The structural and magnetic properties of FeII and CoII complexes with 2-(furan-2-yl)-5-pyridin-2-yl-1,3,4-oxadiazole

Article abstract of DOI:10.3390/molecules25020277

Two novel coordination compounds containing heterocyclic bidentate N,N-donor ligand 2-(furan-2-yl)-5-(pyridin-2-yl)-1,3,4-oxadiazole (fpo) were synthesized. A general formula for compounds originating from perchlorates of iron, cobalt, and fpo can be written as: [M(fpo)₂(H₂O)2](ClO₄)₂ (M = Fe(II) for (1) Co(II) for (2)). The characterization of compounds was performed by general physico-chemical methods—elemental analysis (EA), Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR) in case of organics, and single crystal X-ray diffraction (sXRD). Moreover, magneto-chemical properties were studied employing measurements in static field (DC) for 1 and X-band EPR (Electron paramagnetic resonance), direct current (DC), and alternating current (AC) magnetic measurements in case of 2. The analysis of DC magnetic properties revealed a high spin arrangement in 1, significant rhombicity for both complexes, and large magnetic anisotropy in 2 (D = ?21.2 cm^-1). Moreover, 2 showed field-induced slow relaxation of the magnetization (U_eff = 65.3 K). EPR spectroscopy and ab initio calculations (CASSCF/NEVPT2) confirmed the presence of easy axis anisotropy and the importance of the second coordination sphere.

Full text of DOI:10.3390/molecules25020277

molecules
Article
The Structural and Magnetic Properties of FeII and
CoII Complexes with
2-(furan-2-yl)-5-pyridin-2-yl-1,3,4-oxadiazole
1
2
, Juraj Kuchár 1,3 and Radovan Herchel 1,
*
ˇ
Pavel Zoufalý , Erik Cižmár
1
Department of Inorganic Chemistry, Faculty of Science, Palacký University, 17. Listopadu 12, CZ-771 46
Olomouc, Czech Republic; pavel.zoufaly@upol.cz (P.Z.); juraj.kuchar@upol.cz or juraj.kuchar@upjs.sk (J.K.)
Institute of Physics, Faculty of Science, P.J. Šafárik University in Košice, Park Angelinum 9,
SK-041 54 Košice, Slovakia; erik.cizmar@upjs.sk
Department of Inorganic Chemistry, Institute of Chemistry, Faculty of Science, P.J. Šafárik University in
Košice, Moyzesova 11, SK-041 54 Košice, Slovakia
2
3
*
Correspondence: radovan.herchel@upol.cz; Tel.: +420-58563-4435
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
Academic Editors: Dawid Pinkowicz and Robert Podgajny
Received: 19 December 2019; Accepted: 7 January 2020; Published: 9 January 2020
ꢀꢁꢂꢃꢄꢅꢆ
Abstract: Two novel coordination compounds containing heterocyclic bidentate N,N-donor
ligand 2-(furan-2-yl)-5-(pyridin-2-yl)-1,3,4-oxadiazole (fpo) were synthesized. A general formula
for compounds originating from perchlorates of iron, cobalt, and fpo can be written as:
[M(fpo)2(H2O)2](ClO4)2 (M = Fe(II) for (1) Co(II) for (2)). The characterization of compounds was
performed by general physico-chemical methods—elemental analysis (EA), Fourier transform infrared
spectroscopy (FT-IR), nuclear magnetic resonance (NMR) in case of organics, and single crystal X-ray
diffraction (sXRD). Moreover, magneto-chemical properties were studied employing measurements
in static field (DC) for
and alternating current (AC) magnetic measurements in case of
properties revealed a high spin arrangement in , significant rhombicity for both complexes, and large
magnetic anisotropy in (D = showed field-induced slow relaxation of
21.2 cm−1). Moreover,
1
and X-band EPR (Electron paramagnetic resonance), direct current (DC),
2. The analysis of DC magnetic
1
2
−
2
the magnetization (Ueff = 65.3 K). EPR spectroscopy and ab initio calculations (CASSCF/NEVPT2)
confirmed the presence of easy axis anisotropy and the importance of the second coordination sphere.
Keywords: 1,3,4-oxadiazole; single-molecule magnet; second coordination sphere
1. Introduction
The main focus of magnetochemical research is situated in the study of molecular functional
materials exhibiting bistability in which physical properties can be triggered by a change of temperature,
pressure, light or magnetic field, such as spin crossover compounds (SCO) or single-molecule magnets
(SMM), where the latter are nanomagnets characterized by the slow relaxation of the magnetization.
The widespread basis for the study of these phenomena originates in its potential technological
applications in many areas of everyday life. In this case, SMM compounds are studied as materials for
storage devices, quantum computing, sensors, and spintronics [
may find potential in many applications such as pressure or optical switches [
pressure, or temperature sensors [ ]. Therefore, the magnetic properties of 3d elements are of much
1
,2]. Additionally, SCO compounds
3
], gas sensors [ ],
4
5
interest to the scientific community. Among all of the 3d metals, Co(II) possesses great predispositions
for the formation of compounds exhibiting SMM behavior due to relatively large spin-orbit coupling
and the possibility to vary the coordination numbers from 2 to 8 [6]. Likewise, Fe(II) ranks among
elements whose compounds are commonly known for SCO behavior [
7
]. The tuning of physico-chemical
Molecules 2020, 25, 277; doi:10.3390/molecules25020277
www.mdpi.com/journal/molecules

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properties of both SMM and SCO phenomena can be achieved by means of several factors. In SMMs,
complexes honing of magnetization reversal barrier quantified by Ueff parameter and magnetic
blocking temperature (TB) is achieved by the strengthening of magnetic anisotropy of the easy-axis
type. The modification of Ueff is accomplished by increasing magnitude of the zero-field splitting
parameter D, due to a simple relationship among them, U = |D|(S2
−
1/4) for half-integer spin
and U = |D|(S2) for integer spin, and it holds that values of ZFS (zero-field splitting) parameters
are dictated by the coordination number, the ligand field, and the symmetry of the coordination
polyhedron. In general, higher |D|-values should yield higher Ueff and correspondingly higher
TB [8–11]. However, large zero-field splitting parameter E causes an increase of the tunneling rate of
the magnetization. In SCO compounds, a proper ligand field and cooperativity between neighboring
molecules can allow a transition between low spin (LS) and high spin (HS) state and vice versa, which
is triggered by the change of temperature or pressure, eventually by the light. Since many of these
parameters in both SMM and SCO relate to the coordination environment, search for suitable organic
ligands is crucial for improving properties of these molecular compound classes. Miscellaneous
organic ligands have been employed to study or enhance the magnetic properties of Fe(II) and
Co(II) coordination compounds. Such ligands frequently include five or six-membered heterocycles
containing at least one nitrogen atom. Examples of these heterocycles cover substituted diazoles,
triazoles, tetrazoles, pyridines, pyrimidines, pyrazines, triazines, and others [12–14]. In our search for
suitable ligands, we were inspired by numerous publications with interesting magnetochemical results
encompassing substituted triazoles, and more specifically 4-amino-3,5-di-2-pyridyl-4H-1,2,4-triazole
(abpt). Our co-workers participated in publishing Co(II) field-induced single-ion magnets incorporating
the abpt ligand. The first publication in 2014 introduced [Co(abpt)2(tcm)2] (tcm = tricyanomethanide),
which was identified as the field-induced single-ion magnet with large ZFS parameters and positive
D = 48 cm−1. The energy value for spin reversal barrier Ueff = 86.2 K was the highest at the time among
Co(II) complexes with transversal magnetic anisotropy [15]. The research continued in series of two
publications involving other pseudohalide analogs—[Co(abpt)2(solv)2]X2 (solv = H2O and X = tcap,
solv = H2O and X = nodcm, solv = CH3OH and X = pcp) (tcap = 1,1,3,3-tetracyano-2-azapropenide,
nodcm = nitrodicyanomethanide, pcp = 1,1,2,3,3-pentacyanopropenide), [Co(abpt)2(X)2] (X = nca,
NCSe, ndcm, N3) (nca = nitrocyanamide, ndcm = nitroso-dicyanomethanide). The analysis of DC
measurements revealed strong magnetic anisotropy and D in 31–41.4 cm−1 range with the exception of
[Co(abpt)2(N3)2] where D =
−
24.1cm−1. Subsequent inquiry of AC data showed field-induced slow
16 17]. Thus, we have
relaxation of the magnetization and Ueff in the scope of 71.6–108 cm−1
[
,
decided to explore 1,3,4-oxadiazoles, oxygen analogs of abpt, which are relatively unexplored
in a magnetochemical area. Moreover, the fact that 4H-1,2,4-triazole analog 1,3,4-oxadiazole
contains more electron-withdrawing oxygen instead of nitrogen should have a notable effect
on magnetic properties. Generally, studies involving a 1,3,4-oxadiazole heterocycle focus on
crystal engineering, which covers the design and preparation of building blocks, distinguished
crystal structures, and understanding of intermolecular interactions in supramolecular structure.
Moreover, in these studies, nitrogen of the 1,3,4-oxadiazole ring do not coordinate, and, instead,
substituents (e.g., pyridine, pyrazine) in position 2 or 5 provide suitable donor atoms (Scheme 1).
The structures of [Co(3-bpo)(dca)2]n (dca = dicyanamide, 3-bpo = 2,5-bis(3-pyridyl)-1,3,4-oxadiazole)
and [Co(L1)2(SCN)2(H2O)2](L1)2(H2O)6 (L1 = 2,5-bis(2-pyrazinyl)-1,3,4-oxadiazole) may serve as
a good example [18,19]. Studies comprising of Co(II) complexes with 1,3,4-oxadiazole derivatives in
which cobalt atom is directly bound to a nitrogen atom of 1,3,4-oxadiazole were focused mainly
on biological activities of these compounds, and, sporadically, DC magnetic properties were
also reported [20–25]. Likewise, a few coordination compounds of Fe(II) with 1,3,4-oxadiazole
directly bonded through its nitrogen atom appear in literature, e.g., [Fe(2-bpo)2(H2O)2](ClO4)2
(2-bpo = 2,5-bis(2-pyridyl)-1,3,4-oxadiazole) was prepared and found to be in the high spin state
in the 2-300 K range [26]. Magnetochemically focused study exploring Light-Induced Excited
Spin-State Trapping (LIESST) and SCO in 2,5-bis(2-pyridyl)-1,3,4-chalcadiazoles describes magnetic

Molecules 2020, 25, 277
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properties of [Fe(2-bpo)2(NCS)2]. Despite the fact that the complex contains two NCS coordinated
anions creating stronger ligand field, the occurrence of SCO was not observed in the 2–300 K
temperature scale [27]. The very first SCO compounds based on 1,3,4-oxadiazole were synthesized and,
subsequently, published in 2016 by C. Köhler and E. Rentschler in series of [FeI−I
(
µ
-L)2]Y4 xCH3CN
·
2
(L = 2,5-bis{[(2-pyridylmethyl)amino]-methyl}-1,3,4-oxadiazole; Y = ClO4−, BF4 and CF3SO3−; x = 4
for ClO4 and x = 2 for BF4−, CF3SO3−). The dinuclear complex with the BF4 anion remained in
−
–
−
−
the high-spin (HS) state in the 10–300 K range whereas, in the case of ClO4 and CF3SO3 anions,
SCO was observed. The critical temperature T1/2 occurs somewhere around 150 K for both anions
and, in the case of [FeII
(
µ
-L)2](CF3SO3)4 2CH3CN, steep hysteresis of about 26 K accompanies the
·
2
transition. XRD confirmed the [HS-HS] to [LS-HS] spin transition, where only one of the Fe(II)
atoms undergoes SCO [28]. These complexes were also studied by theoretical methods (DFT and
CASSCF/CASPT2) [29]. The above-mentioned studies show the great potential of 1,3,4-oxadiazole
ligands in the molecular magnetism.
Scheme 1. Structural formulas of studied ligand fpo and ligands discussed in the text comprising
1,3,4-oxadiazole moiety. Various coordination modes are depicted with arrows.
Herein, the 2-(furan-2-yl)-5-pyridin-2-yl-1,3,4-oxadiazole (fpo) was utilized in the preparation
of [M(fpo)2(H2O)2](ClO4)2 (M = Fe(II) for (1) Co(II) for (2)) coordination compounds, which were
characterized by single-crystal X-ray analysis, FT-IR spectroscopy, and EPR spectroscopy. Both static
and dynamic magnetic properties of these complexes were investigated and the analysis was supported
by DFT and CASSCF/NEVPT2 theoretical methods showing the importance of the second coordination
sphere on the zero-field splitting parameters.
2. Results and Discussion
2.1. Synthesis
Preparation of ligand 2-(furan-2-yl)-5-(pyridin-2-yl)-1,3,4-oxadiazole (fpo) is already described in
the literature [30]. However, we obtained fpo through a four-step reaction, which shows simplified
reaction Scheme 2. In the first step, transformation of picolinic acid (PA) to methyl picolinate (I)
occurs via esterification in methanol/sulphuric acid solution. In the second step, production of
picolinic acid hydrazide (II) from I ensues in methanol/hydrazine hydrate solution. The third step
involves the conversion of II into picolinic acid 2-(2-furanylmethylene)hydrazide (III) in a simple
reaction with 2-furaldehyde in methanol. The last step shows oxidative cyclization of the imine
bond in dimethylsulfoxide with iodine as an oxidation reagent and potassium carbonate as the base.
Subsequently, dissolving M(II) perchlorate hexahydrate (M = Fe, Co) in methanol and adding the
solution to fpo in methanol and reflux of the mixture results in the formation of the products. Within
one week, yellow crystals of
recorded in literature, their synthesis was inspired by these protocols [31
of I, II, III, and fpo, the measurements of IR and NMR spectra were employed.
1
and orange crystals of
2
appeared. Since all organic ligands are already
–
33]. To confirm the structure

Molecules 2020, 25, 277
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Scheme 2. Preparation of the fpo ligand in four steps: (
) methanol, hydrazine hydrate, 14 h reflux, ( ) methanol, furfural, 2 h reflux, (
carbonate, 18 h, 120 ◦C.
a
) methanol, sulphuric acid, 24 h reflux,
(
b
c
d
) iodine, potassium
2.2. Description of the Molecular and Crystal Structure
Compounds [Fe(fpo)2(H2O)2](ClO4)2 ( ) and [Co(fpo)2(H2O)2](ClO4)2 (2) are isostructural and
1
crystallize in the monoclinic P21/c space group. In both compounds, the central atoms occupy the
center of the symmetry. Two molecules of the fpo ligand are bidentately coordinated to central atom
via nitrogen (Nox and Npy), and two coordinated oxygens from water molecules are in the apical
positions. This arrangement constitutes {FeN4O2} and {CoN4O2} chromophores. Therefore, both metal
ions are situated in a typical octahedral coordination sphere where distortion occurs mainly due to the
chelate binding of the fpo ligand. The relevant bite angle between pyridine nitrogen (Npy), iron, and
nitrogen of 1,3,4-oxadiazole (Nox) of 1 measures 75.94(9)◦ and 76.88(15)◦ in 2. Thus, the parameter for
P
angular distortion calculated as the sum of deviation of 12 angles from the ideal 90◦ angle yields 61◦
for Fe(II) and 55.6◦ for Co(II). The bond length of Fe(II) and pyridine nitrogen is d(Fe-Npy) = 2.212(3) Å,
whereas lower value of d(Fe-Nox) = 2.165(2) Å appears in 1,3,4-oxadiazole nitrogen and lowest for
aqua oxygen d(Fe-O) = 2.078(3) Å. Similar but shorter distances are found in the Co(II) complex
d(Co-Npy) = 2.159(4) Å, d(Co-Nox) = 2.116(4) Å, and d(Co-O) = 2.064(4) Å. The molecular structures of
1 and 2 are depicted in Figure 1.
Figure 1. The molecular structures of [Fe(fpo)2(H2O)2](ClO4)2 (1, left) and [Co(fpo)2(H2O)2](ClO4)2
(2, right). The most populated part of the perchlorate anion is depicted. Symmetrical codes: i: 1-x,
1-y, 1-z.
Apart from covalent bonds, noncovalent interactions create a rich net, which originates mainly
from oxygen atoms of perchlorates and aqua ligands. Hydrogen bonds between hydrogen atoms of
aqua and perchlorate oxygens form the most prominent structural element. Every hydrogen bond from
aqua links with different perchlorate oxygen including four ClO4− anions in total per [M(fpo)2(H2O)2]2+
unit. The shorter distance of the hydrogen bond between oxygens measures d(O···O) = 2.727(6) Å
and longer 2.807(4) Å in 1, d(O···O) = 2.726(9) Å, and d(O···O) = 2.826(6) Å in 2. The perchlorate anion

Molecules 2020, 25, 277
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creates only two hydrogen bonds and each come with a different aqua ligand. Propagation of the
hydrogen bonds forms a 2D layer as depicted in Figure 2
are shown in Table 1.
. Basic crystallographic data and parameters
Figure 2. The supramolecular 2D network formed through hydrogen bonds among perchlorates and
aqua ligands in the crystal structure of 1. Only atoms involved in the hydrogen bonds are labelled.
Table 1. Crystallographic data and structure refinement parameters of the compounds 1 and 2.
Compound
1
2
Chemical formula
Formula weight
Temperature [K]
λ (Mo, Kα) [Å]
Crystal system
Space group
a [Å]
C22H18Cl2FeN6O14
717.17
293(2)
0.71073
monoclinic
P21/c
C22H18Cl2CoN6O14
720.25
293(2)
0.71073
monoclinic
P21/c
8.8543(18)
13.627(3)
11.840(2)
90
91.26(3)
90
1428.2(5)
2
1.668
8.8827(9)
13.6502(12)
11.7754(10)
90
91.469(7)
90
1427.3(2)
2
1.662
b [Å]
c [Å]
α [◦]
β [◦]
γ [◦]
Volume [Å3]
Z
D [g·cm−3
]
]
µ [mm−1
F (000)
0.80
728
0.86
726
Reflections collected
5761
5605
Independent reflections
3198
3165
Rint
R[F2 > 2σ(F2)]
wR(F2)
0.017
0.049
0.133
1.02
0.065
0.072
0.215
1.02
Σ

Molecules 2020, 25, 277
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2.3. Static and Dynamic Magnetic Properties
The variable temperature and field experimental magnetic data for 1 and 2 are plotted in Figure 3.
The room temperature effective magnetic moments (µeff) adopt values 5.1 µB for 1 and 4.3 µB for
2. These values are a bit higher than the spin-only values for S = 2 (µeff
/µB = 4.9) and for S = 3/2
(
µeff B = 3.9) calculated for g = 2.0, which indicates a significant contribution of the spin-orbit coupling
/µ
to the ground state. On lowering the temperature to 1.9 K, there is only a small decrease of the effective
magnetic moment for 1 down to 4.7 µB, whereas compound 1 exhibit a much larger drop of µeff down
to 3.5 µB. These data suggest that magnetic anisotropy in 2 is much more pronounced in comparison
to 1. This is also reflected in the isothermal magnetization data measured at T = 2 K saturating to
M
M
mol/NAµB = 4.1 for 1 and to Mmol/NAµB = 2.2 for 2, when compared to the theoretical limit values of
4 for 1 and 3 for 2. Moreover, there are no maxima of the molar susceptibility at
mol/NAµB→
g
·S ≈
low temperature for both compounds under study. Thus, we can exclude significant antiferromagnetic
interactions in the solid state.
Figure 3. Magnetic data for 1 and 2 shown as the temperature dependence of the effective magnetic
moment and as the isothermal magnetizations measured at T = 2, 5, and 10 K. The empty circles
represent the experimental data. Red full lines represent the fitted data using Equation (1) with
D =
−
1.23 cm−1, E/D = 0.31, giso = 2.09 for
1
and D =
21.2 cm−1, E/D = 0.32, gxy = 2.13, gz = 2.37 for 2.
−
Therefore, the experimental magnetic data were analyzed with the following spin Hamiltonian
suitable for describing magnetic anisotropy.
ꢀ
ꢁ
ꢀ
ꢁ
ˆ2 ˆ2
ˆ2 ˆ2
ˆ
ˆ
H = D Sz − S /3 + E Sx − Sy + µBBgSa
(1)
where the single-ion ZFS term is described by axial D and rhombic E parameters, and the Zeeman
term is defined in the α-direction of the magnetic field as Bα = B(sin(θ)cos(ϕ), sin(θ)sin(ϕ), cos(θ)) [34].
The molar magnetization (Ma) was then calculated from the partition function (Z) for a given direction
of magnetic field Bα as:
∂ ln Z
Ma = NAkT
(2)
∂Ba
and the integral (orientational) average of the molar magnetization was calculated as:
Z
Z
2ϕ
π
1
4π
Mmol
=
Ma sin θdθdϕ
(3)
ϕ=0 θ=0
to properly simulate experimental powder magnetization data. To obtain trustworthy parameters,
both temperature and field-dependent magnetic data were fitted simultaneously. Moreover, we have
tested both possible signs of the D-parameter for both compounds 1 and 2. The value of |D|~1.2 cm−1
for 1 was found, which is rather small, but, in the case of compound 2, |D| is around 20 cm−1, which

Molecules 2020, 25, 277
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confirms large magnetic anisotropy. In addition, the significant rhombicity was observed in both
compounds (E/D >> 0)—Tables 2 and 3.
Table 2. Comparison of the spin Hamiltonian parameters for 1 derived from CASSCF/NEVPT2
calculations done in ORCA and the experimental magnetic data a.
c
c
δ b
D
E/D
g1
g2
g3
∆
∆
1–6
1–5
[Fe(fpo)2(H2O)2]2+
CAS(6,5)
CAS(6,10)
−7.05
−6.02
0.077
0.082
2.045
2.045
2.076
2.072
2.234
2.225
654
623
29.1
24.9
679
645
{[Fe(fpo)2(H2O)2](ClO4)2}
CAS(6,5)
CAS(6,10)
−1.83
−2.04
0.223
0.158
2.055
2.054
2.084
2.079
2.110
2.104
1267
1238
7.90
6.26
1280
1196
{[Fe(fpo)2(H2O)2](ClO4)4}2−
CAS(6,5)
CAS(6,10)
−2.91
−3.06
0.060
0.061
2.058
2.057
2.073
2.069
2.081
2.076
1761
1727
11.9
12.5
1773
1739
Experimentally Determined Parameters
+1.24
−1.23
0.32
0.31
2.094
2.094
b
2.094
2.094
2.094
2.094
The values of the parameters are in cm−1
.
δ
is the energy of the first excited ligand field term. The
∆
and
a
c
1–5
∆
are energies of the fifth and sixth spin levels, respectively.
1–6
Table 3. Comparison of the spin Hamiltonian parameters for 2 derived from CASSCF/NEVPT2
calculations done in ORCA and the experimental magnetic data a.
c
c
δ b
D
E/D
g1
g2
g3
∆
∆
1–5
1–4
[Co(fpo)2(H2O)2]2+
CAS(7,5)
CAS(7,10)
27.2
27.1
0.076
0.084
2.090
2.091
2.352
2.356
2.374
2.383
2138
1967
54.8
54.7
1964
1825
{[Co(fpo)2(H2O)2](ClO4)2}
CAS(7,5)
CAS(7,10)
23.3
23.3
0.318
0.307
2.092
2.093
2.283
2.291
2.385
2.392
2223
2068
53.2
52.8
2187
2031
{[Co(fpo)2(H2O)2](ClO4)4}2−
CAS(7,5)
CAS(7,10)
−24.9
−24.4
0.212
0.222
2.096
2.098
2.242
2.249
2.390
2.396
2331
2160
53.1
52.3
2331
2160
Experimentally determined parameters
+18.9
−21.2
0.33
0.32
2.31
2.13
b
2.31
2.13
2.02
2.37
The values of the parameters are in cm−1
.
δ
is the energy of the first excited ligand field term. The
∆
and
a
c
1–4
∆
are energies of the fourth and fifth spin levels, respectively.
1–5
The large magnetic anisotropy in 2 encouraged us to measure AC susceptibility data for this
compound. The zero-static magnetic field measurements result in a zero signal of the imaginary
part of AC susceptibility. The application of small static DC field confirmed slow relaxation of
the magnetization (Figure S1). Therefore, small static field (BDC = 0.1 T) was chosen to suppress the
tunneling of the magnetization and to avoid induction of the magnetic dipolar interactions in solid state,
which often cause the appearance of another relaxation pathway. Thus, the temperature-dependent
AC susceptibility data were acquired for the range of frequencies 1–1500 Hz, and these data were
further analyzed with the one-component Debye model based on Equation (4).

Molecules 2020, 25, 277
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χT − χS
χ($) =
+ χS
(4)
1 + (i$τ)1−α
This resulted in isothermal (χT) and adiabatic (χS) susceptibilities, relaxation times (
distribution parameters (
τ
), and
α
) (Table S1), and the Argand (Cole-Cole) plot (Figure 4). Then, the temperature
dependence of the relaxation times was fitted to the combination of two-phonon Raman and Orbach
relaxation processes, which is shown below.
ꢂ
ꢃ
1
τ
1
τ0
Ueff
kT
= CTn +
exp −
(5)
This results in C = 44.8 K−ns−1, n = 2.19,
τ
0 = 1.34
×
10−10 s−1, and Ueff = 65.3 K (Figure 4). It is
worth mentioning that the other combinations of the relaxation processes were tested, like direct and
Raman, or direct and Orbach, but these were unsuccessful. The found spin reversal barrier Ueff = 65.3
K = 45.4 cm−1 is in good agreement with a value of 48.3 cm−1 derived with fitted parameters from DC
magnetic data (D =
21.2 cm−1, E/D = 0.32). The Raman parameter n is shifted to lower values than
−
the theoretical value of 9, which is usually ascribed to the involvement of both acoustic and optical
phonons in the relaxation [35].
Figure 4. AC susceptibility data for 2.
the applied external magnetic field BDC = 0.1 T (full lines are only guides for eyes).
dependence of in-phase χ0 and out-of-phase χ00 molar susceptibilities fitted with one-component
Debye’s model using Equation (4) (full lines). : The Argand (Cole-Cole) plot with full lines fitted with
A
: in-phase χ0 and out-of-phase χ00 molar susceptibilities at
: frequency
B
C
Equation (4) and on the right of the fit of resulting relaxation times
and Orbach processes (red line) using Equation (5).
τ with the combination of Raman
2.4. Electron Paramagnetic Spectroscopy
The X-band spectra of electron paramagnetic resonance (EPR) of complex 2 were measured in the
temperature range from 2 K to 70 K to identify the type of the crystal-field anisotropy. The temperature
evolution of the EPR spectra of complex 2 (Figure 5) shows a typical decrease of the signal intensity
with increasing temperature for Co(II) complexes. Since there is no change of the shape of the spectra
in the whole temperature range, a simplified effective spin Seff = 1/2 model due to the expected strong

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splitting between the ground and excited Kramers doublets was used for the analysis. This model
assumes the mixing of higher excited states with the ground Kramers doublet as the consequence of the
spin-orbit coupling, which yields highly anisotropic effective g-factors. The simulation of EPR spectra
was performed within the EasySpin simulation package [36]. The influence of the unresolved hyperfine
coupling was included at first only by an anisotropic convolutional broadening. To fairly reproduce
the experimental data, the anisotropic hyperfine interaction term A was then included together
with an anisotropic convolutional broadening
with the experiment was obtained using the set g1 = 1.48, g2 = 2.06, g3 = 6.60, A1 = 140 MHz,
A2 = 165 MHz, A3 = 160 MHz, B11 = 22 mT, B12 = 30 mT, and B13 = 7 mT. It should be noted
∆B (full-width at half-height). The best agreement
∆
∆
∆
that the spin-Hamiltonian formalism does not allow us to estimate the value of the D parameter for
large values (only E/D ratio). The fitted g-factors are in good agreement with the parameters derived
from ab initio calculations in Table S2 (vide infra). Our attempts to simulate the EPR spectra using the
spin-Hamiltonian formalism yielded the spectra similar to the experimental ones for both positive
and negative values of the D-parameter with E/D ranging from 0.305 to 0.330. The values of the
effective g-factors from the analysis of the EPR spectra using the effective spin Seff = 1/2 model allow
revealing the type of the anisotropy (easy-axis or easy-plane) when compared with their theoretical
prediction using Griffith-Figgis formalism or with ab initio calculations [37]. Within the Griffith-Figgis
formalism (with axial
components, using a similar approach as in Reference [38], are restricted to g
∆
ax and rhombic
∆
rh crystal field term included), the calculated effective g-factors
2 for the positive
≥
axial field (easy-plane anisotropy). A very high value of g3 = 6.60 is very close to the predicted gz
component in the case of a negative axial field (easy-axis anisotropy) higher than 1500 cm−1. In that case,
the obtained g1, g2, and g3 components of the effective g-factor can be assigned as gx = 2.06, gy = 1.48,
and gz = 6.6. Although, in that case, the predicted difference between the ground and the first excited
Kramers doublet
deformation of the coordination octahedron, e.g., so-called scissoring as present in 2, will yield to
the reduction of 39]. The experimental estimation of the effective g-factors and their anisotropy
∆
is approximately 150 cm−1. The study of Titiš et al. shows that an additional
1–2
∆
[
1–2
agrees better with the results of the ab initio calculations obtained for the second coordination sphere
{[M(fpo)2(H2O)2](ClO4)4}2−, which confirms the existence of the easy-axis anisotropy in 2 (Table S2).
Furthermore, this result is supported by the presence of the Orbach relaxation process with an energy
barrier close to the ∆1–2 (vide infra).
Figure 5. Temperature evolution of the EPR spectra of 2 measured with the excitation frequency of
9.4 GHz (solid lines). The simulation using the effective spin Seff = 1/2 model (black dashed line) with
the parameters described in the text is included.

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2.5. Theoretical Calculations
With the aim to resolve the ambiguity in fitted parameters from DC magnetic data,
the multi-reference ab initio calculations based on the state-averaged complete active space
self-consistent field method (SA-CASSCF) were performed for both complexes 1 and 2.
Herein, the ORCA software package [40,41] was used for these CASSCF calculations with the
active space defined as six electrons in five d-orbitals, CAS(6,5) for FeII complex 1 and seven electrons
in five d-orbitals, CAS(7,5), for CoII complex 2. In addition, the dynamic electronic correlation was
handled by using the N-electron valence state perturbation theory (NEVPT2) [42–44]. These calculations
were undertaken on the mononuclear molecular fragments [M(fpo)2(H2O)2]2+ extracted from the
experimental X-ray structures in which the atomic positions of all hydrogen atoms were optimized using
the BP86 functional [45
The results of CASSCF/NEVPT2 calculations are summarized in Tables 2 and 3. The calculated ZFS
values are far from the experiment, D =
6.02 cm−1, E/D = 0.077 vs. for D = 1.23 cm−1, E/D = 0.31
for 1, and D = 27.2 cm−1, E/D = 0.076 vs. D = 21.2 cm−1, E/D = 0.32 for 2. Thus, we employed the
–47] together with the atom-pairwise dispersion correction method (D3BJ) [48,49].
−
−
−
double-shell effect by enlarging the active space to 10 d-orbitals, having CAS(6,10) for 1 and CAS(7,10)
for 2, but the ZFS parameters changed only slightly. From inspecting the X-ray structures, it is evident
that hydrogen bonds are present among aqua ligands and perchlorate anions in the solid-state, which
forms the second coordination sphere. The perchlorate anions are present in two different distances.
Thus, first, the molecular fragments {[M(fpo)2(H2O)2](ClO4)2} were considered with shorter O . . .
O
distances between the aqua ligand and the perchlorate anion. Again, the analogous calculations were
done, and much better agreement with the experimental ZFS parameters was found, D =
E/D = 0.223 vs. for D =
1.23 cm−1, E/D = 0.31 for 1, and D = 23.3 cm−1, E/D = 0.318 vs. D =
−
1.83 cm−1
21.2 cm−1
,
,
−
−
E/D = 0.32 for 2. Next, the larger second coordination sphere was implemented with four perchlorate
anions, {[M(fpo)2(H2O)2](ClO4)4}2−, which resulted only in much smaller changes in ZFS parameters
than in the previous case. Figure 6 shows molecular fragments employed for calculations. All this
points to the importance of the hydrogen bonds in a solid state in the molecular magnetism. To better
visualize the impact of the second coordination sphere on the electronic structure and ZFS, the ab
initio ligand field theory (AILFT) was utilized to calculate the energies of the d-orbitals in 1 and 2, as
depicted in Figures 7 and 8. Evidently, the change of the molecular fragments {[M(fpo)2(H2O)2]2+
{[M(fpo)2(H2O)2](ClO4)2}
{[M(fpo)2(H2O)2](ClO4)4}2− has a very significant impact on d-orbital’s
→
→
splitting, which is far from ideal Oh symmetry. This is reflected in the energy levels of the ligand-field
terms and, as expected, in ligand-field multiplets showing the zero-field splitting pattern (Figures 7
and 8). The importance and the impact of the second coordination sphere on the magnetism was also
recently studied in other Co(II) complexes [50–52].
Figure 6. Structures used in calculations of ZFS parameters, molecular fragment [M(fpo)2(H2O)2]2+
(left), {[M(fpo)2(H2O)2](ClO4)2} (middle), and {[M(fpo)2(H2O)2](ClO4)4}2− (right).

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Figure 7. The graphical output of the CASSCF/NEVPT2 calculations with CAS(6,5) for the mononuclear
molecular fragments of 1. Plot of the d-orbitals splitting calculated by the ab initio ligand field theory
(AILFT) ( ), low-lying ligand-field terms ( ), and ligand-field multiplets—Kramers doublets ( ).
The quintet, triplet, and singlet spin states are shown in distinct colours.
A
B
C
Figure 8. The graphical output of the CASSCF/NEVPT2 calculations with CAS(7,5) for the mononuclear
molecular fragments of 2. The plot of the d-orbitals splitting calculated by ab initio ligand field theory
(AILFT) (
A), low-lying ligand-field terms (B), and ligand-field multiplets—Kramers doublets (C).
The doublet and quartet spin states are shown in red and green, respectively.

Molecules 2020, 25, 277
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In other words, the perchlorate induced hydrogen bonding alters the axial ligand field
of aqua ligands, and, therefore, we decided to model this phenomenon in an alternative way.
Thus, the bond distances Co-O in [Co(fpo)2(H2O)2]2+ varied from 1.9 to 2.5 Å and, for each geometry,
the CASSCF/NEVPT2 calculations with CAS(7,5) were performed. The results of these calculations
are shown in Figure 9. Evidently, the stronger axial ligand field led to small |D| values. On the
contrary, a weak axial ligand field resulted in a negative D-parameter with small rhombicity. Generally,
the magnetic anisotropy in the modelled system is very sensitive to small changes in the axial ligand
field, which is manifested in abrupt changes of the rhombicity.
Figure 9. The graphical output of the CASSCF/NEVPT2 calculations with CAS(7,5) for the mononuclear
molecular fragments of [Co(fpo)2(H2O)2]2+ of
2 for varying d(Co-O) bond distances. The plot of the
d-orbitals splitting calculated by ab initio ligand field theory (AILFT) (A), four low-lying Kramers
doublets (B), and the zero-field splitting parameters D and E (C and D).
3. Materials and Methods
Chemicals for syntheses were purchased from Sigma-Aldrich (St. Louis, MO, USA),
Across Organics (Geel, Belgium), or Alfa Aesar (Kandel, Germany) and used without any further
purification. Reactions were monitored on aluminium TLC sheets pre-coated with silica gel 60
(SIL G/UV254, 0.2 mm, Macherey-Nagel, Düren, Germany). Ligand fpo was purified by column
chromatography on Merck silica gel 60 (0.015–0.040 nm, Darmstadt, Germany) and the reaction scheme
of organic syntheses drawn by a BIOVIA draw [53]. Elemental analysis of chemical composition was
acquired by the Flash 2000 (Thermo Scientific, Waltham, MA, USA) analyser. IR measurements were
performed on spectrometer Jasco FT/IR-4700, data interpreted in Spectragryph, and assigned with
the help of the table of known characteristic vibration frequencies [54,55]. NMR (Varian, Palo Alto,
CA, USA) experiments 1H and 13C were conducted on 400 MHz Varian spectrometer using CDCl3
or d6-DMSO solvent and processed in the iNMR program [56]. Magnetic measurements on PPMS
Dynacool (Quantum Design, Quantum Design, San Diego, CA, USA) and SQUID magnetometer XL-7
(Quantum Design). X-ray experiments were carried out on a four-circle κ-axis Xcalibur2 diffractometer
equipped with a CCD detector Sapphire2 (Rigaku Oxford Diffraction, Yarnton, UK). The CrysAlis
software package (version 1.171.39.9g, Rigaku Oxford Diffraction, Yarnton, United Kingdom) was used
for data collection and reduction [57]. The structures for both 1 and 2 were solved by the SHELXT
program incorporated in the wingx program package [58,59]. Refinement based on intensities was
performed using the SHELXL program [60]. All non-hydrogen atoms in both 1 and 2 were refined
anisotropically. Perchlorate anions in both compounds were found to be disordered in two positions
and the ratio between both parts was 0.83:0.17 for 1 and 0.82:0.18 for 2. Hydrogen atoms in the
ligand were placed in the calculated positions with isotropic thermal parameters tied with the parent
atoms (U(H) = 1.2U(C)). The positions of the water hydrogen atoms for the aqua ligands were found

Molecules 2020, 25, 277
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in different Fourier maps and their isotropic thermal parameters were tied with the parent atoms
(U(H) = 1.2U(O)). The structural figures were drawn using the Diamond software [61]. Crystal data
and the final parameters of the structural refinements for both 1 and 2 are summarized in Table 1.
The EPR spectra were studied using Bruker ELEXSYS II E500 X–band spectrometer (Bruker BioSpin
GmbH, Rheinstetten, Germany) with an operating frequency of 9.4 GHz equipped with ESR910 helium
flow–type cryostat (Oxford Instruments plc, Abingdon, UK). The powdered sample was mixed with
Apiezon N grease (M&I Materials Ltd, Manchester, UK) and attached to the Suprasil sample holder
(Wilmad-LabGlass, Vineland, NJ, USA).
3.1. Preparation of Organic Ligands
3.1.1. Methyl Picolinate (I)
Picolinic acid (10 g, 41.2 mmol) was put to 100 mL of methanol followed by careful addition
of 12 mL concentrated sulphuric acid. Heating the mixture resulted in dissolving picolinic acid.
After one day of reflux, the solution was concentrated, poured into 400 mL of water and ice mixture,
neutralized with potassium bicarbonate, and extracted five times with 50 mL of chloroform and dried
with MgSO4. Evaporation of chloroform and drying under vacuum for a few hours yielded 93%
of I as a slightly yellow oil, which was used in a subsequent reaction without further purification.
FT-IR (mid ATR, v/cm−1): 3058 v(C-Harom), 2952 v(CH3), 2844 v(CH3), 1742 v(CO), 1719 v(CO), 1582
v(C=C, C=N), 1572 v(C=C, C=N), 1469 v(C=C, C=N), 1442 (CH3), 1431 v(C=C, C=N), 1281 v(COC),
1244 v(OCH3), 1125 v(COC), 995 v(ring, pyridine), 745 v(C-Harom), 705 v(C-Harom), 619 v(ring, pyridine),
1
405 v(ring, pyridine). H NMR (400 MHz, CDCl3)
δ 8.70 (ddd, J = 4.7, 1.7, 0.9 Hz, 1H), 8.09 (dt, J = 7.8,
1.0 Hz, 1H), 7.80 (td, J = 7.8, 1.7 Hz, 1H), 7.43 (ddd, J = 7.6, 4.8, 1.2 Hz, 1H), 3.96 (s, 3H). 13C NMR
(101 MHz, CDCl3) δ 165.73, 149.85, 147.92, 137.12, 127.03, 125.19, 52.97.
3.1.2. Picolinic Acid Hydrazide (II)
Compound I (4.53 g, 33 mmol) was poured into 75 mL of methanolic solution containing 50%–60%
hydrazine hydrate (8.4 g) and the mixture was refluxed for 14 h. Subsequent volume reduction of
solvent and cooling of flask with the product in ice results in the formation of colourless crystalline
solid of II. Further purification was accomplished by dissolving II in hot toluene and quick cooling
by ice. This procedure induces formation of elongated transparent colourless crystals and yield 78%.
EA (calc.%) for C6H7N3O (FW = 137.14): C, 52.55, H, 5.14, N, 30.64, Found: C, 52.34, H, 5.36, N,
30.49. FT-IR (mid ATR, v/cm−1): 3288 v(N-H), 3210 v(NH), 3017 v(C-Harom), 1671 v(C=O), 1622 v(NH2),
1593 v(C=C, C=N), 1568 v(C=C, C=N), 1518 v(C=C, C=N, NH2), 1433 v(C=C, C=N), 1123 v(C-N), 999
1
v(ring, pyridine), 819 v(C-Harom), 750 v(C-Harom), 620 v(ring, pyridine), 410 v(ring, pyridine). H NMR
(400 MHz, DMSO-d6)
δ 9.9 (s, 1H), 8.6 (m, 1H), 8.01–7.93 (m, 2H), 7.58-7.53 (m, 1H), 4.61 (s, 2H).
13C NMR (101 MHz, DMSO-d6) δ 162.68, 149.86, 148.52, 137.67, 126.27, 121.76.
3.1.3. Picolinic Acid 2-(2-furanylmethylene)hydrazide (III)
Furfural (1.5 g, 16 mmol) in 10 mL of methanol was added to II (2 g, 14.6 mmol) in 20 mL of
methanol. After 2 h of boiling the solution, cooling in ice caused colourless crystals to form quickly
from a yellow to an orange solution. Reducing the volume of methanol to a minimum amount
(about one-fifth) yielded an additional amount of product. Repeated washing of crystals with a small
amount of cold methanol yields a pure product (94%). EA (calc.%) for C11H9N3O2 (FW = 215.21): C,
61.39, H, 4.22, N, 19.53, Found: C, 61.31, H, 4.12, N, 19.54. FT-IR (mid ATR, v/cm−1): 3287 v(NH), 3225
v(NH), 3132 v(C-Harom), 3112 v(C-Harom), 3086 v(C-Harom), 3060 v(C-Harom), 3003 v(C-Harom), 1675
v(C=O), 1627 v(C=N, imine), 1590 v(C=C, C=N), 1572 v(C=C, C=N), 1520 v(C=C, C=N), 1434 v(C=C,
1
C=N), 996 v(ring, pyridine), 613 v(ring, pyridine), 594 (ring, furan), and 406 v(ring, pyridine). H NMR
(400 MHz, DMSO-d6)
δ 12.23 (s, 1H), 8.70 (ddd, J = 4.7, 1.5, 1.0 Hz, 1H), 8.56 (s, 1H), 8.12 (dt, J = 7.8,
1.0 Hz, 1H), 8.05 (td, J = 7.8, 1.7 Hz, 1H), 7.86 (d, J = 1.7 Hz, 1H), 7.66 (ddd, J = 7.5, 4.7, 1.3 Hz, 1H),

Molecules 2020, 25, 277
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6.92 (d, J = 3.4 Hz, 1H), 6.64 (dd, J = 3.4, 1.8 Hz, 1H). 13C NMR (101 MHz, DMSO-d6)
δ
160.40, 149.57,
149.51, 148.48, 145.30, 138.78, 138.06, 127.04, 122.73, 113.59, 112.30.
3.1.4. 2-(furan-2-yl)-5-(pyridin-2-yl)-1,3,4-oxadiazole (fpo)
A clear solution of III (500 mg, 2.32 mmol) in 10 mL of dimethylsulfoxide was stirred at room
temperature and potassium carbonate (1284 mg, 9.29 mmol) with iodine (1062 mg, 4.18 mmol) were
slowly added. Dark solution formed and temperature was adjusted to 120 ◦C for 18 h (TLC confirmed
formation of fpo and consumed starting material III, TLC mobile phase cyclohexane:ethylacetate = 2:1).
Afterward, dimethylsulfoxide was evaporated. The solid was dissolved in water containing sodium
metabisulfite and extracted three times with 20 mL of ethylacetate. Combined parts of ethylacetate were
evaporated and re-dissolved in a minimum amount of cyclohexane:ethylacetate (2:1) and subjected to
column chromatography on silica gel with gradient elution (cyclohexane:ethylacetate
≥
5:1, 3:1 and 2:1).
The final product fpo precipitated as white solid in 62% yield after evaporating solvent from joined
parts. Colourless crystals can be obtained by slowly removing cyclohexane/ethylacetate. EA (calc.%)
for C11H7N3O2 (FW = 213.19): C, 61.97, H, 3.31, N, 19.71. Found: C, 61.96, H, 3.01, N, 19.46. FT-IR (mid
ATR, v/cm−1): 3128 v(C-Harom), 3107 v(C-Harom), 3092 v(C-Harom), 3057 v(C-Harom), 3025 v(C-Harom),
3000 v(C-Harom), 1615 (C=C, C=N), 1587 (C=C, C=N), 1574 (C=C, C=N), 1525 (C=C, C=N), 1421 v(C=C,
1
C=N), 995 v(ring, pyridine), 619 v(ring, pyridine), 593 (ring, furan), 400 v(ring, pyridine). H NMR
(400 MHz, DMSO-d6)
δ 8.79 (ddd, J = 4.8, 1.7, 1.0 Hz, 1H), 8.23 (dt, J = 7.8, 1.0 Hz, 1H), 8.11 (dd, J = 1.8,
0.7 Hz, 1H), 8.06 (td, J = 7.8, 1.7 Hz, 1H), 7.64 (ddd, J = 7.6, 4.8, 1.2 Hz, 1H), 7.47 (dd, J = 3.6, 0.7 Hz, 1H),
6.83 (dd, J = 3.6, 1.8 Hz, 1H). 13C NMR (101 MHz, DMSO-d6)
δ 162.76, 157.56, 150.30, 147.30, 142.50,
138.48, 137.88, 126.47, 123.13, 115.17, 112.80.
3.2. Preparation of the Complexes
Preparation of coordination compounds was performed as follows: Fe(II) (with a small amount
of ascorbic acid) or Co(II) perchlorate hexahydrate (0.234 mmol) in 10 mL of methanol was added
dropwise to fpo (100 mg, 0.469 mmol) in 10 mL of methanol. After an hour of reflux, the solution was
filtered and left to evaporate slowly. Square shaped crystals formed within one week.
Caution! Work with perchlorate salts may be dangerous because of the potentially explosive character!
[Fe(fpo)2(H2O)2](ClO4)2 (1). Yellow crystals, 52 mg (31%). EA (calc.%) for C22H18Cl2FeN6O14
(FW = 717.17): C, 36.84, H, 2.53, N, 11.72, Found: C, 37.06, H, 2.18, N, 11.36. FT-IR (mid ATR,
v/cm−1): 3363 v(H2O), 3124 v(C-Harom), 3070 v(C-Harom), 1634 v(H2O), 1575 (C=C, C=N), 1519
(C=C, C=N), 1422 v(C=C, C=N), 1094 v(ClO4), 1048 v(ClO4), 623 v(ClO4), 593 (ring, furan), 414
v(ring, pyridine).
[Co(fpo)2(H2O)2](ClO4)2 (2). Orange crystals, 40.7 mg (24%). EA (calc.%) for C22H18Cl2CoN6O14
(FW = 720.25): C, 36.69, H, 2.52, N, 11.67. Found: C, 36.80, H, 2.45, N, 11.43. FT-IR (mid ATR, v/cm−1):
3384 v(H2O), 3124 v(C-Harom), 3072 v(C-Harom), 1633 v(H2O), 1574 (C=C, C=N), 1517 (C=C, C=N),
1423 v(C=C, C=N), 1094 v(ClO4), 1049 v(ClO4), 623 v(ClO4), 592 (ring, furan), 419 v(ring, pyridine).
4. Conclusions
Our work focused on preparation and magneto-chemical analysis of new 1,3,4-oxadiazole based
coordination compounds. Thus, isolated coordination compounds [M(fpo)2(H2O)2](ClO4)2 (M = Fe(II)
for (1); Co(II) for (2)) may serve as potential building blocks for preparation of polynuclear compounds.
The data showed large rhombicity in both complexes and, in the case of 2, field-induced slow relaxation
of the magnetization with an energy barrier Ueff = 65.3 K was detected. The axial type of the magnetic
anisotropy was also confirmed in 2 by employing X-band EPR. Detailed theoretical investigation
based on the CASSCF/NEVPT2 calculations revealed the importance of the second coordination sphere
formed by the hydrogen bonding between perchlorate anions and aqua ligands on the magnetic

Molecules 2020, 25, 277
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anisotropy parameters D and E. The hydrogen bond tunes the axial ligand field and, thus, this kind
of non-covalent contacts can serve as a trigger between the easy-plane and the easy-axis type of the
magnetic anisotropy.
Supplementary Materials: The following are available online at http://www.mdpi.com/1420-3049/25/2/277/s1.
Figure S1: In-phase χreal and out-of-phase χimag molar susceptibilities for 2 at zero static magnetic field and in
non-zero static field. Table S1: Parameters of one-component Debye model for 2 derived according Eq.4 in main
text. Table S2: The g-values for the ground state Kramers doublet calculated for the effective spin Seff = 1/2 for 2
derived from CASSCF/NEVPT2 calculations. CCDC 1972873-1972874 contain the supplementary crystallographic
data for this paper.
Author Contributions: P.Z. and R.H. conceived and designed the synthesis and performed the analysis of the
ˇ
magnetic properties. J.K. performed X-ray acquisition data and analysis. E.C. performed X-band EPR experiment
and analysis. R.H. performed theoretical calculations. All authors contributed in the writing of the manuscript.
All authors have read and agreed to the published version of the manuscript.
Funding: We acknowledge financial support from the Grant Agency of the Czech Republic (GACR 17-08992S),
the National Programme of Sustainability I (LO1305) of the Ministry of Education, Youth and Sports of the Czech
ˇ
Republic. E.C. is supported by the Slovak Research and Development Agency under contract No. APVV-18-0197
and Scientific Grant Agency of Ministry of Education, Science, Research and Sport of the Slovak Republic under
contract No. VEGA 1/0426/19.
Acknowledgments: The authors also thank Z. Trávnícˇek for initial preliminary X-ray diffraction analysis.
Conflicts of Interest: The authors declare no conflict of interest.
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