Structural characterization and thermal behaviour of some azomethine compounds derived from pyridoxal and 4-aminoantipyrine

Article abstract of DOI:10.1007/s10973-016-5680-7

Eight azomethine-type compounds (1–8) derived from pyridoxal (3-hydroxy-5-(hydroxymethyl)-2-methylpyridine-4-carbaldehyde) and 4-aminoantipyrine, respectively, were prepared and thoroughly characterized from structural and thermal perspective. The structures of the compounds were studied based on IR, ¹H and ¹³C NMR spectroscopy and electrospray ionization mass spectrometry. The formation of the azomethine derivatives is confirmed by the occurence of signals typical for the imine bond. Compounds 6 and 8 feature a high crystallinity, and their structures were resolved by single-crystal X-ray diffraction. The thermal behaviour of all the compounds was studied by non-isothermal chemiluminescence in static air atmosphere at different heating rates. The pattern of the oxidation process is described. Moreover, thermo-gravimetric and differential scanning calorimetry experiments were conducted in order to assess the thermal oxidation stability. The onset oxidation temperatures show a high thermal stability for all the tested azomethine compounds. The crystallinity degree was comparatively evaluated on the basis of melting enthalpy values and discussed in connection with the molecular structure. Compounds 6–8 show the higher thermal stability and crystallinity degree.

Full text of DOI:10.1007/s10973-016-5680-7

J Therm Anal Calorim
DOI 10.1007/s10973-016-5680-7
Structural characterization and thermal behaviour of some
azomethine compounds derived from pyridoxal and
4-aminoantipyrine
1
Reka-S¸tefana Mezey Traian Zaharescu2 Marius Eduard Lungulescu2
•
•
•
´
Virgil Marinescu2 Sergiu Shova3 Tudor Ros¸u1
•
•
Received: 17 January 2016 / Accepted: 30 June 2016
´
´
Ó Akademiai Kiado, Budapest, Hungary 2016
Abstract Eight azomethine-type compounds (1–8) derived
from pyridoxal (3-hydroxy-5-(hydroxymethyl)-2-methyl-
pyridine-4-carbaldehyde) and 4-aminoantipyrine, respec-
tively, were prepared and thoroughly characterized from
structural and thermal perspective. The structures of the
connection with the molecular structure. Compounds 6–8
show the higher thermal stability and crystallinity degree.
Keywords Azomethine compounds Á Pyridoxal Á
4-Aminoantipyrine Á Thermal studies Á Crystal structure
1
compounds were studied based on IR, H and 13C NMR
spectroscopy and electrospray ionization mass spectrome-
try. The formation of the azomethine derivatives is con-
firmed by the occurence of signals typical for the imine
bond. Compounds 6 and 8 feature a high crystallinity, and
their structures were resolved by single-crystal X-ray
diffraction. The thermal behaviour of all the compounds
was studied by non-isothermal chemiluminescence in static
air atmosphere at different heating rates. The pattern of the
oxidation process is described. Moreover, thermo-gravi-
metric and differential scanning calorimetry experiments
were conducted in order to assess the thermal oxidation
stability. The onset oxidation temperatures show a high
thermal stability for all the tested azomethine compounds.
The crystallinity degree was comparatively evaluated on
the basis of melting enthalpy values and discussed in
Introduction
The azomethine compounds as well as their transition
metal complexes represent a field of high interest for both
inorganic chemistry and biochemistry due to their phar-
macological effects [1–4]. The key element of the
azomethine compounds is the imine bond which was
connected with a series of chemical and pharmacological
properties. In particular, due to their easy preparation and
high stability, this category of compounds contributes to
the development of coordinative chemistry by serving as
excellent ligands for the transitional metal ions. Since the
imine bond containing molecules are versatile and possess
electron donor properties, they are able to facilely coordi-
nate to the metal ions via the nitrogen atom of the
azomethine bond. In addition, if the ligand molecule posses
other functional moieties suitable for coordination (like –
OH, –SH, –NO2) close enough to the azomethine group,
the probability to obtain mono- or polychelates rings with
increased kinetic and thermodynamic stability is very high
[5, 6]. Several such complexes derived from azomethine
ligands not only exhibit a superior pharmacological
effect—antimicrobial, anti-inflammatory and analgesic—
compared to the free ligand, but also a high thermal sta-
bility [7, 8].
Electronic supplementary material The online version of this
article (doi:10.1007/s10973-016-5680-7) contains supplementary
material, which is available to authorized users.
´
& Reka-S¸tefana Mezey
reka.mezey@gmail.com
1
Department of Inorganic Chemistry, Faculty of Chemistry,
University of Bucharest, 23 Dumbrava Rosie,
050107 Bucharest, Romania
2
INCDIE ICPE CA, 313 Splaiul Unirii, 030138 Bucharest,
Romania
Starting from these considerations, there are favourable
perspectives to emphasize the development of new organic
compounds for further use as basis for the extension of
3
Institute of Macromolecular Chemistry ‘‘Petru Poni’’, 41A
Grigore Ghica Voda, 400786 Iasi, Romania
123

R. Mezey et al.
application in metal complexes field. The full detailed
characterization of new metal complex structures is
mandatory to qualify the preparation and properties of
these foreseen complexes. Turkyilmaz et al. [9] synthe-
sized, structurally characterized Cu(II), Ni(II) and Pt(II)
azomethine polychelate complexes and identified certain
thermal intervals of stability as well as the thermodynamic
effects which accompany complexes degradation. The
thermal decomposition pattern provides valuable informa-
tion about the presence of smaller fragments (like H2O
molecule, Cl2, inorganic anions) additioned to the metal
complexes and about their position inside or outside of the
coordination sphere [10]. Moreover, Zayed et al. [11]
showed that the start decomposition temperature depends
mainly on the instability of the non-coordinated terminal
aliphatic parts of the ligand, while the decomposition
temperature of the remaining fragment is connected with
the affinity of the metal ion to the organic ligand.
foreseen to be used for the preparation of transition metal
complexes with pharmacological effects. The selection of
these compounds for the synthesis of azomethine com-
pounds is connected with their biological properties.
Pyridoxal is one of the five naturally interconvertible forms
of vitamin B6 which exhibits a large spectrum of applica-
tions in the field of medicine, including its role in the
metabolic decarboxylation and transamination of amino
acids [19], the coenzymatic effect in many biological
processes [20] and the antioxidant capacity [21]. Many of
the biomechanisms which depend on pyridoxal involve the
formation of an azomethine intermediate. On the other
hand, pyridoxal could induce tuning of the electronic
properties of the azomethine ligands and their corre-
sponding complexes by protonation–deprotonation of the
heterocyclic nitrogen atom due to the aldehyde group
attached to the pyridine ring [22].
An extensive number of reports concerning antipyrine
and its derivatives as antiviral, antimicrobial, antitumor and
analgesic effects agents are available [23–26]. The anti-
pyrine-derived complexes are also known for their high
stability [12, 14, 27].
The efficacy of each novel metal complex is highly
dependent either on the electronic or on the structural
properties of the involved ligands [10]. Ideally, the organic
compounds should possess specific thermal and structural
stability. For particular applications, the proper under-
standing of the behaviour of ligands is helpful for the
explanation of the mechanism followed under different
employment conditions [12, 13]. At the same time, because
ligands are intermediates in the metal complex synthesis,
the evaluation of their stability is of a great importance for
the selection of proper conditions for preparation. At the
same time, the interference of degrading secondary com-
pounds can be significantly diminished.
On this direction, the goals of this study are the synthesis
and the detailed structural and thermal characterization of
some compounds belonging to the azomethine category. The
structural characterization of all compounds has been
1
accomplished by elemental analysis, IR, H and 13C NMR
spectroscopy and mass spectrometry. In addition, the crystal
structures of two compounds are reported. Single-crystal
X-ray diffraction was used as an accurate method to char-
acterize the supramolecular systems generated by hydrogen
bonds and aromatic p–p stacking interactions. The thermal
stability is qualified by chemiluminescence, TG and DSC
investigations, which provide basic information on the fur-
ther behaviour of complexes in which they will be used.
Keeping in mind, the use of these compounds as intermedi-
ates in the development of complex molecules, an extensive
study concerning their thermal and thermo-oxidative prop-
erties, was conducted.
The thermal, thermo-oxidative and thermo-gravimetric
methods are background procedures for the characteriza-
tion of complexes stability and their thermodynamic
parameters [12, 14, 15]. Several studies analyse the ther-
modynamic parameters for complexes in connection with
the TG–DTG curves and decomposition model [16]. For
several metal complexes with ligand 2-benzoyl-pyridil-
isonicotinoylhydrazone, the decomposition heats and
DH associated with the exothermal effects were determined
[17]. Abdel-Fattah et al. [18] used the information provided
by the DTG curves in order to determine the kinetic
reaction order of the decomposition of several Schiff base
complexes as well as to calculate the activation entropy,
activation enthalpy and free energy of activation correlated
with the thermal stability.
Experimental
Materials
Pyridoxal hydrochloride, 4-aminoantipyrine, phenylhydrazine
hydrochloride, 2,4-dinitro-phenylhydrazine, 3-hydroxyben-
zoic acid hydrazide, 4-phenyl-3-thiosemicarbazide, 4-benzyl-
While the academic research is focused on the evalua-
tion of physical and thermal properties of metal complexes,
a real interest to the characterization of the uncoordinated
ligands was scarcely observed. In order to add valuable
information to the field of azomethine compounds, we
characterized some imine structures derived from pyri-
doxal and 4-aminoantipyrine, respectively. They are
3-thiosemicarbazide,
4-acetoxy-3-methoxybenzaldehyde,
and 4-(dimethy-
3-benzyloxy-4-methoxybenzaldehyde
lamino)benzaldehyde with high purity were purchased from
Sigma-Aldrich and used as such without any further purifica-
tion step.
123

Structural characterization and thermal behaviour of some azomethine compounds derived from…
Synthesis of the azomethine compounds
under constant stirring. To the obtained pale-yellow mix-
ture were added few drops of glacial acetic acid. The
mixture was stirred at 60 °C for 4 h until its colour changed
to intense yellow. Further, it was concentrated to half of the
initial volume when the fine yellow mass was formed. The
resulting solid compound was filtered, washed with alcohol
and dried under vacuum. Reaction yield: 71 %. ESI-MS
(positive mode, MeOH:H2O) m/z: [LNa]? calculated for
C15H15N3O4Na 324.3, found 324.2.
All the azomethine compounds have been synthesized by
direct condensation reaction. The pyridoxal-derived com-
pounds were obtained by condensation of the named sub-
stance with the corresponding hydrazine, hydrazide and
thiosemicarbazide, in a 1:1 molar ratio. The second cate-
gory of azomethine products have been prepared by 1:1
condensation of 4-aminoantipyrine with the carbonylic
reagents.
Pyridoxal-4-phenyl-3-thiosemicarbazone (4)
Pyridoxal-phenylhydrazone (1)
Compound 4 was prepared following the method described
by Manikandan et al. [28]. To a solution of 0.204 g (1
mmol) of pyridoxal hydrochloride in methanol (10 mL)
was added dropwise a solution of 0.167 g (1 mmol)
4-phenyl-3-thiosemicarbazide dissolved in the minimum
amount of alcohol (5 mL). The yellow solution was
refluxed for 2 h. After concentrating the solution and
cooling at 4 °C, a yellow solid mass was isolated and
purified by recrystallization. Reaction yield: 67 %. ESI-MS
(positive mode, MeOH:H2O) m/z: [LH]? calculated for
C15H17N4O2S 317.3, found 317.2.
A solution of 0.145 g (1 mmol) of phenylhydrazine
hydrochloride in methanol (5 mL) was added slowly to a
solution of 0.204 g (1 mmol) of pyridoxal hydrochloride in
the same solvent (5 mL). The mixture was alkalized to pH
= 7–7.5 by adding few drops of triethylamine (TEA) and
constantly controlling the pH value. The reaction solution
was stirred at 50 °C. After 30 min, a light yellow solid mass
appeared. The mixture was keep stirring for another hour
until the entire azomethine compound precipitated quanti-
tatively. Further, it was filtered, washed with methanol and
dried in vacuum. As a second step, the precipitate was
purified by recrystallization. Reaction yield: 72 %. ESI-MS
(positive mode, MeOH:H2O) m/z: [LH]? calculated for
C14H16N3O2 258.30, found 259.00.
Pyridoxal-4-benzyl-3-thiosemicarbazone (5)
This compound was synthesized in a similar manner as the
previous one. A solution of 0.204 g (1 mmol) of pyridoxal
hydrochloride in methanol (5 mL) was prepared. Separately,
0.181 g (1 mmol) of 4-benzyl-3-thiosemicarbazide was
dissolved in 5 mL methanol and added to the first solution.
The resulting mixture was refluxed at controlled temperature
for 2 h until a light yellow precipitate was obtained. After-
wards, it was collected, purified and dried. Reaction yield: 73
%. ESI-MS (positive mode, MeOH:H2O) m/z: [LH]? cal-
culated for C16H19N4O2S 331.4, found 331.1.
Pyridoxal-2,4-dinitro-phenylhydrazone (2)
To a solution of 0.204 g (1 mmol) of pyridoxal
hydrochloride in methanol was added the methanolic
solution of 0.198 g (1 mmol) of 2,4-dinitro-phenylhy-
drazine. A red–orange solution was formed immediately. A
small amount of TEA was added to increase the pH of the
solution to 7. The final solution was refluxed at controlled
temperature (50 °C) for 1 h until a fine orange precipitate
was isolated. The resulted mass was filtered, washed with
alcohol and dried. The purity of the synthesized compound
was ensured by specific recrystallization. Reaction yield:
53 %. ESI-MS (positive mode, MeOH:H2O) m/z: [LNa]?
calculated for C14H13N5O6Na 370.27, found 370.2; [LH]?
calculated for C14H14N5O6 348.29 found 349.0.
N-(4-amino-1,5-dimethyl-2-phenyl-1,2-dihydro-3H-
pyrazol-3-one)-4-acetoxy-3-methoxybenzaldimine (6)
A solution of 4-aminoantipyrine (0.203 g, 1 mmol) in
chloroform (5 mL) was prepared and added to a solution of
4-acetoxy-3-methoxybenzaldehyde (0.194 g, 1 mmol) in
methanol (5 mL). The mixture was stirred under controlled
temperature for 2 h. Afterwards, the solution was left to
cool at room temperature and slowly evaporated until a
light yellow solid precipitate was formed. In order to obtain
the single crystals suitable for X-ray diffraction, a small
amount of finished compound was re-dissolved in a chlo-
roform:methanol (1:1, v/v) mixture and kept at 4 °C. After
few days, fine crystals were isolated. Reaction yield: 48 %.
ESI-MS (positive mode, MeOH:H2O) m/z: [LH]? calcu-
lated for C21H22N3O4 380.4, found 380.1.
Pyridoxal-3-hydroxybenzoic acid hydrazone (3)
Compound 3 was obtained with good yield by condensa-
tion of pyridoxal hydrochloride with 3-hydroxybenzoic
acid hydrazide. A solution of 0.152 g (1 mmol) of 3-hy-
droxybenzoic acid hydrazide in methanol (5 mL) was
prepared. Parallelly, 0.204 g (1 mmol) of pyridoxal
hydrochloride was dissolved in the same solvent (5 mL).
The resulting solution was added slowly to the hydrazide
123

R. Mezey et al.
Compounds (7) and (8) were obtained in a similar
way as compound (6)
methanol:water mixture and injected directly into the mass
spectrometer’s interface. The determination of thermal
stability of compounds was accomplished by non-isother-
mal chemiluminescence in static air atmosphere at three
heating rates: 5, 10 and 15 °C min-1. The equipment,
LUMIPOL 3, was manufactured by Institute of Polymers,
Slovak Academy of Sciences. Because the evolution of
emission intensities follows the accumulation of carbonyl
intermediates [29], this procedure depicts the progress in
the oxidation state of investigated material. The error of
temperature measurements on the oven is 0.5 °C. Powder
samples of around 3 mg were placed in an aluminium
crucible after pre-weighting. For reliable comparison, the
chemiluminescence intensities were normalized to unit
mass, the results being expressed in Hz g-1, which repre-
sent the number of photons emitted by 1 g of sample. The
listed intensity values are the sums of counted photons over
1 min divided by 60 for their conversion in Hz (counts per
second). Differential scanning calorimetry (DSC) mea-
surements were performed using a Setaram 131 EVO
(Setaram Instrumentation, France), in the following con-
ditions: temperature range, 30–350 °C; heating rate, 10 K
min-1; atmosphere, air (gas flow, 50 mL min-1). Samples
of about 2–3 mg of ligands powder were measured in
aluminium pans of 30 lL. Specific SETARAM software
has been used for data acquisition and processing. Different
parameters characterizing the observed effects (peaks),
such as the oxidation onset temperature (OOT), the peak
temperature (Tm) or the thermal effect (DH), were calcu-
lated from the DSC curves. Thermo-gravimetric study was
performed with an Equipment STA 449 Jupiter, using
crucibles of sinterized Al2O3, in dynamical atmosphere
composed by a flow of 13 mL min-1 oxygen and a flow of
37 mL min-1 nitrogen in sample furnaces. Temperature
program was identical in all the cases, from room tem-
N-(4-amino-1,5-dimethyl-2-phenyl-1,2-dihydro-3H-
pyrazol-3-one)-3-benzyloxy-4-methoxybenzaldimine (7)
A solution of 0.242 g (1 mmol) of 3-benzyloxy-4-
methoxybenzaldehyde in methanol (5 mL) was added to
the solution of 0.203 g (1 mmol) 4-aminoantipyrine dis-
solved in minimum volume of chloroform (5 mL). The
mixture was stirred for 2 h and afterwards left to cool at
room temperature. After 30 min, fine light yellow precip-
itate appeared. Afterwards, it was filtered and washed with
alcohol. Reaction yield: 80 %. ESI-MS (positive mode,
MeOH:H2O) m/z: [LNa]? calculated for C26H25N3O3Na
450.4, found 450.2; [LH]? calculated for C26H26N3O3
428.5, found 428.3.
N-(4-amino-1,5-dimethyl-2-phenyl-1,2-dihydro-3H-
pyrazol-3-one)-4-(dimethylamino)benzaldimine (8)
A solution of 4-aminoantipyrine (0.203 g, 1 mmol) in
chloroform (5 mL) was prepared and added to a solution of
4-(dimethylamino)benzaldehyde (0.149 g, 1 mmol) in the
same solvent (5 mL). The final mixture was stirred at room
temperature for 4 h. After concentrating the mixture, a
shiny orange polycrystalline powder was isolated. To
obtain X-ray single-crystals, a certain amount of compound
was dissolved in 3 mL of methanol in a tube. The same
volume of acetone was slowly added on the tube walls. The
tube was left at 4 °C for few days when crystals were
separated from the solution. Reaction yield: 64 %. ESI-MS
(positive mode, MeOH:H2O) m/z: [LNa]? calculated for
C20H22N4ONa 357.4, found 357.2; [LH]? calculated for
C20H23N4O 335.4, found 335.2.
perature 25–350 °C with a heating rate of 10 °C min-1
.
Analysis/techniques
X-ray diffraction studies
The percentages of C, H and N were determined by ele-
mental analysis using a Carlo-Erba microdosimeter. All
compounds were dried prior analysis at 100 °C. The IR
spectra were measured by using KBr pelletizing technique
and registered with a BioRad FTS 135 spectrophotometer
in the region 600–4000 cm-1. The 1H and 13C NMR
spectra were recorded with a Bruker DRX 400 spectrom-
eter, at room temperature. Chemical shifts were measured
in parts per million using standard tetramethylsilane
(TMS). Solutions of compounds in deuterated dimethyl-
sulfoxide (DMSO) were used for testing. Mass spectra
were obtained with a tandem mass spectrometer operated
in MS mode (mass range of 5–500 m/z). Samples were
delivered via infusion into the electrospray probe. For
testing purposes, each compound was dissolved in
Crystallographic measurements were carried out with an
Oxford-Diffraction XCALIBUR E CCD diffractometer
equipped with graphite-monochromated Mo-Ka radiation.
Single crystals were positioned at 40 mm from the detector,
and 309 and 405 frames were measured each for 30 and 5 s
over 1° scan width for 6 and 8, respectively. The unit-cell
determination and data integration were carried out using
the CrysAlis package of Oxford Diffraction [30]. The
structures were solved by direct methods using Olex2 [31]
software with the SHELXS structure solution program and
refined by full-matrix least-squares on F2 with SHELXL-
97 [32]. Atomic displacements for non-hydrogen atoms
were refined using an anisotropic model. All hydrogen
atoms were refined as riding on their carriers with
123

Structural characterization and thermal behaviour of some azomethine compounds derived from…
Uiso(H) = 1.2eq (CH,) and Uiso(H) = 1.5eq (CH3). Using the
Olex2 [31] program, the molecular plots were obtained.
The main crystallographic data together with refinement
details are summarized in Table 1. CCDC 1440749 (for
compound 6) and CCDC 1440723 (for compound 8) con-
tain the supplementary crystallographic data for this con-
tribution. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (?44) 1223-336-033; or
deposit@ccdc.ca.ac.uk).
containing an amine function. The nature of the aminic
reagent induces wide variation of the electronic and steric
properties of the final condensation product [5, 22]. The
study described here concerns three different types of
azomethine compounds—hydrazones, thiosemicarbazones
and Schiff bases. Although all these compounds possess as
common element the –C=N bond, they differ by the nature
of the donor atoms in the proximity of the imine site. The
structures of compounds 1–8 are presented in Fig. 1. The
molecular formula for each compound was confirmed by
elemental analysis, and the results are indicated in Table 2.
In addition, the molecular mass was confirmed by the ESI-
MS spectra. ESI is as a mild ionization method which
allows the capture of the molecular ion. In this way,
valuable information about the molecular mass of the
azomethine compounds was obtained. The ESI-MS spectra
of compounds 1, 4, 5 and 6 exhibit intense peak corre-
sponding to [LH]? ion at 259.0, 317.2, 331.1 and 380.1 Da,
Results and discussion
The general approach for the preparation of the azomethine
compounds involves the condensation reaction between the
carbonyl group of a molecule and a second reagent
Table 1 Crystallographic data, details of data collection and structure refinement parameters for 6 and 8
6
8
Empirical formula
Formula mass
Temperature/K
Crystal system
Space group
C21H21N3O4
379.41
C20H22N4O
334.42
173(2)
173.00(10)
monoclinic
C2/c
Triclinic
P - 1
˚
a/A
7.0352(5)
9.7036(6)
14.4466(18)
96.406(7)
100.351(8)
102.956(6)
933.55(14)
2
17.8014(16)
6.8034(6)
29.535(3)
90.00
˚
b/A
˚
c/A
a/°
b/°
c/°
101.368(9)
90.00
3
˚
V/A
3506.8(5)
8
Z
D
calc/mg mm-3
1.350
1.267
l/mm-1
0.095
0.081
Crystal size/mm3
0.40 9 0.15 9 0.15
2.9 to 50.04
6956
0.60 9 0.20 9 0.20
4.66 to 50.04
11,871
hmin, h max/°
Reflections collected
Independent reflections
Data/restraints/parameters
Ra1(I [ 2r(I))
3295 [Rint = 0.0333]
3295/0/257
0.0503
3086 [Rint = 0.0535]
3086/0/230
0.0461
wRb2 (all data)
0.1233
0.1132
GOFc
1.022
1.065
-3
˚
Largest diff. peak/hole/e A
0.26/-0.24
0.18/-0.19
a
R1 = R||Fo| - |Fc||/R|Fo|
wR2 = {R[w (F2o - F2c)2]/R[w(Fo2)2]}1/2
GOF = {R[w(F2o - Fc2)2]/(n - p)}1/2, where n is the number of reflections and p is the total number of parameters refined
b
c
123

R. Mezey et al.
O
23
N
O
16
12
15
22
16
12
25
15
HO
20
17
HO
17
17
15
14
14
HO
14
19
19
18
21
18
18
13
5
19
6
6
6
9
5
5
4
13
16
12
NH
N
N
NH
NH
4
4
N
11
N
10
10
O
OH
N
1
2
11
11
N
N
10
20
3
2
9
21
9
22
1
1
O
3
O
3
24
OH
2
13
8
H C
H C
OH
3
7
H C
3
3
OH
7
8
7
1
2
3
8
HO
HO
15
15
22
14
14
23
21
6
6
22
5
9
9
5
21
12
12
17
NH
NH
NH
NH
20
N
N
4
N
N
18
4
11
16
11
16
17
10
1
2
2
1
10
19
3
3
S
S
20
18
13
13
OH
OH
H C
19
H C
3
7
3
7
8
5
8
4
28
25
29
30
27
26
CH
3
CH
3
24
24
17
16
O
18
24
N
23
O
17
16
O
23
H C
H C
3
23
18
18
19
3
25
26
O
19
28
25
17
31
H C
19
O
3
20
32
20
27
15
15
15
21
20
21
16
N
N
CH
21
14
3
14
CH
3
3 13
N
14
4
4
3 13
4
CH
3
3 13
5
N
5
N
O
CH
2
3
O
CH
N
1
2
3
22
5
12
N
N
22
12
O
CH
1
6
2
3
N
6
22
12
7
11
1
6
7
11
7
11
8
10
8
10
9
8
10
9
9
8
6
7
Fig. 1 Structures of compounds 1–8
Table 2 Elemental analysis results for compounds 1–8
Compound
Molecular formula
Found/%
C
Calculated/%
C
H
N
H
N
1
2
3
4
5
6
7
8
C14H15N3O2
66.12
47.80
58.95
57.21
58.30
66.21
72.74
71.24
5.69
3.61
4.89
4.98
5.92
6.06
6.07
7.46
15.85
20.65
14.18
17.43
16.33
10.54
9.22
65.37
48.41
59.80
56.96
58.18
66.49
73.06
71.85
5.83
3.75
4.98
5.06
5.45
5.54
5.85
6.58
16.34
20.17
13.95
17.72
16.96
11.08
9.83
C
C
C
C
C
C
C
14H13N5O6
15H15N3O4
15H16N4O2S
16H18N4O2S
21H21N3O4
26H25N3O3
20H22N4O
16.91
16.77
123

Structural characterization and thermal behaviour of some azomethine compounds derived from…
˚
structure 6 (centroid-to-centroid distance 3.519 A) and C–
respectively. In case of compounds 2, 3, 7 and 8, the for-
mation of Na adducts is shown by the peaks recorded at
349.0, 324.2, 450.2 and 357.2 Da, respectively.
The described compounds can serve as excellent ligands
for the preparation of metal complexes. Based on the
number and position of the heteroatoms, chelate rings
characterized by different dimension and stability can be
obtained.
˚
HÁÁÁp interactions (2.729 A) in 8 are also present.
IR spectra
The signals in the IR spectra have been assigned to the
vibration of the main bonds and fragments in the azome-
thine molecules. The lack of bands characteristic to the
carbonyl and amine groups in the spectra of azomethine
compounds confirm the condensation reaction took place.
Instead, a new well-defined band appears in the region
1590–1630 cm-1. This is a typical signal for the vibration
of the C=N bond [33]. The infrared spectra of compounds 1
and 2 are similar. Both of them emphasize the specific
signal m(C=N) at 1601 cm-1 and 1608 cm-1, respectively.
The broad signal at 3270–3430 cm-1 is assigned to the
hydroxyl group. A sharp signal at 1245 cm-1 and 1217
cm-1, respectively, is characteristic to the vibration of the
–OH group attached to the pyridine ring in the pyridoxal
moiety. Another similar signal can be observed at lower
frequencies (1068 cm-1 and 1035 cm-1), this one being
characteristic to the –CH2-OH group [33]. Lastly, the broad
band visible at high frequencies suggests the establishment
of intra-molecular interactions via hydrogen bond, possible
due to the hydroxyl groups and = N–N-group. The spectra
of compound 3 exhibit similar signals as described above.
In addition, the presence of the –HC=N–NH–C=O group is
emphasized by a series of specific signals between 1047
and 1685 cm-1. Because of this fragment, the hydrazone
molecule is subjected to keto–enol equilibrium phenomena
(Fig. 4a). The decision on the stabilized tautomeric form
was made based on the infrared spectra analysis and
X-ray diffraction
The results of single-crystal X-ray study for compounds 6
and 8 along with the atom numbering scheme are shown in
Figs. 2a and 3a, while the interatomic distances and angles
are summarized in Table 3—supplementary documents.
The crystals of 6 and 8 have a molecular structure built up
from the corresponding neutral entities without any solvate
molecules in the asymmetric part. The studied compounds
exhibit similar structures with closed geometric parameters
(Table 1A—supplementary documents) comprising a pla-
nar fragment formed by pyrazole and benzaldimine moi-
eties (dihedral angles 1.06°, 8.64° for 6 and 8, respectively)
and twisted phenyl substituent (dihedral angles 56.16° and
51.11° for 6 and 8, respectively). In compound 6, the
acetate group is nearly perpendicular to the aromatic ring
(dihedral angle 73.46°). The main crystal structure motif
for compounds 6 and 8 is characterized as one-dimensional
supramolecular architecture, based on the formation of C–
HÁÁÁO intermolecular hydrogen bonds. A view of these
architectures along with the H-bonds parameters for com-
pounds 6 and 8 is shown in Fig. 2b and Fig. 3b, respec-
tively. Significant stacking interactions in the crystal
(a)
(b)
C3
C2
O1
C7
C4
C15
C14
C1
N2
C5
N1
C16
O2
C12
C6
C20
C13
C17
C18
C19
O3
O4
C8
N3
C9
C11
C10
C21
˚
˚
˚
Fig. 2 a X-ray molecular structure of 6. Thermal ellipsoids are drawn
at 50 % probability level; b one-dimensional supramolecular archi-
tecture in the crystal structure of 6. H-bonds parameters: C10-HÁÁÁO1
[O10-H 0.96 A, HÁÁÁO1(1 ? x, y, z) 2.433 A, C10ÁÁÁO1 3.393(4) A,
˚
C10-HÁÁÁO1 178.5°]; C15-HÁÁÁO1 [O10-H 0.93 A, HÁÁÁO1(-x, -y,
˚
˚
-z) 2.392 A, C15ÁÁÁO1 3.272 (4) A, C10-HÁÁÁO1 157.9°]
123

R. Mezey et al.
(a)
(b)
C3
C4
C2
O1
C7
C5
C1
C14
C13
C15
C16
C19
N1
C6
C12
N3
N4
N2
C8
C18
C17
C9
C11
C20
C10
˚
˚
Fig. 3 a X-ray molecular structure of 8. Thermal ellipsoids are drawn
at 50 probability level. One-dimensional supramolecular
architecture in the crystal structure of 8. H-bond parameters: C10-
HÁÁÁO1 [O10-H 0.96 A, HÁÁÁO1(x, -1 ? y, z) 2.405 A, C10ÁÁÁO1
˚
%
b
3.357(4) A, C10-HÁÁÁO1 171.5°]
pertinent bibliographical references [34, 35]. The medium
signal recorded at 1685 cm-1 represents the effect of the
stretching vibration of the –C=O and C–N bonds (amide I
band). Moreover, the IR spectra exhibits two additional
signals which are characteristic to the N–C=O stretching
vibration at 1546 cm-1 (amide II) and C–N stretching
vibration coupled with NH bending effect at 1349 cm-1
(amide III). These signals suggest the stabilization of the
keto tautomeric form. The presence of the hydroxyl group
bonded to the pyridine ring could induce the possibility of a
third tautomeric form in which this hydroxyl group is
converted into a keto one. Still, this hypothesis is not
confirmed by the IR spectra as the presence of the OH
group is clearly emphasized by the signal recorded at 1211
cm-1. The vibration spectra of compounds 4 and 5 are
similar and exhibit the main expected signals for the
desired thiosemicarbazones. The formation of the imines is
proven by the bands associated with the azomethine bond
at 1622 and 1630 cm-1, respectively. Also, the presence of
the aryl-hydroxy group is emphasized by the signal at 1221
and 1274 cm-1, respectively. In case of thiosemicar-
bazones, the thiol–thione equilibrium is frequently
encountered and can lead to different forms in microcrys-
talline powder and liquid phase [36, 37] (Fig. 4b). The
thione form of the compounds 4 and 5 was determined
based on IR and NMR spectra assessment. The IR spectra
of the compound 4 exhibit the band characteristic to
m(C=S?C=N) vibration at 1302 cm-1, as well as a sharp
signal at 756 cm-1 assignable to the C=S bond stretching
[33]. Another important signal observed at 1554 cm-1 is
the one associated to the N–C=S stretching vibration mode
which confirms the thione form. Compound 5 describes a
similar pattern with the target bands recorded at 1325 cm-1
(m(C=S?C=N)), 817 cm-1 (m(C=S)) and 1560 cm-1 (m(N–
C=S)). The IR spectra of compound 6, 7 and 8 show the
signals specific to the main groups in the 4-aminoan-
tipyrine and carbonyl molecules. The conversion of the
reagents is clearly indicated by the medium band recorded
at 1591 cm-1 (6), 1594 cm-1 (7) and 1609 cm-1 (8)
assigned to m(C=N). The signals at 1635 cm-1 (6), 1638
cm-1 (7) and 1643 cm-1 (8) correspond to the keto exo-
cyclic group attached to the pyrazole ring. In case of
compound 6, a special issue is encountered due to the
presence of the C=O bond originating from the acetate
fragment. This fragment is represented in the IR spectra by
the sharp band at the higher frequency—1766 cm-1 [33].
The correct assignment of this band versus the one asso-
ciated to the C=O exocyclic one is important in view of the
(a)
HO
(b)
HO
HO
HO
N
NH
N
N
NH
NH
N
NH
N
N
OH
N
N
R
R
N
OH
N
SH
S
HO
O
OH
OH
OH
OH
H3C
CH2
H3C
H3C
H3C
and
; where R =
Fig. 4 a Tautomeric equilibrium for compound 3; b tautomeric equilibrium for compounds 4 and 5
123

Structural characterization and thermal behaviour of some azomethine compounds derived from…
potential coordination properties of the organic molecule to
transition metal ions and the correct interpretation of the
complex structure.
of the signals in the NMR spectra of compounds 6, 7 and 8
is more difficult due to the complexity of the molecules.
COSY technique was helpful for the determination of the
spins which are coupled to each other. The signals specific
1H and 13C NMR spectra
to the azomethine fragment (d 9.58–9.71 ppm in the H
1
NMR spectra and d 148.44–152.06 ppm in the 13C NMR
spectra) are the proof of the Schiff base formation. More-
over, the signal between d 158.05–161.06 ppm in the 13C
NMR spectra is associated to the carbon involved in the
C=O exocyclic group from the 4-aminoantipyrine moiety.
All the compounds were characterized by means of NMR
spectroscopy in order to confirm the proposed molecular
structures. In addition, the NMR spectra data provide
helpful information about the tautomeric equilibrium in
solution and about the most stable conformation adopted
by the organic molecules. The main 1H NMR and 13C
NMR signals are indicated in Supplementary documents.
Chemiluminescence studies
1
The H NMR spectra of compounds 1 and 2 display a
The thermal stability of inorganic complexes is depicted by
the bonding strength of ligands [40], but the decomposition
of these constitutive moieties brings an important effect on
the material integrity over time or temperature ranges. The
chemical structure, the electronic interactions between
various segments of ligands, the existence environment or
the operation temperature can influence the thermal sta-
bility. The electronic effects of substituents in the benzene
rings of the structures are the consequence of electronic
influence on the N–N bridges. The donating or withdraw-
ing character of substituents modifies the charge distribu-
tion inside the molecules of azomethine-type compounds.
The presence of pyrazole configuration gets supplementary
contribution to the variation in the stability of studied
molecules.
pattern of similar signals. The singlets recorded at d 2.6
ppm and d 4.76–4.86 ppm correspond to methyl and
methylene groups attached to the pyridine ring. This data
are confirmed also by the signals identified in the 13C NMR
spectra at d 38.60–38.67 ppm and d 58.88–59.17 ppm,
respectively. The region between 6.25 and 8.45 ppm shows
a series of multiplets characteristic to the protons of the
aromatic rings. Valuable information about the molecules
fragments containing heteroatoms was obtained by ana-
1
lysing the high region of the H NMR spectra. The reso-
nance at d 8.17 ppm (compound 1) and d 8.90 ppm
(compound 2) is assigned to the azomethine bond proton.
The presence of the imine hydrogen is described also by
the signals at d 142.64 ppm and d 144.03 ppm in the 13C
NMR spectra. The signals observed in the 1H NMR spectra
of compound 1 at d 8.40 ppm and at d 9.16 ppm in the
spectra of compound 2 are associated to the aryl –OH
groups in the pyridoxal moiety. The signal at highest res-
onance can be attributed to the –N–H proton. The shift of
this peak at resonance energies close to d 12 ppm indicates
the interaction of the organic compounds with neighbour
solvent molecules via hydrogen bonds. These data are in
accord with previously reported ones about other hydra-
zones [38]. The assignment of the signals in the NMR
spectra of compound 3, 4 and 5 is more complex in view of
multiple intermolecular interactions and conformation
stabilization. The formation of the azomethine bond is
The onset oxidation temperatures (OOT) for compounds
1–3 indicate a lower thermal stability of the hydrazone-
type molecules.
In Fig. 5, the compound 4, whose structure contains
slightly polarized thio-ketone group, presents one intensity
maximum at 205 °C, which is the consequence of the early
start of oxidation. The ligand 5, which possesses the same
group, is suddenly oxidized at 225 °C. The similar results
were recorded at higher heating rates, when amplified CL
intensities were noticed. The change in the heating rate
caused the modification in the emission intensities of
ligands, which can be ascribed to the vulnerability of
molecules during oxidation. The difference in the thermal
stability of the studied structures may be also ascribed to
the hydrogen bonds established between protons and atoms
with available electrons concerning especially oxygen
atoms. The highest heating rate (b = 15 °C min-1) have not
distinguished behaviour in comparison with the lower
heating conditions (Fig. 1A—supplementary documents).
The OOT (Table 3) and the profiles of CL intensity
dependences on temperatures (Figs. 5, 6 and Fig. 1A—see
supplementary data) depict the contribution of molecular
configurations to the progress of degradation, when the
substituents have different positions and the sterical hin-
drance modifies the distribution of more reactive sites. The
1
confirmed by the signals at d 8.19–8.65 ppm in the H
NMR spectra and the one at d 142.75–143.83 ppm in the
13C NMR spectra. The keto, respectively, thione form
suggested also by the IR spectra is confirmed by the
presence of the signal at d 9.25–10.62 ppm in the 1H NMR
spectra characteristic to the –N–H proton in the amide/
thiosemicarbazide fragment in E conformation [39]. For
compound 3, the same conclusion is supported by the C=O
typical signal observed at d 163.24 ppm in the 13C NMR
spectra. In case of compounds 4 and 5, the stabilization of
the thione form is proven by the signal at d 177–179 ppm
characteristic to the C=S bond. The complete assignment
123

R. Mezey et al.
(a)
(a)
16
12
8
20
15
3
4
10
5
3
2
1
0
4
2
1
0
0
0
50
100
150
200
250
50
100
150
200
250
Temperature/°C
Temperature/°C
50
100
150
200
250
50
100
150
200
250
Temperature/°C
Temperature/°C
(b) 5
(b)
0.5
6
4
2
0
4
3
2
1
0
0.4
0.3
0.2
0.1
50
100
150
200
250
Temperature/°C
0.0
50
100
150
200
250
50
100
150
200
250
Temperature/°C
Temperature/°C
Fig. 6 Non-isothermal CL spectra recorded on the samples a 1–4 and
b 5–8 at the heating rate of 10 °C min-1
Fig. 5 Non-isothermal CL spectra recorded on the samples a 1–4 and
b 5–8 at the heating rate of 5 °C min-1
considered a weaker fragment, where oxidation can start by
bond cleavage.
benzene rings, which have a well-defined electronic
homogeneity, are the supports of chemical interactions
through which certain bonds are weakened. The pyrazole
structure existing in the configurations 6–8 can be
The components of two groups differ by the real
development of oxidation state. While compounds 1–4
have smaller onset oxidation temperature, the structures 6–
Table 3 Onset oxidation temperatures for compounds 1–8
Compound
Onset oxidation temperature/°C
b = 5 °C min-1
b = 10 °C min-1
b = 15 °C min-1
1
2
3
4
5
6
7
8
135
150
142
155
202
211
210
215
148
153
148
158
210
220
215
219
163
158
153
168
215
228
222
224
123

Structural characterization and thermal behaviour of some azomethine compounds derived from…
Table 4 DSC and TG parameters for compounds 1–8
Compound
OOT/°C
DHox/J g-1
Tmax/°C
Tm/°C
DHm/J g-1
Mass loss/%
1
2
3
4
5
6
7
8
245
266
214
218
201
284
301
287
-170
-582
-169
-45
266
267
273
239
215
321
325
320
–
–
11
50
30
22
23
51
41
65
–
–
–
–
–
–
-42
–
–
-83
173
167
221
307
68
221
-401
-217
8 present a slight change in the CL intensities as testing
temperature increases (Fig. 2A—supplementary docu-
ments). The higher thermal stability of compounds 6–8
recommends these structures as the most proper ligands
from the studied series for complexes design.
values of OOT, oxidation enthalpy (DHox), maximum
temperature of the oxidation peak (Tmax) as well as melting
temperature (Tm) and melting enthalpy (DHm) for com-
pounds 6–8 are provided in Table 4.
The thermal oxidation stability of the compounds is
strongly connected with their structure. The values of the
OOT parameter show that all the molecules present a good
stability to oxidation, higher than 200 °C. The stability of
the compounds decreases in the following direction:
Thermal analysis studies
The results of the thermal analysis provide a comprehen-
sive picture of the thermal degradation process and enable
the calculation of specific thermodynamic parameters. The
Compound 7 [ Compound 8 [ Compound
6
[ Compound 2 [ Compound 1 [ Compound 4
[ Compound 3 [ Compound 5
The DSC curves for compounds 1 and 3 (Fig. 7) are
similar. In case of 1, the lower temperature region of the
curve presents three small endothermal peaks at about 87
°C (8.4 J g-1), 104 °C (3.1 J g-1) and 121 °C (1.4 J g-1),
respectively. As observed from TG/DTG curves (Fig. 3A—
supplementary documents), these endothermal peaks are
not being accompanied by a noticeable mass loss.
(a)
12.5
12
Heat : –169.6/Jg–1
Tmax : 256.5/°C
11.5
11
10.5
10
9.5
Heat : 3.17/Jg–1
Tmax : 103.9/°C
9
8.5
8
The curve of compound 3 describes also three
endothermal degradation steps with peaks at 96 °C (7.5 J
g-1), 137 °C (21.1 J g-1) and 153 °C (57.8 J g-1),
respectively. The TG analysis (Fig. 4A—supplementary
Heat : 1.48/Jg–1
Tmax : 121.5/°C
Heat : 8.41/Jg–1
Tmax : 86.5/°C
Exo
7.5
7
OOT : 245.1/°C
250
6.5
6
100
150
200
50
Sample temperature/°C
90
11
10.5
10
(b)
80
Heat : –581.8/Jg–1
Tmax : 267.2/°C
Heat : –168.5/–161.0 –7.5/Jg–1
Tmax 1 : 231.6/°C
Tmax 2 : 272.9/°C
70
9.5
9
8.5
8
60
50
40
Heat : 79.0/21.1 + 57.9/Jg–1
Tmax 1 : 137.8/°C
Tmax 2 : 153.7/°C
7.5
7
6.5
6
30
Heat : 7.5/Jg–1
Tmax : 96.5/°C
Heat : 362.1/Jg–1
Tmax : 121.8/°C
20
Exo
Exo
10
5.5
5
OOT : 214.9/°C
4.5
0
OOT : 265.8/°C
250
4
100
200
50
150
100
200
50
150
250
Sample temperature/°C
Sample temperature/°C
Fig. 7 DSC curves for a 1 and b 3
Fig. 8 DSC curve for compound 2
123

R. Mezey et al.
10
9.5
9
supplementary documents). The oxidation step is associ-
ated with a mass loss of approximately 50 % of the initial
mass. For compound 4, the DSC curve displays two
endothermal peaks at 125 °C (26.5 J g-1) and 161 °C
(26.3 J g-1) (Fig. 9a). The first endothermal peak could be
assigned to the melting of some crystalline domains, not
being accompanied by a mass loss, while the second peak
presents a mass loss of about 19 % due to, probably, the
evaporation of volatile molecules (Fig. 9b) [41]. The DSC
curve of compound 5 also presents two endothermal peaks,
but at higher temperatures: 155 °C (6.6 J g-1) and 195 °C
(11.1 J g-1). The last peak could be assigned to a little
(a)
Heat : –45.1/Jg–1
Tmax : 239.8/°C
8.5
8
Heat : 26.2/Jg–1
Tmax : 160.7/°C
7.5
7
Heat : 26.5/Jg–1
Tmax : 125.1/°C
OOT : 218.1/°C
6.5
6
Exo
5.5
50
100
150
200
250
Sample temperature/°C
DTG/%/min
TG%
10
(a)
100
90
80
70
60
50
0
(1)
Heat : –85.4/J g–1
Tmax : 320.6/°C
(b)
Mass change: –19.15%
8
–1
–2
–3
–4
–5
–6
6
Heat : 30.3/J g–1
Tmax : 173.3/°C
OOT : 284.3/°C
4
2
Mass change: –41.02%
Mass change: –21.88%
Peak: 162.8°C
Exo
0
–2
–7
Peak: 229.5°C
(1)
50
100
150
200
250
300
50
100
300
150
200
250
Sample temperature/°C
Temperature/°C
(b)
Fig. 9 DSC (a) and TG (b) curves for compound 4
15
10
5
Heat : –401.4/J g–1
Tmax : 325.1/°C
9
8.5
8
Heat : 68.1/J g–1
Tmax : 167.5/°C
OOT : 301.05/°C
Heat : –42.1/Jg–1
Tmax : 213.8/°C
7.5
7
Exo
0
6.5
Heat : 11.1/Jg–1
Tmax : 195.8/°C
6
5.5
5
Heat : 6.6/Jg–1
Tmax : 155.5/°C
50
100
150
200
250
300
Sample temperature/°C
Exo
OOT : 201.1/°C
4.5
4
(c)
Heat : –252.4/J g–1
Tmax : 319.9/°C
10
5
50
100
150
200
250
Sample temperature/°C
Heat : 96.1/J g–1
Tmax : 220.8/°C
Fig. 10 DSC curve for compound 5
OOT : 280.8/°C
0
documents) presented a total mass loss of about 5 %, and
these endothermal effects could be associated with the
evaporation of some water molecules or volatile com-
pounds [41].
Exo
–5
The DSC curve of compound 2 exhibits a single, large
endothermal peak with a maximum at 121 °C assigned to
the evaporation of water molecules (Fig. 8). This finding is
supported also by the TG/DTG analysis results (Fig. 5A—
50
100
150
200
250
300
Sample temperature/°C
Fig. 11 DSC curves for 6 (a), 7 (b) and 8 (c)
123

Structural characterization and thermal behaviour of some azomethine compounds derived from…
decomposition of the compound followed by the oxidation
process (Fig. 10). The mass loss generated by the oxidation
process represents 23 % of the initial mass (Fig. 6A—
supplementary documents).
7. Due to their functional moieties and high thermal sta-
bility, these structures can be seen like suitable candidates
as ligands for coordinative complexes development.
The DSC curves recorded on compounds 6–8 are similar
and undertake an endothermal process before oxidation
represented by the peaks at about 173 °C (6, 307 J g-1),
167.5 °C (7, 68.1 J g-1) and 221 °C (8, 92 J g-1),
respectively (Fig. 11). These peaks are due to the melting
of the crystalline structure of the analysed samples. These
results are well correlated with the TG/DTG data (Figs. 7A,
8A, 9A—supplementary documents). In case of each
compound, the oxidation process generates a specific mass
loss: 51 % (6), 41 % (7) and 65 % (8). From the values of
the melting enthalpy, the crystallinity degree of the com-
pounds can be estimated comparatively. The following
descending trend is observed: compound 6 [ compound 8
[ compound 7.
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Conclusions
The aims of this study are to describe and characterize
eight azomethine-type compounds derived from pyridoxal
and 4-aminoantipyrine which could be further used for the
synthesis of metal complexes. The structures of the target
compounds were confirmed by means of elemental analy-
1
sis, IR, H and 13C NMR spectroscopy and mass spec-
trometry. Compounds 6 and 8 were analysed by single-
crystal X-ray diffraction as well. The supramolecular motif
of both organic compounds is defined by one-dimensional
architecture stabilized by multiple intermolecular hydrogen
bond interactions. A comprehensive thermal stability study
was conducted on the basis of non-isothermal chemilumi-
nescence at different heating rates, DSC and TG analyses.
A correlation of the OOT and the molecular configuration
of compounds are observed. The position of different
substituents and the sterical hindrances can be identified as
main molecular parameters influencing electronic charge
distribution and consequently the thermal oxidation sta-
bility. In addition, the presence of intermolecular hydrogen
bonds induces variations in the structure stability. All the
studied compounds possess a high thermal stability with
maximum of onset oxidation temperature recorded for 6–8.
In case of these molecules, the DSC curves show additional
peaks associated with the melting process of the crystalline
structure. This behaviour is confirmed also by the results of
the TG analysis. Based on the melting enthalpy values,
their crystallinity degree was evaluated showing the
decreasing order: compound 6[compound 8[compound
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