Nickel bis{4,6-dibenzhydryl-2-[(arylimino)methyl]phenoxylate} complexes: Synthesis, structures, and catalytic behaviour towards ethylene and norbornene

Abstract

A series of 4,6-dibenzhydryl-2-[(arylimino)methyl]phenol derivatives (L1–L6) and their nickel complexes (Ni1–Ni6) were synthesized and characterized by spectroscopic and elemental analyses. Molecular structures of Ni3 and Ni6 were further confirmed by single-crystal X-ray crystallographic studies. When activated with ethylaluminium sesquichloride (EASC), all nickel pre-catalysts displayed good catalytic activity (up to 2.89 × 10⁶ g mol⁻¹(Ni) h⁻¹) for ethylene dimerization. Furthermore, these nickel complexes showed high activity for norbornene polymerization in the presence of MAO.

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1. Introduction

Since the 1960s, both olefin oligo-/polymerization catalyzed by late transition metal complexes have attracted great attention in both academic and industrial research.¹ In the 1970s, nickel complexes were developed as catalysts for ethylene oligomerization by Keim and co-workers.² Following a further ten years of effort, the well-known SHOP (Shell higher olefin process) was industrialized, thereby allowing access to a whole range of ethylene oligomerization products,³ in particular linear α-olefins of C4 to C20.⁴ In the 1990s, the resurrection of late-transition metal complexes for ethylene oligo-/polymerization was initiated by the α-diiminonickel(II) pre-catalyst systems reported by Brookhart and co-workers.⁵ In addition to ethylene oligo-/polymerization, nickel complexes have been shown to exhibit high activity in the vinyl polymerization of norbornene.⁶ During these polymerization studies, numerous bidentate ligand systems such as O^∧O,⁷ N^∧O,^6,8 N^∧N,^5,9 N^∧P ¹⁰ and P∧P ¹¹ have been utilized. For olefin oligo-/polymerization, nickel complexes bearing bidentate N∧O ligands were also extensively investigated.¹²

In terms of salicylaldimino-based nickel(II) complexes,¹³ the most attractive systems were reported by the Grubbs group, and such nickel pre-catalysts were capable of ethylene polymerization without the need for co-catalysts.^13a Subsequently, research on such systems exploded with numerous research groups involved in varying the substituents of the ancillary ligands present,^13b–j with particular focus on the bulk of the substituents at the ortho-position of the phenoxylate group. However, 3,5-dibenzhydryl-2-hydroxybenzaldehyde has not yet been deployed in salicylaldimine-type ligands. Herein, the synthesis and characterization of 4,6-dibenzhydryl-2-[(arylimino)methyl]phenol derivatives and their nickel complexes are reported along with the catalytic behaviour of the latter towards ethylene oligomerization and norbornene polymerization.

2. Results and discussion

2.1. Synthesis and characterization of 4,6-dibenzhydryl-2-[(arylimino)methyl]phenol derivatives and their nickel complexes

The 4,6-dibenzhydryl-2-[(arylimino)methyl]phenols (L1–L6) were synthesized by the stoichiometric condensation reaction of 3,5-dibenzhydryl-2-hydroxybenzaldehyde with the corresponding anilines in the presence of p-toluenesulfonic acid (Scheme 1). All such organic compounds were characterized by FT-IR spectroscopy, elemental analysis and by ¹H and ¹³C NMR spectroscopy. The organic compounds were then deprotonated using n-BuLi in THF at 0 °C, and were then reacted with 0.5 equivalents of NiBr₂·DME in toluene to afford the corresponding nickel complexes (Ni1–Ni6) as yellow solids. The nickel complexes were characterized by FT-IR spectroscopy and elemental analysis, as well as by single crystal X-ray diffraction studies for the complexes Ni3 and Ni6.

Scheme 1 Synthesis of the 4,6-dibenzhydryl-2-[(arylimino)methyl]phenol derivatives and their nickel complexes.

2.2. Crystal structures

Single crystals of complexes Ni3 and Ni6 suitable for X-ray crystallography were grown by the slow diffusion of n-heptane into their CH₂Cl₂ solutions. The molecular structures of both complexes showed a near square-planar coordination geometry at the nickel center; ORTEP diagrams are shown in Fig. 1 and 2, respectively, along with their selected bond distances and angles. The Ni–O bond lengths of complex Ni3 (1.862(2) Å) and Ni6 (1.844(3) Å) are shorter than the Ni–N bond lengths of complex Ni3 (1.901(2) Å) and Ni6 (1.918(4) Å). The dihedral angle of the phenoxy ring with O(1)–Ni(1)–N(1) plane is 170.0° in Ni3, and 151.2° in Ni6, and indicate the bulky influence of the ortho-benzhydryl substituent.

Fig. 1 ORTEP view of Ni3 with 50% thermal ellipsoids. Hydrogen atoms have been omitted for clarity. Selected bond distances (Å) and angles (°): Ni(1)–O(1A) = Ni(1)–O(1) = 1.862(2), Ni(1)–N(1A) = Ni(1)–N(1) = 1.901(2), O(1)–C(1) = 1.317(4), N(1)–C(7) = 1.310(4), O(1A)–Ni(1)–O(1) = 180.00(12), O(1A)–Ni(1)–N(1A) = O(1)–Ni(1)–N(1) = 92.76(9), O(1)–Ni(1)–N(1A) = O(1A)–Ni(1)–N(1) = 87.24(9), N(1A)–Ni(1)–N(1) = 180.00(16), C(1)–O(1)–Ni(1) = 129.59(19), C(7)–N(1)–Ni(1) = 125.1(2).

Fig. 2 ORTEP view of Ni6 with 50% thermal ellipsoids. Hydrogen atoms have been omitted for clarity. Selected bond distances (Å) and angles (°): Ni(1)–O(1A) = Ni(1)–O(1) = 1.844(3), Ni(1)–N(1A) = Ni(1)–N(1) = 1.918(4), O(1)–C(1) = 1.321(6), N(1)–C(7) = 1.309(6), O(1A)–Ni(1)–O(1) = 174.6(2), O(1A)–Ni(1)–N(1A) = O(1)–Ni(1)–N(1) = 90.89(16), O(1A)–Ni(1)–N(1) = O(1)–Ni(1)–N(1A) = 89.31(16), N(1A)–Ni(1)–N(1) = 175.7(3), C(1)–O(1)–Ni(1) = 125.2(3), C(7)–N(1)–Ni(1) = 124.6(3).

2.3. Ethylene oligomerization

Nickel pre-catalyst Ni1 was used to select the most suitable co-catalyst from methylaluminoxane (MAO), modified methylaluminoxane (MMAO), diethylaluminium chloride (Et₂AlCl) and ethylaluminium sesquichloride (Et₃Al₂Cl₃, EASC) for ethylene oligomerization in toluene over 30 min under 10 atm of ethylene pressure. The results are tabulated in Table 1. The catalytic system using Et₂AlCl (Entry 3 in Table 1) exhibited only poor results, whereas for all the catalytic systems activated by MAO, MMAO, or EASC (Entries 1, 2 and 4 in Table 1), good activities with high selectivity for ethylene dimerization were observed. The system employing EASC as co-catalyst was the most promising, with the best combination of high activity using a minimum amount of co-catalyst.

Table 1 Ethylene dimerization by pre-catalyst Ni1^a

Entry	Co-cat.	Al/Ni	T/°C	t/min	Activity^b	α-C4(%)^c	C4/∑C
a Conditions: 3 μmol Ni, 100 mL toluene, 10 atm ethylene. b 10^5g mol^−1(Ni) h^−1; c Determined by GC and ∑C signifies the total amounts of oligomers.
1	MAO	1500	20	30	2.34	95.3	100
2	MMAO	1500	20	30	2.78	39.1	100
3	Et2AlCl	300	20	30	trace	—	—
4	EASC	300	20	30	3.62	99.1	100
5	EASC	400	20	30	6.38	100	100
6	EASC	500	20	30	6.52	100	100
7	EASC	600	20	30	9.51	100	100
8	EASC	700	20	30	11.5	100	100
9	EASC	800	20	30	14.5	100	100
10	EASC	900	20	30	17.5	100	100
11	EASC	1000	20	30	11.0	95.0	100
12	EASC	1200	20	30	0.87	67.5	100
13	EASC	900	30	30	28.9	95.1	100
14	EASC	900	40	30	29.2	67.5	100
15	EASC	900	50	30	38.2	55.7	100
16	EASC	900	60	30	44.6	27.9	100
17	EASC	900	70	30	18.4	22.7	100
18	EASC	900	80	30	13.1	14.4	100
19	EASC	900	30	10	21.2	84.0	100
20	EASC	900	30	20	45.0	86.1	100
21	EASC	900	30	40	28.2	80.8	100
22	EASC	900	30	60	27.8	66.2	100

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To optimize the reaction parameters, the Al/Ni molar ratio was increased to 1200 (Entries 4–12, Table 1), the highest activity was observed at the Al/Ni ratio of 900 : 1 with high selectivity for 1-butene (entry 10, Table 1). However, with higher Al/Ni ratios (entries 11 and 12, Table 1), the excessive amount of EASC resulted in a decrease of the selectivity for 1-butene.¹⁴

On fixing the Al/Ni at 900, and elevating the reaction temperature from 20 to 80 °C (Entries 13–16, Table 1), the highest activity was observed (4.46 × 10⁶ g mol⁻¹(Ni) h⁻¹) at 60 °C. Although the system continued to perform as a dimerization catalyst, the selectivity for 1-butene decreased on increasing the temperature. At higher temperature (> 60 °C), the catalytic activity was dramatically decreased (Entries 17 and 18, Table 1), consistent with decomposition of the active species at these higher temperatures.¹⁵

To balance the activity and selectivity for 1-butene, the reaction conditions of Al/Ni at 900 and a temperature of 30 °C were fixed, and the lifetime of the active species was monitored over the periods of 10, 20, 30, 40 and 60 min (entries 13, 19–22, Table 1). The activity at 10 min was lower than that observed at 30 min, whilst both were lower than that observed at 20 min. As a consequence, it was believed that there was an initiation period needed to activate these nickel species. On further extending the reaction time, the catalytic activities were maintained over one hour, though the selectivity for 1-butene decreased over time.

Given the above results, the reaction conditions employing Al/Ni 900 at 30 °C over 30 min were employed for all the nickel pre-catalysts Ni1–Ni6, and the results are collected in Table 2. For the ortho-substituents on the arylimino group,^14e,16 it was clear to see that higher activity was observed for less bulky substituents, which resulted in the activity order Ni1 ≈ Ni2 > Ni3, Ni4 > Ni5, however the selectivity for 1-butene exhibited the opposite trend. Considering the active species for these bis(salicylaldiminato)nickel complexes (Ni1–Ni6), upon activation by co-catalysts such as EASC, it is postulated that one chelate ligand will remain together with the newly formed alkyl group and coordinated ethylene at the nickel center. Such mono-ligated nickel pre-catalysts are well known to catalyze ethylene oligomerization.^8,9,13

Table 2 Ethylene dimerization with Ni1–Ni6/EASC^a

Entry	Pre-cat.	Activity^b	α-C4(%)^c	C4/∑C
a Conditions: 3 μmol Ni, 100 mL toluene, 10 atm ethylene. b 10^5g mol^−1(Ni) h^−1; c Determined by GC and ∑C signifies the total amounts of oligomers.
1	Ni1	28.9	95.1	100
2	Ni2	22.7	95.4	100
3	Ni3	16.3	97.3	100
4	Ni4	25.6	86.4	100
5	Ni5	21.8	88.9	100
6	Ni6	15.9	83.5	100

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2.4. Polymerization of norbornene

In order to explore the nickel pre-catalysts for the vinyl polymerization of norbornene, nickel complexes (Ni1–Ni6) were screened in the presence of MAO (Table 3). Similar trends for the catalytic behavior as were found for ethylene oligomerization were observed, i.e. the order was Ni1 > Ni2 > Ni3, and Ni4 > Ni5. In both cases, it could be considered that the electron-withdrawing substituents enhanced the activities.^6b,8c,17 However, the complex Ni6 bearing fluoro-substituents seemed to be the exception to the rule (particularly in the case of ethylene).

Table 3 Norbornene polymerization^a

Entry	Pre-cat.	Activity^b	Yield (%)
a Conditions: 3 μmol Ni, total volume 30 mL toluene, [norbornene]/[Ni] = 20000, MAO/[Ni] = 1000, 20 °C, 30 min. b 10^6 g PNB mol^−1 Ni h^−1.
1	Ni1	2.37	62.8
2	Ni2	2.19	58.2
3	Ni3	0.99	26.2
4	Ni4	2.17	57.7
5	Ni5	2.01	53.3
6	Ni6	2.35	62.5

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3. Conclusion

The nickel bis[4,6-dibenzhydryl-2-(aryliminomethyl)phenoxylate] complexes (Ni1–Ni6) were synthesized and characterized. Square-planar geometries at nickel were confirmed for representative complexes Ni3 and Ni6 using single-crystal X-ray diffraction studies. Upon activation with EASC, all the nickel pre-catalysts showed high activities for ethylene dimerization and with good selectivity for 1-butene. Furthermore, all nickel pre-catalysts showed high activities towards norbornene polymerization. Catalytic activities increased on increasing the bulkiness of the ortho substituents at the arylimino group.

4. Experimental

4.1. General procedure

All manipulations of air and/or moisture-sensitive compounds were carried out under an atmosphere of nitrogen using standard Schlenk techniques. Toluene was refluxed over sodium-benzophenone and distilled under nitrogen prior to use. All aniline derivatives were purchased and used as obtained. Diethylaluminium chloride (Et₂AlCl, 1.7 M in toluene) and ethylaluminum sesquichloride (EASC, 0.87 M in toluene) were purchased from Acros Chemicals. Methylaluminoxane (MAO, 1.46 M solution in toluene) and modified methylaluminoxane (MMAO, 1.93 M in heptane, 3A) were purchased from Akzo Nobel Corp. ¹H and ¹³C NMR spectra were recorded on a Bruker DMX 400 MHz instrument at ambient temperature using TMS as an internal standard. FT-IR spectra were recorded on a Perkin Elmer System 2000 FT-IR spectrometer. Elemental analyses were carried out using a Flash EA 1112 microanalyzer. GC analyses were performed with a Varian CP-3800 gas chromatograph equipped with a flame ionization detector and a 30 m (0.2 mm id., 0.25 mm film thickness) CP-Sil 5 CB column. High-purity ethylene was provided by Beijing Yansan Petrochemical Co. The yield of oligomers was calculated by referencing with the mass of the solvent on the basis of the prerequisite that the mass of each fraction was approximately proportional to its integrated areas in the GC trace. Selectivity for the linear-olefin was defined as (amount of linear-olefin of all fractions)/(total amount of oligomer products) in percent.

4.2. Synthesis of organic compound (L1–L6)

2-[(2,6-Dimethylphenylimino)methyl]-4,6-dibenzhydrylphenol (L1). A solution of 1.46 g (3.22 mmol) 3,5-dibenzhydryl-2-hydroxybenzaldehyde, 0.31 g (3.22 mmol) 2,6-dimethylbenzenamine, and 0.22 g (1.28 mmol) 4-methylbenzenesulfonic acid in 50 mL toluene was stirred for 5 h at 120 °C, following which the crude product was purified by column chromatography on silica with an eluent of petroleum ether/ethyl acetate (v/v, 100 : 1) to afford 1.52 g (2.73 mmol) of a yellow solid of L1 in 88.4% isolated yield. Mp: 145–146 °C. IR (KBr disk, cm⁻¹): 3024, 1623, 1583, 1445, 1262, 1189, 1077, 1016, 919, 741, 696. ¹H NMR (400 MHz, CDCl₃, TMS): δ 13.34 (s, 1H), 8.18 (s, 1H), 7.25–6.49 (m, 23H), 6.90 (d, J = 9.15 Hz, 2H), 6.04 (s, 1H), 5.40 (s, 1H), 2.14 (s, 6H). ¹³C NMR (100 MHz, CDCl₃, TMS): δ 18.74, 48.95, 55.97, 118.11, 124.95, 126.27, 126.44, 128.33, 128.38, 128.43, 129.38, 129.48, 130.94, 132.40, 133.84, 135.75, 143.48, 144.03, 148.25, 157.69, 167.01. Anal. Calcd for C₄₁H₃₅NO: C, 88.29; H, 6.33; N, 2.51. Found: C, 88.16; H, 6.39; N, 2.35.

2-[(2,6-diethylphenylimino)methyl]-4,6-dibenzhydrylphenol (L2). The synthetic procedure was the same as for L1, but using 3,5-dibenzhydryl-2-hydroxybenzaldehyde (1.41 g, 3.11 mmol), 2,6-diethylbenzenamine (0.46 g, 3.11 mmol), and 4-methylbenzenesulfonic acid (0.21 g, 1.22 mmol) to give yellow solid L2 (1.47 g, 81.2%, 2.51 mmol). Mp: 120–121 °C. IR (KBr disk, cm⁻¹): 3027, 1624, 1582, 1492, 1449, 1263, 1178, 1077, 1006, 908, 744, 692. ¹H NMR (400 MHz, CDCl₃, TMS): δ 13.31 (s, 1H), 8.20 (s, 1H), 7.32–7.05 (m, 23H), 6.92 (d, J = 7.50 Hz, 2H), 6.07 (s, 1H), 5.43 (s, 1H), 2.54–2.48 (m, 4H), 1.14 (t, J = 7.52 Hz, 6H). ¹³C NMR (100 MHz, CDCl₃, TMS): δ 14.87, 24.81, 48.80, 55.99, 118.05, 125.27, 126.26, 126.37, 126.44, 128.32, 128.43, 129.38, 129.48, 130.97, 132.36, 133.85, 134.37, 135.79, 143.52, 144.03, 147.49, 157.69, 166.94. Anal. Calcd for C₄₃H₃₉NO: C, 88.17; H, 6.71; N, 2.39. Found: C, 88.09; H, 6.91; N, 2.25.

2-[(2,6-diisopropylphenylimino)methyl]-4,6-dibenzhydrylphenol (L3). The synthetic procedure was the same as for L1, but using 3,5-dibenzhydryl-2-hydroxybenzaldehyde (1.44 g, 3.17 mmol), 2,6-diisopropylbenzenamine (0.56 g, 3.17 mmol), and 4-methylbenzenesulfonic acid (0.22 g, 1.28 mmol) to give yellow solid L3 (1.64 g, 84.5%, 2.67 mmol). Mp: 121–122 °C. IR (KBr disk, cm⁻¹): 3024, 1632, 1583, 1495, 1449, 1255, 1176, 1080, 1009, 908, 791, 744, 695. ¹H NMR (400 MHz, CDCl₃, TMS): δ 13.29 (s, 1H), 8.16 (s, 1H), 7.30–7.10 (m, 18H), 7.08 (d, J = 6.50 Hz, 4H), 7.05 (d, J = 7.16 Hz, 4H), 6.05 (s, 1H), 2.95 (m, 2H), 1.14 (d, J = 6.84 Hz, 12H). ¹³C NMR (100 MHz, CDCl₃, TMS): δ 23.83, 28.08, 48.88, 56.03, 118.05, 123.32, 125.51, 126.28, 126.45, 128.33, 128.44, 129.36, 129.48, 131.02, 132.35, 133.88, 135.86, 138.97, 143.53, 144.02, 146.26, 157.69, 166.38. Anal. Calcd for C₄₅H₄₃NO: C, 88.05; H, 7.06; N, 2.28. Found: C, 88.12; H, 7.19; N, 2.21.

2,4-dibenzhydryl-6-[(mesitylimino)methyl]phenol (L4). The synthetic procedure was the same as for L1, but using 3,5-dibenzhydryl-2-hydroxybenzaldehyde (1.37 g, 3.02 mmol), 2,4,6-trimethylbenzenamine (0.41 g, 3.02 mmol), and 4-methylbenzenesulfonic acid (0.21 g, 1.22 mmol) to give yellow solid L4 (1.40 g, 81.4%, 2.45 mmol). Mp: 119–120 °C. IR (KBr disk, cm⁻¹): 3028, 1621, 1599, 1493, 1446, 1263, 1200, 1075, 1009, 801, 741, 698. ¹H NMR (400 MHz, CDCl₃, TMS): δ 13.51 (s, 1H), 8.21 (s, 1H), 7.28–7.18 (m, 12H), 7.13 (d, J = 7.18 Hz, 4H), 7.07 (d, J = 7.20 Hz, 4H), 6.93-6.89 (m, 4H), 6.07 (s, 1H), 5.44 (s, 1H), 2.29 (s, 3H), 2.15 (s, 6H). ¹³C NMR (100 MHz, CDCl₃, TMS): δ 18.68, 20.89, 48.95, 55.97, 118.18, 126.25, 126.42, 128.32, 128.41, 129.06, 129.37, 129.47, 130.86, 132.34, 133.74, 134.40, 135.60, 143.51, 144.05, 145.73, 157.73, 166.94. Anal. Calcd for C₄₂H₃₇NO: C, 88.23; H, 6.52; N, 2.45. Found: C, 87.99; H, 6.71; N, 2.33.

2-[(2,6-diethyl-4-methylphenylimino)methyl]-4,6-dibenzhydrylphenol (L5). The synthetic procedure was the same as for L1, but using 3,5-dibenzhydryl-2-hydroxybenzaldehyde (1.14 g, 2.51 mmol), 2,6-diethyl-4-methylbenzenamine (0.41 g, 2.51 mmol), and 4-methylbenzenesulfonic acid (0.17 g, 0.99 mmol) to give yellow solid L5 (1.12 g, 74.7%, 1.87 mmol). Mp: 103–104 °C. IR (KBr disk, cm⁻¹): 3028, 1624, 1602, 1495, 1449, 1266, 1197, 1077, 851, 796, 736, 698. ¹H NMR (400 MHz, CDCl₃, TMS): δ 13.43 (s, 1H), 8.19 (s, 1H), 7.32–7.16 (m, 10H), 7.13 (d, J = 7.49 Hz, 4H), 2.33 (s, 3H), 1.13 (t, J = 7.52 Hz, 6H). ¹³C NMR (100 MHz, CDCl₃, TMS): δ 14.99, 21.15, 24.77, 48.89, 55.99, 118.12, 126.24, 126.42, 127.10, 128.30, 128.41, 129.37, 129.47, 130.90, 132.31, 133.74, 134.29, 134.61, 135.66, 143.55, 144.05, 145.02, 157.74, 166.97. Anal. Calcd for C₄₄H₄₁NO: C, 88.11; H, 6.89; N, 2.34. Found: C, 88.21; H, 6.97; N, 2.32.

2-[(2,6-difluorophenylimino)methyl]-4,6-dibenzhydrylphenol (L6). The synthetic procedure was the same as for L1, but using 3,5-dibenzhydryl-2-hydroxybenzaldehyde (0.96 g, 2.11 mmol), 2,6-difluorobenzenamine (0.27 g, 2.11 mmol), and 4-methylbenzenesulfonic acid (0.14 g, 0.81 mmol), which were refluxed for 6 h in 50 mL toluene solution. The products were separated by column chromatography on silica gel (dichloromethane/petroleum ether, 100 : 1) to obtain yellow solid L6 (0.66 g, 55.2%, 1.16 mmol). Mp: 120–121 °C. IR (KBr disk, cm-1): 3063, 1618, 1572, 1476, 1443, 1263, 1200, 1086, 1025, 919, 785, 695. 1H NM R (400 MHz, CDCl₃, TMS): δ 13.19 (s, 1H), 8.72 (s, 1H), 7.26–6.91 (m, 24H), 6.87 (s, 1H), 6.03 (s, 1H), 5.41 (s, 1H). ¹³C-NMR (100 MHz, CDCl₃, TMS): δ 49.03, 55.86, 112.03, 112.27, 118.50, 125.53, 126.30, 126.46, 128.33, 128.44, 129.39, 129.45, 131.57, 132.65, 134.12, 136.49, 143.31, 143.93, 154.70, 157.20, 157.85, 169.17. Anal. Calcd for C₃₉H₂₉F₂NO: C, 82.81; H, 5.17; N, 2.48. Found: C, 82.49; H, 5.36; N, 2.37.

4.3. Synthesis of the nickel complexes (Ni1–Ni6)

An n-butyllithium solution (0.31 mL, 1.6 M) was added dropwise over a 5 min period to a stirred solution of the organic compounds L1–L6 (0.5 mmol) in 15 mL of THF at 0 °C. The solution was stirred for 6 h at room temperature, NiBr₂·DME (0.08 mg, 0.25 mmol) was added and stirred for 12 h at room temperature, and then the toluene was removed in-vacuo at room temperature and 40 mL dichloromethane was added. The mixture was stirred and then filtered before washing thrice with 5 mL amounts of dichloromethane. The solution was evaporated to a small volume, and then 20 mL n-hexane was added. The mixture was stirred for 30 min and filtered, and a yellow solid was obtained and washed with 10 mL n-hexane for three times. Drying in-vacuo produced the desired nickel complexes (Ni1–Ni6).

Nickel bis({2-[(2,6-dimethylphenylimino)methyl]-4,6-dibenzhydryl}phenolate) (Ni1). Yield: 0.11 g, 32.4%. IR (KBr disk, cm⁻¹): 3394, 1601, 1542, 1493, 1438, 1329, 1173, 1028, 851, 736, 698. Anal. Calcd for C₈₂H₆₈N₂NiO₂: C, 84.03; H, 5.85; N, 2.39. Found: C, 83.89; H, 5.69; N, 2.14.

Nickel bis({2-[(2,6-diethylphenylimino)methyl]-4,6-dibenzhydryl}phenolate) (Ni2). Yield: 0.15 g, 41.7%. IR (KBr disk, cm⁻¹): 3415, 1596, 1542, 1495, 1435, 1331, 1173, 1027, 851, 733, 698. Anal. Calcd for C₈₆H₇₈N₂NiO₂: C, 84.10; H, 6.24; N, 2.28. Found: C, 83.99; H, 6.12; N, 2.19.

Nickel bis({2-[(2,6-diisopropylphenylimino)methyl]-4,6-dibenzhydryl}phenolate) (Ni3). Yield: 0.21 g, 57.0%. IR (KBr disk, cm⁻¹): 3426, 1599, 1542, 1495, 1435, 1328, 1173, 1031, 848, 739, 695. Anal. Calcd for C₉₀H₈₄N₂NiO₂: C, 84.17; H, 6.59; N, 2.18. Found: C, 83.81; H, 6.30; N, 2.08.

Nickel bis({2,4-dibenzhydryl-6-[(mesitylimino)methyl]}phenolate) (Ni4). Yield: 0.08 g, 23%. IR (KBr disk, cm⁻¹): 3380, 1598, 1540, 1496, 1437, 1330, 1192, 1131, 853, 741, 698. Anal. Calcd for C₈₄H₇₂N₂NiO₂: C, 84.06; H, 6.05; N, 2.33. Found: C, 84.27; H, 5,89; N, 2.47.

Nickel bis({2-[(2,6-diethyl-4-methylphenylimino)methyl]-4,6-dibenzhydryl}phenolate) (Ni5). Yield: 0.19 g, 48.7%. IR (KBr disk, cm⁻¹): 3380, 1591, 1539, 1495, 1435, 1340, 1189, 1140, 859, 739, 698. Anal. Calcd for C₈₈H₈₀N₂NiO₂: C, 84.13; H, 6.42; N, 2.23. Found: C, 83.97; H, 6.22; N, 1.90.

Nickel bis({2-[(2,6-difluorophenylimino)methyl]-4,6-dibenzhydryl}phenolate) (Ni6). Yield: 0.18 g, 51.4%. IR (KBr disk, cm⁻¹): 3571, 1585, 1536, 1493, 1432, 1334, 1187, 1157, 1009, 848, 730, 695. Anal. Calcd for C₇₈H₅₆F₄N₂NiO₂: C, 78.86; H, 4.75; N, 2.36. Found: C, 78.79; H, 4.44; N, 2.09.

4.4. Procedure for oligomerization at 10 atm ethylene pressure

A 250 mL autoclave stainless steel reactor was equipped with a mechanical stirrer and a temperature control. A 50 mL amount of toluene containing the catalyst precursor was transferred to the fully dried reactor under an ethylene atmosphere. The addition of the aluminium co-catalyst and more toluene maintained a total volume of 100 mL toluene. At the reaction temperature, the reactor was sealed and pressurized to high ethylene pressure, and the ethylene pressure was maintained by feeding of ethylene. After stirring for the desired period of time, the reaction was stopped and cooled to room temperature. After that, the pressure was released, and about 2 mL of the reaction solution was collected and terminated by addition of 10% aqueous hydrogen chloride. The organic layer was analyzed by gas chromatography (GC) to determine the composition and mass distribution of the oligomers. The residual reaction solution was quenched with 5% hydrochloric acid ethanol.

4.5. Procedure for polymerization of norbornene

The nickel complexes (3 μmol) were dissolved in a Schlenk tube (100 mL) in 13 mL toluene under nitrogen and a 15 mL toluene solution of norbornene (4 M) was added via a syringe. After that, a 2.0 mL toluene solution of MAO (1.46 M) was added via syringe to start the polymerization. After 30 min, the polymerization was stopped by the addition of 50 mL acidic ethanol (ethanol/concentrated HCl = 10 : 1, v/v). The precipitated polymer was filtered, washed with methanol, and dried in-vacuo at 60 °C for 12 h. The activity did not lead to a significant warming of the reaction mixture, so, the temperature was basically kept constant.

4.6. X-ray crystallography

Single crystals of complexes Ni3 and Ni6 suitable for X-ray structural determination were grown by the slow diffusion of n-heptane into the respective CH₂Cl₂ solutions. X-ray diffraction studies were carried out on a Rigaku Saturn 724+ CCD with graphite-monochromatic Mo-Kα radiation (k = 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 (Table 4).¹⁸

Table 4 Crystal data and structure refinement details for Ni3 and Ni6

	Ni3	Ni6
Empirical formula	C90H84N2NiO2	C78H56F4N2NiO2
FW	1284.30	1187.96
T/K	173(2)	173(2)
Wavelength (Å)	0.71073	0.71073
Cryst syst.	Triclinic	Monoclinic
Space group	P1̄	C2/c
a/Å	10.515(2)	47.225(9)
b/Å	13.887(3)	16.333(3)
c/Å	15.272(3)	17.278(4)
α (°)	80.61(3)	90
β (°)	71.05(3)	110.26(3)
γ (°)	70.10(3)	90
V/Å^3	1979.8(7)	12503(4)
Z	1	8
D calcd (Mg m^−3)	1.077	1.262
μ/mm^−1	0.291	0.372
F(000)	682	4944
Cryst size/mm	0.25 × 0.22 × 0.20	0.28 × 0.22 × 0.20
θ range (°)	1.41–27.48	0.92–25.00
Limiting indices	−13 ≤ h ≤ 13	−56 ≤ h ≤ 56
	−17 ≤ k ≤ 18	−19 ≤ k ≤ 19
	−19 ≤ l ≤ 19	−17 ≤ l ≤ 20
No. of rflns collected	17 859	34 999
No. unique rflns [R(int)]	9014 (0.0475)	10994 (0.0691)
Completeness to θ (%)	99.3	99.8
Data/restraints/params	9014/0/434	10 994/0/786
Goodness of fit on F^2	1.144	1.175
Final R indices [I > 2σ(I)]	R 1 = 0.0787	R 1 = 0.0974
	wR2 = 0.2086	wR2 = 0.2313
R indices (all data)	R 1 = 0.0985	R 1 = 0.1155
	wR2 = 0.2206	wR2 = 0.2436
Largest diff peak and hole/e Å^−3	0.422 and −0.569	0.674 and −1.363

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Acknowledgements

This work is supported by the MOST 863 program No. 2009AA034601. The EPSRC are thanked for the awarded of a travel grant (to CR).

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Footnote

† CCDC reference numbers CCDC reference numbers 862949 and 862950 for crystallographic data of complexes Ni3 and Ni6. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c2cy20028g

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