Dichloro-Bis(1-Alkyl/Styryl-Benzimidazole)-Cobalt(II) Pre-Catalyst for Ethylene Dimerization

Abstract

A series of five cobalt(II) complexes, dichloro-bis(1-benzyl-benzimidazole)-cobalt(II) (1a), dichloro-bis[1-(4-fluorobenzyl)-benzimidazole]-cobalt(II) (1b), dichloro-bis((Z)-1-styryl-benzimidazole)-cobalt(II) (1c), dichloro-bis[(Z)-1-(2-fluorostyryl)-benzimidazole]-cobalt(II) (1d) and dichloro-bis(1-cinnamyl-benzimidazole)-cobalt(II) (1e), were evaluated in ethylene dimerization. Four of these complexes were described for the first time and fully characterized by IR, elemental analysis, mass and NMR spectroscopy. In the solid state, the cobalt atom exhibited a typical tetrahedral geometry and was found to be coordinated to two chlorine atoms and two benzimidazole rings. In the presence of 20 bar of ethylene and diethylaluminium chloride as a co-catalyst, the complex with styryl substituents on the benzimidazole rings, complex 1c, exhibited the highest activity with a turnover frequency of 3430 mol(ethylene)·mol(Co)⁻¹·h⁻¹.

1. Introduction

Benzimidazole is an aromatic heterocyclic compound that is formed from a benzene ring and an imidazole ring [1]. Consequently, the compound contains two nitrogen atoms. Subsequent to the grafting of a substituent onto one nitrogen atom, the second nitrogen atom can act as a coordinating atom with transition metals such as copper [2], cobalt [3,4], gold [5], iron [6], palladium [7], platinum [8], ruthenium [9] or zinc [3,4].

Following the concurrent discovery in 1998 by the groups of Brookhart and Gibson of [2,6-bis(arylimino)pyridyl]iron and cobalt dihalide complexes, which were found to be effective catalysts for the polymerization of ethylene into high-density polyethylene [10,11], a number of cobalt complexes containing nitrogen ligands have been developed for the oligomerization/polymerization of ethylene [12].

For instance, Solan and co-workers have described the synthesis of the dark green complex dichloro-[(E)-N-(2,4,6-triisopropylphenyl)-1-(1,10-phenanthrolin-2-yl)methan- imine]cobalt(II) (A) (Figure 1), in which the cobalt(II) atom adopted a distorted square pyramidal geometry. Subsequent to activation by methylaluminoxane (MAO), linear α-olefins (89% of formed olefins) were obtained with a Schulz–Flory distribution [13], α = 0.85, and a turnover number (TOF) of 185 mol(ethylene)·mol(A)⁻¹·h⁻¹ [14]. Bianchini and co-workers examined the dichloro-2,6-diisopropyl-N-{(E)-1-[6-{[(1S)-1-(2-naphthyl)ethyl]-ethanimidoyl}-2-pyridinyl]ethylidene}aniline]cobalt(II) (B) (Figure 1) in ethylene oligomerization under a pressure of 4.1 bar at 25 °C. After activation of MAO, the complex exhibited an activity of 1 × 10⁵ mol(ethylene)·mol(B)⁻¹·h⁻¹, which was approximately five times lower than that measured with the iron analogue complex [15].

Gibson and co-workers described the synthesis and characterization of dichloro-2,6-diacetylpyridinebis(N,N-dialkyl/aryl-methylhydrazone)cobalt(II) complexes (Figure 1). The pre-catalysts, when combined with the MAO as co-catalyst, resulted in active catalysts for ethylene oligomerization. The catalytic activities were found to be directly dependent on the substituents of the two nitrogen atoms. The most effective complex, complex C, facilitated the formation of α-olefins, following a Schulz–Flory distribution, α = 0.84, and with an activity of 850 mol(ethylene)·mol(C)⁻¹·h⁻¹ [16].

Conversely, Sun and co-workers have demonstrated that complex dichloro-bis [1-((5-methyl-2-furyl)methylene)-2-(5-methyl-2-furyl)benzimidazole]cobalt(II) (D) (Figure 1) functions as an effective catalyst for ethylene oligomerization. Consequently, at ambient temperature and under 9 bar of ethylene with MAO as a co-catalyst (1000 equivalents per complex), the formation of butenes and hexenes occurred with a TOF of 1.5 × 10⁴ mol(ethylene)·mol(D)⁻¹·h⁻¹ [17].

In the present article, we will continue to develop new cobalt(II) complexes containing benzimidazole aromatic nuclei substituted with alkyl or styryl groups as ligands [18].

The underlying motivation for this research stems from the fact that these complexes can be synthesized in two steps from benzimidazole, an inexpensive compound, with a cost of less than 0.5 EUR per gram. This price was equivalent to that of 2,6-diacetylpyridine or 2,6-dibromopyridine, which were the precursors used for the synthesis of the state-of-the-art Brookhart and Gibson-type complexes. Cobalt(II) chloride is a cost-effective solution, with a market price of less than 2 EUR per gram. In the presence of ethylene, it is hypothesized that the low steric hindrance around the metal center of these complexes will favor chain termination steps against chain propagation, thereby generating short oligomeric chains. To this end, five cobalt complexes (1a–e) will undergo evaluation in the dimerization of ethylene (Figure 2).

2. Materials and Methods

2.1. General

The manipulations were carried out under dry argon with dried solvents. The acquisition of the ¹H, ¹³C{¹H} and ¹⁹F{¹H} spectra was carried out through the utilization of AC 300 and 500 Bruker FT instruments in CDCl₃. The spectra were subsequently referenced to the residual protonated solvent for ¹H (δ = 7.26 ppm) and ¹³C{¹H} (δ = 77.16 ppm) NMR or given relative to external CCl₃F for ¹⁹F{¹H} NMR. Chemical shifts are reported in ppm. Mass spectra were recorded on a Bruker MicroTOF spectrometer (ESI-TOF). UV–visible spectra were performed on an Agilent Technologies Cary 60 UV-Vis spectrometer. The infrared spectra were recorded on a Bruker ATR FT-IR Alpha-P spectrometer. 1-Benzyl-benzimidazole (2a) [19], 1-(4-fluorobenzyl)-benzimidazole (2b) [20], (Z)-1-styryl-benzimidazole (2c) [21] and dichloro-bis(1-cinnamyl-benzimidazole)-cobalt(II) (1e) [18] were prepared according to adapted published procedures.

2.2. Synthesis (Z)-1-(2-Fluorostyryl)-Benzimidazole (2d)

In a Schlenk tube under argon atmosphere, benzimidazole (5 mmol, 0.591 g) was dissolved in N-methyl-2-pyrrolidone (10 mL). Subsequently, potassium hydroxide (2.5 mmol, 0.140 g) was added to the solution, followed by the introduction of 2-fluorophenylacethylene (6 mmol, 0.68 mL). The reaction mixture was subjected to heating at 90 °C for a period of three days. After cooling to room temperature, an aqueous solution of sodium hydrogen carbonate (NaHCO₃, 10 mL) was added and the resulting mixture was extracted with ethyl acetate (10 mL × 3). The combined organic solution was washed with brine (10 mL) and subsequently dried over MgSO₄. The solvent was then removed under reduced pressure, leaving a solid residue. This crude product was purified by column chromatography on silica gel using a mixture of ethylacetate and petroleum ether (3:1 in volume). The target product 2d was obtained in a 22% yield as a white solid. FT-IR: ν(CN) 1486 cm⁻¹; ¹H NMR (300 MHz, CDCl₃): δ = 7.84 − 7.79 (m, 1H, arom CH of C₇H₅N₂), 7.81 (s, 1H, NCHN), 7.38 − 7.30 (m, 3H, arom CH of C₇H₅N₂), 7.28 − 7.21 (m, 1H, arom CH of C₆H₄F), 7.10 − 6.92 (m, 3H, arom CH of C₆H₄F), 6.01 (d, 1H, NCH=CHAr, ³J_HH = 9.0 Hz), 6.62 (d, 1H, NCH=CHAr, ³J_HH = 9.0 Hz); ¹³C{¹H} NMR (126 MHz, CDCl₃): δ = 160.26 (d, CF, ¹J_CF = 250.1 Hz), 143.16, 133.03, 123.97, 123.21, 120.55, 110.30 (6 s, arom Cs), 141.68 (s, NCHN), 130.42 (d, arom CH of C₆H₄F, ³J_CF = 8.4 Hz), 129.47 (d, arom CH of C₆H₄F, ⁴J_CF = 2.8 Hz), 124.61 (d, arom CH of C₆H₄F, ³J_CF = 3.6 Hz), 122.32 (d, NCH=CHAr, ³J_CF = 1.6 Hz), 121.78 (d, arom Cquat of C₆H₄F, ²J_CF = 14.0 Hz), 116.94 (d, NCH=CHAr, ⁴J_CF = 3.5 Hz), 116.16 (d, arom CH of C₆H₄F, ²J_CF = 21.7 Hz); ¹⁹F{¹H} NMR (282 MHz, CDCl₃): δ = −113.96 (s, CF arom) ppm. Elemental analysis (%): calcd for C₁₅H₁₁N₂F (238.26): C: 75.62; H: 4.65; N: 11.76; found C: 75.55; H: 4.56; N: 11.68.

2.3. Synthesis of Cobalt Complexes

In a Schlenk tube under an inert atmosphere of argon, a solution of cobalt(II) chloride (CoCl₂; 0.065 g, 0.50 mmol) and (1-alkyl/vinyl-benzimidazole (1.00 mmol) in ethanol (15 mL) was stirred at room temperature. Following a period of four hours, the blue precipitate that had been formed was filtered, washed with diethylether (3 × 10 mL), and dried under vacuum. This process resulted in the isolation of complexes 1a–e as blue solids.

Dichloro-bis(1-benzyl-benzimidazole)-cobalt(II) (1a): 84% yield (0.230 g); UV–visible (CH₂Cl₂) λ_max = 582, 617, 634 and 660 nm; FT-IR: ν(C=N) 1510 cm⁻¹; ¹H NMR (300 MHz, CDCl₃): δ = 27.68 (s br, 2H), 14.89 (s, 4H), 8.51 (s, 2H), 7.91 (s, 2H), 7.59 (s, 2H), 5.43 (s, 2H), 3.41 (s, 2H), 1.50 (s, 4H), 0.85 (s, 2H), −1.88 (s br, 2H) ppm. MS (ESI-TOF): m/z = 510.10 [M − Cl]⁺ (expected isotopic profiles). Elemental analysis (%): calcd for C₂₈H₂₄N₄CoCl₂ (546.36): C: 61.55; H: 4.43; N: 10.25; found C: 61.47; H: 4.32 N: 10.19.

Dichloro-bis[1-(4-fluorobenzyl)-benzimidazole]-cobalt(II) (1b): 59% yield (0.171 g); UV–visible (CH₂Cl₂) λ_max = 580, 614, 635 and 656 nm; FT-IR: ν(C=N) 1509 cm⁻¹; ¹H NMR (300 MHz, CDCl₃): δ = 82.24 (s br, 2H), 26.98 (s, 2H), 15.27 (s, 4H), 8.41 (s, 4H), 7.55 (s, 4H), 5.32 (s, 2H), 3.04 (s, 2H), 1.47 (s, 2H), −2.34 (s br, 2H); ¹⁹F{¹H} NMR (282 MHz, CDCl₃): δ = −111.46 (s, C₆H₄F) ppm. MS (ESI-TOF): m/z = 546.08 [M − Cl]⁺ and 604.04 [M + Na]⁺ (expected isotopic profiles). Elemental analysis (%): calcd for C₂₈H₂₂N₄CoCl₂F₂ (582.34): C: 57.75; H: 3.81; N: 9.62; found C: 57.66; H: 3.69 N: 9.54.

Dichloro-bis((Z)-1-styryl-benzimidazole)-cobalt(II) (1c): 88% yield (0.251 g); UV–visible (CH₂Cl₂) λ_max = 581, 618, 632 and 659 nm; FT-IR: ν(C=N) 1499 cm⁻¹; ¹H NMR (300 MHz, CDCl₃): δ = 80.77 (s br, 2H), 26.50 (s, 2H), 20.00 (s, 2H), 9.26 (s, 2H), 7.35 (s, 4H), 7.19 (s, 6H), 5.77 (s, 2H), 3.21 (s, 2H), 1.41 (s, 2H) ppm. MS (ESI-TOF): m/z = 534.10 [M − Cl]⁺ (expected isotopic profiles). Elemental analysis (%): calcd for C₃₀H₂₄N₄CoCl₂ (570.38): C: 63.17; H: 4.24; N: 9.82; found C: 63.22; H: 4.19 N: 9.75.

Dichloro-bis[(Z)-1-(2-fluorostyryl)-benzimidazole]-cobalt(II) (1d): 59% yield (0.179 g); UV–visible (CH₂Cl₂) λ_max = 582, 614, 635 and 657 nm; FT-IR: ν(C=N) 1503 cm⁻¹; ¹H NMR (300 MHz, CDCl₃): δ = 81.47 (s br, 2H), 26.40 (s, 2H), 20.05 (s, 2H), 9.10 (s, 2H), 7.82 (s, 2H), 7.53 (s, 2H), 7.36 (s, 2H), 6.99 (s, 2H), 5.69 (s, 2H), 3.13 (s, 2H), 1.48 (s, 2H); ¹⁹F{¹H} NMR (282 MHz, CDCl₃): δ = −111.48 (s, C₆H₄F) ppm. MS (ESI-TOF): m/z = 570.08 [M − Cl]⁺, 611.11 [M − Cl + CH₃CN]⁺ and 628.04 [M + Na]⁺ (expected isotopic profiles). Elemental analysis (%): calcd for C₃₀H₂₂N₄CoCl₂F₂ (606.36): C: 59.42; H: 3.66; N: 9.24; found C: 59.58; H: 3.75 N: 9.36.

Dichloro-bis(1-cinnamyl-benzimidazole)-cobalt(II) (1e): 96% yield (0.184 g).); UV–visible (CH₂Cl₂) λ_max = 583, 617, 638 and 660 nm; FT-IR: ν(C=N) 1511 cm⁻¹; ¹H NMR (300 MHz, CDCl₃): δ = 27.28 (s, 2H), 14.99 (s, 4H), 8.93 (s, 2H), 7.81 − 7.39 (m, 12H), 5.37 (s, 2H), 3.16 (s, 2H), 1.42 (s, 2H), −2.67 (s br, 2H) ppm. MS (ESI-TOF): m/z = 562.14 [M − Cl]⁺ and 620.10 [M + Na]⁺ (expected isotopic profiles). Elemental analysis (%): calcd for C₃₂H₂₈N₄CoCl₂ (598.43): C: 64.23; H: 4.72; N: 9.36; found C: 64.03; H: 4.66 N: 9.23.

2.4. X-Ray Crystal Structure Analysis

Single crystals of dichloro-bis[1-(4-fluorobenzyl)-benzimidazole]-cobalt(II) (1b), suitable for X-ray analysis, were obtained through the slow diffusion of Et₂O into a CH₂Cl₂ solution of the complex. The crystal structure was determined using a Bruker PHOTON-III CPAD with Mo-Kα radiation (λ = 0.71073 Å) at 120(2) K. The final structure was solved with SHELXT-2018/2 [22], which revealed the non-hydrogen atoms of the molecule. Following anisotropic refinement, the location of all hydrogen atoms was ascertained through the use of a Fourier difference map. The structure was refined with SHELXL-2019/3 [23] using the full-matrix least-square technique (use of F square magnitude; x, y, z, and βij for C, Cl, Co, F and N atoms; x, y, and z in a riding mode for H atoms) (

Table 1. Selected crystallographic data for complex 1b.

CCDC depository		2,487,682	Chemical formula		C28H22Cl2CoF2N4
Molar mass (g mol−1)		582.32	Temperature		120(2)
Crystal system		Triclinic	Space group		$P\overline{1}$
Unit cell parameters	a (Å)	9.1062(6)	Unit cell parameters	α (°)	102.004(2)
	b (Å)	9.9570(6)		β (°)	93.637(2)
	c (Å)	16.4854(11)		γ (°)	93.588(2)
Volume (Å3)		1454.71(16)	Z		2
ρcalc. (g·cm−3)		1.329	μ (mm−1)		0.809
F(000)		594	Crystal size (mm)		0.220 × 0.100 × 0.100
Index ranges		−11 ≤ h ≤ 10	θ range for data collection (°)		2.097 ≤ θ ≤ 27.139
		−12 ≤ k ≤ 12	Reflections collected		29,722
		−21 ≤ l ≤ 21	independent/observed		6435/5215
Rint		0.0488	Data		6435
Restraints		0	Parameters		334
Goodness-of-fit on F2		1.005	Tmin, Tmax		0.842, 0.923
Final R indices (I > 2.0 σ(I))		R1 = 0.0371 wR2 = 0.0796	R indices (all data)		R1 = 0.0546 wR2 = 0.0884
Δρmax, Δρmin (e Å−3)		0.342, −0.379

). The Cambridge Crystallographic Data Centre (CCDC) contains the supplementary crystallographic data for the structures. The data can be obtained free of charge via Available online: www.ccdc.cam.ac.uk/structures (accessed on 20 October 2025).

2.5. Dimerization of Ethylene

The dimerization of ethyne was carried out in a glass-lined stainless-steel autoclave (100 mL), under argon, containing a magnetic stirrer bar. Thereafter, a solution of the cobalt complex (5 μmol) in toluene (9.75 mL) and a solution of Et₂AlCl (0.25 mL; 0.9 M in toluene) were added. The autoclave was subjected to three cycles of flushing with ethylene, followed by pressurization to 20 bar. After stirring at 25 °C for one hour, the autoclave was cooled to 5 °C and depressurized over a period of 10 min. The flask containing the reaction mixture was weighed as quickly as possible to minimize the potential evolution of the butene products. The reaction yield was determined by comparing the final mass of the reaction mixture with the mass of the control reaction solution. The mass of the control reaction solution (i.e., the reaction mixture with no catalyst and no co-catalyst) was determined by adding toluene (10 mL) to the autoclave and stirring it under 20 bar of ethylene at 25 °C for one hour [24]. The reactor was cooled to 5 °C, vented to ambient pressure over a period of 10 min, and the flask containing the reaction mixture was weighed. Each catalytic test was repeated three times.

3. Results and Discussion

In the present article the potential of five cobalt(II) complexes, namely of dichloro-bis(1-benzyl-benzimidazole)-cobalt(II) (1a), dichloro-bis[1-(4-fluorobenzyl)-benzimidazole]-cobalt(II) (1b), dichloro-bis((Z)-1-styryl-benzimidazole)-cobalt(II) (1c), dichloro-bis[(Z)-1-(2-fluorostyryl)-benzimidazole]-cobalt(II) (1d) and dichloro-bis(1-cinnamyl-benzimidazole)-cobalt(II) (1e), as pre-catalysts for ethylene dimerization was investigated. First, the synthesis and characterization of the four new complexes 1a–d were described.

3.1. Synthesis of Cobalt(II) Complexes

The four novel cobalt(II) complexes 1a–d were synthesized using the same procedure developed for the synthesis of dichloro-bis(1-cinnamyl-benzimidazole)-cobalt(II) (1e) [18]. The dichloro-bis(1-alkyl/styryl-benzimidazole)-cobalt (II) complexes (1a–d) were obtained in a single step from 1-alkyl/styryl-benzimidazole (2a–d) and the precursor [CoCl₂] with a stoichiometry of 2:1 in ethanol at room temperature for a period of 4 h. The blue precipitates formed are then filtered and washed with diethyl ether and dried, yielding cobalt complexes 1a–d with isolated yields of 59–88% (Scheme 1).

The characterization of these complexes, which exhibited stability in both the solid state and in solution when exposed to air and moisture, involved a combination of analytical techniques. The techniques employed included Fourier-transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance (NMR) spectroscopic analyses, as well as microanalysis and mass spectrometry (see Supplementary Materials).

The formation of cobalt(II) complexes is demonstrated by infrared spectroscopy. An analysis of the infrared spectra of the complexes indicated a shift in the carbon-nitrogen double bond vibration bands, suggesting a modification in the molecular structure. For instance, the ν(C=N) band underwent a shift from 1492 cm⁻¹ for the free 1-benzyl-benzimidazole (2a) to 1510 cm⁻¹ after coordination (complex 1a) (Figure 3).

The 3D configuration of the cobalt(II) cation determined the paramagnetic properties [25] of the blue-formed complexes 1a–d. The ¹H NMR spectra of the high-spin cobalt(II) displayed large signals within the range of −2.5 to 82.2 ppm (Figure 4) [26]. The ¹⁹F{¹H} NMR spectra provided further insight into the coordination of the benzimidazole ligand, exhibiting a slight shift in the signal between the ligands, as observed for the (Z)-1-(2-fluorostyryl)-benzimidazole (2d) at −113.96 ppm, and the corresponding complex 1d at −111.48 ppm.

The presence of two 1-alkyl/styryl-benzimidazole ligands 2a–d in the metal coordination sphere was confirmed by elemental analysis of the complexes. Furthermore, mass spectroscopy revealed the presence of intense peaks at m/z = 510.10, 546.08, 534.10, and 570.08, corresponding to the cations [M − Cl]⁺, with the expected isotopic profiles, for complexes 1a, 1b, 1c and 1d, respectively. A secondary peak at m/z = 604.04 and 628.04, corresponding to [M + Na]⁺, with the expected isotopic profiles, is also discerned for complexes 1b and 1d, respectively.

3.2. Solid-State Structures

X-ray diffraction analyses of single crystals of dichloro-bis[1-(4-fluorobenzyl)-benzimidazole]-cobalt(II) (1b) (Figure 5) and dichloro-bis((Z)-1-styryl-benzimidazole)-cobalt(II) (1c) (Figure S17) complexes definitively confirmed the coordination of two N-alkyl/aryl benzimidazole units at the metal center. Crystals suitable for solid-state analysis were obtained by slow diffusion of diethyl ether into a dichloromethane solution of the cobalt(II) complex. The complexes were found to crystalize in the triclinic space group P$\overline{1}$.

As observed in the case of complexes with the general formula [CoCl₂L₂] (L = substituted benzimidazole), such as dichloro-bis(1-allylbenzimidazole)cobalt(II) [27] and dichloro-bis(1-benzylbenzimidazole)cobalt(II) [28], the cobalt atom adopted a slightly distorted tetrahedral geometry in complexes 1b and 1c with N1-Co1-N3 and Cl1-Co1-Cl2 angles of 107.68(7) and 111.70(2)°, respectively in complex 1b and N1-Co1-N3 and Cl1-Co1-Cl2 angles of 109.99(17) and 119.23(7)°, respectively in complex 1c. The metal center was linked to two chlorine atoms with bond lengths of 2.2461(6)/2.2429(6) and 2.2312(17)/2.2301(17) Å for complexes 1b and 1c, respectively, and two benzimidazole fragments with cobalt-azote bond lengths of 2.0111(17)/2.0151(19) and 2.027(4)/2.009(4) Å for complexes 1b and 1c, respectively. The aforementioned values were in close agreement with those previously reported for this type of cobalt(II) complexes [18,27,28]. The two benzimidazole rings adopt dihedral angles of 82.14 and 64.97° for complexes 1b and 1c, respectively.

3.3. Ethylene Dimerization

The five cobalt(II) complexes 1a–e were evaluated in ethylene dimerization with di-ethylaluminium chloride (AlEt₂Cl) serving as a co-catalyst (Co/Al ratio of 1/45). The reactions were carried out in toluene under 20 bars of ethylene at room temperature for one hour. All pre-catalysts demonstrated activity under these conditions, with TOFs ranging from 2500 to 3430 mol(ethylene)·mol(Co)⁻¹·h⁻¹ (

Table 2. Cobalt-catalyzed dimerization of ethylene ¹.

Entry	Complex	1-Butene (g)	TOF (mol(Ethylene)·mol(Co)−1·h−1)
1	1a	0.350 ± 0.012	2500
2	1b	0.320 ± 0.015	2285
3	1c	0.480 ± 0.015	3430
4	1d	0.460 ± 0.008	3285
5	1e	0.465 ± 0.010	3320

¹ Cobalt(II) complex (5 μmol), Et₂AlCl (0.25 mL; 0.9 M in toluene), ethylene (20 bar), 25 °C, 1 h.

). Interestingly, complexes with a benzyl substituent on the benzimidazole ring exhibited the lowest level of activity, with TOFs of 2500 and 2285 mol(ethylene)·mol(Co)⁻¹·h⁻¹ for pre-catalyst 1a and 1b, respectively (Table 2, entries 1 and 2). Similarly, the presence of fluorine atoms on the benzimidazole ligands has been shown to reduce the reactivity of the catalyst. For instance, substituting a hydrogen atom with a fluorine atom on the styryl ligand resulted in a reduction in the TOF from 3430 to 3285 mol(ethylene)·mol(Co)⁻¹·h⁻¹, complexes 1c and 1d, respectively (Table 2, entries 3 and 4). This phenomenon can be attributed to the inductive attraction effect of the fluorine atom, which resulted in a reduction in the σ-donation capacity of the fluorinated ligand. This phenomenon is substantiated by the solid-state structures of complexes 1a and 1b, wherein the Co-N bond lengths were found to be slightly extended in complexes with fluorine atoms (2.007(5)/1.997(5) Å in 1a [28] versus 2.0111(17)/20151(19) Å in 1b). A higher electron density in the metal has been demonstrated to facilitate the elementary step of β-elimination, otherwise known as a termination reaction. This phenomenon can be seen in the slightly lower activity that has been measured with fluorinated complexes.

A notable observation was the correlation between the distance between the styryl substituent and the benzimidazole ring and the observed activity. Indeed, when the styryl moieties were grafted to the benzimidazole by a methylene, cinnamyl function, a decrease in activity was observed compared to a direct positioning of the styryl on the nitrogen atom, TOFs of 3320 versus 3430 mol(ethylene)·mol(Co)⁻¹·h⁻¹ for pre-catalysts 1e and 1c, respectively (Table 2, entries 3 and 5). This phenomenon can be attributed to the cis conformation of the styryl substituent, which facilitated proximity between the aromatic ring and the metal, thereby offering partial steric protection to the catalytic site.

The ¹H NMR analysis of a dimerization test performed with pre-catalyst 1c in C₆D₆ instead of toluene shows only the formation of 1-butene; no isomerization products (cis- and trans-2-butene) are visible (Figure 6).

4. Conclusions

In the present article, we reported the synthesis of five dichloro-bis[1-alkyl/styryl-benzimidazole]-cobalt(II) complexes in which the benzimidazole ligand was substituted by a benzyl, 4-fluorobenzyl, styryl, 2-fluorostyryl or cinnamyl function and the ability of these complexes to dimerize ethylene. In the presence of Et₂AlCl as co-catalyst, the maximum activity, TOF of 3430 mol(ethylene)·mol(Co)⁻¹·h⁻¹ was observed with the pre-catalyst bearing styryl substituents on the benzimidazole rings. The rigidity of the substituents should, in part, provide steric protection to the reactive center, thereby increasing its lifetime.

Future work will focus on exploiting the selective formation of 1-butene during orthogonal or one-pot polymerization with metallocene complexes to form linear low-density polyethylene [29].