Lanthanide triple helical complexes with a chiral bis(benzimidazole)pyridine derivative

Article abstract of DOI:10.1002/1099-0682(200212)2002:12<3101::AID-EJIC3101>3.0.CO;2-9

The ligand neopentyl 2,6-bis[(1-methylbenzimidazol-2-yl)]-pyridine-4-carboxylate (L¹²) has been synthesised to test the effect of the chiral neopentyl ester group in the 4-position of the pyridine ring on (i) the helical wrapping, (ii) the diastereomeric induction and (iii) the thermodynamic and photophysical properties of the [Ln(L¹²)₃]³⁺ complexes. The crystal structure of ligand L¹² shows the expected trans-trans conformation of the tridentate binding unit. The ligand forms stable 1:3 complexes in anhydrous acetonitrile (logβ₃ in the range 17.3-19.0, logK₃ in the range 2.9-4.6). The triple helical structure in solution is responsible for the four times larger specific rotary dispersion measured in the complexes. Circularly polarised luminescence of the Eu triple helical complex displays a weak effect, suggesting a small diastereomeric excess in solution. Ligand L¹² appears to favour a 37ππ*-to-Ln energy transfer process for Eu, but temperature-dependent nonradiative processes lead to a very small quantum yield. High-resolution luminescence spectra indicate that the Eu complex has a distorted D₃ local symmetry at the metal ion site. Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002).

Full text of DOI:10.1002/1099-0682(200212)2002:12<3101::AID-EJIC3101>3.0.CO;2-9

FULL PAPER
Lanthanide Triple Helical Complexes with a Chiral Bis(benzimidazole)pyridine
Derivative
Gilles Muller,[a,b] James P. Riehl, Kurt J. Schenk, G e´ rard Hopfgartner, Claude Piguet,
[b]
[c]
[d]
[e]
and Jean-Claude G. Bünzli*[
a]
Keywords: Lanthanides / Tridentate ligands / Chirality / Luminescence / Stability constants
The ligand neopentyl 2,6-bis[(1-methylbenzimidazol-2-yl)]-
larger specific rotary dispersion measured in the complexes.
1
2
pyridine-4-carboxylate (L ) has been synthesised to test the
effect of the chiral neopentyl ester group in the 4-position of
the pyridine ring on (i) the helical wrapping, (ii) the diastere-
omeric induction and (iii) the thermodynamic and photo-
Circularly polarised luminescence of the Eu triple helical
complex displays a weak effect, suggesting a small diastere-
1
2
omeric excess in solution. Ligand L appears to favour a
3
ππ*-to-Ln energy transfer process for Eu, but temperature-
1
2
3+
physical properties of the [Ln(L
)
3
]
complexes. The crystal dependent nonradiative processes lead to a very small
1
2
structure of ligand L shows the expected trans-trans con-
quantum yield. High-resolution luminescence spectra indic-
formation of the tridentate binding unit. The ligand forms
3
ate that the Eu complex has a distorted D local symmetry at
stable 1:3 complexes in anhydrous acetonitrile (logβ
3
in the
the metal ion site.
range 17.3−19.0, logK in the range 2.9−4.6). The triple hel-
ical structure in solution is responsible for the four times
3
( Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany,
2002)
Introduction
induced cavities obtained by the self-assembly of carefully
[
15]
tailored multidentate ligands and podands.
A judicious
During the last two decades, lanthanide ions LnIII have design of the ligands results in weak noncovalent in-
proved to be powerful luminescent and magnetic probes in terstrand interactions stabilizing the molecular edifices. We
biology and medicine.[
1Ϫ4]
In the latter field, applications have shown that tridentate aromatic ligands L ϪL , L de-
1
9
11
[5Ϫ7]
range from diagnosis (e.g. fluoroimmunoasssays,
lumin- rived from bis(benzimidazole)pyridine (Scheme 1) lead to
[8,9]
escent specific imaging agents for tissues,
for magnetic resonance imaging[10]) to therapeutic tools.
In addition, these ions are also good catalysts for the select- amic properties can be tuned by varying the substituents
contrast agents triple helical assemblies displaying sizeable ππ-stacking in-
[11]
teractions, whose structural, photophysical and thermodyn-
ive hydrolysis of DNA and RNA.[
These applications require the introduction of the Ln
ions into functional molecular edifices able to preserve or final structure of the complexes, while substitution in the
12,13]
1
2
3 [16Ϫ18]
2
R , R and R .
Substitution in the R position of the
III
benzimidazole rings essentially controls the stability and the
[
14]
1
3
better enhance the metal ion physicochemical properties.
One strategy proposed to meet this goal is the formation of ical properties.
R and R positions affects the electronic and photophys-
[
a]
So far, limited attention has been devoted to the inherent
chirality of triple helical complexes, which often appear as
racemic mixtures both in the solid state and in solution.
Institute of Molecular and Biological Chemistry, Swiss Federal
Institute of Technology
BCH 1402,
1
015 Lausanne, Switzerland
3Ϫ
However, Riehl et al. have shown that the [Ln(dpa)3]
E-mail: jean-claude.bunzli@epfl.ch
[
[
[
b]
c]
Department of Chemistry, University of Minnesota Duluth,
Minnesota 55812Ϫ2496, USA
racemate equilibrium (H dpa ϭ dipicolinic acid) can be
2
[19]
perturbed by adding chiral sugars. Moreover, a small ex-
Institute of Crystallography, BSP, University of Lausanne,
cess of one enantiomer can be induced in the excited state
1
015 Lausanne, Switzerland
d]
[20]
School of Pharmacy, University of Geneva,
with the use of circular polarised excitation light. Chiral
Switzerland,
3
lanthanide complexes are of potential great interest because
0 quai E. Ansermet, 1211 Geneva 4, Switzerland
[
e]
Department of Inorganic, Analytical and Applied Chemistry, of their use as probes for the sensing of chirality in biolo-
University of Geneva,
[21,22]
III
gical substrates.
Since the high lability of the Ln ions
3
0 quai E. Ansermet, 1211 Geneva 4, Switzerland
favours easy racemisation of triple helical complexes, we are
exploring the possibility of producing edifices in which the
Supporting information for this article is available on the
WWW under http://www.ejic.com or from the author.
Eur. J. Inorg. Chem. 2002, 3101Ϫ3110  2002 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim 1434Ϫ1948/02/1212Ϫ3101 $ 20.00ϩ.50/0 3101

G. Muller, J. P. Riehl, K. J. Schenk, G. Hopfgartner, C. Piguet, J.-C. G. Bünz ខl i
FULL PAPER
chirality is brought about by the ligands, a method which
can only be used if the diastereoisomers have a large enough
energy difference.
Working along these lines, we have introduced a chiral
substituent onto the benzimidazole side arms of bis(benz-
11
imidazole)pyridine to yield L . Unfortunately, the bulky
neopentyl group precludes the formation of stable triple
[
18]
helical complexes.
bulky chiral substituent in the 4-position (R ) of a pyridine
,6-dicarboxamide (L*, Scheme 1) results in (i) helical
On the other hand, introducing a
3
2
III
wrapping of the three ligand strands around the Ln ion,
ii) thermodynamically stable 1:3 complexes (logK in ace-
(
3
tonitrile is in the range 5.1Ϫ5.3), and (iii) a small excess of
[
23]
one diastereoisomer. To further our understanding of the
influence of helical wrapping on diastereomeric induction,
we introduce here a chiral neopentyl ester group in the pyr-
3
1
12
idine R position of L (L , Scheme 1), and investigate its
influence on the thermodynamic, chiro-optical and lumin-
III
escent properties of the resulting tris complexes with Ln
ions.
Results and Discussion
Synthesis and Characterisation of the Ligand and its
Complexes
Ligand L12 was synthesised in a 48% yield from 2,6-
bis[(1-methylbenzimidazol-2-yl)]pyridine-4-carboxylic acid
Scheme 1
(6) by esterification with (Ϫ),(S)-2-methyl-1-butanol
Scheme 2), while 6 was initially obtained by a four-step
(
Scheme 2
3102
Eur. J. Inorg. Chem. 2002, 3101Ϫ3110

Lanthanide Triple Helical Complexes with a Chiral Bis(benzimidazole)pyridine Derivative
FULL PAPER
1
2
procedure involving a modified double Phillips coupling re- Table 1. Crystal data for L .
action to form the benzimidazole unit.[ Using an excess
of N-ethyl-2-nitroaniline and 2,4,6-pyridinetricarboxylic Formula
24]
27 27 5 2
C H N O
M
453.54
orthorhombic
P2
acid favoured the formation of by-products such as the tri-
Crystal system
amide, and significantly decreased the yield of the Phillips
Space group
1 1 1
2 2
reaction. Consequently, we turned to a different strategy
˚
˚
a/ A
7.302(5)
16.582(5)
39.001(5)
4722(4)
based on a procedure similar to that reported for the syn- b/ A
7
8 [25]
c/ A˚
V/ A
Z
T/ K
thesis of L and L , and made use of the Kröhnke reac-
˚
3
[
26]
tion for the construction of the 4-substituted central pyr-
8
idine ring. The required pyridinium salt 3 was prepared by
170(1)
0.083
an OrtolevaϪKing reaction[ from 2-acetyl-1-methylbenz- µ(Mo-K )/ mm
27]
Ϫ1
α
imidazole (2), while 4-(1-methylbenzimidazol-2-yl)-4-oxo-2- Reflections measured
15587
6273 (Rint ϭ 0.1013)
4529
Independent reflections
butanoic acid (4) was obtained by a Knoevenagel condensa-
Observed reflections [F
Final R1, wR2 [I Ͼ 2σ(I)]
0 0
Ն 4σ(F )]
[
26,28]
tion.
Finally, 4 was synthesised by treating 2 with a
0.0743, 0.1306
mixture of glyoxylic acid monohydrate, triethylamine and
pyrrolidine, which gave a better yield (30%) and resulted
in an easier isolation process than the originally proposed
method based on UV-irradiation.[
29]
1
2
The specific rotary dispersion of L in degassed anhyd-
2
5
2
Ϫ1
rous acetonitrile, [α] ϭ 8.1 Ϯ 0.4 deg dm mol , confirms
D
the chirality arising from the asymmetric carbon atom. At
1
13
room temperature, the H and C NMR spectra in CDCl3
(
Ϫ3
5 ϫ 10 ) display 13 and 17 signals, respectively, indicat-
ing a local pseudo twofold symmetry for the bis(benzimida-
zole)pyridine part of the molecule (Figure 1, Figure S1).
1
1
These spectra coupled with { H- H} COSY, DEPT-135,
1
13
{
H- C} HSQC experiments and NOE effects observed be-
4
tween H and NϪMe (NOE effects do not exist between
5
12
H and NϪMe), clearly suggest that L has a trans-trans
1
1
conformation, similar to that observed in L . This con-
formation also prevails in the solid state, as demonstrated
by the X-ray crystal structure (Table 1, Figure 2). The unit
cell contains two independent molecules A and B, the bond
lengths and angles of which differ only slightly (Table S1).
All aromatic groups are planar, but the benzimidazole units
are not coplanar with the pyridine group, the interplanar Figure 2. Molecular structure and atom numbering scheme for the
1
2
two types of L molecules found in the unit cell.
angles being either very similar (16.6° and 10.9°) or largely
different (36.3° and 14.2°) in molecules A and B, respect-
ively (Table S2). These angles result in an optimal packing
coefficient. Finally, no noteworthy hydrogen-bonding or π-
stacking interactions exist in the structure, which seems to
be an example of a pure van der Waals packing.
The 1:3 complexes were obtained in 70Ϫ80% yields, by
adding a solution of L12 in CH Cl to solutions of lanthan-
ide perchlorates in a mixture of MeCN and CH Cl . Evid-
ence for complexation is the blue shift of the νCO and the
two ν pyridine and benzimidazole vibrations by 4Ϫ6
cm and 2Ϫ6 cm , respectively. All perchlorate ions are
ionic; hence the complexes are most probably nine-coordin-
ate (see below). Single crystals for X-ray structure deter-
mination could not be obtained.
2
2
2
2
CϭC
Ϫ1
Ϫ1
Interactions between L12 and the LnIII Ions
The various species in solution were first identified by
ESI-MS. Solutions of L12 (10  in anhydrous acetonitrile)
were titrated with Ln(ClO ) ·xH O (x ϭ 0.2Ϫ0.6) at 298 K,
Ϫ4
4
3
2
12
III
for ratios R ϭ [L ] /[Ln ] ϭ 0Ϫ3. The spectra clearly
t
t
show the successive formation of 1:n (n ϭ 1Ϫ4) species,
some of which had solvation molecules and/or were in the
12
Figure 1. Numbering scheme for ligand L .
Eur. J. Inorg. Chem. 2002, 3101Ϫ3110
3103

G. Muller, J. P. Riehl, K. J. Schenk, G. Hopfgartner, C. Piguet, J.-C. G. Bünz ខl i
FULL PAPER
1
2
]3ϩ complexes (n ϭ 1Ϫ3)
n
Table 2. Relative intensities of the ESIMS peaks for solutions with
Table 4. Stability constants of [Ln(L )
as determined by NMR spectroscopy and UV/Vis spectrometry
12
III
[
L ]
t
/[Ln
]
t
ϭ 3 (MeCN, 298 K)
(
µ ϭ 0.1  Et
4
NClO
4
, 298 K, Ϯ 2σ)
Species
La
Eu
Lu
0.0
0.0
0.0
0.0
Log K
Ln UV/Vis NMR
1
log K
UV/Vis NMR
2
log K
UV/Vis[a] NMR
3
12
3ϩ
3ϩ
[
[
[
[
[
[
Ln(L )
3
4
2
3
3
2
]
]
100
16.7
7.5
12.5
0.0
100
17.8
9.2
0.0
0.0
0.0
1
2
Ln(L )
1
2
2ϩ
2ϩ
Ln(L )
(ClO
(ClO
(MeCN)
(MeCN)]
4
)]
)]
La 7.8 Ϯ 0.4 8.0 Ϯ 0.2 6.0 Ϯ 0.5 6.0 Ϯ 0.3 3.8 Ϯ 0.6
3.8 Ϯ 0.3
1
2
[b]
[b]
[b]
Ln(L )
4
Eu 8.0 Ϯ 0.3
6.4 Ϯ 0.4
4.6 Ϯ 0.5
1
2
3ϩ
Ln(L )
4
]
100
75
Lu 8.0 Ϯ 0.3 7.9 Ϯ 0.2 6.4 Ϯ 0.4 6.5 Ϯ 0.3 2.9 Ϯ 0.4
3.1 Ϯ 0.3
1
2
3ϩ
Ln(L )
0.0
[a]
and log K
2
were fixed. [b] Not determined.
Values of log K
1
III
12
To quantify the Ln ϪL interaction further, we have
form of perchlorate adducts (Table 2, Table S3). For La and
Eu, the 1:3 complex (R ϭ 3) is most abundant, while for
Lu, this species coexists in solution with the 1:2 complex.
1
2
performed spectrophotometric titrations of
L
with
Ln(Otf) (Ln ϭ La, Eu, Lu; Otf ϭ trifluoromethanesulfon-
3
1
2
3ϩ
ate) at 298 K in MeCN, in the presence of 0.1  Et
NClO ,
4 4
In parallel, the proportion of the [Ln(L ) ] species is
4
under anhydrous conditions ([H O] Ͻ 30 ppm). Factor ana-
2
relatively important for the larger ions (La, Eu), but this
species is not seen for Lu. Such complexes have been ob-
served for ligands derived from pyridine 2,6-dicarboxamide
and were assigned to an outer-sphere association of a
[
32]
lysis
indicates the presence of three or four absorbing
species and the data can be fitted to Equations (1)Ϫ(3),
where solvation and anion coordination has been omitted.
Ln3 ϩ L12 Ǟ [Ln(L12)]3ϩ logβ1
ϩ
(1)
(2)
(3)
fourth ligand with a 1:3 complex.[
23,30]
The ES-MS data cle-
ǟ
arly point to the presence of stable 1:3 complexes with La
and Eu, while the formation of this species with Lu appears
to be less favoured.
Ln3ϩ ϩ 2 L12 Ǟ
[Ln(L12)
]3ϩ logβ
]3ϩ logβ
ǟ
2
3
2
ϩ
Ln3 ϩ 3 L12 Ǟ
[Ln(L12)
ǟ
3
To confirm the speciation, the titration of L (10Ϫ3  in
1
2
A model including [Ln(L ) ] does not improve the fit,
4
12 3ϩ
III
III
CD CN) with La and Lu ions has been monitored by in agreement with the postulated outer-sphere nature of this
00 MHz H NMR spectroscopy. Chemical shifts are re- species detected in the gas phase. Electronic spectra of the
3
1
4
ported in Table 3, while Figure S2 (see Supporting Informa- three complexes are correlated, which renders the fitting
tion) presents the aromatic part of the spectra obtained process difficult and generates relatively large uncertainties
with Lu. Signals for the 1:1, 1:2, and 1:3 complexes could in the logβ values (Table 4). The stability constants are in
i
clearly be identified for both ions. When R reaches 3, the a good agreement with those extracted from NMR spectro-
12
III
tris species is the major complex in solution for La (Ͼ 68%) scopic data. The interactions between L and the Ln ions
11
and Lu (50%); the other species in solution is the 1:2 com- are stronger than those with L , as indicated by
1
2
11
plex. This observation confirms that the bulky neopentyl ∆logK (L ϪL ) ϭ 2.6 and 3.7 for La and Eu, respect-
3
18]
[
ester group increases the steric constraint in the triple hel- ively. According to previous studies with ligands derived
ical complexes with the smaller lanthanide ions. The from bis(benzimidazole)pyridine, the methyl substituents
stability constants extracted from H NMR spectroscopic bound to the benzimidazole side arms in L favour the
are reported in formation of a tight cavity around the lanthanide ions. This
Table 4. The apparent ease with which the 1:3 complex is not the case for the neopentyl groups in L . This obser-
forms contrasts with the situation observed with L , and vation also confirms the large influence of the alkyl N-sub-
indicates that the substitution in the R position of the cent- stituents on the formation of the 1:3 complexes. However,
[
17]
1
12
ϩ
[31]
data and analysed with MINEQL
11
11
3
ral pyridine is less detrimental to the formation of the hel- the size-discriminating effect along the lanthanide series,
i
3ϩ
ical complexes compared with substitution on the benzimi- and the stability of the [Ln(L ) ] complexes, are largely
3
[
18]
dazole rings.
influenced by the substituents on the benzimidazole and
Table 3. 1H NMR shifts observed (ppm) for L12 in CDCl
, 298 K).
and for its 1:1, 1:2, and 1:3 complexes with LnIII ions in CD
CN (10Ϫ3
Me2
3
3

Species
H1
H2
H3
H4
H5
H6,7
H8
H9,10
Me
Me1
L12
7.90
8.31
7.59
7.74
8.12
7.56
7.82
7.32Ϫ7.43
7.56Ϫ7.65
7.48
7.81
7.20
7.16
7.87
6.98
7.68
8.92
8.78
8.87
8.03
8.91
9.00
8.81
4.23, 4.32
4.36, 4.41
4.40, 4.46
4.31, 4.35
4.39, 4.44
4.45, 4.52
4.25, 4.30
1.95
1.96
2.03
1.92
2.04
2.06
1.94
1.29, 1.56
1.39, 1.62
1.44, 1.66
1.37, 1.56
1.46, 1.52
1.45, 1.68
1.35, 1.58
4.24
4.32
4.34
3.82
4.41
4.24
4.28
1.04
1.10
1.14
1.07
1.12
1.17
1.06
0.97
1.02
1.05
1.02
1.03
1.07
0.99
La 1:1
La 1:2
La 1:3
Lu 1:1
Lu 1:2
Lu 1:3
7.28
7.35
7.67
7.34
7.43
6.91
6.83
7.63
7.09
7.49
3104
Eur. J. Inorg. Chem. 2002, 3101Ϫ3110

Lanthanide Triple Helical Complexes with a Chiral Bis(benzimidazole)pyridine Derivative
FULL PAPER
central pyridine groups; the latter acting on the electron which is particularly well-suited for CPL measurements
density of the pyridyl N atom. The introduction of an elec- since it satisfies the magnetic-dipole selection rule, ∆J ϭ 0,
1
2
tron-attracting neopentyl ester group in L decreases the Ϯ1. The spectrum depends on the polarisation of the excita-
electronic density on the pyridyl N atom, which reduces the tion light, indicating the presence of two species in solu-
1
2
[33]
III
electrostatic interaction, as indicated by ∆logK (L ϪL ) ϭ tion. At the concentrations used, 93% of the Eu ion is
1
3
[
17]
.0Ϫ1.1, but the size-discriminating effect is retained.
in the form of the 1:3 complex, and 7% in the form of the
The specific rotary dispersion of solutions of the 1:2 species, which is expected to generate a smaller chiral
1
2
3ϩ
Ϫ3
[
Ln(L ) ] complexes (10  in anhydrous acetonitrile) effect. The second species could also be a complex in which
3
2
Ϫ1
is 33.5 Ϯ 0.5 (La) and 31.0 Ϯ 0.3 deg dm mol (Eu). This partial decomplexation of a ligand strand is induced by the
is about four times larger than those measured for the free interaction with one water molecule (see below). In conclu-
ligand. We assign the greater rotary dispersion with respect sion, the CPL data are consistent with the presence of a
to that generated by three free ligands to a structural small excess of one diastereoisomer in solution, on the short
contribution such as the formation of triple helical com- time scale of optical measurements, as previously observed
[23]
[23]
plexes in solution, as observed for L*. As a comparison, for the corresponding complex with L*.
2
Ϫ1
relatively little or no (4Ϫ5 deg dm mol ) structural con-
tribution was observed for the 1:1 and 1:2 complexes with Photophysical Properties
1
1 [18]
L .
The circularly polarised luminescence (CPL) spec-
The electronic spectrum of L12 in MeCN displays two
intense bands centred around 33950 and 29590 cm , with
Ϫ3
12 3ϩ
trum of a 5 ϫ 10  solution of [Eu(L ) ] is plotted in
Figure 3 in the spectral range of the D Ǟ F transition,
Ϫ1
3
5
7
0
1
a shoulder at higher energy (Table 5, Figure 4). Irradiation
in the UV region at room temperature yields one broad
Ϫ1
unresolved fluorescence band, centred around 22885 cm
1
and originating from the ππ* level (Figure 5). At 77 K, the
fluorescence band is more structured and emission from the
5
7
12 3ϩ
Figure 3. CPL spectrum of the D
0
Ǟ F
1
transition of [Eu(L )
3
]
Ϫ3
Figure 4. UV/Vis spectra of L and its [Ln(L12)
12
]3ϩ complexes
(
5 ϫ 10  in anhydrous MeCN at 295 K), on ligand excitation
3
Ϫ3
at 386 nm.
(10  in anhydrous MeCN at 293 K).
Table 5. Ligand-centred absorptions in acetonitrile and in the solid state (293 K), ligand-centred singlet- and triplet-state energies as
Ϫ3
12
determined from the emission spectra of the solids (77 K) and of the solutions (10  in acetonitrile at 293 K) for the ligand L and
12
3ϩ
3
its triple helical complexes [Ln(L ) ] .
Ϫ1[a]
1
Ϫ1
E( ππ*)/cm solid state[c]
3
Ϫ1
Compnd
L12
E(xǞπ*)/cm
E( ππ*)/cm
Solution[
b]
solid state
solution
solid state[c]
43480 (4.28) sh[d]
43480 sh
33670
27100
45660 sh
34485
3
2
3950 (4.35)
9590 (4.32)
23570
22960
20920
17670
22885
21210
21100
21645
12
]3ϩ
]3ϩ
[
[
[
La(L )
3
42980 (4.76) sh
3220 (4.66) 27630 (4.46)
3
22100
22350
19750
17990
24940
1
2
Eu(L )
3
43670 (4.89) sh
3115 (4.74) 26255 (4.50)
41490 sh
35715
3
[e]
[e]
24215
1
2
]3ϩ
Tb(L )
3
43945 (4.81) sh
2980 (4.64) 27175 (4.49)
39215 sh
33670
3
24450
22475
21050
[
a]
x ϭ n or π. [b] At the concentration used, 67Ϫ85% of the metal ion is in the form of the 1:3 complex and 15Ϫ33% in the form of the
:2 complex; Logε values are given in parentheses. The 0-phonon transition is given in italics for frozen solutions in acetonitrile. [d]
[c]
1
[
e]
sh ϭ shoulder. not observed because of the L12-to-Ln energy transfer process.
Eur. J. Inorg. Chem. 2002, 3101Ϫ3110
3105

G. Muller, J. P. Riehl, K. J. Schenk, G. Hopfgartner, C. Piguet, J.-C. G. Bünz ខl i
FULL PAPER
olution emission spectra of a solid state sample of
12
[
Eu(L ) ](ClO ) ·2H O (8) at low temperature since this
3 4 3 2
complex is only faintly luminescent at room temperature.
5
The lifetime of the D level, 0.85 Ϯ 0.01 ms, is constant
0
between 13 and 77 K. This is shorter than the lifetimes re-
1
3ϩ
[35]
ported for [Eu(L ) ] (1.85 ms in the range 4Ϫ77 K)
3
11
3ϩ
[36]
and [Eu(L ) ] (2.03 ms at 10 K),
found for [Eu(ClO ) (H O)(L ) ] (0.65 ms at 10 K).
but similar to that
3
11
ϩ
[36]
4
2
2
2
Therefore, we cannot rule out the interaction of one water
molecule in the first coordination sphere. The decrease in
5
the Eu( D ) lifetime from 0.85 ms to 0.27 ms between 77
0
and 295 K indicates temperature-dependent quenching
mechanisms, e.g. a ligand-to-metal charge transfer as dem-
1
3ϩ [37,38]
onstrated for [Eu(L ) ]
Eu complexes
,
and as reported for other
3
III
[39]
or other intermolecular interactions. In
acetonitrile solution (10
Ϫ3
) at 295 K, the lifetime is
longer, reaching 0.96 Ϯ 0.01 ms; since in addition to the
hydration of 8 the solvent contains about 2 m H O, inner-
2
sphere water interaction could occur. Using Horrocks equa-
[
40]
tion, q ϭ 1.05 (kH2OϪkD2O
)
and taking kD2O equal to
1
2
Ϫ1
Figure 5. Top: Time-resolved emission spectra of L and 0.51 ms (average of the values for the anhydrous tris spe-
1
2
3ϩ
[
Ln(L )
3
]
in the solid state at 77 K and recorded with time delays
1
11 [35,36]
cies with L and L ),
we find q ϭ 0.56. This points to
an equilibrium in solution between an anhydrous species
and a monohydrate, as has been observed for triple helical
1
2
of 0.01 (Eu), 0.1 (Tb) and 10 ms (L , La). Bottom: Emission
12
3ϩ
12
Ϫ3
spectra of [Ln(L )
3
]
and L (10  in MeCN at 293 K).
[
41]
complexes with terpyridine derivatives.
Alternatively, it
3ππ* state appears with a maximum around 17670 cmϪ1
The decay of the latter is a single exponential with a lifetime molecules can contribute 0.25 ms to the quenching of the
.
has been demonstrated that fast diffusing outer-sphere H O
2
Ϫ1
III
[42]
of 210 Ϯ 3 ms, in the range of values previously reported Eu ion in complexes with cyclen derivatives.
for similar ligands (160Ϫ323 ms).[16,34]
a similar correction for 8 results in a reduction of q by ap-
The higher energy component observed in the electronic proximately a factor of 2. Therefore, given the uncertainties
(33670 linked to this type of correlation, it is difficult to determine
from the lifetime data whether or not a water molecule in-
ions, while the other band centred at 29590 cm (27100 teracts in the inner coordination sphere. On the other hand,
cm ) undergoes an important red shift (Table 5, Figure 4). it is clear that the introduction of an electron-attracting
This observation reflects the electronic transformations as- group in the R position weakens the LnϪN(py) bond, con-
sociated with the trans-transǞcis-cis conformational sistent with the relatively high energy of the D ǟ F
Applying
1
2
Ϫ1
(
or reflectance) spectrum of L at 33950 cm
Ϫ1
III
cm ) is only slightly modified on complexation with Ln
Ϫ1
Ϫ1
3
5
7
0
0
Ϫ1
change and the complexation of the lanthanide metal ions transition (17258 cm
at 295 K). Using the correlation
[
16,25,34]
[43]
to the tridentate binding unit.
On complexation of proposed by Frey and Horrocks
and the nephelauxetic
ϭ Ϫ15.3 cm that we reported previ-
1
2
III
Ϫ1
L
to La , the 0-phonon transitions of the singlet and parameter δ
triplet states are red-shifted by 1470 and 1170 cm , re- ously,
N(het)
Ϫ1
[44]
Ϫ1
we get δN(py) ϭ Ϫ8.1 cm , a value much lower
spectively (frozen MeCN solution, 77 K, see Figure 5), and than those proposed (Ϫ12 to Ϫ17 cm ).
The emission spectra recorded at 13 K on excitation
the ππ* state of the ligand is still visible for the Eu and Tb through the ligand band, or direct laser excitation of the
D ǟ F transition, display fairly broad bands which are
Emission from the ππ* state disappears for the Eu solu- compatible with a distorted D symmetry environment for
tion, with a concomitant observation of the metal-centred the metal ion (Figure S4). The relative intensities of the D
emission bands. This is not the case for the Tb solution for Ǟ F transitions are 0.07, 1.00, 1.28, Ͻ0.01 and 4.11 for
Ϫ1 [43,45]
3
the ππ* lifetime decreases to 144 Ϯ 1 ms. Emission from
1
5
7
complexes, suggesting an incomplete intersystem crossing.
0 0
3
3
5
0
7
J
5
7
which little or no ligand-to-Tb energy transfer takes place J ϭ 0, 1, 2, 3, and 4, respectively. The D Ǟ F transition
0
1
Ϫ1
because the energy of the ligand donor level (19750 cm
in the La complex at 77 K) is located below the metal D
acceptor level (20350 cm ). Weak emission from the Tb
displays two main components, one of them appearing as a
doublet with a splitting of 17 cm , while the D Ǟ F
transition shows two doublets with a splitting of 13 and 14
5
Ϫ1
5
7
4
III
0 2
Ϫ1
Ϫ1
5
7
ion can only be seen on direct excitation of the metal states cm , respectively. Finally, the D Ǟ F transition dis-
0
4
(
Figure S3, Supporting Information), and the short and plays four main components, three of them having a very
5
temperature-dependent Tb( D ) lifetime (decreasing from weak intensity, again compatible with a distorted D local
4
3
0.38 to 0.12 ms between 13 and 55 K) indicates that energy symmetry.
back transfer occurs in this complex.
To get a better insight into the various energy conversion
12
3ϩ
In order to gain information on the chemical environ- processes occurring in the [Ln(L ) ] complexes, we have
3
III
ment around the Eu ion, we have measured the high-res- determined the quantum yields of both the ligand- and
3106
Eur. J. Inorg. Chem. 2002, 3101Ϫ3110

Lanthanide Triple Helical Complexes with a Chiral Bis(benzimidazole)pyridine Derivative
FULL PAPER
metal-centred luminescence of 10Ϫ3  solutions at 295 K, modulated by varying the substituent in the R position,
3
and the ratio between the integrated triplet and singlet state a diastereomeric excess can be induced in solution if this
emissions, I( /I for the free ligand and its La complex substituent is chiral. The combination of these properties
T) (S)
III
1
2
3ϩ
F
at 77 K. The quantum yield of [La(L ) ] (Q ϭ 5.5%) opens new perspectives for the design of lanthanide triple
3
decreases by a factor of 9 with respect to that of the free helical complexes acting as probes for chiral recognition.
F
ligand (Q ϭ 51%), and the ligand-centred phosphores-
(T) (S)
cence increases, as indicated by I /I which increases from
Ϫ5
Ϫ3
12
III
8
ϫ 10 to 9.8 ϫ 10 on complexation of L with La , Experimental Section
that is by a factor of 122. The better intersystem crossing
isc) efficiency in the La complex may be explained by (i) Solvents and Starting Materials: Acetonitrile, dichloromethane,
III
(
1
3
chloroform, tetrahydrofuran, pyridine and triethylamine were puri-
fied in the usual way.[ Silica gel (Merck 60, 0.04Ϫ0.06 mm) was
used for preparative column chromatography. Other products were
purchased from Fluka AG (Buchs, Switzerland) or Merck and used
the reduced energy gap between the ππ* and ππ* states:
47]
Ϫ1
2
350 cm , determined from the energy of the 0-phonon
Ϫ1
transitions, in contrast to 2650 cm in the free ligand (Fig-
ure 6) and (ii) the spin-orbit mixing of the singlet and triplet
states induced by the complexation with La .
without further purification. 2,4,6-pyridinetricarboxylic acid was
III [46]
The
[48]
prepared according to a literature procedure.
chlorates were obtained as described previously.
Lanthanide per-
quantum yield of the metal-centred luminescence of
[49]
1
2
3ϩ
[
Eu(L ) ]
obtained on ligand excitation remains very
3
Eu
Ϫ3
Caution: Dry perchlorates and their complexes with aromatic
amines may easily explode and should be handled in small quantit-
ies and with extreme caution.[
small, QL ϭ 5 ϫ 10 %, not only because the efficiency
of the isc process stays small, but most probably because of
photoinduced electron transfer processes, as already poin-
50]
1
[37,38]
ted out for the triple helical complex with L .
Spectroscopic and Analytical Measurements: Electronic spectra in
the UV/Vis range were recorded at 293 K with a PerkinϪElmer
Lambda 900 spectrometer and 1.0 and 0.1 cm quartz cells. Re-
flectance spectra were recorded on finely ground powders dispersed
in MgO (5%), with MgO as the reference, on the same spectrometer
equipped with a Labsphere PELA-1000 integration sphere. Specific
Ϫ3
rotary dispersion values were measured from 10  solutions in
degassed anhydrous acetonitrile at 298 K with the help of a JASCO
DIP-370 polarimeter (sodium D line). IR spectra were obtained
from KBr pellets with a Mattson α-Centauri FT-IR spectrometer.
Pneumatically assisted electrospray (ESIMS) mass spectra were re-
corded from MeCN solutions on API III or API 3000 tandem mass
Ϫ1
spectrometers (PE Sciex) by infusion at 10 µL min . The spectra
were recorded under low up-front declustering or collision-induced
12
3ϩ
Figure 6. Schematic energy diagram for [Ln(L )
3
]
complexes.
III
1 1
Data for S and T are those of the La complex in the solid state
dissociation (CID) conditions, typically ∆V ϭ 0Ϫ30 V between the
at 77 K (0-phonon transitions).
1
13
orifice and the first quadrupole of the spectrometer. H, C NMR
spectra and 2-D experiments were recorded at 25 °C on Bruker
AM-360 or Bruker AVANCE 400-DRX spectrometers. Chemical
shifts are reported in parts per million with respect to TMS. Ligand
excitation and emission spectra were recorded on a PerkinϪElmer
LS-50B spectrometer equipped for low temperature (77 K) meas-
urements. The experimental procedures for high resolution, laser-
Conclusion
The introduction of a chiral neopentyl ester group in the
-position of the central pyridine ring of ligand L to yield
1
4
L
1
2
[51]
leads to the formation of thermodynamically stable 1:3 excited luminescence studies have been published previously.
complexes with lanthanide ions in acetonitrile, as shown by Emission spectra are corrected for the instrumental function.
the logK values in the range 2.9Ϫ4.6. Chiro-optical data Quantum yields of the ligand-centred emission were measured rel-
3
2 4
ative to quinine sulfate in 0.05  H SO (A347 ϭ 0.05, absolute
clearly suggest the helical wrapping of the ligand strands
quantum yield: 0.546).[ Quantum yields of the metal-centred
emission were determined as described previously[ at excitation
wavelengths at which (i) the LambertϪBeer law is obeyed and (ii)
52]
III
around the Ln ions. As a comparison, the bulky neopen-
53]
tyl groups grafted onto the benzimidazole side arms in L11
precluded helical wrapping, a fact confirmed by the specific
rotary dispersion measurements.[ Although only a small
excess of one diastereoisomer is induced in solution by the
3
ϩ
the absorption of the reference [Ln(terpy)
3
]
closely matches that
18]
of the sample. CPL measurements were made on an instrument
described previously, operating in a differential photon-counting
1
2
[23]
chiral ligand L , as was observed for complexes with L*,
[33]
mode. Elemental analyses were performed by Dr H. Eder (Mic-
this work reveals that structural chirality may be induced in rochemical Laboratory, University of Geneva).
triple helical complexes by a suitable design of the ligand.
Spectrophotometric Titrations: The electronic spectra in the UV/Vis
The substitution also significantly modifies the photophys-
ical properties of the tris complexes, compared with those
Ϫ4
range were recorded at 298 K from 10  solutions in acetonitrile
containing Et NClO (0.1 ) as the inert electrolyte with a
4
4
1
1 [36]
with L . In particular, the ligand-to-metal energy trans-
PerkinϪElmer Lambda 7 spectrometer connected to an external
computer and in quartz cells of 0.100 cm path length. Solutions
fer is amplified for Eu, but cancelled for Tb.
Therefore, this study demonstrates that, in addition to were prepared in a thermostatted vessel (Metrohm 6.1418.220) and
the electronic and photophysical properties which may be the titrant solution was added with an automated burette from Me-
Eur. J. Inorg. Chem. 2002, 3101Ϫ3110
3107

G. Muller, J. P. Riehl, K. J. Schenk, G. Hopfgartner, C. Piguet, J.-C. G. Bünz ខl i
trohm (6.1569.150 or .210) fitted with an anti-diffusion device. In dried (Na SO ) and the solvents evaporated. The resulting crude
FULL PAPER
2
4
3
12
a typical experiment, 5Ϫ10 cm of L were titrated with a solution
solid was recrystallised from methanol to give 849 mg of 2
(4.87 mmol, 76%) as white crystals. M.p. 72Ϫ74 °C. H NMR
(360 MHz, CDCl ): δ ϭ 2.83 (s, 3 H), 4.11 (s, 3 H), 7.33Ϫ7.37 (m,
3
III
Ϫ4
1
of Ln triflate (10 ) in acetonitrile. After each addition of 0.20
cm and a delay of 2 min, the spectrum was measured and trans-
3
1
3
ferred to the computer. In some instances, reverse titrations, i.e. 1 H), 7.41 (m, 2 H), 7.87 (m, 1 H) ppm. C NMR (360 MHz,
with L12 as the titrating species, were also performed to confirm
the values of the stability constants. All the experiments were con-
ducted in a dry box and the water content of the solutions, meas-
ured at the end of the titration by the Karl Fischer method, was
3
CDCl ): δ ϭ 28.18, 32.38, 110.64, 122.02, 123.82, 126.06, 137.07,
Ϫ1
141.71, 146.28, 193.46 ppm. IR (KBr): ν˜ ϭ 1684 (νCO) cm . ESI-
ϩ
MS (MeOH): m/z ϭ 175.1 [L ϩ H] .
1-{[(Methylbenzimidazol-2-yl)carbonyl]methyl}-1-pyridinium Iodide
always Ͻ30 ppm. Factor analysis and stability constant determina-
tions were carried out with the program SPECFIT, version 2.10.[54]
(3): 2 (211 mg, 1.21 mmol) and I (307 mg, 1.21 mmol) were dis-
2
3
solved in dry distilled pyridine (4 cm ) under an inert atmosphere.
After 20 min at room temp., the mixture was heated at 100 °C for
15 h, then cooled to room temperature. The resulting brown precip-
itate was filtered, washed once with pyridine/diethyl ether (1:1, 5
1
2
Crystal Structure of L : The pale yellowish, lathlike crystals were
directly transferred from the mother liquor into a drop of Hostinert
2
16 oil kept at 210 K. A specimen was selected and placed in a
glass capillary. The data collection, at 170 K, took place on a Stoe
IPDS system equipped with Mo-K radiation (Table 1). The image was recrystallised from ethanol with charcoal to give 410 mg of 3
3
3
cm ) and twice with diethyl ether (10 cm ). The dried crude product
α
1
plate Ϫ crystal distance was set to 80 mm, and a ϕ interval of 1°
was chosen. Two hundred images were then exposed for 20 min
each. Inspection of the reciprocal space revealed that about three
quarters of the diffraction spots could be accounted for by the cell
given in Table 1. The integration was based on an effective mosaic
spread of 0.012 and a profile between 11 and 21 pixels. A peak
overlap of 2 pixels was tolerated in order to retain the maximum
(1.08 mmol, 89%) as dark yellow needles. M.p. 205Ϫ207 °C.
NMR (360 MHz, [D ]DMSO): δ ϭ 4.11 (s, 3 H), 6.54 (s, 2 H), 7.46
(m, 1 H), 7.56 (m, 1 H), 7.84 (m, 1 H), 7.94 (m, 1 H), 8.30 (m, 2
H
6
1
3
H), 8.76 (m, 1 H), 9.03 (m, 2 H) ppm. C NMR (360 MHz,
[D ]DMSO): δ ϭ 31.96, 66.56, 111.86, 121.22, 124.22, 126.47,
6
127.74, 136.65, 141.07, 143.47, 145.56, 146.46, 184.43 ppm. IR
Ϫ1
(KBr): ν˜ ϭ 1705 (νCO) cm . FAB-MS (MeOH): m/z ϭ 252.1 [L
ϩ
number of reflections. Nevertheless, over 2700 reflections were Ϫ I] .
eliminated because of severe peak overlap. The intensities were cor-
4
-(1-Methylbenzimidazol-2-yl)-4-oxobut-2-enoic Acid (4): Glyoxylic
rected for Lorentz and polarisation effects. No absorption correc-
tion could be applied since the crystal was all but invisible in the
frozen oil, but the absorption coefficient appeared to be small. The
3
acid monohydrate (1 g, 10.86 mmol), triethylamine (1.52 cm ,
1
tially added to a solution of 2 (631 mg, 3.62 mmol) in DMF (10
cm ). The mixture was stirred at room temp. for 48 h and 1% hy-
drochloric acid was then added until pH ϭ 1. The solution was
extracted with diethyl ether (3 ϫ 20 cm ) and the combined organic
layers were dried (Na
sulting crude solid was redissolved with 10% NaHCO
3
0.86 mmol) and pyrrolidine (0.33 cm , 3.8 mmol) were sequen-
decay during the measurement was negligible. The structure was
3
with the help of SIR 97.[ The
55]
solved in the space group P2 2 2
1 1 1
oxygen and nitrogen atoms were refined anisotropically and the
carbon and hydrogen atoms isotropically; the latter were allowed
to ride on their associated carbon atoms. The refinement seems to
favour the S configuration for the chirality centre C24 for both
molecules, as expected from the synthesis and the observed rotary
dispersion, but the value of Flack’s parameter (2 Ϯ 2) is not con-
3
2 4
SO ) and the solvents evaporated. The re-
3
3
(5 cm ) to
3
pH ϭ 8 and washed with dichloromethane (2 ϫ 10 cm ). The aque-
ous phase was acidified to pH ϭ 1 by addition of 10% hydrochloric
acid and extracted with diethyl ether (3 ϫ 10 cm ). The combined
3
[
56]
clusive.
CCDC-186842 contains the supplementary crystallo-
3
organic layers were washed with water (2 ϫ 30 cm ), dried
graphic data for this paper, which can be obtained free of charge at
www.ccdc.cam.ac.uk/conts/retrieving.html [or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: (internat) ϩ44Ϫ1223/336Ϫ033; E-mail:
deposit@ccdc.cam.ac.uk].
2 4
(Na SO ) and the solvents evaporated. The resulting crude solid
was recrystallised by slow diffusion of hexane into an ethyl acetate
solution to give 251 mg of 4 (1.09 mmol, 30%) as brownish-red
needles. M.p. 194Ϫ196 °C. H NMR (360 MHz, [D
4
1
6
]DMSO): δ ϭ
3
.15 (s, 3 H), 6.83 (d, J ϭ 15.9 Hz, 1 H), 7.39 (m, 1 H), 7.50 (m,
3
Preparation of the Ligand. (؎)-2-(1-Hydroxyethyl)-1-methylbenzim-
1 H), 7.75 (m, 1 H), 7.88 (m, 1 H), 8.28 (d, J ϭ 15.9 Hz, 1 H)
1
3
6
idazole (1): A mixture of N-Methyl-1,2-phenylenediamine (0.93 ppm. C NMR (360 MHz, [D ]DMSO): δ ϭ 32.18, 111.64, 121.45,
3
3
cm , 8.18 mmol) and (Ϯ)-2-hydroxypropionic acid (0.91 cm ,
2.21 mmol) was refluxed for 45 min in 4  hydrochloric acid (10
123.85, 126.32, 132.46, 136.65, 137.03, 141.22, 146.17, 166.24,
181.66 ppm. IR (KBr): ν˜ ϭ 1707, 1670 (νCO) cm . ESI-MS
Ϫ1
1
3
ϩ
cm ). After cooling, the mixture was neutralised to pH ϭ 7.1 with (MeOH): m/z ϭ 231.2 [L ϩ H] .
4
NH OH (25%). The resulting brown crude solid was filtered,
Ethyl
2,6-Bis[(1-methylbenzimidazol-2-yl)]pyridine-4-carboxylate
washed with water and recrystallised from hot water to give 1.13 g
1
(5): A mixture of 2,4,6-pyridinetricarboxylic acid (1 g, 4.7 mmol),
freshly distilled thionyl chloride (15 cm , 207 mmol) and DMF
of 1 (6.41 mmol, 78%). M.p. 106Ϫ108 °C. H NMR (360 MHz,
3
3
CD
3
OD): δ ϭ 1.73 (d, J ϭ 7.0 Hz, 3 H), 3.94 (s, 3 H), 4.93 (s, 1
3
3
(0.12 cm ) was refluxed for 90 min in dry dichloromethane (40
H), 5.20 (q, J ϭ 7.0 Hz, 1 H), 7.25Ϫ7.34 (m, 2 H), 7.50 (m, 1 H),
3
.66 (m, 1 H) ppm. 1 C NMR (360 MHz, CD
3
cm ), evaporated and dried under vacuum for 60 min. The resulting
7
3
1
3
OD): δ ϭ 21.65,
3
solid was dissolved in dry dichloromethane (45 cm ), and a mixture
0.67, 64.15, 110.79, 119.59, 123.33, 124.06, 137.37, 142.35,
57.45 ppm. IR (KBr): ν˜ ϭ 1105 (νC-OH) cm . ESI-MS (MeOH):
Ϫ1
of 2-nitro-N-ethylaniline (1.08 g, 7.1 mmol) and triethylamine (6.6
3
3
ϩ
cm , 47 mmol) in dry dichloromethane (30 cm ) was added drop-
wise under inert atmosphere. The solution was stirred for 2 h at
room temperature, refluxed for 6 h and the solvents evaporated.
m/z ϭ 177.2 [L ϩ H] .
2
-Acetyl-1-methylbenzimidazole (2): A solution of chromium triox-
3
ide (494 mg, 4.94 mmol) in water (3 cm ) was added dropwise to a The brown residue was partitioned between dichloromethane (3 ϫ
3
3
3
solution of 1 (1.13 g, 6.41 mmol) in glacial acetic acid (5 cm ) at
0 °C. The reaction mixture was heated at 100 °C for 10 min and
100 cm ) and half-saturated aqueous NH
The combined organic phases were dried (Na
4
Cl solution (100 cm ).
SO ) and the solvents
9
2
4
3
poured into water (60 cm ). The resulting precipitate was extracted evaporated. The crude product was dissolved in a mixture of eth-
with dichloromethane (3 ϫ 30 cm ), and the combined extracts
3
3
anol/water (250:55 cm ), and freshly activated iron powder (8 g,
3108
Eur. J. Inorg. Chem. 2002, 3101Ϫ3110

Lanthanide Triple Helical Complexes with a Chiral Bis(benzimidazole)pyridine Derivative
FULL PAPER
/MeOH, 100:0 Ǟ 99:1) and recrys-
3
1
43 mmol) and 25% hydrochloric acid (45.7 cm , 352 mmol) were
tography (silica gel, CH
2
Cl
2
added. The solution was refluxed for 8 h under inert atmosphere.
The excess of iron was filtered off, and the ethanol was distilled off
under vacuum. Water was then added to bring the total volume to
tallised by slow diffusion of hexane into a dichloromethane solu-
1
2
tion to give 245 mg of L (0.54 mmol, 48%) as white crystals. M.p.
1
3
186Ϫ188 °C. H NMR (360 MHz, CDCl
3
): δ ϭ 0.97 (t, J ϭ
3
3
3
1
(
00 cm . The resulting solution was poured into dichloromethane
7.3 Hz, 3 H), 1.04 (d, J ϭ 6.8 Hz, 3 H), 1.29 (m, J ϭ 7.3 Hz, 1
3
3
2
3
2
150 cm ). Na
2 2 2
H edta·2H O (84 g, 236 mmol) in 150 cm water was H, J ϭ 13.6 Hz), 1.56 (m, J ϭ 7.3 Hz, 1 H, J ϭ 13.6 Hz), 1.95
3
3
then added, and the resulting mixture was neutralised to pH ϭ 7
(1 H, pseudo-oct., J ϭ 6.3Ϫ7.3 Hz), 4.23 (dd, 1 H, J ϭ 6.3 Hz),
3
3
with 5  aqueous potassium hydroxide; 30% H
added and the pH adjusted to 8.3. After vigorous stirring for
0 min, the organic layer was separated and the aqueous phase ex- CDCl
2 2
O (1 cm ) was 4.24 (s, 6 H), 4.32 (dd, 1 H, J ϭ 6.3 Hz), 7.35Ϫ7.43 (m, 4 H), 7.48
1
3
(m, 2 H), 7.90 (m, 2 H), 8.92 (s, 2 H) ppm. C NMR (360 MHz,
): δ ϭ 11.35, 16.59, 26.20, 32.68, 34.34, 70.96, 110.15,
120.60, 123.20, 124.10, 124.68, 137.35, 140.51, 142.79, 149.78,
150.79, 164.44 ppm. IR (KBr): ν˜ ϭ 1730 (νCO), 1587, 1564 (νCϭC
)
3
3
3
tracted with dichloromethane (3 ϫ 150 cm ). The combined or-
ganic phases were dried (Na SO ) and the solvents evaporated. The
resulting crude solid was dissolved in dry dichloromethane (40
cm ), then freshly distilled thionyl chloride (15 cm , 207 mmol) and
DMF (0.12 cm ) were added. The mixture was refluxed for 90 min,
2
4
Ϫ1
ϩ
cm
C
.
ESI-MS (MeCN): m/z
·0.11CH Cl : calcd. C 70.3, H 5.9, N 15.1; found C
70.3, H 6.0, N 15.1.
ϭ
454.2 [L
ϩ
H] .
3
3
27
H
27
N
5
O
2
2
2
3
evaporated and dried under vacuum for 60 min. The crude product
was cooled to 0 °C, then dry ethanol (20 cm ) was added dropwise.
3
Preparation of
the
·xH O (La, x ϭ 2, (7); Eu, x ϭ 2, (8); Tb, x ϭ 3,
9)] were prepared by mixing L with stoichiometric amounts of
Ln(ClO ·nH O (Ln ϭ La, Eu, Tb and n ϭ 0.2Ϫ0.6) in dichloro-
methane/acetonitrile. The complexes may be crystallised in
0Ϫ80% yields by slow diffusion of tert-butyl methyl ether
Complexes:
The
1:3
complexes
1
2
[
Ln(L )
3
](ClO
4
)
3
2
The resulting solution was allowed to stand at room temperature,
stirred for 60 min, evaporated and dried under vacuum for 60 min.
1
2
(
3
4
)
3
2
The residue was partitioned between dichloromethane (3 ϫ 40 cm )
3
and half-saturated aqueous NH
bined organic phases were dried (Na
ated. The resulting brownish-red crude solid was purified by col-
4
Cl solution (40 cm ). The com-
7
2
4
SO ) and the solvents evapor-
into an acetonitrile solution. [C81
3.2, 4.6, 11.6, calcd.
Eu·2H
52.7, H 4.6, N 11.4; C81
.5, N 11.1; calcd. C 52.0, H 4.7, N 11.2]. IR (KBr): ν˜ ϭ 1736 (La),
1735 (Eu), 1734 (Tb), (νCO); 1593, 1568 (La), 1591, 1570 (Eu),
H
81
N
15
O18Cl
3
La·2H
2
O: found C
5
H
N
C
53.1,
H
4.7,
N
11.5;
umn chromatography (silica gel, CH
2
1
Cl
2
/MeOH, 100:0 Ǟ 99:1) to
):
C
81
H
81
N
15
O18Cl
3
2
O: found C 52.5, H 4.7, N 11.4, calcd. C
Tb·3H O: found C 51.9, H
give 251 mg of 5 (0.61 mmol, 13%). H NMR (400 MHz, CDCl
3
3
3
H
81
N
15
O
18Cl
3
2
δ ϭ 1.46 (t, J ϭ 7.0 Hz, 3 H), 4.25 (s, 6 H), 4.49 (q, J ϭ 7.0 Hz,
H), 7.39 (m, 4 H), 7.48 (m, 2 H), 7.91 (m, 2 H), 8.95 (s, 2 H)
4
2
1
3
ppm. C NMR (400 MHz, CDCl
3
): δ ϭ 14.97, 33.24, 62.89,
10.69, 121.05, 123.78, 124.66, 125.26, 137.91, 141.03, 143.36,
50.32, 151.33, 164.85 ppm. IR (KBr): ν˜ ϭ 1712 (νCO), 1597, 1562
Ϫ
Ϫ1
4
1589, 1570 (Tb), (νCϭC) and 1092, 625 (ionic ClO ) cm .
1
1
Ϫ1
ϩ
(ν
CϭC) cm . ESI-MS (MeCN): m/z ϭ 412.2 [L ϩ H] .
Acknowledgments
2,6-Bis[(1-methylbenzimidazol-2-yl)]pyridine-4-carboxylic Acid (6).
This work is supported through grants from the Swiss National
Science Foundation. We thank Prof. Pierre Vogel for the use of his
polarimeter, Dr. Anne-Sophie Chauvin for her help in the ligand
synthesis and the Fondation Herbette (Lausanne) for the gift of
spectroscopic equipment.
Procedure A: 3 (410 mg, 1.08 mmol), 4 (226 mg, 0.98 mmol) and
ammonium acetate (3 g, 38.92 mmol) were dissolved in ethanol (25
3
cm ). The mixture was refluxed for 24 h under an inert atmosphere,
and then slowly cooled to room temperature. The resulting white
crystals were separated by filtration and recrystallised from hot
methanol to give 230 mg of 6 (0.60 mmol, 55%). Procedure B: A
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1
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[4]
H NMR (360 MHz, [D
6
]DMSO): δ ϭ 4.37 (s, 6 H), 7.38Ϫ7.47
1
3
(m, 4 H), 7.78 (m, 2 H), 7.87 (m, 2 H), 8.81 (s, 2 H) ppm.
C
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27.10, 128.81, 141.06, 146.18, 146.70, 153.00, 154.41, 166.54 ppm.
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IR (KBr): ν˜ ϭ 1716 (νCO), 1617, 1560 (νCϭC) cm . ESI-MS
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Neopentyl
2,6-Bis[(1-methylbenzimidazol-2-yl)]pyridine-4-carb-
12
oxylate (L ): A mixture of 6 (430 mg, 1.12 mmol), freshly distilled
thionyl chloride (3.62 cm , 50 mmol) and DMF (0.03 cm ) was
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3
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3
residue was redissolved in CHCl
3
(10 cm ). The organic phase was
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washed with water (3 ϫ 20 cm ), then with half-saturated aqueous
1
513Ϫ1531.
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NH
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