Carboxylato-bridged 3D polymeric networks of Pr(III): Synthesis, crystal structure, magnetic property and thermal behavior

Article abstract of DOI:10.1016/j.molstruc.2010.06.019

A novel polymeric carboxylato-bridged praseodymium(III) complex of fumarate, [Pr₂(fum)₃(H₂O)₄] ·5H₂O (fum, fumarate dianion) has been synthesized and characterized by single-crystal X-ray diffraction studies, magnetic measurements and thermal studies. Single crystal X-ray structural determination reveals that it is a new porous three dimensional pillared layer framework with 1D channel for the accommodation of tetrameric water clusters. A preliminary treatment of the variable-temperature magnetic susceptibility using an expression including the ligand field effects and molecular field approximation leads to zJ′ = -1.60 cm^-1, Δ = 0.5 cm^-1 and g = 0.84, suggesting the existence of a weak antiferromagnetic coupling between the praseodymium ions.

Full text of DOI:10.1016/j.molstruc.2010.06.019

Journal of Molecular Structure 979 (2010) 160–164
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Carboxylato-bridged 3D polymeric networks of Pr(III): Synthesis, crystal
structure, magnetic property and thermal behavior
*
Lang Zhao, Gong-Feng Xu, Jinkui Tang
State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Renmin Street 5625, Changchun 130022, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 April 2010
Received in revised form 12 June 2010
Accepted 14 June 2010
Available online 19 June 2010
A novel polymeric carboxylato-bridged praseodymium(III) complex of fumarate, [Pr2(fum)3(H2O)4]ꢀ5H2O
(fum, fumarate dianion) has been synthesized and characterized by single-crystal X-ray diffraction stud-
ies, magnetic measurements and thermal studies. Single crystal X-ray structural determination reveals
that it is a new porous three dimensional pillared layer framework with 1D channel for the accommoda-
tion of tetrameric water clusters. A preliminary treatment of the variable-temperature magnetic suscep-
tibility using an expression including the ligand field effects and molecular field approximation leads to
Keywords:
zJ0 = ꢁ1.60 cmꢁ1
, D
= 0.5 cmꢁ1 and g = 0.84, suggesting the existence of a weak antiferromagnetic
Metal–organic framework (MOF)
Lanthanide (Ln)-containing MOFs
Fumarate ligand
coupling between the praseodymium ions.
Published by Elsevier B.V.
Magnetic property
Antiferromagnetic coupling
Hydrothermal synthesis
1. Introduction
thanide fumarates [13,15]. For example, Gao et al. reported a new
three dimensional (3D) Sm–fum framework, [Sm2(fum)3(H2O)4]ꢀ
3H2O, which possesses a three dimensional structure but has 0D
cavities, and each cavity supports a hexameric water cluster.
Interestingly, the material exhibits a reversible dehydration–
rehydration procedure and framework dynamics [13a].
The present contribution reports the hydrothermal synthesis,
crystal structure, magnetic study and thermal behavior of
the new praseodymium (III) frameworks with fum, namely
[Pr2(fum)3(H2O)4]ꢀ5H2O (complex 1). The compound has the same
Ln/fum ratio of 2:3 as [Sm2(fum)3(H2O)4]ꢀ3H2O [13a], but with dif-
ferent structure. Complex 1, containing a rigid anion, is 3D poly-
meric network of Pr(III) with 10 coordination geometry and has a
small void in the crystal filled with water molecules. Low-temper-
ature magnetic susceptibility measurements show the existence of
a weak antiferromagnetic coupling among the praseodymium ions.
The exponential growth of the field of metal–organic materials,
particularly metal–organic frameworks (MOFs), has been seen over
the past decades due to their fascinating network topologies and
their many potential applications as functional materials [1]. This
structural versatility is reflected in applications to areas as diverse
as catalysis, proton conductivity, ion exchange, intercalation chem-
istry, photochemistry and magnetic materials [2–9]. Even more
remarkably, these applications can be combined and integrated
into individual frameworks to form multifunctional MOFs [10].
Lanthanide (Ln)-containing MOFs have attracted much interest
due to their ability to incorporate both photoluminescent centers
and magnetic properties, making them ideal for developing new
multifunctional materials [11]. Furthermore, the high affinity of
lanthanides for oxygen donor atoms makes carboxylates excellent
candidates as bridging ligands, because they can adopt a variety of
coordination modes and result in diverse multidimensional archi-
tectures [12]. Among the dicarboxylates being employed in the
construction of such rare earth MOFs, fumarate (hereinafter fum)
[13] is unique because of its relatively small central moiety
(CH@CH), compared with others such as the most common ben-
zene–dicarboxylate ligands [14], which allows for more versatile
coordination modes and possible porosity in the constructed
frameworks. Several structural studies have been published on lan-
2. Experimental
2.1. Materials and apparatus
Pr(NO3)3ꢀ6H2O was prepared by dissolving the oxide in dilute
nitric acid and the filtrate was subsequently dried under steam
bath to obtain the nitrate salt. All other chemicals were purchased
and used as received without further purification. Elemental anal-
ysis was carried out on a Perkin–Elmer 2400 elemental analyzer.
TGA 7 thermogravimetric analyzer was used to carry out thermo-
* Corresponding author. Tel./fax: +86 431 85262878.
E-mail address: tang@ciac.jl.cn (J. Tang).
gravimetric analysis in the air with a heating rate of 10 °C minꢁ1
.
0022-2860/$ - see front matter Published by Elsevier B.V.
doi:10.1016/j.molstruc.2010.06.019

L. Zhao et al. / Journal of Molecular Structure 979 (2010) 160–164
161
Variable-temperature magnetic susceptibilitie was measured using
3. Results and discussion
a MPMSXL-7 SQUID magnetometer. The magnetic data were cor-
rected for the sample holder and the diamagnetic contributions.
3.1. Crystal structure description of 1
As shown in Fig. 1, there is only one type of Pr(III) atom environ-
ment in the asymmetric unit of 1. Each praseodymium atom is
coordinated by 10 oxygen atoms, two of which belong to the two
coordinated water molecules, while the remaining eight belong
to six fumarate ligands. There are one and a half crystallographi-
cally independent fumarate molecules in the asymmetrical unit
of 1. Two types of coordination modes of fumarate ligands exist
in complex 1: (i) two chelating/bridging tridentate [Scheme 1a],
the fumarate anions are centrosymmetric and bonded in this mode
to Pr(III) atoms along the a-axis; (ii) two bridging bidentate
[Scheme 1b], the fumarate anions are bonded in this mode to Pr(III)
cations along the b-axis. All the interatomic distances are usual:
The Pr–O distances are in the range of 2.428–2.737 Å and Pr–Pr dis-
tances is 4.370 Å, as well as C–O (1.249–1.271 Å) and C–C distances
(1.314–1.493) Å seemingly typical for praseodymium(III) carboxyl-
ates [15].
2.2. Preparation of [Pr2(fum)3(H2O)4]ꢀ5H2O (1)
Pr(NO3)3ꢀ6H2O (0.5 mmol, 0.218 g) was dissolved in 10 mL
deionized water, to which H2fum (1 mmol, 0.116 g) and glutamic
acid (1 mmol, 0.147 g) were added while stirring. Finally, NaOH
aqueous solution was added to adjust the pH value of the mixture
to about 3.5–4. A homogeneous reaction gel was formed after stir-
ring for 2 h, and was transferred to a Teflon-lined stainless steel
autoclave (15 mL). The vessel was sealed and heated at 150 °C for
5 d under autogenous pressure and then cooled to room tempera-
ture. After filtered, the product was washed with ethanol and then
dried under ambient conditions. Quadrate crystals of the complex
1 were collected. Elemental analysis calcd (%) for 1 in powder
form: C 18.32, H 3.05, O 42.74; found: C 18.21, H 3.12, O 42.51.
Complex 1 possesses three dimensional structure, which is built
up from complex chains of Pr(III) polyhedra, linked in the two
other directions by fumarate. The chains of compound 1 are built
up from the edge-sharing linkage of 10-coordinated Pr(III) polyhe-
dra along the a axis, as shown in Fig. 2a, which are further linked
by fumarate in chelating and bridging mode to give rise to a 2D
sheets parallel to the ac plane (Fig. 2b). The other type fumarate
in syn–syn bridging mode is approximately perpendicular to the
ac plane, pillaring the sheets and a global 3D network is formed
(Fig. 2c). As a consequence, large rhombus 1D channels are formed
2.3. Single-crystal X-ray diffraction
Suitable single crystal with dimensions of 0.15 ꢂ 0.12 ꢂ
0.11 mm3 for compound 1 was selected for single-crystal X-ray dif-
fraction analysis. Crystallographic data were collected at a temper-
ature of ꢁ86 2 °C on a Bruker ApexII CCD diffractometer with
graphite monochromated Mo K
a radiation (k = 0.71073 Å). Data
processing was accomplished with the SAINT processing program.
The structure was solved by the direct methods and refined on F2
by full-matrix least-squares using SHELXTL97 [16] The locations
of Pr atom was easily determined, and O, C, and H atoms were
subsequently determined from the difference Fourier maps. All
non-hydrogen atoms were refined with anisotropic thermal
parameters. Crystal data: [Pr2(fum)3(H2O)4]ꢀ5H2O: Mr = 786.13,
monoclinic system, space group P2(1)/c (No. 14), a = 9.1999(5) Å,
in the direction of
a and c axis with the cavities of ca.
11.525 ꢂ 8.696 Å (Prꢀ ꢀ ꢀPr distance) and ca. 10.046 ꢂ 6.567
(C2ꢀ ꢀ ꢀC2 distance) Å, respectively. The free water molecules,
located in the tunnels, give numerous hydrogen bonds with the
terminal H2O molecules of the skeleton, which point toward the
center of the tunnels. As a comparison, the 3D structure of
[Sm2(fum)3(H2O)4]ꢀ3H2O [13a], containing a different number of
lattice water molecules, originates from a supramolecular isomer.
It comprises of dinuclear entities connected by dicarboxylate an-
ions, rather than containing chains of praseodymium atoms at a
distance of ca. 4.37 Å in complex 1. The structure possesses three
kinds of fum ligands: each has its two COO groups adopting differ-
ent coordination modes. While there are two kinds of fum ligands
and two COO groups of each fum ligand adopting the same
coordination modes in complex 1. Such diversity coordination
b = 15.8570(8) Å, c = 8.6559(5) Å,
a
= 90°, b = 114.4380(10)°,
c
= 90, V = 1149.62(11) Å3, and Z = 2,
l
= 4.29 mmꢁ1
, qcalcd =
2.271 g cmꢁ3. The final wR2 (all data) was 0.0603 and R1 was
0.0333. Experimental details for the structural determination are
described in Table 1. The selected bond distances and bond angles
are given in Tables S1.
Table 1
Crystal data and structure refinement for 1.
Identification code
Empirical formula
Formula weight
1
C
12H24ꢀO21Pr2
786.13
Temperature
187 K
Wavelength
Crystal system, space group
Unit cell dimensions
0.71073 A
P2(1)/c, monoclinic
a = 9.1999(5) Å,
a = 90°
b = 15.857(8) Å, b = 114.438(10)°
c = 8.6559(5) Å,
1149.62 (11)
4, 2.271 Mg/m3
4.286 mmꢁ1
764
c = 90°
Volume
Z, calculated density
Absorption coefficient
F(000)
Crystal size
Theta range for data collection
Limiting indices
Reflections collected/unique
Completeness to theta = 26.02
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2sigma(I)]
R indices (all data)
0.15 ꢂ 0.12 ꢂ 0.11 mm3
2.43–26.02°
11 6 h 6 8, ꢁ19 6 k 6 18, ꢁ10 6 l 6 10
6301/2263 [R(int) = 0.0290]
99.9%
Full-matrix least-squares on F2
2263/12/209
1.050
R1 = 0.0264, wR2 = 0.0579
R1 = 0.0333, wR2 = 0.0603
0.846 and ꢁ0.832e Åꢁ3
Largest diff. peak and hole
Fig. 1. The coordination environments of Pr(III) in complex 1.

162
L. Zhao et al. / Journal of Molecular Structure 979 (2010) 160–164
Scheme 1. Coordination modes of fumarate groups in complex 1.
nated water molecules, and the other three, O(1w), O(2w) and
O(3w) come from the uncoordinated water molecules. They are
self-assembled through hydrogen bonds. The hydrogen bonding
geometry and the water network are shown in Table 2 and
Fig. 3a, respectively. O(1w), O(3w), O(4w), O(5w) forms a tetra-
meric water cluster regarding the short Oꢀ ꢀ ꢀO distances of
2.645–2.861 Å along the ring (Fig. 3b). O(1w) acts as both hydrogen
bond donor and acceptor, connecting with adjacent O(4w)
(O(1w)ꢀ ꢀ ꢀO(4w) 2.809 Å) and O4 (O(1w)ꢀ ꢀ ꢀO4 2.745 Å). O(2w) is
lied on 21 screw axis and the hydrogen atoms of which forms
two kind of C–Oꢀ ꢀ ꢀH hydrogen bonds with C–O of fumarate, the
distances of C–O4ꢀ ꢀ ꢀH–O(2w) and C–O2ꢀ ꢀ ꢀH–O(2w) are 3.062 and
3.276 Å, respectively. O(3w) forms two hydrogen bonds with the
two coordinated water molecules and the distances of
O(3w)ꢀ ꢀ ꢀO(4w) and O(3w)ꢀ ꢀ ꢀO(5w) are 2.839 and 2.861 Å,
respectively.
3.2. Thermal analysis
Thermogravimetric analysis of compound 1 was carried out in
air atmosphere in the range of 35–900 °C. As seen in Fig. 4, the
compound displays mainly two thermal processes. The first weight
loss occurred at ca. 35–240 °C, with a weight loss of 14.5%. This is
the release of the lattice and a part of coordinated water molecules.
The second weight loss process occurred at ca. 400–580 °C, which
due to decomposition of the material. The final residue of 47.8% is a
little high than the calculated 36.9% base on Pr2O3. It can be attrib-
uted that the residue is the admixture of Pr2O3 and PrO2.
3.3. Magnetic measurements
Magnetic susceptibility studies of 1 have been carried out in an
applied magnetic field of 1000 Oe in the temperature range 300–
2 K. The plot of leff vs. T is shown in Fig. 5. At 300 K, the leff value
of 3.58 B.M., which is in fair agreement with the expected value of
Table 2
Hydrogen bonds for 1 [Å and deg].
D–Hꢀ ꢀ ꢀA
d(D–H)
d(Hꢀ ꢀ ꢀA)
d(Dꢀ ꢀ ꢀA)
<(DHA)
OW4–H(6) ꢀ ꢀ ꢀOW3#6
OW4–H(7) ꢀ ꢀ ꢀOW1#1
OW1–H(2) ꢀ ꢀ ꢀO(4)#7
OW5–H(8) ꢀ ꢀ ꢀOW3
OW3–H(4) ꢀ ꢀ ꢀO(2)#8
OW5–H(9) ꢀ ꢀ ꢀOW2
OW3–H(5) ꢀ ꢀ ꢀOW1
OW1–H(1) ꢀ ꢀ ꢀOW3
OW2–H(3) ꢀ ꢀ ꢀO(2)
OW2–H(3) ꢀ ꢀ ꢀO(4)
0.88(2)
0.90(2)
0.88(2)
0.90(2)
0.89(2)
0.89(2)
0.90(2)
0.87(2)
0.88(2)
0.88(2)
2.00(3)
1.85(2)
2.00(3)
1.95(3)
2.57(5)
2.01(3)
2.54(4)
2.57(3)
2.48(5)
2.48(8)
2.861(7)
2.745(5)
2.809(5)
2.839(6)
3.250(7)
2.885(3)
3.402(10)
3.402(10)
3.276(3)
3.062(3)
165(7)
173(7)
152(6)
169(9)
134(6)
164(7)
161(8)
160(6)
151(8)
124(7)
Fig. 2. (a) Polyhedral view of the inorganic chains of complex 1; (b) 2-D network of
1 parallel to the ac plane; (c) view of the structure along c-axis. The pillaring ligand
links the layers between them. The coordinated water molecules point into the
channel. Lattice water molecules are in the channel.
modes of organic ligands induce the synthesis of many novel
structures.
There are five crystallographically independent water mole-
cules in 1. Two of them, O(4w) and O(5w), are from the coordi-
Symmetry transformations used to generate equivalent atoms: (c) #1 x, ꢁy + 1/2,
z ꢁ 1/2; #2 x, ꢁy + 1/2, z + 1/2; #3 x ꢁ 1, y, z ꢁ 1; #4 x + 1, y, z + 1; #5 ꢁx + 1, ꢁy,
ꢁz + 1; #6 ꢁx, y ꢁ 1/2, ꢁz + 1/2; #7 x ꢁ 1, y, z; #8 ꢁx, ꢁy + 1, ꢁz.

L. Zhao et al. / Journal of Molecular Structure 979 (2010) 160–164
163
3.57 B.M. for one uncoupled Pr(III) ions (S = 1, L = 5, 3H4, g = 4/5).
leff gradually decreases until 50 K and then further decreases to
reach a minimum of 0.83 B.M. at 2 K. Indeed, the effects of the ther-
mal depopulation of Pr(III) excited states that result from spin–or-
bit coupling might also be partially responsible for the decrease of
the leff when the temperature is lowered [17]. A fit of the experi-
mental data to a Curie–Weiss law above 2 K leads to the Curie and
Weiss constants of 1.77 cm3 K molꢁ1 and ꢁ18.12 K, respectively.
The negative Weiss (h) constant combined with roughly linearly
shaped isothermal M vs. H plot (see Fig. S1 in the Supporting Infor-
mation), in which M reaches only 0.60 lB at 7 T and 2 K, is clearly
indicative of the presence of some antiferromagnetic interactions
between spin carriers in this complex.
To gain some information concerning the interaction between
Pr(III) ions, as a preliminary treatment, we assumed that owing
to the ligand field effects, Pr(III) ion may exhibit a splitting of mj
2
b
b
energy levels (H ¼
D
Jz ), resulting in a magnetic susceptibility Eq.
(1) previously derived by McPherson and co-workers [18].
2
Ng b2 ½2eðꢁ =kTÞ þ 8eðꢁ4 =kTÞ þ 18eðꢁ9 =kTÞ þ 32eðꢁ16 =kTÞꢃ
D
D
D
D
vPr
¼
ð1Þ
D
=kTÞ
D=kTÞ
D
=kTÞ
D
=kTÞ
kT ½1 þ 2eðꢁ
þ 2eðꢁ4
þ 2eðꢁ9
þ 2eðꢁ16
ꢃ
In this expression,
D is the zero field splitting parameter and N,
g, b and k have their usual meanings. Using the above equation and
considering the molecular field theory with zJ0 as the total ex-
change parameter between Pr(III) ions, we can fit our experimental
data with the following equation.
Fig. 3. (a) The hydrogen bonding network of complex 1. (b) Tetrameric water
clusters formed by the coordinated and uncoordinated water molecules in 1 viewed
along the b-axis.
vPr
v
¼
ð2Þ
1 ꢁ ð2zJ0=Ng2b2ÞvPr
The best fitting of the susceptiꢁb1ility data in the temperature
range 2–300 K gives zJ0 = ꢁ1.60 cm
,
D
= 0.50 cmꢁ1, g = 0.84, and
. This molecular field
P
P
2
R =
ð
vobsd
ꢁ
vcaldÞ = v2obsd = 6.8 ꢂ 10ꢁ3
approximation [19–21] with zJ0 characterizing the total exchange
parameter between the Pr(III) ions, indicates that an overall anti-
ferromagnetic interaction is operative between the lanthanide ions
(since zJ0 < 0), which is probably mediated through carboxylate
groups.
4. Conclusion
In this paper we have presented the synthesis, crystal structure,
and magnetic study of the novel polymeric architectures of praseo-
dymium(III) with dicarboxylate anions. It possesses fumarate
pillared lanthanide–fumarate layers with 1D channels for accom-
modation of guest water. The magnetic investigation reveals weak
antiferromagnetic interactions between metal ions in this
complex.
Fig. 4. TG curve of complex 1.
Acknowledgements
We thank the Changchun Institute of Applied Chemistry (Start-
up) and the National Natural Science Foundation of China
(20871113 and 20921002) for their financial support.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molstruc.2010.06.019.
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