Auto-assembling of ditopic macrocyclic lanthanide chelates with transition-metal ions. Rigid multimetallic high relaxivity contrast agents for magnetic resonance imaging

Article abstract of DOI:10.1021/ic0603050

PhenHDO3A is a ditopic ligand featuring a tetraazacyclododecane unit substituted by three acetate arms and one 6-hydroxy-5,6-dihydro-1,10- phenanthroline group (PhenHDO3A = rel-10-[(5R,6R)-5,6-dihydro-6-hydroxy-1,10- phenantholin-5-yl)-1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid). This ligand was specially designed so as to obtain highly stable heteropolymetallic assemblies. PhenHDO3A has been prepared starting from phenanthroline epoxide and either a triprotected tetraazacyclododecane or tert-butyl triester of N,N′,N″-tetraazacyclododecane-triacetic acid. The latter yields PhenHDO3A in a single step. PhenHDO3A forms kinetically stable lanthanide complexes (acid-catalyzed kinetic constant k_H = (1.2 ± 0.2) × 10^-3 s^-1 M^-1) whose solution structure has been deduced from a quantitative analysis of the paramagnetic shifts and the longitudinal relaxation times of the proton nuclei of YbPhenHDO3A. The alcohol group of the dihydro-phenanthroline unit remains coordinated to the encapsulated metal ion despite the steric crowding brought about by this group. Furthermore, the complexes are monohydrated, as shown by luminescence lifetime measurements on EuPhenHDO3A solutions. Relaxivity titrations at 20 MHz clearly indicate that the phenanthroline unit of GdPhenHDO3A is available for the spontaneous formation of highly stable tris complexes with the Fe²⁺ and Ni ²⁺ ions. The water-exchange times and the rotational correlation times of GdPhenHDO3A and Fe(GdPhenHDO3A)₃²⁺ have been deduced from variable temperature ¹⁷O NMR studies and from nuclear relaxation dispersion curves. Despite rather slow water-exchange rates (τ_m⁰ = 1.0-1.2 × 10^-6 s), relaxivity gains of 90% have been observed upon the formation of the heterometallic tris complexes. The latter rotate about four times more slowly (τ _r⁰ = 398 ps) than the monomeric unit (τ _r⁰ = 105 ps) and their relaxivity is, accordingly, twice as high. The relaxivity of the tris complexes between 10 and 50 MHz is comparable to relaxivities reported for Gd³⁺-containing dendrimers of much higher molecular weights. The high relaxivity of the tris-PhenHDO3A lanthanide complexes is attributed to their internal rigidity.

Full text of DOI:10.1021/ic0603050

Inorg. Chem. 2006, 45, 5092−5102
Auto-Assembling of Ditopic Macrocyclic Lanthanide Chelates with
Transition-Metal Ions. Rigid Multimetallic High Relaxivity Contrast
Agents for Magnetic Resonance Imaging
Je´roˆme Paris, Cristiana Gameiro,§ Vale´rie Humblet, Prasanta K. Mohapatra,† Vincent Jacques, and
Jean F. Desreux*
Coordination and Radiochemistry, UniVersity of Lie`ge, Sart Tilman B16, B-4000 Lie`ge, Belgium
Received February 22, 2006
PhenHDO3A is a ditopic ligand featuring a tetraazacyclododecane unit substituted by three acetate arms and one
6-hydroxy-5,6-dihydro-1,10-phenanthroline group (PhenHDO3A rel-10-[(5R,6R)-5,6-dihydro-6-hydroxy-1,10-
)
phenantholin-5-yl)-1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid). This ligand was specially designed so as
to obtain highly stable heteropolymetallic assemblies. PhenHDO3A has been prepared starting from phenanthroline
epoxide and either a triprotected tetraazacyclododecane or tert-butyl triester of N,N′,N′′-tetraazacyclododecane-
triacetic acid. The latter yields PhenHDO3A in a single step. PhenHDO3A forms kinetically stable lanthanide complexes
3
1
(acid-catalyzed kinetic constant kH
) (1.2 ± 0.2) ×
10- s- M-1) whose solution structure has been deduced from
a quantitative analysis of the paramagnetic shifts and the longitudinal relaxation times of the proton nuclei of
YbPhenHDO3A. The alcohol group of the dihydro-phenanthroline unit remains coordinated to the encapsulated
metal ion despite the steric crowding brought about by this group. Furthermore, the complexes are monohydrated,
as shown by luminescence lifetime measurements on EuPhenHDO3A solutions. Relaxivity titrations at 20 MHz
clearly indicate that the phenanthroline unit of GdPhenHDO3A is available for the spontaneous formation of highly
+
+
stable tris complexes with the Fe2 and Ni2 ions. The water-exchange times and the rotational correlation times
2+
of GdPhenHDO3A and Fe(GdPhenHDO3A)3 have been deduced from variable temperature 17O NMR studies
0
6
and from nuclear relaxation dispersion curves. Despite rather slow water-exchange rates (τm
) 1.0−1.2 ×
10-
s), relaxivity gains of 90% have been observed upon the formation of the heterometallic tris complexes. The latter
rotate about four times more slowly (τr0 ) 398 ps) than the monomeric unit (τr0 ) 105 ps) and their relaxivity is,
accordingly, twice as high. The relaxivity of the tris complexes between 10 and 50 MHz is comparable to relaxivities
reported for Gd3+-containing dendrimers of much higher molecular weights. The high relaxivity of the tris-PhenHDO3A
lanthanide complexes is attributed to their internal rigidity.
Introduction
increase the longitudinal relaxation rate 1/T1 of water protons
in the extracellular space. The first generation of contrast
agents consisted of nonspecific contrast agents that are
carried by blood circulation out to tumors. New paramagnetic
lanthanide chelates are constantly being synthesized in hopes
of reaching the largest possible reduction of the longitudinal
proton relaxation time T1 in vivo and thus the best contrast
in magnetic resonance images.1,3,4 Contrast agents must fulfill
strict requirements concerning toxicity, biodistribution, and
relaxivity (relaxation rate 1/T1 in s-1 and per mmol of Gd3+).
The toxicity of the Gd3+ ion itself has been drastically
Magnetic resonance imaging (MRI) with gadolinium-
containing contrast agents has rapidly evolved into an
important clinical modality that is now used every day in
hospitals.1,2 The contrast of MRI images is improved by the
injection of Gd(III) complexes, which are able to drastically
* To whom correspondence should be addressed. E-mail: jf.desreux@
ulg.ac.be.
§ On leave from the Departamento de Quimica Fundamental, Univer-
sidade Federal de Pernambuco, Recife, PE, Brazil.
† On leave from the Bhabha Atomic Research Center, Trombay, India.
(1) Merbach, A. E., Toth, E., Eds. The Chemistry of Contrast Agents in
Medical Magnetic Resonance Imaging; Wiley: Chichester, U.K., 2001.
(2) Caravan, P.; Ellison, J. J.; McMurry, T. J.; Lauffer, R. B. Chem. ReV.
1999, 99, 2293-2352.
(3) Jacques, V.; Desreux, J. F. Top. Curr. Chem. 2002, 221, 123-164.
(4) Aime, S.; Cabella, C.; Colombatto, S.; Geninatti Crich, S.; Gianolio,
E.; Maggioni, F. J. Magn. Reson. Imaging 2002, 16, 394-406.
5092 Inorganic Chemistry, Vol. 45, No. 13, 2006
10.1021/ic0603050 CCC: $33.50
© 2006 American Chemical Society
Published on Web 06/03/2006

Ditopic Macrocyclic Lanthanide Chelates
Scheme 1
are interesting in their own right, PhenHDO3A is also an
appealing addition to the list of MRI contrast agents that
are responsive to a given stimulus, whether the oxygen
content,9 pH,10,11 a biological molecule,2,3 or biologically
relevant ions such as Ca(II).12 We already published a
preliminary account on PhenHDO3A and showed that the
approach illustrated in Scheme 2 indeed leads to a sizable
increase in relaxivity.13 This account was the source of
inspiration of recent reports by Livramento et al.,14,15 who
investigated a similar system featuring phenanthroline and
a DTPA analogue. A full description of the syntheses16 and
some complexing properties of PhenHDO3A is presented
here. NMR spectroscopy and the dispersion of the nuclear
magnetic relaxation are used to unravel the behavior of
PhenHDO3A, 1, and its complexes.
reduced by encapsulating this ion into polyamino polycar-
boxylic cages, most of which are derived from DTPA (di-
ethylenetriaminepentaacetic acid) or DOTA (1,4,7,10-tet-
raazacyclododecane-N,N′,N′′,N′′′-tetraacetic acid) skeletons
(see Scheme 1).
The DOTA structure ensures that a kinetically inert
complex is formed because its macrocyclic structure hampers
the protonation of the coordinated nitrogen atoms.5 These
small chelates are rapidly tumbling in water, and their
relaxivity essentially depends on their short rotational cor-
relation time. A sizable increase in relaxivity is achieved if
Gd3+ chelates are covalently or noncovalently linked to
macromolecules, because the rotational correlation time
becomes comparable to or larger than the electronic and
water-exchange correlation times. The latter are then the
major factors that govern the relaxivity of these slowly
rotating species.6 Dendrimeric polymers featuring a large
number of metal chelates have thus been synthesized;
however, the achieved relaxivity gains, although large, are
not as high as expected, presumably because the movements
of the Gd3+ chelates are partially independent from those of
the macromolecular units.7 Rigidity is a factor of utmost
importance for reaching very high relaxivities. It is with this
requirement in mind that we decided to synthesize very
compact multimetallic structures that are formed by the self-
association process illustrated in Scheme 2 (where Mnn+ is
a transition-metal ion).
Experimental Section
General. All commercial reagents were used as received unless
otherwise noted. All solvents were dried using common techniques.
1H and 13C NMR spectra were recorded on a Bruker DRX 400
spectrometer, and chemical shifts are reported in parts per million
relative to tetramethylsilane. The longitudinal relaxation times T1
were measured by the inversion recovery pulse sequence. Two-
dimensional NMR spectra of paramagnetic samples were recorded
as reported elsewhere.17,18 Water 17O transverse relaxation times
of GdPhenHDO3A samples (4.5 mM) enriched with H217O (1.3%)
were determined on a Bruker AM250 spectrometer using the
standard Carr-Purcell-Meiboom-Gill spin-echo pulse sequence.
Nuclear magnetic relaxation dispersion data were collected on a
Stelar Spinmaster FFC2000 relaxometer (Stelar, Mede, PV, Italy)
to which a Bruker 2 T permanent magnet had been added. The
Stelar electronic equipment and the Stelar software were used to
record T1 values with both the pulsed and permanent magnets. Time-
resolved luminescence spectra were obtained in water and in D2O
with a Photon Technology International (Birmingham, NJ) spec-
trometer equipped with a N2 laser. Quantitative interpretations of
the 17O, NMRD, and luminescence data were performed with the
Scientist19 and Mathcad20 software programs and with internally
produced computer programs. Melting or decomposition points were
The PhenHDO3A ligand, 1, features two tightly connected
complexing units: first, a DOTA-like unit that is specifically
designed to form kinetically inert lanthanide complexes, as
already reported for triacetic monoalcoholic ligands such as
HP-DO3A,8 and second, a 5,6-dihydro-phenanthroline group
that is well-known to form very stable complexes with Fe2+
or Ni2+ (PhenHDO3A ) rel-10-[(5R,6R)-5,6-dihydro-6-
hydroxy-1,10-phenantholin-5-yl)-1,4,7,10-tetraazacyclodode-
cane-1,4,7-triacetic acid, HPDO3A ) 10-(2-hydroxypropyl)-
1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid, ProHance).
It is also quite possible that the encapsulation of a metal ion
in a complexing subunit could lead to a cooperative rigidi-
fication in another coordinating entity. In addition to being
able to form tightly packed supramolecular structures that
(9) Aime, S.; Ascenzi, P.; Comoglio, E.; Fasano, M.; Paoletti, S. J. Am.
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(12) Li, W.; Parigi, G.; Fragai, M.; Luchinat, C.; Meade, T. J. Inorg. Chem.
2002, 41, 4018-4024.
(13) Comblin, V.; Gilsoul, D.; Hermann, M.; Humblet, V.; Jacques, V.;
Mesbahi, M.; Sauvage, C.; Desreux, J. F. Coord. Chem. ReV. 1999,
185-186, 451-470.
(14) Livramento, J. B.; Toth, E.; Sour, A.; Borel, A.; Merbach, A. E.; Ruloff,
R. Angew. Chem., Int. Ed. 2005, 44, 1480-1484.
(15) (a) Livramento, J. B.; Sour, A.; Borel, A.; Merbach, A. E.; Toth, E.
Chem.sEur. J. 2006, 12, 989-1003, (b) Livramento, J. B.; Weiden-
steiner, C.; Prata, M. I. M.; Allegrini, P. R.; Geraldes, C. F. G. C.;
Helm, L.; Kneuer, R.; Merbach, A. E.; Santos, A. C.; Schmidt, P.;
Toth, E. Contrast Media Mol. Imaging 2006, 1, 30-39.
(16) Desreux, J. F.; Jacques, V.; Humblet, V.; Hermann, M.; Comblin, V.;
Tweedle, M. F. U.S. Patent 6 056 939, 2000.
(5) Toth, E.; Bru¨cher, E.; Lazar, I.; Toth, I. Inorg. Chem. 1994, 33, 4070-
4076.
(6) Aime, S.; Chiaussa, M.; Digilio, G.; Gianolo, E.; Terreno, E. J. Biol.
Inorg. Chem. 1999, 4, 766-774.
(7) Nicolle, G. M.; Toth, E.; Schmitt-Willich, H.; Raduchel, B. E.;
Merbach, A. E. Chem.sEur. J. 2002, 8, 1040-1048.
(8) Kumar, K.; Jin, T. Z.; Wang, X. Y.; Desreux, J. F.; Tweedle, M. F.
Inorg. Chem. 1994, 33, 3823-3829.
(17) Ranganathan, R. S.; Raju, N.; Fan, H.; Zhang, X.; Tweedle, M. F.;
Desreux, J. F.; Jacques, V. Inorg. Chem. 2002, 41, 6856-6866.
(18) Jacques, V.; Desreux, J. F. Inorg. Chem. 1994, 33, 4048-4053.
(19) Scientist 3.0; Micromath: St. Louis, MO; http://www.micromath.com/
scientist.htm.
(20) Mathcad, version 13; Mathsoft: Cambridge, MA; http://www.math-
soft.com/.
Inorganic Chemistry, Vol. 45, No. 13, 2006 5093

Paris et al.
Scheme 2
measured on an Electrothermal IA 9100 apparatus. Electrospray
mass spectra were obtained on a Fisons VG platform and on Bruker
Daltonics micrOTOF spectrometers. Ytterbium(III) trifluorometh-
anesulfonate (99.99%) was purchased from Aldrich. The radio-
chemical tracer 153Gd was obtained from Perkin-Elmer Life and
Analytical Sciences as a chloride salt in dilute HCl. The γ activities
were measured with a Canberra Packard γ detector equipped with
a Ge-Li detector. All pH measurements were performed with a
Schott cg840 pH meter and a Schott N5900A electrode.
by filtration, and the filtrate was concentrated until a second crop
of precipitate was formed. These fractions were combined and dried
in vacuo before being recrystallized in boiling water. Polyamine 6
(5.80 g, 12.1 mmol, 51% yield) was obtained in tris-hydrochloride
form after drying (mp: 258-259 °C). ESI-TOF-MS of the
neutralized amine: calcd for C20H29N6O (M + H+) 369.2397, found
1
369.2392. H NMR (D2O): δ 8.75 (m, 2H), 8.48 (d, 1H, J ) 8
Hz), 8.32 (d, 1H, J ) 8 Hz), 7.92 (m, 1H), 7.84 (m, 1H), 5.24 (d,
1H, J ) 11.6 Hz), 4.7 (d, 1H, J ) 11.6 Hz), 3.05-3.58 (m, 16H).
13C NMR (D2O): δ 148.4, 147.5, 146.9, 144.8, 142.5, 142.3, 139.6,
136.3, 130.3, 129.9, 69.9, 64.6, 49.4, 47.4, 45.9, 44.1.
1a,9b-Dihydro-oxireno[f][1,10]phenanthroline, 3. Na2HPO4
(800 g, 5.6 mol) was dissolved in water (1.2 L) under vigorous
stirring and gentle heating. This solution was poured into 4.5 L of
commercial bleach, and the pH was carefully adjusted to 8.6 with
6 M HCl. This mixture was added to a solution of 1,10-
phenanthroline (40 g, 0.22 mol) in CH2Cl2 (2 L). Tetrabutylam-
monium hydrogensulfate (12 g, 35.3 mmol) was finally added, and
the mixture was stirred for 18 h at room temperature while the pH
of the aqueous phase was strictly maintained at 8.6 with 3 M NaOH.
The organic phase was separated, washed with 10% aq Na2S2O3
(1.6 L) and then water (3 × 1 L), and finally dried over MgSO4.
After evaporation under reduced pressure, the obtained brown solid
was suspended in acetone, filtered, and dried in vacuo to give 35.5
g (0.181 mol, 82% yield) of a light beige solid (mp: 162-163 °C
dec (lit.21 163-165 °C)). ESI-TOF-MS: calcd for C12H9N2O (M
rel-10-[(5R,6R)-5,6-dihydro-6-hydroxy-1,10-phenantho-
lin-5-yl)-1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid,
PhenHDO3A, 7. Synthesis from Polyamine 6. Anhydrous
K2CO3 (4.98 g, 36 mmol) was added to a suspension of compound
6 (3.82 g, 8 mmol) in dry CH3CN (80 mL). The mixture was
refluxed overnight under nitrogen. The hot suspension was filtered,
and the filtrate was evaporated to yield amine 6 in neutral form as
a yellow solid (2.98 g). This compound was dissolved in methanol
(75 mL); a solution of potassium bromoacetate (26.4 mmol; 3.8 g
of bromoacetic acid neutralized with 1.48 g of KOH) in methanol
(60 mL) was added, along with 3.65 g of anhydrous K2CO3 (26.4
mmol). The mixture was heated at 45 °C overnight. After a second
addition of K2CO3 (3.65 g, 26.4 mmol), the temperature was raised
to 65 °C. After 6 h, the inorganic salts were filtered off and washed
with 10 mL of methanol. The filtrate was acidified to pH 2.5-3
with 6 M HCl and was left standing until a precipitate appeared.
This precipitate was collected by filtration and recrystallized in
water to give 2.21 g of product. The resulting filtrate was evaporated
under reduced pressure, and the residue was suspended in 30 mL
of boiling methanol. The inorganic salts were filtered off, and the
filtrate was brought to dryness in vacuo to give 2.08 g of product.
The two fractions were combined, dissolved in 5 mL of water, and
loaded onto a Dowex 50 × 2-200 (H+ form) cation-exchange resin
(70 mL of wet resin). The column was washed with water (350
mL), and the product was recovered by elution with 0.5 M aqueous
NH3 (400 mL). The eluate was evaporated under reduced pressure,
and the remaining light yellow solid (3.02 g) was dissolved in
decarbonated water (5 mL). The pH was brought to 10 with aqueous
NH3, and the solution was transferred onto a Dowex 1 × 2-200
(OH- form) anion-exchange resin (65 mL of wet resin). The column
was washed with decarbonated water (350 mL), and the product
was eluted with aqueous formic acid (0.0075 M, 1 L). The eluate
was evaporated to dryness under reduced pressure. Repetitive
1
+ H+) 197.0709, found 197.0710. H NMR (DMSO-d6): δ 8.79
(dd, 2H, J ) 1.4 Hz, 4.4 Hz), 8.24 (dd, 2H, J ) 1.4 Hz, 7.6 Hz),
7.52 (dd, 2H, J ) 7.6 Hz, 4.4 Hz,), 4.82 (s, 2H). 13C NMR (DMSO-
d6): δ 150, 148.8, 138.4, 129.6, 123.8, 54.5.
6-(1,4,7,10-Tetraazacyclodec-1-yl)-5,6-trans-dihydro-1,10-
phenanthroline-5-ol, 6. Anhydrous LiClO4 (4.99 g, 46.9 mmol in
9 mL of dried CH3CN) was added to a solution of 1a,9b-dihydro-
oxireno[f][1,10]phenanthroline, 3 (4.6 g, 23.46 mmol), in dried
CH3CN (80 mL). A solution of octahydro-5H,9bH-2a,4a,7,9a-tetra-
azacycloocta[cd]pentalene, 422 (4.7 g, 25.7 mmol), in CH3CN (35
mL) was added, and the reaction mixture was refluxed under stirring
for 32 h under nitrogen. The solvent was removed under vacuum
to yield a brown oil. This crude material was dissolved in a solution
of HCl in methanol (40 mL of fuming HCl in 400 mL of methanol).
The mixture was stirred at reflux for 24 h. After being cooled to
room temperature, the reaction vessel was kept in an ice-water
bath until a white solid precipitated. The precipitate was collected
(21) Krishnan, S.; Kuhn, D. G.; Hamilton, G. A. J. Am. Chem. Soc. 1977,
99, 8121-8123.
(22) Atkins, T. J. J. Am. Chem. Soc. 1980, 102, 6364-6365.
5094 Inorganic Chemistry, Vol. 45, No. 13, 2006

Ditopic Macrocyclic Lanthanide Chelates
evaporations after the addition of water were performed in order
to remove formic acid completely. Yield: 2.76 g of PhenHDO3A
(5.08 mmol, 63% yield) (mp: 193-195 °C dec). ESI-TOF-MS:
calcd for C26H35N6O7 (M + H+) 543.2562, found 543.2568. MS
(ES+): m/z 543.1 (M + H+); 272.0 ([(MH2)/2]+). 1H NMR
(D2O): δ 8.80 (d, 1H, J ) 4.8 Hz), 8.75 (d, 1H, J ) 4.8 Hz), 8.57
(broad s, 1H), 8.15 (d, 1H, J ) 7.6 Hz), 7.75-7.6 (m, 2H), 5.41
(s, 1H), 4.97 (s, 1H), 2.2-4.2 (m, 22H). 13C NMR (D2O, 275 K):
δ 177.9, 171.7, 170.2, 149.1, 148.6, 147.6, 145.6, 145.4, 144.5,
137.1, 134.7, 131.6, 130.8, 64.0, 60.8, 58.0, 56.7, 55.7, 55.3, 54.3,
54.0, 52.5, 51.6, 51.2, 48.2, 45.8.
MS: calcd for C26H32N6O7Gd (M + H+) 698.1568, found 698.1573.
Anal. Calcd for C26H31GdN6O7‚1.4H2O (water and Gd content
determined by Karl Fisher titration and inductively coupled plasma
analysis, respectively): C, 43.25; H, 4.72; Gd, 21.78; N, 11.64.
Found: C, 43.10; H, 4.74; Gd, 21.73; N, 11.60. YbPhenHDO3A,
ESI-TOF-MS: calcd for C26H32N6O7Yb (M + H+) 714.1716,
found 714.1705.
Results and Discussion
Syntheses. Two synthetic pathways to PhenHDO3A, 7,
have been tested. The first synthetic approach is presented
in Scheme 3.
Synthesis from Epoxide 3 and Tris(tert-butyl)-1,4,7,10-tet-
raazacyclododecane-1,4,7-triacetate, 8. Tris-tert-butyl-1,4,7,10-
tetraazacyclododecane-1,4,7-tris-acetate hydrobromide was prepared
according to a published procedure.23 The neutral form of this ester
was obtained by percolation of a THF solution of the hydrobromide
salt through a column of A-21 resin (ACROS), followed by
evaporation under reduced pressure. A solution of 1a,9b-dihydro-
oxireno[f][1,10]phenanthroline, 3 (0.55 g, 2.8 mmol), and tris(tert-
butyl)-1,4,7,10-tetraazacyclododecane-1,4,7-triacetate, 8 (2 g, 3.88
mmol), in CH3CN (30 mL) was added dropwise to a stirred solu-
tion of Yb(III) trifluoromethanesulfonate (0.2 g, 0.32 mmol) in
CH3CN (25 mL). The mixture was stirred for 7 days at 65 °C under
nitrogen. The solvent was evaporated in vacuo, and the remaining
oil was dissolved in 250 mL of ethyl acetate. This solution was
washed with water (4 × 100 mL). Drying of the organic layer over
MgSO4 and removal of the solvent in vacuo gave 2.7 g of a light
brown oil containing a mixture of ester 8 and compound 9 (MS
(ES+): m/z 711.4 (M + H+); 356.1 (M + 2H)/2+). This oil was
dissolved in 20 mL of trifluoroacetic acid. The solution was stirred
at room temperature for 48 h and then brought to dryness to obtain
3 g of a light brown oil. This oil was dissolved in water (5 mL, pH
∼2) and loaded onto a Dowex 50 × 2-200 (H+ form) cation-
exchange resin (4 cm × 8 cm). The column was washed with water
(1.3 L), and the sought product was recovered by elution with 0.25
M aqueous NH3 (500 mL). The eluate was evaporated under
reduced pressure, and the light brown solid residue was dissolved
in decarbonated water (5 mL). The pH was brought to 10 with
aqueous NH3, and the solution was transferred onto a Dowex 1 ×
2-200 (formiate form) anion-exchange resin (4 cm × 8 cm). The
column was washed with decarbonated water (1.5 L) and the sought
product was eluted with aqueous formic acid (0.002 M, 2.5 L).
The fractions containing the desired product were combined and
evaporated to dryness under reduced pressure. Repetitive evapora-
tions after the addition of water were done in order to remove formic
acid. The remaining colorless, glassy solid was dissolved in
methanol (30 mL) and left for 1 day at -20 °C. The resulting white
solid was collected by filtration, washed with cold methanol and
acetone, and dried in a vacuum to obtain 624 mg of a white powder.
(yield: 41% from epoxide 3).
Both pathways start with the epoxidation of [1,10]-
phenanthroline. The synthesis of epoxide 3 has been reported
by several authors with yields ranging from 50 to 98%.21,24-26
This reaction has systematically been carried out by treating
a chloroform solution of [1,10]phenanthroline with an
aqueous solution of sodium hypochlorite, to which tetrabu-
tylammonium hydrogensulfate is added as a phase-transfer
catalyst. In our hands, this reaction led to high yields,
provided that chloroform was replaced by dichloromethane,
a large excess of tetrabutylammonium hydrogensulfate was
used, and the pH was strictly maintained at 8.6 during the
reaction. If these conditions were not met, we obtained
mixtures of compounds containing the desired 3 along with
trans-5-chloro-6-hydroxy-5,6-dihydro-1,10-phenanthroline and
unreacted 1,10-phenanthroline in variable proportions. We
needed very large quantities of epoxide 4, and its synthesis
on a large scale is reported in the experimental part. Small
samples of this compound recently became commercially
available.27 Epoxide 3 was reacted with triprotected tetraaza
macrocycle 428 using LiClO4 as catalyst. The resulting con-
densation product 5 was not isolated but was directly treated
with HCl in order to obtain the unprotected tetraazacycle 6.
The ligand PhenHDO3A, 7, was finally obtained by reacting
6 with an excess of bromoacetic acid and was purified by
ion-exchange chromatography.
The second approach to the synthesis of PhenHDO3A is
depicted in Scheme 4.
In this approach, phenanthroline epoxide 3 was reacted
with a small excess of triester 8 to yield PhenHDO3A, 7,
after treatment with trifluoroacetic acid and purification by
ion-exchange. The hydrobromide salt of triester 8 is readily
obtained from 1,4,7,10-tetraazacyclododecane (cyclen) and
3 equiv of tert-butyl bromoacetate in N,N-dimethylacetamide
in the presence of 3 equiv of sodium acetate.23,29 This
compound is also commercially available.30 The first step
in the reaction of 3 with 8 was a neutralization of the
hydrobromide salt of 8 with an Amberlyst A-21 resin. This
step provides the free amine whose enhanced nucleophilicity
Preparation of Metal Complexes. Typically, PhenHDO3A, 7
(30 mg, 0.055 mmol), was dissolved in water (7 mL), and a 5%
stoichiometric excess of a lanthanide trichloride solution (∼0.1 M)
was added dropwise. The solution was stirred at 50 °C while the
pH was continuously maintained at pH 5.5 by adding 1 M KOH
until no more pH variation was observed. The solution was passed
through a column of Chelex 100 resin (Sigma) and eluted with
water to remove the metal ions in excess. Evaporation under reduced
pressure yielded the complex as a glassy, colorless material.
LaPhenHDO3A, ESI-TOF-MS: calcd for C26H32N6O7LaNa (M
+ Na+) 701.1210, found 701.1208. GdPhenHDO3A, 1, ESI-TOF-
(24) Abu-Shqara, E.; Blum, J. Heterocycles 1990, 27, 1197-1200.
(25) Moody, C. J.; Rees, C. W.; Thomas, R. Tetrahedron 1992, 48, 3589-
3602.
(26) Shen, Y.; Sullivan, B. P. Inorg. Chem. 1995, 34, 6235-6236.
(27) Sample available from Aldrich Chemical Co., Milwaukee, WI.
(28) Tweedle, M. F.; Hagan, J. J.; Gaughan, G. T. U.S. Patent EP 292 689,
1998.
(29) Berg, A.; Almen, T.; Klaveness, J.; Rongved, P.; Thomassen, T. U.S.
Patent EP 299 795, 1989.
(23) Schultze, L. M.; Bulls, A. R. U.S. Patent 5 631 368, 1997.
(30) Compound available from Macrocyclics, Dallas, TX.
Inorganic Chemistry, Vol. 45, No. 13, 2006 5095

Paris et al.
Scheme 3a
a a ) NaOCl, Na2HPO4, (n-butyl)4N+HSO4-, 82%; b ) LiClO4; c ) HCl; d ) K2CO3, 63%.
Scheme 4 a
a a ) Yb(OTf)3; b ) TFA, 41% from 3.
is required for the subsequent ring opening of epoxide 3. A
range of activators such as Lewis acids or lithium salts31-33
was then tested to carry out the aminolysis of 3 by 8. We
first evaluated LiClO4 as a catalyst, because this compound
had already been used successfully in the first approach to
the synthesis of PhenHDO3A (Scheme 3). Unfortunately,
all attempts using this catalyst failed and we then examined
the effectiveness of Yb3+ trifluoromethanesulfonate in tet-
rahydrofuran or dichloromethane as proposed by Meguro34
and Chini.35 The aminolysis reaction was first carried out in
dry tetrahydrofuran under reflux as suggested by Meguro,34
with a 2:1 ratio of 8 and 3. After 48 h, the sought product 9
was barely detected by ES-MS with no evolution even after
more than a week. Moreover, performing the reaction with
a 1:2 ratio of 8 and 3, i.e., an excess of epoxide in acetonitrile
or dimethylformamide, also led to very low yields. These
(31) Schoffers, E.; Tran, S. D.; Mace, K. Heterocycles 2003, 60, 769-
771.
(32) Chini, M.; Crotti, P.; Macchia, F. Tetrahedron Lett. 1990, 31, 4661-
(34) Meguro, M.; Asao, N.; Yamamoto, Y. J. Chem. Soc., Perkin Trans.
1 1994, 2597-2601.
(35) Chini, M.; Crotti, P.; Favero, L.; Macchia, F.; Pineschi, M. Tetrahedron
Lett. 1994, 35, 433-436.
4664.
(33) Chini, M.; Crotti, P.; Macchia, F. J. Org. Chem. 1991, 56, 5939-
5942.
5096 Inorganic Chemistry, Vol. 45, No. 13, 2006

Ditopic Macrocyclic Lanthanide Chelates
kd ) k0 + kH[H+]
(1)
for acidities ranging from 0.02 to 0.12 M with kH ) (1.2 (
0.2) × 10-3 s-1 M-1, and no spontaneous dissociation was
observed within the limits of the errors.
The acid-assisted dissociation of GdPhenHDO3A is thus
about 100 times faster than that in the case of GdDOTA (kH
) 8.4 × 10-6 M-1 s-1).36 The dissociation is also five times
faster than in the case of GdHP-DO3A (kH ) 2.6 × 10-4
M-1 s-1), the Gd3+ complex of a triacetic monoalcoholic
derivative of tetraazacyclododecane8,37 currently used in
hospitals for magnetic resonance imaging under the name
ProHance. However, GdPhenHDO3A is still kinetically quite
stable, with a t1/2 value for the dissociation of 65 days at pH
4. The dissociation probably proceeds by the protonation of
one of the carboxylic groups followed by a proton transfer
to one of the cyclic nitrogen atoms, as suggested by earlier
kinetic studies.8,37 The stability of GdPhenHDO3A is also
borne out by relaxation-rate measurements at different pH
levels. The relaxivity is essentially independent of acidity
between pH 3 and 11. At lower pH, a slow dissociation is
taking place and the relaxivity is increasing toward the value
of the free Gd3+ ion, as already observed for GdHP-DO3A.38
Solution Structure. NMR spectroscopy yields information
on the structure and rigidity of the lanthanide complexes, as
shown in a number of studies of dia- and paramagnetic
species.17,39-42 Various NMR techniques are applied here to
the PhenHDO3A chelates. All the resonances exhibited by
an aqueous solution of PhenHDO3A at pD 5.4 are shifted
by the addition of diamagnetic La3+ (see the Supporting
Information, Figure S1). Furthermore, the peaks at 5.36 and
4.90 ppm due to the dihydrophenanthroline (N)CH-CH(OH)
group become much better resolved, and the general ap-
pearance of the broad resonances between 4 and 2 ppm
assigned to the ethylenic and acetic protons is altered. The
spectrum of LaPhenHDO3A is insufficiently resolved to be
amenable to a determination of the solution structure from
the values of the J coupling constants,43 but a quantitative
structural study can be performed in the presence of
paramagnetic ions.
Figure 1. Acid dependence of the first-order rate constant kd for the
dissociation of GdPhenHDO3A.
conditions were selected because 3 and 9 are easily separated
from each other by chromatography, whereas the separation
of triesters 8 and 9 proved more difficult by chromatography
either on alumina or on silica gel under diverse elution
conditions. During the tests of various experimental condi-
tions, it appears that acetonitrile was the most-suitable solvent
for obtaining 9 along with a small excess of nucleophile 8.
The aminolysis step was thus finally conducted with a 1.4:1
ratio of 8 and 3 in acetonitrile, and the resulting amino-
alcohol 9 was used as such in the cleavage of the tert-butyl
ester groups. Pure PhenHDO3A ligand 7 was isolated in 41%
yield from 3 after purification on ion-exchange resins
(Dowex 50 × 2-200 and Dowex 1 × 2-200) followed by
recrystallization in cold methanol.
Kinetic Stability. The motivation of this study was to
design a highly stable gadolinium complex with a ditopic
ligand able to spontaneously form a supramolecular entity
around a transition-metal ion. This structure had to be
extremely compact so that it would rotate as a single entity
without any internal rotations that would make it susceptible
to a lowered relaxivity. One obvious risk of this approach
was that the structure could be too crowded and thus
insufficiently stable. The kinetic stability and the solution
structure of the lanthanide PhenHDO3A complexes were thus
investigated in order to ascertain whether these complexes
are indeed stable and conformationally rigid.
The paramagnetic lanthanide ions induce large NMR shifts
δi that are essentially of dipolar origin and a direct function
of the structure of their chelates in solution, as shown in the
eq 2
The dissociation of GdPhenHDO3A was investigated in
acidic media by a method already reported.36 The dissociation
was followed at regular time intervals by measuring the
activity of acidic solutions of GdPhenHDO3A (ionic strength
1 M NaCl, 25 °C) prepared from GdCl3 spiked with 153Gd.
The Gd3+ liberated during the dissociation of the complex
was completely taken up by a cationic resin previously
conditioned to the same ionic strength and acidity, and the
activity of the solutions was measured by γ spectrometry.
As shown in Figure 1, the dissociation constant kd follows
the simple law
3cos2 θi - 1
sin2 θi cos 2ψi
δi ) Dax
+ Drh
(2)
ri3
ri3
(37) Toth, E.; Kiraly, R.; Platzek, J.; Raduchel, B. E.; Bru¨cher, E. Inorg.
Chim. Acta 1996, 249, 191-199.
(38) Zhang, X.; Chang, C. A.; Brittain, H. G.; Garrison, J. M.; Telser, J.;
Tweedle, M. F. Inorg. Chem. 1992, 31, 5597-5600.
(39) Desreux, J. F. Inorg. Chem. 1980, 19, 1319-1324.
(40) Forsberg, J. H.; Delaney, R. M.; Zhao, Q.; Harakas, G.; Chandran, R.
Inorg. Chem. 1995, 34, 3705-3715.
(41) Aime, S.; Barbero, L.; Botta, M.; Ermondi, G. J. Chem. Soc., Dalton
Trans. 1992, 225-228.
(42) Zhang, S.; Kovacs, Z.; Burgess, S.; Aime, S.; Terreno, E.; Sherry, A.
D. Chem.sEur. J. 2001, 7, 288-296.
(36) Wang, X. Y.; Jin, T. Z.; Comblin, V.; Lopez-Mut, A.; Merciny, E.;
Desreux, J. F. Inorg. Chem. 1992, 31, 1095-1098.
(43) Jacques, V.; Desreux, J. F. Inorg. Chem. 1996, 35, 7205-7210.
Inorganic Chemistry, Vol. 45, No. 13, 2006 5097

Paris et al.
Figure 2. 1H NMR spectrum of YbPhenHDO3A at 279 K, pD 2.4.
where ri, θi, and ψi are the polar coordinates of nucleus i
relative to the principal axes of the magnetic susceptibility
tensor; Dax and Drh are the axial and rhombic anisotropy
terms of the susceptibility tensor, respectively, and are
identical for all nuclei in a complex. This equation yields
reliable quantitative information on the structure of a
lanthanide complex, provided the solution species are slowly
exchanging rigid structures. If this condition is not met, eq
2 is of little use because rapid exchanges between different
labile structures lead to paramagnetic shifts that are no longer
directly related to single geometric and anisotropic factors.
The complexes derived from DOTA are usually very rigid
because of the steric requirements of this macrocycle.
Equation 2 has thus been applied successfully to several
DOTA derivatives.17,39,42 However, a contact contribution to
paramagnetic shifts δi also has to be taken into account, but
its magnitude is difficult to assess.44,45 Quantitative studies
are thus easier to perform with lanthanide ions for which
the contact contributions to the paramagnetic shifts are
essentially negligible in comparison to large dipolar contri-
butions, as in the case of the Yb3+ ion. Figure 2 presents the
1H NMR spectrum of YbPhenHDO3A at 279 K. This
complex displays 30 NMR peaks of equal area, covering
about 240 ppm. Each proton thus gives rise to a separate
resonance. In addition, the spectrum features several less-
shifted minor peaks.
for YbDOTA and its derivatives.18,46 In analogy with the
latter, it is assumed that the intense much-shifted peaks are
due to a square antiprismatic species in which the metal ion
forms five-membered rings with the N-CH2-C(O)-O
groups and with the tetraaza N-CH2-CH2-N units in a
∆(λλλλ) or Λ(δδδδ) arrangement, whereas the minor peaks
arise from a twisted square antiprism species with ∆(δδδδ)
or Λ(λλλλ) geometries.47 YbPhenHDO3A is thus a highly
rigid structure despite the bulkiness of the phenanthroline
group. The proof that this group is firmly coordinated to the
central metal ion through its alcohol function is derived from
a quantitative analysis of the relative magnitude of the para-
magnetic shifts of the major isomer on the basis of eq 2.
A geometric model of GdPhenHDO3A was generated at
the MM2 level using the parameters specifically computed
for lanthanide complexes by Hay48 and Cundari.49 These
parameters allowed these authors to predict the geom-
etries of Gd complexes within 3% of the bond lengths and
bond angles determined by crystallography. They have
already been successfully used for creating models of
macrocyclic lanthanide complexes that were in very good
agreement with NMR studies.17 The minimum-energy struc-
ture of GdPhenHDO3A in a square antiprismatic arrangement
is reproduced in Figure 3. The Gd-N and Gd-O distances
were not restricted during minimization, and the computed
YbPhenHDO3A is thus present in solution in the form of
two rigid, slowly exchanging species, as already observed
(46) Benetollo, F.; Bombieri, G.; Calabi, L.; Aime, S.; Botta, M. Inorg.
Chem. 2003, 42, 148-157.
(47) Howard, J. A. K.; Kenwright, A. M.; Moloney, J. M.; Parker, D.;
Port, M.; Navet, M.; Rousseau, O.; Woods, M. J. Chem. Soc., Chem.
Commun. 1998, 1381-1382.
(44) Bryden, C. C.; Reilley, C. N.; Desreux, J. F. Anal. Chem. 1981, 53,
1418-1425.
(45) Piguet, C.; Edder, C.; Rigault, S.; Bernardinelli, G.; Bu¨nzli, J.-C. G.;
Hopfgartner, G. J. Chem. Soc., Dalton Trans. 2000, 3999-4006.
(48) Hay, B. P. Inorg. Chem. 1991, 30, 2876-2884.
(49) Cundari, T. R.; Moody, E. W.; Sommerer, S. O. Inorg. Chem. 1995,
34, 5989-5999.
5098 Inorganic Chemistry, Vol. 45, No. 13, 2006

Ditopic Macrocyclic Lanthanide Chelates
Figure 3. Molecular structure of GdPhenHDO3A calculated at the MM2
Figure 4. Correlation between the calculated and experimental paramag-
netic dipolar shifts of YbPhenHDO3A (bottom) and between the longitudinal
relaxation rate 1/T1 of the protons and the H-metal distance (top) (279 K,
pD 5.4) Symbols: axial up (~), equatorial up (]‚ ), axial down ($), equatorial
down (_), acetate (K), dihydrophenanthroline (*).17
level using Hay48 and Cundari49 parameters.
values are in good agreement with the values reported in
the Cambridge Structural Database50 (Gd-N ) 2.55-2.65
Å, Gd-O ) 2.35-2.48 Å). The twist angle between the
two square planes formed, respectively, by the four coordi-
nated O and N atoms is 36.6°, as expected for a square
antiprismatic structure.
macrocyclic ligands (R ) [∑(δcalcd - δobs)2/∑δobs2], where
δ is a paramagnetic shift).17,42 Obtaining reliable parameters
for predicting the solution geometry of lanthanide complexes
remains a challenge despite recent progresses,51 and the
parameters we used might not be entirely appropriate for a
chelate featuring a bulky dihydrophenanthroline group. A
quantitative analysis of the longitudinal relaxation times T1
was thus performed to support the paramagnetic shift study.
The electron-nucleus dipolar interaction and the Curie
spin-relaxation mechanism both contribute to the relaxation
rate 1/T1 and are both functions of a 1/ri6 term. A log(1/T1)
vs log ri plot with a slope of -6 must thus be obtained if a
reliable geometric model has been selected. Figure 4 shows
the correlation between the 1/T1 values and the metal-proton
distances in a log-log plot for which the slope is -5.6 (r2
) 0.97) (similar levels of agreement have been reported in
the literature40,41,52 for lanthanide complexes). A particularly
noteworthy aspect of these measurements is that only five
poorly shifted resonances are found with relaxation rates
much shorter than those of all other peaks (Figure 4). These
resonances are due to five of the six aromatic protons in the
dihydrophenanthroline moiety. The sixth proton, indicated
by an arrow in Figure 3, is much more shifted and its
relaxation rate is much higher, as it is closer to the
paramagnetic center.
The NMR analysis of the solution structure of
YbPhenHDO3A was thwarted in part by difficulties in
assigning the featureless NMR peaks and in orienting the
magnetic susceptibility tensor. The NMR resonances of
YbPhenHDO3A were partly assigned by comparison with
the spectrum of YbDOTA and on the basis of the COSY
spectrum that displayed a few cross-peaks. However, we
were unable to obtain COSY cross-peaks for all resonances,
and EXSY cross-peaks were observed only between the
minor and the major species. Longitudinal relaxation times
T1 were thus measured for all resonances in order to make
more-reliable assignments (see below). The susceptibility
tensor (eq 2) was oriented so as to obtain the best agreement
between the calculated and experimental paramagnetic shifts.
It was also verified that the peak assignments were in keeping
with both the 1/T1 values and the paramagnetic shifts
measurements. All solutions that were not in agreement with
the COSY patterns were discarded. Solutions that gave peak
assignments that were too different from the peak ordering
of symmetric Yb3+ macrocyclic complexes such as YbDOTA
were also discarded, as they systematically gave very poor
agreements.
The YbPhenHDO3A thus appears to be a compact
structure featuring a dihydrophenanthroline unit that is
coordinated to the central metal ion with two nitrogen atoms
that are pointing away from the DOTA-like unit and are
available for bonding with a transition-metal ion.
Relaxivity Properties. The gadolinium(III) ion induces
an increase in the relaxation rate 1/T1M of the water protons
through a dipolar interaction that can be accounted for by
The relationship between the calculated and experimental
paramagnetic shifts is presented in Figure 4 (slope ) 0.97,
r2 ) 0.97). The Z axis of the susceptibility tensor was found
to be nearly perpendicular to the mean plane of the nitrogen
atoms (angle ) -1°), and the magnetic susceptibility terms
Dax and Drh were calculated to be 3748 ( 128 and 696 (
275 ppm, respectively. A structural analysis on the basis of
the relative magnitude of induced paramagnetic shifts is not
without flaws, for instance, because of contact contributions
to the chemical shifts or because several protons have very
similar geometric factors.17 In the present study, we could
not obtain an R factor better than 13.6%, whereas lower
values have been systematically obtained for simpler tetraaza
(50) Cambridge Structural Database; Cambridge Crystallographic Data
Centre: Cambridge, U.K.; http://www.ccdc.cam.ac.uk/.
(51) Rocha, G. B.; Freire, R. O.; da Costa, N. B., Jr.; de Sa, G. F.; Simas,
A. M. Inorg. Chem. 2004, 43, 2346-2354.
(52) Dubost, J. P.; Leger, J. M.; Langlois, M. H.; Meyer, D.; Schaefer, M.
C. R. Acad. Sci., Ser. IIb: Mec., Phys., Astron. 1991, 312, 349-354.
Inorganic Chemistry, Vol. 45, No. 13, 2006 5099

Paris et al.
the Solomon-Bloembergen-Morgan equations. The SBM
equations and their limitations have been reported in several
textbooks and papers.1,53,54 A copy of the Mathcad, version
13,55 SBM equations used here for the analysis of relaxation
measurements is included in the Supporting Information
(Figure S2a-c). The longitudinal water relaxation rate 1/T1
per mmol of Gd3+ ion, also called relaxivity, depends on a
number of factors. Among these are qH O, the hydration
2
number of the metal ion; τr and τm, the rotational and solvent
exchange correlation times; T1e, the electronic longitudinal
relaxation time; τv, the correlation time of the modulation
of the zero-field splitting; ∆2, the mean square zero-field
splitting energy; A/p, the hyperfine coupling constant; and
r, the metal-proton distance. The solvent molecules in the
outer coordination sphere around the metal complex also
make a contribution to the relaxivity. This contribution de-
Figure 5. Relaxivity titration of Fe2+ by GdPhenHDO3A (pH 5.5,
25 °C).
pends on the metal-proton distance router and on the diffusion
1,53,54
coefficient Ddiff
.
In SBM theory, the experimental
relaxation rate 1/T1M depends on a correlation time τc that is
itself a function of the times τr, τm, and T1e according to
1
1
1
1
)
+
+
(3)
τc τr τm T1e
Small complexes such as GdPhenHDO3A are tumbling
rapidly in solution. One can safely assume that τr is then the
smallest term in eq 3 and predominates the relaxivity at all
frequencies, because the correlation time τc is then essentially
a function of τr only. In these conditions, the dispersion of
the relaxivity with frequency (NMRD) is a simple S-shape
curve between 0.01 and 80 MHz. Larger species rotate more
slowly and the influence of the τm and T1e factors is likely
to become more significant. It has been shown that this leads
to a relaxivity increase at all frequencies. The NMRD plot
then takes the form of an S-shape curve followed by a
maximum at 20-60 MHz. A slower rotation is expected to
lead to a sizable relaxivity increase in this frequency range.
Figure 6. Relaxivity titration of Ni2+ by GdPhenHDO3A (pH 5.5,
25 °C).
respectively).56 The high stability of the Fe2+ complex is also
supported by the ease with which GdPhenHDO3A extracts
iron from the metal tubing of HPLC equipment (see the
Supporting Information, Figure S3). The small negative slope
observed in the presence of an excess of Fe2+ in Figure 5 is
assigned to the formation of a small proportion of the mono
complex, in agreement with computations of the distribution
of the different forms on the basis of the stability constants
of the Fe2+-phenanthroline complexes. The relaxivity gain
is 90%, a sizable increase obtained by a simple self-assembly
process. A similar relaxivity increase is observed when
GdPhenHDO3A forms a tris complex with the Ni2+ ion, as
shown in Figure 6.
A linear relaxivity increase is again observed until the
[Ni2+]/[GdPhenHDO3A] ratio reaches 0.33 but is followed
by a marked relaxivity decrease. The preference for the
formation of a tris complex with phenanthroline is much
less pronounced for Ni2+ than for Fe2+. The relaxivity
decrease in Figure 6 is thus assigned to the formation of
the mono and bis complexes with Ni2+. The titration curve
Figure 5 presents a 20 MHz relaxivity titration of
GdPhenHDO3A by FeCl2 at 25 °C and pH 5.5. The relaxivity
increases linearly until a [Fe2+]/[GdPhenHDO3A] concentra-
tion ratio of ∼0.33 is reached. A plateau with a small
negative slope is observed at higher concentration ratios. This
titration curve is entirely in agreement with the self-assembly
process illustrated in Scheme 2.
A highly stable tris complex is obviously formed between
Fe2+ and GdPhenHDO3A. The stability constant of this
complex is unknown but is expected to be quite high in view
of the sudden relaxivity break at the expected stoichiometry
and in reference to the strong preference for the formation
of a highly stable tris complex between Fe2+ and phenan-
throline (log ân ) (5.85), (11.15), 21.1 for n ) 1, 2, and 3,
(53) Banci, L., Bertini, I., Luchinat, C., Eds. Nuclear and Electron
Relaxation; VCH: Weinheim, Germany, 1991; pp 1-208.
(54) Relaxometry of Water-Metal Ion Interactions; van Eldik, R., Bertini,
I., Eds; Advances in Inorganic Chemistry, Vol. 57; Elsevier: Am-
sterdam, 2005.
(55) Mathcad, version 13; Mathsoft: Cambridge, MA; http://www.math-
soft.com/.
(56) Smith, R. M.; Martell, A. E., Eds. NIST Critically Selected Stability
Constants of Metal Complexes Database, version 4.0; National Institute
of Standards and Technology: Gaithersburg, Md, 1997.
5100 Inorganic Chemistry, Vol. 45, No. 13, 2006

Ditopic Macrocyclic Lanthanide Chelates
was simulated using the stability constants reported for the
Ni2+-phenanthroline system (log ân ) 8.9, 16.8, 24.4 for n
) 1, 2, and 3, respectively)56 and calculated relaxivities of
8.9, 10.3, and 11.4 s-1 mmol-1 were computed for the 1:1,
1:2, and 1:3 Ni2+:GdPhenHDO3A complexes, respectively.
As expected, the relaxivity increases with the size of the
complexes.
Many of the parameters in the SBM equations are
unknown, and assumptions on their values must be made
or, preferably, have to be measured independently. The
metal-proton distance in the inner and outer coordination
spheres r and router have been set at (3.05 ( 0.5) Å and 3.6
Å, respectively, by reference with literature data.57 The
diffusion coefficient of the complexes has been fixed at (2.2
( 0.2) × 10-9 m2 s-1 and/or was computed by Hindeman’s
Figure 7. Temperature dependence of the reduced 17O transverse relaxation
rate 1/T2r of water in a solution of GdPhenHDO3A (9) and Fe-
[GdPhenHDO3A]3 (b), pD 5.0.
approach.58,59 The hydration number qH O of EuPhenHDO3A
2
was deduced from the measurement of the luminescence
lifetimes τH O and τD O of a EuPhenHDO3A solution in water
2
2
coupling constant was fixed to -3.5 × 106 rad s-1, in keeping
with values found for most gadolinium polyaza polycar-
boxylic complexes.57 Moreover, the activation energy Ev of
the modulation of the zero-field splitting was kept constant
at 1 kJ/mol. The water-exchange correlation times τm of
GdPhenHDO3A and its tris complex with Fe(II) were
deduced from a fitting of the data presented in Figure 7 using
the Swift and Connick equations.57,64 These equations are
reproduced in the Mathcad 13 file included in the Sup-
porting Information. The best agreement between the ex-
perimental and the computed transverse relaxation times of
GdPhenHDO3A and Fe(GdPhenHDO3A)32+ were found for
and in D2O using the corrected Horrocks equation60
1
1
qH O ) 1.2
-
- 0.25
(3)
2
(
τ
τD O
)
[
]
H2O
2
At pH 3.89, the qH O value was 1.41. Both the water protons
2
and the hydroxyl group on the dihydrophenanthroline group
contribute to the quenching of the luminescence of the Eu3+
ion. A similar situation has been reported in the case of
TbHP-DO3A.38 It is thus assumed that qH O is equal to 1.
2
In most cases, the main parameters of the SBM equa-
tions are deduced by fitting simultaneously the tempera-
ture dependence of the 17O nuclear transverse relaxation
rate and the nuclear relaxation dispersion curves (NMRD).
Applying this procedure to both GdPhenHDO3A and
Fe(GdPhenHDOA)32+ did not lead to a sufficiently good fit,
and the 17O and NMRD data were thus interpreted separately.
This difficulty has already been encountered by other
authors,61-63 including a recent study of a self-assembling
process similar to the one reported here.15 The water-
exchange times τm were deduced from variable-temperature
measurements of the 17O transverse relaxation times, as these
data essentially depend on this factor. Following the same
line of reasoning, the rotational correlation times τr were
deduced from the NMRD curves whose shape is essentially
a function of the rotation rate. The water-exchange times
deduced from the 17O measurements were introduced as fixed
parameters in the fitting of the NMRD curves. The hyperfine
∆2 ) 4.7 × 1019 s-2, ∆Hm ) 69 kJ/mol, τv ) 6 ps, τm
)
0
0
(1.2 ( 0.1) × 10-6 s; and ∆2 ) 8.5 × 1019 s-2, ∆Hm ) 62
kJ/mol, τv0 ) 6 ps, τm0 ) (1.0 ( 0.1) × 10-6 s, respectively.
The errors on the water-exchange times were estimated by
taking into account the influence of the parameters A/p, ∆2,
0
∆Hm, and τv . The rate of water exchange is rather slow, as
already observed for a dimeric Gd(III) complex with two
triacetic DOTA ligands featuring an alcohol arm substituted
with an aliphatic ether chain rather than a phenanthroline-
alcohol group.65 Within the error limits, the self-assembling
process around Fe(II) has no or little influence on the water-
exchange time. However, a rigidification of the dihydro-
phenanthroline ring due to the complexation of Fe(II) could
bring about a small change in the water-exchange time.
The NMRD curve of GdPhenHDO3A and its tris complex
with Fe2+ and Ni2+ are reproduced in Figure 8. The curves
of the tris complexes exhibit small maxima at 20-50
MHz, in agreement with Solomon-Bloembergen equations.
Simulations of the NMRD data for GdPhenHDO3A and
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2+
0
Fe(GdPhenHDO3A)3 yielded ∆2 ) 3.1 × 1019 s-2, τv
)
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24 ps, τr0 ) 105 ( 20 ps; and ∆2 ) 1.5 × 1019 s-2, τv0 ) 35
ps, τr° ) 398 ( 40 ps. The formation of the tris complex
brings about a 4-fold increase in the rotational correlation
time, a clear indication that the self-assembling process is
indeed taking place. No quantitative interpretation of the
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Inorganic Chemistry, Vol. 45, No. 13, 2006 5101

Paris et al.
PhenHDO3A exhibits the sought characteristics and is able
to form a stable Gd(III) complex that easily self-assembles
around Fe(II). The formation of tris complexes around Fe-
(II) or Ni(II) results in a 90% relaxivity increase, and the
achieved relaxation enhancements are close to those obtained
with dendrimers of much higher molecular weights but much
lower rigidity.67 Merbach et al. recently15 reported on a
polymetallic complex whose design was built on the same
idea of self-assembling we proposed earlier. The elegant
approach adopted by these authors led to a tetra-anionic
hexagadolinium complex that exhibits a very high relaxivity
between 30 and 60 MHz (33.6 mmol-1 s-1 at 25 °C). This
remarkable achievement was possible because the two
ligands N,N-bis(2-aminoethyl)ethylamine-N′,N′,N′′,N′′-tet-
raacetic acid attached by these authors on bipyridine form a
dihydrated Gd(III) complex with a high water-exchange rate.
This rate is achieved at the cost of a low thermodynamic
stability that is comparable to that of GdEDTA (pGd ) 14.9
vs pGd ) 16.1 for EDTA at pH 7.4, [Gd3+] ) 1 µM, [ligand]
) 10µM).15,68 However, the bis complex is rapidly cleared
by renal elimination in mice.15 The GdPhenHDO3A complex
proposed herein essentially keeps the kinetic inertness of
GdDOTA, but its water-exchange rate is slower. It is as
water-soluble, electronically neutral a species as the com-
mercially available GdHPDO3A (ProHance) and GdDTPA-
BMA (5,8-bis(carboxymethyl)-11-[2-(methylamino)-2-oxo-
ethyl]-3-oxo-2,5,8,11-tetraazatridecan-13-oic acid, Omniscan)
that were developed because of the low osmolality of their
solutions. Finally, GdPhenHDO3A is only monohydrated but
does not suffer from the competition from endogenous ions
such as CO32-, lactate, or amino acids, which are able to
completely remove the inner-sphere water molecules from
complexes in which Gd(III) is only heptacoordinated.10,69
Figure 8. Nuclear magnetic relaxation curves (NMRD) of GdPhenHDO3A
(b), Fe(GdPhenHDO3A)3 ([Fe2+]/[GdPhenHDO3A] ) 0.33) (2), and
2+
2+
Ni(GdPhenHDO3A)3 ([Ni2+]/[GdPhenHDO3A] ) 0.33) (~) at pH 5.5,
25 °C. Calculated NMRD curves are shown for GdPhenHDO3A and Fe-
(GdPhenHDO3A)32+ (-). A simple Cole-Cole70 curve (- - -) is shown for
2+
Ni(GdPhenHDO3A]3 (see text).
NMRD curve of Ni(GdPhenHDO3A)3 was performed, as this
species could not be obtained alone and the influence of
the Ni(II) paramagnetism is not easily taken into account
even if only an outer sphere effect is expected. However,
the NMRD curves of the tris complexes with Fe(II) and
Ni(II) are similar. In keeping with the rather large τm
values, a study of the dependence of the relaxivity of
Fe(GdPhenHDO3A)32+ upon temperature (see the Supporting
Information, Figure S4) indicates that the relaxivity is
essentially independent of temperature between 5 and 30 °C
and decreases between 30 and 50 °C. This behavior is
expected if τm is close to T1M at low temperatures and less
than T1M at high temperatures, as observed for several DTPA
and DOTA derivatives.66 It should be stressed here that the
correlation times deduced from the NMRD curves should
be considered with caution, as several parameters are strongly
Acknowledgment. The authors gratefully acknowledge
the Fonds National de la Recherche Scientifique and the
Institut Interuniversitaire des Sciences Nucle´aires of Belgium
for their financial support. The authors also thank the EC
COST Action D18 “Lanthanide Chemistry for Diagnosis and
Therapy”. C.G. is grateful for support from the Conselho
Nacional de Desenvolvimento Cientifico e Tecnolo´gico of
Brazil for a postdoctoral fellowship and thanks Prof. Severino
Alves Jr. for help and advice.
0
correlated. However, the τm and τr0 factors are considered
to be reliable, as the influence of the other parameters on
these factors is rather limited. It should also be noted that
the Ni(II)- and Fe(II)-centered assemblies are most probably
mixtures of isomers.
Conclusions
Supporting Information Available: 1H NMR spectra of
PhenHDO3A and LaPhenHDO3A, equations used for interpreting
17O and NMRD data in a Mathcad program, HPLC chromatograms
of PhenHDO3A and mixtures with increasing proportions of Fe-
(II), and dependence of the relaxivity of Fe(GdPhenHDO3A)3
on temperature at 20 MHz. This material is available free of charge
via the Internet at http://pubs.acs.org.
The aim of the work reported in this paper was to prepare
stable multimetallic Gd(III) complexes of high relaxivity. It
was anticipated that this goal could be achieved only if all
Gd(III)-containing units would rotate as a single entity. Thus,
all internal movements had to be eliminated. The selected
approach consisted in preparing a ditopic ligand with two
different complexing units firmly connected to each other
that could form rigid polymetallic assemblies around transi-
tion-metal ions. As reported in a preliminary account,13 ligand
2+
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5102 Inorganic Chemistry, Vol. 45, No. 13, 2006