Hydrothermal synthesis and structure of three novel open-framework lanthanide sulfate-oxalates

Article abstract of DOI:10.1016/j.inoche.2010.04.004

Three novel hybrid open-framework lanthanide sulfateoxalates, {[NH ₄][Ln(H₂O)(SO₄)(C₂O ₄)]}_n [Ln=Y (I), La (II), Sm (III)] have been synthesized via hydrothermal reaction, and characterized by single crystal X-ray diffraction, powder X-ray diffraction, infrared spectrophotometry, thermal gravimetric analysis and fluorescence analysis. These three compounds were isostructural, and all crystallized into the monoclinic system with space group of P2₁ /n. In their structures, LnO₄ dodecahedra, SO ₄ tetrahedral and C₂O₄ groups are linked to give rise to a three-dimensional open-framework, which contains two kinds of 12-membered ring channel systems running along the a and b axis, respectively.

Full text of DOI:10.1016/j.inoche.2010.04.004

Inorganic Chemistry Communications 13 (2010) 831–833
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Hydrothermal synthesis and structure of three novel open-framework
lanthanide sulfate–oxalates
a
a
a
Li Li a,b, Ranbo Yu c, Dan Wang a, , Xiaoyong Lai , Dan Mao , Mei Yang
⁎
a
State Key Laboratory of Multi-Phase & Complex Systems, Institute of Processing Engineering, Chinese Academy of Sciences, Beijing 100190, PR China
Graduate University of Chinese Academy of Sciences, Beijing 100049, PR China
Department of physical chemistry, University of Science and Technology Beijing, Beijing 100086, PR China
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Three novel hybrid open-framework lanthanide sulfate–oxalates, {[NH4][Ln(H2O)(SO4)(C2O4)]}n [Ln=Y (I),
La (II), Sm (III)] have been synthesized via hydrothermal reaction, and characterized by single crystal X-ray
diffraction, powder X-ray diffraction, infrared spectrophotometry, thermal gravimetric analysis and
fluorescence analysis. These three compounds were isostructural, and all crystallized into the monoclinic
system with space group of P21 /n. In their structures, LnO8 dodecahedra, SO4 tetrahedral and C2O4 groups
are linked to give rise to a three-dimensional open-framework, which contains two kinds of 12-membered
ring channel systems running along the a and b axis, respectively.
Received 28 January 2010
Accepted 2 April 2010
Available online 12 April 2010
Keywords:
Hydrothermal synthesis
Inorganic–organic hybrid material
Open-framework
© 2010 Elsevier B.V. All rights reserved.
Crystal structure
Crystal engineering of inorganic open-framework materials has
attracted considerable interests due to their fascinating structural
diversity and potential applications [1]. A variety of inorganic
frameworks mostly involving with silicate [2] and phosphate [3]
tetrahedral anionic moieties have been well reported. Nowadays,
great researches are devoted to the design and synthesis of new
inorganic framework materials with different compositions and
structures. While SO4 ions and PO4 ions have different charges, they
are of similar size and shape. It can also be effectively used to
construct materials with new topological structures and interesting
properties [4]. The lanthanide centers have high and variable
coordination numbers and a wide variety of coordination environ-
ments compared with other transition metals. Therefore, it is possible
to lead to unusual topological frameworks [5]. Work reported on
lanthanide sulfate mainly focused on inorganic hydrates and species
of whose structures containing alkali metal, or ammonium ions, or
organic amines template [6,7]. Selection of multidentate ligands as
spacers to link multiple lanthanide ions as nodes under suitable
reaction conditions has been proved to be a powerful methodology,
and important progress has been achieved [8]. However, it is still a
challenge for preparing lanthanide sulfate using the organic compo-
nent as ligand directly coordinated to the lanthanide sulfate
scaffolding to form the organic–inorganic hybrid materials. Oxalic
acid has been well reported to build up a 3D metal-organic framework
[9,10]. Herein, the three-dimensional open-framework lanthanide
sulfate–oxalates {[NH4][Ln(H2O)(SO4)(C2O4)]}n were reported and
the synthesis, structure, and fluorescent properties of these lantha-
nide sulfate–oxalates were discussed.
Colorless columnar crystals of compound I were obtained by
heating a mixture of Y2O3, H2SO4, (NH4)2SO4, oxalic acid, H2O and
ethanol in the molar ratio of 1:8:4:2:833:74 at 160 °C for 3 days.
Syntheses of compounds II and III were similar to that of compound I,
except that La2O3 and Sm2O3 were used to replace Y2O3 respectively.
Crystallographic analysis reveals that all the three compounds are
isostructural [11–13]. Therefore, only the crystal structure of
compound I is described in detail. The single crystal X-ray diffraction
study of the compound showed that the framework structure is made
up of YO8 dodecahedra, SO4 tetrahedra and oxalate groups (Fig. 1).
Each oxalate group links two Y ions in a bidentate mode, showing
a zigzag chain along b axis (Fig. 2a). The sulfate groups adopting η3,
μ3-tridentate coordinated mode connect with YO8 dodecahedra at
three vertices, with the fourth one being a free terminal oxygen, to
form a one-dimensional chain along the a axis as shown in Fig. 2b. The
inorganic [Y2(O–S–O)2] chain is further linked via oxalate groups to
produce an open-framework structure with one-dimensional chan-
nels running along the a and b axes, respectively. Those along the a
axis have a 12-membered window (six YO8, two SO4 and four C2O4)
with a diagonal of 12.6 Å as shown in Fig. 3a. The channel systems
along the b axis also have a 12-membered window (six YO8, four SO4
and two C2O4) (Fig. 3b). It has a pore opening of 7.2 Å×4.0 Å. The pore
is generated by the removal of free ammonium molecules, and the
void volumes are 49.9% (I), 53.1% (II) and 56.1% (III) respectively as
estimated by PLATON [14]. The ammonium cations are located in the
12-membered windows to balance the framework's negative charge.
The Y3+ ions are eight-coordinate and described as a dodecahe-
dron: three oxygen atoms from three SO24− with distances in the range
⁎
Corresponding author. Tel./fax: +86 10 62631141.
E-mail address: danwang@home.ipe.ac.cn (D. Wang).
1387-7003/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2010.04.004

832
L. Li et al. / Inorganic Chemistry Communications 13 (2010) 831–833
Fig. 1. The asymmetric unit of compound I. Color code: Y, azure; S, yellow; N, dark blue; C, green; O, red; and H, gray.
small, as for [O(5)–Y(1)–O(7)], [O(6A)–Y(1)–O(8A)], when the
oxygen atoms are coordinated with carbon atoms of the oxalate
group [17]. All the other bond distances and angles are in the range
expected for this type of bonding and match well with those
reported previously in the literature.
The thermal stability was examined by TGA in a dry air
atmosphere from 20 to 1000 °C. Four distinct mass losses were
observed, as shown in Fig. 1S. The first mass loss (4.50%) occurs
between 80 and 200 °C corresponding to the loss of ammonia
molecule (calcd. 5.50%). The second mass loss of about 37.05% in
the range of 280–700 °C corresponds to the loss of bound water and
oxalate molecules (calcd. 37.22%). The white production after
calcining at 700 °C is Y2(SO4)3 according to the XRD spectra. Between
700 and 900 °C, the TGA curve shows a continuous loss, which is
associated with the loss of SO3 groups. The powder X-ray diffraction
pattern of the sample heated at 1000 °C corresponds to Y2O3 (Fig. 2S),
which indicates the destruction of the framework structure upon
heating as described by the following reaction:
Fig. 2. Structures of 1D zigzag chain formed by Y and oxalate group (a), [Y(SO4)(H2O)]
inorganic chain (b).
of 2.300–2.319 Å, four oxygen atoms from two C2O24− anions with
distances in the range of 2.380–2.416 Å [11] and one terminal water
molecule with distance Y–O(w)=2.372 Å[15]. Each SO24− anion
adopts a tridentate coordination mode bridging three Y3+ ions with
a free S(1)–O(2) of the shortest bond distance 1.460 Å. The average
S–O bond distance and O–S–O angle are 1.472 Å and 109.47°,
respectively [6,7,16]. The O–Y–O bond angle becomes relatively
2f½NH4ꢀ½YðSO4ÞðC2O4ÞðH2OÞꢀgn→2nNH3 þ 3nH2O þ 4nCO2 þ 2nSO2
þ nY2O3
The emission spectra of compounds and oxalic acid are shown in
Fig. 3S. It exhibits strong fluorescence emission bands at 530 and 792
nm (λex =266 nm), which can be assigned to the intraligand
fluorescent emission of oxalate ligands attributed to the π–π*
Fig. 3. The polyhedral diagram of compound I viewed along the a axis (a) and b axis (b), respectively.

L. Li et al. / Inorganic Chemistry Communications 13 (2010) 831–833
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transition. The fluorescent intensity of the compounds is enhanced
significantly compared with oxalic acid. This is probably due to the
coordination of oxalate to Ln, which increases the ligand conforma-
tional rigidity, and reduces the nonradiative decay of the intraligand
excited state.
In summary, three novel lanthanide coordination compounds
{[NH4][Ln(C2O4)(SO4)(H2O)]}n [Ln=Y (I), La (II), Sm (III)] were
prepared by hydrothermal reactions. Due to the coordination of
oxalate to Ln, the fluorescent intensity is significantly enhanced
compared with that free of oxalate ligands. It would be worth
exploring the possibility of introducing other appropriate structure-
directing agents and flexible spacer pillars to prepare new hybrid
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Appendix A. Supplementary material
CCDC 665771, 688417 and 688418 contain the supplementary
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