A new metallostar complex based on an aluminum(iii) 8-hydroxyquinoline core as a potential bimodal contrast agent

Article abstract of DOI:10.1039/c2dt30605k

A ditopic DTPA monoamide derivative containing an 8-hydroxyquinoline moiety was synthesized and the corresponding gadolinium(iii) complex ([Gd(H5)(H ₂O)]^-) was prepared. After adding aluminum(iii), the 8-hydroxyquinoline part self-as

Full text of DOI:10.1039/c2dt30605k

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PAPER
A new metallostar complex based on an aluminum(III) 8-hydroxyquinoline
core as a potential bimodal contrast agent†
Elke Debroye,a Geert Dehaen,a Svetlana V. Eliseeva,b Sophie Laurent,c Luce Vander Elst,c Robert N. Muller,c
Koen Binnemansa and Tatjana N. Parac-Vogt*a
Received 16th March 2012, Accepted 28th June 2012
DOI: 10.1039/c2dt30605k
A ditopic DTPA monoamide derivative containing an 8-hydroxyquinoline moiety was synthesized and
the corresponding gadolinium(III) complex ([Gd(H5)(H2O)]−) was prepared. After adding aluminum(III),
the 8-hydroxyquinoline part self-assembled into a heteropolymetallic triscomplex [(Gd5)3Al(H2O)3]3−
.
The magnetic and optical properties of this metallostar compound were investigated in order to classify it
as a potential in vitro bimodal contrast agent. The proton nuclear magnetic relaxation dispersion
measurements indicated that the relaxivity r1 of [Gd(H5)(H2O)]− and [(Gd5)3Al(H2O)3]3− at 20 MHz and
310 K equaled 6.17 s−1 mM−1 and 10.9 s−1 mM−1 per Gd(III) ion respectively. This corresponds to a
relaxivity value of 32.7 s−1 mM−1 for the supramolecular complex containing three Gd(III) ions. The high
relaxivity value is prominently caused by an increase of the rotational tumbling time τR by a factor of
2.7 and 5.5 respectively, in comparison with the commercially used MRI contrast agent Gd(III)–DTPA
(Magnevist®). Furthermore, upon UV irradiation, [(Gd5)3Al(H2O)3]3− exposes green broad-band
emission with a maximum at 543 nm. Regarding the high relaxivity and the photophysical properties of
the [(Gd5)3Al(H2O)3]3− metallostar compound, it can be considered as a lead compound for in vitro
bimodal applications.
supramolecular complex.1–5 The rotational motion can be
Introduction
reduced after a non-covalent interaction of the ligand with pro-
teins, for instance with human serum albumin (HSA).6–9 Con-
trast agents have also been covalently linked to macromolecular
carriers like linear polymers or dendrimers.10–15 Another way to
achieve higher proton relaxivities is the incorporation of amphi-
philic Gd(III) complexes into slowly tumbling micelles or
liposomes.16–19 More recently, paramagnetic complexes were
assembled in a rigid heteropolymetallic structure with a central
transition metal ion, the so-called metallostars.20,21 On the other
hand, optical imaging is a diagnostic tool which offers high sen-
sitivity, although no high-resolution images can be recorded and
the technique is restricted to thin tissue samples.22–25 The
development of a bimodal reporter with optical as well as mag-
netic properties can lead to a more detailed diagnostic method,
because the advantages of both imaging techniques (optical
imaging and MRI) are assembled in one molecule.26–29 Several
approaches were maintained to create bimodal agents. Deriva-
tives of DTPA or DOTA were functionalized with organic
fluorophores,30–32 transition metal complexes33–35 or a lantha-
nide sensitizer.36–39 Also liposomal structures40–42 and nano-
particles based on iron(III) oxide,43–46 silica47 or a polymer
core48–50 were designed.
Magnetic resonance imaging (MRI) plays a key role in medical
diagnostics as it combines good spatial resolution with deep
tissue penetration so that a true three-dimensional image can be
obtained. Moreover, during the clinical investigation no ionizing
radiation has to be used. Contrast agents increase the water
proton relaxation rate (1/T1) so that the image contrast with sur-
rounding tissue is improved. The relaxivity, r1, or the enhance-
ment of 1/T1 per mM of currently used gadolinium(III)-based
contrast agents is too low to monitor molecular processes and
the efficiency of these contrast agents dramatically drops at
higher magnetic field strength. It is common knowledge that the
low sensitivity is a major drawback of the MRI technique. An
approach to overcome these problems is to lower the molecular
tumbling rate of the contrast agent or to concentrate several para-
magnetic Gd(III) ions in a small volume by organizing them in a
aKU Leuven, Department of Chemistry, Celestijnenlaan 200F, B-3001
Heverlee, Belgium. E-mail: tatjana.vogt@chem.kuleuven.be;
Fax: +32 16 327992; Tel: +32 16 327612
bCentre de Biophysique Moléculaire – CNRS, UPR 4301 Rue Charles
Sadron, 45071 Orléans Cedex 2, France
cDepartment of General, Organic and Biomedical Chemistry, University
of Mons, Place du Parc 23, 7000 Mons, Belgium
Aluminum(III) is known to form highly stable complexes with
the bidentate chelating 8-hydroxyquinoline. In aqueous solu-
tions, three metal–ligand complexes Alq (log β ∼ 8.9), Alq2
(log β ∼ 17.4) and Alq3 (log β ∼ 24.6) are formed with Alq3
being the predominant metal species in a very wide range of pH.
†Electronic supplementary information (ESI) available: 1H (Fig. S1)
and 2D COSY (Fig. S2) NMR spectra of ligand H44, ESI mass spectrum
of [(Gd5)3Al(H2O)3]3− (Fig. S3), absorption spectra of [Gd(H5)(H2O)]−
and [(Gd5)3Al(H2O)3]3− (Fig. S4). See DOI: 10.1039/c2dt30605k
This journal is © The Royal Society of Chemistry 2012
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In the presence of a fully formed complex, only a small amount
of free ligand is produced by complex dissociation, having a
kinetic constant estimated to be 0.2 s−1 51–53
Tris-(8-quinolinate)
.
aluminum(III) complexes and numerous derivatives have been
intensively investigated for their strong green luminescence.54–56
Devices with strong electroluminescent properties for OLED
applications were already successfully prepared. For this
purpose, the high complex stability was assured by dissolving
the Alq3 derivatives in dichloromethane and toluene, avoiding
moisture.55,56 In this work, benzyl protected 5-amino-8-hydroxy-
quinoline was coupled to diethylenetriamine-pentaacetic acid
(DTPA) via a glycine linker to form H44. This ditopic ligand is
able to strongly coordinate to a lanthanide ion with the DTPA
unit, while after deprotection, the 8-hydroxyquinoline self-
assembled around Al(III) ions, resulting in a new metallostar
compound [(Ln5)3Al(H2O)3]3−. The synthesis of the ligand was
confirmed by mass spectrometry, NMR and IR measurements.
Complexation to the diamagnetic La(III) ion allowed recording
1
the H NMR spectrum of [(La5)3Al(H2O)3]3−. Finally, the mag-
netic as well as the photophysical properties of both monomeric
[Gd(H5)(H2O)]− and metallostar [(Gd5)3Al(H2O)3]3− complexes
were investigated.
Results and discussion
Synthesis of ligand and complexes
The synthesis of the 8-hydroxyquinoline DTPA-based ligand
started with the protection of the hydroxyl group of 5-nitro-
8-hydroxyquinoline by a benzyl protecting group, resulting in
5-nitro-8-benzyloxyquinoline (1).
Scheme 1 Synthesis of ligand H44. Conditions: (i) benzyl bromide,
K2CO3, dry DMF; (ii) SnCl2·2H2O, EtOH; (iii) tBoc-glycine, HATU,
DIPEA, dry DCM; (iv) CF3COOH–DCM (2 : 1, v/v); (v) DTPA-precur-
sor, TBTU, DIPEA, dry DMF; (vi) HCl 6 N.
The nitro group of compound 1 was reduced by tin(II) chloride
dihydrate in ethanol. Because of the low stability of this aromatic
amine functional group, the product was immediately coupled
with tBoc-glycine in the presence of ortho-(7-azabenzotriazol-
1-yl)-N,N,N′,N′-tetramethyluronium hexafluorophosphate (HATU)
yielding compound 2. After deprotection by trifluoroacetic acid
(TFA), a more stable amine group was obtained (3) and coupled
further with N,N-bis{N,N-bis[(tert-butoxycarbonyl)methyl]-ethyl-
amine}-glycine to yield the benzyl- and tertbutyl-protected
8-hydroxyquinoline derivative 4. After removal of the tertbutyl
protecting groups, ligand H44 was obtained as a yellow-brownish
solid (Scheme 1). The benzyl protecting group was maintained to
prevent coordination of the lanthanide(III) ion to the 8-hydroxyqui-
noline moiety.57,58
A proton NMR spectrum of ligand H44 was recorded in D2O
and the observed peaks correspond to the proposed structure of
the molecule (Fig. S1 in the ESI†). Further, the ligand was
characterized by a two-dimensional COSY experiment (Fig. S2
in the ESI†), 13C NMR, and CHN analysis. The electrospray
mass spectrum (ESI-MS) in the positive mode showed molecular
peaks [M + H]+ and [M + Na]+ at m/z = 683.4 and 705.2,
respectively. Lanthanide(III) complexes were obtained by reacting
ligand H44 with the corresponding lanthanide(III) chlorides (Ln =
La, Gd) under slightly alkaline conditions (pH = 8). After com-
plexation, the benzyl protecting group was removed by hydro-
genation, resulting in the formation of [Ln(H5)(H2O)]−. All
complexes were purified by Chelex® 100, in order to remove the
free lanthanide ions. The purity of the complexes was verified
with a test with an arsenazo indicator solution.59 Positive mode
ESI-MS of the complexes showed molecular peaks [M + 2H]+,
[M + 2Na]+ and [M + 2Na + H2O]+ at m/z = 729.4, 773.3 and
791.3 corresponding to the La(III) complex and at m/z = 747.8,
791.6 and 809.6 corresponding to the Gd(III) complex. The final
complexes, [(Ln5)3Al(H2O)3]3− (Ln = La, Gd), were obtained
by reacting [Ln(H5)(H2O)]− with anhydrous AlCl3 under
slightly alkaline conditions (pH = 8) (Scheme 2). Positive mode
ESI-MS showed a molecular peak [M + 4Na + 2H + 2H2O]3+ at
m/z = 778.2 and 796.4, corresponding to the La(III) and Gd(III)
complexes (see Fig. S3 in the ESI†). Inductively coupled plasma
optical emission spectrometry (ICP-OES) confirmed a 3 : 1 ratio
of Gd(III) versus Al(III).
1
Fig. 1 shows the H NMR spectra of H44 and the correspond-
ing La(III)–Al(III) complex, [(La5)3Al(H2O)3]3−. The spectrum
shows a significant change in the aliphatic region, indicating
complexation of H44 to La(III). All aliphatic protons, except for
proton f, show line broadening and an increase of proton reso-
nances which is consistent with the occurrence of several inter-
converting isomers characteristic for lanthanide(III) complexes
with DTPA ligands.60 The aluminum(III) ion can coordinate to
three 8-hydroxyquinolinate entities via the oxygen and nitrogen
donor atoms of the ligand. Hereby, [(La5)3Al(H2O)3]3− can exist
as two different isomers with different chirality, i.e. the facial
and meridional isomers.52,61–63 Because of the higher stability of
10550 | Dalton Trans., 2012, 41, 10549–10556
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Fig. 2 Framework molecular model of the complex [(Gd5)3Al-
(H2O)3]3−. Hydrogen atoms have been omitted for clarity.
IR spectral data show a strong absorption at 1636 cm−1 due to
the asymmetric CvO stretching vibration of the deprotonated
acid. A shift of approximately 42 cm−1 to lower energy is
observed for [Ln(H5)(H2O)]−, confirming complexation of the
lanthanide ion by the ligand. Upon complexation of the 8-hydro-
xyquinoline moiety to aluminum(III), the asymmetric CvO
stretching vibration remained unaltered, indicating that the local
environment of the lanthanide(III) ion was not changed.
Although no single crystals suitable for X-ray diffraction ana-
lysis could be grown, the data obtained from IR, ESI-MS
and NMR are consistent with the formation of a supramole-
cular complex with three lanthanide(III) ions and one central
aluminum(III) ion (Fig. 2).
Scheme 2 Formation of the [(Gd5)3Al(H2O)3]3− complex: (i) GdCl3·
6H2O, pyridine; (ii) Pd/C 5%, H2 gas; (iii) anhydrous AlCl3, pyridine.
Photophysical properties
The absorption spectrum of [Gd(H5)(H2O)]− shows an intense
band at 242 nm (ε = 28 100 cm−1 M−1) which is attributed to a
π → π* transition (see Fig. S4 in the ESI†). At lower energy,
a less intense and broader π → π* band is situated at 305 nm
(ε = 4100 cm−1 M−1) and can be ascribed to the protonated qui-
nolinate moiety.65 After coordination with aluminum, the absorp-
tion of [(Gd5)3Al(H2O)3]3− shows a red shift to 255 nm of
the highest energy band (ε = 65 900 cm−1 M−1). The lowest
energy band also red-shifts to 367 nm (ε = 9100 cm−1 M−1).
The position of the bands is typical for aluminum(III) complexes
of 8-hydroxyquinoline.54
In order to investigate the feasibility of [(Gd5)3Al(H2O)3]3− to
act as a bimodal agent, its luminescent properties were further
investigated. 8-Hydroxyquinoline and the aluminum quinolinate
complex are known to exhibit intensive green-blue broad-band
luminescence.54–56 Upon excitation into the π → π* transition
band at 305 nm, [Gd(H5)(H2O)]− shows a blue broad-band
emission in the range of 400–700 nm with a maximum of
454 nm (Fig. 3). After coordination with Al(III), the broad-band
emission of [(Gd5)3Al(H2O)3]3− red-shifts from blue to green
with an emission maximum of 543 nm upon excitation at
367 nm.
Fig. 1 1H NMR spectra of ligand H44 (bottom) and [(La5)3Al(H2O)3]3−
(top) in D2O at 298 K.
the meridional isomer, this form predominates in solution.61,64
The broadening of proton signals, which can be seen in the aro-
matic region, indicates the occurrence of the two isomers after
coordination of 8-hydroxyquinoline to the aluminum(III) ion.
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by several parameters, such as the number of water molecules
coordinated in the first hydration sphere of the complexed ion
(q), the electronic relaxation times of Gd(III) (τS1 and τS2), the
rotational correlation time (τR) and the residence time of the
coordinated water molecules (τM). A fixed τM value of 200 ns
was used to perform the fitting because this value is in good
agreement with other mono-amide derivatives of DTPA–gadoli-
nium(III) complexes.69
The proton nuclear magnetic relaxation dispersion (NMRD)
profiles of [Gd(H5)(H2O)]− and [(Gd5)3Al(H2O)3]3− are shown
in Fig. 4. An enhanced r1 relaxivity up to 10.9 s−1 mM−1
per Gd(III) ion at 20 MHz and 310 K corresponding to
32.7 s−1 mM−1 per metallostar molecule is obtained. The theo-
retical fitting of the NMRD profiles takes into account the inner
and outer sphere contributions to the paramagnetic relaxation
rate. Some parameters were fixed during the fitting procedure:
the distance (d) of closest approach for the outer sphere contri-
bution was set at 0.36 nm, τM was set to 200 ns as described
above, the number of coordinated water molecules was set to one
(q = 1), the relative diffusion constant (D = 3.3 × 10−9 m2 s−1)70
and r, the distance between the Gd(III) ion and the proton nuclei
of water (r = 0.31 nm). The results of these fittings are shown in
Table 1. The plain lines in Fig. 4 correspond to the theoretical
Fig. 3 Emission spectrum (λex = 305 nm) of [Gd(H5)(H2O)]− (top)
and emission spectrum (λex = 367 nm) of [(Gd5)3Al(H2O)3]3− (bottom),
1 × 10−4 M in H2O.
The emission spectrum of [(Gd5)3Al(H2O)3]3− also shows a
shoulder at 456 nm which can be attributed to [Gd(H5)(H2O)]−,
most likely occurring as a result of a change in equilibrium
at low concentrations. The band situated around 425 nm
can be ascribed to a Raman band of water due to excitation
at 367 nm.
The emission maximum of [(Gd5)3Al(H2O)3]3− shows a red-
shift of 18 nm compared to Alq3 (λmax em = 525 nm) upon deri-
vatization with DTPA. This effect is caused by the amide group
situated on the 5-position of the quinolinate ligand because elec-
tron-donating groups located on the 5-position of 8-hydroxyqui-
noline decrease the HOMO–LUMO energy gap of the ligand
and hereby show emission at higher wavelengths.56 The
quantum yield of [(Gd5)3Al(H2O)3]3− was determined with
quinine sulfate in 0.05 M H2SO4 as a standard and equals
0.52%.
Fig. 4 1H NMRD profiles of [Gd(H5)(H2O)]− (open circles),
[(Gd5)3Al(H2O)3]3− (closed circles) and Gd–DTPA (dashed line) in
water at 310 K. The plain line through the experimental data is the result
of the classical fitting of the data. The dashed line corresponds to the
fitting using the Lipari–Szabo approach.
Table 1 Parameters obtained by the theoretical adjustment of the
proton NMRD data in water at 310 K
Relaxometric studies
Parameter
Gd–DTPAa [Gd(H5)(H2O)]− [(Gd5)3Al(H2O)3]3−
The relaxivity of a Gd(III) complex is defined as the efficiency to
enhance the relaxation rate of the neighbouring water protons
and is expressed in s−1 mM−1. It arises from the contributions of
short distance interactions between the paramagnetic Gd(III) ion
and the coordinated water molecules exchanging with bulk
water, the so-called inner sphere interaction,66,67 and from the
long distance interactions related to the diffusion of water
molecules near the paramagnetic Gd(III) center, i.e. the outer
sphere interaction.68 Inner sphere interactions can be described
τM310 [ns]
τR310 [ps]
143
54
200b
147
200b
1
3
295 3 (τRg = 305 1,
τRl = 104 46)c
τSO310 [ps]
τV310 [ps]
r1 [s−1 mM−1
87
25
3
3
80
35
1
2
117 1 (120 2)c
53 2 (40 0.1)c
10.9
] 3.8 0.2 6.17
at 20 MHz
a From ref. 71. b Fixed value. c Fit using the Lipari–Szabo approach.
10552 | Dalton Trans., 2012, 41, 10549–10556
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fittings of the data points. The dashed line corresponds to the
fitting of the Gd–DTPA data. The profile of [(Gd5)3Al(H2O)3]3−
shows increased values compared to the profile of [Gd(H5)-
(H2O)]− and Gd–DTPA as a result of its higher molecular
weight. The τR value of [(Gd5)3Al(H2O)3]3− agrees well with
the size of a supramolecular complex but the agreement between
the experimental data and the fit at high magnetic fields is quite
poor. A better fit of the high field data could be obtained by
using the Lipari–Szabo approach (Fig. 4). This fit results in a
global τR value of 350 1 ps, a local τR of 104 46 ps and S2
equal to 0.86 0.02 showing that this metallostar is quite rigid.
These data are in agreement with the values reported for a larger
metallostar {Fe[Gd2bpy(DTTA)2(H2O)4]3}4− (global τR = 930
50 ps, local τR = 190 15 ps and S2 = 0.6 0.04 at 298 K).21
Samples for the mass spectrometry were prepared by dissolving
the product (2 mg) in methanol (1 mL), then adding 200 μL of
this solution to a water–methanol mixture (50 : 50, 800 μL).
The resulting solution was injected at a flow rate of 5 μL min−1
.
The metal contents were detected on a Varian 720-ES ICP
optical emission spectrometer with reference to Chem-Lab gado-
linium and aluminum standard solutions (1000 μg mL−1, 2–5%
HNO3). Absorption spectra were measured on a Varian Cary
5000 spectrophotometer on freshly prepared aqua solutions in
quartz Suprasil® cells (115F-QS) with an optical path-length of
0.2 cm. Emission data were recorded on an Edinburgh Instru-
ments FS920 steady state spectrofluorimeter. This instrument is
equipped with a 450W xenon arc lamp, a high energy micro-
second flashlamp μF900H and an extended red-sensitive photo-
multiplier (185–1010 nm, Hamamatsu R 2658P). All spectra are
corrected for the instrumental functions. Quantum yields were
determined by a comparative method using a solution of quinine
sulfate (Fluka) in 1 N H2SO4 (Q = 54.6%) as a standard; esti-
mated error 20%.72
Conclusions
A supramolecular metallostar [(Ln5)3Al(H2O)3]3− was syn-
thesized starting from a ditopic ligand with a DTPA and an
8-hydroxyquinoline moiety. The DTPA unit coordinates to a
Gd(III) ion, forming the complex [Gd(H5)(H2O)]−. The rotational
tumbling time τR of this complex is a factor of 2.7 higher in
comparison with that of Gd–DTPA (Magnevist®). This enhances
Model
The model was built using Avogadro, an open-source molecular
builder and visualization tool, version 1.00. The central part con-
taining Al(III) and three 8-hydroxyquinoline molecules and the
arms including Gd(III) were first optimized separately with the
Universal Force Field (UFF).73 The 8-hydroxyquinoline parts of
the central unit and the arms were overlaid and the entire
complex was re-optimized with UFF using Open Babel.
the relaxivity r1 at 20 MHz and 310 K up to 6.17 s−1 mM−1
,
compared to a value of 3.8 s−1 mM−1 for Gd–DTPA. The
8-hydroxyquinoline moiety, in turn, self-assembles around an
Al(III) ion, leading to the formation of the metallostar compound
[(Gd5)3Al(H2O)3]3−. This further increases the rotational tumb-
ling time τR by a factor of 5.5 and results in the relaxivity r1 at
20 MHz and 310 K up to 10.9 s−1 mM−1 per Gd(III) ion, which
corresponds to 32.7 s−1 mM−1 per heteropolymetallic complex.
In addition to the high relaxivity values, [(Gd5)3Al(H2O)3]3−
exhibits green broad-band emission luminescence upon exci-
tation at 367 nm. The favorable relaxometric and photophysical
properties of this metallostar make it an interesting compound
for the further development of bimodal (optical/MR) imaging
agents.
Proton NMRD
Proton nuclear magnetic relaxation dispersion (NMRD)
profiles were measured on a Stelar Spinmaster FFC, fast field
cycling NMR relaxometer (Stelar, Mede (PV), Italy) over a
magnetic field strength range extending from 0.24 mT to 0.7 T.
Measurements were performed on 0.6 mL samples contained in
10 mm o.d. pyrex tubes. Additional relaxation rates at 20, 60 and
300 MHz were respectively obtained on a Minispec mq20, a
Minispec mq60, and a Bruker Avance 300 spectrometer (Bruker,
Karlsruhe, Germany). The proton NMRD curves were fitted
using data-processing software,74,75 including different theore-
tical models describing the nuclear relaxation phenomena
(Minuit, CERN Library).66–68
Experimental
Materials
Reagents were obtained from Aldrich Chemical (Bornem,
Belgium) and Acros Organics (Geel, Belgium), and were used
without further purification. Gadolinium(III) chloride hexahydrate
was obtained from GFS Chemicals (Powell, Ohio, USA).
Synthesis
5-Nitro-8-benzyloxyquinoline (1). Compound (1) was pre-
pared according to a modified literature procedure.76 To a solu-
tion of 5-nitro-8-hydroxyquinoline (10 g, 52.6 mmol) in dry
DMF (220 mL) was added benzyl bromide (15.68 mL,
132 mmol) and K2CO3 (22 g, 159 mmol) and the solution was
stirred for 7 h at 70–80 °C until a brownish red precipitate was
formed. The solvent was removed in vacuo and the solid residue
was triturated three times with diethyl ether. The organic layers
were combined, washed with an aqueous sodium hydroxide
solution (1 M), dried over MgSO4 and evaporated again. The
crude product was purified by column chromatography [silica
Instruments
Elemental analysis was performed by using a CE Instruments
EA-1110 elemental analyzer. 1H and 13C NMR spectra were
recorded by using a Bruker Avance 300 spectrometer (Bruker,
Karlsruhe, Germany), operating at 300 MHz for 1H and 75 MHz
for 13C, or on a Bruker Avance 400 spectrometer, operating at
400 MHz for 1H and 100 MHz for 13C. IR spectra were
measured by using a Bruker Alpha-T FT-IR spectrometer
(Bruker, Ettlingen, Germany). Mass spectra were obtained by
using a Thermo Finnigan LCQ Advantage mass spectrometer.
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gel, DCM–petroleum ether (2 : 1)] resulting in a yellow-orange
solid (10 g, 35.7 mmol, 68%). 1H NMR (300 MHz, CDCl3,
ppm): δ 5.55 (s, 2 H, benzyl CH2), 7.05 (d, 1 H, quinoline CH),
7.33–7.40 (m, 3 H, benzyl CH), 7.50 (d, 2 H, benzyl CH), 7.69
(dd, 1 H, quinoline CH), 8.42 (d, 1 H, quinoline CH), 9.07 (d,
1 H, quinoline CH), 9.22 (d, 1 H, quinoline CH). 13C NMR
(75 MHz, CDCl3, ppm): δ 71.5 (benzyl CH2), 107.3 (quinoline
CH), 123.1 (quinoline C), 124.6, 125.0 (quinoline CH), 127.3,
127.8, 129.1 (benzyl CH), 132.6 (quinoline CH), 136.9 (benzyl C),
139.7, 142.5 (quinoline C), 150.3 (quinoline CH), 160.8
(quinoline C). ESI-MS (C16H12N2O3 [M]): m/z calcd 281.3
([M + H]+); found 281.3 ([M + H]+). Elemental analysis calcu-
lated (%) for C16H12N2O3 (280.3): C 68.56, H 4.32, N 9.99;
found: C 68.87, H 4.35, N 9.75.
obtain trifluoroacetic acid free N-glycine-5-amino-8-benzyloxy-
quinoline (1.1 g, 3.6 mmol, 87%). H NMR (300 MHz, CDCl3,
1
ppm): δ 4.06 (s, 2 H, C(O)CH2NH2), 5.33 (s, 2 H, benzyl CH2),
7.16 (d, 1 H, quinoline CH), 7.35 (m, 3 H, benzyl CH), 7.46 (d,
2 H, benzyl CH), 7.55 (m, 1 H, quinoline CH), 8.31 (d, 1 H, qui-
noline CH), 8.37 (d, 1 H, quinoline CH), 8.74 (d, 1 H, quinoline
CH). 13C NMR (75 MHz, CDCl3, ppm): δ 42.8 (C(O)CH2NH),
70.5 (benzyl CH2), 108.9, 117.4, 122.6 (quinoline CH), 123.5
(quinoline C), 127.3, 127.9, 128.8 (benzyl CH), 131.4 (quinoline
CH), 135.0 (quinoline C), 137.1 (benzyl C), 140.2, 146.7 (qui-
noline C), 149.4 (quinoline CH), 168.5 (C(O)CH2NH). ESI-MS
(C18H17N3O2 [M]): m/z calcd 308.3 ([M + H]+) and 637.6
([2M + Na]+); found 308.7 ([M + H]+) and 637.5 ([2M + Na]+).
Benzyl and tert-butyl protected 8-hydroxyquinoline derivative.
To a stirred solution of (3) (1.68 g, 5.5 mmol) in dry DMF
(70 mL), DIPEA (1.29 mL, 7.5 mmol) was added dropwise
under an argon atmosphere in a first flask. A mixture of N,N-bis
{N,N-bis[(tert-butoxycarbonyl)methyl]-ethylamine}-glycine69
(3.1 g, 5.0 mmol), o-benzotriazol-1-yl-N,N,N′,N′-tetramethyluro-
nium tetrafluoroborate (TBTU) (2.39 g, 7.5 mmol) and DIPEA
(0.86 mL, 5.0 mmol) was also prepared in dry DMF (60 mL)
under an argon atmosphere in a second flask. The solution of
the first flask was added dropwise over a period of 10 min to the
second flask. After 24 h, the DMF was evaporated and the
mixture was redissolved in DCM. The suspension was washed
with a saturated bicarbonate solution (2×), brine (2×) and dried
over MgSO4. After evaporation, the crude brown oil was purified
by MPLC [silica gel, DCM–MeOH (100 : 0) → DCM–MeOH
(100 : 7) over 2 h] resulting in the benzyl and tert-butyl protected
8-hydroxyquinoline derivative (2.66 g, 2.93 mmol, 59%).
1H NMR (300 MHz, CDCl3, ppm): δ 1.36 (s, 36 H, tert-butyl
CH3), 2.60 (t, 8 H, NCH2CH2N), 3.29 (s, 2 H, C(O)CH2N), 3.37
(s, 8 H, NCH2C(O)O), 4.22 (s, 2 H, C(O)CH2NH), 5.43 (s, 2 H,
benzyl CH2), 6.96 (d, 1 H, quinoline CH), 7.34–7.55 (m, 5 H,
benzyl CH), 7.66 (dd, 1 H, quinoline CH), 8.35 (d, 2 H, quino-
line CH), 8.87 (d, 1 H, quinoline CH). 13C NMR (75 MHz,
CDCl3, ppm): δ 28.1 (tert-butyl CH3), 43.9 (C(O)CH2NH), 51.9
(NCH2CH2N), 54.1 (NCH2CH2N), 58.3 (NCH2C(O)O), 58.7
(C(O)CH2N), 70.9 (benzyl CH2), 81.6 (tert-butyl C), 108.5,
117.8, 122.0 (quinoline CH), 122.9 (quinoline C), 127.3, 127.8,
128.7 (benzyl CH), 131.3 (quinoline CH), 133.6 (quinoline C),
137.0 (benzyl C), 140.1, 147.3 (quinoline C), 151.7 (quinoline
CH), 168.9 (NH C(O)CH2NH), 170.6 (C(O)O), 172.3 (C(O)-
CH2N). ESI-MS (C48H70N6O11 [M]): m/z calcd 908.1
([M + H]+) and 930.1 ([M + Na]+); found 907.6 ([M + H]+) and
929.5 ([M + Na]+).
N-(N-tert-Butoxycarbonylglycine)-5-amino-8-benzyloxy-quino-
line (2). To a solution of (1) (2 g, 7 mmol) in ethanol (80 mL)
was added tin(II) chloride dihydrate (6.44 g, 28.5 mmol) and the
mixture was refluxed under an argon atmosphere for 3 h. The
solution was cooled to room temperature and an aqueous solu-
tion of sodium hydrogen carbonate was added dropwise until pH
10 was reached. The reduced product was extracted with DCM,
the combined organic layers were dried over MgSO4 and evapo-
rated to give a dark red oil (1.43 g, 5.7 mmol, 82%). Because of
the low stability of the reduced 5-amino-8-benzyloxyquinoline,
it was immediately redissolved in dry DCM (25 mL) and diiso-
propylethylamine (DIPEA) (1.33 mL, 7.8 mmol) was added
under an argon atmosphere. To a stirred solution of tBoc-glycine
(0.91 g, 5.2 mmol) and o-(7-azabenzotriazol-1-yl)-N,N,N′,N′-
tetramethyluronium hexafluorophosphate (HATU) (2.96 g,
7.8 mmol) in dry DCM under an argon atmosphere was added
dropwise DIPEA (0.89 mL, 5.2 mmol) in a second flask. The
solution of the second flask was added to the first flask over
10 min and the mixture was stirred overnight. The suspension
was washed with an aqueous solution of sodium hydrogen car-
bonate, a saturated sodium chloride solution and dried over
MgSO4. After evaporation, the crude product was purified by
column chromatography [silica gel, DCM–MeOH (100 : 5)]
1
resulting in a yellow oil (2) (1.68 g, 4.1 mmol, 79%). H NMR
(300 MHz, CDCl3, ppm): δ 1.50 (s, 9 H, tert-butyl CH3), 4.02
(s, 2 H, C(O)CH2NH), 5.44 (s, 2 H, benzyl CH2), 6.98 (d, 1 H,
quinoline CH), 7.36 (t, 2 H, benzyl CH), 7.43 (m, 1 H, benzyl
CH), 7.50 (d, 2 H, benzyl CH), 7.60 (d, 1 H, quinoline CH),
8.22 (d, 1 H, quinoline CH), 8.66 (d, 1 H, quinoline CH), 8.98
(d, 1 H, quinoline CH). 13C NMR (75 MHz, CDCl3, ppm): δ
28.3 (tert-butyl CH3), 44.1 (C(O)CH2NH), 70.9 (benzyl CH2),
81.0 (tert-butyl C), 109.4, 117.1, 121.7 (quinoline CH), 124.6
(quinoline C), 127.2, 127.9, 128.7 (benzyl CH), 130.4 (quinoline
CH), 134.1 (quinoline C), 136.7 (benzyl C), 140.4, 147.2 (qui-
noline C), 149.3 (quinoline CH), 156.4 (C(O)O), 169.0 (C(O)-
CH2NH). ESI-MS (C23H25N3O4 [M]): m/z calcd 408.5
([M + H]+) and 430.5 ([M + Na]+); found 408.7 ([M + H]+) and
430.7 ([M + Na]+).
Benzyl protected 8-hydroxyquinoline derivative (H44). The
benzyl and tert-butyl protected 8-hydroxyquinoline derivative
(2.66 g, 2.93 mmol) was dissolved in a 6 N HCl (80 mL) solu-
tion. The mixture was stirred at room temperature for 1 h and
then washed with CH2Cl2 (2×). The deprotected product was
then purified by HPLC (water–acetonitrile) to give H44 as a
yellow-brownish solid (360 mg, 0.53 mmol, 18%). 1H NMR
(400 MHz, D2O, ppm): δ 2.67 (t, 4 H, NCH2CH2N), 2.94 (t,
4 H, NCH2CH2N), 3.42 (s, 2 H, C(O)CH2N), 3.49 (s, 8 H,
NCH2C(O)OH), 4.21 (s, 2 H, C(O)CH2NH), 5.35 (s, 2 H,
benzyl CH2), 6.97 (d, 1 H, quinoline CH), 7.35 (m, 3 H, benzyl
CH), 7.51 (m, 2 H, benzyl CH), 7.62 (dd, 1 H, quinoline CH),
N-Glycine-5-amino-8-benzyloxyquinoline (3). To a mixture
of CF3COOH–DCM 2 : 1 (12 mL) was added dropwise a
solution of (2) (1.68 g, 4.1 mmol) dissolved in DCM (7 mL).
The solvent was removed in vacuo after 2 h and the product was
redissolved three times in DCM and three times in MeOH to
10554 | Dalton Trans., 2012, 41, 10549–10556
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View Article Online
8.01 (t, 1 H, C(O)CH2NH), 8.37 (d, 2 H, quinoline CH), 8.92
(d, 1 H, quinoline CH). 13C NMR (100 MHz, D2O, ppm): δ
42.9 (C(O)CH2NH), 52.1 (NCH2CH2N), 54.2 (NCH2CH2N),
58.3 (C(O)CH2N), 60.9 (NCH2C(O)OH), 71.5 (benzyl CH2),
107.8, 116.7, 121.1 (quinoline CH), 122.6 (quinoline C), 127.6,
128.3, 129.9 (benzyl CH), 131.0 (quinoline CH), 134.9 (quino-
line C), 137.7 (benzyl C), 140.2, 146.6 (quinoline C), 149.1
(quinoline CH), 166.8 (NH C(O)CH2NH), 169.8 (C(O)CH2N),
173.7 (COOH). ESI-MS: (C32H38N6O11 [M]): m/z calcd 683.7
([M + H]+) and 705.7 ([M + Na]+); found 683.4 ([M + H]+) and
705.2 ([M + Na]+). IR (KBr): ν = 1636 (COO− asym. stretch),
1534 (amide II), 1393 (COO− sym. stretch) cm−1. Elemental
analysis calculated (%) for C32H38N6O11·2H2O (718.7): C
53.48, H 5.89, N 11.59; found: C 53.42, H 5.83, N 11.48.
(KBr): ν = 1594 (COO− asym. stretch), 1472 (amide II), 1396
(COO− sym. stretch) cm−1
.
Al(III)–Gd(III) complex [(Gd5)3Al(H2O)3]3−
: Yield: 66%.
ESI-MS (C75H81AlGd3N18O33 [M]): m/z calcd 797.3 ([M + 4Na
+ 2H + 2H2O]3+); found 796.4 ([M + 4Na + 2H + 2H2O]3+). IR
(KBr): ν = 1593 (COO− asym. stretch), 1475 (amide II), 1397
(COO− sym. stretch) cm−1. ICP-OES ratio (Gd/Al): 2.91.
Acknowledgements
E.D., G.D., T.N.P.V. and K.B. acknowledge the IWT Flanders
(Belgium) and the FWO Flanders (project G.0412.09) for
financial support. S.V.E. was a visiting postdoctoral fellow of the
FWO Flanders (project G.0412.09) and now works at the Centre
de Biophysique Moléculaire – CNRS in Orléans. CHN micro-
analysis was performed by Mr Dirk Henot. ESI-MS measure-
ments were done by Mr Dirk Henot and Mr Bert Demarsin and
ICP-OES measurements were performed by Ms Elvira Vassilieva
(Department of Earth and Environmental Sciences). Mr Karel
Duerinckx is acknowledged for his help with the NMR measure-
ments. We also thank Mr Servaas Michielssens for his develop-
ment of the framework molecular model.
Lanthanide complexes. To prevent coordination of the lantha-
nides to the 8-hydroxyquinoline moiety of the ligand,57,58 the
lanthanide(III) complexes were developed starting from the
benzyl protected 8-hydroxyquinoline derivative H44 according
to a general procedure: a solution of hydrated LnCl3 salt
(1.05 mmol) in H2O was added to ligand H44 (1 mmol) dis-
solved in pyridine, and the mixture was heated at 70 °C for 3 h.
The solvent was evaporated under reduced pressure and the
crude product was then refluxed in ethanol for 1 h. After cooling
to room temperature, the complex was filtered off and dried
in vacuo. To allow further complexation with aluminum(III), the
benzyl group was removed according to the following procedure:
the lanthanide(III) complex was dissolved in a mixture of water–
methanol (1 : 1, v/v) and Pd/C 5% was added. The suspension
was stirred over 20 h under a hydrogen atmosphere at room
temperature. The mixture was filtered over Celite and evaporated
to yield the benzyl deprotected lanthanide(III) complex [Ln(H5)-
(H2O)]−. The absence of free lanthanide ions was checked by
using an arsenazo indicator.59
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This journal is © The Royal Society of Chemistry 2012
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This journal is © The Royal Society of Chemistry 2012