Novel coordinating motifs for lanthanide(III) ions based on 5-(2-pyridyl)tetrazole and 5-(2-pyridyl-1-oxide)tetrazole. Potential new contrast agents

Article abstract of DOI:10.1039/b401919a

Water-soluble and neutral Ln(III) and Zn (II) complexes of pyridine- and (pyridine-1-oxide)tetrazole have been synthesized and the Gd derivatives have great potential as high-relaxivity low-osmolarity MRI contrast agents.

Full text of DOI:10.1039/b401919a

Novel coordinating motifs for lanthanide(III) ions based on
5-(2-pyridyl)tetrazole and 5-(2-pyridyl-1-oxide)tetrazole. Potential new
contrast agents†
Antonio Facchetti,*a Alessandro Abbotto,b Luca Beverina,b Silvia Bradamante,c Palma Mariani,d
Charlotte L. Stern,a Tobin J. Marks,a Alberto Vaccad and Giorgio A. Pagani*b
a Department of Chemistry, Northwestern University, 2145 Sheridan Road, 60208, Evanston, IL, USA
b Department of Materials Science, University of Milano-Bicocca, Via Cozzi 53, I-20125, Milano, Italy
c
CNR-Institute of Molecular Science and Technologies, Via Golgi 19, I-20133, Milano, Italy
d Department of Chemistry, University of Florence, Via della Lastruccia 3, I 50019, Sesto Fiorentino, Italy
Received (in Cambridge, UK) 9th February 2004, Accepted 2nd June 2004
First published as an Advance Article on the web 30th June 2004
Water-soluble and neutral Ln(III) and Zn (II) complexes of
pyridine- and (pyridine-1-oxide)tetrazole have been synthesized
and the Gd derivatives have great potential as high-relaxivity
low-osmolarity MRI contrast agents.
employing m-chloroperbenzoic acid in MeOH. This result is
remarkable considering the large number of azine nitrogens present
in 1 and 3. Indeed, other oxidizing agents such as H2O2 and
CH3CO3H afford inseparable mixture of products. The protonation
constant of 1 (pKa = 4.11) and 2 (pKa = 3.55) were obtained via
potentiometric titration. The pKa values are lower than in tetrazole
due to conjugation with the electron-poor pyridine ring. To fully
explore the ligand capacities of both –NNC–C–N2– and O2–
N+NC–C–N2– moieties present in this new class of ligands, anions
12 and 22 were reacted with both a transition metal ion [Zn(II), to
The emergence of magnetic resonance imaging (MRI) techniques
for medical diagnosis has been accompanied by an intensive growth
of interest in the study of paramagnetic metal complexes as contrast
agents (CAs) designed to enhance tissue differentiation.1,2 The
majority of the approved CAs are Gd(III) chelates, the efficiency of
which is given by the proton relaxivity, r1, described as the proton
relaxation rate enhancement in the presence of the CA compared to
a diamagnetic environment. Currently, all Gd(III)-based chelates
approved for use in MRI are nine-coordinate complexes in which a
ligand occupies eight binding sites at the metal center and the ninth
coordination site is occupied by one water molecule. Most of the
investigated CAs are derivatives of a limited number of coordinat-
ing core structures, such as those of DTPA (diethelenetriamine-
N,N,NA,NB,NB-pentaacetic acid) and DOTA (1,4,7,10-tetraazacy-
clododecane-N,NA,NB,NÚ-tetraacetic acid) which have relaxivity
1
allow chelate H NMR analysis] and a set of lanthanide [Gd(III),
Eu(III), and Dy(III)] ions, whereas ligands 32 and 42 were
investigated targeting both Zn(II) and Gd(III) ions.
The synthesis of neutral chelates 1–4 was planned depending on
their expected solubility properties. Poorly or relatively soluble
chelates were prepared by reacting LH with the stoichiometric
amount of K2CO3 in H2O (Scheme 2A). The corresponding anion
L2 was then reacted with concentrated aqueous solutions of the
corresponding metal salts (acetate or chloride) and the resulting
complexes isolated by filtration. For highly soluble Gd chelates, the
ligand anion was prepared by reacting LH with BaCO3 (Scheme
2B). A solution of Gd2(SO4)4 was then added to the barium salt, and
the insoluble BaSO4 filtered off. Evaporation of water afforded the
chelate. All the chelates (Table S1, Supporting Information†) were
purified by crystallization from either H2O or H2O–MeOH.
Single crystals of Gd(12)3, Gd(22)3, and Zn(22)2 were obtained
either by cooling of a saturated solution or slow evaporation of an
aqueous solution of the complex.§ The Gd(III) ion in Gd(12)3 and
Gd(22)3 (Fig. 1) is nona- and octacoordinate by three bidentate
values of ~ 4–5 mM21 21 3
s . Incorporation/functionalization of
these ligand motifs into polymeric, dendrimeric, and protein
structures increases the rotational correlation time and results in
dramatically improved r1 values.3,4 However, the maximum values
are still about half of the possible theoretical limit ( ~ 100
mM21 21) for these cores.1a,4
s
From this intense research effort, all heteroaromatic-based
ligands are practically absent. This is surprising considering the
rich chemistry, stability, and good coordinating properties of many
heterocycles.5 We describe here a new family of ligands (LH =
1–4) based on the pyridine/pyridine-N-oxide–tetrazole skeleton.
Ligands 3 and 4 bearing hydrophilic chains were envisioned to
improve solubility in water. Thanks to the acidity of tetrazole (pKa
= 4.89 in H2O)5 all these systems form stable anions (L2) at
physiological pH range, which act as efficient ligands to transition
[Zn(II)] and lanthanide [Gd (III), Eu(III), Dy(III)] metal ions.
Scheme 1 Synthetic routes to ligands 1–4.
Compound 16 and new ligands 2–4 were prepared according to
Scheme 1.‡ The tetrazole ring in 1 and 3 was synthesized by
reaction of the corresponding cyanopyridine with HN3 (NaN3 +
NH4Cl) in DMF at 130 °C. Regioselective oxidation of the pyridine
nitrogen of 1 and 3 to afford 2 and 4, respectively was achieved
† Electronic supplementary information (ESI) available: chelate analytical
data, crystal structure of Zn(22)2, potentiometric titrations, and relaxivity
plots. See http://www.rsc.org/suppdata/cc/b4/b401919a/
Scheme 2 Synthetic routes to the neutral chelates.
1770
C h e m . C o m m u n . , 2 0 0 4 , 1 7 7 0 – 1 7 7 1
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

mL) was reacted at 130 °C for 2–4 h. After cooling, the inorganic salts were
discarded by filtration and the solvent removed under reduced pressure. The
residue was taken up with dilute HCl (0.1 M, 20 mL) and the formed solid
collected and recrystallized from H2O. 1 (78%): mp 213 °C; 1H NMR (300
MHz, DMSO-d6): d 8.79 (dd, J = 5.0, 1.7, H-6); 8.22 (d, J = 8.0, H-3); 8.08
(td, H-4); 7.63 (dd, J = 8.0, H-5). 3 (76%): mp > 240 °C; 1H NMR (300
MHz, DMSO-d6): d 9.18 (d, J = 2.0, H-2); 8.47 (dd, J = 8.3, 2.0, H-4); 8.44
(d, J = 8.0, –CONH–); 8.32 (d, J = 8.0, H-5); 4.70 (s, OH); 4.01 (dtt, J =
8.0, 5.8, 5.8, CH); 3.60–3.48 (m, CH2). Ligands 2 and 4. A mixture of parent
pyridyltetrazole (6.8 mmol) and m-chloroperbenzoic acid (11.6 mmol) in
MeOH (200 mL) was reacted overnight at room temperature without
stirring. 2 (66%) was isolated by filtration and recrystallized from H2O. mp
> 240 °C; 1H NMR (300 MHz, DMSO-d6): d 8.56 (d, J = 6.4, H-6); 8.41
(dd, J = 7.8, 2.1, H-3); 7.66 (td, H-4); 7.63 (dd, J = 7.7, H-5). 4 (80%) was
isolated by evaporating the solvent and, after washing the solid with CH2Cl2
(300 mL), recrystallized from EtOH/H2O. mp > 240 °C; 1H NMR (300
MHz, DMSO-d6): d 8.97 (s, H-2); 9.49 (d, J = 8.5, H-4); 8.53 (d, J = 8.5,
– CONH–); 7.92 (d, J = 8.5, H-5); 4.70 (s, OH); 4.01 (m, CH); 3.60–3.40
(m, CH2).
Fig. 1 Crystal structure of Gd(12)3 and Gd(22)3.
ligands and three/two H2O molecules, respectively. The different
coordination number probably reflects the dissimilar steric require-
ments3 of a five- versus a six-membered chelation motif for 1 and
2, respectively. Interestingly, three/five additional H2O molecules
are present in the second coordination sphere, suggesting large
relaxivity values. The geometry of Zn(22)2 (Figure S2†) is
octahedral with two equatorial ligands and two H2O molecules in
the axial positions.
Ligands 1–4 and the corresponding Gd(III) chelates are highly
soluble in H2O. Chelate solubility increases going from the
unsubstituted 1 (2.1 g L21) and 2 (4.7 g L21)) to the hydroxyalkyl-
substituted 3 (25.6 g L21) and 4 (21.2 g L21) systems. An
additional requirement for practical application of Gd(III) com-
plexes is thermodynamic stability, to avoid toxicity of the metal.
Potentiometric data obtained by titration of a Gd:LH (LH = 1 and
2, Figs. S3 and S4†) mixture in the pH range of 2–5 (T = 298 K,
I = 0.1 M) can be fitted assuming the stepwise addition of up to
three L2 to the Gd ion,7 leading to a cumulative stability constant
(logb) values of 6.7(2) and 8.66(7) for Gd(12)3 and Gd(22)3,
respectively. As expected, these values are lower than those of
approved CAs based on macrocycle octadentate ligands ( >
15.8).3
Finally, the relaxivity r1 is the key property of a potential contrast
agent. Measurements were performed at pH = 5.7–7.2 and r1
values (mM21 s21) were obtained from the slope of a plot of 1/T1
vs. [GdL3] (concentration range 0.2–1 mM, Fig. S5†) and were
found to be: Gd(12)3 (9.98); Gd(22)3 (9.25); Gd(32)3 (10.79);
Gd(42)3 (17.72). These very high values cannot be explained solely
on the basis of a large number of water molecules probably
coordinating the metal ion in aqueous solution. Rather, very fast
exchange between inner/outer sphere water molecules with the bulk
H2O might be involved, as suggested for complexes of heptadentate
ligands.8
§ Crystal Structure. All measurements were made on a Bruker SMART
CCD diffractometer with graphite monochromated MoKa (0.71073 Å)
radiation. The data were collected at a temperature of 153(2) K and the
structures were solved by direct methods and expanded using Fourier
techniques using SHELXTL. All non-hydrogen atoms were refined
anisotropically. Hydrogen atoms were included in idealized positions and
not refined. Intensities were corrected for absorption. Gd(12)3. Monoclinic,
P2(1)/n, Z = 4. Cell dimensions: a = 9.0273(9) Å; b = 17.714(3) Å; c =
16.7480(19) Å; a = 90°; b = 99.819(9)°; g = 90.00°. V = 2639.0(6) Å3,
rcalcd = 1.794 g cm23, total/independent reflections = 24502/6489, R(int)
= 0.0415, R = 0.0280. Zn(22)2. CCDC 232483.Monoclinic, C2/c, Z = 8.
Cell dimensions: a = 31.01(2) Å; b = 8.561(4) Å; c = 7.546(3) Å; a =
90°; b = 100.65(5)°; g = 90°. V = 1968.8(18) Å3, rcalcd = 1.875 g cm23
,
total/independent reflections = 8819/2401, R(int) = 0.0564, R = 0.0469.
Gd(22)3. CCDC 232481. Monoclinic, P2(1)/n, Z = 4. Cell dimensions: a =
7.847(3) Å; b = 39.783(7) Å; c = 9.092(3) Å; a = 90°; b = 96.50(3)°; g
= 90.00°. V = 2820.0(2) Å3, rcalcd = 1.780 g cm23, total/independent
reflections = 18878/5168, R(int) = 0.0393, R = 0.0762. CCDC 232482.
See http://www.rsc.org/suppdata/cc/b4/b401919a/ for crystallographic data
in .cif or other electronic format.
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In summary, we described here new bidentate ligand motifs
based solely on heteroaromatic units and leading to water-soluble
neutral chelates. The Gd(III) complexes exhibit a large number of
coordinated H2O molecules and Gd(42)3 exhibits the highest r1
values reported to date for a low molecular weight molecule.
Thanks to the facile functionalization of the pyridine ring,
incorporation of these chelating motifs into a single tripodal
hexadentate structure should greatly improve stability (necessary
for application) and retain the intrinsic high relaxivity. Starting
from these large values, r1 can be further improved by ligand
incorporation into slower rotational motion substrates such as
proteins, dendrimers, and polysaccharides,1,2 as demonstrated
successfully for functionalized DTPA-protein bound (r1 ~ 50
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mM21 21 4a
s
)
and DOTA-dendrimer (r1 ~ s )
15–19 mM21 21
systems.4d Efforts in this direction are in progress.
This work was supported by grants MURST II (0103-97) and
NSF CHE-0078998.
5 A. R. Katritzky in, Handbook of Heterocyclic Chemistry, Pergamon
Press, Oxford, 1985.
6 J. M. Mc Manus and R. M. Herbst, J. Org. Chem., 1959, 24, 1462.
7 Potentiometric data were analysed using HYPERQUAD program . P.
Gans, A. Sabatini and A. Vacca, Talanta, 1996, 43, 1739.
Notes and references
‡
Synthetic details. Ligands 1 and 3. A mixture of parent cyanopyridine
8 Y. Bretonniere, M. Mazzanti, J. Pecaut, F. A. Dunand and A. E. Merbach,
Chem. Commun., 2001, 621.
(5.3 mmol), NaN3 (8.0 mmol), NH4Cl (8.0 mmol) in anhydrous DMF (27
C h e m . C o m m u n . , 2 0 0 4 , 1 7 7 0 – 1 7 7 1
1771