New copper(II) dimer with 3-(2-hydroxy-4-nitrophenylhydrazo)pentane-2,4-dione and its catalytic activity in cyclohexane and benzyl alcohol oxidations

Article abstract of DOI:10.1016/j.molcata.2009.11.006

3-(2-Hydroxy-4-nitrophenylhydrazo)pentane-2,4-dione (H₂L, 1) was synthesized by azocoupling of diazonium salts of 2-hydroxy-4-nitroaniline with pentane-2,4-dione and shown to exist in the hydrazone tautomeric form in the free state and in its n

Full text of DOI:10.1016/j.molcata.2009.11.006

Journal of Molecular Catalysis A: Chemical 318 (2010) 44–50
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
New copper(II) dimer with
3-(2-hydroxy-4-nitrophenylhydrazo)pentane-2,4-dione and its catalytic activity
in cyclohexane and benzyl alcohol oxidations
Kamran T. Mahmudova, Maximilian N. Kopylovicha, M. Fátima C. Guedes da Silvaa,b
,
Paweł J. Figiela, Yauhen Yu. Karabacha, Armando J.L. Pombeiroa,∗
a Centro de Química Estrutural, Complexo I, Instituto Superior Técnico, TU Lisbon, Av. Rovisco Pais, 1049-001 Lisbon, Portugal
b Universidade Lusófona de Humanidades e Tecnologias, ULHT Lisbon, Campo Grande 376, 1749-024 Lisbon, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
3-(2-Hydroxy-4-nitrophenylhydrazo)pentane-2,4-dione (H2L, 1) was synthesized by azocoupling of dia-
zonium salts of 2-hydroxy-4-nitroaniline with pentane-2,4-dione and shown to exist in the hydrazone
tautomeric form in the free state and in its new dicopper(II) complex [Cu2(H2O)2(␮-L)2] (2) whose X-ray
crystal structure was determined. Complex 2 acts as a catalyst, under mild conditions, for the peroxidative
oxidation (with H2O2) of cyclohexane to cyclohexanol, cyclohexanone and cyclohexyl hydroperoxide, in
MeCN/H2O, and for the aerobic TEMPO-mediated selective oxidation of benzylic alcohols to the corre-
sponding aldehydes, thus showing that azoderivatives of ␤-diketones can be the suitable ligands for such
types of reactions.
Received 30 September 2009
Received in revised form 6 November 2009
Accepted 9 November 2009
Available online 14 November 2009
Keywords:
3-(2-Hydroxy-4-
nitrophenylhydrazo)pentane-2,4-dione
complexes
© 2009 Elsevier B.V. All rights reserved.
Azoderivatives of ␤-diketones
Copper
X-ray structure
Oxidation of cyclohexane
Oxidation of benzyl alcohol
TEMPO
1. Introduction
forecasting the properties of this class of organonitrogen reagents
[14–16].
The coordination chemistry of azoderivatives of ␤-diketones
(ADB) is still a little explored area of research, notwithstand-
ing their interesting chemical and structural features [1]. Thus,
these compounds having several coordination modes with a
high flexibility, can exist in various tautomeric forms involving
hydrogen bonds (Scheme 1) and potentially can be used, e.g. as
materials with smart hydrogen bonding [2,3] or as bistate molec-
ular switches [4]. Complexes of these ligands or their analogs
have been applied for spectrophotometric determination of some
metal ions [5,6], as photoluminescent materials [7], light trans-
mitters [8] and sensors [9], in high-density recordable optical
storage [10] or spin-coating films [11]; they also can exhibit semi-
conducting [12] and liquid crystal [13] properties. Hence, the
knowledge of characteristics of deprotonation, tautomerization
and complexation in solution as well as the structural details of
coordination with metal ions in solid state is fundamental for
However, the available data on complexes of this interest-
ing type of ligands are still very limited and mainly concern
rather simple examples [17,18], although it was observed [19,20]
that ADB containing
a 2-hydroxyphenyl group can show a
high complexing ability in solution. Thus, in order to further
explore the coordination properties of such a class of com-
pounds, in this work we have synthesized a new ortho-hydroxy
substituted azoderivative of pentane-2,4-dione, i.e. 3-(2-hydroxy-
4-nitrophenylhydrazo)pentane-2,4-dione H2L (1, Scheme 2), inves-
tigated the complexation of this ADB in solution with copper(II)
ion and shed light on structural details and coordination modes to
Cu(II) in the solid state.
Another main aim of the current work concerns the extension
of the application of ADB ligands to a new field, i.e. oxidation catal-
ysis, and the focus on that particular metal (copper) and on 1 is
justified as follows. It is known [21,22] that particulate methane
monooxygenase (pMMO), a multicopper enzyme, catalyzes the
partial oxidation of alkanes to alcohols, and this type of action can
be mimicked by some copper complexes with N,O-type or related
ligands [23–39] that can typically act as catalysts for the peroxida-
∗
Corresponding author. Tel.: +351 21 841 9237; fax: +351 21 846 4455.
E-mail address: pombeiro@ist.utl.pt (A.J.L. Pombeiro).
1381-1169/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2009.11.006

K.T. Mahmudov et al. / Journal of Molecular Catalysis A: Chemical 318 (2010) 44–50
45
Scheme 1. Possible tautomeric equilibria for ADB.
tive oxidation of cyclohexane to cyclohexanol and cyclohexanone
[28–39]. The industrial significance of the partial oxidation of this
cycloalkane to those products is well recognized since the latter find
important applications for the synthesis of Nylon-6,6 and caprolac-
tams [24,40].
analytical Service of the Instituto Superior Técnico. To maintain
the required pH, a commercial volumetric concentrate of HCl (pH
1–2) was used. The acidity of the solutions was measured using
an I-130 potentiometer with an ESL-43-07 glass electrode and an
EVL-1M3.1 silver–silver chloride electrode. The solution of copper
ions was prepared from Cu(NO3)2·2.5H2O. In the aqueous solutions
of the salt, the concentrations of Cu(II) were determined by atomic
absorption spectroscopy. The pH-metric titration of mixtures of 1
with the copper salts was carried out in aqueous-ethanol (40/60 v/v,
%) solution with consideration of the Bates correction [47] at differ-
ent temperatures (298 0.5, 308 0.5 and 318 0.5 K) (for details,
see Supplementary information). Constant temperature was main-
tained within 0.5 K by using an ultrathermostat (Neslab 2 RTE 220).
The conductivities of solutions were measured with a KEL-1M2
conductometer. All of the synthetic work was performed in air and
at room temperature. Chromatographic analyses were undertaken
by using a Fisons Instruments GC 8000 series gas chromatograph
with a DB-624 (J&W) capillary column (FID detector) and the Jasco-
Borwin v.1.50 software. The internal standard method was used to
quantify the organic products.
Different types of N,O- or O,O-ligands (e.g. aminopolyalco-
hols, aminocarboxylates, polycarboxylates, etc.) have been used
for those catalysts but, to our knowledge, ADB ligands have not
yet been reported for such a purpose. Hence, a relevant objec-
tive of the current work was to design an ADB compound that
could act as a N,O- or N,O,O-ligand for a copper complex that
would behave as a catalyst for such an oxidation reaction, i.e. the
peroxidative oxidation of cyclohexane. Moreover, TEMPO radical
(2,2,6,6-tetramethylpiperidine-1-oxyl), combined with a suitable
metal species, is a well-known efficient mediator for aerobic oxi-
dation of alcohols [41–46], and hence it would also be interesting
to associate it to an ADB–Cu complex towards the aerobic selective
oxidation of benzylic alcohols to aldehydes. The choice of those
model reactions was justified by their importance in organic syn-
thesis and chemical industry [40].
2. Experimental
2.2. Synthesis of H2L (1) and [Cu2(H2O)2(ꢀ-L)2] (2)
2.1. Materials and instrumentation
The ligand 1 was synthesized according to the Japp–Klingemann
reaction [48] between the corresponding diazonium salt and
pentane-2,4-dione, while the synthesis of the complex 2 was per-
formed directly by mixing copper(II) nitrate Cu(NO3)2·2.5H2O with
1 in aqueous-ethanol solution in the presence of 0.01 M HCl (for
details see Supplementary information).
All of the chemicals were obtained from commercial sources
and used as received. 1H NMR and l3C NMR spectra were recorded
on a Bruker Avance II+ 300.130 (UltraShieldTM Magnet) spectrom-
eter at ambient temperature using tetramethylsilane as an internal
reference. Infrared spectra (4000–400 cm−1) were recorded on a
BIO-RAD FTS 3000MX instrument in KBr pellets. Carbon, hydrogen,
and nitrogen elemental analyses were carried out by the Micro-
2.3. X-ray structure determination
X-ray quality single crystals of 2 were grown by slow evapora-
tion at room temperature of a water–ethanol solution. They were
immersed in cryo-oil, mounted in a Nylon loop and measured at a
temperature of 150 K. Intensity data were collected using a Bruker
AXS-KAPPA APEX II diffractometer with graphite monochromated
Mo-K␣ (ꢁ 0.71073) radiation. Data were collected using omega
scans of 0.5◦ per frame and full sphere of data were obtained.
Cell parameters were retrieved using Bruker SMART software and
refined using Bruker SAINT [49] on all the observed reflections.
Absorption corrections were applied using SADABS [49]. Structures
were solved by direct methods by using the SHELXS-97 package
[50] and refined with SHELXL-97 [51]. Calculations were performed
Scheme 2. 3-(2-Hydroxy-4-nitrophenylhydrazo)pentane-2,4-dione (H2L, 1).

46
K.T. Mahmudov et al. / Journal of Molecular Catalysis A: Chemical 318 (2010) 44–50
Table 1
3. Results and discussion
Selected distances (Å) and angles (◦) of compound 2.
3.1. Synthesis and characterization of H2L (1) and
[Cu2(H2O)2(ꢀ-L)2] (2)
C2–O1
C3–N1
C4–O2
C11–N2
C16–O3
N1–N2
N2–Cu1
O2–Cu1
1.218(3)
1.354(2)
1.269(2)
1.408(2)
1.332(2)
N1–N2–Cu1
C11–N2–Cu1
C4–O2–Cu1
C16–O3–Cu1
C16–O3–Cu1i
Cu1–O3–Cu1i
O2–Cu1–N2
O2–Cu1–O3
N2–Cu1–O3
O2–Cu1–O3i
N2–Cu1–O3i
O3–Cu1–O3i
O2–Cu1–O10
N2–Cu1–O10
O3–Cu1–O10
O10–Cu1–Cu1i
O3–Cu1–Cu1i
O3–Cu1i–Cu1i
130.74(13)
112.54(12)
127.90(13)
111.49(11)
133.17(12)
99.60(6)
90.52(6)
173.21(6)
84.90(6)
100.59(6)
143.73(6)
80.40(6)
92.21(6)
119.03(5)
94.44(6)
The new 3-(2-hydroxy-4-nitrophenylhydrazo)pentane-2,4-
dione (H2L, 1) ligand was synthesized via the Japp–Klingemann
reaction [48] between the respective aromatic diazonium salt and
pentane-2,4-dione in an ethanolic solution containing sodium
acetate. The 1H NMR spectrum of 1 in dimethylsulfoxide-d6
solution at room temperature shows a signal at ı 14.17 which is
assigned to the proton of the protonated nitrogen atom adjacent
to the aryl unit ( N–NH–hydrazo form, Scheme 1) [17]. The two
methyl groups of the pentane-2,4-dione moiety yield separate sin-
glets in the proton spectrum. These data support the predominance
of the hydrazo form (Scheme 1) with a six-membered H-bonded
ring involving one of the carbonyl groups and the protonated
nitrogen moiety of the hydrazone structure. The IR spectrum of
the isolated compound shows ꢂ(OH) and ꢂ(NH) vibrations at 3461
and 3095 cm−1 respectively, while ꢂ(C O), ꢂ(C O· · ·H) and ꢂ(C N)
are observed at 1638, 1626 and 1598 cm−1, correspondingly, also
supporting the existence of the H-bonded hydrazone structure in
the solid state.
The copper(II) complex [Cu2(H2O)2(␮-L)2] (2) was obtained
upon refluxing for a short time (10 min) an ethanol/water solu-
tion of 1 and copper(II) nitrate hydrate, whereafter it was isolated
in a good yield (67%) as greenish-black crystals formed upon slow
solvent evaporation at room temperature. Its IR spectrum displays
ꢂ(OH), ꢂ(C O), ␯(C O· · ·H) and ꢂ(C N) at 3468, 1652, 1611 and
1602 cm−1, respectively, values that are significantly shifted in rela-
tion to the corresponding ones (see above) of the free ligand. No
bond assignable to ꢂ(NH) was observed, indicating that the ligand
is coordinated in the deprotonated hydrazo form.
1.292(2)
1.9139(16)
1.9062(14)
1.9424(14)
2.0502(14)
3.0503(7)
2.1463(15)
125.90(16)
123.22(17)
112.64(16)
118.34(16)
120.91(17)
O3–Cu1
O3–Cu1i
Cu1–Cu1i
O10–Cu1
N1–C3–C4
O2–C4–C3
N2–C11–C16
O3–C16–C11
N2–N1–C3
95.17(4)
41.51(4)
38.89(4)
iTranslation of symmetry code to equivalent positions: 1 − x, 1 − y, 1 − z.
using the WinGX System-Version 1.80.03 [52]. All hydrogens were
inserted in calculated positions. Least square refinements with
anisotropic thermal motion parameters for all the non-hydrogen
atoms and isotropic for the remaining atoms were employed. Crys-
tallographic and selected structural details are listed in Table S1
and Table 1, respectively. CCDC 734862 contains the supplemen-
tary crystallographic 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.
2.4. Catalytic activity studies
2.4.1. Oxidation of cyclohexane
The reaction mixtures were prepared as follows: to
3.12–20.00 ␮mol of the complex 2 contained in the reaction
flask were added 4 mL MeCN, 0.031–0.200 mmol HNO3, 1.00 mmol
C6H12 and 10.00 mmol H2O2 (30% in H2O), in this order. The
reaction mixture was stirred for 6 h at room temperature (ca.
298 K) and air atmospheric pressure, then 90 ␮L of cycloheptanone
(as internal standard) and 9.0 mL of diethyl ether (to extract the
substrate and the products from the reaction mixture) were added.
The resulting mixture was stirred for 15 min and then a sample
taken from the organic phase was analyzed by GC. Subsequently,
PPh3 was added to the final organic phase (to reduce the cyclo-
hexyl hydroperoxide, if formed), the mixture was analyzed again
to estimate the amount of cyclohexyl hydroperoxide, according to
Shul’pin’s method [53,54]. The GC analyses of the aqueous phase
showed the presence of only traces (less than 0.05%) of oxidation
products. Catalyst recycling experiments were performed accord-
ing to a previously reported technique [55]. Blank experiments
were performed and confirmed that no cyclohexane oxidation
products (or only traces, below 0.3%) were obtained in the absence
of the metal catalyst.
The X-ray structural analysis of 2 showed that the complex
is binuclear in the solid state (Fig. 1) and contains a crystal-
lographically imposed centre of inversion. It crystallizes in the
centrosymmetric triclinic space group P-1 with one molecule per
unit cell and half a molecule in the asymmetric unit. The coordi-
nation sphere of the penta-coordinated copper is best described
as a distorted square–pyramid; the copper atom is 0.337 Å above
the basal O3–O3–O2–N2 plane, shifted towards the apical position
which is occupied by a coordinated water molecule. Each cop-
per atom belongs to three different metallacycles: the Cu2O2 core,
which is the central planar ring of the molecule, and two fused
six- and five-membered metallacycles, Cu1–O2–C4–C3–N1–N2
and Cu1–N2–C11–C16–O3. In agreement, the O2–Cu1–N2 and the
N2–Cu1–O3 angles are of 90.52(6)◦ and 84.90(6)◦, respectively.
Within the Cu(␮-O)2Cu core, the Cu–Cu distance is of 3.0503(7) Å,
the Cu–O distances are of 1.9424(14) and 2.0502(14) Å and the
internal angles are of 38.89(4)◦, 41.51(4)◦ and 80.40(6)◦. The dihe-
dral angles between the central planar Cu2O2 ring and each of
the phenyl rings are of 32.25◦. The crystal lattice is stabilized by
hydrogen bonding interactions between the coordinated water
hydrogens and both the nitro and ketone groups in adjacent units
(see Fig. 1).
2.4.2. Aerobic oxidation of alcohols in aqueous media
The reactions were carried out in 50 mL round-bottom flasks
equipped with condensers under atmospheric pressure of air. Typ-
ically, to a 10 mL of 0.1 mol L−1 aqueous solution of K2CO3 were
added 3.0 mmol of alcohol, 0.03 mmol (1 mol% vs substrate) of cat-
alyst 2 and 0.15 mmol (5 mol% vs substrate) of TEMPO. The reaction
solutions in all cases were vigorously stirred using magnetic stir-
rers, and an oil bath was used to achieve the desired reaction
temperature. After the oxidation reaction, reaction mixtures were
neutralized by 1 mol L−1 HCl, and then extracted with 10 mL of
EtOAc. The organic phase was used for chromatographic analyses
using acetophenone as the internal standard.
The normal dissociation constants of the
1 ligand were
determined, by pH-metric titration, as pK1 = 6.35 0.04 and
pK2 = 10.22 0.01 (for details, see Supplementary information
Tables S1 and S5). The ability to abstract a proton from vari-
ous groups in tautomeric forms of related ligands was evaluated
previously [5,6] and it was considered that pK1 concerns the pro-
ton abstraction from an –OH group in the ortho position to the
hydrazo moiety in the aromatic ring of the molecule, while pK2
refers to the deprotonation of the hydrazone group itself ( N–NH–)
(see Scheme 1, hydrazo form). The thermodynamic parameters of
the dissociation process of 1 were determined by studying the

K.T. Mahmudov et al. / Journal of Molecular Catalysis A: Chemical 318 (2010) 44–50
47
Fig. 1. Molecular structure of 2 with atomic numbering scheme and intermolecular hydrogen bonding interactions (dotted lines). D–H· · ·A [d(D· · ·A) Å; ∠(DHA)◦]:
O10–H10A· · ·O31ii [2.842(2) Å, 170.00◦]; O10–H10B· · ·O1iii [2.705(2) Å, 178.00◦]. Translation of symmetry code to equivalent positions: (i) 1 − x, 1 − y, 1 − z; (ii) 1 + x, y,
−1 + z and (iii) x, 1 + y, z.
dependence of the dissociation constants with the temperature
(Table S5). The analysis of these values reveals that the acidity of
the compound grows with an increase of the temperature and that
the dissociation process is not spontaneous (positive ꢃG◦) and is
endothermic (positive ꢃH◦).
from Fig. S1 that the conductivities decrease after the equivalence
point, what can be associated with protonation of free L2−, HL− or
H2L [15,62].
3.2. Catalytic activity of the dicopper complex 2
Based on the experimental data and calculated values (for
details, see Supplementary information, Table S6), we can conclude
that: (i) the maximum value of the function of formation of n¯ is
two, meaning that the Cu:H2L ratio in the complexes is 2:2 [56–58].
(ii) With increase of temperature the stepwise stability constants
decrease. (iii) The metal complex titration curves are shifted to
down side of the ligand titration curves along the pH axis, indicat-
ing proton release upon complex formation (the large decrease in
pH for the metal titration curves relative to those of ligand titration
indicates the formation of strong metal complexes). (iv) The neg-
ative values of ꢃG◦ show that complex formation is spontaneous
[14–16,59,60] and is mainly contributed by the enthalpy factor (the
process is exothermic and favourable at low temperatures) [61].
(v) The positive change of the entropy contribution conceivably is
caused by the release of water molecules and formation of a com-
plex with a lower charge as compared to the initial ions, as well
as by a lower solvation of the obtained complexes [56], although it
may also depend on the ligand configuration (entropy of tautomeric
equilibrium).
The oxidations of cyclohexane and benzyl alcohol (Scheme 4)
were chosen as model reactions to test the catalytic potential of the
synthesized binuclear copper compound 2 for oxidations of alkanes
and alcohols.
This complex exhibits a good catalytic activity in the perox-
idative oxidation of cyclohexane by aqueous H2O2, under mild
conditions, to afford cyclohexanol (CyOH), cyclohexanone and
cyclohexyl hydroperoxide (CyOOH) (Scheme 4a, Tables 2 and 3).
The total yield, in a single batch, reaches 27%, for a substrate to cat-
alyst molar ratio of 50, which is comparable with those obtained
for other mono- and multinuclear copper(II) complexes with N,O-
ligands [55,63–65]. However, the turnover number (TON) values
(up to 17 moles of products per mole of catalyst, for a single batch)
of our catalyst are not so high as those achieved by other multicop-
per triethanolaminate complexes [28,55]. Nevertheless, the activity
of our catalyst is much higher than that of a simple copper(II) salt
such as Cu(NO3)2 which, under similar conditions, almost does not
show any catalytic activity (TON of 1.4, Table 2, run 9).
Both metal ion coordination and proton elimination occur for
complex formation (Scheme 3) and thus the overall conductivity
of the systems increases due to the higher proton mobility. Com-
parison of the stability constants of the complexes (Table S6b) and
conductivity studies (see Supplementary information, Fig. S1) show
that the formed complex is stable in solution. It can also be seen
The GC analyses (Table 2) were performed with and without
triphenylphosphine (reducer of CyOOH to CyOH) in order to esti-
mate the yields of cyclohexyl hydroperoxide (CyOOH) in the final
reaction mixture, according to a method developed by Shul’pin
[53,54]. Hence, CyOOH produced along the reaction is, at the end,
still present in a considerable amount (3–4% yield) that does not
Scheme 3. Complexation of copper(II) with H2L (1).

48
K.T. Mahmudov et al. / Journal of Molecular Catalysis A: Chemical 318 (2010) 44–50
Scheme 4. Oxidations of cyclohexane and benzyl alcohol using 2 as a catalyst.
Table 2
Peroxidative oxidation of cyclohexane to cyclohexanol and cyclohexanonea.
Entry
Cu catalyst
n(C6H12)/n(Cat)
Yieldb of products, %
Alcohol
TONc
Ketone
Totald
1
2
3
4
2
2
2
2
2
2
2
2
320
200
100
50
320
200
100
50
1.6
2.2
6.0
7.8
3.5
3.8
7.7
9.8
1.6
3.4
5.1
10.2
18.8
1.7
3.8
8.6
16.8
3.8
5.0
7.3
16.2
26.6
5.2
16.0
14.6
16.2
13.3
16.6
15.2
16.3
13.3
1.4
5e
6e
7e
8e
9f
7.6
16.3
26.6
5.4
Cu(NO3)2
25
a
Selected data; reaction conditions: C6H12 (1 mmol), MeCN 4 mL, n(HNO3)/n(Cat) = 10, H2O2 (10.0 mmol added as an aqueous 30% solution), 6 h reaction time, 298 K.
Moles of product/100 moles of C6H12 (alcohol = cyclohexanol and ketone = cyclohexanone).
Overall TON values (moles of products/mole of catalyst).
b
c
d
e
f
Cyclohexanol + cyclohexanone.
GC analysis performed upon addition of PPh3 (conversion of CyOOH into cyclohexanol prior to GC analyses).
For comparative purposes, Cu(NO3)2 was used as the catalyst (25 ␮mol), C6H12 (0.63 mmol), MeCN 3 mL, other conditions as in footnote a.
vary appreciably with the n(C6H12)/n(Cat) molar ratio (Table 3).
(Table 4). This occurred for either a C-centred radical scavenger
(such as CBrCl3 [66] or (CH2)3(CMe2)2NO (TEMPO) [67]) or an
O-centred radical trap (Ph2NH [66]), indicading that the reaction
proceeds mainly via radical paths involving both C- and O-centred
radicals, as for other multi-copper systems [28,30,55,63,65]. Hence,
H-abstraction from cyclohexane conceivably by the hydroxyl rad-
ical HO• (formed by metal-assisted decomposition of H2O2) leads
to the cyclohexyl radical Cy•, and, although the detailed mecha-
nistic pathway is still to be established, it can possibly proceed, as
suggested in other cases, through reaction of Cy• with O2 to give
the organoperoxyl CyOO•, or with a metal-hydroperoxo species
to yield cyclohexyl hydroperoxide (CyOOH) [31,39,68–72]. The
conversions of CyOO• and CyOOH into the products can also be
metal-assisted, conceivably involving dismutation of CyOO• to both
cyclohexanol (CyOH) and cyclohexanone with O2, or homolytic
decomposition of CyOOH to CyOO• (upon O–H bond cleavage)
and the alcoxyl CyO• (upon O–O bond rupture). CyOH could
However, the yields of both cyclohexanol and cyclohexanone are
enhanced upon decreasing that ratio (i.e. increasing the catalyst
amount), while the TON tends to a slight decrease (Table 3).
The ketone (cyclohexanone) always predominates over the alco-
hol (cyclohexanol) which, at the highest n(C6H12)/n(Cat) molar
ratio is not present in the final reaction medium in a significant
amount (Table 3). The measured amount of cyclohexanol is then
essentially derived upon decomposition of CyOOH in the GC or
upon its reduction by PPh3 (when adding this reagent to the final
reaction solution) [53,54]. The catalyst is stable under the reaction
conditions and soluble in the acetonitrile–diethyl ether mixture,
and can be isolated in a crystalline form at the end of the process
via evaporation of the organic phase. Thus, for run 4 (Table 2), ca.
51% of the catalyst was recovered and could be reused.
When the reaction was performed in the presence of a radi-
cal trap, a dramatically fall of the catalytic activity was observed
Table 3
Estimated amounts of cyclohexanol, cyclohexanone and cyclohexyl hydroperoxidea, formed upon peroxidative oxidation of cyclohexaneb.
Entry
n(C6H12)/n(Cat)
Yieldb of products, %
Alcohol
Alcohol/ketone
TON
Ketone
CyOOH
Total
1
2
3
4
320
0
1.6
3.6
8.6
3.5
2.9
3.3
4.1
5.1
7.2
16.2
26.7
0
16.3
14.4
16.2
13.4
200
100
50
0.7
4.3
5.8
0.2
0.5
0.35
16.8
a
Estimated (values before GC analysis) according to Shul’pin’s method [53,54] based on the analytical values (Table 2) measured in the absence of PPh3 and after
addition of this phosphine to the final reaction solution, considering that (i) in the GC spectrometer CyOOH decomposes to 1/2 cyclohexanol + 1/2 cyclohexanone, and (ii)
CyOOH + PPh3 → cyclohexanol + OPPh3.
b
See footnotes a, b and d of Table 2.

K.T. Mahmudov et al. / Journal of Molecular Catalysis A: Chemical 318 (2010) 44–50
49
Table 4
Effects of radical traps on the peroxidative oxidation of cyclohexane to cyclohexanol and cyclohexanonea.
Entry
n(rad. trap)/n(C6H12
)
Radical trapb
Yield of products, %
Alcohol
TONc
Yield drop owing
to radical trapd, %
Ketone
Totale
1
2
2.5
2.5
2.5
0
CBrCl3
TEMPO
Ph2NH
No radical trap
0.4
3.2
1.9
6.0
0.8
1.8
6.0
1.2
5.0
7.9
1.2
5.0
7.9
93
69
51
–
3
4f
10.2
16.2
16.2
a
Selected data; reaction conditions: C6H12 (1 mmol), catalyst precursor (10 ␮mol), MeCN 4 mL, HNO3 (0.10 mmol), H2O2 (10 mmol, added as an aqueous 30% solution), 6 h
reaction time, 25 ◦C.
b
Radical trap (2.5 mmol).
Overall TON (moles of products/mole of catalyst).
c
d
[1 − (total yield with radical trap/total yield without radical trap)] × 100.
e
Cyclohexanol + cyclohexanone.
f
Essay 3 of Table 2.
Table 5
TEMPO-mediated aerobic oxidation of selected alcohols by aira.
Entry
Cu compound
Substrate
Product
Time [h]
Yieldb [%]
1
2
3
4
5
6
7
8
9
2
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
4-Methybenzyl alcohol
Cinnamyl alcohol
1-Phenylethanol
1-Heptanol
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
4-Methylbenzaldehyde
Cinnamaldehyde
Acetophenone
1-Heptanal
6
6
45
20
99
2c
95
62
91
34
5
Cu(NO3)2
2
2
Cu(NO3)2 + H2L (1:1)
Cu(NO3)2 + H2L (1:2)
2
2
2
2
22
22
22
22
22
22
22
22
10
<1
a
Conditions: 3 mmol of the substrate, 1 mol% of 2, 5 mol% of TEMPO, 10 mL of 0.1 M K2CO3 aqueous solution, 1 atm of air, 323 K.
Yields based on GC analyses, selectivities in all cases >97%.
Reaction in the absence of TEMPO.
b
c
also be derived upon H-abstraction, by CyO•, from cyclohexane
[36,53–55,73–77].
system is that of the galactose oxidase type, known for copper
complexes with N,N- or N,O-ligands. This mechanism involves
coordination of the alcohol, followed by copper-centred oxidative
dehydrogenation of the alcohol, where the H-abstraction from the
␣-carbon atom by TEMPO is the rate-determining step of the reac-
tion [79,80,82].
The combination of the TEMPO radical with copper complexes
has been reported as an interesting and efficient catalytic system
for the selective aerobic oxidation of alcohols [42–46,78–83], par-
ticularly in aqueous media [41,84–87]. In the current work we have
tested the catalytic activity of 2 for the aerobic oxidation of alco-
hols in aqueous solutions mediated by TEMPO (Scheme 4b, Table 5).
When benzyl alcohol (3 mmol) was allowed to react with air (1 atm)
in the presence of 2 (1 mol%) and TEMPO (5 mol%) in 0.1 M K2CO3
aqueous solution, for 6 h at 323 K, benzaldehyde (45% yield) was
obtained (Table 5, entry 1); the prolongation of the reaction time
to 22 h gives 99% of benzaldehyde (entry 3). When the reaction
was performed under the same experimental conditions, but with
Cu(NO3)2 instead of 2, only 20% yield of benzaldehyde was achieved
(entry 2). Practically no oxidation of benzyl alcohol was observed
in the absence of TEMPO (2%, entry 4).
4. Final remarks
This work shows that a new azoderivative of pentane-2,4-dione,
i.e. 3-(2-hydroxy-4-nitrophenylhydrazo)pentane-2,4-dione (H2L),
displays a good coordination ability towards complex formation
with copper(II), acting as a suitable ligand to this metal ion to form
a good copper catalyst precursor for oxidations of alkanes and alco-
hols, thus extending for the first time the application of ADB ligands
to these fields of current interest.
As previously reported [86,87], copper catalysts for alcohol oxi-
dation can be prepared in situ, by mixing the ligand and the metal
salt in the reaction medium. This possibility was also examined,
using the combination of copper nitrate and the free ligand (1)
in 1:1 and 1:2 molar ratios (entries 5 and 6). These experiments
revealed nearly the same activity for the Cu/H2L 1:1 ratio (95%
of benzaldehyde yield) as for the pre-synthesized complex (99%).
2 was also found to be an active catalyst for the oxidation of a
substituted benzylic alcohol (4-methylbenzyl alcohol is oxidized
to the corresponding aldehyde with 91% yield, entry 7). Oxidation
of cinnamyl alcohol leads to cinnamaldehyde in a moderate yield
(34%, entry 8), while secondary alcohols (such a 1-phenylethanol)
were barely reactive, as well as simple aliphatic alcohols (such a
1-heptanol) (entries 9 and 10, respectively). This kind of chemose-
lectivity of the present system towards primary benzylic alcohols
shares similarities to previously reported ones [41,46,79,80,86],
and thus we can assume that the plausible mechanism for this
Based on the NMR results it was established that the free ligand
is stabilized in the hydrazo form which is also preserved, in the
solid state, in the new binuclear complex [Cu2(H2O)2(␮-L)2] (2)
with Cu(II) coordination number of 5, as shown by X-rays. It was
established that the dissociation process of H2L is unspontaneous,
endothermic and entropically unfavorable, while the complexation
of copper(II) with H2L is spontaneous, exothermic and entropically
favourable.
The newly synthesized hydroxy-hydrazo-dione (H2L) behaves
(in the basic L2− form) as a convenient N,O,O-ligand to Cu(II) to
form complex 2, that acts as a good catalyst for oxidation processes,
particularly for the peroxidative oxidation, under mild conditions,
of cyclohexane to the corresponding alcohol, ketone and organo-
hydroperoxide (total yield up to 27% in a single batch, the catalyst
being recyclable), as well as for selective oxidation of benzyl alco-
hol to benzaldehyde (up to 99% yield of benzaldehyde with >99%
selectivity).

50
K.T. Mahmudov et al. / Journal of Molecular Catalysis A: Chemical 318 (2010) 44–50
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oxidation reactions, namely of the difficult to oxidize alkane sub-
strates, a still unexplored (but promising as shown by this work)
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and transition metal ions; other catalytic applications of this type
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Acknowledgements
This work has been partially supported by the Foundation for
Science and Technology (FCT), Portugal, and its PPCDT (FEDER
funded) and “Science 2007” programs. K.T.M., P.J.F. and Y.Yu.K.
express gratitude to the FCT for their post-doc and doctoral fel-
lowships. The authors gratefully acknowledge the Portuguese NMR
Network (IST-UTL Center) for providing access to the NMR facility.
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Appendix A. Supplementary data
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the online version, at doi:10.1016/j.molcata.2009.11.006.
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