Synthesis, structure and magnetic characterization of a dinuclear and two mononuclear iron(III) complexes with N,O-donor Schiff base ligands

Article abstract of DOI:10.1016/j.poly.2017.12.040

One centrosymmetric dinuclear and two mononuclear iron(III) Schiff base complexes, [(μ-OMe)₂Fe₂(L¹)₂(N₃)₂] (1) {HL¹ = 2-((2-(dimethylamino)ethylimino)methyl)-6-ethoxyphenol}, [Fe

Full text of DOI:10.1016/j.poly.2017.12.040

Accepted Manuscript
Synthesis, structure and magnetic characterization of a dinuclear and two mon-
onuclear iron(III) complexes with N,O-donor Schiff base ligands
Tanmoy Basak, Kousik Ghosh, Carlos J. Gómez-García, Shouvik
Chattopadhyay
PII:
S0277-5387(18)30033-0
https://doi.org/10.1016/j.poly.2017.12.040
POLY 13010
DOI:
Reference:
Polyhedron
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Accepted Date:
14 October 2017
22 December 2017
Please cite this article as: T. Basak, K. Ghosh, C.J. Gómez-García, S. Chattopadhyay, Synthesis, structure and
magnetic characterization of a dinuclear and two mononuclear iron(III) complexes with N,O-donor Schiff base
ligands, Polyhedron (2018), doi: https://doi.org/10.1016/j.poly.2017.12.040
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Synthesis, structure and magnetic characterization of a dinuclear
and two mononuclear iron(III) complexes with N,O-donor Schiff
base ligands
Tanmoy Basaka, Kousik Ghosha, Carlos J. Gómez-Garcíab and Shouvik Chattopadhyaya,*
a Department of Chemistry, Inorganic Section, Jadavpur University, Kolkata - 700032, India.
Tel: +91-33-24572941, e-mail: shouvik.chem@gmail.com
b Instituto de Ciencia Molecular (ICMol), Parque Científico, Universidad de Valencia,
C/Catedrático JoséBeltrán, 2. 46980 Paterna, Valencia, Spain
Abstract: One centrosymmetric dinuclear and two mononuclear iron(III) Schiff base
complexes, [(µ-OMe)2Fe2(L1)2(N3)2] (1) {HL1 = 2-((2-(dimethylamino)ethylimino)methyl)-6-
ethoxyphenol}, [FeL2(OH2)(N3)]‧CH3OH (2) {H2L2 = N,N'-bis(3-ethoxysalicylidene)propane-
1,3-diamine}
and
[FeL3(H2O)(NCS)]·2H2O
(3)
{H2L3
=
N,N'-bis(3-
methoxysalicylidene)propane-1,3-diamine}, have been synthesized and characterized by
elemental analysis and spectral studies. X-ray diffraction analysis was used to determine the
structures of all three complexes. Variable temperature (2-300 K) magnetic susceptibility (χ) data
of complex 1 show that both the iron(III) centres in the complex are in a high spin configuration
(S = 5/2) and present a moderate antiferromagnetic exchange interaction mediated through the
double methoxido bridges (J = −43.7 cm–1) between the metal centres, that correlated well with
previous magneto-structural correlations in double alkoxido bridged iron(III) dimers.
Keywords: Iron(III); Schiff base; Schiff base; Crystal structure; Antiferromagnetic.
1. Introduction

The high affinity of the iron(III) ion towards oxygen atoms favours the formation of a
large number of oxygen bridged iron(III) species with Fe2O2 cores [1]. Oxide, hydroxide,
alkoxide, acetate etc. have widely been used as bridging ligands by various groups to prepare
many such dinuclear iron(III) species [2-6]. The complexes have been studied extensively for
their potential ability to store and transport dioxygen [7], to oxidise alkanes [8,9], to catalyse the
hydrolysis of phosphate ester [10] etc. There has also been huge interest to synthesize oxo-
bridged dimeric iron(III) species and to check their ability to mimic various important
metalloproteins and metallo-enzymes, including hemerythrin, methane monooxygenase (MMO),
ribonucleotide reductase etc [11-15]. Many complexes of this type have been prepared and
characterized by variable temperature magnetic susceptibility measurements [16] and Mossbauer
spectra [17]. Infrared frequency linked to the oxo-bridge unit has been recognized [18,19].
The use of N2O donor Schiff bases as blocking ligands in preparing such complexes is
relatively less well known. Only a few examples are known where tridentate N2O donor Schiff
bases have been used to prepare dinuclear iron(III) complexes with Fe2O2 cores [20,21].
Tridentate N2O donor Schiff bases usually afford bis-ligand iron(III) complexes [22,23]. In the
present work, we have used a tridentate N2O donor Schiff base ligand, HL1 {2-((2-
(dimethylamino)ethylimino)methyl)-6-ethoxyphenol}, and two tetradentate N2O2 donor Schiff
base ligands, H2L2 {N,N'-bis(3-ethoxysalicylidene)propane-1,3-diamine} and H2L3 {N,N'-bis(3-
methoxysalicylidene)propane-1,3-diamine}, to prepare a methoxido bridged dinuclear and two
mononuclear iron(III) complexes, respectively. The variable-temperature (2-300 K) magnetic
susceptibility data of complex 1 indicates the presence of two high spin (S = 5/2) iron(III)
centers. A moderate antiferromagnetic exchange interaction with J = −43.7 cm–1 is found to be

mediated between the metal centres through the double methoxido bridges. The result agrees
well with previous magneto-structural correlations in double alkoxido bridged iron (III) dimers.
2. Experimental Section
All starting materials and solvents were commercially available, reagent grade and used
as purchased from Sigma-Aldrich without further purification.
Caution!!! Although no problem was encountered in this work, perchlorate salts
containing organic ligands and azide complexes are potentially explosive. Only a small amount
of the materials should be prepared and they must be handled with care.
2.1. Preparation
2.1.1. Preparation of the Schiff base ligands
2.1.1.1. 2-((2-(Dimethylamino)ethylimino)methyl)-6-ethoxyphenol (HL1)
The tridentate N2O donor Schiff base ligand HL1 was prepared by refluxing N,N-
dimethylethane-1,2-diamine (0.10 ml, 1 mmol) with 3-ethoxysalicylaldehyde (166 mg, 1 mmol)
in methanol (20mL) for ca. 1 h. The ligand was not isolated and was used directly for the
synthesis of complex 1.
2.1.1.2. N,N'-bis(3-ethoxysalicylidene)propane-1,3-diamine (H2L2)
The tetradentate N2O2 donor Schiff base ligand H2L2 was prepared by refluxing 1,3-
diaminopropane (0.10 ml, 1 mmol) with 3-ethoxysalicylaldehyde (166 mg, 2 mmol) in methanol
(20mL) for ca. 1 h. The ligand was not isolated and was used directly for the synthesis of
complex 2.
2.1.1.3. N,N'-bis(3-methoxysalicylidene)propane-1,3-diamine (H2L3)

This ligand was prepared using a similar method as that for H2L2, except that 3-
methoxysalicylaldehyde (152 mg, 2 mmol) was used instead of 3-ethoxysalicylaldehyde. The
ligand was not isolated and was used directly for the synthesis of complex 3.
2.1.2. Preparation of the complexes
2.1.2.1. [(µ-OMe)2Fe2(L1)2(N3)2] (1)
A methanol solution (10 mL) of iron(III) perchlorate hydrate (~354 mg, 1 mmol) was
added to a methanol-water (2:1) solution (10 mL) of sodium azide (65 mg, 1 mmol) with
constant stirring. The resulting mixture was then added to a methanol solution (20 mL) of HL1
and the stirring was continued for about 1 h. The solution was filtered and a few drops of
acetonitrile were added to it. The resulting solution was kept for crystallization. Single crystals,
suitable for X-ray diffraction, were obtained after 6-7 days upon slow evaporation of the solution
in an open atmosphere.
Yield: 262 mg (~72%) based on iron(III). Anal. Calc. for C28H44Fe2N10O6 (FW 728.43): C,
46.17; H, 6.09; N, 19.23 % Found: C, 46.1; H, 6.8; N, 19.3 %. IR (KBr, cm-1): 1622 (νC=N); 2076
(νN3); 2855-2965 (υC-H). UV-Vis, λmax (nm), [εmax (L mol-1 cm-1)] (Acetonitrile), 233 (6.7 × 103),
268 (3.6 × 103), 333 (2.4 × 103), 410 (1.4 × 103).
2.1.2.2. [FeL2(H2O)(N3)]ꢀCH3OH (2)
This complex was prepared using a similar method as that for complex 1, except that
H2L2 was used instead of HL1. The resulting solution was kept for crystallization. Single crystals,
suitable for X-ray diffraction, were obtained after 6-7 days upon slow evaporation of the solution
in an open atmosphere.

Yield: 387 mg (~75%) based on iron(III). Anal. Calc. For C22H30FeN5O6 (FW 516.36): C, 51.13;
H, 5.81; N, 13.56 % Found: C, 51.2; H, 5.8; N, 13.6 %. IR (KBr, cm-1): 1612 (νC=N); 2049 (νN3);
2855-2975 (υC-H). UV-Vis, λmax (nm), [εmax (L mol-1 cm-1)] (Acetonitrile), 231 (1.2 × 104), 270
(6.6 × 103), 340 (2.8 × 103), 430 (1.4 × 103), 540 (9.5 × 102)
2.1.2.3. [FeL3(OH2)(NCS)]·2H2O (3)
A methanol solution (10 mL) of iron(III) perchlorate hydrate (~354 mg, 1 mmol) was
added to a methanol-water (2:1) solution (10 mL) of sodium thiocyanate (89 mg, 1 mmol) with
constant stirring. The resulting mixture was then added to a methanol solution (20 mL) of H2L3
and the stirring was continued under heating for about 1 h. The solution was filtered and a few
drops of acetonitrile were added to it. The resulting solution was kept for crystallization. Single
crystals, suitable for X-ray diffraction, were obtained after 6-7 days upon slow evaporation of the
solution in an open atmosphere.
Yield: 362 mg (~74%) based on iron(III). Anal. Calc. for C20H24N3FeO6S (FW 490.34): C,
48.95; H, 4.90; N, 8.57 % Found: C, 48.9; H, 4.9; N, 8.6 %. IR (KBr, cm-1): 1612 (νC=N); 2056
(νNCS); 2835-2965 (υC-H). UV-Vis, λmax (nm), [εmax(L mol-1 cm-1)] (Acetonitrile), 230 (1.3 × 104),
270 (6.8 × 103), 342 (2.4 × 103), 426 (1.5 × 103), 570 (1.9 × 103).
2.2. Physical measurements
Elemental analyses (carbon, hydrogen and nitrogen) were performed using a Perkin
Elmer 240C elemental analyser. IR spectra in KBr (4500-500 cm-1) were recorded with a Perkin
Elmer Spectrum Two spectrophotometer. Electronic spectra in acetonitrile were recorded on a
Perkin Elmer Lambda 35 UV-Vis spectrophotometer. The powder XRD data were collected on a
Bruker D8 Advance X-ray diffractometer using Cu Kα radiation (λ = 1.548 Å) generated at 40 kV

and 40 mA. The PXRD spectra were recorded in the 2θ range 5-50º using a 1-D Lynxeye
detector at ambient conditions.
Magnetic susceptibility measurements were carried out in the temperature range 2-300 K
with an applied magnetic field of 0.5 T on a polycrystalline sample of the title compound (with a
mass of 20.51 mg) with a Quantum Design MPMS-XL-5 SQUID susceptometer. The
susceptibility data were corrected for the sample holder, previously measured using the same
conditions, and for the diamagnetic contributions of the salt as deduced using Pascal´s constant
tables [24].
2.3. X-ray crystallography
Data collection on suitable single crystals of all three complexes was performed on a
Bruker SMART APEX II diffractometer equipped with graphite-monochromated Mo Kα
radiation (λ = 0.71073 Å) at 296 K. The crystal structures were solved using the SHELX-14/7
package [25]. Non-hydrogen atoms were refined using anisotropic thermal parameters. Hydrogen
atoms were placed in their geometrically idealized positions and constrained to ride on their
parent atoms. Multi-scan empirical absorption corrections were applied to the data using the
SADABS program [26]. Crystallographic data and refinement details are given in Table 1.
2.4. Calculation based on the bond valence sum method
The bond valence sum (BVS) method has been used to determine the oxidation state of
the metal ions in the solids based on crystallographically determined metal-ligand bond distances
[27,28]. The bond valences (sij) are calculated according to eq 1:
sij = exp[(r0 - rij)/b]........(eq 1)

where i = donor centre, j = metal centre, r0 = reported bond length between i and j, rij = the
observed bond length, sij = the valence of a bond between two atoms i and j, and b is usually
taken to be 0.37.
The bond valence sum is calculated according to eq 2: The valence sum rule (Pauling's second rule) is defined
zj =
………..(eq 2)
where zj is the valence of atom j connecting i−j bonds with all neighbouring i atoms.
2.5. Hirshfeld surface analysis
The Hirshfeld surfaces [29-31] and the associated two-dimensional (2D) fingerprint [32-
34] plots were calculated using Crystal Explorer, [35] with the bond lengths to hydrogen atoms
set to standard values [36]. Two distances, de (the distance from the point to the nearest nucleus
external to the surface) and di (the distance to the nearest nucleus internal to the surface), are
defined for each point on the Hirshfeld surface. The normalized contact distance (dnorm) based on
de and di is defined as:
=
+
where
and
are the van der Waals radii of the atoms. The dnorm value is negative or
positive depending on intermolecular contacts being shorter or longer than the van der Waals
separations. The parameter dnorm displays a surface with a red-white-blue colour design, where
bright red spots highlight shorter contacts, white areas in the same surface represent contacts
around the van der Waals separation and blue regions are devoid of close contacts. For a given
CIF, the Hirshfeld surface is said to be unique [37].
3. Results and discussion

3.1. Synthesis
The tridentate Schiff base ligand HL1 was synthesized by the condensation of N,N-
dimethylethane-1,2-diamine with 3-ethoxysalicylaldehyde in methanol following the literature
method [38]. The ligand was not isolated and methanol solutions of the ligands were used
directly for the preparation of complex 1. Iron(III) perchlorate hydrate was allowed to react with
sodium azide and the Schiff base ligand HL1 in an equimolar ratio in methanol under stirring to
prepare the dinuclear iron(III) complex [(µ-OMe)2Fe2(L)2(N3)2] (1).
The tetradentate Schiff base ligands H2L2 and H2L3 were prepared by the condensation of
1,3-diaminopropane with 3-ethoxysalicylaldehyde and 3-methoxysalicylaldehyde, respectively.
The ligands were not isolated and methanol solutions of the ligands were used directly for the
preparation of complexes 2 and 3. Iron(III) perchlorate hydrate was allowed to react with sodium
azide, followed by the addition of H2L2, resulting in the mononuclear iron(III) complex
[FeL2(OH2)(N3)]ꢀCH3OH (2). Another mononuclear iron(III) complex, [FeL3(OH2)(NCS)]·2H2O
(3), was prepared by a similar method, except that sodium thiocyanate and H2L3 were used
instead of sodium azide and H2L2 respectively. The whole synthesis was carried out under
aerobic conditions. The synthetic routes to the complexes are shown in Scheme 1.

Scheme 1: Synthetic route to complexes 1-3.
3.2. Description of the structures
3.2.1. [(µ-OMe)2Fe2(L)2(N3)2] (1)
A structure determination reveals that complex 1 crystallizes in the triclinic space group
P . The complex is a centrosymmetric methoxido-bridged iron(III) dimer where both iron(III)
centers have distorted octahedral environments. A perspective view of the complex with the
selective atom numbering scheme is shown in Figure 1. Selected bond lengths and angles are
given in Tables 2 and 3, respectively. Each iron(III) centre is meridionally coordinated to a
deprotonated Schiff base ligand, (L−) through the imine nitrogen atom, N(1), amine nitrogen
atom, N(2) and phenoxo oxygen atom, O(2), together with a terminal azide nitrogen atom, N(3)
and two methoxido ligands, with the atoms O(3) and O(3)a, (a = 1-x,1-y,-z) acting as bridges
between the two iron(III) centres. Thus, the two iron(III) octahedra share two bridging

methoxido groups, forming a Fe2O2 core. The distance between the two iron(III) centres is
3.209(4) Å and the distance between the two bridging oxygen atoms is 2.432(2) Å. The Fe-O-Fe
angle is 105.90(7)º. All these distances and angles are comparable with previously reported
double-methoxido bridged iron(III) complexes [39-41]. The two Fe-µ-OMe bond lengths are
slightly different, making the Fe2O2 core asymmetric, which is a common feature for similar
types of dimer [39,40]. The phenyl rings are almost perpendicular to the Fe2O2 core, with a
dihedral angle of 87.42°. The Fe-Nimine distance {2.136(1) Å} is shorter than the Fe-Namine
distance {2.296(2) Å}, as was also observed in similar type systems [20]. The saturated five
membered chelate ring, [Fe(1)-N(1)-C(10)-C(11)-N(2)], has half chair conformation with
puckering parameters [42], Q = 0.498(2) Å, and ϕ = 122.8(2)º.
A detailed literature survey has been executed to find out similar type previously reported
dinuclear iron(III) complexes with a Fe2O2 core. After an elaborate study only five similar type
complexes have been found. All these complexes, with their structural parameters related to their
magnetic data, are gathered in Table 4. The structures of all these complexes are shown in Figure
2. As there are only five instances of this type of dinuclear iron(III) complex with a Fe2O2 core,
we think that the investigated complex may add a new dimension to this class of complexes.

Figure 1: Perspective view of complex 1 with the selective atom numbering scheme. Hydrogen
atoms are omitted.
Figure 2: Structures of all previously reported similar type dinuclear iron(III) complexes with a
Fe2O2 core. Hydrogen atoms are omitted.
3.2.2. [FeL2(OH2)(N3)]‧CH3OH (2) and [FeL3(OH2)(NCS)]·2H2O (3)
Complex 2 crystallizes in the triclinic space group P , whereas complex 3 crystallizes in
the monoclinic space group C2/c. Complex 2 consists of the discrete mononuclear unit
[FeL2(OH2)(N3)] along with a lattice methanol molecule, while complexes 3 consist of the
discrete mononuclear unit [FeL3(OH2)(NCS)] along with two lattice water molecules. The
corresponding molecular structures of complexes 2 and 3 with selective atom-numbering
schemes are depicted in Figures 3 and 4, respectively. Selected bond lengths and angles are

given in Tables 2 and 3, respectively. The structures of both complexes consist of a hexa-
coordinated central iron atom in an octahedral geometry. In each complex, the central iron(III)
centre, Fe(1), is coordinated by two imine nitrogen atoms, N(1) and N(2), and two phenoxo
oxygen atoms, O(2) and O(3), from one deprotonated Schiff base ligand, [(L2)2¯ in 2 and (L3)2¯
in 3]. The remaining two coordination sites are occupied by an oxygen atom, O(5) of a water
molecule and a nitrogen atom, N(3) of a terminal azide (for 2) or thiocyanate (for 3). The
distortion of the octahedral geometry can be easily observed from the coordinate bond angles
(Table 3), which deviate from the ideal values of 90° for the cis angles and 180° for the trans
angles. In complex 2, the saturated six membered chelate ring [Fe(1)-N(1)-C(9)-C(10)-C(11)-
N(2)] possesses a screw-boat conformation with puckering parameters [42] Q = 0.406(9) Å, θ =
59.6(10)° and ϕ = 147.2(12)°, and in complex 3, the saturated six membered chelate ring [Fe(1)-
N(1)-C(10)-C(11)-C(12)-N(2)] shows a twist-boat conformation with puckering parameters [42]
Q = 0.186(5) Å, θ = 107(2)° and ϕ = 276(2)°. In the case of 2, the terminal azide ligand is quasi-
linear with the N-N-N angle being 175.15(14)°.

Figure 3: Perspective view of complex 2 with the selective atom numbering scheme. Only
selected hydrogen atoms are shown.
Figure 4: Perspective view of complex 3 with the selective atom numbering scheme. Only
selected hydrogen atoms are shown.
3.3. Solid state supramolecular interactions
3.3.1. [(µ-OMe)2Fe2(L)2(N3)2] (1)
The construction of a supramolecular network with interesting non-covalent interactions
as binding forces is observed in complex 1. The hydrogen atom H(14B), attached to the carbon
atom C(14), is involved in a C-H⋅⋅⋅π interaction with the symmetry related (1-x,1-y,-z) chelate
ring Cg(3) [Fe(1)-O(2)-C(8)-C(7)-C(9)-N(1)]. These C-H···π interactions are shown in Figure 5.
On the other hand, the hydrogen atom H(1A), attached to ethoxy carbon atom, C(1), participates
in an inter-molecular C-H⋅⋅⋅π interaction with the symmetry related (1-x,1-y,1-z) phenyl ring

Cg(4) [C(3)-C(4)-C(5)-C(6)-C(7)-C(8)]. Due to this type of inter-molecular C-H⋅⋅⋅π interaction,
a 1D array is formed in the crystal packing of the complex (Figure 6). Details of the geometric
features of the C-H···π interactions are given in Table 6.
The structure of the complex is additionally stabilized by significant π⋅⋅⋅π stacking
interactions. The phenyl ring Cg(4) [C(3)-C(4)-C(5)-C(6)-C(7)-C(8)] undergoes a strong face-to-
face π⋅⋅⋅π stacking interaction with the neighboring phenyl ring Cg(4) [C(3)-C(4)-C(5)-C(6)-
C(7)-C(8)] of symmetry (2-x,1-y,1-z). A one dimensional array is formed because of this type of
π⋅⋅⋅π stacking interaction (Figure 7). Geometric features of the π⋅⋅⋅π stacking interaction are
given in Table 7.
Figure 5: Intra-molecular C-H···π interaction within the dimer. Only the relevant hydrogen
atoms are shown.

Figure 6: One-dimensional supramolecular array in the complex, generated through
intermolecular C-H···π interactions. Only the relevant hydrogen atoms are shown.
Figure 7: Formation of the one dimensional array through π···π stacking interactions in the
complex. Hydrogen atoms are omitted.
3.3.2. [FeL2(OH2)(N3)]‧CH3OH (2) and [FeL3(OH2)(NCS)]·2H2O (3)
Both complexes 2 and 3 show some significant supramolecular interactions. In complex
2, the hydrogen atoms H(1W) and H(2W) attached to the water oxygen atom O(5) participate in
intermolecular hydrogen bonding interactions with the symmetry related phenoxo oxygen atoms,
O(3)a and O(2)a, and ethoxy oxygen atom, O(4)a and O(1)a, respectively {symmetry

transformation a = 1-x,1-y,1-z} of a neighbouring molecule. The combination of these hydrogen
bonding interactions results a supramolecular dimeric structure for complex 2, as shown in
Figure 8. In complex 3, similar intermolecular hydrogen bonding interactions are observed. The
hydrogen atoms H(5A) and H(5B) attached to the water oxygen atom O(5) participate in
intermolecular hydrogen bonding interactions with the symmetry related phenoxo oxygen atoms,
O(3)a and O(2)a, and the methoxy oxygen atoms, O(4)a and O(1)a, respectively {symmetry
transformation a = 1-x,1-y,1-z} of a neighbouring molecule. A supramolecular dimeric structure
is formed due to these intermolecular hydrogen bonding interactions (Figure 9). Details of the
hydrogen bonding interactions are given in Table 5.
The structures of the both complexes are additionally stabilized by some interesting C-
H⋅⋅⋅π and π⋅⋅⋅π interactions. In complex 2, the hydrogen atom H(19A), attached to the ethoxy
carbon atom C(20), participates in an intermolecular C-H⋅⋅⋅π interaction with the symmetry
related (-x,1-y,-z) phenyl ring Cg(5)[C(14)-C(15)-C(16)-C(17)-C(18)-C(19)]. Because of this
intermolecular C-H⋅⋅⋅π interaction, a supramolecular dimer is formed in the crystal packing of
complex 2 (Figure 10).
In complex 2, the phenyl ring Cg(4) [C(3)-C(4)-C(5)-C(6)-C(7)-C(8)] undergoes
a strong face-to-face π⋅⋅⋅π stacking interaction with the neighboring phenyl ring Cg(5) [C(14)-
C(15)-C(16)-C(17)-C(18)-C(19)] of symmetry 1-x,1-y,1-z. A dimeric arrangement is formed
because of this type of π⋅⋅⋅π stacking interaction (Figure 11). Complex 3 also shows similar type
of π⋅⋅⋅π stacking interaction. The phenyl ring, Cg(4) [C(2)-C(3)-C(4)-C(5)-C(6)-C(7)] is
involved in intermolecular π⋅⋅⋅π stacking interaction with the symmetry related (1-x,1-y,1-z)
phenyl ring, Cg(5) [C(13)-C(14)-C(15)-C(16)-C(17)-C(18)], leading to the formation of a

supramolecular dimeric arrangement (Figure 12). Geometric features of all the C-H⋅⋅⋅π and π⋅⋅⋅π
interactions are given in Tables 6 and 7, respectively.
Figure 8: Hydrogen bonded dimeric structure of complex 2. Only the relevant hydrogen atoms
are shown. Symmetry transformation a = 1-x,1-y,1-z.

Figure 9: Hydrogen bonded dimeric structure of complex 3. Only the relevant hydrogen atoms
are shown. Symmetry transformation a = 1-x,1-y,1-z.
Figure 10: C-H···π interactions in complex 2. Only the relevant hydrogen atoms are shown.

Figure 11: Dimeric structure through π···π stacking interactions in complex 2. Only selected
hydrogen atoms are shown.
Figure 12: Dimeric structure through π···π stacking interactions in complex 3. Only selected
hydrogen atoms are shown.

3.4. IR and electronic spectra
The IR and electronic spectra of all three complexes are in good agreement with their
crystal structures. All the complexes exhibit a strong band at around 1620 cm-1, corresponding to
the azomethine (C=N) stretching vibration [46]. The presence of a terminal azide or thiocyanate
ligand is indicated by the sharp band at around 2065 cm-1 [47]. Bands in the range 2965-2835
cm-1 are assigned to alkyl C-H bond stretching vibrations [48]. Sharp bands for the Fe-O
stretching vibration are found around 735 cm-1 [49].
3
2
All three complexes contain a high spin octahedral iron(III) with the t2g eg
configuration. Therefore, d–d transitions are expected to be of very weak intensity as they are
forbidden by both Laporte and Spin selection rules. The electronic spectra of complex 1 shows
high intensity bands at 410 and 321 nm assignable to charge transfer transitions from the pπ
orbitals of the phenoxide oxygen and amine nitrogen atoms of the Schiff base to the half filled
dπ* orbitals of iron(III) ion respectively [50,51]. Similar charge transfer transitions are observed
around 430 and 540 nm (for complex 2) and 426 and 570 nm (for complex 3). The higher energy
transition bands arise from the intra-ligand n-π* and π-π* transitions [52]. IR and electronic
spectra of complexes 1, 2 and 3 are shown in Figures 13, 14 and 15, respectively.

Figure 13: IR (a) and electronic (b) spectra of complex 1.
Figure 14: IR (a) and electronic (b) spectra of complex 2.

Figure 15: IR (a) and electronic (b) spectra of complex 3.
3.5. Bond valance sum calculation
The bond valence sum (BVS) calculation was performed to calculate the oxidation state
of the iron atom in the investigated complexes. The results are in good agreement for an
oxidation state of iron(III). The deviation of the BVS from an integer value usually indicates
either possible steric constraints, excessive thermal motion, problems with the crystal structure
report or some combination of all of these effects. All the parameters and calculated values
related to the BVS calculation are gathered in Table 8.
3.6. Hirshfeld surfaces
The Hirshfeld surfaces of all three complexes are mapped over dnorm (range -0.1 Å to 1.5
Å), shape index and curvedness (Figure 16). Red spots on the Hirshfeld surfaces mapped with
dnorm denote the dominant interactions (Figure 16). The dominant interactions are O⋅⋅⋅H/H⋅⋅⋅O
and N⋅⋅⋅H/H⋅⋅⋅N (for both 1 and 2), O⋅⋅⋅H/H⋅⋅⋅O and S⋅⋅⋅H/H⋅⋅⋅S (for 3). Other visible spots in the

Hirshfeld surfaces correspond to C···H contacts. The intermolecular interactions appear as
distinct spikes in the 2D fingerprint plot (Figure 17).
Figure 16: Hirshfeld surfaces of all three complexes mapped with dnorm (left), shape index
(middle) and curvedness (right).

Figure 17: Fingerprint plot: Full (left-side) and resolved into C⋯H / H⋯C, O···H / H···O (for 1,
2 and 3), N···H / H···N (for 1 and 2), S···H / H···S (for 3) contacts contributing to the total
Hirshfeld Surface area of all three complexes.
3.7. Magnetic properties
3.7.1. [(µ-OMe)2Fe2(L1)2(N3)2] (1)

The thermal variation of the product of the molar magnetic susceptibility times the
temperature (χmT) per iron(III) dimer, shows a room temperature value of ca. 6.5 cm3 K mol-1
(Figure 18). This value is lower than the expected one for two isolated S = 5/2 high spin iron(III)
centres (8.75 cm3 K mol-1, assuming g = 2), indicating the presence of moderate
antiferromagnetic Fe-Fe interactions in the dimer. When the sample is cooled, χmT shows a
progressive decrease from room temperature down to 5 K, where the χmT value reaches a plateau
with a value of ca. 0.06 cm3 K mol-1 that remains constant down to 2K. This behaviour indicates
that the iron(III) dimer shows an antiferromagnetic Fe-Fe interaction through the double
methoxido bridge and that there is a very small amount of paramagnetic impurity attributable to
the presence of iron(III) monomers due to crystal packing defects or co-crystallization
phenomena. Given the dimeric structure shown by our iron(III) complex, we fitted the magnetic
properties of the complex to a simple S = 5/2 antiferromagnetic dimer model, including a
paramagnetic impurity to account for the low χmT plateau at low temperatures [53]. This simple
model reproduces very satisfactorily the magnetic properties of the complex with the following
parameters: g = 2.035, J = -43.7 cm-1 and a paramagnetic impurity, c = 0.7 % (solid line in
Figure 18, the Hamiltonian is written as −JS1S2).

Figure 18: Thermal variation of χmT per iron(III) dimer for complex 1. Inset shows the thermal
variation of χm. Solid line represents the best fit to the model (see text).
The isothermal magnetization (M) at 2 K confirms the presence of antiferromagnetic
interactions. A very small paramagnetic impurity is responsible for the very weak signal (around
0.1 µB at 5 T). The M (µB) vs H (T) plot is shown in Figure 19.
Figure 19: The plot of magnetization vs. field strength at 2 K.

The magnetic coupling of iron(III) dimers through double alkoxido bridges has been
investigated from theoretical and experimental points of view [54]. These magneto structural
correlations have established a dependence of the J parameter only with the Fe−O bond distance,
assuming that the spherical distribution of the electron density in high spin d5 ions should
minimize the angular dependence of the overlap integrals by compensating the different
exchange pathways. Albeit, the proposed correlation: [55] J = A × exp(B × P) (A = −1.08×1013
cm-1, B = −13.9 Å-1, P = average Fe-O distance along the shortest Fe-O-Fe path) results in too
low J values when the FeO2Fe unit is planar [54]. Thus, in our case, this expression gives a J
value of ca. -7.9 cm-1 for an average Fe-O distance of 2.01045 Å, well below the experimental
one (−43.7 cm-1). When the FeO2Fe units are planar (as observed in our complex), the exchange
pathway implies the dx2-y2 orbitals should be the most effective and, therefore, an angular
dependence is expected. By analysing many FeO2Fe complexes, Colacio et al. [54] found a clear
linear correlation between the Fe-O-Fe angle and the J value. This linear correlation fits quite
well with the value found in our complex (complex 1 in Figure 20). Of course, other factors such
as the Fe-O bond distance and the planarity of the FeO2Fe units may also play a role and may be
at the origin of the slightly higher absolute value observed in our iron (III) dimer (Figure 20).

0
-10
-20
-30
-40
-50
a
b
c
d
1
101
102
103
104
105
106
Fe-O-Fe angle (º)
Figure 20: Correlation between the Fe-O-Fe angle and the J parameters in planar FeO2Fe
complexes with double alkoxido bridges. Complex a extracted from reference [56] and
complexes b-d from reference [54].
3.7.2. [FeL2(OH2)(N3)]‧CH3OH (2) and [FeL3(OH2)(NCS)]·2H2O (3)
The magnetic moment of complexes 2 and 3 at room temperature are found to be 5.72 and 5.70
µB, respectively. The experimental magnetic moment is closer to the theoretical value (5.92 µB)
for a magnetically isolated high-spin iron(III) ion (S = 5/2).
4. Concluding remarks
The synthesis and characterization of one dinuclear and two mononuclear iron(III)
complexes with salicylaldimine Schiff base ligands are described in this paper. X-ray crystal
structure determination confirmed their structures. The solid state structures of all three
complexes show the participation of the organic ligand in concurrent hydrogen bonding, C-H⋅⋅⋅π
and π⋅⋅⋅π interactions involving the alkoxy arm and the aromatic ring. In the dinuclear iron(III)

complex the two iron(III) centres are connected together by alkoxide oxygen atoms, forming a
Fe2O2 core, whilst the azide ions remain terminally coordinated. Both the iron(III) centres have a
high spin configuration. Thus, the magnetic characterization shows the presence of a moderate
antiferromagnetic Fe-Fe coupling through the double methoxido bridge which can be well
correlated with the Fe-O-Fe bond angle along the planar FeO2Fe central core. The moderate
antiferromagnetic exchange interaction mediated through the double methoxido bridges with J =
−43.7 cm–1 is in good agreement with that found in similar methoxido bridged complexes.
Acknowledgements
T.B. thanks the CSIR, India for awarding a Junior Research Fellowship [Sanction No.
09/096(0861)/2016-EMR-I]. K.G. thanks the UGC, India, for awarding a Senior Research
Fellowship. Crystallographic data were collected at the DST-FIST, India funded Single Crystal
Diffractometer Facility at the Department of Chemistry, Jadavpur University.
Appendix A. Supplementary data
CCDC 1579669, 1812958 and 1812959 contains the supplementary crystallographic data
of complexes 1, 2 and 3, respectively. This data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.
References
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Acta 353 (2003) 223.