Structural and photophysical properties of Lnin complexes with 2,2'-bipyridine-6,6'-dicarboxylic acid: Surprising formation of a H-bonded network of bimetallic entitiesf

Article abstract of DOI:10.1039/b000436g

The ligand H2L = 2,2'-bipyridine-6,6'-dicarboxylic acid reacts with Ln(NOj)j-.vH2O (.v = 6, Ln = Eu, Tb; x = 5, Ln = Gd) in MeOH/Et3N to give complexes with 1:2 and 2:3 metal : ligand stoichiometry, (Et3NH)[LnL2] and [LnjlHjOl-A'HjO (x = 1, Ln = Eu, Tb; .v = 0, Ln = Gd) which have been isolated and characterised. A sizeable quantum yield is obtained for the 1:2 Eu : Ligand complex in aqueous solution (Euabs = 11.5 ±2.3% at pH = 6.6), pointing to an efficient ligand-to-metal energy transfer. The presence of some inner-sphere interaction with water was deduced from Eu(5D0) lifetime measurements in water (0.86 ±0.01 ms vs. 1.55 ±0.02 ms in the solid state between 10 and 295 K, <7es[ = 0.3-0.4 water molecule). For [TbL2]-, sensitisation of Tbm also occurs ( = 6.3 ± 1.3% at pH = 6.6) but the Tb(5D4) excited level is de-populated at room temperature by a back-transfer process to the ligand. The crystal structure obtained for the 2:3 Tb : ligand complex evidences two distinct terbium sites, one Tb1" being complexed to two ligands affording a mono-anionic complex, itself linked to the second terbium ion with a u-carboxylate bridge; the generic formulation of the crystallised complex is [TbLj-u-TbL(H2O)3]'2H2O ' 2MeOH. Consecutive dimers are linked by an elaborate network of H-bonds involving interstitial solvent molecules. A photophysical study of the 2:3 Eu: Ligand complex in the solid state points to the same structural features, revealing two metal ion sites with essentially no bonded water (q = 0.3, site I) and with 3 co-ordinated water molecules (q = 2.8, site II), respectively. The H2L synthon is therefore an interesting building block for the design of elaborate compartmental ligands and/or of supramolecular functional assemblies. The Royal Society of Chemistry 2000.

Full text of DOI:10.1039/b000436g

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III
Structural and photophysical properties of Ln complexes with
,2Ј-bipyridine-6,6Ј-dicarboxylic acid: surprising formation of a
H-bonded network of bimetallic entities†
2
a
b
b
Jean-Claude G. Bünzli, Loïc J. Charbonnière and Raymond F. Ziessel*
a
Université de Lausanne, Institut de Chimie Minérale et Analytique, BCH-1402,
CH-1015 Lausanne, Switzerland
Laboratoire de Chimie, d’Électronique et de Photonique Moléculaire, ECPM,
UPRES-A 7008 au CNRS, 25 rue Becquerel, 67087 Strasbourg Cedex 02, France
b
Received 17th January 2000, Accepted 13th April 2000
Published on the Web 23rd May 2000
The ligand H L = 2,2Ј-bipyridine-6,6Ј-dicarboxylic acid reacts with Ln(NO ) ؒxH O (x = 6, Ln = Eu, Tb; x = 5,
2
3
3
2
Ln = Gd) in MeOH/Et N to give complexes with 1:2 and 2:3 metal:ligand stoichiometry, (Et NH)[LnL ] and
3
3
2
[
Ln L (H O) ]ؒxH O (x = 1, Ln = Eu, Tb; x = 0, Ln = Gd) which have been isolated and characterised. A sizeable
2 3 2 3 2
Eu
quantum yield is obtained for the 1:2 Eu:Ligand complex in aqueous solution (Q
= 11.5 ± 2.3% at pH = 6.6),
abs
pointing to an efficient ligand-to-metal energy transfer. The presence of some inner-sphere interaction with water
5
was deduced from Eu( D ) lifetime measurements in water (0.86 ± 0.01 ms vs. 1.55 ± 0.02 ms in the solid state
0
Ϫ
III
Tb
between 10 and 295 K, qest = 0.3–0.4 water molecule). For [TbL ] , sensitisation of Tb also occurs (Q
= 6.3 ±
2
abs
5
1
.3% at pH = 6.6) but the Tb( D ) excited level is de-populated at room temperature by a back-transfer process to
4
the ligand. The crystal structure obtained for the 2:3 Tb:ligand complex evidences two distinct terbium sites, one
III
Tb being complexed to two ligands affording a mono-anionic complex, itself linked to the second terbium ion
with a µ-carboxylate bridge; the generic formulation of the crystallised complex is [TbL -µ-TbL(H O) ]ؒ2H Oؒ
2
2
3
2
2
MeOH. Consecutive dimers are linked by an elaborate network of H-bonds involving interstitial solvent
molecules. A photophysical study of the 2:3 Eu:Ligand complex in the solid state points to the same structural
features, revealing two metal ion sites with essentially no bonded water (q = 0.3, site I) and with 3 co-ordinated
water molecules (q = 2.8, site II), respectively. The H L synthon is therefore an interesting building block for the
2
design of elaborate compartmental ligands and/or of supramolecular functional assemblies.
Introduction
lived metal-centred excited state, water solubility and thermo-
dynamic and photophysical stability have to be fulfilled.
Present-day technology in supramolecular chemistry allows a
precise control of the metal co-ordination sphere and permits
a fine-tuning of the energy-transfer processes either in simple
Many biomedical analyses rely on luminescence as a signal
reporter since analytical methods based on this phenomenon
are among the most sensitive available. Luminescent labels are
being extensively developed for the design of efficient fluoro-
immunoassays despite intrinsic limitations due to non-specific
short-lived luminescence from the assay medium. This draw-
back is indeed easily overcome by the use of trivalent lan-
5
devices used as luminescent stains or in more elaborate
6
molecular wires and sensors. The choice of the ligand remains
however a crucial point and molecules containing aromatic
III
moieties are usually found to be good sensitisers for Ln
ions
III
III
thanide ions Ln which have long-lived excited states (e.g. Eu ,
2,4,7
8
but more rarely meet all the above-mentioned criteria.
III
1
Tb ). Since f–f transitions are very weak, excitation of the
metal ion is usually achieved by energy transfer from the
In an effort to bring further understanding to the relationship
between the ligand structure and the efficiency of the energy
transfer process and, also, to unravel suitable synthons for the
design of elaborate compartmental ligands, we have turned
2
bonded ligands, a process termed antenna effect in reference
to the naturally occurring phenomenon encountered in green
3
plants. The higher molar absorbance of the organic shell allows
our attention to 2,2Ј-bipyridine-6,6Ј-dicarboxylic acid, H L.
2
a better collection of the incident photons when compared to
the free metal ions. This is especially true for lanthanide metal
ions, for which the various selection rules make their electronic
transitions strongly forbidden with molar absorption co-
Ϫ1
Ϫ1 4
efficients rarely higher than a few M cm .
Among the prerequisites for a good antenna system with
potential as a label for time-resolved analysis or imaging of
biological materials, criteria such as: high absorptivity of the
compound, efficient energy transfer to the metal ion, shielding
of the latter from vibrational deactivation processes, a long-
Thanks to its aromatic core, combined with a four-site pincer-
shaped co-ordinating pattern, H L appears as an interesting
2
candidate for use as a light-harvesting moiety for lanthanide
cations. Furthermore, the carboxylate functions are expected to
provide suitable thermodynamic stability in water. The relative
†
Electronic supplementary information (ESI) available: emission
III
III
luminescence yield of 1:1 solutions of H L and Eu and Tb
2
spectra for H L and (Et NH)[TbL ], excitation and emission spectra
2
3
2
9
ions has been briefly reported by Mukkala et al. within the
framework of a study relating the influence of the chelating
for (Et NH)[EuL ] and a stacking diagram for [TbL -µ-TbL(H O) ]ؒ
3
2
2
2
3
2
H Oؒ2MeOH. See http://www.rsc.org/suppdata/dt/b0/b000436g/
2
DOI: 10.1039/b000436g
J. Chem. Soc., Dalton Trans., 2000, 1917–1923
This journal is © The Royal Society of Chemistry 2000
1917

View Article Online
groups on the luminescence properties of a series of lanthanide
complexes, but no other data are available and the complexes
were not isolated and characterised in the solid state. In this
paper, we report the isolation and characterisation of two series
of complexes with Ln = Eu, Gd, Tb and the deprotonated
ligand together with the associated structural and photo-
physical data.
Results and discussion
Synthesis and characterisation of (Et NH)[LnL ] complexes
3
2
(
Ln ؍ Eu, Gd, Tb)
The complexes were synthesised by reacting a twofold excess of
the ligand with lanthanide nitrates in methanol and in the pres-
ence of triethylamine as a base, followed by re-crystallisation.
Fig. 1 UV-Vis spectra of solutions of H L (—), (Et NH)[EuL ] (––)
2
3
2
In the case of europium, treatment with KPF in hot water
6
and [Eu L (H O) ]ؒH O (——) in DMSO/H O (5/95, v/v).
2 3 2 3 2 2
resulted in the replacement of the triethylammonium cation by
potassium. IR spectra of the three metal complexes are very
similar, showing the presence of absorption bands character-
Ϫ1
Ϫ1
Ϫ1
cm ) and 315 nm (ε = 20 100 M cm ) for Gd and 308
Ϫ1
Ϫ1
Ϫ1
Ϫ1
(ε = 24 300 M cm ) and 318 nm (ε = 24 750 M cm ) for
Tb. Due to the very low solubility of the complexes, the weak
f–f absorption bands and, in the case of europium, the possible
istic of the pyridine moiety (ν and ν
around 1590 and 1570
at 1637, 1640 and
C᎐C
C᎐N
᎐
᎐
Ϫ1
cm ) and of the carboxylate groups (ν
C᎐O
᎐
Ϫ1
10
15
1
643 cm respectively for Eu, Gd and Tb). The absence of
charge transfer band could not be evidenced.
any absorption band that could be attributed to the nitrate
11
group unambiguously proves the displacement of the anion
Ligand-centred luminescence in H L and (Et NH)[GdL ]
2
3
2
1
during the formation of the complexes. The H-NMR spectrum
In the solid state at 77 K, the protonated ligand presents two
emission bands (Fig. S1, ESI)† upon excitation in the range
40–350 nm (maxima of the excitation spectra). One band
centred at 390 nm is asymmetric with a tail in the long wave-
length range and it disappears as soon as a time delay is
of the europium complex in D O/d -DMSO displays three
2
6
broad signals corresponding to the aromatic protons, with the
expected coupling pattern (two doublets at 7.67 and 6.39 ppm
and a triplet at 7.98 ppm, respectively 8.76, 8.15 and 8.21 for
H L in d -DMSO), and pointing to the presence of two equiv-
3
2
6
1
enforced; we therefore assign it as arising from a ππ* state. The
alent ligands on the NMR time scale. The presence of one tri-
ethylammonium cation is confirmed by the observation of a
quadruplet and a triplet with integral values corresponding to 6
and 9 H respectively, as compared to the value of 4 protons for
each aromatic signal. The de-shielding of the quadruplet at 2.99
other band is structured, with components at 492 nm (20 325
Ϫ1
cm , 0-phonon component), 525 nm (maximum), 558 and
≈
1
600 nm. The main vibrational progression amounts to
220 ± 80 cm and corresponds to a ring breathing mode
Ϫ1
Ϫ1
occurring at 1265 cm in the fundamental state at room tem-
perature, as inferred from the IR spectrum. The luminescence
decay is bi-exponential, with lifetimes of 10.6 ± 0.1 and 2.3 ±
0.1 ms at 10 K. The long luminescence lifetimes and a ring
breathing associated vibrational progression both point to a
ππ* state centred on the bipyridine rings. At room temper-
ature, the emission spectrum remains essentially the same,
except for the expected broadening and a slight red-shift. The
ppm (2.43 ppm for neat Et N) confirms the protonation of the
3
12
1
nitrogen atom. The H-NMR spectrum of the Tb chelate is
stretched over 60 ppm at 298 K as a result of the paramagnetic
13
contribution of the terbium ion. The increase of the trans-
verse relaxation rates for protons in the neighbourhood of the
terbium cation leads to broad peaks and the hyperfine structure
is lost. Nevertheless, the integral values of the peaks allows one
to assign the aromatic protons at 17.88, Ϫ6.10 and Ϫ37.49 ppm
3
L
Ϫ6
luminescence quantum yield Q of a 10 M solution in water,
is small and amounts to 0.76% at pH 9.9 (λexc = 255 nm) and
.0% at pH 2.0 (λexc = 227 nm).
and the ethyl protons of the ammonium cations at 3.38 and
ϩ
1
.44 ppm. The FAB /MS spectrum of (Et NH)[TbL ]ؒ3H O
3
2
2
2
shows an intense peak at m/z = 645 corresponding to the
III
ϩ
Upon complexation to Gd , the room temperature emission
diprotonated complex [Tb(LH) ] as well as peaks at m/z = 746
2
1
from the ππ* state shifts to longer wavelength (391 nm) in the
solid state, while the triplet state emission with lifetimes equal
to 8.2 ± 0.2 and 4.3 ± 0.1 ms appears to be less affected. The
quantum yield of the ligand-centred luminescence decreases to
.36% for a 10 solution of (Et NH)[GdL ] in H O at pH 6.8
λexc = 282 nm). No ligand-centred luminescence is observed for
the Eu and Tb chelates, pointing to an efficient transfer of
the excitation energy onto the metal ions.
and 847, attributed to the same species linked to respectively
one and two triethylamine molecules. Measurements in the
negative mode display a quasi single peak at m/z 643 attributed
Ϫ
to [TbL ] . Both NMR and MS data unambiguously point to
2
Ϫ4
0
3 2 2
the presence of a single species in solution, with a 1:2 metal:
ligand stoichiometry.
(
III III
The absorption spectrum of the protonated ligand H L,
2
recorded in a 95/5 (v/v) water–DMSO mixture at room tem-
Ϫ1
Ϫ1
perature shows a broad band at 297 nm (ε = 6860 M cm )
Metal-centred luminescence in (Et NH)[LnL ] chelates
Ln ؍ Eu, Tb)
Ϫ1
Ϫ1
3
2
with a shoulder at 309 nm (ε = 5800 M cm ) attributed to
π→π* transitions on the pyridine rings (Fig. 1). Upon addition
of triethylamine, the maximum of absorption is gradually shift-
(
The excitation spectrum of a solid sample of (Et NH)[EuL ] at
10 K (Fig. S2, ESI)† displays one broad and structured band
3
2
Ϫ1
Ϫ1
ed to higher energy, appearing at 288 nm (ε = 8610 M cm )
for a large excess of the base, while the position of the shoulder
with components at 331 nm, 342 nm (maximum), 350 and 357
Ϫ1
1
remains essentially unchanged (λ = 310 nm, ε = 2600 M
nm, assigned to the ππ* state of the ligand, as well as charac-
max
Ϫ1
III
5
7
cm ). Upon complexation to the lanthanide cations, the
teristic bands of the Eu ion, at 378.4 ( G ← F ), 382
4 0
5 7 5 7 5 7
π→π* transitions are shifted to lower energy, as often
( G ← F ), 397 and 400 ( L ← F ), 466 ( D ← F ) and 528.5
2 0 6 0 2 0
5 7
observed during the complexation of heteroaromatic systems
nm ( D ← F ). The spectrum at 295 K is essentially identical
1 0
14
with lanthanide cations. The spectra of the three (Et NH)
with, in addition, excitation bands from the thermally popu-
3
7
5
7
[
LnL ] complexes (Ln = Eu, Gd and Tb) are very similar in their
lated F level and a faint D ← F transition (Fig. S2, ESI).†
1 0 0
5 7
2
Ϫ1
Ϫ1
shape, with two maxima at 308 (ε = 23 300 M cm ) and 316
nm (ε = 23 150 M cm ) for Eu (Fig. 1), 306 (ε = 18 800 M
High resolution excitation and emission spectra of the D ← F
0 0
5 7
Ϫ1
Ϫ1
Ϫ1
and D → F transitions (Fig. S3, ESI)† reveal one broad and
0 0
1
918 J. Chem. Soc., Dalton Trans., 2000, 1917–1923

View Article Online
1
At 77 K and upon excitation at both 330 ( ππ*) and 488 nm
5
III
(
D ) a solid state sample of the Tb chelate displays an emis-
4
5
7
sion spectrum dominated by the D → F transition (Fig. S5,
4
5
ESI)† which fades out at room temperature. In parallel, the
5
Tb( D ) lifetime decreases sharply from 1.24 ± 0.01 ms at 77 K
4
to 0.21 ± 0.01 ms at 295 K, pointing to a temperature activated
5
energy back transfer process from the Tb( D ) level to the
4
1
5,18
ligand levels.
This behaviour is understandable since the
3
energy of the 0-phonon component of the ππ* ligand state at
Ϫ1
5
2
0 325 cm is in resonance with the D state, the sub-levels of
4
Ϫ1
which extend from 20 050 to 20 480 cm . As a consequence,
the absolute quantum yield of the metal-centred luminescence
Ϫ4
displayed by a 10 M solution at pH = 6.6 is lower than the one
found for the Eu chelate, amounting to 6.3 ± 1.3%.
III
Synthesis and characterisation of [Ln L (H O) ]ؒxH O
2
3
2
3
2
complexes (x ؍ 1, Ln ؍ Eu, Tb; x ؍ 0, Ln ؍ Gd)
The complexes were crystallised from equimolar solutions
of the ligand and the lanthanide nitrate in a methanol/water
mixture, in the presence of triethylamine. Surprisingly, elemental
analyses point to the formation of compounds with a 2:3
metal-to-ligand ratio and containing respectively four, three
and four water molecules for Eu, Gd and Tb. The IR of the
three complexes are quite similar and display features charac-
Fig. 2 Emission spectra of (Et NH)[EuL ] at 295 K in solution at
3
2
pH = 6.6 (top, λexc = 395 nm) and in the solid state (bottom, λexc
80.74 nm).
=
5
slightly asymmetric component centred at 581.13 nm at 10 K
site I) with a shoulder at 581.28 nm (site II) and 580.76 nm at
95 K. Selective laser excitation on the 0–0 transition at 10 K
yields spectra typical of low symmetry species and dominated
(
2
teristic of the bipyridine ligands (ν , ν
1615 and 1620 cm respectively for Eu, Gd and Tb) but do
and ν
at 1617,
C᎐N
C᎐C
C᎐O
᎐
᎐
᎐
5
7
Ϫ1
by the hypersensitive D → F transition (Fig. S4, ESI).†
0
2
5
7
Relative corrected intensities of the D → F transitions are
not contain bands from nitrate anions. Interestingly, the ν
0
J
C᎐O
᎐
0
.06, 1.00, 5.7, 0.1, and 0.32 for J = 0, 1, 2, 3, and 4, respectively.
absorption bands are observed at weaker energy than in the
The spectra obtained by exciting site I and site II differ slightly
corresponding ML complexes with a difference of about 20
cm . This difference points to a somewhat weakened C᎐O
bond, the electronic density of the carboxylate functions being
displaced toward the metal binding oxygen atoms.
2
5
7
Ϫ1
in the splitting of the D → F transition and in the relative
0
1
5
7
intensity of the components of the D → F transition. These
0
2
differences disappear at room temperature and we assign them
as arising from crystal defects rather than from two different
metal-ion environments. Lifetime measurements have been
When dissolved in d -DMSO or D O the solutions of the
6
2
europium or terbium complexes show a mixture of 1:1 and 1:2
1
5
1
performed exciting both the ligand ππ* level and the D level.
chelates, as demonstrated by the H-NMR for which two sets of
0
Single exponential decays were observed in each case and the
lifetime is constant both with respect to the excitation mode
and to temperature between 10 and 295 K: 1.55 ± 0.08 ms.
These observations point to an Eu environment free of inner
sphere water molecules and fairly rigid.
signals in a 1:2 ratio are observed. For Eu, the signals arising
from the first set, assigned to an ML2 entity is composed of a
Ϫ
triplet (10.20 ppm) and two doublets (13.32 and 7.07 ppm) with
3
coupling constants J = 7.8 Hz; the second set, attributed to an
ϩ
ML entity, is also comprised of a triplet at 4.34 ppm, and two
5
7
Ϫ4
The relative intensities of the D → F transitions of a 10
doublets at 4.52 and Ϫ2.36 ppm, with the same coupling con-
0
J
M solution of the chelate in degassed water (pH 6.6, 295 K) are
similar to those found for the solid state sample (0.004, 1.00,
stant (7.8 Hz). As previously mentioned, the paramagnetic
contribution of Tb leads to a spectrum spread over 160 ppm.
III
6
.81, 0.11 and 0.65 for J = 0, 1, 2, 3 and 4 respectively), but the
Signals from the 1:2 species appear at 17.97, Ϫ22.20 and
Ϫ76.67 ppm while signals from the 1:1 chelate appear at 75.28,
crystal field splitting is much altered, pointing to strong inter-
action with water (Fig. 2). As a result, the Eu( D ) decay time
is shortened to 0.86 ± 0.02 ms, corresponding to kH2O = 1.16
ms , but remains monoexponential. Taking into account the
commonly encountered value of 0.5 ms for the radiative rate
constant kD2O, or the one deduced from the solid state life-
time, and using the newly proposed equation allowing for the
presence of closely diffusing water molecules, q = 1.2 (k Ϫ
5
20.74 and Ϫ4.62 ppm. The comparison of the chemical shifts
0
Ϫ
of the [LnL ] complexes (vide supra) with those of the equiv-
2
Ϫ1
alent sets for the [M L (H O) ] solutions, shows the two entities
2
3
2
3
Ϫ1
to be significantly different in solution. The presence of a
16
second paramagnetic metal atom in the latter case increases the
19
magnetic susceptibility of the bulk. If this point alone is taken
into account and if one assumes that both metallic entities are
weakly coupled, a constant difference ∆δ between the chemical
H O
2
16
kD2O Ϫ 0.15), this decay time reflects the quenching effect of
4
19
0
.4–0.6 water molecules directly bonded to the Eu() ion. This
shifts for a proton in each type of complex is expected, which
may be interpreted as reflecting some inner sphere interaction
with fast exchanging water molecules. The absolute quantum
yield of the metal-centred luminescence of this solution is fairly
large, amounting to 11.5 ± 2.3% and in line with the 0-phonon
component of the ligand triplet state lying about 3000
has not been observed in our case. Furthermore, we note that
Ϫ
the pattern distribution in [EuL ] (triplet, doublet and doublet
2
from low to high fields) is different from that in [Eu L (H O) ]
2
3
2
3
(doublet, triplet and doublet). Two possible explanations can
be formulated: (i) an important magnetic coupling takes place
between the metal ions or (ii) a close contact pairing is retained
Ϫ1
5
17
cm above the Eu( D ) level. In their work on water solutions
0
III
Ϫ
ϩ
with a 1:1 Eu :H L ratio, Mukkala et al. have found a bi-
in solution between the [ML ] and [ML] moieties. In view of
2
2
exponential decay with lifetimes of 0.77 and 0.19 ms at pH 8.5
the known weak magnetic interaction taking place between two
f-metal ions separated by a distance larger than 4 Å and of the
solid state structure of [Tb L (H O) ]ؒH O described below, the
and they assigned the two lifetimes to complexes with different
9
metal-to-ligand ratios. Using the same procedure as above to
2
3
2
3
2
III
estimate q, the lifetimes reported by Mukkala’s group at this
pH correspond to approximately 0.8 and 5.5 inner sphere water
molecules, respectively. Given the difference in the experimental
conditions, our data are in good agreement with those reported
second explanation is favoured. Whatever the metal ion, Eu
III
or Tb , the two sets of signals clearly indicate that no ligand
exchange occurs at room temperature on the NMR time scale,
pointing to species stable in water, with no interconversion
between them, an important prerequisite for biomedical appli-
9
by Mukkala et al. for the 1:2 complex.
J. Chem. Soc., Dalton Trans., 2000, 1917–1923
1919

View Article Online
Table 1 Selected bond lengths (Å) and angles (Њ) in [TbL -µ-TbL(H O) ]ؒ2H Oؒ2MeOH
2
2
3
2
Tb1–O1
Tb1–N1
Tb1–N2
Tb1–O3
Tb1–O5
Tb1–N3
Tb1–N4
Tb1–O7
2.334(2)
2.518(3)
2.494(3)
2.324(3)
2.324(2)
2.503(3)
2.511(3)
2.377(2)
Tb2–O8
Tb2–O9
Tb2–N5
Tb2–N6
Tb2–O11
Tb2–O13
Tb2–O14
Tb2–O15
2.329(2)
2.355(3)
2.524(3)
2.511(3)
2.306(2)
2.372(2)
2.410(3)
2.425(2)
Atom(x)–Tb1–Atom(y)
O1
N1
62.97(9)
129.03(9)
80.05(9)
131.59(9)
139.91(9)
92.00(8)
N2
O3
O5
N3
N4
N1
N2
O3
O5
N3
N4
O7
65.87(9)
128.72(9)
165.08(9)
97.84(9)
85.25(9)
81.29(9)
91.82(9)
66.11(9)
76.97(8)
133.08(9)
141.01(8)
86.66(8)
86.8(1)
83.76(9)
84.74(9)
87.17(9)
65.79(8)
128.76(8)
163.62(8)
63.08(9)
128.55(8)
65.70(8)
O14
Atom(x)–Tb2–Atom(y)
O8
O9
N5
N6
O11
O13
O9
N5
N6
O11
O13
O14
O15
92.93(9)
82.95(9)
76.11(8)
86.76(9)
146.52(8)
145.11(9)
76.19(9)
65.04(9)
127.66(9)
165.69(9)
99.76(9)
79.8(1)
62.89(9)
128.97(8)
74.79(9)
122.7(1)
142.10(9)
66.12(8)
71.59(8)
134.60(8)
138.02(9)
88.13(9)
92.3(1)
81.42(9)
68.14(8)
135.49(9)
84.6(1)
69.19(9)
Fig. 3 The atom numbering scheme for [TbL -µ-TbL(H O) ]ؒ2H Oؒ
2
2
3
2
2
MeOH.
ϩ
cations. The FAB /MS spectrum of [Tb L (H O) ] indicates
2
3
2
3
ϩ
the presence of a [Tb(LH) ] species (m/z = 645) and {[Tb L ؒ
2
2
3
2ϩ
3
H O] ϩ 2H} entities (m/z = 551).
2
Absorption spectra of solutions of the three complexes are
very similar. The π→π* transitions on the bipyridine moieties
are shifted to lower energy with respect to the free ligand. The
absorption bands display two maxima in each case at 308
Fig.
4
View of the dimeric unit in [TbL -µ-TbL(H O) ]ؒ2H Oؒ
2 2 3 2
2
MeOH.
Ϫ1
Ϫ1
(
ε = 34 950 M cm ) and 316 nm (34 900) for Eu, 307 (37 750)
and 318 nm (36 800) for Gd and 307 (37 100) and 316 nm
37 300) for Tb. The shapes of these spectra closely resemble
those of the spectra generated by the (Et NH)[LnL ] chelates
2
.51(1) Å, mean Tb2–O 2.33(2) Å) and completes its co-
ordination sphere by binding three water molecules (mean
Tb2–O 2.40(3) Å) and one O atom from a bridging carboxylate
group attached to a ligand molecule bonded to Tb1 (mean
Tb2–O = 2.33 Å). As a consequence, the Tb1–O(carboxylate)
distance is substantially longer (mean 2.38 Å) than the three
other ones (mean 2.33(1) Å). The correct formulation of the
(
3
2
but the molar absorption coefficients are nearly 1.5 times larger,
as expected on the basis of the number of ligands per
lanthanide cation (Fig. 1).
2
:3 complex is therefore [TbL -µ-TbL(H O) ]ؒ2H Oؒ2MeOH
2 2 3 2
X-Ray crystal structure of [Tb L (H O) ]ؒ2H Oؒ2MeOH
2
3
2
3
2
and the intermolecular Tb1–Tb2 distance amounts to 6.51 Å.
Two consecutive dimers are connected by an elaborate net-
work of hydrogen bonds involving the solvation molecules
(Fig. 5). One methanol molecule (O18) is connected through
H-bonding to a carboxylic O-atom of the ligand bound to Tb2
(O18 ؒ ؒ ؒ O12 = 2.68 Å) while the other methanol molecule
(O19) is H-bonded to both a co-ordinated water molecule
(O19 ؒ ؒ ؒ O14 = 2.70 Å) and, more weakly, to the interstitial O17
water molecule (O19 ؒ ؒ ؒ O17 = 2.96 Å), itself H-bonded to one
carboxylate linked to Tb1 (O17 ؒ ؒ ؒ O1 = 2.85 Å). The second
Selected bond lengths and angles are listed in Table 1 while the
atom-numbering is shown on Fig. 3. The structure is comprised
of discrete and neutral supramolecular dimeric units (Fig. 4).
One of the metal ions is 8-coordinate, being bonded to the four
N atoms of two ligand molecules (mean bond length 2.50(1) Å)
and to the four carboxylate groups (mean bond length 2.34(3)
Å). The two ligand molecules are almost planar and lie per-
III
pendicular to each other. The second 8-co-ordinate Tb ion is
bonded to one de-protonated ligand molecule (mean Tb2–N:
1
920
J. Chem. Soc., Dalton Trans., 2000, 1917–1923

View Article Online
7
Ϫ1
7
Table 2 Identified Eu( F ) crystal field sub-levels (cm , origin F ,
J
0
J = 1–4) in solid samples of (Et NH)[EuL ] and [Eu L (H O) ], as
determined from emission spectra at 295 K
3
2
2
3
2
3
[
Eu L (H O) ]
2 3 2 3
Level
(Et NH)[EuL ]
Site I
Site II
3
2
7
F1
290
273
375
426
274
303
377
3
4
95
16
7
F2
784
921
927
8
9
1029
1048
1832
98
20
943
996
1029
1113
1864
957
1006
1035
1068
1856
1875
Fig. 5 H-bonds network connecting two dimeric units in [TbL -µ-
2
TbL(H O) ]ؒ2H Oؒ2MeOH.
2
3
2
7
F3
1
893
1
939
7
F4
2848
2752
2825
2951
2704
2840
2870
2967
3032
2
3
925
101
3
3
002
085
3
096
Fig.
6
Co-ordination polyhedra in [TbL -µ-TbL(H O) ]ؒ2H Oؒ
2 2 3 2
2
MeOH.
interstitial water molecule bridges two dimeric entities, being
H-bonded to a carboxylate co-ordinated to Tb1 (O16 ؒ ؒ ؒ O2 =
2
2
.71 Å) and to a water molecule bound to Tb2 (O16 ؒ ؒ ؒ O13 =
.66 Å). The dimers are aligned in the crystal, forming parallel
cylinders with no interaction between them (Fig. S6, ESI).†
Within a stack the Tb1–Tb1 and Tb2–Tb2 distances between
metal ions belonging to two consecutive dimers are equal and
amount to 13.7 Å, while the metal–metal distance between
columnar stacks of molecules is much larger (around 24–25 Å).
The co-ordination polyhedron around Tb1 may be described
as a distorted C2v dodecahedron (Fig. 6), the four O-atoms
forming an approximate square plane while the N-atoms of
each ligand sit above and below this plane, respectively, and
are aligned along the two diagonals of the square. The co-
ordination geometry around Tb2 displays lower symmetry and
can be classified as a 1:4:3 polyhedron.
Fig. 7 Emission spectra of [Eu L (H O) ]ؒH O under selective excit-
2
3
2
3
2
ation at 295 K displaying the two different co-ordination environments
III
for Eu . Inset: excitation spectrum of the 0–0 transition.
describing the ability δ of co-ordinating atoms to produce a
nephelauxetic effect:
Photophysical properties of [Eu L (H O) ]ؒH O
20
2
3
2
3
2
5
7
At room temperature, the D ← F excitation spectrum of the
0
0
ν˜
Ϫ ν˜
= C ͚n δ
i
0
CN
i i
solid state Eu compound reveals two broad bands at 580.05 nm
site I) and 579.76 nm (site II). Selective laser excitation of these
two components yields two different spectra dominated by the
(
III
where CCN is a coefficient depending upon the Eu co-
ordination number (1.06 for a co-ordination number (CN) =
5
7
III
D → F transition and typical of Eu in low-symmetry co-
Ϫ1
0
2
8
2
), n is the number of atoms of type i, and ν˜ = 17 374 cm at
i 0
20 20 20
III
ordination sites (Fig. 7). The Eu ions in site I experience a
95 K. Using δOOC = Ϫ17.2, δH2O = Ϫ10.2, and the nephel-
larger crystal field effect than in site II, e.g. a total separation of
auxetic parameter deduced from previous studies on hetero-
Ϫ1
Ϫ1
7
Ϫ1
1
1
53 cm vs. 103 cm for the F sub-levels and of 192 cm vs.
8
Ϫ1
1
cyclic imines, δN᎐C = Ϫ15.3, we predict ν
˜
(site I) = 17 236 cm
Ϫ1
7
5
41 cm for the F sub-level (Table 2). Moreover, the Eu( D )
Ϫ1
2
0
and ν
˜ (site II) = 17 254 cm , in agreement with the measured
lifetime is longer in site I (1.23 ± 0.02 ms) than in site II
0.32 ± 0.01 ms) and taking into account the same radiative rate
Ϫ1
values of 17 240 and 17 248.5 cm , respectively.
(
Upon lowering the temperature, the general features
described above are retained, but the situation becomes more
constant as above, the following number of bonded water
molecules can be calculated: q = 0.3 for site I and 2.8 for site II.
The former value is small and may be accounted for by the
presence of closely diffusing OH oscillators in the second co-
5
7
complicated, with new components appearing on the D ← F
0
0
excitation spectrum, probably arising from rearrangement in
the H-bond network. Further analysis was therefore not carried
out. Moreover, the back transfer process occurring for the
16
ordination sphere of the metal ion. Therefore, the lumin-
escence data point to a structure similar to the one evidenced
III
Tb bimetallic chelate also prevented a detailed analysis of
III
by X-ray crystallography for the Tb bimetallic chelate, site I
5
the Tb( D ) lifetimes.
4
corresponding to Tb1 and site II to Tb2. In addition, the energy
5
7
of the measured D ← F transitions for the two sites is in
0
0
Conclusion
keeping with this explanation. Frey and Horrocks have demon-
strated that a correlation exists between the energy of the 0–0
The ligand 2,2Ј-bipyridine-6,6Ј-dicarboxylic acid in its di-
anionic form appears to be a good building block for designing
5
transition, that is the position of the D level, and parameters
0
J. Chem. Soc., Dalton Trans., 2000, 1917–1923
1921

View Article Online
Table 3 Crystal data and structure refinement for [TbL -µ-TbL-
H O) ]ؒ2H Oؒ2MeOH
extended ligands aimed at complexing and sensitising trivalent
luminescent europium and terbium ions. It efficiently binds the
cations in a planar tetradentate co-ordinating mode, leading to
two series of complexes in which the ligand to lanthanide stoi-
chiometry is different. In the first series labelled (Et NH)[LnL ]
2
(
2
3
2
Formula
Molecular weight
C H N O Tb ؒ2H Oؒ2CH OH
36 24 6 15 2 2 3
1198.59
3
2
Crystal system
Space group
Monoclinic
Ϫ
a single lanthanide species [LnL ] is present in solution, while
2
P1 2 /c 1
1
in the second series labelled [LnL ؒ(H O) ] two lanthanide
3
2
3
a/Å
b/Å
c/Å
β/Њ
V/Å
Z
13.6780(5)
18.930(1)
15.8188(9)
92.227(3)
4092.8(6)
4
ϩ
Ϫ
entities [LnL] and [LnL ] are observed in solution, with some
peculiar interactions deduced from H-NMR. In the solid state,
2
1
these latter moieties formed a neutral bimetallic entity which
was characterised for Tb by an X-ray structure determination
and luminescence spectroscopy. The bipyridine moieties are
3
Ϫ1
µ/mm
3.518
21
confirmed as providing a good antenna effect and the form-
ation of an extended network of H-bonded bimetallic entities
in the 2:3 compounds opens the way for tailoring compart-
T/K
173
25474
Number of data measured
Number of data with I > 3σ(I)
R
15332
0.037
mental ligands bearing H L units or bipyridine monocarboxylic
2
Rw
0.053
acids and programmed for the formation of extended inter-
actions at a nanometric scale.
by centrifugation and was dried under vacuum. [Found: C,
3
3
9.42; H, 2.30; N, 7.41. C H EuN O Kؒ3H O requires C,
24
12
4
Ϫ1
8
2
Experimental
Syntheses and characterisation
9.50; H, 2.49; N, 7.68%]. ν 1655 cm , ν and ν 1597 and
CO
C᎐C
C᎐N
᎐
᎐
Ϫ1
Ϫ1
1571 cm , νCH (aromatic) 778 cm .
Solvents and starting materials were purchased from Fluka,
lanthanide nitrates (99.9%) from Janssen Chimica. 2,2Ј-
Bipyridine-6,6Ј-dicarboxylic acid was prepared in 87% yield by
Ϫ1
(
Et NH)[GdL ]ؒ2H O. Yield 72%. IR (cm ) ν 1640, ν
3 2 2 CO C᎐C
᎐
Ϫ1
and ν
1597 and 1570, νCH (aromatic) 778 cm . (Found: C,
6.09; H, 3.98; N, 8.75. C H GdN O ؒ2H O requires C, 46.20;
C᎐N
᎐
4
30 28 5 8 2
22
oxidation of 6,6Ј-dimethyl-2,2Ј-bipyridine with CrO in con-
3
H, 4.14; N, 8.98%).
23
centrated sulfuric acid, using an adapted literature procedure.
IR spectra were recorded as KBr pellets on a Nicolet 210 spec-
[
Ln L (H O) ]ؒxH O, general procedure
2
3
2
3
2
trometer. UV-Vis spectra of solutions in 95/5 (v/v) H O/DMSO
2
Ϫ4
1
A quantity of 2 × 10 mol of Ln(NO )ؒxH O (x = 6, 5 and 6
3 2
for Eu, Gd and Tb, respectively) was added to a suspension of
H L (2 × 10 mol) in 20 cm MeOH. Then 0.100 cm of tri-
ethylamine in 10 cm water was added and the solution was
stirred for 15 min and filtered. The methanol was evaporated
under reduced pressure and the solution was concentrated to
2 cm and cooled to 4 ЊC. A precipitate formed, which was
collected by centrifugation, washed with cold methanol and
dried under vacuum.
were obtained with a Uvikon 933 spectrophotometer. H-NMR
spectra were recorded at 298 K with a Bruker AC 200 spec-
trometer using the peak of the solvent as internal reference or
Ϫ4
3
3
2
3
tert-BuOH for spectra in D O. Luminescence spectra were
2
24
measured according to previously published procedures.
Absolute quantum yields of the ligand-centred luminescence
3
Ϫ7
were determined with respect to quinine sulfate 2 × 10 M in
25
H SO 0.05 M (absolute quantum yield 0.546), while those
2
4
of the metal-centred luminescence were measured using [Ln-
Ϫ3
(
1
tpy) ](ClO ) 10 M as standards (absolute quantum yields:
3
4 3
[
Eu L (H O) ]ؒH O. Yield 63%. [Found: C, 39.22; H, 2.38; N,
2 3 2 3 2
24
.3 (Eu) and 4.7% (Tb)).
7
7
.62. C H Eu N O ؒH O requires C, 39.45; H, 2.53; N,
36 24 2 6 15 2
3
.53%]. δ (d -DMSO) 13.32 (d, 4 H, J = 7.8 Hz), 10.20 (t, 4 H,
H 6
3 3
(
Et NH)[LnL ], general procedure
3 2
3
J = 7.8 Hz), 7.07 (d, 4 H, J = 7.4 Hz), 4.52 (d, 2 H, J = 7.8
Ϫ4
3
3
3
To a suspension of H L (2 × 10 mol) in 10 cm MeOH, were
Hz), 4.34 (t, 2H, J = 7.8 Hz), Ϫ2.36 (d, 2 H, J = 7.8 Hz).
Ϫ1 Ϫ1 Ϫ1
2
3
Ϫ4
added 3 cm of a solution of Ln(NO )ؒxH O (10 mol, x = 6, 5
νOH 3420 cm , ν 1617 cm , ν and νC᎐N 1592 and 1571 cm ,
CO C᎐C
Ϫ1
3
2
and 6 for Eu, Gd and Tb, respectively) in MeOH. After 15 min
stirring, the solution was evaporated to dryness, suspended in
νCH (aromatic) 775 cm .
3
3
[
Tb L (H O) ]ؒH O. Yield 56%. [Found: C, 39.07; H, 2.61;
3
0 cm water and 0.15 cm neat triethylamine was added. The
2
3
2
3
2
N, 7.49. C H N O Tb ؒH O requires C, 38.73; H, 2.35; N,
7
1
mixture was heated to reflux until all the solid was dissolved,
36 24
6
15
2
2
3
.53%]. δH (d -DMSO) 75.28 (s, 2 H, br), 20.74 (s, 2 H, br),
filtered and concentrated to 2 cm . Upon concentration, a solid
6
3
7.97 (s, 4 H, br), Ϫ4.62 (s, 2 H, br), Ϫ22.20 (s, 4 H, br), Ϫ76.67
formed and 20 cm of THF were added before cooling the solu-
Ϫ1
Ϫ1
(
s, 4 H, br). ν 3415 cm , ν 1620 cm , ν
and νC᎐N 1597
tion to 4 ЊC. The solid was separated by centrifugation and
dried under vacuum.
OH
Ϫ1
CO
C᎐C
Ϫ1
and 1572 cm , ν (aromatic) 773 cm .
CH
[
Gd L (H O) ]. Yield 70%. [Found: C, 39.80; H, 2.48; N,
2 3 2 3
(
Et NH)[TbL ]ؒ3H O. Yield 80%. [Found: C, 44.79; H, 4.02;
3 2 2
7
.67. C H Gd N O requires C, 39.48; H, 2.21; N, 7.67%].
36 24 2 6 15
Ϫ1 Ϫ1
N, 8.61. C H TbN O ؒ3H O requires C, 45.10; H, 4.29; N,
.76%]. δ (D O/tert-BuOH) 17.88 (s, 8 H, br), 3.38 (s, 6 H, br),
30
28
5
8
2
νOH 3405 cm , ν 1615, ν
and ν
1587 and 1560 cm ,
C᎐N
CO
C᎐C
8
1
᎐
᎐
H
2
Ϫ1
νCH (aromatic) 772 cm .
.44 (s, 9 H, br), Ϫ6.10 (s, 8 H, br), Ϫ37.49 (s, 8 H, br). ν 1643
CO
Ϫ1
Ϫ1
cm , ν
and ν
1597 and 1571 cm , ν (aromatic) 780
C᎐C
C᎐N
CH
᎐
᎐
Crystal structure determination of [Tb L ؒ3H O]ؒ2H Oؒ
MeOH
Ϫ1
ϩ
ϩ
2
3
2
2
cm . FABϩ/MS: 645 ([Tb(LH) ] ), 746 ([Tb(LH)LؒEt NH] ),
2
3
2
ϩ
Ϫ
8
47 ([TbL ؒ(Et NH) ] ). FABϪ/MS: 643 ([Tb(L) ] ).
2
3
2
2
Single crystals of [TbL -µ-TbL(H O) ]ؒ2H Oؒ2MeOH were
2
2
3
2
(
Et NH)[EuL ]. Obtained as described previously (yield 86%.
obtained as thin colourless needles from dissolution of the title
compound in hot water followed by slow cooling. A crystal was
mounted in inert oil and transferred to the cold gas stream of
the diffractometer. Crystal data and structure refinement details
are listed in Table 3.
3
2
3
δ (D O/d -DMSO, 50/50 v/v) 7.98 (t, br, 4 H, J = 8.0 Hz), 7.67
H
2
6
3
3
(
d, br, 4 H, J = 7.7 Hz), 6.39 (d, br, 4H, J = 6.6 Hz), 2.99 (q,
3 3 Ϫ1
6
ν
H, J = 7.2 Hz), 1.12 (t, 9H, J = 7.0 Hz). IR (cm ) ν 1637,
CO
and ν
1592 and 1571, νCH (aromatic) 776).
C᎐N
᎐
C᎐C
᎐
K[EuL ]ؒ3H O. Two equivalents of KPF were added to a
CCDC reference number 186/1936.
2
2
6
warm water solution of (Et NH)[EuL ] and the solution was
slowly concentrated and cooled to 4 ЊC. The solid was recovered
See http://www.rsc.org/suppdata/dt/b0/b000436g/ for crystal-
lographic files in .cif format.
3
2
1
922 J. Chem. Soc., Dalton Trans., 2000, 1917–1923

View Article Online
0 R. M. Silverstein and G. C. Bassler, Identification spectrométrique
des composés organiques, Gauthier-Villars, Paris, 1968.
11 K. Nakamoto, Infrared and Raman Spectra of Inorganic and
Coordination Compounds, 4th edition, Wiley, New York, 1986,
p. 254.
1
Acknowledgements
We gratefully acknowledge Ms Véronique Foiret for her
technical assistance in luminescence measurements. We thank
the Fondation Herbette (Lausanne) for a gift of spectroscopic
equipment. This work is supported through grants from the
French Centre National de la Recherche Scientifique and the
Swiss National Science Foundation. Professsor Jean Fischer,
Dr André De Cian and Nathalie Kyritsakas from the Labor-
atoire de Cristallochimie et de Chimie Structurale from
Strasbourg are acknowledged for the X-ray structure deter-
mination.
1
2 C. D. Gutsche, M. Iqbal and I. Alam, J. Am. Chem. Soc., 1987, 109,
4
314.
1
3 I. Bertini and C. Luchinat, NMR of Paramagnetic Molecules in
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14 C. Piguet, J.-C. G. Bünzli, G. Bernardinelli, G. Hopfgartner and
A. F. Williams, J. Am. Chem. Soc., 1993, 115, 8197.
5 L. J. Charbonnière, C. Balsiger, K. J. Schenk and J.-C. G. Bünzli,
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6 A. Beeby, I. M. Clarkson, R. S. Dickins, S. Faulkner, D. Parker,
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