Positional isomeric effect of phenylenediacetate on the construction of mixed-ligand CdII coordination frameworks

Article abstract of DOI:10.1016/j.inoche.2011.01.029

Three Cd^II coordination polymers merging two types of flexible ligands, phenylenediacetate isomers (o/m/p-pda = 1,2/1,3/1,4-phenylenediacetate) and 1,4-bis(imidazol-1-ylmethyl)benzene (bix), have been hydrothermally synthesized and characterized. [Cd(o-pda)(bix)]_n (1) and [Cd(m-pda)(bix)_0.5(H₂O)]_n (2) have 2D coordination networks containing hetero-helical subunits, in which 1 features a (3,5)-connected (4^2.6)(4^2.6^7.8) topology, but 2 shows a (3,4)-connected V₂O₅-type net with (4 ²·6)(4²·6³·8) topology. Different from 1 and 2, [Cd(p-pda)(bix)]_n (3) displays a 5-fold interpenetrating dia framework, indicating structure-directing effect of the positions of acetate groups in pda. Crown Copyright

Full text of DOI:10.1016/j.inoche.2011.01.029

Inorganic Chemistry Communications 14 (2011) 578–583
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Positional isomeric effect of phenylenediacetate on the construction of mixed-ligand
CdII coordination frameworks
⁎
Qiu-Fen He, Dong-Sheng Li , Jun Zhao, Xi-Jun Ke, Cai Li, Yi-Qiang Mou
College of Mechanical & Material Engineering, Functional Materials Research Institute, China Three Gorges University, Yichang 443002, China
a r t i c l e i n f o
a b s t r a c t
Three CdII coordination polymers merging two types of flexible ligands, phenylenediacetate isomers (o/m/p-
pda=1,2/1,3/1,4-phenylenediacetate) and 1,4-bis(imidazol-1-ylmethyl)benzene (bix), have been hydro-
thermally synthesized and characterized. [Cd(o-pda)(bix)]n (1) and [Cd(m-pda)(bix)0.5(H2O)]n (2) have 2D
coordination networks containing hetero-helical subunits, in which 1 features a (3,5)-connected (42.6)
(42.67.8) topology, but 2 shows a (3,4)-connected V2O5-type net with (42·6)(42·63·8) topology. Different
from 1 and 2, [Cd(p-pda)(bix)]n (3) displays a 5-fold interpenetrating dia framework, indicating structure-
directing effect of the positions of acetate groups in pda.
Article history:
Received 15 December 2010
Accepted 25 January 2011
Available online 3 February 2011
Keywords:
Cd(II) complexes
Phenylenediacetate
Isomeric effect
Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.
Luminescent properties
The world-wide interest in metal–organic frameworks (MOFs)
focuses not only on seeking their potential application in gas storage,
ion-exchange, catalysis, luminescence, magnetic and electronic
devices, but also for the intriguing variety of architectures and
topologies [1,2]. As a well known fact, the judicious choice of
appropriate metal ions and tailor-made organic precursors is the
dominating factor to building well-defined coordination frameworks.
In this point, the flexibility of conformational freedom ligands favors
the formation of alluring topological structures [3]. Despite the lack of
predictability, some flexible N/O-donor ligands with multiform
conformations have been used as excellent candidates for rationally
constructing attractive networks. Among them, pyridyl/imidazolyl/
triazolyl ligands with alkyl spacers and flexible polycarboxylate
ligands are outstanding examples [4]. However, the research about
the positional isomeric effect of such flexible ligands on structural
assemblies of coordination networks remains less explored yet [5].
As for the aromatic dicarboxyl ligands, the rigid isomers such as 1,2-,
1,3-, and 1,4-benzene-dicarboxylicate, have been extensively investi-
gated [6]. In contrast, the flexible analogs, especially phenylenediace-
tate isomers with conformationally flexible pendants, have been
seldom used for the construction of coordination polymers [4c,d,5].
Most recently, using phenylenediacetate isomers and flexible dipyridyl-
ligands, we have successfully synthesized a series of ZnII and CdII
complexes, where single or mixed conformations of such two types of
flexible ligands are found [7]. As an extension of our work, herein, we
report three CdII coordination polymers with three phenylenediacetate
isomers (o/m/p-pda=1,2/1,3/1,4-phenylenediacetate) and a flexible
co-ligand 1,4-bis(imidazol-1-ylmethyl) benzene (bix), [Cd(o-pda)
(bix)]n (1), [Cd(m-pda) (bix)0.5(H2O)]n (2), and [Cd(p-pda)(bix)]n
(3). Their corresponding structural patterns vary from (3,5)- or (3,4)-
connected 2D nets to a 5-fold interpenetrating 3D diamond motif,
which demonstrate the positional isomeric effect of phenylenediacetate
isomers on forming such coordination architectures. Further, the
luminescent properties and thermal stabilities of 1–3 were also
discussed.
Crystalline products of 1, 2 and 3 were hydrothermally prepared
by reacting Cd(CH3COO)2·2H2O (0.1 mmol), o/m/p-pda (0.1 mmol),
bix (0.05 mmol), NaOH (8.0 mg, 0.20 mmol) and H2O (10 mL) at
160 °C [8], respectively, and characterized by elemental analysis, IR,
PXRD and single crystal X-ray diffraction [9]. Single crystal X-ray
diffraction reveals that 1 has a double-layer coordination motif. As
illustrated in Fig. 1a, the asymmetric unit of 1 consists of one CdII ion,
one bix ligand, and one fully deprotonated o-pda ligand. Each CdII
center is seven—coordinated by five carboxylate oxygen donors from
three individual μ3-o-pda and two nitrogen atoms from two trans-bix
ligands, taking a pentagonal bipyramid (CdN2O5) geometry. The bond
angles around CdII range from 50.84(5) to 162.29(6)°. The Cd–O
(2.295(9)–2.546(2) Å) and Cd–N (2.281(0)–2.301(8) Å) bond lengths
are in the normal range [10].
Interestingly, the two carboxylate groups of o-pda in 1 are situated
at the two sides of the benzene ring with the trans-configuration (see
Fig. S1a) and a dihedral angle of 10.4°. This kind of distortion between
the separated flexible functional groups is mostly inclined to form
helical structures [5a,7a,d11]. As expected, the o-pda ligands with
chelating carboxylate groups connect the CdII centers into the left-
and right-handed helical subunits that are parallel to the [010]
direction (see Fig. 1b), with the Cd···Cd separation of 9.543 Å (length
of the b axis). Furthermore, two hereto-handed helical arrays are
⁎
Corresponding author. Tel./fax: +86 717 6397516.
E-mail address: lidongsheng1@126.com (D.-S. Li).
1387-7003/$ – see front matter. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.01.029

Q.-F. He et al. / Inorganic Chemistry Communications 14 (2011) 578–583
579
Fig. 1. Views of 1. (a) Coordination environment of CdII (50% probability displacement ellipsoids; H-atoms being omitted for clarity). Symmetry codes: #1=−1+x, 1+y, 1+z;
#2=x, 1+y, z. (b) The left- (plum) and right- (olive) handed helical chains based on o-pda and CdII. (c) 1D double-helical chains (left) containing 4-membered rings and 18-
membered rings (right). (d) 2D double-layer. (e) 3D supramolecular network formed by π···π stacking. (f) Topological view of the 2D network (teal for CdII and olive for o-pda).
bridged via O3 atoms of carboxylates to form a double-helical array,
which contains alternate 4-membered and 18-membered rings with
the Cd∙∙∙Cd distances of 3.970 and 7.111 Å, respectively (see Fig. 1c).
On the other hand, the bidentate bix ligands adopt the trans-
conformation to pillar these double helical chains into a 2D double-
layer (see Fig. 1d), in which the Cd···Cd distance across the bix spacer
is 15.636 Å. The adjacent 2D patterns exhibit a parallel stacking along
[101] in an ABAB fashion (see Fig. 1e), with the presence of π···π
interactions resulting from interlayer imidazole rings of bix (centroid-
to-centroid distance = 3.780 Å and dihedral angle = 21.9°).

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Q.-F. He et al. / Inorganic Chemistry Communications 14 (2011) 578–583
Topologically, each o-pda acts as a 3-connected node, and the CdII ion
can be regarded as a 5-connected node. Thus, the 2D layer shows a
novel (3,5)-connected topology (see Fig. 1f) with the point symbol of
(42.6)(42.63.8), as analyzed by TOPOS program [12].
Complex 2 displays a wave-like 2D coordination layer, and the
asymmetric unit contains one CdII ion, one fully deprotonated m-pda,
one coordinated water molecule, and half a bix ligand. The CdII center
adopts a distorted octahedral geometry, provided by two μ3-m-pda
Fig. 2. Views of 2. (a) Coordination environment of CdII (50% probability displacement ellipsoids; H-atoms being omitted for clarity). Symmetry codes: #1=−x, 1−y, 1−z; #2=1+x, y,
−1+z; #3=1−x, 2−y, −1−z. (b) 1D double-helical chains (left) containing 8-membered rings and 20-membered rings (right). (c) 2D wave-like layer. (d) 3D architecture formed
byO–H···Obonds. (e) Topologicalviewofthe2D V2O5-type net (tealfor CdII and olive for m-pda). (f)Topological viewshowing the3D trinodal (3,5,5)-connected net (tealfor CdII, olive for
o-pda, and plum for aqua ligand).

Q.-F. He et al. / Inorganic Chemistry Communications 14 (2011) 578–583
581
ligands (Cd–O=2.252(2)–2.406(2) Å) as well as one water and one
bix ligands (Cd–O=2.348(2) and Cd–N=2.263(2) Å), as depicted in
Fig. 2a. Similar to o-pda of 1, the m-pda ligand shows the μ3-linking
mode and takes the trans-configuration (see Fig. S1b) but with the
dihedral angle between two carboxylate groups of 60.3°. Therefore,
the o-pda ligands also connect the CdII ions to form double-helical
arrays containing both 8-membered and 20-membered rings, with
the Cd∙∙∙Cd distances of 4.697 and 7.907 Å, respectively (see Fig. 2b).
Compared with 1, due to the presence of water ligands occupying the
coordination sites in 2, the trans-conformation bix ligands alternately
bridge these double chains to result in a 2D wave-like layer, with the
Cd···Cd distance of 14.824 Å separated by bix (see Fig. 2c).
Remarkably, O–H···O H-bonds are observed between the water
ligands (O5W) and carboxylate oxygen atoms (O2 and O4) (for
intralayer: O5W–H···O2, 2.748 Å and 152.7°; for interlayer: O5W–
H···O4, 2.747 Å and 172.2°), which not only stabilize the 2D layers
but also extend them to form a 3D architecture (see Fig. 2d).
Topological analysis reveals that 2 has a (3,4)-connected V2O5-type
coordination net with the point symbol of (42.6)(42.63.8), with m-pda
and CdII serving as 3- and 4-connected nodes, respectively (see
Fig. 2e). Further, if the H-bonds are taken into account, the water
ligands can be regarded as 3-connected nodes, while both CdII and m-
pda2− can be treated as 5-connected nodes. Thus, the overall 3D
network is trinodal (3,5,5)-connected with the point symbol of (3.4.5)
(3.42.5.64.7.8)(3.43.52.62.72) (see Fig. 2f).
Complex 3 has a 5-fold interpenetrating diamond framework, and
the fundamental building unit of 3 is composed of one CdII ion, a
disordered p-pda and an unordered bix ligand. Each CdII ion takes a
highly distorted octahedral sphere, completed by four carboxylate
oxygen atoms (Cd–O1=2.48(4) and Cd–O2=2.293(2) Å) from two
p-pda anions and two imidazole nitrogen atoms from two bix ligands
(Cd–N=2.217(4) Å), as shown in Fig. 3a. Each p-pda adopts the trans-
bis(chelating) binding mode to bridge the CdII centers into 1D infinite
chains along the [001] axis, with the Cd···Cd separation of 12.048 Å.
Further, the trans-conformation bix tectons extend such 1D chains
(Cd···Cd distance across the bix spacer of 15.782 Å) to afford a 3D dia
network(see Fig. 3b). Notably, the large adamantanoid cages in a
single dia network have the maximum dimension of
17.811×35.610×39.477 Å3 (see Fig. 3c), which allows the other
selfsame dia nets to penetrate and thus, affords a 5-fold interpene-
trating dia framework (see Fig. 3d). Analysis of the interpenetrating
fashion in 3 with TOPOS suggests the so-called ‘normal’ type for
diamondoid frames [13], which belongs to class Ia with the
interpenetration vector being equal to the b axis [14].
Structural diversification of 1–3 may mainly arise from two factors:
(i) the phenylenediacetate isomers with different orientations, torsions,
Fig. 3. Views of 3. (a) Coordination environment of CdII (50% probability displacement ellipsoids; H-atoms being omitted for clarity). Symmetry codes: #1=−x, y, 0.5−z; #2=−x,
2−y, −z; #3=0.5−x, −0.5−y, 1−z. (b) 3D network (left) and a single 3D dia net (right). (c) A single dia cage unit. (d) Topological representation of 5-fold interpenetrating
framework (left) and 5-fold interpenetrating nets viewed along the [010] direction (right).

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Q.-F. He et al. / Inorganic Chemistry Communications 14 (2011) 578–583
and binding modes of the carboxylate groups (see Fig. S1); (ii) the
diverse coordination geometries of CdII (7-coordinated in 1 and 6-
coordinated in 2/3). In 1 and 2, the μ3-o/m-pda ligands with trans-
conformation and suitable torsion of the carboxylate groups link the
CdII centers to form double-helical chains, in which the carboxylate
oxygen atoms occupy the equatorial planes of CdII spheres. Therefore,
the exo-bidentate bix ligands extend these chains from the apical
positions CdII spheres to result in different 2D networks in 1 and 2. In
contrast, the trans-bis(chelating) p-pda ligands in 3 act as the linkers to
extend the six-coordinated CdII centers into 1D infinite arrays, and
further, the trans-bix connectors bridge these chains by holding the two
remanent sites of CdII spheres to form a dia framework.
will enrich the current research of isomeric effect of flexible ligands on
constructing coordination polymers and provide new insights into its
application in designing such crystalline materials with desired
structures and properties.
Acknowledgments
This work was financially supported by the NSF of China
(21073106, 20773104).
Appendix A. Supplementary Data
For the PXRD patterns for 1, 2 and 3(see Fig. S2), although the
experimental patterns have a few unindexed diffraction peaks and
some are slightly broadened in comparison with those simulated from
the single-crystal diffraction data, it can still be regarded that the bulk
as-synthesized materials represent the pure phase of 1, 2 and 3.
Thermogravimetric analysis (see Fig. S3) of 2 indicates that the weight
loss between 130 and 180 °C corresponds to the release of
coordination water molecules (obsd 4.26% and calcd 4.08%), and the
anhydrous composition begins to decompose at 289 °C. The TGA
curves of 1 and 3 exhibit similar weight loss stage. There are no lattice/
coordination water molecules therein and thus, decomposition of the
organic components occurs at 295 °C for 1 and 303 °C for 3,
respectively.
CCDC 787445–787447 contain the supplementary crystallographic
data for complexes 1, 2 and 3. 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 section of supporting information.
Supplementary data to this article can be found online at doi:10.1016/
j.inoche.2011.01.029.
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