International Journal of Chemical Engineering, ISSN:2051-6051, Vol.31, Issue.2
1179
Oxalate-Bridged Binuclear Iron (III) Complexes of
3, 5-Dimethylpyrazole Ligands: Synthesis,
Structure, Spectral and Electrochemical Properties
Wafa Selmi
Laboratory of Materials and Crystallochemistry, Faculty of Science,
University of Tunis El-Manar, 2092 Tunis, Tunisia
Jawher Abdelhak
Laboratory of Materials and Crystallochemistry, Faculty of Science,
University of Tunis El-Manar, 2092 Tunis, Tunisia
Aïcha Arfaoui
Laboratory of Selective Organic Synthesis and Biological Activity, Faculty of Science,
University of Tunis, El-Manar, 2092 Tunis, Tunisia
Hassen Amri
Laboratory of Selective Organic Synthesis and Biological Activity, Faculty of Science,
University of Tunis, El-Manar, 2092 Tunis, Tunisia
Khaled Boujlel
Laboratory of Analytical Chemistry and Electrochemistry, Faculty of Science,
University of Tunis El-Manar, 2092 Tunis, Tunisia
Mohamed Faouzi Zid
Laboratory of Materials and Crystallochemistry, Faculty of Science,
University of Tunis,El-Manar, 2092 Tunis, Tunisia
Ahmed Driss
Laboratory of Materials and Crystallochemistry, Faculty of Science,
University of Tunis El-Manar, 2092 Tunis, Tunisia
ABSTRACT
1. INTRODUCTION
A new binuclear iron (III) complex has been synthesized
and characterized by physico-chemical methods. The
compound [Fe(oxalato)Cl2]3(3,5-dimethylpyrazole)2(3,5dimethylpyrazolium)2 has been prepared by slow
evaporation at room temperature and its structure was
elucidated using single-crystal X-ray diffraction. The
compound has been characterized by IR and UV–visible
spectroscopy and cyclic voltammetry analysis.
The oxalate group (dianion of the ethanedioic acid, ox2-) is
a classical ligand in coordination chemistry and in
magneto-structural studies, the negative charge and good
donor ability due to the presence of four oxygen atoms
make this ligand very attractive to build coordination
polymers by chelating metallic cations. Oxalato-bridged
complexes have been intensively studied because of the
versatile abilities of the bidentate oxalate ligand to mediate
magnetic coupling between paramagnetic metal centers
separated by more 5Å [1-9]. To our best knowledge, there
are many reports of oxalate bridged metal complexes with
Cu, Cr or Co [10-14], but only few examples dealing the
oxalato-bridged dinuclear iron complexes have been
published [6]. Among these hybrid materials, the
bimetallic oxalate-based networks have provided many
examples of multifunctional compounds [7-9]. They are
formed by polymeric anionic networks with magnetic ions
linked through oxalate ligands with cooperative magnetic
properties (ferro- ferri- or canted antiferromagnetism), and
a bulky charge-compensating molecular cation, which
templates the network formation and add a second
physical property of interest [13-14]. The insertion of
different cations into oxalate networks has led to
compounds combining the longrange magnetic ordering
from the oxalate network with a second property such as
paramagnetism,
antiferromagnetic
[15-16]
and
ferromagnetic [17-18].
In crystal, the metal Fe (III) ions are six-coordinated by
four oxygen atoms from two bischelating oxalate ligands
and the two terminal Cl- ions. The crystal structure
analysis reveals that the structure can be described as a
succession 1-D anionic [Fe(ox2)Cl2]- zigzag chains parallel
to the plane (001) which are inserted between them
organic cation (C5H9N2)+ and molecules (C5H8N2).
Structural cohesion is established essentially by π–π
interactions between the rings of dimethylpyrazole and
intermolecular N-H…Cl hydrogen bonds connecting the
ionic entities and the organic cation, thus forming a threedimensional framework. The iron center in the complex
undergoes quasi-reversible electrochemical oxidation at
low potentials owing the stability of the as-prepared
complex.
Keywords - Iron (III) complexes, 3,5-dimethylpyrazole,
crystal structure,
voltammetry.
UV-visible
spectroscopy,
cyclic
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Pyrazole and its derivatives are important heterocyclic
molecules many with biologic activity and that exhibit the
ability to coordinate to metal centers and participate in
hydrogen bonding interactions. Up to now, a variety of
complexes containing 3,5-dimethylpyrazole ligands have
been synthesized and employed in coordination chemistry
and organometallic chemistry [19-21]. There can be two
advantages in using 3,5-dimethylpyrazole (Hdmpz) as a
ligand, firstly, 3,5-dimethylpyrazole ligand has ability to
form dative bond through one of the nitrogen lone pair
[18-20], secondly, the free NH bond of the ring may be
utilized to create a supramolecular environment through
hydrogen bonding.
Based on these principles, iron complex-containing
oxalate anion has drawn attention. Our research has been
directed to the preparation and characterization of mixed
Fe(III) complexes using oxalate dianion and the Hdmpz
nitrogen donor . In this paper, we reported its synthesis
and structural characterization as well as the spectroscopic
and electrochemical investigations of a new compound
[Fe(ox)Cl2]3(C5H9N2)2(C5H8N2)2.
2. EXPERIMENTAL SECTION
2.1. Materials and physical measurements
The X-ray data was collected on an Enraf-Nonius CAD-4
diffractometer. The FT-IR spectra were recorded from
KBr pellets in the range 4000-400 cm-1 on a Mattson
Alpha-Centauri spectrophotometer, and UV-visible
spectrum was recorded on a Unico Spectro Quest 2802
UV/VIS spectrophotometer, in the range 250–800 nm.
Voltammetric measurements were made with a computercontrolled electrochemical system (Autolab PGSTAT101).
A platinum electrode with a surface area of 1 mm was
used as a working electrode, a platinum wire served as the
counter electrode and Ag/AgCl (KCl 3M) was used as the
reference electrode. The electrochemical curves were
recorded
in
0.1
mol.L−1
tetrabutylammonium
tetrafluoroborate/acetonitrile
(TBAF/ACN)
as
a
supporting electrolyte solution in room temperature and
the scan rate is 50 mVs-1. The solution was deoxygenated
by bubbling nitrogen gas through them.
2.2. Synthesis of 3,5-dimethylpyrazole
The 3,5-dimethylpyrazole [22] was synthesized by
adopting the following method. Acetylacetone (3.12 g,
31.21 mmol) was added dropwise to hydrazine
monohydrate (1 g, 20 mmol) in 50 ml of ethanol cooled to
0 °C. After the addition is complete, the mixture was
stirred for 30 min, and the off-white solid was collected by
filtration and recrystallized with petroleum ether. IR
spectral (KBr pellet, ν/cm-1): 3200 (m), 1593(s), 1486(s)
[m, medium; s, strong]. m.p. 100-102 °C .
1180
The 1H NMR spectral data of the free pyrazole showed
one single signal for two methyl groups, i.e., they have the
same environments, due to the fact that this moiety exists
in the following two tautomeric forms.
H3 C
H3C
4
5
3
N
H
CH3
N
1
2
A
5
4
3
N
1
CH3
N2
B H
Similarly, the 13C NMR spectral data of this ligand
showed single signal for the carbon atom of both methyl
groups, as well as single signal for carbon-3 and-5, again
because both methyl groups and both carbon-3 and-5 have
the same environments.
2.3. Synthesis of [Fe (ox) Cl2]3(3,5dimethylpyrazole)2(3,5-dimethylpyrazolium)2
An aqueous solution of 3,5-dimethylpyrazole (2.0 mmol)
was added dropwise to aqueous solution of oxalic acid
dehydrate (2.0 mmol) and FeCl3.6H2O (1.0 mmol). The
resulting solution was then stirred for 2 h and allowed to
evaporate at room temperature. After two months, green
crystals suitable for X-ray analysis were obtained.
2.4. X-ray Crystallography
A parallelepiped crystal of compound (0.3×0.25×0.2 mm)
was selected for the structural analysis. The compound
crystallizes in the monoclinic system, C2/c space group.
Diffraction data were collected at 293(2) K with Enraf–
Nonius CAD4 automatic four-circle equipped with
graphite monochromator using MoKα (λ = 0.71073 Å)
radiation. Unit-cell parameters and orientation matrix of 1
were determined by least-squares treatment of the setting
angles of 25 reflections on the range 10°< θ <15°.
Empirical absorption corrections were applied using the
program. The structures were solved by Patterson methods
and were refined with the full-matrix least-squares method
on F2 267 refined parameters, with anisotropic thermal
parameters for all non-hydrogen atoms. Lorentzpolarization and empirical absorption corrections through
the Ψ-scan program were applied for the compound.
The computations were performed with SHELXS 97 [23]
and SHELXL 97 [24]. All non hydrogen atoms were
treated anisotropically and the N-bound H atoms and CH2
were located in a difference Fourier map and refined
isotropically.
The molecular plots were drawn with the program
Diamond 3.0 [25]. The summary of the crystal data,
experimental details and refinement results for (1) is
summarized in Table 1.
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1181
Table 1 Crystal and structure refinement data for [Fe
(ox)Cl2]3(C5H9N2)2(C5H8N2)2.
Formula
C26H32Cl6Fe3N8O12
Formula weight
1028.85
Crystal system
Monoclinic
λ (Å)
0.71073
Space group
C2/c
3
Volume (Å )
4058.20 (5)
Z
4
a (Å)
19.527 (1)
b (Å)
11.867 (1)
c (Å)
19.429 (1)
β(°)
115.66 °
−3
ρ (gcm )
1.684
−1
μ (mm )
1.52
θ range (°)
2.1 -27.0
Index ranges
-24 ≤ h ≤ 7, -1≤ k ≤ 1,
-24 ≤ l ≤ 24
Total data collected
6634
Independent reflections
4389
Reflections with I > 2σ(I)
2711
Rint
0.044
Goodness-of-fit on F2
R [I > 2σ(I)]
0.0531
b
Largest difference peak and
hole (eÅ−3)
a
The anion complex [Fe2(C2O4)2Cl4]2- formed a zigzag
chain 1D , parallel to the (001) plan (Fig 2). As far as we
know, there are a few 1D Fe(III)-ox chain compounds
reported up to now [6]. In these chains, the central atom of
all anion is hexa-coordinated by two Cl- and four
carboxilate-oxygen atoms from two bidentate oxalato
ligands. The Fe-O (2.049-2.167 Å), Fe-Cl distances (2.296
to 2.333 Å) and the distance between the adjacent Fe
atoms bridged by ox ligands is 5.48 Å range, are similar to
those reported in oxalato-bridged iron(III) complexes in
the literature [6,8,9].
1.027
a
wR [I > 2σ(I)]
Fig.1. Molecular structure of
[Fe(ox)Cl2]3(C5H9N2)2(C5H8N2)2
0.1630
0.83 and −0.70
R = Σ||F0| − |Fc||/Σ|F0|
b
wR = [Σw(|F0|2 −|Fc|2)2/Σw|F0|2]1/2
3. RESULTS AND DISCUSSION
3.1. Crystal structure
Study of the oxalato-bridged heterobimetallic complex of
formula
[Fe(C2O4)Cl2]3(C5H8N2)2(C5H9N2)2 is performed .The title
compound
tri(dichloro
bisoxalatoferrate(III))di(3,5dimethyl-1H-pyrazole)di(3,5-dimethyl-4H-pyrazolium), is
formed by the [Fe2(C2O4)2Cl4]2- anion complex, (C5H9N2)
+
cation and (C5H8N2) neutral molecule as showed in
Figure 1. The anion is generated by a crystallographic axis
and the inversion center. The middle of the binding C3C3i oxalate group is located on an inversion center and the
iron atom (Fe2) is located on a two fold axis.
Fig.2. View of the molecular structure of
[Fe(ox)Cl2]3(C5H9N2)2(C5H8N2)2, showing the hydrogen
bonds between the chains.
The best equatorial planes around the Fe1 and Fe2 atoms
are defined by the (O2, O3, O6 and Cl2) and (O4, O7, O4ii
and Cl4) respectively. The largest deviation from the mean
plan is 0.0182 Å of Fe1. The distortion from the ideal
octahedral geometry of the metal Fe1 environment is due
to the reduced angle of the oxalate ligand [values varying
in the range 77.9(1)-161.3(1) Å]. Selected bond distances
and angles for each octahedron are given in Table 2.
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1182
Table 2 Selected bond lengths (Å) and angles (°) for [FeO4Cl2].
Iron1 (III) coordination sphere
Distances (Å)
Fe1-O6
2.049 (3)
Fe1-O3
2.133 (3)
Fe1-Cl1
2.325 (1)
Fe1-O2
2.081 (3)
Fe1-O5
2.136 (3)
Fe1-Cl2
2.296 (1)
Iron2 (III) coordination sphere
Distances (Å)
Fe2-O4
2.073 (3)
Fe2-O7 i
2.167 (4)
Fe2-Cl4 i
2.333 (1)
Fe2-O4 i
2.073 (3)
Fe2-O7
2.167 (4)
Fe2-Cl4
2.333 (1)
Angles (°)
O6-Fe1-O2
O2-Fe1-O3
O6-Fe1-O3
O2-Fe1-O5
O6-Fe1-Cl2
O5-Fe1-Cl2
O6-Fe1-Cl1
O2-Fe1-Cl1
O6-Fe1-O5
O3-Fe1-Cl1
O3-Fe1-O5
O2-Fe1-Cl2
O5-Fe1-Cl1
O3-Fe1-Cl2
161.3 (1)
85.6 (1)
78.5 (1)
77.9 (1)
95.3 (1)
90.2 (1)
99.3(1)
90.9 (1)
89.9 (1)
92.1 (1)
82.0 (1)
98.9 (1)
167.8 (1)
170.1 (1)
Angles (°)
O4-Fe2-O4 i
O4 i -Fe2-O7 i
O4-Fe2-O7 i
O4 i -Fe2-O7
O4-Fe2-O7
O4 i -Fe2-Cl4 i
O7 i -Fe2-O7
O4 i -Fe2-Cl4
O7-Fe2-Cl4
O4-Fe2-Cl4 i
O7 i -Fe2-Cl4 i
O7-Fe2-Cl4 i
O4-Fe2-Cl4
O7 i -Fe2-Cl4
Cl4 i -Fe2-Cl4
159.9 (1)
87.4 (1)
77.3 (1)
77.3 (1)
87.4 (1)
100.4 (1)
81.6 (2)
92.6 (1)
167.5 (1)
92.6 (1)
167.5 (1)
90.6 (1)
100.4 (1)
90.6 (1)
98.6 (1)
Codes of symmetry i : 0,5 + x ; 0,5 + y ; z
The C-C bond distance in the oxalate ligands is as
expected for a single C-C bond [1.543(2)A° for C1-C2 and
C3-C3i]. The bond length values of the peripheral and
inner C-O bonds compare well with those reported for
other oxalate complexes, the shorter values being due to
the greater double bond character of the free C-O bonds
[26].
centroid–centroid distance between two adjacent organic
rings of 3,5-dimethylpyrazole, Cg1..Cg1= 4.986Å Cg1 is
the centroid of the N1-N2-C10-C11-C12 (Fig. 3).
The Hdmpz ligands are planar and the average C–C
(1.487Å), C–N (1.359Å) and N-N (1.356Å) bond lengths,
and the average angles (110°) within the rings are in good
agreement with those currently given in the literature for
Hdmpz-coordinated metal complexes [27-30].
Based on the structural description, between the 1-D
anionic chains are the 3,5-dimethylpyrazolium cations and
the 3,5-dimethylpyrazole neutral molecules, forming a
cationic zigzag chains. Within each cationic layer, the 3,5dimethylpyrazolium are placed face-to-face giving rise to
π-π interactions between such cycles.
The cohesion between cationic layers is assumed by π–π
stacking interactions established between parallel cations
leads to chains along the crystallographic c axis. The
Fig.3. Fragments of the molecular structure of
[Fe(ox)Cl2]3(C5H9N2)2(C5H8N2)2 the π–π stacking
interactions between the neighboring Hdmpz ligands are
drawn as dashed lines.
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In this compound, the [Fe(C2O4)Cl2]22- anion and
uncoordinated pyrazole molecules are joined through NH…Cl hydrogen bonds [length d(D…A) and angle < (D–
H…A) are 3.165(6)Å and 136(4)°, respectively] into 3D
supramolecular networks (Fig 4, Table 3). The N-H…Cl
hydrogen bonds are located between uncoordinated
pyrazole molecules N4 as donor atoms and chlore acceptor
atoms Cl1 from anion complex.
Fig. 4. Fragments of the molecular structure of
[Fe(ox)Cl2]3(C5H9N2)2(C5H8N2)2 showing hydrogen
bonding interactions.
1183
located at 520 and 462 cm−1 are assigned to ν(Fe-O) and
ν(Fe-Cl), respectively[32].
3.4. Electronic transition spectrum
Electronic spectroscopic data (Fig. 5) for the compound
was obtained from an aqueous solutions. The choice of
solvent is necessary to avoid interference with any
phenomenon to mask the desired properties. The solvent
used for the study is the same as that already used for the
preparation.
First we examined the spectroscopic properties of solution
containing iron in the form of Fe(Cl3).6H2O . The
appearance of a band at 260 nm accompanied by a slight
bathochromic shift is mainly observed, this is
characteristic of the formation of a complex where the
ferric ion is hexacoordinate. The absorption spectra of the
complexe (1) show bands in 261 nm that band can be
attributed probably to oxalate-to- FeIII charge transfer [33].
The high-frequency absorption bands at 281 nm is
assigned as π–π* transitions of the Hdmpz ligand [34-35].
In the bibliography, intense band (not shown) is found at
200 nm, which can be assigned to Hdmpz n–π* [36]. For a
complex of Co (III), these transitions are due to the
excitonic effect under the π band of Hdmpz [37].
The presence of intermolecular N-H…Cl interactions adds
a new dimension to the crystal structure of the complex
which may thus be described as an overall 3-D
supramolecular network.
Table 3 Hydrogen-bond geometry (Å).
D - H… A
D-H(Å) H…A
D…A
N4-H1…Cl1
1.01
3.16 (6)
2.36(5)
D-H…A(°)
136(4)
D: donor; A: acceptor
3.2 IR Spectroscopy study
The IR absorptions of the oxalate group in the spectrum
[1635 and 1382 cm-1 are assigned respectively to νs(CO)
and νas(CO) stretching vibrations, and 792 cm-1 δ(O-C-O)
vibrations] suggest the presence of chelating and bischelating oxalato [14], a feature that has been
demonstrated by the X-ray structure for this compound.
3.5 Electrochemistry
The region of the to νas(CO) and νs(CO) stretching
vibrations of the oxalate group often shows slight
differences owing to the diverse coordination modes. The
split bands are generally characteristic of the bidentate
oxalate groups as terminal ligands [31]. The two peaks
The cyclic voltammogram shows two oxidation and two
reduction peaks related of the ion complex redox activity.
The thermodynamic data are gathered in Table 4.
Fig.5. UV/Vis spectra of [Fe (ox)Cl2]3(C5H9N2)2(C5H8N2)2
in water
The CV of complex, recorded in 0.1 M TBAF-ACN
solutions and the potential range comprised between -1.5
to 1.5, is depicted in Fig. 6.
Table 4 Electrode peak potentials (in V) for oxidation and reduction of [Fe(ox)Cl2]3(C5H9N2)2 (C5H8N2)2 measured at
T= 298 K in TBAF-ACN solution. The scan rate 50 mVs-1
Complex
E1ox /V
E2ox / V
E1red / V
E2red / V
ΔE/ V
E1/2/ V
i1ox/ V
i1red/ V
r=ired/iox/ V
1
0.06
1.14
-0.44
-0.03
0.03
0.11
0.18
0.14
0.78
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1184
-5
8x10
ox
E2
-5
6x10
-5
4x10
ox
E1
-5
2x10
I/A
0
-5
-2x10
-5
red
-4x10
E2
-5
-6x10
-5
red
-8x10
E1
-1,5
-1,0
-0,5
0,0
0,5
1,0
1,5
E/V(vs.Ag/AgCl)
Fig.6. Cyclic voltammogram of [Fe(ox)Cl2]3(C5H9N2)2(C5H8N2)2 obtained in TBAF-ACN at 298 K (scan rate 50 mVs-1).
The E1ox and E2red located at E1/2=30 mV are related to the
redox activity of the fer(III). The oxidation of iron occurs
at low potential due to the formation of very stable
iron/oxalate complex [38,40]. Moreover, the peak-to-peak
separation 110 mV and ired-to-iox ratio is close to the unity
are consistent with monoelectronic quasi-reversible iron
system [39,40].
The two other irreversible peaks could be attributed to
oxidation the N-H group and the reduction of the NH2+
group form respectively from the pyrazole and pyrazolium
moieties.
4. CONCLUSION
In conclusion, this paper describes a new binuclear
oxalate-bridged
iron
(III)
complex
of
3,5dimethylpyrazole ligands have been synthesized and
characterized by X-ray crystallography. X-ray studies
showed that complexes have binuclear structures, in which
iron(III) centers are bridged with oxalate dianion
oxygen’s. The iron-iron separation is 5.48 Å. In addition
to π–π interactions between the rings of 3,5dimethylpyrazole groups, the cations and chloride are
connected through hydrogen bonds into a 3D
supramolecular framework. Uv-vis and IR spectroscopy
we confirmed the oxidation state and to elucidate the
coordination sphere of the iron.
5. SUPPLEMENTARY MATERIAL
Crystallographic data and full lists of bond lengths and
angles have been deposited with the Cambridge
Crystallographic Data Centre, CCDC No. 977800. Copies
of this information may be obtained free of charge from
The Director, CCDC, 12 Union Road, CAMBRIDGE CB2
1EZ,
UK
(fax:
+44-1223-336-033;
e-mail:
deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk).
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