Synthesis and spectroscopy of anionic tridentate benzimidazole-pyridine carboxylate and tetrazolate chromophore ligands

Article abstract of DOI:10.1016/j.ica.2014.11.037

We report on seven new anionic benzimidazole-pyridine carboxylate and tetrazolate tridentate N^N^O and N^N^N ligands that are modified with chromophore (phenyl, biphenyl, naphthyl) and solubilizing groups. The ligands are UV chromophores with the lowest-energy absorption maxima at 312-335 nm and with the molar absorption coefficients of (20-25) × 10³ M^-1 cm^-1 in DMSO solution. The ligands form neutral complexes with trivalent lanthanides and sensitize the red luminescence of europium. The triplet state energies of the deprotonated ligands, which were measured from the phosphorescence spectra of their lanthanum complexes at 77 K, are in the range of (18.8-21.1) × 10³ cm^-1. We also describe synthesis of non-symmetric pyridines that are 2,6- and 2,4,6-substituted with hydroxymethyl, carboxaldehyde, and carbonitrile groups.

Full text of DOI:10.1016/j.ica.2014.11.037

Inorganica Chimica Acta 427 (2015) 81–86
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Synthesis and spectroscopy of anionic tridentate benzimidazole-
pyridine carboxylate and tetrazolate chromophore ligands
⇑
Nail M. Shavaleev , Svetlana V. Eliseeva
École Polytechnique Fédérale de Lausanne, Institut des Sciences et Ingénierie Chimiques, Avenue Forel 2, BCH, CH-1015 Lausanne, Switzerland
a r t i c l e i n f o
a b s t r a c t
Article history:
We report on seven new anionic benzimidazole-pyridine carboxylate and tetrazolate tridentate N^N^O
and N^N^N ligands that are modified with chromophore (phenyl, biphenyl, naphthyl) and solubilizing
groups. The ligands are UV chromophores with the lowest-energy absorption maxima at 312–335 nm
and with the molar absorption coefficients of (20–25) ꢀ 103 Mꢁ1 cmꢁ1 in DMSO solution. The ligands
form neutral complexes with trivalent lanthanides and sensitize the red luminescence of europium.
The triplet state energies of the deprotonated ligands, which were measured from the phosphorescence
spectra of their lanthanum complexes at 77 K, are in the range of (18.8–21.1) ꢀ 103 cmꢁ1. We also
describe synthesis of non-symmetric pyridines that are 2,6- and 2,4,6-substituted with hydroxymethyl,
carboxaldehyde, and carbonitrile groups.
Received 30 September 2014
Received in revised form 21 November 2014
Accepted 23 November 2014
Available online 9 December 2014
Keywords:
Ligand
Benzimidazole
Carboxylic acid
Tetrazole
Ó 2014 Elsevier B.V. All rights reserved.
Lanthanide
Luminescence
1. Introduction
and the N-methylene aryl group are not oxidized during this
synthesis.
Luminescent lanthanide(III) complexes are used in lighting and
analytical applications [1–3]. Polydentate carboxylate [4–20] and
tetrazolate [21–24] ligands are strong chelators for lanthanide ions
and efficient sensitizers of their luminescence. Here, we report on
synthesis and spectroscopy of anionic tridentate carboxylate
and tetrazolate benzimidazole-pyridine ligands HL1–HL7 that are
functionalized with chromophore and solubilizing groups
(Schemes 1 and 2). We developed these tridentate ligands to make
neutral homoleptic tris-complexes of the lanthanides. The lantha-
nide ion in this type of complexes is expected to be nine-coordinate
[7–12,23]. We also report on the synthesis of non-symmetric pyri-
dines P2–P4 that are 2,6- or 2,4,6-substituted with hydroxymethyl,
carboxaldehyde, and carbonitrile groups (Scheme 3).
The two new tetrazolate N^N^N ligands HL6 and HL7 were pre-
pared in four steps [23]. The first two steps were the same as for
the carboxylate ligands. In the third step, the carboxaldehyde
was converted into the carbonitrile with NH2OHꢂHCl in formic acid
[27]. Finally, reaction of the carbonitrile with NaN3 in DMF gave
the target tetrazoles [28].
We introduced the phenyl, biphenyl, and naphthyl chromoph-
ores via the N-methylene linker into the ligands HL1–HL5. The
N-methylene linker prevents conjugation of the aryl chromophore
with the benzimidazole. If required, any chromophore can be
attached in the same way. We previously reported N-alkyl and
N-aryl analogs of HL1–HL5 [23].
The ligands HL3–HL7 were modified with an n-octyloxy group
to increase the solubility of the ligands and their complexes in
organic solvents. If required, the n-octyloxy group can be replaced
with other solubilizing and chromophore alkyloxy- and aryloxy-
groups. Chart 1 shows the three known reference ligands HR1–
HR3 [23,29].
New 2,4,6-trisubstituted pyridine P2 was obtained by mono-
oxidation of bis-methanol 3 (itself prepared from chelidamic acid
in three multi-gram steps by modified literature procedures [5–
7,30–32]) with SeO2 in dioxane (Scheme 3) [12]. Until now, only
one report described an analogue of P2 as a by-product of bis-oxi-
dation of a substituted bis-methanol-pyridine by SeO2 [33]. We
note that mono-oxidation of bis-methanol-pyridines to carboxal-
dehyde can also be achieved with MnO2 as oxidant [34].
2. Results and discussion
The five new carboxylate N^N^O ligands HL1–HL5 were
prepared in three steps (Scheme 1) [23]. The formation of the
benzimidazole heterocycle by the reaction of the carboxalde-
hyde-6-hydroxymethylpyridine P1 or P2 with o-nitroaniline [25]
was followed by step-wise oxidation of the pyridine-2-methanol
first to the carboxaldehyde with SeO2 [23] and then to the
carboxylic acid with H2O2 in formic acid [26]. The n-octyloxy chain
⇑
Corresponding author. Tel.: +7 (347) 254 84 38.
E-mail address: shava@mail.ru (N.M. Shavaleev).
http://dx.doi.org/10.1016/j.ica.2014.11.037
0020-1693/Ó 2014 Elsevier B.V. All rights reserved.

82
N.M. Shavaleev, S.V. Eliseeva / Inorganica Chimica Acta 427 (2015) 81–86
R
R
N
R
N
R
N
C8H17
C8H17
N
R2
R2
N
a
a
HN
O2N
HN
O2N
N
+
+
OH
O
OH
N
N
OH
O
OH
R1
R1
R1
R1
P1: R = H
P2: R = OC8H17
P1: R = H
P2: R = OC8H17
R1 = H, OC8H17
R1 = H, OC8H17, NPh2
R2 = CH2Ar
R
R
N
R
C8H17
N
C8H17
N
b
c
R2
N
N
b
c
N
N
O
N
N
R1
R1
O
N
R1
OC8H17
C8H17
N
C8H17
N
d
N
N
N
N
N
N
NH
N
N
NH
N
N
O
O
O
OC8H17
N
N
N
N
HL6
HL7
OH
N
N
OH
OH
N
N
Scheme 2. Synthesis of tetrazoles HL6 and HL7: (a) Na2S2O4, 2-methoxyethanol/
H2O or DMF/H2O, under N2, 110–120 °C; (b) SeO2, dioxane, under N2, 110 °C; (c)
NH2OHꢂHCl, sodium formate, formic acid, under N2, 120 °C; (d) NaN3, NH4Cl, DMF,
under N2, 110 °C.
HL1
HL2
OH
N
OH
N
O
O
N
N
N
N
N
OH
OH
OC8H17
OC8H17
a
c
b
HL3
HL4
O
O
O
O
O
OH
OC2H5
OH
OC2H5
OC8H17
1
OC8H17
OC8H17
OC8H17
N
N
d
O
HL5
N
N
N
OC2H5
OC2H5
OH
OH
OH
O
Scheme 1. Synthesis of carboxylic acids HL1–HL5: (a) Na2S2O4, DMF/H2O, under
N2, 110–120 °C; (b) SeO2, dioxane, under N2, 110 °C; (c) H2O2, formic acid, under air,
2
3
P2
0 °C (this reaction does not work when R, R1, and R2 are H, NPh2, and C8H17
,
respectively).
The known 2,6-disubsituted pyridines P3 [35–37] and P4 [36–
38], which we planned to use in the synthesis of tetrazoles, were
prepared from P1 (we described its multi-gram synthesis before
[12]) by conversion of the carboxaldehyde into the carbonitrile
P3 with NH2OHꢂHCl in DMSO [39], and by oxidation of the pyri-
dine-2-methanol to the carboxaldehyde P4 with SeO2 (Scheme 3).
The N-substituted o-nitroanilines were prepared by reaction of
2-halonitrobenzenes with an excess of amine in DMSO on heating
(Scheme 3).
Br
F
HN
Br
O2N
N
e
+
O2N
4
R2
HN
Hal
R1
R1
Hal = F, Cl, Br
f
R1 = H, OC8H17, NPh2
R2 = C8H17, CH2Ar
O2N
O2N
The intermediate 4 was obtained by a non-catalyzed exothermic
reaction of 2-bromo-4-fluoronitrobenzene with diphenylamine in
the presence of potassium tert-butoxide in DMSO (Scheme 3)
[40]. This reaction takes advantage of the high reactivity of C–F
bond in electron-deficient arenes [40–42]. An attempt to make car-
boxylate ligand ‘HL8’ from 4 worked for the first three steps (see the
intermediates L8-NO2, L8-CH2OH, L8-CHO and the structure of
ligand ‘HL8’ in the Supplementary material); however, oxidation
of the carboxaldehyde L8-CHO to the carboxylic acid ‘HL8’ with
H2O2 in formic acid [26] resulted in oxidative decomposition, prob-
ably because of the presence of electron-rich diphenylamino group
(Supplementary material). The benzimidazole-pyridines L8-CH2OH
and L8-CHO (Supplementary material) can be used as ligands them-
selves and as precursors to polydentate ligands [20]. The ligands
derived from 4 are likely to exhibit low-energy intra-ligand diphe-
nylamine-to-benzimidazole charge-transfer absorption transition.
g
h
N
N
N
N
N
OH
O
OH
O
P1
P3
P4
Scheme 3. Synthesis of precursors: (a) ethanol, H2SO4, 100 °C; (b) 1-octylbromide,
K2CO3, DMF, under N2, 60 °C; (c) NaBH4, CH3OH/THF, under N2, room temperature;
(d) SeO2, dioxane, under N2, 60 °C; (e) KOtBu, DMSO, under N2, room temperature;
(f) amine, DMSO, under N2, 90–100 °C; (g) NH2OHꢂHCl, DMSO, under N2, 100 °C; (h)
SeO2, dioxane, under N2, 110 °C.
Tris-complexes of the ligands HL1–HL7 with lanthanum(III) and
europium(III) were obtained as air- and moisture-stable solids from
hot ethanol/water solutions with a 3:3:1 M ratio of the ligand, NaOH
(base), and LnCl3ꢂnH2O. Elemental analysis indicates that these

N.M. Shavaleev, S.V. Eliseeva / Inorganica Chimica Acta 427 (2015) 81–86
83
Table 1
OCH3
Absorption spectra of the ligands.a
kabs/nm (e )
/103 Mꢁ1 cmꢁ1
R
N
HL1
315 (23)
314 (23), 298 (22)
334 (25)
334 (24), 298 (12), 286 (11)
312 (21)
335 (24)
OCH3
O
N
N
HL2
HL3
HL4
HL5
HL6
HL7
HR3b
N
N
N
OH
N
NH
N
N
HR1
HR2: R = CH3
HR3: R = C8H17
314 (20)
320 (22)
Chart 1. Reference ligands [23,29].
a
See Fig. 1. kabs and
at 298 K. Errors: 1 nm for kabs
e
in DMSO solution at 250–500 nm at (1.25–1.96) ꢀ 10ꢁ4
M
; 5% for e.
b
Data from [23].
complexes have the composition Ln(Ligand)3ꢂnH2OꢂmC2H5OH,
where n = 0.5–4 and m = 0–1. The complexes are soluble in
dichloromethane.
The new europium complexes exhibit the line-like red f–f lumi-
nescence from the 5D0 excited state of europium (Fig. S2, Supple-
mentary material). The photophysical study of the neat solid
europium complexes indicate that they are a mixture of species
because (a) the luminescence decays are bi-exponential at 298 K
(short and long components of 0.05–0.87 ms and 0.24–2.52 ms;
ligand-sensitized quantum yield 0.3–61%) and at 10 K (short and
long components of 0.43–0.83 ms and 1.65–2.77 ms), and (b) the
5D0 7F0 transition in the high resolution excitation spectrum
exhibits multiple, and often broad, components [3]. This mixture
L7
L6
L5
L4
L3
ε
L2
L1
is probably made of the hydrated complex [Eu(j -
3-Ligand)2(j1
Ligand)(H2O)x], where one of the ligands is monodentate-bound to
250
300
350
400
the europium through the anionic carboxylate oxygen or the tetraz-
olate nitrogen, and of the anhydrous complex [Eu(j
3-Ligand)3].
Wavelength [nm]
Both types of these lanthanide complexes are known for tridentate
N^N^O and N^N^N ligands [12,23]. The excited states of europium
are quenched by water [14,15]. The hydrated and the anhydrous
complexes give rise to the short and the long components in the
luminescence decay, respectively. The europium complexes were
not studied further, because it is difficult to make a quantitative
characterization of europium-centred luminescence for mixtures
of species.
Fig. 2. Absorption spectra of lanthanum complexes in dichloromethane. The unit
on the vertical axis is 20 ꢀ 103 Mꢁ1 cmꢁ1
.
Table 2
Ligand-centered absorption and phosphorescence of lanthanum complexes.
a
kabs/nm (
e
/103 Mꢁ1 cmꢁ1
)
ET(0–0)/103 cmꢁ1b
The new lanthanum complexes are probably a mixture of
species too; however, this should not change the ligand-centred
spectroscopy of these complexes, which is discussed below. The
ligands HL1–HL7 may provide pure anhydrous tris-complexes with
La(L1)3ꢂ2H2O
317 (52), 251 (82)
20.5
19.2 (sh)
18.9
La(L2)3ꢂ3H2OꢂC2H5OH
La(L3)3ꢂ2H2O
316 (53), 295 (53), 285 (51)
340 (50), 256 (88)
338 (50), 295 (39), 284 (38),
273 (33), 263 (35)
311 (52)
350 (52), 258 (38)
319 (52), 244 (90)
322 (68), 269 (45)d
La(L4)3ꢂ3H2O
18.8
lanthanides [Ln(j
3-Ligand)3] when the synthesis is conducted in
La(L5)3ꢂ2H2O
La(L6)3ꢂ4H2O
La(L7)3ꢂ4H2O
[La(R3)3]ꢂ2.5H2Oc
20.8
18.9 (sh)
21.1
anhydrous conditions.
The absorption spectra of the ligands HL1–HL7 in DMSO solution
exhibit the lowest-energy band centred at 312–335 nm with molar
absorption coefficient of (20–25)ꢀ103 Mꢁ1 cmꢁ1, which corre-
20.7
a
See Fig. 2. kabs and
e in dichloromethane solution at 250–500 nm at (3.79–
5.79) ꢀ 10ꢁ5 M at 298 K. Errors: 1 nm for kabs
; 5% for e.
sponds to
p ?
p⁄ transition of the benzimidazole chromophore
b
See Fig. 3. Triplet state energies, ET(0–0), from 0–0 transition of phosphores-
(Fig. 1 and Table 1). For the lanthanum and europium complexes,
cence spectra of neat solid complexes at 77 K. Error: 200 cmꢁ1
.
c
Data from [23].
In DMSO solution.
d
HL7
HL6
the lowest-energy ligand-centred absorption maxima and molar
absorption coefficients are 311–352 nm and (46–53)ꢀ103 Mꢁ1
cmꢁ1 in dichloromethane solution (Fig. 2 and Table 2, and Fig. S1
and Table S1, Supplementary material).
The ligand-to-lanthanide energy transfer in lanthanide com-
plexes usually occurs from the triplet state of the ligand [13].
The triplet state energies of the ligands (ET) were determined to
be (18.8–21.1) ꢀ 103 cmꢁ1 from the 0–0 transition of the phospho-
rescence spectra of their neat solid lanthanum complexes at 77 K
(Fig. 3 and Table 2).
HL5
HL4
HL3
ε
HL2
HL1
280 300 320 340 360 380 400
Wavelength [nm]
The ligands HL1–HL7 exhibit intense UV absorption and their
triplet state energies are sufficiently high to sensitize the red lumi-
nescence of europium [13]; therefore, these ligands can be used as
sensitizer chromophores in luminescent lanthanide complexes
Fig. 1. Absorption spectra of ligands HL1–HL7 in DMSO. The unit on the vertical
axis is 5 ꢀ 103 Mꢁ1 cmꢁ1
.

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Commercial reagents were used without purification. Chroma-
tography was performed on a column with an i.d. of 30 mm on sil-
ica gel 60 (Fluka, Nr 60752). The progress of reactions and the
elution of products were followed on TLC plates (silica gel 60
F254 on aluminum sheets, Merck).
Caution: Tetrazole derivatives and other nitrogen-rich heterocy-
cles are known to be explosive hazard. We did not encounter any
problems in the handling of small quantities of the new tetrazole
ligands and their complexes; however, we did not perform stress
tests on these materials.
L7
L6
L5
L4
L3
L2
L1
3.1. Synthesis of pyridines
400 450 500 550 600 650 700
Wavelength [nm]
P2: The reaction was performed under nitrogen. SeO2 (182 mg,
1.64 mmol; 99.8%, Acros) and 3 (900 mg, 3.37 mmol, small excess)
were stirred in dioxane (30 mL; neither dried nor degassed) at
60 °C (bath temperature) for 48 h (we note that the use of higher
temperature and of the excess of SeO2 in this reaction will lead
to undesired bis-oxidation). The mixture was filtered through Cel-
ite to remove precipitated selenium. Celite was washed with CH2-
Cl2. Chromatography was performed on silica (20 g) with CH2Cl2 to
remove the bis-aldehyde and with 2–2.5% CH3OH in CH2Cl2 to
recover the product. The product shows one spot on TLC (silica;
ethyl acetate). It is soluble in organic solvents, and it remains sol-
uble even after long storage as a solid (in contrast to the non-
substituted analog P1 [12]). Pale yellow solid: 804 mg (3.03 mmol;
90%; C15H23NO3; MW 265.35). 1H NMR (400 MHz, DMSO-d6):
d = 9.89 (s, 1H, CHO), 7.3–7.2 (m, 2H), 5.61 (s, br, 1H, OH), 4.60
(s, 2H, CH2OH), 4.14 (t, J = 6.4 Hz, 2H), 1.79–1.68 (m, 2H), 1.46–
1.20 (m, 10H), 0.85 (t, J = 7.2 Hz, 3H) ppm.
P3: For alternative syntheses, see [35–37]. The reaction was per-
formed under nitrogen. Hydroxylamine hydrochloride (600 mg,
8.63 mmol; excess) and P1 (750 mg, 5.47 mmol) were stirred in
DMSO (3 mL; 99.7%, degassed, extra dry, over mol. sieve, water
<50 ppm, Acros) at 100 °C (bath temperature) for 2 h to give dark
red solution. It was extracted with CH2Cl2 and sat. aq. solution of
NaCl. The organic layer was extracted with sat. aq. solution of NaCl
to remove the residual DMSO. The aqueous extracts were back-
extracted with CH2Cl2. The aim of the extraction is to remove DMSO
without losing the product (it is soluble in water). The combined
organic extracts were evaporated and purified by chromatography
on silica (20 g) with CH2Cl2 to remove the impurities and with 0.5%
CH3OH in CH2Cl2 to recover the product. Pale yellow solid: 446 mg
(3.32 mmol; 61%; C7H6N2O; MW 134.14). It can be checked for the
presence of DMSO by recording 1H NMR (400 MHz, CDCl3): d = 7.85
(t, J = 8.0 Hz, 1H), 7.63 (d, J = 7.6 Hz, 1H), 7.55 (d, J = 8.0 Hz, 1H), 4.83
(d, J = 5.2 Hz, 2H, CH2OH), 3.13 (t, J = 5.2 Hz, 1H, OH) ppm.
Fig. 3. Phosphorescence spectra of lanthanum complexes in neat solids at 77 K.
(Fig. S2, Supplementary material) [20]. The lowest-energy absorp-
tion transition and the triplet state of the new ligands and their
lanthanum complexes exhibit the following trends (Figs. 1–3 and
Tables 1 and 2):
(a) the absorption transition is unchanged, but the triplet state
is red-shifted when biphenyl at the N-methylene is replaced
by naphthyl from HL1 to HL2, because the triplet state
energy of the naphthyl is lower than that of the biphenyl
and benzimidazole [43];
(b) both are unchanged when the biphenyl at the N-methylene
is replaced by naphthyl from HL3 to HL4 (the only changes
in the absorption spectra are observed at <310 nm), because
the triplet state energy of the alkoxy-benzimidazole in HL3
and HL4 is lower than that of the biphenyl and naphthyl;
(c) both are red-shifted when electron-donor n-octyloxy group
is attached to the benzimidazole from HL1 to HL3, from
HL2 to HL4, and from HR3 to HL6 because of generation of
low-energy benzimidazole-centered ‘‘alkoxy’’-to-imine
charge-transfer excited state;
(d) both are blue-shifted when n-octyloxy group is attached to
the 4-position of pyridine, from HL1 to HL5 and from HR3
to HL7.
In conclusion, the new tridentate anionic ligands HL1–HL7,
which combine ‘hard’ oxygen and ‘soft’ nitrogen donor atoms and
which can be modified with solubilizing and chromophore groups,
can be used for lanthanides and actinides [4–24], for lanthanide/
actinide separation [24], and for any ‘hard’ or ‘soft’ metal ion with
square-planar or octahedral coordination geometry [44–46].
P4: For alternative syntheses, see [36–38]. The reaction was
performed under nitrogen. SeO2 (386 mg, 3.48 mmol) and P3
(933 mg, 6.96 mmol) were stirred in dioxane (30 mL; neither dried
nor degassed) at 110 °C (bath temperature) for 24 h to give a dark
yellow solution and black precipitate of selenium. The mixture was
filtered through Celite to remove selenium. Celite was washed with
CH2Cl2. Purification by chromatography (10 g of silica; CH2Cl2)
gave pale yellow eluate of the product. White solid: 692 mg
(5.24 mmol; 75%; C7H4N2O; MW 132.12). 1H NMR (400 MHz,
CDCl3): d = 10.07 (s, 1H), 8.16 (dd, J = 8.0, 1.2 Hz, 1H), 8.07 (td,
1H), 7.92 (dd, J = 7.6, 0.8 Hz, 1H) ppm.
3. Experimental
Elemental analyses were performed by Dr. E. Solari, Service for
Elemental Analysis, Institute of Chemical Sciences and Engineering
(EPFL). ESI MS were recorded on a Q-TOF Ultima (Waters) or
TSQ7000 (Thermo Fisher) spectrometers at the Mass Spectrometry
Service of the Institute of Chemical Sciences and Engineering
(EPFL). 1H and 13C NMR spectra were recorded on Bruker Avance
DRX 400 MHz and Bruker Avance II 800 MHz spectrometers.
Absorption spectra were measured on a Perkin-Elmer Lambda
900 UV/Vis/NIR spectrometer. Luminescence was measured with
a previously described instrumental setup [23]. Spectroscopic
studies were conducted in optical cells of 2-mm path length or in
2-mm i.d. quartz capillaries under air. The solutions in DMSO
(Fluka, >99.9%, ACS spectrophotometric grade) and in CH2Cl2
(Fisher Scientific, analytical reagent grade) were freshly prepared
before each experiment.
3.2. Synthesis of carboxylic acids
The reaction was performed under air [26]. The required alde-
hyde (its synthesis is described in the Supplementary material)
was dissolved in formic acid at room temperature to give a pale
yellow solution (it might be cloudy due to the presence of red solid,
which is residual Se from the previous synthetic step). It was

N.M. Shavaleev, S.V. Eliseeva / Inorganica Chimica Acta 427 (2015) 81–86
85
cooled to 0 °C. Cold H2O2 was added (30% wt. solution in water).
The mixture was stirred at 0 °C for 6 h and kept overnight in the
refrigerator at 3 °C to give colorless or pale yellow solution. Ice-
cold water (30 mL for HL1 and HL2; 15 mL for HL3; 20 mL for
HL4 and HL5) was added drop-wise to precipitate the product.
The suspension was stirred for 1 h at 0 °C. It was filtered. The prod-
uct was washed with water and organic solvent [ether for HL1 and
HL2; hexane, hexane/ether (1/1, 50 mL), and, again, hexane for
HL3; hexane for HL4 and HL5]. It was dried under vacuum. Further
details are provided below.
HL1: Aldehyde (642 mg, 1.65 mmol), formic acid (6 mL, 7.32 g,
0.16 mol), and H2O2 (0.9 mL, 300 mg of H2O2, 8.81 mmol) gave
white solid: 585 mg (1.44 mmol, 87%). Anal. Calc. for C26H19N3O2
(MW 405.45): C, 77.02; H, 4.72; N, 10.36. Found: C, 76.97; H,
5.16; N, 10.32%. 1H NMR (400 MHz, DMSO-d6): d = 8.58 (dd,
J = 7.6, 1.2 Hz, 1H), 8.17 (t, J = 7.6 Hz, 1H), 8.13 (dd, J = 8.0, 1.6 Hz,
1H), 7.79 (d, J = 7.6 Hz, 1H), 7.74 (d, J = 7.6 Hz, 1H), 7.55 (d,
J = 7.2 Hz, 2H), 7.51 (d, J = 8.4 Hz, 2H), 7.43–7.24 (m, 7H), 6.46 (s,
2H) ppm; CO2H proton not observed.
HL2: Aldehyde (528 mg, 1.45 mmol), formic acid (6 mL, 7.32 g,
0.16 mol), and H2O2 (0.8 mL, 266 mg of H2O2, 7.83 mmol) gave
white solid: 349 mg (0.92 mmol, 63%). Anal. Calc. for C24H17N3O2
(MW 379.41): C, 75.97; H, 4.52; N, 11.08. Found: C, 76.06; H,
4.66; N, 10.98%. 1H NMR (400 MHz, DMSO-d6): d = 8.58 (d,
J = 8.0 Hz, 1H), 8.31 (d, J = 8.0 Hz, 1H), 8.13 (td, J = 8.0, 1.2 Hz,
1H), 8.02 (dd, J = 8.0, 1.2 Hz, 1H), 7.93 (d, J = 8.0 Hz, 1H), 7.84 (d,
J = 7.6 Hz, 1H), 7.73 (d, J = 8.4 Hz, 1H), 7.65–7.53 (m, 2H), 7.46 (d,
J = 8.0 Hz, 1H), 7.35–7.22 (m, 2H), 7.19 (t, J = 7.6 Hz, 1H), 6.98 (s,
2H), 6.34 (d, J = 7.2 Hz, 1H) ppm; CO2H proton not observed.
HL3: Aldehyde (665 mg, 1.28 mmol), formic acid (4 mL, 4.88 g,
0.11 mol), and H2O2 (0.75 mL, 250 mg of H2O2, 7.34 mmol) gave
white solid: 512 mg (0.96 mmol, 75%). Anal. Calc. for C34H35N3O3
(MW 533.66): C, 76.52; H, 6.61; N, 7.87. Found: C, 76.55; H, 6.66;
N, 7.84%. 1H NMR (400 MHz, DMSO-d6): d = 8.47 (d, J = 7.2 Hz,
1H), 8.12–8.02 (m, 2H), 7.63 (d, J = 8.8 Hz, 1H), 7.53 (d, J = 7.6 Hz,
2H), 7.48 (d, J = 8.0 Hz, 2H), 7.37 (t, J = 8.0 Hz, 2H), 7.30 (d,
J = 7.2 Hz, 1H), 7.28–7.21 (m, 3H), 6.89 (dd, J = 8.8, 2.4 Hz, 1H),
6.41 (s, 2H), 4.00 (t, J = 6.4 Hz, 2H), 1.75–1.65 (m, 2H), 1.44–1.15
(m, 10H), 0.82 (t, J = 6.8 Hz, 3H) ppm; CO2H proton not observed.
HL4: Aldehyde (227 mg, 0.46 mmol), formic acid (4 mL, 4.88 g,
0.11 mol), and H2O2 (0.35 mL, 117 mg of H2O2, 3.43 mmol) gave pale
orange solid: 154 mg (0.30 mmol, 66%). Anal. Calc. for C32H33N3O3
(MW 507.62): C, 75.71; H, 6.55; N, 8.28. Found: C, 75.19; H, 6.67;
N, 8.12%. 1H NMR (400 MHz, DMSO-d6): d = 8.49 (dd, J = 8.0,
1.2 Hz, 1H), 8.30 (d, J = 8.4 Hz, 1H), 8.07 (t, J = 8.0 Hz, 1H), 7.96 (dd,
J = 7.6, 0.8 Hz, 1H), 7.92 (d, J = 8.4, 1.6 Hz, 1H), 7.75–7.67 (m, 2H),
7.64–7.53 (m, 2H), 7.19 (t, J = 7.6 Hz, 1H), 7.01 (d, J = 2.0 Hz, 1H),
6.94 (s, 2H), 6.91 (dd, J = 8.8, 2.4 Hz, 1H), 6.33 (d, J = 7.2 Hz, 1H),
3.85 (t, J = 6.8 Hz, 2H), 1.67–1.56 (m, 2H), 1.37–1.15 (m, 10H), 0.83
(t, J = 7.2 Hz, 3H) ppm; CO2H proton not observed.
>99.8% GC, over molecular sieves, Fluka) at 110 °C (bath tempera-
ture) for 24 h to give yellow suspension. It was cooled to room
temperature. Water (30 mL) was added to give a suspension of
the product. Further details are provided below.
HL6: The reaction was performed with 2-pyridinecarbonitrile
(503 mg, 1.09 mmol), NaN3 (78 mg, 1.20 mmol), and NH4Cl
(64 mg, 1.20 mmol). pH of the suspension was adjusted to 3–4. It
was extracted with H2O/CH2Cl2 and purified by chromatography
(7 g of silica; with 0.3% CH3OH in CH2Cl2 to remove the impurities;
with 2.5% CH3OH in CH2Cl2 to recover the crude product). The
product was dissolved in CH2Cl2 (20 mL). Hexane (20 mL) was
added. CH2Cl2 was rotor-evaporated to leave a suspension of the
pure product in hexane. The suspension was filtered. The product
was washed with ice-cold hexane. White solid: 370 mg
(0.73 mmol; 67%). Anal. Calc. for C29H41N7O (MW 503.68): C,
69.15; H, 8.20; N, 19.47. Found: C, 69.07; H, 8.05; N, 19.72%. 1H
NMR (400 MHz, DMSO-d6): d = 8.39 (d, J = 7.2 Hz, 1H), 8.29–8.17
(m, 2H), 7.61 (d, J = 8.8 Hz, 1H), 7.24 (d, J = 2.0 Hz, 1H), 6.90 (dd,
J = 8.8, 2.0 Hz, 1H), 4.95 (t, J = 6.8 Hz, 2H), 4.07 (t, J = 6.4 Hz, 2H),
1.82–1.72 (m, 2H), 1.66–1.55 (m, 2H), 1.52–1.40 (m, 2H), 1.40–
1.22 (m, 8H), 1.18–0.92 (m, 10H), 0.87 (t, J = 6.8 Hz, 3H), 0.74 (t,
J = 7.2 Hz, 3H) ppm; NH proton not observed. 13C NMR (200 MHz,
DMSO-d6): d = 156.7, 151.4, 148.1, 143.8, 139.6, 137.8, 137.0,
126.3, 123.2, 120.7, 113.4, 95.0, 68.5, 44.8, 31.7, 31.4, 29.9, 29.3,
29.24, 29.18, 28.9, 28.7, 26.3, 26.1, 22.6, 22.4, 14.4, 14.3 ppm;
one of the aromatic carbons not observed. ESI+ TOF MS: m/z
504.5 {M+H}+.
HL7: The reaction was performed with 2-pyridinecarbonitrile
(406 mg, 0.88 mmol), NaN3 (63 mg, 0.97 mmol), and NH4Cl
(54 mg, 1.01 mmol). The suspension was stirred for 1 h. The solid
was filtered. It was thoroughly washed with water and with ice-
cold hexane (2 ꢀ 10 mL). It was dried under vacuum. It was puri-
fied by chromatography (6 g of silica; with 0.3% CH3OH in CH2Cl2
to remove the impurities; with 3% CH3OH in CH2Cl2 to recover
the product). Pink oil that crystallizes to white solid: 273 mg
(0.54 mmol; 62%). Anal. Calc. for C29H41N7O (MW 503.68): C,
69.15; H, 8.20; N, 19.47. Found: C, 69.16; H, 8.15; N, 19.28%. 1H
NMR (400 MHz, DMSO-d6): d = 7.89 (d, J = 1.6 Hz, 1H), 7.77 (d,
J = 2.0 Hz, 1H), 7.75 (d, J = 8.0 Hz, 1H), 7.73 (d, J = 7.6 Hz, 1H),
7.35 (t, J = 8.4 Hz, 1H), 7.29 (t, J = 8.0 Hz, 1H), 4.98 (t, J = 7.2 Hz,
2H), 4.29 (t, J = 6.8 Hz, 2H), 1.86–1.76 (m, 2H), 1.69–1.58 (m, 2H),
1.52–1.41 (m, 2H), 1.41–1.21 (m, 8H), 1.18–0.92 (m, 10H), 0.86
(t, J = 7.2 Hz, 3H), 0.74 (t, J = 7.2 Hz, 3H) ppm; NH proton not
observed. ESI+ TOF MS: m/z 504.3 {M+H}+.
3.4. Synthesis of lanthanide complexes
The reactions were performed under air with a 3:3:1 M ratio of
the ligand (HL1 to HL7), NaOH (base), and LnCl3ꢂnH2O (Ln = La, Eu).
The ligand was suspended in hot ethanol (65–75 °C, 5 mL; the
same temperature was kept throughout the reaction). A solution
of NaOH in water was added (0.5–1 mL, used as a stock solution
with 100 mg of NaOH per 10 mL of water). The mixture was stirred
for 5 min to give a solution. A solution of LnCl3ꢂnH2O (Ln = La, n = 7;
Ln = Eu, n = 6; 99.9%, Aldrich) in water (2 mL) was added drop-wise
over 5 min. The mixture was stirred for further 5 min. Usually, a
solid or an oily precipitate of the complex appeared on stirring. If
it was required, water was added to induce and complete precipi-
tation of the complex or to crystallize the oily precipitate. The sus-
pension was further stirred for 5 min at 65–75 °C. It was allowed to
cool to 40–50 °C. It was filtered while it was warm. The product
was washed with ethanol/water (1/1) and either ether or hexane.
It was dried under vacuum at room temperature. The complexes
are white or pale colored solids. They are soluble in dichlorometh-
ane and DMSO. They are insoluble in hexane and water. Elemental
analysis indicates that the complexes have the composition
HL5: Aldehyde (490 mg, 1.11 mmol), formic acid (4 mL, 4.88 g,
0.11 mol), and H2O2 (0.65 mL, 217 mg of H2O2, 6.36 mmol) gave
pale yellow solid: 397 mg (0.87 mmol, 78%). Anal. Calc. for
C
28H31N3O3 (MW 457.56): C, 73.50; H, 6.83; N, 9.18. Found: C,
73.31; H, 7.06; N, 9.06%. 1H NMR (400 MHz, DMSO-d6): d = 8.02
(d, J = 2.4 Hz, 1H), 7.79–7.73 (m, 1H), 7.72–7.66 (m, 1H), 7.60 (d,
J = 2.4 Hz, 1H), 7.35–7.26 (m, 2H), 7.24–7.12 (m, 5H), 6.39 (s, 2H),
4.24 (t, J = 6.4 Hz, 2H), 1.83–1.72 (m, 2H), 1.50–1.22 (m, 10H),
0.86 (t, J = 6.8 Hz, 3H) ppm; CO2H proton not observed.
3.3. Synthesis of tetrazoles
The reaction was performed under nitrogen [28]. The required
2-pyridinecarbonitrile (its synthesis is described in the Supple-
mentary material), NaN3 (small excess), and NH4Cl (small excess)
were stirred in dry degassed DMF (2.5 mL, absolute, puriss

86
N.M. Shavaleev, S.V. Eliseeva / Inorganica Chimica Acta 427 (2015) 81–86
Ln(Ligand)3ꢂnH2OꢂmC2H5OH, where n = 0.5–4 and m = 0–1. The
new lanthanide complexes are mixtures of species (see the main
text for discussion). The complexes did not give suitable crystals
for X-ray structure analysis. 1H NMR spectra of lanthanum com-
plexes in CD2Cl2 (a non-coordinating solvent) at room temperature
are broad and non-informative. Further details for the lanthanum
complexes are provided below. The europium complexes are
described in the Supplementary material.
Appendix A. Supplementary material
Synthesis of intermediates and europium complexes; absorp-
tion, luminescence, and 1H NMR spectra. Supplementary data asso-
ciated with this article can be found, in the online version, at http://
dx.doi.org/10.1016/j.ica.2014.11.037.
References
La(L1)3ꢂ2H2O: The complex precipitated on addition of water
(1 mL). It was washed with ethanol/water (1/1) and ether. White
solid: 33 mg (0.024 mmol, 58%) from HL1 (50 mg, 0.123 mmol),
NaOH (4.93 mg, 0.123 mmol), and LaCl3ꢂ7H2O (15.3 mg,
0.041 mmol). Anal. Calc. for C78H54LaN9O6ꢂ2H2O (MW 1388.26): C,
67.48; H, 4.21; N, 9.08. Found: C, 67.73; H, 4.31; N, 8.93%.
La(L2)3ꢂ3H2OꢂC2H5OH: The complex precipitated on addition of
water (3 mL). It was washed with ethanol/water (1/1) and ether.
Pale pink solid: 28 mg (0.020 mmol, 46%) from HL2 (50 mg,
0.132 mmol), NaOH (5.27 mg, 0.132 mmol), and LaCl3ꢂ7H2O
(16.3 mg, 0.044 mmol). Anal. Calc. for C72H48LaN9O6ꢂ3H2OꢂC2H5OH
(MW 1374.23): C, 64.68; H, 4.40; N, 9.17. Found: C, 65.07; H,
4.41; N, 8.89%.
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Anal. Calc. for C102H102LaN9O9ꢂ2H2O (MW 1772.89): C, 69.10; H,
6.03; N, 7.11. Found: C, 69.24; H, 6.25; N, 7.05%.
La(L4)3ꢂ3H2O: The complex precipitated as oil on mixing of the
reagents and on addition of water (1 mL). It crystallized on cooling
and scratching. It was washed with ethanol/water (1/1) and hex-
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O (12.2 mg, 0.033 mmol). Anal. Calc. for C96H96LaN9O9ꢂ3H2O (MW
1712.80): C, 67.32; H, 6.00; N, 7.36. Found: C, 67.36; H, 6.03; N,
7.26%.
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water (1 mL). It crystallized after decanting of the mother liquor
and after washing with ethanol/water (1/1). It was washed with
ethanol/water (1/1) and hexane. White solid: 32 mg (0.021 mmol,
58%) from HL5 (50 mg, 0.109 mmol), NaOH (4.37 mg, 0.109 mmol),
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O9ꢂ2H2O (MW 1544.60): C, 65.32; H, 6.13; N, 8.16. Found: C,
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Acknowledgments
This work was performed in the Laboratory of Lanthanide
Supramolecular Chemistry (LCSL EPFL) and was funded by the
Swiss National Science Foundation (Grant 200020_119866/1). We
are grateful to Prof. Jean-Claude G. Bünzli, the head of LCSL EPFL,
for guidance and support.