(Phenoxyimidazolyl-salicylaldimine)iron complexes: Synthesis, properties and iron catalysed ethylene reactions

Article abstract of DOI:10.1039/c4dt01886a

The reaction of 2-{[2-(1H-imidazol-4-yl)-ethylimino]-methyl}-phenol (L1), 2,4-di-tert-butyl-6-{[2-(1H-imidazol-4-yl)-ethylimino]-methyl}-phenol (L2) or 4-tert-butyl-2-{[2-(1H-imidazol-4-yl)-ethylimino]-methyl}-phenol (L3) with iron(ii) precursors produced either iron(ii) or iron(iii) complexes, depending on the nature of the anions in the iron(ii) precursor and the ligand. When the anion is chloride and the ligand L1, the product is [(L1)₂Fe] [FeCl₄] (1), but when the anion is triflate (OTf^-) and the ligand is L2, the product is [(L2)₂Fe][OTf]₂ (2). With iron(ii) halides and tert-butyl groups on the phenoxy ligands L2 and L3, the iron(iii) complexes [(L2)FeX₂] {where X = Cl (3), Br (4) and I = (5)} and [(L3)FeCl₂] (6) were formed. Complexes 1-6 were characterised by a combination of elemental analyses, IR spectroscopy and mass spectrometry; and in selected cases (3 and 4) by single crystal X-ray crystallography. The crystal structures of 3 and 4 indicated that the iron(ii) precursors oxidised to iron(iii) in forming complexes 3-6; an observation that was corroborated by the magnetic properties and the ⁵⁷Fe Moessbauer spectra of 3 and 4. The iron(iii) complexes 3-6 were used as pre-catalysts for the oligomerisation and polymerisation of ethylene. Products of these ethylene reactions depended on the solvent used. In toluene ethylene oligomerised mainly to 1-butene and was followed by the 1-butene alkylating the solvent to form butyl-toluenes via a Friedel-Crafts alkylation reaction. In chlorobenzene, ethylene oligomerised mainly to a mixture of C₄-C₁₂ alkenes. Interestingly small amounts of butyl-chlorobenzenes and hexyl-chlorobenzenes were also formed via a Friedel-Crafts alkylation with butenes and hexenes from the oligomerisation of ethylene.

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(Phenoxyimidazolyl-salicylaldimine)iron
complexes: synthesis, properties and iron
Cite this: DOI: 10.1039/c4dt01886a
catalysed ethylene reactions†
Margaret Yankey,a Collins Obuah,a Ilia A. Guzei,a,b Emmanuel Osei-Twum,c
Giovanni Hearne*d and James Darkwa*a
The reaction of 2-{[2-(1H-imidazol-4-yl)-ethylimino]-methyl}-phenol (L1), 2,4-di-tert-butyl-6-{[2-(1H-
imidazol-4-yl)-ethylimino]-methyl}-phenol (L2) or 4-tert-butyl-2-{[2-(1H-imidazol-4-yl)-ethylimino]-
methyl}-phenol (L3) with iron(II) precursors produced either iron(II) or iron(III) complexes, depending on
the nature of the anions in the iron(II) precursor and the ligand. When the anion is chloride and the ligand
L1, the product is [(L1)2Fe][FeCl4] (1), but when the anion is triflate (OTf −) and the ligand is L2, the product
is [(L2)2Fe][OTf]2 (2). With iron(II) halides and tert-butyl groups on the phenoxy ligands L2 and L3, the iron(III)
complexes [(L2)FeX2] {where X = Cl (3), Br (4) and I = (5)} and [(L3)FeCl2] (6) were formed. Complexes
1–6 were characterised by a combination of elemental analyses, IR spectroscopy and mass spectrometry;
and in selected cases (3 and 4) by single crystal X-ray crystallography. The crystal structures of 3 and 4
indicated that the iron(II) precursors oxidised to iron(III) in forming complexes 3–6; an observation that
was corroborated by the magnetic properties and the 57Fe Mössbauer spectra of 3 and 4. The iron(III)
complexes 3–6 were used as pre-catalysts for the oligomerisation and polymerisation of ethylene.
Products of these ethylene reactions depended on the solvent used. In toluene ethylene oligomerised
mainly to 1-butene and was followed by the 1-butene alkylating the solvent to form butyl-toluenes via a
Friedel–Crafts alkylation reaction. In chlorobenzene, ethylene oligomerised mainly to a mixture of C4–C12
alkenes. Interestingly small amounts of butyl-chlorobenzenes and hexyl-chlorobenzenes were also
formed via a Friedel–Crafts alkylation with butenes and hexenes from the oligomerisation of ethylene.
Received 23rd June 2014,
Accepted 23rd July 2014
DOI: 10.1039/c4dt01886a
www.rsc.org/dalton
polymerisation reactions leading to highly linear products.
Depending on the substituents on the bis(arylimino)pyridine
Introduction
Since the pioneering work by Brookhart and co-workers on ligand, these iron and cobalt complexes are excellent cata-
α-diimines nickel(II) and palladium(II) complexes as catalysts lysts for ethylene oligomerisation reactions that have high
for the oligomerisation and polymerisation of ethylene and activity and selectivity for making α-olefins, but when poly-
other α-olefins,1,2 there has been renewed focus on late tran- merisation occurs the products are mainly linear high density
sition metals as ethylene transformation catalysts as indicated polyethylene.4b,5a,c
by several reports in this area.3 Development of iron and
Iron as a metal for producing catalysts is gaining a lot of
cobalt catalysts in particular was sparked by the independent attention in various organic transformations due to its earth
reports4–6 of Gibson and Brookhart on bis(arylimino)pyridine- abundance, low toxicity and low cost. With N^N^N bis(aryl-
iron and cobalt as catalysts for ethylene oligomerisation and imino)pyridine ligands, iron(II) form complexes that catalyse
ethylene oligomerisation.7 These (N^N^N)iron(II) catalysts have
been modified through variations of substituents on the
aDepartment of Chemistry, University of Johannesburg, Auckland Park Kingsway
ligand backbone which have in several instances improved the
Campus, Auckland Park, 2006, South Africa. E-mail: jdarkwa@uj.ac.za
activity of these catalysts.8–10 However, unlike the α-diimine
bDepartment of Chemistry, University of Wisconsin-Madison, 1101 University Avenue,
nickel and palladium catalysts11 the active species from bis-
Madison, WI 53706, USA
cSaudi Basic Industries Corporation, P.O. Box 42503, Riyadh, 11551, Saudi Arabia
(arylimino)pyridineiron pre-catalysts are not well understood.
EPR and Mössbauer studies have revealed that a bis(arylimino)-
dDepartment of Physics, University of Johannesburg, Auckland Park Kingsway
Campus, Auckland Park, 2006, South Africa
pyridineiron(II) pre-catalyst gets oxidised to iron(III) species
†Electronic supplementary information (ESI) available. CCDC 998101 (3),
upon activation with MAO as co-catalyst.12 In fact iron(II) and
998102 (3, Fig. S2) and 998103 (4). For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c4dt01886a
iron(III) pre-catalysts of the same bis(arylimino)pyridine ligand
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have been reported to show similar catalytic activities for the product (Scheme 2). To test this hypothesis, L2 was reacted
production of polyethylene.5b This similarity in the activities with the iron(II) chloride in the presence of a large excess of
suggests that the same active species are involved in reactions Et3N. This led to the isolation of the proposed intermediate
that use iron(II) and iron(III) pre-catalysts, although the exact [(L2)FeCl] (7), confirming that all the iron(III) complexes, 3–6,
nature of these species is unknown.5b,13 Other reports have were formed via the intermediate in Scheme 2. The acidic phe-
shown that bis(arylimino)pyridineiron(III) complexes are good nolic-OH was deprotonated by the Et3N and formed an
catalysts for the oligomerisation of ethylene to C4–C12 linear ammonium salt with the chloride, thereby preventing any oxi-
α-olefins.14
dation of the iron(II) to iron(III) by HCl formed from the depro-
The above narration suggests that imine containing ligands tonation of the phenolic proton. Gibson and co-workers have
produce excellent catalysts with iron. One such ligand type is prepared similar iron(II) complexes as catalysts in ATRP.16a
salicylaldimine which has been used in making excellent tita- Reacting iron(III) chloride with L2 under the same reaction
nium and zirconium ethylene polymerisation catalysts.15 conditions used to prepare 3 gave a compound that analysed
However, there are very few salicylaldimine-iron(II) or iron(III)16 exactly as complex 3 (Fig. S2†).
catalysts. The few that are reported are used in atom transfer
Complexes 2, 4, 5 and 7 were purified by recrystallisation
radical polymerisation (ATRP).16a We have, therefore, investi- from toluene and hexane, while 1, 3 and 6 were purified by
gated O^N^N salicylaldimineiron(III) complexes as catalysts for recrystallisation from dichloromethane and hexane. All the
ethylene oligomerisation and polymerisation reactions. The products were isolated in moderate to high yields (44–73%) as
products formed from the ethylene reaction catalysed by these dark blue solids except complexes 5 and 7 which were isolated
iron(III) catalyst are highly dependent on the solvent in which as dark green solids.
the reaction is run.
The infrared spectra of the complexes showed ν(CvN)
peaks in the range of 1599–1619 cm−1 (Fig. S3–S9†) which are
lower and weaker in intensity compared to the peaks in the
ligands in the range 1630–1631 cm−1. These observations
support the presence of imine groups in the iron complexes
and also the binding of the ligands to the iron centres.17 The
nature of complexes 3–6 was first established from the single
crystal X-ray structures of 3 and 4 as iron(III) halide complexes
(Fig. 1 and 2). Microanalyses were subsequently used to
confirm that these complexes indeed have two halides bound
to the iron metal centre and were pure. These observations,
therefore, are enough evidence that 3–6 were formed via the
oxidation of iron(II) intermediates by in situ formed HX (X = Cl,
Br or I). Similar formation of nickel(III)18 complexes from
nickel(II) by strong acids have been reported in literature.
Results and discussion
Synthesis of iron complexes
The reaction of iron(II) chloride with L1 formed the iron(II)
complex [(L1)2Fe][FeCl4] (1) (Scheme 1), whereas L2 and L3
reacted with iron(II) halides to form [(L2)FeX2] {where X = Cl
(3), Br (4) and I = (5)} and [(L3)FeCl2] (6) (Scheme 2). A similar
iron(II) complex, [(L2)2Fe][OTf]2 (2) (Fig. S1†), was formed
when the iron(II) precursor was iron(II) trifluoromethanesulfo-
nate. The formation of complexes 3–6 suggested that in
forming these complexes intermediate iron(II) complexes are
formed, which are oxidised by HX (a by-product) to the iron(III)
Scheme 1 Synthesis of bis(O∧N∧N salicylaldimine)iron(II) complexes.
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Scheme 2 Synthesis of O∧N∧N salicylaldimineiron(III) complexes from iron(II) precursors.
Fig. 1 Molecular structure of 3 with the 50% probability ellipsoids.
Selected bond lengths (Å) and angles (°) are: Fe1–O1, 1.860(17); Fe1–N1,
2.094(2); Fe1–N3, 2.130(2); Fe1–Cl1, 2.277(7); Fe1–Cl2, 2.316(7); O1–
C12, 1.324(3); N3–C6, 1.289(3); O1–Fe1–N1, 137.16(8); O1–Fe1–N3,
85.44(7); N1–Fe1–N3, 85.63(7); O1–Fe1–Cl2, 113.02(6); N1–Fe1–Cl2,
108.85(6); N3–Fe1–Cl2, 90.30(6); O1–Fe1–Cl(1), 91.25(5); N1–Fe1–Cl1,
91.07(6); N3–Fe1–Cl1 170.86(6); Cl2–Fe1–Cl1, 98.84(3); C5–N3–C6,
116.0(2).
Fig. 2 Molecular structure of 4 with the 50% probability ellipsoids.
Selected bond lengths (Å) and angles (°) are: Fe1–O1, 1.851(2); Fe1–N1,
2.126(2); Fe1–N2, 2.095(3); Fe1–Br2, 2.471(7); Fe1–Br1, 2.415(7); O(1)–C(1),
1.319(3); N1–C7, 1.286(4); O1–Fe1–N1, 85.49(9); O1–Fe1–N2, 138.96(10);
N1–Fe1–N2, 85.54(9); O1–Fe1–Br2, 90.20(6); N1–Fe1–Br2, 169.90(7);
N2–Fe1–Br2, 91.78(7); O1–Fe1–Br1, 112.10(8); N1–Fe1–Br1, 90.98(7);
N2–Fe1–Br1, 108.01(7); Br2–Fe1–Br1, 99.11(3); C7–N1–C16, 116.2(2).
The formation of complex 2 is the result of the inability of
triflic acid, the expected by-product from this reaction, to
oxidise the intermediate in Scheme 1. A molecular structure of
complex 2 (Fig. S1†) was obtained but with a high R factor. As
such the bond lengths and angles are not discussed. The mole-
cular structure however confirmed the connectivity of the sali-
cylaldimine ligand L2 to the iron(II) precursor bearing weak
counterion. This salt consists of two cationic molecules of L2
with two triflate ions as counterions. The iron metal centre is
coordinated through the donor oxygen atoms, imine-N atoms
and imidazolyl-N atoms. On the other hand the lack of steric
complexes have been described for various bis(arylimino)pyridine
compounds.14b,c,20 Mass spectrometry was used in the charac-
terization of both 1 and 2. For instance with 1, negative ion
high resolution electrospray ionization mass spectrometry
identified the anion as [FeCl4]2− (m/z = 195.8104) while
the corresponding cation was identified by positive ion
high resolution electrospray ionization mass spectrometry as
[Fe(L3)2]2+ (m/z = 484.1312) (Fig. S10†).
Molecular structures of 3 and 4
bulk for L1 leads to the formation of complex 1. In fact the tri- Dark blue single crystals of complexes 3 and 4 suitable for the
dentate 2,6-bis(1-phenylimino)ethyl)pyridine ligand, without X-ray structural determination were obtained by slow evapor-
substituents on the iminophenyl ring, is reported to form an ation of toluene and hexane solutions of these complexes at
ionic compound comprising of [(L)2Fe] and [FeCl4] ions (L = room temperature. Crystallographic data are provided in
2,6-bis(1-phenylimino)ethyl)pyridine).19 Similar ion-pair iron(II) Table 1, whereas Fig. 1 and 2 show the molecular structures of
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Magnetic behaviour and 57Fe Mössbauer effect spectroscopy
(MES) of iron(III) complexes 3 and 4
Table 1 Crystal data collection and structural refinement parameters
for 3 and 4
To further investigate the spin state of the iron(III) complexes 3
and 4, variable-temperature magnetic susceptibility measure-
ments were conducted on powder samples of 3 and 4 in the
temperature range 2–300 K on a SQUID magnetometer. The
susceptibility values were calculated in cgs units. The data are
presented as χMT versus T plots. The χMT value for 3 at 300 K is
4.35 cm3 mol−1 K (Fig. 3) corresponding to a μeff of 5.90 BM.
This value is consistent with HS iron(III).16b,23 At high tempera-
tures up to 300 K, μeff is temperature-independent (Fig. 3
inset). As the temperature is lowered from 300 K, χMT remains
constant until approximately 50 K then decreases sharply to χM
T = 2.25 cm3 mol−1 K at 2 K. No maximum of the suscepti-
bility is observed in the temperature range studied. The sharp
drop at low temperatures is likely due to a zero-field splitting
(spin–orbit coupling effect) or very weak intermolecular mag-
netic interactions.24 The former is likely, as this is expected to
be dominant at low temperatures, taking into account the
mononuclear nature of the complex and the anticipated
absence of intermolecular iron–iron interactions.
Parameters
3
4
Empirical formula
Formula weight
Temperature/K
Wavelength/Å
Crystal system
Space group
a/Å
b/Å
C
453.2
100(1)
1.54178
Orthorhombic
P212121
7.7909(3)
14.1052(5)
19.9130(7)
90
20H28Cl2FeN3O
C20H28Br2FeN3O
542.12
296(2)
0.71073
Orthorhombic
P212121
7.931(2)
14.457(4)
20.040(7)
90
c/Å
α/°
β/°
90
90
90
90
γ/°
Volume/Å3
2188.28(14)
4
1.376
7.882
948
2297.8(12)
4
1.567
4.148
1092
Z
Density (calculated)/Mg m−3
Absorption coefficient/mm−1
F(000)
Goodness-of-fit on F2
Final R indices [I > 2σ(I)]
1.038
1.049
R1 = 0.0294,
wR2 = 0.0685
R1 = 0.0335,
wR2 = 0.0702
R1 = 0.0247,
wR2 = 0.0617
R1 = 0.0273,
wR2 = 0.0629
R indices (all data)
At the lowest temperature of measurement (2 K) the χMT
value is 2.25 cm3 mol−1 K which corresponds to μeff of 4.22
BM. The value is lower than that for HS and higher than that
for LS. It is however in the range of χMT (1.8–2.8 cm3 mol−1 K)
and μeff (3.9–4.5 BM) for IS iron(III) (S = 3/2) with 3 unpaired
electrons.24 57Fe Mössbauer spectroscopy at liquid helium
temperatures would be necessary to confirm this. The spin
pairing would lead to a large increase in quadrupole splitting,
EQ, parameter in comparison to the HS electronic scenario.25
Plot of 1/χcgs against temperature (Fig. 4) shows expected lin-
earity typical of paramagnetic behaviour. It however does not
extrapolate to zero obeying the Curie-Weiss law with a negative
Weiss temperature. The hint of a downturn at low tempera-
tures as well as this negative Weiss temperature is typical of
ferrimagnetic behaviour. It is interesting that such a hint of
ferrimagnetism occurs in a metal complex having only a single
transition-metal component. All of the reported compounds
that exhibit ferrimagnetic behaviour include at least two
these compounds. Central metals in complexes 3 and 4 are
pentacoordinate and have a distorted geometry about the
metal centre consistent with a complex nature of metal–ligand
binding. According to the criterion τ developed by Addison
et al.21 the structure is square pyramidal when τ = 0 and trigo-
nal pyramidal when τ = 1. For compounds 3 and 4 the values
of τ are 0.56 and 0.52, indicating a transitional conformation
between the two idealized geometries.
Complex 3 has been obtained by two synthetic routes
(Scheme 2). This indicates that starting from either iron(II) or
iron(III) metal source results in the formation of the same iron(III)
complex for this ligand L2, thus confirming that iron(II)
starting precursors are indeed oxidised to iron(III) products. In
3, one of the chlorides, Cl1, occupies the apical position of the
square pyramid with the Fe–Cl bond lengths noticeably
unequal. That is the bond to the apical chloride is significantly
longer (2.316(7) Å) than that of its basal counterpart (2.277(7)
Å). The Fe–N(imino) distance in 3 (2.130(2) Å) is longer than the
Fe–N(imidazole) 3 (2.094(2) Å) probably as a consequence of
satisfying the tri-dentate chelating constraints of the ligand as
described by Gibson et al. for diimine iron complexes.5a The
fifth bond, Fe1–O1 (1.860(17) Å) falls in the usual range. The
bond angles around the iron(III) centre O1–Fe1–Nimino, 85.44(7)°,
N1–Fe1–Nimino 85.63(7)°, O1–Fe1–Cl1, 91.25(5)° and Nimidazole
–
Fe1–Cl1, 91.07(6)° show deviations from the expected 90°
angles for a square pyramidal geometry.
In complex 4 the Fe–Nimino bond length (2.126(2) Å) is simi-
larly longer than the Fe–Nimidazole (2.095(3) Å) and Fe–O1
(1.851(2) Å) bond length compares well to that in 3. It is also
interesting to note that there is no significant effect on the
imine bond length in the coordinated ligands in either complex
(i.e. N3–C6 in 3 and N1–C7 in 4 measure 1.289(3) Å and 1.286(4) Å
respectively and are typical of the HCvN bond).22
Fig. 3 χMT product versus T plot for 3 (inset = μeff versus T plot).
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Fig. 4 1/χcgs (cm3 mol−1 −1
versus T plot for 3 showing a hint of ferri-
)
Fig. 6 1/χcgs (cm3 mol−1 −1
magnetic behaviour.
)
versus T plot for 4 showing antiferro-
magnetic behaviour.
magnetic components: bimetallic complexes or metal com-
plexes with organic radicals.26
observed corroborating the magnetic susceptibility data. As is
characteristic of transition-metal complexes displaying spin-
transition behaviour, the higher spin state is favoured at high
temperatures while the spin paired (IS or LS) state is favoured
at low temperatures. Mössbauer measurements were also
taken at 15 K on complex 4. This was on a sample that had
been in storage for several months. Some degradation was
evident in the spectrum that was re-measured at room temp-
erature. The measured EQ values of the sub-components in
this sample do not increase substantially upon cooling to 15 K
and are in the order of ≈1.4 mm s−1. A spin state transition
would involve a much bigger increase compared to the high
temperature (HS) value, EQ ≈ 1.1 mm s−1. Based on the suscep-
tibility results shown in Fig. 5, the precipitous decrease in μeff
is ascribed to antiferromagnetic coupling of molecular units.
The 57Fe Mössbauer spectra of complexes 3 and 4 provide
further evidence that these iron complexes are in the +3 oxi-
dation states. 57Fe Mössbauer spectra were measured for com-
plexes 3 and 4 at 300 K and after fitting the spectra we
observed the spectra were asymmetric doublets with isomer
shifts of δ = 0.36 mm s−1 for 3 and 0.38 mm s−1 for 4 (Table 2).
These isomer shifts are consistent with high spin iron(III) ions
in an N^O coordination environment (where δ ≈ 0.35 mm
s−1).28 The asymmetry in the intensities of 57Fe Mössbauer
spectra (doublets) can be attributed to spin–spin relaxation
effects. The 57Fe Mössbauer spectra for 3 and 4 were also
measured at 157 K (Fig. S11†). The degree of asymmetry
decreased with decreasing temperature as reported in the
literature.29 We also observed a change in isomer shift
For complex 4, the χMT at high temperature (300 K) is
4.54 cm3 mol−1 K and 0.70 cm3 mol−1 K at low temperature
of 2 K. These values correspond to a μeff of 6.02 (HS) and 2.4
(LS) BM respectively, which are a little higher than the antici-
pated spin-only values for both HS and LS iron(III) complexes.
Nonetheless, they are typical values observed for other similar
HS and LS iron(III) complexes of this kind.27 In Fig. 5, the χMT
is constant at high temperatures from 300 K until approxi-
mately 50 K when it drops rapidly till 2 K. A broad maximum
is observed in the range of 6–13 K and then the susceptibility
decreases slowly until 100 K (Fig. 5 inset). The plot of 1/χcgs
against T does not extrapolate to zero as well, indicative of
some degree of antiferromagnetic behaviour (Fig. 6).
Mössbauer spectroscopy was used as a further tool to inves-
tigate the spin state of the iron(III) in complexes 3 and 4. The
Mössbauer data for both complexes 3 and 4 (vide infra) show
only a single doublet at both 300 K and 157 K. The quadrupole
splitting (EQ) calculated for both complexes are consistent with
EQ values (0.6–1.3 mm s−1) for HS iron(III). It is therefore
evident that at these temperatures only the HS state is
Table 2 57Fe Mössbauer parameters obtained at variable temperatures
Complex
δ (mm s−1
)
EQ (mm s−1
)
3a
4a
3b
4b
0.36
0.38
0.44
0.46
1.15
1.19
1.27
1.32
a 300 K. b 157 K. The isomer shift, δ, is quoted relative to α-Fe metal at
room temperature.
Fig. 5 χMT product versus T plot for 4 with the low-T region of the
temperature dependent molar susceptibility expanded as inset.
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Δδ = 0.08 mm s−1 for both complexes due to the second-order ethylene to 1-butene and via Friedel–Crafts alkylation gave
Doppler Effect.30 Quadrupole splitting (EQ) values are weakly ethyl- and butyl- o-/p-C6H5. Such alkylation products have been
temperature dependent, are relatively small and this also reported by Darkwa and co-workers using nickel(II) (pyrazol-1-
corroborates the high spin nature of the iron(III) complexes. ylmethyl)pyridine,34 (3,5-dimethylpyrazol-1-yl)methylbenzene35
The quadrupole splitting values in 4 are systematically higher pyrazolylpyridylamine and pyrazolylpyrroleamine36 catalysts.
than in 3 (Table 2). The difference in the quadrupole splitting The ethylene reaction also produced a small amount of poly-
between complexes 3 and 4 can be explained by the different ethylene (0.39 g; entry 5, Table 3). The polymer was character-
halides on the iron(III) metal centre. This influences the elec- ized by 13C{1H} NMR spectroscopy (Fig. S14†) and GPC, which
tric field gradient at the iron(III) nucleus, hence the difference.
showed unimodal molecular weight distribution (Fig. S16†)
with Mw = 60.4 × 103 g mol−1 and Mw/Mn of 2.2. This GPC data
confirms the single-site nature of the iron catalysts.
Catalysis
Using chlorobenzene as solvent gave three types of pro-
ducts. The first type of product was a mixture of C4–C12
(Fig. S17†) ethylene oligomers that followed a Schulz–Flory dis-
tribution, with α coefficient in the range of 0.33–0.93. The
major component of the product ethylene oligomers were
butenes and hexenes, but at high Schulz–Flory coefficients
there was higher decene and dodecene components in the
mixture (Table 3).37 The second type of products were oils with
Ethylene oligomerisation and polymerisation using com-
plexes 3–6. The iron(III) complexes 3–6 produced active cata-
lysts upon activation with EtAlCl2 for the oligomerisation and
polymerisation of ethylene. The generation of the active cata-
lysts was highly exothermic, with reactor temperatures
immediately reaching 50–80 °C upon activation with the co-
catalyst. The temperatures, however, dropped for the remain-
der of the reaction time to 25 °C. Similar exothermic processes
have been reported for other transition metal catalysed ethy-
lene oligomerisation reactions.31
1
carbon content >C30). Typical H (Fig. S18†) and 13C{1H} NMR
spectroscopic data (Fig. 7) showed these are highly branched
olefins.38 Their ESI-MS data showed these oils have low mole-
cular weights (Table 3), suggesting that they are oligomers
with high carbon content and not polymers. The third type of
products is small amounts of alkyl-chlorobenzenes in the
product mixture. These alkyl-chlorobenzenes were character-
ised by GC (Fig. S17†) and GC-MS (Fig. S19, S20 and S21†) and
identified as mono- and di- butyl-chlorobenzenes and mono-
hexyl-chlorobenzene which were formed via Friedel–Crafts
alkylation of chlorobenzene by butenes and hexenes from the
oligomerisation of ethylene. Although Friedel–Crafts alkylation
of deactivated aromatic solvents such as chlorobenzene is less
likely compared to alkylation of activated aromatic solvents
such as toluene, there have been reports of halo-benzenes that
have been alkylated with polychlorinated methanes.39 To the
best of our knowledge, there is no report on Friedel–Crafts
alkylation of halo-benzenes with alkenes.
All four complexes catalysed the oligomerisation and poly-
merisation of ethylene and exhibited high catalytic activities for
these reactions (0.30–9.55 × 106 g mol Fe−1 h−1) (Table 3). The
activities obtained are comparable with those of similar iron(II)
(13.9 × 106 g mol Fe−1 h−1 17a
)
and iron(III) (2.16 × 105 g mol Fe−1
catalysts. Most iron complexes that have been reported in
h−1 32
)
literature produce both ethylene oligomers and polymer (waxes)
but with MAO as the co-catalyst.17a,33 In the case of the O^N^N
salicylaldimineiron(III) pre-catalysts used in this work, the type
of oligomers and polymers produced were solvent dependent.
The effect of solvent on the catalytic activity and the products
obtained was thus investigated in detail with pre-catalyst 3.
Effect of solvent on the catalytic activity and products formed
Using toluene as the solvent, three types of products were
obtained: 1-butene, C2 and C4 alkyl-toluenes and polyethylene.
The C2 and C4 alkyl-toluenes were identified and characterised
Effect of reaction parameters on catalytic activity of 3/EtAlCl2
by GC (Fig. S12†) and GC-MS (Fig. S13 and S14†). So the for- The pre-catalyst 3 with EtAlCl2 was used to investigate the
mation of 1-butene is inferred from the formation of butyl- effect of reaction parameters such as ethylene pressure, Al : Fe
toluenes. It is believed that the reaction between the pre-cata- ratio and reaction time. The data obtained from these
lyst 3 and EtAlCl2 produced a tandem catalyst that catalysed variations is summarized in Table 4. Kinetic studies have
a
Table 3 Oligomerisation and polymerisation of ethylene with 3–6/EtAlCl2
Product distributionb (%)
Oligomer
yield (g)
Polymer
yield (g)
Entry
Pre-catalyst
C4
C6
C8
C10
C12
F–C
α
Activityc × 106
m/zd
1
2
3
4
5
3
4
5
35
43
49
28
—
30
38
39
21
—
12
10
5
16
—
12
6
3
15
—
7
2
2
14
—
4
1
2
6
100
0.58
0.33
0.67
0.90
—
9.55
2.00
2.55
3.64
0.79
98.35
17.33
24.65
36.59
8.28
6.65
4.67
3.35
3.41
0.39
758.22
850.26
809.67
833.24
60.40f
6
3e
a Reaction conditions: chlorobenzene (70 mL), Al : Fe (400), reaction temperature (30 °C), reaction time (1 h) and ethylene pressure (10 bar).
b Determined by GC. c Activity = g mol Fe−1 h−1
.
d ESI-MS of polymer. e Toluene (70 mL). f Mw determined by GPC, ×103 g mol−1. α = Schulz–Flory
coefficient.
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Fig. 7 Typical 13C {1H} NMR spectrum of oils obtained using 3/EtAlCl2.
a
Table 4 Ethylene oligomerisation and polymerisation data using 3/EtAlCl2
Product distributionb (%)
Pressure
(bar)
Time
(min)
Oligomer
yield (g)
Polymer
yield (g)
Entry
Al : Fe
C4
C6
C8
C10
C12
F–C
α
Activityc × 106
m/zd
708.38
743.66
758.22
998.29
673.58
—
—
1072.29
—
924.29
758.23
1
2
3
4
5
6
7
8
300
350
400
450
500
400
400
400
400
400
400
10
10
10
10
10
5
15
20
10
10
10
60
60
60
60
60
60
60
60
30
44
42
35
36
26
44
64
28
42
59
41
34
32
30
33
33
35
26
26
35
34
29
8
11
12
12
15
8
6
24
9
6
7
12
10
12
7
3
14
6
2
4
7
7
9
4
1
13
5
1
6
4
4
2
5
2
1
5
3
—
3
0.33
0.57
0.58
0.70
0.75
0.57
0.33
0.93
0.83
0.50
0.78
4.00
5.00
9.55
5.55
4.55
2.55
7.00
5.27
5.09
3.77
1.67
39.52
50.22
98.35
53.79
42.08
28.00
76.28
57.44
21.46
76.01
48.73
4.48
4.78
6.65
7.21
7.92
—
0.72
0.56
6.54
6.99
6.27
9
10
11
120
180
5
14
2
9
7
a Reaction conditions: chlorobenzene (70 mL), pre-catalyst 3 loading (11 μmol), reaction temperature 30 °C. b Determined by GC. c Activity = g mol
Fe−1 h−1 d ESI-MS of polymer. α = Schulz–Flory coefficient.
.
shown that increasing the pressure of ethylene increases the activity decreased when Al : Fe ratio was further increased
rate of oligomerisation.40 Therefore ethylene oligomerisation beyond 400 : 1 (entries 1–3, Table 4). The highest oligomerisa-
reactions were performed at varying ethylene pressures tion activity (9.55 × 106 g mol Fe−1 h−1) was obtained with
(entries 3, 6, 7 and 8, Table 4). At ethylene pressure of 5 bar, Al : Fe ratio of 400 : 1 and ethylene pressure of 10 bar (entry 3,
catalyst activity was 2.55 × 106 g mol Fe−1 h−1 (entry 6, Table 4), Table 4). Reaction time was also varied from 30 to 180 min
which increased as the pressure was increased to 10 bar (entries 3, 9–11, Table 4) leading to optimum activity at 60 min
(0.61–9.55 × 106 g mol Fe−1 h−1, entry 3, Table 4). However, the but activity decreased as reaction time was increased to
catalyst activity decreased with increasing pressure at 15 bar 180 min. This decrease in activity is indicative of catalyst de-
(0.07–7.00 × 106 g mol Fe−1 h−1, entry 7, Table 4) and 20 bar activation with reaction time.
(0.05–5.27 × 106 g mol Fe−1 h−1, entry 8, Table 4). The
reduction in activity as pressure reached 15 bar could be due
to saturation of ethylene in the reaction mixture.41
Conclusions
The amount of co-catalyst used was also found to have sub-
stantial effect on catalyst activity. Catalytic activity increased
when the Al : Fe ratio was increased from 300 : 1 to 400 : 1, but
Ligand dependent reactions of (imidazol-4-yl)-ethylimino]-
methyl}phenol ligands with iron(II) halide precursors gave two
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types of products, namely bis{(imidazol-4-yl)-ethylimino]- structures were solved by direct methods and refined by least
methylphenoxy}iron(II) and O^N^N {(imidazol-4-yl)-ethyl- squares techniques using OLEX2: a complete structure solu-
imino]-methylphenoxy}iron(III) halides. The formation of the tion, refinement and analysis program.45 All non-hydrogen
iron(III) complexes is driven by the oxidation of iron(II) inter- atoms were refined with anisotropic displacement coefficients.
mediates by HX (X = Cl, Br, I) by-products. These iron(III) com- All hydrogen atoms were included in the structure factor calcu-
plexes can be activated with EtAlCl2 to produce catalysts for lation at idealized positions and were allowed to ride on the
ethylene reactions that produce oligomers or polymers neighbouring atoms with relative isotropic displacement
depending on the solvent used. In toluene and chlorobenzene coefficients. The refined structure was subjected to the check-
alkyl-aromatics via Friedel–Crafts alkylation of the solvent with CIF criteria of the International Union of Crystallography
ethylene, butene and hexene lead to ethyl-, butyl- and hexyl- (IUCr) and no major alerts came up.
toluenes or chlorobenzenes.
General procedure for ethylene oligomerisation and
polymerisation reactions
The oligomerisation was carried out in a 450 mL high pressure
reactor which was preheated for 1 h at 120 °C under vacuum
and cooled under nitrogen to the desired temperature before
Experimental
Materials and instrumentation
All procedures were performed under dry Argon using stan- use. To an appropriate amount of pre-catalyst in a Schlenk
dard Schlenk techniques. All solvents were of analytical grade tube was added degassed toluene (35 mL) and the solution
and were dried and distilled under nitrogen prior to use. Ethy- transferred into the reactor by a cannula. The reactor was then
lene (99.9%) was purchased from AFROX (South Africa) and charged with ethylene (3 bar) for 5 min. In a second Schlenk
used as received. The co-catalyst ethylaluminium dichloride tube containing degassed toluene (35 mL), an appropriate
(EtAlCl2), 1.0 M in hexane was purchased from Sigma-Aldrich amount of ethylaluminium dichloride (Al : Fe = 300 : 1–500 : 1)
and used as received. The O^N^N ligands, 2-{[2-(1H-imidazol- was added with a syringe. The solution was then transferred
4-yl)-ethylimino]-methyl}-phenol (L1),42 2,4-di-tert-butyl-6- via a cannula into the reactor. Immediately after the addition
{[2-(1H-imidazol-4-yl)-ethylimino]-methyl}-phenol (L2)43 and of the EtAlCl2 solution, the reactor was charged with ethylene
4-tert-butyl-2-{[2-(1H-imidazol-4-yl)-ethylimino]-methyl}-phenol while the reactor temperature was maintained at the desired
(L3)43 were synthesised according to literature procedures.
temperature. An appropriate amount of ethylene was continu-
The nuclear magnetic resonance (NMR) spectra were ously supplied to maintain a constant pressure in the reactor
recorded on a Bruker 400 MHz instrument (1H at 400 MHz during the polymerisation reaction. After the set reaction time,
and 13C{1H} at 100 MHz) at room temperature. Chemical shifts excess ethylene was vented and the reaction mixture quenched
are reported in δ (ppm) and referenced to the residual proton by adding 2 M HCl solution. The product was sampled for gas-
and (7.24 and 77.0 ppm for CDCl3 and 132.36 for o-dichloro- chromatography (GC) analysis.
benzene-d4). Infrared (IR) spectra were recorded on a Bruker
Tensor 27 equipped with a diamond ATR. Elemental analyses
Synthesis of salicylaldimine iron complexes
were performed on a Vario Elementar III microcube CHNS A solution of [FeX2], (X = Cl, Br, I or OTf), in either dichloro-
analyser. Mass spectra were recorded on a Waters API Quattro methane (complexes 1, 3, 4, 6, and 7) or tetrahydrofuran
Micro spectrophotometer. Variable-temperature (2–300 K) (complex 5) was added to a solution of the ligand in either
magnetic susceptibility measurements were carried out on a dichloromethane or tetrahydrofuran. The resulting blue, green or
Quantum Design MPMS SQUID magnetometer after cooling in maroon solution was stirred for 18 h. The solvent was evaporated
zero field (ZFC) and measuring on warming in a magnetic after the reaction time and the pure product recrystallised from a
field of 100 Oe. Diamagnetic corrections were performed in solution of either dichloromethane and hexane (complexes 1, 3
the usual way using Pascal’s constants. Variable-temperature and 6) or toluene and hexane (complexes 2, 4, 5 and 7). All the
Mössbauer spectra were recorded on a conventional spectro- complexes were synthesised in moderate to good yields.
meter equipped with a Janis cryostat, using a Lakeshore temp-
erature controlling device and a calibrated silicon diode phenol)iron(II)] [iron(II)tetrachloride] (1). Complex 1 was syn-
thermometer. 57Co(Rh) was used as the source of radiation.
thesized using L1 (0.18 g, 0.84 mmol) and [FeCl2] (0.11 g,
Synthesis of [bis(2-{[2-(1H-imidazol-4-yl)-ethylimino]-methyl}-
The crystal evaluation and data collection were performed 0.84 mmol) as dark blue powder. Yield: 0.25 g, (44%). Anal.
on a Bruker SMART APEXII diffractometer with either Cu Kα Calc. for C24H24Fe2Cl4N6O2·1.25CH2Cl2·2C6H14: C, 46.58; H,
(λ = 1.54178 Å) or Mo Kα (λ = 0.71073 Å) radiation and the dif- 5.72; N, 8.75%. Found: C, 46.47; H, 5.45; N, 9.13%. MS (ESI)
fractometer to crystal distance of 4.03 cm or 4.96 cm respect- positive: m/z (%) = 484.1312 (100) [(L1)2Fe]+. MS (ESI) negative:
ively. The initial cell constants were obtained from three series m/z (%) = 195.8104 (80) [FeCl4]−. IR (ATR) (cm−1): 1610(s) ν(CvN).
of ω scans at different starting angles. The reflections were suc-
Synthesis of [bis(2,4-di-tert-butyl-6-{[2-(1H-imidazol-4-yl)-
cessfully indexed by an automated indexing routine built in ethylimino]-methyl}-phenol)iron(II)] [triflate] (2). Complex 2
the APEXII program. The absorption correction was based was synthesized using L2 (0.15 g, 0.45 mmol) and [Fe(OTf)2]
on fitting a function to the empirical transmission surface (0.16 g, 0.45 mmol) as dark blue crystals. Yield: 0.11 g (22%).
as sampled by multiple equivalent measurements.44 The Anal. Calc. for C42H56F6N6O8FeS2: C, 50.10; H, 5.61; N, 8.35;
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S, 6.37%. Found: C, 49.91; H, 5.79; N, 9.37; S, 4.36%. MS (ESI):
m/z (%) = 708.3819 (100) [(L2)2Fe]+. IR (ATR) (cm−1): 1599 (s) ν(CvN).
Synthesis of (2,4-di-tert-butyl-6-{[2-(1H-imidazol-4-yl)-ethyl-
imino]-methyl}-phenol)iron(III) chloride (3). Complex 3 was
synthesized using L2 (0.16 g, 0.48 mmol) and [FeCl2] (0.06 g,
0.48 mmol) as dark blue crystals. Yield: 0.14 g (67%). Anal.
Calc. for C20H28N3Cl2OFe: C, 53.00; H, 6.23; N, 9.27%. Found:
C, 52.92; H, 6.45; N, 9.25%. MS (ESI): m/z (%) = 417 (55)
[M − Cl]+, 382 (92) [M − 2Cl]+. IR (ATR) (cm−1): 1607 (s) ν(CvN).
Synthesis of (2,4-di-tert-butyl-6-{[2-(1H-imidazol-4-yl)-ethyl-
imino]-methyl}-phenol)iron(III) chloride (3). Complex 3 was
synthesized using L2 (0.24 g, 0.72 mmol) and [FeCl3] (0.12 g,
0.72 mmol) as dark blue crystals. Yield: 0.18 g (55%). Anal.
Calc. for C20H28N3Cl2OFe·0.125CH2Cl2: C, 52.11; H, 6.14;
N, 9.06%. Found: C, 52.13; H, 6.91; N, 9.25%. IR (ATR) (cm−1):
1600 (s) ν(CvN).
Synthesis of (2,4-di-tert-butyl-6-{[2-(1H-imidazol-4-yl)-ethyl-
imino]-methyl}-phenol)iron(III) bromide (4). Complex 4 was
synthesized using L2 (0.43 g, 1.31 mmol) and [FeBr2] (0.28 g,
1.31 mmol) as dark blue crystals. Yield: 0.52 g (73%). Anal.
Calc. for C20H28N3Br2OFe: C, 44.31; H, 5.21; N, 7.75%. Found:
C, 44.44; H, 5.41; N, 7.70%. MS (ESI): m/z (%) = 463.0747 (5)
[M − Br]+, 382.1570 (5) [M − 2Br]+, 328.2385 (15) (L2). IR (ATR)
(cm−1): 1601 (s) ν(CvN).
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(100) (ligand). IR (ATR) (cm−1): 1619 (s) ν(CvN).
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1.27 mmol) and Et3N (0.18 mL, 1.27 mmol) as dark green
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57.50; H, 6.76; N, 10.06%. Found: C, 57.98; H, 6.23; N, 10.10%.
MS (ESI): m/z (%) = 708.3811 (100) [(L2)2Fe]+, 354.6942 (60)
[M/2]+, 382.1606 (10) [M − Cl + H]+, 327.2177 (10) (L2). IR
(ATR) (cm−1): 1601 (s) ν(CvN).
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Acknowledgements
We acknowledge the DST-NRF Centre of Excellence in Catalysis
(c*change) and the University of Johannesburg for financial
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