[1,2,3]Triazolo[1,5-a]pyridine derivatives as molecular chemosensors for zinc(ii), nitrite and cyanide anions

Article abstract of DOI:10.1039/b906992e

Three recently prepared tridentate ligands TPF, TPS and TPT, based on the triazolopyridine-pyridine nucleus possessing fluorescent properties, have been tested as chemosensors for metal ions. A particular response is obtained in the case of ZnTPT⁺²

Full text of DOI:10.1039/b906992e

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PAPER
www.rsc.org/njc | New Journal of Chemistry
[1,2,3]Triazolo[1,5-a]pyridine derivatives as molecular chemosensors
for zinc(II), nitrite and cyanide anionsw
a
b
b
´
Rafael Ballesteros-Garrido, Belen Abarca,* Rafael Ballesteros,
a
b
Carmen Ramırez de Arellano, Frederic R. Leroux,* Franc
´
´ ´
and Enrique Garcı
¸
oise Colobert*a
´
a-Espan
˜
a*c
Received (in Montpellier, France) 7th April 2009, Accepted 15th June 2009
First published as an Advance Article on the web 27th July 2009
DOI: 10.1039/b906992e
Three recently prepared tridentate ligands TPF, TPS and TPT, based on the
triazolopyridine–pyridine nucleus possessing fluorescent properties, have been tested as
chemosensors for metal ions. A particular response is obtained in the case of ZnTPT+2
.
The Zn2+ TPT 1 : 1 complex has proved to be an efficient chemosensor for anions especially
nitrite and cyanide.
now report on the luminescence properties of three potentially
Introduction
tridentate triazolopyridine molecules.9–12 Triazolopyridine–
pyridine rearrangements offer an interesting strategy for
preparing tridentate ligands (Scheme 1). Ring–chain–ring
isomerization between A and B derivatives depends on the
electronic properties of the R substituent.13 Compounds B
have structures that permit their tridentate coordination provided
that the R substituents have an additional donor atom.
As an essential component of many proteins, zinc plays
an important role in hydrolytic enzymes and has structural
functions in transcription factors and related proteins.1 In
addition to bound Zn2+, there are other amounts of relatively
free Zn2+ located in the brain and nervous system which
are believed to play key roles in neurotransmission and
neurodegeneration.2 Alterations in zinc homeostasis are
claimed to be implicated in diseases such as Alzheimer’s.1,3
Therefore, the detection of trace concentrations of Zn2+ is a
research goal of great current interest.4 In this sense,
fluorescent methods offer the possibility of lowering the
detection limit and require simple equipment available in
many research laboratories.
Therefore, to prepare the new ligands we undertook the
strategy depicted in Scheme 2. The first step consisted of direct
regioselective lithiation of 3-pyrid-20-yl-[1,2,3]triazolo[1,5-a]-
pyridine 1 at position C-7.14 The chiral alcohol 2 (TPF) was
prepared by adding (ꢀ)-fenchone to lithiated 1.9 The sulfoxide
4 (TPS) was obtained by Pd catalyzed10,11 reaction using iodo
derivative 3, obtained by quenching the lithio derivative with
iodine.13 Synthesis of ligand 6 (TPT) was possible by quenching
with ethyl picolinate giving 5, followed by reaction with
tosylhydrazine in NaOH (Scheme 2).12
In recent years we have been developing different
[1,2,3]triazolo[1,5-a]pyridine compounds and we have studied
their synthesis,5 coordination capacity,6 and fluorescence
behavior.7–8 In this respect, we have recently reported on a
triazolopyridine-based molecule which reveals very important
changes in its optical properties upon coordination to Zn2+ 8
.
Results and discussion
Additionally, we proved that coordination of anionic species
to the preformed Zn2+ complex generated photo-induced
charge transfer processes (PCT) that could be advantageously
used for the detection of the bound anions.8 In order to
improve this behavior by reaching lower detection limits we
TPF, TPS and TPT are soluble in organic solvents like EtOH,
MeOH, CH2Cl2 and are highly fluorescent. The fusion of a
triazole (electron-donating group) and a pyridine moiety
(electron acceptor) provides extremely interesting fluorescence
properties to these three ligands. The close spectral character-
istics and quantum yields of the three compounds can be
attributed to the fact that the fluorophoric behavior is marked
by the triazolopyridine–pyridine system. The fluorescence
of 98 : 2 v/v ethanol–water solutions of TPS and TPT
keeps constant with time. The TPF fluorescence decreases
a
´ ´
´
Laboratoire de Stereochimie, CNRS, Universite de Strasbourg
(ECPM). 25 rue Becquerel, 67089 Strasbourg, France.
E-mail: frederic.leroux@ecpm.u-strasbg.fr,
Francoise.colobert@unistra.fr
´
Departamento de Quımica Organica, Facultad de Farmacia,
Universidad de Valencia, Avda. Vicente Andres Estelles s/n,
b
c
´
´
´
46100 Burjassot, Valencia, Spain. E-mail: Belen.abarca@uv.es
´
´
Departamento de Quımica Inorganica, ICMOL, Universidad de
Valencia, Edificio de Institutos, Apdo. Correos 22085,
46071 Valencia, Spain. E-mail: Enrique.garcia-es@uv.es
w Electronic supplementary information (ESI) available: UV-Vis spectra
(Fig. S1 and S2), fluorescence spectra/data (Fig. S3–S23, S26), LOD
(Fig. S24 and S25), X-ray refinement details and packing (Fig. S27),
1H-NMR spectra comparison (Fig. S28). CCDC 736124. For ESI
and crystallographic data in CIF or other electronic format see
DOI: 10.1039/b906992e
Scheme 1 The triazolopyridines–pyridine ring rearrangement.
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Scheme 2 Procedure for the synthesis of triazolopyridine derivatives TPF, TPS and TPT. (a) tert-butyl-3-(p-tolylsulfinyl)propanoate (1 eq.),
Pd2dba3 (0.05 eq.), Xantphos (0.1 eq.), KOH (10 eq.), toluene–water (1 : 1), 3 h, reflux.
significantly, which can be associated with the occurrence of
some precipitation.
Table 1 Fluorescent properties of compounds TPS and TPT and
their corresponding complexes with Zn2+
Zn2+–L complexes were prepared by the reaction of zinc(II)
chloride etherate with each one of the three ligands. FAB or
ESI-MS supported, in all cases, the formation of 1 : 1
[Zn(L)]+–Cl aggregates. However, in the system Zn2+–TPT,
ESI-MS also shows the presence of a complex of [Zn(TPT)2Cl]+
stoichiometry. As a matter of fact, it was possible to get
suitable crystals for X-ray diffraction of the complex
[Zn(TPT)2](ClO4)2ꢂ1/2CH3CN (7) by slow evaporation of
CH3CN solutions of TPT and Zn(ClO4)2. The asymmetric unit
of 7 ꢀconsists of two almost equivalent [Zn(TPT)2]2+ cations,
ClO4 counter-anions and one CH3CN molecule. The cationic
complexes (Fig. 1) display distorted octahedral geometry with
meridional arrangement of the bis(triazolopyridino)pyridine
moieties, which behave as tridentate ligands through the
nitrogen donor atom of the central pyridine ring and
the nitrogens placed at the 2-positions of the triazole rings.
The bond distances of the metal ion with the pyridine nitrogens
(average distance 2.13 A) are slightly shorter than those with the
triazolo nitrogens (average distance 2.20 A). The ligand is
slightly domed with a mean angle between the triazolopyridine
units of ca. 101 (see Fig. 1). The crystal packing shows
p–p-stacking interactions between the cationic units which give
rise to a sort of pillar-like chains (see Fig. S26 in the ESIw).
TPS
ZnTPS2+
TPT
ZnTPT2+
Quantum yielda (%)
2
2
2
359
412
23
359
404
l
l
absb/nm
emsb/nm
340
384
401b
340
384
403b
a
b
10ꢀ5 solution of the corresponding ligand were used. Most intense
signal.
The spectroscopic characteristics of the Zn2+ complexes
were analyzed in 98 : 2 v/v ethanol–water solution of
Zn(ClO4)2 and the ligands TPS and TPT (see Table 1). The
system Zn2+–TPF was not analyzed due to the above
mentioned precipitation of this compound.
Although addition of Zn2+ to TPS produced very slight
changes in the fluorescence intensity and quantum yield
values, addition of Zn2+ to TPT solutions leads to very
significant chelation enhancements of fluorescence (CHEF)
(Fig. 2 and Table 1). The intensity and quantum yield increase
after addition of one equivalent of Zn2+ to TPT very
significantly (Table 1, Fig. 3). The fluorescence increase is
accompanied by
a small (8 nm) hypsochromic effect.
These fluorescence properties can be explained by a PCT
mechanism.15 The shift in the position of the band is smaller
than those we have previously reported for the triazolo-
pyridine molecule in previous works.8 While in that system,
coordination of Zn2+ occurs at the electron-donating pyridine
fragments, in the present systems Zn2+ coordination involves
both the electron-accepting pyridine fragment and the electron-
donating triazolopyridine ring compensating the effect.
The less significant changes observed for TPS can probably
be due to the high electron withdrawing properties of the
sulfoxide.
The fluorescence behavior of TPS and TPT was also
checked in the presence of the divalent transition metal ions
Co2+, Ni2+ and Cu2+ and of the post-transition ones Zn2+
,
Cd2+ and Pb2+ (Fig. 3). As observed for Zn2+, the fluorescence
changes are, in general, larger for TPT than for TPS. Although
not so large as in the case of Zn2+, significant CHEF effects
are also produced in the system Cd2+–TPT.16 Quenching
Fig.
1
X-Ray structure of Zn(TPT)2ꢂ(ClO4)2ꢂ1/2CH3CN, (7)
asymmetric unit (red oxygen, blue nitrogen, green chlorine, gray
carbon and white hydrogen atoms).
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detect anionic species.17 We have checked the interaction of
solutions containing Zn2+ and TPT in 1 : 1 molar ratio for
which the 1 : 1 complex is the predominant species in solution,
with different monovalent anions (Fꢀ, Clꢀ, Brꢀ, Iꢀ, CNꢀ
,
SCNꢀ, NO2ꢀ, NO3ꢀ) by adding solutions of tetrabutyl-
ammonium salts of all anions (except NaNO2) to Zn(TPT)2+
solutions. In all cases, quenching of the emission was produced.
The analysis of the fluorescence spectra with the program
SPECFIT18 has allowed determination of the stability
constants shown in Table 2 for the formation of mixed
Zn(TPT)2+–anion complexes. Only complexes of 1 : 1
stoichiometry were inferred from the analysis of the data.
The constants shown for the halide anions reveal the
stability sequence Fꢀ 4 Clꢀ E Brꢀ o Iꢀ. The CNꢀ binding
constant is one order of magnitude lower than SCNꢀꢀwhile
ꢀ
NO2 anions interact much more strongly than NO3 with
Zn(TPT)2+
.
However, as is often observed, the magnitudes of the
binding constants do not correlate exactly with the extension
of the quenching phenomena.19 Particularly, in spite of having
a lower binding constant, CNꢀ anions exert a much more
marked quenching than SCNꢀ anions. It is also interesting to
remark the neat thermodynamic and sꢀensing discrimination
ꢀ
that our system produces between NO2 and NO3 (Fig. 4).
The p-acceptor character of these two ligands CNꢀ and NO2
ꢀ
may be contributing to these quenching effects. Taking into
account the sensitivity of the equipment and a plot of the
fluorescence intensity variation with respect to the concentration
ꢀ
of the substrate detection limits for NO2 of 3.2 ppb and
below 1 ppb for CNꢀ were calculated (see ESIw).20
Fig. 2 Fluorescence spectra of TPT (8 ꢃ 10ꢀ5 M) in ethanol–water
98 : 2, and changes produced upon Zn(II) addition.
Conclusions
In conclusion we have reported the fluorescence properties of
the triazolopyridine–pyridine TPT system. These systems act
as fluorophores and coordinating molecules; revealing TPT as
a
suitable chemosensor for Zn(II); and the complex
Zn(TPT)2+ as a sensor for anions especially cyanide and
nitrite. Encouraged by our preliminary results, we are
currently investigating the recognition of amino acids and
the development of new structures with similar fluorescent
characteristics.
Fig. 3 Fluorescent responses upon addition of equimolar amounts
M2+ solutions of TPT and TPS. Quenching effects are represented
with negative intensity.
Experimental
Synthesis of ligands TPF, TPS and TPT
phenomena were observed with Cu2+, Ni2+ and Co2+
However, even large excesses of Na+, K+, Ca2+ and Mg2+
.
,
A detailed description has been given in ref. 11, 10 and 12,
respectively.
which are most biologically relevant potentially competing
mobile ions, do not perturb the fluorescence of TPT. At this
pH, the extent of formation of the complexes of these metal
ions is very limited and therefore the changes in fluorescence
are not significant.
Zn(TPT)2+ 1 : 1 complex has a coordinatively unsaturated
coordination sphere. Three positions are occupied by ancillary
ligands, likely solvent molecules, that can be readily replaced
by anionic ligands. Such substitutions should affect the
emissive properties of the system and can thereby be used to
Anhydrous sample preparation for the ESI-MS and NMR
analysis
The general procedure for the preparation of solid Zn2+
complexes for MS analysis was in all cases as follows: to a
solution of the corresponding ligand (0.14 mmol) in dichloro-
methane (20 mL), a solution of ZnCl2 in ether (0.15 mL, 1 M)
was added. The yellow-white solution formed was then stirred
(30 min) at room temperature. Evaporation of the solvent gave
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Table 2 Logarithms of the binding constants18 (log K) for the formation of Zn(TPT) (anion) complexes in ethanol–water (98 : 2 v/v)
Fꢀ
Clꢀ
Brꢀ
Iꢀ
CNꢀ
SCNꢀ
NO2
5.07 ꢄ 0.03
NO3
3.50 ꢄ 0.02
ꢀ
ꢀ
log K
4.85 ꢄ 0.05
4.08 ꢄ 0.04
4.23 ꢄ 0.02
4.58 ꢄ 0.02
3.87 ꢄ 0.09
4.88 ꢄ 0.04
0.24 ꢃ 0.06 ꢃ 0.04 mm size, monoclinic, Pc, a = 12.045(2),
b = 26.990(5), c = 11.652(2) A, b = 103.01(3)1, V =
3690.7(13) A3, Z = 4, rcalcd = 1.640 g cmꢀ3, ymax = 27.10,
MoKa (l = 0.71073 A), o-scan, diffractometer Nonius
KappaCCD, T = 150(2) K, 26 633 reflections collected of
which 14 522 were independent (Rint = 0.068), multi-scan
absorption correction (Tmin/Tmax
= 0.836/0.969), direct
primary solution and refinement on F2 (SHELXS-97 and
SHELXL-9722), 1129 refined parameters, residual electron
density tentatively refined as a disordered CH3CN molecule,
disordered atoms refined isotropically, hydrogen atoms refined
as riding, enantiomeric twin with components 0.405(12) and
0.595(12),23 possible higher symmetry (P21/c space group) has
been closely examined (see ESIw), R1[I 4 2s(I)] = 0.0529,
wR2(all data) = 0.1229.
Fig. 4 Plot of I0/I of Zn(TPT)2+ (8 ꢃ 10ꢀ5 M) in 98 : 2 v/v
EtOH–H2O upon successive additions of nitrite (up) and nitrate
(down) (lexc = 359 nm).
yellow-white solids that were washed with hot ethyl acetate
and characterized by ESI-MS.
Acknowledgements
ESI-MS [ZnTPSCl]+ 432 (100), 433 (28), 434 (95), 435 (34),
We are very grateful to the Ministerio de Educacion y Ciencia
´
436 (59), 436 (18), 440 (5).
ESI-MS [ZnTPTCl]+ 412 (100), 413 (22), 414 (89), 415 (89),
415 (38), 416 (60), 417 (19), 418 (18), 419 (6).
ESI-MS [Zn(TPT)2Cl]+ 725 (100), 726 (40), 727 (89), 728
(Spain) (Project CTQ2006-15672-C05-03 and 01) for its
financial support, and to the SCSIE for the recording of the
ESI-MS spectra. CRA thanks Consolider Ingenio 2010
(CSD2007-00006) and Generalitat Valenciana (GV07/087)
for financial support. R.B.-G. wants to thank Ministere de
(45), 729 (65), 730 (25), 731 (19), 732 (7) 1.
At rt ZnCl2 in ether (0.1 mL, 1 M) was added to a solution
of TPT (0.1 mmol) in CDCl3–D3COD (1 mL 80 : 20 v/v was
added providing [ZnTPTCl2].21 1H-NMR (300 MHz, CDCl3)
d = 8.61 (d, J = 6.9 Hz, 2H, 7H), 8.09 (d, J = 8.9 Hz, 2H, 6H),
7.91 (t, J = 8.1 Hz, 1H, 40H), 7.79 (d, J = 8.1 Hz, 2H, 30H),
´
l’Education Nationale, de la Recherche et de la Technologie
(France) for a doctoral fellowship.
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2106 | New J. Chem., 2009, 33, 2102–2106