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Article

On-Purpose Oligomerization by 2-t-Butyl-4-arylimino-2,3-dihydroacridylnickel(II) Bromides

by
Song Zou
1,
Zheng Wang
1,2,
Yizhou Wang
1,
Yanping Ma
1,
Yang Sun
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
College of Science, Hebei Agricultural University, Baoding 071001, China
*
Author to whom correspondence should be addressed.
Catalysts 2024, 14(6), 342; https://doi.org/10.3390/catal14060342
Submission received: 21 April 2024 / Revised: 22 May 2024 / Accepted: 23 May 2024 / Published: 24 May 2024
(This article belongs to the Section Catalysis in Organic and Polymer Chemistry)
Browse Figures
Versions Notes

Abstract

:
In this study, 2-t-butyl-4-arylimino-2,3-dihydroacridylnickel dibromides were synthesized by nickel-template one-pot condensation, and well characterized along with the single-crystal X-ray diffraction to one representative complex, revealing a distorted tetrahedral geometry around nickel. When activated with modified methylaluminoxane (MMAO), all nickel complexes exhibited high activities (up to 1.91 × 106 g mol−1 (Ni) h−1) toward major trimerization of ethylene. When activated with ethylaluminum dichloride (EtAlCl2), however, the title complexes performed good activities (up to 1.05 × 106 g mol−1 (Ni) h−1) for selective dimerization of ethylene. In comparison to analogous nickel complexes, higher activities were achieved with the substituent of t-butyl group, especially in the rare case of nickel complexes performing trimerization of ethylene.

Graphical Abstract

1. Introduction

In 1995, Brookhart’s group first reported ethylene polymerization using α-diimino-Ni(II) (A, Scheme 1) and Pd(II) complexes [1,2,3]. Subsequently, modifications of α-diimine derivatives for their nickel catalysts were developed [4,5,6,7,8,9,10]. Moreover, other N^N bidentate nickel complexes, including 2-iminopyridines [11,12,13] (B, Scheme 1), 2-imidazol pyridines [14,15], 2-iminoquinolines [16,17,18,19] (C, Scheme 1), and cycloalkyl-fused iminopyridines [20,21,22,23,24,25,26,27,28,29,30,31,32] (D, Scheme 1), were developed in parallel toward ethylene oligomerization and/or polymerization. Regarding the model catalysts [33,34,35], characteristic differences have been observed for their products, indicating major polyethylenes for model A [4,5,6,7,8,9,10] and both polyethylenes and oligomers for models B [11,12,13], C [16,17,18,19], and D [20,21,22,23,24,25,26,27,28,29,30,31,32]; however, only oligomers were indicated for model E [36,37]. Torches are commonly used by refinery companies due to the carbohydrates in refinery gases, in which ethylene accounts for more than one-fifth of the contents. Driven to selectively transfer ethylene into higher-order carbohydrate compounds, it is worthwhile to further investigate model E and its derivative catalysts in order to explore its scope and find the practicing catalyst.
The 4-arylimino-1,2,3-trihydroacridylnickel complexes (E, R = H, Scheme 1), activated by trimethylaluminum, oligomerized ethylene to form oligomers in the range of C4 to C16 [36], being less active than model D [22]. Following that, the 2-propyl-4-arylimino-1,3-dihydroacridylnickel complexes (E, R = n-Pr, Scheme 1) were developed, but they resulted in a negative effect on the oligomerization activity [37]. With the aim of extensively studying the influence of the substituent, expecting better solubility using tert-butyl, here, the 2-t-butyl-4-arylimino-1,3-dihydroacridylnickel bromides (E, R = t-Bu, Scheme 1) are synthesized and investigated toward ethylene oligomerization.

2. Results and Discussion

2.1. Synthesis and Characterization of 2-Tert-butyl-4-arylimino-2,3-dihydroacridylnickel(II) Bromides

The 2-t-butyl-1,3,4-trihydroacridine was synthesized through the coupling reaction of 2-aminobenzaldehyde and 4-tert-butylcyclohexanone using a ruthenium catalyst [38], according to the reported process [39]. Then, 4-benzylidene-2-(tert-butyl)-1,2,3,4-tetrahydroacridine, formed by the condensation reaction of 2-t-butyl-1,3,4-trihydroacridine with benzaldehyde, was oxidized with ozone in a solution of dichloromethane and MeOH at −40 °C to produce 2-t-butyl-2,3-dihydroacridine-4-one in good yield [40,41] (Scheme 2; the synthesis and characterization are available in the Supplementary Materials).
The condensation reaction of 2-t-butyl-2,3-dihydroacridine-4-one and anilines did not result in the formation of the desired products; therefore, the template reaction of 2-t-butyl-2,3-dihydroacridine-4-one and the corresponding anilines with (DME)NiBr2 was conducted in acetic acid to prepare the title nickel complexes, as in previous studies (Scheme 2) [36,37]. All nickel complexes showed their FT-IR spectra in the range of 1616–1621 cm−1, indicating the C=N stretching vibration [36,37] and consistent with the elemental analysis.
Single crystals of NiL1 suitable for X-ray analysis were obtained by the slow diffusion of diethyl ether into the NiL1 dichloromethane solution at ambient temperature. The molecular structure is shown in Figure 1 and the selected bond lengths and angles are listed in Table 1.
As shown in Figure 1, there was a distorted tetrahedral geometry around its nickel(II) center coordinated with the N^N ligand and two bromides, similar to its nickel analogues [36,37]. The bond lengths of Ni-Br were, individually, 2.3396(8) Å and 2.3441(7) Å. The N1-C13-C12-N2 coordination plane and C2-C3-C4-C5 quinoline plane were almost coplanar, with a dihedral angle of 1.68°; meanwhile, the dihedral angle between the C9-C10-C11 plane and the coordination plane was about 35.11°, reflecting the twisted cyclohexyl ring along with its remote, stretched t-butyl group.

2.2. Ethylene Oligomerization

2.2.1. Exploring the Suitable Co-Catalysts

Inspired by nickel pre-catalysts [2,3,17,18,19,36,37], potential co-catalysts include diethylaluminium chloride (Et2AlCl), ethylaluminum sesquichloride (EASC), ethylaluminum dichloride (EtAlCl2), methylaluminoxane (MAO), and modified methylaluminoxane (MMAO). They were compared with isostructural Ni(II) catalysts using model E. Model E (E, R = H, Scheme 1) was activated by trimethylaluminum and oligomerized ethylene to form oligomers in the range of C4 to C16 [36]. Model E (E, R = n-Pr, Scheme 1) was developed but resulted in a negative effect on the oligomerization activity [37]. In this system (runs 1–6, Table 2), the advantage is that all nickel complexes bearing a bulky t-butyl group showed high activities toward ethylene on-purpose oligomerization compared with previous similar structures, providing more interesting results for researchers who work on nickel-catalyzed ethylene oligomerization. More precisely, when activated with EtAlCl2, the dimerization was highly selective for all nickel pre-catalysts; however, major trimerization with high activity was rarely achieved with the co-catalyst MMAO, which is a rare case for nickel complexes controlling ethylene on-purpose oligomerization. In short, tiny variations in the nickel complex backbone structure are meaningful for the fine control of ethylene oligomerization product distribution.

2.2.2. Dimerization by the Catalytic System Ni1–Ni5/EtAlCl2

To optimize the parameters of NiL1/EtAlCl2, fixing the Al/Ni ratio at 300 and 30 min under 10 atm of ethylene, dimerization was carried out at different temperatures, such as 20, 30, 40, and 50 °C (Runs 1–4, Table 3), confirming the highest activity of 10.50 × 105 g mol−1 (Ni) h−1, observing high selectivity of butene along with α-C4 at >99.0% (Run 2, Table 3). Furthermore, examples of GC under the optimized reaction conditions of NiL1/EtAlCl2 are presented in the Supplementary Materials (page 4), showing high selectivity of butene. When the temperature was up to 100 °C, the activity decreased to 0.83 × 105 g mol−1 (Ni) h−1 (Run 5, Table 3). Upon verifying the amounts of co-catalysts (Runs 6–9, Table 3), both fewer and more co-catalysts somewhat deactivated the system, affirming our decision to use an Al/Ni ratio of 300 as the best choice. Regarding the reaction period (Runs 2 and 10–12, Table 3), the shorter the period, the higher the activity observed, which means that the catalytic reaction quickly occurred without an initial period, and the active species were gradually deactivated as its analogues [36,37].
Using the optimal conditions established for NiL1/EtAlCl2 (i.e., Al:Ni molar ratio = 300:1, reaction temperature = 30 °C, and PC2H4 = 10 atm), all other nickel pre-catalysts, Ni2L–NiL5, were investigated (Runs 13–16, Table 3), and they exhibited higher activities, ranging from 12.14 × 105 g mol−1 (Ni) h−1 to 16.92 × 105 g mol−1 (Ni) h−1. Their activities were in the order: NiL5 > NiL4 > NiL2 > NiL3 > NiL1. The para-methyl substituent was helpful in enhancing the catalytic system: NiL4 (R2 = Me) > NiL1 and NiL5 (R2 =Me) > NiL2, due to its better solubility. The ortho-substituents affected the catalytic performance: Ortho-ethyl (NiL2) was higher than its analogues, ortho-methyl (NiL1) and ortho-isopropyl (NiL3), as the electronic influence was reflected in the better activity, with higher electron-withdrawing of ethyl than methyl and isopropyl [31,32]. Isopropyl (NiL3) was still higher than methyl (NiL1) due to the soluble influence, consistent with the observation of para-methyl substituents.

2.2.3. Major Trimerization Using NiL1–NiL5/MMAO

Unlike chromium pre-catalysts, commonly resulting in on-purpose oligomerization [42,43], only a few cases of nickel pre-catalysts achieving major trimerization of ethylene though nickel-promoted dimerization have been reported [25,26,27]. Therefore, it is worthwhile to investigate the MMAO system in detail using the title complexes. To find the optimal condition on the basis of NiL1/MMAO, the Al/Ni molar ratio was verified from 1000 to 5000 at 30 °C (Runs 1–5, Table 4) under 10 atm of ethylene, and its highest activity was observed at 1.907 × 106 g mol−1 (Ni) h−1 with the Al/Ni ratio of 4000 (Run 4, Table 4). Examples of GC under the optimized reaction conditions of NiL1/MMAO are presented in the Supplementary Materials (page 4), showing high selectivity of hexenes over 80%. Subsequently, the reaction temperature was set at 20, 30, 40, and 50 °C (Runs 4 and 6–8, Table 4), finding the best at 30 °C (Run 4, Table 4). At 100 °C, the activity was maintained at 5.87 × 105 g mol−1 (Ni) h−1 (Run 9, Table 4). Again, the activities gradually decreased along with the prolonged reaction time (Runs 4 and 10–12, Table 4), consistent with the EtAlCl2 system and reported analogues. In all cases, the products remained, with fewer butenes and major hexenes over 80%, confirming that a rare case of characteristic trimerization occurred. There is no better explanation, as there is no direct evidence for the intermediates. The co-catalyst MMAO indeed interacted with nickel species, which could react with two ethylene molecules in forming relatively stable Ni-heterocyclopentane as its intermediates. Such intermediates would further react with another ethylene to break its cyclic intermediates and form new unstable intermediates, quickly cleaving for hexenes [43].
Other NiL2–NiL5/MMAO systems were extensively conducted under the optimal conditions (i.e., Al/Ni = 4000, 30 °C, 30 min, and 10 atm C2H4) (Runs 13–16, Table 4), achieving high activities. Their activities were in the order: NiL2 > NiL1 > NiL5 > NiL4 > NiL3 (Figure 2). Strangely, a negative influence of the complexes with an additional para-methyl group was observed [36,37]. The influences of their substituents were generally considered as both electronic and steric, in addition to the solubility of the complexes, but all factors weakly accumulated for the final performance. Importantly, the selectivity of the C6 fraction was significantly affected by the steric influence of the substituents, which are generally considered steric. The NiL3 with substituent iPr achieved selectivity of C6 up to 95.4% (Run 14, Table 4). In general, high selectivity for ethylene trimerization to hexenes was observed by all five nickel complexes activated by MMAO, indicating a rare model of nickel pre-catalysts for ethylene trimerization.

3. Experimental Section

3.1. General Procedures

All air- or moisture-sensitive compounds were obtained under an atmosphere of nitrogen using a nitrogen-filled glovebox or standard Schlenk techniques. The solvent (toluene) was dried over sodium and distilled under nitrogen for 8 h prior to use. Methylaluminoxane (1.42 M solution in toluene), modified methylaluminoxane (2.46 M solution in n-heptane), ethylaluminum sesquichloride (EASC 3.30 M in toluene), ethylaluminum dichloride (EtAlCl2, 1.20 M in hexane), and diethylaluminum chloride (Et2AlCl, 1.17 M in hexane) were purchased from Anhui Botai Electronic Materials Co. (Chuzhou, China). High-purity ethylene was purchased from Beijing Yanshan Petrochemical Corp. (Beijing, China) and used as received. Other reagents were purchased from Aldrich (St. Louis, MO, USA), Acros (Fukuoka, Japan), or local suppliers. FT-IR spectra were recorded on a PerkinElmer System 2000 FT-IR spectrometer (PerkinElmer, Waltham, MA, USA), and elemental analyses were determined on a Flash EA 1112 microanalyzer (EA Consumables, Marlton, NJ, USA). Gas chromatography (GC) analysis was performed with a VARIAN CP-3800 (Varian, Palo Alto, CA, USA) gas chromatograph equipped with a flame ionization detector and a 30 m (0.2 mm internal diameter and 0.25 μm film thickness) DM-1 silica capillary column.

3.2. Synthesis and Characterization of 2-t-Butyl-2,3-dihydroacridine-4-one

Based on previous work [38,39], 2-aminobenzyl alcohol (15.1 g, 123 mmol), 4-tert-butylcyclohexanone (20.0 g, 129 mmol, 1.04 eq.), t-BuOK (13.44 g, 120 mmol), and [fac-PNN]RuH(PPh3)(CO) [38] (44 mg, 0.06 mmol, 0.05 mol%), in a mixture of THF (50 mL) and toluene (250 mL), were stirred and heated to reflux under an atmosphere of nitrogen. After stirring for 24 h at this temperature, the resultant solution was cooled to room temperature, and water (300 mL) was added. The mixture was stirred for 0.5 h at room temperature and then extracted with ethyl acetate (2 × 300 mL). The combined organic layers were washed with water (2 × 100 mL) and dried over anhydrous NaSO4. Evaporation of the solvent yielded a red oil (30.0 g), which was dissolved in dichloromethane (20 mL), and hexane (100 mL) was introduced. After cooling the mixture to 0 °C, the resulting precipitate was filtered, affording 1 as a yellow solid (24.9 g, 85%). Then, 1 was directly used in the next step.
Under nitrogen atmosphere, 2-t-butyl-1,3,4-trihydroacridine (16.0 g, 66 mmol), benzaldehyde (10.6 g, 100 mmol, 1.5 eq.), and acetic anhydride (10.3 g, 100 mmol, 1.5 eq.) were mixed in a bottle and refluxed for 6 h at 160 °C (monitored by TLC). When the reaction was completed, the acetic anhydride was removed by rotary evaporation. Then, distilled water (100 mL) was added for rotary evaporation to remove excess benzaldehyde. More distilled water was added to wash the residue solid and then it was filtered to obtain the yellow crude product. The crude product was recrystallized in methanol and filtered to obtain the pale-yellow product, 4-benzylidene-2-(tert-butyl)-1,2,3,4-tetrahydroacridine (18.8 g, 87% yield). Without further purification, the 4-benzylidene-2-(tert-butyl)-1,2,3,4-tetrahydroacridine (15.0 g, 45 mmol) was dissolved with dichloromethane (400 mL) and methanol (400 mL). After cooling to −40 °C, the O3/O2 gas was added for the ozone cleavage reaction. After 5 h, the solution gradually became clear and the O3/O2 gas was stopped. Then, the reaction solution was gradually raised to room temperature and dimethyl sulfide (6 mL) was added and stirred overnight. The reaction liquid was concentrated under reduced pressure to obtain a pale-yellow oil. Distilled water (60 mL) was added for rotary evaporation to remove benzaldehyde, then it was filtered to obtain a light-yellow solid, and slurried with methanol (100 mL) to yield the white product, 2-t-butyl-2,3-dihydroacridine-4-one (4.7 g, 51% yield). 1H NMR (400 MHz, CDCl3) δ 8.29 (d, J = 8.6 Hz, 1H), 8.08 (s, 1H), 7.75 (dd, J = 8.2, 1.3 Hz, 1H), 7.68 (ddd, J = 8.5, 6.8, 1.4 Hz, 1H), 7.56 (ddd, J = 8.0, 6.8, 1.1 Hz, 1H), 3.18 (dt, J = 15.8, 3.2 Hz, 1H), 3.06 (dt, J = 16.6, 3.0 Hz, 1H), 2.94 (ddd, J = 15.9, 12.4, 1.4 Hz, 1H), 2.51 (dd, J = 16.6, 13.8 Hz, 1H), 2.03 (ddt, J = 13.6, 12.4, 3.4 Hz, 1H), 1.00 (s, 9H); 13C NMR (100 MHz, CDCl3) δ 198.2, 148.2, 147.4, 136.6, 136.6, 135.6, 131.2, 129.7, 129.5, 128.8, 126.7, 44.9, 42.5, 32.5, 31.2, 27.0, 27.0. Anal. Calcd. for C17H19NO (253.15): C, 80.60; H, 7.56; N, 5.53. Found: C, 80.75; H, 7.62; N, 5.38%.

3.3. Synthesis and Characterization of Complexes NiL1–NLi5

[4-(2,6-Dimethylphenylimino)-2-tert-butyl-1,3-dihydroacridine nickel dibromides] NiL1
NiBr2(DME) (0.1 g, 0.4 mmol) and 2-(tert-butyl)-2,3-dihydroacridin-4(1H)-one (0.1 g, 0.4 mmol) were suspended in glacial acetic acid (5 mL). Then, 2,6-dimethylaniline (0.05 g, 0.4 mmol) was added, and the reaction mixture was refluxed under stirring at 130 °C for 6 h. After that, the solution was allowed to cool to room temperature, and a green solid precipitated. The solid was separated by filtration and washed with diethyl ether to remove the remaining acetic acid and aniline. The solid was dried to obtain a green powder (0.16 g, 91.4% yield). FT-IR (cm−1): 3344(w), 2846(w), 2160(m), 1974(m), 1711(w), 1662(s), 1620(m, vC=N), 1570(m, vC=N), 1489(m), 1458(m), 1357(m), 1335(m), 1275(m), 1192(m), 1154(m), 1027(m), 924(m), 853(w), 805(m), 768(vs), 705(vs). Anal. Calcd. for C25H28Br2N2Ni (572.00): C, 52.22; H, 4.91; N, 4.87. Found: C, 52.68; H, 5.38; N, 4.62%.
[4-(2,6-Diethylphenylimino)-2-tert-butyl-1,3-dihydroacridine nickel dibromides] NiL2
Using the same procedure as described above for the synthesis of NiL1, NiL2 was obtained as a green powder (0.21 g, 88.6% yield). FT-IR (cm−1): 3322(m), 2954(m), 2864(m), 1976(w), 1664(s), 1616(s, vC=N), 1585(w, vC=N), 1499(w), 1458(m), 1401(w), 1364(w), 1342(m), 1304(w), 1237(w), 1205(m), 1144(m), 1050(w), 1005(w), 919(w), 826(w), 787(s), 756(vs), 670(m). Anal. Calcd. for C27H32Br2N2Ni (600.03): C, 53.77; H, 5.35; N, 4.65. Found: C, 53.48; H, 5.15; N, 4.51%.
[4-(2,6-Diisopropylphenylimino)-2-tert-butyl-1,3-dihydroacridine nickel dibromides] NiL3
Using the same procedure as described above for the synthesis of NiL1, NiL3 was obtained as a green powder (0.14 g, 55.4% yield). FT-IR (cm−1): 3015(w), 2963(s), 2656(m), 2549(m), 2524(m), 2161(w), 1972(w), 1666(s), 1616(s, vC=N), 1586(m, vC=N), 1549(m), 1500(s), 1460(s), 1390(w), 1366(w), 1344(m), 1289(w), 1238(w), 1207(m), 1152(m), 1035(w), 1009(w), 969(w), 918(w), 850(w), 828(w), 800(m), 756(s), 673(m). Anal. Calcd. for C29H36Br2N2Ni (628.06): C, 55.19; H, 5.75; N, 4.44. Found: C, 55.58; H, 5.89; N, 4.79%.
[4-(2,4,6-Trimethylphenylimino)-2-tert-butyl-1,3-dihydroacridine nickel dibromides] NiL4
Using the same procedure as described above for the synthesis of NiL1, NiL4 was obtained as a brown powder (0.13 g, 57.7% yield). FT-IR (cm−1): 3129(w), 2956(m), 2850(w), 2161(w), 1974(w), 1665(s), 1618(s, vC=N), 1587(w, vC=N), 1566(w), 1502(w), 1460(s), 1398(w), 1344(m), 1325(m), 1310(w), 1288(w), 1238(w), 1206(m), 1152(s), 1052(w), 1008(w), 973(w), 851(w), 828(w), 790(m), 755(s), 718(w), 672(m). Anal. Calcd. for C26H30Br2N2Ni (586.01): C, 53.02; H, 5.13; N, 4.76. Found: C, 53.43; H, 5.35; N, 4.51%.
[4-(2,6-Diethyl-4-methylphenylimino)-2-tert-butyl-1,3-dihydroacridine nickel dibromides] NiL5
Using the same procedure as described above for the synthesis of NiL1, NiL5 was obtained as a brown powder (0.11 g, 44.6% yield). FT-IR (cm−1): 3129(w), 2954(m), 2850(w), 2162(w), 1975(w), 1665(vs), 1621(s, vC=N), 1587(w, vC=N), 1566(w), 1501(w), 1460(s), 1398(w), 1344(m), 1325(m), 1309(w), 1288(w), 1238(w), 1206(s), 1152(s), 1052(w), 1007(w), 971(w), 880(w), 850(w), 828(w), 790(m), 754(s), 672(m). Anal. Calcd. for C28H34Br2N2Ni (614.04): C, 54.50; H, 5.55; N, 4.54. Found: C, 54.31; H, 5.23; N, 4.30%.

3.4. X-ray Diffraction Studies

The single crystal of NiL1 that qualified for XRD studies was obtained by diffusing diethyl ether onto a dichloromethane solution containing the corresponding complex. A suitable crystal of NiL1 was selected and mounted on an XtaLAB Synergy R (Rigaku Corporation, Tokyo, Japan), HyPix diffractometer, incorporating a graphite-mono-chromated Cu-Kα radiation (λ = 1.54184 Å) source and a nitrogen cold stream. The crystal was kept at 169.99(10) K during data collection. Using Olex2-1.3, the structures were solved with the ShelXT [44] structure solution program using Intrinsic Phasing and refined with the ShelXL [45] refinement package using Least Squares minimization. The X-ray structure determination and refinement details are presented in Table S1.

3.5. General Ethylene Oligomerization Procedures

For the general procedure of ethylene oligomerization at 10 atm of ethylene pressure, a 0.25 L stainless-steel autoclave equipped with a mechanical stirrer and a temperature controller was prepared. Before the reaction, the reactor was heated in a vacuum at 110 °C and recharged with ethylene three times. The nickel pre-catalyst was dissolved in 50 mL of toluene using standard Schlenk techniques and injected into the reactor under an ethylene atmosphere. When the temperature was reached, the required amounts of co-catalyst and the residual toluene (the total volume of toluene was 100 mL) were injected into the reactor. Ethylene at the desired pressure was introduced to start the reaction, and ethylene was continuously fed at a constant pressure during the reaction. When the reaction was completed, 1 mL of the reaction solution was quenched by the addition of 1 M hydrogen chloride (HCl). The organic layer was collected to be analyzed by gas chromatography (GC) to determine the distribution of the obtained oligomers.

4. Conclusions

A series of 2-t-butyl-4-arylimino-2,3-dihydroacridylnickel(II) bromides were prepared through a nickel-induced template reaction of 2-t-butyl-2,3-dihydroacridine-4-one and a corresponding aniline with (DME)NiBr2. Besides routine and full analysis, the molecular structure of NiL1 was illustrated as a distorted tetrahedral geometry around its nickel(II) center, coordinated with the N^N ligand and two bromides. More importantly, all nickel complexes showed high activities toward ethylene on-purpose oligomerization. When activated with EtAlCl2, the dimerization was highly selective for all nickel pre-catalysts; however, major trimerization with high activity was rarely achieved with the co-catalyst MMAO. The roles of ligands’ substituents were illustrated differently: both solubility and electronic influence occurred in the system with EtAlCl2, but likely mainly electronic influence for the system with MMAO. Though dimerization by nickel commonly occurs, trimerization by nickel is rare but promising when using the title complexes. On that basis, the on-purpose oligomerization by nickel pre-catalysts would be an interesting topic for chemists, both in extending catalytic applications and finding alternative catalysts to replace chromium catalysts. As a promising conceptual procedure, the ethylene within refinery gases could be transformed into butenes and hexenes, such as liquefied petroleum gas (LPG).

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal14060342/s1, Figure S1: 1H NMR for 2-t-butly-2,3-dihydroacridine-4-one [38,39]. Figure S2: 13C NMR for 2-t-butyl-2,3-dihydroacridine-4-one. Figure S3. GC of NiL1 (run 2, Table 3). Figure S4. GC of NiL1 (run 4, Table 4). Table S1: Crystallographic data and refinement details for complex NiL1.

Author Contributions

S.Z., synthesis and oligomerization, validation, data curation, and writing—original draft preparation; Z.W., synthesis, methodology, and investigation; Y.W., writing—review and editing; Y.M., software; Y.S., single-crystal XRD determination; W.-H.S., conceptualization, methodology, supervision, funding acquisition, and project administration. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Beijing National Laboratory for Molecular Sciences [2022BMS20084].

Data Availability Statement

The data presented in this work are available in this article.

Conflicts of Interest

The authors declare no conflicts of interest.

References

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Catalysts 14 00342 sch001
Scheme 1. Representative N^N bidentate nickel complexes A–E toward ethylene oligomerization and/or polymerization.
Scheme 1. Representative N^N bidentate nickel complexes A–E toward ethylene oligomerization and/or polymerization.
Catalysts 14 00342 sch001
Catalysts 14 00342 sch002
Scheme 2. Synthetic route of the 2-t-butyl-2,3-dihydroacridine-4-one, and the one-pot method for the 2-t-butyl-4-arylimino-1,3-dihydroacridylnickel bromides complexes.
Scheme 2. Synthetic route of the 2-t-butyl-2,3-dihydroacridine-4-one, and the one-pot method for the 2-t-butyl-4-arylimino-1,3-dihydroacridylnickel bromides complexes.
Catalysts 14 00342 sch002
Catalysts 14 00342 g001
Figure 1. ORTEP diagram of NiL1, with the thermal ellipsoids shown at the 30% probability level. All hydrogen atoms have been omitted for clarity.
Figure 1. ORTEP diagram of NiL1, with the thermal ellipsoids shown at the 30% probability level. All hydrogen atoms have been omitted for clarity.
Catalysts 14 00342 g001
Catalysts 14 00342 g002
Figure 2. A bar chart exhibiting the catalytic activities and C6 fraction in total oligomers for five nickel complexes activated with MMAO (Table 4, Runs 4 and 13–16).
Figure 2. A bar chart exhibiting the catalytic activities and C6 fraction in total oligomers for five nickel complexes activated with MMAO (Table 4, Runs 4 and 13–16).
Catalysts 14 00342 g002
Table 1. Selected bond lengths (Å) and angles (°) for NiL1.
Table 1. Selected bond lengths (Å) and angles (°) for NiL1.
NiL1
Bond lengths (Å)
Ni(1)-N(1)2.036(3)
Ni(1)-N(2)2.057(3)
Ni(1)-Br(1)2.3396(8)
Ni(1)-Br(2)2.3441(7)
N(1)-C(13)1.328(5)
N(1)-C(1)1.365(5)
N(2)-C(14)1.448(5)
N(2)-C(12)1.285(5)
Bond angles (°)
N(1)-Ni(1)-N(2)80.57(12)
N(1)-Ni(1)-Br(1)109.36(10)
N(1)-Ni(1)-Br(2)108.58(9)
N(2)-Ni(1)-Br(1)112.21(9)
N(2)-Ni(1)-Br(2)112.56(9)
Br(1)-Ni(1)-Br(2)124.62(3)
Table 2. Ethylene oligomerization results by NiL1 with different co-catalysts a.
Table 2. Ethylene oligomerization results by NiL1 with different co-catalysts a.
RunCo-CatalystAl/NiT (°C)t (min)Activity [105 g mol−1 (Ni) h−1]Oligomers b (%)
C4/ΣCα-C4/C4C6/ΣC
1Et2AlCl30030303.4899.998.4-
2EASC30030307.8199.6>99.00.4
3EtAlCl2300303010.5099.999.1-
4MAO200030301.2915.066.785.0
5MMAO200030306.1017.5>99.082.5
6None c-3030No---
a Conditions: 2.0 μmol of NiL1, 100 mL of toluene, and 10 atm of ethylene. b Determined by GC. ΣC denotes the total amount of oligomers. c Oligomerization without co-catalyst.
Table 3. Ethylene oligomerization results by NiL1-NiL5/EtAlCl2 a.
Table 3. Ethylene oligomerization results by NiL1-NiL5/EtAlCl2 a.
RunPre-CatalystCo-CatalystAl/NiT (°C)t (min)Activity [105 g mol−1
(Ni) h−1]		Oligomers b (%)
C4/ΣCα-C4/C4C6/ΣC
1NiL1EtAlCl230020307.7099.9>99.9-
2NiL1EtAlCl2300303010.5099.999.1-
3NiL1EtAlCl230040304.1499.988.1-
4NiL1EtAlCl230050301.5999.986.1-
5NiL1EtAlCl2300100300.8399.979.6-
6NiL1EtAlCl210030300.7099.997.6-
7NiL1EtAlCl220030302.4199.997.7-
8NiL1EtAlCl240030302.7399.998.5-
9NiL1EtAlCl250030300.6799.998.2-
10NiL1EtAlCl2300301515.2499.995.1-
11NiL1EtAlCl230030457.8099.999.3-
12NiL1EtAlCl230030605.6399.987.5-
13NiL2EtAlCl2300303015.4399.987.9-
14NiL3EtAlCl2300303012.1499.994.7-
15NiL4EtAlCl2300303016.4199.990.9-
16NiL5EtAlCl2300303016.9299.990.3-
a Conditions: 2.0 μmol of pre-catalyst, 100 mL of toluene, and 10 atm of ethylene. b Determined by GC. ΣC denotes the total amount of oligomers.
Table 4. Ethylene oligomerization results by NiL1-NiL5/MMAO a.
Table 4. Ethylene oligomerization results by NiL1-NiL5/MMAO a.
RunPre-CatalystCo-CatalystAl/NiT (°C)t (min)Activity [105 g mol−1 (Ni) h−1]Oligomers b (%)
C4/ΣCα-C4/C4C6/ΣC
1NiL1MMAO100030303.607.581.392.5
2NiL1MMAO200030306.1017.5>99.082.5
3NiL1MMAO3000303014.1815.066.785.0
4NiL1MMAO4000303019.0713.178.686.9
5NiL1MMAO5000303014.4111.673.588.4
6NiL1MMAO400020306.9312.972.687.1
7NiL1MMAO4000403014.8212.188.887.9
8NiL1MMAO4000503013.5018.066.682.0
9NiL1MMAO4000100305.8723.156.376.9
10NiL1MMAO4000301535.9113.288.986.8
11NiL1MMAO4000304511.6414.175.985.9
12NiL1MMAO4000306010.2116.986.983.1
13NiL2MMAO4000303021.2323.058.377.0
14NiL3MMAO4000303015.719.595.490.5
15NiL4MMAO4000303015.8017.561.282.5
16NiL5MMAO4000303018.9216.368.683.7
a Conditions: 2.0 μmol of pre-catalyst, 100 mL of toluene, and 10 atm of ethylene. b Determined by GC. ΣC denotes the total amount of oligomers.
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Zou, S.; Wang, Z.; Wang, Y.; Ma, Y.; Sun, Y.; Sun, W.-H. On-Purpose Oligomerization by 2-t-Butyl-4-arylimino-2,3-dihydroacridylnickel(II) Bromides. Catalysts 2024, 14, 342. https://doi.org/10.3390/catal14060342

AMA Style

Zou S, Wang Z, Wang Y, Ma Y, Sun Y, Sun W-H. On-Purpose Oligomerization by 2-t-Butyl-4-arylimino-2,3-dihydroacridylnickel(II) Bromides. Catalysts. 2024; 14(6):342. https://doi.org/10.3390/catal14060342

Chicago/Turabian Style

Zou, Song, Zheng Wang, Yizhou Wang, Yanping Ma, Yang Sun, and Wen-Hua Sun. 2024. "On-Purpose Oligomerization by 2-t-Butyl-4-arylimino-2,3-dihydroacridylnickel(II) Bromides" Catalysts 14, no. 6: 342. https://doi.org/10.3390/catal14060342

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