Thermal properties of the hybrid compound [Cu(en)2(VO3)2]?3H2O — phase analysis upon heating and cooling. Crystal structure characterization of its two polymorphic modifications

Article abstract of DOI:10.1016/j.molstruc.2021.129930

The inorganic-organic hybrid compound [Cu(en)₂(VO₃)₂]?3H₂O (1) was prepared by crystallization at room temperature from the CuSO₄?5H₂O – KVO₃ – ethylenediamine – H₂O system. Upon cooling 1, phase transition occurred at 176 K. Single-crystal diffraction analysis of the high- temperature and low-temperature phases has shown both phases to be polymeric, incorporating metavanadate chains (VO₃)_n^n– coordinated via oxygen atoms to copper(II) atoms. The four remaining positions of the copper atoms with octahedral environment are taken by nitrogen atoms of two ethylenediamine ligands. The low-temperature phase exhibits displacive modulation in its structure. The band gap of the high-temperature phase is 2.95 eV. The compound is paramagnetic (1.79 μ_B) and exhibits no distinct magnetic interactions. A detailed phase analysis upon heating of 1 to 600 °C revealed the final products of the thermal decomposition to be monoclinic Cu(VO₃)₂, Cu₂V₂O₇ as ziessite and V₂O₅. After cooling to room temperature, monoclinic Cu(VO₃)₂ transformed to its triclinic polymorph, while ziessite became transformed to blossite.

Full text of DOI:10.1016/j.molstruc.2021.129930

Journal of Molecular Structure 1230 (2021) 129930
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstr
•
Thermal properties of the hybrid compound [Cu(en)2(VO3)2] 3H2O —
phase analysis upon heating and cooling. Crystal structure
characterization of its two polymorphic modifications
Gabriela Stehlíkováa, Róbert Gyepesb, Roman Bystrickýc, Martin Škrátekd, Prˇemysl Vaneˇke,
Jozef Tatierskya,∗
a Department of Inorganic Chemistry, Faculty of Natural Sciences, Comenius University, Ilkovicˇova 6, 842 15 Bratislava, Slovakia
b Department of Inorganic Chemistry, Charles University, Hlavova 2030, Prague 2, 128 40, Czech Republic
c Institute of Inorganic Chemistry, Slovak Academy of Sciences, Dúbravská cesta 9, Bratislava, 845 36, Slovakia
d Department of Magnetometry, Institute of Measurement Science, Slovak Academy of Sciences, Dúbravská cesta 9, Bratislava, 841 04, Slovakia
e Department of Dielectrics, Institute of Physics ASCR, v.v.i., Na Slovance 2, Praha 8, 182 21, Czech Republic
a r t i c l e i n f o
a b s t r a c t
Article history:
The inorganic-organic hybrid compound [Cu(en)2(VO3)2]•3H2O (1) was prepared by crystallization at
room temperature from the CuSO4•5H2O – KVO3 – ethylenediamine – H2O system. Upon cooling 1,
phase transition occurred at 176 K. Single-crystal diffraction analysis of the high- temperature and low-
Received 25 September 2020
Revised 17 December 2020
Accepted 23 December 2020
Available online 13 January 2021
n–
temperature phases has shown both phases to be polymeric, incorporating metavanadate chains (VO3)n
coordinated via oxygen atoms to copper(II) atoms. The four remaining positions of the copper atoms
with octahedral environment are taken by nitrogen atoms of two ethylenediamine ligands. The low-
temperature phase exhibits displacive modulation in its structure. The band gap of the high-temperature
phase is 2.95 eV. The compound is paramagnetic (1.79 μB) and exhibits no distinct magnetic interac-
tions. A detailed phase analysis upon heating of 1 to 600 °C revealed the final products of the thermal
decomposition to be monoclinic Cu(VO3)2, Cu2V2O7 as ziessite and V2O5. After cooling to room temper-
ature, monoclinic Cu(VO3)2 transformed to its triclinic polymorph, while ziessite became transformed to
blossite.
Keywords:
Copper vanadate
Ethylenediamine ligand
Modulation
Phase transition
Magnetic measurement
© 2021 Elsevier B.V. All rights reserved.
1. Introduction
optic, magnetic or dielectric properties, electrical mobility, ther-
mal stability and mechanical hardness, whereas organic part adds
During recent years, enormous interest has been payed to hy-
brid organic-inorganic compounds. [1,2,3] amongst of the vana-
dium oxides are those with organic components which might
be directly coordinated to the inorganic scaffoldings or to sec-
ondary metal centres. [4] Their composition corresponds to the
formula {M/L/VxOy}, where M = transition metal, L = organic
ligand. [5] The structure of [VxOy]n− ranges from monomeric
structural diversity, luminescence properties and polarizability. [9]
Some of the hybrid inorganic-organic vanadium compounds
contain in addition to vanadium also some secondary metal. One
of the first-row transition metals used often is copper, due to the
coordination plasticity of CuII. [10] Within complexes incorporat-
ing copper as the central atom, nitrogen donor ligands are mostly
used, like derivatives of pyridine, pyrazine and aliphatic amines.
The great progress in researching organic-inorganic hybrid com-
pounds incorporating copper is encouraged by their interesting
structure and properties, having application to sorption, electrical
conductivity, ion exchange, catalysis, magnetism, and photochem-
istry. [7]
6−
[VO4]3−, oligomeric [V2O7]4−, [V4O12
]
4−, [V6O18
]
units to poly-
n−
meric [VO3]n
chains and [VxOy]n− networks. [6,7,8] Vanadates
are potential candidates for secondary cathode materials amongst
others. [8]
Both inorganic and organic parts of hybrid compound bring
interesting properties to the final compound. The inorganic part
of the hybrid inorganic-organic vanadium compounds may supply
Based on the CCDC [11] records about 770 compounds contain
Cu and ethylenediamine (en); 69 of them contain also the vana-
dium:
∗
i) compounds
e.:
containing
]
mixed
[12],
valent
vanadium,
i.
Corresponding author.
E-mail addresses: jozef.tatiersky@uniba.sk, jozef.tatiersky@uniba.sk (J. Tatiersky).
Cu(en)2[V6O14
[Cu(en)2]2[V10O25
]
[13],
https://doi.org/10.1016/j.molstruc.2021.129930
0022-2860/© 2021 Elsevier B.V. All rights reserved.

G. Stehlíková, R. Gyepes, R. Bystrický et al.
Journal of Molecular Structure 1230 (2021) 129930
IV
[Cu(en)2]4[Na2VV
V
2O42(VIVO4)]•2H2O [14] and {Cu(en)2}4
different temperatures: 2 and 300 K. Temperature dependence of
magnetic susceptibility was measured in two ways: first the sam-
ple was cooled from room temperature (RT) down to 1.8 K without
applying any magnetic field (using ultra low field option to set zero
field) and then heated back to RT under applied magnetic field of
50 Oe (zero field cooled − ZFC curve); afterwards, under the same
field the sample was cooled down again to 1.8 K, following the
same steps as before (field cooled − FC curve).
14
[Cl⊂V15O36
ii) compounds
state,
[{Cu(en)2}(V10O28)]n•2n[Cu(en)2(H2O)]•2n(H3BO3)•
2n(H2O) [10,17], (H2en)2[Cu(en)2(H2O)2][V10O28]•4H2O,
(H3O)2[{Cu(en)2(H2O)}2V10O28]•3H2O,
[{Cu(en)2}3(V10O28)]•6H2O [18],
[19], [{Cu(bpy)(en)}{Cu(bpy)(H2O}
[Cu(C5H5N)2(C2H8N2)][(VO3)2] [20],
[21], Cu(H2N(CH2)2NH2) [V2O6] [22,21],
iii) heterometallic compounds, e. g: Na8[Cu(en)2]2[V12B18 O60H6]
]
•12H2O [15],
containing
vanadium
in
fully
oxidized
i.e.:
[Cu(en)2H2O]2[H2V10O28]•12H2O [16],
{[Cu(en)2]2V2O7}•4H2O
7
Pyris Diamond DSC (PerkinElmer) with liquid nitrogen CryoFill
cooling was used for differential scanning calorimetry. PerkinElmer
Pyris software, version 13.1.1., was used for control and evaluation.
The temperature and heat flow were calibrated using phase tran-
sition temperature and melting point of cyclohexane. The sample
(33 mg) was placed in a hermetically closed aluminium pan, the
sample holder was purged by helium. The sample was heated from
93 K to 323 K by a rate of 10 K/min and then cooled to 103 K by
the same rate. Two measurements were performed.
{VO3}4]n
,
[Cu(en)(Im)2][(VO3)2]
(NO3)2•14.7H2O,
Na7[Cu(en)2]2[V12B18 O60H6](NO3)•15.5H2O
(NH4)5n{[Cu(en)2][PMo8V4O40]}n•9nH2O
Cu(en)2]
{[Cu(en)2]2[MoVI5MoV3VIV8O40
[23],
[24,
(PO4)]}•4H2O
[25],
[Cu(en)2(H2O)]4
[Cu(en)2]3.5[PMoVI8VIV6O42 Cu(en)(1,10-phen)]3•14H2O [26],
´
[Cu(en)2(H2O)]{[PMoVI8VIV6O42Cu(en)2][Cu0.5(en)]3}•5.5H2O
[27], [Cu(en)2]1.5 [H3As6V15O42(H2O)]•3H2O
[Cu(en)2(H2O)]2[Cu(en)2]2[AsWVI2WV7VIV7O44]•2H2O
[28],
[29],
2.2. X-ray crystallography
[Cu(en)2(H2O)]{PMoVI8VIV6O42[Cu(en)2][Cu0.5(en)]3}•5.5H2O,
H3{VVMoVI8VIV6O42[Cu(en)2]4}[MoO4]2•14H2O [30].
Diffraction data for 1 were collected on a Bruker D8 VENTURE
Kappa Duo diffractometer using Cu K α radiation at 195(2) K for
the HT phase or at 155 K for the LT phase. The data collection for
both phases was carried out at different times using different sam-
ple specimens. After collecting all data for the LT phase, the sample
temperature was raised to 195 K and the disappearance of satellite
reflections confirmed the loss of modulation in the lattice. Even
after cooling the sample and heating it up again, no splitting of
diffraction spots could be observed, thus the phase transition oc-
curred reversibly without inflicting any damage to the lattice. This
observation was confirmed also after removing the specimen from
the diffractometer and inspecting it under a polarization micro-
scope, where no visible damage was detected. Data reduction on
the collected frames for both phases was carried out by the diffrac-
tometer software.
Herein we report the synthesis, structural and spectroscopic
characterization (IR, UV–Vis), powder X-ray diffraction analysis at
room temperature and during heating, thermal analysis and mag-
netic properties of compound [Cu(en)2(VO3)2]•3H2O (1).
2. Experimental
2.1. Materials and measurements
Chemicals
CuSO4•5H2O
(CentralChem),
ethylenediamine
(Fluka), KOH (Reachem), ethanol (Diplomat v.o.s.) were used
as obtained without further purification. V2O5 was prepared by
thermal decomposition (500 °C) of previously purified NH4VO3.
[31] KVO3 was synthesised by reaction of an aqueous solution
KOH and V2O5 at pH = 8. [32] The pH of the aqueous solution
was measured on a HANNA pH 213 (HANNA Instruments).
Elemental analyses C, H, N were determined on a Vario MICRO
cube (Elementar).
The phase problem for the HT phase was solved by intrin-
sic phasing and the structure model was refined by full-matrix
least-squares based on F2 using the SHELX program suite. [33] All
non-hydrogen atoms were refined with anisotropic displacement
parameters. The V(1)–O(4A) and V(1)–O(4B) bond distances were
constrained to the same free variable and with standard uncer-
tainty set to 0.002. Hydrogen atoms residing on carbon atoms were
placed in idealized positions and refined with the riding model
using isotropic thermal parameters set to 1.2∗Ueq of their pivotal
atoms. All other hydrogen atoms refined with no constraints and
isotropically.
Infrared spectra were recorded on a Nicolet 6700 FT-IR spec-
trometer (Thermo Fisher Scientific) in KBr pellets in the range
4000 – 400 cm−1
.
Thermal analysis (DTA-TG) was performed with Derivatograph
Q-1500 D, (MOM, Hungary) with heating rate of 5 °C/min.
Reflectance spectrum was measured on spectrofluorometer FSP
920 (Edinburgh Instruments, UK). The sample was placed in in-
tegrating sphere. BaSO4 was used as a reference material. ab-
sorbence was calculated by the device according to equations:
R(λ) = ESam(λ)/ERef(λ), A(λ) = log10 (ERef(λ)/ESam(λ).
The phase problem for the LT phase was solved by charge flip-
ping using SUPERFLIP. [34] The structure was refined on F using
Jana2006. [35] The type of modulation for all atoms were deter-
mined after inspecting the de Wolff sections and the displacive
modulations of all atoms were described by harmonic waves. All
organic hydrogen atoms were put into idealized positions. For all
hydrogen atoms of the crystal water molecules, their respective O–
The crystalline phases present in the samples were identified
using X-ray diffraction (XRD) (Panalytical Empyrean, Netherlands,
˚
H interatomic distances were restrained to 0.82(3) A.
CuK radiation) as well as PW 1050 diffractometer (Philips, CuK
α
α
The PLATON program [36] was used to generate all solid-state
structure plots. Crystal data are given in Table 1.
radiation). High temperature measurements were performed on
Anton Paar high-temperature chamber HTK-16 N using Pt heat-
ing strip. The heating rate during non-isothermal segments was
10 °C/min, the duration of isothermal segments during which the
patterns were collected was 60 s, the temperature step when col-
lecting the individual patterns was 10 °C. After cooling down sam-
ple was measured once again to identify possible phase transfor-
mation during cooling.
CCDC 2018195 and 2018200 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre.
2.3. Synthesis of 1
Magnetic properties were measured using a MPMS-XL 7AC
SQUID magnetometer (Quantum Design, San Diego, CA, USA) op-
erating in the 1.8 - 300 K range. Isothermal magnetization vs. ap-
plied field curves, M(H), were measured up to 1 T field at two
CuSO4•5H2O (1.00 g, 4.01 mmol) was dissolved in water
(2.82 ml) and under vigorous stirring ethylenediamine (en, 1.08 ml,
8.01 mmol) was added. To the purple solution obtained was added
KVO3 (1.11 g, 8.01 mmol) and 150 ml of H2O. Solution was left to
2

G. Stehlíková, R. Gyepes, R. Bystrický et al.
Journal of Molecular Structure 1230 (2021) 129930
Table 1
Selected crystallographic data for 1 (HT – high temperature, LT –
low temperature polymorphic modification).
Compound
HT
LT
Empirical formula
Crystal system
Space group
C4H22CuN4O9V2
monoclinic
C2/c
C2H11Cu0.5N2O4.5
monoclinic
Cc(α0γ )s
13.4444(3)
10.9794(2)
11.3765(3)
90
V
˚
a [A]
13.4734(6)
10.9947(5)
11.3858(5)
90
˚
b [A]
˚
c [A]
α [°]
β [°]
γ [°]
114.6750(10)
90
114.687(3)
90
3
˚
Volume [A ]
1532.64
4
1525.82
2
Z
−3
•
ρ [g cm
μ [mm−1
ꢀmax [°]
R
]
1.888
1.897
]
12.076
12.130
72.375
79.370
0.0356
0.0678
wR2
0.0838
0.0968
Table 2
Selected bands in the IR spectrum of 1.
Bands (cm–1) and Intensity
Assignment
528 s
νs(V–O–V)
νas(V–O–V)
631 vs
907 s
νt(V=O)
δ(HOH)
δ(HNH), δ(HOH)
ν(C–
1597 s
1638 m
2937 m
2885 m
3248 vs
3421 vs
H)
ν(N–H)
ν(OH)
Fig. 1. The basic monomeric unit of 1 residing in the asymmetric part of the unit
cell. The two unterminated equatorial bonds from Cu(1) connect to another en lig-
and, while the apical bond links to the symmetry-related O(1) of the next vanadate.
Atom O(3) and both positions of O(2) continue to two vanadium neighbours. O(2)
and O(2)’ are related by the centre of symmetry located halfway on their intercon-
nection. Atom Cu(1) resides in special position with site occupancy 0.5.
crystalize at room temperature and dark purple crystals were iso-
lated after 6 weeks in about 64% yield (based on Cu). Anal. Calcd.
for C4H22CuN4O9V2: C, 11.03; H, 5.09; N, 12.86. Found: C, 10.82; H,
5.03; N, 12.85. The theoretical pattern is in a good agreement with
the measured XRD data (Supplementary data Fig. S1). The assign-
ment of the vibrational bands (Table 2) in IR spectrum (Fig. S2) is
in accordance with data published in literature. [37,38,39,40]
been modelled in the HT phase by splitting it into two distinct
ones – O(4A) and O(4B), which are mutually unrelated by any sym-
metry operation.
3. Results and discussion
Propagating the monomeric unit through O(3) and either
through O(2) or its symmetry equivalent (O(2)’) creates a poly-
3.1. Synthesis and solid-state structure of 1
meric vanadate layer in solid state. These vanadate layers become
2+
Compound 1 (high temperature phase, HT) was prepared by
crystallization from CuSO4•5H2O – KVO3 – ethylenediamine – H2O
system at room temperature as well as at 5 °C.
crosslinked by Cu(en)2
fragments that act as formal bridges be-
tween two distinct anionic layers. The copper of one cation be-
comes simultaneously linked to O(1) on one layer and to the sym-
metry equivalent of O(1) on another layer. Such interconnection of
The solid-state structure of 1 (HT phase) is polymeric that is
built from the monomeric unit by chaining it in three directions.
The monomeric unit incorporates all atoms of 1 except its crys-
tal water molecules (Fig. 1) and generates the polymeric motif
by sharing its Cu(1), both symmetry equivalents of O(2) (see be-
low) and O(3) with the neighbouring monomeric units related by
group symmetry. The crystal water molecules occupy cavities in
the polymeric network and are interacting with the main frame-
work through hydrogen bridges.
the anionic layers leads to the creation of distinct vanadate stripes,
2+
which become interspaced by Cu(en)2
monomeric units (Figs. 2,
S5, S6).
Within the HT phase, the positional disorder of O(2) and the
splitting of O(4) follows no precise ordering and was assumed to
be stochastic. Some positional disorder, albeit less severe, was ob-
served also for O(1) and O(3), apparently as the consequence of
O(2) scattered between two positions, although the bridging atoms
O(1) and O(3) are less prone to positional displacement than their
terminal counterpart O(4). Since diffraction frames of the HT phase
suggested no modulation to be present whatsoever, this structure
model was refined using site occupancy factors of O(2), O(4A) and
O(4B) fixed to 0.5. Attempts to refine O(4) in a single position
yielded a structure model of poor quality, which was abandoned.
Splitting O(4) into two positions improved the model quality con-
siderably, but even then the presence of further electron maxima
in the proximity to the ones refined already were suggested on dif-
ference Fourier maps. The partitioning of O(4) into more positions
The copper atoms (Cu1) possess an octahedral coordination ge-
ometry: two ethylenediamine molecules are bidentately coordi-
nated in equatorial positions (N1, N2) and two oxygen atoms (O1)
of vanadate chain occupy axial positions.
Atoms Cu(1) and O(3) reside in special positions, while atom
O(2), which resides in general position becomes split into two
symmetrically equivalent positions coupled through the group in-
version centre located in its proximity. The vanadate tetrahedron
becomes complete with the terminal atom O(4). This atom suffers
from positional disorder in the crystal lattice and its position has
3

G. Stehlíková, R. Gyepes, R. Bystrický et al.
Journal of Molecular Structure 1230 (2021) 129930
as for the HT phase. Subsequent data reduction was carried out
on all data and provided an incommensurate modulation vector
q = (0.1835, 0.0000, 0.2314) for the LT phase.
The LT phase model refinement was guided by inspecting the
de Wolff sections computed during various stages of the refine-
ment and these evidenced displacive modulation to be present be-
neath the temperature of phase transition. In consistency with C2/c
space group of the HT phase, the supergroup C2/c(α0γ )0 s was ini-
tially chosen for refinement. As no discontinuities were suggested
on the density maps, the positional modulation of all atoms was
described using harmonic waves.
The average structural model obtained after final refinement re-
sembled closely the HT model, although two important distinctions
were observed. The first difference was the presence of modulative
displacement of atoms in the structure. Although some modula-
tion was observable for all atoms, it was prevalent for the vana-
date oxygen atoms and even amongst those, the terminal O(4)
was affected mostly. The second difference between the HT and
LT models was the description of the O(4) atom position. In the
LT model, this atom could successfully be refined without splitting
it into two (or even more) positions. One single position and full
site occupancy was employed for its refinement and its modulative
wave provided a suitable description for its displacement within
the crystal lattice.
Fig. 2. Perspective view of the solid-state structure of 1 along [100]. Crystal water
The LT model could successfully be refined to satisfactory qual-
ity and the resulting structural model was very similar to the HT
phase, even though it offered a more proper description for the
position of the terminal O(4). However, the assumption of O(4) oc-
cupying a single site triggered a further shortcoming within the
LT model. This shortcoming came from the remnant splitting of
O(2), which now yielded unphysical bond angles within the vana-
date residue. This can be illustrated by the O(2)–V(1)–O(4) bond
angle that would become either 88.552(1)° or 127.594(1)° depend-
ing on which symmetry equivalent of O(2) is chosen. These values
are in contrast with the remaining O–V–O vanadate angles, whose
values acquired the expected range: O(1)–V(1)–O(3) 109.581(1)°;
O(1)–V(1)–O(4) 108.109(1)° and O(3)–V(1)–O(4) 112.302(1)°.
In analogy with the HT phase, the splitting of O(2) into two
positions occurred because of the inversion centre present in the
(super)group. This however suggested, that apart from triggering
modulation of atomic positions, the phase transition could also
generate changes within the crystal lattice that would lead to the
decrease of its symmetry. Such decrease should be achieved by
cancelling the presence of the inversion centre, which would now
allow to resolve the bridging O(2) into symmetrically independent
positions. This possibility has been verified by subsequent refine-
ment of the structure model employing the supergroup Cc(α0γ )s.
This model with no inversion centre could successfully describe
the bridging oxido ligand O(2) residing in two symmetrically in-
dependent positions and provided a model with all O–V–O angles
approaching values close to ideal tetrahedral.
molecules are omitted for clarity.
Table 3
˚
Selected bond distances (A) and angles (°) for the HT phase of 1.
V(1)–O(1)
V(1)–O(2)
V(1)–O(2)#1
V(1)–O(3)
V(1)–O(4A)
V(1)–O(4B)
1.614(3)
1.873(4)
1.778(4)
1.8020(12)
1.648(4)
1.617(4)
Cu(1)–N(1)
2.019(3)
2.030(3)
2.5139(1)
1.318(10)
1.510(4)
1.479(4)
1.476(4)
42.2(3)
Cu(1)–N(2)
Cu(1)–O(1)
O(2)–O(2)#1
C(1)–C(2)
N(1)–C(1)
C(2)–N(2)
O(1)-V(1)-O(4A)
O(1)-V(1)-O(4B)
O(1)-V(1)-O(2)#1
O(4A)-V(1)-O(2)#1
O(1)-V(1)-O(3)
98.8(2)
O(2)#1-V(1)-O(2)
O(3)-V(1)-O(2)
O(2)#1-O(2)-V(1)#1
O(2)#1-O(2)-V(1)
V(1)#1-O(2)-V(1)
N(1)#2-Cu(1)-N(2)
C(1)-N(1)-Cu(1)
C(2)-N(2)-Cu(1)
N(1)-Cu(1)-N(2)
N(1)-Cu(1)-N(2)#2
N(2)-C(2)-C(1)
119.4(2)
121.6(2)
102.2(2)
109.54(12)
107.55(18)
116.8(2)
114.62(17)
93.6(2)
101.86(17)
72.7(4)
65.0(3)
137.8(3)
95.53(11)
107.96(19)
109.12(19)
84.47(11)
95.53(11)
107.9(3)
O(4A)-V(1)-O(3)
O(4B)-V(1)-O(3)
O(2)#1-V(1)-O(3)
O(1)-V(1)-O(2)
O(4B)-V(1)-O(2)
O(4A)-V(1)-O(2)
V(1)#4-O(3)-V(1)
111.8(2)
141.9(2)
138.28(18)
Symmetry transformations used to generate equivalent atoms.
#1 -x + 2,-y + 1,-z.
#2 -x + 3/2,-y + 1/2,-z.
was prevented by serious numerical difficulties with such models
and the final refinement was carried out by keeping two atomic
positions for this atom and restraining the bond distances V(1)–
O(4A) and V(1)–O(4B) to the same value with standard uncertainty
0.002. Atoms O(1) and O(3) were refined without splitting them
into two positions. Selected interatomic bond distances and angles
for the HT phase are given in Table 3.
The assumption of the lower supergroup resulted in more
atoms to be present within the asymmetric part of the unit cell
and for coherence, the atom labelling scheme followed the one
of the HT phase. The newly generated symmetrically independent
atoms were marked by appending a trailing s to the label of their
original symmetrical equivalents, while the two symmetrically in-
dependent positions of bridging O(2) were denoted as O(2a) and
O(2b) (Fig. 3). The original labelling of the HT phase could not
however be fully retained, as no atom was residing in a special po-
sition in the new supergroup. Selected bond distances and angles
are listed in Table 4.
The reproducible and reversible indistinct lambda-type DSC
anomaly at 176 K (Fig. S4) with low (obviously instrumental) tem-
perature hysteresis indicates the second order phase transition typ-
ical for the insignificant structural change such as the transition
to a phase with modulated structure on cooling. It is in accor-
dance with the crystal structure of LT phase of 1, as a single-
crystal specimen of 1 cooled to 155 K exhibited well discernible
satellite diffractions, which suggested modulation to occur in the
solid-state lattice. The unit cell was determined using the stronger
(main) reflections and yielded cell parameters very similar to those
After observing that the less symmetric supergroup provides
a more physical description of the structure model, the possibil-
ity of a lower spacegroup for the HT phase has also been exam-
ined. However, for the HT phase, the splitting of O(2) remained
4

G. Stehlíková, R. Gyepes, R. Bystrický et al.
Journal of Molecular Structure 1230 (2021) 129930
Fig. 3. The vanadate chain part in the asymmetric part of the unit cell for the LT
phase of 1. Atoms O(1) and O(1 s) are connecting to copper atoms which are not
drawn for clarity.
Table 4
˚
Average bond distances (A) and angles (°) for the LT phase of 1.
Fig. 4. Determination of the optical bandgap of 1 using the Kubelka-Munk method
Cu(1)–N(1)
2.036(7)
2.022(10)
2.011(8)
2.038(9)
1.635(8)
1.817(8)
1.871(10)
1.68(2)
V(1 s)–O(1 s)
V(1 s)–O(2a)
V(1 s)–O(3 s)
V(1 s)–O(2b)
N(1)–C(1)
1.603(9)
1.794(8)
1.60(2)
and direct Tauc plot.
Cu(1)–N(2)
Cu(1)–N(1 s)
Cu(1)–N(2 s)
1.761(11)
1.491(16)
1.492(13)
1.461(13)
V(1)–O(1)
V(1)–O(2a)
C(1)–C(2)
V(1)–O(2b)#1
V(1)–O(3)
C(2)–N(2)
N(1)–Cu(1)–N(2)
N(1)–Cu(1)–N(1 s)
N(1)–Cu(1)–N(2 s)
N(2)–Cu(1)–N(1 s)
N(2)–Cu(1)–N(2 s)
N(1 s)–Cu(1)–N(2 s)
O(1)–V(1)–O(2)
O(1)–V(1)–O(2b)
O(1)–V(1)–O(2a)
O(3)–V(1)–O(2a)
84.2(4)
O(2a)–V(1)–O(2b)
O(3 s)–V(1 s)–O(2a)
O(3 s)–V(1 s)–O(1 s)
O(2a)–V(1 s)–O(2b)
Cu(1)–N(1)–C(1)
101.2(4)
114.5(7)
114.4(10)
115.9(4)
106.7(5)
108.4(8)
107.5(8)
109.4(6)
107.4(8)
108.3(8)
108.8(6)
178.6(3)
95.2(3)
96.3(3)
178.1(3)
84.3(3)
N(1)–C(1)–C(2)
102.8(10)
94.6(4)
C(1)–C(2)–N(2)
Cu(1)–N(2)–C2
110.3(5)
109.3(9)
N(1 s)–C(1 s)–C(2 s)
C(1 s)–C(2 s)–N(2 s)
Cu(1)–N(2 s)–C(2 s)
Symmetry transformations used to generate the equivalent atom of O(2b).
#1 x1,-x2+1,x3+1/2,x4+1/2.
Fig. 5. DTA-TG curves for 1.
present in the Fourier maps even upon considering either noncen-
trosymmetric space groups C2 or Cc. This result confirmed the cor-
rectness of the original assignment of the centrosymmetric space
group C2/c for the HT phase and no better description for the dis-
order present could be found.
After liberation of ethylenediamine molecules, Cu2V2O7 (zies-
site) becomes formed together with Cu0.261V2O5. With further in-
crease of temperature the diffraction peaks of Cu0.261V2O5 are
gradually fading until the temperature reaches 440 °C. Above this
temperature, ziessite partially decomposes to triclinic Cu(VO3)2
and V2O5.
3.2. Reflectance UV–Vis spectroscopy
The final thermal decomposition product obtained at 600 °C
is a mixture of monoclinic Cu(VO3)2 as a major phase [41] and
V2O5 [42] as well as ziessite as minor phases. After cooling to the
room temperature, ziessite is transforming to Cu2V2O7 (blossite)
and monoclinic Cu(VO3)2 is transforming to triclinic (Fig. 7).
The optical bandgap was established applying the Kubelka-
Munk method and direct Tauc plot (Fig. 4). The bandgap value is
2.95 eV, thus the compound is an isolator. An absorption detected
at 560 nm can be referred to a d-d transition of Cu2+ (Fig. S3).
3.4. Magnetic properties
3.3. Thermal analysis and in situ XRD measurements
Fig. 8 shows M(H) curves measured for compound 1 at 2 and
300 K. These curves are linear with positive slope (for enlargement
of 300 K curve see inset of Fig. 8) and no hysteresis, these charac-
teristics are typical for paramagnetism.
Compound 1 decomposes in several steps (Fig. 5). The initial
step starts at around 50 °C and the mass loss is 12% (calculated
12.3%). This is due to the release of three water molecules per for-
mula unit of the 1. This process is expressed on the DTA curve by
three endothermic signals. The XRD pattern (Fig. 6) shows that this
process is accompanied by amorphization of the 1. Next, deami-
nation takes place and consists of two part-stages. The first stage
(two exothermic signals at Tmax = 262 °C and Tmax = 270 °C) is fol-
lowed by weight loss of 16%. The second stage occurs in the tem-
perature range 285 – 380 °C and is accompanied by a weight loss
of 12%. The combined weight loss of these stages is 28%, which
corresponds to a theoretical release of two molecules of ethylene-
diamine (calcd. 27.6%).
ZFC/FC susceptibility plots could be seen in Fig. 9. Plots are
overlapping in whole temperature region, which mean that no
magnetic phase change occurs in the sample. The effective mag-
netic moment μeff = 1.79 μB of 1 at 275 K, determined from equa-
tion:
−1/2
)
μeff = 2.828 χ T
(1)
(
m
is consistent with that expected for an isolated paramagnetic sys-
tem with S = 1/2 (μeff = 1.73 μB). Similar results were obtained for
the Cu(tn)2(VO3)2 compound (tn = 1,3-diaminopropane). [43]
5

G. Stehlíková, R. Gyepes, R. Bystrický et al.
Journal of Molecular Structure 1230 (2021) 129930
Fig. 6. XRD patterns made at different temperatures during thermal decomposition of compound 1.
Fig. 7. Phase analysis during heating and cooling of compound 1 (T – triclinic, M – monoclinic).
Inverse susceptibility (χm–1) vs T plot (Fig. 9) show almost lin-
ear dependence in whole temperature range suggesting paramag-
netism in the sample. Fitting this to Curie-Weiss law:
χm = χCW + χ0
(2)
χCW = C/ T + ꢀ
(3)
(
)
Here χ0 is temperature independent contribution, C = NCWNAμB/kB
is the Curie constant, ꢀ is the Curie-Weiss temperature. Assuming
the spin S = ½ and the g factor g = 2, the best fit of the data
yields to χ0 = 5.6 × 10−5 emu/mole and ꢀ = 0.061 K. The very
small ꢀ demonstrates that no distinct magnetic interactions exist.
This is in line that the nearest Cu…Cu distance in the structure is
˚
˚
greater than 6.7654(2) A (HT) and 6.7223(3) A (LT), and the direct
exchange couplings thus can be excluded.
Fig. 8. M(H) curves for compound 1 measured at 2 and 300 K. Inset shows enlarge-
ment of the 300 K dependence.
6

G. Stehlíková, R. Gyepes, R. Bystrický et al.
Journal of Molecular Structure 1230 (2021) 129930
Fig. 9. Molar susceptibility (left scale) ZFC/FC plots for compound 1 measured at field of 50 Oe. Right scale belongs to inverse susceptibility plot (black circles) and its linear
fit (red line) to Curie-Weiss law. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
4. Conclusions
public (VEGA no. 1/0507/17, no. 2/0152/18 and no. 2/0164/17,
APVV-0540–15) and Charles University Centre of Advanced Ma-
terials (CUCAM) (OP VVVExcellent Research Teams), project num-
ber CZ.02.1.01/0.0/0.0/15_003/0000417. Authors thank RNDr. Marek
Cigánˇ, PhD. (Department of Organic Chemistry, Faculty of Natu-
ral Sciences, Comenius University) for the reflectance UV–Vis mea-
surement.
To summarize, we prepared an inorganic-organic hybrid com-
pound [Cu(en)2(VO3)2]•3H2O that crystallizes in monoclinic lat-
tices and has two phases – a high-temperature (HT) and a low-
temperature (LT) one. The phase transition occurs at 176 K and
below this temperature, displacive modulation was observed in
the structure. The solid-state structure incorporates octahedrally
coordinated copper atoms having two bidentately coordinated
ethylenediamine ligands in equatorial positions, while the axial po-
sitions are taken by oxygen atoms from the metavanadate chains.
Phase analysis during heating up to 600 °C was performed. Prod-
ucts obtained at 600 °C were: Cu(VO3)2 in monoclinic phase,
Cu2V2O7 as ziessite and V2O5. After cooling to the room temper-
ature the monoclinic phase of Cu(VO3)2 is transformed to triclinic
and ziessite is transformed to blossite (both Cu2V2O7). Band gap
value of the compound is 2.95 eV. Prepared sample is paramag-
netic with no distinct magnetic interactions; its effective magnetic
moment is 1.79 μB.
Supplementary materials
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.molstruc.2021.129930.
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•
´
8