Hydrothermal assembly of a novel three-dimensional framework formed by [GdMo12O42]9- anions and nine coordinated GdIII cations

Article abstract of DOI:10.1021/ja017782w

The three-dimensional framework of [Gd(H₂O)₃]₃[GdMo₁₂O₄₂]vdt·3H₂O, which was synthesized from hydrothermal reaction, is built up from Silverton-type anions linked by nine coordinated gadolinium(III) cations. It is the first time that the paramagnetic lanthanide(III) cation is introduced into the Silverton-type anionic center in the heteropolymolybdate chemistry, and it acts as 18 dentate ligands coordinating to six Gd^III atoms and linking up six other neighboring [Mo₁₂GdO₄₂]^9- units in a staggered manner to generate a remarkable three-dimensional framework. The magnetic susceptibility measurement suggests that the weak antiferromagnetic behavior results from the transformation of O-Mo-O units, which has been confirmed by EPR spectra. Copyright

Full text of DOI:10.1021/ja017782w

Published on Web 03/20/2002
Hydrothermal Assembly of a Novel Three-Dimensional Framework Formed by
[GdMo12O42]9- Anions and Nine Coordinated GdIII Cations
Chuan-De Wu, Can-Zhong Lu,* Hong-Hui Zhuang, and Jin-Shun Huang
The State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter,
The Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China
Received December 17, 2001
There has been extensive interest in heteropolyoxometalates,
owing to their fascinating properties and great potential applications
in many fields (such as, catalysis, material science, medicine, and
magnetochemistry) as well as their unusual topological properties.1-4
Although various preparation methods have been proposed to obtain
such polynuclear complexes, a “rational” synthetic method is still
one of the major challenges in heteropolyoxometalatic chemistry.
One of the challenging tasks in polyoxometalatic chemistry is
to find some novel units and then to link them up into larger clusters,
or to one-, two-, even three-dimensional extended solid frameworks
in appropriate ways. Lanthanide ions and their complexes exhibit
unique spectroscopic and magnetic properties, which have wide
potential applications.5-10 It has been well-known that the rare earth
elements are highly oxophilic, and many polymolybdates have
Figure 1. Structure of the [GdMo12O42]9- anion: {Mo12} fragment is in
oxygen-rich compositions, suggesting that the combination of them
may generate some interesting novel complexes. However, the
heteropolymolybdates containing lanthanide elements remain largely
polyhedral representation and central 12-coordinated GdIII cation in ball-
and-stick representation.
unexplored. These results do not mean the polymolybdate and the
lanthanide ions cannot coexist together. Contrarily, as the oxygen
atoms on the surface of the polymolybdates are rather reactive, some
lanthanide ions can readily incorporate into polymolybdates, which
in most cases only precipitate instead of crystallize. One of the
effective methods is by reacting rare earth metal salts with
traditional polymolybdate or by using protecting methods using
appropriate ligands.11-17 However, the synthesis of higher-
dimensional extended solids based on lanthanides and polymolyb-
dates is still one of the great challenges facing chemists. Recent
elaboration has proven that hydrothermal reaction, which causes a
reaction shifting from kinetic to thermodynamic domain compared
to traditional aqueous reactions, provides a powerful tool for the
synthesis of higher-dimensional oxide frameworks.18 Thus, one-
pot hydrothermal synthesis provides an alternative way to the above-
mentioned step-by-step aggregation process for the design of
heteropolymolybdate frameworks containing lanthanide. In this
communication we report the synthesis and characterization of the
first three-dimensional framework, [Gd(H2O)3]3[GdMo12O42]‚3H2O
1, containing Silverton-type anions linked by gadolinium(III)
cations.
A mixture of gadolinium(III) chloride (0.55 mmol, prepared from
Gd2O3 dissolved in 35% HCl) and (NH4)6Mo7O24‚4H2O (0.18 g,
0.146 mmol) in 18 mL of water at low pH (0.80) was heated at
170 °C for 5 days, and the colorless crystals of complex 1 were
obtained. The structural analysis19 suggests that the structure of
complex 1 is a three-dimensional framework of Silverton-type
anions linked by nine coordinated gadolinium(III) atoms. Two
strong bands at 966 and 931 cm-1 in the infrared spectrum of 1
are ascribed to ν(ModO). Multiple bands attributed to the bridging
(Mo-O-Mo(Gd))-group absorptions are found in the 908-409
cm-1 region.
X-ray structural analysis suggests that the central anion has D3d
symmetry. The anion consists of six Mo2O9 units and a 12-
coordinate GdIII atom at the center, which is a Silverton-type anion,
such as [Ce4+Mo12O42].8-20 To the best of our knowledge, it is the
first example that a paramagnetic lanthanide(III) cation is introduced
into the Silverton-type anionic center in the heteropolymolybdate
chemistry. As shown in Figure 1, each Mo2O9 is formed by two
octahedra by sharing a face, and then six such Mo2O9 units are
linked to each other through corner-sharing to build up the anion.
Two corners are on the shared face, while the other two are the
innermost ones opposite to the shared face. The Mo-O bond
lengths can be divided into three different classes according to the
manner of their coordination. The octahedral geometry around each
molybdenum in the {GdMo12O42} unit is defined by two terminal
oxygen atoms with Mo-O distances within 1.70(1)-1.72(1) Å,
three inner shared-face oxygen atoms involvied in corner-sharing
(1.88(1)-2.30(1) Å), and only one oxygen atom in the shared face,
not involved in corner-sharing with the Mo-O distance of about
1.94(1)-1.95(1) Å. The 12 molybdenum atoms form a slightly
distorted icosahedron around the central GdIII atom. In other words,
the Gd atom is 12-coordinate, and the coordination polyhedron is
an icosahedron, in which the 12 oxygen atoms are from six Mo2O9
units. The two independent Gd-O distances are 2.50(1) and 2.53-
(1) Å.
As shown in Figure 2, the crystallization-independent GdIII cation,
in a slightly disordered tricapped trigonal prism, is situated at the
outer sphere of the [Mo12GdO42]9- anion. The coordination oxygen
atoms around the GdIII cation can be divided into two classes
according to different sources: the first one contains the three aqua
ligands with Gd-O distances within 2.39(2)-2.48(1) Å; the other
contains the six terminal oxygen atoms of molybdenum (2.36(1)-
2.48(1) Å). These coordination patterns cause the Mo-O distances
9
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C O M M U N I C A T I O N S
netic molybdenum(VI) is very weak. The EPR spectra of poly-
crystalline 1 give broad signals with g ) 1.9915, ∆H ) 1541 G at
room temperature, and g ) 1.9958, ∆H ) 1458 G at 77 K. These
results are almost consistent with the magnetic susceptibility
measurements, which reveal that the overall interactions between
the paramagnetic GdIII centers are transferred by O-Mo-O units.
In conclusion, although hydrothermal synthetic methods have
been extensively used to prepare polymolybdate-containing inorganic/
organic hybrid materials,22 the synthesis of heteropolymolybdate-
containing lanthanides by a hydrothermal synthetic method has
fallen far behind. The successful synthesis of the novel three-
dimensional framework of 1 suggests that the hydrothermal
synthesis method is an effective method to construct some higher-
dimensional frameworks for novel heteropolymolybdates containing
lanthanide elements.
Figure 2. Polyhedral representation of the [GdMo12O42]9- anion and ball-
and-stick representation of the nine-coordinated GdIII cations down along
the c axis.
Acknowledgment. This work was supported by 973 Program
of the MOST (001CB108906), National Natural Science Foundation
of China (20073048), NSF of Fujian, and the Chinese Academy of
Sciences.
Supporting Information Available: Tables of crystal data, struc-
ture solution and refinement, atomic coordinates, bond lengths, angles,
anisotropic thermal parameters, variable-temperature magnetic suscep-
tibilities and supplementary figures for 1 (PDF). X-ray crystallographic
file (CIF) This material is available fee of charge via the Internet at
http://pubs.acs.org.
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7
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