Stability, structure and dynamics of cationic lanthanide(III) complexes of N,N′-bis(propylamide)ethylenediamine-N,N′-diacetic acid

Article abstract of DOI:10.1039/b211060a

The cationic [Ln(EDTA-PA₂)]⁺ complexes (EDTA-PA₂ = EDTA-bispropylamide) have been characterized by a multinuclear NMR study. ⁸⁹Y and ¹³C NMR data indicate the formation of 1:1 and 1:2 (Ln:ligand) complexes in aqueous solution. The stability constants of these complexes, as determined by potentiometric measurements, are log K_GdL = 10.3 and log K_GdL2 = 14.3. ¹³C Relaxation times of the Nd³⁺ complex show hexadentate binding of the organic ligand via the two amines, the two carboxylates and the two amide oxygen atoms. The complexes are present in solution as a mixture of three isomers: two trans forms and a cis one. Luminescence measurements demonstrate that both Eu³⁺ and Tb³⁺ complexes are nona-coordinated at low concentrations (～10^-3 M). Three water molecules then complete the coordination sphere. At higher concentrations, the complexes exist in solution as a mixture of nona- and octa-coordinated species, the relative concentration of the latter increases with increasing concentration as a consequence of intermolecular interactions operating in aqueous solutions. Data sets obtained from variable-temperature ¹⁷O NMR at 7.05 T and variable-temperature ¹H nuclear magnetic relaxation dispersion (NMRD) on the Gd³⁺ complex were fitted simultaneously to give insight into the parameters governing the water ¹H relaxivity. Fast rotation limits the relaxivity at 10-40 MHz.

Full text of DOI:10.1039/b211060a

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Stability, structure and dynamics of cationic lanthanide(III)
complexes of N,NЈ-bis(propylamide)ethylenediamine-N,NЈ-diacetic
acid
Carlos Platas-Iglesias,a,b Daniele M. Corsi,a Luce Vander Elst,c Robert N. Muller,c
Daniel Imbert,d Jean-Claude G. Bünzli,d Éva Tóth,e Thomas Maschmeyera and
Joop A. Peters*a
a Laboratory of Applied Organic Chemistry and Catalysis, Delft University of Technology,
Julianalaan 136, 2628 BL Delft, The Netherlands. E-mail: j.a.peters@tnw.tudelft.nl;
Fax: (ϩ31) 15 2784289
b Departamento de Química Fundamental, Universidade da Coruña, Campus da Zapateira s/n,
15071 A Coruña, Spain
c NMR Laboratory, Department of Organic Chemistry, University of Mons-Hainaut,
7000 Mons, Belgium
d Laboratory of Supramolecular Lanthanide Chemistry, Institute of Molecular and Biological
Chemistry, Swiss Federal Institute of Technology Lausanne, BCH, CH-1015 Lausanne,
Switzerland
e Laboratory of Inorganic and Bioinorganic Chemistry, Institute of Molecular and Biological
Chemistry, Swiss Federal Institute of Technology Lausanne, BCH, CH-1015 Lausanne,
Switzerland
Received 11th November 2002, Accepted 18th December 2002
First published as an Advance Article on the web 28th January 2003
The cationic [Ln(EDTA-PA2)]ϩ complexes (EDTA-PA2 = EDTA-bispropylamide) have been characterised by a
multinuclear NMR study. 89Y and 13C NMR data indicate the formation of 1 : 1 and 1 : 2 (Ln : ligand) complexes
in aqueous solution. The stability constants of these complexes, as determined by potentiometric measurements, are
logKGdL = 10.3 and logKGdL = 14.3. 13C Relaxation times of the Nd3ϩ complex show hexadentate binding of the
2
organic ligand via the two amines, the two carboxylates and the two amide oxygen atoms. The complexes are present
in solution as a mixture of three isomers: two trans forms and a cis one. Luminescence measurements demonstrate
that both Eu3ϩ and Tb3ϩ complexes are nona-coordinated at low concentrations (∼10Ϫ3 M). Three water molecules
then complete the coordination sphere. At higher concentrations, the complexes exist in solution as a mixture of
nona- and octa-coordinated species, the relative concentration of the latter increases with increasing concentration
as a consequence of intermolecular interactions operating in aqueous solutions. Data sets obtained from variable-
temperature 17O NMR at 7.05 T and variable-temperature 1H nuclear magnetic relaxation dispersion (NMRD) on
the Gd3ϩ complex were fitted simultaneously to give insight into the parameters governing the water 1H relaxivity.
Fast rotation limits the relaxivity at 10–40 MHz.
Introduction
Gadolinium complexes with poly(aminocarboxylate) ligands
present considerable interest since they are commonly used as
contrast agents in magnetic resonance imaging (MRI).1–3 Cur-
rently, about a third of all MRI scans are made after adminis-
tration of a Gd3ϩ-based contrast agent. Contrast agents
enhance the image contrast by preferentially influencing the
relaxation efficiency of the water proton nuclei in the target
tissue. The efficiency of a contrast agent is evaluated in terms of
the relaxivity, which is defined as the relaxation-rate enhance-
ment of water proton nuclei per mM concentration of the metal
ion. These complexes contain at least one Gd3ϩ-bound water
molecule that rapidly exchanges with the bulk water of the
body; this imparts an efficient mechanism for the longitudinal
and transverse-relaxation (T 1 and T 2) enhancement of water
protons. Around a paramagnetic ion, the relaxation rate of the
bulk water protons is enhanced due to long-range interactions
(outer-sphere relaxation) and short-range interactions (inner-
sphere relaxation). According to the standard Solomon–
Bloembergen–Morgan model, the latter process is governed by
four correlation times: the rotational correlation time of the
complex (τR), the residence time of a water proton in the inner
coordination sphere (τm), and the electronic longitudinal and
transverse relaxation rates (1/T 1e and 1/T 2e) of the metal centre.
Scheme 1
The two most studied ligand frameworks for Gd3ϩ complexes
with poly(aminocarboxylates) are based on DOTA and
DTPA1–3 (see Scheme 1). An important point that has to be
understood when one wants to design new potential Gd-based
contrast agents is the relationship between the structure and the
parameters governing the relaxivity. Therefore, the study of
the solution structure as well as the parameters governing the
proton relaxivity of any Gd3ϩ polycarboxylate complex is
interesting for the rational design of novel and more efficient
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 3
D a l t o n T r a n s . , 2 0 0 3 , 7 2 7 – 7 3 7
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Table 1 Ligand protonation constants and thermodynamic stability
constants of EDTA-PA2 and its Gd3ϩ complexes as determined by
pH-potentiometry (I = 0.1 M (CH3)4NCl). Data reported previously for
related systems are provided for comparison
contrast agents for MRI. For instance, the ideal value of the
residence time of water molecules in the inner coordination
sphere, τm, can be calculated to be about 30 ns at 25 ЊC, but
both Gd(DOTA)Ϫ and Gd(DTPA)2Ϫ, the two reference com-
pounds that first became available for clinical use, have τm
values of about 300 ns.4 This value further increases by an
order of magnitude for derivatives in which carboxylate groups
of the ligands are replaced by carboxamide moieties.4 In the
search for a better understanding of the relationship between
the exchange rate of the coordinated water and the structure
and overall electric charge of the complex we report here a
detailed study of the structure and dynamics in aqueous
solution of cationic Ln3ϩ complexes with the polycarboxylate
ligand EDTA-PA2 (Scheme 1). Although lanthanide com-
plexes of EDTA4Ϫ have been studied extensively in the
past,5–7 less is known about the coordination chemistry of
functionalised EDTA derivatives.8,9 In the present study the
structures and dynamics of the EDTA-PA2 complexes was
a, c
b, c
EDTA-PA2
EDTA-IPA2
EDTA-TBA2
EDTAd
pK1
pK2
pK3
pK4
βML
βML
7.15 0.01
3.59 0.01
1.96 0.03
7.36
3.66
1.99
7.19
3.81
1.95
10.17
6.11
2.68
1.95
17.32
10.3 0.1
14.3 0.2
12.79
18.96
12.76
20.35
2
a EDTA-bisisopropylamide. b EDTA-bis-tert-butylamide. c From ref. 9.
d From ref. 10.
These data are collected in Table 1 and compared with those
of similar ligands.9,10 They clearly show that the stability of the
[GdL]ϩ complexes (where L = EDTA-bisamide) is lower than
that of [Gd(EDTA)]Ϫ.
2
investigated by potentiometry, multinuclear (89Y, 13C, 17O, H)
NMR, UV-visible and luminescence spectroscopy to give
detailed information on the structure, the dynamics, and the
stability of the species in solution. Nuclear magnetic reson-
ance dispersion (NMRD) investigations and variable-
temperature 17O NMR measurements of the Gd(EDTA-PA2)ϩ
complex were conducted in order to assess its relaxation
enhancement abilities and to gain insight into the parameters
which govern the relaxation process in this positively charged
complex.
Coordination of the EDTA-PA2 ligand
The 13C NMR spectra of the diamagnetic La3ϩ and Lu3ϩ
EDTA-PA2 (1 : 1) complexes in D2O at 25 ЊC display two sets of
signals for the 13C nuclei of the EDTA backbone. The spectra
of the paramagnetic complexes (Ce, Pr and Nd) show also a
doubling of the number of resonances for the 13C nuclei in the
propyl side chains (Table 2). These data point to the presence of
at least two different isomers in aqueous solution.
Information on the coordination of the isomers by the Ln3ϩ
ions was obtained from Nd3ϩ induced relaxation rate enhance-
ments in the 13C nuclei of the EDTA-PA2 ligand. Among the
lighter Ln3ϩ ions (Ln = Ce Eu), Nd3ϩ has the longest electron
relaxation times,11,12 and therefore this cation is very suitable for
obtaining structural information of lanthanide complexes in
solution.13–15 The Nd3ϩ-induced 13C NMR relaxation enhance-
ments for EDTA-PA2 were measured at 7.05 T and 25 ЊC
(Table 3). In order to correct for diamagnetic contributions, the
relaxation rates for the corresponding La3ϩ complex were
subtracted from the measured values of the Nd3ϩ complex (see
Table 3).
Under the conditions employed, the signals for each of the
two isomers existing in solution were well resolved for some of
the carbons, while for the carbons in the α-position with respect
to the carbonyl groups and for the carbons of the ethylene
bridge only one signal is observed. Therefore, the relaxation
rates obtained in the latter case are averaged relaxations for the
structures of the two isomers. Since the outer-sphere contribu-
tion (1/T 1,OS) becomes significant only for remote nuclei this
contribution was neglected. From the electron relaxation for
Nd3ϩ (T 1e ≈ 10Ϫ13 s) it can be estimated that the contact contri-
bution to the paramagnetic relaxation is negligible for this
Ln3ϩ ion. Two contributions are of importance: the “classical”
dipolar relaxation and the Curie relaxation. Eqn. (3) can be
Results and discussion
Chemical speciation in aqueous solution, protonation constants
and thermodynamic stability constants
The chemical properties and the ionic radius of Y3ϩ for a
coordination number of 9 (1.08 Å) are comparable to those of
the Ln3ϩ ions (1.22–1.03 Å), and thus Y3ϩ complexes have
coordination numbers and geometries that are closely related to
those of the Ln3ϩ complexes. The formation of the Y3ϩ com-
plexes with EDTA-PA2 was monitored by 89Y NMR in a 0.15 M
YCl3 solution in D2O at pD 6.0. The spectrum recorded on a
solution with ρЈ = 0.5 (where ρЈ is the molar ratio EDTA-PA2 :
Y3ϩ) displays a signal at about 0 ppm due to free Y3ϩ as well as
two sharp signals at 100.6 and 102.3 ppm that can be assigned
to the presence of two [Y(EDTA-PA2)]ϩ isomers in aqueous
solution. Upon increase of the amount of ligand at ρЈ = 1, these
two peaks coalesce to a single broad peak at 101.4 ppm, while
the peak due to uncomplexed Y3ϩ is no longer observable.
When more ligand is added to the solution the peak at 101.4
progressively shifts to higher fields until 90.5 ppm for ρЈ = 2
reflecting the formation of 1 : 2 (Y3ϩ : L) species in solution, as
observed before for related systems.8,9 13C NMR data confirmed
the formation of both 1 : 1 and 1 : 2 species in aqueous solution:
when increasing amounts of NdCl3ؒ6H2O were added to a D2O
solution of EDTA-PA2 at 25 ЊC (pD = 6.0), the 13C resonances
of the ligand became broader, reflecting exchange processes
between different species in solution, while for ρЈ in the range
0.5–1.0, the 13C resonances gradually became sharper. Once the
1 : 1 stoichiometry was reached, addition of more NdCl3ؒ6H2O
did not result in further changes in the spectrum.
(3)
derived from a simplified Solomon–Bloembergen equation16
and the equation for the Curie relaxation.17,18 In this equation,
the first term between the brackets represents the “classical”
dipolar contribution, and the second term describes the Curie
relaxation. Here, µ0/4π is the magnetic permeability in a
vacuum, µ is the effective magnetic moment of the lanthanide
ion, γI is the gyromagnetic ratio of the nucleus under study,
β is the Bohr magneton, T 1,e is the electron spin relaxation
time, r is the distance between the 13C nucleus in question and
the lanthanide ion, H0 is the magnetic field strength, k is
the Boltzmann constant, T is the temperature, and τR is the
The ligand protonation constants Ki and the stability
constants of the Gd3ϩ complexes βML were determined by
pH-potentiometry:
(1)
(2)
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Table 2 13C NMR shifts for the [Ln(EDTA-PA2)]ϩ complexes in 0.165 M D2O solutions at 25 ЊC and pD = 6.5
COO
CONH
CH2COO
CH2CONH
NCH2CH2N
CH2CH2CH3
CH2CH2CH3
CH2CH2CH3
La
Lu
Ce
Pr
181.2
180.0
181.1
181.0
185.4
179.8
196.6
181.1
180.9
174.1
175.5
175.4
176.9
63.2
61.5
61.0
62.2
62.0
52.8
55.9
55.6
59.4
43.1
43.6
23.2
23.0
22.9
12.2
12.2
64.0
63.6
59.2
59.4
59.1
45.7
54.6
171.6
167.2
168.7
154.2
167.7
161.0
37.1
36.1
15.7
16.0
22.1
45.7
44.5
47.6
43.9
46.5
44.8
24.4
23.6
25.2
23.2
24.3
23.3
13.3
12.8
13.7
12.7
13.2
12.7
57.3
41.0
45.5
Nd
Table 3 13C NMR relaxation data (sϪ1) for 165 mM solutions of Ln3ϩ
complexes in D2O at 25 ЊC and pD = 6
two different stable conformations, δ and λ.19 Therefore, in
principle, eight enantiomeric forms are possible in the static
situation. Between 0 and 85 ЊC, only two sets of resonances
were observed in the 13C spectra. Most likely, the wagging
motion between the δ and λ form is rapid on the NMR time
scale under these conditions. Moreover, it is likely that the R,S/
S,R-isomers have a geometry with nearly C2 symmetry and,
therefore, cannot be discriminated by NMR. So, three “time-
averaged” complex geometries remain to be considered (see
Fig. 1): two trans (R,R and S,S) and one cis (R,S). The two
trans isomers are mirror images and are thus not distinguishable
by NMR.
a
1/T 1(Nd)
1/T 1(La)
1/T 1(La–Nd)
r/Å
COO
6.636
7.189
5.379
5.388
10.526
0.355
0.294
0.341
1.852
3.20
3.35
3.16
3.53
3.36
5.10
CONH
1.085
1.217
3.805
CH2COO
CH2CONH
NCH2CH2N
CH2CH2CH3
3.340
3.486
3.448
3.759
3.990
4.078
1.766
7.262
8.977
2.160
3.593
2.945
4.205
4.440
1.800
a Relaxation data (sϪ1) for the 13C NMR resonances of [La(EDTA-
PA2)]ϩ in the presence of one equivalent of [Nd(EDTA-PA2)]ϩ. The
total concentration of Ln3ϩ ions was 170 mM.
rotational tumbling time of the complex. The contribution of
the Curie spin mechanism to the total relaxation becomes
significant for larger molecules (τR increases), particularly at
higher fields. If τR and T 1,e are known, the absolute distances
between Nd3ϩ and the ligand nuclei can be calculated from the
relaxation rates by using eqn. (3).
The relative Nd3ϩ–C distances for [Nd(EDTA-PA2)]ϩ were
calculated from the paramagnetic relaxation rates (see Table 3)
by using eqn. (3). A τR value of 171 ps, as determined from the
deuterium longitudinal relaxation rate of the diamagnetic
[La(EDTA-PA2)]ϩ complex with the deuterated ligand (see
below), was used in these calculations. From these data it is
clear that the organic ligand is bound in a similar fashion as
EDTA, i.e. through the two carboxylate groups acting as
monodentate anions, the two amide oxygen atoms, and the
two nitrogen atoms. The distances presented in Table 3 are
normalised with respect to a distance between the carbon
atoms of the carboxylate groups and Nd3ϩ, which was
assumed to be 3.2 Å. By fixing this distance, a T 1,e value of 4.22
× 10Ϫ14 s was calculated from eqn. (3). This is close to the
T 1,e data reported by Alsaadi et al.11 on Nd3ϩ complexes, par-
ticularly if one considers that, as a result of the r6 depend-
ence shown in eqn. (3), an error of 5% in the distances leads to
an error of 34% in T 1,e. The propyl carbon atoms at position β
and γ with respect to the amide function are relatively far
away from the Nd3ϩ ion. Consequently, their Nd3ϩ-induced
relaxation rate enhancement is small causing the correspond-
ing Nd3ϩ–C distances to be inaccurate; hence, these data
were not considered. All calculated distances are in the range
previously observed for Nd3ϩ complexes of DTPA-bis-
amides.14,15
Fig. 1 Schematic representation of the isomers of the Ln(EDTA-
PA2)ϩ complexes: R = CH2CH2CH3. Only the enantiomers with a
δ-configuration of the ethylenediamine unit are depicted.
Upon increasing the temperature of a sample of the dia-
magnetic [La(EDTA-PA2)]ϩ complex from 25 to 85 ЊC,
a substantial increase of the line widths was observed in
the 13C spectrum, and the two sets of signals coalesced at 85 ЊC.
If it is assumed that the exchange process associated with
this line broadening (before coalescence) is slow on the NMR
time scale, then the exchange rate between the cis and trans
isomers (k) can be calculated from the observed line widths
(∆ν1/2):
Isomer interconversion in [Ln(EDTA-PA2)]؉
k = π(∆ν Ϫ ∆ν (0))
(4)
½
½
Upon hexadentate binding of EDTA-PA2 to Ln3ϩ ions inver-
sion of the two nitrogen atoms of the ligand backbone is pre-
cluded. Consequently, the two nitrogen atoms become chiral.
Furthermore, the coordinated ethylenediamine unit may adopt
where ∆ν1/2(0) is the linewidth in the absence of exchange. A
plot of k/T versus 1/T [k = (kbT /h)exp((∆S‡/R) Ϫ (∆H‡/RT ))]
(where kb and h are the Boltzmann and Planck constants,
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respectively, T is the absolute temperature and k is the rate
constant) yields the activation parameters for the inter-
conversion process (∆G‡ = 78 5 KJ molϪ1, ∆H‡ = 84 5 KJ
molϪ1, ∆S‡ = 19 16 J molϪ1 KϪ1, k = 0.12 0.02 sϪ1 at 298 K).
The calculated ∆G‡ is comparable to those determined for
the inversion of the terminal diethylenetriamine N-atoms
in lanthanide complexes with DTPA bis-amide derivatives,
which requires partial decoordination of the ligand.14,15 This
confirms that the two isomers of [Ln(EDTA-PA2)]ϩ evidenced
in the 13C NMR spectra are the cis and trans forms that
interconvert by inversion of the terminal ethylenediamine
N-atoms.
Self-association
The value of the rotational correlation time, τR, can be esti-
mated from the analysis of the deuterium longitudinal
relaxation rate of the deuterated ligand complexed to the
diamagnetic La3ϩ ion.20 In diamagnetic systems the deuterium
relaxation is fully controlled by quadrupolar interactions.
Under the extreme narrowing condition (ωτR Ӷ 1), it is given by
eqn. (5).
Fig. 3 13C NMR shifts induced in [La(EDTA-PA2)]ϩ upon addition of
[Tm(EDTA-PA2)]ϩ: CH2COOϪ (᭿,᭹); –CH2CH2– (᭺); –CH2CONH–
(ᮀ); –CH2CONH– (ꢀ); CH2CH2CH3 (᭞); CH2CH2CH3 (᭝);
CH2CH2CH3 (᭛). The concentration of [La(EDTA-PA2)]ϩ was 179
mM at pD = 6.
the NMR timescale. Assuming that no covalent interactions are
involved and that the diamagnetic shifts are insignificant, only
pseudo-contact contributions have to be considered.
(5)
The relative large induced shifts indicate that the nuclei of the
molecule must approach the paramagnetic Ln3ϩ ion rather
closely and must have a preferential orientation. Without a pre-
ferred orientation, the nuclei would experience all possible
orientations equally and the pseudo-contact shift would aver-
age to zero. A preferred location is supported by the different
signs and magnitudes of the induced shifts for the various
nuclei in the [La(EDTA-PA2)]ϩ complex, which shows that
the various nuclei have different location with respect to the
lanthanide ion and the magnetic axes.2
The quadrupolar coupling constant (e2qQ/ប) depends on the
degree of hybridisation of the C–2H bond, and takes a value of
2
21,22
3
approximately 170ؒ2π kHz for a C(sp )– H bond.
The T 1
values for aqueous solutions of the [La(EDTA-PA2-d4)]ϩ com-
plex appeared to increase upon dilution of the solution, while
the calculated τR values decrease linearly (Fig. 2). This effect was
The 13C resonance of the carboxylic functions immediately
disappeared upon addition of small amounts of Tm3ϩ complex
to the La3ϩ one, probably because it is too broad to be detected.
This indicates that the [La(EDTA-PA2)]ϩ complex is interacting
with [Tm(EDTA-PA2)]ϩ through the carboxylic groups. This is
confirmed by the longitudinal 13C relaxation enhancement
effects induced in [La(EDTA-PA2)]ϩ by addition of an equi-
molar amount of [Nd(EDTA-PA2)]ϩ (Table 3). The relaxation
rate of the 13C resonance of the carbonyl group was enhanced
by a factor of 5.2, that of the carbonyl group of the amide
functions was enhanced by a factor of 3.2, whereas the relax-
ation rates for other carbon resonances of the ligand were
nearly not affected. This suggests that the carboxyl groups
are involved in the formation of the intermolecular adducts,
most likely via intermolecular bridging of the monomeric
complexes.
Fig. 2 Rotational correlation times at 298 K (τ298R) obtained with H2O
solutions of [La(EDTA-PA2-d4)]ϩ at different concentrations.
Hydration numbers
previously observed for different polycarboxylate chelates,20
and it has been attributed to an increase of intermolecular
Emission and absorption spectra. The emission spectrum of
a 10Ϫ3 M solution of [Eu(EDTA-PA2)]ϩ in D2O at pD 6 and
295 K, obtained under excitation at 25 253 cmϪ1, displays the
typical 5D0 7FJ transitions. The spectrum is dominated by the
transition to 7F2, as shown by the integrated and corrected
relative intensities: 0.41, 1.00, 1.76 and 0.47 for J = 0, 1, 2 and 4.
interactions and viscosity in concentrated solutions.
A
rotational correlation time of 104 ps was determined for the
[La(EDTA-PA2-d4)]ϩ complex at a concentration close to those
generally used for NMRD studies (4 mM). This rotational
correlation time is longer than those reported in the literature
for small Ln3ϩ complexes (66 ps for [Ln(DTPA-BMA)], BMA =
bismethylamine; 58 ps for [Ln(DTPA)]2Ϫ and 66 ps for [La-
(EDTA)]Ϫ 4,23 and, therefore, we suspected that self-association
occurs in this system. To further explore the possibility of
intermolecular interactions in the [Ln(EDTA-PA2)]ϩ com-
plexes, we have performed a titration of a solution of the dia-
magnetic [La(EDTA-PA2)]ϩ with the paramagnetic [Tm(EDTA-
PA2)]ϩ. Upon addition of increasing amounts of Tm3ϩ complex
to the [La(EDTA-PA2)]ϩ solution most of the 13C resonances of
the La3ϩ complex shift linearly to lower fields (Fig. 3). The
exchange between the La3ϩ and the Tm3ϩ complex was slow on
The 5D0
7F0 excitation spectrum of the same solution
recorded by monitoring in the maximum of the transition to 7F2
produces a single band with a maximum at 17 253 cmϪ1. This
band is fairly sharp (full width at half-height fwhh = 8.7 cmϪ1
)
and presents a faint shoulder on the low energy side. Moreover,
the excitation spectra recorded by analysing at different emis-
sion wavelengths are virtually identical to that one, which is
consistent with the presence of a single main environment for
Eu3ϩ in the complex.
The emission lifetimes of the Eu(5D0) and Tb(5D4) excited
levels have been measured in D2O and H2O (10Ϫ3 M solutions)
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Table
4
Thermodynamic parameters determined by UV-visible
spectrophotometry for equilibrium (9) at pH = 6
and were used to calculate the number of coordinated water
molecules q by use of eqns. (6) and (7):
[Eu3ϩ]/M
Ligand
K298
0.44 0.04
0.35 0.04
0.23 0.03
0.15 0.04
0.031 0.009
0.085 0.008
0.59 0.05
∆HЊa
∆SЊb
q = 1.05(∆kobs
)
(6a)
(6b)
(6c)
(7a)
(7b)
0.1
0.05
0.025
0.0125
0.009
0.00625
0.0201
EDTA-PA2
q = 1.2(∆kobs Ϫ 0.25 Ϫ 0.075qN)
q = 1.11(∆kobs Ϫ 0.31 Ϫ 0.075qN)
22
4
46 11
q = 4.2(∆kobs
)
EDTA
17.7 0.5
54.9 1.6
a Data in kJ molϪ1 b Data in J KϪ1 molϪ1
. .
q = 5.0(∆kobs Ϫ 0.06)
where kobs = 1/τobs, ∆kobs = kobs(H2O) – kobs(D2O), kobs is given
in msϪ1 and qN is the number of NH oscillators when amide
groups are coordinated to the metal ion. The measured emis-
sion lifetimes in H2O solutions (τobs(H2O)) are 0.28 0.01 (Eu)
and 0.98 0.01 ms (Tb), while the τobs(D2O) values amount to
2.33 0.01 (Eu) and 3.04 0.01 ms (Tb). By using eqns. (6a)
and (7a),24 we obtain q = 3.3 and 2.9 for Eu and Tb, respectively,
with an estimated uncertainty on q of 0.5. These equations
were established from crystalline complexes in which inter-
actions generated by molecules of water in the second coordin-
ation sphere are absent. Consequently, we have also made use
of eqns. (6b) and (7b), proposed by Beeby et al.25 for solutions
of polyaminocarboxylate complexes with q ≤ 1 (including
cyclen derivatives), with qN = 2 and obtained q = 3.3 and 3.2
for Eu and Tb, respectively. Therefore, eqns. (6a,b) and (7a,b)
give similar results with an average number q = 3.3 (Eu) and
3.0 (Tb). Quite recently, a refined equation (6c),26 has been
proposed for Eu complexes in solution, with an estimated
uncertainty on q of
0.1. In our case, this equation yields
q = 3.0 for the Eu complex. All these results point to the
complexes having three coordinated water molecules in
aqueous solutions at this concentration.
The absorption spectrum of a 6.25 × 10Ϫ3 M solution of
[Eu(EDTA-PA2)]ϩ recorded in the 5D0
7F0 region presents a
main band centred at about 17260 cmϪ1 as well as a faint
shoulder on the low energy side (Fig. 4(a)), in agreement with
the excitation spectrum. On the basis of the emission lifetimes
of the Eu and Tb complexes, the absorption band appearing at
high energy can be safely assigned to the species with q = 3. The
presence of two absorption bands in this region has been previ-
ously observed for other Eu3ϩ chelates such as [Eu(EDTA)]Ϫ,7
and has been attributed to an equilibrium between species with
two and three inner-sphere water molecules. Several evidences
suggest that this is also the case for the [Eu(EDTA-PA2)]ϩ
chelate: (i) The absorption spectra of [Eu(EDTA-PA2)]ϩ are
nearly identical to those reported for [Eu(EDTA)]Ϫ;7 (ii) The
variation with temperature of the UV-visible absorption
spectra of [Eu(EDTA-PA2)]ϩ can be explained very well with an
equilibrium between a nine-coordinate species containing three
inner-sphere water molecules and an eight-coordinate species
with two inner-sphere water molecules: the intensity of the
band at 17260 cmϪ1 decreases with increasing temperature
while that of the band at 17246 cmϪ1 increases; (iii) The low
energy band is assigned to [Eu(EDTA-PA2)(H2O)2]ϩ in line with
the smaller nephelauxetic effect for this species compared to
[Eu(EDTA-PA2)(H2O)3]ϩ.27 Therefore, an equilibrium between
[Eu(EDTA-PA2)(H2O)3]ϩ and [Eu(EDTA-PA2)(H2O)2]ϩ species
appears to exist also in the case of the EDTA-PA2 (L)
derivatives:
Fig. 4 UV-visible spectra of the Eu3ϩ 5D0
7F0 transition in
[EuEDTA-PA2]ϩ recorded at different concentrations at 298 K and pH
= 5.8: (a) 6.25 × 10Ϫ3 M; (b) 0.025 M and (c) 0.1 M. The dotted lines
correspond to the best least-squares fit of the experimental data as
described in the text. Spectrum (d): Excitation spectrum of the 5D0
7F0 transition in [EuEDTA-PA2]ϩ (analysis wavelength: 16257 cmϪ1
concentration: 0.1 M). Vertical scale: arbitrary units.
;
centration dependence of the number of inner-sphere water
molecules has been observed. This phenomenon has to be
traced back to the presence of intermolecular interactions in
aqueous solutions at higher concentrations (vide supra). A pre-
viously reported least squares fitting procedure7 was used to
determine the equilibrium constants according to eqn. (8) at
different concentrations. Furthermore, the temperature depend-
ence of the absorption spectrum at a concentration of 0.009 M
was used to determine the reaction enthalpy, ∆HЊ, and reaction
entropy, ∆SЊ. The results are compiled in Table 4. For com-
parison, previously reported data for EDTA7 are included in
this Table. It should be noted that the intermolecular inter-
actions observed between the monomeric chelates may also
lead to a decrease in the hydration number. Therefore, one
could argue that the second absorption band in the UV-Vis
spectrum is to be attributed to the aggregated species. However,
the variable temperature UV-Vis study performed at 0.009 M
concentration and 1 : 1 metal/ligand ratio seems to disprove
this. The lower intensity absorption band increases with increas-
ing temperature, whereas the self-aggregation should be more
important at lower temperatures resulting in an opposite
temperature dependence.
[Ln(L)(H2O)3]ϩ
[Ln(L)(H2O)2]ϩ ϩ H2O
(8)
Upon increasing the concentration the relative intensity of
the absorption bands changes dramatically (Fig. 4), the absorp-
tion assigned to the [Eu(EDTA-PA2)(H2O)2]ϩ species grows at
the expense of that attributed to [Eu(EDTA-PA2)(H2O)3]ϩ. This
is, to the best of our knowledge, the first time that such a con-
D a l t o n T r a n s . , 2 0 0 3 , 7 2 7 – 7 3 7
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Table 5 Lanthanide-induced water 17O shifts (ppm) for 0.2 M
[Ln(EDTA-PA2)]ϩ complexes in D2O at pD 6
Ln3ϩ
δ
a, b/ppm
δ
a, c/ppm
Ln3ϩ
δ
a, b/ppm
δ
a, c/ppm
La
Ce
Pr
Nd
Tb
Dy
252
473
838
Ϫ221
0
260
430
Ϫ5229
Ϫ4699
Ho
Er
Tm
Yb
Lu
Ϫ4777
Ϫ2620
Ϫ1218
Ϫ205
241
Ϫ4370
Ϫ2620
Ϫ1461
Ϫ656
1157
Ϫ6336
Ϫ5531
Ϫ321
a The values are extrapolated to a molar ratio of Ln3ϩ/water (ρw) = 1.
b Values obtained at 25 ЊC. c Values obtained at 73 ЊC.
Ln3؉-Induced water 17O shifts. Previously, we have shown
that the Dy3ϩ-induced water 17O shift is almost independent of
the other ligands coordinated to the Dy3ϩ ion.28–31 Conse-
quently the Dy3ϩ-induced shift can be used to determine the
hydration numbers of the Dy3ϩ complexes. The Dy3ϩ-induced
shift as measured for the [Dy(EDTA-PA2)]ϩ system at 25 ЊC
(extrapolated to a molar ratio of Dy3ϩ/water (ρw = 1)) amounts
5531 ppm. The Dy3ϩ-induced shift determined for a DyCl3
solution under the same conditions amounts to 20358 ppm.
If it is assumed that in the absence of organic ligands Dy3ϩ
is coordinated to eight water ligands, it can be concluded from
these data that q = 2.2. From the data reported in Table 4, we
can estimate a K298 ≈ 0.5–0.6 at the concentration used for
investigating the Ln3ϩ-induced 17O NMR shifts (0.2 M). There-
fore, the averaged number of bound molecules is estimated to
be in the range 2.6–2.7. This is in good agreement with the
experimental (averaged) value of 2.2 since the eight coordinated
species is expected to be more favoured upon the decrease of
Fig. 5 Plot of ∆/CD versus 〈Sz〉/CD for the 17O signal in the [Ln(EDTA-
PA2)]ϩ system at 25 (᭺) and 73 ЊC (ᮀ).
lanthanide complexes at both temperatures (Fig. 5). By means
of a multiple least-squares method the values of qF were
determined to be Ϫ206(3) and Ϫ155(5) at 25 and 73 ЊC, respect-
ively, while the values of qG were 3(2) and 2(2), respectively. The
qF values are proportional to the inverse of the temperatures, as
should be expected for contact shifts.31 It may be concluded that
the q value remains fairly constant along the lanthanide series
and does not change much with the temperature. However,
we cannot exclude that a small and gradual change in the
hydration number occurs along the lanthanide series, which
cannot be detected in the plots according to eqn. (10).
The parameters determining the relaxivity
The relaxivity describes the efficiency of magnetic dipolar
coupling occurring between the solvent nuclei and the para-
magnetic metal ion and represents a measure of the efficacy of
the complex as a contrast agent. The relaxation rates of the
bulk water protons in the vicinity of a paramagnetic ion are
enhanced due to long-range interactions (outer-sphere relax-
ation) and short-range interactions (inner-sphere relaxation).
The latter are governed by the rotational correlation time of the
complex (τR), the residence time of a water proton in the inner
coordination sphere (τm), and the electronic longitudinal and
transverse relaxation rates (1/T 1e and 1/T 2e) at the metal centre.
As pointed out previously,4 it is difficult to determine the
parameters determining the relaxivity of a given compound
from nuclear magnetic resonance dispersion (NMRD) profiles
without obtaining independent information of at least some of
the most important parameters. We therefore carried out vari-
able-temperature 17O NMR measurements in solutions of the
Gd3ϩ complex to obtain information about the water exchange
kinetics. Furthermore, the rotational correlation times were
determined independently from the 2H transversal relax-
ation rates in the La3ϩ complexes of the deuterated ligand (see
above).
the ionic radius upon going from Eu3ϩ to Dy3ϩ
.
In order to evaluate the number of inner-sphere water
molecules in the other [Ln(EDTA-PA2)]ϩ complexes, a more
rigorous treatment of the Ln3ϩ-induced water 17O shifts is
required. The induced shifts (∆) are a combination of dia-
magnetic (∆d), contact (∆c) and pseudo-contact (∆p) shifts.
The value of ∆d was estimated from the induced shift of the
La3ϩ complex for the lighter lanthanide complexes (Ce Eu)
and from the induced shift of the Lu3ϩ complex for the heav-
ier lanthanide complexes (Tb Yb). The contact contribution
results from a through-bond transmission of unpaired density
of the Ln3ϩ cation in question, whereas the pseudo-contact
shift arises from a through-space dipolar interaction between
the magnetic moments of the unpaired electrons of the Ln3ϩ
electrons and any NMR active nucleus. Both ∆c and ∆p can
be expressed as the product of a term characteristic of the
Ln3ϩ ion but independent of the complex structure (〈Sz〉 and
CD, respectively) and a second term characteristic of the
complex but independent of the Ln3ϩ ion (F and G, respect-
ively):32–34
∆Ј = ∆ Ϫ ∆d = ∆c ϩ ∆p = 〈Sz〉F ϩ CDG
(9)
Variable-temperature 17O NMR measurements. A 37 mM
aqueous solution of the [Gd(EDTA-PA2)]ϩ chelate was investi-
gated by variable-temperature 17O NMR shifts and relaxation
measurements. From the measured 17O NMR relaxation
rates and angular frequencies of the [Gd(EDTA-PA2)]ϩ solu-
tion, 1/T 1, 1/T 2 and ω, and of the acidified water reference
1/T 1A, 1/T 2A and ωA, one can calculate the reduced relaxation
rates and chemical shifts, 1/T 1r, 1/T 2r, and ωr, which may be
written as eqns. (11)–(13):35–38
Values for 〈Sz〉 and CD are tabulated in the literature.34 When
the various Ln3ϩ complexes are isostructural, eqn. (9) can be
rearranged in a linear form:32–34
(10)
So, when a series of Ln3ϩ complexes yields a linear plot of ∆Ј/
CD versus 〈Sz〉/CD, this is an indication that they are isostruc-
tural.2 The water 17O shifts obtained for the various [Ln(EDTA-
PA2)]ϩ systems at 25 and 73 ЊC were extrapolated to ρw = 1 (see
Table 5). The values obtained correspond to q∆ where q is the
number of inner-sphere water molecules. Plots according to
eqn. (10) give single straight lines for the whole series of
(11)
(12)
D a l t o n T r a n s . , 2 0 0 3 , 7 2 7 – 7 3 7
732

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useful to consider the simplified eqns. (14) and (15), where the
contribution of ∆ωm in eqn. (12) has been neglected.
(13)
(14)
(15)
where 1/T 1m, 1/T 2m are the relaxation rates in the bound water,
∆ωm is the chemical shift difference between the bound and
bulk water (in the absence of a paramagnetic interaction with
the bulk water), Pm is the mole fraction of bound water, and τm
is the residence time of water molecules in the inner coordin-
ation sphere. The total outer-sphere contributions to the
reduced relaxation rates and chemical shift are represented by
1/T 1os, 1/T 2os and ∆ωos. The number of coordinated water mole-
cules affects directly the value of Pm, and therefore to calculate
the reduced relaxation rates and chemical shifts, 1/T 1r, 1/T 2r
and ωr, the number of coordinated water molecules in the range
of temperatures studied must be known. Although in diluted
solutions (∼10Ϫ3 M) the only species present in solution is the
nona-coordinated one, this is not the case in the range of con-
centrations required for the 17O NMR measurements. From the
data reported in Table 4 we estimate q = 2.8 in a 0.037 M solu-
tion at 298 K. Furthermore, we assume that q is not changing
with the temperature at a given concentration. This assumption
is supported by the thermodynamic parameters reported in
Table 4 at a concentration of 9 × 10Ϫ3 M. Under these condi-
tions, the concentration of octa-coordinated species varies from
0.01% at 278 K to ∼10% at 358 K, which corresponds to q
values ranging from 3.0 to 2.9. Therefore, the assumption of
constancy of q with the temperature produces an error of only
about 3%. The temperature dependence of the reduced relax-
ation rates and chemical shifts for [Gd(EDTA-PA2)]ϩ is shown
in Fig. 6.
Since τm decreases, while T 1m and T 2m generally increase with
increasing temperature, the sign of the temperature dependence
of 1/T 1r and 1/T 2r will depend on which term dominates in the
denominator of eqns. (14) and (15). When a changeover exists
from the “fast exchange” limit at high temperature, where T 2m
is the principal term in the denominator of eqn. (15), to the
slow exchange limit at low temperatures, where τm is the princi-
pal term, a maximum is observed in the temperature depend-
ence of both 1/T 1r and 1/T 2r. However, in the case of
[Gd(EDTA-PA2)]ϩ 1/T 1r and 1/T 2r decrease with increasing the
temperature, which indicates that T 1m and T 2m dominate the
denominator of eqns. (14) and (15). This is characteristic of
species in the fast exchange regime in the whole range of tem-
peratures studied. Under these conditions, the inner-sphere
contribution to ∆ωr is given by the chemical shift of the bound
water molecules, which is determined by the hyperfine inter-
action between the Gd3ϩelectron spin and the 17O nucleus via
eqn. (16),41
(16)
where gL is the isotropic Landé g-factor (gL = 2.0 for Gd3ϩ), S is
the electron spin (S = 7/2 for Gd3ϩ), A/ប is the hyperfine or
scalar coupling constant, and B is the magnetic field. We
assume that the outer-sphere contribution to ∆ωr has temper-
ature dependence similar to ∆ωm and is given by:
∆ωOS = COS ∆ωm
(17)
where COS is an empirical constant.4
The 17O longitudinal relaxation rates in Gd3ϩ solutions are
dominated by the dipole-dipole and quadrupolar mechanisms39
and, to a good approximation,40 may be expressed by:
(18)
where γS = gLµB/ប is the electron gyromagnetic ratio (γS = 1.76 ×
1011 rad sϪ1 TϪ1 for gL = 2.0), γI is the nuclear gyromagnetic ratio
(γI = Ϫ3.626 × 107 rad sϪ1 TϪ1 for 17O), rGdO is the distance
between the electron charge and the 17O nucleus (the metal–
oxygen distance in the point dipole approximation), τR is the
rotational correlation time for the Gd3ϩ–O vector, τdiϪ1 = τmϪ1 ϩ
Ϫ1
T ie ϩ τRϪ1, I is the nuclear spin (I = 5/2 for 17O), χ is the
quadrupolar coupling constant, and η is an asymmetry
parameter.
Fig. 6 Temperature dependence of (a) the reduced 17O transverse (᭺)
and longitudinal (ᮀ) relaxation rates of [Gd(EDTA-PA2)]ϩ, expressed
as ln(1/T 1,2r), and the reduced chemical shifts, ∆ωr (∆), and (b) NMRD
profiles of [Gd(EDTA-PA2)]ϩ at 37 (∆), 25 (᭺) and 5 ЊC (ᮀ).
We assume that the rotational correlation time, τR, has a
simple exponential temperature dependence as in eqn. (19),
298
where τR is the correlation time at 298.15 K and ER is the
activation energy.
It has been shown that the outer-sphere contributions in
eqns. (11) and (12) can be neglected.39,40 Although the full eqns.
(12) and (13) were used in the fit of the experimental data, it is
(19)
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The 17O transverse relaxation rates in Gd3ϩ bound water
where rGdH is the effective distance between the gadolinium
molecules are dominated by the scalar relaxation mechanism42
electronic spin and the water protons and τϪdi1 = τϪM1 ϩ τϪR1 ϩ T Ϫ1
ie
Ϫ1
and obey to a very good approximation (eqn. (20)) where τis
=
(i = 1, 2). The rotational correlation time (τR) now refers to the
τm ϩ T ieϪ1. It should be noted that the scalar mechanism is
unimportant in the longitudinal relaxation and in proton
relaxations.
rotation of the Gd3ϩ–water proton vector.
Ϫ1
The outer-sphere contribution to the relaxivity, arising
from diffusing water molecules external to the chelate com-
plex for a 1 mM solution, can be expressed by eqns. (26) and
(27),48 where j = 1,2 and NA is the Avogadro’s number, aGdH
is the distance of the closest approach of a second-sphere
water molecule to Gd3ϩ, and the correlation time τGdH corre-
sponds with aGdH2/DGdH. The diffusion coefficient DGdH is
assumed to obey an exponential temperature dependence
(eqn. (28)),
(20)
The residence time (or exchange rate, kex) of water molecules
in the inner-sphere is assumed to obey the Eyring equation as
written in eqn. (21), where ∆H‡ is the enthalpy of activation for
the exchange process and kex298 is the exchange rate at 298.15 K.
(28)
(21)
298
where DGdH is the diffusion coefficient at 298.15 K and ED
GdH
is the activation energy.
The electronic rates (1/T ie) were approximated using eqns. (22)
The temperature dependence of the NMRD profile usually
gives a good indication of which parameter limits the proton
relaxivity. If the high field value (>10 MHz) increases with
increasing temperature, relaxivity is limited by slow water
exchange, whereas in the opposite case fast rotation is the limit-
ing factor. For [Gd(EDTA-PA2)]ϩ, the relaxivity increases with
decreasing the temperature which shows that the relaxivity is
dominated by the fast rotation, as is usually observed for small
Gd3ϩ chelates.3,49
and (23):41,43
(22)
(23)
Simultaneous fitting of NMRD and 17O NMR data. A simul-
taneous fitting of the NMRD and 17O NMR data of
[Gd(EDTA-PA2)]ϩ was performed according to eqns. (11)–
(28). The following strategy was adopted: aGdH, the distance
of closest approach for the outer-sphere contribution was set
at 0.35 nm and, following previous NMRD studies, the dis-
tance between the protons of the coordinated water molecules
and the Gd3ϩ ion, rGdH was fixed at 0.31 nm.49,50 The rGdO
distance was also fixed at 0.25 nm. The number of water mol-
ecules in the first coordination sphere of Gd3ϩ,q, was taken as
3.0 for the NMRD equations. However, an effective q = 2.8
was used to determine the 17O reduced relaxation rates and
chemical shifts (see above). One might argue that a simul-
taneous fitting of the NMRD and 17O NMR data is not the
best approach in the present case, since some of the param-
eters (q and τR) have been shown to be concentration depend-
ent (vide supra). However, independent fits of the 17O NMR
data have been shown to be quite insensitive to several fitting
parameters due to the nearly linear dependence of the
reduced relaxation rates and chemical shifts with the temper-
ature. The validity of a simultaneous treatment of the
NMRD and 17O NMR data in this case is therefore based on
the assumption that the residence time of water molecules in
the inner coordination sphere of Gd3ϩ, τm, does not change
significantly in the concentration range 1–37 mM. Because of
the concentration dependence of the rotational correlation
time, τR, observed from deuterium longitudinal relaxation rate
measurements, we used two different rotational correlation
times for the Gd–H (τRH) and Gd–O (τRO) vectors during the
fitting procedure. Furthermore, it has been demonstrated,
where ωS is the Larmor frequency, ∆2 is the trace of the square
of the ZFS tensor, and τv is the correlation time for the modu-
lation of ZFS. The modulation of the ZFS may be due to the
modulation of transient distortions,44 and we assume that the
correlation time has Arrhenius behaviour (eqn. (24)).
(24)
Variable-temperature NMRD measurements. The [Gd-
(EDTA-PA2)]ϩ chelate was investigated by water 1H longi-
tudinal relaxation time measurements at 5, 25 and 37 ЊC
and magnetic field strengths varying between 2.16 × 10Ϫ3 and
0.47 T (NMRD), and the curves obtained are included in
Fig. 6. Longitudinal proton relaxation enhancements in
NMRD studies are commonly expressed in relaxivities (r1, in
sϪ1 mMϪ1). The relation between r1 and the inner- and outer-
sphere contributions is given by eqn. (11) with Pm = (cq)/55.5,
where c is the Gd3ϩ concentration in mol LϪ1 and q is the
number of inner-sphere water molecules (q = 3 for [Gd(EDTA-
PA2)]ϩ at a concentration of 10Ϫ3 M). The longitudinal relax-
ation rate of the inner-sphere water molecules is dominated
by the dipolar interaction and is given by the Solomon–
Bloembergen equation (eqn. (25)):45–47
(25)
(26)
(27)
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Table 6 Parameters obtained from the simultaneous analysis of 17O
τR, and the number of inner-sphere water molecules, q. We
NMR and NMRD for the [Gd(EDTA-PA2)]ϩ complex
believe that these intermolecular interactions may also oper-
ate in other cationic Gd3ϩ chelates and, therefore, caution is
needed when different techniques that require a different con-
centration range are used to study the parameters governing
Parameter
Gd(EDTA-PA2)
Gd(DTPA-BMA)a
kex298/106 sϪ1
∆H‡/kJ molϪ1
A/ប/106 rad sϪ1
τRH298/ps
44
29
4
1
0.45 0.01
47.6 1.1
Ϫ3.8 0.2
66 11
1
the H relaxivity. At 37 ЊC, the fast rotation is the limiting
factor for the inner-sphere relaxivity of the [Gd(EDTA-
Ϫ3.93 0.05
113
18
117
13
PA2)]ϩ complex.
5
2
8
1
4
ERH/kJ molϪ1
τRO298/ps
21.9 0.5
c
ERO/kJ molϪ1
τV298/ps
c
Experimental
36
25
1
EV/kJ molϪ1
∆2/1020 sϪ2
1b
3.9 1.4
0.41 0.02
2
1H (300 MHz), 13C (75.5 MHz), 17O (40.7 MHz) and H (46.1
0.16 0.02
DGdH298/10Ϫ10 m2 sϪ1
EDGdH/kJ molϪ1
18
52
3
4
23
2
MHz) NMR spectra were recorded on a Varian INOVA-300
spectrometer with 5 mm or 10 mm tubes (2H). Chemical
shifts are reported as δ values. For measurements in D2O,
tert-butyl alcohol was used as an internal standard with the
methyl signal calibrated at δ = 1.2 ppm (1H) or 31.2 (13C).
D2O (100%) was used as an external chemical shift reference
for 17O resonances. Samples of the Ln3ϩ complexes for the
13C and 17O NMR measurements were prepared by mixing of
equimolar amounts of EDTA-PA2 and hydrated LnCl3
(Aldrich Chemical Co.) in D2O, followed by the adjustment
of the pD to ca. 6 with a solution of NaOD in D2O. 89Y
(19.6 MHz) NMR spectra were recorded on a Varian
VXR-400 S spectrometer using 5 mm sample tubes. A 2 M
YCl3 solution was used as external reference. The pH of the
solutions was measured at room temperature with a cali-
brated microcombination probe purchased from Aldrich
Chemical Co. The pH values given are direct meter readings
without correction for D-isotope effects. The solvent (water)
for deuterium longitudinal relaxation rates was deuterium
depleted. Longitudinal relaxation rates (1/T 1) were deter-
mined by the inversion-recovery method,55 and the transverse
relaxation rates (1/T 2) were obtained by the Carr–Purcell–
Meiboom–Gill spin–echo technique.56
The experimental details for high-resolution laser excited
luminescence measurements were previously described.57
Lifetimes are averages of at least 3–5 independent determin-
ations. The 1/T 1 nuclear magnetic relaxation dispersion
(NMRD) profiles of the solvent protons at 5, 25 and 37 ЊC
and 1 mM concentration were obtained on a Spinmaster
FFC Fast Field Cycling NMR relaxometer (Stelar), covering
a continuum of magnetic fields from 7 × 10Ϫ4 to 0.47 T
(corresponding to a proton Larmor frequency range of
0.03–20 MHz). UV-Visible spectrophotometric measure-
ments were made on a Perkin-Elmer Lambda 19 double beam
spectrophotometer and the spectra were recorded in digital
form. A thermostatable cylindrical cell with a long optical
path (10 cm) was used, in order to maximise the observed
absorption.
12.9 2.1
½
χ(1 ϩ η2) /MHz
7.58b
7.58b
a The data listed for [Gd(DTPA-BMA)] have been reported previously
in ref. 4 and are provided here for comparison. b Parameters fixed dur-
ing the fitting procedure. c Only one rotational correlation time for both
Gd–H and Gd–O vectors was used.
both experimentaly and by molecular dynamics simulations,
that τRH/τRO = 0.65–0.75 in several Gd3ϩ complexes of
poly(aminocarboxylates).51,52 The results of the fits are pre-
sented in Table 6 and are also shown in Fig. 6.
It has been observed that the water-exchange rate decreases
dramatically from [Gd(H2O)8]3ϩ to the chelate complexes with
one inner-sphere water molecule.4 In the case of the [Gd-
(EDTA-PA2)]ϩ complex the water-exchange rate is about 18
times smaller than for [Gd(H2O)8]3ϩ, but about 100 times faster
than for [Gd(DTPA-BMA)], which contains a single inner-
sphere water molecule (see Table 6). The water exchange rate
determined for [Gd(EDTA-PA2)]ϩ is also ca. 4.5 times faster
than that reported for [Er(EDTA)]Ϫ (9.8 × 106 sϪ1).7 These data
contrast with previous observations for related systems which
showed that an increase of the net charge from Ϫ1 to ϩ1 is
accompanied by a decrease in the water exchange rate of one
order of magnitude.53 Apparently, other factors than the net
charge of the complex have a more important impact on the
water exchange rates.54
From the data reported in Fig. 2, we estimate rotational
correlation times of 106 and 120 ps at concentrations of 1 mM
and 37 mM respectively, in excellent agreement with the values
298
obtained from the simultaneous fitting (τRH = 113 ps and
τRO298 = 117 ps).
The good quality of the fittings obtained from the simul-
taneous treatment of the NMRD and 17O NMR data and the
reasonable parameters obtained gives us confidence that the
water exchange rate does not vary much with the concentration
in the range 1–37 mM. Moreover, the correctness of the
assumption of a hydration number of q = 2.8 for the 17O NMR
data is confirmed by the value obtained for the scalar coupling
constant (A/ប), which is similar to values reported for other
Gd3ϩ polyaminocarboxylate complexes.1
Potentiometry measurements
The ligand protonation constants and the stability constant of
the Gd3ϩ complex were determined by pH-potentiometry at a
constant ionic strength (0.1 M (CH3)4NCl). The titrations were
Conclusions
carried out in a thermostated vessel (25
0.2 ЊC) using
This study shows that the hydration number, q, of [Ln(EDTA-
PA2)]ϩ complexes in aqueous solutions is dependent the con-
centration of the complex. The substitution of two carboxyl-
ates of EDTA with amide functionalities strongly influences
some properties of the Ln3ϩ complexes including their stabil-
ity. 13C NMR studies demonstrate that the complexes exist in
solution as a mixture of cis and trans isomers. The presence
of intermolecular interactions, that are especially important
in concentrated solutions, have been demonstrated by differ-
ent NMR techniques. These interactions are responsible for
the concentration dependence of some parameters that affect
the proton relaxivity such as the rotational correlation time,
(CH3)4NOH as titrant solution dosed with a Metrohm Dosimat
665 automate burette. A combined glass electrode (C14/02-SC,
reference electrode Ag/AgCl in 3 M KCl, Moeller Scientific
Glass Instruments, Switzerland) connected to a Metrohm 692
pH/ion-meter was used to measure pH. The titrated solution
(3 mL) was stirred with a magnetic stirrer and bubbled with a
constant N2 flow. Protonation and stability constants were
determined at 0.002 M ligand concentration from 3–4 parallel
titrations. Since the Gd3ϩ complex is almost completely formed
at low pH, the stability constant was obtained by ligand
competition between EDTA-PA2 and EDTA, at a metal :
EDTA : EDTA-PA2 ratio of 1 : 1 : 1. The hydrogen ion concen-
D a l t o n T r a n s . , 2 0 0 3 , 7 2 7 – 7 3 7
735

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36, 1316.
9 Y.-M. Wang, Y.-J. Wang and Y-L. Wu, Polyhedron, 1998, 18, 109.
10 IUPAC Stability constants. Academic Software, K. J. Powell, 1999.
11 B. M. Alsaadi, F. J. C. Rossotti and R. J. P. Williams, J. Chem. Soc.,
Dalton Trans., 1980, 2151.
tration was calculated from the measured pH values using the
correction method suggested by Irving et al.58 All protonation
and stability constants were computed with the program
PSEQUAD.59
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Synthesis of EDTA-PA2
Ethylenediaminetetraacetic acid (20.0 g, 68 mmol) was com-
bined with acetic anhydride (32 g, 312 mmol) and dry pyrid-
ine (34 mL) in a round bottom flask equipped with stirrer bar
and condenser. The mixture was heated at 65 ЊC for 72 h and
then cooled to room temperature. The suspension obtained
was filtered and the solids were washed with diethyl ether.
After drying in vacuo over KOH, a 16 g amount of EDTA-
bis(anhydride) (62.4 mmol) was obtained, which was dis-
solved in 125 mL of DMF. To the stirred solution, two
equivalents of triethylamine was added. The propylamine (7.4
g, 125 mmol) was added dropwise and the stirring was con-
tinued for 24 h at ambient temperature. Then the mixture was
concentrated by rotary evaporation leaving a brown solid
residue. The crude product was treated with isopropyl alco-
hol, which dissolved the coloured impurities, leaving a white
solid. The solid was collected, washed with isopropyl alcohol
and dried in vacuo yielding 15.3 g of EDTA-PA2 (66%); mp
153–154 ЊC; [Found: C, 51.0; H, 7.9; N, 14.8. Calc. for
C16H30N4O6: C, 51.3; H, 8.1; N, 15.0%]. δH (D2O, pH 7): 0.92
(6 H, t), 1.57 (4 H, m,), 3.25 (8 H, m), 3.55(4 H, s) and
3.78(4 H, s); δC (D2O, pH 7): 12.2, 23.4, 42.8, 53.5, 58.5, 58.9,
171.2, 176.3.
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Synthesis of EDTA-PA2-d4
Deuteration of EDTA-PA2 at the α-position respect to carbonyl
groups of the acetate pendant arms was carried out by a similar
procedure to that described in the literature:60 0.5 mmol of
ligand was dissolved in 20 mL of D2O, the pD was adjusted to
10.6 by addition of solid K2CO3, and the mixture was stirred
and heated to reflux for 24 h. The pH was then adjusted to
2 with a 25% HCl solution, the solution of the ligand
concentrated to 10 mL and 10 mL of EtOH were added. The
precipitated KCl was filtered off, and the solution concen-
trated to dryness. The resultant solid residue was treated with
10 mL of MeOH, the solution filtered and the filtrate concen-
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Acknowledgements
This research was performed within the framework of the
EU COST Action “Lanthanide Chemistry for diagnosis and
therapy” (D18).
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