Synthesis, structure, and photoluminescent property of a trinuclear Cd II complex based on semi-rigid bis(imidazole-4,5-dicarboxylate) ligand

Article abstract of DOI:10.1016/j.inoche.2014.01.004

A new 3D complex, {[Cd₃(HL)₂(H₂L)(H ₂O)₄]·2H₂O}_n (1) (H ₄L = 1,1′-(1,4-phenylenebis(methylene))bis(1H-imidazole-4,5- dicarboxylic acid), has been hydrothermally synthesized and characterized by elemental analyses, IR, TG, and X-ray single-crystal diffraction. Complex 1 is the first framework based on Cd^II ion and semi-rigid bis-IDC ligand. The (6,6)-net architecture of 1 is built from H₂L^{2 -} ligands linking 1D chain-like [Cd₃(HL)₂]_n SBUs, which formed by HL^{3 -} anions bridging two kinds of metal centers. In addition, complex 1 was demonstrated to display strong blue-violet fluorescence emission in the solid state at room temperature.

Full text of DOI:10.1016/j.inoche.2014.01.004

Inorganic Chemistry Communications 42 (2014) 15–19
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis, structure, and photoluminescent property of a trinuclear CdII
complex based on semi-rigid bis(imidazole-4,5-dicarboxylate) ligand
b
a
a
b,
Gang Yuan a, , Kui-Zhan Shao , Xiang-Rong Hao , Ya-Ru Pan , Zhong-Min Su
⁎
⁎
a
Faculty of Chemistry, Tonghua Normal College, Tonghua 134002, China
Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal University, Changchun 130024, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A new 3D complex, {[Cd3(HL)2(H2L)(H2O)4]·2H2O}n (1) (H4L = 1,1′-(1,4-phenylenebis(methylene))bis(1H-
imidazole-4,5-dicarboxylic acid), has been hydrothermally synthesized and characterized by elemental analyses,
IR, TG, and X-ray single-crystal diffraction. Complex 1 is the first framework based on CdII ion and semi-rigid
bis-IDC ligand. The (6,6)-net architecture of 1 is built from H2L2− ligands linking 1D chain-like [Cd3(HL)2]n
SBUs, which formed by HL3− anions bridging two kinds of metal centers. In addition, complex 1 was
demonstrated to display strong blue-violet fluorescence emission in the solid state at room temperature.
Crown Copyright © 2014 Published by Elsevier B.V. All rights reserved.
Received 3 September 2013
Accepted 5 January 2014
Available online 11 January 2014
Keywords:
Hydrothermal reaction
CdII complex
Bis(imidazole-4,5-dicarboxylate) ligand
SBUs
Topology
The design and synthesis of coordination polymers (CPs), often re-
ferred to as metal–organic frameworks (MOFs), have been extensively
studied not only stemming from their appealing structural and topolog-
ical novelty but also owing to their tremendous potential applications in
microelectronics, fluorescence, nonlinear optics, magnetism, porous
materials and heterogeneous catalysis [1–4]. It is well known that the
self-assembly of CPs is highly influenced by the structural character of
the ligands, coordination trend of the metal ions, and other factors
such as the temperature, templates, counterions, pH value of the
solution, and solvent system [5,6]. Among them, tactical synthesis
or selection of the organic ligand and controlling reaction condition
are key factors for achieving expected CPs [7]. The syntheses
of new N-heterocyclic carboxylic acid ligands are a long-standing
fascination of chemists, and so far, many pyridine-based, imidazole-
based, pyrazine-based and triazole-based carboxylic acids have been
reported, but the study of their analog and derivative ligands is still
underdeveloped [8,9].
In recent years, imidazole-4,5-dicarboxylic acid (H3IDC) and its
2-position substituted analog ligands have attracted upsurging research
interest owing to the excellent performance on the construction of
nanostructures and MOFs [10]. However, the derivate ligands of H3IDC
with two IDC groups remain largely unexplored. Prompted by this
interesting finding, we design and synthesis a new H3IDC derivative
ligand, namely 1,1′-(1,4-phenylenebis(methylene))bis(1H-imidazole-
4,5-dicarboxylic acid) (H4L), by using 1,4-dibenzyl group linking two
identical H3IDC molecules [11]. We are interested in the use of H4L as
organic linker in combination with CdII ion adopting d10 configuration
to construct new CPs based on the following considerations: (i) H4L
and its anions possess multiple potential coordination sites from two
IDC groups and can be partially or fully deprotonated at different
pH values, therefore, providing various acid-base type coordination
modes; (ii) the oxygen-rich and nitrogen-rich backbone of H4L may
help in constructing supramolecular networks via hydrogen bonds
and π–π aromatic interactions; and (iii) H4L has bigger size and two
IDC groups can freely twist around the –CH2– spacers to meet the
requirements of the coordination geometries in the assembly process.
In this work, we reported the synthesis and characterization of a new
trinuclear d10 metal polymer, {[Cd3(HL)2(H2L)(H2O)4]·2H2O}n, which
exhibits 3D 6-connected framework assembly of bis-deprotonated
ligands linking 1D chain-like SBUs by the employment of H4L.
Crystalline products of 1 was hydrothermally synthesized by
reacting Cd(NO3)2·4H2O (0.1 mmol), H4L (0.05 mmol), dimethylamine
hydrochloride (0.2 mmol) and H2O (10 mL) at 150 °C [12], and char-
acterized by elemental analysis, IR and single crystal X-ray diffrac-
tion, and the phase purity of the bulk sample was identified by
powder X-ray diffraction (PXRD) (Fig. S1).
The single crystal X-ray analysis [13] indicates that complex 1
crystallizes in the triclinic system space group P − 1 and the asym-
metrical unit of 1 contains two crystallography independent CdII
ions, one HL3− ligand, a half of H2L2− ligand, two coordination
water molecules and a lattice water molecule. As shown in Fig. 1a,
Cd1 ion lies at an inversion center and is six-coordinated by two ni-
trogen atoms (Cd1–N5, 2.273(4) Å; Cd1–N5A, 2.273(4) Å) and two
oxygen atoms (Cd1–O9, 2.408(4) Å; Cd1–O9A, 2.408(4) Å) from
two separate H2L2− anions, and two oxygen atoms (Cd1–O4,
2.272(4) Å; Cd1–O4A, 2.272(4) Å) from two isolated HL3− anions
in a slightly distorted octahedral environment. Different from Cd1,
⁎
Corresponding authors. Tel.: +86 431 85099108.
E-mail address: zmsu@nenu.edu.cn (Z.-M. Su).
1387-7003/$ – see front matter. Crown Copyright © 2014 Published by Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.inoche.2014.01.004

16
G. Yuan et al. / Inorganic Chemistry Communications 42 (2014) 15–19
Fig. 1. (a) View of the coordination environment of CdII ions in 1 with thermal ellipsoids drawn at the 50% probability level. All hydrogen atoms are omitted for clarity. Symmetry
codes: A, −x, −y, −z; B, −x + 1, −y + 1, −z; C, −x + 1, −y, −z; D, −x + 2, −y, −z + 1. (b) The 1D chain formed by HL3− bridging two different metal centers. (c) View of
four 1D chain-like SBUs connected by the H2L2− anion.
Cd2 adopts a pentagonal bipyramid coordination geometry and
surrounded by two nitrogen atoms (Cd2–N1, 2.278(4) Å; Cd2–N4D,
2.297(4) Å) and two oxygen atoms (Cd2–O1, 2.491(4) Å; Cd2–O8D,
2.461(4) Å) from two individual HL3− ligands, and one oxygen atom
(Cd2–O9C, 2.408(4) Å) from one H2L2− ligand as well as two terminal
water molecules (Cd2–O1W, 2.293(4) Å; Cd2–O2W, 2.633(5) Å).
In complex 1, the H4L ligands were deprotonated into HL3− and
H2L2− fashions in the presence of dimethylamine, respectively, and
they exhibit two different types of coordination modes, as depicted
in Scheme 1. One acts as a tridentate connector adopting μ3-kN,O: kN′,
O′: kO″ coordination mode linking three CdII ions (Scheme 1a) in bis-
N,O-chelating and O-monodentate fashions, in which the dihedral
angle between two imidazole rings and the benzene ring are 75.86°
and 76.30°. The other serves as tetradentate ligand bridging four CdII
centers and displays a μ4-kN,O: kN′,O′: kO: kO′ coordination mode
via bis-N,O-chelating and bis-O-monodentate fashions with the
dihedral angle of 83.10° between the aromatic rings. It is worth not-
ing that all carboxylate groups and the attaching aromatic rings are
almost coplanar in two different coordination modes.
Interestingly, two adjacent Cd2 metal centers were interconnected
by two HL3− ligands with the cis–gauche conformation leading to the
formation of a Cd2(HL)2 ring, in which the Cd2⋯Cd2 separation is
9.650 Å. The neighboring Cd2(HL)2 rings link with each other by sharing
the Cd1 nodes giving rise to a one-dimensional (1D) infinite [Cd3(HL)2]n
chain with the shortest Cd1⋯Cd1 distance of 21.169 Å (Fig. 1b). Each 1D
[Cd3(HL)2]n chain can be considered as a secondary building unit (SBU).
The four identical chain-like [Cd3(HL)2]n SBUs are further connected by
the H2L2− anion with trans conformation (Fig. 1c) resulting to a three-
dimensional (3D) co-valent framework (Fig. 2). Although a large num-
ber of complexes based on the new designed N-heterocyclic carboxylic
acid ligands have been reported. To the best of our knowledge, such
complex constructed from semi-rigid H3IDC derivative ligand bear-
ing bis-IDC groups have not been observed in coordination polymers.
To further understand the structure of 1, topological analysis by
reducing multidimensional structure to simple node-and-linker net
was performed. Three CdII ions were μ2-bridged by two caboxylate
oxygen atoms from two H2L2− ligands to form a trinuclear cluster.
Each trinuclear cluster is surrounded by six trinuclear clusters and can
be considered as a six-connected node (Fig. 3a). On the basis of the sim-
plification above, complex 1 possesses a classical non-interpenetrating
6-connected a-Po net with the Schläfli symbol of (412 · 63) based
on the trinuclear cadmium nodes, which is assigned to the pcu net
(Fig. 3b).
The thermal stability of 1 has been examined by using thermogra-
vimetric (TG) analyzer from 40 to 1000 °C under flowing nitrogen at
a heating rate of 10 °C/min. From the TG curve (Fig. S2), 1 shows a
three-step weight loss. The first weight loss of 2.05% (cal. 2.14%) is
assigned to the liberation of two lattice water molecules which oc-
curred in the range of approximate 40–70 °C. The second weight
loss of 4.12% (cal. 4.28%) between 100 and 150 °C is ascribed to the
Scheme 1. Coordination modes of the HL3− and H2L2− ligands in complex 1.

G. Yuan et al. / Inorganic Chemistry Communications 42 (2014) 15–19
17
Fig. 2. Polyhedral view of the 3D framework of complex 1.
loss of four coordination water molecules. The last loss of 70.03% (cal.
70.66%) occurs from 315 to 810 °C, which corresponds to the com-
plete decomposition of the samples. After the decomposition, the
final product may be CdO.
all [16], suggesting that the emission band of complex can be attributed
to ligand-to-ligand charge transfer (LLCT). The enhancement in lumi-
nescence may be ascribed to an increase in the conformational rigidity
of the H4L ligand due to their coordination to the CdII ion, resulting in
a decrease in the nonradiative decay of the intraligand excited states.
In conclusion, an interesting 3D CdII complex, {[Cd3(HL)2(H2L)
(H2O)4]·2H2O}n, has been successfully synthesized and structurally
characterized, which is constructed by two different configuration
ligands linking metal centers. In this work, we explore the coordina-
tion characteristics of bis-IDC system ligand for the first time. More-
over, complex 1 displays blue–violet fluorescent property indicating
that the polymer may have potential applications in optical mate-
rials. Further work on this ligand and its metal complexes is still
explored in our laboratory.
The photoluminescent properties of the free H4L ligand and CdII
complex 1 in the solid state were investigated at room temperature
and relevant emission spectra are shown in Fig. 4. The free H4L ligand
shows an emission band centered at 435 nm upon excitation at
364 nm, which can be assigned to the π → π∗ transitions [14]. In con-
trast to the free ligand, complex 1 has an emission at 432 nm under
the same conditions and merely blue-shifted by ~2 nm. This emission
is assignable as an intraligand transition of the H4L ligand. Since the
heteroatoms of the heterocyclic aromatic ligand will have decreased π
and π∗ orbital energies (the HOMO and LOMO, as well as orbitals with
energies close to these, may not be significantly contributed by the Cd
metal atoms, so that LMCT can be excluded from consideration) [15].
The features of the highest occupied (HOMO) and the lowest unoccu-
pied (LUMO) frontier orbitals are shown in Fig. S3. Obviously, the elec-
tron densities of the singlet state of HOMO and LUMO reside the ligands
Fig. 3. (a) View of 6-connected node in 1. (b) A schematic representation of a-Po
network of 1.
Fig. 4. The solid-state photoluminescent spectra of the free H4L and complex 1 at
room temperature.

18
G. Yuan et al. / Inorganic Chemistry Communications 42 (2014) 15–19
(c) G. Li, Z. Lei, Q.-M. Wang, Luminescent molecular Ag−S nanocluster
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This work was financially supported by the National Science
Foundation of China (No. 20701006), the Foundation for Excellent
Youth of Jilin, China (No. 20070103), PhD Station Foundation
of Ministry of Education for New Teachers (No. 20070200014/
20070200015), and Science and Technology Research Foundation
of Education Bureau of JiLin Province in the National “12th 5-year
Plan”, China (No. 2014559).
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Appendix A. Supplementary material
Crystallographic data for the structural analysis have been deposited
to the Cambridge Crystallographic Data Centre, CCDC No. 687253 for
complex 1. The data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Supplementary data to this article can be found online at http://dx.doi.
org/10.1016/j.inoche.2014.01.004.
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NaOH(1.32 g, 33 mmol) in DMF (30 mL) was stirred at 70 °C for 2 h, and the
1,4-bis(chloromethyl)benzene (2.62 g, 15 mmol) was added. The mixture was
cooled to room temperature after being stirred at 70 °C for 24 h and poured into
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100.855(5)°, V = 1520.2(11) Å3, Z = 1, R1 = 0.0439 (I N 2δ(I)). Intensity
data were collected on a Bruker SMART APEXII CCD diffractometer using Mo
Kα radiation (λ = 0.71073 Å) at room temperature. The structure was solved
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19
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[16] Theoretical calculations: All calculations in this work were carried out with the
Gaussian03 program. The parameters of the molecular structure for calculation
were all from the experimental data of the complex. On the basis of this geometry,
density functional theory (DFT) calculations using the B3LYP hybrid functional and
the LANL2DZ basis set were performed.