Synthesis, crystal structure, and fluorescence of an unexpected dialkoxo-bridged dinuclear copper(II) complex with bis(salen)-type tetraoxime

Article abstract of DOI:10.1080/00958972.2012.677836

An unexpected dinuclear Cu(II) complex, [Cu₂(L²) ₂] (H₂L²=3-methoxysalicylaldehyde O-(2-hydroxyethyl)oxime), has been synthesized via complexation of Cu(II) acetate monohydrate with H₄L

Full text of DOI:10.1080/00958972.2012.677836

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Synthesis, crystal structure, and
fluorescence of an unexpected
dialkoxo-bridged dinuclear copper(II)
complex with bis(salen)-type
tetraoxime
Lan-Qin Chai a , Gang Wang a , Yin-Xia Sun a , Wen-Kui Dong a , Li
Zhao a & Xiong-Hou Gao b
a School of Chemical and Biological Engineering, Lanzhou Jiaotong
University , Lanzhou 730070 , P.R. China
b Lanzhou Petrochemical Research Center , PetroChina , Lanzhou
730060 , P.R. China
Published online: 05 Apr 2012.
To cite this article: Lan-Qin Chai , Gang Wang , Yin-Xia Sun , Wen-Kui Dong , Li Zhao & Xiong-
Hou Gao (2012) Synthesis, crystal structure, and fluorescence of an unexpected dialkoxo-bridged
dinuclear copper(II) complex with bis(salen)-type tetraoxime, Journal of Coordination Chemistry,
65:9, 1621-1631, DOI: 10.1080/00958972.2012.677836
To link to this article: http://dx.doi.org/10.1080/00958972.2012.677836
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Journal of Coordination Chemistry
Vol. 65, No. 9, 10 May 2012, 1621–1631
Synthesis, crystal structure, and fluorescence of an unexpected
dialkoxo-bridged dinuclear copper(II) complex with
bis(salen)-type tetraoxime
LAN-QIN CHAIy, GANG WANGy, YIN-XIA SUNy, WEN-KUI DONG*y,
LI ZHAOy and XIONG-HOU GAOz
ySchool of Chemical and Biological Engineering, Lanzhou Jiaotong University,
Lanzhou 730070, P.R. China
zLanzhou Petrochemical Research Center, PetroChina, Lanzhou 730060, P.R. China
(Received 27 December 2011; in final form 28 February 2012)
An unexpected dinuclear Cu(II) complex, [Cu2(L2)2] (H2L2 ¼ 3-methoxysalicylaldehyde
O-(2-hydroxyethyl)oxime), has been synthesized via complexation of Cu(II) acetate mono-
hydrate with H4L1. Catalysis by Cu(II) results in unexpected cleavage of two N–O bonds in
H4L1, giving
a dialkoxo-bridged dinuclear Cu(II) complex possessing a Cu–O–Cu–O
four-membered ring core instead of the usual bis(salen)-type tetraoxime Cu3–N4O4 complex.
Every complex links six other molecules into an infinite-layered supramolecular structure via 12
intermolecular C–H ꢀ ꢀ ꢀ O hydrogen bonds. Furthermore, Cu(II) complex exhibits purple
emission with maximum emission wavelength ꢀmax ¼ 417 nm when excited with 312 nm.
Keywords: Bis(salen)-type tetraoxime ligand; Cu(II) complex; Synthesis; Crystal structure;
Fluorescence behavior
1. Introduction
Macrocyclic ligands play an important role in selective strong binding with metal ions
in coordination chemistry. The difference between acyclic and cyclic ligands during
coordination mainly arises from preorganization of the coordinating atoms for the
metal ions (macrocyclic effect) [1]. Complexes of transition and non-transition metals
with Schiff bases have been investigated because the complexes have many applications
[2–6]. Salen-type N2O2 ligands coordinate tetradentate to various transition-metal ions
to produce stable complexes [7–9]. Some of these complexes serve as catalysts of organic
reactions [10], models of the catalytic centers of metalloenzymes [11], nonlinear optical
materials [12], and metallomesogens [13]. Salen complexes are also used as a building
block of supramolecular structures [14] or sensing of chiral organic molecules [15].
A molecule containing two or more salen moieties may have cooperation of several
*Corresponding author. Email: dongwk@126.com
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2012 Taylor & Francis
http://dx.doi.org/10.1080/00958972.2012.677836

1622
L.-Q. Chai et al.
metal centers. Studies of Cu(II) complexes with salen-type ligands have made great
progress [16–20], but cleavage of two N–O bonds of bis(salen)-type tetroximes has not
been observed when the ligands react with metal salts.
In this article, we report the synthesis, structural characterization, and fluorescence of
a dinuclear Cu(II) complex with H2L2 which is formed in the course of complexation of
H4L1 at Cu(II) acetate monohydrate, [Cu2(L2)2]. The product is an unexpected
dialkoxo-bridged dinuclear Cu(II) complex possessing a Cu–O–Cu–O four-membered
ring core instead of the bis(salen)-type tetraoxime Cu3–N4O4 complex. Fluorescence of
the Cu(II) complex in DMF is also discussed.
2. Experimental
2.1. Materials and physical measurements
All chemicals were of analytical reagent grade and used without purification. C, H, and
N analyses were obtained using a GmbH VarioEL V3.00 automatic elemental analysis
instrument. Elemental analysis for Cu was determined by an IRIS ER/S ꢀ WP–1 ICP
atomic emission spectrometer. UV-Vis absorption and fluorescence spectra were
recorded on a Shimadzu UV-2550 spectrometer and Perkin-Elmer LS-55 spectrometer,
respectively. Electrolytic conductance measurement was made with a DDS–11D type
conductivity bridge using a 1.0 ꢁ 10ꢂ3 mol Lꢂ1 solution in DMF at room temperature.
1H NMR spectra were determined on a German Bruker AVANCE DRX-400
spectrometer. Single-crystal X-ray structure determination was carried out on a
Bruker Smart Apex CCD diffractometer. Melting points were obtained by use of a
microscopic melting point apparatus made by Beijing Taike Instrument Limited
Company and are uncorrected.
2.2. Preparation of H4L and its complex
2.2.1. Preparation of bis(salen)-type tetraoxime ligand (H4L1). The synthetic route of
bis(salen)-type tetraoxime (H4L1) is shown in scheme 1.
CHO
CHO
CHO
1) n-BuLi, TMEDA,Et O
2
1) BBr , CH Cl
2
OH
OH
3
2
2) DMF
OMe
OMe
OMe
OMe
2) H O
3) H O
O
N
2
2
O
N
HO
CHO
OH
OEt
OEt
CHO
O
N
O
OH
NH2
OH
OEt
HO
N
O
H2N O
O
NH2
OH
N
O
OEt
Scheme 1. Synthetic route of tetraoxime H4L1.

Dialkoxo-bridged dinuclear copper(II)
1623
2,3-Dimethoxybenzene-1,4-dicarbaldehyde was synthesized from 1,2-dimethoxyben-
zene (0.166 g, 1 mmol), N,N,N0,N0-tetramethylenediamine (0.67 mL, 5 mmol) and
n-butyllithium (2.66 mol Lꢂ1 hexane solution, 1.88 mL, 5 mmol) in 32.2% yield as
1
pale yellow crystals [16]; m.p. 100–101ꢃC, H NMR (400 MHz, CDCl3) ꢁ 4.06 (s, 6H),
7.65 (s, 2H), 10.46 (s, 2H). Anal. Calcd for C10H10O4 (Mw 194.18) (%): C, 61.85;
H, 5.19. Found (%): C, 61.96; H, 5.05.
2,3-Dihydroxybenzene-1,4-dicarbaldehyde was synthesized from 2,3-dimethoxyben-
zene-1,4-dicarbaldehyde (0.398 g, 2 mmol) and boron tribromide (0.78 mL, 8.2 mmol) in
90% yield as yellow crystals after recrystallization (chloroform/hexane) [21]; m.p.
140.5–142.5ꢃC, 1H NMR (400 MHz, CDCl3) ꢁ 7.26 (s, 2H), 10.00 (s, 2H), 10.88 (s, 2H).
Anal. Calcd for C8H6O4 (Mw 166.13) (%): C, 57.84; H, 3.64. Found (%): C, 57.58;
H, 3.76.
1,2-Bis(aminooxy)ethane was synthesized according to a method reported earlier
[22, 23]. Yield, 75.2%. Anal. Calcd for C2H8N2O2 (Mw 92.1) (%): C, 26.08; H, 8.76; N,
1
30.42. Found (%): C, 25.98; H, 8.90; N, 30.38. H NMR (400 MHz, CDCl3): 3.79
(s, 4H), 5.52 (s, 4H).
Monooxime compound was synthesized from 2-hydroxy-3-ethoxybenzaldehyde
(0.166 g, 1.0 mmol) and 1,2-bis(aminooxy)ethane (0.0921 g, 1.0 mmol) in 52.3% yield.
The sample was purified by column chromatography (SiO2, chloroform/ethyl acetate,
1
50 : 1) to give colorless crystals [24]; m.p. 168–169ꢃC, H NMR (400 MHz, CDCl3) ꢁ
3.92 (s, 3H), 3.98 (t, J ¼ 4.4 Hz, 2H), 4.13 (t, J ¼ 4.4 Hz, 2H), 4.36 (t, J ¼ 4.4 Hz, 2H),
5.51 (brs, 2H), 6.82 (dd, J ¼ 7.7, 1.6 Hz, 1H), 6.87 (t, J ¼ 7.7 Hz, 1H), 6.92 (dd, J ¼ 7.7,
1.6 Hz, 1H), 8.24 (s, 1H), 9.88 (s, 1H). Anal. Calcd for C11H16N2O4 (Mw 240.26) (%):
C, 54.99; H, 6.71; N, 11.66. Found (%): C, 55.30; H, 6.56; N, 12.47.
H4L1 was synthesized from an ethanol solution (75 mL) of 2,3-dihydroxybenzene1,
4-dicarbaldehyde (0.0632 g, 2.21 mmol) and monooxime (0.25 g, 4.64 mmol) at 60ꢃC,
and the solution was stirred for 6 h at the same temperature. After the solution was
allowed to stand overnight at room temperature, precipitates were collected on a
suction filter to afford H4L1 (0.21 g, 83%) as yellow crystals; m.p. 128–129ꢃC. 1H NMR
(400 MHz, CDCl3) ꢁ 3.93 (s, 6H), 4.46–4.53 (m, 8H), 4.63 (dt, J ¼ 5.4, 1.5 Hz, 4H), 6.77
(s, 2H), 6.78–6.86 (m, 4H), 6.93 (dd, J ¼ 7.2, 2.2 Hz, 2H), 8.25 (s, 2H), 8.28 (s, 2H), 9.60
(s, 2H), 9.69 (s, 2H). Anal. Calcd for C30H34N4O10 (Mw 610.61) (%): C, 59.01; H, 5.61;
N, 9.18. Found (%): C, 59.15; H, 5.57; N, 8.96.
2.2.2. Preparation of [Cu2(L2)2]. A solution of Cu(II) acetate monohydrate (0.00323 g,
0.016 mmol) in ethanol (3 mL) was added dropwise to a solution of H4L1 (0.00303 g,
0.005 mmol) in acetone (1 mL). The mixture turned brown immediately. Stirring was
continued for 2 h at room temperature. The mixture was filtered and the filtrate was
allowed to stand at room temperature for 3 weeks. The solvent was partially evaporated
and yellowish-brown needle-like single crystals (yield: 36%) suitable for X-ray
crystallographic analysis were obtained. Anal. Calcd for C22H26Cu2N2O8 [Cu2(L2)2]
(Mw 573.53) (%): C, 46.07; H, 4.57; N, 4.88; Cu, 22.16. Found (%): C, 46.01; H, 4.65;
N, 4.78; Cu, 22.29.
2.3. X-ray structure determination of [Cu2(L2)2]
The single crystal of [Cu2(L2)2] with approximate dimensions of 0.30 ꢁ 0.11 ꢁ 0.03 mm3
was selected on a Bruker Smart Apex CCD area detector. The reflections were collected

1624
L.-Q. Chai et al.
Table 1. Crystallographic data and data collection parameters for [Cu2(L2)2].
Empirical formula
Formula weight
Temperature (K)
C22H26Cu2N2O8
573.53
298(2)
0.71073
Monoclinic
P21/c
˚
Wavelength (A)
Crystal system
Space group
Unit cell dimensions (A,
ꢃ
˚
)
a
b
c
12.571(1)
17.963(2)
5.1349(5)
95.595(1)
1154.0(2), 2
1.651
1.894
ꢃ
Volume (A ), Z
3
˚
Calculated density (Mg mꢂ3
)
Absorption coefficient (mmꢂ1
F(000)
)
588
Crystal size (mm3)
ꢄ range for data collection (ꢃ)
Index ranges
0.30 ꢁ 0.11 ꢁ 0.03
2.79–25.02
ꢂ14 ꢄ h ꢄ 14;
ꢂ15 ꢄ k ꢄ 21;
ꢂ6 ꢄ l ꢄ 6
Reflections collected
Independent reflection
Completeness to ꢄ ¼ 25.02 (%)
Data/restraints/parameters
Goodness-of-fit on F2
5984
2029 [R(int) ¼ 0.0924]
99.6
2029/0/155
1.029
Final R indices [I 4 2ꢅ(I)]
R indices (all data)
Largest difference peak and hole (e A
R1 ¼ 0.0513, wR2 ¼ 0.0860
R1 ¼ 0.0910, wR2 ¼ 0.0939
0.604 and ꢂ0.404
ꢂ3
˚
)
˚
using graphite monochromated Mo-Kꢂ radiation (ꢀ ¼ 0.71073 A) at 298(2) K. Data
reduction and cell refinement were performed using SAINT [25]. The structure was
solved by the direct method (SHELXS-97) and subsequent difference-Fourier map
revealed 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 [26, 27]. Anisotropic thermal
parameters were assigned to all non-hydrogen atoms. The hydrogen atoms were
generated geometrically. Crystallographic data and structural refinements for the
Cu(II) complex are listed in table 1.
3. Results and discussion
3.1. Crystal structure of [Cu2(L2)2]
The desired bis(salen)-type tetraoxime complex [Cu3(L1)] was not formed, but an
unexpected complex [Cu2(L2)2] was obtained which was formed in the course of
complexation of H4L1 by Cu(II) acetate monohydrate. Because of the catalysis of
copper(II) [28], complexation resulted in cleavage of two N–O bonds in H4L1, giving a
new O–N–O tridentate ligand H2L2, which coordinates to Cu(II) forming a dialkoxo-
bridged dinuclear Cu(II) complex with a Cu–O–Cu–O four-membered ring core instead
of the desired bis(salen)-type tetraoxime Cu3–N4O4 complex [29, 30] (figure 1).

Dialkoxo-bridged dinuclear copper(II)
1625
(a)
O
N
O
N
Cu
O
O
O
N
OEt
OEt
O
N
Cu
1
HO
Cu L
3
O
OH
OEt
OEt
Cu(OAC)2 H2O
O
Cu
N
O
N
O
OH
HO
EtO
N
O
N
O
N
O
2
Cu (L )
2
O
O
2
Cu
Cu
O
O
N
O
OEt
(b)
O
N
OH
OH
OEt
H2L2
Figure 1. (a) Complexation of H4L1 with Cu(II) acetate and (b) chemical structure of H2L2.
The crystal structure and atom numbering of the Cu(II) complex are shown in
figure 2. Selected bond lengths and angles are listed in table 2.
Single-crystal X-ray structure analysis reveals that the Cu(II) complex is a
centrosymmetric neutral homobinuclear entity, [Cu(L2)]2, crystallizing in the mono-
clinic space group P21/c. The ORTEP shows that two [Cu(L2)] moieties are linked via
two alkoxy bridges, each of which comes from H2L2. The whole complex resides on a
crystallographic center of inversion and has two four-coordinate Cu(II) centers. Their
geometry can be best described as slightly distorted square-planar with CuN1O3
coordination (one oxime nitrogen N1, one phenolic oxygen O3, and two bridging
alkoxy oxygen atoms O2 and O2#1) as shown in figure 2. Thus, a planar Cu2O2 core is
formed by two divalent coppers and two bridging alkoxy oxygen atoms with a Cu–Cu
˚
separation of 2.9916(1) A which is too long to represent intramolecular Cu–Cu
bonding. The bridging Cu–O bonds are slightly asymmetric with one being short, Cu1–
#1
˚
˚
O1 ¼ 1.906(3) A, and one long at Cu1–O1 ¼ 1.914(3) A. The bond lengths Cu1–O3
#1
˚
and Cu1–N1 are 1.875(3) and 1.930(4) A, respectively. The two planes O1–Cu1–O1
and O1–Cu1#1–O1#1 are parallel to each other, indicating that Cu1, Cu1#1, O1, and
O1#1 are exactly coplanar. The angles Cu1–O1–Cu1#1 and O1–Cu1–O1#1 are 103.08ꢃ
and 76.92ꢃ, respectively. The dihedral angle of the two planes O1–Cu1–O1#1 and O3–
#1
ꢃ
˚
Cu1–N1 is 2.51 . Cu1 deviates by 0.004 A from the mean plane defined by O1, O1 ,
O3, and N1.

1626
L.-Q. Chai et al.
Figure 2. Molecular structure and atom numbering of the Cu(II) complex.
ꢃ
˚
Table 2. Selected bond distances (A) and angles ( ) for the Cu(II) complex.
Cu1–O3
Cu1–O1
Cu1–O1#1
1.875(3)
1.906(3)
1.914(3)
Cu1–N1
Cu1–Cu1#1
1.930(4)
2.9916(1)
O3–Cu1–O1
O3–Cu1–O1#1
O1–Cu1–O1#1
O3–Cu1–N1
O1–Cu1–N1
O1#1–Cu1–N1
171.5(1)
C3–N1–Cu1
O2–N1–Cu1
C1–O1–Cu1
C1–O1–Cu1#1
Cu1–O1–Cu1#1
125.5(3)
94.8(1)
76.9(1)
93.1(2)
95.2(2)
171.9(2)
122.3(3)
126.4(3)
130.0(3)
103.1(1)
#1
Symmetry transformations used to generate equivalent atoms: ꢂx þ 1, ꢂy þ 1, ꢂz þ 1.
The Cu(II) complex is stabilized by four pairs of intermolecular hydrogen bonds, C2–
H2B ꢀ ꢀ ꢀ O3 and C2–H2B ꢀ ꢀ ꢀ O4, linking both molecules into a 1-D infinite chain parallel
to the c-axis (figure 3). The molecules are further linked by four intermolecular C3–
H3 ꢀ ꢀ ꢀ O2 hydrogen bonds between the –C3H3 unit of the oxime and oxygen (O2) of the
oxime to form a 2-D infinite planar motif (figure 4). Every complex molecule links six
other molecules into an infinite-layered supramolecular structure via 12 intermolecular
C–H ꢀ ꢀ ꢀ O hydrogen bonds (table 3, figure 5).
3.2. Molar conductance
[Cu2(L2)2] is soluble in DMF and DMSO, slightly soluble in CHCl3 but not soluble in
EtOH, MeOH, MeCN, THF, acetone, and ethyl acetate. The complex displays good
stability in air at room temperature. H4L1 is soluble in the aforementioned solvents.
Molar conductance values of [Cu2(L2)2] of 1.0 ꢁ 10ꢂ3 mol Lꢂ1 DMF solution are
3.4 S cm2 molꢂ1, indicating a non-electrolyte.

Dialkoxo-bridged dinuclear copper(II)
1627
Figure 3. View of the 1-D chain motif of Cu(II) complexes along the c-axis (hydrogen atoms except those
forming hydrogen bonds are omitted for clarity).
Figure 4. View of the 2-D layered motif of the Cu(II) complexes (hydrogen atoms except those forming
hydrogen bonds are omitted for clarity).

1628
L.-Q. Chai et al.
ꢃ
˚
Table 3. Hydrogen-bonding distances (A) and angles ( ) for the Cu(II) complex.
D–H ꢀ ꢀ ꢀ A
d(D–H)
d(Hꢀ ꢀ ꢀA)
d(Dꢀ ꢀ ꢀA)
ffD–Hꢀ ꢀ ꢀA
C2–H2B ꢀ ꢀ ꢀ O3
C2–H2B ꢀ ꢀ ꢀ O4
C3–H3 ꢀ ꢀ ꢀ O2
0.97
0.97
0.93
2.60
2.59
2.56
3.514(3)
3.240(3)
3.415(3)
156
125
154
Figure 5. View of the 2-D supramolecular structure of the Cu(II) complex (hydrogen atoms except those
forming hydrogen bonds are omitted for clarity).
3.3. UV-Vis absorption spectra
UV-Vis absorption spectra of H4L1 and the complex were determined in
5 ꢁ 10ꢂ5 mol Lꢂ1 DMF solution. Absorptions of Cu(II) complex are obviously different
from those of H4L1 (figure 6). Compared with the Cu(II) complex, an important feature
of the absorption spectrum of H4L1 is that two absorptions are observed at 273 nm and
310 nm, respectively. The former is absent in the spectrum of Cu(II) complex. The other
absorption at 310 nm of H4L1 shifts to 315 nm in Cu(II) complex, and a new absorption
peak at 439 nm was observed in Cu(II) complex, indicating coordination of Cu.
3.4. Fluorescence spectra
The emission spectrum of the Cu(II) complex in dilute DMF at room temperature is
shown in figure 7. The shape of excitation spectrum of the Cu(II) complex strongly

Dialkoxo-bridged dinuclear copper(II)
1629
Figure 6. UV-Vis absorption spectra: H4L1 (- - -) and the Cu(II) complex (—) in DMF (5 ꢁ 10ꢂ5 mol Lꢂ1).
Figure 7. Emission spectrum of the Cu(II) complex in dilute DMF at room temperature (c ¼ 5 ꢁ 10ꢂ5 mol
L
ꢂ1, ꢀem ¼ 417 nm).
resembles that of UV-Vis absorption spectrum and intense excitation peak is observed
at 312 nm. The emission peak of Cu(II) complex appears at 417 nm. The Stokes shift
between the maximum wavelength of the fluorescence emission and the fluorescence
excitation spectrum is 105 nm. This red-shift might be related to the head-to-tail dimeric
structure, which make the conjugated system larger, and also indicates it may be a
purple device.

1630
L.-Q. Chai et al.
4. Conclusion
An unexpected dinuclear Cu(II) complex, [Cu2(L2)2], bearing a 2 : 2 ligand : metal
stoichiometry, was synthesized from H4L1 in moderate yield. Cu(II) is four coordinate
by two monooxime (L2)2ꢂ units. Cu(II) caused an unexpected cleavage of two N–O
bonds, giving a dialkoxo-bridged Cu(II) complex possessing a Cu–O–Cu–O four-
membered ring core instead of the expected bis(salen)-type tetraoxime Cu3–N4O4
complex. The molecules are linked to six other molecules in an infinite-layered
supramolecular network via 12 intermolecular C–H ꢀ ꢀ ꢀ O hydrogen bonds. Meanwhile,
Cu(II) exhibits purple emission with maximum emission wavelength ꢀem ¼ 417 nm when
excited at ꢀex ¼ 312 nm.
Supplementary material
Further details of the crystal structure investigation(s) may be obtained from the
Cambridge Crystallographic Data Centre, Postal address: CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK. Telephone: (44) 01223 762910; Facsimile: (44) 01223 336033;
E-mail: deposit@ccdc.cam.ac.uk on quoting the depository number CCDC 852419.
Acknowledgments
This work was supported by the ‘‘Jing Lan’’ Talent Engineering Funds of Lanzhou
Jiaotong University, which are gratefully acknowledged.
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