Luminescent neutral lanthanide complexes of 5-aryl-2,2′-bipyridine-6-carboxylic acids, synthesis and properties

Article abstract of DOI:10.1016/j.poly.2015.09.055

Luminescent lanthanide complexes of 4-aryl-1-(2-pyridyl)-6,7-dihydro-5H-cyclopenta[c]pyridine-3-carboxylic acids and 5-tolyl-2,2′-bipyridine-6-carboxylic acid were prepared. The ligands were synthesized using the "1,2,4-triazine" methodology. In one case, the opportunity of copper complex preparation, followed by exchange of the metal cation from copper to europium to overcome synthetic difficulties, was demonstrated. The ability of tuning the properties of the complexes by means of varying the structure of the ligands was demonstrated. In particular, the introduction of the cyclopentene moiety allowed the preparation of highly soluble complexes in non-polar organic solvents.

Full text of DOI:10.1016/j.poly.2015.09.055

Accepted Manuscript
Luminescent neutral lanthanide complexes of 5-aryl-2,2’-bipyridine-6-carbox-
ylic acids, synthesis and properties
Alexey P. Krinochkin, Dmitry S. Kopchuk, Dmitry N. Kozhevnikov
PII:
S0277-5387(15)00565-3
DOI:
http://dx.doi.org/10.1016/j.poly.2015.09.055
POLY 11569
Reference:
Polyhedron
To appear in:
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Accepted Date:
29 July 2015
15 September 2015
Please cite this article as: A.P. Krinochkin, D.S. Kopchuk, D.N. Kozhevnikov, Luminescent neutral lanthanide
complexes of 5-aryl-2,2’-bipyridine-6-carboxylic acids, synthesis and properties, Polyhedron (2015), doi: http://
dx.doi.org/10.1016/j.poly.2015.09.055
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Luminescent neutral lanthanide complexes of 5-aryl-2,2'-bipyridine-6-carboxylic acids,
synthesis and properties
Alexey P. Krinochkin,a Dmitry S. Kopchuk,a,b∗ Dmitry N. Kozhevnikova,b
a Ural Federal University, 19, Mira St., 620002 Yekaterinburg, Russian Federation
b
Postovsky Institute of Organic Synthesis of RAS (Ural Division), 22/20, S. Kovalevskoy/Akademicheskaya St.,
620990 Yekaterinburg, Russian Federation
Abstract
Luminescent
lanthanide
complexes
of
4-aryl-1-(2-pyridyl)-6,7-dihydro-5H-
cyclopenta[c]pyridine-3-carboxylic acids and 5-tolyl-2,2'-bipyridine-6-carboxylic acid were
prepared. The ligands were synthesized using the “1,2,4-triazine” methodology. In one case, the
opportunity of copper complex preparation, followed by exchange of the metal cation from
copper to europium to overcome synthetic difficulties, was demonstrated. The ability of tuning
the properties of the complexes by means of varying the structure of the ligands was
demonstrated. In particular, the introduction of the cyclopentene moiety allowed the preparation
of highly soluble complexes in non-polar organic solvents.
Keywords: 5-aryl-2,2'-bipyridine-6-carboxylic acids; luminescence; lanthanide
complexes; aza-Diels-Alder reaction; 1,2,4-triazine.
1. Introduction
Nowadays, luminescent neutral lanthanide complexes represent a considerable practical
interest,1 and one of the important spheres of their applications is the creation of new
electroluminescent materials (OLEDs). For this particular application, in addition to the most
attractive features of luminescent lanthanide compounds, such as line-like emissions with high
quantum efficiency, some specific additional properties are beneficial. Namely, such complexes
have to be steady, capable of being sublimated in a vacuum or being soluble in non-polar organic
solvents in order to dope semi-conductive polymers. In order to achieve these properties the
formation of a neutral lanthanide complex with the whole inner coordination sphere (the
coordination number of the lanthanide ion is 9) being occupied by organic ligands is required. As
a consequence the quenching of the complex luminescence by the presence of water molecules
∗Corresponding author. Tel.: +7-343-375-4501; fax: +7-343-374-0458; e-mail: dkopchuk@mail.ru

in the inner coordination sphere due to non-radiative deactivation via interaction with O-H
oscillators of the lanthanide cation is avoided.1a
Among the luminescent lanthanide coordination compounds, β-diketonate complexes are
the most popular and the most intensively investigated2 due to the commercial availability of β-
diketones, easy synthesis and excellent luminescence properties of these complexes.
Unfortunately, they usually suffer from poor photostability upon UV irradiation.3 In addition,
2,2'-bipyridine-6-carboxylic acid derivatives or their aza-analogs are potentially useful as
components for OLEDs since they form neutral complexes, [Ln(L)3] (L = 2,2’-bipyridine-6-
carboxylate), with a high quantum efficiency of lanthanide luminescence.4,5 However, according
to the literature, such complexes suffer from poor solubility in organic solvents. Therefore
luminescent properties for some of these complexes were only studied in the solid state. Indeed,
all of these facts considerably constrained the practical use of such chelates.
We analyzed the existing literature methods for the preparation of 2,2'-bipyridine-6-
carboxylic acids from the viewpoint of possibly overcoming above mentioned disadvantages. In
particular, 2,2'-bipyridine-6-carboxylic acids and their aza-analogues may be prepared directly
using various heterocyclization reactions,6 in particular by the Krönke method,7 via the oxidation
of methyl,8 acetyl9 or aldehyde10 groups as well as 2-furyl moiety11 in the C2 position of the
pyridine ring, by the direct functionalization of 6-bromobipyridines,12 and, finally, by the
hydrolysis of cyanogroups.13,14
Analysis of these data showed that the most appropriate way to achieve our goal is the
approach to 2,2'-bipyridine-6-carboxylic acids via their 1,2,4-triazine precursors, namely 6-aryl-
3-(2-pyridyl)-5-cyano-1,2,4-triazines. Further synthesis can be carried out by the described
earlier convenient methods,14, 15 namely by means of the conversion of the 1,2,4-triazine ring
into a pyridine one and the hydrolysis of the cyano group in acid medium. Fig. 1 shows the
structure of the ligands that will be used for the neutral lanthanide complexes.
R
O
N
OH
N
Fig. 1. The proposed structure of the ligands to produce neutral lanthanide complexes

The current approach was chosen among others for the following reasons: in this case, the
preliminary introduction of various aromatic substituents in the C5 position of the 2,2’-
bipyridine core can be used as a convenient tool for the tuning of the photophysical properties of
the final complexes. Besides, increasing the conjugation degree of the whole ligand system and
thus more effective intermolecular π-π–interactions improves energy transfer in the resultant
OLED devise. The synthesis of these 1,2,4-triazine rings requires the use of commonly available
reagents and simple synthetic procedures. It is worth mentioning that during the conversion of
the 1,2,4-triazine ring into the pyridine one, by means of a very well-proven synthetic
technique16, the further functionalization of positions C3 and C4 of the newly formed pyridine
ring is possible. For instance, in the case of enamines, used as dienophiles, the direct introduction
of a fused cycloalkane moiety into the pyridine core is possible.14,17 It should be noted that the
synthesis of cycloalkene-annelated pyridine systems is very difficult using other synthetic
approaches. In most cases such compounds have a much better solubility in organic solvents
compared with their non-annelated pyridine analogues. Thus, it becomes possible to overcome
the main disadvantage of most lanthanide complexes, namely low solubility in organic solvents.
2. Experimental
All chemicals and solvents were received from Aldrich/Merck as reagent grade and used
without any further purification. NMR spectra were recorded on a Bruker Avance-400
spectrometer, at 298 K, digital resolution 0.01 ppm and using TMS as an internal standard.
Infrared spectra were measured on a Bruker Alpha FTIR spectrometer with an ATR accessory
(ZnSe). UV-Vis spectra were recorded on a Lambda 45 spectrophotometer (Perkin Elmer).
Luminescence spectra were recorded on a Cary Eclipse spectrofluorometer (Varian). Mass
spectra were recorded on a MicrOTOF-Q II mass spectrometer (Bruker Daltonics) with
electrospray ionization (acetonitrile). Elemental analysis was performed on a РЕ 2400 II CHN-
analyzer (Perkin Elmer). Compounds 2a and 3b were synthesized as described in the literature.14
6-(4-Fluorophenyl)-3-(2-pyridyl)-1,2,4-triazine-4-oxide (1d) was synthesized in
accordance with the literature method14 for similar compounds. Yield 50%. M.p. 217-219 °C. 1Н
NMR (DMSO-d6), δ, ppm: 7.34 (m, 2H, CHarom.), 7.58 (m, 1Н, Н-5 (Py)), 8.00 (m, 2Н, Н-3,4
(Py)), 8.33 (m, 2Н, CHarom.), 8.78 (dd, 1Н, 3J = 4.9 Hz, 4J = 1.7 Hz, Н-6 (Py)), 9.41 (s, 1H, H-5).
ESI-MS, m/z: found 269.08 (М-Н)+, required 269.08. Found, %: С 62.55, Н 3.19, N 20.59.
С14Н9FN4O calculated, %: С 62.69, Н 3.38, N 20.89.

5-Cyano-6-(4-fluorophenyl)-3-(2-pyridyl)-1,2,4-triazine (6d) was synthesized in
accordance with the literature method14 for similar compounds. Yield 80%. M.p. 124-126 °C. 1Н
3
NMR (DMSO-d6), δ, ppm: 7.45 (m, 2H, CHarom.), 7.64 (m, 1Н, Н-5 (Py)), 8.08 (ddd, 1Н, J =
7.3, 7.3 Hz, 4J = 1.7 Hz, Н-4 (Py)), 8.18 (m, 2Н, CHarom.), 8.59 (dd, 1Н, 3J = 7.3 Hz, 4J = 1.2 Hz,
Н-3 (Py)), 8.87 (dd, 1Н, 3J = 4.9 Hz, 4J = 1.7 Hz, Н-6 (Py)). ESI-MS, m/z: found 278.08 (М-Н)+,
required 278.08. Found, %: С 64.80, Н 2.77, N 24.98. С15Н8FN5 calculated, %: С 64.98, Н 2.91,
N 25.26.
4-Aryl-1-(2-pyridyl)-6,7-dihydro-5H-cyclopenta[c]pyridines (2) were synthesized in
accordance with the literature method14 for similar compounds.
1-(2-Pyridyl)-4-tolyl-6,7-dihydro-5H-cyclopenta[c]pyridine (2b). Yield 78%. M.p.
3
169-171 °C. 1Н NMR (СDСl3), δ, ppm: 2.12 (m, 2Н, СН2-6), 2.44 (s, 3Н, Ме), 2.91 (t, 2Н, J =
7.6 Hz, СН2-7), 3.58 (t, 2Н, 3J = 7.6 Hz, СН2-5), 7.34 (m, 5Н, H-5 (Py), Tol), 7.85 (ddd, 1Н, 3J
= 7.3, 7.3 Hz, 4J = 1.7 Hz, Н-4 (Py)), 8.34 (dd, 1Н, 3J = 7.3 Hz, 4J = 1.2 Hz, Н-3 (Py)), 8.70 (dd,
3
4
1Н, J = 4.9 Hz, J = 1.7 Hz, Н-6 (Py)). ESI-MS, m/z: found 312.15 (М-Н)+, required 312.15.
Found, %: С 83.62, Н 6.21, N 9.54. С20Н18N2 calculated, %: С 83.88, Н 6.34, N 9.78.
4-(4-Methoxyphenyl)-1-(2-pyridyl)-6,7-dihydro-5H-cyclopenta[c]pyridine (2c). Yield
1
3
84%. M.p. 164-166 °C. Н NMR (СDСl3), δ, ppm: 2.12 (m, 2Н, СН2-6), 2.91 (t, 2Н, J = 7.6
Hz, СН2-7), 3.57 (t, 2Н, 3J = 7.6 Hz, СН2-5), 3.88 (s, 3Н, ОМе), 7.05 (m, 2Н, С-Нarom), 7.33 (m,
3
4
1Н, Н-5 (Py)), 7.41 (m, 2Н, С-Нarom.), 7.85 (ddd, 1Н, J = 7.3, 7.3 Hz, J = 1.7 Hz, Н-4 (Py)),
3
4
3
4
8.33 (dd, 1Н, J = 7.3 Hz, J = 1.2 Hz, Н-3 (Py)), 8.71 (dd, 1Н, J = 4.9 Hz, J = 1.7 Hz, Н-6
(Py)). ESI-MS, m/z: found 328.14 (М-Н)+, required 328.14. Found, %: С 79.19, Н 5.77, N 9.13.
С20Н18N2O calculated, %: С 79.44, Н 6.00, N 9.26.
4-(4-Fluorophenyl)-1-(2-pyridyl)-6,7-dihydro-5H-cyclopenta[c]pyridine (2d). Yield
85%. M.p. 174-176 °C. 1Н NMR (СDСl3), δ, ppm: 2.12 (m, 2Н, СН2-6), 2.89 (t, 2Н, 3J = 7.6 Hz,
СН2-7), 3.59 (t, 2Н, 3J = 7.6 Hz, СН2-5), 7.22 (m, 2H, CHarom.), 7.34 (m, 1Н, Н-5 (Py)), 7.45 (m,
3
4
3
2Н, CHarom.), 7.86 (ddd, 1Н, J = 7.3, 7.3 Hz, J = 1.7 Hz, Н-4 (Py)), 8.34 (dd, 1Н, J = 7.3 Hz,
3
4
4J = 1.2 Hz, Н-3 (Py)), 8.70 (dd, 1Н, J = 4.9 Hz, J = 1.7 Hz, Н-6 (Py)). ESI-MS, m/z: found
316.13 (М-Н)+, required 316.13. Found, %: С 78.41, Н 5.36, N 9.31. С19Н15FN2 calculated, %: С
78.60, Н 5.21, N 9.65.
General method for the synthesis of acids 4 and 5
The corresponding cyanobipyridine 2 or 3 (3 mmol) was suspended in sulfuric acid
(50%, 6 mL) and the resulting mixture was stirred at 140 °C for 10 h. The mixture was then
cooled to room temperature and water (20 mL) was added. The obtained precipitate was filtered

off, washed with water and dissolved in an aqueous solution of NaOH (1M, 5 mL). The
undissolved part was filtered off, then hydrochloric acid (5N) was added to the filtrate to adjust
the pH value to 2. The resulting precipitate was filtered off, washed with water and dried under
vacuum.
4-Phenyl-1-(2-pyridyl)-6,7-dihydro-5H-cyclopenta[c]pyridine-3-carboxylic acid (4a).
1
Yield 0.55 g (1.74 mmol, 58%). M.p. 209-210 °C. Н NMR (DMSO-d6), δ, ppm: 2.11 (m, 2Н,
СН2-6), 2.84 (t, 2Н, 3J = 7.6 Hz, СН2-7), 3.48 (t, 2Н, 3J = 7.6 Hz, СН2-5), 7.39 (m, 5H, Ph), 7.82
3
(m, 1H, H-5 (Py)), 8.41 (m, 2H, H-3,4 (Py)), 8.88 (d, 1Н, J = 4.8 Hz, Н-6 (Py)). IR, υ/cm-1:
1736 (СО). ESI-MS, m/z: found 315.12 (М-Н)-, required 315.11.
1-(2-Pyridyl)-4-tolyl-6,7-dihydro-5H-cyclopenta[c]pyridine-3-carboxylic acid (4b).
1
Yield 0.6 g (1.82 mmol, 61%). M.p. 195-197 °C. Н NMR (DMSO-d6), δ, ppm: 2.12 (m, 2Н,
СН2-6), 2.41 (s, 3Н, Ме), 2.85 (t, 2Н, 3J = 7.6 Hz, СН2-7), 3.47 (t, 2Н, 3J = 7.6 Hz, СН2-5), 7.23
3
(m, 4Н, Tol), 7.71 (m, 1Н, Н-5 (Py)), 8.27 (m, 1Н, Н-4 (Py)), 8.45 (d, 1Н, J = 8.0 Hz, Н-3
3
(Py)), 8.84 (d, 1Н, J = 4.8 Hz, Н-6 (Py)). 13C NMR (DMSO-d6), δ, ppm: 21.3, 25.1, 32.6, 33.4,
125.3, 125.8, 128.9, 129.5, 133.5, 133.6, 137.9, 141.6, 142.9, 145.0, 146.1, 148.8, 152.2, 157.2,
167.9 (СО). IR, υ/cm-1: 1737 (СО). ESI-MS, m/z: found 329.13 (М-Н)-, required 329.13.
4-(4-Fluorophenyl)-1-(2-pyridyl)-6,7-dihydro-5H-cyclopenta[c]pyridine-3-
1
carboxylic acid (4d). Yield 0.55 g (1.65 mmol, 55%). M.p. 208-210 °C. Н NMR (DMSO-d6),
3
δ, ppm: 2.12 (m, 2Н, СН2-6), 2.84 (t, 2Н, 3J = 7.6 Hz, СН2-7), 3.50 (t, 2Н, J = 7.6 Hz, СН2-5),
7.20 (m, 2Н, С-Нarom), 7.36 (m, 2Н, С-Нarom), 7.71 (m, 1Н, Н-5 (Py)), 8.26 (m, 1Н, Н-4 (Py)),
3
3
8.46 (d, 1Н, J = 8.0 Hz, Н-3 (Py)), 8.84 (d, 1Н, J = 4.4 Hz, Н-6 (Py)). IR, υ/cm-1: 1735 (СО).
ESI-MS, m/z: found 333.10 (М-Н)-, required 333.10.
5-Tolyl-2,2'-bipyridine-6-carboxylic acid (5). Yield 0.5 g (1.72 mmol, 57%). M.p. >
1
250 °C. Н NMR (DMSO-d6), δ, ppm: 2.41 (s, 3Н, Ме), 7.27 (m, 2Н, Tol), 7.38 (m, 2Н, Tol),
3
7.64 (m, 1Н, Н-5’), 8.01 (d, 1Н, J = 8.4 Hz, Н-3), 8.18 (m, 1Н, Н-4’), 8.64 (m, 2Н, Н-4,3’),
3
8.76 (d, 1Н, J = 4.4 Hz, Н-6’). IR, υ/cm-1: 1737 (СО). ESI-MS, m/z: found 289.10 (М-Н)-,
required 289.10.
General method for the synthesis of the lanthanide complexes
The corresponding acid (0.35 mmol) was suspended in methanol (40 mL), then sodium
hydroxide (14 mg, 0.35 mmol) was added and the resulting mixture was refluxed until a clear
solution was obtained. The mixture was cooled to room temperature and the chloride of the
corresponding lanthanide (0.117 mmol) was added. The mixture was kept at room temperature
for 2 h. Solvent was removed under reduced pressure, then water (20 mL) was added to the
residue. The precipitate was filtered off, washed with water, dried under vacuum and dissolved

in a mixture of methanol-DCM (1:1, 30 mL). The undissolved part was filtered off and solvents
were removed under reduced pressure. The resulting product was dried under a vacuum.
Eu*4a. Yield 80 mg (0.067 mmol, 57%). IR, υ/cm-1: 1643 (СО). ESI-MS, m/z: found
1099.26 (М+Н)+, required 1099.27. Found, %: С 61.47, Н 4.01, N 6.90. С60Н45EuN6О6*CH2Cl2 ,
calculated, %: С 61.94, Н 4.00, N 7.10.
Eu*4b. Yield 90 mg (0.076 mmol, 64%). IR, υ/cm-1: 1644 (СО). ESI-MS, m/z: found
1141.32 (М+Н)+, required 1141.32. Found, %:
С63Н51EuN6О6·0.5CH2Cl2 calculated, %: С 64.50, Н 4.43, N 7.11.
С
64.44,
Н
4.29,
N
7.23.
Eu*4d. Yield 80 mg (0.065 mmol, 57%). ESI-MS, m/z: found 1153.25 (М+Н)+, required
1153.24. Found, %: С 58.78, Н 3.12, N 6.67. С60Н42EuF3N6О6·CH2Cl2 calculated, %: С 59.23,
Н 3.59, N 6.79.
Tb*4d. Yield 85 mg (0.071 mmol, 61%). ESI-MS, m/z: found 1159.24 (М+Н)+, required
1159.24. Found, %: С 60.31, Н 3.49, N 6.91. С60Н42TbF3N6О6·0.5CH2Cl2 calculated, %: С
60.48, Н 3.61, N 7.00.
Eu*5. Yield 70 mg (0.063 mmol, 55%). ESI-MS, m/z: found 1021.22 (М+Н)+, required
1021.22. Found, %: С 59.38, Н 3.53, N 7.21. С54Н39EuN6О6·CH2Cl2 calculated, %: С 59.79, Н
3.74, N 7.61.
Cu*4с. Cyanobipyridine 1c (175 mg, 0.54 mmol) was dissolved in an ethanol (30 mL),
solution of CuCl2·2H2O (92 mg, 0.54 mmol) in water (30 mL) was added and the resulting
mixture was refluxed for 8 h. Solvents were removed under reduced pressure, then water (20
mL) was added to the residue. The precipitate was filtered off, washed with water and dried.
Yield 150 mg (0.34 mmol, 63%). ESI-MS, m/z: found 443.04 (М+), required 443.02. Found, %:
С 56.88, Н 3.53, N 6.07. С21Н17ClCuN2О3 calculated, %: С 56.76, Н 3.86, N 6.30.
Eu*4c. Complex Cu*4с (135 mg, 0.3 mmol) was dissolved in a mixture of water-
ethanol (1:1, 20 mL). A mixture of acetone cyanohydrin (0.28 mL, 3 mmol), KOH (170 mg, 3
mmol) and water (5 mL) was added and the resulting mixture was kept at 50 °C for 30 min. The
solution was then cooled to room temperature, followed by adding EuCl3·6H2O (39 mg, 0.11
mmol). The resulting mixture was kept at room temperature for 1 h. The product was extracted
with DCM (3 х 20 mL) and dried with anhydrous sodium sulfate. Solvent was removed under
reduced pressure. Yield 55 mg (0.045 mmol, 45%). ESI-MS, m/z: found 1189.30 (М+Н)+,
required 1189.30. Found, %: С 59.89, Н 3.94, N 6.47. С63Н51EuN6О9·CH2Cl2 calculated, %: С
60.38, Н 4.20, N 6.60.

3. Results and discussion
As starting compounds for the preparation of the target ligands, the previously described
6-aryl-3-(2-pyridyl)-1,2,4-triazine-4-oxides 114 were obtained. The subsequent nucleophilic
substitution of a hydrogen atom by the cyanide anion generated in situ and the following aza-
Diels-Alder reaction with 2,5-norbornadiene or 1-morpholinocyclopentene, used as dienophiles,
afforded the cyano-substituted precursors 2 and 314 of the target ligands. The final products, the
5-aryl-2,2’-bipyridine-6-carboxylic acids 4 and 5, were obtained by the acidic hydrolysis of the
cyano group in a media of 50% sulfuric acid. Such reaction conditions allow the one-step
synthesis without isolation of the intermediate amide, unlike in the previously reported method.14
At the final step, the in situ prepared sodium salts of acids 4 and 5 were reacted with
europium(III) or terbium(III) chloride in a solution of methanol at room temperature to form the
corresponding complexes Ln*4 and Ln*5 (Scheme 1, Table. 1).
Scheme 1. Synthesis of neutral lanthanides chelates of acids 4 and 5. Reagents and
conditions: i) Acetone cyanohydrin, DCM, 40 °C, 30 min; ii) 1-morpholinocyclopentene,
toluene, 110 °C, 1.5 h, then acetic acid, 118 °C, 2 min; iii) 2,5-norbornadiene, toluene, 110 °C, 7
h; iv) H2SO4 (50%), 140 °C, 10 h; v) NaOH (1 eq.), ethanol, 78 °C, 5 min, then LnCl3·6H2O
(0.33 eq.), 20 °C.

In the case of the 2,2’-bipyridine 2c, containing a strong electron donor methoxy group,
during the hydrolysis of the cyano-group in 50% sulfuric acid a side reaction of sulfonation of
the 4-methoxyphenyl moiety, as well as the demethylation reaction, occurred. Attempts at the
hydrolysis of 2c under basic conditions, even after boiling for several hours, did not result in the
formation of the carboxylic acid either. However, according to the literature, the formation of
complexes with transition metals cations considerably facilitates the reaction of nitriles with
various nucleophiles.18 In particular, such effects were previously reported for 2-
cyanopyridines19 and 5-aryl-6-cyano-2,2'-bipyridines.20 The hydrolysis of the cyano group in the
latter case takes place in aqueous ethanol under reflux in the presence of 1 equivalent of copper
dichloride. The main disadvantage of this method is the formation of a sufficiently stable copper
complex as the reaction product.
Scheme 2. Preparation of Eu*4c. Reagents and conditions: i) CuCl2·2H2O, EtOH/H2O
(1:1), 85 ºС, 8 h; ii) KCN, EtOH/H2O (1:1), 50 ºC, 30 min; iii) EuCl3·6H2O, EtOH/H2O (1:1), 20
ºС, 2 h.

This approach was used in the current work for the hydrolysis of cyanopyridine 2c, and
by reacting of 2c with CuCl2·2H2O the copper complex of bipyridine carboxylic acid, Cu*4c,
was isolated (Scheme 2). The extremely high stability of the complex prevented the isolation of
the acid 4c in its pure form. On the other hand, the dissociation of Cu*4c in the presence of in
situ generated KCN followed by the addition of europium(III) chloride to the reaction mixture
afforded the target complex Eu*4c, after the extraction of the aqueous reaction mixture with
methylene chloride.
The structures of the obtained complexes were confirmed by elemental analysis, mass
spectrometry (ESI-MS) and IR spectroscopy. In particular, upon chelating of the europium
cation the ligand carbonyl group vibration band shifts from 1720 to 1640 cm-1, which directly
indicates the participation of the carboxyl group in the complexation of the lanthanide cation. In
addition, assignment of peaks and their intensities by mass spectrometry match the expected
isotopic distribution of the europium complexes.
The complexes Eu*4 and Eu*5 show the typical photoluminescence of europium
chelates (Table 1). At room temperature in solutions of methylene chloride, upon excitation of
these complexes at the absorption maximum, i.e. in the range 294-309 nm, which corresponds to
the π−π* transitions of the ligand, a strong luminescence is observed. The luminescence spectra
show the typical narrow bands of the europium cation emission (591, 615 and 695 nm). The
europium cation luminescence quantum yields of these complexes are in the range 3.2-11%. The
data obtained are summarized in Table 1 and the absorption and emission spectra are shown in
Fig. 2.
Table 1. Photophysical properties of the lanthanide complexes of ligands 4 and 5
Lanthanide
complex
Eu*4a
Absorption, λmax, nm
(ε, 10-3 M-1 см-1)a
309 (41.9)
Ligand structure
Ar
Фb
τc, ms
Ph
0.064
0.11
Tol
309 (41.0)
Eu*4b
Ar
4-MeOC6H4
4-FC6H4
4-FC6H4
232 (34.5), 295 (17.3) 0.057
232 (42.6), 294 (20.7) 0.042
232 (42.7), 296 (21.8)
1.1
Eu*4c
N
HOOC
N
0.95
Eu*4d
Tb*4d
Ar
N
Tol
300*
0.032
0.8
Eu*5b
HOOC
N
b
aIn a solution of CH2Cl2 at room temperature; Europium cation luminescence quantum yields were
c
measured in CH2Cl2 solution using [Ru(bpy)3]Cl2 as a standard21; Europium cation luminescence
lifetime; *Due to the low solubility, the extinction coefficient could not be measured

350
300
250
200
150
100
50
0.5
0.4
0.3
0.2
0.1
0.0
0
580
600
620
640
660
680
700
720
250
300
350
400
Wavelength, nm
Wavelength, nm
a)
b)
Fig. 2. The absorption spectra (a) and europium cation luminescence spectra (b) of complexes Eu*4
(Eu*4a – solid lines; Eu*4b – dash lines; Eu*4c – dot lines; Eu*4d – dash dot lines) and Eu*5 (dash dot
dot lines) in CH2Cl2 solution at room temperature.
The D2d site symmetry of the Eu3+ cation was determined from the luminescent spectra,
5
5
in accordance with the number of components in the transitions D0→7F0, D0→7F2 and
5D0→7F4.22
Complex Tb*4d showed a very weak terbium cation luminescence in CH2Cl2 solution at
room temperature, with two major bands at 490 and 545 nm. Apparently, an increase of the
conjugation system of the ligand due to the introduction of an aromatic substituent reduces the
energy of the excited state of the ligand, which greatly complicates the energy transfer to the
higher in energy 5D4 level of the terbium cation, followed by sensitization of the terbium cation
luminescence. In the europium complexes, the ligand-to-metal energy transfer takes place on the
5D0 level, which is much lower in energy, therefore significant sensitization of europium cation
luminescence is observed. The absorption spectra of the complexes Eu*4d and Tb*4d (Fig. 3a)
actually do not differ in shape. The terbium cation luminescence spectrum of the complex Tb*4d
is shown in Fig. 3b.

0.3
0.2
0.1
0.0
2.0
1.5
1.0
0.5
0.0
500
550
600
250
300
350
400
Wavelength, nm
Wavelength, nm
a)
b)
Fig. 3. The absorption spectra of the complexes Eu*4d (solid line) and Tb*4d (dash line) (a) and
the terbium cation luminescence spectrum of the complex Tb*4d (b) in CH2Cl2 solution at room
temperature.
As was expected, analysis of the photophysical data clearly demonstrates the significant
influence of the nature of the aromatic substituent in position C5 of the pyridine ring on the
efficiency of the luminescence of the europium cation. Thus, complex Eu*4b showed the most
intense luminescence, having a ligand with the tolyl moiety.
A comparison of the solubility of complexes Eu*5b and Eu*4 in non-polar solvents
showed that it was much higher for the latter case (up to 10g/L in CH2Cl2), which is due to the
annelution of the cyclopentene fragment. Thus, the “1,2,4-triazine" approach to 2,2’-bipyridine
ligands can largely eliminate the low solubility drawback of the majority of such complexes.
Especially notable is the fact that the europium cation luminescence of complex Eu*5b is less
effective compare to Eu*4.
Compare to the previously reported photophisycal data for similar neutral lanthanide
complexes, i.e. a luminescence quantum yield of 85%4b and a luminescence lifetime of 2.81
ms5b, both the luminescence quantum yields and the luminescence lifetimes of europium
complexes Eu*4 and 5 are lower in value (up to 11% and up to 1.1 ms, respectively). However,
the good solubility of complexes Eu*4 in organic non-polar solvents suggests that this work
approach is potentially promising from the standpoint of obtaining lanthanide complexes with
acceptable properties, and further research is required in this direction for improving the
photophysical properties.

4. Conclusions
In summary, in this work the synthetic approach to 4-aryl-1-(2-pyridyl)-6,7-dihydro-5H-
cyclopenta[c]pyridine-3-carboxylic acids and 5-aryl-2,2’-bipyridine-6-carboxylic acids has been
optimized, as compared with the previously described methods. The range of compounds was
substantially expanded, and at first they were used as ligands for neutral lanthanide complexes. It
is shown that upon varying the structure of the ligands (i.e. by varying of the nature of the
aromatic substituent in the C5 position of the pyridine ring, and by the introduction of a fused
cyclopentene fragment into the pyridine core) the turning of the photophysical properties of the
lanthanide complexes, namely the luminescence efficiency, is possible. In addition, due to the
direct annulation of the cyclopentene moiety onto the central pyridine ring, the solubility of such
complexes in non-polar solvents has been improved dramatically compared with the previously
reported analogs. Despite the lower values of both the luminescence quantum yields and the
luminescence lifetimes of the obtained lanthanide complexes, further research for new synthetic
possibilities to 2,2’-bipyridine ligands using the “1,2,4-triazine” methodology seems promising,
since only in this case may highly soluble cycloalkane-annulated bipyridine ligands be obtained,
which are useful for the preparation of highly soluble lanthanide chelates that are valuable for
practical use.
Acknowledgments
This work was supported by the Russian Scientific Found (Grant # 15-13-10033).
Supplementary data
Supplementary data associated with this article can be found, in the online version.
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Graphical abstract

Luminescent neutral lanthanide complexes of 5-aryl-2,2'-bipyridine-6-carboxylic acids,
synthesis and properties
Alexey P. Krinochkin,a Dmitry S. Kopchuk,a,b∗ Dmitry N. Kozhevnikova,b
a Ural Federal University, 19, Mira St., 620002 Yekaterinburg, Russian Federation
b
Postovsky Institute of Organic Synthesis of RAS (Ural Division), 22/20, S. Kovalevskoy/Akademicheskaya St.,
620990 Yekaterinburg, Russian Federation
Abstract
Phosphorescent lanthanide complexes of 4-aryl-1-(2-pyridyl)-6,7-dihydro-5H-
cyclopenta[c]pyridine-3-carboxylic acids and 5-tolyl-2,2'-bipyridine-6-carboxylic acid were
prepared. The ligands were synthesized using the “1,2,4-triazine” methodology. In one case, the
opportunity for copper complex preparation, followed by exchange of the metal cation from
copper to europium to overcome synthetic difficulties, was demonstrated. The ability of tuning
the properties of the complexes by means of varying the structure of the ligands was
demonstrated. In particular, the introduction of the cyclopentene moiety allowed the preparation
of highly soluble complexes in non-polar organic solvents.
∗Corresponding author. Tel.: +7-343-375-4501; fax: +7-343-374-0458; e-mail: dkopchuk@mail.ru