Accessible, Highly Active Single-Component β-Ketiminato Neutral

Apr 27, 2010 - Synopsis. A series of newly designed neutral nickel catalysts bearing readily available β-ketiminato ligands have been synthesized and...
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Organometallics 2010, 29, 2306–2314 DOI: 10.1021/om100075u

Accessible, Highly Active Single-Component β-Ketiminato Neutral Nickel(II) Catalysts for Ethylene Polymerization Dong-Po Song,†,‡ Ji-Qian Wu,† Wei-Ping Ye,† Hong-Liang Mu,†,‡ and Yue-Sheng Li*,† †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China, and ‡Graduate School of the Chinese Academy of Sciences, Changchun Branch, Changchun, People’s Republic of China Received January 29, 2010

A series of novel neutral nickel complexes based on cyclic β-ketiminato ligands, [(2,6-iPr2C6H3)Nd CHC6H8O]Ni(Ph)(PPh3) (3b), [(2,6-iPr2C6H3)NdCHCnþ3H2nþ2O(C6H4)]Ni(Ph)(PPh3) (6a-c), and [(2,6-iPr2C6H3)NdCHCnþ3H2nþ2O(C6H4)]Ni(CH3)(Py) (7a-c: a, n = 0; b, n = 1; c, n = 2) have been synthesized and characterized. These conveniently accessible complexes proved to be highly active catalysts for ethylene polymerization without an activator. Under the optimized conditions, an activity of 71.4 kg of PE/((mol of Ni) h atm) was observed using 6c as a catalyst. Particularly, it is of great interest that a bulky substituent proximate to the oxygen atom of the β-ketiminato complex is no longer a prerequisite to attain high catalytic efficiency, which is much different from the case for salicylaldiminato neutral nickel catalysts. This is effectively supported by the lower activation enthalpy changes of complexes 3b and 6a-c relative to values for the classic salicylaldiminato complexes. Moreover, for complexes 6a-c and 7a-c, the degree of conjugation between the phenylene ring and the corresponding nickel chelate of the complex can be tuned via changes in the ligand structure, which remarkably influence the molecular weights and the microstructures of the resulting polyethylenes. In comparison with the highly conjugated complexes 6a and 7a, complexes 6b,c and 7b,c, with a low degree of conjugation, produced polyethylenes with much higher molecular weights and lower branch contents. X-ray crystallographic analysis of 3b and 6a-c provides detailed information about the differences among these structures, and various typical angles and bond distances produce evidence of the conjugation modulation effect. Introduction In the past several years, interest in late-transition-metal catalysts for olefin polymerization has mushroomed, due to a major advance which came with Brookhart’s discovery of cationic Ni(II) and Pd(II) catalysts.1-9 Subsequently, numerous *To whom correspondence should be addressed. Fax: þ86-43185262039. E-mail: [email protected]. (1) (a) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. Rev. 2000, 100, 1169. (b) Boffa, L. S.; Novak, B. M. Chem. Rev. 2000, 100, 1479. (c) Gibson, V. C.; Spitzmesser, S. K. Chem. Rev. 2003, 103, 283. (d) Nakamura, A.; Ito, S.; Nozaki, K. Chem. Rev. 2009, 109, 5215. (2) Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc. 1995, 117, 6414. (3) (a) Johnson, L. K.; Mecking, S.; Brookhart, M. J. Am. Chem. Soc. 1996, 118, 267. (b) Mecking, S.; Johnson, L. K.; Wang, L.; Brookhart, M. J. Am. Chem. Soc. 1998, 120, 888. (c) Li, W.; Zhang, X.; Meetsma, A.; Hessen, B. J. Am. Chem. Soc. 2004, 126, 12246. (d) Chen, G.; Guan, Z. B. J. Am. Chem. Soc. 2004, 126, 2662. (e) Luo, S.; Jordan, R. F. J. Am. Chem. Soc. 2006, 128, 12072. (f) Drent, E.; van Dijk, R.; van Ginkel, R.; van Oort, B.; Pugh, R. I. Chem. Commun. 2002, 744. (4) Gates, D. P.; Svejda, S. A.; Onate, E.; Killian, C. M.; Johnson, L. K.; White, P. S.; Brookhart, M. Macromolecules 2000, 33, 2320. (5) Leatherman, M. D.; Brookhart, M. Macromolecules 2001, 34, 2748. (6) Cotts, P. M.; Guan, Z. B.; McCord, E. F.; McLain, S. J. Macromolecules 2000, 33, 6945. (7) Guan, Z. B.; Cotts, P. M.; McCord, E. F.; McLain, S. J. Science 1999, 283, 2059. (8) Mecking, S.; Held, A.; Bauers, F. M. Angew. Chem., Int. Ed. 2002, 41, 545. (9) Leatherman, M. D.; Svejda, S. A.; Johnson, L. K.; Brookhart, M. J. Am. Chem. Soc. 2003, 125, 3068. pubs.acs.org/Organometallics

Published on Web 04/27/2010

such kinds of catalysts have been explored for olefin polymerization.10-13 Recently, Grubbs and co-workers found that introducing a bulky substituent at the ortho position of the phenoxy group not only blocks the axial faces of the nickel center and retards the rate of chain termination but also enhances the catalytic activity of the neutral nickel salicylaldiminato catalyst by accelerating triphenylphosphine dissociation and decreases the rate of catalyst deactivation.14,15 For example, (10) (a) Camacho, D. H.; Guan, Z. B. Organometallics 2005, 24, 4933. (b) Leung, D. H.; Guan, Z. B. J. Am. Chem. Soc. 2008, 130, 7538. (11) (a) Speiser, F.; Braunstein, P. Inorg. Chem. 2004, 43, 1649. (b) Kermagoret, A.; Braunstein, P. Organometallics 2008, 27, 88. (c) Chavez, P.; Braunstein, P. Organometallics 2009, 28, 1776. (12) (a) Rose, G. M.; Coates, G. W. J. Am. Chem. Soc. 2006, 128, 4186. (b) Rose, G. M.; Coates, G. W. Macromolecules 2008, 41, 9548. (13) (a) Huang, Y. B.; Jin, G. X. Organometallics 2008, 27, 259. (b) Han, F. B.; Sun, X. L.; Tang, Y. Organometallics 2008, 27, 1924. (c) Dorcier, A.; Basset, J. M. Organometallics 2009, 28, 2173. (d) Brasse, M.; Campora, J. Organometallics 2008, 27, 4711. (e) Gao, R.; Sun, W. H. Organometallics 2008, 27, 5641. (f) Long, J. M.; Gao, H. Y.; Wu, Q. Eur. J. Inorg. Chem. 2008, 4296. (g) Noda, S.; Nozaki, K. Organometallics 2009, 28, 656. (h) Azoulay, J. D.; Schneider, Y.; Galland, G. B.; Bazan, G. C. Chem. Commun. 2009, 6177. (14) (a) Wang, C.; Friedrich, S.; Younkin, T. R.; Li, R. T.; Grubbs, R. H.; Bansleben, D. A.; Day, M. W. Organometallics 1998, 17, 3149. (b) Younkin, T. R.; Connor, E. F.; Henderson, J. I.; Friedrich, S. K.; Grubbs, R. H.; Bansleben, D. A. Science 2000, 287, 460. (c) Connor, E. F.; Younkin, T. R.; Henderson, J. I.; Hwang, S.; Grubbs, R. H.; Rorberts, W. P.; Litzau, J. J. J. Polym. Sci., Polym. Chem. Ed. 2002, 40, 2842. (d) Connor, E. F.; Younkin, T. R.; Henderson, J. I.; Waltman, A. W.; Grubbs, R. H. Chem. Commun. 2003, 2272. (e) Waltman, A. W.; Younkin, T. R.; Grubbs, R. H. Organometallics 2004, 23, 5121. r 2010 American Chemical Society

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Chart 1. Neutral Nickel Salicylaldiminato Complexes Reported Previously

Chart 2. Neutral Nickel Complexes Containing Five-Membered Nickel Chelates Reported Previously

catalyst B (Chart 1), with a bulky 9-anthracenyl substituent at the ortho position of the phenoxy group, displays a much higher activity (134 kg of PE/(mol of Ni) h atm)) than catalyst A under optimized conditions.16 This greatly stimulated the area of

neutral nickel catalysts for olefin polymerization, and a large number of such catalysts have been explored.17-24 For instance, the nickel methyl salicylaldiminato pyridine catalysts reported by Mecking’s group, bearing substituted aryls at the 2,6positions of the N-aryl moiety, displayed high efficiency for ethylene (co)polymerization even when the polymerization was carried out in water.17,18 Our group found that the binuclear complex C (Chart 1), in which each nickel unit acts as the bulky ortho hindrance of the other, also exhibited high catalytic activity and produced high-molecular-weight polyethylene.19 Typically, all these neutral nickel catalysts include a six-membered nickel chelate containing coordinated N and O, a hindered N-aryl group, and complete conjugation between the N and O. To date, the majority of investigations on neutral nickel catalysts have focused on salicylaldiminato ligand based studies, because of the facility for introducing various substituents on the backbone to enhance the activity and control the microstructure of the polymer. However, Brookhart and co-workers also demonstrated that anilinotropone- and anilinoperinaphthenone-based neutral nickel catalysts (catalysts D and E in Chart 2), containing five-membered chelates instead of the six-membered ones, exhibited high activities toward ethylene polymerization and catalyst D showed a much higher activity than the parent catalyst A, probably due to the smaller chelate size.20 Recently, β-ketiminato ligand based neutral nickel complexes were found to have a great potential for olefin polymerization. For example, Brookhart and Mecking reported that electron-poor neutral nickel enolatoimine catalysts were highly active for ethylene polymerization under nonaqueous or aqueous conditions.21 Our group also reported that a series of modified neutral nickel β-ketiminato catalysts showed high activities toward ethylene polymerization in the presence of B(C6F5)3.22b Unfortunately, cocatalysts or activators are generally required in a great number of olefin polymerization systems in order to obtain maximum activity, which increases the cost of industrial use to some extent. Consequently, there is a great need to develop new catalyst systems that can provide high catalytic activity with no need for a cocatalyst. Herein, we report a series of highly active, single-component neutral nickel(II) catalysts for ethylene polymerization prepared from readily available cyclic β-ketiminato ligands. Interestingly, the degree of conjugation between the phenylene groups and the corresponding nickel chelates of complexes 6a-c and 7a-c was found to have a great influence on the molecular weight and microstructure of the resulting polymers.

(15) Bansleben, D. A.; Grubbs, R. H.; Wang, C.; Friedrich, S.; Younkin, T. R.; Li, R. T. New Late Transition Metal Catalysts for the Polymerization of Ethylene; Proceedings of MetCon'98: “Polymers in Transition”; June 10-11, 1998, Houston, TX; Paper II.3. (16) In 58 °C and 400 psig of ethylene, B can produce PE with the highest activity of 134 kg of PE/((mol of Ni) h atm), but the Mw value is only 85.9.14a (17) (a) Zuideveld, M.; Wehrmann, P.; R€ ohr, C.; Mecking, S. Angew. Chem., Int. Ed. 2004, 43, 869. (b) Kuhn, P.; Semeril, D.; Jeunesse, C.; Matt, D.; Neuburger, M.; Mota, A. Chem. Eur. J. 2006, 12, 5210. (c) G€ottkerSchnetmann, I.; Wehrmann, P.; R€ohr, C.; Mecking, S. Organometallics 2007, 26, 2348. (d) Yu, S. M.; Berkefeld, A.; G€ottker-Schnetmann, I.; M€uller, G.; Mecking, S. Macromolecules 2007, 40, 421. (e) Bastero, A.; G€ottkerSchnetmann, I.; R€ ohr, C.; Mecking, S. Adv. Synth. Catal. 2007, 349, 2307. (f) Guironnet, D.; Mecking, S. Chem. Commun. 2008, 4965. (g) Berkefeld, A.; Mecking, S. J. Am. Chem. Soc. 2009, 131, 1565. (18) (a) Held, A.; Bauers, F. M.; Mecking, S. Chem. Commun. 2000, 301. (b) Bauers, F. M.; Mecking, S. Macromolecules 2001, 34, 1165. (c) Bauers, F. M.; Mecking, S. Angew. Chem., Int. Ed. 2001, 40, 3020. (d) Bauers, F. M.; Chowdhry, M. M.; Mecking, S. Macromolecules 2003, 36, 6711. (e) Bauers, F. M.; Thomann, R.; Mecking, S. J. Am. Chem. Soc. 2003, 125, 8838. (f) Kolb, L.; Monteil, V.; Thomann, R.; Mecking, S. Angew. Chem., Int. Ed. 2005, 44, 429–432. (g) Wehrmann, P.; Mecking, S. Macromolecules 2006, 39, 5963. (h) Wehrmann, P.; Zuideveld, M. A.; Thomann, R.; Mecking, S. Macromolecules 2006, 39, 5995. (i) G€ottkerSchnetmann, I.; Korthals, B.; Mecking, S. J. Am. Chem. Soc. 2006, 128, 7708. (j) Weber, C. H. M.; Chiche, A.; Krausch, G.; Rosenfeldt, S.; Ballauf, M.; Harnau, L.; G€ ottker-Schnetmann, I.; Tong, Q.; Mecking, S. Nano Lett. 2007, 7, 2024. (19) Hu, T.; Tang, L. M.; Li, X. F.; Li, Y. S.; Hu, N. H. Organometallics 2005, 24, 2628. (20) (a) Hicks, F. A.; Brookhart, M. Organometallics 2001, 20, 3217. (b) Jenkins, J. C.; Brookhart, M. Organometallics 2003, 22, 250. (c) Hicks, F. A.; Jenkins, J. C.; Brookhart, M. Organometallics 2003, 22, 3533. (d) Jenkins, J. C.; Brookhart, M. J. Am. Chem. Soc. 2004, 126, 5827. (21) (a) Zhang, L.; Brookhart, M.; White, P. S. Organometallics 2006, 25, 1868. (b) Yu, S. M.; Mecking, S. Macromolecules 2007, 40, 421. (22) (a) Li, X. F.; Li, Y. G.; Li, Y. S.; Chen, Y. X.; Hu, N. H. Organometallics 2005, 24, 2502. (b) Song, D. P.; Ye, W. P.; Wang, Y. X.; Liu, J. Y.; Li, Y. S. Organometallics 2009, 28, 5697. (23) (a) Soula, R.; Broyer, J. P.; Llauro, M. F.; Tomov, A.; Spitz, R.; Claverie, J.; Drujon, X.; Malinge, J.; Saudemont, T. Macromolecules 2001, 34, 2438. (b) Gibson, V. C.; Tomov, A.; White, A. J. P.; Williams, D. J. Chem. Commun. 2001, 719. (c) Tian, G. L.; Boone, H. W.; Novak, B. M. Macromolecules 2001, 34, 7656. (d) Li, X. F.; Li, Y. S. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 2680. (e) Li, Y. S.; Li, Y. R.; Li, X. F. J. Organomet. Chem. 2003, 667. (f) Zhang, D.; Jing, G. X. Organometallics 2003, 22, 2851. (g) Liang, H.; Liu, J. Y.; Li, X. F.; Li, Y. S. Polyhedron 2004, 23, 1619. (h) Zhang, D.; Jin, G. X. Inorg. Chem. Commun. 2006, 9, 1322. (24) (a) Sujith, S.; Dae, J. J.; Na, S. J.; Park, Y. W.; Choi, J. H.; Lee, B. Y. Macromolecules 2005, 38, 10027. (b) Na, S. J.; Lee, B. Y. J. Organomet. Chem. 2006, 691, 611. (c) Zeller, A.; Strassner, T. J. Organomet. Chem. 2006, 691, 4379. (d) Okada, M.; Shiono, T. J. Organomet. Chem. 2007, 692, 5183. (e) Chen, Q.; Yu, J.; Huang, J. Organometallics 2007, 26, 617. (f) Wei, W. P.; Huang, B. T. Inorg. Chem. Commun. 2008, 11, 487. (g) Shen, M.; Sun, W. H. J. Organomet. Chem. 2008, 693, 1683. (h) Li, W. F.; Sun, H. M.; Shen, Q. J. Organomet. Chem. 2008, 693, 2047. (i) Chandran, D.; Kim, I. J. Organomet. Chem. 2009, 694, 1254. (j) Rodriguez, B. A.; Delferro, M.; Marks, T. J. Organometallics 2008, 27, 2166. (k) Rodriguez, B. A.; Delferro, M.; Marks, T. J. J. Am. Chem. Soc. 2009, 131, 5902.

Results and Discussion Synthesis and Characterization of the Neutral Nickel Complexes. A general synthetic route for the neutral nickel complexes 3b and 6a-c used in this study, bearing β-ketiminato chelate ligands, is shown in Scheme 1. β-Diketones were first

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Scheme 1. General Synthetic Route of the Nickel Phenyl Phosphine Complexes 3b and 6a-c

Scheme 2. General Synthetic Route of the Nickel Methyl Pyridine Complexes 7a-c

prepared via the reaction between ethyl formate and cyclohexanone, 1-indanone, 1-tetralone, and 1-benzosuberone, respectively, with the help of a strong base, such as potassium tert-butoxide, in anhydrous diethyl ether. β-Keto imines 2 and 5a-c were prepared in good yields by the condensation of the corresponding β-diketones with 2,6-diisopropylaniline in ethanol containing a small amount of formic acid as a catalyst. The deprotonation of free ligands 2 and 5a-c readily proceeded with excess sodium hydride in anhydrous THF for 4 h at room temperature, and the isolated sodium salts then reacted with a equivalent amount of trans-PhNi(PPh3)2Cl for 12 h in toluene to afford the neutral nickel complexes 3b and 6a-c, respectively. According to the literature,17c the nickel methyl pyridine complexes 7a-c were prepared in high yields by adding (pyridine)2NiMe2 to toluene solutions of ligands 5a-c with vigorous stirring at room temperature (Scheme 2). The neutral nickel complexes 3b, 6a-c, and 7a-c, bearing β-ketiminato ligands, are clearly characterized by 1H and 13C NMR spectra. To further confirm the structures of these complexes, crystals of 3b and 6a-c suitable for X-ray crystallographic analysis were grown from a toluene-hexane solution. The data collection and refinement data of the analysis are summarized in Table S1 (see the Supporting Information), and the ORTEP diagrams are shown in Figures 1-4, respectively. In the solid state, these complexes adopt a near squareplanar coordination geometry, and the triphenylphosphine group is trans to the N-aryl group, just as for the neutral nickel complexes reported previously, because of the steric effects of the N-aryl and triphenylphosphine groups. Selected bond distances and angles are summarized in Table 1. It is noteworthy that complex 6b exhibits Ni-O and Ni-P bond distances relatively longer than those of its parent complex 3b, though they display similar Ni-N bond distances and the

Figure 1. Molecular structure of complex 3b. Thermal ellipsoids are drawn at the 30% probability level, and H atoms as well as a hexane molecule are omitted for clarity.

Figure 2. Molecular structure of complex 6a. Thermal ellipsoids are drawn at the 30% probability level, and H atoms are omitted for clarity.

close Ni-C bond distances. There are nearly no differences in the O-C(39), N-C(37), C(37)-C(38), and C(38)-C(39) bond distances of 6b and 3b. Analogously, the angles around the nickel center of the two complexes are almost the same (see Table 1). However, the intriguing differences with regard to the torsion angles N-C(37)-C(38)-C(39) and C(37)C(38)-C(39)-O cannot be ignored. Complex 6b shows torsion angles (-0.91 and 1.77°) much smaller than those of its parent complex 3b (-4.91 and 3.24°), which indicates that the degree of conjugation between O and N atoms can be enhanced by a phenylene group. Among complexes 6a-c, 6a exhibits the longest Ni-O and Ni-N bond distances, due to the relatively wider O-Ni-N angle compensated by narrower O-Ni-P, C(1)-Ni-N and C(1)-Ni-P angles, while complex 6c shows the shortest Ni-N bond distance. As shown in Table 1, complexes 6a,b

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Table 1. Selected Bond Distances (A˚) and Angles (deg) catalyst 3b

6a

6b

6c

1.897(3) 1.923(3) 1.897(4) 2.1913(11) 1.285(4) 1.320(5) 1.401(5) 1.379(5)

1.9061(19) 1.915(2) 1.887(3) 2.1855(8) 1.296(3) 1.318(3) 1.402(4) 1.385(4)

179.47(11) 173.04(14) 86.79(8) 92.72(12) 93.29(14) 87.18(12)

176.69(8) 171.86(11) 87.20(6) 92.60(9) 93.15(11) 87.39(9)

14.21 -0.91 1.77

38.97 -6.40 11.32

Bond Distances (A˚) Ni-O Ni-N Ni-C Ni-P O-C(39) N-C(37) C(37)-C(38) C(38)-C(39)

1.885(2) 1.920(2) 1.894(3) 2.1883(9) 1.291(3) 1.318(4) 1.400(4) 1.372(4)

1.923(2) 1.929(3) 1.893(4) 2.1755(11) 1.284(4) 1.324(4) 1.388(5) 1.377(5)

Bond Angles (deg) N-Ni-P O-Ni-C(1) O-Ni-P O-Ni-N C(1)-Ni-N C(1)-Ni-P

Figure 3. Molecular structure of complex 6b. Thermal ellipsoids are drawn at the 30% probability level, and H atoms are omitted for clarity.

179.44(8) 172.88(11) 87.71(6) 92.19(9) 93.06(11) 87.08(9)

176.83(9) 172.15(13) 85.93(8) 95.04(11) 92.81(14) 86.22(11)

Torsion Angles (deg) O-C(39)-C(40)-C(41) N-C(37)-C(38)-C(39) -4.91 C(37)-C(38)-C(39)-O 3.24

1.16 -0.65 -2.43

Scheme 3. Generation of Active Sites for Ethylene Polymerization

Figure 4. Molecular structure of complex 6c. Thermal ellipsoids are drawn at the 30% probability level, and H atoms are omitted for clarity.

exhibit similar Ni-C bond distances, slightly longer than those of complex 6c. In comparison with complex B (Ni-P = 2.174 A˚),14a complexes 6a-c exhibit slightly longer Ni-P bond distances (2.1755(11), 2.1913(11), and 2.1855(8) A˚, respectively), which must be closely related to the phosphine dissociation reaction (catalyst activation) during the ethylene polymerization process. As shown in Figures 2-4, the torsion angle of O-C(39)C(40)-C(41) deciding the spatial location of the phenylene group increases from 1.16° through 14.21° to 38.97° (see Table 1) by changing the structure from 6a through 6b to 6c, leading to different degrees of conjugation between the phenylene group and the corresponding nickel chelate ring. In addition, complex 6c exhibits much larger N-C(37)C(38)-C(39) and C(37)-C(38)-C(39)-O torsion angles relative to complexes 6a and 6b, resulting in a significantly reduced degree of conjugation between O and N.

Ethylene Polymerization with Neutral Nickel(II) Complexes. The generation of catalytically active sites (Scheme 3) for ethylene polymerization using the neutral nickel complexes is modeled by phosphine or pyridine dissociation reactions.25a The enthalpy change (ΔH) with regard to the reaction of our systems as well as catalyst B (Chart 1) was calculated by DFT to find the differences among the different structures. As shown in Figure 5, the result for complexes 3b and 6b clearly suggests that an extra phenylene substituent has no strong effect on the phosphine dissociation enthalpy change. Therefore, the steric effect of the phenylene group can be ignored in these complexes. Among complexes 6a-c, 6b displays the lowest enthalpy change of 29.38 kcal/mol, which is consistent with the longest Ni-P bond distance (see Table 1). Interestingly, although there is no bulky substituent at the close position of the coordinating oxygen atom, these β-ketiminato complexes show lower enthalpy changes than the salicylaldiminato complex B (ΔH = 33.08 kcal/mol) bearing a bulky (25) (a) Mary, S. W.; Deng, L. Q.; Ziegler, T. Organometallics 2000, 19, 2714. (b) Michalak, A.; Ziegler, T. Organometallics 2003, 22, 2069.

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Figure 5. Enthalpy change for the phosphine and pyridine dissociation reaction evaluated by DFT methods.

9-anthracenyl at the ortho position of the phenoxy group. The electron effect of the β-ketiminato backbone is considered to be the main factor responsible for this, which is similar to the case for the anilinotropone-based catalyst D (Chart 2) also having a lower enthalpy change than catalyst B.25b As a result, the activation of these β-ketiminato complexes will become more facile than for the salicylaldiminato system due to their distinguishing electronic features such as the more negative nitrogen of the β-ketiminato ligand.20a In addition, substituting the pyridine and methyl groups (7b) for the phosphine and phenyl groups (6b) results in a much higher enthalpy change (ΔH = 33.39 kcal/mol), indicating that the generation of active species from catalyst 6b will be much easier than for catalyst 7b, which proved to be reasonable by subsequent experimental results. The neutral nickel complexes 3b, 6a-c, and 7a-c were investigated as the catalysts for ethylene polymerization in toluene without cocatalysts. The typical results are summarized in Table 2. The data in Table 2 indicate that ligand structure significantly affects catalytic activity and polymer microstructure along with properties. Catalyst 3b was synthesized for comparison and showed a moderate activity of 10.2 kg of PE/((mol of Ni) h atm) (entry 1) under the typical conditions (63 °C, ethylene pressure 50 atm). In comparison with catalyst 3b (Figure 1), there is an extra phenylene group (C40-C45) in the molecule of catalyst 6b (Figure 3) as a distinguishing feature. Interestingly, catalyst 6b became more stable in the polymerization process and exhibited a much higher activity (37.6 kg of PE/((mol of Ni) h atm), entry 4) than catalyst 3b under the same conditions, suggesting that the phenylene group must have played a critical role in stabilizing the active center and enhancing the catalytic activity. The enhanced degree of conjugation between N and O atoms by the phenylene group may be mainly responsible for this. Considering that further modulation of the conjugation between the phenylene ring and the corresponding nickel chelate may give birth to some other improvements in catalytic properties, we prepared catalysts 6a,c, having different degrees of conjugation. Among catalysts 6a-c, highly conjugated 6a showed an activity of 35.2 kg of PE/((mol of Ni) h atm) (entry 2), and similar activities (entries 4 and 8) were also observed using the less conjugated catalysts 6b,c under the same conditions. The minor difference between these activities may be attributed to the small variations in ethylene pressure and reaction temperature. Reaction conditions, such as reaction temperature and ethylene pressure, also dramatically influence the catalytic

Song et al.

activity. For 6c, the catalytic activity was greatly enhanced from 2.10 to 40.8 kg of PE/((mol of Ni) h atm) via increasing reaction temperature from 53 to 63 °C (entries 6 and 8). On the other hand, the catalytic activity can also be greatly increased up to 22.6 kg of PE/((mol of Ni) h atm) (entry 7) by adding B(C6F5)3 to promote activation of the catalyst at 53 °C. However, further elevating the temperature to 73 °C caused a lower catalytic activity of 14.1 kg of PE/((mol of Ni) h atm) (entry 9). The lower equilibrium concentration of ethylene in solution at 73 as compared to that at 53 °C may be mainly responsible for the difference. Under the optimized conditions, a high activity of 71.4 kg of PE/((mol of Ni) h atm) (entry 10) was obtained using catalyst 6c. As shown in Table 2, the amounts of polymer produced by catalysts 6a,c are greatly influenced by ethylene pressure. Much lower productivities (entries 3 and 11) were observed by decreasing the ethylene pressure from 50 to 25 atm. Distinctively, a similar amount of polymer was obtained using 6b (entry 5), which was less dependent on ethylene pressure. This behavior suggests that the phosphine group of catalyst 6b does not compete effectively with ethylene for binding to the metal center even at reduced pressure.26 In comparison with the nickel phenyl phosphine catalysts, the nickel methyl pyridine catalysts 7a-c, bearing the same ligands, were also prepared to explore the differences in the two types of catalysts. The typical results listed in Table 2 indicate that complexes 7a-c are less active catalyst precursors toward ethylene polymerization under the same reaction conditions. For example, catalyst 7a showed a lower activity (entry 12) than catalyst 6a (entry 3) when 25 atm of ethylene pressure was used in the absence of an activator. Similarly, lower activities of catalysts 7b,c were also observed relative to catalysts 6b,c. This has been effectively supported by the investigation of the catalyst activation concerning the two types of nickel catalysts using DFT methods, and the intriguing difference in activation energies of catalysts 6b (29.38 kcal/mol) and 7b (33.39 kcal/mol) provides the best explanation of the catalytic behaviors observed. In addition, much higher activities of 7a-c (entries 13, 17, and 19 in Table 2) were attained by adding B(C6F5)3 to promote the dissociation of pyridine from the nickel center (catalyst activation process), indicating that the rate-limiting step of the ethylene polymerization reaction is the generation of active species. The data given in Table 2 also indicate that the ligand structure considerably affects the molecular weight and microstructure of the polyethylene obtained. Under the typical conditions (63 °C, ethylene pressure 50 atm), the weightaverage molecular weights (Mw) of the polymers produced by 3b and 6b increase from 33.7 to 46.8, while the branching numbers decrease from about 37 to 26 branches per 1000 carbon atoms (entries 1 and 4). The suppression of the “chain walking” process by the extra phenylene group of catalyst 6b should be mainly responsible for the observed polymerization behaviors. Moreover, it was found that the degree of conjugation between the phenylene groups and the corresponding nickel chelates of complexes 6a-c led to a great influence on the molecular weight and microstructure of the resulting polymer. As shown in Table 2, the rigid and highly conjugated catalyst 6a produced polyethylene with much lower molecular (26) (a) Vela, J.; Lief, G. R.; Shen, Z. L.; Jordan, R. F. Organometallics 2007, 26, 6624. (b) Guironnet, D.; Roesle, P.; Runzi, T.; Mecking, S. J. Am. Chem. Soc. 2009, 131, 422.

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Table 2. Results of Ethylene Polymerization Reactionsa entry

complex (amt (μmol))

temp (°C)

pressure (atm)

amt of polymer (g)

activityb

Tm (°C)

Mwc

Mw/Mnc

branchesd/1000C

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

3b (20) 6a (20) 6a (20) 6b (20) 6b (20) 6c (20) 6c (20) 6c (20) 6c (20) 6c (10) 6c (10) 7a (20) 7a (20) 7b (20) 7b (20) 7b (20) 7b (20) 7c (20) 7c (20)

63 63 63 63 63 53 53 63 73 63 63 63 63 63 63 63 63 63 63

50 50 25 50 25 50 50 50 50 50 25 25 25 50 25 50 25 25 25

3.40 11.7 5.20 12.5 12.3 0.70 7.50 13.6 4.70 11.9 5.80 3.60 6.70 6.40 3.50 11.9 10.4 3.80 6.40

10.2 35.2 31.2 37.6 74.0 2.10 22.6 40.8 14.1 71.4 69.6 21.6 40.4 19.2 21.0 35.8 62.4 22.8 38.4

100 87 81 106 90 112 109 106 101 105 93

33.7 9.81 7.54 46.8 28.9 47.5 42.4 47.1 23.9 46.5 28.9 7.78 8.24 52.2 38.5 56.9 42.7 38.4 41.6

2.1 1.9 1.9 2.0 2.0 1.9 2.2 2.2 2.3 2.1 1.8 2.0 2.1 2.1 2.0 1.9 2.1 1.9 2.1

37 54 69 26 79 21 25 28 40 27 42 72 77 27 51 28 55 43 47

g g

105 98 105 94 94 96

a Reaction conditions: 100 mL of toluene, polymerization for 20 min. b In units of kg of PE/((mol of Ni) h atm). c Determined by GPC vs polystyrene standards. d Determined by 1H NMR according to the literature.4 e Polymerization in 70 mL of toluene. f Two equivalents of B(C6F5)3 was added as the cocatalyst (B/Ni molar ratio 2). g Completely amorphous polymer.

Figure 6. 13C NMR of polyethylenes with different branching numbers and styles produced by catalysts 6a,b (entries 2, 4, and 5 in Table 2). Spectra were assigned according to the literature.31

weight (MW) and higher branching number (entry 2) relative to that with the flexible, less conjugated catalysts 6b,c (entries 4 and 8) under the typical conditions. Differences between the molecular structures of 6a and 6b,c provide evidence of the conjugation modulation effect. For example, catalyst 6a displays longer Ni-O and shorter Ni-P bond distances than 6b, c (shown in Table 1), suggesting that the electronic nature of the nickel center of 6a should be different from that of 6b,c due to the highly conjugated structure. The higher phosphine dissociation energy of catalyst 6a relative to those of 6b,c provides insight into the correlation of the degree of conjugation and the electronic nature of the metal center. The microstructures of the typical polyethylenes were analyzed according to 13C NMR. As shown in Figure 6, only methyl branches are observed in the polymers produced by 6b under the typical conditions (entry 4), while not only methyls but also ethyls and longer branches are found in the polyethylene produced by 6a (entry 2), indicating that the control of polyethylene microstructure can be realized simply by variation of the ligand structure. Great differences also exist in MWs and branching numbers of the polyethylenes produced by catalysts 7a-c. Similar to the catalysts 6a-c, the more flexible and less conjugated 7b,c produced polyethylenes with much higher MWs and lower branch contents (entries 12-19 in Table 2) than the rigid and

Figure 7. DSC curves of the polyethylenes produced by catalysts 7a (lines a and b, entry 13) and 7c (lines c and d, entry 19).

highly conjugated catalyst 7a, indicating again that the degree of conjugation between the phenylene group and the corresponding nickel chelate ring is the determining factor for controlling the MW and microstructure of the resulting polymer. For example, completely amorphous polyethylenes with MWs less than 10.0 kg/mol and branching numbers more than 70 branches per 1000 carbon atoms (entries 12 and 13) have been produced by catalyst 7a. As shown in Figure 7, the melting point of the polymer disappears when the branching number reaches up to 77 branches per 1000 carbon atoms (entry 13). In comparison with 7a, catalysts 7b,c produced semicrystalline polyethylenes with MWs more than 38.0 kg/ mol and moderate branching numbers from about 43 to 55 branches per 1000 carbon atoms (see lines c and d of Figure 7). In addition, in comparison with 6b, catalyst 7b produced polyethylenes with higher MWs under the same conditions (entries 14 vs 4 and 15 vs 5), suggesting that the methyl group resembles the growing polymer chain much more closely than a phenyl group, resulting in the easier insertion of ethylene momoners.17a As shown in Table 2, the MWs of the polyethylenes produced by catalyst 6c decreased from 47.5 to 23.9 kg/mol by increasing the reaction temperature from 53 to 73 °C, and

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Organometallics, Vol. 29, No. 10, 2010

the branch contents were enhanced from 21 to 40 branches per 1000 carbon atoms (entries 6-9). A similar phenomenon has also been reported concerning cationic nickel R-diimine catalysts as well as the neutral nickel salicylaldiminato systems,2,14a demonstrating that the rate of chain migration and termination in our systems also increased at elevated temperature, yielding higher branched and lower MW polymer. Ethylene pressure also dramatically affects the microstructure of the resulting polymer. It is noteworthy that polyethylene with much lower molecular weight can be obtained by changing the pressure from 50 to 25 atm (from 46.8 to 28.9 kg/mol, entries 4 and 5) at 63 °C using catalyst 6b, which suggests that β-hydride elimination is the major chain transfer mechanism in this case.27 On the other hand, the branching number increases with the decrease of ethylene pressure (from 26 to 79 branches/1000C, entries 4 and 5), which can be best explained by the acceleration of “chain walking” under a lower ethylene concentration. According to 13C NMR (Figure 6), methyl branches are the predominant branching style in the polymers produced under 50 atm (entry 4), while the small signal visible at about 14.1 ppm suggests the presence of other longer branches in the polyethylene produced under reduced pressure (25 atm, entry 5).4 The melting points of polymers decrease with an increase of branching number (Table 2), because branches can restrain the crystallization of the polymers. The relatively narrow PDI (Mw/Mn = 1.8-2.1) of the obtained polyethylene indicates that these neutral nickel complexes are favorable homogeneous singlesite catalysts.

Song et al.

General Procedures and Materials. All work involving airand/or moisture-sensitive compounds was carried out under a

dry nitrogen atmosphere by using standard Schlenk techniques or under a dry argon atmosphere in an MBraun glovebox, unless otherwise noted. All solvents used were purified from an MBraun SPS system. The NMR data of ligands and complexes were obtained on a Bruker 300 MHz spectrometer at ambient temperature with CDCl3 or C6D6 as the solvent. The NMR analyses of polymers were performed on a Varian Unity 400 MHz spectrometer at 135 °C, using o-C6D4Cl2 as the solvent. The differential scanning calorimetric (DSC) measurements were performed with a PerkinElmer Pyris 1 DSC differential scanning calorimeter at a rate of 10 °C/min. The molecular weights and the polydispersities of the polymer samples were determined at 150 °C by a PL-GPC 220 type high-temperature chromatograph equipped with three PLgel 10 μm Mixed-B LS type columns. 1,2,4-Trichlorobenzene (TCB) was employed as the solvent at a flow rate of 1.0 mL/min. The calibration was made by the polystyrene standard EasiCal PS-1 (PL Ltd.). Cyclohexanone, 1-indanone, 1-tetralone, and 1-benzosuberone were purchased from Aldrich Chemicals and directly used without purification. 2,6-Diisopropylaniline and NaH were obtained from Acros. Potassium tert-butoxide was purchased from Aldrich Chemicals. trans-PhNi(PPh3)2Cl and Py2Ni(CH3)2 were prepared according to the literature methods.28 Commercial ethylene was used without further purification. Synthesis of Ligands 2 and 5a-c. To a slurry of 3.3 g of potassium tert-butoxide (1.5 equiv) in anhydrous diethyl ether (40 mL) were added 1.9 g of cyclohexanone (20 mmol) and 2.9 g of ethyl formate (2.0 equiv) at 0 °C. Immediately a large amount of white solid appeared in the reaction bottle, and the mixture was stirred for 30 min at 0 °C. Then the resulting suspension was warmed to room temperature and stirred for about 10 h. The white solid was separated by filtration and dried under reduced pressure. Formic acid in ethanol was added to the solid until the pH was