Two air oxidation copper(II) complexes of salicylaldehyde derivatives obtained by in situ copper(II) ion catalysis and complexation

Article abstract of DOI:10.1016/j.inoche.2011.09.025

A new salicylaldehyde derivative 1, i.e. 5-chloro-3-(ethoxymethyl)-2- hydroxybenzaldehyde, has been prepared and structurally characterized. A novel dinuclear copper(II) complex of its air-oxidized product 2 has been successfully yielded from the in situ copper(II) ion catalysis and complexation. Additionally, another control experiment has been carried out by using 3,5-dibromo-2-hydroxybenzaldehyde as the starting material, and a similar mononuclear air oxidation copper(II) complex 3 is obtained, where 3,5-dibromo-2-hydroxybenzaldehyde has also been in situ transformed to the divalent anion of 3,5-dibromo-2-hydroxybenzoic acid.

Full text of DOI:10.1016/j.inoche.2011.09.025

Inorganic Chemistry Communications 14 (2011) 1978–1981
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Two air oxidation copper(II) complexes of salicylaldehyde derivatives obtained by
in situ copper(II) ion catalysis and complexation
⁎
Jiao Geng, Tao Tao, Ke-Hua Gu, Gang Wang, Wei Huang
State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Microstructures, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093,
P. R. China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 July 2011
Accepted 15 September 2011
Available online 23 September 2011
A new salicylaldehyde derivative 1, i.e. 5-chloro-3-(ethoxymethyl)-2-hydroxybenzaldehyde, has been pre-
pared and structurally characterized. A novel dinuclear copper(II) complex of its air-oxidized product 2 has
been successfully yielded from the in situ copper(II) ion catalysis and complexation. Additionally, another
control experiment has been carried out by using 3,5-dibromo-2-hydroxybenzaldehyde as the starting ma-
terial, and a similar mononuclear air oxidation copper(II) complex 3 is obtained, where 3,5-dibromo-2-
hydroxybenzaldehyde has also been in situ transformed to the divalent anion of 3,5-dibromo-2-hydroxybenzoic
acid.
Keywords:
Copper(II) complexes
In situ copper (II) ion catalysis
Air oxidation
© 2011 Elsevier B.V. All rights reserved.
Salicylaldehyde derivatives
The oxidation of aldehydes to their corresponding carboxylic
acids has been a longstanding issue in synthetic organic chemistry
[1]. To date, the most popular method is using the Jones reagent
[2], however, the Cr-based side products can be regarded as environ-
mental hazards. Other efficient reagents can also achieve such trans-
formation; for example, oxone [3], calcium hypochlorite [4] and 2-
hydroperoxyhexafluoro-2-propanol [5], urea-hydrogen peroxide
complex [6], etc. Furthermore, some convenient approaches, such
as Baeyer–Villiger oxidation [7], Cannizzaro reaction [8], and cata-
lyzed oxidation [9] have also been widely investigated. In catalyzed
oxidation, different metal sources including [Ni(acac)2], NiCl2, Hg
(OAc)2, Cs6H2P2W17O61Co·H2O, AgNO3, Au/C, Pt/C, [Fe(TPP)Cl], and
CuX [10], have been utilized to promote the reactions in the presence
of some oxidants such as molecular oxygen, hydrogen peroxide, dioxy-
gen, tert-butyl hudroperoxide, and so on. We have also reported a series
of cyano-bridged mixed-valent copper complexes (mono-, di-, tri-, and
tetranuclear) obtained from in situ cleavage of C−C bond in acetonitrile
and their effective catalysis on the oxidation of benzaldehyde by perox-
ide [11].
of air-oxidized products because two coordination sites of copper(II)
center have been previously occupied by bpy molecule which will
greatly simplify the structures of final copper(II) complexes. In this
work, we focus on the air oxidation of salicylaldehyde derivatives to
corresponding acids under a mild experimental condition and in situ
catalyzed by a [Cu(bpy)(NO3)2] (bpy = 2,2′-bipyridine) complex as
well as the formation of resultant mononuclear and dinuclear copper(II)
complexes of air-oxidized divalent anionic carboxylates.
New organic compound 1 is prepared according to a previously
reported method [12] and full characterizations have been carried
out. Copper(II) complexes
2 and 3 are synthesized by the
treatment of equal molar ratio of [Cu(bpy)(NO3)2] and 5-chloro-
3-(ethoxymethyl)-2-hydroxybenzaldehyde in 2 and 3,5-dibromo-
2-hydroxybenzaldehyde in 3 [13], as shown in Scheme 1, where
the air-oxidized products of aldehydes are formed and they are in situ
coordinated with the copper(II) ions as bidentate dianionic ligands. It
should be mentioned that no additional oxidant is introduced during
the reaction and only the oxygen in air can serve as an oxidant. It is
also certain to us from the reference experiment that the aldehyde can-
not be oxidized so easily without the presence of a CuII ion catalyst. Rou-
tine analyses including 1H NMR spectrum have been for characterizing
related compounds 1–3 and their UV–vis spectra in methanol (Fig. 1)
show typical intraring π–π* absorptions and charge transfer between
the C=O chromophores and their conjugated phenol rings. In addition,
X-ray single-crystal diffraction method has been used for characterizing
the structures of complexes 1, 2 and 3 [14].
Although many reagents can oxidize aldehydes to their carboxylic
acids, it is much more difficult to achieve the transformation under
very mild procedures and isolate the in situ formed metal carboxylate
complexes. To our knowledge, direct air oxidation of aldehydes to
their carboxylic acids under the catalysis of [Cu(bpy)]2+ species has
not been reported before. By using a [Cu(bpy)]2+ catalyst instead of a
free copper ion, it is possible for us to isolate the copper(II) complexes
Molecular structure of compound 1 with the atom-numbering
scheme is shown in Fig. 2a. It crystallizes in the monoclinic P21/c
space group and each asymmetric unit contains one 5-chloro-3-
(ethoxymethyl)-2-hydroxybenzaldehyde molecule. The C–O bond
⁎
Corresponding author. Tel.: +86 2583686526; fax: +86 2583314502.
E-mail address: whuang@nju.edu.cn (W. Huang).
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.09.025

G. Jiao et al. / Inorganic Chemistry Communications 14 (2011) 1978–1981
1979
Scheme 1. Air oxidation of salicylaldehyde derivatives by in situ CuII ion catalytic and complexation.
lengths of phenolic and aldehyde units are 1.343(3) Å (C6–O1) and
1.212(3) Å (C7–O2), respectively, indicative of the right assignment
of corresponding functional groups. However, no π–π stacking inter-
actions can be observed between contiguous aromatic rings in the
crystal packing of 1 mainly because of the steric hindrance of ethoxy-
methyl group.
(5)°, showing the presence of a typical carboxylic structure. The bpy
and the phenol planes in 3 are almost parallel with the dihedral
angle of 1.1(2)°. In this case, typical π–π stacking interactions are
found between neighboring phenol ring and one of the pyridyl rings
of bpy molecule with the centroid-to-centroid separation of 3.689
(5) Å, forming a dimeric packing fashion with two coordination etha-
nol molecules pointing outside of the unit.
Molecular structure of dinuclear complex 2 with the atom-
numbering scheme is shown in Fig. 2b. It crystallizes in the monoclinic
P21/c space group and the coordination geometry of each copper(II)
center is square pyramidal, where two nitrogen atoms from the che-
lating 2,2′-bipyridine ligand and two oxygen atoms from the dianion
of 5-chloro-3-(ethoxymethyl)-2-hydroxybenonic acid are located at
the basal coordination plane. The apical position is occupied by an-
other phenolic oxygen atom of adjacent dianion of air-oxidized prod-
uct of aldehyde 1, and the Cu–O and Cu–N bond lengths are very
analogous in the range of 1.869(4) –1.985(5) Å. The phenolic oxygen
atom O3 serves as a μ2-oxo bridging unit and the carboxylic group
adopts a monodentate coordination mode with the O1–C11 and O2–
C11 bond lengths of 1.293(7) and 1.224(7) Å and the O2–C11–O1
angle of 123.0(6)°, indicative of the formation of a typical carboxylate
unit in 2. The dihedral angle between the bpy and the phenol planes
within the molecule is 20.4(2)° and the centroid-to-centroid separa-
tions between the phenol ring and two adjacent pyridyl rings of the
same bpy molecule are 4.145(4) and 4.312(4) Å, respectively.
Molecular structure of complex 3 with the atom-numbering
scheme is shown in Fig. 3. Different from the dinuclear complex 2, it
crystallizes in the triclinic P 1 space group and each asymmetric
unit contains one mononuclear copper(II) structure. The coordination
geometry of copper(II) center is elongated pyramidal, in which two
nitrogen atoms from 2,2-bipyridine and two oxygen atoms from
3,5-dichloro-2-hydroxybenzaldehyde constitute the basal coordina-
tion plane while another coordination ethanol molecule occupies
the apical position. The axial Cu1-O4 bond length [2.516(4) Å] is
much longer than those Cu–O [1.995(4) and 2.002(4) Å] and Cu–N
[1.884(3) and 1.897(3) Å] bond lengths in the coordination plane, in-
dicative of typical Jahn–Teller distortions for the central copper(II)
ion. It is also noted that the bond lengths of C7–O3 and C7–O2 are
1.235(5) and 1.279(5) Å and the bond angle of O3–C7–O2 is 122.4
In summary, we have firstly used the [Cu(bpy)]2+ catalytic sys-
tem in the study of in situ oxidation from aldehydes to their corre-
sponding carboxylic acids in air under mild reaction. It is proved to
be an efficient approach and two [Cu(bpy)]2+ based complexes 2 and
3 have been successfully obtained by in situ catalysis and complexation.
Two salicylaldehyde derivatives, namely 5-chloro-3-(ethoxymethyl)-2-
hydroxybenzaldehyde and 3,5-dibromo-2-hydroxybenzaldehyde, un-
dergo the aforementioned air oxidation to their corresponding salicylic
acid derivatives, where steric hindrance effects of different substituent
groups are believed to be responsible for the formation of a mononu-
clear complex in 2 and a dinuclear complex in 3 with double μ2-oxo
Fig. 1. UV–vis spectra of complexes 1, 2 and 3 in methanol.

1980
G. Jiao et al. / Inorganic Chemistry Communications 14 (2011) 1978–1981
Fig. 2. ORTEP diagrams of (a) molecular structure of complex 1 and (b) molecular structure of complex 2 (A: –x, 2-y, 1-z) with the atom-numbering scheme. Hydrogen atoms are
omitted for clarity. Selected bond lengths (Å) and bond angles (°): 1: C6–O1, 1.343(3); C7–O2, 1.212(3); O2–C7–C1, 124.5(3); O2–C7–H7, 117.8; O1–C6–C1, 122.3(2); O1–C6–C5,
117.5(2). 2: Cu1–O1, 1.869(4); Cu1–O3, 1.893(4); Cu1–N1, 1.976(5); Cu1–N2, 1.985(5); O1–C11, 1.293(7); O2–C11, 1.224(7); O3–C17, 1.331(6); O1–Cu1–O3, 93.87(16); O1–Cu1–
N1, 167.97(19); O3–Cu1–N1, 93.13(19); O1–Cu1–N2, 92.1(2); O3–Cu1–N2, 173.6(2); N1–Cu1–N2, 80.6(2); O2–C11–O1, 123.0(6); O2–C11–C12, 118.7(6); O1–C11–C12, 118.3(6);
O3–C17–C12, 123.9(5); O3–C17–C16, 117.0(6).
bridges. The main limitation of this study is the use of a stoichiomet-
ric catalyst. Further work is being undertaken on the effective oxida-
tion reactions of similar aldehyde derivatives having certain chiral
units in the presence of small amount of catalysts as well as similar
oxidation reactions catalyzed by other complex catalysts, for exam-
ple, [Cu(bpy)2]2+ or [Ru(bpy)2]2+ species.
Appendix A. Supplementary data
CCDC 833072-833074 contain the supplementary crystallographic
data for 1, 2 and 3. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crys-
tallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
(+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data to this article can be found online at doi:10.
1016/j.inoche.2011.09.003.
Acknowledgements
We acknowledge the Major State Basic Research Development
Program (Nos. 2011CB933300, 2007CB925101 and 2011CB808704),
the National Natural Science Foundation of China (Nos. 20871065
and 21171088) and the Jiangsu Province Department of Science and
Technology (No. BK2009226) for financial aids.
References
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Appendix A. Supplementary material
Supplementary data to this article can be found online at doi:10.
1016/j.inoche.2011.09.025.
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[13] Preparation of C10H11ClO3 (1), [Cu(bpy)(C10H11ClO4)]·CH3OH (2) and [Cu(bpy)
(C7H2Br2O3)(C2H5OH)] (3). 1: To a solution of 5-chloro-3-(chloromethyl)-2-
hydroxybenzaldehyde (1.000 g, 4.88 mmol) in ethanol (50 cm3) was added a few
drops of ethane-1,2-diamine. The mixture was heated to reflux for 2 h and then
cooled down to the room temperature. After the solvent was removed, the residue
was subjected to column chromatography (SiO2; eluent, ethyl acetate:petroleum
ether=1:20) to afford compound 1 in a yield of 0.832 g (79.5% based on phenol).
Fig. 3. ORTEP diagram of molecular structure of complex 3 with the atom-numbering
scheme. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and bond
angles (°): 3: Cu1–O1, 1.884(3); Cu1–O2, 1.897(3); Cu1–N2, 1.995(4); Cu1–N1, 2.002
(4); Cu1–O4, 2.516(4); C7–O3, 1.235(5); C7–O2, 1.279(5); C1–O1, 1.309(5); O1–
Cu1–O2, 94.9(1); O1–Cu1–N2, 91.1(2); O2–Cu1–N2, 170.9(2); O1–Cu1–N1, 170.3(2);
O2–Cu1–N1, 92.8(2); N2–Cu1–N1, 80.6(2); O1–Cu1–O4, 96.0(2); O2–Cu1–O4, 91.6
(2); N2–Cu1–O4, 94.7(2); N1–Cu1–O4, 89.8(2); O1–C1–C6, 126.2(4); O1–C1–C2,
117.3(4); O3–C7–O2, 122.4(5); O3–C7–C6, 118.3(4); O2–C7–C6, 119.3(4).

G. Jiao et al. / Inorganic Chemistry Communications 14 (2011) 1978–1981
1981
Main FT-IR absorptions (KBr pellets, cm−1): 3419 (m), 2862 (w), 1617 (s), 1536 (s),
1416 (s), 1122 (s), 732 (m). Elemental Anal. Calcd. for C10H11ClO3: C 55.96; H 5.17%.
Found: C 55.84; H 5.44%. 1H NMR (500 MHz, CDCl3): δ=11.18 (1H, s), 9.88 (1H, s),
7.68 (1H, s), 7.50 (1H, s), 4.60 (2H, s) 3.64–3.68 (2H, m), 1.31–1.34 (3H, t). UV–vis in
methanol: λmax =342, 258 and 220 nm. EI-TOF-MS: m/z=140.1 [PhCH2ClOH]
(100%) and 170.1 [PhCH2ClOHCHO] (29.9%). 2: A solution of [Cu(bpy)(NO3)2]
(w), 1099 (w). Elemental Anal. Calcd. for C19H16Br2CuN2O4: C 40.77; N 5.01; H
2.88%. Found: C 40.43; N 4.76; H 3.07%. UV–vis in methanol: λmax=299, 268 and
222 nm. Positive ESI-MS in methanol: m/z=513.75 [M-C2H5OH]+ (100%).
[14] Single crystal samples of 1, 2 and 3 were glue-covered and mounted on the top of
the glass fibers and used for data collection on a Bruker SMART 1K CCD diffractom-
eter at 291(2) K using graphite mono-chromated Mo Kα radiation (λ=0.71073 Å).
Crystallographic data for C10H11ClO3 (1): M=214.64 g/mol, light green, needle,
0.10×0.12×0.14 mm, monoclinic, space group P21/c (No. 14), a=13.804(13),
b=9.238(9), c=8.435(6) Å, α=90°, β=103.31(3)°, γ=90°, V=1046.8(16) Å3,
(80.2 mg, 0.233 mmol) in ethanol (20 cm3) was added into
a solution of 1
(50.0 mg, 0.233 mmol) in ethanol (20 cm3). After refluxed for 2 h, the mixture
was cooled to the room temperature and then left for slow evaporation in air. Finally,
dark green crystals suitable for X-ray diffraction determination were obtained after
several days. Yield: 47.5 mg (41.3% based on metal). Main FT-IR absorptions
(KBr pellets, cm–1): 3419 (w), 1617 (s), 1417 (m), 1122 (m), 732 (m). Elemen-
tal Anal. Calcd. for C42H42Cl2Cu2N4O10: C 52.50; N 5.83; H 4.41%. Found: C 52.22;
N 5.99; H 4.74%. UV–vis in methanol: λmax =405, 299, 245 and 221 nm. Positive
ESI-MS in methanol: m/z =401.15 [Cu(bpy)C6H2COOClCO]+. 3: Complex 3 was
prepared in the same method as complex 2. The only difference is using 3,5-
dibromo-2-hydroxybenzaldehyde (50.0 mg, 0.179 mmol) instead of 5-chloro-
3-(ethoxymethyl)-2-hydroxybenzaldehyde and the mass of [Cu(bpy)(NO3)2]
was reduced to 61.6 mg (0.179 mmol). Yield: 42.7 mg (42.7% based on metal).
Main FT-IR absorptions (KBr pellets, cm–1): 3415 (s), 1599 (m), 1445 (m), 1244
Z=4, Dc=1.362 g/cm3, μ=0.343 mm−1
, F(000)=448, S=0.98. R1=0.0439,
wR2=0.1127. For C42H42Cl2Cu2N4O10 (2): M=960.78 g/mol, dark green, needle,
0.10×0.11×0.11 mm, monoclinic, space group P21/c (No. 14), a=8.057(3),
b=23.761(9), c=13.076(4) Å, α=90°, β=117.390(18)°, γ=90°, V = 2222.7
(14) Å3, Z = 2, Dc = 1.436 g/cm3, μ = 1.136 mm-1, F(000)=988, S=0.79.
R1=0.0581, wR2=0.1529. For C19H16Br2CuN2 O4 (3): M=559.70 g/mol, dark
green, block, 0.10×0.11×0.12 mm, triclinic, space group P 1 (No. 2), a=9.614(7),
b=10.725(8), c=11.723(8) Å, α=63.870(9)°, β=81.907(10)°, γ=71.562(10)°,
V=1029.6(13) Å3, Z=2, Dc=1.805 g/cm3, μ=4.969 mm−1
S=0.63. R1=0.0382, wR2=0.0580.
, F(000)=550,