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Ethylene oligomerization promoted by group 8 metal complexes containing 2-(2-pyridyl)quinoxaline ligands
Catalysis Communications 3 (2002) 405–410 www.elsevier.com/locate/catcom
Ethylene oligomerization promoted by group 8 metal complexes containing 2-(2-pyridyl)quinoxaline ligands Changxing Shao, Wen-Hua Sun *, Zilong Li, Youliang Hu, Lingqin Han
1
State Key Laboratory of Engineering Plastics, The Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China Received 4 February 2002; received in revised form 26 June 2002; accepted 26 June 2002
Abstract NiII , CoII and FeII complexes of 2-(2-pyridyl)quinoxaline were synthesized and used in the ethylene oligomerization with methyl-aluminoxane (MAO) as cocatalyst. The nickel complex system mainly produced a-olefins with good activity, while the cobalt and iron complexes showed only marginal activity. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: 2-(2-Pyridyl)quinoxaline; Late transition metal; Ethylene oligomerization
1. Introduction In recent years, there has been an increasing interest in the development of late transition metal complexes as catalysts for the polymerization and/ or oligomerization of ethylene [1–5]. The merit of such catalysts has lied in carefully tuning of the ligands to vary their corresponding catalytic products’ composition from lower oligomers to polymer. The most notable are the catalysts of the shell higher olefin process (SHOP) [6,7] and the imine-coordinated [ArN@C(R)C(R)@NAr]MCHþ 3 (M ¼ Ni(II), Pd(II), Fe(II)) [8–16]. As the new fascinating homogeneous catalysts, late transition *
Corresponding author. Fax: +86-10-62559373. E-mail address: [email protected] (W.-H. Sun). 1 Visiting student from College of Chemistry and Environmental Sciences, Hebei University, Baoding 071002, China.
metal complexes have great potential in oligomerization, polymerization and copolymerization [17]. It was believed that b-hydrogen elimination reaction on the late transition metal made the strong propensity to undergo ethylene oligomerization. By now, transition-metal-promoted ethylene oligomerization has become a very important process, which converts the basic feedstock of ethylene into useful fine chemicals, such as a-butene, a-hexene and other linear olefins. For instance, the SHOP process [6,18–21], which is highly selective for producing linear a-olefins within the C4 –C30 range by ethylene oligomerization, produced approximately one million ton capacity in 1990. In the SHOP process, the ligands often consist of organic phosphine [22], while the improved catalysts contain diimine ligands, such as [ArN@C(R)C(R)@NAr]NiBr2 [16] and f½ð2-ArN@CðMeÞÞ2 C5 H3 NFeCl2 g [8–14], which
1566-7367/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 1 5 6 6 - 7 3 6 7 ( 0 2 ) 0 0 1 5 8 - 9
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C. Shao et al. / Catalysis Communications 3 (2002) 405–410
FT-IR Perkin–Elmer system. Element analysis was recorded on Carlo Erba 1106 Microanalyzer. 2.2. Materials
Scheme 1.
were also reported with high activity and selectivity. Recently, we have devoted to designing new catalysts without phosphine for ethylene oligomerization. In the previous research, 8-aminoquinoline nickel complexes performed the high activity for ethylene oligomerization [23]. In present work, 2-(2-pyridyl)quinoxaline reacted with NiCl2 6H2 O, (1,2-dimethoxyethane)nickel bromide, CoCl2 and FeCl2 4H2 O, respectively, to form the corresponding complexes 2a, 2b, 2c and 2d in excellent yields (Scheme 1). The results show that the nickel complexes 2a and 2b performed good catalytic activity for ethylene oligomerization under ambient pressure of ethylene. The predominant products were a-olefins, and the catalytic activities were primarily relied on the cocatalyst (MAO) ratio and reaction temperature. Herein, we report the brief results of ethylene oligomerization using the late transition metal complexes containing 2-(2-pyridyl)quinoxaline.
All solvents were treated under nitrogen atmosphere. Toluene and tetrahydrofuran (THF) were refluxed with sodium and distilled from sodium benzophenone under nitrogen. CH2 Cl2 was dried by refluxing with P2 O5 and distilled prior to use. 2Acetylpyridine and (DME)NiBr2 were purchased from Acros Organics, MAO from Arbemarle, and other materials were commercially obtained from Beijing Chemicals. 2.3. Synthesis procedure
2.1. General
2.3.1. Synthesis of 2-(2-pyridyl)quinoxaline o-Phenylenediamine (1.025 g, 9.5 mmol), 2-acetylpyridine (0.288 g, 10.4 mmol) and TsOH (2 mg) were dissolved in 10 ml of methanol in a round-bottom flask equipped with a magnetic stirrer, and the solution was refluxed for 24 h. After that, the resultant solution was cooled to room temperature for a while and some precipitate appeared which was filtered subsequently. After washed with the mixture of diethyl ether and petroleum ether and dried under vacuum, the product was obtained as a white needle crystals (0.59 g, yield: 50%), m.p. 118:5 120:0 °C. 1 H NMR ðCDCl3 Þ : d ¼ 10:0ðs; H3 Þ, 8.82(d, H15 ), 8.63(d, H12 ), 8.20(m, H6;9 , 2H), 7.94(t, H13 ), 7.82(m, H7;8 , 2H), 7.44(q, H14 ). 13 C NMRðCDCl3 Þ : d ¼ 154:4 ðC2 Þ, 150.0(C11 ), 149.3(C3 ), 144.0(C15 ), 142.4(C10 ), 141.7(C5 ), 137.0(C12 ), 130.1(C9 ), 130.0(C6 ), 129.6 (C8 ), 129.2(C7 ), 124.5(C13 ), 121.9(C14 ). IR (KBr) at 618.87 and 405:97 cm 1 is assigned to the in-plane and out-of-plane absorption bands of the 2-substitued pyridine ring.
All manipulations of moisture-sensitive compounds were performed under nitrogen atmosphere using standard Schlenk tubes. 1 H and 13 C NMR were recorded on Bruker MX (300 MHz) with TMS as the internal standard. The oligomers of ethylene were analyzed by GC–MS method on Shimatsu GC/MS-QP5050A spectrometer. The IR spectra were recorded on 2000
2.3.2. Synthesis of complex 2a 2-(2-Pyridyl)quinoxaline (0.059 g), NiCl2 6H2 O (0.064 g) and THF (5 ml) were combined in a Schlenk tube under nitrogen atmosphere. After stirring the mixture at room temperature for 5 h, a yellow solid was precipitated. The solid was isolated by filtration, washed with diethyl ether and dried in vacuo; yield: 0.086 g (96%). ðC13 H9 N3
2. Experimental
C. Shao et al. / Catalysis Communications 3 (2002) 405–410
NiCl2 Þ2 ð336:83Þ2 : Calcd. C 46.36; H 2.69; N 12.48; Found C 45.70; H 2.67; N 12.08. 2.3.3. Synthesis of complex 2b (DME)NiBr2 (0.184 g) and 2-(2-pyridyl)quinoxaline (0.128 g) were added to 5 ml of dry dichloromethane in a Schlenk tube under nitrogen atmosphere. The mixture was stirred at room temperature for 10 h. The same procedure as 2a gave 0.24 g (95%) of complex 2b. ðC13 H9 N3 NiBr2 Þ2 ð425:73Þ2 : Calcd. C 36.68; H 2.13; N, 9.87; Found C 36.55; H 2.20; N 9.79. 2.3.4. Synthesis of complex 2c Analogous to the preparation of 2b, complex 2c was prepared from 2-(2-pyridyl)quinoxaline (0.093 g) and CoCl2 (0.058 g) in CH2 Cl2 (5 ml). The product is a purple-black powder; yield 0.13 g (86%). ðC13 H9 N3 CoCl2 Þ2 ð337:07Þ2 : Calcd. C 46.32; H 2.69; N 12.40; Found C 46.06; H 2.51; N 12.33. 2.3.5. Synthesis of complex 2d Analogous to the preparation of 2b, complex 2d was prepared from 2-(2-pyridyl)quinoxaline (0.12 g) and FeCl2 4H2 O (0.10 g) in CH2 Cl2 (5 ml). The purple powder was obtained; yield 0.19 g (90%). ðC13 H9 N3 FeCl2 Þ2 ð333:98Þ2 : Calcd. C 46.75; H 2.72; N 12.50; Found C 45.97; H 2.72; N 12.33. 2.3.6. General procedure for ethylene oligomerization Employing a three-neck flask as the reactor, magnetic stirrer and injector were dried prior to use. Toluene (50 ml) was injected into the reactor under ethylene atmosphere, then the proper amount of MAO was injected into the toluene. The solution was saturated by ethylene after the mixture was stirred for about 10 min, and then the catalyst precursor was induced into the mixture.
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The amount of ethylene absorbed was recorded every 5 min, and the kinetic curves were plotted according to the data. After 30 min, adding hydrochloric acid ethanol solution into the catalytic reactor stopped the reaction. The products were detected by GC–MS.
3. Results and discussion Preparation of catalysts. Reactions of 2-(2-pyridyl)quinoxaline with late transition metal halides, including nickel, cobalt and iron, were reported to form their complexes [24–26]. In the present case, the nickel (II) dichloride complex 2a was prepared by the reaction of a slight excess of the free 2-(2pyridyl)quinoxaline (L) with NiCl2 6H2 O in THF (Scheme 2), and the crude nickel complex precipitated from the solution in several hours under mild condition. After the excess ligand was washed away with diethyl ether, the pure product was obtained as a yellow powder. Other complexes shown in Scheme 1 were similarly prepared by the reaction of 2-(2-pyridyl)quinoxaline with (DME)NiBr2 ; CoCl2 or FeCl2 4H2 O, respectively, in CH2 Cl2 . To investigate the structure of complexes, 2a was determined by single-crystal X-ray structure analysis (Fig. 1) [27]. Ethylene oligomerization. Treatment of catalyst precursors 2a, 2b, 2c and 2d with MAO, respectively in toluene produced active oligomerization catalysts, which were subsequently employed for ethylene oligomerization reaction. The results show that the catalytic activity of the titled complexes is fairly dependent on the species of the metal cations. In general, the nickel complexes 2a and 2b, its catalytic species can consist as the 14electron configuration, displayed good catalytic activities. However, the cobalt and iron analogues
Scheme 2. Synthesis of the complex 2a.
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Fig. 1. The crystal structure of the complex 2a.
(2c and 2d) gave only marginal activity, which may be attributed to the improper electronic environment provided by coordinated cobalt and iron centers [28]. The resultant data are agreed with suggested electronic configuration principle [2]. Table 1 collected the corresponding results of oligomerization reactions by using four complexes with different ratio of MAO. Among the examined species, the nickel complexes showed good activities for ethylene oligomerization. When cobalt and
iron are employed, however, no activity was observed for ethylene oligomerization. Additionally, a threshold amount of cocatalyst is necessary to scavenge impurities that may poison the active catalyst [21], so all oligomerization experiments were performed with at least of 500 equivalents of cocatalyst (MAO). In the 2a/MAO catalytic system, the best catalytic activity was obtained with a Al/Ni ratio 1000. When changing the halide anion of complexes from chloride (2a) to bromide (2b), the promising results can be observed. The catalytic activity of 2b gradually enhanced by increasing the amount of cocatalyst MAO, showing the highest activity up to 7:1 105 g C2 H4 =ðmol h atmÞ at the Al/Ni ratio 3000. The dynamitic data of ethylene oligomerization by 2b/MAO in toluene at 20 °C with atmospheric pressure were shown in Fig. 2. According to Fig. 2, the initial stage is necessary for every entry and all catalytic systems reached the highest activity within 10 min. In the cases of Al/Ni ratio 500 or 1000, only one period of high activity can be observed, however, two periods for high activities appeared when Al/Ni ratio enhances to 1500 or 2000. The influence of the temperature on activity and lifetime of the catalysts has also been studied. As shown in Table 2, the highest catalytic activity of 2b/MAO system is obtained at 30 °C, while a
Table 1 Ethylene oligomerization by 2-(2-pyridyl)quinoxaline–metal/MAO system Catalyst
Temp (°C)
Al/Ma
Activityb
C4 c
a-C4 d
C6 c
a-C6 d
2a/MAO 2a/MAO 2a/MAO 2a/MAO 2b/MAO 2b/MAO 2b/MAO 2b/MAO 2b/MAO 2c/MAO 2d/MAO
20 20 20 20 20 20 20 20 20 30 30
500 1000 2000 3000 500 1000 1500 2000 3000 500 500
1.5 4.4 2.7 2.0 1.7 2.0 2.7 3.5 7.1 n.ae n.ae
85.3 88.7 80.0 83.9 100 89.3 89.2 90.3 77.5 – –
100 100 100 100 100 100 100 100 100 – –
14.7 11.3 19.6 16.1 – 10.7 10.8 9.7 22.5 – –
71.4 17.9 – 22.2 – – 44.3 – 12 – –
a
Molar ratio of MAO/complex. 105 g C2 H4 =ðmol M h atmÞ. c Weight percentage of olefins. d a-Olefin percentage in its corresponding C4 or C6 . e No activity detected. b
C. Shao et al. / Catalysis Communications 3 (2002) 405–410
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tene, a-hexene and other hexene isomers. Changing the reaction parameters such as its temperature and Al/Ni molar ratio merely affects the product distribution in a limited range, which implies that there may be strong b-hydrogen elimination of the reaction intermediate. On the axial face of the catalyst, there is no hindrance, which favors the chain transfer and chain isomerization [29,30] as shown in the catalysis using (2,20 -pyridyl)NiBr2 [14]. There are no significant differences for the oligomer distributions between the catalysts 2a and 2b according to Table 1, which indicates analogous active specie working for both the catalysts. This can be elucidated with the proposed activation mechanism in which the catalytic active cationic species generated in the reaction with MAO and subsequent methylation of the metal center. Thus, the effect of different halogen ligands is quenched by incorporation into the bulky MAO counter ion. In the same process, the precursor compound, nickel complex, is transferred into the active monomer nickel species while the bridged bonds through halides are broken [28,31].
Fig. 2. Activity/treatment time relationship of MAO/2b system. (Al/2b ratio for B, C, D and E: 500, 1000, 1500 and 2000, respectively) at 20 °C in toluene with ambient ethylene.
decrease of activity will be observed at either higher or lower temperature. This phenomenon shows the thermal reliability of its activity. The higher temperature will cause the decrease of the ethylene concentration in toluene, and lower temperature may be unfavorable to the formation of the active species. Thus the optimal temperature for each system will depend on the balance among the factors mentioned above. Furthermore, the temperature can also influence the lifetime of catalysts. It was found that the lower temperature can extend the lifetime of catalyst. For example, at 5 °C, the catalyst can keep its good activity over an hour, while a higher temperature (at 60 °C) shortens the catalyst lifetime to only 10 min. The distribution of the oligomer products was observed as three fractions, major product a-bu-
4. Conclusions In summary, comparing with the cobalt and iron complexes which show marginal activity, 2-(2pyridyl)quinoxaline nickel(II) complexes show good activity and selectivity in ethylene oligomerization for a-olefins. The nickel bromide complex performed the highest activity at Al/Ni ratio 3000, while the nickel chloride complex at Al/Ni ratio
Table 2 Oligomerization of ethylene using complex 2b Temp (°C)
MAO/Nia
Activityb
C4
a-C4 d
C6 c
a-C6 d
5 20 30 40 60
1000 1000 1000 1000 1000
1.1 2.0 3.1 0.69 0.1
85.2 89.3 82.1 100 100
100 100 100 100 100
14.8 10.7 17.9 – –
26.8 – 19.1 – –
a
Molar ratio. 105 g C2 H4 =ðmol M h atmÞ. c Weight percentage. d a-Olefin percentage in its corresponding C4 or C6 . b
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1000. In addition, the reaction temperature affected the catalytic activity obviously, and the optimal temperature for the polymerization is 30 °C.
Acknowledgements We are grateful to the Chinese Academy of Sciences for the financial supports from under ‘‘One Hundred Young Talents’’ and Core Research for Engineering Innovation KGCX203-2.
References [1] S.D. Ittle, L.K. Johnson, M. Brookhart, Chem. Rev. 100 (2000) 1169. [2] G.J.P. Britovsek, V.C. Gibson, D.F. Wass, Angew. Chem. 111 (1999) 448. [3] E.P. Talsi, D.E. Babushkin, N.V. Semikolenom, V.N. Zudin, V.N. Panchenko, V.A. Zakharov, Macromol. Chem. Phys. 202 (2001) 2046. [4] L. Shultz, D.J. Tempel, M. Brookhart, J. Am. Chem. Soc. 123 (2001) 11539. [5] T.R. Younkin, E.F. Connor, J.I. Henderson, S.K. Friedrich, R.H. Grubbs, D.A. Bansleben, Science 287 (2000) 460. [6] W. Keim, Angew. Chem. Int. Ed. Engl. 29 (1990) 235. [7] J. Heinicke, M. He, A. Dal, H.-F. Klein, O. Hetche, W. Keim, U. Fl€ orke, H.-J. Haupt, Eur. J. Inorg. Chem. (2000) 431. [8] L.K. Johnson, C.M. Killian, M. Brookhart, J. Am. Chem. Soc. 117 (1995) 6414. [9] L.K. Lohnson, S. Mecking, M. Brookhart, J. Am. Chem. Soc. 118 (1996) 267. [10] S. Mecking, L.K. Johnson, L. Wang, M. Brookhart, J. Am. Chem. Soc. 120 (1998) 888. [11] G.J.P. Britovsek, V. Gibson, B.S. Kimberley, P.J. Maddox, S.J. Mctavish, G.A. Solan, A.J.P. White, D.J. Williams, Chem. Commun. (1998) 849.
[12] C.M. Killian, D.J. Tempel, L.K. Johnson, M. Brookhart, J. Am. Chem. Soc. 118 (1996) 11664. [13] B.L. Small, M. Brookhart, J. Am. Chem. Soc. 120 (1998) 7143. [14] C.M. Killian, L.K. Johnson, M. Brookhart, Organometallics 16 (1997) 2005. [15] J. Feldman, S.J. McLain, A. Parthasarathy, W.J. Marshall, J.C. Calabrese, S.D. Arthur, Organometallics 16 (1997) 1514. [16] S.A. Svejda, M. Brookhart, Organometallics 18 (1999) 65. [17] C. Wang, S. Friedrich, T.R. Younkin, R.T. Li, R.H. Gruggs, D.A. Bansleben, M.W. Day, Organometallics 17 (1998) 3149. [18] W. Keim, F.H. Kowaldt, R. Godard, C. Kr€ uger, Angew. Chem. Int. Ed. Engl. 17 (1978) 466. [19] W. Keim, B. Hofman, R. Lodewick, M. Peuckert, G. Schmitt, J. Fleischhauer, U. Meier, J. Mol. Catal. 6 (1979) 79. [20] M. Peucket, W. Keim, Organometallics 2 (1983) 594. [21] S. Mecking, Angew. Chem. Int. Ed. 40 (2001) 534. [22] M. Peockert, W. Keim, S. Storp, R.S. Weber, J. Mol. Catal. 20 (1983) 115. [23] Z. Li, W.-H. Sun, Z. Ma, Y. Hu, C. Shao, Chin. Chem. Lett. 12 (2001) 691. [24] S. Kasselouri, A. Garoufis, A. Katehanakis, G. Kalkanis, S.P. Perlepes, N. Hadjiliadis, Inorg. Chim. Acta. 207 (1993) 255. [25] S.P. Perlepes, S. Kasselouri, A. Garoofis, F. Lutz, R. Bau, N. Hadjiliadis, Polyhedron 14 (1995) 1461. [26] J.C. Plakatouras, N. Hadjiliadis, S.P. Perlepes, A. Albinati, G. Kalkanis, Polyhedron 12 (1993) 2069. [27] C. Shao, W.-H. Sun, Y. Chen, R. Wang, C. Xi, Inorg. Chem. Commun. 5 (2002) 667. [28] T.V. Laine, K. Lappalainen, J. Liimatta, E. Aitola, B. L€ ofgren, M. Leskel€a, Macromol. Rapid Commun. 20 (1999) 487. [29] B.L. Small, M. Brookhart, A.M.A. Bennett, J. Am. Chem. Soc. 120 (1998) 4049. [30] T.-J.J. Kinnunen, M. Haukka, T.T. Pakkanen, T.A. Pakkanen, J. Organomet. Chem. 613 (2000) 257. [31] T.V. Laine, M. Klinga, M. Leskel€a, Eur. J. Inorg. Chem. (1999) 959.
Ethylene oligomerization promoted by group 8 metal complexes containing 2-(2-pyridyl)quinoxaline ligands Changxing Shao, Wen-Hua Sun *, Zilong Li, Youliang Hu, Lingqin Han
1
State Key Laboratory of Engineering Plastics, The Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China Received 4 February 2002; received in revised form 26 June 2002; accepted 26 June 2002
Abstract NiII , CoII and FeII complexes of 2-(2-pyridyl)quinoxaline were synthesized and used in the ethylene oligomerization with methyl-aluminoxane (MAO) as cocatalyst. The nickel complex system mainly produced a-olefins with good activity, while the cobalt and iron complexes showed only marginal activity. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: 2-(2-Pyridyl)quinoxaline; Late transition metal; Ethylene oligomerization
1. Introduction In recent years, there has been an increasing interest in the development of late transition metal complexes as catalysts for the polymerization and/ or oligomerization of ethylene [1–5]. The merit of such catalysts has lied in carefully tuning of the ligands to vary their corresponding catalytic products’ composition from lower oligomers to polymer. The most notable are the catalysts of the shell higher olefin process (SHOP) [6,7] and the imine-coordinated [ArN@C(R)C(R)@NAr]MCHþ 3 (M ¼ Ni(II), Pd(II), Fe(II)) [8–16]. As the new fascinating homogeneous catalysts, late transition *
Corresponding author. Fax: +86-10-62559373. E-mail address: [email protected] (W.-H. Sun). 1 Visiting student from College of Chemistry and Environmental Sciences, Hebei University, Baoding 071002, China.
metal complexes have great potential in oligomerization, polymerization and copolymerization [17]. It was believed that b-hydrogen elimination reaction on the late transition metal made the strong propensity to undergo ethylene oligomerization. By now, transition-metal-promoted ethylene oligomerization has become a very important process, which converts the basic feedstock of ethylene into useful fine chemicals, such as a-butene, a-hexene and other linear olefins. For instance, the SHOP process [6,18–21], which is highly selective for producing linear a-olefins within the C4 –C30 range by ethylene oligomerization, produced approximately one million ton capacity in 1990. In the SHOP process, the ligands often consist of organic phosphine [22], while the improved catalysts contain diimine ligands, such as [ArN@C(R)C(R)@NAr]NiBr2 [16] and f½ð2-ArN@CðMeÞÞ2 C5 H3 NFeCl2 g [8–14], which
1566-7367/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 1 5 6 6 - 7 3 6 7 ( 0 2 ) 0 0 1 5 8 - 9
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FT-IR Perkin–Elmer system. Element analysis was recorded on Carlo Erba 1106 Microanalyzer. 2.2. Materials
Scheme 1.
were also reported with high activity and selectivity. Recently, we have devoted to designing new catalysts without phosphine for ethylene oligomerization. In the previous research, 8-aminoquinoline nickel complexes performed the high activity for ethylene oligomerization [23]. In present work, 2-(2-pyridyl)quinoxaline reacted with NiCl2 6H2 O, (1,2-dimethoxyethane)nickel bromide, CoCl2 and FeCl2 4H2 O, respectively, to form the corresponding complexes 2a, 2b, 2c and 2d in excellent yields (Scheme 1). The results show that the nickel complexes 2a and 2b performed good catalytic activity for ethylene oligomerization under ambient pressure of ethylene. The predominant products were a-olefins, and the catalytic activities were primarily relied on the cocatalyst (MAO) ratio and reaction temperature. Herein, we report the brief results of ethylene oligomerization using the late transition metal complexes containing 2-(2-pyridyl)quinoxaline.
All solvents were treated under nitrogen atmosphere. Toluene and tetrahydrofuran (THF) were refluxed with sodium and distilled from sodium benzophenone under nitrogen. CH2 Cl2 was dried by refluxing with P2 O5 and distilled prior to use. 2Acetylpyridine and (DME)NiBr2 were purchased from Acros Organics, MAO from Arbemarle, and other materials were commercially obtained from Beijing Chemicals. 2.3. Synthesis procedure
2.1. General
2.3.1. Synthesis of 2-(2-pyridyl)quinoxaline o-Phenylenediamine (1.025 g, 9.5 mmol), 2-acetylpyridine (0.288 g, 10.4 mmol) and TsOH (2 mg) were dissolved in 10 ml of methanol in a round-bottom flask equipped with a magnetic stirrer, and the solution was refluxed for 24 h. After that, the resultant solution was cooled to room temperature for a while and some precipitate appeared which was filtered subsequently. After washed with the mixture of diethyl ether and petroleum ether and dried under vacuum, the product was obtained as a white needle crystals (0.59 g, yield: 50%), m.p. 118:5 120:0 °C. 1 H NMR ðCDCl3 Þ : d ¼ 10:0ðs; H3 Þ, 8.82(d, H15 ), 8.63(d, H12 ), 8.20(m, H6;9 , 2H), 7.94(t, H13 ), 7.82(m, H7;8 , 2H), 7.44(q, H14 ). 13 C NMRðCDCl3 Þ : d ¼ 154:4 ðC2 Þ, 150.0(C11 ), 149.3(C3 ), 144.0(C15 ), 142.4(C10 ), 141.7(C5 ), 137.0(C12 ), 130.1(C9 ), 130.0(C6 ), 129.6 (C8 ), 129.2(C7 ), 124.5(C13 ), 121.9(C14 ). IR (KBr) at 618.87 and 405:97 cm 1 is assigned to the in-plane and out-of-plane absorption bands of the 2-substitued pyridine ring.
All manipulations of moisture-sensitive compounds were performed under nitrogen atmosphere using standard Schlenk tubes. 1 H and 13 C NMR were recorded on Bruker MX (300 MHz) with TMS as the internal standard. The oligomers of ethylene were analyzed by GC–MS method on Shimatsu GC/MS-QP5050A spectrometer. The IR spectra were recorded on 2000
2.3.2. Synthesis of complex 2a 2-(2-Pyridyl)quinoxaline (0.059 g), NiCl2 6H2 O (0.064 g) and THF (5 ml) were combined in a Schlenk tube under nitrogen atmosphere. After stirring the mixture at room temperature for 5 h, a yellow solid was precipitated. The solid was isolated by filtration, washed with diethyl ether and dried in vacuo; yield: 0.086 g (96%). ðC13 H9 N3
2. Experimental
C. Shao et al. / Catalysis Communications 3 (2002) 405–410
NiCl2 Þ2 ð336:83Þ2 : Calcd. C 46.36; H 2.69; N 12.48; Found C 45.70; H 2.67; N 12.08. 2.3.3. Synthesis of complex 2b (DME)NiBr2 (0.184 g) and 2-(2-pyridyl)quinoxaline (0.128 g) were added to 5 ml of dry dichloromethane in a Schlenk tube under nitrogen atmosphere. The mixture was stirred at room temperature for 10 h. The same procedure as 2a gave 0.24 g (95%) of complex 2b. ðC13 H9 N3 NiBr2 Þ2 ð425:73Þ2 : Calcd. C 36.68; H 2.13; N, 9.87; Found C 36.55; H 2.20; N 9.79. 2.3.4. Synthesis of complex 2c Analogous to the preparation of 2b, complex 2c was prepared from 2-(2-pyridyl)quinoxaline (0.093 g) and CoCl2 (0.058 g) in CH2 Cl2 (5 ml). The product is a purple-black powder; yield 0.13 g (86%). ðC13 H9 N3 CoCl2 Þ2 ð337:07Þ2 : Calcd. C 46.32; H 2.69; N 12.40; Found C 46.06; H 2.51; N 12.33. 2.3.5. Synthesis of complex 2d Analogous to the preparation of 2b, complex 2d was prepared from 2-(2-pyridyl)quinoxaline (0.12 g) and FeCl2 4H2 O (0.10 g) in CH2 Cl2 (5 ml). The purple powder was obtained; yield 0.19 g (90%). ðC13 H9 N3 FeCl2 Þ2 ð333:98Þ2 : Calcd. C 46.75; H 2.72; N 12.50; Found C 45.97; H 2.72; N 12.33. 2.3.6. General procedure for ethylene oligomerization Employing a three-neck flask as the reactor, magnetic stirrer and injector were dried prior to use. Toluene (50 ml) was injected into the reactor under ethylene atmosphere, then the proper amount of MAO was injected into the toluene. The solution was saturated by ethylene after the mixture was stirred for about 10 min, and then the catalyst precursor was induced into the mixture.
407
The amount of ethylene absorbed was recorded every 5 min, and the kinetic curves were plotted according to the data. After 30 min, adding hydrochloric acid ethanol solution into the catalytic reactor stopped the reaction. The products were detected by GC–MS.
3. Results and discussion Preparation of catalysts. Reactions of 2-(2-pyridyl)quinoxaline with late transition metal halides, including nickel, cobalt and iron, were reported to form their complexes [24–26]. In the present case, the nickel (II) dichloride complex 2a was prepared by the reaction of a slight excess of the free 2-(2pyridyl)quinoxaline (L) with NiCl2 6H2 O in THF (Scheme 2), and the crude nickel complex precipitated from the solution in several hours under mild condition. After the excess ligand was washed away with diethyl ether, the pure product was obtained as a yellow powder. Other complexes shown in Scheme 1 were similarly prepared by the reaction of 2-(2-pyridyl)quinoxaline with (DME)NiBr2 ; CoCl2 or FeCl2 4H2 O, respectively, in CH2 Cl2 . To investigate the structure of complexes, 2a was determined by single-crystal X-ray structure analysis (Fig. 1) [27]. Ethylene oligomerization. Treatment of catalyst precursors 2a, 2b, 2c and 2d with MAO, respectively in toluene produced active oligomerization catalysts, which were subsequently employed for ethylene oligomerization reaction. The results show that the catalytic activity of the titled complexes is fairly dependent on the species of the metal cations. In general, the nickel complexes 2a and 2b, its catalytic species can consist as the 14electron configuration, displayed good catalytic activities. However, the cobalt and iron analogues
Scheme 2. Synthesis of the complex 2a.
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Fig. 1. The crystal structure of the complex 2a.
(2c and 2d) gave only marginal activity, which may be attributed to the improper electronic environment provided by coordinated cobalt and iron centers [28]. The resultant data are agreed with suggested electronic configuration principle [2]. Table 1 collected the corresponding results of oligomerization reactions by using four complexes with different ratio of MAO. Among the examined species, the nickel complexes showed good activities for ethylene oligomerization. When cobalt and
iron are employed, however, no activity was observed for ethylene oligomerization. Additionally, a threshold amount of cocatalyst is necessary to scavenge impurities that may poison the active catalyst [21], so all oligomerization experiments were performed with at least of 500 equivalents of cocatalyst (MAO). In the 2a/MAO catalytic system, the best catalytic activity was obtained with a Al/Ni ratio 1000. When changing the halide anion of complexes from chloride (2a) to bromide (2b), the promising results can be observed. The catalytic activity of 2b gradually enhanced by increasing the amount of cocatalyst MAO, showing the highest activity up to 7:1 105 g C2 H4 =ðmol h atmÞ at the Al/Ni ratio 3000. The dynamitic data of ethylene oligomerization by 2b/MAO in toluene at 20 °C with atmospheric pressure were shown in Fig. 2. According to Fig. 2, the initial stage is necessary for every entry and all catalytic systems reached the highest activity within 10 min. In the cases of Al/Ni ratio 500 or 1000, only one period of high activity can be observed, however, two periods for high activities appeared when Al/Ni ratio enhances to 1500 or 2000. The influence of the temperature on activity and lifetime of the catalysts has also been studied. As shown in Table 2, the highest catalytic activity of 2b/MAO system is obtained at 30 °C, while a
Table 1 Ethylene oligomerization by 2-(2-pyridyl)quinoxaline–metal/MAO system Catalyst
Temp (°C)
Al/Ma
Activityb
C4 c
a-C4 d
C6 c
a-C6 d
2a/MAO 2a/MAO 2a/MAO 2a/MAO 2b/MAO 2b/MAO 2b/MAO 2b/MAO 2b/MAO 2c/MAO 2d/MAO
20 20 20 20 20 20 20 20 20 30 30
500 1000 2000 3000 500 1000 1500 2000 3000 500 500
1.5 4.4 2.7 2.0 1.7 2.0 2.7 3.5 7.1 n.ae n.ae
85.3 88.7 80.0 83.9 100 89.3 89.2 90.3 77.5 – –
100 100 100 100 100 100 100 100 100 – –
14.7 11.3 19.6 16.1 – 10.7 10.8 9.7 22.5 – –
71.4 17.9 – 22.2 – – 44.3 – 12 – –
a
Molar ratio of MAO/complex. 105 g C2 H4 =ðmol M h atmÞ. c Weight percentage of olefins. d a-Olefin percentage in its corresponding C4 or C6 . e No activity detected. b
C. Shao et al. / Catalysis Communications 3 (2002) 405–410
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tene, a-hexene and other hexene isomers. Changing the reaction parameters such as its temperature and Al/Ni molar ratio merely affects the product distribution in a limited range, which implies that there may be strong b-hydrogen elimination of the reaction intermediate. On the axial face of the catalyst, there is no hindrance, which favors the chain transfer and chain isomerization [29,30] as shown in the catalysis using (2,20 -pyridyl)NiBr2 [14]. There are no significant differences for the oligomer distributions between the catalysts 2a and 2b according to Table 1, which indicates analogous active specie working for both the catalysts. This can be elucidated with the proposed activation mechanism in which the catalytic active cationic species generated in the reaction with MAO and subsequent methylation of the metal center. Thus, the effect of different halogen ligands is quenched by incorporation into the bulky MAO counter ion. In the same process, the precursor compound, nickel complex, is transferred into the active monomer nickel species while the bridged bonds through halides are broken [28,31].
Fig. 2. Activity/treatment time relationship of MAO/2b system. (Al/2b ratio for B, C, D and E: 500, 1000, 1500 and 2000, respectively) at 20 °C in toluene with ambient ethylene.
decrease of activity will be observed at either higher or lower temperature. This phenomenon shows the thermal reliability of its activity. The higher temperature will cause the decrease of the ethylene concentration in toluene, and lower temperature may be unfavorable to the formation of the active species. Thus the optimal temperature for each system will depend on the balance among the factors mentioned above. Furthermore, the temperature can also influence the lifetime of catalysts. It was found that the lower temperature can extend the lifetime of catalyst. For example, at 5 °C, the catalyst can keep its good activity over an hour, while a higher temperature (at 60 °C) shortens the catalyst lifetime to only 10 min. The distribution of the oligomer products was observed as three fractions, major product a-bu-
4. Conclusions In summary, comparing with the cobalt and iron complexes which show marginal activity, 2-(2pyridyl)quinoxaline nickel(II) complexes show good activity and selectivity in ethylene oligomerization for a-olefins. The nickel bromide complex performed the highest activity at Al/Ni ratio 3000, while the nickel chloride complex at Al/Ni ratio
Table 2 Oligomerization of ethylene using complex 2b Temp (°C)
MAO/Nia
Activityb
C4
a-C4 d
C6 c
a-C6 d
5 20 30 40 60
1000 1000 1000 1000 1000
1.1 2.0 3.1 0.69 0.1
85.2 89.3 82.1 100 100
100 100 100 100 100
14.8 10.7 17.9 – –
26.8 – 19.1 – –
a
Molar ratio. 105 g C2 H4 =ðmol M h atmÞ. c Weight percentage. d a-Olefin percentage in its corresponding C4 or C6 . b
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1000. In addition, the reaction temperature affected the catalytic activity obviously, and the optimal temperature for the polymerization is 30 °C.
Acknowledgements We are grateful to the Chinese Academy of Sciences for the financial supports from under ‘‘One Hundred Young Talents’’ and Core Research for Engineering Innovation KGCX203-2.
References [1] S.D. Ittle, L.K. Johnson, M. Brookhart, Chem. Rev. 100 (2000) 1169. [2] G.J.P. Britovsek, V.C. Gibson, D.F. Wass, Angew. Chem. 111 (1999) 448. [3] E.P. Talsi, D.E. Babushkin, N.V. Semikolenom, V.N. Zudin, V.N. Panchenko, V.A. Zakharov, Macromol. Chem. Phys. 202 (2001) 2046. [4] L. Shultz, D.J. Tempel, M. Brookhart, J. Am. Chem. Soc. 123 (2001) 11539. [5] T.R. Younkin, E.F. Connor, J.I. Henderson, S.K. Friedrich, R.H. Grubbs, D.A. Bansleben, Science 287 (2000) 460. [6] W. Keim, Angew. Chem. Int. Ed. Engl. 29 (1990) 235. [7] J. Heinicke, M. He, A. Dal, H.-F. Klein, O. Hetche, W. Keim, U. Fl€ orke, H.-J. Haupt, Eur. J. Inorg. Chem. (2000) 431. [8] L.K. Johnson, C.M. Killian, M. Brookhart, J. Am. Chem. Soc. 117 (1995) 6414. [9] L.K. Lohnson, S. Mecking, M. Brookhart, J. Am. Chem. Soc. 118 (1996) 267. [10] S. Mecking, L.K. Johnson, L. Wang, M. Brookhart, J. Am. Chem. Soc. 120 (1998) 888. [11] G.J.P. Britovsek, V. Gibson, B.S. Kimberley, P.J. Maddox, S.J. Mctavish, G.A. Solan, A.J.P. White, D.J. Williams, Chem. Commun. (1998) 849.
[12] C.M. Killian, D.J. Tempel, L.K. Johnson, M. Brookhart, J. Am. Chem. Soc. 118 (1996) 11664. [13] B.L. Small, M. Brookhart, J. Am. Chem. Soc. 120 (1998) 7143. [14] C.M. Killian, L.K. Johnson, M. Brookhart, Organometallics 16 (1997) 2005. [15] J. Feldman, S.J. McLain, A. Parthasarathy, W.J. Marshall, J.C. Calabrese, S.D. Arthur, Organometallics 16 (1997) 1514. [16] S.A. Svejda, M. Brookhart, Organometallics 18 (1999) 65. [17] C. Wang, S. Friedrich, T.R. Younkin, R.T. Li, R.H. Gruggs, D.A. Bansleben, M.W. Day, Organometallics 17 (1998) 3149. [18] W. Keim, F.H. Kowaldt, R. Godard, C. Kr€ uger, Angew. Chem. Int. Ed. Engl. 17 (1978) 466. [19] W. Keim, B. Hofman, R. Lodewick, M. Peuckert, G. Schmitt, J. Fleischhauer, U. Meier, J. Mol. Catal. 6 (1979) 79. [20] M. Peucket, W. Keim, Organometallics 2 (1983) 594. [21] S. Mecking, Angew. Chem. Int. Ed. 40 (2001) 534. [22] M. Peockert, W. Keim, S. Storp, R.S. Weber, J. Mol. Catal. 20 (1983) 115. [23] Z. Li, W.-H. Sun, Z. Ma, Y. Hu, C. Shao, Chin. Chem. Lett. 12 (2001) 691. [24] S. Kasselouri, A. Garoufis, A. Katehanakis, G. Kalkanis, S.P. Perlepes, N. Hadjiliadis, Inorg. Chim. Acta. 207 (1993) 255. [25] S.P. Perlepes, S. Kasselouri, A. Garoofis, F. Lutz, R. Bau, N. Hadjiliadis, Polyhedron 14 (1995) 1461. [26] J.C. Plakatouras, N. Hadjiliadis, S.P. Perlepes, A. Albinati, G. Kalkanis, Polyhedron 12 (1993) 2069. [27] C. Shao, W.-H. Sun, Y. Chen, R. Wang, C. Xi, Inorg. Chem. Commun. 5 (2002) 667. [28] T.V. Laine, K. Lappalainen, J. Liimatta, E. Aitola, B. L€ ofgren, M. Leskel€a, Macromol. Rapid Commun. 20 (1999) 487. [29] B.L. Small, M. Brookhart, A.M.A. Bennett, J. Am. Chem. Soc. 120 (1998) 4049. [30] T.-J.J. Kinnunen, M. Haukka, T.T. Pakkanen, T.A. Pakkanen, J. Organomet. Chem. 613 (2000) 257. [31] T.V. Laine, M. Klinga, M. Leskel€a, Eur. J. Inorg. Chem. (1999) 959.