Mn(II), Fe(II), Co(II), Ni(II), Cu(II) and Zn(II) transition metals isonicotinate complexes: Thermal behavior in N2 and air atmospheres and spectroscopic characterization

Article abstract of DOI:10.1016/j.tca.2018.06.010

Solid-state M(IN)₂·nH₂O complexes, where M stands for bivalent transition metals (Mn, Fe, Co, Ni, Cu and Zn), IN is isonicotinate and n = 0.5 to 4.0 H₂O, were synthesized. Characterization and thermal behavior of the compounds were performed employing elemental analysis (EA), complexometric titration with EDTA, powder X-ray diffraction (PXRD), infrared spectroscopy (FTIR), simultaneous thermogravimetry and differential scanning calorimetry (TG–DSC) in dynamic dry air and nitrogen atmospheres, differential scanning calorimetry (DSC) and TG–DSC coupled to FTIR. The thermal behavior of isonicotinic acid and its sodium salt was also investigated in both atmospheres. The dehydration of these compounds occurs in a single step in both atmospheres. In air atmosphere, the thermal decomposition of the anhydrous compounds also occurs in a single step, except for the copper compound where two steps are observed. In N₂ the thermal decomposition of the anhydrous compounds occurs in two consecutive steps, except for iron compound, where three steps are observed. The main gaseous products of thermal decomposition/pyrolysis of the compounds were identified as CO, CO₂ and Pyridine. Mn, Co, Cu and Zn compounds show a physical transformation process in DSC curves. The ligand coordinates through the pyridine nitrogen atom to the metal and for the Zn compound, the carboxylate group also participates in the coordination. The IR absorption profile of hydrated and dehydrated compounds suggest that there is a probable change in the coordination mode of the ligand upon dehydration. This change needs to be further investigated, once it is not possible to ensure only with infrared spectroscopy data.

Full text of DOI:10.1016/j.tca.2018.06.010

Accepted Manuscript
Title: Mn(II), Fe(II), Co(II), Ni(II), Cu(II) and Zn(II)
transition metals isonicotinate complexes: Thermal behavior
in N and air atmospheres and spectroscopic characterization
2
Authors: Wilhan Donizete Gon c¸ alves Nunes, Jos e´ Augusto
Teixeira, Bruno Ekawa, Andr e´ Luiz Carneiro Soares do
Nascimento, Massao Ionashiro, Fl a´ vio Caires Junior
PII:
DOI:
Reference:
S0040-6031(18)30408-8
https://doi.org/10.1016/j.tca.2018.06.010
TCA 78023
To appear in:
Thermochimica Acta
Received date:
Revised date:
Accepted date:
16-2-2018
16-6-2018
19-6-2018
Please cite this article as: Nunes WDG, Teixeira JA, Ekawa B, do Nascimento
ALCS, Ionashiro M, Caires F, Mn(II), Fe(II), Co(II), Ni(II), Cu(II) and
Zn(II) transition metals isonicotinate complexes: Thermal behavior in N and
2
air atmospheres and spectroscopic characterization, Thermochimica Acta (2018),
https://doi.org/10.1016/j.tca.2018.06.010
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Mn(II), Fe(II), Co(II), Ni(II), Cu(II) and Zn(II) transition metals isonicotinate
complexes: Thermal behavior in N
characterization
2
and air atmospheres and spectroscopic
a
a
a
Wilhan Donizete Gonçalves Nunes , José Augusto Teixeira , Bruno Ekawa , André Luiz
a
a
a,b*
Carneiro Soares do Nascimento , Massao Ionashiro , Flávio Junior Caires
.
aUniversidade Estadual Paulista (Unesp), Instituto de Química, Araraquara, CP 355, 14801-970
Araraquara, SP, Brazil
b
Universidade Estadual Paulista (Unesp), Faculdade de Ciências, Bauru, 17033-260 Bauru, SP, Brazil
*
Corresponding author.
Tel.: +55 16 3301 9617; fax: +55 16 3322 7932.
E-mail address: flavio.caires@unesp.br (F. J. Caires)
Highlights


The stoichiometry and hydration degree of compounds were determined.
Previously unreported thermal behavior of the complexes has been
determined.


Through TG/DSC-FTIR thermal decomposition mechanism was proposed.
Physical transformations were investigated by PXRD and IR
spectroscopy.

Water, CO, CO
2
and Pyridine gases were identified by EGA
.
Abstract
Solid-state M(IN)
metals (Mn, Fe, Co, Ni, Cu and Zn), IN is isonicotinate and n = 0.5 to 4.0 H
2
·nH
2
O complexes, where M stands for bivalent transition
O, were
2
synthesized. Characterization and thermal behavior of the compounds were performed
employing elemental analysis (EA), complexometric titration with EDTA, powder X-ray
diffraction (PXRD), infrared spectroscopy (FTIR), simultaneous thermogravimetry and
differential scanning calorimetry (TG–DSC) in dynamic dry air and nitrogen
atmospheres, differential scanning calorimetry (DSC) and TG–DSC coupled to FTIR.
The thermal behavior of isonicotinic acid and its sodium salt was also investigated in both
atmospheres. The dehydration of these compounds occurs in a single step in both
atmospheres. In air atmosphere, the thermal decomposition of the anhydrous compounds
also occurs in a single step, except for the copper compound where two steps are
observed. In N
2
the thermal decomposition of the anhydrous compounds occurs in two
1

consecutive steps, except for iron compound, where three steps are observed. The main
gaseous products of thermal decomposition/pyrolysis of the compounds were identified
as CO, CO and Pyridine. Mn, Co, Cu and Zn compounds show a physical transformation
2
process in DSC curves. The ligand coordinates through the pyridine nitrogen atom to the
metal and for the Zn compound, the carboxylate group also participates in the
coordination. The IR absorption profile of hydrated and dehydrated compounds suggest
that there is a probable change in the coordination mode of the ligand upon dehydration.
This change needs to be further investigated, once it is not possible to ensure only with
infrared spectroscopy data.
Keywords: Isonicotinate, thermal behavior, bivalent transition metals, pyrolysis,
evolved gas analysis.
1
. Introduction
A literature survey of coordination compounds with organic ligands results in a wide
variety of papers describing applications for this type of material. The investigations
focus mainly in synthesize and characterize functional materials for gas storage, catalysis,
drug delivery, luminescent materials, molecular sieves, improved biological activity of
pharmaceuticals, etc. When it comes to the ligand isonicotinate (4-pyridinecarboxilate) it
is very interesting for obtaining such materials due to its ability to coordinate by two sites
(pyridine nitrogen atom, and carboxylate group), forming polymeric porous materials [1–
5
].
The formation of complexes between bivalent transition metals and isonicotinate
ligand was also already described as a possible method for gravimetric determination of
these elements [6]. In such application, knowing its stoichiometry and thermal stability is
crucial, once the precipitate in such method needs to be dried at elevated temperatures.
Concerning the structure of some complexes, depending on the synthetic
conditions employed the isonicotinate ligand can coordinate by both the carboxylate
group and pyridyl N atom (under solvothermal conditions [7]) or just by the pyridyl N
atom (in aqueous solution) [8].
Besides the already mentioned applications, the study of the thermal behavior of metal
complexes can lead to the identification of stable intermediates during the thermal
decomposition. These intermediates can be interesting in some applications, and therefore
obtaining it by thermal methods could be an alternative way for producing compound that
sometimes would be difficult to obtain by other routes. Some studies described the
thermal behavior of isonicotinate complexes and proposed a degradation mechanism in
nitrogen atmosphere [9,10]
2

Although the literature on the isonicotinate coordination compounds shows a
variety of approaches for the compound’s characterization, there is a lack of an extensive
study on the thermal behavior of isonicotinate transition metal complexes in different
atmosphere conditions. So, the focus of the current research was to investigate the thermal
behavior (employing TG-DSC, DSC and EGA/TG-FTIR) of some bivalent transition
metals isonicotinate complexes in dynamic dry air and nitrogen atmospheres to determine
its minimal formula, thermal stability, and gaseous products evolved during oxidation
(air) and pyrolysis (N
2
). Also, a spectroscopic investigation by FTIR and PXRD were
conducted to obtain structural information.
2
. Experimental
Isonicotinic acid with 99% purity was obtained from Aldrich and used as received.
–
1
Aqueous solution of sodium isonicotinate 0.1 mol L was prepared by neutralization of
–
1
an aqueous solution of isonicotinic acid with sodium hydroxide solution 0.1 mol L .
–
1
Aqueous 0.1 mol L solutions of bivalent transition metal ions were prepared by
dissolving the correspondent chloride [Mn(II), Co(II), Ni(II)] or sulphate [Fe(II), Cu(II),
Zn(II)].
The solid-state compounds were obtained by adding slowly with stirring 100 mL
–
1
of sodium isonicotinate 0.1 mol L to 50 mL of the respective metal ions solutions 0.1
–
1
mol L heated up to near ebullition. The system was cooled down to room temperature
–
and the precipitates were filtered off, washed with distilled water until chloride (Cl ) or
2–) ions were eliminated (tested with AgNO
/HNO
solution for Cl ions or
–
sulphate (SO
4
3
3
2
–
BaCl
2
for SO
4
ions) dried at 50 °C in a forced circulation air oven for 12 h and kept in
a desiccator over anhydrous calcium chloride. To avoid the oxidation of Fe(II) to Fe(III),
all the solutions, as well as the water used for washing the precipitate, were purged with
nitrogen gas, even during the heating to near ebullition and cooling to room temperature.
For the compounds in the solid state the metal, hydration water and isonicotinate
contents were determined from TG curves obtained in dry air atmosphere. The metal ions
content were also determined by complexometry with standard EDTA solution after
calcinating the compounds in a furnace to obtain the respective oxides and dissolving in
hydrochloric acid solution [11].
Carbon, hydrogen and nitrogen contents were determined by microanalytical
procedures with a CHN elemental analyzer from Perkin Elmer, model 2400 and from TG
3

curves (dry air atmosphere), since the ligand lost in the thermal decomposition occurs
with the formation of the respective oxides with known stoichiometry as final residue.
Simultaneous TG–DSC curves were obtained in a Mettler Toledo thermal
analysis system, model TG–DSC 1. As purge gas, dynamic dry air or nitrogen (99%
–
1
purity), with a flow rate of 50 ml min in both cases, were used. When nitrogen
atmosphere was employed, the system was purged for 20 min before starting the analysis.
–
1
The heating rate was set as 10 °C min , with samples weighting about 10 mg. As sample
support, 150 L alumina (–Al ) crucibles were used.
2
O
3
The evolved gas analysis (EGA/TG–FTIR) experiments were performed coupling
the thermogravimetric analyzer to the Nicolet FTIR spectrophotometer equipped with a
gas cell, using DTGS detector with KBr window. The furnace outlet was couplet to the
heated (at 250 °C) spectrophotometer gas cell were coupled through a heated (at 225 °C)
1
20 cm stainless steel transfer line with diameter of 3 mm, both purged dry air and N
2
(50
–
1
mL min ). The FTIR spectra were recorded with 16 scans per spectrum at a resolution
–
1
of 4 cm .
The DSC experiments were performed in a TA Instruments model Q10
differential scanning calorimeter. Dry air or nitrogen was employed as purge gas, both at
–
1
–1
a flow rate of 50 mL min . The heating rate of 10 °C min and sample mass of about 2
mg were adopted. Aluminum crucible with perforated cover was used as sample support.
The attenuated total reflectance (ATR) infrared spectra for the compounds were
run in a Nicolet iS10 FTIR spectrophotometer, using an ATR accessory with Ge window.
The spectrophotometer was equipped with a DTGS detector. Each spectrum was acquired
–
1
with 32 scans and resolution set to 4 cm .
The powder X-ray diffractograms (PXRD) were obtained using a Siemens X-ray
diffractometer, model D-5000, employing CuK radiation ( = 1.541 Å), operating
voltage of 40 kV, current of 20 mA. Glass sample supports were used, and the samples
were exposed to the radiation with incidence angles (2θ) swept from 5º to 80º, at a step
of 0,02º with collection time of 2 s.
3
. Results and Discussion
.1. TG-DSC of Isonicotinic acid and Sodium Isonicotinate
The TG–DSC curves of isonicotinic acid (HIN) in dynamic dry air and nitrogen
3
atmospheres are shown in Fig. 1. In both conditions, the TG–DSC curves show total mass
4

loss in a single step between 165 – 260 °C, attributed to sublimation of the compound. It
was confirmed by heating the isonicotinic acid in a glass tube up to 260 °C. A white solid,
deposited on the glass wall, were collected, and by IR analysis it was identified as
isonicotinic acid.
Insert Fig. 1
For sodium isonicotinate (NaIN), the TG–DSC curves in air and N atmospheres
2
are also shown in Fig. 1. These curves show mass losses in three consecutive steps, in
both atmospheres, with thermal events corresponding to these losses, even though in some
cases thermal events cannot be observed. The first mass loss between 50 – 95 °C (air, N
with mass loss of 2.45% and 2.29%, respectively, is attributed to dehydration with loss
of 0.2 H O (Δmcalcd. = 2.42%). No thermal event corresponding to the dehydration is
2
)
2
observed in the TG curve in both atmospheres. This is attributed to the fact that the
dehydration occurs so slowly that a low heat flow rate is involved in this step, therefore
it does not produce a noticeable thermal event.
The anhydrous compound is stable up to 430 °C in both atmospheres. Above this
temperature the thermal decomposition (air) or pyrolysis (N
steps between 430 – 530 °C and 530 – 790 °C (air) or 430 – 520 °C and 520 – >1000 °C
). In the first step the endo and exothermic peaks at 445 °C and 470 °C (air) or
endothermic peaks at 450 and 460 °C (N ), with mass loss of 43.62% (air) and 44.32%
) are attributed to the thermal decomposition and oxidation of the organic matter
and/or of the gaseous products evolved during thermal decomposition (air) and pyrolysis
) with formation of sodium carbonate and carbonaceous residue (air) and
carbonaceous residue (N ). The last step with mass loss of 15.37% (air) and 22.40% with
mass loss still being observed (N ) is attributed to oxidation and pyrolysis of the
2
) occur in two consecutive
(N
2
2
(N
2
(N
2
2
2
carbonaceous residue, respectively. In the last step, no thermal event is also observed,
probably because it occurs slowly, and the heat involved is not sufficient to produce
thermal event, as mentioned for the first step (dehydration).
3
.2. Analytical results for isonicotinate metal complexes
For the synthesized compounds the analytical and thermoanalytical (TG) results
are presented in Table 1. From these results the stoichiometry of the compounds was
established, which agree with the general formula M(IN) ·nH O, where M stands for
2
2
5

bivalent transition metals, IN is isonicotinate and n = 3 (Mn), 3.5 (Fe, Cu) and 4 (Co, Ni,
Zn).
Insert Table 1
3
.3. TG-DSC of isonicotinate metal complexes
Simultaneous TG–DSC curves in dynamic dry air and N atmospheres for the
2
synthesized compounds are shown in Fig. 2 a-f, and a*-f*, respectively. These curves
show mass losses in two or three steps (air), and three or four steps (N
events corresponding to these losses or due to physical phenomenon.
Insert Fig. 2
2
), and thermal
From these curves it was possible to conclude that the thermal stability of the
hydrated compounds (I) depends only on the nature of the metal ion. Differently, the
stability of the anhydrous compounds (II) and the final temperature of thermal
decomposition (III) depend on the nature of the metal ion and of the purge gas used:
(
(
(
I) Air and N
Zn (40 °C)
II) Air: Co = Zn (365 °C) > Mn (340 °C) > Ni (305 °C) > Cu = Fe (260°C)
: Mn (380 °C) > Co (370 °C) > Zn (365 °C) > Ni (295 °C) > Fe = Cu (255 °C)
III)Air: Zn (460 °C) > Mn (450 °C) > Co (430 °C) > Ni (410 °C) > Fe (400 °C) > Cu
375 °C)
: Mn (>1000 °C) > Zn (900°C) > Ni (890°C) > Cu (810 °C) > Co (765°C) > Fe
720 °C)
As the thermal behavior of the compounds depends on the nature of the metal ion
2
: Ni (125 °C) > Co (110 °C) > Fe (100 °C) > Cu (90 °C) > Mn (80 °C) >
N
2
(
N
2
(
and atmosphere used, the features of each compound are discussed individually and for
each atmosphere separately.
3
.3.1. TG–DSC in air atmosphere
Manganese compound
The simultaneous TG–DSC curves for manganese compound are shown in Fig.
a. The first mass loss between 80 and 190 °C, corresponding to an endothermic peak at
20 °C is due to dehydration with loss of 3 H
2
1
2
O (mcalcd. = 15.30%; mTG = 15.26%).
The anhydrous compound is stable up to 340 °C and above this temperature the thermal
decomposition occurs in a single step between 340 and 450 °C with mass loss of 63.43%,
corresponding to a large exothermic peak attributed to oxidation of the organic matter
6

and/or of the gaseous products evolved during the thermal decomposition, with the
formation of Mn as final residue. The small mass gain (mTG = 0.48%), between 450
and 470 °C is attributed to partial oxidation reaction of Mn to Mn and the mass
loss between 910 and 930 °C (mTG = 0.82%) corresponding to an endothermic peak at
15 °C is attributed to the reduction reaction of Mn to Mn [12]. The endothermic
3
O
4
3
O
4
2
O
3
9
2
O
3
3
O
4
peak at 250 °C without mass loss in the TG curve is attributed to phase transition process
and is further discussed in the PXRD section.
Iron Compound
The simultaneous TG–DSC curves for iron compound are shown in Fig. 2b. These
curves show mass losses in two consecutive steps (TG) although the DSC curve suggests
that the second step occurs through two overlapping ones. The first mass loss between
1
00 and 160 °C, corresponding to an endothermic peak at 145 °C is attributed to
dehydration with loss of 3.5 H O (mcalcd. = 17.37%; mTG = 17.35%). The anhydrous
2
compound is stable up to 260 °C and above this temperature the thermal decomposition
occurs between 260 and 400 °C with loss of 60.73%, corresponding to exothermic peaks
at 365 and 390 °C attributed to oxidation of Fe(II) to Fe(III) and oxidation of the organic
matter and/or of the gaseous products evolved during the thermal decomposition with
formation of Fe
2
3
O as final residue (mcalcd. = 78.01%; mTG = 78.08%).
Cobalt Compound
The simultaneous TG–DSC curves for cobalt compound are shown in Fig. 2c.
These curves show mass losses in three steps and thermal events corresponding to these
losses or due to physical phenomenon. The first mass loss between 110 and 174 °C,
corresponding to an endothermic peak at 155 °C is attributed to dehydration with loss of
4
H
2
O (mcalcd. = 19.21%; mTG = 19.36%). The anhydrous compound is stable up to 365
C and above this temperature the thermal decomposition occurs in a single step between
65 and 430 °C, with loss 59.35%, corresponding to a large exothermic peak at 425 °C
°
3
attributed to oxidation of the organic matter and/or of the gaseous products evolved during
the thermal decomposition. The total mass loss up to 430 °C agrees with the formation of
Co
3
O
4
as final residue (mcalcd. = 78.61%; mTG = 78.71%). The last mass loss of 1.39%,
between 890 and 910 °C corresponding to an endothermic peak at 900 °C, is due to
7

reduction of Co
3
4
O to CoO and in agreement with the literature [12]. The exothermic peak
at 260 °C, without mass loss in the TG curve is attributed to phase transition.
Nickel Compound
The simultaneous TG–DSC curves for nickel compound are shown in Fig. 2d. The
first mass loss between 125 – 185 °C corresponding to an endothermic peak at 170 °C is
attributed to dehydration with loss of 4 H O (mcalcd. = 19.22%; mTG = 19.37%). The
2
anhydrous compound is stable up to 305 °C and above this temperature the thermal
decomposition occurs in a single step between 305 and 410 °C, with loss of 63%,
corresponding to an exotherm with two peaks at 390 and 400 °C attributed to oxidation
of the organic matter and/or the gaseous products evolved during the thermal
decomposition with the formation of a mixture of Ni° to NiO. At higher temperatures
there is oxidation of Ni° to NiO, and the mass loss up to 800 °C agrees with the formation
of NiO as final residue (mcalcd. = 80.10%; mTG = 79.95%).
Copper Compound
The simultaneous TG–DSC curves for copper compound are shown in Fig. 2e.
These curves show mass losses in three consecutive steps and thermal events
corresponding to these losses or due to physical phenomenon. The first mass loss between
9
0 and 160 °C, corresponding to an endothermic peak at 145 °C is due to dehydration
with loss of 3.5 H O (mcalcd. = 17.01%; mTG = 17.22%). The anhydrous compound is
2
stable up to 260 °C and above this temperature the thermal decomposition occurs in two
consecutive steps between 260 – 375 °C, with losses of 46.03% and 15.34%,
corresponding to an exothermic peak at 350 °C, respectively, attributed to the oxidation
of organic matter and/or of the gaseous products evolved during the thermal
decomposition. The total mass loss up to 375 °C agrees with the formation of CuO as
final residue (mcalcd. = 78.55%; mTG = 78.59%). The exothermic peak at 245 °C,
without mass loss in the TG curve is attributed to phase transition.
Zinc Compound
The simultaneous TG–DSC curves for zinc compound are shown in Fig. 2f. The
first mass loss between 40 and 140 °C, without thermal event in the DSC curve is
attributed to dehydration with loss of 0.5 H
2
O (mcalcd. = 2.83%; mTG = 3.01%). No
8

thermal event due to dehydration is observed, probably because the small mass loss
occurs slowly, and the heat involved is not sufficient to produce the endothermic peak.
The anhydrous compound is stable up to 365 °C and between 365 – 460 °C the thermal
decomposition occurs in a single step with loss of 71.21%, corresponding to a large and
sharp exothermic peak at 450 °C attributed to the oxidation of organic matter and/or of
the gaseous products evolved during the thermal decomposition. The total mass loss up
to 460 °C agrees with the formation of zinc oxide, ZnO, as final residue (mcalcd.
4.46%; mTG = 74.22%). The small exothermic peak at 240 °C, without mass loss in the
TG curve is attributed to phase transition process.
=
7
3
.3.2. TG–DSC in N
2
atmosphere
atmospheres are shown in Fig. 2 a*–f*. These curves
The TG–DSC curves in N
2
show mass losses in three steps, (except for iron compound where four steps are observed)
and endothermic peaks attributed to dehydration and thermal decomposition, exothermic
peak attributed to physical phenomenon or without thermal event. A thermal
decomposition mechanism was proposed for each compound based on mass losses and
the gaseous evolved products identified by TG coupled to FTIR (see next section).
Manganese compound
The TG–DSC curves of the compound are shown in Fig. 2a*. These curves show
mass losses in three consecutive steps, with mass loss still being observed up to 1000 °C.
the first mass loss between 75 and 140 °C corresponding to an endothermic peak at 125
°
C is attributed to dehydration with loss of 3 H
4.68%).
2
O (mcalcd. = 15.30% (Eq. 1a); mTG =
1
The anhydrous compound is stable up to 380 °C and above this temperature the
mass losses occur in two consecutive steps of loss mass between 380 – 1000 °C. The first
step between 280 – 460 °C (fast process), corresponding to the endothermic peak at
4
35 °C, is attributed to the thermal decomposition of the compound leading to the
formation of Mn and carbonized residue, with release of CO2, pyridine (C N) (both
identified by EGA), N and H (not detectable by IR), according to the Eq. 1b (mcalcd.
7.05% (Eq. 1b), mTG = 45.04%).
2
O
3
5
H
5
2
2
=
9
4

The last step of mass loss between 460 – >990 °C is attributed to the slow
oxidation of the carbonized material formed in the previous step (mcalcd. = 19.83% (Eq.
1
c), mTG = 18.88%). The total mass loss up to 1000 °C suggests the formation of MnO
and a small amount of remaining carbonaceous material (mcalcd. MnO = 79.91%, mcalcd.
MnO + 0.75C = 77.37%); mTG = 78.60%)
,
as final residue, with release of CO.
Mn(IN)
2
·3H
Mn(IN) (s) ⟶ 2Mn
Mn (s) + 13C(s) + 4O
Residual O in the N purge gas
2
O (s) ⟶ Mn(IN)
2
(s) + 3H
2
O (g)
(Eq. 1a)
4
2
*
2
2 3
O (s) + 5CO
2
(g) + 5C
5
H
5
N(g) + 13C(s) + 3/2N
2
(g) + 7/2H
2
(g) (Eq. 1b)
O
2 3
2
(g)* ⟶ 4MnO(s) + 3C(s) + 10CO(g)
(Eq. 1c)
2
2
Iron compound
The TG–DSC curves of the compound are shown in Fig. 2b*. These curves show
mass losses in four steps between 100 and 720 °C and mass gain between 720 – > 1000
°
C. The first mass loss between 100 and 160 °C, corresponding to an endothermic peak
at 140 °C is attributed to dehydration with loss of 3.5 H O (mcalcd. = 17.37% (Eq. 2a);
TG = 17.60%).
The anhydrous compound is stable up to 255 °C and above this temperature the
2

m
mass losses occur in three consecutive steps, being the first two overlapping ones,
between 55 – 370 °C (mTG = 8.57%) and 370 – 455 °C (mTG = 38.80%), corresponding
to endothermic peaks at 360 °C and 440 °C. These two steps suggest a decomposition
mechanism with release of CO2, (C
detectable in IR) (mcalcd..= 49.89% (Eq. 2b), mTG = 47.37%) resulting in the formation
of Fe and carbonized organic matter, as evidenced by visual inspection and PXRD of
5
H
5
N) (both identified by EGA), N
2
and H (not
2
2
O
3
the sample heated to 455 ° C (see Fig. S1 in supplementary material), and is in
disagreement with that reported in the literature[13].
The third mass loss step between 455 - 720 °C, associated with the endothermic
peak at 640 °C, is attributed to the oxidation-reduction reaction between the iron oxide
0
and the carbonized material with formation of Fe and CO. (mcalcd. = 15.43 (Eq. 2c),
0
m
TG = 16.50%). The total mass loss up to 720 °C agrees with the formation of Fe and
0
0
a small amount of C (mcalcd. Fe = 84.62%, mcalcd. Fe + 1.25C = 80.49% (Eq. 2c), mTG
=
82.78%). The mass gain above 720 °C up to 1000 °C is attributed to oxidation of the
0
Fe to FeO (mcalcd. = 4.41% (Eq. 2d), mTG = 5.05%) [14,15].
1
0

Fe(IN)
2
·3.5H
Fe(IN) (s) ⟶ 2Fe
(s) + 13C(s) + O
(g)* ⟶ 2FeO(s)
in the N purge gas.
2
O(s) ⟶ Fe(IN)
2
(s) + 3.5H
2
O(g)
(g) + 6C
(Eq. 2a)
(Eq. 2b)
(Eq. 2c)
(Eq. 2d)
4
2
2
*
2
2 3
O (s) + 13C(s) + 5CO
2
5
H
5
N(g) + N
2
(g) + H (g)
2
0
Fe
2
O
3
2
(g)* ⟶ 4Fe (s) + 5C(s) + 8CO(g)
0
Fe (s) + O
2
Residual O
2
2
Cobalt compound
The TG–DSC curves of the compound are shown in Fig. 2c*. The first mass loss
between 105 and 175 °C, corresponding to an endothermic peak at 155 °C is attributed to
dehydration with loss of 4 H O (mcalcd. = 19.21% (Eq. 3a); mTG = 19.29%).
2
The anhydrous compound is stable up to 370 °C and above this temperature the
mass losses occur in two consecutive steps between 370 - 765 °C. The first step of mass
loss between 370 - 450, associated to the endothermic peak at 440 ° C in the DSC curve,
is attributed to the thermal decomposition of the compound, with release of CO2, (C
5
5
H N)
(
both identified by EGA), N and H (not detectable in IR), leading to the formation of
2
2
CoO and carbonized material (mcalcd. = 51.22% (Eq. 3b), mTG = 48.66%). The second
step of mass loss, without thermal events in the DSC curve, is attributed to the slow
oxidation-reduction reaction between CoO and carbonized material with the formation of
0
Co and CO (mcalcd. = 14.93% (Eq. 3c), mTG = 15.49%). The total mass loss up to 765
0
0
°
C, suggest the formation of a mixture of Co and carbonaceous residue (mcalcd. Co =
8
7
4.29%, mcalcd. Co + 1.5C = 79.49%, mTG = 83.45%). The mass gain observed between
0
65 – 1000 °C is attributed to the oxidation of the Co to CoO (mCalcd. = 4.26% (Eq. 3d),
mTG = 3.07%) [14,15].
Co(IN)
2
·4H
2
O(s) ⟶ Co(IN)
2
(s) + 4 H
2
O(g)
(Eq. 3a)
(g) (Eq. 3b)
(Eq. 3c)
4
4
4
*
Co(IN)
2
(s) ⟶ 4CoO(s)·+ 12C(s) + 6CO
2
(g) + 6C
5
H
5
N(g) + N
2
(g) + H
2
0
CoO(s) + 12C(s) + 2O
2
*(g) ⟶ 4Co (s) + 6C(s) + 8CO(s)
0
Co (s) + 2O
2
*(g) ⟶ 4CoO (s)
(Eq. 3d)
Residual O
2
in the N purge gas.
2
Nickel compound
The TG–DSC curves of the compound are shown in Fig. 2d*. The first mass loss
between 115 and 190 °C, corresponding to an endothermic peak at 170 °C is attributed to
dehydration with loss of 4 H O (mcalcd. = 19.22% (Eq. 4a); mTG = 19.38%).
2
1
1

The anhydrous compound is stable up to 295 °C and above this temperature the
mass losses occur in two consecutive steps, being the first a fast process and the second
a slow one. The first step between 295 and 400 °C corresponding to endothermic peaks
at 332 °C and 368 °C suggesting overlapping steps, with mass loss of 49.26% (mcalcd.
1.25% (Eq. 13)) is attributed to thermal decomposition, with release of CO2, N,
both identified by EGA), N and H (not detectable in IR). The second step between 400
=
5
C
5
H
5
(
2
2
and 890 °C, corresponding to a small (480 °C) and an indicium (640 °C) of endothermic
peaks in the DSC curve, is attributed to the oxidation-reduction reaction between the
nickel oxide and the carbonized material leading to the formation of Ni° and CO (mcalcd.
=
14.94% (Eq. 4c), mTG = 14.51%). The total mass loss up to 890 °C also suggest the
0
formation of a mixture of Ni and carbonaceous residue, in no simple stoichiometric
0
relation (mcalcd. Ni = 84.34%, mcalcd. Ni + C = 81.68%, mTG = 83.42%), as confirmed
by XRD and visual inspection. The small mass gain (mTG = 1.27%) above 890 °C is
0
attributed to the partial oxidation of the Ni in the mixture to NiO[14,15].
Ni(IN)
2
·4H
2
O(s) ⟶ Ni(IN)
2
(s) + 4 H
2
O(g)
(Eq. 4a)
(Eq. 4b)
(Eq. 4c)
(Eq. 4d)
4
4
4
*
Ni(IN)
2
(s) ⟶ 4NiO(s) + 12 C(s) + 6 CO
2
(g) + 6 C
5
H
5
N(g) + N
2
(g) + H (g)
2
0
NiO(s) + 12C(s) + 2O
2
(g)*  4Ni (s) + 4C(s) + 8CO(g)
0
Ni (s) + xO
2
(g)*  (4 – 2x)Ni (s) + (2x)NiO(s)
Residual O
2
in the N purge gas
2
Copper compound
The TG–DSC curves are shown in Fig. 2e*. The first mass loss between 80 and
1
60 °C, corresponding to an endothermic peak at 145 °C, is attributed to dehydration with
loss of 3.5 H
2
O (mcalcd. = 17.22% (Eq. 5a); mTG = 17.68%).
The anhydrous compound is stable up to 255 °C and above this temperature the
mass losses occur in two consecutive steps, being the first a fast process, followed by a
slow one. The first step between 255 and 290 °C with loss of 51.24% (mcalcd. = 51.82%
(
Eq. 5b)), corresponding to an endothermic peak at 285 °C in the DSC curve, is attributed
to thermal decomposition of the compound, with release of CO2, N, (both identified
by EGA), N and H (not detectable by IR).
C
5
H
5
2
2
The second step between 290 and 810 °C, without thermal event in the DSC curve,
is attributed to the oxidation-reduction reaction between the CuO and the carbonized
0
material leading to the formation of Cu and CO (mcalcd. = 13.22% (Eq. 5c), mTG
=
1
2

1
2.93%). The total mass loss up to 810 °C agrees with the formation of a mixture of Cu0
0
and carbonaceous residue, in no simple stoichiometric relation (mcalcd. Cu = 82.86%,
0

m
m
calcd. Cu + C = 79.62%, mTG = 81.61%). The mass gain (mcalcd. = 2.16% (Eq. 5d);

TG = 1.53%) above 810 °C and up to 1000 °C is attributed to oxidation reaction of the
0
Cu to Cu
2
O.
Cu(IN) ·3.5H
2
2
O(s) ⟶ Cu(IN)
2
(s) + 3.5 H
2
O(g)
(g) + 6C
(Eq. 5a)
(g) (Eq. 5b)
(Eq. 5c)
4
4
4
*
Cu(IN)
2
(s) ⟶ 4CuO(s) + 12C(s) + 6CO
2
5
H
5
N(g) + N
2
(g) + H
2
0
CuO(s) + 12 C(s) + 3/2O
2
(g) ⟶ 4Cu (s) + 5C(s) + 7CO(g)
0
*
Cu (s) + 3/2 O
2
(g) ⟶ 2 Cu
2
O(s)
(Eq. 5d)
Residual O in the N
2
2
purge gas
Zinc compound
The TG–DSC curves of the zinc compound are shown in Fig. 2f*. The first mass
loss between 40 and 140 °C, without thermal event in the DSC curve is attributed to
dehydration with loss of 0.5 H O (mcalcd. = 2.83%; mTG = 2.28%):
2
The anhydrous compound is stable up to 365 °C and above this temperature the
mass losses occur in two consecutive steps. The first step between 365 and 658 °C occurs
through a fast process with mass loss of 59.09% (mcalcd. = 60.32% (Eq. 19)),
corresponding to an endothermic peak at 445 °C, is attributed to thermal decomposition
of the compound, with release of CO2,
C
5
H
5
N (both identified by EGA), N
2
and H (not
2
detectable by IR). The second step occurs slowly between 400 and 900 °C, associated
with endothermic peaks at 760 °C and 845 °C, is attributed to the oxidation-reduction
reaction between the ZnO and the carbonized material leading to the formation of Zn0
0
and carbon monoxide (mcalcd. Zn = 17.58% (Eq. 6c), mTG = 18.53%). The last
observed endothermic peak at 945 °C can be attributed to the slow evaporation of Zn0
[
14–17].
Zn(IN) ·0.5H
2
2
O (s) ⟶ Zn(IN)
2
(s) + 0.5H
2
O (g)
(g) + 6C
(Eq. 6a)
(Eq. 6b)
(Eq. 6c)
4
4
Zn(IN)
2
(s) ⟶ 4ZnO(s) + 12C(s) + 6CO
2
5
H
5
N(g) + N
2
(g) + H (g)
2
0
ZnO(s) + 12C(s) ⟶ 4Zn (s) + CO(g)
The exothermic peaks at 255 °C (Mn), 275 °C (Fe), 260 °C (Co), 235 °C (Cu) and
40 °C (Zn), without mass loss in the TG curve is attributed to a phase transition, which
2
was further investigated by means of X-ray diffractometry.
1
3

4
. Evolved Gas Analysis (TG–FTIR)
The gaseous products evolved during the thermal decomposition of the compounds
studied in this work, in both atmospheres, were monitored by FTIR and are presented in
Table 2. In both atmospheres the same gaseous species are produced. However, in air
atmosphere pyridine is detected in lower amounts, probably due to oxidation in the
furnace and/or heated transfer line during the diffusion to FTIR gas cell. The water vapor
was omitted in Table 2, but it was identified in all analyzed compounds due to the
dehydration step in TG curve (except Zn), and due to oxidation of organic material from
ligand. Even in oxidative conditions (air atmosphere) CO could be detected for Ni
compound in sensible amounts, which is usual in EGA experiments [18,19]
Insert Table 2
FTIR spectra of the gaseous products evolved for manganese compound in nitrogen
atmosphere was chosen as representative for the series of compounds. A contour map plot
of absorbance versus time of analysis is shown in Fig. 3. The dashed red squares highlight
the regions where characteristic absorptions for each of the gaseous species identified
occur. Reference spectra [20] are also presented in Fig. 3 to facilitate comparison.
Individual spectra of evolved gases in both atmospheres for each compound are available
in supplementary material.
Insert Fig. 3
5
. Differential Scanning Calorimetry
The DSC curves of the compounds in dynamic dry air atmosphere are shown in
Fig. 4. The endothermic peak at 125 °C (Mn), 147 °C (Fe), 154 °C (Co) 170 °C (Ni) and
1
1
42 °C (Cu) is attributed to dehydration and the calculated dehydration enthalpy was
–
1
59.3, 174.6, 268.6, 268.8 and 224.4 kJ mol , respectively. The exothermic peak at 251
°
C (Mn), 256 °C (Co), 244 °C (Cu) and 237 °C (Zn) is attributed to phase transition
–
1
process and the enthalpy found was 13.7, 17.8, 18.8 and 17.0 kJ mol , respectively.
Insert Fig. 4
Cyclic DSC experiments (heating/cooling/heating) in air atmosphere were
performed with the compounds that presented exothermic events attributed to physical
transformation. The cyclic curve for Manganese compound is shown in Fig. 5. It was
concluded that this process is irreversible, once it was not possible to observe endothermic
1
4

events in the DSC curve during the cooling cycle, and no exothermic event was observed
on the second heating cycle. Besides that, to evaluate the long-term reversibility of this
phenomenon, the aluminum crucible used to perform the DSC experiment was allowed
to rest for a few days in a desiccator, at ambient temperature and atmospheric pressure.
A new DSC experiment was then performed, and no exothermic event was observed,
confirming its irreversibility.
Insert Fig. 5
6
. Infrared spectroscopy
The IR spectra of isonicotinic acid, sodium isonicotinate and its compounds with
the bivalent metal ions considered in this work are shown in Fig. 6 and the assignments
of the vibrational modes associated with the main bands are presented in Table 3. These
assignments were based on literature data [4,21–24] and by comparison of the spectra of
−
1
the compounds. For isonicotinic acid the ν(C=O) occurs in 1712 cm and it is absent in
the sodium salt and complexes’ spectra. In the sodium salt spectrum two new bands
–
–
−1
(asCOO and symCOO ) appears in 1587 and 1409 cm respectively, indicating the total
deprotonation of the acid. This also confirms that the complexes do not present any
protonated acid contamination.
Insert Table 3
Insert Fig. 6
The infrared spectra were also collected for the compounds heated at different
temperatures, chosen according to events observed in the TG–DSC analysis. The FTIR
spectra were recorded after dehydration and physical transformation of some compounds.
The changes in intensity and wavenumbers of some absorption bands suggest that in both
cases the compound’s structures are altered, resulting in new crystalline arranges,
intermolecular interactions and coordination mode.
–
For the sodium isonicotinate the asymmetric (asCOO ) and symmetric
–
–1
–1
(
[
(
symCOO ) stretching frequencies are locate at 1542 cm and 1410 cm , respectively
–
–
24,25]. Analysis of the difference in the wavenumbers of asCOO and symCOO bands
Δν = as –sym) are generally used as an indicative of coordination mode assumed by the
carboxylate group in metal complexes [26,27]. The Δν value observed the hydrated
–
1
–1
compounds is much larger (~ 204 cm ) then for sodium salt (178 cm ), although a close
value was expected, because in this case the carboxylate acts generally as ion
1
5

(uncoordinated)[5,6,13,28]. The hydrogen bonds formed with coordinated water
molecules by this group causes a change in the charge distribution between the pyridine
ring and the carboxylate group. This reflects in the vibration modes diverging from the
observed for the sodium salt, making ambiguous the attribution of ionic carboxylate just
by the analysis of symmetric and asymmetric stretching [27]. For the Zn compound, once
–
1
it was obtained in an almost anhydrous state, the Δν value (253 cm ) suggests that the
carboxylate group also participates in the coordination with the metal in a unidentate
coordination mode, once it is much larger than the observed for the sodium salt and the
other compounds [26].
According to the crystalline structure described in the literature for these compounds,
in which the two ligands coordinates to one metal center through the pyridine nitrogen
atom in a trans configuration, and four coordinated water molecules, resulting in an
approximately octahedral geometry [5,29] the dehydration process was expected to cause
such change. This is because the crystalline lattice is held by hydrogen bonds formed
between the oxygen atoms from the carboxylate group and the hydrogen atoms from
coordinated water molecules. Upon dehydration, the crystalline lattice must reorganize to
produce a new set of intermolecular interactions, which would affect mostly the
carboxylate group [27]. This phenomenon reflects in the vibrational modes of this group
in the FTIR spectra. In Fig. 7, the symmetric and asymmetric stretching of carboxylate
group are slightly shifted to different wavenumbers in the spectra of the Co compound
heated up to 200 °C when compared to the hydrated one. This could be related to a
structure rearrangement and possibly the coordination of carboxylate oxygen to the metal
ion upon dehydration, to maintain the coordination number of the metal [4]. However,
just the IR data are not sufficient to undoubtedly assign the coordination of carboxylate
upon dehydration.
Insert Fig. 7
After the physical transformation temperature (280 °C for Co compound) a different
–
1
infrared absorption profile is observed, mainly in the range of 1000 – 675 cm . This
region is associated with the C=C ring stretching, so it can be suggested that the
dehydration and physical transformation lead to a change in the charge distribution
around the pyridine ring, which reflects in the shift of absorption bands. This observation
provides further evidence that the exothermic event, observed in DSC curve, is attributed
to a phase transition in the sample.
1
6

7
. PXRD
The powder X-ray diffractograms in the Fig. 8 show that all the compounds were
obtained in a crystalline phase. The great similarity of diffraction peaks in these
diffractograms is evidence that these compounds form an isomorphous series.
Insert Fig. 8
Comparing the diffractograms with those previously reported in the literature for
bivalent transition metals isonicotinate complexes [5], we can conclude that the
compound synthesized in this work have the same stoichiometry and crystalline structure.
Besides that, experimental mass loss and EA results agree with this conclusion.
Also, as previously discussed, the DSC curves of Mn, Co, Cu and Zn compounds
show an exothermic event with no mass loss in the TG curve. To further investigate this
physical process, the compounds were heated to a temperature above those indicated by
the exothermic events in the DSC curve (280 °C for Mn, Co and Zn; 260 °C for Cu), and
a diffractogram was acquired for each compound. The results are presented in Fig. 9. It
can be seen a significative change in the diffraction pattern, with the disappearance of
diffraction peaks and decrease of its intensity for all compounds. As expected, these
results indicate a phase transition for the compounds, probably associated with the
collapse/rearrange of the original crystal structure. However, these are qualitative
observations that need further investigation, ideally by single crystal structural
determination for the heated compounds.
Insert Fig. 9
8
. Conclusions
This study aimed to synthesize some bivalent transition metal complexes with
isonicotinate (4-pyridinecarboxylate) ligand and to investigate the thermal behavior under
air and N atmospheres, to determine the minimal formula, identify the evolved gases
2
during thermal decomposition and pyrolysis, and to correlate structural characteristic with
spectroscopic techniques.
From TG–DSC experiment and analytical results (EA and EDTA complexometry) it
was possible to stablish a minimal formula for the compounds as M(IN)
2
·nH O and to
2
determine the purity of the synthesized compounds. Furthermore, from the TG–DSC
curves it was possible to obtain previously unreported information about the thermal
1
7

behavior (thermal stability, dehydration characteristics, profile and steps of thermal
decomposition, intermediate compounds and final residue) of these compounds in both
conditions employed.
The results of TG–DSC and EGA show that the thermal stability and the initial
thermal decomposition mechanism are little influenced by the conditions used (air or N ),
2
but it significantly influences the later stages, which suggests that the intermediate
compounds formed are sensitive to conditions of analysis and / or the heat released in the
oxidation of the volatiles contributes to accelerate the oxidation process of the organic
matter.
The main gaseous products of thermal decomposition/pyrolysis of the compounds
were identified as water vapor, CO, CO and Pyridine, from which, together with the TG
2
data, it was possible to suggest a thermal decomposition mechanism under N
for the compounds.
2
atmosphere
Mn, Co, Cu and Zn compounds show a physical transformation process in DSC
curves, which were further investigated through PRXD analysis that indicates a phase
transition.
The infrared spectroscopic data was not capable of indicating the carboxylate
coordination character. However, comparing the results with the ones previously reported
in the literature we could conclude that the ligand coordinates through the pyridine
nitrogen atom to the metal. For the Zn compound, the carboxylate group also participates
in the coordination to the metal. Comparing the IR absorption profile of hydrated with
dehydrated compounds it was possible to suggest that there is a probable change in the
coordination mode of the ligand upon dehydration. This change needs to be further
investigated, once it is not possible to ensure only with infrared spectroscopy data.
Acknowledgments
The authors thank FAPESP (Proc. 2017/14936-9), CNPq (Proc. 421469/2016-1) and
CAPES foundations (Brazil) for financial support.
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2
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Figure captions
Fig. 1. Isonicotinic acid (HIN) and sodium isonicotinate (NaIN) TG–DSC curves in air
and N atmospheres.
2
Fig. 2. TG–DSC curves of the synthesized metal complexes in air (a-f) and N (a*-f*)
2
atmospheres.
Fig. 3. Contour map of FTIR spectra collected during Mn compound thermal
decomposition in N
products
2
atmosphere, and reference spectra for the identified evolved gaseous
Fig. 4. DSC curves obtained in air atmosphere, showing the dehydration and phase
transition temperatures
Fig. 5. Cyclic DSC (heating/cooling/heating) obtained for Mn compound, evidencing the
irreversible character of the phase transition
Fig. 6. FTIR spectra collected for isonicotinic acid, sodium isonicotinate and bivalent
metal complexes.
Fig. 7. Cobalt isonicotinate FTIR spectra obtained for hydrated, dehydrated (after heating
to 200 °C) and phase transition temperature (after heating to 280 °C)
Fig. 8. PRXD patterns for the isonicotinate complexes
Fig. 9. PRXD patterns for Mn, Co, Cu and Zn complexes at room temperature (RT) and
heated above the transition temperature.
2
2

2
3

2
4

2
5

2
6

2
7

Table 1. Analytical and thermoanalytical (TG*) data for M(IN)
2
·nH
2
O compounds.
IN
Lost)/%
Metal oxide/%
Calc EDT
Water/%
C/%
Calc
N/%
H/%
Final
Resid
ue
(
Compounds
Calc
d.
Calc
d.
Calc
d.
Calc
d.
TG
TG
TG
EA TG
EA TG
8.5 7.9
EA TG
3.4 3.9
d.
A
d.
Mn(IN)
2
·3H 21.5 21.4 21.7
63.1 62.9
15.3 15.2
40.8 41.2 40.7
Mn O
3
7
7
7
7
7
.96
.72
.49
.47
.56
4.00
4.17
4.31
4.31
4.04
2
O
9
1
9
1
5
0
6
0
5
0
5
4
8
9
4
Fe(IN)
2
·3.5
21.9 22.4 21.9
60.6 60.7
17.3 17.3
39.6 39.1 39.7
7.3 7.7
4.2 4.1
Fe
2
O
3
H
2
O
9
1
2
4
3
7
5
9
2
3
0
3
5
7
Co(IN)
2
·4H
2
21.3 21.6 21.2
59.4 59.3
19.2 19.3
38.4 38.8 38.4
7.3 7.5
4.5 4.3
Co O
3
O
9
0
9
0
5
1
6
1
5
6
5
0
0
2
4
Ni(IN)
2
·4H
2
19.9 20.1 20.0
60.8 60.5
19.2 19.3
38.4 38.7 38.3
7.8 7.4
4.0 4.3
NiO
CuO
O
0
5
5
8
8
2
7
3
1
7
0
6
2
0
Cu(IN)
2
·3.5 21.4 21.8 21.4
61.5 61.3
17.0 17.2
38.8 38.7 38.8
7.8 7.5
3.7 4.0
H
2
O
5
2
1
4
7
1
2
6
2
8
5
6
3
9
2
8

Zn(IN)
2
·0.5 25.5 25.2 25.7
71.6 71.2
45.2 44.8 45.0
9.1 8.7
2.7 2.8
2
.83 3.01
8.79
2.85
ZnO
H
2
O
4
8
8
3
1
4
2
9
1
6
1
4
*
TG in air atmosphere; IN = isonicotinate; M = bivalent transition metal
2
9