Complexation of europium(III) with 2,2′-Dipyridyl in Eu(An) 3-2,2′-dipyridyl ethyl acetate systems

Article abstract of DOI:10.1134/S003602361203031X

Complexation of europium(III) with 2,2′-dipyridyl in Eu(An) ₃-2,2′-dipyridyl-ethyl acetate systems, where An is acetylacetonate, trichloroacetate, or trifluoroacetate ions, was studied by spectrophoto-metric methods. The stability constants of

Full text of DOI:10.1134/S003602361203031X

ISSN 0036ꢀ0236, Russian Journal of Inorganic Chemistry, 2012, Vol. 57, No. 3, pp. 367–372. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © V.P. Smagin, T.G. Vovk, O.A. Varnavskaya, E.V. Yudina, 2012, published in Zhurnal Neorganicheskoi Khimii, 2012, Vol. 57, No. 3, pp. 419–424.
COORDINATION COMPOUNDS
Complexation of Europium(III) with 2,2'ꢀDipyridyl
in Eu(An)3–2,2'ꢀDipyridyl Ethyl Acetate Systems
V. P. Smagin, T. G. Vovk, O. A. Varnavskaya, and E. V. Yudina
Altai State University, pr. Lenina 61, Barnaul, 656049 Russia
Received April 11, 2011
Abstract—Complexation of europium(III) with 2,2'ꢀdipyridyl in Eu(An)3–2,2'ꢀdipyridylꢀethyl acetate sysꢀ
tems, where An is acetylacetonate, trichloroacetate, or trifluoroacetate ions, was studied by spectrophotoꢀ
metric methods. The stability constants of the complexes at different temperatures were determined, and
Gibbs energy, enthalpy, and entropy changes in complexation were calculated. The influence of the initial
organic ligands on complexation was elucidated.
DOI: 10.1134/S003602361203031X
Lanthanide complexes are characterized by unorꢀ
dinary spectroluminescent properties. They are used
in optoꢀ and microelectronics, lighting engineering,
and other fields for manufacturing light sources and
diverse functional materials, including those based on
optic polymers [1, 2]. The efficient control of properꢀ
ties of functional materials requires control over proꢀ
cesses that occurr during their synthesis and perforꢀ
mace. Complexation is one of the most important proꢀ
cesses, but the direct study of complexation in
monomers is hampered by their propensity to polyꢀ
merize even at room temperatures. A considerable
amount of information can be obtained by studying
the complexation in solvents whose physical characꢀ
teristics are close to those of optic monomers, which
are lowꢀpolarity organic compounds. The processes
occurring in lowꢀpolarity solvents are rather complex.
In solutions, mixedꢀligand complexes are formed.
EXPERIMENTAL
Europium haloacetates were synthesized by the
method described in [6] via the reaction of europium
oxide with the corresponding acid in an aqueous soluꢀ
tion (1 : 1) and the subsequent separation of crystalline
salts. Europium acetylacetonate was prepared via the
reaction of ammonium acetylacetonate with europium
chloride in an aqueous solution [7] and used immediꢀ
ately as synthesized. Europium chloride was prelimiꢀ
nary synthesized via the reaction of the oxide with
hydrochloric acid and subsequent concentration of
the solution on a water bath until the water and trace
hydrochloric acid evaporated.
The compounds synthesized were identified by IR
spectroscopy [8, 9]. The number of water molecules in
crystal hydrates was determined by indirect methods
from the results of trilonometric titration in the presꢀ
ence of Xylenol Orange in an acetate buffer [10].
The absorption spectra of solutions were recorded
on a Specord UV VIS spectrophotometer within the
range of 240–350 nm, and the optical density within
the absorption band of a complex at wavelengths of
301, 307, 309, and 312 nm was measured on an SFꢀ46
spectrophotometer relative to ethyl acetate within a
temperature range of 268–323 K. The photometric
scale of the SFꢀ46 spectrophotometer was calibrated
as described in [11]. Solutions were preliminary
allowed to stay in an ML W U8 thermostat.
The objective of this work is to establish the influꢀ
ence of different organic anions on the complexation
of europium(III) with 2,2'ꢀdipyridyl (2,2'ꢀDipy) in
ethyl acetate (EA).
Europium acetylacetonate (Eu(acac)3
fluoroacetate (Eu(CF3COO)3 3H2O), and trichloroꢀ
acetate (Eu(CCl3COO)3 2Н2O) were selected to be
⋅
3H2O), triꢀ
⋅
⋅
studied. These compounds are quite well soluble in
lowꢀpolarity organic solvents, including ethyl acetate
and methyl methacrylate. They are used as modifiers
of organic polymers to introduce different metals into
their composition [3, 4]. Interest in 2,2'ꢀDipy is due to
its efficient sensitizing effect on the luminescence of
europium(III). The study was conducted by the isoꢀ
molar series, saturation, and Benesi–Hildebrand
spectrophotometric methods, as in [5].
RESULTS AND DISCUSSION
In studying the absorption spectra of solutions, we
discovered a number of specific features. An absorpꢀ
tion band with a peak at 252 nm appeared in the specꢀ
trum of a europium trifluoroacetate solution (Fig. 1).
The apparent molar absorption coefficient at the peak
367

368
SMAGIN et al.
A
0.4
A
264 nm
80 : 1
252
(b)
0.2
A
60 : 1
1.0
250
280
282
0 : 1
λ
, nm
20 : 1
40 : 1
100 : 1
80 : 1
60 : 1
1.0
40 : 1
(а)
80 : 1
252
60 : 1
40 : 1
1 : 1
0.5
20 : 1
3 : 1
5 : 1
40 : 1
20 : 1
10 : 1
7 : 1
100 : 1
80 : 1
0.5
60 : 1
0 : 1
7 : 1
5 : 1
20 : 1
10 : 1
280
310
, nm
250
λ
3 : 1
1 : 1
Fig. 1. Absorption spectra for (a) Eu(CF СОО) –2,2'
ꢀ
3
3
Dipy–EA system with different Eu(III) : 2,2'ꢀDipy molar
ratios and (b) Eu(CF СОО) –EA system with different
salt concentrations.
3
3
250
280
310
, nm
λ
of this band is ~80 L/(mol cm). A similar band in the
spectrum of a europium trichloroacetate solution
shifts towards the longꢀwave region. Its peak correꢀ
sponds to 264 nm (Fig. 2). This band is characterized
by a higher molar absorption coefficient at the peak.
The intensity of these spectral bands in the systems
grew proportionally to the increase in the salt concenꢀ
tration in solutions. These bands arise from the carꢀ
boxyl group of the anions bidentately coordinated to
europium. Their different positions in the spectrums
are explained by the presence of fluorine or chlorine
atoms, different values of the inductive effect with
respect to the carboxyl group and, as a consequence,
Fig. 2. Absorption spectra for the Eu(CCl COO) –2,2'
Dipy–EA system with different Eu(III) : 2,2'ꢀDipy molar
ratios.
ꢀ
3
3
absorption band modified. This was most pronounced
in the system containing europium trichloroacetate
(Fig. 2). A shoulder was observed in the descending
branch of the spectral signal, the intensity of which
grew with an increase in the concentration of the comꢀ
plex. The specific features of the spectral signal were
nullified at a great excess of the salt in solution. Hence,
these bands were additive. Their additivity was associꢀ
ated with the absorption of anions and the change in
their absorption upon complexation. These changes
are produced by the insertion of 2,2'ꢀDipy into the
sphere of influence of europium(III). It is problematic
whether the optical density measured within the
region of maximum of these absorption bands may be
used to determine the composition of complexes and
their stability constants.
by the different influence on the energy of excited
electron states.
π
*ꢀ
An absorption band with a peak at 282 nm
appeared in the absorption spectrum of a 2,2'ꢀDipy
solution in ethyl acetate (Fig. 1, ratio is 0 : 1). The
molar absorption coefficient of this band at the peak is
~1.4
ꢀ → π *
After europium compounds were introduced into
×
104 L/(mol cm). The band was assigned to the
ꢀelectron transition in 2,2'ꢀDipy.
In the spectra of solutions, an absorption band was
2,2'ꢀDipy solutions, the spectra experienced substanꢀ observed at wavelengths near 300 nm. This band
tial evolution. In the spectra of Eu(CCl3COO)3 appeared only when 2,2'ꢀDipy and europium comꢀ
2H2O–2,2'ꢀDipy–EA and Eu(CF3COO)3 3H2O– pounds were simultaneously present in solutions. As
⋅
⋅
2,2'ꢀDipy–EA, the envelope of the shortꢀwave the Eu(III) : 2,2'ꢀDipy molar ratio grew, the intensity
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 57 No. 3 2012

COMPLEXATION OF EUROPIUM(III)
369
у
= 0.00 + 0.0728
x; 0.973
of this band increased, and the intensity of the initial
absorption band of 2,2'ꢀDipy (282 nm) decreased. The
complete saturation of the optical density measured
within this absorption band did not occur even if
Eu(III) : 2,2'ꢀDipy molar ratios exceeded 80 : 1. For
the solutions prepared with the use of europium
trichloroacetate, this was related more to the additive
contribution of the shortꢀwave absorption band,
which was manifested to a considerable degree at
molar ratios Eu(III) : 2,2'ꢀDipy > 20 : 1 (Fig. 2).
Isobestic points were observed in the spectra. The
appearance of new absorption bands in the spectra and
their dependence on the Eu(III) : 2,2'ꢀDipy molar
ratio confirm the formation of complexes in solutions.
A
0.8
0.6
0.4
x
= 10
y
= 0.70 + 0.0026x; 0.973
0.2
0.1
0
10 20 30 40 50 60 70 80
cEu(III) c2,2'ꢀDipy
/
Fig. 3. Optical density of solutions vs Eu(III) : 2,2'ꢀDipy
molar ratio in the Eu(CCl COO) –2,2'ꢀDipy–EA sysꢀ
tem; λ = 309 nm.
3
3
Owing to a considerable absorption of 280–340ꢀnm
radiation by europium acetylacetonate, the solutions
of its complexes were studied by differential differenꢀ
tial method. Their absorption spectra were recorded
relative to a europium acetylacetonate solution. In the
spectrum we distinguished the only analytical absorpꢀ
tion band of the complex with a peak at 301 nm.
different temperatures from solution optical densities
measured at different wavelengths at the peak of the
absorption band of a complex, and the average stability
constants of the complexes determined at different
temperatures. The coincidence within the limits of
error between the stability constants estimated from
optical densities measured at different wavelengths
verifies the validity of this method for the determinaꢀ
tion of stability constants as applied to the systems
under consideration. The other systems were studied
in a similar manner. The average stability constants of
complexes at different temperatures for all studied sysꢀ
tems are given in Table 3. As a result, the stability conꢀ
stants of europium(III) complexes with 2,2'ꢀDipy
were found to grow as an organic anion was changed in
the sequence of europium acetylacetonate–europium
trifluoroacetate–europium trichloroacetate. Comꢀ
plexes of this type are characterized by low stability,
which is decreased with increasing temperature.
For each system,
А = f(Eu(III) : 2.2'ꢀDipy) plots
λ
were constructed (Fig. 3). The change of a europium
compound dissolved in ethyl acetate led to the shift of
a saturation point in these plots. The average abscissa
values corresponding to this point in the plots for difꢀ
ferent series of solutions are 28, 18, and 12 for
europium acetylacetonate, europium trifluoroacetate,
and europium trichloroacetate, respectively. The shift
of this point corresponds to the change in the stability
of 2,2'ꢀDipy complexes in this sequence of comꢀ
pounds.
The compositions of complexes were determined
by the isomolar series method. Isomolar diagrams for
different systems were similar. The plots in the diaꢀ
grams each had a single extreme point at an equimolar
ratio of the components. Consequently, complexes
with a Eu(III) : 2,2'ꢀDipy ratio of 1 : 1 are formed in
the systems.
Using the stability constants at different temperaꢀ
tures and assuming the heat effect to be constant
within the selected temperature range, we calculated
the Gibbs energy, enthalpy, and entropy changes in the
complexation of europium(III) with 2,2'ꢀDipy. The
Gibbs energy changes were calculated by the equation
−RT lnK.
=
The heat of complexation reactions
was determined graphically from ln (1/ plots.
The values of were calculated from the slopes of
ΔGT
K
=
f
Т)
The stability constants of complexes were deterꢀ
mined by the Benesi–Hildebrand method. The
detailed description of the method used to estimate
them and the results of studying the system containing
europium trifluoroacetate are found in [5]. As an
ΔH
these plots. The entropy changes were calculated by
ΔH − ΔG
ΔS
the Gibbs’ equation
=
[12]. The details
T
example, Table 1 lists data for the Eu(CCl3COO)3
2H2O–2,2'ꢀDipy–EA system, namely the optical
densities ( ) of solutions measured at the peak of the
absorption band of the complex, the equations and
⋅
of estimating these parameters for the Eu(CF3COO)3
⋅
3H2O–2,2'ꢀDipy–EA system are found in [5]. The
initial data and results of calculation for the
А
Eu(CCl3COO)3
⋅
2H2O–2,2'ꢀDipy–EA system are
c
2,2−Dipy ⋅ ꢀ
listed in Table 4, and the corresponding parameters for
all the systems studied are given in Table 5.
1
ε
1
1
coefficients of
=
linear
,
+
A
K ⋅ εcEu(III)
λ
In summary, mixedꢀligand complexes with the
ratio Eu(III) : 2,2'Dipy = 1 : 1 have been found to form
in ethyl acetate. The thermodynamic characteristics of
the reaction of europium(III) with 2,2'Dipy were calꢀ
culated from the thermal stability constants of comꢀ
relationships, and the stability constants of the comꢀ
plexes calculated from the optical densities measured
for the series of solutions in question. Table 2 displays
the stability constants calculated for the complexes in
the Eu(CCl3COO)3 2H2O–2,2'ꢀDipyꢀEA system at
⋅
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370
SMAGIN et al.
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 57 No. 3 2012

COMPLEXATION OF EUROPIUM(III)
371
Table 2. Stability constants of europium(III) complexes with 2,2'ꢀDipy at different temperatures in the Eu(CCl3COO)3–2,2'ꢀ
Dipy–EA system
Stability constants
T
, K
307 nm
309 nm
312 nm
Kst
268
295
303
313
(4.4 0.2) × 103
(3.3 0.1) × 103
(2.6 0.1) × 103
(2.4 0.1) × 103
(4.0 0.2) × 103
(3.1 0.1) × 103
(2.7 0.1) × 103
(2.3 0.1) × 103
(3.5 0.2) × 103
(2.7 0.1) × 103
(2.4 0.1) × 103
(2.2 0.1) × 103
(4.0 0.2) × 103
(3.0 0.1) × 103
(2.6 0.1) × 103
(2.3 0.1) × 103
Table 3. Stability constants of europium(III) complexes with 2,2'ꢀDipy at different temperatures in the EuL3–2,2'ꢀDipy–
EA systems, where L is acetylacetonate, trifluoroacetate, or trichloroacetate ion
K × 10−2
Stability constants,
T
, K
Eu(acac)3
⋅
2,2'ꢀDipy
Eu(CF3COO)3 ⋅ 2,2'ꢀDipy Eu(CCl3COO)3 ⋅ 2,2'ꢀDipy
268
295
298
303
308
313
318
323
–
–
–
–
40
30
2
1
2.94 0.09
2.86 0.09
2.61 0.08
2.45 0.08
–
5.7 0.2
5.6 0.2
5.3 0.2
4.6 0.2
4.2 0.2
3.99 0.12
–
26
23
1
1
–
–
–
2.34 0.07
Table 4. Gibbs energy changes (
um(III) with 2,2'ꢀDipy in the Eu(CCl3COO)3–2,2'ꢀDipy–EA system
Δ
G
), enthalpy changes (
Δ
H), and entropy changes (ΔS) in the complexation of europiꢀ
T,
K
268
295
303
313
1/
T
, 10–3, 1/K
× 10–3
3.73
4.0
3.39
3.0
3.30
2.6
3.19
2.3
Kst
8.29
–19
40
8.01
–20
40
7.86
–20
38
7.74
–20
37
lnK
Δ
Δ
G
, kJ/mol
S
, J/(mol K)
1
T
lnK = f
y
= 4.54 + 1.01 × 103x, 0.991
Equation of
, kJ/mol
, linearity coefficient (r)
)
(
Δ
H
–8.4
plexes. The stability constants of europium(III) comꢀ with the decrease in Gibbs energy. The character of the
plexes with 2,2'ꢀDipy were shown to grow in the order change in the enthalpy component of the process vioꢀ
of Eu(acac)3
Eu(CCl3COO)3
⋅
2,2'ꢀDipy–Eu(CF3COO)3
⋅
⋅
2,2'Dipy
–
lates the above order, but correlates with the anion–
2
,2'ꢀDipy. Their growth correlates europium(III) interaction strength, which grows in
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 57 No. 3 2012

372
SMAGIN et al.
), enthalpychanges ( ), and entropy changes ( S) in the complexation of europium(III)
with 2,2'ꢀDipy in the EuL3–2,2'ꢀDipy–EA systems, where L is acetylacetonate, trifluoroacetate, or trichloroacetate ion
Table 5. Gibbs energy changes (
Δ
G
Δ
H
Δ
Compound
Parameter
Eu(acac)3
–14
–7.7
21
⋅
2,2'ꢀDipy
Eu(CF3COO)3
⋅
2,2'ꢀDipy Eu(CCl3COO)3 2,2'ꢀDipy
⋅
Δ
Δ
Δ
G
, kJ/mol
, kJ/mol
, J/(mol K)
–16
–12
12
–20
–8.4
39
H
S
6. V. V. Serebrennikov, The Chemistry of RareꢀEarth Eleꢀ
ments (Izd–vo Tomsk. Gos. Univ., Tomsk, 1959), Vol. 1
[in Russian].
the order Eu(CF3COO)3–Eu(CCl3COO)3–Eu(acac)3
.
The entropy component of the process favors the
enhanced stability of europium(III) complexes with
2,2'ꢀDipy in the Eu(CCl3COO)3–2,2'ꢀDipy–EA sysꢀ
tem (Table 5).
7. K. B. Yatsimirskii, N. A.Kostromina, Z. A. Sheka,
et al., The Chemistry of Complexes of RareꢀEarth Eleꢀ
ments (Naukova Dumka, Kiev, 1966) [in Russian].
8. K. Nakamoto, Infrared and Raman Spectra of Inorganic
and Coordination Compounds (Wiley, New York, 1986;
Mir, Moscow, 1991).
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