Synthesis and thermal decomposition of mixed 2,4′-bipyridine-oxalato complexes with Mn(II), Co(II), Ni(II) and Cu(II)

Article abstract of DOI:10.1023/A:1010193106676

The mixed 2,4′-bipyridine-oxalato complexes of the formulae M(2,4′-bipy)₂C₂O₄·2H₂O (M(II) = Mn, Co, Ni, Cu; 2,4′-bipyridine = 2,4′-bipy or L; C₂O₄² = ox) have been prepared and c

Full text of DOI:10.1023/A:1010193106676

Journal of Thermal Analysis and Calorimetry, Vol. 60 (2000) 145–151
SYNTHESIS AND THERMAL DECOMPOSITION OF
MIXED 2,4’-BIPYRIDINE-OXALATO COMPLEXES
WITH Mn(II), Co(II), Ni(II) AND Cu(II)
D. Czakis-Sulikowska, J. Radwañska-Doczekalska and M. Markiewicz
Institute of General and Ecological Chemistry, Technical University of £ódŸ, 90-924 £ódŸ, Poland
Abstract
The mixed 2,4’-bipyridine-oxalato complexes of the formulae M(2,4’-bipy)2C2O42H2O (M(II)=Mn,
Co, Ni, Cu; 2,4’-bipyridine=2,4’-bipy or L; C2O2– =ox) have been prepared and characterized. IR data
4
show that the 2,4’-bipy coordinated with these metals(II) via the least hindered (4’)N atom; that oxalate
group acts as bidentate chelating ligand. Room temperature magnetic moments are normal for the or-
bital singlet states. The thermal decomposition of these complexes was investigated by TG, DTA and
DTG in air. The endothermic or exothermic character of the decomposition of ML2(ox)2H2O was dis-
cussed.
Keywords: 2,4’-bipyridine-oxalato complexes, IR spectra, thermal decomposition, transition
metals(II)
Introduction
Various workers [1–9] have reported the thermal decomposition of oxalates of transition
metals and mixed metal oxalates of the types M’ (or M’’) [M(ox)n]H2O (where
ox=C2O42– ; M’=K+, NH+4 , Na+; M’’=Fe++, Co++, Ni++; M=other metals(II)). In the litera-
ture, very little is known about thermal stability of mixed amine-oxalato complexes
[10–13]. 2,4’-Bipyridine (2,4’-bipy or L) complexes with metal oxalates are unknown.
The work presented here is a continuation of our studies on the thermal decomposition of
metal complexes with bipyridine isomers [14–17]. The paper includes the results of the
synthesis and properties of 2,4’-bipy compounds with Mn(II), Co(II), Ni(II) and Cu(II)
oxalates. Thermal decomposition of these complexes in order to find their thermal stabil-
ity, intermediate solids and final products was analysed and discussed.
Experimental
Preparation and analyses of the complexes
The mixed 2,4’-bipyridine-oxalato complexes of Mn(II), Co(II), Ni(II) and Cu(II) of the
type M(2,4’-bipy)2(ox)2H2O, were prepared as follows: 2.6 mmol of Mox was dissolved
in 50 cm3 of 0.3 molar water solution of Na2(ox) (containing 0.1 cm3 of 0.5 molar H2(ox)
solution) and gradual residue of M(ox) was filtered off. This solution was mixed with
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Table 1 Selected bands (cm–1) in IR spectra for the complexes ML2(ox)2H2O
Assignment
MnL2(ox)2H2O
CoL2(ox)2H2O
NiL2(ox)2H2O
CuL2(ox)2H2O
Coordinated 2,4’-bipy modes
ν(C=N)(4-sub)
1610.5vs
1415.7m
1012.6s
1616.2vs
1612.4vs
1415.5s
1016.4m
1606.3vs
1417.6s
1022.2m
ν
ring (4-sub)
1411.8m
1014.5m
breathing (4-sub)
Coordinated oxalate modes
1749.3, 1733.9, 1749.3, 1733.9,
1749.3, 1733.4,
1716.5vw
νas(C=O)
ν7
1716.5vw
1662.5w
1390.0sh
1716.5vw
1662.5,
1652.9sh
1668.3,
1647.1m
νasC=O)
ν1
ν2
ν8
1670.0sh
1365.0vw
νS(C–O) + ν(C–C)
νS(C–O) + δ(O–C=O)
1390.0sh
1390.2vw
1353.9, 1338.5w,
1309.6s
1357.8m,
1340.4w
1352.0m,
1338.0vw
δ(O–C=O) + ν(M–O)
ν(M–O) + ν(C–C)
ν9
ν4
798.5w
529.0vw
488.6s
802.3s
516.9w
486.8s
806.2m
528.5w
486.0s
505.0w
418.5w
ring def. + δ(O–C=O)
ν10
ν(M–O) + ring def.
ν11
418.5w
418.5s
418.6w
s, m, v, w, sh – strong, medium, very, weak, shoulder; νas – asymmetric, νs – symmetric stretch, δ – bending

CZAKIS-SULIKOWSKA et al.: 2,4’-BIPYRIDINE-OXALATO COMPLEXES
147
10 cm3 of water solution (with a few drops of 95 v/v% EtOH) containing 7.7 mmol of
2,4’-bipy. The reaction mixture was heated on a water-bath for 10 min. The resulting
solid product was filtered off, washed with a mixture of EtOH and Et2O (1:1) and dried at
room temperature. The isolated compounds and solid thermal decomposition products
(sinters) were analysed by standard methods. The theoretical composition was confirmed
by analysis inside 0.0÷0.3%. The intermediate products of decomposition were also iden-
tified by X-ray diffraction. The sinters of the obtained complexes were prepared under
conditions similar to those used in thermal analysis.
Physical measurements
The thermal investigations were carried out with a derivatograph Q-1500 in air atmo-
sphere. Each sample (100 mg) was heated from room temperature to 1000°C at a rate
of 5°C min–1; Al2O3 was used as the reference, other apparatus and measuring condi-
tions were the same as described in our papers [15, 16].
Results and discussion
All complexes are air stable. The solubility of these compounds in water at 21°C was
found to be in the range of 10–2–10–3 mol l–1.
The room temperature effective magnetic moments of Mn(II), Co(II), Ni(II) and
Cu(II) complexes are 5.75, 4.94, 2.89 and 1.85 M.B., respectively, being normal for
the orbital singlet states [18–20] for pseudo-octahedral coordination of the metal
atom in these complexes.
Infrared spectra
Selection IR bands of 2,4’-bipy and oxalate group as well as assignments [3, 21, 22] of
their frequencies are presented in Table 1. Upon coordination with M(ox), the IR spec-
trum of free 2,4’-bipy undergoes a change only in the ring vibration modes of
4-substituted pyridine (4-sub). Such an effect has been observed previously for com-
pounds of 2,4’-bipy with other of ndm metal salts [15–17]. IR spectra of these complexes
suggest that 2,4’-bipy is coordinated to these metals(II) via the least hindered (4’)N atom
[15–17, 21]. In addition to the 2,4’-bipy frequencies IR spectra of ML2(ox)2H2O com-
plexes exhibit new absorption bands (Table 1), which are identified as ν7,ν1, ν2, ν8, ν9, ν4,
ν10 andν11 modes of the oxalate group, respectively. The frequency values of these bands
suggest that the oxalate group is bonded as normal chelating ligand [3, 22–25]. The bands
with the maximum at 3600–3400 cm–1 confirm the presence of water molecules. Peak in
the water bending region is superimposed on oxalato bands.
Thermal decomposition
Thermoanalytical curves of ML2(ox)2H2O (M(II)=Ni, Cu) in air are shown, as an ex-
ample in Figs 1a and 1b. The results of the thermal decomposition studies of ML2(ox)
2H2O are given in Table 2. All the complexes decompose progressively.
J. Therm. Anal. Cal., 60, 2000

148
CZAKIS-SULIKOWSKA et al.: 2,4’-BIPYRIDINE-OXALATO COMPLEXES
Table 2 Characteristic of thermal decomposition of the complexes in air
Mass loss/%
Trange
°C
/
DTApeak
°C
/
Intermediate
and residue
Complex
found
calcd.
7.33
MnL2(ox)2H2O
60–95
170–240
>700
90endo
220exo
8.0
MnL2(ox)
Mn(ox) with ca 4%
mixture of metal oxides
67.5
63.57
300exo
280exo
8.0
46.0
36.5
2.0
13.57*
46.68
37.13
1.08
pure Mn3O4
CoL0.75(ox)
pure Co3O4
CoO
CoL2(ox)2H2O 160–320
320–390
>750
365exo, vs
NiL2(ox)2H2O
CuL2(ox)2H2O
138–365 305endo, br
71.0
15.0
70.36
14.54
Ni(ox)
365–410
>430
380exo, vs
NiO with traces of Ni
pure NiO
60–158
70endo
170exo
3.6
3.64
CuL2(ox)H2O
158–190
190–260
35.0
34.88
CuL(ox)
mixture of Cu2O+Cu(ox)
(1:1) with traces of Cu
230endo
42.0
3.0
41.91
3.73
260–330 260, 310exo
pure CuO
* Theoretical calculation for pure Mn(ox)
The thermal stability of complexes and their further thermal decomposition are
different. CoL2(ox)2H2O and NiL2(ox)2H2O complexes are thermally the most sta-
ble. They start to decompose at 160 and 138°C, respectively. The intermediate prod-
ucts 2Co(ox)L1.5 (the fragmentary elimination of amine was observed in other com-
plexes [26, 27]) and Ni(ox) are formed. These products are converted quantitatively
in pure Co3O4 and into NiO with Ni (as an example Fig. 1c). Forming of such NiO
with Ni during pyrolysis of Niox has been reported in the literature [1, 28]. With in-
creasing of the temperature pure NiO is formed. The DTA curve showed weak endo-
thermic peak at 280 (Co) and 305°C (Ni); a strong exothermic peak was seen at 365
(Co) and 400°C (Ni). The remaining complexes are stable only till 60°C. MnL2(ox)
2H2O loses all its water within the temperature range 60–95°C (clearly separated step
with endothermic peak at 90°C). In the temperature range 170–240°C Mn(ox) (with
ca 4% other products of decomposition) is formed; exothermic peak at 220°C. Next,
the process of Mn(ox) decomposition takes place. At ca 450°C we identified Mn3O4,
traces of Mn2O3 and other amorphous products. DTA curve showed strong exother-
mic peak at 300°C. A horizontal mass level of Mn3O4 began at ca 700°C. However,
the decomposition of Mn(ox) yields a series of different oxide products depending on
experimental conditions. The product in air is one of the manganese oxides: MnO2,
Mn2O3 or Mn3O4 [1]. In the first step of decomposition CuL2(ox)2H2O loses 1 mol
H2O at 60–158°C (endo peak at 70°C). Between 158–190°C CuL(ox) is formed. In
the last deamination step at 190–260°C the remaining 2,4’-bipy is eliminated, accom-
J. Therm. Anal. Cal., 60, 2000

CZAKIS-SULIKOWSKA et al.: 2,4’-BIPYRIDINE-OXALATO COMPLEXES
149
Fig. 1 Thermoanalytical curves of a – NiL2(ox)2H2O; b – CuL2(ox)2H2O and
c – X-ray diffraction patterns of decomposition products of NiL2(ox)2H2O
heated up to 380°C
panied by oxalate partially decomposition, and mixture of Cuox and Cu2O (ca 1:1)
with traces of Cu is formed. A horizontal level of CuO begins at 330°C. Exothermic
peaks are observed at 170, 260 and 310°C; endothermic – at 230°C.
The general Scheme of the thermal decomposition for ML2(ox)2H2O com-
pounds can be thus showed as follows:
where: M(II)=Mn, Co, Ni, C; a) with ca 4% mixture of metal oxides; b) with a trace of
Cu; c) transitionally with Ni
J. Therm. Anal. Cal., 60, 2000

150
CZAKIS-SULIKOWSKA et al.: 2,4’-BIPYRIDINE-OXALATO COMPLEXES
Some processes of elimination of water and 2,4’-bipy, as well as of elimination of
2,4’-bipy and decomposition of oxalate partially overlap for Co(II), Ni(II) and Cu(II)
compounds, without no clearly separated steps.
Generally the intermediate compounds (Mn(ox), Ni(ox), Cu(ox) and 2Co(ox)L1.5)
decompose in our experimental conditions with an exothermic effect. The decarboxy-
lation peak in air is endothermic, however DTA signals are more complicated in result of
the oxidization reactions.
* * *
We thank Dr. J. Ka³u¿na-Czapliñska for performing some investigations.
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