catalytic trimerization of ethylene with highly active half-sandwich titanium complexes bearing...

Chinese Journal of Chemistry, 2006, 24, 1397—1401 Full Paper

* E-mail: [email protected]; Tel.: 0086-21-5428-2375; Fax: 0086-21-5428-2375 Received April 10, 2006; revised May 17, 2006; accepted June 23, 2006. Project supported by the National Natural Science Foundation of China (No. 20372022) and the Major State Basic Research Development Program

of China (No. 2005CB623801).

© 2006 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Catalytic Trimerization of Ethylene with Highly Active Half-sandwich Titanium Complexes Bearing Pendant

p-Fluorophenyl Groups

WANG, Chen(王晨) HUANG, Ji-Ling*(黄吉玲)

Laboratory of Organometallic Chemistry, East China University of Science and Technology, Shanghai 200237, China

Two new complexes [η5-C5H4CMe2-(p-fluorophenyl)]TiCl3 (1) and [η5-C5H4C(cyclo-C5H10)-(p-fluorophenyl)]TiCl3 (2) were synthesized and characterized. Their activities and selectivities for trimerization of ethylene were investigated. The introduction of fluorine atom greatly weakened the arene coordination, but this disadvanta-geous factor can be eliminated by introduction of a bulky substituent, such as cyclo-C5H10, to the bridging carbon linked to the Cp ring. The combinative effect of the fluorine substitute and the bridging unit can make complex 2 as a highly active and selective catalyst for ethylene trimerization. Its productivity and selectivity for 1-hexene can reach 1024.0 kg•mol－1•h－1 and 99.3% respectively.

Keywords ethylene, trimerization, coordination, cyclopentadienyl, titanium

Introduction

The oligomerization of ethylene is of considerable academic and industrial interest in the synthesis of α- olefins.1-4 Compared to conventional oligomerization processes, the selective oligomerization of ethylene is highly desirable, because it would avoid the production of unwanted olefins. 1-Hexene, which is used as a co-monomer for synthesis of linear low-density polyethyl-ene (LLDPE),5-7 can be produced by catalytic trimeriza-tion of ethylene. Although most catalytic trimerization systems are based on chromium compounds,8,9 the re-search on non-chromium based trimerization systems is still in process, not only for potential productivity, but also for pollution reasons.10

In 2002, Hessen and co-workers10 reported that the half-sandwich titanium complexes [η5-C5H4-(bridge)-Ar]- TiCl3, activated by methylalumoxane (MAO), formed a family of highly active catalysts for the trimerization of ethylene, giving 1-hexene as main product. This dis-covery has triggered chemist’s interest in developing group 4 metal complexes used in ethylene trimerization. However, the further research of this class of catalysts has been focused on computational DFT (Density Func-tional Theory) study.11,12 To the best of our knowledge, up to now, there is no other experiments of trimerization catalyzed by group 4 metals reported, except for the discovery by our group that Cp’TiCl3 complexes bear-ing pendant thienyl or ethereal groups can trimerize ethylene with high selectivity.13,14

It should be noted that although the cyclopentadienyl

group is one of the most ubiquitous ligands in or-ganometallic chemistry, only a few such ligands substi-tuted by fluorine have been reported.15 In order to study the highly electron-withdrawing substituent effects on ethylene trimerization, herein, we report the synthesis of two new half-sandwich titanium complexes bearing pendant p-fluorophenyl groups and some first results on their catalytic activities and selectivities in ethylene trimerization.

Results and discussion

Synthesis of catalyst precursors

The half-sandwich titanium complexes used in this study can be described by the general formula [η5-C5H4- (bridge)-(p-fluorophenyl)]TiCl3 [bridge＝CMe2 or C(cyclo- C5H10)]. The synthetic routes employed for complexes 1 and 2 are summarized in Scheme 1.

Cyclopentadienyl ligands with [-(bridge)-(p-fluoro-phenyl)] substituents were prepared from the reaction of 6,6-dialkylfulvenes16,17 with the p-fluorophenyl lith-ium.10 The resulting lithium cyclopentadienides [C5H4- (bridge)-(p-fluorophenyl)]Li, reacted with trimethylsilyl chloride to afford the corresponding [C5H4-(bridge)-(p- fluorophenyl)]SiMe3 as yellow oils. Then, complexes 1 and 2 were prepared via Me3SiCl elimination upon reaction of the corresponding (trimethylsilyl)cyclopentadienyl intermediates with TiCl4. The complexes were characterized by 1H NMR, IR, MS spectra and elemental analysis or HRMS.

1398 Chin. J. Chem., 2006, Vol. 24, No. 10 WANG & HUANG


Scheme 1 The synthetic route of complexes 1 and 2

Polymerization results

To evaluate the activities and selectivities of the new catalysts, the catalytic system 3/MAO reported by Hes-sen10 was used as reference. The oligomerization prod-ucts were analyzed by GC. No other oligomer was de-tected, except for 1-hexene and a small portion of poly-mer. The oligomerization results are listed in Table 1.

Effect of fluorine substituent

The reason that [η5-C5H4-(bridge)-(p-fluorophenyl)]- TiCl3/MAO system can prompt the trimerization of eth-ylene, according to the mechanism proposed by Hes-sen,10 is that the pendant aryl group can exhibit hemi-labile behaviour. In the course of formation of active species, the arene coordination can provide additional stabilization to the Ti(II) state. Subsequently, the η

6-coordinated aryl moiety can dissociate or ring slip-page from the titanium center to yield a less elec-tron-rich metal center that can capture and insert the third ethylene molecule. Hessen found that making the pendant phenyl group more electron-rich, by adding one or two methyl substituents respectively, significantly diminishes the activity of the catalyst. He presumed that if the arene coordination binds too strongly it could slow the catalytic reaction. To the best of our knowl-edge, up to now, only the alkyl substitution has been studied. The effect of electron-withdrawing substitution is still unknown.

In our experiment, we found that the activity of complex 1 bearing a pendant p-fluorophenyl group was remarkably lower than that of its analogous complex 3.

Table 1 Trimerization of ethylene catalyzed by [η5-C5H4-(bridge)-Ar]TiCl3/MAO systemsa

Entry Catalyst T/℃ Pressure/1.01×105 Pa Al∶Ti 1-Hexene/g Polymer/g Productivity of 1-hexeneb Selectivity of 1-hexene/%

1 0 5 1000 0.0131 0.0015 17.5 89.8

2 30 5 1000 0.0461 0.0080 61.5 85.2

3 1 80 5 1000 0.0128 0.0100 17.1 56.2

4 30 8 1000 0.0146 0.0043 19.5 77.3

5 30 5 2000 0.0143 0.0092 19.1 60.9

6 0 5 1000 0.2881 0.0020 384.2 99.3

7 30 5 1000 0.3747 0.0104 499.6 97.3

8 2 80 5 1000 0.1008 0.0875 134.4 53.5

9 30 8 1000 0.7680 0.0566 1024.0 93.1

10 30 5 2000 0.4684 0.0299 624.5 94.0

11 0 5 1000 0.3371 0.0173 449.5 95.1

12 30 5 1000 0.4455 0.0295 594.1 93.8

13 3 80 5 1000 0.0605 0.0450 80.6 57.3

14 30 8 1000 0.6795 0.0471 906.0 93.5

15 30 5 2000 0.1575 0.0194 210.0 89.0 a Reaction conditions: 1.5 µmol of catalyst, 20 mL of toluene, 30 min run time. b In kg•mol－1

•h－1.

Half-sandwich titanium complex Chin. J. Chem., 2006 Vol. 24 No. 10 1399


This indicates that the p-fluorophenyl group could hardly exhibit hemilabile behavior efficiently. Fluoro (F) substituent is highly electron-withdrawing. Introduction of fluorine might greatly reduce the electron density of phenyl ring. Thus, the arene coordination is weakened. Combined with the experiment by Hessen, it might im-ply that neither too strong arene coordination nor too weak arene coordination is favorable to the trimeriza-tion of ethylene. There seems to be an optimized state between them.

Effect of the bridging unit

From Table 1, it can be seen that the activity of complex 2 is extraordinarily high, though it contains a p-fluorophenyl moiety. It should be noted that the activ-ity of complex 2 is even higher than that of the non-fluoro-substituted complex 3 under the condition of 8.08×105 Pa ethylene pressure. The result indicates that the bridging unit between the Cp ring and the aryl moi-ety plays a more important role in trimerization than the fluorine substitution.

Two factors may influence the hemilabile behavior. One is the electron density of the phenyl ring, as we discussed above. The other is the degree of in-tramolecular coordination between the phenyl ring and the titanium center. When the bridging carbon, which is linked to the Cp ring, is substituted by bulky cyclo- C5H10 group, the phenyl ring is forced to move closely to the titanium center. This is reflected by the 1H NMR spectra of complexes 1 and 2 (Figure 1).

Figure 1 The phenyl and cyclopentadienyl region of 1H NMR spectra of complexes 1 and 2.

According to 1H NMR spectra, if the phenyl ring is near the highly electrophilic titanium center, the de-shielding effect will lead to the NMR signal occurring at a lower magnetic field. Bochmann18 reported that, in

cationic complexes, the phenyl group could coordinate to titanium center, wherein, the chemical shifts of ArH moved to lower magnetic field (about δ 8.5—8.7). From Figure 1, it can be seen that the difference between the chemical shift of the m-ArH (the Ar-H adjacent to the fluorine atom) and the o-ArH is about δ 0.20 in complex 1, but in complex 2, the difference is about δ 0.35. This might imply that the phenyl rings of complexes 1 and 2 are in a different state. In addition, the chemical shift of m-ArH in complex 1 (δ 7.21—7.14), compared with that of L1 (δ 7.29—7.21), moved to higher magnetic field. In contrast, the chemical shift of the m-ArH in complex 2 (δ 7.41—7.39), compared with that of L2 (δ 7.36—7.30), moved to lower magnetic field. This somewhat implies that the phenyl group of complex 2 is much more prone to arene coordination than that of complex 1. It is known that the phenyl group might not coordinate to the titanium center in normal oxidative state, but, when the Ti(IV) is reduced to Ti(III) or Ti(II) upon ac-tivation by MAO, the phenyl group can coordinate to the titanium center more easily. Meanwhile, the bulky cyclo-C5H10 substituent on the bridging carbon will strengthen the intramolecular arene coordination. Thus, the activity of p-fluoro-substituted catalyst 2 is in-creased.

Several reports have proved that the substituents on the bridging carbon linked to the Cp ring played a cru-cial role in the reaction and the polymerization.19 We previously reported that, in the preparation of o-MeO- containing benzyl-substituted cyclopentadienyl titanium complexes, when the bridging carbon linked to Cp ring was substituted by ethyl or bigger group, the reaction always gave titanoxacycle complexes.20 This is also due to that the increasing steric bulkiness of the substituents on the bridging unit can increase the tendency of in-tramolecular coordination.

Effect of trimerization condition

The activity and selectivity of these catalysts for ethylene trimerization greatly depend on the tempera-ture, which is consistent with the typical polymerization system containing intramolecular coordination.19

Within the range of 0—80 ℃, the selectivity of all these catalysts increased with the lowering of tempera-ture [n(Al)∶n(Ti)＝1000, 5.05×105 Pa]. The highest selectivity of 1-hexene was obtained at 0 ℃ by com-plex 2 (99.3%, Entry 6). In contrast, the polymer product increased with the temperature. This indicates that high temperature is advantageous for polymerization. On the other hand, all the catalysts exhibit lower productivity of 1-hexene at 80 ℃, probably because the arene coordination is weakened at high temperature.

Increasing the pressure of ethylene is advantageous for trimerization. It should be noted that the highest productivity of 1-hexene can reach 1024.0 kg•mol－1•h－1 by complex 2 under pressure 8.08×105 Pa. Compared to the temperature and pressure, the effect of molar ratio of Al/Ti is more complicated. The trimerization activity

1400 Chin. J. Chem., 2006, Vol. 24, No. 10 WANG & HUANG


of complex 2 increased with the increase of molar ratio of Al/Ti, but that of complex 3 decreased with the in-crease of Al/Ti ratio.

Experimental

Materials and instruments

Complex 3 was prepared according to Ref. 10 All manipulations were carried out under a dry argon

atmosphere using standard Schlenk techniques. Solvents were purified by distillation over sodium benzophenone (diethyl ether, tetrahydrofuran (THF), toluene and n-hexane) and P2O5 (dichloromethane). MAO (toluene solution, 1.53 mol•L－1) was produced by Witco GmbH. Polymerization grade ethylene was used without further purification.

Mass spectra were measured on an HP5989A spec-trometer. IR spectra were recorded on a Nicolet FT-IR 5SXC spectrometer. 1H NMR spectra was measured on a Bruker AVANCE-500 MHz spectrometer using tetra- methylsilane (TMS) as an internal standard. Elemental analyses were performed on an EA-1106 spectrometer. GC was performed on Fuli 9790 instrument equipped with Agilent J&W HP-5 GC column.

General procedure for ethylene oligomerization

A 100 mL stainless steel reactor equipped with a magnetic stirrer was heated under vacuum for 1 h over 80 ℃, then allowed to cool to the required reaction temperature under ethylene atmosphere, and charged with 20 mL of toluene. The reactor was sealed and pressurized to 5.05×105 Pa of ethylene pressure. Stir-ring for about 30 min ensured that the solution was equilibrated and the required reaction temperature was established. The ethylene pressure was then released. Then MAO and catalyst in 1 mL of toluene were added. The reactor was sealed and pressurized to required eth-ylene pressure. After the reaction was carried out for 0.5 h, the pressure was released and 1 mL of ethanol was injected immediately to stop the reaction. Heptane (100 µL) was added as internal standard. The polymer was filtered, washed with acidified ethanol (0.5% HCl), and dried under vacuum at 60 ℃ to a constant weight. The solution part was analyzed by GC to determine the con-tent of oligomers.

Synthesis of catalysts

Synthesis of C5H4(SiMe3)CMe2C6H4F (L1): To the solution of 2.04 g (20 mmol) of p-fluorophenyl lithium in 50 mL of Et2O, was added dropwise 2.1 g (20 mmol) of 6,6-dimethylfulvene at 0 ℃ . The mixture was warmed to room temperature and stirred overnight. A white precipitate formed. The precipitate was separated and redissolved in 50 mL of THF. The solution was then cooled to 0 ℃ and 2.1 g (20 mmol) of trimethyl-silyl chloride were added. After it was warmed to room temperature, the mixture was stirred for 6 h. The reac-tion mixture was poured into 20 mL of ice-water. The

water layer was extracted with 2×20 mL of Et2O , after which the combined organic layers were dried over MgSO4. The volatiles were removed in vacuo to yield yellow oil. 4.1 g (15 mmol, 74% yield). 1H NMR (CDCl3, 500 MHz) δ: 7.29—7.21 (m, 2H), 6.97—6.93 (t, J＝8.6 Hz, 2H), 6.42 (s, 1H), 6.32 (d, J＝4.7 Hz, 1H), 6.17 (s, 1H), 3.28 (s, 1H), 1.55 (s, 6H), 0.00 (s, 9H).

Synthesis of [η5-C5H4CMe2C6H4F]TiCl3 (1): To a solution of 1.38 g (7.3mmol) of titanium tetrachloride in 20 mL of methylene chloride, cooled to －60 ℃, were added 2.0 g (7.3 mmol) of C5H4(SiMe3)CMe2C6H4F (L1). The reaction mixture was warmed to room tem-perature and was stirred overnight. The volatiles were removed in vacuo, and the residue was extracted with 20 mL of n-hexane. Cooling the filtrate to －30 ℃ yielded yellow crystals. Yield 0.68 g, (1.9 mmol, 26%). 1H NMR (CDCl3, 500 MHz) δ: 7.21—7.14 (m, 2H), 7.01—6.95 (m, 2H), 6.94—6.93 (m, 1H), 6.89—6.87 (m, 2H), 6.75 (t, J＝2.7 Hz, 1H), 1.83 (s, 6H); IR (KBr) ν: 3102, 2975, 1598, 1507, 1231, 1047, 833, 593 cm－1; HRMS calcd for C14H14Cl3FTi 353.9625, found 353.9621.

Synthesis of C5H4(SiMe3)C(cyclo-C5H10)C6H4F (L2): The same procedure as described for L1 was used. p-Fluorophenyl lithium 2.04 g (20 mmol), 6,6-(cyclo- C5H10)-fulvene 2.9 g (20 mmol), and trimethylsilyl chloride 2.1 g (20 mmol) were used to give C5H4- (SiMe3)C(cyclo-C5H10)C6H4F. Yield 3.5 g (11 mmol, 56%). 1H NMR (CDCl3, 500 MHz) δ: 7.36—7.30 (m, 2H), 6.97—6.94 (t, J＝8.5 Hz, 2H), 6.59—6.08 (m, 3H), 3.23 (s, 1H), 2.23—0.83 (m, 10H), 0.00 (s, 9H).

Synthesis of [η5-C5H4C(cyclo-C5H10)C6H4F]TiCl3 (2): The same procedure as described for 1 was used. Titanium tetrachloride 1.2 g (6.3 mmol) and C5H4- (SiMe3)C(cyclo-C5H10)C6H4F 2.0 g (6.3 mmol) were used to give [η5-C5H4C(cyclo-C5H10)C6H4F]TiCl3. Yield 0.78 g (1.9 mmol, 31%). 1H NMR (CDCl3, 500 MHz) δ: 7.41—7.39 (m, 2H), 7.05—7.04 (t, J＝8.6 Hz, 2H), 6.87 (t, J＝2.7 Hz, 2H), 6.77 (t, J＝2.7 Hz , 2H), 2.72—1.33 (m, 10H); IR (KBr) ν: 3104, 2929, 1602, 1510, 1465, 1235, 1013, 840, 542 cm－1; MS (70 eV) m/z (%): 394 (M＋, 6.5), 322 (100). Anal. calcd for C17H18Cl3FTi: C 51.62, H 4.59; found C 51.58, H 4.71.

Conclusion

The introduction of fluorine greatly reduced the electron density of phenyl ring, leading to the result that arene coordination is weakened remarkably. But, by introduction of a bulky substituent, such as cyclo-C5H10, to the bridging carbon linked to the Cp ring, the p-fluorophenyl group can be forced to move closely to the titanium center, thus the intramolecular arene coor-dination is strengthened. The combinative effect of the fluorine substitute and the bridging unit can make com-plex 2 as a highly active and selective catalyst for eth-ylene trimerization. Its productivity and selectivity for 1-hexene can reach 1024.0 kg•mol－1•h－1 and 99.3% respectively.

Half-sandwich titanium complex Chin. J. Chem., 2006 Vol. 24 No. 10 1401


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(E0604105 ZHAO, C. H.; LING, J.)

catalytic trimerization of ethylene with highly active half-sandwich titanium complexes bearing...

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