Existence of stronger C[sbnd]H···π(chelate ring) interaction compared to C[sbnd]H···π(arene) interactions in the supramolecular assembly of dinuclear iron(III) Schiff base complexes: A theoretical insight

Article abstract of DOI:10.1016/j.ica.2020.120081

Two centrosymmetric dinuclear iron(III) complexes, [(μ-O)(FeL)₂] (1) and [Fe₂L₂(N₃)₂] (2) {H₂L = 1,2-Bis[[1-(2-hydroxyphenyl)ethylidene]amino]ethane} have been synthesized and characterized

Full text of DOI:10.1016/j.ica.2020.120081

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Research paper
Existence of stronger C–H· · · π(chelate ring) interaction compared to C–H· ·
· π(arene) interactions in the supramolecular assembly of dinuclear iron(III)
Schiff base complexes: A theoretical insight
Tanmoy Basak, Antonio Frontera, Shouvik Chattopadhyay
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DOI:
Reference:
S0020-1693(20)31281-0
https://doi.org/10.1016/j.ica.2020.120081
ICA 120081
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Inorganica Chimica Acta
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27 July 2020
10 October 2020
10 October 2020
Please cite this article as: T. Basak, A. Frontera, S. Chattopadhyay, Existence of stronger C–H· · · π(chelate ring)
interaction compared to C–H· · · π(arene) interactions in the supramolecular assembly of dinuclear iron(III)
Schiff base complexes: A theoretical insight, Inorganica Chimica Acta (2020), doi: https://doi.org/10.1016/j.ica.
2020.120081
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Existence of stronger C–H···π(chelate ring) interaction compared to C–
H···π(arene) interactions in the supramolecular assembly of dinuclear
iron(III) Schiff base complexes: A theoretical insight
Tanmoy Basaka, Antonio Fronterab,*, Shouvik Chattopadhyaya,*
aDepartment of Chemistry, Inorganic Section, Jadavpur University, Kolkata- 700032, India.
E-mail: shouvik.chem@gmail.com
bDepartamento de Química, Universitat de les Illes Balears, Crta. deValldemossa km 7.5, 07122
Palma, Baleares, Spain. E-mail: toni.frontera@uib.es
Abstract: Two centrosymmetric dinuclear iron(III) complexes, [(-O)(FeL)2] (1) and [Fe2L2(N3)2]
(2) {H2L= 1,2-Bis[[1-(2-hydroxyphenyl)ethylidene]amino]ethane} have been synthesized and
characterized by X-ray crystallographic studies. The structural correlation of these two
complexes has been reported. The energies of interesting C–H···π interactions have been
estimated by DFT calculations. Three different types of C–H···π interactions have been observed
in the solid state of complexes 1 and 2, which involve a classical π-system (arene), and two non-
classical ones (chelate ring and azide co-ligand). Moreover, the NCI plot index has been used to
characterize these interactions. The C–H···π(arene) interactions in complex 2 have been found
to be weaker than the C–H···π(Chelate Ring) interaction in complex 1, thus confirming the
higher ability of the chelate ring to participate in C–H···π interactions.
Keywords: Iron(III), Schiff base, dinuclear, DFT study, C–H···π interactions.

Introduction
Multimetallic complexes may have a wide range of applications in metal clusters,
material science and magnetism, catalysis, transport and activation of small molecules, bio-
inorganic chemistry etc [1]. We have been synthesizing a lot of such complexes using simple
salen and half-salen type Schiff bases since long [2]. Many of these complexes showed
interesting molecular and crystalline architectures [3], attracting magnetic properties [4],
remarkable phto-sensitivity [5], enzyme mimicking activity [6] etc. Different conventional and
non-conventional supramolecular interactions in many such complexes have also been
investigated [7]. DFT calculations were employed to estimate their energies of these
interactions [8].
To explore the supramolecular architecture in coordination complexes is a growing area
of research and is receiving vast attention for its potential uses in a variety of fields [9].
Hydrogen bonding, π···π, C-H···π, cation-π, anion-π interactions etc are the major driving forces
in assembling metal complexes [10]. The electrostatic attraction among the positive hydrogen
atoms and the π-electron cloud of aromatic rings are the driving force to generate C−H···π
interactions. The π-electron cloud of N3/ NCS/ NCO are also found to be involved in C−H···
interactions now-a-days. These interactions are gradually becoming important players in
supramolecular chemistry and crystal engineering [11].
Focussing to iron, chemistry of iron(III) Schiff base complexes have been dominated by
oxygen-bridged species due to its high oxygen affinity [12]. Many polynuclear iron(III)
complexes have been prepared with salen type Schiff bases by several research groups. Many
such complexes were utilized as magnetic materials and many were utilized as catalysts [13]. In

the
present
work,
a
Schiff
base
ligand,
H2L
[1,2-Bis[[1-(2-
hydroxyphenyl)ethylidene]amino]ethane], has been used to prepare two dinuclear iron(III)
complexes, [(-O)(FeL)2] (1) and [Fe2L2(N3)2] (2). Complex 1 is a oxo-bridged dimer with
Fe−O−Fe core having Fe−O−Fe= 155.9(3)° and Fe···Fe distance = 3.47(7) Å and complex 2 is
diphenoxo bridged dimer with Fe2O2 core having Fe−O−Fe= 103.54(6)° and Fe···Fe distance =
3.26(4) Å. Both complexes have been characterized by elemental and spectral analysis. Single
crystal X-ray diffraction study has confirmed their structures. Similar dimeric iron(III) complex
are important and interesting to magnetic chemists for their magnetic properties [14]. There
are many examples of ferromagnetic complexes with similar dimeric iron(III) building blocks.
Catalytic activities of these types of complexes, [e.g. phenoxazinone synthase or catechol
oxidase mimicking activity etc], are also very common [15]. Recently, we have shown that
similar complexes may be used as semiconductor to fabricate electronic devices [16]. However,
in this work, we have studied in details the supramolecular interactions in the solid state
structures of both complexes.
Three different types of C–H···π interactions have been observed in the solid state of
complexes 1 and 2, which involve a classical π-system (arene), and two non-classical ones
(chelate ring and azide co-ligand). The energies of these interesting C–H···π interactions have
been estimated and compared by using calculations with the purpose to investigate their
relative importance. Moreover, the NCI plot index has been used to characterize these
interactions.

Experimental
Materials
All other chemicals were of reagent grade and used as purchased from Sigma-Aldrich
without further purification.
Caution!!! Although no problems were encountered in this work, organic ligands in the
presence of azide are potentially explosive. Only a small amount of the material should be
prepared and it should be handled with care.
Preparation
Preparation of the ligand
The condensation of ethylenediamine (~2 mmol, 0.1 mL) with 2-hydroxy-1-
acetophenone (~4 mmol, 0.280 g) in 1:2 molar ratio produces a Schiff base ligand, 1,2-bis[[1-(2-
hydroxyphenyl)ethylidene]amino]ethane (H2L). The ligand was not isolated and the acetonitrile
solution of the ligand was used for the synthesis of complexes 1 and 2.
Preparation of the complexes
[(-O)(FeL)2] (1)
An acetonitrile solution (5 mL) of ethylenediamine (~2 mmol, 0.12 mL) with 2-hydroxy-1-
acetophenone (~4 mmol, 0.280 g) in 1:2 molar ratio was refluxed for ca. 1 h. An acetonitrile
solution of iron(III) chloride hydrate (~2 mmol, 0.330 g) was added slowly with stirring and after

that the solution was refluxed for ca. 30 min. It was cooled in room temperature and a small
amount of precipitate was filtered off. The filtrate was kept in open atmosphere for 3-4 days to
obtain dark green crystalline product which was collected by filtration. X-ray quality single
crystals were collected from this crystalline product.
Yield: 0.522 g, 73% (based on iron). Anal. Calc. for C36H36Fe2N4O5 (716.39): C, 60.33; H,
5.07; N, 7.82%. Found: C, 60.2; H, 4.9; N, 7.9 %. FT-IR (KBr, cm−1): 2996-2928(νC−H); 1595 (νC=N);
831; 870 (νas, Fe–O–Fe); λmax (nm) [εmax(lit mol-1 cm-1)] (acetonitrile): 480 (4.3 x 102), 330 (2.3 x 103),
230 (8 x 103). ESI-MS m/z = 349.9630 [FeL]+ and 716.9677 [(-O)(FeL)2(H)]+. Magnetic moment=
5.76 BM.
[Fe2L2(N3)2] (2)
An acetonitrile solution (5 mL) ethylenediamine (~2 mmol, 0.1 mL) with 2-hydroxy-1-
acetophenone (~4 mmol, 0.280 g) in 1:2 molar ratio was refluxed for ca. 1 h. An acetonitrile
solution of iron(III) chloride hydrate (~2 mmol, 0.330 g) was added slowly and after that an
aqueous solution of sodium azide (~2 mmol, 0.130 g) was then added to it with continue
stirring. The resulting solution was refluxed further for ca. 30 min. The reaction mixture was
allowed to cool at room temperature and kept in open atmosphere for 7 days. Dark green
crystalline product was collected by filtration. X-ray quality single crystals were collected from
this crystalline product.
Yield: 0.541 g, 69% (based on iron). Anal. Calc. for C36H36Fe2N10O4 (784.45): C, 55.12; H,
4.63; N, 17.86%. Found: C, 55.0; H, 4.5; N, 17.9 %. FT-IR (KBr, cm-1): 2965-2850 (νC−H); 2048(νN3);

1595 (νC=N), 873; 830 (νas, Fe–O–Fe); λmax (nm) [εmax(lit mol-1 cm-1)] (acetonitrile): 478 (9.0 x 102),
315 (2.7 x 103), 228 (9 x 103). ESI-MS m/z = 350.0318 [FeL]+. Magnetic moment= 5.82 BM
Physical measurements
Elemental analysis (carbon, hydrogen and nitrogen) was performed using a Perkin-Elmer
240C elemental analyzer. IR spectrum in KBr (4500-500 cm-1) was recorded with a Perkin-Elmer
Spectrum Two spectrophotometer. Electronic spectra in acetonitrile were recorded on a
Shimadzu UV-1800 spectrophotometer. Electrospray ionization mass spectra were recorded
using Waters QTOF Micro YA263. Powder X-ray diffraction was performed on a Bruker D8
instrument with Cu-Kα radiation (λ = 1.548 Å) generated at 40 kV and 40 mA. In this process,
the complexes were ground with a mortar and pestle to prepare fine powders. The powders
were then dispersed with alcohol onto a zero background holder (ZBH). The alcohol was
allowed to evaporate to provide a nice, even coating of powder adhered to the sample holder.
The powder XRD spectrum was recorded in a 2θ range of 5–50 using 1D Lynxeye detector at
ambient conditions. Thermogravimetric analysis of complex 1 was performed up to 200C in
nitrogen atmosphere with a rate of 2C per minute in a PerkinElmer Diamond (TG/DTA)
instrument using Pyris software.
X-ray Crystallography
Suitable crystals of both complexes were used for data collection using a ‘Bruker D8
QUEST area detector’ diffractometer equipped with graphite-monochromated Mo Kα radiation
( = 0.71073 Å). Molecular structures were solved by direct method and refined by full-matrix
least squares on F2 using the SHELX-18 package [16]. Non-hydrogen atoms were refined with

anisotropic thermal parameters. All other 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 program SADABS [17]. The details of
crystallographic data and refinements have been given in Table 1.
Hirshfeld surface analysis
Hirshfeld surfaces [18] and the associated two-dimensional (2D) fingerprint [19] plots
were calculated using Crystal Explorer[20], with bond lengths to hydrogen atoms set to
standard values [21]. 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 rivdw and revdware 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 [22].
Theoretical Methods
The energetic features of both complexes included in this study have been calculated at
the B3LYP-D/def2-SVP level of theory using the crystallographic coordinates. The binding

energies of the supramolecular assemblies have been computed by subtracting the sum of
energies of the isolated monomers (X-ray geometries) to the total energy of the assembly. This
methodology has been used to study a great deal of interactions [23]. For the calculations, the
GAUSSIAN-09 program has been used [24]. The Grimme’s dispersion correction has also been
used as implemented in GAUSSIAN-09 program since it is adequate for the evaluation of non-
covalent interactions where dispersion effects are relevant. The basis set superposition error
for the calculation of interaction energies has been corrected using the counterpoise method
[25]. Molecular electrostatic potential (MEP) surfaces have been computed at the same level of
theory and represented using the 0.001 a.u. isosurface [26]. The NCI plot index [27]
computational tool has been used to characterize non-covalent interactions using the B3LYP-
D/def2-SVP wave function. They correspond to both favorable and unfavorable interactions, as
differentiated by the sign of the second density Hessian eigen value and defined by the
isosurface color. The color scheme is a red-yellow-green-blue scale with red for ρ+cut (repulsive)
and blue for ρ−cut (attractive).
Results and discussion
Synthesis
The tetradentate N2O2 donor Schiff base ligand (H2L) was produced by condensation of
ethylenediamine and 2-hydroxy-1-acetopheone in 1:2 molar ratio, following the literature
method [28]. The ligand was not isolated and the yellow coloured acetonitrile solution was
used directly for the synthesis of both complexes. Complex 1 was prepared on adding iron(III)
chloride hydrate to the acetonitrile solution of the ligand in open atmosphere. Here the source

of oxo bridge is the water molecule of iron(III) chloride hydrate. However the reaction was
carried out in inert atmosphere also and found the same reaction product as reported. So it can
be concluded that the oxide ion comes from the water molecule of iron(III) chloride hydrate not
from air. On the other hand, addition of sodium azide and iron(III) chloride hydrate into the
acetonitrile solution of the ligand produced complex 2. The synthetic route for both complexes
was shown in Scheme 1.
It is to be noted here, that a different research group prepared complex 1 in a different
procedure starting from FeIIILCl and Ag2O, dissolved in a solution of CH2Cl2 [29]. It is also to be
noted here that another dinucldear iron(III) complex, iso-structural with complex 1, was
reported in literature [30]. The complex was, however, crystallized in a different space group.
The magnetic properties of both the complexes were studied in detail but the supramolecular
interactions in the solid state structures of the complexes were not analyzed by DFT calculation.
In this work, however, we are interested to focus light on the supramolecular interactions in its
solid state structure. The C–H···π (chelate ring) interaction in complex 1 is found to be stronger
than the C–H···π (arene) interactions in complex 2 (vide infra).

Scheme 1. Synthetic route for the ligand, H2L, and complexes 1 and 2.
IR and electronic spectra and magnetic moments
In the IR spectrum of each complex exhibit a strong band at 1595 cm-1, corresponding to
the azomethine (C=N) stretching vibration [31]. A sharp band arises around 2048 cm-1 for
complex 2, due to the presence of terminal azide co-ligand [32]. The C−H stretching vibrations
for both complexes are in the range of 2942−2968 cm−1 [33]. It is well known that the
antisymmetric stretching mode namely νas(Fe–O–Fe) of a linear or bent Fe–O–Fe system usually
occurs in the 900–800 cm-1 region [34]. The 870–830 cm-1 absorptions found for both
complexes 1 and 2 indicate the existence of Fe–O–Fe bonds.
The electronic spectra of both complexes were recorded in acetonitrile medium. Each
complex exhibit intense absorption bands around 230 nm which can be assigned as π→π*
electronic transition. Another bands observed at 330 nm (for 1) and 315 nm (for 2) may arise

from n→π* electronic transition [14b]. A band arises in visible region around 480 nm which can
be assigned as charge transfer transition from pπ orbital of phenoxido oxygen to the dπ orbitals
of the iron(III) [15].
The magnetic moments of 1 and 2 at room temperature are 5.76 and 5.82 BM,
respectively. These values are very close to the theoretical value (5.92 MB) for a magnetically
isolated high-spin iron (III) ion (S = 5/2).
Description of structure
[(-O)(FeL)2] (1)
Complex 1 crystallizes in the monoclinic space group C2/c. It is a centrosymmetric dimer
in which the oxygen atom resides on the center of inversion. A perspective view of the complex
with selective atom-numbering scheme is shown in Fig. 1. Important bond lengths and bond
angles in the coordination sphere of iron(III) centers are listed in Tables 2 and 3, respectively.
The iron(III) center, Fe(1), is penta-coordinated and consist of square pyramidal
geometry which is confirmed by the Addison parameter(τ) value (τ= 0.07) [35]. The equatorial
positions are occupied by two imine nitrogen atoms, N(1) and N(2) and two phenoxo oxygen
atoms, O(1) and O(2) of the deprotonated Schiff base ligand (L)2−. The axial position is occupied
by one oxygen atom, O(3), of an oxide ion, which acts as a bridge between two [FeL] units. The
bridging angle Fe(1)−O(3)−Fe(1)a {a= 1−x,y,1.5−z} is 155.9(3)°. The deviations of the coordinating
atoms N(1), N(2), O(1) and O(2) from the least-square mean plane through them are −0.103(3)
Å, −0.168(4) Å, −0.139(3) Å and −0.068(3) Å, respectively, and that of Fe(1) from the same plane

is 0.479(5) Å. The trans angles, N(1)−Fe(1)−O(2) and N(2)−Fe(1)−O(1), around Fe(1) are found to
be 142.2(1)° and 146.5(1)°, respectively. The conformation of the saturated five-membered ring
Fe(1)−N(1)−C(9)−C(10)−N(2) is closer to envelope with the puckering parameters [36], q =
0.557(4) Å, and φ = 224.8(4)°.
Fig. 1. Perspective view of complex 1 with selective atom numbering scheme. Hydrogen atoms
have been omitted for clarity. Symmetry transformation: a= 1−x,y,1.5−z.
[Fe2L2(N3)2] (2)
Single crystal X-ray crystallographic data analysis reveals that the complex 2 crystallizes
in the monoclinic space group P21/n. A perspective view of the complex with selective atom-
numbering scheme is shown in Fig. 2. It is also a centrosymmetric dimer and the asymmetric
unit of the complex contains [FeL(N3)] unit. Important bond lengths and bond angles in the
coordination sphere of the iron(III) centers are listed in Tables 2 and 3, respectively.

The geometry around the iron(III) center, Fe(1), is octahedral. The basal plane of iron(III)
center, Fe(1), consists of two imine nitrogen atoms, N(1) and N(2), and two phenoxo oxygen
atoms, O(1) and O(2), of the deprotonated Schiff base ligand, L2−. The deviations of the
coordinating atoms, N(1), N(2), O(1) and O(2), from the least-square mean plane through them
are −0.092(14) Å, 0.015(15) Å, 0.035(16) Å and −0.089(16) Å, respectively and that of Fe(1) from
the same plane is 0.131(3) Å. The trans angles, N(1)−Fe(1)−O(2) and N(2)−Fe(1)−O(1), around
Fe(1) are found to be 167.12(6)° and 161.06(6)°, respectively. The fifth coordination site of
iron(III) is coordinated by one nitrogen atom, N(3), of a terminal azide. One phenoxo oxygen
atom, O(1)b {b= 1−x,1−y,1−z}, of another deprotonated Schiff base ligand from a symmetry
related molecule of the complex coordinates in sixth coordination site to complete the
octahedral geometry of iron(III) with the formation of a dinuclear species having Fe2O2 core.
The Fe(1)−O(1)−Fe(1)b angle in the Fe2O2 core is 103.54(6)°. The conformation of the saturated
five-membered ring Fe(1)−N(1)−C(9)−C(10)−N(2) is closer to envelope with the puckering
parameters [36], q = 0.388(2) Å, and φ = 76.4(2)°.

Fig. 2. Perspective view of complex 2 with selective atom numbering scheme. Hydrogen atoms
have been omitted for clarity. Symmetry transformation: b= 1−x,1−y,1−z.
Table 4 gathers some selected structural parameters of some similar dinuclear iron(III)
complexes with salen type Schiff base ligands, previously reported in literature [37].
ESI-MS study
The electron spray ionization mass spectra (ESI-MS positive) of both complexes
were recorded in acetonitrile medium and the results are shown in Figs. 3 and 4, respectively. A
molecular ion peak for complex 1 was observed at m/z = 716.9677 [(-O)(FeL)2(H)]+. The
dinuclear complexes were found to dissociate into two mononuclear moieties; the molecular
ion peak for complex 1 was observed at m/z = 349.9630 (theoretical m/z = 350.07) and might
be assigned to [FeL]+. Similar type of molecular ion peak in the mass spectrum of complex 2,
which was observed at m/z = 350.0318 ([FeL]+, theoretical m/z = 350.07).

Fig. 3. ESI-MS positive spectrum of the complex 1
Fig. 4. ESI-MS positive spectrum of the complex 2

Hirshfeld surfaces
The Hirshfeld surfaces of the both complexes are mapped over dnorm (range −0.1 Å to 1.5
Å), shape index and curvedness (Fig. 5). Red spots on the Hirshfeld surfaces mapped with dnorm
denote the dominant interactions (Fig. 5). The dominant interactions are C···H/H···C and
O···H/H···O (for 1), C···H/H···C and N··H/H···N (for 2). The intermolecular interactions appear as
distinct spikes in the 2D fingerprint plot (Fig. 6).
Fig. 5. Hirshfeld surfaces of both complexes, mapped with dnorm (left), shape index (middle) and
curvedness (right).

Fig. 6. The proportion of O···H/H···O interactions comprise 15.1% of the Hirshfeld surfaces for 1
and N···H/H···N interaction comprise 27.3% of the Hirshfeld surfaces for 2. The proportion of
C···H/H···C interactions comprise 20.6% and 16.0% of the Hirshfeld surfaces for complex 1 and
2, respectively.
Powder X-ray Diffraction and TGA
The experimental PXRD patterns of the bulk products of both complexes are in good
agreement with the simulated XRD patterns from single crystal X-ray diffraction results,
indicating consistency of the bulk sample. The simulated patterns of both complexes 1 and 2
are calculated from the single crystal structural data (cif) using CCDC Mercury software.
The TGA of complex 1 shows that that the complex is stable up to 120oC and then
decomposition starts. Complex 2, however, contains azide and so its TGA study could not be
performed to avoid potential explosion during experiment.

DFT study on supramolecular interactions
Two different types of C–H···π interactions are observed in the crystal packing of
complexes 1 and 2, where in both the C–H bond donor is the methyl group of the Schiff-base
ligand and the acceptor is a conventional aromatic ring in 2 and an unconventional chelate ring
in 1. Both supramolecular assemblies observed are represented in Fig. 7. In particular, complex
1 forms self-assembled dimers where two symmetrically equivalent C–H···π (CR) are established
involving the six-membered chelate ring. Instead, complex 2 forms supramolecular 1D infinite
chains where conventional C–H···π(arene) are relevant propagating the chain. The theoretical
study is dedicated to analyze the energy associated to these interactions in order to compare
the ability of the chelate and phenyl rings to participate in C–H···π interactions as electron
donors. Moreover, in complex 2, additional C–H···π(azide) interactions are also described and
studied (vide infra).
Fig. 7. Partial views of the X-ray structures of 1 (a) and 2 (b). The C–H···π interactions have been
represented using dashed lines. H-atoms omitted for clarity apart from those participating in
the C–H···π interactions.
The molecular electrostatic potential (MEP) surface of complex 1 is shown in Fig. 8a. It
can be observed that the MEP value over the center of the six membered ring is more negative

(–26 kcal/mol) than that over the phenyl ring (–22 kcal/mol) thus it is expected to form
strongest interactions with C–H bonds. The MEP surface also reveals that the values at the H-
atoms of the methyl group are positive (+14 kcal/mol) likely due to the inductive effect
provoked by the N-coordination to the iron(III) ion. Therefore, the MEP surface analysis
anticipates a favorable electrostatic attraction between the methyl group and the chelate ring.
The interaction energy of the self-assembled dimer of complex 1 (see Fig. 8b) is large and
negative (ΔE1 = –34.4 kcal/mol), due to the formation of several van der Waals interactions in
addition to both C–H···π(CR) contacts, which are very short (2.57 Å). In an effort to estimate the
contribution of the C–H···π(CR) interactions, we have computed an additional theoretical model
where the methyl groups have been changed by H-atoms (see small arrows in Fig 9c). As a
result, the interaction energy reduces to ΔE2= –23.4 kcal/mol and, consequently, the
contribution of both C–H···π(CR) interactions is –11 kcal/mol (–5.5 kcal/mol each interaction),
which is quite strong for this type of interactions. The NCI plot (Fig 9d) shows the existence of
two large and green isosurfaces between the methyl groups and the chelate rings, thus
characterizing the C–H···π(CR) interactions. Moreover, it also reveals the existence of weak van
der Waals interactions between both molecules that also contribute to the stabilization of the
supramolecular dimer.

Fig. 8. (a) MEP surface of complex 1 (isosurface = 0.001). The energies at selected points of the
surface are indicated in kcal/mol; (b) Self-assembled dimer observed in the X-ray solid state
structure of complex 1; (c) theoretical model where the methyl groups were removed and
substituted with hydrogen atom. Distances in Å; (d) NCI surface of the self-assembled dimer of
complex 1. The gradient cut-off is s = 0.35 au, and the color scale is −0.04 <ρ< 0.04 au.
The molecular electrostatic potential (MEP) surface of complex 2 is shown in Fig. 9a. It
can be observed that six-membered chelate ring is not accessible due to the presence of the
azide ligand. The MEP value over the center of the six membered phenyl ring is negative and
similar to that found in complex 1 (–17 kcal/mol) therefore adequate for interacting with C–H
bonds. The MEP surface also reveals that the values at the H-atoms of the methyl group are

positive (+15 kcal/mol) and very similar to that found in complex 1. As shown in Fig. 9b, this
complex forms an infinite 1D assembly in the solid state. We have retrieved a dimer and
computed the interaction energy, which is shown in Fig. 9b, ΔE3 = –26.5 kcal/mol. In addition to
the C–H···π (arene) interactions, a close examination of the dimer reveals that one aromatic C–
H bond on one molecule is pointing to the π-system of the azide co-ligand (H···N distances 2.78
and 2.86 Å) of the adjacent molecule, and vice versa. In an effort to estimate the contribution
of the C–H···π (arene) interactions, we have computed an additional theoretical model where
the methyl groups were removed and substituted with hydrogen atom to H-atoms (see small
arrows in Fig 9c). As a result, the interaction energy reduces to ΔE4 = –20.1 kcal/mol and,
consequently, the contribution of both C–H···π(arene) interactions can be estimated by
difference, that is –6.4 kcal/mol (–3.2 kcal/mol each one), which is weaker than the C–H···π(CR)
estimated for complex 1, thus confirming the higher ability of the chelate ring to participate in
C–H···π interactions. The NCI plot (Fig.9d) shows the existence of two large and green
isosurfaces between the methyl groups and the phenyl rings, thus characterizing the C–H···π
(arene) interactions. Moreover, it also confirms the existence of C–H···π (azide) interactions,
which are characterized by several small isosurfaces between the azide ligands and the
aromatic proton.

Fig. 9. (a) MEP surface of complex 2 (isosurface = 0.001). The energies at selected points of the
surface are indicated in kcal/mol; (b) Dimer retrieved from the X-ray solid state 1D infinite chain
in complex 2; (b) Theoretical model where the methyl groups were removed and substituted
with hydrogen atom. Distances in Å; (c) NCI surface of the dimer of complex 2. The gradient cut-
off is s = 0.35 au, and the color scale is −0.04 <ρ< 0.04 au.
Concluding remarks
Two dinuclear iron(III) Schiff base complexes have been synthesized and structurally
characterized by X-ray crystallography. Two iron(III) centers are bridged by an oxo group in
complex 1 and they are bridged by is phenoxo groups in complex 2. C–H···π(arene), C–H···π(CR)
and unconventional C–H···π(azide) interactions present in the solid state structures of these

complexes were estimated by DFT calculations and further characterized by the NCI plot index
computational tool. The MEP surface plots reveal that both phenyl and chelate ring present
negative values over the center of the ring and, consequently they are well suited for
interacting with electron deficient atoms. The contribution of C–H···π(arene) interactions in the
solid state packing of complex 2 has been estimated (–3.2 kcal/mol for each interaction), which
is weaker than the C–H···π(CR) interaction (estimated as –5.5 kcal/mol for each interaction) in
complex 1, thus confirming the higher ability of the chelate ring to participate in C–H···π
interactions.
Acknowledgements
T.B. thanks the CSIR, India, for awarding a Senior Research Fellowship [Sanction No.
09/096(0861)/2016-EMR-I]. S.C. gratefully acknowledges UGC-CAS II program, Department of
Chemistry,
Jadavpur
University,
for
financial
support
under
the
head
[Chemicals/Consumables/Glassware]. A. Frontera acknowledges the financial support of this
work by the MINECO/AEI of Spain (project CTQ2017-85821-R, FEDER funds). He thanks the CTI
(UIB) for free allocation of computer time.
Appendix A. Supporting Information
CCDC 1948760 and 1948761 contain the supplementary crystallographic data for
complexes 1 and 2, respectively.

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