Self-assembly of a trinuclear luminescent Europium complex

Article abstract of DOI:10.1002/chem.200802676

A study was conducted to demonstrate the self-assembly of a trinuclear luminescent europium complex. The study also involved preparation, structure, and characterization of the europium complex with bis(6-carboxypyridine-2- carbonyl)amine. It was observed

Full text of DOI:10.1002/chem.200802676

COMMUNICATION
DOI: 10.1002/chem.200802676
Self-Assembly of a Trinuclear Luminescent Europium Complex
Soumaila Zebret,[a] Nathalie Dupont,[b] Gꢀrald Bernardinelli,[c] and Josef Hamacek*[a]
Functional molecules containing lanthanides have already
found many applications in medicine and biology, especially
for medical imaging purposes (MRI contrast agents)[1] and
luminescent probes.[2] However, the development of new
lanthanide complexes combining both responsive features is
of importance for the enhancement of imaging and diagnos-
tic procedures. The design of suitable receptors for lantha-
nides is challenging because it must take into account sever-
al antagonistic requirements. The application of LnIII com-
plexes as MRI contrast agents requires high stability and the
presence of exchangeable water molecules to exhibit a good
relaxivity. Efficient ligands for GdIII coordination are there-
fore derived from macrocyclic ligands providing a high ther-
modynamic and kinetic stability and one or two positions
for water coordination.[1] Good luminescent properties of
lanthanide complexes are achieved with organic ligands pos-
sessing suitable chromophores for efficient sensitization of
the lanthanide luminescence. One of the most investigated
type of ligands for LnIII complexation are receptors bearing
the b-diketonate structural motif, and various ligands were
recently reviewed by Binnemans.[3] Suitable diketonate com-
plexes can be advantageously used for fluoroimmunologic
assays[4] and for responsive systems based on luminescence
quenching.[5] Diketonate-containing ligands are often com-
bined with other ligands (bipy, terpy, phen)[6] 1) to complete
the coordination sphere and 2) to enhance luminescent
properties of new materials.[7] Pyridyl-substituted diketo-
nates provide an additional coordination within one recep-
tor to give rise to binuclear complexes.[8] Analogous ligands
(bis(2-pyridylcarbonyl)amine (Hbpca) and bis(2-pyrimidyl-
carbonyl)amine (Hbpmca)) can be obtained upon replace-
ment of the connecting carbon with an amidic nitrogen. This
structural motif was successfully used for the preparation of
coordination complexes with different transition-metal ions,
in which the carbonyl groups may bridge neighboring cat-
ions.[9] The only example of lanthanide-containing com-
pounds in which LnIII is coordinated by carbonyl groups, is
reported for binuclear FeIII–DyIII complexes.[10]
To better satisfy a high coordination number of LnIII, we
have prepared a new ligand by introducing terminal carbox-
ylic groups on the Hbpca backbone to achieve the LnIII co-
ordination in the pentacoordinate cavity (Scheme 1). The
Scheme 1. Synthesis of L1 and L2. Reagents: i) SOCl2, DMF, CH2Cl2/tol-
uene; ii) NH4Cl/SiO2, TsCl, NEt3.
[a] S. Zebret, Dr. J. Hamacek
Department of Inorganic, Analytical and Applied Chemistry
University of Geneva
30 quai E. Ansermet, 1211 Geneva (Switzerland)
Fax : (+41)22-379-6830
E-mail: Josef.Hamacek@unige.ch
topology of this coordination site can be compared to some
pentadentate ligands, namely derivatized dicarbazone[11] and
terpyridine ligands[12] used for fluorimetric assays.[4] As
shown previously for transition metals, the amidic carbonyl
groups point to the opposite side of the cavity, which makes
them available for some additional complexation. After de-
protonation of the amidic and carboxylic groups, the nega-
tively charged ligand compensates the positive charge of
lanthanides, which is supposed to increase the thermody-
[b] N. Dupont
Department of Physical Chemistry, University of Geneva
30 quai E. Ansermet, 1211 Geneva (Switzerland)
[c] Dr. G. Bernardinelli
Laboratory of X-ray Crystallography, University of Geneva
24 quai E. Ansermet, 1211 Geneva (Switzerland)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200802676.
Chem. Eur. J. 2009, 15, 3355 – 3358
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3355

namic stability of the resulting complexes. In this work, we
report on the preparation, structure, and characterization of
the europium complex with bis(6-carboxypyridine-2-carbon-
yl)amine (L2). The luminescent properties of this neutral
complex are reported in view of possible applications in
imaging techniques.
The L1 precursor was synthesized by adapting the condi-
tions usually used for the synthesis of diacylamide com-
pounds (Scheme 1, see the Supporting Information). The
1H NMR spectrum of L1 in CD3CN (see Figure S1 in the
Supporting Information) shows well-resolved peaks of three
aromatic protons and a singlet for the terminal methyl pro-
tons, in agreement with C2 symmetry. Attempts to transform
L1 to L2 by hydrolysis of the ester group in basic media led
to the hydrolysis of the amidic group. To isolate the EuIII
complexes with L2, the solution of L1 in DMF was treated
with NaH. The DMF solution containing one equivalent of
[EuACHTUNGTRENNUNG(Otf)3] was slowly added to the resulting mixture under
stirring. The hydrolysis of the terminal methyl ester groups
occurs by the concerted complexation of the ester group to
the EuIII cation acting as a Lewis acid and the action of
NaH, as was described for the alkaline hydrolysis of tert-
butyl esters.[13] Finally, diffusion of water into a filtered
DMSO solution of the complex provides X-ray quality cubic
crystals showing a strong red-orange photoluminescence
upon the UV light irradiation.
Figure 1. Crystal structure of [Eu3(L2)3ACTHNUTRGNEU(NG H2O)6]. View of the trinuclear
complex a) along and b) perpendicular to the threefold axis.
ACHTUNGTRENNUNG[Eu3(L2)3ACHTUNGTRENNUNG(H2O)6] crystallizes in the cubic system (space
¯
group Pa3) and contains eight trimeric complexes in the unit
cell.[14] The X-ray crystal structure of [Eu3(L2)
CTHUNGTRENNUNG
shows that this complex is formed by three molecules of L2
interconnected with three europium cations around a crys-
tallographic threefold axis (Figure 1 and S2 in the Support-
ing Information). Each europium cation in the crystal struc-
ture is nine-coordinated by five donor atoms of one ligand
(pentacoordinated cavity), two oxygen atoms of the amidic
carbonyl groups of the neighboring ligand, and the two re-
maining positions are occupied by water molecules
(Figure 2). The coordination sphere of EuIII can be de-
scribed as a distorted mono-capped square antiprismatic
site, in which one water molecule caps the rectangular face
formed by two carbonyl and two carboxylate oxygens. The
final edifice is electroneutral due to the full compensation
of the europium positive charge by the two carboxylates and
the deprotonated amide nitrogen. A view of the crystal
packing is given in Figures S3 and S4 in the Supporting In-
Figure 2. View of the coordination environment around EuIII cations in
[Eu3(L2)3ACHTUNTRGNEUNG(H2O)6] with the atomic numbering scheme.
coordination bonds are provided by two ligands and the two
remaining positions are occupied by water molecules. How-
ever, the intermetallic distance of 3.745 ꢁ is much shorter
than in [Eu3(L2)3ACHTUNTRGNEG(UN H2O)6], in which the distance between Eu
cations is 6.4969(4) ꢁ.
1
ꢀ
ꢀ
formation. The Eu O and Eu N bond lengths in [Eu3(L2)3-
(H2O)6] are with the expected range (see Table S1 in the
The H NMR spectrum of the complex [Eu3(L2)
N
U
in DMSO (Figure 3) shows three different aromatic signals,
which are shifted upfield and broadened due to the para-
magnetic contribution of EuIII cations. The ES-MS spectrum
of the DMSO/CH3CN solution (7ꢂ10ꢀ5 m) clearly confirms
the presence of the trinuclear edifice in solution with peaks
at m/z 1393.9 ([Eu3(L2)3 +H+]+) and m/z 697.8 ([Eu3(L2)3
+2H+]2+).
Supporting Information). The bond lengths in the CO-N-
ꢀ
CO group are symmetrical with the C O bonds shorter than
in lanthanide complexes with real b-diketonate ligands.
These data suggest the presence of the diketo form of the
N-deprotonated L2 with a lower p-electron delocalization.
The triangular arrangement of europium cations can be
compared with the arrangement in trinuclear sandwich com-
plexes obtained from LnIII and cis-inositol,[15] in which lan-
thanides form similar two-dimensional arrays. In the com-
plex with EuIII, the cations are only eight-coordinated: six
The trinuclear complex was investigated by using EuIII as
a structural probe to correlate the coordination environment
in the solid state and in solution. The high-resolution excita-
tion and emission spectra of isolated [Eu3(L2)3ACHTNURGTNE(UNG H2O)6] crys-
3356
www.chemeurj.org
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 3355 – 3358

Trinuclear Luminescent Europium Complex
COMMUNICATION
!
The energy of the EuACUTHGNTRNENUG
(5D0 7F0) transitions is modified by
the composition of the europium coordination sphere (the
nephelauxetic effect) and can be calculated according to
Frey and Horockꢃs equation [Eq. (1)], in which n˜0 =
!
17374.0 cmꢀ1 is the energy of the Eu(5D0
7F0) transition
in the free metal ion, CCN is equal to 1.0 for the nine-coordi-
nate EuIII cation, ni is the number of atoms of type i, and di
is the capacity of the atom i to accept the electronic density
of the metal ion.[16] The coordination sphere of each europi-
um cation is considered as that of the crystal structure: two
heterocyclic nitrogens, two oxygen atoms of carboxylic
groups, two water molecules, two carbonyl oxygens, and one
negatively charged nitrogen of the amide group.
Figure 3. 1H NMR spectrum of [Eu3(L2)
3ACHTNUTRGNE(UNG H2O)6].
X
~
~
n ¼ n0 þ CCN
:
nidi
ð1Þ
tals were recorded at temperatures from 10 to 300 K. The
emission spectrum (Figure 4a) obtained upon irradiation
If we consider di =ꢀ15.3 for the deprotonated nitrogen,
through the ligand-centered excited states (lexc =280 nm) is
the application of Equation (1) gives the value 17241.5 cmꢀ1
7
7
dominated by the intense hypersensitive Eu
U
(580 nm). The position of the EuACHNUTGTRENNUNG
found to be 17227 cmꢀ1 at 10 K, which corresponds to
17239 cmꢀ1 at 295 K by taking into account the temperature
correction 1 cmꢀ1 per 24 K.[17] The resulting difference of
2.5 cmꢀ1 suggests that the negatively charged nitrogen has a
stronger nephelauxetic effect than expected above. The
straightforward calculation gives di =ꢀ17.8 cmꢀ1, which is in
line with a general effect of anionic ligands. The alternative
consideration of b-diketonate oxygens instead of carbonyl
ones for the prediction of n˜ gives the value 17245.1 cmꢀ1,
which differs from that of the experiment by 6.1 cmꢀ1. This
comparison is consistent with the diketo configuration of L2
discussed above.
The EuACHTNUGRTNEUNG
(5D0) lifetimes in the solid state were obtained
upon irradiation through ligand-centered excited states
(28169 cmꢀ1) at temperatures between 10 and 300 K. Mono-
exponential luminescent decays support the presence of
only one type of the EuIII site. The average lifetime for dif-
ferent transitions (see Table S3 and Figure S5 in the Sup-
porting Information) at 10 K is 0.47(4) ms and slightly de-
creases with temperature to 0.41(2) ms at 300 K. Such a life-
time is comparable to the one of other EuIII complexes con-
taining two water molecules in the first coordination
sphere.[18]
Figure 4. The emission spectra of [Eu3(L2)3ACHTUNRTGNEUNG(H2O)6 ] in the solid state at
a) 10 K (lex =280 nm), b) 295 K (lex =280 nm), and c) in DMSO (lex
=
279 nm, 295 K).
sition at 616 nm and does not significantly change with tem-
perature (Figure 4a,b). The excitation profiles upon moni-
The excitation spectrum of [Eu3(L2)3] in DMSO (see Fig-
ure S7 in the Supporting Information) coincides with the ab-
sorption spectrum with the maximum absorbance at 284–
286 nm as found in the solid state. Except for some broaden-
ing, the europium-centered emission spectra in DMSO,
D2O, and H2O show the same pattern as in the solid state,
which confirms the presence of the same trinuclear structure
in solution (Figure 4c and Figure S8 in the Supporting Infor-
mation). Compared to the results in the solid state,
[Eu3(L2)3] in DMSO shows a considerable lengthening of
the luminescent lifetime (t=1.57 ms). This can be explained
toring different EuACHTUNGTRENNUNG
(5D0) transitions can be superimposed
and show a broad band with the maximum at 284 nm with a
significant tail up to 400 nm (see Figure S5 in the Supporting
Information), which allows efficient ligand (pp)* ! EuIII
!
energy transfers. The very weak EuACTUHGNTRNENGU
(5D0 7F0) transition is
unique and indicates a single type of environment around
the EuIII cations related by a crystallographic threefold axis.
7
7
The considerable splitting of the F1 (3 transitions), F2 (5
7
transitions), and F4 (9 transitions) states is compatible with
the low local Cs-symmetry of the EuIII site found in the crys-
by the replacement of water molecules containing O H os-
ꢀ
tal structure of [Eu3(L2)
N
cillators with DMSO in the first coordination sphere of
porting Information).
EuIII. The absolute quantum yield, Fabs, of the [Eu3(L2)3] lu-
Chem. Eur. J. 2009, 15, 3355 – 3358
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3357

J. Hamacek et al.
cal Magnetic Resonance Imaging (Ed.: A. E. Merbach, E. Toth),
Wiley, New York, 2001.
[2] a) J.-C. G. Bꢄnzli, C. Piguet, Chem. Soc. Rev. 2005, 34, 1048–1077;
b) J.-C. G. Bꢄnzli in Lanthanide Probes in Life, Chemical and Earth
Sciences, Elsevier, Amsterdam, 1989, Chapter 7.
[3] K. Binnemans in Handbook on the Physics and Chemistry of Rare
Earths (Eds.: K. A. Gschneidner, J. C. G. Bꢄnzli, V. K.Pecharsky),
Elsevier, Amsterdam, 2005, pp. 107–272.
[4] a) Y. Y. Xu, I. Hemmilꢅ, Talanta 1992, 39, 759–763; b) I. Hemmilꢅ,
T. Stꢆhlberg, P. Mottram in Bioanalytical Applications of Labelling
Technologies, Wallac Oy, Turku, 1995.
[5] J. P. Leonard, C. M. G. dos Santos, S. E. Plush, T. McCabe, T. Gunn-
laugsson, Chem. Commun. 2007, 129–131.
[6] a) L. R. Melby, N. J. Rose, E. Abramson, J. C. Caris, J. Am. Chem.
Soc. 1964, 86, 5117–5125; b) R. C. Holz, L. C. Thompson, Inorg.
Chem. 1988, 27, 4640–4644; c) C. R. De Silva, J. R. Maeyer, A.
Dawson, Z. Zheng, Polyhedron 2007, 26, 1229–1238.
[7] a) P. Lenaerts, K. Driesen, R. Van Deun, K. Binnemans, Chem.
Mater. 2005, 17, 2148–2154; b) L. Puntus, K. J. Schenk, J.-C. G.
Bꢄnzli, Eur. J. Inorg. Chem. 2005, 4739–4744; c) A. A. Knyazev,
Y. G. Galyametdinov, B. Goderis, K. Driesen, K. Goossens, C. Gorl-
ler-Walrand, K. Binnemans, T. Cardinaels, Eur. J. Inorg. Chem. 2008,
756–761.
[8] S. Brꢄck, M. Hilder, P. C. Junk, U. H. Kynast, Inorg. Chem.
Commun. 2000, 3, 666–670.
[9] H. Tanaka, T. Kajiwara, Y. Kaneko, S. Takaishi, M. Yamashita, Poly-
hedron 2007, 26, 2105–2109.
[10] M. Ferbinteanu, T. Kajiwara, K.-Y. Choi, H. Nojiri, A. Nakamoto,
N. Kojima, F. Cimpoesu, Y. Fujimura, S. Takaishi, M. Yamashita, J.
Am. Chem. Soc. 2006, 128, 9008–9009.
[11] a) R. C. Palenik, K. A. Abboud, S. P. Summers, L. L. Reitfort, G. J.
Palenik, Inorg. Chim. Acta 2006, 359, 4645–4650; b) M. Albrecht, S.
Mirtschin, O. Osetska, S. Dehn, D. Enders, R. Froehlich, T. Pape,
F. E. Hahn, Eur. J. Inorg. Chem. 2007, 3276–3287.
minescence in DMSO at 295 K was estimated to be about
37%. The solution structure of [Eu3(L2)3], especially the
number of water molecules in the first coordination sphere,
was evaluated by using Horrocks and Sudnickꢃs equation[18]
[Eq. (2)] corrected for closely diffusing oscillators.[19]
q ¼ 1:1ðkH O ꢀ kD O ꢀ 0:31Þ
ð2Þ
2
2
The lifetimes measured for [Eu3(L2)3] in water (0.42 ms)
and deuterated water (2.41 ms) (see Table S3 in the Sup-
porting Information) give q=1.8(1), which confirms that 1)
two water molecules are present in the first coordination
sphere and 2) the trinuclear complex is maintained in solu-
tion. Although the quantum yield in water falls to about
13%, this value is still compatible with potential applica-
tions in sensing devices. The decrease of Fabs by about 70%
compared to value in the DMSO solution is accompanied by
the decrease of the luminescent lifetime (tH O/tDMSO =0.3,
2
Table S3).
In summary, the self-assembly of the new receptor L2
with EuIII results in the formation of the trinuclear discrete
complex. Its crystal structure reveals the two-dimensional
arrangement of europium cations with an unusual topology,
in which each EuIII is nine-coordinated by two neighboring
ligands and two water molecules. Despite the coordination
of water molecules to the EuIII cations in [Eu3(L2)
3ACHTNUTRGNE(UNG H2O)6],
the trinuclear complex exhibits remarkable luminescent
properties and the presence of three metal ions in proximity
may constitute a significant advantage for designing homo-
and heterometallic sensing systems. Current investigations
are focused on the analogous trinuclear complexes with par-
amagnetic lanthanides, which are of a special interest as po-
tential MRI contrast agents.
[12] L. J. Charbonniere, S. Mameri, D. Flot, F. Waltz, C. Zandanel, R. F.
Ziessel, Dalton Trans. 2007, 2245–2253.
[13] S. Paul, R. R. Schmidt, Synlett 2002, 1107–1108.
[14] Crystal data for of the [Eu(C14H6N3O6)ACHTUNGTNERU(NGN H2O)2] CTHUNRTGENNUG(C2H6OS)3ACHTUNGTRENNUNG(H2O)12.
3A
M=1951.5, cubic, space group Pa3, a=24.2325(9) ꢁ, V=
14229.7(16) ꢁ3, 1calcd =1.822 gcmꢀ1 m=2.8 mmꢀ1
Z=8, l=
¯
,
,
0.7107 ꢁ, T=150 K, 152788 reflections collected, 4649 independent
reflections (Rint =0.062), and 3375 observed reflections [F0 >3s(F0)],
346 refined parameters, R=0.032, wR=0.032. CCDC-712769 con-
tains the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[15] R. Hedinger, M. Ghisletta, K. Hegetschweiler, E. Toth, A. E. Mer-
bach, R. Sessoli, D. Gatteschi, V. Gramlich, Inorg. Chem. 1998, 37,
6698–6705.
Acknowledgements
Financial support from the University of Geneva and SNF is gratefully
acknowledged. We thank P. Perrottet for recording the ESI-MS spectra
and H. Eder for performing the elemental analyses. We thank Prof A.
Hauser for fruitful discussions.
[16] S. T. Frey, W. d. Horrocks, Inorg. Chim. Acta 1995, 229, 383–390.
[17] M. Albin, W. D. Horrocks, Jr., Inorg. Chem. 1985, 24, 895–900.
[18] W. D. Horrocks, Jr., D. R. Sudnick, J. Am. Chem. Soc. 1979, 101,
334–340.
[19] R. M. Supkowski, W. D. Horrocks, Jr., Inorg. Chim. Acta 2002, 340,
44–48.
Keywords: europium
supramolecular chemistry · trinuclear complexes
· luminescence · self-assembly ·
[1] a) P. Caravan, J. J. Ellison, T. J. McMurry, R. B. Lauffer, Chem. Rev.
1999, 99, 2293–2352; b) The Chemistry of Contrast Agents in Medi-
Received: December 18, 2008
Published online: February 20, 2009
3358
www.chemeurj.org
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 3355 – 3358