Synthesis, characterization and crystal structure of 2D Copper(II) complex with 6-Ethoxy-4',6'-dichloro-2,2'-[ethylenedioxybis(nitrilomethylidyne)]diphenol

Article abstract of DOI:10.14233/ajchem.2014.15902

A new Cu(II) complex with asymmetric Salamo-type ligand, 6-ethoxy-4',6'-dichloro-2,2'-[ethylenedioxybis(nitrilomethylidyne)]diphenol (H₂L) has been synthesized and characterized by elemental analyses, FT-IR, UV-visible spectra and single crystal X-ray crystallography. The X-ray crystal structure of the Cu(II) complex reveals it to be a mononuclear structure, which contains one Cu(II) atom and one deprotonated L²- unit. The center Cu(II) atom is tetra-coordinated, showing that the coordination sphere can be described as a square planar geometry distorted tetrahedrally by 33.07(4)°. In addition, the Cu(II) complex is linked to form an infinite 2D layer supramolecular structure through intermolecular C- H···O and C-H···Cl hydrogen bonds and π···π stacking interactions.

Full text of DOI:10.14233/ajchem.2014.15902

Asian Journal of Chemistry; Vol. 26, No. 8 (2014), 2337-2340
ASIAN JOURNAL OF CHEMISTRY
http://dx.doi.org/10.14233/ajchem.2014.15902
Synthesis, Characterization and Crystal Structure of 2D Copper(II) Complex with
6-Ethoxy-4',6'-dichloro-2,2'-[ethylenedioxybis(nitrilomethylidyne)]diphenol
*
W.-K. DONG , W.-M. ZHOU, X.-Y. ZHANG, M.-M. ZHAO, G. LI and X.-Y. DONG
School of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou 730070, P. R. China
*Corresponding author: E-mail: dongwk@126.com
Received: 12 June 2013;
Accepted: 22 July 2013;
Published online: 15 April 2014;
AJC-15021
A new Cu(II) complex with asymmetric Salamo-type ligand, 6-ethoxy-4',6'-dichloro-2,2'-[ethylenedioxybis(nitrilomethylidyne)]diphenol
(H2L) has been synthesized and characterized by elemental analyses, FT-IR, UV-visible spectra and single crystal X-ray crystallography.
The X-ray crystal structure of the Cu(II) complex reveals it to be a mononuclear structure, which contains one Cu(II) atom and one
deprotonated L2- unit. The center Cu(II) atom is tetra-coordinated, showing that the coordination sphere can be described as a square
planar geometry distorted tetrahedrally by 33.07(4)º. In addition, the Cu(II) complex is linked to form an infinite 2D layer supramolecular
structure through intermolecular C-H···O and C-H···Cl hydrogen bonds and π···π stacking interactions.
Keywords: Asymmetric Salamo-type ligand, Cu(II) complex, Synthesis, Crystal structure.
INTRODUCTION
EXPERIMENTAL
Salen-type ligands and its derivatives are well-known
chelating ligands in modern coordination chemistry1-3. During
the past few decades, transition metal complexes of Salen-
type ligands, consisting of two oxygen and two nitrogen donors
are of particular interest4,5. Factors that have contributed to
the widespread and continued interest in Salen-type ligands
are the initial recognition that metal complexes containing these
types of ligands can reversibly bind oxygen6, the similarity
between Salen-type ligands and heme, hence their use in model
complexes7 and finally, the more recent discovery of Salen-
type complexes as efficient chiral catalyst, a field which conti-
nues to be at the forefront of research efforts8,9. Furthermore,
Salen-type Cu(II) complexes have been shown to their relevance
as active site structures of metal-proteins and enzymes10 and
partly because of attempts to understand the relationship
between the structure and the spectroscopic properties11.
Moreover, some Salen-type complexes have strong activity
with DNA12 and can be used as elemental building blocks
for construction of chain- or cyclic-like coordinated supra-
molecular structures13.
2-Hydroxy-3-ethoxybenzaldehyde (> 99 %) and 2-
hydroxy-3,5-dichlorobenzaldehyde (> 99 %) were purchased
from Alfa Aesar and used without further purification. The
others are the same as literature early14.
Synthesis of ligand H2L: Synthetic route to the asymme-
trical Salamo-type ligand H2L is shown in Scheme-I. The
ligand H2L was synthesized with a slightly modified method
reported literature14g. Yield 75.9 %. m.p. 379-381 K. Anal.
Calcd. for C18H18N2O5Cl2 (%): C, 52.31; H, 4.39; N, 6.78.
Found: C, 52.38; H, 4.47; N, 6.69.
CHO
O
N
O
OH
Cl
CHO
O
N
O
N
NH2
OH
H2
N
O
O
NH2
Cl
OH
Ethanol
OH
HO
Cl
OEt
Ethanol
OEt
OEt
Cl
H2L
Scheme-I: Synthetic route to 6-ethoxy-4',6'-dichloro-2,2'- [ethylenedioxybis
-(nitrilomethylidyne)]diphenol (H2L)
Synthesis of Cu(II) complex:A solution of Cu(OAc)2·H2O
(2 mg, 0.01 mmol) in methanol (2 mL) was added dropwise
to a solution of H2L (4.20 mg, 0.01 mmol) in acetone (4 mL)
at room temperature. The colour of the mixing solution turned
to dark green immediately, then stirred for 45 min at room
Herein, we report the synthesis, characterization and
crystal structure of a Cu(II) complex with the new asymmetric
Salamo-type ligand 6-ethoxy-4',6'-dichloro-2,2'-[ethylene-
dioxybis(nitrilomethylidyne)]diphenol (H2L).

2338 Dong et al.
Asian J. Chem.
temperature. The mixture was filtered off and the filtrate was
allowed to stand at room temperature for about six weeks, the
solvent was partially evaporated and obtained brown rhombic
single crystals suitable for X-ray crystallographic analysis.
Anal. Calcd. for C18H16N2O5Cl2Cu (%): C, 45.54; H, 4.40; N,
5.90; Cu, 13.38. Found: C, 45.47; H, 4.49; N, 5.79; Cu, 13.42.
X-Ray crystal structure determination: X-Ray structure
determination is the same as literature early14h. The crystal data
and structure refinement for the Cu(II) complex are given in
Table-1.
stretching band occurs at 1242 cm-1 for H2L, whereas that at
1211 cm-1 for the Cu(II) complex as reported for similar Salen-
type ligands17. This shift ofAr-O stretching frequency indicates
that the M-O bonds are formed between the Cu(II) ion and the
phenolic O atoms of the deprotonated L2- unit14. Meanwhile, a
O-H stretching band of the free ligand H2L at 3395 cm-1 dis-
appears in the Cu(II) complex, indicating the phenolic hydroxyl
of the Cu(II) complex has been completely deprotonated and
coordinated to the Cu(II) ion. In the 1570-1456 cm-1 region,
the observed bands were attributed to aromatic C=C vibrations.
UV-visible spectra of H2L and its Cu(II) complex: The
UV-visible spectra of H2L and its corresponding Cu(II) complex
in diluted DMF solution are presented in Table-3. The spectrum
of the Cu(II) complex is different from the spectrum of the
ligand H2L. The UV-visible spectrum of the free ligand H2L
exhibits two intense absorption peaks at 282 nm and 321 nm.
The former absorption peak can be assigned to the π-π*
transition of the benzene rings, while the latter one at 321 nm
can be attributed to the intra-ligand π-π* transition of the C=N
bonds18.
TABLE-1
CRYSTAL DATA AND STRUCTURE REFINEMENT
FOR Cu(II) COMPLEX
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
C18H16N2O5Cl2Cu
474.77
293(2)
0.71073
Monoclinic
Space group
P2(1)/c
Cell dimensions, (Å, deg)
a = 11.3917(14), b =
10.7909(10),
For the Cu(II) complex, the π-π* transition of the benzene
rings of benzaldehyde is slightly shifted in the Cu(II) complex
and appears at 277 nm. Meanwhile, the absorption peak
disappears at 321 nm due to coordination of the L2- unit in the
Cu(II) complex, indicating that the oxime nitrogen atoms are
involved in coordination to the Cu(II) atom19. In addition, a
new moderate absorption peak appears at 376 nm in the Cu(II)
complex, which is assigned to the n-π* charge transfer transi-
tion from the filled pπ orbital of the bridging phenolic oxygen
to the vacant d-orbital of the Cu(II) ion20.
c = 15.331(2), β = 94.962(2)
1877.5(4)
Volume (Å3)
Z
4
Density (calculated) (mg/m3)
Absorption coefficient (mm-1)
F(000)
1.680
1.483
964
Crystal size (mm)
Index ranges
0.25 × 0.24 × 0.11
-13 < h < 8, -6 < k < 12, -18 < l
< 16
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness of fit indicator
R [I > 2σ(I)]
6434/3310 [R(int) = 0.0594]
1290
3310/0/254
TABLE-3
1.029
UV-VIS SPECTRA DATA OF H2L AND
ITS Cu(II) COMPLEX
R1 = 0.0617, wR2 = 0.1461
Largest diff. peak and hole (e Å-3) 0.969 and -0.664
First band
Second band
C
Comp.
(×10-5 mol L-1)
λmax (nm)
282
277
λmax (nm)
321
376
RESULTS AND DISCUSSION
H2L
[Cu(L)]
5.00
5.00
IR spectra of H2L and its Cu(II) complex: The FT-IR
spectra of H2L and its corresponding Cu(II) complex exhibit
various bands in the 4000-400 cm-1 region. The most impor-
tant FT-IR bands for H2L and its Cu(II) complex are given in
Table-2.
Crystal structure of Cu(II) complex: The structure of
the Cu(II) complex was determined by X-ray crystallography,
revealing that the formation of a mononuclear structure (Fig.
1). Selected bond lengths and angles are listed in Table-4.
X-ray crystallographic analysis reveals that the Cu(II) atom
lies in the N2O2 coordination sphere and crystallizes in the
monoclinic system, space group P2(1)/c and the unit cell
contains four Cu(II) complex molecules.
Each Cu(II) complex molecule contains one tetra-coordi-
nated center Cu(II) atom and one deprotonated L2- unit. The
dihedral angle between the coordination plane of O3-Cu-N1
and that of O4-Cu-N2 is 33.07(4)º, indicating the coordination
sphere can be described as a square planar geometry distorted
tetrahedrally with N2 and O3 up average by 0.375(4) and
0.405(5) Å and with N1 and O4 below average by 0.368(5)
and 0.412(3) Å, respectively. Meanwhile, the Cu(II) atom is
displaced by 0.030(3) Å out of the basal plane of the donor
(N1, N2, O4 and O3) atoms. The C1 and C2 atoms of the O-
alkyl chain are also buckled asymmetrically from the Cu-N1-
TABLE-2
IR SPECTRAL DATA FOR H2LAND
ITS Cu(II) COMPLEX (cm-1)
ν (C=C) benzene
ring skeleton
1570, 1495, 1471
Comp.
ν(C=N) ν(Ar-O)
ν(O-H)
3395
—
H2L
Cu(II)
complex
1611
1604
1242
1211
1541, 1475, 1456
The characteristic C=N stretching band of the free ligand
H2L appears at 1611 cm-1, while the C=N band of the Cu(II)
complex is observed at 1604 cm-1. The C=N stretching
frequency is shifted by about 7 cm-1 upon complexation, indi-
cating a decrease in the C=N bond order due to the coordination
of the Cu(II) ion to the oxime nitrogen lone pair14-16. TheAr-O

Vol. 26, No. 8 (2014)
Synthesis, Characterization and Crystal Structure of 2D Copper(II) Complex 2339
TABLE-4
SELECTED BOND LENGTHS (Å) AND ANGLES (°) FOR THE Cu(II) COMPLEX
Bond
Cu1-O3
Cu1-N1
Bond
O3-Cu1-O4
O3-Cu1-N2
O4-Cu1-N2
O3-Cu1-N1
Lengths
1.900(3)
2.016(4)
Angles
90.9(2)
158.0(2)
91.0(2)
90.1(2)
Bond
Cu1-O4
Lengths
1.912(4)
Bond
Cu1-N2
Lengths
1.913(4)
Bond
Angles
155.0(2)
97.3(2)
123.8(4)
126.9(3)
Bond
Angles
129.1(4)
117.5(3)
127.7(3)
129.0(3)
O4-Cu1-N1
N2-Cu1-N1
C3-N1-Cu1
O1-N1-Cu1
C10-N2-Cu1
O2-N2-Cu1
C5-O3-Cu1
C12-O4-Cu1
N2 plane, with the displacement for C1 being 1.376(2) Å
toward the plane and for C2 being only 0.528(3) Å in the same
direction. It is noteworthy that the Cu-N bond lengths, 2.016(4)
and 1.913(4) Å, are considerably longer than the Cu-O bond
lengths, 1.912(4) and 1.900(3) Å, respectively.
Fig. 2. View of 1D two-layer chain motif of the Cu(II) complex units along
the b axis (hydrogen atoms, except those forming hydrogen bonds,
are omitted for clarity)
Fig. 3. View of the π···π stacking motif of neighboring six-membered rings
of the Cu(II) complex
Fig. 1. Molecule structure of the Cu(II) complex with atom numbering
scheme. Displacement ellipsoids for non-hydrogen atoms are drawn
at the 30 % probability level
Intermolecular interactions of Cu(II) complex: The
Cu(II) complex contains four intermolecular hydrogen bonds,
π···π stacking interactions of neighboring six-membered rings.
Hydrogen bond data are summarized in Table-5.
TABLE-5
DATA FOR INTERMOLECULAR
HYDROGEN-BONDING INTERACTIONS (Å, °)
D-H···A
d(D-H)
0.97
0.97
0.97
0.97
d(H···A)
2.54
2.53
2.70
2.91
d(D···A)
3.110(7)
3.210(7)
3.388(6)
3.543(6)
∠D-H···A
118
C1-H1B···O3
C2-H2A···O4
C1-H1A···O5
C1-H1B···Cl1
127
129
124
Fig. 4. View of the 2D layer supramolecular motif of the Cu(II) complex
units on the bc crystallographic plane (hydrogen atoms, except those
forming hydrogen bonds, are omitted for clarity)
Each Cu(II) complex links two other molecules in the
same direction by four pairs of intermolecular C1-H1B···O3,
C2-H2A···O4, C1-H1A···O5 and C1-H1B···Cl1 hydrogen-
bonding interactions into an infinite 1D two-layer chain
along the b axis (Fig. 2). The methylene -C1H1B group of the
O-alkyl chain is respectively bound to the phenolic oxygen
O3 atom and chloro Cl1 atom, as well as the methylene groups
(-C2H2A and -C1H1A) of the O-alkyl chain are respectively
bound to the phenolic oxygen O4 atom of the adjacent Cu(II)
complex molecule and the ethoxy O5 atom.
Meanwhile, there exists three π···π stacking interactions
between the six-membered rings (Cg4) containing Cu(II)
atoms and the benzene ring (Cg6) of the L2- unit in the crystal
structure, resulting in the Cu(II) complex comes in pairs, as
shown in Fig. 3. Thus, every pair of the Cu(II) complexes link
four other complex molecules to form an infinite 2D layer
supramolecular structure (Fig. 4) through intermolecular
C-H···O, C-H···Cl hydrogen bonds and π···π stacking inter-
actions.

2340 Dong et al.
Asian J. Chem.
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ACKNOWLEDGEMENTS
This work was supported by the Foundation of Gansu
Province Traffic Transportation Dust Control Engineering
Technology Research Center (513001-1), the Fundamental
Research Funds for the Gansu Province Universities (212086)
and the science and technology support funds of Lanzhou
Jiaotong University (ZC2012003), which are gratefully acknow-
ledged.
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