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Article

2-(5-Phenylpyrazol-3-yl)-8-arylimino-5,6,7-trihydroquinolyliron Chlorides as Precatalysts for Ethylene Oligomerization

by
Jiahao Gao
1,
Yaling Fang
2,
Yanping Ma
1,
Liqun Jin
2,*,
Yizhou Wang
1,
Xinquan Hu
2,
Wenjuan Zhang
3 and
Wen-Hua Sun
1,*
1
Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
2
State Key Laboratory of Green-Chemical Synthesis and Conversion Technology, College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310032, China
3
Beijing Key Laboratory of Clothing Materials R&D and Assessment, Beijing Engineering Research Center of Textile Nanofiber, School of Materials Science and Engineering, Beijing Institute of Fashion Technology, Beijing 100029, China
*
Authors to whom correspondence should be addressed.
Catalysts 2025, 15(9), 898; https://doi.org/10.3390/catal15090898
Submission received: 30 August 2025 / Revised: 14 September 2025 / Accepted: 16 September 2025 / Published: 17 September 2025
(This article belongs to the Special Issue Innovative Catalytic Approaches in Polymerization)
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Abstract

A series of 2-(5-phenylpyrazol-3-yl)-8-arylimino-5,6,7-trihydroquinolyliron chlorides (Fe1–Fe6) have been prepared and characterized by FT-IR spectra and elemental analysis. In single-crystal X-ray crystallography, Fe4 shows a distorted trigonal–bipyramidal geometry along with a self-assembling network through intermolecular NH···HO and OH···Cl hydrogen bonding. In the presence of MAO, all iron complexes perform with good activity and high selectivity for 1-butene. In addition, mixed solvents with different ratios of methylcyclohexane and toluene have been explored with a view to optimizing the catalytic performance.

1. Introduction

α-Olefins are increasingly in demand due to their essential applications as co-polymer commoners, lubricants, plasticizers, oilfield chemicals, etc. [1,2,3,4,5,6,7]. Therefore, there is continuous interest by both academic and industrial scientists in exploring efficient processes for ethylene oligomerization through developing new complex catalysts [8,9,10,11,12,13]. Ethylene oligomerization is typically either on-purpose or full-range [14,15,16,17]. On-purpose oligomerization selectively produces short-chain α-olefins [18,19,20,21], indicating dimerization [22,23,24,25,26,27], trimerization [28,29,30,31], and tetremerization [32,33,34]; meanwhile, full-range oligomerization has largely been operated with a Ziegler catalyst [35,36,37], the SHOP process [38,39,40,41], and recently, an iron catalyst, 2-imino-1,10-phenanthrolinyliron catalyst [42]; the latter has been commercialized at two factories with an annual scale of 300 Kt α-olefins in China. There are also our extensive investigations [43] of the 2,6-bis(imino)pyridylmetal catalysts pioneered by Brookhart [44] and Gibson [45]. Moreover, the 2,6-bis(imino)pyridylchromium complexes were found to act in both on-purpose [46,47,48,49] and full-range oligomerization of ethylene [50,51,52,53,54,55]. Today, there is still interest in developing iron catalysts for the on-purpose oligomerization of ethylene. In subsequent works to 2-imino-1,10-phenanthrolinylmetal complexes (Scheme 1A) [42], there have been various tridentate N,N,N-ligand frameworks, including heterocyclic benzimidazoles (Scheme 1B) [56,57,58] or/and cycle-fused pyridines ([59,60], Scheme 1C,D). Impressively, all iron complexes of model B performed ethylene oligomerization for α-olefins [56,57,58]; meanwhile, the cycle-fused model C, without heterocyclic dentate, resulted in iron catalysts with better thermostability and higher activities contributing toward ethylene polymerization [59]. However, when benzimidazole was included along with cycle-fused model D, the cobalt analogues selectively performed dimerization and trimerization of ethylene [60]. Therefore, herein, the imidazole unit is introduced for model E, attractively; their iron complexes have been found to be active, serving mainly to oligomerize ethylene. By changing the parameters of the co-catalyst, temperature, and solvents, their catalytic performance is investigated, along with the identification of the resultant products.

2. Results and Discussion

2.1. Synthesis and Characterization of Fe1–Fe6

The iron chloride complexes, [2-(C9H7N2H)-8-(ArN)C10H8-N]FeCl2 (Ar = 2,6-Me2C6H3 Fe1; 2,6-Et2C6H3 Fe2; 2,6-iPr2C6H3 Fe3; 2,4,6-Me3C6H2 Fe4; 2,6-Et2-4-MeC6H2 Fe5; 2-Me-6-EtC6H2 Fe6] were obtained in a reasonable yield (56–69%) by mixing 2-(5-phenyl-1H-pyrazol-3-yl)-5,6,7-dihydroquinolin-8-one (6) with the corresponding aniline in the presence of iron dichloride in acetic acid and n-butanol at reflux (Scheme 2). The synthetic procedure of 2-(5-phenyl-1H-pyrazol-3-yl)-5,6,7-dihydroquinolin-8-one (6) is shown in the Supporting Information (SI). It is noteworthy that related one-pot approaches have been applied elsewhere to synthesize similar species [61,62,63,64]. All iron complexes have been characterized by FT-IR spectroscopy (Supporting Information) as well as by elemental analysis. In addition, the single crystals of complex Fe4 were grown and have been analyzed by X-ray diffraction.
Single crystals of Fe4 suitable for the X-ray determinations were grown by layering diethyl ether onto a solution of the corresponding complex in a methanol/dichloromethane mixture. Perspective views are shown in Figure 1, and selected bond lengths and angles are tabulated in Table 1. The iron center is surrounded by two chlorides and three nitrogen atoms belonging to the chelating 2-(5-phenyl-1H-pyrazol-3-yl)-8-arylimino-5,6,7-trihydroquinoline (aryl = 2,4,6-Me3Ph Fe4), to complete a geometry that can be best described as a trigonal–bipyramidal geometry [65,66,67].
For the three iron–nitrogen distances, the one involving the central pyridine is the shortest [Fe(1)-N(2) 2.1243(2) Å] while the exterior nitrogen donors show some variation. In particular, the Fe-Nbenzimidazole distance is noticeably shorter [Fe(1)-N(3) 2.2002(2) Å] than the Fe-Nimine distance [Fe(1)-N(1) 2.2416(2) Å], reflecting the superior donor properties of benzimidazole–nitrogen. The Fe-Cl bond distances display non-significant differences with Fe(1)-Cl(1) (2.3179(9) Å) and Fe(1)-Cl(2) (2.3132(7) Å). Within each five-membered chelate ring, the bite angles show modest variation, with that involving Nimine [N(1)-Fe(1)-N(2) 73.85(8)°] slightly larger than that with Nbenzimidazole [N(2)-Fe(1)-N(3) 72.78(1)°], which presumably derives from the constraints of the five-membered imidazole ring. The imine bond lengths [C(17)-N(3) 1.276(4) Å] are typical of this functionality, while the N-aryl group is inclined almost orthogonally with respect to the imine vector. In this structure, the sp3-hybridized C(15) atom belonging to the trihydroquinoline unit is folded away from the plane of the neighboring unsaturated heterocycle. Due to the presence of both NH groups and H2O (from the solvent), an intermolecular hydrogen bridging both compounds is a feature of this structure. In particular, H2O undergoes hydrogen bonding interactions via Cl···H-OH (Cl(1)···H-O(1) 2.077 Å, Cl(2)’···H-O(1) 2.192 Å) and H2O···H-N (O(1)···H-N(4) 1.998 Å, O(1)’···H-N(4) 2.021 Å) to connect two neighboring molecules.
The FT-IR spectra of Fe1–Fe6 reveal imine stretching frequencies in the range of 1600–1604 cm−1, which are characteristic features of nitrogen atoms involved in imine coordination; no absorption bands corresponding to complexed C=O groups or to ketone 1 could be detected. In addition, the elemental analysis data confirmed that the complexes have the composition LMCl2.

2.2. Ethylene Oligomerization

Inspired by pre-catalysts, potential co-catalysts including diethylaluminium chloride (Et2AlCl), ethylaluminum sesquichloride (EASC), ethylaluminum dichloride (EtAlCl2), methylaluminoxane (MAO), and modified methylaluminoxane (MMAO) were tested. They were compared with isostructural Fe catalysts in toluene at 10 atm C2H4 over 30 min (runs 1–5, Table 2 and Table S1) and oligomerized ethylene to form oligomers in the range of C4 to C12. However, when using MAO as the activator, the dimerization oligomer is the major product with high catalytic activity. Although the catalyst exhibits greater activity than other co-catalysts and high selectivity for C4 when activated with MMAO, significant amounts of polyethylene wax are observed. These may arise because different active species are formed besides the species for ethylene oligomerization [42,56,57,58,68].
To explore the effect of different conditions, oligomerization studies were performed using methylaluminoxane (MAO) as the activator. Typically, the oligomerization experiments were performed in toluene at 10 atm C2H4 over 30 min, and the selectivity for dimerization, trimerization, as well as specific α-olefins was determined by gas chromatography (GC); the results are listed in Table 3.
Using Fe4 as the test pre-catalyst, the effects of temperature, the Al: Fe molar ratio, the reaction time, and different solvents were investigated (entries 1–11, Table 3). First, the Al:Fe molar ratio was fixed from 500 to 1250 and the activity in each case measured after 30 min at 20 °C (entries 1–4, Table 3). The optimum activity of 2.70 × 105 g·mol−1 (Fe)·h−1 for Fe4/MAO was observed at 20 °C with 57.0% selectivity for ethylene dimerization (C4) and 32.5% selectivity for ethylene trimerization (C6), with α-olefin being the major component (90%). However, as we enhanced the Al:Fe molar ratio, the major product tended to be C4 with the increase in the catalytic activity (until Al:Fe = 1000 showed the highest catalytic activity) (Figure 2 and Figure S1). The result suggested that the addition of co-catalyst can stabilize the active center and thus increase the catalytic activity, and excessive co-catalyst increases the chain transfer process, which is why the product tended to be C4 [57].
In addition, the influence of temperature on the activity and selectivity of the catalyst can be particularly observed with the temperature maintained at 20 °C (entries 2, 5–9, Table 3). When increasing the reaction temperature from 20 °C to 100 °C (entries 2, 6–9, Table 3), the catalytic activity decreased (2.70 × 105 g mol−1 (Fe) h−1 at 20 °C to 0.11 × 105 g mol−1 (Fe) h−1 at 100 °C), with higher selectivity for C4 (from 51.8% to 96%) (Figures S2 and S3). At the same time, polyethylene wax was also detected when the reaction temperature reached 80 °C, consistent with previous reports [69]. However, after 40 °C, the catalyst showed very high selectivity for C4 (88.4–96.0% for C4, 95–99% for α-olefin). With the optimized conditions established as temp = 20 °C, Al:Fe = 1000, and PC2H4 = 10 atm, the remaining six iron pre-catalysts were similarly screened (entries 13–17, Table 3). In terms of activity (range: 2.70–0.87 × 105 g·mol−1 (Fe) h−1), the six pre-catalysts were found to fall in the following order: Fe4 > Fe5 > Fe1 ~ Fe6 > Fe2 > Fe3 (Figures S4 and S5). This suggests that both electronic and steric hindrance effects play a combined role in affecting performance, with trimetyl-containing Fe4 the most active and the 2,4-diisopropyl-Fe3 the least. In particular, para-methyl Fe4 and Fe5 were more active than para-H Fe1, Fe2, and Fe3, highlighting the importance of the electron donating para-substituent. On the other hand, the most sterically encumbered Fe3 likely impedes ethylene coordination, leading to the lowest activity. In general, the current model catalyst demonstrates higher catalytic activity than the reported catalysts (B, R = Me, catalytic activity was 0.026 × 105 g (PE) mol−1 (Fe) h−1 when using 5 µmol of catalyst under 20 atm of ethylene at 20 °C; D, catalytic activity was 0.49 × 105 g (PE) mol−1 (Fe) h−1 when using 3 µmol of catalyst under 10 atm of ethylene at 30 °C) under similar conditions.
Considering that methylcyclohexane (MCH) is often used as the oligomerization solvent in industry, we also tried to apply it to this catalyst system (entry 10, Table 3). Compared with toluene as the solvent, the system obtained similar results in methylcyclohexane. In order to understand the influence of solvent change on oligomerization, we tried to test the influence of gradient change on the oligomerization reaction of two solvents. Surprisingly, after mixing the two solvents, not only the catalytic activity but also the selectivity for C4 was improved (entries 1–6, Table 4). In particular, the polymerization activity and selectivity reached the highest at 50% methylcyclohexane and 50% toluene (3.92 × 105 g·mol−1 (Fe) h−1, 91.6% for C4, Figure 3 and Figure 4).

3. Experimental Section

3.1. General Considerations

All manipulations of air- and/or moisture-sensitive operations were undertaken in a nitrogen atmosphere using standard Schlenk techniques. Toluene was refluxed over sodium and distilled under nitrogen immediately prior to use. Methylaluminoxane (MAO, 1.46 M solution in toluene) and modified methylaluminoxane (MMAO, 2.5 M in n-heptane, containing 20–25% Al(i-Bu)3) were purchased from Anhui Botai Electronic Materials Co. (Chuzhou, China). Diethylaluminum chloride (Et2AlCl, 1.0 M in toluene), ethylaluminum sesquichloride (EASC, 1.0 M in toluene), and ethylaluminum dichloride (EtAlCl2, 1.0 M in toluene) were purchased from Acros Chemicals. High-purity ethylene was purchased from Beijing Yanshan Petrochemical Co. and used as received. Other chemical reagents were purchased from Concord Technology Co., Ltd. (Tianjin, China), Macklin Biochemical Technology Co., Ltd. (Shanghai, China), and Innochem Technology Co., Ltd. (Beijing, China) and used as received: diethyl ether (GR., Concord, Tianjin, China), n-hexane (GR., Concord, Tianjin, China), glacial acetic acid (AR., Concord, Tianjin, China), n-butanol (AR., Concord, Tianjin, China), ferrous chloride tetrahydrate (98% purity, Macklin, Shanghai, China), and anilines (99% purity, Innochem, Beijing, China). NMR spectra were recorded on Bruker DMX 400 MHz and Bruker DMX 500 MHz instruments at ambient temperature using TMS as an internal standard. The FT-IR spectra were recorded on a PerkinElmer System 2000 FT-IR spectrometer (PerkinElmer Scientific, Waltham, MA, USA). The elemental analyses were conducted using a Flash EA 1112 microanalyzer (Thermo Fisher Scientific, Waltham, MA, USA). GC analyses were performed with a Varian CP-3800 gas chromatograph (Beijing, China) equipped with a flame ionization detector and a 30 m (0.2 mm i.d., 0.25 mm film thickness) CP-Sil 5 CB column.

3.2. Synthesis of 2-(5-Phenyl-1H-pyrazol-3-yl)-8-arylimino-5,6,7-trihydroquinoline Iron Chloride Complexes (Fe1–Fe6)

Ar = 2,6-Me2C6H3 (Fe1). Glacial acetic acid (0.1 mL) and n-butanol (2.0 mL) were added to a mixture of 2-(5-phenyl-1H-pyrazol-3-yl)-8-arylimino-5,6,7-trihydroquinoline (0.14 g, 0.50 mmol), 2,6-dimethylaniline (0.068 g, 0.55 mmol), and iron chloride tetrahydrate (0.12 g, 0.50 mmol). After stirring at reflux for 12 h, the mixture was cooled to room temperature. Diethyl ether (30 mL) was added to precipitate the solid, which was then filtered and collected. Recrystallization from a mixture of dichloromethane and diethyl ether gave a blue green powder (0.14 g, 65%). FT-IR (KBr cm−1): 3101 (w), 3055 (w), 2960 (w), 2885 (w), 1664 (s), 1600 (s, vC=N), 1571 (w), 1485 (s), 1462 (w), 1402 (m), 1373 (w), 1327 (w), 1295 (w), 1276 (m), 1206 (s), 1149 (w), 1115 (w), 1094 (w), 1041 (w), 981 (s), 955 (w), 919 (w), 886 (w), 852 (m), 822 (s), 768 (s), 741 (w), 693 (s). Anal. Calcd for C26H24Cl2FeN4: C, 60.14, H, 4.66, N, 10.79. Found: C, 59.66, H, 4.29, N, 10.98%.
Ar = 2,6-Et2C6H3 (Fe2). Using a similar procedure and molar ratios as that described for Fe1, Fe2 was obtained as a blue green powder (0.16 g, 69%). FT-IR (KBr cm−1): 3116 (w), 3068 (w), 2955 (w), 2870 (w), 1673 (s), 1601 (s, vC=N), 1561 (w), 1532 (w), 1490 (w), 1455 (s), 1427 (w), 1396 (m), 1352 (w), 1280 (s), 1215 (s), 1148 (w), 1104 (m), 1037 (w), 997 (s), 918 (w), 868 (m), 822 (s), 770 (s), 743 (w), 717 (w), 693 (m). Anal. Calcd for C28H28Cl2FeN4: C, 61.45, H, 5.16, N, 10.24. Found: C, 60.96, H, 5.29, N, 10.58%.
Ar = 2,6-iPr2C6H3 (Fe3). Using a similar procedure and molar ratios as that described for Fe1, Fe3 was obtained as a blue green powder (0.13 g, 67%). FT-IR (KBr cm−1): 3224 (w), 3121 (w), 2957 (w), 2877 (w), 1660 (s), 1604 (m, vC=N), 1564 (w), 1538 (w), 1493 (w), 1457 (s), 1434 (w), 1393 (s), 1355 (w), 1284 (s), 1218 (s), 1105 (m), 1073 (m), 1041 (m), 997 (s), 924 (w), 870 (m), 830 (s), 773 (s), 693 (s). Anal. Calcd for C30H32Cl2FeN4: C, 62.63, H, 5.61, N, 9.74. Found: C, 62.34, H, 6.03, N, 9.97%.
Ar = 2,6-Et2-4-MeC6H2 (Fe4). Using a similar procedure and molar ratios as that described for Fe1, Fe4 was obtained as a blue green powder (0.09 g, 59%). FT-IR (KBr cm−1): 3101 (w), 3059 (w), 2955 (w), 2885 (w), 1664 (s), 1601 (s, vC=N), 1569 (w), 1494 (m), 1485 (s), 1457 (w), 1400 (m), 1372 (w), 1324 (w), 1295 (w), 1275 (w), 1207 (s), 1150 (w), 1116 (w), 1089 (w), 1041 (w), 1001 (w), 980 (s), 951 (w), 919 (w), 888 (w), 853 (w), 823 (m), 768 (s), 742 (w), 692 (s). Anal. Calcd for C27H26Cl2FeN4: C, 60.81, H, 4.91, N, 10.51. Found: C, 60.57, H, 5.11, N, 10.17%.
Ar = 2,6-Et2-4-MeC6H2 (Fe5). Using a similar procedure and molar ratios as that described for Fe1, Fe5 was obtained as a blue green powder (0.09 g, 56%). FT-IR (KBr cm−1): 3117 (w), 3062 (w), 2955 (w), 2875 (w), 1672 (s), 1600 (s, vC=N), 1557 (w), 1528 (w), 1488 (w), 1454 (s), 1426 (m), 1395 (m), 1351 (w), 1280 (s), 1210 (s), 1145 (w), 1100 (w), 1040 (w), 995 (s), 916 (w), 857 (m), 817 (s), 769 (s), 690 (s). Anal. Calcd for C29H30Cl2FeN4: C, 62.05, H, 5.39, N, 9.98. Found: C, 62.34, H, 5.01, N, 9.72%.
Ar = 2-Me-6-EtC6H2 (Fe6). Using a similar procedure and molar ratios as that described for Fe1, Fe6 was obtained as a blue green powder (0.14 g, 66%). FT-IR (KBr cm−1): 3117 (w), 3063 (w), 2956 (w), 2873 (w), 1672 (s), 1600 (s, vC=N), 1560 (w), 1532 (w), 1489 (w), 1456 (s), 1426 (m), 1396 (m), 1351 (w), 1323 (w), 1280 (s), 1214 (s), 1149 (w), 1102 (w), 1070 (w), 1040 (w), 996 (s), 917 (w), 869 (m), 821 (s), 769 (s), 742 (w), 692 (s). Anal. Calcd for C27H26Cl2FeN4: C, 60.81, H, 4.91, N, 10.51. Found: C, 61.14, H, 5.23, N, 10.66%.

3.3. X-Ray Crystallographic Studies

Single crystals of Fe4 suitable for the X-ray diffraction studies were grown by slow diffusion of diethyl ether into a solution of the corresponding complex in a mixture of methanol and dichloromethane at room temperature. Data collection for both X-ray determinations was carried out on a Rigaku MM007-HF Saturn 724 + CCD diffractometer with confocal mirror monochromated Cu-Kα radiation (λ = 1.54184). 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 direct methods and refined by full-matrix least-squares on F2. All hydrogen atoms were placed in calculated positions. Structural solution and refinement were performed by using the SHELXL-97 package [69,70]. Details of the X-ray structural determination and refinements are provided in Table S2.

3.4. General Ethylene Oligomerization Procedures

The ethylene oligomerizations were carried out in a 250 mL stainless-steel autoclave equipped with an ethylene pressure control system, a mechanical stirrer, and a temperature controller. The autoclave was evacuated and refilled with ethylene three times. Toluene (50 mL), the desired amount of co-catalyst (MAO, MMAO etc.), and a solution of the pre-catalyst (Fe1–Fe6, 2 μmol) in toluene (50 mL) were successively added by syringe under an ethylene atmosphere, taking the total volume of solvent to be 100 mL. When the desired reaction temperature was reached, stirring commenced, and the ethylene pressure was increased to 10 atm and maintained at this level by a constant feed of ethylene. After 30 min, the reaction was stopped by cooling the reactor in an ice bath, and the excess pressure was then slowly released. A small amount of this cooled reaction solution was collected (ca. 1 mL) and 5% aqueous hydrogen chloride added to terminate the reaction. A sample of this mixture (0.02 μL) was then immediately injected into the GC instrument to determine the distribution of oligomers obtained. The mass of C4 and C6 was calculated based on the ratio of the corresponding peaks to the toluene peak in the gas chromatogram. Since the volume of the toluene is fixed, the amount of butenes and hexenes can be determined. The catalytic activity was based on the mass of all oligomer fractions obtained.

4. Conclusions

Six 2-(5-phenyl-1H-pyrazol-3-yl)-8-arylimino-5,6,7-trihydroquinoline iron chloride complexes (Fe1–Fe6) were successfully synthesized and characterized using single crystal X-ray diffraction. Upon activation with methylaluminoxane, these iron complexes exhibited remarkable catalytic activities contributing toward ethylene oligomerization. Using MAO as a co-catalyst, all iron complexes efficiently promoted selective ethylene dimerization, in which 1-butene accounted for the major product (catalytic activities up to 2.70 × 105 g mol−1 (Fe) h−1 for Fe4). Under the same conditions, changing the system solvents (toluene, methylcyclohexane, hexane) has little effect on the catalytic activity and selectivity. In particular, the catalytic activity of the catalyst in a mixed solvent of methylcyclohexane and toluene and the selectivity for ethylene dimerization have been significantly enhanced. Therefore, this kind of iron pre-catalyst has potential for the production of oligomerization in industry.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal15090898/s1, Scheme S1. Template synthetic route to 2-(5-phenyl-1H-pyrazol-3-yl)-5,6,7-dihydroquinolin-8-one; Table S1. Ethylene oligomerization by Fe4 with different co-catalysts; Figure S1. The influence of the proportion of co-catalysts on catalytic activity when using Fe4/MAO as catalyst (entries 1–4, Table 3); Figure S2. The influence of temperature on catalytic activity when using Fe4/MAO as catalyst (entries 2, 5–9, Table 3); Figure S3. The influence of temperature on distribution of oligomer products when using Fe4/MAO as catalyst (entries 2, 5–9, Table 3); Figure S4. The influence of different catalysts on catalytic activity (entries 2, 13–17, Table 3); Figure S5. The influence of the different catalysts on distribution of oligomer products (entries 2, 13–17, Table 3); Figure S6. The 1H NMR spectrum of 1-(6,7-dihydro-5H-spiro[quinoline-8,2′-[1,3]dioxolan]-2-yl)-3-hydroxy-3-phenylprop-2-en-1-one (4); Figure S7. The 13C NMR spectrum of 1-(6,7-dihydro-5H-spiro[quinoline-8,2′-[1,3]dioxolan]-2-yl)-3-hydroxy-3-phenylprop-2-en-1-one (4); Figure S8. The 1H NMR spectrum of 2-(5-phenyl-1H-pyrazol-3-yl)-6,7-dihydro-5H-spiro[quinoline-8,2′-[1,3]dioxolane] (5); Figure S9. The 13C NMR spectrum of 2-(5-phenyl-1H-pyrazol-3-yl)-6,7-dihydro-5H-spiro[quinoline-8,2′-[1,3]dioxolane] (5); Figure S10. The 1H NMR spectrum of 2-(5-phenyl-1H-pyrazol-3-yl)-8-arylimino-5,6,7-trihydroquinoline (6); Figure S11. The 13C NMR spectrum of 2-(5-phenyl-1H-pyrazol-3-yl)-8-arylimino-5,6,7-trihydroquinoline (6); Figure S12. The FT-IR spectra of Fe1; Figure S13. The FT-IR spectra of Fe2; Figure S14. The FT-IR spectra of Fe3; Figure S15. The FT-IR spectra of Fe4; Figure S16. The FT-IR spectra of Fe5; Figure S17. The FT-IR spectra of Fe6; Table S2. Crystal data and structural refinement details for Fe4; Figure S18. The example of GC chromatogram for Fe4/MAO (entry 2, Table 3); Figure S19. The example of GC chromatogram for Fe4/MAO (entry 8, Table 3). Scheme S2. Typical structures of methylaluminoxane (MAO); Scheme S3. Possible reaction scheme of the alkylation and ion pair formation of iron complex and MAO.

Author Contributions

J.G.: methodology, investigation, data curation, writing—original draft preparation, and writing—review and editing; Y.F.: methodology and data curation; Y.M.: software, and writing—review and editing; L.J.: validation, supervision, and writing—review and editing; Y.W.: data curation and X-ray diffraction; X.H.: software and supervision; W.Z.: supervision, and writing—review and editing; 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 received no external funding.

Data Availability Statement

CCDC reference number 2469522 contains Supplementary Crystallographic Information for Fe4, and the data presented in this study are available in this article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Catalysts 15 00898 sch001
Scheme 1. Developments in unsymmetrical N,N,N-chelating ligands for iron chlorides, ((A) 2-imino-1,10-phenanthrolinyl model; (B) heterocyclic benzimidazoles model; (C) cycle-fused pyridines model; (D) cycle-fused benzimidazole model; (E) This work).
Scheme 1. Developments in unsymmetrical N,N,N-chelating ligands for iron chlorides, ((A) 2-imino-1,10-phenanthrolinyl model; (B) heterocyclic benzimidazoles model; (C) cycle-fused pyridines model; (D) cycle-fused benzimidazole model; (E) This work).
Catalysts 15 00898 sch001
Catalysts 15 00898 sch002
Scheme 2. Template synthetic route to Fe1–Fe6.
Scheme 2. Template synthetic route to Fe1–Fe6.
Catalysts 15 00898 sch002
Catalysts 15 00898 g001
Figure 1. ORTEP representation of Fe4·H2O with the thermal ellipsoids set at the 30% probability level (left), and NH···H2O and Cl···H2O hydrogen-bonding interactions between neighboring molecules of Fe4·H2O (right).
Figure 1. ORTEP representation of Fe4·H2O with the thermal ellipsoids set at the 30% probability level (left), and NH···H2O and Cl···H2O hydrogen-bonding interactions between neighboring molecules of Fe4·H2O (right).
Catalysts 15 00898 g001
Catalysts 15 00898 g002
Figure 2. The influence of the Al/Fe ratios on distribution of oligomer products obtained using Fe4/MAO (entries 1–4, Table 3).
Figure 2. The influence of the Al/Fe ratios on distribution of oligomer products obtained using Fe4/MAO (entries 1–4, Table 3).
Catalysts 15 00898 g002
Catalysts 15 00898 g003
Figure 3. The influence of the methylcyclohexane/toluene ratio (MCH:TOL) in solvent on distribution of oligomer products obtained using Fe4/MAO (entries 1–6, Table 4).
Figure 3. The influence of the methylcyclohexane/toluene ratio (MCH:TOL) in solvent on distribution of oligomer products obtained using Fe4/MAO (entries 1–6, Table 4).
Catalysts 15 00898 g003
Catalysts 15 00898 g004
Figure 4. The influence of the concentration of methylcyclohexane in solvent (MCH/(MCH + TOL) × 100%) on catalytic activity (entries 1–6, Table 4).
Figure 4. The influence of the concentration of methylcyclohexane in solvent (MCH/(MCH + TOL) × 100%) on catalytic activity (entries 1–6, Table 4).
Catalysts 15 00898 g004
Table 1. Selected bond lengths (Å) and angles (°) for Fe4.
Table 1. Selected bond lengths (Å) and angles (°) for Fe4.
Bond Lengths
Fe(1)-N(1)2.2416(2)N(1)-C(17)1.276(4)
Fe(1)-N(2)2.1243(2)N(2)-C(10)1.340(3)
Fe(1)-N(3)2.2002(2)N(2)-C(18)1.347(3)
Fe(1)-Cl(1)2.3179(9)N(3)-C(9)1.341(3)
Fe(1)-Cl(2)2.3132(7)N(3)-N(4)1.340(3)
Bond Angles
Cl(1)-Fe(1)-Cl(2)114.45(3)Cl(2)-Fe(1)-N(2)124.56(6)
Cl(1)-Fe(1)-N(1)103.81(6)Cl(2)-Fe(1)-N(3)95.18(6)
Cl(1)-Fe(1)-N(2)120.43(6)N(1)-Fe(1)-N(2)73.85(8)
Cl(1)-Fe(1)-N(3)95.83(6)N(1)-Fe(1)-N(3)146.46(8)
Cl(2)-Fe(1)-N(1)100.89(6)N(2)-Fe(1)-N(3)72.78(7)
Table 2. Ethylene oligomerization by Fe4 with different co-catalysts a.
Table 2. Ethylene oligomerization by Fe4 with different co-catalysts a.
EntryCatCo-CatalystAl/FeOligomers b (wt.%)Wax d
C4/ƩCC6/ƩCC8/ƩC ≥ C10/ƩCα-Olefin (%)Activity c
1Fe4Et2AlCl30025.421.853.7->970.21-
2Fe4EtAlCl230026.355.96.211.5>970.63-
3Fe4EASC30021.959.318.3->980.50-
4Fe4MAO100057.033.56.33.2>902.70-
5Fe4MMAO100097.22.70.1->956.781.19
a Conditions: 2.0 µmol Fe4, 100 mL toluene, and 10 atm ethylene, 20 °C, 30 min. b Determined by GC. ΣC denotes the total amount of oligomers. c Activity for oligomer 105 g mol−1 (Fe) h−1. d Activity for polyethylene wax: 105 g (PE) mol−1 (Fe) h−1.
Table 3. Ethylene oligomerization by titled catalysts with MAO a.
Table 3. Ethylene oligomerization by titled catalysts with MAO a.
EntryCatAl/FeTemp
(°C)		Oligomers b (wt.%)Wax d
C4/ƩCC6/ƩCC8/ƩC ≥ C10/ƩCα-Olefin (%)Activity c
1Fe412502086.310.62.11.0>972.19-
2Fe410002057.033.56.33.2>902.70-
3Fe47502046.642.38.72.4>961.25-
4Fe45002023.553.620.82.1>980.61-
5Fe410001051.832.412.23.6>931.00-
6Fe410004090.21.56.31.8>951.45-
7Fe410006088.41.110.30.2>980.74-
8Fe410008096.00.92.30.6>990.311.13
9Fe4100010094.80.14.80.3>990.113.65
10 eFe410002055.932.79.02.3>972.30-
11 fFe410002063.322.610.43.7>961.82-
12 gFe410002035.356.74.93.0>990.251.74
13Fe110002077.815.73.62.9>952.04-
14Fe210002065.19.419.16.4>951.04-
15Fe310002072.417.28.12.3>960.87-
16Fe510002062.423.111.03.5>982.37-
17Fe610002079.317.12.51.1>992.04-
a Conditions: 2.0 µmol catalyst, 100 mL toluene, and 10 atm ethylene, 30 min. b Determined by GC. ΣC denotes the total amount of oligomers. c Activity for oligomer 105 g mol−1 (Fe) h−1. d Activity for polyethylene wax: 105 g (PE) mol−1 (Fe) h−1. e Methylcyclohexane as solvent. f hexane as solvent. g 60 min.
Table 4. Ethylene oligomerization by using Fe4 with MAO in the mixture of methylcyclohexane (MCH) and toluene (TOL) a.
Table 4. Ethylene oligomerization by using Fe4 with MAO in the mixture of methylcyclohexane (MCH) and toluene (TOL) a.
EntrySolvent
(MCH:TOL)		Oligomers b (wt.%)
C4/ƩCC6/ƩCC8/ƩC ≥ C10/ƩC α-Olefin (%)Activity c
1100:055.932.79.02.3>972.30
295:576.115.16.72.1>972.66
375:2590.52.68.80.4>953.45
450:5091.60.77.10.5>983.92
525:7565.117.017.20.6>973.15
60:10057.033.56.33.2>902.70
a Conditions: 2.0 µmol Fe4, Al:Fe = 1000, and 10 atm ethylene, 20 °C, 30 min, total 100 mL solvent. b Determined by GC. ΣC denotes the total amount of oligomers. c Activity for oligomer 105 g mol−1 (Fe) h−1.
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Gao, J.; Fang, Y.; Ma, Y.; Jin, L.; Wang, Y.; Hu, X.; Zhang, W.; Sun, W.-H. 2-(5-Phenylpyrazol-3-yl)-8-arylimino-5,6,7-trihydroquinolyliron Chlorides as Precatalysts for Ethylene Oligomerization. Catalysts 2025, 15, 898. https://doi.org/10.3390/catal15090898

AMA Style

Gao J, Fang Y, Ma Y, Jin L, Wang Y, Hu X, Zhang W, Sun W-H. 2-(5-Phenylpyrazol-3-yl)-8-arylimino-5,6,7-trihydroquinolyliron Chlorides as Precatalysts for Ethylene Oligomerization. Catalysts. 2025; 15(9):898. https://doi.org/10.3390/catal15090898

Chicago/Turabian Style

Gao, Jiahao, Yaling Fang, Yanping Ma, Liqun Jin, Yizhou Wang, Xinquan Hu, Wenjuan Zhang, and Wen-Hua Sun. 2025. "2-(5-Phenylpyrazol-3-yl)-8-arylimino-5,6,7-trihydroquinolyliron Chlorides as Precatalysts for Ethylene Oligomerization" Catalysts 15, no. 9: 898. https://doi.org/10.3390/catal15090898

APA Style

Gao, J., Fang, Y., Ma, Y., Jin, L., Wang, Y., Hu, X., Zhang, W., & Sun, W.-H. (2025). 2-(5-Phenylpyrazol-3-yl)-8-arylimino-5,6,7-trihydroquinolyliron Chlorides as Precatalysts for Ethylene Oligomerization. Catalysts, 15(9), 898. https://doi.org/10.3390/catal15090898

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