Fluorinated Sterically Bulky Mononuclear and Binuclear 2-Iminopyridylnickel Halides for Ethylene Polymerization: Effects of Ligand Frameworks and Remote Substituents

Abstract

In the present work, four new mono(imino)pyridine ligands, 2-((2,4-bis(bis(4-R-phenyl)methyl)-6-fluorophenylimino)methyl)pyridine (R = H, L1; R = OCH₃, L2; R = F, L3) and 2-((2-(bis(4-fluorophenyl)methyl)-4-((3-(bis(4-fluorophenyl)methyl)-4-amine-5-fluoro-phenyl)(phenyl)methyl)-6-fluorophenylimino)methyl)pyridine (L4), have been designed in good yields. Additionally, three novel benzhydryl-bridged bis(imino)pyridine ligands, 2-(2-(bis(4-R-phenyl)methyl)-6-fluoro-phenylimino)pyridine (R = H, L5; R = OCH₃, L6; R = F, L7), were also prepared for comparison. All these organic compounds have been characterized by FT-IR analysis, ¹H/¹³C NMR spectroscopy, and elemental analysis. The treatment of L1–L7 with nickel halides afforded the corresponding monometallic (Ni1–Ni4) and bimetallic (Ni5–Ni7) nickel complexes in moderate to good overall yields. Upon activation with methylaluminoxane (MAO), Ni4_Cl showed the highest activity up to 8.3 × 10⁶ g of polyethylene (PE) (mol of Ni)⁻¹ h^–1 among Ni1–Ni7 for ethylene polymerization. In all cases, unsaturated PEs with low molecular weights (0.7–13.3 kg mol^–1) were produced effectively. The introduction of remote para-substituents into the benzhydryl groups showed a beneficial effect on catalytic activity with the overall activities following the order of Ni–F > Ni–OCH₃ > Ni–H. In addition, these para-substituents were also found to affect not only the catalytic performance of catalysts but also the branching content of the PE product. Generally, the resultant PE waxes were moderately branched and contained both terminal vinyls (−CH=CH₂) and internal vinylenes (−CH=CH−) while with different ratios of vinyls to vinylenes. Notably, the polymers produced using para-methoxy-substituted Ni2/MAO and Ni6/MAO possessed the least branching content and uniquely high vinyl contributions.

Introduction

Because N,N-bidentate nickel complexes were discovered to produce highly branched polyethylenes (PEs) through the unique “chain walking” mechanism,¹ explosive investigations have been carried out to develop new nickel catalytic systems by modifying the substituents on the parent ligand set,² or employing newly designed bidentate ligands.³ Over the years, 2-iminopyridylnickel complexes as representative N,N-bidentate precatalysts have been explored for their catalytic potential toward ethylene oligomerization or polymerization.^4,5 In order to overcome the disadvantages of easy decomposition of 2-iminopyridylnickel complexes and undesired production of polymers of low molecular weights at elevated reaction temperatures, benzhydryl-substituted anilines were developed by successfully modifying 2-iminopyridyl-based bidentate ligands, and the resulting nickel precatalysts showed an obvious enhancement in both their catalytic activity and thermal stability.^4d,4e,6 Interestingly, by utilizing a sterically bulky benzhydryl-substituted 1-aminonaphthalene (A, Chart 1)^4d,4e or phenylamine (B, Chart 1)^6a derivatives, the resulting nickel catalysts were highly active in ethylene polymerization with excellent activities of up to 10⁷ g of PE (mol of Ni)⁻¹ h^–1.

Chart 1. Pyridylarylimine Family of Mononickel Complexes (A, B, G, and H), Aryl-Linked Binuclear Nickel Complexes (C, D), Methylene-Bridged Binuclear Nickel Complexes (E), and Benzhydryl-Bridged Binuclear Nickel Complexes (I).

In addition, the neighboring multinuclear centers in the bimetallic complexes were also reported to have improved catalytic performances via cooperative metal–metal interactions imparted by the close proximity of the two metal centers.^5,7 The recent progress in bridged bis(pyridinylimino) nickel complexes together with other transition-metal systems has been summarized in numerous review articles.⁸ Aryl-linked, methylene-bridged multidentate binucleating ligands have been proved to be compatible supports for active dinickel catalysts;⁵ additionally, the steric bulk of the corresponding bridged bis(pyridinylimino) nickel complexes, which depends on the type of the bridging group, conversely affects the catalytic activity and polymer properties. By changing the types of imino-carbon substituents on the pyridinylimino moiety and the complexity of bridged groups including the variation of N-aryl groups and the connection mode between pyridinylimino moieties, several examples of bridged bis(pyridinylimino) nickel complexes (C, D, and E, Chart 1) were found to exhibit different catalytic performances in ethylene oligomerization and polymerization.^{5a−5d,5f,5f} However, in all cases, the bridged bis(pyridinylimino) ligands were formed exclusively through N-aryl linkages or methylene-bridged N-aryl linkages with no electron-withdrawing substituents on the N-aryl groups.

In consideration of the beneficial effects of the benzhydryl and electron-withdrawing substituents on the late-transition-metal catalysts,⁹ and in order to further investigate the scope of the bridged bis(pyridinylimino) nickel complexes and their catalytic activities, we have designed in this work a benzhydryl-bridged structure (I, Chart 1) with the fluorine atoms and para-R-substituted benzhydryl groups at 2- and 6- positions, respectively, of the N-aryl group. For the purpose of comparison, a series of mononuclear nickel complexes (G and H, Chart 1) with almost half the structure of binuclear nickel complexes have also been designed and applied in ethylene polymerization. Full details of the synthesis and characterization of all seven fluorinated ligands and their sterically mononuclear and binuclear 2-iminopyridylnickel halides are reported, along with an investigation of their catalytic behavior. In particular, the electronic effects of remote para-substituents (H, OCH₃, and F) in benzhydryl groups on the catalytic performance have been systematically evaluated, enabling the efficient modulation of the polymerization and microstructures of PE products.¹⁰

Results and Discussion

Synthesis and Characterization of L1–L7 and Ni1–Ni7

A family of aniline (A1–A3) along with three type of dianiline (A4–A6) bridged by benzhydryl group was successfully synthesized by following the method reported elsewhere,^6a,11 with the specific synthesis steps included in the Supporting Information. Treatment of 2-acetylpyridine with A1–A4 at a stoichiometry of 3:2 for A1–A3 and 1:2 for A4 in toluene at the refluxing temperature in the presence of a catalytic amount of p-TsOH produced corresponding ligands [2-N{2-F-4-R²-6-((4-R¹C₆H₄)₂CH)}C₇H₇N] (R¹ = H, R² = Θ, L1; R¹ = OCH₃, R² = Θ, L2; R¹ = F, R² = Θ, L3; R¹ = F, R² = Θ′, L4). While the condensation reactions of 2-acetylpyridine with A4–A6 at a molar ratio of 5:2 provided other three ligands [CH(C₆H₅){2-(2-F-6-((4-R¹C₆H₄)₂CH)C₆H₂N=CMe)}₂] (R¹ = H, L5; R¹ = OCH₃, L6; R¹ = F, L7) in good to moderate yield. The detailed synthetic routes are illustrated in Scheme 1. All organic compounds were characterized with FT-IR, ¹H NMR, and ¹³C NMR spectra (Figures S1–S14) as well as by elemental analysis. The corresponding monometallic (Ni1–Ni4) and bimetallic (Ni5–Ni7) nickel complexes were formed via the coordination of the obtained ligands with 1 or 2 equiv of NiCl₂·6H₂O or (DME)NiBr₂ in good to high yield (54–86%), respectively (Scheme 1), and were characterized with FT-IR spectroscopy and elemental analysis. In addition, the crystal of Ni3_Cl·1/3H₂O has been characterized with single-crystal X-ray diffraction.

Scheme 1. Synthetic Procedures of the Ligands and Their Nickel Complexes.

The crystal (Ni3_Cl·1/3H₂O) suitable for the X-ray structure determination was grown by layering diethyl ether onto the dichloromethane (DCM) solutions at room temperature over a long period of time (several months). The molecular structure of Ni3_Cl·1/3H₂O showed two independent molecules (A and B) in one cell with minimal differences between them. A view of Ni3_Cl·1/3H₂O (molecule A) is depicted in Figure 1 with selected bond lengths and angles listed in Table S1. It should be noted that, due to the long crystallization time required, some of the Ni complexes might have decomposed and the crystals could be formed from the more stable Ni complexes resulting from the decomposition after long standing.

Figure 1.

(a) ORTEP representation of Ni3_Cl·1/3H₂O and (b) a view with the N-aryl groups removed. The thermal ellipsoids are shown at the 30% probability level and the hydrogen atoms have been omitted for clarity.

The molecular structure of Ni3_Cl·1/3H₂O is based on a trinuclear core, Ni₃Cl₂(μ₂-Cl)₃(μ₃-Cl), in which Cl2 triply bridges all three nickel centers, while Cl3, Cl4, and Cl5 act as bridging ligands between two adjacent metals; monodentate Cl1 and Cl6 are additionally bound to Ni1 and Ni3, respectively. Besides, six nitrogen atoms belonging to three neutral chelating ligands along with a water molecule coordinated with a nickel center to complete the coordination spheres. Such a coordination mode leads to a distorted octahedral geometry at the nickel center Ni1, while Ni2 and Ni3 are located in a seven-coordination environment (Figure 1b). The Ni–N_pyridine distances [2.042(4) Å for Ni1–N1, 2.040(4) for Ni2–N3, and 2.042(4) Å for Ni3–N5] in Ni3_Cl are shorter than the corresponding Ni–N_imine distances [2.088(4) Å for Ni1–N2, 2.076(4) for Ni2–N4, and 2.101(4) Å for Ni3–N6], as a result of the good donor properties of the central pyridine; similar structural features have been reported elsewhere.^4d,4e,6 In addition, the imine vectors are essentially coplanar with the pyridine ring, whereas the 2-F-4,6-difluorobenzhydrylphenylimino group is almost perpendicular to the chelate plane [the dihedral angles are 82.52° for Ni1, 83.68° for Ni2, and 78.46° for Ni3]. Stronger intramolecular hydrogen-bonding interactions between the coordinated water and Cl4 and Cl6 [OH···Cl(4) 2.459 Å, OH···Cl(6) 2.221 Å] were also observed in this case compared with previous work. There are no intermolecular contacts of note. According to the FT-IR spectra, the C=N stretching vibrations in complexes Ni1–Ni7 shift to lower wavenumbers in the region 1631–1621 cm^–1 with weaker intensities compared to those at 1645–1640 cm^–1 for the free ligands L1–L7, indicating the effective coordination between the nitrogen atoms and the nickel center.

Catalytic Behavior toward Ethylene Polymerization

In order to determine the most suitable cocatalyst, various alkylaluminum reagents, such as methylaluminoxane (MAO), modified methylaluminoxane (MMAO), dimethylaluminum chloride (Me₂AlCl), and ethylaluminum sesquichloride (Et₃Al₂Cl₃, EASC) were used to activate the nickel precatalyst Ni1 for ethylene polymerization at 30 °C under 10 atm of ethylene with a run time of 30 min (see Table 1). All the cocatalysts were efficient in activating Ni1 with Ni1/MAO giving the highest activity up to 2.6 × 10⁶ g of PE (mol of Ni)⁻¹ h^–1. As such, MAO has been used for all following investigations. Tables 2 and 3 summarize all the polymerization runs with various nickel precatalysts in the presence of MAO. In particular, Ni1_Cl/MAO and Ni5_Cl/MAO have been chosen as the catalyst systems to further optimize the polymerization parameters, such as the molar ratio of Al/Ni, reaction time, and reaction temperature.

Table 1. Ethylene Polymerization Using Ni1_Cl with a Range of Different Cocatalysts^a.

entry	co-cat.	Al/Ni	act.	Mwb	Mw/Mnb	Tmc
1	MAO	2500	2.6	0.9	1.6	91.5, 103.8
2	MMAO	2500	1.2	1.0	1.6	96.4, 106.1, 113.5
3	Me2AlCl	400	1.7	0.8	1.4	75.6, 99.82
4	EASC	400	1.0	0.8	1.4	75.6, 99.6

^aGeneral conditions: 2.0 μmol of Ni1_Cl, 100 mL of toluene, 10 atm of ethylene, 30 min, 30 °C, and activity in units of 10⁶ g of PE (mol of Ni)⁻¹ h^–1.

^bM_w in units of kg mol^–1, determined by GPC.

^cT_m in units of °C, determined by DSC.

Table 2. Catalytic Properties of Mononuclear Nickel Complexes Ni1_Cl–Ni4_Cl and Ni1_Br–Ni4_Br Activated with MAO^a.

entry	pre-cat.	T	t	Al/Ni	act.	Mwb	Mw/Mnb	Tmc
1	Ni1Cl	30	30	1000	1.7	1.1	1.6	85.1, 104.7
2	Ni1Cl	30	30	1500	1.8	1.0	1.6	93.1, 104.3
3	Ni1Cl	30	30	2000	2.1	1.0	1.6	93.4, 105.1
4	Ni1Cl	30	30	2500	2.6	1.0	1.6	91.5, 103.8
5	Ni1Cl	30	30	3000	2.4	0.9	1.5	80.5, 103.1
6	Ni1Cl	20	30	2500	0.5	1.4	1.7	102.2, 112.3
7	Ni1Cl	40	30	2500	1.9	0.7	1.5	72.1, 95.3
8	Ni1Cl	50	30	2500	1.6	0.7	1.3	75.3
9	Ni1Cl	30	5	2500	2.0	0.9	1.4	81.1, 103.3
10	Ni1Cl	30	15	2500	2.9	1.0	1.5	90.8, 104.2
11	Ni1Cl	30	45	2500	2.5	1.0	1.5	93.4, 104.2
12	Ni1Cl	30	60	2500	2.4	1.0	1.6	90.8, 103.3
13	Ni2Cl	30	30	2500	4.8	10.8	10.3	123.6
14	Ni3Cl	30	30	2500	7.0	0.8	1.6	72.3, 100.4
15	Ni4Cl	30	30	2500	8.3	1.4	2.2	96.5, 116.2
16	Ni1Br	30	30	2500	2.6	1.0	1.6	93.5, 104.9
17	Ni2Br	30	30	2500	2.8	11.7	10.5	124.2
18	Ni3Br	30	30	2500	3.7	1.1	1.7	96.5, 106.5
19	Ni4Br	30	30	2500	5.9	1.5	2.5	97.1, 115.9

^aGeneral conditions: 2.0 μmol of Ni, 100 mL of toluene for 10 atm of ethylene, T in units of °C, t in units of min, and activity in units of 10⁶ g of PE (mol of Ni)⁻¹ h^–1.

^bM_w in units of kg mol^–1, determined by GPC.

^cT_m in units of °C, determined by DSC.

Table 3. Catalytic Properties of Binuclear Nickel Complexes Ni5_Cl–Ni7_Cl and Ni5_Br–Ni7_Br Activated with MAO^a.

entry	pre-cat.	T	t	Al/Ni	act.	Mwb	Mw/Mnb	Tmc
1	Ni5Cl	30	30	1000	1.1	3.1	3.1	80.2, 103.3, 121.5
2	Ni5Cl	30	30	1500	2.6	2.3	2.7	73.7, 100.4, 119.4
3	Ni5Cl	30	30	2000	2.7	1.8	2.3	100.7, 117.3
4	Ni5Cl	30	30	2500	3.0	1.6	2.1	98.5, 115.2
5	Ni5Cl	30	30	3000	2.8	1.5	1.9	110.3, 116.6
6	Ni5Cl	20	30	2500	1.9	2.4	2.4	104.7, 115.3, 120.2
7	Ni5Cl	40	30	2500	2.2	1.3	2.0	77.3, 95.3, 113.9
8	Ni5Cl	50	30	2500	1.9	1.1	2.0	72.3, 96.8, 115.0
9	Ni5Cl	30	5	2500	2.2	1.3	1.7	83.1, 104.7, 116.4
10	Ni5Cl	30	15	2500	3.6	1.5	1.8	106.5, 119.0
11	Ni5Cl	30	45	2500	2.7	1.6	2.2	98.1, 115.7
12	Ni5Cl	30	60	2500	2.7	1.9	2.5	99.9, 117.7
13	Ni6Cl	30	30	2500	3.0	11.8	8.9	124.7
14	Ni7Cl	30	30	2500	4.8	1.2	1.7	99.0, 105.6, 116.8
15	Ni5Br	30	30	2500	1.3	1.7	1.9	77.4, 104.6, 117.7
16	Ni6Br	30	30	2500	1.5	13.3	8.9	125.3
17	Ni7Br	30	30	2500	3.2	1.5	2.0	98.8, 106.1, 116.8

^aGeneral conditions: 2.0 μmol of Ni, 100 mL of toluene for 10 atm of ethylene, T in units of °C, t in units of min, and activity in units of 10⁶ g of PE (mol of Ni)⁻¹ h^–1.

^bM_w in units of kg mol^–1, determined by GPC.

^cT_m in units of °C, determined by DSC.

Ethylene Polymerization with Mononuclear Ni1–Ni4/MAO Systems

First, with Ni1_Cl/MAO at a fixed temperature of 30 °C, the effect of the Al/Ni molar ratio between 1000 and 3000 was investigated (entries 1–5, Table 2). Examination of the data indicates that the catalytic activity was enhanced upon the increase of the Al/Ni ratio from 1000 up to 3000 with the highest activity of 2.6 × 10⁶ g of PE (mol of Ni)⁻¹ h^–1 reached at the optimum Al/Ni ratio of 2500 (entry 4, Table 2). A further increase of the Al/Ni molar ratio to 3000 led to a slight decrease in activity (entry 5, Table 2), suggesting the higher rate of chain termination than that of chain propagation at the molar ratio of Al/Ni.^9b,12 The same reason could be used to explain the downward trend in the molecular weight of the resulting polymers from 1.1 to 0.9 kg mol^–1 upon increasing the ratio from 1000 to 3000 (Figure 2a). Meanwhile, the corresponding polymer samples showed narrow dispersities in the range of 1.5–1.6, highlighting the presence of single-site active species during the polymerization process.

Figure 2.

(a) Effects of the Al/Ni molar ratio on the catalytic activity and molecular weight (M_w) of PEs generated using Ni1_Cl/MAO (entries 1–5, Table 2); (b) GPC curves of PEs produced at various Al/Ni molar ratios.

Second, to understand the thermal stability of this catalytic system, reactions were carried out at various temperatures from 20 to 50 °C (entries 4 and 6–8, Table 2) with the Al/Ni ratio fixed at 2500 and the reaction time set at 30 min. The peak catalytic activity of 2.6 × 10⁶ g of PE (mol of Ni)⁻¹ h^–1 was reached at 30 °C (entry 4, Table 2). Further elevating the temperature from 30 to 50 °C led to a gradual decrease in the activity down to 1.6 × 10⁶ g of PE (mol of Ni)⁻¹ h^–1 at 50 °C (entry 8, Table 2), which can be ascribed to the partial deactivation of the active species and lowered ethylene solubility in toluene at elevated temperatures.^6b,13,14 In terms of the molecular weight of the resultant polymer, lower molecular weights were achieved at higher reaction temperatures in line with enhanced chain transfer/chain termination at the higher temperature compared to chain propagation (Figure 3).¹⁵ Similar trends have been previously reported for nickel precatalysts.^14,16 Additionally, the values of M_w/M_n gradually became narrower from 1.7 to 1.3 with the increasing temperature (Figure 3a), suggesting that the higher temperature was favorable for the formation of a single active site.

Figure 3.

(a) Effects of temperature on the catalytic activity and molecular weight of PEs produced using Ni1_Cl/MAO (entries 4, 6–8, Table 2); (b) GPC curves of PEs produced at various temperatures.

Third, with the temperature set at 30 °C and the Al/Ni ratio at 2500, the lifetime of active species was investigated by conducting the polymerization between 5 and 60 min (entries 4, 9–12, Table 2). As seen from Figure 4a, there is a long induction period required to form the active species following the addition of MAO, with the highest activity of 2.9 × 10⁶ g of PE (mol of Ni)⁻¹ h^–1 observed at a reaction time of 15 min (entry 10, Table 2). With the reaction time extended, the activities slightly decreased and reached the lowest value of 2.4 × 10⁶ g of PE (mol of Ni)⁻¹ h^–1 at a reaction time of 60 min (entry 12, Table 2) due to the deactivation of some active species over time (Figure 4).¹² A slight drop of the activity also highlights the good stability of these active species in this catalytic system.^4d,4e,17 Consequently, the resultant PEs show higher molecular weights with increasing reaction times, with the highest M_w of 1.0 kg mol^–1 achieved at 60 min. In all cases, narrow molecular weight distributions were again observed [polymer dispersity index (PDI): 1.4–1.6].

Figure 4.

(a) Effects of the reaction time on the catalytic activity and the molecular weight of PEs produced using Ni1_Cl/MAO (entries 4, 9–12, Table 2); (b) GPC curves of PEs produced at different reaction times.

Finally, in order to investigate the effects of remote para-substituents of benzhydryl groups and the ligand structure on the catalytic properties, the remaining precatalysts, Ni2–Ni4, were also evaluated in combination with MAO using the optimized conditions established for Ni1_Cl [that is, Al/Ni ratio = 2500, run temp = 30 °C, and run time = 30 min]. In general, all the nickel chloride complexes showed higher activities than the counter nickel bromide complexes while producing PEs of lower molecular weights. Inspection of the polymerization results reveals the overall activity of mononuclear nickel chloride complexes decreases in the order of Ni4_Cl [F–NH₂] > Ni3_Cl [F] > Ni2_Cl [OCH₃] > Ni1_Cl [H] (see Figure 5), with the most sterically bulky Ni4_Cl showing the highest activity up to 8.3 × 10⁶ g of PE (mol of Ni)⁻¹ h^–1. The difference between Ni3_Cl and Ni4_Cl lies in the type of para-substituents on the N-aryl groups, illustrating that the presence of an elongated benzene chain containing amino group on the para-position of N-aryl groups promoted chain propagation. Clearly, the electronic properties of the para-substituent on benzhydryl groups had great effects on catalytic activity and polymer properties. However, it should be noted that the NH₂ group in Ni4_Cl may react with MAO, affording metal amide by deprotonation.

Figure 5.

Comparison of catalytic activities and molecular weights of the PEs generated using mononuclear nickel complexes (Ni1–Ni4) with MAO as the cocatalyst (entries 4, 13–19, Table 2).

Similar to those findings in a previous work,¹⁸ both electron-withdrawing and electron-donating remote substituents are beneficial to the improvement of catalytic activity. In this work, nickel complexes containing the di(4-methoxyphenyl)methyl group were less active than those containing the di(4-fluorophenyl)methyl group while affording PEs of a much broader molecular weight distribution, indicating the multisite behavior of the active species.^18c,18d (See Table 2). The notably high melting temperature (T_m = 123.6 °C) of the PE generated by Ni2_Cl is in line with its high weight-average molecular weight. Interestingly, mononuclear nickel complexes other than Ni2 produced PEs with relatively lower weight-average molecular weights than those obtained by structurally related nickel complexes in previous works.^14,16 This may result from the presence of ortho-fluorine, which can neither prevent the elimination of β-H by the interaction between β-H and fluorine nor protect the active center sterically, thus promoting the production of lower-molecular-weight PEs.¹⁹ Moreover, the nickel bromide complexes exhibit the same order of activities as the nickel chloride complexes in accordance with the electronic nature of R¹ (Figure 5).

Ethylene Polymerization with the Ni5–Ni7/MAO System

To compare with mononuclear nickel complexes, binuclear nickel complexes Ni5_Cl were evaluated for optimum conditions. Following the above screening sequence, the Al/Ni ratio was first varied from 1000 up to 3000 (entries 1–5, Table 3), and the most suitable Al/Ni ratio was 2500 (entry 4, Table 3), with the highest activity of 3.0 × 10⁶ g of PE (mol of Ni)⁻¹ h^–1. The profile in Figure S15 displays the decreasing molecular weights of resulting PEs with increasing Al/Ni molar ratios, which is ascribed to enhanced chain transfer relative to chain propagation with increasing aluminum concentration.²⁰ This finding is commonly observed for late transition metal complex precatalysts.²¹ In comparison with the mononuclear nickel systems, the Ni5_Cl catalytic system is more active and the resultant PEs exhibit relatively higher molecular weights as well as higher melting points. These enhanced molecular weights may result from the cooperative interaction between the growing polymer chain and the metal center that efficiently inhibits chain transfer or the deactivation of the active center.²² The much higher PDI values of the polymers obtained with Ni5_Cl compared to Ni1_Cl are partly due to the low solubility of binuclear nickel complex Ni5_Cl in toluene.^22b

In order to explore the difference of thermal stability between binuclear nickel complexes and their corresponding mononuclear ones, the range of reaction temperature was still set from 20 to 50 °C (entries 4 and 6–8, Table 3). The results showed that the same optimum temperature of 30 °C was observed in Ni5_Cl/MAO catalytic system as Ni1_Cl/MAO, with the highest activity of 3.0 × 10⁶ g of PE (mol of Ni)⁻¹ h^–1. The decrease in activity on elevating the reaction temperature from 40 to 50 °C is probably due to the active species being partly unstable at higher temperatures (entries 7 and 8, Table 3).^13,14 For each temperature, a higher activity was observed with the Ni5_Cl/MAO catalytic system than that with Ni1_Cl/MAO, suggesting the better thermal stability of the binuclear nickel active centers. The gel permeation chromatography (GPC) curves in Figure S16b exhibit the same trend as those with the Ni1_Cl/MAO system, with lower-molecular-weight PEs obtained at higher temperatures. Under the same conditions, namely the same temperature and Al/Ni ratio, the molecular weight of the resultant PE obtained with binuclear nickel complexes is much higher than that obtained with the mononuclear nickel counterpart, reflecting the improved stability of binuclear metal catalysts. Moreover, the narrow dispersity (M_w/M_n < 2.4) provided evidence in support of the single-site nature of the active species.

To explore the effect of time on the performance of Ni5_Cl/MAO, the polymerization was quenched within different periods between 5 and 60 min (entries 4 and 9–12, Table 3) with the temperature and the Al/Ni ratio kept at 30 °C and 2500, respectively. The polymerization activity rose first reaching the highest activity of 3.6 × 10⁶ g of PE (mol of Ni)⁻¹ h^–1 at 15 min and then gradually decreased as the run time was extended due to the deactivation of active species. Nevertheless, after 60 min, the polymerization activity still remained relatively high at 2.7 × 10⁶ g of PE (mol of Ni)⁻¹ h^–1 highlighting the long lifetime of these bimetallic catalysts. On prolonging the reaction time, the obtained PEs showed higher molecular weights and wider polydispersities (M_w/M_n: from 1.7 to 2.5), which is consistent with the observations for the reported mononuclear nickel analogues bearing the 2-iminopyridine ligand system (Figure S17).^4d,6a,23

Based on the above results, the optimum conditions with an Al/Ni ratio of 2500 at 30 °C were utilized to explore the catalytic behavior of all the other five binuclear nickel precatalysts (entries 13–17, Table 3). With the various ligands bearing different remote substituents, the catalytic performance of the bimetallic systems is highly similar to that of the monometallic analogues, with the activity following the order of Ni7 [F] > Ni6 [OCH₃] > Ni5 [H]. This indicates the positive role of electron-withdrawing groups in improving the catalytic activity despite the remoteness (Figure 6). In general, the bimetallic systems exhibited a slightly higher activity and produced higher-molecular-weight PEs than the monometallic ones. Such a finding can be accredited to the stronger interaction between the metal center and growing polymer and the bulkier substituent as the bridge-groups on the para-position of N-aryl groups impede chain transfer, leading to more effective propagation in the bimetallic systems.^8c,13,18 Again binuclear nickel bromide complexes were found to be less active for ethylene polymerization than binuclear nickel chloride analogues, consistent with the observations of mononuclear nickel complexes. As seen with the polymers formed using monometallic systems Ni1, Ni2, and Ni4, the PEs produced with bimetallic Ni5 and Ni7 also showed low polydispersities (1.7–2.0), indicating their single site nature. In line with previous studies,^18b the nickel complexes containing methoxy in this work (Ni2 and Ni6) produced PEs with the highest molecular weights, the broadest molecular weight distributions, as well as the highest T_m’s in each set.

Figure 6.

Comparison of catalytic activities and molecular weights of PEs produced with binuclear nickel complexes (Ni5–Ni7) with MAO as the cocatalyst (entries 4, 13–17, Table 3).

Microstructural Analysis of the PEs

The melting points of all resultant PEs in this study exhibited lower T_m values in the range 72.1–125.3 °C and differential scanning calorimetry (DSC) curves of most of PE products showed several broad melting absorption peaks, suggesting the formation of crystalline lamellas of different thicknesses owing to the presence of polymers of different branching densities and molecular weights.²⁴ To evaluate the influence of the ligand structure on the PE microstructural properties, high-temperature ¹H and ¹³C NMR spectroscopic measurements were carried out on a series of polymer samples obtained with nickel chloride complexes (Ni1_Cl–Ni7_Cl) (entries 4, 13–15 in Table 2; entries 4, 13, and 14 in Table 3) and Ni4_Br (entry 19, Table 2). The ¹H NMR spectra show that all the PE products contain two types of unsaturated groups. One is the vinyl (−CH=CH₂) end group identified at δ 5.0 (H_a) and 5.9 (H_b) (an integration ratio of 1:2) and the other is the vinylene group with characteristic −CH=CH– peaks at δ 5.5 (H_c/H_c′) (Figures 7 and 8). This finding is also backed up by the presence of characteristic downfield resonances (a, b, c, and c′) for the corresponding alkenic carbon atoms in their ¹³C NMR spectra (Figures 9, S18–S24).

Figure 7.

¹H NMR spectra of PEs obtained with mononuclear nickel chloride complexes Ni1–Ni4/MAO at 30 °C (entries 4, 13–15, Table 2); recorded in C₂D₂Cl₄ at 100 °C.

Figure 8.

¹H NMR spectra of PEs produced with mononuclear nickel chloride complexes Ni5–Ni7/MAO at 30 °C (entries 4, 13, and 14, Table 3); recorded in C₂D₂Cl₄ at 100 °C.

Figure 9.

¹³C NMR spectrum of the PE produced with Ni1_Cl/MAO at 30 °C (entry 4, Table 2); recorded in C₂D₂Cl₄ at 100 °C.

A closer inspection of the ¹H NMR spectra reveals that the ratio of vinyl to vinylene (H₂C=CH−)/(−CH=CH−) was notably affected by the type of remote substituents and the number of metal centers in the nickel complexes. With regard to mononuclear nickel complexes, the vinyl group content followed the order of Ni2(OCH₃) > Ni1(H) ∼ Ni4(F_NH2) > Ni3(F), suggesting that the electron-donating methoxy group favored the formation of vinyl as the dominant end group. With binuclear nickel complexes, Ni5 and Ni7, the proportion of vinylene was doubled compared to their mononuclear analogues with the ratio of vinyl/vinylene decreased from 1:0.83 to 1:1.75 and from 1:1 to 1:2, respectively (Figures 7 and 8). Meanwhile, Ni6 with methoxy groups afforded polymers containing slightly more vinylene groups than its mononuclear analogue Ni2. This is likely due to the steric hindrance of benzhydryl-bridged substituents in binuclear catalysts, which suppress β-H elimination and favor chain-walking and hence increase the likelihood of forming a vinylene end group. However, the general order of the vinyl proportion for mononuclear nickel complexes was found to be the same as that of binuclear ones, highlighting the determinative influence of the ligand structure on the polymer microstructure.

Furthermore, the upfield peaks between 11 and 40 ppm in the ¹³C NMR spectrum of the polymer produced with Ni1_Cl/MAO (entry 4, Table 2) also give information about the branches in the polymer (Figure 9, as well as Figures S18–S24 for other polymers). The key findings of the branching analysis are summarized in Table 4, with further details included in Tables S2–S9. Generally, 2-iminopyridylnickel complexes in this work produced PE of relatively lower branches but with quite similar branch types (methyl, butyl, and longer-chain branches) in comparison with structurally related nickel precatalysts.^14,16 On inspection of the results listed in Table 4, it is clear that the branching degree and the branching content are highly influenced by the remote substituents in nickel complexes. Following the quantification according to the literature,²⁵ the polymer prepared using Ni1_Cl/MAO had a moderate branching density of 31 branches per 1000 Cs, including methyl (53.1%), butyl (6.7%), and longer branches (40.2%) (see Figure 9 and Table S2). The polymers obtained using Ni2_Cl/MAO had just 17 branches per 1000 Cs, with the least long branches in connection to the high molecular weight as mentioned above (Figure S18). While the polymers formed by nickel complexes containing bis(4-F-phenyl)methyl groups (Ni3_Cl, Ni4_Cl, and Ni4_Br) were found to possess relatively higher branches among the mononuclear nickel complexes (Ni1–Ni4). To sum up, the degree of branching is enhanced with the increase of the electron-withdrawing ability of the remote substituent, which is consistent with the observations for the bis(imino)acenaphthene–nickel catalysts.¹⁸ Similarly, the trend of change in the branch density in polymers produced with Ni1–Ni3 also applies to those with the binuclear complexes (Ni5–Ni7), with a slight decrease in branches observed upon the increase of the electron-donating ability of the remote substituent. The samples produced with Ni5–Ni7 had relatively more methyl and butyl branches while with fewer longer branches when compared with those by mononuclear analogues, highlighting again the close relationship between the branching architectures and the catalyst structure.

Table 4. Branching Analysis of PE Samples Produced with Mononuclear (Entries 4, 13–15, and 19 in Table 2) and Binuclear Nickel Complexes (Entries 4, 13, and 14 in Table 3) Activated by MAO^a.

	branching with respect to total (%)
sample	NM′	NE′	NP′	NB′	NA′	NL′	Rb	branchesc
PE-Ni1Cl	53.1	0	0	6.7	0	40.2	2.91	31
PE-Ni2Cl	56.7	0	0	8.4	0	34.9	3.27	17
PE-Ni3Cl	50.5	0	0	9.0	0	40.5	2.77	35
PE-Ni4Cl	50.9	0	0	9.6	0	39.5	2.41	43
PE-Ni4Br	44.4	0	0	11.0	0	44.7	2.37	40
PE-Ni5Cl	63.9	0	0	10.8	0	25.3	3.48	30
PE-Ni6Cl	62.7	0	0	12.4	0	24.9	2.17	17
PE-Ni7Cl	51.0	0	0	11.3	0	36.7	2.84	34

^aData determined from their ¹³C NMR spectra using approaches described elsewhere;²⁵N_x^′ refers to the relative percentage of methyl, ethyl, propyl, butyl, amyl, and longer-chain branches.

^bBranching (% R) can be calculated with respect to the total ethylene units XδδCH₂PE present in the polymer: R (%) = N_M + N_E + N_P + N_B + N_A + N_M(1,4) + N_M(1,5) + N_M(1,6) + N_L + N_L(1,4); N_x refers to the results described in Tables S2–S9.

^cExpressed as per 1000 Cs.

Experimental Part

General Considerations

All manipulations involving air- and moisture-sensitive compounds were carried out under a nitrogen atmosphere using the standard Schlenk technique or in a nitrogen-filled glovebox. Solvents (toluene and DCM) were distilled under nitrogen from appropriate drying agents and degassed prior to use. Cocatalysts MAO (1.46 M in toluene) and MMAO (1.93 M in heptane) were provided by AkzoNobel Corporation, while dimethylaluminum chloride (Me₂AlCl, 1.00 M in toluene) and EASC (0.87 M in toluene) were obtained from Acros Organics. High-purity ethylene was purchased from Beijing Yanshan Petrochemical Co. and used as received. Other reagents were purchased from Aldrich, Acros, or local suppliers. ¹H and ¹³C NMR spectra of these newly synthesized compounds were recorded with a Bruker DMX 400 MHz at room temperature using CDCl₃ as a solvent and tetramethylsilane (TMS) as an internal standard; chemical shifts (ppm) were referred to the residual protic solvent peaks, and chemical shifts were in hertz (Hz). Elemental analyses were recorded on a FlashEA 1112 microanalyzer and the FT-IR spectra were obtained on a PerkinElmer system 2000 FT-IR spectrometer. A PerkinElmer TA-Q2000 DSC analyzer was used to obtain the T_m value of the PE samples under a nitrogen atmosphere. The testing program was set as follows: a sample (4.0–6.0 mg) was heated to 160 °C at a rate of 20 °C min^–1 and kept at 160 °C for 5 min to remove the thermal history and cooled to −20 °C at a rate of 20 °C min^–1. High-temperature GPC was performed on an Agilent PL-GPC 220 equipped with a refractive index detector. M_w and M_w/M_n values of PEs were measured at 150 °C using 1,2,4-trichlorobenzene as the solvent. For NMR characterizations, a prescribed amount of PE sample (80–100 mg) was dissolved in 1,1,2,2-tetrachloroethane-d₂ (2 mL) in a 5 mm high-temperature nuclear magnetic tube under heat. ¹H and ¹³C NMR spectra were recorded on a Bruker DMX 300 spectrometer at 100 °C with TMS as an internal standard. The spectrometer frequency for the ¹³C spectra was 75.47 MHz, while it was 300.13 MHz for the ¹H spectra. Operating conditions used for the ¹³C spectra are the following: a spectral width of 18.8324 kHz; an acquisition time of 0.87 s; and a relaxation delay of 2.0 s. Operating conditions used for the ¹H spectra are as follows: a spectral width of 14.9701 kHz; an acquisition time of 2.1889 s; and a relaxation delay of 1.0 s.

Experimental Procedure for the Synthesis of Ligands (L1–L7)

(E)-N-(2,4-Dibenzhydryl-6-fluorophenyl)-1-(pyridin-2-yl)ethan-1-imine (L1)

A mixture of 1-(pyridin-2-yl)ethan-1-one (0.36 g, 3 mmol) and 2,4-dibenzhydryl-6-fluoroaniline (0.89 g, 2 mmol) was completely dissolved in toluene (20 mL) at 120 °C under stirring. Then, p-toluenesulfonic acid was added to the solution as the catalyst, and the solution was further refluxed for 10 h. Upon completion of the reaction, as confirmed by silica thin-layer chromatography (TLC), toluene was wholly removed by using a rotary evaporator. The residual solid was purified by basic alumina column chromatography using petroleum ether/ethyl acetate (50/1 v/v) as an eluent, affording L1 as a light-yellow powder (0.33 g, 30%). ¹H NMR (400 MHz, CDCl₃, TMS): δ 8.61 (d, J = 4.0 Hz, 1H, Py-H_o), 8.26 (d, J = 8.0 Hz, 1H, Py-H_m), 7.77 (t, J = 8.0 Hz, 1H, Py-H_p), 7.35 (t, J = 6.0 Hz, 1H, Py-H_m), 7.28–7.24 (m, 5H, aryl-H), 7.21 (d, J = 8.0 Hz, 2H, aryl-H), 7.15–7.12 (m, 5H, aryl-H), 7.06 (d, J = 8.0 Hz, 4H, aryl-H), 6.97 (d, J = 4 Hz, 4H, aryl-H), 6.74 (d, J = 8.0 Hz, 1H, aryl-H(o-F)), 6.60 (s, 1H, aryl-H), 5.57 (s, 1H, CHPh₂), 5.42 (s, 1H, CHPh₂), 1.77 (s, 3H, N=CCH₃). ¹³C NMR (100 MHz, CDCl₃, TMS): δ 170.93, 156.10, 152.06, 149.64, 148.64, 144.18, 143.83, 142.66, 141.98, 140.14, 140.08, 137.59, 136.38, 135.19, 135.05, 129.57, 129.42, 129.33, 128.70, 128.43, 128.32, 128.29, 126.87, 126.49, 126.35, 126.29, 124.99, 121.58, 114.97, 114.76, 56.31, 52.27, 17.01, 16.98. Anal. Calcd for C₃₉H₃₁FN₂ (546.69): C, 85.68; H, 5.72; N, 5.12%. Found: C, 85.33; H, 5.91; N, 5.03%. FT-IR (cm^–1): 3040 (w), 2996 (w), 2919 (w), 2858 (w), 1645 (ν(C=N), m), 1601 (m), 1578 (m), 1509 (s), 1470 (m), 1433 (m), 1367 (m), 1290 (m), 1225 (s), 1153 (m), 1136 (w), 1108 (m), 1049 (w), 1023 (m), 999 (m), 824 (m), 780 (m), 736 (m).

(E)-N-(2,4-Bis(bis(4-methoxyphenyl)methyl)-6-fluorophenyl)-1-(pyridin-2-yl)ethan-1-imine (L2)

Using the same procedure above for the synthesis of L1 but with a different ratio of 30/1 for petroleum ether/ethyl acetate in the purification by column chromatography, L2 was prepared as a light-yellow powder (0.42 g, 31%). ¹H NMR (400 MHz, CDCl₃, TMS): δ 8.61 (d, J = 4.0 Hz, 1H, Py-H_o), 8.26 (d, J = 8.0 Hz, 1H, Py-H_m), 7.77 (t, J = 8.0 Hz, 1H, Py-H_p), 7.34 (t, J = 8.0 Hz, 1H, Py-H_m), 6.96 (d, J = 8.0 Hz, 4H, aryl-H), 6.86 (d, J = 8.0 Hz, 4H, aryl-H), 6.80 (d, J = 8.0 Hz, 4H, aryl-H), 6.70–6.67 (m, 5H, aryl-H), 6.54 (s, 1H, aryl-H), 5.45 (s, 1H, CH(Ph(p-OCH₃))₂), 5.32 (s, 1H, CH(Ph(p-OCH₃))₂), 3.79 (s, 6H, OCH₃), 3.74 (s, 6H, OCH₃), 1.55 (s, 3H, N=CCH₃). ¹³C NMR (100 MHz, CDCl₃, TMS): δ 170.71, 158.39, 158.14, 158.00, 156.17, 149.11, 148.61, 140.83, 140.76, 137.96, 136.93, 136.75, 136.40, 136.35, 135.25, 134.39, 130.37, 130.28, 130.25, 130.15, 127.17, 126.16, 125.97, 124.93, 121.76, 121.54, 114.60, 114.39, 114.02, 113.85, 113.76, 113.64, 55.33, 55.29, 54.64, 50.55, 17.02. Anal. Calcd for C₄₃H₃₉FN₂O₄ (666.79): C, 77.46; H, 5.90; N, 4.20%. Found: C, 77.17; H, 5.96; N, 4.11%. FT-IR (cm^–1): 3037 (w), 2998 (w), 2928 (w), 2834 (w), 1644 (ν(C=N), m), 1607 (m), 1582 (m), 1506 (s), 1463 (m), 1434 (m), 1363 (m), 1298 (m), 1241 (s), 1173 (m), 1107 (m), 1031 (m), 993 (m), 830 (m), 811 (m), 778 (m), 744 (m).

(E)-N-(2,4-Bis(bis(4-flurophenyl)methyl)-6-fluorophenyl)-1-(pyridin-2-yl)ethan-1-imine (L3)

Using the same procedure above for L1, L3 was prepared as a light-yellow powder (0.36 g, 29%). ¹H NMR (400 MHz, CDCl₃, TMS): δ 8.62 (d, J = 4.0 Hz, 1H, Py-H_o), 8.22 (d, J = 8.0 Hz, 1H, Py-H_m), 7.77 (t, J = 12.0 Hz, 1H, Py-H_p), 7.36 (t, J = 6.0 Hz, 1H, Py-H_m), 6.98–6.96 (m, 8H, aryl-H), 6.90–6.83 (m, 8H, aryl-H), 6.72 (d, J = 6.0 Hz, 1H, aryl-H(o-F)), 6.39 (s, 1H, aryl-H), 5.53 (s, 1H, CH(Php-F)₂), 5.39 (s, 1H, CH(Php-F)₂), 1.83 (s, 3H, N=CCH₃). ¹³C NMR (100 MHz, CDCl₃, TMS): δ 171.09, 162.92, 162.77, 160.48, 160.33, 155.84, 152.15, 149.72, 148.79, 140.06, 139.20, 139.17, 138.18, 137.53, 136.44, 135.29, 130.83, 130.75, 130.67, 125.78, 125.18, 121.45, 115.50, 115.35, 115.29, 115.13, 115.04, 114.83, 54.68, 50.68, 17.19. Anal. Calcd for C₃₉H₂₇F₅N₂ (618.65): C, 75.72; H, 4.40; N, 4.53%. Found: C, 75.63; H, 4.56; N, 4.62%. FT-IR (cm^–1): 3038 (w), 2998 (w), 2930 (w), 2869 (w), 1645 (ν(C=N), m), 1603 (m), 1572 (m), 1506 (s), 1475 (m), 1431 (m), 1369 (m), 1297 (m), 1227 (s), 1157 (m), 1130 (w), 1100 (m), 1047 (w), 1021 (m), 997 (m), 829 (m), 783 (m), 739 (m).

(E)-2-(Bis(4-fluorophenyl)methyl)-4-((3-(bis(4-fluorophenyl)methyl)-5-fluoro-4-((1-(pyridin-2-yl)ethylidene)amino)phenyl)(phenyl)methyl)-6-fluoroaniline (L4)

A mixture of 1-(pyridin-2-yl)ethan-1-one (0.12 g, 1 mmol) and 4,4′-(phenylmethylene)bis(2-(bis(4-fluorophenyl)methyl)-6-fluoroaniline) (1.43 g, 2 mmol) was dissolved in toluene (20 mL) at 120 °C under stirring. Then, p-toluenesulfonic acid was added as the catalyst, and the solution was further refluxed for 10 h. Upon the completion of the reaction, as confirmed by silica TLC, toluene was removed by using a rotary evaporator. The residual solid was purified by basic alumina column chromatography using petroleum ether/ethyl acetate (100/1 v/v) as an eluent, affording L4 as a light-yellow powder (0.24 g, 29%). ¹H NMR (400 MHz, CDCl₃, TMS): δ 8.63 (d, J = 4.0 Hz, 1H, Py-H_o), 8.24 (d, J = 8.0 Hz, 1H, Py-H_m), 7.78 (t, J = 8.0 Hz, 1H, Py-H_p), 7.36 (t, J = 8.0 Hz, 1H, Py-H_m), 7.22–7.18 (m, 3H, aryl-H), 6.97–6.94 (m, 10H, aryl-H), 6.87–6.84 (m, 8H, aryl-H), 6.64 (t, J = 12.0 Hz, 2H, aryl-H(o-F)), 6.34 (s, 1H, aryl-H), 6.08 (s, 1H, aryl-H), 5.51 (s, 1H, CH(Ph(p-F))₂), 5.40 (s, 1H, CH(Ph(p-F))₂), 5.18 (s, 1H, CHPh₃), 3.46 (s, 2H, NH₂), 1.82 (s, 3H, N=CCH₃). ¹³C NMR (100 MHz, CDCl₃, TMS): δ 170.93, 163.07, 162.73, 160.62, 160.29, 155.90, 153.36, 152.02, 148.76, 143.33, 140.58, 140.51, 138.26, 137.57, 137.47, 137.12, 136.41, 135.02, 134.88, 133.75, 133.68, 131.02, 130.84, 130.81, 130.76, 130.73, 129.10, 128.41, 126.58, 126.00, 125.76, 125.12, 121.46, 115.80, 115.75, 115.59, 115.54, 115.28, 115.07, 114.95, 114.74, 114.42, 114.22, 55.34, 50.63, 17.13. Anal. Calcd for C₅₂H₃₇F₆N₃ (817.88): C, 76.36; H, 4.56; N, 5.14%. Found: C, 76.23; H, 4.70; N, 5.28%. FT-IR (cm^–1): 3053 (w), 2921 (m), 1640 (ν(C=N), m), 1601 (m), 1576 (m), 1505 (s), 1431 (m), 1364 (w), 1294 (m), 1222 (s), 1157 (m), 1128 (m), 1100 (m), 998 (m), 963 (w), 833 (m), 781 (m), 740 (m), 702 (m).

(1E,1′E)-N,N′-((Phenylmethylene)bis(2-benzhydryl-6-fluoro-4,1-phenylene))bis(1-(pyridin-2-yl)ethan-1-imine) (L5)

A mixture of 1-(pyridin-2-yl)ethan-1-one (0.61 g, 5 mmol) and 4,4′-(phenylmethylene)bis(2-benzhydryl-6-fluoroaniline) (1.29 g, 2 mmol) was dissolved in toluene (20 mL) at 120 °C under stirring. Then, p-toluenesulfonic acid was added and the solution was refluxed for 10 h. Upon the completion of the reaction, as confirmed by silica TLC, toluene was removed by using a rotary evaporator. The residual solid was purified by basic alumina column chromatography using petroleum ether/ethyl acetate (100/1 v/v) as an eluent, affording L5 as a light-yellow powder (0.62 g, 37%). ¹H NMR (400 MHz, CDCl₃, TMS): δ 8.61 (d, J = 4.0 Hz, 2H, Py-H_o), 8.28 (d, J = 8.0 Hz, 2H, Py-H_m), 7.78 (t, J = 8.0 Hz, 2H, Py-H_p), 7.35 (t, J = 6.0 Hz, 2H, Py-H_m), 7.26–7.14 (m, 15H, aryl-H), 7.02–6.96 (m, 10H, aryl-H), 6.68 (d, J = 6.0 Hz, 2H, aryl-H(o-F)), 6.54 (s, 2H, aryl-H), 5.57 (s, 2H, CHPh₂), 5.30 (s, 1H, CHPh₃), 1. 75 (s, 6H, N=CCH₃). ¹³C NMR (100 MHz, CDCl₃, TMS): δ 170.91, 152.04, 149.62, 148.65, 143.31, 142.68, 142.58, 140.02, 139.95, 137.61, 136.38, 135.28, 135.14, 129.58, 129.54, 129.26, 128.45, 128.33, 128.31, 126.59, 126.42, 126.02, 125.00, 121.57, 114.77, 114.56, 55.66, 52.29, 17.03. Anal. Calcd for C₅₉H₄₆F₂N₄ (849.04): C, 83.46; H, 5.46; N, 6.60%. Found: C, 83.43; H, 5.56; N, 6.62%. FT-IR (cm^–1): 3058 (w), 3025 (w), 2922 (w), 1644 (ν(C=N), m), 1604 (m), 1567 (m), 1494 (m), 1473 (m), 1454 (m), 1427 (m), 1364 (m), 1296 (m), 1241 (m), 1205 (m), 1105 (m), 1077 (w), 1031 (m), 997 (m), 872 (m), 781 (m), 739 (m), 697 (s).

(1E,1′E)-N,N′-((Phenylmethylene)bis(2-(bis(4-methoxyphenyl)methyl)-6-fluoro-4,1-phenylene))bis(1-(pyridin-2-yl)ethan-1-imine) (L6)

Using the same procedure above for L5 but using a half molar ratio of reactants and with a different ratio of 50/1 for petroleum ether/ethyl acetate in the purification by column chromatography, L6 was prepared as a light-yellow powder (0.29 g, 30%). ¹H NMR (400 MHz, CDCl₃, TMS): δ 8.62 (s, 2H, Py-H_o), 8.27 (d, J = 8.0 Hz, 2H, Py-H_m), 7.77 (t, J = 8.0 Hz, 2H, Py-H_p), 7.36 (d, J = 4.0 Hz, 2H, Py-H_m), 7.23–7.19 (m, 3H, aryl-H), 7.01 (d, J = 8.0 Hz, 2H, aryl-H(o-F)), 6.86–6.84 (m, 8H, aryl-H), 6.70–6.66 (m, 10H, aryl-H), 6.54 (s, 2H, aryl-H), 5.45 (s, 2H, CH(Ph(p-OCH₃))₂), 5.30 (s, 1H, CHPh₃), 3.73 (s, 12H, OCH₃), 1.81 (s, 6H, N=CCH₃). ¹³C NMR (100 MHz, CDCl₃, TMS): δ 170.77, 158.08, 156.21, 148.66, 143.42, 140.03, 138.12, 136.37, 135.29, 135.17, 135.03, 130.40, 129.31, 128.42, 126.53, 125.97, 124.97, 121.59, 113.74, 55.71, 55.35, 50.57, 17.08. Anal. Calcd for C₆₃H₅₄F₂N₄O₄ (969.15): C, 78.08; H, 5.62; N, 5.78%. Found: C, 77.93; H, 5.76; N, 5.82%. FT-IR (cm^–1): 3058 (w), 3006 (w), 2929 (w), 1643 (ν(C=N), m), 1608 (m), 1572 (m), 1508 (s), 1468 (m), 1431 (m), 1363 (m), 1297 (m), 1243 (s), 1175 (m), 1107 (m), 996 (m), 874 (m), 833 (m), 780 (m), 741 (m), 705 (m).

(1E,1′E)-N,N′-((Phenylmethylene)bis(2-(bis(4-fluorophenyl)methyl)-6-fluoro-4,1-phenylene))bis(1-(pyridin-2-yl)ethan-1-imine) (L7)

Using the same procedure above for L5 but a half molar ratio of reactants, L7 was prepared as a light-yellow powder (0.21 g, 23%). ¹H NMR (400 MHz, CDCl₃, TMS): δ 8.62 (d, J = 4.0 Hz, 2H, Py-H_o), 8.24 (d, J = 8.0 Hz, 2H, Py-H_m), 7.78 (t, J = 8.0 Hz, 2H, Py-H_p), 7.37 (t, J = 6.0 Hz, 2H, Py-H_m), 7.24–6.98 (m, 3H, aryl-H), 6.86 (d, J = 8.0 Hz, 2H), 6.73–6.70 (m, 16H, aryl-H), 6.72 (d, J = 6.0 Hz, 2H, aryl-H(o-F)), 6.42 (s, 2H, aryl-H), 5.52 (s, 2H, CH(Ph(p-F))₂), 5.31 (s, 1H, CHPh₃), 1.83 (s, 6H, N=CCH₃). ¹³C NMR (100 MHz, CDCl₃, TMS): δ 171.04, 162.77, 160.33, 155.91, 152.11, 149.68, 148.79, 142.99, 140.18, 140.12, 138.35, 138.19, 137.33, 136.43, 135.26, 135.12, 130.85, 130.78, 129.17, 128.55, 126.80, 125.76, 125.15, 121.49, 115.35, 115.34, 115.14, 115.12, 115.04, 114.83, 55.63, 50.66, 17.15, 17.12. Anal. Calcd for C₅₉H₄₂F₆N₄ (921.00): C, 76.94; H, 4.60; N, 6.08%. Found: C, 76.63; H, 4.56; N, 6.12%. FT-IR (cm^–1): 3055 (w), 3003 (w), 2958 (w), 1643 (ν(C=N), m), 1602 (m), 1567 (m), 1505 (s), 1472 (m), 1428 (m), 1363 (m), 1294 (m), 1223 (s), 1157 (m), 1101 (m), 997 (m), 969 (w), 876 (m), 834 (m), 780 (s), 740 (m), 703 (m).

Experimental Procedure for the Synthesis of Nickel Complexes (Ni1–Ni7)

Synthesis of Ni1_Cl

DCM (5 mL) and methanol (5 mL) were added to a mixture of L1 (0.055 g, 0.10 mmol) and NiCl₂·6H₂O (0.024 g, 0.10 mmol). After stirring for 12 h at room temperature, the mixture becomes yellowish-brown in color. The solvent was concentrated to ca. 3 mL, and diethyl ether (10 mL) was added to induce precipitation. The solid was collected by filtration and washed with diethyl ether to afford Ni1_Cl as a green solid (0.036 g, 54%). Anal. Calcd for C₃₉H₃₁Cl₂FN₂Ni (676.28): C, 69.27; H, 4.62; N, 4.14. Found: C, 69.03; H, 4.87; N, 4.21%. FT-IR (cm^–1): 3065 (w), 2975 (w), 1621 (ν(C=N), m), 1599 (m), 1576 (m), 1494 (m), 1445 (m), 1429 (m), 1371 (m), 1312 (m), 1258 (m), 1076 (m), 1027 (w), 778 (m), 742 (m), 700 (s).

Synthesis of Ni2_Cl

Using the same procedure and molar ratio of reactants as above for the synthesis of Ni1_Cl but with L2 as the ligand, Ni2_Cl was prepared as a green solid (0.063 g, 79%). Anal. Calcd for C₄₃H₃₉Cl₂FN₂NiO₄ (796.39): C, 64.85; H, 4.94; N, 3.52. Found: C, 64.76; H, 5.26; N, 3.60%. FT-IR (cm^–1): 2996 (w), 2833 (w), 1630 (ν(C=N), m), 1605 (m), 1579 (m), 1507 (s), 1462 (m), 1435 (m), 1374 (w), 1300 (m), 1244 (s), 1175 (m), 1109 (w), 1030 (m), 814 (m), 776 (m).

Synthesis of Ni3_Cl

Using the same procedure and molar ratio of reactants as above for the synthesis of Ni1_Cl but with L3 as the ligand, Ni3_Cl was prepared as a green solid (0.061 g, 82%). Anal. Calcd for C₃₉H₂₇Cl₂F₅N₂Ni (748.24): C, 62.60; H, 3.64; N, 3.74. Found: C, 62.47; H, 3.89; N, 3.82%. FT-IR (cm^–1): 3074 (w), 2984 (m), 1630 (ν(C=N), m), 1600 (m), 1574 (m), 1506 (s), 1476 (m), 1434 (m), 1374 (m), 1298 (m), 1227 (s), 1158 (m), 1098 (m), 1020 (w), 998 (m), 832 (m), 779 (m).

Synthesis of Ni4_Cl

Using the same procedure and molar ratio of reactants as above for the synthesis of Ni1_Cl but with L4 as the ligand, Ni4_Cl was prepared as a green solid (0.051 g, 54%). Anal. Calcd for C₅₂H₃₇Cl₂F₆N₃Ni (947.47): C, 65.92; H, 3.94; N, 4.44. Found: C, 65.69; H, 4.21; N, 4.55%. FT-IR (cm^–1): 3065 (w), 2925 (m), 1628 (ν(C=N), m), 1599 (m), 1576 (m), 1505 (s), 1435 (m), 1372 (w), 1319 (m), 1261 (m), 1223 (s), 1157 (m), 1099 (w), 1015 (m), 877 (w), 827 (m), 777 (m), 743 (w), 703 (m).

Synthesis of Ni5_Cl

DCM (5 mL) and methanol (5 mL) were added to a mixture of L5 (0.17 g, 0.20 mmol) and NiCl₂·6H₂O (0.094 g, 0.40 mmol). After stirring for 12 h at room temperature, the mixture becomes green in color. The solvent was concentrated to ca. 3 mL, and diethyl ether (20 mL) was added to induce precipitation. The solid was collected by filtration and washed with diethyl ether to form Ni5_Cl as a green solid (0.15 g, 68%). Anal. Calcd for C₅₉H₄₆Cl₄F₂N₄Ni₂ (1108.23): C, 63.94; H, 4.18; N, 5.06. Found: C, 63.79; H, 4.39; N, 5.15%. FT-IR (cm^–1): 3060 (w), 2923 (m), 1627 (ν(C=N), m), 1598 (m), 1579 (m), 1494 (m), 1444 (m), 1374 (m), 1312 (m), 1261 (m), 1029 (m), 779 (m), 743 (m), 701 (s).

Synthesis of Ni6_Cl

Using the same procedure and molar ratio of reactants as above for the synthesis of Ni5_Cl but with L6 as the ligand, Ni6_Cl was prepared as a green solid (0.19 g, 77%). Anal. Calcd for C₆₃H₅₄Cl₄F₂N₄Ni₂O₄ (1228.33): C, 61.60; H, 4.43; N, 4.56. Found: C, 61.29; H, 4.67; N, 4.63%. FT-IR (cm^–1): 3069 (w), 2963 (m), 1630 (ν(C=N), m), 1603 (m), 1579 (m), 1509 (s), 1468 (m), 1436 (m), 1374 (m), 1301 (m), 1247 (s), 1177 (m), 1108 (m), 1029 (m), 836 (m), 815 (m), 779 (m), 750 (m), 706 (m).

Synthesis of Ni7_Cl

Using the same procedure and molar ratio of reactants as above for the synthesis of Ni5_Cl but with L7 as the ligand, Ni7_Cl was prepared as a green solid (0.17 g, 72%). Anal. Calcd for C₅₉H₄₂Cl₄F₆N₄Ni₂ (1180.19): C, 60.05; H, 3.59; N, 4.75. Found: C, 59.84; H, 3.79; N, 4.80%. FT-IR (cm^–1): 3069 (w), 2994 (w), 1628 (ν(C=N), m), 1599 (m), 1579 (m), 1506 (s), 1477 (m), 1433 (m), 1374 (m), 1315 (m), 1260 (m), 1224 (s), 1159 (m), 1100 (m), 1023 (w), 879 (w), 840 (m), 780 (m).

Synthesis of Ni1_Br

Under a nitrogen atmosphere, DCM (10 mL) was added to a mixture of L1 (0.055 g, 0.10 mmol) and (DME)NiBr₂ (0.031 g, 0.10 mmol). After stirring for 12 h at room temperature, the mixture become yellowish-brown in color. The solvent was concentrated to ca. 3 mL, and diethyl ether (10 mL) was added to induce precipitation. The solid was collected by filtration and washed with diethyl ether to afford Ni1_Br as a yellow solid (0.055 g, 72%). Anal. Calcd for C₃₉H₃₁Br₂FN₂Ni (765.19): C, 61.22; H, 4.08; N, 3.66. Found: C, 61.04; H, 4.33; N, 3.81%. FT-IR (cm^–1): 3058 (w), 1625 (ν(C=N), m), 1597 (m), 1578 (m), 1490 (m), 1445 (m), 1373 (m), 1310 (m), 1259 (m), 1111 (w), 1078 (w), 1029 (m), 873 (m), 779 (m), 743 (m), 701 (m).

Synthesis of Ni2_Br

Using the same procedure and molar ratio of reactants as above for the synthesis of Ni1_Br but with L2 as the ligand, Ni2_Br was isolated as a yellow solid (0.058 g, 66%). Anal. Calcd for C₄₃H₃₉Br₂FN₂NiO₄ (885.29): C, 58.34; H, 4.44; N, 3.16. Found: C, 58.20; H, 4.62; N, 3.32%. FT-IR (cm^–1): 3035 (w), 3001 (w), 2937 (w), 2904 (w), 2835 (w), 1631 (ν(C=N), m), 1605 (m), 1579 (m), 1508 (s), 1462 (m), 1436 (m), 1372 (m), 1301 (m), 1245 (s), 1175 (m), 1109 (m), 1028 (s), 830 (m), 814 (m), 778 (m), 671 (w).

Synthesis of Ni3_Br

Using the same procedure and molar ratio of reactants as above for the synthesis of Ni1_Br but with L3 as the ligand, Ni3_Br was isolated as a yellow solid (0.052 g, 62%). Anal. Calcd for C₃₉H₂₇Br₂F₅N₂Ni (837.15): C, 55.96; H, 3.25; N, 3.35. Found: C, 54.77; H, 3.41; N, 3.31%. FT-IR (cm^–1): 3041 (w), 2991 (m), 1628 (ν(C=N), m), 1599 (m), 1569 (m), 1505 (s), 1475 (m), 1432 (m), 1372 (m), 1314 (m), 1262 (m), 1224 (s), 1158 (m), 1098 (m), 1054 (w), 1019 (m), 830 (m), 783 (m).

Synthesis of Ni4_Br

Using the same procedure and molar ratio of reactants as above for the synthesis of Ni1_Br but with L4 as the ligand, Ni4_Br was isolated as a yellow solid (0.065 g, 63%). Anal. Calcd for C₅₂H₃₇Br₂F₆N₃Ni (1036.38): C, 60.26; H, 3.60; N, 4.05. Found: C, 60.11; H, 3.81; N, 4.27%. FT-IR (cm^–1): 3062 (w), 2975 (m), 1628 (ν(C=N), m), 1600 (m), 1576 (m), 1505 (s), 1435 (m), 1374 (w), 1315 (m), 1260 (m), 1225 (s), 1158 (m), 1099 (w), 1017 (m), 878 (w), 837 (m), 780 (m), 744 (w), 704 (m).

Synthesis of Ni5_Br

Under an atmosphere of nitrogen, DCM (10 mL) was added to a mixture of L5 (0.17 g, 0.20 mmol) and (DME)NiBr₂ (0.12 g, 0.40 mmol). After stirring for 12 h at room temperature, the mixture becomes green. The solvent was concentrated to ca. 3 mL and diethyl ether (20 mL) added to induce precipitation. The solid was collected by filtration and washed with diethyl ether to afford Ni5_Br as a yellow solid (0.22 g, 86%). Anal. Calcd for C₅₉H₄₆Br₄F₂N₄Ni₂ (1286.04): C, 55.10; H, 3.61; N, 4.36%. Found: C, 55.03; H, 3.76; N, 4.32%. FT-IR (cm^–1): 3058 (w), 1625 (ν(C=N), m), 1597 (m), 1578 (m), 1490 (m), 1445 (m), 1373 (m), 1310 (m), 1259 (m), 1111 (w), 1078 (w), 1029 (m), 873 (m), 779 (m), 743 (m), 701 (s).

Synthesis of Ni6_Br

Using the same procedure and molar ratio of reactants for the synthesis of Ni5_Br but with L6 as the ligand, Ni6_Br was isolated as a yellow solid (0.22 g, 78%). Anal. Calcd for C₆₃H₅₄Br₄F₂N₄Ni₂O₄ (1406.15): C, 53.81; H, 3.87; N, 3.98%. Found: C, 53.63; H, 3.96; N, 3.82%. FT-IR (cm^–1): 3056 (w), 2980 (m), 1627 (ν(C=N), m), 1604 (m), 1579 (m), 1509 (s), 1485 (m), 1435 (m), 1374 (m), 1303 (m), 1249 (s), 1178 (m), 1110 (w), 1029 (m), 1029 (m), 882 (w), 837 (m), 781 (m), 725 (w), 710 (m).

Synthesis of Ni7_Br

Using the same procedure and molar ratio of reactants for the synthesis of Ni5_Br but with L7 as the ligand, Ni7_Br was isolated as a yellow solid (0.23 g, 85%). Anal. Calcd for C₅₉H₄₂Br₄F₆N₄Ni₂ (1358.01): C, 52.18; H, 3.12; N, 4.13%. Found: C, 51.98; H, 3.46; N, 4.31%. FT-IR (cm^–1): 3066 (w), 2970 (m), 1624 (ν(C=N), m), 1600 (m), 1574 (m), 1506 (s), 1480 (m), 1433 (m), 1373 (m), 1314 (m), 1260 (m), 1224 (s), 1159 (m), 1099 (m), 1056 (w), 1022 (m), 879 (w), 839(m), 779 (m), 745 (w), 706 (m).

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. The oven-dried autoclave was evacuated by a vacuum pump and back-filled two times with nitrogen and once with ethylene. Due to the poor solubility of the nickel complex, the corresponding precatalyst dissolved in a mixture of toluene (25 mL) and DCM (3 mL) was injected into the autoclave containing ethylene (ca. 1 atm), followed by the addition of more toluene (25 mL) to clean the injection tube. Upon increasing the operating temperature to 10 °C higher than the set polymerization temperature, the required amount of cocatalyst (MAO, MMAO, EASC, or Me₂AlCl) and additional toluene (50 mL) were successively added using a syringe, making the total volume to 100 mL. Then, ethylene at the desired pressure (10 atm) was introduced to start the reaction under stirring. After the required reaction time, the reactor was cooled with a water bath and ethylene was vented. The residual solution was quenched with HCl-acidified ethanol (10%), the precipitated PE was collected by filtration, washed with ethanol, dried in vacuum at 40 °C until constant weight, weighed, and finally characterized.

X-ray Crystallographic Studies

The single crystal of Ni3_Cl·1/3H₂O suitable for X-ray diffraction analyses was obtained by slow diffusion of diethyl ether into its DCM solution at room temperature. X-ray studies were carried out on a XtaLAB Synergy-R single-crystal diffractometer with mirror-monochromatic Cu Kα radiation (λ = 1.54184 Å) at 169.99(10) K, and 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. Structure elucidation by direct methods (and Patterson methods) was performed by using the SHELXT (Sheldrick, 2015),^26a and structure refinement using full matrix least squares on F² was performed by using SHELXL (Sheldrick, 2015).^26b All hydrogen atoms were placed in calculated positions, and all non-H atoms were refined with anisotropic displacement parameters. The Squeeze option and TwinRotMat option of the crystallographic program PLATON were used to remove the disordered solvent from the structure and search for the possible twin orientation, respectively.²⁷ Graphical representations of the molecular structures were generated with the ORTEP program.²⁸ Details of the X-ray structure determinations and refinements are provided in Table S10.

Conclusions

In this study, we designed a series of benzhydryl-bridged dinuclear nickel complexes (Ni5–Ni7) containing different remote p-substituents on benzhydryl groups ranging from electron-donating substituents (OCH₃, H) to the electron-withdrawing substituent (F), along with their mononuclear analogues (Ni1–Ni4). Upon activation with MAO, all the nickel precatalysts exhibited moderate to high activities toward ethylene polymerization and produced PEs with low-molecular-weight (1–2 kg mol^–1 or ca. 10 kg mol^–1) and narrow molecular weight distributions. In particular, mononuclear nickel complex Ni4 with amino groups showed highest activities up to 8.3 × 10⁶ g of PE (mol of Ni)⁻¹ h^–1 among Ni1–Ni7, while complexes containing methoxy groups (Ni2 and Ni6) delivered polymers with the highest molecular weights (>10 kg mol^–1). Generally, the bimetallic systems exhibited a slightly higher activity and produced higher-molecular-weight PES than monometallic nickel systems, reflecting the better catalytic performance of the former. Moreover, the nature of the remote substituents greatly affected the branching content and the types of end unsaturation in the resultant PEs. All the PEs were moderately branched with the branching degree decreasing in the order of Ni_F > Ni_H > Ni_OCH3, while possessing the same branching types including methyl, butyl, and longer-chain branches with different distributions. In addition, both vinyl and vinylene unsaturations were observed in the NMR spectra of resultant polymers. Benzhydryl-bridged binuclear nickel complexes were initially expected to improve the thermal stability of catalysts and produce high-molecular-weight PE. Compared with mononuclear nickel analogues, the catalytic properties of binuclear nickel complexes have not been greatly improved, which may be due to the presence of ortho-fluorine groups. While the binuclear ones generated polymers containing more vinylene groups, those with the vinyl proportion decreased in the order of vinyl proportion as Ni_OCH3 > Ni_H > Ni_F, which is opposite to the order of branching degree. The results highlight the important role of ligand structures in tuning the microstructure of resultant polymers and point toward the direction for the future catalyst design.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.1c05418.

• ¹H NMR and ¹³C NMR spectra of the prepared compounds; effects of the Al/Ni molar ratio, temperature, and time on the catalytic activity and M_w of PEs; branching analysis of the PE samples; and crystal data and structure refinement for Ni3_Cl·1/3H₂O (PDF)

• Crystallographic data (CIF)

Accession Codes

X-ray crystallographic data for Ni3_Cl·1/3H₂O. CCDC: 2089716 (Ni3_Cl·1/3H₂O). These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44 1223 336033.

The authors declare no competing financial interest.