Synthesis and luminescent properties of coordination compounds of europium(III), gadolinium(III), and terbium(III) with para-alkyloxybenzoic acids

Full text of DOI:10.1134/S1070363216050418

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ISSN 1070-3632, Russian Journal of General Chemistry, 2016, Vol. 86, No. 5, pp. 1209–1211. © Pleiades Publishing, Ltd., 2016.
Original Russian Text © F.A. Kolokolov, A.N. Kulyasov, M.A. Magomadova, Kh.K. Shapieva, I.E. Mikhailov, G.A. Dushenko, V.A. Panyushkin, 2016,
published in Zhurnal Obshchei Khimii, 2016, Vol. 86, No. 5, pp. 873–875.
LETTERS
TO THE EDITOR
Synthesis and Luminescent Properties of Coordination
Compounds of Europium(III), Gadolinium(III),
and Terbium(III) with para-Alkyloxybenzoic Acids
F. A. Kolokolova, A. N. Kulyasova, M. A. Magomadovab, Kh. K. Shapievab,
I. E. Mikhailovc, G. A. Dushenkoc, and V. A. Panyushkina
a Kuban State University, ul. Stavropol’skaya 149, Krasnodar, 350040 Russia
e-mail: kolokolov@chem.kubsu.ru
b Chechen State University, Grozny, Russia
c Southern Scientific Center of the Russian Academy of Sciences, Rostov-on-Don, Russia
Received September 17, 2015
Keywords: para-alkoxybenzoic acid, lanthanide, luminescence
DOI: 10.1134/S1070363216050418
It is well known that coordination compounds of
lanthanides with aromatic carboxylic acids display
efficient luminescent properties in the visible and near-
infrared ranges [1]. We have earlier prepared a number
of complexes with promising luminescent properties
[2, 3]. However, the majority of the compounds were
poorly soluble in organic media; that limited their
possible application. One of the ways to enhance the
complexes solubility in nonpolar organic solvents is
the use of aromatic carboxylic acids with alkyl
substituents as the ligands. Moreover, the improved
film-forming properties and the amphiphilic nature of
the compounds containing long-chain substituents will
open the possibility of the molecular films preparation
via the Langmuir–Blodgett technology.
In view of the above, we synthesized coordination
compounds of europium(III), gadolinium(III), and
terbium(III) with para-dodecyloxybenzoic (HL1) and
para-octadecyloxybenzoic (HL2) acids. The ligands
were prepared via the reaction of the corresponding
alkyl bromides with para-oxybenzoic acid in the
presence of potassium hydroxide, followed by hyd-
rolysis of the intermediate ester (Scheme 1).
Scheme 1.
RO
HO
O
O
+
2RBr
+ 2KOH
+ 2KBr + 2H2O
OH
RO
OR
O
O
HO
+ HCl
+ RCl
OR
OR
R = C12H25 (HL1), C18H37 (HL2).
1209

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1210
KOLOKOLOV et al.
Scheme 2.
RO
HO
3
O
O
O
O
O
Ln
O
OR
+ LnCl3 + 3KOH
+ 3KCl + 3H2O
O
OR
RO
R = C12H25, Ln = Eu3+ (1); R = C12H25, Ln = Tb3+ (2); R = C12H25, Ln = Gd3+ (3); R = C18H37, Ln = Eu3+ (4); R = C18H37,
Ln = Tb3+ (5); R = C18H37, Ln = Gd3+ (6).
Synthesis of complexes 1–6 was performed via
reaction of the prepared para-alkoxybenzoic acids with
europium(III), gadolinium(III), or terbium(III) chloride
in an alkaline medium. Yield of the complexes was of
55–71% (Scheme 2).
coordination of the acid anions could be suggested [4].
The spectra of the synthesized complexes 1–6 con-
tained no band of coordinated water hydroxyl; the
absence of water was further confirmed by the data of
elemental analysis. The anhydrous nature of the
complexes could be explained by the hindrance effect
of the long-chain substituents making impossible
introduction of water molecules at the inner coordina-
tion sphere of the lanthanide ions.
The prepared coordination compounds and the
starting acids were white powders, readily soluble in
chloroform. According to the elemental analysis data
(Table 1), composition of complexes 1–6 matched the
general formula LnL3.
Phosphorescence spectra of gadolinium complexes
has been generally used for the determination of the
energy of excited triplet levels (T1) of the ligands,
since phosphorescence of the Gd3+ ion is caused by the
transfer of the ligand molecule from excited triplet
state to the ground singlet [1]. From the luminescence
spectra of the gadolinium complexes with para-do-
decyloxybenzoic and para-octadecyloxybenzoic acids,
the energies of excited triplet levels of the acid anions
were of 20441 and 20047 cm–1, respectively.
In contrast to the ligands, IR spectra of the prepared
complexes (see table) did not contain a group of the
bands at 2800–2400 cm–1 assigned to the dimers of
carboxylic acids. Moreover, the band of stretching
vibrations of C=O bond (1686 cm–1) was also absent,
evidencing anionic form of the acids in the complex.
Since the difference (Δν) between the asymmetric and
symmetric vibrations of the carboxylic group stretch-
ing was of 156–183 cm–1 (<220 cm–1), bidentate
Yields and the data of elemental analysis and IR spectroscopy for compounds 1–6
Found, %
Calculated, %
Comp. Yield,
IR spectrum, cm–1
Formula
no.
%
С
Н
Ln
С
Н
Ln
1
65.3 1606 [νas(COO–)], 1425 [νs(COO–)] C57H87O9Eu 64.15 8.25
68.4 1610 [νas(COO–)], 1435 [νs(COO–)] C57H87O9Tb 63.75 8.17
71.2 1605 [νas(COO–)], 1423 [νs(COO–)] C57H87O9Gd 63.89 8.23
55.4 1609 [νas(COO–)], 1429 [νs(COO–)] C75H123O9Eu 68.32 9.41
57.5 1612 [νas(COO–)], 1431 [νs(COO–)] C75H123O9Tb 67.97 9.32
61.8 1601 [νas(COO–)], 1417 [νs(COO–)] C75H123O9Gd 68.05 9.35
14.32
14.90
14.75
11.58
12.02
11.97
64.10
63.69
63.81
68.23
67.87
67.98
8.15
8.10
8.12
9.33
9.28
9.29
14.25
14.80
14.65
11.52
11.99
11.86
2
3
4
5
6
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 86 No. 5 2016

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SYNTHESIS AND LUMINESCENT PROPERTIES OF COORDINATION COMPOUNDS
1211
The energy difference between the T1 term of the
of the ligand (3 mmol) to pH 6. Then, lanthanide
chloride (1 mmol) in 5 mL of a 1 : 1 ethanol–water
mixture was added dropwise to the reaction mixture at
vigorous stirring. The solution with the formed
precipitate was stirred during 1 h. The precipitate was
filtered off, sequentially washed with distilled water
and with ethanol, and dried at 80°C to constant mass.
1H NMR spectra were registered using a JNM ECA
400 (JEOL) spectrometer. IR spectra were recorded
using a Vertex 70 (Bruker) Fourier spectrometer
equipped with an ATR attachment (4000–400 cm–1).
Elemental analysis (C, H) was performed using a
Vario EL III (Elementar) analyzer; the metal content
was determined via complexonometric titration.
Luminescent spectra were obtained with a Flyuorat-02-
Panorama spectrofluorimeter.
5
ligand and the D4 one of Tb3+ ion (20500 cm–1) in the
2500–4000 cm–1 range has been found optimal for the
intermolecular transfer of excitation energy from
organic ligand to lanthanide ion [5]. In the studied
case, the resonance term of Tb3+ ion was higher in
energy than the triplet levels of para-dodecyloxy-
benzoate and para-octadecyloxybenzoate anions; there-
fore, luminescence was not observed for those complexes.
Similarly, for Eu3+ complexes, the optimal energy
gap between the T1 term of the ligand and the 5D0 term
(17200 cm-1) of Eu3+ ion should fall in the 2500–
3500 cm–1 range [5]. In the studied case, it was of
3241 cm–1 for europium(III) para-dodecyloxybenzoate
and of 2847 cm–1 for europium(III) para-octadecyloxy-
benzoate, matching the indicated limits. Therefore, the
efficient excitation energy transfer from the ligands to
Eu3+ ion should be operative for those complexes; that
was confirmed by the experimental luminescence spectra.
Moreover, the integral intensity of luminescence of
europium(III) complex with para-dodecyloxybenzoic
acid was 1.5 times higher than of that with para-
octadecyloxybenzoate acid and 4.3 times higher than
for europium(III) benzoate used as reference.
ACKNOWLEDGMENTS
This work was financially supported by the Russian
Foundation for Basic Research (grant no. 14-03-
00830) and the Ministry of Education and Science in
the frame of the governmental contract (no. 007-
01114-16 PR 0256-2014-0009).
Synthesis of para-alkoxybenzoic acids was per-
formed as described elsewhere [6]. The corresponding
alkyl bromide was added to a solution of para-
hydroxybenzoic acid (1.18 g) in 30 mL of ethanol, and
then a solution of KOH (1.69 g in 60 mL of ethanol)
was added dropwise. The reaction mixture was stirred
at 60°C during 14 h. The precipitate was filtered off
and dried at 80°C to constant mass.
para-Dodecyloxybenzoic acid (HL1). Yield 58.9%.
IR spectrum, ν, cm–1: 2700–2500 (O–H), 1673 (C=O).
1Н NMR spectrum, δ, ppm: 10.00 s (1Н, COOH), 8.05
d (2H, HAr, J = 8.8 Hz), 6.94 (2H, HAr, J = 8.8 Hz),
4.04 t (2H, OCH2, J = 6.6 Hz), 1.27–1.84 m (20H,
CH2), 0.88 t (3H, CH3, J = 6.8 Hz). Found, %: С 74.55;
Н 9.75. C19H30O3. Calculated, %: С 74.51; Н 9.80.
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