Synthesis, characterization and ethylene polymerization behavior of nickel dihalide complexes bearing bulky unsymmetrical α-diimine ligands

Abstract

A series of nickel(II) dihalide complexes (C1–C10) bearing unsymmetrical α-diimine ligands of the type 2,4-dibenzhydryl-N-(2-phenyliminoacenaphthylenylidene)-6-methylbenzenamine (L1–L5) were synthesized and fully characterized. Single-crystal X-ray diffraction revealed a distorted tetrahedral geometry around the nickel center in the complexes C3, C5 and C9. Upon activation with modified methylaluminoxane (MMAO), all nickel pro-catalysts performed with high activities in ethylene polymerization, producing highly branched polyethylene products.

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1. Introduction

The use of late-transition metal complexes as catalysts for ethylene oligomerization and polymerization has undergone remarkable progress in the past two decades, especially since the milestone of the discovery of α-diimine nickel and palladium pro-catalysts by the Brookhart group.¹ Significant influence on the catalytic activities of the metal complexes was achievable by the variation of the nature of the ancillary ligands present,² and as a consequence the development of new pro-catalyst systems has relied heavily on the design of multi-dentate ligands such as those binding as N⁁N,³P⁁N,⁴O⁁N,⁵O⁁N⁁N,⁶N⁁P⁁N,⁷P⁁N⁁P,⁸P⁁O⁁P,⁹P⁁N⁁N¹⁰ and N⁁N⁁Nchelates.¹¹ In general, α-diimine derivatives have been accessed via modification of the ligand frameworks ¹² and in particular, via variation of the substituents on the arylimino groups.^12,13 A unique feature of the resultant polyethylene obtained by using such nickel pro-catalysts is the high degree of branching in the resultant PEs,^1a,13f,14 which is due to “chain-walking” on the active nickel species.^14,15 Significant influences were observed through varying the substituents on the arylimino group by a number of researchers,¹⁶ and we have also noted that modification of a series of α-diimine nickel dihalides derived from the precursor 2,6-dibenzhydryl-6-methylaniline provided not only highly active pro-catalysts, but also highly branched PEs.^13f However, the use of bulky anilines with benzhydryl substituents has not fully been explored. The limited studies to date have included the use of 2,6-dibenzhydryl-6-methylaniline as a precursor for preparing iron pro-catalysts,¹⁷ and, as mentioned previously, nickel pro-catalysts ligated by α-diimine ligands derived from 2,6-dibenzhydryl-6-methylaniline.^13f There is thus scope for the fine-tuning of α-diimine ligands containing dibenzhydryl substituents with a view to further exploitation in nickel-based ethylene polymerization systems. Herein, the title nickel complexes are synthesized, characterized, and their performance in ethylene polymerization investigated.

2. Results and discussion

2.1. Synthesis and characterization

According to our reported procedure,^13f the stoichiometric condensation of acenaphthylene-1,2-dione with 2,4-dibenzhydryl-6-methylaniline produced the 2-(2,4-dibenzhydryl-6-methylphenylimino)acenaphthylenone, which was further reacted with a second aniline to afford the 2,4-dibenzhydryl-N-(2-phenyliminoacenaphthylenylidene)-6-methylbenzenamine ligands (L1–L5) (Scheme 1). All the newly synthesized organic compounds were characterized by FT-IR and NMR spectroscopy, and the formulae were consistent with their elemental analyses.

Scheme 1 Synthesis of ligands (L1–L5) and nickel complexes (C1–C10).

All 2,4-dibenzhydryl-N-(2-phenyliminoacenaphthylenylidene)-6-methylbenzenamine ligands (L1–L5) acted as α-diimine type ligands and readily reacted with an equivalent of (DME)NiBr₂ in dichloromethane to afford the corresponding nickel dibromide complexes (C1–C5), whilst on reaction with NiCl₂·6H₂O, the α-diimine ligands (L1–L5) produced the corresponding nickel chlorides (C6–C10) (see Scheme 1).

2.2. Crystal structures

The molecular structures are shown in Fig. 1–3, with the selected bond lengths and angles tabulated in Table 1. As shown in Fig. 1, the coordination geometry of C3 can be best described as a distorted tetrahedron, with the three atoms N1, N2 and Br1 forming the basal plane, and the Br2 atom occupying the apical position. The nickel atom is 0.818 Å out of the basal plane. Due to the bulky asymmetric nature of the ligand set, the plane composed of N1, N2 and Ni1 and the plane composed of Br1, Br2 and Ni1 form a dihedral angle of 82.26°. The dihedral angles between the aryl ring on N1 and the plane comprising N1, N2 and Ni1 is 86.54°, but the aryl ring on N2 and the plane composed of N1, N2 and Ni1 is 82.36°. There is a fused five-membered ring comprising the nickel and the ligand with an acute angle N1–Ni1–N2 of 83.14(15)°, and associated bond distances of Ni1–N1 2.028(4) Å and Ni1–N2 2.037(4) Å. The imino bond length N1–C12 is 1.285(6) Å, whereas N2–C1 is slightly longer at 1.295(6) Å.

Fig. 1 ORTEP structure of C3. Thermal ellipsoids are shown at 30% probability level. Hydrogen atoms have been omitted for clarity.

Fig. 2 ORTEP structure of C5. Thermal ellipsoids are shown at 30% probability level. Hydrogen atoms have been omitted for clarity.

Fig. 3 ORTEP structure of C9. Thermal ellipsoids are shown at 30% probability level. Hydrogen atoms have been omitted for clarity.

Table 1 Selected bond lengths and angles for C3, C5 and C9

	C3	C5	C9
Bond lengths (Å)
Ni(1)–N(1)	2.028(4)	2.018(4)	2.018(5)
Ni(1)–N(2)	2.037(4)	2.037(4)	2.046(5)
Ni(1)–Br(1)	2.3188(10)	2.3232(9)	2.205(2)
Ni(1)–Br(2)	2.3398(10)	2.3265(11)	2.204(2)
N(1)–C(12)	1.285(6)	1.287(6)	1.308(7)
N(1)–C(46)	1.442(5	1.440(6)	1.459(7)
N(2)–C(1)	1.295(6)	1.283(6)	1.284(7)
N(2)–C(13)	1.435(5)	1.444(5)	1.442(7)
Bond angles (°)
N(2)–Ni(1)–N(1)	83.14(15)	82.50(15)	82.34(19)
N(2)–Ni(1)–Br(1)	114.60(12)	124.26(10)	120.88(15)
N(1)–Ni(1)–Br(1)	118.56(11)	109.68(11)	109.14(15)
N(2)–Ni(1)–Br(2)	107.48(12)	102.21(10)	101.48(15)
N(1)–Ni(1)–Br(2)	102.40(11)	112.71(11)	110.63(15)
Br(1)–Ni(1)–Br(2)	123.15(4)	119.70(4)	124.49(8)
C(12)–N(1)–C(46)	119.1(4)	119.4(4)	120.5(5)
C(1)–N(2)–C(13)	121.3(4)	119.7(4)	119.9(5)

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In the structure of C5, the nickel atom is coordinated by an N-(2-(2,4-dibenzhydryl-6-methylphenylimino)acenaphthylenylidene)-2,6-diethyl-4-methylbenzenamine ligand, and two terminal bromides. Similarly, the geometry of the four-coordinate complex can be described as a distorted tetrahedron, with the plane comprising the three atoms N1, N2 and Br2 as the basal plane and the Br1 atom occupying the apical position. The plane composed of N1, N2 and Ni1 and the plane composed of Br1, Br2 and Ni1 form a dihedral angle of 80.23°, slightly smaller than that observed for C3. Interestingly, the sterically bulky group [CH(Ph)₂] is positioned above the square planar metal center, whereas for C3 the situation is reversed, and so these metal complexes can as being like a pair of ‘stereoisomers’.

As shown in Fig. 3, the structure of C9 more closely resembles that of C3. The plane composed of N1, N2 and Ni1 and the plane composed of Cl1, Cl2 and Ni1 form a dihedral angle of 81.87°. The dihedral angles between the aryl ring on N1 and the plane composed of N1, N2 and Ni1 is 83.46°, whereas the aryl ring on N2 and the plane composed of N1, N2 and Ni1 is 79.07°, both slightly smaller than observed in C3. There is a fused five-membered ring comprising nickel and the ligand with an acute angle N1–Ni1–N2 at 82.34(19)°. The imino bond lengths N1–C12, N2–C1 are 1.308(7) Å and 1.284(7) Å, both typical of C=N double bond character.

2.3. Ethylene polymerization

The current focus of this study is the catalytic behavior of nickel complexes ligated by 2,4-dibenzhydryl-N-(2-phenyliminoacenaphthylenylidene)-6-methylbenzenamine ligands. The two groups of nickel(II) bromide and chloride complexes were investigated separately, and evaluated to determine the optimum conditions for catalysis. Initially, the pro-catalyst C5 was employed with various alkylaluminum reagents as co-catalyst/activator at room temperature under 10 atm of ethylene. Treatment with either ethylaluminum sesquichloride (Et₃Al₂Cl₃, EASC) or diethylaluminum chloride (Et₂AlCl) afforded inactive systems (Table 2). However, in the presence of modified methylaluminoxane (MMAO), the pro-catalyst C5 exhibited outstanding catalytic activity, and as a consequence, modified methylaluminoxane was selected as the preferred co-catalyst for further investigation.

Table 2 Ethylene polymerization by pro-catalyst C5 with various co-catalysts^a

Entry	Catalyst	Co-catalyst	Al/Ni^b	Activity^c	M w ^d ^, ^e	M w/Mn^d	T m ^f (°C)
a Reaction conditions: 1.5 μmol; 20 °C; 30 min; 10 atm ethylene; 100 mL toluene. b Molar ratio of Al/Ni. c 10^3 kg mol^−1(Ni) h^−1. d Determined by GPC. e 10^4 g mol^−1. f Determined by DSC.
1	C5	MMAO	1000	7.67	3.46	2.75	118.4
2	C5	Et2AlCl	200	Trace
3	C5	EASC	200	Trace

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2.3.1. Ethylene polymerization with C1–C5/MMAO system. The catalytic behavior of complex C5 was evaluated for optimum conditions in the presence of MMAO; reaction parameters such as molar ratios of MMAO to nickel and the reaction temperature are collected in Table 3. From the data, it was observed that the catalytic activity was enhanced on increasing the Al/Ni ratio from 1000 up to 3000 (entries 1–3 in Table 3), and the molecular weights of resultant polyethylenes also increased with the Al/Ni ratio, whereas the molecular weight distributions of the resultant polyethylenes became narrower. On further increasing the Al/Ni ratio up to 4000, both the activity and the polyethylene molecular weight decreased. The temperature had a great influence both on the catalytic activity and the molecular weight. As for the former α-diimine-based nickel catalytic systems, our novel bulky unsymmetrical α-diimine nickel pro-catalysts also performed better at low temperature. When elevating the temperature from 20 °C to 60 °C, a marked decrease in activity as well as in the molecular weight was observed. Consequently, for the C5/MMAO catalyst system, the highest activity and molecular weights were obtained at the Al/Ni ratio of 3000 and at 20 °C (entry 3 in Table 3). The other pro-catalysts were also found to have good activity for ethylene polymerization and the data is summarized in Table 3. The polymerization activity increased in the order of C1 [2,6-di(Me)] < C2 [2,6-di(Et)] < C3 [2,6-di(i-Pr)], C4 [2,4,6-tri(Me)] < C5 [2,6-di(Et)-4-Me], C1 [2,6-di(Me)] < C4 [2,4,6-tri(Me)], C2 [2,6-di(Et)] < C5 [2,6-di(Et)-4-Me], indicating slightly better activities for nickel complexes containing ligands bearing longer alkyl substituents or more methyl groups, which presumably provided better solubility, resulting in enhanced activity.

Table 3 Ethylene polymerization with C1–C5/MMAO^a

Entry	Catalyst	Al/Ni	T ^b (°C)	Yield (g)	Activity^c	M w ^d ^, ^e	M w/Mn^d	T m ^f (°C)
a Reaction conditions: MMAO; 1.5 μmol; 30 min; 10 atm ethylene; 100 mL toluene. b Reaction temperature. c 10^3 kg mol^−1(Ni) h^−1. d Determined by GPC. e 10^4 g mol^−1. f Determined by DSC.
1	C5	1000	20	5.75	7.67	3.46	2.75	118.4
2	C5	2000	20	5.97	7.96	4.97	2.42	117.3
3	C5	3000	20	6.19	8.25	6.63	1.97	126.3
4	C5	4000	20	5.39	7.19	5.46	2.01	122.6
5	C5	3000	40	5.17	6.89	3.50	2.67	94.6
6	C5	3000	60	4.60	6.12	1.92	2.78	70.2
7	C5	3000	80	2.44	3.25	1.62	2.67	53.8
8	C1	3000	20	3.16	4.21	5.07	2.47	127.6
9	C2	3000	20	5.58	7.44	5.10	2.11	121.9
10	C3	3000	20	6.71	8.95	6.03	1.91	124.3
11	C4	3000	20	5.79	7.72	4.51	2.70	122.3

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As for the other α-diimine catalytic systems, our new unsymmetrical bulky α-diimine catalyst yielded polyethylene with a high degree of branching, especially at higher temperature. As shown in Fig. 4, the number of branches was calculated according to the literature,¹⁸ and it was found that polyethylene with 166 branches/1000 carbons was obtained at 80 °C (entry 7 in Table 3), which was much lower than reported previously for polyethylenes obtained using analogous catalysts based on a series of unsymmetrical ligands using 2,6-dibenzhydryl-4-methylanilines.^13f Meanwhile, polyethylene with 14 branches/1000 carbons was obtained at 20 °C (entry 3 in Table 3), consistent with the observation of obtaining higher branched polyethylenes on increasing the reaction temperature.^1a Moreover, ¹³C NMR data showed that almost all of the branches present were methyl branches (Fig. 5). Indeed, a higher order of branches in the polyethylenes was achieved at the elevated reaction temperature (Fig. 4).

Fig. 4 ¹³C NMR spectrum of polyethylene prepared at 80 °C (entry 7 in Table 3).

Fig. 5 ¹³C NMR spectrum of polyethylene prepared at 20 °C (entry 3 in Table 3).

2.3.2. Ethylene polymerization with C6–C10/MMAO systems. The nickel(II) chloride pro-catalysts (C6–C10) also showed good catalytic activities towards ethylene polymerization upon treatment with MMAO, and the data is tabulated in Table 4. From Table 4, activities slightly increased when the Al/Ni molar ratio was elevated from 1000 to 2000 (entries 1–2 in Table 4). On further increasing the Al/Ni molar ratio to 4000, an obvious decrease in the activity was observed. As a consequence, the optimum conditions employed used an Al/Ni molar ratio of 2000. Further elevating the temperature from 20 °C to 60 °C led to a marked decrease of activity (entries 2 and 5–7 in Table 4) and lower molecular weights of the resultant polyethylenes. Employing the conditions with an Al/Ni molar ratio of 2000 and at 20 °C, all the other nickel chloride pro-catalysts were screened; results are tabulated in Table 4. Catalytic performances by these chlorides were similar to their nickel(II) bromide analogs (Table 4). The polymerization activity increased in the order of C6 [2,6-di(Me)] < C7 [2,6-di(Et)] < C8 [2,6-di(i-Pr)], C9 [2,4,6-tri(Me)] < C10 [2,6-di(Et)-4-Me], C6 [2,6-di(Me)] < C9 [2,4,6-tri(Me)], C7 [2,6-di(Et)] < C10 [2,6-di(Et)-4-Me].

Table 4 Ethylene polymerization with C6–C10/MMAO^a

Entry	Catalyst	Al/Ni	T (°C)	t ^b (min)	Yield (g)	Activity^c	M w ^d ^, ^e	M w/Mn^d	T m ^f (°C)
a Reaction conditions: MMAO; 1.5 μmol; 30 min; 10 atm ethylene; 100 mL toluene. b Reaction time. c 10^3 kg mol^−1(Ni) h^−1. d Determined by GPC. e 10^4 g mol^−1. f Determined by DSC.
1	C9	1000	20	30	5.61	7.48	3.55	2.85	120.6
2	C9	2000	20	30	6.51	8.68	3.76	2.75	124.3
3	C9	3000	20	30	6.35	8.47	4.94	2.41	119.6
4	C9	4000	20	30	5.15	6.87	4.28	2.58	117.3
5	C9	2000	40	30	5.67	7.56	1.88	3.37	94.7
6	C9	2000	60	30	4.15	5.53	1.31	2.58	93.2
7	C9	2000	80	30	2.66	3.55	0.81	2.76	89.9
8	C9	2000	20	5	1.83	14.6	4.70	2.31	118.4
9	C9	2000	20	10	4.38	17.5	3.73	3.46	117.1
10	C9	2000	20	60	8.72	5.81	4.20	2.85	116.5
11	C6	2000	20	30	5.75	7.67	4.54	2.92	119.8
12	C7	2000	20	30	6.58	8.77	5.38	2.14	118.8
13	C8	2000	20	30	7.04	9.39	4.97	2.17	116.7
14	C10	2000	20	30	7.12	9.49	5.20	2.20	117.4

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Regarding the lifetime of the catalytic system, polymerizations using the C9/MMAO system were conducted over different time periods, namely 5, 10, 30 and 60 min (entries 2 and 8–10 in Table 4). On prolonging the reaction time from 5 to 30 min, the production of polyethylene followed an exponential increase, whereas on further prolonged reaction time, a distinct decrease of output was observed, suggesting that the active species suffered from severe deactivation at reaction times of over 30 min.

3. Conclusion

The synthesis and characterization of a new class of nickel-based ethylene polymerization pro-catalysts ligated by new bulky unsymmetrical α-diimines is described. Despite there been many reported α-diimine systems in the literature, the unsymmetrical α-diimine family presented herein represents a new avenue of research for α-diimine catalytic systems. On treatment with the co-catalyst MMAO, these complexes afforded good catalytic activity, and the polyethylenes obtained exhibited a high degree of branching; the degree of branching varied at different temperatures. The use of the ligand systems described herein with other metals will be reported separately.

4. Experimental section

4.1. General considerations

All manipulations of air and/or moisture sensitive compounds were carried out under a nitrogen atmosphere using standard Schlenk techniques. Toluene was refluxed over sodium–benzophenone and distilled under nitrogen prior to use. Modified methylaluminoxane (MMAO, 1.93 M in heptane, 3A) was purchased from Akzo Nobel Corp. Diethylaluminum chloride (Et₂AlCl, 0.79 M in toluene), and ethylaluminum sesquichloride (EASC, 0.87 M in toluene) were purchased from Acros Chemicals. High-purity ethylene was purchased from Beijing Yansan Petrochemical Co. and used as received. Other reagents were purchased from Aldrich, Acros, or local suppliers. NMR spectra were recorded on a Bruker DMX 400 MHz instrument at ambient temperature using TMS as an internal standard. IR spectra were recorded on a Perkin-Elmer System 2000 FT-IR spectrometer. Elemental analysis was carried out using a Flash EA 1112 microanalyzer. Molecular weights and molecular weight distributions (MWD) of polyethylene were determined by a PL-GPC 220 at 150 °C, with 1,2,4-trichlorobenzene as the solvent. DSC trace and melting points of polyethylene were obtained from the second scanning run on a Perkin-Elmer DSC-7 at a heating rate of 10 °C min⁻¹.

4.2. Synthesis and characterization

4.2.1. Preparation of the organic compounds and nickel complexes.
2-(2,4-dibenzhydryl-6-methylphenylimino)acenaphthylenone . A mixture of 2,4-diphenylmethyl-6-methylaniline (1.30 g, 2.96 mmol), acenaphthylene-1,2-dione (0.53 g, 2.91 mmol) and a catalytic amount of p-toluenesulfonic acid in toluene (80 mL) was refluxed for 3 h. After solvent evaporation at reduced pressure, the crude product purified by column chromatography on silica with the eluent of petroleum ether/dichloromethane (V : V = 10 : 1) to afford 0.84 g of the red product in 47% isolated yield. Mp: 98–99 °C. IR (KBr; cm⁻¹): 3056.2(w), 3023.6(w), 1728.3(s), 1651.9(m), 1596.5(m), 1492.3(m), 1443.3(m), 1274.2(m), 1223.5(m), 1073.6(m), 1025.9(s), 909.5(m), 829.1(m), 805.0(s), 740.9(s), 696.3(vs). Anal. Calcd. for C₄₅H₃₃NO (603.75): C, 89.52; H, 5.51; N, 2.32; Found; C, 89.77; H, 5.61; N, 2.04. ¹H NMR (400 MHz, CDCl₃, TMS): δ 8.06 (t, J = 6.81, 2H), 7.85 (d, J = 8.34, 1H), 7.74 (t, J = 7.54, 1H), 7.32–7.17 (m, 7H), 7.13–7.07 (m, 7H), 6.95 (d, J = 7.42, 2H), 6.91 (s, 1H), 6.77 (d, J = 7.63, 2H), 6.68 (s, 1H), 6.41 (t, J = 6.36, 3H), 6.19 (t, J = 7.39, 1H), 5.54 (s, 1H), 5.50 (s, 1H), 1.98 (s, 3H). ¹³C NMR (100 MHz, CDCl₃, TMS): δ 189.84, 161.59, 146.75, 144.28, 142.95, 142.69, 141.74, 139.68, 132.95, 131.94, 130.51, 130.44, 129.82, 129.49, 129.42, 129.09, 128.96, 128.31, 128.02, 127.54, 126.31, 126.08, 125.14, 124.59, 122.93, 121.77, 56.42, 52.57, 17.82.
Synthesis of 2,4-dibenzhydryl-N-(2-phenyliminoacenaphthylenylidene)-6-methylbenzenamine (L1–L5).
2,4-dibenzhydryl-N-(2-(2,6-dimethylphenylimino)acenaphthylenylidene)-6-methylbenzenamine (L1). A solution of 2-(2,4-dibenzhydryl-6-methylphenylimino)acenaphthylenone (0.68 g, 1.13 mmol), 2,6-dimethylaniline (0.16 g, 1.32 mmol) and a catalytic amount of p-toluenesulfonic acid in toluene (50 mL) was mixed and refluxed for 8 h. The solution was evaporated at reduced pressure. The residual solids was further purified by silica column chromatography (V petroleum ether : V ethyl acetate = 15 : 1) to afford L1 0.22 g (yellow, 28% yield). Mp: 147–148 °C. IR (KBr; cm⁻¹): 3023.8(w), 2956.7(w), 1667.7(m), 1644.0(m), 1594.2 (m), 1492.6(m), 1442.1(m), 1277.6(w), 1232.0(m), 1203.7(m), 1077.2(m), 1034.4(m), 924.0(m), 830.9(m), 740.7(s), 696.4(vs). Anal. Calcd. for C₅₃H₄₂N₂ (706.91): C, 90.05; H, 5.99; N, 3.96. Found; C, 90.33; H, 6.12; N, 3.55. ¹H NMR (400 MHz, CDCl₃, TMS): δ 7.78 (d, J = 8.27, 1H), 7.673 (d, J = 8.24, 1H), 7.32–7.27 (m, 5H), 7.24–7.18(m, 3H), 7.16–7.06(m, 10H), 6.99 (d, J = 7.12, 2H), 6.90 (d, J = 5.14, 2H, 6.88 (s, 1H), 6.70 (s, 1H), 6.58 (d, J = 7.05, 1H, 6.45 (t, J = 7.41, 2H , 6.38 (d, J = 7.06, 1H), 6.24 (t, J = 7.19, 1H), 5.73 (s, 1H), 5.51 (s, 1H), 2.28 (s, 3H), 2.06 (s, 3H), 2.04 (s, 3H). ¹³C NMR (100 MHz, CDCl₃, TMS): δ 162.58, 161.29, 149.34, 147.51, 144.51, 143.46, 141.83, 140.32, 139.01, 133.28, 130.45, 129.55, 129.48, 129.10, 128.83, 128.54, 128.45, 128.31, 127.99, 127.92, 127.48, 126.26, 126.00, 125.19, 125.07, 124.94, 124.83, 123.79, 123.07, 122.10, 56.45, 52.26, 18.30, 17.86, 17.77.
2,4-dibenzhydryl-N-(2-(2,6-diethylphenylimino)acenaph-thylenylidene)-6-methylbenzenamine (L2). Using the same procedure as for the synthesis of L1, L2 was obtained as a yellow powder in 32.8% (0.38 g) yield. Mp: 224–225 °C. IR (KBr; cm⁻¹): 3023.0(w), 2964.0(w), 1675.6(m), 1650.0(m), 1596.6(m), 1492.5(s), 1438.7(s), 1278.6(w), 1235.1(w), 1192.4(m), 1079.3(m), 1031.8(m), 924.0(m), 832.9(m), 740.8(s), 697.1(vs). Anal. Calcd. for C₅₅H₄₆N₂ (734.97): C, 89.88; H, 6.31; N, 3.81. Found; C, 90.21; H, 6.55; N, 3.59. ¹H NMR (400 MHz, CDCl₃, TMS): δ 7.76 (d, J = 8.24, 1H), 7.71 (d, J = 8.23, 1H), 7.36–7.28 (m, 5H), 7.27–7.22 (m, 3H), 7.20–7.10 (m, 10H), 6.99 (d, J = 7.20, 2H), 6.90 (d, 3H), 6.72 (s, 1H), 6.57 (d, J = 7.06, 1H), 6.47 (t, J = 7.32, 2H), 6.35 (d, J = 7.07, 1H), 6.25 (t, J = 7.23, 1H), 5.75 (s, 1H), 5.52 (s, 1H), 2.73 (m, 1H), 2.71–2.49 (m, 2H), 2.35 (m, 1H), 2.04 (s, 3H), 1.25 (t, J = 7.28, 3H), 1.05 (t, J = 7.44, 3H). ¹³C NMR (100 MHz, CDCl₃, TMS): δ 162.55, 161.33, 148.56, 147.60, 144.49, 143.58, 141.85, 140.36, 139.02, 133.31, 131.00, 130.70, 130.45, 129.98, 129.56, 129.49, 129.31, 129.26, 129.13, 128.74, 128.52, 128.29, 127.98, 127.75, 127.54, 126.51, 126.40, 126.26, 125.99, 125.24, 124.92, 124.13, 123.07, 122.59, 56.47, 55.58, 24.89, 24.65, 17.73, 14.51, 13.83.
2,4-dibenzhydryl-N-(2-(2,6-diisopropylphenylimino)acenaphthylenylidene)-6-methylbenzenamine (L3). Using the same procedure as for the synthesis of L1, L3 was obtained as a yellow powder in 27% (0.34 g) yield. Mp: 243–244 °C. IR (KBr; cm⁻¹): 3022.7(w), 2956.0(m), 1678.7(m), 1653.4(m), 1596.7(m), 1492.8(m), 1439.0 (m), 1277.67(w), 1248.1(m), 1187.5(w), 1078.7(w), 1032.1(m), 925.6(m), 833.1(m), 741.3(s), 6967.7(vs). Anal. Calcd. for C₅₇H₅₀N₂ (763.02): C, 89.72; H, 6.60; N, 3.67. Found; C, 89.87; H, 6.96; N, 3.30. ¹H NMR (400 MHz, CDCl₃, TMS): δ 7.75 (d, J = 8.25, 1H), 7.70 (d, J = 8.23, 1H), 7.35–7.19 (m, 10H), 7.16–7.05 (m, 8H), 7.00 (d, J = 7.31, 2H), 6.92 (s, 1H), 6.89 (d, J = 7.52, 2H), 6.71 (s, 1H), 6.52 (d, J = 7.12, 1H), 6.46 (t, J = 7.44, 2H), 6.32 (d, J = 7.16, 1H), 6.24 (t, J = 7.31, 1H), 5.75 (s, 1H), 5.52 (s, 1H), 2.21 (m, 1H), 2.91 (m, 1H), 2.04 (s, 3H), 1.33 (d, J = 6.72, 3H), 1.20 (d, J = 6.76, 3H), 1.13 (d, J = 6.80, 3H), 0.86 (d, J = 6.76, 3H). ¹³C NMR (100 MHz, CDCl₃, TMS): δ 162.62, 161.59, 147.63, 147.33, 144.52, 143.60, 141.79, 140.43, 139.00, 135.72, 135.54, 133.34, 130.45, 129.97, 129.56, 129.49, 129.11, 128.75, 128.51, 128.29, 127.97, 127,56, 127.50, 126.26, 125.98, 125.26, 124.92, 124.48, 123.70, 123.46, 123.08, 123.01, 56.46, 52.55, 28.70, 28.62, 23.91, 23.69, 23.50, 23.20, 17.70.
2,4-dibenzhydryl-N-(2-(2,4,6-trimethylphenylimino)acenaphthylenylidene)-6-methylbenzenamine (L4). Using the same procedure as for the synthesis of L1, L4 was obtained as a yellow powder in 32% (0.39 g) yield. Mp: 164–165 °C. IR (KBr; cm⁻¹): 3023.5(w), 2967.6(w), 1667.4(m), 1640.8(m), 1596.7(m), 1493.2(m), 1441.8(m), 1278.4(w), 1232.7(m), 1207.3(mw), 1075.8(m), 1032.2(m), 922.4(m), 829.6(m), 738.4(s), 696.5(vs). Anal. Calcd. for C₅₄H₄₄N₂ (720.94): C, 89.96; H, 6.15; N, 3.89. Found; C, 90.11; H, 6.54; N, 3.62. ¹H NMR (400 MHz, CDCl₃, TMS): δ 7.77 (d, J = 8.25, 1H), 7.71 (d, J = 8.24, 1H), 7.32–7.26 (m, 4H), 7.24–7.19 (m, 3H), 7.17–7.07 (m, 8H), 6.99 (d, J = 6.69, 3H), 6.95 (s, 1H), 6.89 (d, J = 8.46, 3H), 6.70 (s, 1H), 6.65 (d, J = 7.10, 1H), 6.45 (t, J = 7.41, 2H), 6.37 (d, J = 7.14, 1H), 6.23 (t, J = 7.28, 1H), 5.73 (s, 1H), 5.51 (s, 1H), 2.38 (s, 3H), 2.24 (s, 3H), 2.04 (s, 6H). ¹³C NMR (100 MHz, CDCl₃, TMS): δ 162.64, 161.46, 147.58, 146.85, 144.54, 143.50, 141.84, 140.31, 138.96, 133.30, 132.99, 130.46, 130.04, 129.56, 129.49, 129.33, 129.16, 129.10, 129.03, 128.71, 128.51, 128.30, 127.98, 127.91, 127.46, 126.26, 125.98, 125.18, 124.97, 124.84, 124.60, 123.02, 122.13, 56.46, 52.62, 21.05, 18.24, 17.77.
2,4-dibenzhydryl-N-(2-(2,6-diethyl-4-methylphenylimino)-acenaphthylenylidene)-6-methylbenzenamine (L5). Using the same procedure as for the synthesis of L1, L5 was obtained as a yellow powder in 39.5% (0.49 g) yield. Mp: 192–193 °C. IR (KBr; cm⁻¹): 3022.7(w), 2967.5(w), 1674.4(m), 1649.3(m), 1597.4(m), 1492.5(m), 1442.3(m), 1277.9(w), 1233.1(w), 1205.1(w), 1076.3(m), 1030.4(m), 922.8(m), 833.3(m), 745.4(s), 697.6(vs). Anal. Calcd. for C₅₆H₄₈N₂ (748.99): C, 89.80; H, 6.46; N, 3.74. Found; C, 89.67; H, 6.33; N, 3.69. ¹H NMR (400 MHz, CDCl₃, TMS): δ 7.75 (d, J = 8.17, 1H), 7.70 (d, J = 8.16, 1H), 7.30–7.26 (m, 5H), 7.24–7.20 (m, 2H), 7.18–7.09 (m, 8H), 7.04 (s, 1H), 7.00 (d, J = 6.43, 3H), 6.90 (d, J = 8.81, 3H), 6.71 (s, 1H), 6.63 (d, J = 6.99, 1H), 6.46 (t, J = 7.20, 2H), 6.34 (d, J = 7.01, 1H), 6.25 (t, J = 7.07, 1H), 5.75 (s, 1H), 5.51 (s, 1H), 2.70 (m, 1H), 2.58–2.45 (m, 2H), 2.42 (s, 3H), 2.32 (m, 1H), 2.03 (s, 3H), 1.23 (t, J = 7.40, 3H), 1.03 (t, J = 7.36, 3H). ¹³C NMR (100 MHz, CDCl₃, TMS): δ 162.67, 161.52, 147.67, 146.05, 144.54, 143.61, 141.84, 140.34, 138.96, 133.34, 133.28, 130.84, 130.54, 130.44, 130.00, 129.58, 129.50, 129.35, 129.11, 128.63, 128.49, 128.31, 127.98, 127.74, 127.54, 127.44, 127.31, 127.18, 126.27, 125.98, 125.23, 124.95, 123.02, 122.62, 56.48, 52.57, 24.90, 24.66, 21.34, 17.74, 14.70, 13.96.

4.2.2. Synthesis of tridentate nickel complexes C1–C10. The complexes C1–C5 were synthesized by the reaction of (DME)NiBr₂ with the corresponding ligands in dichloromethane. A typical synthetic procedure for C1 is described as the following: the ligand L1 (0.101 g, 0.143 mmol) and (DME)NiBr₂ (0.048 g, 0.155 mmol) were added to a Schlenk tube together with 10 ml of dried dichloromethane. The reaction mixture was then stirred for 8 h at room temperature and absolute diethyl ether (10 ml) was added to precipitate the complex. The precipitate was washed with diethyl ether and dried in vacuo to obtain a red powder in 81.1% (0.109 g) yield. IR (KBr; cm⁻¹): 3026.0(w), 2974.7(w), 1643.6(w), 1598.6(m), 1577.5(m), 1491.0 (m), 1447.6(m), 1293.4(m), 1191.6(m), 1032.4(m), 826.6(m), 776.7(s), 744.1(s), 700.9(vs). Anal. Calcd. for C₅₃H₄₂Br₂N₂Ni (925.42): C, 68.79; H, 4.57; N, 3.03. Found; C, 68.55; H, 4.21; N, 3.27.

Data for C2 Yield: 83.7%, red powder. IR (KBr; cm⁻¹): 3023.9(w), 2969.5(w), 1646.0(w), 1620.2(m), 1580.2(m), 1492.7(m), 1444.3(m), 1294.6(m), 1184.5(w), 1030.9(w), 825.0(m), 774.9(s), 745.9(s), 699.5(vs). Anal. Calcd. for C₅₅H₄₆Br₂N₂Ni (953.47): C, 69.28; H, 4.86; N, 2.94. Found; C, 69.46; H, 4.93; N, 2.58.

Data for C3 Yield: 85.8%, red powder. IR (KBr; cm⁻¹): 3023.0(w), 2963.9(w), 1651.0(w), 1622.4(m), 1581.3(s), 1493.7(s), 1442.0(s), 1293.2(m), 1181.0(m), 1030.7(m), 831.9(m), 776.2(s), 741.8(s), 696.5(vs). Anal. Calcd. for C₅₇H₅₀Br₂N₂Ni (981.52): C, 69.75; H, 5.13; N, 2.85. Found; C, 69.66; H, 5.21; N, 2.60.

Data for C4 Yield: 77.9%, red powder. IR (KBr; cm⁻¹): 3023.3(w), 2968.6(w), 1644.7(w), 1619.7(m), 1579.4(s), 1492.8(s), 1446.4(s), 1294.1(m), 1200.9(m), 1030.7(m), 827.7(m), 773.5(s), 743.8(s), 699.8(vs). Anal. Calcd. for C₅₄H₄₄Br₂N₂Ni (939.44): C, 69.04; H, 4.72; N, 2.98. Found; C, 69.33; H, 4.87; N, 2.75.

Data for C5 Yield: 79.5%, red powder. IR (KBr; cm⁻¹): 3024.1(w), 2968.1(w), 1644.7(m), 1620.2(m), 1580.0(s), 1492.6(s), 1451.4(s), 1293.4(m), 1200.5(m), 1030.9(m), 827.6(m), 774.7(s), 744.0(s), 699.0(vs). Anal. Calcd. for C₅₆H₄₈Br₂N₂Ni (967.5): C, 69.52; H, 5.00; N, 2.90. Found; C, 69.62; H, 5.20; N, 2.67.

The chloride complexes C6–C10 were synthesized by the reaction of NiCl₂·6H₂O with the corresponding ligands in dichloromethane. As a typical synthetic procedure complex C6 is described as follows: the solution of 0.080 g (0.113 mmol) ligand L1 and 0.03 g (0.126 mmol) of NiCl₂·6H₂O in 10 mL dichloromethane was stirred for 8 h at room temperature. The precipitate was washed with diethyl ether and dried in vacuo to obtain a brown solid in 0.078 g (82.8%) yield. IR (KBr; cm⁻¹): 3024.6(w), 2964.8(w), 1657.6(w), 1626.5(m), 1578.2(s), 1493.4(s), 1444.0(m), 1289.9(m), 1190.3(m), 1032.9(s), 829.4(m), 772.7(s), 741.1(s), 697.6(vs). Anal. Calcd. for C₅₃H₄₂Cl₂N₂Ni (836.51): C, 76.10; H, 5.06; N, 3.35. Found; C, 76.27; H, 5.22; N, 3.19.

Data for C7 Yield: 86.0%, red powder. IR (KBr; cm⁻¹): 3025.9(w), 2965.7(w), 1659.8(w), 1627.9(m), 1592.3(s), 1492.9(s), 1444.8(s), 1288.8(m), 1184.9(m), 1035.1(s), 827.1(m), 772.6(s), 740.4(s), 698.0(vs). Anal. Calcd. for C₅₅H₄₆Cl₂N₂Ni (864.57): C, 76.41; H, 5.36; N, 3.24. Found; C, 76.53; H, 5.59; N, 3.11.

Data for C8 Yield: 72.7%, red powder. IR (KBr; cm⁻¹): 3025.1(w), 2963.4(w), 1653.7(w), 1624.0(m), 1585.4(s), 1493.4(s), 1442.5(s), 1289.8(m), 1182.8(m), 1032.3(s), 829.8(m), 777.3(s), 741.9(s), 697.4(vs). Anal. Calcd. for C₅₇H₅₀Cl₂N₂Ni (892.62): C, 76.70; H, 5.65; N, 3.14. Found; C, 76.88; H, 5.72; N, 3.01.

Data for C9 Yield: 80.3%, red powder. IR (KBr; cm⁻¹): 3025.0(w), 2967.2(w), 1655.7(w), 1626.6(m), 1583.3(s), 1493.9(m), 1444.8(m), 1290.6 (m), 1193.4(w), 1031.8(m), 830.0(m), 775.0(s), 740.8(s), 698.7(vs). Anal. Calcd. for C₅₄H₄₄Cl₂N₂Ni (850.54): C, 76.25; H, 5.21; N, 3.29. Found; C, 76.44; H, 5.37; N, 2.91.

Data for C10 Yield: 77.8%, red powder. IR (KBr; cm⁻¹): 3024.3(w), 2963.6(w), 1654.1(w), 1624.4(m), 1586.9(m), 1490.8(m), 1445.7(m), 1288.9(m), 1200.4(m), 1031.7(m), 829.6(m), 774.9(s), 740.4(s), 679.7(vs). Anal. Calcd. for C₅₆H₄₈Cl₂N₂Ni (878.59): C, 76.55; H, 5.51; N, 3.19. Found; C, 76.77; H, 5.87; N, 2.92.

4.3. General procedure for ethylene polymerization

Ethylene polymerization at 10 atm ethylene pressure was performed in a 0.3 L stainless steel autoclave equipped with a mechanical stirrer, a temperature controller and gas ballast through a solenoid clave for continuous feeding of ethylene at constant pressure. The pro-catalyst was dissolved in 30 mL toluene and injected into the autoclave; then the required co-catalyst toluene solution and additional toluene (to maintain the total volume of 100 mL) were added. When the required temperature was reached, the ethylene (10 atm pressure) was continuously introduced to initiate the polymerization. After the desired period of reaction, the autoclave was sealed and cooled in an ice–water bath for one hour. Then the resultant solution was quenched with 10% hydrochloric acid ethanol. The precipitated polyethylene was collected by filtration, and washed with ethanol and water, and then dried in a vacuum at 60 °C until of constant weight.

4.4. X-Ray crystallographic studies

Single crystals of C3, C5 and C9 suitable for X-ray diffraction analysis were obtained by laying diethyl ether on their dichloromethane solutions at room temperature. With graphite-monochromatic MoK_α radiation (λ = 0.71073 Å) at 173(2) K, 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. The structures were solved by direct methods and refined by full-matrix least squares on F². All hydrogen atoms were placed in calculated positions. Structure solution and refinement were performed by using the SHELXL-97 package.¹⁹ Details of the X-ray structure determinations and refinements are provided in Table 5.

Table 5 Crystallographic data and refinement details for complexes C3, C5 and C9

	C3	C5	C9
Formula	C57H50Br2N2Ni	C56H48Br2N2Ni	C54H44Cl2N2Ni
Formula weight	981.52	967.49	850.52
Temperature (K)	173(2)	173(2)	173(2)
Wavelength (Å)	0.71073	0.71073	0.71073
Crystal system	Monoclinic	Monoclinic	Monoclinic
Space group	P21/n	P21/c	P21/c
a (Å)	10.378(2)	15.352(3)	15.330(3)
b (Å)	16.008(3)	22.551(5)	22.285(5)
c (Å)	31.765(6)	15.380(3)	14.954(3)
α (°)	90	90	90
β (°)	95.61(3)	106.96(3)	106.33(3)
γ (°)	90	90	90
Volume (Å^3)	5251.8(18)	5093.0	4902.6(17)
Z	4	4	4
D calc (Mg m^−3)	1.241	1.262	1.152
μ (mm^−1)	1.927	1.986	0.540
F(000)	2016	1984	1776
Crystal size (mm)	0.43 × 0.16 × 0.15	0.22 × 0.21 × 0.03	0.22 × 0.17 × 0.02
θ range (°)	1.29–26.34	1.39–25.05	1.38–27.47
Limiting indices	−8 ≤ h ≤ 12,	−18 ≤ h ≤ 17,	−19 ≤ h ≤ 19,
	−19 ≤ k ≤ 19,	−23 ≤ k ≤ 26,	−23 ≤ k ≤ 28,
	−36 ≤ l ≤ 37	−18 ≤ l ≤ 18	−17 ≤ l ≤ 19
No. of reflections collected	36252	35074	39551
No. of unique reflections	10463	9011	11189
R int	0.0606	0.0898	0.0957
Completeness to θ (%)	97.8 (θ = 26.34°)	99.8 (θ = 25.05°)	99.7 (θ = 27.47°)
No. of parameters	559	550	532
Goodness-of-fit on F^2	1.175	1.105	1.155
Final R indices [I > 2σ(I)]	R 1 = 0.0696	R 1 = 0.0683	R 1 = 0.1173
	wR 2 = 0.1778	wR 2 = 0.1528	wR 2 = 0.2780
R indices (all data)	R 1 = 0.0886	R 1 = 0.0937	R 1 = 0.1619
	wR 2 = 0.1894	wR 2 = 0.1664	wR 2 = 0.3059

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Acknowledgements

This work was supported by MOST 863 program No. 2009AA033601. CR wishes to thank the EPSRC for an overseas travel grant (EP/H031855/1).

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Footnote

† Electronic supplementary information (ESI) available: CCDC reference numbers 831465, 831466 and 831467 for crystallographic data of complexes C3, C5 and C9 respectively.

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