Catalytic activity of some new copper(II) azo-complexes

Article abstract of DOI:10.1080/00958972.2012.719079

Copper(II) complexes [Cu(L)2]nH2O, where L is 3-(p-X-)-4-hydroxy-l,2- naphthoquinone (for L1, X=H; L2, X=CH3; L3, X=Cl; L4, X=Br; and L5, X=NO2), have been synthesized and characterized by analytical, electrochemical, spectroscopic (IR, UV-Vis, ESR, and <

Full text of DOI:10.1080/00958972.2012.719079

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Catalytic activity of some new
copper(II) azo-complexes
Abdel Ghany F. Shoair a
a Department of Chemistry, Faculty of Science , Damiatta
University , 34517 Damiattta , Egypt
Accepted author version posted online: 07 Aug 2012.Published
online: 23 Aug 2012.
To cite this article: Abdel Ghany F. Shoair (2012) Catalytic activity of some new
copper(II) azo-complexes, Journal of Coordination Chemistry, 65:20, 3511-3518, DOI:
10.1080/00958972.2012.719079
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Journal of Coordination Chemistry
Vol. 65, No. 20, 20 October 2012, 3511–3518
Catalytic activity of some new copper(II) azo-complexes
ABDEL GHANY F. SHOAIR*y
Department of Chemistry, Faculty of Science, Damiatta University, 34517 Damiattta, Egypt
(Received 12 January 2012; in final form 27 June 2012)
Copper(II) complexes [Cu(L)2] ꢀ nH2O, where L is 3-(p-X-)-4-hydroxy-l,2-naphthoquinone
(for L1, X ¼ H; L2, X ¼ CH3; L3, X ¼ Cl; L4, X ¼ Br; and L5, X ¼ NO2), have been synthesized
and characterized by analytical, electrochemical, spectroscopic (IR, UV-Vis, ESR, and
1H NMR), and magnetic methods. From the data obtained, square-planar geometry has
been assigned for all the complexes. [CuL1] ꢀ H2O exhibits catalytic activity for oxidation of
benzyl alcohol, piperonyl alcohol, and cinnamyl alcohol into their respective aldehydes in the
presence of H2O2 as co-oxidant and in CH3CN and H2O as solvents at room temperature.
Keywords: Copper(II) complexes; Catalytic oxidation; Aromatic alcohols
1. Introduction
Copper(II) complexes have interesting spectroscopic and catalytic properties [1].
The development of highly active catalysts for selective oxidation of aromatic alcohols
to their corresponding carbonyl compounds is important [2, 3]. Although various
catalytic methods based on transition metal complexes and H2O2 as co-oxidant have
been developed, synthesis of cheap and benign catalysts remains a major challenge
[4–9]. We reported the catalytic oxidation of some aromatic alcohols by ruthenium
complexes [10–12].
In this study, we report synthesis and characterization of a new series of copper(II)
complexes, [Cu(L)2] ꢀ nH2O and investigate the catalytic oxidation of benzyl alcohol,
piperonyl alcohol, and cinnamyl alcohol to their respective aldehydes by [CuL1] ꢀ H2O
with H2O2 as co-oxidant in CH3CN and H2O at room temperature.
2. Experimental
2.1. Chemicals and general methods
Syntheses were performed in air at room temperature. Chemicals were supplied by
Sigma-Aldrich Chemicals Company (USA). All other chemicals and solvents were used
without additional purification. Microanalyses were performed using a Perkin-Elmer
*Email: shoairaksm@hotmail.com
yDepartment of Chemistry, Faculty of Science, 2097 Gazan University, KSA.
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2012 Taylor & Francis
http://dx.doi.org/10.1080/00958972.2012.719079

3512
A.G.F. Shoair
CHN 2400 microanalysis unit at Cairo University (Cairo, Egypt). IR spectra were
recorded as KBr discs from 4000–400 cmꢁ1 on a JASCO 410 spectrophotometer.
Electronic absorption spectra were obtained in DMF on a Perkin-Elmer Lambda 2
spectrophotometer. Magnetic moments were measured by Gouy’s method at room
temperature. 1H NMR spectra were recorded on a Varian Unit Plus 300 MHz
spectrometer in DMSO-d6 and chemical shifts are reported in parts per million (ppm)
relative to the internal standard Me4Si. ESR measurements of the polycrystalline
samples were taken on a Bruker ECS 106 EPR spectrometer using a quartz Dewar at
room temperature. All spectra were calibrated with DPPH (g ¼ 2.0027). Copper content
was determined by using atomic absorption technique after destruction of the
complexes with concentrated HNO3. The electrochemical behavior of the complexes
was studied using an electrochemical analyzer CHI 610 A (HCH Instrument).
The electrochemical cell that was used in this work contains three electrodes: platinum
wire as a working electrode, Agþ/AgCl as a reference electrode, and a platinum wire as
a counter electrode.
2.2. Synthesis
2.2.1. Preparation of the ligand. Aniline or p-substituted aniline (10 mmol)
was dissolved in 10 mL of hydrochloric acid (5 mol Lꢁ1). The produced aniline salt
was diazotized below ꢁ5ꢂC with a solution of sodium nitrite (0.85 g, 10 mmol) in
30 mL distilled water. The produced diazonium salt was coupled with 10 mL ethanol
solution of 2-hydroxy-l,4-naphthoquinone (1.74 g, 10 mmol). The produced crude dye
was collected by filtration, recrystallized from ethanol, and then dried in vacuo
over P4O10.
2.2.2. Preparation of copper(II) complexes. CuCl2 ꢀ 2H2O or CuSO4 ꢀ 5H2O (1 mmol)
was dissolved in 10 mL of hot methanol, then 10 mL of hot methanol solution of the
ligand (2.5 mmol) was added. The reaction mixture was refluxed on a water bath with
stirring. The complexes were completely formed and precipitated after 3 h. In each case,
the complex was filtered off while hot, washed several times with hot methanol, and
then dried in vacuo over P4O10 (Yield 60–70%).
2.2.3. General procedure for catalytic oxidation of alcohols. [Cu(L1)2] ꢀ H2O (0.0064 g,
0.01 mmol) was dissolved in CH3CN (2 mL) and H2O (10 mL), then alcohol (2 mmol)
was added with stirring for 30 min. Hydrogen peroxide (2.3 mL, 30%, 10 mmol) was
then added dropwise and the reaction mixture was stirred for 2 h, then reduced in vacuo.
The residues were collected in diethylether (3 ꢃ 10 mL), filtered through a bed of silica
gel, and dried over anhydrous MgSO4. The produced aldehydes were quantified as their
2,4-dinitrophenylhydrazone derivatives. Similarly, large-scale oxidation for 20 mmol of
benzyl alcohol was carried out using 10-fold excess of the complex, hydrogen peroxide
(30%), and solvents and the reaction was stirred for 5 h.

Copper(II) azo-complexes
3513
Table 1. Elemental analysis, melting point, and color data for copper(II) complexes.
Analytical data – Found (Calcd)
Complex
Color
Melting point (ꢂC)
%C
%H
%N
%Cu
HL1
[Cu (L1)2] ꢀ H2O
Orange
Brown
Orange
Brown
Red
Brown
Red-orange
Brown
Yellow
brown
234
279
208
230
236
257
225
259
269
294
73 (71.52)
3.5 (3.31)
10.1 (9.27)
–
64 (63.02) 3.33 (3.20)
69.5 (68.35) 3.10 (2.79)
63 (62.33) 3.72 (3.55)
65.5 (64.19) 2.94 (2.67)
60.2 (58.65) 2.66 (2.44)
59.1 (58.23) 2.65 (2.42)
52 (51.34) 2.45 (2.13)
8.55 (8.16) 9.60 (9.26)
9.21 (8.86)
8.03 (7.65) 8.67 (8.68)
8.53 (8.32)
8.11 (7.60) 8.60 (8.62)
8.13 (7.54)
6.85 (6.65) 7.50 (7.54)
HL2
[Cu (L2)2] ꢀ 2H2O
HL3
–
–
[Cu (L3)2]
HL4
–
[Cu (L4)2] ꢀ H2O
HL5
[Cu (L5)2]
63.5 (62.27) 2.66 (2.59) 12.33 (12.10)
57.2 (56.95) 2.45 (2.37) 11.56 (11.07) 8.50 (8.37)
–
3. Results and discussion
3.1. Synthesis
The copper(II) complexes [Cu(L)2] ꢀ nH2O were prepared by refluxing a methanol
solution of CuCl2 ꢀ 2H2O or CuSO4 ꢀ 5H2O with a methanol solution of the ligand in
1 : 2 molar ratio. The complexes were isolated as brown stable solids, soluble in DMF,
CH3CN, and DMSO. The analytical data (table 1) are in a good agreement with the
proposed molecular formulae in scheme 2.
3.2. 1H NMR spectra of ligand
1
The H NMR spectra of the prepared ligands were recorded in DMSO-d6 using TMS
as reference. They are all similar and showed three mean signals. The first was
7.38–7.47 ppm with integration area corresponding to four protons; this signal is
assigned to the phenylazo protons. Two medium signals were observed at 8.02 ppm and
7.81 ppm. The first signal is assigned to H5 and H8, while the second is assigned to H6
and H7 in the naphthoquinone ring.
3.3. IR Spectra
The most important bands occurring in the IR spectra of the free ligands and their
copper(II) complexes have been compared to ascertain the bonding sites in the chelating
ligand (table 2). IR spectra of the ligands display bands at 3428–3448 cmꢁ1, assignable
to intramolecular hydrogen-bonding between the C-4 hydroxyl and ꢀ-nitrogen of the
azo linkage (scheme 1). IR spectra of the ligands show two medium bands at
1280ꢁ1290 cmꢁ1 and 1500–1515 cmꢁ1. These bands are attributable to ꢁ(N¼N) and
phenolic ꢁ(C–O) and shifted to lower frequencies in IR spectra of the corresponding
copper(II) complexes. The mode of bonding to copper(II) is further supported by
appearance of two new absorption bands at 420–440 cmꢁ1 and 500–530 cmꢁ1 ranges,
which are not observed in spectra of the free ligands. These bands are assignable to

3514
Table 2. Spectroscopic (IR and UV-Vis) and magnetic data for copper(II) complexes.
IR data (cmꢁ1
ꢁ(N¼N) ꢁ(C–O) ꢁ(OH) ꢁ(Cu–N) ꢁ(Cu–O) ꢃeff (BM)
A.G.F. Shoair
)
Compound
UV-Vis data (cmꢁ1
)
HL1
[Cu(L1)2] ꢀ H2O
1503m
1490m
1507m
1488m
1510m
1489m
1500m
1492m
1515m
1495m
1290m 3428b
1278m
1288m 3430b
1270m
1289m 3435b
1275m
1285m 3442b
1275m
1280m 3448b
1273m
–
420w
–
440w
–
440w
–
435w
–
430w
–
500w
–
520w
–
530w
–
515w
–
513w
–
1.82
–
1.99
–
2.1
–
1.68
–
25,641
26,315
25,316
27,777
25,974
28,169
25,706
28,985
25,316
26,315
47,169
–
22,222
46,411
22,727
45,045
22,988
43478
22,727
44,444
22,573
HL2
[Cu(L2)2] ꢀ 2H2O
HL3
–
[Cu(L3)2]
HL4
–
[Cu(L4)2] ꢀ H2O
–
HL5
[Cu(L5)2]
–
1.92
ꢁ(Cu–N) and ꢁ(Cu–O), respectively [13]. IR spectra of the copper(II) complexes display
two bands at 1673–1693 cmꢁ1 and 1623–1670 cmꢁ1, characteristic of uncoordinated
1,2-quinone carbonyls of the ligand [14]. IR spectra of the heated complexes (up to
130ꢂC) did not show any ꢁ(OH) for water or MeOH. These assignments indicate that
coordination of these ligands to copper(II) takes place via the azo-nitrogen and phenolic
oxygen of the C-4 hydroxyl group of the ligand (scheme 2).
3.4. Electronic absorption spectra
Electronic absorption spectra of the ligands and their copper complexes were measured
at room temperature in DMF, and the spectral data are given in table 2. Electronic
absorption spectra of the ligands display two electronic absorptions at 25,169–25,974
and 43,478–47,169 cmꢁ1, which are assigned to n ! ꢂ* and intraligand ꢂ ! ꢂ*
transitions, respectively [15]. The electronic spectra of the copper(II) complexes display
two electronic absorptions at 22,222–26,315 and 22,988–28,985 cmꢁ1, assigned to the
2
2
2
2
two spin allowed transitions B1g ! A1g (dxy ! dxz) and B1g ! Eg (dyz ! dxz),
respectively [16]. These spectral features support the square-planar geometry around
copper(II) [17]. Additional bands are present in the high-energy region, assignable to
(O ! Cu) and (N ¼ N ! Cu) charge-transfer transitions [18]. These spectral features
are in agreement with other square-planar copper(II) complexes [19].
3.5. Magnetic measurements and ESR spectra
The copper(II) complexes show normal room temperature magnetic moments
ꢃeff ¼ 1.68–2.1 BM, close to the spin-only value of 1.73 BM [19]. X-band ESR spectra
of the polycrystalline samples of synthesized copper(II) complexes were measured at
room temperature. The spectra are quite similar and exhibit the following features:
(a) All complexes show spectral parameters characteristic of axial symmetry (g|| 4 g ).
?
(b) The unpaired electron is located in the dx2ꢁy2 orbital of copper(II) ions and the trend
g|| 4 g for these complexes indicates the orbital d
is in the ground state [16].
x2ꢁy2
?

Copper(II) azo-complexes
3515
O
OH
N
N
O
R
O
O
H
N
R
N
O
O
O
N
O
N
R
H
Scheme 1. General formulae of the ligands and their intramolecular hydrogen-bonding (R ¼ H, CH3, Cl, Br,
and NO2).
(c) Values of the G-parameter are greater than four, indicating negligible exchange
interaction between Cu(II) centers in the solid state. ESR spectral features of the
copper(II) complexes are similar and resemble those reported for axial-type
copper(II) complexes that have square-planar stereochemistry [20–23].
Representative spectra are shown in ‘‘Supplementary material’’ section.
3.6. Electrochemical properties
Electrochemical behavior of the copper(II) complexes was studied in DMF containing
0.1 mol Lꢁ1 (TBAC) as supporting electrolyte. Cyclic voltammograms are similar and

3516
A.G.F. Shoair
O
O
N
O
R
N
O
Cu
R
N
N
O
O
Scheme 2. The proposed structures of the [Cu(L)2] ꢀ nH2O complexes (L ¼ L1, L2, L3, L4 and L5).
show one-electron irreversible reduction wave (DE ¼ 0.111–0.123 V) on the negative side
and one-electron irreversible oxidation wave (DE ¼ 0.076–0.150 V) on the positive side
against Agþ/AgCl electrode. The reduction wave is assigned to Cu(II)/Cu(I) reduction
and the oxidation wave is assigned to Cu(II)/Cu(III) oxidation. The marked shift of DE
for these two waves from 0.059 V (oxidation potential for one electron) is probably due
to geometrical rearrangement around the copper center while undergoing reduction and
oxidation. A representative cyclic voltammogram is shown in ‘‘Supplementary
material’’ section.
3.7. Catalytic oxidation of alcohols
Many transition metals Cu [24], Cr [25], Ni [26], Fe [27], V [28], Pd [29], Ru [30], W [31],
and [32] Mn have been used as catalysts for oxidation of alcohols to their respective
carbonyl compounds with different co-oxidants. Vijayaraj et al. [24] reported the
kinetics of oxidation of pyrocatechol to o-quinone by a series of copper(II) Schiff-base
complexes, [CuL] and found the rate of oxidation of catecholase to o-quinone increases
as the chain length of the Schiff base increased. Saha et al. [25] reported the kinetics and
mechanism of oxidation of ethanol to acetaldehyde by a Cr(VI)-phen complex with the
anionic surfactant (sodium dodecyl sulfate) and suggested the complex Cr(VI)-phen as
the active catalyst.
The kinetics and mechanism of oxidation of the polyhydric alcohols, D-mannitol,
and fucoidan by [Ni(III)(Me6-[14]aneN4)H2O]3þ have been investigated, showing
that coordination of these alcohols to the Ni(III) complex enhances the antioxidant
response [26].

Copper(II) azo-complexes
3517
However, catalytic methods based on transition metals need stringent reaction
conditions and high cost of the ligand, or have disadvantages such as long reaction time
and high temperature. Therefore, development of practical, inexpensive, simple, and
green chemical processes for oxidations is still needed. We are interested in the use of
hydrogen peroxide, since it is cheap and sufficiently environment-friendly to be used on
a commercial scale.
Herein, we investigate the catalytic oxidation of three alcohols by
[Cu(L1)2] ꢀ H2O/H2O2 at room temperature. As a typical reaction procedure, oxidation
experiments have been performed as follows: 10 mmol of 30% H2O2 was added
dropwise to solution of the complex (0.01 mmol) and alcohol (2 mmol) in a mixture of
CH3CN and H2O and the reaction mixture was stirred for 2 h. The color of the reaction
mixture changes from brown to green, probably due to coordination of alcohol to
copper(II).
Comparing our catalyst system with other systems in the literature [24–32], it was
found that the catalyst systems, Fe(III)-Schiff-base/H6IO5 [27] and Pd(II)-bisquinoline/
O2 [29] oxidized benzyl alcohol to benzaldehyde in 80% at 70ꢂC and in 60% yield at
125ꢂC, respectively; in the latter procedure the reaction time was 27 h. After refluxing
for 4 h in CH3CN, benzyl alcohol was oxidized to benzaldehyde in 100% and 20
turnovers by [bmim]5[PW11ZnO39] ꢀ 3H2O/H2O2 (bmim ¼ 1-n-butyl-3-methylimidazo-
lium) [31]. The yields of benzaldehyde in these systems are nearly comparable with the
yield obtained by our catalyst system, [Cu(L1)2] ꢀ H2O/H2O2.
Oxidation of benzyl alcohol and piperonyl alcohol gave benzaldehyde and
piperonaldehyde in 90% and 80% yield with turnovers 180 and 160, respectively.
Oxidation of cinnamyl alcohol gave cinnamaldehyde in 75% yield and 150 turnover.
In this reaction, benzoic acid was not detected, indicating the catalyst system does not
cleave the double bond in cinnamyl alcohol. Similarly, large-scale oxidation of 20 mmol
of benzyl alcohol gave benzaldehyde in 90% yield and 180 turnovers.
The catalytic oxidation of benzyl alcohol as a model substrate has been performed in
the absence of [Cu(L1)2] ꢀ H2O and in the presence of some other co-oxidants like
NaIO4, K2S2O8, NaBrO3, and NaOCl instead of H2O2 and gave very low yields of
benzaldehyde (less than 5%). This is probably due to formation of solid precipitates
NaIO3, K2SO4, NaBr, and NaCl, which remain at the end of the reaction and make
work-up difficult. We conclude that the use of H2O2 is preferred as co-oxidant for
oxidation of these substrates in the presence of [Cu(L1)2] ꢀ H2O since it works at room
temperature and gives high yields and turnovers in short reaction times.
4. Conclusion
Five copper(II) azo-complexes have been synthesized and characterized by physico-
chemical techniques. Electronic spectra of [Cu(L)2] ꢀ H2O indicate square-planar
geometry. Cyclic voltammograms exhibit one-electron irreversible reduction wave
and one-electron irreversible oxidation wave due to Cu(II)/Cu(I) reduction and
Cu(II)/Cu(III) oxidation, respectively. The catalyst system, [Cu(L1)2] ꢀ H2O/H2O2 was
used for catalytic oxidation of benzyl alcohol, piperonyl alcohol, and cinnamyl alcohol
to their respective aldehydes. Although some transition complexes have been found in

3518
A.G.F. Shoair
recent literature [24–32], our catalyst system oxidizes these substrates in short reaction
times with good yields and turnovers at room temperature.
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