Magnetic properties of novel dendrimeric spin crossover iron(III) complex

Article abstract of DOI:10.1016/j.ica.2015.10.024

The synthesis and magnetic properties of novel dendrimeric spin crossover Fe(III) complex of formula [Fe(L)₂]⁺PF₆^-, where L = 3,5-di(3,4,5-tris(tetradecyloxy)benzoyloxy)benzoyl-4-oxy-salicylidene-N′-ethyl-N-ethy

Full text of DOI:10.1016/j.ica.2015.10.024

Inorganica Chimica Acta 439 (2016) 186–195
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Magnetic properties of novel dendrimeric spin crossover iron(III)
complex
a
b
c
c
Natalia Domracheva a, , Valerya Vorobeva , Andrew Pyataev , Rui Tamura , Katsuaki Suzuki ,
⇑
Matvey Gruzdev d, Ulyana Chervonova d, Arkadij Kolker d
a Zavoisky Kazan Physical-Technical Institute, Russian Academy of Science, Sibirsky Tract 10/7, 420029 Kazan, Russia
b Kazan Federal University, Kremlyovskaya St. 18, 420008 Kazan, Russia
c Graduate School of Human and Environmental Studies, Kyoto University, Kyoto 606-8501, Japan
d G.A. Krestov Institute of Solution Chemistry, Russian Academy of Science, Akademicheskaya St. 1, 153045 Ivanovo, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 June 2015
Received in revised form 16 October 2015
Accepted 16 October 2015
Available online 2 November 2015
The synthesis and magnetic properties of novel dendrimeric spin crossover Fe(III) complex of formula [Fe
(L)2]+PFꢀ6 , where L = 3,5-di(3,4,5-tris(tetradecyloxy)benzoyloxy)benzoyl-4-oxy-salicylidene-N0-ethyl-N-
ethylenediamine have been studied for the first time by magnetic susceptibility, Electron Paramagnetic
Resonance (EPR) and Mössbauer spectroscopy in the wide (2–300 K) temperature range. EPR showed that
the compound is magnetically inhomogeneous, consists of two magnetic sub-lattices, displays a partial
spin crossover (S ¼ 5=2 () 1=2) of ꢁ25% of the Fe(III) molecules above 160 K and undergoes the antifer-
romagnetic (AF) ordering below 10 K. High-spin (HS, S = 5/2) Fe(III) centers with weakly distorted octa-
hedral environment most probably form chains in layers. The dimeric molecules, formed from low-spin
(LS, S = 1/2) centers and HS centers with strongly distorted octahedral environment are likely located
between the layers and are involved in the spin crossover. EPR has shown the presence of AF dynamical
spin clusters in the high temperature (70–300 K) range, which are visible in the short time scale (10ꢀ10 s)
and could not be registered in the static magnetic measurements. Mössbauer spectra demonstrated in a
paramagnetic state of the compound a quadrupole doublet with average isomer shift of 0.35 mm/s and
splitting 0.72 mm/s corresponding to HS Fe(III) centers. Below 60 K, the spectra displayed the appearance
of magnetic hyperfine structure, whose relaxation nature testifies the collective spin flips of small
clusters in the material. Mössbauer spectroscopy confirmed the existence of AF ordering in the Fe(III)
dendrimeric complex at 5 K.
Dedicated to the memory of Dr. R.A.
Manapov.
Keywords:
Iron(III) complex
EPR spectroscopy
Mössbauer spectroscopy
Spin-crossover
Magnetic susceptibility
Dendrimers
Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction
by another. In this context, cationic spin-crossover (SCO) iron(III)
complexes are suitable for this purpose. Iron(III) centers can adopt
Designing magnetic materials with two or more potential
functions arising from different physical properties has become a
strong topic in chemical and materials science [1,2]. A wide choice
of constituent molecules could allow the appearance in the same
compound of unusual combination of physical properties. A suit-
able approach to obtain such a multifunctional material is the
hybrid approach in which the material structure is constructed
through self-assembly of two different molecular fragments having
distinct properties. An interesting example is the multifunctional
material that exhibits spin-crossover behavior with other physical
properties [3–5]. Such synergic approach makes possible the
production of systems in which one property may be controlled
two different spin states, that is, low-spin (LS) and high-spin (HS)
states that are interconvertible under an external stimulus such
as temperature, pressure, light irradiation or solvents [6]. Consid-
erable research efforts are focused to utilize the SCO phenomenon
together with the non-linear optical properties [4], chirality [7],
electrical conductivity [8] or long-range order [9].
The motivation for this work came from our desire to create a
novel multifunctional material, in which spin-crossover phenom-
ena and magnetic ordering coexist. This strategy may open a way
to design switching magnets in which the magnetic ordering of
the system could be tuned, thus taking advantage of the possibility
to induce the SCO phenomenon by applying the external stimuli
such as light or pressure. However, this challenging goal requires
first the preparation of such a material which is able to display
the coexistence of spin crossover and magnetic ordering.
⇑
Corresponding author. Tel.: +7 (843) 2319065.
E-mail address: ndomracheva@gmail.com (N. Domracheva).
http://dx.doi.org/10.1016/j.ica.2015.10.024
0020-1693/Ó 2015 Elsevier B.V. All rights reserved.

N. Domracheva et al. / Inorganica Chimica Acta 439 (2016) 186–195
187
Until now, work in this direction was carried out in the follow-
ing way. The material structures have been constructed through
the self-assembly of two different magnetic sub-lattices of
transition metal ions having distinct properties. Namely, spin
crossover (SCO) metal complexes have been integrated into 1D,
2D or 3D magnetically ordered sub-lattice of the other metal com-
plexes. Examples of coexistence of the spin crossover behavior
with magnetic ordering are very few today [10–12].
We suggest another way to obtain such bi-functional system:
embedded SCO iron(III) complex into the focal point (core) of den-
drimer molecule. This bi-functional system will be prepared by
only one type of metal complexes. It is known that dendritic mole-
cules can self-assemble into ordered supramolecular structures
[13]. So, it can be expected that the self-organization of dendrons
will generate a magnetic ordering in a system as it has been in
the case of dendrimer-functionalized gold nanoparticles (NPs)
showing ferromagnetism at room temperature [14].
permeation chromatography, elemental analysis, FT-IR, NMR spec-
troscopy and MALDI-ToF-MS method. The results of these methods
permit us to construct the schematic model of the complex given in
Scheme 1. Moreover, these data allowed one to conclude that the
compound is a monocationic bis(ligand) Fe(III) complex of formula
[Fe(L)2]+PFꢀ6 , where iron(III) ion has an octahedral geometry with
N4O2 donor atoms formed by three atoms (ONN) of tridentate
ligand (L). X-ray diffraction data obtained for similar [Fe(Saen)2]+
Clꢀ complex but without dendritic periphery [15] showed that
bis-tridentate ligands are arranged in a mertidional (mer) configu-
ration. DFT calculations [16] carried out for the full complex with
dendritic environment confirmed that mer configuration of ligands
is energetically most preferable. The optimized structure is shown
in Fig. 1a.
Such a model of coordination polyhedron allows the formation
of dimeric molecules, where iron ions are coupled by weak inter-
molecular interactions (H-bonds between the NRH fragments of
the neighboring complexes and the anions) [15]. The possibility
of the formation of such kind of dimeric molecules is also con-
firmed by quantum chemical calculations (Fig. 1b). Furthermore,
X-ray diffraction data showed that [FeIII(Saen)2]+ cations are
In the present paper, we report about the unusual magnetic
properties of spin-crossover dendrimeric iron(III) complex by
EPR, Mössbauer spectroscopy and magnetic susceptibility, which
demonstrate the coexistence of a partial spin crossover, magnetic
ordering and AF dynamical spin clusters in the system.
packed in
a
chain forming layered structures [17] for
[FeIII(Saen)2]+ClO4ꢀꢂ0.5H2O compound and linked into a dimer
[15,17] for complex [FeIII(Saen)2]+ClOꢀ4 , wherein anions are located
between the cationic [Fe(L)2]+ fragments. It is not possible to grow
a crystal and to get accurate crystallographic data for such a highly
branched dendrimeric iron complex. Therefore, the X-ray powder
diffractogram was measured on a sample at 300 K, which is shown
in Fig. 2. The diffractogram demonstrates one intense low-angle
peak and two broad halos in the region of the high angles.
According to the results of studies of a similar iron mesogenic
complex, this single low-angle peak probably indicates a layered
structure [18].
2. Results and discussion
2.1. Synthesis and Characterization of the dendritic iron(III) complex
The synthesis of iron(III) bis[3,5-di(3,4,5-tris(tetradecyloxy)ben-
zoyloxy)benzoyl-4-oxy-salicylidene-N0-ethyl-N-ethylenediamine]
hexafluorophosphate complex was performed in accordance with
Scheme 1; the details of synthesis are given in the Experimental
Section. The synthesized compound was characterized by gel
Scheme 1. Schematic representation of the synthetic procedure and the model of Fe(III) complex with PFꢀ6 counter ion.

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Fig. 1. The optimized structures of (a) Fe(III) complex and (b) dimeric molecule formed by two Fe(III) complexes and two chloride anions.
Fe(III) ions in an octahedral environment with weak (D ꢄ 0.3 cmꢀ1
,
E = 0) distorted crystal field (II-type of HS centers). The axial
resonance with g\ = 2.208, g|| = 1.933 belongs to low-spin (LS,
2
S = 1/2) Fe(III) centers that have the T2g ground state. It is impor-
tant to note that the observation of distinct HS and LS EPR signals
implies that the spin-state interconversion rates in the region of
4.2–300 K are slower relative to the X-band EPR time scale
(ca. 10ꢀ10 s).
The temperature dependence of the EPR lines integrated inten-
sity (I) is one of the sources of information about the spin transi-
tion process. The magnetic behavior of the compound reflected
by the temperature dependencies of I and I ꢅ T product of the
whole EPR spectrum is shown in Fig. 4. One can see that the tem-
perature dependence of I has complicated, two-step behavior:
reaches the maximum at 10 K and then drops to the minimal value
at about 70 K in the first temperature interval (4.2–70 K) and
further begins to grow in the second temperature interval
(70–300 K) (Fig. 4a). Variation of I ꢅ T product from temperature
(Fig. 4b) shows that the direct (4.2–300 K) and the inverse
(300–4.2 K) thermal cycles practically coincide.
Let us try to understand the origin of the observed anomaly in
our compound. For this purpose the temperature dependencies
of the EPR lines integrated intensity were examined for each type
of Fe(III) centers separately. The analysis of the EPR signals for each
type of iron centers was based on the procedure of the model spec-
trum fitting to the observed experimental one. To fit the EPR spec-
tra written in numerical format, we used the standard EasySpin-
EPR spectrum simulation program. The parameters of the fit were
the g- and D-tensor components, the line shape and the widths of
Fig. 2. X-ray diffractogram for the powder sample at room temperature.
The liquid crystalline properties of the compound were studied
by differential scanning calorimetry and optical polarizing micro-
scopy. It has been shown that above 317.3 K the complex exhibits
mesomorphic properties and turns into isotropic liquid at 414.9 K.
2.2. Electron paramagnetic resonance
The temperature profile of the X-band EPR spectra
(hm
= 0.3 cmꢀ1) for the iron(III) dendrimeric complex measured in
the region of 4.2–300 K are illustrated in Fig. 3. Three signals are
observed in EPR spectra, one low-field signal with geff = 4.2 and
two high-field signals: the broad line with geff = 2 and the axial
signal with g\ = 2.208, g|| = 1.933.
The effective g values for high-spin (HS, S = 5/2) Fe(III) centers
are determined by zero-field splitting (ZFS) of the 6A electronic
state. Wickman [19] and Aasa [20] have used the Hamiltonian (1)
the individual components
DH.
The first step is to turn our attention to the estimation of the
fine structure parameter (D) of the broad signal belonging to HS
Fe(III) ions of the II-type (octahedral iron centers with weak axial
distortion). A computer simulation of the broad signal indicates
that the value of D should vary in the range 0.02 cmꢀ1 < |D|
< 0.03 cmꢀ1. We used for the model spectrum fitting the absolute
value of D = 0.025 cmꢀ1, but a large number of different D values,
belonging to the indicated range, can describe the broad signal
(see Fig. 6b).
ꢀ
ꢁ
1
3
2
^
2
^
2
^
^
^^
H ¼ D Sz ꢀ SðS þ 1Þ þ EðSx ꢀ SyÞ þ gbHS;
ð1Þ
to describe the combined effects of axial ZFS (D), rhombic ZFS (E)
and the Zeeman interaction on the 6A electronic term. The positions
of the fine structure lines in the spectrum depend on the value of
The simulated EPR spectra of the compound are depicted in
Fig. 3 (see dashed lines) which have been calculated with the
the ratio between the microwave quantum h
m
and ZFS-parameters,
following
magnetic
parameters:
g = 2.0,
D = 0.421 cmꢀ1
,
which are induced by distortions of the crystal field in the
E = 0.109 cmꢀ1 for the I-type HS Fe(III) ions; g = 2.0,
D = 0.025 cmꢀ1 for the II-type of HS centers and gx,y = 2.208,
gz = 1.933 for the LS Fe(III) ions. The individual Lorentzian line
shape was used for the simulation. As can be seen, a satisfactory
compound. The geff = 4.2 signal belongs to HS iron ions with strong
(D ꢃ h
m
= 0.3 cmꢀ1) low-symmetry (E/D ꢁ 1/3) crystal field (I-type
of HS centers), whereas the broad geff = 2 signal corresponds to HS

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189
Fig. 3. The temperature dependence of the EPR spectra of the compound from (a) 4.2 to 120 K and (b) 120 to 281 K. Spectra in the temperature range (b) are recorded at
higher amplification (ꢅ7.5) in comparison with spectra in the range (a). The dashed lines show the theoretical EPR spectra.

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Fig. 5. (a) The temperature dependence of the EPR lines integrated intensity of the
I-type HS iron(III) centers. (b) The temperature dependence of the g-factor and the
width of the individual line of the I-type HS iron(III) centers.
Fig. 4. (a) The temperature dependence of the EPR lines integrated intensity of the
whole EPR spectrum. (b) The I ꢅ T vs T plot.
layered structure. The number of these HSII centers at room
temperature is about 97%.
agreement is obtained between the experimental and theoretical
spectra.
Let us analyze the results obtained for the other two iron cen-
ters. In accordance with the integrated intensity ratio, the number
of LS to HS centers of the I-type is approximately 1:1 and is about
25% at T = 4.2 K. The appearance of maximum on the curves ILS and
IHS_I shows that LS–LS and HSI–HSI iron centers are coupled by
antiferromagnetic (AF) exchange interactions. The EPR magnetic
data ILS and IHS_I vs. T between 4.2 and 90 K were analyzed with
the spin–spin interaction model based on the exchange Hamilto-
nian H ¼ ꢀJSA ꢂ SB. In the framework of a dimer model, the
exchange constants JLS and JHS_I between LS-LS and HSI-HSI centers
were estimated with the help of Bleaney–Bowers equation [23]
and Eq. (1) in [24], respectively. The best fit to the experimental
data is displayed in Fig. 8a and b, that was obtained with
JLS = ꢀ13.1 K and JHS_I = ꢀ10.2 K, respectively. Negative coupling
constant J confirms the presence of the antiferromagnetic interac-
tions between iron centers.
If we normalize the integrated intensity of each molecule
fraction and superimpose these curves, the identical magnetic
behavior for the LS and HS centers of the I-type is observed up to
160 K (see Fig. 8c), which follows the Curie–Weiss law above TN.
Above 160 K, the temperature dependencies of ILS and IHS_I deviate
from the Curie–Weiss law: the number of HS centers of the I-type
increases, while the number of LS centers decreases relative to the
Curie–Weiss law. This fact means that a partial spin transition
occurs in the compound between 160 and 300 K, where in a
spin crossover (HS () LS) takes part of ꢁ25% of the Fe(III)
molecules. Since the behavior of LS and HS centers of the I-type
Now let us analyze the EPR spectra of each type of iron(III)
centers independently. Figs. 5–7 show the temperature variation
of the EPR lines integrated intensities (I), g,
DH and ZFS-parameters
values for the two types of HS and one LS centers, respectively.
Note that the EPR lines integrated intensity (I), which was obtained
by numerical double integration of the first-derivative EPR
spectrum, is proportional to the magnetic susceptibility. As can
be seen, the temperature dependence of I for HS centers of the
II-type (IHS_II) behaves in the opposite way compared with the
EPR lines integrated intensities for the LS (ILS) and HS centers of
the I-type (IHS_I). IHS_II value grows with increasing temperature
in the second (70–300 K) temperature interval and shows a broad
maximum at about Tmax ꢁ 259 K, while the EPR lines integrated
intensities of the other two iron(III) centers exhibit a sharp maxi-
mum at TN = 10 K and above TN, ILS and IHS_I follow the Curie–Weiss
law up to 160 K. Such different behavior of iron centers indicates
that there are two different magnetic sub-systems in the com-
pound. According to the literature [21,22], a broad maximum in
the magnetic susceptibility characterizes one-dimensional Heisen-
berg antiferromagnetic behavior, and it reflects the influence of the
magnetic coupling along the chain of iron ions. It should be noted
that the position of a broad maximum in the curve I vs. T
essentially depends on the type of anion and the broad maximum
shifts to lower temperatures Tmax ꢁ 100 K for Fe(III) complex with
Clꢀ counterion. Thus, we can assume that the broad resonance line
at g ꢆ 2 for the HS centers with weakly distorted coordination
sphere arises from [FeIII(L)2]+ cations, packed in a chain forming a

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191
Fig. 6. (a) The temperature dependence of the EPR lines integrated intensity of the
II-type HS iron(III) centers. (b) The range of D-values variation and the width of the
individual line for the II-type HS iron(III) centers.
Fig. 7. (a) The temperature dependence of the EPR lines integrated intensity of the
LS iron(III) centers. (b) The temperature dependence of the g-factor and the width of
the individual line of the LS iron(III) centers.
differs cardinally from the behavior of HS centers of the II-type, we
can conclude that these centers are likely located between the
layers.
A comparison of the temperature dependence of the static mag-
netic susceptibility (Fig. 9a) and the EPR integrated intensity
(Fig. 4a) shows a large difference, especially in the high tempera-
ture range (70–300 K). These differences can be explained by
regarding that IHS_II, in contrast to the static magnetic susceptibil-
ity, reveals dynamic spin clusters at the EPR spectrometer fre-
quency (ꢁ9.4 GHz). These antiferromagnetic (AF) spin clusters of
short-range ordered spins are observed only on a short time scale
Thus, the EPR results show that the studied magnetic
[Fe(L)2]+PFꢀ6 system is inhomogeneous and consists of two mag-
netic sub-lattices. The HS Fe(III) centers with weakly distorted
octahedral environment most probably form chains in layers.
LS–LS and HS–HS centers with strongly distorted octahedral
environment form dimers, that are likely arranged between the
layers and these centers are involved in a spin crossover. Example
of such inhomogeneous magnetic system based on Fe(III) mesogen
is known in the literature [25].
(s
= 1/m
ꢁ 10ꢀ10 s) and could not be detected by method of static
magnetic measurements. We suppose that the dynamic spin clus-
ters represent nano-regions with antiferromagnetically correlated
spins. Such nano-regions exist in the paramagnetic phase and
behave themselves like superparamagnetic particles in the mag-
netic resonance spectrum. More clearly their behavior is observed
in the iron(III) dendrimeric complex with chlorine counterion and
this problem will be considered in our next publication.
Application of the theory Raikher–Stepanov developed to super-
paramagnetic particles allow us to estimate the size and value of
the anisotropy field of such nano-regions with antiferromagneti-
cally correlated spins.
Note that for the first time the concept of dynamic spin clusters
was proposed for the polycrystalline sample of Zn3Fe4V6O24, where
the temperature dependence of the EPR integrated intensity of the
broad line at g ꢆ 2.0 showed a marked anomaly at about 220 K in
contrast to the static magnetic susceptibility, where no such irreg-
ularity has been observed [26]. The authors explained the differ-
2.3. Magnetic susceptibility
The static magnetic susceptibility
v for the powder sample was
measured in a magnetic field of 5000 Oe as a function of tempera-
ture in the range 2–300 K. The measured data were corrected
for the diamagnetic susceptibility (ꢀ0.000199 emu) and for the
sample holder contribution. Fig. 9a shows the temperature depen-
dencies of the magnetic susceptibility and the effective magnetic
moment (leff), which demonstrates a decrease of the value from
5.8 lB at 300 K to 2.4 lB at 2.4 K. leff value at room temperature
corresponds to the high-spin (HS) value of the Fe(III) ion (S = 5/2,
leff = 5.9 lB). The behavior of the inverse magnetic susceptibility
versus T is presented in Fig. 9b. As can be seen, the Curie–Weiss
law prevails above 10 K with constant h = ꢀ9.9 K. The negative sign
of h clearly indicates that antiferromagnetic interactions operate
between Fe(III) ions.
ence in behavior of I(T) and
v(T) in the high temperature range

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Fig. 8. The temperature dependence (4.2 K 6 T 6 90 K) of the EPR integrated intensity of the LS (a) and HS centers of the I-type (b). The solid lines represent the best fit to the
experimental data with coupling constants mentioned in the text. (c) The temperature dependence of the EPR normalized integrated intensity of the LS and HS centers of the
I-type at 4.2 K 6 T 6 300 K.
by means of AF dynamic spin clusters of short-range ordered spins
existing only in the time scale of the EPR method of observation.
To confirm the concept of dynamic spin clusters which are
present in our material, the Mössbauer spectroscopy studies were
carried out, where the time scale of this method is ꢁ10ꢀ7 s.
iron(III) centers. A good agreement between the experimental
results and computer simulations can only be obtained by taking
into account the distribution of the isomer shifts and quadruple
splittings. The distribution of the latter is the result of different dis-
tortions of Fe (III) coordination polyhedrons due to flexibility of
dendrimeric ligands or due to anion site disorder between the
neighboring iron centers.
2.4. Mössbauer spectroscopy study
One could expect to find a superposition of the quadrupole dou-
blets of two spin states (HS and LS) at lower temperatures, as
observed in the structurally similar complexes in [16]. However,
only one quadrupole doublet with a broad line width is observed
in the whole temperature range. This phenomenon indicates that
the electronic relaxation time between the HS and LS state is faster
than the lifetime of the nuclear excited states of iron-57
(0.98 ꢅ 10ꢀ7 s) and the iron nucleus ‘‘sees” an average of the prop-
erties of both states, and that the values of the Mössbauer param-
eters are averaged and depend on the population of each form. The
examples of such fast spin interconversion have been observed for
many spin-crossover iron(III) complexes [27,28].
Mössbauer spectra were measured in the temperature range
5–310 K. Spectrum recorded at 310 K (Fig. 10) consists of one
broad doublet with parameters determined by the center of gravity
method: isomer shift dFe = 0.35 mm/s with respect to
a-Fe and
quadrupole splitting QS = 0.72 mm/s, which correspond to the
high-spin (HS) state of the Fe(III) ion in an octahedral environment.
It is known that the magnitude of the quadrupole splitting depends
on the value of electric field gradient (EFG) on the 57Fe nucleus. EFG
is formed by two contributions, the first one arises from the asym-
metry of the electrons on the d-shell of Fe(III) ion. The second con-
tribution comes from the atoms forming the coordination sphere of
the iron ion. Fe(III) in HS state (S = 5/2) has a symmetric, half-filled
electron d-shell. Therefore, QS = 0.72 mm/s value for our complex is
mainly caused by distortions of the coordination polyhedron of
At the temperature below 60 K, a magnetic hyperfine structure
(Hobs = 454.5 kG at 5 K) appears in the spectrum and becomes more
pronounced with further decrease of the temperature (Fig. 10).

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193
Fig. 9. (a) Temperature dependence of magnetic susceptibility
v and effective
magnetic moment for the studied compound. (b) Plot of the inverse magnetic
susceptibility vs. temperature for the iron(III) complex. The solid line corresponds
to the Curie–Weiss fit with h = ꢀ9.9 K.
This indicates a process of magnetic spin correlations with fre-
quency
Fig. 10. Mössbauer transmission spectra of the compound at various temperatures.
m
< 107 Hz of the spin fluctuations. At the same time, a
small part of the quadrupole doublet remains in the spectra at
5 K. In the temperature range between 60 and 10 K, the hyperfine
structure of spectra has a superparamagnetic-type of behavior
[29]. The relaxation character of the hyperfine components is the
result of collective spin flips of small clusters of the material
[30]. So, below 60 K spectra demonstrate a magnetic phase transi-
tion from the paramagnetic state to antiferromagnetic (AF) type of
ordering with a wide temperature range of superparamagnetism
existence.
in a paramagnetic phase consists of HS Fe(III) centers. Observation
of distributions of the isomer shifts and quadruple splittings indi-
cates the distortions of Fe (III) coordination polyhedrons. Below
60 K, the magnetic hyperfine structure appears in Mössbauer spec-
tra and its relaxation nature testifies the collective spin flips of
small clusters in the material. Antiferromagnetic ordering in the
compound is observed at 5 K. The coexistence of magnetic ordering
and a partial spin crossover has been detected for the first time in
iron(III) dendrimeric complex.
3. Conclusions
EPR, DC magnetic susceptibility and Mössbauer spectroscopy
have been used to study the magnetism of spin-crossover den-
drimeric iron(III) complex, which exhibits mesogenic properties.
The system contains three types of magnetically active iron centers:
one S = 1/2 low-spin (LS) and two S = 5/2 high-spin (HS) centers
with strong (D = 0.421 cmꢀ1, E = 0.109 cmꢀ1) low-symmetry and
weak (0.02 cmꢀ1 < |D| < 0.03 cmꢀ1) distorted octahedral environ-
ments. EPR has shown that the compound is magnetically heteroge-
neous and consists of two magnetic sub-lattices. The HS Fe(III)
centers with weakly distorted octahedral environment most prob-
ably form chains in layers. The dimeric molecules, formed from
LS–LS centers and HS–HS centers with strongly distorted octahedral
environment, are likely located between the layers and are involved
in the spin crossover that occurs in the range 160–300 K. EPR
revealed the presence of the dynamical spin clusters in the high
temperature (70–300 K) range, which are not detected by the DC
magnetic susceptibility. Mössbauer spectra showed that the sample
4. Experimental
4.1. Synthesis of dendrons derived from 3,4,5-tris(tetradecyloxy)
benzoic acid
The ligand precursor 3,5-di[3,4,5-tris(tetradecyloxy)benzoyloxy]-
benzoyl-4-oxy-2-hydroxybenzaldehyde was prepared in accordance
with reference [31,32] and Scheme A given in the Supporting Infor-
mation. 3,4,5-Tris(tetradecyloxy)benzoic (1) acid was selected as
the initial compound, and was synthesized in compliance with a
standard method [33]. For the regular branching, the method of
doubling the molecular size was employed, using the benzyl ester
of 3,5-dihydroxybenzoic acid [34,35], which was synthesized by
Chervonova et al. [34]. The esterification was carried out by inter-
action between the carboxyl and hydroxyl groups with the aid of
dicyclohexylcarbodiimide in dichloromethane. The benzyl group
was selectively removed by hydrogenolysis using 5% Pd/C as

194
N. Domracheva et al. / Inorganica Chimica Acta 439 (2016) 186–195
catalyst in dry 1,4-dioxane [32,34,36]. The aldehyde was obtained
by means of esterification of the corresponding acid and para-
hydroxysalicylic aldehyde.
4.3. Physical measurements
The EPR experiments were carried out on the powder sample.
EPR studies were performed using X-band (9.41 GHz) CW-EPR
EMXplus Bruker spectrometer that was provided with the helium
ER 4112HV and the digital ER 4131VT temperature control sys-
tems. The accuracy of the reported magnetic parameters for LS iron
4.2. Sample preparation and characterization
The synthesis of the complex was performed in accordance with
Scheme 1. The reaction was carried out in binary solvent (benzene/
ethanol). The formation of azomethine in solution and complexa-
tion by addition of iron(III) salt were confirmed by FT-IR spec-
troscopy and mass-spectrometry. Iron enriched to ꢁ6% in 57Fe
isotope was used to synthesize the complex. The substitution of
the counter ion was carried out through the exchange reaction
by adding a double excess of the potassium salt.
complexes was
Dg = 0.005 and 0.005 for the fine structure
parameters. All Mössbauer spectra were obtained using a conven-
tional constant-acceleration spectrometer where 57Co in Rh matrix
was used as the source. The velocity calibration was obtained by
using the
a-Fe spectrum. The isomer shift data were reported rel-
ative to the isomer shift of the
continuous-flow cryostat was used for the temperature measure-
a-Fe spectrum. A liquid nitrogen
ments in the range of 80–302 K. The lower temperature measure-
4.2.1. Synthesis of iron(III) dendrimeric complex [31]
ments were made using a helium cryostat. The Mössbauer
A
portion of 3,5-di[3,4,5-tris(tetradecyloxy)benzoyloxy]ben-
absorber, which was prepared as a thin layer of powder between
two aluminum foil discs, was mounted on the copper sample
holder of the helium cryostat. The higher temperature measure-
ments were made using a Mössbauer furnace. The temperature
measurements and control were carried out by a type-T thermo-
couple and a heater above 50 K with the accuracy of 0.2 K. A car-
bon-resistance thermometer, calibrated at the liquid nitrogen
and liquid helium temperatures, was used for the temperature
measurements and control below 50 K with the accuracy of 0.5 K.
Magnetic susceptibility measurements were carried out at heating
rates of 2 K minꢀ1 in a 0.5 T magnetic field by means of a Quantum
Design MPMS2 SQUID magnetometer. The experimental data were
corrected for the diamagnetic contribution.
zoyl-4-oxy-2-hydroxy-benzaldehyde (0.5 g, 0.28 mmol) was dis-
solved in mixture of benzene and ethanol under constant
stirring. N’-Ethyl-N-ethylenediamine (0.025 g, 0.28 mmol) and
KOH (0.071 g, 1.28 mmol) dissolved in alcohol were added. Then
the alcoholic solution of Fe(NO3)3ꢂ9H2O (0.057 g, 0.14 mmol) was
added slowly drop by drop. After 15 min of stirring KPF6 (0.103 g,
0.56 mmol) dissolved in mixture of EtOH a few drops of water were
slowly poured in it. The stirring was continued for 4 h. The precip-
itate was filtered off on a glass filter, washed by ethanol, and
freeze-dried from benzene. The product is a finely dispersed brown
powder. Yield is 0.46 g.
Found: C, 71.99; H, 10.13; N, 1.67. Calc. for C232H390O26N4
FeꢂPF6: C, 72.33; H, 10.20; N, 1.45.
FT-IR spectrum of complex,
t
max/cmꢀ1
:
3095w (Ph-H),
4.4. DFT calculations
2921–2849s (Alk-H), 1729s (CO), 1591s (CN), 1002vs (NH), 857
w (PFꢀ6 ).
Geometry optimizations were performed within the PBE frame-
work using the so-called 3z basis set of TZ2P quality (PRIRODA,
version 5.0 package) [16].
1H NMR spectrum of the complex registered in CDCl3 is param-
agnetic which is registered as a few broad single peaks [37].
The complex was analyzed by gel permeation chromatography
to establish of the chemical purity and homogeneity of the sample.
Dry tetrahydrofuran was used as eluent. The chromatogram
Acknowledgments
shows two peaks with different retention time (
s
beg1 = 8.27 min,
We gratefully acknowledge the financial support for this work
by RAS Presidium program No. 24, in part by the RFBR, project
No 11-03-01028, grant of RFBR No. 14-03-31280-mol_a and the
grant of the President of the Russian Federation (No. MK-
70.2014.3). We thank Prof. V.F. Tarasov and Dr. R.A. Manapov for
fruitful discussions and remarks. EA, NMR, FT-IR spectroscopy
and DSC analysis were carried out by the instrumentality of equip-
ment of Interlaboratory scientific center ‘‘Verhnevolzhskij region
center of physicochemical researching”.
s
fin1 = 8.72 min; beg2 = 8.73 min, fin2 = 9.27 min) (see Fig. S1 in
s
s
Supporting Information). The peak with s1 corresponds to
supramolecular aggregates formed from the compound. Their for-
mation can be explained by intermolecular interactions, where
azomethine protons and PFꢀ6 counter ion are the bridging units.
The second signal (
target compound.
s2) is identified as the peak of the individual
The presence of Schiff base and coordinated iron(III) ion in the
structure of the complex was established by FT-IR spectroscopy.
Two bands at ꢁ5840 and ꢁ5700 cmꢀ1 correspond to vibrations
of tertiary and secondary amines [38] (see Fig. S2 in Supporting
Information). A strong band at ꢁ1585 cmꢀ1 is evidence of the
ACH@NA bond occurrence which lies close to the absorption band
of the carboxyl group C@O (1715–1730 cmꢀ1). Stretching vibra-
tions of the Fe–O bond (422 cmꢀ1) and stretching vibrations of
the Fe-N bond (584 cmꢀ1) are observed in the far range of infrared
spectra [39–41].
Appendix A. Supplementary material
The results of synthesis of 3,5-di[3,4,5-tris(tetradecyloxy)ben-
zoyloxy]benzoyl-4-oxy-2-hydroxybenzaldehyde, gel permeation
chromatography, FT-IR spectroscopy, differential scanning
calorimetry and optical polarizing microscopy are available. Sup-
plementary data associated with this article can be found, in the
online version, at http://dx.doi.org/10.1016/j.ica.2015.10.024.
A chelate compound with a high degree of branching does not
give stable molecular ions, but it is fully fragmented within the
range of m/z from 200 to 2000. The fragmentary ion with the value
of m/z ꢁ 1861 [L+K]+ indicates Schiff base formation. Most inten-
sity stable molecular ions are not observed even at change of
matrix.
The liquid–crystalline properties of the compound were investi-
gated by differential scanning calorimetry and optical polarizing
microscopy. These data are available in the Supporting Information
(see Fig. S3).
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