A dinuclear copper(ii) complex based on the bisoxime ligand 5,5′-dimethoxy-2,2′-[(ethylene)dioxybis(nitrilomethylidyne)] diphenol: Synthesis, crystal structure and spectral properties

Article abstract of DOI:10.1515/znb-2012-0104

A dinuclear Cu(II) complex, [Cu₂(L²)₂] (H₂L² = 4-methoxysalicylaldehyde O-(2-hydroxyethyl)-oxime) , has been synthesized through the complexation of Cu(II) acetate monohydrate with the ligand H_2<

Full text of DOI:10.1515/znb-2012-0104

A Dinuclear Copper(II) Complex Based On the Bisoxime Ligand
5,5ꢀ-Dimethoxy-2,2ꢀ-[(ethylene)dioxybis(nitrilomethylidyne)]diphenol:
Synthesis, Crystal Structure and Spectral Properties
Wen-Kui Donga, Guo-Hua Liua, Yin-Xia Suna, Xiu-Yan Donga, and Xiong-Hou Gaob
a School of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou,
Gansu 730070, P. R. China
b Lanzhou Petrochemical Research Center, PetroChina, Lanzhou 730060, P. R. China
Reprint requests to Prof. Wen-Kui Dong. E-mail: dongwk@126.com
Z. Naturforsch. 2012, 67b, 17 – 22; received January 1, 2012
A dinuclear Cu(II) complex, [Cu2(L2)2] (H2L2 = 4-methoxysalicylaldehyde O-(2-hydroxyethyl)-
oxime), has been synthesized through the complexation of Cu(II) acetate monohydrate with the ligand
H2L1 (H2L1 = 5,5ꢀ-dimethoxy-2,2ꢀ-[(ethylene)dioxybis(nitrilomethylidyne)]diphenol), and charac-
terized by elemental analyses, IR, UV/Vis and emission spectra. The crystal structure of the Cu(II)
complex has been determined by single-crystal X-ray diffraction. The catalysis by Cu(II) ions results
in the unexpected cleavage of the N–O bonds in the ligand H2L1, giving a novel dialkoxo-bridged
dinuclear Cu(II) complex possessing a Cu-O-Cu-O four-membered ring core instead of the expected
salen-type bisoxime Cu-N2O2 complex.
Key words: Bisoxime Ligand, Cu(II) Complex, Synthesis, Crystal Structure
Introduction
Experimental Section
Materials and physical measurements
Oxime-type ligands and their complexes have been
playing an important role in the development of co-
ordination chemistry [1 – 4]. The development of new
metal-organic complexes can provide new topolo-
gies for functional materials, in which coordination
forms and functionality are important variables [5, 6].
Polynuclear complexes in which the metal centers are
bridged through organic ligands are of much current
interest, because of their enormous variety of struc-
tural topologies as well as their potential applications
in optoelectronics and catalysis [7 – 14]. This subject
is currently being pursued in our research, including
the use of functional oxime groups and a study of
the magnetic, photoelectric and catalytic properties of
such complexes.
4-Methoxy-2-hydroxybenzaldehyde (≥ 98 %) from Alfa
Aesar was used without further purification. The other
reagents and solvents were of analytical grade from Tian-
jin Chemical Reagent Factory. IR spectra were recorded on
a VERTEX70 FT-IR spectrophotometer, with samples pre-
pared as KBr (500 – 4000 cm−1) and CsI (100 – 500 cm−1
)
pellets. Fluorescence spectra were obtained from a 970CRT
spectrofluoro-photometer. 1H NMR spectra were recorded
on a Bruker DRX 400 spectrometer in CDCl3 solution with
TMS as internal standard. Melting points were measured by
the use of a ×10 microscopic melting point apparatus made
by Beijing Taike Instrument Limited Company, and are un-
corrected.
1
Preparation of H L
2
In this paper, we report the synthesis, structural
characterization and spectral properties of an unex-
pected dinuclear Cu(II) complex with the new ligand
H2L2 (H2L2 = 4-methoxysalicylaldehyde O-(2-hydr-
oxyethyl)oxime) which is formed in the course of the
complexation of H2L1 at Cu(II) acetate monohydrate.
The product is a dialkoxo-bridged dinuclear Cu(II)
complex possessing a Cu-O-Cu-O four-membered ring
core instead of the usually expected salen-type com-
plex [15, 16].
5,5ꢀ -Dimethoxy-2,2ꢀ -[(ethylene)dioxybis(nitrilomethyl-
idyne)]diphenol (H2L1) was synthesized according to an
analogous method reported earlier [17 – 19]. To an ethanol
solution (5 mL) of 4-methoxysalicylaldehyde (393.7 mg,
2.59 mmol) was added an ethanol solution (3 mL) of 1,2-
bis(aminooxy)ethane (118.0 mg, 1.28 mmol). The mixture
was stirred at 55 ◦C for 5 h. After cooling to r. t., the precip-
itate was filtered and washed successively with ethanol and
ethanol-hexane (1 : 4). The product was dried in vacuo, and
355.3 mg of a colorless flocculent crystalline solid was ob-
c
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W.-K. Dong et al. · A Dinuclear Copper(II) Complex Based On a Bisoxime Ligand
˚
Table 1. Crystal data and numbers pertinent to data collection Table 2. Selected bond lengths (A) and angles (deg)
and structure refinement for [Cu2(L2)2].
for [Cu2(L2)2] with estimated standard deviations in
parenthesesa.
Molecular formula
Mr
C20H22Cu2N2O8
545.48
brown, needle-like
0.40×0.11×0.08
monoclinic
P21/c
Distances
Color, habit
Cu1–O3
1.882(2)
1.9258(19)
1.9258(19)
Cu1–O2
Cu1–N1
1.913(2)
1.943(2)
Crystal size, mm3
Crystal system
Space group
Cu1–O2#1
O2–Cu1#1
Angles
˚
a, A
5.2380(4)
O2–Cu1–O2#1
O3–Cu1–O2
O2–Cu1–O2#1
O2–Cu1–N1
C3–N1–Cu1
C2–O2–Cu1
C5–O3–Cu1
a
76.63(8)
170.70(8)
76.63(9)
95.45(9)
125.9(2)
125.81(17)
128.39(18)
Cu1–O2–Cu1#1
O3–Cu1–O2#1
O3–Cu1–N1
103.37(9)
94.37(8)
93.64(9)
171.70(9)
122.29(17)
129.96(18)
˚
b, A
15.3909(15)
13.0131(12)
100.210(1)
1032.47(16)
2
1.76
298(2)
2.1
556
−6 ≤ h ≤ 6,−18 ≤ k ≤ 16,
−10 ≤ l ≤ 15
2.65 – 25.02
5341 / 1819 / 0.0367
1809 / 0 / 146
0.0306 / 0.0816
0.0374 / 0.0853
1.010
˚
c, A
β, deg
O2#1–Cu1–N1
O1–N1–Cu1
3
˚
V, A
Z
C2–O2–Cu1#1
Dcalcd, g cm−3
T, K
µ(MoKα ), mm−1
F(000), e
hkl range
Symmetry transformation used to generate equivalent atoms:
#1
1−x, 1−y, 1−z.
were collected using graphite-monochromatized MoKα radi-
ation (λ = 0.71073 A) at 298(2) K. The structure was solved
˚
θ range for data collection, deg
Reflections collected / unique / Rint
Data / restraints / parameters
Final R1 / wR2 [I ≥ 2σ(I)]a
Final R1 / wR2 (all data)b
using the program SHELXS-97 and Fourier difference tech-
niques, and refined by the full-matrix least-squares method
on F2 with SHELXL-97. All hydrogen atoms were added in
calculated positions. Crystal data and numbers pertinent to
data collection and structure refinement are given in Table 1.
CCDC 832645 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data request/cif.
Goodness-of-fit (F2)c
Largest diff. peak / hole, e A
−3
˚
0.346 / −0.218
a R1 = ΣꢁFo|−|Fcꢁ/Σ|Fo|; b wR2 = [Σw(Fo2−Fc2)2/Σw(F 2)2]1/2
,
=
−
w
=
[σ2(Fo2) + (0.0499P)2 + 0.2789P]−1
,
whereo
P
c
(Max(Fo2,0) + 2Fc2)/3;
GoF = [Σw(Fo2 − Fc2)2/(nobs
n
param)]1/2
.
tained. Yield 78.9 %. M. p. 97 – 98 ◦C. – 1H NMR (400 MHz, Results and Discussion
CDCl3): δ (ppm) = 3.79 (s, 6H), 4.41 (s, 4H), 6.46 (d, J =
The crystal structure of [Cu2(L2)2]
2.8 Hz, 2H), 6.49 (d, J = 2.4 Hz, 2H), 7.04 (d, J = 8.8 Hz,
2H), 8.17 (s, 2H), 9.94 (s, 2H). – C18H20N2O6 (360.36):
calcd. C 59.99, H 5.59, N 7.77; found C 59.89, H 5.75,
N 7.60.
For the synthesis of the copper(II) complex, the
same synthetic route was used as for the mono-nuclear
complex [Cu(H2L)]·2H2O according to our previous
work [17]. However, it is remarkable that the de-
sired salen-type bisoxime complex [Cu(L1)] was not
formed, but an unexpected dinuclear copper(II) com-
plex [Cu2(L2)2] was obtained which was formed in the
course of the complexation of H2L1 by copper(II) ac-
etate. The results show that because of the catalysis
of copper(II) ions [20, 21], the complexation resulted
in a cleavage of one of the N–O bonds in H2L1, giv-
ing a new O-N-O tridentate ligand H2L2, which coor-
dinates Cu(II) ions forming a dialkoxo-bridged dinu-
clear Cu(II) complex [22, 23] with a Cu-O-Cu-O four-
membered ring core instead of the expected salen-type
bisoxime Cu-N2O2 complex (Scheme 1).
2
Preparation of [Cu (L ) ]
2
2
A solution of Cu(II) acetate monohydrate (3.99 mg,
0.02 mmol) in ethanol (2 mL) was added dropwise to a so-
lution of H2L1 (7.20 mg, 0.02 mmol) in dichloromethane
(4 mL) at r. t. The color of the mixture turned to dark brown
immediately. Stirring was continued for 1 h at r. t. The mix-
ture was filtered, and the filtrate was allowed to stand at
r. t. for about one week. The solvent was partially evapo-
rated, and single crystals suitable for X-ray crystallographic
analysis were obtained. Yield 25.6 %. – C20H22Cu2N2O8
(545.49): calcd. C 44.04, H 4.07, N 5.14, Cu 23.30; found
C 44.08, H 4.10, N 5.17, Cu 23.35.
2
X-Ray crystallography of [Cu (L ) ]
2
2
The molecular structure and crystallographic atom
numbering of the complex [Cu2(L2)2] are shown in
Fig. 1. Selected bond lengths and bond angles are listed
A suitable single crystal of [Cu2(L2)2] with approxi-
mate dimensions of 0.40 × 0.11 × 0.08 mm3 was placed on
a Bruker Smart Apex CCD diffractometer. The reflections in Table 2. The structure determination revealed that
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W.-K. Dong et al. · A Dinuclear Copper(II) Complex Based On a Bisoxime Ligand
19
a)
b)
Scheme 1. (a, left) Reaction of the ligand [H2L1] with Cu(II) acetate monohydrate, (b, right) chemical formula of H2L2
(4-methoxysalicylaldehyde O-(2-hydroxyethyl)oxime).
˚
by 0.002 A from the mean plane defined by the
atoms O2, O2#1 O3, N1.
Intermolecular interactions of [Cu2(L2)2]
Of special interest is the self-assembly of the Cu(II)
complex in the crystal by twelve intermolecular hydro-
gen bonds. The hydrogen bond data are summarized in
Table 3.
In the crystal structure, four intermolecular hydro-
gen bonds, C1–H1A···O3, are formed between the
methylene unit (C1–H1A) of the O-alkyl chain and the
phenolic oxygen atom (O3) of the (L2)2− unit, which
link the neighboring molecules into an infinite chain
parallel to the a axis (Fig. 2).
Furthermore, the complex molecules are stabilized
in their position by four pairs of hydrogen bonds,
C3–H3···O4 and C10–H10B···O1, leading to an in-
finite 2D layer structure along the crystallographic
ac plane (Fig. 3). The C3–H3···O4 hydrogen bonds
are formed between the -C3H3 unit of the oxime
groups and the oxygen atom (O4) of the methoxy
group, while the hydrogen bonds C10–H10B···O1 are
formed between a -C10H10B unit of the methoxy
group and the oxygen atom (O1) of the oxime groups.
Thus, each complex is linked to ten other molecules
into an infinite 3D supramolecular network via inter-
molecular C–H···O hydrogen bonds.
Fig. 1. ORTEP-style drawing of the title complex [Cu2(L2)2].
the complex is a neutral homobinuclear entity with
crystallographic inversion symmetry. Two [Cu(L2)]
moieties are linked via two alkoxo bridges, each of
which comes from the individual new ligand [L2]2−
.
The whole complex resides on a center of inversion
and has two tetra-coordinated Cu(II) centers. Their
geometry can be best described as slightly distorted
squareplanar with CuN1O3 coordination (one oxime
nitrogen atom N1, one phenoxo oxygen atom O3 and
two bridging alkoxo oxygen atoms O2 and O2#1) as
shown in Fig. 1. Thus, a planar Cu2O2 core is formed
by two divalent copper(II) ions and their bridging
two alkoxo oxygen atoms with a Cu···Cu separation
˚
of 3.0118(6) A which is too long to be considered as
representing intramolecular Cu–Cu bonding.
The bridging Cu–O bonds are slightly asym-
˚
metric, one (Cu1–O2) with 1.913(2) A, while the
#1
˚
other (Cu1–O2 ) is 1.9258(19) A. The bond lengths
˚
Cu1–O3 and Cu1–N1 are 1.882(2) and 1.943(2) A, re-
IR spectra of H2L1 and [Cu2(L2)2]
spectively. The four-membered ring Cu1–O2–Cu1#1–
O2#1 is exactly planar as required by symme-
The neutral free ligand H2L1 exhibits character-
try. The angles Cu1–O2–Cu1#1 and O2–Cu1–O2#1 istic C=N stretching bands at 1635 cm−1, while
are 103.37(9)◦ and 73.63(8)◦, respectively. The di- the C=N bond of the Cu(II) complex was observed
hedral angle of the two planes O2–Cu1–O2#1 and at 1614 cm−1. Thus, the C=N stretching frequency
O3–Cu1#1–N1 is 3.32◦, and the Cu1 atom deviates is shifted to lower frequency by ca. 21 cm−1 upon
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W.-K. Dong et al. · A Dinuclear Copper(II) Complex Based On a Bisoxime Ligand
Fig. 2 (color online). View of the
1D chain motif of the complex
units along the a axis (hydro-
gen atoms, except those forming
hydrogen bonds, are omitted for
clarity).
Fig. 3 (color online). View of the 2D
layer motif along the ac plane (hydro-
gen atoms, except those forming hydro-
gen bonds, are omitted for clarity).
complexation, indicating a decrease in the C=N bond reported for M(II) complexes with N2O2 chelate lig-
order due to the coordination of the Cu(II) ion to ands [30, 31].
the oxime nitrogen lone pair [24 – 27]. In the 143 –
The far-infrared spectrum of the Cu(II) complex
1519 cm−1 region, the observed bands were attributed was obtained in the region 500 – 100 cm−1 in order
to aromatic C=C vibrations. Upon coordination these to identify frequencies due to the Cu–O and Cu–N
bands are shifted to lower frequencies for the Cu(II) vibrations. ν(Cu-N) and ν(Cu-O) vibrations were found
at 472 cm−1 and 431 cm−1, respectively. As pointed
out by Percy and Thornton [32], the metal-oxygen
and metal-nitrogen frequency assignments are at times
very difficult.
complex [28].
Ar-O stretching vibrations appear as strong bands
in the range 1263 – 1213 cm−1 as reported for simi-
lar ligands [29]. This vibration occurs at 1231 cm−1
for the free ligand H2L1, and at 1238 cm−1 for the
Cu(II) complex. Thus the Ar-O stretching vibration is
shifted to a higher frequency upon complexation. This
shift clearly results from the metal-phenolic oxygen
Emission spectrum of [Cu2(L2)2]
The emission spectrum of the Cu(II) complex in di-
interaction upon complexation and is similar to that lute DMF solution at r. t. is shown in Fig. 4. An intense
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W.-K. Dong et al. · A Dinuclear Copper(II) Complex Based On a Bisoxime Ligand
21
the Cu(II) atom to the ligand and a head-to-tail dimeric
structure, which make the conjugated system larger.
Conclusion
We have reported the synthesis and structural char-
acterization of a new dinuclear Cu(II) complex. In this
complex the Cu(II) ions are tetra-coordinated by two
monooxime [L2]2− units. In a catalysis by the Cu(II)
ions an unexpected cleavage of the N–O bonds in the
ligand H2L1 occurs, giving a dialkoxo-bridged Cu(II)
complex possessing a Cu-O-Cu-O four-membered
ring core instead of the usually expected salen-type
bisoxime Cu-N2O2 complex. The molecules are linked
to ten other molecules into an infinite supramolecular
network via intermolecular C–H···O hydrogen bonds.
The Cu(II) complex exhibits blue emission with the
maximum emission wavelength λem = 441 nm when
excited at λex = 340 nm.
Fig. 4. Emission spectrum of the Cu(II) complex in dilute
DMF solution at room temperature (c = 5 × 10−5 mol L−1
λex = 340 nm, λem = 441 nm).
,
excitation peak is observed at 340 nm. The emission
peak appears at around 441 nm. The Stokes shift be-
tween the maximum wavelength of the fluorescence
emission and the fluorescence excitation is 101 nm.
This red-shift might be related to the coordination of
Acknowledgement
This work was supported by the ‘Jing Lan’ Talent Engi-
neering Funds of Lanzhou Jiaotong University.
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