Ni(II) and Co(II) complexes with 2,4-dichlorophenoxyacetic acid and 2-(2,4-dichlorophenoxy)-propionic acid. Synthesis, properties and thermal decomposition

Article abstract of DOI:10.1023/A:1026398307803

Nickel(II) and cobalt(II) complexes with the commercial herbicides 2,4-dichlorophenoxyacetic acid (2,4D; C₈H₆O₃Cl₂) and 2-(2,4-dichlorophenoxy)-propionic acid (2,4DP; C₉H₈O₃Cl

Full text of DOI:10.1023/A:1026398307803

Journal of Thermal Analysis and Calorimetry, Vol. 74 (2003) 237–250
Ni(II) AND Co(II) COMPLEXES WITH
2,4-DICHLOROPHENOXYACETIC ACID AND
2-(2,4-DICHLOROPHENOXY)-PROPIONIC ACID
Synthesis, properties and thermal decomposition
B. PtaszyÕski* and A. ZwoliÕska
Institute of General and Ecological Chemistry, Technical University, 90-924 ˜ódü, Poland
(Received November 8, 2002; in revised form March 11, 2003)
Abstract
Nickel(II) and cobalt(II) complexes with the commercial herbicides 2,4-dichlorophenoxyacetic
acid (2,4D; C8H6O3Cl2) and 2-(2,4-dichlorophenoxy)-propionic acid (2,4DP; C9H8O3Cl2) were
prepared and characterized. On the basis of the results of elemental analysis and Ni and Co deter-
mination, the following molecular formulae were proposed for the obtained compounds:
Ni(C8H5O3Cl2)2⋅6H2O, Co(C8H5O3Cl2)2⋅6H2O, Ni(C9H7O3Cl2)2⋅2H2O and Co(C9H7O3Cl2)2⋅2H2O.
X-ray powder analysis was carried out. The IR, electronic (VIS) spectra and conductivity data were
discussed. Water solubility of the synthesized complexes at room temperature was examined.
Thermal decomposition of the compounds was studied. Dehydration processes occur during heat-
ing in air. The anhydrous compounds decompose via different intermediate products to oxides.
TG/MS studies indicate formation of gaseous mass fragments of decomposition including H2O+,
OH+, CO+2, HCl+, Cl+2 , CH3Cl+, CH2O+, C6H+6 and other.
Keywords: complexes, 2,4-dichlorophenoxyacetic acid, 2-(2,4-dichlorophenoxy)-propionic acid,
IR spectra, mass spectrometry, thermal decomposition, VIS spectra, X-ray powder
diffraction
Introduction
2,4-dichlorophenoxyacetic acid (2,4D; C8H6O3Cl2) and 2-(2,4-dichlorophenoxy)-
propionic acid (2,4DP; C9H8O3Cl2) are commonly used herbicides which belong to
the group of arylcarboxyl agents. 2,4D and 2,4DP may, due to the presence of the car-
boxyl group in their molecules, coordinate with metal cations.
The review of the available literature indicates that there have been no studies on
the simple complexes of 2-(2,4-dichlorophenoxy)-propionic acid with metals. Two
papers have been found on mixed complexes of 2,4DP with copper and 2,2’-
bipyridine [1, 2].
*
Author for correspondence: E-mail: ptaszynski@lodz.msk.pl
1388–6150/2003/ $ 20.00
Akadémiai Kiadó, Budapest
© 2003 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

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The metal(II) compounds of 2,4-dichlorophenoxyacetic acid and its mixed com-
plexes with N-bases [2–7], α-aminoacids [6] and bovine serum albumin [8] were de-
scribed.
Few papers concern investigation of simple complexes of 2,4D with metals(II)
[9–15]. The thermal properties of their compounds have not been extensively studied so
far. The thermal decomposition of 2,4D complexes with Mn(II), Zn(II), Co(II), Cu(II),
Ni(II), Cd(II) and Fe(III) was carried out by Shulgin et al. [13, 14] and Ristici [15].
The present paper describes the preparation, water solubility at room temperature,
IR and VIS spectra, X-ray powder studies, molar conductivity investigations and thermal
decomposition of Ni(C8H5O3Cl2)2⋅6H2O (Ni-2,4D), Co(C8H5O3Cl2)2⋅6H2O (Co-2,4D),
Ni(C9H7O3Cl2)2⋅2H2O (Ni-2,4DP) and Co(C9H7O3Cl2)2⋅2H2O (Co-2,4DP). The thermal
decomposition was studied in air. A coupled TG/MS system was used to analyse the
principal gaseous products involved during pyrolysis of the obtained compounds
[16–19]. The solid intermediate and final products of decomposition were identified by
X-ray diffraction.
Experimental
Chemicals
2,4-dichlorophenoxyacetic acid and 2-(2,4-dichlorophenoxy)-propionic acid were
delivered to the authors by the Chemical Industrial Establishment ‘Rokita S.A.’ in
Brzeg Dolny. They were purified by double crystallization from water–ethanol (1:1)
solutions. 2,4D and 2,4DP purity was tested by measuring their melting point (m.p.
for 2,4D=140.5°C; m.p. for 2,4DP=117.5–118.1°C).
Cobalt(II) nitrate (Co(NO3)2⋅H2O) and nickel(II) nitrate (Ni(NO3)2⋅6H2O) p.a.
were obtained from Fluka; methanol (anhydroscan) was from Lab-Scan. Other chem-
icals were analytically pure products from POCh–Gliwice.
Physical measurements: techniques and apparatus
X-ray powder diffraction patterns were recorded on a Siemens D5000 diffractometer,
using CuKα radiation monochromatized by means of a secondary graphite monochro-
mator. The curves were recorded over the 2θ range 2–90° with scan step of 0.04° and
time of scan step 1 s.
IR spectra were recorded on FTIR-8501 Shimadzu spectrophotometer over the
range 4400–400 cm–1 using KBr discs technique.
The VIS spectra were obtained in a Nujol mull by means of M-40 Specord In-
strument over the range 29500–11500 cm–1.
Conductivity measurements were performed at 25 0.05°C on an OK-102/1
conductometer with an OK902 electrode. The cell constant was determined by use of
KCl standard aqueous solution. The molar conductivity (ΛM) of the complexes was
measured using 1.0·10–3 mol dm–3 solution in methanol.
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239
The thermal decomposition processes of the prepared complexes were studied by
thermogravimetry (TG, DTG, DTA) and TG/MS technique. Thermal curves were ob-
tained on derivatograph OD-102/1500 in air over the temperature ranges 25–1000°C at a
heating rate 10°C min–1. The mass of the samples was 100 mg. α-Al2O3 was used as a
reference material.
The TG/MS system (thermoanalyser TG/DTA-SET SYS-16/18, mass spectrom-
eter ThermoStar from Balzers, corundum crucible) was used for the characterization
of gaseous products thermal decomposition of the Ni and Co complexes (mass of the
samples of Ni-2,4D, Ni-2,4DP, Co-2,4D and Co-2,4DP were 4.01, 2.19, 3.34 and
5.27 mg, respectively). The solid intermediate products of decomposition obtained
under conditions similar to those used in thermal analysis were identified by X-ray
diffraction (using Powder Diffraction File) [20].
Synthesis of the complexes
The reaction of 2,4D and 2,4DP with cobalt(II) and nickel(II) nitrates(V) were per-
formed using amounts of substrates corresponding with molar ratio M:2,4D and
M:2,4DP=1:2 (where M=Ni and Co) and at pH=7.
As 2,4D and 2,4DP are sparingly soluble in water, whereas their sodium salts are
readily soluble, the water solutions of these compounds (20 mmol in 150 mL of the
water) were obtained by dissolving their weighed samples with the addition of solid
NaOH in water heated to about 60°C (equimolar amounts of NaOH and acids). Next,
pH of the acid solution was brought to pH=7 by adding 1 mol dm–3 NaOH. The solu-
tions were filtered and slowly, while stirring vigorously added to a solution contain-
ing Co(II) or Ni(II) (10 mmol in 80 mL of the water). The precipitates formed imme-
diately. The Ni compounds were green while these of Co – pink. The precipitated
complexes were filtered off after 30 min and dried in air at room temperature.
Analyses of the complexes
The composition of the synthesized complexes was established on the basis of deter-
mination of the metals, carbon, hydrogen and chlorine. Nickel and cobalt in mineral-
ized samples were determined by complexometric titration using EDTA [21] vs.
murexide whereas carbon, hydrogen and chlorine by elementary analysis. The analyt-
ical results are shown in Table 1.
Water solubility of the complexes under study was determined at room tempera-
ture (about 21°C). About 1.800 g of the compound was weighed, 50 mL of distilled
water were added and the sample was shaken for 6 h. Next the solution was filtered.
In the saturated water solution of the complex the content of the metal was deter-
mined by complexometric titration using EDTA vs. pyrocatechol violet [21] and by
the spectrophotometric method with 4-(2-pyridylazo)-resolcinol (PAR) [22, 23]. The
average values of water solubility of studied complexes are presented in Table 1.
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PTASZYÑSKI, ZWOLIÑSKA: Ni(II) AND Co(II) COMPLEXES
Results and discussion
The analytical results indicate the following molecular formulae for the obtained
complexes: Ni(C8H5O3Cl2)2⋅6H2O, Co(C8H5O3Cl2)2⋅6H2O, Ni(C9H7O3Cl2)2⋅2H2O
and Co(C9H7O3Cl2)2⋅2H2O. All compounds are stable in air. Their solubility in water
at 21°C is of the order of 10–2 mol dm–3. They dissolve fairly in ethanol and methanol.
The molar conductivities in methanol (Table 1) indicate that the complexes
show intermediate behaviour between those of non- and 1:1 electrolytes [24].
Table 1 Analytical data, solubility S [mol dm–3] in water at 21°C and molar conductivity ΛM
[Ω–1cm2 mol–1] in methanol (concentration 1.0⋅10–3) at 25°C of the nickel and cobalt
complexes with 2,4D and 2,4DP
Analysis: found (calculated)/%
S 10–2
Λ M
Complexes
M
C
H
Cl
Ni-2,4D
9.57(9.67)
10.02(10.43)
9.77(9.71)
9.97(10.47)
31.40(31.66)
38.17(38.41)
31.91(31.65)
38.23(38.39)
3.47(3.66)
3.22(3.23)
3.68(3.66)
3.26(3.23)
23.30(23.37)
25.44(25.19)
23.36(23.36)
24.48(25.18)
2.01
4.08
1.28
5.51
62.6
61.6
60.6
65.2
Ni-2,4DP
Co-2,4D
Co-2,4DP
Fig. 1 Diffractograms of a – Co-2,4D; b – Ni-2,4D; c – Co-2,4DP and d – Ni-2,4DP
The analysis of the diffractograms (Fig. 1) shows that Ni-2,4D and Co-2,4D are
crystalline compounds, while Ni-2,4DP and Co-2,4DP are either amorphous or their
degree of crystallinity is very low.
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241
Electronic and IR spectra
Due to the presence of carboxyl group in their molecules, 2,4-dichlorophenoxyacetic
acid and 2-(2,4-dichlorophenoxy)-propionic acid may coordinate with metal cations.
IR spectrophotometry (over the range 4400–400 cm–1) confirms that 2,4D and 2,4DP
combine with nickel and cobalt through the oxygen atoms of the carboxylate group.
Table 2 presents only the fundamental frequencies of vibrations of the carboxylate
group for acids and prepared complexes [25, 26].
The stretching band of the undissociated carboxyl group ν(C=O), present in the
2,4D and 2,4DP spectra, does not appear in the spectra of their complexes. Instead,
two new stretching bands, ν(OCO)sym and ν(OCO)asym appear, representing the vibra-
tions of the dissociated carboxylate group.
On the basis of ∆ν value calculated as the difference between the frequencies of
the asymmetric and symmetric vibrations of the dissociated carboxylate group and its
comparison with the results obtained for sodium salts, the type of coordination of the
carboxylate group was established [27–29].
Kennard et al. studied crystal and molecular structures of Zn(II) and Cu(II) com-
plexes of 2,4-dichlorophenoxyacetic acid [10, 11]. They found out that the molecular
units in the Zn-2,4D compound consist of both octahedral [Zn(H2O)4(2,4D)2] and tet-
rahedral [Zn(H2O)2(2,4D)2] complexes linked only by hydrogen bonding between the
non-complexed carboxyl oxygens and the complexed aqua ligands. The carboxylate
groups are unidentate in these complexes. In the case of Cu-2,4D,
[Cu2(2,4D)4(H2O)2⋅2H2O] forms centrosymmetric dimeric units with two copper cen-
tres bridged by four carboxylate groups of 2,4-dichlorophenoxyacetic acid.
Therefore the carboxylate group in 2,4D compounds may be both unidentate and
bidentate.
Unequivocal assertion of nature of carboxylate group is very difficult without
supplementary structural studies. In this paper separation (∆ν) of ν(OCO) for Ni and
Co complexes are lower than ∆ν values for sodium salts of the studied acids but dif-
ference is not large.
Waddington et al. [29] suggest that if difference between ∆ν for the complexes
and ∆ν for the sodium salt is small, carboxylate group is bridging. If this difference is
large, the carboxylate group is chelating. With Waddington’s criterion accepted, the
carboxylate group in our complexes is bridging. The obtained values of molar con-
ductivity in methanol confirm our presumptions with regard to coordination of
carboxylate groups.
The spectra of all the complexes of 2,4D and 2,4DP under study show strong and
broad bands in the water stretching region (ca. 3600–3100 cm–1). The bands in the water
bending region (1650–1620 cm–1) are overlaped with the streching bands (ν(OCO)asym
)
of carboxylate group. The ν(M–O)+δ(OCO) vibrations are observed within the interval
ca. 516.5–466.7 cm–1.
VIS spectra of the solid nickel and cobalt compounds with 2,4D and 2,4DP acids
was obtained in Nujol mulls within the 29500–11500 cm–1 region. d-d transition
bands present in the electronic spectra of the studied complexes are all consistent
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PTASZYÑSKI, ZWOLIÑSKA: Ni(II) AND Co(II) COMPLEXES
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PTASZYÑSKI, ZWOLIÑSKA: Ni(II) AND Co(II) COMPLEXES
243
with a distorted octahedral environment around the Ni(II) and Co(II) atom
[16–18, 30]. Table 2 shows ligand field spectral data for Ni and Co complexes with
2,4D and 2,4DP. The nickel complexes exhibit only two bands: at ca. 15000 cm–1
which correspond to the transition 3A2g(F)→3T1g(F) and at ca. 25000 cm–1 connected
with 3A2g(F)→3T1g(P) transition. The ligand field spectra of cobalt complexes present
multiple band at ca. 19500 cm–1. This band may be assigned to the 4T1g(F)→4T1g(P)
transition. It is apparently asymmetrical band and has a shoulder in the region
20620 cm–1 for Co-2,4D and 20860 cm–1 for Co-2,4DP.
Thermal analysis
The thermoanalytical curves Ni(II) and Co(II) complexes are presented in Figs 2
and 3 and the thermal decomposition data are listed in Table 3.
Fig. 2 Thermoanalytical curves (TG, DTG, DTA) of a – Ni-2,4D and b – Co-2,4D
Fig. 3 Thermoanalytical curves (TG, DTG, DTA) of a – Ni-2,4DP and b – Co-2,4DP
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All the complexes are hydrated and lose water molecules between 40–145°C in
one stage (except for Co-2,4D which loses them in two steps). The dehydration pro-
cesses of the compounds are connected with endothermic effects between 80–100°C.
Anhydrous complexes decompose to oxides (NiO and CoO) with formation of dif-
ferent intermediate products, which may be identified by the X-ray analysis. All the com-
plexes decompose progressively. The TG curves for Ni and Co compounds with the stud-
ied acids exhibit multiple mass loss steps. Several endo- and exothermic peaks on the
DTA curves are observed. The further mass loss on the TG curves within the temperature
range 200–470°C (Ni-2,4D), 180–370°C (Co-2,4D), 240–450°C (Ni-2,4DP) and
210–370°C (Co-2,4DP) is caused by the pyrolysis of organic ligands. In these stages sev-
eral exothermic peaks on the DTA curves are observed. The next stages are caused by
farther oxidation of organic fragments and very strong and broad exothermic effects on
DTA accompany those steps (probably decomposition of carboxylate anions and de-
struction of benzene rings take place – TG/MS measurements).
The anhydrous Ni-2,4D complex decompose in two stages connected with con-
siderable mass losses and oxidation of organic fragments. On the DTA curve weak
exothermic effects at 200–470°C and very strong at 470–650°C are observed. At
470°C the mixture of Ni-2,4D (where Ni:2,4D=1:1), Ni, NiCl2 and NiO was identi-
fied. The final products of Ni-2,4D decomposition (at 650°C) is NiO.
The Ni-2,4DP complex decompose to NiO (at 660°C) via the formation of unde-
fined intermediate products. On the TG curve three stages of pyrolysis of this compound
are observed. The exothermic effects on DTA curve at 160–450°C are weak while at
450–660°C very strong and broad. Loss of mass is the largest in the range 240–450°C. In
this stage, oxidation processes of intermediate products proceed very quickly.
The anhydrous Co-2,4D decompose to CoO. The first stage of thermal decom-
position occurs within the range 180–370°C. In this stage 1 mol of 2,4D liberates and
Co3O4 begins to form. The loss of mass in the range 370–650°C is attributed to the
pyrolysis of the remaining 2,4D. The final product of this stage is Co3O4. At
650–950°C the cobalt(II) oxide CoO is formed. This process is accompanied by the
endothermic peak on DTA at 920°C.
Co-2,4DP decompose in four stages. The first two stages are connected with
decompositon of 1 mol 2,4DP. At 370°C the mixture of Co-2,4DP (where
Co:2,4DP=1:1), CoCl2 and Co3O4 are formed. On the DTA curve weak exothermic
peaks are observed. In the range 370–600°C oxidation of the organic ligand takes
place. The DTA curve shows very strong and broad exothermic effects. The final
product of this stage is Co3O4. At 600–950°C it transforms into CoO with endother-
mic peak at 910°C.
In this paper, a coupled TG/MS system was used to analyse of gaseous products
evolved during the thermal decomposition of Co and Ni complexes with 2,4-
dichlorophenoxyacetic and 2-(2,4-dichlorophenoxy)-propionic acids in air atmo-
sphere. Both decomposition and MS data of these compounds are similar. Figures 4
and 5 (as an example) present relationship of the ion current for m/z detected in the
mass spectrometry vs. time of decomposition (heating rate=10°C min–1) for Ni-2,4D
and Co-2,4DP. In these figures the profiles for MS are not presented, which curves
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PTASZYÑSKI, ZWOLIÑSKA: Ni(II) AND Co(II) COMPLEXES
Fig. 4 Ion current for m/z detected in the mass spectrometer vs. time for Ni-2,4D (mass
sample 4.01 mg, heating rate 10°C min–1)
Fig. 5 Ion current for m/z detected in the mass spectrometer vs. time for Co-2,4DP
(mass sample 5.27 mg, heating rate 10°C min–1)
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247
Fig. 6 TG curve for Ni-2,4D and ion current detected by the MS for mass fragments
m/z: 1 – 17; 2 – 18; 3 – 45; 4 – 46 (a); 1 – 30; 2 –50; 3 – 74; 4 – 78 (b)
Fig. 7 TG curve for Co-2,4DP and ion current detected by the MS for mass fragments
m/z: 1 – 18; 2 – 38; 3 – 44; 4 – 46 (a); 1 – 30; 2 –50; 3 – 74; 4 – 94 (b)
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PTASZYÑSKI, ZWOLIÑSKA: Ni(II) AND Co(II) COMPLEXES
are shown in Figs 6 and 7, where TG/MS data are listed. In the mass spectrum differ-
ent intensities of signal ions are detected.
In all cases, the first decomposition products detected were H2O+ (m/z=17, 18).
The strong maximum, while the elimination of H2O+ appears (crystalline or coordi-
nated), occurs at 139, 137, 157 and 130°C for Co-2,4D, Co-2,4DP, Ni-2,4D and
Ni-2,4DP, respectively. Next, the H2O is a product of oxidation of organic ligands at
higher temperature.
The mass spectra suggested that at lower temperature (to 370°C) the several
gaseous products are liberated: C2H+2 , CH2O+, CO+2 , C+, CH+, CH+3 , Cl+, HCl+, Cl+2 ,
CH3Cl+, H+2 (m/z=26, 30, 44, 45, 46, 12, 13, 15, 35, 37, 36, 38, 70, 74, 50, 2, respec-
tively) and CCl+ for Co and Ni with 2,4DP. In air atmosphere CO+ was not detected.
The profiles observed for m/z=44, 45, 46 (CO+2 and its isotopes) are similar. CO+2 is
mainly formed as a result of decomposition of carboxylate groups. At the same time,
chlorine compounds are liberated. These compounds and CO+2 are presented at
higher temperatures (above 370°C) too. In the range 370–570°C, benzene and its
products of destruction appear additionally (among other: C6H6+ , C6H5OH+,
C6H4ClOH+ where m/z=78, 94, 129 and 131).
Both the simple carbon chains and chlorine bonding crack at low temperatures
whereas the benzene ring cracks at above 400°C.
TG coupled with MS data for some decomposition gaseous products are shown
in Figs 6 and 7 (as an example) for Co-2,4DP and Ni-2,4D.
Conclusions
2,4-dichlorophenoxyacetic acid and 2-(2,4-dichlorophenoxy)-propionic acid form
with cobalt and nickel complexes of the molar ratio M:2,4D and M:2,4DP=1:2
(where M=Co, Ni). The complexes contain crystallisation or coordinated water and
have
the
following
molecular
formulae:
Ni(C8H5O3Cl2)2⋅6H2O,
Co(C8H5O3Cl2)2⋅6H2O, Ni(C9H7O3Cl2)2⋅2H2O and Co(C9H7O3Cl2)2⋅2H2O. The com-
plexes are sparingly soluble in water at room temperature and their solubility in-
creases in the orders:
Co-2,4D<Ni-2,4D and Ni-2,4DP<Co-2,4DP
The 2,4DP complexes are more readily soluble in water than 2,4D complexes
(for the same metal).
The powder X-ray diffraction patterns of the studied compounds demonstrate
that Co-2,4D and Ni-2,4D have a crystalline structure. Co-2,4DP and Ni-2,4DP are
amorphous or exhibit very low degree of crystallinity. The IR spectroscopy confirms
that 2,4D and 2,4DP acids combine with Co and Ni through the oxygen atoms of the
carboxylate group. On the basis of the values of ∆ν=ν(OCO)asym–ν(OCO)sym and con-
ductance measurements in methanol a conclusion can be drawn that in compounds
studied carboxylate group is bidentate bridging. The electronic spectra (VIS) indicate
pseudooctahedral environment around the Co and Ni.
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249
The thermal decomposition of all obtained hydrated complexes begins with the
release of water. Thermal stability of anhydrous complexes increases in the following
order:
Co-2,4DP<Ni-2,4DP<Co-2,4D<Ni-2,4D
(130°C) (160°C)
(180°C) (200°C)
The anhydrous 2,4D complexes are thermally more stable than 2,4DP com-
plexes. All compounds decompose via intermediate products to oxides (NiO and
CoO). The gaseous products liberated during pyrolysis of studied complexes are sim-
ilar. During the heating of the complexes the decomposition of the organic ligands
and oxidation processes take place. Molecular ions: H2O+, CO+2 , HCl+, Cl2 , CH3Cl+,
+
CH2O+, C6H6+ , C2H2+ , C+, CH+, CH3 , Cl+, Cl2O+, C6H5Cl+, C6H5OH+, C6H4ClOH+
+
were detected.
* * *
The authors wish to thank the Chemical Industrial Establishment ‘Rokita S.A.’ in Brzeg Dolny for
providing 2,4D and 2,4DP acids used in our studies.
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