Coordination-directed one-dimensional coordination polymers generated from a new unsymmetrical oxadiazole bridging ligand and transition metal ions

Article abstract of DOI:10.1016/j.inoche.2010.03.032

Three one-dimensional coordination polymers, namely [ZnL ₂(NO₃)₂]n (1), [CdL₂(NO ₃)₂]n (2) and {[ZnL₂ (H₂O) ₂](ClO₂)₂?2L?2(H₂

Full text of DOI:10.1016/j.inoche.2010.03.032

Inorganic Chemistry Communications 13 (2010) 809–813
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Coordination-directed one-dimensional coordination polymers generated from a
new unsymmetrical oxadiazole bridging ligand and transition metal ions
⁎
Zhi-Gang Song, Cui-Hong Geng, Jian-Ping Ma, Yu-Bin Dong
College of Chemistry, Chemical Engineering and Materials Science, Key Lab of Molecular and Nano Probes Engineering Research Center of Pesticide and Medicine Intermediate Clean
Production, Ministry of Education, Shandong Normal University, Jinan, 250014, P.R. China
a r t i c l e i n f o
a b s t r a c t
Article history:
Three one-dimensional coordination polymers, namely [ZnL2(NO3)2]n (1), [CdL2(NO3)2]n (2) and {[ZnL2
Received 12 January 2010
Accepted 18 March 2010
Available online 29 March 2010
(H2O)2](ClO4)2•2L•2(H2O)}n (3), have been successfully synthesized based on
a new unsymmetrical
oxadiazole bridging ligand 2-[2-((3-pyridyl)methoxy)phenyl]-5-[(4-pyridyl)]-1,3,4-oxadiazole (L). Com-
pounds 1–3 feature a similar one-dimensional infinite chain that consists of M2L2 building block (M=Zn(II)
and Cd(II)). In 3, the uncoordinated L ligands are located between the [M2L2]n chains and serve as the agents
to cross-link the chains by weak π–π and H-bonding interactions into a 2D network. In addition, the
luminescent properties of L and 1–3 were primarily investigated in the solid state.
Keywords:
Unsymmetrical oxadiazole ligand
Crystal structures
One-dimensional chain
Photoluminescence
© 2010 Elsevier B.V. All rights reserved.
During the past decades, pronounced interest has been focused on
new coordination polymers due to their interesting structures and
potential applications in sensing, photoluminescence, ion exchange,
separations, gas storage, or catalysis [1,2]. In this context, the design
and synthesis of specific organic ligands are foundational issues,
because they are some of the most important factors in determining
the ultimate structures that are related to their corresponding
physical and chemical properties [3].
Recently, our research group has provided a series of bent rigid and
flexible organic ligands bridged by five-membered heterocycles for
the assembly of polymeric and discrete metal–organic assemblies [4].
As these bent organic spacers possess of variational conformations
(cis, trans, or any intermediary conformations between them), this
alternative ligand-directed approach has resulted in various coordi-
nation metal–organic frameworks with novel patterns not easily
achievable by linear ligands. Moreover, heteroatoms such as N and O
with free electron pairs on the five-membered heterocyclic rings
could be considered as potential active coordination sites and/or
hydrogen bond acceptors to expand the polymeric frameworks with
additional coordinating and/or hydrogen-bonding interactions to
higher dimensionality.
X-ray powder (Fig. 6a) and single-crystal diffractions [6]. In addition,
the photoluminescence properties of 1–3 are primarily investigated in
the solid state.
Single-crystal structure revealed that 1 contains only one
crystallographically independent Zn(II) atom. As shown in Fig. 1,
each Zn(II) center adopts a 4+2 pseudooctahedral {ZnN4O2}
coordination geometry with the equatorial sites occupied by four N-
donors from L; the axial positions are occupied by two monodentate
coordination nitrate anions. The Zn–N distances range from 2.166(3)
Å to 2.197(3) Å, while the axial Zn–O distance is 2.207(3) Å. The Zn–N
and Zn–O bond lengths are comparable to those of related compounds
[7].
The Zn(II) centers in 1 are connected to each other by L to form an
infinite 1D chain that consists of a 30-membered bimetallic macro-
cycle Zn2L2 along the c-axis. In each rectangle-like ring, the Zn(II) Zn
(II) distance is around 13.1 Å (Fig. 2). These 1D chains stack to each
other to generate a 2D sheet via interchain π–π interactions
(dπ–π =∼3.4 Å) along the direction which is perpendicular to the
chains (Fig. 3a). The interchain π–π interactions are resulted from the
parallel stacking of the central oxadiazole rings. The shortest
interchain Zn(II) Zn(II) distance is ∼7.9 Å. Additionally, these π–π
interaction-driven 2D sheets are stack in a -ABAB- fashion and linked
together through interlayer π–π interaction (dπ–π =∼3.73 Å), which is
resulted from the parallel stacking of one of the terminal pyridyl rings
(Fig. 3b), into a 3D network. The shortest interlayer Zn(II) Zn(II)
distance is ∼9.1 Å (Fig. 3b). Notably, no significant H-bonding
interactions have been found in 1.
As an in-depth analysis and part of our systemic investigation of
self-assembly based on the bent ligands of this type, we herein
present three new zinc(II) and cadmium(II) coordination polymers
based on a new unsymmetrical ligand L [5]. All three new compounds
have been fully characterized by IR spectroscopy, elemental analysis,
2 and 1 are isostructural, and they feature the same 1D infinite
chain that consists of M2L2 building block. The Cd–N distances range
⁎
Corresponding author.
E-mail address: yubindong@sdnu.edu.cn (Y.-B. Dong).
1387-7003/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2010.03.032

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Z.-G. Song et al. / Inorganic Chemistry Communications 13 (2010) 809–813
Fig. 1. The coordination environment of Zn(II) atom in 1, drown with 30% probability ellipsoids. Hydrogen atoms are omitted for clarity.
Fig. 2. The 1D double chain of 1 (running along the c-axis).
from 2.335(3) to 2.371(3) Å, while the axial Cd–O distances are 2.363
(5) and 2.371(3) Å. The found Cd–N and Cd–O bond lengths in 2 are
comparable to those of reported compounds. The corresponding
centroid-to-centroid distance between parallel central oxadiazole
rings is ca. 3.32 Å, while the centroid-to-centroid distance between
the parallel pyridyl rings is ca. 3.74 Å [7] (Fig. 4).
Fig. 3. (a) The 2D sheet driven by interchain π–π interactions. (b) The 3D network driven by the interlayer π–π interactions.

Z.-G. Song et al. / Inorganic Chemistry Communications 13 (2010) 809–813
811
Fig. 4. ORTEP figure of 3 (displacement ellipsoids drawn at 30% probability level).
For studying the impact of various counterions on the self-
assembly based on L, the weakly coordinated ClO−4 was used instead
of NO−3 . Crystallization of L with ZnClO4 in the same solvent system at
room temperature produced compound 3. Compared to 1, the Zn(II)
center in 3 adopts a similar distorted octahedral {ZnN4O2} coordina-
tion sphere. The axial positions, however, are occupied by two
coordinated water molecules instead of coordinated anions found in
1. The Zn–N distances (ranging from 2.174(3) Å to 2.218(3) Å) are
slightly longer that those of 1, while the Zn–O distance (2.152(3) Å) is
slightly shorter than that of 1. Again, a similar 1D chain composed of
the Zn2L2-macroring was found in 3 (Fig. 5a). The uncoordinated ClO4−
counterions are located between the chains and hydrogen bonded to
the uncoordinated water molecules (Fig. 5a).
The most important structural feature in 3 is that there are two
(per formula) uncoordinated crystallographically independent L
spacers. It is worthwhile to point out that, in this specific reaction,
the product does not depend on the ligand-to-metal ratio. As shown
in Fig. 5a, these 1D chains are linked together by the interchain
hydrogen-bonding system which is consisting of the coordinated
water molecules, free L ligands and uncoordinated water molecules.
Fig. 5. (a) The hydrogen bonds system between free L, H2O, ClO−4 and coordinated H2O in 3. (b) The π–π interactions between the chains and free L in 3.

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Z.-G. Song et al. / Inorganic Chemistry Communications 13 (2010) 809–813
Table 1
Geometrical parameters of hydrogen bonds in 3.
Atom involved
D–H/Å
H⋅⋅⋅A/Å
D⋅⋅⋅A/Å
Angle of D–H⋅⋅⋅A/°
O(10)–H(2O1)...O(9′)v
O(10)–H(2O1)...O(7)v
O(10)–H(1O1)...N(8)vi
O(5)–H(5A)...N(6)vii
O(5)–H(5B)...O(10)viii
0.84
0.84
0.86
0.87
0.79
2.25
2.04
2.14
1.98
1.93
3.039(12)
2.841(10)
2.945(6)
2.853(4)
2.673(5)
155.7
158.3
155.6
177.5
155.3
Symmetry codes: (i) x,y−1,z; (ii) −x+2, −y+2, −z; (iii) x,y+1,z; (iv) −x+2, −y
+1, −z; (v) −x+1, −y+1, −z+1; (vi) x−1,y,z; (vii) −x+2, −y+1, −z+1; (viii)
x+1,y+1,z.
The corresponding hydrogen bonds parameters for 3 are listed in
Table 1. Furthermore, the interchain π–π interactions (dπ–π =3.7–
3.8 Å) also exist in 3, which is composed of the coordinated and
uncoordinated L ligands. In 3, the free L organic spacers serve as the
effective agents that allow weak π–π stacking and hydrogen-bonding
interactions to expand the dimensionality of 3 from one to two.
It is noteworthy that compounds 1–3 are obtained as pure phase,
which is well confirmed by the X-ray powder diffraction. As shown in
Fig. 6, the XRPD patterns of 1–3 obtained from the bulk crystalline
solid are identical to those of simulated ones based on the single
crystals.
Fig. 7. Solid-state photoinduced emission spectra of L and 1–3 at room temperature.
Acknowledgments
We are grateful for financial support from the National Natural
Science Foundation of China (grant no. 20871076), the Shandong
Natural Science Foundation (grant no. JQ200803), and the Ph.D.
Programs Foundation of the Ministry of Education of China (grant no.
200804450001).
Syntheses of inorganic–organic coordination polymers by the
judicious choice of conjugated organic spacers and transition metal
centers have been proven to be an efficient method for obtaining new
types of luminescent materials [8]. The photoluminescence property
of these three new compounds as well as the free ligand was
examined in the solid state at room temperature. Upon excitation at
λ=348 nm, L exhibits one emission maximum at 409 nm, while 1–3
provide their emission maxima at 457, 429 and 460 nm, respectively
(Fig. 7). The red-shift luminescence of 1–3 originates from ligand-to-
metal-charge transfer (LMCT). Notably, in the cases of 2 and 3, almost
identical emission bands are observed, thus, the different emission
colors of 1–3 might be caused by the different involved metal ions.
We are currently expanding the results presented here by
preparing new unsymmetrical ligands of this type with different
substituted organic functional groups. We anticipate this approach to
be useful for the construction of a variety of new coordination
polymers with interesting fluorescent properties.
Appendix A. Supplementary data
CCDC 760754, 760755 and 760756 contain the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
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C
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