Effect of anions on cadmium(II) complexes with ligand 2-(pyrazin-2-yl)-1H- benzimidazole

Article abstract of DOI:10.1016/j.ica.2011.07.011

Three new supramolecular complexes based on a 2-(pyrazin-2-yl)-1H- benzimidazole (Hpbi) and a series of Cd(II) salts have been solvothermally synthesized and structurally characterized by single-crystal X-ray diffraction analysis. Reaction of CdCl₂·2.5H₂O with Hpbi afforded a one-dimensional chain [Cd(Hpbi)Cl₂] (1), which exhibits a three-dimensional (3-D) supramolecular architecture through intermolecular X-H?Cl (X = N and C) hydrogen bonds and π-π stacking interactions. When using CdBr₂·4H₂O instead of CdCl ₂·2.5H₂O under similar reaction conditions, a bisnuclear complex [Cd(Hpbi)₂Br₂] (2) is obtained, which obviously exhibits intermolecular X-H?Br (X = N and C) hydrogen bonds and π-π stacking interactions. When CdI₂ take place of CdCl ₂·2.5H₂O, a mononuclear complex, [Cd(Hpbi) ₂I₂] (3), is isolated, which shows a 3D supramolecule framework formed by intermolecule hydrogen bonds and π-π packing interactions. Interestingly, the Hpbi ligand exhibits the same coordination modes in complexes 1-3. It is noteworthy that the radius of anions plays an important role in affecting the structures and luminescent intensity of the final products. The TGA for 1-3 have been investigated and discussed in detail.

Full text of DOI:10.1016/j.ica.2011.07.011

Inorganica Chimica Acta 376 (2011) 492–499
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Inorganica Chimica Acta
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Effect of anions on cadmium(II) complexes with ligand
2-(pyrazin-2-yl)-1H-benzimidazole
⇑
Yong-Tao Wang , Shi-Chen Yan, Gui-Mei Tang, Chao Zhao, Tian-Duo Li, Yue-Zhi Cui
Department of Chemical Engineering, Shandong Provincial Key Laboratory of Fine Chemicals, Shandong Polytechnic University, Jinan 250353, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Three new supramolecular complexes based on a 2-(pyrazin-2-yl)-1H-benzimidazole (Hpbi) and a series
of Cd(II) salts have been solvothermally synthesized and structurally characterized by single-crystal
X-ray diffraction analysis. Reaction of CdCl2Á2.5H2O with Hpbi afforded a one-dimensional chain
Received 1 March 2011
Received in revised form 30 June 2011
Accepted 11 July 2011
[Cd(Hpbi)Cl2] (1), which exhibits
a three-dimensional (3-D) supramolecular architecture through
Available online 23 July 2011
intermolecular X–HÁÁÁCl (X = N and C) hydrogen bonds and
CdBr2Á4H2O instead of CdCl2Á2.5H2O under similar reaction conditions,
[Cd(Hpbi)2Br2] (2) is obtained, which obviously exhibits intermolecular X–HÁÁÁBr (X = N and C) hydrogen
bonds and
stacking interactions. When CdI2 take place of CdCl2Á2.5H2O, a mononuclear complex,
[Cd(Hpbi)2I2] (3), is isolated, which shows a 3D supramolecule framework formed by intermolecule
hydrogen bonds and packing interactions. Interestingly, the Hpbi ligand exhibits the same coordina-
p
–p
stacking interactions. When using
a
bisnuclear complex
Keywords:
Anion-dependent
2-(Pyrazin-2-yl)-1H-benzimidazole
Cadmium complexes
Fluorescence
p–p
p–p
tion modes in complexes 1–3. It is noteworthy that the radius of anions plays an important role in affect-
ing the structures and luminescent intensity of the final products. The TGA for 1–3 have been investigated
and discussed in detail.
Luminescent intensity
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
provide further insights in developing new specified functional
materials.
The rational design and synthesis of metal complexes have at-
tracted considerable attention not only due to their intriguing vari-
ety of architectures [1,2] but also owing to their useful properties,
such as nonlinear optics [3], catalysis [4,5], ferroelectricity [6,7],
luminescence [8], magnetism [9], gas adsorption [10–13], and
molecular recognition [14,15]. Despite some significant advance-
ment, the ability to rationally predict and prepare the final archi-
Recently, benzimidazole and its derivatives have exclusively
been developed in areas of the coordination chemistry, materials
and medicine [40–46]. So far, more studies on metal complexes
based on ligands containing both pyridine and benzimidazole have
been established, which exhibit beautiful structures and fascinating
properties with luminescence [47–49]. However, little attention
has been paid to metal complexes with rigid ligands possessing
both pyrazine and benzimidazole, 2-(pyrazin-2-yl)-1H-benzimid-
azole (Hpbi) [50,51].
As a section of our program for constructing of intriguing and
functional coordination polymers with the asymmetric rigid or
flexible ligands, the design and control of the self-assemblies of
coordination polymers and supramolecular complexes have been
investigated [52–54]. In this contribution, we have chosen and
synthesized a bridging ligand Hpbi, which probably results in the
versatile structure of supramolecules with the desired properties.
It has characteristics based on the following advantages: (i) it has
four potential binding sites; (ii) it has a strong capability of forming
hydrogen bonding and stacking interactions along aromatic rings
(Scheme 1). To the best of our knowledge, these Cd(II)-based
supramolecular complexes constructed by this ligand have not
been reported. Herein, we report the synthesis, characterization
and structures of three new Cd(II)-based coordination complexes
containing rigid Hpbi ligand, [Cd(Hpbi)Cl2] (1), [Cd(Hpbi)2Br2] (2)
tectures remains still
a great challenge. There exist several
factors affecting the supramolecular assemblies, such as the coor-
dination geometry of the central metal ions [16–20], ligand struc-
ture [21,22], metal–ligand ratio [23,24], reaction conditions
(solvents [25,26], pH value [27–30], and temperature [31,32]),
and counterions [33–36]. Among these factors, the anions play
an important role in controlling synthesis of the molecular archi-
tecture. Additionally, the noncovalent forces, such as hydrogen-
bonding (X–HÁÁÁY) (X = C, N or O atoms; Y = N, O, Cl, Br or I ones),
C–HÁÁÁ
p
,
p p stacking, and other interactions, obviously affect
ÁÁÁ
the supramolecular topology and dimensionality although they
are weaker than the covalent force [37–39]. With this understand-
ing, one key aim of this contribution is to investigate the influence
of counterions on the structural self-assemblies, which may
⇑
Corresponding author. Tel./fax: +86 0531 89000551.
E-mail address: ceswyt@sohu.com (Y.-T. Wang).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.07.011

Y.-T. Wang et al. / Inorganica Chimica Acta 376 (2011) 492–499
493
suitable for X-ray diffraction were obtained in a yield of ca. 40%. Anal.
calc. for C22H16Br4Cd2N8: C, 28.20; H, 1.72; N, 11.96. Found: C, 28.37;
H, 1.73; N, 11.91%. IR (KBr, cmÀ1): 3184(br), 3143(w), 3061(w),
2981(w), 2681(w), 2331(w), 1956(w), 1800(w), 1616(w), 1589(m),
1531(w), 1483(m), 1431(s), 1394(m), 1315(s), 1231(m), 1173(s),
1141(w), 1114(w), 1053(m), 1030(s), 978(m), 945(w), 853(m),
819(m), 760(s), 717(m), 656(m), 628(w), 570(w), 543(w), 505(w),
436(w), 410(w).
Scheme 1. The ligand Hpbi.
and [Cd(Hpbi)2I2] (3), in which the effect of counter-anions on the
luminescent intensity is observed, and the bridging character of
halide deceases down the radius of counter anions. Additionally,
the TGA for complexes 1–3 have also been presented and discussed
in detail.
2.5. Synthesis of [Cd(Hpbi)2I2] (3)
Complex 3 was synthesized by a similar solvothermal procedure
using CdI2 in the place of CdCl2Á2.5H2O. Orange plate crystals suit-
able for X-ray diffraction were obtained in a yield of ca. 47%. Anal.
calc. for C22H16CdI2N8: C, 34.83; H, 2.13; N, 14.77. Found: C,
34.73; H, 2.14; N, 14.81%. IR (KBr, cmÀ1): 3408(br), 3183(w),
3140(w), 3049(w), 3016(2), 2854(w), 2762(w), 1933(w), 1807(w),
1591(m), 1531(m), 1481(m), 1431(s), 1317(m), 1296(w), 1255(w),
1231(w), 1167(m), 1145(w), 1057(m), 1024(m), 978(m), 908(w),
835(m), 768(s), 754(w), 681(m), 570(w), 546(w), 505(w), 436(m),
413(w).
2. Experimental
2.1. Materials and physical measurements
The reagents and solvents employed were commercially avail-
able and used as received without further purification. The C, H,
and N microanalyses were carried out with a Perkin-Elmer 240 ele-
mental analyzer. The FT-IR spectra were recorded from KBr pellets
in the range 4000–400 cmÀ1 on a Bruker spectrometer. Thermo-
gravimetric analysis (TGA) data were collected with a TA SDTQ600
analyzer in N2 flow at a heating rate of 15 °C minÀ1. Powder X-ray
diffraction (PXRD) patterns were recorded on a D8 ADVANCE dif-
2.6. X-ray crystallography
Single crystal X-ray diffraction measurements of 1–3 were
carried out with a Bruker Smart APXII CCD diffractometer equipped
with a graphite crystal monochromator situated in the incident
beam for data collection at 296(2) K. Data collections were
fractometer with Cu Ka radiation (k = 1.5409 Å) at a scanning rate
of 4° minÀ1 with 2h ranging from 5° to 60°. The emission/excitation
spectra were recorded on an Edinburgh FLS920 spectrometer
equipped with a continuous Xe900 Xenon lamp and an nF900
nanosecond flash lamp.
performed with Mo Ka radiation (k = 0.71073 Å) by x-scan mode.
No evidence was found for crystal decay during data collection of
complexes 1–3. All the measured independent reflections were
used in the structural analysis, and semi-empirical absorption
corrections were applied using the SADABS program. The program
SAINT [56] was used for integration of the diffraction profiles. All
the structures were solved by direct methods using the SHELXS
program of the SHELXTL package and refined with SHELXL [57]. Metal
atoms were located from the 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 all the non-
hydrogen atoms on F2. All the hydrogen atoms were first found
in difference electron density maps, and then placed in the calcu-
lated sites and included in the final refinement in the riding model
approximation with displacement parameters derived from the
parent atoms to which they were bonded. A summary of the crys-
tallographic data and structure refinements are listed in Table 1.
2.2. Synthesis of ligand 2-(pyrazin-2-yl)-1H-benzimidazole (Hpbi)
Ligand Hpbi was prepared by modifying a reported literature
procedure [55]. The pyrazine-2-carboxylic acid (2.604 g, 21 mmol)
and o-phenylenediamine (2.16 g, 20 mmol) were mixed. Then
polyphosphate acid (20 ml) was added to the above mixture. The
reaction mixture was irradiated in microwave oven. After 10 min,
the reaction mixture was cooled to room temperature, poured in
the iced water (100 ml), and neutralized by 10% NaOH. The solid
compound obtained was filtered, washed with water, and recrys-
tallized from aqueous ethanol. Yield: 62.63% (2.455 g). The spectra
analysis of Hpbi is identical to that of the reported compound.
2.3. Synthesis of [Cd(Hpbi)Cl2] (1)
CdCl2Á2.5H2O (22 mg, 0.1 mmol) and Hpbi (20 mg, 0.1 mmol)
were mixed with a mixture solvents of methanol (3 mL) and water
(3 mL). The mixture was sealed in a 25 mL Teflon-lined autoclave,
heated at 160 °C for 2 days, and then slowly cooled to room
temperature. Orange plate crystals suitable for X-ray diffraction
were obtained in a yield of ca. 85%. Anal. calc. for C11H8CdCl2N4: C,
34.81; H, 2.12; N, 14.76. Found: C, 34.31; H, 2.14; N, 14.79%. IR
(KBr, cmÀ1): 3217(br), 3151(w), 3093(w), 3062(w), 2922(w),
2852(w), 2638(w), 2507(w), 2331(w), 1964(w), 1917(w), 1805(w),
1616(w), 1593(m), 1531(m), 1483(m), 1431(s), 1392(m), 1314(m),
1305(m), 1240(m), 1171(m), 1149(m), 1122(w), 1028(m), 978(m),
943(w), 858(m), 821(w), 762(s), 725(m), 652(m), 503(m), 434(m),
410(w).
3. Results and discussion
3.1. Preparation of Hpbi and complexes 1–3
The ligand Hpbi was synthesized as a pale-brown solid by the
condensation reaction of pyrazine-2-carboxylic acid and o-phenyl-
enediamine in polyphosophoric acid under microwave radiation.
The ligand possesses four potential donor sites, two nitrogen donor
atoms from the pyrazine rings, two nitrogen atoms from the
imidazole (Scheme 1). The one-dimensional chain, binuclear and
mononuclear complexes were readily isolated in moderate yield
by the reaction of Hpbi with a series of Cd(II) with different halo
anions in 1:1 mol ratio in methanol solution, respectively. The
structures of these complexes were not obviously changed though
the ligand/metal ratios changed from 2:1 to 1:2. The chelated
complexes were always obtained, which were confirmed by their
PXRD (Figs. S1–S3). The present results indicate that the final
products are independent of the molar ration of ligand/metal salts.
2.4. Synthesis of [Cd(Hpbi)2Br2] (2)
Complex 2 was synthesized by a similar solvothermal procedure
using CdBr2Á4H2O in the place of CdCl2Á2.5H2O. Orange plate crystals

494
Y.-T. Wang et al. / Inorganica Chimica Acta 376 (2011) 492–499
Table 1
Crystal data and structure refinement parameters for 1–3.
1
2
3
Empirical formula
Formula weight
T (K)
C
11H8CdCl2N4
C22H16Br4Cd2N8
936.85
296
C22H16CdI2N8
758.64
296
379.52
296
k (Å)
0.71073
monoclinic
P21/c (No. 14)
7.0841(7)
18.7054(19)
9.5569(10)
90
97.492(1)
90
1255.6(2)
4
0.71073
triclinic
P1 (No. 2)
0.71073
orthorhombic
Pccn (No. 56)
16.9291(17)
9.8548(10)
14.5912(14)
90
90
90
2434.3(4)
4
3.461
Crystal system
Space group
a (Å)
b (Å)
c (Å)
ꢀ
8.1346(12)
8.8032(13)
9.6850(14)
76.676(2)
78.926(2)
76.945(2)
650.22(17)
1
a
(°)
b (°)
c
(°)
V (Å3)
Z
l
(mmÀ1
)
2.149
7.810
Crystal size (mm)
0.10 Â 0.10 Â 0.30
0.10 Â 0.10 Â 0.30
0.10 Â 0.10 Â 0.40
q
(g cmÀ3
)
2.008
2.393
2.070
Reflections collected
10 747
5741
20 081
Independent reflections
2869
2930
2808
Reflections [I > 2
r(I)]
2608
2462
2291
Rint
0.027
0.028
0.036
h (°)
2.2/27.5
1.36
2.2/27.5
1.09
2.4/27.6
1.04
Goodness-of-fit (GOF) on F2
Largest hole and peak (e ÅÀ3
)
À0.57/0.54
À1.01/0.89
À0.85/0.97
a
R1 [I > 2
r(I)]
0.0341
0.0312
0.0301
b
wR2 (all data)
0.0974
0.0927
0.0838
P
P
a
R1
=
||Fo| À |Fc||/ |Fo|.
P
P
b
2
wR2 = [ [w(Fo À Fc2)2]/ [w(Fo2)2]]1/2
.
These complexes give satisfactory microanalytical data, and the
crystal structures show that the ligand Hpbi in complexes
possesses the same chelated form. Complexes 1–3 are air stable
and can retain their structural integrity at room temperature for
a considerable length of time.
2.721(1) Å (Table 2), which are similar to those found in the re-
ported complexes [58]. The adjacent Cd(II) centers are bridged by
two l2-Cl atoms into a 1-D zig-zag type chain with a dihedral angle
of 78.9° between two adjacent Cd2Cl2 planes. In this plane unit,
the interachain Cd–Cl–Cd angle is 96.41(4)°, and the interchain
CdÁÁÁCd spearation is 3.972(1) Å, obviously larger than that found
in complexes [59]. There exists the torsion angle of À5.96(5)° be-
tween the benzimidazole ring and pyrazine one in Hpbi ligand,
which is different from that found in the document [51].
3.2. Description of crystal structures
Reactions of CdX2 (X = ClÀ, BrÀ, IÀ) and the ligand Hpbi yield the
structures of [Cd(Hpbi)(l2-Cl)2] (1), [Cd(Hpbi)2(l2-Br)2] (2), and
[{Cd(Hpbi)I2] (3), respectively. Chloride serves as a bridging ligand
to generate one-dimensional complex 1. Bromide acts as both a
The Hpbi molecules arrange alternately on both sides of the par-
ent chain because the Hpbi rings of adjacent CdCl2 chains are in-
ter-ditigated through the
p–p stacking interactions with a
distance of 3.630(3) and 3.688(3) Å along the b axial direction
(Fig. 1b). Additionally, along the b axis direction, the interchain
hydrogen bonds are formed through the nitrogen atom and carbon
one from each Hpbi ligand with the Cl atoms (N4–H4ÁÁÁCl1i
3.364(4) Å, 164(5)°; C10–H10AÁÁÁCl2i 3.517(5) Å, 158°, i = x, y,
1 + z), which extends these 1-D chains into a 2-D grid-like supra-
molecular sheet (Fig. 1b). Meanwhile, there exist the intrachain
weak hydrogen bonds (C7–H7AÁÁÁCl2 3.568(5) Å). The carbon atom
from each Hpbi ligand is connected to uncoordinated nitrogen
atom from pyrazine group on the neighboring layers via the hydro-
gen bonds (C4–H4AÁÁÁN2i 3.319(8), 141°, i = x, 1/2 À y, À1/2 + z),
which propagates finally the 2-D supramolecular layers into a
simple coordinating ligand and
a bridging ligand to form
bromide-bridged dinuclear complex 2. Iodide only plays at a
simple coordinating ligand to produce a mononuclear complex 3.
From the present results, it is interestingly observed that bridging
ability of halides decreases from chloride to iodide (Chart 1).
X-ray single-crystal determination indicates that complex 1 is
one-dimensional chain. As shown in Fig. 1a, in compound 1, the
crystallographically unique Cd atom with a distorted octahedral
geometry bonds to four Cl atoms and two N atoms from a Hpbi
molecule. The Cd–N distance is normal 2.334(3)–2.455(4) Å, but
the Cd–Cl distances spin a rather wider range from 2.552(1) to
3-D supramolecular architecture along the
(Fig. 1c and Table 3).
a axial direction
Unlike 1, complex 2 crystallizes in the orthorhombic space
group Pccn and consists of a bisnuclear unit. As shown in Fig. 2a,
an asymmetric unit contains a Cd atom, a Hpbi ligand, two
bromides, and whole dinuclear molecule has been generated
through the inversion center in the middle of the molecule. Each
Cd(II) ion coordinates two nitrogen atoms of the chelating Hpbi
ligand, two bridging bromides, and a simple coordinating bromide
occupying an apical position, adopting an distorted square pyramid
geometry (CdN2Br3). The Cd–N bond distances are 2.300(4) and
2.419(4) Å, similar to those found in the literature (Table 2) [59].
Chart 1.

Y.-T. Wang et al. / Inorganica Chimica Acta 376 (2011) 492–499
495
Fig. 1. (a) Local coordination environment around Cd atom for 1. (b) 2-D supramolecular layer extended by hydrogen bonds along the b axis in 1. Thin purple and red lines
denote hydrogen bonds and stacking interactions, respectively. (c) 3-D packing diagram generated by hydrogen bonds along the a axis in 1. Thin red lines stand for hydrogen
bonds. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this paper.)
The Cd–
l
2-Br distances are 2.6842(8) and 2.7211(7) Å, respec-
hydrogen-bonding interactions, including N4–H4ÁÁÁBr1i (3.475(4)
Å, 160(5), i = 2 À x, Ày, Àz), between the nitrogen atom from imid-
azole group and a simple coordinated bromide from adjacent
dimer unit, and C2–H2AÁÁÁBr1i (3.724(6) Å, 156°, i = 2 À x, Ày, Àz)
between the carbon atom from pyrazine ring and a simple coordi-
nated bromide atom from adjacent dimer unit. In addition, the
tively, which are much longer than the Cd–Brno
distances
bridge
(2.5836(8) Å). This implies that the bridging capacity of the bro-
mide is weaker than that of the chloride. In the dinuclear unit in
2, the Cd–Br–Cd angle is 93.87(2)°, being much smaller than that
of complex 1. Though so, the CdÁÁÁCd distance is 3.949(1) Å, which
is comparable to that in the chloride-bridged complex 1. Both
benzimidazole and pyrazine ring little distorts with the torsion
angle N1–C1–C5–N3 of À5.73(1)°, being slightly equal to the
corresponding value in 1. Alike 1, there exist two kinds of the weak
p p stacking interactions (3.614(3) Å) occur between the imidaz-
ÁÁÁ
ole ring and pyrazine one from the adjacent dimer unit. These
interactions give rise to a 1-D chain. Strong face-to-face stack-
ÁÁÁ
ing interactions (minimum centroid-to-centroid distances 3.599(3)
p
p

496
Y.-T. Wang et al. / Inorganica Chimica Acta 376 (2011) 492–499
Table 2
iodide is much weaker than that of the chloride and the bromide.
Selected bond lengths (Å) and bond angles (°) for 1–3.
The benzimidazole ring and the pyrazine one in the ligand is some-
what tilted with the torsion angle N1–C1–C5–N3 of À8.81(1)°,
which is different from the corresponding value in 1 and 2.
1
Cd1–Cl1
Cd1–Cl2
Cd1–N1
2.6058(11)
2.5518(12)
2.455(4)
101.41(3)
96.57(9)
161.71(9)
92.83(3)
83.59(3)
159.02(9)
94.45(9)
86.41(3)
Cd1–N3
2.334(3)
2.6754(12)
2.7210(12)
98.59(4)
69.93(12)
81.98(9)
94.04(9)
97.23(9)
85.10(9)
174.34(4)
Cd1–Cl2a
Analysis of the crystal packing of 3 reveals the existence of
intermolecular hydrogen bonds N4–H4ÁÁÁN2i (2.987(5) Å, i = 1 À x,
1/2 + y, 3/2 À z) between nitrogen atom from imidazole rings and
nitrogen atom from pyrazine rings from adjacent complex mole-
cules, which results in the formation of 2D supramolecular layers
(Fig. 3b). Similar to 1 and 2, the weak hydrogen-bonding (C8–
H8AÁÁÁI1i, 3.865(5) Å, i = x, 1 + y, z) is also present, which is compa-
rable to those of Cd(II) complexes documented previously [58,59].
Cd1–Cl1b
Cl1–Cd1–Cl2
Cl1–Cd1–N1
Cl1–Cd1–N3
Cl1–Cd1–Cl2a
Cl1–Cd1–Cl1b
Cl2–Cd1–N1
Cl2–Cd1–N3
Cl2–Cd1–Cl2a
Cl1b–Cd1–Cl2
N1–Cd1–N3
Cl2a–Cd1–N1
Cl1b–Cd1–N1
Cl2a–Cd1–N3
Cl1b–Cd1–N3
Cl1b–Cd1–Cl2a
Additionally, the
p p stacking interactions (3.635(2) Å) occur at
ÁÁÁ
2
phenyl rings and imidazole rings from adjacent mononuclear com-
plexes, arranging and stabilizing of the supramolecular networks 3
(Fig. 3c).
Cd1–Br1
Cd1–Br2
Cd1–N1
Br1–Cd1–Br2
Br1–Cd1–N1
Br1–Cd1–N3
Br1–Cd1–Br2a
Br2–Cd1–N1
2.5836(8)
2.6842(8)
2.419(4)
105.98(2)
100.96(10)
104.25(10)
110.92(2)
152.87(10)
Cd1–N3
2.300(4)
2.7211(7)
Cd1–Br2a
Br2–Cd1–N3
Br2–Cd1–Br2a
N1–Cd1–N3
Br2a–Cd1–N1
Br2a–Cd1–N3
98.89(10)
86.13(2)
71.03(13)
87.23(9)
141.57(9)
3.3. Powder X-ray diffraction
3
To examine the phase purity of these complexes, the PXRD pat-
terns of the compounds were investigated at room temperature.
The peak positions of the simulated PXRD patterns are identical
to that of experimental ones (Figs. S1–S3), which indicates that
the purity of the products is single phase. The tiny discrepancy in
intensity may be attributable to the preferred orientation of the
microcrystalline powder samples.
I1–Cd1
Cd1–N1
2.8372(5)
2.605(3)
88.60(7)
98.69(7)
99.83(1)
171.29(7)
108.90(7)
Cd1–N3
2.312(3)
I1–Cd1–N1
I1–Cd1–N3
I1–Cd1–I1a
I1–Cd1–N1a
I1–Cd1–N3a
N1–Cd1–N3
N1–Cd1–N1a
N1–Cd1–N3a
N3–Cd1–N3a
67.28(10)
83.06(9)
80.45(10)
136.72(10)
Symmetry codes: for 1: aÀx, 1 À y, Àz; b1 À x, 1 À y, Àz; cx, y, À1 + z for 2: a1 À x, Ày,
1 À z; bx, y, 1 + z for 3: a1/2 À x, 1/2 À y, z; bx, À1 + y, z; c1 À x, 1 À y, 1 À z.
3.4. Thermogravimetric analysis
In order to estimate the thermal stability of complexes 1–3,
thermogravimetric analyses (TGA) were carried out. TGA of com-
plexes 1–3 were performed under N2 atmosphere with a heating
rate of 20 °C minÀ1. TGA curves for 1–3 indicate complexes are sta-
ble up to 176, 350, and 330 °C, respectively (Figs. S4–S6). The
weight loss occurs at the temperature range of 180–700 °C for 1,
360–700 °C for 2, and 332–580 °C for 3, respectively, correspond-
ing to the loss of Hpbi ligand, halide anion and the decomposition
of the structure.
Table 3
Hydrogen bond geometries in the crystal structure of 1–3.
Complex
D–HÁÁÁAa
HÁÁÁA (Å)
DÁÁÁA (Å)
D–HÁÁÁA (°)
1
N4–H4ÁÁÁCl1a
C4–H4AÁÁÁN2b
C7–H7AÁÁÁCl2
C10–H10AÁÁÁCl2a
2.64(6)
2.5400
2.7800
2.6400
3.364(4)
3.319(8)
3.568(5)
3.517(5)
164(5)
141.00
143.00
158.00
2
3
N4–H4ÁÁÁBr1a
C2–H2AÁÁÁBr1a
C3–H3AÁÁÁBr2b
2.53(6)
2.8600
2.8800
3.475(4)
3.724(6)
3.768(6)
160(5)
156.00
159.00
3.5. Luminescent properties
N4–H4ÁÁÁN2a
C8–H8AÁÁÁI1b
2.27(6)
3.0100
2.987(5)
3.865(5)
139(4)
153.00
As illustrated Fig. 4, the solid-state room temperature photolu-
minescence of 1 exhibits the emission peak at 497 nm upon an
excitation maximum at 280 nm. The solid-state photolumines-
cence spectrum of 2 at room temperature is observed with the
main emission at 507 nm (kex = 280 nm). Complex 3 also shows
broad emission bands with kmax at 517 nm upon 280 nm excita-
tion. To further estimate the relative luminescence of between
the free ligand and its complexes, the luminescent property of
the free Hpbi ligand was also measured, indicating the maximum
emission at ca. 548 nm (kex = 280 nm) (Fig. S7). By comparison
with the free ligand Hpbi, the emission maxima of complexes 1–
3 are obviously blue-shifted about 51, 41 and 31 nm, respectively,
which may assign to the coordination effects of the ligand Hpbi to
cadmium ions. It is difficult to oxidize or to reduce the Cd(II) ions,
which is owing to their d10 configuration. For 1–3, the similarity of
luminescent profiles is probably attributable to the ligand-centred
emission [60]. Notably, it is observed that the luminescent inten-
sity for 1–3 increases because of effective increase of the rigidity
of the ligand and reduction of the loss of energy by nonradiative
decay [61]. Additionally, it is noteworthy that the photolumines-
cent intensity increase from 1 to 3, which may be assigned to their
difference in polarization [60].
Symmetry codes: for 1: ax, y, 1 + z; bx, 1/2 À y, À1/2 + z; for 2: a2 À x, Ày, Àz; b1 + x,
À1 + y, z for 3: a1 À x, 1/2 + y, 3/2 À z; bx, 1 + y, z.
and 3.642(3) Å, respectively) between 1-D chains form a 2-D
supramolecular layer (Fig. 2b), which further extend to 3-D
supramolecular networks through the weak C3–H3AÁÁÁBr2i
(3.768(6) Å, i = x, 1 + y, z) hydrogen bonds interactions (Fig. 2c
and Table 3).
In contrast to the complexes 1 and 2, [Cd(Hpbi)2I2] (3) is a mono-
nuclear complex (Fig. 3a). In the centrosymmetric mononuclear
compound 3, each Cd(II) ion, locating at the inversion center, coor-
dinates to four nitrogen atoms from two distinct Hpbi molecues and
two iodides, adopting a distorted octahedral geometry (CdN4I2).
The Cd–N distances are 2.312(3) and 2.605(3) Å, respectively,
which falls in normal range and are similar to those found in com-
plexes published before [58,59]. The Cd–I length is 2.8372(5) Å,
which is much shorter than those of Cd(II) complexes reported in
the literature [59]. It indicates that the bridging capacity of the

Y.-T. Wang et al. / Inorganica Chimica Acta 376 (2011) 492–499
497
Fig. 2. (a) Local coordination environment around Cd atom for 2. (b) 2-D supramolecular layer formed by hydrogen bonds and stacking interactions along the b axis in 2. Thin
purple and red lines represent intermolecular hydrogen bonds and stacking interactions, respectively. (c) The packing perspective along the c axis in 2. (For interpretation of
the references to color in this figure legend, the reader is referred to the web version of this paper.)
of CdII with different anions. The present results demonstrate
4. Conclusions
that the structures and luminescent intensity of final product
are affected by the radius and polarization of anions. It is
noteworthy that the ligand Hpbi in these complexes is coordina-
tion unsaturated, which can be acted as ‘‘complex ligands’’ to
In summary, three new coordination supramolecular com-
plexes have been generated from an N-heterocycle organic
ligand 2-(pyrazin-2-yl)-1H-benzoimidazole (Hpbi) and a series

498
Y.-T. Wang et al. / Inorganica Chimica Acta 376 (2011) 492–499
Acknowledgements
This work was financially supported by the Project of Shandong
Province Higher Educational Science and Technology Program
(J09LB03) and the Starting Funding of Shandong Polytechnic Uni-
versity (to Dr. Y.-T. Wang).
Appendix A. Supplementary material
CCDC 795430, 795431 and 795432 contain the supplementary
crystallographic data for complexes 1, 2 and 3, respectively. These
data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2011.07.011.
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