Equilibria and structure of the lanthanide(III)-2-hydroxy-1,3- diaminopropane-N,N,N′,N′-tetraacetate complexes: Formation of alkoxo-bridged dimers in solid state and solution

Article abstract of DOI:10.1021/ic0517321

The complex formed between 1,3-diamino-2-hydroxypropane-N,N,N′, N′-tetraacetic acid (H₄L-OH) and Nd³⁺ at pH 7.5 was found to be a dinuclear dimer in the solid state by X-ray crystallography. In the complex K₄[Nd₂(L-O)₂-(H₂O) ₂]-14H₂O each ligand is coordinated to both Nd ³⁺ atoms with an iminodiacetate group (the Nd³⁺-Nd ³⁺ distance is 3.9283(8) A). The alcoholic OH groups are deprotonated, and the alkoxo oxygens are coordinated to both Nd³⁺ in a bridging position. The Nd³⁺ ions are nine-coordinated with one water molecule per Nd(III) ion in the inner sphere. The complex K ₄[Nd₂(L-O)₂(H₂O)₂] ·14H₂O has an inversion center, and the space group is P1. Two of the K⁺ counterions are six-coordinated, while the other two K⁺ ions are eight-coordinated; polar polymeric water-K⁺ layers are formed between the apolar ligand layers via the bridging water molecules. The dinuclear dimer complexes are also present in aqueous solution. The proton relaxivities of the Gd³⁺ complex decrease with the increase of pH, and at pH > 6, the low relaxivity values indicate the probable absence of H₂O in the inner sphere and the predominance of the eight-coordinated dimer species [Gd₂(L-O)₂]. ^4- The results of ESI-TOF MS studies of the complexes of La ³⁺, Nd³⁺, and Lu³⁺ proved the formation of dinuclear dimers in dilute (0.25 mM) solutions. pH-potentiometric titrations indicate the formation of complexes with 1:1 (Ln(L-OH)^-, Ln(HL-OH), and Ln₂(L-O)₂^4-) and 2:1 (Ln ₂(L-O)⁺) metal-to-ligand ratios. The stability constants of the Ln(L-OH)^- species increase from La³⁺ (log K = 10.19) to Lu³⁺ (log K = 14.08). The alcoholic OH group of the Ln(L-OH)^- species dissociates at unusually low pH values. The pH range of dissociation shifts to lower and lower pH's with the increasing atomic number of the lanthanides. This pH range is about 4-7 for the La³⁺ complex and 1-4 for the Lu³⁺ complex. The results of 1D and 2D ¹H and ¹³C NMR studies of the La³⁺ complex, the number and multiplicity of signals, and the values of coupling constants are in agreement with the dinuclear dimer structure of the complex in solution.

Full text of DOI:10.1021/ic0517321

Inorg. Chem. 2006, 45, 4951−4962
Equilibria and Structure of the
Lanthanide(III)-2-hydroxy-1,3-diaminopropane-N,N,N
′
,N′
-tetraacetate
Complexes: Formation of Alkoxo-Bridged Dimers in Solid State and
Solution
Gyula Tircso´,*,‡,§ Attila Be´nyei,† Erno˜ Brucher,*,‡ Anita Kis,‡ and Ro´bert Kira´ly‡
1
Department of Chemistry, Laboratory for X-ray Diffraction, Department of Inorganic and
Analytical Chemistry, UniVersity of Debrecen, Egyetem te´r 1, Debrecen H-4010, Hungary
Received October 6, 2005
+
The complex formed between 1,3-diamino-2-hydroxypropane-N,N,N′,N′
-tetraacetic acid (H4L-OH) and Nd3 at pH
7.5 was found to be a dinuclear dimer in the solid state by X-ray crystallography. In the complex K4[Nd2(L-O)2-
+
(H2O)2]
‚
14H2O each ligand is coordinated to both Nd3+ atoms with an iminodiacetate group (the Nd3
−
Nd3+ distance
is 3.9283(8) Å). The alcoholic OH groups are deprotonated, and the alkoxo oxygens are coordinated to both Nd3
in a bridging position. The Nd3 ions are nine-coordinated with one water molecule per Nd(III) ion in the inner
+
+
sphere. The complex K4[Nd2(L-O)2(H2O)2]
‚
14H2O has an inversion center, and the space group is P1
h
. Two of the
K+ counterions are six-coordinated, while the other two K+ ions are eight-coordinated; polar polymeric water
−
K+
layers are formed between the apolar ligand layers via the bridging water molecules. The dinuclear dimer complexes
are also present in aqueous solution. The proton relaxivities of the Gd3+ complex decrease with the increase of pH,
and at pH > 6, the low relaxivity values indicate the probable absence of H2O in the inner sphere and the
-
predominance of the eight-coordinated dimer species [Gd2(L-O)2].4 The results of ESI-TOF MS studies of the
+
complexes of La3+, Nd3+, and Lu3 proved the formation of dinuclear dimers in dilute (0.25 mM) solutions. pH
−
4
potentiometric titrations indicate the formation of complexes with 1:1 (Ln(L-OH)-, Ln(HL-OH), and Ln2(L-O)2 -) and
2:1 (Ln2(L-O)+) metal-to-ligand ratios. The stability constants of the Ln(L-OH)- species increase from La3 (log K
+
+
)
10.19) to Lu3 (log K
)
14.08). The alcoholic OH group of the Ln(L
−
OH)- species dissociates at unusually low
pH values. The pH range of dissociation shifts to lower and lower pH’s with the increasing atomic number of the
lanthanides. This pH range is about 4
−
7 for the La3+ complex and 1
−
4 for the Lu3+ complex. The results of 1D and
2D H and 13C NMR studies of the La3 complex, the number and multiplicity of signals, and the values of coupling
constants are in agreement with the dinuclear dimer structure of the complex in solution.
1
+
Introduction
chromatographic separations.1,2 Lactic acid and particularly
R-hydroxy-isobutyric acid have recently been used in
analytical separations by ion exchange chromatography or
capillary electrophoresis.3-6 The successful use of these
ligands for the separations has attracted considerable interest
in the study of the equilibrium properties of the lanthanide-
The complexes formed between lanthanide(III) ions and
hydroxycarboxylic acids played an important role in the
development of ion-exchange separation methods and the
coordination chemistry of lanthanides. Citric acid was one
of the first eluents used for the radiochemical and preparative
(1) Tompkins, E. R.; Mayer, S. W. J. Am. Chem. Soc. 1947, 69, 2859.
(2) Spedding, F. H.; Fulmer, E. I.; Butler, T. A.; Gladrow, E. M.; Gobush,
M.; Porter, P. E.; Powell, J. E.; Wright, J. M. J. Am. Chem. Soc. 1947,
69, 2812.
(3) Choppin, G. R.; Silva, R. J. J. Inorg. Nucl. Chem. 1956, 3, 153.
(4) Nervik, W. E. J. Phys. Chem. 1955, 59, 153.
(5) Sawatari, H.; Asano, T.; Hu, X. C.; Saizuka, T.; Itoh, A.; Hirose, A.;
Haraguchi, H. Bull. Chem. Soc. Jpn. 1995, 68, 898.
(6) Shi, Y. C.; Fritz, J. S. J. Chromatogr. 1993, 640, 473.
* To whom correspondence should be addressed. E-mail: gyulat@
gmail.com (G.T.); ebrucher@delfin.klte.hu (E.B.). Tel: (+1) 972-883-
2666 (G.T.); (36) 52-512-900/2374 (E.B.). Fax: (+1) 972-883-2925 (G.T.);
(36) 52-489-667 (E.B.).
† Department of Chemistry, Laboratory for X-ray Diffraction.
‡ Department of Inorganic and Analytical Chemistry.
§ Present address: University of Texas at Dallas, Department of
Chemistry, P.O. Box 830660, Richardson, Texas 75080.
10.1021/ic0517321 CCC: $33.50
Published on Web 05/25/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 13, 2006 4951

Tircso´ et al.
The stability constants of the Ln3+ complexes formed with
dpta-OH are very close to those of the Ln(dpta) (H4dpta )
1,3-diaminopropane-N,N,N′,N′-tetraacetic acid).13,15 On the
basis of these findings, Powell at al. concluded that the
alcoholic OH group of L-OH does not participate in complex
formation because the 2-hydroxy-1,3-diaminopropane back-
bone is not sufficiently flexible and the dissociation of the
alcoholic OH may occur at higher pH values.13 The
complexes of L-OH formed with Fe3+, Al3+, and Cu2+ have
been studied by several groups. The formation of mono-
nuclear, dinuclear, and tetranuclear complexes with an
ionized alcoholic hydroxyl group has been demonstrated.16-18
In the di- and tetranuclear complexes, the oxygen of the
ionized alcoholic OH has a µ-alkoxo bridging position. The
formation of dinuclear lanthanide(III)-L-OH complexes in
solution has not been detected, but the complexes formed
with La3+ or Ce4+ at 2:1 metal to L-OH concentration ratio
can carry out double-strand cleavage of plasmid DNA. The
authors reported no information on the complexes formed
in these systems. 19
More recently it has been found by Miyashita et al. that
the L-OH ligand forms dimer complexes with some lan-
thanides in the solid state. The X-ray diffraction studies
revealed that, in the Na4[Yb2(µ-L-O)2] complex, the ligands
are coordinated to both Yb3+ with an iminodiacetate group
and the two Yb3+ ions are bridged by two alkoxo oxygens.
Conductivity measurements indicated that the aqueous solu-
tion of the complex behaves like a 4:1 electrolyte.20 The
formation of a similar alkoxo-bridged dimer was found
recently in the reaction of Zr(IV) and L-OH by X-ray
diffraction and NMR spectroscopy.21
Chart 1. Ligands Studied and Used for Comparison in This Work
(III) (Ln3+) complexes. However, despite the detailed studies,
the possible role of the alcoholic hydroxyl group in the
complex formation has remained a disputed problem for a
long time. The stability constants of the complexes formed
with, for example, glycolate, lactate, and R-hydroxy-isobu-
tyrate are higher than those formed with acetate, propionate
and isobutyrate, which indicates the role of the alcoholic
hydroxyl in the complexation, but the dissociation of the
coordinated OH group is disputed.7,8 In R-hydroxy-carboxylic
acids, the OH group has a terminal position, and the presence
of one carboxylate group promotes the coordination of the
oxygen atom of the alcoholic OH group in the complexes.
In the complexes of citrate and some other hydroxy-
polycarboxylates, two carboxylate groups assist not only the
coordination of the alcoholic OH but also, because of the
formation of two chelate rings, in the dissociation of the
hydrogen ion and the coordination of the alkoxo group. The
dissociation of the alcoholic OH in the La3+-glycolate
complex occurs at pH > 8, as was determined by NMR
spectroscopy,9 while in the Gd3+-citrate complex, the
dissociation takes place at pH > 5,10 indicating the combined
effect of the two coordinated carboxylates. The dissociation
of the alcoholic OH in the complexes of lanthanides formed
with some polyhydroxy-carboxylates has also been detected
by 13C NMR spectroscopy.11
The first experiments in our study indicated the dissocia-
tion and coordination of the alcoholic OH group (formation
of Ln(L-O)2- complexes) by pH potentiometry, and we could
also detect the formation of dinuclear complexes (Ln2(L-
O)+), which were not observed earlier.13,14 On the basis of
these interesting results, we made a detailed equilibrium
study for the complexes of the whole series of lantahanides.
The stoichoimetry of the complexes formed in solution was
verified with ESI-TOF MS. The structures of the complexes
were studied in solution by 1H and 13C NMR spectroscopy.
The structure of the Nd3+ complex in the solid state has been
studied by X-ray diffraction.
Alcoholic hydroxyl groups are also attached to some
polyamino-polycarboxylate ligands. For the complexes of
N-2-hydroxyethyl-ethylenediamine-N,N′,N′-triacetate formed
with La3+, Lu3+, and Y3+, the dissociation of the alcoholic
OH group was demonstrated by NMR spectroscopy.12
However, in the lanthanide(III) complexes of the H4edta
analogue ligand, 2-hydroxy-1.3-diaminopropane-N,N,N′,N′-
tetraacetic acid (H4L-OH), which contains a linker CH(OH)
group between two methyliminodiacetate groups (Chart 1),
the dissociation of the alcoholic OH has not been detected
Experimental Section
Preparation of Solutions. LnCl3 stock solutions were prepared
by dissolving lanthanide(III) oxides (99.9% Fluka) in 6 M HCl,
and the excess HCl was evaporated. The concentrations of the LnCl3
1
either by pH potentiometry or H NMR spectroscopy.13,14
(14) Balgavy, P.; Novomesky, P.; Majer, J. Inorg. Chim. Acta 1980, 39,
233.
(15) Anderegg, G.; Wenk, F. HelV. Chim. Acta 1971, 54, 216.
(16) Kawata, S.; Iwaizumi, M.; Kosugi, H.; Yokoi, H. Chem. Lett. 1987,
2321.
(17) Kato, M.; Yamada, Y.; Inagaki, T.; Mori, W.; Sakai, K.; Tsubomura,
T.; Sato, M.; Yano, S. Inorg. Chem. 1995, 34, 2645.
(18) Schmitt, W.; Jordan, P. A.; Henderson, R. K.; Moore, G. R.; Anson,
C. E.; Powell, A. K. Coord. Chem. Rev. 2002, 228, 115.
(19) Branum, M. E.; Que, L. J. Biol. Inorg. Chem. 1999, 4, 593.
(20) Miyashita, Y.; Sanada, M.; Yamada, Y.; Fujisawa, K.; Okamoto, K.
Chem. Lett. 2002, 840.
(7) Choppin, G. R.; Chopoorian, J. A. J. Inorg. Nucl. Chem. 1961, 22,
97.
(8) Seyb, K. E. J. Inorg. Nucl. Chem. 1964, 26, 231.
(9) Vanwestrenen, J.; Peters, J. A.; van Bekkum, H.; Rizkalla, E. N.;
Choppin, G. R. Inorg. Chim. Acta 1991, 181, 233.
(10) Jackson, G. E.; Dutoit, J. J. Chem. Soc., Dalton Trans. 1991, 1463.
(11) Giroux, S.; Aury, S.; Henry, B.; Rubini, P. Eur. J. Inorg. Chem. 2002,
1162.
(12) Bru¨cher, E.; Kostromina, N. A. Teoret. Eksper. Khim. (in Russian)
1972, 8, 210.
(13) Powell, J. E.; Ling, D. R.; Tse, P. K. Inorg. Chem. 1986, 25, 585.
4952 Inorganic Chemistry, Vol. 45, No. 13, 2006

Lanthanide(III) Complexes
solutions were determined by complexometry with standardized
Na2H2edta and xylenol orange indicator. The ligand stock solutions
were prepared by dissolving H4dpta-OH and H4dpta (Sigma) with
known amounts of KOH (pH ≈ 4.5). The concentration of the
ligand solutions was determined by pH-potentiometric titrations in
the absence and presence of a 50-fold excess of CaCl2. The KCl
used for keeping a constant ionic strength was recrystallyzed from
double distilled water.
using direct methods with SIR-92 software, and refined on F2 by
full-matrix least-squares methods using SHELX-97.24,25 All non-
hydrogen atoms were refined anisotropically. Hydrogen atoms of
the C-H were included using a riding model. Some of the water
hydrogen atoms were found on the difference electron density map,
while others were placed into calculated positions for strong
hydrogen bonds. For the final refinement, O-H bond distances were
restrained for 0.85 Å. Publication material was prepared with the
WINGX-97 suite.26 The residual electron density of 1.162-1.453
e/Å is close to the neodimium atom (<1 Å).
Measurement of Relaxation Times. The longitudinal relaxation
times of the water protons (T1) in the presence of Gd(L-OH)- and
Gd(dpta)- were measured with an MS-4 NMR spectrometer
(Institute Jozef Stefan, Ljubljana) at 9 MHz. The temperature of
the cell holder was controlled with a thermostated air stream. The
T1 values were measured by the inversion recovery method (180°-
τ-90°) with five to nine measurements at six to eight different τ
values at 25 °C. The concentration of the Gd3+ complex solution
was 1.0 mM.
Equilibrium Measurements. The pH-potentiometric titrations
were carried out with a Radiometer PHM-85 pH meter, using
pHG211 glass and K401calomel electrodes in vessels thermostated
at 25 °C. The titrations were made with an ABU80 autoburet. The
titrated samples (20 mL) were stirred with a magnetic stirrer, and
argon gas was bubbled throughout the measurements. For the
calibration of the electrode system, KH-phthalate (pH ) 4.005)
and Borax (pH ) 9.180) buffers were used. To obtain H+
concentrations from the measured pH values, the method proposed
by Irving et al. was used.22 A 0.01 M HCl solution (1.0 M for
KCl) was titrated with standardized KOH solution. The differences
between the measured and calculated pH values were used to
calculate the H+ concentrations from the pH values, measured in
the titration experiments. The ionic product of water (log Kw) was
found to be 13.89. The concentration of the ligand solutions titrated
in the presence of LnCl3 varied between 0.002 and 0.01 M. The
metal to ligand concentration ratio was 4, 2, 1, 0.67, 0.5, and 0.25
for La, Eu, Ho, Er, Tm, Lu, and Y; 4, 2, 1, and 0.25 for Nd, Gd,
Tb, Dy, and Yb; and 2, 1, and 0.5 for the Ce, Pr, and Sm. The
number of the fitted milliliters of titrant-pH ([H+]) data points
used for the equilibrium calculations was between 150 and 520.
For the titrations, 0.2 M KOH, which was kept under argon
atmosphere, was used. The ionic strength of the titrated solutions
was constant, 1.0 M for KCl. The protonation constants of the ligand
ant stability constants of the complexes were calculated with the
use of the computer program PSEQUAD.23
Electrospray Ionization Time-of-Flight Mass Spectrometry
(ESI-TOF MS). ESI-TOF MS measurements were performed on
a BioTOF II instrument (Bruker Daltonics, Billerica, MA). The
solutions were introduced directly into the ESI source by a syringe
pump (Cole-Parmer Ins. Co.) at a flow rate of 2 µL/min. The
temperature of drying gas (N2) was maintained at 100 °C. The
voltages applied on the capillary entrance, capillary exit, and the
first and the second skimmers were -4500, 120, 40, and 30 V,
respectively. The spectra were accumulated and recorded by a
digitizer at a sampling rate of 2 GHz.
NMR Spectroscopy. The solutions of complexes were prepared
by mixing equivalent amounts of the K2H2L-OH and LnCl3
solutions. After evaporation of the H2O, the solid material was
dissolved in D2O. The pD of the solutions was monitored by the
addition of KOD or DCl (Cambridge Isotope Laboratories) solu-
tions, and the readings were corrected for the deuterium isotope
effect using the relationship pH ) 0.4 + pD.27 Both one- and two-
dimensional NMR spectra were recorded mainly on a Bruker
Avance DRX-360 spectrometer operating at 360.0 and 90.5 MHz
Spectrophotometric measurements were made with a Cary 1E
UV-vis spectrophotometer using 5 cm cells in a thermostated cell
holder.
Preparation of the K4[Nd2(L-O)2(H2O)2]‚14H2O Complex.
Equivalent amounts (0.3 mmol) of the K2H2L-OH ligand and NdCl3
stock solutions were mixed together, and the pH of the mixture
was increased to about 7.5 with the addition of KOH solution. On
the basis of the equilibrium studies at this pH, only the [Nd2(L-
O)2]4- species is present in the solution. Absolute ethanol was added
to the solution of the complex when the complex quickly solidified
in the form of a pinkish powder. The precipitate was filtered and
washed twice with anhydrous ethanol (10 mL) and diethyl ether
(10 mL) and dried in air to a constant weight. A weighted amount
(0.047 g, 0.1 mmol) of the dried complex was redissolved in 0.5
mL of doubled distilled water and placed into a narrow tube.
Anhydrous ethanol was carefully layered on the aqueous phase,
and the tube was placed into refrigerator and maintained at around
4 °C. Pink block crystals suitable for X-ray crystallography grew
in three weeks.
1
for H and 13C, respectively. Chemical shifts were referenced to
the signal of DSS (DSS ) 2,2-dimethyl-2-silapentane-5-sulfonate)
in D2O as an internal standard. For variable-temperature measure-
ments, a Bruker DRX-500 spectrometer was used. It was equipped
with Eurotherm variable-temperature unit ((0.1 K), which was
calibrated using the methanol method.28 Two-dimensional 1H-1H
correlation spectroscopy (COSY-45, DQF-COSY, NOESY, and
1
EXSY) and H-13C heteronuclear shift correlation spectra (HET-
COR and COLOC) were recorded using standard pulse sequences
in direct mode with a 5 mm QNP probehead. The repetition times
between two transients were as long as possible but not less than
1
5 and 30 s for H and 13C nuclei, respectively, to get the proton
and carbon atoms relaxed. 13C NMR spectra were recorded in
J-modulated or inverse-gated decoupling mode. The NOESY and
EXSY spectra were recorded in TPPI mode (TPPI ) time-
X-ray Crystallography. A pink block crystal of K4[Nd2(L-O)2-
(H2O)2]‚14H2O was mounted into a capillary to prevent drying and
deterioration of the crystal. Data were collected at 293(1)K on an
Euraf Nonius MACH3 diffractometer using monochromated Mo
KR radiation (λ ) 0.71073 Å) and ω-2θ motion. Absorption
corrections were made using ψ scans. The structure was solved
(23) Ze´ka´ny, L.; Nagypa´l, I. Computational Methods for Determination of
Formation Constants; Legett, D. J., Ed.; Plenum Press: New York,
1985.
(24) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A. J. Appl.
Crystallogr. 1993, 26, 343.
(25) Sheldrik, G. M.; SHELX-97, A Program for Crystal Structure
Refinement; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(26) Farrugia, L. J.; WINGX-97 system; University of Glasgow: Glasgow,
U.K. 1996.
(27) Mikkelsen, K.; Nielsen, S. O. J. Phys. Chem. 1960, 64, 632.
(28) Van Geet, A. L. Anal. Chem. 1970, 42, 679.
(21) Zhong, W. Q.; Parkinson, J. A.; Parsons, S.; Oswald, L. D. H.; Coxall,
R. A.; Sadler, P. J. Inorg. Chem. 2004, 43, 3561.
(22) Irving, H. M.; Miles, M. G.; Pettit, L. D. Anal. Chim. Acta 1967, 38,
475.
Inorganic Chemistry, Vol. 45, No. 13, 2006 4953

Tircso´ et al.
Table 2. Selected Bond Lengths (Å) and Angles (deg) for
K4[Nd2(L-O)2(H2O)2]‚14H2Oa
Nd1-Nd1i
Nd1-O1
Nd1-O2
Nd1-O4
Nd1-O6
Nd1-O8
K2-K2iii
3.9283(8)
Nd1-O1w
Nd1-O1i
Nd1-N1
Nd1-N2
O1-C2
2.559(6)
2.325(4)
2.691(5)
2.802(6)
1.403(8)
4.560(2)
2.379(5)
2.476(5)
2.497(5)
2.468(5)
2.475(5)
3.656(4)
Nd1-K1ii
O1-Nd1-O1i
Nd1-O1-Nd1i
O1-Nd1-N1
O1-Nd1-N2
O1-Nd1-O1w
66.8(2)
113.2(2)
64.9(2)
125.3(2)
74.2(2)
O1-Nd1-O4
O1-Nd1-O6
O1-Nd1-O8
N1-Nd1-N2
O1w-Nd1-Nd1i
109.45(2)
140.1(2)
142.4(2)
136.6(2)
76.5(1)
a For numbering scheme see Figure 1. Symmetry transformation used
to generate equivalent atoms: i 1 - x, 1 - y, -z; ii 1 - x, -y, -z; iii -x,
-y, 1 - z.
Figure 1. ORTEP view of the structure of K4[Nd2(L-O)2(H2O)2] at the
35% probability level with a partial numbering scheme. Solvent water
molecules and potassium ions are omitted for clarity.
However, in our case, the solid-state structure of the Nd3+
complex has lower symmetry with inversion center and the
space group is P1h, triclinic. The lack of higher symmetry
can be explained as the two imino-diacetate groups of the
ligand are not equivalent having different electrostatic
interactions and hydrogen-bond networks (see later). As a
result of its ionic radius, the coordination number of Nd3+
is 9 (i.e., apart from two imino-diacetate fragments and two
bridging alkoxy groups a water molecule (O1w) is coordi-
nated). Because of steric reasons, the Nd-N distances differ
for the two amino-polycarboxylate ligands giving 2.691(5)
and 2.802(6) Å. Altogether the coordination of neodimium
is highly disordered, the Nd-Nd distance being 3.9283(8)
Å. It is worth to mention that in a similar dimeric vanadyl
complex of the same ligand the V-V distance is 3.343(2)
Å,29 while the same data for an iron complex is 3.305(2)
Å.30 Miyashita et al. crystallized and collected X-ray data
for the holmium and ytterbium complexes too, giving 3.646-
(2) and 3.716(5) Å, respectively, as the metal-metal
distances for the dimers.20 These numbers indicate that the
(L-O)5- ligand is quite relaxed in our Nd3+ complex. Strong
electrostatic interactions, as well as a hydrogen-bond net-
work, stabilize the structure. Data for the hydrogen-bond
network are given as Supplementary Information (Table S1).
One of the potassium counterions has distorted octahedral
coordination to the non-Nd-coordinated oxygen atoms of the
acetate arms and bridging water molecules to the symmetry-
related potassium (K-K ) 3.656(4) Å), while there are 8
potential donor atoms around the second potassium ion with
bridging O1w, O4, and O6 to the nearest neodimium atom
(K-Nd ) 4.560(2) Å). Altogether the structure is a
complicated polymeric one with polar water-potassium
regions between apolar ligand layers.
Table 1. Details of X-ray Data Collection and Structure Refinement
for K4[Nd2(L-O)2(H2O)2]‚14H2O
chemical formula
mol wt
C11H29K2N2O17Nd
683.8
cryst habit
cryst size (mm3)
cryst syst
space group
a (Å)
pink block
0.40 × 0.35 × 0.32
triclinic
P1h
9.5072(10)
10.2779(10)
13.497(2)
78.69(1)
71.95(1)
82.48(1)
1226.1(3)
2
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
T (K)
293
1.852
2.534
686
F
calcd (g cm-3
)
µ (mm-1
F (000)
)
2θmax (deg)
no. reflns collected
50.74
4854
no. unique reflns with I > 2σ(I)
4495
346
0.0327
0.1365
params
Ra
R
w (all data)b
GOF on F2
1.503
a R ) ∑|Fo - Fc|/∑Fo. b Rw ) {∑[w(Fo2 - Fc2).2]/∑[w(Fo ).2]} 1/2, where
w ) 1/[σ2(Fo2 + (aP)2 + bP)], P ) (Fo2 + 2Fc2)/3, and a and b are constants
given in the Supporting Information.
2
proportional phase incrementation) and at least 3 times with various
mixing times to obtain acceptable noesy spectra.
Data processing was performed mainly with the Bruker WinNMR
program package. For the preparation of the electronic Supporting
Information, the program MestReC (beta version) was used.
Results and Discussion
Equilibrium Studies. The protonation constants of the
ligand L-OH (KiH ) [HiL]/[Hi-1L][H+]) have been deter-
mined by pH-potentiometric titration of 0.005 and 0.01 M
solutions. The log KHi values were calculated with the use
of 284 data points obtained in the pH range of 1.6-11.0.
The log KHi values (and the standard deviations) are pre-
X-ray Crystal Structure. The structure of K4[Nd2(L-O)2-
(H2O)2]‚14H2O has been established by single-crystal X-ray
diffraction studies (Figure 1 and Tables 1 and 2). The solid-
state structure inevitably shows deprotonation of the alco-
cholic OH group and formation of a dimeric complex in
which the two (L-O)5- ligands are coordinated to both Nd3+
with an imda group and the bridging alkoxo oxygen. The
similar [Yb2(L-O)2]4- complex is also a dimer having
crystallographic mirror plane between the two metal atoms.20
(29) Robles, J. C.; Matsuzaka, Y.; Inomata, S.; Shimoi, M.; Mori, W.;
Ogino, H. Inorg. Chem. 1993, 32, 13.
(30) Schmitt, W.; Anson, C. E.; Pilawa, B.; Powell, A. K. Z. Anorg. Allg.
Chem. 2002, 628, 2443.
4954 Inorganic Chemistry, Vol. 45, No. 13, 2006

Lanthanide(III) Complexes
Table 3. Protonation Constants of the Ligands L-OHa and dptab
thanides, the pH values obtained in the presence of heavier
lanthanides (Er3+ and Lu3+ in Figure 2) show a smooth
increase, and the base consumption is finished at a pH of
around 4-5. At such low pH values, the hydrolysis of
lanthanides cannot occur, thus the fifth equivalent of base is
used to neutralize the protons released by the ionization of
the alcoholic OH group. The neutralization of the alcoholic
OH occurs in lower and lower pH ranges as the atomic
number of the lanthanides increases. For La(L-OH), it occurs
in the pH range of about 4-7, while for the Lu(L-OH), the
neutralization is practically finished at around pH 4. Ac-
cording to these observations the interaction between the
Ln3+ ion and the alkoxo oxygen O- is electrostatic in nature
which becomes stronger with the decrease of the size of the
Ln3+ ions. The dissociation of the alcoholic OH in the L-OH
complexes was not taken into account by Powell et al., even
at pH values of about 4-5, and they assumed the formation
log KiH
L-OHa
L-OHb
dptac
H
log K1
log K2
log K3
log K4
9.19 (0.01)
6.87 (0.01)
2.46 (0.01)
1.76 (0.01)
1.5 (0.01)
9.49
7.04
2.62
1.47
10.46
8.02
2.47
1.88
H
H
H
H
log K5
a t )25 °C, I ) 1.0 M KCl. b 25 °C, 0.1 M KNO3, ref 13. c I ) 0.1 M
KNO3, ref 15.
13
of the Ln(L-OH)- species only.
To obtain information on the composition of the complexes
formed in the Ln3+-L-OH systems, spectrophotometric
studies were performed. The narrow absorption band of Nd3+
(aq) observed at 427.1 nm (2P1/2-4I9/2 transition) is very
sensitive for the changes in its surroundings.31 At a 1:1
concentration ratio of Nd3+ and L-OH at pH values above
2, a new band appears at 428.4 nm. The intensity of this
band increases with increasing pH, and at about pH 3, another
band starts to appear at 430.2 nm. The position of the first
band is close to that of the Nd(mimda)+ band (428.3 nm),
which indicates that at about pH 2-3 a protonated complex
is formed when an iminodiacetate group and probably the
oxygen of the alcoholic OH is coordinated to the Nd3+ while
the other iminodiacetate group is free. The band at 430.2
nm is presumably characteristic of the formation of the
species Nd(L-OH)-, in which all the donor atoms of the
ligand are coordinated, since, in the case of Nd(edta)-, the
band appears at 429.5 nm and the coordination of the
alcoholic OH may result in a red shift. These results indicate
the formation of the Nd(L-OH)- complex in the pH range
of about 3-5. Above pH 4.5, a new band appears at about
430.8 nm. The shape and position of this band does not
change between pH 6 and 13, indicating that the Nd(L-
O)2- complex, formed by the ionization of the alcoholic OH
group, exists in this broad pH range.
In the presence of excess Nd3+ (0.01 M Nd3+ and 0.005
M L-OH), the intensity of the band characteristic for the Nd3+
(aq) decreases with the increase of pH and practically
disappears at about pH > 6.3, indicating the formation of
dinuclear complexes with a broad absorption band at about
429 nm. From the solution at a pH of ∼6.3, nearly the total
amount of the complex was precipitated in several hours.
The composition of the precipitate was studied pH poten-
tiometrically and found to be Nd2(L-O)(OH). The precipitate
could be dissolved by adding KOH at about a pH of 9-10,
and the solution obtained was stable even at pH around 12.
Figure 2. Titration curves of the H4L-OH ligand (2.0 × 10-3 M) in the
absence (1) and presence of equivalent amounts of La3+ (2), Nd3+ (3), Gd3+
(4), Er3+ (5), and Lu3+ (6).
sented in Table 3. For comparison, the protonation constants
reported by Powell at al.13 and the log KiH values of the
analogous ligand dpta15 are also shown in Table 3. The log
KH values obtained at lower ionic strength (0.1 M KNO3)
i
are about 0.2-0.3 log K unit higher than those determined
by us in 1.0 M KCl. These discrepancies are caused by the
difference in the ionic strengths and the more prominent
complexation with K+ at 1.0 M concentration. The first two
protonation constants of L-OH are significantly lower than
those of the structurally similar dpta (Chart 1). The lower
basicity of the two nitrogen atoms of L-OH is probably the
result of the electron-withdrawing effect of the oxygen atom
of the alcoholic OH.
The dissociation of the alcoholic OH group of the free
ligand could not be detected by 1H NMR spectroscopy. The
effect of increasing OH- ion concentration on the 1H NMR
spectra of the ligand have been studied in the range of 0.01-
2.5 M (the sum of the KOH and KCl concentration was 3.0
M). The signals, involving that of the methyne proton (CH-
OH) and methylene protons of the acetate arms (N-CH2-
COO-), were both shifted very slightly upfield with increas-
ing OH- concentration, which indicated that the dissociation
of the alcoholic hydroxyl group of the free ligand does not
take place even in 2.5 M KOH.
The pH-potentiometric titration curves of the L-OH ligand
obtained in the presence and absence of the Ln3+ ions are
shown in Figure 2. The titration data indicate that 5 equiv
of KOH were needed for the neutralization of all the
dissociable protons of L-OH at a 1:1 metal-to-ligand ratio.
While the addition of the fifth equivalent of base results in
a weaker jump in the titration curves of the lighter lan-
(31) (a) Yatsimirskii, K. B.; Davidenko, N. K.; Kostromina, N. A.;
Ternovaya, T. V. Teoret. Eksper. Khim. (in Russian) 1965, 1, 100.
(b) Kostromina, N. A.; Slovatsevskaya, E. M. Zh. Inorg. Chem. (in
Russian) 1968, 13, 1105.
Inorganic Chemistry, Vol. 45, No. 13, 2006 4955

Tircso´ et al.
Table 4. Stability Constants (log K) of Complexes Formed in the
The solution had a broad absorption band with a maximum
at 429.8 nm.
Ln3+-L-OH Systema
4-
Ln Ln(L-OH)- Ln(HL-OH) Ln(L-O)- Ln2(L-O)2
Ln2(L-O)+
In the presence of a large (20-fold) ligand excess at
different pH values, the Nd3+ spectra were identical to those
obtained at a 1:1 metal-to-ligand ratio, indicating that
complexes with 1:2 metal-to-ligand ratios were not formed.
The formation of ternary complexes between Nd(L-O)- and
excess imda, glycinate, or salicylate ligands could not be
detected by spectrophotometry. The absence of ternary
complex formation indicates that the number of free coor-
dination sites on the Nd3+ is lower than two: it is either one
or zero. This is in good agreement with the number of
coordinated water molecules (q), which was found to be 0.50
La 10.19(0.01) 4.39(0.02) 5.71(0.02) 9.01(0.03)
Ce 10.93(0.03) 3.77(0.05) 5.18(0.03) 8.03(0.06)
4.22(0.03)
4.53(0.10)
4.68(0.07)
4.02(0.05)
4.28(0.11)
4.20(0.05)
3.82(0.06)
3.61(0.06)
3.49(0.06)
2.80(0.05)
2.30(0.06)
2.48(0.05)
2.48(0.02)
1.69(0.03)
1.66(0.07)
Pr
11.87(0.03) 3.28(0.05) 4.99(0.02) 7.61(0.05)
Nd 12.08(0.02) 3.12(0.04) 4.73(0.02) 7.13(0.05)
Sm 12.56(0.04) 2.94(0.06) 4.26(0.03) 6.12(0.07)
Eu 12.86(0.02) 2.75(0.04) 4.17(0.02) 5.99(0.05)
Gd 12.82(0.02) 2.56(0.06) 4.09(0.02) 5.89(0.05)
Tb 13.24(0.02)
Dy 13.56(0.02)
Ho 13.57(0.02)
Er 13.71(0.02)
Tm 13.84(0.04)
Yb 14.38(0.03)
Lu 14.08(0.01)
3.78(0.02) 5.21(0.05)
3.54(0.02) 4.70(0.05)
3.24(0.02) 4.01(0.04)
2.87(0.02) 3.31(0.04)
2.38(0.03) 2.52(0.04)
2.07(0.01) 2.08(0.02)
2.09(0.01) 2.12(0.01)
3.11(0.01) 3.91(0.03)
4-
for the Eu2(L-O)2 complex, determined by fluorescence
Y
12.79(0.02)
lifetime measurements using the method proposed by Beeby
a t ) 25 °C, I ) 1.0 M KCl.
et al.,32 and also with the unusually low relaxivity value
4-
obtained for the Gd2(L-O)2 complex (vide infra).
diprotonated species Ln(H2L-OH)+ leads to better fitting
parameters and lower standard deviation values. For the
complexation of Nd3+, the spectrophotometric data and the
calculated species distributions agreed only if the formation
of the diprotonated species was taken into consideration by
the calculations.
The experimental data obtained by ESI-TOF MS and NMR
spectroscopy indicated that parallel with the dissociation of
the alcoholic OH group, dimer complexes of Ln2(L-O)2
were formed, and the dimerization was practically complete
even in dilute (0.25 mM) solutions. The dissociation of the
alcoholic OH group and the formation of dimer complexes
can be characterized with the equilibrium as follows:
On the basis of the results obtained by pH potentiometry
and spectrophotometry, we assumed the formation of pro-
tonated Ln(HL-OH) and nonprotonated Ln(L-OH)- com-
plexes. The dissociation of the alcoholic OH group may lead
to the formation of the Ln(L-O)2- species. In the presence
of excess Ln3+, dinuclear Ln2(L-O)+ complexes can be
formed. Since the solubility of the dinuclear complexes in
the pH range of about 6.5-9 is low and the formation of
some hydroxo species is slow, we only calculated the stability
constant of the Ln2(L-O)+ complex, which is formed at pH
< 6. The stability constants characterizing the assumed
equilibria are as follows:
4-
[Ln(L-OH)-]
[Ln3+][L-OH4-]
2Ln(L-OH)- h Ln2(L-O)24- + 2H+
(5)
KLn(L-OH)
)
(1)
(2)
-
This equilibrium is characterized by the equilibrium constant
[Ln(HL-OH)]
[Ln(L-OH)-][H+]
KLn(HL-OH)
)
K
4-
Ln2(L-O)2
[Ln2(L-O)24-][H+]2
Because the dissociation constant characterizing the ion-
ization of the alcoholic OH group of the ligand cannot be
determined, the stability constant of the species Ln(L-O)2-
cannot be calculated. The formation of this species is
characterized with the reciprocal of the dissociation constant
of the species Ln(L-OH)-
KLn (L-O)
)
(6)
4-
2
[Ln(L-OH)-]2
2
The equilibrium constant KLn (L-O)
is a composite
4-
2
constant that expresses the propensity2 of the Ln(L-OH)-
complexes for both the dissociation of the alcoholic OH and
the structural rearrangement to form a dimer species. The
dimerization may also promote the dissociation of the OH
[Ln(L-OH)-]
[Ln(L-O)2-][H+]
KLn(L-O)2-
)
(3)
group. Thus the equilibrium constant KLn (L-O) (defined
4-
2
2
2-
by eq 6) has a physical meaning, while KLn(L-O) is a formal
2-
constant. However, the KLn(L-O) values can be used for
The stability constant of the dinuclear complex is defined
as
practical aims such as to express, for example, more directly
the extent of dissociation of the alcoholic OH at different
pH values, and so with the other equilibrium constants, we
[Ln2(L-O)+]
[Ln(L-O)2-][Ln3+]
KLn (L-O)
)
(4)
+
2-
2
also present the log KLn(L-O) values in Table 4. It is quite
2-
surprising that the log KLn(L-O) values are unusually low
and the Ln(L-OH)- complexes behave like medium-weak
The formation of the protonated complexes is important
only for the lighter lanthanides (to Tb3+). However, for the
description of the equilibria for the La3+, Ce3+, Pr3+, and
Nd3+ complexes, the assumption for the formation of
2-
acids. The significant decrease in the log KLn(L-O) values
from La to Lu indicates that the interaction between the
alkoxo oxygen and the Ln3+ ions increases with the decrease
of the ionic size of lanthanides. The low log KLn(L-O) values
are probably the result of the cooperative effect of all the
functional groups of the ligand. The two coordinated
2-
(32) Beeby, A.; Clarkson, I. M.; Dickins, R. S.; Faulkner, S.; Parker, D.;
Royle, L.; de Sousa, A. S.; Williams, J. A. G.; Woods, M. J. Chem.
Soc., Perkin Trans. 2 1999, 493.
4956 Inorganic Chemistry, Vol. 45, No. 13, 2006

Lanthanide(III) Complexes
Gd3+ exchange rapidly with the bulk water. The outer-sphere
relaxivity is the result of the diffusion of the water molecules
in the proximity of the Gd3+-containing species. The
relaxation effect of the Gd3+ complexes strongly depends
on the number of water molecules coordinated in the inner
is
sphere of Gd3+ (r1 ). The numbers of inner-sphere water
molecules in the complexes Gd(dpta)-, Gd(dtpa)2- and
Gd(ttha)3- (H6ttha ) triethylene-tetramine hexaacetic acid)
are two, one, and zero, respectively, and the relaxivity values
(9 MHz, 25 °C) are 9.1, 5.8, and 2.9 mM-1 s-1, respec-
tively.34,35
The relaxivity values obtained for the Gd3+-L-OH and
Gd3+-dpta systems at different pH values at 9 MHz (25
°C) are shown in Figure 5. The formation of Gd(dpta)-
occurs in the pH range of about 3-5, where the relaxivity
values significantly decrease. In the Gd3+-L-OH system,
the drop in the relaxivity values starts at about pH 2,
indicating the formation of the Gd(HL-OH) and Gd(L-OH)-
species. However, at somewhat higher pH values, the
dissociation of the alcoholic OH group starts and finishes at
about pH 6 when the ligand L-O5- coordinates to the Gd3+
by its seven donor atoms. By the formation of a Gd3+complex
with a heptadentate ligand, the relaxivity value should be
higher than that of the Gd(dtpa)2- in which the ligand is
octadentate and there is only one water molecule in the inner
sphere. Surprisingly, at pH > 6 when the predominance of
the Gd(L-O)2- species is expected, the relaxivity value is
around 3.4 mM-1s-1, which is significantly lower than that
of Gd(dtpa)2-. This unexpectedly low relaxivity value cannot
be interpreted by assuming the slow exchange of the
coordinated water(s), as was observed for the Gd3+complexes
formed with some dtpa- and dota-amide derivatives (H4dota
) 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid),
because the increase of temperature results in a smooth
decrease in the relaxivity values of Gd(L-O)2-, similar to
that of Gd(dtpa)2- and Gd(dota)- (Figure S1).36,37 The
interpretation of the low relaxivity is possible by assuming
the formation of dimer complexes, Gd2(L-O)24-, in solution,
in which there are no water molecule in the inner sphere of
Figure 3. Species distribution in the Lu3+-L-OH-edta system (the
concentrations of all reacting species were 5 mM). Numbers refers to the
4-
species Lu3+ (1), Lu(edta)- (2), and Lu2(L-O)2 (3) (the concentrations
of other species in the equilibrium model were too low to appear).
iminodiacetate groups in Ln(L-OH)- pull the alcoholic OH
close to the Ln3+ ion, and the formation of two chelate rings
with the N-CH2-CH(OH)-CH2-N amine chain leads to
a strong electrostatic interaction between the OH oxygen and
the Ln3+ ion, resulting in the weakening of the O-H bond
and the dissociation of the H+.
4-
The increase in the relative stability of the Ln2(L-O)2
complexes resulting from the coordination of the ionized
alcoholic OH group can be demonstrated by calculating the
species distributions in the presence of the Ln3+ ion, L-OH,
and edta at a 1:1:1 concentration ratio. As can be seen in
Figure 3, for the complexation of Lu3+ the species, Lu(edta)-
predominates between pH 2 and 6 since its stability constant,
-
log KLu(edta) ) 19.8 is significantly higher than that of
Lu(L-OH)- (Table 4).33 However at pH > 5, the (L-O)5-
ligand starts to replace the edta, and at pH > 8, the Lu2(L-
O)24- species dominates. In the case of lanthanides of lower
atomic number, the displacement of edta occurs at a higher
pH range.
The stability constants of the Ln(L-OH)- complexes
4-
Gd3+, similar to Yb2(L-O)2 found in solid state by
determined by Powell et al.13 are somewhat higher for the
Miyashita et al.20 The relaxivity values increase linearly with
the increase of the concentration of the Gd3+ complex in
the range of 0.5-10 mM at pH 7.8, which indicates that the
-
whole series of lanthanides than the log KLn(L-OH) values
obtained by us (Figure 4). The differences are partly from
the higher protonation constants (Table 3), but the increase
in the differences upon going from La3+ to Lu3+ is the
consequence of the dissociation of the alcoholic OH group
which was not taken into account by Powell et al.13
4-
composition of the Gd2(L-O)2 complex does not change
in this concentration range (Figure S2).
ESI-TOF MS Studies. Electrospray mass spectrometry
is increasingly used to study the stoichiometry of metal
complexes formed in dilute solutions by transferring and
identifying them in the gas phase. The passage into the gas
phase is a gentle process where the metal-ligand composi-
tion of the solution species does not change. Anionic
Relaxometric Studies. The relaxation rate of water
protons, 1/T1 (T1 is the longitudinal relaxation time),
significantly increases in the presence of paramagnetic Gd3+
complexes. The increase in the 1/T1 values produced by the
increase in the concentration of the Gd3+ complex by 1.0
mM is called the relaxivity (r1, mM-1 s-1) of the solution.
(34) Southwood-Jones, R. V.; Earl, W. L.; Newman, K. E.; Merbach, A.
E. J. Chem. Phys. 1980, 73, 5909.
is
The relaxivity is composed of the inner-sphere (r1 ) and
outer-sphere (r1os) terms. The inner-sphere relaxivity occurs
(35) Caravan, P.; Ellison, J. J.; McMurry, T. J.; Lauffer, R. B. Chem. ReV.
1999, 99, 2293.
as the water molecules coordinated in the inner sphere of
(36) Aime, S.; Botta, M.; Fasano, M.; Paoletti, S.; Analli, P. L.; Mggeri,
F.; Virtuani, M. Inorg. Chem. 1994, 33, 4707.
(37) Dunand, F. A.; Aime, S.; Merbach, A. E. J. Am. Chem. Soc. 2000,
122, 1506.
(33) Martell, A. E.; Smith, R. M. Critical Stability Constants, Vol. 1;
Plenum: New York, 1974.
Inorganic Chemistry, Vol. 45, No. 13, 2006 4957

Tircso´ et al.
Figure 4. Comparison of the stability constants of complexes formed with L-OH [(1) this work, (3) ref 13] and dpta [(2) ref 15].
Figure 6. ESI-TOF MS spectrum of the solution containing equimolar
amounts of La3+ and L-OH (0.25 mM and pH ) 8.47). The insert spectra
shows (a) the isotope distribution calculated for the dimer species K3La2-
(L-O2)- and (b) the isotope distribution calculated for the dimer species
K2La2(L-O)22-. The isotope distributions for the other parts of the spectra
are also satisfactory.
Figure 5. Relaxivities of water protons in the Gd3+-L-OH (1) and Gd3+
-
dpta (2) systems at 9 MHz and 25 °C (the Gd3+ and ligand concentrations
were 1 mM).
complexes may interact in the gas phase with positive ions,
like alkali metals or protons. The interaction with protons is
more probable if the solution is acidic or neutral. The
complexes formed with polycarboxylate ligands can undergo
decarboxylation, when one or more CO2 molecules are lost.38
The dimerization of the Ln(III)-aminopolycarboxylate
complexes in solution, which had to be assumed for the
interpretation of the results of relaxivity measurements, is
quite unusual. To obtain information with an independent
method, detailed ESI-TOF MS studies were made on the
La3+, Nd3+, and Lu3+ complexes to find out if the dimer
structure is retained in solution. In the samples studied, the
concentration of the Ln3+ ion and the L-OH ligand were 0.25
mM, and the pH was around 8, where the dissociation of
the alcoholic OH group was practically complete for all
complexes studied and the presence of single dimer species
was expected in solution on the basis of equilibrium studies.
Because of the negative charge of the complexes, the ESI
MS spectra were recorded in negative ion mode.
The spectrum obtained for the La complex (Figure 6)
shows six peak groups in the region of 400-1100 m/z with
a characteristic isotopic distribution for the species containing
4-
the La2(L-O)2 unit. The peaks were assigned as follows:
peaks at around 1028.7-1031.7 m/z with frequency of 1 m/z
-
belong to K3La2(L-O)2 (Fw ) 1029.59). The second peak
group showing the highest intensity in the region 494.9-
496.4 m/z with a frequency of 0.5 m/z units belongs to the
2-
double-charged species K2La2(L-O)2 (Fw ) 990.49). The
next two peak groups at lower m/z values are associated with
protonated species. The signals at around 475.9-477.5 m/z
can be assigned as the double-charged (H)KLa2(L-O)22- (Fw
) 952.40) species and the peaks in the range of 456.5-
2-
457.99 m/z presumably belong to the (H)2La2(L-O)2 (Fw
) 914.30) species. The peaks in the 450.9-452.4 m/z region
are the result of the fragmentation of the main complex in
the gas phase (K2La2(L-O)22--CO2), and species formed by
the loss of a second molecule of CO2 were also detected in
the 406.9-408.4 m/z range. Species with a higher charge
(3- or 4-) were not observed for either La3+ or the other
lanthanides.
(38) Deery, M. J.; Fernandez, T.; Howarth, O. W.; Jennings K. R. J. Chem.
Soc., Dalton Trans. 1998, 2177.
4958 Inorganic Chemistry, Vol. 45, No. 13, 2006

Lanthanide(III) Complexes
The ESI-TOF MS spectrum obtained for the Nd3+ complex
shows four peak groups in the 400-500 m/z region with a
peak frequency of 0.5 m/z (Figure S3). The signals in this
region are characteristic for the double-charged dimer
structures K2Nd2(L-O)22-, (H)KNd2(L-O)22-, and (H)2Nd2-
(L-O)22- with m/z values of 497.9-502.9 (Fw ) 1001.15),
478.8-483.9 (Fw ) 963.06), and 459.9-464.9 (Fw
)
924.94), respectively. Similar to the La3+ complex, decar-
2-
boxylation of the species K2Nd2(L-O)2 was also detected
in the 450.9-455.9 m/z region. The signal of the mono-
-
charged ion K3Nd2(L-O)2 was not found in the spectrum
in this case. In the spectra of the Lu3+ complex, three signal
groups were found which were assigned as the following
species: K2Lu2(L-O)22- (m/z 530.96-531.95, Fw ) 1062.61),
(H)KLu2(L-O)22- (m/z 511.97-512.99, Fw ) 1024.52), and
2-
Figure 7. High-resolution 1H NMR spectrum of the La2(L-O)24- complex
at pD 7.73, and the coupling scheme determined from the cross-peaks of
the 2D COSY spectrum (Figure S8, Ccomplex ) 0.1 M, 298 K).
(H)2Lu2(L-O)2 (m/z 492.85-494.01, Fw ) 986.42).
The results of the ESI-TOF MS studies unambiguously
show that between the L-OH ligand and Ln3+ions at a 1:1
metal-to-ligand ratio and a pH of around 8, dimer complexes,
Ln2(L-O)24-, are preferentially formed in solution even if
the concentration is about 10 times lower than in the samples
used for the pH potentiometric titration experiments.
findings, we concluded that the composition of the complex
formed does not change in this concentration range (there is
no association or dissociation), and the dinuclear species,
La2(L-O)24-, exists throughout the entire concentration range
studied.
NMR Spectroscopic Studies. The relaxation rates of
water protons were found to be directly proportional to the
concentration of Gd2(L-O)24- in a broad concentration range
which is possible only if the composition of the Gd3+
complex does not change by dilution. On the other hand,
the results of the ESI-TOF MS studies indicated the presence
of dimer complexes even in very dilute (0.25 mM) solutions.
The equilibrium data could also be described by assuming
the predominance of the dimer species in solution at about
pH > 7 for the La3+ and at pH > 4 for the Lu3+ complexes.
These data strongly suggest that the complexes have similar
dinuclear dimer structures both in solution and the solid state.
So, for the interpretation of the results obtained by 1D and
2D 1H and 13C NMR spectroscopy, we assumed the existence
of dimer species (e.g., La2(L-O)24-). In the interpretation of
If the structure of the La2(L-O)24- complex in solution is
similar to the solid-state structure of Nd2(L-O)24-, because
of the presence of the inversion center in this structure, 12
carbon signals (8 aliphatic and 4 carbonyl) and 14 proton
resonances can be expected as found for Eu2(L-O)24- at low
1
temperatures (Figure S5). At room temperature in the H
NMR spectrum, only the water signal was found; the other
signals were too broad to detect them. However, with the
decrease of temperature to 273 K (frozen structures), 14
1
resonances were detectable in the H NMR spectrum, but
the signals were still broad (Figure S5). With the increase
of the temperature, the peaks broadened, and close to the
higher temperature limit (353 K), 8 peaks were observed.
1
4-
the results of earlier H (1D) NMR spectroscopic studies,
The 360 MHz proton NMR spectrum of La2(L-O)2
the dissociation of the alcoholic OH group of L-OH and the
formation of dimer complexes was not taken into consider-
ation, so the assignment of the spectra was not correct.14
consists of 10 signals in the range of 2.3-4.5 ppm (Figure
7). The J-modulated 13C{1H} NMR spectrum shows 8
aliphatic (Figures S6 and S7) and 4 carbonyl resonances.
Two carbon resonances of the J-modulated 13C{1H} spectrum
at 73.61 and 73.48 ppm can be assigned to the C2 and C2_2
atoms, respectively. On the basis of the 1H-13C{1H}
HETCOR spectrum (Figure S7), the proton signals, H2_2
(4.36 ppm) and H2 (4.19 ppm), which integrate to one proton
each, can easily be assigned to the two different methyne
protons. The labeling for carbon atoms is the same as the
crystallographic labels for the K4[Nd2(L-O2)] complex, while
the labeling for protons is H for methyne and Ha and Hb for
the methylene protons attached to the questionable carbon
atom (Scheme 1). Six sets of methylene signals were
observed for which the protons are magnetically nonequiva-
lent as indicated by single (H) or pairs (Ha and Hb) of proton
signals associated with single carbon resonances (Scheme
1
The H NMR spectrum of the deprotonated (L-OH)4-
ligand at pD ) 11.85 consists of three multiplets. A triplet
of triplets (ABX pattern) of the methyne proton is at 3.81
ppm. Signals at 3.26 ppm (A) and 3.18 ppm (B) are from
the AB pattern of eight chemically equivalent acetate
methylene protons (acAB). The signals (ABX pattern) of
the four chain methylene protons are at 2.64 ppm (A) and at
2.45 ppm (B) (Figure S4a and Table S2). In the 13C NMR
spectrum of the ligand, there are three signals. The acetate
methylene and chain carbon atoms show resonances at 61.64
ppm, the methyne carbon signal appears at 69.01 ppm, and
the signal of the carbonyls at 182.36 ppm (Figure S4b).
1
The H and 13C NMR spectra of the (L-OH)4- ligand in
the presence of equivalent amounts of La3+ (pD ) 7.74) are
much more complicated (Figure 7 and Table S2), but the
line width of the proton signals do not change in a broad
concentration range of 0.001-0.2 M. On the basis of these
1
1). On the basis of the H-1H COSY experiment (Figure
S8), it was easy to assign the protons of the chain methylene
groups because of their coupling with the methyne (H2 and
Inorganic Chemistry, Vol. 45, No. 13, 2006 4959

Tircso´ et al.
Scheme 1. Numbering Scheme Used to Label the Proton and Carbon
4-
Atoms in the Complex Ion La2(L-O)2
1
4-
Figure 8. 2D H-1H NOESY spectrum of the La2(L-O)2 complex at
pD 7.73, acquired with a mixing time of 400 ms at 298 K (Ccomplex ) 0.1
M, the most important nOe peaks needed for the assignment are boxed).
(NOESY). Figure 8 shows the 2D NOESY spectra of the
complex acquired with the mixing time of 400 ms. When
this quite long mixing time was used for the evaluation of
the nOe effect, some residual chemical exchange peaks
(detectable on the acetate proton signals) are still apparent,
but the major part of the data consists of through-space nOe
peaks (major and most important peaks for the correct
assignment are boxed). The appearance of negative cross-
peaks (box 1) between the methyne proton H2_2 and one of
the acetate protons shows a strong interaction between the
spins of these protons. This is possible if these protons are
on the same ligand and close to each other in space. This
acetate proton is probably the Ha8 proton (Scheme 1) with
Scheme 2. Torsion Angle Between Chain Protons (a) and Dihedral
Angle for the R-CH2-COO- Fragment (b)
H2_2) protons. In this way, the “pseudotriplet” centered at
2.44 ppm was assigned to the Ha3 proton (2JHH{Hb3} ) 11.6
1
a chemical shift of 3.47 ppm. However, from the H-13C-
3
and JHH{H2_2} ) 10.0 Hz) on the C3 carbon atom with
{1H} HETCOR spectrum (Figure S7), both the proton
coupled to Ha8 and the carbon atom to which they are
attached to can be easily determined. The signal at 3.27 ppm
can be assigned to the Hb8 proton with geminal coupling
2JHH{Ha8} ) 17.4 Hz which is an exceptionally large value,
while the signal at 63.15 ppm in the 13C NMR spectrum can
be attributed to the C8 atom. However, there is a rule
supported with experimental data and quantum chemical
calculations for the R-CH2-COO- fragments predicting the
chemical shift 67.38 ppm. The torsion angle between the
Ha3 and H2_2 protons on the basis of Karplus relationship
is close to axial (ca. 170° with a large coupling constant)
(Scheme 2a). The doublet centered at 2.65 ppm belongs to
the signal of the Hb3 proton (2JHH{Ha3} ) 11.6 Hz and 3JHH
-
{H2_2} ≈ 1.7 Hz) which is in the equatorial position to the
H2_2 proton (ca. 50° with small coupling constant). These
findings are in a good agreement with the X-ray crystal-
lographic data of the [Nd2(L-O)2]4- complex anion, where
for these angles, values of 178.9, -163.6, 59.7, and -45.2°,
respectively, were found. Similarly, the signals of protons
on the C1 carbon atom (66.60 ppm) were observed as a
“pseudotriplet” centered at 2.53 ppm, assigned to the proton
Ha1 (2JHH{Hb1} ) 11.6 Hz and 3JHH{H2} ) 10.0 Hz, axial),
and a doublet signal centered at 2.60 ppm for the proton
2
variation of the JHH with the dihedral angle of the R-C
bond and COO- plane. According to the literature data, if
the coupling constant is around 12 Hz, the angle is about
90°, while if it is in the range of -18 to -19 Hz, the angle
is close to 0 or 180°.39-41 The large coupling constant is the
result of the coordination which stabilizes the conformation
with a dihedral angle of around 180°. The coordination mode
is demonstrated in Scheme 2b. Indeed, it is easy to see from
3
Hb1 (2JHH{Ha1} ) 11.6 Hz and JHH{H2} ≈ 1.7 Hz,
equatorial). These findings are consistent with the results of
selective homonuclear decoupling experiments, where the
signals of H2_2 at 4.36 ppm and H2 at 4.19 ppm were
irradiated and then the spectrum was recorded. The com-
parison of these spectra shows the coupled pair of the
irradiated protons (Figures S8 and S9). The signals of Ha3
and Ha1 appear as triplets because of the large geminal and
vicinal couplings with protons H2 and H2_2, respectively.
The assignment of the acetate protons was achieved with
the use of through-space nOe correlation spectroscopy
4-
the X-ray structure of the complex Nd2(L-O)2 (Figure 1)
that, for instance, the atoms N2-C8-C9-O6 are almost in
the same plane.
Identically to the previously described results, the proton
signal of Ha4 can be recognized from the nOe spectra because
(39) Moore, G. J.; Sillerud, L. O. J. Magn. Reson. 1994, 103, 87.
(40) Barfield, M.; Grant, D. M. J. Am. Chem. Soc. 1963, 85, 1899.
(41) Barfield, M.; Hruby, V. J.; Meraldi, J. P. J. Am. Chem. Soc. 1976, 98,
1308.
4960 Inorganic Chemistry, Vol. 45, No. 13, 2006

Lanthanide(III) Complexes
of the presence of an nOe cross-peak (box 2, Figure 8). The
signal at 3.40 ppm can be attributed to the Ha4 proton, and
therefore, the coupled proton at 3.34 ppm must be Hb4 on
the same C4 carbon atom with a chemical shift of 64.47
ppm. Moreover, these protons must be on the other ligand.
The geminal coupling between the protons is exactly the
2
same, and for Ha4, JHH{Hb4} ) 17.4 Hz. The remaining
1
signals in both the H and 13C NMR spectra were assigned
on the basis of the analysis of nOe peaks between the chain
and acetate proton signals (box 3, Figure 8). As a result, the
singlet signal at 3.39 ppm and the carbon signal at 65.69
ppm are assigned to Hab10 and C10, respectively. The Hab-
10 protons give nOe cross-peaks with Ha and Hb protons on
the C3 carbon atom. This is only possible if the aforemen-
tioned protons are present on the same ligand. Similarly, the
signals at 3.43 and 66.84 ppm in the 13C NMR spectrum
can be assigned to Hab6 and C6, respectively.
Figure 9. Methylene part of the 2D 13C-13C EXSY spectrum of the La2-
4-
(L-O)2 complex at pD 7.73, acquired with a mixing time of 200 ms at
298 Κ (Ccomplex ) 0.1 M, the exchange peaks are boxed).
Usually, important structural information can be extracted
from three-bond heteronuclear couplings (3JCH).42 In our case,
because of the exchange processes, the signals were quite
broad (10-12 Hz), and it was practically impossible to obtain
useful information from these coupling constants (Figure S6).
The coupling constants can be determined only for some
carbonyl signals: 4.6 Hz for separate C7 and C11 carbonyl
resonances. The assignment was made on the basis of 2D
1H-13C COLOC data set. Figure S10 shows an expansion
between the chain methylene (C1 and C3) carbon atoms are
in good agreement with our speculations. The carbonyl region
of the 13C-13C EXSY spectra measured with a mixing time
of 600 ms is presented in the Figure S11. The exchange peaks
marked with boxes 1 and 2 result from the exchange process
between carbonyls C5-C7 and C9-C11, but cross-peak 3
can be the result of the ligand interconversion process
(exchange C7-C11) which occurs with a slower rate.
1H NMR spectroscopy was used to monitor the temper-
ature-dependent behavior of the La2(L-O)24- complex anion
in D2O over the range of 273-353 K (Figure S12). With
the increase of temperature, the signals broadened indicating
the increase of the exchange rates at higher temperatures.
The signal of H2_2 at 333 K in the 1H NMR spectrum could
1
of the carbonyl-to-acetate proton region of the 2D H-13C
COLOC spectrum. According to the spectrum, the singlet
peak of Hab6 correlates with carbonyl resonance at 182.64
ppm and can be assigned to the C7 atom. The proton signal
of Hab10 correlates with carbonyl resonance at 183.18 ppm
and can be assigned to carbon atom C11. The remaining
carbonyl resonances at 183.65 and 183.69 ppm correlate with
the AB pattern of the acetate methylene protons on the C4
and C8 carbon atoms and can be dedicated to the C9 and
C5 carbon atoms, respectively.
The peaks of Hab10 and Hab6 protons are singlets, which
may be attributed to the chemical exchange process in
solution. During the chemical exchange, the Ha8 and Hb8
protons take on the identity of Hab10 and vice versa, and
similar exchanges for Ha4 and Hb4 protons with Hab6 protons
on the second ligand can also occur. This is an exchange
between protons of two acetate groups attached to the same
nitrogen atom. However, in the case of ligand interconver-
sion, a similar exchange process might be present between
chain protons (Ha1 to Ha3 and Hb1 to Hb3) and carbon atoms
(C1 to C3). This exchange process requires the rupture of
the La-alkoxy O bonds, which must be a slow process, since
these bonds exhibit a long lifetime on the NMR time scale.
This is in good agreement with the 13C-13C EXSY spectra
measured with various mixing times. The methylene region
of the 13C-13C EXSY spectrum measured with a mixing time
of 200 ms is shown in Figure 9. Cross-peaks 1 and 2 in
Figure 9 can be assigned to the exchange of acetate (C4-
C6 and C8-C10) methylene carbon atoms. The presence of
exchange peaks on the acetate and the absence of exchange
1
not be detected, but by 2D H-1H COSY, it was found to
overlap the water signal (cross-peak with water in Figure
S13). At 353 K, the signal of H2 behaves similarly.
Surprisingly, even at the highest temperature, there was no
coalescence of the signals, and the AB multiplet pattern was
observed both for the chain and acetate methylene reso-
nances.
Conclusions
The presence of the alcoholic OH group in the aminopo-
lycarboxylate ligand H4L-OH significantly changes its com-
plexation properties in comparison with those of the analo-
gous H4dpta or H4edta.The stability constants of the
lanthanide(III) complexes formed with the heptadentate (L-
OH)4- are somewhat lower than those of the hexadentate
(dpta),4- probably because of the lower basicities of the
nitrogen atoms of (L-OH)4-. However, the coordination of
the two iminodiacetate groups of the ligand leads to strong
interactions between the alcoholic oxygen and the Ln3+ ion,
and the alcoholic OH group dissociates at unexpectedly low
pH values. The pH range of the dissociation shifts to more
acidic regions with the decrease of the size of the Ln3+ ion,
which indicates the electrostatic nature of the metal-ligand
interactions.
The coordinated heptadentate (L-O)5- ligand, containing
an alkoxo oxygen, shows a strong propensity to form
(42) Thomas, W. A. Prog. Nucl. Magn. Reson. Spectrosc. 1997, 30, 183.
Inorganic Chemistry, Vol. 45, No. 13, 2006 4961

Tircso´ et al.
dinuclear dimer complex anions, Ln2(L-O)24-, both in
solution and solid state. In the dinuclear dimers the ligands
are coordinated to both Ln3+ ions with an iminodiacetate
group, and the alkoxo oxygens are in bridging positions. The
coordination number of the Ln3+ ions is 9 in the complexes
of the lighter elements, with one water molecule per Ln(III)
ion in the inner sphere (e.g., the solid K4[Nd2(L-O)2(H2O)2]‚
14H2O). The structure of the dinuclear dimers of the heavier
elements is highly stable with 8 coordinated Ln3+ ions (Ho
and Lu complexes).20 The probable absence of the water
molecule in the inner sphere of the Gd3+ complex is indicated
by the unusually low relaxivity value of Gd2(L-O)24-, which
also shows the formation of dimer species with the hepta-
dentate ligand (L-O)5-.
The geometries of the dimer complexes of the lighter and
heavier lanthanides are different. In the complexes of the
heavier elements in the solid state, there is a symmetry
plane,20 while in the Nd3+ dimer, the only symmetry element
is a center of symmetry.
The solution structure of the La2(L-O)24- complex studied
by 1D and 2D NMR spectroscopy is similar to the solid-
state structure of the Nd2(L-O)24- anion, obtained by X-ray
diffraction studies.
To¨ro¨k for their help in the collection of ESI-TOF MS data
and to Judit Vanka and Be´la Ro´zsa for their assistance in
the collection of pH potentiometric titration data.
Supporting Information Available: Tables of additional
crystallographic and refinement parameters in CIF format, geometry
data of hydrogen bonds in K4[Nd2(L-O)2(H2O)2]‚14H2O, 1H NMR
chemical shifts and coupling constants for the L-OH4- ligand and
La2(L-O)24- complex (Tables S1 and S2), temperature dependence
4-
of the relaxivity (r1) of Gd2(L-O)2 complex (Figure S1), proton
4-
relaxation rates versus total concentration of Gd2(L-O)2 (Figure
S2), ESI-TOF MS spectrum of the solution containing equimolar
1
amounts of Nd3+ and L-OH (0.25 mM) (Figure S3), H and 13C
NMR spectra of the free ligand (Figure S4), temperature dependence
4-
of the 1H NMR spectra of Eu2(L-O)2 (Figure S5), 13C{1H}
J-modulated and proton-coupled 13C NMR spectrum of the La2-
(L-O)24- complex (Figure S6), 1H-13C{H} HETCOR and 1H-1H
4-
COSY spectra of the La2(L-O)2 anion (Figures S7 and S8,
4-
respectively), selective decoupling experiment for the La2(L-O)2
complex (Figure S9), carbonyl-to-acetate proton region of the 2D
4-
1H-13C COLOC spectrum of the La2(L-O)2 anion at pD 7.73
optimized for 4.6 Hz long-range couplings (Figure S10), carbonyl
4-
region of the 2D 13C-13C EXSY spectrum of the La2(L-O)2
1
complex (Figure S11), temperature dependence of the H NMR
spectra of the La2(L-O)24- complex (Figure S12), and the 2D 1H-
1H COSY spectrum of the La2(L-O)24- complex at 333 K and 500
MHz (Figure S13). This material is available free of charge via
the Internet at http://pubs.acs.org.
Acknowledgment. Financial support from the Hungarian
Scientific Research Found (OTKA T 043365 and T 038364)
are gratefully acknowledged. A.B. is grateful for the Hungar-
ian Academy of Sciences for Istva´n Sze´chenyi Fellowhip.
The authors are indebted to Dr. Sa´ndor Ke´ki and Dr. Ja´nos
IC0517321
4962 Inorganic Chemistry, Vol. 45, No. 13, 2006