Synthesis, thermal study and some properties of N2O4—donor Schiff base and its Mn(III), Co(II), Ni(II), Cu(II) and Zn(II) complexes

Article abstract of DOI:10.1007/s10973-016-5804-0

The new hexadentate N₂O₄ Schiff base ligand from condensation of 2,4-hydroxybenzophenone and 1,3-propanediamine has been prepared in methanol and ethanol solutions. The Schiff base synthesized from EtOH contains half molecule of amin

Full text of DOI:10.1007/s10973-016-5804-0

J Therm Anal Calorim
DOI 10.1007/s10973-016-5804-0
Synthesis, thermal study and some properties of N2O4—donor
Schiff base and its Mn(III), Co(II), Ni(II), Cu(II) and Zn(II)
complexes
Agata Bartyzel1
Received: 30 May 2016 / Accepted: 27 August 2016
Ó The Author(s) 2016. This article is published with open access at Springerlink.com
Abstract The new hexadentate N2O4 Schiff base ligand
from condensation of 2,4-hydroxybenzophenone and 1,3-
propanediamine has been prepared in methanol and ethanol
solutions. The Schiff base synthesized from EtOH contains
half molecule of amine per one molecule of ligand in its
structure. The complexes were synthesized from the direct
reaction of this ligand and Mn(II), Co(II), Ni(II), Cu(II) and
Zn(II) acetates. The Schiff base and its complexes were
characterized by elemental analysis, X-ray crystallographic
techniques, spectroscopic (UV–Vis, IR) and thermal (TG,
TG-FTIR) methods, molar conductance and magnetic
measurements. The complexes, except for Ni(II) com-
pound, similar to the Schiff base are amorphous solids. The
Ni(II) complex crystallizes in the monoclinic space group
P21/c. The N2O2 coordination geometry around nickel(II) is
slightly tetrahedrally distorted square planar. Variable
temperature magnetic studies of the paramagnetic com-
plexes show the existence of weak antiferromagnetic [for
complexes of manganese(III) and cobalt(II)] and weak
ferromagnetic [for copper(II)] interactions. The complexes
are stable at ambient temperature. After heating at first they
lose solvent molecules (water or methanol), and after that
organic part undergoes defragmentation and combustion.
Based on the obtained data from the TG-FTIR analysis, the
main gaseous products resulting from thermal degradation
in nitrogen atmosphere are CO2, CO, NH3, acetic acid,
methane, C6H6, C6H5CN and C6H5CH3.
Keywords Schiff base Á Metal complexes Á Crystal
structure Á Spectroscopic studies Á Thermal studies
Introduction
The metal complexes with polydentate ligands have been
of great interest for the last few decades. It is well known
that N and O donors play a key role in the coordination
chemistry because they give the possibility of obtaining
chelate compounds with interesting structures and proper-
ties [1–6]. The O,N-donor Schiff bases have often been
used as chelating ligands in the field of coordination
chemistry because they readily form stable complexes with
most of the transition metals [4–7]. The Schiff base ligands
and their complexes have numerous applications. The
formation of stable complexes with metals gives the
opportunity to design new systems selective to specific
metallic ions. For this reason, they can be used in different
extraction methods for preconcentration and separation of
trace amount of metal ions or for the preparation of ion-
selective electrodes for determination of anions and cations
in analytical samples [8–10]. Metallic complexes of Schiff
bases are efficient catalysts in oxidation and polymeriza-
tion of organic compounds [8, 11–15]. Schiff base com-
plexes are also noted for their significant biological
activities such as antifungal, antibacterial, antimalarial,
antitumor, antiproliferative, antiinflammatory, antiviral,
antioxidant and antipyretic properties [6, 8, 11, 16, 17].
Special attention is paid to the aromatic Schiff bases with
OH group in the ortho position to the amino group. This
type of Schiff base compounds shows thermo- and
Electronic supplementary material The online version of this
article (doi:10.1007/s10973-016-5804-0) contains supplementary
material, which is available to authorized users.
& Agata Bartyzel
agata.bartyzel@poczta.umcs.lublin.pl
1
Department of General and Coordination Chemistry, Maria
Curie-Skłodowska University, Maria Curie-Skłodowska Sq.
2, 20-031 Lublin, Poland
123

A. Bartyzel
Preparation of 4,40-{(1,3-
photochromism providing potential applications as data
storage, information processing, telecommunications and
optical switching [10, 17–19].
propanediyl)bis[nitrilophenylmethylidene]}di(1,3-
benzenodiol) (SchB)
As for numerous applications of Schiff base ligands
and their metal complexes, there have been synthesized a
salen-type Schiff base ligand derived from 2,4-dihydrox-
ybenzophenone and 1,3-diaminopropane as well as its
metal complexes of Mn(III), Co(II), Ni(II), Cu(II) and
Zn(II). Hydroxybenzophenones (e.g. 2,4-dihydroxyben-
zophenone) have the ability to absorb and to dissipate UV
light A, and therefore, they are used as UV stabilizers in
plastic surface coatings on food packaging and sunscreens
for humans [20]. The synthesized Schiff base and its
metal complexes have been characterized on the basis of
spectroscopic (IR, UV–Vis), X-ray crystallographic tech-
niques and thermal (TG, TG-FTIR) methods as well as
elemental analyses, molar conductance and magnetic
measurements.
The Schiff base ligand was synthesized (Scheme 1) using a
similar method described in previous papers [8, 21, 22].
Mixtures of 10 mmol of 2,4-dihydroxybenzophenone and
5 mmol of 1,3-propanediamine in 40 mL of methanol or
ethanol were refluxed for 2 h. The excess of the solvents
(ca. 30 mL) was then evaporated. After cooling to 4 °C
yellow solids were formed in both solvents. The products
were collected by filtration, washed with methanol or
ethanol and dried in air.
Synthesis of complexes 2–6
The syntheses of complexes were performed in methanol
solution with a ratio of metal(II) acetate to the Schiff base
2:1. To a stirred hot solution of the Schiff base ((1b),
0.43 mmol, 0.2 g) in MeOH (40 mL), 0.86 mmol of
appropriate acetate dissolved in methanol (10 mL) was
added. The obtained mixtures were refluxed for 2 h, and
after this time about 30 mL of methanol was evaporated. In
all the reaction mixtures with the exception of the nickel
one, solid products were formed. Precipitates were col-
lected by filtration and washed several times with metha-
nol. The yields of the purified complexes are given in
Table 1. In the case of nickel(II) complex, the reaction
mixture was allowed to cool to room temperature. After 1 h
the first crystal started to appear. The reaction mixture was
left in the fridge and allowed to evaporate slowly until the
volume was reduced to ca. 5 mL. Then, the solution was
decanted from crystals and the crystals were washed sev-
eral times with methanol.
Experimental
Materials
The solvents (MeOH—methanol, EtOH—ethanol, DMF—
N,N-dimethylformamide) and reagents (i.e. 1,3-propanedi-
amine (1,3-PDA), 2,4-hydroxybenzophenone (K), acetates:
Cu(CH3COO)2ÁH2O,
Co(CH3COO)2Á4H2O,
Mn(CH3
COO)2Á4H2O, Ni(CH3COO)2Á4H2O, Zn(CH3COO)2Á2H2O)
used for synthesis and studies were commercially available
from Aldrich Chemical Company, Alfa Aesar Company
and Polish Chemical Reagents. All solvents and chemicals
were of reagent grade and used without further purification.
Scheme 1 Synthesis of Schiff
base in ethanol (1a) and
methanol (1b)
1a
Schb·(1,3-PDA)0.5
EtOH
N
N
HO
NH2
OH
HO
OH
O
H2N
T = 65-75 °C
t = 2h
0.5
HO
OH
K
1b
SchB
MeOH
H2N
NH2
1,3-PDA
N
HO
N
OH
OH
HO
123

Synthesis, thermal study and some properties of N2O4—donor Schiff base and its Mn(III)…
Table 1 Elemental analysis and physicochemical properties of the ligands (1a–b) and complexes (2–6)
Compound
M. Wt./
g mol-1
Colour
Yield/ Found (Calcd.)/%
%
Kam/
X-1 cm2 mol-1
C/%
H/%
N/%
8.24
C29H26N2O4Á0.5 C3H6(NH2)2
C29H26N2O4
(1a) 503.59
(1b) 466.53
Dark yellow
Light yellow
Dark brown
91.60 72.93
6.65
2.69b
1.07c
7.44b
4.39b
0.51c
0.87b
0.80b
(72.74)
62.32 74.84
(74.66)
92.46 60.10
(60.59)
37.60 50.66
(50.49)
45.40 63.11
(63.40)
50.25 60.63
(60.65)
39.61 63.03
(63.11)
(6.20)
5.77
(5.62)
5.09
(5.08)
4.82
(5.08)
5.32
(5.49)
4.87
(5.09)
4.67
(4.78)
(8.34)
6.01
(6.00)
5.02
(4.56)
3.71
(3.36)
4.95
(4.77)
5.01
(4.72)
4.86
(4.75)
Mn(C29H24N2O4)(CH3COO)(H2O)2
Co2(C29H25N2O4)(CH3COO)3(H2O)4
Ni(C29H24N2O4)(CH3OH)2
(2)
(3)
(4)
614.52
832,58
587.29
1188.23
589.97
Light green
(mint)
Red-brown
Cu2(C29H24N2O4)2(CH3COOH)(H2O)4 (5)
Dark olive
green
Zn(C29H25N2O4)(CH3COO)
(6)
Pale yellow
(cream)
a
Used concentration 10-3 mol L-1
b
Measurement was taken in DMSO
c
Measurement was taken in DMF
Methods and physical measurements
complexes were determined by means of a Setaram Setsys
16/18 derivatograph, recording TG, DTG and DSC curves.
The samples (6.38–7.77 mg) were heated in a ceramic
crucible in the temperature range of 30–700 (Cu), 30–800
(Ni, Zn, ligand 1b), 30–850 (ligand 1a) and 30–1000 °C
Elemental analysis for C, H, N was performed using a
PerkinElmer CHN 2400 analyser. Magnetic susceptibilities
for the powdered samples were measured over the tem-
perature range 1.8–300 K at the magnetic field 0.1 T using
a Quantum Design SQUID-VSM magnetometer. Field
dependences of magnetization were measured at 2 K in an
applied field up to 5 T. The corrections were based on
subtracting the sample—holder signal and contribution of
vD estimated from the Pascal’s constants [23]. The molar
conductivities were measured at 25 °C using a PHYWE
13701.93 conductivity metre in the DMF solvent (except
for 1a and 2, which were dissolved in DMSO). The molar
conductivities reported in Table 1 are corrected values.
The solvent conductivity was subtracted from that of the
appropriate solution in order to obtain the real conductivity
of the electrolyte. The infrared spectra were recorded on a
Nicolet 6700 FTIR using the ATR accessory on the ZnSe
crystal for pressed powder samples. The data were col-
lected between 4000 and 600 cm-1, with a resolution of
4 cm-1 for 16 scans. The most important bands are given
in Table 2, and all FTIR spectra are presented in Supple-
mentary material (Figs. S1-S8). The UV–Vis spectra of
Schiff base ligands (1a and 1b) and metal complexes were
recorded in 10 9 10 mm quartz cells over the range
200–900 nm using a Genesys 10S UV–Vis spectropho-
tometer (DMF solution 2.5 9 10– 5 mol L-1, except for 1a
and 2, which were dissolved in DMSO), see Table 3.
Thermal stability and the decomposition of the analysed
(Co, Mn) under flowing air atmosphere (v = 1 dm3 h-1
)
with a heating rate of 10 °C min-1. TG-infrared spec-
trometry (TG-FTIR) of the title compounds was performed
using a TG Q5000 analyser interfaced to the Nicolet 6700
FTIR spectrophotometer. About 11.53–24.13 mg of com-
pounds was put in an open platinum crucible and heated
from an ambient temperature (*22 to 23 °C) to 1000 °C.
The analysis was made at a heating rate of 20 °C min-1
under nitrogen atmosphere at a flow rate of 25 mL min-1
.
In order to reduce the possibility of gases condensing along
the transfer line, the temperature in the gas cell and transfer
line was set to 250 and 240 °C, respectively. The FTIR
spectra were recorded in the spectral range of
600–4000 cm-1 with a resolution of 4 cm-1 and 6 scans
per spectrum.
The crystallographic measurements were taken using an
Oxford Diffraction Xcalibur CCD diffractometer with the
˚
graphite-monochromated Mo Ka radiation (k = 0.71073 A).
Data sets were collected at 100 K using the x scan technique,
with an angular scan width of 1.0°. The programs CrysAlis
CCD[24] and CrysAlisRed[24]wereusedfordatacollection,
cell refinement and data reduction. A multi-scan absorption
correction was applied. The structures were solved by direct
methods using SHELXL-2013 [25] and refined by the full-
matrix least squares on F2 using SHELXL-97 [25] both
123

A. Bartyzel
Table 2 Selected infrared absorption bands of the ligands and complexes 2–6 (cm-1
)
Compound
m(O–H)
d(N–H)
m(C=O)
m(C=N)
mas(COO-)
mas(COO-)
d(O–H)
m(C–O)
1a
1b
2
2513
2502
3190
3544
3333
3202
3613
3275
3139
3625
3143
3056
1671
–
–
–
–
1580
1584
1580
1578
–
–
1339
1337
1341
1358
1344
1245
1242
1245
1257
–
–
–
–
–
1537
1557
1390
1440
3
4
–
–
1563
–
–
1359a
1275b
1238a
1071b
5
6
–
–
1676
–
1578
1565
–
–
1342
1341
1246
1565
1483
1288
1253
a
Phenoxide
b
Methanol
Table 3 Electronic spectral data
k )
max/nm (e 9 10-4/L mol-1 cm-1
Results and discussion
Compound
1a
1b
2
258(1.28); 283(1.93); 310(1.784); 381(0.39)
262(1.94); 282(2.34); 309(1.96); 384(0.35)sh
260(2.20); 318(1.60); 357 (1.34)sh
267(4.00); 347(1.37)sh; 563(0.07)sh,a
273(7.28); 312(2.42)sh 343(1.61)sh; 409(1.08),
584(0.01)sh,a
The synthesis of Schb was conducted from two different
alcoholic solutions [ethanol (1a) or methanol (1b)]. In
both cases solid, amorphous products differing in colour
and composition were obtained (Table 1). The Schiff base
synthesized from ethanol contains in its structure amine
(SchbÁ(1.3-PDA)0.5) which causes that the ligand 1a is
less soluble than 1b. Therefore, the compound 1b was
used in synthesis of complexes. The synthesized Schiff
base has in its structure six possible coordination sites, i.e.
two nitrogen atoms of azomethine groups and four oxy-
gen atoms of the phenolic groups. For this reason, the
synthesis of metal complexes was conducted through the
reaction between metal ions and the ligand at 2:1 M:SchB
ratio. The analysis of the power diffraction patterns of
complexes (apart from 4) reveals that they, just like
ligands 1a and 1b, are amorphous compounds. Despite
several attempts, I was unable to obtain single crystal for
them. The nickel(II) complex was obtained as crystalline
solid. Compounds 1a–b and complexes 2–6 are stable in
air in solid state. The molar conductivities of complexes
were measured in DMF (3–6) or DMSO (2) solvents at
the concentration 10-3 mol L-1. The obtained values are
very low and indicate the non-electrolytic nature of
complexes [31]. The elemental analyses and some
physicochemical properties of the ligands (1a–b) and
complexes (2–6) are shown in Table 1.
3
4
5
6
267(6.21); 296(7.82); 358(3.43); 604(0.07)a
266(2.23); 284(2.39); 320 (1.22)sh; 346(1.18)
sh
Shoulder
The band recorded at a concentration 10-3 mol L-1
a
operating under WinGX package program [26]. All non-H
atoms were refined with anisotropic displacement parameters.
Hydrogen atoms residing on carbon atoms were positioned
geometrically and refined applying the riding model [with
Uiso(H) = 1.2 or 1.5 Ueq(C)]. All remaining ones were found
in the difference Fourier maps and refined with isotropic
displacement parameters. The summary of crystal data,
experimental details and refinement results is given in
Table 4. The selected bond distances and angles are presented
in Table S1 (Supplementary material). The molecular plots
were drawn with ORTEP–3 for Windows [27, 28] and Mer-
cury [29]. The geometrical calculations were performed using
the PLATON program [30]. The CIF file for 4 refinement can
be retrieved from the Cambridge Crystallographic Data
Centre (CCDC) (deposition number CCDC 1450297).
123

Synthesis, thermal study and some properties of N2O4—donor Schiff base and its Mn(III)…
[4, 10–12, 17]. In the spectrum of compound 1a this vibration
Table 4 Crystal data and structure refinement for 4
is coupled with the m(C=C) vibration and shifted towards
lower wavelengths. For this reason, it is not observed as an
independent band, but it appears as a shoulder peak at
1580 cm-1. In the case of 1b this band is better separated and
observed at 1584 cm-1. In structure of 1a there is also one of
the substrates, i.e. amine. The primary amines exhibit basi-
cally two stretching vibration bands due to the asymmetric
and symmetric stretching vibration N–H situated in the
3500- to 3200-cm-1 region and one band due to in-plane
bending vibration of N–H group at 1650–1550 cm-1. In the
FTIR spectrum of 1a, the N–H stretching bands are not
observed, but at 1671 cm-1 a week band is found. This band
is shifted to higher values probably due to hydrogen bonding
between 1,3-PDA and Schb.
Formula
[Ni(C29H24N2O4)]Á2CH3OH
Temperature K
Crystal system
Space group
100(2)
Monoclinic
P21/c
˚
a/A
10.172(1)
˚
b/A
27.496(2)
˚
c/A
10.349(1)
b/°
Volume/A
109.44(1)
3
˚
2729.6(4)
Z
4
Calculated density/g cm-3
l/mm-1
1.429
0.759
Absorption correction
F(000)
Multi-scan
1232
The infrared spectra of complexes 2–5 exhibit a wide
medium intense band in the range 3610–2500 cm-1, which
points to the presence of hydrogen-bonded water or
methanol molecules, and phenolic groups in their structures
(Figs. S4-S8 in Supplementary material). In the case of 6
the broad band with the maximum at *3056 cm-1 is also
observed which is due to the O–H stretching vibrations of
the phenolic groups involved in the hydrogen bonding
between them as well as the phenolic group coordinated to
zinc(II) ion. In the spectra of all complexes the band
assigned to the vibration of azomethine groups, m(C=N), is
shifted to lower wavenumbers indicating participation of
nitrogen atoms in coordination to metal ions. The m(C–O)
vibrations are shifted to higher frequencies (apart from 4)
which suggests that coordination of metal ions takes place
via phenolate and/or phenolic ortho-oxygen atoms as it is
was observed for some transition metal complexes of o-
hydroxy Schiff base [6, 11]. In the case of 4, the Ni(II)
centre is also coordinated by phenoxide oxygen, but m(C–
O) vibration occurs at a bit lower wavenumber compared to
the free ligand which is quite common for compounds of
this type [4, 12, 13, 17]. In the structure of complexes 2, 3
and 6 also acetate ions occur. The bands characteristic of
deprotonated carboxylic group, asymmetric and symmetric
stretching vibration COO are observed at 1637–1565 and
1390–1483 cm-1, respectively. It is noteworthy that in the
zinc complex spectrum the band derived from mas(COO) is
coupled with m(C=N) vibration. In the IR spectrum of
copper(II) compound at 1676 cm-1 a week band which is
not found in the spectra of other complexes is observed.
This IR region corresponds to the stretching C=O vibration
indicating that acetic acid appears in the structure of 5. In
general the C=O group of saturated aliphatic carboxylic
acid absorbs in the region of 1740–1700 cm-1. The shift of
this band to a lower wavelength may imply that probably
carbonyl oxygen atom coordinates to copper(II) ion like in
the molybdenum complexes reported by Bortoluzzi et al.
[33]. It is also worth mentioning that the sharp, weak peak
Crystal size/mm
h range/°
Index ranges
0.25 9 0.08 9 0.05
2.59–27.48
-7 B h B 13
-22 B k B 35
-13 B l B 10
12,353/6248
0.0684
Reflections collected/unique
Rint
Data/restraints/parameters
GooF on F2
6248/0/379
0.825
Final R indices [I [ 2r(I)]
R indices (all data)
R1 = 0.0503, wR2 = 0.0787
R1 = 0.1067, wR2 = 0.0890
Infrared spectroscopy
The FTIR spectra of the free ligands (1a and 1b) and the
complexes exhibit various bands in the 600–400 cm-1
region (see Supplementary material Figs. S1-S3). The most
characteristic bands and their assignments are given in
Table 2. The O–H stretching frequency of 1a and 1b is
expected at 3500–3300 cm-1. In the o–hydroxy Schiff
bases, this band is generally moved to lower frequencies
(3200–2500 cm-1
) due to the presence of strong
intramolecular hydrogen bond O–HÁÁÁN=C [6, 10, 11, 32]. In
the studied Schiff base, the m(O–H) vibration appears as
broad bands at 2513 cm-1 for 1a and 2502 cm-1 for 1b
which indicates the existence of a strong hydrogen bond
between hydroxyl and azomethine groups. The medium
peaks at 1339 cm-1 (1a) and 1337 cm-1 (1b) have been
assigned to in-plane bending O–H vibration. Very strong
bands observed at 1245 cm-1 (1a) and 1243 cm-1 (1b) are
probably due to the C–O stretching vibration as reported for
this type of compounds [6, 11–13, 16, 32]. The FTIR spectra
of Schiff base also show a strong absorption band in the range
1690–1590 cm-1 which is characteristic of the C=N group
123

A. Bartyzel
is observed at 3625 cm-1 which is assigned to stretching
vibration of free O–H group [32]. This band can be due to
the presence of free hydroxyl group of acetic acid which
for all complexes with exception of 4 [13, 16, 17, 37–41].
In the case of nickel(II) complex the CT transitions are
observed at 343 and 409 nm [5, 15, 36]. The bands char-
acteristic of the d–d transitions are not observed on spectra
of 2–5 at the applied concentration due to their low
intensity. In more concentrated solutions (10-3 mol L-1),
the low intensity bands assigned to d–d transitions appear
for 3 (575 nm), 4 (584 nm) and 5 (604 nm). The presence
of only one d–d band at these wavelengths indicates that
the coordination environment of copper(II) and nickel(II)
ions in solution is square planar, while geometry of
cobalt(II) is tetrahedral [7, 15, 36, 40, 42].
ˆ
was observed for monomer of acetic acid by Mac¸oas et al.
[34]. However, it cannot be ruled out that one of the phe-
nolic groups in the para position may not be involved in
any hydrogen bonds and also shows vibrations in this
region [32, 35].
Electronic absorption spectra
The electronic absorption spectra were recorded in DMF
(for 1b and 3–6) or DMSO (for 1a and 2, because of their
lower solubility in N,N-dimethylformamide) solutions. The
data are summarized in Table 3 and presented at Fig. S9
(see Supplementary material). As can be seen from the
table, the spectra of 1a and 1b exhibit four bands and
according to the literature [9, 15–17] these bands can be
assigned as follows. The absorption bands at 256 and
283 nm for 1a and 262 and 282 nm for 1b are most
probably due to the p ? p* transition of the aromatic
rings. The bands at 310 nm (1a) and 312 nm (1b) are
assigned to the p ? p* transition of the azomethine group.
The last bands at 381 and 385 nm (1a and 1b, respectively)
have much lower molar extinction coefficients than the
previous ones and are the n ? p* transition involving the
promotion of a lone pair electron on the nitrogen to the
antibonding p-orbital associated with the azomethine
group. It is worth mentioning that differences in the posi-
tions of ligands maxima are the result of used solvents. In
the spectrum of a saturated solution of 1a in DMF, the
bands occur at exactly the same wavelength as for the 1b.
The spectra of the complexes show changes in relation
to the position and intensity of characteristic bands in
comparison with the spectrum of free ligand. The new
bands, although sometimes not well defined, are also pre-
sent. The p ? p* transition of the aromatic ring is
observed for all complexes and occurs as one (for 2–4, at
260–273 nm) or two (for 5 and 6, at 266–267 and
284–296 nm) bands. In the case of complexes where
p ? p* transition of the aromatic ring is present as a single
band, the shift of maxima towards shorter wavelengths is
observed, while for 5 and 6 both absorption maxima show a
negligible red shift. Moreover, the bands at 318, 312 and
320 nm, respectively, for the complexes of Mn(III), Ni(II)
and Zn(II), can be assigned to p ? p* transition of the
azomethine group by analogy with related metal complexes
of Schiff base [12, 15, 36]. In the electronic spectra of the
transition metal bands associated with a charge transfer
(CT) and d–d transitions are also observed. The charge
transfer vibration is combined with n ? p*/p ? p* tran-
sition of (C=N) and observed as one bands at 346–430 nm
Structure of nickel(II) complex
Complex 4 was obtained as a crystalline substance which
made it possible to carry out a single-crystal X-ray analy-
sis. The molecular structure of complex 4 is displayed in
Fig. 1. The crystallographic and refinement data for 2 are
summarized in Table 4. The selected bond distances and
angles are listed in Table S1, and the hydrogen bond
parameters are given in Table S2 (see Supplementary
material). The nickel(II) complex crystallizes in the mon-
oclinic space group P21/c. In this complex, the Ni(II) metal
centre is coordinated by a pair of phenolate O (O1 and O3)
and imine N (N1 and N2) atoms from the Schiff base
ligand. The square-planar N2O2 coordination geometry is a
slightly tetrahedrally distorted as indicated by less than
linear bond angles (171.97(10) O(1)–Ni1–N(2) and
171.78(9) O(3)–Ni1–N(1)) and the relatively small dihe-
dral angle between the two N–Ni–O planes (h = 9.09°).
The Ni(II) atom lies practically in the plane of the N2O2
˚
core (with a small rms deviation of 0.005(1) A). In the
C11
C12
C20
C10
C21
C19
C15
N2
C13
C14
N1
C9
C16
C22
C23
C8
C18
C7
C17
C6
C1
Ni1
C29
C5
C24
O1
O3
C31
O6
C28
C2
C25
C4
C3
C27
C26
O2
O5
O4
C30
Fig. 1 ORTEP view with the atom numbering scheme of 4.
Displacement ellipsoids are drawn at the 50 % probability level
123

Synthesis, thermal study and some properties of N2O4—donor Schiff base and its Mn(III)…
˚
coordination plane, Ni–O (1.834(2), 1.844(2) A) and Ni–N
˚
(1.881(2), 1.885(2) A) bond distances are similar to the
intermolecular C–HÁÁÁO hydrogen bonds and C–HÁÁÁp(ring)
interactions. The presence of weak intermolecular C–HÁÁÁO
hydrogen bonds leads to the formation of a three-dimen-
sional network, see Fig. S12 in Supplementary material.
The intermolecular C–HÁÁÁCg contact (Table S2 given in
Supplementary material) occurs between the centroid of
the phenyl ring (Cg4, centroid of the phenyl formed by C1–
C6) and phenyl hydrogen atom [C13-H13ÁÁÁCg6iv, sym-
metry code (iv): -x, 1 - y, 1 - z] and stabilizes structure
by creating other dimeric units.
corresponding distances observed in other Schiff base
nickel(II) complexes with square-planar environment of
metal centre [43–46]. It is worth mentioning that in the
case of the increasing coordination number, the increase in
Ni–O and Ni–N bond lengths is observed; e.g. the Ni–O
˚
and Ni–N bond distances 1.936(2)–1.963(3) A and
˚
1.953(2)–2.000(3) A [47, 48] for a square pyramidal and
˚
˚
1.980(11)–2.039(2) A and 2.032(15)–2.071(2) A [5, 49, 50]
for octahedral coordination geometries have been reported.
Methanol molecules are linked to Ni(C29H24N2O4) mole-
cule via intermolecular O–HÁÁÁO hydrogen bonds with
phenoxy and phenolic oxygen atoms as proton acceptors
(Table S2 in Supplementary material). What is more, one
methanol molecule is engaged in the formation of dimeric
units through O2–H2PÁÁÁO6 and O6–H6 MÁÁÁO1 hydrogen
bonds where it is both the donor and acceptor of proton,
generating cyclic hydrogen-bonded motifs with the graph-
set notation R44(16) [51], see Fig. S10 in Supplementary
material. These dimeric units are further linked via another
O4–H4P–O3i (symmetry code (i): x, -y ? 1/2, z - 1/2)
hydrogen bonds, forming a layer parallel to the bc plane
(Figure S11). The structure is also stabilized by weak
Thermal analysis
Thermal analyses of studied compounds (1a–b) and com-
plexes (2–6) were carried out by the TG-DSC (air) and TG-
FTIR (nitrogen) techniques. The TG-DSC curves providing
information about thermal properties of the compounds are
shown in Figs. 2 and 3.
Ligands 1a and 1b
Compounds 1a and 1b have high thermal stability which
depends on the alcohol used for the synthesis. In the case of
(c)100
(a)
(1a)
100
80
30
20
10
0
Exo
DTG
80
60
60
40
40
20
0
DSC
TG
0
200
400
600
800
1000
0
200
400
800
600
Temperature/°C
Temperature/°C
(1b)
Exo
(d)
(b)100
0.15
0.10
1,3-PDA
NH3
CO
2 H2O
20
15
10
5
0.30
0.25
DTG
H2
1,3-PDA
O
0.05
80
60
40
4000
3000
2000
1000
0.20
0.15
t = 9 min
T = ∼ 201 °C
0.10
0.05
0.00
40.0
30.0
20.0
DSC
0
20
0
–5
4000
TG
3000
10.0
2000
0
200
400
800
600
1000
Temperature/°C
Fig. 2 a TG, DTG and DSC curves of 1a in air, b TG, DTG and DSC curves of 1b in air, c TG curve of 1a in nitrogen, d FTIR spectra of
gaseous products evolved during the decomposition of 1a
123

A. Bartyzel
(b)
(a)100
(2)
(3)
Exo
Exo
40
30
20
10
0
100
80
DTG
30
20
10
0
DTG
80
60
40
20
60
40
DSC
TG
DSC
TG
20
0
0
0
200
400
600
800
1000
200
1000
0
400
600
800
Temperature/°C
Temperature/°C
40
30
20
10
0
(c)
(4)
(d)100
(5)
Exo
DTG
Exo
100
40
30
20
10
0
DTG
80
60
40
20
80
60
40
20
DSC
TG
DSC
TG
200
0
400
600
800
200
400
600
800
Temperature/°C
Temperature/°C
(e)100
(6)
(f)
Exo
100
80
DTG
2
30
20
10
0
3
4
5
6
80
60
40
20
60
DSC
40
TG
200
0
200
400
600
800
1000
400
600
800
Temperature/°C
Temperature/°C
Fig. 3 a–e TG, DTG and DSC curves of 2–6 in air, f TG curves of 2–6 in nitrogen
compound 1a the first change on the TG curve recorded in
air atmosphere has been found at 195 °C with an initial
mass loss of 7.21 % which is in good agreement with the
theoretical value of 7.36 % attributed to the loss of mole-
cule 1,3-PDA, Fig. 2a. The deamination process is con-
nected with an endothermic effect recorded on the DSC
curve of 1a. The enthalpy of this process is equal to
57.39 0.35 kJ mol-1. The same process is also observed
on the TG curve recorded in nitrogen atmosphere (see
Fig. 2c), but the initial mass loss is a bit higher (8.06 %)
than the calculated one. This may suggest that in an inert
atmosphere the partial decomposition of the Schiff base
also appeared. The FTIR spectra of the evolved gas phase
confirm that 1,3-PDA and its products as well as partial
SchB decomposition (CO2, NH3 and H2O) are released in
this stage (Fig. 2d). Further thermal decomposition of 1a in
air proceeds in two stages. The first stage, the partial
destruction and combustion of the Schiff base, starts at
265 °C, and intermediate, unstable products are formed.
This step occurs together with the melting process. The
DSC peak attributed to melting accompanied by the neg-
ligible mass loss is recorded at 264 °C, the melting point
determined from the DSC curve is 268 °C, and the
enthalpy of fusion is 30.90 0.12 kJ mol-1. The second
stage above 395 °C corresponds to the complete destruc-
tion and combustion of the remaining parts of the com-
pound. Compound 1a fully decomposed at 678 °C. The
thermal decomposition of 1a in nitrogen after the deami-
nation process is a bit different from the one observed in
air. The first step of compounds decomposition starts at
270 °C, and in this stage more than fifty per cent of mass
loss is observed (see Fig. 2d; Table 5). Similar to thermal
123

Synthesis, thermal study and some properties of N2O4—donor Schiff base and its Mn(III)…
associated with the pyrolysis of the Schiff base fragment
Table 5 Thermal analysis data for ligands 1a–b and complexes (2–6)
containing amine and benzene rings with phenolic groups.
At a higher temperature (*335 °C), the weak bands
characteristic of benzonitrile are recorded, and the bands
corresponding to C6H5CN are observed in the range
3100–3000 (m(C–H)), 2275–2200 (m(C:N)) and
825–650 cm-1(d(Car = Car), c(CC:N)) [17]. At the end
of the first step of decomposition around 17 min
(*350 °C) several bands characteristic of toluene were
recorded. The bands assigned to toluene occur in the ranges
3200–2800 cm-1 (with four maxima at 3072, 3032, 2933
and 2885 cm-1) and 800–600 cm-1 (with the maxima at
728 and 692 cm-1). The other bands characteristic of
toluene in the range of 2000–1000 cm-1 do not have sharp
marked maxima because these vibrations are overlapped
with those of water and ammonia molecules. As it was
mentioned previously, the first step is connected with for-
mation of unidentified, unstable product which immedi-
ately undergoes further thermal decomposition. The second
stage of ligand pyrolysis starts around 430 °C, and initially
it is associated with the release of similar products as they
were observed at the end of the first stage, i.e. H2O, CO2,
NH3 and toluene. At a higher temperature (ca. 485 °C) the
peaks characteristic of CO (double peaks at
2275–2050 cm-1 [16, 17, 52, 54]) and benzene (in the
ranges 3150–300, 1550–1450 and 1050–1000, and at 692,
672 and 653 cm-1 [52]) bands are recorded. With the
increasing temperature, the intensity of the bands associ-
ated with the vibration of C6H6 and C6H5CH3 in the range
3200–2800 cm-1 decreases and the bands characteristic of
methane are noted. The bands characteristic of CH4 are
observed at *525 °C in the range of 3175–2900 cm-1
with a characteristic maximum at 3016 cm-1 [16, 17, 54].
Probably methane is released at a lower temperature, but
the vibrations are covered by the bands of benzene and
toluene. At the end of this step (above 600 °C) in the FTIR
spectra the bands characteristic of CO, CO2, H2O, NH3 and
CH4 are only recorded, but ammonium and methane
vibrations disappear below 800 °C. In contrast to the
thermal decomposition in air atmosphere, here the
decomposition process of 1a is not finished. The residue
remaining after the analysis at 1000 °C is 32.71 %, and
probably this is a mixture of carbon with hydrocarbons.
Thermal behaviour of 1b was studied in air atmosphere
(see Fig. 2b). The TG-DSC curves show that the studied
compound is thermally stable under air stream up to
*280 °C. The compound shows its first DSC peak (at-
tributed to melting accompanied by negligible mass loss) at
282 °C. This endothermic peak is connected with melting
process; the melting point and the enthalpy of fusion are
284 °C and 37.06 0.23 kJ mol-1, respectively. This
DSC peak is sharp which indicates that this is a pure
substance. The decomposition of 1b in air is a two-step
Compound Atm Temperature
range/°C
Tpeak/°C
Mass
loss/%
1a
Air 195–233
268–395
212
327
590
206
308
553
319
594
87
7.21
21.93
70.66
8.06
40.14
19.09
29.35
70.60
5.37
81.80
0.56
5.25
–
395–647
N2
181–225
275–430
430–858
1b
2
Air 282–430
430–662
Air 60–140
140–476
303; 378
954
79
927–970
N2
40–110
110–
270; 365; 450;
540
3
Air 49–116
137–175
96
6.69
1.88
72.39
1.53
6.45
1.66
15.65
–
157
177–386
226; 310
919
907–930
N2
38–110
120–150
150–318
318–
83
140
225; 270
459; 544; 853
103
4
5
Air 51–143
296–482
5.60
80.40
7.12
48.75
386
N2
58–251
233
320–975
364; 406; 430;
674; 906
Air 49–100
246–370
83
6.12
13.98
66.83
5.87
1.13
–
290; 343
425; 480
83
373–537
N2
37–107
190–248
270–
228
286; 344; 550
326; 381
327; 403
886
6
Air 260–495
86.19
57.46
–
N2
260–770
770–
Atm, atmosphere of analysis; Tpeak, DTG peak temperature (maxi-
mum change of weight)
decomposition of 1a in air, this step is connected with
formation of intermediate, unstable products. At the
beginning of the first stage of decomposition process the
main gaseous products emitted at around 245–330 °C are
identified as: H2O (4000–3450 and 1950–1300 cm-1
[52–54], CO2 (peaks at 2450–2300 and 750–600 cm-1
)
)
[16, 17, 54] and NH3 (several bands with two characteristic
maxima at 966 and 931 cm-1) [16, 17, 54], Table 6. These
gaseous products indicate that probably this step can be
123

A. Bartyzel
Table 6 Gaseous products evolved during thermal decomposition of ligand 1a and complexes 2–6 in a nitrogen atmosphere
Gaseous species
T/°C
1a
2
3
4
5
6
1,3-PDA
H2O
185–240
185–1000
–
–
–
–
–
–
45–1000
–
43–1000
–
160–1000
60–350
320–680
160–1000
390–1000
–
45–1000
–
340–1000
–
CH3OH
NH3
200–700
185–1000
485–1000
–
150–620
150–1000
420–1000
110–420
380–520
–
362–555
150–1000
330–1000
150–430
–
270–530
190–1000
380–700
190–290
–
330–790
340–1000
365–1000
265–390
455–540
455–580
365–530
–
CO2
CO
Acetic acid
Toluene
Benzene
Benzonitrile
CH4
350–590
485–600
335–405
525–795
–
390–540
390–560
390–540
540–760
–
405–520
405–505
360–495
405–530
360–410
290–410
470–720
360–410
340–500
400–660
–
Phenol
–
(a)
0.30 (b)
0.15
0.25
0.20
0.15
0.10
0.10
0.05
40.0
30.0
40.0
30.0
0.05
0.00
20.0
10.0
20.0
10.0
0.00
4000
4000
3000
3000
2000
2000
1000
1000
Fig. 4 a FTIR spectra of gaseous products evolved during the decomposition of 2, b FTIR spectra of gaseous products evolved during the
decomposition of 6
process. The first mass loss step is observed at the tem-
perature 282 °C and occurred together with the melting
process. This stage is connected with the partial destruction
and combustion of 1b. Within this first step of 1b
decomposition less than thirty per cent of mass loss is
observed (Table 5), and an intermediate, unstable product
is formed. The second stage above 430 °C corresponds to
the complete destruction and combustion of the remaining
parts of the compound. Compound 1b fully decomposed at
a temperature of approximately 660 °C.
stage corresponds to the desolvation process. On the basis
of the TG-FTIR analysis, it was found that in this stage
water molecules are released. A calculated mass loss due to
removal of all water molecules is 5.86 %, while the mea-
sured mass loss is 5.37 %. The enthalpy of dehydration
process is equal to 47.29 0.42 kJ mol-1. The anhydrous
compound is unstable and immediately undergoes thermal
decomposition into Mn2O3 (found/calcd. overall mass
losses: 87.17/87.15 %). At
a
high temperature
(927–970 °C), this oxide is transformed into Mn3O4 as it
can been observed in the previous work [55]. Thermal
behaviour of 2 was also studied in nitrogen atmosphere
together with the infrared spectroscopy analysis of gaseous
decomposition products (Figs. 3f, 4a). The first step is
analogous to that recorded in air atmosphere and corre-
sponds to the loss of water molecules (found mass loss
5.25 %). As it has already been mentioned the FTIR
Complexes 2–6
The thermal degradation of manganese(III) complex in air
atmosphere starts with an initial well-separated mass loss
in the temperature range of 60–140 °C with an endother-
mic effect recorded on the DSC curve, see Fig. 3a. This
123

Synthesis, thermal study and some properties of N2O4—donor Schiff base and its Mn(III)…
spectra of the evolved gaseous products confirm the pres-
ence of water molecules in the composition of Mn(II)
complex, see Fig. 4a. The characteristic valence and
deformation vibration bands of water molecules appear at
45 °C. As in the case of thermal decomposition in air,
anhydrous compound is immediately pyrolysed. At the
beginning of the first stage of decomposition process in
nitrogen the elimination of acetate anions as acid molecules
is observed. The weak bands characteristic of acetic acid
(m(OH): 3595, 3580 and 3566 cm-1; m(CH3): 3044, 2997
and 2969 cm-1; m(C=O): 1797 and 1774 cm-1; d(CH3):
bands characteristic of water molecules are only observed.
The anhydrous complex is unstable and immediately
decomposes. The thermal decomposition proceeds in two
steps, and this process is not completed under measurement
conditions (Fig. 4f). The first stage of pyrolysis is con-
nected with the elimination of acetate anions as the acid
whose vibrations are noted on the FTIR spectra together
with trace products of its decomposition, i.e. H2O and CO2.
Here, in contrast to thermal decomposition in air, this step
is better marked on the TG curve and a greater part of
acetate ions is eliminated (found mass loss: 15.65 %). The
second step of thermal degradation of complex starts above
318 °C and initially is related to the removal of the
remaining parts of acetate ion. The first bands indicating
the Schiff base degradation, i.e. the bands of NH3 and CH4
molecules, appear on the FTIR spectra above 360 °C.
Further heating causes the increase in the emission of these
gaseous species, and new products are observed, i.e. ben-
zene, benzonitrile and phenol. The most characteristic,
qualitative bands for C6H5OH are observed at 3700–3600,
3150–3000, 1650–1100 and 850–650 cm-1 [17]. Above
560 °C only CO2, H2O and CO molecules are present in
gaseous products.
1398 cm-1; m(C–O): 1266 cm-1; d(COH): 1177 cm-1
;
c(CH3): 1087 and 999 cm-1; d(OH): 979 cm-1; m(C–C):
859 and 835 cm-1; s(C–O): 656 and 642 cm-1 [56, 57])
appear in the FTIR spectra of the gaseous products at
110 °C. With the increasing temperature the intensity of
the bands associated with the release of acid grows, but at
150 °C the bands characteristic of CO2 are also observed.
The appearance of these vibrations with a simultaneous
increase in the intensity of the bands derived from water
molecules may be due to degradation of acetic acid as well
as decomposition of the Schiff base. This is confirmed by
the presence of bands characteristic of ammonia, and
associated with defragmentation of SchB, which are visible
when the intensity of the acetic acid bands decreases
(*150 °C). At a higher temperature, the bands character-
istic of benzonitrile (340 °C), toluene (380 °C) and
methane (400 °C) are also recorded. It can be observed
from the TG curve that the decomposition process was not
completed in nitrogen atmosphere.
Decomposition of nickel(II) complex in air is a two-step
process as visible on the TG curves, see Fig. 3c. Thermal
degradation of compound 4 starts with an initial well-
separated mass loss of 5.60 %, between 50 and 143 °C and
corresponding to the loss of one methanol molecule per
formula unit (calcd. 5.45 %). The desolvation process is
connected with an endothermic effect recorded on the DSC
curve, and the enthalpy of this process is equal to
52.50 0.41 kJ mol-1. The last methanol molecule pre-
sent in the structure of 4 is removed at a higher temperature
(297 °C) together with the degradation and combustion of
the Schb ligand. The final product was a grey-black pow-
der which indicates that Ni2O3 was formed (found/calcd.
overall mass losses: 86.01/85.92 %) [58, 59]. The thermal
behaviour of 4 in nitrogen together with the analysis of
gaseous products was also investigated. As can be seen in
Fig. 3f, the first stage starts at a slightly higher temperature
than in the air and takes longer to 250° C. What is more, a
greater loss in mass, 7.12 %, is registered on the TG curve.
However, this value is still lower than those calculated for
the loss of two molecules of methanol, i.e. 10.45 %. The
next stage on the TG curve is observed in the range
330–975 °C and corresponds to removal of the remaining
part of CH3OH from the crystal structure as well as
destruction of the organic ligand. The desolvation process
at such a high temperature was confirmed by the FTIR
analysis of gaseous products. The traces of methanol as
well as its decomposition products are recorded on the
FTIR spectra up to about 350 °C. The characteristic
vibration bands of CH3OH are observed in the
The complex of cobalt(II) is stable at room temperature.
The first change of mass on the TG curve of 3 recorded in
air is found above 49 °C, see Fig. 3b. The observed mass
loss for this step is 6.69 % and is connected with an
endothermic
peak
on
a
DSC
curve
(88.23 0.45 kJ mol-1). Similar to the previously
described complex this stage is related to the dehydration
process. In this stage three molecules of water are lost
(calcd. value 6.48 %). The remaining molecule of water is
eliminated above 137 °C (found/calcd. mass losses: 1.88/
2.16 %). The anhydrous compound is very unstable and
immediately undergoes degradation into Co3O4 (found/-
calcd. overall mass losses: 80.90/80.74 %). Within this
stage on the TG curve around 236 °C there can be seen an
inflection point which corresponds to the loss of 9.11 %.
This can be associated mainly with the partial decompo-
sition of acetate anion (calcd. mass loss: 7.09, 14.17 and
21.25 % for a loss of 1, 2 and 3 acetate molecules,
respectively). Above 907 °C Co3O4 transforms into CoO.
Thermal behaviour of 3 in nitrogen begins, as in the air,
with the two-step dehydration. The first stage is recorded at
38–110 °C, the second one at 120–150 °C. On the FTIR
spectra of the evolved gaseous products up to 150 °C the
123

A. Bartyzel
wavenumber ranges 3750–3600, 3150–2750 and
1100–950 cm-1 [16, 17]. The beginning of the second
stage is also associated with the release of ammonia
molecules. The most characteristic, qualitative bands for
NH3 are already observed at 320 °C. The FTIR spectra of
the gas phase products indicate that further decomposition
of 4 is connected with release of H2O, CO2, CO, toluene,
benzene, benzonitrile and methane molecules as a conse-
quence of ligand defragmentation, see Table 6. The final
stable product has not been identified. It is formed at
975 °C, and this can be a mixture of nickel phenolates. The
residue left after decomposition is 44.13 %, while the
values calculated for the individual compounds are as
released in this stage. Further decomposition of 5 in
nitrogen is observed at a lower temperature (190 °C)
compared to decomposition in air (248 °C). On the basis of
the FTIR spectra analysis of the evolved gas phase, it was
found that in this stage part of acetic acid molecules and its
decomposition products (CO2 and H2O) are released. The
found mass loss is 1.13 %, and this is about 25 % of total
amount of acetic acid present in structure of 5. The next
stage is observed at 270 °C, and at the beginning it is
connected with further elimination of remaining parts of
acid. The degradation of the Schiff base is observed above
300 °C. The gaseous products, which are released and
recorded on the FTIR spectra during decomposition of
Schb, are as follows: CO2, H2O, NH3, benzonitrile, ben-
zene, phenol, CO and CH4. Most of these gases disappear
below *550 °C, and on the FTIR spectra of evolved
volatile pyrolysis products mainly bands of carbon oxide
(up to *700 °C), methane (up to *720 °C), water and
carbon dioxide were recorded (Table 6). Similarly to the
other complexes, thermal decomposition is incomplete
under measurement conditions; the found total mass loss at
1000 °C is 59.86 %. The zinc(II) complex is stable at room
temperature, Fig. 3e. In air atmosphere, the decomposition
process of 6 proceeds in one step and starts at 260 °C. The
final product is ZnO (found/calcd. overall mass loss: 86.19/
86.21 %). Thermal stability of 6 in nitrogen is similar to
that in air; i.e. the decomposition process starts at the same
temperature. The first stage begins with the release of
molecule of acetic acid which is the only degradation
product of the complex up to 330 °C, Fig. 4b. Above this
temperature on the FTIR spectra bands characteristic of
ammonia and associated with the thermal decomposition of
Schiff base can be observed. The beginning of the frag-
mentation of the Schiff base is also associated with the
release of carbon dioxide (340 °C), water (340 °C), carbon
monoxide (365 °C) and benzonitrile (365 °C). At the end
of this step (above 455 °C) in FTIR spectra the bands
characteristic of toluene and benzene are also recorded. On
the FTIR spectra of gaseous decomposition products of 6,
contrary to the other compounds, bands characteristic of
methane were not observed. However, it cannot be defi-
nitely stated that there is no methane in the evolved gas
because its vibrations may be overlapped by C6H6,
C6H5CN and C6H5CH3 bands.
follows:
20.19 %—NiCO3,
30.75 %—Ni(HCO3)2,
41.65 %—Ni(C6H5O)2 and 47.09 %—Ni(HOC6H4O)2.
The copper(II) complex has comparable thermal stabil-
ity as the other hydrates. The dehydration process starts
around 49 °C and occurs in one step during which all water
molecules are lost (found/calcd. mass loss: 6.12/6.06 %).
This process is connected with an endothermic effect
recorded on the DSC curve of 5, Fig. 5d. The enthalpy of
this process is equal to 237.64 0.42 kJ mol-1. Further
thermal decomposition takes place in two stages. The first
step is observed at 246–370 °C. At the beginning, this is
probably related to elimination of acetate ion from the
structure (calcd. mass loss: 4.96 %), but with the increasing
temperature, combustion and decomposition of the Schiff
base ligand are also observed (found mass loss: 13.98 %),
as evidenced by two maxima on the DTA curve. The
second stage starts at 373 °C and corresponds to a com-
plete degradation and combustion of the Schiff base ligand.
The final product CuO is formed at 537 °C (found/calcd.
total mass losses: 86.93/86.96 %). The first change in the
mass on the TG curve recorded in nitrogen is found above
37 °C, and similar to the thermal behaviour of 5 in air, it is
related to the dehydration process. The FTIR spectra of the
evolved gas phase confirm that water is the only product
5.50
4.50
μ
3.50
μ
Magnetic properties
2.50
The magnetic behaviours of paramagnetic complexes, i.e.
Mn(III), Co(II) and Cu(II), are shown in Fig. 5. Tempera-
ture-dependent molar susceptibility measurements of
powdered samples 2, 3 and 5 were taken in an applied
magnetic field of 0.1 T over the temperature range
1.8–300 K. The plot of reciprocal magnetic moment
1.50
0
50
100
150
200
250
300
T/K
Fig. 5 Temperature dependence of the magnetic moment for 2
(circle), 3 (square) and 5 (triangle)
123

Synthesis, thermal study and some properties of N2O4—donor Schiff base and its Mn(III)…
against temperature of 2 is shown in Fig. 5. At room tem-
peraturethe magnetic moment is 5.94 lB, and it is higher than
the expected value for manganese(III) (4.90 lB), but such
height values were observed before [60–62]. As the tem-
perature was reduced, the magnetic moment was roughly
constant ca. 180 K, then decreased slowly to 5.58 lB (70 K)
and after that drop abruptly reached the value 3.73 lB at
1.8 K. The decrease of leff at low temperature is due to the
zero-field splitting term of manganese(III) ion and/or an
intermolecular antiferromagnetic interactions [60–62]. The
vMT value keeps decreasing, while the temperature decrea-
ses from 300 to 1.8 K which indicates that a possible weak
antiferromagnetic interaction exists between Mn(III) ions.
The corresponding v versus T curve is Curie–Weiss like in
shape, with an inflection occurring at ca. 7 K. The Weiss and
Curie constants are h = -12 K and C = 4.60 cm3 -
K mol-1. The negative value of the Weiss constant h con-
firms the antiferromagnetic interactions between Mn(III)
ions [63]. The magnetic moment of 3 at room temperature is
4.55 lB (300 K). This value is characteristic of high-spin
complex of Co(II) and decreases smoothly up to 3.84 lB
(1.8 K). The magnetic moment value of 3 at room temper-
ature differs slightly from the spin-only moment which is
observed for tetrahedral symmetry. The octahedral com-
plexes typically have a moment of 4.7–5.2 lB [63]. The
vMT value decreases slowly with the decrease of temperature
from 300 to 1.8 K which indicates the existence of antifer-
romagnetic coupling. The negative value of Weiss constant h
(-5.9 K) confirms the antiferromagnetic interactions
between Co(II) ions [63]. The temperature dependence of the
magnetic moment of 5 is shown in Fig. 5. The leff value at
room temperature (300 K) is 1.90 lB, and it is a little higher
than the expected, spin-only value for uncoupled copper(II)
(1.73 lB) ions and is indicative of the presence of ferro-
magnetic intradimer interactions [63–65]. Moreover, as the
temperature was decreased, the magnetic moment increased
slowly to a maximum of 2.26 at 12 K and then decreased
slightly to 2.19 at 1.8 K which confirms weak ferromagnetic
coupling [64]. Compound 5 shows also behaviour typical of
ferromagnetic coupling, with the vMT versus T (K) profiles
which involves an increase in the vMT value as temperature is
lowered, followed by a rapid drop at low temperatures
(*7 K) [63, 65].
techniques. The ligands (1a and 1b) and complexes (2–6)
are stable at room temperature. The degradation of the
Schiff base occurs in consecutive reactions where probably
the destruction of azomethine double bonds firstly takes
place which is confirmed by the TG-FTIR analysis; the
ammonia bands appear as the first on the FTIR spectra of
evolved gases. In the case of complexes (2–5), the first step
is connected with the desolvation process. The TG-FTIR
analysis confirms the presence of water (for 2, 3 and 5) or
methanol (for 4) molecules in their structures. Next, the
acetate ions (2, 3 and 6) or acetic acid (5) is removed from
the structures of complexes which is confirmed by the
presence of acetic acid bands in the FTIR spectra of gas-
eous products. At the end the Schiff base undergoes
defragmentation and combustion (air) or pyrolysis (nitro-
gen). The heating in nitrogen resulted in evolution of: CO2,
CO, NH3, acetic acid, methane, benzene, acetonitrile and
toluene/phenol molecules as volatile products of Schiff
base ligand thermal decomposition. Variable temperature
magnetic studies of the paramagnetic complexes exhibit
the existence of antiferromagnetic (for 2 and 3) or ferro-
magnetic (for 5) interactions.
Open Access This article is distributed under the terms of the Creative
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commons.org/licenses/by/4.0/), which permits unrestricted use, distri-
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