Ethylene Polymerization Characteristics of an Electron-Deficient

Ethylene Polymerization Characteristics of an Electron-Deficient Nickel(II) Phenoxyiminato Catalyst Modulated by Non-Innocent Intramolecular Hydrogen ...
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Organometallics 2010, 29, 5040–5049 DOI: 10.1021/om100251j

Ethylene Polymerization Characteristics of an Electron-Deficient Nickel(II) Phenoxyiminato Catalyst Modulated by Non-Innocent Intramolecular Hydrogen Bonding† Massimiliano Delferro, Jennifer P. McInnis, and Tobin J. Marks* Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113 Received March 31, 2010

The synthesis and characterization of the neutrally charged electron-deficient nickel(II) phenoxyiminato catalyst {2-(hydroxydiphenylmethyl)-4-tert-butyl-[6-(2,6-diisopropylphenyl)]salicylaldiminato}methyl(trimethylphosphine)nickel(II) (1b) with an intramolecular hydrogen bond directed toward the active catalytic site is reported. At room temperature, catalyst 1b exhibits 2.5 greater ethylene polymerization activity and 2 greater polyethylene product branching than an analogous catalyst without the hydrogen bond (2b). Furthermore, catalyst 1b produces substantially greater polyethylene yields in the presence of polar additives such as ethyl ether, acetone, and water than does 2b under identical conditions. This enhanced polymerization activity in the presence of polar additives suggests that the hydrogen bonding proximate to the metal center significantly modifies the relative rates of competing enchainment and chain transfer processes.

Introduction In the past several years, late-transition-metal catalysts for olefin polymerization, in particular Ni(II) complexes,1 have attracted much research interest.2 Due to their functional group tolerance, such catalysts can effect ethylene polymerization in the presence of polar additives3 and in aqueous emulsions4 or can readily copolymerize ethylene with polar † Part of the Dietmar Seyferth Festschrift. In honor of Professor Dietmar Seyferth for his outstanding contributions in the field of Organometallic Chemistry. *To whom correspondence should be addressed. Tel: (þ1) 847 491 5658. Fax: (þ1) 847 491 2990. E-mail: [email protected]. (1) (a) Guironnet, D.; Friedberger, T.; Mecking, S. Dalton Trans. 2009, 41, 8929–8934. (b) Guironnet, D.; Goettker-Schnetmann, I.; Mecking, S. Macromolecules 2009, 42, 8157–8164. (c) Rodriguez, B. A.; Delferro, M.; Marks, T. J. J. Am. Chem. Soc. 2009, 131, 5902–5919. (d) Rodriguez, B. A.; Delferro, M.; Marks, T. J. Organometallics 2008, 27, 2166–2168. (e) Wehrmann, P.; Mecking, S. Organometallics 2008, 27, 1399–1408. (f) Goettker-Schnetmann, I.; Wehrmann, P.; Roehr, C.; Mecking, S. Organometallics 2007, 26, 2348–2362. (g) Chen, Q.; Yu, J.; Huang, J. Organometallics 2007, 26, 617–625. (h) Hu, T.; Tang, L. M.; Li, X. F.; Li, Y. S.; Hu, N. H. Organometallics 2005, 24, 2628–2632. (i) Zuideveld, M. A.; Wehrmann, P.; Roehr, C.; Mecking, S. Angew. Chem., Int. Ed. 2004, 43, 869–873. (j) Connor, E. F.; Younkin, T. R.; Henderson, J. I.; Hwang, S.; Grubbs, R. H.; Roberts, W. P.; Litzau, J. J. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 2842–2854. (k) Wang, C.; Friedrich, S. K.; Younkin, T. R.; Li, R. T.; Grubbs, R. H.; Bansleben, D. A.; Day, M. W. Organometallics 1998, 17, 3149–3151. (2) (a) Takeuchi, D. Dalton Trans. 2010, 39, 311–328. (b) Domski, G. J.; Rose, J. M.; Coates, G. M.; Bolig, A. D.; Brookhart, M. Prog. Polym. Sci. 2007, 32, 30–92. (c) Zhang, J; Wang, X.; Jin, G. X. Coord. Chem. Rev. 2006, 250 (1-2), 95–109. (d) Durand, J.; Milani, B. Coord. Chem. Rev. 2006, 250, 542–560. (e) Gibson, V. C.; Spitzmesser, S. K. Chem. Rev. 2003, 103, 283–316. (f) Guan, Z. Chem. Eur. J. 2002, 8, 3086–3092. (g) Bauers, F. M.; Mecking, S. Angew. Chem., Int. Ed. 2001, 40, 534–540. (h) Mecking, S. Coord. Chem. Rev. 2000, 203, 325–351. (i) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. Rev. 2000, 100, 1169–1203. (j) Britovsek, G. J. P.; Gibson, V. C.; Wass, D. F. Angew. Chem., Int. Ed. 1999, 38, 428. (k) Boffa, L. S.; Novak, B. M. Chem. Rev. 2000, 100, 1479.10. (3) Younkin, T. R.; Connor, E. F.; Henderson, J. I.; Friedrich, S. K.; Grubbs, R. H.; Bansleben, D. A. Science 2000, 287, 460–462.

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comonomers such as acrylates.5 Our laboratory previously demonstrated that cooperative effects in single-site6 bimetallic olefin polymerzation catalysts (group 47 and group 101c,d) selectively enhance bulky/polar comonomer enchainment and branch formation versus the corresponding monometallic catalysts. The nature and density of branching in (4) (a) Padavattan, G.; Jaekel, C.; Steinke, T.; Reimer, V.; DiazValenzuela, M. B.; Crewdson, P.; Rominger, F. J. Organomet. Chem. 2010, 695, 673–679. (b) Yu, S. M.; Berkefeld, A.; Goettker-Schnetmann, I.; Mueller, G.; Mecking, S. Macromolecules 2007, 40, 421–428. (c) Korthals, B.; Goettker-Schnetmann, I.; Mecking, S. Organometallics 2007, 26, 1311– 1316. (d) Wehrmann, P.; Mecking, S. Macromolecules 2006, 39, 5963–5964. (e) Wehrmann, P.; Zuideveld, M.; Thomann, R.; Mecking, S. Macromolecules 2006, 39, 5995–6002.(f) Gottker-Schnetmann, I.; Korthals, B.; Mecking, S. J. Am. Chem. Soc. 2006, 128, 7708–7716. (g) Kolb, L.; Monteil, V.; Thomann, R.; Mecking, S. Angew. Chem., Int. Ed. 2005, 44, 429–432. (5) Reviews: (a) Nakamura, A.; Ito, S.; Nozaki, K. Chem. Rev. 2009, 109, 5215–5244. (b) Chen, E. Y. X. Chem. Rev. 2009, 109, 5157–5214. (6) For recent reviews of single-site olefin polymerization, see: (a) Amin, S. B.; Marks, T. J. Angew. Chem., Int. Ed. 2008, 47, 2006–2025. (b) Suzuki, N. Top. Organomet. Chem. 2005, 8, 177–216. (c) Alt, H. G. Dalton Trans. 2005, 20, 3271–3276. (d) Kaminsky, W. J. Polym. Sci. Polym. Chem. 2004, 42, 3911–3921. (e) Wang, W.; Wang, L. J. Polym. Mater. 2003, 20, 1–8. (f) Delacroix, O.; Gladysz, J. A. Chem. Commun. 2003, 6, 665–675. (g) Kaminsky, W.; Arndt-Rosenau, M. Applied Homogeneous Catalysis with Organometallic Compounds, 2nd ed.; Wiley-VCH: Weinheim, Germany, 2002. (h) Lin, S.; Waymouth, R. M. Acc. Chem. Res. 2002, 35, 765–773. (i) Schweier, G.; Brintzinger, H.-H. Macromol. Symp. 2001, 173, 89–103. (j) Gladysz, J. A. Chem. Rev. 2000, 100, 1167–1168 and contributions therein. (k) Chen, E. Y-X.; Marks, T. J. Chem. Rev. 2000, 100, 1391–1434. (7) (a) Salata, M. R.; Marks, T. J. Macromolecules 2009, 42, 1920– 1933. (b) Salata, M. R.; Marks, T. J. J. Am. Chem. Soc. 2008, 130, 12–13. (c) Guo, N.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 2008, 130, 2246–2261. (d) Amin, S.; Marks, T. J. J. Am. Chem. Soc. 2007, 129, 2938–2953. (e) Li, H.; Marks, T. J. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15295–15302and references therein. (f) Amin, S.; Marks, T. J. J. Am. Chem. Soc. 2006, 128, 4506–4507. (g) Li, H.; Stern, C. L.; Marks, T. J. Macromolecules 2005, 38, 9015–9027. (h) Li, H.; Li, L.; Schwartz, D. J.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 2005, 127, 14756–14768. (i) Li, H.; Li, L.; Marks, T. J. Angew. Chem., Int. Ed. 2004, 43, 4937–4940. (j) Guo, N.; Li, L.; Marks, T. J. J. Am. Chem. Soc. 2004, 126, 6542–6543. (k) Li, H.; Li, L.; Marks, T. J.; Liable-Sands, L.; Rheingold, A. L. J. Am. Chem. Soc. 2003, 125, 10788–10789. r 2010 American Chemical Society

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Scheme 1. General Pathway for Nickel-Catalyzed Ethylene Polymerization

Ni(II)-mediated polymerization processes is known to depend on competition between chain propagation and reversible β-H elimination/addition (Scheme 1),8 which in turn modifies the product microstructure and, hence, processing and mechanical properties.9 To facilitate β-hydrogen elimination/re-insertion during the propagation and thus favor highly branched polyolefins, several approches have been demonstrated: (1) decrease the ethylene concentration,1c (2) increase the reaction temperature,2j (3) use cationic complexes,8m,o,p,10 and (4) decrease the electron density at the metal center by introducing electron-withdrawing ligand substituents.11 In this regard, electron deficiency at the Ni center significantly increases ethylene polymerization rates,12 due (8) (a) McCord, E. F.; McLain, S. J.; Nelson, L. T. J.; Ittel, S. D.; Tempel, D.; Killian, C. M.; Johnson, L. K.; Brookhart, M. Macromolecules 2007, 40, 410–420. (b) Zhang, L.; Brookhart, M.; White, P. S. Organometallics 2006, 25, 1868–1874. (c) Collins, S.; Ziegler, T. Organometallics 2007, 26, 6612–6623. (d) Cherian, A. E.; Rose, J. M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 2005, 127, 13770–13771. (e) Liu, W.; Brookhart, M. Organometallics 2004, 23, 6099–6107. (f) Jenkins, J. C.; Brookhart, M. J. J. Am. Chem. Soc. 2004, 126, 5827–5842. (g) Leatherman, M. D.; Svejda, S. A.; Johnson, L. K.; Brookhart, M. J. Am. Chem. Soc. 2003, 125, 3068–3081. (h) Gottfried, A. C.; Brookhart, M. Macromolecules 2003, 36, 3085–3100. (i) Shultz, L. H.; Tempel, D. J.; Brookhart, M. J. Am. Chem. Soc. 2001, 123, 11539–11555. (j) Shultz, L. H.; Brookhart, M. Organometallics 2001, 20, 3975–3982. (k) Gottfried, A. C.; Brookhart, M. Macromolecules 2001, 34, 1140–1143. (l) Tempel, D. J.; Johnson, L. K.; Huff, R. L.; White, P. S.; Brookhart, M. J. Am. Chem. Soc. 2000, 122, 6686–6700. (m) Gates, D. P.; Svejda, S. A.; Onate, E.; Killian, C. M.; Johnson, L. K.; White, P. S.; Brookhart, M. Macromolecules 2000, 33, 2320–2334. (n) Guan, Z.; Cotts, P. M.; McCord, E. F.; McLain, S. J. Science 1999, 283, 2059–2062. (o) Killian, C. M.; Tempel, D. J.; Johnson, L. K.; Brookhart, M. J. Am. Chem. Soc. 1996, 118, 11664–11665. (p) Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc. 1995, 117, 6414–6415. (9) (a) Brookhart, M. S.; Johnson, L. K.; Killian, C. M.; Arthur, S. D.; Feldman, J.; McCord, E. F.; McLain, S. J.; Kreutzer, K. A.; Bennett, M. A.; Coughlin, E. B.; Ittel, S. D.; Parthasarathy, A.; Tempel, D. J. WO 9623010, 1996. (b) Simpson, D. M.; Vaughan, G. A. Ethylene Polymers: LLDPE. In Encyclopedia of Polymer Science and Technology; Mark, H. F., Ed.; Wiley: New York, 2003; Vol. 2, pp 441-482. (10) (a) Leatherman, M. D.; Brookhart, M. Macromolecules 2001, 34, 2748. (b) Mecking, S.; Johnson, L. K.; Wang, L.; Brookhart, M. J. Am. Chem. Soc. 1998, 120, 888. (c) Johnson, L. K.; Mecking, S.; Brookhart, M. J. Am. Chem. Soc. 1996, 118, 267. (11) Kuhn, P.; Semeril, D.; Jeunesse, C.; Matt, D.; Neuburger, M.; Mota, A. Chem. Eur. J. 2006, 12, 5210–5219. (12) Soula, R.; Broyer, J. P.; Llauro, M. F.; Tomov, A.; Spitz, R.; Claverie, J.; Drujon, X.; Malinge, J.; Saudemont, T. Macromolecules 2001, 34, 2438–2442 and references therein.

Chart 1

to a lowered ethylene-insertion barrier, and enhances β-agostic interactions with Ni-alkyl species, as suggested by DFT calculations.13 New ways to manipulate electron density at the Ni center would be of great interest, and in the present contribution we present structural and solution studies along with olefin polymerization catalytic chemistry of a novel phenoxyiminato Ni(II) catalyst with a non-innocent intramolecular hydrogen bond14 directed at the active catalytic center (1b; Chart 1). This catalyst exhibits 2.5 increased polymerization activity in comparison to the control phenoxyiminato Ni(II) catalyst analogue without an intramolecular hydrogen bond (2b). Furthermore, this enhanced activity is mantained in the presence of polar additives, such as water, acetone, and diethyl ether.

Experimental Section Materials and Methods. All manipulations of air-sensitive materials were performed with rigorous exclusion of oxygen and moisture in flamed Schlenk-type glassware on a dualmanifold Schlenk line, interfaced to a high-vacuum line (10-5 Torr), or in a nitrogen-filled Vacuum Atmospheres glovebox with a high-capacity recirculator ( 2σ(I)) no. of params R indices (I > 2σ(I))a

C36H41NO2 C40H52NNiO2P 519.72 668.49 triclinic triclinic P1 P1 170(2) 100(2) 9.6595(2) 11.6789(1) 11.1247(2) 12.5998(2) 14.6231(3) 14.3988(2) 100.354(1) 109.437(1) 102.299(1) 106.588(1) 95.643(1) 95.916(1) 1494.98(5) 1868.34(4) 2 2 1.154 1.188 560 716 0.33  0.11  0.09 0.32  0.27  0.09 0.070 0.595 18 359 42 449 6075 10 836 3110 8730 356 421 R1 = 0.0490 R1 = 0.0351 (wR2 = 0.0995) (wR2 = 0.0826) a R1 = 0.1063 R1 = 0.0495 R indices (all data) (wR2 = 0.1418) (wR2 = 0.0892) P P P P a 2 R1 = ||Fo| - |Fc||/ |Fo|. wR2 = [ [w(Fo - Fc2)2]/ [w(Fo2)2]].

and 2,6-diisopropylaniline (1.33 mL, 7.1 mmol). The mixture was stirred for 2 h at reflux, and then overnight at room temperature. The volatiles were next removed in vacuo to yield a yellow oil. MeOH (10 mL) was added and the solution left in the refrigerator at -20 C overnight. A yellow solid was formed. The solid was collected by filtration, washed with cold methanol, and dried under vacuum. Yield: 60%. Anal. Calcd for C37H43NO2 (M = 533.74): C, 83.26; H, 8.12; N, 2.62. Found: C, 83.17; H, 8.30; N, 2.73. 1H NMR (25 C, CDCl3, 500 MHz): δ 13.44 (s, 1H, Ph-OH), 8.39 (s, 1H, CHN), 8.14-7.37 (m, 13H, Ph), 3.21 (s, 1H, -OMe), 3.02 (sept., 2H, CHiso, 3JH,H = 6.5 Hz), 1.25 (d, 12H, CH3iso, 3JH,H = 6.5 Hz), 1.49 (s, 9H, CH3tert) ppm. 13C NMR (25 C, CDCl3, 125 MHz): δ 166.8, 157.0, 146.2, 142.6, 140.9, 138.8, 130.2, 128.9, 128.2, 127.4, 126.9, 125.3, 123.2, 118.4, 85.9, 52.0, 34.3, 31.6, 28.1, 23.6 ppm. Preparation of 2-(Hydroxydiphenylmethyl)-4-tert-butyl-[6-(2,6diisopropylphenyl)]salicylaldiminate Li(THF)þ Salt (1a). Compound 1 (2 g, 3.85 mmol) and LiCH2TMS (0.398 g, 4.23 mmol) were combined in a reaction flask in the glovebox. Dry THF (15 mL) was added at -78 C and the mixture was stirred for 30 min, followed by stirring at room temperature overnight. The volatiles were next removed in vacuo and the residue was recrystallized from pentane to give 2-(hydroxydiphenylmethyl)-4-tertbutyl[6-(2,6-diisopropylphenyl)]-salicylaldiminate lithium salt (1a) as a white powder. Yield, 89%. Anal. Calcd for C40H48LiNO3 (M = 597.76): C, 80.37; H, 8.09; N, 2.34. Found: C, 80.62; H, 7.83; N, 2.66. 1H NMR (25 C, C6D6, 500 MHz): δ 8.06 (s, 1H, CHN), 7.18-6.77 (m, 13H, Ph), 5.24 (s, br, 1H, C-OH), 3.41 (m, 4H, THF), 3.02 (sept., 2H, CH(iso), 3JH,H = 7.0 Hz), 1.35 (m, 4H, THF), 1.16 (s, 9H, CH3(tert)), 1.04 (d, 6H, CH3(iso), 3JH,H = 7.0 Hz), 0.855 (d, 6H, CH3(iso), 3JH,H = 7.0 Hz) ppm. 13C NMR (25 C, C6D6, 125 MHz): δ 169.1, 168.3, 151.1, 147.3, 140.2, 136.7, 132.7, 132.5, 132.3, 127.9, 127.4, 125.2, 124.0, 122.3, 84.9, 68.1 (THF), 34.8, 32.0, 28.5, 26.1 (THF), 25.8 ppm. Preparation of 2-(Methoxydiphenylmethyl)-4-tert-butyl-[6-(2,6diisopropylphenyl)]salicylaldiminate Liþ Salt (2a). Compound 2 (1.63 g, 3.05 mmol) and LiCH2TMS (0.316 g, 3.36 mmol) were combined in a reaction flask in the glovebox. Dry THF (15 mL) was added at -78 C, and the mixture was stirred for 30 min, followed by stirring at room temperature overnight. The volatiles were next removed in vacuo, and the residue was recrystallized

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Delferro et al. Scheme 2. Ligand Synthesis

from pentane to give 2-(methoxydiphenylmethyl)-4-tert-butyl[6-(2,6-diisopropylphenyl)]salicylaldiminate lithium salt (2a) as a white powder. Yield: 80%. Anal. Calcd for C37H42LiNO2 (M = 539.68): C, 82.34; H, 7.84; N, 2.60. Found: C, 82.62; H, 7.59; N, 2.61. 1H NMR (25 C, C6D6, 500 MHz): δ 8.19 (s, 1H, CHN), 7.47-6.85 (m, 13H, Ph), 3.26 (sept., 2H, CH(iso), 3JH,H = 6.5 Hz), 2.78 (s, 3H, -OMe), 1.24 (s, 9H, CH3(tert)), 1.42 (d, 12H, CH3(iso), 3JH,H = 6.5 Hz) ppm. 13C NMR (25 C, C6D6, 125 MHz): δ 169.7, 167.6, 151.6, 144.3, 139.7, 134.1, 132.5, 131.9, 129.9, 126.5, 124.9, 123.6, 122.1, 90.4, 52.8, 34.3, 31.6, 28.4, 22.6 ppm. Preparation of {2-(Hydroxydiphenylmethyl)-4-tert-butyl-[6-(2,6diisopropylphenyl)]salicylaldiminato}methyl(trimethylphosphine)nickel(II) (1b). A solution of 7a (0.400 g, 0.67 mmol) in benzene (30 mL) was added dropwise at room temperature to a solution of trans-[NiMeCl(PMe3)2] (0.175 g, 0.67 mmol) in benzene (25 mL). The orange mixture was stirred for 6 h at room temperature. After this time, the reaction mixture was filtered by cannula. The filtrate was then evaporated in vacuo, and the solid residue was washed with pentane. A light orange microcrystalline powder of {2-(hydroxydiphenylmethyl)-4-tert-butyl-[6-(2,6-diisopropylphenyl)]salicylaldiminato}methyl(trimethylphosphine)nickel(II) (1b) was obtained. Yield: 92%. Orange crystals of 1b suitable for X-ray studies were obtained by slow evaporation of pentane solutions. Anal. Calcd for C40H52NiNO2P (M = 668.51): C, 71.87; H, 7.84; N, 2.10. Found: C, 71.68; H, 7.81; N, 2.08. 1H NMR (25 C, C6D6, 500 MHz): δ 8.22 (s, 1H, C-OH), 7.89 (s, 1H, CHN), 7.62-6.98 (m, 13H, Ph), 3.88 (m, 2H, CH(iso), 3 JH,H = 6.5 Hz), 1.31 (d, 6H, CH3(iso), 3JH,H = 6.5 Hz), 1.11 (s, 9H, CH3(tert)), 0.97 (d, 6H, CH3(iso), 3JH,H = 6.5 Hz), 0.59 (d, 9H, PMe3, 4JP,H = 9 Hz), -1.14 (s, 3H, Ni-CH3) ppm. μeff = 1.08 μB in CD2Cl2. 13C NMR (25 C, C6D6, 125 MHz): δ 166.3, 163.7, 149.0, 141.3, 136.9 136.2, 134.2, 129.3, 126.9, 123.7, 119.8, 83.5, 33.8, 31.4, 28.6, 25.0, 23.2, 13.3 (d, PMe3, 1JC,P = 28 Hz), -12.5 (d, Ni-CH3, 2JC,P = 43 Hz). 31P{1H} NMR (25 C, C6D6, 162 MHz): δ -15.7 ppm. Preparation of {2-(Methoxydiphenylmethyl)-4-tert-butyl-[6-(2,6diisopropylphenyl)]salicylaldiminato}methyl(trimethylphosphine)nickel(II) (2b). A solution of 2a (0.500 g, 0.92 mmol) in benzene (30 mL) was added dropwise at room temperature to a solution of trans-[NiMeCl(PMe3)2] (0.242 g, 0.92 mmol) in benzene (25 mL). The orange mixture was stirred for 2 days at room temperature.

Figure 1. ORTEP plot of the molecular structure of ligand 1. Thermal ellipsoids are drawn at the 50% probability level. Selected distances (A˚): C1-O1 = 1.361(3), C24-O2 = 1.443(3), C11N1 = 1.278(3). After this time, the reaction mixture was filtered by cannula. The filtrate was then evaporated in vacuo, and the solid residue was washed with pentane. A light orange microcrystalline powder of {2-(methoxydiphenylmethyl)-4-tert-butyl-[6-(2,6-diisopropylphenyl)]salicylaldiminato}methyl(trimethylphosphine)nickel(II) (2b) was obtained. Yield: 89%. Anal. Calcd for C41H54NiNO2P (M = 682.54): C, 72.15; H, 7.97; N, 2.05. Found: C, 72.47; H, 7.92; N, 1.99. 1H NMR (25 C, C6D6, 500 MHz): δ 7.98 (d, 1H, CHdN, 4 JP,H =9.5 Hz), 7.59-6.97 (m, 13H, Ph), 4.02 (sept., 2H, CH(iso), 3 JH,H = 7 Hz), 3.32 (s, 3H, C-OMe), 1.38 (d, 6H, CH3(iso), 3 JH,H = 7 Hz), 1.20 (s, 9H, CH3(tert)), 1.09 (d, 6H, CH3(iso), 3 JH,H = 7 Hz), 0.71 (d, 9H, PMe3, 4JP,H = 9.5 Hz), -1.13 (d, 3H, Ni-CH3, 3JP,H = 7.5 Hz) ppm. 13C NMR (25 C, C6D6, 125 MHz): δ 165.7, 149.9, 147.5, 141.4, 137.5, 134.2, 131.2, 126.1, 123.6, 119.9, 88.2, 53.0, 33.8, 31.4, 28.6, 25.0, 23.2, 12.9 (d, PMe3, 1 JC,P = 26 Hz), -13.5 (d, Ni-CH3, 2JC,P = 52 Hz). 31P{1H} NMR (25 C, C6D6, 162 MHz): δ -8.7 ppm.

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Figure 2. ORTEP plots of the molecular structure of 1b: (A) view perpendicular to the molecular plane; (B) view in the molecular plane. Thermal ellipsoids are drawn at the 50% probability level. H atoms are omitted for clarity, except for the H2 atom, which is involved in a hydrogen bond. Selected distances (A˚) and angles (deg): Ni1-O1 = 1.925(1), Ni1-C1 = 1.932(1), Ni1-N1 = 1.937(1), Ni1-P1 = 2.147(4), O2-H2 = 0.81(2), N1-C11 = 1.305(2), C5-O1 = 1.312(1), C28-O2 = 1.440(2); — O1-Ni1-C1 = 164.4(1), — O1-Ni1-N1 = 93.6(1), — C1-Ni1-N1 = 94.5(1), — O1-Ni1-P1 = 88.3(1), — C1-Ni1-P1 = 87.0(1), — N1-Ni1-P1 = 166.0(1), — C28-O2-H2 = 104.2(2), — C11-N1-Ni1 = 123.6(1), — O2-H2 3 3 3 O1 = 145.1(1). Scheme 3. Synthesis of Catalysts 1b and 2b

X-ray Data Collection, Structure Solution, and Refinement. Intensity data for compounds 1 and 1b were collected at 170 and 100 K, respectively, on a Bruker AXS Smart 100021 singlecrystal diffractometer equipped with an area detector using graphite-monochromated Mo KR radiation (λ = 0.710 73 A˚). Crystallographic and experimental details of the structure are summarized in Table 1. An empirical correction for absorption was made. The structures were solved by direct methods and refined by full-matrix least-squares procedures (based on Fo2),22 first with isotropic thermal parameters and then with anisotropic thermal parameters in the last cycles of refinement for all the non-hydrogen atoms. The hydrogen atoms, introduced into the geometrically calculated positions, were refined riding on the corresponding parent atoms, except for the hydrogen H2 in 1b that was found in the ΔF2 maps and refined isotropically. General Procedure for Ethylene Polymerization by Ni Catalysts. A 350 mL glass pressure vessel (dried in a 120 C oven overnight prior to use) was loaded with a solution of 10 μmol of catalyst and 20 μmol of [Ni(cod)2] in 25 mL of dry toluene inside the glovebox. A large magnetic stir bar was added to the vessel, and it was sealed, taken out of the glovebox, and attached to a high-pressure/high-vacuum line. The solution was then degassed and brought to the required temperature with an external bath, and the temperature of the bath was monitored by a (21) SAINT Software Users Guide, Version 6.0; Bruker Analytical X-ray Systems: Madison, WI, 1999. (22) Sheldrick, G. M. SADABS; Bruker Analytical X-ray Systems, Madison, WI, 1999. Sheldrick, G. M. SHELXL-97, Program for Crystal Structure Refinement; University of G€ottingen, G€ottingen, Germany, 1997.

Chart 2

thermocouple to ensure minimal mass transfer effects. The reactor was then quickly charged with 8.0 atm of ethylene and polymerization was allowed to proceed for 40 min with rapid stirring. After the desired run time, the reactor was vented, and the reaction mixture was quickly quenched with 10% HCl in methanol. The precipitated polymer was stirred for several hours, collected by filtration, and washed with methanol. It was then dried under high vacuum at 80 C overnight. General Procedure for Ethylene Polymerization by Ni Catalysts in Polar Solvents. A 350 mL glass pressure vessel (dried in a 120 C oven overnight prior to use) was loaded with a solution of 10 μmol of catalyst and 20 μmol of [Ni(cod)2] in 25 mL of dry toluene inside the glovebox. A large magnetic stir bar was added to the vessel, and it was sealed, taken out of the glovebox, and attached to a high-pressure/high-vacuum line. The solution was than degassed and brought to the required temperature with an external bath, and the temperature of the bath was monitored by a thermocouple to ensure minimal mass transfer effects. The reactor was pressurized with 1.0 atm of ethylene, and 1500 equiv

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Table 2. Ethylene Homopolymerization Data at 25 C and at 50 C with Catalysts 1b, 2b, and 3 a

entry

catalyst

T (C)

time (min)

polymer yield (g)

activityb

Mwc

Mw/Mn

branches/1000 Cd

Tm (C)e

1 2 3 4 5 6

1b 2b 3 1b 2b 3

25 25 25 50 50 50

40 40 40 40 40 40

7.82 3.12 3.50 5.21 6.32 6.72

148(1) 59(2) 65(3) 99(5) 120(3) 125(2)

6700 7400 62700 1400 3100 5600

1.8 2.0 2.8 2.4 2.2 1.9

60 44 27 104 77 77

75 77 105 31 33 31

a Polymerizations carried out with 10 μmol of catalyst and 2.0 equiv of cocatalyst/Ni at 25 C in 25 mL of toluene at 8.0 atm ethylene pressure. Each entry is the average of at least two runs. b In units of kg of polyethylene/((mol of Ni) h atm). c Determined by GPC (triple detection). d Determined by 1 H NMR.20 e Melting temperature determined by DSC.

of the desired rigorously degassed polar solvent additive was quickly syringed in. The pressure was then swiftly brought to 8.0 atm of ethylene for 40 min with rapid stirring. After the desired run time, the reactor was vented, and the reaction mixture was quickly quenched with 10% HCl in methanol. The precipitated polymer was stirred for several hours, collected by filtration, and washed with methanol. It was then dried under high vacuum at 80 C overnight. Decomposition Mechanism Experiment. A J. Young NMR tube was charged with catalyst 1b (0.007 g, 10.5 μmol), ligand 1 (0.0055 g, 10.5 μmol), and C6D6 (0.7 mL) at room temparature. Conversion to the bis-ligation product at 50 C was not observed via 1H NMR spectroscopy, only the formation of free ligand, Ni(0), and a trace amount of unidentified products.

Results The goal of this research was to investigate the effects of installing a hydrogen bond directed toward the active site of a Ni(II) phenoxyiminato olefin polymerization catalyst (1b) and to compare the ethylene polymerization properties with those of analogous catalysts without intramolecular hydrogen bonding (catalyst 2b). Ethylene homopolymerizations were also carried out in the presence of polar additives, and it is found that introduction of the proximate -OH group substantially increases catalyst activity tolerance. Catalysts Synthesis and Characterization. The ligand 2-(hydroxydiphenylmethyl)-4-tert-butyl-[6-(2,6-diisopropylphenyl)]salicylaldimine (1) is efficiently synthesized by condensation of 5-tert-butyl-2-hydroxy-3-(hydroxydiphenylmethyl)benzaldehyde18 with 2,6-diisopropylaniline in methanol (Scheme 2). The analogous ligand 2-(methoxydiphenylmethyl)4-tert-butyl-[6-(2,6-diisopropylphenyl)]salicylaldimine (2), designed as a non-hydrogen-bonded control, is prepared in a similar manner via selective deprotection of the MOM-protected phenol by stirring with dilute HCl in MeOH for 50 min at room temperature (see Scheme 2). Yellow crystals of 1 suitable for X-ray diffraction were obtained by layering hexane onto a CH2Cl2 solution. The ORTEP plot of the structure of 1 is depicted in Figure 1, and experimental details are summarized in Table 1. In the solid state, two intramolecular hydrogen bonds are present in ligand 1 (O1-H1 3 3 3 N1 and O2-H2 3 3 3 O1), which form two six-membered pseudorings.23 The distance between phenol atom O1 and phenyl methanol O2 is unexceptional (2.781(2) A˚), while that between the phenol O1 atom and imine N1 atom is 2.617(2) A˚.24 These two hydrogen bonds persist in solution, as confirmed by the 1H NMR displacement (23) Filarowski, A.; Koll, A.; Sobczyk, L. Curr. Org. Chem. 2009, 13, 172–193. (24) In the Cambridge Crystallographic Data Base, the mean distance (A˚) for an intramolecular hydrogen bond O(H) 3 3 3 O is 2.44 A˚ and for O(H) 3 3 3 N is 2.68 A˚.

Table 3. Ethylene Homopolymerization Data at 50 C with Catalyst 1b entry

catalysta

T (C)

time (min)

polymer yield (g)

activityb

4 5 6 7 8

1b 1b 1b 1b 1b

50 50 50 50 50

10 20 30 40 60

1.87 3.22 4.33 5.21 6.84

140(3) 120(4) 108(4) 99(5) 85(4)

a Polymerizations carried out with 10 μmol of catalyst and 2.0 equiv of cocatalyst/Ni at 50 C in 25 mL of toluene at 8.0 atm ethylene pressure. Each entry is the average of at least two runs. b In units of kg of polyethylene/((mol of Ni) h atm).

to low field of the hydroxyl protons (δ 14.22 (O1H1) and 5.92 (O2H2) ppm in CDCl3). The 1H NMR spectrum of 2 shows a similar chemical shift for the phenol proton (δ 13.44 ppm in CDCl3), again suggesting significant intramolecular hydrogen bonding. Monodeprotonation of 1 and 2 with 1.1 equiv of LiCH2TMS in THF affords the 2-(hydroxydiphenylmethyl)-4-tert-butyl[6-(2,6-diisopropylphenyl)]salicylaldiminate lithium salt 1a and 2-(methoxydiphenylmethyl)-4-tert-butyl-[6-(2,6-diisopropylphenyl)]salicylaldiminate lithium salt 2a, respectively (Scheme 3). Deprotonation is selective at the 1 phenol due to the large pKa difference of the two hydroxyl groups.25 The 1H NMR spectrum of 1a in C6D6 still exhibits the hydroxyl proton H2 resonance at δ 5.239 ppm, and two new signals at δ 3.412 and 1.349 ppm are assigned to a single THF molecule coordinated to the Liþ ion. Instead, in compound 2a, no THF molecules are coordinated to the lithium atom, as confirmed by 1H NMR spectroscopy. In both cases, salts 1a and 2a were isolated prior to reaction with the Ni(II) reagent. Reaction of lithium salt 1a and 2a with 1.0 equiv of trans-[NiMeCl(PMe3)2] in benzene yields the Ni(II) phenoxyiminato complexes 1b and 2b, respectively (Scheme 3). The solid-state structure of complex 1b was determined by X-ray diffraction, and experimental details are summarized in Table 1. ORTEP views of the crystal structure (Figure 2) reveal that the phenoxyiminato ligand chelates the Ni center in a N,O-κ2 fashion. The structure of 1b can be compared (vide infra) to these of related hydrogen-bonded complexes (Chart 2) {[Ph2PCHCO)(2-OH-C6H4)]Ni(PPh3)Ph} (4),26 (25) The pKa of phenol in water at 25 C is 9.95: Torres-Lapasio, J. R.; Garcia-Alvarez-Coque, M. C.; Bosch, E.; Roses, M. J. Chromatogr. A 2005, 1089, 170–186. In contrast, the pKa of triphenylmethanol is 12.73: Nishiura, Y.; Omatsu, T.; Nakayama, H.; Matsufuji, A.; Takahashi, O.; Nimura, S. PCT Int. Appl. WO2006016667 A1, 2006. (26) Braunstein, P.; Chauvin, Y.; Mercier, S.; Saussine, L.; De Cian, A.; Fischer, J. J. Chem. Soc., Chem. Commun. 1994, 2203–2204. This is a SHOP-type ethylene oligomerization catalyst; the presence of an intramolecular hydrogen bond influences the product oligomer molecular mass distribution: C4-C8 = 95-97%.

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Scheme 4

Table 4. Ethylene Homopolymerization Data at 25 C in the Presence of 1500 Equiv of Polar Additive entry

catalysta

polar additive

polymer yield (g)

activityb

Mwc

Mw/Mn

branches/1000 Cd

Tm (C)e

1 2 3 4 5 6 7 8 9

1b 2b 3 1b 2b 3 1b 2b 3

ethyl ether ethyl ether ethyl ether acetone acetone acetone water water water

5.93 2.20 2.83 2.77 1.20 2.11 1.93 0.62 0.45

112(2) 42(2) 60(4) 52(1) 23(2) 39(2) 37(3) 12(2) 10(1)

5300 7900 53800 4900 8000 58000 2600 8100 66200

2.1 2.1 2.8 1.7 2.1 2.6 2.1 2.3 1.6

59 48 33 54 52 26 50 41 30

66 73 104 61 69 105 79 75 108

a Polymerizations carried out with 10 μmol of catalyst and 2.0 equiv of cocatalyst/Ni at 25 C and 1500 equiv of polar additive for 40 min in 25 mL of toluene at 8.0 atm ethylene pressure. b In units of kg of polyethylene/((mol of Ni) h atm). Each entry is the average of at least two runs. c Determined by GPC (triple detection). d Determined by 1H NMR.20 e Melting temperature determined by DSC.

{(2,6-iPr2-C6H3)NdCH[2-(20 -(OH)-C6H4)C6H3O)]Ni(PPh3)Ph} (5),27 and [NiMe(OPh)(HOPh)(PMe3)2] (6).28 Note, however, that the Ni-O bond in 1b (1.925(1) A˚) is significantly longer than those found in the Cambridge Structural Database for analogous compounds (mean value 1.900(1) A˚), suggesting that the proximate hydrogen bond weakens the O1 donor power. The above structural results are also in accord with the DFT calculations of Ziegler for N,O-chelated Ni catalysts, which argue that reduced electron density at the O atom depresses the ethylene insertion barrier,13 and the experimental results of Matt et al. with Ni(II) phosphanylenolates having electron-withdrawing substituents.11 The intramolecular O2H2 3 3 3 O1 hydrogen bond is preserved in 1b, with — O2H2 3 3 3 O1 = 145.0(1). The H2 atom is located ∼0.81(1) A˚ from O2 of the hydroxyl group and 1.98(1) A˚ from O1 of the salicylideniminato ligand. The distance between the O1 (hydroxyl group) and O2 (salicylideniminato ligand) is 2.68(1) A˚. The intramolecular hydrogen-bonding parameters are significantly different from those found in the crystal structure of complex 5 (O-H 3 3 3 O bond distance 2.62 A˚; — O-H 3 3 3 O = 158)27 and complex 6 (O-H 3 3 3 O bond distance 2.61 A˚; — O-H 3 3 3 O = 165).28 With regard to complex 5, the distance between the two oxygen atoms is shorter than in 1b, while — O-H 3 3 3 O is greater. These variations in bond distance and angle, as well as the higher acidity of the free hydroxyl group in 5 versus 1b,25 correlate with notable differences in ethylene polymerization properties (vide infra). The trimethylphosphine and the Ni-methyl group are displaced approximately 0.515(2) and 0.443(3) A˚, (27) Hu, T.; Li, Y. G.; Liu, J. Y.; Li, Y. S. Organometallics 2007, 26, 2609–2615. The ethylene polymerization activity of this salicylaldiminebased Ni(II) catalyst is reported to be 3.33 kg/((mol of Ni) h atm) at 25 C. (28) Kim, Y. J.; Osakada, K.; Takenaka, A.; Yamamoto, A. J. Am. Chem. Soc. 1990, 112, 1096–1104.

respectively, from the Ni1-O1-N1 mean plane. This distortion is due to the intramolecular hydrogen bond and is accompanied by a slight paramagnetism in solution, as evidenced by 1H NMR (μeff ≈ 1.0 μB in CD2Cl2 at room temperature).19 However, complex 2b is diamagnetic in solution, which suggests that the distortion caused by the intramolecular hydrogen bonding in 1b is responsible for the paramagnetism. The 1H-1H 2D NOESY NMR spectrum of 1b (C6D6 at 25 C) displays cross-peaks between the Ni-CH3 group and one iPr group (CH3 and CH) (see Figure S1 in the Supporting Information). Close proximity between the OH group and trimethylphosphine is also evident and is consistent with the solid-state distorted-square-planar coordination geometry. Furthermore, the 1H NMR spectrum exhibits a large downfield OH signal displacement from δ 5.239 ppm in 1a to 8.292 ppm in 1b. Complex 2b shows the same spatial arrangement of the PMe3 and Me groups as in compound 1b, as is evident in the 1H-1H 2D NOESY NMR spectrum. Ethylene Homopolymerization Experiments. Ethylene homopolymerizations were carried out at room temperature with catalysts 1b, 2b, and 3 in the presence of the phosphine scavenger/cocatalyst Ni(cod)2, using conditions minimizing mass transport and exotherm effects.7e,g-j Most notable is that catalyst 1b exhibits, at room temperature, 2.5 greater polymerization activity than do 2b and 3 (Table 2). The microstructures of the polyethylenes produced by all catalysts were characterized by 1H and 13C NMR spectroscopy.20 The 13C NMR spectra show prominent non polyethylene backbone resonances assignable to methyl and hexyl branches (see Supporting Information), except for polyethylenes produced by catalyst 1b, where small amounts of ethyl branches are also evident. 1H NMR spectra of the polyethylenes produced by catalysts 1b and 2b also exhibit vinyl end groups -CH2CHdCH2, internal olefinic subunits -CHdCH-, and vinylidene groups -CH2C(Me)dCH2. The polyethylene

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Figure 3. Plot of activity (kg of PE/((mol Ni) h atm)) vs time (min) for ethylene polymerization with 1b at 50 C.

Figure 4. Ethylene polymerization activity (kg of PE/((mol Ni) h atm)) in toluene solution with the indicated cosolvents, mediated by 1b (blue b), 2b (red [), and 3 (green 2) Ni phenoxyiminato catalysts.

product branch density by 1H NMR20 obtained with 1b is 2 greater than that achieved by catalysts 2b and 3 under identical reaction conditions, which is also supported by melting temperature depression data (DSC). Ethylene Homopolymerization as a Function of Temperature. Homopolymerizations at higher temperatures (g50 C) yield polyethylenes with lower molecular weights and higher branching densities (Table 2). Only in the case of complex 1b does precipitation of black Ni(0) occur, and ethylene consumption falls after ca. 10 min (Table 3).29 Accordingly, catalyst 1b deactivation experiments were conducted at 50 C. Treatment of complex 1b with an equimolar amount of ligand 1 in an NMR tube at 50 C does not form the inactive species A, only Ni(0) and the free ligand (Scheme 4). Ethylene Homopolymerization in Presence of Polar Additives. Catalysts 1b, 2b, and 3 are also polymerization active in the presence of polar additives. Ethylene homopolymerizations (29) Berkefeld, A.; Mecking, S. J. Am. Chem. Soc. 2009, 131, 1565– 1574 and references therein.

were carried out in toluene solutions containing 1500 equiv of polar additives (Table 4). Polymerization activities decrease in the order toluene > ethyl ether > acetone > water (Figure 4). Note that catalyst 1b remains ∼4 more active in the presence of water than do catalysts 2b and 3 and retains significantly greater branch density.

Discussion Ethylene Homopolymerization. Comparing the ethylene homopolymerization characteristics of catalyst 1b with those of 2b and 3 reveals significant differences that are attributed to the intramolecular hydrogen bond directed toward the catalyst active site. At room temperature, the 1b catalyst activity and product polyethylene branch density are 2.5 and 2.0 greater, respectively, than that achieved by analogous catalysts 2b and 3 under identical conditions. The significant differences in activity and microstructure reasonably reflect decreased electron density at the Ni center, which should decrease the ethylene insertion barrier.13 Catalyst 1b

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Scheme 5. Possible Influence of Hydrogen Bonding on the Interactions of Polar Additives (S) with a Ni Catalytic Center

produces polyethylenes with molecular weights comparable to those afforded by catalyst 2b, but dissimilar by 10 from those obtained with catalyst 3 (1b, Mw = 6700; 3, Mw = 62 700); all polydispersities are consistent with single-site processes. This 1b vs 3 Mw difference is presumably a consequence of catalyst 1b kinetically favoring reversable β-hydrogen elimination versus chain propagation and/or the different steric hindrance around the metal centers. At high temperatures (g50 C) all of the present catalysts yield polyethylenes with lower molecular weights and higher branching densities (Table 2). A shorter catalyst lifetime (ca. 10 min) is observed with 1b (Figure 3), which corresponds to the formation of Ni(0) and free ligand detected by 1H NMR. On the other hand, negligible Ni(0) and bis-ligating deactivation processes are observed for 2b and 3 at 50 C, indicating that the intramolecular hydrogen bonding reduces the thermal stability of catalyst 1b. The decomposition mechanism for 1b presumably involves the formation of a Ni-H intermediate, which can either undergo ethylene insertion and reinitiate the catalytic cycle or reductively eliminate Ni(0) and free ligand (Scheme 1). In addition, free ligand in the reaction medium could react with the Ni active species to afford the inactive bis-ligated Ni(II) complex. In the case of compound 1b, the formation of the Ni-H intermediate is more favorable than for 2b and 3, as reflected in the polyethylene microsctructures obtained. At higher temperatures, reductive elimination dominates, but the formation of the bis-ligated species is not observed. Ethylene homopolymerization experiments in presence of polar additives reveal that catalyst 1b is more polar additive tolerant than catalysts 2b and 3. Polymerization activities decline in the order toluene > diethyl ether > acetone > water (Figure 4). While the polar additives significantly

reduce activity, note that catalyst 1b remains 2 more active than catalysts 2b and 3. The polar additives (S) in the reaction medium may act as ligands, coordinating to the metal center, thus competing with ethylene and slowing the chain propagation. We suggest that the intramolecular hydrogen bond directed toward the catalyst active site acts as a local scavenger for the polar additives, shifting the equilibrium toward chain walking scenarios (Scheme 5).

Conclusions These results show that hydrogen-bonded complex 1b displays high catalytic activity for ethylene polymerizations with respect to the analogous non-hydrogen-bonded complexes 2b and 3. The intramolecular hydrogen bond directed toward the catalyst active site promotes fine tuning of product branch density and appears to accelerate β-hydrogen elimination/re-insertion processes. Moreover, the polar functionality proximate to the Ni center significantly stabilizes catalyst activity during ethylene homopolymerization in the presence of polar additives. Further studies are underway to clarify the scope and mechanisms of such directed electrophile effects on olefin activation and catalytic enchainment.

Acknowledgment. Financial support by the DOE (Grant 86ER13511) is gratefully acknowledged. We thank Mr. M. P. Weberski, Jr., and Mr. C. J. Stephenson for helpful discussions. Supporting Information Available: Figures giving the 1H-1H 2D NOESY NMR spectrum of 1b and NMR (1H and 13C) spectra of polymer samples and CIF files giving crystallographic data for 1 and 1b. This material is available free of charge via the Internet at http://pubs.acs.org.