Targeting polyethylene waxes: 9-(2-cycloalkylphenylimino)-5,6,7,8-tetrahydrocycloheptapyridylnickel halides and their use as catalysts for ethylene polymerization

Abstract

A series of 9-(2-cycloalkylphenylimino)-5,6,7,8-tetrahydrocycloheptapyridine derivatives (L1–L3) was synthesized, and reacted with nickel halides to form their corresponding nickel complexes (bromide: Ni1–Ni3; chloride: Ni4–Ni6). All organic compounds and nickel complexes were well characterized. The structure of a representative complex Ni1 was determined by a single crystal X-ray study, revealing a distorted trigonal bipyramidal geometry at the nickel centre. Upon activation with either modified methylaluminoxane (MMAO) or diethylaluminium chloride (Et₂AlCl), all nickel complexes showed high activities toward ethylene polymerization. The obtained polymers were confirmed to be polyethylene waxes with low molecular weights (in the range of 1.83 to 6.78 kg mol⁻¹) and narrow polydispersity (PDI: 1.38–1.78); moreover, the obtained polyethylenes were highly branched ones. These polyethylene waxes have potential application as functional adducts of lubricants or pour-point depressants.

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Introduction

One hundred years of developments of organo-nickel compounds was reviewed for its historical significance by Wilke about three decades ago.¹ In addition to catalytic performances towards hydrogenation and C–C coupling cyclization, ethylene insertion induced by nickel (nickel effect) was recognized as pioneering work resulting in Ziegler catalysts for the massive polyolefin industry.² In fact, nickel catalysts have made a significant contribution in the petrochemical industry; in addition to olefin polymerization, ethylene oligomerization has been achieved by nickel complex pre-catalysts as a commercialized “SHOP” process for α-olefins as basic substances for the production of fine chemicals.³ The discovery of α-diiminometal (Ni²⁺ or Pd²⁺) complex pre-catalysts for ethylene polymerization resurrected the use of late-transition metal pre-catalysts for olefin polymerization in 1995,⁴ and extensive investigations have been made on this useful catalytic system.⁵ The late-transition metal complex pre-catalysts have fixed necessary requirements of petrochemical companies regarding catalytic efficiency; besides catalytic activities, the unique properties of the newly obtained polyolefin materials is an important issue. Therefore useful polymers with advanced properties are in demand in order to attract further investment to potentially commercialize the process.

Within the nickel complex pre-catalysts exploited,^5a–c two models of the α-diiminonickel (A, Scheme 1)^4,6,7 and 2-iminopyridylnickel (B, Scheme 1)⁸ complexes have been extensively investigated. The α-diimino ligands (model A) have been developed from either simple diketones^4,6 or acenaphthylene-1,2-dione as well as its analogues,^4,7 in which pre-catalysts using rigid ligands generally showed higher catalytic activities. To create the rigid ligands on the basis of the 2-iminopyridyl frame (model B), pyridine-based frames fused with cyclic ketones have been designed and used to form 8-arylimino-5,6,7-trihydroquinolylnickel (C, Scheme 1)^9,10 and 9-aryliminocycloheptapyridylnickel (D, Scheme 1) complexes;¹¹ and these new pre-catalysts (C and D) showed better performance in ethylene polymerization than their analogues of 2-iminopyridylnickel complexes (B).⁸ Moreover, using cycloalkyl-substituted anilines instead of anilines, 8-(2-cycloalkylphenylimino)-5,6,7-trihydroquinolylnickel pre-catalysts (model C)¹⁰ produced polyethylenes with lower molecular weights and narrower polydispersity than those from their analogues (C) without cycloalkyl-substituents,⁹ which are in high demand in the market as polyethylene waxes. Until now, there have been only a few examples of metal complexes developed from cycloalkyl-substituted anilines^10,12 because no cycloalkyl-substituted anilines are commercially available; however, a conveniently synthetic procedure for cycloalkyl-substituted anilines has been developed and potentially scaled up to meet industrial requirements. Subsequently, reactions of 5,6,7,8-tetrahydrocycloheptapyridine-9-one with 2-cycloalkylanilines were conducted to form a series of 9-(2-cycloalkylphenylimino)-5,6,7,8-tetrahydrocycloheptapyridine derivatives, which further reacted with nickel halides to form the title complexes. Upon activation with either MMAO or Et₂AlCl, all nickel complexes gave high activities towards ethylene polymerization; more importantly, the obtained polyethylenes possessed lower molecular weights and narrower polydispersity.

Scheme 1 Typical bidentate nickel complex pre-catalysts.

Results and discussion

Synthesis and characterization

The condensation reactions of 5,6,7,8-tetrahydrocycloheptapyridine-9-one with a series of 2-cycloalkylanilines were conducted to form 9-(2-cycloalkyl phenylimino)-5,6,7,8-tetrahydrocycloheptapyridine derivatives (L1–L3, including their corresponding enamine analogues L1′–L3′, Scheme 2) as yellowish compounds according to our reported procedure.^9b,10 The mixture of the two isomer ligands was reacted with NiCl₂ or DME·NiBr₂ in ethanol and dichloromethane as solvent at ambient temperature for 12 hours forming complexes (Ni1–Ni6) in reasonable to excellent yields (71–94%, Scheme 2). All organic compounds were identified by ¹H/¹³C NMR measurements, FT-IR spectroscopy as well as elemental analysis, and the corresponding nickel complexes were also characterized by FT-IR spectroscopy and elemental analysis. As reported in previous work,^9b,10 all the ligands contained two isomers, forming enamine (L1′) and Schiff-base (L) (Scheme 2) due to the migration of the double bond from the imino part to the cycloheptane. Furthermore, the structure of a representative complex Ni1 was confirmed by single-crystal X-ray diffraction.

Scheme 2 Synthetic procedures for 9-(2-cycloalkylphenylimino)-5,6,7,8-tetrahydrocycloheptapyridines and their nickel halides.

Single-crystal X-ray diffraction studies

A single crystal of Ni1 suitable for X-ray diffraction analysis could be obtained within two days by layering diethyl ether onto a mixed solution of dichloromethane and methanol at room temperature. The molecular structure of complex Ni1 is shown in Fig. 1, accompanied by selected bond lengths and angles in Table 1.

Fig. 1 ORTEP drawing of the molecular structure of Ni1·CH₃OH. Thermal ellipsoids are shown at the 30% probability level. Hydrogen atoms were omitted for clarity.

Table 1 Selected bond lengths (Å) and angles (°) for Ni1·CH₃OH

Bond lengths (Å)		Bond angles (°)
Ni1–N1	2.038(3)	O1–Ni1–N2	94.68(12)
Ni1–N2	2.027(3)	O1–Ni1–N1	170.80(13)
Ni1–O1	2.053(3)	N2–Ni1–N1	79.20(12)
Ni1–Br1	2.4705(9)	O1–Ni1–Br2	95.95(9)
Ni1–Br2	2.4117(9)	N2–Ni1–Br2	112.15(8)
N1–C1	1.327(5)	N1–Ni1–Br2	92.75(10)
N1–C5	1.338(5)	O1–Ni1–Br1	86.89(9)
N2–C6	1.279(4)	N2–Ni1–Br1	110.15(8)
N2–C11	1.449(4)	N1–Ni1–Br1	88.78(10)
		Br2–Ni1–Br1	137.18(3)

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As shown in Fig. 1, the nickel atom exists in penta-coordination with a N,N-bidentate ligand, two bromides and an assisted coordination of a methanol molecule. Furthermore, the molecular structure of Ni1 reveals a distorted trigonal bipyramidal geometry at the nickel atom centre consistent with its analogues.^13,14 The equatorial plane is made up of N1, N2 and O1 along with a 0.129 Å deviation of the nickel metal atom, and the axial plane is made up of three atoms (N2, Br1 and Br2). The length of the Ni–N_imino (Ni1–N2, 2.027(3) Å) is slightly shorter than the Ni–N_pyridine (Ni1–N1, 2.038(3) Å), suggesting stronger electron donation from N_imino to the nickel core, which is actually opposite to the observation in the previous work without cycloalkyl-substituents;¹¹ in other words, the Ni–N_imino bond is enhanced by coordination between the nickel and the rich-electronic N_imino atom due to the electron-donating cycloalkyl substituents. The dihedral angle between the plane of Ni1, N1 and N2 and phenyl plane (C1, C2, C3, C4, C5 and N1) is 1.90°. As shown in Table 1, the bond angles of O1–Ni1–N2 and N2–Ni1–N1 are at 94.67° and 79.20°, respectively.

Ethylene polymerization

Based on our previous work,⁹ ethylene polymerization was carried out with nickel complex (Ni2) using different co-catalysts (MAO, MMAO, Et₂AlCl, Me₂AlCl and EASC) to select a suitable co-catalyst. Obviously, co-catalyst Et₂AlCl (entry 3, Table 2) showed the highest activity towards ethylene polymerization among these alkylaluminium reagents. Meanwhile, MMAO (entry 2, Table 2) also showed reasonable activity within various aluminoxane reagents. Hence, MMAO and Et₂AlCl, were selected for the further investigations.

Table 2 Selection of suitable alkylaluminiums based on Ni2^a

Entry	Co-cat.	Al/Ni	Yield (g)	Act.^b	Mw^c (kg mol^−1)	Mw/Mn^c	Tm^d (°C)
a Conditions: 3 μmol of Ni2; 30 min; 30 °C 10 atm of ethylene; total volume 100 mL.b Values in units of 10^6 g(PE) mol(Ni)^−1 h^−1.c Determined by GPC.d Determined by DSC.
1	MAO	1000	0.06	0.04	4.51	1.49	71.2
2	MMAO	1000	1.08	0.72	4.88	1.66	68.3
3	Et2AlCl	200	4.80	3.20	3.89	1.76	54.6
4	Me2AlCl	200	Trace	Trace	—	—	—
5	EASC	200	Trace	Trace	—	—	—

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Ethylene polymerization in the presence of Et₂AlCl

Ethylene polymerizations were performed using Et₂AlCl as co-catalyst at elevated ethylene pressure (10 atm). Complex Ni2 was used to screen the optimum conditions including the Al/Ni molar ratio, temperature and reaction time, and the corresponding results are tabulated in Table 3.

Table 3 Ethylene polymerization by Ni1–Ni6/Et₂AlCl^a

Entry	Cat.	Al/Ni	t (min)	T (°C)	Yield (g)	Act.^b	Mw^c (kg mol^−1)	Mw/Mn^c	Tm^d (°C)
a Conditions: 3 μmol of Ni 10 atm of ethylene; total volume 100 mL.b Values in units of 10^6 g(PE) mol^−1(Ni) h^−1.c Determined by GPC.d Determined by DSC.e 5 atm of ethylene.
1	Ni2	200	30	30	4.80	3.20	3.89	1.76	54.6
2	Ni2	300	30	30	5.45	3.63	3.87	1.69	56.3
3	Ni2	400	30	30	7.45	4.97	3.64	1.46	32.7
4	Ni2	500	30	30	6.08	4.05	3.63	1.67	35.1
5	Ni2	600	30	30	5.96	3.97	3.61	1.53	33.7
6	Ni2	700	30	30	4.43	2.95	3.58	1.64	52.7
7	Ni2	400	30	20	4.62	3.08	5.32	1.70	75.7
8	Ni2	400	30	40	2.02	1.35	2.37	1.49	17.4
9	Ni2	400	30	50	0.32	0.21	2.25	1.38	76.0
10	Ni2	400	15	30	3.62	4.83	3.41	1.60	32.3
11	Ni2	400	45	30	9.80	4.36	3.70	1.76	31.0
12	Ni2	400	60	30	11.03	3.68	3.77	1.78	35.5
13	Ni1	400	30	30	4.62	3.08	2.36	1.53	52.9
14	Ni3	400	30	30	9.41	6.27	1.91	1.65	53.0
15	Ni4	400	30	30	5.20	3.47	2.32	1.64	51.9
16	Ni5	400	30	30	7.14	4.76	3.21	1.53	33.4
17	Ni6	400	30	30	6.25	4.17	1.83	1.66	53.1
18^e	Ni2	400	30	30	1.82	1.21	2.93	1.52	18.9

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On changing the molar ratio of Al/Ni from 200 to 700 (entries 1–6, Table 3), the best activity was observed with a molar ratio of 400 at 4.97 × 10⁶ g(PE) mol⁻¹(Ni) h⁻¹ (entry 3, Table 3). With the increase of Al/Ni ratios (entries 1–6, Table 3), the molecular weights of the obtained polyethylene exhibited a tendency to slightly decrease, which was attributed to the increase of the aluminum concentration leading to more chain transfers.¹⁵ Impressively, the resulting polyethylenes possessed narrow molecular weight distributions in the range of 1.46 to 1.76, illustrating the well-defined single-site catalysis.

Regarding the thermo-stability, the ethylene polymerization was conducted under a reaction temperature setting from 20 to 50 °C (entries 3 and 7–9, Table 3). In all cases, the real temperature was commonly higher than the setting temperature due to the exothermal reaction of the ethylene polymerization. Maximum activity was obtained at 30 °C (entry 3, Table 3). The thermo-stability of this system was better than those obtained from the 9-arylamino-5,6,7-trihydrocycloheptapyridylnickel complexes.¹¹ According to the GPC curves in Fig. 2, a lower molecular weight of polyethylene was attained at a higher temperature, which is interpreted in being from both deactivation of the active species at elevated temperature and fast chain transfer at higher temperature.^4,15 These phenomena were consistent with observations of their analogs.^10,11 All GPC curves of the resulting polyethylenes (Fig. 2) show narrow polydispersity, which indicates the typical single-site behavior of the system.

Fig. 2 The GPC curves of polyethylene obtained at different temperatures (entries 3, 7–9, Table 3).

In addition, the ethylene polymerization was investigated at different reaction times. On prolonging the reaction time from 15 to 60 min (entries 3 and 10–12, Table 3), the obtained polyethylenes accumulated higher molecular weights. Interestingly, a higher activity of 4.97 × 10⁶ g(PE) mol(Ni)⁻¹ h⁻¹ (entry 3, Table 3) was seen at 30 min, meanwhile a lower polyethylene activity was obtained with 3.41 × 10⁶ g(PE) mol(Ni)⁻¹ h⁻¹ at 15 min (entry 10, Table 3), indicating the induction time required in the current system. This phenomenon was also observed in the previous nickel precatalysts derived from cycloalkyl-substituted anilines;¹⁰ this means that the cycloalkyl-substituents occupied the space around the nickel core and caused the slowly activating metal complex to form active species. For the longer reaction times (entries 11 and 12, Table 3), the amounts of polyethylene obtained were increased, however, the catalytic activities were decreased along with prolonged polymerization time; which indicated that some of the active species are being deactivated during the ethylene polymerization.

On the basis of the above information, the optimum conditions (Al/Ni ratio of 400 at 30 °C) were employed to explore the catalytic behavior of other nickel analogs (Ni1 and Ni3–Ni6), and the best activity was achieved by Ni3/Et₂AlCl (up to 6.27 × 10⁶ g(PE) mol⁻¹(Ni) h⁻¹). Regarding the influence of anionic halides, their catalytic activities were in the order as Ni1 [Br, 2,6-di(cyclohexyl)] < Ni4 [Cl, 2,6-di(cyclohexyl)], Ni2 [Br, 2,6-di(cyclopentyl)] > Ni5 [Cl, 2,6-di(cyclopentyl)] and Ni3 [Br, 2-methyl-6-cyclohexyl] > Ni6 [Cl, 2-methyl-6-cyclohexyl], indicating random results; however, the bromide positively affected catalytic performances for complexes containing less bulky substituents. For bromide complexes, their activities decreased in the order Ni3 [2-methyl-6-cyclohexyl] > Ni2 [2,6-di(cyclopentyl)] > Ni1 [2,6-di(cyclohexyl)] (entries 3, 13–14, Table 3), which is ascribed to the steric hindrance by the bulky substituents. Moreover, the polyethylene produced by Ni2 possessed higher molecular weight and narrower polydispersity, but lower T_m, indicating higher branched polyethylene being obtained. For chloride complexes, the order of their activities is Ni5 [2,6-di(cyclopentyl)] > Ni6 [2-methyl-6-cyclohexyl] > Ni4 [2,6-di(cyclohexyl)] (entries 15–17, Table 3), indicating the synergic influence of the substituents linked on the phenyl bridged to N_imino and chloride in both a steric and electronic manner. Similar to its bromide analogues, the precatalyst Ni5 produced polyethylene having a higher molecular weight, narrower polydispersity and lower T_m.

Compared with 8-(2-cycloalkylphenylimino)-5,6,7-trihydroquinolylnickel complexes,¹⁰ these catalytic systems exhibited much higher activities. By contrast to 9-arylamino-5,6,7-trihydrocycloheptapyridylnickel complexes,¹¹ these catalytic systems showed slightly narrower polydispersity, which is probably due to the bulky substituents stabilizing the active species that made the narrower PDIs. It could also be confirmed by the GPC curves (Fig. 3).

Fig. 3 The GPC curves of polyethylenes obtained using the pre-catalysts (entries 3, 13–17, Table 3).

In general, both bromide and chloride nickel pre-catalysts produced polyethylene waxes with narrow polydispersity, low molecular weights and low melting points (T_m). The low melting points (T_m) of the obtained polyethylenes were generally caused by high branching. For example, the polyethylenes obtained at 40 and 50 °C possessed similar molecular weights and polydispersity (entries 8 and 9, Table 3), but lower T_m, for the polyethylene formed at 40 °C indicating higher branching. Therefore the ¹³C NMR measurement was conducted in deuterated tetrachloroethane for the polyethylene from Ni2/Et₂AlCl at 40 °C (entry 8, Table 3, Fig. 4), and indicated a branch number of 95 per 1000 carbons which is interpreted according to the literature;¹⁶ and the main branches were methyl (55.1%), ethyl (17.6%), propyl (5.9%) as well as longer chains (21.4%).

Fig. 4 ¹³C NMR spectrum of the polyethylene from Ni2/Et₂AlCl at 40 °C (entry 8, Table 3).

Ethylene polymerization in the presence of MMAO

Different from the previous works^10,11 the catalyst activated by MMAO showed higher polyethylene activity with an aluminoxane reagent as co-catalyst. The influence of reaction conditions on the ethylene polymerization with Ni2/MMAO (Table 4) was investigated in detail, and showed slightly different results to the Ni2/Et₂AlCl system regarding the Al/Ni molar ratios and reaction temperatures.

Table 4 Ethylene polymerization by Ni1–Ni6/MMAO^a

Entry	Cat.	Al/Ni	t (min)	T (°C)	Yield (g)	Act.^b	Mw^c (kg mol^−1)	Mw/Mn^c	Tm^d (°C)
a Conditions: 3 μmol of Ni 10 atm of ethylene; total volume 100 mL.b Values in units of 10^6 g(PE) mol^−1(Ni) h^−1.c Determined by GPC.d Determined by DSC.e 5 atm of ethylene.
1	Ni2	1000	30	30	1.08	0.72	4.88	1.66	68.3
2	Ni2	1500	30	30	2.41	1.61	4.08	1.62	53.3
3	Ni2	1750	30	30	4.05	2.70	3.94	1.66	53.2
4	Ni2	2000	30	30	3.91	2.61	3.93	1.53	61.9
5	Ni2	2250	30	30	3.15	2.10	4.24	1.67	53.6
6	Ni2	2500	30	30	2.74	1.83	4.39	1.67	53.5
7	Ni2	1750	30	20	5.53	3.69	5.80	1.76	80.2
8	Ni2	1750	30	40	1.65	1.10	2.48	1.50	19.0
9	Ni2	1750	30	50	0.17	0.11	2.16	1.34	82.2
10	Ni2	1750	15	20	2.25	3.00	5.73	1.74	80.2
11	Ni2	1750	45	20	8.03	3.57	5.99	1.73	76.7
12	Ni2	1750	60	20	9.42	3.14	6.15	1.78	76.9
13	Ni1	1750	30	20	3.50	2.33	4.11	1.64	79.8
14	Ni3	1750	30	20	5.67	3.78	4.68	1.68	92.8
15	Ni4	1750	30	20	3.55	2.37	4.15	1.62	78.8
16	Ni5	1750	30	20	3.38	2.25	6.78	1.66	85.6
17	Ni6	1750	30	20	7.45	4.97	3.96	1.51	86.3
18^e	Ni2	1750	30	20	1.58	1.05	4.73	1.56	38.7

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Changing the Al/Ni molar ratios from 1000 to 2500 (entries 1–6, Table 4), the highest activity of 2.70 × 10⁶ g(PE) mol⁻¹(Ni) h⁻¹ was observed with an Al/Ni ratio of 1750 (entry 3, Table 4). On further increasing the Al/Ni molar ratios (entries 4–6, Table 4), the catalytic system showed slightly lower activities and produced polyethylenes with negligible differences between molecular weights and polydispersity, which is similar behavior to the system Ni1/MAO of 8-(2-cycloalkylphenylimino)-5,6,7-trihydroquinolylnickel complexes.¹⁰

Conducting the polymerization between 20 to 50 °C (entries 3 and 7–9, Table 4), unlike the Ni2/Et₂AlCl system, the best activity was achieved at 20 °C (entry 7, Table 4); when a higher reaction temperature was applied, lower activities were obtained, indicating the deactivation of the active species at the higher temperature, which may be decomposed due to generate Ni-hydride species.^7c The resultant polyethylenes showed gradually lower molecular weights with reaction temperature elevation (Fig. 5).

Fig. 5 The GPC curves of polyethylene obtained at different temperatures (entries 3, 7–9, Table 4).

The trend for the lifetime of the pre-catalysts (entries 7 and 10–12, Table 4) was detected to be the same as for the Ni2/Et₂AlCl system (entries 3 and 10–12, Table 3). Meanwhile, prolonging the reaction time can also obtain the polyethylenes with higher molecular weights and narrow polydispersity.

Under the optimum conditions with an Al/Ni ratio of 1750 at 20 °C over 30 min (entry 7, Table 4), the other complexes were investigated for ethylene polymerization (entries 13–17, Table 4). In all cases, these nickel complex pre-catalysts exhibited high activities towards ethylene polymerization, producing polyethylenes with low molecular weights and narrow polydispersity. Moreover, these polyethylene samples possessed narrower polydispersity, indicating well-behaved single-site catalysis.

Compared with Ni/Et₂AlCl systems, the catalytic systems with MMAO generally showed slightly lower activities, but produced polyethylenes having higher molecular weights and higher T_m values; most of the resultant polyethylenes possessed molecular weights around 5.0 kg mol⁻¹, which were higher than those obtained by Ni/Et₂AlCl systems and their analogues.^10,11 In addition, the lowest T_m value was observed for the polyethylene obtained at 40 °C (entry 8, Table 4), consistent with the observation with the Ni/Et₂AlCl system. Although a higher T_m value for polyethylene was obtained from Ni3/MMAO (entry 14, Table 4), the ¹³C NMR spectrum of the polyethylene shown in Fig. 6 indicated a branch number of 53 per 1000 carbons, in which the main branches were methyl (84.4%), ethyl (5.3%) and longer chains (10.3%).¹⁶

Fig. 6 ¹³C NMR spectrum of the polyethylene by Ni3/MMAO (entry 14, Table 4).

Conclusion

9-(2-Cycloalkylphenylimino)-5,6,7,8-tetrahydrocycloheptapyridylnickel complexes (Ni1–Ni6) were successfully synthesized and characterized. The influence of these pre-catalysts and various reaction conditions have been extensively investigated for ethylene polymerization. The complex Ni1, as confirmed by single crystal X-ray, showed a distorted trigonal bipyramidal geometry around the nickel center. Upon activation by either MMAO or Et₂AlCl, all the nickel complexes exhibited well-behaved single-site catalysis with high activities (up to 6.27 × 10⁶ g(PE) mol(Ni)⁻¹ h⁻¹). The resultant polyethlyenes were highly branched with low molecular weights and narrow molecular weight distributions, which correspond to the unique properties of polyethylene waxes for use as additives to lubricants and pour-point depressants. Therefore these typical polyethylenes are potentially useful and can be considered for industrial application.

Experimental

General considerations

All manipulations involving air- and moisture-sensitive compounds were performed under a nitrogen atmosphere using standard Schlenk techniques. Toluene was refluxed over sodium and distilled under nitrogen prior to use. Methylaluminoxane (MAO, 1.46 M solution in toluene), modified methylaluminoxane (MMAO, 1.93 M in heptane), ethylaluminum sesquichloride (EASC, 0.87 M in toluene) and dimethylaluminium chloride (Me₂AlCl, 1.0 M solution in toluene) were purchased from Akzo Nobel Corp. Diethylaluminium chloride (Et₂AlCl, 1.17 M in toluene) was 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 using a Perkin-Elmer System 2000 FT-IR spectrometer. Elemental analysis was carried out using a Flash EA 1112 micro-analyzer. Molecular weights and molecular weight distribution (MWD) of polyethylene were determined by PL-GPC220 at 150 °C, with 1,2,4-trichlorobenzene as the solvent. The melting points of polyethylene were measured from the second scanning run using a Perkin-Elmer TA-Q2000 differential scanning calorimetry (DSC) analyzer under a nitrogen atmosphere. In the procedure, a sample of about 4.0 mg was heated to 140 °C at a rate of 20 °C min⁻¹ and kept for 2 min at 140 °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 using a Bruker DMX 400 MHz instrument at ambient temperature in deuterated 1,1,2,2-tetrachloroethane with TMS as an internal standard.

Synthesis of organic compounds

9-(2,6-Dicyclohexylphenylimino)-5,6,7,8-tetrahydrocycloheptapyridine (L1) and 9-(2,6-dicyclohexylphenylamino)-5,6,7-trihydrocycloheptapyridine (L1′). 5,6,7,8-Tetrahydrocycloheptapyridin-9-one (0.37 g, 2.5 mmol) and 2,6-dicyclohexylaniline (0.51 g, 2 mmol) were stirred with a catalytic amount of p-toluenesulfonic acid dissolved in 100 mL chlorobenzene. The solution was refluxed for 4 hours. Then the solvent was evaporated at reduced pressure, the mixture was isolated by silica gel column chromatography (V_{petroleum ether} : V_{ethyl acetate} = 25 : 1) to get the target product (yellow powder L1 : L1′ = 91 : 9, 0.32 g, 40% yield). ¹H NMR (400 MHz, CDCl₃, TMS): δ 8.64 (d, J = 3.6 Hz, 1H, L1–Py–H), 8.54 (d, J = 3.6 Hz, L1′–Py–H), 7.55 (s, L1′–Py–H), 7.51 (d, J = 7.6 Hz, 1H, L1–Py–H), 7.26 (t, J = 3.8 Hz, 1H, L1–Py–H), 7.24 (s, L1′–Py–H), 7.19–7.15 (m, L1′–Ar–H), 7.13 (d, J = 7.2 Hz, 2H, L1–Ar–H), 7.07 (d, J = 6.4 Hz, 1H, L1–Ar–H), 7.04 (s, L1′–Ar–H), 6.24 (s, L1′–NH), 4.57 (t, J = 6.8 Hz, L1′–CH), 3.01 (t, J = 8.0 Hz, L1′–CH₂), 2.89 (t, J = 6.2 Hz, 2H, L1–CH₂), 2.69 (t, J = 6.8 Hz, L1′–CH₂), 2.59 (t, J = 12 Hz, 2H, L1–CH₂), 2.31 (t, J = 6.2 Hz, 2H, L1–CH₂ and L1′–CH₂), 2.04–2.00(m, L1′–CH₂), 1.96 (d, J = 11.6 Hz, 2H, L1–CH₂), 1.69–1.55 (m, 10H, L1–CH₂ and L1′–CH₂), 1.40–1.15 (m, 10H, L1–CH₂ and L1′–CH₂). ¹³C NMR (100 MHz, CDCl₃, TMS): δ 172.8, 157.2, 148.5, 146.1, 136.6, 134.8, 134.2, 123.8, 123.7, 123.3, 38.8, 34.2, 33.2, 31.9, 31.5, 27.3, 27.0, 26.4, 25.9, 23.1. FT-IR (KBr, cm⁻¹): 2923, 2849, 1632, 1568, 1442, 1301, 1257, 1225, 1175, 1096, 1019, 965, 889, 853, 799, 779. Anal. calcd for C₂₈H₃₆N₂ (400): C, 83.95; H, 9.06; N, 6.99. Found: C, 83.63; H, 8.89; N, 7.21%.

9-(2,6-Dicyclopentylphenylimino)-5,6,7,8-tetrahydrocycloheptapyridine (L2) and 9-(2,6-dicyclopentylphenylamino)-5,6,7-trihydrocycloheptapyridine (L2′). This compound was synthesized using the same procedure as described for L1. L2 and L2′ were obtained (yellow oil, L2 : L2′ = 91 : 9, 0.60 g, 80% yield). ¹H NMR (400 MHz, CDCl₃, TMS): δ 8.64 (d, J = 3.6 Hz, 1H, L1–Py–H), 8.53 (d, J = 3.6 Hz, L1′–Py–H), 7.56 (d, J = 7.2 Hz, 1H, L1–Py–H), 7.51 (d, J = 7.2 Hz, L1′–Py–H), 7.26 (t, J = 6.2 Hz, 1H, L1–Py–H and L1′–Py–H), 7.20 (s, L1′–Ar–H), 7.15 (d, J = 7.6 Hz, 2H, L1–Ar–H), 7.05 (t, J = 7.6 Hz, 1H, L1–Ar–H), 6.96–6.89 (m, L1′–Ar–H), 6.21 (s, L1′–NH), 4.61 (t, J = 6.8 Hz, L1′–CH), 3.05 (t, J = 8.8 Hz, 2H, L1–CH₂ and L1′–CH₂), 2.84 (t, J = 6.4 Hz, 2H, L1–CH₂), 2.66 (t, J = 6.4 Hz, L1′–CH₂), 2.33 (t, J = 6.0 Hz, 2H, L1–CH₂ and L1′–CH₂), 2.15–2.12 (m, 2H, L1–CH₂ and L1′–CH₂), 1.91–1.88 (m, L1′–CH₂), 1.82–1.73 (m, 8H, L1–CH₂), 1.65–1.61 (m, 8H, L1–CH₂), 1.59–1.57 (m, L1′–CH₂). ¹³C NMR (100 MHz, CDCl₃, TMS): δ 172.8, 156.8, 148.4, 147.6, 136.9, 134.5, 133.9, 123.9, 123.7, 123.4, 39.8, 34.8, 34.3, 31.8, 31.2, 26.1, 25.8, 25.6, 22.4. FT-IR (KBr, cm⁻¹): 2946, 2864, 1635, 1568, 1447, 1297, 1185, 1094, 766. Anal. calcd for C₂₆H₃₂N₂ (372): C, 83.82; H, 8.66; N, 7.52. Found: C, 83.64; H, 8.75; N, 7.65%.

9-(2,4-Dimethyl-6-cyclohexylphenylimino)-5,6,7,8-tetrahydrocycloheptapyridine (L3) and 9-(2,4-dimethyl-6-cyclohexyl phenylamino)-5,6,7-trihydrocycloheptapyridine (L3′). This compound was synthesized using the same procedure as described for L1. L3 and L3′ were obtained (yellow oil, L3 : L3′ = 93 : 7, 0.41 g, 59% yield). ¹H NMR (400 MHz, CDCl₃, TMS): δ 8.64 (d, J = 3.6 Hz, 1H, L1–Py–H), 8.53 (d, J = 3.6 Hz, L1′–Py–H), 7.51 (d, J = 7.6 Hz, 1H, L1–Py–H), 7.34 (d, J = 6.8 Hz, L1′–Py–H), 7.26 (t, J = 3.6 Hz, 1H, L1–Py–H), 7.24 (s, L1′–Py–H), 7.04 (s, L1′–Ar–H), 6.97 (s, L1′–Ar–H), 6.93 (s, 1H, L1–Ar–H), 6.86 (s, 1H, L1–Ar–H), 6.07 (s, L1′–NH), 4.55 (t, J = 6.8 Hz, L1′–CH), 2.89–2.84 (m, 2H, L1–CH₂), 2.69–2.65 (m, L1′–CH₂), 2.30 (s, 3H, L1–CH₃), 2.28 (s, L1′–CH₃), 2.13 (s, 3H, L1–CH₃), 2.11 (s, L1′–CH₃), 1.92–1.50 (m, 12H, L1–CH₂ and L1′–CH₂), 1.38–1.26 (m, 4H, L1–CH₂ and L1′–CH₂). ¹³C NMR (100 MHz, CDCl₃, TMS): δ 173.1, 157.4, 148.3, 144.7, 136.8, 135.0, 134.3, 132.1, 128.5, 124.6, 124.5, 123.8, 38.6, 34.0, 33.6, 31.6, 31.5, 27.2, 26.9, 26.3, 25.8, 23.2, 21.0, 18.2. FT-IR (KBr, cm⁻¹): 2945, 2864, 1635, 1567, 1447, 12 297, 1185, 1094, 766. Anal. calcd for C₂₄H₃₀N₂ (346): C, 83.19; H, 8.73; N, 8.08. Found: C, 83.38; H, 8.69; N, 8.12%.

Synthesis of nickel complexes

General procedure: NiCl₂·6H₂O or DME·NiBr₂ (0.4 mmol) was dissolved in 5 mL ethanol and added dropwise to the corresponding ligand (0.4 mmol) which was dissolved in 10 mL dichloromethane. The mixture was stirred at room temperature for 12 h, and then diethyl ether was added to the mixture to precipitate the complex. The precipitant was collected by filtration, washed with diethyl ether, and dried under vacuum.

9-(2,6-Dicyclohexylphenylimino)-5,6,7,8-tetrahydrocycloheptapyridylnickel bromide (Ni1). (Yellow, 0.23 g, 94% yield) FT-IR (KBr, cm⁻¹): 3318, 2925, 2850, 1606, 1574, 1445, 1342, 1265, 1085, 1045, 881, 775. Anal. calcd for C₂₈H₃₆Br₂N₂Ni (616): C, 54.32; H, 5.86; N, 4.52. Found: C, 54.65; H, 6.34; N, 4.45%.

9-(2,6-Dicyclopentylphenylimino)-5,6,7,8-tetrahydrocycloheptapyridylnickel bromide (Ni2). (Yellow, 0.18 g, 76% yield) FT-IR (KBr, cm⁻¹): 3315, 2946, 2863, 2361, 1609, 1576, 1449, 1339, 769. Anal. calcd for C₂₆H₃₂Br₂N₂Ni (588): C, 52.83; H, 5.46; N, 4.74. Found: C, 52.35; H, 5.64; N, 4.66%.

9-(2,4-Dimethyl-6-cyclohexylphenylimino)-5,6,7,8-tetrahydrocycloheptapyridylnickel bromide (Ni3). (Yellow, 0.17 g, 76% yield) FT-IR (KBr, cm⁻¹): 2926, 2850, 2361, 1604, 1568, 1447, 1274, 1205, 1119, 848, 819. Anal. calcd for C₂₄H₃₀Br₂N₂Ni (562): C, 51.02; H, 5.35; N, 4.96. Found: C, 50.88; H, 5.47; N, 4.92%.

9-(2,6-Dicyclohexylphenylimino)-5,6,7,8-tetrahydrocycloheptapyridylnickel chloride (Ni4). (Yellow, 0.15 g, 71% yield) FT-IR (KBr, cm⁻¹): 3327, 2924, 2851, 2361, 1608, 1575, 1446, 1345, 1269, 1048, 883, 775. Anal. calcd for C₂₈H₃₆Cl₂N₂Ni (528): C, 63.43; H, 6.84; N, 5.28. Found: C, 62.95; H, 7.31; N, 5.21%.

9-(2,6-Dicyclopentylphenylimino)-5,6,7,8-tetrahydrocycloheptapyridylnickel chloride (Ni5). (Yellow, 0.18 g, 90% yield) FT-IR (KBr, cm⁻¹): 3307, 2947, 2865, 2361, 1610, 1576, 1449, 1339, 770. Anal. calcd for C₂₆H₃₂Cl₂N₂Ni (500): C, 62.19; H, 6.42; N, 5.58. Found: C, 61.71; H, 6.56; N, 5.21%.

9-(2,4-Dimethyl-6-cyclohexylphenylimino)-5,6,7,8-tetrahydrocycloheptapyridylnickel chloride (Ni6). (Yellow, 0.17 g, 87% yield) FT-IR (KBr, cm⁻¹): 2928, 2852, 2361, 1606, 1571, 1448, 1208, 850, 826. Anal. calcd for C₂₄H₃₀Cl₂N₂Ni (474): C, 60.54; H, 6.35; N, 5.88. Found: C, 60.41; H, 6.57; N, 5.93%.

X-ray crystallographic study

A single crystal of the nickel complex Ni1·CH₃OH suitable for X-ray diffraction was obtained by layering diethyl ether onto the mixed solution of dichloromethane and methanol at room temperature. X-ray studies were carried out using a Rigaku Saturn 724 + CCD diffractometer with MoKα 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.¹⁷ Details of the X-ray structure determinations and refinements are provided in Table 5.

Table 5 Crystal data and structure refinement for Ni1·CH₃OH

Ni1·CH3OH
Crystal colour	Colourless
Empirical formula	C29H40Br2N2NiO
Formula weight	651.16
Temperature (K)	446(2)
Wavelength (Å)	0.71073
Crystal system	Triclinic
Space group	P1
a (Å)	10.561(2)
b (Å)	10.773(2)
c ​ (Å)	14.329(3)
α (°)	79.82(3)
β (°)	88.35(3)
γ (°)	63.18(3)
Volume (Å^3)	1429.6(5)
Z	2
D calcd (mg m^−3)	1.513
μ (mm^−1)	3.498
F (000)	668
Crystal size (mm)	0.57 × 0.37 × 0.10
θ range (°)	2.15–27.51
Limiting indices	−13 ≤ h ≤ 13
	−13 ≤ k ≤ 13
	−18 ≤ l ≤ 18
No. of rflns collected	13 984
No. of unique rflns	6369
Rint	0.0682
Completeness to θ (%)	97 (θ = 27.51)
Goodness-of-fit on F^2	1.046
Final R indices [I > 2σ(I)]	R1 = 0.0556
	wR2 = 0.1474
R indices (all data)	R1 = 0.0621
	wR2 = 0.1549
Largest diff. peak and hole (e Å^3)	0.722 and −1.409

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General procedure for ethylene polymerization

Ethylene polymerizations were carried out in a 250 mL stainless steel autoclave equipped with a mechanical stirrer and a temperature controller. And the autoclave was evacuated by a vacuum pump and back-filled three times with N₂ and once with ethylene. When the required temperature was reached, 30 mL toluene (freshly distilled) was added under ethylene atmosphere, and another 20 mL toluene which dissolved the nickel pre-catalyst was injected. The required amount of co-catalyst (MMAO or Et₂AlCl) and additional toluene (maintaining the total volume as 100 mL in the reactor) were added by syringe. The reaction mixture was intensively stirred for the desired time under the desired ethylene pressure and maintained at this level by constant feeding of ethylene. The reaction was quenched by addition of acidic ethanol. The precipitated polymer was washed with ethanol several times and dried in vacuum until of constant weight.

Acknowledgements

This work is supported by National Natural Science Foundation of China (NSFC No. 21374123, 51411130208 and U1362204).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Appendix A. Crystallographic data for Ni1. CCDC 1415743. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra15806k

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