, 0.9 mmol) in THF (30 mL) was added concentrated HCl (5 mL, 12 M, 60 mmol) at 0 °C. Then the mixture was slowly warmed to room temperature and stirred overnight. After removal of solvent under reduced pressure the residue was extracted with CH2Cl2. Upon concentration, addition of hexane, and cooling, 0.413 g (90%) of 2 was obtained as a red solid. Mp: >300 °C. Anal. Calcd for C20H23Cl3OSiTi2: C, 47.14; H, 4.55. Found: C, 47.08; H, 4.44. 1H NMR (400 MHz, 293 K, CDCl3): δ 7.06 (s, 2H, C5H3), 6.97 (s, 1H, C5H3), 6.89 (s, 1H, C5H3), 6.52 (s, 6H, C5H3, C5H5), 6.06 (s, 1H, C5H3), 1.69 (s, 3H, C-Me), 1.47 (s, 3H, C-Me), 0.75 (s, 3H, Si-Me), 0.40 (s, 3H, Si-Me) ppm. 13C NMR (100 MHz, 293 K, CDCl3): δ 154.1, 147.6, 139.0, 126.4, 124.0, 122.0, 120.5, 119.4, 117.4, 109.8, 109.1, 38.8, 36.5, 26.0, 1.7, -2.3 ppm. MS (EI): m/z 508 (14, M+), 443 (100, M+ - C5H5), 309 (8, [(Me2C)(Me2Si)(C5H3)2TiCl]+), 148 (8, [CpTiCl]+), 65 (13, [C5H5]+). Synthesis of 3. Using a procedure similar to that described above for 2, the reaction of 1 (0.485 g, 0.9 mmol) with concentrated HBr (19) Nifant’ev, I. E.; Yarnykh, V. L.; Borzov, M. V.; Mazurchik, B. A.; Mstyslasky, V. I.; Roznyatovsky, V. A.; Ustynyuk, Y. A. Organometallics 1991, 10, 3739. (20) Gorsich, R. D. J. Am. Chem. Soc. 1960, 82, 4211.

Notes (5 mL, 7 M) gave complex 3 (0.255 g, 44%) as dark red crystals. Mp: >300 °C. Anal. Calcd for C20H23Br3OSiTi2: C, 37.36; H, 3.61. Found: C, 37.28; H, 3.55. 1H NMR (400 MHz, 293 K, CDCl3): δ 7.37 (s, 1H, C5H3), 7.02 (s, 2H, C5H3), 6.67 (s, 1H, C5H3), 6.63 (s, 6H, C5H3, C5H5), 5.90 (s,1H, C5H3), 1.65 (s, 3H, C-Me), 1.47 (s, 3H, C-Me), 0.80 (s, 3H, Si-Me), 0.39 (s, 3H, Si-Me) ppm. 13C NMR (100 MHz, 293 K, CDCl3): δ 154.0, 147.6, 137.7, 125.7, 124.2, 122.9, 120.4, 119.2, 117.9, 110.1, 108.9, 38.8, 36.3, 26.1, 2.0, -1.6 ppm. MS (EI): m/z 577 (100, M+ - C5H5), 562 (22, M+ - Br), 354 (6, [(Me2C)(Me2Si)(C5H3)2TiBr]+), 192 (9, [CpTiBr]+), 65 (8, [C5H5]+). Crystallographic Studies. Single crystals of the complexes (CpTiCl2)2O, 1, and 3 suitable for X-ray diffraction were obtained from n-hexane/CH2Cl2 solutions at -20 °C. Data collections of (CpTiCl2)2O and 1 were performed with a Bruker SMART 1000 diffractometer at 294(2) K, whereas that of 3 was performed with a Rigaku Saturn 70 diffractometer equipped with a rotating anode system at 113(2) K by using graphite-monochromated Mo KR radiation (ω-2θ scans, λ ) 0.710 73 Å). Semiempirical absorption corrections were applied for all complexes. The structures were solved by direct methods and refined by full-matrix least squares. All calculations were performed by using the SHELXL-97 program system. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were assigned idealized positions and were included in structure factor calculations. The molecular structure of 1 contains a CH2Cl2 molecule of solvation. The crystal data and a summary of the X-ray data collection are presented in the Supporting Information. Polymerization of Ethylene. Ethylene polymerization experiments were carried out in a 250 mL Schlenk flask with magnetic stirring. A prescribed amount of MAO was added to 40 mL of toluene saturated with ethylene (1 atm) in the flask, and the reactor was placed in an oil bath at the desired temperature. Then the titanium catalyst (5 µmol in 10 mL of toluene) was injected into the flask and the polymerization was performed at 60 °C for 1 h. The polymerization was terminated by addition of 10% HCl in methanol. The precipitated polymer was washed three times with methanol and dried at 100 °C in vacuo to a constant weight.

Acknowledgment. We are grateful to the National Natural Science Foundation of China (Nos. 20702026, 20672058, and 20721062) and the Research Fund for the Doctoral Program of Higher Education of China (No. 20070055020) for financial support. Supporting Information Available: A table of crystal data and summary of X-ray data collection for (CpTiCl2)2O, 1, and 3 and CIF files giving X-ray structural information for (CpTiCl2)2O, 1, and 3. This material is available free of charge via the Internet at http://pubs.acs.org. OM900089W