Synthesis, characterization, and ethylene polymerization of 1-[2,4-bis(bis(4-fluorophenyl)methyl)naphthylimino]-2-aryliminoacenaphthylnickel bromides: influences of polymerization parameters on polyethylenes

Abstract

A series of 1-[2,4-bis(bis(4-fluorophenyl)methyl)naphthylimino]-2-aryliminoacenaphthylene derivatives (L1–L5) was prepared and used to react with (DME)NiBr₂ to form the title complexes (C1–C5). The organic compounds were characterized by ¹H/¹³C NMR measurements, FT-IR spectra and elemental analysis. Meanwhile, the nickel complexes were analyzed by FT-IR spectra, elemental analysis, and single-crystal X-ray diffraction of the representative complex C1, which indicated a distorted tetrahedral geometry around the nickel center. Upon activation with diethylaluminum chloride (Et₂AlCl), all the title nickel complexes were highly active for ethylene polymerization, resulting in polyethylenes with high molecular weights ranging from 0.86–5.58 × 10⁵ g mol⁻¹ and narrow polydispersity (1.22–1.99). Moreover, significant influences of polymerization parameters on the resultant polyethylenes were examined and are discussed.

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Introduction

Late-transition metal precatalysts in ethylene polymerization have been extensively investigated since the emergence of α-diiminonickel complexes which produced polyethylenes with high molecular weights.¹ Research progress has been summarized in timely review articles.^2,3 N,N-Bidentate nickel complex precatalysts have been extensively investigated including derivatives of α-diiminonickel complexes^2,4,5 and alternative nickel complex models bearing ligands of 2-aryliminopyridines,⁶ 8-arylamino-5,6,7-trihydroquinolines,⁷ and 9-arylamino-5,6,7-trihydrocycloheptapyridines.⁸ Though polymerization parameters have been generally emphasized regarding catalytic activities of complex precatalysts, influences on polymer properties are not commonly considered. Targeting the current catalytic systems in industry, the most important factors are the properties and scope of applications of obtained polyethylenes, which are reflected by the structural information of obtained polyethylenes such as their molecular weights and polydispersity. Our efforts are to explore new and highly active complex precatalysts of α-diiminonickel complexes; therefore, the polymerization parameters and resultant polyethylenes were investigated.

To enhance the catalytic behaviors of α-diiminonickel complexes,¹ bulky benzhydryl-substituted 1,2-diiminoacenaphthylene⁹ and 2,3-diiminobutane derivatives¹⁰ were developed and they showed positive efficiencies as well as naphthyl-substituted α-diiminoacenaphthylene derivatives.¹¹ In addition, positive effects of ligands containing electron-withdrawing substituents were confirmed by experimental results^9f–h,10b and computational simulation.¹² An extensive series of 1-[2,4-bis(bis(4-fluorophenyl)methyl)naphthylimino]-2-aryliminoacenaphthylene derivatives (L1–L5) was synthesized and used to prepare their nickel complexes (C1–C5). All the nickel complexes, when activated by Et₂AlCl, exhibited high activities towards ethylene polymerization; in addition, the polymerization parameters heavily affected properties of the obtained polyethylenes. Herein, synthesis and characterization of the 1-[2,4-bis(bis(4-fluorophenyl)methyl)naphthylimino]-2-aryliminoacenaphthylene derivatives and their corresponding nickel complexes are systematically investigated, as well as their catalytic behaviour toward ethylene polymerization and properties of the resultant polyethylenes.

Results and discussion

Synthesis and characterization of 1-[2,4-bis(bis(4-fluorophenyl) methyl)naphthylimino]-2-aryliminoacenaphthylene derivatives (L1–L5) and their nickel bromide complexes (C1–C5)

According to the literature procedure (Scheme 1),⁹ the condensation reaction of acenaphtylen-1,2-dione with 2,4-bis[bis(4-fluorophenyl)methyl]naphthylamine was conducted to form 1-[2,4-bis(bis(4-fluorophenyl)methyl)naphthylimino]acenaphthylen-2-one, which further reacted with various anilines to form 1-[2,4-bis(bis(4-fluorophenyl) methyl)naphthylimino]-2-arylimino-acenaphthylene derivatives (L1–L5). These compounds (L1–L5) reacted with a stoichiometric amount of (DME)NiBr₂ in a mixture of dichloromethane and ethanol, respectively; and the corresponding nickel complexes, 1-[2,4-bis(bis(4-fluorophenyl)methyl)naphthylimino]-2-aryliminoacenaphthylnickel bromides (C1–C5) were obtained in good yields (Scheme 1). All organic compounds were characterized by ¹H/¹³C NMR, FT-IR, and elemental analysis; meanwhile, the nickel complexes were analyzed by FT-IR, and elemental analysis without NMR measurement due to their paramagnetic nature. According to their FT-IR spectra, the C=N stretching vibrations of complexes C1–C5 were observed in the region 1620–1652 cm⁻¹ with weaker intensity in comparison with those vibrations of corresponding organic compounds in the range of 1635–1675 cm⁻¹, indicating effective coordination between the imino-N atom with the cationic nickel center. Furthermore, the molecular structure of complex C1 was confirmed by single crystal X-ray diffraction.

Scheme 1 Synthetic procedures of ligands (L1–L5) and nickel complexes (C1–C5).

X-ray crystallographic study

Crystals of the complex C1 suitable for the X-ray diffraction were grown by the diffusion of heptane into its dichloromethane solution at room temperature. The molecular structure of complex C1 is shown in Fig. 1, and selected bond lengths and angles are tabulated in Table 1.

Fig. 1 ORTEP drawing of C1. Thermal ellipsoids are shown at the 30% probability level. Hydrogen atoms and free dichloromethane molecules have been omitted for clarity.

Table 1 Selected bond lengths and angles for C1

Bond lengths (Å)		Bond angles (°)
Ni(1)–N(1)	2.023(2)	N(1)–Ni(1)–N(2)	82.79(10)
Ni(1)–N(2)	2.025(3)	N(1)–Ni(1)–Br(2)	101.44(7)
Br(1)–Ni(1)	2.305(6)	N(2)–Ni(1)–Br(2)	118.62(2)
Br(2)–Ni(1)	2.347(6)	N(1)–Ni(1)–Br(1)	119.57(7)
C(1)–N(1)	1.282(4)	N(2)–Ni(1)–Br(1)	122.44(8)
C(2)–N(2)	1.281(4)	Br(2)–Ni(1)–Br(1)	118.62(2)
C(13)–N(1)	1.442(4)	C(1)–N(1)–C(13)	119.90(3)
C(49)–N(2)	1.439(4)	C(2)–N(2)–C(49)	120.70(3)

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As shown in Fig. 1, the nickel center is coordinated with two imino-nitrogen atoms of the organic ligand and two bromides, adopting a distorted tetrahedron geometry around the nickel center in which the N1, N2, and Br2 atoms form its basal plane along with the apical position occupied by the Br1 atom. Such structural geometry is consistent to its analogous complexes.⁹ In complex C1, the nickel atom deviated by 1.072 Å from the basal plane and the Ni–N bonds are quite similar with the values of 2.023(2) Å (Ni1–N1) and 2.025(3) Å (Ni1–N2), indicating a highly equal coordination ability between the nickel with two nitrogen atoms (N1 and N2); meanwhile, the C=N bonds are also very similar with bond lengths such as C1–N1 for 1.282(4) Å and C2–N2 for 1.281 (4) Å. The dihedral angles between the basal plane comprising N1, N2, and Br2 with the naphthyl ring on the N1-aryl ring, and the aryl ring on N2-aryl ring, are 55.2° and 53.7°, respectively; this observation is similar to its previous nickel analogue containing the naphthyl substituents.¹¹

Ethylene polymerization

To find a suitable co-catalyst for ethylene polymerization, the complex C3 was screened with different alkylaluminum reagents such as methylaluminoxane (MAO), modified methylaluminoxane (MMAO), diethylaluminum chloride (Et₂AlCl) and ethylaluminum sesquichloride (Et₃Al₂Cl₃, EASC) at 30 °C under 10 atm. of ethylene (entries 1–4 in Table 2). Most catalytic systems produced trace polyethylene; however, the system C3/Et₂AlCl exhibited high activity toward ethylene polymerization. Therefore, the Et₂AlCl was further explored to activate these nickel complexes for ethylene polymerization as well as detailed investigations of polymerization parameters and properties of the obtained polyethylenes.

Table 2 Ethylene polymerization by C3 with various co-catalysts^a

Entry	Co-cat.	Al/Ni	Yield (g)	Act.^b	Mw^c (10^5 g mol^−1)	Mw/Mn^c	Tm^d (°C)
a Conditions: 1.5 μmol of Ni; 30 min; 30 °C; 10 atm. of ethylene; total volume 100 mL.b In units of g of PE per mmol of Ni per h per bar.c Determined by GPC.d Determined by DSC.
1	MAO	1000	Trace	—	—	—	—
2	MMAO	1000	Trace	—	—	—	—
3	EASC	600	Trace	—	—	—	—
4	Et2AlCl	600	2.89	385	3.68	1.73	116.8

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To optimize the conditions of ethylene polymerization in the presence of Et₂AlCl, the catalytic performance of complex C3 was extensively investigated by changing reaction parameters of the Al/Ni ratio, reaction temperature, reaction time as well as ethylene pressure, and results were collected in Table 3. Along with increasing the Al/Ni ratio from 300 to 700 (entries 1–5 in Table 3), the activities were observed to gradually increase up to 4.79 × 10⁶ g of PE per mol of Ni per h at the Al/Ni ratio of 700 (entry 5 in Table 3); however, with the Al/Ni ratio up to 800, a significant decrease appeared with the activity as 2.76 × 10⁶ g of PE per mol of Ni per h (entry 6 in Table 3). The higher the ratio of Al/Ni used (entries 2–6 in Table 3), the slightly higher molecular weights of the polyethylenes resulted (Fig. 2); however, a narrower polydispersity of the polyethylenes was achieved. This case is different from the common observation of its nickel analogs:^9,10 the catalytic system with the higher ratio of co-catalyst produced polyethylenes with lower molecular weight because of a higher possibility of a chain transfer from the nickel species to aluminum for termination at a higher ratio of co-catalyst. In current complexes incorporating the bulky-substituent of a 2,4-bis[bis(4-fluorophenyl)methyl]naphthyl group, the cationic nickel active species is well stabilized, and more co-catalyst makes the single-site active species more uniform for the resultant polyethylene with narrower polydispersity. In other words, the chain propagations are generally superior to chain migrations for producing polyethylenes with higher molecular weights; this phenomenon was previously observed with binuclear nickel complex analogs.^5a

Table 3 Ethylene polymerization by C1–C5/Et₂AlCl^a

Entry	Cat.	Al/Ni	t (min)	T (°C)	Yield (g)	Activity ^b	Adjusted activity^c	Mw^d (10^5 g mol^−1)	Mw/Mn^d	Tm^e (°C)
a Conditions: 1.5 μmol of Ni; 30 min; 10 atm. of ethylene; total volume 100 mL.b In units of g of PE per mmol of Ni per h per bar.c In units of g of PE per mmol of Ni per h per Cethylene per bar.d Determined by GPC.e Determined by DSC.f 5 atm. of ethylene.g 1 atm. of ethylene; total volume 30 mL.
1	C3	300	30	30	Trace	—	—	—	—	—
2	C3	400	30	30	0.14	18	14	3.20	1.99	116.20
3	C3	500	30	30	2.05	274	218	3.63	1.74	119.84
4	C3	600	30	30	2.89	385	306	3.68	1.73	116.78
5	C3	700	30	30	3.59	479	381	3.73	1.72	119.93
6	C3	800	30	30	2.07	276	219	4.80	1.70	126.50
7	C3	700	30	20	0.55	73	50	5.58	1.48	129.06
8	C3	700	30	40	4.08	545	496	2.11	1.29	95.88
9	C3	700	30	50	3.43	458	474	1.44	1.67	85.25
10	C3	700	30	60	1.57	209	244	0.86	1.40	63.16
11	C3	700	30	80	Trace	—	—	—	—	—
12	C3	700	15	40	2.51	669	609	2.10	1.66	106.48
13	C3	700	45	40	4.84	430	392	2.13	1.89	105.32
14	C3	700	60	40	5.13	342	311	2.30	1.82	102.82
15^f	C3	700	30	40	2.56	342	623	1.58	1.33	87.60
16^g	C3	700	30	40	0.34	46	419	1.27	1.84	55.25
17	C1	700	30	40	3.45	461	420	0.96	1.22	112.05
18	C2	700	30	40	2.68	358	326	2.17	1.81	114.31
19	C4	700	30	40	3.35	446	406	0.92	1.99	108.28
20	C5	700	30	40	3.75	500	455	2.17	1.97	108.33

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Fig. 2 GPC curves for polyethylene obtained using the C3/Et₂AlCl system with various Al/Ni ratios (entries 2–6 in Table 3).

Keeping the Al/Ni ratio at 700, the reaction temperature was changed from 20 to 80 °C (entry 5 and entries 7–11 in Table 3), and the highest activity was observed with 5.45 × 10⁶ g of PE per mol of Ni per h at 40 °C (entry 8 in Table 3). However, the optimum temperature of 40 °C is still generally higher than the optimum temperature of nickel precatalysts previously reported.^1,4,9,11 Considering the correlation of ethylene concentration in toluene at different temperatures,¹³ the adjusted activities were additionally obtained through calculations and shown in Table 3. These indicated close similar activities at 40 and 50 °C (entries 8 and 9 in Table 3), but a significant decrease of activity occurred at 60 °C (entry 10 in Table 3), perhaps due to deactivation of active species at the higher temperature. The GPC curves significantly showed that the resultant polyethylenes had gradually lower molecular weights with elevated reaction temperatures (entries 5, 7–10 in Table 3 and Fig. 3); this is ascribed to more chain transfers and terminations at the higher temperatures.^4,9

Fig. 3 GPC curves for polyethylene obtained using the C3/Et₂AlCl system at different temperatures (entries 5, 7–10 in Table 3).

Pondering over effects of reaction time of the C3/Et₂AlCl system, ethylene polymerization was conducted at different periods within 15, 30, 45, and 60 min (entries 8 and 12–14 in Table 3). Along with prolonging reaction time, the observed activities gradually decreased, indicating no initiation time was required for this catalytic reaction. A partly deactivated species resulted and the obtained polyethylenes possessed higher molecular weights (Fig. 4), which is consistent with observations of its analogs.^4,9 Besides the deactivation of active species, increased viscosity of the reaction medium due to the formation of polyethylene disfavors the diffusion of ethylene into the solution and less coordination of ethylene onto active species, also resulting in lower activity.

Fig. 4 GPC curves for polyethylene obtained using the C3/Et₂AlCl system with different reaction times (entries 8, 12–14 in Table 3).

Another factor that significantly affects the ethylene concentration in solution is ethylene pressure. Thus, polymerization of ethylene was conducted under different pressures such as 1 atm., 5 atm., and 10 atm. (entries 8, 15–16 in Table 3); higher activities were always achieved with a higher pressure of ethylene. Moreover, higher ethylene pressure led to producing polyethylenes with higher molecular weights, as clearly reflected by their GPC curves (Fig. 5).

Fig. 5 GPC curves for the polyethylene obtained using the C3/Et₂AlCl system at different pressures of ethylene (entry 8 and entries 15–16 in Table 3).

Under optimum conditions of an Al/Ni ratio of 700 at 40 °C and using 10 atm. pressure of ethylene, other nickel complexes were investigated and all showed high activities toward ethylene polymerization (entries 17–20 in Table 3) (Fig. 6). In general, their activities were closely similar, but with differences in the order: C3 [2,6-di(i-Pr)] > C1 [2,6-di(Me)] > C2 [2,6-di(Et)], C5 [2,6-di(Et)-4-Me] > C4 [2,4,6-tri(Me)], which were somewhat random regarding different substituents. These variations from anilines with relative less bulky substituents made the differences through their bulkiness and electronic affects; moreover, the solubility of complex precatalysts would be affected with substituents. In fact, the same substituent of the bulky 2,4-bis[bis(4-fluorophenyl)methyl]naphthyl group played a more important role of steric influence around the nickel center, resulting in closer activities.

Fig. 6 ¹³C NMR spectrum of polyethylene by C3/Et₂AlCl at 60 °C (entry 10, Table 3).

Compared with polyethylenes obtained by their analogs,⁹ the obtained polyethylenes possessed relatively lower molecular weights and narrower polydispersity; meanwhile, their T_m values were verified from low to high. A polyethylene with a lower T_m value indicates higher branching. To confirm that highly branched polyethylenes formed, the ¹³C NMR spectrum of the polyethylene by C3/Et₂AlCl at 60 °C (entry 10, Table 3) was obtained; it indicated 164 branches/1000 carbons including methyl (41.7%), ethyl (6.28%), propyl (15.4%), butyl (9.72%) and long branches (26.9%) according to the interpretation method.¹⁴

Conclusions

The series of 1-[2,4-bis(bis(4-fluorophenyl)methyl)naphthylimino]-2-aryliminoacenaphthylene derivatives (L1–L5) and their nickel bromide complexes (C1–C5) were synthesized and well characterized. When activated with Et₂AlCl, all the nickel precatalysts exhibited high activities toward ethylene polymerization (up to 5.45 × 10⁶ g per mol(Ni) per h), as with a similar range of their analogs,^9,11 but the optimum temperature was 40 °C which is higher than the optimum temperatures observed with their analogs.^9,11 In addition, the polymerization parameters heavily affected the properties of obtained polyethylenes, finely controlling their molecular weights and branching (as reflected with their T_m values). Therefore, the resulting polyethylenes could be tailored through using different parameters as well as with modified-substituents within ligands.

Experimental

General considerations

All manipulations of air- and/or moisture-sensitive compounds were carried out under a nitrogen atmosphere using standard Schlenk techniques.¹⁵ Toluene was refluxed over sodium benzophenone and distilled under nitrogen prior to use. Methylaluminoxane (MAO, 1.46 M solution in toluene) and modified methylaluminoxane (MMAO, 1.93 M in heptane, 3A) were purchased from Akzo Nobel Corp. Diethylaluminum chloride (Et₂AlCl, 1.17 M in toluene) and ethylaluminum sesquichloride (Et₃Al₂Cl₃, 0.87 M in hexane) were purchased from Acros Chemicals. High-purity ethylene was purchased from Beijing Yansan Petrochemical Co. and used as received. 2,4-Bis(bis(4-fluorophenyl)methyl)naphthyl amine was prepared according to our previous literature.^6h 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. Melting points of polyethylenes were measured from a second scanning run on a Perkin-Elmer DSC-7 differential scanning calorimetry (DSC) analyzer under a nitrogen atmosphere; in the procedure, a sample of about 3.8–4.5 mg was heated to 180 °C at a rate of 20 °C min⁻¹ and kept for 5 min at 180 °C to remove the thermal history and then cooled at a rate of 20 °C min⁻¹ to −40 °C. ¹³C NMR spectra of the polyethylenes were recorded on a Bruker DMX-300 MHz instrument at 135 °C in deuterated 1,1,2,2-tetrachloroethane with TMS as an internal standard.

Synthesis and characterization

1-[2,4-Bis(bis(4-fluorophenyl)methyl)naphthylimino]acenaphthylen-2-one. A mixture of acenaphthylen-1,2-dione (0.73 g, 4.0 mmol), 2,4-bis(bis(4-fluorophenyl)methyl)naphthylamine (2.19 g, 4.0 mmol) and a catalytic amount of p-toluenesulfonic acid (0.15 g, 0.80 mmol) was dissolved in 40 mL of CH₂Cl₂ and 2 mL of EtOH and stirred for 24 h at room temperature. After solvent evaporation at reduced pressure, the residue was purified by column chromatography on silica gel with an eluent of petroleum ether–ethyl acetate (v/v = 100 : 1) to afford a red powder (0.80 g) in 53% isolated yield. ¹H NMR (400 MHz, CDCl₃, TMS): δ 8.12 (t, J = 8.8 Hz, 2H), 7.93 (d, J = 8.4 Hz, 1H), 7.85–7.77 (m, 2H), 7.67 (d, J = 8.0 Hz, 1H), 7.39 (t, J = 7.6 Hz, 1H), 7.29–7.27 (m, 1H), 7.06–7.00 (m, 5H), 6.94–6.87 (m, 4H), 6.80 (d, J = 7.2 Hz, 4H), 6.67–6.64 (m, 2H), 6.47 (s, 1H), 6.20 (s, 1H), 6.13 (t, J = 8.4 Hz, 2H), 5.78 (d, J = 7.6 Hz, 1H), 5.66 (s, 1H).

1-[2,4-Bis(bis(4-fluorophenyl)methyl)naphthylimino]-2-(2,6-dimethylphenylimino)acenaphthylene (L1). A mixture of 1-[2,4-bis(bis(4-fluorophenyl)methyl)naphthylimino]acenaphthylen-2-one (1.27 g, 1.78 mmol), 2,6-dimethylphenylamine (0.215 g, 1.78 mmol) and a catalytic amount of p-toluenesulfonic acid (0.068 g, 0.35 mmol) in toluene (50 mL) was refluxed for 10 h. After solvent evaporation at reduced pressure, the residue was purified by column chromatography on silica gel with an eluent of petroleum ether–ethyl acetate (v/v = 200 : 1) to afford a red powder (0.21 g) in 14% isolated yield. mp: 149 °C. IR (KBr, cm⁻¹): 2864 (w), 2730 (w), 1888 (w), 1669 (m), 1635 (m), 1600 (s), 1504 (vs), 1225 (vs), 1156 (s), 1017 (m), 821 (vs), 771 (vs), 723 (m). Anal. calcd for C₅₆H₃₈F₄N₂ (814.91): C, 82.54; H, 4.70; N, 3.44. Found: C, 82.15; H, 4.66; N, 3.65. ¹H NMR (400 MHz, CDCl₃, TMS): δ 7.93 (d, J = 8.4 Hz, 1H), 7.81 (d, J = 8.4 Hz, 2H), 7.72 (d, J = 8.0 Hz, 1H), 7.41–7.28 (m, 4H), 7.20 (t, J = 8.2 Hz, 2H), 7.00 (t, J = 8.6 Hz, 3H), 6.96–6.89 (m, 5H), 6.86–6.81 (m, 5H), 6.76 (t, J = 7.0 Hz, 2H), 6.65 (d, J = 7.2 Hz, 1H), 6.47 (s, 1H), 6.21 (s, 1H), 6.17 (d, J = 8.6 Hz, 2H), 5.83 (s, 1H), 5.78 (d, J = 7.2 Hz, 1H), 2.27 (s, 3H), 2.18 (s, 3H). ¹³C NMR (100 MHz, CDCl₃, TMS): δ 164.0, 162.4, 161.3, 140.5, 139.4, 130.5, 129.1, 128.3, 127.1, 126.0, 124.5, 123.7, 122.4, 115.1, 114.3, 51.4, 50.8, 18.2, 18.0.

1-[2,4-Bis(bis(4-fluorophenyl)methyl)naphthylimino]-2-(2,6-diethylphenylimino)acenaphthylene (L2). Using the same procedure as for the synthesis of L1, L2 was obtained as a red powder in 36% yield (0.30 g). mp: 160 °C. IR (KBr, cm⁻¹): 2963 (w), 2927 (w), 2870 (w), 1890 (w), 1668 (m), 1636 (m), 1599 (s), 1504 (vs), 1224 (vs), 1157 (s), 1016 (m), 820 (vs), 772 (vs), 723 (m). Anal. calcd for C₅₈H₄₂F₄N₂ (842.96): C, 82.64; H, 5.02; N, 3.32. Found: C, 82.23; H, 5.06; N, 3.71. ¹H NMR (400 MHz, CDCl₃, TMS): δ 7.93 (d, J = 8.4 Hz, 1H), 7.80 (d, J = 8.0 Hz, 2H), 7.71 (d, J = 8.4 Hz, 1H), 7.39 (t, J = 7.4 Hz, 1H), 7.34–7.29 (m, 2H), 7.27–7.20 (m, 2H), 7.07 (t, J = 6.0 Hz, 2H), 7.01 (t, J = 8.4 Hz, 2H), 6.97–6.87 (m, 6H), 6.83 (t, J = 6.2 Hz, 4H), 6.74 (t, J = 6.6 Hz, 2H), 6.63 (d, J = 7.2 Hz, 1H), 6.48 (s, 1H), 6.22 (s, 1H), 6.18 (t, J = 8.2 Hz, 2H), 5.84 (s, 1H), 5.76 (d, J = 7.2 Hz, 1H), 2.77–2.68 (m, 1H), 2.66–2.55 (m, 2H), 2.50–2.40 (m, 1H), 1.25 (t, J = 7.2 Hz, 6H). ¹³C NMR (100 MHz, CDCl₃, TMS): δ 164.1, 162.7, 161.2, 160.3, 148.2, 145.2, 140.5, 139.5, 138.6, 135.1, 130.6, 129.4, 128.8, 127.1, 126.2, 124.3, 123.6, 115.2, 114.3, 51.4, 50.8, 24.8, 24.6, 14.4, 14.1.

1-[2,4-Bis(bis(4-fluorophenyl)methyl)naphthylimino]-2-(2,6-diisopropylphenylimino)acenaphthylene (L3). Using the same procedure as for the synthesis of L1, L3 was obtained as a red powder in 17% yield (0.30 g). mp: 231 °C. IR (KBr, cm⁻¹): 2925 (w), 2927 (w), 2866 (w), 1893 (w), 1675 (m), 1646 (w), 1600 (m), 1505 (vs), 1225 (vs), 1157 (s), 1016 (w), 822 (s), 772 (vs), 721 (m). Anal. calcd for C₆₀H₄₆F₄N₂ (871.01): C, 82.74; H, 5.32; N, 3.22. Found: C, 82.49; H, 5.53; N, 3.27. ¹H NMR (400 MHz, CDCl₃, TMS): δ 7.94 (d, J = 8.4 Hz, 1H), 7.80 (d, J = 6.0 Hz, 2H), 7.71 (d, J = 8.4 Hz, 1H), 7.40 (t, J = 7.4 Hz, 1H), 7.32 (t, J = 7.4 Hz, 5H), 7.09 (t, J = 6.4 Hz, 2H), 7.02 (t, J = 8.4 Hz, 2H), 6.96–6.80 (m, 9H), 6.76 (t, J = 6.4 Hz, 2H), 6.60 (d, J = 7.2 Hz, 1H), 6.49 (s, 1H), 6.23 (s, 1H), 6.17 (t, J = 8.4 Hz, 2H), 5.84 (s, 1H), 5.74 (d, J = 7.2 Hz, 1H), 3.23–3.16 (m, 1H), 3.07–3.00 (m, 1H), 1.34 (d, J = 6.8 Hz, 3H), 1.28 (d, J = 6.8 Hz, 3H), 1.13 (d, J = 6.8 Hz, 3H), 0.97 (d, J = 6.8 Hz, 3H). ¹³C NMR (100 MHz, CDCl₃, TMS): δ 164.1, 162.7, 162.6, 161.6, 161.4, 160.3, 160.2, 147.0, 145.2, 140.6, 139.5, 135.4, 130.7, 130.4, 128.8, 128.4, 126.5, 126.0, 124.5, 124.3, 123.6, 123.3, 115.3, 114.6, 51.4, 50.8, 28.7, 28.6, 23.8, 23.6, 23.5, 23.2.

1-[2,4-Bis(bis(4-fluorophenyl)methyl)naphthylimino]-2-(2,4,6-trimethylphenylimino)acenaphthylene (L4). Using the same procedure as for the synthesis of L1, L4 was obtained a red powder (0.17 g) in 21% isolated yield. mp: 140 °C. IR (KBr, cm⁻¹): 2956 (w), 2920 (w), 2857 (w), 1889 (w), 1668 (m), 1636 (m), 1600 (m), 1504 (vs), 1224 (vs), 1157 (s), 1016 (m), 820 (s), 775 (s), 722 (m). Anal. calcd for C₅₇H₄₀F₄N₂ (828.93): C, 82.59; H, 4.86; N, 3.38. Found: C, 82.23; H, 5.17; N, 3.53. ¹H NMR (400 MHz, CDCl₃, TMS): δ 7.85 (d, J = 8.4 Hz, 1H), 7.73 (d, J = 8.4 Hz, 2H), 7.64 (d, J = 8.0 Hz, 1H), 7.29 (t, J = 6.2 Hz, 1H), 7.23–7.18 (m, 2H), 7.00 (t, J = 6.8 Hz, 2H), 6.95–6.90 (m, 4H), 6.88–6.81 (m, 5H), 6.79–6.73 (m, 4H), 6.69 (t, J = 6.8 Hz, 2H), 6.64 (d, J = 7.2 Hz, 1H), 6.39 (s, 1H), 6.14 (s, 1H), 6.08 (t, J = 8.4 Hz, 2H), 5.76 (s, 1H), 5.69 (d, J = 7.2 Hz, 1H), 2.32 (s, 3H), 2.16 (s, 3H), 2.07 (s, 3H). ¹³C NMR (100 MHz, CDCl₃, TMS): δ 164.2, 162.8, 162.6, 161.7, 160.4, 159.2, 146.6, 145.3, 140.5, 139.1, 137.4, 133.3, 132.4, 130.8, 129.2, 127.1, 126.0, 124.6, 123.8, 115.3, 114.5, 51.5, 50.9, 21.0, 19.3, 18.2.

1-[2,4-Bis(bis(4-fluorophenyl)methyl)naphthylimino]-2-(2,6-diethyl-4-methylphenylimino)acenaphthylene (L5). Using the same procedure as for the synthesis of L1, L5 was obtained a red powder (0.27 g) in 32% isolated yield. mp: 221 °C. IR (KBr, cm⁻¹): 2963 (w), 2926 (w), 2867 (w), 1892 (w), 1660 (m), 1635 (m), 1598 (m), 1504 (vs), 1223 (vs), 1158 (s), 1016 (w), 821 (vs), 770 (vs), 722 (m). Anal. calcd for C₅₉H₄₄F₄N₂ (856.99): C, 82.69; H, 5.18; N, 3.27. Found: C, 82.47; H, 5.22; N, 3.50. ¹H NMR (400 MHz, CDCl₃, TMS): δ 7.93 (d, J = 8.4 Hz, 1H), 7.80 (d, J = 8.4 Hz, 2H), 7.70 (d, J = 8.4 Hz, 1H), 7.40–7.34 (m, 2H), 7.32–7.25 (m, 1H), 7.09–6.98 (m, 6H), 6.95–6.89 (m, 5H), 6.86–6.81 (m, 4H), 6.78–6.74 (m, 2H), 6.70 (d, J = 6.8 Hz, 1H), 6.48 (s, 1H), 6.21 (s, 1H), 6.17 (t, J = 8.4 Hz, 2H), 5.83 (s, 1H), 5.74 (d, J = 6.8 Hz, 1H), 2.73–2.64 (m, 2H), 2.62–2.53 (m, 2H), 2.44 (s, 3H), 1.23 (t, J = 7.6 Hz, 3H), 1.14 (t, J = 7.4 Hz, 3H). ¹³C NMR (100 MHz, CDCl₃, TMS): δ 164.3, 163.0, 161.6, 160.4, 146.0, 140.7, 138.9, 135.3, 133.7, 131.0, 130.7, 129.2, 127.4, 124.5, 115.3, 114.7, 51.7, 51.0, 25.0, 24.8, 21.4, 14.7, 14.1.

1-[2,4-Bis(bis(4-fluorophenyl)methyl)naphthylimino]-2-(2,6-dimethylphenylimino)acenaphthylnickel bromide (C1). The 1-[2,4-bis(bis(4-fluorophenyl)methyl)naphthylimino]-2-(2,6-dimethylphenylimino)acenaphthylene (L1) (0.114 g, 0.14 mmol) was dissolved in 10 mL CH₂Cl₂ and then added to a solution of (DME)NiBr₂ (0.043 g, 0.14 mmol) in 5 mL ethanol. The mixture was stirred for 24 h, and then diethyl ether was poured into the mixture to precipitate the complex. The precipitate was collected by filtration, washed with diethyl ether (3 × 5 mL), and dried in vacuum to obtain a deep red powder of C1 (0.12 g) in 82% yield. IR (KBr, cm⁻¹): 2800 (w), 2771 (w), 1889 (w), 1649 (w), 1622 (m), 1602 (s), 1507 (vs), 1227 (vs), 1155 (s), 1017 (m), 830 (s), 776 (vs), 737 (m). Anal. calcd for C₅₆H₃₈Br₂F₄N₂Ni (1033.41): C, 65.09; H, 3.71; N, 2.71. Found: C, 62.22; H, 3.72; N, 2.60.

1-[2,4-Bis(bis(4-fluorophenyl)methyl)naphthylimino]-2-(2,6-diethylphenylimino)acenaphthylnickel bromide (C2). Yield: 72% (0.08 g), deep red powder. IR (KBr, cm⁻¹): 2974 (w), 2937 (w), 2876 (w), 1908 (w), 1647 (w), 1621 (m), 1600 (s), 1505 (vs), 1225 (vs), 1157 (s), 1016 (m), 835 (s), 776 (m), 723 (m). Anal. calcd for C₅₈H₄₂Br₂F₄N₂Ni (1061.46): C, 65.63; H, 3.99; N, 2.64. Found: C, 65.55; H, 4.31; N, 2.43.

1-[2,4-Bis(bis(4-fluorophenyl)methyl)naphthylimino]-2-(2,6-diisopropylphenylimino)acenaphthylnickel bromide (C3). Yield: 93% (0.15 g), deep red powder. IR (KBr, cm⁻¹): 2968 (w), 2930 (w), 2867 (w), 1889 (w), 1650 (w), 1621 (m), 1602 (m), 1505 (vs), 1227 (s), 1154 (s), 1018 (w), 827 (s), 772 (s), 723 (w). Anal. calcd for C₆₀H₄₆Br₂F₄N₂Ni (1089.52): C, 66.14; H, 4.26; N, 2.57. Found: C, 66.25; H, 4.47; N, 2.26.

1-[2,4-Bis(bis(4-fluorophenyl)methyl)naphthylimino]-2-(2,4,6-trimethylphenylimino)acenaphthylnickel bromide (C4). Yield: 32% (0.03 g), deep red powder. IR (KBr, cm⁻¹): 2950 (w), 2913 (w), 2853 (w), 1900 (w), 1652 (w), 1624 (m), 1600 (s), 1503 (vs), 1220 (vs), 1157 (s), 1015 (m), 827 (vs), 777 (s), 713 (w). Anal. calcd for C₅₇H₄₀Br₂F₄N₂Ni (1047.44): C, 65.36; H, 3.85; N, 2.67. Found: C, 65.45; H, 3.49; N, 2.43.

1-[2,4-Bis(bis(4-fluorophenyl)methyl)naphthylimino]-2-(2,6-diethyl-4-methylphenylimino)acenaphthylnickelbromide (C5). Yield: 93% (0.10 g), deep red powder. IR (KBr, cm⁻¹): 2971 (w), 2934 (w), 2874 (w), 1910 (w), 1647 (w), 1620 (w), 1599 (m), 1505 (vs), 1225 (vs), 1158 (s), 1015 (w), 828 (s), 777 (m), 717 (w). Anal. calcd for C₅₉H₄₄Br₂F₄N₂Ni (1075.49): C, 65.89; H, 4.12; N, 2.60. Found: C, 65.52; H, 4.27; N, 2.58.

General procedure for ethylene polymerization

Ethylene polymerization at ambient pressure. The precatalyst was dissolved in toluene using standard Schlenk techniques, and the reaction solution was stirred with a magnetic stir bar under ethylene atmosphere (1 atm.) with a steam bath for controlling the desired temperature. Finally, the required amount of co-catalyst (Et₂AlCl) was added by a syringe. After the reaction was carried out for the required time, the reaction solution was collected and terminated by the addition of 10% hydrochloric acid. The precipitated polymer was collected, washed with ethanol, and finally dried.

Ethylene polymerization at elevated pressure (5 or 10 atm.). A 300 mL stainless steel autoclave, equipped with a mechanical stirrer and a temperature controller, was employed for the reaction. First, 50 mL toluene (freshly distilled) was injected into the autoclave which was full of ethylene. When the required temperature was reached, another 30 mL toluene which dissolved the complex (1.5 μmol of nickel), the required amount of co-catalyst (Et₂AlCl), and the residual toluene were successively added by syringe. The reaction mixture was intensely stirred for the desired time under corresponding pressure of ethylene through the entire experiment. The reaction was terminated and analyzed using the same procedure as above for ethylene polymerization.

X-ray crystallographic studies

Crystals of C1 suitable for X-ray diffraction analysis were obtained by the slow diffusion of heptane into dichloromethane solution at room temperature. With graphite-monochromatic Mo K_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 refinements were performed by using the SHELXL-97 package.¹⁶ Details of the X-ray structure determinations and refinements are provided in Table 4.

Table 4 Crystal data and structure refinements for C1

C1
Empirical formula	C58H42Br2Cl4F4N2Ni
Formula weight	1203.27
Temperature (K)	173(2)
Wavelength (Å)	0.71073
Crystal system	Triclinic
Space group	P1̄
a (Å)	11.224
b (Å)	11.341
c (Å)	22.484
α (°)	92.60
β (°)	96.06
γ (°)	113.86
V (Å^3)	2590.5
Z	2
Dcalc (mg m^−3)	1.543
μ (mm^−1)	2.179
F(000)	1212
Crystal size (mm)	0.39 × 0.23 × 0.18
θ range (°)	0.91–27.49
Limiting indices	−14 ≤ h ≤ 14
	−14 ≤ k ≤ 14
	−29 ≤ l ≤ 29
No. of reflections collected	36 070
No. of unique reflections	11 811
Rint	0.0333
Completeness to θ (%)	99.4 (θ = 27.49°)
No. of parameters	642
Goodness-of-fit on F^2	1.069
Final R indices [I > 2σ(I)]	R1 = 0.0619
	wR2 = 0.1776
R indices (all data)	R1 = 0.0655
	wR2 = 0.1818

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Acknowledgements

The authors thank the National Natural Science Foundation of China (Nos. 21101101, 21374123 and U1362204) and Shanxi Province Natural Science Fund (No. 2012021007-1).

Notes and references

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Footnote

† CCDC 1435888 for C1. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra23644d

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