Supramolecular luminescent lanthanide dimers for fluoride sequestering and sensing

Article abstract of DOI:10.1002/anie.201404847

Lanthanide complexes (Ln=Eu, Tb, and Yb) that are based on a C ₂-symmetric cyclen scaffold were prepared and characterized. The addition of fluoride anions to aqueous solutions of the complexes resulted in the formation of dinuclear supramolecular compounds in which the anion is confined into the cavity that is formed by the two complexes. The supramolecular assembly process was monitored by UV/Vis absorption, luminescence, and NMR spectroscopy and high-resolution mass spectrometry. The X-ray crystal structure of the europium dimer revealed that the architecture of the scaffold is stabilized by synergistic effects of the Eu-F-Eu bridging motive, πstacking interactions, and a four-component hydrogen-bonding network, which control the assembly of the two [EuL] entities around the fluoride ion. The strong association in water allowed for the luminescence sensing of fluoride down to a detection limit of 24nM.

Full text of DOI:10.1002/anie.201404847

Angewandte
Chemie
DOI: 10.1002/anie.201404847
Supramolecular Sensors
Supramolecular Luminescent Lanthanide Dimers for Fluoride
Sequestering and Sensing**
Tao Liu, Aline Nonat, Maryline Beyler, Martꢀn Regueiro-Figueroa, Katia Nchimi Nono,
Olivier Jeannin, Franck Camerel, FranÅois Debaene, Sarah Cianfꢁrani-Sanglier,
Raphaꢂl Tripier,* Carlos Platas-Iglesias,* and Loꢃc J. Charbonniꢄre*
Dedicated to Professor Jean-Marie Lehn on the occasion of his 75th birthday
Abstract: Lanthanide complexes (Ln = Eu, Tb, and Yb) that
are based on a C2-symmetric cyclen scaffold were prepared and
characterized. The addition of fluoride anions to aqueous
solutions of the complexes resulted in the formation of
dinuclear supramolecular compounds in which the anion is
confined into the cavity that is formed by the two complexes.
The supramolecular assembly process was monitored by UV/
Vis absorption, luminescence, and NMR spectroscopy and
high-resolution mass spectrometry. The X-ray crystal structure
of the europium dimer revealed that the architecture of the
selective sensing of Fꢀ in aqueous solutions a real challenge.
Whereas numerous examples dealt with the sensing of
fluoride in organic solvents or aqueous/organic mixtures,[7]
far fewer have been reported for the sensing in pure water.[8]
The analytical means of choice for Fꢀ sensing in water remain
ion-selective electrodes,[9] with a detection limit of approx-
imately 20 ppb (10ꢀ6 m).
Fluoride is a relatively strong Lewis base, and its
coordination to metals that are described as hard Lewis
acids according to Pearsonꢁs HSAB classification[10] appears
as a potential attractive way of sensing, provided that the
coordination can be easily monitored. In our approach to
fluoride sensing by metal coordination, we hypothesized that
hard lanthanide cations (Ln) could be the targets of choice,
and we intended to take advantage of their well-documented
luminescence that is sensitive to water quenching[11] or anion
binding.[12] However, the interaction of fluoride anions
with lanthanide complexes in water was reported to be
weak (logK ꢁ 2–4)[13] and hardly compatible with a high
sensitivity. We recently demonstrated that europium com-
plexes of DOTA derivatives (DOTA = 1,4,7,10-tetraazacy-
clododecane-1,4,7,10-tetraacetic acid) having a water mole-
cule at the ninth coordination site could be very effective and
selective fluoride sensors;[14] the complexation of fluoride
displaces the coordinated water molecule with a concomitant
enhancement of the europium-based luminescence. In one
case,[14b] a crystal structure determination of the complex
revealed that it assembled into dimers in the solid state, with
a fluoride anion sandwiched between the two Eu complexes.
ꢀ ꢀ
scaffold is stabilized by synergistic effects of the Eu F Eu
bridging motive, p stacking interactions, and a four-component
hydrogen-bonding network, which control the assembly of the
two [EuL] entities around the fluoride ion. The strong
association in water allowed for the luminescence sensing of
fluoride down to a detection limit of 24 nm.
F
luoride anions play an important role in numerous health
concerns, such as dentistry,[1] water depollution,[2] metabolism
disorders,[3] or for the in vivo imaging of 18F by positron
emission tomography (PET).[4] Whereas fluoride can have
beneficial impacts on teeth and bones, the exposure to larger
amounts of fluoride can lead to serious health hazards, so that
the World Health Organization recommends its content to be
lower than 1.5 ppm in drinking water.[5] A particularity of
fluoride is its similarity with the isoelectronic hydroxide
anions, which have the same charge and a very similar ionic
radius (1.285 ꢀ for Fꢀ vs. 1.32 ꢀ for OHꢀ).[6] This similarity,
combined with a very large hydration enthalpy, makes the
[*] Dr. T. Liu, Dr. A. Nonat, Dr. K. Nchimi Nono, Dr. L. J. Charbonniꢀre
LIMAA, IPHC, UMR 7178 CNRS, Universitꢁ de Strasbourg, ECPM
25 rue Becquerel, 67087 Strasbourg Cedex (France)
E-mail: l.charbonn@unistra.fr
263 Avenue du Gꢁnꢁral Leclerc, CS 74205, 35042 Rennes Cedex
(France)
Dr. F. Debaene, Dr. S. Cianfꢁrani-Sanglier
LSMBO, IPHC, UMR 7178 CNRS-Universitꢁ de Strasbourg, ECPM
25 rue Becquerel, 67087 Strasbourg (France)
Dr. M. Beyler, Prof. R. Tripier
Universitꢁ de Bretagne Occidentale, UMR-CNRS 6521/IFR148
ScInBioS
6 avenue Victor le Gorgeu, C.S. 93837, 29238 Brest Cedex 3 (France)
E-mail: Raphael.Tripier@univ-brest.fr
[**] This work was supported by the French Centre National de la
Recherche Scientifique, the Universities of Strasbourg and Brest
(France) and of A Coruna (Spain). L.J.C. and C.P. gratefully
acknowledge support from the European COST action EuFen.
Dr. M. Regueiro-Figueroa, Dr. C. Platas-Iglesias
Departamento de Quꢂmica Fundamental, Universidade da CoruÇa
Alejandro de la Sota 1, 15008A CoruÇa (Spain)
E-mail: carlos.platas.iglesias@udc.es
Supporting information for this article, including full details of the
synthesis and characterization of the complexes, UV/Vis and
spectrofluorimetric titrations of the Eu, Tb, and Yb complexes, the
1H NMR spectrum of [EuL(H2O)]+, details of the NMR analysis of
the Yb complex, and DFTmodelling, is available on the WWW under
http://dx.doi.org/10.1002/anie.201404847.
Dr. O. Jeannin, Dr. F. Camerel
MaCSE, Institut des Sciences Chimiques de Rennes (ISCR)
UMR-CNRS 6226
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Communications
pointing to a square antiprismatic (SAP)
geometry around the Eu cation.[17] The
decay profile of the emission intensity
(Figure 1, inset) is perfectly mono-expo-
nential with a lifetime of 0.58 ms, character-
istic of monohydrated Eu complexes.[18] In
deuterated water, this value increased to
1.95 ms. It was calculated that one water
molecule is directly coordinated to the
europium center in the first coordination
sphere.[11b] Table 1 summarizes the main
spectroscopic properties of the Eu, Tb, and
Yb complexes. The interaction of the com-
plex [EuL(H2O)](ClO4) with various anions
was monitored in aqueous solution by
a combination of UV/Vis absorption and
spectrofluorimetric titrations. Whereas the
addition of up to 200 equivalents of Clꢀ,
Scheme 1. Synthesis of ligand L and of its Ln complexes. a) LiAlH4, THF, 70%; b) Boc2O,
Et3N, DMAP, THF, 50%; c) PBr3, DMF, 60%; d) 1) K2CO3, CH3CN, 2) TFA, CH2Cl2,
3) NaOH, MeOH, 50% over three steps; e) LnX3·H2O, 85–95%. Boc=tert-butoxycarbonyl,
DMAP=4-dimethylaminopyridine, TFA=trifluoroacetic acid.
2ꢀ
Brꢀ, HCO3ꢀ, AcOꢀ, or HPO4 anions
In solution, only the monomeric species could be observed,
and the formation of the dimer in the solid state was related to
the presence of p–p stacking interactions between the
resulted in negligible perturbations of the emission intensity
of the europium complex (Supporting Information, Fig-
ure S1), the introduction of fluoride anions led to drastic
changes. Figure 2 presents the evolution of the emission
spectra upon fluoride addition; the evolution of the intensities
of the peaks at 610 and 690 nm during the titration is shown in
the inset. Corresponding data obtained by UV/Vis titration
can be found in Figure S2. In strong contrast to the previously
studied complexes,[14] important changes in the emission
intensity could be observed even at very low concentrations of
ꢀ
pyridine rings of two facing monomers, assisted by weak C
H···F hydrogen-bonding interactions with the hydrogen atom
adjacent to the coordinating nitrogen atoms of the pyridyl
rings.
Considering our prior results,[14] we hypothesized that Ln
complexes of ligand L (Scheme 1) may fulfill the criteria for
the formation of supramolecular fluoride-containing dimers
in water. The large aromatic indazolyl moieties were expected
to enforce the p–p stacking interactions in a dimeric complex,
while increasing the UV/Vis absorption properties of the
complex. Concomitantly, the NH groups of the indazole rings
were envisaged to strengthen the hydrogen-bonding inter-
actions in a putative dimer. Finally, we anticipated that the
two carboxylate moieties should stabilize the coordination of
the Ln cations through electrostatic interactions, while
decreasing the overall net charge of each complex from
+ III to + I, which might decrease the electrostatic repulsion
associated with the formation of a dimeric adduct. Herein, we
demonstrate the successful implementation of this strategy,
showing that the LnIII complexes (Ln = Eu, Tb, and Yb) of
ligand L can strongly bind fluoride in aqueous solutions,
forming the dimeric supramolecular complex [Fꢂ(LnL)2]+.
The synthesis of ligand L and its Ln complexes is depicted
in Scheme 1. Experimental details of the synthesis and
characterization of the compounds can be found in the
Supporting Information. Figure 1 shows the UV/Vis absorp-
tion and fluorescence spectra of a solution of the complex
[EuL(H2O)](ClO4) in water. The absorption spectrum is
composed of two main absorption bands with maxima at 293
and 255 nm, which were attributed to p!p* transitions
centered on the indazolyl groups.[15]
Figure 1. UV/Vis absorption (left) and emission (right, lexc =293 nm)
spectra of an aqueous solution of [EuL(H2O)](ClO4) (3.5ꢃ10ꢀ5 m).
Inset: Decay of the emission intensity with time (lexc =293 nm,
lem =616 nm, pH 6.4).
Table 1: Main spectroscopic parameters of the Eu, Tb, and Yb complexes
in water.
lmax (e)[a]
FH O
tH O (tD O)
2 2
q[d]
[b]
[c]
2
[EuL(H2O)]+
[TbL(H2O)]+
[YbL(H2O)]+
255 (10170)
293 (10080)
255 (10080)
293 (10220)
255 (10000)
293 (9850)
0.002
0.58 (1.95)
1.46 (2.34)
1
0.17
1
–
Upon excitation into these absorption bands, the emission
spectra displayed the typical pattern associated with the
1.6ꢃ10ꢀ4
<0.004
5
7
europium-centered D0! FJ transitions,[16] with sharp emis-
[a] lmax in nm, e in mꢀ1 cmꢀ1. [b] FH O measured in water relative to
sion bands spanning from 580 to 710 nm. The region from 585
2
7
to 602 nm, which corresponds to the 5D0! F1 transition, only
[Ru(bipy)3]Cl2 (Ref. [19a]) for Eu, to TbL (Ref. [19b]) for Tb, and relative to
IR125 (Ref. [19c]) for Yb. [c] In ms. [d] Calculated according to Ref. [11b].
displayed two bands with a large splitting of 255 cmꢀ1,
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the spectrum displayed two main species (Figure 3a), which
correspond to the [EuL]+ monomer without bonded water
molecules, as found in the spectrum of pure [EuL(H2O)]-
(ClO4) in water (maximum at m/z 699.190), and a new species,
disclosed as a new peak with a maximum at m/z 1415.378. As
confirmed by the calculated isotopic distribution pattern in
Figure 3b, this peak can be attributed to a [Fꢂ(EuL)2]+
cation. Aside from these two peaks, three minor species
could be observed with maxima at m/z 708.193, 719.184 and
727.171. From the isotopic patterns, it could be deduced that
these peaks correspond to doubly charged cations and to
[Fꢂ(EuL)2 + H]2+, [Fꢂ(EuL)2 + Na]2+, and [Fꢂ(EuL)2 + K]2+
species, respectively.
Upon slow concentration of an aqueous solution of the
[EuL(H2O)]Cl complex containing half an equivalent of
fluoride, platelet-like crystals suitable for X-ray diffraction
appeared within a few weeks. The compound crystallized in
the P1 group with one complex per asymmetric unit
(Table S1–S4). The complex is composed of two [EuL]
moieties that are almost linearly bridged by a fluoride anion
(Eu1-F-Eu2 = 178.58; Figure 4). In each [EuL] unit, the Eu is
Figure 2. Steady-state emission spectroscopic titration of a solution of
[EuL(H2O)](ClO4) (c=4.9ꢃ10ꢀ5 m, lex =295 nm, pH 6.5) as a function
of the number of fluoride equivalents. Inset: Fluorescence intensity at
~
^
690 nm ( ) and 610 nm ( ) as a function of the number of added
NaF equivalents and the corresponding fits.
Fꢀ. A large increase in the intensity was observed up to the
addition of 0.5 equivalents, which slowly leveled off when
excess amounts of Fꢀ were added. This behavior unambigu-
ously pointed to the formation of a com-
pound with a [EuL] to fluoride ratio of 2:1.
The evolution of the absorption and emis-
sion titrations were fitted with SpecFit
software.[20] The evolving factor analysis
pointed to the formation of only one new
species, and the titrations were fitted to the
following model:
2 ½EuLðH2OÞꢃþ þ Fꢀ $ ½F ꢂ ðEuLÞ2ꢃþ þ 2 H2O
2
½F ꢂ ðEuLÞ2ꢃ½H2Oꢃ
with : bEu
¼
2
½EuLðH2OÞꢃ ½Fꢀꢃ
ð1Þ
assuming that the concentration of free
water was constant (55.55mꢀ1). Excellent
fitting of the data could be obtained with
logbEu = 13.0(3). It is noteworthy that the
lack of evidence for the formation of
a
mononuclear fluoride intermediate
([EuLF]) points to a strong positive coop-
erative effect during the self-assembly pro-
cess. The excited-state lifetime of Eu in the
complex increased to 1.84 ms in the dimer,
which is in agreement with the replacement
of the water molecule by Fꢀ in the first
coordination sphere of Eu. The lumines-
cence quantum yield of the dimer was
4.9%, indicating an impressive 22-fold
increase upon Fꢀ binding. Using spectral
variations observed by fluorimetry, an
unoptimized detection limit of 24 nmolLꢀ1
(0.46 ppb) at 3s could be calculated in our
assay.
The presence of the fluoride-containing
dimer was further questioned by means of
electrospray mass spectrometry (ES/MS) in
Figure 3. ES/MS spectrum of the europium complex in the presence of fluoride (top), and
measured and calculated spectra for [EuL(H2O)]+ (bottom left) and [Fꢂ(EuL)2]+ (bottom
water. In the presence of fluoride anions, right).
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Figure 4. Crystal structure of the [Fꢂ(EuL)2]+ dimer viewed perpendic-
ular to (a and b) and along (c) the main pseudo-C2 axis. Representa-
tion of the square antiprismatic coordination polyhedron of Eu (d)
ꢀ
along the Eu F axis. Hydrogen atoms were omitted for clarity, except
for those engaged in hydrogen-bonding interactions (dashed blue
lines). C gray, Eu green, F orange, H white, N blue, O red.
Figure 5. 1H NMR spectrum (300 MHz, 298 K, D2O, pD 7.0) of
[YbL(H2O)]+ recorded before (top) and after (bottom) the addition of
0.5 equivalents of NaF.
nonacoordinated by the four nitrogen atoms of the cyclen
ring, two oxygen atoms of the carboxylate moieties, two
nitrogen atoms of the indazole groups, and the bridging
spectroscopy according to published methods.[22] The spectra
recorded for the Yb complex in D2O indicated the presence of
a single species with an effective C2 symmetry in solution
(Figure 5, top). The analysis of the paramagnetic shifts of
[YbL(H2O)]+ was accomplished by using the DFT geometries
of these system optimized in aqueous solution (TPSSh
functional). An excellent agreement was observed when
using the optimized geometries corresponding to the SAP
isomers (Table S5 and Figure S9–S11). The addition of
0.5 equivalents of Fꢀ to a solution of the [YbL(H2O)]+
complex in D2O had a significant effect on the YbIII-induced
NMR shifts (Figure 5 bottom), while addition of an excess of
the anion did not induce further changes. The huge decrease
in the pseudocontact shifts of the axial protons H7ax and H5ax
is in line with previous studies, which correlated the magni-
tude of the pseudocontact shifts of these protons with the
polarization brought by the axial ligand in Yb DOTA-like
complexes, in agreement with the changes observed in the
splitting of the DJ = 1 emission band of the Eu complex upon
fluoride binding.[23] The analysis of the pseudocontact shifts
was performed by using two different models: a monomeric
[YbLF] structure and a [Fꢂ(YbL)2]+ dimer. The agreement
between experimental and calculated shifts (AFj) improved
significantly from 16.7% to 12.1% for the monomer and
dimer, respectively (Figure S10 and S11). An analysis of the
pseudocontact shifts using the dimer model indicates that for
most protons, the pseudocontact shifts are dominated by the
contribution of the Yb center to which the ligand nuclei are
bound. However, we noticed that in the DFT structure of
[Fꢂ(YbL)2]+, the H15 protons lie closer to the Yb center of
the neighboring YbL unit (5.58 ꢀ) than to the Yb ion bound
to the corresponding ligand unit (6.30 ꢀ). The overall general
improvement of the agreement between experimental and
calculated pseudocontact shifts is particularly important for
fluoride anion (dEu F = 2.27 ꢄ 0.02 ꢀ). The coordination of
ꢀ
the DOTA-like ligand leads to a square antiprismatic (SAP)
geometry,[21] with a twisting angle of approximately 388
between the square defined by the four nitrogen atoms of
the cyclen ring and the square containing the two coordinated
N atoms of the indazole groups and the two coordinated
O atoms of the acetate arms. In a dimer, both [EuL]
monomers have the same helicity. Aside from the bridging
fluoride anion, the supramolecular edifice is stabilized by p–p
stacking interactions between facing indazole groups of each
monomer, placed at a distance of 3.46 ꢄ 0.04 ꢀ. Finally, the
overall stability of the dimer is further reinforced by the
presence of four hydrogen bonds between the H atoms of the
indazolyl moieties of one monomer and the coordinated
O atoms of the carboxylate moieties of the second monomer
ꢀ
ꢀ
(d(N O) = 2.80 ꢄ 0.06 ꢀ, d(NH O) = 2.08 ꢄ 0.07 ꢀ, angle
(N-H-O) = 141 ꢄ 98). Interestingly, all complexes in the
structure are of the same chirality (L in our case), which is
in agreement with the chiral space group of the structure (P1).
As for the Eu complex, the interactions of the fluoride
ions with the Tb and Yb complexes were also determined by
UV/Vis and fluorescence titration experiments. High-resolu-
tion emission spectroscopy of a solution of the Tb complex
(Figure S5 and S6) gave an association constant of logbTb
=
12.5(1.0), which is very similar to that of the Eu complex. In
the case of the Yb complex, which emits in the near-infrared
(NIR) region, the variations were very large (Figure S7 and
S8), showing the same trends as for the Eu complex and
resulting in a stability constant of logbYb = 12.6(1.0).
Considering that the contribution of the paramagnetic Yb
cations to the proton chemical shifts of proximal bonded
ligands is largely dipolar in nature, we also investigated the
formation of the [Fꢂ(YbL)2]+ complex in D2O by NMR
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In summary, the interaction of fluoride anions with the
[EuL(H2O)]+ complex in water resulted in the formation of
a supramolecular [Fꢂ(EuL)2]+ dimer, in which the fluoride
anion is encapsulated into the cavity formed by two [EuL]+
complexes. The formation of the [Fꢂ(EuL)2]+ complex can be
monitored by absorption and fluorescence spectroscopy,
revealing a remarkable 22-fold increase in the emission
intensity upon fluoride binding and a very strong association
constant for the formation of the dimer. This association is
selective towards Clꢀ, Brꢀ, AcOꢀ, and HCO3ꢀ, and very
sensitive for fluoride detection, with an unoptimized detec-
tion limit of 0.46 ppb (24 nm) of fluoride. The X-ray crystal
structure of the dimer showed that in addition to the bridging
of the EuL complexes by the fluoride anion, the self-assembly
process is assisted by the synergistic effect of two p–p stacking
interactions and four hydrogen bonds, which stabilize the
association of the two complexes. The same strong association
was observed for complexes of smaller Ln cations (Tb and
Yb). The self-assembled supramolecular complex and the
strength of the observed interactions open up direct perspec-
tives for fluoride sensing in water and might enable the use of
such complexes as fluoride carriers in positron emission
tomography with 18Fꢀ.
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Keywords: dimers · fluoride · lanthanides · luminescence ·
sensing
.
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