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Article

Electronic Tuning of Sterically Encumbered 2-(Arylimino)Pyridine-Nickel Ethylene Polymerization Catalysts by Para-Group Modification

by
Zahra Hosseinzadeh
1,
Ming Liu
1,
Qiuyue Zhang
1,
Tongling Liang
1,
Gregory A. Solan
2,*,
Yanping Ma
1 and
Wen-Hua Sun
1,*
1
Key Laboratory of Engineering Plastics and Beijing National Laboratory for Molecular Sciences, Institute of Chemistry Chinese Academy of Sciences, Beijing 100190, China
2
Department of Chemistry, University of Leicester, University Road, Leicester LE1 7RH, UK
*
Authors to whom correspondence should be addressed.
Catalysts 2022, 12(12), 1520; https://doi.org/10.3390/catal12121520
Submission received: 31 October 2022 / Revised: 19 November 2022 / Accepted: 23 November 2022 / Published: 25 November 2022
(This article belongs to the Special Issue Metal-Organic Catalyst for High Performance Materials)
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Abstract

:
A collection of five related 2-(arylimino)pyridines, 2-{(2,6-(CH(C6H4-p-F)2)2-4- RC6H2)N=CMe}C5H4N, each ortho-substituted with 4,4′-difluorobenzhydryl groups but distinct in the electronic properties of the para-R substituent (R = Me L1, Et L2, i-Pr L3, F L4, OCF3 L5), were prepared and combined with (DME)NiBr2 to form their corresponding LNiBr2 complexes, Ni1–Ni5, in high yields. All the complexes were characterized by FT-IR, 19F NMR spectroscopy and elemental analysis, while Ni5 was additionally the subject of an X-ray determination, revealing a bromide-bridged dimer. The molecular structure of bis-ligated (L4)2NiBr2 (Ni4’) was also determined, the result of ligand reorganization having occurred during attempted crystallization of Ni4. On activation with either EtAlCl2 or MMAO, Ni1–Ni5 exhibited high catalytic activities (up to 4.28 × 106 g of PE (mol of Ni)−1 h−1 using EtAlCl2) and produced highly branched polyethylene exhibiting low molecular weight (Mw range: 2.50–6.18 kg·mol−1) and narrow dispersity (Mw/Mn range: 2.21–2.90). Notably, it was found that the type of para-R group impacted on catalytic performance with Ni5 > Ni4 > Ni3 > Ni1 > Ni2 for both co-catalysts, underlining the positive influence of electron withdrawing substituents. Analysis of the structural composition of the polyethylene by 1H and 13C NMR spectroscopy revealed the existence of vinyl-end groups (–CH=CH2) and high levels of internal unsaturation (–CH=CH–) (ratio of vinylene to vinyl, range: 3.1:1–10.3:1) along with various types of branch (Me, Et, Pr, Bu, 1,4-paired Me, 1,6-paired Me and LCBs). Furthermore, reaction temperature was shown to greatly affect the end group type, branching density, molecular weight and in turn the melting points of the resulting polyethylenes.

1. Introduction

The application of group 10 metal (Ni, Pd) catalyzed ethylene polymerization has been thoroughly studied for more than 25 years [1,2,3], following the pioneering disclosure that their α-diimine complexes can promote the formation of high molecular weight polyethylene (PE) [3,4,5,6,7,8,9]. Moreover, their low oxophilicity when compared to early transition metal complexes and their unique ability to mediate a “chain-walking” pathway to form branched PE makes this class of polymerization catalyst particularly attractive [4,10,11,12,13,14]. Furthermore, through a choice of the N,N-ligand, reaction parameters and group 10 metal center, there offers a means to precisely control the polymer architecture (linear, hyperbranched or dendritic PEs) and in turn access some practically important polymeric materials [4,15,16,17,18].
With particular regard to the α-diimine ligand structure, structural modifications in the form of steric and electronic variations to the ligand backbone and/or the N-aryl substituents can have a significant impact on the activity of the metal catalyst and resulting polymer properties [19,20,21]. One early example relates to nickel precatalysts bearing 2-(arylimino)pyridines (A, Figure 1), where the steric hindrance imparted by the ortho-substituents of the N-aryl rings (e.g., Me, i-Pr) can engender good activity on the ethylene polymerization catalyst [22]. In related work, the presence of electron-withdrawing groups at the para-position of the N-aryl ring have been shown to increase the catalytic performance, whilst also affecting molecular weight and dispersity of the polymer [23,24]. Recent developments involving 2-iminopyridyl-nickel catalysts have seen the incorporation of even more sterically hindered benzhydryl (CHPh2) substituents to the 2,7-positions on an N-naphthyl ring (B, Figure 1). By comparison, the introduction of dibenzocyclopentyl substituents to the para position of the N-aryl group displayed poorer performance and lower molecular weight (C, Figure 1). Alternatively, by fusion of a cyclooctyl group to the iminopyridine backbone (D, Figure 1), the activity of the resulting nickel catalyst decreased while the molecular weight of the polymer increased. More recent developments involving 2-iminopyridyl-nickel catalysts have seen the addition of sterically hindered dibenzosuberyl substituents to the 2,4-positions of the N-aryl group in E (Figure 1) [24] resulting in high catalytic activity and high molecular weight polymer [25,26,27,28]. Furthermore, our group has recently studied the impact of electronic variations made to the ortho-R group in E (Figure 1) and have demonstrated the positive effects of introducing electron-withdrawing substituents (e.g., R = Cl, F) [23]. Elsewhere, the benefits of para-substituted trifluoromethoxy (OCF3) groups have seen the disclosure of both highly active nickel catalysts [25,29], along with improved thermal stability.
On account of the positive effects of introducing fluorine and fluorine-containing substituents to the N,N-ligand manifold [26], herein we report a new family of 2-(arylimino)pyridine-nickel precatalysts containing ortho-benzhydryl groups decorated with fluorine atoms at the para-positions of their aryl rings (F, Figure 1). Specifically, five examples of F are disclosed in which the para-R group of the N-aryl group is systematically varied to include methyl, ethyl, isopropyl, fluoro and trifluoromethoxy. An in-depth polymerization study is then performed with the aim to explore any correlations between the para-R substituent and catalytic activity and polymer properties. Furthermore, to understand how the structural variations reported in this study compare with previous reports, the data generated for F are compared with C, D and E. In addition, full synthetic details for the new ligands and complexes developed in the work are described.

2. Results and Discussion

2.1. Synthesis and Characterization of L1–L5 and Their Complexes Ni1–Ni5

Five novel examples of 4,4′-difluorobenzhydryl-substituted 2-(arylimino)pyridines, 2-{(2,6-(CH(C6H4-p-F)2)2-4-RC6H2)N=CMe}C5H4N (R = Me L1, Et L2, i-Pr L3, F L4, and OCF3 L5), were synthesized by a two-step procedure involving firstly the reaction of 2-acetylpyridine with 2,6-(CH(C6H4-p-F)2)2-4-RC6H2NH2 (R = Me A1, Et A2, i-Pr A3, F A4, OCF3 A5) in the presence of zinc(II) chloride, and secondly demetallation of the resulting zinc intermediate with aqueous potassium carbonate (Scheme 1). Following work-up, L1–L5 could be isolated in reasonable yields (55–74%) and characterized by FT-IR, 1H, 19F and 13C NMR spectroscopies and elemental analysis.
Next, the interaction of L1–L5 with (DME)NiBr2 (DME = 1,2-dimethoxyethane) in a mixture of ethanol and dichloromethane at room temperature gave [2-{(2,6-(CH(C6H4-p-F)2)2-4-RC6H2)N=CMe}C5H4N]NiBr2 (R = Me Ni1, Et Ni2, i-Pr Ni3, F Ni4, OCF3 Ni5) in good yield (82–91%) (Scheme 1). All complexes proved air-stable and were amenable to characterization by FT-IR, 19F NMR spectroscopy and elemental analysis. In addition, Ni5 and a derivative of Ni4 have been the subject of single crystal X-ray diffraction studies.
Single crystals of Ni5 suitable for X-ray determination were grown by slow diffusion of diethyl ether into a dichloromethane solution containing the complex at room temperature. Unexpectedly, a similar slow diffusion approach using Ni4 resulted in the formation of a few crystals of (L4)2NiBr2 (Ni4’); it is presumed that ligand reorganization occurred during the crystallization process. Both structures are distinct (Figure 2 and Figure 3) and will be discussed separately; selected bond distances and angles for each are given in Table 1.
The structure of Ni5 has been generated by symmetry and comprises an Ni(μ-Br)2Ni core, with each nickel center additionally coordinated by a bromide ligand and an N,N-bound L5; the two metals are positioned at a distance of 3.749 Å apart (Figure 2). The geometry about each nickel center can be best described as trigonal bipyramidal, with Br1, Br2 and N2 defining the equatorial belt and Br2i and N1 defining the axial sites. The Ni-Br distances show some variation (Ni1-Br1 2.5196(8) vs. Ni1-Br2 2.3857(8) Å) on account of their distinct bonding modes (bridging vs. terminal), while their Ni-N distances are comparable (Ni1-Npyridine 2.042(3) vs. Ni1-Ni2imine 2.055(3) Å). The five-membered chelate ring formed by Ni1, N1, N2, C5 and C6 is almost co-planar (max. deviation from plane = 0.019 Å), while the planes of the pyridine and N-aryl group are near perpendicular (dihedral angle = 87.72°). Related dimeric structures of the type (N,N)NiX(μ-X)2NiX(N,N) are fairly common in the literature for this class of complex [16]. In terms of the para-OCF3 group, this adopts a skew conformation with respect the adjacent arene ring (C–C–O–C dihedral angle 66.8°), while the distance between the CF3 carbon and the neighboring oxygen atom in [1.312(5) Å] suggests some double bond character [30]. There are no intermolecular contacts of note.
For Ni4’, the structure was also generated by symmetry and consisted of a single nickel center surrounded by two N,N-chelating L4 ligands and two trans-disposed bromide ligands to form a distorted octahedral geometry; related bis-chelated structural types have been previously reported [29,31,32]. Dissimilar to that in Ni5, the Ni-N bond lengths show some variation with Ni-Npyridine (2.020(4) Å), noticeably shorter than the Ni-Nimine (2.365(3) Å), an observation that is presumably attributable to the higher coordination number in Ni4’ and the steric properties exerted by the ortho-4,4′-difluorobenzydryl groups. Nonetheless, the five-membered chelate ring formed by Ni1, N1, C5, N2 and C6 is essentially co-planar, with C5 and C6 atoms deviating by 0.043 Å and 0.041 Å from co-planarity. As with Ni5, the planes of the pyridine ring and the N-aryl plane are nearly perpendicular with a dihedral angle of 89.51°, as is commonly found for 2-(arylimino)pyridyl-nickel(II) halide structures reported elsewhere [23].
The 19F NMR spectra of Ni1–Ni5 all gave two closely located resonances for the ortho-4,4′-difluorobenzhydryl groups, which would suggest some restricted rotation in the CH(C6H4-F)a(C6H4-F)b groups on account of the close proximity of the imine C-methyl group and the NiBr2 unit. For Ni4 and Ni5, additional resonances were seen for the para-F (δ -119.6 ppm) and para-OCF3 (δ -57.8 ppm) groups. In the IR spectra of Ni1–Ni5, bands typical of the (C=N) absorptions are seen around 1600 cm−1 which compares with 1644–1637 cm−1 in the free ligands, L1–L5.

2.2. Catalytic Evaluation for Ethylene Polymerization

2.2.1. Co-Catalyst Screening

To ascertain the most suitable activator, Ni1 was employed as the test precatalyst and assessed with five different alkyl aluminum co-catalysts, including MAO (methylaluminoxane), MMAO (modified methylaluminoxane), EtAlCl2, Et2AlCl and EASC (ethylaluminium sesquichloride). All runs were performed in toluene under PC2H4 of 10 atm over 30 min with the temperature set at 30 °C (runs 1–5, Table 2). Inspection of the results indicated that all Ni1/co-catalyst combinations were effective in generating polyethylene, with activities spanning a fairly narrow range of 2.31–2.86 × 106 g of PE (mol of Ni)−1 h−1 with the relative level falling in the order: EtAlCl2 > EASC > Et2AlCl > MMAO > MAO. With the aim to focus on just two types of co-catalyst, the most effective ethyl aluminum chlorides (EtAlCl2) and aluminoxanes (MMAO) were selected for further optimization of the polymerization conditions.

2.2.2. Ethylene Polymerization Using Ni1–Ni5 in the Presence of EtAlCl2

To achieve an effective set of polymerization conditions that could be employed to evaluate all five nickel precatalysts, Ni1/EtAlCl2 was optimized by subjecting it to a systematic investigation examining Al:Ni molar ratio, reaction temperature and run time (Table 3). In the first instance, the Al:Ni molar ratio was modified between 300:1 and 600:1 with the temperature kept at 30 °C (runs 1–6, Table 3). Scrutiny of the results revealed the highest value of 3.20 × 106 g of PE (mol of Ni)−1 h−1 was achieved at 500:1 (run 4, Table 3). At ratios in excess of 500:1, the activity decreased, as is demonstrated by the run performed at 600:1 that gave the lowest activity of 2.45 × 106 g of PE (mol of Ni)−1 h−1 (run 6, Table 3). In terms of the polymer molecular weight, no dramatic variation could be identified with respect to changes in the molar ratio, though it could be viewed that some modest overall decline was evident with the Mw value dropping from 5.39 to 4.53 kg·mol−1 in line with the onset of chain transfer [33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48]. In any case, the polymer dispersity (Mw/Mn range: 2.56–2.88) remained narrow and unimodal, as shown in the GPC curves (Figure 4).
After that, we explored the influence of temperature on the performance of Ni1/EtAlCl2 by varying it from 20 to 50 °C with the Al:Ni molar ratio maintained at 500:1 (runs 4, 7–9, Table 3). The highest activity of 3.20 × 106 g of PE (mol of Ni)−1 h−1 was obtained at 30 °C, while further increasing the temperature saw the activity rapidly decline, reaching a low point of 5.0 × 105 g of PE (mol of Ni)−1 h−1 at 50 °C (run 9, Table 3). Such a downward trend in activity can be interpreted in terms of deactivation of the active species and the lower solubility of ethylene at elevated temperature [24,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50]. With respect to the resulting polymer, lower run temperature led to higher molecular weight PE (Mw = 9.59 kg·mol−1 at 20 °C) with a relatively high melting temperature (Tm = 102.1 °C). By comparison, with the run temperature at 50 °C, the molecular weight and melting point of the polymer dropped to their lowest values of 2.63 kg·mol−1 and 58.5 °C, respectively (run 9, Table 3). It is assumed that higher temperature increases the rate of chain termination compared to chain propagation, resulting in lower molecular weight PE [51]. Furthermore, the polymer dispersity remained narrow (Mw/Mn range: 2.2–3.0) and unimodal at the various temperatures in accordance with the single-site nature of the catalyst (Figure 5). As a final remark, the appearance of the polymer visually changed from powder to wax due to its lower melting temperature (Tm range: 58.5–102.1 °C), lower molecular weight and likely higher branching content (vide infra).
To gauge the effectiveness of Ni1/EtAlCl2 over time, the polymerization tests were conducted at various intervals between 5 and 60 min at 30 °C with the Al:Ni molar ratio fixed at 500:1 (runs 4, 10–13, Table 3). As a key observation, the catalytic activity displayed a modest upward trend in the first 30 min, culminating in a maximum activity of 3.20 × 106 g of PE (mol of Ni)−1 h−1 (run 4, Table 3). However, over the following 30 min the activity then gradually dropped to lower levels. It would seem likely that this decline in activity is attributable to the mass transfer limitations or to partial deactivation of the active species [52]. On the other hand, the molecular weight of the polymer increased and reached a maximum of 6.18 kg·mol−1 after 1 h, while the melting temperature of the polymer remained in a similar range (Tm range: 63.9–68.9 °C). As for the polymer dispersity, this remained narrow (Mw/Mn range: 2.6–2.9) and unimodal as is depicted in the GPC curves (Figure 6). As a final point, the catalytic activity of Ni1/EtAlCl2 was also affected by ethylene pressure, with much lower activity observed at 5 atm, whereas at 1 atm only a trace amount of polymer was obtained (runs 14, 15, Table 3).
Following the optimization studies performed on Ni1/EtAlCl2, the most effective conditions were established as Al:Ni molar ratio = 500:1, reaction temp. = 30 °C and run time = 30 min. By employing these conditions, the remaining precatalysts were assessed with EtAlCl2 as co-catalyst. As a general point, all precatalysts showed high activities ranging between 4.28 and 2.88 × 106 g of PE (mol of Ni)−1 h−1 (runs 16–19, Table 3) with the relative order being: Ni5 > Ni4 > Ni3 > Ni1 > Ni2. Significantly, the precatalysts bearing electron withdrawing para-R substituents displayed a positive effect on catalytic activity. For example, when R = OCF3 (Ni5), the activity of 4.28 × 106 g of PE (mol of Ni)−1 h−1 was higher than that observed for R = Me (Ni1) of 3.20 × 106 g of PE (mol of Ni)−1 h−1. The explanation for this observation would likely stem from the increased electrophilicity of the metal center caused by the electron-withdrawing substituent which in turn increases the rate of ethylene insertion and higher activities [42,43,44,45,46,47,48]. Similar conclusions have been drawn from computational calculations that reveal favorable electronic effects of F or OCF3 substitution on the net charge of the active species [53,54]. In terms of the polymer molecular weight, all polymers displayed low molecular weight in the range 4.61–5.83 kg·mol−1, with no clear correlations with the para-R group evident. Nonetheless, all catalysts formed narrowly dispersed PE (Mw/Mn range: 2.5–2.9) (Figure 7), findings that are characteristic of single-site active center.

2.2.3. Ethylene Polymerization Using Ni1–Ni5 in the Presence of MMAO

With MMAO now utilized as the co-catalyst, Ni1 was again initially deployed to probe the effect of Al:Ni molar ratio, run temperature, run time and ethylene pressure. First the Al:Ni molar ratio was evaluated at 30 °C by varying it between 1500:1 and 3500:1 (runs 1–5, Table 4). From the data, it was evident that at 2000:1, the catalytic activity reached a peak value of 2.51 × 106 g PE (mol of Ni)−1 h−1 (run 2, Table 4), which then gradually dropped as the ratio was increased beyond 2000:1 (runs 3–5, Table 4). On the other hand, the molecular weight of the PE gradually decreased from the outset, which can be related to the higher rates of chain transfer as compared to chain propagation at higher molar ratios of co-catalyst [52]. As has been noted throughout this study, the polymerizations are well-controlled with their dispersities (Mw/Mn), falling in this case between 2.21 and 2.54 (see Figure S1 for the GPC curves).
Next, with the Al:Ni molar ratio retained at 2000:1, the influence of the run temperature on the performance of Ni1/MMAO was investigated by performing the polymerizations between 20 and 50 °C (runs 2, 6–8, Table 4). As with EtAlCl2 study, the results showed that the optimum temperature was at 30 °C (run 4, Table 4), above which deactivation of the active species occurred, leading to a rapid decrease in performance (e.g., 3.0 × 105 g PE (mol of Ni)−1 h−1 at 50 oC, run 8, Table 4); the lower concentration of ethylene at elevated temperature could also be a contributing factor [19,20,21,22,23,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51]. The molecular weight of the PE also decreased from 7.32 to 2.50 kg·mol−1 (runs 2, 6–8, Table 4), which can be ascribed to a higher rate of chain termination when compared to chain propagation at elevated reaction temperature [17,45,46,47,48,49,55]. Nonetheless, the single site nature of the catalyst was maintained across the temperature range with narrow dispersities seen in all cases (Mw/Mn range: 2.63–2.03, see Figure S2). The melting temperatures (Tm) of the polymers displayed similar trends to those observed for Ni1/EtAlCl2 with values dropping as the temperature was raised.
The time/activity profile of Ni1/MMAO was then investigated over various run times, typically 5, 15, 45, and 60 min with the Al:Ni molar ratio fixed at 2000:1 and the run temperature at 30 °C (runs 2, 9–12, Table 4). Similar to that seen with Ni1/EtAlCl2, the catalytic activity reached a maximum after 30 min, in this case attaining a slightly lower value of 2.51 × 106 g of PE (mol of Ni)−1 h−1 (run 2, Table 4). Likewise, the molecular weight of the polymer displayed an upward trend over time (from 3.30 kg·mol−1 after 5 min to 3.75 kg·mol−1 after 1 h), though less pronounced than that seen with Ni1/EtAlCl2. As would be expected, the melting points of the polymers remained almost constant over the different run times (Tm range: 69.1–76.6 °C) and the dispersities narrowed (Mw/Mn range: 2.0–2.6). The effect of varying ethylene pressure mirrored that seen with EtAlCl2, with catalytic activity and polymer molecular weight dropping as the pressure was reduced (runs 4, 14, 15, Table 4).
On the strength of the optimum conditions established for Ni1/EtAlCl2, the four remaining nickel precatalysts Ni2–Ni5 were similarly screened (Al:Ni molar ratio = 2000:1, reaction temp. = 30 °C and run time = 30 min); the results are collected in Table 4, runs 15–18. In general, comparable trends were found to that seen with Ni1–Ni5/EtAlCl2, though lower catalytic activities were apparent (3.15–2.05 × 106 g of PE (mol of Ni)−1 h−1 (MMAO) vs. 4.28–2.88 × 106 g of PE (mol of Ni)−1 h−1 (EtAlCl2)), which highlights the important role played by the alkyl-aluminum activator. Moreover, lower amounts of EtAlCl2 are required for the nickel catalyst to reach optimal performance.

2.2.4. Microstructural Studies of the Polyethylenes

On inspection of Table 3 and Table 4, the melting points (Tm) for the polyethylenes obtained with either EtAlCl2 or MMAO were found to fall in the range of 58.5–102.8 °C, values that reflect both their level of branching and molecular weight. To provide further insight into the microstructural properties of the PEs, the 1H and 13C NMR spectra were recorded for selected Pes, including those generated using Ni1/EtAlCl2 at 20, 30 and 50 °C (runs 7, 4 and 9, Table 3) and Ni1/MMAO at 30 °C (run 2, Table 4).
The 1H NMR spectra of the PEs obtained using Ni1/EtAlCl2 at the three different temperatures, 20, 30 and 50 °C (runs 7, 4, 9, Table 3), displayed the presence of both vinyl and vinylene functional groups, with characteristic signals at δ 5.02 ppm (Ha) and 5.84 (Hb) for the –CH=CH2 protons with an integral ratio of 2:1 along with a signal at δ 5.40 ppm (Hc/Hc′) for the –CH=CH– protons. On closer examination of the spectra, it was evident that the vinylene to vinyl ratio, (–CH=CH–):(–CH=CH2), was greatly affected by the reaction temperature (Figure 8). In particular, on increasing the temperature from 20 to 50 °C, this ratio increased from 3.1:1 to 11.8:1, which suggests that raising the polymerization temperature increased the rate of alkene isomerization.
In the 13C NMR spectrum of the PE produced using Ni1/EtAlCl2 at 20 °C (run 7, Table 3), characteristic resonances for the alkenic-carbons for the vinyl and vinylene functional groups, denoted Ca, Cb, Cc and Cc’, were seen in the downfield region (see inset in Figure S5). In the upfield region (main spectrum in Figure 9), peaks for the (–CH2–) repeat unit along with less intense resonances corresponding to the various types of branch can be seen. Using approaches described in the literature [56,57,58], the assignment and relative percentage of each branch along with the branching density can be determined. Specifically, the analysis of this sample revealed 67 branches/1000 Cs, including methyl (76.3%), ethyl (1.6%), propyl (2.3%), butyl (4.8%), 1,4-paired methyl (2.5%), 1,6-paired methyl (2.9%) and longer chain branches (9.3%) (see Figure S5).
By comparison, for the PE sample generated at 30 °C (run 4, Table 3), the number of branches increased to 91/1000 Cs, which included methyl (70.4%), ethyl (2.0%), propyl (2.9%), butyl (2.2%), 1,4-paired methyl (7.6%), 1,6-paired methyl (4.2%) and longer chain branches (10.3%) (see inset in Figure 9). This higher branching density was supported by a lower Tm value for this PE sample; similar results have been reported in the literature [23,59,60,61]. Conversely, the 13C NMR spectrum of the PE generated at 50 °C (run 8, Table 3) showed the branching density increased to 135/1000 Cs and was based on methyl (25.8%), ethyl (4.8%), propyl (3.4%), butyl (8.1%), amyl (6.0%), 1,4-paired methyl (15.9%), 1,5-paired methyl (4.1%), 1,6-paired methyl (21.4%) and longer chain branches (10.1%) (see Figure S6).
To examine the effect of the co-catalyst on the microstructural properties of the polymer, the PE sample obtained using Ni1/MMAO at 30 °C (run 2, Table 4) was also characterized by 1H and 13C NMR spectroscopy. Interestingly, little difference in appearance of their spectra (see Figures S7 and S8, respectively) could be detected when compared with that obtained using Ni1/EtAlCl2 at the same temperature. Indeed, the branching density was similar at 92 branches/1000 Cs, as was the branching composition: methyl (70.0%), ethyl (3.0%), propyl (2.2%), butyl (1.6%), amyl (1.7%), 1,4-paired methyl (6.3%), 1,6-paired methyl (4.3%) and longer chain branches (10.5%).

2.2.5. Comparison between Current and Reported Analogues Precatalysts

With the intent to shed some light on the impact of the ortho-substituted 2,6-bis(4,4′-difluorobenzhydryl) groups as well as the variations in the electronic properties of the para-substituent on catalyst performance, the activity of F (Ni5 in this work) and molecular weight of the polymer were compared with those of previously reported 2-iminopyridine-containing C, D and E (Figure 10); all polymerization runs were performed under optimized conditions of 10 atm of ethylene, 30 °C and with EtAlCl2 as a co-catalyst [22,24,25,26,27,28]. As a notable observation, F displayed the highest activity of this series, which supports the view that the incorporation of an electron withdrawing OCF3 group at the para-position has a positive effect on catalytic performance. Furthermore, F (Ni4) produced polymer with the highest molecular weight which highlights the beneficial role played by the 4,4′-difluorobenzhydryl groups at the ortho positions on chain propagation.

3. Experimental Section

3.1. General Consideration

All manipulations of air or moisture-sensitive compounds were undertaken under an atmosphere of nitrogen using standard Schlenk techniques. The toluene used for the polymerization studies was dried over sodium for 8 h and distilled under a nitrogen atmosphere immediately prior to use. The ethylene monomer was purchased from Beijing Yanshan Petrochemical Company (Beijing, China) and utilized directly for the polymerization studies without any further purification. All other reagents were provided from Acros, Aldrich, or local suppliers. The co-catalysts MMAO (1.93 M solution in n-heptane) and MAO (methylaluminoxane, 1.30 M solution in toluene) were purchased from Anhui Botai Electronic Materials Co., (Chuzhou, China) while EtAlCl2 (ethylaluminumdichloride), Et2AlCl (diethylaluminumchloride) and EASC (ethylaluminumsesquichloride, 40% solution in n-hexane) were provided by Yantai Lianli Chemical Co (Yantai, China). The NMR spectra were recorded on a Bruker AVANCE III (400 MHz instrument) (Bruker, Fällanden, Switzerland) using TMS as an internal standard at ambient temperature. In addition, 19F NMR spectra of the ligands and nickel complexes were recorded on a Bruker AVANCE III 500 MHz instrument at ambient temperature. A Perkin Elmer System 2000 FT-IR spectrometer (Perkin-Elmer, Waltham, MA, USA) was used to record the FT-IR spectra, while elemental analysis was performed by a Flash EA1112 microanalyzer. Molecular weight (Mw) and molecular weight distributions (Mw/Mn) of the polyethylenes were determined with a PL-GPC220 instrument (Beijing, China) operating at 150 °C with 1,2,4-trichlorobenzene as eluting solvent. Differential scanning calorimetry (DSC, TA2000) (Perkin-Elmer, Waltham, MA, USA) was performed under a nitrogen atmosphere to record the melting temperatures of the polyethylenes. The temperature program was set as follows: a sample (4.0–6.0 mg) was heated to 130 °C at a heating rate of 20 °C per min and held for 5 min at the same temperature to remove its thermal history, then cooled to −50 °C at the same heating rate, held for 5 min at −50 °C and then heated to 130°C at a heating rate of 20 °C per min. A Bruker Avance III 500 MHz spectrometer was used to record the 13C NMR spectra of the polyethylenes and these were run in a 5 mm standard glass tubes at 100 °C. The operating conditions used were as follows: spectral width 14.9701 kHz, acquisition time 2.1889 s, relaxation delay 1.0 s, number of scans at around 1024. Inverse gated 13C NMR spectra were recorded at 110 °C on a Bruker Neo 700 NMR spectrometer equipped with a cryo BBO probe. Sample preparation involved dissolving 80–100 mg of polymer at 100 °C in 1,1,2,2-tetrachloroethane-d2 (2 mL) containing TMS as an internal standard. The branching content of the polymer was calculated from the integration of the corresponding peaks in the 13C NMR spectra according to literature approaches [59,60]. The anilines, 2,6-(bis(4,4′-difluorodibenzhydyl)-R-phenylamine (R = Me A1, Et A2, i-Pr A3, F A4, OCF3 A5), were synthesized based on reported methods [45,61,62,63] and described in the SI.

3.2. Synthesis of 2-{(2,6-(CH(C6H4-p-F)2)2-4-RC6H2)N=CMe}C5H4N (L1 − L5)

3.2.1. R = Me (L1)

A mixture of 2-acetylpyridine (0.25 g, 2.07 mmol) and ZnCl2 (0.336 g, 2.5 mmol) in glacial acetic acid (3 mL) was stirred and heated to reflux for 20 min. Aniline A1 (1.05 g, 2.07 mmol) was added to the reaction mixture and the stirring continued for a further 4 h at reflux. Once cooled to room temperature, a bright orange-red solid precipitated. The solid was filtered and washed with acetic acid (3 × 3 mL) and diethyl ether (5 × 6 mL) and then dried under reduced pressure to give the intermediate complex (L1)ZnCl2 as a bright orange-red solid [64,65]. This solid was then dissolved in dichloromethane (3 mL) and demetallation of the zinc intermediate was undertaken by introducing 3 mL of aqueous potassium carbonate (0.553 g, 3.0 mmol) and vigorous stirring the reaction mixture for 1 h. The organic and aqueous phases were separated, and the organic layer washed with water (3 × 10 mL) and dried with MgSO4. Following filtration, the solvent was removed under reduced pressure to afford the crude product which was then recrystallized from dichloromethane and n-hexane to afford L1 as an orange powder (0.80 g, 65%). FT–IR (cm−1): 2919 (w), 1858 (w), 1644 (m), 1600 (m), 1505 (s), 1465 (w), 1361 (w), 1301 (w), 1217 (s), 1157 (s), 1099 (w), 1038 (w), 1016 (w), 869 (w), 835 (s), 783 (m), 726 (w). 1H NMR (400 MHz, CDCl3, TMS): δ 8.60 (d, J = 6.40 Hz, 1H), 7.93 (d, J = 8.00 Hz, 1H), 7.72 (t, J = 8.0 Hz, 1H), 7.34 (t, J = 6.4 Hz, 1H), 6.86-6.96 (m, 16H), 6.61 (s, 2H), 5.20 (s, 2H), 2.18 (s, 3H), 1.20 (s, 3H). 13C NMR (100 MHz, CDCl3, TMS): 169.4, 162.6, 162.5, 160.2, 155.7, 148.6, 145.7, 139.1, 138.2, 138.1, 136.1, 132.0, 131.9, 131.0, 130.7, 130.6, 128.5, 124.8, 121.1, 115.3, 115.1, 114.9, 114.7, 50.5, 21.2, 17.0. 19F NMR (470 MHz, CDCl3): δ –116.29, –117.05. Anal. calcd for C40H30F4N2 (614.69): C, 88.16; H, 6.92; N, 4.56. Found: C, 87.76; H, 6.83; N, 4.59.

3.2.2. R = Et (L2)

By employing the same method and molar ratios as described for L1, L2 was isolated as a yellow powder (0.92 g, 73%). FT-IR (cm−1): 2962 (w), 2285 (w), 2082 (w), 1640 (m), 1600 (m), 1505 (m), 1460 (w), 1367 (w), 1301 (w), 1258 (s), 1215 (m), 1155 (w), 1013 (m), 873 (w), 788 (s), 731 (w), 667 (w). 1H NMR (400 MHz, CDCl3, TMS): δ 8.59 (d, J = 4.8 Hz, 1H), 7.96 (d, J = 7.8 Hz, 1H), 7.72 (t, J = 7.5 Hz, 1H), 7.35 (t, J = 5.4 Hz, 1H), 6.85-6.96 (m, 16H), 6.63 (s, 2H), 5.21 (s, 2H), 2.46 (q, J = 5.4 Hz, 2H), 1.19 (s, 3H), 1.05 (s, 3H). 13C NMR (100 MHz, CDCl3, TMS): 168.3, 162.0, 158.7, 154.6, 147.6, 144.9, 138.2, 138.1, 137.4, 137.1, 135.1, 130.9, 130.0, 129.9, 129.7, 129.6, 126.3, 123.8, 120.1, 114.3, 114.0, 113.9, 113.6, 49.5, 27.4, 16.0, 14.7. 19F NMR (470 MHz, CDCl3): δ −116.32, −117.14. Anal. calcd for C41H32F4N2 (628.71): C, 78.33; H, 5.13; N, 4.46. Found: C, 78.73; H, 5.15; N, 4.48.

3.2.3. R = i-Pr (L3)

By employing the same method and molar ratios as described for L1, L3 was isolated as a yellow powder (0.88 g, 68%). FT-IR (cm−1): 2959 (w), 2932 (w), 1973 (w), 1637 (C=N, m), 1599 (m), 1503 (s), 1464 (w), 1366 (w), 1301 (w), 1218 (s), 1155 (m), 1097 (m), 1015 (w), 832 (s), 782 (m), 729 (m), 664 (w). 1H NMR (400 MHz, CDCl3, TMS): δ 8.59 (d, J = 4.8 Hz, 1H), 7.97 (d, J = 8.0 Hz, 1H), 7.72 (t, J = 7.6 Hz, 1H), 7.35 (t, J = 5.2 Hz, 1H), 6.66-6.99 (m, 16H), 6.47 (s, 2H), 5.21 (s, 2H), 2.71 (m, 1H), 1.21 (s, 3H), 1.07 (d, J = 10.0 Hz, 6H). 13C NMR (100 MHz, CDCl3, TMS): δ 162.5, 160.2, 160.1, 155.7, 148.6, 145.9, 143.0, 139.2, 139.2, 138.2, 138.2, 136.1, 131.7, 131.0, 130.9, 130.7, 130.6, 125.9, 124.8, 121.1, 115.3, 115.1, 114.9, 114.6, 58.4, 50.6, 33.5, 24.0, 18.2, 17.1. 19F NMR (470 MHz, CDCl3): δ −116.35, −117.13. Anal. calcd for C42H34F4N2 (642.74): C, 78.49; H, 5.33; N, 4.36. Found: C, 78.03; H, 5.29; N, 4.32.

3.2.4. R = F (L4)

By employing the same method and molar ratios as described for L1, L4 was isolated as a yellow powder (0.18 g, 75%). FT-IR (cm−1): 3070 (w), 2962 (w), 2282 (w), 2110 (w), 1642 (C=N, m), 1599 (m), 1505 (s), 1466 (w), 1441 (m), 1363 (w), 1302 (m), 1259 (m), 1219 (s), 1155 (m), 1095 (m), 1016 (m), 873 (w), 760 (s), 748 (w), 722 (w), 681 (w). 1H NMR (400 MHz, CDCl3, TMS): δ 8.61 (d, J = 4.4 Hz, 1H), 7.94 (d, J = 7.6 Hz, 1H), 7.73 (t, J = 1.6 Hz, 1H), 7.37 (t, J = 1.6 Hz, 1H), 6.88-6.94 (m, 16H), 6.55 (d, J = 9.6 Hz, 2H), 5.21 (s, 2H), 1.20 (s, 3H). 13C NMR (100 MHz, CDCl3, TMS): δ 170.2, 162.8, 162.7, 160.3, 157.7, 155.4, 148.7, 144.1, 138.2, 137.4, 137.3, 136.2, 134.2, 134.1, 131.0, 130.9, 130.6, 130.5, 125.0, 121.1, 115.5, 115.3, 115.2, 114.9, 114.7, 50.5, 17.0. 19F NMR (470 MHz, CDCl3): δ −115.67, −116.41, −119.70. Anal. calcd for C39H27F5N2 (618.65): C, 75.72; H, 4.40; N, 4.53. Found: C, 75.42; H, 4.42; N, 4.55.

3.2.5. R = OCF3 (L5)

By employing the same method and molar ratios as described for L1, L5 was isolated as a yellow powder (1.10 g, 80%). FT-IR (cm−1): 3044 (w), 2914 (w), 1892 (w), 1643 (C=N, m), 1602 (m), 1583 (w), 1567 (w), 1505 (s), 1465 (w), 1442 (m), 1366 (m), 1303 (w), 1254 (m), 1224 (w), 1209 (w), 1195 (w), 1172 (w), 1154 (w), 1098 (m), 1043 (w), 1014 (m), 993 (w), 875 (w), 833 (s), 794 (w), 783 (w), 740 (m), 656 (m). 1H NMR (400 MHz, CDCl3, TMS): δ 8.59 (d, J = 4.0 Hz, 1H), 7.93 (d, J = 7.6 Hz, 1H), 7.72 (t, J = 7.6 Hz, 1H), 7.35 (t, J = 6.4 Hz, 1H), 6.88–6.94 (m, 16H), 6.67 (s, 2H), 5.19 (s, 2H). 13C NMR (100 MHz, CDCl3, TMS): δ 160.3, 148.7, 146.7, 138.0, 137.1, 136.2, 134.0, 130.9, 130.8, 130.6, 130.5, 125.1, 121.1, 120.8, 115.6, 115.4, 115.2, 115.0, 50.5, 17.2. 19F NMR (470 MHz, CDCl3): δ –58.22, –115.53, –116.29. Anal. calcd for C40H27F7N2O (684.66): C, 70.17; H, 3.98; N, 4.09. Found: C, 69.78; H, 4.01; N, 4.08.

3.3. Synthesis of [2-{(2,6-(CH(C6H4-p-F)2)2-4-RC6H2)N=CMe}C5H4N]NiBr2 (Ni1 − Ni5)

3.3.1. R = Me (Ni1)

Under a nitrogen atmosphere, L1 (0.30 g, 0.52 mmol) and NiBr2(DME) (0.08 g, 0.26 mmol) were added to a mixture of dichloromethane (10 mL) and ethanol (10 mL) and stirred for 12 h at room temperature. All the volatiles were then evaporated under reduced pressure and diethyl ether added to the residue to induce precipitation. The precipitate was filtered, washed with diethyl ether (3 × 10 mL) and dried to afford Ni1 as an orange powder (0.20 g, 58%). FT-IR (cm−1): 3353 (w), 3063 (w), 2921 (w), 1630 (w), 1598 (C=N, m), 1571 (w), 1505 (s), 1450 (w), 1372 (w), 1318 (w), 1253 (w), 1221 (s), 1158 (s), 1097 (w), 1061 (w), 1021 (w), 982 (w), 837 (s), 783 (m), 727 (w). 19F NMR (470 MHz, CDCl3): δ −116.13, −117.52. Anal. calcd for C40H30Br2F4N2Ni (833.19): C, 57.66; H, 3.63; N, 3.36. Found: C, 57.63; H, 3.56; N, 3.57.

3.3.2. R = Et (Ni2)

By utilizing a similar procedure and molar ratios to those described for Ni1, Ni2 was isolated as an orange powder (0.20 g, 63%). FT-IR (cm−1): 3061 (w), 2970 (w), 2894 (w), 1627 (w), 1597 (m), 1571 (w), 1505 (s), 1455 (w), 1425 (w), 1373 (w), 1318 (w), 1220 (s), 1158 (m), 1095 (w), 1051 (w), 1020 (w), 878 (w), 836 (s), 783 (m), 726 (w). 19F NMR (470 MHz, CDCl3): δ −115.44, −116.82. Anal. calcd for C41H32Br2F4N2Ni (847.22): C, 58.13; H, 3.81; N, 3.31. Found: C, 58.02; H, 3.84; N, 3.34.

3.3.3. R = i-Pr (Ni3)

By utilizing a similar procedure and molar ratios to that described for Ni1, Ni3 was isolated as an orange powder (0.22 g, 60%). FT-IR (cm−1): 3063 (w), 2962 (w), 1598 (m), 1574 (w), 1505 (s), 1449 (w), 1372 (w), 1317 (w), 1256 (w), 1223 (s), 1158 (m), 1134 (w), 1097 (w), 1020 (w), 877 (w), 837 (s), 783 (m), 750 (w), 726 (w). 19F NMR (470 MHz, CDCl3): δ −115.45, −116.82. Anal. calcd for C42H34Br2F4N2Ni (861.24): C, 58.57; H, 3.98; N, 3.25. Found: C, 58.53; H, 4.03; N, 3.42.

3.3.4. R = F (Ni4)

By utilizing a similar procedure and molar ratios to that described for Ni1, Ni4 was isolated as an orange powder (0.27 g, 77%). FT-IR (cm−1): 3358 (w), 3070 (w), 1912 (w), 1630 (w), 1597 (C=N, m), 1574 (w), 1505 (s), 1448 (m), 1373 (w), 1320 (w), 1263 (w), 1224 (s), 1157 (w), 1100 (w), 1055 (w), 1007 (w), 942 (w), 869 (w), 839 (s), 781 (w), 725 (w), 684 (w), 651 (w). 19F NMR (470 MHz, CDCl3): δ −115.66, −116.41, −119.6. Anal. calcd for C39H27Br2F5N2Ni (837.15): C, 55.96; H, 3.25; N, 3.35. Found: C, 55.85; H, 3.18; N, 3.28.

3.3.5. R = OCF3 (Ni5)

By utilizing a similar procedure and molar ratios to that described for Ni1, Ni5 was isolated as an orange powder (0.36 g, 84%). FT-IR (cm−1): 3761 (w), 3062 (w), 2161 (w), 2028 (w), 1971 (w), 1624 (w), 1598 (m), 1574 (w), 1505 (s), 1449 (w), 1373 (w), 1317 (w), 1263 (m), 1221 (s), 1190 (w), 1156 (m), 1098 (w), 1017 (w), 982 (w), 879 (w), 834 (s), 784 (m), 730 (w). 19F NMR (470 MHz, CDCl3): δ −57.84, −114.76, −115.72. Anal. calcd for C40H27Br2F7N2NiO (903.16): C, 53.20; H, 3.01; N, 3.10. Found: C, 53.06; H, 3.03; N, 2.89.

3.4. Ethylene Polymerization Evaluation

3.4.1. The Polymerizations at PC2H4 = 5 or 10 atm Were Performed in a Stainless Steel Autoclave (250 mL Capacity) Equipped with a Mechanical Stirrer and an Ethylene Pressure and Temperature Control System

The autoclave was evacuated and backfilled with nitrogen (×3) and once with ethylene (×1). The nickel precatalyst (Ni1–Ni5, 2.0 μmol) was dissolved in toluene (50 mL) and injected into the autoclave at the desired reaction temperature followed by freshly distilled toluene (25 mL). The required amounts of co-catalyst (EtAlCl2, Et2AlCl, EASC, MAO or MMAO) and toluene (50 mL), were successively injected using a syringe, taking the total volume to 100 mL. The autoclave was immediately pressurized to the required ethylene pressure (5 or 10 atm) with rapid stirring. After a predetermined reaction time, the flow of ethylene was stopped, and the reactor cooled in a water bath. Once the temperature had reached ambient, the reactor was vented, and the reactor opened. The resulting mixture was poured into a 10% solution of HCl in ethanol, filtered and washed with ethanol several times before drying under reduced pressure until of constant weight.

3.4.2. Polymerizations at PC2H4 = 1 atm

The Polymerizations at 1 atm were conducted in a schlenk vessel. The nickel precatalyst (0.002 mmol) was added to a Schlenk vessel equipped with a stir bar, followed by freshly distilled toluene (30 mL). The required amount of aluminum co-catalyst was then added by syringe. The reaction mixture was stirred at 1 atm of ethylene pressure at room temperature. After 30 min, the supply of ethylene was ceased, and the reaction vessel vented. Following the addition of 10% hydrochloric acid in ethanol, the polymer was filtered and washed with ethanol and then dried under reduced pressure until of constant mass.

3.5. X-ray Crystallographic Studies

Single crystals of Ni4’ and Ni5 suitable for an X-ray diffraction determination were grown by slow diffusion of diethyl ether into the corresponding dichloromethane solutions at room temperature. Data collection was performed on a Rigaku Saturn724+ CCD with graphite-monochromatic Cu-Kα radiation (λ = 1.54184 Å) at 169(2) K. The 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 applying direct methods and refined by full-matrix least-squares on F2. All hydrogen atoms were placed in the calculated positions. Structure solution and refinement were performed using the Olex2 1.2 package [66]. The structure was solved with the SHELXT [67] and structure solution program using Intrinsic Phasing and refinement with the SHELXL [68] refinement package using Least Squares minimization. Crystal data and structure refinements for Ni4’ and Ni5 are provided in Table S1.

4. Conclusions

A series of novel 2-(2,6-bis(bis(4,4′-difluorobenzhydryl)-4-R-phenylimino)ethyl) pyridines (R = Me (L1), Et (L2), i-Pr (L3), F (L4), OCF3 (L5), together with their corresponding LNiBr2 complexes, Ni1–Ni5, were successfully prepared and fully characterized by a range of techniques, including FT-IR, 1H, 19F, 13C NMR spectroscopy and elemental analysis. In addition, structural characterization of Ni5 and a derivative of Ni4, (L4)2NiBr2 (Ni4’), was undertaken. Following an initial screen of a range of aluminum-alkyl co-catalysts, EtAlCl2 and MMAO were taken forward to investigate the effects of the para-R substitution on ethylene polymerization. All the catalysts exhibited high activity in the range of 0.50 to 4.28 × 106 g of PE (mol of Ni)−1 h−1 (EtAlCl2) and 0.30 to 3.15 × 106 g of PE (mol of Ni)−1 h−1 (MMAO), generating highly branched PE with low molecular weight and narrow dispersity, along with a high content of unsaturated end groups (ratio of vinylene to vinyl, range: 3.1:1–10.3.1). Significantly, catalysts bearing electron withdrawing 4-substituents (e.g., F and OCF3) showed higher activities than their electron donating counterparts. Furthermore, the catalytic activity, type of polymer unsaturation, molecular weight and branching densities were greatly influenced by the reaction temperature. Finally, by comparison with a series of structurally related nickel catalysts, the effect of para-OCF3 substitution was to enhance catalytic activity, while the ortho-4,4′-difluorobenzhydryl groups augmented polymer molecular weight.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/catal12121520/s1, Synthesis of the organic compounds A1–A5 as ligands according to literature [62,63] and characterization, 19F NMR of ligands and complexes, the GPC curves of the polyethylene samples produced with the Ni1/MMAO, 13C NMR and 1H NMR spectra of the polyethylene samples generated using Ni1/MAO and Ni1/EtAlCl2 (PDF). In addition, crystallographic information for Ni4’ (CCDC 2215182) and Ni5 (CCDC 2215183) (CIF) are provided in the supporting information. Scheme S1 Synthetic route to A1–A5; Figure S1 GPC curves for the polyethylene produced using Ni1/MMAO at different Al:Ni molar ratios (runs 1–5, Table 4); Figure S2 GPC curves for the polyethylene produced using Ni1/MMAO, at different reaction temperatures (runs 2, 6–8, Table 4); Figure S3 GPC curves for the polyethylene produced using Ni1/MMAO at different reaction times (runs 2, 9–12, Table 4); Figure S4 GPC curves for the polyethylene produced using Ni1–Ni5/MMAO under optimized conditions (runs 2, 15–18, Table 4); Figure S5 13C NMR spectrum of the polyethylene sample produced using Ni1/EtAlCl2 at 20 °C (run 7, Table 3), including an inset of the alkenic region and a segment of the assigned polymer backbone; recorded at 100 °C in d-C2D2Cl4; Figure S6 13C NMR spectrum of the polyethylene sample produced using Ni1/EtAlCl2 at 50 °C (run 9, Table 3), including an inset of the alkenic region and a segment of the assigned polymer backbone; recorded at 100 °C in d-C2D2Cl4; Figure S7 1H NMR spectrum of the polyethylene sample produced using Ni1/MMAO at 30 °C (run 2, Table 4); recorded at 100 °C in d-C2D2Cl4; Figure S8 13C NMR spectrum of the polyethylene sample produced using Ni1/MMAO at 30 °C (run 2, Table 4), including an inset of the alkenic region and a segment of the assigned polymer backbone; recorded at 100 °C in d-C2D2Cl4; Figures S9–S18 19F NMR spectrum of L1, L2, L3, L4, L5, Ni1, Ni2, Ni3, Ni4, Ni5 (recorded in CDCl3 at room temperature); Figure S19 DSC curve Ni/MMAO (recorded at room temperature); Table S1 Crystal data and structure refinement for Ni4’ and Ni5.

Author Contributions

Design of the study by W.-H.S.; synthesis of the organic compounds and the nickel complexes by Z.H.; characterization by Z.H and W.-H.S.; X-ray study by M.L., Q.Z. and T.L.; catalytic study by Z.H.; characterization of polyethylene by Z.H.; writing and editing Z.H., G.A.S., Y.M. and W.-H.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (21871275).

Data Availability Statement

The data of this manuscript are available from the corresponding authors upon request.

Acknowledgments

Z.H. is grateful to the Chinese Academy of Sciences President’s International Fellowship Initiative (no. 2020PM0055). G.A.S. thanks the Chinese Academy of Sciences for a President’s International Fellowship for Visiting Scientists.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 12 01520 g001 550
Figure 1. 2-(Arylimino)pyridine-nickel(II) halide (A), its substituted derivatives, (B–E), and the target of this work, (F).
Figure 1. 2-(Arylimino)pyridine-nickel(II) halide (A), its substituted derivatives, (B–E), and the target of this work, (F).
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Catalysts 12 01520 sch001 550
Scheme 1. Synthetic route to 2-(arylimino)pyridines, L1–L5 and their corresponding nickel(II) bromide complexes, Ni1–Ni5.
Scheme 1. Synthetic route to 2-(arylimino)pyridines, L1–L5 and their corresponding nickel(II) bromide complexes, Ni1–Ni5.
Catalysts 12 01520 sch001
Catalysts 12 01520 g002 550
Figure 2. ORTEP drawing of Ni5 with thermal ellipsoids shown at a probability level of 30%. All the hydrogen atoms have been omitted for clarity. The atoms labelled with an ‘i’ superscript have been generated by symmetry.
Figure 2. ORTEP drawing of Ni5 with thermal ellipsoids shown at a probability level of 30%. All the hydrogen atoms have been omitted for clarity. The atoms labelled with an ‘i’ superscript have been generated by symmetry.
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Catalysts 12 01520 g003 550
Figure 3. ORTEP drawing of bis-ligated Ni4′ with thermal ellipsoids shown at a probability level of 30%. All the hydrogen atoms have been omitted for clarity. The atoms labelled with an ‘i’ superscript have been generated by symmetry.
Figure 3. ORTEP drawing of bis-ligated Ni4′ with thermal ellipsoids shown at a probability level of 30%. All the hydrogen atoms have been omitted for clarity. The atoms labelled with an ‘i’ superscript have been generated by symmetry.
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Catalysts 12 01520 g004 550
Figure 4. GPC curves of the PE produced using Ni1/EtAlCl2 at different Al:Ni molar ratios (runs 1–6, Table 3).
Figure 4. GPC curves of the PE produced using Ni1/EtAlCl2 at different Al:Ni molar ratios (runs 1–6, Table 3).
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Catalysts 12 01520 g005 550
Figure 5. GPC curves of the PE produced using Ni1/EtAlCl2 at different reaction temperatures (runs 4, 7–9, Table 3).
Figure 5. GPC curves of the PE produced using Ni1/EtAlCl2 at different reaction temperatures (runs 4, 7–9, Table 3).
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Catalysts 12 01520 g006 550
Figure 6. GPC curves of the PE produced using Ni1/EtAlCl2 at different reaction times (runs 4, 10–13, Table 3).
Figure 6. GPC curves of the PE produced using Ni1/EtAlCl2 at different reaction times (runs 4, 10–13, Table 3).
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Catalysts 12 01520 g007 550
Figure 7. GPC curves of the PE produced using Ni1–Ni5 in combination with EtAlCl2 under optimized conditions (runs 4, 16–19, Table 3).
Figure 7. GPC curves of the PE produced using Ni1–Ni5 in combination with EtAlCl2 under optimized conditions (runs 4, 16–19, Table 3).
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Catalysts 12 01520 g008 550
Figure 8. 1H NMR spectra of the PE samples produced using Ni1/EtAlCl2 at (a) 20 oC, (b) 30 oC and (c) 50 °C (runs 7, 4 and 9, Table 3); recorded at 100 °C in d-C2D2Cl4.
Figure 8. 1H NMR spectra of the PE samples produced using Ni1/EtAlCl2 at (a) 20 oC, (b) 30 oC and (c) 50 °C (runs 7, 4 and 9, Table 3); recorded at 100 °C in d-C2D2Cl4.
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Catalysts 12 01520 g009 550
Figure 9. 13C NMR spectrum of the PE produced using Ni1/EtAlCl2 at 30 °C (run 4, Table 3) recorded at 100 °C in d-C2D2Cl4.
Figure 9. 13C NMR spectrum of the PE produced using Ni1/EtAlCl2 at 30 °C (run 4, Table 3) recorded at 100 °C in d-C2D2Cl4.
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Catalysts 12 01520 g010 550
Figure 10. Comparison of F with previously reported non-fluorinated C, D and E under optimal conditions at 10 atm of C2H4.
Figure 10. Comparison of F with previously reported non-fluorinated C, D and E under optimal conditions at 10 atm of C2H4.
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Table
Table 1. Selected bond lengths (Å) and angles (°) for Ni5 and Ni4’.
Table 1. Selected bond lengths (Å) and angles (°) for Ni5 and Ni4’.
Ni5Ni4’
Bond lengths (Å)
Ni1-Br12.5196 (8)Ni1-Br12.5176 (5)
Ni1-Br22.3857 (8)Ni1-N12.020 (4)
Ni1-N12.042 (3)Ni1-N22.365 (3)
Ni1-N22.055 (3)N2-C61.286 (6)
N2-C61.298 (5)N2-C81.449 (5)
N2-C81.438 (5)
O1-C471.312( 5)
Bond angles (o)
N1-Ni1-N279.24 (13)N1-Ni1-N275.86 (14)
Br2-Ni1-Br1128.92 (3)N1-Ni1-Br190.96 (10)
Br2-Ni1-Br1i95.80 (3)N2-Ni1-Br191.92 (9)
Br1-Ni1-Br1i84.55 (3)N1i-Ni1-Br189.04 (10)
N2-Ni1-Br2110.91 (10)N2i-Ni1-Br188.08 (9)
N2-Ni1-Br1119.64 (10)N1-Ni1-N2i104.14 (14)
N2-Ni1-Br1i97.98 (10)N1-Ni1-N1i180.00 (19)
N1-Ni1-Br291.29 (10)N2i-Ni1-N2180.00 (16)
The atoms labelled with an ‘i’ superscript have been generated by symmetry.
Table
Table 2. Assessment of the effectiveness of the co-catalyst using Ni1 as precatalyst a.
Table 2. Assessment of the effectiveness of the co-catalyst using Ni1 as precatalyst a.
RunCo-cat.Al:NiActivity bMwcMw/MncTm (°C) d
1EASC 4002.713.702.2757.5
2Et2AlCl4002.673.272.1162.2
3EtAlCl24002.864.762.5668.8
4MAO 20002.315.362.2365.4
5MMAO20002.513.682.2573.4
a Conditions: 2.0 μmol of Ni1, 100 mL of toluene, 10 atm C2H4, 30 °C, 30 min; b values in units of 106 g(PE)(mol Ni)–1 h–1; c Mw: kg·mol–1, determined by GPC; d determined by DSC.
Table
Table 3. Polymerization results using Ni1–Ni5 and EtAlCl2 a.
Table 3. Polymerization results using Ni1–Ni5 and EtAlCl2 a.
RunPrecat.T (°C)t (min)Al:NiActivity bMwcMw/MncTm (°C) d
1Ni130303001.375.222.6867.4
2Ni130304002.864.762.5668.8
3Ni130304503.104.842.8869.8
4Ni130305003.205.392.6567.8
5Ni130305502.654.532.6466.1
6Ni130306002.454.792.7066.3
7Ni120305002.769.593.02102.1
8Ni140305002.233.462.5758.2
9Ni150305000.502.632.2157.4
10Ni130055002.814.272.1863.9
11Ni130155003.024.972.6972.1
12Ni130455003.065.743.4268.4
13Ni130605002.746.183.4368.9
14 eNi130305001.143.982.2558.0
15 fNi13030500trace---
16Ni230305002.884.612.8968.3
17Ni330305003.534.852.5370.1
18Ni430305003.875.832.9068.1
19Ni530305004.285.412.7377.3
a Conditions: 2.0 μmol Ni, 100 mL toluene, 10 atm of C2H4. b 106g of PE (mol of Ni)−1 h−1. c Mw: kg·mol−1, determined by GPC. d Determined by DSC. e 5 atm of C2H4. f 1 atm of C2H4.
Table
Table 4. Polymerization results using Ni1–Ni5 and MMAO a.
Table 4. Polymerization results using Ni1–Ni5 and MMAO a.
RunPrecat.T (°C)t (min)Al:NiActivity bMwcMw/MncTm (°C) d
1Ni1303015000.704.142.2179.1
2Ni1303020002.513.682.2573.4
3Ni1303025002.403.502.2472.1
4Ni1303030002.003.592.3769.8
5Ni1303035001.863.722.2574.9
6Ni1203020002.077.322.63102.8
7Ni1403020001.182.622.2663.1
8Ni1503020000.302.502.0358.4
9Ni1300520001.873.302.0269.1
10Ni1301520002.403.552.2771.3
11Ni1304520001.763.732.1976.6
12Ni1306020001.523.752.4073.5
13 eNi1303020001.012.922.0158.5
14 fNi130302000trace---
15Ni2303020002.053.692.2874.5
16Ni3303020002.623.492.3674.4
17Ni4303020002.843.982.3872.5
18Ni5303020003.153.832.3877.4
a Conditions: 2.0 μmol Ni, 100 mL toluene, 10 atm of C2H4. b 106g of PE (mol of Ni)−1 h−1. c Mw: kg·mol−1, determined by GPC. d Determined by DSC. e 5 atm of C2H4. f 1 atm of C2H4.
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Hosseinzadeh, Z.; Liu, M.; Zhang, Q.; Liang, T.; Solan, G.A.; Ma, Y.; Sun, W.-H. Electronic Tuning of Sterically Encumbered 2-(Arylimino)Pyridine-Nickel Ethylene Polymerization Catalysts by Para-Group Modification. Catalysts 2022, 12, 1520. https://doi.org/10.3390/catal12121520

AMA Style

Hosseinzadeh Z, Liu M, Zhang Q, Liang T, Solan GA, Ma Y, Sun W-H. Electronic Tuning of Sterically Encumbered 2-(Arylimino)Pyridine-Nickel Ethylene Polymerization Catalysts by Para-Group Modification. Catalysts. 2022; 12(12):1520. https://doi.org/10.3390/catal12121520

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Hosseinzadeh, Zahra, Ming Liu, Qiuyue Zhang, Tongling Liang, Gregory A. Solan, Yanping Ma, and Wen-Hua Sun. 2022. "Electronic Tuning of Sterically Encumbered 2-(Arylimino)Pyridine-Nickel Ethylene Polymerization Catalysts by Para-Group Modification" Catalysts 12, no. 12: 1520. https://doi.org/10.3390/catal12121520

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