New Chromium(III) Complexes with ImineâCyclopentadienyl Ligands

Organometallics 2011, 30, 433–440 DOI: 10.1021/om1005935

433

New Chromium(III) Complexes with Imine-Cyclopentadienyl Ligands: Synthesis, Characterization, and Catalytic Properties for Ethylene Polymerization Lei Zhang, Wei Gao, Xin Tao, Qiaolin Wu, Ying Mu,* and Ling Ye The State Key Laboratory for Supramolecular Structure and Materials, School of Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, People’s Republic of China Received June 17, 2010

A series of new half-sandwich chromium(III) complexes chelated with (2-((arylimino)methyl)phenyl)tetramethylcyclopentadienyl ligands, 2-(ArNdCH)C6H4Me4CpCrCl2 (Ar = 2,6-Me2C6H3 (1), 2, 6-Et2C6H3 (2), 2,6-iPr2C6H3 (3), 4-MeC6H4 (4)), have been synthesized from the reaction of CrCl3 with the lithium salt of the corresponding ligand 2-(ArNdCH)C6H4Me4CpLi (Ar = 2,6-Me2C6H3 (LiL1), 2,6-Et2C6H3 (LiL2), 2,6-iPr2C6H3 (LiL3), 4-MeC6H4 (LiL4)). Free ligands HL1-HL4 were prepared by the condensation reaction of 2-(tetramethylcyclopentadienyl)benzaldehyde with 2,6-dialkylaniline. The free ligands were characterized by 1H NMR spectroscopy, while the chromium(III) complexes were characterized by elemental analyses and single-crystal X-ray crystallography. The X-ray crystallographic analysis indicates that the imine N atom in these complexes coordinates to the central chromium atom. Upon activation with AlR3 and Ph3CB(C6F5)4, these complexes exhibit high catalytic activity for ethylene polymerization and produce polyethylene with moderate to high molecular weights. These chromium(III) complexes can also be activated with AlR3 alone. In the latter case, they show slightly lower catalytic activity for ethylene polymerization in comparison to the AlR3/ Ph3CB(C6F5)4 activated catalyst systems. The effects of ligand structure, polymerization temperature, AlR3, and Al/Cr molar ratio on the catalytic behavior of these complexes were examined.

Introduction Heterogeneous chromium-based catalysts are an important class of catalysts for producing polyethylenes in industry. The silica-supported chromium-based Phillips1 and Union Carbide catalysts2 have been used for the industrial production of high-density polyethylene for decades. The development of homogeneous chromium-based catalysts for olefin polymerization has also attracted intensive research interest in the past decade.3-7 Among the homogeneous

chromium-based catalysts with various ligands, monocyclopentadienyl chromium(III) complexes have been extensively studied for ethylene homo- or copolymerization.8-24 We

*To whom correspondence should be addressed. Tel: (86)-43185168376. Fax: (86)-431-85193421. E-mail: [email protected]. (1) (a) Hogan, J. P.; Banks, R. L. U.S. Patent 2,825,721, 1958. (b) Hogen, J. P. J. Polym. Sci., Part A 1970, 8, 2637. (2) (a) Karapinka, G. L. U.S. Patent 3,709,853, 1973. (b) Karol, F. J.; Karapinka, G. L.; Wu, C.; Dow, A. W.; Jonson, N.; Carrick, W. L J. Polym. Sci., Part A 1972, 10, 2621. (3) (a) Theopold, K. H. Acc. Chem. Res. 1990, 23, 263. (b) Theopold, K. H. Eur. J. Inorg. Chem. 1998, 15. (4) (a) Gibson, V. C.; Spitzmesser, S. K. Chem. Rev. 2003, 1, 283. (b) Gibson, V. C.; Redshaw, C.; Solan, G. A. Chem. Rev. 2007, 107, 1745. (5) Groppo, E.; Lamberti, C.; Bordiga, S.; Spoto, G.; Zecchina, A. Chem. Rev. 2005, 105, 115. (6) (a) Esteruelas, M. A.; L opez, A. M.; Mendez, L.; Olivan, M.; O~ nate, E. Organometallics 2003, 22, 395. (b) Small, B. L.; Carney, M. J.; Holman, D. M.; O'Rourke, C. E.; Halfen, J. A. Macromolecules 2004, 12, 4375. (7) (a) Britovsek, G. J. P.; Gibson, V. C.; Wass, D. F. Angew. Chem., Int. Ed. 1999, 38, 428. (b) McGuinness, S. S.; Gibson, V. C.; Wass, D. F.; Steed, J. W. J. Am. Chem. Soc. 2003, 125, 12716. (c) Jones, D. J.; Gibson, V. C.; Green, S. M.; Maddox, P. J.; White, A. J. P.; Williams, D. J. J. Am. Chem. Soc. 2005, 127, 11037. (d) Tomov, A. K.; Chirinos, J. J.; Long, R. J.; Gibson, V. C.; Elsegood, M. R. J. Am. Chem. Soc. 2006, 128, 7704.

(8) Emrich, R.; Heinemann, O.; Jolly, P, W.; Kruger, C.; Verhovnik, G. P. J. Organometallics 1997, 16, 1511. (9) D€ ohring, A.; G€ ohre, J.; Jolly, P. W.; Kryger, B.; Rust, J.; Verhovnik, G. P. J. Organometallics 2000, 19, 388. (10) Jensen, V. R.; Angermund, K.; Jolly, P. W.; Børve, K. J. Organometallics 2000, 19, 403. (11) Dohring, A.; Jensen, V. R.; Jolly, P. W.; Thiel, W.; Weber, J. C. Organometallics 2001, 20, 2234. (12) Kotov, V. V.; Avtomonov, E. V.; Sundermeyer, J.; Aitola, E.; Repo, T.; Lemenovskii, D. A. J. Organomet. Chem. 2001, 640, 21. (13) Fernandez, P.; Pritzkow, H.; Carb o, J. J.; Hofmann, P.; Enders, M. Organometallics 2007, 26, 4402. (14) Enders, M.; Fernandez, P.; Mihan, S.; Pritzkow, H. J. Organomet. Chem. 2003, 687, 125. (15) Enders, M.; Kohl, G.; Pritzkow, H. Organometallics 2004, 23, 3823. (16) Enders, M. Macromol. Symp. 2006, 236, 38. (17) Liang, Y. F.; Yap, G. P. A.; Rheingold, A. L. Organometallics 1996, 15, 5284. (18) Zhang, H.; Ma, J.; Qian, Y. L.; Huang, J. Organometallics 2004, 23, 5681. (19) Ogata, K.; Nakayama, Y.; Yasuda, H. J. Polym. Sci., Part A 2002, 40, 2759. (20) Ikeda, H.; Monoi, T.; Ogata, K.; Yasuda, H. Macromol. Chem. Phys. 2001, 202, 1806. (21) Bazan, G. C.; Rogers, J. S.; Fang, C. C. Organometallics 2001, 20, 2059. (22) Endres, M.; Fernandez, P.; Ludwig, G.; Pritzkow, H. Organometallics 2001, 20, 5005. (23) Randoll, S.; Jones, P. G.; Tamm, M. Organometallics 2008, 27, 3232.

r 2010 American Chemical Society

Published on Web 12/31/2010

pubs.acs.org/Organometallics

434

Organometallics, Vol. 30, No. 3, 2011

Zhang et al.

Figure 1. Functionalized cyclopentadienyl chromium(III) complexes with a pendant N-donor group.

have recently focused our research attention on the development of new half-sandwich chromium(III) catalysts.25 Of the monocyclopentadienyl chromium(III) complexes, the socalled constrained-geometry monocyclopentadienyl chromium(III) complexes with a pendant nitrogen donor, such as dialkylamino (a),9 isopropylimino (b),9 N,N-dimethylanilinyl (c),22 alkylamido (d),17 pyridyl (e),18 quinolinyl (f),13 and imidazolin-imino (g),23 as shown in Figure 1, have attracted particular attention due to their structural features. Monocyclopentadienyl chromium(III) complexes with a dialkylamino group attached to the Cp ring were reported to show good catalytic activity for ethylene polymerization and ethylene/R-olefin copolymerization.8,9,11 Similar chromium(III) complexes with a pyridyl-cyclopentadienyl,18 quinolinyl-cyclopentadienyl13 or N,N-dimethylanilinyl-cyclopentadienyl22 ligand were also found to be efficient catalysts for ethylene polymerization. The typical constrained-geometry complex d was reported to show only moderate catalytic activity for ethylene polymerization when activated with MAO or fluorinated borate cocatalyst. 26 Imidazolin-imino group functionalized cyclopentadienyl chromium(III) complexes g were recently reported to show moderate catalytic activity for ethylene polymerization as well.23 The isopropylimino functionalized cyclopentadienyl chromium(III) complex b was reported as having been synthesized and tested as an ethylene polymerization catalyst, but its (24) (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. (c) Heinemann, O.; Jolly, P. W.; Kruger, C.; Verhovnik, G. P. J. J. Organomet. Chem. 1998, 553, 477. (d) Mani, G.; Gabbai, F. P. Angew. Chem., Int. Ed. 2004, 43, 2263. (e) Huang, Y.; Yu, W.; Jin, G. Organometallics 2009, 28, 4598. (25) (a) Xu, J.; Gao, W.; Zhang, Y.; Li, J.; Mu, Y. J. Organomet. Chem. 2007, 692, 1505. (b) Xu, T.; Mu, Y.; Gao, W.; Ni, J.; Ye, L.; Tao, Y. J. Am. Chem. Soc. 2007, 129, 2236. (26) (a) Okuda, J.; Eberle, T. In Metallocenes; Togni, A., Halterman, R. L., Eds.; Wiley-VCH: Weinheim, Germany, 1998; Vol. 1, p 415. (b) Chum, P. S.; Kruper, W. J.; Guest, M. J. Adv. Mater. 2000, 12, 1759. (c) Cano, J.; Kunz, K. J. Organomet. Chem. 2007, 692, 4411.

Scheme 1. Synthetic Route for Complexes 1-4

structure cannot be confirmed in the absence of any crystallographic data.9 Considering that many late-transition-metal complexes with a diimine ligand show good catalytic performance for ethylene polymerization and their catalytic properties can be easily tuned by changing the substituent on the imine groups,4,27 it would be interesting to systematically synthesize imino group functionalized cyclopentadienyl chromium(III) complexes similar to complex b and investigate their catalytic properties. In comparison to the pyridyl and dialkylamino functional groups, the imino group attached to the cyclopentadienyl ligand should have the superiority of relatively strong coordination ability to the central metal and easily modified bulkiness of the substituent on the N atom. On the basis of these considerations and the experiences accumulated from previous research work, we have developed a number of new chromium(III) complexes of the type h, as shown in Figure 1, and found that these complexes exhibit high catalytic activity for ethylene polymerization upon activation with AlR3 (R=Me, Et, iBu) alone or AlR3 and Ph3CB(C6F5)4 together. In this paper, we report the synthesis and characterization of these complexes as well as their catalytic properties for ethylene polymerization reactions. (27) (a) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. Rev. 2000, 100, 1169. (b) Mecking, S. Angew. Chem., Int. Ed. 2001, 40, 534. (c) Park, S.; Han, Y.; Kim, S. K.; Lee, J.; Kim, H. K.; Do, Y. J. Organomet, Chem. 2004, 689, 4263.

Article


Figure 2. Perspective view of 1 with thermal ellipsoids drawn at the 30% probability level. Hydrogens are omitted for clarity.

435


Results and Discussion 1. Synthesis of Ligands and Complexes. The synthetic route for the free ligands (2-((arylimino)methyl)phenyl)tetramethyl cyclopentadiene, 2-(ArNdCH)C6H4Me4CpH (Ar = 2, 6-Me2C6H3 (HL1), 2,6-Et2C6H3 (HL2), 2,6-iPr2C6H3 (HL3), 4-MeC6H4 (HL4)) and their chromium(III) complexes ((2-((arylimino)methyl)phenyl)tetramethylcyclopentadienyl)chromium(III) dichloride, 2-(ArNdCH)C6H4Me4CpCrCl2 (Ar = 2,6-Me2C6H3 (1), 2,6-Et2C6H3 (2), 2,6-iPr2C6H3 (3), 4-MeC6H4 (4)) is illustrated in Scheme 1. The free ligands HL1-HL4 were prepared by the condensation reaction of 2-(tetramethylcyclopentadienyl)benzaldehyde with the corresponding aniline derivatives in methanol in the presence of formic acid and 4 A˚ molecular sieves. These free ligands were found to be sensitive to water and moisture. To obtain them in high yields (80-90%), it is necessary to carry out the condensation reaction under an inert atmosphere with anhydrous reagents and solvents. We have also tried to synthesize (2-((alkylimino)methyl)phenyl)tetramethylcyclopentadiene derivatives from the condensation reaction of 2-(tetramethylcyclopentadienyl)benzaldehyde with alkylamines. However, these attempts were not successful, due probably to the poor stability of the alkylimine products. 1H NMR spectroscopic analysis of the free ligands indicates that they all exist as mixtures of three isomers, owing to the variation in the locations of the two CdC double bonds in the cyclopentadienyl ring. These free ligands were obtained in about 95% purity and were used without further purification. Treatment of the free ligands with 1.05 equiv of n-butyllithium in THF at -78 °C led to the deprotonation of their cyclopentadiene rings to form the lithium salts of the ligands LiL1-LiL4, which was accompanied by a color change from pale yellow to brown. The chromium(III) complexes 1-4 were synthesized in high isolated yields (60-65%) by the reaction of CrCl3(THF)3 with the corresponding lithium salt of the ligand in THF at room temperature. During the period of the reaction, the color of the reaction mixture changes from purple to deep green, indicating the formation of the chromium(III) complexes. These complexes are found to be thermally stable even heated to melting temperature under an inert atmosphere, but are air and moisture sensitive. These complexes are fairly


soluble in CH2Cl2, less soluble in toluene, and almost insoluble in common alkanes. They form green solutions in CH2Cl2 and blue solutions in toluene, due probably to the difference in coordination ability of the solvent molecules. All complexes were characterized by means of elemental analyses and infrared spectroscopy, and the molecular structures of complexes 1-3 were determined by X-ray diffraction. Crystals suitable for an X-ray structural determination were obtained by recrystallization from n-hexane/CH2Cl2 mixed solvent systems. 2. Crystal Structures of Complexes 1-3. The molecular structures of complexes 1-3 with the atom-numbering schemes are shown in Figures 2-4, and selected bond lengths and angles are given in Table 1. Crystallographic data indicate that crystals of complexes 1 and 3 belong to the monoclinic system and P21/n space group, while complex 2 crystallizes in the orthorhombic system and P212121 space group. All three complexes possess a three-legged pianostool geometry with a distorted-octahedral coordination environment around the central chromium atom. As can be seen from their crystal structures, the coordination of the

436


Zhang et al.

Table 1. Selected Bond Lengths (A˚) and Bond Angles (deg) for Complexes 1-3 1

2

3

Cr(1)-N(1) Cl(1)-Cr(1) Cl(2)-Cr(1) Cr(1)-CCp (range) Cr(1)-CCp (av) Cr(1)-Cp(ct) C(16)-N(1)

2.128(5) 2.293(2) 2.303(2) 2.197(5)-2.274(6) 2.238(9) 1.886 1.290(7)

2.117(2) 2.2928(9) 2.2877(9) 2.185(2)-2.284(3) 2.237(3) 1.886 1.279(3)

2.1313(15) 2.2840(7) 2.2755(8) 2.195(2)-2.269(2) 2.238(2) 1.884 1.289(2)

Cl(1)-Cr(1)-Cl(2) N(1)-Cr(1)-Cl(1) N(1)-Cr(1)-Cl(2) Cp(ct)-Cr-N(1) C(16)-N(1)-Cr(1) C(17)-N(1)-Cr(1) C(16)-N(1)-C(17) N(1)-C(16)-C(15)

94.87(9) 98.53(15) 99.18(13) 118.7 130.9(4) 116.1(3) 113.1(4) 127.6(5)

96.89(4) 96.54(7) 99.09(6) 119.3 129.05(17) 117.83(16) 113.1(2) 128.4(2)

96.72(4) 95.86(5) 98.48(5) 120.6 126.11(13) 120.54(11) 113.33(15) 130.13(17)

imine nitrogen atom to the central metal in these complexes builds a six-membered chelating ring in a vertical position to the cyclopentadienyl ring with the aryl group at the imine nitrogen atom being placed close to the metal center, which constructs a relatively crowded coordinating environment surrounding the central chromium atom. The Cr-N distances in complexes 1-3 are 2.128(5), 2.117(2), and 2.131(2) A˚, respectively, being shorter than the corresponding Cr-N distances found in complexes a (2.17 A˚),9 f (2.17 A˚),13 and c (2.25 A˚)22 but longer than those in complexes d (1.920 A˚),17 e (2.108 A˚),18 g (2.088 A˚),23 and b (2.04 A˚)9 shown in Figure 1. Complex 3 has the longest Cr-N bond distance among the complexes 1-3, which is reasonable since the aryl group at the imine N atom in complex 3 is the largest, 2,6-iPr2Ph. The Cr-N bond length in complex 2 being about 0.01 A˚ shorter than that in complex 1 is abnormal, which may be caused by packing forces. The Cr-Cl bond distances (2.276-2.303 A˚) in these complexes are in agreement with those observed for related amino- or pyridyl-functionalized cyclopentadienyl chromium dichloride complexes9,17,18,22 and change in the order 1 > 2 > 3. The Cr-Cp(ct) distances (ct = centroid) are close to each other for complexes 1-3 at 1.886 A˚ for 1 and 2 and 1.884 A˚ for 3. Similarly, the Cr-CCp(av) (av = average) distances (2.238 A˚ for 1 and 3 and 2.237 A˚ for 2) are also close to each other. The individual Cr-CCp bond distances range from 2.185 to 2.284 A˚, with the Cr-C3 and Cr-C4 distances being obviously longer than the remaining Cr-CCp bond lengths, indicating that the central chromium atom is not located exactly below the center of the Cp ring due to the coordination of the imine N atom. The imino CdN bonds in these complexes retain their double-bond character, being 1.290(7) A˚ for 1, 1.279(3) A˚ for 2, and 1.289(2) A˚ for 3. The N-Cr-Cp(ct) angles of complexes 1 (118.7°), 2 (119.3°), and 3 (120.6°) are obviously influenced by the bulkiness of the aryl groups at the imine N atoms. Similar effects on the N-Cr-Cl angles (98.53 and 99.18° for 1, 96.54 and 99.09° for 2, 95.86 and 98.48° for 3) can also be observed. However, little influence from the bulkiness of these aryl groups on the Cl-Cr-Cp(ct) (120.4 and 120.1° for 1, 120.5 and 119.5° for 2, 120.0 and 119.7° for 3) and Cl-Cr-Cl (94.87° for 1, 96.89° for 2, and 96.72° for 3) angles can be seen. The Cr-N-C(17) angles (116.1° for 1, 117.8° for 2, and 120.5° for 3) are slightly less than the corresponding N-Cr-Cp(ct) angles of these complexes, which causes the aryl group at the imine N atom to be located in a nearly vertical direction to the cyclopentadienyl ring. The angles

between the Cp ring and the attached phenyl ring (77.9° for 1, 73.8° for 2, and 75.3° for 3) are less than 90°, indicating that the six-membered chelating rings in these complexes are somewhat distorted. The angles between the Cp ring and the aryl ring at the imine N atom (87.7° for 1, 87.8° for 2, and 84.6° for 3) are near 90°. The bulkiness and location of the aryl group in these complexes may be important factors for affecting their catalytic performance by influencing the rate of ethylene molecule coordination to the chromium atom and insertion into the growing polyethylene chain, as well as the rate of polymer chain termination.28 3. Ethylene Polymerization. The chromium(III) complexes 1-4 have been tested as precatalysts for ethylene polymerization and copolymerization with 1-hexene under different conditions, and it was found that these complexes show catalytic activity only for ethylene polymerization upon activation with either AlR3 (R = Me, Et, iBu) alone or AlR3 and Ph3CB(C6F5)4 together. The polymerization results of ethylene catalyzed by 1-4/AlR3 systems and 1-4/ AlR3/Ph3CB(C6F5)4 systems are summarized in Tables 2 and 3, respectively. Upon activation with AlR3, complexes 1-4 all exhibit moderate to high catalytic activities for ethylene polymerization. Their catalytic activities ((0.5-3.0) 106 g of PE (mol of Cr)-1 h-1) are obviously higher than those reported for most N-containing group functionalized cyclopentadienyl chromium complexes,10,14,18,19 except for the N, N-dimethylanilinyl functionalized cyclopentadienyl chromium complexes.22 By comparison of the polymerization results under similar conditions in Table 2, it can be seen that the steric effect of the Ar group at the imine N atom of these complexes and the R group in the AlR3 activator on their catalytic properties is remarkable. For example, the catalytic activity of complexes 1-4 decreases in the order 1 > 2 > 4 > 3 under similar conditions on activation with AlMe3, while the catalytic activity is in the order 1 > 4 > 2 > 3 and 4 > 1 > 2 > 3 on activation with AlEt3 and AliBu3, respectively. The decrease in the catalytic activity of complexes 1-3 with an increase in the bulkiness of the aryl group may be attributed to the fact that a bulkier aryl group at the imine N atom would make the coordination sphere of the catalytic active species in these systems more crowded and thus slow down the ethylene coordination and insertion rate relatively. The fact that complex 4, with the smallest aryl group, does not show the highest catalytic activity in the AlMe3 and AlEt3 (28) (a) Alt, H. G.; Koppl, A. Chem. Rev. 2000, 100, 1205. (b) McKnight, A. L.; Waymouth, R. M. Chem. Rev. 1998, 98, 2587.

Article


437

Table 2. Polymerization Results of Ethylene Catalyzed by Complex 1-4/AlR3 Systemsa entry

cat./amt (μmol)

AlR3

Al/Cr

temp (°C)

yield (g)

activityb

10-4Mηc

10-4Mnd

Mw/Mn

Tme (°C)

Xcf (%)

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

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

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

60 60 60 60 120 120 120 120 30 180 240 300 400 120 120 120 120 120 120 120 120 120 120 120 120

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

2.21 1.49 0.65 0.81 2.75 1.80 0.97 1.19 trace 2.80 2.81 2.74 2.70 3.04 1.21 0.55 2.55 0.70 1.42 0.58 0.75 0.48 1.43 0.85 1.52

2210 1490 650 810 2750 1800 970 1190

26.3 38.9 45.7 114.1 19.8 23.1 26.9 77.7

17.6 25.4 29.3

2.37 2.47 2.55

14.1 16.2 17.9

2.34 2.38 2.36

133.3 134.5 135.3 138.8 131.8 133.2 134.3 137.1

64.7 65.9 60.8 68.4 62.0 67.4 67.9 65.7

2800 2810 2740 2700 3040 1210 550 2550 700 1420 580 750 480 1430 850 9120

16.7 13.2 11.6 10.9 21.2 17.3 13.0 28.6 58.3 31.9 104.1 59.1 114.8 23.2 36.1 10.3

11.7 8.4 7.5 6.8 15.6 11.3 8.2 18.1

2.35 2.33 2.31 2.34 2.41 2.76 3.23 2.38

19.4

2.43

15.6 23.8 6.3

2.42 2.48 2.29

131.5 130.8 130.5 129.6 132.2 131.0 130.0 133.7 136.6 134.2 138.4 137.3 138.9 132.2 134.4 130.1

61.6 68.6 64.5 63.2 66.5 58.8 66.7 60.2 69.5 68.7 70.5 68.8 54.5 67.2 69.5 65.6

Polymerization conditions: solvent 60 mL of toluene, ethylene pressure 5 bar, amount of catalyst 2 μmol. b In units of kg of PE (mol of Cr)-1 h-1. Measured in decahydronaphthalene at 135 °C. d Measured by GPC analysis. e Determined by DSC at a heating rate of 10 °C min-1. f Crystallinity Xc = ΔHf/ΔHf0; ΔHf0 = 273 J/g for polyethylene. g Polymerization time 5 min. a

c

Table 3. Polymerization Results of Ethylene Catalyzed by Complex 1-4/AlR3/Ph3CB(C6F5)4 Systemsa entry

cat.

AlR3

Al/Cr

temp (°C)

time (min)

yield (g)

activityb

10-4Mηc

10-4Mnd

Mw/Mn

Tme (°C)

Xcf (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

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

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

120 120 120 30 60 180 240 120 120 120 120 120 120 120 120 120 120 120 120 120 120 120 120

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

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

2.50 1.46 0.59 trace 1.47 2.12 2.08 1.41 0.68 1.71 0.55 1.65 2.55 1.30 1.77 2.25 2.72 0.91 0.58 0.65 0.32 2.64 0.93

5000 2920 1180

31.3 45.5 106.5

19.2 28.7

2.41 2.59

134.3 135.3 138.1

64.2 62.4 68.5

2940 4240 4160 2820 1360 3420 1100 3300 5100 15600 10620 6750 2720 1820 580 650 320 5280 1860

46.2 29.8 21.3 31.9 34.2 19.7 21.4 27.6 39.8 10.2 19.3 28.5 34.6 69.3 236.7 76.7 114.2 11.8 267.3

29.2 18.6 15.4 18.8 22.5 13.9 12.8 16.3 25.6 6.1 13.3 17.7 20.4

2.54 2.35 2.33 2.38 2.42 2.37 3.24 2.88 2.31 2.34 2.36 2.39 2.47

135.7 133.8 132.8 134.6 134.8 131.1 132.5 133.3 135.2 130.3 131.8 135.2 134.7 136.5 140.6 137.8 138.2 129.6 140.8

63.5 68.6 67.0 68.4 65.1 57.5 62.1 72.7 64.0 62.0 66.6 68.1 67.4 62.3 64.5 67.8 62.1 71.5 61.1

a Polymerization conditions: solvent 60 mL of toluene, ethylene pressure 5 bar, B/Cr molar ratio 1.2/1, amount of catalyst 1 μmol. b In units of kg of PE (mol of Cr)-1 h-1. c Measured in decahydronaphthalene at 135 °C. d Measured by GPC analysis. e Determined by DSC at a heating rate of 10 °C min-1. f Crystallinity Xc = ΔHf/ΔHf0; ΔHf0 = 273 J/g for polyethylene.

activated systems seems to imply that, except for the aforementioned steric effects, the electronic effects of the aryl group may also play an important role in these systems. With the more bulky AliBu3 cocatalyst, the 4/AliBu3 system shows the highest catalytic activity in the 1-4/AliBu3 systems, indicating that the steric effect of the aryl group plays a dominant role in controlling the catalytic activity of the AliBu3 activated catalyst systems. Upon activation with different AlR3, the catalytic activity of complexes 1-3 decreases in the order AlMe3 >AlEt3 > AliBu3. The variation of catalytic activity caused by trialkylaluminum may be

attributed to the interaction between the chromium catalyst and the aluminum cocatalyst molecules by forming a bridged heterobimetallic chromium-aluminum complex, as reported previously for similar catalyst systems.29 The effect (29) (a) Mani, G.; Gabba, F. P. Angew. Chem., Int. Ed. 2004, 43, 2263. (b) Britovsek, G. J. P.; Gibson, V. C.; Kimberley, B. S.; Maddox, P. J.; McTavish, S. J.; Solan, G. A.; White, A. J. P.; Williams, D. J. Chem. Commun. 1998, 849. (c) Britovsek, G. J. P.; Bruce, M.; Gibson, V. C.; Kimberley, B. S.; Maddox, P. J.; Mastroianni, S.; McTavish, S. J.; Redshaw, C.; Solan, G. A.; Str€omberg, S.; White, A. J. P.; Williams, D. J. J. Am. Chem. Soc. 1999, 121, 8728.

438


of Al/Cr molar ratio on the catalytic activity was also examined for the complex 1/AlMe3 system. It was found that the catalytic activity increases rapidly with the increase in Al/Cr molar ratio from 30 to 120, while further increase in the Al/Cr molar ratio from 120 to 400 results in limited impact on the catalytic activity. The effect of polymerization temperature on the catalytic activity of the complex 1/AlMe3 system was studied as well (entries 5 and 14-16 in Table 2). The catalytic activity of these reactions was found to decrease remarkably with an increase in polymerization temperature from 0 to 60 °C, which is indicative of the weak thermal stability of the catalytically active species formed in this catalyst system. In addition, as observed for most of the olefin polymerization catalyst systems, the catalytic activity of this catalyst system also decreases obviously with an increase in polymerization time. When complexes 1-4 were activated with AlR3 and Ph3CB(C6F5)4 together, it was found that these chromium complexes show much higher catalytic activity for ethylene polymerization than the AlR3 activated systems under similar conditions. The high catalytic activity of these AlR3/ Ph3CB(C6F5)4 activated systems should be attributed to the fact that the catalytically active species in these systems are positively charged.30 Attempts to isolate or observe the cationic species from stoichiometric reactions have not been successful so far. However, Ph3CCH3 was observed by 1H NMR from the reactions of complexes 1-4 with AlMe3 and Ph3CB(C6F5)4, which may indirectly demonstrate the formation of the catalytically active cationic species in these systems. In a way similar to that for the AlR3 activated systems, the catalytic activity of complexes 1-4 decreases in the order 1 > 4 > 2 > 3 under similar conditions on activation with AlMe3/Ph3CB(C6F5)4, while the catalytic activity is in the order 4 > 1 > 2 > 3 on activation with AlEt3/Ph3CB(C6F5)4 and AliBu3/Ph3CB(C6F5)4. For complexes 1-3, their catalytic activity was found to decrease in the order AlMe3 > AlEt3 > AliBu3 upon activation with different AlR3/Ph3CB(C6F5)4 cocatalysts. As discussed above, such results might result from the interaction between the chromium catalyst and AlR3 cocatalyst. As noted above in the AlR3 activated catalyst system, the catalytic activity of complex 4 also changes in the order AlEt3 > AlMe3 > AliBu3 on activation with different AlR3/Ph3CB(C6F5)4 cocatalysts. Similar to the case for the AlR3 activated catalyst systems, the catalytic activity of the AlR3/Ph3CB(C6F5)4 activated catalyst systems was found to increase with an increase in Al/Cr molar ratio from 30 to 120, while a further increase in the Al/Cr molar ratio from 120 to 240 results in a decrease in the catalytic activity. It is understandable that excess AlR3 would slow down the ethylene coordination rate, and probably consume some Ph3CB(C6F5)4 cocatalyst as well.31 The catalytic activity of the AlR3/Ph3CB(C6F5)4 activated catalyst systems was also found to decrease with an increase in polymerization temperature from 0 to 60 °C, which is similar to the case for the AlR3 activated catalyst systems. In addition, it was noticed that the lifetime of these catalyst systems is relatively long at 20 °C and the polymerization reaction can last for more than (30) (a) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1994, 116, 10015. (b) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1991, 113, 3623. (c) Bochmann, M.; Lancaster, S. J. Angew. Chem., Int. Ed. 1994, 33, 1634. (d) Chen, E. Y.; Marks, T. J. Chem. Rev. 2000, 100, 1391. (31) Bochmann, M.; Sarsfield, M. J. Organometallics 1998, 17, 5908.

Zhang et al.

30 min, although the catalytic activity decreases quickly with time (entries 1 and 14-17 in Table 3). The molecular weights of the polyethylenes produced by these catalyst systems are quite different ((10-267) 104 Da), depending on the catalyst structure and polymerization conditions. By examination of the viscosity-average molecular weight (Mη) data, it can be seen that the molecular weight of the polyethylene is affected by several factors. In general, catalysts with a bulky Ar group seem to produce polyethylenes with relatively high molecular weight (on comparison of the Mη data of the samples from catalysts 1-3), while polymerization reactions carried out at high temperatures or with high Al/Cr molar ratios give polyethylenes with relatively low molecular weight. These results are normal for metallocene catalyst systems,32 and similar results were also observed for related half-sandwich chromium(III) catalysts.9,18 However, there are more factors affecting the polyethylene molecular weight in the studied catalyst systems worth noting. One is that the polyethylenes produced from the AlR3/Ph3CB(C6F5)4 activated catalyst systems generally possess higher molecular weight than the polyethylenes obtained from the AlR3 activated catalyst systems under similar conditions, which could result from the higher catalytic activity of the former systems. The other is that the molecular weight of the obtained polyethylenes is strongly dependent on the alkylaluminum cocatalyst and decreases in the order AliBu3 > AlEt3 > AlMe3 for catalysts 1-3, which might be attributed to the fact that both the polymer chain transfer to the alkylaluminum cocatalyst and the β-H elimination reaction taking place at the catalytically active species in a bulkier AlR3 activated catalyst system should be slower, considering the interaction between the catalyst and cocatalyst molecules as discussed above. Another thing worth noting is that the molecular weight of the polyethylene produced by these catalysts increases with time up to about 60 min, although it increases more rapidly in the beginning 20 min of the reaction (entries 1 and 14-17 in Table 3), which implies that at least a part of the polymer chains in these catalyst systems grow over a long period of time. The polyethylene samples with low molecular weight have been analyzed by gel permeation chromatography (GPC). The molecular weight distribution is basically unimodal and narrow, being characteristic for metallocene polyolefins. All polyethylene samples were analyzed by differential scanning calorimetry (DSC), and the melting transition temperature and crystallinity data are summarized in Tables 2 and 3. Most of the samples show a sharp endothermic melting peak in the range of 129-140 °C in their DSC diagrams with crystallinity in the range of 60-80%. 13C NMR spectra of the polyethylene samples obtained at 130 °C in C6D4Cl2 show no signals for branches,33 which indicates that the polyethylenes are linear. Powder X-ray diffraction (XRD) analysis on some typical polyethylene samples was also carried out, and the observed diffraction peaks in the 2θ region of 19-25° in the XRD diagrams can be assigned to the polyethylene’s orthorhombic (110 and 200) reflections at (32) Zhang, Y.; Mu, Y.; Lu, C.; Li, G.; Xu, J.; Zhang, Y.; Zhu, D.; Fen, S. Organometallics 2004, 23, 540. (33) (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.

Article


21.32 and 23.72° and monoclinic (001, 200, and 201) reflections at 19.22, 22.94, and 24.96°.34

Conclusion A series of new constrained-geometry chromium(III) complexes with chelated (2-((arylimino)methyl)phenyl)tetramethylcyclopentadienyl ligands have been synthesized in good yields by the reaction of CrCl3(THF)3 with the lithium salt of the corresponding ligand in THF. X-ray crystallographic analysis indicates that these half-metallocene chromium(III) complexes adopt a pseudo-octahedral coordination environment with the imine N coordinated to the Cr metal center. Upon activation with AlR3 (R = Me, Et, iBu) or AlR3/ Ph3CB(C6F5)4, these complexes exhibit good to high catalytic activity for ethylene polymerization and produce polyethylene with high molecular weight under mild conditions. The catalytic activity of these complexes and the molecular weight of the produced polyethylene vary in a broad range with variation in the Ar group and the AlR3 cocatalyst. Catalysts with a bulkier Ar group show lower catalytic activity and produce polyethylene with higher molecular weight. In addition, the AlR3/Ph3CB(C6F5)4 activated catalyst systems show higher catalytic activity in comparison to the AlR3 activated catalyst systems.

Experimental Section General Considerations. All manipulations for air- and watersensitive compounds were performed under an inert atmosphere of nitrogen using standard Schlenk or glovebox techniques. Solvents were purified and dried by known procedures and distilled under nitrogen prior to use. Polymerization grade ethylene was further purified by passage through columns of 5 A˚ molecular sieves and MnO. CrCl3(THF)3,35 Ph3CB(C6F5)4,36 and 2-(tetramethylcyclopentadienyl)benzaldehyde37 were prepared according to published procedures. AlMe3, AlEt3, AliBu3, 2,6-dimethylaniline, 2,6-diethylaniline, and 2,6-diisopropylaniline were purchased from Aldrich or Acros and used as received. NMR spectra were measured using a Varian Mercury-300 NMR spectrometer, and elemental analysis was performed on a Perkin-Elmer 2400 analyzer. 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:38 [η] = (6.77 10-4)Mη0.67. Molecular weight and molecular weight distribution of the low-molecular-weight polymer samples were measured on a PL-GPC 220 at 140 °C with 1,2,4-trichlorobenzene as the solvent. The melting transition temperature (Tm) and crystallinity (Xc) of the polymers were measured with a Model 204 differential scanning calorimeter (DSC). The samples (5-10 mg) were heated from 35 to 160 °C at a rate of 10 °C/min, and the data from the second heating cycle were used. 1H and 13C NMR spectra of polyethylene samples were measured on a Varian Unity 400-MHz (34) (a) Russell, K. E.; Hunter, B. K.; Heyding, R. D. Polymer 1997, 38, 1409. (b) Bartczak, Z.; Galeski, A. Polymer 1996, 37, 2113. (c) Vickers, M. E. Polymer 1995, 36, 2667. (d) Joo, Y. L.; Han, O. H.; Lee, H. K.; Song, J. K. Polymer 2000, 41, 1355. (e) Baker, A. M. E.; Windle, A. H. Polymer 2001, 42, 667. (35) Boudjouk, P.; So, J.-H. Inorg. Synth. 1992, 29, 108. (36) (a) Massey, A. G.; Park, A. J. J. Org. Chem. 1962, 2, 245. (b) Massey, A. G.; Park, A. J. J. Organomet. Chem. 1966, 5, 218. (c) Chein, J. C. W.; Tsai, W. M.; Rasch, M. D. J. Am. Chem. Soc. 1991, 113, 8570. (37) Matharu, D. S.; Morris, D. J.; Kawamoto, A. M.; Clarkson, G. J.; Wills, M. J. Org. Chem. 2005, 7, 5489. (38) Francis, P. S.; Cooke, R. C.; Elliott, J. H. J. Polym. Sci. 1958, 31, 453.

439

spectrometer at 130 °C in o-C6D4Cl2. Powder XRD was performed on a Rigaku D/Max-2550 diffractometer using Cu KR radiation operating at 50 kV and 200 mA, and the data were collected in the 2θ range of 15-30° with a scanning rate of 1 ° min-1. Synthesis of 2-(2,6-Me2C6H3NdCH)C6H4Me4CpH (HL1). To a solution of 2-(tetramethylcyclopentadienyl)benzylaldehyde (4.37 g, 19.3 mmol) in dried methanol (80 mL) was added 2,6-dimethylaniline (2.57 g, 21.2 mmol), 4 A˚ molecular sieves (3 g), and formic acid (4 drops). After the reaction mixture was stirred for 8 h, the molecular sieves were filtered off and the solution was concentrated under reduced pressure to give the crude product, which was purified by column chromatography on silica gel using petroleum ether/ethyl acetate (98/2) as the eluent to afford the product (5.70 g, 17.3 mmol, 89.6%) in about 95% purity as an orange oil. 1H NMR (300 MHz, CDCl3): δ 8.37 (s, 1H, ArHCdN), 8.12 (d, J = 5.2 Hz, 1H, ArH), 7.50-7.37 (m, 2H, ArH), 7.16 (d, J = 7.8 Hz, 1H, ArH), 7.05 (d, J = 7.5 Hz, 2H, ArH), 6.90-6.97 (m, 1H, ArH), 3.29-3.11 (m, 1H, CpH), 2.09 (s, 6H, ArCH3), 1.87 (s, 3H, CpCH3), 1.79 (s, 3H, CpCH3), 1.68 (s, 3H, CpCH3), 0.89 (d, J = 6.9 Hz, 3H, CpCH3). MS: m/z 330 [M þ H]. Synthesis of 2-(2,6-Et2C6H3NdCH)C6H4Me4CpH (HL2). Compound HL2 was synthesized in the same manner as HL1 with 2,6-diethylaniline (3.13 g, 21.0 mmol) as starting material. The product (6.41 g, 17.9 mmol, 85.4%) was obtained in about 95% purity as an orange oil. 1H NMR (300 MHz, CDCl3): δ 8.38 (s, 1H, ArHCdN), 8.16 (d, 1H, ArH), 7.52-7.42 (m, 2H, ArH), 7.22-7.14 (m, 1H, ArH), 7.09 (m, 2H, ArH), 6.92 (m, 1H, ArH), 3.29-3.10 (m, 1H, CpH), 2.52-2.43 (m, 4H,CH2CH3), 1.88 (s, 3H, CpCH3), 1.80 (s, 3H, CpCH3), 1.70 (s, 3H, CpCH3), 1.17-1.04 (m, 6H, CH2CH3), 0.90 (d, J = 6 Hz, 3H, CpCH3). MS: m/z 358 [M þ H]. Synthesis of 2-(2,6-iPr2C6H3NdCH)C6H4Me4CpH (HL3). Compound HL3 was synthesized in the same manner as HL1 with 2,6-diisopropylaniline (3.73 g, 21.0 mmol) as starting material. The product (6.53 g, 16.9 mmol, 80.7%) was obtained in about 95% purity as an orange oil. 1H NMR (300 MHz, CDCl3): δ 8.39 (s, 1H, ArHCdN), 8.15 (d, J = 8.4 Hz, 1H, ArH), 7.50-7.43 (m, 3H, ArH), 7.22-7.01 (m, 3H, ArH), 3.20-3.10 (m, 1H, CpH), 2.99-2.91 (m, 2H, CH3CH), 1.82 (s, 3H, CpCH3), 1.68 (s, 3H, CpCH3), 1.53 (s, 3H, CpCH3), 1.11 (d, J = 7.2 Hz, 12H, CHCH3), 0.86 (d, J = 7.2 Hz, 3H, CpCH3). MS: m/z 386 [M þ H]. Synthesis of 2-(4-MeC6H3NdCH)C6H4Me4CpH (HL4). Compound HL4 was synthesized in the same manner as HL1 with 4-methylaniline (2.25 g, 21.0 mmol) as starting material. The product (5.82 g, 18.5 mmol, 87.9%) was obtained in about 95% purity as an orange oil. 1H NMR (300 MHz, CDCl3): δ 8.35 (s, 1H, ArHCdN), 8.25 (d, 1H, ArH), 7.97 (d, 1H, ArH), 7.60-7.55 (m, 1H, ArH), 7.46-7.42 (m, 1H, ArH), 7.18-7.15 (d, 2H, ArH), 7.06-7.03 (d, 2H, ArH), 3.23-3.17 (m, 1H, CpH), 2.36 (s, 3H, ArCH3), 1.93 (s, 3H, CpCH3), 1.82 (s, 3H, CpCH3), 1.72 (s, 3H, CpCH3), 0.95 (d, J = 6 Hz, 3H, CpCH3). MS: m/z 316 [M þ H]. Synthesis of Complex 1. To a solution of free ligand HL1 (750 mg, 2.28 mmol) in 20 mL of THF was added dropwise a solution of butyllithium (1.50 mL, 2.40 mmol) in THF at -78 °C. The reaction mixture was warmed to room temperature and stirred for 1 h. The resulting solution was then added to a suspension of CrCl3(THF)3 (900 mg, 2.40 mmol) in 30 mL of THF at -78 °C. The reaction mixture was warmed to room temperature and stirred for another 3 h. During the reaction, the color of the reaction mixture changed from purple to deep blue. After the solvent was removed under vacuum, the residue was extracted with 15 mL of dichloromethane to remove the insoluble impurities. Pure product 1 was obtained by recrystallization from CH2Cl2/n-hexane ((1-2)/10 v/v) as blue crystals (680 mg, 1.38 mmol, 60.4%). Anal. Calcd for C24H26NCrCl2 3 0.5CH2Cl2(493.82): C, 59.59; H, 5.51; N, 2.84. Found: C, 59.51; H, 5.58; N, 2.88. MS: m/z 452, 474 [M þ H, M þ Na].

440


Synthesis of Complex 2. Complex 2 was synthesized in the same manner as complex 1 with compound HL2 (820 mg, 2.29 mmol), n-BuLi (1.50 mmol), and CrCl3(THF)3 (900 mg, 2.40 mmol) as starting materials. Pure product 2 (670 mg, 1.44 mmol, 62.8%) was obtained as dark green crystals. Anal. Calcd for C26H30NCrCl2 (479.42): C, 65.14; H, 6.31; N, 2.92. Found: C, 65.09; H, 6.37; N, 2.89. MS: m/z 480, 502 [M þ H, M þ Na]. Synthesis of Complex 3. Complex 3 was synthesized in the same manner as complex 1 with compound HL3 (880 mg, 2.28 mmol), n-BuLi (2.40 mmol) ,and CrCl3(THF)3 (900 mg, 2.40 mmol) as starting materials. Pure product 3 (750 mg, 1.48 mmol, 64.7%) was obtained as dark green crystals. Anal. Calcd for C28H34NCrCl2 (507.48): C, 66.27; H, 6.75; N, 2.76. Found: C, 66.24; H, 6.79; N, 2.74. MS: m/z 508, 530 [M þ H, M þ Na]. Synthesis of Complex 4. Complex 4 was synthesized in the same manner as complex 1 with compound HL4 (720 mg, 2.28 mmol), n-BuLi (2.40 mmol), and CrCl3(THF)3 (900 mg, 2.40 mmol) as starting materials. Pure product 4 (620 mg, 1.42 mmol, 62.2%) was obtained as dark green crystals. Anal. Calcd for C23H24NCrCl2 (437.34): C, 63.16; H, 5.53; N, 3.20. Found: C, 63.08; H, 5.47; N, 3.15. MS: m/z 508, 530 [M þ H, M þ Na]. X-ray Crystallographic Studies. Single crystals of complexes 1-3 suitable for X-ray structure analysis were obtained from the CH2Cl2/n-hexane mixed solvent system. The data were collected at 293 K on a Rigaku R-AXIS PAPID IP diffractometer with (39) SHELXTL PC; Siemens Analytical X-ray Instruments, Madison, WI, 1993. (40) Sheldrick, G. M. SHELXTL Structure Determination Programs, Version 5.0 PC; Siemens Analytical Systems, Madison, WI, 1994.

Zhang et al. Mo KR radiation (λ = 0.710 73 A˚). All structures were solved by direct methods39 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.40 Ethylene Polymerization Procedure. 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) (together with a solution of Ph3CB(C6F5)4 in toluene (10 mL) for the AlR3/Ph3CB(C6F5)4 activated polymerization). The vessel was repressurized to the needed pressure with ethylene immediately, and the pressure was maintained by a continuous feed of ethylene. After a certain period of time, the polymerization was quenched by injecting acidified methanol (HCl (3 M)/methanol 1/1). The polymer was then collected by filtration, washed with water and methanol, and dried at 60 °C in vacuo to a constant weight.

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

New Chromium(III) Complexes with ImineâCyclopentadienyl Ligands

Recommend Documents

New Chromium(III) Complexes with ImineâCyclopentadienyl Ligands