Luminescent 2D supramolecular network constructed from tubular coordination polymer based on H-bonding and π-π interactions

Article abstract of DOI:10.1016/j.molstruc.2005.12.032

A novel cadmium(II) coordination polymers, [Cd(bitmb)₂·H₂O]_n (1) (Hbitmb=4-(2-benzimidazolethiomethyl)benzoic acid) has been synthesized under hydrothermal conditions, and characterized by elemental analysis and IR spectra

Full text of DOI:10.1016/j.molstruc.2005.12.032

Journal of Molecular Structure 789 (2006) 128–132
www.elsevier.com/locate/molstruc
Luminescent 2D supramolecular network constructed from tubular
coordination polymer based on H-bonding and p–p interactions
Lei Han, Yaqiong Gong, Daqiang Yuan, Maochun Hong
*
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter,
Chinese Academy of Sciences, Fuzhou, Yang Qia, West Road 155, Fujian 350002, People’s Republic of China
Received 14 November 2005; received in revised form 19 December 2005; accepted 20 December 2005
Available online 14 February 2006
Abstract
A novel cadmium(II) coordination polymers, [Cd(bitmb) $H O] (1) (HbitmbZ4-(2-benzimidazolethiomethyl)benzoic acid) has been
2
2
n
synthesized under hydrothermal conditions, and characterized by elemental analysis and IR spectra. X-ray diffraction analyses show that 1 is a 1D
tubular structure, which is further assembled to 2D supramolecular network by H-bonding and p–p interactions. 1 is luminescent and displays an
emissive maximum at 450 nm in the solid state at room temperature.
q 2006 Elsevier B.V. All rights reserved.
Keywords: Coordination polymer; Cd(II) complex; Crystal structure; Luminescence
1
. Introduction
a greater understanding of the factors leading to this structural
variety.
Luminescent coordination polymers have recently attracted
We have been investigating the use of flexible ligands based
on aromatic spacers linked to thioether–pyridine arms to build
coordination polymer arrays. Assembly of these type ligands
much research interest primarily because of their potential
applications in photoactive materials [1–3]. Compared with the
luminescent organic polymers, the emission wavelength of
most metal–organic coordination polymers can be tuned or
changed by adjusting the length or conjugation of ligand since
luminescent emission is mostly dominated by the coordinated
ligand transition. In addition, luminescent metal–organic
coordination polymers have many more advantages—such as
high thermal stability, good mechanical properties and
relatively high emitting intensity—over pure organic materials.
Attempts to control the dimensionality and topology of these
materials have focused on judicious selection of functional
ligands with metal ions of the appropriate coordination
geometry [4–8]. Recently, the use of flexible ligands with
multiple donor sites to engineer coordination polymer arrays
encounters a number of inherent challenges, such as difficult to
predict and to form high dimensionality [9,10]. However, the
propensity toward structural diversity in such systems brings
opportunities to probe their conditions of formation and to gain
1
0
and d transition metal ions produced some interesting
luminescent complexes [11–13]. In our continuous investi-
gations on self-assembly with flexible bridging ligands, herein,
we wish to report on the exploitation of a new bridging ligand,
4-(2-benzimidazolethiomethyl)benzoic acid (Hbitmb)
(Scheme 1). This ligand is designed and synthesized partly
because the asymmetrical feature of it containing the twisted
binding sites that can induce the formation of novel structures
when linked by advisable metal nodes; but more importantly,
our new strategy lies in the recognition that the Hbitmb ligand
with a benzimidazole ring and a carboxylate group can serve as
a strong N–H$$$O hydrogen bonding donor–acceptor site, and
generate secondary chain-to-chain interaction in addition to the
coordination and p–p interaction. In nature, the protein that is
found in silk fibers epitomizes this strategy: the primary
polypeptide chains are further linked by a secondary inter-
chain hydrogen bonding interaction to generate a b-pleated
sheet topology, which provides the flexibility yet very
strong and resistant to stretching [14]. Thus, the assembly of
2
C
*
Corresponding author. Tel.: C86 591 83792460; fax: C86 591 83714946.
E-mail address: hmc@ms.fjirsm.ac.cn (M. Hong).
Hbitmb with Cd
afforded a luminescent 1D tubular coordination polymer,
Cd(bitmb) $H O] (1), which is further assembled to 2D
ions under hydrothermal conditions
0
022-2860/$ - see front matter q 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2005.12.032
2
2
n
supramolecular network by H-bonding and p–p interactions.

L. Han et al. / Journal of Molecular Structure 789 (2006) 128–132
129
2.4. Crystal structural determination
A colorless single crystal of 1 was mounted on a glass fiber
and used for data collection, respectively. All intensity data
were collected on a Bruker CCD diffractometer by u scan
technique at room temperature using graphite-monochromated
˚
Mo Ka (lZ0.71073 A) radiation. All empirical absorption
Scheme 1.
corrections were applied by using the SADABS program [15].
The structures were solved by direct methods and refined using
the SHELXL-97 program package [16]. Metal atoms in each
complex were located from E-maps, and 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-
2
. Experimental
2
.1. Materials and measurements
All the reagents for syntheses were commercially available
and employed without further purification or with purification
by standard methods prior to use. Elemental analyses (C, H, N)
were performed on a Perkin–Elmer model 240C automatic
instrument. IR spectra were recorded on a Magna 750 FT-IR
2
hydrogen atoms on F . The positions of H atoms were
generated geometrically, assigned isotropic thermal par-
ameters, and allowed to ride on their parent carbon atoms
before the final cycle of refinement. The important crystal data
are summarized in Table 1, and the selected bond lengths and
angles for the complexes are given in Table 2. Crystallographic
data for the complex 1 have been deposited at the Cambridge
Crystallographic Data Center with CCDC Number 256264.
Copies of the data can be obtained free of charge on application
to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax:
C44 122 333 6033; e-mail: deposit@ccdc.cam.ac.uk).
1
spectrophotometer with KBr pellet. H NMR was measured on
a Bruker AM500 spectrometer with SiMe as the internal
4
reference. Thermogravimetric analysis was performed on a
NETZSCH STA 449C thermai analyzer under flowing N with
2
a heating rate of 10 8C/min. Emission spectra were recorded on
a Perkin–Elmer LS55 Luminescence spectrometer.
2.2. Synthesis of 4-(2-benzimidazolethiomethyl)benzoic
acid (Hbitmb)
2
-Mercaptobenzimidazole (1.502 g, 10 mmol) was added to
a stirred solution of KOH (0.56 g, 10 mmol) in H O (20 mL).
2
Table 1
Crystallographic data for 1
The mixture was warmed to reflux, then a-bromo-p-toluic acid
(
2.15 g, 10 mmol) was added and the mixture was refluxed for
1
about 10 h with vigorous stirring. After being cooled, the
mixture was left to stand overnight. The precipitate was filtered
off and washed with water, giving a fine white powder in 95%
yield. Anal. Calcd. for C H N O S: C 63.36, H 4.25, N 9.85;
Empirical formula
FW
30 4 5 2
C H24CdN O S
697.05
293(2)
0.71073
Triclinic
P-1
Temperature (K)
˚
Wavelength (A)
1
5 12 2 2
Crystal system
Space group
˚
a (A)
˚
b (A)
c ( A˚ )
found: C 63.31, H 4.21, N 9.89. IR (KBr) n: 3065 (w), 2984
(
m), 2916 (m), 2668 (m), 1704 (vs), 1609 (s), 1520 (s), 1423
s), 1368 (vs), 1247 (s), 1176 (s), 1020 (m), 842 (m), 745 (vs),
8.631
10.814
15.833
89.86
77.12
76.25
1397.3
2
(
K1
01 (m) cm ; H NMR (500 MHz, DMSO-d ): d 4.631 (s,
1
6
2
5
6
H, CH ), 4.762 (s, 1H, NH), 7.118–7.134 (t, J Z3 Hz, J Z
a (8)
b (8)
g (8)
2
1
2
Hz, 2H, bi), 7.556–7.571 (d, JZ7.5 Hz, 2H, ph), 7.862–7.878
(
d, JZ8 Hz, 2H, ph), 7.919–7.934 (d, JZ7.5 Hz, 2H, bi),
3
˚
V (A )
1
2.616 (s, 1H, COOH).
Z
K3
D
c
, (g cm
K1
)
1.657
0.980
704
m (mm
F(000)
)
2
.3. Synthesis of [Cd(bitmb) $H O] (1)
2
2
n
Crystal size (mm)
q range (8)
Limiting indices
0.15!0.12!0.05
1.94–25.03
A mixture of CdCl $2.5H O (23 mg, 0.1 mmol) with
2
Hbitmb (57 mg, 0.2 mmol) in 10 mL of H O was sealed in a
2
2
K10%h%9, K12%k%12,
K18%l%17
8659/4892 [R(int)Z0.0327]
99.0%
stainless-steel reactor with a Teflon liner and heated at 110 8C
for 72 h. A quantity of colorless single crystals was obtained
after the solution was cooled to room temperature at a rate of
Reflections collected/unique
Completeness to qZ25.03
Refinement method
2
Full-matrix least-squares on F
4892/0/379
10 8C/h. The yield of 1 is ca. 70% based on Hbitmb. Anal.
Calcd. for [C H CdN O S ]: C 51.69, H 3.47, N 8.04; found:
C 51.62, H 3.42, N 8.01. IR (KBr) n: 3377 (m), 3112 (w), 3097
Data/restraints/parameters
2
Goodness-of-fit on F
1.105
3
0
24
4 5 2
Final R indices [IO2s(I)]
R indices (all data)
R
R
1
Z0.0503, wR
Z0.0690, wR
2
Z0.0831
Z0.0916
1
2
(
(
w), 1609 (s), 1587 (vs), 1533 (vs), 1404 (vs), 1276 (s), 1227
K1
K3
˚
0.718 and K0.483 e A
Largest diff. peak and hole
s), 1017 (m), 861 (m), 743 (s), 639 (m) cm
.

130
L. Han et al. / Journal of Molecular Structure 789 (2006) 128–132
Table 2
˚
Selected bond lengths (A) and angles (8) for complex 1
a
1
Cd(1)–O(2)
2.228(3)
2.278(4)
Cd(1)–O(4B)
2.290(4)
2.409(4)
Cd(1)–N(3)
Cd(1)–O(3B)
Cd(1)–N(1A)
2.290(4)
Cd(1)–O(1)
2.628(4)
O(2)–Cd(1)–N(3)
O(2)–Cd(1)–N(1A)
N(3)–Cd(1)–N(1A)
O(2)–Cd(1)–O(4B)
N(3)–Cd(1)–O(4B)
N(1A)–Cd(1)–O(4B)
O(2)–Cd(1)–O(3B)
O(3B)–Cd(1)–O(1)
100.42(13)
90.47(14)
99.94(13)
130.38(15)
125.87(15)
97.27(13)
123.87(16)
80.16(14)
N(3)–Cd(1)–O(3B)
N(1A)–Cd(1)–O(3B)
O(4B)–Cd(1)–O(3B)
O(2)–Cd(1)–O(1)
N(3)–Cd(1)–O(1)
N(1A)–Cd(1)–O(1)
O(4B)–Cd(1)–O(1)
83.90(13)
144.44(15)
54.89(14)
52.71(12)
127.22(14)
121.47(13)
82.57(14)
Fig. 2. The 1D structure of 1 with 2!2 and 4!4 grids in its backbone.
distorted octahedral geometry. The average Cd–O and Cd–N
˚
distances are 2.388 and 2.278 A, respectively, which are
a
Symmetry transformations used to generate equivalent atoms are: A: KxC
, KyC2, Kz; B: xC1, yK1, z; C: xK1, yC1, z.
1
similar to normal Cd–O and Cd–N coordination bonds [17].
Each ligand coordinates to two Cd(II) atoms and the
3
. Results and discussion
carboxylate group adopts a chelating mode. Owing to the
3
ligand is flexible by way of sp configurations of –CH S–
2
3
.1. Synthesis and crystal structure of 1
The ligand Hbitmb was conveniently prepared from the
spacer, the dihedral angles between phenyl and benzimidazole
groups are 97.9 and 88.18, respectively. The array of ligands
linked by Cd centers gives a 1D polymer, as shown in Fig. 2.
The structural characteristic of this chain is that there are two
kinds of grids (2!2 and 4!4) within its backbone.
Furthermore, these 2!2 grids form a 1D channel which
results in tubular coordination polymer for 1 (Fig. 3). As
expected, these chains are parallel each other and further
assembled to 2D supramolecular network via hydrogen
bonding and p–p interactions. Fig. 4 indicates the 2D
H-bonding network with two types: N–H$$$Ocarboxylate and
N–H$$$Owater (Table 3). Fig. 5 displays two types of weak p–
substitution reaction between 2-mercaptobenzimidazole and
a-bromo-p-toluic acid under strong alkaline (KOH) conditions
with higher yield. The resulting white powder was character-
1
ized by elemental analyses, IR and H NMR. The complex 1
was obtained as colorless needle crystals by hydrothermal
2
C
reaction (110 8C, 72 h) between Hbitmb and Cd
salts and
characterized by spectroscopic data as well as X-ray
crystallography. It is interestingly noted that 1 could also be
isolated according to the same synthetic procedure, however,
using Cd(X) (XZNO , Cl, OAc) as the source of metals,
2
3
p interactions: pphenyl–pbenzimidazole and pbenzimidazole
–
which indicates that the final products are independent of the
counter-anions of the metal salts. Furthermore, 1 is air stable
and insoluble in water or common organic solvents.
pbenzimidazole (Table 4). It can be concluded that the designed
chain-to-chain strong H-bonding and p–p interaction play an
important role on the formation and stability of supramolecular
network. Thermogravimetric analysis (TGA) shows that
compound 1 undergoes two-step weight losses. As shown in
Fig. 6, the first step occurs from 100 to 150 8C where the weight
loss is 2.62%, which corresponds to the loss of water molecules
The complex 1 crystallizes in the triclinic unit cell space
group P-1 consisting of neutral Cd(bitmb) asymmetric unit
2
and a dissociative water molecule. As shown in Fig. 1, each
Cd(II) center binds to four carboxylate oxygen atoms and two
nitrogen atoms from four bitmb ligands to form a severely
(
expected, 2.58%). The weight remains almost unchanged in
the plateau from 150 to 310 8C. The second occurs at the range
from 340 to 510 8C, and a weight loss of 42.1% showing that
the compound begins to decompose.
Fig. 1. The asymmetric unit representation of 1 showing the local geometry
around the metal center and ligand. Thermal ellipsoids were drawn at the 50%
probability level.
Fig. 3. View of 1D tubular structure of 1.

L. Han et al. / Journal of Molecular Structure 789 (2006) 128–132
131
Table 4
p–p interaction parameters for 1
Distance
Dihedral angle
Symmetry code
between ring
˚
centroids (A)
(8)
Ring(1)–ring(4)
Ring(3)–ring(3)
Ring(3)–ring(4)
Ring(2)–ring(2)
3.711(3)
3.785(3)
3.668(3)
3.680(3)
7.07
0.00
0.00
6.10
2Kx, 2Ky, Kz
2Kx, 2Ky, Kz
1Kx, 3Ky, Kz
2Kx, 2Ky, Kz
Ring (1): N(1)–C(9)–N(2)–C(10)–C(15). Ring (2): C(2)–C(3)–C(4)–C(7)–
C(6)–C(5). Ring (3): C(10)–C(11)–C(12)–C(13)–C(14)–C(15). Ring (4):
C(17)–C(18)–C(19)–C(22)–C(21)–C(20)
Fig. 4. View of H-bondings in the 2D supramolecular network of 1.
Table 3
˚
Hydrogen bond lengths (A) and bond angles (8) for 1
˚
D–H (A) H$$$A
D–H$$$A
D$$$A
˚
(A)
:DHA
(8)
Symmetry
code
˚
A)
(
N(2)–
0.88
0.88
1.96
1.99
2.827(6) 167
2.767(6) 147
2Kx,
2Ky,Kz
x,1Cy,z
H(2A)$$$O(1)
O(5)–
H(5A)$$$O(4)
3
.2. Luminescent spectrum
The emission spectrum of compound 1 in the solid state at
room temperature was measured. Excitation of solid samples of
at 330 nm produces a strong blue emission with peak
Fig. 6. The TG curve of 1.
4. Conclusion
1
maximum at 450 nm, as shown in Fig. 7. According to the
luminescent Cd(II) coordination polymers reported in litera-
tures [4,17,18], the emission band might be attributable to the
ligand-to-metal charge transfer (LMCT). This observation
indicates that compound 1 may be candidate for blue
photoactive materials.
In conclusion, we have successfully demonstrated a feasible
way in obtaining luminescent 2D supramolecular network from
tubular polymer by employing strong chain-to-chain hydrogen
bonding and p–p interactions to enhance the framework
rigidity. Other results based on the Hbitmb ligand are in
progress and will be published very soon.
Fig. 5. View of p–p interactions in the 2D supramolecular network of 1.
Fig. 7. The emission spectrum of 1 in the solid state at room temperature.

132
L. Han et al. / Journal of Molecular Structure 789 (2006) 128–132
Acknowledgements
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2
We are thankful for the financial support of the National
Nature Science Foundation of China (No. 20231020) and the
Nature Science Foundation of Fujian Province.
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