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Neodymium(III) D(–)-citramalate: a chiral three-dimensional
framework with water-filled channels
Pierre Thuéry
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DOI: 10.1039/b000000x [DO NOT ALTER/DELETE THIS TEXT]
Reaction of neodymium nitrate with enantiopure D(–)-citramalic
acid (H3citml) in the presence of NaOH under hydrothermal
conditions
gave
the
complex
{[Nd7(Hcitml)8(NO3)(H2O)8](CO3)2·4H2O}n, which crystallizes as
a three-dimensional framework with a complex hexagonal
network of channels.
Among the numerous metal-organic frameworks involving f
element ions presently described,1 the complexes formed by
lanthanide ions and citric acid have recently been the subject of
several reports which evidence the intricacy of the assemblages
formed, as expected for metal ions with large coordination numbers
and multi-functional ligands.2 However, polyacids closely related
to citric acid have not been investigated, although they can give
very different structures, as previously observed in the case of
uranyl ion complexation by citric and tricarballylic acids.3 D(–)Citramalic acid [D(–)-2-hydroxy-2-methylbutanedioic acid,
H3citml] differs from citric acid by replacement of one of the –
CH2–COOH groups by a –CH3 group (Scheme 1). Very few crystal
structures of citramalate complexes are known, these being
restricted to one molybdenum carbonyl mononuclear species4 and
several dimeric and polymeric uranyl complexes.5 In the latter, its
behaviour is not much different from that of citric acid, one of the
-carboxylic groups of which is uncoordinated, but more radical
changes could be expected with the less constrained coordination
geometry of lanthanide ions. In the lanthanide citrates
[Ln(Hcit)(H2O)]n (Ln = La, Nd, Eu, Tb)2a,d,e and
[Ln(Hcit)(H2O)2·H2O]n (Ln = La, Nd, Gd),2b,c (H4cit = citric acid),
ladder-like monodimensional,2b,c two-2d and three-dimensional2a,e
frameworks were observed, the latter very compact.
OH
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in the chiral, hexagonal space group P6122, with a very large unit
cell c parameter of 85.294(4) Å. The asymmetric unit comprises
four independent neodymium atoms and four Hcitml2– ligands,
labelled A–D (Fig. 1). All metal atoms are in general position,
except for Nd1, which is located on the binary axis (x, –x, 11/12).
These four metal atoms are in quite different environments. Nd1 is
bound to four Hcitml2– ligands, two of them (A, A') monodentate
and the two others (B, B') chelating through the hydroxyl, one and one -carboxylate oxygen atoms, and to a disordered water
molecule (O1), which gives a nine-coordinate environment of very
distorted capped square antiprismatic geometry. Nd2, Nd3 and Nd4
are bound to six, five and four Hcitml2– ligands, respectively, with
an additional water molecule for Nd3 and two and a half water
molecules and half a bidentate nitrate ion (disordered) for Nd4,
resulting in coordination numbers of 10, 9 and an average of 9.5,
respectively, and very distorted coordination geometries.
O4C#
O4A
O5A
O3D# O5D
O5C#
O1A
O4"
Nd2
O2A
O1D
O5B'
O3A
O1B'
O1
O2B'
O1B
Nd4
O2
O1C
O4
O3B
O6
O4B
O3A'
OH
O5A*
O3C
O4B'
O2B
O3D
O4C
O2D
Nd3
Nd1
O5C
O4D
O2C
O3
O5
N1
O7
O5B
HO2CH2C
CH2CO2H
HO2CH2C
CH3
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40
CO2H
CO2H
Citric acid
D(
)-Citramalic acid
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Scheme 1
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Reaction of neodymium nitrate hexahydrate with D(–)-citramalic
acid in water at either 90°C or 180°C, in the presence of NaOH,
resulted in the deposition of colourless crystals of
{[Nd7(Hcitml)8(NO3)(H2O)8](CO3)2·4H2O}n, {1}n,† which were
characterized by X-ray crystallography.‡ Compound 1 crystallizes
CEA/Saclay, DSM/DRECAM/SCM (CNRS URA 331), Bât. 125, 91191
Gif-sur-Yvette, France. E-mail: pierre.thuery@cea.fr
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Fig. 1 View of the four independent neodymium atoms and their
environment in 1. Counterions, solvent water molecules and hydrogen
atoms have been omitted. Displacement ellipsoids are drawn at the 40%
probability level. Symmetry codes: ' = 1 – y, 1 – x, 11/6 – z; " = 1 + x – y,
1 – y, 2 – z; # = x – y, 1 – y, 2 – z; * = 1 + x, y, z. Selected bond lengths
(Å): Nd1–O3A 2.433(6), Nd1–O1B 2.510(8), Nd1–O2B 2.534(7), Nd1–
O4B 2.537(7), Nd1–O1 2.641(10), Nd2–O1A 2.432(7), Nd2–O2A
2.600(6), Nd2–O2C 2.375(9), Nd2–O4D 2.509(7), Nd2–O5D 2.609(9),
Nd2–O4A" 2.459(8), Nd2–O5A" 2.955(8), Nd2–O4C# 2.513(8), Nd2–
O5C# 2.606(10), Nd2–O3D# 2.425(8), Nd3–O2A 2.554(8), Nd3–O2B
2.483(7), Nd3–O3C 2.491(8), Nd3–O1D 2.498(8), Nd3–O2D 2.528(8),
Nd3–O4D 2.413(7), Nd3–O4B' 2.536(7), Nd3–O5B' 2.495(9), Nd3–O2
2.476(8), Nd4–O3B 2.414(8), Nd4–O1C 2.498(9), Nd4–O3C 2.629(8),
Nd4–O4C 2.504(7), Nd4–O2D 2.543(9), Nd4–O5A* 2.397(8), Nd4–O3
2.611(9), Nd4–O4 2.555(12), Nd4–O5 2.637(16), Nd4–O6 2.583(14).
Several coordination modes of Hcitml2– are present, with three
occurrences of the chelating tridentate mode, four of the [JOURNAL], 200X, 00, 0000 | 1
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carboxylate chelating mode and one of the hydroxy/-carboxylate
chelating mode. Each Hcitml2– ligand is thus bound to four or five
metal atoms with, more unusual, no oxygen atom left
uncoordinated. Eight oxygen atoms being bridging (one - and one
-carboxylate atom in each ligand), a very intricate assembly is
formed. The average Nd–O bond lengths, 2.49(3) and 2.52(11) Å
for hydroxy and carboxylate groups, respectively, are in agreement
with the average values of 2.53(5) and 2.53(9) Å for neodymium
citrates.2b,e The average values for mono- and bidentate carboxylate
oxygen atoms, 2.47(8) and 2.54(12) Å, respectively, are not
significantly different from one another. The four carbon atoms
joining the carboxylate groups are nearly planar in ligand A, with a
torsion angle around the central bond of 165(1)°, whereas this angle
is gauche, 59(1)–61(1)°, in ligands B–D, which are all three
chelating in a tridentate mode.
a
b
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structure since they are connected via the c-directed cylindrical
channels and the triangular ones in the ab plane. Compound 1 thus
presents a continuous network of channels, with a smallest aperture
of about 3.5 Å. The overall volume of the channels represents about
22% of the total volume (estimation with PLATON6).
Only some of the lanthanide citrates previously reported present
three-dimensional structures, and these are quite compact, as
indicated by the calculated density of 2.662 g cm–3 for
[Nd(Hcit)(H2O)]n,2e to be compared with 2.127 g cm–3 in 1 (while
the neodymium mass content amounts to 41 and 39%,
respectively).
Even
in
the
one-dimensional
[Nd(Hcit)(H2O)2·H2O]n,2b the density of 2.501 g cm–3 is larger than
in 1. In contrast with the predictability achieved when designing
hybrid nanoporous structures, and even chiral ones, based on dblock metal atoms,7 those involving f elements and flexible multifunctional ligands appear quite serendipitous. However, open
structures involving lanthanide elements and polycarboxylate
ligands have been reported,1 among which some are chiral, with
potential applications in enantioselective catalysis or separation.8
Compound 1 is a novel example of such an open framework with
in-built chirality and an unprecedented topology.
Notes and references
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Fig. 2 View showing (a) the cylindrical channels parallel to the c axis, (b)
the channels arrangement in the ac plane. Solvent water molecules are
omitted in (b). Cyan: Nd, red: O, black: N, C.
The structural motif described above gives rise to a chiral threedimensional, hexagonal framework which displays a complex
network of channels. When viewed down the c axis, the
arrangement presents narrow cylindrical channels, with a diameter
of about 3.5 Å, centred on the 61 screw axis of symmetry and
containing water solvent molecules (Fig. 2a). Considering the
orthogonal directions, the same pattern of channels is apparent
down the [1 0 0], [1 1 0] and [0 1 0] directions, with a translation of
1/6 of the c parameter for each 60° rotation in the ab plane (Fig.
2b). This pattern comprises large and rather flat channels directed
along a, b or the ab diagonal, with a size of ca. 3.5  18 Å (the
largest dimension along c), which intersect the channels along the c
axis at right angles. There are also narrower channels of somewhat
triangular section which run between the c-directed ones. The
elongated channels being larger than c/6, some overlapping should
be observed between those running at 60° from one another.
However, when taking into account van der Waals radii, these
channels are only contiguous. They nevertheless build an open
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† Synthesis of 1. Nd(NO3)3·6H2O (241 mg, 0.550 mmol), D(–)-citramalic
acid (82 mg, 0.547 mmol) and NaOH (22 mg, 0.550 mmol) were
dissolved in demineralized water (2.5 mL). The solution was placed in a
20 mL tightly closed vessel and heated at 180°C under autogenous
pressure. Colourless crystals of compound 1 mixed with a white powder
appeared within ten days (55 mg, 27% yield on the basis of Nd atom;
however, the crystals could not be removed from the powder, which had
seemingly a slightly different composition, as indicated by unsatisfying
chemical analyses (calculated/measured discrepancies of about 1.3% at
most). Recording of the 1H NMR spectrum was prevented by the low
solubility of 1 in organic solvents. Identical crystals were also obtained by
heating at 90°C. The presence of carbonate ions in the structure is likely
due to partial decomposition of citramalic acid.
‡ Crystal data for 1: C42H72NNd7O61, M = 2576.69, hexagonal, space
group P6122, a = b = 12.7819(6), c = 85.294(4) Å, V = 12068.1(10) Å3, Z
= 6, T = 100(2) K. Refinement of 542 parameters on 7613 independent
reflections out of 127725 measured reflections (Rint = 0.074) led to R1 =
0.052, wR2 = 0.127, S = 1.001, min = –1.79, max = 1.69 e Å–3, Flack
parameter = 0.01(3). Data were collected on a Nonius Kappa-CCD areadetector diffractometer and processed with HKL2000.9 Absorption effects
were corrected with the program SCALEPACK.9 The structure was
solved by direct methods and refined by full-matrix least-squares on F2
with SHELXTL.10 All non-hydrogen atoms were refined with anisotropic
displacement parameters, with restraints for some badly behaving atoms.
One water molecule (O1) was affected with a 0.5 occupancy factor since
it is too close to its own image by symmetry and four others because of
their too large displacement parameters. The nitrate group has also been
affected with a 0.5 occupancy factor so as to account for its too large
displacement parameters and for charge equilibrium, and it has been
supposed to be disordered with a coordinated, unresolved water molecule.
The hydrogen atoms bound to oxygen atoms were not found, nor
introduced. All the other H atoms were introduced at calculated positions
and treated as riding atoms. CCDC reference number. See
http://www.rsc.org:suppdata/cc for crystallographic data in CIF format.
C. L. Cahill, D. T. de Lill and M. Frisch, CrystEngComm, 2007, 9,
15 and references therein.
(a) R. Baggio and M. Perec, Inorg. Chem., 2004, 43, 6965; (b) R.
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Inorg. Chem., 2005, 44, 8979; (c) G. Vanhoyland, J. Pagnaer, J.
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Chem. Commun., 2005, 8, 328; (e) D. Q. Yuan, Y. Q. Xu, M. Y. Wu
and M. C. Hong, Acta Crystallogr., Sect. E, 2005, 61, m108.
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A. L. Spek, PLATON, University of Utrecht, The Netherlands,
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(a) Y. Cui, H. L. Ngo, P. S. White and W. Lin, Chem. Commun.,
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S. Huang, Dalton Trans., 2003, 3192; (c) S. Thushari, J. A. K. Cha,
H. H. Y. Sung, S. S. Y. Chui, A. L. F. Leung, Y. F. Yen and I. D.
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Z. Otwinowski and W. Minor, Methods Enzymol., 1997, 276, 307.
G. M. Sheldrick, SHELXTL, Version 5.1, Bruker AXS Inc.,
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Graphical Abstract:
The complex formed by neodymium(III) with D(–)-citramalate ligands crystallizes as a hexagonal three-dimensional framework with
in-built chirality and a complex network of channels.
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