Phenoxyalkanoic acid complexes. Part II. Complexes of selected bivalent metals with 2,4-dichlorophenoxyacetic acid (2,4D) and 2-(2,4-dichlorophenoxy)propionic acid (2,4DP)

Article abstract of DOI:10.1016/j.tca.2008.10.005

Seven new solid complexes have been obtained: Hg(II), Fe(II), Ca(II) and Mg(II) complexes with 2,4D and Hg(II), Fe(II) and Ca(II) complexes with 2,4DP as well as previously described Cu(II), Zn(II), Cd(II), Pb(II), Mn(II), Co(II) and Ni(II) complexes with

Full text of DOI:10.1016/j.tca.2008.10.005

Thermochimica Acta 482 (2009) 49–56
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Phenoxyalkanoic acid complexes Part II. Complexes of selected bivalent
metals with 2,4-dichlorophenoxyacetic acid (2,4D) and
2-(2,4-dichlorophenoxy)propionic acid (2,4DP)
J. Kobyłecka∗, A. Turek, L. Sieron´
Institute of General and Ecological Chemistry, Technical University, 90-924 Lodz, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 July 2008
Received in revised form 2 October 2008
Accepted 6 October 2008
Available online 17 October 2008
Seven new solid complexes have been obtained: Hg(II), Fe(II), Ca(II) and Mg(II) complexes with 2,4D and
Hg(II), Fe(II) and Ca(II) complexes with 2,4DP as well as previously described Cu(II), Zn(II), Cd(II), Pb(II),
Mn(II), Co(II) and Ni(II) complexes with both ligands. The composition of ML2·nH2O complexes, where
n = 0–6, was established by chemical analysis methods. Their water solubility has been determined and X-
ray, IR and thermal analyses have been carried out. It has been found out that the most sparingly soluble
mercury, lead, copper and cadmium compounds precipitate also from a solution of a herbicide which
contains 2,4-dichlorophenoxyacetic acid (2,4D) as an active substance. The analysis of powder diffraction
patterns of the complexes under study shows that they are crystalline, but have different structures. Their
IR spectra have been interpreted in relation to the structural data available. It has been found out that
metals are coordinated by the carboxylate groups of the ligands. The IR analysis does not allow to establish
precisely the mode of coordination of the metals with the carboxylate groups. The thermal decomposition
of all the compounds proceeds in a similar way. In the case of the hydrated complexes first the 1–3 step
dehydration process takes place. The decomposition of the anhydrous compounds is a two step process
with a complicated course. Total mass losses for the mercury, lead and cadmium complexes are close to
100%, thus in the process of thermal decomposition the metals form volatile compounds.
Keywords:
Metals(II)
Phenoxylic herbicides (2,4D and 2,4DP)
Complexes
X-ray diffractometry
IR analysis
Thermal analysis
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
are not so well known, although they have also been described in
literature [5,8–11]. Since not all the literature data on complexes
Phenoxyalkanoic acids, containing chlorine and/or methyl
group in 2 and 4 position of benzene ring, show herbicidal prop-
erties. These compounds are selective herbicides because they
destruct effectively broadleaf plants whereas the monocotyle-
donous plants are resistant to them. 2,4-dichlorophenoxyacetic
(2,4D), 2-(2,4-dichlorophenoxy)propionic (2,4DP), 4-chloro-2-
methylphenoxyacetic (MCPA) and 4-chloro-2-methylphenoxy-
propionic (MCPP) acids are widely used for protection of crops and
water plants control. Due to the presence of the carboxylate group
in their structure, the compounds can react with metal ions to form
complexes which are sparingly soluble in water.
In Part I, complexes of Pb(II), Cd(II) and Cu(II) with MCPA
were studied [1]. Synthesis and properties of solid complexes of
2,4D with Cu(II) [2–5], Zn(II) [2,4,6–8], Cd(II) [2,4,9], Pb(II) [4,9],
Mn(II) [2,7,10], Co(II) [2,11,12] and Ni(II) [2,11] have been hitherto
described. Complexes of the above mentioned metals with 2,4DP
with 2,4D are consistent, and the complexes with 2,4DP have been
investigated only by Ptaszynski and Zwolinska [5,8–11], it seemed
useful to carry out their syntheses and comparative studies on the
properties of these complexes.
The present work concerns complexes of 2,4D and 2,4DP with
metals present in soil and water environments. Among the met-
als under study are microelements and macroelements Cu(II),
Zn(II), Mn(II), Co(II), Ni(II), Fe(II), Ca(II), Mg(II) necessary for plants
and toxic heavy metals Cd(II), Hg(II), Pb(II). The new compounds
obtained in our study are mercury, iron and calcium complexes
with 2,4D and 2,4DP and the magnesium complex with 2,4D. Ele-
mental, IR, X-ray and thermal analyses were carried out to examine
them.
The current interest in investigating these complexes results
from the pollution of the environment with heavy metals and pes-
ticides. It was assumed that similar complex formation can also
occur between phenoxyacetic herbicides and metals present in the
soils and waters. These herbicides are usually applied in the form
of a suspension at the time when the crops are sprouting. After
spraying, the herbicides get also into the soil where they undergo
∗
Corresponding author.
E-mail address: joankob@p.lodz.pl (J. Kobyłecka).
0040-6031/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2008.10.005

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J. Kobyłecka et al. / Thermochimica Acta 482 (2009) 49–56
Table 1
Chemical analyses and solubility of 2,4D complexes.
Compound
Analysis: found (calculated) (%)
S × 103 (mol L−1
)
Metal
C
H
Cl
H2O
Hg(C8H5O3Cl2)2
30.23 (31.31)
9.81 (9.69)
9.84 (9.83)
9.99 (9.71)
9.68 (9.67)
8.06 (7.78)
4.56 (4.38)
10.66 (11.04)
11.48 (11.69)
19.73 (19.10)
30.88 (31.15)
29.26 (29.97)
34.07 (33.89)
33.45 (33.83)
32.18 (31.66)
32.16 (31.47)
36.92 (37.20)
35.59 (34.63)
33.05 (33.39)
34.33 (34.35)
31.50 (32.66)
27.65 (28.89)
1.78 (1.56)
3.26 (3.20)
3.01 (3.19)
3.58 (3.65)
3.96 (3.62)
2.55 (2.71)
4.15 (3.61)
2.88 (3.13)
2.80 (2.86)
2.26 (2.38)
1.75 (1.80)
0.01
11
4.2
5.3
6.0
8.6
Mn(C8H5O3Cl2)2·4H2O
Fe(C8H5O3Cl2)2·4H2O
Co(C8H5O3Cl2)2·6H2O
Ni(C8H5O3Cl2)2·6H2O
Ca(C8H5O3Cl2)2·2H2O
Mg(C8H5O3Cl2)2·5H2O
Cu(C8H5O3Cl2)2·4H2Oa
Zn(C8H5O3Cl2)2·3H2Oa
Cd(C8H5O3Cl2)2·2H2Oa
Pb(C8H5O3Cl2)2·H2Oa
25.29 (25.01)
25.47 (24.97)
23.23 (23.36)
22.81 (23.37)
25.06 (27.47)
26.04 (25.58)
23.84 (24.64)
25.85 (25.35)
21.46 (24.11)
18.25 (21.32)
12 (12.70)
12 (12.68)
18 (17.8)
17 (17.8)
7 (6.97)
17 (16.23)
13 (12.51)
10 (11.89)
5.5 (5.23)
3 (2.71)
1.1
7.0
2.8
0.15
a
As published in ref. [4].
complex reactions leading to their complete biodegradation. The
half-life period in soil of 2,4D and MCPA is considered to be 2–4
weeks [13]. The solubility of metal–herbicide complexes may affect
the mobility of the metal in the soil and resistance of herbicides
to biodegradation. Our experiments indicate that spraying of the
wheat with 2,4D and MCPA decreases the uptake of Cu, Zn and Mn
by the wheat stems [14]. Widespread use of phenoxylic herbicides
causes accumulation of hazardous waste (unused and outdated her-
bicides, pesticide packaging). This waste has to be disposed of in a
suitable way, for instance in the processes of thermal decomposi-
tion. The results of our thermal analyses may be used to develop
safe technologies to utilize mixed waste containing herbicides and
heavy metal compounds.
2.2. Synthesis and chemical analysis
The complexes under study were prepared by reaction of sodium
salts of 2,4D or 2,4DP with metal nitrates in a molar ratio 2:1.
The 2,4D and 2,4DP acids are sparingly soluble in water and they
were used in the synthesis after transformation into soluble sodium
salts. Samples of 2,4D or 2,4DP (0.01 mol) were mixed with 80 mL
of distilled water and then a 1 mol L−1 NaOH solution was added
dropwise while stirring to complete dissolution of acid. If the pH
of obtained solutions was alkaline it was adjusted to pH about 7
with a 1 mol L−1 HNO3 (pH-meter Mettler Delta 350). Samples of
metal nitrates (0.005 moles) were dissolved in 20 mL of distilled
water (in the case of mercury in 1 mol L−1 HNO3) and added to
Na-2,4D or Na-2,4DP solutions, which causing precipitation of the
compounds under study. The precipitates were filtered after 24 h,
washed with a small amounts of distilled water and dried in air
at room temperature. The complexes were obtained with yields
60–90%.
2. Experimental
2.1. Chemicals
The composition of the complexes was established on the basis
of metal, carbon, hydrogen, chlorine and water contents determina-
tion (Tables 1 and 2). The metals (except mercury) were determined
by complexometric titration [16] after decomposition of complexes
using concentrated HNO3 (UniClever BM-1z mineralizer, Plazma-
tronika) while carbon, hydrogen and chlorine by elemental analysis
methods (CHN analyser, Carlo Erba 1108). To determine the content
of mercury, weighed samples of the complexes were decomposed
by means of 1 mol L−1 NaOH, the released HgO was dissolved in
1 mol L−1 HNO3 and mercury was determined in the obtained solu-
tion by complexometric back-titration.
2,4-dichlorophenoxyacetic acid (2,4D) and 2-(2,4-dichloro-
phenoxy)propionic acid (2,4DP), (98% and 95%, respectively) were
obtained from Aldrich. They were purified by double crystalliza-
tion from toluene. Their melting temperatures after purification
were 141 ◦C (theoretical melting point 140.5 ◦C) and 118 ◦C (the-
oretical melting point 117.5–118.1 ◦C), respectively [15]. Pesticide
preparation “Aminopielik 450 SL” (Rokita S.A. Poland). Cu(II), Zn(II),
Cd(II), Hg(II), Pb(II), Mn(II), Co(II), Ni(II), Ca(II), Mg(II) nitrates(V)
and Fe(II) chloride (GR for analysis, Merck). Stock solutions of
the same metals (1000 mg L−1 as nitrate salts in 0.5 mol L−1 nitric
acid) were purchased from Merck. Working solutions were pre-
pared by appropriate dilution of standard solutions. The other
chemicals are p.a. from POCh-Poland or GR for analysis from
Merck.
Attempts were also made to precipitate the compounds under
study from the herbicidal preparation “Aminopielik” which con-
tains 2,4-dichlorophenoxyacetic acid in the form of the ammonium
salt. Solutions of the metals were added to the 100 times diluted
Table 2
Chemical analyses and solubility of 2,4DP complexes.
Compound
Analysis: found (calculated) (%)
S × 103 (mol L(1
)
Metal
C
H
Cl
H2O
Hg(C9H7O3Cl2)2
29.25 (29.99)
9.03 (8.70)
9.31 (9.36)
9.40 (9.28)
9.19 (9.25)
32.69 (32.30)
34.02 (34.25)
35.30 (36.27)
34.66 (34.04)
33.55 (34.06)
38.98 (39.70)
38.41 (39.33)
34.42 (35.69)
34.58 (35.69)
33.40 (32.00)
2.08 (2.09)
4.03 (4.15)
3.51 (3.72)
3.99 (4.13)
4.04 (4.13)
3.24 (3.31)
2.74 (2.93)
3.02 (3.63)
2.94 (2.92)
2.29 (2.07)
0.03
24
Mn(C9H7O3Cl2)2·6H2O
Fe(C9H7O3Cl2)2·4H2O
Co(C9H7O3Cl2)2·6H2O
Ni(C9H7O3Cl2)2·6H2O
Ca(C9H7O3Cl2)2·2H2O
Cu(C9H7O3Cl2)2·H2O
Zn(C9H7O3Cl2)2·4H2O
Cd(C9H7O3Cl2)2·2H2O
Pb(C9H7O3Cl2)2
22.32 (23.46)
22.69 (23.79)
22.18 (22.30)
20.97 (22.34)
23.25 (26.05)
24.04 (25.80)
21.96 (23.43)
20.82 (23.00)
18.66 (21.01)
16.5 (17.11)
10 (12.08)
18 (17.01)
17.5 (17.01)
7 (6.61)
2.5 (3.27)
14 (11.89)
7 (5.22)
2.4
5.8
7.0
4.6
6.4
15
7.64 (7.37)
11.38 (11.56)
10.82 (10.80)
17.78 (18.24)
29.07 (30.69)
8.6
1.0

J. Kobyłecka et al. / Thermochimica Acta 482 (2009) 49–56
51
Table 3
Yield of the synthesis and metal contents in the precipitates obtained in the reaction of metal nitrate with “Aminopielik”.
Metal
Yield (%)
Metal content (%)
Found
Calculated
Hg
Cu
Cd
Pb
100
100
60
29.75
8.58
19.40
31.73
31.33
11.04
19.10
31.15
100
“Aminopielik” solution (pH about 7) recommended for spraying
plants maintaining 2:1 molar ratio. The precipitates appeared
immediately in the case of Cu(II), Pb(II) and Hg(II) and after a
week in the case of Cd(II). Metal content was determined in the
precipitates (Table 3). The compounds of Cu(II), Pb(II) and Hg(II)
precipitated also from more diluted (250, 500 and 1000 times)
“Aminopielik” solutions.
Water solubility of the complexes (at 293 K) was established on
the basis of the content of metal determined by complexometric
[16], spectrophotometric [17] or CV-AAS (mercury) methods in the
saturated water solutions (Tables 1 and 2).
metal = 2:1. In the case of the mercury compounds it was necessary
to acidify the solution of mercury nitrate to avoid precipitation
of HgO. The hydration state of the compounds is different. Both
complexes of mercury and the complex of lead with 2,4DP are anhy-
drous and one water molecule is attached to Pb-2,4D and Cu-2,4DP.
The other compounds contain from 2 to 6 water molecules.
Water solubility of the obtained complexes varies considerably
(Tables 1 and 2) and according to the increasing S value they can be
put in the following way:
2,4D
Hg < Pb < Cu < Cd < Fe < Co < Ni < Zn < Ca < Mn < Mg
Hg < Pb < Fe < Ca < Co < Cu < Ni < Cd < Zn < Mn
2,4DP
2.3. X-ray analysis
In both series the compounds of mercury and lead are defi-
nitely the most sparingly soluble and the manganese compound
– the most readily soluble. The compound of magnesium with
2,4D exhibits the highest solubility. Its analogue with 2,4DP was
not obtained, probable because it is more soluble than magnesium
compound with 2,4D. In most cases the salts of the same metals
with 2,4DP were more readily soluble than those with 2,4D. Sim-
ilar observations were made in refs. [9,11]. The complexes of iron
and calcium are an exception.
The above presented series of solubility of the 2,4D complexes
explains the result of the experiment with the use of the herbicide
“Aminopielik” which contains 2,4D. In solution of “Aminopielik”
precipitates appear immediately after the addition of mercury, lead
and copper salts and after a week in the case of cadmium salt. The
content of Hg, Pb and Cd determined in these precipitates is similar
to the content established in the complexes under study (Table 3).
The solid obtained with “Aminopielik” contained a low percentage
of copper in comparison with Cu(2,4D)2·4H2O. It can be caused
by co-precipitation of the “Aminopielik” components. Under the
same conditions the other metals did not appear in the form of
precipitates.
Powder diffraction patterns of the complexes under study were
recorded on a Siemens 5000 diffractometer using CuK␣ radiation
monochromatized by means of a secondary graphite monochroma-
tor within the 2Â range 2–90◦.
2.4. IR spectra
IR spectra of 2,4D or 2,4DP, their sodium salts and com-
plexes under study were recorded on a FTIR-8501 spectrometer
(Shimadzu), within frequency range 4000–400 cm−1 in KBr discs
(Table 4).
2.5. Thermal analysis
The thermal decomposition curves of complexes under study
were obtained on OD-102/1500 derivatograph. The samples of
100 mg were heated in corundum crucible in static air atmosphere.
␣-Al2O3 was used as standard material. All thermal investigations
were carried out within the temperature range 20–1000 ◦C at a
heating rate of 10◦/min.
3. Results and discussion
3.2. X-ray analysis
3.1. Composition and solubility of the complexes
Powder diffraction patterns indicate that the obtained com-
plexes are crystalline and have different structures. The number of
the peaks proves that the complexes of Zn(II), Cd(II), Pb(II), Mn(II),
Co(II), Ni(II), Ca(II) and Mg(II) have higher degree of crystallinity
than the Cu(II), Hg(II) and Fe(II). Fig. 1 presents examples of diffrac-
tion patterns of new Ca complexes with 2,4D and 2,4DP and the Mg
complex with 2,4D.
Monocrystals of Co, Ni, Ca and Mg compounds with 2,4D and
Mn with 2,4DP sufficient for structural studies were obtained and
their structures were resolved [18]. The crystallographic structures
of Cu and Zn complexes with 2,4D have been described earlier
by Smith and Kennard [3,6]. The structures of the other com-
plexes under study are unknown. The copper complex is a dimer
of a formula [Cu2(2,4D)4(H2O)2]·2H2O, in which bidentate bridg-
ing carboxylate groups are present [3]. The zinc complex is also a
dimer {[Zn(2,4D)2(H2O)4] [Zn(2,4D)2(H2O)2]}, in which an octa-
hedral and tetrahedral complexes are bound by hydrogen bonds
and carboxylate groups are unidentate ligands [6]. The Co, Ni and
Twenty one solid compounds of the general formula ML2·nH2O,
where M = Cu(II), Zn(II), Cd(II), Hg(II), Pb(II), Mn(II), Fe(II), Co(II),
Ni(II), Ca(II), Mg(II), L = 2,4D or 2,4DP and n = 0–6 were obtained.
The series with 2,4D consists of 11 complexes (Table 1) and that
with 2,4DP – 10 (Table 2), because it was not possible to obtain
the magnesium complex with this ligand in the solid form. The
complexes of Hg(II), Fe(II) and Ca(II) with 2,4D and 2,4DP and the
complex of Mg(II) with 2,4D were obtained for the first time so
more attention will be given to them in the discussion. Analogous
general composition of the complexes was reported in literature
[1–12], with the exception of [8], where the composition of zinc
complex with 2,4D was found to be M2L3. In some cases differences
also appear in the number of water molecules attached.
All of the compounds were precipitated from neutral solutions
of sodium salts of 2,4D or 2,4DP, to which water solutions of nitrates
of respective metals were added maintaining the molar ratio ligand:

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J. Kobyłecka et al. / Thermochimica Acta 482 (2009) 49–56
Fig. 1. X-ray diffraction patterns of: (a) Mg(2,4D)2·5H2O, (b) Ca(2,4DP)2·2H2O, (c) Ca(2,4D)2·2H2O.
Mg compounds with 2,4D which we investigated are complex salts
containing two unidentate carboxylate groups and water molecules
in their coordination spheres. The Co and Ni complexes are poly-
meric structure with bridges formed by one of the water molecules
and Mg compound is monomer. In the Ca complex with 2,4D each
carboxylate group is bidentate chelating in relation to one metal
atom and unidentate in relation to the other. The bonds with car-
boxylate groups acting as unidentate ligands cause formation of
polymer chains which are connected by hydrogen bonds, thus giv-
ing a lamellar structure. An analogous structure of Pb(MCPA)2·H2O
was established by Kruszynski et al. [19]. The ligand in this com-
plex is 4-chloro-2-methylphenoxyacetic acid which differs from
2,4D with one substituent in the benzene ring. The structures
of several other metals with MCPA have also been determined.
Kruszynski and Turek [20] found that Cd(MCPA)2·2H2O is a poly-
mer whose chains are formed by bidentate bridging carboxylate
groups and polymer net is stabilized by hydrogen bonds. Shulgin
and Konnik [21] found out that Cu(MCPA)2·4H2O has an analogous
polymer structure but postulated a formation of a bond between
copper and ether oxygen. Smith et al. [22] examined the structure
of the magnesium compound and found it to be a monomer of
the formula [Mg(MCPA)2·4H2O]·2H2O and ruled out a bond of the
metal with ether oxygen. Tangoulis et al. [23] pointed out that the
manganese complex is a polymer with a layer structure in which
bidentate bridging carboxylate groups are bound with manganese
atoms [Mn(MCPA)2·(H2O)2]n. The only metal complex with 2,4DP,
whose structure is known and was solved by us, is a polymeric
manganese compound of a formula [Mn(2,4DP)2·(H2O)3]·3H2O)]n
[18], where manganese is attached to two unidentate carboxylate
groups and four water molecules. In the polymer chain there are
Table 4
Principal IR bands for OCO groups in sodium salts and obtained complexes.
Compound
ꢀCOOH (cm−1
)
ꢀ(OCO)as (cm−1
)
ꢀ(OCO)s (cm−1
)
ꢁꢀ = ꢀas − ꢀs (cm-1
)
2,4D
1738.9
Na(2,4D)
Hg(2,4D)2
1622.0
1618.2
1589.2
1584.0
1575.7
1580.2
1591.2
1595.0
1635.5
1616.2
1618.2
1618.2
1429.2
1427.2
1423.4
1425.3
1415.7
1416.0
1430.1
1427.2
1429.2
1425.3
1429.2
1438.8
192.8
191.0
160.6
158.7
160.0
164.2
161.1
167.8
206.3
190.9
198.0
179.4
Mn(2,4D)2·4H2O
Fe(2,4D)2·4H2O
Co(2,4D)2·6H2O
Ni(2,4D)2·6H2O
Ca(2,4D)2·2H2O
Mg(2,4D)2·5H2O
Cu(2,4D)2·4H2O
Zn(2,4D)2·3H2O
Cd(2,4D)2·2H2O
Pb(2,4D)2·H2O
2,4DP
1716.5
Na(2,4DP)
Hg(2,4DP)2
1616.0
1608.5
1589.2
1591.2
1591.2
1587.3
1575.7
1624.0
1590.0
1568.5
1593.1
1417.6
1408.0
1415.7
1419.5
1413.7
1419.5
1432.0
1419.5
1417.6
1418.5
1417.6
198.4
200.5
173.5
171.7
177.5
167.8
143.7
204.5
172.4
150.0
175.5
Mn(2,4DP)2·6H2O
Fe(2,4DP)2·4H2O
Co(2,4DP)2·6H2O
Ni(2,4DP)2·6H2O
Ca(2,4DP)2·2H2O
Cu(2,4DP)2·H2O
Zn(2,4DP)2·4H2O
Cd(2,4DP)2·2H2O
Pb(2,4DP)2·H2O

J. Kobyłecka et al. / Thermochimica Acta 482 (2009) 49–56
53
Fig. 2. Thermal analysis curves of metal complexes with 2,4D: (a) Ni, (b) Ca, (c) Mg.
bridges formed by one of the water molecules between manganese
atoms.
[25,26]. According to these criteria, the highest ꢁꢀ values should
accompany unidentate coordination, lower-bidentate bridging and
the lowest-bidentate chelating. The direction of shifts of ꢀ(OCO)as
and ꢀ(OCO)s frequencies in relation to ionic bond in sodium salts
should be successive:
3.3. IR spectra
In the spectra of metal complexes with carboxylic acid, bands
of the valence vibrations of nondissociated COOH group (over the
range 1760–1690 cm−1) are not observed. On the contrary there
are bands of asymmetrical (1610–1550 cm−1) and symmetrical
(1420–1300 cm−1) vibrations of dissociated OCO groups [24]. The
magnitude of separation ꢁꢀ = ꢀ(OCO)as − ꢀ(OCO)s and the direction
of the shifts of these bands in relation to the respective frequencies
in the spectrum of the sodium salt of the acid are often used as
spectroscopic criteria to determine the mode of carboxylate binding
(1) unidentate – ꢀ(OCO)as higher, ꢀ(OCO)s lower;
(2) unsymmetrical bidentate bridging – ꢀ(OCO)as higher, ꢀ(OCO)s
almost the same;
(3) symmetrical bidentate bridging – ꢀ(OCO)as higher, ꢀ(OCO)s
higher;
(4) unsymmetrical bidentate chelating – ꢀ(OCO)as almost the same,
ꢀ(OCO)s higher;
Fig. 3. Thermal analysis curves of metal complexes with 2,4DP: (a) Zn, (b) Hg, (c) Fe.

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J. Kobyłecka et al. / Thermochimica Acta 482 (2009) 49–56
(5) symmetrical bidentate chelating – ꢀ(OCO)as lower, ꢀ(OCO)s
shifts of the bands are the same as in the Ca complexes but ꢀ(OCO)as
shift is much smaller. This is partly in agreement with the results of
the structural analysis of Ca compounds with 2,4D [18] and Pb com-
pounds with MCPA [19], which show that they are polymers with
a unsymmetrical bidentate chelating and unidentate carboxylate
group. In the spectra of Co, Ni and Zn complexes with 2,4D ꢀ(OCO)as
bands appear at lower and ꢀ(OCO)s bands at higher frequencies
than in the case of the sodium salt, which results in low values
ꢁꢀ and indicates the presence of bidentate chelating carboxylate
groups. This disagrees with the results of structural analysis which
indicate that Co and Ni salts are polymers and the Zn compound
is a dimer with unidentate carboxylate groups. In the spectra of
the other complexes under study, ꢀ(OCO)as bands are shifted in the
direction of lower frequencies and ꢀ(OCO)s bands also in the direc-
tion of lower values or remain the same as for the sodium salt. This
is against the assumptions of the spectroscopic criterion. The struc-
tures of only two complexes: a Mg monomer with 2,4D [18] and a
Mn polymer with 2,4DP [18] are known. Both compounds contain
unidentate carboxylate groups. Zn(II), Pb(II), Fe(II), Co(II) and Ni(II)
complexes with 2,4DP exhibit ꢀ(OCO)as and ꢀ(OCO)s vibration fre-
quencies similar to those of Mn complex with 2,4DP and similar ꢁꢀ
values. It can be supposed that their structures are analogous to the
higher.
In the IR analysis of the complexes under study (Table 4), the
information resulting from the above-discussed X-ray analyses was
taken into account. The structures of six of the metal complexes
with 2,4D (Cu, Zn, Co, Ni, Ca, Mg) and three metal complexes with
its analogue MCPA (Cd, Pb, Mg) are known. The spectroscopic crite-
rion could be used to determine the mode of coordination in only a
few of the 21 investigated complexes. In the spectrum of Cu com-
plex with 2,4D, ꢀ(OCO)as is shifted towards higher frequencies and
ꢀ(OCO)s remains the same, so it can be supposed to be unsymmet-
rical bidentate bridging coordination, which is in agreement with
the structure of the compound [3]. It is similar in the spectrum of Cu
complex with 2,4DP, but the ꢀ(OCO)s band is shifted toward higher
frequencies, which may indicate symmetrical bidentate bridging
coordination. In the spectra of Ca complexes with 2,4D and 2,4DP
a big shift of ꢀ(OCO)as in the direction of lower frequencies is
observed while ꢀ(OCO)s shifts toward higher frequencies. It fol-
lows that coordination in both complexes is symmetrical bidentate
chelating. A similar carboxylate group coordination mode follows
from the spectrum of Pb complex with 2,4D, where the directions of
Table 5
Thermal analysis of 2,4D complexes.
Compound
Temperature range of decomposition stages (◦C)
Mass loss (%)
Temperature of DTA peaks and thermal effects (◦C)
Found (calc.)a,b
Total
100
Hg(2,4D)2
160–335
335–800
86 (50.55)b
14
590 exo
Mn(2,4D)2·4H2O
60–85
85–130
240–360
360–660
6 (6.35)a
6 (6.35)a
41 (57.11)b
34
87
80 endo
130 endo
390 exo
520 exo
Fe(2,4D)2·4H2O
Co(2,4D)2·6H2O
45–100
180–300
300–550
12 (12.68)a
55 (57.02)b
18.5
85.5
89
95 endo
440 exo
500 exo
40–90
90–140
260–340
340–640
12.5 (11.86)a
5.5 (5.93)a
42 (53.25)b
29
90 endo
140 endo
350 exo
560 exo
Ni(2,4D)2·6H2O
Ca(2,4D)2·2H2O
Mg(2,4D)2·5H2O
50–110
280–500
500–650
17 (17.79)a
51 (53.25)b
18
88
83
95
90 endo
390 exo
570 exo
80–140
270–480
480–700
7 (6.97)a
60 (62.74)b
16
130 endo
440 exo
570 exo
60–90
90–120
120–170
280–470
470–740
7.0 (6.50)a
3.5 (3.25)a
6.5 (6.50)a
56 (58.41)b
22
80 endo
110 endo
160 endo
430 exo
540 exo
Cu(2,4D)2·4H2O
Zn(2,4D)2·3H2O
Cd(2,4D)2·2H2O
60–100
220–340
340–880
13 (12.51)a
55 (56.26)b
23
91
88
99
99
105 endo
270 exo
470 exo
60–100
260–340
340–620
10 (11.89)a
58 (57.88)b
20
90, 250 endo
340 exo
580 exo
95–130
235–360
360–770
5.5 (5.23)a
43.5 (55.00)b
50
130 endo
380 exo
540 exo
Pb(2,4D)2·H2O
100–140
210–360
360–810
3 (2.71)a
49 (48.68)b
47
130, 220 endo
380 exo
570 exo
a
Calculated for the number of water molecules as found in the complex structure.
Calculated for two –O–C6H3Cl2 leaving groups.
b

J. Kobyłecka et al. / Thermochimica Acta 482 (2009) 49–56
55
structure of the manganese compound. However, full interpretation
of the mode of coordination between metal(II) and carboxylates in
the obtained compounds will be possible after the determination
of their crystal structure.
two, then one, and finally the other two molecules of water (Fig. 2).
Similarly, the complex of Fe with 2,4DP which contains 4 molecules
of H2O is dehydrated in a one step process, while the dehydra-
tion of the tetrahydrous Zn compound with 2,4DP proceeds in two
steps and the compound loses two water molecules in each of them
(Fig. 3). Fig. 3 also presents thermal analysis curves of the anhydrous
Hg complex with 2,4DP.
3.4. Thermal analysis
TG, DTG and DTA curves (Figs. 2 and 3 and Tables 5 and 6) show
that the thermal decomposition of the complexes proceeds in a
similar way. The first step of the decomposition of the hydrated
compounds is dehydration. Thermal decomposition of anhydrous
compounds consists of two steps with a complex course.
The temperature ranges at which the compounds under study
are dehydrated are different. The one step dehydration of Cu, Fe and
Ni with either ligand, Zn with 2,4D and Mn and Co with 2,4DP begins
at 45–60 ◦C and ends at 100–110 ◦C, whereas the dehydration of
Cd, Pb and Ca complexes with 2,4D and Ca complex with 2,4DP
occurs at higher temperatures, from 80–100 ◦C to 130–140 ◦C. In
the case of complexes of Zn and Cd with 2,4DP and Mn and Co with
2,4D which release water in two steps, the first step of dehydration
begins at 20–60 ◦C and ends at 70–95 ◦C and the second step ends
at 120–150 ◦C. The three step dehydration of the Mg complex with
2,4D ends at the highest temperature (170 ◦C), which indicates a
strong bond with two water molecules in the complex.
3.4.1. Dehydration
As they are heated, most of the compounds lose water in
one step, but dehydration of Mn(2,4D)2·4H2O, Co(2,4D)2·6H2O,
Zn(2,4DP)2·4H2O and Cd(2,4DP)2·2H2O proceeds in two, and that
of Mg(2,4D)2·5H2O in three steps. On the basis of loss in mass in the
process, the number of water molecules in the obtained complexes
was determined. The loss in mass showing in the TG curve corre-
sponds with an appropriate number of endothermic peaks in the
DTA curve and distinct peaks in the DTG curve. The number of dehy-
dration steps does not depend on the number of water molecules
in the complex. Thus, for instance, Ni and Ca complexes with 2,4D,
containing 6 and 2 H2O molecules, respectively, lose water in one
step while dehydration of the Mg pentahydrate complex is a pro-
cess consisting of three steps, in which the compound loses first
3.4.2. Decomposition of the anhydrous compounds
The course of TG and DTG curves indicates that the thermal
decomposition of all the anhydrous complexes can be divided into
two steps (Figs. 1 and 2, Tables 4 and 5). In the first step the
compounds decompose rapidly with mass losses within the range
40–60% and in the case of the mercury complexes reaching up
Table 6
Thermal analysis of 2,4DP complexes.
Compound
Temperature range of decomposition stages (◦C)
Mass loss (%)
Found (calc.)
Temperature of peaks and thermal effects (◦C)
Total
100
Hg(2,4DP)2
180–320
320–800
85 (48.42)a
15
265 exo
510 exo
Mn(2,4DP)2·6H2O
50–100
240–370
370–740
16 (17.11)b
50 (51.31)a
21
87
90 endo
380 exo
510 exo
Fe(2,4DP)2·4H2O
Co(2,4DP)2·6H2O
Ni(2,4DP)2·6H2O
Ca(2,4DP)2·2H2O
Cu(2,4DP)2·H2O
Zn(2,4DP)2·4H2O
50–130
200–310
310–505
11 (12.08)b
54 (54.33)a
19
84
90 endo
580 exo
50–110
280–370
370–560
16.5 (17.01)b
49 (50.99)a
21
86.5
89.5
83
105 endo
360 exo
480, 540 exo
50–110
285–375
375–655
17.5 (17.01)b
48 (50.99)a
24
100 endo
350 exo
580, 630 exo
100–140
270–445
445–840
7 (6.4)b
58.5 (59.5)a
17.5
150 endo
420 exo
585 exo
60–95
230–310
310–940
3 (3.27)b
60 (58.91)a
27
90
80 endo 280 exo
460 exo
40–85
85–120
260–350
350–610
6.5 (5.95)b
7.5 (5.95)b
54.5 (53.43)a
17
85.5
70 endo
100 endo
400 exo
530 exo
Cd(2,4DP)2·2H2O
50–95
95–120
260–320
320–760
3 (2.61)b
3.5 (2.61)b
47.5 (52.52)a
36
91
99
80 endo
110 endo
350 exo
535 exo
Pb(2,4DP)2
235–350
350–950
49 (47.95)a
50
190 endo
320, 590 exo
a
Calculated for two –O–C6H3Cl2 leaving groups.
Calculated for the number of water molecules as found in the complex structure.
b

56
J. Kobyłecka et al. / Thermochimica Acta 482 (2009) 49–56
The temperature of the beginning of the DTG peak associated
with the first decomposition step was taken as the criterion of ther-
mal stability of the complexes under investigation. Fig. 4 presents
the respective comparison for the two series of metal complexes
with 2,4D and 2,4DP. Mercury complexes are characterized by the
lowest thermal stability and it should be stressed that already at
160–180 ◦C highly toxic compounds appear in the air. A correla-
tion was also observed between the temperature of the onset of
the decomposition of the anhydrous compounds and the solubility
of the complexes. The complexes of the metals under study with
2,4DP which were found to be more readily soluble in water than
the complexes with 2,4D, usually show higher thermal stability.
The least stable Hg complexes, on the other hand, exhibit also the
lowest solubility.
3.5. Conclusions
In the metal complexes with phenoxyalkanoic acids the car-
boxylate group may be either unidentate or bidentate bridging or
chelating. The complexes may be monomers, dimers or polymers.
Dimeric and polymeric structures are formed by bidentate bridg-
ing carboxylate groups, but metal atoms may be bridged by water
molecules and dimers may be formed by hydrogen bonds. Hydro-
gen bonds are present in all complexes with phenoxyalkanoic acid
anions and they stabilize their structure. Due to such great diversity
of the structures of the complexes under study: (i) spectroscopic
criterion ꢁ cannot be used for explicit determination of the mode of
coordination between metal ions and carboxylate groups; (ii) ther-
mal stability of these complexes does not depend on the properties
of the central ion.
Fig. 4. Thermal stabilities of all anhydrous complexes under study.
to 86%. The peaks in the DTG curves are intensive and sharp, but
also unsymmetrical or split, which indicates the complexity of the
decomposition process. In this step pyrolysis of the organic lig-
ands probably proceeds and chlorophenoxy groups –O–C6H3Cl2 or
phenyl groups –C6H3Cl2 are split off. It is known from the avail-
able structural studies that the ether group oxygen bond with the
benzene ring in 2,4D and 2,4DP and their complexes with metals
are shorter than the bond with the carbon of a respective alkane
acid [3,6,18]. Thus –O–C6H3Cl2 groups are more likely to split off.
The mass losses in the first step of decomposition, calculated with
the assumption that the complex molecule loses two chlorophe-
noxy groups, are in agreement (within the range 5%) with the
experimental data (Tables 5 and 6), except for the mercury com-
plexes for which they are almost twice higher and the compounds
of Cd, Mn and Co with 2,4D and of Cd with 2,4DP for which they are
lower by about 10–30%. The split off organic fragments are com-
busted. In the DTA curves a broad exothermic peak begins from
the start of the first step of decomposition and ends as the sec-
ond is finished. It has two maxima and that corresponding with
the second decomposition step is usually more intensive. The sec-
ond step of decomposition is slower, the mass losses are smaller
and the corresponding DTG peaks are broad and not intensive. The
remaining organic substance is released, and the gaseous decompo-
sition products are combusted. The final decomposition products
are oxides of respective metals (identified by X-ray diffractometry),
with the exception of mercury, cadmium and lead, for which mass
losses are close to 100%. It follows that under the conditions of ther-
mal decomposition, Hg, Cd and Pb form volatile compounds. In the
case of Cd and Pb complexes, they can be volatile chlorides, while
in the case of decomposing Hg compounds, the release of mer-
cury vapour, the formation of volatile chlorides and metaloorganic
volatile derivatives, such as the very toxic methylmercury CH3Hg+,
have to be taken into account. The thermal analysis curves of the
mercury complexes show a considerably bigger loss of mass in the
first step of decomposition and less visible exothermic effects than
the derivatograms of the other compounds under study (Fig. 3). It
followsthatvolatilemercuryderivativesare oxidizedgradually dur-
ing the whole decomposition process and do not undergo complete
combustion.
Acknowledgements
This work was financially supported by the State Committee for
Scientific Research (grant no. 3 T09B 141 28)
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