Furopyridines and furopyridine-Ni(II) complexes : SSynthesis, thermal and spectral characterization

Article abstract of DOI:10.1007/s10973-005-0769-4

The thermal decomposition of the complexes Ni(SCN)₂(fp) ₄?2H2O (I), Ni(SCN)₂(mfp)₄ (II) and Ni(SCN)₂(dmfp)₃ (III) (where fp=furo[3,2-c]pyridine, mfp=2-methylfuro[3,2-c]pyridine and dmfp=2,3

Full text of DOI:10.1007/s10973-005-0769-4

Journal of Thermal Analysis and Calorimetry, Vol. 81 (2005) 211–215
FUROPYRIDINES AND FUROPYRIDINE-Ni(II) COMPLEXES
Synthesis, thermal and spectral characterization
S. C. Mojumdar1*, J. Miklovi¹2, A. Krutošíková2, D. Valigura3 and J. M. Stewart4
1National Research Council of Canada, M-20, 1200 Montreal Road, Ottawa, Ontario K1A 0R6, Canada
2Department of Chemistry, Faculty of Natural Sciences, University of St. Cyril and Methodius, SK-917 01 Trnava, Slovakia
3Department of Inorganic Chemistry, Slovak University of Technology, Radlinského-9, SK-81237 Bratislava, Slovakia
4Department of Biology and Biochemistry, Mount Allison University, Sackville, NB, E4L 1G8, Canada
The thermal decomposition of the complexes Ni(SCN)2(fp)4⋅2H2O (I), Ni(SCN)2(mfp)4 (II) and Ni(SCN)2(dmfp)3 (III) (where
fp=furo[3,2-c]pyridine, mfp=2-methylfuro[3,2-c]pyridine and dmfp=2,3-dimethylfuro[3,2-c]pyridine) have been investigated in
dynamic air from room temperature to 1000oC by means of TG, DTG and DTA. The chemical composition of the complexes, solid
intermediates and the resultant products of thermolysis have been identified by means of elemental analysis and complexometric ti-
tration. The results revealed that NiO was left as residue at the end of the thermal degradation experiments. IR data suggested that
fp, mfp and dmfp were coordinated to Ni(II) through the N atom of the respective heterocyclic ring.
Keywords: DTA, FTIR, furopyridine-Ni(II) complexes, furopyridines, TG, UV-VIS
Introduction
Experimental
Synthesis of furo[3,2-c]pyridines
Investigations of quinoline isosters (furopyridines) in
which the benzene ring is replaced by furan rings
have resulted in discovering many biologically active
compounds. New pharmacophores with potential
antipsychotic activity possess the furo[3,2-c]pyridine
ring system [1]. Therefore, efficient synthetic meth-
ods for these types of heterocycles are of a great inter-
est. Recently, the syntheses of the furo[3,2-c]pyri-
dines and pyrrolo[2´,3´:4,5]furo[3,2-c]pyridines have
been reported in [2–5]. The study of the influence of
metal and ligand nature on the process of thermal de-
composition is of a certain interest. Therefore, many
authors have investigated many heterocyclic com-
pounds as ligands in coordination complexes of sev-
eral central atoms and also examined their thermal
and spectral properties [6–31]. In our previous pa-
pers, we described the thermoanalytical properties of
Mg(II), Cu(II), Co(II) and Fe(III) complexes with
pyridine and its derivatives [32–44]. Up to our best
knowledge furo[3,2-c]pyridine (fp), 2-methylfuro
[3,2-c]pyridine (mfp) and 2,3-dimethylfuro[3,2-c]
pyridine (dmfp) were never used as ligands for coor-
dination compounds with Ni(II). This fact has
prompted us to undertake a systematic study in
complexation reactions of these heterocyclic ligands.
This paper describes the preparation of Ni(II) com-
plexes with fp, mfp and dmfp along with thermal anal-
yses and spectral investigation of the complexes.
Starting from furan-2-carbaldehydes 1a–c via
Doebner condensation we synthesized the corre-
sponding acids 2a–c (Fig. 1). The acylazides were
prepared by treatment of 2a–c with ethyl
chloroformate and sodium azide in a one-pot reaction
and then were transformed by thermal cyclisation in
Dowtherm with tributylamine to 4,5-dihydropyri-
dines 3a–c. Refluxing of 3a–c with phosphorus
oxychloride and the reduction of the obtained
chloroderivatives led to furo[3, 2-c]pyridines 4a–c.
Fig. 1 The scheme for the synthesis of furo[3,2-c]pyridines.
For compounds a – R=R1=H; b – R=CH3, R1=H and
c – R=R1=CH3
*
Author for correspondence: subhash.mojumdar@nrc-cnrc.gc.ca
1388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
© 2005 Akadémiai Kiadó, Budapest

MOJUMDAR et al.
Synthesis of Ni(II) complexes
X-ray powder diffraction was carried out using a
MIROMETA II instrument, using CuK radiation.
α
The records were compared with ASTM tables.
Complexes I–III were prepared by treating
(0.002 mol) fp, mfp or dmfp, respectively with
Ni(SCN)2·6H2O (0.001 mol) in methanol (50 mL). The
solutions were kept at room temperature. The fine pre-
cipitated microcrystals were filtered out, washed with
cold methanol and air-dried at room temperature.
Results and discussion
The composition of the complexes I–III, the solid in-
termediates and the resultant final products of
thermolysis have been identified by elemental analy-
sis and complexometric titration. The data of elemen-
tal analysis and complexometric titration of com-
plexes I–III are given in Table 1.
The data for the thermal decomposition of com-
plexes I–III are summarized in Table 2. The thermal
decomposition of the compounds is a multi-stage pro-
cess. The subsequent detachment of the ligands was
observed. The final solid products were NiO. The TG
and DTA curves for Ni(SCN)2(dmfp)3 (III) are
shown in Fig. 2 as an example.
Measurements
Elemental analyses (C, H, N) were carried out by
means of Carlo Erba 1106 Analyser and the content of
Ni(II) was determined by complexometric titration.
The IR spectra were obtained on Philips analyti-
cal PU9800 FTIR spectrometer by using Nujol mulls
in the range 400–4000 cm–1.
Electronic spectra in the T-region, 10–2 ·103 K was
measured with a PerkinElmer 450 spectrophotometer.
Thermal decomposition studies were carried out
using a TA Instruments (model SDT 2960) in dy-
namic air atmosphere (flow rate 90 cm3 min–1) by us-
ing a platinum crucible with a sample mass of 20 mg
from room temperature to 1000°C. A heating rate of
10°C min–1 was chosen for all measurements.
The
TG
curve
for
the
complex
Ni(SCN)2(fp)4·2H2O (I) indicates that it is stable at
temperature up to 60°C, where the slow decomposi-
tion to Ni(SCN)2(fp)4 begins. The TG curve shows
four mass losses at 140, 190, 340 and 420°C. They
correspond to the presence of four intermediate de-
composition products: Ni(SCN)2(fp)4, Ni(SCN)2(fp)2,
Table 1 Elemental analysis and complexometric titration data of the complexes I–III
Theoretical/%
Experimental/%
H
Complex
C
H
N
Ni
C
N
Ni
Ni(SCN)2(fp)4·2H2O (I)
Ni(SCN)2(mfp)4 (II)
Ni(SCN)2(dmfp)3 (III)
52.43
57.73
56.52
3.50
3.96
4.38
12.23
11.89
11.37
8.55
8.30
9.53
52.51
57.68
56.45
3.48
3.97
4.39
12.20
11.91
11.40
8.53
8.32
9.55
Table 2 Thermal decomposition data of the complexes I–III
DTA
TG
Complex
Peak/°C
135
180
232
379
513
186
254
369
519
222
273
391
520
Process
endo
endo
endo
exo
Temp. interval/°C
60–140
∆m(found/calc.)/%
5.50/5.24
Lost component
Residue
2H2O
2fp
140–190
190–340
340–450
450–800
110–207
207–350
350–400
400–770
150–240
240–335
335–420
420–840
35.00/34.67
34.50/34.67
12.00/12.22
2.25/2.33
I
2fp
SCN, CN
S
exo
NiO
NiO
NiO
endo
endo
exo
41.00/41.03
40.50/41.03
9.00/8.67
2mfp
2mfp
SCN, CN
S
II
III
exo
2.00/1.64
endo
endo
exo
48.00/47.77
24.00/23.88
13.50/13.63
2.50/2.60
2dmfp
dmfp
SCN, CN
S
exo
212
J. Therm. Anal. Cal., 81, 2005

FUROPYRIDINES AND FUROPYRIDINE-Ni(II) COMPLEXES
NiO as the final solid product (oxygen is from the air
atmosphere). The thermal reaction of complex II can
be represented as:
Ni(SCN)2(mfp)4→Ni(SCN)2(mfp)+3mfp
Ni(SCN)2(mfp)→Ni(SCN)2+mfp
Ni(SCN)2→NiS+decomposition product
NiS+O2→NiO+decomposition product
The DTA curve for the complex II shows two
endotherms and two exotherms (Table 2). The two en-
dothermic peaks observed at 186 and 254°C corre-
sponding to the loss of 3mfp and mfp, respectively.
The two exothermic peaks observed at 369 and 519°C
corresponding to decomposition reactions of Ni(SCN)2
and NiS, respectively. NiO, which was identified by
X-ray diffraction measurements, was to be final solid
residue of these thermal decompositions.
Fig. 2 TG and DTA curves of Ni(SCN)2(dmfp)3 (III)
Ni(SCN)2 and NiS. The chemical composition of the
solid intermediates and the resultant final products of
thermolysis have been identified by means of elemen-
tal analysis and complexometric titration. NiS and
NiO were also identified by X-ray diffraction. The
most probable thermal decomposition reaction of
complex I can be represented as:
As summarized in Table 2, the TG curve of
complex Ni(SCN)2(dmfp)3 (III) indicates that it is
stable at temperature up to 150°C and four mass loss
steps (Fig. 2) were observed. The first step between
150–240°C is accompanied by 48.0% mass loss and
the second step between 240–335°C is accompanied
by 24.0% mass loss. Both steps are attributed to the
decomposition of 2,3-dimethylfuro[3,2-c]pyridine
(2dmfp and dmfp, respectively). The third step be-
tween 335–420°C is accompanied by 13.5% mass
loss. It is attributed to the decomposition of the
Ni(SCN)2 to NiS. The fourth step took place between
420–840°C is accompanied by 2.5% mass loss. It is
attributed to the oxidation reaction of NiS with atmo-
spheric oxygen to NiO. The thermal decomposition
reaction of complex III can be represented as:
Ni(SCN)2(fp)4·2H2O→Ni(SCN)2(fp)4+2H2O
Ni(SCN)2(fp)4→Ni(SCN)2(fp)2+2fp
Ni(SCN)2(fp)2 →Ni(SCN)2+2fp
Ni(SCN)2 →NiS+decomposition product
NiS+O2→NiO+decomposition product
The DTA curve for the complex I shows three
endotherms and two exotherms (Table 2). The three
endothermic peaks observed at 135, 180 and 232°C
corresponding to the loss of 2H2O, 2fp and 2fp, re-
spectively. The two exothermic peaks observed at
379 and 513°C corresponding to decomposition reac-
tions of Ni(SCN)2 and NiS, respectively. NiO, which
was identified by X-ray diffraction measurements,
was found to be final solid residue of these thermal
decompositions.
Ni(SCN)2(dmfp)3→Ni(SCN)2(dmfp)+2dmfp
Ni(SCN)2(dmfp)→Ni(SCN)2+dmfp
Ni(SCN)2 →NiS+decomposition product
NiS+O2 →NiO+decomposition product
As summarized in Table 2, the TG curve for the
complex Ni(SCN)2(mfp)4 (II) indicates that mass loss
becomes observable at 110°C , and four mass loss
steps were observed. The first step between
110–207°C is accompanied by 41.0% mass loss. It is
attributed to the decomposition of 2mfp. The second
step took place between 207–350°C and is accompa-
nied by 40.5% mass loss. It is attributed to the decom-
position of the remaining 2mfp. The third step took
place between 350–400°C and is accompanied by
9.0% mass loss. It is attributed to the decomposition
of the Ni(SCN)2 to NiS. The fourth step took place be-
tween 400 –770°C and is accompained by 2.0% mass
loss. It is attributed to the oxidation reaction of NiS to
The DTA curve for the complex III shows two
endotherms and two exotherms (Table 2). The two
endothermic peaks observed at 222 and 273°C as-
cribed to the loss of 2dmfp and dmfp, respectively.
The two exothermic peaks observed at 391 and 520°C
ascribed to decomposition reactions of Ni(SCN)2 and
NiS, respectively. NiO, which was identified by
X-ray diffraction measurements, was to be final solid
residue of these thermal decompositions.
The solid state electronic spectra of complexes
I–III exhibit ligand field bands with maxima at
17800–18500 cm–1. These bands were identified as d-d
transitions of nickel atoms and are typical for tetragonal
distorted octahedral Ni(II) complexes [45, 46].
J. Therm. Anal. Cal., 81, 2005
213

MOJUMDAR et al.
Table 3 IR spectral data (400–4000 cm–1) of complexes I–III
Assignment
ν(CN)
I
II
III
1608
660
1610
650
1605
630
601
958
1608
865
γ(CCC)
613
605
ν(C–C)
ν(C–C)
ν(C–H)ring
ν(OH)
929
928
1605
862
1610
864
3472
1615
δ(HOH)
a=asymmetric, s=symmetric
The modes of the coordinated ligands in the
complexes have been investigated by means of IR
spectra. The most important IR frequencies attributed
to the vibrations of the complexes I–III are reported
in Table 3. IR spectrum of complex I showed a broad
absorption band at 3472 cm–1 (antisymmetric and
symmetric OH stretching) and 1615 cm–1 (HOH bend-
ing). These bands clearly confirm the presence of wa-
ter of crystallization in the compound I. The presence
of water as water of crystallization in these com-
pounds was further shown by the thermal decomposi-
tion data [47, 48]. The stretching vibration of the C=N
in the pyridine ring appeared at 1590 cm–1 [29]. Upon
complex formation the band shifts to higher frequen-
cies. The bands in the range (1605–1610 cm–1) in
complexes I–III may suggest the bond formation by
the Ni(II) with the nitrogen atom of the respective
pyridine ring. The band shifts to higher frequencies
for the dipolar contribution of C=N+ in the
heterocyclic ring [50].
described for the different components. However,
spectroscopic and analytical data available enable us
to predict the structures and we can also use thermal
decomposition studies to help us.
Acknowledgements
We wish to thank the Slovak Grant Agency (project-VEGA
1/6249/99) and National Research Council of Canada (NRC)
for the financial support.
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215