Applied Catalysis A: General 246 (2003) 11–16

Ethylene oligomerization by hydrazone Ni(II) complexes/MAO

Liyi Chen, Junxian Hou, Wen-Hua Sun∗State Key Laboratory of Engineering Plastics and Center for Molecular Sciences, Institute of Chemistry, The Chinese Academy of Sciences,

Beijing 100080, PR China

Received 26 October 2002; received in revised form 9 December 2002; accepted 9 December 2002

Abstract

The ethylene oligomerization was investigated by using new hydrazone nickel complexes Ni(NˆO)2Cl2 (2a, NˆO =4,5-diazafluorene-9-one-benzoylhydrazone;2b, NˆO = 4,5-diazafluorene-9-one-4-nitrobenzoylhydrazone;2c, NˆO = 4,5-diazafluorene-9-one-3-nitrobenzoylhydrazone) with methylaluminoxane (MAO) in toluene. Those catalytic systems mainlyassisted the dimerization of ethylene with good catalytic activity (105 to 104 g mol−1 h−1) at ambient pressure. The reactionconditions, such as the ratios of Al/Ni, reaction temperature and reaction time, were investigated. The best catalytic activityfor the three complexes was observed for complex2a without nitro-substituent on its aryl ring.© 2002 Elsevier Science B.V. All rights reserved.

Keywords:Ethylene oligomerization; Homogeneous catalysts; Hydrazone nickel(II) complexes

1. Introduction

Ethylene oligomerization is a versatile processused to convert ethylene into value-added usefulfine-chemicals, such as environmentally friendly fu-els, feedstock for linear low-density polyethylene,lubricants, plastics, surfactants and detergents. Thecommercially practiced Shell High Olefin Process(SHOP) based on the pioneering research of Keimproduced olefins in the range of C4–C30 at the rate ofmore than one million tons every year[1]. The SHOPcatalyst employed the catalyst of nickel complex bear-ing P̂O bidentate ligands (Scheme 1) and gave highselectivity, with 99% linear olefin containing 98%�-olefin [2]. However, it was reported that the SHOPprocess is performed at 80–120◦C and 70–140 bar[3]; such reaction conditions were sometimes harsh.

∗ Corresponding author. Tel.:+86-10-6255-7955;fax: +86-10-6256-6383.E-mail address:whsun@infoc3.icas.ac.cn (W.-H. Sun).

Recently, cationic nickel�-diimine catalysts werereported by Brookhart’s group[4,5]. Following thereport, independently from Brookhart’s and Gibson’sgroups, 2,6-bis(imino)pyridine Fe(II) and Co(II) cat-alysts were reported with very high activities[6,7].It seems promising to use late transition metal com-plexes as the promising next generation catalyst forethylene oligomerization or polymerization at mildconditions.

It is supposed that late transition metal complexeshave a strong propensity of undergoing�-hydrogenelimination process on the central metal, which wouldinduce ethylene oligomerization[8]. Adjustments ofboth steric and electronic environment around thecentral metal through tailoring ligands could improvethe catalytic activity and olefins’ distribution[1]. Pre-viously, we have designed series of catalysts, whichcontain P̂N, NˆNˆN and N̂N moieties, for ethyleneoligomerizations[9–12]. In order to make compar-isons among different ligands affecting the catalyticsystem, we designed hydrazone ligands bearing NˆO

0926-860X/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.doi:10.1016/S0926-860X(02)00660-9

12 L. Chen et al. / Applied Catalysis A: General 246 (2003) 11–16

Scheme 1. Model of SHOP catalysts.

bidentate according to the SHOP model catalysts.Each ligand contains a non-coordinating N atombearing a lone electron pair as a method of adjustingthe electronic effect. It was found that the hydra-zone nickel(II) complexes2 could perform ethyleneoligomerization by using methylaluminoxane (MAO)as co-catalyst under ambient atmosphere. Several re-action parameters influencing the catalytic activityand selectivity have been evaluated.

2. Experimental parts

We used Schlenk techniques to operate all moisture-sensitive manipulations. Toluene was newly distilledover sodium under a dry nitrogen atmosphere. Allthe other chemical reagents were commercially pur-chased and used without further purification. Thecomplexes were prepared as explained in the refer-ence [13]. 1H-NMR spectra were obtained on theBRUKER DMX 300. Mass spectra were obtainedusing either fast atom bombardment (FAB), elec-

Scheme 2. Synthesis of complexes.

tron ionization (EI), ESI or MALDI-TOF. Elementanalyses were performed on Flash EA1112 or CarloErba1106 (Scheme 2).

2.1. Preparation of ligands

Synthesis of 4,5-diazafluorene-9-one (dafo) wassynthesized according to the literature method[14].The condensation reaction ofdafo with arylhy-drine gave the ligands for coordinating with nickelchloride.

2.1.1. Synthesis of 4,5-diazafluorene-9-one-benzoylhydrazone (1a)

4,5-Diazafluorene-9-one (0.36 g, 2 mmol) wasadded to a solution of benzoylhydrine (0.27 g, 2 mmol)in absolute ethanol (30 ml). After the addition ofp-toluene sulfonic acid (catalytic eq.), the solutionwas refluxed for 6 h. The ethanol was partly removedon vacuum line and the remainder was kept in a coolplace over night. Yellow crystalline materials cameout; the product was filtered, washed with ethanoland dried in a vacuum oven overnight. Yield: 85%.1H-NMR (CD3SOCD3): δ = 7.50–7.69 (m, 5H), 8.03(d, J = 6 Hz, 2H), 8.245 (d,J = 9 Hz, 1H), 8.545(d, J = 9 Hz, 1H), 8.74–8.77 (t, 2H), 12.07 (NH). EIMS—m/z: 300 [M]+. C18H12N4O·CH3CH2OH cal-culated: C (69.35%), H (5.24%), N (16.17%); found:C (69.24%), H (5.16%), N (16.49%).

L. Chen et al. / Applied Catalysis A: General 246 (2003) 11–16 13

2.1.2. Synthesis of 4,5-diazafluorene-9-one-4-nitrobenzoylhydrazone (1b)

The procedure was the same as that of 2.1.2and golden yellow powder is obtained. Yield: 92%.1H-NMR (CD3SOCD3): δ = 7.50–7.60 (m, 2H),8.26–8.28 (d, 3H), 8.415 (d,J = 9 Hz, 2H), 8.61(d, J = 6 Hz, 1H), 8.76–8.79 (t, 2H), 12.33 (NH).FAB MS—m/z: 344 [M − H]+. C18H11N5O3 calcu-lated: C (62.61%), H (3.21%), N (20.28%); found: C(62.64%), H (3.18%), N (20.08%).

2.1.3. Synthesis of 4,5-diazafluorene-9-one-3-nitrobenzoylhydrazone (1c)

The procedure was the same as that of 2.1.2, andyellow-greenish powder is obtained. Yield: 94%.1H-NMR (CD3SOCD3): δ = 7.50–7.60 (m, 2H),7.88–7.93 (t, 1H), 8.245 (s, 1H), 8.47–8.52 (t, 2H),8.61 (d, J = 6 Hz, 1H), 8.76–8.79 (t, 2H), 8.84(s, 1H), 12.34 (NH). MALDI-TOF MS—m/z: 346[M + H]+. C18H11N5O3 calculated: C (62.61%), H(3.21%), N (20.28%); found: C (62.45%), H (3.14%),N (20.05%).

2.2. Preparation of complexes

2.2.1. Synthesis of 4,5-diazafluorene-9-one-benzoylhydrazone nickel(II) complex (2a)

4,5-Diazafluorene-9-one-benzoylhydrazone (60 mg,0.2 mmol) was dissolved in hot ethanol (20 ml) atreflux. A ethanol solution (5 ml) of NiCl2·6H2O(24 mg, 0.1 mmol) was dropwise added. The productwas refluxed for 24 h and yellow-greenish precipitatecame out. The product was filtered, washed with hotethanol and dried in a vacuum oven. Yield: 53%.ESI MS—m/z: 695 [M − Cl]+, 660 [M − 2Cl]+.C36H24N8O2NiCl2·H2O calculated: C (57.79%), H(3.50%), N (14.98%); found: C (57.82%), H (3.41%),N (14.73%).

2.2.2. Synthesis of 4,5-diazafluorene-9-one-4-nitrobenzoylhydrazone nickel(II)complex (2b)

NiCl2·6H2O (48 mg, 0.2 mmol) was dissolved inethanol (10 ml), and then the solution was dropwiseadded to a suspension of 4,5-diazafluorene-9-one-4-nitrobenzoylhydrazone (138 mg, 0.4 mmol) in reflux-ing ethanol (100 ml). The mixture was refluxed for24 h. During this period, the mixture first became an

orange solution, then the precipitate came out as or-ange powder. The product was filtered, washed withhot ethanol and dried in a vacuum oven. Yield: 80%.MALDI-TOF MS—m/z: 928 [M + 6H2O]+, 910[M+5H2O]+, 892 [M+4H2O]+, 874 [M+3H2O]+,856 [M + 2H2O]+. C36H22N10O6NiCl2·4H2O calcu-lated: C (48.46%), H (3.39%), N (15.70%); found: C(48.53%), H (2.74%), N (15.23%).

2.2.3. Synthesis of 4,5-diazafluorene-9-one-3-nitrobenzoylhydrazone nickel(II)complex (2c)

The procedure was the same as that of 2.2.2and an earthy yellow powder is obtained. Yield:20%. MALDI-TOF MS—m/z: 749 [M − 2Cl]+C36H22N10O6NiCl2 calculated: C (52.72%), H(2.70%), N (17.08%); found: C (52.53%), H (2.96%),N (16.94%).

2.3. Ethylene oligomerization

2.3.1. Preparation of active species from nickelcatalysts precursor

Typically 5�mol of nickel(II) complex was addedinto a Schlenk tube, and then 5 ml newly distilledtoluene under dry nitrogen was charged into the tubewith magnetic stirring. Methylaluminoxane (MAO)toluene solution (1.4 M) of desired amount was in-jected into the suspension above. Immediately thecolor of the solution was changed, and the activecatalytic species in toluene solution was available.

2.3.2. Oligomerization of ethyleneThe oligomerization reaction was carried out in a

100 ml three-neck flask. After evacuation and flush-ing with nitrogen three times, then with ethylene twotimes, the flask was charged with 25 ml toluene andmagnetically stirred under ambient ethylene atmo-sphere. When the desired reaction temperature wasestablished by oil bath, the solution of nickel cata-lyst was injected into the reactor. Typically 30 minlater, the reaction solution was quickly cooled downto −20◦C and then quenched by adding 6 M HCl.Finally, 0.5 ml n-pentane was added as an internalstandard.

Oligomers were analyzed by SHIMADZU GCMS-QP505A with a DB-5MS column (30 m× 0.25 mm).The program was set the initial temperature 40◦C

14 L. Chen et al. / Applied Catalysis A: General 246 (2003) 11–16

(hold 2 min) and finishing temperature 220◦C (hold10 min) with a heating rate of 10◦C/min.

3. Results and discussion

3.1. The effect of Al/Ni ratios

Just as with the SHOP catalytic system, the catalysts2 show an electronic conjugated system between co-ordinated atoms. Adapting Al/Ni ratios, the catalyticsystem obtained the moderate to high catalytic ac-tivities for ethylene oligomerization. This may be at-tributed to the hemilibility of the N̂O bidentate, whichcan be flexible in different circumstances, resultingin space for ethylene coordination and insertion. Dueto electron-withdrawing nitro-group in two precursors2b and 2c, ethylene is firmly coordinated on metal

Table 1The effect of Al/Ni ratios on activity and selectivity of complex2a

Entry Cata Al/M b (mol/mol) Activity (g mol−1 h−1) Product distribution (%) �-Olefinc (%)

C4 C6 C8 �-C4 �-C6 �-C8

1 2a 500/1 1.23× 105 87.0 11.5 1.5 100 6.9 16.22 2a 800/1 1.46× 105 83.4 14.8 1.8 100 6.1 87.83d 2a 1000/1 1.59× 105 65.4 27.0 7.6 100 32.3 5.64 2a 1200/1 1.06× 105 86.6 12.6 0.8 100 – –5 2a 1500/1 5.18× 104 98.4 1.6 – 100 – –

a Catalyst, entry 3 10�mol, and the others 5�mol each.b Al/M, the molar ratio of MAO and Ni catalyst.c The percentage of�-olefin in its analogue olefins.d Reaction time, entry 3 1 h, others 30 min.

Table 2The effect of Al/Ni ratios on activity and selectivity of complexes2b and 2c

Entry Cata Al/M b (mol/mol) Activity (g mol−1 h−1) Product distribution (%) �-Olefinc (%)

C4 C6 C8 �-C4 �-C6 �-C8

1 2b 500/1 3.16× 104 97.7 2.3 – 100 – –2 2b 800/1 7.79× 104 92.5 7.5 – 100 29.8 –3 2b 1000/1 5.58× 104 94.0 6.0 – 100 – –4 2b 1500/1 5.14× 104 92.9 5.3 1.8 100 100 –5 2c 800/1 1.28× 104 92.4 7.6 – 100 24.0 –6 2c 1000/1 2.00× 104 100 – – 100 – –7 2c 1200/1 1.03× 104 100 – – 100 – –8 2c 1500/1 1.18× 104 92.8 – 7.2 100 – –

Reaction time 30 min.a Catalyst 5�mol each.b Al/M, the molar ratio of MAO and Ni catalyst.c The percentage of�-olefin in its analogue olefins.

center and migratory insertion is also hindered. There-fore, catalyst2a performed the best activities amongthem. The detailed results are collected inTables 1and 2.

The data in the tables indicate that their catalyticactivities did not always rise with increasing Al/Niratio; the same phenomena were observed in our pre-vious works[11,12]. Take complex2a as a typicalexample: the catalytic activity reached its highestpoint 1.59× 105 g (ethylene) mol−1 (Ni) h−1 with theratio of Al/Ni in 1000/1. Subsequently, its activitydropped; 5.18 × 104 g (ethylene) mol−1 (Ni) h−1 wasobtained with the Al/Ni ratio of 1500/1. One possi-ble reason was that a threshold amount of MAO asco-catalyst was needed to effectively activate the cat-alyst precursor, but large amounts of MAO containedtoo many trialkylaluminum impurities, which mightcause such degradation.

L. Chen et al. / Applied Catalysis A: General 246 (2003) 11–16 15

Table 3The effect of temperature on activity and selectivity of complex2a

Entry Cata Temperature (◦C) Activity (g mol−1 h−1) Product distribution (%) �-Olefinb (%)

C4 C6 C8 �-C4 �-C6 �-C8

1 2a 0 5.50× 104 84.5 15.5 – 100 – –2 2a 30 1.59× 105 65.4 27.0 7.6 100 32.3 5.63 2a 50 4.90× 104 86.7 13.1 0.2 100 – –4 2a 70 4.33× 104 78.8 21.2 – 100 79.6 –5 2a 90 – – – – – – –

Al/M (the molar ratio of MAO and Ni catalyst)= 1000/1. Reaction time, entry 2 1 h, entry 3 40 min, others 30 min.a Catalyst, entry 2 10�mol, and the others 5�mol each.b The percentage of�-olefin in its analogue olefins.

In the term of the products’ distribution, the lowerAl/Ni ratio seemed to produce more�-olefins for C6and C8, which is probably caused with the competitionbetween the chain transfer and the chain propagation.The Al/Ni ratio of 1000/1 was also the optimum ratiofor the production of C6 and C8.

3.2. The effect of reaction temperature

Further investigation of complex2a was per-formed at different reaction temperatures (Table 3).Like the complexes in the 2-(2-pyridyl) quinoxa-line [11] and 8-(diphenylphosphino) quinoline[12]systems, the catalytic activities of complex2a weresensitive to reaction temperatures. Lower tempera-tures are generally favorable, however, the activityof 5.18× 105 g (ethylene) mol−1 (Ni) h−1 was best at30◦C. The low temperature is unfavorable because itmay hinder the formation of the active species. Therewas no activity for ethylene oligemerization obtainedat 90◦C, due to a decrease of ethylene solubility in

Table 4The effect of reaction time on activity and selectivity of complex2a

Entry Cata Time (min) Activity (g mol−1 h−1) Product distribution (%) �-Olefinb (%)

C4 C6 C8 �-C4 �-C6 �-C8

1 2a 10 1.29× 105 92.2 7.0 0.8 100 18.5 –2 2a 20 4.75× 104 92.6 5.7 1.7 100 – –3 2a 40 4.90× 104 86.7 13.1 0.2 100 – –

Al/M (the molar ratio of MAO and Ni catalyst)= 1000/1. Temperature 50◦C.a Catalyst 5�mol each.b The percentage of�-olefin in its analogue olefins.

toluene and the deactivation of the catalyst. In addi-tion, the trend of variations of�-olefins distributionis nearly the same as the trend of catalytic activity.

3.3. The effect of reaction time

In the industrial processes, the lifetime of a catalystplays an important role. Therefore, ethylene oligomer-ization by complex2a proceeded at 50◦C withindifferent periods (Table 4). The hydrazone Ni(II) sys-tem2a showed very similar catalytic dynamic perfor-mances to those observed for 2-(2-pyridyl)quinoxalinenickel complexes[11]. A deactivation of catalystoccurred about 10 min after initiating, however, its ac-tivities vary a little during the periods over 20–40 min.With the prolonged reaction time, products of C6 andC8 were increased while the amount of C4 decreased.The amount of C8 was raised more quickly comparedto C6 in 20 min. The double bond immigration con-trolled by thermodynamics resulted in the decrease of�-olefins.

16 L. Chen et al. / Applied Catalysis A: General 246 (2003) 11–16

4. Conclusions

Ethylene oligomerization by hydrazone Ni(II) com-plexes with MAO as co-catalyst was investigated. Theresults demonstrated that those catalysts performedhigh or moderate catalytic activities. The nickel(II)complex 2a indicated higher catalytic activity com-pared with other two analogues and there was anoptimum situation for the ethylene oligomerizationcatalyzed by complex2a under normal pressure.

Acknowledgements

We are grateful to the Chinese Academy of Sciencesfor financial support under the Fund of One HundredTalents and Core Research for Engineering InnovationKGCX203-2, and National Natural Science Founda-tion of China No. 20272062.

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