Preparation, characterization, DFT calculations and ethylene oligomerization studies of iron(II) complexes bearing 2-(1H-benzimidazol-2-yl)-phenol derivatives

Article abstract of DOI:10.1080/00958972.2018.1446527

Five 2-(1H-benzimidazol-2-yl)-phenol derivatives including 1H (HL₁), 5-chloro-(HL₂), 5-methyl-(HL₃), 5,6-dichloro-(HL₄), and 5,6-dimethyl-(HL₅) were synthesized by the reaction of their corresponding

Full text of DOI:10.1080/00958972.2018.1446527

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Preparation, characterization, DFT calculations
and ethylene oligomerization studies of iron(II)
complexes bearing 2-(1H-benzimidazol-2-yl)-
phenol derivatives
Marzieh Haghverdi, Azadeh Tadjarodi, Naeimeh Bahri-Laleh & Mehdi
Nekoomanesh Haghighi
To cite this article: Marzieh Haghverdi, Azadeh Tadjarodi, Naeimeh Bahri-Laleh & Mehdi
Nekoomanesh Haghighi (2018): Preparation, characterization, DFT calculations and ethylene
oligomerization studies of iron(II) complexes bearing 2-(1H-benzimidazol-2-yl)-phenol derivatives,
Journal of Coordination Chemistry, DOI: 10.1080/00958972.2018.1446527
To link to this article: https://doi.org/10.1080/00958972.2018.1446527
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Publisher: Taylor & Francis
Journal: Journal of Coordination Chemistry
DOI: http://doi.org/10.1080/00958972.2018.1446527
Preparation, characterization, DFT calculations and ethylene oligomerization
studies of iron(II) complexes bearing 2-(1H-benzimidazol-2-yl)-phenol
derivatives
MARZIEH HAGHVERDI†, AZADEH TADJARODI*†, NAEIMEH BAHRI-LALEH‡ and
MEHDI NEKOOMANESH HAGHIGHI‡
†Research Laboratory of Inorganic Materials Synthesis, Department of Chemistry, Iran University of Science
and Technology, Tehran, Iran
‡Polymerization Engineering Department, Iran Polymer and Petrochemical Institute (IPPI), Tehran, Iran
Five 2-(1H-benzimidazol-2-yl)-phenol derivatives including 1H (HL1), 5-chloro- (HL2),
5-methyl- (HL3), 5,6-dichloro- (HL4) and 5,6-dimethyl- (HL5) were synthesized by the reaction
of their corresponding benzene-1,2-diamine precursors and 2-hydroxybenzaldehyde which
subsequently was employed in complexation with Fe(II) to prepare complexes C1-C5,
respectively. Indeed, in all complexes, the ligands were coordinated as bidentate, via the C=N
nitrogen and hydroxy oxygen atom of benzimidazole moiety and phenol ring, respectively. The
compounds were characterized by FT-IR, UV-vis, 1H- and 13C-NMR spectropscopy, ICP and
elemental analysis (C, H and N). The purity of these compounds was determined by melting
point (m.p (and TLC. The synthesized ligands and complexes were geometrically optimized by
Gaussian09 software at B3LYP/TZVP level of theory and satisfactory theoretical-experimental
agreement was achieved for analysis of IR data of the compounds. Catalytic behavior of the
iron(II) complexes was investigated for ethylene reactivity. On activation with diethylaluminum
chloride (Et2AlCl), iron(II) complex (C4) showed the highest activity (1686 kg
oligomers.mol-1(Fe).h-1) for ethylene oligomerization when it contains chlorine substituents and
exhibits good selectivity for linear 1-butene. The steric and electronic effects of ligands were
investigated in detail on the influence of their catalytic activities.
Keywords: Benzimidazolyl-phenols; Iron(II) complexes; DFT Calculations; Ethylene
oligomerization; Substituent effects
*Corresponding author. Email: tajarodi@iust.ac.ir

1. Introduction
Ethylene oligomerization is a major industrial process for producing linear α-olefins, about six
millions of tons productivities annually, which provide basic feedstocks in the preparation of
detergents, synthetic lubricants, plasticizers, alcohols and comonomers used for branched
polyolefins [1]. The iron and cobalt catalysts showed unique properties for high selectivity of
vinyl type oligomers and polyolefins (polyolefin waxes) produced, which was initially found by
research groups of Brookhart and Gibson using bis(imino)pyridyl iron and cobalt complexes
[2-4]. In the recent decade, the intensive researches have explored numerous model complexes of
late-transition metals as catalysts toward ethylene reactivity [5, 6]. However, a few models of
iron catalysts showed good catalytic activities toward ethylene oligomerization and
polymerization in comparison with various models of late-transition metals complexes.
Nevertheless, there are some significant advantages for iron catalysts, such as high catalytic
activity, high α-olefin selectivity in ethylene oligomerization, and lower toxicity than other
metals [7]. Therefore, relative research works have been focused on new models of iron
complexes are still highly attractive, and the comprehensive review papers are reported.
Alternative models of iron complex catalysts bearing bi- and tri-dentate ligands for ethylene
reactivity have been intensively explored [8-11]. There are reports about the use of
benzimidazolyl-phenol and its derivatives in the role of bidentate ligands with different transition
metals. Given the easy preparation, good stability and rich coordination chemistry of these
ligands, the synthesis of their transition metal complexes has been deeply explored [12]. Metal
complexes with these ligands are important because of their high thermal stability, good catalytic
performance and superior optical properties [13].
Herein, we discuss our investigations on the synthesis and characterization of Fe(II)
complexes having 2-(1H-benzimidazol-2-yl)-phenol derivatives as ligands. The optimized
geometry and vibrational frequency calculations of these compounds were derived from
theoretical calculation in B3LYP/TZVP level on Gaussian09 program. Also, the catalytic
behavior of Fe(II) complexes in ethylene reactivity was investigated. They showed good
catalytic activities toward ethylene oligomerization in the presence of diethylaluminum chloride
(Et2AlCl). The relationship between the structures of the complexes, their catalytic activities and
product properties are discussed in detail.

2. Experimental
2.1. General methods and materials
All manipulations of air- and moisture-sensitive compounds were carried out under nitrogen
using standard Schlenk techniques. Toluene was refluxed over sodium-benzophenone and
distilled under nitrogen prior to use. Diethylaluminum chloride (Et2AlCl) was purchased from
Aldrich Chemicals. All other chemicals were obtained commercially and used without
purification. Elemental analyses were performed on a Perkin Elmer model 2400 series 2 (made
in USA) automatic carbon, hydrogen, nitrogen analyzer. Determination of metal percent was
performed using ICP (3410ARL model made in Swiss). NMR spectra (300 MHz for 1H and
75 MHz for 13C) were measured with a Bruker AVANCE-300 instrument (made in Germany) in
dimethylsulfoxide (DMSO-d6) using tetramethylsilane (TMS) as internal standard. IR spectra
were recorded in KBr disks on a Shimadzu FT-IR 8400 spectrometer (made in Japan) from 400-
4000 cm-1. UV-vis absorption spectra were recorded using a Shimadzu, UV-1700
spectrophotometer (made in Japan) in DMSO solution. GC analyses were performed with a
Varian CP-3800 gas chromatograph equipped with a flame ionization detector and a 100 m
(0.25 mm i.d., 0.5 µm film thickness) CP-Sil PONA CB. The yield of oligomers was calculated
by referencing with the mass of the solvent on the basis of the prerequisite that the mass of each
fraction was approximately proportional to its integrated areas in the GC trace. Selectivity for
linear 1-alkenes was defined as (amount of linear 1-alkenes of all fractions)/(total amount of
oligomer products) in percentage. Melting points were determined using an electrothermal
melting point apparatus.
2.2. Synthesis of the ligands: General procedure
The ligands (five 2-(1H-benzimidazol-2-yl)-phenol derivatives including 1H- (HL1), 5-chloro-
(HL2), 5-methyl- (HL3), 5,6-dichloro- (HL4) and 5,6-dimethyl- (HL5)) were prepared according
to our previous literature procedures [14]. For instance, 2-(5,6-dichloro-1H-benzimidazol-2-yl)-
phenol (HL4) was synthesized by reacting 2-hydroxybenzaldehyde (1.83 g, 15.0 mmol) with an
equivalent amount of sodium metabisulfide (1.60 g, 8.5 mmol) at room temperature in EtOH
(20 mL) for several minutes. The reaction mixture was stirred vigorously and 10 mL ethanol was
added. The mixture was kept in a refrigerator overnight. The mixture was treated with

4,5-dichlorobenzene-1,2-diamine (0.71 g, 4.0 mmol) in 10 mL dimethylformamide (DMF) and
was gently refluxed for 4 h. The completion of reaction was checked by TLC. Then the reaction
mixture was poured into 100 mL icy water, filtered and crystallized from ethanol. This method
was selected because of its ease, high yield and short synthesis time. This obtained ligand was
used for the synthesis of complex C4. Molecular structure of the synthesized ligands is given in
scheme 1.
2.3. Synthesis of complexes C1-C5
Complexes C1-C5 were synthesized by the reaction of FeCl2·4H2O (0.049 g, 0.25 mmol) with
the corresponding ligands and KOH (0.028 g, 0.5 mmol) in methanol. A typical synthetic
procedure for C1 can be described as follows: the ligand HL1 (0.106 g, 0.5 mmol), KOH
(0.028 g, 0.5 mmol) and FeCl2·4H2O (0.049 g, 0.25 mmol) were added to a Schlenk tube,
followed by the addition of freshly distilled methanol (15 mL) with rapid stirring at room
temperature. The color of solution changed immediately. The completion of reaction was
monitored by TLC. The reaction mixture was stirred for 24 h, and then the precipitate was
washed with diethylether several times and dried to give the pure product as a brown-orange
powder in 69% yield. FT-IR (KBr disc, cm-1), 3415 v(N-H), 3184 v(C-H), 1623 v(C=N), 1531
v(C=C), 1259 v(C-O), 472, 549 v(Fe-O), 426 v(Fe-N). Anal. Calcd. for C26H26N4O6K2Cl2Fe: C,
45.00; H, 3.74; N, 8.05; Fe, 8.03. Found: C, 45.09; H, 4.02; N, 8.47; Fe, 7.86 %. UV-vis
(DMSO, nm), 510.
Data for C2 are as follows: Brown-orange powder, yield: 51%, FT-IR (KBr disc, cm-1),
3417 v(N-H), 3174 v(C-H), 1628 v(C=N), 1558 v(C=C), 1257 v(C-O), 470, 534 v(Fe-O), 424
v(Fe-N). Anal. Calcd. for C26H18N4O3K2Cl4Fe: C, 43.95; H, 2.53; N, 7.88; Fe, 7.86. Found: C,
43.87; H, 2.43; N, 7.52; Fe, 7.90 %. UV-vis (DMSO, nm), 480.
Data for C3 are as follows: Brown-orange powder, yield: 59%, FT-IR (KBr disc, cm-1),
3421 v(N-H), 3058 v(C-H), 1631 v(C=N), 1558 v(C=C), 1261 v(C-O), 486, 578 v(Fe-O), 418
v(Fe-N). Anal. Calcd. for C28H22N4O2K2Cl2Fe: C, 51.62; H, 3.38; N, 8.60; Fe, 8.57. Found: C,
51.24; H, 3.00; N, 8.86; Fe, 8.87 %. UV-vis (DMSO, nm), 500.
Data for C4 are as follows: Black grey powder, yield: 57%, FT-IR (KBr disc, cm-1), 3419
v(N-H), 3112 v(C-H), 1623 v(C=N), 1558 v(C=C), 1257 v(C-O), 461, 520 v(Fe-O), 426 v(Fe-N).

Anal. Calcd. for C26H14N4O2Cl4Fe: C, 50.99; H, 2.28; N, 9.15; Fe, 9.12. Found: C, 50.33; H,
2.06; N, 8.96; Fe, 9.82 %. UV-vis (DMSO, nm), 510.
Data for C5 are as follows: Black grey powder, yield: 50%, FT-IR (KBr disc, cm-1), 3406
v(N-H), 3058 v(C-H), 1631 v(C=N), 1560 v(C=C), 1259 v(C-O), 436, 516 v(Fe-O), 409 v(Fe-N).
Anal. Calcd. for C32H34N4O4K2Cl2Fe: C, 51.69; H, 4.57; N, 7.53; Fe, 7.51. Found: C, 51.17; H,
4.02; N, 7.72; Fe, 7.15 %. UV-vis (DMSO, nm), 504.
2.4. Computational details
All structures have been optimized with density functional theory (DFT) method using the
B3LYP (hybrid GGA functional of Becke-Lee, Parr, and Yang [15, 16]) functional and triple-ζ
valence TZVP [17] basis set, since as reported before, better agreement with the experimental
results can be obtained with B3LYP and it is a widely used functional in the molecular
simulation of transition metal catalytic systems [18-22]. All the calculations have been carried
out with the Gaussian09 package [23].
For all the computed complexes, the quintet, containing four unpaired electrons, was the
electronic ground state and thus open shell calculations were performed [23]. In order to ensure
that the optimized geometries represent the global minima and that there are only positive eigen-
values, the vibrational frequency calculations were performed [24, 25]. The optimized geometry
of compounds furnished the total energy E.
The energy values obtained by using the DFT calculations are: HL1 (-686), HL2 (-1146),
HL3 (-726), HL4 (-1606), HL5 (-765), C1 (-3556), C2 (-4475), C3 (-3635), C4 (-4474) and C5
(-3713). Values inside parentheses are in a.u.
2.5. General procedure for ethylene oligomerization
Ethylene oligomerization was done in a 1-L autoclave stainless steel reactor equipped with a
mechanical stirrer and a temperature controller. At first, reactor charged with toluene, the desired
amount of cocatalyst, and toluene solution of catalytic precursor under a nitrogen atmosphere,
the total volume was 110 mL. At the reaction temperature, the reactor was sealed and pressurized
to 20 atm of ethylene pressure, and the ethylene pressure was kept with feeding of ethylene.
After the reaction was carried out for the required time, the pressure was released and a small
amount of the reaction solution was collected and then analyzed by gas chromatography (GC) for

determining the composition and mass distribution of oligomers obtained. Then the residual
reaction solution was quenched with 5% hydrochloric acid ethanol in order to collect
polyethylene obtained. However, the polyethylene was not formed.
3. Results and discussion
1H-Benzimidazol-2-yl-phenol derivatives (HL1-HL5) were prepared according to our reported
procedures [14]. The iron(II) complexes were synthesized by treating the corresponding ligand
with the methanol solution of FeCl2·4H2O. The color of iron(II) complexes was brown-orange or
black grey and also, they had good yields (50-69%) with high purity. These complexes are stable
in both solution and solid state. The analytical data and physical properties of the ligands and
their complexes are summarized in table S1. The formation of complexes was further supported
by the results of FT-IR and UV-vis spectra, ICP and elemental analysis (C, H and N). Elemental
analysis showed the general formula for complexes C1 and C2 were as K2[Fe(L1)2Cl2]∙4H2O and
K2[Fe(L2)2Cl2]∙H2O, respectively, while in complexes C3, C4 and C5, it was obtained as
K2[Fe(L3)2Cl2], [Fe(L4)2] and K2[Fe(L5)2Cl2]∙2(CH3OH), respectively. During the procedure, it
was difficult to achieve the single-crystals of all complexes, although the complexes were
prepared in several different solvents. Besides, the few obtained crystals were not suitable for
X-ray diffraction, as well. Therefore, it was decided to determine the molecular structures of
complexes by using DFT calculations. Theoretical calculations involving geometry optimization,
vibrational frequencies, stability of the compounds, as well as the various bond lengths and
angles generated from the optimized structure was also performed on the ligands and complexes
which will be discussed further in the following sections.
3.1. Geometrical parameters of the ligands and complexes
The optimized geometry of HL5 is shown in figure 1 as a representative structure. The various
bond lengths and angles generated from the structures of the optimized ligand (HL5) and
complex (C5) are obtained. They showed good similarity with the X-ray structural investigations
of analogous ligands and complexes reported [26-29]. The computed bond lengths for HL5
O(1)-C(6), O(1)-H(24), N(3)-C(7), N(3)-C(9), N(8)-C(7), N(8)-C(2), are 1.34, 0.99, 1.38, 1.34,
1.32 and 1.38 Å, respectively. The computed bond angles, C(6)-O(1)-H(24), C(7)-N(3)-C(9),
C(7)-N(8)-C(2) and N(8)-C(7)-N(3) in HL5 are 108.47°, 107.70°, 106.49°, 111.20°, respectively.

The computed bond lengths Fe(49)-O(1), Fe(49)-O(7), Fe(49)-N(8), Fe(49)-N(31) (30), Fe(49)-
Cl(47) and Fe(49)-Cl(48) in complex C5 are 2.11, 2.11, 2.23, 2.23, 2.62 and 2.60 Å,
respectively. The computed bond angles in complex C5, such as N(8)-Fe(49)-O(1) (=97.95°),
N(8)-Fe(49)-O(7) (=82.11°), N(8)-Fe(49)-Cl(47) (=88.65°), N(8)-Fe(49)-Cl(48) (=91.33°),
N(8)-Fe(49)-N(31) (=177.32°), N(31)-Fe(49)-O(1) (=82.11°), N(31)-Fe(49)-O(7) (=97.94°),
N(31)-Fe(49)-Cl(47) (=88.66°), N(31)-Fe(49)-Cl(48) (=91.33°), O(1)-Fe(49)-O(7) (=177.27°),
O(1)-Fe(49)-Cl(47) (=91.36°), O(1)-Fe(49)-Cl(48) (=88.63°), O(7)-Fe(49)-Cl(47) (=91.35°),
O(7)-Fe(49)-Cl(48) (=88.63°) and Cl(47)-Fe(49)-Cl(48) (=179.99°) suggest octahedral structure.
The optimized structure of C5 is shown in figure 2.
3.2. FT-IR spectroscopy of the ligands and complexes
Vibrational properties of the ligands and complexes have been done by using the DFT
calculations and FT-IR instrument. The striking bands in the FT-IR spectra of the complexes are
given in table S2. As an example, the measured and calculated FT-IR spectra of the HL5 ligand
and its related complex, i.e. C5, are compared in figures 3(a,b) and 4(a,b), respectively. In our
earlier work [14], vibration frequencies of the ligands HL1-5 were noted and were in agreement
with the theoretical calculations. Considering their FT-IR spectra, the vibrational modes of N-H
(~1479 cm-1), C=C (~1558 cm-1), C=N (~1631 cm-1) and C-O (~1259 cm-1) bonds in C1-C5
shifted to lower wavenumbers [30], in comparison with their corresponding free ligands HL1-5
(푣(N–H): ~1490 cm-1, 푣(C=C): ~1591 cm-1, 푣(C=N): ~1637 cm-1 [31] and 푣(C-O): ~1298 cm-1).
The vibrational frequency of N-H, C=C, C=N, C-O in predicted theoretical spectra of the
complexes were calculated at ~1480 cm-1, ~1552, ~1633 and ~1255 cm-1, respectively. In the
FT-IR spectra of complexes C1-C5, the imine nitrogen coordination could also be confirmed by
appearance of a weak band located at the low wavenumber of (~426 cm-1) which may belong to
(Fe-N) [32, 33]. In high wavenumber region, bands at 2854-3184 cm-1 are attributed to the
stretching vibrations of aromatic C-H for complexes C1-C5 [34, 35], and the corresponding peak
in theoretical spectra were calculated at the range of 3001-3163 cm-1. The sharp or medium
bands in the complexes in the range of ~750-968 cm–1 are due to the out-of-plane deformation
bands for the aromatic C-H [36]. In complexes C1-C5, deprotonation and subsequent
involvement of the phenoxyl group in metal coordination could also be supported by the

appearance of new peaks at a lower wavenumber of (~461, ~549 cm-1) region assignable to
(Fe-O). These results are in accord with the theoretical calculations.
The experimental and calculated FT-IR spectra of ligands and their complexes showed
good agreement in terms of their peak frequencies, band intensities, and the shapes of the bands.
3.3. Electronic absorption spectra of the complexes
The UV-vis spectra were recorded from 200 to 800 nm in DMSO. UV-vis spectral data of HL1-5
ligands were noted in our previous work [14]. The observed bands at lower wavelength bands
(210-300 nm) correspond to π→π* transitions of the aromatic rings. The bands in the range of
310-360 nm are due to n→π* transitions [31]. The electronic spectra of complexes C1-C5 show
a broad band at 510, 480, 500 and 504 nm, respectively, which may be assigned to
5T2g(D)→5Eg(D) transition. They are characteristic for high-spin octahedral geometry for C1-C3
and C5. In the electronic spectrum of C4, there is a band at 510 nm that may be due to the
5E(D)→5T2(D) transition [37, 38]. This characteristic is related to distorted tetrahedral geometry.
3.4. NMR spectra of the ligands
1H-NMR spectra data of the ligands and the assignments of the peaks were reported in our
previous work [14]. For example, the 1H NMR spectra of HL3, HL4 and HL5 ligands showed
only a single broad band for the OH and the amine NH protons (13.14, 12.5 and 13.11 ppm for
HL3, HL4 and HL5, respectively). This combination is because of intramolecular hydrogen
bonding between the O–H hydrogen and the C=N nitrogen atoms [39]. The ligands HL1 and HL2
gave two singlets for OH and NH protons (13.07 and 9.44 ppm for HL1, 13.26 and 12.76 ppm for
HL2, respectively). In the 1H-NMR spectra of HL3 (including one methyl group) and HL5
(including two methyl groups), a singlet was observed at 2.42 and 2.32 ppm, respectively.
Position of protons and the assignments of the peaks in benzimidazole moiety and phenolic ring
moiety in ligands HL1-5 are various, due to the different functional groups in the benzimidazole
ring. As our previous report, the chemical shift values of H1, H3 and H4 protons of HL2 (having
one chloro-substituent at the 5-position on the benzimidazole ring) were at 7.64, 8.04 and
7.37 ppm, and of HL4 ligand (having two chloro-substituents at the 5,6-positions on the
benzimidazole ring) were at 7.87, 8.05 and 7.38 ppm, respectively. Also, the chemical shift
values of H1, H3 and H4 protons of HL3 ligand (having one methyl substituent at the 5-position

on the benzimidazole ring) were at 7.43, 8.03 and 7.35 ppm, and in the HL5 ligand (having two
methyl substituents at the 5, 6-positions on the benzimidazole ring) were 7.40, 8.00 and
7.33 ppm, respectively. It is known that the chloro groups, electron withdrawing, on aromatic
ring move the resonance of the H1, H3 and H4 protons downfield, while the methyl groups,
electron donating, on ring move the resonance of these protons upfield. 1H-NMR spectra of the
ligands are shown in figure S1.
As already mentioned, in the 13C-NMR spectra of the synthesized ligands HL1-5
(figure S2), there are signals around 157.71-158.98 and 150.83-153.77 ppm for carbon atoms
bonded to the hydroxyl oxygen atom and imidazole C=N carbon atom, respectively. In the
13C-NMR spectra of HL1-5, the observed signal around 132 ppm is assigned to C8 and C13
carbon atoms. Also the carbon atoms bonded to the chloro- and/or methyl- groups appear in the
range of 125-133 ppm. The other signals belong to the benzimidazole benzene and phenol rings
carbon atoms.
In our earlier work, 1H- and 13C-NMR chemical shift calculations of HL1-5 ligands were
performed using the same functional and basis set as used for geometry optimizations. The
results showed extreme agreement between experimental and computed chemical shifts (table 1).
3.5. Theoretical calculation and stability
According to the DFT calculations, Fe(II) complexes C1 (-3556 a.u.), C2 (-4475 a.u.), C3
(-3635 a.u.), C4 (-4474 a.u.) and C5 (-3713 a.u.) have lower energy values than HL1-5 ligands
(HL1: -686 a.u., HL2: -1146 a.u., HL3: -726 a.u., HL4: -1606 a.u., HL5: -765 a.u.). These numbers
prove that ligands and the complexes containing the chloro- derivatives have higher stability than
the methyl- derivatives when compared to each other. According to the DFT calculations, HL1,
as a ligand, and C1, as a complex, have the lowest stability among the ligands and the
complexes, the synthesized compounds, respectively. Also, it is observed that C2, with one
chloro-substituent, is slightly more stable (total energy: -4475 a.u.) than C4, containing two
chloro-substituents (-4474 a.u.) (figure 5) and this lower stability of C4 (despite having two
chloro atoms) is due to the absence of chloro atoms attached to Fe atom. According to the
geometry optimization of the structure of C4 with DFT calculations, Fe is located on a distorted
tetrahedral center with the imine nitrogen and hydroxy oxygen atoms of benzimidazole moiety
and phenol ring, respectively, and for C1-C3 and C5 structures Fe is located on a distorted

octahedral center with one imine nitrogen and the phenolate oxygen atom along with two
chloride ions (molecular symmetry: D2h for C1-C3, C5 and C2v for C4 complexes).
3.6. Ethylene oligomerization
All the synthesized iron(II) complexes, when activated with diethylaluminum chloride (Et2AlCl),
show high catalytic activities for ethylene oligomerization with high selectivity for 1-butenes at
20 atm of ethylene pressure. Also, the distribution of oligomers does not follow Schulz-Flory
rules. No oligomer with odd carbon number was found in the GC analysis [40]. The detailed
results are summarized in table 2.
3.6.1. Ethylene oligomerization by iron(II) complexes C1-C5. It should be pointed out that the
selectivity for linear α-olefins of oligomers was varied with the ligand environments. However,
no unambiguous trend could be identified. The oligomers are only obtained and measured by
GC. Iron(II) complexes were initially studied for their catalytic activities in ethylene
oligomerization at 20 atm of ethylene pressure. The effects of the ligand environment were
studied in the ethylene reactivity. All the synthesized iron(II) complexes, when activated with
Et2AlCl, display modest-to-high catalytic activities for ethylene oligomerization with high
selectivity for α-olefins. Apart from C1 complex (R1 = R2 = H), which generated dimer and
trimer oligomeric products, C2-C5 complexes generated only dimer as main oligomeric product.
The detailed results are summarized in table 2. The variation of the R-substituent on the
benzimidazole resulted in changes of the catalytic performance. Among iron catalysts, complex
C1 (R1 = R2 = H) showed the lowest catalytic activities and C4 complex (R1 = R2 = Cl) showed
the highest activity among their analogues. According to table 2 (entries 1-5), the
oligomerization activities decreased in the order C4 > C2 > C3 > C5 > C1. Comparing the
catalysts activity, it can be found that complexes C2 and C4 containing electron-withdrawing
halogen groups showed high catalytic activity. On the contrary, relatively lower productivity was
observed with the complexes bearing electron-donating alkyl groups such as complexes C3 and
C5, despite their good selectivity toward 1-butene. It can be concluded that the catalytic
activities ascend with an increase in the number of Cl atoms in the ligand, probably ascribed to
the electron-withdrawing nature of chlorine atom, which leads to increase of the electrophilic
iron center and decrease of the electron density in the benzimidazole rings [41] as well as leads

to increased selectivity for α-olefins. It can be concluded that the d orbital of iron is almost
empty and therefore it has strong bond with the ligand, which causes little space for ethylene
coordination, and the ethylene monomer does not reach the active center well and the activity is
reduced. For this reason, when electron-withdrawing Cl groups are placed on the ligand, it
causes the electron density to drop around the metal and the ethylene monomer interacts easily
with the active center and increases its activity. In versus, the methyl groups at the
benzimidazole ring of complexes C3 and C5 may prevent the access of ethylene to the active
center in the catalytic system, therefore resulting in the decrease of catalytic activity. Also,
activity decrease can probably be attributed to the increasing nucleophilicity of the metal center
with higher number of methyl groups on the phenyl ring, which weakens the interaction between
the iron atom and the π-electrons of ethylene monomer and decreases the rate of ethylene
insertion in the chain-growth steps [26]. Furthermore, incorporating an electron-donating methyl
group on the benzimidazole rings might push more electrons to the iron atom, reduce the net
charge on the iron center, and therefore, resulted in lower catalytic activity for ethylene
oligomerization [42]. It is a general notion that decrease in the donor ability of the substituents,
(i.e. increase in the electrophilicity of the metal center) should decrease the insertion barrier,
thus, resulting in higher activity [43].
Briefly, iron catalysts generally produce ethylene dimer and have high selectivity for
1-butene, in other words, with higher catalytic activity, the content of C4 is obviously increased,
because β-hydrogen elimination was faster than ethylene propagation and the distribution of
obtained oligomers did not resemble Schulz-Flory rules.
4. Conclusion
Fe(II) complexes containing 2-(2-hydroxyaryl)-1H-benzimidazole derivatives were synthesized
and characterized. In conclusion, the structures of the compounds were verified by FT-IR, 1H-,
13C-NMR and UV-vis spectroscopy, ICP and elemental analysis. According to the DFT
calculations, in all complexes, the ligands are coordinated bidentately via one imine nitrogen and
the phenolate oxygen atoms. Furthermore, in all iron(II) complexes, the structure with four
unpaired electrons had the minimum energy among other electronic structures which shows that
the complexes were basically in high-spin mode. All the analytical and spectral data are
consistent with theoretical calculations. These complexes could be easily activated and exhibited

good activities for ethylene oligomerization in the presence of Et2AlCl as cocatalyst, even though
the amount of cocatalyst was very small and the main products were 1-butenes of ethylene.
Supplementary material
Cartesian coordinates of all the species discussed in the text to together with spectral data for the
synthesized ligands are available free of charge via the Internet at the publisher’s website.
Acknowledgement
This work was supported by Iran University of Science and Technology, Iran Polymer and
Petrochemical Institute and Petrochemical Research & Technology Company. Naeimeh Bahri-
Laleh thanks MOLNAC (www.molnac.unisa.it) for its computer facilities.
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Figure captions
Figure 1. Optimized structure of ligand HL5 at B3LYP/TZVP level of theory.
Figure 2. Optimized structure of complex C5 at B3LYP/TZVP level of theory.
Figure 3. Comparison of (a) experimental FT-IR and (b) theoretical FT-IR calculated at
B3LYP/TZVP level, spectra for HL5.
Figure 4. Comparison of (a) experimental FT-IR and (b) theoretical FT-IR calculated at
B3LYP/TZVP level, spectra for complex C5.
Figure 5. Optimized structure of complex C4 at B3.

Graphical abstract

Table 1. Theoretical assignment of 1H-NMR and 13C-NMR data for the ligands optimized by B3LYP/TZVP level: the
chemical shift values (δH, ppm).
Proton
1H-NMR
1H-NMR
Carbon
13C-NMR
13C-NMR
Ligands
HL1
assignment
(Experimental)
(Theoretical)
assignment
(Experimental)
(Theoretical)
NH
OH
NH
OH
9.44
8.45
C=N
151.35
158.98
152.85
157.87
126.59
151.40
157.98
131.54
21.29
154.20
167.80
156.00
168.00
143.00
154.00
167.00
137.00
21.00
13.07
12.76
13.26
13.20
8.40
C-OH
C=N
HL2
HL3
12.90
C-OH
C-Cl
NH
OH
CH3
13.14
13.14
2.42
8.35
13.20
2.50
C=N
C-OH
C-CH3
-CH3
C=N
HL4
HL5
NH
OH
12.50
12.50
8.25
153.77
157.71
124.99
150.83
157.92
131.31
19.96
156.00
167.70
140.50
153.80
167.00
136.00
20.10
12.62
C-OH
C-Cl
NH
OH
CH3
13.11
13.11
2.32
8.30
13.20
2.35
C=N
C-OH
C-CH3
-CH3

Table 2. Results of ethylene oligomerization with C1–C5/ Et2AlCla.
Oligomer distributionc (%)
C4 C6 α-C4 α-C6
93.11 6.89 91.91 100
Entry
Precatalyst
Yield (g)
Activityb
1
2
3
4
5
C1
C2
C3
C4
C5
1.72
8.32
8.20
8.43
8.13
344
1664
1640
1686
1626
100
100
100
100
-
-
-
-
62.16
100
-
-
-
-
100
84.21
a General conditions: 10 µmol precatalyst, 100 mL toluene as solvent, [Al]/[Fe] = 200,
20 atm of ethylene, reaction time 0.5 h, reaction temp. 30 C. b In units of kg
oligomers.mol-1(Fe).h-1. c Determined by GC. ΣC denotes the total amounts of
oligomers.