Copper(ii) complexes with a new carboxylic-functionalized arylhydrazone of β-diketone as effective catalysts for acid-free oxidations

Article abstract of DOI:10.1039/c2nj40210f

The aquasoluble [Cu(H₂O)((CH₃)₂NCHO)(HL)] (2) and [Cu₂(CH₃OH)₂(μ-HL)₂] (3) Cu^II complexes were prepared by reaction of Cu^II nitrate hydrate with the new 3-(2-hydroxy-4-carboxyphenylhydrazone)pentane-2,4-dione (H₃L, 1), in the presence (for 2) or absence (for 3) of (n-C ₄H₉)₂SnO, and characterized by elemental analysis, IR spectroscopy and X-ray single crystal diffraction. Magnetic susceptibility measurements, in compound 3, reveal strong antiferromagnetic coupling between the Cu^II ions through the μ₂- phenoxido-O atoms, J = -203(1) cm^-1. Complexes 2 and 3 act as catalyst precursors for the acid-free peroxidative oxidation of cyclohexane to the mixture of cyclohexyl hydroperoxide (primary product), cyclohexanol and cyclohexanone (TONs and yields up to 163 and 14.4%, respectively), as well as for the selective aerobic oxidation of benzyl alcohols to benzaldehydes in aqueous solution, mediated by a TEMPO radical, under mild conditions (TONs and yields up to 390 and 94%, respectively). In the alkane oxidations, 2 and 3 appear to behave as dual role catalysts combining, in one molecule, an active metal centre and an acidic promoting group, to provide a high activity of the system even without any acid promoter.

Full text of DOI:10.1039/c2nj40210f

View Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
NJC
Cite this: New J. Chem., 2012, 36, 1646–1654
www.rsc.org/njc
PAPER
Copper(II) complexes with a new carboxylic-functionalized arylhydrazone
of b-diketone as effective catalysts for acid-free oxidationsw
Maximilian N. Kopylovich,a Malgorzata J. Gajewska,a Kamran T. Mahmudov,ab
a
a
ac
´
Marina V. Kirillova, Pawe$ J. Figiel, M. Fatima C. Guedes da Silva,
´
Beatriz Gil-Hernandez, Joaquin Sanchiz and Armando J. L. Pombeiro*
d
d
a
Received (in Victoria, Australia) 16th March 2012, Accepted 23rd May 2012
DOI: 10.1039/c2nj40210f
The aquasoluble [Cu(H2O)((CH3)2NCHO)(HL)] (2) and [Cu2(CH3OH)2(m-HL)2] (3)
CuII complexes were prepared by reaction of CuII nitrate hydrate with the new 3-(2-hydroxy-4-
carboxyphenylhydrazone)pentane-2,4-dione (H3L, 1), in the presence (for 2) or absence (for 3) of
(n-C4H9)2SnO, and characterized by elemental analysis, IR spectroscopy and X-ray single crystal
diffraction. Magnetic susceptibility measurements, in compound 3, reveal strong antiferromagnetic
coupling between the CuII ions through the m2-phenoxido-O atoms, J = ꢀ203(1) cmꢀ1
.
Complexes 2 and 3 act as catalyst precursors for the acid-free peroxidative oxidation of
cyclohexane to the mixture of cyclohexyl hydroperoxide (primary product), cyclohexanol and
cyclohexanone (TONs and yields up to 163 and 14.4%, respectively), as well as for the selective
aerobic oxidation of benzyl alcohols to benzaldehydes in aqueous solution, mediated by a
TEMPO radical, under mild conditions (TONs and yields up to 390 and 94%, respectively).
In the alkane oxidations, 2 and 3 appear to behave as ‘‘dual role catalysts’’ combining, in
one molecule, an active metal centre and an acidic promoting group, to provide a high
activity of the system even without any acid promoter.
within the AHBD complexes could be expected to promote
Introduction
catalytic reactions which require specific acidic conditions
(see below). On the other hand, the carboxylic groups can
significantly improve the solubility of complexes in water
media, a property of a great significance for the development
of green homogeneous metal catalysis.3 Reactions carried out
in water can also provide the opportunity to finely tune the
pH, which can lead to changes in reactivity and selectivity.
Thus, it would be attractive to prepare new a AHBD bearing
those groups (Scheme 1), synthesize derived water-soluble
complexes and study their catalytic and magnetic properties.
It is known that CuII is a good candidate for coordination
with AHBD, as well as for magnetic and catalytic studies.1,2
Therefore, we have chosen this metal ion for the current study,
Arylhydrazones of b-diketones (AHBD, Scheme 1 for the particular
example of this study, see below) are compounds which bear
diketo- and hydrazone-moieties in one molecule. One can
expect that AHBD would combine the advantages of both
the above-mentioned types of species, thus being attractive
candidates for a wide range of applications and further
chemical transformations.1 Complexes of AHBD have
recently been intensively investigated regarding their structural
properties and applications, e.g., in catalysis.2 The addition to
AHBD of groups other than diketo- and hydrazone-moieties
(e.g., hydroxyl or carboxyl moieties) even further expands
their coordination ability with possible formation of diverse
types of complexes. Moreover, acidic (e.g., carboxylic) groups
a
´
Centro de Quımica Estrutural, Complexo I, Instituto Superior
Tecnico, Technical University of Lisbon, Av. Rovisco Pais,
´
1049-001 Lisbon, Portugal. E-mail: pombeiro@ist.utl.pt;
Fax: +351-21-846-4455; Tel: +351 218419237
b Baku State University, Department of Chemistry, Z. Xalilov Str. 23,
Az 1148 Baku, Azerbaijan
c Universidade Luso´fona de Humanidades e Tecnologias, ULHT
Lisbon, Campo Grande 376, 1749-024 Lisbon, Portugal
d Departamento de Quimica Inorganica, Universidad de La Laguna,
38200 La Laguna, Tenerife, Spain
w Electronic supplementary information (ESI) available. CCDC
866372 (for 2) and 853676 (for 3). For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c2nj40210f
Scheme 1 3-(2-Hydroxy-4-carboxyphenylhydrazone)pentane-2,4-dione
(H3L, 1).
c
1646 New J. Chem., 2012, 36, 1646–1654
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012

View Online
but employed a synthetic strategy different from that described
before,2 hoping to obtain new complexes with interesting
properties.
shows two separate singlets for the methyl groups of the pentane-
2,4-dione moiety at d 26.49 and 31.27. These data are consistent
with the predominance of the hydrazone form with a six-
membered H-bonded ring involving one of the carbonyl groups
and the QN–NH– of the hydrazone structure (Scheme 1). The
IR spectrum of 1 shows n(OH) and n(NH) vibrations at 3468 and
3083 cmꢀ1, respectively, while n(CQO), n(CQOꢁꢁꢁH), d(OH)
and n(CQN) are observed at 1731 and 1669, 1635, 1631 and
1605 cmꢀ1, correspondingly, supporting the existence of the
H-bonded hydrazone structure in the solid state.
One of the known applications of copper complexes concerns
their use as catalysts in various oxidation reactions, whereas
cyclohexane oxidation is an important and widely studied catalytic
transformation, which is carried out by industrial chemical
processes but with low conversions and poor selectivities.4 There-
fore, it is of great practical interest to develop a more efficient, as
well as easily synthesizable, reusable and environment-friendly
catalyst for this oxidation process.
Upon attempting to prepare a heteronuclear Cu–Sn complex by
reaction of CuII nitrate with 1 and dibutyltin oxide (1:1:1 molar
ratios) in dry methanol, the brown solid precipitated during the
reaction was recrystallized from N,N-dimethylformamide (DMF)
to give [Cu(H2O)((CH3)2NCHO)(HL)] (2) (Fig. 1, Scheme 2, for
details see Experimental part). Suitable crystals of 2 for X-ray
structural analysis were obtained upon concentration of a DMF
solution of the crude precipitate.
The selective oxidation of alcohols to the corresponding
aldehydes or ketones is another fundamental transformation
in both laboratory and industrial syntheses, being frequently
used e.g. for the preparation of pharmaceuticals, flavours and
fragrances.5 In this case, the development of new catalytic
oxidation systems with a high selectivity using cheap and green
oxidants (e.g., molecular oxygen) is also very attractive. Hence,
we have selected these two oxidation processes for consideration
in the current study.
The structure of complex 2 shows the Cu atom in a square
pyramidal environment formed, in the basal plane, by two O
and one N atoms from HL2ꢀ [O1, O3 and N1] and the aqua
O10 atom; the apical position is taken by O6 of a formamide
ligand. However, the linkage of neighbouring units via inter-
molecular contacts gives rise to the formation of dimeric
copper aggregates (Fig. 1b). In such a situation, each copper
metal can be considered as taking up an octahedral geometry
where the intermolecular Cu1ꢁ ꢁ ꢁO2 bond distances assume
values of 2.9001(19) A, which are considerably longer than the
other metal–oxygen bond distances of the complex (in the
1.9300(14)–2.5260(16) A range). In these units the Cu atoms
are 6.7305(4) A apart. Additionally, each monomer of 2 is
associated with neighbouring units via intermolecular hydrogen
interactions involving the coordinated water molecules as H-donor
atoms, the O6 from dmf as an acceptor, and the carboxylate
group as both donor and acceptor (Fig. 1a). When the phenolate
O1 atoms act as acceptors, planar (Cu1–O10–H10A–O1)2 rings
are generated, leading to a CuꢁꢁꢁCu distance of 5.0748(3) A, the
minimum inter-metal distance that could be found in 2; an
inversion centre lies in the middle of the thus engendered eight-
membered ring. The copper(II) cation belongs to fused six- and
five-membered metallacycles. Due to the coordination of O3
and to the hydrogen bonding it is involved in, the C11–O3
bond length is longer than that of the uncoordinated C9–O2
(1.268(2) A and 1.222(3) A, respectively). Interestingly, the
metal–O bond distances in both metallacycles are quite similar
[1.9370(14) and 1.9300(14) A] and considerably shorter than
that involving the aqua ligand (2.5260(16) A).
The CuII complex 3 (Scheme 2) was prepared upon refluxing
for a short time (5 min) a methanol solution of 1 and CuII
nitrate hydrate, whereafter it was isolated in a good yield
(47%) as dark brown crystals formed upon slow solvent
evaporation at room temperature. The crystal structure shows
that 3 consists of binuclear [Cu2(CH3OH)2(m-HL)2] molecules.
The copper atoms are bridged by the phenolate oxygen atoms
of the chelating ligands HL2ꢀ thus giving rise to a Cu2O2 core
with the Cuꢁ ꢁ ꢁCu separation of 3.0137(8) A; an inversion
centre lies in the middle of this core. The geometry around
the copper atoms is that of basal edge-sharing highly distorted
square pyramids (t5 = 0.40), the copper atom being shifted
Recent years have also witnessed much interest in molecule-
based multifunctional materials, including photomagnetic and
magneto-optical materials, and magnetic conductors.5a Great
efforts are currently devoted to the development of organic–
inorganic hybrid building blocks, in which long-range magnetic
coupling involving p–d interactions is expected.6b,c Although the
magnetic properties of b-diketonates of transition metals have
been investigated,6d,e to our knowledge there are no reported
data about magnetic properties of AHBD complexes.
Therefore, the main objectives of this work are as follows: (i)
to synthesize a new AHBD bearing both a carboxylic and an
hydroxo group, i.e., with an acidic function and a chelating
ability, namely 3-(2-hydroxy-4-carboxyphenylhydrazone)pentane-
2,4-dione (H3L, 1, Scheme 1); (ii) to prepare water soluble CuII
complexes with this species; (iii) to study the magnetic properties
of the synthesized CuII–AHBD complexes; (iv) to evaluate the
catalytic activity of such complexes for the mild oxidations of
cyclohexane to cyclohexanol and cyclohexanone, and of benzyl
alcohols to the corresponding aldehydes.
Results and discussion
Syntheses and characterization of H3L (1) and its
[Cu(H2O)((CH3)2NCHO)(HL)] (2) and [Cu2(CH3OH)2(l-HL)2]
(3) complexes
3-(2-Hydroxy-4-carboxyphenylhydrazone)pentane-2,4-dione
(H3L, 1) was synthesized via the Japp–Klingemann reaction2
between the respective aromatic diazonium salt and pentane-
2,4-dione in water solution containing sodium hydroxide, and
characterized by IR and NMR spectroscopies and elemental
analysis. It is known that AHBD can theoretically exist in
three tautomeric forms (enol-azo, keto-azo and hydrazone),1,2
but experimentally it has been observed that all the studied
arylhydrazones prepared from symmetric b-diketones exist in
solution and in the solid state only in the hydrazone form.2 In
agreement, the 1H and 13C-NMR spectra of 1 support the hydra-
zone formulation.2 The hydrazo QN–NH– signal is observed at
1
d 14.56 in the H-NMR spectrum, while the 13C-NMR spectrum
c
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012
New J. Chem., 2012, 36, 1646–1654 1647

View Online
Fig. 1 ORTEP diagrams of complex 2 (a) with atomic numbering scheme and showing the hydrogen bonding interactions, and (b) showing the
linkage of neighbouring molecules via intermolecular Cuꢁ ꢁ ꢁO contact interactions generating octahedral dinuclear aggregates. Ellipsoids are drawn
at 50% probability. Selected bond distances (A) and angles (1): O1–C1 1.342(2), O3–C11 1.268(2), N1–N2 1.289(2), N2–C10 1.353(3),
Cu1–O1 1.9370(14), Cu1–O3 1.9300(14), Cu1–O6 2.5260(16), Cu1–O10 1.9712(14), Cu1–N1 1.9153(16), Cu1–O2iv 2.9001(19); O1–Cu1–O3
173.42(6), C12–N3–C14 121.8(2), O1–Cu1–O6 98.12(6), O1–Cu1–O10 96.35(6), O1–Cu1–N1 85.83(6), O3–Cu1–O6 86.77(6), O3–Cu1–O10
88.45(6), O3–Cu1–N1 89.13(6), O6–Cu1–O10 84.59(6), O6–Cu1–N1 98.29(7), O10–Cu1–N1 176.12(6), O6–Cu1–O2iv 174.04(5). Selected hydrogen
bonds [d(Hꢁ ꢁ ꢁA), +(DHA)]: O5–H5ꢁ ꢁ ꢁO6 2.666(2), 1741; O10–H10Aꢁ ꢁ ꢁO1 2.636(2), 173(3)1; O10–H10Bꢁ ꢁ ꢁO4 2.838(2), 176(2)1. Symmetry
operations used to generate equivalent atoms: (i) ꢀ1 + x, 1 + y, z; (ii) 1 + x, ꢀ1 + y, z; (iii) ꢀx, 1 ꢀ y, 1 ꢀ z; (iv) 1 ꢀ x, 1 ꢀ y, ꢀz.
Scheme 2 Schematic representations of complexes 2 and 3.
0.284 A above the fitted basal O1–O1i–O2–N1 plane towards the
apical position occupied by a coordinated methanol molecule
(Fig. 2). While the anionic HL2ꢀ ligands are nearly planar,
they make angles of ca. 25.471 with the central Cu2O2 core. The
b-diketone fragments act as O,N,O donors to the CuII atoms, with
the phenolate oxygen bridging between the two metal cations.
The copper atoms belong 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–C9–
C10–N2–N1 and Cu1–N1–C2–C1–O1, with O2–Cu1–N1
and O1–Cu1–N1 angles of 90.96(12)1 and 85.24(12)1, respec-
tively. The [Cu2(CH3OH)2(m-HL)2] units form an extended
array by hydrogen bonding contacts (Fig. 2) leading to the
formation of a 2D network: one network flows along the [100]
direction by means of the interactions between carboxyl groups of
vicinal units, and another network runs along the [0-1-1] path and
results from the connections between the methanol hydroxyls and
the ketone groups. The elemental analysis supports the formula-
tion, while the IR spectrum of 3 displays n(OH), n(CQO), d(OH)
and n(CQN) at 3448, 1672 and 1638, 1629 and 1594 cmꢀ1
,
respectively, values that are significantly shifted in relation to the
corresponding ones of the free ligand.
Both complexes 2 and 3 are soluble in water and in
acetonitrile, features that are of significance towards their
eventual application in catalysis in water and in aqueous
NCMe media (see below).
c
1648 New J. Chem., 2012, 36, 1646–1654
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012

View Online
Fig. 3 Temperature dependence of the molar magnetic susceptibility,
M, of compound 3. The solid line is the best fit to the Bleaney–Bowers
expression, see text.
w
The magnetic susceptibility data show a good agreement with
the dinuclear model (Fig. 3).
The nature and the strength of the magnetic exchange
coupling in alkoxide and hydroxide bridged CuII dinuclear
complexes are mainly affected by the Cu–O–Cu bond angles
(Y). Generally antiferromagnetic coupling is observed for
Y 4 981, whereas ferromagnetic couplings are observed for
Y o 981.7b However, other parameters such as the trigonal
distortion of the CuII environment, the out-of-plane shift of
the carbon atom of the alkoxide bridge or the hinge distortion
of the Cu2O4 core modulate the coupling7c and ferromagnetic
couplings can be observed for slightly larger Y angles.7d
Compound 3 conforms this correlation displaying strong
antiferromagnetic couplings for large Cu–O–Cu angles and
ferromagnetic couplings for angles smaller than 99.81.
Fig. 2 ORTEP diagram of complex 3, with atomic numbering
scheme and showing the hydrogen bonding interactions. Ellipsoids
are drawn at 50% probability. Selected bond distances (A) and angles
(1): N1–N2 1.277(4), N1–Cu1 1.912(3), O1–Cu1 1.939(2), O1–Cu1i
2.007(3), O2–Cu1 1.899(3), O20–Cu1 2.204(3), Cu1–O1i 2.007(3),
Cu1–Cu1i 3.0137(10); O2–Cu1–N1 90.96(12), O2–Cu1–O1
173.52(12), N1–Cu1–O1 85.24(12), O2–Cu1–O1i 100.44(11),
N1–Cu1–O1i 149.36(13), O1–Cu1–O1i 80.45(12), O2–Cu1–O20
95.34(11), N1–Cu1–O20 113.43(13), O1–Cu1–O20 90.99(11),
O1–Cu1–O20i 93.88(12), O2–Cu1–Cu1i 139.47(9), N1–Cu1–Cu1i
121.31(9). Selected hydrogen bonds [d(Hꢁ ꢁ ꢁA), +(DHA)]:
O5–H5ꢁ ꢁ ꢁO4 2.618(5), 168.211; O20–H20Oꢁ ꢁ ꢁO3 2.691(5), 165.591.
Symmetry operations used to generate equivalent atoms: (i) ꢀx, ꢀy,
1 ꢀ z; (ii) ꢀ1 ꢀ x, ꢀy, 1 ꢀ z; (iii) 1 ꢀ x, ꢀy, 1 ꢀ z; (iv) ꢀ1 ꢀ x, 1 ꢀ y, 2
ꢀ z; (v) ꢀ1 + x, y, z; (vi) 1 + x, y, z; (vii) 1 + x, ꢀ1 + y, ꢀ1 + z.
Catalytic activity of the copper complexes
Mild peroxidative oxidation of alkanes. Following the interest
of our group in the Cu-catalyzed functionalization of alkanes
under mild conditions,2,8 we have tested in detail the catalytic
behaviour of complexes 2 and 3 in the oxidation of cyclohexane
(CyH, model alkane substrate, Scheme 3). A couple of other
alkanes (methylcyclohexane and n-heptane) were also tested, in
less detail, for particular selectivity studies (see below). The
catalytic tests were undertaken by reacting, at 50 1C in MeCN/
H2O medium, an alkane with aqueous hydrogen peroxide
(50%) in the presence of 2 or 3, either in the absence or in
the presence of an acid promoter (trifluoroacetic acid, TFA). In
some cases (with CyH) the reaction mixture was analyzed twice,
before and after addition of PPh3, and the observed difference
in alcohol/ketone compositions allowed us to infer the presence
of cyclohexyl hydroperoxide as the primary oxidation product.9
But for the determination of the final oxygenate amounts, only
data obtained after treatment of the reaction sample with PPh3
(in order to reduce CyOOH to CyOH) were usually used.
Both complexes 2 and 3 catalyze the oxidation of cyclohexane
(CyH) to cyclohexyl hydroperoxide (CyOOH) as the primary
product, which is converted in the course of the reaction to more
stable cyclohexanol and cyclohexanone. One can assume that the
predominant formation of cyclohexanol over cyclohexanone
after the treatment of the reaction mixture with PPh3 indicates9
Magnetic properties
The temperature dependence of the molar magnetic suscepti-
bility, wM, for compound 3 is shown in Fig. 3. The shape of the
plot is typical for a CuII dinuclear complex displaying a rather
strong intramolecular antiferromagnetic coupling. Having in
mind the dinuclear structure of compound 3, the magnetic
data were analyzed by means of the Bleaney–Bowers expression
for two interacting spin doublets, eqn (1) (where J is the
intramolecular magnetic exchange coupling constant, g is the
Lande factor, b is the Bohr magneton and k is the Boltzman’s
´
constant), which considers the isotropic spin Hamiltonian
H = ꢀJ(S1S2).7a
ꢀ
ꢁ
Nb2g2
3kT 3 þ expðꢀJ=kTÞ
6
ð1Þ
wM
¼
The best fit parameters to eqn (1) are g = 2.09(1), J =
ꢀ203(1) cmꢀ1 and R = 7.27ꢂ10ꢀ6. R is the agreement factor
defined as Si[(wM)obs(i) – (wM)calc(i)]2/Si[(wM)obs(i)]2. Paramagnetic
impurities as mononuclear species are found to be r = 0.04.
c
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012
New J. Chem., 2012, 36, 1646–1654 1649

View Online
Scheme 3 Oxidation of cyclohexane catalyzed by 2 and 3.
the formation of CyOOH (Table 1). The catalyst 2 was initially Nevertheless, we have tested the influence of a trifluoroace-
tested in a lower loading (30 ꢂ 10ꢀ5 mol Lꢀ1) which allowed
reaching higher turnover numbers (TONs) (Table 1). These
gradually increase from 8 to the maximum of 163 (corre-
sponding to 10.6% yield) on prolonging the reaction time
from 15 to 190 min (Table 1, entries 1–6). It is important to
highlight that these levels of activity have been reached in the
absence of any acid promoter, thus constituting a remarkable
feature of the present catalytic system. Such a behaviour of 2
contrasts to many of the previously reported Cu-containing
catalytic systems,8,9 which exhibit a good activity towards
the oxidation of alkanes by H2O2 only in the presence of an
acid additive. However, such Cu-catalyzed oxidations with
tert-butyl hydroperoxide (TBHP) usually do not require the
presence of an acid.8d
tic acid (TFA) additive on the oxidation of cyclohexane.
Interestingly, in the presence of 2 the addition of an excess
of TFA accelerates to some extent the oxidation, leading to
higher TONs within the first 60 min of the reaction (Table 1,
entries 7–9 vs. 1–3). However, the final TON of 115 (entry 12)
achieved in the presence of an acid promoter is inferior to that
of 163 (entry 6) obtained in the acid-free system. A similar
behaviour was observed when a higher amount of catalyst 2
(Table 1, entries 13–20, Fig. S1, ESIw) was used, with
maximum TONs of 21 and 18 (or 13.4 and 11.8% yields) in
acid-free and TFA-promoted systems, respectively (entries 16
and 20). The fact that 2 does not require an acid additive
conceivably concerns the presence of uncoordinated acidic
COOH groups of the AHBD ligands within the molecule of
Table 1 Oxidation of cyclohexane to cyclohexanol and cyclohexanone catalyzed by complexes 2 and 3a
Products, mmol Lꢀ1b (after PPh3)
Catalyst
amount, mol Lꢀ1
Acid additive
(TFA), mol Lꢀ1
Total TONc
Entry
Time (min)
[total yield, %]d
Cyclo-hexanol
Cyclo-hexanone
Total
Catalyst 2
1
2
3
4
5
6
7
8
30 ꢂ 10ꢀ5
—
15
30
60
90
140
190
15
30
60
1.7
3.5
5.7
18.4
28.9
42.3
3.4
4.0
8.8
16.4
22.3
31.3
21.7
24.6
42.3
44.2
0.6
0.9
2.6
3.2
7.3
5.8
0.9
1.1
1.4
1.5
3.7
3.2
3.2
4.7
8.1
17.6
2.3
4.4
8.3
21.6
36.2
48.1
4.3
8
15
28
72
121
163 [10.6]
14
17
34
60
87
115 [7.5]
8
10
17
21 [13.4]
30 ꢂ 10ꢀ5
0.0035
5.1
9
10.2
17.9
26.0
34.5
24.9
29.3
50.4
61.8
10
11
12
13
14
15
16
90
140
190
60
120
240
330
30 ꢂ 10ꢀ4e
30 ꢂ 10ꢀ4e
5 ꢂ 10ꢀ4e
5 ꢂ 10ꢀ4e
—
17
18
19
20
0.01
—
60
120
240
330
40.2
44.1
49.9
33.7
10.1
9.3
9.5
20.7
50.3
53.4
59.4
54.4
17
18
20
18 [11.8]
Catalyst 3
21
22
23
24
25
26
27
28
29
30
20
40
60
120
270
20
40
65
10.1
19.0
27.1
48.9
49.0
6.3
23.8
34.5
33.3
35.9
0.0
1.9
8.4
9.0
17.3
1.1
1.1
6.8
9.7
9.5
10.1
20.9
35.5
57.9
66.3
7.4
24.9
41.3
43.0
45.4
20
42
71
116
133 [14.4]
15
50
83
86
0.01
120
270
91 [9.9]
a
Reaction conditions, unless stated otherwise: [cyclohexane]0 = 0.46 mol Lꢀ1, [total H2O2]0 = 2.2 mol Lꢀ1 (50% aqueous), [total H2O]0
=
6.4 mol Lꢀ1, MeCN up to 5 mL total volume, 50 1C. TFA = CF3COOH. Based on GC analysis, after treatment with PPh3. Total TON
corresponds to moles of products per mol of catalyst. Values in square brackets correspond to total yields (moles of products/100 moles of
cyclohexane). [Total H2O]0 = 4.2 mol Lꢀ1
b
c
d
e
.
c
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012
1650 New J. Chem., 2012, 36, 1646–1654

View Online
Table 2 Selectivity parameters in the oxidation of n-heptane and methylcyclohexane (MCH)a
Regioselectivity (n-C7H16
C(1) : C(2) : C(3) : C(4)
)
Bond selectivity
(MCH) 11 : 21 : 31
Entry
Catalytic system
1
2
3
2/H2O2/CH3CN
2/TFA/H2O2/CH3CN
3/H2O2/CH3CN
3/TFA/H2O2/CH3CN
Cu4/TFA/H2O2/CH3CN
Cu4/t-BuOOH/CH3CN
1 : 5 : 5 : 5
1 : 6 : 6 : 5
1 : 5 : 5 : 5
1 : 6 : 6 : 5
1 : 8 : 7 : 5
1 : 34 : 23 : 21
1 : 2 : 6
1 : 2 : 7
1 : 3 : 7
1 : 3 : 9
1 : 5 : 14
1 : 16 : 128
4
5b
6b
a
Reaction conditions: catalyst [2]0 or [3]0 = 30 or 5 ꢂ 10ꢀ4 mol Lꢀ1, [TFA]0 = 0.01 mol Lꢀ1, [substrate]0 = 0.46 mol Lꢀ1, [H2O2]0 = 2.2 mol Lꢀ1
(50% aqueous), [total H2O]0 = 4.2 mol Lꢀ1, MeCN up to 5 mL total volume, 50 1C. All parameters were measured after reduction of the reaction
mixtures with PPh3 before GC analysis and calculated based on the ratios of isomeric alcohols. The calculated parameters were normalized,
i.e., recalculated taking into account the number of H atoms at each carbon. Parameters C(1) : C(2) : C(3) : C(4) are relative reactivities of
hydrogen atoms at carbons 1, 2, 3 and 4 of the n-heptane chain. Parameters 11 : 21 : 31 are relative normalized reactivities of the hydrogen atoms at
b
primary, secondary and tertiary carbons of methylcyclohexane. Compound Cu4 is the [OCu4{N(CH2CH2O)3}4(BOH)4][BF4]2 complex; for this
system see ref. 8a, 9d.
the complex which thus can be considered as a ‘‘dual role
catalyst’’. Considering the prior background8b,f,9d on the use
of acid co-catalysts in Cu-catalyzed alkane oxidations, one
should expect that the role of an acid additive (TFA) or
uncoordinated COOH groups of the AHBD ligands consists
in participating in proton-transfer steps, activating the catalyst
via protonation of ligands, promoting unsaturation of the
copper centres and enhancing the oxidizing properties of metal
complex intermediates and hydrogen peroxide.
present alkane oxidations proceed via a free radical mechanism8,9
through the generation of hydroxyl radicals as active oxidizing
species.
In summary, the obtained results indicate that in the alkane
oxidations with hydrogen peroxide both complexes 2 and 3
behave as ‘‘dual role catalysts’’ which combine the active metal
centre with acidic promoting groups in the molecule. This
allows achieving rather high values of TONs (up to 163) even
without any acid promoter, what constitutes a remarkable and
unusual feature of the present catalytic systems.
The dicopper(II) complex 3 also catalyzes the oxidation of
cyclohexane (Table 1, entries 21–30, Fig. S2, ESIw), showing
(for the complex concentration of 5 ꢂ 10ꢀ4 mol Lꢀ1) a similar
catalytic performance to that of the monocopper(II) compound 2,
leading to maximum TON and yield values of 133 and 14.4%,
respectively, in the acid-free system. This apparently suggests the
involvement of analogous types of catalytically active species.
Such a possibility was also indicated by the study of the oxidation
of n-heptane and methylcyclohexane (MCH), with the estimate
of the regioselectivity C(1) : C(2) : C(3) : C(4) and bond selectivity
11 :21 :31 parameters for their respective oxidations (Table 2).
Both catalysts 2 and 3 appear to show rather similar regio- and
bond selectivity parameters, either in the absence or presence of
TFA. Their typical values range from 1 : 5 : 5 : 5 to 1 : 6 : 6 : 5 for
regioselectivity, and from 1 : 2 : 6 to 1 : 3 : 9 for bond selectivity
(Table 2, entries 1–4). These rather low selectivity parameters
are well comparable to those previously reported for other
multicopper(II) catalytic systems (e.g. Table 2, entry 5) that
operate with participation of free hydroxyl radicals.9 In fact,
alkane oxidations that do not involve hydroxyl radicals
are expected to show rather distinct selectivity profiles (for
comparison, see entry 6, Table 2). Thus, based on the literature
background8,9 and the above mentioned features regarding (i) the
formation of CyOOH as a primary intermediate product and
(ii) the observed selectivity parameters, we can conclude that the
Aerobic oxidation of alcohols in aqueous solutions. We have
recently reported the activity of some CuII complexes with
N,N- and N,O-ligands as catalysts for the aerobic oxidation of
benzylic alcohols mediated by TEMPO in aqueous media.2a,5,10,11
Therefore, as a continuation of these studies, we have now applied
the aquasoluble complexes 2 and 3 as catalysts for this reaction
(Scheme 4, Table 3), and used previously optimized reaction
conditions,2a although now with significantly lower amounts
of catalysts.
Thus, aerobic oxidation of benzyl alcohol (1.5 mmol) in
basic (0.1 M K2CO3) aqueous solution, in the presence of a
catalytic amount of complex 2 (0.015 mmol, 1 mol% vs.
substrate) and TEMPO (0.075 mmol, 5 mol% vs. substrate)
results in 20% yield of benzaldehyde after 4 h of reaction
(Table 3, run 1). The reaction under the same conditions but
with the dimeric complex 3 results in 9% of the product, the
triple increase of the amount of 3 does not rise the yield
substantially (runs 2 and 3). Prolongation of the reaction time
to 22 h resulted in a very good yield of benzaldehyde in the case
of 2 (93%, with TON of ca. 93, run 4), but in a lower one for 3
(62%, run 5), while a triple amount of 3 increases slightly the
yield to 65% (run 6). Reaction in the presence of 2 but at the
lower temperature of 60 1C results in a lower yield (77%, run 7).
Scheme 4 Aerobic oxidation of benzylic alcohols in water.
c
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012
New J. Chem., 2012, 36, 1646–1654 1651

View Online
Table 3 Aerobic oxidation of selected alcohols to the corresponding aldehydesa
Run Complex Substrate-to-catalyst ratio Substrate
Product
Temperature [1C] Time [h] Yieldb [%] TON
1
2
3
4
5
6
7
2
3
3
2
3
3
2
2
2
2
2
2
2
2
100
600
200
100
600
200
100
100
100
100
100
100
100
100
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
2-Me-benzyl alcohol
3-Me-benzyl alcohol
4-Me-benzyl alcohol
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
80
80
80
80
80
80
60
80
80
80
80
80
4
4
4
20
9
20
54
26
93
372
390
77
3
13
93
62
65
77
3
22
22
22
22
22
22
22
22
22
22
22
8c
9d
10
11
12
13
14
2
2
2-Me-benzaldehyde
3-Me-benzaldehyde
4-Me-benzaldehyde
43
72
82
94
70
43
72
82
94
70
4-MeO-benzyl alcohol 4-MeO-benzaldehyde 80
4-Cl-benzyl alcohol 4-Cl-benzaldehyde 80
a
Conditions: 1.5 mmol of the substrate, 0.0025–0.015 mmol (0.17–1.0 mol%) of catalyst and 0.075 mmol (5 mol%) of TEMPO in 5 mL of 0.1 mol Lꢀ1
b
c
d
K2CO3 aqueous solution, 1 atm air. Yields based on GC analyses, selectivity in all cases 499%. Reaction without K2CO3. Reaction
without TEMPO.
The presence of a base (here K2CO3), as well as of a
TEMPO radical, is crucial for the efficiency of this process
(runs 8 and 9). Substituted benzylic alcohols can also be
converted to the corresponding aldehydes with rather good
yields. Thus, oxidation of 1-, 2- or 3-methylbenzyl alcohols
results in 43, 72 and 82% of the corresponding aldehydes,
revealing a hampering effect of the substituents on ortho- and
to lesser extent on meta-positions (runs 10–12). 4-Methoxy-
benzyl alcohol is oxidized to the corresponding aldehyde
with identical yield (94%) and TON (564) to those of the
unsubstituted alcohol (run 13 vs. 4), whereas 4-methyl and
4-chlorobenzyl alcohols are oxidized less effectively (runs 12
and 14). The catalytic system is expected to involve the
coordination of alcohol (and TEMPO) to the copper centre,
deprotonation of alcohol by base and b-hydrogen abstraction
from the benzyloxide ligand by TEMPO resulting in a radical-
TEMPOH-copper species. Intramolecular one-electron transfer
from the radical to CuII leads to the aldehyde, TEMPOH and
CuI species. The initial CuII complex is regenerated by the
TEMPO-mediated oxidation of CuI to CuII, whereas TEMPO
is regenerated by the aerobic oxidation of TEMPOH.4,10,11
general trend for alkoxo-bridged CuII complexes, displaying
antiferromagnetic couplings for large Cu–O–Cu bridging angles
and ferromagnetic couplings for the angles smaller than 99.81.
The strategy used in this study deserves to be extended to
the synthesis of other carboxyl functionalized AHBD and
derived water soluble complexes to be applied as convenient
‘‘dual role catalyst’’ precursors in green catalysis, under acid-free
aqueous conditions.
Experimental
Materials and instrumentation
1D [1H, 13C{1H}] NMR spectra were recorded on Bruker
Avance II+ 300.130 (UltraShieldt Magnet) and Bruker
Avance II+ 400 MHz (UltraShield Magnet) spectrometers
at ambient temperature using tetramethylsilane as an internal
reference. Infrared spectra (4000–400 cmꢀ1) were recorded on
a BIO-RAD FTS 3000MX instrument in KBr pellets. Carbon,
hydrogen, and nitrogen elemental analyses were carried out by
Microanalytical Service of Instituto Superior Tecnico. The
´
synthetic work of ligand 1 and copper complex 3 was
performed in air and at room temperature, whereas the mono-
nuclear complex 2 was synthesized in flame-dried glassware
under a dinitrogen atmosphere by using inert gas flow and
vacuum-line techniques. All reagents were obtained from
commercial sources (Aldrich), whereas methanol was purified
by conventional distillation over magnesium powder. Chromato-
graphic analyses were undertaken by using a Fisons Instruments
GC 8000 series gas chromatograph (He as carrier gas) with
a DB-624 (J&W) capillary column (FID detector) and the
Jasco-Borwin v.1.50 software, or a Perkin Elmer Clarus 500
gas chromatograph (He as carrier gas, FID) with a BP20
capillary column (SGE). Internal standards (nitromethane and
acetophenone) were used to quantify the organic products in
the catalytic experiments. Magnetic susceptibility measure-
ments on a polycrystalline sample were carried out at the
temperature range of 55–310 K, by means of a Quantum
Design SQUID magnetometer operating with an applied
magnetic field of 10.000 Oe. Diamagnetic corrections of the
constituent atoms were estimated from Pascal’s constants.
Conclusions
A new carboxyl containing arylhydrazone of pentane-2,4-dione,
i.e. 3-(2-hydroxy-4-carboxyphenylhydrazone)pentane-2,4-dione
(H3L, 1), is shown to display a good synthetic potential towards
complex formation with copper(II), acting as a suitable ligand to
create water soluble CuII complexes (2 and 3).
It is established that 2 and 3 act as active catalysts for
oxidation processes, particularly for the peroxidative oxidation,
in NCMe/H2O and under mild conditions, of cyclohexane to
the mixture of cyclohexyl hydroperoxide, cyclohexanol and
cyclohexanone, as well as for the selective oxidation, in water,
of benzyl alcohols to the corresponding benzaldehydes. In the
alkane peroxidative oxidations, 2 and 3 behave as ‘‘dual role
catalysts’’ combining, in one molecule, an active metal centre
and an acidic promoting group, thus providing a high activity
of the system even without any acid promoter. The magnetic
study reveals that the magnetic behaviours of 3 follow the
c
1652 New J. Chem., 2012, 36, 1646–1654
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012

View Online
Experimental susceptibilities were also corrected for the tempera-
ture-independent paramagnetism [60 ꢂ 10ꢀ6 cm3 molꢀ1 per CuII]
and the magnetization of the sample holder.
Synthesis of [Cu2(CH3OH)2(l-HL)2] (3)
To a 50 mL methanol solution of 1 (0.0264 mg, 0.1 mmol),
0.0233 mg (0.1 mmol) of Cu(NO3)2ꢁ2.5 H2O was added. The
mixture was stirred for 5 min and left for slow evaporation.
Dark brown crystals started to form in the reaction mixture
after five days at room temperature; they were filtered off and
dried in air. Yield 47% (based on Cu). Calcd. for C26H28Cu2-
N4O12 (M = 715.6): C, 43.64; H, 3.94; N, 7.83; Found: C,
43.36; H, 3.76; N, 7.84%. MS (ESI): m/z: 651 [M ꢀ 2CH3OH +
H+]. IR, cmꢀ1: 3448 n(OH), 1672 and 1638 n(CQO), 1629
d(OH), 1594 n(CQN).
Synthesis of H3L (1)
Diazotization. A 3.825 g, 0.025 mol portion of 2-hydroxy-4-
carboxyaniline was dissolved in 50 mL of water upon addition
of 1.000 g of crystalline NaOH. The solution was cooled in an
ice bath to 0 1C and 1.725 g, 0.025 mol of NaNO2 was added
with subsequent addition of 5 mL of 33% HCl in portions of
0.2 mL for 1 h under vigorous stirring. During the reaction
the temperature of the mixture must not exceed 278 K. A suspen-
sion of the unstable 2-hydroxy-4-carboxyphenyldiazonium chloride
[NRN{CC(OH)CHC(COOH)CHCH}]+Clꢀ was obtained which
then was used as such for the next stage.
X-Ray structure determination
Crystals of 2 and 3 were grown by slow evaporation at room
temperature of DMF (2) and methanol (2) solutions, respec-
tively. 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 diffracto-
meter with graphite monochromated Mo–Ka (l = 0.71073)
radiation. Data were collected using omega scans of 0.51 per
frame and full sphere of data were obtained. Cell parameters
were retrieved using Bruker SMART software and refined using
Bruker SAINT12a on all the observed reflections. Absorption
corrections were applied using SADABS.12a Structures were
solved by direct methods by using the SHELXS-97 package12b
and refined with SHELXL-97.12c Calculations were performed
using the WinGX System-Version 1.80.03.12d All hydrogen
atoms were inserted at calculated positions. Crystallographic
details are listed in Table S1 (ESI).w
Azocoupling. 1.000 g, 0.025 mol NaOH were added to a
mixture of 2.55 mL, 0.025 mol of pentane-2,4-dione with
50 mL of water–ethanol (1 : 1, v/v). The solution was cooled
on an ice bath, and a suspension of 2-hydroxy-4-carboxyphenyl-
diazonium chloride was added in two equal portions under
vigorous stirring for 1 h. On the next day, the formed precipitate
of 3-(2-hydroxy-4-carboxyphenylhydrazone)pentane-2,4-dione (1)
was filtered off, washed with water, recrystallized from ethanol
and dried in air. The identification of 1 was carried out by IR,
1
element analysis, H and 13C NMR spectroscopies.
Yield 87.0% (based on pentane-2,4-dione), dark brown
powder, soluble in ethanol, acetone, THF and methanol,
and insoluble in water and chloroform. Calcd. for
C12H12N2O5 (M = 264.2): C, 54.55; H, 4.58; N, 10.60; Found:
C, 54.52; H, 4.60; N, 10.56%. IR, cmꢀ1: 3537 (s, br) n(OH),
3468 (s, br) n(OH), 3083 (s, br) n(NH), 1731 and 1669 (s)
n(CQO), 1635 (s) n(CQOꢁ ꢁ ꢁH), 1631 d(OH), 1605 (s)
Catalytic activity studies
Peroxidative oxidation of alkanes. The alkane oxidations
were typically carried out in air in thermostated (50 1C) Pyrex
cylindrical vessels or round bottom flasks with vigorous
stirring and using MeCN as solvent (up to 5.0 mL total
volume). Typically, the catalyst precursor 2 or 3 was
introduced into the reaction mixture as a solid or in the form
of a stock solution in aqueous (10% vol) acetonitrile (7.5 or
1.25 ꢂ 10ꢀ4 mol Lꢀ1, respectively, for 2 or 3). Then TFA
(optional) was added in the form of a stock solution in MeCN
(0.44 mol Lꢀ1). The alkane substrate, typically cyclohexane
(0.25 mL, 2.3 mmol) was then introduced, and the reaction
started when hydrogen peroxide (50% in H2O, 0.68 mL,
11 mmol) was added in one portion. The final concentrations
of the reactants in the reaction mixture were as follows:
catalyst precursor 2 or 3 (30 ꢂ 10ꢀ4–5 ꢂ 10ꢀ5 mol Lꢀ1),
TFA (0–0.01 mol Lꢀ1), substrate (0.46 mol Lꢀ1) and H2O2
(2.2 mol Lꢀ1). The reactions were stopped by cooling and
analyzed by GC. Attribution of peaks was made by compar-
ison with chromatograms of authentic samples. Nitromethane
(0.05 mL) was used as GC internal standard.
1
n(CQN). H NMR (300 MHz, DMSO-d6, Me4Si): d = 2.27
(s, 3H, CH3), 2.45 (s, 3H, CH3), 7.47–7.64 (m, 3H, Ar–H),
11.81 (s, 1H, OH), 14.56 (s, 1H, NH). 13C{1H} (75.468 MHz,
DMSO-d6, Me4Si): d = 26.49 (CH3), 31.27 (CH3), 114.13
(Ar–H), 116.48 (Ar–H), 121.40 (Ar–COOH), 128.84 (Ar–H),
132.72 (CQN), 134.02 (Ar–NH–N), 146.08 (Ar–OH), 167.14
(CQO), 196.36 (CQO), 196.75 (CQO).
Synthesis of [Cu(H2O)((CH3)2NCHO)(HL)] (2)
Addition of solid Cu(NO3)2 (159.2 mg, 0.68 mmol) to a stirred
suspension of 1 (181.5 mg, 0.68 mmol) in 30 mL of dry
methanol at room temperature (20 1C) gave a brown precipitate.
The mixture was further stirred under nitrogen for 1 h. Solid
Bu2SnO (170.1 mg, 0.68 mmol) was then added and the reaction
mixture was left stirring vigorously overnight at 40 1C in an inert
atmosphere. The brown solid that formed was collected by
filtration, washed with cold methanol and dried under vacuum.
The crystals of 2 suitable for X-ray structural analysis were
obtained by slow evaporation of a DMF solution of the
crude brown solid. Yield 47% (based on Cu). Calcd. for
C15H19CuN3O7 (M = 416.87): C, 43.22; H, 4.59; N, 10.08;
Found: C, 43.06; H, 4.46; N, 10.04%. MS (ESI): m/z: 418
[M + H+]. IR, cmꢀ1: 3259 n(OH), 1676 n(CQO), 1645
n(CQO), 1596 n(CQN).
Since the oxygenation of alkanes usually gives rise to the
formation of the corresponding alkyl hydroperoxides as the
main primary products, the quantification was performed by a
method developed by Shul’pin.9a–c For precise determination
of oxygenate concentrations only data obtained after reduction
of the reaction sample with PPh3 were usually used, taking into
account that the original reaction mixture typically contained
c
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012
New J. Chem., 2012, 36, 1646–1654 1653

View Online
the three products: alkyl hydroperoxide (as the primary product),
ketone and alcohol.
J. Mol. Catal. A: Chem., 2006, 251, 200; (d) R. A. Sheldon and
I. W. C. E. Arends, Adv. Synth. Catal., 2004, 346, 1051;
(e) F. Minisci, F. Recupero, G. F. Pedulli and M. Lucarini,
J. Mol. Catal. A: Chem., 2003, 204–205, 63.
Aerobic oxidation of alcohols. The reactions were carried out
in 50 mL round-bottom flasks equipped with condensers under
atmospheric pressure of air. Typically, to a 10 mL of 0.1 mol Lꢀ1
aqueous solution of K2CO3 were added 3.0 mmol of alcohol,
0.03 mmol (1 mol% vs. substrate) of catalyst 2 or 3, and
0.15 mmol (5 mol% vs. substrate) of TEMPO. The reaction
solutions in all cases were vigorously stirred using magnetic
stirrers, and an oil bath was used to achieve the desired reaction
temperature. After the oxidation reaction, reaction mixtures were
neutralized by 1 mol Lꢀ1 HCl, and then extracted with 10 mL of
ethylacetate. The organic phase was used for chromatographic
analysis using acetophenone as the internal standard.
6 (a) E. Coronado and P. Day, Chem. Rev., 2004, 104, 5419;
(b) L. Ouahab and T. Enoki, Eur. J. Inorg. Chem., 2004, 933;
(c) C. Faulmann, S. Dorbes, B. G. de Bonneval, G. Molnar,
A. Bousseksou, C. J. Gmez-Garcia, E. Coronado and L. Valade,
Eur. J. Inorg. Chem., 2005, 3261; (d) P. A. Vigato, V. Peruzzo and
S. Tamburini, Coord. Chem. Rev., 2009, 253, 1099; (e) G. Aromi,
P. Gamez and J. Reedijk, Coord. Chem. Rev., 2008, 252, 964.
7 (a) O. Kahn, Molecular Magnetism, VCH, New York, 1993;
(b) V. H. Crawford, H. W. Richardson, J. R. Wasson, D. J.
Hodgson and W. E. Hatfield, Inorg. Chem., 1976, 15, 2107;
(c) E. Ruiz, P. Alemany, S. Alvarez and J. Cano, J. Am. Chem.
Soc., 1997, 119, 1297; (d) A. M. Kirillov, Y. Y. Karabach,
M. Haukka, M. F. C. Guedes da Silva, J. Sanchiz,
M. N. Kopylovich and A. J. L. Pombeiro, Inorg. Chem., 2008,
47, 162.
8 (a) A. M. Kirillov, M. N. Kopylovich, M. V. Kirillova,
M. Haukka, M. F. C. Guedes da Silva and A. J. L. Pombeiro,
Angew. Chem., Int. Ed., 2005, 44, 4345; (b) K. R. Gruenwald,
A. M. Kirillov, M. Haukka, J. Sanchiz and A. J. L. Pombeiro,
Dalton Trans., 2009, 2109; (c) M. V. Kirillova, A. M. Kirillov,
M. L. Kuznetsov, J. A. L. Silva, J. J. R. Frausto da Silva and
A. J. L. Pombeiro, Chem. Commun., 2009, 2353; (d) M. V.
Kirillova, A. M. Kirillov, D. Mandelli, W. A. Carvalho, A. J. L.
Pombeiro and G. B. Shul’pin, J. Catal., 2010, 272, 9;
(e) G. B. Shul’pin, J. Gradinaru and Y. N. Kozlov, Org. Biomol.
Chem., 2003, 1, 3611; (f) M. V. Kirillova, Y. N. Kozlov,
L. S. Shul’pina, O. Y. Lyakin, A. M. Kirillov, E. P. Talsi, A. J. L.
Pombeiro and G. B. Shul’pin, J. Catal., 2009, 268, 26; (g) A. M. Kirillov,
M. V. Kirillova, L. S. Shul’pina, P. J. Figiel, K. R. Gruenwald,
M. F. C. Guedes da Silva, M. Haukka, A. J. L. Pombeiro and
G. B. Shul’pin, J. Mol. Catal. A: Chem., 2011, 350, 26.
Acknowledgements
This work has been partially supported by the Foundation
for Science and Technology (FCT), Portugal, and its PTDC/
QUI-QUI/102150/2008 and PEst-OE/QUI/UI0100/2011
projects and ‘‘Science 2007’’ program. M.N.K., M.J.G., K.T.M.,
M.V.K. and P.J.F. express gratitude to the FCT for their working
contract and post-doc fellowships. The authors gratefully acknow-
ledge the Portuguese NMR Network (IST-UTL Center) for
providing access to the NMR facility.
References
9 (a) G. B. Shul’pin, Mini-Rev. Org. Chem., 2009, 6, 95;
(b) G. B. Shul’pin, J. Mol. Catal. A: Chem., 2002, 189, 39;
(c) G. B. Shul’pin, C. R. Chim., 2003, 6, 163; (d) A. M. Kirillov,
M. N. Kopylovich, M. V. Kirillova, Y. Y. Karabach, M. Haukka,
M. F. C. Guedes da Silva and A. J. L. Pombeiro, Adv. Synth.
Catal., 2006, 348, 159; (e) G. B. Shul’pin, Y. N. Kozlov,
L. S. Shul’pina, A. R. Kudinov and D. Mandelli, Inorg. Chem.,
2009, 48, 10480; (f) G. B. Shul’pin, Y. N. Kozlov, L. S. Shul’pina
and P. V. Petrovskiy, Appl. Organomet. Chem., 2010, 24, 464.
1 E. V. Shchegol’kov, Y. V. Burgart, O. G. Khudina, V. I. Saloutin
and O. N. Chupakhin, Russ. Chem. Rev., 2010, 79, 31.
2 (a) M. N. Kopylovich, K. T. Mahmudov, M. F. C. Guedes da Silva,
M. L. Kuznetsov, P. J. Figiel, Y. Y. Karabach, K. V. Luzyanin and
A. J. L. Pombeiro, Inorg. Chem., 2011, 50, 918; (b) M. N. Kopylovich,
A. C. C. Nunes, K. T. Mahmudov, M. Haukka, T. C. O. Mac Leod,
L. M. D. R. S. Martins, M. L. Kuznetsov and A. J. L. Pombeiro,
Dalton Trans., 2011, 40, 2822; (c) M. N. Kopylovich, T. C. O. Mac
Leod, K. T. Mahmudov, M. F. C. Guedes da Silva and A. J. L.
Pombeiro, Dalton Trans., 2011, 40, 5352.
3 (a) K. H. Shaughnessy, Chem. Rev., 2009, 109, 643; (b) P. T. Anastas
and M. M. Kirchhoff, Acc. Chem. Res., 2002, 35, 686;
(c) D. G. Blackmond, A. Armstrong, V. Coombe and A. Wells,
Angew. Chem., Int. Ed., 2007, 46, 3798; (d) Organic Reactions in
Water: Principles, Strategies and Applications, ed. U. M. Lindstrom,
Blackwell Publishing, Oxford, 2007.
4 (a) A. E. Shilov and G. B. Shul’pin, Chem. Rev., 1997, 97, 2879;
(b) G. B. Shul’pin, in Oxidations of C–H Compounds Catalyzed by
Metal Complexes, in Transition Metals for Organic Synthesis,
ed. M. Beller and C. Bolm, Wiley–VCH, Weinheim/New York, 2nd
edn, 2004, pp. 215–242, vol. 2; (c) H. Fiege, Ullmann’s Encyclopedia of
Industrial Chemistry, Wiley-VCH, Weinheim, 6th edn, 2002.
10 (a) P. J. Figiel, A. Sibaouih, J. U. Ahmad, M. Nieger, M. T. Raisanen,
¨
¨
M. Leskela and T. Repo, Adv. Synth. Catal., 2009, 351, 2625;
¨
(b) P. J. Figiel, M. Leskela and T. Repo, Adv. Synth. Catal., 2007,
349, 11739.
¨
11 (a) P. Gamez, I. W. C. E. Arends, R. A. Sheldon and J. Reedijk,
Adv. Synth. Catal., 2004, 346, 805; (b) P. Gamez, I. W. C.
E. Arends, J. Reedijk and R. A. Sheldon, Chem. Commun., 2003,
2414; (c) L. Lin, M. Juanjuan, J. Liuyan and W. Yunyang, Catal.
Commun., 2008, 9, 1379.
12 (a) Bruker, APEX2 & SAINT. Bruker, AXS Inc., Madison, Wisconsin,
USA, 2004; (b) G. M. Sheldrick, Acta Crystallogr., Sect. A: Found.
Crystallogr., 1990, 46, 467; (c) G. M. Sheldrick, Acta Crystallogr., Sect.
A: Found. Crystallogr., 2008, 64, 112; (d) L. J. Farrugia, J. Appl.
Crystallogr., 1999, 32, 837; (e) A. L. Spek, Acta Crystallogr., Sect. A:
Found. Crystallogr., 1990, 46, C34.
5 (a) T. Vogler and A. Studer, Synthesis, 2008, 1979; (b) M. V. N. De
Souza, Mini-Rev. Org. Chem., 2006, 3, 155; (c) R. A. Sheldon,
c
1654 New J. Chem., 2012, 36, 1646–1654
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012