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. 2023 Sep 12;8(38):34549–34556. doi: 10.1021/acsomega.3c03356

Chromium Ethylene Tri-/Tetramerization Catalysts Supported by Iminophosphine Ligands

Xing Zhao 1, Jihe Wang 1, Dongchang Liu 1, Weihuan Kong 1, Jun Zhang 1,*
PMCID: PMC10536060  PMID: 37780000

Abstract

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A new class of highly active ethylene tri-/tetramerization chromium catalysts supported by iminophosphine ligands has been studied. The impact of electronic and steric changes of these ligands on selectivity and activity has been investigated by varying P- and/or N-substituents. Upon activation with MMAO, the ligand bearing a P-cyclohexyl group displayed a high activity of 307 kg/(g Cr/h) with a high trimerization selectivity of 92.6%. Decreasing the steric hindrance of N-aryl group led to a decrease in 1-hexene selectivity (74.5%), producing more 1-octene (10.3%). X-ray diffraction analysis verifies that the ligands coordinate with the chromium center in a κ2-P,N mode.

1. Introduction

The past two decades have seen an increasing demand for linear α-olefins (LAOs) mainly due to the extensive use of 1-hexene and 1-octene as comonomers with ethylene for the production of linear low-density polyethylene (LLDPE).1−27 Generally, LAOs are produced with a broad range of olefins from C4 to C20 based on the Schulz–Flory distributions manufactured by Shell, Ineos, SABIC, and Chevron–Phillips.12 In order to meet market demand growth by avoiding the production of less valuable or lower-volume LAOs, selective ethylene oligomerization catalysts are highly desirable and have been developed for use on an industrial scale.28−30 Considerable success has been achieved by employing chromium-based catalysts in developing highly active ethylene trimerization systems for manufacturing 1-hexene,31−54 including the Chevron–Phillips’ pyrrole,31,32 the BP diphosphinoamine (PNP),33 and the Sasol mixed heteroatomic systems.34

The first efficient ligand for promoting ethylene tetramerization is PNP diphosphinoamine reported by Sasol researchers,55 which was initially reported by BP researchers in the Cr-catalyzed selective ethylene trimerization.33 Further extensive ligand modification of diphosphines led to the development of a few highly active catalytic systems with promising tri-/tetramerization selectivity. However, selective ethylene tri-/tetramerization catalysts using bidentate ligand frameworks other than diphosphine-based systems are still very scarce.1−4,56,57 Replacing a P-donor of diphosphine ligands with an N-donor is expected to offer greater flexibility for steric and electronic structure tuning. In recent years, several P,N bidentate ligands with different scaffolds have been applied in Cr-catalyzed ethylene tri/tetramerization.58−63 Those P,N-ligands with a 1- or 2-atom spacer, such as PCN (B)62 and PNCN (A)58,63 ligands showed very high activities and are among the most active ethylene tri/tetramerization catalyst systems (Figure 1). They are much more active than other P,N-ligands where P- and N-donor atoms are linked by more than two atoms.59−61 Both A and B achieve a total selectivity (≥95%) toward valuable 1-hexene and 1-octene. Depending on the choice of ligand substituents, the selectivities can be switched from trimerization to mixed trimerization/tetramerization. Recognizing the excellent catalytic performance of A- and B-based catalyst systems, we focus our study on iminophosphine ligands PCCN ligands (C). C has been employed in Ni-catalyzed both ethylene oligomerization64 and ethylene polymerization,65 but showed poor catalytic performance in both cases. Herein, we present the application of the PCCN ligands in Cr-catalyzed ethylene tri-/tetramerization. The ligand bearing a P-cyclohexyl group exhibited high activities toward trimerization, achieving a high 1-hexene selectivity of 92.6%. Decreasing the steric hindrance of the N-aryl group of the ligands led to a decrease in 1-hexene selectivity (74.5%) and the production of more 1-octene (10.3%).

Figure 1.

Figure 1

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Selected P,N-ligands (A, B) and targeted iminophosphine ligands (C) for Cr-catalyzed selective ethylene tri-/tetramerization.

2. Results and Discussion

Known P-aryl-substituted PCCN ligand L1 was prepared from commercially available aniline and acetophenone by a two-step synthesis according to a literature method (Scheme 1).66 Very recently, we reported a kind of PCCP diphoshpine ligands in Cr-catalyzed ethylene tri-/tetramerization.19 The PCCP ligands bearing a P-alkyl group feature more electron density to the Cr center than their diphenylphosphine counterparts, thus showing better catalytic performance. It is a little surprising that the PCCN ligands of type C having a P-alkyl group are very rare.65 Using the above-mentioned method,66 five unknown P-alkyl-substituted PCCN ligands L2–L6 were synthesized successfully by the treatment of the corresponding imines 2a–6a with tert-butyl lithium, followed by the coupling with ClPR’2. Based on the NMR spectra of L2–L6, these ligands exist as mixtures of cis/trans isomers, and no detectable amount of enamine tautomer was observed, which is consistent with other PCCN ligands.66

Scheme 1. Synthesis of PCCN Ligands L1–L6.

Scheme 1

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Treatment of L1–L6 with CrCl3(THF)3 yielded the corresponding chromium complexes 1–6 as blue or green solids (Scheme 2). Those complexes are moisture-sensitive and show moderate solubility at room temperature in acetonitrile and THF and poor solubility in toluene.

Scheme 2. Synthesis of Chromium Complexes 1–6.

Scheme 2

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To study the effect of N- and P-substituents of the ligands on catalyst performance, Cr complexes 1–6 were then taken for catalytic testing. Upon activation with 800 molar excess MMAO in the presence of 40 bar ethylene at 60 °C, the precatalysts 1–6 produced active, selective 1-hexene/1-octene catalysts (Table 1). The ligand structure significantly influences the 1-hexene/1-octene ratio as well as catalytic activity. Although achieving high 1-octene selectivity (33.5%), precatalyst 1 supported by L1 with a P-phenyl group led to low activity and produced a large amount of polyethylene (Table 1, entry 1). Replacing the P-phenyl group (in 1) with a bulky P-alkyl group, i.e., cyclohexyl, (in 3–6) significantly increased the activity and reduced the polymer formation (Table 1, entries 3∼6). Compared with L4, L2 with a less bulky P-isopropyl group led to poor activity and similar selectivity and produced more polymer (Table 1, entries 2 and 4). For precatalysts 3–5, 1-hexene/1-octene ratio is strongly dependent on the steric bulk of N-substituents. Smaller N-aryl groups (−NAr) produced more 1-octene: 3 (Ar = o-tolyl, 10.3 wt %); 4 (Ar = mesityl, 2.1 wt %); 5 (2-ethyl-6-methylphenyl, 1.3 wt %) (Table 1, entries 3–5). Precatalyst 5 exhibited the highest catalytic activity (307 kg/g Cr/h) and the highest 1-hexene selectivity (92.6%) (Table 1, entry 5). However, further increasing the steric bulk of the N-aryl group from 2-ethyl-6-methylphenyl in 5 to N-2,6-diethylphenyl in 6 resulted in worse catalytic performance, giving a decreased activity and producing more polyethylene (Table 1, entries 5 and 6). It is probably because the increased steric bulk in ligand affects the coordination of ethylene to the active catalyst center.

Table 1. Comparison of the Precatalysts 1–6 for Ethylene Oligomerizationa.

    oligomer distribution (wt %)
  
entry (precatalyst) activity (kg/g Cr·h) 1-C6 (wt %)b 1-C6 in C6 (%) cy-C6 (wt %)b 1-C8 (wt %)b 1-C8 in C8 (%)b C10+ (wt %)b PE (wt %)c
1(1) 15 17.2 26.4 47.9 33.5 95.9 1.4 29.7
2(2) 48 89.7 97.6 2.2 3.4 88.0 4.7 8.5
3(3) 243 74.5 85.4 12.7 10.3 95.8 2.5 1.3
4(4) 282 90.4 95.1 4.6 2.1 96.0 2.9 1.2
5(5) 307 92.6 96.1 3.8 1.3 93.6 2.3 1.1
6(6) 181 90.6 93.3 6.6 1.6 91.7 1.2 3.6
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a

General conditions: 120 mL reactor, 2 μmol of precatalyst, 800 equiv. of MMAO-3A, 40 bar of ethylene, 30 mL of methylcyclohexane, 60 °C, and 30 min.

b

wt % of liquid products (oligomers).

c

wt % of total product (oligomers + polymer).

Aimed at establishing the coordination mode of the PCCN ligands, we tried to obtain the single crystals of complexes 1–6 suitable for X-ray diffraction analysis, but all attempts failed, resulting in amorphous materials. Therefore, we prepared the chromium carbonyl complexes 7 and 8, ligated by L3 and L4, respectively, via treatment of Cr(CO)6 with the corresponding PCCN ligands in toluene at reflux (Scheme 3). The structures of these two complexes were determined by single-crystal X-ray diffraction. Both 7 (Figure 2) and 8 (Figure 3) display a distorted-octahedral geometry with an expected Cr–P–C–C–N ring. Compared with a related Cr complex 9 with the PCN ligand,627 and 8 have similar Cr–P and Cr–N bond lengths. As expected, expanding the size of the chelate ring leads to larger bite angles in 7 (78.72°) and 8 (79.14°), which are larger than that in 9 (65.64°) (Figure 4). The observation offers an explanation for the fact that 9 achieved a high 1-octene selectivity of up to 61.2%.15,62

Scheme 3. Synthesis of Complexes 7 and 8.

Scheme 3

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Figure 2.

Figure 2

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Molecular structure of 7 with 20% probability. H atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Cr(1)-P(1) 2.3773(8); Cr(1)-N(1) 2.159(2); Cr(1)-C(28) 1.841(3); Cr(1)-C(29) 1.888(3); Cr(1)-C(30) 1.824(3); Cr(1)-C(31) 1.899(3); P(1)-C(9) 1.842(3); C(8)-C(9) 1.503(4); C(8)-N(1) 1.288(4); N(1)-Cr(1)-P(1) 78.71(6); C(9)-P(1)-Cr(1) 99.85(10); C(8)-C(9)-P(1) 110.5(2); N(1)-C(8)-C(9) 118.7(5).

Figure 3.

Figure 3

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Molecular structure of 8 with 20% probability. H atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Cr(1)-P(1) 2.3758(15); Cr(1)-N(1) 2.187(5); Cr(1)-C(30) 1.900(7); Cr(1)-C(31) 1.852(6); Cr(1)-C(32) 1.879(6); Cr(1)-C(33) 1.828(6); P(1)-C(1) 1.844(6); C(1)-C(2) 1.501(7); C(2)-N(1) 1.285(7); N(1)-Cr(1)-P(1) 79.14(12); C(1)-P(1)-Cr(1) 100.83(19); C(2)-C(1)-P(1) 112.0(4); N(1)-C(2)-C(1) 119.3(5).

Figure 4.

Figure 4

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Comparison of the molecular structures between complexes 7–9.

Given that among the precatalysts 2–6, 3 gave the highest 1-octene selectivity and 4 yielded promising activity, the two precatalysts were chosen for further reaction condition optimization, focusing on the influences of n(Al)/n(Cr) ratio, temperature, and ethylene pressure.

Precatalyst 3 was first tested under various reaction conditions, and the results are shown in Table 2. At 60 °C, increasing the Al/Cr molar ratio from 600 to 1000 slightly enhanced the activity (Table 2, entries 1–3). However, under the n(Al)/n(Cr) ratio of 1000, a decrease in 1-octene selectivity to 7.4% was observed (Table 2, entry 3). Lowering the temperature to 40 °C resulted in an increased 1-octene selectivity and a decreased 1-hexene selectivity (Table 2, entry 4). The result is as expected; elevating temperature would increase 1-hexene selectivity at the expense of 1-octene selectivity due to lower ethylene concentrations and reduced stability of metallacyclononane intermediate at higher temperatures.67 Elevating reaction temperature from 40 °C to 60 °C gave an increased activity, but at 80 °C, a decrease in activity was observed, probably due to poor thermal stability of the precatalyst 3 at higher temperature (Table 2, entries 1, 4, and 5). Lowering the ethylene pressure decreases the ethylene concentration in the solvent, reduces the chain propagation rate, and thus causes poor catalytic activity as well as lower 1-octene selectivity (Table 2, entries 1, 6, and 7). Moreover, either lowering pressure or reaction temperature led to an increase in the amount of cyclic C6 byproducts (Table 2, entries 1, 4, 6, and 7).

Table 2. Ethylene Oligomerization by Precatalyst 3 and 4 under Various Conditionsa.

        oligomer distribution (wt %)
  
entry (precatalyst) T (°C) Al/Cr activity (kg/g Cr·h) 1-C6 (wt %)b 1-C6 in C6 (%) cy-C6 (wt %)b 1-C8 (wt %)b 1-C8 in C8 (%)b C10+ (wt %)b PE (wt %)c
1(3) 60 800 243 74.5 85.4 12.7 10.3 95.8 2.5 1.3
2(3) 60 1000 287 79.3 87.7 11.1 7.4 98.1 2.2 1.4
3(3) 60 600 215 74.3 85.5 12.7 10.9 93.9 2.1 1.1
4(3) 40 800 167 59.1 69.7 25.3 14.6 93.4 1.0 1.5
5(3) 80 800 196 80.7 88.7 10.2 6.1 97.0 3.0 2.7
6(3)d 60 800 139 72.8 81.2 16.9 8.1 94.3 2.2 1.5
7(3)e 60 800 84 72.6 77.7 20.6 3.6 87.0 3.2 2.7
8(4) 60 800 282 90.4 95.1 4.6 2.1 96.0 2.9 1.2
9(4) 60 1000 325 90.7 93.2 6.6 1.5 92.6 1.1 0.8
10(4) 60 600 256 92.5 94.3 5.6 1.4 94.1 0.6 1.3
11(4) 40 800 169 88.1 95.5 8.1 2.3 93.5 1.5 4.1
12(4) 80 800 272 92.3 95.7 4.1 1.0 92.2 2.6 1.8
13(4)d 60 800 149 80.2 84.1 15.2 1.2 80.6 3.4 0.7
14(4)e 60 800 95 71.5 74.7 24.0 0.5 55.7 4.0 2.1
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a

General conditions: 120 mL reactor, 2 μmol of precatalyst, 40 bar of ethylene, 30 mL of methylcyclohexane, and 30 min.

b

wt % of liquid products (oligomers).

c

wt % of total product (oligomers + polymer).

d

20 bar of ethylene.

e

10 bar of ethylene.

Precatalyst 4 was also tested under the above reaction conditions. We find similar activity and selectivity trends as observed for precatalyst 3. The highest activity of 325 kg/(g Cr/h) was obtained using 1000 equivalents of MMAO-3A at 60 °C (Table 2, entries 8–10). Lowering the reaction temperature to 40 °C slightly increased the 1-octene selectivity, and a significant decrease in activity was observed (Table 2, entries 8 and 11). Lowering either the pressure or reaction temperature caused an increase in cyclic C6 byproducts (Table 2, entries 8, 11, 13, and 14).

3. Conclusions

In summary, we have demonstrated that PCCN/Cr(III) complexes, upon activation with MMAO, are highly active catalysts for selective ethylene tri/tetramerization. By modifying the amino and phosphine substituents, we have investigated the influence of ligand variations on activity and selectivity. The ligand L3 bearing a P-cyclohexyl group displayed a high activity with a high trimerization selectivity of 92.6%. Decreasing the steric hindrance of the N-aryl group led to a decrease in 1-hexene selectivity (74.5%) and produced more 1-octene (10.3%). Ongoing experiments aim at the development of other didentate phosphine ligand systems to promote ethylene tetramerization with high selectivity and activity.

4. Experimental Section

4.1. General Information

Unless otherwise stated, all reactions and manipulations were performed under a dry atmosphere of nitrogen using standard Schenk-line techniques. All solvents were purified by distillation prior to use applying the standard procedures. All commercially available reagents were used as received without further purification. MMAO-3A (modified methylaluminoxane) (7 wt % in heptane solution) was purchased from Akzo-Nobel. NMR spectra were recorded by using a Bruker 400 MHz spectrometer. Chemical shifts are reported in ppm from tetramethylsilane with the solvent resonance as the internal standard (1H NMR CDCl3: 7.26 ppm; 13C NMR CDCl3: 77.16 ppm). Elemental analyses were performed by an in-house microanalytical laboratory. Quantitative gas chromatographic analysis of the products of ethylene oligomerization was carried out on an Agilent 6890 series GC instrument with a J&W DB-1HT column working at 36 °C for 10 min and then heating at 10 °C min–1 until 250 °C. n-Nonane was used as an internal standard. Known compounds 1a,68 3a,69 and ligand L1(68) were prepared according to the literature methods.

4.1.1. N-(2,4,6-Trimethyl)phenyl-1-phenylethylideneamine (2a)

2a was synthesized following a standard procedure by the condensation of acetophenone (5.4 g, 44.4 mmol) and 2,4,6-trimethylaniline (5.0 g, 37 mmol) in the presence of a small amount of p-toluenesulfonic acid (637.1 mg, 3.7mmol) in boiling toluene (70 mL) in a Dean–Stark apparatus with azeotropic removal of water. After being refluxed for 10 h, toluene was removed under reduced pressure and the residue was eluted with petroleum ether on an alumina column. The eluent was concentrated to give 2a (6.5 g, 74%) as a yellow oil. 1H NMR (400 MHz, CDCl3) δ = 8.06–7.99 (m, 2H), 7.51–7.44 (m, 3H), 6.88 (s, 2H), 2.29 (s, 3H), 2.07 (s, 3H), 2.00 (s, 6H). 13C NMR (101 MHz, CDCl3) δ = 165.54, 146.67, 139.40, 131.99, 130.58, 130.52, 130.44, 128.86, 128.63, 128.55, 128.42 127.38, 127.07, 125.67, 21.30, 21.06, 20.81, 20.56, 18.19, 17.99, 17.63, 17.40.

4.1.2. 2-(Diisopropylphosphaneyl)-N-mesityl-1-phenylethan-1-imine (L2)

Caution: tert-butyllithium is extremely pyrophoric and care must be taken to avoid exposure to air. To a solution of 2a (310 mg, 1.3 mmol) in tetrahydrofuran (15 mL) was added 1.6 M tBuLi (1.1 mL, 1.7 mmol) dropwise over 10 min at −78 °C. The stirring was continued for 30 minutes and then the cooling bath was removed, allowing the solution to warm to room temperature and stirred for another 1 h. Then, chlorodiisopropylphosphine (0.21 mL, 1.3 mmol) in tetrahydrofuran (5 mL) was added dropwise to the solution. After stirring for 5 h, the solvent was removed under vacuum and the residue was then extracted with n-hexane (15 mL) and the extract was filtered through Celite. The eluent was concentrated to afford L2 (390.3 mg, 85%) as a yellow oil that can be stored at 5 °C under an Ar atmosphere. 1H NMR (600 MHz, CDCl3) δ = 7.89 (d, J = 2.4 Hz, 2H), 7.44 (s, 3H), 6.86 (s, 2H), 2.68 (s, 2H), 2.27 (s, 3H), 2.13 (s, 6H), 1.55–1.45 (m, 2H), 0.94–0.74 (m, 12H). 13C NMR (101 MHz, CDCl3) δ = 168.62, 168.52, 145.68, 140.32, 132.21, 130.01, 128.80, 128.77, 128.74, 128.35, 127.84, 126.13, 26.15, 25.87, 23.86, 23.71, 20.86, 20.80, 19.28, 18.91, 18.65, 18.62. 31P NMR (243 MHz, CDCl3) δ = 8.95 (s, 81%), 3.78 (s, 19%). HRMS (ESI): [M + H]+m/z calcd for [C23H33NP]+: 354.2351, found, 354.2345.

4.1.3. 2-(Dicyclohexylphosphaneyl)-1-phenyl-N-(o-tolyl)ethan-1-imine (L3)

L3 was prepared and purified by the same procedure and under experimental conditions as those employed for L2, using 3a (400 mg, 1.9 mmol), tBuLi (2.5 mmol, 1.6 mL), and dicyclohexylchlorophosphine (1.9 mmol, 0.42 mL). L3 was obtained as a yellow oil (662.2 mg, 86%). Due to the instability and the presence of cis/trans isomers, 13C NMR spectra of L3 are highly complex, precluding further assignment. Although the 1H NMR spectrum is also complex and exhibits significant line broadening, resonances corresponding to L3 are able to be extracted as follows: 1H NMR (400 MHz, CDCl3) δ = 7.93 (2H), 7.44 (3H), 7.35–6.60 (4H), 2.91 (2H), 1.82–0.98 (22H). 31P NMR (162 MHz, CDCl3) δ = 2.28 (s, 85%), −5.56 (s, 15%). HRMS (ESI): [M + H]+ m/z calcd for [C27H37NP]+: 406.2664, found, 406.2658.

4.1.4. 2-(Dicyclohexylphosphaneyl)-N-mesityl-1-phenylethan-1-imine (L4)

L4 was prepared and purified by the same procedure and under experimental conditions as those employed for L2, using 2a (310 mg, 1.3 mmol), tBuLi (1.7 mmol, 1.1 mL), and dicyclohexylchlorophosphine (1.3 mmol, 0.29 mL). L4 was obtained as a yellow oil (478.8 mg, 85%). 1H NMR (400 MHz, CDCl3) δ = 7.91–7.80 (m, 2H), 7.45–7.40 (m, 3H), 6.85 (s, 2H), 2.65 (d, J = 1.2 Hz, 2H), 2.27 (s, 3H), 2.12 (s, 6H), 1.67–0.84 (m, 22H). 13C NMR (101 MHz, CDCl3) δ = 168.96, 168.84, 145.86, 140.28, 132.08, 129.90, 128.79, 128.71, 128.25, 127.79, 126.10, 34.02, 33.85, 29.99, 29.86, 29.02, 28.92, 27.31, 27.21, 27.12, 27.03, 26.34, 20.80, 20.73, 18.68, 18.65. 31P NMR (162 MHz, CDCl3) δ = 0.77 (s, 83%), −4.51 (s, 17%). HRMS (ESI): [M + H]+ m/z calcd for [C29H41NP]+: 434.2977, found, 434.2971.

4.1.5. N-(2-Ethyl-6-methy)phenyl-1-phenylethylideneamine (5a)

5a was prepared and purified by the same procedure and under experimental conditions as those employed for 2a, using 6-ethyl-o-toluidine (6 g, 40.2 mmol), acetophenone (5.8 g, 48.2 mmol), and p-toluenesulfonic acid (692.2 mg, 4.0 mmol). 5a was obtained as a yellow oil (7.1 g, 74%). 1H NMR (400 MHz, CDCl3) δ = 8.10–8.03 (m, 2H), 7.57–7.45 (m, 3H), 7.16–7.05 (m, 2H), 6.99 (t, J = 7.2 Hz, 1H), 2.51–2.31 (m, 2H), 2.11 (s, 3H), 2.05 (s, 3H), 1.17 (t, J = 7.6 Hz, 3H). 13C NMR (101 MHz, CDCl3) δ = 165.11, 148.63, 139.20, 131.90, 130.54, 128.50, 128.02, 127.97, 127.90, 127.23, 127.15, 126.15, 126.12, 126.05, 125.56, 123.09, 123.04, 122.98, 24.81, 18.15, 18.08, 17.75, 17.67, 14.03, 13.91.

4.1.6. 2-(Dicyclohexylphosphaneyl)-N-(2-ethyl-6-methylphenyl)-1-phenylethan-1-imine (L5)

L5 was prepared and purified by the same procedure and under experimental conditions as those employed for L2, using 5a (310 mg, 1.3 mmol), tBuLi (1.7 mmol, 1.1 mL), and dicyclohexylchlorophosphine (1.3 mmol, 0.29 mL). L5 was obtained as a pale yellow oil (495.7 mg, 88%). 1H NMR (400 MHz, CDCl3) δ = 7.95–7.86 (m, 2H), 7.49–7.42 (m, 3H), 7.11 (d, J = 4.8 Hz, 1H), 7.07 (d, J = 4.8 Hz, 1H), 6.98 (t, J = 5.2 Hz, 1H), 2.68 (s, 2H), 2.66–2.40 (m, 2H), 2.19 (s, 3H), 1.26 (t, J = 5.2 Hz, 3H), 1.86–0.88 (m, 22H). 13C NMR (101 MHz, CDCl3) δ = 168.58, 168.47, 147.84, 140.15, 131.98, 129.93, 128.25, 128.02, 127.80, 125.94, 125.78, 123.07, 77.48, 77.16, 76.84, 34.04, 33.98, 33.87, 30.08, 29.94, 29.82, 29.10, 28.99, 28.87, 28.77, 27.14, 27.05, 26.30, 24.93, 18.85, 13.47. 31P NMR (162 MHz, CDCl3) δ = 0.30 (s, 83%), −4.34 (s, 17%). HRMS (ESI): [M + H]+ m/z calcd for [C29H41NP]+: 434.2977, found, 434.2971.

4.1.7. N-(2,6-Diethyl)phenyl-1-phenylethylideneamine (6a)

6a was prepared and purified by the same procedure and under experimental conditions as those employed for 2a, using 2,6-diethylaniline (6 g, 40.2 mmol), acetophenone (5.8 g, 48.2 mmol), and p-toluenesulfonic acid (692.2 mg, 4.0 mmol). 6a was obtained as a yellow oil (6.8 g, 67%). 1H NMR (400 MHz, CDCl3) δ = 8.17–8.08 (m, 2H), 7.59–7.50 (m, 3H), 7.18 (d, J = 7.6 Hz, 2H), 7.10 (dd, J = 8.0, 6.8 Hz, 1H), 2.57–2.35 (m, 4H), 2.16 (s, 3H), 1.22 (t, J = 7.6 Hz, 6H). 13C NMR (101 MHz, CDCl3) δ = 164.88, 148.20, 139.20, 131.58, 130.55, 128.54, 127.28, 127.15, 126.09, 126.03, 125.94, 123.27, 123.20, 123.12, 24.89, 24.74, 24.56, 18.04, 17.94, 17.84, 17.74, 14.00, 13.83.

4.1.8. 2-(Dicyclohexylphosphaneyl)-N-(2,6-diethylphenyl)-1-phenylethan-1-imine (L6)

L6 was prepared and purified by the same procedure and under experimental conditions as those employed for L2, using 6a (312 mg, 1.2 mmol), tBuLi (1.6 mmol, 1.0 mL), and dicyclohexylchlorophosphine (1.2 mmol, 0.27 mL). L6 was obtained as a yellow oil (488.5 mg, 91%). 1H NMR (400 MHz, CDCl3) δ = 7.93–7.88 (m, 2H), 7.49–7.41 (m, 3H), 7.12 (d, J = 5.2 Hz, 2H), 7.04 (t, J = 5.2 Hz, 1H), 2.69 (d, J = 1.2 Hz, 2H), 2.65–2.39 (m, 4H), 1.26 (t, J = 5.2 Hz, 6H), 1.93–0.87 (m, 22H). 13C NMR (101 MHz, CDCl3) δ = 168.40, 168.28, 147.36, 140.16, 131.67, 129.89, 128.25, 127.81, 125.62, 123.17, 34.05, 33.88, 30.09, 29.96, 28.99, 28.89, 27.34, 27.24, 27.15, 27.07, 26.31, 24.90, 13.43. 31P NMR (162 MHz, CDCl3) δ = −0.30 (s, 83%), −4.48 (s, 17%). HRMS (ESI): [M + H]+ m/z calcd for [CH43NP]+: 448.3133, found, 448.3128.

4.1.9. [N-(2,6-Dimethylphenyl)-2-(diphenylphosphaneyl)-1-phenylethan-1-imine](THF)CrCl3 (1)

[CrCl3(THF)3] (106 mg, 0.28 mmol) was added to a solution of L1 (138 mg, 0.34 mmol) in dry THF (10 mL), and the resulting mixture was stirred at room temperature for 1 h. The solvent was removed under reduced pressure, and 10 mL of n-hexane was added to complete precipitation. The product was collected by filtration, washed with 15 mL of n-hexane, and dried in vacuo, yielding 1 (153 mg, 86%) as a green powder. Anal. Calcd for C32H34Cl3CrNOP·1.5H2O (%): C, 57.80; H, 5.61; N, 2.11. Found: C, 57.44; H, 5.61; N, 1.73.

4.1.10. [2-(Diisopropylphosphaneyl)-N-mesityl-1-phenylethan-1-imine](THF)CrCl3 (2)

2 was prepared and purified by the same procedure and experimental conditions as those employed for 1, using L2 (120 mg, 0.34 mmol) and [CrCl3(THF)3] (106 mg, 0.28 mmol). Complex 2 was obtained as a blue powder (147 mg, 90%). Anal. Calcd for C27H40Cl3CrNOP·0.5THF (%): C, 56.18; H, 7.15; N, 2.26. Found: C, 56.51; H, 7.49; N, 1.98.

4.1.11. [2-(Dicyclohexylphosphaneyl)-1-phenyl-N-(o-tolyl)ethan-1-imine](THF)CrCl3 (3)

3 was prepared and purified by the same procedure and experimental conditions as those employed for 1, using L4 (105 mg, 0.26 mmol) and [CrCl3(THF)3] (81 mg, 0.22 mmol). Complex 3 was obtained as a blue powder (103 mg, 74%). Anal. Calcd for C31H44Cl3CrNOP (%): C, 59.37; H, 7.40; N, 1.98. Found: C, 59.51; H, 7.29; N, 2.08.

4.1.12. [2-(Dicyclohexylphosphaneyl)-N-mesityl-1-phenylethan-1-imine](THF)CrCl3 (4)

4 was prepared and purified by the same procedure and experimental conditions as those employed for 1, using L3 (100 mg, 0.23 mmol) and [CrCl3(THF)3] (72 mg, 0.19 mmol). Complex 4 was obtained as a blue powder (107 mg, 85%). Anal. Calcd for C33H48Cl3CrNOP·H2O (%): C, 58.11; H, 7.39; N, 2.05. Found: C, 58.01; H, 7.69; N, 1.83.

4.1.13. [2-(Dicyclohexylphosphaneyl)-N-(2-ethyl-6-methylphenyl)-1-phenylethan-1-imine](THF)CrCl3 (5)

5 was prepared and purified by the same procedure and experimental conditions as those employed for 1, using L5 (100 mg, 0.26 mmol) and [CrCl3(THF)3] (81 mg, 0.22 mmol). Complex 5 was obtained as blue powder (121 mg, 83%). Anal. Calcd for C31H44Cl3CrNOP·3H2O (%): C, 55.19; H, 7.58; N, 1.95. Found: C, 55.44; H, 7.29; N, 1.80.

4.1.14. [2-(Dicyclohexylphosphaneyl)-N-(2,6-diethylphenyl)-1-phenylethan-1-imine](THF)CrCl3 (6)

6 was prepared and purified by the same procedure and experimental conditions as those employed for 1, using L6 (100 mg, 0.24 mmol) and [CrCl3(THF)3] (74 mg, 0.20 mmol). Complex 6 was obtained as blue powder (108 mg, 80%). Anal. Calcd for C31H44Cl3CrNOP·THF (%): C, 59.37; H, 7.40; N, 1.98. Found: C, 59.51; H, 7.29; N, 2.08.

4.1.15. [2-(Dicyclohexylphosphaneyl)-1-phenyl-N-(o-tolyl)ethan-1-imine]Cr(CO)4 (7)

To a solution of L3 (230 mg, 0.57 mmol) in dry toluene (10 mL) was added [Cr(CO)6] (137.4 mg, 0.62 mmol), and the resulting mixture was stirred under reflux for 48 h. The solvent was evaporated, and the crude product was purified by chromatography on short silica gel to yield 7 as a yellow solid (123.3 mg, 38%). 1H NMR (400 MHz, CDCl3) δ = 7.23–7.09 (m, 5H), 6.99 (d, J = 7.2 Hz, 2H), 6.95–6.86 (m, 2H), 3.28 (ddd, J = 26.2, 17.3, 7.0 Hz, 2H), 1.89 (s, 3H), 2.26–1.23 (m, 22H). 13C NMR (600 MHz, CDCl3) δ = 230.76, 223.50, 221.35, 221.26, 220.25, 220.17, 178.84, 178.78, 152.06, 136.21, 130.87, 129.50, 127.97, 127.21, 125.76, 125.29, 125.13, 122.30, 41.95, 41.88, 38.20, 38.07, 36.30, 36.22, 29.70, 29.54, 28.39, 28.35, 27.73, 27.66, 27.37, 27.30, 27.11, 27.07, 27.01, 26.71, 26.63, 26.11, 25.81, 17.71. 31P NMR (162 MHz, CDCl3) δ = 77.27 (s). Anal. Calcd for C31H36CrNO4P (%): C, 65.37; H, 6.37; N, 2.46. Found: C, 64.96; H, 6.35; N, 2.28.

4.1.16. [2-(Dicyclohexylphosphaneyl)-N-mesityl-1-phenylethan-1-imine]Cr(CO)4 (8)

To a solution of L4 (200 mg, 0.46 mmol) in dry toluene (10 mL) was added [Cr(CO)6] (111.4 mg, 0.51 mmol), and the resulting mixture was stirred under reflux for 48 h. The solvent was evaporated, and the crude product was purified by chromatography on short silica gel to yield 8 as a yellow solid (118.1 mg, 43%).1H NMR (400 MHz, CDCl3) δ = 7.23–7.11 (m, 3H), 7.04–6.90 (m, 2H), 6.65 (s, 2H), 3.31 (d, J = 6.8 Hz, 2H), 2.17 (s, 6H), 2.14 (s, 3H), 2.09–1.25 (m, 22H). 13C NMR (600 MHz, CDCl3) δ = 230.11, 230.04, 223.09, 221.00, 220.91, 179.14, 179.08, 149.13, 136.79, 136.76, 134.39, 129.71, 129.19, 128.19, 126.66, 126.09, 43.11, 43.04, 38.79, 38.69, 29.47, 29.37, 27.73, 27.65, 27.32, 27.26, 26.19, 20.76, 19.32. 31P NMR (162 MHz, CDCl3) δ = 70.78 (s). Anal. Calcd for C33H40CrNO4P (%): C, 66.32; H, 6.75; N, 2.34. Found: C, 66.03; H, 6.80; N, 2.10.

4.2. Oligomerization of Ethylene

A 120 mL stainless steel reactor was heated (120 °C) under vacuum for 3 h and then cooled to reaction temperature. Precatalysts were weighted into a Schlenk vessel, and methylcyclohexane was added. Then, a co-catalyst was transferred into the Schlenk vessel. The pre-activated catalyst solution was stirred for 1 min and immediately transferred to the reactor. The autoclave was pressurized. After the specified reaction time, the oligomerization was stopped by closing the gas supply, cooling the system (0 °C), and depressurizing. Also, the reactor contents were quenched by the addition of 30 mL of 10% HCl (aq). A small sample of the upper-layer solution was taken, filtered through a layer of Celite, and analyzed by GC using nonane as the internal standard. The individual oligomer products were identified by GC–MS. The solid polymeric products were collected, suspended in 10% HCl (aq) and stirred for 24 h, dried under reduced pressure, and weighed.

Acknowledgments

Financial support from the National Key R&D Program of China (2017YFB0306701), the Fundamental Research Funds for the Central Universities (222201717003), and the National Natural Science Foundation of China (22071055) is greatly acknowledged.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.3c03356.

  • Experimental procedures and spectral data for all new compounds (PDF)

  • Crystallographic data 1 (CIF)

  • Crystallographic data 2 (CIF)

The authors declare no competing financial interest.

Supplementary Material

ao3c03356_si_001.pdf (1.5MB, pdf)
ao3c03356_si_002.cif (788.9KB, cif)
ao3c03356_si_003.cif (823.6KB, cif)

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Supplementary Materials

ao3c03356_si_001.pdf (1.5MB, pdf)
ao3c03356_si_002.cif (788.9KB, cif)
ao3c03356_si_003.cif (823.6KB, cif)

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