Halide/pseudohalide complexes of cadmium(II) with benzimidazole: Synthesis, crystal structures and fluorescence properties

Article abstract of DOI:10.1016/j.molstruc.2017.07.013

Two new dinuclear Cd(II) complexes, [CdL¹Cl₂]₂·H₂O (1) and [CdL¹(N₃)₂]₂·CH₃OH (2) and one dicyanamide bridged one-dimensional polynuclear network [CdL¹(μ_1,5-dca)dca]_n (3) of the potentially tridentate NNN-donor Schiff base 2-((1H-benzimidazol-2-yl-ethylimino)-methyl)pyridine (L¹) and another dinucler Cd(II) complex [CdL²Cl(dca)]₂ (4) of a similar NNN-donor Schiff base ligand 2-((1H-benzimidazol-2-yl-propylimino)-methyl)pyridine (L²), have been synthesized and characterized by elemental analyses, IR and single crystal X-ray crystallography. The ligands L¹ and L² are [1 + 1] condensation products of pyridine-2-carbaldehyde with 2-aminoethyl-1H-benzimidazole and 2-aminopropyl-1H-benzimidazole, respectively. In the complexes 1 and 4 the two Cd(II) centers are held together by the bridged chloride ligands, while in 2 the two Cd(II) centers are bridged by μ_1,1-azide ions. Complex 3 has a one-dimensional infinite chain structure in which Cd(II) ions are bridged by single dicyanamide groups in end-to-end fashion. All the metal centers have a distorted octahedral geometry and H-bonding or π?π interactions are operative to bind the complex units in the solid state. Furthermore, these complexes have been investigated by thermogravimetric analyses and fluorescence spectra.

Full text of DOI:10.1016/j.molstruc.2017.07.013

Accepted Manuscript
Halide/pseudohalide complexes of cadmium(II) with benzimidazole: Synthesis, crystal
structures and fluorescence properties
Hai-Yan Zhao, Fu-Li Yang, Na Li, Xiao-Jing Wang
PII:
S0022-2860(17)30934-1
10.1016/j.molstruc.2017.07.013
MOLSTR 24040
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 11 November 2016
Revised Date: 3 June 2017
Accepted Date: 9 July 2017
Please cite this article as: H.-Y. Zhao, F.-L. Yang, N. Li, X.-J. Wang, Halide/pseudohalide complexes of
cadmium(II) with benzimidazole: Synthesis, crystal structures and fluorescence properties, Journal of
Molecular Structure (2017), doi: 10.1016/j.molstruc.2017.07.013.
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ACCEPTED MANUSCRIPT
Halide/pseudohalide
complexes
of
cadmium(II)
with
benzimidazole: Synthesis, crystal structures and fluorescence
properties
Hai-Yan Zhao1*, Fu-Li Yang2 , Na Li1, Xiao-Jing Wang1
1
College of Science, Hebei University of Science and Technology, Shijiazhuang
050018, China
2Department of Chemical and Environmental Engineering, Hebei Chemical
Pharmaceutical College, Shijiazhuang, Hebei 050026,China
&
*Corresponding author:
Hai-Yan Zhao. Tel: +86-0312-88168532. E-mail: hbhaiyanzh@163.com
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ABSTRACT
Two new dinuclear Cd(II) complexes, [CdL1Cl2]2·H2O (1)
and
[CdL1(N3)2]2·CH3OH (2) and one dicyanamide bridged one-dimensional polynuclear
network [CdL1(ꢀ1,5-dca)dca]n (3) of the potentially tridentate NNN-donor Schiff base
2-((1H-benzimidazol-2-yl-ethylimino)-methyl)pyridine (L1) and another dinucler
Cd(II) complex [CdL2Cl(dca)]2 (4) of a similar NNN-donor Schiff base ligand 2-
((1H-benzimidazol-2-yl-propylimino)-methyl)pyridine (L2), have been synthesized
and characterized by elemental analyses, IR and single crystal X-ray crystallography.
The ligands L1 and L2 are [1+1] condensation products of pyridine-2-carbaldehyde
with 2-aminoethyl-1H-benzimidazole and 2-aminopropyl-1H-benzimidazole,
respectively. In the complexes 1 and 4 the two Cd(II) centers are held together by the
bridged chloride ligands, while in 2 the two Cd(II) centers are bridged by ꢀ1,1-azide
ions. Complex 3 has a one-dimensional infinite chain structure in which Cd(II) ions
are bridged by single dicyanamide groups in end-to-end fashion. All the metal centers
have a distorted octahedral geometry and H-bonding or πꢁꢁꢁπ interactions are operative
to bind the complex units in the solid state. Furthermore, these complexes have been
investigated by thermogravimetric analyses and fluorescence spectra.
Keywords:
Benzimidazole-based-Schiff base, Cadmium(II) complexes, Crystal structures,
Fluorescence
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Graphical abstract
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Three new dinuclear Cd(II) complexes, [CdL1Cl2]2·H2O, [CdL1(N3)2]2·CH3OH
and [CdL2Cl(dca)]2 and one polynuclear network [CdL1(ꢀ1,5-dca)dca]n have been
prepared and characterized by single-crystal X-ray crystallography using the
benzimidazole derived primary Schiff base ligands L1and L2. The fluorescent
properties of two ligands as well as four compounds were also studied.
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1. Introduction
Recently, complexes of Cd(II) with tridentate and tetradentate Schiff base
ligands have growing interest, not only because of their fascinating structures, but also
their antibacterial, antitumor activities, photocatalytic activity, adsorption,
electrochemical luminescence and fluorescent properties , as well as other potential
applications [1–33] . In these complexes Cd(II) ions can exhibit a wide variety of
geometries and coordination numbers along with mono-, di-, polynuclear coordination
molecules and 1D, 2D and 3D supramolecular frameworks due to its larger radius and
d10 electronic configuration [8]. The coordination geometry of these complexes also
depends upon the coordination behaviors of Schiff bases, the coordination ability and
size of counteranions, as well as the reaction temperature and pH [1–4, 34–35]. In
additions, multiple weak non-covalent forces like hydrogen bonding, π···π and C–
H···π interactions play an important role in construction of these architectures [3–5,
11–14, 36–39]. Halides and pseudohalides such as thiocyanate (SCN−), dicyanamide
−
(dca), and azide (N3 ) are suitable inorganic building blocks which can link one or
more metal ions in versatile coordination modes for creating different mono-, di-,
polynuclear coordination molecules and supramolecular entities. Many different
cadmium(II) compounds with interesting molecular or supramolecular structures
using organic ligands in combination with halides/pseudohalides are well-studied in
the literatures [8–12, 38–46].
For the present work, we chose two tridentate N-donor Schiff bases, L1(2-((1H-
benzimidazol-2-yl-ethylimino)-methyl)pyridine) and L2 (2-((1H-benzimidazol-2-yl-
propylimino)-methyl)pyridine )(Scheme 1), which differ by only one –CH2– group in
their ligating backbone to construct Cd(II) complexes in combination with various
−
anion co-ligands including chloride (Cl−), azide (N3 ) and dicyanamide (dca): (i) Due
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to presence of NH group and large aromatic system of the two Schiff bases, they can
act as hydrogen bonds donors and provide potential supramolecular recognition sites
for π···π stacking interactions leading to supramolecular architecture. Furthermore,
Cd(II) complexes of the two Schiff bases which contain conjugated π systems may
-
have fluorescent properties; (ii) The anion co-ligands chloride (Cl-), azide (N3 ) and
dicyanamide (dca) can tune the coordination numbers and geometry of the Cd(II)
centers through coordinate to the Cd(II) centers, which may influence the properties
of these resulting complexes. In addition, these anions can also act as the hydrogen-
bonded acceptor to extend the complexes into high-dimension networks.
Based on the above considerations, we report herein the synthesis, structures,
supramolecular architectures, thermal stability and fluorescent properties of four
distinct Cd(II) Schiff base complexes. All the Cd(II) ions in the four complexes are
six coordinate, however, very interesting structural diversities occurred on varying the
anion co-ligands. Complexes 1 [CdL1Cl2]2·H2O and 2 [CdL1(N3)2]2·CH3OH are
binuclear complexes, which are bridged by chloride and azide groups, respectively.
Complex 3 [CdL1(ꢀ1,5-dca)dca]n is a one dimensional chain through the ꢀ1, 5-bridged
dca anions. D
ca and chloro anions produce complex 4 [Cd(L2)(ꢀ2-Cl)(dca)]2, which is
also a chroride bridged binuclear complex and one of the axial positions is occupied
by the dca anions. All the complexes reported here exhibit 3D supramolecular
frameworks in 1, 3, and 4 and 2D supramolecular frameworks in 2 via hydrogen
bonding and π···π interactions.
2. Experimental
2-Aminoethyl-1H-benzimidazole dihydrochloride and 2-aminopropyl-1H-
benzimidazole dihydrochloride were prepared by the method reported by Cescon et al
[47]. A solution of 10.8 g (0.1 mol) of o-phenylenediamine and 13.4g (0.15 mol) of β-
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alanine in 100 mL of 5.5 mol·L-1 hydrochloric acid was refluxed for 24 h. The
solution was cooled overnight and the hydrochloride removed by filtration. It was
recrystallized from 90% ethyl alcohol with the aid of decolorizing carbon. 2-
Aminopropyl-1H-benzimidazole dihydrochloride was prepared by the same procedure
as that of 2-Aminoethyl-1H-benzimidazole dihydrochloride, but using γ-aminobutyric
acid 10.3 g (0.1 mol) in place of p-alanine. All of the other reagents were of AR grade
and were obtained from commercial sources and used without further purification.
2.1. Preparations
2.1.1. Preparation of ligands
2.1.1.1. Synthesis of L1 (2-((1H-benzimidazol-2-yl-ethylimino)-methyl)pyridine).
The ligand L1 was synthesized by a condensation reaction between 2-aminoethyl-
1H-benzimidazole dihydrochloride (1.170g, 5mmol), previously neutralized with
K2CO3 (0.828g, 6mmol), and pyridine-2-carbaldehyde (0.535g, 5mmol) in 25 mL of
methanol. The reaction mixture was stirred at room temperature for approximately 2h.
The resulting brown solution was removed on a rotatory evaporator and the light
brown product was then filtered and recrystallized from EtOH for two times. Yield:
1.06 g (85%). m.p.: 155-156 ℃. IR (KBr pellet, cm−1): 1635(νC=N), 1437(νC=C–C=N). 1H
NMR (500MHz, d6-DMSO, ppm) δ: 3.22–3.25(t, 2H, –CH2–), 4.05–4.08 (t, 2H, –
CH2–), 7.42–8.97(m, 8H, Ar–H), 8.98(s, 1H, –CH=N–), 12.61(br s, 1H, NH). UV-Vis
(methanol, λmax, nm): 204, 248, 276, 281. Anal. Calc. for C15H14N4: C, 71.98; H, 5.64;
N, 22.38. Found: C, 71.86; H, 5.57; N, 22.29%.
2.1.1.2. Synthesis of L2 (2-((1H-benzimidazol-2-yl-propylimino)-methyl)pyridine).
The ligand L2 was prepared following the same procedure as that of L1, but using
2-aminopropyl-1H-benzimidazole dihydrochloride (1.24 g, 5 mmol) in place of 2-
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aminoethyl-1H-benzimidazole dihydrochloride. Yield: 1.08 g (82%). m.p.: 118-119 ℃.
1
IR (KBr pellet, cm−1): 1635(νC=N), 1437(νC=C–C=N). H NMR (500MHz, d6-DMSO,
ppm) δ: 2.26–2.32(m, 2H, –CH2–), 2.99–3.02(t, 2H, –CH2–), 3.69–3.71 (t, 2H, –CH2–
), 7.43–8.97(m, 8H, Ar–H), 8.98(s, 1H, –CH=N–), 12.64(br s, 1H, NH). UV-Vis
(methanol, λmax, nm): 204, 239, 273, 280. Anal. Calc. for C16H16N4: C, 72.70; H, 6.10;
N, 21.20. Found: C, 72.52; H, 6.05; N, 21.08%.
2.1.2. Synthesis of the complexes
2.1.2.1. Synthesis of [CdL1Cl2]2·H2O (1)
To a methanol solution (10 mL) of L1 (0.025g, 0.1 mmol), a solution of
CdCl2·2.5H2O (0.023g, 0.1mmol) in the same solvent was slowly added. The stirring
was continued for addition 2 h and filtered. The light yellow solution was kept at
room temperature, which produced yellow crystals suitable for X-ray diffraction after
7 days. The crystals were isolated by filtration and air-dried. Yield: (0.055g, 61%).
IR (KBr pellet, cm−1): 3416(νOH), 3200-2700(νNH), 1624(νC=N), 1451(νC=C–C=N). UV-
Vis (methanol, λmax, nm): 201, 203, 232, 273, 279. Anal. Calc. for C30H32Cd2Cl4N8O2:
C, 39.89; H, 3.57; N, 12.41%. Found: C, 39.84; H, 3.49; N, 12.30%.
2.1.2.2. Synthesis of the [CdL1(N3)2]2·CH3OH (2)
To a methanol solution (10 mL) of L1 (0.025g, 0.1 mmol), a solution of
Cd(ClO4)·6H2O (0.042g, 0.1 mmol) in the same solvent was slowly added, followed
by drop addition of a 1 mL aqueous solution of NaN3 (0.013g, 0.2 mmol). The stirring
was continued for addition 2 h and filtered. The light yellow solution was kept at
room temperature, which produced yellow crystals suitable for X-ray diffraction after
7 days. The crystals were isolated by filtration and air-dried. Yield: (0.062, 65%). IR
(KBr pellet, cm−1):
3587(νOH), 3400-2800(νNH), 1629(νC=N), 1453(νC=C–C=N),
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2076(νN3). UV-Vis (methanol, λmax, nm): 201, 203, 233, 273, 279. Anal. Calc. for
C32H36Cd2N20O2: C, 40.14; H, 3.79; N, 29.25. Found: C, 40.05; H, 3.76; N, 29.05%.
2.1.2.3. Synthesis of the [CdL1(ꢀ1,5-dca)dca]n (3)
Complex 3 was prepared by the same procedure as that of complex 2, but using
NaN(CN)2 (0.018 g, 0.2 mmol) in place of NaN3 .Yield: (0.033g, 67%). IR (KBr
pellet, cm−1): 3230-2825(νNH), 1628(νC=N), 1450(νC=C–C=N), 2301, 2208, 2250, 2237,
2164, 2146(νdca). UV-Vis (methanol, λmax, nm): 201, 204, 235, 273, 279. Anal. Calc.
for C19H14Cd N10: C, 46.12; H, 2.85; N, 28.31. Found: C, 46.08; H, 2.80; N, 28.25%.
2.1.2.4. Synthesis of [CdL2Cl(dca)]2 (4)
To a methanol solution (10 mL) of L2 (0.026g, 0.1 mmol), a solution of
CdCl2·2.5H2O (0.023g, 0.1mmol) in the same solvent was slowly added, followed by
drop addition of a 1 mL aqueous solution of NaN(CN)2 (0.018 g, 0.2 mmol). The
stirring was continued for addition 2 h and filtered. The light yellow solution was kept
at room temperature, which produced yellow crystals suitable for X-ray diffraction
after 7 days. The crystals were isolated by filtration and air-dried. Yield: (0.060g,
63%). IR (KBr pellet, cm−1): 3260-2740(νNH), 1624(νC=N), 1450(νC=C–C=N), 2276,
2215, 2159(νdca). UV-Vis (methanol, λmax, nm): 202, 204, 237, 272, 279. Anal. Calc.
for C36H32Cd2Cl2N14: C, 45.21; H, 3.37; N, 20.50. Found: C, 45.17; H, 3.34; N,
20.38%.
2.2. Physical measurements
1
Elemental analyses were taken using a Perkin-Elmer 240C analyzer. H NMR
spectra were recorded on a Bruker Avance500MHz spectrometer at 25 °C in DMSO
with tetramethylsilane as the internal reference. The infrared spectra in KBr pellets
were obtained on a ThermoFisher Nicolet 6700 spectrometer in the 4000–400 cm-1
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region. A Shimadzu UV-2550 spectrophotometer was used to record the electronic
spectra. Fluorescent spectra were measured with a Hitachi F-4600 spectrophotometer.
The powder XRD data was collected in a 2θ range of 5–50° using a Rigaku D/MAX-
2500 X-ray diffractomete at ambient conditions.
2.3. X-ray crystallography
Diffraction intensities for complexes 1, 3 and 4 were collected on a Bruker Smart
1000 CCD area detector using graphite-monochromatized Mo-Kα radiation (λ =
0.71073 Å) with ω scan mode at 298(2) K for 1 and 3 and 293(2) K for 4, repectively.
Diffraction intensities for complex 2 was collected on a Bruker FRAMBO CCD area
detector using graphite-monochromatized Cu-Kα radiation (λ = 1.54178 Å) with ω
scan mode at 298(2) K. Unit cell dimensions were obtained with least-squares
refinements and semi-empirical absorption corrections were applied using SADABS
program [48]. All the structures were solved by direct method and non-hydrogen
atoms were obtained in successive difference Fourier syntheses. The final refinements
were performed by full-matrix least-squares methods on F2 by SHELXL-97 program
package [49]. Hydrogen atoms were included in calculated positions and refined with
fixed thermal parameters riding on their parent atoms. Further crystallographic data
and experimental details for structural analyses are summarized in Table 1.
(Insert Table 1 here)
3. Results and discussion
3.1. Synthesis
Two unsymmetrical Schiff base ligands L1 and L2 were respectively synthesized
by the condensation of pyridine-2-carbaldehyde with 2-aminoethyl-1H-benzimidazole
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and 2-aminopropyl-1H-benzimidazole in MeOH, in which both L1 and L2 are light
brown crystalline solid. Complex 1 was obtained by mixing CdCl2·2.5H2O and L1 in a
1:1 ratio in methanol. Complexes 2, 3 and 4 were obtained by the reaction of the
ligands L and the respective Cd(II) salts and different pseudohalids taken in a 1:1:2
ratio (where L = L1 (2 and 3) or L2 (4)). X-ray quality crystals of 1-4 were obtained
upon slow evaporation of the suitable reaction mixtures at room temperature. The
complexes are soluble in common organic solvents such as EtOH, MeOH, DMSO and
DMF. The analytical and spectral data are completely consistent with the proposed
formulation.
3.2. Crystal structures of the complexes
The molecular structures of the complexes 1-4 are displayed in Fig.1-8. The
selected bond lengths and bond angles relevant to the coordination sphere are listed in
Table 2.
(Insert Table 2 here)
3.2.1. Molecular and supramolecular structures of [CdL1Cl2]2·H2O ( 1)
A perspective view of complex 1, with the atomic numbering scheme, is shown
in Fig. 1. Complex 1 crystallizes in the space group Pbca. The structure of 1 consists
of one water molecule and one centrosymmetric [Cd(L1)(ꢀ2-Cl)Cl]2 dimer containing
two cadmium ions bridged by two chloride ligands. Each Cd(II) cation shows a
distorted octahedral geometry. The equatorial plane is formed by N1 from azomethine,
N2 from benzimidazole, N4 from pyridine and the bridging Cl1 atom, while the axial
positions are occupied by Cl1A atom and the terminal Cl2 atom. The Cd atoms are
shifted by a distance of 0.2338 Å downward from the equatorial plane towards the
terminal Cl2 atom. The Cd–N distances fall within the range 2.267(4)–2.363(5) Å,
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while the two Cd–Cl (bridging) bond lengths are 2.4907(13) and 2.9345(15) Å and
the Cd–Cl (non-bridging) bond length is 2.5833(15) Å, as were observed in other
reported complex [15]. The cisoid and transoid angles in the equatorial plane are in
the range of 70.26(17)–107.95(12)° and 152.98(16)–162.26(12)°, respectively,
indicating the distortion from an ideal octahedral geometry. In this dimeric unit, the
Cd···Cd separation is 2.935 Å. Furthermore, through the inter- and intramolecular
hydrogen bonds N3–H3···O1, O1–H1E···Cl2 and O1–H1F···Cl2, the dinuclear unit
is assembled into a 3D supramolecular network (Table 3, Fig. 2). The lattice water
molecules act as both donors and acceptors, together with the NH of the
benzimidazole ring and the terminal Cl-, all of which play an important role in
constructing those hydrogen bonds.
(Insert Fig.1 and Fig.2 here)
3.2.2. Molecular and supramolecular structures of [Cd(L1)(N3)2]2·CH3OH (2)
A perspective view of complex 2, with the atomic numbering schemes, is shown
in Fig 3. Complex 2 crystallizes in the space group P21/c. The structure of 2 consists
of a methanolic molecule and a centrosymmetric [Cd(L1)(ꢀ1,1-N3)(N3)]2 dimer,
bridged by two end-on bridging azide groups. Here the ligand L1 acts as a neutral
tridentate NNN donor. Each Cd atom has a distorted octahedral geometry. Three
nitrogen atoms, N3 from azomethine, N1 from benzimidazole, N4 from pyridine and
N5A from the bridging azide group form the equatorial plane, while N5 from the
other bridging azide ligand and N8 of the terminal azide ligand are situated in the
axial positions. The Cd atom is shifted by a distance of 0.2829Å downward from the
equatorial plane towards the dangling azide unit. The Cd–N bond distances are
ranging from 2.236(10) to 2.545(11) Å, as were comparable to other similar
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systems[12, 39]. The bridging azide is asymmetric [N5–N6 = 1.199(11) and N6–N7 =
1.141(12) Å], while two N–N bond distances of the dangling zaide are almost the
same [N8–N9 = 1.161(14) and N9–N10 = 1.162(14) Å]. In this dimeric unit, the
Cd···Cd separation is 3.697 Å. In addition, there are intermolecular π···π interactions
in which each pyridine ring Cg1 is offset-stacked with the phenyl ring Cg2 within the
dimer with the centroid-to-centroid separation of 3.972 Å. The complex units are
further connected by the inter- and intramolecular hydrogen bonds N–H···O, O–
H···N and C–H···N between the methanolic molecule and the dimer to form a 2D
supramolecular sheets parallel to ab-plane (Tables 3 and 4, Fig. 4).
(Insert Fig.3 and Fig.4 here)
3.2.3. Molecular and supramolecular structures of [Cd(L1)(ꢀ1,5-dca)dca]n (3)
A perspective view of a short segment of the polynuclear architecture of complex
3, with the atomic numbering schemes, is shown in Fig 5. Complex 3 crystallizes in
the space group Pī. The coordination polyhedron around each Cd center in 3 is best
described as a distorted octahedron with a CdN6 chromophore. Three nitrogen atoms
(N1, N2 and N4) of the tridentate ligand L1 and one nitrile N atom (N5) of the
terminal dca occupy the equatorial plane. The axial positions are occupied by one
nitrile nitrogen N8 and a symmetry related (x –1, y, z) nitrile nitrogen N10A of the ꢀ1,
5-bridged dca. The ꢀ1, 5-bridged dca connects two neighboring Cd(II) centers in a non-
ending fashion and produces the 1D chain structure. The Cd–N distances fall within
the range 2.243(3)–2.454(4) Å. The intrachain Cd···Cd separation is 7.740 Å. The
C≡N–Cd bond angles are 161.0(3)°, 156.7(3)° and 135.6(3)°, which deviate from
linearity, indicating that both the terminal and the ꢀ1, 5-bridged dca anions coordinate
to Cd(II) in a bending fashion. The N7–C17 distance (1.141(6) Å) is some longer than
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N5–C16 distance (1.136(5) Å), indicating coordination of N6 to Cd(II) for the
terminal dca. The larger dca anions are non-linearity as reflected in the skeletal bond
angles N–C–N (173.7(5)°, 175.1(6)°, 173.5(4)°, 173.9(5)°) and C–N–C(119.5(4)° and
123.8(4)°)
The 1D chains in 3 are further connected by two symmetry-independent π···π
stacking interactions between the pyridine rings C3g of one unit and the phenyl rings
C4g of the other with centroid-to-centroid distance of 3.612 and 4.168 Å, respectively,
to give rise to a 2D supramolecular network parallel to ab-plane(Table 4 and Fig. 6a).
These 2D supramolecular sheets are further connected by N–H···N and C–H···N
hydrogen bonding interactions to give rise to a 3D suparmolecular network (Table 3
and Fig. 6b).
(Insert Fig.5 and Fig.6 here)
3.2.4. Molecular and supramolecular structures of [Cd(L2)(ꢀ2-Cl)(dca)]2( 4)
A perspective view of complex 4, with the atomic numbering schemes, is shown
in Fig 7. Complex 4 crystallizes in the space group Pī. Similar to the structure of 1,
the structure of 4 consists of a centrosymmetric [Cd(L2)(ꢀ2-Cl)(dca)]2 dimer
containing two cadmium ions bridged by two chloride ligands. Each cadmium(II)
cation shows a distorted octahedral geometry. The equatorial plane is formed by N1
from azomethine, N2 from benzimidazole, N4 from pyridine and the bridging Cl1
atom, while the axial positions are occupied by Cl1A atom and the nitrile nitrogen N5
of the terminal dca. The Cd atoms are nearly in the mean basal plane (0.0020 Å). The
Cd–N distances fall within the range 2.277(5)–2.411(6) Å. The equatorial Cd–Cl
bond length is 2.5653(19) Å and the axial atoms are less tightly bound with bond
lengths of 2.7407(19) Å. In this dimeric unit, the Cd···Cd separation is 2.741 Å.
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The discrete molecules of complex 4 are further connected by π···π stacking
interactions between the pyridine rings C5g of one molecule and the pyridine rings of
the other, to give rise to a 1D supramolecular chain along the crystallographic a-axis
(Table 4 and Fig. 8a). These 1D supramolecular chains are further connected by N–
H···N and C–H···N hydrogen bonding interactions to give rise to a 3D
suparmolecular network (Table 3 and Fig. 8b).
(Insert Fig.7 and Fig.8 here)
(Insert Talbe 3 and Table 4 here)
3.3. Powder X-ray diffraction and thermal analysis
To confirm whether the crystal structures are truly representative of the as-
synthesized materials, the powder XRD patterns for complexes 1–4 have been
investigated at room temperature. The simulative powder diffraction patterns were
based on the single crystal structural data (cif file) using CCDC Mercury software. As
shown in Fig.9, the main peaks of experimental spectra of 1−4 are basically consistent
with their simulated ones except for the relative intensity variation because of
preferred orientations of the crystals. This suggests that all the complexes are single
phase.
(Insert Fig.9 here)
To characterize the thermal stabilities of compounds 1−4, their thermal behaviors
were studied by thermogravimetric analyses (TGA) under N2 atmosphere(Fig.9).
Complex 1 undergoes dehydration in the temperature range of 249-309 °C (obsd
3.42%, calcd 3.98%). Decomposition of one Schiff base occurs from 309 to 506 °C
(obsd 28.92%, calcd 27.68%). The weight loss of 41.39% from 506 to 783 °C is
attributed to departure of the four chlorides and the other Schiff base (calcd 43.40% ).
The remaining weight is 26.27%, which may correspond to the formation of CdO
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(calcd 28.42%). For complex 2, the first weight loss from 157 to 222 °C is assigned to
the loss of one methanol molecule (obsd 1.95%, calcd 1.67%). The second weight
loss in the temperature range of 222-508 °C is due to the loss of the other methanol
molecule and four coordinated azido anions (obsd 23.96%, calcd 19.53%). The third
weight loss is associated with the decomposition of one Schiff base between 508-
784 °C (obsd 24.32%, calcd 26.32%). Complex 3 shows thermal stability up to
316 °C and from this temperature decomposition of the coordinated dca anions and
Schiff base occurs up to 785 °C, followed by a drastic weight loss of 61.71%.
However, it is difficult to identify the released fragments, which may be due to the
unstability of the intermediate products. For complex 4, the weight loss corresponding
to the release of two bridging chlorides and two dca anions is observed from 192 to
508 °C (obsd 19.17%, calcd 21.22%). Decomposition of one Schiff base occurs from
508 to 735 °C (obsd 31.36%, calcd 27.60%). The thermogravimetric analyses of the
complexes show that the complex 3 with 1D-polymeric structure is more stable than
the other three dinuclear complexes.
(Insert Fig.10 here)
3.4. IR, electronic and fluorescence spectra
In both of the two free Schiff bases, two bands at 1635 and 1437 cm-1 are
assigned as the ν(C=N) of the imine and the νa(–C=N–C=C–) of the benzimidazole
ring stretching vibrations, respectively [50]. However, in all the four complexes the
1635 cm-1 band shifts to lower frequencies by 6-11 cm-1 whereas the 1437 cm-1 band
shifts to higher frequencies by 13-16 cm-1, indicating the coordination of the nitrogen
atoms to Cd(II) ions. This is further supported by the band around 485 cm-1
corresponding to the ν(Cd–N) vibration. Several medium intensity bands spreading
between 3200 and 2700 cm-1 in all the four complexes indicate that the NH groups of
16

ACCEPTED MANUSCRIPT
the benzimidazole rings are involved in hydrogen bonding with other electronegative
atoms [51].
Broad bands at 3416 and 3587cm-1 are assigned as ν(O–H) stretching vibrations
of the hydrate water molecules in complex 1 and methanol molecules in complex 2,
respectively, which are involved in hydrogen bonding in the crystals of the two
complexes. A distinct band at 2076 cm-1 is indicative of the presence of coordinated
azide in complex 2. The free dca shows three characteristic IR bands at 2287, 2229
and 2181 cm-1, which are assigned to νas+νs(C≡N), νas(C≡N) and νs(C≡N) modes [52].
In complex 3 three bands at 2237, 2208, 2146 cm-1 indicate the presence of terminal
coordination mode of dca. In complex 4 these bands are split into two parts of which
one part (2301, 2250, 2164 cm-1) indicates 1, 5 bridging coordination mode of dca,
whereas the other part (2276, 2216 cm-1, 2146cm-1) indicates terminal dca
coordination [53]. This is confirmed by X-ray structure determinations of 3 and 4.
The electronic absorption spectra of the free Schiff base ligands and the four
complexes in methanol were recorded at room temperature. UV bands around 275 and
283 nm are observed for the free Schiff bases, which are characteristic of the
benzimidazole group and arise from a π–π* transition. These are blue-shifted upon
coordination, showing clear evidence of C=N coordination to Cd(II) atoms [54].
Similarly, the ligand chromophore (C=N) (azomethine) absorbs at 248 nm for L1 and
239 nm for L2, respectively, and shifts to a shorter wavelength by 2-12 nm upon
complexation indicating the coordination of azomethine (C=N).
The fluorescent behaviors of complexes 1-4 and free Schiff bases L1and L2 are
investigated in the solid state at room temperature (Fig.11). The free Schiff bases L1
and L2 show a similar green emission band at 567 and 569 nm upon excitation at
280nm, respectively. The complexes 1-4, also upon excitation at 280nm, exhibit the
17

ACCEPTED MANUSCRIPT
green emissions at 573, 571, 575 and 577nm, respectively. The emission bands for
complexes 1, 2 and 4 are similar to those of the dinuclear Cd-Schiff base complexes
[Cd(L1)Cl]2, [Cd(L1)(NO3)]2, [Cd(L2)(NO3)](CH3OH)2 and [Cd(L2)(NO3)](H2O)2,
where the maximum emission wavelength is at 564, 561, 549 and 548 nm. The
emission band for complex 3 is similar to those of 1D polymer Cd-Schiff base
complexes [Cd2(L1)2](SCN)2]n and [Cd(L2(SCN)2]n, where the maximum emission
wavelength is at 562 and 551nm (HL1 = 2-((2-(dimethylamino-)ethylimino)
methyl)phenol, HL2 = methyl-2-pyridylmethylidenehy-drazinecarbodithioate). It is
apparent that the emission spectra of 1-4 resemble those of the corresponding Schiff
bases, which may be assigned to the intraligand π* → π transition. Compared with the
emission spectra of the free corresponding Schiff bases, slightly red shifts of 6 nm in
1, 4 nm in 2, 8nm in both 3 and 4 can be observed, which may be mainly caused by
the coordination of metal atom to ligand [3]. Moreover, we can find that the emission
intensity of the four complexes is weaker than that of the corresponding Schiff bases,
which are comparable to other Cd-organic complexes [41-42, 55]. This may be
attributed to the heavy metal effect of Cd(II) ions and the quenching by the
pseudohalide and halogens [42, 55]. In addition, the emission intensity of 1-4 were
found to be 1≈3>4≈2. It is possibly explained that 1 and 3 have bigger conformational
rigidity for their 3D supramolecular network constructed by the cooperative
association of coordination bonds as well as hydrogen bonds and π···π packing
interactions than 2 which has a 2D supramolecular network. Complex 4, though also
having 3D supramolecular network, has the weaker emission intensity than 1 and 3
and compared to 2, which may be caused by the weaker non-covalent interactions and
the appearance of both the pseudohalide (dca) and halogens (chloride).
(Insert Fig.11 here)
18

ACCEPTED MANUSCRIPT
4. Conclusion
The benzimidazole derived NNN donor Schiff bases L1 and L2 form four new
Cd(II) complexes in the same reaction conditions, which are two dinuclear complexes
and one 1D polymer with L1, and one dinuclear complex with L2. Compared the
−
structures of the four complexes, we can find that the counter anions (Cl−, N3 and dca)
-
play an important role in determining the resulting structures. The anions Cl-, N3 and
dca have strong coordination ability and can present both terminal and bridging
−
coordination mode. In complexes 1 and 2, both Cl− and N3 adopt terminal and µ1, 1
bridging coordination mode, resulting in the formation of dinuclear complexes 1 and
2. In complex 3, the counter anion dca with larger size and steric hindrance adopts
terminal and µ1, bridging coordination mode and extends the SBUs, namely
5
[CdL1dca]+ to one direction. So a 1D chain structure is formed in 3. It is surprising
that complex 4 is also a dinuclear structure containing dca and Cl−. In 4, the dca is
coordinated to Cd(II) ions in the terminal mode, and Cl− is coordinated to the Cd(II)
ion as a bridging ligand, which can be contributed to the competition and compromise
between the two anions in the system, though both of them can act as bridging
ligands . The crystal structures of these complexes have 3D networks in 1, 3 and 4,
and 2D network in 2 due to the existence of hydrogen bonds and π···π interactions.
Thermal analysis reveals that the complex 3 with 1D polymeric chains is more stable
than complexes 1, 2 and 4, which are dinuclear structures. The fluorescent spectra of
all the four complexes are near to their corresponding Schiff bases excluding the
slightly red shifts and weaker emission intensity.
Acknowledgment
19

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This work was supported by the Department of education of Hebei province
Youth Foundation (Nos. QN2015009 ) and the National Natural Science Foundation
of P. R. China (Grant No. B060902).
Appendix A. Supplementary material
CCDC 1507280–83 contain the supplementary crystallographic data for
complexes 1–4. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or
e-mail: deposit@ccdc.cam.ac.uk.
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Table1 Crystallographic data and refinement parameters for 1–4.
1
2
3
4
Formula
C30H32Cd2Cl4N8O2
903.24
298(2)
0.71073
Orthorhombic
Pbca
C32H36Cd2N20O2
957.61
298(2)
1.54178
Monoclinic
P21/c
C19H14Cd N10
494.80
298(2)
0.71073
Triclinic
C36H32Cd2Cl2N14
956.46
293(2)
0.71073
Triclinic
Pī
M(g mol-1)
Temperature (K)
λ (Å)
Crystal System
Space group
Unit cell dimensions
a (Å)
b (Å)
c (Å)
P
ī
9.3942(6)
18.0449(14)
20.2242(16)
90
90
90
3428.4(4)
4
1.594
8.4191(9)
10.9282(11)
22.441(2)
90
105.912(2)
90
1985.6(3)
2
9.062
7.7400(6)
9.1690(8)
15.4790(13)
77.1490(10)
89.614(2)
66.7310(10)
979.84(14)
2
8.6625(4)
8.9921(5)
13.6146(12)
71.7030(10)
89.019(2)
70.6300(10)
945.40(11)
1
α (º)
β (º)
γ (º)
Volume ( Å3)
Z
ꢀ(mm-1)
1.144
1.314
F(000)
1792
960
492
476
Crystal size (mm)
Theta range (º)
Index ranges
0.47 x 0.21 x 0.10
2.47– 25.02
–11/10,–21/20, –22/24 ±9, –12/11 , –9/26
0.25 x 0.23 x 0.16
4.10 –66.19
0.42 x 0.26 x 0.20
2.53–25.02
±9, –6/10, –17/18
1.677
0.22 x 0.17 x 0.10
2.50–25.02
–10/9, –9/10, ±16
1.680
D
calc (g cm-3)
1.750
1.602
Reflections collected
Independent reflections
R1, wR2(I>2σ(I))
16040
3030
0.0372, 0.0786
0.0872, 0.0959
1.014
6476
3434
0.0788, 0.0995
0.1631, 0.1331
0.964
4901
3363
0.0333, 0.0729
0.0422, 0.0775
1.058
4822
3276
0.0622, 0.1351
0.0832, 0.1429
1.045
2
R1, wR2(for all Fo )
Goodness-of-fit on F2