Formation of novel dinuclear lanthanide luminescent samarium(III), europium(III), and terbium(III) triple-stranded helicates from a C 2-symmetrical pyridine-2,6-dicarboxamide-based 1,3-xylenediyl-linked ligand in MeCN

Article abstract of DOI:10.1002/hlca.200900213

The synthesis of the C₂-symmetrical ligand 1 consisting of two naphthalene units connected to two pyridine-2,6-dicarboxamide moieties linked by a xylene spacer and the formation of Ln^III-based (Ln = Sm, Eu, Tb, and Lu) dimetallic hel

Full text of DOI:10.1002/hlca.200900213

Helvetica Chimica Acta – Vol. 92 (2009)
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Formation of Novel Dinuclear Lanthanide Luminescent Samarium(III),
Europium(III), and Terbium(III) Triple-Stranded Helicates from a
C2-Symmetrical Pyridine-2,6-dicarboxamide-Based 1,3-Xylenediyl-Linked
Ligand in MeCN
by Steve Combya), Floriana Stomeoa), Colin P. McCoyb), and Thorfinnur Gunnlaugsson*a)
a) School of Chemistry, Centre for Synthesis and Chemical Biology, Trinity College Dublin, IE-Dublin 2
(phone: þ 35318963459; fax: þ 35316712826; e-mail: gunnlaut@tcd.ie)
b) School of Pharmacy, Queenꢀs University of Belfast, 97 Lisburn Road, UK-Belfast, BT97BL
Dedicated to Professor Jean-Claude Bꢀnzli on the occasion of his 65th birthday
The synthesis of the C2-symmetrical ligand 1 consisting of two naphthalene units connected to two
pyridine-2,6-dicarboxamide moieties linked by a xylene spacer and the formation of LnIII-based (Ln ¼
Sm, Eu, Tb, and Lu) dimetallic helicates [Ln2 · 13] in MeCN by means of a metal-directed synthesis is
described. By analyzing the metal-induced changes in the absorption and the fluorescence of 1, the
formation of the helicates, and the presence of a second species [Ln2 · 12] was confirmed by nonlinear-
regression analysis. While significant changes were observed in the photophysical properties of 1, the
most dramatic changes were observed in the metal-centred lanthanide emissions, upon excitation of the
naphthalene antennae. From the changes in the lanthanide emission, we were able to demonstrate that
these helicates were formed in high yields (ca. 90% after the addition of 0.6 equiv. of LnIII), with high
binding constants, which matched well with that determined from the changes in the absorption spectra.
The formation of the LuIII helicate, [Lu2 · 13], was also investigated for comparison purposes, as we were
unable to obtain accurate binding constants from the changes in the fluorescence emission upon
formation of [Sm2 · 13], [Eu2 · 13], and [Tb2 · 13].
Introduction. – The use of metal-directed synthesis in the construction of
complexed supramolecular structures has recently become a very active research area
[1 – 4]. To date, such structures have been mostly developed by using various transition-
metal ions and organic ligands possessing defined coordination motives that match or
complement the coordination requirements of the given transition-metal ion [5 – 7].
This strategy has resulted in the formation of many elegant examples of complex
supramolecular structures, some of which possess the inherent physical properties of
the coordinating ligands, in addition to those associated with the d-metal ions alone
[8][9]. In contrast, the use of the lanthanide-metal ions in such supramolecular
synthesis has been much less investigated [10 – 12]. However, as many of these ions
possess both unique photophysical and magnetic properties, their use in metal-directed
synthesis has grown, resulting in the publication of some elegant examples, notably in
the area of lanthanide-directed helicate synthesis [13][14], as well as in the formation
of lanthanide complexes possessing high symmetry [15]. This work has, in particular,
been spearheaded by the pioneering work of Bꢀnzli [2][16], who has developed
varieties of examples of lanthanide-based helicates, and mixed f-d-metal-ion-based
ꢁ 2009 Verlag Helvetica Chimica Acta AG, Zꢂrich

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helicates (some in collaboration with the group of Piguet), and studied their
photophysical properties in solution and in the solid state [17][18]. Inspired by this
work, we have recently embarked on the developments of functional lanthanide and
mixed f-d-metal-ions self-assembly [19 – 22], as well as on using the f-metal ions for the
formation of chiral luminescent self-assemblies, or ꢃcoordination bundlesꢀ (sliotars),
that possess a high degree of symmetry and display strong lanthanide luminescence
upon excitation of the ligand antennae [23]. In this article, we present some recent
results of our investigation into the development of novel structurally ꢃsimpleꢀ C2-
symmetry-based bis[pyridine-2,6-dicarboxamide] ligands, such as 1, possessing two
chiral naphthalene moieties connected to two pyridine-2,6-dicarboxamide ꢃwingsꢀ
linked by a 1,3-xylene spacer, that can be employed in the synthesis of dinuclear
lanthanide-based helicates [24]. The formation of the three dimetallic triple-stranded
helicates [Sm2 · 13], [Eu2 · 13], and [Tb2 · 13] from 1 was analysed in MeCN solution by
carrying out various spectroscopic titrations where the changes in the absorption and
the fluorescence emission of 1 was monitored as well as the evolution of their
respective lanthanide-centred emissions. For comparison, we also studied the
formation of [Lu2 · 13] in solution; however, unlike those above, this helicate does
not give rise to metal-centred luminescence. Our results clearly show that all these
lanthanide ions form helicates in MeCN solution, and that [Sm2 · 13], [Eu2 · 13], and
[Tb2 · 13] all give rise to characteristic sensitized lanthanide emissions upon excitation of
the naphthalene antenna. We also demonstrate by fitting the various spectroscopic
changes observed, by means of nonlinear regression analysis, that the desired metal/
ligand 2 :3 stoichiometry is the dominant species in solution for all of these helicates,
the formation of dinuclear species, e.g., [Ln2 · 12], only occurring at higher ion
concentrations.
Results and Discussion. – 1. Synthesis of Ligand 1. The synthesis of 1 was achieved
in a few steps starting from the commercially available pyridine-2,6-dicarboxylic acid,
by first forming the corresponding pyridine-2,6-dicarboxylic acid monobenzyl ester [25]
which was then treated with (1S)-1-(1-naphthyl)ethylamine in anhydrous THF, in the
presence of HOBt, EDCI · HCl, and Et3N (Scheme). This gave 2 in 81% yield after
aqueous acid/base workup. Ester hydrolysis of 2 to give 3 was achieved in 98% yield in
EtOH with 10% Pd/C catalyst (10% loading) under 3 atm pressure of H2. Compound 3
was then subjected to the same peptide coupling reaction as employed for 2, which gave
the desired product 1 after workup in 25% yield as an off-white solid (see Exper. Part).
Compound 1 was characterized by conventional methods (see Exper. Part). The high-
resolution ESI-MS showed the presence of a species at m/z 763, which matched the
1
calculated isotopic distribution pattern for [M þ Na]þ, while the H-NMR (600 MHz,
CDCl3; Fig. 1) clearly demonstrated the presence of a single species with C2 symmetry,
where the NꢀH resonances appeared as d at d 8.70 and 8.53 – 8.44, respectively, and the
H-atoms in a-position at d 4.20 and 3.72 as a dd. The structure was fully assigned by
1
means of H,1H-, 13C,1H-, and 14N,1H-COSY NMR experiments (see Exper. Part).
2. Absorption and Fluorescence Spectroscopic Investigation of 1 in the Presence of
Lanthanide Ions. The absorption and the fluorescence-emission spectra of 1 were first
recorded in MeCN solution. The absorption spectra consisted of two main bands, a high
energy one at l 223 nm and a longer-wavelength band with lmax at 281 nm (e ¼

Helvetica Chimica Acta – Vol. 92 (2009)
Scheme. Synthesis of Ligand 1
2463
Fig. 1. Partial 1H-NMR spectrum (600 MHz, CDCl3) of 1
21055 mꢀ1 cmꢀ1), with a small shoulder at 293 nm. Excitation at the 281 nm transition
gave rise to a fluorescence-emission spectrum consisting of two main bands at 333 and
427 nm. We next monitored the changes in the ground and the excited states upon
titrating 1 (1.0 · 10ꢀ5 m) with SmIII, EuIII, and TbIII (as their corresponding Ln(SO3CF3)3
stock solutions). The overall changes observed in the absorption spectra of 1 for the
titration with EuIII are shown in Fig. 2, which demonstrates significant changes upon
binding of EuIII to 1. The long-wavelength absorption band was particularly affected
(see inset in Fig. 2), where the absorption was bathochromically shifted and a new

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Helvetica Chimica Acta – Vol. 92 (2009)
shoulder was formed at longer wavelength, with a pseudo-isosbestic point at 296 nm. A
clearer isosbestic point was also observed at 265 nm. These changes were analyzed by
fitting the global changes in Fig. 2 by means of the nonlinear regression analysis
program SPECFIT (Fig. 3,a). The speciation-distribution diagram resulting from these
fits showed the presence of three major species in solution (Fig. 3,b). Of these, within
the addition of 0.2 ! 0.8 equiv. of EuIII, the most predominant one was a ligand/metal
3 :2 species, being formed in over 80% at 0.67 equiv. of EuIII. The stoichiometry of this
species corresponds to the formation of a dimetallic triple-stranded helicate, where
Fig. 2. Changes in the absorption spectra of 1 (1.0 · 10ꢀ5 m) upon titrating with Eu(SO3CF3)3 (0 ! 4
equiv.) in MeCN at room temperature. Inset: Changes in the long-wavelength absorption band.
Fig. 3. a) Experimental binding isotherms for the UV/VIS titration of 1 (1.0 · 10ꢀ5 m) with Eu(SO3CF3)3
(0 ! 4 equiv.) in MeCN at room temperature and their corresponding fit by means of SPECFIT (—). b)
Speciation-distribution diagram obtained from the fit.

Helvetica Chimica Acta – Vol. 92 (2009)
2465
each of the EuIII ion would be located within a nine-coordinated environment, bound to
the two carboxamide groups and the central pyridine N-atom of each ligand giving
[Eu2 · 13]. However, at higher concentration, the formation of the ligand/metal 2 :2
species [Eu2 · 12] became dominant in solution, being formed in over 90% after the
addition of 3 equiv. of EuIII. Similarly, the titrations of 1 with SmIII and TbIII both gave
rise to identical changes in the absorption spectra as seen for EuIII in Fig. 2, from which
speciation-distribution diagrams showed again the presence of two main metallic
species, corresponding to the formation of [Ln2 · 13] and [Ln2 · 12], respectively.
From the above results, we determined the stability constants (expressed as
log bM/L) for the formation of all of the [Ln2 · 13] and [Ln2 · 12] complexes. In the case of
EuIII, the [Eu2 · 13] helicate was formed with a high binding constant log b2 :3 ¼ 27.3
(ꢁ0.3), while the [Eu2 · 12] complex was formed with a log b2 :2 ¼ 20.5 (ꢁ0.2). These are
of a magnitude comparable to those reported by Bꢀnzli and co-workers for triple-
stranded helicates, based on the use of bis-benzimidazole ligands [16]. In a similar
manner, binding constants for the titrations of SmIII and TbIII were determined and
found to be of similar magnitude to that seen for EuIII (Table).
Table. Binding Constants Obtained by Fitting Various Spectroscopic Data
Absorption
Luminescencea)
Species
log bM/L
% Species for M/L 2 : 3
log bM/L
% Species for M/L 2 : 3
SmIII
EuIII
TbIII
LuIII
log b2 : 2
log b2 : 3
log b2 : 2
log b2 : 3
log b2 : 2
log b2 : 3
log b2 : 2
log b2 : 3
log b2 : 1
20.2 ꢁ 0.2
26.4 ꢁ 0.3
20.5 ꢁ 0.2
27.3 ꢁ 0.3
20.3 ꢁ 0.2
26.8 ꢁ 0.3
19.0 ꢁ 0.3
25.8 ꢁ 0.4
–
23
65
13
80
17
73
10
81
–
21.8 ꢁ 0.2
28.3 ꢁ 0.3
21.7 ꢁ 0.2
28.7 ꢁ 0.3
19
72
10
84
b
b
)
)
)
b
b
)
19.8 ꢁ 0.3
27.4 ꢁ 0.4
12.7 ꢁ 0.2
4
92
< 1
a) Obtained by fitting the changes in the metal-centred emission, except for LuIII where the fluorescence
b
emission was used. ) The evolution of the TbIII luminescence could not be fitted satisfyingly.
Having investigated the changes in the absorption spectra of 1, we next investigated
the changes in the fluorescence-emission spectra of 1 upon titration with SmIII, EuIII,
and TbIII. The overall changes observed for the titration with EuIII are shown in Fig. 4,
and clearly establish significant changes in the fluorescence upon formation of the EuIII
helicates. The long-wavelength emission was particularly affected, being almost 80%
quenched after the addition of ca. 0.7 equiv. of EuIII, which corresponds to the expected
metal/ligand 2 :3 stoichiometry and, hence, to the formation of the [Eu2 · 13] helicate.
However, unlike that seen in the absorption spectra above, we were unable to obtain
reliable binding constants for the formation of both the [Eu2 · 13] and [Eu2 · 12]
complexes upon fitting these fluorescence changes with SPECFIT. In a similar manner,
the fitting of the changes observed in the fluorescence emission of 1 upon titration with
SmIII and TbIII only resulted in binding constants carrying an unacceptable error.

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Fig. 4. a) Changes in the fluorescence emission of 1 (1.0 · 10ꢀ5 m) upon titrating with Eu(SO3CF3)3 (0 ! 4
equiv.) in MeCN at room temperature. b) Experimental binding isotherms of the fluorescence-emission
intensity at l 333 and 428 nm vs. the equiv. of Eu(SO3CF3)3.
Because of this, we also carried out a titration of 1 with LuIII, which should result in
the formation of helicates displaying no metal-centred emission. Hence, the formation
of either the [Lu2 · 13] or [Lu2 · 12] complex would not be expected to perturb the singlet
excited state of 1 in the manner to that observed above for SmIII, EuIII, or TbIII. The
results from this titrations are shown in Fig. 5, and demonstrate that the fluorescence of
1 was dramatically affected (Fig. 5,a), and unlike that seen above, where the lmax at
425 nm was quenched, the concomitant formation of a new red-shifted band with lmax at
ca. 513 nm, was observed. Furthermore, from the changes in the emission spectra at
various wavelengths (Fig. 5,b), we were able to verify the successful formation of [Lu2 ·
13] in solution. Moreover, fitting the changes observed in Fig. 5,a, with SPECFIT gave
log b2 :3 ¼ 27.4 (ꢁ0.4) and log b2 :2 ¼ 19.8 (ꢁ0.3) for [Lu2 · 13] and [Lu2 · 12], respectively,
which are in good agreement with that observed from the ground-state titration of EuIII
above. In contrast, the changes in the absorption spectra were identical to those seen
for EuIII in Fig. 2, and analysis of the changes with SPECFIT indeed gave similar
binding constants to those obtained from the absorption spectra of 1 upon titration with
Fig. 5. a) Changes in the fluorescence emission of 1 (1.0 · 10ꢀ5 m) upon titrating with Lu(SO3CF3)3 (0 ! 4
equiv.) in MeCN at room temperature. b) Experimental binding isotherms of the fluorescence-emission
intensity at l 334, 428, and 514 nm vs. the equiv. of Lu(SO3CF3)3

Helvetica Chimica Acta – Vol. 92 (2009)
2467
SmIII, EuIII, and TbIII. These results clearly demonstrate that even in the absence of an
energy acceptor such as SmIII, EuIII, and TbIII, the fluorescence emission is dramatically
affected, giving rise to the formation of the long-wavelength emission, which we assign
to the formation of the helical structure in solution. Hence we next evaluated the
evolution of the lanthanide emission in solution upon formation of [Ln2 · 13] and [Ln2 ·
12] complexes.
3. Observing the Formation of [Ln2 · 13] and [Ln2 · 12] by Monitoring the
Lanthanide-Centred Emission. In a similar manner to that described above, the
evolution in the delayed lanthanide luminescence arising from the SmIII, EuIII, and TbIII
ions was monitored upon formation of these helical species with 1, upon exciting the
naphthalene antennae at 281 nm. The changes observed for EuIII are shown in Fig. 6,a,
monitoring the characteristic emission bands appearing at 580, 595, 616, 650, and
Fig. 6. a) Changes in the EuIII phosphorescence spectra upon titrating 1 (1.0 · 10ꢀ5 m) with Eu(SO3CF3)3
(0 ! 4 equiv.) in MeCN at room temperature. b) Experimental binding isotherms of the EuIII phosphores-
7
7
7
cence-emission intensity at l 595 (5D0 ! F1), 616 (5D0 ! F2), and 696 nm (5D0 ! F4) vs. the equiv. of
Eu(SO3CF3)3. c) Formation of [Eu2 · 13] and red emission of its solution under a UV/VIS lamp.

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Helvetica Chimica Acta – Vol. 92 (2009)
696 nm, respectively, and assigned to the deactivation of the 5D0 excited state to the 7FJ
7
ground states (J ¼ 0, 1, 2, 3 and 4). Of these, the 5D0 ! F1, 7F2, and 7F4 were the most
intense. Analysis of their emission intensity as a function of the equiv. of EuIII added
(Fig. 6,b) showed that the EuIII emission evolves rapidly up to the addition of 0.6 – 0.7
equiv., after which the emission decreases until it reaches a plateau at ca. 1.2 equiv. of
EuIII. This suggests the initial formation of the desired [Eu2 · 13] helicate, after which the
[Eu2 · 12] becomes the dominant species in solution. The helicate [Eu2 · 13] was also
synthesized in situ by mixing the correct stoichiometry of 1 with EuIII ([[Eu2 · 13]] ¼
1.33 · 10ꢀ5 m), and the changes in the EuIII emission were monitored over 5 h. The
results from this experiment demonstrated the immediate formation of the helicate in
solution (within seconds) and that the EuIII emission was unchanged over this time
period, clearly indicating the kinetic stability of the [Eu2 · 13] species in solution.
Moreover, this solution displayed the characteristic red emission under a UV/VIS lamp,
as demonstrated in Fig. 6,c. We also measured the decay of the 5D0 excited state upon
excitation at 281 nm, at different ligand-to-metal ratios and fitting the data with both
mono- and bi-exponential functions. The results demonstrate that, up to 0.6 equiv. of
EuIII, the decay was best fitted to mono-exponential decay, with a lifetime of 1.57 ms
measured for the [Eu2 · 13] helicate. At higher concentrations of EuIII, the decay was,
however, best fitted to the bi-exponential function, which is in accordance to our
observation that a second minor species, [Eu2 · 12], is formed in solution (which
becomes a dominant contributor at higher concentrations).
The overall results from the titrations of 1 with SmIII and TbIII are shown in Fig. 7,a
4
and b, respectively, and also demonstrate the effective sensitization of their G5/2 and
5D4 excited states, respectively, and the evolution of a line-like emission bands at long
wavelengths upon excitation of the antennae. Analysis of the changes in the SmIII
6
transitions (4G5/2 ! H5/2, 6H7/2, 6H9/2, and 6H11/2) are shown in Fig. 8,a, and demonstrate
the initial formation of triple-stranded helicate [Sm2 · 13] followed by the formation of
the [Sm2 · 12] complex at higher concentration. In contrast to these results, the changes
in the TbIII emission were strikingly different (Fig. 8,b) which demonstrates that only
very minor changes were observed up to the addition of 0.6 equiv. of TbIII, after which
Fig. 7. a) Changes in the SmIII phosphorescence spectra upon titrating 1 (1.0 · 10ꢀ5 m) with Sm(SO3CF3)3
(0 ! 4 equiv.) in MeCN at room temperature. b) Changes in the TbIII phosphorescence spectra upon
titrating 1 (1.0 · 10ꢀ5 m) with Tb(SO3CF3)3 (0 ! 12 equiv.) in MeCN at room temperature.

Helvetica Chimica Acta – Vol. 92 (2009)
2469
Fig. 8. a) Experimental binding isotherms of the SmIII phosphorescence-emission intensity at l 564
6
6
6
6
(4G5/2 ! H5/2), 604 (4G5/2 ! H7/2), 645 (4G5/2 ! H9/2), and 710 nm (4G5/2 ! H11/2) vs. the equiv. of
Sm(SO3CF3)3. b) Experimental binding isotherms of the TbIII phosphorescence-emission intensity at l 490
7
7
7
7
(5D4 ! F6), 545 (5D4 ! F5), 585 (5D4 ! F4), and 622 nm (5D4 ! F3) vs. the equiv. of Tb(SO3CF3)3.
the TbIII emission intensity rapidly increased between 0.6 ! 2.5 – 3 equiv. of TbIII. When
these measurements were repeated with samples that were equilibrated over 24 h, the
same behaviour was observed. Hence, this difference is not due to slow kinetics.
Furthermore, the formation of the desired dimetallic TbIII helicate was ascertained by
monitoring the changes observed in the UV/VIS absorption spectra; consequently, we
attribute these changes to a most likely occurring energy back transfer from the metal
to the ligand. We are in the process of investigating this phenomenon further.
The changes in the EuIII and the SmIII emissions shown above were fitted with
SPECFIT. The results for EuIII (Fig. 9,a) demonstrate that an excellent fit was
observed, from which the binding constants for the formation of the [Eu2 · 13] helicate
and [Eu2 · 12] were determined as log b2 :3 ¼ 28.7 (ꢁ0.3) and log b2 :2 ¼ 21.7 (ꢁ0.2),
respectively, which are in good agreement with that determined from the changes in the
absorption spectra. These values are summarized in the Table. From these, the
speciation-distribution diagram was deduced (Fig. 9,b), which again matched that
Fig. 9. a) Experimental binding isotherms for the changes in the EuIII phosphorescence-emission spectra
upon titrating 1 (1.0 · 10ꢀ5 m) with Eu(SO3CF3)3 (0 ! 4 equiv.) in MeCN at room temperature and their
corresponding fit by means of SPECFIT (—). b) Speciation-distribution diagram obtained from the fit.

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Helvetica Chimica Acta – Vol. 92 (2009)
derived from the absorption changes (Fig. 3,b). In a similar manner, the fitting of the
changes observed for SmIII also resulted in an excellent fit, from which binding
constants log b2 :3 ¼ 28.3 (ꢁ0.3) and log b2 :2 ¼ 21.8 (ꢁ0.2) for the formation of the
[Sm2 · 13] helicate and [Sm2 · 12], respectively, were determined. These are in excellent
agreement with that found for EuIII. From these values, the speciation-distribution
diagram was constructed, which showed a comparable trend to that shown in Fig. 9,b.
Overall, these results clearly demonstrate that these helicates are formed in high yield
and with high binding constants.
Conclusions. – The synthesis of a C2-symmetrical ligand 1 was undertaken and
achieved in few and high-yielding steps. We showed that upon titrating 1 with various
lanthanide ions, significant changes were observed in the absorption and the
fluorescence-emission spectra of 1. Analysis of the changes in the absorption spectrum
revealed that significant changes occurred upon the addition of 0 ! 0.7 equiv. of LnIII
ions, suggesting the formation of a metal/ligand 2 :3 stoichiometry between LnIII and 1.
Further changes were observed up to 1 equiv. of LnIII. Analysis of these global changes
with the nonlinear regression analysis program SPECFIT confirmed the formation of
dinuclear triple-stranded helicates for all LnIII (Ln ¼ Sm, Eu, Tb, and Lu) with average
binding constants log b ꢂ 27, and the formation of a 2 :2 species at higher concen-
trations of these ions with log b ꢂ 20 (see details in the Table). The changes in the
fluorescence emission also confirmed the formation of two species in solution;
however, we were unable to fit the changes induced by SmIII, EuIII, and TbIII, all of
which can give rise to metal-centred luminescence. However, we were able to analyse
the fluorescence changes seen for LuIII. While these changes were somewhat different
to that observed for the above ions, the analysis resulted in a good fit to these changes,
which confirmed the formation of [Lu2 · 13] and [Lu2 · 12] in solution. In a similar
manner, the delayed lanthanide emission was also monitored. Only minor changes
were seen in the TbIII emission up to the addition of 0.7 equiv. of TbIII, after which the
emission gradually increased up to the addition of 2.5 – 3 equiv. Consequently, we were
unable to determine from these data the binding constants and then the speciation
distribution in solution for the formation of [Tb2 · 13] and [Tb2 · 12]. However, in the
case of SmIII and EuIII, their respective lanthanide emission was ꢃswitched onꢀ upon
formation of [Sm2 · 13] and [Eu2 · 13], as well as [Sm2 · 12] and [Eu2 · 12], respectively,
with binding constants that were determined to be of similar magnitude to that
determined from the changes observed in the UV/VIS absorption spectra. In summary,
we have developed novel lanthanide luminescent dimetallic triple-stranded helicates
from a simple ligand 1, and demonstrated that these helicates are formed in solution in
high yield, while possessing high binding constants. We are currently in the process of
developing other related systems and conjugates for the formation of novel supra-
molecular architectures.
We thank the SFI (Science Foundation Ireland), HEA (Higher Education Authority; for PRTLI
Cycle 3 CSCB funding to T. G. and a scholarship to F. S. ), IRCSET (The Irish Research Council for
Science, Engineering & Technology; for a postdoctoral fellowship to S. C.), Queenꢀs University of Belfast
and Trinity College Dublin for financial support, and Dr. John E. OꢁBrian (Trinity College Dublin) for
performing NMR measurements.

Helvetica Chimica Acta – Vol. 92 (2009)
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Experimental Part
General. All chemicals were purchased from commercial sources and unless specified used without
further purification. Stock solns. of lanthanides at a concentration around 5 · 10ꢀ3 m were prepared just
before use in HPLC-grade MeCN from the corresponding Ln(CF3SO3)3 salts (Ln ¼ Sm, Eu, Tb, and Lu).
The lanthanide triflate salts were dried under vacuum over P2O5 prior to use. IR Spectra: Perkin-Elmer-
Spectrum-One FT-IR spectrometer equipped with a Universal-ATR sampling accessory; solid samples
were recorded directly as neat samples; in cmꢀ1. NMR Spectra: Bruker-DPX-400-Avance spectrometer
(400.13 (1H) and 100.6 MHz (13C)), or Bruker-AV-600 spectrometer (600.13 (1H) and 150.2 MHz (13C));
in commercially available deuterated solvents; d in ppm rel. to SiMe4 (¼0 ppm) referenced rel. to the
internal solvent signals, J in Hz; data were processed with Bruker Win-NMR 5.0 and Topspin 2.1
softwares. ESI-MS: Micromass-LCT spectrometer, running MassLynx NT V 3.4 on a Waters-600
controller connected to a 996 photodiode array detector with HPLC-grade MeCN as carrier solvents; all
experiments in positive-ion mode; accurate molecular masses by a peak-matching method, with leucine
enkephaline (H-Tyr-Gly-Gly-Phe-Leu-OH) as the standard internal reference (m/z 556.2771), and
reported within 5 ppm. Elemental analyses were carried out at the Microanalytical Laboratory, School of
Chemistry and Chemical Biology, University College Dublin.
UV/VIS-Absorption and Emission Data. Absorption spectra were measured in 1-cm quartz cuvettes
with a Varian-Cary-50 spectrophotometer. Baseline correction was applied for all spectra. Emission
spectra and lifetimes were measured with a Varian-Cary-Eclipse luminescence spectrometer. In both
experiments, the temp. was kept constant at 298 K throughout the measurements by using a thermostated
unit block. In a typical spectrophotometric titration, 2.7 ml of ligand 1 at a concentration of 1.0 · 10ꢀ5
m
was titrated with a ca. 5 · 10ꢀ4 m LnIII soln. (Ln ¼ Sm, Eu, Tb, or Lu). Treatment of the resulting data with
the nonlinear least-squares regression-analysis program SPECFITꢄ then allowed the determination of
the conditional stability constants for the formation of the different metal complexes.
6-{{[(1S)-1-(Naphthalen-1-yl)ethyl]amino}carbonyl}pyridine-2-carboxylic Acid Benzyl Ester (2). To
a stirred soln. of (1S)-1-(naphthalen-1-yl)ethylamine (0.630 ml, 3.9 mmol) in anh. THF (30 ml), HOBt
(¼1-hydroxy-1H-benzotriazole; 0.527 g, 3.9 mmol) and pyridine-2,6-dicarboxylic acid 2-benzyl ester
(1 g, 3.9 mmol) were added. This soln. was then stirred for further 30 min at 08 under Ar. To this soln.,
EDCI · HCl (¼1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide hydrochloride; 0.785 g, 4.1 mmol) and
Et3N (0.571 ml, 4.1 mmol) were added, and the mixture was stirred at 08 for 30 min and then at r.t. for
18 h. The insoluble residue was filtered off and the filtrate concentrated. CH2Cl2 was added to the crude
oil, the soln. washed twice with 1m HCl, sat. NaHCO3 soln., and H2O, dried, and concentrated, and the
crude product, if necessary, purified by flash column chromatography (neutral silica gel): 2 (1.26 g, 79%).
Pale yellow oil. IR (NaCl): 3381, 1718, 1651, 1588, 1305, 1155, 1077, 966, 722. 1H-NMR (400 MHz, CDCl3,
298.2 K): 8.51 (d, J ¼ 8.5, NH); 8.44 (d, J ¼ 7.5, 1 pyr-H); 8.23 (d, J ¼ 7.5, 1 nap-H, 1 pyr-H); 8.00 (t, J ¼
7.5, 1 pyr-H); 7.89 (d, J ¼ 8.5, 1 nap-H); 7.83 (d, J ¼ 8.5, 1 nap-H); 7.64 (d, J ¼ 7.0, 1 nap-H); 7.57 – 7.38 (m,
3 nap-H, Ph); 6.22 – 6.15 (m, MeCH); 5.42 (s, CH2); 1.82 (d, J ¼ 7.0, Me). 13C-NMR (100 MHz, CDCl3,
298.2 K): 163.7; 161.9; 149.7; 146.0; 138.0; 137.9; 134.9; 133.5; 130.6; 128.4; 128.2; 128.0; 127.8; 127.7;
126.8; 126.0; 125.3; 125.1; 124.9; 122.9; 122.2; 67.0; 44.5; 20.9. HR-ESI-MS: 433.1549 ([M þ Na]þ,
C26H22N2NaOþ3 ; calc. 433.1528).
6-{{[(1S)-1-(Naphthalen-1-yl)ethyl]amino}carbonyl}pyridine-2-carboxylic Acid (3). The free acid 3,
was obtained by hydrogenation of 2 (1.22 g) in MeOH in the presence of 10% Pd/C (0.1 equiv.) under
3 atm of H2. The resulting mixture was passed through a Celite filter and the filtrate concentrated: 3
(0.85 g, 90%). Yellow solid. M.p. 116 – 1208. IR (neat): 3282, 3049, 2977, 2936, 1751, 1651, 1524, 1452,
1345, 1255, 1172, 1118, 1076, 1000, 921, 846, 799, 776, 745, 681. 1H-NMR (400 MHz, CDCl3, 298.2 K): 8.49
(d, J ¼ 8.5, 1 NH); 8.44 (d, J ¼ 7.5, 1 pyr-H); 8.23 (d, J ¼ 8.5, 1 nap-H); 8.19 (d, J ¼ 7.5, 1 pyr-H); 7.99 (t,
J ¼ 7.5, 1 pyr-H); 7.88 (d, J ¼ 8.0, 1 nap-H); 7.82 (d, J ¼ 8.0, 1 nap-H); 7.65 (d, J ¼ 7.0, 1 nap-H); 7.56 (t, J ¼
7.0, 1 nap-H); 7.50 – 7.45 (m, 2 nap-H); 6.21 – 6.15 (m, MeCH); 1.83 (d, J ¼ 7.0, Me). 13C-NMR (100 MHz,
CDCl3, 298.2 K): 163.3; 161.5; 148.8; 144.5; 139.1; 137.2; 133.4; 130.6; 128.4; 128.1; 126.4; 126.2;
126.2; 125.4; 124.8; 122.7; 122.4; 44.6; 20.3. HR-ESI-MS: 343.1061 ([M þ Na]þ, C19H16N2NaO3þ ; calc.
343.1059). Anal. calc. for C19H16N2O3 · 0.5 H2O: C 69.29, H 5.20, N 8.51; found: C 69.23, H 5.13, N
8.34.

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Helvetica Chimica Acta – Vol. 92 (2009)
N2,N2’-[1,3-Phenylenebis(methylene)]bis[N6-[(1S)-1-(naphthalen-1-yl)ethyl]pyridine-2,6-dicarbox-
amide (1). To a mixture of 1,3-xylenediamine (¼ benzene-1,3-methanamine; 62 ml, 0.47 mmol) and 3
(0.3 g, 0.94 mmol), HOBt (0.1324 g, 0.98 mmol) in THF (ca. 25 ml), EDCI · HCl (0.187 g, 0.98 mmol),
Et3N (0.136 ml, 0.98 mmol), and DMAP (N,N-dimethylpyridin-4-amine; 68.4 mg, 0.56 mmol) were
added after cooling the mixture at 08. The mixture was stirred for 24 h and then filtered through a Celite
pad. The filtrate was concentrated, the crude oil dissolved in CH2Cl2, the soln. washed three times with
aq. 1m HCl, sat. NaHCO3 soln., and H2O, dried (MgSO4), and concentrated, and the residue dried under
vacuum to give a yellow oil (186 mg, 54% yield). This oil was dissolved in few ml of CH2Cl2/CHCl3 and
added dropwise to swirling 100 ml of dry Et2O. The off-white precipitate that was formed was collected
by filtration through a Hirsch funnel: 1 (87 mg, 0.119 mmol, 25%). Yellow solid. IR (neat): 3300, 3053,
2976, 2933, 2084, 1658, 1598, 1521, 1444, 1411, 1398, 1373, 1339, 1309, 1238, 1174, 1118, 1076, 1000, 920,
1
844, 800, 777, 750, 676. H-NMR (600.1 MHz, CDCl3, 298 K): 8.70 (d, J ¼ 8.8, 2 NH); 8.53 – 8.44 (m,
2 NH); 8.26 (d, J ¼ 7.3, 2 pyr-H); 8.12 (d, J ¼ 8.8, 2 nap-H); 8.10 (d, J ¼ 7.3, 2 pyr-H); 7.81 (t, J ¼ 7.7, 2 pyr-
H); 7.64 (d, J ¼ 8.0, 2 nap-H); 7.49 (d, J ¼ 8.1, 2 nap-H); 7.45 (t, J ¼ 7.7, 2 nap-H); 7.41 – 7.34 (m, 4 nap-H);
7.13 (t, J ¼ 7.7, 2 nap-H); 6.88 (t, J ¼ 7.7, 1 Ar-H); 6.08 – 6.04 (m, MeCH); 4.18 – 4.22 (m, CH2); 3.72 (m,
CH2); 1.57 (d, J ¼ 6.6, 2 Me). 13C-NMR (150.9 MHz, CDCl3, 298 K): 164.3; 163.0; 149.2; 148.7; 139.0;
138.6; 138.4; 134.2; 131.6; 129.1; 128.5; 126.9; 126.6; 126.1; 125.7; 125.3; 125.2; 123.8; 123.4; 45.3; 43.2;
20.7. HR-ESI-MS: 763.2815 ([M þ Na]þ, C46H40N6NaOþ4 ; calc. 763.3009). Anal. calc. for C46H40N6O4 ·
0.27 CHCl3: C 71.89, H 5.25, N 10.87; found: C 71.89, H 5.35, N 10.84.
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Received June 4, 2009