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SYNTHESIS AND POTENTIOMETRIC ANALYSIS OF
ORGANOPHOSPHOROUS COMPLEXONES AND COMPLEXONATES
OF METALS ON THE BASIS OF 2-AMINOTHIAZOLE DERIVATIVES
Lyazat K. Salkeeva, Yelena V. Minaeva, Larissa M. Sugralina, Ekaterina K.
Taishibekova
E.A. Buketov’ Karaganda State University
Kazakhstan, 100028, Karaganda, 28;Universitetskaya str.
e-mail: [email protected]
Conditions of synthesis of 4-phenyl-2-amidothiazolylacetate and 2-thiourido-4-diethylphosphonothiazole
investigated for complexing ability in formation mixed ligand complexes were developed. Optimum conditions
and thermodynamic parameters of process of a complex formation were determined. Thermodynamic parameters
of reactions of mixed ligand complex formation with copper (II) and lead (II) ions were determined by
potentiometric method, composition of forming complexes on the basis of 4-phenyl-2-amidothiazole and its
phosphorylated derivatives was determined.
Synthesis of phosphorylated derivatives of thiazoles represents significant theoretical and
practical interest, remaining, nevertheless, practically unstudied field of research. Earlier we
showed
complexing
ability
of
2-[diethylamido-(4'-phenylthiazolyl-2'-
amido)phosphono]propylaldehyde with various cations of metals [1].
The basic regularities of a complex formation of phosphorylated ligands were revealed
by M.I. Kabachnik and Н.М. Dyatlova et al. on an example of compounds which structure
includes PO3N2 and РО2Н2 groups as fragments: completely phosphorylated derivatives of
amines, complexones combining two types of complexing groupings: both phosphonic and
carboxylic, oxyethyl or mercaptoethyl ones [2-4].
Presence of polybase salt forming groups and high base atom of nitrogen in structure of
phosphorylated complexone provides their consecutive dissociation in a wide interval of рН
that considerably widens borders of complexes existence for the most of cations. Phosphorous
containing complexones are known to form strong complexes bonded with hydrogen at
interaction with transitive metals, that is not characteristic for carboxyl containing prototypes.
Potential ability of aminoalkylphosphonic acids to exhibit increased dentisity by means
of oxygen of phosphonic groups favours to a complex formation with rare earth elements
which are known to have high coordination number and the big affinity to oxygen.
Alkylenepolyaminopolyalkylphosphonic acids form high-strength mononuclear complexes
bonded with hydrogen.
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We consider derivative of 2-amino-4-phenylthiazole (1), namely 4-phenyl-2amidothiazolylacetate (2), obtained by reaction of (1) with acetylchloride in the presence of
triethylamine to be interesting as compexing agent.
Compound (2) has acetyl group in its structure capable to enolization, and because of
+М-effect of amino-group the electronic density on oxygen atom of a carbonyl group is
increased, that, obviously, shifts balance to enol form.
Thus, at studying 4-phenyl-2-amidothiazolylacetate (2) as comlexing agent we
considered possibility of subject investigated to formation of intramolecular hydrogen bonds
both in a keto- form and in enol one.
Ph
N
S
(1)
O Et N
3
NH2 + CH3C
Cl
Ph
N
S
Ph
O
NH C CH3
N
S
(2)
OH
NH C CH2
The choice of subjects for studying complex formation that contain atom of phosphorus
or representing various functional derivatives of 2-amino-4-phenylthiazole (1) without atom
of phosphorus is caused by that the combination phosphorylated and unphosphorylated
ligands is possible for obtaining of complexes with wide range of properties.
Continuing research to obtain new phosphorylated derivatives of thiazole, we attempted
to get aminothiazole containing phosphorus component directly from the thiazole ring, based
on the classical method of closing the thiazole cycle known Hantzsch reaction scheme:
O
CH3
(EtO)2P C
O
+ H2N-C-NH2
O
S
J2
(EtO)2P
N
NH2
S
(3)
Synthesized 4 - (diethylphosphono)-2-aminothiazole (3) is a white crystalline solid,
soluble in water and organic solvents. The IR spectrum of (3) contains the characteristic
absorption bands in the 1040 cm-1 and 1206 cm-1 related to the vibrations of P-O-C and P = O,
respectively, and the fluctuations of the free amino groups correspond to two absorption
bands in the 3375-3380 cm-1.
Analogically phosphorylated thiuridothiazole (4), potentially possessing complexing
properties was synthesized on the basis of the aminothiazole (3):
2
O
O
(EtO)2P
HCl, H2O
N
S
(EtO)2P
N
NH2
NH2HCl
S
(3)
O
O
(EtO)2P
N
(EtO)2P
NH4SCN
NH
NH2HCl
S
NH2
N
S
C
S
(4)
The resulting 2-thiourido-4-diethylphosphonothiazol (4) is a white crystalline solid,
soluble in water, alcohol and other polar solvents. The structure of the resulting
phosphorylated thiourea derivative suggests the potential ability to form complexes at the
expense of the thiazole ring nitrogen atoms and polar thiocarbonyl group.
Complexing ability of compounds (2) and (4) was investigated by studying the reactions
with copper acetate (II) in various water - organic media. Alcohol, acetone, dioxane,
dimethylformamide were used as solvents mixed with water. The experiments showed that
colored compounds of greenish-blue color formed only in water-alcohol medium. All further
studies were carried out using ethanol. Based on the results obtained by studying the structure
of the complex (5, 6) we proposed the structure shown at the scheme:
CH3
NH
S
O
N
Ph
N
S
Cu(CH3COO)2
O
O
Ph
Ph
Cu
N
NH C CH3 C2H5OH : C6H6. 50:50
NH
H3C
(2)
S
(5)
H2N
O
(EtO)2P
NH2
N
NH
S
C
S
(4)
N
S
CH
Cu(CH3COO)2
C2H5OH : C6H6. 50:50
S
O
(EtO)2 P
N
2+
Cu
N
P(OEt)2
O
S
CH
S
N
NH2
(6)
Physical and chemical study of complexation of transition metal ions with
organophosphorus ligands is not only of fundamental significance but also has applied aspect.
Practical use of complexones and their effect on biochemical processes require thorough
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study of the thermodynamic parameters of complexation reactions of these compounds in
solution. Let’s note that the compounds synthesized may act as agents for the analysis of
metal ions in different media, where by the quantitative side of the issue plays an important
role. This is confirmed by our investigation of the thermodynamic parameters of complexforming process of some compounds synthesized.
Experimental part
The IR spectra of the synthesized compounds were recorded by means of the
spectrometer «Nicolet Avatar-360" (KBr tablets, measurement error is 0.2 cm-1). 1H NMR
spectra were recorded by the spectrometer «Bruker DRX500» of operating frequency of 500
MHz in DMSO-d6 solution relatively to internal standard (TMS) (measurement error is ± 0.10.2 ppm). Melting point was detemined on the device «Boetius» (measurement error is ± 0.1
°C). Progress of all reactions and individuality of compounds were monitored by thin layer
chromatography on standard plates «Silufol UV-254» in the system of benzene - ethyl alcohol
= 6:1. Chromatograms were developed by iodine vapor.
Thermodynamic parameters of complexation reactions with copper (II) and lead (II) ions
in a water-alcohol solution at 298.15 - 318.15 K and ionic strength I = 0.1; 0.5; 1.0 (KNO3)
were determined by potentiometric method. Potentiometric determinations were carried on by
ionometer, as a measuring device there was used pH meter I-500 c with the rating
measurement error  2,5 mV using glass and silver chloride electrodes under temperaturecontrolled conditions. Silver chloride reference electrode EVL-1 M was always separated
from the working solution by electrolytic bridge filled with 0.1 M KCl solution. The potential
of the reference electrode was specified with respect to quinhydrone electrode. The solutions
were stirred by a magnetic stirrer. Titrations were performed in stages, namely approximately
and exactly by drops method, end point was found by integral and differential curves. The
working solution of complexonate was prepared using accurately weighed reagent. KNO3 of
"analytical grade" mark used for creation of the ionic strength was purified by double
recrystallization from water. The concentration of test solutions in all experiments was 0.01
mol/l. The ratio of M: L was varied from 1:1 to 1:3.
The experimental data were treated by the PHMETR program designed for the
calculation of equilibrium constants in systems with an arbitrary number of reactions
according to the measured equilibrium concentration of the particle. Potentiometric
experimental data processing according to PHMETR program has allowed identifying all
possible particles forming in the system. We can establish a number of mixed ligand ions and
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calculate the estimated values of the thermodynamic parameters of complex formation
reactions. Stability constants of the metal ions were determined on the base of results of
potentiometric titration in aqueous-alcoholic medium:
The curves: 1-0 I; 2 - 0.1 I; 3 - 0.5 I; 4 - 1.0 I;
Fig. 1. The dependence of the stability constants (lg β) of complexes on temperature: 4phenyl-2-amidothiazolylacetate
(2)
+
2-thiouridodiethylphosphonothiazole-4-(4)
with
Cu(CH3COO)2
As it can be seen from the temperature dependence of the stability constants (lgβ) of
mixed-ligand complexes of thiazole derivatives with copper ions, temperature increase leads
to a decrease in the stability of the compounds. This is primarily due to changes in the
hydrated environment of metal ion of complexing agent. However, the above data on the
equilibrium constants are not sufficiently informative. Therefore, we calculated the basic
thermodynamic parameters of the processes studied, the results of which are shown in Table
1.
Thermodynamic stability constants of the complexes were calculated by extrapolating the
concentration constants to zero ionic strength. Important for understanding the stability of the
complexes is an analysis of the change in entropy. Entropy change of systems at
complexation is connected with several factors such as the destruction of the hydration shells
of ligand groups, the displacement of water molecules from the first coordination sphere of
the metal ion and the formation of chelate structures. The positive contribution of the first two
factors in the reduction of systems ordering may not recover the decrease in entropy due to
the loss of configuration entropy due to increased stiffness of the chain in the implementation
of functional groups in the first coordination sphere of copper and lead ions to form a
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sufficiently stable multichelate structures. Table 1 shows the thermodynamic parameters of
the complex formation.
Table 1
The thermodynamic characteristics of complex formation in the system of 4-phenyl-2amidothiazolylacetate (2) + 2-thiouridodiethylphosphonothiazole-4-(4) with Cu(CH3COO)2
o
  r H 298
o
  r H 308
o
  r H 318
o
  r G298
o
  r G308
o
  r G318
o
  r S 298
o
  r S308
o
  r S318
The observed changes in the Gibbs free energy depend on the enthalpy and entropy
factors. The relative contribution of each of these factors depends on the nature of the ligand
and the central metal ion, the nature of indifferent salt (KNO3), creating an ionic strength of
the solution as well as its value. In other words, joining of each metal ion is considered as a
cross-linking of molecule. For the processes investigated a slight change in the Gibbs energy
proceeds with increase of temperature.
Currently one of the most urgent problems of modern analytical chemistry is control for
the content of the various chemical elements in natural and industrial subjects.
Spectrophotometric determination of metals in the form of complexes with organic reagents
by extraction methods is widespread in practice. However, the extraction is characterized by
difficulty and usage of toxic solvents. So the development of new off-exaction methods for
the determination of metals is of great interest.
We have studied the interaction of copper (II) ions with synthesized thiazole derivatives
(2) and (4) in various water-organic media. Alcohol, acetone, dioxane, dimethylformamide
were used as solvents mixed with water. The experiments showed that greenish-blue colored
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compounds were formed only in water-alcohol medium. All further studies were carried out
using ethanol.
The absorption spectra of the complexes (5) and (6) has two maxima in the UV(λ 350
and 340 nm) and in the visible (λ 705 and 700 nm) regions, respectively. The absorption
spectrum of water-alcohol solutions of thiazoles (2) and (4) has a single maximum of
absorption in the UV region at λ 360 and 355 nm, respectively. Graphic dependence of the
optical density (A) of the synthesized compounds (2), (5), (4) and (6) on the wavelength is
shown in Figures 2 and 3.
Figure 2 - Electronic absorption spectra of compounds (2) (curve 1) and
(5) (curve 2)
Figure 3 - Electronic absorption spectra of compounds (4) (curve 3) and
(6) (curve 4)
The study of complex formation in the established system is carried on in the visible
region because initial reactants have significant absorption in UV region of the spectrum.
To find the optimal conditions for the interaction of ions of copper (II) with a solution of
synthesized thiazoles (2) and (4) we investigated the influence of various factors such as the
percentage of organic solvent, reagent concentration, the acidity of the solution on the
complexation process.
The dependence of the optical density at the wavelength of the absorption maxima of the
initial reagents (2), (4) and obtained complexes (5) and (6) on the content of the ethanol was
stated to have a form of curve with a maximum shown in Figure 4. As it can be seen, the
greatest value of the optical density is observed when the ethanol content is 50% by volume
for both cases.
Curve 1 - Dependence for the (2, 4)
Curve 2 - Dependence for the (5, 6)
Figure 4 - Dependence of the optical density at the wavelength of the absorption maxima
of the initial reagents (2) and (4) (curve 1) and the obtained complexes (5) and (6) (curve 2)
on the ethanol content in the mixed solvent
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Figure 5 shows the effect of medium acidity on the strength of the forming complexes (5)
and (6), expressed in terms of change of the optical density of the solution from pH. The
results obtained show that the maximum degree of complexation is achieved in the 8-8.5 pH
range.
Figure 5 - Dependence of the optical density of the solutions on pH
(Curve 1 - compound (5), curve 2 - compound (6))
The study of the effect of concentration of reagents (2) and (4) necessary for complete
binding of analyzed ion in colored compound is shown in Figure 6.
As it was seen on Figure 6 the concentration of the reagent is 1.25 mol∙l-1, and (4) - 1.5
mol l-1 for compound (2).
Figure 6 - Dependence of the optical density of the system (2) (5) and (4) (6) at the
wavelengths of the absorption maxima of the complexes on concentrations of reactant (2)
(curve 1) and reactant (4) (curve 2)
Composition of the synthesized complexes (5) and (6) was determined by the Harvey
Manning’s slope ratio method-used for investigation of the different reactions when forming
strong complex MmRn: mM + nR MmRn.
Two series of experiments were conducted. In the first series the initial concentration of
the reactants (2), CR, and (4) CR1, remains constant and is much greater than varied
concentration of central ion M (where M = Cu2+), CR> CM.
Because of the large excess of reagent R (2) and (4) we can assume that the ion M will
completely migrate to the complex MmRn, the concentration of which is 1/m∙CM. Optical
density A1 is: A1=k1CM/m (where k1 is coefficient of proportionality) and the slope coefficient
of this line is found using tgα1 = k1/m.
In the second series keeping the concentration of component M (CM) constant and
changing the concentration of the reagent R, under similar conditions, we obtained graphical
dependence of the optical density A2on the concentration of R: A2 = k2CR/n and determined
the slope coefficient of the line: tgα2 = k2/n.
Applying the basic law of light absorption to the expressions for the optical density of the
A1 and A2 we obtain next equations:
A1 = k1CM/m = tgα1 ∙ CM = CMεl/m and A2 = k2CR/n = tgα2 ∙ CR = CRεl/n.
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Hence tgα1 = εl/m and tgα2 = εl/n.
Dividing one equation to another we got ratio of angle coefficients equal to the ratio of
stoichiometric coefficients in the formula of MmRn complex in accordance with (1).
tgα1/tgα2 = n/m (1)
According to data obtained by us as a result of spectrophotometric study and presented in
Tables 2 and 3 slopes coefficients characterizing the dependence of A1 on CM and A2 on CR in
accordance with the formulas (2-5) were calculated by method of the least squares.
Table 2
The concentrations of reagent (2) and the optical density system (2) (5)
C1M
mol / L A1 C2R1
mol / L A2
2.2 * 10-6 0.103 0.43 * 10-5 0.091
4.3 * 10-6 0.215 0.86 * 10-5 0.196
6.5 * 10-6 0.316 1.30 * 10-5 0.296
8.6 * 10-6 0.407 1.71 * 10-5 0.388
Notes
1 When CR = const;
2 When CM = const.
Table 3
The concentrations of the reagent (4) and the optical density
system (4) (6)
Consequently, the ratio of components M: R in the complexes (5) and (6) is 1:2.
Conclusions
Convenient preparative methods for the synthesis of multifunctional derivatives of 2aminothiazole possessing evident complex-forming ability were worked out.
Potentiometric method was used for determination of thermodynamic parameters of the
reactions of mixed ligand complex forming of compounds obtained with ions of copper (II)
and lead (II) in a water-alcohol solution and it was found that the increase in temperature
leads to a decrease in the stability of the complexes formed.
References
[1] Salkeeva L.K., Amerkhanova Sh.K., Shlyapov R.M., Minaeva Ye.V., Uali
A.S.//Russian Journal of General Chemistry. - 2010. T. 80. № 6. Pp. 10-33.
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[2] Dyatlova N.M., Temkina V.Ya., Popov K.I. Complexones and complexonates of
metals. –M.: Khimiya, 1988. - 565 p.
[3] IM Dyatlova//Journal of the WMO them. DI Mendeleev. 1984. T. 29. Number 3. S.
358.
[4] Kabachnik M.I., Medved .T. Ya., Dyatlova N.M.//Usp. 1974. T. 43. Number 4. Pp.
1554-1574.
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