3,3′-Disubstitued 2,2′-bipyridines as carboxylate receptors: Conformational regulation of the bipyridine moiety

Article abstract of DOI:10.1002/ejoc.200701005

Two bipyridine derivatives were synthesized and characterized, and their ability to act as sensors for carboxylates was evaluated by UV/Vis and fluorescence studies. The influence of the substituents of the thiourea groups on the stoichiometry of the resu

Full text of DOI:10.1002/ejoc.200701005

FULL PAPER
DOI: 10.1002/ejoc.200701005
3,3Ј-Disubstitued 2,2Ј-Bipyridines as Carboxylate Receptors: Conformational
Regulation of the Bipyridine Moiety
Ana M. Costero,*[a] Salvador Gil,[a] Margarita Parra,[a] Nuria Huguet,[a] Zouhir Allouni,[a,b]
Rajae Lakhmiri,[b] and Ahmed Atlamsani[b]
Keywords: Biaryls / Molecular recognition / Fluorescent probes / Receptors / Hydrogen transfer
Two bipyridine derivatives were synthesized and charac-
terized, and their ability to act as sensors for carboxylates
was evaluated by UV/Vis and fluorescence studies. The in-
fluence of the substituents of the thiourea groups on the stoi-
chiometry of the resulting dicarboxylate complexes was es-
tablished. Conformational changes in the bipyridine moiety
under different conditions were evaluated.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
supramolecular inorganic architectures.[7] Furthermore,
chemical modification of the 2,2Ј-bipyridine ligand by the
introduction of additional functionality at the 4-, 5- and 6-
positions is well known, and the coordination chemistry of
such compounds has been actively explored.
However, much fewer 3,3Ј-disubstituted bipyridines are
available, and the coordination chemistry of only a small
number of such compounds has been studied. Nevertheless,
small substituents in the 3- and 3Ј-positions such as methyl
groups,[8] carboxylic acids[9] and methyl esters[10] have all
been reported. Furthermore, in the early 1980s, Rebek et al.
successfully exploited the 3,3Ј-substitution positions for the
preparation of a novel crown ether derivative that bound
transition-metal ions at the bipyridine moiety or alkali-
metal ions at the crown ether.[11]
Introduction
The construction of multicomponent molecular systems
represents an important first step in the development of
novel supramolecular systems for catalysis, light-to-energy
conversion and chemical sensing.[1] Chemical sensing is an
interesting area of research, and the application, design and
synthesis of chemosensors for a wide range of substrates
have become active fields in supramolecular chemistry.[2]
Sensing of anionic guests has recently become an area of
focus because anions play a fundamental role in many bio-
logical and chemical processes.[3] In recent years, we have
been working towards the design and application of chemo-
sensors based on the binding site–signalling unit approach.
Thus, we prepared ligands derived from biphenyl, as a
transducer subunit, which are able to act as fluorescent or
colorimetric sensors for cations and anions.[4] The results of
studies on these biphenyl derived ligands have demon-
strated that conformational changes induced by modifica-
tion of the dihedral angle of the biphenyl moiety have a
strong influence on the fluorescent properties of the sen-
sor.[5]
We report herein the synthesis of two 2,2Ј-bipyridine
units with thiourea groups in the 3- and 3Ј-positions to act
as a receptor subunit. We also present the results of com-
plexation studies with selected carboxylic acids, as well as
findings on their ability to coordinate to transition-metal
cations.
The development of new sensor systems has prompted
us to synthesize new ligands. As a result of its interesting
characteristics[6] and, more importantly, its analogy to the
biphenyl system, bipyridine seemed an interesting target for
a transducer subunit.
Results and Discussions
Synthesis
The bidentate ligand 2,2Ј-bipyridine is one of the sim-
plest but most widely used organic linkers. In recent years,
this ligand has been incorporated into a diverse range of
The new bipyridine-based ligands are presented in Fig-
ure 1. Compounds 1 and 2 were prepared from disubsti-
tuted bipyridyl 3 (Scheme 1). 3,3Ј-Diamino-2,2Ј-bipyridine,
3, was prepared from 3,3Ј-dinitro-2,2Ј-bipyridine, which
was obtained following the Ullman coupling procedure first
described by MacBride et al.[12] However, as the reaction
conditions described by MacBride proved to be problem-
atic, we chose to follow the procedure of Kaczmarek et
al.[13] This method involves the Ullman coupling of 3-ace-
[a] Departamento de Química Orgánica, Universitat de València,
Dr. Moliner 50, 46100-Bujassot. Valencia, Spain
[b] Departement de Chimie, Faculté des sciences, Université Abdel-
malek Essaadi,
Essaadi, Morocco
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
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A. M. Costero et al.
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tylamino-2-chloropyridine, followed by acidic hydrolysis to
afford 3,3Ј-amino-2,2Ј-bipyridine in improved yields. The
reaction of 3 with the corresponding isothiocyanate af-
forded ligands 1 and 2.
Figure 1. Bipyridine-based ligands.
Scheme 1. Synthesis of 1 and 2.
Protonation and Complexation with Transition-Metal
Cations
Figure 3. UV spectra of free, protonated (6 h after protonation)
and Ni2+-complexed ligand 1 (top); fluorescence spectra of free,
protonated (immediately after protonation) and Ni2+-complexed li-
gand 1 (bottom).
Ligands 1 and 2 are mainly present in solution in the s-
trans conformation, which is stabilized through the forma-
tion of hydrogen bonds between the bipyridyl N atoms and
the amide NH of the thiourea groups at the 3- and 3Ј-posi-
tions of the bipyridine moiety.[14] Thus, a planar bipyridine
system is expected (Figure 2).
In the literature, it is established that the absorption band
of bipyridine system shows a clear bathochromic shift after
monoprotonation, which arises due to a conformational
change between the s-trans and the s-cis forms.[15a,16] By
contrast, ligands 1 and 2 showed different behaviour; in
both cases, the UV spectra showed no changes upon pro-
tonation. This suggests that protonation takes place on the
sulfur atom instead of on the pyridine rings, which is in
agreement with the evolution of the system with time. Thus,
after six hours at room temperature, a green colour attrib-
uted to the presence of a mixture of hydrolysis products
was observed. As expected, the UV spectrum under these
conditions showed a new band at 446 nm, which was the
origin of the green colour.
Figure 2. Conformation calculated by PcModel for ligand 1.
The fluorescence spectra of both protonated and non-
protonated systems were also studied. The emission spec-
The coplanarity of the pyridine groups in these ligands trum of free ligand 1 shows an emission band at 420 nm
is in agreement with the absorption maximum in the UV [λexc (DMSO) = 290 nm], and the protonated system shows
spectra [λmax (DMSO) = 294 nm and λmax (DMSO) = emission at 426 nm [λexc (DMSO) = 290 nm]. However, a
280 nm for ligands 1 and 2, respectively].[15] In addition, large enhancement (130%) in the fluorescence was observed
ligand 1 shows absorption at 260 nm, which corresponds to after protonation, probably due to an increase in the rigid-
the naphthalene group (Figure 3, top).
ity of the system (Figure 3, bottom).
1080
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Eur. J. Org. Chem. 2008, 1079–1084

3,3Ј-Disubstitued 2,2Ј-Bipyridines as Carboxylate Receptors
Complexation experiments with NiCl2 were also carried the appropriate anion in DMSO solutions. UV/Vis studies
out.[17] The obtained complexes were studied by UV, fluo- reveal that 1 and 2 are able to complex dicarboxylates and
1
rescence and H NMR spectroscopy. The UV spectrum of acetate but the stoichiometries of the resulting complexes
1·NiCl2 shows a shoulder at λ = 350 nm that is not present depend on the nature of both the ligand and the carboxyl-
in the spectrum of free ligand 1 (Figure 3, top). This obser- ate (Figure 4). Ligand 1 gave rise to 1:2 complexes with
vation is in agreement with the bathochromic shift expected both dicarboxylates and acetate, whereas ligand 2 formed
for the conformational change between the s-trans and the 1:2 complexes with acetate and 1:1 complexes with dicarb-
s-cis forms, which is induced by complexation (Scheme 2). oxylates. These observations might be related to the dif-
This conformational change also has a strong influence on ferent substituents on the thiourea moieties of 1 and 2. The
the fluorescence emission and gives rise to almost total observed stoichiometries along with the corresponding
quenching (Figure 3, bottom).
complexation constants are summarized in Table 1.
Scheme 2. Conformational change induced by complexation with
NiCl2.
Additionally, strong differences were observed between
the 1H NMR spectra of free ligands 1 and 2 and their corre-
sponding Ni2+ complexes. Thus, the aromatic NH of the
thiourea group is shifted from 11.6 to 9.9 ppm and from
12.00 to 11.00 ppm for 1 and 2, respectively. By contrast,
the second NH proton does not experience any significant
shift. These observations are in agreement with conforma-
tional changes that result in cleavage of the hydrogen bonds
present in the free ligand. On the other hand, interesting
effects were also observed at the bipyridine moiety; thus,
protons Hc are shifted from 8.88 and 8.85 ppm in the free Figure 4. UV titration of ligand 1 plus TBA oxalate (top) and li-
gand 2 plus TBA oxalate (bottom) both in DMSO (3ϫ10–4 , in-
ligand to 8.05 and 7.9 ppm in the complexes for 1 and 2,
creasing addition until 4 equiv. at 25 °C). For the other anions
studied, see the Supporting Information.
respectively. These observations are in agreement with the
proposed conformational change that places the thiourea
groups out of the bipyridine plane and reduces the interac-
Table 1. Stoichiometry and overall binding constant (logβ) (calcu-
lated by SpecFit software) in DMSO for 1 and 2 with various
anions measured by UV/Vis spectroscopy at 25 °C.
tion of Hc with the sulfur atom.
1
2
Complexation Studies with Mono- and Dicarboxylates
Anion[a]
logβ
L:A
logβ
L:A
The ability of these new ligands to act as receptors for
carboxylates was studied. A series of dicarboxylates (oxal-
ate, malonate and succinate), all as their tetrabutylammo-
nium (TBA) salts, were used. These anions were chosen to
evaluate the effect of the aliphatic chain length on the com-
plexation constants. We also tested both acetate as a simple
model and TMAOH to ensure that the observed behaviour
Oxalate
7.2Ϯ0.2
7.7Ϯ0.1
7.0Ϯ0.2
7.4Ϯ0.1
1:2
1:2
1:2
1:2
6.0Ϯ0.7
5.7Ϯ0.6
6.2Ϯ0.7
8.3Ϯ0.1
1:1
1:1
1:1
1:2
Malonate
Succinate
Acetate
[a] All anions were used as their TBA salts.
Titration experiments showed that the addition of in-
is not due to the deprotonation of the ligand (see Support- creasing amounts of anion to ligand 1 gave rise to an en-
ing Information). hancement in the fluorescence emission (for TBA oxalate,
The carboxylates were prepared from the corresponding see Figure 5). These results corroborate an increase in the
carboxylic acids and commercially available TBA hydroxide rigidity of the system, but not changes in the bipyridine
(2.0 equiv.) in DMSO. All studies were carried out by using conformation; an s-trans to s-cis conformational change
1
UV, fluorescence and H NMR spectroscopy.
should produce a strong quenching of the fluorescence (see
The anion binding ability of receptors 1 and 2 was ini- above). Consequently, the s-trans conformation present in
tially evaluated by UV/Vis titration of each receptor with the free ligand is kept in the 1:2 complex. Similar experi-
Eur. J. Org. Chem. 2008, 1079–1084
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A. M. Costero et al.
FULL PAPER
ments carried out with ligand 2 showed a quenching of the
The difference in the behaviours of the ligands should be
fluorescence when increasing amounts of anion were pres- related to the substituents on the thiourea moiety. Greater
ent in the solution (Figure 5), which indicates that an s- steric hindrance between the naphthyl groups in the s-cis
trans to s-cis conformational change is likely. These results conformation probably precludes the formation of 1:1 com-
allow us to propose the complexation types shown in Fig- plexes of ligand 1 with dicarboxylates.
ure 6 for the 1:1 and the 1:2 complexes.
To obtain additional information about the structure of
these complexes, H NMR spectroscopic experiments were
1
carried out. Unfortunately, broad signals were observed in
the spectra; however, the signal corresponding to Hc of the
ligands, which resonates at ca. 9 ppm, remained essentially
unchanged after complexation of ligand 1, but it shifted to
7.61 ppm upon complexation of ligand 2. These observa-
tions are in agreement with the conformational change that
gives rise to the 1:1 complex.
On the other hand, the bipyridine conformation in the
Ni2+ complex places both thiourea groups close to each
other, and as such, complexation with ligand 1 seems to be
inhibited. Thus, UV and fluorescence studies carried out
with 1 and TBA acetate demonstrated that no change was
produced after the addition of increasing amounts of the
anion. Therefore, this nickel complex exhibits negative allo-
steric cooperativity in the complexation of anions.
Conclusions
Two new bipyridine ligands were prepared. These ligands
are able to complex anions through the thiourea groups or
Ni2+ through the bipyridine moiety. They are present in
DMSO solution as the s-trans conformer. This conforma-
tion allows two types of hydrogen bonding: (i) between the
NH of the thiourea group in one ring and the pyridine ni-
trogen in the other ring and (ii) between Hc and the sulfur
atom of the thiourea. The latter interaction is responsible
1
for the anomalous δ value for Hc in the H NMR spectra.
Protonation experiments demonstrated that it is the sulfur
atom, and not the nitrogen atom, which is bound to the
proton, and under these conditions, hydrolysis takes place.
By contrast, complexation with NiCl2 induces a conforma-
tional change from the s-trans conformation to the s-cis.
Ligands 1 and 2 show different behaviour as anion recep-
tors. Ligand 1 gives rise to complexes with a 1:2 stoichiome-
try with carboxylates, whereas the corresponding complexes
of ligand 2 exist in a 1:1 stoichiometry with dicarboxylates.
Finally, a clear negative allosteric cooperativity was ob-
Figure 5. Titration of ligand 1 with TBA oxalate [λexc (DMSO) =
340 nm] (top) and titration of ligand 2 with TBA oxalate [λexc served for these ligands. Thus, the formation of the Ni2+
(DMSO) = 435 nm] (bottom).
complex precludes coordination with the studied anions.
Experimental Section
General Procedures and Materials: Compound 3 was prepared ac-
cording to a literature procedure.[14] All reagents were commercially
available and used without purification. THF was distilled from
Na/benzophenone under an atmosphere of argon prior to use.
Water-sensitive reactions were performed under an atmosphere of
argon. Column chromatography was performed with silica gel 60
(230–400 mesh, Merck) and with neutral alumina (pH = 7.0Ϯ0.7)
with a particle size of 0.05–0.2 nm (ref. 2210017, Sds). Silica gel
Figure 6. Complexation types of the 1:1 and 1:2 complexes.
1082
upholded on a 0.2-mm thick aluminium foil DC-60 F (254) (ref.
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3,3Ј-Disubstitued 2,2Ј-Bipyridines as Carboxylate Receptors
5554, Merck) or neutral alumina on a similar support (ref. 105550,
also Merck) were used for TLC. 1H and 13C NMR spectra were
recorded with either a Bruker Avance 300, 400 or 500 MHz spec-
trometer with the deuterated solvent as the lock and residual sol-
vent as the internal reference. High-resolution mass spectra (FAB)
were recorded in the positive ion mode with a Fisons VG-Autoes-
pec. Melting points were determined with a microscope equipped
with a heating slide (Cambridge Instruments). Infrared spectra
were recorded with a Bruker Equinox 55 FTIR with a screen width
of 4000 to 400 cm–1. All spectra were recorded after 10 scans and
with a step wavelength of 0.5 cm–1. Samples were submitted for
IR spectroscopy as KBr discs. Values correspond to the maximum
absorbance for the most characteristic bands of the compounds.
UV spectra were recorded at room temperature with a Shimadzu
UV-2102 PC. Steady-state fluorescence measurements were carried
out with a Varian Cary Eclipse Fluorimeter.
aged dicarboxylates (as their TMA salts) in DMSO and recording
the UV/Vis spectrum after each addition. Logβ was calculated by
fitting all spectrophotometric titration curves with the SPECFIT
program [SPECFIT/32 Global Analysis System v. 3.0, Spectrum
Associates (Marlborough, MA, USA)].
Supporting Information (see footnote on the first page of this arti-
cle): UV spectra of 1, protonated 1, 2 and 2·NiCl2 and their corre-
sponding titration curves with TBA succinate, malonate, acetate
and oxalate. Fluorescence spectra of 2, protonated 2 and 2·NiCl2.
Acknowledgments
This research was supported by A.E.C.I (ref. 803/2006). We
acknowledge the Universitat de Valencia (S.C.I.E.) for most of the
equipment employed. We thank the Spanish Ministerio de Ciencia
y Tecnología for financial support (Project CTQ2006-15456-02)
and the Generalitat Valenciana for the projects ACOMP07-080 and
GV/2007/061.
Syntheses
1: Naphthylisothiocyanate (0.35 g, 1.93 mmol) was added to a solu-
tion of 3,3Ј-diamino-2,2Ј-bipyridine (0.17 g, 0.96 mmol) in THF
(5 mL). The mixture was then heated at reflux under an atmosphere
of argon for 36 h. After cooling to room temperature, 1 was precipi-
tated from dioxane as a brown-yellow solid (0.37 g, 69%). M.p.
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195–197 °C. H NMR (500 MHz, [D6]DMSO): δ = 11.64 (s, 2 H,
NH), 10.64 (s, 2 H, NH), 8.89 (d, J = 8.5 Hz, 2 H, Ar-H), 8.06 (d,
J = 8.3 Hz, 4 H, Ar-H), 7.83 (d, J = 8.3 Hz, 2 H, Ar-H), 7.67 (t, J
= 7.5 Hz, 2 H, Ar-H), 7.55–7.52 (m, 10 H, Ar-H) ppm. IR (KBr)
ν = 3124, 2922, 1593, 1567, 1524, 1397, 1193, 1072, 942, 772, 732,
˜
635 cm–1. HRMS (FAB+): calcd. for C32H25N6S2 [M + H]+
557.1582; found 557.1584.
2: Following the same procedure as that described for 1, 2 was
synthesized from 3,3Ј-diamino-2,2Ј-bipyridine (0.16 g, 0.86 mmol)
and phenylisothiocyanate (0.20 mL, 1.72 mmol) as a white precipi-
tate (0.36 g, 78%). M.p. 116–118 °C. 1H NMR (400 MHz, [D6]-
DMSO): δ = 12.21 (s, 2 H, NH), 10.78 (s, 2 H, NH), 9.02 (d, J =
7.5 Hz, 2 H, Ar-H), 7.66–7.56 (m, 14 H, Ar-H) ppm. 13C NMR
(100.5 MHz, [D6]DMSO): δ = 179.2, 145.2, 143.0, 139.9, 136.2,
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134.8, 129.6, 126.5, 125.0, 123.5 ppm. IR (KBr) ν = 3160, 2360,
˜
1593, 1496, 1438, 1392, 1235, 1179, 1071, 938, 751, 691, 515 cm–1.
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1·NiCl2: Ligand
1 (0.12 g, 0.22 mmol) and NiCl2 (0.35 g,
0.27 mmol) were suspended in EtOH (50 mL). The suspension was
heated at reflux for 10 min to obtain a clear solution. The tempera-
ture was then kept around 80 °C for 2 h. After this time, the reac-
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240 °C (dec.). H NMR (500 MHz, [D6]DMSO): δ = 9.90 (br. s, 4
H, NH), 8.05 (d, J = 8.6 Hz, 1 H), 7.97 (d, J = 8.8 Hz, 1 H), 7.86
(d, J = 7.7 Hz, 1 H), 7.71 (m, 1 H), 7.57 (m, 1 H), 7.40 (m, 1 H),
7.07 (d, J = 9.0 Hz, 1 H) ppm.
2·NiCl2: Complex 2 was prepared by the same procedure as that
described for 1, and it was isolated as a brown wax (0.09 g, 73.3%).
1H NMR (500 MHz, [D6]DMSO): δ = 11.00 (s, 2 H, NH), 10.21
(s, 2 H, NH), 7.93 (br. m, 2 H), 7.55 (d, J = 7.6 Hz, 2 H), 7.45
(br. s, 2 H), 7.32 (br. s, 4 H), 7.21–7.16 (m, 6 H) ppm. 13C NMR
(100.5 MHz, [D6]DMSO): δ = 177.2, 149.6, 140.5, 130.2, 129.5,
125.5, 125.0, 124.2, 122.8, 115.3 ppm.
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Eur. J. Org. Chem. 2008, 1079–1084
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