Two rod-based 3-D lead(II) tetrafluoroterephthalate coordination frameworks with sra topology: Syntheses, structures, and properties

Article abstract of DOI:10.1016/j.inoche.2009.06.029

Two three-dimensional (3-D) Pb^II coordination frameworks [Pb(BDC-F₄)(CH₃OH)]_n (1) and [Pb(BDC-F₄)(DMF)(CH₃OH)]_n (2) have been prepared by the reactions of Pb(NO₃)

Full text of DOI:10.1016/j.inoche.2009.06.029

Inorganic Chemistry Communications 12 (2009) 835–838
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Two rod-based 3-D lead(II) tetrafluoroterephthalate coordination frameworks with
sra topology: Syntheses, structures, and properties
b
a
a
c,
*
Sheng-Chun Chen a, Zhi-Hui Zhang a, Qun Chen a, , Hai-Bo Gao , Qi Liu , Ming-Yang He , Miao Du
*
a Key Laboratory of Fine Petro-chemical Technology, Jiangsu Polytechnic University, Changzhou 213164, PR China
b College of Biological Sciences and Technology, Beijing Forestry University, Beijing 10083, PR China
c College of Chemistry and Life Science, Tianjin Key Laboratory of Structure and Performance for Functional Molecule, Tianjin Normal University, Tianjin 300387, PR China
a r t i c l e i n f o
a b s t r a c t
Two three-dimensional (3-D) PbII coordination frameworks [Pb(BDC-F4)(CH3OH)]n (1) and [Pb(BDC-
F4)(DMF)(CH3OH)]n (2) have been prepared by the reactions of Pb(NO3)2 with a rigid dicarboxyl com-
pound tetrafluoroterephthalic acid (H2BDC-F4) in different solvents. Single crystal X-ray diffraction
reveals that both complexes show the unusual rod-based coordination networks with sra topology,
although they crystallize in different space groups (C2/c and P1). Their spectroscopic, thermal, and fluo-
rescence properties have also been studied.
Article history:
Received 18 May 2009
Accepted 30 June 2009
Available online 3 July 2009
ꢀ
Keywords:
Crystal structure
Metal–organic framework
PbII tetrafluoroterephthalate
Rod-based network
Photoluminescence
Ó 2009 Elsevier B.V. All rights reserved.
The design and construction of inorganic–organic hybrid coor-
dination materials, especially high-dimensional metal–organic
frameworks (MOFs), is currently attracting considerable attention
because of their intriguing structural topologies and tailor-made
applications as functional solids [1–5]. Up to date, a large number
of 3-D coordination frameworks have been prepared and charac-
terized by using a variety of organic aromatic polycarboxylate li-
gands with high symmetry, such as benzene-1,4-dicarboxylate
(BDC) [6], benzene-1,3,5-tricarboxylate (BTC) [7], and benzene-
1,2,4,5-tetracarboxylate (BTEC) [8]. Recent developments have
also shown that the analogues of BDC-type building blocks with
substituents such as methyl [9], chlorine [10], or bromine [11]
groups are also significant to assemble such polymeric frame-
works (for example, the avoidance of net interpenetration) with
unusual thermal and chemical stability as well as stereochemical
characteristics. In this context, tetrafluoroterephthalic acid
(H2BDC-F4) with fluorinated side groups, is a good candidate for
the construction of supramolecular materials with robust net-
work structures and peculiar properties. For instance, Kitagawa
et al. have successfully synthesized a 3-D jungle-gym-like open
framework based on tetrafluoroterephthalic acid [12]. Chen
et al. have reported a 3-D rod-packing Er(III) coordination poly-
mer with a 4,6-connected binodal net topology, and revealed that
the luminescence intensity of near-infrared emissive MOFs can be
significantly enhanced by incorporation of the fluorinated organic
linkers [13]. On the other hand, it is well known that the PbII ion,
bearing a stereochemically active electron lone-pair and large io-
nic radius, can adopt variable coordination number and geometry,
and thus may be used to construct novel coordination frame-
works [14]. Moreover, the known PbII complexes with structural
diversity have shown several potential applications in the fields
of electroluminescence [15], fluorescent sensors [16], and organic
light-emitting diodes [17]. In this contribution, we will present
two 3-D PbII coordination frameworks with the rigid BDC-F4
tecton, i.e. [Pb(BDC-F4)(CH3OH)]n (1) and [Pb(BDC-F4)(DMF)
(CH3OH)]n (2), which exhibit similar rod-based sra type networks.
The spectroscopic, thermal, and fluorescence properties of both
complexes will also be described.
Complexes 1 [18] and 2 [19] were similarly prepared under
general conditions by the reactions of equimolar Pb(NO3)2 and
H2BDC-F4 in CH3OH/H2O and CH3OH/DMF media, respectively.
The IR spectra of 1 and 2 are quite similar due to their comparable
compositions and network structures. The absence of the band
around 1709 cmꢀ1 for the carboxyl group shows that the H2BDC-
F4 ligand is completely deprotonated, which is also confirmed by
the X-ray analysis. The asymmetric and symmetric stretching of
* Corresponding authors. Tel./fax: +86 519 86330251 (Q. Chen), tel.: +86 22
23540241; fax: +86 22 23766556 (M. Du).
E-mail addresses: chenqunjpu@yahoo.com (Q. Chen), dumiao@public.tpt.tj.cn
(M. Du).
1387-7003/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2009.06.029

836
S.-C. Chen et al. / Inorganic Chemistry Communications 12 (2009) 835–838
carboxylate appear in the range of 1640–1590 cmꢀ1 and 1450–
1470 cmꢀ1, respectively, being shifted to higher frequencies com-
pared with that of carboxyl for the free ligand.
X-ray diffraction studies of 1 and 2 [20] reveal that they exhibit
similar rod-based coordination frameworks with sra topology in
despite of their different space groups and solvent ligands (one
additional terminal DMF in 2). Therefore, only the crystal structure
of 1 will be described in detail. In 1, the 3-D coordination frame-
work is built from infinite rod-shaped secondary building units
(SBUs), which are interconnected by the tetrafluophenyl skeletons
of the ligands. The infinite SBU formulated as [–Pb–CH3OH–COO–
(ca. 12.5 ꢁ 6.3 Å2, considering the van der Waals radii of the
channel ‘walls’), as depicted in Fig. 2b. Further analysis of the
crystal packing reveals that each rod is connected to four neigh-
boring rods to form a sra type network topology [24], as illus-
trated in Fig. 2c. In addition, due to the existence of fluorine
groups and solvent ligands, only very small voids are found in
the overall 3-D crystalline lattice (61.8 Å3, 4.1% of the unit cell
volume), as calculated by PLATON [25].
Thermal stability of coordination polymers 1 and 2 has been
investigated by TG-DSC technique and the thermogravimetric
curves are shown in Fig. S1 (see Supplementary material). For 1,
TG analysis shows that the CH3OH molecule is eliminated when
heating from room temperature to ca. 95 °C (calculated: 6.3%;
found: 6.7%) with one endothermic peaking appearing at 85 °C in
DSC. The remaining framework is decomposed with three consec-
utive weight losses starting at 240 °C and not ending at 800 °C. In
the case of 2, the weight loss of 18.6% occurring from room temper-
ature to ca. 280 °C corresponds to the release of methanol and DMF
ligands (calculated: 19.1%). Then, a sharp weight loss occurs in the
temperature range of 280–350 °C, indicating pyrolysis of the or-
ganic components with the corresponding endothermic peak at
329 °C in DSC. Further heating to 800 °C reveals a continuous and
slow weight loss.
Photoluminescent properties of both complexes were studied
in solid sate at room temperature. Complex 2 shows strong fluo-
rescent emission with a maximum at 464 nm (see Fig. 3), upon
the irradiation of ultraviolet light at 338 nm, which may be ten-
tatively assigned to the intraligand transfer, by comparison with
other lumiescent d10 metal coordination polymers [26]. Notably,
the presence of a heavy metal ion such as lead(II) can facilitate
the intersystem crossing to successfully complete the radiation-
less deactivations of the triplet state. However, no obvious
fluorescence was found for 1, probably due to the quenching
effect of high-energy C–H and/or O–H oscillators from the coor-
dinated MeOH solvent [13]. The emission in the blue region
indicates that complex 2 may serve as a potential blue-light-
emitting material.
]
consists of PbII ions, bridging carboxylate groups and terminal
1
methanol solvents. Each PbII center is eight-coordinated by two
bidentate chelating and two monodentate carboxylate groups from
four BDC-F4 ligands as well as two terminal methanol ligands,
adopting an approximately dodecahedral sphere (see Fig. 1a). Each
BDC-F4 takes a
l
4-bridging fashion to connect four adjacent PbII
ions to constitute a 3-D network. Notably, the BDC-F4 ligand in 1
possesses a familiar bis-tridentate bridging mode, however, the
coordination fashions of BDC-F4 in other known complexes are
bis-monodentate bridging [21], bis-bidentate bridging [22], and
tridentate-bidentate chelating/bridging fashions [13], respectively.
Within the BDC-F4 ligand, the tetrafluorinated phenyl ring is in-
clined to the carboxylate group with a dihedral angle of 50.1(2)°,
which can be attributed to the repulsion between fluorine and
carboxylate.
The detail for the 8-coordinated geometry of PbII is described
as follows. Firstly, two symmetry-related carboxylate groups from
BDC-F4 coordinate to PbII in a bidentate chelating mode. The Pb–O
lengths (2.610(3)–2.651(3) Å) are close to those in {[Pb(BDC)
(C2H5OH)](C2H5OH)}n (2.485(3)–2.682(3) Å) [23]. Secondly, the
bridging Pb–O distance [2.602(4) Å] is slightly shorter than the
chelating ones. Thirdly, each PbII ion is coordinated by two cen-
trosymmetric terminal methanol molecules with the Pb–O
distance of 2.727(4) Å, which is obviously longer than those of
the Pb–Ocarboxylate bonds and indicates some pendant oxygen
characteristics. With such a complicated connectivity, these dis-
torted dodecahedra are connected to afford the infinite rod-
shaped SBUs in corner-sharing and edge-sharing fashion (see
Fig. 2a). These rods are distributed in parallel along the [0 0 1]
direction and interconnected by the –C6F4– backbones of BDC-F4
to afford the resulting 3-D network with 1-D rhombic channels
In summary, two lead(II) metal–organic frameworks have been
synthesized by the reactions of lead(II) nitrate and tetrafluorote-
rephthalic acid under different solvent conditions. Both complexes
have similar Pb–O–C rod-shaped secondary building units, which
are further extended by the backbones of BDC-F4 connectors to re-
Fig. 1. A portion view showing the coordination environment of PbII and binding mode of BDC-F4 in 1. Symmetry codes: (A) ꢀx + 3/2, ꢀy + 1/2, ꢀz + 1; (B) ꢀx + 2, ꢀy, ꢀz + 1;
(C) ꢀx + 2, y, ꢀz + 3/2; (D) x, ꢀy, z + 1/2; (E) x + 1/2, y ꢀ 1/2, z.

S.-C. Chen et al. / Inorganic Chemistry Communications 12 (2009) 835–838
837
Fig. 3. Solid-state emission and excitation (inset) spectra of complex 2.
thank Prof. Jianping Lang (Suzhou University) for helpful
discussions.
Appendix A. Supplementary material
CCDC 727775 and 727776 contain the supplementary crystallo-
graphic data for complexes 1 and 2. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data
associated with this article can be found, in the online version, at
doi:10.1016/j.inoche.2009.06.029.
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