Synthesis, characterization and ethylene oligomerization behaviour of 8-(1-aryliminoethylidene)quinaldinylnickel dihalides

Abstract

A series of 8-(1-aryliminoethylidene)quinaldines and the nickel halides thereof were synthesized and characterized, and the molecular structures of two representative nickel complexes were confirmed by single-crystal X-ray diffraction studies. Upon treatment with diethylaluminium chloride (Et₂AlCl), the nickel pro-catalysts exhibited high activity for ethylene oligomerization (1.24–1.83 × 10⁶ g mol⁻¹(Ni) h⁻¹) with good thermal stability at 60 °C under 10 atm of ethylene. The influence of the reaction parameters on the catalytic behaviour was investigated for these nickel-based systems, including variation of Al/Ni molar ratio and reaction temperature. Furthermore, the effect of the ancillary ligand Ph₃P was also probed.

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1. Introduction

The Shell higher olefin process (SHOP), which employs a nickel-based catalytic system, is the major route in providing linear α-olefins as industrial chemical substrates.¹ In recent years, attention has focused on the use of nickel complexes as pro-catalysts towards ethylene reactivity, primarily due to the discovery of α-diiminonickel complexes as highly active pro-catalysts in ethylene polymerization as reported by Brookhart et al.² This, together with further successes with other late transition metal systems, has stimulated extensive research, much of which has been the focus of recent reviews.⁴ In the case of nickel-based pro-catalysts, much of the research has related to the variation of different ligand sets at the metal centre, with bidentate chelate ligands of the type P⁁O,⁵P⁁P,⁶N⁁O,⁷P⁁N,⁸ and N⁁N,⁹ and some tridentate ligands of N⁁N⁁O,¹⁰N⁁P⁁N,¹¹P⁁N⁁N,¹² and N⁁N⁁N,¹³etc., proving to be popular. However, a critical problem remains within late-transition metal pro-catalysts for ethylene activation, and that is the correlation between the activity and the reaction temperature. It is usually the case that the catalytic activity decreases on elevating the reaction temperature. Such a phenomenon was discussed in the cases of the iron and cobalt-based pro-catalysts,¹⁴ but for nickel complexes, only those bearing quinaldine derivatives as bidentate ligands have been evaluated versus temperature.^8b,9d As a consequence, here a series of 8-(1-aryliminoethylidene)quinaldines has been synthesized, reacted with nickel dihalides, and the resultant pro-catalysts investigated. Upon activation with Et₂AlCl, all the nickel pro-catalysts reported herein showed high activity towards ethylene oligomerization, and the affects of reaction parameters on their catalytic performance has been investigated in detail.

2. Results and discussion

2.1 Synthesis and characterization.

The stoichiometric reaction of 8-acetylquinaldine with various anilines afforded the 8-(1-aryliminoethylidene)quinaldines in moderate to good isolated yields (Scheme 1). All the ligands were characterized by FT-IR, ¹H and ¹³C NMR measurements as well as by elemental analysis. Further reaction of the 8-(1-aryliminoethylidene)quinaldines with an equivalent of a nickel halide [(DME)NiBr₂ or NiCl₂·6H₂O] in ethanol afforded the title nickel complexes (Scheme 1: C1–C5, bromides; C6–C10, chlorides) in high yields. All nickel complexes were characterized by FT-IR spectroscopic and elemental analyses, and were found to be highly stable either in the solid-state or in solution.

Scheme 1 Synthetic procedure.

In the FT-IR spectra, the C=N stretching frequencies of the nickel complexes shifted to lower values (1600–1620 cm⁻¹) with weaker intensity compared with those of the corresponding free ligand. Such changes are in line with the coordination between imino-nitrogen and nickel. The molecular structures of the nickel complexes with C3 and C5 were confirmed by single-crystal X-ray diffraction.

2.2 Crystal structures

Single crystals of the nickel complexes C3 and C5 suitable for X-ray diffraction studies were obtained by slow diffusion of diethyl ether into methanol solution. The molecular structures are shown in Fig. 1 and 2, respectively, with selected bond lengths and angles given in the caption.

Fig. 1 Molecular structure of C3. Thermal ellipsoids are shown at the 30% probability level. Hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (°): Br(1)–Ni(1), 2.3196(11); Br(2)–Ni(1), 2.3850(12); Ni(1)–N(2), 1.958(5); Ni(1)–N(1), 1.994(5); N(2)–C(11), 1.290(8); N(2)–C(13), 1.457(8); N(1)–C(1), 1.331(9); N(1)–C(10), 1.378(8); N(2)–Ni(1)–N(1), 91.2(2); N(2)–Ni(1)–Br(1), 109.25(15); N(1)–Ni(1)–Br(1), 128.58(15); N(2)–Ni(1)–Br(2), 108.35(15); N(1)–Ni(1)–Br(2), 96.19(15); Br(1)–Ni(1)–Br(2), 119.06(4). The dihedral angle of quinolinyl and aryl planes is 64.5°.

Fig. 2 Molecular structure of C5. Thermal ellipsoids are shown at the 30% probability level. Hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (°): Br(1)–Ni(1), 2.3486(12); Br(2)–Ni(1), 2.3867(13); Ni(1)–N(2), 1.966(5); Ni(1)–N(1), 2.026(5); N(1)–C(1), 1.336(9); N(1)–C(9), 1.383(8); N(2)–C(11), 1.288(8); N(2)–C(13), 1.454(8); N(2)–Ni(1)–N(1), 89.6(2); N(2)–Ni(1)–Br(1), 118.76(17); N(1)–Ni(1)–Br(1), 132.60(17); N(2)–Ni(1)–Br(2), 104.67(16); N(1)–Ni(1)–Br(2), 98.82(16); Br(1)–Ni(1)–Br(2), 108.17(5).

In comparison with the molecular structure of complex C3, the complex C5 also possessed a distorted tetrahedral geometry at nickel (Fig. 2) with associated bond distances Ni–N(1) (2.026(5) Å), Ni–N(2) (1.966(5) Å) Br(1)–Ni(1), (2.3486(12) Å) and Br(2)–Ni(1) (2.3867(13) Å). The slight differences in the bond distances and angles around the nickel centre were caused by the presence of the different substituents on the ligands, which also affects the catalytic behaviour of the nickel complexes.

2.3 Ethylene oligomerization

Various reaction parameters, including the type of alkylaluminium employed as co-catalyst, the molar ratio of co-catalyst to nickel complex (Al/Ni) and reaction temperature, all affect the catalytic activities of the nickel complexes. With regard to the halides involved, the nickel pro-catalysts can be divided into two groups, namely bromide compounds [(L)NiBr₂ (C1–C5)] and chloride compounds [(L)NiCl₂ (C6–C10)].

2.3.1 Ethylene oligomerization by nickel bromide complexes. The influence of the various alkylaluminium reagents was evaluated for C3 by employing the co-catalysts methylaluminoxane (MAO), modified methylaluminoxane (MMAO) or diethylaluminium chloride (Et₂AlCl)—see Table 1. Ethylene oligomerization was observed in all cases, with the catalytic system using Et₂AlCl exhibiting the better activity. In light of these findings, Et₂AlCl was chosen as the co-catalyst for further investigation.

Table 1 Oligomerization of ethylene with pro-catalyst C3^a

Entry	Co-catalyst	Al/Ni	Activity^b	Oligomer distribution^c
				α-C4	C4/∑C	C6/∑C
a Conditions: 5 μmol Ni, 10 atm C2H4, 30 min, 20 °C, 100 mL toluene. b Activity, 10^5g mol^−1(Ni) h^−1. c Determined by GC, ∑C denotes the total a mount of oligomers (%).
1	MAO	1000	0.13	95.2	95.5	4.5
2	MMAO	1000	1.33	70.5	92.4	7.6
3	Et2AlCl	200	5.89	95.8	97.8	3.2

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In the C3/Et₂AlCl system, the molar ratio of Et₂AlCl to the nickel pro-catalyst (Al/Ni) greatly affected the catalytic activity (entries 1–4, Table 2), and the optimum Al/Ni molar ratio at 20 °C was found to be 200 : 1. Upon changing the Al/Ni molar ratio, the selectivity for α-olefins together with the distribution of oligomers varied only slightly. Fixing the catalytic system of C3 with 200 equiv. of Et₂AlCl, the influence of reaction temperature was investigated (entries 2 and 5–10, Table 2), and a high activity of 1.83 × 10⁶ g mol⁻¹(Ni) h⁻¹ was observed at 60 °C. Compared with other nickel pro-catalysts which more commonly perform better at lower reaction temperature,¹⁵ the current system demonstrates an optimum reaction temperature of 60 °C, which is consistent with our previous observations.^13f Interestingly, the content of C₆ oligomers was highest at 60 °C, implying more chain propagation and enhanced thermal stability of the active species. Upon the addition of 25 equivalents of PPh₃ to the catalytic system, the catalytic activity of C3 was greatly increased, with activities observed as high as 1.72 × 10⁷ g mol⁻¹(Ni) h⁻¹ (entry 11, Table 2), which is almost an order of magnitude higher than that without PPh₃ (entry 8, Table 2). In addition, the selectivity for α-C₄ was dramatically decreased. It is believed that the auxiliary PPh₃ protects the active species during the catalytic reaction, and induces isomerization along with β-hydrogen elimination. Such phenomena are consistent with previous results.^9k,16,17

Table 2 Oligomerization of ethylene with C3/Et₂AlCl^a

Entry	T/°C	Co-catalyst	Al/Ni	Activity^b	Oligomer distribution^c (%)
					α-C4	C4/∑C	C6/∑C
a Conditions: 5 μmol Ni, 10 atm C2H4, 30 min, 100 mL toluene. b Activity, 10^5 g mol^−1(Ni) h^−1. c Determined by GC, ∑C denotes the total a mount of oligomers (%). d 25 equiv. of Ph3P, 20min.
1	20	Et2AlCl	100	1.61	95.8	96.9	3.1
2	20	Et2AlCl	200	5.89	95.8	97.8	3.2
3	20	Et2AlCl	300	1.30	92.5	96.7	3.3
4	20	Et2AlCl	400	1.27	76.1	91.3	8.7
5	30	Et2AlCl	200	5.90	93.1	94.2	5.8
6	40	Et2AlCl	200	5.92	95.1	95.1	4.9
7	50	Et2AlCl	200	7.26	78.4	95.4	4.6
8	60	Et2AlCl	200	18.3	63.4	86.5	13.5
9	70	Et2AlCl	200	8.62	72.3	89.5	10.5
10	85	Et2AlCl	200	1.61	85.5	92.1	7.9
11^d	60	Et2AlCl	200	113	17.2	94.4	5.6

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To compare the influence of the ligand environment on the catalytic behaviour of the nickel pro-catalysts, nickel bromide pro-catalysts C1–C5 were investigated under optimum reaction condition of molar ratio Et₂AlCl/Ni at 200 : 1, 60 °C and 10 atm ethylene (see Table 3). High activities were observed for all nickel pro-catalysts, which varied in the order C3 > C1 > C2 > C4 > C5. The catalytic activities were likely affected by both the steric and electronic influences of the ligands. An exception is the complex C3 bearing the bulky 2,6-diisopropyl substituted ligand, which protects the active species and affords highest activity. Despite the ethyl group being bulkier than the methyl group, the rotation of the σ-bond in the ethyl group reduces the steric hindrance for the ethyl compared with the methyl group. The activity order C1 > C2, C4 > C5, C1 > C4 and C2 > C5 seems to be governed by the electronic influences of the ligands. The presence of the additional methyl group in the ligands enhances the solubility, which can be helpful for activating the complexes,^9k,13f,18 but can also cause lower activity due to a lower net charge at the nickel center.¹⁹

Table 3 Ethylene oligomerization by nickel bromide pro-catalyst C1–C5^a

Entry	Pro-catalyst	Activity^b	Oligomer distribution^c (%)
			α-C4	C4/∑C	C6/∑C
a Conditions: 5 μmol Ni, 200 equiv. Et2AlCl, 60 °C, 10 atm C2H4, 30 min, 100 mL of toluene. b Activity, 10^5 g mol^−1(Ni) h^−1. c Determined by GC. ∑C denotes the total amount of oligomers.
1	C1	15.9	66.4	88.3	11.7
2	C2	13.6	66.4	93.4	6.6
3	C3	18.3	63.4	86.5	13.5
4	C4	10.5	68.1	92.7	7.3
5	C5	8.61	70.4	92.5	7.5

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2.3.2 Ethylene oligomerization by nickel chloride complexes. In a similar procedure, complex C8 was used in exploring suitable co-catalysts (entries 1–3 in Table 4) for optimum oligomerization. Both the catalytic systems with MMAO and Et₂AlCl exhibited highest activities, however the system with Et₂AlCl used less aluminium and exhibited higher ratios of C₆ fractions and selectivity for α-C₄. Given this, the co-catalyst Et₂AlCl was further explored. In the range of Al/Ni molar ratio from 100 to 500 : 1, highest activity was observed at 200 : 1 (entries 3–6 in Table 4). On varying the reaction temperature from 20 to 70 °C (entries 3 and 7–10 in Table 4), the highest activity value was obtained at 60 °C, consistent with the bromide analogues above. Similarly, on adding 25 equivalents of PPh₃ to the catalytic system, the activity greatly increased (up to 8.43 × 10⁶ g mol⁻¹(Ni) h⁻¹).

Table 4 Oligomerization of ethylene with pro-catalyst C8^a

Entry	T/°C	Co-catalyst	Al/Ni	Activity^b	Oligomer distribution^c (%)
					α-C4	C4/∑C	C6/∑C
a Conditions: 5 μmol Ni, 10 atm C2H4, 30 min, 100 mL toluene. b Activity, 10^5 g mol^−1(Ni) h^−1. c Determined by GC, ∑C denotes the total amount of oligomers. d 25 equiv. of Ph3P, 20 min.
1	20	MAO	1000	2.01	94.4	98.2	2.8
2	20	MMAO	1000	0.79	60.7	86.7	13.3
3	20	Et2AlCl	200	1.81	96.1	94.9	5.1
4	20	Et2AlCl	100	0.51	98.2	91.3	8.7
5	20	Et2AlCl	300	1.50	93.4	95.3	4.7
6	20	Et2AlCl	500	1.03	87.9	96.0	4.0
7	30	Et2AlCl	200	2.12	93.2	94.8	5.2
8	50	Et2AlCl	200	2.91	93.0	94.6	5.4
9	60	Et2AlCl	200	11.0	73.5	90.7	9.3
10	70	Et2AlCl	200	2.19	84.8	93.3	6.7
11^d	60	Et2AlCl	200	84.3	17.3	93.9	6.1

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In a similar manner to the bromide analogues, ethylene oligomerization by the C6–C10/Et₂AlCl systems was investigated under optimum condition (Al/Ni molar ratio of 200 : 1 at 60 °C—see Table 5). Relatively lower activities were exhibited relative to those of the bromide analogues (Table 3), presumably because of the better solubility associated with the latter. A similar trend for catalytic activities was displayed in regard to the nature of the ligands present, such that the order C8 > C6 ≈ C9 > C7 > C10 was observed. It was also noted that the methyl group at the arylimino para-position of ligands exerted a stronger influence on the catalytic activities for the chloride pro-catalysts (compared with their bromide analogues).

Table 5 Ethylene oligomerization by nickel pro-catalyst C6–C10^a

Entry	Pro-catalyst	Activity^b	Oligomer distribution^c (%)
			α-C4	C4/∑C	C6/∑C
a Conditions: 5 μmol Ni, 200 equiv. Et2AlCl, 60 °C, 10 atm C2H4, 30 min, 100 mL of toluene. b Activity, 10^5 g mol^−1(Ni) h^−1. c Determined by GC. ∑C denotes the total amount of oligomers.
1	C6	6.89	78.1	94.1	5.9
2	C7	1.88	90.8	93.4	6.6
3	C8	11.0	73.5	90.7	9.3
4	C9	6.74	76.1	99.7	0.3
5	C10	1.24	74.9	90.0	10.0

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3. Conclusion

A series of nickel(II) halide complexes bearing 8-(1-aryliminoethylidene)quinaldines was synthesized and fully characterized by FT-IR and elemental analyses. Upon activation with Et₂AlCl, all the nickel(II) complexes showed good activity in ethylene oligomerization with enhanced thermal stability at 60 °C and high selectivity for α-C4. The better thermo-stable property provides a potential advantage for such nickel pro-catalysts for industrial considerations. Solubility and the electronic influence of the ligand substituents played an important role and pro-catalysts bearing ligands with more electron-donating features had lower activity. The addition of PPh₃ dramatically increased the catalytic activity of the system for ethylene oligomerization.

4. Experimental

4.1 General considerations

All operations of air- and moisture-sensitive compounds were manipulated under an atmosphere of nitrogen using standard Schlenk techniques. Toluene was refluxed over sodium benzophenone and distilled under argon prior to use. Methylaluminoxane (MAO, 1.46 M solution in toluene) and modified ethylaluminoxane (MMAO, 1.93 M in heptane) were purchased from Akzo Nobel Corporation. Diethylaluminium chloride (Et₂AlCl, 1.7 M in toluene) was purchased from Acros Chemicals. Other reagents were purchased from Aldrich or Acros Chemicals. The ¹H and ¹³C NMR spectra were recorded on Bruker DMX 400 MHz instruments at ambient temperature using TMS as an internal standard. FT-IR spectra were recorded on a Perkin-Elmer System 2000 FT-IR spectrometer. Elemental analyses was carried out using a Flash EA 1112 microanalyzer. GC analyses were performed with a Varian CP-3800 gas chromatograph equipped with a flame ionization detector and a 30 m (0.2 mm i.d., 0.25 μm film thickness) CP-Sil 5 CB column. The yield of oligomers was calculated by referencing with the mass of the solvent on the basis of the prerequisite that the mass of each fraction was approximately proportional to its integrated areas in the GC trace. Selectivity for the linear α-olefin was defined as (amount of linear α-olefin of all fractions)/(total amount of oligomer products) in percent.

4.2 Syntheses

(E)-2,6-dimethyl-N-(1-(2-methylquinoline-8-yl)ethylidene)benzenamine L1. A mixture of 8-acetylquinaldine (1.542 g, 0.008 mol) and 2,6-dimethylbenzenamine (0.7922 g, 0.008 mol, 1 equiv.) with excess p-toluenesulfonic acid monohydrate (0.200 g, 0.006 mol) was stirred at 140 °C for 24 h in 100 mL toluene. After the mixture was cooled to room temperature, the solvent was evaporated under reduced pressure. Following purification by column chromatography (alumina column 200 : 1, petroleum ether (60–90)-ethyl acetate), L1 (0.92 g, 41% yield) was obtained as a white powder (Found: C 83.29, H 7.01, N 9.70. C₂₀H₂₀N₂ requires C 83.30, H 6.99, N 9.71%); Mp: 60–61 °C; δ_H (400 MHz; CDCl₃; Me₄Si) 8.06 (d, 1 H, J = 8.4 Hz, quinoline⁷), 7.81 (d, 1 H, J = 8.4 Hz, quinoline⁴), 7.81 (d, 1 H, J = 8.2 Hz, quinoline⁵), 7.54 (t, 1 H, J = 7.6 Hz, quinoline⁶), 7.30 (d, 1 H, J = 8.4 Hz, quinoline³), 7.09 (d, 2 H, J = 7.5 Hz, m-Ar), 6.95 (t, 1 H, J = 6.9 Hz, p-Ar), 2.72 (s, 3 H, –CH₃), 2.27 (s, 6 H, 2 × –CH₃), 2.18 (s, 3 H, –CH₃); δ_C (100 MHz; CDCl₃; Me₄Si) 171.2 (N=C), 158.9, 153.3, 144.3, 135.6, 131.3, 130.5, 130.2, 130.1, 126.0, 127.4, 127.1, 126.0, 126.6, 122.1, 25.2, 16.0. FT-IR (KBr disc, cm⁻¹) 3042, 1645, 1611, 1594, 1567, 1498, 1465, 1438, 1357, 1280, 1253, 1183, 1094, 834, 755.

(E)-2,6-diethyl-N-(1-(2-methylquinoline-8-yl)ethylidene)benzenamine L2. Using a similar procedure as for the synthesis of L1, L2 was obtained as a white powder in 46% yield (Found: C 83.49, H 7.65, N 8.84. C₂₂H₂₄N₂ requires C 83.50, H 7.64, N 8.85%); Mp: 66–67 °C;δ_H (400 MHz; CDCl₃; Me4Si) 8.07 (d, 1 H, J = 8.4 Hz, quinoline⁷), 7.82 (d, 2 H, J = 7.4 Hz, quinoline^4,5), 7.53 (t, 1 H, J = 7.6 Hz, quinoline⁶), 7.31 (d, 1 H, J = 8.4 Hz, quinoline³), 7.29 (d, 2 H, J = 7.5 Hz, m-Ar), 7.13 (t, 1 H, J = 6.9 Hz, p-Ar), 2.71 (s, 3 H, –CH₃), 2.60–2.54 (m, 4 H, –CH₂–), 2.30 (s, 3 H, –CH₃), 1.27 (s, 6 H, 2 × –CH₃); δ_C (100 MHz; CDCl₃; Me4Si) 171.7 (N=C), 158.7, 147.9, 146.0, 141.6, 136.1, 132.4, 128.5, 127.7, 126.7, 126.2, 125.7, 123.4, 122.2, 25.72, 24.82, 23.55, 14.41. FT-IR (KBr disc, cm⁻¹) 3049, 2970, 1656, 1610, 1598, 1571, 1498, 1450, 1439, 1367, 1350, 1254, 1183, 1059, 970, 890, 832, 590.

(E)-2,6-diisopropyl-N-(1-(2-methylquinoline-8-yl)ethylidene)benzenamine L3. Using a similar procedure as for the synthesis of L1, L3 was obtained as a white powder in 56% yield (Found: C 83.49, H 8.21, N 8.10. C₂₄H₂₈N₂ requires C 83.68, H 8.19, N 8.13%); Mp: 93–94 °C; δ_H (400 MHz; CDCl₃; Me₄Si) 8.07 (d, 1 H, J = 8.4 Hz, quinoline⁷), 7.81 (d, 1 H, J = 7.4 Hz, quinoline⁴), 7.79 (d, 1 H, J = 8.4 Hz, quinoline⁵), 7.51 (t, 1 H, J = 7.6 Hz, quinoline⁶), 7.31 (d, 1 H, J = 8.4 Hz, quinoline³), 7.21 (d, 2 H, J = 7.5 Hz, m-Ar), 7.13 (t, 1 H, J = 6.9 Hz, p-Ar), 3.31–3.12 (m, 2 H, –CH), 2.71 (s, 3 H, –CH₃), 2.30 (s, 3 H, –CH₃), 1.33 (s, 6 H, 2 × –CH₃),1.21 (d, 6 H, J = 6.8 Hz, –CH₃); δ_C (100 MHz; CDCl₃; Me₄Si) 171.8 (N=C), 158.5, 146.1, 145.7, 141.5, 136.9, 135.8, 128.2, 127.2, 126.4, 125.5, 123.5, 122.9, 121.9, 27.9, 25.4, 23.5, 23.4, 23.3, 22.4. FT-IR (KBr disc, cm⁻¹) 3057, 2960, 1656, 1605, 1571 1497, 1460, 1434, 1357, 1326, 1279, 1183, 1034, 933, 793, 657.

(E)-2,4,6-trimethyl-N-(1-(2-methylquinoline-8-yl)ethylidene)benzenamine L4. Using a similar procedure as for the synthesis of L1, L4 was obtained as a white powder in 28% yield (Found: C 83.38, H 7.37, N 9.21. C₂₁H₂₂N₂ requires C 83.40, H 7.33, N 9.26%); Mp: 70–71 °C;δ_H (400 MHz; CDCl₃; Me₄Si) 8.06 (d, 1 H, J = 8.4 Hz, quinoline⁷), 7.87 (d, 1 H, J = 8.4 Hz, quinoline⁴), 7.81 (d, 1 H, J = 8.4 Hz, quinoline⁵), 7.54 (t, 1 H, J = 7.6 Hz, quinoline⁶), 7.30 (d, 1 H, J = 8.4 Hz, quinoline³), 6.91 (d, 2 H, J = 7.5 Hz, m-Ar), 2.70 (s, 3 H, –CH₃), 2.30 (s, 3 H, –CH₃), 2.26 (s, 3 H, –CH₃), 2.22 (s, 3 H, –CH₃), 2.18 (s, 3 H, –CH₃); δ_C (100 MHz; CDCl₃; Me₄Si) 171.4 (N = C), 158.9, 150.3, 144.3, 136.7, 135.6, 131.3, 130.4, 130.1, 130.2, 129.2, 126.6, 126.0, 122.1, 25.2, 24.9, 16.3, 16.0. FT-IR (KBr disc, cm⁻¹): 3057, 2960, 1656, 1605, 1571, 1497, 1460, 1434, 1357, 1326, 1279, 1183, 1034, 933, 793, 657.

(E)-2,6-diethyl-4-methyl-N-(1-(2-methylquinoline-8-yl)ethylidene)benzenamine L5. Using a similar procedure as for the synthesis of L1, L5 was obtained as a yellow powder in 49% yield (Found: C 83.49, H 7.97, N 8.45. C₂₃H₂₆N₂ requires C 83.59, H 7.93, N 8.48%); Mp: 75–76 °C; δ_H (400 MHz; CDCl₃; Me₄Si) 8.06 (d, 1 H, J = 8.4 Hz, quinoline⁷), 7.83 (d, 2H, J = 7.4 Hz, quinoline^4,5), 7.54 (t, 1 H, J = 7.6 Hz, quinoline⁶), 7.31 (d, 1 H, J = 8.4 Hz, quinoline³), 7.25 (s, 2 H, J = 7.5 Hz, m-Ar), 2.71 (s, 3 H, –CH₃), 2.60–2.54 (m, 4 H, –CH₂), 2.35 (s, 3 H, –CH₃), 2.30 (s, 3 H, –CH₃), 1.27 (s, 6 H, 2 × –CH₃); δ_C (100 MHz; CDCl₃; Me₄Si) 171.7 (N=C), 158.5, 145.8, 145.2, 141.6, 135.9, 132.3, 132.2, 128.3, 127.5, 126.8, 126,5, 125.5, 122.0, 25.54, 24.64, 23.30, 21.18, 14.36. FT-IR (KBr disc, cm⁻¹) 2962, 2928, 1635, 1600, 1567, 1498, 1431, 1257, 1204, 1186, 1147, 951, 898, 836, 767, 630, 557.

[(E)-2,6-dimethyl-N-(1-(2-methylquinoline-8-yl)ethylidene)benzenamine]dibromonickel C1. To a solution of the ligand (E)-2,6-dimethyl-N-(1-(2-methylquinoline-8-yl)ethylidene)benzenamine (0.288, 1.0 mmol) in ethanol (5 mL), (DME)NiBr₂ (0.308 g, 1.0 mmol) was added. The reaction mixture was stirred at room temperature for 12 h to afford a brown precipitate from the reaction mixture (C1, 0.491 g, 82% yield) (Found: C 47.45, H 3.97, N 5.51. C₂₀H₂₀Br₂N₂Ni requires C 47.39, H 3.98, N 5.53%). FT-IR (KBr disc, cm⁻¹) 1620, 1596, 1567, 1506, 1466, 1370, 1282, 1207, 862, 843, 794.

[(E)-2,6-diethyl-N-(1-(2-methylquinoline-8-yl)ethylidene)benzenamine]dibromonickel C2. Using the same procedure as for the synthesis of C1, C2 was obtained in 86% yield (Found: C 49.42, H 4.56, N 5.21. C₂₂H₂₄Br₂N₂Ni requires C 49.40, H 4.52, N 5.24%). FT-IR (KBr disc, cm⁻¹): 3409, 1620, 1593, 1563, 1509, 1439, 1309, 1277, 1206, 1178, 855, 774, 665.

[(E)-2,6-diisopropyl-N-(1-(2-methylquinoline-8-yl)ethylidene)benzenamine]dibromonickel C3. Using the same procedure as for the synthesis of C1, C3 was obtained in 93% yield (Found: C 50.99, H 5.10, N 4.93. C₂₄H₂₈Br₂N₂Ni requires C 51.20, H 5.01, N 4.98%). FT-IR (KBr disc, cm⁻¹): 3048, 1619, 1596, 1585, 1508, 1445, 1381, 1361, 1277, 1204, 1149, 1056, 935, 846, 806, 771.

[(E)-2,4,6-trimethyl-N-(1-(2-methylquinoline-8-yl)ethylidene)benzenamine]dibromonickel C4. Using the same procedure as for the synthesis of C1, C4 was obtained in 88% yield (Found: C 48.53, H 4.27, N 5.32. C₂₁H₂₂Br₂N₂Ni requires C 48.42, H 4.26, N 5.38%). FT-IR (KBr disc, cm⁻¹) 2970, 1618, 1593, 1563, 1509, 1431, 1371, 1276, 1206, 1150, 850, 767.

[(E)-2,6-diethyl-4-methyl-N-(1-(2-methylquinoline-8-yl)ethylidene)benzenamine]dibromonickel C5. Using the same procedure as for the synthesis of C1, C5 was obtained in 89% yield (Found: C 60.34, H 4.79, N 5.07. C₂₃H₂₆Br₂N₂Ni requires C 60.32, H 4.77, N 5.10%). FT-IR (KBr disc, cm⁻¹) 2962, 1612, 1594, 1565, 1505, 1458, 1371, 1278, 1147, 1031, 875, 798, 483.

[(E)-2,6-dimethyl-N-(1-(2-methylquinoline-8-yl)ethylidene)benzenamine]dichloronickel C6. To a solution of the ligand (E)-2,6-dimethyl-N-(1-(2-methylquinoline-8-yl)ethylidene)benzenamine (0.288 g, 1.0 mmol) in ethanol (5 mL), NiCl₂·6H₂O (0.236 g, 1 mmol) was added. The reaction mixture was stirred at room temperature for 12 h to afford a light yellow precipitate from the reaction mixture. C6 was obtained in 76% yield (Found: C 57.52, H 4.86, N 6.64. C₂₀H₂₀Cl₂N₂Ni requires C 57.47, H 4.82, N 6.70%). FT-IR (KBr disc, cm⁻¹) 3391, 1606, 1536, 1365, 1260, 1191, 1143, 848, 503.

[(E)-2,6-diethyl-N-(1-(2-methylquinoline-8-yl)ethylidene)benzenamine]dichloronickel C7. Using the same procedure as for the synthesis of C6, C7 was obtained in 75% yield (Found: C 59.14, H 5.45, N 6.21. C₂₂H₂₄Cl₂N₂Ni requires C 59.24, H 5.42, N 6.28%). FT-IR (KBr disc, cm⁻¹) 3406, 1607, 1534, 1408, 1259, 1190, 1147, 853.

[(E)-2,6-diisopropyl-N-(1-(2-methylquinoline-8-yl)ethylidene)benzenamine]dichloronickel C8. Using the same procedure as for the synthesis of C6, C8 was obtained in 87% yield (Found: C 60.89, H 5.94, N 5.89. C₂₄H₂₈Cl₂N₂Ni requires C 60.80, H 5.95, N 5.91%). FT-IR (KBr disc, cm⁻¹) 3424, 1609, 1557, 1503, 1468, 1429, 1365, 1339, 1224, 1193, 1147, 952, 844, 772, 755, 717, 480.

[(E)-2,4,6-trimethyl-N-(1-(2-methylquinoline-8-yl)ethylidene)benzenamine]dichloronickel C9. Using the same procedure as for the synthesis of C6, C9 was obtained in 79% yield (Found: C 68.35, H 5.17, N 6.42. C₂₁H₂₂Cl₂N₂Ni requires C 58.38, H 5.13, N 6.48%). FT-IR (KBr disc, cm⁻¹) 3411, 2360, 2341, 1606, 1535, 1486, 1409, 1259, 1210, 1150, 1032, 851, 768, 668.

[(E)-2,6-diethyl-4-methyl-N-(1-(2-methylquinoline-8-yl)ethylidene)benzenamine]dichloronickel C10. Using the same procedure as for the synthesis of C6, C10 was obtained in 80% yield (Found: C 60.00, H 5.71, N 6.05. C₂₃H₂₆Cl₂N₂Ni requires C 60.04, H 5.70, N 6.09%). FT-IR (KBr disc, cm⁻¹) 3397, 3075, 2968, 2359, 1604, 1532, 1457, 1425, 1258, 1222, 1191, 1143, 1025, 852, 763, 602.

4.3 General procedure for ethylene oligomerization

Ethylene oligomerization at 10 atm ethylene pressure was performed in a stainless steel autoclave (0.25 L capacity) equipped with a mechanical stirrer, a temperature controller and gas ballast through a solenoid clave for continuous feeding of ethylene at constant pressure. The catalyst precursor was dissolved in 50 mL toluene in a Schlenk tube stirred with a magnetic stirrer and injected into the reactor under an ethylene flux. With Et₂AlCl, 50 mL toluene was added. When the reaction temperature had been reached, ethylene with the desired pressure was introduced to initiate the reaction. After the reaction mixture had been stirred for the desired period of time, the reaction was stopped and about 1 mL of the reaction solution was collected, terminated by the addition of 10% aqueous hydrogen chloride. The organic layer was analyzed by gas chromatography (GC) for determining the composition and mass distribution of the oligomers obtained.

4.4 Crystal structure determinations

Diffraction data of C3 and C5 were collected on a Rigaku R-AXIS Rapid IP diffractometer with graphite monochromated Mo-Kα radiation (λ = 0.71073 Å). Cell parameters were obtained by global refinement of the positions of all collected reflections. Intensities were corrected for Lorentz and polarization effects and empirical absorption. Data were collected using phi-scans and the structures were solved by direct methods (SIR-97) using the SHELX-97 software²⁰ and the refinement was by full-matrix least squares on F². All non-hydrogen atoms were refined anisotropically. Crystal data and processing parameters for pro-catalysts C3 and C5 are summarized in Table 6.

Table 6 Crystal data and structure refinement for the pro-catalysts C3 and C5

	C3	C5
Crystal colour	Brown	Green
Empirical formula	C24H28Br2N2Ni	C23H26Br2N2Ni
FW	563.01	548.99
T/K	293(2)	293(2)
λ/Å	0.71073	0.71073
Crystal system	Monoclinic	Monoclinic
Space group	C2/c	P21/n
a/Å	29.331(6)	15.834(3)
b/Å	10.775(2)	8.6441(17)
c/Å	15.226(3)	17.665(4)
α/°	90	90
β/°	105.30(3)	114.28(3)
γ/°	90	90
V/Å^3	4641.8(16)	2203.9(8)
Z	8	4
D c/mg m^−3	1.611	1.655
μ/mm^−1	4.292	4.517
F(000)	2272	1104
Crystal size/mm	0.20 × 0.20 × 0.20	0.12 × 0.11 × 0.04
θ range/°	1.44–27.48	1.46–27.52
Limiting indices	−30 ≤ h ≤ 37	−20 ≤ h ≤ 20
	−13 ≤ k ≤ 13	−11 ≤ k ≤ 10
	−19 ≤ l ≤ 19	−22 ≤ l ≤ 22
No. of reflections collected	18 631	26 210
No. unique reflections [Rint]	5284(0.0635)	5043(0.0730)
Completeness to θ (%)	99.2%	99.3%
Absorption correction	None	None
data/restraints/parameters	5284/0/262	5043/0/253
Goodness of fit on F^2	1.062	1.368
Final R indices [I > 2σ(I)]	R 1 = 0.0755	R 1 = 0.0855
	wR2 = 0.1768	wR2 = 0.1510
R indices (all data)	R 1 = 0.0899	R 1 = 0.0954
	wR2 = 0.1857	wR2 = 0.1581
Largest diffraction peak and hole/e Å^−3	0.485 and −0.659	0.526 and −0.623

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Acknowledgements

This work is supported by MOST 863 program No. 2009AA033601. The EPSRC is thanked for the award of a travel grant (to CR).

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Footnote

† CCDC reference numbers 793554 and 793555 for crystallographic data of complexes C3 and C5. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c0cy00002g

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