Three different configurations of d10 complexes based on benzoxazole pyridyl ligand: Synthesis, structures and properties

Article abstract of DOI:10.1016/j.poly.2017.06.042

Three different coordination complexes containing 2-(3′-pyridyl)-benzoxazole (3-PBO) and 2-(4′-pyridyl)-benzoxazole (4-PBO) ligands, namely [Cd(3-PBO)₂(NO₃)₂(H₂O)₂]CH₃CN (1), [Zn(4-PBO)

Full text of DOI:10.1016/j.poly.2017.06.042

Polyhedron 134 (2017) 336–344
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
1
0
Three different configurations of d complexes based on benzoxazole
pyridyl ligand: Synthesis, structures and properties
⇑
Shanshan Mao, Han Zhang, Kesheng Shen, Yuling Xu, Xinkui Shi, Huilu Wu
School of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou, Gansu 730070, PR China
a r t i c l e i n f o
a b s t r a c t
0
0
Article history:
Three different coordination complexes containing 2-(3 -pyridyl)-benzoxazole (3-PBO) and 2-(4 -pyri-
dyl)-benzoxazole (4-PBO) ligands, namely [Cd(3-PBO) (NO (H O) ]CH CN (1), [Zn(4-PBO) (NO ] (2)
Ag(4-PBO) Pic] (Pic = Picric acid) (3) have been synthesized and characterized. Single crystal X-ray
Received 3 May 2017
Accepted 24 June 2017
Available online 4 July 2017
2
3
)
2
2
2
3
2
3 2
)
[
2
n
structure analysis reveals that complex 1 is distorted seven-coordinated pentagonal-bipyramidal geom-
etry, and complex 2 is a twisted six-coordinate octahedral structure, while complex 3 is a four-coordi-
nated one-dimensional linear coordination polymer. A wide range of hydrogen bonding (of the
Keywords:
Transition metal complexes
Synthesis
Crystal structure
Fluorescence
Electrochemical studies
O–Hꢀ ꢀ ꢀO and O–Hꢀ ꢀ ꢀN types) and
p–p stacking interactions are also present in the crystal structure.
These arrangements lead to the formation of three supramolecular structures. By careful inspection of
the structures of 1 and 2, the ion radius of metal centers adopting various coordination modes is a crucial
factor for the formation of the different structures. Cyclic voltammograms of 3 indicate a quasi-reversible
+
Ag /Ag couple. Moreover, the fluorescence properties of the ligands and complexes 1–3 were studied in
solid state. Strong photoluminescence is observed in complexes 1 and 2 at room temperature and the
complexes may be good candidates for potential luminescence materials.
Ó 2017 Elsevier Ltd. All rights reserved.
1
. Introduction
playing important roles in many newer functional materials [12–
1
4]. Zinc-containing compounds are particularly noticeable as
Over the past decade, the rational design and assembly of
luminescent materials for organic light-emitting diodes [15], and
promote the various structures that can be used for sensory mate-
rials to detect alkaloids [16] and nitro aromatics [17]. Cd (II) is usu-
ally used in the field of optics such as fluorescent probes and
nonlinear optical materials [18–21]. Ag(I) ions are more compati-
ble with the electrical, photographic, imaging and pharmaceutical
industries [22,23], the complexes of which exhibit different coordi-
nation geometries and luminescent properties [24].
metal–organic frameworks have extensive interest in coordination
and supramolecular chemistry because of its intriguing structure
and potential applications in functional materials [1–3]. However,
the ability to predict and control supramolecular assembly is still a
long-term challenge the due to the fact that the self-assembly pro-
cess is frequently influenced by various factors, including inbeing
of the metal ions and the predesigned organic ligands as well as
other factors such as medium, template, temperature and counte-
rion [4,5]. With regard to the efforts to pursue the supramolecular
In our previous work, we have investigated a series of V-shaped
bis-benzimidazole ligands and their complexes [25]. However, lit-
tle is known on the use of the title ligands. Hence, in this work, we
wish to report the synthesis, structure, fluorescence studies, and
framework synthesis strategy, some noncovalent forces, such as
p–
p
stacking interactions and hydrogen bonding interactions, also
1
0
intensively impact the supramolecular topology and dimensional-
ity [6,7]. The ligands containing 2-substituded benzimidazoles/
benzoxazoles have a wide range of interest for their antiviral activ-
ity [8], luminescent properties [9,10], multifunctional coordination
patterns, and the potential to form supramolecular aggregates
electrochemical properties of three d metal complexes contain-
ing benzoxazole pyridyl ligands.
2. Experimental details
through p–p stacking interactions [10,11].
1
0
Complexes of d metal ions such as zinc(II), cadmium(II), and
2.1. Materials and methods
silver(I) have recently drawn global attention due to these metals
All chemicals and solvents were reagent grade and were used
without further purification. The C, H and N elemental analyses
were performed using a Carlo Erba 1106 elemental analyzer. The
⇑
Corresponding author.
E-mail address: wuhuilu@163.com (H. Wu).
http://dx.doi.org/10.1016/j.poly.2017.06.042
277-5387/Ó 2017 Elsevier Ltd. All rights reserved.
0

S. Mao et al. / Polyhedron 134 (2017) 336–344
337
ꢁ
ꢁ
thermal analysis of the complexes 1–3 were carried out under
nitrogen atmosphere with a heating rate of 10 °C/min on METTLER
TOLEDO TGA1. The IR spectra were recorded in the 4000–400 cm
region with a Nicolet FT-VERTEX 70 spectrometer using KBr pel-
lets. Electronic spectra were taken on a Lab-Tech UV Bluestar spec-
trophotometer. Absorbance was measured with the Spectrumlab
cmꢁ1): 1617
1240 (CAO). UV–Vis (in DMF), kmax (nm): 303.
[Zn(4-PBO) (NO ] (2), Yield: 54%. Elemental analysis for C24
Zn: calculated (%): C, 49.55; H, 2.77; N, 14.44. Found (%):
m
(C@C), 1401
m(C@N), 1300
m(NO
), 1041
m
(NO
),
m
ꢁ1
2
3
)
2
-
16 6 8
H N O
ꢁ1
C, 49.45; H, 2.65; N, 14.31. Selected-IR (KBr; cm ): 1628
m
(C@C),
ꢁ
3
m(C@N), 1346 m(NO ), 1032 m(NO ), 1216 m(CAO). UV–Vis
ꢁ
3
1413
(in DMF), kmax (nm): 302.
[Ag(4-PBO) Pic] (3), Yield: 51%. Elemental analysis for C30
AgN : calculated (%): C, 49.47; H, 2.49; N, 13.46. Found (%): C,
49.33; H, 2.44; N, 13.55. Selected-IR (KBr; cm ): 1589
1395 (C@N), 1336 (ArANO ), 1232 (CAO). UV–Vis (in DMF),
max (nm): 303 and 380.
7
22sp spectrophotometer at room temperature. 1H NMR spectra
were recorded on a Varian VR300-MHz spectrometer with TMS
as an internal standard. Fluorescence measurements were per-
formed on a 970-CRT spectrofluorophotometer. Electrochemical
measurements were performed on a LK2005A electrochemical
analyser under nitrogen at 283 K. A glassy carbon working elec-
trode, a platinum-wire auxiliary electrode and a Ag/AgCl reference
2
n
18
H -
7 9
O
ꢁ1
m
(C@C),
m
m
2
m
k
ꢁ
electrode ([Cl ] = 1.0 mol/L) were used in the three-electrode
2.3. X-ray crystallography
ꢁ
3
measurements. The electroactive component was at 1.0 ꢂ 10
ꢁ3
molꢀdm
TBAP) (0.1 molꢀdm ) used as the supporting electrolyte in DMF.
Ag(Pic) is obtained by reacting silver carbonate with picric acid.
concentration with tetrabutylammonium perchlorate
Suitable single crystals of complexes 1–3 were mounted on a
glass fiber, and the intensity data were collected on a Bruker APEX
II area detector with graphite-monochromated Mo Ka radiation
ꢁ
3
(
(k = 0.71073 Å) at 296(2) K. Data reduction and cell refinement
were performed using the SMART and SAINT programs [27]. The
absorption corrections are carried out by the empirical method.
The structure was solved by direct methods and refined by full-
2
.2. Synthesis of the ligands and complexes 1–3
The ligands 3-PBO and 4-PBO (Scheme 1) were synthesized
2
matrix least squares against F of data using SHELXTL software. [28]
according to the reported method [26].
Three complexes were prepared using a similar procedure.
Complex 1 was prepared by the following reaction. A solution of
All H atoms were found in different electron maps and were subse-
quently refined in a riding-model approximation with C–H dis-
tances ranging from 0.95 to 0.99 Å. Information concerning the
crystallographic data collection and structural refinements is sum-
marized in Table 1. The relevant bond lengths and angles are listed
in Table 2.
3
-PBO (0.0392 g, 0.2 mmol) in 3 mL of dichloromethane was very
carefully placed on the bottom of the tube. 1 mL of dichloro-
methane as the buffer was slowly layered onto the ligand solution.
A clear solution of Cd(NO
acetonitrile was then very carefully layered on the top of the buffer
solution. After 1 week, colorless block crystals of 1 were collected
by filtration and dried in vacuo. The synthesis of complex 2 is sim-
3
)
2
2
ꢀ4H O (0.0308 g, 0.1 mmol) in 3 mL of
3. Results and discussion
ilar to 1 except for using 4-PBO (0.0392 g, 0.2 mmol) and Zn(NO
6H O (0.0291 g, 0.1 mmol) instead of 3-PBO and Cd(NO
ꢀ4H
Complex 3 was synthesized similar to 2, but using Ag(Pic)
0.0336 g, 0.1 mmol) instead of Zn(NO O.
ꢀ6H
Cd(3-PBO) (NO (H O) ]CH CN (1), Yield: 52%. Elemental
analysis for C26 23CdN 10: calculated (%): C, 44.24; H, 3.28; N,
3.89. Found (%): C, 44.33; H, 3.41; N, 13.76. Selected-IR (KBr;
3
)
2
O.
2
-
Synthetic routes towards ligands and d10 metal complexes are
exhibited in Scheme 1. The three complexes were obtained by
the reaction of Cd(NO ) , Zn(NO ) and Ag(Pic) with two ligands,
3 2 3 2
namely, 3-PBO and 4-PBO, in acetonitrile and dichloromethane.
They are soluble in polar aprotic solvents such as DMF, DMSO
and MeCN, slightly soluble in ethanol, methanol, ethyl acetate, ace-
ꢀ
2
3
)
2
(
3
)
2
2
[
2
3
)
2
2
2
3
H
7
O
1
2
tone and chloroform, and insoluble in Et O and petroleum ether.
Scheme 1. Synthesis of ligands and complexes 1–3.

3
38
S. Mao et al. / Polyhedron 134 (2017) 336–344
Table 1
Crystal and structure refinement data for complexes 1–3.
Complex
1
2
3
Empirical formula
Molecular weight (gmꢁ1)
Crystal system
Space group
Unit cell dimensions
a (Å)
C
26
H
23 Cd N
7
O
10
C
24
H
16
N
6
O
8
Zn
C
30 7 9
H18 Ag N O
705.91
triclinic
P1ꢀ
581.82
monoclinic
C2/c
728.38
triclinic
P1ꢀ
7.2224(11)
13.4212(18)
15.367(2)
98.219(2)
103.490(2)
102.207(2)
1386.5(3)
2
11.6344(18)
8.3896(12)
24.534(4)
90
94.987(2)
90
2385.6(6)
4
1.620
8.3903(9)
b (Å)
c (Å)
10.5988(10)
17.1211(16)
101.068(2)
100.4900(10)
99.197(2)
1439.0(2)
2
1.681
0.771
732
a
(°)
b (°)
(°)
c
3
V (Å )
Z
D
calc (g cm 3
ꢁ
)
1.691
0.859
712
Absorption coefficient (mmꢁ1)
F(000)
1.094
1184
Crystal size (mm)
0.43 ꢂ 0.34 ꢂ 0.30
0.46 ꢂ 0.33 ꢂ 0.31
0.47 ꢂ 0.38 ꢂ 0.31
h range for data collection (°)
Reflections collected
Independent reflections
Index ranges
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
1.39–25.50
3.00–25.48
1.24–25.50
7289
6122
2221 [Rint = 0.0203]
ꢁ13 ꢃ h ꢃ 14, ꢁ10 ꢃ k ꢃ 7, ꢁ28 ꢃ l ꢃ 29
Full-matrix least-squares on F2
2221/1/177
7619
5299 [Rint = 0.0216]
ꢁ10 ꢃ h ꢃ 10, ꢁ12 ꢃ k ꢃ 12, ꢁ19 ꢃ l ꢃ 20
Full-matrix least-squares on F2
5299/0/425
5116 [Rint = 0.0159]
ꢁ8 ꢃ h ꢃ 6, ꢁ11 ꢃ k ꢃ 16, ꢁ18 ꢃ l ꢃ 18
2
Full-matrix least-squares on F
5116/2/414
1.119
1.091
1.056
Finial R
R indices (all data)
Largest diff. peak and hole (e Åꢁ3)
1
and wR
2
[I > 2
r
(I)]
R
R
1
= 0.0316, wR
= 0.0356, wR
2
2
= 0.0989
= 0.1097
R
R
1
= 0.0380, wR
= 0.0407, wR
2
= 0.1087
= 0.1110
R
R
1
= 0.0351, wR
= 0.0429, wR
2
2
= 0.0931
= 0.0996
1
1
2
1
0.695 and ꢁ0.663
0.427 and ꢁ0.371
0.353 and ꢁ0.639
Table 2
cadmium atom in complex 1 is a seven-coordinated cadmium
complex and has a distorted pentagonal-bipyramidal geometry
(the common coordination geometry for seven-coordinated metal
Selected bond distances (Å) and angles (1) for complexes 1–3.
Complex 1
Cd(1)–N(2)
Cd(1)–O(10)
2.314(3)
2.332(3)
Cd(1)–N(4)
Cd(1)–O(9)
2.308(3)
2.334(3)
(
II) complexes) [29]. As shown in Fig. 1, the equatorial sites are
occupied by the five oxygen atoms, namely, two oxygen atoms of
the water molecules (O9 and O10), two oxygen atoms of the
chelating nitrate ligand (O6 and O8) and one oxygen atom of
unidentate nitrate ion (O5). This arrangement leads to the forma-
tion of an approximately planar pentagon. The deviation from
the perfect planar coordination mode (72°) is best illustrated by
the O6-Cd1-O8 angle of 49.35(8)° due to the chelating nitrate
ligand. 3-PBO ligands occupied the axial positions. The Cd–O bond
distances of Cd(1)-O(10) = 2.332(3) Å, Cd(1)-O(9) = 2.334(3) Å, Cd
(1)-O(5) = 2.401(3) Å, Cd(1)-O(6) = 2.530(3) Å, Cd(1)-O(8) = 2.579
Cd(1)–O(5)
Cd(1)–O(6)
2.401(3)
2.530(3)
Cd(1)–O(6)#1
Cd(1)–O(8)
2.530(3)
2.579(3)
N(4)–Cd(1)–N(2)
N(2)–Cd(1)–O(10)
N(2)–Cd(1)–O(9)
N(4)–Cd(1)–O(5)
O(10)–Cd(1)–O(5)
N(4)–Cd(1)–O(6)#1
O(10)–Cd(1)–O(6)#1
O(5)–Cd(1)–O(6)#1
179.36(8)
93.06(13)
86.00(11)
86.64(10)
93.57(12)
89.82(10)
73.23(11)
166.50(10)
N(4)–Cd(1)–O(10)
N(4)–Cd(1)–O(9)
O(10)–Cd(1)–O(9)
N(2)–Cd(1)–O(5)
O(9)–Cd(1)–O(5)
N(2)–Cd(1)–O(6)#1
O(9)–Cd(1)–O(6)#1
N(4)–Cd(1)–O(6)
87.23(13)
93.56(11)
164.67(12)
92.77(10)
71.21(11)
90.81(9)
122.06(10)
89.82(10)
Complex 2
Zn(1)–N(2)
Zn(1)–O(3)#2
Zn(1)–O(4)#2
N(2)–Zn(1)–N(2)#2
N(2)#2–Zn(1)–O(3)#2
N(2)#2–Zn(1)–O(3)
N(2)–Zn(1)–O(4)#2
2.037(2)
2.056(2)
2.380(2)
103.54(12)
105.63(9)
103.01(8)
158.73(8)
Zn(1)–N(2)#2
Zn(1)–O(3)
Zn(1)–O(4)
N(2)–Zn(1)–O(3)#2
N(2)–Zn(1)–O(3)
O(3)#2–Zn(1)–O(3)
N(2)#2–Zn(1)–O(4)#2
2.037(2)
2.056(2)
2.380(2)
103.01(8)
105.63(9)
132.90(12)
90.18(9)
(
3) Å and CdAN bond lengths of Cd(1)-N(2) = 2.314(3) Å, Cd(1)-N
(
4) = 2.308(3) Å are close to those reported in the literatures [30–
3
2].
The crystal packing of 1 is dominated by hydrogen bonding
interactions between co-crystal water molecule and complex,
and also stacking interactions between oxazole rings defined
p–p
Complex 3
Ag(1)–N(4)
Ag(1)–O(9)
N(4)–Ag(1)–N(2)
N(2)–Ag(1)–O(9)
by atoms C5/C6/O1/C7/N1 and C17/C18/O2/C19/N3 (Fig. 2). The
structure shows the neighboring chains are connected by OAHꢀ ꢀ ꢀO,
OAHꢀ ꢀ ꢀN and CAHꢀ ꢀ ꢀO hydrogen bonds (Table 3), in addition, the
2.179(3)
2.646(2)
173.15(9)
95.09(9)
Ag(1)–N(2)
Ag(1)–O(4)
N(4)–Ag(1)–O(9)
O(4)–Ag(1)–O(9)
2.189(3)
2.808(6)
89.64(9)
125.39(6)
neighboring oxazole rings are stabilized by p–p stacking with cen-
troid distances of 3.630 and 3.634 Å, thus generating an infinite 2-
D layer (Fig. 2).
Symmetry transformations used to generate equivalent atoms: #1 x,y,z, #2 ꢁx + 1,
y,ꢁz + 1/2.
Complex 2 crystallizes in the monoclinic space group C2/c, and
its ORTEP structure along with the atomic numbering scheme is
shown in Fig. 3. The crystallographic data reveal that the molecular
unit is centrosymmetric and is made up of equivalent halves. The
symmetry transformations #1 ꢁx + 1, y, ꢁz + 1/2 are used to gener-
ate equivalent atoms. All the equivalent bond lengths and angles
around Zn(II) are equal (Table 2). The complex 2 comprises [Zn
3.1. X-Ray structure determination of complexes
The structures of 1–3 were confirmed by X-ray diffraction. The
crystal structures are shown in Figs. 1, 3 and 5(a), respectively.
ꢀ
Complex 1 crystallizes in the triclinic space group P1, and its
2
(4-PBO) ] cation and two bidentate nitrate anions. The central zinc
ORTEP structure along with the atomic numbering scheme is
shown in Fig. 1. According to the determined molecular structure,
atom in complex 2 is six-coordinated structure consisting of N O
2 4
(two N2 atoms from two 4-PBO ligands, four oxygen atoms from
two bidentate nitrate ions) as shown in Fig. 3. The coordination
geometry of the zinc(II) may be best described as distorted octahe-
complex 1 consists of one neutral molecule, [Cd(3-PBO)
2 3 2
(NO )
(
H
2
O) ] and one uncoordinated acetonitrile molecule. The central
2

S. Mao et al. / Polyhedron 134 (2017) 336–344
339
Fig. 1. Molecular structure and atom numberings of complex 1 showing displacement ellipsoids at the 30% probability level. Hydrogen atoms were omitted for clarity.
Fig. 2. 2-D layer generated by O–Hꢀ ꢀ ꢀO, O–Hꢀ ꢀ ꢀN and C–Hꢀ ꢀ ꢀO hydrogen bonding and
p–p stacking in complex 1, respectively. Some atoms are omitted for clarity.
Table 3
Intermolecular hydrogen bonds for complex 1.
D–Hꢀ ꢀ ꢀA
d(D–H)
d(Hꢀ ꢀ ꢀA)
d(Dꢀ ꢀ ꢀA)
\(DHA)
C(14)–H(14)ꢀ ꢀ ꢀO(3)#3
O(9)–H(20)ꢀ ꢀ ꢀO(3)#4
O(9)–H(19)ꢀ ꢀ ꢀN(7)#1
O(10)–H(18)ꢀ ꢀ ꢀO(8)#5
0.93
2.51
3.395(5)
2.822(4)
2.831(6)
3.001(5)
158.4
0.84(6)
0.83(5)
0.66(6)
1.98(6)
2.00(5)
2.39(6)
175(5)
175(5)
156(7)
Symmetry transformations used to generate equivalent atoms: #1 x,y,z, #3 x + 1,y + 1,z, #4 x + 1,y,z, #5 x ꢁ 1,y,z.
dral with (O3A,O4,O3,N2A) providing the equatorial plane. The
maximum deviation distance (O3) from the least squares plane cal-
culated from the four coordination atom atoms is 0.190(5) Å, and
the zinc atom is out of this plane by 0.410(7) Å.
shown in Fig. 5. The asymmetric unit of 3 contains one Ag atom,
two 4-PBO ligands and one picrate anion. As illustrated in Fig. 5,
the Ag(I) ion is four-coordinated (N2 and N4 atoms from two 4-
PBO ligands, two oxygen atoms from picrate anions) with a seesaw
The bond length between the zinc atom and the apical atoms
conformation ( is defined as [360-(
s
= 0.44). The parameter
s
a
(
N2, O4A) are 2.037 and 2.380 Å, respectively. The bond angle of
+ b)]/141 [where b = N(2)–Ag(1)–N(4), a = O(4)-Ag(1)-O(9)] and
the two atoms (N2–Zn1–O4A) in axial positions is 158.726(87)°.
Therefore, compared with the regular octahedral, the geometry
around the Zn(II) is a relatively distorted octahedron [33–36].
Besides, in the crystal structure a parallel arrangement between
its value varies from 0 (in regular square planar geometry) to 1
(tetrahedral) [37]. The picrate anion ligand bridges Ag atoms with
a one-dimensional linear polymer and the aromatic rings of the 4-
PBO ligands paralleling each other. Furthermore, there are two
the oxazole rings is realized indicating
p–p
stacking of them with
kinds of face-to-face p–p stacking in 3. One is between pyridine
a distance between them of around 3.545 and 3.638 Å (Fig. 4).
Complex 3 crystallizes in the triclinic space group P1, and its
ORTEP structure along with the atomic numbering scheme is
rings of adjacent layers with the centroid–centroid distance of
3.6187(3) Å and interplanar separation being 3.331(3) Å, and the
other is between oxazole and benzene rings of benzoxazole of
ꢀ

3
40
S. Mao et al. / Polyhedron 134 (2017) 336–344
Fig. 3. Molecular structure and atom numberings of complex 2 showing displacement ellipsoids at the 30% probability level. Hydrogen atoms were omitted for clarity.
Fig. 4. 2D supramolecular layer of 2 viewed along the [101] plane, showing the p–p stacking interactions as purple broken lines. (Color online.)
the 4-PBO ligands with the centroid–centroid length of 3.7587(3) Å
and interplanar separation of 3.516(3) Å, yielding 3D
related to those of the free ligand 3-PBO. The spectrum of 3-PBO
ꢁ1
a
shows a strong band at 1454 cm , which is assigned to
m(C@N)
supramolecular framework through the two types of strong
interactions.
p–p
[38]. In comparison with the IR spectra of the ligands, all the metal
complexes exhibit blue shift 41–59 cm of m(C@N) indicating the
ꢁ1
A close look into the structures of complexes 1 and 2 reveals
that the metal centers play key roles on the frameworks of the
complexes. Since Zn(II) and Cd(II) are d10 systems that do not pos-
sess crystal field stabilization energy and the effect of steric repul-
sion between different ligands (3-PBO, 4-PBO) might be slight, the
coordination numbers of Zn(II) and Cd(II) ions are mainly deter-
mined by the radius of central metal ion. The Cd(II) ion radius is
larger than that of Zn(II) ion, therefore the coordination numbers
of Cd(II) ion in 1 are 7 and Zn(II) ion in 2 are 6.
participation of imine nitrogen atoms in the coordination to the
metal ion. Moreover, complexes 1 and 2 appear two new bands
ꢁ1
ꢁ1
at 1300, 1041 cm and 1346, 1032 cm respectively, which indi-
cate that nitrate are coordinated to the metal ions center [38]. This
deduction agrees with the results of the X-ray crystal structure
determinations.
The electronic spectra of the ligands and metal complexes were
recorded in DMF solution at room temperature. The UV band of 3-
PBO (301 nm) is only marginally red-shifted about 2 nm for com-
plex 1, which is evidence of C@N (pyridine) coordination to the
metal center. Analogously, the UV bands of 4-PBO (300 nm) are
red-shifted about 2–3 nm in complex 2 and complex 3 respec-
tively. This phenomenon also shows that C@N is involved in coor-
3.2. IR and UV spectra
The IR spectral data for the free ligands and three metal com-
plexes with their relative assignments have been studied to char-
acterize their structures. The IR spectra of 4-PBO are closely
dination to the metal center. The picrate bands of complex 3
⁄
(
observed at 380 nm) are assigned to
p
?
p
transitions [39].

S. Mao et al. / Polyhedron 134 (2017) 336–344
341
Fig. 5. (a), (b) Molecular structure and atom numberings of complex 3 showing displacement ellipsoids at the 30% probability level. (Hydrogen atoms were omitted for
clarity), (c), (d) 2D supramolecular layer of 3 viewed along in the ab plane, showing the stacking interactions. (d) The 3D supramolecular networkformed by two types
green and blue) of stacking interactions of 3. (Color online.)
p–p
(
p–p
3.3. Thermal analyses of complexes 1–3
3.4. Luminescent properties of the free ligands and their complexes
To study the stabilities of the three complexes, the thermogravi-
Luminescent compounds are of great current interest because
of their various applications in chemical sensors, photochemistry,
and electroluminescent display [40].
The fluorescent emission spectra of the ligands (3-PBO and 4-
PBO) and their three complexes 1–3 were measured in a solid state
at room temperature as shown in Fig. 7(a), (b). Fig. 7(c) depicts the
colour and luminescence changes for these compounds under
ambient light and UV light irradiation (365 nm). The free ligands
3-PBO and 4-PBO show purple fluorescent luminescence with the
metric analysis under nitrogen atmosphere was performed with a
heating rate of 10 °C/min. As shown in Fig. 6, the complex 1
revealed obvious two-step mass-loss process. Firstly, the weight
loss of 10.3% in the range 87–210 °C corresponds to the release
of the two lattice water and acetonitrile molecules (calcd 10.8%).
The successive mass loss at the temperature range of 210 to above
8
00 °C may be attributed to the gradual removal of ligand, nitrate
radicals and the rest of CdO. Complex 2 displayed a first weight loss
region from 108 to 210 °C, assigned to release of the nitrate
radicals of crystallization molecules (obsd 10.9%, calcd 10.7%).
Then, there was formation of a plateau region observed from 210
to 252 °C. The subsequent mass loss at the temperature range of
maximum wavelengths at 392 nm and 406 nm when excited at
⁄
340 nm and 358 nm, respectively, significantly assigned to
p–p
transition fluorescence. When complex 1 is excited at 340 nm, a
strong emission band is observed at 404 nm, which is red shifted
in comparison to its related ligand 3-PBO. In addition, complexes
2 exhibits intense fluorescent emission bands at 429 nm upon
excitation at 358 nm, which is also red shifted in comparison to
its related ligand 4-PBO. To our knowledge, because Zn(II) and Cd
2
52 to above 800 °C is explained on the basis of the stepwise
removal of ligand and the remainder of ZnO. Complex 3 could
resist decomposition until to 211 °C, and then undergoes a large
loss-weight process from 211 to 403 °C with a weight-loss of 72%
corresponding to the release of all ligands (calculated value 71%).
The successive mass loss from 403 to above 800 °C may be
the result of the gradual removal of ligand and the remainder of
AgO.
1
0
(II) are d systems, it is really difficult for these to undergo oxida-
tion or reduction process. The spectral behavior of these complexes
is neither due to metal-to-ligand nor ligand-to-metal charge trans-
⁄
fer transition. Hence, metal perturbed intraligand
p
–p
charge

3
42
S. Mao et al. / Polyhedron 134 (2017) 336–344
Fig. 6. TG curves of complexes 1–3.
Fig. 7. (a), (b) The solid-state fluorescent emission spectra of the ligands and the complexes. (c) Photographic images of ligands and the complexes under ambient light and
UV light irradiation (365 nm).
transfer transition is responsible for the emission behavior of the
reported complexes [41]. Moreover the enhancement and shift of
Table 4
the emissions for complexes 1 and 2 compared to 3-PBO and 4-
PBO may be attributed to the fact that the coordination of metal
ions increases the ligand conformational rigidity and reduces the
loss of energy by radiationless decay of the intraligand emission
excited state [42]. During complexation, quenching of fluorescence
of complexes 3 is a somewhat common phenomenon, which is
Electrochemical data of complex 3.
Complex
E
pc
E
pa
D
E
p
E
1/2
i
pa
i
pc
I
3
0.373
0.621
0.248
0.497
57.498
ꢁ17.835
3.224
D
E = Epa ꢁ Epc; E1/2 = (Epa + Epc)/2; I = ipa/ipc
.

S. Mao et al. / Polyhedron 134 (2017) 336–344
343
Appendix A. Supplementary material
CCDC 1519464, 1519465 and 1519466 contains the supplemen-
tary crystallographic data for complexes 1–3. 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-
3
36-033; or e-mail: deposit@ccdc.cam.ac.uk.
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