Self-assembly of three cadmium(II) complexes based on 5-methylisophthalic acid and flexible bis(imidazole) ligands with different spacer lengths

Article abstract of DOI:10.1016/j.ica.2013.07.053

Three different cadmium(II) coordination polymers, [Cd₂(mip) ₂(1,2-bie)] (1), [Cd(mip)(1,3-bip)] (2), {[Cd(mip)(1,4-bib)(H ₂O)]·H₂O}₂ (3), where H₂mip = 5-methylisophthalic acid, 1,2-bie = 1,2-bis(imidazole)ethane, 1,3-bip = 1,3-bis(imidazole)propane and 1,4-bib = 1,4-bis(imidazole)butane, were synthesized under hydrothermal conditions. Their structures have been characterized by elemental analysis, IR and single-crystal X-ray crystallography. Complex 1 displays a 3D network with a (4,5)-connected (4 ².7⁴)(4².6².7⁶) topology. Complex 2 features a 2D layer structure with a (3,5)-connected (4 ².6)(4².6⁷.8) topology. Complex 3 has two crystallographically distinct 2D (4,4) polymeric motifs, which are further extended by hydrogen-bonding interactions to form a 2D double layer. These structural analyses indicate that the flexible bis(imidazole) ligands with different spacer lengths play important roles in the formation of the final structures. In addition, the photoluminescent properties of 1-3 in the solid state have been investigated.

Full text of DOI:10.1016/j.ica.2013.07.053

Inorganica Chimica Acta 407 (2013) 153–159
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Self-assembly of three cadmium(II) complexes based on
5-methylisophthalic acid and flexible bis(imidazole) ligands with
different spacer lengths
b
b,c,
Fei-Fei Li a, Zhen-Zhen Shi a,b, Lu-Fang Ma b, , Xiao-Jie Wu , Li-Ya Wang
⇑
⇑
a Department of Physics and Chemistry, Henan Polytechnic University, Jiaozuo 454000, PR China
b College of Chemistry and Chemical Engineering, Luoyang Normal University, Luoyang 471022, PR China
c College of Chemistry and Pharmaceutical Engineering, Nanyang Normal University, Nanyang 473061, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Three different cadmium(II) coordination polymers, [Cd2(mip)2(1,2-bie)] (1), [Cd(mip)(1,3-bip)] (2),
{[Cd(mip)(1,4-bib)(H2O)]ꢀH2O}2 (3), where H2mip = 5-methylisophthalic acid, 1,2-bie = 1,2-bis(imidaz-
ole)ethane, 1,3-bip = 1,3-bis(imidazole)propane and 1,4-bib = 1,4-bis(imidazole)butane, were synthe-
sized under hydrothermal conditions. Their structures have been characterized by elemental analysis,
IR and single-crystal X-ray crystallography. Complex 1 displays a 3D network with a (4,5)-connected
Received 1 November 2012
Received in revised form 5 July 2013
Accepted 28 July 2013
Available online 3 August 2013
(42.74)(42.62.76) topology. Complex
2 features a 2D layer structure with a (3,5)-connected
Keywords:
(42.6)(42.67.8) topology. Complex 3 has two crystallographically distinct 2D (4,4) polymeric motifs, which
are further extended by hydrogen-bonding interactions to form a 2D double layer. These structural anal-
yses indicate that the flexible bis(imidazole) ligands with different spacer lengths play important roles in
the formation of the final structures. In addition, the photoluminescent properties of 1–3 in the solid state
have been investigated.
5-Methylisophthalic acid
Bis(imidazole) ligands
Spacer lengths
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
mum when coordinating to the metal centers, which often cause
structural diversities [32].
The rational design, construction and characterization of new
metal-organic frameworks (MOFs) is rapidly expanding in crystal
engineering and materials chemistry, not only because of their po-
tential applications in gas absorption [1–5], luminescence [6–8],
magnetism [9–12], sensing [13–15] and so on, but also because
of their fascinating structural diversities and intriguing topological
features [16–18]. In principle, the control of coordination poly-
meric structure is complicated and still remains a major challenge
in this field due to the fact that the self-assembly process
influenced by many factors, such as the number, type and spatial
disposition of coordination sites by organic ligands [19,20], the
stereoelectronic preferences of the metal ions [21,22], metal–li-
gand ratio [23–25], solvent system [26,27], reaction temperature
[28,29], pH value [30,31]. Up to now, the selection of suitable or-
ganic ligands is crucial for constructing extended coordination
frameworks. Among them, flexible ligands can bend or rotate to
adopt the appropriate conformation and hold the energetic mini-
Flexible bis(imidazole) ligands, such as 1,2-bis(imidazole)eth-
ane (1,2-bie), 1,3-bis(imidazole)propane (1,3-bip), 1,4-bis(imidaz-
ole)butane (1,4-bib), 1,5-bis(imidazole)pentane (1,5-bip), 1,6-bis
(imidazole)hexane (1,6-bih) and 1,4-bis(imidazol-1-ylmethyl)ben-
zene (bix), with –(CH2)n– (n P 2) spacers as their flexible back-
bones, have attracted considerable attention recently [33–37].
Their bis(imidazole) rings can freely twist around the –(CH2)n–
spacer group to meet the requirements of the coordination confor-
mations (such as cis- and trans-) of metal atoms in the assembly
process. Moreover, due to the presence of flexible –(CH2)n–, the
bis(imidazole) ligands can act as bridges of different lengths to
construct high dimensional frameworks and have different flexibil-
ity and conformational freedom, which may provide different
capacities of spatial extension [31]. These features can induce the
construction of diverse frameworks [38–43].
With this analysis in mind, we employed 5-methylisophthalic
acid with Cd(II) salt and three bis(imidazole) ligands to give rise
to three different structures, namely [Cd2(mip)2(1,2-bie)] (1),
[Cd(mip)(1,3-bip)] (2) and {[Cd(mip)(1,4-bib)(H2O)]ꢀH2O}2 (3).
Herein, we study syntheses, crystal structures and topological
analysis of these three complexes. The influence of the spacer
length in the flexible bis(imidazole) ligands on the structures of
⇑
Corresponding authors at: College of Chemistry and Chemical Engineering,
Luoyang Normal University, Luoyang 471022, PR China. Fax: +86 379 65526089.
E-mail addresses: mazhuxp@126.com (L.-F. Ma), wlya@lynu.edu.cn (L.-Y. Wang).
0020-1693/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2013.07.053

154
F.-F. Li et al. / Inorganica Chimica Acta 407 (2013) 153–159
these complexes were also discussed. In addition, the photolumi-
3. Results and discussion
nescent properties of 1–3 in the solid state were also investigated.
3.1. Description of crystal structures
2. Experimental
3.1.1. [Cd2(mip)2(1,2-bie)] (1)
X-ray single crystal structure analysis reveals that the asym-
metric unit of 1 consists of one crystallographically independent
Cd(II), one mip anion, and half of a 1,2-bie molecule. As shown in
Fig. 1(a), the Cd(II) ion is six-coordinated by one nitrogen atom
of a 1,2-bie molecule and five carboxylate oxygen atoms from four
separate mip ligands. The Cd–N bond length is 2.257(3) Å, and the
Cd–O bond lengths are in the range of 2.291(2)–2.518(2) Å, respec-
tively. Both of the carboxylate groups of mip are deprotonated, in
agreement with the IR data in which no characteristic absorption
bands of the –COOH group at 1700–1750 cmꢁ1 are observed. The
2.1. Materials and general methods
All chemicals purchased were used as received without further
purification. All the products were highly stable in air at ambient
conditions. Elemental analyses for carbon, hydrogen and nitrogen
were performed on a Thermo Science Flash 2000 elemental ana-
lyzer. The infrared spectra (4000–600 cmꢁ1) were recorded on a
NICOLET 6700 FT-IR spectrometer. Fluorescent analyses were per-
formed on a Hitachi F-4500 analyzer.
carboxylate groups of mip adopt l4-j
: O, O, O0, O00, O000 coordination
2.2. Synthesis
modes (Scheme 1(a)), linking the Cd atoms to result in an infinite
1D chain with the adjacent Cdꢀ ꢀ ꢀCd distances of 3.8294(4) and
3.8796(4) Å along the c axis (see Fig. 1(b)). These chains are parallel
to each other and are cross-linked by mip linkages to form a 3D
framework with two kinds of channels (Fig. S1). The –CH3 groups
of the mip ligands protrude inside the voids of one type of channel
and the other type of channel encapsulates gauche 1,2-bie ligands,
which are bonded with Cd atoms through Cd–N coordination
bonds (Fig. 1(c)). The 1,2-bie ligand adopts a trans conformation
with a dihedral angle between two imidazole rings of 72.23°.
Analysis of the network topology of 1 reveals that each mip li-
gand acts as a 4-connected node, and Cd atom plays a 5-connected
role. Thus, a (4,5)-connected 3D array with the Schläfli symbol of
(42.74) (42.62.76) is formed (see Fig. 1(d)).
When 5-methoxyisophthalic acid (CH3O-H2ip) is selected as
starting materials, {[Cd(CH3O-ip)(1,2-bie)0.5(H2O)2]ꢀH2O}n (4) [46]
was obtained. 4 exhibits a 1D ladder-like chain, in which Cd(II)
ion is seven-coordinated with a pentagonal bipyramid and CH3O-
H2ip takes the bidentate-chelating coordination mode to bridge
the neighboring Cd(II) ions.
2.2.1. Synthesis of [Cd2(mip)2(1,2-bie)] (1)
A mixture of 5-methylisophthalic acid (18.0 mg, 0.1 mmol), 1,2-
bie (16.2 mg, 0.1 mmol), Cd(OAc)2ꢀ2H2O (26.6 mg, 0.1 mmol), and
KOH (5.6 mg, 0.1 mmol) were added to water (12 mL) in a 25 mL
Teflon-lined stainless steel vessel. The mixture was heated at
413 K for 3 d, and then slowly cooled down to room temperature.
Colorless block crystals of 1 were obtained. Yield: 17.5 mg, 47%
(Based on Cd). Anal. Calc. for C26H22Cd2N4O8: C, 41.83; H, 2.97; N,
7.51. Found: C, 41.78; H, 2.79; N, 7.64%. IR (cmꢁ1): 3131 m, 1608
m, 1541 s, 1412 m, 1352 s, 1243 m, 1096 m, 771 s, 731 s, 653 s.
2.2.2. Synthesis of [Cd(mip)(1,3-bip)] (2)
The reaction condition is similar to those described in 1 except
that 1,2-bie was replaced by 1,3-bip (17.6 mg, 0.1 mmol). Colorless
block crystals of 2 were obtained. Yield: 19.6 mg, 42% (Based on
Cd). Anal. Calc. for C18H18CdN4O4: C, 46.32; H, 3.89; N, 12.00.
Found: C, 46.28; H, 3.93; N, 11.96%. IR (cmꢁ1): 3484 m, 3118 m,
1615 m, 1546 s, 1370 s, 1090 s, 780 s, 730 s, 659 s.
2.2.3. Synthesis of {[Cd(mip)(1,4-bib)(H2O)]ꢀH2O}2 (3)
The reaction condition is similar to those described in 1 except
that 1,2-bie was replaced by 1,4-bib (19.0 mg, 0.1 mmol). Colorless
block crystals of 3 were obtained. Yield: 23.3 mg, 45% (Based on
Cd). Anal. Calc. for C38H48Cd2N8O12: C, 44.16; H, 4.68; N, 12.84.
Found: C, 44.23; H, 4.78; N, 12.77%. IR (cmꢁ1): 3446 m, 3128 m,
2591 m, 1548 s, 1439 s, 1369 s, 1111 s, 774 s, 730 s, 657 s.
3.1.2. [Cd(mip)(1,3-bip)] (2)
The asymmetric unit of 2 contains one mip anion, one Cd(II) ion
and one 1,3-bip molecule. As illustrated in Fig. 2(a), Cd centre is
six-coordinated with a distorted octahedral geometry by two cis
nitrogen atoms from two 1,3-bip and four oxygen atoms from
three mip. The apical positions are occupied by one nitrogen atom
of 1,3-bip and one oxygen atom from mip with the N–Cd–O3 angle
of 165.73(6)°. The Cd–N bond distances are 2.2471(16) and
2.3512(17) Å, and the Cd–O ones are in the range of 2.308(5)–
2.3. X-ray crystallographic data collection and structural
determination
2.524(4) Å, respectively. The carboxylate groups of mip adopt l3
j
-
The crystallographic diffraction data for 1–3 were collected on a
: O, O0, O00, O000 coordination modes (Scheme 1(b)) to coordinate
Bruker SMART APEX CCD diffractometer with graphite-monochro-
with three Cd atoms. The adjacent Cd ions are bridged by mip li-
gands to form a 1D ribbon chain. The infinite chain comprises 8-
and 16-membered rings with the Cdꢀ ꢀ ꢀCd distances of 4.3275(3)
and 8.0760(6) Å, respectively (Fig. 2(b)). Such 1D ribbon chains
are further interlinked by 1,3-bip to form a 2D layer, in which
the Cdꢀ ꢀ ꢀCd distance across the 1,3-bip spacer is 11.4265(8) Å
(Fig. 2(c)). Notably, the bidentate 1,3-bip ligand adopts the cis-con-
formation with a dihedral angle between two imidazole rings of
89.20° (see Fig. 2(c)).
From the topological perspective, the Cd(II) center and mip an-
ion can be considered as a 5-connected node and a 3-connected
node, respectively. Then the framework of 2 features a binodal
(3,5)-connected network with a Schläfli symbol of (42.6)(42.67.8)
topology (see Fig. 2(d)).
mated Mo K
a radiation (k = 0.71073 Å) at room temperature using
the scanning technique. All the structures were solved using
u/x
direct methods and successive Fourier difference synthesis, and
refined using the full-matrix least-squares method on F2 with
anisotropic thermal parameters for all non-hydrogen atoms using
SHELXS-97 [44]. An empirical absorption correction was applied
using the SADABS program [45]. The hydrogen atoms were placed
in calculated positions and refined using a riding on attached
atoms model with isotropic thermal parameters 1.2 times those
of their carrier atoms. Corrections for Lp factors were applied and
all non-hydrogen atoms were refined with anisotropic thermal
parameters. Basic information pertaining to crystal parameters
and structure refinements are summarized in Table 1. Selected
bond lengths and angles for 1–3 are listed in Table S1, hydrogen
bonding distance and angle data for 3 are listed in Table S2, shown
in Supporting Information.
When 5-methoxy isophthalic acid (CH3O-H2ip) are selected as
starting material, {[Cd(CH3O-ip)(1,3-bip)]ꢀH2O}n (5) [37] was ob-
tained. 5 exhibits a similar binodal (3,5)-connected network as that

F.-F. Li et al. / Inorganica Chimica Acta 407 (2013) 153–159
155
Table 1
Crystal and structural refinement data of 1–3.
Complex
1
2
3
Empirical formula
Formula weight
T (K)
C
26H22Cd2N4O8
C18H18CdN4O4
466.76
296(2)
C38H48Cd2N8O12
1033.64
296(2)
743.28
296(2)
Crystal size (mm)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
0.34 ꢂ 0.30 ꢂ 0.29
tetragonal
P42212
18.2560(7)
18.2560(7)
7.7060(6)
90
2568.3(2)
4
1.922
0.35 ꢂ 0.21 ꢂ 0.19
monoclinic
P21/n
10.3038(8)
11.4265(8)
17.3227(13)
104.3150(10)
1976.2(3)
4
0.21 ꢂ 0.08 ꢂ 0.05
orthorhombic
Pna21
24.608(3)
10.2320(12)
18.294(2)
90
4606.3(9)
4
1.490
b (°)
V (Å3)
Z
Dcalc (g/cm3)
Absorption coefficient
1.569
1.135
1.715
0.988
(mmꢁ1
F(000)
h (°)
)
1464
2.49–25.49
936
2.43–25.50
2096
2.28–25.50
Limiting indices
Reflections collected/
unique
ꢁ22 6 h 6 22, ꢁ22 6 k 6 22, ꢁ9 6 l 6 9
ꢁ12 6 h 6 12, ꢁ13 6 k 6 13, ꢁ20 6 l 6 20
ꢁ29 6 h 6 29, ꢁ12 6 k 6 12, ꢁ22 6 l 6 22
19626/2389
14723/3685
33940/8530
Rint
0.0196
0.0159
0.0945
Maximum and
minimum
0.6361, 0.5932
0.8133, 0.6921
0.8330, 0.3797
transmission
Data/restraints/
parameters
Goodness-of-fit (GOF)
on F2
2389/0/182
1.077
3685/0/245
1.061
8530/1/543
1.002
R1
0.0205
0.0184
0.0480
wR2 [I > 2r(I)]
0.0462
0.0476
0.0790
R1 (all data)
0.0221
0.0207
0.0770
wR2 (all data)
0.0471
0.0478
0.0872
D
qmax and
D
qmin
0.690, ꢁ0.271
0.329, ꢁ0.279
0.345, ꢁ0.571
(e Åꢁ3
)
in 2 except that Cd(II) ion takes a distorted trigonal bipyramidal
geometry.
and free water molecules with Oꢀ ꢀ ꢀO distances of 2.855(7) and
2.846(7) Å; (iii) hydrogen bonding between free water molecules
and carboxylate oxygen atoms with Oꢀ ꢀ ꢀO distances in the range
of 2.722(11)–3.036(7) Å. The hydrogen-bonding metrics and bond
parameters are shown in Table S2, Supporting Information.
When 5-nitroisophthalic acid (5-O2N-H2ip), 5-hydroxylisoph-
thalic acid (5-HO-H2ip) and Cd(II) salt are selected as starting
materials, [Cd(1,4-bib)0.5(5-O2N-ip)(H2O)]n (6), and {[Cd(1,4-
bib)0.5(5-HO-ip)](H2O)}n (7) were generated [47]. In 6, the 5-O2N-
H2ip ligand links Cd(II) centers into a 1D double chain motif with
alternating 8-membered and 16-membered rings. The 1,4-bib
ligand as an exo-bidentate linkage extends the adjacent chain into
a 2D layer. Complex 7 is a porous framework with guest water
molecules, which arises from a 1D cadmium-carboxylate chain
stretched by HO-ip and 1,4-bib. When 4,40-bis(1-imidazolyl)biben-
zene (bimb) and 1,4-bis(2-methylimidazol-1-yl)butane (bmib) are
selected as starting materials [Cd(mip)(bimb)]n (8) [48] and
[Cd(bmib)(mip)]n (9) [49] are synthesized. Complex 8 is a 3D
framework in which 2D [Cd4(mip)4]n layers are interlinked by
bimb. Complex 9 is a twofold interpenetrated 3D pcu topology con-
taining binuclear Cd units.
3.1.3. {[Cd(mip)(1,4-bib)(H2O)]ꢀH2O}2 (3)
Single-crystal X-ray characterization indicates that complex 3
comprises two crystallographically distinct 2D polymeric motifs,
that is, two puckered (4,4) layers of [Cd(mip)(1,4-bib)(H2O)]. As
shown in Fig. 3(a), there are two crystallographic independent
Cd(II) atoms, and both of which are six-coordinated with a dis-
torted{CdN2O4} octahedral environment with three oxygen atoms
from two different mip and a coordinated water molecule as well
as two nitrogen atoms from two 1,4-bib. The apical positions are
occupied by two nitrogen atoms with the N1#1–Cd1–N4 and
N5–Cd2–N8#3 angles of 177.6(2) and 175.3(3)°. The Cd–N dis-
tances are 2.219(7) to 2.296(7) Å, and the Cd–O distances range
from 2.308(5) to 2.524(4), respectively.
Both of the carboxylate groups of mip are deprotonated. As
shown in Fig. 3(b), the carboxylate groups of mip adopt l2-j: O,
O0, O00 coordination modes (Scheme 1(c)) to link the adjacent Cd(II)
ions to form a 1D linear chain in the two motifs. These 1D linear
chains are further linked by l2-1,4-bib to generate two 2D frame-
works parallel to the ab plane (Fig. 3(c) and (d)). The two crystallo-
graphically distinct 2D polymeric motifs are further packed into a
2D double layered structure by O–Hꢀ ꢀ ꢀO hydrogen bonding
interactions. Free water molecules are fixed by hydrogen bonding,
located in the channels of the double layers (Fig. 3(e)). These
hydrogen-bonding interactions bring further stability to the struc-
ture of 3.
Three kinds of hydrogen bonds are present: (i) hydrogen bond-
ing between coordinated water molecules and carboxylate oxygen
atoms with Oꢀ ꢀ ꢀO distances of 2.777(7) and 2.716(6) Å, respec-
tively; (ii) hydrogen bonding between coordinated water molecule
3.2. Effect of bis(imidazole) ligands
To investigate the influence of the spacer length and flexibility
of bis(imidazole) ligands on the complex structures, three typical
bis(imidazole) ligands, 1,2-bie, 1,3-bip and 1,4-bib were used in
terms of their differences in shape and flexibility. The synthetic
strategy for the Cd(II)-H2mip/ bis(imidazole) system is schemati-
cally depicted in Scheme 2.
From the structural descriptions above, we can see that
the bis(imidazole) ligands can bend and rotate freely when

156
F.-F. Li et al. / Inorganica Chimica Acta 407 (2013) 153–159
Fig. 1. (a) Coordination environments of the Cd(II) ion in 1, hydrogen atoms are omitted for clarity. Symmetry codes: #1, x, y, ꢁz; #2, ꢁx + 3/2, y ꢁ 1/2, z + 1/2; #3, ꢁx + 3/2,
y ꢁ 1/2, ꢁz + 1/2. (b) View of the 1D [Cdn(mip)n] chain along the c axis in 1. (c) Perspective view of the 3D coordination network along the ab plane in 1. (d) Schematic view of
the (4,5)-connected (42.74)(42.62.76) topology in 1. (green nodes for 5-connected Cd(II) and pink ones for mip). (For interpretation of the references to colour in this figure
legend, the reader is referred to the web version of this article.)
coordinating to the central Cd ion due to the flexible nature of the
CH3
CH3
spacers between the two imidazole rings. In 1, the methylene
groups of neutral 1,2-bie ligand display ‘‘1’’ conformations, with
the distances between neighboring Cd centers linked by 1,2-bie
being 9.4933(4) Å. In 2, the methylene groups of neutral 1,3-bip li-
gand display ‘‘3’’ formed conformations, with the distances be-
tween neighbouring Cd centers linked by 1,3-bip being
11.4265(8) Å. In 3, the neutral ligand is 1,4-bib, and the methylene
groups display ‘‘S’’ conformations, with the distances in two similar
polymeric motifs between neighboring Cd centers linked by 1,4-
bib being 12.4611(16) and 12.3281(17) Å, respectively. The dihe-
dral angles between the two imidazole rings are 72.23° for 1,
89.20° for 2 and 12.09° for 3, respectively.
M
M
M
O
O
O
O
M
O
O
M
O
O
M
M
(a)
(b)
CH3
As a result, a slight change in bis(imidazole) ligands has an im-
pact on 1–3. In 1, the mip anions bridge the Cd(II) ions to form a 3D
framework with two kinds of channels and gauche 1,2-bie ligands
encapsulated inside the voids of one type of channel. In 2, the mip
anions bridge the Cd(II) ions to form a 1D ribbon [Cd(mip)]2n
chains, which are further extended by the 1,3-bip ligands to gener-
ate a 2D layer with a (42.6)(42.67.8) topology. Whereas, in 3, the
O
O
M
M
O
O
(c)
Scheme 1. Dicarboxylate coordination modes found in 1–3.

F.-F. Li et al. / Inorganica Chimica Acta 407 (2013) 153–159
157
Fig. 2. (a) Coordination environments of the Cd(II) ion in 2, hydrogen atoms are omitted for clarity. Symmetry codes: #1, x + 1, y, z; #2, x, y + 1, z; #3, ꢁx + 1, ꢁy + 1, ꢁz. (b)
View of the 1D [Cd(mip)]n chain in 2. (c) Polyhedral View of the 2D layer in 2. (d) Schematic view of the (3,5)-connected (42.6)(42.67.8) topology in 2. (green nodes for 5-
connected Cd(II) and pink ones for mip). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
[Cdn(mip)n] chains are linked by 1,4-bib to generate two 2D frame-
works with both (4,4) topologies. And these two crystallographi-
cally distinct 2D polymeric motifs are further packed into a 2D
double layered structure by O-Hꢀ ꢀ ꢀO hydrogen bonding
interactions.
cence properties of the free dicarboxylic acid ligands and Na salts
for the dicarboxylate ligands were also measured, upon excitation
at ca. 280 nm for H2mip, and 300 nm for Na2mip, which show the
similar emissions at ca. 350 nm for H2mip, and 328 nm for Na2mip.
In comparison with the free H2mip and Na2mip, the emission max-
ima of complexes 1–3 have changed and showed red shifts. Since
the Cd(II) ions are difficult to oxidize or reduce, these bands may
The spacer lengths of the bis(imidazole) ligands have also ex-
erted a significant influence on the coordination modes of the car-
boxylate groups of mip ligands. There are three types of
originate from the intraligand (p ?
p⁄) fluorescent emissions that
coordination modes in 1–3: l4
-j
: O, O, O0, O00, O000 for 1, l3
-j
: O,
are tuned by the metal–ligand interactions and deprotonated effect
of the organic carboxylic ligands [50–52].
O0, O00, O000 for 2 and l2 : O, O0, O00 for 3, respectively.
-
j
3.3. Photoluminescent properties
4. Conclusion
In the present paper, the photoluminescent properties of com-
plexes 1–3 were studied in the solid state at room temperature,
as depicted in Fig. 4. The emission spectra have broad peaks with
maxima at 463 and 498 nm (kex = 407 nm) for 1, 435 nm
(kex = 316 nm) for 2, 430 nm (kex = 318 nm) for 3, respectively. In
order to understand the nature of such emission bands, the fluores-
In summary, three new complexes based on 5-methylisophtha-
lic acid and flexible bis(imidazole) ligands have been prepared un-
der hydrothermal conditions. These three complexes are all based
on the [Cdn(mip)n] chains. However, by changing the spacer length
of bis(imidazole)-based ligands (–(CH2)2–, –(CH2)3– and –(CH2)4–),
three complexes with different structures were constructed. 1

158
F.-F. Li et al. / Inorganica Chimica Acta 407 (2013) 153–159
Fig. 3. (a) Coordination environments of the Cd1 and Cd2 ions in 3, hydrogen atoms are omitted for clarity. Symmetry codes: #1, x ꢁ 1/2, ꢁy + 3/2, z; #2, x, y + 1, z; #3, x ꢁ 1/2,
ꢁy + 5/2, z. (b) View of the 1D [Cd(mip)]2n chain in 3. (c) Perspective view of the single 2D coordination network in 3. (d) Polyhedral View of the 2D double layer along c axis in
3. (e) View of the 2D double layer via hydrogen bonds in 3. Hydrogen bonds are represented as light yellow dash line. (For interpretation of the references to colour in this
figure legend, the reader is referred to the web version of this article.)
N
N
N
(CH2)2
N
3D net with (42·74)(42·62·76) topology
(1)
CH3
N
N
N
N
N
N
(CH2)3
(CH2)4
N
N
Cd(OAC)2·2H2O
2D layer with (42·6)(42·67·8) topology
2D double layer with (44) topology
(2)
(3)
HOOC
COOH
Scheme 2. Representation of synthesis of 1–3.
displays a 3D structure with a (42.74) (42.62.76) topology. 2 shows a
2D layer with a (42.6) (42.67.8) topology, while 3 possesses a 2D
double layer with a (4,4) topology. These structural analyses reveal
that the spacer length and flexibility of bis(imidazole) ligands have

F.-F. Li et al. / Inorganica Chimica Acta 407 (2013) 153–159
159
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Appendix A. Supplementary material
CCDC 876260–876262 contains the supplementary crystallo-
graphic data for complexes 1–3. 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 associ-
ated with this article can be found, in the online version, at http://
dx.doi.org/10.1016/j.ica.2013.07.053.
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