From rational octahedron design to reticulation serendipity. A thermally stable rare earth polymeric disulfonate family with CdI2-like structure, bifunctional catalysis and optical properties

Article abstract of DOI:10.1039/b202639b

A new family of lanthanide disulfonates Ln(OH)(NDS)- (H₂O), (Ln = La, Pr and Nd; NDS = 1,5-naphthalenedisulfonate) was designed and hydrothermally synthesized; this is the first example of a disulfonate ligand coordinated to six different Ln atoms; these materials, with high thermal stability, act as active and selective bifunctional catalysts in oxidation and epoxide ring opening; strong luminescence from the optically active Nd center was observed.

Full text of DOI:10.1039/b202639b

From rational octahedron design to reticulation serendipity. A thermally
stable rare earth polymeric disulfonate family with CdI2-like structure,
bifunctional catalysis and optical properties
Natalia Snejko, Concepción Cascales, Berta Gomez-Lor, Enrique Gutiérrez-Puebla, Marta Iglesias,
Caridad Ruiz-Valero and M. Angeles Monge*
Instituto de Ciencia de Materiales de Madrid, CSIC Cantoblanco, E-28049 Madrid, Spain.
E-mail: amonge@icmm.csic.es
Received (in Cambridge, UK) 19th March 2002, Accepted 17th May 2002
First published as an Advance Article on the web 5th June 2002
A new family of lanthanide disulfonates Ln(OH)(NDS)-
(H2O), (Ln = La, Pr and Nd; NDS = 1,5-naphthalenedisulfo-
nate) was designed and hydrothermally synthesized; this is
the first example of a disulfonate ligand coordinated to six
different Ln atoms; these materials, with high thermal
stability, act as active and selective bifunctional catalysts in
oxidation and epoxide ring opening; strong luminescence
from the optically active Nd center was observed.
framework, (denoted LnPF-1). An X-ray powder diffraction
pattern showed that the Pr compound has the same structure. In
LnPF-1 the lanthanide ion is octacoordinated to two m2-OH
groups, one water molecule and five oxygen atoms of different
NDS ligands (Fig. 1). Every two of these (LnO8) polyhedra
share an OH–OH edge, giving rise to dimers, which are isolated
in the b direction and joined through S atoms in the a direction.
Connections along the c direction are made via whole NDS
ligands. Hence, at a first sight the structure can be thought of as
formed by Ln2O14 dimers that give rise to (Ln2O14)–NDS–
(Ln2O14)–NDS... infinite chains, kept together through the NDS
connectors. Regarding the connectors, there are two differently
coordinated NDS molecules, which are perpendicularly ori-
ented along the [2210] and the [210] directions. One of them
(NDS-1) (blue ligand in Fig. 2) coordinates, as expected to six
distinct Ln atoms, whereas the other (NDS-2) (grey in Fig. 2) is
With such a great number of known purely inorganic structures
based on octahedra, we entertained the idea of using this motif
as a secondary building unit (SBU) for the creation of
supramolecular assemblies that, while leading to known
inorganic structure types, may have the properties of the hybrid
organo-inorganic frameworks. The first such structure, based on
3d
octahedral cages, an A62aA6 assembly, has recently been
reported.1 The formation of a hybrid octahedral SBU through
one only central hexacoordinated ligand, made us establish the
following conditions. For the donor connector: (i) capability of
linking six different metals (hexatopic connector); (ii) the
presence of an inversion center, this condition being funda-
mental to avoid the formation of a trigonal prism; (iii) volume
and geometry such as to minimize steric hindrance among the
metal coordination spheres. Regarding the acceptor, i.e. the
metal cation, a metal with high coordinative capability is
envisioned in order to favor the reticulation of octahedra into an
infinite network through the same or different links. All these
requirements, together with the remarkable magnetic, optical
and catalytic properties of the lanthanides,2–8 prompted us to
investigate the possibility of synthesizing sulfonates of rare
earth elements (Ln). This combination in a metal coordination
polymer could result in a multifunctional catalyst, considering
the strong catalytic activity of the rare earth elements and the
relatively strong acid character of sulfonate groups. Moreover,
in contrast with the rich chemistry of open framework
coordination polymers formed by phosphonates and carboxy-
lates, to the best of our knowledge, only one 3D mixed-ligand
sulfonate and carboxylate-containing coordination polymer of
rare earth elements has been reported.9 We considered that
1,5-naphthalenedisulfonic acid (H2NDS), with its rigid struc-
ture and two centrosymmetric, functionally active groups in the
two extreme positions, could meet all the demands to engineer
a material with the required structural and physico-chemical
properties.
Fig. 1 ORTEP plot of the building unit showing more than the asymmetric
unit.
The compounds were obtained by treating an aqueous
solution containing a mixture of equimolar amounts of
Ln(NO3)3·6H2O (Ln = La, Pr and Nd) and Na2NDS under
hydrothermal reaction conditions (24 h, 170 °C). The crystalline
product was separated by suction filtration, washed with water
and dried in air, and its purity checked by X-ray powder
diffraction, by comparison with the simulated pattern on the
basis of the single crystal data.
Crystals of the La and Nd compounds were selected and
mounted in a diffractometer equipped with a CCD detector.
Upon determining the crystal structure,10 the composition was
found to be Ln(OH)(NDS)(H2O), a lanthanide polymeric
Fig. 2 Formation of the crystal structure of Ln(OH)(NDS)(H2O) (in blue)
based on octahedra and idealised I2Cd structure type (in yellow).
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bonded only to four lanthanide atoms, leaving thus, one free
oxygen atom for each sulfonate group. By considering the
centre of the NDS-1 ligand, which is an inversion centre of the
organic molecule and of the unit cell, as a hypothetical atom A
coordinated to the six rare-earth atoms that are bonded to this
ligand (Fig. 2(b) and (c)), an ALn6 polyhedron arises. Given that
the six Ln atoms are 3 to 3 related by the inversion centre,
trigonal prismatic geometry is not present, and therefore it is an
irregular octahedron, which defines the SBU (Fig. 2(d)).
In doing that, the structure could be also described in terms of
layers of ALn6 octahedra, each sharing an edge with each of six
adjacent ones (Fig. 2(e)). This geometrical disposition corre-
sponds to the CdI2 structure,11 with the lanthanide ions
occupying the place of the I ions and the centre of the NSD-1
ligands (A) those of the cadmium ions. These layers that are
perpendicular to the b direction are linked to each other through
the four coordinated oxygen atoms of the NDS-2 connectors, in
a manner closely reminiscent of other known inorganic
structures based on CdI2-like layers11 (Fig. 2(f)).
The presence of hydroxy groups and water can be clearly
observed in the IR spectrum (bands at ca. 3600, 3500–3100 and
1530 cm21). Besides, the aromatic C–H stretching (3080–2870
cm21 ) and both nas and ns of the SO3 groups are detected at
1240–1250 and 1180 (nas) and 1044 cm21 (ns). Thermogravi-
metric analysis, and differential thermal analyses (TGA–DTA)
carried out under N2 (50 ml min21) for LnPF-1 (Ln = La, Pr
and Nd) show a weight loss between 240 and 310 °C,
accompanied by an endothermic effect corresponding to the loss
of the coordinated water molecule per formula unit (calculated
1.9%, found 2%). No further weight loss was detected within
the temperature interval 310–510 °C. Above the latter tem-
perature both frameworks completely decomposed.
Fig. 3 10 K PL spectra of NdPF-1.
of Nd3+ in this host are split in to the maximum number of Stark
components, Kramers doublets, 7, 6 and 5 for 4I13/2, 4I11/2, Fig.
3, and I9/2, respectively, in agreement with the crystal field
4
symmetry of its site. On the other hand, the moderate measured
phonon energies of this material suggests adequate non-
radiative multiphonon decay rates among Nd3+ energy levels,
and consequently satisfactory radiative efficiencies for this
polymeric disulfonate matrix.
In summary, a novel designed polymeric structure is
presented. This new family of compounds represents the first
example of a disulfonate ligand coordinated to six different
lanthanide atoms. These materials, with high thermal stability,
act as active and selective bifunctional catalysts in oxidation
and epoxide ring opening. Further studies on their optical,
catalytic and magnetic properties are in progress.
Notes and references
The presence in these materials of active metal (Ln), and acid
(sulfonate groups) sites forming part of the framework,
prompted us to test the catalytic activity of the three LnPF-1
compounds in the oxidation of linalool 1. Linalool oxides are
extensively found in nature, and are used as fragrances. They
also appear to have a strong biological significance in certain
pollination systems, acting as insect attractants. Although
numerous multistep organic synthesis of pyranoid and furanoid
linalool oxides have been reported,12 some of them highly
stereoselective,13,14 they are usually obtained by extraction
from natural products or by transformation of linalool. The
samples were used as catalysts (20 mg) for the transformation of
linalool (330 mg, 2.1 mmol) by H2O2 (3 mmol) in acetonitrile
(10 ml) in a glass reactor at 343 K. The samples were active for
the oxidation of linalool to hydroxy ethers (furanoid and
piranoid form, 2, 3 in Scheme 1), with yields after 24 h of 100,
94 and 75%, for La, Pr and Nd, respectively. This activity is
comparable to that of microporous bifunctional titanium
aluminosilicates.15 The similar 2+3 ratios found in all cases
(1.8, 2.1 and 2.2 for La, Pr and Nd compounds, respectively),
show that all materials behave similarly. It seems plausible that
the process involves firstly the epoxidation at the metal site of
the 2,3 double bond followed by intramolecular opening of the
epoxide ring by the hydroxy group at positions 6 or 7, the latter
reaction being catalyzed by the acid sites. All the catalysts are
very selective and no other reaction products were detected.Op-
tical properties of active Ln ions were studied on NdPF-1.16
Well-resolved strong photoluminescence spectra at 10 K
from the lowest energy Stark component of 4F3/2 unequivocally
confirms the existence of a single optically active ordered site
for Nd3+ in the NdPF-1 crystal host. Moreover, The J manifolds
1 D. W. Johnson, J. Xu., R. W. Saalfrank and K. N. Raymond, Angew.
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2 T. M. Reineke, M. Eddaoudi, M. Fehz, D. Kelley and O. M. Yaghi, J.
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and O. M. Yaghi, Acc. Chem. Res., 2001, 34, 319.
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7 J.-M. Lehn, Angew. Chem., Int. Ed., 1990, 29, 1304.
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9 R. G. Xiong, J. Zhang, Z. F. Chen, X. Z. You, C. M. Che and H. K. Fun,
J. Chem. Soc., Dalton Trans., 2001, 780.
10 Crystal data for LaPF-1 [NdPF-1]: Ln3+(OH)2[C10H6O6]22·H2O:
¯
triclinic, space group P1, a = 5.7263(2) [5.6745(8)], b = 10.6104(4)
[10.551(2)], c = 11.5533(8) [11.450(2)] Å, a = 76.615(1) [76.456(2)],
b = 77.619(1) [77.484(2)], g = 87.937(1) [88.034(2)]°, V = 666.94(4)
[650.5(2)] Å3, T = 296 K, Z = 2, Mw = 460.2 [465.3], Dc = 2.291
[2.377] Mg cm23, m(Mo-Ka) = 3.55 [4.35] mm21. CCDC reference
numbers 182916 and 182917. See http://www.rsc.org/suppdata/cc/b2/
b202639b/ for crystallographic data in CIF or other electronic format.
Patent Registry No. 200200850.
11 A. F. Wells, Structural Inorganic Chemistry, Oxford Science Publica-
tions, 5th edn., 1991, p. 259.
12 Ch. Fournier-Nguefack, P. Lhoste and D. Sinou, Tetrahedron, 1997, 53,
4353 and references therein.
13 G. Vidari, A. Di Rosa, G. Zanoni and C. Bicchi, Tetrahedron, 1999, 10,
3547.
14 G. Vidari, A. Di Rosa, F. Castronovo, G. Zanoni and C. Bicchi,
Tetrahedron: Asymmetry, 2000, 11, 981.
15 A. Corma, M. Iglesias and F. Sanchez, J. Chem. Soc., Chem. Commun.,
1995, 1635.
16 10 K photoluminescence spectra of the Nd-sample were recorded by
exciting the 4F5/2 + 2H9/2 multiplets with a Ti-sapphire laser (l = 799
± 0.5 nm), and collecting the luminescence from 4F3/2. The emission was
dispersed with a 500 M spex monochromator and the signal recorded
with suitable detectors, a Hamamatsu R636 photomultiplier (4I9/2), and
a cooled Ge photodiode (4I11/2 4I13/2), using the lock-in amplifying
,
technique. Raman spectra were obtained with a Reninshaw Ramascope
2000 equipped with a CCD detector. The samples were excited with the
514.5 nm line of an Ar+ laser. The spectra were recorded in the
backscattering geometry at room temperature, and corrected by the
spectral response of the experimental set-up.
Scheme 1
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