6-Arylimino-2-(2-(1-phenylethyl)naphthalen-1-yl)-iminopyridylmetal (Fe and Co) Complexes as Highly Active Precatalysts for Ethylene Polymerization: Influence of Metal and/or Substituents on the Active, Thermostable Performance of Their Complexes and Resultant Polyethylenes

A series of 6-arylimino-2-(2-(1-phenylethyl)naphthalen-1-yl)iminopyridines and their iron(II) and cobalt(II) complexes (Fe1–Fe5, Co1–Co5) were synthesized and routinely characterized as were Co3 and Co5 complexes, studied by single crystal X-ray crystallography, which individually displayed a distorted square pyramidal or trigonal bipyramid around a cobalt center. Upon treatment with either methyluminoxane (MAO) or modified methyluminoxane (MMAO), all complexes displayed high activities regarding ethylene polymerization even at an elevated temperature, enhancing the thermostability of the active species. In general, iron precatalysts showed higher activities than their cobalt analogs; for example, 10.9 × 106 g(PE) mol−1 (Co) h−1 by Co4 and 17.0 × 106 g(PE) mol−1 (Fe) h−1 by Fe4. Bulkier substituents are favored for increasing the molecular weights of the resultant polyethylenes, such as 25.6 kg mol−1 obtained by Co3 and 297 kg mol−1 obtained by Fe3. A narrow polydispersity of polyethylenes was observed by iron precatalysts activated by MMAO, indicating a single-site active species formed.


X-Ray Crystallographic Studies
Single crystals of Co3 and Co5 (Figure 1) of suitable quality for the X-ray determinations were grown by the slow diffusion of diethyl ether into a solution of the corresponding complex in dichloromethane at room temperature. Co3 and Co5 comprise a single cobalt center surrounded by three nitrogen atoms of a tridentate ligand (N1, N2, and N3) and two chlorides (Cl1 and Cl2) to form a pentacoordinate geometry. Three nitrogen atoms and a Cl2 atom of Co3 formed a basal plane, and the cobalt atom lay at a distance of 0.586 Å above this basal plane ( Figure 1a). The dihedral angle of the naphthyl plane and the basal plane is 74.71°, and 80.75° for the angle between the benzene plane and the basal plane. Co5 displayed C2 molecular symmetry (Figure 1b), and two naphthyl planes were close vertical to the pyridyl frame plane with dihedral angles are 78.50° and 85.39°, respectively. Two benzene substituents planes on naphthyl planes. Similar to many bis(imino)pyridine-cobalt complexes, the Co-Npyridyl bond length of two complexes [Co3: 2.050(4) Å; Co5: 2.021(4) Å] is shorter than their exterior Co-Nimino ones [2.170(4)-2.294(4) Å], indicating good donor property of the central pyridine [13,14]. Other selected bond lengths and angles are listed in Table 1 and more detail can be seen in Supplementary Materials.

X-ray Crystallographic Studies
Single crystals of Co3 and Co5 (Figure 1) of suitable quality for the X-ray determinations were grown by the slow diffusion of diethyl ether into a solution of the corresponding complex in dichloromethane at room temperature. Co3 and Co5 comprise a single cobalt center surrounded by three nitrogen atoms of a tridentate ligand (N1, N2, and N3) and two chlorides (Cl1 and Cl2) to form a pentacoordinate geometry. Three nitrogen atoms and a Cl2 atom of Co3 formed a basal plane, and the cobalt atom lay at a distance of 0.586 Å above this basal plane ( Figure 1a). The dihedral angle of the naphthyl plane and the basal plane is 74.71 • , and 80.75 • for the angle between the benzene plane and the basal plane. Co5 displayed C 2 molecular symmetry (Figure 1b), and two naphthyl planes were close vertical to the pyridyl frame plane with dihedral angles are 78.50 • and 85.39 • , respectively. Two benzene substituents planes on naphthyl planes. Similar to many bis(imino)pyridine-cobalt complexes, the Co-N pyridyl bond length of two complexes [Co3: 2.050(4) Å; Co5: 2.021(4) Å] is shorter than their exterior Co-N imino ones [2.170(4)-2.294(4) Å], indicating good donor property of the central pyridine [13,14]. Other selected bond lengths and angles are listed in Table 1

X-Ray Crystallographic Studies
Single crystals of Co3 and Co5 (Figure 1) of suitable quality for the X-ray determinations were grown by the slow diffusion of diethyl ether into a solution of the corresponding complex in dichloromethane at room temperature. Co3 and Co5 comprise a single cobalt center surrounded by three nitrogen atoms of a tridentate ligand (N1, N2, and N3) and two chlorides (Cl1 and Cl2) to form a pentacoordinate geometry. Three nitrogen atoms and a Cl2 atom of Co3 formed a basal plane, and the cobalt atom lay at a distance of 0.586 Å above this basal plane ( Figure 1a). The dihedral angle of the naphthyl plane and the basal plane is 74.71°, and 80.75° for the angle between the benzene plane and the basal plane. Co5 displayed C2 molecular symmetry (Figure 1b), and two naphthyl planes were close vertical to the pyridyl frame plane with dihedral angles are 78.50° and 85.39°, respectively. Two benzene substituents planes on naphthyl planes. Similar to many bis(imino)pyridine-cobalt complexes, the Co-Npyridyl bond length of two complexes [Co3: 2.050(4) Å; Co5: 2.021(4) Å] is shorter than their exterior Co-Nimino ones [2.170(4)-2.294(4) Å], indicating good donor property of the central pyridine [13,14]. Other selected bond lengths and angles are listed in Table 1 and more detail can be seen in Supplementary Materials.

Ethylene Polymerization
Previous achievements have approved the efficiency of both MAO and MMAO cocatalysts in activating the iron or cobalt precatalysts for ethylene polymerization [40][41][42][43]. Therefore, the title metal complexes have been screened for their catalytic performances towards ethylene polymerization with the assistance of either methyluminoxane (MAO) or modified methyluminoxane (MMAO).

Ethylene Polymerization by Co1-Co5
Co2/MAO was first selected as a precatalyst system to optimize ethylene polymerization parameters and the results were collected ( Table 2). The temperature was the first considered parameter. The polymerization reactivity was significantly increased from 1.89 to 8.32 × 10 6 g(PE) mol −1 (Co) h −1 as the temperature elevated from 50 to 70 • C (run 1-3, Table 2), due to the reactant molecule becoming more active at higher temperatures. The polymerization reactivity was then decreased to 4.16 × 10 6 g(PE) mol −1 (Co) h −1 (run 4-5, Table 2) with a temperature that continued to increase to 90 • C at a lower concentration of ethylene in toluene and decomposed cobalt active species at this temperature [44].The molecular weight of the polyethylenes obtained reduced from 26.4 to 14.6 kg mol −1 as the polymerization temperature increased (run 1-5, Table 2), indicating a faster termination of polymer chains at the increased temperature [45,46]. Table 2. Polymerization screening using Co1-Co5 a .

Run
Precat. In comparison to the previous cobalt models described in Scheme 1 (A, 60 • C; B, 40 • C; D, 30 • C), the Co2 shows a higher thermostability with the optimum temperature (70 • C) due to the positive influence of the bulky substituent at N-aryl groups. Moreover, the molecular weights of the resultant polyethylenes are well maintained, with only 7% decreasing within the temperature from 60 to 80 • C.
The parameter of the Al/Co molar ratio was then optimized. The catalytic activity was increased from 1.72 to 8.32 × 10 6 g(PE) mol −1 (Co) h −1 as the Al/Co increased from 2000 to 2500, but slightly decreased to 6.98 × 10 6 g(PE) mol −1 (Co) h −1 as the ratio further increased to Al/Co = 3000, resulting in the optimized molar ratio of Al/Co = 2500 (run 3, 6-9, Table 2). The molecular weight of the obtained polyethylenes decreased from 24.0 kg mol −1 (Al/Co = 2000) to 10.1 kg mol −1 (Al/Co = 3000), and the result was similar to previous reports [47,48].
The polymerization was conducted over different running times at 70 • C (run 3, 10-13, Table 2), in order to evaluate the lifetime of the Co2/MAO. The catalytic reactivity was quite prominent (13.8 × 10 6 g(PE) mol −1 (Co) h −1 ) at 5 min, suggesting the active species were quickly formed [49]. Then the catalytic activity gradually decreased as the polymerization time prolonged, but still reached up to 4.82 × 10 6 g(PE) mol −1 (Co) h −1 at 60 min.
Other precatalysts (Co1, Co3-Co5) were employed to catalyze ethylene polymerization at the optimized polymerization conditions (run 14-17, Table 2). All the complexes showed high activity during ethylene polymerization at 70 • C with an order Co4 > Co1 > Co5 > Co2 > Co3, which was affected by both the electron effect and steric effect [50,51]. Co4 had most electron-donating methyl groups but the lowest steric hindrance around the metal center resulted in the highest activity while Co3 with bulky isopropyl groups displayed the lowest activity [30]. Co5 showed a closed activity with Co4 and Co1 for bulky o-sec-phenyl-substituted aryl groups in ligands and donated more electrons [52][53][54]. The molecular weight of the polyethylenes obtained by cobalt complexes was also affected by their ligand structure. The molecular weight of polyethylenes catalyzed by cobalt complexes decreased in the order Co3 > Co5 > Co2 > Co1 > Co4, which was consistent with the literature, which previously reported that the bulky steric substituent of the N-aryl group usually produces a higher molecular weight of polyethylenes because bulky substituents prevent the deactivation of active species [46,55]. Moreover, the current model of the cobalt precatalysts displays a higher activity than previous models of cobalt complexes in Scheme 1. For example, the activity of Co4/MAO goes up to 10.9 × 10 6 g(PE) mol −1 (Co) h −1 , while the activity of its analog (D) (Scheme 1) is 4.15 × 10 6 g(PE) mol −1 (Co) h −1 at their optimized conditions.
The system of the Co1-Co5/MMAO was employed to further explore the catalytic behavior during the ethylene polymerization process. All of these complexes displayed a high activity at an elevated temperature (70 • C). For the Co1-Co5/MMAO system, the catalytic activity decreased in the order Co4 > Co1 > Co2 > Co5 > Co3, and the molecular weight of the polyethylenes obtained decreased in the order Co3 > Co5 > Co2 > Co1 > Co4. The melting temperature of all the polyethylenes was around 130 • C, suggesting that polyethylenes had a high linear microstructure [56,57]. To confirm the microstructure of the polyethylenes obtained, the 1 H NMR and 13 C NMR of the polyethylenes were carried out and the polyethylenes showed a high linear microstructure with the vinyl-end group ( Figure 2). This vinyl-end group was formed by β-H eliminate during ethylene polymerization.  Table 2).

Ethylene Polymerization by Fe1-Fe5
Subsequently, the ethylene polymerizations were conducted by the iron complexes with MMAO as a cocatalyst. Initially, the Fe2/MMAO system was used to optimize the polymerization conditions (run 1-4, Table 3). The polymerization activity first increased and then decreased by  Table 2).

Ethylene Polymerization by Fe1-Fe5
Subsequently, the ethylene polymerizations were conducted by the iron complexes with MMAO as a cocatalyst. Initially, the Fe2/MMAO system was used to optimize the polymerization conditions (run 1-4, Table 3). The polymerization activity first increased and then decreased by varying the polymerization temperature from 60 to 90 • C, and the highest activity (16.5 × 10 6 g(PE) mol −1 (Fe) h −1 ) was observed at 70 • C. A better thermostability with an optimized temperature of 70 • C would be a favorable advantage for industrial considerations [57]. From the GPC curves (Figure 3a), the molecular weight decreased as the polymerization temperature increased, and all samples displayed a unimodal distribution of molecular weight except for one sample obtained at 60 • C. The molecular distribution index demonstrated that there was a narrow distribution of molecular weights obtained at a higher temperature (PDI = 1.60-2.45, run 2-4, Table 3) but a broad one observed at a lower temperature (PDI = 4.62, run 1, Table 3). Such a phenomenon was common in catalytic systems-i.e., that there are multi-active species at lower temperatures while single-active species at higher temperatures [58,59]-including bulky substituents. Table 3. Polymerization screening using Fe1-Fe5 a .

Run
Precat. The polymerization conditions were further optimized by varying the Al/Fe molar ratio from 2000 to 3000 (run 2, 5-7, Table 3). Similar to the previous cobalt complex behavior, the catalytic activity initially increased from 15.8 to 16.5 × 10 6 g(PE) mol −1 (Fe) h −1 as the Al/Fe molar ratio increased from 2000 to 2500, and then decreased to 13.0 × 10 6 g(PE) mol −1 (Fe) h −1 as the Al/Fe molar ratio further increased to 3000. The molecular weight decreased from 6.64 to 4.31 kg mol −1 , and all the samples displayed a narrow molecular weight distribution (Figure 3b), highlighting the single active site of the Fe2/MMAO system at 70 • C.
The lifetime of Fe2/MMAO was assessed by varying the polymerization time from 5 to 60 min (run 6, 8-11, Table 3). It was worth mentioning that a high activity (up to 56.9 × 10 6 g (PE) mol −1 (Fe) h −1 ) was observed at 5 min, indicating that the active species was formed very quickly [57]. By prolonging the polymerization time from 5 to 30 min, the high activity was significantly decreased to 16.5 × 10 6 g (PE) mol −1 (Fe) h −1 ). Additionally, the activity of Fe2/MMAO declined to 8.81 × 10 6 g (PE) mol −1 (Fe) h −1 , further extending the polymerization time to 60 min. Such a result would be attributed to the higher viscosity of the polymerization solution which restricted active species to coordinate with ethylene. as cocatal.
The polymerization conditions were further optimized by varying the Al/Fe molar ratio from 2000 to 3000 (run 2, 5-7, Table 3). Similar to the previous cobalt complex behavior, the catalytic activity initially increased from 15.8 to 16.5 × 10 6 g(PE) mol −1 (Fe) h −1 as the Al/Fe molar ratio increased from 2000 to 2500, and then decreased to 13.0 × 10 6 g(PE) mol −1 (Fe) h −1 as the Al/Fe molar ratio further increased to 3000. The molecular weight decreased from 6.64 to 4.31 kg mol −1 , and all the samples displayed a narrow molecular weight distribution (Figure 3b), highlighting the single active site of the Fe2/MMAO system at 70 °C.  Table 3).
The lifetime of Fe2/MMAO was assessed by varying the polymerization time from 5 to 60 min (run 6, 8-11, Table 3). It was worth mentioning that a high activity (up to 56.9 × 10 6 g (PE) mol −1 (Fe) h −1 ) was observed at 5 min, indicating that the active species was formed very quickly [57]. By prolonging the polymerization time from 5 to 30 min, the high activity was significantly decreased to 16.5 × 10 6 g (PE) mol −1 (Fe) h −1 ). Additionally, the activity of Fe2/MMAO declined to 8.81 × 10 6 g (PE) mol −1 (Fe) h −1 , further extending the polymerization time to 60 min. Such a result would be attributed to the higher viscosity of the polymerization solution which restricted active species to coordinate with ethylene.

General Considerations
All the experimental manipulations of air-and/or moisture-sensitive compounds were carried out under an atmosphere of nitrogen by the use of standard Schlenk techniques. Freshly distilled toluene was used for the polymerization runs that had previously been dried over sodium for approximately 10 h before distillation under a nitrogen atmosphere. Methylaluminoxane (MAO, 1.46 M in toluene) and modified methylaluminoxane (MMAO, 1.93 M in heptane) were provided by Albemarle Corp (Charlotte, NC, USA). High purity ethylene was provided by the Beijing Yanshan Petrochemical Company and used as received. Other reagents were purchased from Alderich (Beijing, China), Acros (Beijing, China), or local suppliers. The 1 H and 13 C NMR spectroscopic measurements for the organic compounds were performed on Bruker DMX 400 MHz instruments (Beijing, China) at room temperature using tetramethylsilane (TMS) as an internal standard. All the chemical shifts and coupling constants are given in ppm and Hz, respectively. Elemental analyses (C, H, and N) were performed on a Flash EA 1112 microanalyzer (Beijing, China). The FT IR spectra were recorded using a PerkinElmer System 2000 FT IR spectrometer (Beijing, China). The molecular weights and molecular weight distributions (M w /M n ) of the polyethylenes were measured using a PL-GPC220 instrument (Beijing, China) at 150 • C with 1,2,4-trichlorobenzene as the solvent. Data collection and processing were performed using Cirrus GPC Software (Agilent PL-Cirrus Software, Beijing, China) and Multi-Detector Software (Agilent PL-Cirrus Software, Beijing, China). The calibrants employed for construction of the conventional calibration (Polystyrene Calibration KitS-M-10) were provided by PL Company (Beijing, China). The true average molecular weights of the polyethylenes were attained by inputting the Mark-Houwink constants of polyethylenes; K (0.727) and α (40.6) were provided by PL Company (Beijing, China). The sample was dissolved at a concentration of 1.0 mg mL −1 . The DSC traces and melting points of the polyethylenes were obtained from the second scanning run on a PerkinElmer TA-Q2000 DSC analyzer (Beijing, China) under a nitrogen atmosphere. During the procedure, a sample of about 4.0-6.0 mg was heated to 160 • C at a heating rate of 20 • C min −1 , followed by 5 min at 150 • C to remove the thermal history and then cooled at a rate of 20 • C min −1 to −20 • C. The compound 2-arylimino-6-acetylpyridines (S1-S4) was prepared according to our previous reports [36][37][38]. In this reaction, S1 (3.4 mmol) and 2-phenethyl-1-naphthylamine (2.9 mmol) were added into a flask with 20 mL of toluene. When the temperature of the reactor reached 110 • C, 0.17 g p-TsOH was added into this reactor. After 6 h, the product was purified by column chromatography on aluminum oxide to afford the product as yellow solid in 14% yield. 1 13   Similar to the synthesis of L1, L3 was obtained as a yellow solid in a 15% yield. 1 36, 40.45, 39.09, 28.40, 28.38, 28.34, 23.29, 17.16.  31, 169.62, 146.57, 145.73, 138.74, 137.38, 137.02, 133.11, 132.84, 128.73, 128.47, 128.17, 127.94, 127.71, 127.52, 126.38, 125.80, 125.49, 125.23, 124.94, 124.68, 124.09, 123.39, 122.83, 122.69, 122.55, 120.43, 118.45, 40.61, 40.31, 25.65, 21.78, 16.

X-ray Crystallographic Studies
Single crystals of Co3 and Co5 suitable for the X-ray diffraction analysis were obtained by layering a dichloromethane solution of the corresponding complex with ethyl ether at room temperature under a nitrogen atmosphere. With graphite monochromated Mo-Kα radiation (λ = 0.71073 Å) at 170.00(10)K or CuKα (λ = 1.54184 Å) at 169.99 (14)K, the cell parameters were obtained by the global refinement of the positions of all collected reflections. The intensities were corrected for Lorentz and polarization effects and empirical absorption. The structures were solved by direct methods and refined by full-matrix least-squares on F 2 . All hydrogen atoms were placed in calculated positions. Structure solution and structure refinement were performed using the SHELXT-97 package [61,62]. The free solvent molecules present within the crystal structures were removed by using the SQUEEZE option of the crystallographic program PLATON [61,62]. Detail of the X-ray structure determinations and refinements are provided in Table 4. X-ray crystallographic data in Calibration Index File (CIF) for the Cambridge Crystallographic Data Centre (CCDC) 2024895 (Co2) and 2024896 (Co5) are available free of charge from the Cambridge Crystallographic Data Centre.

General Procedure for Ethylene Polymerization under 10 Atm Pressure
The polymerization at an ethylene pressure of 10 atm was carried out in a 250 mL stainless steel autoclave (Dalian Sanling Electronic Manufacture, Dalian, China) equipped with an ethylene pressure control system, a mechanical stirrer, and a temperature controller. The autoclave was evacuated and backfilled with ethylene three times. When the required temperature was reached, the precatalyst (2.0 µmol) was dissolved in toluene (25 mL) in a Schlenk tube and injected into the autoclave containing ethylene (1 atm) followed by the addition of more toluene (25 mL). The required amount of cocatalyst (MAO and MMAO) and additional toluene was 100 mL. The autoclave was immediately pressurized with an ethylene pressure of 10 atm and the stirring commenced. After the required reaction time, the reactor was cooled with a water bath and the ethylene pressure vented. Following quenching of the reaction with 10% hydrochloric acid in ethanol, the polymer was collected and washed with ethanol and dried with a vacuum oven at 60 • C and weighed.

Conclusions
A series of iron and cobalt complexes supporting by 6-arylimino-2-(2-(1-phenylethyl) naphthalen-1-yl)iminopyridine ligands (L1-L5) were synthesized and characterized. On the activation with either MMAO or MAO, cobalt precatalysts showed high activities up to 10.9 × 10 6 g(PE) mol −1 (Co) h −1 at an elevated temperature. Their activities are in the order as Co4 > Co1 > Co5 > Co2 > Co3, meanwhile the molecular weights of the resultant polyethylenes decreased in the order Co3 > Co5 > Co2 > Co1 > Co4, which was interpreted as the electron-donating ligands enhancing the catalytic activity and the steric hindrance decreasing the catalytic activity, producing polyethylene with a higher molecular weight. In comparison to the cobalt precatalysts, their iron analogs Fe1-Fe5/MMAO displayed higher activities up to 17.0 × 10 6 g(PE) mol −1 (Fe) h −1 when polymerizing ethylene and produced highly linear polyethylenes with lower molecular weights in the range of kg mol −1 and with a narrow polydispersity. In the case of using MAO as the cocatalyst, the polyethylenes obtained by Fe1-Fe5/MAO showed higher molecular weights up to 291 kg mol −1 with boarder polydispersity, indicating the multispecies of active sites. All precatalysts achieved a good thermostability and were positively influenced by the new modifications of the tridentate ligands.