Novel multinuclear transition metal(II) complexes based on an asymmetric Salamo-type ligand: Syntheses, structure characterizations and fluorescent properties

Article abstract of DOI:10.1016/j.ica.2016.01.034

Three multinuclear complexes, [{Cu(HL)(OAc)Cu}₂]·CH₃OH, [Zn(HL)(OAc)Zn(CH₃CH₂OH)] and [{Cd(HL)(OAc)Cd(CH₃CH₂OH)}₂]·2CHCl₃, have been synthesized with an asymmetric Salamo-type ligand H₄L, and characterized by FT-IR, UV–vis spectra and X-ray crystallography. The Cu(II) complex contains two crystallographically independent but chemically identical dinuclear complexes (molecules 1 and 2). Meanwhile, self-assembled infinite 1D, 2D and 3D structures are formed by intermolecular hydrogen bonds and C–H?π interactions of neighboring complexes in the solid state. The Zn(II) complex also forms a novel dinuclear structure. And a self-assembled infinite 2D structure is formed by C–H?π interaction of neighboring complexes in the solid state. Otherwise, the Cd(II) complex forms an infinite 1D structure by C–H?π interaction of the neighboring complex molecules. In addition, the photophysical properties of the Cu(II), Zn(II) and Cd(II) complexes have also been discussed.

Full text of DOI:10.1016/j.ica.2016.01.034

Accepted Manuscript
Novel multinuclear transition metal(II) complexes based on an asymmetric Sal-
amo-type ligand: syntheses, structure characterizations and fluorescent proper-
ties
Wen-Kui Dong, Jing Zhang, Yang Zhang, Na Li
PII:
S0020-1693(16)00045-1
http://dx.doi.org/10.1016/j.ica.2016.01.034
ICA 16862
DOI:
Reference:
Inorganica Chimica Acta
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19 September 2015
16 January 2016
20 January 2016
Please cite this article as: W-K. Dong, J. Zhang, Y. Zhang, N. Li, Novel multinuclear transition metal(II) complexes
based on an asymmetric Salamo-type ligand: syntheses, structure characterizations and fluorescent properties,
Inorganica Chimica Acta (2016), doi: http://dx.doi.org/10.1016/j.ica.2016.01.034
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Novel multinuclear transition metal(II) complexes based
on an asymmetric Salamo-type ligand: syntheses,
structure characterizations and fluorescent properties
Wen-Kui Dong*, Jing Zhang, Yang Zhang, Na Li
School of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou 730070, P. R. China
ABSTRACT
Three
multinuclear
complexes,
[{Cu(HL)(OAc)Cu}2]·CH3OH,
[Zn(HL)(OAc)Zn(CH3CH2OH)] and [{Cd(HL)(OAc)Cd(CH3CH2OH)}2]·2CHCl3, have
been synthesized with an asymmetric Salamo-type ligand H4L, and characterized by FT-IR,
UV-vis spectra and X-ray crystallography. The Cu(II) complex contains two
crystallographically independent but chemically identical dinuclear complexes (molecules 1
and 2). Meanwhile, self-assembled infinite 1D, 2D and 3D structures are formed by
intermolecular hydrogen bonds and C–H···π interactions of neighboring complexes in the
solid state. The Zn(II) complex also forms a novel dinuclear structure. And a self-assembled
infinite 2D structure is formed by C–H···π interaction of neighboring complexes in the
solid state. Otherwise, the Cd(II) complex forms an infinite 1D structure by C–H···π
interaction of the neighboring complex molecules. In addition, the photophysical properties
of the Cu(II), Zn(II) and Cd(II) complexes have also been discussed.
Keywords: Salamo-type ligand; Transition metal(II) complex; Synthesis; Crystal structure;
Fluorescent property
1. Introduction
Transition metal(II) complexes with Salen-type ligands have been studied for many
decades, which have been attracted much interest for their catalytic activities [1,2], medical
imaging, optical materials [3,4], etc. Photoluminescence has been one of the highlights in

modern coordination chemistry owing to potential applications in luminescent device [5,6],
which have been also observed for Salen-type compounds and their transition metal(II)
complexes [7,8]. Noticeably, several excellent works have been devoted to synthesize and
characterize mononuclear, homo- or heteropolynuclear transition metal(II) complexes
bearing Salamo-type ligand or its derivatives [9-15].
Compared with symmetric salamo-type ligands, the asymmetric configuration would
afford the opportunities for a greater structural variation and infinite coordination polymers,
which would be expected to lead to novel characteristics [16]. As selective synthesis of
asymmetrical Salamo-type ligands is important because electronic and steric effect of the
ligands on Salen-metal-assisted catalysis may be controlled by introduction of different
substituents into the two benzene rings [17]. In order to study the structural feature and
fluorescent characteristics of the transition metal(II) complexes with asymmetric
Salamo-type ligands, herein we have synthesized a new asymmetric Salamo-type ligand
and its corresponding Cu(II), Zn(II) and Cd(II) complexes. And the structures of the Cu(II),
Zn(II) and Cd(II) complexes have been characterized by X-ray single crystal diffraction.
2. Experimental
2.1. Materials and instruments
2,3-Dihydroxybenzaldehyde,
2-hydroxy-3-aminoacetophenone
and
2-hydroxy-3-methoxybenzaldehyde were purchased from Alfa Aesar and used without
further purification. The other reagents and solvents are of analytical grade from Tianjin
Chemical Reagent Factory.
C, H, and N analyses were obtained using a GmbH VarioEL V3.00 automatic elemental
analysis instrument. Elemental analyses for Cu(II), Zn(II) and Cd(II) ions were detected by
an IRIS ER/S·WP–1 ICP atomic emission spectrometer. FT-IR spectra were recorded on a
VERTEX70 FT-IR spectrophotometer, with samples prepared as KBr (400-4000 cm-1) and
CsI (100-500 cm-1) pellets. UV-Vis absorption spectra in the 200-600 nm range were
recorded on a Hitachi UV-3010 spectrophotometer in chloroform solution. 1H NMR spectra
were determined by German Bruker AVANCE DRX-400 spectrometer. Melting points were

obtained by use of an X4 microscopic melting point apparatus made in Beijing Taike
Instrument Limited Company and were not corrected. Fluorescent spectra were taken on a
LS-55 fluorescence photometer. X-ray single crystal structures were determined on a
Bruker Smart Apex CCD diffractometer.
2.2. Synthesis of the ligand H4L
1,2-Bis(phthalimidoxy)ethane was synthesized according to the literature [8,11].
1,2 -Bis(aminooxy)ethane was prepared according to an analogous method reported
earlier [11,16].
The asymmetric Salamo-type ligand H4L was synthesized by a modified method
reported in the earlier literature (Scheme 1) [18,19].
Scheme 1. Synthetic route to H4L.
To an ethanol solution (30 ml) of 2-hydroxy-3-methoxybenzaldehyde (304.3 mg, 2
mmol) was added an ethanol solution (20 ml) of 1, 2-bis(aminooxy)ethane (276.0 mg, 3
mmol). The mixture solution was heated at 50 – 55 °C for 6 h. The solution was
concentrated in vacuo and the residue was purified by column chromatography (SiO2,
chloroform/ethyl acetate, 10:1) to afford colorless flocculent crystalline solid of
2-hydroxy-3-methoxybenzaldehyde O-(2-(aminooxy)ethyl) oxime (327.2 mg, 1.45 mmol).
Yield, 72.3%. M.p. 91-92 °C. Anal. Calc. for C10H14N2O4: C, 53.09; H, 6.24; N, 12.38.
Found: C, 53.12; H, 6.22; N, 12.37%.
A solution of 2-hydroxy-3-methoxybenzaldehyde O-(2-(aminooxy)ethyl) oxime (452.4
mg, 2 mmol) in ethanol (10 mL) was added dropwise to a solution of
2-hydroxy-3-aminoacetophenone (303.2 mg, 2 mmol) in ethanol (10 mL), and the mixture
solution was heated at 55 – 60 °C for 24 h. After cooling to room temperature, brownish
intermediate products (359.9 mg, 1 mmol) were collected on a suction filter. Yield, 50%.
M.p. 123-124 °C. Anal. Calc. for C18H21N3O5: C, 60.16; H, 5.89; N, 11.69. Found: C, 60.27;
H, 5.78; N, 11.54%.
A solution of above intermediate products (719.8 mg, 2 mmol) in ethanol (10 mL) was
added to a solution of 2,3-dihydroxybenzaldehyde (276.2 mg, 2 mmol) in ethanol (10 mL),

and the mixture solution was heated at 50 – 55 °C for 4 h. Takeing advantage of heat
filtration, red precipitates of H4L (796.8 mg, 1.6 mmol) were collected on a suction filter.
1
Yield, 80%. M.p. 136-137 °C. H NMR (400 MHz, CDCl3): δ 2.37 (s, 3H, CH3), 3.90 (s,
3H, CH3), 4.52 (s, 4H, CH2), 6.17 (s, 1H, ArH), 6.70 (m, 1H, ArH), 6.91 (m, 6H, ArH),
7.35 (dd, J = 8, 2H, ArH), 8.37 (s, 1H, CH=N), 8.65 (s, 1H, CH=N), 9.72 (s, 1H, OH),
11.89 (s, 1H, OH), 15.13 (s, 1H, OH). Anal. Calc. for C25H25N3O7 (H4L): C, 62.62; H, 5.26;
N, 8.76. Found: C, 62.75; H, 5.07; N, 8.65%.
2.3. Synthesis of the Cu(II) complex
A solution of copper(II) acetate monohydrate (1.99 mg, 0.01 mmol) in methanol (4 ml)
was added to a solution of H4L (2.50 mg, 0.005 mmol) in dichloromethane (2 ml) at room
temperature. The color of the mixing solution turned to dark-green immediately, the
mixture solution was filtered and the filtrate was allowed to stand at room temperature for
about one week. The solvent was partially evaporated and dark-brown block-shaped single
crystals suitable for X-ray crystallographic analysis were obtained (0.699 mg, 0.001 mmol).
Yield, 20.6%. Anal. Calc. for C55H54Cu4N6O19: C, 48.67; H, 4.01; N, 6.19; Cu, 18.73.
Found: C, 48.68; H, 4.03; N, 6.17; Cu, 18.74%.
2.4. Synthesis of the Zn(II) complex
A solution of zinc(II) acetate dihydrate (2.19mg, 0.01mmol) in ethanol (6 ml) was
added to a solution of H4L (2.50 mg, 0.005 mmol) in dichloromethane (2 ml) at room
temperature. The color of the mixing solution turned to yellow immediately, the mixture
solution was filtered and the filtrate was allowed to stand at room temperature for about one
week. The solvent was partially evaporated and several yellow block-shaped single crystals
suitable for X-ray crystallographic analysis were obtained (0.837 mg, 0.001 mmol). Yield,
23.5%. Anal. Calc. for C29H31Zn2N3O10: C, 48.90; H, 4.39; N, 5.90; Zn, 18.36. Found: C,
48.91; H, 4.38; N, 5.88; Zn, 18.39%.
2.5. Synthesis of the Cd(II) complex
A solution of cadmium(II) acetate dihydrate (2.66 mg, 0.01 mmol) in ethanol (3 ml)
was added dropwise to a solution of H4L (2.50 mg, 0.005 mmol) in chloroform (3 ml) at
room temperature. The color of the mixing solution turned to yellow immediately, the
mixture solution was filtered and the filtrate was allowed to stand at room temperature for

about several weeks. The solvent was partially evaporated and several yellow block-shaped
single crystals suitable for X-ray crystallographic analysis were obtained (1.190 mg, 0.001
mmol). Yield, 25.7%. Anal. Calc. for C60H64Cd4Cl6N6O20: C, 38.92; H, 3.48; N, 4.54; Cd,
24.28. Found: C, 38.94; H, 3.49; N, 4.52; Cd, 24.26%.
2.6. X-ray Crystallography
X-ray diffraction data (Table 1) for the Cu(II), Zn(II) and Cd(II) complexes were
collected on a Bruker Smart Apex CCD diffractometer at 293.42(10), 290.92(10) and
293.42(10) K, respectively. Using graphite monochromatized Mo Kα radiation (λ =
0.071073 nm). Unit cell parameters were determined by least squares analysis. The LP
factor and Semi-empirical absorption corrections were applied to the intensity data. The
structure was solved by the direct methods (SHELXS-97) and subsequent
difference-Fourier map revealed the positions of the remaining atoms, and all hydrogen
atoms were added theoretically. All non-hydrogen atoms were refined anisotropically using
a full-matrix least-squares procedure on F2 with SHELXL-97.
Table 1 X-ray crystallographic data collection, solution and refinement parameters for the
Cu(II), Zn(II) and Cd(II) complexes.
3 Result and discussion
3.1 Crystal structures
3.1.1 The Cu(ΙΙ) complex
The crystal structure and atom numbering of the Cu(ΙΙ) complex is given in Fig. 1.
Selected bond distances and angles of the Cu(ΙΙ) complex are listed in Table 2.
X-ray crystallographic analysis of the Cu(ΙΙ) complex reveals a rare dinuclear structure.
The Cu(ΙΙ) complex crystallizes in the triclinic system, space group P-1, and the unit cell
contains two crystallographically independent but chemically identical dinuclear complexes
(molecules 1 and 2, Fig. 1) consisting of two Cu(ΙΙ) atoms, one (HL)3- unit and one
coordinated acetate ion as expected from the analytical data. The Cu1 atom is
penta-coordinated by two nitrogen atoms (N1, N2) and two oxygen atoms (O2, O5) of

(HL)3- unit and one oxygen atom (O15) of coordinated acetate ion. And the phenolic
oxygen atoms (O2 and O5) and the oxime nitrogen atoms (N1 and N2) of the (HL)3- unit
constitute together the basal plane (Cu1–O2, 1.919(4) Å; Cu1–O5, 1.944(5) Å; Cu1–N1,
1.944(6) Å and Cu1–N2, 2.024(5) Å), the oxygen atom (O15) of coordinated acetate ion is
occupying the axial position (Cu1···O15, 2.178(5) Å), the coordination geometry around
the Cu1 center can be regarded as a distorted square pyramidal geometry which is deduced
by calculating the value of τ = 0.147 [20]. The Cu2 atom is tetra-coordinated by one oxime
nitrogen atom (Cu2–N3, 1.952(5) Å), two phenolic oxygen atoms (Cu2–O5, 1.959(4) Å;
Cu2–O6, 1.890(4) Å) of (HL)3- unit and one oxygen atom (Cu2–O16, 1.963(5) Å) of
coordinated acetate ion, and has a slightly distorted square geometry. Therefore, two Cu(ΙΙ)
atoms of [{Cu(HL)(OAc)Cu}2]·CH3OH have two different coordination geometries.
In the Cu(ΙΙ) complex, the Cu1 atom is displaced 0.2380(4) Å from the mean plane
N2O2. The coordination planes of O2–Cu1–N1 and O5–Cu1–N2 have a dihedral angle of
19.22(3)º, giving a nonplanar configuration. The Cu2 atom is displaced 0.1182(3) Å from
the mean plane NO3. The dihedral angle between the coordination planes of O6–Cu2–N3
and O5–Cu2–O16 is 12.83(3)º. In addition, the ethylenedioxime carbons C1 and C2 in the
Cu(ΙΙ) complex are buckled asymmetrically from the Cu1-N1-N2 plane, with the
displacement for C1 being 1.311(4) Å toward the plane and for C2 being only 1.889(4) Å in
the same direction.
Table 2. Selected bond distances (Å) and angles (º) for the Cu(II), Zn(II) and Cd(II)
complexes.
Fig. 1. Molecular structure of the Cu(II) complex with the atom numbering. The
hydrogen atoms are omitted for clarity.
In the crystal structure, the Cu(II) complex molecules 1 and 2 are linked by
intermolecular hydrogen bond interactions. Three pairs of intermolecular hydrogen bonds,
C17−H17···O8, C42−H42···O1 and C42−H42···O2 are formed (Fig. S1). One of the
protons (C17−H17) of the benzene ring is hydrogen-bonded to the methoxy oxygen atom

(O8) of the (HL)3- unit. The other one (C42-H42) is hydrogen-bonded to one of the
phenolic oxygen atoms (O2) and the methoxy oxygen atom (O1) of the (HL)3- unit,
respectively (Table 2). Consequently, these hydrogen-bonding interactions have stabilized
the Cu(II) complex molecules 1 and 2. The hydrogen bond data are summarized in Table 3.
As shown in Fig. S1, the Cu(II) complex molecules are interlinked by
C9–H9B···Cg18 (C29-C34) interactions into a 1D chain structure along the b axis [21]. In
addition, the adjacent chains along the b axis are held together to form an infinite 2D
structure on the ab crystallographic plane by three pairs of intermolecular C23–H23···O11,
C30–H30···O14 and C51–H51···O4 hydrogen bonds (Fig. S2), which are formed between
the C23–H23 unit of the benzene ring, the C30–H30 unit of the benzene ring and the
C51–H51 unit of the benzene ring and the oxygen atoms of oxime group (O11), phenolic
oxygen (O14), acetic oxygen (O4) of each adjacent Cu(II) complex molecules, respectively
[22, 23]. The planes are further interlinked by C39–H39C···O12 intermolecular hydrogen
bond interaction to form an infinite 3D structure (Fig. S3) [24].
Thus, a self-assembled infinite 3D structure is formed by seven intermolecular
hydrogen bonds and C–H···π interaction of neighboring Cu(II) complex molecules.
Table 3. Hydrogen-bonding interactions [Å, º] for the Cu(II), Zn(II) and Cd(II) complexes.
3.1.2 The Zn(ΙΙ) complex
The structural features of the Zn(II) complex are very similar to those of the Cu(II)
complex mentioned above, the crystal structure and atom numbering of the Zn(II) complex
is presented in Fig. 2 and selected bond distances and angles of the Zn(II) complex are
shown in Table 2.
The Zn(II) complex crystallizes in the monoclinic space group P2(1)/c, which consists
of two Zn(II) atoms, one (HL)3- unit, one coordinated acetate ion and one coordinated
ethanol molecule. Moreover, the Zn(II) complex also forms a novel 1:2 ((L)3- : Zn(II))
dinuclear structure which is different from the structures of 1:1 [25,26], 2:3 [23], 2:4 [8],
4:8 [12] (L: Zn(II)) Salamo-type Zn(II) complexes reported earlier. The Zn1 atom is
penta-coordinated by two nitrogen atoms (N1, N2) and two oxygen atoms (O1, O5) of

(HL)3- unit and one oxygen atom (O8) of coordinated acetate ion. Moreover the phenolic
oxygen atom (O1), the oxime nitrogen atom (N2) of the (HL)3- unit and the oxygen atom
(O8) of coordinated acetate ion constitute together the basal plane (Zn1–O1, 1.972(3) Å;
Zn1–O8, 2.020(3) Å and Zn1–N2, 2.059(3) Å), one phenolic oxygen atoms (O5) and one
oxime nitrogen atoms (N1) are occupying the axial position (Zn1–O5, 2.016(3) Å; Zn1–N1,
2.136(3) Å). The Zn2 atom is also penta-coordinated by two phenolic oxygen atoms (O5,
O6), one oxygen atom (O9) of coordinated acetate ion, one Schiff base nitrogen atom (N3)
and one oxygen atom (O10) of coordinated ethanol molecule. Moreover the oxygen atom
(O9) of coordinated acetate ion, the Schiff base nitrogen atom (N3) and the oxygen atom
(O10) of coordinated ethanol molecule constitute together the basal plane (Zn2–O9,
1.979(3) Å; Zn2–O10, 2.034(3) Å and Zn2–N3, 2.029(3) Å), the two phenolic oxygen
atoms (O5, O6) are occupying the axial position (Zn2–O5, 2.100(3) Å; Zn2–O6, 1.981(3)
Å). As we know, the coordination number of Zn(II) is generally four or six and the
coordination geometry around Zn(II) is tetrahedron or octahedron [25]. However, there are
only a few reports of the five-coordinated Salamo-type Zn(II) complexes having distorted
square–pyramidal or trigonal–bipyramidal configurations [12]. Here, this dinuclear Zn(II)
complex obtained in which the Zn1 and Zn2 atoms are both five-coordinated with trigonal
bipyramidal geometries which are deduced by calculating the values of τ1 = 0.848, τ2 =
0.764, respectively [20].
The Zn(II) complex molecules form a dimer structure by π-π stacking interaction of
neighboring benzene rings. As illustrated in Fig. S4, the Zn(II) complex molecules are
interlinked by Cg7 (C12-C17)···Cg8 (C19-C24) π-π stacking interaction into a dimer.
Consequently, two π-π stacking interactions of neighboring benzene rings have stabilized a
pair of the Zn(II) complex molecules to form a dimer. As shown in Fig. S5, the dimers are
interlinked by C14–H14···Cg6 (C1-C6) and C21–H21···Cg7 (C12-C17) interactions into a
infinite 2D network structure along the bc axis, which are formed between the C14–H14
units of the benzene rings, the C21–H21 units of the benzene rings and the neighboring
benzene rings of each adjacent dimers, respectively [18]. Thus, a self-assembled infinite 2D
structure is formed by C–H···π interaction of neighboring dimmers in the solid state. It is
interesting to note that there are strong intermolecular π-π stacking interactions between the

nearest parallel benzene rings. Two neighboring parallel benzene planes of the Zn(II)
complex molecules form π-π stacking interaction at a distance of 3.808(4) Å.
Fig. 2. Molecular structure of the Zn(II) complex with the atom numbering. The hydrogen
atoms are omitted for clarity.
3.1.3 The Cd(ΙΙ) complex
The crystal structure and atom numbering of the Cd(II) complex are drawn in Fig. 3.
The selected bond distances and bond angles are summed in Table 2.
The Cd(II) complex is composed of four Cd(II) atoms, two (HL)3- units, two
coordinated acetate ions and two coordinated ethanol molecules. Two Cd(II) atoms of
[{Cd(HL)(OAc)Cd(CH3CH2OH)}2]·2CHCl3 have two different coordination geometries.
The Cd2 atom is hexa-coordinated by two nitrogen atoms (N2, N3) and two oxygen atoms
(O3, O6) of (HL)3- unit and two oxygen atoms (O9, O10) of coordinated acetate ion.
Meanwhile, the phenolic oxygen atom (O3), the oxime nitrogen atom (N3) and the oxygen
atoms (O9, O10) of coordinated acetate ion constitute together the basal plane (Cd2–O3,
2.235(3) Å; Cd2–O9, 2.374(4) Å; Cd2–O10, 2.331(4) Å and Cd2–N3, 2.354(4) Å), the
phenolic oxygen atom (O6) and the oxime nitrogen atom (N2) are occupying the axial
position (Cd2–O6, 2.221(3) Å; Cd2–N2, 2.332(4) Å). The coordination geometry around
the Cd2 center can be regarded as a distorted octahedral geometry. The Cd1 atom is
hepta-coordinated by four phenolic oxygen atoms (O3, O6, O2, O2#), one oxygen atom (O7)
of methoxy group of (HL)3-, one Schiff base nitrogen atom (N1) and one oxygen atom (O8)
of coordinated ethanol molecule. Moreover, the oxygen atom (O7) of methoxy group of
(HL)3-, the Schiff base nitrogen atom (N1) and the phenolic oxygen atoms (O2, O3, O6)
constitute together the basal plane (Cd1–O3, 2.348(3) Å; Cd1–O2, 2.250(3) Å; Cd1–O7,
2.559(3) Å; Cd1–O6, 2.254(3) Å and Cd1–N1, 2.318(4) Å), the phenolic oxygen atoms
(O2#) and the oxygen atom (O8) of coordinated ethanol molecule are occupying the axial
position (Cd1–O8, 2.374(4) Å; Cd1–O2#, 2.330(3) Å). The coordination geometry around
the Cd1 center can be regarded as a distorted pentagonal bipyramidal geometry. This
hepta-coordinated Cd(II) complex forms a novel 2:4 ((L)3- : Cd) tetranuclear structure

which is different from the structure of 1:1 (L : Cd) [25] reported earlier in the Salamo-type
Cd(II) complexes.
The Cd(II) complex forms a infinite 1D structure by C–H···π interaction of
neighboring complex molecules. As shown in Fig. S6, the Cd(II) complex molecules are
interlinked by C15–H15A···Cg5 (C8-C13) interaction into a infinite 1D chain structure
along the c axis.
Fig. 3. Molecular structure of the Cd(II) complex with the atom numbering. The hydrogen
atoms are omitted for clarity.
3.2 FT-IR spectral analysis
The FT-IR spectra of H4L and its corresponding Cu(II), Zn(II) and Cd(ΙΙ) complexes
exhibit various bands in the 400–4000 cm-1 region (Fig. 4). The most important FT-IR
bands for H4L and its Cu(II), Zn(II) and Cd(ΙΙ) complexes are given in Table 4. The Cu(II),
Zn(II) and Cd(ΙΙ) complexes have similar FT-IR spectra which indicate that they have
similar structures. The free ligand H4L shows a broad characteristic band of the OH group
at 2935-3065 cm-1. This band is weakened in the FT-IR spectra of the Cu(II), Zn(II) and
Cd(ΙΙ) complexes, which is indicative of the fact that the phenolic OH groups of H4L have
been deprotonized and coordinated to the Cu(II), Zn(II) and Cd(ΙΙ) atoms [27]. For the
Cu(II), Zn(II) and Cd(ΙΙ) complexes, these bands weakened are expected due to the
substitution of hydrogen atoms for the Cu(II), Zn(II) and Cd(ΙΙ) atoms upon coordination
[28].
Table 4. The most important FT-IR bands for the ligand and its Cu(II), Zn(II) and Cd(ΙΙ)
complexes (cm-1).
The free ligand H4L exhibits characteristic C=N stretching band at 1613 cm-1, while the
C=N stretching bands of the Cu(II), Zn(II) and Cd(ΙΙ) complexes are observed in the 1605
cm-1, 1605 cm-1 and 1598 cm-1, respectively. The C=N stretching frequencies are shifted to
lower frequencies by ca. 8, 8 and 15 cm-1 upon complexation, indicating that the Cu(II),

Zn(II) and Cd(ΙΙ) atoms are coordinated by N2O2 donor atoms of (HL)3- units. Thus, it
provides evidence for the coordination of H4L with Cu(II), Zn(II) and Cd(ΙΙ) atoms. In the
1438-1466 cm-1 region, the observed bands are attributed to aromatic C=C vibration.
Upon coordination these bands shift to lower frequencies for the Cu(II), Zn(II) and Cd(ΙΙ)
complexes [29].
The Ar–O stretching frequency appears at 1257 cm-1 for the ligand H4L, while the Ar–O
stretching frequencies of the Cu(II), Zn(II) and Cd(ΙΙ) complexes are observed in the 1244,
1244 and 1240 cm-1, respectively. The Ar–O stretching frequencies are shifted to a lower
frequencies, indicating that the Cu-O or Zn–O or Cd–O bonds are formed between the
Cu(II), Zn(II) and Cd(ΙΙ) ions and oxygen atoms of phenolic groups [30].
The far-infrared spectra of the Cu(II), Zn(II) and Cd(ΙΙ) complexes were also obtained
in the region 550-100 cm-1 in order to identify frequencies due to the M-O and M-N bonds.
The bands at 422 cm-1 in the Cu(II) and Zn(II) complexes are assigned to νCu–O and νZn–O
,
while the bands at 526 cm-1 are assigned to νCu–N and νZn–N, respectively. The FT-IR
spectrum of the Cd(ΙΙ) complex shows νCd–O at 413 cm-1, and νCd-N is very weak, so it isn’t
marked out in the spectrum of Cd(ΙΙ) complex. These new bands are observed for the Cu(II),
Zn(II) and Cd(ΙΙ) complexes and are not present in the spectrum of the free ligand H4L. As
pointed out by Percy and Thornton [31], the metal–oxygen and metal–nitrogen frequency
assignments are at times very difficult.
3.3 UV–vis absorption spectral study
The UV-vis absorption spectra of H4L and its Cu(II), Zn(II) and Cd(II) complexes were
determined in chloroform solution (c = 1×10-5 mol⋅L-1), are shown in Fig. 5. The absorption
peaks of the Cu(II), Zn(II) and Cd(II) complexes are obviously different from those of the
ligand.
UV-Vis spectrum of the free ligand H4L exhibits two absorption bands at 271 and 364
nm. The former absorption band at 271 nm can be assigned to the π-π* transition of the
benzene rings and the latter one at 364 nm can be attributed to the π-π* transition of the
oxime groups [32]. Upon coordination of the ligand, the absorption bands of the Zn(ΙΙ) and

Cd(ΙΙ) complexes appear at ca. 269 nm, and absorption intensities are weakened compared
with the free ligand H4L, which indicate that the oxime nitrogen atoms are involved in
coordination to the Zn(ΙΙ) or Cd(ΙΙ) atoms. Upon coordination of the ligand, the intraligand
π-π* transition of the benzene ring of salicylaldehyde is disappeared in the Cu(ΙΙ) complex
[33]. Moreover, the new bands are observed at 387, 407 and 368 nm in the Cu(ΙΙ), Zn(ΙΙ)
and Cd(ΙΙ) complexes respectively, which is assigned to the n–π* charge transfer transition
from the filled pπ orbital of the bridging phenolic oxygen to the vacant d-orbital of the
metal(II) ions [34].
3.4 Fluorescence spectral study
The fluorescent properties of H4L and its corresponding Cu(II), Zn(II) and Cd(II)
complexes were measured at room temperature (Fig. 6). The Cu(II), Zn(II) and Cd(II)
complexes display enhanced emission intensities compared to the corresponding ligand
(H4L) when excited with the some amount of energy. Enhancement of fluorescence through
complexation is of much interest as it creates the opportunity for photochemical
applications of these complexes [35, 36].
The fluorescence of the free ligand is probably quenched by the occurrence of a
photoinduced electron transfer process due to the presence of a lone pair of nitrogen atoms.
Such the process is prevented by the complexation of the ligand with metal atoms. Thus the
fluorescence intensity may be effectively enhanced by the coordination of Cu(II), Zn(II)
and Cd(II) atoms [37].
The free ligand exhibits a weak emission peak at 443 nm upon excitation at 407 nm.
The Cu(II) complex shows an intense photoluminescence with maximum emission at 444
nm upon excitation at 407 nm (Fig. 6). Since the emission peak position of the Cu(II)
complex is similar to that of the free ligand, the emission peak of the Cu(ΙΙ) complex may
also arise from the intraligand transition.
The emission spectra shows that the emission spectral shape of the Zn(II) complex
closely resemble that of the Cd(II) complex. In comparison with the corresponding H4L
with the maximum emission wavelength at 443 nm when excited with 407 nm, the Zn(ΙΙ)
and Cd(ΙΙ) complexes exhibits red-shift with the maximum emission at 533 and 537 nm

when excited with 407 nm, which could be assigned to ligand-to-metal charge transfer
(LMCT) [38].
Conclusions
According to the data and discussion above, three new complexes with the chemical
formulae
[{Cu(HL)(OAc)Cu}2]·CH3OH,
[Zn(HL)(OAc)Zn(CH3CH2OH)]
and
[{Cd(HL)(OAc)Cd(CH3CH2OH)}2]·2CHCl3, have been synthesized and structurally
characterized. The Cu(II) complex forms a dimer unit by intermolecular hydrogen bonds. In
addition, a self-assembled infinite 1D, 2D, 3D structure are formed by intermolecular
hydrogen bonds and C–H···π interaction of neighboring dimmers in the solid state. The
Zn(II) complex molecules form a dimer structure by π-π stacking of neighboring benzene
rings. In addition, a self-assembled infinite 2D structure is formed by C–H···π interaction
of neighboring dimmers in the solid state. The Cd(II) complex forms a infinite 1D structure
by C–H···π interaction of the neighboring complex molecules. In the FT-IR spectra of the
Cu(II), Zn(II) and Cd(II) complexes, the ν(M-O) and ν(M-N) vibrational absorption
frequencies have been observed. Meanwhile, the Cu(II), Zn(II) and Cd(II) complexes
exhibit blue emission with the maximum emission wavelength at 444, 533, 537 nm when
excited with 407 nm.
Acknowledgements
This work was supported by the National Natural Science Foundation of China
(21361015), Graduate Student Guidance Team Building Fund of Lanzhou Jiaotong
University (260001) and the science and technology support funds of Lanzhou Jiaotong
University (ZC2012003), which are gratefully acknowledged.

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Table 1 X-ray crystallographic data collection, solution and
refinement parameters for the Cu(II), Zn(II) and Cd(II) complexes.
CCDC deposit number+
Empirical formula
Formula weight
Crystal size (mm)
Crystal system
Space group
a (Å)
1424749
1424750
1424748
C55H54Cu4N6O19
1357.20
C29H31Zn2N3O10
712.31
C60H64Cd4Cl6N6O20
1851.46
0.25 × 0.30 × 0.32 0.22 × 0.26 × 0.28 0.25 × 0.31 × 0.32
triclinic
P-1
monoclinic
P2(1)/c
triclinic
P-1
9.8721(4)
15.5004(8)
19.2091(8)
95.063(4)
97.008(4)
94.171(4)
2
14.8512(5)
13.4320(7)
15.7579(7)
90
9.6876(10)
12.9281(10)
14.2601(10)
104.768(6)
94.128(7)
93.940(7)
1
b (Å)
c (Å)
α (º)
111.368(5)
90
β (º)
γ (º)
4
Z
Cell volume (Å3)
2895.7(2)
2927.3(2)
1.616
1715.4(3)
1.792
Density calculated (g cm– 3) 1.557
µ (mm– 1) /
1.528
1.702
1.532
Collected reflections
20620
11588
11994
11315
5760
6744
Unique reflections (Rint)
[Rint = 0.046]
11315/0/767
1.044
[Rint = 0.054]
5760/3/405
1.042
[Rint = 0.033]
6744/0/442
1.072
Data/restraints/parameters
Goodness-of-fit on F2
R1 = 0.0713,
wR2 = 0.1838
R1 = 0.1033,
wR2 = 0.2141
4.36 and -0.57
R1 = 0.0531,
wR2 = 0.0797
R1 = 0.0982,
wR2 = 0.1024
0.61 and -0.47
R1 = 0.0426,
wR2 = 0.0843
R1 = 0.0657,
wR2 = 0.1023
0.94 and -0.87
Final R1 Indices [I>2σ(I)]
R Indices (all data)
Residuals peak/hole (e / Å3)
Data for these compounds can be obtained free of charge from the CCDC via www.ccdc.cam.ac.uk/data_request/cif.
Wen-kui Dong et al.

Table 2. Selected bond distances (Å) and angles (º) for the Cu(II), Zn(II) and Cd(II)
complexes.
Cu(II) complex
Cu1-N1
1.944(6)
2.024(5)
1.890(4)
102.3(2)
95.2(2)
Cu1-O2
1.919(4)
2.178(5)
1.963(5)
91.54(19)
91.0(2)
Cu1-O5
1.944(5)
1.959(4)
1.952(5)
170.2(2)
83.9(2)
Cu1-N2
Cu1-O15
Cu2-O5
Cu2-O6
Cu2-O16
Cu2-N3
N1-Cu1-O15
N1-Cu1-N2
N2-Cu1-O15
O2-Cu1-O5
N3-Cu2-O16
O6-Cu2-O16
O2-Cu1-O15
O2-Cu1-N1
O2-Cu1-N2
O5-Cu1-O15
O5-Cu2-O16
O6-Cu2-N3
O5-Cu1-N1
O5-Cu1-N2
N1-Cu1-O15
N1-Cu1-N2
O6-Cu2-O5
N3-Cu2-O5
104.2(2)
87.04(18)
167.0(2)
88.4(2)
161.4(2)
87.39(19)
95.09(19)
93.7(2)
102.3(2)
95.2(2)
176.46(19)
82.75(19)
Zn(II) complex
Zn1-O1
1.972(3)
2.020(3)
2.100(3)
2.034(3)
94.07(12)
Zn1-O5
Zn2-N3
Zn2-O6
2.016(3)
2.029(3)
1.981(3)
Zn1-N1
Zn1-N2
Zn2-O9
2.136(3)
2.059(3)
1.979(3)
Zn1-O8
Zn2-O5
Zn2-O10
O1-Zn1-O5
O1-Zn1-N2
O5-Zn1-N2
N2-Zn1-N1
O6-Zn2-N3
O9-Zn2-O10
N3-Zn2-O5
O1-Zn1-O8
111.93(13)
95.34(12)
86.73(13)
170.69(11)
94.46(12)
124.46(14)
124.87(14)
O1-Zn1-N1
O5-Zn1-N1
O8-Zn1-N2
O6-Zn2-O10
O9-Zn2-O6
O10-Zn2-O5
87.45(14)
176.75(13)
125.86(15)
93.90(13)
93.44(13)
88.07(12)
122.06(14) O5-Zn1-O8
85.15(12)
91.61(14)
92.69(13)
O8-Zn1-N1
O6-Zn2-O5
O9-Zn2-O5
109.65(13) O9-Zn2-N3
78.77(12)
N3-Zn2-O10
Cd(II) complex
Cd1-O2
2.250(3)
2.559(3)
2.330(3)
2.331(4)
2.690(6)
Cd1-O3
Cd1-O8
Cd2-O6
Cd2-N2
Cd2-O3
2.348(3)
Cd1-O6
Cd1-N1
Cd2-O9
Cd2-N3
2.254(3)
2.318(4)
2.374(4)
2.354(4)
Cd1-O7
Cd1-O2#
2.374(4)
2.221(3)
Cd2-O10
2.332(4)
Cd2-C28
2.235(3)
O2-Cd1-O3
O2-Cd1-O8
O3-Cd1-O8
O6-Cd1-O8
N1-Cd1-O2
N1-Cd1-O8
144.49(12) O2-Cd1-O6
106.14(14) O2-Cd1-N1
139.28(11)
79.56(13)
72.76(11)
141.15(13)
69.96(13)
88.78(13)
O2-Cd1-O7
O3-Cd1-O7
O6-Cd1-O7
O8-Cd1-O7
N1-Cd1-O7
O3-Cd2-O10
78.50(11)
136.72(11)
64.89(11)
82.55(15)
147.23(13)
143.36(14)
86.78(14)
86.91(14)
O6-Cd1-O3
O6-Cd1-N1
106.20(12) N1-Cd1-O3
80.52(15) O3-Cd2-O9

O3-Cd2-N2
O6-Cd2-O3
O6-Cd2-N2
O9-Cd2-C28
O10-Cd2-N3
N2-Cd2-N3
N3-Cd2-C28
76.49(14)
75.56(11)
O3-Cd2-N3
O6-Cd2-O9
122.56(13)
91.99(13)
76.96(13)
54.97(15)
27.00(16)
107.58(16)
O3-Cd2-C28
O6-Cd2-O10
O6-Cd2-C28
O10-Cd2-N2
N2-Cd2-O9
N3-Cd2-O9
116.79(17)
107.54(15)
99.64(15)
147.50(14) O6-Cd2-N3
28.04(16)
92.95(15)
O10-Cd2-O9
O10-Cd2-C28
104.76(16)
103.68(15)
141.49(16)
105.05(16) N2-Cd2-C28
116.90(18)
Wen-kui Dong et al.

Table 3. Hydrogen-bonding interactions [Å, º] for the Cu(II), Zn(II) and Cd(II) complexes.
Symmetry code
D-H
d(D-H)
A
d(H···A) d(D···A) ∠D-H···A
Cu(II) complex
C17-H17
C42-H42
C42-H42
C23-H23
C30-H30
C51-H51
C39-H39C
C9-H9B
0.930
0.930
0.930
0.930
0.930
0.930
0.960
0.970
2.480
2.540
2.570
2.410
2.560
2.560
2.520
2.680
3.313(5) 150
O8
O1
3.316(7) 141
3.366(4) 144
O2
3.322(2) 168.00
3.484(6) 170.00
3.426(7) 155.00
3.454(8) 165.00
3.477(8) 140.00
O11
O14
O4
[ 2-x, 1-y, 1-z ]
[ -x, 1-y, -z ]
[ -x, -y, -z ]
O12
Cg18
[ 1-x, 1-y, 1-z ]
[ x, y-1, z ]
Zn(II) complex
C14-H14
0.930
0.930
2.910
2.990
3.748(5) 150.00
3.336(6) 104.00
Cg6 [ 1-x, -1/2+y, 1/2-z ]
C21-H21
Cg7
[ x, 1/2-y, 1/2+z ]
Cd(II) complex
C15-H15A
0.960
2.960
3.786(7) 145.00
Cg5
[ -x, -y, 1-z ]
Note: Cg18=C29-C34; Cg6=C1-C6; Cg7=C12-C17; Cg5=C8-C13.
Wen-kui Dong et al.

Table 4. The most important FT-IR bands for the ligand and its Cu(II), Zn(II) and Cd(ΙΙ)
complexes (cm-1).
Compound
H4L
νC=N νAr-O νM-N νM-O νO-H νC=C
1613 1257 3065 1466
—
—
Cu(II) complex 1605 1244 526 422
Zn(II) complex 1605 1244 526 422
—
—
—
1448
1448
1438
Cd(II) complex 1598 1240
413
Wen-kui Dong et al.

Figures legends:
Scheme 1. Synthetic route to H4L.
Fig. 1. Molecular structure of the Cu(II) complex with the atom numbering. The hydrogen atoms are
omitted for clarity.
Fig. 2. Molecular structure of the Zn(II) complex with the atom numbering. The hydrogen atoms are
omitted for clarity.
Fig. 3. Molecular structure of the Cd(II) complex with the atom numbering. The hydrogen atoms are
omitted for clarity.
Fig. 4. Infrared absorption spectra of H4L and its Cu(II), Zn(II) and Cd(II) complexes.
Fig. 5. UV–vis absorption spectra of H4L and its Cu(II), Zn(II) and Cd(II) complexes in chloroform
-1
solution (c = 1×10-5 mol⋅L ).
Fig. 6. Emission spectra of H4L and its Cu(II), Zn(II) and Cd(II) complexes in chloroform solution (c =
1×10-5 mol⋅L-1).
Wen-kui Dong et al.

O
O
N
O
O
N
O
N
OH
CHO
OH
OCH3
NH2
H2N
O
O
NH2
NH2
HO
OH
OH
H2N
OCH3
OCH3
CHO
O
N
O
N
OH
OH
OH HO
N
OCH3
OH
HO
Scheme 1.
Wen-kui Dong et al.

Fig. 1.
Wen-kui Dong et al.

Fig. 2.
Wen-kui Dong et al.

Fig. 3.
.
Wen-kui Dong et al.

Cd(II) complex
Zn(II) complex
Cu(II) complex
H L
4
3500
3000
2500
2000
1500
1000
500
Wavenumber,cm-1
Fig. 4.
Wen-kui Dong et al.

H L
4
Cu(II) complex
Zn(II) complex
Cd(II) complex
0.4
0.3
0.2
0.1
0.0
271
269
269
364
387
407
368
300
400
Wavelength (nm)
500
Fig. 5.
Wen-kui Dong et al.

H L
4
600
400
200
0
Cu(II) complex
Zn(II) complex
Cd(II) complex
444
536
534
443
500
600
700
Wavelength(nm)
Fig. 6.
Wen-kui Dong et al.

Graphical Abstract-pictogram:
Wen-kui Dong et al.
Graphical Abstract
An novel asymmetric Salamo-type ligand H4L and its three novel supramolecular
multinuclear Cu(II), Zn(II) and Cd(II)complexes have been synthesized and characterized
by FT-IR, UV-vis spectra and X-ray crystallography. The obtained crystal structures are
very interesting. In addition, the photophysical properties of the Cu(II), Zn(II) and Cd(II)
complexes have also been discussed.