Synthesis, characterization and application of iron N-substituted imidazole complexes with the motif ClFeL4OFeCl3

Article abstract of DOI:10.1016/j.inoche.2014.10.038

The activation of molecular oxygen was investigated applying a mixture of iron(II) chloride and N-substituted imidazole ligands. The resulting complexes were characterized and investigated by various techniques, pointing out bimetallic complexes with the

Full text of DOI:10.1016/j.inoche.2014.10.038

Inorganic Chemistry Communications 51 (2015) 4–8
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis, characterization and application of iron N-substituted
imidazole complexes with the motif ClFeL4OFeCl3
Peter Döhlert a, Elisabeth Irran b, Robert Kretschmer c, Stephan Enthaler a,
⁎
a
Technische Universität Berlin, Institute of Chemistry: Cluster of Excellence “Unifying Concepts in Catalysis”, Straße des 17. Juni 135/C2, D-10623 Berlin, Germany
Technische Universität Berlin, Institute of Chemistry: Metalorganics and Inorganic Materials, Straße des 17. Juni 135/C2, D-10623 Berlin, Germany
b
c
Technische Universität Berlin, Institute of Chemistry, Str. des 17. Juni 135/C4, D-10623 Berlin, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
The activation of molecular oxygen was investigated applying a mixture of iron(II) chloride and N-substituted
imidazole ligands. The resulting complexes were characterized and investigated by various techniques, pointing
out bimetallic complexes with the general Cl(imidazole)4FeOFeCl3 motif. The complexes contain two different
iron centers, on the one hand iron with an octahedral geometry and on the other hand iron with a tetrahedral
geometry were observed. Both units are connected via a bridging oxido-ligand. Moreover, the complexes have
been applied as precatalyst in iron-catalyzed epoxidation of stilbene.
Received 17 September 2014
Received in revised form 30 October 2014
Accepted 31 October 2014
Available online 4 November 2014
Keywords:
Iron
© 2014 Elsevier B.V. All rights reserved.
Nitrogen ligands
Oxygen activation
Catalysis
Epoxidation
The activation and chemical transformation of molecular oxygen or
oxygen equivalents to valuable products is one of the major challenges
in current research, especially with respect to a greener and more sustain-
able chemistry [1]. For instance nature developed a range of iron-based
enzymes for such transformation [2]. Based on that, throughout the last
decades various artificial model complexes (heme and non-heme sys-
tems) have been accounted to mimic the structural functions and to
study the catalytic activity in oxidation chemistry [3]. Particularly, iron
complexes modified by polydentate pyridine ligands have received
great attention as non-heme models [4,5]. In more detail, iron pyridine
complexes were allowed to react with molecular oxygen to form dimeric
complexes containing an [Fe\O\Fe] motif with oxygen serving as an
oxido-bridge [4,5]. Interestingly, depending on the ligand structure and
the electronic properties different geometries were feasible. Moreover,
several [Fe\O\Fe] complexes demonstrated activity in catalytic oxida-
tion reactions [4,5]. In contrast, the synthesis, isolation and characteriza-
tion of [Fe\O\Fe] complexes with monodentate nitrogen-based
ligands have been scarcely investigated [6,7]. Advantageously, due to
the easy accessibility of monodentate ligands (e.g., pyridines, imidazoles),
the electronic and steric properties of the [Fe\O\Fe] complexes could be
easily tuned and in consequence effects on catalysis can be easily exam-
ined. In addition, the monodentate coordination mode allows a higher
flexibility compared to rigid polydentate ligands. Based on that, we re-
cently investigated the binding of monodentate pyridines towards iron
and different motifs have been observed depending on the substitution
pattern of the pyridines (Scheme 1, 1). In more detail, the [Fe(III)\
O\Fe(III)] pyridine complexes were accessible via two different path-
ways. One approach is the use of iron(III) chloride/ligand and water as
source for the oxido-bridge; the corresponding ligand hydrochloride is
formed as side product. On the other hand the mixture of iron(II)
chloride/ligand can activate molecular oxygen to access the [Fe(III)\
O\Fe(III)] complexes in a more cleaner fashion with molecular oxygen
as oxido-bridge source. Besides pyridine ligands, imidazole ligands have
been discussed as an interesting biomimetic ligand class [8]. Recently,
Beller and coworkers described the synthesis of two iron imidazole com-
plexes (Scheme 1, 2e–f), which proceeds most likely via the water activa-
tion pathway [9]. In contrast, the activation of molecular oxygen by iron
complexes modified by monodentate imidazole ligands has not been
accounted so far. Based on that, we report herein the synthesis, isolation,
characterization and application of [Fe\O\Fe] complexes modified by
monodentate imidazole ligands.
The complexes 2a–d were accessible by the reaction of anhydrous
iron(II) chloride with N-substituted imidazole 3 in dry THF or
acetonitrile at room temperature (Scheme 2). The solution was subse-
quently cooled to −40 °C and the dinitrogen atmosphere was replaced
⁎
Corresponding author.
E-mail address: stephan.enthaler@tu-berlin.de (S. Enthaler).
http://dx.doi.org/10.1016/j.inoche.2014.10.038
1387-7003/© 2014 Elsevier B.V. All rights reserved.

P. Döhlert et al. / Inorganic Chemistry Communications 51 (2015) 4–8
5
Scheme 1. [Fe\O\Fe] complexes with monodentate pyridine and monodentate imidazole ligands.
by dry dioxygen. Immediately a color change from off-white to deep red
along with the formation of a red powder was observed. The mixture
was warmed to room temperature and after removal of the solvent,
the residues were crystallized from acetonitrile. Red crystals suitable
for X-ray diffraction analysis were obtained from concentrated acetoni-
trile solutions of the corresponding complexes 2a, 2b, 2c, and 2d,
respectively.
The solid state structures of the complexes 2a–d revealed a dinu-
clear [Fe\O\Fe] motif with two different iron centers connected via
oxygen (Figs. 1–4). On one hand, an octahedral iron center with four
imidazole ligands coordinated in a plane with a Fe\N bond distance
in the range of 2.132–2.162 Å was observed (Table 1). In the axial po-
sition a chlorido and an oxido-ligand are located showing an almost
linear arrangement Cl1\Fe1\O1: 177.83–177.81°. This observation
is contrary to the geometry described for complexes containing
polydentate pyridine ligands. Here a cis-arrangement of the chlorido
to the oxido-ligand was reported, probably caused by the shielding
of the polydentate ligands [4,5]. The Fe2\O1 bond (1.745–1.758 Å)
is to some extent shorter than the Fe1\O1 (1.827–1.841 Å) bond.
The angle of the included [Fe\O\Fe] motif is deviated from a linear
arrangement (160.96–174.55°), which is among the large scope of
complexes with polydentate pyridine ligands (139–180°) [4,5]. On
the other hand, the second iron center has a tetrahedral coordination
geometry with three chlorido ligands and one oxido-ligand. Note-
worthy, no significant relationship was observed for the different
N-substitution. In order to get more information on the electronic
abilities especially the basicity of the nitrogen of the imidazole li-
gands, DFT calculations (MO62X/TZVP) were performed [10]. In
more detail, the following basicities were calculated 955.8 kJ/mol
(2a), 975.8 kJ/mol (2b), 969.0 kJ/mol (2c) and 972.0 kJ/mol (2d).
Afterwards the catalytic abilities of the complexes were investigated
in epoxidation reactions in accordance to the in situ system composed
Scheme 2. Synthesis of dinuclear iron complexes 2a–d.

6
P. Döhlert et al. / Inorganic Chemistry Communications 51 (2015) 4–8
Fig. 3. Molecular structure of complex 2c-(CH3CN). Thermal ellipsoids are drawn at the
50% probability level. Hydrogen atoms, one molecule 2c and solvent molecules are omit-
ted for clarity. Selected bond lengths (Å) and angle (°): Fe(1)\O(1): 1.841(3), Fe(2)\
O(1): 1.749(3), Fe(1)\N(1): 2.161(3), Fe(1)\N(3): 2.144(3), Fe(1)\Cl(1): 2.3973(11),
Fe(2)\O(1)\Fe(1): 172.37(17), O(1)\Fe(1)\N(1): 90.23(12), O(1)\Fe(1)\N(3):
93.26(13), N(3)\Fe(1)\N(1): 86.17(12), N(1)\Fe(1)\N(5): 178.55(13), N(7)\
Fe(1)\N(1): 89.38(13), O(1)\Fe(1)\Cl(1): 178.38(10), N(1)\Fe(1)\Cl(1): 90.67(9),
O(1)\Fe(2)\Cl(2): 110.33(10).
Fig. 1. Molecular structure of complex 2a-(CH3CN)3. Thermal ellipsoids are drawn at the
50% probability level. Hydrogen atoms and solvent molecules (3× acetonitrile) are omit-
ted for clarity. Selected bond lengths (Å) and angle (°): Fe(1)\O(1): 1.831(2), Fe(2)\
O(1): 1.745(2), Fe(1)\N(1): 2.146(3), Fe(1)\N(3): 2.132(3), Fe(1)\Cl(1): 2.4342(10),
Fe(2)\O(1)\Fe(1): 167.02(18), O(1)\Fe(1)\N(1): 91.76(12), O(1)\Fe(1)\N(3):
93.67(13), N(3)\Fe(1)\N(1): 88.90(13), N(7)\Fe(1)\N(1): 92.09(12), N(5)\Fe(1)\
N(1): 173.63(12), O(1)\Fe(1)\Cl(1): 177.54(9), N(1)\Fe(1)\Cl(1): 87.77(9), O(1)\
Fe(2)\Cl(2): 110.58(9).
of FeCl3/imidazoles reported by Beller et al. (Scheme 3). In more detail,
2.5 mol% of 2 was reacted with trans-stilbene 4 and 3.0 equivalents of
aqueous H2O2 (30 wt.%) at room temperature for 24 h in tert-amyl alco-
hol. With the complexes (2a and 2b) containing N-alkyl substituted im-
idazole ligands low yields (b4%) of the corresponding epoxide 5 were
obtained, while higher yields up to 52% were achieved with N-aryl
substituted imidazoles with selectivities up to 98%. Importantly, in the
absence of complexes 2 no conversion of the starting material was
observed.
In summary, we present the synthesis, isolation and characterization
of dinuclear iron complexes with an [Fe\O\Fe] structural motif. The
complexes are easily accessible by dioxygen activation applying a mix-
ture of FeCl2 and monodentate imidazoles. On the one hand, an octahe-
dral iron center with four imidazole ligands coordinated in a plane and
on the other hand, one is observed with a tetrahedral geometry with
three chlorido ligands. Moreover, both iron centers are connected via
an oxido-bridge, which derived from molecular oxygen. In addition, cat-
alytic studies showed activity in oxidation reactions of olefins to pro-
duce epoxides with yields up to 52%.
Acknowledgment
Financial support from the Cluster of Excellence “Unifying Concepts
in Catalysis” (EXC 314/1; www.unicat.tu-berlin.de; funded by the
Deutsche Forschungsgemeinschaft and administered by the Technische
Universität Berlin) is gratefully acknowledged. R.K. thanks the Alexan-
der von Humboldt Foundation for a Feodor Lynen Research Fellowship.
We thank the Institut für Mathematik of TU Berlin for the allocation of
computing time.
Fig. 2. Molecular structure of complex 2b-(CH3CN). Thermal ellipsoids are drawn at the
50% probability level. Hydrogen atoms and one solvent molecule are omitted for clarity.
Selected bond lengths (Å) and angle (°): Fe(1)\O(1): 1.8338(19), Fe(2)\O(1):
1.7466(19), Fe(1)\N(1): 2.162(2), Fe(1)\N(3): 2.136(2), Fe(1)\Cl(1): 2.4112(7),
Fe(2)\O(1)\Fe(1): 160.96(11), O(1)\Fe(1)\N(1): 89.22(8), O(1)\Fe(1)\N(3):
90.00(8), N(3)\Fe(1)\N(1): 90.83(8), N(5)\Fe(1)\N(1): 90.41(8), N(7)\Fe(1)\
N(1): 177.03(8), O(1)\Fe(1)\Cl(1): 177.81(6), N(1)\Fe(1)\Cl(1): 89.38(6), O(1)\
Fe(2)\Cl(2): 111.04(7).
Appendix A. Supplementary material
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.inoche.2014.10.038.

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Fig. 4. Molecular structure of complex 2d-(CH3CN). Thermal ellipsoids are drawn at the
50% probability level. Hydrogen atoms and one solvent molecule are omitted for clarity.
Selected bond lengths (Å) and angle (°): Fe(1)\O(1): 1.8278(14), Fe(2)\O(1):
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Table 1
Comparison of bond lengths and angles of complexes 2a–f.
2a
1.831(2)
1.745(2)
2.4342(10)
2.146(3)
167.02(18)
88.90(13)
173.63(12)
91.76(12)
2b
2c
1.841(3)
1.749(3)
2.3973(11)
2.161(3)
172.86(17)
86.17(12)
178.55(13)
90.23(12)
2d
2ea
2fa
Fe1\O1 [Å]
Fe2\O1 [Å]
Fe1\Cl1 [Å]
Fe1\N1 [Å]
Fe1\O1\Fe2 [°]
N1\Fe1\N3 [°]
N1\Fe1\N5 [°]
N1\Fe1\O1 [°]
1.8338(19)
1.7466(19)
2.4112(7)
2.162(2)
160.96(11)
90.83(8)
1.8278(14)
1.7581(14)
2.4222(6)
2.1538(17)
174.55(9)
96.92(7)
1.8265(16)
1.7511(16)
2.4010(7)
2.142(2)
172.89(12)
90.03(9)
1.8184(19)
1.7651(18)
2.4548(8)
2.138(2)
176.20(12)
90.66(8)
177.03(8)
89.22(8)
170.42(7)
94.02(7)
176.16(8)
92.23(8)
174.17(9)
91.89(8)
a
2e: R = Ph, 2f: R = 2,6-di-iso-propylphenyl values taken from reference [9].

8
P. Döhlert et al. / Inorganic Chemistry Communications 51 (2015) 4–8
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