Synthesis of the Binuclear Half-Metallocene Chromium(III) Aryloxides

Feb 1, 2011 - Oxidatively Induced Reductive Elimination from a Chromium(III) Bis(aryl) Complex. K. Cory MacLeod , Brian O. Patrick , and Kevin M. Smit...
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Organometallics 2011, 30, 669–675 DOI: 10.1021/om100112x

669

Synthesis of the Binuclear Half-Metallocene Chromium(III) Aryloxides [Cp0 Cr(OAr)Cl]2 and Their Catalytic Properties for Ethylene Polymerization in the Presence of Alkylaluminum Cocatalyst Mingtai Sun, Ying Mu,* Yang Liu, Qiaolin Wu, and Ling Ye State Key Laboratory for Supramolecular Structure and Materials, School of Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, People’s Republic of China Received February 11, 2010

A number of new chloride-bridged binuclear half-metallocene chromium(III) aryloxide complexes [Cp0 Cr(OAr)Cl]2 (Cp0 = C5H5 (1, 2), C5Me5 (3, 4); Ar = 2,6-iPr2C6H3 (1, 3), 2,6-tBu2C6H3 (2, 4)) have been synthesized from the reaction of Cp0 CrCl2(THF) with the lithium salt of the corresponding aryloxide ligand in THF. All complexes have been characterized by elemental analyses, 1H NMR, IR, and UV-vis spectroscopy. The molecular structures of complexes 1 and 3 were determined by X-ray crystallography. Upon activation with a small amount of AlR3, all complexes 1-4 show high catalytic activity for ethylene polymerization, producing high-molecular-weight polyethylene. The effects of polymerization conditions such as polymerization temperature, alkyl aluminum cocatalyst, and Al/Cr molar ratio on the catalytic activity and properties of the produced polyethylene were investigated. Introduction Chromium-based olefin polymerization catalysts have attracted intensive attention in the past decades due to their applications in polyolefin industry.1-3 Heterogeneous chromiumbased olefin polymerization catalysts of the Phillips and Union Carbide systems (Cr/SiO2) have been widely used in the commercial production of polyolefins.4 To understand the nature of the active species of the Phillips and Union Carbide catalysts, a considerable amount of effort has been *To whom correspondence should be addressed. Tel: (86)-43185168376. Fax: (86)-431-85193421. E-mail: [email protected]. (1) (a) Theopold, K. H. Eur. J. Inorg. Chem. 1998, 15. (b) MacAdams, L. A.; Buffone, G. P.; Incarvito, C. D.; Rheingold, A. L.; Theopold, K. H. J. Am. Chem. Soc. 2005, 127, 1082. (c) Gibson, V. C.; Spitzmesser, S. K. Chem. Rev. 2003, 103, 283. (2) (a) Rogers, J. S.; Bu, X. H.; Bazan, G. C. Organometallics 2000, 19, 3948. (b) Enders, M.; Fernandez, P.; Ludwig, G.; Pritzkow, H. Organometallics 2001, 20, 5005. (c) Gibson, V. C.; Maddox, P. J.; Newton, C.; Redshaw, C.; Solan, G. A.; White, A. J. P.; Williams, D. J. Chem. Commun. 1998, 1651. (d) D€ ohring, A.; G€ ohre, J.; Jolly, P. W.; Kryger, B.; Rust, J.; Verhovnik, G. P. J. Organometallics 2000, 19, 388. (e) Tomov, A. K.; Chirinos, J. J.; Jones, D. J.; Long, R. J.; Gibson, V. C. J. Am. Chem. Soc. 2005, 127, 10166. (3) (a) Vidyaratne, I.; Scott, J.; Gambarotta, S.; Duchateau, R. Organometallics 2007, 26, 3201. (b) Junges, F.; Kuhn, M. C. A.; dos Santos, A. H. D. P.; Rabello, C. R. K.; Thomas, C. M.; Carpentier, J.-F.; Casagrande, O. L. Organometallics 2007, 26, 4010. (c) Albahily, K.; AlBaldawi, D.; Gambarotta, S.; Koc-, E.; Duchateau, R. Organometallics 2008, 27, 5943. (d) Vidyaratne, I.; Nikiforov, G. B.; Gorelsky, S. I.; Gambarotta, S.; Duchateau, R.; Korobkov, I. Angew. Chem., Int. Ed. 2009, 48, 6552. (e) McGuinness, D. S. Organometallics 2009, 28, 244. (4) (a) Clark, A. Catal. Rev. 1969, 3, 145. (b) Karol, F. J.; Karapinka, G. L.; Wu, C.; Dow, A. W.; Johnson, R. N.; Carrick, W. L. J. Polym. Sci., Part A: Polym. Chem. 1972, 10, 2621. (c) Hogan, J. P. J. Polym. Sci., Part A: Polym. Chem. 1970, 8, 2637. (5) (a) MacAdams, L. A.; Kim, W. K.; Liable-Sands, L. M.; Guzei, I. A.; Rheingold, A. L.; Theopold, K. H. Organometallics 2002, 21, 952. (b) Albahily, K.; Koc-, E.; Al-Baldawi, D.; Savard, D.; Gambarotta, S.; Burchell, T. J.; Duchateau, R. Angew. Chem., Int. Ed. 2008, 47, 5816. r 2011 American Chemical Society

dedicated to design well-defined chromium-based catalysts and to explore their catalytic performance for homogeneous model systems.1a It was reported that chromium(III) complexes5,6 bearing β-diketiminate ligands and salicylaldiminato ligands are active catalysts for ethylene polymerization and exhibit lower catalytic activity upon activation with MAO compared to systems activated with alkylaluminum chloride or trialkylaluminum (AlR3). Chromium(III) complexes bearing triazacyclohexane,7a diphosphine ligands,7b and other neutral donor ligands7c were also reported to show good catalytic activity for ethylene polymerization, and some chromium(III) complexes with diphosphine ligands7d are extremely active and selective ethylene trimerization catalysts. In addition, cyclopentadienyl-amine,8a,b cyclopentadienyl-phosphine,8c (6) (a) Gibson, V. C.; Mastroianni, S.; Newton, C.; Redshaw, C.; Solan, G. A.; White, A. J. P.; Williams, D. J. Dalton Trans. 2000, 1969. (b) Gibson, V. C.; Newton, C.; Redshaw, C.; Solan, G. A.; White, A. J. P.; Williams, D. J. Eur. J. Inorg. Chem. 2001, 1895. (c) Gibson, V. C.; Newton, C.; Redshaw, C.; Solan, G. A.; White, A. J. P.; Williams, D. J. J. Chem. Soc., Dalton Trans. 1999, 827. (7) (a) Kohn, R. D.; Haufe, M.; Mihan, S.; Lilge, D. Chem. Commun. 2000, 1927–1928. (b) Carter, A.; Cohen, S. A.; Cooley, N. A.; Murphy, A.; Scutt, J.; Wass, D. F. Chem. Commun. 2002, 858. (c) Wei, P.; Stephan, D. W. Organometallics 2002, 21, 1308. (d) Wass, D. F. Pat. Appl. WO 02/04119, 2002 (to BP Chemicals). (8) (a) Emrich, R.; Heinemann, O.; Jolly, P. W.; Kr€ uger, C.; Verhownik, G. P. J. Organometallics 1997, 16, 1511. (b) Jensen, V. R.; Angermund, K.; Jolly, P. W.; Børve, K. J. Organometallics 2000, 19, 403. (c) D€ ohring, A.; Jensen, V. R.; Jolly, P. W.; Thiel, W.; Weber, J. C. Organometallics 2001, 20, 2234. (d) Liang, Y.; Yap, G. P. A.; Rheingold, A. L.; Theopold, K. H. Organometallics 1996, 15, 5284. (9) (a) Thomas, B. J.; Theopold, K. H. J. Am. Chem. Soc. 1988, 110, 5902. (b) Thomas, B. J.; Noh, S. K.; Schulte, G. K.; Sendlinger, S. C.; Theopold, K. H. J. Am. Chem. Soc. 1991, 113, 893. (10) (a) Mani, G.; Gabbaı¨ , F. P. Angew. Chem., Int. Ed. 2004, 43, 2263. (b) Mani, G.; Gabbaı¨ , F. P. Organometallics 2004, 23, 4608. (11) (a) Xu, T.; Mu, Y.; Gao, W.; Ni, J.; Ye, L.; Tao, Y. J. Am. Chem. Soc. 2007, 129, 2236. (b) Huang, Y.; Jin, G. J. Chem. Soc., Dalton Trans. 2009, 767. (c) Huang, Y.; Yu, W.; Jin, G. Organometallics 2009, 26, 4598. Published on Web 02/01/2011

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Scheme 1. Synthetic Procedure for Complexes 1-4

8d

cyclopentadienyl-amido, and other half-metallocene type chromium(III) catalyst systems9-11 have also been reported. Recently, we have reported a number of salicylaldiminatochelated half-sandwich chromium(III) complexes, Cp*Cr[2,4t Bu2-6-(CHdNR)C6H2O]Cl (R= tBu, Ph, 2,6-iPr2C6H3),11a which show high catalytic activity for ethylene polymerization upon activation with only a small amount of AlR3 and produce ultrahigh-molecular-weight polyethylene. The catalytic activity of some of these catalysts reaches the level of cationic metallocene catalysts of titanium and zirconium.12 To further develop the half-sandwich Cr-based catalysts and study the relationship between the molecular structure of the catalysts and their catalytic activity, we have synthesized a number of new chromium(III) complexes, [Cp0 Cr(OAr)Cl]2 (Cp0 = C5H5 (1, 2), C5Me5 (3, 4); Ar = 2,6-iPr2C6H3 (1, 3), 2,6-tBu2C6H3 (2, 4)), by replacing the salicylaldiminato ligand with an aryloxy ligand. Herein we report the synthesis and structural characterization of these new complexes, as well as their catalytic performance for ethylene polymerization.

Results and Discussion Synthesis and Structural Analysis. The chromium complexes 1-4 were synthesized in high yields as green solids from the reaction of Cp0 CrCl2(THF) with the lithium salt of the corresponding aryloxide in THF, as shown in Scheme 1. The starting materials Cp0 CrCl2(THF) were synthesized in situ by the reaction of CrCl3 with the corresponding Cp0 Li species. These complexes could also be obtained from the reaction of [Cp0 CrCl2]2 with the lithium salt of the corresponding aryloxide in THF, toluene, or Et2O. However, the isolated yields were found to be lower than those from the reaction with the in situ formed Cp0 CrCl2(THF) as the starting material. Complexes 3 and 4 with the bulky Cp* ligand were obtained in lower yields (65-70%) than the Cp chelated complexes 1 and 2 (78-81%). Similarly, complexes 1 and 3 with a 2,6-diisopropylphenoxy ligand were isolated in higher yields in comparison to complexes 2 and 4, which have a bulkier 2,6-di-tert-butylphenoxy ligand. Chromium complexes 1-4 were all characterized with 1H NMR, IR and UV-vis spectroscopy, as well as elemental analyses. Their melting points and effective magnetic moments were determined. 1H NMR analysis shows that the resonances of protons in the iPr and tBu groups in these complexes are slightly influenced by the paramagnetic chromium atom and all major signals can be observed and assigned. The (12) (a) Chen, Y. X.; Marks, T. J. Organometallics 1997, 16, 5958. (b) Zhang, Y.; Mu, Y.; Lu, C.; Li, G.; Xu, J.; Zhang, Y.; Zhu, D.; Fen, S. Organometallics 2004, 23, 540.

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

Figure 2. Perspective view of 3 with thermal ellipsoids drawn at the 30% probability level. Hydrogens are omitted for clarity.

molecular structures of complexes 1 and 3 were determined by single-crystal X-ray diffraction analysis, which indicates that both complexes exist in a dimeric form. The effective magnetic moment (μeff) of these complexes is in the range 3.90-4.32 μB at 298 K, corresponding to an average of approximately three unpaired electrons per dimer. This behavior has been attributed to intramolecular coupling of the spins of the unpaired d electrons.13 Deep green single crystals of complexes 1 and 3 suitable for X-ray diffraction analysis were grown from an n-hexane/ dichloromethane (10/1) mixed solvent system at room temperature. The ORTEP drawings of complexes 1 and 3 are shown in Figures 1 and 2, respectively. Selected bond lengths and angles for complexes 1 and 3 are summarized in Table 1. Complexes 1 and 3 both exist in a binuclear complex form in which the two Cp0 Cr(OAr)Cl fragments are held together by the two bridging chlorides. Both the two phenoxy ligands and the two Cp0 ligands in these binuclear complexes are located in positions trans to each other. Since the binuclear molecules of both complexes 1 and 3 lie on a crystallographic inversion center, only half of the two molecules have been labeled in Figures 1 and 2. The geometry of complexes 1 and 3 can be described as an edge-sharing bioctahedron, with the (13) Richeson, D. S.; Mitchell, J. F.; Theopold, K. H. Organometallics 1989, 8, 2570. (b) Richeson, D. S.; Hsu, S.-W.; Fredd, N. H.; Duyne, G. V.; Theopold, K. H. J. Am. Chem. Soc. 1986, 108, 8273. (14) Kohler, F. H.; Lachmann, J.; Muller, G.; Zeh, H.; Brunner, H.; Pfauntsch, J.; Watchter, J. J. Organomet. Chem. 1989, 365, 15. (15) (a) Bhandari, G.; Kim, Y.; McFarland, J. M.; Rheingold, A. L.; Theopold, K. H. Organometallics 1995, 14, 738. (b) Heintz, R. A.; Leelasubcharoen, S.; Liable-Sands, L. M.; Rheingold, A. L.; Theopold, K. H. Organometallics 1998, 17, 5477.

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Table 1. Selected Bond Lengths (A˚) and Bond Angles (deg) for Complexes 1 and 3 Complex 1 Cr(1)-C(1) Cr(1)-C(2) Cr(1)-C(3) Cr(1)-C(4) Cr(1)-C(5) Cp(cent)-Cr(1)

2.189(5) 2.231(5) 2.223(5) 2.187(4) 2.187(4) 1.874

O(1)-Cr(1)-Cl(1) C(6)-O(1)-Cr(1) Cp(cent)-Cr(1)-Cl(1) Cp(cent)-Cr(1)-Cl(1A)

93.29(7) 131.34(17) 122.0 120.6

Cr(1)-C(1) Cr(1)-C(2) Cr(1)-C(3) Cr(1)-C(4) Cr(1)-C(5) Cp*(cent)-Cr(1) O(1)-Cr(1)-Cl(1) C(11)-O(1)-Cr(1) Cp*(cent)-Cr(1)-Cl(1) Cp*(cent)-Cr(1)-Cl(1A)

2.2514(17) 2.2387(17) 2.2288(18) 2.2340(18) 2.2488(17) 1.886 92.46(4) 142.39(11) 121.8 120.9

Cr(1)-O(1) Cr(1)-Cl(1) Cr(1)-Cl(1A) Cr(1)-Cr(1A) O(1)-C(6) Cl(1)-Cr(1)-Cl(1A) Cr(1)-Cl(1)-Cr(1A) O(1)-Cr(1)-Cl(1A) Cp(cent)-Cr(1)-O(1)

1.869(2) 2.3878(10) 2.3808(11) 3.328 1.352(3) 91.50(4) 88.50(4) 92.72(7) 127.6

Complex 3

Cp0 ligand occupying one trigonal face of an octahedron, which is analogous to that of known related Cp0 CrCl complexes.10b,13-15 The Cr-C bond lengths in both complexes 1 and 3 are in line with those observed in related monocyclopentadienyl chromium(III) complexes.9,15b,16 The Cr-Cl bond lengths in complex 3 (2.420-2.438 A˚) are longer than that in complex 1 (2.381-2.388 A˚) and also longer than those observed in related chromium(III) chloridebridged complexes such as [CpCr(Me)(μ-Cl)]2 (average 2.356 A˚),13b [Cp*Cr(Bz)(μ-Cl)]2 (average 2.383 A˚),15a [Cp*Cr(CH2SiMe3)(μ-Cl)]2 (average 2.394 A˚),15b and [Cp*Cr(C6F5)(μ-Cl)]2 (average 2.375 A˚).10b The Cr-O bond length in complex 1 (1.869(2) A˚) with a Cp ligand is slightly shorter than that in complex 3 (1.8932(12) A˚) with a Cp* ligand, while both of them are shorter than those reported for salicylaldiminato11a (1.927(3) A˚) and hydroxyindanimine11c (1.935(2) A˚) chromium(III) complexes. The chromiumchromium distances (3.328 A˚ for 1 and 3.551 A˚ for 3) are consistent with the absence of any direct metal-metal bonding. The Cp*(cent)-Cr(1)-O(1) (131.4)° and Cr(1)O(1)-C(11) (142.39(11)°) angles in complex 3 are larger than the corresponding data in complex 1 (127.6 and 131.34(17)°, respectively), which is obviously a result of the repellency of the bulky Cp* ligand to the phenoxy ligand. On the other hand, the Cp(cent)-Cr(1)-Cl(1) (122.0°) and Cp(cent)-Cr(1)-Cl(1A) (120.6°) angles in complex 1 are close to those in complex 3 (121.8 and 120.9°, respectively), showing limited steric effects from the Cp ring. Ethylene Polymerization. The chromium(III) complexes 1-4 have been tested as precatalysts for ethylene polymerization under different conditions. It was found that these complexes show very low catalytic activity for ethylene polymerization upon activation with methylaluminoxane (MAO), and only small amounts of polymer (less than 100 mg) were obtained from polymerization reactions with 5 μmol of catalyst and an Al/Cr molar ratio of 500 or 1000. However, these complexes show moderate catalytic activity for ethylene polymerization when activated with AlR3 (16) Heinemann, O.; Jolly, P. W.; Kruger, C.; Verhovnik, G. P. J. J. Organomet. Chem. 1998, 553, 477.

Cr(1)-O(1) Cr(1)-Cl(1) Cr(1)-Cl(1A) Cr(1)-Cr(1A) O(1)-C(11) Cl(1)-Cr(1)-Cl(1A) Cr(1)-Cl(1)-Cr(1A) O(1)-Cr(1)-Cl(1A) Cp*(cent)-Cr(1)-O(1)

1.8932(12) 2.4200(5) 2.4382(5) 3.551 1.329(2) 86.071(16) 93.929(16) 92.29(4) 131.4

(R = Me, Et, iBu). The results of ethylene polymerization using complexes 1-4 as precatalysts and AlR3 as cocatalysts under different conditions are summarized in Table 2. Upon activation with a small amount of AlR3, complexes 1-4 show reasonable to good catalytic activity for ethylene polymerization, producing polyethylene with high molecular weight. Under similar conditions, the catalytic activity of these complexes for ethylene polymerization decreases in the order 3 > 4 > 1 > 2 (entries 1-12 in Table 2), which indicates that the catalytic activity of these catalysts is obviously affected by the nature of the Cp0 and aryloxy ligands. Complexes 3 and 4 with a Cp* ligand exhibit obviously higher catalytic activity than the corresponding Cp-containing complexes 1 and 2, which is in agreement with the results observed for other half-metallocene type chromium(III) catalyst systems.11c These results might be attributed to the fact that the introduction of electrondonating groups into the cyclopentadienyl ring could stabilize the catalytically active species17 and thus improve the catalytic activity. The result of complexes 1 and 3 with a 2,6diisopropylphenoxy ligand showing higher catalytic activity than complexes 2 and 4 with a bulky 2,6-di-tert-butylphenoxy ligand may imply that complexes 2 and 4 are less stable under the polymerization conditions. Similar results have previously been observed for titanium analogues.17 These chromium(III) complexes can be effectively activated with only a small amount of AlR3 cocatalyst, and their catalytic activity reaches a maximum with an Al/Cr molar ratio of about 50. Similar results were also observed in other related half-metallocene chromium(III) catalyst systems.10,11 The catalytic activity of these catalysts is also dependent on the AlR3 cocatalysts and decreases in the order AlMe3 > AlEt3 > AliBu3 under similar conditions (entries 1-12 in Table 2), which may result from either a slower initiation process with the larger AlR3 or a weak interaction between the catalyst and cocatalyst molecules, as reported previously for similar catalyst systems.11a To investigate the effect of (17) (a) Nomura, K.; Naga, N.; Miki, M.; Yanagi, K.; Imai, A. Organometallics 1998, 17, 2152. (b) Nomura, K.; Naga, N.; Miki, M.; Yanagi, K. Macromolecules 1998, 31, 7588. (c) Nomura, K.; Fujii, K. Organometallics 2002, 21, 3042.

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Table 2. Summary of Ethylene Polymerization Catalyzed by Complexes 1-4 Activated with AlR3a entry

complex

AlR3

Al/ Cr

time/ min

T/° C

yield/g

activityb

10-5Mηc

Tm/° Cd

Xc/ %e

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

1 1 1 2 2 2 3 3 3 4 4 4 3 3 3 3 3 3 3 3 3 3 3 [CpCrCl2]2 [Cp*CrCl2]2

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

50 50 50 50 50 50 50 50 50 50 50 50 12 25 100 50 50 50 50 50 50 50 50 50 50

30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 5 10 20 45 60 30 30

20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 0 40 60 20 20 20 20 20 20 20

1.44 0.98 0.68 1.23 0.86 0.58 2.62 1.86 0.85 1.88 1.18 0.75 trace 1.71 1.49 2.40 1.89 1.15 0.39 0.86 1.78 3.01 3.14 trace 0.08

576 392 272 492 344 232 1048 744 340 752 472 300

5.1 4.6 4.2 6.5 5.7 4.5 8.6 7.4 6.3 9.0 8.1 6.7

138.0 138.2 138.1 138.5 137.9 138.4 139.7 139.2 138.6 138.8 138.4 138.2

60.0 57.3 56.4 64.2 61.2 58.4 66.6 53.2 54.3 55.1 58.6 53.7

684 596 960 756 460 936 1032 1068 903 628

7.0 6.6 8.4 7.6 5.4 2.6 5.3 7.2 9.4 9.5

137.3 137.7 139.2 139.2 137.0 137.1 137.5 138.6 139.4 139.8

44.2 46.1 60.0 52.7 50.5 51.2 54.6 58.8 65.4 67.1

32

17

136.4

45.4

Polymerization conditions: solvent 60 mL of toluene, catalyst 5 umol, time 30 min, ethylene pressure 5 bar. In units of kg of PE (mol of Cr) h-1. Measured in decahydronaphthalene at 135 °C. d Determined by DSC at a heating rate of 10 K min-1. e Crystallinity Xc = ΔHf/ΔHf°; ΔHf° = 273 J g-1 for completely crystalline PE. a

b

-1

c

polymerization temperature on the catalytic activity of these catalyst systems, ethylene polymerization experiments with the 3/AlMe3 catalyst system were carried out at 0, 20, 40, and 60 °C (entries 1 and 16-18 in Table 2). It was found that the 3/AlMe3 catalyst system shows the highest catalytic activity at 20 °C and the catalytic activity decreases despite an increase or decrease in the polymerization temperature. This result is also similar to those observed for other related halfmetallocene chromium(III) catalyst systems.10,11 The lifetime of the 3/AlMe3 catalyst system was also examined by conducting the polymerization experiments for different times. It was found that the yield and the molecular weight of the polyethylene produced by this catalyst system increase with time up to about 60 min, although the catalytic activity decreases rapidly after 20 min (entries 7 and 19-23 in Table 2). The catalytic properties of complexes [CpCrCl2]2 and [Cp*CrCl2]2 for ethylene polymerization activated with AlMe3 were also examined under similar conditions, and the results are given in Table 2 (entries 24 and 25). Both the catalytic activity (∼32 kg of PE (mol of Cr)-1 h-1) of the two complexes and the molecular weight of the obtained PE are much lower than those obtained with the [Cp0 Cr(OAr)Cl]2/ AlR3 catalyst systems. These results demonstrate that the aryloxide group in these [Cp0 Cr(OAr)Cl]2 complexes plays an important role in improving the catalytic activity of these complexes and the PE molecular weight. However, it is unclear whether or not the aryloxide group still binds to the chromium atom in the catalytically active species during the polymerization. Attempts to isolate or trap the catalytically active species by adding CH2dCHCH2SCH3, pyridine, and PMe3 to the [Cp0 Cr(OAr)Cl]2/AlMe3 reaction mixtures were not successful. The catalytic activity of the [Cp0 Cr(OAr) Cl]2/AlR3 catalyst system is lower than that of the salicylaldiminato-chelated half-sandwich chromium(III) catalyst system (4106 g of PE (mol of Cr)-1 h-1) reported previously11a but higher than those for other similar monocyclopentadienyl

chromium(III) catalyst systems bearing β-ketoiminato, β-diketiminate,11b hydroxyindanimine,11c and other ligands.10,16 In order to get some information on the activation procedure and the catalytically active species of these catalyst systems, the reaction mixtures of complex 4 with AlMe3 were studied by 1H NMR spectroscopy, and the 1H NMR spectra of complex 4 and the 4/AlMe3 mixtures at different concentrations and Al/Cr molar ratios are shown in Figure 3. As can be seen from Figure 3A,D, complex 4 in 1  10-2 M solution shows three singlets at 1.03, 1.34, and 1.43 ppm (peak width at half height ∼6 Hz) and a broad signal at -47.8 ppm (peak width at half height ∼400 Hz), while the same complex in 1  10-3 M solution shows only a singlet at 1.03 ppm. Considering that the tBu protons of the 2,6-tBu2C6H3O group are far (six σ-bonds) away from the Cr(III) metal center and thus may be less affected by the paramagnetism of the Cr(III) atom, it is possible that the two singlets at 1.34 and 1.43 ppm are for the tBu protons of the 2,6-tBu2C6H3O group and the broad signal at -47.8 ppm is for the CH3 protons of the Cp* group in the dimeric form of 4 (labeled as a in Figure 3). The singlet at 1.03 ppm may be from either the tBu group in a monomeric form of 4 or an impurity. After reaction with AlMe3, the reaction mixture of 4 (1  10-2 M)/AlMe3 in an Al/Cr molar ratio of about 3 shows four broad signals in the high-field region at -9.3, -11.9, -42.7, and -47.8 ppm (Figure 3B), while the reaction mixture of 4/AlMe3 in a Al/Cr molar ratio of about 10 shows just two broad signals at -9.3 and -28.1 ppm (Figure 3C). Furthermore, a dilute solution of 4 (1  10-3 M)/AlMe3 with an Al/Cr molar ratio of about 10 gives only the signal at -9.3 ppm in this region (Figure 3E). The peak width at half-height for signals at -9.3 and -11.9 ppm is about 240 Hz, and that for the signals at -28.1, -42.7, and -47.8 ppm is about 400 Hz. These broad resonances are probably all for the CH3 protons of the Cp* group in different species, since their integrations are basically in agreement with that of the signals for the tBu

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Figure 3. 1H NMR spectra of (A) complex 4 (1  10-2 M), (B) complex 4 (1  10-2 M)/AlMe3 mixture (1/3), (C) complex 4 (1  10-2 M)/ AlMe3 mixture (1/10), (D) complex 4 (1  10-3 M), (E) complex 4 (1  10-3 M)/AlMe3 mixture (1/10).

protons of the 2,6-tBu2C6H3O group and are proportional to that of the added AlMe3 in the related spectra. The resonances of the Cr-CH3 protons may be too weak and broad or may be shifted too far away to be detected. If this is the case, on the basis of the reaction conditions, it might be reasonable to make assignments for the resonances at -42.7, -11.9, -9.3, and -28.1 ppm to species b-e, respectively, as shown in Figure 3. The supposed structures for b and c are similar to the solid-state structure of complex 4, and the heterobimetallic species d and e are similar to the previously reported catalyst/AlR3 bimetallic species for chromium18 and zirconium19 catalyst systems. Of course, for such a complicated paramagnetic system, it is difficult to know the exact structures of possible catalytically active species on the basis of limited 1H NMR information. On the other hand, considering that the aryloxide group may be removed from the catalyst molecule by alkylaluminum in the activation reaction to form 2,6-tBu2C6H3OAlMe2, (2,6-tBu2C6H3O)2AlMe, and 2,6-tBu2C6H3OAlMe3-, variations of the t Bu proton resonance in several related reactions have been examined. The reaction mixture 4 (1  10-2 M)/AlMe3 (Al/ Cr ≈ 10) shows two broad signals at 1.58 and 1.66 ppm for the tBu protons, as shown in Figure 4A. In order to know if these signals are from the tBu protons in the aforementioned alkylaluminum aryloxide compounds, 2,6- tBu2C6H3OH (2 equiv) and 2,6-tBu2C6H3OLi (1 equiv) were added to (18) Bazan, G. C.; Rogers, J. S.; Fang, C. C. Organometallics 2001, 20, 2059. (19) (a) Bochmann, M.; Lancaster, S. J. Angew. Chem., Int. Ed. Engl. 1994, 33, 1634. (b) Bochmann, M.; Lancaster, S. J. J. Organomet. Chem. 1995, 497, 55.

the 4/AlMe3 mixture separately to produce 2,6-tBu2C6H3OAlMe2, (2,6-tBu2C6H3O)2AlMe, and 2,6-tBu2C6H3OAlMe3- in situ. As a result, new signals at 1.49 and 1.34 ppm and at 1.51 and -0.58 ppm were observed in Figure 4B,C, respectively. It seems that the aforementioned alkylaluminum aryloxide compounds may not be formed in the activation reaction between 4 and AlMe3. However, it is still difficult to conclude whether or not the aryloxide group has been removed from the catalyst molecule in the activation reaction, due to the complication of the paramagnetic system. In fact, the in situ observed NMR signals from Figure 4B are quite different from those seen in the direct reaction of 2,6-tBu2C6H3OH and AlMe3, as shown in Figure 4D. The viscosity-average molecular weight (Mη) of the obtained polyethylenes was determined in decahydronaphthalene at 135 °C, and the results are given in Table 2. The polyethylenes produced with these catalysts possess relatively high molecular weight ((4-9.5)  105 g mol-1), and their molecular weight is apparently dependent on the structure of the catalyst. It can be seen that a catalyst with bulkier Cp0 and/or aryloxy ligands produces polyethylene with higher molecular weight, which can be attributed to the fact that a catalyst with bulkier coordination environment would slow the rate of the chain transfer reaction. On the other hand, the molecular weight of the polyethylene is also affected by the cocatalyst AlR3 and decreases in the order AlMe3 > AlEt3 >AliBu3 under similar conditions. For most olefin polymerization catalyst systems, the molecular weight of the obtained polyethylene decreases with an increase in polymerization temperature and Al/Cr molar ratio. 13C NMR analysis of the polymer samples indicates that the polyethylenes produced by

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Figure 4. 1H NMR spectra of (A) complex 4 (1  10-2 M)/AlMe3 mixture (1/10), (B) complex 4 (1  10-2 M)/AlMe3/2,6-tBu2C6H3OH mixture (1/10/2), (C) complex 4 (1  10-2 M)/AlMe3/2,6-tBu2C6H3OLi mixture (1/10/1), (D) AlMe3/2,6-tBu2C6H3OH mixture (1/1.5).

these catalysts are linear and have no branches.20 The melting transition temperature (Tm) of the obtained polyethylenes is in the range 137-140 °C, and the crystallinity of the polymer samples is in the range 44-67%.

Conclusion A number of new chloride-bridged binuclear half-metallocene chromium(III) aryloxide complexes 1-4 have been synthesized in high yields from the reaction of Cp0 CrCl2(THF) with the lithium salt of the corresponding aryloxide in THF, and the structures of complexes 1 and 3 were determined by X-ray crystallography. When they are activated with a small amount of AlR3, these complexes exhibit good catalytic activity for ethylene polymerization, producing high-molecular-weight polyethylene under mild conditions. The catalytic activity of these new half-metallocene chromium(III) complexes and the molecular weight of the resultant polyethylene are remarkably influenced by the aryloxy and cyclopentadienyl ligands, polymerization temperature, and Al/Cr molar ratio, as well as the type of AlR3 cocatalyst. The complex 3/AlMe3 system shows the highest catalytic activity under similar conditions.

Experimental Section General Considerations. All manipulations for air- and watersensitive compounds were performed under an inert atmosphere (20) (a) Wang, W.; Yan, D.; Zhu, S.; Hamielec, A. E. Macromolecules 1998, 31, 8677. (b) Kooko, E.; Lehmus, P.; Leino, R.; Luttikhedde, H. J. G.; Ekholm, P.; N€asman, J. H.; Sepp€al€a, J. V. Macromolecules 2000, 33, 9200. (c) Malmberg, A.; Kokko, E.; Lehmus, P.; L€ ofgren, B.; Sepp€ al€ a, J. V. Macromolecules 1998, 31, 8448.

of nitrogen using standard Schlenk or glovebox techniques. Solvents were dried and purified by known procedures and distilled under nitrogen prior to use. CrCl3(THF)321 and Cp*Li22 were prepared according to the literature procedures. CpLi and LiOAr were prepared from the reactions of nBuLi with CpH or the corresponding phenol in hexane. All other reagents were purchased from Aldrich or Acros and used as received. 1H NMR spectra were measured using a Varian Mercury-300 NMR spectrometer, and the elemental analyses were performed on a Perkin-Elmer 2400 analyzer. Magnetic susceptibilities were measured with a SQUID magnetometer (Quantum Design, MPMS-LS). The intrinsic viscosity (η) was measured in decahydronaphthalene at 135 °C using an Ubbelohde viscometer. Viscosity average molecular weight (Mη) values of polyethylenes were calculated by the following equation:23 [η] = (6.77  10-4)Mη0.67. The melting transition temperature (Tm) and crsytallinity (Xc) of the polymers were measured with a 204 differential scanning calorimeter (DSC). The samples (5-10 mg) were heated from 40 to 180 °C at a rate of 10 °C/min, and the data from the second heating cycle were used. Synthesis of [CpCr(O-2,6-iPr2C6H3)Cl]2 (1). A suspension of CpLi (0.144 g, 2.00 mmol) in THF (10 mL) was slowly added to a purple suspension of CrCl3(THF)3 (0.750 g, 2.00 mmol) in THF (20 mL) at 0 °C. The mixture was warmed to room temperature and stirred overnight. Then a powder of LiO-2,6-iPr2C6H3 (0.368 g, 2.00 mmol) was added slowly to the above reaction mixture at -15 °C. The reaction mixture was warmed to room temperature and stirred overnight. Removal of the solvents under reduced pressure gave a dark green residue, followed by (21) Shamir, J. Inorg. Chim. Acta 1989, 156, 163. (22) Threlkel, R. S.; Bercaw, J. E.; Seidler, P. E.; Stryker, J. M.; Bergman, R. G. Org. Synth. 1987, 65, 42. (23) Francis, P. S.; Cooke, R. C.; Elliott, J. H. J. Polym. Sci. 1958, 31, 453.

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extraction with toluene (20 mL) to remove the insoluble impurities. Removal of toluene under reduced pressure gave a dark green residue. Pure product 1 was obtained by recrystallization from CH2Cl2/n-hexane ((1-2)/10 v/v) as green crystals (0.538 g, 16.3 mmol, 81.5%). Mp: 142-144 °C. IR (cm-1): 3100 (w), 2960 (m), 2870 (m), 1651 (m), 1610 (s), 1557 (m), 1505 (s), 1456 (m), 1384 (s), 1361 (m), 1258 (m), 822 (m), 747 (m), 667 (s), 523 (w). Anal. Calcd for C34H44Cl2Cr2O2 (659.59): C, 61.91; H, 6.72; O, 4.85. Found: C, 61.64; H, 6.55; O, 4.90. 1H NMR (CDCl3): δ 1.18 (d, 12H, CHMe2), 2.98 (2H, CHMe2), 3.45 (5H, Cp H). UV/ vis (dichloromethane): 327, 427 nm. Magnetic measurement: μeff(298 K) = 4.25 μB. Synthesis of [CpCr(O-2,6-tBu2C6H3)Cl]2 (2). Complex 2 was synthesized in the same manner as 1 with LiO-2,6-tBu2C6H3 (0.425 g, 2.00 mmol), CpLi (0.144 g, 2.00 mmol), and CrCl3(THF)3 (0.750 g, 2.00 mmol) as starting materials. Pure 2 (0.562 g, 15.7 mmol, 78.5%) was obtained by recrystallization from CH2Cl2/n-hexane (1/5 v/v) as green crystals. Mp: 155-157 °C. IR (cm-1): 3080 (w), 2961 (m), 2870 (m), 1650 (m), 1608 (s), 1557 (m), 1501 (s), 1458 (m), 1380 (s), 1361 (m), 1258 (m), 823 (m), 747 (m), 667 (s), 524 (w). Anal. Calcd for C38H52Cl2Cr2O2 (715.72): C, 63.77; H, 7.32; O, 4.47 Found: C, 63.41; H, 7.41; O, 4.54. 1H NMR (CDCl3): δ 1.40 (s, 9H, CMe3), 1.48 (s, 9H, CMe3), 3.60 (5H, Cp H). UV/vis (dichloromethane): 320, 425 nm. Magnetic measurement: μeff(298 K) = 3.90 μB. Synthesis of [Cp*Cr(O-2,6-iPr2C6H3)Cl]2 (3). Complex 3 was synthesized in the same manner as 1 with OLi-2,6-iPr2C6H3 (0.368 g, 2.00 mmol), CpLi (0.144 g, 2.00 mmol), and CrCl3(THF)3 (0.750 g, 2.00 mmol) as starting materials. Pure 3 (0.560 g, 14.0 mmol, 70.0%) was obtained as green crystals. Mp: 172-173 °C. IR (cm-1): 3050 (w), 2960 (m), 2920 (s), 2870 (m), 1602 (s), 1557 (m), 1502 (s), 1455 (m), 1377 (s), 1330 (m), 1264 (m), 862 (m), 749 (m), 667 (s), 534 (w). Anal. Calcd for C44H64Cl2Cr2O2 (799.85): C, 66.07; H, 8.06; O, 4.00 Found: C, 65.86; H, 8.14; O, 4.16. 1H NMR (CDCl3): δ 1.20 (d, 12H, CHMe2), 2.99 (2H, CHMe2), 1.49 (s, 15H, Cp*-Me). UV/vis (dichloromethane): 331, 428 nm. Magnetic measurement: μeff(298 K) = 4.32 μB. Synthesis of Cp*Cr(O-2,6-tBu2C6H3)Cl (4). Complex 4 was synthesized in the same manner as 1 with LiO-2,6-tBu2C6H3 (0.425 mg, 2.00 mmol), CpLi (0.144 g, 2.00 mmol), and CrCl3(THF)3 (0.750 g, 2.00 mmol) as starting materials. Pure 4 (0.556 g, 13.2 mmol, 66.0%) was obtained as green crystals. Mp: 184-185 °C. IR (cm-1): 3052 (w), 2956 (m), 2912 (s), 2860 (m), 1634 (m), 1600 (s), 1557 (m), 1485 (m), 1428 (m), 1377 (s), 1230 (m), 1072 (m), 1024 (w), 883 (m), 803 (m), 745 (m), 667 (s), 529 (w). Anal. Calcd for for C48H72Cl2Cr2O2 (855.98): C, 67.35; H, 8.48; O, 3.74 Found: C, 67.11; H, 8.51; O, 3.78. 1H NMR

(CDCl3): δ 1.06 (s, 9H, CMe3), 1.34 (s, 9H, CMe3), 1.43 (s, 15H, Cp* Me). UV/vis (dichloromethane): 404, 425 nm. Magnetic measurement: μeff(298 K) = 4.17 μB. 1 H NMR Spectroscopic Study. A 1H NMR spectrum of the binuclear complex 4 (0.5 or 5 μmol) in C6D6 (0.4 mL) was obtained on a Varian Mercury-300 spectrometer. To this solution in an NMR tube was added about 3 or 10 equiv of AlMe3 in C6D6 (0.1 mL) in a glovebox. After the solution changed from green to purple, an NMR spectrum of the complex 4/AlMe3 reaction mixture was recorded. To the mixture was added about 2 equiv of 2,6-tBu2C6H3OH (10 μmol) or 1 equiv of 2,6-tBu2C6H3OLi (5 μmol) in a glovebox, and then the NMR spectrum was recorded. X-ray Crystallographic Study. Single crystals of complexes 1 and 3 suitable for X-ray crystal structural analysis were obtained from a CH2Cl2/n-hexane (1/10 v/v) mixed solvent system. The data were collected on a Siemens P4 four-circle diffractometer for complex 1 and a Bruker SMART-CCD diffractometer for complex 3 (graphite-monochromated Mo KR radiation: λ = 0.710 73 A˚). Details of the crystal data, data collections, and structure refinements are summarized in Table S1 in the Supporting Information. Both structures were solved by direct methods24 and refined by full-matrix least squares on F2. All non-hydrogen atoms were refined anisotropically, and the hydrogen atoms were included in idealized positions. All calculations were performed using the SHELXTL crystallographic software packages.25 Ethylene Polymerization Experiments. A dry 250 mL steel autoclave with a magnetic stirrer was charged with 60 mL of toluene and saturated with ethylene (1.0 bar) at 20 °C. The polymerization reaction was started by injection of a mixture of AlR3 and a catalyst in toluene (10 mL). The vessel was repressurized to the needed pressure with ethylene immediately, and the pressure was maintained by continuous feeding of ethylene. After a certain period of time, the polymerization was quenched by injecting acidified methanol (1/1 HCl (3 M)/methanol), and the polymer was collected by filtration, washed with water and methanol, and dried at 60 °C in vacuo to a constant weight.

(24) SHELXTL; Siemens Analytical X-ray Instruments, Madison, WI, 1993.

(25) Sheldrick, G. M. SHELXTL Structure Determination Programs, Version 6.12; Siemens Analytical Systems, Madison, WI, 1994.

Acknowledgment. This work was supported by the Natural Science Foundation of China (Nos. 20772044 and 21074043). Supporting Information Available: A table and CIF files giving X-ray crystallographic data for complexes 1 and 3. This material is available free of charge via the Internet at http:// pubs.acs.org.