Tuning the stoichiometry of lanthanide complexes with calixarenes: Bimetallic complexes with a calix[6]arene bearing ether-amide pendant arms

Article abstract of DOI:10.1002/ejic.200300824

A de-tert-butylated calix[6]arene (A₆L⁶) fitted with six etheramide pendant arms was synthesized and characterized in solution. NMR spectroscopic data point to the six phenoxide units adopting an average D _6h conformation on the NMR time scale. According to MM3 molecular mechanics, and MOPAC quantum mechanical calculations, A₆L⁶ is a ditopic ligand featuring two nonadentate coordination sites, each built from three pendant arms, and extending in opposite directions with one above, and the other under the main ring. A₆L⁶ reacts with Ln^III ions (Ln = La, Eu) in acetonitrile to successively form 1:1 and 2:1 complexes. The isolated Eu^III 2:1 complex is luminescent (quantum yield: 2.5% in acetonitrile, upon ligand excitation). In the solid state, the luminescence decay is bi-exponential, with lifetimes equal to 1.66 and 0.46 ms (upon direct metal excitation), pointing to the presence of two differently coordinated metal ions, one with essentially no bound water molecules, and the other one with two ligated water molecules. According to molecular mechanics calculations, the more stable isomer is indeed asymmetric with two nine-coordinated metal ions. Both Eu^III ions are bound to three bidentate arms, and one monodentate triflate anion, but one metal ion completes its coordination sphere with two phenoxide oxygen atoms, and the other one with two water molecules, consistent with IR spectroscopic and luminescence data. In acetonitrile solution, the two metal ion sites become equivalent, and the relatively long lifetime (1.35 ms) is indicative of a coordination environment free of water molecules. This work demonstrates, that the stoichiometry of lanthanide complexes with calixarenes can be tuned by a suitable choice of the narrow and/or wide rim substituents. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.

Full text of DOI:10.1002/ejic.200300824

FULL PAPER
Tuning the Stoichiometry of Lanthanide Complexes with Calixarenes:
Bimetallic Complexes with a Calix[6]arene Bearing Ether-Amide Pendant Arms
[a,b]
[c]
[a,d]
´
´
`
¨
Flor de Marıa Ramırez,
Loıc Charbonniere, Gilles Muller,
and
Jean-Claude G. Bünzli*[a]
Keywords: Lanthanides / Calixarenes / Europium / Luminescence
A de-tert-butylated calix[6]arene (A6L6) fitted with six ether-
amide pendant arms was synthesized and characterized in
no bound water molecules, and the other one with two lig-
ated water molecules. According to molecular mechanics cal-
solution. NMR spectroscopic data point to the six phenoxide culations, the more stable isomer is indeed asymmetric with
units adopting an average D6h conformation on the NMR
time scale. According to MM3 molecular mechanics, and
MOPAC quantum mechanical calculations, A6L6 is a ditopic
ligand featuring two nonadentate coordination sites, each
two nine-coordinated metal ions. Both EuIII ions are bound
to three bidentate arms, and one monodentate triflate anion,
but one metal ion completes its coordination sphere with two
phenoxide oxygen atoms, and the other one with two water
built from three pendant arms, and extending in opposite dir- molecules, consistent with IR spectroscopic and lumines-
ections with one above, and the other under the main ring.
cence data. In acetonitrile solution, the two metal ion sites
become equivalent, and the relatively long lifetime (1.35 ms)
A6L6 reacts with LnIII ions (Ln = La, Eu) in acetonitrile to suc-
cessively form 1:1 and 2:1 complexes. The isolated EuIII 2:1 is indicative of a coordination environment free of water mo-
complex is luminescent (quantum yield: 2.5% in acetonitrile,
upon ligand excitation). In the solid state, the luminescence
decay is bi-exponential, with lifetimes equal to 1.66 and 0.46
ms (upon direct metal excitation), pointing to the presence of
two differently coordinated metal ions, one with essentially
lecules. This work demonstrates, that the stoichiometry of
lanthanide complexes with calixarenes can be tuned by a
suitable choice of the narrow and/or wide rim substituents.
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2004)
Introduction
at using calixarenes for the development of separation pro-
cesses,[10] and the nuclear industry is particularly interested
in these ligands for the separation of highly radioactive acti-
nides from non-radioactive lanthanides before nuclear wa-
ste confinement.[11Ϫ13]
Calixarenes act as versatile building blocks in supramol-
ecular and coordination chemistry, since both their lower
and upper rims may be derivatized to induce specific func-
tionalities.[1,2] They are currently being used in the design
of chemical sensors,[3] catalysts,[4] and selective ligands for
the complexation of d[5] and f[6,7] metal ions, as well as for
the self-assembly, through π-stacking, H-bonding or coordi-
nation interactions, of elaborate supramolecular struc-
tures.[8,9] As a consequence, many studies have aimed
In recent years, we have been developing model calixar-
ene molecules for the study of energy migration mecha-
nisms in luminescent lanthanide-containing compounds,
and of magnetic 4fϪ4f interactions.[7] We have also been
interested in calixarenes having 4fϪ5f separation capabili-
ties. This leads to the formation of luminescent 4f-species
which can be easily analyzed during the extraction process.
With this in mind, we have focused on two series of
branched calixarenes, one bearing ether-amide pendant
arms A, and the other having phosphanylyl groups B (see
Scheme 1). The ether-amide substituent was designed so
that it can act as a bidentate ligand forming a five-mem-
bered cycle upon coordination of its ether and amide func-
tions to trivalent lanthanide ions LnIII. We have shown that
A4bL4 acts as an inorganic-organic receptor, encapsulating
both a lanthanide ion and a solvent molecule. In addition,
the crystal structure of the LuIII 1:1 complex shows the co-
ordination cavity predisposed to further accommodate one
[a]
Institute of Molecular and Biological Chemistry, Swiss Federal
Institute of Technology,
BCH 1402, 1015 Lausanne, Switzerland
E-mail: jean-claude.bunzli@epfl.ch
[b]
Instituto Nacional de Investigaciones Nucleares, Departamento
´
de Quımica,
´
Km 36.5 Carretera Mexico-Toluca, Apartado Postal 111,
´
Lerma, Edo. de Mexico, C.P. 52045, Mexico
[c]
´
`
´
Ecole de Chimie, Polymeres et Materiaux, Laboratoire de
´
Chimie Moleculaire, UMR 7008 CNRS,
25 rue Becquerel, 67087 Strasbourg Cedex 2, France
Department of Chemistry, University of Minnesota Duluth,
MN 55812-3020, USA
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
[d]
2348
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/ejic.200300824
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Tuning the Stoichiometry of Lanthanide Complexes with Calixarenes
FULL PAPER
water molecule hydrogen-bonded to the oxygen atoms of were performed to help understand the structures of both
the phenol units, which rigidifies the induced cavity, and the free ligand and the complex.
binds the metal ion.[14] The phosphanylyl-substituted calix-
arenes BnbLn (n ϭ 4, 6) form complexes with 1:1 and 1:2
LnIII ligand ratios having significant stability in aceto-
nitrile,[15,16] and interesting luminescent[15,16] and 4fϪ5f sep-
aration[17] properties. Several derivatized calix[6]arenes have
been tested in extraction studies[18Ϫ20] and, in our case,
B6bL6 proved to be a better extracting agent than B4bL4.[17]
Despite that calix[6]arene complexes with lanthanide ions
have also been proposed as luminescent probes,[21,22] little
is known about the interaction between calix[6]arenes and
LnIII ions. The reaction of europium nitrate with p-tert-bu-
tylcalix[6]arene leads to the formation of a 1:2 (EuIII/L)
compound in which only one calix[6]arene molecule is
bound to the 8-coordinate metal ion through a single phen-
oxide unit, as shown by its X-ray structure.[23] Replacing
nitrate with trifluoromethanesulfonate (triflate) results in
the formation of bimetallic 2:1 species (Ln ϭ La, Sm, Yb)
in which the two metal cations are anchored on both sides
of the macrocycle in the 1,2,3-alternate conformation,
which acts as a bidentate ligand, while the coordination
sphere of the 7-coordinate LnIII ions (Ln ϭ Sm, Yb) is com-
pleted by one monodentate triflate and one nitrogen donor
atom from the pyridine and/or terpyridine.[24]
Results and Discussion
Synthesis and Characterization of the Substituted
Calixarene A6L6
The branched calix[6]arene A6L6 (Scheme 1) was ob-
tained in two steps via a Williamson reaction (Scheme 2)
from the brominated pendant arm ABr, synthesised as de-
scribed previously,[14] and the calix[6]arene H6L6. The hexa-
sodium derivative of calix[6]arene was prepared in toluene,
demonstrating that this salt can be formed in a non-polar
solvent despite the high fluxionality of calix[6]arene, pro-
vided adequate experimental conditions are used i.e. heat-
ing, vigorous stirring, and a nitrogen atmosphere to avoid
oxidation and protonation of the phenoxide groups. The
macrocycle is soluble in CH2Cl2, CHCl3, CH3CN, toluene,
methanol, and ethanol. Its IR spectrum displays an intense
band at 1639 cmϪ1 typical of the carbonyl groups in an
N,NЈ-disubstituted amide (1646 cmϪ1 in ABr). The absorp-
tion band associated with contributions from the aliphatic
ether groups appears at 1126 cmϪ1 (1119 cmϪ1 in ABr),
medium bands at 1218 and 1045 cmϪ1 are typical of the
phenoxide groups, while the three neighboring aromatic
CϪH groups generate a characteristic symmetric out-of-
plane bending at 765 cmϪ1. In the UV/Vis spectrum, the
structured band at 270 nm (37040 cmϪ1) with a shoulder at
278 nm (35970 cmϪ1) indicates complete functionalization
of the phenoxide groups. This structured absorption band is
associated with πǞπ* transitions involving orbitals mainly
located on the phenyl rings. A band corresponding to tran-
sitions primarily located on the carbonyl groups can be seen
at 223 nm (44840 cmϪ1) in the diffuse reflectance spectrum.
Scheme 2
Scheme 1
In this paper, we discuss how the removal of the p-tert-
The 1H NMR spectrum of A6L6 in CD3CN at room tem-
butyl groups influences the conformation of the calix[6]- perature is shown in Figure 1 together with its complete
arene derivatized with pendant arms A and, consequently, assignment (see Scheme 1 for atomic numbering). The lim-
the stoichiometry of the LnIII complexes. Removal of the ited number of signals points to a high average symmetry
tert-butyl groups should result in easier conformational in- on the NMR time scale. This may be interpreted as a conse-
terconversion, and a relief in steric hindrance is expected to quence either of fast interconversion processes between the
favor the formation of the 2:1 complexes, contrary to what different conformers on the NMR timescale, or of the pres-
has been observed for B6bL6, which forms 1:2 complexes. ence of a single species of high symmetry, such as the 1,3,5
The calixarene A6L6, with tert-butyl groups removed, was alternate conformation belonging to the S6 point group.
therefore synthesized, and characterized both in the solid While the introduction of long complexing arms on the p-
state and in solution, as well as its 2:1 complex with EuIII
Molecular mechanics and quantum mechanical calculations oxygen-through-the-annulus rotation,[25,26] removal of the
.
tert-butylcalix[6]arene moiety may be expected to freeze the
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FULL PAPER
bulky tert-butyl groups favors the aromatic-through-the-an- rings are in an anti conformation. Only four signals are ob-
nulus rotation. Since the latter mechanism has been ob- served for the aromatic carbon atoms, indicating the equiv-
served with O-substituted p-tert-butylcalix[6]arenes,[26] it is alence of the six aromatic rings on the NMR time scale,
reasonable to assume that the lack of substitution on the and an average hexagonal (D6h) symmetry for the calixarene
phenol groups allows for rapid aromatic-through-the-annu- core. All the carbon atoms of the pendant arms also give
lus rotation and, hence, the apparent high symmetry ob- rise to one signal, except those of the ethyl groups which
served is the result of fast interconversion between the dif- are split into two components pointing to a somewhat re-
ferent conformers.
stricted rotation around the amide function.
To substantiate the interpretation of the NMR spectra,
we turned to molecular mechanics calculations, and evalu-
ated the A6L6 structure using a MOPAC PM3 procedure,
taking into account solvent effects (water). The conformer
with the lowest energy is shown in Figure 2. It displays a
pinched ‘‘chair’’ conformation of the 1,2,3-alternate type.
Three consecutive phenol groups are positioned in such a
way, that the pendant arms form two cavities predisposed
towards metal ion coordination, each built up of three
pendant arms, and extending on opposite sides of the
macrocycle.
1
Figure 1. H NMR spectrum of A6L6 10Ϫ3  in CD3CN at 295 K
A detailed comparison of the chemical shifts of the pro-
ton atoms of the complexing arms in A6L6 and A4bL4
further enlightens an unusual point. While the ethyl groups
of the amide functions are found essentially at the same
chemical shifts in both ligands, signals corresponding to the
H1 and H2 protons are shifted significantly upfield (more
than 0.4 ppm) in A6L6 compared with A4bL4.[14] 1H NMR
spectroscopic data of A4bL4 indeed showed the compound
adopting a cone conformation in solution, in which the four
complexing arms are held together in closed proximity,
thereby providing a suitable octadentate coordination site
for lanthanide cations.[14] The X-ray crystal structure of the
lutetium complex further showed that a water molecule was
encapsulated in the cavity formed by the four dioxo-ethyl-
ene bridges (OϪC1ϪC2ϪO), with H-bonding interactions
between this water molecule and the oxygen atoms. Interest-
ingly, an elemental analysis of free A4bL4 revealed the pres-
ence of one water molecule, even after the drying process.
It was suspected, that the H-bonding interactions with the
water molecule are also present in the free ligand in solu-
tion, leading to a decrease of the electronic density on the
ethylene bridges, and a concomitant upfield shift of the cor-
responding signals in the 1H NMR spectrum. For the larger
and more flexible calix[6]arene, NMR spectroscopic data
indicate that the cone conformation is not the most favor-
Figure 2. (Top) optimized geometry of A6L6 as determined by MO-
PAC PM3 calculations taking into account the solvent effect
(water); (bottom) View of the calixarene core only
Interaction of A6L6 with LaIII and EuIII in Acetonitrile
To gain information on the interaction between lantha-
able conformation, leading to chemical shifts similar to nide ions and the calixarene A6L6, solutions of the latter
those found in the brominated ABr molecule or its hydroxy (typically 10Ϫ3 ) were titrated with solutions of lanthanide
precursor. The same reasoning can be applied to the corre- triflates Ln(OTf)3·6H2O (typically 10Ϫ3 , Ln ϭ La, Eu)
sponding carbon atoms (C1 and C2), one of which is shifted for ratios R ϭ [LnIII]t/[A6L6]t varying from 0 to 3. The spec-
upfield by 1.1 ppm in A4bL4 compared with A6L6.
tra are shown in Figure 3 (S2, and S3, see Supporting Infor-
13C{1H} NMR spectra in both CDCl3 and CD3CN con- mation). In all cases, the spectra display broad overlapping
firm the fast exchange hypothesis (see Figure S1 in the Sup- signals, probably as a result of fast exchange reactions of
porting Information; see also the footnote on the first page the complexed species. This prevented a quantitative treat-
of this article) since δ(C13) ϭ 30.95 ppm points to a syn ment. For R Ͻ 1, the free ligand and a complex species
orientation of adjacent phenol rings,[27] thus excluding the coexist in solution. For R ϭ 1, a distinct AB spin system
hypothetical 1,3,5-alternate conformation in which adjacent can be observed (Ln ϭ La) with two doublets at 4.45 and
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Tuning the Stoichiometry of Lanthanide Complexes with Calixarenes
FULL PAPER
3.60 ppm for the methylene proton atoms bridging two phe- groups of the pendant arms can be ascertained from the IR
nolic units. This strongly suggests an average cone-like con- data (obtained after drying the pellet at 118 °C), the νCϭO
formation for the 1:1 LnA6L6 complex. The broadness of vibration being red-shifted by 25 cmϪ1 compared with the
the peaks corresponding to the protons of the arms may be ligand, pointing to a strong metal-to-ligand interaction.
attributed to exchange processes, possibly between forms in Moreover, the intensity of the band arising from the ali-
which these arms are complexed or not complexed to the phatic ether functions of the arm is strongly reduced, in line
metal cation. For R Ͼ 1, the peaks of the non-coordinated with the coordination of these moieties to the metal ion.
ligand disappear, and signals corresponding to at least two The vibration involving contribution from the aromatic
species with different LnA6L6 stoichiometries can be ident- ether could not be identified, probably being masked by
ified. Since the spectra do not change further when R is another band, contrary to what we reported for the
increased from 2 to 3, we conclude that two species form [Ln(A4bL4)]3ϩ complexes, for which (i) this band is little
successively in solution, with 1:1 and 2:1 metal-to-ligand affected by the complexation, and (ii) the crystal structure
ratios. This is in line with the calculated structure of A6L6 of [Lu(A4bL4)(H2O)](OTf)3·2Et2O does not show evidence
which shows two sets of three pendant arms extending on of any coordination of aromatic ethers.[14] In our case, this
opposite sides of the calixarene (cf. Figure 2). Attempts to suggests some change in the ligand conformation upon
establish a more quantitative speciation by ES-MS failed complexation and, possibly, coordination of these groups to
due to hydrolysis at high dilution in the capillary during the the metal ion. Other ligand vibrations, for instance the
analysis (T ϭ 200 °C). As a result, only monometallic spec- bending CH2 mode (1450 cmϪ1), and the out-of-plane ring
ies were detected for R values in the range 1Ϫ3, in addition bending mode (760 cmϪ1), experience small, but character-
to free A6L6 (Table 1).
istic changes upon complexation. In addition, two strong
bands at 1030 and 638 cmϪ1 can be assigned to the νs(SO2)
and νas(SO3) modes of coordinated triflate,[28] while the
band at 1321 cmϪ1 is evidence for ionic triflate. Finally,
water coordination can be deduced from a sharp band near
520 cmϪ1 [29]
Attempts to grow single-crystals by slow
.
vapor diffusion of diethyl ether or diisopropyl ether in
acetonitrile, THF or MeOH, were unsuccessful.
1H NMR spectra obtained during the titration of A6L6
with europium triflate (Figure S3, see Supporting Infor-
mation) revealed that complexation induces freezing of the
ligand conformation, resulting in a very complex pattern,
which is difficult to interpret. Variable temperature NMR
spectra of a Eu2(OTf)6(A6L6)·9H2O solution in CD3CN
were recorded up to 75 °C. Although the overall pattern
becomes somewhat simplified, the spectrum remains com-
plex. Higher temperatures could not be reached, because
the complex experiences de-metallation in DMSO, a solvent
in which, for instance, B6bL6 undergoes complete intercon-
version of its isomers on the NMR time scale at 132 °C.[16]
Figure 3. Part of the 1H NMR spectra obtained during the titration
of A6L6 by La(OTf)3·6H2O in CD3CN at room temperature; R ϭ
[La]t/[A6L6]t
Photophysical Properties of A6L6 and Its 2:1 EuIII Complex
The photophysical properties of A6L6, and of the 2:1
Isolation and Characterization of the 2:1 EuIII Complex
EuIII complex have been determined both for solid state
The complex can be isolated in good yield from stoichio- samples (295 and 77 K), and for frozen acetonitrile solu-
metric mixtures of A6L6 and europium triflate in aceto- tions (77 K). The excitation spectrum of the ligand is simi-
nitrile. The elemental analysis is consistent with the formula lar to the absorption and reflectance spectra discussed
Eu2(OTf)6(A6L6)·9H2O. Coordination of the carbonyl above. Excitation of a solid sample of A6L6 at 275 nm
Table 1. Main species observed in the ES-MS spectra of solutions containing A6L6 and Eu(Otf)6·6H2O in various proportions (see text)
Species
Obsd. m/z
Rel. Int.
Calcd. m/z
[Eu(OH)3(A6L6)(H2O)5 ϩ H]ϩ
[Eu(OH)(CF3SO3)(A6L6)(CH3CN)]ϩ
[Eu(OH)(CF3SO3)(A6L6)(H2O)(CH3CN)]ϩ
[Eu(OH)(CF3SO3)(A6L6)(H2O)3(CH3CN)]ϩ
[A6L6 ϩ H]ϩ
1873.4
1940.3
1958.4
1993.8
1580.8
Ͻ 15%
Ͻ 15%
15Ϫ20%
25Ϫ50%
100%
1873.9
1939.1
1957.1
1993.2
1581.0
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(36360 cmϪ1) and 77 K yields two emission bands centered q directly bound to the LnIII ion, either in the solid state[32]
at 305 nm (32790 cmϪ1), which can be assigned as arising or in solution.[33] Interpreting our lifetime data along these
from the 1ππ* state, and at 446 nm (22420 cmϪ1), attributed lines, we note that one lifetime, τ1, is long. It decreases, un-
to the 3ππ* state, as ascertained by the lifetime, which der direct excitation, from 1.66 ms (10 K) to 0.86 ms
amounts to 104 Ϯ 8 ms in frozen acetonitrile solution (2 ϫ (295 K). Such a lifetime, at least at low temperature, is con-
10Ϫ3 ). This can be compared with 83 Ϯ 4 ms for B6bL6 sistent with the inner coordination sphere containing no
under the same experimental conditions.[16]
water molecules. The other lifetime, τ2, is much shorter,
The absorption spectrum of the 2:1 complex in aceto- decreasing from 0.46 ms (10 K) to 0.28 ms (295 K), under
nitrile (4.74 ϫ 10Ϫ5 ) displays three bands at 268 nm direct excitation of the EuIII ion. Taking τ1 ϭ τref as a first
[32]
(37310 cmϪ1, ε ϭ 9490 molϪ1·cmϪ1), 276 nm (36230 cmϪ1
,
approximation, and q ϭ 1.05 ∆kobsd.
,
we calculate, that
ε ϭ 8650 molϪ1·cmϪ1), and 293 nm (34130 cmϪ1, ε ϭ 3380 the number of bound water molecules lies between 1.6
molϪ1·cmϪ1). The first two bands are blue-shifted 265 cmϪ1 (10 K) and 2.5 (77 and 295 K). The apparent increase in q
compared with those in the uncomplexed ligand. The third with increasing temperature could be simply due to outer
band is only present in the complex. The excitation spec- sphere water molecules hydrogen-bonded to the complex,
trum of the complex, obtained at 10 K with concomitant which become more labile when the temperature increases,
monitoring of the D0Ǟ7F2 transition, reveals the usual f-f or to other vibrational temperature-dependent deactivation
5
transitions as well as a broad band in the range of processes affecting the de-excitation of the two metal ions
320Ϫ450 nm with a maximum at 372 nm (26880 cmϪ1), differently.
which could possibly be associated with a ligand-to-metal
charge transfer (LMCT) band. Excitation in the ligand
5
Table 2. Lifetimes (ms) of the D0(Eu) level in a solid state sample
levels yields a luminescence spectrum displaying both li-
gand-centered and metal-centered 5D0Ǟ7FJ emission
bands, pointing to incomplete ligand-to-metal energy trans-
fer. The high-resolution laser-excited excitation spectrum in
the range of the 5D0ǟ7F0 transition, monitored via the
5D0Ǟ7F2 transition, results in a single, broad component at
17267 cmϪ1 at 10 K (full width at half height fwhh ϭ 37
cmϪ1), and 17280 cmϪ1 at 295 K (fwhh ϭ 24 cmϪ1), indica-
tive of a sample with several slightly different chemical en-
vironments for the EuIII ion, possibly arising from different
conformations of the macrocycle and associated pendant
arms.[30] Excitation in either the 5D0ǟ7F0 or 5L6ǟ7F0 tran-
sitions yields relatively broad emission spectra dominated
by the hypersensitive 5D0Ǟ7F2 transition, which are charac-
teristic of a chemical environment with low symmetry (Fig-
ure S4, Supp. Inf.). The relative and corrected intensities of
of Eu2(OTf)6(A6L6)·9H2O under various experimental conditions
T (K)
ν˜ex (cmϪ1
)
τ1 (ms)
τ2 (ms)
10
77
295
10
32468
32468
32468
17268
17268
17280
25445
2.01 Ϯ 0.05
1.71 Ϯ 0.03
1.09 Ϯ 0.02
1.66 Ϯ 0.01
1.19 Ϯ 0.04
0.86 Ϯ 0.02
Ϫ
0.97 Ϯ 0.04
0.48 Ϯ 0.01
0.36 Ϯ 0.01
0.46 Ϯ 0.02
0.31 Ϯ 0.01
0.28 Ϯ 0.01
1.38 Ϯ 0.02
77
295
295[a]
Acetonitrile solution 2 ϫ 10Ϫ3 ; single exponential.
[a]
In solution, the 5D0 luminescence decay becomes a single
exponential, with a value (1.38 ms) compatible with the ab-
sence of OH oscillators directly bound to EuIII. Dissolution
of the complex results, therefore, in a change in the coordi-
nation environment of the EuIII ions, since the lifetime is
now larger than those observed for the two different sites
in the solid state. The quantum yield measured in aceto-
nitrile upon ligand excitation QELu amounts to 2.5%, a value
5
the D0Ǟ7FJ transitions are 1.0, 7.4, 0.1, and 1.9 (10 K)
and 1.0, 7.2, 0.2, and 2.0 (295 K) for J ϭ 1, 2, 3, and 4,
respectively.
5
The lifetimes of the excited D0 level were measured at
10, 77, and 295 K both upon ligand and direct metal exci-
tation (Table 2). The luminescence decays of the solid state
sample are bi-exponential and temperature dependent,
decreasing roughly by a factor of two between 10 K and
room temperature. This is indicative of vibrational and/or
similar to that found for [Eu(B6bL6)2]3ϩ
.
Optimized Structure of the EuIII 2:1 Complex
In order to confirm the feasibility of our interpretation,
electronic radiationless de-activation mechanisms. Here, we resorted to molecular mechanics calculations to evaluate
both processes are probably operating in view of the a potential structure for the 2:1 complex. Several systems
fluxional nature of the calixarene, and of the LMCT band were tested, taking into account the experimental data,
apparent in the excitation spectrum. Lifetimes measured which suggested that the isolated compound has coordi-
upon ligand excitation are longer than those obtained nated triflate anions and water molecules (IR), and two dif-
through direct excitation of the 5D0 level, pointing to a ferent metal ion sites with q ഠ 0 and 2 (luminescence life-
feeding level with a long lifetime, namely the triplet state. times). It turned out, that the conformer with the lowest
High energy vibrations such as those generated by OϪH energy, as calculated by the MM3 and Conflex procedures,
groups are known to efficiently de-excite LnIII excited and shown in Figure 4 (see also Table S1 in the Supporting
levels,[31] and phenomenological equations have been pro- Information), has indeed an asymmetric structure with two
posed to relate ∆kobsd. ϭ 1/τobsd. Ϫ1/τref where τref is the 9-coordinate EuIII ions. One ion (site I) is bound to two
lifetime measured on the corresponding deuterated sample phenolic oxygen atoms, three bidentate arms, and one mon-
or in deuterated water, and the number of water molecules odentate triflate anion. The coordination environment of
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Tuning the Stoichiometry of Lanthanide Complexes with Calixarenes
FULL PAPER
the second ion (site II) is somewhat different, with two Yb).[24] The absence of substituents on the narrower rim of
water molecules replacing the bonding to the phenolic ether H6L6 allows easy interconversion between different con-
atoms. Therefore, the suggested formulation of the 2:1 com- formers, through either one of the ‘‘tert-butyl through the
plex is [Eu2(OTf)2(A6L6)(H2O)2]4ϩ. Generally speaking, the annulus’’ or ‘‘narrower rim through the annulus’’ path-
LnϪO distances are too long compared with the expected ways,[35] facilitating the formation of the 2:1 complexes. On
ones for 9-coordinate EuIII, which lie in the range the other hand, p-tert-butyl-substituted B6bL6 displays a
[34]
˚
˚
2.45Ϫ2.60 A [averages: 2.72(17) A for site I and 2.83(18) less labile alternate in-out conformation in which the six
˚
A for site II]. Relative distances, however, can be discussed. phosphanylyl pendant arms are located on the same side of
The coordination in site I is comprised of six short bonds the molecule, and no 2:1 complexes are obtained with this
(three carbonyl groups, one ether function, and the two receptor but, instead, compounds with 1:1 and 1:2 stoichio-
phenoxide moieties), and three longer bonds (in 13%) in- metries can be isolated.[16] This demonstrates once again
volving two ether functions and the triflate anion. The situ- the versatility of the calixarene frameworks, since a confor-
ation for site II is somewhat different in that it features only mation favoring a given stoichiometry of the LnIII com-
three short bonds (carbonyl groups) identical to those of plexes can be induced by modifying either the wider or the
Site I, all the other distances being 13% longer. It is note- narrower rim of the ligand, a property which should prove
worthy, that site I builds a more protective cavity around useful in the development of convenient extraction systems.
the metal ion than site II does. Moreover, contrary to what
was inferred from the ligand design, the phenoxide groups
participate more than expected in the bonding (Site I), with
LnϪO distances similar to those with carbonyl functions.
Experimental Section
Solvents and Starting Materials: Tetrahydrofuran and toluene were
distilled from Na, and acetonitrile was distilled from CaH2.[36] Di-
glycolic anhydride, BH3·Me2S (2  solution in THF), Et2NH, Et3N,
formalin (36% solution), butylphenol, KOH, AlCl3 phenol, chloro-
form, acetone, and SOBr2 were purchased from Fluka AG (Buchs,
Switzerland) or Merck, and used without further purification.
Syntheses
Preparation of the Pendant Arm: The brominated ether-amide arm
ABr was synthesized as described previously (see Scheme 1).[14] It
˚
was dried for 3 h over 4-A molecular sieves, which had been pre-
viously dried at 240 °C under high vacuum (24 h, 10Ϫ5 Torr).
Calix[6]arene H6L6: p-tert-Butylcalix[6]arene (H6bL6) was synthe-
sized according to the procedure reported by Gutsche et al.,[37] and
its purity checked by TLC analysis: Rf ϭ 0.65 in CH2Cl2/petroleum
ether (1:1, v/v), Rf ϭ 0.36 in acetone/petroleum ether (1:9, v/v). The
compound was further characterized by IR, 1H, and 13C NMR
spectroscopy, and EI mass spectrometry. The tert-butyl groups were
then removed following an AlCl3 catalyzed retro-FriedelϪCrafts
reaction in the presence of phenol.[38] TLC analysis yielded Rf ϭ
0.61 in CH2Cl2/petroleum ether (1:1, v/v), and Rf ϭ 0.31 in acetone/
Figure 4. Optimized geometry of the EuIII 2:1 complex with A6L6
as determined by MM3 Augmented and Conflex CAChe pro-
cedures
1
petroleum ether (1:9, v/v). It was further characterized by IR, H,
and 13C NMR and EI mass spectrometry.
Functionalization of Calix[6]arene: All operations were performed
under strictly anhydrous conditions under nitrogen. Dried calix[6]-
arene (135 mg, 0.21 mmol) and NaH (60% in mineral oil, 149 mg,
3.73 mmol) were placed into a 100-mL round-bottomed flask fitted
with a nitrogen inlet and a condenser, and dried toluene (60 mL)
was added. The turbid mixture was heated with stirring, becoming
almost transparent at 70 °C. The temperature increased spon-
taneously to 76 °C, hydrogen evolved, and a white emulsion
formed. The temperature was slowly increased to 90 °C, and the
mixture maintained at this temperature until no fluorescent UV
spot corresponding to H6L6 was observed on the TLC plate (ap-
proximately 1 h). The hot emulsion was quickly poured into a 100-
mL three-necked round-bottomed flask containing a solution of
dried brominated arm ABr (808 mg, 3.4 mmol) in dry toluene
(5 mL), which had been previously heated with stirring at 115 °C
for 2 h. More toluene was added (10 mL), and the temperature was
Conclusion
The calix[6]arene A6L6 exhibits a 1,2,3-alternate confor-
mation in which two sets of three arms extend on opposite
sides of the phenoxide framework, henceforth leading to a
ditopic ligand able to bind to two metal ions. Complexes
with a 2:1 (Ln/A6L6) stoichiometry could effectively be ob-
served in solution by H and 13C NMR spectroscopy, and
1
the EuIII complex was isolated. The calculated structure of
this complex confirms the results of the luminescence study,
and resembles those found experimentally for the 2:1 com-
plexes with p-tert-butylcalix[6]arene H6bL6 (Ln ϭ Sm, increased to 130 °C. As the reaction progressed the solution became
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´
`
F. de M. Ramırez, L. Charbonniere, G. Muller, J.-C. G. Bünzli
FULL PAPER
turbid and its color changed from brownish-orange to reddish-or-
ange. The reaction was stopped after 6 days. The solid was sepa-
rated at room temp. by vacuum filtration, and washed with warm
toluene until the filtrate became colorless. The filtered organic solu-
tion was washed several times with doubly distilled water until the
pH of the latter remained unchanged, and no AgBr formed upon
addition of 2% AgNO3 to the aqueous phase. During the washing,
a white emulsified intermediate phase formed, which disappeared
quickly. The organic phase was dried with anhydrous magnesium
sulfate, filtered, concentrated at 40 °C, and finally evaporated under
(360 MHz) or DRX Advance 400 (400 MHz) spectrometers; chemi-
cal shifts δ are given with respect to TMS or CD3CN. Low resolu-
tion luminescence spectra of solid state samples (295 and 77 K)
were recorded with a PerkinϪElmer LS-50B spectrofluorimeter.
Lifetimes of solutions (data reported are averages of at least five
determinations), and quantum yields were measured with the same
instrument. The quantum yield of the metal-centered luminescence
was determined in degassed and anhydrous CH3CN at room tem-
perature with respect to [Eu(L)]3ϩ 3 ϫ 10Ϫ5  in acetonitrile
(Qabs ϭ 23% [39]) where L is a cyclen derivative {1,3,7,10-tetrakis[N-
vacuum (10Ϫ2 Torr) at 60Ϫ70 °C. A deep orange glassy-solid was (4-phenylacetyl)carbamoylmethyl]-1,4,7,10-tetraazacyclododecane}
obtained (ca. 150 mg) which was ground up. n-Hexane (20 mL) was
added, the mixture was stirred and heated to 45 °C for 30 min, and
the yellow solution was separated. This step was repeated until the
solution became colorless. The bulk of the solvent was separated,
the remaining solvent removed under vacuum (10Ϫ2 Torr), and the
orange waxy solid dried at 70 °C under high vacuum (10Ϫ5 Torr)
for 1 day. It was then re-dissolved in a minimum amount of meth-
anol, and diisopropyl ether was added until cloudiness was ob-
served. This mixture was left in the refrigerator overnight to induce
precipitation. Small and slightly yellow, brilliant glassy crystals
formed after 2 days, which were collected and dried at 100 °C under
high vacuum (10Ϫ5 Torr) for 48 h. The final deliquescent com-
pound (66 mg, 0.042 mmol, 20% yield) was then characterized,
C90H126N6O18 (Mol wt. 1580.02). IR (KBr, cmϪ1): ν˜ ϭ 1639
using the following formula:
Qx/Qr ϭ [Ar(λ)/Ax(λ)]·[Dx/Dr]·[n2x/n2r]
where the subscript r stands for the reference, and x for the sample.
A is the absorbance at the excitation wavelength λ, n the refractive
index (1.344 for solution in CH3CN and 1.333 for the solution
in water), and D the integrated luminescence intensity. The same
excitation wavelength (279 nm) and absorbance (0.27) were used
for the sample and the reference, and a bandpass filter (350 nm or
390 nm) was inserted to eliminate both the Rayleigh diffusion band
and second order spectra. High resolution laser excited lumi-
nescence spectra were recorded using published procedures.[40] Life-
time data for solid samples were measured at different temperatures
under pulsed excitation provided by a FL-3001 dye laser from
Lambda Physik coupled to an excimer XeCl laser EMG-101-MSC
from Lambda Physik. Elemental analyses were performed by Dr
H. Eder (Microchemical Laboratory, University of Geneva).
(νCϭO), 1249 (νCϪOϪC), 1126 (νOϪCH ), 1218 and 1045 (νArϪO), 765
2
(νCϪH). UV/Vis (dried and degassed spectroscopic grade CH3CN):
λ ϭ 270, 278 nm. Diffuse reflectance (5% in MgO): 44840 (CϭO)
and 36630 (phenyl) cmϪ1. ES-MS (10Ϫ4 , CH3CN): m/z ϭ 1580.82
([M ϩ H]ϩ, calcd. 1581.03), 1602.86 ([M ϩ Na]ϩ calcd. 1603.02).
Semi-Empirical Calculations: The optimized structures and energy
minima of the free ligand, and of the 2:1 complex were calculated
by using MM3 augmented and CONFLEX procedures, respec-
tively, from the CAChe Pro 5.02 or 5.04 program package for Win-
dows (Fujitsu Ltd., 2000Ϫ2002). The structure of A6L6 was
further optimized with respect to its enthalpy of formation by MO-
PAC PM3 molecular orbital calculations using the same program
package. The effect of solvent (water) was taken into account using
the COSMO procedure built into CAChe Pro. Experimental data
for both the free ligand and the EuIII complex (IR, NMR, lumi-
nescence) were taken into account for optimizing the calculations.
2
1H NMR (10Ϫ3 , CD3CN, 400 MHz): δ ϭ 6.94 (d, J ϭ 7.44 Hz,
12 H, H8, H10), 6.79 (t, 3J ϭ 7.44 Hz, 6 H, H9), 4.11 (s, 12 H, H3),
4.00 (s, 12 H, H13), 3.72 (broad, 12 H, H1), 3.56 (broad, 12 H, H2),
3
3.35Ϫ3.25 (m, 24 H, H5, H5Ј), 2.14 (s, 4 H, 2H2O), 1.14 (t, J ϭ
3
7.1 Hz, 18 H, H6, H6Ј), 1.07 (t, J ϭ 7.1 Hz, 18 H, H6, H6Ј) ppm.
13C{1H} NMR (1 ϫ 10Ϫ3 , CD3CN, 50 MHz): δ ϭ 168.9 (C4),
155.8 (C12), 135.7 (C7, C11), 129.8 (C9), 124.8 (C8, C10), 73.1, 71.3
(C1, C2), 71.0 (C3), 41.9, 40.5 (C5, C5Ј), 31.0 (C13), 14.7, 13.3 (C6,
C6Ј) ppm.
Synthesis of the Complex: Eu(CF3SO3)3·6H2O (21 mg, 0.026 mmol)
was dissolved in dry MeCN (2 mL) at 36 °C with stirring under
N2, and A6L6 (20.5 mg, 0.013 mmol) in dry MeCN (4 mL) was
added dropwise. The resultant mixture was stirred for 1 h, cooled to
room temp., and stirred for a further 4 h. The solvent was partially
evaporated under vacuum, and diisopropyl ether was added drop-
wise until cloudiness appeared. The mixture was left overnight at
Ϫ20 °C. The precipitate was centrifuged, washed with diisopropyl
ether (2 mL), and dried under vacuum (10Ϫ2 Torr) at room temp.,
and then at 80 °C for 24 h under high vacuum (10Ϫ5 Torr), yielding
Acknowledgments
This work was supported through grants from the Swiss National
´
Science Foundation, and by Conacyt (Mexico), project Nr. 36689-
´
E. We thank Veronique Foiret for her help in gathering the lumi-
nescence data.
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