Two-dimensional CuII and first PbII coordination polymers based on a flexible 1,4-cyclohexanedicarboxylate ligand displaying different conformations and coordination modes

Article abstract of DOI:10.1016/j.ica.2005.07.006

Two novel metal-organic hybrid coordination polymers {[Cu(bpo)(chdc)(H ₂O)](H₂O)_0.5}_n (1) and [Pb(chdc)(H₂O)]_n (2) have been synthesized under different conditions and structurally characterized by single-crystal X-ray diffraction technique, where H₂chdc refers to a flexible 1,4- cyclohexanedicarboxylic acid ligand and bpo is 2,5-bis(4-pyridyl)-1,3,4- oxadiazole. Complex 1 has a two-dimensional (2-D) grid-like [11.28 × 13.63 A?²] framework in which the Cu^II centers are extended via bidentate bridging ligands bpo and e,e-trans-chdc along two directions, exhibiting large porous cavities. Coordination polymer 2 represents the first Pb^II complex of H₂chdc in which the larger Pb^II centers are connected by e,a-cis-chdc anions to afford a 2-D close-knit structure.

Full text of DOI:10.1016/j.ica.2005.07.006

Inorganica Chimica Acta 358 (2005) 4034–4038
www.elsevier.com/locate/ica
Note
Two-dimensional CuII and first PbII coordination polymers based
on a flexible 1,4-cyclohexanedicarboxylate ligand displaying
different conformations and coordination modes
*
Miao Du , Hua Cai, Xiao-Jun Zhao
College of Chemistry and Life Science, Tianjin Normal University, Tianjin 300074, PR China
Received 16 January 2005; received in revised form 17 June 2005; accepted 5 July 2005
Available online 10 August 2005
Abstract
Two novel metal–organic hybrid coordination polymers {[Cu(bpo)(chdc)(H2O)](H2O)0.5
}
(1) and [Pb(chdc)(H2O)]n (2) have
n
been synthesized under different conditions and structurally characterized by single-crystal X-ray diffraction technique, where
H2chdc refers to a flexible 1,4-cyclohexanedicarboxylic acid ligand and bpo is 2,5-bis(4-pyridyl)-1,3,4-oxadiazole. Complex 1 has
2
a two-dimensional (2-D) grid-like [11.28 · 13.63 A ] framework in which the CuII centers are extended via bidentate bridging ligands
˚
bpo and e,e-trans-chdc along two directions, exhibiting large porous cavities. Coordination polymer 2 represents the first PbII com-
plex of H2chdc in which the larger PbII centers are connected by e,a-cis-chdc anions to afford a 2-D close-knit structure.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Crystal structure; Metal–organic framework; Flexible ligand; Conformation; Copper (II) and lead (II) complexes
1. Introduction
three-dimensional, 3-D) coordination frameworks [4–
9]. Of further interests, only a few coordination poly-
mers based on flexible ligands have been documented
so far probably due to their low symmetry and the flex-
ibility of the backbones, which make them more difficult
to forecast and control the final coordination networks
[1–3]. 1,4-Cyclohexanedicarboxylic acid (H2chdc) is a
flexible diacid ligand that usually adopts chair-type
backbone but with different cis- or trans-conformations
(see Scheme 1) and varied coordination modes of car-
boxylate groups upon metal complexation under appro-
priate conditions [10–13]. Very recently, a well written
work [14] on the successful separation of cis- and
trans-conformations of H2chdc within a CdII/H2chdc/
1,10-phenanthroline reaction system, predominantly
relying on the temperature and pH value, has been re-
ported. Thus, we anticipate that the incorporation of
both distinctive dicarboxylic- and dipyridyl-type bridg-
ing ligands would lead to the formation of interesting
coordination frameworks when binding to the metal
The rapid development in metal–organic hybrid coor-
dination polymers with fascinating structural topologies
and functions has received considerable attention [1,2].
The rational design via self-assembly driven by coordi-
nation and/or hydrogen-bonded force has been proven
to be an effective and facile synthetic strategy in this
realm [3], which depends on a variety of parameters,
basically including the suitable pre-designed organic li-
gands (building blocks) and metal centers with versatile
coordination geometries (nodes). Recently, we have in-
tensely devoted to the self-assemblies based on the unu-
sual angular dipyridyl ligands containing oxadiazole
spacer, typically, 2,5-bis(4-pyridyl)-1,3,4-oxadiazole
(bpo, Scheme 1), which could potentially provide both
discrete and divergent (from one-dimensional, 1-D to
*
Corresponding author. Tel./fax: +86 22 23540315.
E-mail address: dumiao@public.tpt.tj.cn (M. Du).
0020-1693/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2005.07.006

M. Du et al. / Inorganica Chimica Acta 358 (2005) 4034–4038
4035
M
M
N
N
bpo
O
N
N
H
COOH
H
COOH
HOOC
HOOC
H
H
e,e-trans-H2chdc
e,a-cis- H2chdc
Scheme 1.
ions. In this contribution, we will report our recent re-
sults on the preparation and structural analysis of two
new two-dimensional (2-D) coordination polymers,
49.53; H, 4.36; N, 11.55%. Found: C, 49.34; H, 4.78;
N, 11.71%.
{[Cu(bpo)(chdc)(H2O)](H2O)0.5 n
}
(1) possessing large
2.2.2. [Pb(chdc)(H2O)]n (2)
porous cavities and [Pb(chdc)(H2O)]n (2) representing
A mixture of Pb(NO3)2 (33 mg, 0.1 mmol), H2chdc
(34 mg, 0.2 mmol) and bpo (22 mg, 0.1 mmol) in water
(10 mL) was heated at 140 ꢁC for three days in a sealed
Teflon-lined stainless steel vessel (20 mL) under autoge-
nous pressure. After the reaction system was slowly
cooled down to the room temperature at a rate of 5
ꢁC/h, colorless prism crystals of 2 were obtained in
75% yield (based on Pb(NO3)2 salt). IR (KBr, cmꢀ1):
3433 b, 2932 s, 2856 m, 1528 vs, 1415 vs, 1300 m,
1215 w, 1146 w, 1036 w, 929 m, 847 w, 774 m, 733 m,
652 m, 587 m, 531 m, 469 m. Anal. Calc. for C8H12PbO5:
C, 24.30; H, 3.06%. Found: C, 24.21; H, 2.89%.
the first PbII complex with H2chdc.
2. Experimental
2.1. Materials and general methods
With the exception of the ligand 2,5-bis(4-pyridyl)-
1,3,4-oxadiazole (bpo), which was synthesized as de-
scribed in our previous publications [4–9], all of the
starting materials and solvents were obtained commer-
cially and used as received without further purification.
The raw flexible ligand H2chdc is a mixture of cis- and
trans-conformations in a ratio of 3:2. Fourier transform
(FT)-IR spectra (KBr pellets) were taken on an AVA-
TAR-370 (Nicolet) spectrometer. Elemental (carbon,
hydrogen and nitrogen) analyses were performed on a
CE-440 (Leemanlabs) analyzer.
2.3. X-ray crystallographic data collection and structural
determination
Single crystal X-ray diffraction data collections of
complexes 1 (0.23 · 0.12 · 0.06 mm) and 2 (0.20 · 0.16 ·
0.11 mm) were performed on a Bruker Apex II CCD
diffractometer at 293(2) K with Mo Ka radiation
˚
2.2. Syntheses of the complexes
(k = 0.71073 A). A total of 17253 (for 1) and 2758 (for
2) reflections were collected, of which 5394 (for 1) and
953 (for 2) are independent. Both structures were solved
by direct methods and refined by full-matrix least-
squares with SHELXTL package (semi-empirical absorp-
tion corrections were applied using SADABS program).
Metal atoms in each structure were located from the
E-maps and the other non-hydrogen atoms were located
in successive difference Fourier syntheses. The final
refinement was performed by full-matrix least-squares
methods with anisotropic thermal parameters for non-
hydrogen atoms on F2. H atoms of organic ligands were
generated with assigned isotropic thermal parameters,
and those of coordinated water were first found in
difference electron density maps, and then placed in
2.2.1. {[Cu(bpo)(chdc)(H2O)](H2O)0.5}n (1)
A solution of H2chdc (29 mg, 0.17 mmol) in methanol
(15 mL) was added to an aqueous solution (10 mL) of
Cu(OAc)2 Æ H2O (43 mg, 0.2 mmol). After ca. 5 min of
vigorous mixing, a methanol solution (5 mL) of bpo
(22 mg, 0.1 mmol) was added. The mixture was stirred
thoroughly for 30 min and the resultant filtrate was al-
lowed to evaporate slowly under ambient conditions.
Lamellar blue single crystals of 1 suitable for X-ray
analysis were obtained after 10 days in 50% yield (based
on bpo). IR (KBr, cmꢀ1): 3444 b, 1697 s, 1650 s, 1620 m,
1560 vs, 1539 vs, 1512 s, 1420 m, 1336 w, 1223 w, 839 m,
716 m, 670 m. Anal. Calc. for C20H21CuN4O6.5: C,

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M. Du et al. / Inorganica Chimica Acta 358 (2005) 4034–4038
the calculated sites and included in the final refinement.
H atoms of disordered guest water molecules in 1 were
not located.
2.3.1. Crystallographic data for C20H21CuN4O6.5 (1)
Mr = 484.95, monoclinic, space group C2/c (No. 15),
˚
a = 22.670(8), b = 27.223(10), c = 10.787(4) A, b =
3
ꢀ3
˚
108.674(5)ꢁ, V = 6307(4) A , Z = 8, Dc = 1.021 g cm
,
l = 0.725 mmꢀ1
,
F(0 0 0) = 2000, S = 0.991, R =
0.0688 and wR = 0.1464.
2.3.2. Crystallographic data for C8H12PbO5 (2)
Mr = 395.37, monoclinic, P21/m (No. 11), a =
˚
7.971(2), b = 7.204(2), c = 8.664(3) A, b = 94.956(3)ꢁ,
3
V = 495.6(2) A , Z = 2, Dc = 2.649 g cmꢀ3, l = 17.013
˚
mmꢀ1, F(0 0 0) = 364, S = 0.990, R = 0.0366 and wR =
0.0981.
3. Results and discussion
Complex 1 was synthesized by the reaction of CuII
salt and mixed chdc/bpo ligands at room temperature.
Single crystal X-ray diffraction analysis shows that 1
has a 2-D neutral coordination network with large por-
ous cavities. As shown in Fig. 1(a), each CuII center is
coordinated to two N atoms from distinct bpo mole-
cules, two O atoms awarded by the monodentate car-
boxylate groups of two anionic chdc ligands with the
most stable e,e-trans-conformational isomer and one ax-
ial water ligand. Hence, the coordination polyhedron
around the CuII center could be best described as a
slightly distorted square-based pyramid (see archived
CIF file for detailed bond geometries), with a s para-
meter of 0.11 (s = 0 for an ideal square pyramidal geo-
metry and s = 1 for an ideal trigonal bipyramidal) [15].
Fig. 1(b) shows the 2-D lamellar network of 1. Along
the crystallographic b-axis, the adjacent CuII centers are
linked via the bridging bpo ligands in trans arrangement
to form 1-D chains. The chdc anions further extend
these chains along the a direction via two unidentate
carboxylate groups, resulting in the final 2-D grid-type
2
˚
layer, with the dimensions of 13.63 · 11.28 A for each
repeating unit (see Fig. S1). Analysis of the crystal pack-
ing of 1 reveals that these 2-D layers adopt the closely
stacking mode (see Fig. S2) with an A(–A)A(–A) se-
quence along the [0 0 1] direction, creating large quad-
rate cavities, as depicted in Fig. 1(c). Although this
packing mode clearly reduces the size of the porous
channels compared with the parallel overlap-fashion, a
computation [16] of the voids of the host networks of
Fig. 1. (a) A portion view of 1 showing the coordination sphere of CuII
and conformation of chdc with atom labeling of the asymmetric unit.
(b) 2-D grid-like network structure of 1. Hydrogen atoms are omitted
for clarity. (c) Space-filling model of the 3-D packing diagram of 1
showing the large porous cavities.
3
˚
1 still shows a value of 2805 A (44.5% of the unit cell).
Allowing for the volume of the included water mole-
3
˚
cules, there still remains 2319 A of void space, corre-
sponding to 36.8% of the unit-cell volume. The porous

M. Du et al. / Inorganica Chimica Acta 358 (2005) 4034–4038
4037
nature of 1 can also be reflected from its rather low crys-
tal density (1.021 g cmꢀ3). Additionally, this structure is
further stabilized by the interlayer O6–H6Aꢁ ꢁ ꢁO4i and
O6–H6Bꢁ ꢁ ꢁO3i (i = x, ꢀy, z ꢀ 1/2) hydrogen bonds
order to further explore the potential coordination pos-
sibility of chdc, the first PbII–chdc complex [Pb(chdc)-
(H2O)]n (2) was prepared under hydrothermal condition
and structurally determined by X-ray diffraction tech-
nique. Although the starting ligand bpo is not involved
in the final product, it is a necessary element to afford
the coordination polymer 2. As shown in Fig. 2(a), the
local crystallographic unit, with a mirror symmetry
passing through C5–C1–C4–C6, comprises one PbII
center, a doubly deprotonated chdc anionic ligand and
a water ligand. The PbII ion is hepta-coordinated to
six carboxylate O atoms of chdc with e,a-cis-conforma-
tion and one water molecule, adopting an unusual
distorted capped trigonal prismatic geometry. The
Pb–O bond distances are in the range of 2.463(7)–
˚
(bond geometries: Hꢁ ꢁ ꢁO, 1.870 and 1.927 A; Oꢁ ꢁ ꢁO,
˚
2.699 and 2.761 A; O–Hꢁ ꢁ ꢁO, 163.8 and 168.5ꢁ), between
each coordinated water molecule and two uncoordi-
nated carboxylate oxygen atoms of chdc from the adja-
cent 2-D layer.
PbII is a favorable component for the construction of
unusual coordination polymers or polynuclear com-
plexes due to its large ion radius, a variety of coordina-
tion geometries and the possible occurrence of a
stereochemically active lone pair of electrons [17]. In
˚
2.673(7) A (see archived CIF file for detailed bond
geometries). Each chdc synchronously binds to four PbII
atoms, and two different coordination modes of the car-
boxylate groups, including symmetric bidentate chelate
for O1–C5–O1i and l-O,O-g-O,O0-l-O0,O for O2–C6–
O2i (i = x, ꢀy + 3/2, z), are observed. As a consequence,
the PbII centers are connected by chdc ligands, leading
to the formation of a 2-D close-knit network as illus-
trated in Fig. 2(b). Another alternative description,
perhaps more revealing from the structural viewpoint
of supramolecular network, is to consider as follows.
The neighboring PbII centers (Pbꢁ ꢁ ꢁPb separation of
˚
8.664(3) A) are linked by chdc with two carboxylate
groups (symmetric bidentate chelate modes) along the
[0 0 1] direction to form 1-D zigzag chains, which are
connected to afford a 2-D net via further Pb–O coordi-
nation interactions along the [0 1 0] direction. In the
resulting Pb2O2 dimeric subunit, the Pbꢁ ꢁ ꢁPb distance
˚
is 4.152(2) A and the Pb–O–Pb bridging angle is
107.8(3)ꢁ. Investigation of the crystal packing of 2
suggests that these 2-D layers are in parallel stacking
fashion (see Fig. 2(c) and Fig. S3) and there are no
solvent-accessible voids in the crystal lattice.
4. Conclusion and remarks
As illustrated in the previous study [13,14], the flexi-
ble backbone of chdc may favor e,e-trans-conformation
at higher temperature and pH value. However, in this
work, we can conclude that chdc takes the e,e-trans-
conformation in the 2-D open porous network 1 and
e,a-cis-conformation in the 2-D close-knit architecture
2, although the former was prepared at room tempera-
ture and the latter under hydrothermal condition at
140 ꢁC. Furthermore, the rather diverse coordination
modes of the carboxylate groups of chdc are observed
in both cases. These differences may result from the nat-
ure of the PbII center, such as the larger radius and thus
higher coordination numbers compared with that of
CuII, which could affect the conformation and binding
Fig. 2. (a) A portion view of 2 showing the coordination sphere of PbII
and conformation/coordination modes of chdc with atom labeling of
the asymmetric unit. (b) 2-D close-knit layered structure of 2.
Hydrogen atoms and water ligands are omitted for clarity. (c) 3-D
parallel packing diagram of 2 viewing from the [0 0 1] direction.

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M. Du et al. / Inorganica Chimica Acta 358 (2005) 4034–4038
modes of chdc in the resulting coordination polymers.
This work may provide helpful information for con-
structing other relative metal–organic coordination
frameworks and further investigations on such an inter-
esting system are still in progress in our laboratory.
associated with this article can be found, in the online
version at doi:10.1016/j.ica.2005.07.006.
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Science Foundation (No. 043804111) and the Starting
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Crystallographic data (excluding structure factors)
have been deposited with the Cambridge Crystallo-
graphic Data Center (CCDC Nos. 257634 and
257635). Copies of the data can be obtained free of
charge on application to The Director, CCDC, 12 Un-
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Three color figures including (Fig. S1) Space-filling
model of the 2-D open network in 1, (Fig. S2) Space-filling
model of 3-D packing diagram of 1 with an A(–A)A
sequence, and (Fig. S3) 3-D parallel packing diagram of
2 viewing from the [0 1 0] direction. Supplementary data
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